Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals
The review and some general comments devoted to scientific problems of high temperature wetting and adhesion in systems liquid metals—solid ceramic materials were done including the analysis of data received in a last time. The theoretical treatment of high temperature wetting processes and mechanis...
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Інститут проблем матеріалознавства ім. І.М. Францевича НАН України
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irk-123456789-1254582017-10-28T03:03:49Z Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals Naidich, Y.V. The review and some general comments devoted to scientific problems of high temperature wetting and adhesion in systems liquid metals—solid ceramic materials were done including the analysis of data received in a last time. The theoretical treatment of high temperature wetting processes and mechanism based on traditional thermodynamic notions and new approach on atomic and atomic-electron level were developed. As ceramics various substrates namely substrates with predominated ionic interatomic bonds (oxides for example) which usually hardly wetted by liquid metals are considered first of all. The contribution of nonequilibrium and equilibrium parts of adhesion to wetting process are considered and analysed. The ”active“ metals viz metals with high enough chemical affinity for solid phase atoms or iones were divided on transition metals (d-electron orbitals are partly occupied by electrons) and nontransition metals (dorbitals are empty or completely full by electrons). Зроблено огляд та подані деякі загальні коментарі, що стосуються наукових проблем високотемпературного змочування та адгезії у системах рідкі метали—тверді керамічні матеріали, включаючи аналіз одержаних останнім часом даних. Розроблені теоретична трактовка процесів високотемпературного змочування та механізм, що базується на традиційних термодинамічних уявленнях, і також новий підхід на атомному та атомно-електронному рівні. У першу чергу як кераміка розглянуті різні субстрати, а саме, субстрати, де переважають іонні міжатомні зв’язки (наприклад, оксиди), котрі звичайно погано змочуються рідкими металами. Розглянуто й проаналізовано внесок у процес змочування нерівноважної та рівноважної складових адгезії. “Активні” метали, а саме, метали з досить високою хімічною спорідненістю до атомів твердої фази або іонів розділені на перехідні метали (d-електронні орбіталі частково заповнені електронами) та неперехідні метали (d-орбіталі не заповнені або цілком заповнені електронами). Сделан обзор и представлены некоторые общие комментарии, касающиеся научных проблем высокотемпературной смачиваемости и адгезии в системах жидкие металлы-твердые керамические материалы, включая анализ полученных в последнее время данных. Разработаны теоретическая трактовка процессов высокотемпературной смачиваемости и механизм, основанный на традиционных термодинамических представлениях, и новый подход на атомном и атомно-электронном уровне. В первую очередь в качестве керамики рассмотрены различные субстраты, а именно, субстраты с преобладающими ионными межатомными связями ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 53 (например, оксиды), которые обычно плохо смачиваются жидкими металлами. Рассмотрен и проанализирован вклад в процесс смачивания неравновесной и равновесной составляющих адгезии. “Активные” металлы, а именно, металлы с достаточно высоким химическим сродством к атомам твердой фазы или ионам разделены на переходные металлы (d-электронные орбитали частично заполнены электронами) и непереходные металлы (d-орбитали не заполнены или полностью заполнены электронами). 2013 Article Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals / Y.V. Naidich // Адгезия расплавов и пайка материалов. — 2013. — Вып 46. — С. 3-62. — Бібліогр.: 75 назв. — рос. 0136-1732 http://dspace.nbuv.gov.ua/handle/123456789/125458 532.64:666.3/7:669 en Адгезия расплавов и пайка материалов Інститут проблем матеріалознавства ім. І.М. Францевича НАН України |
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The review and some general comments devoted to scientific problems of high temperature wetting and adhesion in systems liquid metals—solid ceramic materials were done including the analysis of data received in a last time. The theoretical treatment of high temperature wetting processes and mechanism based on traditional thermodynamic notions and new approach on atomic and atomic-electron level were developed. As ceramics various substrates namely substrates with predominated ionic interatomic bonds (oxides for example) which usually hardly wetted by liquid metals are considered first of all. The contribution of nonequilibrium and equilibrium parts of adhesion to wetting process are considered and analysed. The ”active“ metals viz metals with high enough chemical affinity for solid phase atoms or iones were divided on transition metals (d-electron orbitals are partly occupied by electrons) and nontransition metals (dorbitals are empty or completely full by electrons). |
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Naidich, Y.V. |
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Naidich, Y.V. Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals Адгезия расплавов и пайка материалов |
| author_facet |
Naidich, Y.V. |
| author_sort |
Naidich, Y.V. |
| title |
Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals |
| title_short |
Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals |
| title_full |
Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals |
| title_fullStr |
Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals |
| title_full_unstemmed |
Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals |
| title_sort |
advance in the theory of ceramics/liquid metal systems wettability. peculiarity of contact processes for transition and non—transition metals |
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Інститут проблем матеріалознавства ім. І.М. Францевича НАН України |
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2013 |
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http://dspace.nbuv.gov.ua/handle/123456789/125458 |
| citation_txt |
Advance in the theory of ceramics/liquid metal systems wettability. Peculiarity of contact processes for transition and non—transition metals / Y.V. Naidich // Адгезия расплавов и пайка материалов. — 2013. — Вып 46. — С. 3-62. — Бібліогр.: 75 назв. — рос. |
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Адгезия расплавов и пайка материалов |
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AT naidichyv advanceinthetheoryofceramicsliquidmetalsystemswettabilitypeculiarityofcontactprocessesfortransitionandnontransitionmetals |
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2025-07-09T03:11:53Z |
| last_indexed |
2025-07-09T03:11:53Z |
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1837137359001354240 |
| fulltext |
УДК 532.64:666.3/7:669
Y. V. Naidich*
ADVANCE IN THE THEORY OF CERAMICS/LIQUID METAL
SYSTEMS WETTABILITY. PECULIARITY OF CONTACT
PROCESSES FOR TRANSITION AND NON—TRANSITION
METALS
Abstract
The review and some general comments devoted to scientific problems
of high temperature wetting and adhesion in systems liquid metals—solid
ceramic materials were done including the analysis of data received in a
last time. The theoretical treatment of high temperature wetting processes
and mechanism based on traditional thermodynamic notions and new
approach on atomic and atomic-electron level were developed. As
ceramics various substrates namely substrates with predominated ionic
interatomic bonds (oxides for example) which usually hardly wetted by
liquid metals are considered first of all.
The contribution of nonequilibrium and equilibrium parts of adhesion
to wetting process are considered and analysed.
The ”active“ metals viz metals with high enough chemical affinity for
solid phase atoms or iones were divided on transition metals (d-electron
orbitals are partly occupied by electrons) and nontransition metals (d-
orbitals are empty or completely full by electrons).
* Ю. В. Найдіч — професор, завідуючий відділом, Інститут проблем
матеріалознавства ім. І. М. Францевича НАН України, Київ.
Ю. В. Найдіч, 2013
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 4
The wettability in first case (transition metals) is much more
intensive than that for nontransition metals inspite of thermodynamic
factors (chemical affinity of metal for atoms of solid phase are closed).
Many experimental data illustrate these regularity.
The Ligand field theory is proposed to explane and to give treatment
on electron level superiority of transition metals in adhesion activity.
An adhesion activity of nontransition metals to solid substrates
(oxides) is much low or moderate and dewetting phenomena are
frequently observed in this case. This especial feature is explained by
formation at the interface nonstable of intermediate valency compounds
which can be subjected to reaction of disproportionation.
Some practical recommendations related to regulation and governing
wetting processes in various technological procedures were done.
Keywords: high temperature wettability, adhesion, ceramics, transition
and nontransition metals, d-electron orbitals and ligand field theory,
dewetting, reaction of disproportionation.
Introduction
The wettability of ceramic materials by liquid metals is important factor
for high technology development in various area of advanced materials
manufacture including composites processing, ceramic—to—metal
joining, many metallurgical problems. The wettability, as a part of
surface and interface phenomena is important for developing of
nanosystems science.
Ceramics (ionic or ionocovalent substrates) — metal systems as the
most contrasted in viewpoint of difference in interatomic bonds nature in
contact pair are of special.
The wettability and adhesion at liquid metal—ceramic interfaces are
intensively studied in the last thirty or forty years. The data received
before 1980 are discussed in the review [5]. In the last time numerous
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 5
new data were obtained, which were considered at the International
Conferences on High Temperature Capillarity (HTC-94, -97, -2000, -
2004, -2007, -2009, -2012) and at the NATO workshop (1997).
That demands further development of a theoretical concept
describing high temperature wettability processes, taking into account
that a number of wettability process peculiarities remain not clear or
hardly understandable and debatable; and in the last HTC-international
conferences and in the most publications generalized review or special
invited lectures devoted to the problem questions of the high temperature
capillarity discipline are practically absent.
The aim of this paper is to consider some key questions in the theory
of high temperature wettability and interface structure for liquid metal-
solid nonmetal substrates and make further step to the better
understanding of high temperature wettability phenomena. The new
approach is proposed to consideration of interface processes and
adhesion mechanism including treatment of wettability process on
atomic—electron level.
Some general comments
The classic thermodynamics of surface phenomena [1—4] is a
background for high temperature wettability processes.
Among the interface parameters which describe the contact systems
solid-liquid-gas, namely surface energies at the boundary
liquid—gas σlg
solid—gas σsg
liquid—solid σls,
only liquid—gas surface tension σlg and contact angle θ can be measured
experimentally with sufficient accuracy. As to σsg and σls, we have to
recognize that numerous attempts to obtain them were in the whole not
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 6
successful in the case of rigid solids, and now, unfortunately, in the most
scientific works no attention is paid to this important question.
The difficulty in obtaining of solid (rigid!) surface energy consists in
not only absence of accurate reliable experimental method but also in
complexity of the very notion of the surface energy of solids. A solid
phase barierred by surface in the most cases is not in the state of
thermodynamic equilibrium (in difference from liquid phase). Also we
must account the specific solid surface structure, phenomena of
relaxsation of surface atoms after surface arised, formation of specific
surface crystal lattice and symmetry differented from bulk structure [23,
24]. Now these processes are under intensive study — theoretically (ab
initio calculations) [26, 28] and experimentally — e. g. Low energy
electronic diffraction method (see foot-note as well in page 5).
As to the experimental methods of solid surface energy obtaining by
microscopic studies of the morphology of an interface area closed to
triple junction liquid—solid—gas with dihedral angle measurements (or
so colled ”sine rule“) like in earlier Kingery's team works (see e. g. [45])
in which the measurements of σsg for Al2O3 and ZrO2 ceramics and
interfacial energies of these ceramics contacted to liquid Ni were carried
out or in the last separated works [61—63], we can say that all these
measurements have drawbacks inasmuch as we cannot have garantee,
that equilibrium state of contact system is reached and interface solid/gas
remains flat up to the triple line of liquid drop (this late relates especially
to [62, 63]).
Nevertheless the scientific works in this important direction (solid
surface energy obtaining) should be continued. Perhaps it is necessary to
performe systematic investigations of separated stage of complicated
methodology for measuring the surface energy of solid body.
Some examples of such investigations can be the works [64, 65]
where dihedral angles formation processes were studied experimentally
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 7
and analyzed for couples of metalsolid — metalliquid (policrystal Ni and Pb,
and bicrystal Mo — NiMo alloy [64]. Such systems are of more simple
(it is lightly to reach equlibrium state) than ceramic – metal systems. But
even in these (metal/metal) systems (Fe/Mn—Cu [65]) surface energy of
solid substrate could be evaluated with the aid of some theoretical
calculations.
At the same time σlg and contact angle θ values allow to calculate
work of adhesion using Young’s equation
WA = σж (1 + cosθ). (1)
It is work of adhesion can be related directly to solid—liquid
attraction forces and bonding energy. WA value is always positive, some
attraction between solid and liquid always exists. So called “work of
immersion”: product σlg ⋅ cosθ (really “adhesion tension” — the
parameter and terminology introduced by Freindlich in 1924 to describe
the behaviour of a liquid in capillary space), is sign changed value and,
we should consider, not suitable to characterize the liquid—solid
energetic processes of bonding and interaction as it proposed to be
considered in some works [6—8].
But of course we should note that due to complication of atomic
structure of a solid surface, particularly the “relaxation” of the surface
atoms after surface is formed, in some theoretical works (ab initio
calculations) correction of WA is performed by introduce definition
WA-separation (Wsep). In the last time both WA values are calculated (see
first of all Finnis works [27, 28].
There are many difficulties when transferring the classical capillarity
concepts, which are stated on and operate in low temperature
equilibrium systems, to high temperature reactive systems area. For
nonequilibrium systems and nonreversible contact processes in many
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 8
cases one can believe that the equilibrium (pcevdoequilibrium) is reached
in narrow interface region and thermodynamic consideration can be
applied. *
Ceramics are usually badly wetted by liquid metals, substances with
high values of surface tension, which cannot be overcome to spread by
weak physical van der Waals liquid—solid interaction. A commonly
accepted point of view is that a good enough wettability is realized in
contact systems which are deviated from equilibrium state and
conditioned by chemical forces, chemical interface reactions. Historically
this assertion was pointed out in different forms by many authors:
A. Levin (1952 [9]), Kingery, (1956 [10]), our investigation (1954—
1958 [11, 12]), Jordan and Lane (1962 [13]), Zhuchovitsky (1964 [14]),
Ono and Kondo (1960 [15]) , Aksay, Hoge, Pask (1974 [16]).
A classification of the contact systems liquid metal/solid, dividing
them on equilibrium and nonequilibrium ones and analyses of wetting
process in each type of the systems were done in [12] (1968). According
to [12] for any contact systems
WA = WA nonequil + WA equil, (2)
where WA nonequil = ( )S
i
L
if µ−µ ; SL
i
,µ is chemical potential of i-
component in liquid and solid phases, and S
i
L
i µ≠µ for nonequlibrium
contact systems. WA equil is function of difference in nature and
properties of phases, which are in contact: PL and PS,
WA equil = ϕ( PL − PS); the difference of phase properties is the very
reason for positive interface tension. WA = is equal WA equil for the case
when the phase L a phase S are closed on chemical nature and are in
* The thermodynamic of nonequilibrium processes can be used if system deviates not
far from equilibrium state (linear Onzager equations); But these theory is developed
not enough for use it in real contact systems.
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 9
conditions of thermodynamic equilibrium at their contact (of course
temperature and pressure in each phases mean to be equal). *
Chemical interface reaction gives directly an energetic contribution to
wettability and adhesion. This last assertion is sometimes debatable and
only the role of a new intermediate phase which are formed at the
interface is declared [17—19]. But free energy of chemical reaction is
identically equal to the energy of chemical bonds established (in our case
interfacial bonds). So this assertion as a matter of fact is evident and
confirmed by many experimental data, namely by the correlations
between wetting and energy change ∆G (Gibbs potential change when
reaction occurs) for various systems. Let us consider some examples.
On Fig. 1 data for simple (monocomponent) solid (carbon—
graphite)/liquid metal systems wettability are presented [25]. The
wettability was measured for alloys, containing transition metals Ti, Ta,
Nb, V, Cr, Mn dissolved in Au, and Ge. The enumerated elements are
characterized by high enough (but different!) chemical affinity for carbon
and form carbide layers at the interface.
There is a good correlation between wettability and ∆G for reaction
of carbide formation Me + C → MeC + ∆G. The more ∆G value the
better wettability degree. (Action of nonequilibrium, reactive
contribution to WA value). Separate deviations from strict succession in
∆G — contact angle dependence can be explained by influence of
different thermodynamic activity of elements Ti, Ta, V, Nb, Cr, Mn in a
solution of these metals in Au, Ge. The data for thermodynamic activity
in many cases are not known, but cunsideration of corresponding phase
diagrams of alloys allows to remove some contradiction.
* For values of PL and PS various characteristics of liquid and solid phass (tipe of
chemical bonds — e. g. share of ionicity of bonds, electronegatiuty, electrocouductivity,
equlibrium concentration of one of components at given temperature of phase diagram,
e. c.) can be taken.
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 10
Fig. 1. Wettability of graphite by Au—melts (a) and Ge—melts (b):
concentration dependencies of contact angle. T = 1200 °C. Gibbs
potential changes when metal—carbon reaction occurs and carbides are
formed -∆GkJ/g-at C: for Mn (3,5), Cr (12), V (27), Nb (32),
Ta (36), Ti (47)
0 10 20 30 40 60 80
0
30
60
90
120
150
% (at.)
Θo
Nb
Ti
Ta
V
Cr Mn
Ge
Mn,Cr,V,
Nb,Ta,Ti
0 10 20 30 60 70 80
0
25
50
75
100
125
150
% (at.)
Θo
V
NbTa
Ti
Cr
Mn
Au
Mn,Cr,V,
Nb,Ta,Ti
a
b
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 11
(Thermodynamic activity of Mn in Mn—Au alloys is available, and aMn
at concentration region 0 ≅ 30% (at.) is very low and corresponding
curve for Mn in Fig. 1, a will be really displaced to left and down in
Fig. 1, but succession of ∆Go values for different metals and corellation
wettability — ∆Go will be nevertheless observed).
It should be added that adhesiactive elements investigated form at the
interface alloy/graphite the layer of metallike carbides, which itself can
be wetted by liquid metals (action of equilibrium part in total WA). But in
this case it is hard to find the correlation (if it do exists) between
wettability degree and properties of corresponding carbide. As a rule
thermodynamically stable carbides (like TiC) are wetted worse than
chromium or tungsten carbides.
Another example clearly shows important role of reactive
nonquilibrium part of WA (see table 1 [25, p. 62—64]).
The metals of VIII group of periodic systems — Ni, Co, Fe and Pt,
Pd wet graphite surface only due to dissolution process of carbon in these
metals.
The dissolution of carbon in liquid Ni and other enumerated metals is
chemical process, chemical reaction and while it occurs high enough
wetting degree takes place. The metals saturated by carbon up to limit
concentration (at the given temperature conditions) don’t wet graphite
surfaces (θ > 90°). In this case the chemical potentials of carbon in liquid
and solid phase are equal and contribution of nonequilibrium term in the
total work of adhesion is zero. Ni, containing ~12% (at.) C
(T ~1500 °C) is hardly closed on properties to pure graphite so that
equilibrium term to WA total cannot be significant (it is possible — on
the level of van der Waals energy interaction).
The next point. It was revealed [58] that metal melt spreading (alloy
Cu46Sn4Ti30 and other Ti-containing melts) on graphite surface is very
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 12
T a b l e 1. Wettability of graphite by liquid metal—pure and
saturated by carbon [25]*
Liquid
metal
T, °C
θ°
Liquid
metal
saturated by
carbon,
% (at.)
T, °C
θ°
Fe
1550
37
Fe + 15,7 C
1550
107
Co 1500 48 Co + 15,5 C 1550 120
Ni 1500 45 Ni + 13 C 1550 115
Pd 1560 44 Pd + C** 1560 116
Pt
1800 87 — — —
* Experiments were performed by sessile drop method in high vacuum
with the aid of special set up, which allows to heat metal sepcimen and
graphite without contact of one to another to temperature we need and
then to place drop on graphite surface. Experiments were performed with
G. A. Kolesnichenko.
**Carbon concentration is closed to saturation one.
fast process. After the moment of liquid—solid contact occurs, wetting
angle dropping from 160—180° to 20—30° takes only ~10-2 sec
(experiments were performed with the aide of super high speed photo
camera: 1500—5000 frames/sec). In such case it is hard to expect
formation of 3D layer of titanium carbide. One can say about adsorption
process of titanium on the graphite surface. In [19] the process of
chemical adsorption of one of component of liquid phase on solid
substrate surface is treated as essentially different from chemical
interface reaction. But chemical bonds (including “adsorption” bonds)
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 13
are formed in both cases, and energy of these bonds gives direct
contribution to the total adhesion at the liquid/solid contact.
Further the most contrast systems as to nature of interatomic bonds
namely liquid metals and ionic solid phases (oxides with predominated
ionic bonds) will be considered.
In liquid metal—oxide contact systems when oxidation-reduction
interfacial reaction produces a layer of a new intermediate oxide (oxide
of liquid metal phase) the greatest wettability degree will take place for
oxides in which metallic properties are the most pronounced. In
particular for Ti-containing melts contacted to oxides Al2O3, MgO, SiO2
this is clearly illustrated experimentally [30]. Data are presented on
Table 2. If TiO oxide is formed at the interface (group I in the Table 2)
adhesion is the much more (circa by factor 2) than adhesion in case of
Ti2O3 formation (group II in the Table 2). Simple explanation of the fact
is as follows: TiO-oxide is the more metallike compound in comparison
with Ti2O3-oxide (last compound is closed to semiconductive substrate,
in which electroconductinity of metal type is manifested at high
temperature. The change of Gibbs potential for TiO formation
(-387,7 kJ/g.at.O) only some more than that for Ti2O3 (-380,0 kJ/g.at.O); so
that in this case one can talk about significant role in wetting process of
equilibrium part of total WA.
Nevertheless at consideration on the whole dependence of contact
angles on Ti concentration (system Cu—Ti/Al2O3) one can observe
intencive action of nonequilibrium, reactive factor as well. Moreover
there is possibility to divide the action of nonequilibrium and equilibrium
factor in wetting process. This illustrates Fig. 2, where wettability — Ti
concentration dependence is presented for systems CuTi—Al 2O3 and
CuTi—TiO1,14.
* For CuTi—Al2O3 total contact angle dropping when Ti-
* TiO1,14 oxide has been specially synthesised, this substrate is closed on composition to
TiO oxide.
ISSN 0136-1732. Адгезия расплавов и пайка материалов, 2013. Спецвып. 46 14
0 2 4 6 8
0
20
40
60
80
100
120
140
CuTi - TiO
1,14
CuTi - Al
2
O
3
Θ
o
Ti, % (at.)
Fig. 2. Wetting of Al2O3 and TiO1.14 by CuTi alloys, T = 1150 °C
concentration increases from 0 to 8% (at.) is of 129 – 15 = 114°, it is
caused by action of both factors—reactive and equilibrium ones.
For pure Cu contact angle wetting of TiO1,14 is 82° viz the contact
angle dropping (comparatively to Al2O3) is 129 – 82 = 47° and such a
contact angle change must be conditioned by action of equilibrium factor.
Further not so significant contact angle dropping as we can see on the
Fig. 2 for CuTi—TiO1,14 system can be explained by some remaining
reactivity (TiO1,14 contains some more oxygen than TiO) and changing
liquid phase composition from pure Cu to Cu—Ti alloys. We should pay
attention, that steepness of the curve of contact angle dropping for
reactive systems CuTi— Al2O3 in interval 0—2% (at.) Ti is much more
than that for Cu—TiO1,14 systems, closed to equilibrium one.
In sum as to Eustathopoulos point of view [17] that reactiv term role
in adhesion and wettability is negligible one can say that such assertion
cannot be accepted. Authors [17] insists that … “irreversible
contribution to wetting would be effective only in the case of very
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intensive reactions locallised at the triple line” (linear boundary liquid—
solid—gas). Such formulation causes many questions and containes
vagueness. For example: there are to much conditions for manifestation
of irreversible contribution to wetting process.
Besides author in [17] evaluate the action of reactive factor by
thickness of interface layer — Ti2O3 arised at the liquid/solid boundary in
system CuPd—Ti(15% (at.)) contacted to Al2O3, mullite, SiO2; 1475° K.
But it is clear that thickness of an intermediate layer in experiment [17]
and on the whole, say e. g. thickness of oxide phase on an solid metal
surface at oxidation of metal is ”kinetic“ factor, determined (excluding
may be for noble metals) by diffusion mobility of components in
compounds, structure of and defects in solids substrate (new and base
one) and cannot be cryterium of reactive, chemical ”ability“ of
components at the interface.
So at the end we need to take account of the both contributions to
wetting processes: nonequilibrium (reactive) contribution and
equilibrium one.
It should be noted that the processes considered above for Ti-
containing melts—oxide systems are especial interested. Titanium is a
transition metal, that is metal in which its atoms are characterized by
partly filled d-electron orbitals. Only transition metals can form the
oxides with significant share of metallic interatomic bonds and metallic
properties.
The question about the role of d-electron orbitals in the adhesion
processes for contact systems metal melt—solid oxides will be discussed
below in details on atomic—electron level.
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T a b l e 2. The work of adhesion at the interface of liquid—solid in
systems Ti — containing melts (2% (at.) Ti) — Al2O3, MgO, SiO2 and
chemical composition of interface layer [30]
Group
Systems
T, °C
WA,
mJ/m2
Interface
layer
Method of
identification
of interface
compound
Cu—Ti—Al2O3
1150
2100
TiO
x — ray*
Cu—Ti—MgO 1150 1894 TiO visual**
I
Cu—Ti—SiO2 1150 2185 TiO visual
Au—Ti—Al 2O3
1150
1100
Ti2O3
x — ray
Ni—Ti—Al 2O3 1500 1220 Ti2O3 x — ray
Ni—Mo—Ti—
Al 2O3
1500 <1500 Ti2O3 x — ray
II
Sn—Ti—SiO2 1150 867 Ti2O3 visual
* The identification of different Ti oxides was performed by investigation
of its crystallographic structure with the aid of x-ray debyegramms which
were received on sapphire and other oxides as solid phase. The seamples
of oxides were prepared in shape of cylindrical pivots (∅ ~1 mm, 10—
15 mm long) with well polished surface. Such pivots were immersed in
metal melt and then extracted from one. Temperature—time—high
vacuum regime was the same in the all manipulations. Due to not full
wettability of solid surface covered by TiO or Ti2O3 oxide layer by metal
melt (θ ~30—40°), after pivot extraction surface of seamples was bared
and “dry”. Then seample (cylindrical pivot) is placed in x-ray camera and
debyegramm was produced at revolving of seample.
** The colour of TiO and Ti2O3 is strongly different. So identification of
such oxides can be perfomed by visual observation as well.
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The atomic mechanism of interface bonds formation in metal
melts/solid substrates (mainly—oxide compounds)
The oxides surfaces* randomly oriented are formed mainly by oxygen
ions, which are of significantly more dimensions and higher
polarisability than that for cations.
When crystal is breaking on some plane the cations on surface due to
its low dimention and more mobility under action of force field of crystal
are displaced beneath the geometrical crystal surface level, in the depth
of crystal. It means that liquid metal at contact to oxide surface interacts
mainly with oxygen (anion) on oxide surface. All these assertions are
based on many works, beginning with Madelung [20], Dent [21], Weyl
[22], Adam [4]. Really the surface atoms due to lack of neighbors and
under action of force field of crystal are displaced from its position in a
bulk of crystal. These phenomena as mentioned above are known as
atomic relaxation and surface reconstruction (see e. g. [23, 24]).
The formation of interfacial strength bonds can be imaged of follows.
Terminal metal atoms of liquid phase contacting directly to solid
surface (oxides) have to participate simultaneously: a) in the ionic
bonds to the oxide, giving a part of their valent electrons to p-level of
oxygen in initial solid oxide (oxidation-reduction chemical process which
leads to formation of a new oxide, viz oxide of metal of liquid phase) and
b) in metallic bonds to other, more deeply positioned liquid metal phase
atoms.
This last bond will be weakened because of no participation in it of
electrons leaving terminal metal atoms and accepted by oxygen of solid
phase. The resultant bond at the interface liquid metal/solid oxide and
* We talk about so called ”common oxides“— like Al2O3, MgO (oxides of nontransition
metals). It is possible to enlarge these considerations on the other ionic or ionocovalent
substrates, viz galogenides (fluorides), some sulfides, soltlike compounds.
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wettability will be conditioned by weak link in the chaine: metal atom in
the depths of melt — terminal metal atom — oxygen in solid oxides, and
two cases can be realized:
i. If metal of liquid phase is of low chemical affinity for oxygen (noble
metals for example) interface (in atomic treatment) remains between
terminal layer of metal phase atoms and solid oxides (weak ionic or
only van-der Waals bonds will take place).
ii. If metal of liquid phase is of high chemical affinity for oxygen,
strength ionic bond will be formed between terminal metal atoms and
oxygen of solid oxides. That will result in significant weakening of
bond of terminal layer of liquid metal phase to other metal atoms in
the depths of melt due to excluding of a part of walent electrons from
metal-metal bonds. Displacement of an interface will occur from
initial position of first metal layer/oxide surface to position of
first/second metal layer of liquid phase.
Such a hypothesis, formulated early (1980—81) [5, 25], is confirmed
by “ab initio” calculations for contact systems metal—oxide [26, 27]. In
this sense Finnis works are especially interested. For Nb—Al2O3 system
Finnis [27] shows that bond of terminate Nb atoms monolayer to oxygen
layer on Al2O3 surface (ionic bonds), is strong, stronger than Nb—Nb
and Nb—Al bonds. In the atomic chain
O-- — Nb terminate — Nb volume — Nb volume
the most weak link is
Nb terminate — Nb volume bond.
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It is beneficial for good adhesion when only part of valent electrons
are excluded from metal—metal bond, i. e. boundary atoms of metal
phase (now — are ions!) must be in the state of intermediate valency.
The stable state of intermediate valency is characteristic of transition
metals.
Transition elements — element with partly occupied d-electron
orbitals are characterized by easily moveable electronic structure, an
ability to form the different electronic configurations with close energy
(hybrid Sd and Spd — orbitals) or in chemical language — ability to
form a number of stable compounds with different intermediate valency
or in many cases with significant deviation from stochiometry, arising of
vacancies in crystallographic lattice and even wide enough area of
homogenety in constitutional diagrams of alloys*.
Taking all that into account let us consider some experimental data
including received in the last time for thermodynamic stable oxides (like
Al 2O3, SiO2, MgO) wettability by liquid metals. As liquid phases alloys
containing transition metals — Ti, V, Zr, Cr, Sc will be considered. And
for comparing nontransition metals Al, Si, Mg and its alloys will be
considered as well. All listed metals possess significant (although
somewhat different) affinity to oxygen.
The wettability process for transition metals
Pure Al, Si, Mg wet (but moderately) oxide surface, contact angles are
70—80° (see Table 3).
The addition of these metals (silicon, aluminium, magnesium) to
inactive metals, viz metals with very low energy of oxide formation, like
Cu, Au, Sn, Pd and others at concentration 5—10% leads to contact
* As it is said in [31] “Transition elements manifest the much more variety of chemical
bond types than any element of the main subgroups of the periodical system. They can
exist in many different oxidation states”. The same is in [56] “For all transition elements
a great number of their oxidation states is characteristic of …”.
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angles dropping from 120—130° to 60—80° [5, 19, 29, 52] (Fig. 3 ), viz
∆θ ~50—60°.
According to [19] wetting action of Mg in liquid Pd (system Pd—
Mg—Al 2O3, 1565 °C) is as follows: at Mg concentration ~4—10% (at.)
contact angle drops from 117 ± 5° (pure Pd) to 84°; ∆θ ~40°.
So one can say that reactive (nonequilibrium) wetting action is
significant, but some restricted, for systems considered.
The wetting of Ti, V, Zr is much more strong. At more lower
concentration of these metals — 2—15% wettability of Al 2O3 by alloys
on base of Cu, Sn, Au and others is high; contact angles are 10—30°,
very high of adhesion activity is observed for Sc as well (see Fig. 3 and
Fig. 6).
T a b l e 3. Wetting of oxides by pure nontransition metals
Systems T, °C
θ°
Refer.
Al 2O3—Si
1450
82
[44]
Al 2O3—Si 1417 80 [40]
MgO—Si 1450 101 [44]
BeO—Si 1450 76 [44]
Al 2O3—Al 1600 61—70 [50]
Al 2O3—Mg 720 78 [46]
Al 2O3—Mg 700 65—70 [47]
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Fig. 3. Comparative data for wettabilitty of Al2O3 by alloys Cu, Au, Ge,
Ga with nontransition metal (Si, 1150—1200 oC) and transition metals
Ti, V, Zr [28, 30, 48]. (The data for Au—Si alloys are presented more
exactly on Fig. 8)
Fig. 4. Comparative data for wetting activity of Al [29] and Ti
[30], dissolved in Sn at contact to Al2O3, T = 1000 oC
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The same one can see at comparing of a weak wetting activity of Al
and strong activity of Ti, dissolved in liquid Sn (Fig. 4). So transition
elements possess much more adhesioactivity, than nontransition
elements.
The main reason for it according to mentioned above is as follows:
transition metal atoms (in difference from nontransition ones) can
participate simultaneously in interatomic bonds of different kinds —
metal, ionic, covalents; strict valent relations of components are not so
importent in this case.
So, if terminate atomic layer of liquid metal and formed by him of
mono- or several atomic dimentions layer of a new interface compound
(oxide in this case) to consider as a link, bonding liquid metal and solid
ionic substrate, it is necessary to allow that metal atoms of this layer take
part in bonds of different type – ionic and metallic. Both bonds — ionic
and metallic have to be of high strong in order to wettability would be
high enough. The transition metals mitt these conditions on the whole.
But it is necessary to define this rule with more accuracy end explain it
on atomic — electron level.
To understand the nature and reasons for arising of metal type
bonding in a ionic substrates, electronic structure of an intermediate
oxides of low valency at the interface in relation to wetting processes
have to be considered. Ligand field theory [31—33] can be propose to
use for it.
Ligand field theory as it is known was firstly developed for complex
compounds of transition elements and then enlarged on various refractory
compounds by american works (Morin [34] and especially Gudinough
[35]) and by russians (Geld-Shveikin works [36—38]) particularly for
transition metals oxides.
The essence of this theory (for oxides particularly) is as follows:
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• In monoxides (MeO) of Sc, Ti, V, Nb, Zr, Hf with NaCl
structure;
• In oxides (Me2O3) with corundum structure;
• In bioxides (MeO2) with rutile structure;
transition metal cation is in an octahedral (or distorted octahedral)
interstice of an oxygen anion sublattice. The electrostatic interaction*
between negative oxygen ions (Ligands) and positive transition metal
ions causes a splitting of energy of d-cation electron level (initialy five
multiple degenerated) to a more stable triply degenerated t2g level
(three-dxy, dyz, dxy atomic orbitals) directed away from neighboring
oxygen anions, and a less stable double degenerative eg level (two-dz2,
dx2 –y2 orbitals) directed towards neighboring anions (Fig. 5). In all
above mention oxides the splitting ∆ (difference between energy of t2g
and eg levels) is smaller than the intra-atomic electron exchange energy
Eex; viz ∆<Eex (Fig. 5, a). So d-electrons, at least if n≤5 in according to
Hund’s rule, will in a high-spin state and will “ready“ to coupling and
bonding neighboring cations (Fig. 5, b); (n is number of d-electrons in
cation).
The direct cation—cation interaction via overlapping cation d-
electron wave functions of neiboring cations will take place and
intercations bonds (including metal—like ones!) can arise if distance
between cations is lower than some critical value.
It means that lower oxides of transition elements, situated at the left
part of periods of periodic system (n < 5) can possess metallic properties.
Especially favourable case is when d-electrons number in transition
metal cation n ≤ 3; the electrons occupy only t2g levels, and cation-cation
* Really covalent interaction due to overlapping of p-orbitals of oxygen and d-orbitals of
metal and electron-spin interaction (exchange interaction) take place; in our case we
neglect these effects (for simplicity). So that theory of “crystal field” in essence is
under consideration.
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a b
Fig. 5. Spliting of energy of d-cation electron level (initialy five multiple
degenerated) on more stable triply degenerated T2g level (three dxy, dxz,
dyz orbitals) and less stable double degenerated eg level
(two 2z
d , 22 yx
d
−
orbitals) — a, and metal bond formation between
neighboring metal iones (b)
interaction is very strong (stronger than the cation-anion-cation
interaction). Point is that: eg-orbitals directed towards neighboring anions
(Ligands) and electron on these orbitals cause a repulsion colomb* forces,
increasing parameter of crystal lattice of compound and cation-cation
distance, and overlapping of its orbitales decreases. An absence of
eg electrons allows a minimal separation between neighboring cations
and maximal overlapping of t2g electron wave functions, and if cations
have half—or—less—filled cation orbitals viz partly—filled
electroconductive zone, such compounds have to possess really the metal
properties: electroconductivity of metallic type, weak Pauli
paramagnetizm, metal luster.
* Really repulsion can be caused by exchanged energy of electrons as well.
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Metalic preperties of intermediate phase are very beneficial for good
wetting in contact systems with liquid metals.
Quantitative numerical parameter determining intensity of cation-
cation interaction is integral of overlapping of cation-d-electron wave
functions∫ dvjiψψ , where ψI and ψj are wave functions (dxy, dxz, dzy in
our case) for neighboring cations dv—element of volume, which was
calculated by Shveikin and coauthors [36—38] for various oxides of d-
transition elements.
The overlapping integral ∫ dvjiψψ characterizes strength of
cation-cation metal bonds in ionic compound of transition metals and can
be consider as electronic criterium of metalicity of ionic substrates. In
our case we are talking about compound (intermediate oxide), arised at
the interface of metal melt, containing transition metal—solid oxide as a
result of chemical interaction between metal melt and base solid oxide.
So in adhesion and wetting in system metal/oxide this parameter can be
of key factor (of course beside chemical thermodynamic factor—affinity
of metal of liquid phase for oxygen (Gibbs potential change ∆G when the
chemical reaction metal—oxide occurs).
As above mentioned in our earlier works [30] more high wettability
degree was revealed for systems in which at the interfaces lower TiO
oxide arises comparing to systems where Ti2O3 was identified
(see Table 2). This fact has been explained on the simple
phenomenogical consideration: TiO is more metallike compound than
Ti2O3.
The Ligand field theory allows to give the more deep theoretical
treatment of these phenomena. On the Table 4 electronic properties of
titanium oxides put in conformity to wetting behavior of titanium in
different alloys which are in the contact to Al2O3, MgO, SiO2.
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For TiO interaction cation—cation is strong and metal type bonds
are provided by two d-electrons (dE electrons), overlapping integral is
0,02 (it is not so little value). The wettability in systems with TiO at the
interface is as follows: contact angle drops from 129—133° for pure Cu
as liquid phase to 14—16° for alloy Cu—8% (at.) Ti. In oxide Ti2O3
cation has only one d-electron; there is moderate cation-cation
interaction; overlapping integral is lesser, namely 0,012; Ti2O3 — is
semiconductor with narrow energy gap, metal type conductivity is
observed only at high temperature. The wettability in systems where
Ti2O3 arises at the interface is less intensive; Contact angle drops at the
same concentrations of Ti — 8% (at.) to 65—80°. Meanwhile in both
type systems (with TiO and Ti2O3 at the interface) nonequilibrium
(reactive) and equilibrium contributions to adhesion work act in the
same favorable direction increasing the wettability.
The work of adhesion for the first case (”TiO case“) is much more
than that for ”Ti2O3 case“. (These data were presented in Table 2).
TiO2 — dielectric, insulator; d-electrons for cation—cation
interaction bonding are absent; overlapping integral is O; Bonds between
cations is realized only through anion oxygen viz Ti+ 4 — O2
- - — Ti+ 4.
For such a material only nonequilibrium reactive contribution can act
(first term of equation 2).
The experimental data for wettability by Ti — containing melt for
case of TiO2 as intermediate phase formed are absent. But there are data
[39] for systems where the intermediate phase Ti5O9 is formed (closed by
composition to TiO2). For such system (Ni55Pd45—Ti/Al2O3) Ti5O9 was
identified at the interface, and contact angle drops from 112 to 92° only.
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Thus on the whole there is correlation between electronic structure of
intermediate phase formed at the liquid/solid boundary and wettability in
system.
The decrease of the value∫ ψψ dvji causes decrease of metallic
bounds in compound. For oxides of various valence for one and the same
transition metal, metallic properties are the greatest for lower oxides and
decrease down to those which inherent insulator substances (with
maximum valence of metal).
In distinction from data of Table 4, where relation of wetting activity
for one and the same metal (Ti) in different valence state, electronic
structure and overlapping integral for d-electron wave functions is
considered, we can try to ascertain how much the wetting activity of
different transition metals (at least for the same period) in the same
oxidation state Me++ (monoxides) relates to electronic properties of
interfacial oxides and corresponding overlapping integrals.
(Geld-Shveikin [36—38] calculated overlapping integral values for
monooxides MeO, where Me—transition metals in 3d, 4d and 5d periods
of periodic systems; Table 5).
The experimental data-namely concentration dependences of the
wettability in the systems with 3d elements — Sc, Ti, V, Cr (as additions
to nontransition metals) at contact to oxides are available and fitted to
compare [41, 42]. Let us consider these data.
Sn—Sc/Al2O3, In—Sc/Al2O3 systems can be compared with the same
systems with titanium. The wetting activity of Ti in Ga-melts contacted
Al 2O3 and SiO2 can be also compared with that of vanadium and
chromium (Fig. 6, 7, Table 6).
Scandium being dissolved in indium and tin was shown to decrease
sharply the contact angle in contact systems with Al2O3 and SiO2 (see
Fig. 6 a, b). Wetting action of scandium is stronger comparing to the
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T a b l e 5. Electronic structure of monoxids (MeO) of 3d-transition
metals—overlapping integrals of d-electronic wave functions
∫ dvjiψψ . (Geld’s—Shveikin’s calculation). For comparing the
data for 4d- and 5d- transition metals are presented
3d-metals
Metal
Sc
Ti
V
Cr
Mn
…….Fe
∫ dv21ψψ
—
0,02
0,008
—
—
0,0004
4d-metals
Metal
—
Zr
Nb
……………………………..
∫ dv21ψψ
—
0,077 0,057 ……………………………..
5d-metals
Metal
—
Hf
Ta
.……………………………..
∫ dv21ψψ
—
0,128*
0,108*
……………………………..
* The values for hypothetic monooxides.
T a b l e 6. The wettability (contact angles, deg) of Al2O3 and SiO2
(solids) by In—melts containing additions of Sc and Ti
Solid substrates
Melt
composition,
% (at.)
T, °C
Al 2O3
SiO2
600
52
—
In—2% Ti
700 48 64
600 35 35
In—2% Sc
700 10 12
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Fig. 6. Wetting of Al2O3 (a) and SiO2 (b) by Sn—Sc, and Sn—Ti
melts [41]
action of titanium. There are data [60] showing that ScO is of cubic
structure (NaCl type a = 4,54
o
A ) and possesses metallic properties—
metallic electroconductivity evidently similar to TiO*. Integral of
* The question about existence of two-valence Sc compounds is under discussion. In
[48 (1964)] one believes that only Sc''' valence compounds exist, but in more later
works [60 (1974)]Sc'' valence compounds (ScO) was declared to obtaine; its
crystallographic structure and some properties were identified. Evidently we can follow
to these later work.
900 °°°°C
900 °°°°C
600 °°°°C
900 °°°°C
900 °°°°C
% (at.) Sc, Ti
a
600 °°°°C
% (at.) Sc, Ti
b
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Fig. 7. Al2O3 wettability by Ga base melts,
T = 1050 °C [48]
overlapping of wave d-electron functions is not know for ScO. But we
can suggest that this integral would be higher than that for TiO in
accordance with the general tendency considered earlier. That can
explain more higher Sc wetting activity.
Ti promotes wettability much more than V (Fig. 7). The contact
angle decrease ∆θ for Ga—Ti melt/Al2O3 at 1050 °C is near 60° when
Ti concentration is ~2% (at.); in the same time ∆θ ~30° for Ga—V melt
at the same conditions. This is in accordance with much higher value for
the overlapping integral of 3dε-electronic orbitals for Ti++-iones in TiO
oxide (0,02); the such value for vanadium oxide (VO) is equal 0,008.
Thus the electronic structure of VO is less beneficial for the wettability
than the structure of TiO. The difference in wetting activity of Ti and V
is also conditioned by different affinity of these metals for oxygen (∆H298
for TiO and VO equal 124 kcal/mol and 98 kcal/mol respectively). This
difference is not so significant and it is probably not enough to explain
the markable difference in the wetting behavior of Ti- and V- containing
alloys (taking also into account that thermodynamic activity of Ti in Ga-
melt must be much lower than that for V. This can be deduced from
corresponding constitutional diagrams Ga—Ti and Ga—V alloys). At
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least we can consider that the change of the overlapping degree of 3dε -
electron orbitals does not contradict to the change in wettability. So
value of ∫ ψψ dvji is important factor for characteristic of metallic
interatomic bonds formation and metallic properties of lower oxides of
transition elements. Higher value ∫ ψψ dvji is favourable for the good
wettability of solids oxides by liquid metals.
Adhesion activity of chromium is low (Fig. 7). Contact angle
decreases only by ~5—7° for the system Ga—Cr/Al2O3 at Cr-
concentration 10% (at.) and at temperature 1050 °C.* At the same
conditions contact angle for the systems with vanadium decreases by
~30°. It is hard to explain such data only by lesser affinity of chromium
for oxygen (1/3 ∆H298 for Cr2 O3 is 90 kcal/g. atom O2; this value for
vanadium oxide ∆H298 for VO is 98 kcal/mol). Reason for such weak
wettability of chromium containing melts must be in the peculiarity of
electronic structure of chromium and its lower oxides, which can formed
at the interface.
Analysis of Geld’s-Shveikin’s calculation data, concerning to the
overlapping integrals for the wave d-electron functions for the oxides of
3d (and 4d and 5d metals), Table 5, shows that the overlapping integrals
are the greatest for the elements in the beginning of a period and decrease
with increase of the element atomic number along the period. In other
words there is a tendency for the overlapping integral to decrease from
the left part to the right one of the period of periodic system.
(Unfortunately we cannot say that such dependence is strongly
* Some more chromium wetting activity was found in [43]. For alloy Ni—19,3Fe—
20,7Cr (% (at.)) which was in contact to Al2O3 at temperature 1470 °C contact angle
decreases from 112° for Ni—Fe alloy without cromium to 90° for alloy with cromium.
It contradicts mentioned above but can be conditioned by very high temperature and
significant greater chromium concentration.
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monotonous; extremal points are possible for middle of period — when
d- cation electrons number are in range n ≥ 5).
As it has been pointed out above wetting activity depends also on the
thermodynamic factors, first of all on the chemical affinity of liquid
metals for oxygen. The affinity of metal to oxygen is also changed along
the period from the left to the right part of the row of elements. The
greatest most values ∆H, ∆G for oxide formation inherent for elements
which are disposed at the beginning of period, decreasing to the right part
of period. We have to accent, that “affinity factor” produces the
nonequilibrium, reactive part of adhesion (first term in the equation (1);
In prolonging discussion about Cr—adhesioactivity it should note an
electron—valent state of chromium compound.
Many two-valence chromium (Cr'') compounds are known [48, 59].
The lower chromium oxide CrO exists (black chromium suboxide [59])
and is characterized as chemically inert compound at temperature
≤ 700 °C. Compound CrO at heating to T > 700 °C in vacuum conditions
is transformed to Cr2O3. But if we consider interface of chromium
containing melts/Al2O3 in conditions of oxiden lack (due to high
thermodynamic stability of Al2O3 and strong Al—O bonds) we can
suggest that chromium suboxide (CrO) can exist at the interface at the
more high temperature. In this case we can discuss electronic properties
of CrO.
CrO crystallographic structure is not known. We can believe that it is
cubic structure of NaCl type, like TiO and VO structure. But in any case
for all (almost) compounds of Cr(II) (two valence chromium), chromium
cationes are in a octahedral (or partly distorted octahedral) interstice of
anion sublattice — O- - ligands [48, page 232]. Coordination number for
the cation is 6. According to magnetic measurements for almost all
Cr (II) compounds high spin electronic state takes place. Therefore
electronic configuration of Cr++ cation must be T2
3
g eg
1 (but not T2g
4).
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That means that one of d-electrons will fill the higher dγ level (eg). So
distance between the cations must increase due to repulsion forces
between electrons on the eg level orbitals directed to ligands O- - and the
overlapping d- electronic orbitals must be small. Unfortunately the
overlapping integral for CrO is not known now. According to the general
tendency and mention above considerations, we can expect that the
overlapping integral for chromium cations electronic orbitals must be
small and lesser then that for vanadium (and titanium) monoxide and
metallic properties of CrO should be express faintly. That can explain
poor wetting activity of chromium.
As to mechanism of bonding of terminal atoms of liquid phase
containing transition metal (Ti for example) with the layer of
intermediate (lower) oxide formed at the interface, we can allow
possibility formation besides ionic bonds with anion of oxide also metal-
bonds with cations in this oxide. As usual this last interaction is weak or
absent for nontransition metal oxides due to particularly dimensional
reasons – great anion radius (O-2) comparatively to the cation radius. For
Al 2O3 e. g. value
.37,0~
A 32,1
A 5,0
~ o
o
anion
cation
r
r
For oxides of the transition metals (TiO) this relation is more
favorably by factor ~2
.,~
,
r
r
anion
cation 610
A 1,32
A 80
o
o
≅
So bond between positive metal ion of Me+ in oxide and neutral metal
atom in liquid phase can exist. We consider interatomic transition metal
bonds on the same concept — splitting the d-level on t2g and eg
sublevels, high spin state of electronic systems and overlapping of the d-
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electron wave function with creation of the bonds between transition
metal atoms.
The structure of d-electron orbitals and their role in an interatomic
bond for the transition metals is under discussion during many years.
Perhaps beginning with Pauling (and not withstanding by Mott)
“atomic” treating of d-orbital electrons as localized near atom framework
(Heitler-London approaching) is declared. Such orbitals are positioned
inside atomic sphere and almost not overlapped with d-orbitals of
neighboring atoms. Friedel [57] argued it and presented a number of facts
which confirm d-d-interaction between the nearest atoms. Many of these
facts are evident: contribution of d-electrons to electruconductivity and
electronic heat capacity at low temperature; known correlation between
degree of occupation of d-orbitals by electrons and bonding energy, with
the maximum of bonds value when d-electrons number equaled 5 and
others. F. Cotton and G. Wilkinson emphasize that …”d-orbitals is
placed in space, extending far away on periphery of atoms or ions” [48,
page 11]. According to Biltz [49] d-electrons of Me-Me bonds
particularly in lower titanium compounds of such a tipe, provide the
greatest long-range interaction between ions and determin
electroconductivity and magnetic properties of substances.
The sum inference can be formulated as follows: the wettability in the
most contrast (as to nature of interatomic bonds) contact systems metal
melt/solid substrate with predominate ionic bonds will be the most high
when melts containe transition metals with electronic structure d1-d5, and
these metals are of high chemical affinity for electronegative elements—
oxygen, sulfur, fluorides or complex anion like SO4
--, PO3
--- and others.
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The wettability process for nontransition metals
Below the contact systems nontransition melt/ionic or ionocovalent solid
substrate (for example typical oxide like Al2O3) will be considered.
If chemical affinity of metal for oxygen (or other electronegative
element) is high enough, chemical interface reaction occurs and new
phase — compound at contact boundary is formed. New interatomic
bonds arise and their energy — chemical reaction energy — gives the
nonequilibrium part of adhesion WA nonequil (equation 2).
As pure nontransition metals Si, Al, Mg are considered, for which
wettability data are available and which are of high enough chemical
affinity for oxygen to interact with oxide of solid phase.
These pure metals wet (moderately!) thermodynamically stable
oxides — Al2O3, MgO, BeO. The data are presented in the table 3. As
one can see the contact angles are 70—80° — sometime 100°.
The contact angles (for Si, Al, Mg) don’t attain essentially low
values (say 20—25°) as much as new phases arising at the liquid
metal—solid boundary (oxides, spinels) are not possessed metallic
properties, it means that WA equilibr term in equation (2) is almost not
“working” in this case. The such behavior is observed in system Pd—
Mg alloys contacted Al2O3 [19]: Mg additions ~10—12% (at.) to Pd—
melt at 1565 °C allow to decrease the contact angles from value ~110—
120° (pure Pd) to ~85° only, although the chemical affinity of Mg for
oxygen is very high.
The data in detail were obtained in our works for binary alloys
containing nontransition metals (mainly silicon) as additions to base
metals (Cu, Au, Ge, Ni e. c.) contacted oxides (Al2O3, SiO2). Systems
investigated are of: Al2O3/Me—Si, Me—Au, Cu, Ge, Pd, Ni; SiO2/Me—
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Si, Me—Au, Ni, Pd* [52, 53]. Alloys Al—Ni contacted Al2O3 were
studied as well [50].
In literature there are data for Si-adhesioactivity in Al2O3/Au—Si
system [66], separate single data for Au—31% (at.) Si alloy [68] and
Ni—63% (at.) Si [69] contacted Al2O3. In all cases final contact angles
were in the range 70—85°.
So general assertion can be done as follows.
The adhesioactivity of metals investigated (nontransition metals Si,
Al, Mg) is significantly lower than that for transition metals, say titanium
whose chemical affinity for oxygen is not so greater than that for silicon
and close to chemical affinity for oxygen of Al and Mg.
Interesting peculiarity observed at systematic investigations of Si-,
Al- containing alloys in our works [52, 53] and still earlier in one work
[66] for Si—Au alloys contacted Al2O3.
At heating such alloys and after total melting (closed to liquidus line)
there is no wetting, θ ~120—130°. Then at further heating contact angles
decrease and at 1100—1150 °C its values are ~65—75°. For systems:
Au—Si, Pd—Si, Ni—Si contacted Al2O3 or SiO2 contact angles after
primary decreasing and after some time holding at constant temperature
attain minimal value (θmin) and then begin increase by circa 15—20° viz
dewetting phenomenon occurs.
The wettability data for systems in which anomalous phenomena —
dewetting processes are exhibited in Fig. 8—11. It is important that at
increase of wetting angle the reduction in liquid—solid contact square
(“retreat” of the melt) occurs but drop volume remain constant. The
process of “retreat” of melt is shown on the photo (Fig. 11 ) for Au—Si.
* Pd, Ni are transition metals, but with almost completely full d-electronic orbitals and
low (or very low) chemical affinity for oxygen.
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Fig. 8. Temperature-time dependence for wetting angle in the system
(Au—Si) — Al2O3. (The solid line is the change in wetting angle with
time at constant temperature (1100 °C); the broken line is the
temperature-time dependence of contact angle. Initial contact angle at
T <1000—1100 °C is more than 90°; these data are not presented
excluding first above left picture)
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It should noted that C. Marumo and J. Pask [67] in investigation of
Al 2O3 (monocrystal) by alloy Au—31% (at.)Si (only one composition
was studied) in vacuum and at continious heating (800—1200 °C) found
only contact angle dropping (from 150 to 78°) when temperature
increases.
Treatment and explanation of all these processes can be done as
follows.*
Fig. 9. Time dependence of dewetting process for an Al2O3
surface with melts of Au—30% (at.) Si (1) and Au—60%
(at.) Si (2) at 1150 °C and Pd—58% (at.) Si (3) at 1200 °C
* In some cases an evaporation of liquid phase (decrease of volume of liquid drop on
solid substrate surface) and pinning of the liquid phase perimeter on a triple line can
simulate wetting — dewetting processes: at drop evaporation visible decreasing contact
angle is observed (“wetting” increases) and then an equilibrium shape of drop is
restored contact angle increases (pseudo-dewetting behaviour). Such “geometrical”
process we don’t consider, but chemical processes at the solid—liquid interface resulted
in wetting change are under study.
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Fig. 10. Time-temperature dependence for wetting angle in
systems Ni—60%Si — SiO2, Pd—45%Si — SiO2, Au—
60%Si — SiO2
Fig. 11. Photograph of a Au—Si drop solidified on
Al 2O3 surface. Concentration of Si is 60% (at.),
T = 1150—1200 °C
5 10 15 20 25 30 35 40 45 50
70
75
80
85
90
95
100
105
1200
o
C
1100 oC
Au-60%Si
Pd-45%Si
Ni-60%Si
W
et
tin
g
an
gl
e,
d
eg
Temperat.-time, min
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As pointed above for high wettability of iono – or ionocovalent
substrates (oxides) is favorable when metal in a new compound arised at
the interface is in the state of intermediate valence, lower than group
valence in period system. Such ionic compounds are frequently (but not
always!) lightly evaporated (sublimated).
J. A. Chempion et al. [70] which at first performed experimental
investigations of contact system liquid aluminium/ aluminium oxide
(Al 2O3 monocrystal) observed reaction rings formation around the Al
drop at temperature 1350 °C in vacuum. The reaction rings arise
periodically on the solid surface and periodically contact angles are
jumplike changed.
Increase in the contact angle occurs by strong jumps from 55—60° up
to ~80°. This process cannot be called as ”dewetting“ in the sense
mentioned before, nevertheless it can be beneficial to consider this
process for understanding in the whole speciality of interface interaction
of nontransistion metals Al, Si contacted solid oxides.
J. A. Chempion et al. [70] believe (postulate) that the volatile oxide
of aluminum, Al2O is formed at the contact region Al—Al2O3 (reference
to Brewer and Searcy work [71] (1951)) and progressive decrease in drop
volume of aluminum melt contacted Al2O3 occurs due to intensive
evaporation of both Al2O and Al. But the more later works M. Hoch and
Jonston [72] (1954), Cochran [73] (1955), A. E. Vol [74] (1959)showed
that Al2O oxide dose not evaporate in vacuum <1 Pa (10-2 tor) up to
temperature 1700 °C. So at interaction of Al with Al2O3 at high
temperature (1350 °C) only Al is evaporated very intensive. The volume
of the drop Al decreases by more then 0,6 of initial value (1,0) when
holding time was 60 min.
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Thus for Al and at the conditions of high temperature (1350 °C) and
intensive Al evaporation the observation of dewetting phenomena is hard
or impossible.
Nevertheless in special work [50] for Al—Ni alloys (not for Al pure)
contacted Al2O3 the very process of dewetting can be reproduced and
investigated. The Al—Ni alloys (15% (at.) Ni and 30% (at.) Ni contacted
Al 2O3 were used. The temperature was 1600—1700 °C, gas media:
purified argon, P ~1 atm was used.
After holding time ~30—60 min the contact angles from minimal
values ~60—65° increase: 64→76°; 66→78° (30% (at.) Ni) and
68→76°(15% (at.) Ni).
Strong Ni—Al bonds and low thermodynamic activity of components
in Ni—Aluminid compounds prevent evaporation of aluminium and drop
volume of alloys are practically no changed, in spite of changes in
contact angles and drop diameter occur periodically. All these
phenomena we can consider as dewetting processes.
The main question is: what is the reason for and physicochemical
nature of dewetting processes?
Let us consider once more the literature data mentioned above
concerning wetting of oxides by silicon and aluminium melts in view
point of question formulated.
B. Drevet, D. Chatain, N. Eustathopoulos [66] observed increase in
the contact angle by ~20° (dewetting) in Au—Si/ Al2O3 systems at Si-
concentration XSi ≅ 0,13; 0,25; 0,3; 0,5 at almost constant temperature
~1000—1100 °C at holding time 60—120 min.
Contact angles were as follows:
at XSi = 0,13; θmin = 80°; θfinal = 94°;
at XSi = 0,30; θmin = ~65°; θfinal = ~85°;
at XSi = 0,50; θmin = ~63°; θfinal = 83°;
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According to authors opinion … “a likely explanation of this
processes lies in a variation in the concentration of a trace elements
(impurity) probably oxygen”. Oxygen is really active element influencing
on liquid—gas and liquid—solid boundary, but mechanism of the very
dewetting process stay not clear. C. Marumo and J. Pask [68] which
investigated wettability of alumina by alloy Au—31% (at.) Si at
continions heating (800—1200 °C) observed only contact angles
dropping — from 150 to 78°.
According B. Drevet and N. Eustathopoulos [69] for system
Al 2O3/alloy Ni—63% (at.) Si at temperature 1200 °C contact angle
decreases from 112 °C (t = 0 sec) to 95° (t ~20 sec) and then exhibits
”small oscillations“ around this later value. Amplitude of the oscillation
was of 2—3° with the period 5—10—20 sec.
An oscillation of contact angle was explained by special topography
of alumina surface contacted Si—Ni alloy. This phenomenon evidently
does not relate to dewetting processes. Note, authors of this work-review,
published in 2012 don’t refer to their more earlier paper (1990, [66]), in
which increase in the contact angle (dewetting) was observed and
discussed.
So in this contradictious situation and the question mentioned above
other approach to solve the problem can be proposed.
Point is: there is another factor influenced up on the wettability and
contact properties in metal—oxide systems. Namely, the ionic
compounds of intermediate valence are inclined to reaction of
disproportionation, at which the compound of the highest valence (higher
oxides) and metal of zero valence are formed.
For system Al—Al2O3 when liquid Al interacts with Al2O3 at high
enough temperature (~1200—1300 °C) low aluminium oxide Al2O (and
AlO as well) is formed.
Al2O is formed by reaction: 4Al(liq) + Al2O3(solid) ↔ 3Al2O.
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This oxide at the definite temperature — time conditions
(~1000 °C) is under went by reaction of disproportionation:
3Al2
+ O-- → 4Alo + Al2
+++ O3
--.
This reaction will occure, when Al2O will be accumulated at the
interface and the equilibrium of the reaction ( ) will be displaced from
right to left side.
It is especially important that at this process zerovalence aluminium
is formed, therefore some part of aluminium atoms will be bound to
Al 2O3 by week not chemical van der Waals forces; that leads up to
adhesion decrease, contact angle increase and dewetting phenomena will
occure.
The phenomenon of reaction of disproportionation should consider
as a guide factor in the treatment of wetting/deweting processes, and it
allows to explain many peculiarities of contact, capillary, adhesion
behavior in systems with nontransition metals.*
About Al2O oxide and its properties mentioned above. When silicon
(liquid or solid) reacts to SiO2 oxide at temperature ~1150—1250 °C in
vacuum, chemical reaction occurs [67]
Si + SiO2 ↔ 2SiO
The same tipe reaction for systems Si—Al2O3 oxide must be as
follows:
2Si + Al2O3 ↔ 2SiO + Al2O
That is at such interaction low valence oxides are formed.
* The conception of reaction of dispropotionation is used in many areas of chemistry,
including transformation in ionic or ionocovalent structures, particularly, explains
nonstability of monogalogenides of the alkaline earth metals (see e. g. [54, 55]). It
should be noted that the main reason for nonstability of the lower ionic (ionocovalent)
compounds of nontransition metals is a low energy of the crystall latice of compound
(thermodynamic general considiration), which in its turn conditioned by abcence in a
compound of the direct bonds between metal atoms (cations): intercation bonds
accomlished only trough anions in distinction from transition metal compounds of lower
valence, where strong enough direct metal bonds between cations exist (atomic level
consideration).
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SiO is stable compound in gaseous state at T ≥ 1000 °C. At some
lower temperature crystals of SiO were obtained: cubic structure,
a = 5,16
o
A , density is 2,13 g/cm3. (This later assertion is sometimes
under discussion). When temperature decrease up to ~700 °C SiO
undergo by intensive reaction of dispproportionation:
2Si++O-- → Sio + Si++++O2,
and zero valence Si(Sio) is formed like as it was declared for Al
containing alloys. For SiO investigation in detail was performed by
P. Held and M. Kochnev [51], which suppose that SiO is real chemical
compound, but to keep it in solid state according [59] very rapid cooling
is necessary.
The situation related to state, stability of SiO, or its inclination to
reaction of disproportionation can be some other, when not pure Si but
Si-alloys contacted any oxides are under experimental study, especially
for alloys where thermodynamic activity of Si is low [75] and there are
strong bonds Si/base metal. Just these cases were realized in system Si—
Au alloys and Al—Ni alloys considered and discussed above. In these
cases we can believe that interface reactions (including reaction of
disproportionation) must be retarded.
The reaction of disproportianation can hinder to optimal (in view
point of high adhesion in system metal/ionic compound) distribution of
the valent electrons of metal, at which optimal part of the valent electrons
are accepted by electronegative element (oxygen for oxide as solid
phase).
Summarizing all data, we can consider that the dewetting processes in
oxide/nontransition metal systems can be determined in significant
degree by reaction of disproportionation.
Nevertheless chemical composition of Al-or Si-alloys contacted
Al 2O3 or SiO2 at high temperature should control after dewetting
process occured possible changing in concentration of
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elements (Al, Si) due to its evaporation, or evaporation of its lower
oxides.
In our experiments chemical analysis was performed for Pd—Si
melts contacted Al2O3. Two alloys with initial concentration of silicon
45% (at.) and 58% (at.) (18% (mass.) and 27% (mass.) Si accordingly)
were used.
Analysis was performed after initial wetting process occurred (θ dec-
reases from ~120° to ~75°) and then θ increases to ~85—90° (dewetting).
The results of analysis are presented on Table 7. The data received
showed an increase (not decrease!) of Si content in melt (in zone closed
to interface): from 18% (mass.) to 32,56% (mass.) Si, and for other alloy
from 27% (mass.) to 36,0% Si. Evaporation of SiO if it really took place
could be resulted in decreasing Si-content in melt. Aluminum
concentration in Pd—Si melts (45% (at.) and 58% (at.) Si) being initially
zero, after contact processes of melt with Al2O3 becomes 4,75 and 6,42%
(mass.) accordingly. These data (related to Al) denote the partial
dissolution of alumina in Pd—Si melt (chemical interface interaction).
The process of “retreat” of the melt is shown on the photo (Fig. 11)
for Au—Si drop on the Al2O3 surface. The same picture is observed for
Pd—Si drop on Al2O3 surface.
Profilographic investigations of Al2O3 surface after removal of Pd—
Si drop show that same very small (~1—1,5 µm) Pd—Si alloy goes deep
in to Al2O3 substrate.
So it is possible that increase of Si-content in Pd—Si alloy observed
in experiments can be conditioned by some segregation of silicon at the
interface Pd—Si alloy/Al2O3 (polymolecular adsorption, aluminum
silicate layer formation). Part of this can be near to zone of action of
instrument VG-900 (see figure in the table No. 7).
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T a b l e 7. Results of mass-spectrometric analysis (instrument
VG-9000) of chemical composition of Pd—Si drops which contacted
Al 2O3 during 60 min at T = 1200 °°°°C
Content of element
(Pd—45% (at.) Si)/(Pd—58% (at.) Si)
C, ppm
O, ppm
Mg, ppm
Al, % (mass.)
Si, % (mass.)
Fe, ppm
Ni, ppm
Mo, ppm
Pd, % (mass.)
W, ppm
17,998/0,126
41,597/12,316
752,530/854,650
4,75/6,424
32,562/35,998
3284,800/4036,900
1160,900/1328,700
4499,800/11196,000
60,966/53,117
5087,200/18971,000
1 ppm = 10-4 % (mass.)
The scheme of mass-spectrometer analysis
Pd—Si drop, separated from sapphire
Bottom surface of a metal drop (separation was purely adhesive).
The base of a drops after separation from the Al2O3 was analyzed in a
mass spectrometer in a glow discharge (instrument VG-9000) at the
depth ~4—6 µm.
Initial depths in metal melt at
which analysis was performed
(4—6 µm)
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So most facts and theirs discussion bear the witness that main reason
for dewetting processes observed in systems ionic substrates
(oxides)/metal melts containing nontransition elements (Si, Al) is the
reaction of disproportionation — disintegration of low (intermediate)
nonstable compounds arised at the interface solid—liquid and formation
of zero valence metal and high (group) valence oxide.
On the whole nontransition metals (pure or as additions) even with
high enough affinity for anion of solid ionic or ionocovalent compounds
can provide only moderate wetting degree.
The practical recommendations related to wetting process control and
regulation in systems liquid—metal/ceramic materials
The wettability degree is key factor in many technological operations-
joining of dissimilar materials, composite materials manufacture,
deposition of metal coating on ceramics et. c.
The considerations of the physico-chemical nature of contact
processes at the boundary liquid metals/solid nonmetal substrates allow
to formulate the methods in order to govern and change a wettability
degree in direction we need. In the most cases the high enough
wettability and adhesion must be reached.
We will talk about the most contrast systems related to chemical
nature and interatomic bonds in contact pair viz metal/ionic or
ionocovalent ceramic materials.
Typical interface interaction at the liquid—solid boundary are as
follows:
1. Dissolution of solids in liquid melt.
2. Diffusion of liquid phase atoms in solid substrate.
3. New interface compound formation.
This last process should be considered as the most perspective for
wettability regulation and it will be discussed below.
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In this case wettability degree will depend in the whole on agregate
state of a new phase — solid, liquid, gas. For solid state of a new phase
wettability will depend on chemical nature (type of interatomic bonds in
interface compound and its properties — ionic, covalent, metalic
substrates).
General requirement for high wettability is faivourable combination
of great nonequilibrium part in adhesion work and significant equilibrium
contribution to adhesion liquid/solid boundary, viz both terms of
equation (1) must be high enough. It means that an intensive chemical
interface interaction mast be occurred with great ∆G — Gibbs potential
change, high chemical affinity of liquid metal for solid substrate (oxygen
for oxides) that will result in formation at the interface intermediate
oxide. It is very favourable if this oxide will possess metallike properties.
The Ligand field theory explanes the possibility of existence metalic
interatomic bonds in ionic structures (oxides). As mentioned above
significant d-electronic orbitals overlapping of neibouring cations, high
value of overlapping integral ∫ dvjiψψ have to lead up to strong direct
cationi— cationj bonds of metalic nature between atoms (cations) and
metalic properties in new oxide at the interface.
Such a situation is realised if interfacial oxides are formed by
transition elements of 3d, 4d, 5d periods of periodic system and such
elements are placed at the left part of periods of periodic systems viz
namber of delectrons have to be n ≤ 5 (preferably n ≤ 3). These elements
are: Sc, V, Zr, Hf, Nb and some others. These elements are as a rule used
as adhesioactive additions to nontransition metals like: Sn, Cu, Ag, Au
and others.
By other words for high wettability of stable oxides of type Al2O3,
BeO, MgO, SiO2 by liquid metals favourable combinations of
thermodynamic ∆G—Gibbs potential changing, when reaction occures
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and electronic (∫ dvjiψψ ) factors for contact system are necessary.
Such a combination is possible for transition metals.
The nontransition metals in pure state or as additions even with high
affinity to oxygen allow to get only low or moderate wettability of above
mentioned oxides.
The fulfillment of given instructions results in high enough
“chemical” stength of liquid metal/solid ceramic interface (high adhesion
and wettability values).
High wettability (contact angle value must be ~10—30°) determins
very possibility of formation of brazing joint or composites processing by
liquid phase sintering or infiltration.
For practice we need to get high enough mecanical contact strength
as well. It is special questions, which demands separate detale
consideration. Now we can say that high chemical interface strength is
nesessary (but not enough) condition to reach high mecanical strength,
nevertheless one can say, that at least at abidance of the elementary
technological conditions, for example, if the coefficients of thermal
expantion of dissimilar materials to be joined would be different not so
significantly, correlation between mechanical and chemical strength of
brazing joint will be take place.
Another point. For ceramic/metal joining by brazing process brazing
alloys have to containe adhesioactive elements — Ti, Zr, Nb and others.
These metals are caracteristic of high chemical affinity for oxygen,
nitrogen, sulfur and other nonmetal atoms. So technological process must
be performed in high vacuum (10-3—10-4 Pa) or in gas media — helium,
argon of high purity.
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Conclusion
The review concerning to high temperature wetting processes in
ceramics-liquid metal systems including analysis of the new data
received in the last time are presented. New theoretical conceptions are
proposed to treat wettability phenomena.
In the whole case for nonequilibrium contact systems and when
chemical interaction at the interface occures, work of adhesion can be
written down by equation
WA = WA equilibr. + WA nonequilibr. (3)
Where WA terms are of equilibrium and nonequilibrium parts of total
work of adhesion.
WA nonequilibr. – is direct contribution of chemical interface reaction to
wetting. As this assertion in some works up to now is denyed or is
debateble, special consideration of this key question has been performed
for various contact systems. It has been shown that really free energy of
chemical interface reaction is identically equal to the energy of chemical
interface bonds established and gives direct contribution to liquid/solid
adhesion.
The consideration of atomic mechanism of interface bonds formation
especially for the most contrasted as to nature of interatomic bonds in
substrates contacted – metal melt/ionic or ionocovalent compounds
(oxides first of all, like Al2O3, MgO, SiO2, BeO) allows to drow
conclusion as follows: for high enough wettability terminate atomic layer
of liquid metal atoms must participate simultaneously in interatomic
bonds of different types – metallic bonds with deeply placed metal atoms
and ionic bonds with solid substrate (oxide in this case). Transition
elements – elements with partly occupied d-electron orbitals are
characterized by easily moveable electronic structere, ability to form the
different electronic configurations and to be in the stable state of
intermediate valency mitt these requirements.
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The both bonds—ionic and metallic have to be of high strength in
order to adhesion and wettability would be high enough. So at the
interface liquid metal/solid oxide intermediate low valency oxide of
metal of liquid phase is formed. It is especially favourably when such
oxides are possessed of metallike properties. All these processes are
considered on atomic-electron level. Ligand field theory was proposed to
use for it. Quantitative numerical parameter determining intensity of
cation—cation interection and stregth of metal interaction bonds is
integral of overlapping of cation-d-electron wave functions ∫ ,ψψ dvji
where Ψi an Ψj are wave functions of electrons for neiboring cations.
The chemical compounds of intermediate valency (low then group
one) for nontransition metals are as the rule nonstable substrates, inclined
to reaction of disproportionation (metal of zerovalency and compound of
the highest valency are formed). The dewetting process occures, contact
angle increases, and wettability degree in this case is low or moderate.
Bassed on all above mention regularities practical recommendations
to control and govern wettability can be used in various technological
procedures.
РЕЗЮМЕ. Сделан обзор и представлены некоторые общие
комментарии, касающиеся научных проблем высокотемпературной
смачиваемости и адгезии в системах жидкие металлытвердые
керамические материалы, включая анализ полученных в последнее
время данных. Разработаны теоретическая трактовка процессов
высокотемпературной смачиваемости и механизм, основанный на
традиционных термодинамических представлениях, и новый подход
на атомном и атомно-электронном уровне. В первую очередь в
качестве керамики рассмотрены различные субстраты, а именно,
субстраты с преобладающими ионными межатомными связями
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(например, оксиды), которые обычно плохо смачиваются жидкими
металлами.
Рассмотрен и проанализирован вклад в процесс смачивания
неравновесной и равновесной составляющих адгезии.
“Активные” металлы, а именно, металлы с достаточно высоким
химическим сродством к атомам твердой фазы или ионам разделены
на переходные металлы (d-электронные орбитали частично
заполнены электронами) и непереходные металлы (d-орбитали не
заполнены или полностью заполнены электронами).
Смачиваемость в первом случае (переходные металлы) намного
интенсивнее, чем для непереходных металлов, несмотря на
термодинамические факторы (химическое сродство металлов к
атомам твердой фазы близко). Множество экспериментальных
данных иллюстрирует эту закономерность.
Для объяснения и трактовки на электронном уровне
превосходства переходных металлов в плане адгезионной
активности предложена теория поля лигандов.
Адгезионная активность непереходных металлов к твердым
субстратам (оксидам) намного ниже или является умеренной, и в
этом случае часто наблюдаются явления десмачивания. Эта
характерная особенность объясняется образованием на поверхности
раздела нестабильных соединений промежуточной валентности,
которые могут подвергаться реакции диспропорционирования.
Даны некоторые практические рекомендации, касающиеся
регулирования и управления процессами смачивания в различных
технологических процедурах.
Ключевые слова: высокотемпературное смачивание, адгезия,
керамика, переходные и непереходные металлы, d-электронные
орбитали и теория поля лигандов, десмачивание, реакция
диспропорционирования.
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НАЙДІЧ Ю. В.
ПРОГРЕС У ТЕОРІЇ ЗМОЧУВАННЯ У СИСТЕМАХ
КЕРАМІКА/РІДКИЙ МЕТАЛ. ОСОБЛИВІСТЬ КОНТАКТНИХ
ПРОЦЕСІВ ДЛЯ ПЕРЕХІДНИХ ТА НЕПЕРЕХІДНИХ
МЕТАЛІВ
Зроблено огляд та подані деякі загальні коментарі, що стосуються
наукових проблем високотемпературного змочування та адгезії у
системах рідкі металитверді керамічні матеріали, включаючи
аналіз одержаних останнім часом даних. Розроблені теоретична
трактовка процесів високотемпературного змочування та механізм,
що базується на традиційних термодинамічних уявленнях, і також
новий підхід на атомному та атомно-електронному рівні. У першу
чергу як кераміка розглянуті різні субстрати, а саме, субстрати, де
переважають іонні міжатомні зв’язки (наприклад, оксиди), котрі
звичайно погано змочуються рідкими металами.
Розглянуто й проаналізовано внесок у процес змочування
нерівноважної та рівноважної складових адгезії.
“Активні” метали, а саме, метали з досить високою хімічною
спорідненістю до атомів твердої фази або іонів розділені на
перехідні метали (d-електронні орбіталі частково заповнені
електронами) та неперехідні метали (d-орбіталі не заповнені або
цілком заповнені електронами).
Змочування у першому випадку (перехідні метали) набагато
інтенсивніше, аніж для неперехідних металів, попри термодинамічні
чинники (хімічна спорідненість металів до атомів твердої фази
близька). Багато експериментальних даних ілюструють цю
закономірність.
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Для пояснення та надання трактовки на електронному рівні
переваги перехідних металів у плані адгезійної активності
запропоновано теорію поля лігандів.
Адгезійна активність неперехідних металів до твердих
субстратів (оксидів) набагато нижча або помірна, і в цьому випадку
часто спостерігаються явища дезмочування. Ця характерна
особливість пояснюється утворенням на поверхні розділу
нестабільних сполук проміжної валентності, котрі можуть
піддаватись реакції диспропорціонування.
Подані деякі практичні рекомендації щодо регулювання та
керування процесами змочування у різних технологічних
процедурах.
Ключові слова: високотемпературне змочування, адгезія, кераміка,
перехідні та неперехідні метали, d-електронні орбіталі та теорія
поля лігандів, дезмочування, реакція диспропорціонування.
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