- Email: [email protected]
Synthesis of silicon carbide powders by a CW CO2 laser
52
Applied Surface Science 36 (1989) 52-58 North-Holland, Amsterdam
S Y N T H E S I S O F S I L I C O N C A R B I D E P O W D E R S BY A C W C O z L A S E R F. C U R C I O , G. G H I G L I O N E *, M. M U S C I CISE Tecnologie Innouative S.p.A., P.O. Box 12081, 201.74 Milano, Italy and
C. N A N N E T ] ' I ENEA, Dip. TIB, Divisione Chimicn, CRE Casaccin, Ror~ Italy
Received 2 June 1988; accepted for publication 13 July 1988
Ultrafme SiC ceramic powders have been produced by irradiating silane and acetylene mixtures w/th a CW CO, laser. The work is mainly concerned with the evaluation of the parameters affecting the material production efficiency: laser power and laser intensity, pressure in the reaction chamber, reactant and carrier gas flow rates. The characterization of the produced material refers to particle composition, size and shape and crystalline structure. Sintering tests have been made in order to evaluate the performances of laser-produced ceramic powders. Preliminary measurements aimed at the evaluation of the feasibility of process scaling-up have been also carried out.
L h~'oduc~on It is well established that laser sources are useful tools for the p r o d u c t i o n o f sol/d materials. Pz~rticularly I R C O 2 lasers have been successfully experim e n t e d for production of b o t h thin fihns a n d ultrafine powders. T h e basic p r h a d p l e in b o t h cases is the r e s o n a n t absorption o f the laser radiation b y suitable reactant gases: ha the deposition processes the p h o t o p r o d u c e d radicals d/ffus~ towards a solid substrate a n d grow a film; ha powder photosynthesis the comsions a m o n g rz~dicals are the main effect that ~ v e s rise to particle growth ha the gas phase. T h e latter process is particularly attractive from the point o f view of b o t h p~wder characteristics a n d expected production cost. In comparison with other emerg/ng techniques for the p r o d u c t i o n of ultrafhae powders, ha fact, the laser induced processes allow finer, purer a n d more monodispersed particles to b e obtained; moreover, because of the high spatial confhaement of the reaction zone permitted b y the laser b e a m coherence, reaction effidenc/es close to 100~ have been checked experimentaUy [1]. • CISE Guest. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsev/er S d e n c e Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
F. Curcio et al. / Synthesis of SiC powders by C W COz laser
53
Most work performed up to now concerns the synthesis of silicon carbide and silicon nitride powders [1-4]; in fact, the applicati.on of these materials to high temperature structural ceramics has led to renewed interest in the
production of improved powders. This paper reports the experimental results on CW CO 2 laser driven synthesis of SiC powders starting from mixtures of silane and acetylene as reactant gases. The work is mahdy concerned ~ith the evaluation of the parameters affecting the material production efficiency, the characterization of the produced powders and their sintering performances. It includes a first stage performed at low laser power and low reactant flow rates and a second stage where prelhnmary measurements, aimed at the evaluation of the feasibility of process s e a . g - u p , were carried out.
2. F_~:~ri~ntal The experimental apparatus essen~ally consie,ts of a gas distribution system, a reaction chamber, a filter for powder collection and a double vacuum system for high prelhninary vacuum in the:. reaction ckmmber and for continuous gas flow during irradiation, respectively. The. schematic of the reaction chamber is shown in fig. 1: the laser beam ~)erpendicularly intersects the reactant gas stream which enters the cell through a 1 ~ n di~u~eter nozzle; the injection of an inert g,as, both through a coaxial tube and an inlet near the front window, allows the spatial confinement of the reaet~at gases and of the produced powders to be obtained. Because of the suction of the rotary pump, the powders are collected in a cylindrical borostL;cate fiber filter, set between the reaction cell and the vacuum pump. When the laser beam irradiates the reactant strc~Lrna bright flame appears, whose ,~emperature is measured by an optical pyrometer. sl H,*% ~ . ~
SZ.~AL
I ° ,I
LA ER I L ` ER ] _t.. . . . . . .L--
....
He
~
GAU~;~NO
:l:~°~R
v TO FILTER AND ROTARY PUMP
Fig. 1. Schematic of the reaction chamber for SiC photosynthesis.
54
F. Curcio et al. / Synthesis of SiC powders by C W CO2 laser
In the fLrst stage of the work the maximum laser power ha the reaction zone was about 70 W and the maximum s[lane flow rate was 50 SCCM; the silane to acetylene flow rate ratio was 2 in order to obta/n the stolchiometric composition. A more powerful laser source (Rofin-Sinar RSS00) with emitted power up to 550 W, was employed in the second stage of the work along with mass flow controllers of a few thousand SCCM. Different techniques were used for the powder characterizations: composit/on was determined through I R spectroscopy and ~.hem/cal an~dyses; s;ze and shape through transmission electron microscopy and BET analysis; crystaifine structure through X-ray and eleckron diffraction. 3. R ~ t s
~
disc~.ion
3.1. Process a n d p o w d e r characterization
The most important process parameters were varied ha order to evaluate their influence on productivity and on particle characteristics. A proper injection of an inert gas is essential to spatially confine the reaction zone, to avoid re~'~.ctant gases and energy losses, and to efficiently collect the stream of produced particles. To this aim, argon was tested as fhe most su/table cartier gas, because of its low thermal conductivity. The best results were achieved using flow rates about one order of magn/tude l~Jgher than the reactant gases. The quantity of collected powder linearly increases with reactant flow rate, as shown ha fig. 2 and does not give any evidence of saturation. Tl-ds suggests that more powder could be produced with the same laser power by hacreashag the gas flow rates, without a decrease ha the reaction efficiency. Meanwhile, the temperature of the flame, due to the radiation emiCted by the hot SiC particles [1] seems to saturate near 1500 ° C. The material productivity does not change by increas/ng the radiation density through focal/zafion of the beam, or varying the reaction celt pressure in the range 100-500 Torr either. These results indicate that the material production is linfited by the comp|ete depletion of reactants rather than by the insufficient radiation supply or by the short residence time under the laser beam. The most interesting result, however, as for the process productivity, is the constancy of oollected powders even if the laser power is lowered down to 20 W, for SIH 4 flow rates up to 50 SCCM. This means that the energy requirements are very low and therefore the scaling up to massive material production might be quite advantageous. All the varied parameters, ha the previously mentioned ranges, do not substantially affect the reaction efficiency but only the particle nucleation and growth kinetics. The characterization of the produced powders was concerned with composition, size and shape of the particles, and crystagl:me structure. The composition
F. Curcio ,'.t aL / Synthesis of SiC powders
55
by C W CO z laser
~000
2
P
i
1500
5OO
0o
i0 ~0
20
30
40
50
8 ~ 4 (SGGM)
Fig. 2. Quantity of collected SiC powder against Silla flow rate; the incident laser power was about 70 W. The flame temperature is also repot ted.
was determined through infrared spectroscopy and chemical analysis; the S i / C atomic ratio ranged between 0.98 and 1.00 for ~SiH4/OC2H2 = 2 and unfocused beam irradiation while some silicon deficiency ( S i / C = 0.92) was found under focused beam irradiation. The specific surface was dete~:nined through BET analysis and typical values are 125 m2//g f o r ~[)SiH4 = 20 S e e m and 160 m 2 / g for ~)siH4= 40 S C C M with 70 W laser power. Fig. 3a shows a typical transmission electron microscopy (TE,~D picture from which is evident the spherical shape of the partieles, othe narrow range of sizes and the values of the~ diameters, typically 200 A. The powders are crystalllne with the cubic ~ structure, as proved by X-ray and electron diffraction analyses. Sinterlng tests were performed on a number of boron-doped specimens. They were dry pressed in steel dies and then sintered in a graphite element resistance furnace at 2000 o C for 20 rain under flowing inert gas. By properly choosing the types and the anlounLs of slntering aids high density slntered specimens were obtained (up to 98.5% to the theoretical density). Higher green and sintered densities, up to 99% of the theoretical density, could be achieved by pre-treating the powders in vacuum at moderate temperature.
56
F. Curcio et ol. / Synthesis of SiC powders by C W CO2 laser
Fig. 3. Transmissionelectronn,:~ographof typicalSiC powder. 3.2. Process scaling-up
The subsequent step of the work was devoted to prelhninary measurements aimed at testing the possiblity of process scaling up, using a more powerful laser and higher flow rates. A plot of the productivity against the reactant flow rate gain exhibits a linear trcnd, however its slope is lower in comparison with the previous measurements as if in the presence of a progressive reaction effidency decrease. The flame temperature rapidly increases by enhancing either the flow rates or the incident power, and in both cases the colour of the powders becomes darker and darker thus suggesting an increasing free carbon content. This hypothesis is confirmed by TEM analysis that demonstrates substantial amounts of graphific carbon, probably due to a partial silicon evaporation from SiC heated to temperatures higher than 1700°C. This phenomenon, rather than the incomplete reaction of gases, might explain the lack in the produced material. Another important parameter, in view of both energy balance evaluation and understanding of the process mechanisms, is the absorbed laser power. The trend of the absorbed power against reactant flow rate is reported in fig. 4. For OSiH,= 100 SCCM only 4 W are absorbed while the material yield, as evaluated by weighing the collected powders, is ~ = 0.7; this means that for every reacted Sill4 molecule about 8 photons must be supplied. At higher flow rates the energy absorbed for promoting a single event further decreases so
57
F.. Curcio et aL / Synthesis of SiC powders by C W COz laser
AP{W] 15
"
~ ql= .7
tL'~
"
-
~ / 6A,=200o ,10o sccM
r Pot~5o w I
200
I
300
! ~
400
='=""~
500
t 600
~S"~4(SCCM)
Fig. 4. Absorbed laser power for SiH~ +CzH 2 mixture and for pure silane, agaiust reactant flow rate.
that for ~ s i . , = 600 SCCM it is reduced by about a factor 2. The very small energy requirement is certai;dy due to the strong reaction exothermicity in the temperature range involved in the process (AH~s00o c = - - 8 5 k c a i / m o l ) together with the very low losses of the released energy. In fact, the high spatial confinement of the reaction zone and the low the~Tnal conductivity of argon, allows the released reaction heat to sustain the reaction itself. For comparison fig. 4 also reports the absorbed energy for the endothermic (,~H~200oc = + 15 k c a l / m o l ) dissociation of pure silane in the process of silicon powder synthesis: for OSiH~= 100 SCCM the absorbed power is more than doubled in comparison with the Sill 4 + C2H 2 mixture while the reaction efficiency is = 0.43. Accurate measurements with C2H 4 instead of C2H 2 will be carried out in order to compare the behaviour of the SiC photosynthesis process starting from quite a similar although less exothermic (AH!s0ooc = --32 k c a l / m o l ) reaction. Anyway, some prelimina_,T tests show that ethylene is by far less advantageous than acetylene for material yield, energy balance and stoichiometry of powders. As for the powder characteristics, the main differences in comparison whh the previous ones refer to particle sizes and compositions. Both measurements through BET analysis and transmission electron microscopy evidence a strong reduction in particle size at the higher laser powers and flow rates: the particle diameter for P0 = 200 W and Osia, = 100 S C C M lowers down to 60 ,~ with 242 m 2 / g specific surface area. Despite their very small dimensions, these powders maintain the cubic crystalline structure; moreover, they appear rather dark and T E M analyses show the presence of big (few microns) platdets of polycrystalline graphitic carbon.
58
F. Curcio et al. / Synthesis of SiC powders by C W CO2 laser
4. Com'.leslons
The CO2 laser induced synthesis of SiC powders is a very efficient and energy effective process when silane and acetylene mixtures are used as reactant gases. This is a consequenc~~ of the strong exothermicity of the reaction combined v~th the strict spatial and thermal confinement of the reaction zone allowed by the photon source coherence. In tiffs ldn:l of process, the exotherm/dty prov/des a preferential requ/rement in the choice of the reactants, because the released energy can substantially contribute to sustain the reacton itself. In fact, though ethylene resonantly absorbs the laser radiation, nfixtures of Sill4 and C2H4 are less favourable to SiC synthesis, because of the lower e×othermicity of the involved reaction. The produced powders are spherical in shape, monodispersed and ultrafine, their specific surface area ranging from 120 m2/g, for low laser powers (50 W) and low flow rates (20 SCCM), to 240 m 2 / g for higher laser powers (200 W) and higher flow rates (100 SCCM). Prel/minary measurements for the process scaling-up showed some problems relatexl to the too high local temperatures, which cause the silicon evaporation and nucleation of carbon platele~ts. A w/dening of the reaction zone and a larger beam cross section will probably be necessary in order to fully exploit ~ e production potential of the process. Sintering performances of the produced powders were successfully tested and further work is still underway.
Acknowledgments Thanks are due to N. Ricci and A. Zoboli for TEM and X-ray analyses, and to R. SantorelE for helpful discussions on chem/cal reaction thermodynamics. Work st~pported by ENEA/CISE under contract No. 09297.
References [1] W.R.Cannon,S.C. Danforth,J.H. Flint,J.S. Haggertyand R.A. Marra,J. Am. Ceram.Soc.65 (1982) 324.
[2] Y. Suyarna,R.M. Marra, 3.S. Haggerty and H.K. Bowen,Am. Ceram. So(:. Bull. 64 (1985) 1356. [3] M. C.auchefier,O. Croix,M. Luce, M. Michon,J. Parisand S. Tistchenko,Mater.Sci.Monogr. 38A (1986) 545 [Proc. 6th Intern.MeetingMod. Cer. Tech.]. [4] Y. Kizaki,T. Kandoriand Y. Fujitani,Japan. J. AppL Phys.24 (1985)800.
Applied Surface Science 36 (1989) 52-58 North-Holland, Amsterdam
S Y N T H E S I S O F S I L I C O N C A R B I D E P O W D E R S BY A C W C O z L A S E R F. C U R C I O , G. G H I G L I O N E *, M. M U S C I CISE Tecnologie Innouative S.p.A., P.O. Box 12081, 201.74 Milano, Italy and
C. N A N N E T ] ' I ENEA, Dip. TIB, Divisione Chimicn, CRE Casaccin, Ror~ Italy
Received 2 June 1988; accepted for publication 13 July 1988
Ultrafme SiC ceramic powders have been produced by irradiating silane and acetylene mixtures w/th a CW CO, laser. The work is mainly concerned with the evaluation of the parameters affecting the material production efficiency: laser power and laser intensity, pressure in the reaction chamber, reactant and carrier gas flow rates. The characterization of the produced material refers to particle composition, size and shape and crystalline structure. Sintering tests have been made in order to evaluate the performances of laser-produced ceramic powders. Preliminary measurements aimed at the evaluation of the feasibility of process scaling-up have been also carried out.
L h~'oduc~on It is well established that laser sources are useful tools for the p r o d u c t i o n o f sol/d materials. Pz~rticularly I R C O 2 lasers have been successfully experim e n t e d for production of b o t h thin fihns a n d ultrafine powders. T h e basic p r h a d p l e in b o t h cases is the r e s o n a n t absorption o f the laser radiation b y suitable reactant gases: ha the deposition processes the p h o t o p r o d u c e d radicals d/ffus~ towards a solid substrate a n d grow a film; ha powder photosynthesis the comsions a m o n g rz~dicals are the main effect that ~ v e s rise to particle growth ha the gas phase. T h e latter process is particularly attractive from the point o f view of b o t h p~wder characteristics a n d expected production cost. In comparison with other emerg/ng techniques for the p r o d u c t i o n of ultrafhae powders, ha fact, the laser induced processes allow finer, purer a n d more monodispersed particles to b e obtained; moreover, because of the high spatial confhaement of the reaction zone permitted b y the laser b e a m coherence, reaction effidenc/es close to 100~ have been checked experimentaUy [1]. • CISE Guest. 0 1 6 9 - 4 3 3 2 / 8 9 / $ 0 3 . 5 0 © Elsev/er S d e n c e Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
F. Curcio et al. / Synthesis of SiC powders by C W COz laser
53
Most work performed up to now concerns the synthesis of silicon carbide and silicon nitride powders [1-4]; in fact, the applicati.on of these materials to high temperature structural ceramics has led to renewed interest in the
production of improved powders. This paper reports the experimental results on CW CO 2 laser driven synthesis of SiC powders starting from mixtures of silane and acetylene as reactant gases. The work is mahdy concerned ~ith the evaluation of the parameters affecting the material production efficiency, the characterization of the produced powders and their sintering performances. It includes a first stage performed at low laser power and low reactant flow rates and a second stage where prelhnmary measurements, aimed at the evaluation of the feasibility of process s e a . g - u p , were carried out.
2. F_~:~ri~ntal The experimental apparatus essen~ally consie,ts of a gas distribution system, a reaction chamber, a filter for powder collection and a double vacuum system for high prelhninary vacuum in the:. reaction ckmmber and for continuous gas flow during irradiation, respectively. The. schematic of the reaction chamber is shown in fig. 1: the laser beam ~)erpendicularly intersects the reactant gas stream which enters the cell through a 1 ~ n di~u~eter nozzle; the injection of an inert g,as, both through a coaxial tube and an inlet near the front window, allows the spatial confinement of the reaet~at gases and of the produced powders to be obtained. Because of the suction of the rotary pump, the powders are collected in a cylindrical borostL;cate fiber filter, set between the reaction cell and the vacuum pump. When the laser beam irradiates the reactant strc~Lrna bright flame appears, whose ,~emperature is measured by an optical pyrometer. sl H,*% ~ . ~
SZ.~AL
I ° ,I
LA ER I L ` ER ] _t.. . . . . . .L--
....
He
~
GAU~;~NO
:l:~°~R
v TO FILTER AND ROTARY PUMP
Fig. 1. Schematic of the reaction chamber for SiC photosynthesis.
54
F. Curcio et al. / Synthesis of SiC powders by C W CO2 laser
In the fLrst stage of the work the maximum laser power ha the reaction zone was about 70 W and the maximum s[lane flow rate was 50 SCCM; the silane to acetylene flow rate ratio was 2 in order to obta/n the stolchiometric composition. A more powerful laser source (Rofin-Sinar RSS00) with emitted power up to 550 W, was employed in the second stage of the work along with mass flow controllers of a few thousand SCCM. Different techniques were used for the powder characterizations: composit/on was determined through I R spectroscopy and ~.hem/cal an~dyses; s;ze and shape through transmission electron microscopy and BET analysis; crystaifine structure through X-ray and eleckron diffraction. 3. R ~ t s
~
disc~.ion
3.1. Process a n d p o w d e r characterization
The most important process parameters were varied ha order to evaluate their influence on productivity and on particle characteristics. A proper injection of an inert gas is essential to spatially confine the reaction zone, to avoid re~'~.ctant gases and energy losses, and to efficiently collect the stream of produced particles. To this aim, argon was tested as fhe most su/table cartier gas, because of its low thermal conductivity. The best results were achieved using flow rates about one order of magn/tude l~Jgher than the reactant gases. The quantity of collected powder linearly increases with reactant flow rate, as shown ha fig. 2 and does not give any evidence of saturation. Tl-ds suggests that more powder could be produced with the same laser power by hacreashag the gas flow rates, without a decrease ha the reaction efficiency. Meanwhile, the temperature of the flame, due to the radiation emiCted by the hot SiC particles [1] seems to saturate near 1500 ° C. The material productivity does not change by increas/ng the radiation density through focal/zafion of the beam, or varying the reaction celt pressure in the range 100-500 Torr either. These results indicate that the material production is linfited by the comp|ete depletion of reactants rather than by the insufficient radiation supply or by the short residence time under the laser beam. The most interesting result, however, as for the process productivity, is the constancy of oollected powders even if the laser power is lowered down to 20 W, for SIH 4 flow rates up to 50 SCCM. This means that the energy requirements are very low and therefore the scaling up to massive material production might be quite advantageous. All the varied parameters, ha the previously mentioned ranges, do not substantially affect the reaction efficiency but only the particle nucleation and growth kinetics. The characterization of the produced powders was concerned with composition, size and shape of the particles, and crystagl:me structure. The composition
F. Curcio ,'.t aL / Synthesis of SiC powders
55
by C W CO z laser
~000
2
P
i
1500
5OO
0o
i0 ~0
20
30
40
50
8 ~ 4 (SGGM)
Fig. 2. Quantity of collected SiC powder against Silla flow rate; the incident laser power was about 70 W. The flame temperature is also repot ted.
was determined through infrared spectroscopy and chemical analysis; the S i / C atomic ratio ranged between 0.98 and 1.00 for ~SiH4/OC2H2 = 2 and unfocused beam irradiation while some silicon deficiency ( S i / C = 0.92) was found under focused beam irradiation. The specific surface was dete~:nined through BET analysis and typical values are 125 m2//g f o r ~[)SiH4 = 20 S e e m and 160 m 2 / g for ~)siH4= 40 S C C M with 70 W laser power. Fig. 3a shows a typical transmission electron microscopy (TE,~D picture from which is evident the spherical shape of the partieles, othe narrow range of sizes and the values of the~ diameters, typically 200 A. The powders are crystalllne with the cubic ~ structure, as proved by X-ray and electron diffraction analyses. Sinterlng tests were performed on a number of boron-doped specimens. They were dry pressed in steel dies and then sintered in a graphite element resistance furnace at 2000 o C for 20 rain under flowing inert gas. By properly choosing the types and the anlounLs of slntering aids high density slntered specimens were obtained (up to 98.5% to the theoretical density). Higher green and sintered densities, up to 99% of the theoretical density, could be achieved by pre-treating the powders in vacuum at moderate temperature.
56
F. Curcio et ol. / Synthesis of SiC powders by C W CO2 laser
Fig. 3. Transmissionelectronn,:~ographof typicalSiC powder. 3.2. Process scaling-up
The subsequent step of the work was devoted to prelhninary measurements aimed at testing the possiblity of process scaling up, using a more powerful laser and higher flow rates. A plot of the productivity against the reactant flow rate gain exhibits a linear trcnd, however its slope is lower in comparison with the previous measurements as if in the presence of a progressive reaction effidency decrease. The flame temperature rapidly increases by enhancing either the flow rates or the incident power, and in both cases the colour of the powders becomes darker and darker thus suggesting an increasing free carbon content. This hypothesis is confirmed by TEM analysis that demonstrates substantial amounts of graphific carbon, probably due to a partial silicon evaporation from SiC heated to temperatures higher than 1700°C. This phenomenon, rather than the incomplete reaction of gases, might explain the lack in the produced material. Another important parameter, in view of both energy balance evaluation and understanding of the process mechanisms, is the absorbed laser power. The trend of the absorbed power against reactant flow rate is reported in fig. 4. For OSiH,= 100 SCCM only 4 W are absorbed while the material yield, as evaluated by weighing the collected powders, is ~ = 0.7; this means that for every reacted Sill4 molecule about 8 photons must be supplied. At higher flow rates the energy absorbed for promoting a single event further decreases so
57
F.. Curcio et aL / Synthesis of SiC powders by C W COz laser
AP{W] 15
"
~ ql= .7
tL'~
"
-
~ / 6A,=200o ,10o sccM
r Pot~5o w I
200
I
300
! ~
400
='=""~
500
t 600
~S"~4(SCCM)
Fig. 4. Absorbed laser power for SiH~ +CzH 2 mixture and for pure silane, agaiust reactant flow rate.
that for ~ s i . , = 600 SCCM it is reduced by about a factor 2. The very small energy requirement is certai;dy due to the strong reaction exothermicity in the temperature range involved in the process (AH~s00o c = - - 8 5 k c a i / m o l ) together with the very low losses of the released energy. In fact, the high spatial confinement of the reaction zone and the low the~Tnal conductivity of argon, allows the released reaction heat to sustain the reaction itself. For comparison fig. 4 also reports the absorbed energy for the endothermic (,~H~200oc = + 15 k c a l / m o l ) dissociation of pure silane in the process of silicon powder synthesis: for OSiH~= 100 SCCM the absorbed power is more than doubled in comparison with the Sill 4 + C2H 2 mixture while the reaction efficiency is = 0.43. Accurate measurements with C2H 4 instead of C2H 2 will be carried out in order to compare the behaviour of the SiC photosynthesis process starting from quite a similar although less exothermic (AH!s0ooc = --32 k c a l / m o l ) reaction. Anyway, some prelimina_,T tests show that ethylene is by far less advantageous than acetylene for material yield, energy balance and stoichiometry of powders. As for the powder characteristics, the main differences in comparison whh the previous ones refer to particle sizes and compositions. Both measurements through BET analysis and transmission electron microscopy evidence a strong reduction in particle size at the higher laser powers and flow rates: the particle diameter for P0 = 200 W and Osia, = 100 S C C M lowers down to 60 ,~ with 242 m 2 / g specific surface area. Despite their very small dimensions, these powders maintain the cubic crystalline structure; moreover, they appear rather dark and T E M analyses show the presence of big (few microns) platdets of polycrystalline graphitic carbon.
58
F. Curcio et al. / Synthesis of SiC powders by C W CO2 laser
4. Com'.leslons
The CO2 laser induced synthesis of SiC powders is a very efficient and energy effective process when silane and acetylene mixtures are used as reactant gases. This is a consequenc~~ of the strong exothermicity of the reaction combined v~th the strict spatial and thermal confinement of the reaction zone allowed by the photon source coherence. In tiffs ldn:l of process, the exotherm/dty prov/des a preferential requ/rement in the choice of the reactants, because the released energy can substantially contribute to sustain the reacton itself. In fact, though ethylene resonantly absorbs the laser radiation, nfixtures of Sill4 and C2H4 are less favourable to SiC synthesis, because of the lower e×othermicity of the involved reaction. The produced powders are spherical in shape, monodispersed and ultrafine, their specific surface area ranging from 120 m2/g, for low laser powers (50 W) and low flow rates (20 SCCM), to 240 m 2 / g for higher laser powers (200 W) and higher flow rates (100 SCCM). Prel/minary measurements for the process scaling-up showed some problems relatexl to the too high local temperatures, which cause the silicon evaporation and nucleation of carbon platele~ts. A w/dening of the reaction zone and a larger beam cross section will probably be necessary in order to fully exploit ~ e production potential of the process. Sintering performances of the produced powders were successfully tested and further work is still underway.
Acknowledgments Thanks are due to N. Ricci and A. Zoboli for TEM and X-ray analyses, and to R. SantorelE for helpful discussions on chem/cal reaction thermodynamics. Work st~pported by ENEA/CISE under contract No. 09297.
References [1] W.R.Cannon,S.C. Danforth,J.H. Flint,J.S. Haggertyand R.A. Marra,J. Am. Ceram.Soc.65 (1982) 324.
[2] Y. Suyarna,R.M. Marra, 3.S. Haggerty and H.K. Bowen,Am. Ceram. So(:. Bull. 64 (1985) 1356. [3] M. C.auchefier,O. Croix,M. Luce, M. Michon,J. Parisand S. Tistchenko,Mater.Sci.Monogr. 38A (1986) 545 [Proc. 6th Intern.MeetingMod. Cer. Tech.]. [4] Y. Kizaki,T. Kandoriand Y. Fujitani,Japan. J. AppL Phys.24 (1985)800.