Rapid and low temperature synthesis of high purity Ti2SC powder by microwave hybrid heating

Ti₂SC powders were successfully synthesised at relatively low temperature by a microwave hybrid heating method. The mixtures of Ti, C and S or TiS₂ with different molar ratios were heated to investigate the formation of Ti₂SC phase. It was confirmed that Ti₂SC with high purity was achieved using TiS₂ as sulphur source. The results indicated that the synthesis temperature of Ti₂SC was ～500°C lower, and holding time was only one-twentieth compared with those reported by other methods. The high purity Ti₂SC powders from Ti/TiS₂/C with the molar ratio of 2.85:1.15:2 obtained at 1100°C for 3?min were uniform, and the average particle size was 1–2?μm.     

Ti2SC粉体的快速合成及其反应机理

以TiS2、Ti、石墨粉(C)为原料,在200～1100 ℃温度范围内,用微波混合加热法合成Ti2SC 陶瓷粉体。探讨了反应温度对Ti2SC制备的影响,并在1000～1400 ℃温度下用无压烧结进行对比研究。采用XRD、SEM、DSC和热力学对样品的物相、形貌和反应机理进行分析。研究结果表明：利用无压烧结到1400 ℃保温15 min,样品中主相是C,只合成了少量目标相Ti2SC。而用微波混合加热, 400 ℃时,样品中主相为Ti2SC,含有大量的C。升温到800 ℃,目标相衍射峰增强,C衍射峰减弱。在 1100 ℃保温3 min,合成了单相Ti2SC 粉体材料,其颗粒平均尺寸为2-5 mm。在TiS2-Ti-C体系中,400 ℃以下,TiS2和Ti反应生成Ti-S化合物;400 ℃及以上,Ti、C和Ti-S化合物反应生成Ti2SC。     

Preparation and characterization of nano-sized LiFePO4 by low heating solid-state coordination method and microwave heating

The precursors of LiFePO₄ were prepared by low heating solid-state coordination method using lithium acetate, ammonium dihydric phosphate, ferrous oxalate and citric acid as raw materials. Olivine phase LiFePO₄ as a cathode material for lithium-ion batteries was successfully synthesized by microwave heating in a few minutes. X-ray diffraction (XRD) and transmission electron microscope (TEM) were used to characterize its structure and morphology. Cyclic voltammetry (CV) and charge-discharge cycling performance were used to characterize its electrochemical properties. The results showed that the grain size of the optimal sample was about 40-50 nm, and the as-prepared particles were homogeneous. The nano-sized LiFePO₄ obtained has a high electrochemical capacity (125 mAh g⁻¹) and stable cycle ability.     

低温固相反应合成碳酸锶纳米粉体

固相化学反应无污染、工艺简单、操作方便、易于实现工业化生产。应用低温固相化学反应原理，以氯化锶和碳酸铵为原料，经固相反应合成碳酸锶纳米粉体，研究了研磨时间、温度、沉淀剂及表面活性剂对反应的影响。试验结果用XRD与TEM进行表征，证明固相反应进行得很完全，产品为纯碳酸锶，其粒度为50－80nm。     

Synthesis of fine and high purity Cr2AlC powders by novel method

Cr₂AlC powders using Cr/Al/C and Cr/Al/Cr₃C₂ systems as raw materials were successfully synthesised by a microwave hybrid heating method for the first time. The mixtures of Cr, Al and graphite or Cr₃C₂ with different molar ratios were used to investigate the formation of Cr₂AlC phase. For Cr/Al/C with the molar ratios of 2:(1.0–1.2):1 system, Cr₂AlC with a small amount of Cr₇C₃ and Cr₂Al was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. For Cr/Al/Cr₃C₂ with the molar ratio of 1:2:1 system, high purity Cr₂AlC powders was synthesised at 1000°C for 3 min, and the average particle size was ?1?μm. The synthesis of high purity Cr₂AlC powders for short time was attributed to the combination of the hybrid heating effect and the introduction of Cr₃C₂ as carbon source. Microwave hybrid heating is a promising method for the preparation of various other MAX phases.     

Synthesis of basic aluminum sulfate assisted by microwave heating

The synthesis of basic aluminum sulfate (BAS) was promoted by the microwave heating of a mixture of aluminum sulfate, aluminum nitrate, and sodium hydroxide. The heating process was facilitated by microwaves set at different temperatures and reaction time durations. The obtained products were characterized by X-ray diffraction and scanning electronic microscopy. Crystallographic and morphological analysis revealed BAS, boehmite, or a mixture of both products, depending on the reaction conditions.     

低热固相法制备纳米氧化镍

比较了四水合乙酸镍与三种有机酸(草酸、酒石酸和乙二胺四乙酸)在室温条件下发生固相化学反应的速度,选取反应速度最快的草酸(和四水合乙酸镍)为原料,用室温固相化学反应合成出前驱配合物,进而热分解得到纳米氧化镍粉末,产物用X射线粉末衍射和透射电镜进行了表征。结果表明,产物纳米氧化镍为球形立方晶系结构,平均粒径为15nm,并初步探讨了分解温度对产物粒径大小、分散性的影响及低热固相反应制备纳米粒子的机理。     

Inorganic gels as precursors of TiO2 photocatalysts prepared by low temperature microwave or thermal treatment

A simple procedure for preparing active TiO₂ photocatalysts is presented. The starting materials were unusual TiO₂ gels formed from TiCl₄. The use of microwaves for a very short time enhanced the TiO₂ crystallinity preventing an increase of particle size and minimizing the decrease of specific surface area. This result makes this preparation very attractive. The formation of the gels was monitored through measurements of viscosity. All the samples were characterized by means of X-ray diffraction, diffuse reflectance spectroscopy and BET specific surface area measurements. The photoactivity of the samples was evaluated using the photodegradation of 4-nitrophenol in liquid–solid regimen and gaseous 2-propanol as probe reactions. Commercial TiO₂ Degussa P25 was used for the sake of comparison.     

Synthesis and photocatalytic properties of H2La2Ti3O10/TiO2 intercalated nanomaterial

H₂La₂Ti₃O₁₀/ TiO₂ intercalated nanomaterial was fabricated by successive intercalation reactions of H₂La₂Ti₃O₁₀ with n-C₆H₁₃NH₂/C₂H₅OH mixed solution and acid TiO₂ sol, followed by irradiating with a high-pressure mercury lamp. The intercalated materials possess a gallery height of 0.46 nm and a specific surface area of 31.58 m²·g⁻¹, which indicate the formation of a porous material. H₂La₂Ti₃O₁₀/TiO₂ shows photocatalytic activity for the decomposition of organic dye under irradiation with visible light and the activity of TiO₂ intercalated material was superior to the unsupported one.     

Carbonation reaction of lime, kinetics at ambient temperature

The uptake of carbon dioxide due to the carbonation reaction of Ca(OH)₂ in ambient temperature of approximately 20 °C has been studied. Different types of lime have been used and the CO₂ concentration has been varied to identify the influence of different variables on the kinetics of the reaction. A closed loop system has been developed and validated that allows measurement of the carbonation progress directly from monitoring CO₂ uptake. Thermal analysis (TA) was used to verify the degree of carbonation that reached up to 83%. Factor analysis on the data set has demonstrated that reaction speed is not dependent upon the CO₂ concentration within the limits tested. Carbonation speed depends on the specific surface of the lime. The results of this study contribute to research carried out on lime mortar carbonation models and on the carbonation process in general.     

Synthesis and reaction mechanism of Ti3SiC2 ternary compound by carbothermal reduction of TiO2 and SiO2 powder mixtures

The present study aims to investigate synthesis of Ti₃SiC₂ from TiO₂ and SiO₂ powder mixtures by carbothermal reduction method. Equilibrium TiO₂–SiO₂–C ternary phase diagram was used to predict the conditions for the formation of Ti₃SiC₂ at 1800 K under Ar atmosphere. A reactant mixture with a TiO₂:SiO₂ molar ratio of 1.5 and a C content of 68.75 mol% (26.86 wt%) was initially selected among the thermodynamically favorable reactant compositions for the experimental studies. Two different C sources, graphite flakes and pyrolytic C coating, were used to synthesize Ti₃SiC₂ at 1800 K under Ar atmosphere. When graphite flakes were used, the products contained a trace amount of Ti₃SiC₂ phase along with major TiC and minor SiC phases. Whereas, pyrolytic C coating on the oxide particles resulted in the products with much higher Ti₃SiC₂ contents owing to the close contact between the reactants. Optimal C concentration for the C coated oxide mixtures with a TiO₂:SiO₂ molar ratio of 1.5 was determined to be 30.05 wt% under the experimental conditions studied. Ti₃SiC₂ content of the products obtained from this reactant was observed to increase with reaction time to 31 wt% at 75 min beyond which it gradually decreased. XRD studies indicated that the product with the highest ternary carbide content also contained TiC and a trace amount of SiC. SEM-EDS analyses showed that this sample essentially consisted of spherical fine TiC particles and Ti₃SiC₂ nanolaminates. Equilibrium thermodynamic analysis of the TiO₂–SiO₂–C system suggested that the reaction of solid Ti₂O₃ with SiO and CO gases may play a dominant role in the formation of Ti₃SiC₂.     

Synthesis of resins as alpha-alumina precursors by the Pechini method using microwave and infrared heating

A polymeric method based on the Pechini process was successfully used to synthesize alpha-alumina. Different resins have been prepared in order to compare the effect of the heating source (microwave or conventional IR radiation) during the polyesterification reaction, the importance of the starting aluminium salt and the ratios between reactants. The resin precursor and the product powders were fully characterized in order to investigate the polymer decomposition and the characteristic of the resulting alumina. It is concluded that the control of the ethylene glycol to citric acid ratio is important to obtain weak agglomerated particles, which are required to achieve soft porous precursors. Comparing with a conventional infrared source, the 2.45 GHz-microwave heating reduces the polyesterification reaction time to a great extent followed by a solid polymeric resin formation; however, the reaction mechanisms do not change. The best final alumina powder was synthesised by applying the microwave radiation to a precursor resin obtained from the thermal reaction of the aluminium lactate with small amounts of ethylene glycol as the polymerizant agent.     

Ti3SiC2 friction material prepared by novel method of infiltration sintering

Titanium silicon carbide (Ti₃SiC₂) is a remarkable friction material for its combination of the best properties of metals and ceramics. The high purity Ti₃SiC₂ ceramic has been prepared by infiltration sintering (IS), and the effect of a small amount of Si on Ti₃SiC₂ ceramic formation was investigated. The results show that the purity of Ti₃SiC₂ ceramic could be increased significantly and the sintering time for Ti₃SiC₂ could be decreased remarkably when proper amount of Si was added in the starting mixture. The Ti₃SiC₂ sintered compact with a purity of 99.2?wt-% and a relative density of 97% was obtained by the IS from a starting mixture composed of n(Ti):n(Si):n(TiC)?=?1:0.3:2 at 1500°C with holding time of 2/3?h.     

Design of double-layer ceramic absorbers for microwave heating

We propose a guide for designing double-layer ceramic absorbers in microwave heating by optimizing the thickness based on the analysis of reflection loss (RL) of a double-layer absorber consisting of a high-loss SiC layer and a low-loss Al₂O₃ layer. The calculated reflection losses for individual layers of SiC and Al₂O₃ show that the former with a thickness of 0.0054 m has the maximum microwave absorption while the latter in the thickness range up to 0.1 m is identified as a poor microwave absorbing material with RL larger than −0.4 dB. By using a 0.0054-m-thick SiC layer as the susceptor, the absorption in the Al₂O₃ layer and of the entire double-layer absorber increases significantly. The results demonstrate that high microwave absorption throughout the heating process can only be achieved in a sample with a small thickness in which a slight absorption peak shift during heating (less than one eighth-wavelength in the medium) occurs.     

A rate equation for Ca(OH)2 and CO2 reaction in a spouted bed reactor at low gas concentrations

In the present study, the carbonation reaction of hydrated lime in semi-dry condition is investigated experimentally in a laboratory-scale spouted bed reactor. Results show that for operating conditions where the concentration of CO₂ is low, the capture efficiency is raised by increasing the inlet CO₂ concentration. Additionally, because of the inconsistency between the experimental reaction rate and the calculated values based on the previous proposed equations, a new rate equation is introduced that considers the dependency of CO₂ concentration too. To validate the proposed equation, its predictions were compared with another set of experimental data.     

LiFePO4/C cathode materials synthesized by co-precipitation and microwave heating

LiFePO₄/C cathode materials were synthesized by a combination of co-precipitation and microwave heating using polyethylene glycol (PEG) as a carbon resource and the influence of microwave heating time on the structure and electrochemical performance of the materials was also discussed. The samples were characterized by X-ray diffraction (XRD), TEM, particle-size analysis and constant current charge-discharge experiment. The results show that the LiFePO₄/C heated for 9 min has a pure olive-type phase and excellent electrochemical performance. The initial discharge capacities of this sample are 154.3, 139.7, 123.9 mAh/g at the rates 0.1C, 0.2C, 1C at room temperature, respectively, and after 20 cycles remain 152.3, 134.3, 118.5 mAh/g, respectively. __________ Translated from New Chemical Materials, 2008, 36(2): 21–24 [译自:化工新型材料]     

Microstructure and mechanical properties of Zn based composites reinforced by Ti3AlC2

30?vol.-% Ti₃AlC₂ particle reinforced Zn based (Zn–27?wt-% Al, denoted as ZA27) composites have been prepared via three different mechanically activated sintering technologies: pressureless sintering, hot pressing and combination of pressureless sintering and hot pressing (two-step sintering) technology. The relationship between sintering conditions, microstructure and mechanical properties of the 30?vol.-% Ti₃AlC₂/ZA27 composites has been investigated. The Ti₃AlC₂/ZA27 composite with enhanced tensile strength of 335?MPa and bending strength of 570?MPa was achieved by the two-step technology. The improved mechanical properties are attributed to the good bonding between reinforcing particles and matrix as well as the fine grained microstructure. Raising the sintering temperature is more efficient to improve the mechanical properties of the composites than applying pressure.     

Interface structure and wetting behaviour of Cu/Ti3SiC2 system

Wetting behaviour of a Cu/Ti₃SiC₂ system was investigated by the sessile drop technique under a vacuum atmosphere. Contact angles between Cu and Ti₃SiC₂ changed from 95 to 15° as temperatures increased from 1089 to 1270°C. Two distinct reaction layers consisting of different contents of Cu, TiC_x, Ti₃SiC₂ and Cu_xSi_y intermetallics were formed at the interface of Cu and Ti₃SiC₂. The formation of the interface layers contributes to the improvement of the wettability of the system. The dissolution of Si from theTi₃SiC₂ into the molten Cu at high temperature plays a dominant role in the wetting behaviour of Cu/Ti₃SiC₂ systems.     

Photocatalytic reduction of N2O on metal-supported TiO2 powder at room temperature in the presence of H2O and CH3OH vapor

Ag- and Cu-supported TiO₂ photocatalysts showed high activity for the reduction of N₂O to N₂ at room temperature in the presence of CH₃OH and H₂O vapor. The suppression by H₂O on the activity was not observed in the present photocatalyst system. The remarkable behavior of the Ag and Cu co-catalysts for TiO₂ photocatalysts agreed well with that of electro- and thermal catalyses. This revised version was published online in July 2006 with corrections to the Cover Date.