X-ray diffraction studies of thermal properties of the core and surface shell of isolated and sintered SiC nanocrystals (original) (raw)
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Journal of Materials Research, 2007
Microstructure of sintered nanocrystalline SiC is studied by x-ray line profile analysis and transmission electron microscopy. The lattice defect structure and the crystallite size are determined as a function of pressure between 2 and 5.5 GPa for different sintering temperatures in the range from 1400 to 1800°C. At a constant sintering temperature, the increase of pressure promotes crystallite growth. At 1800°C when the pressure reaches 8 GPa, the increase of the crystallite size is impeded. The grain growth during sintering is accompanied by a decrease in the population of planar faults and an increase in the density of dislocations. A critical crystallite size above which dislocations are more abundant than planar defects is suggested.
The thermal expansion of 3C–SiC in TRISO particles by high temperature X-ray diffraction
Journal of Nuclear Materials, 2013
The lattice parameter change of SiC in TRISO particles prepared by chemical vapour deposition (CVD) was measured using high temperature X-ray diffraction, across a temperature range of 25 to 1400°C. Al 2 O 3 was used as the internal standard and the SiC temperature corrections were calibrated using its two independent lattice parameter values along the a-and c-axes. Experimental unit cell values of SiC at low temperatures corresponded well with those published in previous literature, but deviated systematically at higher temperatures. Thermal expansion coefficients of the CVD prepared SiC shell material are considered the most accurate and follow a linear trend with increasing temperature (α 11 = 2.7706 x 10-9 T + 3.3048 x 10-6 K-1). The TRISO particles are described best using non-linear expansion coefficients. Apparent is the deviation in the SiC lattice constants of the shell material and the TRISO particles from which it originated. This could indicate a residual strain in the TRISO particles or a difference in sample displacement between the TRISO particles and the surrounding alumina standard. The room temperature lattice constant for the shell material is 4.36030Å (SD 0.00006Å) as compared with that of the TRISO sample of 4.35835Å (SD 0.00006Å) after adjusting the sample displacement to get alumina lattice constants as close to the accepted values. Residual stress of ~300MPa is calculated from the lattice constant differences.
2021
αand β-SiC starting powders of similar particle sizes were used to investigate the effect of initial α-phase content on the electrical, thermal, and mechanical properties of pressureless solid-state sintered (PSS) SiC ceramics with B4C and C. For β-SiC starting powders, a coarse-grained microstructure with elongated platelet grains formed by the 3C to 6H to 4H-SiC phase transformation was obtained. In contrast, α-SiC powders exhibited a fine-grained microstructure with platelet grains. The electrical resistivity decreased by an order of magnitude with increasing initial α-phase content presumably due to (1) an increased 6H-SiC content causing a decrease in bandgap energy and (2) the low soluble impurity content (Fe and V) of the α-SiC powders. The thermal conductivity increased by approximately 32% with increasing initial α-phase content due to (1) an increased 6H-SiC content, which has a higher intrinsic thermal conductivity compared to 4H and (2) the low impurity content of the α-...
The Prediction of the Bulk Modulus and its Temperature-Derivative of the Crystalline β-SiC Ceramic
The prediction of the isothermal bulk modulus BT and the temperature-derivative (∂BT/∂T)p of crystalline silicon carbide (β -SiC) from experimental data are presented in this work. Using the experimental measurements of the coefficient of thermal expansion found by Z. Li and R. C. Bradt and the empirical value of the Anderson parameter, the variations in bulk modulus as a function of temperature and pressure are traced. Both BT and its temperature-derivative were calculated within the temperature range of 273-1573K. The bulk modulus has been found to decrease by 2.4% from its value at 273K upon heating to a temperature of 1573K. The temperature derivative of the bulk modulus decreases from − 0.003 GPa/K at 273K to a minimum of -0.0046 at Debye's temperature (1173K), and then increases again as the temperature increases. The bulk modulus has been also calculated by thermodynamical equations for the purpose of comparison, where the empirical and the thermodynamical results match t...
Various Thermal Coefficients İnvestigation of 3C-Sic Nanoparticles at the Different Heating Rates
2021
Several thermal parameters were analyzed for nanocrystalline silicon carbide (3C-SiC) particles at the performed depending on the thermal processing rate. The hydroxyl groups on the surface of nanocrystalline 3C-SiC particles have been investigated as a function of temperature and heating rate. Specific heat capacity and Gibbs energy of silicon carbide nanoparticles have been determined in the temperature range of 300 ÷ 1270K at the various heating rates. The enthalpy and the entropy were calculated at different thermal processing rates (theoretical calculations are confirmed based on experimental results). Experminetal results obtained for all thermophysical parameters were comparatively studied at different thermal processing rates.PACS: 61.46.+w, 65.80.+n, 67.80.Gb
Zeitschrift für Kristallographie Supplements, 2006
Thermal atomic motions of nanocrystalline SiC were characterized by two temperature atomic factors B core and B shell. With the use of wide angle neutron diffraction data it was shown that at the diffraction vector of above 15 Å-1 the Wilson plot gives directly the temperature factor of the grain interior (B core). At lower Q values the slope of the plot provides information on the relative amplitudes of vibrations of the core and shell atoms.
Journal of Nuclear Materials, 1997
In this, the second part of a theoretical study of the thermal properties of crystalline b-SiC, the thermal conductivity is calculated by using molecular dynamics simulation to evaluate directly the heat current correlation function and thus, obtain the conductivity through the Green-Kubo expression in linear response theory. Adopting the same empirical potential model and the temperature scaling method as in part one, we predict absolute conductivity values for a perfect crystal which are in Ž . satisfactory agreement with available data, except in the low-temperature region below 400 K where quantum effects become dominant. The effects of carbon and silicon vacancies and antisite defects are studied by introducing a single defect into the simulation cell, allowing the atomic configuration to relax, and then performing heat capacity, thermal expansion and conductivity calculations. We find that the heat capacity and thermal expansion coefficient are affected very little by point defects even at a high concentration of 0.5%. On the other hand, the thermal conductivity is observed to degrade markedly as a result of the greatly enhanced decay of the heat current correlation, clearly attributable to the dominant mechanism of defect scattering of phonons. The defect simulations also reveal that the conductivity becomes essentially temperature independent. Both characteristics appear to have correspondence with observations on conductivity behavior in neutron-irradiated specimens. q
Monolithic nanocrystalline SiC ceramics
Journal of the European Ceramic Society, 2016
Additive-free -SiC nanopowders were densified by using high-pressure "anvil-type with hollows" apparatus at the pressure of 4 GPa in the range of 1500-1900 • C. The starting powder with average particle size of 10 nm was synthesized by a sol-gel process. Crystallite size and lattice parameters of the samples have been studied at room temperature by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that the size of the crystallites gradually increases from 16 to 51 nm with increasing sintering temperature (1500 to 1900 • C). Fully densified sample (>99%) was obtained at a sintering temperature of 1900 • C for 60 s. This sample exhibits nano-hardness and Young's modulus of elasticity of 35 GPa and 450 GPa, respectively. Modified vibratory cavitation test method was used for laboratory testing of the cavitation resistance. A very low erosion level with mass loss 0.1% after 10 h was exhibited during the cavitation test.
Thermophysical properties of porous SiC ceramics fabricated by pressureless sintering
Science and Technology of Advanced Materials, 2007
Highly porous SiC with approximately 30-41% porosity was fabricated by pressureless sintering without sintering additives at temperatures in the range 1700-2000 1C. Thermal diffusivities, specific heats, thermal conductivities and thermal resistivities of sintered samples are reported for temperatures from room temperature to 1000 1C. The thermal diffusivities and thermal conductivities of all samples decreased significantly with increasing temperature over this range, whereas specific heats and thermal resistivities increased. At any given temperature, the greater the porosity of the SiC, the lower the thermal conductivity.