Thermal expansion

About: Thermal expansion is a research topic. Over the lifetime, 21040 publications have been published within this topic receiving 349407 citations. The topic is also known as: heat expansion.

The thermal expansion of wood cellulose crystals

Abstract: We have investigated tension wood cellulose obtained from Populus maximowiczii using X-ray diffraction at temperatures from room temperature to 250 °C. Three equatorial and one meridional d-spacings showed a gradual linear increase with increasing temperature. For temperatures above 180 °C, however, the equatorial d-spacing increased dramatically. Thus, the linear and volume thermal expansion coefficients (TECs) below 180 °C were determined from the d-spacings. The linear TECs of the a-, b-, and c-axes were: α a = 13.6 × 10−5 °C−1, α b = −3.0× 10−5 °C−1, and α c =0.6× 10−5 °C−1, respectively, and the volume TEC was β = 11.1× 10−5 °C−1. The anisotropic thermal expansion in the three coordinate directions was closely related to the crystal structure of the wood cellulose, and it governed the macroscopic thermal behavior of solid wood.

Ground-state properties and high-pressure behavior of plutonium dioxide: Density functional theory calculations

Abstract: Plutonium dioxide is of high technological importance in nuclear fuel cycle and is particularly crucial in long-term storage of Pu-based radioactive waste. Using first-principles density-functional theory, in this paper we systematically study the structural, electronic, mechanical, thermodynamic properties, and pressure induced structural transition of PuO$_{2}$. To properly describe the strong correlation in the Pu $5f$ electrons, the local density approximation$+U$ and the generalized gradient approximation$+U$ theoretical formalisms have been employed. We optimize the $U$ parameter in calculating the total energy, lattice parameters, and bulk modulus at the nonmagnetic, ferromagnetic, and antiferromagnetic configurations for both ground state fluorite structure and high pressure cotunnite structure. The best agreement with experiments is obtained by tuning the effective Hubbard parameter $U$ at around 4 eV within the LDA$+U$ approach. After carefully testing the validity of the ground state, we further investigate the bonding nature, elastic constants, various moduli, Debye temperature, hardness, ideal tensile strength, and phonon dispersion for fluorite PuO$_{2}$. Some thermodynamic properties, e.g., the Gibbs free energy, volume thermal expansion, and specific heat, are also calculated. As for cotunnite phase, besides the elastic constants, various moduli, and Debye temperature at 0 GPa, we have further presented our calculated electronic, structural, and magnetic properties for PuO$_{2}$ under pressure up to 280 GPa. A metallic transition at around 133 GPa and an isostructural transition in pressure range of 75-133 GPa are predicted.

Development of rapid heating and cooling systems for injection molding applications

Abstract: The injection molding process has several inherent problems associated with the constant temperature mold. A basic solution is the rapid thermal response molding process that facilitates rapid temperature change at the mold surface thereby improving quality of molded parts without increasing cycle time. Rapid heating and cooling systems consisting of one metallic heating layer and one oxide insulation layer were investigated in this paper. Design issues towards developing a mold capable of raising temperature from 25°C to 250°C in 2 seconds and cooling to 50°C within 10 seconds were discussed. To reduce thermal stresses in the layers during heating and cooling, materials with closely matched low thermal expansion coefficient were used for both layers. Effects of various design parameters, such as layer thickness, power density and material properties, on the performance of the insert were studied in detail with the aid of heat transfer simulation and thermal stress simulation. Several rapid thermal response mold inserts were constructed on the basis of the simulation results. The experimental heating and cooling response agrees with the simulation and also satisfies the target heating and cooling requirement.

Positron trapping in solid and liquid metals

Abstract: Measurements of the peak counting rate in the angular correlation curve of positron annihilation radiation have been performed in the solid and liquid phase of indium, lead, and aluminum, and in the solid phase of magnesium, as a function of temperature. In the solid phase In, Pb, and Al show at low temperatures the thermal-expansion effect and at higher temperatures in addition the positron-trapping effect. At the melting point the peak counting rate increases abruptly and stays then constant with temperature. This behavior in the liquid phase is interpreted as "saturation trapping," i.e., all positrons annihilate from a trapping site and the thermal expansion of the lattice is not effective for these positrons. Trapped positrons are effectively "shielded" from the thermal expansion of the lattice. Therefore, it is possible to separate the thermal-expansion effect from the vacancy-trapping effect and very accurate values for the monovacancy formation energy can be obtained. A difference was found in the nature of trapping sites in solid and liquid aluminum. Magnesium shows only the thermal-expansion effect and no vacancy-trapping effect.