Role of some technological parameters during carburizing titanium dioxide

The influence of technological parameters is followed during the carbothermal synthesis of titanium carbide from the dioxide. The carbon grain size, the homogeneity of the carbon/oxide mixtures and the ventilation of the powders' beds are the most important conditions for a rapid reaction. The oxide grain size, the mixing method and the compactness of the mixture has no influence, or very little. These results are explained by the carburizing mechanism where the Boudouard's reaction: C+CO₂→2CO plays a central role. This reaction is more rapid when fine carbon is used and when the carbon monoxide is eliminated as soon as formed. These conditions are those required for a complete synthesis of the carbide by: TiO₂+3C→TiC+2CO for mixtures having the stoichiometric composition.     

Preparation of a titanium carbide coating on carbon fibre using a molten salt method

A method for preparing protective titanium carbide (TiC) coatings on carbon fibres has been developed using a molten salt synthesis method. The TiC coatings were formed on the surface of carbon fibres in a reaction medium consisting of Ti powder in a mixture of molten LiCl-KCl-KF salts under an argon atmosphere at 900 and 950 °C. The structure and morphology of the TiC coatings were characterized by XRD, SEM and energy dispersive X-ray (EDX) analyses. The coatings consisted of homogeneous single phase cubic TiC with thicknesses in the range of 60-800 nm. Variation of the synthesis time and reaction mixture was found to significantly affect the thickness and integrity of the TiC coating although variation of the reaction temperature had little effect. The coating thickness was closely related to the composition of the molten salts and to the molar ratio between the carbon fibre and titanium.     

Combustion of the gasless system Ti + 0.5C in a nitrogen coflow

Combustion of mixtures of titanium with carbon black of bulk density placed in a quartz tube with nitrogen purging (concurrent filtration) is examined. The nitrogen flow is provided by a fixed pressure difference not exceeding 1 atm at the ends of the mixture. The burning rate is determined as a function of the amount of titanium nitride added to the initial mixture and of the pressure at the ends of the sample. The tests show that a nitration front is formed during combustion of a Ti + 0.5C (carbon black) powder mixture in the nitrogen flow, in addition to a carbidization front. Various modes of propagation of the carbidization and nitration fronts are found and described. A classification of combustion modes of the Ti + 0.5C (carbon black) powder mixture in the nitrogen flow is proposed.     

Fabrication of core-shell structured MWCNT-Ti(TiC) using a one-pot reaction from a mixture of TiCl3, TiH2, and MWCNTs

Multi-walled carbon nanotubes (MWCNTs) coated with metal titanium or carbide layers were prepared by heating a mixture of TiCl₃, TiH₂, and MWCNTs under vacuum. The resulting MWCNT-Ti(TiC) materials had a uniform and conformal core-shell structure. The thickness of the Ti(TiC) layer was determined by the deposition temperature and time. The amount of TiC in the shell depended primarily on the deposition temperature. Whether a metal or carbide coating was obtained was determined by controlling the deposition temperature. Ti(TiC)-coated MWCNTs showed better field emission properties than did pristine MWCNTs. This deposition method should prove to be a versatile route for fabricating other one-dimensional core-shell materials.     

Experimental investigation of combustion of a gasless pelletized mixture of Ti + 0.5C in argon and nitrogen coflows

Combustion of a pelletized mixture of titanium and carbon black placed in a quartz tube and exposed to a flow of argon or nitrogen is studied. The gas flow (cocurrent filtration) is provided by a fixed pressure gradient at the inlet and outlet of the tube, which did not exceed 1 atm. The possible modes of combustion of pelletized mixtures related to the presence of a more complex hierarchy of scales (micro, macro, and meso) compared to that of powder mixtures (micro, macro) are analyzed. A comparison is made of the burning rates of powder and pelletized mixtures. An increase in the burning rate when using pelletized mixtures was found experimentally. It is shown that the gas coflow through the pelletized mixture of Ti + 0.5C leads to an increase in the burning rate. It is established that the propagation of the flame front of the pelletized mixture of Ti + 0.5C in flows of nitrogen and argon is controlled by different reactions. In contrast to combustion of powder mixtures of Ti + 0.5C, in combustion of pelletized mixtures of Ti + 0.5C in a nitrogen flow, only one front is observed. It is proved that radiation plays a significant role in the propagation of the combustion front in the pelletized mixture of Ti + 0.5C.     

Formation of Titanium Carbide/Aluminum Oxide Nanocomposite Powder by High-Energy Ball Milling and Subsequent Heat Treatment

The preparation of titanium carbide/aluminum oxide (TiC/Al₂O₃) nanocomposite powders from a mixture of titanium, carbon, and Al₂O₃ powders via a high-energy ball milling process and subsequent heat treatment was investigated. The microstructure development of the powder mixtures was monitored by X-ray diffraction and transmission electron microscopy. The ball milling of an elemental carbon, titanium, and Al₂O₃ powder mixture at ambient temperature resulted in the formation of TiC within 15 h of milling. With further milling of up to 25 h, the resulting powder mixture was composed of nanosized TiC particles and nanocrystalline carbon, titanium, and Al₂O₃. The nanocrystalline titanium and carbon were transformed into nanosized TiC particles after subsequent heat treatment. The final product was composed of nanosized TiC and microcrystalline Al₂O_3. Most of the nanosized TiC particles were located within Al₂O₃ grains.     

Bimodal Microstructure and Reaction Mechanism of Ti2SnC Synthesized by a High-Temperature Reaction Using Ti/Sn/C and Ti/Sn/TiC Powder Compacts

High purity of titanium tin carbide (Ti₂SnC) powder was fabricated by pressureless sintering two types of mixtures of Ti/Sn/C and Ti/Sn/TiC powders under different conditions. A bimodal microstructure of Ti₂SnC with plate-like and rod-like forms was first observed, which is determined by the grain growth rate in different planes, the C particle's size, and the growth environment. Based on the microstructure observation, a reaction model was proposed to understand the reaction mechanism for the formation of Ti₂SnC. Further investigation of the thermal stability of Ti₂SnC demonstrates that Ti₂SnC decomposes to TiC and Sn in vacuum atmosphere at 1250°C.     

Influence of TiB2 content on microstructure and hardness of TiB2-B4C composite

In this study a composite ceramic powder mixture using fine powders of titanium diboride (TiB₂) and boron carbide (B₄C) was consolidated to theoretical density using the technique of plasma pressure compaction. Achieving rapid consolidation of the composite powder mixture is an essential requirement for achieving microstructural control and better mechanical properties in the consolidated end product. The variables chosen in making the composite samples was the titanium diboride (TiB₂) content. The microstructure and hardness of the ceramic composite sample made by consolidating various powder mixtures at a temperature of 1700 °C are compared. Microhardness measurements reveal a gradual increase in hardness with an increase in TiB₂ content in the starting powder mixture. The specific role of TiB₂ content in influencing microstructural development and hardness is presented and discussed.     

Effect of heat release conditions on the phase composition of the combustion products of a Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript>/Al/C thermite mixture

An X-ray method was used to study the phase composition of the combustion products of a Fe₂O₃/TiO₂/Al/C thermite composite mixture, in particular, during combustion of samples under cooling by water. The data of the experiments are compared with the results of thermodynamic calculations. A probable explanation of the reasons of the angular displacement of the line of iron in the X-ray spectrum of the combustion products and a mechanism for the crystallization of an eutectic TiC + α-Fe mixture from the melt of the Ti/Fe/C ternary system. The composition of the combustion products was found to be different from the equilibrium one, which is manifested in the presence of defects in the carbon sublattice of titanium carbide. A probable explanation of the deficiency of carbon is proposed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 4, pp. 39–43, July–August, 2008.     

Characterization of the structure of TiB2/TiC nanocomposite powders fabricated by high-energy ball milling

TiB₂/TiC nanocomposite powders were successfully prepared by high-energy ball milling of the powder mixtures of Ti and B₄C. X-ray diffraction analysis showed that the TiC phase was not produced until the milling time was up to 24 h and only a minimal amount of TiB₂ was generated, even after 48 h of milling. The critical grain size of Ti milled for the reaction between Ti and B₄C was 31.2 nm. Transmission electron microscopy clearly indicated that the resulting powder mixture obtained after milling for 48 h and annealing at 800 °C for 30 min was composed of nanosized TiC and TiB₂ particles.     

Sintering of mixtures of the MgO-MgCr2O4-MeC (Me=Si,Ti) system

A thermodynamic calculation of the probable reactions of interaction in mixtures of the MgO-MgCr₂O₄-MeC (Me=Si, Ti) system is presented. Sintering of MgO+MgCr₂O₄ mixture with additions of titanium and silicon carbides is investigated. It is established that the maximum density (96% of the theoretical value) of sintered specimens of MgO+MgCr₂O₄ mixture is attained at a 0.5% mass fraction of titanium or silicon carbide and a firing temperature of 1850°C.Translated from Ogneupory, No. 11, pp. 11–13, November, 1995.     

Ignition of a Ti–Al–C System by an Electron Beam

This paper describes the optimal modes of initiation of self-propagating hightemperature synthesis with the help of an electron beam on the example of a Ti–Al–C powder mixture. A pulsed electron beam with a particle energy of tens of kiloelectronvolts and a duration of hundreds of microseconds is used. Morphology, structure, and elemental composition of formed products in the form of Ti₃AlC₂ and TiC are studied.     

添加碳化钛对超细Ti(C,N)-Ni金属陶瓷显微结构和力学性能的影响

用传统粉末冶金法制备了添加碳化钛(TiC)的Ti(C,N)基金属陶瓷.为了得到超细晶粒的显微结构,主要硬质相[Ti(C,N),TiC和TiN]的初始粉末粒度为纳米、亚微米级.通过扫描电子显微镜观察,发现了一种新的白芯/灰壳结构,揭示了其形成机制.此外,通过能谱仪分析,获得了各相的化学成分.用X射线衍射仪并通过计算得出了各相的点阵参数.对室温下该材料的力学性能进行了测试,并尝试把它们与金属陶瓷的原始成分和显微结构的特点联系起来.     

An alternate approach to synthesize TiC powder through thermal plasma processing of titania rich slag

This paper reports the preparation of fine titanium carbide (TiC) powder by using titania rich slag as a cheap raw material. The mixture of titania rich slag and activated charcoal was reacted in a thermal plasma reactor for 30 min under argon flowing atmosphere. The reaction product, a fused mass of Fe-TiC composite, was milled to fine powder at ambient atmosphere for 10 h then chemically leached for the removal of iron and other minor impurities. The obtained TiC powder after leaching was characterized by X-ray diffraction (XRD), Raman spectroscopy, electron probe micro analysis (EPMA), field emission scanning electron microscopy (FESEM) and particle size analysis (PSA). XRD, Raman spectroscopy results confirmed the formation of TiC. EPMA, EDS results indicated the synthesized product obtained after leaching to be free from iron and other minor impurities. Particle size analysis result revealed the average particle size of TiC powder to be 2.54 µm.     

Phase Constitution and Dynamic Properties of Spark Plasma‐Sintered Alumina–Titanium Composites

Al₂O₃/Ti composites of various metal to ceramic ratios were fabricated by the spark plasma sintering (SPS) technique. The effects of titanium concentration in the initial mixture on phase composition, and on the static and dynamic (planar impact testing) mechanical properties of the SPS‐processed composites were investigated. It was observed that the significant alumina dissolution in titanium takes place during SPS treatment. The composites fabricated from starting alumina/titanium powder mixtures with a mass fraction of titanium less than 0.8 consisted of two phases, alumina and a solid solution of oxygen and aluminum in titanium. For starting mixtures with higher titanium content, the presence of a Ti₃Al intermetallic phase with a relatively low fraction of dissolved oxygen was detected. Changes in phase composition could explain the effect of titanium content in the starting mixture on physical and mechanical properties of the composites. Mechanisms governing the dynamic response of the composites under loading of different intensities are discussed.     

不同原料体系对无压烧结合成Ti3SiC2的影响及其腐蚀行为

通过液相磁力搅拌混合原料粉末,压片后无压烧结合成了三元Ti3SiC2,研究不同原料配比Ti/Si/C,Ti/SiC/C/和TiC/Si/Ti对合成Ti3SiC2的影响,同时为了比较,在相同条件下加入少量Al或Sn,研究其对Ti3SiC2的合成过程及最终产物的影响,并探讨Ti3SiC2的合成机理.结果表明:3Ti/1.2Si/2C/0.1Al在1400 ℃无压烧结合成了较高纯度的Ti3SiC2,Al粉的加入可以降低混合粉末的起始反应温度,有利于三元层状化合物Ti3SiC2的合成和纯度的提高,其合成机制为,在铝粉形成的熔池中,经形核钛和硅反应生成钛硅金属间化合物,钛与石墨反应生成碳化钛,随后扩散,长大,随着温度的升高,反应生成三元层状Ti3SiC2.而以TiC或SiC为Ti或Si源制备的Ti3SiC2含杂质较多,不适用于无压烧结合成Ti3SiC2.合成的Ti3SiC2在HF溶液中经200 ℃溶剂热反应后,产物主要为两种不同晶型的SiC和AlF3立方体,且随着反应时间的延长,AlF3的含量增加,结晶更完善.     

Microstructural Evolution during the Sintering of TiC–Mo–Ni Cermets

TiC-Ni-Mo cermet specimens were prepared by using a mixture of fine (1.5 μm) and coarse (30 μm) TiC powders. When the fraction of fine TiC particles was 80%, a (Ti,Mo,Ni)C complex carbide phase was observed deposited on the coarse TiC particles and resulted in a typical cored structure. As the fraction of fine TiC particles decreased, the coarse TiC particles exhibited a unique microstructural evolution with the development of a concave interface. This microstructural change of the coarse TiC grains can be explained in terms of the coherency strain energy.     

Formation of Al2O3–TiC Composite Nano-Particles Synthesized from Carbon-Coated Precursors

The formation of nano-sized alumina–titanium carbide (Al₂O₃–TiC) composite powders from a carbon-coated titanium dioxide–aluminum (TiO₂–Al) mixture was investigated. The carbon-coated TiO₂–Al mixture altered the mechanism of the reaction, compared with standard mixtures, to produce high-quality nano-sized Al₂O₃–TiC powders. Data synthesized from intermediate temperatures indicate that these products form via Ti₂O₃ and Al₃Ti. TEM images of the Al₂O₃–TiC powders showed fine size (50–100 nm), narrow size distribution, and lack of agglomeration. DSC data for the carbon-coated TiO₂–Al mixture showed a single endothermic and four successive weak exothermic reactions as the carbon coating moderated the heat release during the reaction.     

The use of plastic waste as carbon raw materials to obtain TiC-based powders

Plastic pollution is one of the most pressing environmental problems, and a huge amount of effort and money is directed towards solving it. The existing processing methods are ineffective. The main component of all plastic materials (C_xH_y) is carbon. High-purity fine titanium carbide was obtained using plastic waste (polyethylene terephthalate – C₁₀H₈O₄) as a carbon raw material. Combustion processes, phase composition and structure of the obtained materials were studied. A probable mechanism for the formation of titanium carbide during the combustion of the (Ti + C₁₀H₈O₄) mixture was proposed. During the synthesis, polyethylene terephthalate decomposes into carbon, hydrogen and oxygen. Carbon and oxygen react with titanium and form titanium oxycarbide. Titanium oxycarbide is saturated with carbon to form titanium carbide and carbon dioxide. The remaining hydrogen and carbon dioxide are released from the synthesis products, which leads to self-purification of the synthesis products. The obtained results will create the basis for the development of a fundamentally new, cost-effective technology for recycling plastic waste into carbides and carbide-containing materials.     

Electrolytic synthesis of TiC/SiC nanocomposites from high titanium slag in molten salt

The molten salt electrolytic method for the preparation of titanium carbide and silicon carbide composites has been subjected to a systematic investigation by experimental analyses and thermodynamic calculations. It has been confirmed that the electrolysis of high titanium slag in the presence of mixed graphite powders generates intermediates CaTiO₃, Ti₂O₃, TiO, Fe₃Si and objective carbonous products TiC/SiC. It has been furthermore found that the deoxidization process depends critically on a number of process parameters, namely, electrolyte composition, graphitic regime, reaction temperature, cell voltage and reaction time. After careful optimization of these parameters, TiC/SiC nanocomposites with particle sizes of 10–174 nm has been produced by electrolysis of high titanium slag and graphite mixtures in molar ratio of 1:2 referred to Ti:C under 3.2 V at 900 °C for 6 h in 1 mol%CaO-CaCl₂-NaCl molten salt and with particle sizes of 12 nm~207 nm in 1 mol%CaO-CaCl₂ electrolyte.