Papers by Keyword: Thermal Expansion

Abstract: High speed ball-screw system has serious friction heat to form thermal expansion to each component. An analyzing model considering with contact deformation and thermal expansion is established in realizing positioning error for a high speed ball-screw system. A finite element model of nut is also built in calculating elongation of nut. Surface strain of nut is measured by strain gages in order to confirm with data obtained from finite element model. Temperature of nut and screw were also measured by thermal couples and are used in calculation of elongation by the use of linear elongation equation. The tendency of positioning error is well estimated by the analyzing model. The model can be used in feedback positioning control factor and develop precision high speed ball-screw system.

Abstract: Examples of UNiAlD_2.1 and UCoGe illustrate that the soft crystallographic direction coincides quite generally with the shortest U-U links in U intermetallics. It leads to a simple rule, that the easy magnetization direction and the soft crystallographic direction (in the sense of highest compressibility under hydrostatic pressure) must be mutually orthogonal.

Abstract: Low-carbon Al2O3-C refractories were prepared using white fused corundum, α-Al2O3 powders and flake graphite as main raw materials. The critical particle sizes of corundum selected in this experiment were 0.5mm, 1mm and 2mm. The effects of corundum critical particle size on physical, mechanical and thermo-mechanical properties of low-carbon Al2O3-C refractories were investigated. The results show that the increase of critical particle size is conducive to the improvement of thermal shock resistance and fracture energy, but little effect on thermal expansion. The cold modulus of rupture after thermal shock test of samples using 0.5mm critical particle size corundum was 2.09MPa, while using 2mm critical particle size corundum was 2.98MPa. And the fracture energy increased from 265N/mm to 588N/mm when the critical particle size increased from 0.5mm to 2mm. The effects of critical particle size on apparent porosity, bulk density and modulus of rupture were insignificant.

Abstract: Different dosages of BN-B₄C compound additive have been introduced for preparing specimen with fused quartz granule (d₅₀=10μm). Fused quartz ceramic materials were fabricated in reduction atmosphere at various temperatures for 1h. The characterizations were carried out by thermal expansion ratio, apparent porosity, cold modulus of rupture (CMOR) and SEM as well as by X-ray diffraction (XRD). The effect of BN-B₄C on the sintering, crystallization and properties of fused quartz was investigated. The result reveals that with increase of the temperature, the apparent porosity and CMOR of specimens decrease. The specimen C sintered at 1300°C possessed a maximum CMOR (22.31 MPa). The specimen C sintered at 1350°C has more compact microstructure. The lowest thermal expansion ratio of the specimens sintered at various temperatures was obtained when the dosages of compound additive is 3%. With increasing BN-B₄C content, the role on inhibiting crystallization was improved. The specimens containing 3 wt% BN-B₄C had obvious effect on inhibiting crystallization of the specimens sintered at various temperatures.

Abstract: Fused quartz granule (d50=10μm) was used as raw material, and Al₂O₃-AlN (1:1, in mass) was used as additive with dosages of 1 wt%, 2 wt% and 3 wt%. The crystallization characteristics of fused quartz ceramic fabricated in reduction atmosphere at 1300°C, 1350°C and 1400°C for 1h has been investigated by thermal expansion ratio and XRD. The results showed that the crystallization characteristics were markedly improved by 1 wt% AI₂O₃-AIN compound additive. 1 wt% Al2O3-AIN had obvious effect on inhibiting crystallization of the samples sintered at various temperatures. The apparent porosity, bending strength and SEM of the samples were also examined. The results showed that the samples with 1 wt% AI₂O₃-AIN had the lowest apparent porosity among the samples containing Al₂O₃-AlN, the highest bending strength, and the sample AA1 sintered at 1350°C had more compact microstructure, which indicated that 1 wt% Al₂O₃-AlN was conducive to sintering of fused quartz ceramic materials. It can be deduced that the 1 wt% Al₂O₃-AIN compound additive plays the excellent role on inhibiting crystallization and promoting sintering of fused quartz ceramic materials.

Abstract: ZrO2-ZrW2O8 diphasic composites with controllable coefficients of thermal expansion (CTEs) are synthesized by rapid in-situ reactive sintering with ZrO2 and WO3 as reactants. High density of ZrO2-ZrW2O8 composites without decomposition of ZrW2O8 is obtained with Y2O3 sintering additive. The CTEs of specimen with ZrO2 to ZrW2O8 mass ratio 1:1.0, 1:1.3, 1:1.5 and 1:2.0 are measured to be about 1.20×10−6, 0.31×10−6, -0.78×10−6 and -1.13×10−6 K−1, respectively. Raman mappings demonstrate homogenous dispersions of ZrO2 and ZrW2O8 in the ZrO2-ZrW2O8 composites. In addition to the role as sintering additive, some Y3+ cations enter the lattice to substitute Zr4+ in ZrW2O8, leading to an increase in disorder and a decrease in phase transition temperature of ZrW2O8 in the composites.

Abstract: For a long time since the anisotropy basically confined to a single crystal, used as a polycrystalline ceramic materials generally considered to be isotropic. In this paper, the anisotropic mechanical, thermal expansion coefficient and thermal conductivity of a double perovskite slab-rocksalt layer Nd₂SrAl₂O₇ was studied by first principles as an example. The method is using density functional theory (DFT) and crystal parameters, which has been used to calculate the modulus of elasticity of anisotropic in a three-dimensional space. While combined with traditional thermal conductivity theory, we have obtained the tensor of thermal diffusion, thermal conductivity in 3D space for the first time in the world within no. The results are in good agreement with the experiment. The advantage of method is avoiding the difficulty of experimental measurement, reducing the time and obtaining relatively accurate results.

Abstract: The anti-perovskite structured Mn₃XN(X=Cu,Al,Ag,Zn,Ga,Sn,In) have wide perspective and practicability with unique advantages compared with other materials as a new negative thermal expansion (NTE) material. Because of its simple preparation and unique properties of NTE, this kind of compounds aroused scientists’ attention. The metallic nitrides Mn₃Zn_1-xSn_xN (x=0.1, 0.2, 0.3, 0.4, 0.5) were prepared by solid-state sintering. The anti-perovskite compound Mn₃Zn_1-xSn_xN has a cubic crystal structure with space group Pm3m. It shows that Zn element is partial replaced by Sn element. The Sn doping in Mn₃Zn_1-xSn_xN compound can cause the thermal expansion behavior of the compound to change between positive and negative by analyzing the curve of thermal expansivity with the temperature. Mn₃Zn_0.7Sn_0.3N shows a very strong NTE. Its negative thermal expansion coefficients were -4.39×10^-4/K from 345.4 °C to 476.2 °C. In addition, the variation of the thermal expansion curve for Mn₃Zn_0.8Sn_0.2N is almost negligible with the increasing of temperature to 600 °C, exhibiting nearly zero thermal expansion behavior. Therefore, the thermal expansion of Mn₃Zn_1-xSn_xN could be tuned via different contents of Sn in Mn₃ZnN.

Abstract: The C-C stretch vibration associated with the dicarbon antisite in 4H SiC has been observed out to the fifth harmonic in the low temperature photoluminescence spectrum. The anharmonicity is accounted for reasonably well by fits to the data based on the Morse potential. We combine the observations from experiment, the analytically tractable Morse potential, and results obtained from first principles calculations on this defect to obtain an estimate of the thermal expansion coefficient of the C-C bond. This local thermal expansion coefficient is considerably smaller than the linear thermal expansion coefficient of bulk 4H SiC, in striking contrast with the recent result for the nitrogen-vacancy center in diamond that the local thermal expansion coefficient is larger than the bulk value.

Abstract: In this work, the results of a dilatometric study of alloys on the base of the Al-Fe system were presented. Thermal expansion investigations of the alloys of the Al-Fe system with concentration of Al equal to - 38, 48, 58 at.% were also presented. The alloys were obtained by classical casting technique. The thermal expansion studies of the alloys were carried out by dilatometric analysis method using a Setsys thermal analyzer made by Setaram. A linear thermal expansion coefficient α was calculated using standard methods. A temperature dependence of the α coefficient was noted. The results are an important supplement of knowledge on the alloys of the Al-Fe system.