RMCs can be produced in one of several ways. WCl6 and MoCl5 are produced through the chlorination of crude tungsten or molybdenum metal powder in a stream of chlorine above 600°C. A surplus of chlorine in the process is necessary to prevent the generation of the lower chlorides. With their low boiling points of 346°C (WCl6) and 268°C (MoCl5), W(VI)- and Mo(V)- chlorides are gaseous under these conditions. The gaseous chlorides are cooled and crystallized as violet (WCl6) or dark green to black (MoCl5) crystals. The chlorination takes place under a nitrogen atmosphere, due to the air sensitivity of these chlorides (see Figure 1).
2 Mo + 5 Cl2
FeTa/Nb + 7 NaFeCl4 + NaCl
The volatile chlorides evaporate out of the melt, and the tetrachlorides of silicon, tin and titanium (which have boiling points between 57 and 136°C) are separated by cooling from TaCl5, NbCl5 and WOCl4 (which have boiling points between 228 and 248°C, respectively). The chlorides of Nb and Ta are purified via a distillation process. NbCl5 (yellow crystals) and TaCl5 (colorless crystals) from this process are very pure. The total amount of metallic impurities is less than 50 µg/g.
Applications A simple literature or patent database search clearly demonstrates the widespread use of RMCs in a variety of industries, including semiconductors, catalysts, energy and nanotechnology.
Semiconductors Due to the shrinking dimensions of integrated chips (ICs) and the increasing aspect ratios in semiconductor devices, the role of alternative deposition techniques, such as chemical vapor deposition (CVD) and atomic layer deposition (ALD), is expected to increase. This is because the current deposition method, physical vapor deposition (PVD), has certain limitations that can be overcome by CVD and ALD. However, CVD and ALD require a new set of deposition precursors, and this is where the RMCs come into play.
Two major groups of metalorganic precursors-metal alkoxides and metal amides-currently being tested by semiconductor companies are synthesized from RMCs through reactions with alcohols and amines, respectively. The products include compounds such as Ta-ethoxide, PDMAT (pentakis dimethylamino tantalum) and TBTDET (t butylimino tris diethylamino tantalum), which can be used to generate Ta2O5 high-k films in memory devices (using Ta-ethoxide)1 and TaN diffusion barriers.
Although much of the industry has focused on chloride-derived products for precursor development, it is worth mentioning that the chlorides themselves can be used directly as precursors in their high-purity form. As an example, U.S. patent no. 6,900,129 describes a CVD method for depositing high-quality conformal Ta and tantalum nitride (TaNx) films from TaCl5 and other inorganic tantalum pentahalide (TaX5) precursors at 300-500°C.2
The use of Nb or Mo chlorides has also been described in the production of lithium secondary batteries. In one example, a high-voltage lithium secondary battery's performance is enhanced by selecting alternative materials for the positive electrode.6 Such alternative materials include lithium-manganese oxides, in which manganese is partially substituted by, among others, Nb or Mo, for which NbCl5 and MoCl5, respectively, were used in the preparation.
Pulp and Paper. TaCl5 has been used as a precursor material in the production of the oxide electrode Ti/Ta2O5-IrO2 used in the oxidation of sulfides to treat waste products in the pulp and paper industry.10
Ceramic Matrix Composites. Precursor-polymers that were produced from refractory metal halides (such as TaCl5) and organo-transition metal complexes have been used to manufacture a ceramic matrix composite (such as TaC) by means of polymer infiltration pyrolysis. The technique leads to the decomposition of the polymer, followed by the formation of refractory metal carbides or borides; the resulting material offers extremely high melting points (3880°C for TaC). Such composites could be used for multistage nozzles in rocket motors or for high-temperature coatings. This method delivers carbides with superior properties compared to those manufactured by the common chemical vapor infiltration method.11
Optical. Niobium is commonly used as a refractive index adjuster in optical applications. Accordingly, niobium oxide optical thin films can be generated to obtain a specific refractive index by immersing a substrate in a NbCl5-containing mixture.12 Metal alkoxides, similar to those used as CVD/ALD precursors in semiconductor applications, can also be used for optical applications, such as in optical thin film transparent layers used for electrochromic devices.13 Additionally, the thermal management of buildings requires intelligent glass coatings. For example, thermochromic layers on glass can be prepared from tungsten doped vanadium oxide and can be deposited at atmospheric pressure using WCl6 as a precursor for WO3.14
Biomedical. According to a Japanese study, the bone-building ability (osteoconductivity) of transplants was enhanced through a chemical treatment with hydrogen peroxide solution containing tantalum chloride.14
RMC powders can be synthesized in various grain sizes, allowing the optimization of the desired material properties. Depending on the application, different grain sizes offer different advantages. Larger grain sizes (1-10 mm) improve flow properties and ease the material handling and its dosage, while also reducing moisture sensitivity. On the other hand, smaller grain sizes (<0.25 mm) increase the material's reactivity and solubility.
Editor's note: References can be found with this article online at www.ceramicindustry.com.
2. Tokyo Electron Ltd., U.S. Patent No. 6,900,129 (2005).
3. AlliedSignal Inc., U.S. Patent No. 6,268,540 (2001).
4. BASF Aktiengesellschaft, U.S. Patent No. 6,524,996 (2003).
5. Eltron Research Inc., U.S. Patent No. 6,899,744 (2005).
6. Sanyo Electric Co., Ltd., U.S. Patent No. 6,337,158 (2002).
7. Wang, Yi; Cui, Zoulin; and Zhang, Zhikun, "Synthesis and Phase Structure of Tantalum Nanoparticles," Materials Letters, Vol. 58, 2004, pp. 3017-3020.
8. Zhu, Hongmin and Sadoway, Donald R., "Synthesis of Nanoscale Particles of Ta and Nb3Al by Homogeneous Reduction in Liquid Ammonia," J. Mater. Res., Vol. 16, No. 9, 2001, pp. 2544-2549.
9. Shi, Liang; Gu, Yunle; Chen, Luyang; Yang, Zeheng; Ma, Jianhua; and Qian, Yitai, "Synthesis and Oxidation Behavior of Nanocrystalline Niobium Carbide," Solid States Ionics, Vo. 176, 2005, pp. 841-843.
10. Miller, Brad and Chen, Aicheng, "Oscillatory Instabilities During the Electrochemical Oxidation of Sulfide on Ti/Ta2O5-IrO2 Electrodes," presented at the 207th Electrochemical Society Meeting, Quebec City, 2005.
11. Southwest Research Institute, U.S. Patent No. 6,815,006 (2004).
12. Denglas Technologies, L.L.C, U.S. Patent No. 6,811,901 (2004).
13. Sustainable Technologies Australia Limited, U.S. Patent No. 6,355,821 (2002).
14. Manning, T.D. and Parkin, I.P. "Atmospheric pressure chemical vapour deposition of tungsten doped vanadium(IV) oxide from VOCl3 and WCl6," J. Mater. Chem., Vol. 14, 2004, pp. 2554-2559.
15. Kim, Taeseong; Suzuki, Masahiko; Ohtsuki, Chikara; Masuda, Kimihiko; Tamai, Hiroshi; Watanabe, Eiichirou; Osaka, Akiyoshi; Moriya, Hideshige, "Enhancement of Bone Growth in Titanium Fiber Mesh by Surface Modification with Hydrogen Peroxide Solution Containing Tantalum Chloride," Journal of Biomedical Materials Research Part B: Applied Biomaterials, Wiley Periodicals, Inc., Vol. 64B, No. 1, 2003, pp. 19-26.