Influences of Powder Source Porosity on Mass Transport during AlN Crystal Growth Using Physical Vapor Transport Method

We developed a two-dimensional (2D) transport model to investigate mass transport during bulk AlN crystal growth via the physical vapor transport (PVT) process using the finite element method (FEM), taking the powder source porosity, buoyancy, and vapor diffusion into account. The porosity effects of the powder source on mass transport under various growth conditions were investigated in detail. The simulation results show that the porosity of the powder source significantly affects the mass transport process during AlN sublimation growth. When the porosity of the powder source decreases, the growth rate becomes more uniform along the seed deposition surface, although the sublimation rate and crystal growth rate decrease, which can be attributed to the reduced specific surface area of the powder source and the reduced flow rate of Al vapor in the powder source. A flat growth interface can be achieved at a porosity of 0.2 under our specific growth conditions, which in turn facilitate the growth of high-quality AlN crystals and better yield. The decomposition of the powder source and the transport of Al vapor in the growth chamber can be suppressed by increasing the pressure. In addition, the AlN growth rate variation along the deposition surface can be attributed to the Al vapor pressure gradient caused by the temperature difference in the growth chamber.

New Criterion for Shutter Designing by Dry Pluviation Method

Dry pluviation is a technique applied for preparation of sandy soil samples for physical modeling. In this method, soil particles rainfall in the mold after passing through the mesh with certain opening sizes. In this study, a dry pluviation device was designed and manufactured to examine the effect of shutter properties including the hight of fall (HF), deposition intensity (DI), and gradation on relative density (RD). The results indicate that HF and DI have a direct and indirect relation with RD, respectively, but RD is constant for HF ≥ 1200 mm. In order to investigate the impact of shutter properties on DI and RD, α ratio (total area of the shutter holes to the area of the deposition surface) was defined and a linear relationship is proposed between DI and α. The results revealed an indirect relation between RD and α, but for α ≥ 0.130 RD is constant. In order to assess the simultaneous effect of grain size and shutter properties on RD, an independent grain size variable (i.e., α/D50) was defined and a linear relationship was established between α/D50 and RD. The results showed that the variable α/D50 can be considered as a useful criterion for designing a shutter to reconstitute sandy soils with different gradations.

Simulation of Multilayer Nanosystems Interface Formation Process for Spintronics

Modeling the processes of forming contact regions (interface) of the multilayer niobium-cobalt nanosystem is carried out. The morphology and composition of a multilayer nanosystem interface is investigated. The layer boundaries morphology is shown to depend on the deposition substrate temperature and, largely, is determined by preparing the surface for deposition. The work considers the deposition surface modification by removing its defects. Simulation showed that surface preparation significantly affects the morphology and composition of a multilayer nanosystem interface, depending on the type of deposited atoms and atoms forming the deposition surface.

Finite Element Analysis of An Evaporation System to Synthesize Kesterite thin Films

Currently, there is an interest within the scientific community in thin-film solar cells with a Kesterite (Cu2ZnSnS4) type absorber layer, since they report a theoretical efficiency greater than 32 %. The synthesis of Kesterites by evaporation has allowed for efficiencies at the laboratory level of 11.6 %. Although these are good results, the design of the evaporation chamber and the distribution of the electrodes is essential to control synthesis parameters and evaporate each precursor in the corresponding stage. This project seeks to design an evaporation chamber that can achieve a vacuum of 10-5 mbar, increase the deposition surface and avoid each precursor evaporation in a non-corresponding stage. This last objective was studied using Comsol multiphysics R. (licensed product) software, with the adequate disposition of metallic precursors (zinc, copper, and tin) determined by analyzing heat distribution. It was concluded that the lower the evaporation temperature of the precursor, the smaller the height of the copper electrode in the system. This is because, with a lower height the concentration of heat in the container is lower.

Investigation of the Influence of Microdroplets on the Coatings Nanolayer Structure

The paper presents the results of studies focused on the specific features typical for the formation of the shape and structure of microdroplets embedded into the structure of the Ti–TiN–(Ti,Cr,Mo,Al)N and Ti–TiN–(Ti,Al,Nb,Zr)N coatings during their deposition. Three main microdroplet shapes—a sphere, a tear, and a lens—have been considered. The specific features typical for the formation of secondary layered structures on the surface of some microdroplets have also been examined. As a result of the conducted investigations, with the use of scanning and transmission electron microscopy, the influence of microdroplets on the distortion of the nanolayer structure of the coatings was studied. A hypothesis has been proposed concerning a relationship between the microdroplet shape and the presence or absence of secondary structures and the microdroplet sizes and weight, as well as the conditions in the unit chamber during the movement of a microdroplet from a cathode to the deposition surface. Based on the study focused on the shape of the microdroplet core and the specific features typical for the formation of the secondary structure around it, a hypothesis has been proposed, according to which, for some microdroplets, it takes much more time than previously assumed for the movement from a cathode to the deposition surface.

Лимитирующие режимы роста III-V нитевидных нанокристаллов

Theoretical analysis is presented for vapor-liquid-solid growth of III-V nanowires in the presence of three competing processes of the group V deposition, surface diffusion of group III adatoms and nucleation of islands at the liquid-solid interface. A generalized equation for the nanowire growth rate is obtained which can be limited of one of the three processes depending on the growth environment. Different regimes of vapor-liquid-solid growth of III-V nanowires are analyzed depending on the group III and V influxes and nanowire radius.

Expression of apoptotic and proliferation factors in gastric mucosa of patients with systemic sclerosis correlates with form of the disease

Despite high prevalence of patients with gastric disease in systemic sclerosis (SSc), its pathogenesis is still poorly understood. We immunohistochemically analysed biopsies of gastric mucosa (GM) in 5 controls and 15 patients with different forms of SSc: limited cutaneous (lc), diffuse cutaneous moderate (sys1) and severe (sys2). The number of positive cells was analysed by a Kruskall-Wallis test, P < 0.05 was considered statistically significant. Percentage of proliferating (Ki-67 positive) cells was highest in sys1 (3% in superficial and 4,6% in deeper parts of GM), which dropped to 1% in sys2. Percentage of α-smooth muscle actin (α-SMA) positive cells was 5% in controls, 9% in superficial GM, while in deeper GM rose from 7% to 19% in sys1 and sys2, thus indicating increased myofibroblast population. Caspase-3 positive apoptotic cells characterized 1,5–2% of controls, 8% of superficial and 6% of deeper GM cells in sys1. In sys2, apoptosis affected 50% of surface epithelial and gland cells and 30% of deeper glands, and correlated with increased fibrosis and decreased syndecan-1 expression. Our data demonstrate that sys1 is the most „active” proliferating form of SSc. Sys2 characterize collagen deposition, surface epithelium defects, extensive apoptosis and low proliferation, GM atrophy and loss of function.