Biomorphic SiC from peas and beans
Biomorphic porous SiC ceramics produced by impregnation with liquid or vapor silicon of carbon matrices derived from peas (Pisum sativum L.) and beans (Phaseolus) precursors were investigated. Optical and scanning electron microscopy was used to study the structure of ceramics. It was shown that...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2012
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nasplib_isofts_kiev_ua-123456789-1187172025-06-03T16:28:46Z Biomorphic SiC from peas and beans Kiselov, V.S. Yukhymchyk, V.A. Poludin, V.I. Tryus, M.P. Belyaev, A.E. Biomorphic porous SiC ceramics produced by impregnation with liquid or vapor silicon of carbon matrices derived from peas (Pisum sativum L.) and beans (Phaseolus) precursors were investigated. Optical and scanning electron microscopy was used to study the structure of ceramics. It was shown that SiC ceramics made from endosperm of peas and beans seeds has inherited the alveolate structure and possesses many hierarchical pores with diameters varying between 20 to 100 µm. Raman spectroscopy investigations showed that the 3C polytype is formed at a synthesis temperature of about 1550 ⁰C, and that both 3C and 6H-SiC are formed at temperatures of about 1800 ⁰C. It is shown possibilities of production of ceramic articles of various forms from seeds. 2012 Article Biomorphic SiC from peas and beans / V.S. Kiselov, V.A. Yukhymchyk, V.I. Poludin, M.P. Tryus, A.E. Belyaev // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2012. — Т. 15, № 4. — С. 305-309. — Бібліогр.: 18 назв. — англ. 1560-8034 PACS 77.84.Bw, 82.80.Gk https://nasplib.isofts.kiev.ua/handle/123456789/118717 en Semiconductor Physics Quantum Electronics & Optoelectronics application/pdf Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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English |
| description |
Biomorphic porous SiC ceramics produced by impregnation with liquid or
vapor silicon of carbon matrices derived from peas (Pisum sativum L.) and beans
(Phaseolus) precursors were investigated. Optical and scanning electron microscopy was
used to study the structure of ceramics. It was shown that SiC ceramics made from
endosperm of peas and beans seeds has inherited the alveolate structure and possesses
many hierarchical pores with diameters varying between 20 to 100 µm. Raman
spectroscopy investigations showed that the 3C polytype is formed at a synthesis
temperature of about 1550 ⁰C, and that both 3C and 6H-SiC are formed at temperatures
of about 1800 ⁰C. It is shown possibilities of production of ceramic articles of various
forms from seeds. |
| format |
Article |
| author |
Kiselov, V.S. Yukhymchyk, V.A. Poludin, V.I. Tryus, M.P. Belyaev, A.E. |
| spellingShingle |
Kiselov, V.S. Yukhymchyk, V.A. Poludin, V.I. Tryus, M.P. Belyaev, A.E. Biomorphic SiC from peas and beans Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Kiselov, V.S. Yukhymchyk, V.A. Poludin, V.I. Tryus, M.P. Belyaev, A.E. |
| author_sort |
Kiselov, V.S. |
| title |
Biomorphic SiC from peas and beans |
| title_short |
Biomorphic SiC from peas and beans |
| title_full |
Biomorphic SiC from peas and beans |
| title_fullStr |
Biomorphic SiC from peas and beans |
| title_full_unstemmed |
Biomorphic SiC from peas and beans |
| title_sort |
biomorphic sic from peas and beans |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2012 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/118717 |
| citation_txt |
Biomorphic SiC from peas and beans / V.S. Kiselov, V.A. Yukhymchyk, V.I. Poludin, M.P. Tryus, A.E. Belyaev // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2012. — Т. 15, № 4. — С. 305-309. — Бібліогр.: 18 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
| work_keys_str_mv |
AT kiselovvs biomorphicsicfrompeasandbeans AT yukhymchykva biomorphicsicfrompeasandbeans AT poludinvi biomorphicsicfrompeasandbeans AT tryusmp biomorphicsicfrompeasandbeans AT belyaevae biomorphicsicfrompeasandbeans |
| first_indexed |
2025-11-27T04:18:59Z |
| last_indexed |
2025-11-27T04:18:59Z |
| _version_ |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 305-309.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
305
PACS 77.84.Bw, 82.80.Gk
Biomorphic SiC from peas and beans
V.S. Kiselov, V.A. Yukhymchyk, V.I. Poludin, M.P. Tryus, A.E. Belyaev
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
E-mail: vit_kiselov@ukr.net
Abstract. Biomorphic porous SiC ceramics produced by impregnation with liquid or
vapor silicon of carbon matrices derived from peas (Pisum sativum L.) and beans
(Phaseolus) precursors were investigated. Optical and scanning electron microscopy was
used to study the structure of ceramics. It was shown that SiC ceramics made from
endosperm of peas and beans seeds has inherited the alveolate structure and possesses
many hierarchical pores with diameters varying between 20 to 100 µm. Raman
spectroscopy investigations showed that the 3C polytype is formed at a synthesis
temperature of about 1550 C, and that both 3C and 6H-SiC are formed at temperatures
of about 1800 C. It is shown possibilities of production of ceramic articles of various
forms from seeds.
Keywords: biomorphic SiC ceramics, Raman scattering, SEM.
Manuscript received 03.07.12; revised version received 26.09.12; accepted for
publication 17.10.12; published online 12.12.12.
1. Introduction
In the recent decade, great efforts were devoted to the
development of different kinds of biotechnologies,
which is caused by a wide spectrum of problems in
medicine, pharmacology and industry. One of the ways
of preparation of new porous biomaterials is the method
of biotemplating. It used different biological objects for
synthesis of new porous materials. These materials are
pseudomorphic to the initial biological objects on the
micro-, meso-, and macroscales. Biotemplating is a
relatively new way of production of ceramic materials
with a complicated porous structure. This alternative
technique is very simple and cost-effective as compared
to traditional lithography that is widely used in
microelectronics. Cellular structures available in nature
are particularly interesting as natural replica templates,
mainly due to their specific pore morphology and
intricate microstructures, which might be difficult to
produce artificially. Materials containing predetermined
porosity exhibit special properties and features. They are
interesting due to their particular properties such as low
density, low thermal conductivity, thermal stability,
high-surface area, and high permeability. Nowadays,
except for medicine, such porous materials find many
applications as filters, diesel particulate filters, catalyst
and membrane supports. There are known works on
using millet [1], egg shell membrane, butterfly wings
and sea urchin [2-4], marine algae and plants [5] for
producing biomorphic SiC materials. Following the
same biomimetic approach, many investigators have
studied the transformation of wood cellular structures
into macro porous ceramics [ 116 ].
In this work, we obtain and investigate porous SiC
ceramics prepared from the family of leguminous
cultures – peas and beans.
2. Experimental details
2.1. The first step. Production and investigation of
carbon matrix
In our investigations, we used seeds of dicotyledonous
(Licotyledoneae) metsperm plants – peas (Pisum sativum
L.) and beans (Phaseolus). Preparation of biomorphic
SiC includes two steps. First, a carbon template is
prepared using the pyrolysis of natural specimens in
inert atmosphere. The specimens were heated for 6-8 h
in a horizontal silica pipe furnace in argon flow
(�10 cm3/min). To avoid the collapse of the specimen
structure, it was slowly heated up to 400 C with the rate
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 305-309.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
306
1-2 C/min. Subsequently, the temperature was increased
up to 900 C with the rate 5-6 C/min to obtain a porous
carbon template. In some special cases, we realize
pyrolysis up to 2000 C. During the pyrolysis, H2O, CO2,
acids, carbonyl groups and alcohols are released due to
decomposition of biopolymer structures at temperatures
above 600 C. Meanwhile, the major biopolymer
constituents of cell wall materials are rearranged and
converted into carbon struts.
Under pyrolysis at the initial stage (Т ≤ 400 С), the
seeds of peas, beans, millets and wheat are increased in
their sizes. Subsequent growth of the temperature leads
to shrinkage up to 20-25%. Therefore, we carry out this
operation by two ways: in closed volume and in open
volume. In the first way, we obtained a cemented carbon
matrix having the form of closed volume used under
pyrolysis. In the second way, we produced a carbon
matrix from individual seeds. The unique anatomical
features of the native beans tissue can be retained during
pyrolysis, which yields a porous template composed
primarily of carbon.
Peas and beans define the presence of two separate
cotyledons. In Fig. 1а, the structure of the seeds is
shown. It consists of endosperm (1), embryo (2), seed
coat (3) and inner shells (4). The SEM images of the
surfaces of seed coat and inner shells are shown in
Fig. 1b. To investigate the inner microstructure of the
carbon template, a complete carbon matrix was split
across in three cross directions A-A, B-B and C-C.
Fig. 2a shows the SEM images of the cross section of
bean. From these images, it can be seen that the carbon
template has indeed inherited the alveolate structure of
endosperm and possesses many hierarchical pores with
diameters varying between 20 to 100 µm. These pores
are almost uniformly distributed. The size of pores in
matrix increases with sizes of the initial seeds.
Fig. 1. The structure of the carbon matrix made from seeds of
dicotyledonous (Licotyledoneae) metsperm plants (а) and the
photo of the surface of seed coats and inner shells (b).
2.2. The second step.
Impregnation by Si and synthesis of SiC
At the second step, the carbon template is transformed
into bio-SiC by impregnation with Si and subsequent
synthesis of SiC. For impregnation, we removed seed
coats and inner coats from the seeds. The REDMET-30
industrial furnace was used for thermal operations.
Carbon matrix and silicon were added to individual
crucibles from tight graphite. Impregnation was
realized under a variable pressure of He within the
temperature range T = 1600–1900 °C and duration ts =
10–30 min. At high temperatures, liquid or vapor
silicon spontaneously infiltrated into the carbon
template and produced SiC ceramics. It should be noted
that due to small sizes and irregular shape of
specimens, it was difficult to provide impregnation
only with liquid or vapor silicon. In certain cases on the
surface of ceramics we can observe areas obtained by
impregnation with liquid and vapor silicon.
Biomorphic SiC ceramics depending on the
impregnation method with liquid or vapor silicon can
be crystalline or amorphous. As a rule, the ceramics
prepared using impregnation with liquid silicon
consists of SiC faceted microcrystals with the size 4–
5 µm, Fig. 3a. It was ascertained that the mechanical
properties, structure and color of biomorphic SiC
depend on the carbon template infiltration method [12].
Melt impregnation resulted in final material with high
mechanical strength and green-brown color, while
impregnation with Si vapor resulted in final material of
white color with amorphous structure, Fig. 3b. If
required, excess carbon in final products was burn out
in the furnace in oxygen atmosphere at T = 900 °C for
2 h.
Fig. 2. The surface SEM image of the carbon matrix from
beans ((a), (b)), peas (с). Dependence of pore size d on
longitudinal sizes of carbon matrices (d).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 305-309.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
307
Fig. 3. SEM image of surface of the biomorphic SiC ceramics:
(а) peas, impregnation with liquid silicon, (b) peas,
impregnation with vapor silicon, and (с) beans, impregnation
with vapor silicon.
Fig. 4. Typical ceramic samples obtained by pyrolysis in
closed volume and subsequent impregnation with liquid
silicon (a); SEM image of surface of the biomorphic SiC
ceramics from parts of Agarics: underside of cap (pileus) (b),
plates from stipe (c), and that from potato plates ((d), (f)).
In Fig. 4a, it is shown the ceramic sample obtained
by pyrolysis in closed volume and subsequent
impregnation with liquid silicon. So, this process open
possibilities for production of ceramic articles with
various forms. In addition, it must be noted that by this
method biomorphic SiC materials can be obtain from
many foodstuffs. In particular, we produce such porous
ceramics from parts of common mushroom (Agaricus)
(Figs. 4b and 4c) and potato plates (Figs. 4d and 4e).
3. Results and discussion
The structure and composition of the materials obtained
were investigated by Raman spectroscopy. Fig. 5 shows
Raman spectra of the carbon matrix that was obtained
directly after pyrolysis (curve 1) and after additional
high-temperature annealing (curves 2 and 3). The
quantitative characterization of the structural
imperfection of graphite by using Raman spectroscopy
was first done in the work [13]. The authors [13]
suggested estimating the value of the correlation length
La in the plane of the graphite sheet (in other words – the
areal size of the graphite cluster) based on the intensity
ratio typical for D and G bands:
aL
C
GI
DI )(
)(
)(
, (1)
where С(λ) = 4.4 nm for λ = 514.5 nm. After
deconvolution of the experimental Raman bands by
components and calculation of their integrated intensity,
we obtained the aL based on Eq. (1) equal to 1.5 and
4.1 nm for spectra 2 and 3, respectively. This analysis
shows, therefore, that the minimum size of the graphite
clusters increased after the thermal treatment. Note that
Eq. (1) is valid for the quantitative estimation of a
confined sp2 structure only when its imperfection is
related with confinement in the graphite sheet plane.
Moreover, it was assumed in [13] that the graphite
nanocrystallites are monomodal with respect to the aL -
dimension. At the same time, for an ensemble of
crystallites with the size distribution, in which the
portion of nanocrystallites with size aiL comprises the
iX part of the cluster, the effective value of effaL , is
given by [14]:
ai
N
i
i
effa L
X
L
11
,
. (2)
In this case, the correlation length calculated in accord
with Eq. (1) is dominated by the smallest Laі. As to other
kinds of distortion of the graphite spatial symmetry (for
example due to a high concentration of impurities), for
each of them С(λ) in (1) will be different.
In addition to the intensity ratio of D and G bands,
a degree of disorder is frequently estimated from their
frequency shifts and bandwidths (full width at half
maximum, FWHM). For the structures under study here,
the FWHM decreases after annealing, indicating the
reduced concentration of defects in carbon matrix. Note
that since during the synthesis of bio-SiC, the carbon
matrices formed by pyrolysis are kept for a certain time
within the given temperature range (1450-2000 C), they
were not additionally annealed.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 305-309.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
308
Fig. 5. Raman spectra of carbon template after pyrolysis at
900 C (1) and after additional high-temperature
annealing (2, 3).
Fig. 6. Raman spectra of bio-SiC ceramics produced by
impregnation with gaseous silicon (1) and liquid silicon (2).
The inset shows deconvolution of TO bands in spectra of 6H
and 3C polytypes by separate Lorenz components.
In Fig. 6, the Raman spectra of formed SiC
ceramics are shown. Curves 1 and 2 correspond to the
SiC structures formed by impregnation of carbon matrix
with gaseous and liquid silicon, respectively. It can be
concluded from observation of the transversal optical
mode of SiC ( 1
TO cm796 ) that silicon carbide was
formed in both cases. In addition, the band related with
SiSi vibrations, 1
Si-Si cm520 , is observed, and it
indicates to some unreacted silicon remaining. We
showed earlier [15] that the phase composition of
synthesized ceramics depends on the mass ratio of initial
silicon and carbon matrix, ψ = Si/C. It can be concluded
from the higher intensity of the SiSi mode in the
Raman spectra (Fig. 6) that the total volume of the
unreacted silicon is larger in the case of impregnation
with liquid silicon. The D and G bands related with the
carbon matrix were not detected, which can be caused by
additional annealing of the synthesized structures at
700 C in air (spectra are not shown). The spectrum of
SiC synthesized by impregnation of carbon matrix with
gaseous silicon (Fig. 6, curve 1) reveals the intense TO
and weak unstructured LO bands. Analysis of the TO
band shows that it is a superposition of TO modes 3C
and 6H SiC polytypes. This is in agreement with our
previous study [15], where bio-SiC formed from wood
was shown to contain only 3C polytype for synthesis
temperatures up to Tmaх 1550 C, while for
temperatures up to 1800 C both 3C and 6H polytypes
coexist.
As to the weak intensity of the LO bands, this fact
is due to the different mechanisms of the Raman
scattering by LO and TO phonons – the Frölich and
deformation potential coupling mechanisms. In the work
[16], the Raman scattering cross section, , was shown
to depend on the excitation quanta energy, , as
1 gE for deformation potential and as
3 gE for Frölich mechanism, with gE being
the band gap. It follows from the above relations that the
scattering by LO phonons will weaken much faster with
increasing the separation between the excitation energy
and electronic transition in the structure, as compared to
the scattering by TO phonons. Thus, the LO mode can
be observed only under resonance or near-resonant
excitation conditions. The predictions of the work [16]
were supported experimentally in the works [17, 18]
which studied the excitation wavelength dependence of
the Raman scattering in SiC nanorods. On the other
hand, the broad unstructured band between 800 and
1cm950 in the spectra in Fig. 6 can be an indication of
the SiC being partly in the amorphous phase.
The Raman spectrum of bio-SiC formed by
impregnation of carbon matrix with liquid silicon
(Fig. 6, curve 2) differs from that corresponding to
impregnation with gaseous silicon. The TO band has one
intense peak that corresponds to the contribution of 3C
polytype dominating over that of the 6H. Thus, we
conclude that under this synthesis conditions the portion
of the 6H polytype is smaller than that in 3C. The
presence of LO peak evidences a less amorphous phase
in this sample.
It should be noted that for both SiC structures we
observed a strong photoluminescence under excitation
with 514.5 nm laser line. Further investigations are
needed, however, in order to ascertain whether this
luminescence is related with synthesized bio-SiC or with
nanocrystalline Si in the pores of the structure.
4. Conclusion
Biomorphic SiC ceramics produced by impregnation
with liquid or vapor silicon of carbon matrices derived
from peas (Pisum sativum L.) and beans (Phaseolus)
precursors have been investigated. The structure and
composition of the materials obtained were investigated
using different techniques such as optical microscopy,
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 305-309.
© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
309
scanning electron microscopy, and Raman spectroscopy.
It has been shown that the carbon matrix and
consequently SiC ceramics made from endosperm has
indeed inherited the alveolate structure and possesses
many hierarchical pores with diameters varying from 20
up to 100 µm. Raman spectroscopy investigations
showed that the 3C polytype is formed at the synthesis
temperature close to 1550 C, and that both 3C and 6H-
SiC are formed at temperatures up to 1800 C.
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P.M. Lytvyn, V.I. Poludin, V.O. Yukhymchuk,
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2012. V. 15, N 4. P. 305-309.
PACS 77.84.Bw, 82.80.Gk
Biomorphic SiC from peas and beans
V.S. Kiselov, V.A. Yukhymchyk, V.I. Poludin, M.P. Tryus, A.E. Belyaev
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine
E-mail: vit_kiselov@ukr.net
Abstract. Biomorphic porous SiC ceramics produced by impregnation with liquid or vapor silicon of carbon matrices derived from peas (Pisum sativum L.) and beans (Phaseolus) precursors were investigated. Optical and scanning electron microscopy was used to study the structure of ceramics. It was shown that SiC ceramics made from endosperm of peas and beans seeds has inherited the alveolate structure and possesses many hierarchical pores with diameters varying between 20 to 100 µm. Raman spectroscopy investigations showed that the 3C polytype is formed at a synthesis temperature of about 1550 (C, and that both 3C and 6H-SiC are formed at temperatures of about 1800 (C. It is shown possibilities of production of ceramic articles of various forms from seeds.
Keywords: biomorphic SiC ceramics, Raman scattering, SEM.
Manuscript received 03.07.12; revised version received 26.09.12; accepted for publication 17.10.12; published online 12.12.12.
1. Introduction
In the recent decade, great efforts were devoted to the development of different kinds of biotechnologies, which is caused by a wide spectrum of problems in medicine, pharmacology and industry. One of the ways of preparation of new porous biomaterials is the method of biotemplating. It used different biological objects for synthesis of new porous materials. These materials are pseudomorphic to the initial biological objects on the micro-, meso-, and macroscales. Biotemplating is a relatively new way of production of ceramic materials with a complicated porous structure. This alternative technique is very simple and cost-effective as compared to traditional lithography that is widely used in microelectronics. Cellular structures available in nature are particularly interesting as natural replica templates, mainly due to their specific pore morphology and intricate microstructures, which might be difficult to produce artificially. Materials containing predetermined porosity exhibit special properties and features. They are interesting due to their particular properties such as low density, low thermal conductivity, thermal stability, high-surface area, and high permeability. Nowadays, except for medicine, such porous materials find many applications as filters, diesel particulate filters, catalyst and membrane supports. There are known works on using millet [1], egg shell membrane, butterfly wings and sea urchin [2-4], marine algae and plants [5] for producing biomorphic SiC materials. Following the same biomimetic approach, many investigators have studied the transformation of wood cellular structures into macro porous ceramics [
11
6
-
].
In this work, we obtain and investigate porous SiC ceramics prepared from the family of leguminous cultures – peas and beans.
2. Experimental details
2.1. The first step. Production and investigation of carbon matrix
In our investigations, we used seeds of dicotyledonous (Licotyledoneae) metsperm plants – peas (Pisum sativum L.) and beans (Phaseolus). Preparation of biomorphic SiC includes two steps. First, a carbon template is prepared using the pyrolysis of natural specimens in inert atmosphere. The specimens were heated for 6-8 h in a horizontal silica pipe furnace in argon flow (∼10 cm3/min). To avoid the collapse of the specimen structure, it was slowly heated up to 400 (C with the rate 1-2 (C/min. Subsequently, the temperature was increased up to 900 (C with the rate 5-6 (C/min to obtain a porous carbon template. In some special cases, we realize pyrolysis up to 2000 (C. During the pyrolysis, H2O, CO2, acids, carbonyl groups and alcohols are released due to decomposition of biopolymer structures at temperatures above 600 (C. Meanwhile, the major biopolymer constituents of cell wall materials are rearranged and converted into carbon struts.
Under pyrolysis at the initial stage (Т ≤ 400 (С), the seeds of peas, beans, millets and wheat are increased in their sizes. Subsequent growth of the temperature leads to shrinkage up to 20-25%. Therefore, we carry out this operation by two ways: in closed volume and in open volume. In the first way, we obtained a cemented carbon matrix having the form of closed volume used under pyrolysis. In the second way, we produced a carbon matrix from individual seeds. The unique anatomical features of the native beans tissue can be retained during pyrolysis, which yields a porous template composed primarily of carbon.
Peas and beans define the presence of two separate cotyledons. In Fig. 1а, the structure of the seeds is shown. It consists of endosperm (1), embryo (2), seed coat (3) and inner shells (4). The SEM images of the surfaces of seed coat and inner shells are shown in Fig. 1b. To investigate the inner microstructure of the carbon template, a complete carbon matrix was split across in three cross directions A-A, B-B and C-C. Fig. 2a shows the SEM images of the cross section of bean. From these images, it can be seen that the carbon template has indeed inherited the alveolate structure of endosperm and possesses many hierarchical pores with diameters varying between 20 to 100 µm. These pores are almost uniformly distributed. The size of pores in matrix increases with sizes of the initial seeds.
Fig. 1. The structure of the carbon matrix made from seeds of dicotyledonous (Licotyledoneae) metsperm plants (а) and the photo of the surface of seed coats and inner shells (b).
2.2. The second step.
Impregnation by Si and synthesis of SiC
At the second step, the carbon template is transformed into bio-SiC by impregnation with Si and subsequent synthesis of SiC. For impregnation, we removed seed coats and inner coats from the seeds. The REDMET-30 industrial furnace was used for thermal operations. Carbon matrix and silicon were added to individual crucibles from tight graphite. Impregnation was realized under a variable pressure of He within the temperature range T = 1600–1900 °C and duration ts = 10–30 min. At high temperatures, liquid or vapor silicon spontaneously infiltrated into the carbon template and produced SiC ceramics. It should be noted that due to small sizes and irregular shape of specimens, it was difficult to provide impregnation only with liquid or vapor silicon. In certain cases on the surface of ceramics we can observe areas obtained by impregnation with liquid and vapor silicon.
Biomorphic SiC ceramics depending on the impregnation method with liquid or vapor silicon can be crystalline or amorphous. As a rule, the ceramics prepared using impregnation with liquid silicon consists of SiC faceted microcrystals with the size 4–5 µm, Fig. 3a. It was ascertained that the mechanical properties, structure and color of biomorphic SiC depend on the carbon template infiltration method [12]. Melt impregnation resulted in final material with high mechanical strength and green-brown color, while impregnation with Si vapor resulted in final material of white color with amorphous structure, Fig. 3b. If required, excess carbon in final products was burn out in the furnace in oxygen atmosphere at T = 900 °C for 2 h.
Fig. 2. The surface SEM image of the carbon matrix from beans ((a), (b)), peas (с). Dependence of pore size d on longitudinal sizes of carbon matrices (d).
Fig. 3. SEM image of surface of the biomorphic SiC ceramics: (а) peas, impregnation with liquid silicon, (b) peas, impregnation with vapor silicon, and (с) beans, impregnation with vapor silicon.
Fig. 4. Typical ceramic samples obtained by pyrolysis in closed volume and subsequent impregnation with liquid silicon (a); SEM image of surface of the biomorphic SiC ceramics from parts of Agarics: underside of cap (pileus) (b), plates from stipe (c), and that from potato plates ((d), (f)).
In Fig. 4a, it is shown the ceramic sample obtained by pyrolysis in closed volume and subsequent impregnation with liquid silicon. So, this process open possibilities for production of ceramic articles with various forms. In addition, it must be noted that by this method biomorphic SiC materials can be obtain from many foodstuffs. In particular, we produce such porous ceramics from parts of common mushroom (Agaricus) (Figs. 4b and 4c) and potato plates (Figs. 4d and 4e).
3. Results and discussion
The structure and composition of the materials obtained were investigated by Raman spectroscopy. Fig. 5 shows Raman spectra of the carbon matrix that was obtained directly after pyrolysis (curve 1) and after additional high-temperature annealing (curves 2 and 3). The quantitative characterization of the structural imperfection of graphite by using Raman spectroscopy was first done in the work [13]. The authors [13] suggested estimating the value of the correlation length La in the plane of the graphite sheet (in other words – the areal size of the graphite cluster) based on the intensity ratio typical for D and G bands:
a
L
C
G
I
D
I
)
(
)
(
)
(
l
=
,
(1)
where С(λ) = 4.4 nm for λ = 514.5 nm. After deconvolution of the experimental Raman bands by components and calculation of their integrated intensity, we obtained the
a
L
based on Eq. (1) equal to 1.5 and 4.1 nm for spectra 2 and 3, respectively. This analysis shows, therefore, that the minimum size of the graphite clusters increased after the thermal treatment. Note that Eq. (1) is valid for the quantitative estimation of a confined sp2 structure only when its imperfection is related with confinement in the graphite sheet plane. Moreover, it was assumed in [13] that the graphite nanocrystallites are monomodal with respect to the
a
L
-dimension. At the same time, for an ensemble of crystallites with the size distribution, in which the portion of nanocrystallites with size
ai
L
comprises the
i
X
part of the cluster, the effective value of
eff
a
L
,
is given by [14]:
ai
N
i
i
eff
a
L
X
L
1
1
,
å
=
.
(2)
In this case, the correlation length calculated in accord with Eq. (1) is dominated by the smallest Laі. As to other kinds of distortion of the graphite spatial symmetry (for example due to a high concentration of impurities), for each of them С(λ) in (1) will be different.
In addition to the intensity ratio of D and G bands, a degree of disorder is frequently estimated from their frequency shifts and bandwidths (full width at half maximum, FWHM). For the structures under study here, the FWHM decreases after annealing, indicating the reduced concentration of defects in carbon matrix. Note that since during the synthesis of bio-SiC, the carbon matrices formed by pyrolysis are kept for a certain time within the given temperature range (1450-2000 (C), they were not additionally annealed.
Fig. 5. Raman spectra of carbon template after pyrolysis at 900 (C (1) and after additional high-temperature annealing (2, 3).
Fig. 6. Raman spectra of bio-SiC ceramics produced by impregnation with gaseous silicon (1) and liquid silicon (2). The inset shows deconvolution of TO bands in spectra of 6H and 3C polytypes by separate Lorenz components.
In Fig. 6, the Raman spectra of formed SiC ceramics are shown. Curves 1 and 2 correspond to the SiC structures formed by impregnation of carbon matrix with gaseous and liquid silicon, respectively. It can be concluded from observation of the transversal optical mode of SiC (
1
TO
cm
796
-
=
w
) that silicon carbide was formed in both cases. In addition, the band related with
Si
Si
-
vibrations,
1
Si
-
Si
cm
520
-
=
w
, is observed, and it indicates to some unreacted silicon remaining. We showed earlier [15] that the phase composition of synthesized ceramics depends on the mass ratio of initial silicon and carbon matrix, ψ = Si/C. It can be concluded from the higher intensity of the
Si
Si
-
mode in the Raman spectra (Fig. 6) that the total volume of the unreacted silicon is larger in the case of impregnation with liquid silicon. The D and G bands related with the carbon matrix were not detected, which can be caused by additional annealing of the synthesized structures at 700 (C in air (spectra are not shown). The spectrum of SiC synthesized by impregnation of carbon matrix with gaseous silicon (Fig. 6, curve 1) reveals the intense TO and weak unstructured LO bands. Analysis of the TO band shows that it is a superposition of TO modes 3C and 6H SiC polytypes. This is in agreement with our previous study [15], where bio-SiC formed from wood was shown to contain only 3C polytype for synthesis temperatures up to Tmaх ( 1550 (C, while for temperatures up to 1800 (C both 3C and 6H polytypes coexist.
As to the weak intensity of the LO bands, this fact is due to the different mechanisms of the Raman scattering by LO and TO phonons – the Frölich and deformation potential coupling mechanisms. In the work [16], the Raman scattering cross section,
s
, was shown to depend on the excitation quanta energy,
w
h
, as
(
)
1
-
w
-
µ
s
h
g
E
for deformation potential and as
(
)
3
-
w
-
µ
s
h
g
E
for Frölich mechanism, with
g
E
being the band gap. It follows from the above relations that the scattering by LO phonons will weaken much faster with increasing the separation between the excitation energy and electronic transition in the structure, as compared to the scattering by TO phonons. Thus, the LO mode can be observed only under resonance or near-resonant excitation conditions. The predictions of the work [16] were supported experimentally in the works [17, 18] which studied the excitation wavelength dependence of the Raman scattering in SiC nanorods. On the other hand, the broad unstructured band between 800 and
1
cm
950
-
in the spectra in Fig. 6 can be an indication of the SiC being partly in the amorphous phase.
The Raman spectrum of bio-SiC formed by impregnation of carbon matrix with liquid silicon (Fig. 6, curve 2) differs from that corresponding to impregnation with gaseous silicon. The TO band has one intense peak that corresponds to the contribution of 3C polytype dominating over that of the 6H. Thus, we conclude that under this synthesis conditions the portion of the 6H polytype is smaller than that in 3C. The presence of LO peak evidences a less amorphous phase in this sample.
It should be noted that for both SiC structures we observed a strong photoluminescence under excitation with 514.5 nm laser line. Further investigations are needed, however, in order to ascertain whether this luminescence is related with synthesized bio-SiC or with nanocrystalline Si in the pores of the structure.
4. Conclusion
Biomorphic SiC ceramics produced by impregnation with liquid or vapor silicon of carbon matrices derived from peas (Pisum sativum L.) and beans (Phaseolus) precursors have been investigated. The structure and composition of the materials obtained were investigated using different techniques such as optical microscopy, scanning electron microscopy, and Raman spectroscopy. It has been shown that the carbon matrix and consequently SiC ceramics made from endosperm has indeed inherited the alveolate structure and possesses many hierarchical pores with diameters varying from 20 up to 100 µm. Raman spectroscopy investigations showed that the 3C polytype is formed at the synthesis temperature close to 1550 (C, and that both 3C and 6H-SiC are formed at temperatures up to 1800 (C.
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© 2012, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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