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Nanoscale Si-C and Al-O-(N,C) ceramic powders by laser synthesis from gaseous precursors
NanoStrueturcd Materials. Vol. 6, pp. 341-344, 1995 Copyright © 1995 Elsevier Science Ltd Prin~d in the USA. All rights rc~erved 0965-9773/95 $9.50 + .00
Pergamon 0965-9773(95)00066-6
N A N O S C A L E S I - C A N D A L - O - ( N , C ) C E R A M I C P O W D E R S BY L A S E R SYNTHESIS FROM GASEOUS PRECURSORS E. Borsella, S. Botti, M. C. Cesile*, S. Martelli and R. Alexandrescu** ENEA- C.R.E Frascati, Dept. 1NN-FIS P.O.Box 65, 00044 Frascati, Rome (Italy) R. Giorgi, C.A. Nannetti, S. Turt~* and G. Zappa ENEA-CR E Casaccia, Dept. INN-NUMA P OBox 2400, 00100 Rome (Italy) Abstract--Laser induced photosynthesis has been employed to produce nanosized aluminium oxide powders. The dependence of the reaction outcomes oll the process parameters, in particular on reactant gases (trimethylaluminiunl TAL'-I and nitrous oxide) relative concentration, has been investigated in several different conditions. The experimental results" indicate two main reaction paths. Low TA£4 relative concentration leads to the formation of nanocrystalline y-Al203 with traces of Al303N compounds. Increasing the TMA concentration the synthesis of aluminium oxicarbide 6~120C) is obseta,ed Only the first reaction path is able to produce, after calcining at 1400°(; nanosized a-Al20 3 powder. Prelimina~. attempts in sintering laser produced silicon carbide have shown, that, due to the nanometric dimension of the powder particles, it is possible to retain the fl-SiC low tenlperature phase even for sintering tenlperature up to 2150 °C.
INTRODUCTION Laser assisted synthesis of ceramic powders from the gas phase, firstly introduced by Haggerl3' and co-workers in 198111] has already proved one of the most promising route for producing ideal nanosized ceramic powders. Up to now several ceramic compounds have been produced bv this technique, especially silicon based ceramics, like SiCI2] and SixNa[3 ]. The umque properties of laser synthesis permits the formation not only of equilibrium~co~hpounds, but also the attainment of novel intermediate silicon carbonitrides{4]. However, to our knowledge, very little work has been done on laser processing of oxide-ceramics and on processing and sintering of these nanoscale powders. Nanoscale ~-AI,~O3 powder has already, been successfully synthesized[5], but the process yield was still unsatist'aclory. One reason for the poor productivity can be inferred to the lack of CO2 laser radiation absorption by the used gaseous precursors. The reaction needs the addition of a sensitizer gas to be ignited and sustained, and becomes very sensitive to the correct choice of process parameters. * ENEA Fellow **Permanent address: Atom Physic Institute MG6 Bucharest (Romania) 341
342
E BORSELLA ET AL.
RESULTS AND DISCUSSION
As in the pre~fous investigations[5] the outcoming alumina powders are dark in colour, due to the large carbon content as measured by chemical analysis. The considerable diffraction peak broadening (Fig. 1) suggests that the powder is constituted by nanosized particles ( _< 10 nm), In order to eliminate the carbon excess the powder batches were calcined at temperature ranging between 900 °C and 1400 °C. In table 1 the chemical analysis of the two outermost runs, among the several synthesis experiments, are reported. They are, hereafter referred as low yield conditions (l.y.) and high )field conditions (h.y.). Figure 1: X-ray diffraction patterns of the TAI203 based powders, a) Low yield condition. J~.. ° o b) The diffraction line at 20~57 ° is originated , only by the AI303N, all the other AI303N + = AI30~N ~ ~ peaks are hidden beneath the ~-A1203 ones. b) high yield condition. From the relative intensities a comparable amount of T-AI203 and A12OC phases can be deduced.
4., ~ a)__
.~,*=AI20C, ~
20
30
40
20
50
60
70
O0
Treatments at moderate temperature (900 °C) produce a considerable powder weight loss of ~ 30 wt.% for the l.y. sample, while this effect is reduced dowxl to 9 ~¢t% in the h.y. powder. Comparing the quantitative chemical analysis one observes that the carbon content is drastically reduced by oxidation in both samples, but in the h.y. powder it is replaced by oxygen, which may explain the reduced weight loss. TABLE 1 Chemical anab sis of the ~owders before and after calcining (~xt%). AI N O C Phases WAXS analysis Fig. 1 low yield (as synth. ) 40.7 0.7 41. 17.2 ?-AI)O~+AI-~O~N 10 grams/hour (major) (minor) 900 °C 60 0.4 39. 0.4 as before high yield (as synth.) 47 0.5 28. 24.3 7-AI20~+A19OC >__20 grams/hour 900 °C 55.4 0.4 44 0.2 7-A1203 Combining together the results of the characterisation techniques, two main synthesis paths can be recognised: - low TMA concentration (~VlA/~N20 _<0.2, chamber pressure: 400 torr, T flame: 1570 °C). A reduced powder production yield (10 g/h) is observed together ~ith an almost total cracking of the ethylene molecule. Only a small fraction of the inlet C2H 4 amount (8.5%) is measured by mass spectrometry after the reaction process. The huge amount of free carbon is probably due both to the TMA dissociation and C2H4 cracking. The excess of N20 with respect to TMA in the starting mixture is reasonably responsible for the N dilution in the synthesized compound, leading to the mixed formation of T-AI203, the major phase, and small amounts of AI303N,
NANOSCALESi-C and AI-O-(N,C) CERAMICPOWDERS
343
The present investigation is intended to explore different process conditions for laser assisted photosynthesis of aluminium oxide with the aim of optimizing the reaction yield. Since the final purpose is to obtain ceramic pieces, handling and sintering procedures should be developed. To this end, more assessed SiC powders have been firstly chosen, to transfer, in a second time, the acquired experience to newly produced ceramic powders. Some results on preliminary attempts of sintering laser synthesized silicon carbide are presented.
EXPERIMENTAL
The experimental apparatus for laser synthesis of nanosized powder has already been described in details[4]. The reactant precursors used for alumina based powder synthesis are AI(CH3) 3 trimethylaluminium (TMA) and N20 nitrous oxide. Both precursors do not absorb the radiation emitted by the CO 2 laser and, moreover, the liquid Al organometallic precursor has a very. low vapour pressure at room temperature (~ l0 torr). TMA vapor was injected inside the reaction chamber by bubbling Ar gas through the liquid A1 donor. To reach the activation temperature, ethylene gas (C2H4) was chosen as sensitizer, because of its resonant absorption at the CO2 laser emission wavelength and its higher dissociation energy with respect to the other precursors (C2H4 --~ 7.2 eV, TMA --~ 2.9 eV, N20 --~ 1.67 eV). The induced photochemical reaction is accompanied by a visible flame, whose temperature is measured by an optical pyrometer. Different ratios between TMA and N20 concentrations were obtained 193 heating the liquid Al organometallic precursor, in order to rise its equilibrium vapour pressure. The flow rate of N20 was kept constant between 100 - 150 s.c.c.m, whereas the flow rate of the sensitizer ethylene gas (C2H4) was adjusted according to the Ar flow and to the total pressure of the reaction chamber in order to reach a flame optical temperature between 1400 - 1500 °C. The ratio between C2H4 and the total flow of the reactant gases (dPN20+dp TMA) was ~ 0.3. Several analytical techniques were used to identify both gaseous and solid final products: mass spectrometr3 ~and I.R. absorption spectrophotometry were used as on line diagnostics in order to monitor the gas composition during the photo induced reactions; powders have been characterised by I.R. spectrophotometr3.', chemical analysis, x-ray photoelectron spectroscopy (XPS). x-ray induced Auger electron spectroscopy (XAES). wide angle x-ray diffraction scattering (WAXS). scanning (SEM) and transmission (TEM) electron microscopy. Calcining has been performed in a tubular oven under controlled 02 atmosphere up to 1400 °C. Sintering tests on SiC powders have been performed by using conventional ceramic techniques, i.e. cold pressing and pressureless sintering. The used powder had the following characteristics: specific surface area 53 m2/g. total carbon 30.6 ~I.%, total ox3'gen 0.9 ~t.%. Some excess carbon has been added to counteract oxidation occurring in the processing steps and to optimize the amount of sintering aids (B, C). Boron has been added as high surface amorphous powder in fixed amount (0.5 %). Green pellets have been uniaxially pressed in steel dies at 200 MN/m 2. Green densit3 ~was quite satisfactory for this kind of powders (45-47 % of t.d.). Sintering tests have been performed under flowing helium at temperatures in the range 2050- 2150 °C. The sintered samples have been characterized in terms of sintered densib" and x-ray diffraction.
344
E BORSELLAET AL.
as proved by the very low N (0.7 wt.%) content measured by chemical analysis. - high TMA concentration (~VIA/dPN20 ~ 1, chamber pressure: 250 torr, T flame: 1450 °C) with considerable production yield (> 20 g/h). Mass spectrometry analysis reports equal amounts of C2H4 in the starting and residual gas mixture, attesting that ethylene remains almost unaffected. The x-ray diffraction pattern, which shows the diffraction peaks of the AI2OC compound, and the I.R. spectra, which show an absorption band due to the vibration of the A1G.3 skeleton, confirm the bonded state of carbon in the synthesized powder, mainly due to TMA dissociation. Calcining up to 1400 °C induces in all samples the transition to ~-AI203. The carbon content is reduced in all samples to less than 0.1 vet.%. The l.y. powder is able to retain the nanosized features, as it can be deduced by specific surface measurements, which give a value of about 70 m2/g. This is reasonably due to the presence of free carbon which avoids the physical contact of the powder particles inhibiting in this way the particles coalescence during the ~--~x transition. On the contrary enhanced coalescence phenomena with particles in the pan size are shown by TEM investigation in the h.y. powder containing mostly bonded carbon. As far it concerns the sintering of laser produced SiC powder, by using an optimized amount of carbon content, a sintered density up to 97.5% of the theoretical density has been obtained. A particular and rather unique feature of this sintered SiC material is, that it completely retains the cubic low temperature 13-structure up to 2150 °C as shown by x-ray diffraction (Fig.2), while samples from commercial powders present a I% to m-phase transition at much lower temperature (~1900 °C). This behaviour can be reasonably inferred to the interface energy, required to form the s-phase, which is proportional to the specific surface of the powder particles. p- s i c
Figure 2: X-ray diffraction pattern of SiC sintered from laser powder.
J
~0
.
40
.
.
50
I L
60
.
7O
B0
2O
REFERENCES
1. J. S. Haggerty and R. W. Cannon, "Laser Induced Chemical Processes", Ed. J. I. Steinfeld, Plenunm Press, New York, 1981. 2. W. R. Cannon, S. C. Danforth, J. H. Flint, J. S. Haggerty and R. A. Marra, J. Am. Ceram. Soc. 65, 1982, 324 3. W.R. Cannon, S. C. Danforth, J. S. Haggerty and R. A. 'Marra, J. Am. Ceram. Soc. 65, 1982, 330. 4. E. Borsella, S. Botti, R. Fantoni, R. Alexandrescu, I. Morjan, C. Popescu, T. Dikonimos, R. Giorgi and S. Enzo, J. Mat. Res. ,7, 1992, 2257. 5. E. Borsella, S. Botti, R. Giorgi, S. Martelli, S. Turt~ and G. Zappa, Appl. Phys. Lett., 63, 1345.
Pergamon 0965-9773(95)00066-6
N A N O S C A L E S I - C A N D A L - O - ( N , C ) C E R A M I C P O W D E R S BY L A S E R SYNTHESIS FROM GASEOUS PRECURSORS E. Borsella, S. Botti, M. C. Cesile*, S. Martelli and R. Alexandrescu** ENEA- C.R.E Frascati, Dept. 1NN-FIS P.O.Box 65, 00044 Frascati, Rome (Italy) R. Giorgi, C.A. Nannetti, S. Turt~* and G. Zappa ENEA-CR E Casaccia, Dept. INN-NUMA P OBox 2400, 00100 Rome (Italy) Abstract--Laser induced photosynthesis has been employed to produce nanosized aluminium oxide powders. The dependence of the reaction outcomes oll the process parameters, in particular on reactant gases (trimethylaluminiunl TAL'-I and nitrous oxide) relative concentration, has been investigated in several different conditions. The experimental results" indicate two main reaction paths. Low TA£4 relative concentration leads to the formation of nanocrystalline y-Al203 with traces of Al303N compounds. Increasing the TMA concentration the synthesis of aluminium oxicarbide 6~120C) is obseta,ed Only the first reaction path is able to produce, after calcining at 1400°(; nanosized a-Al20 3 powder. Prelimina~. attempts in sintering laser produced silicon carbide have shown, that, due to the nanometric dimension of the powder particles, it is possible to retain the fl-SiC low tenlperature phase even for sintering tenlperature up to 2150 °C.
INTRODUCTION Laser assisted synthesis of ceramic powders from the gas phase, firstly introduced by Haggerl3' and co-workers in 198111] has already proved one of the most promising route for producing ideal nanosized ceramic powders. Up to now several ceramic compounds have been produced bv this technique, especially silicon based ceramics, like SiCI2] and SixNa[3 ]. The umque properties of laser synthesis permits the formation not only of equilibrium~co~hpounds, but also the attainment of novel intermediate silicon carbonitrides{4]. However, to our knowledge, very little work has been done on laser processing of oxide-ceramics and on processing and sintering of these nanoscale powders. Nanoscale ~-AI,~O3 powder has already, been successfully synthesized[5], but the process yield was still unsatist'aclory. One reason for the poor productivity can be inferred to the lack of CO2 laser radiation absorption by the used gaseous precursors. The reaction needs the addition of a sensitizer gas to be ignited and sustained, and becomes very sensitive to the correct choice of process parameters. * ENEA Fellow **Permanent address: Atom Physic Institute MG6 Bucharest (Romania) 341
342
E BORSELLA ET AL.
RESULTS AND DISCUSSION
As in the pre~fous investigations[5] the outcoming alumina powders are dark in colour, due to the large carbon content as measured by chemical analysis. The considerable diffraction peak broadening (Fig. 1) suggests that the powder is constituted by nanosized particles ( _< 10 nm), In order to eliminate the carbon excess the powder batches were calcined at temperature ranging between 900 °C and 1400 °C. In table 1 the chemical analysis of the two outermost runs, among the several synthesis experiments, are reported. They are, hereafter referred as low yield conditions (l.y.) and high )field conditions (h.y.). Figure 1: X-ray diffraction patterns of the TAI203 based powders, a) Low yield condition. J~.. ° o b) The diffraction line at 20~57 ° is originated , only by the AI303N, all the other AI303N + = AI30~N ~ ~ peaks are hidden beneath the ~-A1203 ones. b) high yield condition. From the relative intensities a comparable amount of T-AI203 and A12OC phases can be deduced.
4., ~ a)__
.~,*=AI20C, ~
20
30
40
20
50
60
70
O0
Treatments at moderate temperature (900 °C) produce a considerable powder weight loss of ~ 30 wt.% for the l.y. sample, while this effect is reduced dowxl to 9 ~¢t% in the h.y. powder. Comparing the quantitative chemical analysis one observes that the carbon content is drastically reduced by oxidation in both samples, but in the h.y. powder it is replaced by oxygen, which may explain the reduced weight loss. TABLE 1 Chemical anab sis of the ~owders before and after calcining (~xt%). AI N O C Phases WAXS analysis Fig. 1 low yield (as synth. ) 40.7 0.7 41. 17.2 ?-AI)O~+AI-~O~N 10 grams/hour (major) (minor) 900 °C 60 0.4 39. 0.4 as before high yield (as synth.) 47 0.5 28. 24.3 7-AI20~+A19OC >__20 grams/hour 900 °C 55.4 0.4 44 0.2 7-A1203 Combining together the results of the characterisation techniques, two main synthesis paths can be recognised: - low TMA concentration (~VlA/~N20 _<0.2, chamber pressure: 400 torr, T flame: 1570 °C). A reduced powder production yield (10 g/h) is observed together ~ith an almost total cracking of the ethylene molecule. Only a small fraction of the inlet C2H 4 amount (8.5%) is measured by mass spectrometry after the reaction process. The huge amount of free carbon is probably due both to the TMA dissociation and C2H4 cracking. The excess of N20 with respect to TMA in the starting mixture is reasonably responsible for the N dilution in the synthesized compound, leading to the mixed formation of T-AI203, the major phase, and small amounts of AI303N,
NANOSCALESi-C and AI-O-(N,C) CERAMICPOWDERS
343
The present investigation is intended to explore different process conditions for laser assisted photosynthesis of aluminium oxide with the aim of optimizing the reaction yield. Since the final purpose is to obtain ceramic pieces, handling and sintering procedures should be developed. To this end, more assessed SiC powders have been firstly chosen, to transfer, in a second time, the acquired experience to newly produced ceramic powders. Some results on preliminary attempts of sintering laser synthesized silicon carbide are presented.
EXPERIMENTAL
The experimental apparatus for laser synthesis of nanosized powder has already been described in details[4]. The reactant precursors used for alumina based powder synthesis are AI(CH3) 3 trimethylaluminium (TMA) and N20 nitrous oxide. Both precursors do not absorb the radiation emitted by the CO 2 laser and, moreover, the liquid Al organometallic precursor has a very. low vapour pressure at room temperature (~ l0 torr). TMA vapor was injected inside the reaction chamber by bubbling Ar gas through the liquid A1 donor. To reach the activation temperature, ethylene gas (C2H4) was chosen as sensitizer, because of its resonant absorption at the CO2 laser emission wavelength and its higher dissociation energy with respect to the other precursors (C2H4 --~ 7.2 eV, TMA --~ 2.9 eV, N20 --~ 1.67 eV). The induced photochemical reaction is accompanied by a visible flame, whose temperature is measured by an optical pyrometer. Different ratios between TMA and N20 concentrations were obtained 193 heating the liquid Al organometallic precursor, in order to rise its equilibrium vapour pressure. The flow rate of N20 was kept constant between 100 - 150 s.c.c.m, whereas the flow rate of the sensitizer ethylene gas (C2H4) was adjusted according to the Ar flow and to the total pressure of the reaction chamber in order to reach a flame optical temperature between 1400 - 1500 °C. The ratio between C2H4 and the total flow of the reactant gases (dPN20+dp TMA) was ~ 0.3. Several analytical techniques were used to identify both gaseous and solid final products: mass spectrometr3 ~and I.R. absorption spectrophotometry were used as on line diagnostics in order to monitor the gas composition during the photo induced reactions; powders have been characterised by I.R. spectrophotometr3.', chemical analysis, x-ray photoelectron spectroscopy (XPS). x-ray induced Auger electron spectroscopy (XAES). wide angle x-ray diffraction scattering (WAXS). scanning (SEM) and transmission (TEM) electron microscopy. Calcining has been performed in a tubular oven under controlled 02 atmosphere up to 1400 °C. Sintering tests on SiC powders have been performed by using conventional ceramic techniques, i.e. cold pressing and pressureless sintering. The used powder had the following characteristics: specific surface area 53 m2/g. total carbon 30.6 ~I.%, total ox3'gen 0.9 ~t.%. Some excess carbon has been added to counteract oxidation occurring in the processing steps and to optimize the amount of sintering aids (B, C). Boron has been added as high surface amorphous powder in fixed amount (0.5 %). Green pellets have been uniaxially pressed in steel dies at 200 MN/m 2. Green densit3 ~was quite satisfactory for this kind of powders (45-47 % of t.d.). Sintering tests have been performed under flowing helium at temperatures in the range 2050- 2150 °C. The sintered samples have been characterized in terms of sintered densib" and x-ray diffraction.
344
E BORSELLAET AL.
as proved by the very low N (0.7 wt.%) content measured by chemical analysis. - high TMA concentration (~VIA/dPN20 ~ 1, chamber pressure: 250 torr, T flame: 1450 °C) with considerable production yield (> 20 g/h). Mass spectrometry analysis reports equal amounts of C2H4 in the starting and residual gas mixture, attesting that ethylene remains almost unaffected. The x-ray diffraction pattern, which shows the diffraction peaks of the AI2OC compound, and the I.R. spectra, which show an absorption band due to the vibration of the A1G.3 skeleton, confirm the bonded state of carbon in the synthesized powder, mainly due to TMA dissociation. Calcining up to 1400 °C induces in all samples the transition to ~-AI203. The carbon content is reduced in all samples to less than 0.1 vet.%. The l.y. powder is able to retain the nanosized features, as it can be deduced by specific surface measurements, which give a value of about 70 m2/g. This is reasonably due to the presence of free carbon which avoids the physical contact of the powder particles inhibiting in this way the particles coalescence during the ~--~x transition. On the contrary enhanced coalescence phenomena with particles in the pan size are shown by TEM investigation in the h.y. powder containing mostly bonded carbon. As far it concerns the sintering of laser produced SiC powder, by using an optimized amount of carbon content, a sintered density up to 97.5% of the theoretical density has been obtained. A particular and rather unique feature of this sintered SiC material is, that it completely retains the cubic low temperature 13-structure up to 2150 °C as shown by x-ray diffraction (Fig.2), while samples from commercial powders present a I% to m-phase transition at much lower temperature (~1900 °C). This behaviour can be reasonably inferred to the interface energy, required to form the s-phase, which is proportional to the specific surface of the powder particles. p- s i c
Figure 2: X-ray diffraction pattern of SiC sintered from laser powder.
J
~0
.
40
.
.
50
I L
60
.
7O
B0
2O
REFERENCES
1. J. S. Haggerty and R. W. Cannon, "Laser Induced Chemical Processes", Ed. J. I. Steinfeld, Plenunm Press, New York, 1981. 2. W. R. Cannon, S. C. Danforth, J. H. Flint, J. S. Haggerty and R. A. Marra, J. Am. Ceram. Soc. 65, 1982, 324 3. W.R. Cannon, S. C. Danforth, J. S. Haggerty and R. A. 'Marra, J. Am. Ceram. Soc. 65, 1982, 330. 4. E. Borsella, S. Botti, R. Fantoni, R. Alexandrescu, I. Morjan, C. Popescu, T. Dikonimos, R. Giorgi and S. Enzo, J. Mat. Res. ,7, 1992, 2257. 5. E. Borsella, S. Botti, R. Giorgi, S. Martelli, S. Turt~ and G. Zappa, Appl. Phys. Lett., 63, 1345.