Papers by Keyword: Sublimation

Abstract: Herein, we report an original adaptation of an XRD set-up in order to measure the miscut angle values in our 4H-SiC on axis substrates with a high precision of ± 0.02°. This study also reveals a correlation between the formation of wide steps on 4H-SiC(0001) and the relative orientation of the SiC crystalline planes versus the gas flow direction. These two results paves the way towards the reproducible growth of graphene over wide SiC steps.

Abstract: We investigated the relation between the nucleation of dislocations and the lattice misfits by nitrogen concentration difference between seed and grown crystal during the initial stage of growth. 4H-SiC single crystals were grown with various nitrogen gas flow rates introduced into the crystal growing chamber under the same temperature and pressure to minimize the effect of thermal stress on the nucleation of dislocations. The nitrogen atomic concentrations of grown crystals depended on the introduced nitrogen gas ratios and they highly increased at the very early stage of growth. The generation of new threading dislocations at the interface also was affected by the nitrogen atomic concentrations differences between seed and grown crystals. Very few generated threading dislocations were observed in low nitrogen atomic concentration samples, however nucleation of threading dislocations at the interface were found in high nitrogen atomic concentrations samples. At initial stages of PVT growth process, the generation of threading dislocations induced by lattice misfits originated from nitrogen concentration difference between seed and grown crystals were investigated and found the appropriate nitrogen gas flow rates and profile at the heating and depressurized stage.

Abstract: The structural changes of condensed fullerenes C₆₀ and C₂₈ at a temperature increase from 200 K to 2000 K have been studied by computational methods using the TERRA software for carbon-argon systems. The processes of destruction of fullerenes C₆₀ and C₂₈ molecules are presented, and the temperature ranges of their thermal stability are determined: up to 1000 K and up to 400 K, respectively. The following thermophysical parameters of the C₆₀-Ar and C₂₈-Ar systems are considered: specific volume, entropy, total enthalpy, total internal energy, equilibrium specific heat, molar mass of the gas phase, gas constant, and mass fraction of the condensed phase. A comparative analysis of their changes with increasing temperature is carried out. The results obtained in the course of thermodynamic modeling are similar to the results of a full-scale experiment conducted under similar conditions. In the future, the obtained data can be used to determine the explosive and fire-hazardous properties of fullerenes as a dispersed solid.

Abstract: The generation and transformation of dislocations in 4H-SiC crystals grown by PVT were investigated. Experiments were carried out in two stages for more comprehensive observation on dislocation behaviors. For the first stage known as initial growth, we investigated mainly the seed and grown interface. The behavior and transition of the dislocations in grown crystal were observed along the length of the crystal at second stage. The formation of threading edge dislocations (TEDs) strongly depends on the surface morphologies related with internal temperature gradients during crystal growth. The basal plane dislocation (BPDs) and threading screw dislocation (TSDs) cause from the seed crystal and formed at the initial stage of growth were gradually decreased in number along the length of the crystal and under certain conditions such as distorted stresses, dislocations were converted into other types of dislocations.

Abstract: The heating temperature of the silicon carbide sublimation growth crucible is changed by adjusting the output power of the medium frequency induction coil, and the sintering experiments were carried out using NaCl and Al₂O₃ to observe the morphological changes after sintered under different output power, the corresponding temperature was determined, and the corresponding relationship between the output power and the heating temperature was obtained, the precise temperature control was realized. The results of temperature measurement were compared with that of the infrared photoelectric pyrometer. Based on this, the SiC grains were prepared according to the temperature measurement results. The Raman spectroscopy result shows that the SiC polytype was 6H, the SiC grains distributions are homogeneous, and the size of the SiC grains is uniform and dense.

Abstract: The authors present the results of investigations of degradation processes that occur in the structure of heat-resistant coating of the Si-TiSi₂-MoSi₂-B-Y system in hypersonic flows of air plasma. It is found that coating operating capacity at surface temperatures T_w ≤ 1820÷1830°C is provided by the structural-phase state of its microcomposite main layer and formation on the coating surface of a heterogeneous passivating protective film. It is based on borosilicate glass reinforced by rutile microneedles. The mechanism of coating destruction at T_w ≥ 1850÷1860°C is erosion loss of oxide film as well as generation and growth of gas-filled cavities at the "coating main layer–oxide film" interface. As the pressure of saturated vapor of gaseous oxidation products (SiO, CO, MoO₃ and B₂O₃) exceeds that of the ambient, the oxide film integrity is disrupted and oxidation process becomes active. The rates of erosion loss and sublimation grow as operating temperature increases and ambient pressure decreases.

Abstract: Effects of proton irradiation in n-3C-SiC grown by sublimation on a 4H-SiC substrate have been studied by the Hall effect and photoluminescence methods. It was found that the carrier removal rate (Vd) reaches a value of ~110 cm^-1. The full compensation of samples with an initial concentration of (1-2) x 10¹⁸ cm ^-3 was estimated to occur at doses of about 6 x 10¹⁵ cm ^-2. Compared with 4H and 6H silicon carbide, no significant increase in the intensity of so-called "defective" photoluminescence was observed in 3C-SiC.

Abstract: This paper examines a model for coupled heat and mass transfer for freezing in a porous matrix with Dirichlet and convective boundary conditions. Variables include porosity, heat transfer coefficients, thermal and mass diffusivity, density, latent heat and boundary temperatures. It is shown that heat and mass transfer balance at the interface can affect stability. The effect of boundary conditions on the velocity of freezing is computed for some cases, and applications to physical problems highlighted

Abstract: The paper describes the advantages and characteristics of the fluoride technology applicable for the recovery of oxides of rare earth elements with the help of a fluorinating agent of elemental fluorine. A metallurgical mixture (2.5 m²/g) of rare earth elements oxides of the cerium subgroup, containing % wt.: CeO₂ - 50-55; La₂ O₃ - 23-25; Pr₆O₁₁ - 6-9, and Nd₂O₃ - 13-16 were studied. It has been shown, that using fluoride technology allows recovering mixture of rare earth elements with outlet no less than 98.9 %.

Abstract: The paper studies the iodine-123 extraction process from the targets of enriched Tellurium-122 irradiated by deuterons at the cyclotron R-7M. We researched the regularities of radionuclide accumulation in the absorbent solution tank, depending on the temperature and air pumping regime through the system. As a result, we suggested a separate temperature treatment of the targets with the purpose of their preliminary purification from impurities and subsequent obtaining of "Na¹²³I" preparation with radiochemical yield 97%.