Patoka V. Investigation of refractory metals and their alloys evaporation parameters in ultrahigh vacuum

Patoka V. I. Investigation of refractory metals and their alloys evaporation parameters in ultrahigh vacuum. – Qualifying scientific work as the manuscript. Thesis for a Candidate of Physical and Mathematical Sciences (PhD in Physics and Mathematics) from specialties 01.04.07 – Solid State Physics. – G. V. Kurdyumov Institute for Metal Physics of the N.A.S. of Ukraine, Kyiv, 2020. Refractory transition metals and their alloys are known as the basis of key elements of modern energy devices and high-current vacuum technology operating under extreme conditions of high temperatures and vacuum, such as cathodes of thermionic converters (TECs) and protective shell of thermonuclear reactors. Studies of sublimation processes, which determine a thermal stability of these materials, allow to predict the service life of the respective products and to optimize their other characteristics. A method for reliable determination of sublimation characteristics of solids by purposefully reducing the number of poorly controlled high-temperature experiments conditions was proposed in this paper for the first time. Ultra-high vacuum conditions were provided and a number of pre-process heat treatments of samples were performed. The influence of factors that depend on the properties of the sample (chemical composition, concentration and diffusion of impurities, anisotropy of the crystal lattice, mono- and polycrystallinity) and experimental conditions (its geometry, vapour pressure and temperature) on the sublimation characteristics was studied. Reliable values of vapour pressure, rate and energy of evaporation of a number of transition metals (W, Mo, Re, Ir, Os, Pt, Fe), including refractory ones, in the pressure range of their saturated vapour of 10-3–10-5 Torr was determined for the first time. It is shown that the greatest influence on these characteristics have interstitial impurities, especially carbon atoms, which cannot be removed by any pre-treatments. Physical mechanisms of the influence of impurities introduction on the sublimation properties of refractory transition metals and their alloys are determined. These mechanisms take into account both the carbon transition between carbide and solid solution having the different rates of evaporation, and the influence of carbon on the electronic structure of the metal and through it on the sublimation properties, in particular on their anisotropy. Key words: refractory transition metals, alloys of transition metals, interstitial impurities, defects of crystal structure, electronic structure, speed and energy of sublimation, saturated vapour pressure, thermionic energy converter, protective shell of thermonuclear reactor, high-current vacuum devices, high temperatures.