| Abstract: | The aim of this thesis entitled “Influence o f doping on thermal diffusivity dependence on temperature of single crystals of YAG, YVOj i GdCOB’’ was to determine the thermal diffusivity of YAG, YVC>4, and GdCOB single crystals as a function of temperature, and to investigate an influence of dopants on these dependencies. To realize this purpose new experimental setup for the determination of thermal diffusivity dependence on
temperature of transparent samples was built. The study was carried out for crystals which belong to different crystal systems: the yttrium aluminum garnet (YAG) having cubic structure, yttrium orthovanadate (YV04) of tetragonal structure and gadolinium calcium oxoborate (GdCOB) crystallizing in the monoclinic structure. The crystals were doped with calcium ions, rare earth ions such as ytterbium, neodymium, thulium, and transition metal vanadium. The influence of doping and temperature in rangę from 30 °C to 300 °C on the
thermal diffusivity was analyzed. An anisotropy of thermal conductivity of investigated samples was also taken into account. Obtained results confirmed, that the influence of doping on the thermal diffusivity of
investigated materials strongly depends on temperature. The thermal diffusivity of all
investigated samples decreases with increasing of sample temperature from 30 °C to 300 °C,
but the drop in the thermal diffusivity is the highest for pure single crystals. The introduction
of dopant ions into a crystal lattice leads to a significant reduction in the thermal diffusivity at
lower temperatures in comparison with pure crystals. However, the influence of dopants
becomes less pronounced with increasing temperature, and in the case o f weakly doped
crystals it becomes negligible at higher temperatures. The interpretation of obtained thermal
diffusivity dependence on temperature for pure and doped single crystals was based on the
Debye model of lattice thermal conductivity of solids. It was assumed that the decrease in
thermal diffusivity with temperature and increasing concentration of impurities is caused by shortening of the phonons mean free path due to phonon-phonon and phonon-point defect scatterings. Dopant ions create additional phonon scattering centers which cause a drop of thermal diffusivity of doped crystals in comparison with pure ones. At the same time an increase of crystal temperature lowers the thermal diffusivity because of increasing freąuency of phonon-phonon scattering. This mechanism at higher temperatures prevails on phonon
scattering on dopants, what is evident in the measured thermal diffusivity dependency on temperature for the doped crystals. All the aims and scopes of this thesis have been completed, the results presented at
conferences, and published in international journals. |