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21.
Weihua Shen Qing Ge Kaijie Gu Yingying Nie Linyu Jiao Zhiqing Zhu Yunjin Fang 《化学工程与技术》2020,43(12):2359-2364
The transesterification of dimethyl carbonate with phenol to methyl phenyl carbonate (MPC) was investigated on novel catalysts such as titanium diisopropoxide bis(ethyl acetoacetate) and titanium dibutoxide bis(ethyl acetoacetate) in a closed batch reactor at 185–206 °C under high pressure. The produced methanol could be removed efficiently by reactive distillation in order to overcome the equilibrium. The prepared catalysts have higher resistance to water than titanium alkoxides. Phenol conversion as high as 86.4 % with an MPC selectivity of 99.4 % was achieved under optimal reaction conditions within 9 h. Most of the catalytic activity was retained after repeated use for ten times. 相似文献
22.
I. Shepa E. Mudra M. Vojtko P. Tatarko V. Girman O. Milkovic T. Sopcak V. Medvecka J. Dusza 《Ceramics International》2018,44(15):17925-17934
The aim of the present contribution is to study the influence of the post-spinning heat - treatment of single TiO2/PVP precursor fibers on the properties and morphology of the final titanium-based microfibers. The post-spinning treatment conditions were: calcination in air at 450–600?°C and pyrolysis in argon at 1000–1700?°C. Calcination resulted in a production of anatase-rich and pure rutile fibers. The use of an alternative sintering method, the low-temperature plasma treatment, led to the crystallization of the composite Magnéli phases/polymer fibers. As a result of the same one precursor, pyrolysis at 1000?°C, the Carbon/TiO2 composite fibers were obtained. Rising the treatment temperature in inert atmosphere led to the formation of the titanium carbide fibers. The formation process and all the obtained products were characterized by differential scanning calorimetry accompanied with thermogravimetric analysis (DSC/TGA), scanning and transmission electron microscopy (SEM, TEM), X-ray diffraction (XRD), and image analysis techniques. 相似文献
23.
Zhongren Zhou Yingjie Zhang Peng Dong Yixin Hua Qibo Zhang Ding Wang Jianguo Duan Zhen Zhang 《Ceramics International》2018,44(4):3596-3605
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. 相似文献
24.
Mylena M.M. Carrijo Hannes Lorenz Carlos R. Rambo Peter Greil Nahum Travitzky 《Ceramics International》2018,44(7):8116-8124
This paper describes the development and fabrication of pastes suitable for screen printing process using Ti3SiC2 as the ceramic filler and ethyl cellulose as the binder. With the aim of obtaining high quality screen printed films, the influence of different amounts of Ti3SiC2 filler (20–40?vol%) and binder (0–5?vol%) on the rheological properties of the pastes was investigated. Samples with higher viscosity, such as pastes containing 30?vol% and 40?vol% Ti3SiC2 filler, regardless of the amount of ethyl cellulose, showed a higher printing quality compared to the samples with other compositions. The different paste compositions were screen printed onto paper-derived Al2O3 substrates containing 28.6 ± 4.8% open porosity and sintered for 1?h under an argon atmosphere at 1600?°C. X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM) analysis showed that the sintered films contained TiC as a primary phase and Ti3SiC2 as a secondary phase. The partial decomposition of Ti3SiC2 after sintering can be attributed to residual carbon from the organic additives, which decreases the thermal stability of this material. 相似文献
25.
Titanium carbide (TiC) has been widely used as reinforcement in metal matrix composites and is known to exist over a wide range of stoichiometry. In this study, the effect of C/Ti ratio on the densification kinetics, grain size, lattice parameter, hardness and elastic modulus of TiCx prepared by reactive spark plasma sintering (RSPS) is presented. Commercial purity titanium was ball milled with 5, 7.5, 10, 12.5, 15 and 17.5 wt% carbon black powder for 5 h and subjected to RSPS to prepare TiCx samples with different C/Ti ratio. Dense TiCx samples with ‘x′ ranging from 0.34 to 0.78 could be prepared by RSPS at 1400 °C. Increasing C/Ti ratio was found to increase the activation energy thereby reducing the rate of sintering and also resulted in finer grain size. The lattice parameter and the ratio of intensities of (200) to (111) peaks were correlated with the C/Ti ratio. The hardness and elastic modulus were shown to increase significantly with increase in C/Ti ratio. 相似文献
26.
Namhoon Kim Changkyu Kim Soyoung Jung Youngae Park Youngju Lee Juyeon Jo 《Food additives & contaminants. Part A, Chemistry, analysis, control, exposure & risk assessment》2018,35(7):1238-1246
Food-grade titanium dioxide (TiO2) is a common and widespread food additive in many processed foods, personal care products, and other industrial categories as it boosts the brightness and whiteness of colours. Although it is generally recognised as safe for humans, there is a growing interest in the health risks associated with its oral intake. This study quantified and identified TiO2 nanoparticles present in confectionery foods, which are children’s favourite foods, with inductively coupled plasma optical emission spectrometry (ICP-OES) and transmission electron microscopy (TEM). A reliable digestion method using hot sulphuric acid and a digestion catalyst (K2SO4:CuSO4 = 9:1) was suggested for titanium analysis. Validations of the experimental method were quite acceptable in terms of linearity, recoveries, detection limits, and quantification limits. Of all the 88 analysed foods, TiO2 was detected in 19 products, all except three declared TiO2 in their labelling. The mean TiO2 content of candies, chewing gums, and chocolates were 0.36 mg g?1, 0.04 mg g?1, and 0.81 mg g?1, respectively. Whitish particles isolated from the confectionery foods were confirmed as TiO2 nanoparticles via TEM and energy dispersive X-ray spectroscopy (EDX), in which nanosized particles (<100 nm) were identified. 相似文献
27.
Ultrasonic elliptical vibration cutting of titanium alloy Ti–6Al–4V is investigated in this research. Because products made of Ti–6Al–4V alloy are usually designed for possessing low-rigidity structures or good-quality cut surfaces, machining requirements such as low cutting forces and slow rate of tool wear need to be fulfilled for realization of their precision machining. Therefore, the ultrasonic elliptical vibration cutting is applied as a novel machining method for those products. Machinability of Ti–6Al–4V alloy by the ultrasonic elliptical vibration cutting with cemented carbide tools is examined to figure out suitable cutting conditions for precision machining of Ti–6Al–4V alloy. As experimental results, generated chips, cutting forces, and profiles of cut surfaces are indicated. A forced vibration problem occurred due to the segmented chip formation, which is also well-known in the ordinary non-vibration cutting. Therefore, characteristics of the forced vibration due to the chip segmentation are investigated in this research. Through the experiments, it is found that the frequency and magnitude of the forced vibration have relation with the average uncut chip thickness and cutting width. Especially, it is found that the averaging effect can suppress the forced vibration, i.e. the chip segmentation tends to occur randomly over the large cutting width, and hence the force fluctuations with random phases tend to cancel each other as the cutting width increases relatively against the average uncut chip thickness. Based on the investigations, a new practical strategy to suppress the forced vibration due to chip segmentation is proposed and verified. Using the proposed method significantly decreased cutting forces and good quality of surfaces are obtained when the forced vibration is suppressed compared to the ordinary non-vibration cutting results. Therefore, the results suggest that the precision machining can be realized without sacrificing the machining efficiency by increasing the width of cut and decreasing the average uncut chip thickness. 相似文献
28.
The Friction Stir Welding process for producing corner and T-joints out of 6 mm Ti–6Al–4V was developed in this effort using previous work on butt weld joints as a starting point. A limited number of corner joints were also subjected to a bending fatigue test to preliminarily assess the applicability of the process in producing fatigue critical structures. These results were also compared to predictions made by applying stress concentration factors to previously generated uniaxial butt joint test data. While additional testing is still required to obtain a higher degree of confidence in the conclusions of this study, it was found that the performance of these corner joints in fatigue could be compared to butt joint data when a geometrically based stress concentration factor is applied. Furthermore, these welded joints possessed equivalent fatigue performance relative to identical test specimens machined from wrought product forms, both bar and extrusion. Thus, from the perspective of fatigue design, this study has shown that Friction Stir Welding is able to produce structures with the same performance as currently made from wrought materials. 相似文献
29.
Fatigue crack growth behaviour of Ti–6Al–2Zr–1.5Mo–1.5V (VT-20 a near-α Ti alloy) was studied in lamellar, bimodal and acicular microstructural conditions. Fatigue crack growth tests at both increasing and decreasing stress intensity factor range values were performed at ambient temperature and a loading ratio of 0.3 using compact tension samples. Lamellar and acicular microstructures showed lower fatigue crack growth rates as compared to the bimodal microstructure due to the tortuous nature of cracks in the former and the cleavage of primary α in the latter. The threshold stress intensity factor range was highest for acicular microstructure. 相似文献
30.
《Ceramics International》2020,46(2):1668-1679
In the present study, a NiCr–Cr3C2 powder mixture was prepared by mechanical alloying and then coated on titanium aluminide substrates by the powder-fed laser cladding process using a set of optimum parameters. The high temperature oxidation behavior of the substrate and coating was studied by isothermal annealing at 900 °C for 5 h. It was found that the microstructure of the coating is composed of γ solid solution with different chromium carbide phases (Cr3C2, Cr7C3 and Cr23C6). The presence of different chromium carbides in the microstructure of coating can be attributed to the partial melting of primary Cr3C2 and the formation of non-equilibrium carbide phases during rapid cooling of laser cladding. The NiCr-chromium carbide laser cladded coating samples showed superior oxidation resistance compared to the substrate. The oxidation mechanism of both coating and substrate follow the parabolic law, where the parabolic rate constant of the coating was 20% of that of the substrate at 900 °C. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and Grazing Angle X-Ray Diffraction (GAXRD) analysis revealed that the surface of the oxide layer formed on the NiCr-chromium carbides coating and the substrate is mostly composed of Cr2O3 and TiO2, respectively. 相似文献