Gas-solid displacement reactions in the Ti–W–C system

Displacement reactions between binary and ternary ceramics in the Ti–W–C system and reactive gaseous atmospheres are investigated in this work. Specifically, WC and 50:50 wt% TiC:WC solid solution powders were exposed to flowing hydrogen gas, or equilibrated against an excess of titanium in the presence of iodine, to form metallic tungsten and TiC solid products. In the case of pure WC reacting with hydrogen, transformation to metallic tungsten occurred as a result of removal of chemically bound carbon as gaseous hydrocarbons. In the case of pure WC reacting with titanium iodide vapors, transformation was accompanied by the appearance of TiC as a solid product formed at the gas-solid interface. In the case of 50:50 wt% TiC:WC solid solution powders, hydrogen was generally found to be an ineffective displacing reagent, whereas reaction with titanium iodide vapors was observed to proceed virtually to completion, resulting in a two phase product mixture comprising metallic tungsten and TiC. For the latter case, a variety of microstructures could be observed within a given batch, including tungsten platelets and/or lamellae in a TiC matrix, or coarse tungsten grains interspersed with TiC grains. These morphological variations are speculated to arise from compositional variation in the starting material and the occurrence of local rapid coarsening along fast diffusion pathways within reacting agglomerates and polycrystalline primary particles. The observed reaction products and relative efficacy of gaseous reagents to promote displacement reactions in the Ti–W–C system are rationalized on the basis of thermodynamic predictions. The reaction between 50:50 wt% TiC:WC solid solution powders and titanium iodide vapors constitutes the first known report of an internal displacement reaction proceeding via gaseous intermediates in a nonoxide ceramic system.     

Modeling of thermal explosion in constrained dies for B4C–Ti and BN–Ti powder blends

The process of reactive in-situ synthesis of dense particulate reinforced TiB₂/TiC and TiB₂/TiN ceramic matrix composites from B₄C–Ti and BN–Ti powder blends with and without the addition of Ni has been modeled. The objective of modeling was the determination of optimal thermal conditions preferable for production of fully dense ceramic matrix composites. Towards this goal heat transfer and combustion in dense and porous ceramic blends were investigated during heating at a constant rate. This process was modeled using a heat transfer–combustion model with kinetic parameters determined from the differential thermal analysis of the experimental data. The kinetic burning parameters and the model developed were further used to describe the process of combustion synthesis in a constrained die under pressure. It has been shown that heat removal from the reaction zone affects the ignition temperature of thermal explosion.     

Structure and phase formation in the Ti–Al–Nb system in the thermal explosion mode

This paper presents the results of the structure and phase formation in the Ti/Nb/2Al, Ti/Nb/2.5Al and Ti/Nb/3Al systems in the thermal explosion mode of self-propagating hightemperature synthesis. The morphology, phase composition, microstructure, and physical properties have been studied. It has been found that compounds with the highest content of aluminum have the most homogeneous composition and the lowest porosity. The main phase of the synthesis product is a phase based a solid solution of Nb in γ-TiAl.     

SHS-Produced Al–Ti–B Master Alloys: Performance in Commercial Al Alloy

Al–Ti–B master alloys (MAs) were SHS-produced from the elements taken in the amounts that would ensure the preparation of final combustion products (MAs) containing about 20, 25, and 30 mol % TiB₂. Combustion products and the samples of commercial Al alloy doped with 0.2 wt % combustion-synthesized MAs were characterized by optical microscopy, XRD, SEM, and EDS. The effect of added MAs was explored by optical microscopy using Clemex Vision software to determine the ASTM grain size number (G) and percentage of intermetallic compounds in doped Al alloys. A higher microhardness (97.64 H_V 0.05) was observed for the Al specimens doped with MA nominally containing 25 mol % TiB₂.     

Reaction mechanism in Ti–SiC–C powder mixture during pulse discharge sintering

During pulse discharge sintering (PDS) of Ti/SiC/C powder mixture, combustion synthesis reactions occurred at heating rates above 20 °C/min. With an increase in heating rate, combustion synthesis occurred at higher temperatures. The essential of this combustion reaction is the liquid reaction between Ti and formed Ti₅Si₃. The exothermic TiC formation during PDS process promotes this liquid reaction. We have found that the combustion reactions alone did not finish the formation reactions for Ti₃SiC₂, and further heating following the combustion reactions is necessary for the synthesis process of Ti₃SiC₂.     

Anodic oxidation behaviour of Al–Ti alloys in acidic media

The anodic oxidation behaviour of Al–Ti alloys in several acidic solutions was investigated. The influence of conditions such as alloy composition, electrolytic solution, electrolyte temperature and formation current density on the formation rate of oxides on Al–Ti alloys and the dielectric properties of the anodic films were analysed. It was shown that the oxide formation rate for the Al–Ti alloy containing 54 at % aluminium was the highest and the dielectric property of its anodic oxide was also the best. In addition, by means of several surface analytical techniques, the chemical composition of the films were determined as (TiO₂) _n (Al₂O₃) _m . AES (Auger electron spectroscopy) profiling analysis data showed that Al–Ti alloys had preferential oxidation behaviour, that is, the aluminium was oxidized preferentially.     

Investigation of Alumina Wetting by Fe–Ti,Fe–P and Fe–Ti–P Alloys

Abstract

The wetting of alumina substrates by Fe–Ti, Fe–P and Fe–Ti–P alloys has been investigated using sessile drop experiments conducted under an inert gas atmosphere in the temperature range of 1550 to 1620°C. The surface and interfacial structures have been explored by scanning electron microscopy and energy dispersive X-ray spectroscopy. Substantial additions of titanium are known to induce steel melts to wet alumina due to the formation of a Ti-rich reaction product at the alloy/ceramic interface, but the present work has shown that even low Ti concentrations can induce a reactive wetting process leading to an improvement of the wettability of alumina by Fe alloys. The contact angle of molten steel containing phosphorus on alumina decreased with increasing P content. The improvement of the wetting behaviour in this system was attributed solely to the adsorption of P onto the surface of the Fe melt. The addition of P as a ternary alloying element to the system Fe–Ti proved to be beneficial to the wetting behaviour. The measured contact angles were much lower than those in the binary systems Fe–Ti and Fe–P. This effect was related to the fact that P enhances the activity of Ti in the Fe melt. According to experimental observations, it turns out that the wettability of liquid Fe-based alloys, when an Al2O3 surface is present, is not only a property of the metal/oxide couple but is also dependent on the oxygen partial pressure, whereas temperature variations bring about a comparatively small effect. This work is of interest in understanding the phenomena pertaining to inclusion engineering and steel– refractory interactions, such as the clogging of submerged entry nozzles by agglomerated alumina particles during the continuous casting process.     

The Effect of Si Substitution for SiC on SHS in the Ti–Si–C System

Investigated was the effect of Si substitution for SiC on SHS in the Ti–Si–C system. Starting powders were intermixed to obtain 3Ti–SiC–C and 3Ti–Si–2C green mixtures and then green compacts by uniaxial pressing. The influence of heating rate, reactor temperature, and replacement of SiC by Si was studied by XRD, SEM, and TEM. In combustion products obtained in optimized conditions, Ti₃SiC₂ was found to be predominant. In comparison with conventional methods, our products obtained in a one-step low-temperature process contained minimal amounts of undesired impurities and required no finishing processes such as chemical purification.     

Evaluation of corrosion behaviour of Ti–25Mo alloy in chloride medium

The corrosion behaviour of Ti–25Mo alloy in 0.9 wt% NaCl was evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and compared with that of commercially pure titanium (CP-Ti). The corrosion behaviour of Ti–25Mo alloy has been reported for the first time in this paper. The microstructure and structural characteristics were also examined using optical microscopy and X-ray diffraction (XRD), respectively. The study reveals that Ti–25Mo alloy possesses a β-phase microstructure. The Ti–25Mo alloy exhibits higher passivation range, lower average passive current density (i_pass) and higher charge transfer resistance (R_ct) compared to that of CP-Ti. Based on the corrosion protection ability, Ti–25Mo alloy can be a suitable alternative material for orthopaedic implant applications.     

Extraction of Ti Powder from Ti–MgO–Mg(–CaO) Cakes Produced by Magnesiothermic Reduction

The extraction of Ti powder from SHS-produced Ti–MgO–Mg(–CaO) cakes by treatment in leaching solutions (HNO₃, HCl, and NH₄Cl) was explored and optimized in relation to such factors as concentration of leaching agent, leaching temperature, chemical resistance of target Ti powder, and extent of byproducts extraction. The type of leaching solution was found to affect the size, structure, and morphology of resultant Ti powder. Best results were obtained at 70°C with aqueous solutions of: (1) the nitric acid taken in a 6-fold excess to the Mg content of combustion product and (2) the ammonium chloride taken in a 20-fold excess to nominal Mg content.     

Ablation behavior of (Hf–Ta–Zr–Nb–Ti)C high-entropy carbide and (Hf–Ta–Zr–Nb–Ti)C–xSiC composites

The ablation behavior of (Hf–Ta–Zr–Nb–Ti)C high-entropy carbide (HEC-0) was investigated using a plasma flame in air for different times (60, 90, and 120 s) at about 2100°C. The effect of SiC content on the ablation resistance of HEC–xSiC composites (x = 10 and 20 vol%) was also studied. The linear ablation rate of HEC-0 decreases with increasing ablation time, showing the positive role of the oxide layer with a complex composition. The linear ablation rate of HEC–10 vol% SiC (0.3 µm s⁻¹) is only a 10th of that of HEC-0, showing a significant improvement in ablation resistance, probably due to the formation of a protective oxide layer containing melted SiO₂ and refractory Hf–Zr–Si–O oxides.     

Ti和NiTi表面Ni–Ti–O纳米层的显微结构及电催化性能

采用水热合成法在钛片和镍钛片表面制备了多种形貌的Ni–Ti–O纳米层。采用扫描电镜(SEM)和透射电镜(TEM)观察Ni–Ti–O纳米层的微观形貌。用循环伏安法初步研究了Ni–Ti–O纳米层对甲醇的催化氧化能力。结果表明:水热合成过程中溶液的Ti/Ni浓度比和基体材料种类对产物的微观形貌有重要影响。随着溶液中镍浓度的升高,纯Ti片上纳米层颗粒逐渐变小,分布变得均匀;NiTi片表面的纳米层则从纳米线束状结构转变为Ni–Ti–O纳米片结构。当溶液中无TiCl_4时,Ti片表面的产物为垂直于其生长的纳米薄片。与纯钛片相比,以镍钛合金片作为基体制备的电极对甲醇的电催化性能更好。     

Cathodes for chlorate electrolysis with nanocrystalline Ti–Ru–Fe–O catalyst

Cathodes for chlorate electrolysis were prepared by mixing nanocrystalline Ti–Ru–Fe–O catalyst powder with small amounts of Teflon and subsequent hot pressing on a carbon–Teflon sublayer. Initially, the electrode materials were characterized by SEM, EDX, XRD and BET measurements. The behaviour of electrodes with catalyst loadings from 300 mg cm^–2 reduced to 10 mg cm^–2 was investigated in chlorate electrolyte with pH 6.5 and in part, for comparison, in 1 M sodium hydroxide solution at 70 C. Several methods have been used: cyclic voltammetry for the determination of double layer capacitance, Tafel plot analysis, cathodic potentiodynamic polarization and potentiostatic tests at i = –250 mA cm^–2. The as-milled catalyst powder electrodes showed a high activity for the HER in chlorate electrolyte particularly expressed in low overpotentials of about 580 mV at –250 mA cm^–2 for catalyst loadings down to 20 mg cm^–2 and high double layer capacitances in the freshly prepared state. These electrodes show increased activity at low polarization. The long-term stability during electrolysis was also analysed.     

Corrosion Behavior of PM Processed Ti–Ca–P Bioceramic Composites in Hank's Balanced Salt Solution Using Potentiodynamic Studies

Ti–Ca–P bioceramic composites for load bearing implants developed by a new powder metallurgy processing technique were studied for their electrochemical corrosion properties. For determining corrosion behavior of such composites having in situ formed bioactive Ca–P phases, potentiodynamic and studies were conducted in simulated body fluid namely Hank's balanced salt solution. Potentiodynamic polarization tests showed no evidence of pitting corrosion. Corrosion potentials (−0.27 to −0.53 V) and corrosion rates (0.17–4.46 mills per year) of Ti–Ca–P bioceramic composite samples were superior to earlier reported results for coated Ti implants due to the formation of passive layer of bone-like calcium phosphate on the sample surface.     

Hydrogen-storage properties of MgH2–10Ni–2NaAlH4–2Ti–2CNT milled in a planetary ball mill under H2

A sample with a composition of 84 wt% MgH₂–10 wt% Ni–2 wt% NaAlH₄–2 wt% Ti–2 wt% CNT (named MgH₂–10Ni–2NaAlH₄–2Ti–2CNT) was prepared by milling in a planetary ball mill under H₂. Activation of the sample was not required. At the first cycle, the sample absorbed 3.75 wt% H for 10 min, and 4.17 wt% H for 60 min at 593 K under 12 bar H₂. Reactive mechanical grinding of Mg with Ni, NaAlH₄, Ti, and CNT is thought to create defects on the surface and in the interior of Mg, as well as to reduce Mg particle size.     

Selective oxidation of ethane over hydrothermally synthesized Mo–V–Al–Ti oxide catalyst

Mo–V-based oxide catalysts are known highly active for various hydrocarbon selective oxidations. Particularly those which are monophasic giving XRD diffraction at d=4 Å are extremely active for alkane oxidations. We succeeded to synthesize this unique monophasic material by hydrothermal method and obtained Mo₆V₂Al₁O_x mixed oxide catalysts which showed activities for gas-phase ethane oxidation to ethene and acetic acid. The addition of titanium to the Mo₆V₂Al₁O_x oxide catalyst was found to result in a marked increase of the activity for the ethane selective oxidation, which was due to the morphological change of the catalyst particles and the increase of surface area by the addition of titanium. During the heat-treatment above 550°C under a nitrogen stream, the structural phase of the Mo₆V₂Al₁Ti_0.5O_x catalyst giving the XRD diffraction at d=4 Å transferred to (Mo_0.93V_0.07)₅O₁₄-like phase with drastic decreases of the oxidation activity and the surface area. On the basis of kinetic data and the fact that the lower reaction temperature and the existence of water vapor in the feed facilitated the formation of acetic acid, it is concluded that the breaking of C–H bond of ethane is the rate determining of the oxidation and acetic acid do not form through ethene.     

无压烧结制备Ti–Al–C系三元层状陶瓷

以TiC粉、Al粉、Ti粉为原料,采用无压烧结工艺制备高纯Ti–Al–C三元层状陶瓷,探究了烧结温度、烧结时间、烧结助剂等对Ti–Al–C系三元层状陶瓷制备的影响。结果表明:在一定范围内提高烧结温度和烧结时间能减少杂质相的产生,不添加助剂情况下在1 400℃下保温3 h能得到80%(质量分数)以上的Ti–Al–C系三元层状陶瓷,该条件下掺入少量Si粉或Sn粉能得到高纯Ti–Al–C系三元层状陶瓷。TiC、Al、Ti和Si质量比为2.0:1.2:1.0:0.1的原料粉末在1 400℃保温3 h能得到纯度99%以上的Ti_3AlC_2陶瓷,TiC、Al、Ti和Sn质量比为2.0:1.2:1.0:0.1与TiC、Al、Ti和Sn质量比为1.0:1.2:1.0:0.1的原料粉末在1 400℃保温3 h均能制备出纯度99%的以Ti_3AlC_2为主晶相的Ti_3AlC_2/Ti_2AlC复相陶瓷。     

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.