(198g) Preparation of Tantalum Nanopowders by Hydrogen Reduction of Tacl5 Vapor

INTRODUCTION Nanosized tantalum powders find applications in bio-mechanical micro-devices, high-grade capacitors, explosively formed penetrators and so on. An aerosol synthesis of tantalum nanopowders by hydrogen reduction of TaCl5 vapor was patented earlier by Brutvan et al. [1]. Then, a pilot-scale production of tantalum nanopowders by the same method was reported by Winter [2]; very limited data were provided, however. Later, Tsukihashi and Iijima [3] reported the effects of reaction temperature, hydrogen partial pressure and evaporation temperature of TaCl5 on the distribution of particle size, morphology and reaction degree. In their experiments, the evaporation rate of the precursor, which is required to determine the precursor concentration in the gas entering the reactor, was not available. No data on the crystalline structures were provided, either. Only few reports on the production of tantalum powders by hydrogen reduction of TaCl5 have thus appeared. In the present work, the mass of the precursor is instantaneously measured during vaporization by a load cell so that the precursor concentration can be determined and controlled, and the crystalline structures of produced particles were investigated with varying reactor temperature. The effects of the reactor temperature and the evaporation rate on the particle size and size distribution are discussed. EXPERIMENTAL The experimental set-up consists of a deoxidizer, a drying column, a precursor evaporator, a tubular reactor, and a particle collector. The precursor evaporator is made of Pyrex, 2 cm in inner diameter and 30 cm in length, heated by a heating coil, equipped with a thermocouple to measure the precursor temperature, and connected to a reaction tube. A glove box measuring 55 cm by 34 cm by 24 cm is installed overhead for loading the solid precursor, TaCl5 (Aldrich, > 99.99%), to a boat in a moisture-free environment; the precursor is very sensitive to moisture. About 0.3 g of the precursor was used for a run. The boat is suspended by a thin rod hooked up through a flexible bellows to a load cell (Minebea Co., Type UL-20GR). The precursor vaporization begins as soon as the precursor-laden boat is lowered to the evaporator which has already been heated to a desired temperature. During vaporization, the mass of the boat with precursor is measured instantaneously by the load cell. The reaction tube is made of alumina, 2.7 cm in inner diameter and 54 cm in length, and heated by an electric furnace (Lenton Co., Tube Furnace). Produced particles were collected at the exit of the reactor with a Teflon membrane filter (Cole-Parmer, Model Zix 90C), the pore size of which is 200 nm. The gas free of particles was then bubbled through a NaOH solution for removal of the HCl formed by the hydrogen reduction, before being exhausted to the atmosphere. RESULTS AND DISCUSSION The x-ray diffraction patterns of produced tantalum powders were investigated with reactor temperature varied from 1000 to 1400° C holding the evaporator temperature at 230° C and the hydrogen flow rate at 2 l/min. The vaporization rate of TaCl5 was 0.0031 g/s. At the reactor temperature of 1000° C, the phase was amorphous. Peaks indicating the evolution of crystalline structures appeared at 1200° C, and those peaks became sharper as the temperature increased to 1400° C. Compared with the peaks of tantalum provided in JCPDS no. 04-0788 [4], observed peaks shifted slightly towards the lower scattering angles. This is probably due to a formation of hydrides by hydrogen dissolution in the tantalum matrix. It is known that a tantalum absorbs hydrogen, at temperatures higher than 450° C, to the extent of 0.74 hydrogen atoms per tantalum atom [5]. The expansion of the inter-atomic distance by hydrogen dissolution may explain the shift. The reactor temperature was fixed at 1400° C because the crystallinity was poor at lower temperatures. The evaporator temperature and the hydrogen flow rate were then varied from 210 to 230° C and from 0.4 to 2 l/min, respectively, to study the effects of these variables on particle morphology and size. Produced powders were all in the form of aggregates composed of nanosized primary particles with necking between them. Their average primary-particle size was 17 nm at 210° C, 21 nm at 220° C, and 56 nm at 230° C. The increase of particle size with increasing evaporator temperature was probably due to an increase in the vaporization rate of TaCl5. The vaporization rate increased from 0.0014 to 0.0017 and 0.0031 g/s as the evaporator temperature was increased from 210 to 220 and 230° C. The geometric standard deviation was 1.2 to 1.3, exhibiting little difference between 210 and 230° C, as determined from the log-probability plot. At the same reactor temperature of 1400° C, Tsukihashi and Iijima [3] reported a particle size of about 40 nm at a vaporization zone temperature of 250° C, smaller than 56 nm that was obtained currently at the evaporator temperature lower by 20° C. The analysis of the particle size difference could not be made because the vaporization rate or the TaCl5 vapor concentration was not available from their work. The hydrogen flow rate was varied with the evaporator temperature constant at 230° C. The primary particle size increased with an increase in hydrogen flow rate: 14 nm at the flow rate of 0.4 l/min, 16 nm at 0.7 l/min, and 56 nm at 2 l/min. With the present apparatus, an increase in hydrogen flow rate not only decreased the residence time but also increased the vaporization rate. The increase of vaporization rate may be due to an influence of the gas velocity on the thickness of the boundary layer across which vaporized precursor must diffuse toward the hydrogen bulk stream. The decrease in the residence time would reduce the chance for particles to grow, resulting in a decrease in primary particle size. While, the increase in vaporization rate must have acted towards particle size increase. The two effects in opposite direction competed, giving the overall increase of particle size with increasing hydrogen flow rate. At the reactor temperature of 1400° C, the evaporator temperature of 230° C and the hydrogen flow rate of 2 l/min, the amount of TaCl5 vaporized and transported to the reactor was measured at 0.72 g or 0.002 mol, and that of the HCl absorbed in the NaOH solution was determined at 0.007 mol from the volume of the solution and the Cl concentration measured by an ion-selective electrode. By stoichiometry that 5 mol of HCl is produced from one mole of TaCl5, the amount of HCl absorbed in the NaOH solution is equivalent to a conversion of 0.0014 mol of TaCl5 . From the amounts of the TaCl5 admitted to the reactor and converted therein, the conversion was calculated at 70%. This is comparable to about 75% reported by Tsukihashi and Iijima [3] at a similar operating condition. Considering that some of the HCl produced may have adsorbed on the powders in the filter, actual conversion may be higher than that proposed. The conversion needs to be determined more accurately in the future. REFERENCES [1] G. Winter: ?Nanosized Tantalum Powders,? International Symposium on Tantalum and Niobium, Goslar, Germany, September, 1995, pp. 495-515. [2] D.R. Brutvan, R.L. Ripley and H.V. Seklemian: Can. Pat. 702, 612 (Jan. 26, 1965). [3] F. Tsukihashi and H. Iijima: ?Production of Fine Powder of Tantalum by Reduction of Tantalum Chloride Vapor with Hydrogen,? 2nd International Conference on Processing Materials for Properties, Edited by B. Mishra and C. Yamaguchi, TMS (The Minerals, Metals & Materials Society), 2000. [4] International Centre for Powder Diffraction Source, JCPDS 1997. [5] Kirk Othmer: Encyclopedia of Chemical Technology, Wiley, 1978, pp 541-564.