Moscow State University physics alumni and the Soviet Atomic Project

Abstract

We discuss the important aspects of synthesis and crystal growth of MgB 2 under high pressure (P) and temperature (T) in the Mg-B-N system, including the optimization of P-T conditions for reproducible crystal growth, the role of liquid phases in this process, the temperature dependence of crystal size and the effect of growing instabilities on single-crystal morphology. Extensive experiments have been carried out on single crystals with slightly different lattice constants and defect concentrations, which have revealed the possible effects of Mg deficiency and lattice strain on the superconducting properties of MgB 2 (T c , J c , residual resistivity ratio, anisotropy, etc).

Oxygen (3P) atom recombination on a Pyrex surface in an O2 plasma

“…In the case of dielectric surfaces (such as Pyrex, as studied here) the chemical nature of the sites is quite different, and chemisorption usually only occurs at surface defects. This localized character of the interaction can lead to a wide variation of the density of chemisorption sites on dielectrics, typically from ~10 12 to ~10 14 cm -2 [54]. For semiconductors the situation is intermediate and the density of chemisorption sites can vary from what is typical for dielectrics up to that of metals.…”

“…After exposure of a surface to the atmosphere, all chemisorption sites will generally be occupied. For hydrophilic surfaces (such as silica-based glasses) they are typically occupied by hydroxyl (OH) radicals, along with some hydrocarbons [54]. Whereas hydrocarbons can, as rule, be removed by cleaning procedures (for instance, by O 2 plasma treatment), OH removal is difficult and requires high temperatures; under typical discharge conditions full OH surface coverage will be established.…”

“…).Temperatures above ~500C are required to start removal of surface hydroxyl groups. The characteristic density of surface -OH groups estimated by different methods (EPR, IR absorbance) is ~(1-5)10 14 for silica-based glasses [54] and directly correlates with the density of chemisorption sites [1,5,17,52].…”

Surface recombination of oxygen atoms in O2 plasma at increased pressure: I. The recombination probability and phenomenological model of surface processes

“…The deeper wells are the chemisorption sites with a higher desorption energy. It is known [11] that on a silica surface (including also quartz) the chemisorption sites are most often occupied by so-called 'coordinated water' if the special cleaning methods are not applied. The measurements of the number of coordinated OH bonds on a quartz surface by the paramagnetic electron resonance technique [11] give the estimation of OH surface density that in the first approximation can be correlated with the chemisorption site density [S 0 ] ∼ (1-5) × 10 13 cm −2 .…”

“…It should be noted that the given value of k approximately corresponds to the rate constant of gasphase kinetic collisions for oxygen at room temperature. Thus, supposing the adsorption probability is close to unity, k O F and k O 2 F can be estimated simply by the rate constant of kinetic collisions k. The desorption frequencies can be written in the usual form: for O atoms and O 2 molecules as ∼4000-6000 K (∼0.35-0.5 eV) and 2000-3000 K (∼0.17-0.26 eV), respectively (in accordance with [4,8,10,11,13]), the [O] f and [O 2 ] f parameters for a quartz surface at a temperature of 300 K will be approximately (1-5) × 10 16 and (0.2-1) × 10 19 cm −3 , respectively. At low pressures when [O] [O] f and [O 2 ] [O 2 ] f almost all the physisorption sites are free and O atom loss probability γ corresponds to the surface recombination only with the chemisorbed atoms (i.e.…”