Thermal Expansion of 3C-SiC Obtained from In-Situ X-ray Diffraction at High Temperature and First-Principal Calculations

In situ X-ray crystallography powder diffraction studies on beta silicon carbide (3C-SiC) in the temperature range 25–800 °C at the maximum peak (111) are reported. At 25 °C, it was found that the lattice parameter is 4.596 Å, and coefficient thermal expansion (CTE) is 2.4 ×10−6/°C. The coefficient of thermal expansion along a-direction was established to follow a second order polynomial relationship with temperature (α11=−1.423×10−12T2+4.973×10−9T+2.269×10−6). CASTEP codes were utilized to calculate the phonon frequency of 3C-SiC at various pressures using density function theory. Using the Gruneisen formalism, the computational coefficient of thermal expansion was found to be 2.2 ×10−6/°C. The novelty of this work lies in the adoption of two-step thermal expansion determination for 3C-SiC using both experimental and computational techniques.     

2008—2011年上海市5岁儿童患龋情况调查报告

目的:调查2008—2011年上海市5岁儿童患龋状况,为分析评价儿童口腔健康状况和服务水平提供依据。方法:采用等容量随机抽样方法,从每个区县随机抽取1所幼儿园5岁儿童(男女各半),2008、2009年各950名,2010、2011年各900名,检测患龋率、龋均、龋失补充填构成比、SiC指数。应用SPSS13.0软件包进行独立样本t检验、方差分析和χ2检验。结果:2008-2011年5岁儿童患龋率分别为63.47%、64.00%、64.89%和64.44%,龋均分别为2.96、2.99、3.23和3.09,SiC指数分别为7.10、7.20、7.74和7.37,龋失补充填构成比分别为10.93%、12.76%、16.96%和20.63%。患龋率及龋均差异无统计学意义(P>0.05),龋失补充填构成比逐年提高(P<0.01),SiC指数是龋均的2倍多。各年间的患龋率及龋均市区均低于郊区(P<0.01),龋失补充填构成比市区显著高于郊区(P<0.01),乳牙龋集中发生在1/3儿童中。结论:上海市5岁儿童乳牙患龋率、龋均指数变化不大,龋失补充填构成比逐年上升,应采取更为积极的措施,控制儿童龋高危人群,进一步改善儿童口腔健康水平。     

Sintering and Mechanical Properties of (SiC + TiCx)p/Fe Composites Synthesized from Ti3AlC2, SiC,and Fe Powders

Ceramic-particle-reinforced iron matrix composites (CPR-IMCs) have been used in many fields due to their excellent performance. In this study, using the fast resistance-sintering technology developed by our team, iron matrix composites (IMCs) reinforced by both SiC and TiC_x particles were fabricated via the addition of SiC and Ti₃AlC₂ particles, and the resulting relative densities of the sintering products were up to 98%. The XRD and EDS analyses confirmed the in situ formation of the TiC_x from the decomposition of Ti₃AlC₂ during sintering. A significant hybrid reinforcing effect was discovered in the (SiC + TiC_x)_p/Fe composites, where the experimental strength and hardness of the (SiC + TiC_x)_p/Fe composites were higher than the composites of monolithic SiC_p/Fe and (TiC_x)_p/Fe. While, under the condition of constant particle content, the elongation of the samples reinforced using TiC_x was the best, those reinforced by SiC was the lowest, and those reinforced by (SiC + TiC_x) fell in between, which means the plastic response of (SiC + TiC_x)_p/Fe composites obeyed the rule of mixture. The successful preparation of IMCs based on the hybrid reinforcement mechanism provides an idea for the optimization of IMCs.     

Precise Modeling of Thermal and Strain Rate Effect on the Hardening Behavior of SiC/Al Composite

Temperature and strain rate have significant effects on the mechanical behavior of SiC/Al 2009 composites. This research aimed to precisely model the thermal and strain rate effect on the strain hardening behavior of SiC/Al composite using the artificial neural network (ANN). The mechanical behavior of SiC/Al 2009 composites in the temperature range of 298–623 K under the strain rate of 0.001–0.1 s⁻¹ was investigated by a uniaxial tension experiment. Four conventional models were adopted to characterize the plastic flow behavior in relation to temperature, strain rate, and strain. The ANN model was also applied to characterize the flow behavior of the composite at different strain rates and temperatures. Experimental results showed that the plastic deformation behavior of SiC/Al 2009 composite possesses a coupling effect of strain, strain rate, and temperature. Comparing the prediction error of these models, all four conventional models could not provide satisfactory modeling of flow curves at different strain rates and temperatures. Compared to the four conventional models, the suggested ANN structure dramatically improved the prediction accuracy of the flow curves at different strain rates and temperatures by reducing the prediction error to a maximum of 4.0%. Therefore, the ANN model is recommended for precise modeling of the thermal and strain rate effect on the flow curves of SiC/Al composites.     

Investigation of SiC Trench MOSFETs’ Reliability under Short-Circuit Conditions

In this paper, the short-circuit robustness of 1200 V silicon carbide (SiC) trench MOSFETs with different gate structures has been investigated. The MOSFETs exhibited different failure modes under different DC bus voltages. For double trench SiC MOSFETs, failure modes are gate failure at lower dc bus voltages and thermal runaway at higher dc bus voltages, while failure modes for asymmetric trench SiC MOSFETs are soft failure and thermal runaway, respectively. The shortcircuit withstanding time (SCWT) of the asymmetric trench MOSFET is higher than that of the double trench MOSFETs. The thermal and mechanical stresses inside the devices during the short-circuit tests have been simulated to probe into the failure mechanisms and reveal the impact of the device structures on the device reliability. Finally, post-failure analysis has been carried out to verify the root causes of the device failure.     

Design and Manufacturing of Si-Based Non-Oxide Cellular Ceramic Structures through Indirect 3D Printing

Additive manufacturing of Polymer-Derived Ceramics (PDCs) is regarded as a disruptive fabrication process that includes several technologies such as light curing and ink writing. However, 3D printing based on material extrusion is still not fully explored. Here, an indirect 3D printing approach combining Fused Deposition Modeling (FDM) and replica process is demonstrated as a simple and low-cost approach to deliver complex near-net-shaped cellular Si-based non-oxide ceramic architectures while preserving the structure. 3D-Printed honeycomb polylactic acid (PLA) lattices were dip-coated with two preceramic polymers (polyvinylsilazane and allylhydridopolycarbosilane) and then converted by pyrolysis respectively into SiCN and SiC ceramics. All the steps of the process (printing resolution and surface finishing, cross-linking, dip-coating, drying and pyrolysis) were optimized and controlled. Despite some internal and surface defects observed by topography, 3D-printed materials exhibited a retention of the highly porous honeycomb shape after pyrolysis. Weight loss, volume shrinkage, roughness and microstructural evolution with high annealing temperatures are discussed. Our results show that the sacrificial mold-assisted 3D printing is a suitable rapid approach for producing customizable lightweight highly stable Si-based 3D non-oxide ceramics.     

微核实验评价碳化硅陶瓷的潜在致突变性

目的通过微核实验检测自制碳化硅陶瓷对小鼠的致突变性以评价其生物相容性。方法将小鼠分成高、中、低剂量组及阴、阳性对照组,并与纯钛作比较,每组10只,共80只,按1ml/20g体重口服同剂量的浸提液,生理盐水作为阴性对照,阳性诱变剂:注射用环磷酰胺(CP)(75mg/kg)作为阳性对照,24小时后处死小鼠,观察P/N值。结果各实验组的微核率与阴性对照比较,无显著差异,并与纯钛组相近。结论自制碳化硅对小鼠无致突变性。     

Effects of SiC nanoparticles orally administered in a rat model: Biodistribution, toxicity and elemental composition changes in feces and organs

Background

Silicon carbide (SiC) presents noteworthy properties as a material such as high hardness, thermal stability, and photoluminescent properties as a nanocrystal. However, there are very few studies in regard to the toxicological potential of SiC NPs.

Objectives

To study the toxicity and biodistribution of silicon carbide (SiC) nanoparticles in an in vivo rat model after acute (24 h) and subacute (28 days) oral administrations. The acute doses were 0.5, 5, 50, 300 and 600 mg·kg^− 1, while the subacute doses were 0.5 and 50 mg·kg^− 1.

Results

SiC biodistribution and elemental composition of feces and organs (liver, kidneys, and spleen) have been studied by Particle-Induced X-ray Emission (PIXE). SiC and other elements in feces excretion increased by the end of the subacute assessment. SiC did not accumulate in organs but some elemental composition modifications were observed after the acute assessment. Histopathological sections from organs (stomach, intestines, liver, and kidneys) indicate the absence of damage at all applied doses, in both assessments. A decrease in the concentration of urea in blood was found in the 50 mg·kg^− 1 group from the subacute assessment. No alterations in the urine parameters (sodium, potassium, osmolarity) were found.

Conclusion

This is the first study that assesses the toxicity, biodistribution, and composition changes in feces and organs of SiC nanoparticles in an in vivo rat model. SiC was excreted mostly in feces and low traces were retrieved in urine, indicating that SiC can cross the intestinal barrier. No sign of toxicity was however found after oral administration.     

Microstructure Evolution and Mechanical Properties of 20%SiCp/Al Joint Prepared via Laser Welding

SiC particles-reinforced Al matrix composites (SiC_p/AMCs) have been widely used in the aerospace structural components. In this work, 20 vol% SiC_p/2A14 joint was fabricated by laser welding technology. The effects of different laser power/welding velocity on the 20 vol% SiC_p/2A14 joint forming, microstructure evolution and mechanical properties were studied in detail. The results showed that, under the same heat input, the high power/high welding velocity was beneficial to reduce the porosity of SiC_p/2A14 joint and inhibited the formation of brittle phase of Al₄C₃. At 8 kW-133 mm/s welding parameters, the maximum tensile strength of the SiC_p/2A14 joint reached 199 MPa, which is ~64% higher than that of the SiC_p/2A14 joint prepared at 4 kW-66 mm/s welding parameters. By analyzing the fracture morphology and SEM image of SiCp/2A14 joint section, it is was found that the porosity of weld and Al₄C₃ brittle phase were the important factors limiting the strength of SiCp/2A14 joint. This work provides a reference for the process window design of laser welding SiC_p/2A14 composites.     

Effect of Reinforcement Size on Mechanical Behavior of SiC-Nanowires-Reinforced 6061Al Composites

In the present study, the effects of SiC nanowires (SiCnws) with diameters of 100 nm, 250 nm and 450 nm on the microstructure and mechanical behavior of 20 vol.% SiCnws/6061Al composites prepared by pressure infiltration were studied. It was found that the interface between SiCnws and Al matrix was well bonded, and no interface product was found. The thicker SiCnws are beneficial to improve the density. In addition, the bamboo-like and bone-like morphologies of SiCnws produce a strong interlocking effect between SiCnws and Al, which helps to improve the strength and plasticity of the material. The tensile strength of the composite prepared by SiCnws with a diameter of 450 nm reached 544 MPa. With a decrease in the diameter of SiCnws, the strengthening effect of SiCnws increases. The yield strength of SiCnws/6061Al composites prepared by 100 nm is 13.4% and 28.5% higher than that of 250 nm and 450 nm, respectively. This shows that, in nano-reinforced composites, the small-size reinforcement has an excellent improvement effect on the properties of the composites. This result has a guiding effect on the subsequent composite structure design.