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High-temperature interaction between silicon and carbon
Ceramics hTternational 19 (1993) 129-132
H i g h - T e m p e r a t u r e Interaction between Silicon and Carbon V. I. G o r o v e n k o , V. A. Knyazik & A. S. S h t e i n b e r g Institute for Structural Macrokinetics, Academy of Sciences of Russia, 142432, Chernogolovka, Moscow Region, Russia (Received 5 May 1992; accepted 8 June 1992)
Abstract: The ETE method was used for the investigation of high-temperature interaction in the silicon-carbon system and for the manufacture of compacted silicon carbide. The samples were ignited after silicon melting, independently of the heating rate. Maximum values of temperature realized in ETE were 2330-2430°C. The intensity of chemical heat evolution after the melting of silicon depends upon temperature exponentially, with the apparent activation energy equal to 55 _+4 kcal/mole. The conclusion was drawn that a limiting stage of the process is the liquid-phase diffusion of carbon in silicon occurring because of a difference in carbon concentration in the melt near the surface of carbon particles and carbide particles. The ETE technique allows both the synthesis of SiC and the manufacture of constructional materials to be accomplished simultaneously. The ETEmanufactured SiC product is represented mainly by its fl-phase. The relative density of the product is about 0.8~3"9.
INTRODUCTION
tested in the course of study of the high-temperature interaction of titanium and tantalum with carbon. 4'5 In the present work, the ETE method was used for investigation of the kinetic regularities of hightemperature reaction in a silicon-carbon system and for the manufacture of compacted silicon carbide. Silicon carbide is one of the most interesting refractories from both the theoretical and practical points of view. In contrast to powder mixtures of transition metals with carbon, the Si-C system cannot react in the gasless-combustion mode at the initial temperature of the room because it does not possess a sufficiently high heat content, which is only 16"5 kcal/mole. 6 Nevertheless, this reaction may occur in the thermal-explosion mode, 7 in particular, on heating a reaction mixture by means of an electric current. 8
Investigation of the macrokinetics of hightemperature exothermic reactions in powder mixtures appears to be one of the most urgent problems in the light of recent developments in the theory and implementation of gasless combustion, including those of self-propagating high-temperature-synthesis (SHS) processes. The so-called classical, i.e. isothermic, methods of chemical kinetics are known ~ to be essentially inapplicable for determination of the rates of these reactions, which is due both to major difficulties in thermostatting samples in a practically important temperature region (up to 2000-3500°C) and the transient nature of the processes (a duration of 10-2-10-1 s) under study. A new non-isothermic technique, called the electrothermal explosion (ETE), has recently come into use for the solution of these problems. The ETE is based on the recording of thermograms of thermal explosion in a specimen pressed from an electrically conductive exothermic powder mixture of reactants brought to explosion by passing electric current through the sample. 2'3 The ETE technique was
MACROKINETICS OF HIGHTEMPERATURE INTERACTION BETWEEN SILICON AND CARBON ETE experiments on the Si-C system were carried out with the use of equipment described in detail 129
130
V. I. Gorovenko, V. A. K n y a z i k , A. S. Shteinberg 9
Table 1. M i x t u r e s studied
Powder dispersity (/~m)
Mixture
Silicon No. 1 No. 2
Graphite
5<6<16
8
3<63
--
.ca
63<6<90
7 ¢.J
elsewhere.9 Initial samples of silicon and graphite powders, thoroughly mixed in alcohol (Table 1), were pressed in a steel mould up to a density of 1.75_+ 0-5 g/cm 3 to become cylinders 10mm in diameter and 15 mm in height. To study the heat evolution accompanying the high-temperature reaction of silicon with carbon, a time-dependence of the luminance temperature of a sample surface at different rates of sample-heating was recorded in the course of ETE development. Typical thermograms obtained in the ETE of the silicon-carbon system are shown in Fig. 1. Analysis of the thermograms indicates that the samples were ignited after silicon-melting, independently of heating rate. Maximum values of temperature realized in ETE of both mixtures were 2330°C to 2430°C. Figure 2 gives the Arrhenius representation of the intensity of chemical-heat evolution in the reaction of silicon with graphite of various degrees of dispersity as a function of temperature, obtained by treatment of ETE thermograms. The procedure of ETE thermogram treatment is similar to that used in thermal analysis and is described in detail by Knyazik e t al. 4 On considering these curves, it is clear that the ETE process in the silicon-carbon system is ofa stepwise nature. At the initial stage, the intensity of chemical-heat evolution after melting of silicon depends exPonentially upon temperature, with the apparent activation energy, E, equal to 55 _+ 4 kcal/mole for both mixtures No. 1 and No. 2. At a temperature of about 1930°C, the intensity of 2500 •
=
'"
"
=
S
2000 0
1500 Tmelt
1000
0
0.5
1.0
1.5
t,sec Fig. 1. Typical ETE thermograms of silicon-carbon system.
. t:2e
v
i
5 4.0
I 4.5
i 5,0
I
5.5
6.0
104/T, K - t Fig. 2. Temperature-dependence of chemical-heat evolution during the interaction of silicon with graphite of various degrees of dispersity: A - - m i x t u r e No. l i , - - m i x t u r e No. 2.
heat evolution reaches its maximum for both graphite dispersities. Furthermore, the intensity of heat evolution decreases• Though the problem of silicon carbide synthesis is dealt with in many studies, the issue of the reaction mechanism in the silicon-carbon system is still far from being clear• Measurements of the solubility of carbon in liquid silicon have shown 1° that this quantity depends exponentially on temperature, with an enthalpy of 59kcal/mole, which agrees within the limits of experimental error with the value of apparent activation energy obtained in the present work. Almost the same value of activation energy, 56.2 kcal/mole, was obtained in the DTA study of the interaction of liquid silicon with carbon filaments near the melting point of silicon in the temperature range of 1422-1436°C. 11 These results cannot be explained on the basis of the Brantov model, 12 assuming that the diffusion of carbon through the SiC layer is the limiting stage of the process. The mechanism of successive endothermic carbon-dissolving and exothermic silicon carbide precipitation and of related temperature oscillation was suggested. The results of our study show that the mechanism of interaction in the Si-C system after the melting of silicon is somewhat different. During electrothermal explosion in the Si-C system, its temperature increases monotonically, so it seems natural to assume that the dissolving of carbon particles and the size growth of silicon carbide particles take place simultaneously. A limiting stage of the process is the liquid-phase diffusion of carbon in silicon occurring
131
High-temperature interaction between silicon and carbon
because of the difference in the carbon concentration in the melt near the surface of carbon particles and carbide particles. The solubility of carbon in liquid silicon reported by Scace and Slack 1° probably corresponds to the concentration of carbon in the melt at the silicon carbide surface, since the carbon surface has had time to be coated with a carbide layer during an experimental run. As long as the temperaturedependence of the intensity of chemical-heat evolution in ETE is close to that of the equilibrium concentration of carbon in liquid silicon in the vicinity of the SiC surface, it may be concluded that the concentration of carbon at the carbon-particle surface in liquid silicon increases with temperature in accordance with the same law and the same activation energy but a different pre-exponential factor. The maximum intensity of heat evolution during ETE in the Si-C system is attained at the m o m e n t when the product formed begins to retard further occurrence of the reaction.
P R E P A R A T I O N OF O N E - P H A S E C O M P A C T E D SILICON C A R B I D E This study also had a technological objective, i.e. the estimation of the possibility of manufacturing onephase compacted silicon carbide by means of the ETE method. The conventional procedure for the manufacture of silicon-carbide-based structural materials involves two stages. The first stage is the preparation of SiC powder, and the second is the manufacture of constructional materials themselves from this powder by using activated-sintering or hot-compaction techniques. The ETE technique dealt with in this paper, in principle, allows both of these processes to be operated simultaneously. Experiments on the preparation of compacted SiC were carried out with the following charge compositions: the stoichiometric mixture, the mixture with silicon deficiency, the mixture with excess silicon, and the stoichiometric charge of silicon and carbon with aluminum and boron additives. Graphite and PM-16E carbon black were used as a source of carbon. To prepare a charge, the powders were mixed in alcohol for 10 hours and then dried in vacuo. Cylindrical samples of 2.5g in mass, 1 6 m m in height, and 12 m m in diameter, which corresponded to the initial density of 1"38 g/cm 3, were pressed from this mixture. ETE experiments were carried out in an argon atmosphere and at a mechanical load of 150 kgf/cm 2 in a quartz shell with steel banding (Fig. 3). In the experimental run, the time-dependences of
2 1
3 4
2
Fig. 3. Schematicdiagram of experimentalsystemfor manufacture of compacted SiC by means of ETE: l--reacting mixture; 2--electrodes; 3--quartz shell; 4--steel banding. the voltage applied to the electrodes and the electric current passing through a specimen were measured. The typical appearance of these dependences as well as the change in electrical resistivity and temperature of the sample in the course of the experiment is shown in Fig. 4. Let us consider these curves. Schematically, the whole of the process of interaction between silicon and carbon in ETE may be split into three periods, i.e. the first period as a stage of sample-heating, the second period as the intensive occurrence of chemical reactions, and the third one as sample-cooling. During the first period, the joule heating of the sample takes place. Chemical transformation is insignificant at this stage; the current flowing through the sample increases gradually, and the electric power increases. The second period, which lasts for about 0.2 s, is characterized by a dramatic decrease in the electrical resistivity of the sample. I,A U,V
p,Ohm//cm T,°C
-2500
.12-
-8 200 -
-2000
-1500
100-
-1000
o.o, 500
0
I
I
I
I
2
3
t, sec Fig. 4. Typical time-dependences of voltage applied to electrodes (l), electric current passing through a specimen (2), electrical resistivity (3), and temperature of sample {4), in the course of experiment.
V. L Gorovenko, V. A. Knyazik, A. S. Shteinberg
132 Table 2. Initial and final composition and density of samples
Initial
Final composition
Density
composition of samples
of samples according
of samples
to X-ray phase
(g/cm 3)
analysis Stoichiometric mixture 26% excess of Si 20% lack of Si
Stoichiometric mixture + 5% B Stoichiometric mixture + 5% AI
//-SIC, 10 + 20% Si, 1% of graphite //-SIC, 25% Si 1% of graphite //-SIC, 10% :<-SIC 5 + 10% Si 5% of graphite //-SIC, B, 5 + 16% Si 1 + 2% of graphite //-SIC, AI, 10% Si 3% of graphite
The manufacture of compacted silicon carbide by means of the ETE technique has been developed. Such an approach allows both the synthesis of SiC and the compacting of constructional materials to be accomplished simultaneously. The ETEmanufactured SiC product is represented mainly by its fl-phase.
2.82 + 3.03 2.68 + 2.97 2.62 + 2.87
2.71 + 2.96
ACKNOWLEDGMENTS The authors gratefully acknowledge R. Pampuch, U. I. Goldshleger, and V. A. Kudryashov for very useful discussions during the course of this work.
2.75 + 2.98
The voltage on the sample decreases, and the current increases drastically. The sample temperature increases up to its maximum value under these conditions, which is an indication of the high-speed occurrence of the reaction with the evolution of a large amount of heat. During the third period, the resistivity of the sample increases and tends to reach a certain value (0.1 f~), the voltage on the sample is restored, and the current decreases. From this, it can be concluded that the chemical interaction of silicon and carbon is over and the formation of the final product, silicon carbide, has taken place. After the experiments, the specimens were studied by an X-ray phase-analysis technique. The corresponding data and sample densities are listed in Table 2. The density of single-crystal SiC is 3-2 g/cm 3. It may be seen from Table 2 that the ETEmanufactured SiC product is represented mainly by its fl-phase, which agrees rather well with data reported by Kieffer et al.l 3 The density of sintered specimens of SiC is about 80-90% of that for singlecrystal SiC. CONCLUSIONS It has been shown that the high-temperature interaction in a powder mixture of silicon and carbon under the electrothermal explosion conditions occurs after silicon-melting and is probably limited by the liquid-phase diffusion of carbon in silicon occurring because of the difference in carbon concentration in the melt near the surface of carbon particles and carbide particles.
REFERENCES 1. MERZHANOV, A. G., Non-isothermal methods in chemical kinetics, Fiz. Goreniya i Vzryva, 9 (1973) 4-36. 2. SHTEINBERG, A. S. & ULYBIN, V. B., Thesis presented to the Seminar-School, Theo O' and Practice q[" SHSProcesses, Arzakan, 1985. 3. KNYAZIK, V. A. & SHTEiNBERG, A. S., The investigation of high-speed reaction kinetics by the electrothermal explosion technique. In Heat and Mass Trans.f~,r in Chemically Reacting Systems (Proceedings ~[" the biternational Seminar-School), 1, ITMO, Minsk, 1988, pp. 143-52. 4. KNYAZIK, V. A., MERZHANOV, A. G. & SHTEINBERG, A. S., Mechanism of combustion in the titanium-carbon system, Dokl. Akad. Nauk. SSSR, 301 (1988) 899-902. 5. KNYAZIK, V. A. & SHTEINBERG, A. S., Electrothermal explosion in heterogeneous systems. In Proceedings of the Joint Meeting of the Soviet and Italian Sections of the Combustion Institute, Pisa, 1990, 4.4. 6. SHICK, H. L., Thermodynamics of Certain Re]ractory Compounds. New York and London, 1966, 1, p. 2. 7. PAMPUCH, R., STOBIERSKI, L., LIS, J. & RACZKA, M., Solid combustion synthesis of beta-SiC powders. Mater. Res. Bull., 22 (1987) 1225-31. 8. YAMADA, O., MIYAMOTO, Y. & KOIZUMI, M., Selfpropagating high-temperature synthesis of SiC. J. Mater. Res., I (1986) 275-9. 9. K N Y A Z I K , V. A., DENISENKO, A. E., CHERNOMORSKAYA, E. A. & SHTEINBERG, A. S., Automised set-up for the investigation of the selfpropagating high-temperature synthesis reaction kinetics. Prib. i Tekh. Exp., 4 (1991) 164-7. 10. SCACE, R. I. & SLACK, G. A., Solubility of carbon in silicon and germanium. J. Chem. Phys., 30 (1959) 1551-5. 11. PAMPUCH, R., BIALOSKORSKI, J. & WALASEK, E., Mechanism of reaction in the Si + C system and the self-propagating high-temperature synthesis of silicon carbide, Ceram. Int., 13 (1987) 63-8. 12. B R A N T O V , S. K., Z A K H A R O V , Yu. N., TATARCHENKO, V. A. & EPELBAUM, B. M., Contact interaction between silicon and carbon filaments. Izv. AN SSSR, Neorg. Mat., 21 (1985) 1032. 13. KIEFFER, A. R., ETTMAYER, P. & GUGEL, E. & SCHMIDT, A., Phase stability of silicon carbide in the ternary system Si-C-N. In Silicon Carbide--1968 (Proceedings q[ the International Conference on Silicon Carbide), Mater. Res. BulL, 4 (1969) 153-66.
H i g h - T e m p e r a t u r e Interaction between Silicon and Carbon V. I. G o r o v e n k o , V. A. Knyazik & A. S. S h t e i n b e r g Institute for Structural Macrokinetics, Academy of Sciences of Russia, 142432, Chernogolovka, Moscow Region, Russia (Received 5 May 1992; accepted 8 June 1992)
Abstract: The ETE method was used for the investigation of high-temperature interaction in the silicon-carbon system and for the manufacture of compacted silicon carbide. The samples were ignited after silicon melting, independently of the heating rate. Maximum values of temperature realized in ETE were 2330-2430°C. The intensity of chemical heat evolution after the melting of silicon depends upon temperature exponentially, with the apparent activation energy equal to 55 _+4 kcal/mole. The conclusion was drawn that a limiting stage of the process is the liquid-phase diffusion of carbon in silicon occurring because of a difference in carbon concentration in the melt near the surface of carbon particles and carbide particles. The ETE technique allows both the synthesis of SiC and the manufacture of constructional materials to be accomplished simultaneously. The ETEmanufactured SiC product is represented mainly by its fl-phase. The relative density of the product is about 0.8~3"9.
INTRODUCTION
tested in the course of study of the high-temperature interaction of titanium and tantalum with carbon. 4'5 In the present work, the ETE method was used for investigation of the kinetic regularities of hightemperature reaction in a silicon-carbon system and for the manufacture of compacted silicon carbide. Silicon carbide is one of the most interesting refractories from both the theoretical and practical points of view. In contrast to powder mixtures of transition metals with carbon, the Si-C system cannot react in the gasless-combustion mode at the initial temperature of the room because it does not possess a sufficiently high heat content, which is only 16"5 kcal/mole. 6 Nevertheless, this reaction may occur in the thermal-explosion mode, 7 in particular, on heating a reaction mixture by means of an electric current. 8
Investigation of the macrokinetics of hightemperature exothermic reactions in powder mixtures appears to be one of the most urgent problems in the light of recent developments in the theory and implementation of gasless combustion, including those of self-propagating high-temperature-synthesis (SHS) processes. The so-called classical, i.e. isothermic, methods of chemical kinetics are known ~ to be essentially inapplicable for determination of the rates of these reactions, which is due both to major difficulties in thermostatting samples in a practically important temperature region (up to 2000-3500°C) and the transient nature of the processes (a duration of 10-2-10-1 s) under study. A new non-isothermic technique, called the electrothermal explosion (ETE), has recently come into use for the solution of these problems. The ETE is based on the recording of thermograms of thermal explosion in a specimen pressed from an electrically conductive exothermic powder mixture of reactants brought to explosion by passing electric current through the sample. 2'3 The ETE technique was
MACROKINETICS OF HIGHTEMPERATURE INTERACTION BETWEEN SILICON AND CARBON ETE experiments on the Si-C system were carried out with the use of equipment described in detail 129
130
V. I. Gorovenko, V. A. K n y a z i k , A. S. Shteinberg 9
Table 1. M i x t u r e s studied
Powder dispersity (/~m)
Mixture
Silicon No. 1 No. 2
Graphite
5<6<16
8
3<63
--
.ca
63<6<90
7 ¢.J
elsewhere.9 Initial samples of silicon and graphite powders, thoroughly mixed in alcohol (Table 1), were pressed in a steel mould up to a density of 1.75_+ 0-5 g/cm 3 to become cylinders 10mm in diameter and 15 mm in height. To study the heat evolution accompanying the high-temperature reaction of silicon with carbon, a time-dependence of the luminance temperature of a sample surface at different rates of sample-heating was recorded in the course of ETE development. Typical thermograms obtained in the ETE of the silicon-carbon system are shown in Fig. 1. Analysis of the thermograms indicates that the samples were ignited after silicon-melting, independently of heating rate. Maximum values of temperature realized in ETE of both mixtures were 2330°C to 2430°C. Figure 2 gives the Arrhenius representation of the intensity of chemical-heat evolution in the reaction of silicon with graphite of various degrees of dispersity as a function of temperature, obtained by treatment of ETE thermograms. The procedure of ETE thermogram treatment is similar to that used in thermal analysis and is described in detail by Knyazik e t al. 4 On considering these curves, it is clear that the ETE process in the silicon-carbon system is ofa stepwise nature. At the initial stage, the intensity of chemical-heat evolution after melting of silicon depends exPonentially upon temperature, with the apparent activation energy, E, equal to 55 _+ 4 kcal/mole for both mixtures No. 1 and No. 2. At a temperature of about 1930°C, the intensity of 2500 •
=
'"
"
=
S
2000 0
1500 Tmelt
1000
0
0.5
1.0
1.5
t,sec Fig. 1. Typical ETE thermograms of silicon-carbon system.
. t:2e
v
i
5 4.0
I 4.5
i 5,0
I
5.5
6.0
104/T, K - t Fig. 2. Temperature-dependence of chemical-heat evolution during the interaction of silicon with graphite of various degrees of dispersity: A - - m i x t u r e No. l i , - - m i x t u r e No. 2.
heat evolution reaches its maximum for both graphite dispersities. Furthermore, the intensity of heat evolution decreases• Though the problem of silicon carbide synthesis is dealt with in many studies, the issue of the reaction mechanism in the silicon-carbon system is still far from being clear• Measurements of the solubility of carbon in liquid silicon have shown 1° that this quantity depends exponentially on temperature, with an enthalpy of 59kcal/mole, which agrees within the limits of experimental error with the value of apparent activation energy obtained in the present work. Almost the same value of activation energy, 56.2 kcal/mole, was obtained in the DTA study of the interaction of liquid silicon with carbon filaments near the melting point of silicon in the temperature range of 1422-1436°C. 11 These results cannot be explained on the basis of the Brantov model, 12 assuming that the diffusion of carbon through the SiC layer is the limiting stage of the process. The mechanism of successive endothermic carbon-dissolving and exothermic silicon carbide precipitation and of related temperature oscillation was suggested. The results of our study show that the mechanism of interaction in the Si-C system after the melting of silicon is somewhat different. During electrothermal explosion in the Si-C system, its temperature increases monotonically, so it seems natural to assume that the dissolving of carbon particles and the size growth of silicon carbide particles take place simultaneously. A limiting stage of the process is the liquid-phase diffusion of carbon in silicon occurring
131
High-temperature interaction between silicon and carbon
because of the difference in the carbon concentration in the melt near the surface of carbon particles and carbide particles. The solubility of carbon in liquid silicon reported by Scace and Slack 1° probably corresponds to the concentration of carbon in the melt at the silicon carbide surface, since the carbon surface has had time to be coated with a carbide layer during an experimental run. As long as the temperaturedependence of the intensity of chemical-heat evolution in ETE is close to that of the equilibrium concentration of carbon in liquid silicon in the vicinity of the SiC surface, it may be concluded that the concentration of carbon at the carbon-particle surface in liquid silicon increases with temperature in accordance with the same law and the same activation energy but a different pre-exponential factor. The maximum intensity of heat evolution during ETE in the Si-C system is attained at the m o m e n t when the product formed begins to retard further occurrence of the reaction.
P R E P A R A T I O N OF O N E - P H A S E C O M P A C T E D SILICON C A R B I D E This study also had a technological objective, i.e. the estimation of the possibility of manufacturing onephase compacted silicon carbide by means of the ETE method. The conventional procedure for the manufacture of silicon-carbide-based structural materials involves two stages. The first stage is the preparation of SiC powder, and the second is the manufacture of constructional materials themselves from this powder by using activated-sintering or hot-compaction techniques. The ETE technique dealt with in this paper, in principle, allows both of these processes to be operated simultaneously. Experiments on the preparation of compacted SiC were carried out with the following charge compositions: the stoichiometric mixture, the mixture with silicon deficiency, the mixture with excess silicon, and the stoichiometric charge of silicon and carbon with aluminum and boron additives. Graphite and PM-16E carbon black were used as a source of carbon. To prepare a charge, the powders were mixed in alcohol for 10 hours and then dried in vacuo. Cylindrical samples of 2.5g in mass, 1 6 m m in height, and 12 m m in diameter, which corresponded to the initial density of 1"38 g/cm 3, were pressed from this mixture. ETE experiments were carried out in an argon atmosphere and at a mechanical load of 150 kgf/cm 2 in a quartz shell with steel banding (Fig. 3). In the experimental run, the time-dependences of
2 1
3 4
2
Fig. 3. Schematicdiagram of experimentalsystemfor manufacture of compacted SiC by means of ETE: l--reacting mixture; 2--electrodes; 3--quartz shell; 4--steel banding. the voltage applied to the electrodes and the electric current passing through a specimen were measured. The typical appearance of these dependences as well as the change in electrical resistivity and temperature of the sample in the course of the experiment is shown in Fig. 4. Let us consider these curves. Schematically, the whole of the process of interaction between silicon and carbon in ETE may be split into three periods, i.e. the first period as a stage of sample-heating, the second period as the intensive occurrence of chemical reactions, and the third one as sample-cooling. During the first period, the joule heating of the sample takes place. Chemical transformation is insignificant at this stage; the current flowing through the sample increases gradually, and the electric power increases. The second period, which lasts for about 0.2 s, is characterized by a dramatic decrease in the electrical resistivity of the sample. I,A U,V
p,Ohm//cm T,°C
-2500
.12-
-8 200 -
-2000
-1500
100-
-1000
o.o, 500
0
I
I
I
I
2
3
t, sec Fig. 4. Typical time-dependences of voltage applied to electrodes (l), electric current passing through a specimen (2), electrical resistivity (3), and temperature of sample {4), in the course of experiment.
V. L Gorovenko, V. A. Knyazik, A. S. Shteinberg
132 Table 2. Initial and final composition and density of samples
Initial
Final composition
Density
composition of samples
of samples according
of samples
to X-ray phase
(g/cm 3)
analysis Stoichiometric mixture 26% excess of Si 20% lack of Si
Stoichiometric mixture + 5% B Stoichiometric mixture + 5% AI
//-SIC, 10 + 20% Si, 1% of graphite //-SIC, 25% Si 1% of graphite //-SIC, 10% :<-SIC 5 + 10% Si 5% of graphite //-SIC, B, 5 + 16% Si 1 + 2% of graphite //-SIC, AI, 10% Si 3% of graphite
The manufacture of compacted silicon carbide by means of the ETE technique has been developed. Such an approach allows both the synthesis of SiC and the compacting of constructional materials to be accomplished simultaneously. The ETEmanufactured SiC product is represented mainly by its fl-phase.
2.82 + 3.03 2.68 + 2.97 2.62 + 2.87
2.71 + 2.96
ACKNOWLEDGMENTS The authors gratefully acknowledge R. Pampuch, U. I. Goldshleger, and V. A. Kudryashov for very useful discussions during the course of this work.
2.75 + 2.98
The voltage on the sample decreases, and the current increases drastically. The sample temperature increases up to its maximum value under these conditions, which is an indication of the high-speed occurrence of the reaction with the evolution of a large amount of heat. During the third period, the resistivity of the sample increases and tends to reach a certain value (0.1 f~), the voltage on the sample is restored, and the current decreases. From this, it can be concluded that the chemical interaction of silicon and carbon is over and the formation of the final product, silicon carbide, has taken place. After the experiments, the specimens were studied by an X-ray phase-analysis technique. The corresponding data and sample densities are listed in Table 2. The density of single-crystal SiC is 3-2 g/cm 3. It may be seen from Table 2 that the ETEmanufactured SiC product is represented mainly by its fl-phase, which agrees rather well with data reported by Kieffer et al.l 3 The density of sintered specimens of SiC is about 80-90% of that for singlecrystal SiC. CONCLUSIONS It has been shown that the high-temperature interaction in a powder mixture of silicon and carbon under the electrothermal explosion conditions occurs after silicon-melting and is probably limited by the liquid-phase diffusion of carbon in silicon occurring because of the difference in carbon concentration in the melt near the surface of carbon particles and carbide particles.
REFERENCES 1. MERZHANOV, A. G., Non-isothermal methods in chemical kinetics, Fiz. Goreniya i Vzryva, 9 (1973) 4-36. 2. SHTEINBERG, A. S. & ULYBIN, V. B., Thesis presented to the Seminar-School, Theo O' and Practice q[" SHSProcesses, Arzakan, 1985. 3. KNYAZIK, V. A. & SHTEiNBERG, A. S., The investigation of high-speed reaction kinetics by the electrothermal explosion technique. In Heat and Mass Trans.f~,r in Chemically Reacting Systems (Proceedings ~[" the biternational Seminar-School), 1, ITMO, Minsk, 1988, pp. 143-52. 4. KNYAZIK, V. A., MERZHANOV, A. G. & SHTEINBERG, A. S., Mechanism of combustion in the titanium-carbon system, Dokl. Akad. Nauk. SSSR, 301 (1988) 899-902. 5. KNYAZIK, V. A. & SHTEINBERG, A. S., Electrothermal explosion in heterogeneous systems. In Proceedings of the Joint Meeting of the Soviet and Italian Sections of the Combustion Institute, Pisa, 1990, 4.4. 6. SHICK, H. L., Thermodynamics of Certain Re]ractory Compounds. New York and London, 1966, 1, p. 2. 7. PAMPUCH, R., STOBIERSKI, L., LIS, J. & RACZKA, M., Solid combustion synthesis of beta-SiC powders. Mater. Res. Bull., 22 (1987) 1225-31. 8. YAMADA, O., MIYAMOTO, Y. & KOIZUMI, M., Selfpropagating high-temperature synthesis of SiC. J. Mater. Res., I (1986) 275-9. 9. K N Y A Z I K , V. A., DENISENKO, A. E., CHERNOMORSKAYA, E. A. & SHTEINBERG, A. S., Automised set-up for the investigation of the selfpropagating high-temperature synthesis reaction kinetics. Prib. i Tekh. Exp., 4 (1991) 164-7. 10. SCACE, R. I. & SLACK, G. A., Solubility of carbon in silicon and germanium. J. Chem. Phys., 30 (1959) 1551-5. 11. PAMPUCH, R., BIALOSKORSKI, J. & WALASEK, E., Mechanism of reaction in the Si + C system and the self-propagating high-temperature synthesis of silicon carbide, Ceram. Int., 13 (1987) 63-8. 12. B R A N T O V , S. K., Z A K H A R O V , Yu. N., TATARCHENKO, V. A. & EPELBAUM, B. M., Contact interaction between silicon and carbon filaments. Izv. AN SSSR, Neorg. Mat., 21 (1985) 1032. 13. KIEFFER, A. R., ETTMAYER, P. & GUGEL, E. & SCHMIDT, A., Phase stability of silicon carbide in the ternary system Si-C-N. In Silicon Carbide--1968 (Proceedings q[ the International Conference on Silicon Carbide), Mater. Res. BulL, 4 (1969) 153-66.