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Carbothermal reduction TiO_2 micron titanium carbide powder prepared experimental study

Author: SenWei
Tutor: DaiYongNian；YangBin
School: Kunming University of Science and Technology
Course: Materials Physics and Chemistry
Keywords: TiC powders Carbothermal reduction TiO2 Vacuum High-frequency induction Quantum chemical calculations
CLC: TF123.2
Type: Master's thesis
Year: 2011
Downloads: 168
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Abstract

Titanium carbide has been widely used in many fields (such as ceramic materials, cutting tools, wear resistance tools and aerial materials) because of its outstanding properties:high melting temperature, wear resistance, corrosion resistance, high electrical conductivity and thermal conductivity. Seeking for a low-energy method to produce TiC powders with high purity, uniform grain size, low agglomeration and near stoichiometry, has been extensively studied by researchers in recent years.Firstly, in the process of preparation of TiC powders by carbothermal reduction of TiO2 method, the reaction enthalpy and the initial reaction temperature of reaction equations under different pressures was thermodynamically calculated and analyzed, respectively. The results indicate that:the heat for generating TiC up to 500KJ/mol. So higher temperatures and a certain holding time are required in order to provide sufficient heat and prepare pure TiC powders. Preparation of TiC in vacuum is easier than that at atmospheric pressure (noble gas atmosphere) on thermodynamics. What’s more, large amounts of TiC form easily for the gas CO escapes fast from raw material (TiO2 and charcoal) in vacuum.On the basis of thermodynamic calculations, author systematically studied the method that preparation of fine TiC powders by carbothermal reduction of titania/charcoal at vacuum condition. The calculations and experimental results illustrate that the formation sequence of products prepared in vacuum should be Magneli phase (Ti4O7)→Ti3O5→Ti2O3→TiCxO1-x→TiC. As the holding time extended at the same temperature, the endothermic product transformed into titanium oxide with lower valence, and at last converted into titanium carbide. At a higher temperature, the time of the product transformed into single-phase titanium carbide is shorter. Through the research of different material ratio, different temperatures and different time, the optimizing process parameters for fine TiC powders are:the temperature is 1450℃, holding time is 8h and molar ratio is 1:4 (TiO2:C). SEM micrograph analysis shows loose structure, homogeneous morphology and low agglomeration are observed on the surface. The grain size is 2.05μm (D50). The lattice parameter calculation indicates that the powders are stoichiometric TiC1.0 with low impurities, and its single peaks and crystal plane are similar with that of industrial TiC.Formation of ultrafine TiC powders by carbothermal reduction of titania and charcoal in high-frequency induction furnace was also investigated because of its outstanding properties:induction coil can make the materials self-heating and the materials can be heated to high temperatures in a short time. The result indicates that ultrafine TiC powders with low impurities was prepared at temperature range 1490～1510℃(monitored by infrared thermometer, internal temperature of materials is higher than 1600℃) for a short time (10-30min). The surface of TiC particles is homogeneous and loose agglomeration while the grain size (D50) is about 1-10μ.m. Lattice parameter calculation indicates that sub-stoichiometric TiCx (x<1) should exist in TiC powders when TiO2:C (molar ratio) =1:3. But stoichiometric TiC1.0 was prepared when TiO2:C (molar ratio)=1:4.Corrosion resistance test indicate that:the acid corrosion resistance of titanium carbide powders prepared by above mentioned methods is better than industrial powders. It is hardly dissolved in HCl, H2SO4, HNO3, HF, HClO4 and aqua regia, and slightly dissolved in mixed solution HF+HNO3. As heated at air atmosphere, titanium carbide powders is gradually oxidized at 345℃～917℃, and the product may be titanium dioxide and titanium oxides with lower valence. When the temperature rose to 494～551℃, a large number of titanium carbide powders is oxidized. And when the temperature rose to 673～688℃, except the exothermic oxidation of titanium carbide powders, the free carbon in it is also oxidized and transformed into gas CO2 What’s more, less free carbon is contained in TiC powders prepared by high-frequency induction method.Finally, quantum chemical calculations and dynamics simulation results show that:the initial reaction of TiO2 and C are mainly solid reaction; at 1450℃and 50Pa, CO gas is generated easily from C and O (TiO2), While TiCxOy product is generated from Ti and a small number of C.

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