Direct electrochemical preparation of nanostructured silicon carbide and its nitridation behavior

Article abstract of DOI:10.1149/2.0591814jes

Silicon carbide was synthesized from mixtures of SiO2 and graphite by applying the concept of the FFC-Cambridge process and several fundamental aspects of the synthesis route were investigated. Porous disks composed of powders of SiO2 and graphite in molar ratios of 1:0.5, 1:1 and 1:1.5 were prepared by sintering in inert atmosphere and subjected to electro-deoxidation in molten CaCl2 at 1173 K under a range of experimental conditions. Disks of molar ratio 1:1.5, reduced at an applied voltage of 2.8 V for a duration of 6 h, yielded exclusively phase-pure SiC of nanowire morphology as the reaction product, while the other precursor compositions provided significant amounts of calcium silicides. Voltages lower than 2.8 V gave mixtures of SiC with elemental Si and graphite, and voltages higher than that gave CaSi alone. Shorter electro-deoxidation times led to incomplete reduction and allowed for the identification of CaSiO3 as a transient phase. Based on the experimental results a multipath reaction mechanism is proposed, consisting of the electrochemical reduction of SiO2 and CaSiO3 to Si and the subsequent in-situ carbonization of the Si formed to SiC. The effect of N2 at high temperature on the electrochemically synthesized SiC was investigated and the formation of nanowire Si2N2O was observed. Overall, the process presented is a facile single-step and low-temperature method for the synthesis of SiC with possible commercial prospects.

Full text of DOI:10.1149/2.0591814jes

Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
D731
Direct Electrochemical Preparation of Nanostructured Silicon
Carbide and Its Nitridation Behavior
1
,2,z
1,2
1
3
D. Sri Maha Vishnu,
Jagadeesh Sure,
Hyun-Kyung Kim, Ji-Young Kim,
1
1,2,∗
R. Vasant Kumar, and Carsten Schwandt
1
Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom
Department of Materials Science and Metallurgy, University of Nizwa, 616 Nizwa, Sultanate of Oman
Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 02792, Korea
2
3
Silicon carbide was synthesized from mixtures of SiO2 and graphite by applying the concept of the FFC-Cambridge process and
several fundamental aspects of the synthesis route were investigated. Porous disks composed of powders of SiO2 and graphite in
molar ratios of 1:0.5, 1:1 and 1:1.5 were prepared by sintering in inert atmosphere and subjected to electro-deoxidation in molten
CaCl2 at 1173 K under a range of experimental conditions. Disks of molar ratio 1:1.5, reduced at an applied voltage of 2.8 V for
a duration of 6 h, yielded exclusively phase-pure SiC of nanowire morphology as the reaction product, while the other precursor
compositions provided significant amounts of calcium silicides. Voltages lower than 2.8 V gave mixtures of SiC with elemental
Si and graphite, and voltages higher than that gave CaSi alone. Shorter electro-deoxidation times led to incomplete reduction and
allowed for the identification of CaSiO3 as a transient phase. Based on the experimental results a multipath reaction mechanism is
proposed, consisting of the electrochemical reduction of SiO2 and CaSiO3 to Si and the subsequent in-situ carbonization of the Si
formed to SiC. The effect of N2 at high temperature on the electrochemically synthesized SiC was investigated and the formation of
nanowire Si2N2O was observed. Overall, the process presented is a facile single-step and low-temperature method for the synthesis
of SiC with possible commercial prospects.
©
The Author(s) 2018. Published by ECS. This is an open access article distributed under the terms of the Creative Commons
Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any
medium, provided the original work is properly cited. [DOI: 10.1149/2.0591814jes]
Manuscript submitted August 20, 2018; revised manuscript received October 15, 2018. Published October 30, 2018.
Silicon carbide is an important non-oxide ceramic material because
of its desirable properties, such as high melting point, high hardness
the electrolyte. Many metal oxides and mixtures of metal oxides have
been electro-deoxidized in this way to form their parent metals and
alloys. The scope and the versatility of the FFC-Cambridge process
concerning the oxide precursors used and the metal products made
1
and strength, and excellent wear resistance and chemical inertness.
3
SiC has a relatively low density, of 3.21 g/cm , compared with other
1
9–11
refractory metal carbides, and it is a wide-bandgap semiconductor.
have been reviewed extensively.
Due to this superior blend of thermal, mechanical, chemical and elec-
trical properties, SiC finds diverse applications, as heating elements,
as components in automobile brakes and clutches, as ceramic plates
in bulletproof vests, as an abrasive, and in electronic devices. SiC is
also used as an additive in a variety of composite materials. A more
recent research focus has been on nanostructured SiC because of its
attractive applications in electrical and energy devices and as a support
The FFC-Cambridge process can be modified for the synthesis of
metal carbides, by employing a precursor material as the cathode that
comprises a mixture of metal oxide and carbon of suitable ratio. Under
such conditions, the metal released from its oxide under the influence
of the cathodic potential reacts in-situ with the carbon present in the
cathode to form the respective metal carbide. Various carbides have
12,13
14,13
15,13
16
been made like this, including TiC,
ZrC,
TaC, WC, and SiC.20 For the latter, the elec-
trolytic cell can be represented as follows.
HfC,
3 2
Cr C ,
2
17
18,13
13
19
material for catalysts.
Cr
7
C
3
,
NbC,
SiC is commercially synthesized by carbothermic reduction of
2
SiO at very high temperature. The commonly used method is the
Acheson process in which silica sand is reacted with petroleum coke
(
+) C |CaCl
2
| SiO
2
, C (−)
[1]
1
,3
at above 2773 K in a graphite electric resistance furnace. The
method is laborious and energy intensive and yields a bulky mate-
The metal carbides made by electro-deoxidation were typically in
the form of powders and consisted of particles in the nanometer range.
Zou et al. reported the preparation of SiC from SiO
molten CaCl using an yttria-stabilized zirconia solid oxide membrane
as the anode thereby avoiding a carbon anode. The same group also
reported the formation of SiC nanowires from SiO /C via a combined
1
rial with relatively low surface area and many impurities. SiC can
also be prepared by various alternative routes. Some of these are gas-
phase reactions at high temperature, for instance, the decomposition of
2
/C precursors in
2
organosilicon compounds such as methyltrichlorosilane (CH
3
SiCl
in the pres-
, and the reduction-carburization from Si powder and
3
)
21
4
2 4 4
in the presence of H , the reaction of SiCl and CCl
5
2
ence of H
CCl with Na as a reductant. Further methods are mechanical alloy-
2
22
solid-state reduction and dissolution-deposition mechanism and the
further processing of these wires into mesoporous structures for use
6
4
1
ing, liquid phase sintering, arc melting, and sol-gel precipitation.
However, all these methods have drawbacks in that they require
high energy input, multiple process steps and/or the presence of a
substrate.
23
in supercapacitors.
Despite the earlier reports there are still many uncertainties that
prevent the straightforward implementation of the FFC-Cambridge
process for the synthesis of SiC at a large scale and with controled mor-
phology. One aspect concerns the optimum precursor composition.
This is because it has been found in general that the carbon content of
the oxide/carbon precursor has to be higher than the quantity required
There is a growing demand for processes that allow the synthesis
of metal carbides, particularly SiC, at temperatures lower than those of
7
8
the conventional production method. The FFC-Cambridge process is
a promising approach in this regard. In this, a metal oxide or semimetal
oxide serves as the cathode in an electrolytic cell that furthermore
14,15,17,18
stoichiometrically due to loss of carbon during processing,
20
21
although Zhao et al. and Zou et al. have reported that SiC could
be made from SiO /C of molar ratio 1:1. Another issue concerns the
2
contains a molten salt electrolyte of typically CaCl and an anode
2
of typically carbon. The cell is then polarized such that the potential
of the cathode is negative enough to cause expulsion of oxides ions
into the electrolyte, but positive enough to preclude electrolysis of
electrochemical process parameters. This is because the main and the
side reactions are sensitive to the cathodic potential applied.
2
The intended cathodic reaction is the electro-deoxidation of SiO to
Si followed by the immediate in-situ reaction of the Si with the carbon
in the cathode to yield SiC. The reactions read as follows, with the
standard electrode potentials calculated from thermodynamic data,
∗
Electrochemical Society Member.
E-mail: smvd2@cam.ac.uk; vishnu@unizwa.edu.om
z
24
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D732
Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
depending on whether they are written in one step,
−
2−
SiO
2
+ 4 e + C = SiC + 2O
E01173 K = 0.999 V vs. Ca/Ca
2+
[2]
or in two steps,
−
2−
SiO
2
+ 4 e → Si + 2 O
E01173 K = 0.837 V vs. Ca/Ca
2+
[3]
[4]
Si + C → SiC
The undesired cathodic reactions are characterized by the involve-
ment of calcium from the electrolyte. These include the deposition of
Figure 1. Schematic of competing reactions occurring at a SiO2/graphite sam-
ple under cathodic polarization in CaCl2 melt.
2
+
Ca, the in-situ reaction of Ca ions with SiO
such as CaSi and Ca Si, and the in-situ reaction of Ca ions with C
to calcium carbide CaC
2
to calcium silicides
2
+
2
.
The objective of the current study has been to investigate the
electrochemical synthesis of SiC with the aims of optimizing precursor
composition, understanding the cathodic reactions, and correlating
product morphology with process conditions. A further goal has been
to examine the nitridation behavior of the electrochemically prepared
SiC and to identify the composition and morphology of the product.
2
2
+
−
E01173 K = 0 V vs. Ca/Ca2+
Ca + 2 e → Ca
[5]
[6]
[7]
[8]
2
+ Ca + + 6 e → CaSi + 2 O
−
2−
SiO
2
E01173 K = 0.783 V vs. Ca/Ca
2+
Experimental
+ 2 Ca + + 8 e → Ca
Si + 2 O
2
−
2−
SiO
2
2
2
Powders of SiO (99% purity, 0.5–5 μm, Sigma Aldrich) and
E01173 K = 0.651 V vs. Ca/Ca
2+
graphite (<20 μm, Sigma Aldrich) were mixed in the molar ratios
of 1:0.5, 1:1 and 1:1.5. The mixtures were made into slurries by
adding isopropanol together with 1 mass% of polyvinyl alcohol and
2
+
−
Ca + 2 e + 2 C → CaC
2
0
.5 mass% of polyethylene glycol as binder and plasticiser. The slur-
E01173 K = 0.470 V vs. Ca/Ca
2+
ries were ground in a ball mill and dried at 373 K in air. The process
was repeated several times so as to attain a uniform powder mixture.
The dry powder was uniaxially pressed into disks of 0.5 g in mass and
13 mm in diameter. The disks were sintered at 1523 K in a stream
If the Si formed by electro-deoxidation does not immediately react
further to SiC but is temporarily available for other reactions, the
following two reactions may occur too.
of Ar with 5 vol% H for 3 h to provide sufficient strength for fur-
2
2
+
−
ther processing. The open porosities of the disks were measured by
Si + Ca + 2 e → CaSi
E
Archimedes’ method.
0
2+
1173 K = 0.673 V vs. Ca/Ca
[9]
Anhydrous CaCl
Aldrich) by drying at 423 K in air and then at the same tempera-
ture in vacuum for 48 h. The dried CaCl was loaded into an alumina
2
was obtained from CaCl
2
· 2H
2
O (Sigma
2
2
+
−
Si + 2 Ca + 4 e → Ca
2
Si
crucible that was placed inside a gastight Inconel retort with provision
for the insertion of electrodes from the top. The retort was brought
to the operating temperature of 1173 K and constantly flushed with
0
2+
E
1173 K = 0.464 V vs. Ca/Ca
[10]
In view of the above, Ca2 discharge on a SiO
+
-containing cathode
2
Ar. The CaCl melt was subjected to pre-electrolysis at 2.8 V for sev-
2
will take place at potentials far less negative to that on an inert cathode
such as W or Mo. Qualitatively this has already been demonstrated
by Jin et al. and Xiao et al. in cyclic voltammetric studies on the
electro-deoxidation of pure SiO , although there is only vague infor-
mation about the precise voltages at which such reactions dominate.
Ca discharge on a C-containing cathode is also thermodynamically
possible at potentials less negative than that on an inert cathode. This
has been confirmed experimentally by Fan et al. and Peng et al.,
eral hours, using a Ni coil as the cathode and a high-density graphite
rod (Tokai Carbon, UK), degassed at 400 K in vacuum for 12 h,
as the anode, until all redox-active impurities had been removed. A
2
5
26
2
sintered SiO /graphite disk, tied with Ni wire, was employed as the
2
cathode in the electro-deoxidation, using the same graphite anode as
2
+
before. A Ni/NiO assembly was used to monitor the potentials of
29
the individual electrodes. The SiO /graphite disks were subjected to
2
13
27
electro-deoxidation under different experimental conditions, includ-
14
while it has been claimed by Dai et al. that graphite does not react
ing (i) cathodes of three compositions as detailed above, (ii) applied
voltages of 2.5, 2.8 and 3.1 V, and (iii) process durations of 20 min, 3 h
and 6 h, all as compiled in Table I. Cyclic voltammetry was carried out
with a W wire as an inert working electrode and with a graphite rod as a
reactive working electrode. All potentials were quoted with respect to
2
+
cathodically with Ca . Fig. 1 illustrates schematically the compet-
ing reactions that are possible under cathodic polarization of SiO /C
samples in a CaCl melt.
Si -bonded SiC composites are of considerable interest due to
their attractive mechanical and thermal properties. Si
tionally produced by nitridation of a mixture of Si and SiC through
heating in N gas at temperatures where the nitridation of silicon is
thermodynamically favored over the nitridation of SiC.
2
2
3
N
4
28
2+
3
N
4
is conven-
the Ca/Ca potential. This was identified by extrapolating the linear
region of the cathodic branch to zero current, thus eliminating any am-
biguity originating from the gradual increase of the current prior to Ca
2
28
30
deposition due to electronic conduction through the polarized melt.
After completion of each experiment, the electrodes were lifted
above the melt and cooled to room temperature. The products were
thoroughly rinsed with distilled water until free from salt and dried
in vacuum at room temperature. The crystallographic phases, mi-
crostructures and elemental compositions of the samples before and
after electro-deoxidation were identified by X-ray diffraction analy-
sis (XRD) (Philips PW 1830), scanning electron microscopy (SEM)
3
Si (s) + 2 N
2
(g) = Si
3 4
N
(s)
0
ꢀ
G
1473 K = −269.642 kJ/mol
[11]
[12]
3
SiC (s) + 2 N
2
(g) = Si
3
N
4
(s) + 3 C (s)
0
ꢀ
G
1473 K = −89.817 kJ/mol
(FEI Nova NanoSem 450) and energy-dispersive X-ray spectroscopy
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Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
D733
Table I. Electro-deoxidation experiments of SiO2/graphite disks carried out in CaCl2 melt at 1173 K under argon.
Experimental conditions
SiO2:graphite (in moles)
Voltage and time
Phases formed
Residual oxygen
1
1
1
:1
:1
:1
2.5 V for 6 h
2.8 V for 6 h
3.1 V for 6 h
2.5 V for 6 h
2.8 V for 6 h
3.1 V for 6 h
2.8 V for 6 h
2.8 V for 20 min
2.8 V for 3 h
graphite (M), Si (SH), SiC (m)
SiC (M), Si5C3, CaSi, Ca2Si
CaSi
16,300 ppm
-
-
1:1.5
1:1.5
1:1.5
1:0.5
1:1.5
1:1.5
graphite (M), SiC (SH)
SiC
81,400 ppm
22,600 ppm
13.20 %
CaSi
CaSi (M), SiC (SH), Ca2Si (trace)
graphite (M), SiO2 (M), CaSiO3 (SH)
graphite (M), SiO2 (M), SiC, CaCO3 (m)
-
-
-
M - major, SH - second highest, m - minor.
discharge on graphite at potentials less negative to that on W may
be attributed to the formation of Ca-C compounds, most likely CaC
2
,
as expressed in Eq. 8. Based on the present results, graphite has to
be considered a reactive electrode when polarized cathodically in a
CaCl
2
melt. Comparing the linear regions of the cathodic branches of
in
the cyclic voltammograms, the electrochemical window of CaCl
2
the presence of a graphite electrode is lower by ∼450 mV than in the
presence of an inert electrode.
13
In line with the observations of the present study, Fan et al. and
27
Peng et al. likewise suggested the formation of CaC
2
from graphite
melt. In contrast,
Dai et al. found in cyclic voltammetry studies with a metallic cavity
2
+
and Ca during cathodic polarization in a CaCl
2
14
2
+
electrode at 1123 K that graphite did not react with Ca during ca-
thodic polarization in CaCl but behaved more like an inert electrode.
2
Sintering behavior and microstructure of SiO
mixtures.—SiO /graphite disks, made from mixtures of the SiO
graphite powders in the molar ratios of 1:0.5, 1:1 and 1:1.5 and sin-
tered at 1523 K in Ar with 5 vol% H for 3 h, possessed open porosities
of about 40%. Figs. 3a–3c show the XRD patterns of the SiO and
graphite powders along with that of a sintered SiO /graphite disk of
2
/graphite
2
2
and
2
2
Figure 2. Cyclic voltammograms with W as the working electrode on the
cathodic side and with high-density graphite as the working electrode on the
cathodic and the anodic sides in CaCl2 melt at 1173 K at a scan rate of 20 mV/s.
Graphite rods and Ni coils were used as the counter electrodes in the former
and latter cases, respectively. Ni/NiO was used as the reference electrode.
2
◦
◦
◦
molar ratio 1:1.5. The peaks at 2θ of 20.95 , 26.73 and 50.20 seen
in the XRD patterns for the pure SiO and the sintered SiO /graphite
mixture originate from the diffractions of the (100), (011) and (112)
2
2
◦
◦
planes of quartz, and those at 26.5 and 54.6 in the XRD pattern for
the pure graphite and the sintered SiO /graphite mixture are from the
002) and (004) planes of graphite. The XRD analysis confirms that
2
(
(
EDX) (Bruker X Flash 6I100). In selected cases, the reduced sam-
ples were further characterized by transmission electron microscopy
TEM) (FEI Tecnai G2-F20). The residual oxygen content of the
reduced samples was determined with the hot extraction method
ELTRA ONH 2000).
The effect of N at elevated temperature on the electrochemically
prepared SiC was investigated by heating a SiC sample of about 20 mg
under a continuous flow of N of 100 mL/min at 1473 K for 8 h,
the starting materials were phase pure and that, in line with thermo-
dynamic expectations, they underwent no reaction during sintering.
(
Figs. 3d–3i show SEM images and EDX spectra of the SiO
2
and
graphite powders. The micrographs reveal that the SiO consisted of
2
(
particles of irregular morphology with size in the range of 0.15–5 μm,
while the graphite consisted of particles of flaky morphology with size
in the range of 2–12 μm; and the elemental analysis confirmed that
there were no significant impurities. Figs. 4a–4f show SEM images
2
2
and subsequently studying the phase composition and morphology of
the reaction product. The thermokinetic behavior was examined by
subjecting a SiC sample of 200 mg to simultaneous thermogravimetric
and differential scanning calorimetric analysis (TG/DSC) (SDT Q600,
TA Instruments) using a heating rate of 20 K/min from 298 to 1473 K
2
of the cross-sections from the sintered SiO /graphite disks of the
three different compositions. The two types of particles can be clearly
identified by their morphologies and compositions, evidencing that
they formed an intimate near-uniform mixture. Elemental maps for
Si, O and C of selected regions from the cross-sections are given in
Fig. S1 (Supplementary Material).
2
in a stream of N .
Electro-deoxidation of SiO
deoxidation of SiO /graphite of molar ratio 1:1 at different
voltages.—SiO /graphite disks of molar ratio 1:1 were subjected
to electro-deoxidation at applied voltages of 2.5, 2.8 and 3.1 V for
durations of 6 h in order to investigate the effect of polarization. The
current vs. time curves are shown in Fig. S2. The results of the XRD
analysis of the three products are compiled in Table I. At 2.5 V, Si
and SiC were observed together with unreacted graphite. At 3.1 V,
only CaSi was found. At the intermediate voltage of 2.8 V, SiC was
2
/graphite mixtures.—Electro-
Results and Discussion
Electrochemical behavior of graphite during cathodic polariza-
tion in CaCl melt.—Fig. 2 shows cyclic voltammograms recorded
in a CaCl melt, with a W working electrode on the cathodic side,
2
2
2
2
and with a high-density graphite working electrode on the cathodic
and the anodic sides. The reduction of Ca2+ on W was found to be-
gin at 0.00 V, which confirmed its inertness. In contrast, appreciable
2
+
reduction on graphite progressed from +1.059 V onwards. The Ca
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D734
Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
Figure 3. XRD patterns of (a) as-received SiO2 powder, (b) as-received graphite powder, and (c) SiO2/graphite disk (molar ratio 1:1.5) sintered at 1523 K in
Ar with 5 vol% H2 for 3 h. (d) SEM image and (e) high-resolution SEM image of SiO2 powder with (f) corresponding EDX analysis. (g) SEM image and (h)
high-resolution SEM image of graphite powder with (i) corresponding EDX analysis.
the major phase, but minor amounts of Si
also present.
5
C
3
, CaSi and Ca
2
Si were
cathode potential vs. time curves are shown in Figs. 5a, 5b. In all three
cases, the currents initially peaked and then declined toward stable
residual values, thus indicating the progression of the cathodic reac-
tions. As expected, the current was higher at higher voltage, while the
main characteristics of the curves were similar. The cathode potentials
vs. Ca/Ca evolved with distinct features that could be directly cor-
related with the current behavior. The potentials shifted in the more
negative direction with increasing reaction time, with stable values
reached faster at higher voltage.
The XRD analysis thus demonstrates that, at the low voltage, all
SiO was electro-deoxidized, and even though some SiC was formed,
2
its rate of formation was low so that not all graphite was consumed. At
the high voltage, undesired reactions involving Ca prevailed. At the
intermediate voltage, the complete consumption of the graphite was
achieved, but Ca-based by-products were formed. The results there-
2
+
2
fore indicate that equal quantities of SiO and graphite are insufficient
to arrive at a stoichiometric SiC product, presumably because some
graphite is lost due to side reactions.
The three products obtained after electro-deoxidation were all in
the form of powder, as seen from the insets of Fig. 5a. The XRD
patterns of the products are shown in Figs. 5c–5e and the phases
identified are given in Table I. At 2.5 V, the product was a mixture of
SiC and graphite, and at 3.1 V, the product was again pure CaSi. At
2.8 V, however, phase-pure SiC was now obtained. The SiC existed
Electro-deoxidation of SiO
ent voltages.—Based upon the above findings, SiO
2
/graphite of molar ratio 1:1.5 at differ-
/graphite disks of
2
molar ratio 1:1.5 were subjected to electro-deoxidation at applied volt-
ages of 2.5, 2.8 and 3.1 V for durations of 6 h. The current vs. time and
◦
◦
◦
in its cubic β modification, with the peaks at 35.5 , 41.1 , 60.0 and
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Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
D735
Figure 4. SEM images of cross-sections of SiO2/graphite disks of molar ratios (a) 1:0.5 (b) 1:1 and (c) 1:1.5, sintered at 1523 K in Ar with 5 vol% H2 for 3 h,
showing SiO2-rich and graphite-rich regions. (d), (e), (f) High-resolution SEM images with EDX analyses of the indicated areas.
◦
7
(
1.6 stemming from the diffractions of its (111), (200), (220) and
about 2.7 mass% was found too. Fig. 8 shows a high-resolution TEM
image that clearly indicates the nanoscopic nature of the SiC with
good crystallinity and an average particle size of about 20–30 nm.
The lattice spacing was measured as approximately 0.25 nm for the
SiC (111) plane (inset in Fig. 8) which is coincident with literature
311) planes. It has been reported that β-SiC forms preferentially at
process temperatures below 1973 K while the α-type prevails at higher
temperatures.
Figs. 6a–6f show SEM images of the three products. The mi-
crostructure of the product at 2.5 V contained wire-like entities with
lengths from a few tens to a few hundreds of nanometers (Figs. 6a,
d). EDX analysis revealed that they were composed of Si and C,
thereby suggesting them to be SiC. In addition, some flaky particles
with diameters of a few micrometers were observed that were similar
31
21,22
values.
The occurrence of the different reaction products can be readily
understood in view of Eqs. 2 to 8. An applied voltage of about 2.8 V is
required to ensure a sufficiently fast formation of the SiC, including the
6
electro-deoxidation of the SiO
2
to Si and its in-situ reaction with the
to those in the SiO
2
/graphite precursor before processing (Figs. 6a, 6d,
graphite in the cathode. However, at this voltage the formation of CaC
2
2
+
Fig. S3). EDX analysis confirmed them to be from carbon, thereby
identifying them as unreacted graphite. The microstructure of the
product at 3.1 V contained comparatively bulky particles with di-
ameters in the range of several micrometers (Figs. 6c, 6f, Fig. S4).
EDX analysis showed them to consist of primarily Ca and Si, thus
identifying them as CaSi. Neither SiC nanowires nor graphite flakes
were present, and the significant charging during imaging was be-
cause of the sample’s poor electronic conductivity. Significant lev-
els of O were found (Fig. S5), which were likely due to absorption
of and reaction with water in the washing step. The microstructure
of the product at 2.8 V was very different (Figs. 6b, 6e), featuring
exclusively SiC nanowires. Fig. 7 shows a low-magnification SEM
image of the sample, along with the EDX elemental maps for Si
and C and a typical EDX analysis. A small quantity of oxygen of
through the reaction of the graphite in the cathode with Ca ions from
the electrolyte is not negligible, and this leads to a partial consumption
of the graphite. The arising Si surplus then manifests itself in the
formation of Si
and Ca Si, as seen with the SiO
notion was further corroborated by electro-deoxidation experiments
with SiO /graphite disks of molar ratio 1:0.5 under otherwise the same
5
C
3
as a carbon-deficient silicon carbide and of CaSi
2
2
/graphite disks of molar ratio 1:1. This
2
conditions (Fig. S6). In such acathode, the carbondeficit exists straight
from the beginning, and the main compound found in the reduced
samples was CaSi (Table I). Consequently, to achieve stoichiometric
SiC, a sufficient excess of graphite in the SiO
necessary to compensate for its partial loss as CaC
2
/graphite precursor is
. Usefully, CaC
2
2
reacts with water in the washing step and does not accrue as a by-
product in the cathode.
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D736
Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
Figure 5. (a) Current vs. time curves and (b) cathodic potential vs. time curves during electro-deoxidation of SiO2/graphite disks (molar ratio 1:1.5) in CaCl2
melt at 1173 K for 6 h at 2.5, 2.8, and 3.1 V. Insets in (a) show photographs of the three products. (c), (d), (e) XRD patterns of the products obtained at the three
different voltages.
The above would also explain the observation of a carbon loss, or
the need of a carbon surplus, in related studies. For example, in the
9b show the current vs. time and the cathodic potential vs. time curves
from these experiments along with curves from a 6 h run.
The product obtained after 20 min was in the form of a disk, and
that after 3 h was a powder, as seen from the insets of Fig. 9a. Figs. 9c,
9d show the XRD patterns of the 20 min and 3 h samples, respectively,
preparation of ZrC from ZrO
achieve a phase-pure product, and in the preparation of HfC from
2
, an excess of carbon was required to
1
4
15
HfO
effect may be further compounded by the large difference in particle
size of the SiO and the graphite in the present case, as this leads
2
, a small loss of carbon happened during the process. The
and demonstrate that the main transient phase was CaSiO
its amount diminished as the amount of SiC product grew.
3
and that
2
to longer diffusion distances in the in-situ reaction of Si with carbon
and enhances the possibility of carbon loss via side reactions. For
Fig. 10a shows a low-resolution SEM image of the cross-section of
the sample obtained after 20 min of electro-deoxidation, and Fig. 10b
shows SEM image of a smaller area of this cross-section, in which
the interface between two separate regions can be clearly discerned.
As seen from Figs. 10c, 10d, one region had bulky morphology with
example, in the preparation of Cr
carbon particles led to a slow rate of carbonization, while in the
preparation of NbC from Nb , the use of carbon nanoparticles
improved the extent of carbonization.
7 3 2 3
C from Cr O , the use of large
17
2
O
5
18
2
particles of SiO and carbon similar to that in the starting material,
indicating that practically no reduction had taken place here, while
the other region had SiC nanowires together with large particles and
flakes, indicating that reduction had commenced but remained incom-
plete. Separate SEM images of these regions with their dissimilar
microstructures are shown in Fig. S7, and an EDX analysis further
indicated that the partially reduced regions still had significant quan-
tities of oxygen. Figs. 10e, 10f show the microstructure of the sample
Electro-deoxidation of SiO
ferent durations.—In order to investigate the nature of any transient
phases occurring in the course of the reduction of SiO in the presence
of graphite, partially reduced samples were prepared, by subjecting
SiO /graphite disks of molar ratio 1:1.5 to electro-deoxidation at an
applied voltage of 2.8 V for durations of only 20 min and 3 h. Figs. 9a,
2
/graphite of molar ratio 1:1.5 for dif-
2
2
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Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
D737
Figure 6. SEM images of the various products obtained from SiO2/graphite (molar ratio 1:1.5) by electro-deoxidation in CaCl2 melt at 1173 K for 6 h at (a) 2.5 V,
b) 2.8 V, and (c) 3.1 V. (d), (e), (f) High-resolution SEM images of the indicated areas.
(
C.32 Such direct conversions are likewise known for the reduction of
Fe O and UO2.
2 3
obtained after 3 h of electro-deoxidation. The individual particles
were identified as SiO , graphite and CaSiO , and the amount of
33
34
2
3
SiC nanowires had increased compared with the shorter experiment.
Further SEM images and an EDX analysis are shown in Fig. S8.
Reduction of SiO via electrochemical formation of CaSiO .—The
2
3
transient presence of CaSiO
3
in the cathode in the short experiments
Mechanistic considerations.—Ideas about the reaction mecha-
at intermediate applied voltage may indicate a two-step reduction
3
process, which involves the combined formation of Si and CaSiO ,
nism of the electro-deoxidation of SiO
2
in the presence of graphite in
a CaCl melt can be derived from the transient phases, Si and CaSiO
2
3
,
3
followed by the electro-deoxidation of the CaSiO .
found in some of the experiments. The following reaction sequences
are conceivable.
+ 2 Ca2+ + 4 e → Si + 2 CaSiO
−
3
SiO
2
3
E01173 K = 1.308 V vs. Ca/Ca
2+
[15]
2
Direct reduction of SiO .—The transient presence of free elemental
Si in the cathode, in combination with an amount of SiC, in the exper-
iments at low applied voltage indicates that the electro-deoxidation
−
2+
2−
CaSiO
3
+ 4 e → Ca + Si + 3 O
= 0.602 V vs. Ca/Ca2+
E0
[16]
[17]
of SiO
2
directly to Si is one of the reactions occurring. This is then
1173 K
followed by the in-situ reaction of the Si with C to SiC.
−
2−
SiO
2
+ 4 e → Si + 2 O
Si + C → SiC
E01173 K = 0.837 V vs. Ca/Ca
2+
[13]
[14]
The reduction of SiO
CaSiO , has indeed been reported when only small amounts of elec-
trolyte were present. Reaction sequences of this kind are also well
2
to Si, with the temporary formation of
3
35
36
30
29
known from the reduction of TiO
2
,
Cr
2
O
3
2 5
and Nb O . It is noted
Si + C → SiC
2
to Si, without the temporary formation of
that Ca metallates tend to give very strong peaks in the XRD pattern,
so that those of the corresponding metals or suboxides are sometimes
obscured.
The reduction of SiO
other phases, has also been reported to happen in the absence of
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D738
Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
Figure 7. (a) SEM image with EDX elemental maps and (b) EDX spectrum of the SiC product obtained from SiO2/graphite (molar ratio 1:1.5) by electro-
deoxidation in CaCl2 melt at 1173 K for 6 h at 2.8 V.
Reduction of SiO
sient presence of CaSiO
formation from SiO and CaO dissolved in the CaCl
2
via chemical formation of CaSiO
in the cathode may also be due to its chemical
. The CaO orig-
3
.—The tran-
3
2
2
inates from insufficient drying of the CaCl
during the electro-deoxidation process.
2
and forms in the melt
SiO
2
+ CaO → CaSiO
3
[18]
−
2+
2−
CaSiO
3
+ 4 e → Ca + Si + 3 O
E01173 K = 0.602 V vs. Ca/Ca2+
Si + C → SiC
[19]
[20]
The chemical formation of Ca metallates with CaO from the elec-
37
2 5
trolyte has also been found in the reduction of Ta O .
Reduction of CaSiO
reaction sequences, the reduction of the CaSiO
as a solid-state reaction. However, there is significant solubility on
3
via dissolution and deposition.—In the above
3
has been written
3
8
the percent level of CaSiO
Therefore, it is possible that at least a portion of the CaSiO
entered into the CaCl
was then electro-deposited back onto the cathode.
3 2
in CaCl melts with CaO additions.
3
may have
2
melt during the experiment, from which the Si
2
+
2−
CaSiO
3
→ Ca + SiO
3
[21]
Figure 8. High-resolution TEM image of the SiC product obtained from
SiO2/graphite (molar ratio 1:1.5) by electro-deoxidation in CaCl2 melt at
2−
−
2−
SiO
3
+ 4 e → Si + 3 O
1
173 K for 6 h at 2.8 V. Inset shows the lattice spacing of the (111) plane of
E0
= 0.602 V vs. Ca/Ca
2+
[22]
SiC.
1
173 K
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Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
D739
Figure 9. (a) Current vs. time curves and (b) cathodic potential vs. time curves during electro-deoxidation of SiO2/graphite disks (molar ratio 1:1.5) in CaCl2
melt at 1173 K for 20 min, 3 h and 6 h at 2.8 V. Insets in (a) show photographs of the three products. XRD patterns of the products obtained after (c) 20 min and
(
d) 3 h.
This type of SiO
one by Xiao et al. and Zou et al.
Overall, the electro-deoxidation of SiO
2
in a CaCl melt is highly complex and likely to progress via several
parallel reaction sequences. These are dependent on the precursor mor-
phology and on the applied voltage and may therefore differ between
individual experiments. This highlights the necessity of empirical pro-
cess optimization as has been done in the present study.
2
reduction has been considered to be the dominant
followed by a mass gain, which is rather slow up to 1120 K and then
becomes quite steep.
38
22
39
2
in the presence of graphite
Tishchenko et al. studied the thermokinetic behavior of SiC in air
by TG/DSC and reported that the mass loss from room temperature
to about 467 K was due to the presence of moisture. In line with
that, the small endothermic mass loss of the SiC sample up to 475 K
40
may be attributed to the removal of adsorbed moisture. Roy et al.
reported that the subsequent oxidation of SiC has two components,
active oxidation with oxygen at low activity according to Eq. 23,
involving a net mass loss, and passive oxidation with oxygen at high
activity according to Eq. 24, involving a net mass gain.
Nitridation of SiC nanowires.—Fig. 11a shows the XRD pattern
of the material obtained after heating the electrochemically prepared
SiC + 2 O (residual in SiC) → SiO (g) + CO (g)
[23]
SiC in N
2
at 1473 K for 8 h. The product contained Si
2 2
N O together
with the cristobalite phase of SiO
2
. Figs. 11b–11d show SEM images
and an EDX analysis of the product, notably demonstrating that it
had retained its nanowire morphology. The present study is there-
fore believed to be the first report on the synthesis of nanowire-type
SiC + 2 O (surrounding gas) → SiO (s) + CO (g)
2
2
2
[24]
Si
2
N
2
O.
Fig. 11e shows the mass change and heat flow vs. temperature
curves recorded in the TG/DSC experiments of the electrochemically
made SiC in N atmosphere. The TG curve presents two distinct
stages, one involving mass loss (stage I), and one involving mass gain
stage II). The curve begins with a mass loss, which is quite steep
Due to the high level of residual oxygen in the electrochemically
prepared SiC material (Table I), the mass loss of the SiC up to 870 K
may be attributed to active oxidation, with the oxygen evolving in the
form of CO and SiO gases.
2
The endothermic mass gain above 870 K may be ascribed to the re-
(
action of the SiC with N
as evidenced by the XRD analysis of the nitrided sample. Mechanisti-
cally, the formation of Si O may include both the residual oxygen
2 2 2
. This must involve the formation of Si N O,
from 25 to about 710 K and then becomes rather insignificant. The
minimum mass of 99.98% of the original is reached at 870 K. This is
2
N
2
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D740
Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
Figure 10. (a) SEM images of the product obtained from SiO2/graphite (molar ratio 1:1.5) by electro-deoxidation in CaCl2 melt at 1173 K for 20 min at 2.8 V,
with (b) enlargement of the indicated area in (a) showing the interface of regions with no reduction and with commencing reduction. High-resolution image of
(
c) region with no reduction, and (d) region with commencing reduction. (e) SEM images of the product obtained from SiO2/graphite (molar ratio 1:1.5) by
electro-deoxidation in CaCl2 melt at 1173 K for 3 h at 2.8 V, with (f) enlargement of the indicated area in (e).
in the SiC as well as the trace oxygen in the N
2
gas.
O + 3 C
competing reactions were found to occur in cathodically polarized
SiO /graphite, (i) the electro-deoxidation of SiO to Si and the sub-
2
2
2
SiC + CO (g) + N
2
(g) → Si
2 2
N
sequent in-situ reaction of the Si with graphite to form SiC, (ii) the
0
2+
reaction of Si with Ca ions to form calcium silicides such as CaSi
ꢀ
G
1473 K = −109.577 kJ/mol
[25]
[26]
2
+
and Ca
The successful synthesis of pure SiC requires an optimized combina-
tion of the composition of the SiO /graphite precursor, the magnitude
2
Si, (iii) the reaction of graphite with Ca ions to form CaC .
2
1
2
SiC + /
2
O
2
(g) + N
2
(g) → Si
2
N
2
O + 2 C
2
0
of the applied voltage, and the duration of the polarization, while de-
viations from such conditions lead to the incomplete reaction of the Si
with the graphite or to the preferential formation of calcium silicide.
A significant excess of graphite in the starting material is required to
compensate for the loss of carbon through inevitable side reactions.
Partially reacted SiO /graphite samples contained Si or CaSiO , which
ꢀ
G
1473 K = −351.029 kJ/mol
Munro and Dapkunas41 reported that when pure SiC is heated in
the presence of pure N , it slowly converts into Si . They further
2
3 4
N
reported that the additional presence of CO would lead to the formation
of oxynitrides. This is in line with the current findings where CO is
thought to be released in the active oxidation.
2
3
enabled detailed considerations of possible reaction paths.
Overall, the observed mass changes and the formation of Si
2
N
2
O
Nitridation experiments of the electrochemically made SiC at ele-
vated temperature showed the formation of Si N O. TG/DSC studies
and SiO can be explained by the oxidation and nitridation of the
2
2
2
electrochemically made SiC. Some stoichiometric considerations to
corroborate this notion are given in the Appendix.
with the SiC in N atmosphere indicated distinct stages, attributable
to the loss of moisture, the partial oxidation with residual oxygen in
2
the material, and the reaction with N
Notably, the SiC is in its β-phase and has nanowire morphology,
and the Si O retains this nanowire morphology. This may make
2
.
Conclusions
2
N
2
Silicon carbide with nanowire morphology was electrochemically
synthesized from SiO /graphite mixtures in a CaCl melt. Three
2 2
possible the application of the materials in the energy and other sectors
as will be elaborated in future reports.
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Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
D741
Figure 11. (a) XRD pattern of the Si2N2O/SiO2 product obtained from SiO2/graphite (molar ratio 1:1.5) by electro-deoxidation in CaCl2 melt at 1173 K for 6 h at
.8 V and subsequent heat-treatment in N2 flow at 1473 K for 8 h. (b) SEM image and (c) high-resolution SEM image of the same sample with (d) corresponding
EDX analysis. (e) TG/DSC curves of a 200 mg sample of SiC in N2 flow of 100 mL/min at a scan rate of 20 K/min.
2
Acknowledgment
discussed in the main text, and can be expressed as follows.
SiC + 2 O (residual in SiC) → SiO (g) + CO (g)
This study was partly funded through the Research Chair Grant
Programme of The Research Council of the Sultanate of Oman.
[A1]
[A2]
Appendix: Stoichiometric Calculations on the Formation of
2
SiC + CO (g) + N2 (g) → Si2N2O + 3 C
2 2
Si N O from SiC
It is suggested that the formation of Si2N2O from SiC can progress via parallel
reaction paths.
One reaction path is based on information from the literature concerning the mech-
anisms of active oxidation with residual oxygen and nitridation with gaseous N2, as
Each experiment involved 20 mg of electrochemically prepared SiC, with a residual
oxygen content of 2.7 mass%. The mass of oxygen in the SiC was 0.54 mg, corresponding
to 0.27 mg of oxygen present in the CO and the other half in the SiO. This would have
allowed for the formation of 1.7 mg of Si2N2O.
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D742
Journal of The Electrochemical Society, 165 (14) D731-D742 (2018)
It may alternatively be assumed that all the residual oxygen remained in the sample
and that no SiO was released.
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SiC + O (residual in SiC) + N2 (g) → Si2N2O + 2 C
[A3]
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9
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−
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−5
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.0 · 10 moles of O2 and 1.2 · 10 moles of H2O passed through the reaction vessel,
2
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This cannot be quantified, but may have been significant considering that the SiC was
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Overall, the formation of milligram quantities of Si2N2O from the SiC is consistent
with the experimental conditions applied in the nitridation experiments. The results there-
fore suggest that, by carefully determining the residual oxygen content in the SiC reactant
and by carefully introducing further oxygen via the gas phase, the controlled synthesis of
Si2N2O will become possible. An enhanced reaction rate should moreover be achievable
by passing the gas stream through, rather than above, the SiC powder.
2
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