versão On-line ISSN 0104-6632
Braz. J. Chem. Eng. v. 16 n. 1 São Paulo Mar. 1999
PREPARATION OF TANTALUM CARBIDE FROM AN ORGANOMETALLIC PRECURSOR
C. P. SOUZA1, C. FAVOTTO, P. SATRE, A.L' HONORÉ and M. ROUBIN
1Laboratório de Termodinâmica e Reatores, CT - DEQ/PPGEQ, Universidade Federal do Rio Grande do Norte, Campus Universitário, 59072-970, Natal, RN - Brazil, Phone: 084-215-3769, Fax: 084-215-3770, e-mail: firstname.lastname@example.org
Laboratoire de Physicochimie du Matériaux et du Milieu Marin, LPCM3, Equipe des Matériaux à Finalité Spécifique, Université de Toulon et du Var B.P. 132, 83957 La Garde Cedex, France, Phone and Fax: 04 94 14 24 29, e-mail: email@example.com
(Received: June 12, 1997 ; Accepted: April 28, 1998)
Abstract - In this work we have synthesized an organometallic oxalic precursor from tantalum oxide. This oxide was solubilized by heating with potassium hydrogen sulfate. In order to precipitate Ta2O5.nH2O, the fused mass obtained was dissolved in a sulfuric acid solution and neutralized with ammonia. The hydrated tantalum oxide precipitated was dissolved in an equimolar solution of oxalic acid/ammonium oxalate. The synthesis and the characterization of the tantalum oxalic precursor are described. Pyrolysis of the complex in a mixture of hydrogen and methane at atmospheric pressure was studied. The gas-solid reaction made it possible to obtain tantalum carbide, TaC, in the powder form at 1000oC. The natural sintering of TaC powder in an inert atmosphere at 1400°C during 10 hours, under inert atmosphere made it possible to densify the carbide to 96% of the theoretical value.
Keywords: organometallic, tantalum carbide, precursor.
The area of materials is a developing field of research in which chemistry plays an important part. Unlike the classical processes of chemistry solid state, new processes of preparation of inorganic material called soft chemical processes, allow us to obtain homogeneous solids at low temperatures. The inorganic material can be obtained in different forms (powder, fine layers, fibers). In the field of ceramics, these methods generally lead to fine powders and permit a considerable lowering of sintering temperatures. Depending on the nature of the atmosphere in which the precursor is decomposed, a gas-solid reaction, can result in different types of materials. In this work, tantalum carbide is obtained by the decomposition of an oxalic precursor, in a methane and hydrogen atmosphere.
The objective of this work is to synthesize an organometallic complex in order to get tantalum carbide.
Tantalum oxide was used as the raw material to prepare the tantalum organometallic precursor.
Preparation Of Hydrated Tantalum Oxide From Ta2O5
The tantalum oxide is solubilized by potassium hydrogen sulfate (KHSO4 ) at high temperatures, using the method of Powell and Schoeller (Powell and Schoeller, 1925; Edmister and Albritton,1932). The oxide and the potassium hydrogen sulfate are mixed and ground in the proportions of 1g and 10g, respectively. The mixture is heated in a silicon crucible with the use of a Bunsen burner until the solid becomes a homogeneous liquid. After cooling, the fused mass obtained, K2O.Ta2O5, (Bagshave, 1955) is ground and dissolved in hot concentrated H2SO4.
During the fusion, the potassium hydrogen sulfate decomposes as shown in the following equation:
2KHSO4 Þ 2K+ + 2HSO4- Þ 2K+ + H2O + S2O72- Þ H2O + K2S2O7 (1)
and the potassium pyrosulfate decomposes as follows:
K2S2O7 Þ K2O + 2SO3 (2)
The dissolution of the fused mass in sulfuric acid results in the formation of the following species : K+, SO42- and Ta2O5.nH2O. In order to precipitate the Ta2O5.nH2O after cooling, the solution obtained is diluted and neutralized using ammonia. Neutralization is achieved by the gradual addition of NH4OH, while monitoring the pH. The reaction is exothermic; therefore, it is necessary to cool the solution during the neutralization step. The white, gelatinous precipitate obtained is separated from the solution by filtration and then washed with a hot acetic acid solution (1% vol.) to eliminate the K+, SO42- and NH4+ ions (Rohmer, 1941; Bagshave, 1955; Clar, 1964).
Preparation of the Ammonium Tantalum Oxalate Complex
Recently precipitated tantalum oxide becomes dehydrated in time and its solubility in H2C2O4 decreases (Marignac, 1866). Due to this fact, it must be dissolved immediately in an equimolecular solution of H2C2O4 /(NH4)2C2O4 by ebullition. The crystals of the ammonium tantalum oxalate complex, (NH4)3[TaO(C2O4)3].1.5H2O, and ammonium hydrogen oxalate, (NH4)HC2O4, are obtained by slow evaporation of this solution. This complex is stable only in the presence of an excess of oxalic acid, H2C2O4, and/or ammonium oxalate (Clar, 1964; Thibaudon, 1970). The X-ray diffraction patterns of the precursor are shown in the Figure 1.
Figure 1: X-ray patterns of the precursor
In the case of the preparation of tantalum carbide, there is no need for the separation of the complex (precursor) of ammonium hydrogen oxalate, (NH4)HC2O4.
Thermal Decomposition of the Complex
Thermal decomposition of the mixture (precursor and (NH4)HC2O4)) was studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) using a SETARAM TG -DTA 92. Figure 2 shows the curves obtained during the heating of the organometallic precursor from 250oC to 1000oC at a rate of 10 Kmin-1. The thermogravimetric curve shows the decomposition at various stages associated with the endothermic phenomena (DTA), corresponding to the elimination of H2O, NH3, CO and CO2, leading to the formation of amorphous tantalum oxide at 320oC. The exothermic peak at 713oC characterizes the crystallization of Ta2O5.
Figure 2: TD-TGA curves of the precursor
Carburation of the Precursor By Gas-Solid Reaction
Carburation is carried out in a furnace by the mixture of hydrogen and methane. The methane is a source of carbons whereas hydrogen is the reducer which permits the displacement of dissociation equilibrium CH4 Û C + 2H2.
The composition of the mixture is selected as a function of the carburation temperature. At a temperature of 1000oC, the percentage of methane used is 1% (Clar, 1964). The mixture of the two gases is obtained with the help of Brooks mass flow meters. The experimental apparatus is controlled by a program in a personal computer. Before being used in the reaction, the gases with very high degrees of purity (O2 below 1 ppm) are dried in a column of phosphorous pentoxide.
Carburation is a gas-solid reaction. In order to have a very high yield, it is necessary to have a thorough exchange between the condensed and gaseous phases. A small quantity (3.0 g) of the organometallic complex is in a crucible and is placed inside the furnace. The whole experimental unit is purified 15 minutes before the heating process. The heating rate is 3 Kmin-1. Once the pre-established temperature is reached (1000oC), the furnace is maintained at this temperature for one hour to obtain carburation and cooled in a mixture of methane and hydrogen to room temperature. The final product, obtained in powder form, is characterized by means of X-ray diffraction, SEM, and laser granulometry. The natural sintering of the powder at 1400°C during 10 hours, in a inert atmosphere makes it possible to densify the carbide to 96% of the theoretical value.
Characterization of the Carbide
X -ray Diffraction
The characterization of the tantalum carbide by X-ray diffraction was carried out in a SIEMENS D5000 diffractometer. X-ray patterns in Figure 3 show a centered cubic face structure (cfc). The corresponding values for q , d and relative intensity are shown in Table 1.
Figure 3: X-ray patterns of the tantalum carbide TaC
Table 1: Values of q , d and relative intensity for the X-ray analysis of TaC
|2 theta|| |
The granulometric distributions were measured in an apparatus of the MALVERN 2000/3000 type. The analysis was carried out in aqueous solution and a curve for the distribution of the particles is shown in Figure 4. It shows grain size distribution with a diameter range of 0.3 to 290 to 50 µm.
Figure 4: Distribution curve of the particles of TaC
Scanning electron microscopy (SEM)
The characterization of the tantalum carbide by SEM was carried out in a PHILIPS XL30 apparatus. The micrographs in Figure 5 show agglomerate particles with a sponge like aspect. The agglomerate size is smaller than 10 m m.
Figure 5 : SEM micrographs of tantalum carbide powder
In order to obtain TaC, we synthesized an organometallic presursor from Ta2O5. This precursor was obtained by a complexation reaction in oxalate solution after fusion of Ta2O5 with KHSO4. Once the hydrated tantalum oxide was obtained, it was solubilized in concentrated sulfuric solution and precipitated by ammoniac. The pyrolysis of the complex at 350°C led to amorphous oxide and at 713°C to the crystal. The carburation reaction, which is a solid-gas type reaction, was conducted in a CH4/H2 gas atmosphere by heating to 1000°C. The parameters of this reaction, time, temperature, rate and composition of the gas were optimized. The carbide obtained in powder form was characterized by means of X-ray diffraction, SEM and laser granulometry. The natural sintering of TaC powder at 1400°C during 10 hours, in a inert atmosphere (argon) made it possible to densify the carbide to 96% of the theoretical value.
In industry, materials are obtained by high temperature processes generally operating above 1500°C. The soft chemical process used in this work will be applied to increase the value of the tantalite mineral with the formation of a solid solution between tantalum and niobium carbides.
This investigation was supported by CNPq-RHAE, Brazil and the Université de Toulon et du Var, Laboratoire de Physicochimie du Matériaux et du Milieu Marin, LPCM3, La Garde Cedex, France.
SYMBOLS AND UNITS
|q||diffraction angle, in degrees|
|d||interplanar distance, in Å ngstroms|
|°C||temperature in degrees Celsius|
|K||temperature in Kelvin|
|g||Unit mass in grams|
|ppm||parts per million|
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Edmister, F. H. and Albritton, G. G., J. Am. Chem. Soc., 54, 438 (1932). [ Links ]
Marignac, M. C., Ann. Chim. Phys., 4e série, 8, 62 (1866). [ Links ]
Powell, A. R. and Shoeller, W. R., Analyst, 50, 485 (1925). [ Links ]
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Thibaudon, D., Ph. D. diss., Université de Lyon (1970).