Modification of Liquid Silicone Rubber by Octavinyl-polyhedral Oligosilsesquioxanes and Silicon Sol

To develop an efficient and bio-compatible way to improve the thermal and mechanical properties of addition type liquid silicone rubber（LSR）, a series of modified LSR samples were prepared by introducing octavinyl-polyhedral oligosilsesquioxanes（VPOSS） and high purity silicon sol singly or in combination before vulcanization. Significant correlation was found between the loading rate of VPOSS and thermal properties. However, mechanical properties were negatively correlated with VPOSS content within the range experimented, which may be ascribed to material defect caused by uneven distribution and aggregation. Furthermore, test results approved that the introducing of silicon sol indeed affected the stabilities of the polymer by restraining the material defect caused by the aggregation of POSS molecules and improving cross link density. For example, adding 10%-20% of silicon sol into VPOSS（1.0%） modified LSR will increase tear resistance by 43.9%-85.7%, elongation at break by 31.7%-57.3%, residue at 800 ℃ in N2 atmosphere by 32.0%-37.9%, residue at 650 ℃ in air atmosphere by 70.9%-91.6%, respectively. This work proves that, to incorporate VPOSS into LSR by hydrosilylation, and to use silicon sol as dispersant and reinforce filler can become an efficient way to improve the mechanical property, thermal stability and bio-compatibility of LSR in the future.

Influence of Yb_2O_3-MgO on Mechanical Properties and Thermal Conductivity of Silicon Nitride Ceramics via Gas Pressure Sintering

In this work,Yb2O3 and Mg O were used as sintering aids in preparing silicon nitride ceramics by gas pressure sintering（ 0. 6 MPa N2atmosphere） to investigate how the amounts of Yb2O3- Mg O influence the mechanical properties and thermal conductivity of silicon nitride ceramics. The total contents of Yb2O3- Mg O added were 1 mol%,2 mol%,4 mol%,6 mol%,8 mol%,10 mol%,12 mol%,14 mol%,keeping the Yb2O3-Mg O molar ratio of 1 ∶ 1 steadily. Curves of the relative density,thermal conductivity and bending strength plotted against the aids content present a ‘mountain＇shape with a maximum at nearly 10 mol% aids. The fracture toughness increased with the amounts of additives up to10 mol% and decreased slightly thereafter. The mechanical properties and thermal conductivity were almost proportional to the amount of the additives before10 mol%. When the content of aids exceeded 10 mol%,it would weaken the mechanical properties and thermal conductivity of the ceramics. The optimum content of Yb2O3- Mg O was 10 mol% by gas pressure sintering（ 0. 6MPa） at 1 850 ℃ for 4 h,which led to a relative density of 98. 9%,a flexural strength of（ 966 ± 38）MPa as well as a fracture toughness of（ 6. 29 ± 0. 29）MPa·m1 /2and thermal conductivity of 82 W /（ m·K）.

Effects of thermal transport properties on temperature distribution within silicon wafer

A combined conduction and radiation heat transfer model was used to simulate the heat transfer within wafer and investigate the effect of thermal transport properties on temperature non-uniformity within wafer surface. It is found that the increased conductivities in both doped and undoped regions help reduce the temperature difference across the wafer surface. However, the doped layer conductivity has little effect on the overall temperature distribution and difference. The temperature level and difference on the top surface drop suddenly when absorption coefficient changes from 104 to 103 m-1. When the absorption coefficient is less or equal to 103 m-1, the temperature level and difference do not change much. The emissivity has the dominant effect on the top surface temperature level and difference. Higher surface emissivity can easily increase the temperature level of the wafer surface. After using the improved property data, the overall temperature level reduces by about 200 K from the basis case. The results will help improve the current understanding of the energy transport in the rapid thermal processing and the wafer temperature monitor and control level.

Thermoelastic stresses in SiC single crystals grown by the physical yapor transport method

A finite element-based thermoelastic anisotropic stress model for hexagonal silicon carbide polytype is developed for the calculation of thermal stresses in SiC crystals grown by the physical vapor transport method. The composite structure of the growing SiC crystal and graphite lid is considered in the model. The thermal expansion match between the crucible lid and SiC crystal is studied for the first time. The influence of thermal stress on the dislocation density and crystal quality is discussed.

Microstructure and properties of electronic packaging shell with high silicon carbide aluminum-base composites by semi-solid thixoforming

The electronic packaging shell with high silicon carbide aluminum-base composites was prepared by semi-solid thixoforming technique. The flow characteristic of the Si C particulate was analyzed. The microstructures of different parts of the shell were observed by scanning electron microscopy and optical microscopy, and the thermophysical and mechanical properties of the shell were tested. The results show that there exists the segregation phenomenon between the Si C particulate and the liquid phase during thixoforming, the liquid phase flows from the shell, and the Si C particles accumulate at the bottom of the shell. The volume fraction of Si C decreases gradually from the bottom to the walls. Accordingly, the thermal conductivities of bottom center and walls are 178 and 164 W·m-1·K-1, the coefficients of thermal expansion(CTE) are 8.2×10-6 and 12.6×10-6 K-1, respectively. The flexural strength decreases slightly from 437 to 347 MPa. The microstructures and properties of the shell show gradient distribution.

Preparation and properties of polycrystalline silicon seed layers on graphite substrate

Polycrystalline silicon（poly-Si） seed layers were fabricated on graphite substrates by magnetron sputtering. It was found that the substrate temperature in the process of magnetron sputtering had an important effect on the crystalline quality,and 700℃was the critical temperature in the formation of Si（220） preferred orientation. When the substrate temperature is higher than 700℃,the peak intensity of X-ray diffraction（XRD） from Si（220） increases distinctly with the increasing of substrate temperature.Moreover,the XRD measurements indicate that the structural property and crystalline quality of poly-Si seed layers are determined by the rapid thermal annealing （RTA） temperatures and time.Specifically,a higher annealing temperature and a longer annealing time could enhance the Si（220） preferred orientation of poly-Si seed layers.