October 2002 dAS EEs ELSEVIER Materials Letters 56(2002)279-283 www.elsevier.com/locate/matlet Preparation of different morphology of TaCr whiskers Yong-Jun Chen", Jian-Bao Li, Qiang-Min Wei, Hua-Zhang Zhai State Key Laboratory of New Ceramics and Fine Processing Tsinghua University, Beijing 100 084, People s Republic of Received 1 October 2001; accepted 1 December 2001 Abstract Tantalum carbide (TaCx)whiskers were successfully synthesized via material systems of Ta2Os-NaCl-C-Ni, Ta2O5-NaF C-C12H22O11(sucrose)and Ta2Os-KCI-C-Ni systems It was found that whi d via different material systems have different morphologies. The lattice parameter and x(C/Ta ratio) increase w of the reaction temperature. All material systems were proceeded under the condition of about 0. 1 MPa argon atmosph eaction temperatures ranging from 1150 to 1350C. The composition, morphology, crystal structure and the growth were investigated. C 2002 Elsevier Science B V. All rights reserved Keywords: TaCr whiskers; Synthesis; Morphology; Growth mechanism Tantalum carbide(Tac)is an undoubtedly promis- 3-5] and NbC whiskers [6, 7] and others have been ing material among transition metal carbides. It is char- studied in detail to some extent. However, there is little acterized by high hardness (9-10, Mohshardness at work dealing with synthesis of TaC whiskers except 25C), high melting point(approximately 3880 for one reported by Johnsson and Nygren [8] in 1997 excellent electronic conductivity (42.1 un2 cm at In that paper, Johnsson and Nygren discussed mainly 25C)[1], good resistance to chemical attack and ther- about the Ta2Os-NaCI-C-Ni system and confirmed mal shock and high resistance to oxidation [2], which that the growth mechanism is VLs. The discussion makes it very attractive for high-temperature and elec- about morphology was lacking. In this letter, we syn tronic applications. However, in order to meet the thesized Tac whiskers not only via Ta2O5-NaCI currently increasing needs from the microelectron in-(KCD)-C-Ni system as Johnsson did, but also via dustry, it is very imperative to prepare proper morpho- Ta2Os-NaF-C-C12H22O11(sucrose) material sys- logy of Tac whiskers due to their almost integrated tem, which is firstly reported by us. Meanwhile, we crystal structure. For example, TaC whiskers are regar- found that the whiskers mostly grew via VLS mech- ded as good candidate materials for they interconnect in anism, but there are also some whiskers likely via other a very-large-scale-integrated (VLSI circuit. Amongst unknown growth mechanism. In addition, the mor- the transition metal carbide whiskers, TiC whiskers phology of whiskers was discussed Analytical-grade Ta2Os with the purity of.99% and excess activated carbon( to acquire nearly mon- Corresponding author. Fax: +86-10-6278-2753 tantalum) containing 21% wt. volatiles were chosen E-mailaddress:chyj99@mails.tsinghua.edu.cn(Y.-J.Chen). as the main raw material. small amounts of nacl 0167-577X/02/S- see front matter o 2002 Elsevier Science B v. All rights reserved. P:S0167-577X(02)00455-X
Preparation of different morphology of TaCx whiskers Yong-Jun Chen*, Jian-Bao Li, Qiang-Min Wei, Hua-Zhang Zhai Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100 084, People’s Republic of China Received 1 October 2001; accepted 1 December 2001 Abstract Tantalum carbide (TaCx) whiskers were successfully synthesized via material systems of Ta2O5 –NaCl –C –Ni, Ta2O5 –NaF – C–C12H22O11 (sucrose) and Ta2O5 –KCl –C –Ni systems. It was found that whiskers prepared via different material systems have different morphologies. The lattice parameter and x (C/Ta ratio) increase with the rise of the reaction temperature. All material systems were proceeded under the condition of about 0.1 MPa argon atmosphere and reaction temperatures ranging from 1150 to 1350 jC. The composition, morphology, crystal structure and the growth mechanism were investigated. D 2002 Elsevier Science B.V. All rights reserved. Keywords: TaCx whiskers; Synthesis; Morphology; Growth mechanism Tantalum carbide (TaC) is an undoubtedly promising material among transition metal carbides. It is characterized by high hardness (9– 10, Mohshardness at 25 jC), high melting point (approximately 3880 jC), excellent electronic conductivity (42.1 AV cm at 25 jC) [1], good resistance to chemical attack and thermal shock and high resistance to oxidation [2], which makes it very attractive for high-temperature and electronic applications. However, in order to meet the currently increasing needs from the microelectron industry, it is very imperative to prepare proper morphology of TaC whiskers due to their almost integrated crystal structure. For example, TaC whiskers are regarded as good candidate materials for they interconnect in a very-large-scale-integrated (VLSI) circuit. Amongst the transition metal carbide whiskers, TiC whiskers [3– 5] and NbC whiskers [6,7] and others have been studied in detail to some extent. However, there is little work dealing with synthesis of TaC whiskers except for one reported by Johnsson and Nygren [8] in 1997. In that paper, Johnsson and Nygren discussed mainly about the Ta2O5 –NaCl –C –Ni system and confirmed that the growth mechanism is VLS. The discussion about morphology was lacking. In this letter, we synthesized TaC whiskers not only via Ta2O5 –NaCl (KCl) –C –Ni system as Johnsson did, but also via Ta2O5 –NaF –C –C12H22O11 (sucrose) material system, which is firstly reported by us. Meanwhile, we found that the whiskers mostly grew via VLS mechanism, but there are also some whiskers likely via other unknown growth mechanism. In addition, the morphology of whiskers was discussed. Analytical-grade Ta2O5 with the purity of 99.99% and excess activated carbon (to acquire nearly montantalum) containing 21% wt. volatiles were chosen as the main raw material. Small amounts of NaCl 0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S0167-577X(02)00455-X * Corresponding author. Fax: +86-10-6278-2753. E-mail address: chyj99@mails.tsinghua.edu.cn (Y.-J. Chen). www.elsevier.com/locate/matlet October 2002 Materials Letters 56 (2002) 279 – 283
.. Chen et al./ Materials Letters 56(2002)279-283 (99.5%)and KCL (99.5%)and NaF (99.5%)were ac were analyzed by X-ray energy dispersive spectrom- ded as precursor of halogenation and Ni (99.5%)pow- eter(EDS, Link IsIS-300)equipped with the SEM der added as catalyst. Furthermore, NaF and sucrose Fig. I shows whiskers prepared via different mate- can also act as additives to decrease the eutectic point rial system under suitable condition. They formed on of the system. Powders of different mole ratios were the original powder bed and around the wall of the mixed in an agate mortar for 45 min, then placed into graphite crucible. When the raw materials have pre covered graphite crucible and housed in a graphite scriptions of Ta2O /C/KCL (NaCI/Ni=1: 7: 0.5: 0.08 resistance furnace which was heated up to the desired and Ta205/C/NaF/C12H22O11(sucrose)=1: 11: 4: 4, the temperature of 1150-1350C in argon atmosphere at yield of whiskers is the highest about 0. 1 MPa pressure. The temperature increasing The powder XrD patterns of the synthesized whis rate is approximately 1000C/h, and these samples kers via different system are shown in Fig. 2. It can be ere held at the synthesis temperature for 2h. seen that they matched the data reported by the JCPDS The composition, structures and the lattice param-(card no. 35-801) very well and so we can consider eters of obtained whiskers were identified by X-ray that these whiskers are mainly Tac phase diffraction(XRD)method (nickel-filtered Cuk a radi- As shown in Fig. 1, whiskers exhibit mainly four ation, scan step 0.25/min). With scanning electron morphologies when synthesized via different material microscopy(SEM, JSM-6301F), their morphologies system. Fig. la and b are the whiskers synthesized via were investigated, and their chemical compositions Ta2O5-C-KCI-Ni system Some whiskers have fair (b) 1303 Fig 1 SEM of whiskers synthesized via different material system.(a, b)Ta205-C-KCl-Ni system(c)Ta2Os-C-NaCl-Ni system (d) Ta2O5-C-NaF-C12H22On1(sucrose) system
(99.5%) and KCL (99.5%) and NaF (99.5%) were added as precursor of halogenation and Ni (99.5%) powder added as catalyst. Furthermore, NaF and sucrose can also act as additives to decrease the eutectic point of the system. Powders of different mole ratios were mixed in an agate mortar for 45 min, then placed into covered graphite crucible and housed in a graphite resistance furnace which was heated up to the desired temperature of 1150– 1350 jC in argon atmosphere at about 0.1 MPa pressure. The temperature increasing rate is approximately 1000 jC/h, and these samples were held at the synthesis temperature for 2 h. The composition, structures and the lattice parameters of obtained whiskers were identified by X-ray diffraction (XRD) method (nickel-filtered CuK a radiation, scan step 0.25j/min). With scanning electron microscopy (SEM, JSM-6301F), their morphologies were investigated, and their chemical compositions were analyzed by X-ray energy dispersive spectrometer (EDS, Link ISIS-300) equipped with the SEM. Fig. 1 shows whiskers prepared via different material system under suitable condition. They formed on the original powder bed and around the wall of the graphite crucible. When the raw materials have prescriptions of Ta2O5/C/KCL (NaCl)/Ni = 1:7:0.5:0.08 and Ta2O5/C/NaF/C12H22O11 (sucrose) = 1:11:4:4, the yield of whiskers is the highest. The powder XRD patterns of the synthesized whiskers via different system are shown in Fig. 2. It can be seen that they matched the data reported by the JCPDS (card no. 35-801) very well and so we can consider that these whiskers are mainly TaC phase. As shown in Fig. 1, whiskers exhibit mainly four morphologies when synthesized via different material system. Fig. 1a and b are the whiskers synthesized via Ta2O5 –C –KC1 –Ni system. Some whiskers have fairFig. 1. SEM of whiskers synthesized via different material system. (a, b) Ta2O5 –C – KCl – Ni system. (c) Ta2O5 –C – NaCl – Ni system. (d) Ta2O5 –C – NaF –C12H22O11 (sucrose) system. 280 Y.-J. Chen et al. / Materials Letters 56 (2002) 279–283
Y. Chen et al. Materials Letters 56(2002)279-283 indicates that these whiskers grew via VLS process Fig 3 shows the EDS of the whiskers. Fig. 3a shows that Ta element is the principal composition in whisker body and Fig 3b indicates that Ni element is the prin- cipal composition in whisker tip, which means that Ni 1250℃ powder is the catalyst that promotes the VLs pr Actually, carbon is also the predominant element in 350℃ whisker but it can not be detected by the apparatus due to its light mass. From this, we can also draw a con- clusion that the whiskers comprise Tac phase. Their 20 /Degree diameters are about 0.6 um and their lengths are 3 um Many whiskers, however, exhibit a shape of square conical body with zigzag tip. Their length are 1-10 um. Because their tips have no obvious spherical drop- lets, the growth mechanism can not be easily deter- Ic shows the whiskers synthesized via Ta2O5 I-Ni system. They are straight and mostly are pillar-shaped with spherical droplets at their tips. Thi strongly indicates these whiskers formed via VLS pro- cess. The diameters varied in a range of 0.2-0.5 um and the length in a range of 5-10 um. Fig. Id is the whiskers prepared via Ta2O5-NaF-C-C12H22O11 (sucrose)system. They grew in the form of cluster and are fairly straight. The surfaces are very smooth and some tips are capped with spherical droplets, which verified the growth mechanism is VLs. Accord- ing to the whisker diameter, these whiskers can be classified as two types: thin whisker and thick whisker. The thin whiskers have diameters of 0. 1-0.5 um and lengths of 10-50 um. The thick whiskers have diam- eters of 1-9 um. The aspect ratio was in the range of 20-100. Since via different material systems and even the same material systems, the whiskers grown have dif- ferent morphologies, there must be some factors rela- ted to raw material and growth condition which affect whisker morphology. In order to obtain the desired morphology and even control the morphology of whis- Fig. 2. The powder XRD pattems of the synthesized whiskers syn- ker, it is, therefore, necessary to investigate this rela esized via(a) Ta205-C-KCl-Ni system at different temper tionship and growth mechanism, which is the further atures,(b)Ta2O5-C-NaCl-Ni system at 1350C and (c) Ta205- work of our research group C-NaF-C12H22On1(sucrose)system at 1350C, respectively On the other hand, it is well known that metal carbides are always nonstoichiomet ly perfect pillar-shaped bodies with a rectangular pounds, and without exception for tantalum cross-section and spherical droplet at whisker tip, In our work, the whiskers synthesized are TaCx and the and there are striations distributed at equal intervals values of x are smaller than 1. In order to determine the along the lateral surfaces of the whiskers. This also lattice parameter a and the values of x(C/Ta ratio),we
ly perfect pillar-shaped bodies with a rectangular cross-section and spherical droplet at whisker tip, and there are striations distributed at equal intervals along the lateral surfaces of the whiskers. This also indicates that these whiskers grew via VLS process. Fig. 3 shows the EDS of the whiskers. Fig. 3a shows that Ta element is the principal composition in whisker body and Fig. 3b indicates that Ni element is the principal composition in whisker tip, which means that Ni powder is the catalyst that promotes the VLS process. Actually, carbon is also the predominant element in whisker but it can not be detected by the apparatus due to its light mass. From this, we can also draw a conclusion that the whiskers comprise TaC phase. Their diameters are about 0.6 Am and their lengths are 3 Am. Many whiskers, however, exhibit a shape of square conical body with zigzag tip. Their length are 1 –10 Am. Because their tips have no obvious spherical droplets, the growth mechanism can not be easily determined. Fig. 1c shows the whiskers synthesized via Ta2O5 – C–NaC1 –Ni system. They are straight and mostly are pillar-shaped with spherical droplets at their tips. This strongly indicates these whiskers formed via VLS process. The diameters varied in a range of 0.2 –0.5 Am and the length in a range of 5– 10 Am. Fig. 1d is the whiskers prepared via Ta2O5 –NaF–C–C12H22O11 (sucrose) system. They grew in the form of cluster and are fairly straight. The surfaces are very smooth and some tips are capped with spherical droplets, which verified the growth mechanism is VLS. According to the whisker diameter, these whiskers can be classified as two types: thin whisker and thick whisker. The thin whiskers have diameters of 0.1 –0.5 Am and lengths of 10 – 50 Am. The thick whiskers have diameters of 1– 9 Am. The aspect ratio was in the range of 20– 100. Since via different material systems and even the same material systems, the whiskers grown have different morphologies, there must be some factors related to raw material and growth condition which affect whisker morphology. In order to obtain the desired morphology and even control the morphology of whisker, it is, therefore, necessary to investigate this relationship and growth mechanism, which is the further work of our research group. On the other hand, it is well known that transition metal carbides are always nonstoichiometric compounds, and without exception for tantalum carbides. In our work, the whiskers synthesized are TaCx and the values of x are smaller than 1. In order to determine the lattice parameter a and the values of x (C/Ta ratio), we Fig. 2. The powder XRD patterns of the synthesized whiskers synthesized via (a) Ta2O5 –C – KCl – Ni system at different temperatures, (b) Ta2O5 –C – NaCl –Ni system at 1350 jC and (c) Ta2O5 – C – NaF –C12H22O11 (sucrose) system at 1350 jC, respectively. Y.-J. Chen et al. / Materials Letters 56 (2002) 279–283 281
Y Chen et al. Materials Letters 56 (2002)279-283 Fig 3. EDS of the rectangular pillar shape whisker synthesized via Ta2O5-C-KCl-Ni system (a) Ta is the principal element in the whisker(b) performed XRD tests for the product prepared via chiometric tantalum carbide, the reaction temperature Ta2O5-C-KCI-Ni system with very slow scan step should be higher. However, the optimal temperature of 0. 25/min. The lattice parameters of the cubic Tac, for preparation of completely stoichiometric Tac is were firstly determined by a least-squares fit of the unknown because the related work is lacking five observed and calculated spacing. Then we can alculate the value of x according to the relationship between x and a [9]. Table I shows the lattice para Acknowledgements eter a and composition x versus synthesis temper atures. From it, we found both parameters increase This work was supported by the Natural Scienc as the synthesis temperature increases from 1150 to Foundation of China(NSFC) and the Chinese Edu- 1350C. This indicates that with the increase of syn- cation Ministry. The authors would like to acknow thesis temperature, the number of defect in the crystal ledge the technical assistance of Xiao-Jun guo decreases. Therefore, if we want to obtain nearly stoi Table 1 References prepared via Ta205-C-NaCl-Ni system at different [Q.-w. Guo, Tantalum cor edia of Chemical temperatures try, vol. 15, Chemical Industry Press, Beijing, 1997 Synthesis Lattice 2]N. Ahlen, M. Johnsson, M. Nygren, Oxidation behaviour of temperature(°C) parameter(nm) TayTil-C and TaxTil-CNI-y whiskers, Thermochim. Acta 336(1999)11 0.44463 3]Y. Yuan, J. Pan, The effect of vapor phase on the growth of TiC 0.4474 hikers prepared by chemical vapor deposition, J. Cryst. Growth193(1998)585
performed XRD tests for the product prepared via Ta2O5 –C –KC1 –Ni system with very slow scan step of 0.25j/min. The lattice parameters of the cubic TaC, a, were firstly determined by a least-squares fit of the five observed and calculated spacing. Then we can calculate the value of x according to the relationship between x and a [9]. Table 1 shows the lattice parameter a and composition x versus synthesis temperatures. From it, we found both parameters increase as the synthesis temperature increases from 1150 to 1350 jC. This indicates that with the increase of synthesis temperature, the number of defect in the crystal decreases. Therefore, if we want to obtain nearly stoichiometric tantalum carbide, the reaction temperature should be higher. However, the optimal temperature for preparation of completely stoichiometric TaC is unknown because the related work is lacking. Acknowledgements This work was supported by the Natural Science Foundation of China (NSFC) and the Chinese Education Ministry. The authors would like to acknowledge the technical assistance of Xiao-Jun Guo. References [1] Q.-W. Guo, Tantalum compounds, Encyclopedia of Chemical Industry, vol. 15, Chemical Industry Press, Beijing, 1997. [2] N. Ahlen, M. Johnsson, M. Nygren, Oxidation behaviour of TaxTi1 xC and TaxTi1 xCyN1 y whiskers, Thermochim. Acta 336 (1999) 111. [3] Y. Yuan, J. Pan, The effect of vapor phase on the growth of TiC whiskers prepared by chemical vapor deposition, J. Cryst. Growth 193 (1998) 585. Fig. 3. EDS of the rectangular pillar shape whisker synthesized via Ta2O5 –C – KCl – Ni system. (a) Ta is the principal element in the whisker. (b) Ni is the principal element in the spherical droplet. Table 1 The variation of lattice parameters and values of x in TaCx whiskers prepared via Ta2O5 –C –NaCl – Ni system at different synthesis temperatures Synthesis temperature (jC) Lattice parameter (nm) x in TaCx 1150 0.44425 0.907 1250 0.44463 0.935 1350 0.44474 0.943 282 Y.-J. Chen et al. / Materials Letters 56 (2002) 279–283
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[4] T. Hashishin, H. Yaana, T. Kanawa, Y. Kaneko, Synthesis of titanium carbide whiskers from potassium fluorotitanate, J. Ceram. Soc. Jpn. (Int. Ed.) 105 (1997) 537. [5] A. Niklas, J. Mats, N. Mats, Carbothermal synthesis of TiC whiskers via a vapor –liquid – solid growth mechanism, J. Am. Ceram. Soc. 79 (1996) 2803. [6] J.-B. Li, G.-Y. Xu, E.Y. Sun, Y. Huang, P.F. Becher, Synthesis and morphology of niobium monocarbide whiskers, J. Am. Ceram. Soc. 81 (1998) 1689. [7] G.-Y. Xu, J.-B. Li, Y. Huang, Z.-P. Xie, Fabrication and morphology of different color NbCx whiskers, J. Cryst. Growth 200 (1999) 143. [8] M. Johnsson, M. Nygren, Carbothermal synthesis of TaC whiskers via a vapor –liquid – solid growth mechanism, J. Mater. Res. 12 (1997) 2419 – 2427. [9] E.K. Storms, The Refractory Carbides, Academic Press, New York, 1967. Y.-J. Chen et al. / Materials Letters 56 (2002) 279–283 283
