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Effect of Raw Material Characteristics on the Carbothermal Reduction Of Materials Transactions, Vol. 44, No. 10 (2003) pp. 2145 to 2150 #2003 The Japan Institute of Metals Effect of Raw Material Characteristics on the Carbothermal Reduction of Titanium Dioxide Gil-Geun Lee1 and Byoung-Kee Kim2 1Division of Materials Science & Engineering, Pukyong National University, Yongdang-dong, Nam-gu, Busan 608-739, Korea 2Korea Institute of Machinery & Materials, Sangnam-dong, Changwon, Kyungnam 641-010, Korea In the present study, the focus is on the effect of the particle characteristics of titanium dioxide powder in the carbothermal reduction of the titanium dioxide/carbon system. Four types of titanium dioxide powders with various phase structures and mean particle sizes were mixed with carbon black. These mixtures were heat treated under a flowing argon atmosphere. The changes in the phase structure and thermal gravity of the mixtures during heat treatment were analyzed using XRD and TG-DTA. Titanium dioxide powders with 100% anatase phase structure exhibit a higher titanium carbide (TiC) formation ability than the titanium dioxide powders with the mixed phase structure of the anatase and rutile phase structures. It was concluded that the phase structure of the titanium dioxide plays a more important role than the particle size on the carbothermal reduction of the titanium dioxide/carbon system. (Received April 18, 2003; Accepted August 13, 2003) Keywords: titanium dioxide, titanium carbide, carbothermal reduction, phase structure, particle size, powder 1. Introduction formation, in the carbothermal reduction method is very important to obtain an optimized titanium carbide powder. Tool materials were generally composed of hard materials The manufacturing of the ultrafine titanium carbide powder such as WC, TiC, TaC, TiCN and others, and soft binder with a nonagglomerated state by this method should be materials such as Co and Ni.1) Among the rest of those hard known to be the reaction mechanism. The chemical mech- materials, titanium carbide (TiC) has a high melting point anism of this reaction is often unknown, however, this is (3533 K), Young’s modulus (450 GPa) and hardness (Vick- highly dependent on the conditions of the process parameters ers; 29 GPa) with a low density (4:93  103 kg/mÀ3). It also such as atmosphere, pressure and temperature.8,9) It has been exhibits a high resistance to both oxidation and corrosion.2) recently verified that the carbon particle (grain) size, the Therefore, titanium carbide is extensively used for cutting homogeneity of the titanium dioxide/carbon mixture and the tools, grinding wheels, coating cutting tips and coating steel ventilation of the powder’s bed are important parameters for press tools, in the powder form. a low-temperature reaction.10,11) However, the parameters The tool materials manufactured by the powder metallurgy related to the particle characteristics of the starting material process have a composite microstructure of hard particles of the titanium dioxide, such as the particle (grain) size and dispersed in a soft binder matrix. The mechanical properties phase structure, have been rarely studied exhaustively. In the of the tool materials depend not only on the constituent present study, the focus is on their role in the carbothermal materials but also on their microstructure.3) The hardness, reaction of the titanium dioxide/carbon system. fracture toughness and wear properties of the tool materials were strongly influenced by the size of the hard particles and 2. Experimental Procedure the distance of the mean free path among the hard particles at the same constitution composition. These mechanical prop- The four kinds of starting titanium dioxide powders (TiO2) erties increased simultaneously with decreasing particle size used have different characteristics. They are reported in and the distance of the mean free path. To manufacture high- Table 1, which gives, in particular, the mean particle performance tool material with an ultrafine microstructure by the powder metallurgy process, raw powder materials with an ultrafine particle size should be used. Table 1 Characteristics of the starting titanium dioxide powders. A number of processes exist for synthesizing titanium Mean diameter Crystalline size carbide powders, such as carbothermal reduction of titanium Variety Supplier (nm) (nm) dioxide,4) direct carbarization of titanium,5) chemical reac- 6) Japan High tion of titanium chloride (TiCl4), self-propagating high- 95% rutile Rutile:66 7) TiO2(I) 700 Purity temperature synthesis, amongst others. Each method has + 5% anatase Anatase:70 varying characteristics of particle size and distribution, Chemical 15% rutile Rutile:22 morphology, state of agglomeration, chemical purity, and TiO2(II) 45 Daeggusa stoichiometry. The titanium carbide powder is commercially + 85% anatase Anatase:20 produced primarily by the carbothermal reduction of titanium TiO2(III) 100% anatase 30 16 Nanotech dioxide by carbon, especially carbon black, over a temper- Japan High ature range of 17002100 K.4) The reaction behavior of the TiO2(IV) 100% anatase 300 47 Purity titanium dioxide/carbon mixture, especially titanium carbide Chemical 2146 G.-G. Lee and B.-K. Kim (a) (b) 1µm 100nm (c) (d) 100nm 0.5µm Fig. 1 SEM micrographs of (a) TiO2(I), (b) TiO2(II), (c) TiO2(III) and (d) TiO2(IV). diameter measured by microscopy, the phase structure and 100 the crystalline size measured by X-ray powder diffractometry with Cu-K radiation, and the supplier. Figure 1 shows that 95 their morphologies are very similar to each other. The TiO2(I) powder has an almost stable rutile phase structure 90 (about 95% rutile + 5% anatase) with the largest particle 85 size, and the TiO2(II) powder has an almost metastable anatase phase structure (about 15% rutile + 85% anatase) 80 with a nanoscale particle size. The TiO2(III) and TiO2(IV) powders have a 100% anatase phase structure, but these 75 TiO2( I ) Mass Fraction (%) powders show different particle sizes. These four types of TiO2( II ) titanium dioxide powders are each mixed with carbon black 70 TiO2( III ) TiO ( IV ) (mean particle size: 0.5 mm) using an agate mortar. The 2 65 quantities of titanium dioxide and carbon black were those of 273 473 673 873 1073 1273 1473 1673 the reaction: Temperature, T / K TiO2 þ 3C ! TiC þ 2CO ð1Þ Fig. 2 Change in the mass fraction of the mixtures of titanium dioxide and The mixtures were placed in a graphite crucible and then heat carbon black with temperature during carbothermal reduction. treated at a temperature from 1173 to 1472 K for 30 minutes in a tube furnace under a flowing stream of argon atmosphere. The weight of sample is 3 g for the heat treatment. After heat initial mass (mo) according to eq. (2). treatment, the samples were observed with a SEM and ¼ðÁm=m Þ100 ð2Þ analyzed by X-ray diffraction. The changes in the thermal o gravity of these powder mixtures during heat treatment from The theoretical mass fraction of the reaction (1) was 51.68% room temperature to 1673 K under a flowing stream of argon after complete carbothermal conversion of the titanium atmosphere were analyzed by TG-DTA with a heating rate of dioxide by the reaction. The mass fraction of the powder 5 K/min. mixtures, in the present study, should have a value of 51.68% after complete carbothermal conversion of the titanium 3. Results and Discussion dioxide powder. As shown in Fig. 2, the values of the mass fraction of the mixtures at a 1673 K were higher than 51.68%, Figure 2 shows the change in the mass fraction of the which means that the present titanium oxide in the mixtures mixture of titanium dioxide and carbon black with the could not be completely converted to titanium carbide during temperature during carbothermal reduction under a flowing the analysis by TG-DTA. The TiO2(I) and TiO2(II) powders stream of argon atmosphere for the various types of the which have a mixed phase structure of anatase and rutile starting titanium dioxide powder. After weighting, the mass structures demonstrated similar carbothermal reduction be- fraction was obtained by the ratio of the mass loss (Ám) to its havior. These powder mixtures showed a remarkable Effect of Raw Material Characteristics on the Carbothermal Reduction of Titanium Dioxide 2147 : TiO2(rutile) : TiO2(anatase) : TiO (anatase) : TiO (rutile) 2 2 : TiC : TiC : Ti O (230606) 3 5 : Ti 3O5(110217) : Ti 3 O 5(230606) 1673K Heat treatment 1673K Heat treatment 1573K Heat treatment 1573K Heat treatment 1398K Heat treatment 1398K Heat treatment Intensity (arb. units) Intensity (arb. units) As-mixing As-mixing 20 30 40 50 60 70 80 20 30 40 50 60 70 80 θ 2 2θ Fig. 3 X-ray diffraction patterns of the mixture of the TiO (I) and carbon 2 Fig. 4 X-ray diffraction patterns of the mixture of the TiO (II) and carbon black heat treated at a specified temperature for 30 minutes. 2 black heat treated at a specified temperature for 30 minutes. decrease in the mass fraction at about 1523 K. The TiO2(IV) dioxide and carbon black during the phase transformation of powder which has a 100% anatase phase structure with a the titanium dioxide could not be found. mean particle size of 300 nm showed a remarkable decrease Figure 4 shows the X-ray diffraction patterns of reaction in the mass fraction at about 1473 K. The TiO2(III) powder products from the TiO2(II) mixed with carbon black and heat which has also a 100% anatase phase structure with a smaller treated at a specified temperature for 30 minutes in the tube mean particle size then the value of the TiO2(IV) powder furnace under a flowing stream of argon. In the mixing state, showed a striking decrease in the mass fraction occurring at the X-ray diffraction pattern exhibits only TiO2 peaks which about 1373 K.
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