Search results for "Thermal Conductivity"
showing 10 items of 160 documents
Reduced thermal conductivity of TiNiSn/HfNiSn superlattices
2015
Diminution of the thermal conductivity is a crucial aspect in thermoelectric research. We report a systematic and significant reduction of the cross-plane thermal conductivity in a model system consisting of DC sputtered TiNiSn and HfNiSn half-Heusler superlattices. The reduction of $\kappa$ is measured by the 3$\omega$ method and originates from phonon scattering at the internal interfaces. Heat transport in the superlattices is calculated based on Boltzmann transport theory, including a diffusive mismatch model for the phonons at the internal interfaces. Down to superlattice periodicity of 3 nm the phonon spectrum mismatch between the superlattice components quantitatively explains the re…
Alloy-like behaviour of the thermal conductivity of non-symmetric superlattices
2017
In this work, we show a phenomenological alloy-like fit of the thermal conductivity of (A)d1:(B)d2 superlattices with d1 /= d2, i.e. non-symmetric structure. The presented method is a generalization of the Norbury rule of the summation of thermal resistivities in alloy compounds. Namely, we show that this approach can be also extended to describe the thermal properties of crystalline and ordered-system composed by two or more elements, and, has a potentially much wider application range. Using this approximation we estimate that the interface thermal resistance depends on the period and the ratio of materials that form the superlattice structure
Refrigeration bound of heat-producing cylinders by superfluid helium
2019
In this paper we go ahead in our studies on refrigeration of nanosystems by superfluid helium, as an appealing subject for future applications to computers or astronautical precision nanodevices. We first recall the effective thermal conductivity in laminar counterflow superfluid helium through arrays of mutually parallel cylinders and we discuss the conditions for the appearance of quantum turbulence around the heat-producing cylinders. We then consider the cooling of an array of heat-producing cylindrical nanosystems by means of superfluid-helium counterflow. We discuss the upper bound on heat removal set by avoidance of quantum turbulence and avoidance of phase transition to normal He I,…
In-flight temperature of solid micrometric powders during cold spray additive manufacturing
2020
Abstract During cold dynamic gas spray additive manufacturing, the thermal field within particles is generally simplified by a state of an instantaneous uniform distribution over the particle's media at any position inside and outside the De-Laval nozzle. This paper addresses critical discussions using analytical and computational analysis of the transient heat transfer within the solid particles due to the convective exchange with the flowing gas. An analytical criterion depicts conditions of instantaneous uniform temperature over the range of typical cold spray data including various particle thermal conductivity, particle size range, dragging velocities, gas nature and gas setting condit…
A study of hydrothermal convection in saturated porous media
1993
Abstract Because of its relevance to many geological and technical problems, hydrothermal convection is investigated here mainly with the aid of numerical models by a systematic analysis of the properties of this type of convection for a range of super-critical Rayleigh numbers. Calculations were performed for two-dimensional models with constant properties in a region of aspect ratio 2. The principal results in the case of temperature fixed at the impermeable top and bottom are the following for the Nusselt number Nu, the cell aspect ratio a, and the boundary layer thickness δ: Nu ≈ 1.7 R0.5, a ≈ 1.3 R−0.4, δc ≈ 0.4 R−0.4 for 2.5 R = R f /R f ∗ and Rf and R f ∗ are the ambient and critical…
Design and Experimental Test of a Thermomagnetic Motor
2012
Abstract This paper presents a Thermomagnetic Motor. The design of the motor is based on a thermal-magnetic coupled dynamic model, which is obtained by assuming the use of a ferromagnetic material working at temperatures near the curie point. The motor is modeled in terms of both its magnetic as well thermal properties (magnetic permeability and thermal conductivity) and the thermal processes are supposed to be influenced by the thermal conductivity, the convection and the advection. An analytical expression of the generated torque, which links this quantity to the magnetic, thermal and geometrical parameters of the generated torque is given. A design of a machine, based on this theory is p…
Fine tuning of thermoelectric performance in phase-separated half-Heusler compounds
2015
Two successful recipes to enhance the thermoelectric performance, namely carrier concentration optimization and reduction of thermal conductivity, have been combined and applied to the p-type (Ti/Zr/Hf)CoSb1−xSnx system. An intrinsic micrometer-scale phase separation increases the phonon scattering and reduces the lattice thermal conductivity. A substitution of 15% Sb by Sn optimizes the electronic properties. Starting from this, further improvement of the thermoelectric properties has been achieved by a fine tuning of the Ti to Hf ratio. The microstructuring of the samples was studied in detail with high-resolution synchrotron powder X-ray diffraction and element mapping electron microscop…
Epitaxial growth and thermoelectric properties of TiNiSn and Zr0.5Hf0.5NiSn thin films
2011
Abstract Due to their exceptional thermoelectric properties Half-Heusler alloys like MNiSn (M = Ti,Zr,Hf) have moved into focus. The growth of single crystalline thin film TiNiSn and Zr 0.5 Hf 0.5 NiSn by dc magnetron sputtering is reported. Seebeck and resistivity measurements were performed and their dependence on epitaxial quality is shown. Seebeck coefficient, specific resistivity and power factor for Zr 0.5 Hf 0.5 NiSn at room temperature were measured to be 63 μV K − 1 , 14.1 μΩ m and 0.28 mW K − 2 m − 1 , respectively. Multilayers of TiNiSn and Zr 0.5 Hf 0.5 NiSn are promising candidates to increase the thermoelectric figure-of-merit by decreasing thermal conductivity perpendicular …
Thermal conductivity of half-Heusler superlattices
2014
Thin films and superlattices (SLs) of TiNiSn and ZrHfNiSn layers have been grown by dc magnetron sputtering on MgO (100) substrates to reduce the thermal conductivity, aiming for improvement of the thermoelectric figure of merit ZT. The thermal conductivity of 1 Wm−1K−1 was measured by the differential 3ω method for an SL with a periodicity of 8.8 nm. In addition to x-ray diffraction analysis of the SL crystal structure, smooth interfaces were confirmed by scanning/transmission electron microscopy.
Thermal effects during adsorption of n-butane on a slilicalite-1 membrane. A non-equilibrium molecular dynamics study
2007
Abstract Non-equilibrium molecular dynamic (NEMD) simulations have been used to study the kinetics of adsorption of n-butane molecules in a silicalite membrane. We have chosen this simple well-known process to demonstrate that the process is characterized by two stages, both non-isothermal. In the first stage the large chemical driving force leads to a rapid uptake of n-butane in all the membrane and a simultaneous increase in the membrane temperature, explained by the large enthalpy of adsorption, Δ H = − 61.6 kJ / mol butane. A diffusion coefficient for transport across the external surface layer is calculated from the relaxation time; a value of 3.4 × 10 −9 m 2 / s is found. During the a…