Heat pulse experiments have been made on Al₂O₃ single crystals in the temperature range 3.8° K to 35°K with the aim of gaining further insight into the nature of heat transport in solids at low temperatures. Short heat pulses were produced by heating a thin metal film evaporated on to one end of the crystal. The thermal pulse arriving at the other end of the crystal was detected by an indium film thermometer placed in a coil connected to a sensitive radio-frequency bridge, so that the variation of resistance was finally displayed on an oscilloscope. The pulses received at low temperatures (3.8°K to 8°K) show two quite separate parts, an initial sharp rise followed by a slow rise, starting at a definite delay time corresponding to the phonon velocity in the medium. The results up to 18°K do not show appreciable variation in delay time, showing that the heat pulse propagation has not entered a second sound region. As the temperature increases, the amplitude of the initial phonon pulse decreases very much compared with the amplitude of the slow rise. Above 18°K, the small sharp rise can no longer be seen clearly so that the delay time is no longer well defined, and at 30°K only the slow rise is observed. It is found that the conventional theory of heat conduction is inadequate to interpret our results at low temperatures, as it fails to predict the finite delay of the initial rise of the received pulse. A phenomenological approach is taken, using a modified heat equation which has an electrical transmission line analogy. Using Laplace transforms, a solution is obtained and the results calculated with a computer are compared with the experimental curves. It is found that the pulse shape can be interpreted quite satisfactorily, especially at the lowest temperatures. The thermal diffusivity, D, for different temperatures is found, and the apparent thermal conductivity, K, is calculated and compared with Herman's (1955) results. The solution of the modified heat equation is also calculated for liquid He II at 0.25°K and compared with the heat pulses observed by Kramers et al (1954); very good agreement is obtained. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Research into efficient thermoelectric materials has gained traction recently be cause of their applications in converting waste heat into electricity. Chalcogenide and transition metals are among emerging classes of materials for potentials thermoelectric applications. In this work, we employ first-principle calculations and the Boltzmann transport equation along with self-consistent phonon theory to evaluate the thermoelectric performance of barium chalocgenide perovskites BaBX3 (B= Zr, Hf and X= S, Se) in the orthorhombic perovskite phase, as well as BaZreS3 in the needle-like phase. Vibrational properties were investigated through the phonon dispersion, mode Gruneisen parameters and lattice thermal conductivity to understand and measure the anharmonicity in the systems. The carrier transport properties including the Seebeck coefficient, electric conductivity and the electron contribution to thermal conductivity were evaluated. We report ultra low lattice thermal conductivity of κl = 1.23W/mK for BaHSe3 at T=300K by including high order phonon scattering events. A maximum power factor of 1.16 mW/mK2 was achieved at high n-doping concentration, resulting in a thermoelectric figure of merit zT = 0.2 for BaHSe3.

No description available.

No description available.

The interest in one dimensional (1D) magnetism has been strongly renewed with the synthesis of many magnetic compounds which exhibit a quasi one dimensional magnetic behaviour. One of the peculiarities of this 1D system is the absence of a long range magnetic ordered phase at any finite temperature for the ideal 1D system with short range interaction. Tetramethylammonium manganese chloride (CH3)4NMnCl3(TMMC) exhibits the properties of an ideal one dimensional antiferromagnets for temperature above 1 K, the transition to a three dimensional (3D) long range ordered state only occurs at 0.84K. In addition to its magnetic transition, TMMC exhibits structural phase transition due to the ordering of the tetramethylammonium (TMA) ions which makes also this compound very attractive from a lattice dynamical point of view. Structural phase transitions of tetramethylammonium manganese chloride (TMMC), tetramethylammonium manganese bromide (TMMB) and tetramethylammonium manganese chloride doped with 8% Cu (TMMC:Cu) of the hexagonal type compounds are investigated using the Brillouin scattering method. These crystals show pronounced acoustic anomalies in the region of the structural phase transition. The acoustic anomalies were observed by measuring sound velocity and hence the elastic constant can be deduced. The phase transition temperatures were observed at 129.6K and 388.6K (TMMC), 114.6K and 377.6K (TMMB) and at 108.6K and 359.6K (TMMC:Cu). The elastic constant at room temperature were C11 = 2.10 (TMMC) and C11 = 1.59 (TMMB) in units of 1010 Nm-2. The phase transition of these compounds were further investigated macroscopically using the Differential Scanning Calorimetry (DSC) method. Activation energies of TMMC, TMMC:Cu, TMMB and deuterated TMMB at the phase transition were determined using this method. The values are 70.612 kJ/mol (TMMC), 49.224 kJ/mol (TMMC:Cu), 51.747 kJ/mol (TMMB) and 69.909 kJ/mol (d12-TMMB). The elastic constant of the linear chain antiferromagnet CsNiCl3 and RbNiCl3 was also determined using the Brillouin scattering method. The room temperature measurements give C11 = 3.77 (3.71) and C33 = 5.62 (5.42) in units of 1010 Nm·2 for CsNiCl3 and RbNiCl3 respectively. The phonon dispersion curves at room temperature in the hexagonal CsFeBr3 have been studied using the inelastic neutron scattering technique. From the initial slope of the dispersion curve, the sound velocity was deduced which enable us to calculate the elastic constant of CsFeBr3 at room temperature. The values obtained are C11 = 7.33, C66 = 1.01, C33 = 2.58 and C44 = 0.56 in units of 1010 Nm·2.

The dispersion relation between frequency and wavevector of atomic vibrations, or phonons, can be succinctly described by the adiabatic bond charge model, first developed by Weber. The model employs as few as four parameters to fit experiment. We investigated this model in order to better unify the description of the technologically relevant group IV elemental semiconductors (e.g. diamond, silicon, germanium, and gray tin) by replacing an ad hoc parameter introduced by Weber with one arising from quadrupolar interactions between the bond charges, and by fitting the parameters to density functional theory calculations. We also illustrate constant frequency surfaces embedded in wavevector space for the various modes of vibration for the first time. The bond charge model allows for rapid calculation of various quantities related to the interaction of phonons with electrons and photons as compared to density functional theory, especially in structures with little symmetry and for macroscopic structures, thus enabling the design of complicated electronic and photonic devices much more accurately.

R.D. Mattuck, M.W.P. Strandberg. / "October 19, 1959"--Cover. "Reprinted from The Physical review, vol.119, no.4, 1204-1217, August 15, 1960." / Includes bibliographical references. / Army Signal Corps Contract DA36-039-sc-78108. Dept. of the Army Task 3-99-20-001 and Project 3-99-00-000.

Reproduction de : Thèse de doctorat : Energie, Thermique, combustion : Poitiers : 2008. / Titre provenant de l'écran-titre. Bibliogr. 121 réf.

Thèse de doctorat : Physique théorique : Strasbourg : 2009. / Thèse soutenue sur un ensemble de travaux. Titre provenant de l'écran-titre. Bibliogr. 8 p.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Zhang, Zhuomin; Committee Member: Kumar, Satish; Committee Member: Peterson, G. P. Part of the SMARTech Electronic Thesis and Dissertation Collection.