E驅≈3S Journal of the European Ceramic Society 20(2000)2607-2618 On the carbothermal vapour-liquid-solid (Vls) mechanism for Tac, TiC, and Ta_c whisker growth Niklas ahlen. Mats Johnsson a, Ann-Kristin larsson a. Bo Sundman b a Department of Inorganic Chemistry, Stockholm University, S-10691 Stockholm. Swede dEpartment of Material Science and Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden Received 9 December 1999: received in revised form 10 April 2000; accepted 13 April 2000 Abstract The growth of TaC, TiC and TaxTil-C whiskers has been studied in some detail. The whiskers were synthesised via a vapour- liquid-solid (VLS)growth mechanism in the temperature range 1220-1400oC. The starting materials were Ta2Os, TiO2, C, NaCl and a catalyst metal (Ni, Co, Fe, and Cu were tested). The main reaction during synthesis was a carbothermal reduction of Ta O and TiO2, and NaCl was added to form the oxochlorides and chlorides of Ta and Ti that account for the transport to the catalyst metal. The syntheses were made in a protecting Ar atmosphere. From experiments interrupted after different times at the synthesis temperature it is clear that sodium tantalates form as intermediate products, whereas sodium titanates cannot be identified. Only metals that are able to dissolve the elements building up the whiskers work as catalysts. Whisker growth starts either from a catalys droplet in contact with carbon or from an oxide particle in contact with both catalyst metal and carbon. For Tac and TaxTil-xC the only growth direction observed is [100], while TiC may grow either along [100] or along [lll]. 2000 Elsevier Science Ltd. All rights reserved Keywords: Carbides; TaC; TiC; VLS process; Whiskers 1. ntroduction can be favoured by optimising the molar ratios, the particle sizes of the precursor materials and the reaction This article presents a study of the carbothermal temperature. The temperature should be raised as higl vapour-liquid-solid (VLS) mechanism for whisker as possible in order to reduce the residual amount of owth. The whisker materials we have studied are TaC, oxygen. There is a compromise to be achieved, however TiC and Ta Ti-C. The Vls growth mechanism because at a too high synthesis temperature the solid involves all three aggregation states: a vapour phase state reaction between the oxide precursor and carbon transport of one or more of the whisker components to may dominate over the VLs growth process. Further- a melted catalyst droplet where the desired whisker more, at high temperatures the gaseous chloride and grows. The catalyst must be able to dissolve the whisker oxochloride species that are responsible for transport of components and the vls mechanism is only operative Ta and/ or Ti show a tendency to escape from the reac- at temperatures above the melting point of the catalyst tion chamber before reacting at the catalyst. Ar nother (i.e. the eutectic temperature of the multi-component problem at too high synthesis temperatures is that system catalyst-whisker), which therefore constitutes the whiskers may start to sinter together, forming agglom lower temperature limit for whisker synthesis via this erates growth mechanism. The catalyst metal must also be able wo main types growth mechanisms can reactions takia Components. There are two defined in general: (i)all mass transport takes place in perature: the VLs growth of whiskers and the direct acteristic for growth of whiskers via chemical vapour reaction between carbon and the transition metal oxides deposition, CVD), and(i) as in the present case solid resulting in carbide particles. The VLS growth mechanism carbon dissolves into the catalyst at the droplet inter face, while the other whisker component are transported in the vapour phase to the catalysts(characteristic for the carbothermal VLS mechanism).2,3 0955-2219/00/S-see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(00)00121-7
On the carbothermal vapour±liquid±solid (VLS) mechanism for TaC, TiC, and TaxTi1ÿxC whisker growth Niklas AhleÂn a , Mats Johnsson a,*, Ann-Kristin Larsson a , Bo Sundman b a Department of Inorganic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden bDepartment of Material Science and Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden Received 9 December 1999; received in revised form 10 April 2000; accepted 13 April 2000 Abstract The growth of TaC, TiC and TaxTi1ÿxC whiskers has been studied in some detail. The whiskers were synthesised via a vapour± liquid±solid (VLS) growth mechanism in the temperature range 1220±1400C. The starting materials were Ta2O5, TiO2, C, NaCl, and a catalyst metal (Ni, Co, Fe, and Cu were tested). The main reaction during synthesis was a carbothermal reduction of Ta2O5 and TiO2, and NaCl was added to form the oxochlorides and chlorides of Ta and Ti that account for the transport to the catalyst metal. The syntheses were made in a protecting Ar atmosphere. From experiments interrupted after dierent times at the synthesis temperature it is clear that sodium tantalates form as intermediate products, whereas sodium titanates cannot be identi®ed. Only metals that are able to dissolve the elements building up the whiskers work as catalysts. Whisker growth starts either from a catalyst droplet in contact with carbon or from an oxide particle in contact with both catalyst metal and carbon. For TaC and TaxTi1ÿxC the only growth direction observed is [100], while TiC may grow either along [100] or along [111]. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Carbides; TaC; TiC; VLS process; Whiskers 1. Introduction This article presents a study of the carbothermal vapour±liquid±solid (VLS) mechanism for whisker growth. The whisker materials we have studied are TaC, TiC and TaxTi1ÿxC. The VLS growth mechanism involves all three aggregation states: a vapour phase transport of one or more of the whisker components to a melted catalyst droplet where the desired whisker grows. The catalyst must be able to dissolve the whisker components and the VLS mechanism is only operative at temperatures above the melting point of the catalyst (i.e. the eutectic temperature of the multi-component system catalyst-whisker), which therefore constitutes the lower temperature limit for whisker synthesis via this growth mechanism. The catalyst metal must also be able to dissolve the whisker components. There are two competing reactions taking place at the synthesis temperature: the VLS growth of whiskers and the direct reaction between carbon and the transition metal oxides resulting in carbide particles. The VLS growth mechanism can be favoured by optimising the molar ratios, the particle sizes of the precursor materials and the reaction temperature. The temperature should be raised as high as possible in order to reduce the residual amount of oxygen. There is a compromise to be achieved, however, because at a too high synthesis temperature the solidstate reaction between the oxide precursor and carbon may dominate over the VLS growth process. Furthermore, at high temperatures the gaseous chloride and oxochloride species that are responsible for transport of Ta and/or Ti show a tendency to escape from the reaction chamber before reacting at the catalyst. Another problem at too high synthesis temperatures is that whiskers may start to sinter together, forming agglomerates. Two main types of VLS growth mechanisms can be de®ned in general: (i) all mass transport takes place in the vapour phase to the vapour±liquid interface (characteristic for growth of whiskers via chemical vapour deposition, CVD),1 and (ii) as in the present case solid carbon dissolves into the catalyst at the droplet interface, while the other whisker component are transported in the vapour phase to the catalysts (characteristic for the carbothermal VLS mechanism).2,3 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00121-7 Journal of the European Ceramic Society 20 (2000) 2607±2618 * Corresponding author. E-mail address: matsj@inorg.su.se (M. Johnsson)
2608 N. Ahlen et al. Journal of the European Ceramic Society 20(2000)2607-2618 The carbothermal vls mechanism thus involves four Within a certain temperature range, however, this main steps:(1)dissolution of carbon at the interface direct carbothermal reaction is not kinetically favoured between the catalyst droplet and solid carbon, (2)mass in the presence of NaCl and Ni. The reactions that transport of whisker elements in the vapour phase to the actually take place in the carbothermal VLS mechanism vapour-liquid interface (3)chemical reaction at the are complicated to stud vapour-liquid interface, (4) dissolution into and diffu There must be a gas phase transport of Ta and Ti to sion through the liquid alloy phase, and (5) precipita- the catalyst metal. Thermodynamic calculations with tion at the liquid-solid interface he computer program HSC that uses the Gibbs energy The overall chemical reaction for the synthesis of minimisation method suggest the following reactions to TaxTil-C from TiO2 and Ta2O5 is a straightforward take place at the synthesis temperature carbothermal reduction of the oxides 0.5xTa2O5(s)+(1-x)TiO2(s)+(3+0.5x)C(s) Raw materials used in the synthesis ->TaTil-.C(S)+(2+0.5x)co(g) Substance Purity Manufacturer Particle Comment (wt.%) 99.9 Crac 99.9 Aldrich Anatase Degussa ng Carbon black Fw200) Ta, c agglomerates 21 wt. volatiles) Nacl 99.5 Akzo 99.9 Cerad <325 mesh Co 99.9 Cerad <325 mesh 9 Aldrich <325 mesh 99.5 Aldrich <150 mesh P(red) 99 Kebo Table 2 5 The molar ratios in the starting mixtures that were found to give the Fig. 1. A simplified sequence for the carbothermal VLs growth highest whisker yield for TaC, TiC and TaosTio. Ca mechanism. The reactions at the catalyst droplet can be described as TaC TIC allows:(I) the catalyst particle(Ni) in contact with carbon start to dissolve the whisker constituents: C, Ta, and Ti. (2)The droplet is TarOs 1(60.00g) supersaturated and a Ta,Ti-C whisker is nucleated. (3)The surface TiOz 1(20.00g) tension balance at the interface between the whisker and the catalyst 13(14.24g) droplet determines the whisker diameter. (4)As the reactants are con- Nacl 0.503.97g) tinuously being dissolved into the catalyst, the whisker grows, and the Ni 0.05(0.40g) 0.075(1.13g) arbon particles are consumed. (5)When all carbon in contact with the catalyst droplet has been consumed, the whisker growth terminates. a Weighed- in amounts are given within parentheses. Gas Out Reactor g Element Gas In Fig. 2. An SEM micrograph of Tao.s Tio sC whiskers. The yield is estimated to be 80 vol % The whisker diameter is 0.2-0.6 um and the length 10-30 Fig 3. The furnace set-up with the gas flow around the reacto
The carbothermal VLS mechanism thus involves four main steps: (1) dissolution of carbon at the interface between the catalyst droplet and solid carbon, (2) mass transport of whisker elements in the vapour phase to the vapour±liquid interface (3) chemical reaction at the vapour±liquid interface, (4) dissolution into and diusion through the liquid alloy phase, and (5) precipitation at the liquid±solid interface. The overall chemical reaction for the synthesis of TaxTi1ÿxC from TiO2 and Ta2O5 is a straightforward carbothermal reduction of the oxides. 0:5xTa2O5 s 1 ÿ x TiO2 s 3 0:5x C s ! TaxTi1ÿxC s 2 0:5x CO g 1 Within a certain temperature range, however, this direct carbothermal reaction is not kinetically favoured in the presence of NaCl and Ni. The reactions that actually take place in the carbothermal VLS mechanism are complicated to study. There must be a gas phase transport of Ta and Ti to the catalyst metal. Thermodynamic calculations with the computer program HSC4 that uses the Gibbs energy minimisation method suggest the following reactions to take place at the synthesis temperature: Fig. 1. A simpli®ed sequence for the carbothermal VLS growth mechanism. The reactions at the catalyst droplet can be described as follows: (1) the catalyst particle (Ni) in contact with carbon start to dissolve the whisker constituents: C, Ta, and Ti. (2) The droplet is supersaturated and a TaxTi1ÿxC whisker is nucleated. (3) The surface tension balance at the interface between the whisker and the catalyst droplet determines the whisker diameter. (4) As the reactants are continuously being dissolved into the catalyst, the whisker grows, and the carbon particles are consumed. (5) When all carbon in contact with the catalyst droplet has been consumed, the whisker growth terminates. Fig. 2. An SEM micrograph of Ta0:5Ti0:5C whiskers. The yield is estimated to be 80 vol.%. The whisker diameter is 0.2±0.6 mm and the length 10±30 mm. Table 1 Raw materials used in the synthesis Substance Purity (wt.%) Manufacturer Particle size Comment Ta2O5 99.9 Cerac ÿ325 mesh Ceramic grade TiO2 99.9 Aldrich 0.2±0.6 mm Anatase C ± Degussa (FW200) 13 nm forming ¯occulent agglomerates Carbon black (containing 21 wt.% volatiles) NaCl 99.5 Akzo <5 mm Ni 99.9 Cerac <325 mesh Co 99.9 Cerac <325 mesh Fe 97 Aldrich <325 mesh Cu 99.5 Aldrich <150 mesh P(red) 99 Kebo Table 2 The molar ratios in the starting mixtures that were found to give the highest whisker yield for TaC, TiC and Ta0:5Ti0:5Ca TaC TiC Ta0:5Ti0:5C Ta2O5 1 (60.00 g) ± 1 (30.00 g) TiO2 ± 1 (20.00 g) 2 (10.85 g) C 7.2 (14.86 g) 3.3 (12.56 g) 13 (14.24 g) NaCl 0.5 (3.97 g) 0.5 (7.31 g) 1.5 (5.95 g) Ni 0.05 (0.40 g) 0.075 (1.13 g) 0.15 (0.60 g) a Weighed-in amounts are given within parentheses. Fig. 3. The furnace set-up with the gas ¯ow around the reactor. 2608 N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618
N. Ahlen et al. / Journal of the European Ceramic Society 20(2000)2607-2618 2Ta2O5(s)+6CI(g)+3C(s)-2TaOCl3(g)+3CO(g) (2) TaOCl3 (g)+C(s)+ Ni-Ti-C()- Ni-Ta-Ti-CO +Co(g)+3cl(g) TiO2(s)+3Cl(g)+2C(s)- TiCl3 (g)+ 2C0(g) (3) Ni-Ta-Ti-C()- TaxTil-C(s)+NiO) Other gas-phase species that also form by similar A simplified sequence for the carbothermal VLS eactions but in lower concentrations are TaO, CI(g), growth mechanism is outlined in Fig. I TaCls(g), and TiCl(g), and these may also to some For some years, we have been studying growth of extent be responsible for the transport of Ta and Ti different transition-metal carbide and carbonitride The melting temperature of the catalyst metal must whiskers 2.3, 5-7 We have achieved control of the synth match the temperature range where TiCl3(g) and esis so as to gain a high whisker yield(see Fig. 2), but TaOCl3(g)form in high concentrations. If a catalyst we have not obtained a detailed understanding of the metal with a too high melting point is chosen, then growth mechanism. In the present article we have per direct carbothermal reaction, resulting in particles, may formed a detailed study of the growth mechanisms of dominate over the whisker formation by the VLS Tax Til-xC whiskers with 0<x<I designed to explore mechanism. The following reactions are expected to some essential features of the VLs mechanism occur at the catalyst(Ni-C, etc, denote that C, Ta, and Ti are dissolved in the Ni catalyst) 2. Experimental C(s)+Ni()→Ni-C() 2.1. Synthesis The starting materials used in the preparation of TiCl3(g)+Ni-CO->Ni-Ti +3cl(g) (5) Ta Ti-C whiskers are listed in Table 1. The carbon Table 3 Chemical analysis data and measured weight loss for the series with interrupted TaC synthesis experiments" eight loss(wt % O(wt % C(wt % Na(wt % Ni(wt % N(wt % Ta+Cl (wt % nalys sis method LECO LECO ng mixture 6.8 23.5 30.6 00000 33 33.0 6.3 33.3 6.4 Table 4 Phases present ding to powder X-ray diffraction data(XRD) after different reaction times for the series of interrupted Tac-whisker syntheses Starting mixture 15 90 20 240 a Na]Ta,O 5xxxx 4xxxx not the experiments were interrupted after different holding times, from 0 to 240 min, at the synthesis temperature 1220oC.NaCl,Ni,and Ccould
2Ta2O5 s 6Cl g 3C s ! 2TaOCl3 g 3CO g 2 TiO2 s 3Cl g 2C s ! TiCl3 g 2CO g 3 Other gas-phase species that also form by similar reactions but in lower concentrations are TaO2Cl(g), TaCl5(g), and TiCl4(g), and these may also to some extent be responsible for the transport of Ta and Ti. The melting temperature of the catalyst metal must match the temperature range where TiCl3(g) and TaOCl3(g) form in high concentrations. If a catalyst metal with a too high melting point is chosen, then a direct carbothermal reaction, resulting in particles, may dominate over the whisker formation by the VLS mechanism. The following reactions are expected to occur at the catalyst (Ni±C, etc., denote that C, Ta, and Ti are dissolved in the Ni catalyst). C s Ni l ! NiÿC l 4 TiCl3 g NiÿC l ! NiÿTiÿC I 3Cl g 5 TaOCl3 g C s NiÿTiÿC l ! NiÿTaÿTiÿC l CO g 3Cl g 6 NiÿTaÿTiÿC l ! TaxTi1ÿxC s Ni l 7 A simpli®ed sequence for the carbothermal VLS growth mechanism is outlined in Fig. 1. For some years, we have been studying growth of dierent transition-metal carbide and carbonitride whiskers.2,3,5ÿ7 We have achieved control of the synthesis so as to gain a high whisker yield (see Fig. 2), but we have not obtained a detailed understanding of the growth mechanism. In the present article we have performed a detailed study of the growth mechanisms of TaxTi1ÿxC whiskers with 04x41 designed to explore some essential features of the VLS mechanism. 2. Experimental 2.1. Synthesis The starting materials used in the preparation of TaxTi1ÿxC whiskers are listed in Table 1. The carbon Table 3 Chemical analysis data and measured weight loss for the series with interrupted TaC synthesis experimentsa Weight loss (wt.%) O (wt.%) C (wt.%) Na (wt.%) Ni (wt.%) N (wt.%) Ta+Cl (wt.%) Analysis method LECO LECO AAS AAS LECO Balance Starting mixture 0 16.2 15.0 2.4 0.8 0.2 65.1 0 6.8 13.7 14.3 1.9 0.8 0.2 69.1 5 8.6 13.2 14.0 1.6 0.9 0.1 70.2 15 12.2 12.3 13.4 1.3 0.9 0.1 72.0 25 16.7 10.8 12.6 0.8 0.7 0.1 74.9 45 21.2 8.4 11.3 0.3 1.3 0.2 78.5 60 23.5 7.3 10.7 0.2 1.1 0.1 80.6 90 25.8 5.5 9.7 0.2 1.0 0.2 83.5 120 28.1 3.7 8.6 0.2 1.1 0.3 86.2 150 30.6 2.0 8.6 0.2 1.1 0.2 88.0 180 33.0 0.4 6.5 0.2 1.1 0.2 91.6 210 33.0 0.09 6.3 0.02 1.2 0.2 92.2 240 33.3 0.1 6.4 0.01 1.2 0.2 92.1 a The experiments were quenched after dierent holding times, from 0 to 240 min, at the synthesis temperature 1220C. Ta and Cl were not analysed. No information on the number of vacancies is available. Table 4 Phases present according to powder X-ray diraction data (XRD) after dierent reaction times for the series of interrupted TaC-whisker synthesesa Starting mixture 0 5 15 25 45 60 90 120 150 180 210 240 Ta2O5 ± ± Na2Ta4O11 ± ±± ±±± NaTaO3 ± ±±± ± ± TaC ± a The experiments were interrupted after dierent holding times, from 0 to 240 min, at the synthesis temperature 1220C. NaCl, Ni, and C could not be detected. N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618 2609
10 N. Ahlen et al. Journal of the European Ceramic Society 20 (2000)2607-2618 source contained 21 wt. volatile components that nace(Thermal Technology) at a rate of 1000C/h to were burned off during the heating-up period. The choice the synthesis temperature in an Ar atmosphere. All of carbon source has a strong influence on the whisker experiments were conducted at atmospheric pressure ield. It has been shown in earlier studies that carbon and at temperatures in the range 1220-1400C Previous black with a volatile part retains its fluffy consistency after studies have revealed the optimum temperature for tac heat treatment, in contrast to carbon powder without and Tax Til- C whisker growth to be 1220-1250C3, 5 volatiles, and we believe that this is essential in improv- and 1400C for TiC whiskers. The whis sers o ng the ease with which the volatile Ta and Ti species were 0. 2-0. 6 um in diameter and 5-30 um in length. The reach the catalyst.2,3 The weighed-in TiO2/Ta2Os molar ratio can be varied in order to yield whiskers with dif- ferent x-values. The molar ratios of the different pre- Table 5 cursor materials used in this study for preparation of TaC, Chemical analysis data for the series of interrupted Tic synthesis TiC and Tao. Tios C are listed in Table 2 experiments The starting materials were mixed to homogeneity in Total o c Na Ni a blender. Portions of about 10-20 g were placed in weight (wt %)(wt %)(wt %)(wt %)(wt %)(wt % graphite crucible covered with a lid with a number of holes to allow controlled gas exchange between the (wt%) reactor chamber and the surrounding atmosphere(see LECO LECO AAs AAS Fig. 3). The mixture was then heated in a graphite fur 597.52.9 12.519.324.77.63.2 333.1 23418.32506.23.69938.1 34.216.72534.14.06944. 43.114.225.82.24.8 48.112.226.00.45.3 9725.30.085.30.0158.9 52.58.224.60.055.5 G0.4 a The experiments were quenched after different holding times, from 0 to 960 min, at the synthesis temperature 1250 C. The compo- sition of the starting mixture is not analysed it is calculated from the 总02 weight in amounts of the different starting materials. Chemical analysis data for the series of interrupted TiC synthesis Total o C Ni weight (wt %)(wt %)(wt %)(wt %)(wt %)(wt % 08 (wt.%) :2 LECO LECO AAS AAS Balance 0.925.97.2 11.631.3 谷04 23.717.723.86.53.510.937.5 47.31102501.7 51.39.224.80.4 53.27624.30.085.6 968 58.8 61.2 00250 3.86.623.80.026.2 1062.5 55.1 5.3 .700164 1.164.0 time(min) 14.623.60.016.0 240 3 0.0096.2 0.01658 Fig 4.(a) Observed weight change as a fraction of the expected vs. 960 57.5 2.1 23. 1 0.006 6.2 0 reaction time for the series of interrupted Tac whisker synthesis experiments at 1220.C;(b)observed weight change of the different The experiments were quenched after different holding times, Na,Cl, and Ni disappear from the reactor during synthesi that all O, sition of the starting mixture is not analysed it is calculated from o- elements as a fraction of the expected vs reaction time. Based on the from 0 to 960 min, at the synthesis temperature 1400.C. The com assumption that the starting mixture transforms to Tac and weight in amounts of the different starting materials
source contained 21 wt.% volatile components that were burned o during the heating-up period. The choice of carbon source has a strong in¯uence on the whisker yield. It has been shown in earlier studies that carbon black with a volatile part retains its ¯uy consistency after heat treatment, in contrast to carbon powder without volatiles, and we believe that this is essential in improving the ease with which the volatile Ta and Ti species reach the catalyst.2,3 The weighed-in TiO2=Ta2O5 molar ratio can be varied in order to yield whiskers with different x-values. The molar ratios of the dierent precursor materials used in this study for preparation of TaC, TiC and Ta0:5Ti0:5C are listed in Table 2. The starting materials were mixed to homogeneity in a blender. Portions of about 10±20 g were placed in a graphite crucible covered with a lid with a number of holes to allow controlled gas exchange between the reactor chamber and the surrounding atmosphere (see Fig. 3). The mixture was then heated in a graphite furnace (Thermal Technology) at a rate of 1000C/h to the synthesis temperature in an Ar atmosphere. All experiments were conducted at atmospheric pressure and at temperatures in the range 1220±1400C. Previous studies have revealed the optimum temperature for TaC and TaxTi1ÿxC whisker growth to be 1220±1250C3,5 and 1400C for TiC whiskers.2 The whiskers obtained were 0.2±0.6 mm in diameter and 5±30 mm in length. The Fig. 4. (a) Observed weight change as a fraction of the expected vs. reaction time for the series of interrupted TaC whisker synthesis experiments at 1220C; (b) observed weight change of the dierent elements as a fraction of the expected vs. reaction time. Based on the assumption that the starting mixture transforms to TaC and that all O, Na, Cl, and Ni disappear from the reactor during synthesis. Table 5 Chemical analysis data for the series of interrupted TiC synthesis experimentsa Total weight loss (wt.%) O (wt.%) C (wt.%) Na (wt.%) Ni (wt.%) Cl (wt.%) Ti (wt.%) Analysis method LECO LECO AAS AAS Balance Starting mixture 20.9 25.9 7.5 2.9 11.6 31.3 0 12.5 19.3 24.7 7.6 3.2 12.3 33.1 30 23.4 18.3 25.0 6.2 3.6 9.9 38.1 60 34.2 16.7 25.3 4.1 4.0 6.9 44.3 90 43.1 14.2 25.8 2.2 4.8 3.8 50.8 120 48.1 12.2 26.0 0.4 5.3 2.2 55.0 180 50.8 9.7 25.3 0.08 5.3 0.01 58.9 240 52.5 8.2 24.6 0.05 5.5 0 60.7 960 55.7 5.0 22.8 0 6.5 0 64.4 a The experiments were quenched after dierent holding times, from 0 to 960 min, at the synthesis temperature 1250C. The composition of the starting mixture is not analysed it is calculated from the weight in amounts of the dierent starting materials. Table 6 Chemical analysis data for the series of interrupted TiC synthesis experimentsa Total weight loss (wt.%) O (wt.%) C (wt.%) Na (wt.%) Ni (wt.%) Cl (wt.%) Ti (wt.%) Analysis method LECO LECO AAS AAS Balance Starting mixture ± 20.9 25.9 7.5 2.9 11.6 31.3 0 23.7 17.7 23.8 6.5 3.5 10.9 37.5 10 39.5 13.8 24.5 4.3 4.5 8.7 47.0 20 47.3 11.0 25.0 1.7 5.2 3.9 55.0 30 51.3 9.2 24.8 0.4 5.5 1.6 58.8 40 53.2 7.6 24.3 0.08 5.6 1.8 61.2 60 53.8 6.6 23.8 0.02 6.2 1.0 62.5 120 55.1 5.3 23.7 0.01 6.4 1.1 64.0 180 56.1 4.6 23.6 0.01 6.0 0.02 65.2 240 56.3 3.7 23.1 0.009 6.2 0.01 65.8 960 57.5 2.1 23.1 0.006 6.2 0 68.3 a The experiments were quenched after dierent holding times, from 0 to 960 min, at the synthesis temperature 1400C. The composition of the starting mixture is not analysed it is calculated from the weight in amounts of the dierent starting materials. 2610 N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618
N. Ahlen et al. Journal of the European Ceramic Society 20(2000)2607-2618 reactions were completed after 4 h at the respective composition and the phase composition changes during optimum synthesis temperature. Three series with synthesis; one series with Tac whisker synthesis at experiments interrupted after different times from o to 960 1220C and two series with Tic whisker synthesis at min were performed in order to study how the chemical 1250 and 1400 C, respectively. Only a few whiskers terminated with a catalyst dro- (a)1.0 plet after completed reaction, indicating that there is a rapid redistribution of Ni in the reactor after a whisker has grown to full length. The search for droplets in experiments interrupted before completed whisker growth was a time-consuming task. It was observed that 1400°C small additions of red phosphorus to the starting mix- --1250C ture increased the occurrence of nickel droplets. After 0.4 solidification of the droplets small nickel phosphide particles were found to have precipitated on the droplet surface. No effect on the whisker yield was observed however. Therefore, experiments were done where red 0200060800010 phosphorous was added to the starting mixture in order to increase the possibility to find whiskers terminated lyst dre 22. characterisation The products were characterised by their X-ray powder diffraction patterns(XRD), obtained with a Guinier 806 Hagg focusing camera. CuKaI radiation (=1.54060 A) sed. and fine ed silicon a=5. 430880(35)AI were evaluated in an automatic film scanner Orded films was added as an internal standard. The reco The morphology and composition of the whiskers and catalyst droplets were investigated using a high-resolu tion scanning electron microscope(SEM, JEOL 880) equipped with an energy-dispersive spectrometer 200 (LINK ISIS), which allows detection of boron and heavier elements. A high-resolution transmission elec tron microscope (TEM, JEOL 3010)and a TEM JEOL 2000FX) equipped with EDS (LINK AN10000) were used for the electron diffraction work to determine whisker growth directions and for studies of the catalyst droplets 806/ 0.4 Table 7 0.2 Phases present, according to powder X-ray diffraction data (XRD) after different reaction times for the 1250C series of interrupted TiC whisker synthesis experiments 0 Starting03060120180240 mixture time(min) Fig. 5.(a)Observed weight change as a fraction of the expected vs TiO,(rutile) eaction time for the two temperature series(1250 and 1400oC)of TiO,(anatase) nterrupted TiC whisker synthesis experiments; (b)observed weight ange of the different elements as a fraction of the expected vs reac. ion time for TiC whisker synthesis at 1250C. Based on the assump- ion that the starting mixture transforms to TiC and that all O, Na, CL. and Ni disappear from the reactor during synthesis; (c)same as(b)but The experiments were interrupted after different holding times for TiC whisker synthesis at 1400oC. from o to 240 min. Ni and c could not be detected from Xrd data
reactions were completed after 4 h at the respective optimum synthesis temperature. Three series with experiments interrupted after dierent times from 0 to 960 min were performed in order to study how the chemical composition and the phase composition changes during synthesis; one series with TaC whisker synthesis at 1220C and two series with TiC whisker synthesis at 1250 and 1400C, respectively. Only a few whiskers terminated with a catalyst droplet after completed reaction, indicating that there is a rapid redistribution of Ni in the reactor after a whisker has grown to full length. The search for droplets in experiments interrupted before completed whisker growth was a time-consuming task. It was observed that small additions of red phosphorus to the starting mixture increased the occurrence of nickel droplets. After solidi®cation of the droplets small nickel phosphide particles were found to have precipitated on the droplet surface. No eect on the whisker yield was observed, however. Therefore, experiments were done where red phosphorous was added to the starting mixture in order to increase the possibility to ®nd whiskers terminated with a catalyst droplet. 2.2. Characterisation The products were characterised by their X-ray powder diraction patterns (XRD), obtained with a GuinierHaÈgg focusing camera. CuKa1 radiation (l=1.54060 AÊ ) was used, and ®nely powdered silicon [a=5.430880(35) AÊ ] was added as an internal standard. The recorded ®lms were evaluated in an automatic ®lm scanner.8 The morphology and composition of the whiskers and catalyst droplets were investigated using a high-resolution scanning electron microscope (SEM, JEOL 880) equipped with an energy-dispersive spectrometer (LINK ISIS), which allows detection of boron and heavier elements. A high-resolution transmission electron microscope (TEM, JEOL 3010) and a TEM (JEOL 2000FX) equipped with EDS (LINK AN10000) were used for the electron diraction work to determine whisker growth directions and for studies of the catalyst droplets. Fig. 5. (a) Observed weight change as a fraction of the expected vs. reaction time for the two temperature series (1250 and 1400C) of interrupted TiC whisker synthesis experiments; (b) observed weight change of the dierent elements as a fraction of the expected vs. reaction time for TiC whisker synthesis at 1250C. Based on the assumption that the starting mixture transforms to TiC and that all O, Na, Cl, and Ni disappear from the reactor during synthesis; (c) same as (b) but for TiC whisker synthesis at 1400C. Table 7 Phases present, according to powder X-ray diraction data (XRD), after dierent reaction times for the 1250C series of interrupted TiC whisker synthesis experimentsa Starting mixture 0 30 60 120 180 240 TiC ± TiO2 (rutile) ± ± TiO2 (anatase) ± Ti2O3 ± ±±± Ti3O5 ± ±± ± ± NaCl ±±± a The experiments were interrupted after dierent holding times from 0 to 240 min. Ni and C could not be detected from XRD data. N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618 2611