To the issue of the ultimate purification of metals
The paper considers issues related to the ultimate content of impurity elements in metals in the processes of their refining by evaporation and distillation in vacuum, zone recrystallization and electromigration. It is shown that at all refining methods, with an increase in the number of refining cy...
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2022
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| Series: | Вопросы атомной науки и техники |
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nasplib_isofts_kiev_ua-123456789-1958222025-02-09T21:21:31Z To the issue of the ultimate purification of metals До питання про гранічне очищення металів К вопросу о предельной очистке металлов Kovtun, G.P. Solopikhin, D.A. Shcherban, A.P. Pure materials and vacuum technologies The paper considers issues related to the ultimate content of impurity elements in metals in the processes of their refining by evaporation and distillation in vacuum, zone recrystallization and electromigration. It is shown that at all refining methods, with an increase in the number of refining cycles and the duration of the process, a “saturation effect” arises, associated with the achievement of the ultimate concentrations of impurity elements in refined metals. The reasons for the impossibility to produce absolutely pure metals have been discussed. Розглянуті питання, пов'язані з граничним вмістом домішкових елементів у металах у процесах їх рафінування, методами випаровування та дистиляції у вакуумі, зонної перекристалізації та електроперенесення. Показано, що при всіх методах рафінування зі збільшенням числа циклів рафінування та тривалості процесу виникає «ефект насичення», пов'язаний з досягненням граничних концентрацій домішкових елементів у рафінованих металах. Обговорено причини неможливості одержання абсолютно чистих металів. Рассмотрены вопросы, связанные с предельным содержанием примесных элементов в металлах в процессах их рафинирования, методами испарения и дистилляции в вакууме, зонной перекристаллизации и электропереноса. Показано, что при всех методах рафинирования с увеличением числа циклов рафинирования и длительности процесса возникает «эффект насыщения», связанный с достижением предельных концентраций примесных элементов в рафинированных металлах. Обсуждены причины невозможности получения абсолютно чистых металлов. 2022 Article To the issue of the ultimate purification of metals / G.P. Kovtun, D.A. Solopikhin, A.P. Shcherban // Problems of Atomic Science and Technology. — 2022. — № 1. — С. 3-5. — Бібліогр.: 9 назв. — англ. 1562-6016 DOI: https://doi.org/10.46813/2022-137-003 https://nasplib.isofts.kiev.ua/handle/123456789/195822 546.47:669.054 en Вопросы атомной науки и техники application/pdf Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Pure materials and vacuum technologies Pure materials and vacuum technologies Kovtun, G.P. Solopikhin, D.A. Shcherban, A.P. To the issue of the ultimate purification of metals Вопросы атомной науки и техники |
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The paper considers issues related to the ultimate content of impurity elements in metals in the processes of their refining by evaporation and distillation in vacuum, zone recrystallization and electromigration. It is shown that at all refining methods, with an increase in the number of refining cycles and the duration of the process, a “saturation effect” arises, associated with the achievement of the ultimate concentrations of impurity elements in refined metals. The reasons for the impossibility to produce absolutely pure metals have been discussed. |
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Kovtun, G.P. Solopikhin, D.A. Shcherban, A.P. |
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Kovtun, G.P. Solopikhin, D.A. Shcherban, A.P. |
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Kovtun, G.P. |
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To the issue of the ultimate purification of metals |
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To the issue of the ultimate purification of metals |
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To the issue of the ultimate purification of metals |
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To the issue of the ultimate purification of metals |
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To the issue of the ultimate purification of metals |
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to the issue of the ultimate purification of metals |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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2022 |
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Pure materials and vacuum technologies |
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https://nasplib.isofts.kiev.ua/handle/123456789/195822 |
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To the issue of the ultimate purification of metals / G.P. Kovtun, D.A. Solopikhin, A.P. Shcherban // Problems of Atomic Science and Technology. — 2022. — № 1. — С. 3-5. — Бібліогр.: 9 назв. — англ. |
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Вопросы атомной науки и техники |
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ISSN 1562-6016. ВАНТ. 2022. №1(137) 3
SECTION 1
PURE MATERIALS AND VACUUM TECHNOLOGIES
https://doi.org/10.46813/2022-137-003
UDC 546.47:669.054
TO THE ISSUE OF THE ULTIMATE PURIFICATION OF METALS
G.P. Kovtun
1,2
, D.A. Solopikhin
1
, A.P. Shcherban
1
1
National Science Center “Kharkov Institute of Physics and Technology”, Kharkiv, Ukraine;
2
V.N. Karazin Kharkiv National University, Kharkiv, Ukraine
E-mail: gkovtun@kipt.kharkov.ua
The paper considers issues related to the ultimate content of impurity elements in metals in the processes of their
refining by evaporation and distillation in vacuum, zone recrystallization and electromigration. It is shown that at all
refining methods, with an increase in the number of refining cycles and the duration of the process, a “saturation
effect” arises, associated with the achievement of the ultimate concentrations of impurity elements in refined metals.
The reasons for the impossibility to produce absolutely pure metals have been discussed.
In a high-purity state, metals acquire qualitatively
new, previously unknown properties, what cause their
widespread use both for fundamental research and for
practical purposes. There are various methods for deep
refining of metals, which are continuously being
expanded and improved [1–5, 8, 9]. In the purest
chemical elements (silicon, germanium, gallium,
mercury), the content of the total impurity elements is
around the 10
-6
at.%, and a number of individual
impurities – 10
-7
…10
-9
at.% [6].
An increase in purity is associated with an increase
in material costs which grow nonlinearly in the area of
ultimate cleaning [7]. Therefore, issues almost always
arise, which related to the determination of the ultimate
content of impurity elements in the processes of metal
refining, the possibility of assessing this content, and the
establishment of external factors affecting the content of
impurities. These and other issues are considered in the
article concerned refining metals by the methods of
evaporation and distillation in vacuum, directed
crystallization and electromigration.
The simple way to remove volatile impurities from a
metal is to evaporate them in a vacuum. The time
dependence of the distillation rate of highly volatile
impurities in a binary alloy was considered in [1]. The
main conclusions from this consideration are as follows.
First, the content of the volatile component during
vacuum distillation decreases exponentially with time,
and, second, the complete distillation of the volatile
impurity in a finite time (t → ∞) from the alloy is
impossible.
With an increase in the number of refining cycles or
the duration of the process, the purity of metals, as a
rule, increases, but at the same time, for all methods, the
so-called “saturation effect” is observed, that is, the
achievement of the ultimate content of an impurity
element.
The issue remains undecided, namely can the
concentration of the volatile component reach an
ultimate value during evaporation and will not change
further with an increase in the distillation time?
In the case of a binary alloy consisting of the main
nonvolatile component A and an impurity volatile
component B, the total equilibrium pressure of the melt
(Ptot) according to Dalton's law will be equal to the sum
of the partial pressures of components A and B.
Рtot = РA+ РB, where РА and РВ – are the equilibrium
partial pressures of components A and B.
According to Raoult's law, the equilibrium partial
pressures for real systems are expressed by the ratios:
AAАA Npp 0 and
ВВВВ Npp 0 ,
where γА and γВ are the activity coefficients;
0
Ap and
0
Вp are equilibrium vapor pressures of pure components
A and B; NA and NB are mole fractions, respectively, for
components A and B.
As the distillation proceeds, the mole fractions of
components A and B will change, and the mole fraction
of the volatile component (NB) will change faster. With
a change in the molar fractions of components A and B,
their equilibrium partial vapors pressures will also
change, which, upon reaching certain (final)
concentrations of the components, will be equal to each
other, that is, PA = PB. In this case:
К
АAАA Npp 0 and
К
ВВВВ Npp 0 ,
where
К
АN and К
ВN are equilibrium mole fractions of
components A and B, the ratio of which will not be
changed with increasing sublimation time.
Achieving equality of the partial vapor pressures of
the main and impurity components makes it possible to
estimate the final (ultimate) content of the volatile
component in the process of distilling the melt.
A number of assumptions can be made for estimated
calculations. Let us assume that during the evaporation
of a highly volatile impurity, the mass of the main
component does not change. In this case, from the
equality condition
AAАA Npp 0 and
К
ВВВВ Npp 0 ,
the final value of the molar fraction of the volatile
component has the form:
mailto:gkovtun@kipt.kharkov.ua
4 ISSN 1562-6016. ВАНТ. 2022. №1(137)
0
0
BB
AAAK
B
P
NP
N
.
For dilute solutions (γА→1) and taking into account
that К
В A N-1N , then
00
0
ABB
AK
B
PP
P
N
.
For the case of an ideal system (γВ → 1), the final
(ultimate) content of the molar fraction of the volatile
component will be determined by the expression
00
0
AB
AK
B
PP
P
N
.
In the process of multiple distillation, there will be
occur also a change in the partial pressures of the
impurity element and the main component. When
equality between them is reached, the content of the
volatile component will not change either in the melt or
in the condensate.
The process is more complex where during the
evaporation of a highly volatile component, partial
evaporation of the main component also occurs. In this
case, we can use the expression to calculate the change
in the concentration of alloy components during
evaporation [8].
0
0 0
lg 1 lg .В В В A
В B Bi A
C p M W
C p M W
(1)
Here
0
ВC and
ВC are the concentration of a highly
volatile impurity, respectively, before and after
evaporation, МА and МВ are molecular weights of
components A and B,
0
AW and
AW are the masses of the
main component before and after evaporation. The
expression (1) makes it possible to estimate the content
of volatile impurity in the melt taking into account the
mass loss of the main component.
In the case of directional crystallization of metals, a
“saturation effect” is also observed. After multiple
passes of the liquid zone, the distribution of impurities
along the length of the ingot approaches the steady-state
final state which characterizes the maximum attainable
separation of impurities [9].
Fig. 1 shows an approximate distribution of the
concentration of impurity C with a distribution
coefficient K < 1 after one pass and multiple passes.
Fig. 1. Distribution of impurity concentration C, with
the distribution coefficient K < 1 along the length of the
ingot L, after one pass (1) and multiple passes (2).
C0 is the concentration of impurities in the charge.
The zone moves from left to right
Additional passes make the start section deeper,
heighten the end section and shorten the length of the
horizontal part of the curve. As a result, all three areas
are encompassed by a single, relatively smooth curve.
After multiple passes through the zone, the distribution
of the impurity approaches to the steady state which
characterizes the final, maximum attainable separation
of the impurity.
In this case, the convection flow of the impurity,
caused by the crystallization action of the zone, meets
the equal opposition of the reverse flow due to the
accumulation of the impurity in the final section.
A similar (final) distribution of impurities is
observed in the process of refining metals using the
method of electromigration – the movement of ions of
impurity elements in solid or liquid metals under the
applied a constant electric field [5]. The movement of
the impurity ion is carried out under the action of the
force F = Zeff·E, where Zeff is the effective charge of the
ion; E is the electric field strength.
The quantitative characteristic of the refining
process during electromigration follows from the
relationship between the speed of movement of the
impurity under the applied electric field and its reverse
movement in consequence of diffusion caused by the
appearance of a concentration gradient.
To describe this process, one uses the equation of
the matter flow (I) generated by an electric field [5].
,UCE
dx
dC
DI (2)
where С is the impurity concentration at a distance x
from the beginning of the sample; D is a self-diffusion
coefficient of impurity ion; U – ion mobility; E is the
electric field strength.
For a sufficiently long time of electric field
application, the first term of the flux, characterizing the
reverse diffusion, and the second term, due to the action
of the electric field, balance each other, giving zero flux
and, consequently, the maximum achievable degree of
purification (Fig. 2).
Fig. 2. Distribution of impurities along the length of the
sample L under the influence of a direct current for
different periods of time t
The “saturation effect” is inherent in other methods
of deep cleaning of metals. In this regard, the question is
natural – is it possible to obtain an absolutely pure
substance? In our opinion, it is impossible.
The “saturation effect” at the refining of metals is
conditioned by the mechanisms of separation of
length of the ingot
ISSN 1562-6016. ВАНТ. 2022. №1(137) 5
impurity elements, or rather, their limiting ability in
deep refining of metals.
On the other hand, the impossibility to obtain an
absolutely pure substance is associated with technical
difficulties due to contamination of the substance to be
purified by construction materials and the environment.
The impossibility to obtain an absolutely pure
substance is, apparently, of a fundamental nature. The
realization of any process in the system is associated
with a decrease in the Gibbs energy. The basic Gibbs
energy equation is G = H – T∙S, where H – enthalpy;
S – entropy, T – temperature. The process in the system
is possible at ∆G < 0. The achievement of absolutely
pure substance in the process of refining metals can be
considered as an approximation to the state of matter
with zero entropy, which is impossible in principle.
REFERENCES
1. V.A. Pazukhin, A.Ya. Fisher. Separation and
refining of metals in a vacuum. M.: “Metallurgy”, 1969,
204 p.
2. A.I. Belyaev. Physicochemical basics of
purification of metals and semiconductor materials. M.:
“Metallurgy”, 1973, 224 p.
3. G.G. Devyatykh, Yu.E. Elliev. Introduction to
the theory of deep purification of substances. M.:
“Nauka”, 1981, 320 p.
4. G.F. Tikhinsky, G.P. Kovtun, V.M. Azhazha.
Obtaining ultrapure rare metals. M.: ”Metallurgy”,
1986, 160 p.
5. High pure substances. M.: “Nauchny Mir”, 2018,
996 p.
6. G.G. Devyatykh, Yu.A. Karpov, A.I. Osipova.
Exhibition-collection of substances of special purity.
M.: “Nauka”, 2003, 236 p.
7. L.A. Niselson, C.V. Kopecky. The problem of
purity of materials in electronics. // Vysokochystyje
veshchestva. 1988, N 2, p. 20-30 (in Russian).
8. G.F. Zaboronok, T.I. Zelentsov, A.S. Ronzhin,
B.G. Sokolov. Electronic melting of metals. M.:
“Metallurgy”, 1972, p. 251-260.
9. V. Pfan. Zone melting. M.: ”Mir”, 1970, 366 p.
Article received 08.10.2021
К ВОПРОСУ О ПРЕДЕЛЬНОЙ ОЧИСТКЕ МЕТАЛЛОВ
Г.П. Ковтун, Д.А. Солопихин, А.П. Щербань
Рассмотрены вопросы, связанные с предельным содержанием примесных элементов в металлах в
процессах их рафинирования, методами испарения и дистилляции в вакууме, зонной перекристаллизации и
электропереноса. Показано, что при всех методах рафинирования с увеличением числа циклов
рафинирования и длительности процесса возникает «эффект насыщения», связанный с достижением
предельных концентраций примесных элементов в рафинированных металлах. Обсуждены причины
невозможности получения абсолютно чистых металлов.
ДО ПИТАННЯ ПРО ГРАНИЧНЕ ОЧИЩЕННЯ МЕТАЛІВ
Г.П. Ковтун, Д.О. Солопіхін, О.П. Щербань
Розглянуті питання, пов'язані з граничним вмістом домішкових елементів у металах у процесах їх
рафінування, методами випаровування та дистиляції у вакуумі, зонної перекристалізації та
електроперенесення. Показано, що при всіх методах рафінування зі збільшенням числа циклів рафінування
та тривалості процесу виникає «ефект насичення», пов'язаний з досягненням граничних концентрацій
домішкових елементів у рафінованих металах. Обговорено причини неможливості одержання абсолютно
чистих металів.
|