Papers by Keyword: Impurities

Abstract: In this work, an investigation of the mechanically exfoliated MoS₂ under the influence of heat treatment was carried out. Optical and atomic force microscopy techniques were applied to determine the number of layers. Resonant Raman investigation was performed, which clearly showed systematic layer-dependent spectral features. The surface morphology of MoS₂ was investigated with the STM. Atomic-resolution images of MoS₂ is were obtained. Three types of atomic defects were identified as substitutions of donor and acceptor atoms in the Mo atomic layer below the topmost sulfur layer.

Abstract: During crystal growth processes, growth sectors are formed due to growth along different crystallographic directions. Although the crystal structure in the different growth sectors is unchanged, strain induced topography contrast is observed by synchrotron X-ray topography. In this study, synchrotron monochromatic beam X-ray topography (SMBXT), synchrotron X-ray plane wave topography (SXPWT), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS) are used to characterize growth sectors in gallium nitride (GaN) substrate wafers grown by patterned hydride vapor phase epitaxy (HVPE). The SMBXT images reveal the boundaries of {0001} and {1122} type growth sectors. Strain maps generated from SXPWT shows that the out-of-plane strains in different growth sectors have a difference of the order of 10^-5. SEM images from SE2 signal shows no contrast of growth sector boundaries while images from Robinson detector (RBSD) show different growth sectors as different grey scale contrast, indicating a strain effect. SIMS analysis shows that the different oxygen impurity levels in the growth sectors, which is the origin of the strain. A formation mechanism of growth sectors in patterned HVPE grown GaN wafers is proposed.

Abstract: This research focuses on purifying the biodiesel provided by the Davao City Biodiesel Plant using a modified fractional distillation. The researchers considered six variables to determine raw and purified biodiesel properties and characteristics: density, kinematic viscosity, flashpoint, fire point, soap content (NaOH and KOH), and water content. The experimentation results were verified using ASTM D6751 – Standards Specification for Biodiesel Fuel (B100). For the purified biodiesel, an increase in flashpoint and fire point temperatures has been observed using the modified process, thus exceeding the standard limit. Although it affects biodiesel quality, an increase in flashpoints and fire points may be better for fuel handling, transportation, and storage safety reasons. These barely influence the overall performance of biodiesel. In conclusion, the modified process improved the quality of the raw biodiesel from 50% to 83% of the variables' set standard with the optimum conditions of 3.5 inHg at 2 hours and 2.5 inHg at 1 hour.

Abstract: One of the prime mechanical properties of any steel is impact strength, which may change dramatically due to the thermal treatments. The present investigation covers the influence of different thermally treated conditions on the impact strength of AISI 1020 steel. The thermal conditions included are: as rolled, oil quenched & tempered, air quenched, normalized and annealed condition. To evaluate the characteristics of the steel in different thermal conditions; optical microscopy, scanning electron microscopy, hardness testing and room temperature impact testing were carried out. Experimental results elaborated the existence of a narrow temperature range (250 ± 50 °C), which may decrease the impact strength drastically, when the steel is heat treated. In this temperature range toughness scavengers i.e., As, S, Sb etc. are precipitated on the grain boundary, which cause the noticeable decrease in impact strength. Optimum impact strength could be achievable where this critical temperature range was avoided.

Abstract: Metallurgical processes in Me-O-C systems cannot be studied and controlled using conventional analytical and physicochemical techniques. This is due to the complex physical and chemical interactions occurring in high-temperature metallurgical systems with the formation of various compounds. To study such processes, mathematical (thermodynamic) simulation and other techniques are widely used, among which graphical diagrams illustrating the equilibrium between individual phases or their systems occupy a special place. Diagrams have been proposed that are more common, informative, and herewith universal to study pyrometallurgical processes. The main research objective was to determine the equilibrium composition of the Me-O-C system, i.e., the amount or ratio of its various compounds such as carbides, oxides, pure substances, and, possibly, ternary compounds. The equilibrium compositions of the system were obtained to plot diagrams from not only experimental studies but also the chemical equilibria mathematical simulation results using the Selector software package. The diagrams proposed herein allow establishing a quantitative relationship between the temperature, the ratio of independent components (metal, oxygen, and carbon), on the one hand, and the complete equilibrium composition of the system, which implies data on the amount and type of condensed and gas phases in the system and their ratios, on the other hand. This mechanism is a new tool to study chemical conversions in complex metallurgical processes, which will be very useful for metallurgists, chemists, and technologists.

Abstract: A wide range of intermetallic compounds of the Al-Ti, Al-Ni, Al-Zr, Al-Cr, Al-Fe, Al-V systems has been examined. The enthalpy of formation of intermetallic compounds in a wide temperature range has been calculated. The obtained values are attributed to one mole of the chemical compound. The dependences of the obtained values on the stoichiometric composition of chemical compounds with regard to their position on the state diagram are established. A comparative assessment of the stability of intermetallic compounds for each system has been conducted. A method of approximate calculation of the enthalpy of intermetallic compounds has been proposed.

Abstract: The surface structure of the WSe₂ were studied using scanning tunneling microscopy. Exfoliation method in an ultra-high-vacuum chamber method is used to obtain a clean surface of WSe₂ samples with atomically smooth terraces and multi-layer steps. Atomic-resolution images revealed two types of atomic defects of surface or near surface. These defects have been identified as the defects in the tungsten atom layer just below the topmost selenium layer.

Abstract: Silicon production by silica-containing raw material reduction by carbon in ore-smelting furnaces (OSF) is a complex pyro-metallurgic process. Silicon is produced in a system consisting of charge, furnace electrodes, air, melt, slag, and gases at temperatures above 2000°C, which generates silicon monoxide and carbide unavoidably. The finished product contains impurities (Fe, Ca, Al, Ti, etc.) in small amounts; however, it is still unknown for sure how impurities, contained in raw materials, behave and interact with each other and with silicon. To analyze the distribution of charge components during smelting, the research team used thermodynamic (mathematical) modeling in Selector software. The constructed four-tank silicon production model describes adequately the process. The assumed tank temperatures correspond to the OSF reaction zones (400°C, 1530°C, 2200°C, and 2000°C). Modeling involves 15 charge-carried elements: quartzite from the Cheremshanskoye deposit, charcoal and black coal, petroleum coke, wood chips, and OSF coal electrodes. According to the model, silicon recovery (with a 97.15 wt% silicon content in the melt) is 75%, which is consistent with literature and production data (AO Kremniy, Shelekhov, Irkutsk Oblast).

Abstract: The effect of Li and Sr impurities on the kinetics and structure formation of alloys in the Sn - Pb, In - Zn, Sn - Zn systems used in the soldering of instrument nodes in electronics has been investigated. The XPS method showed that under the given experimental conditions and the indicated concentrations of lithium impurities in lead and strontium in zinc, there are no prerequisites for the formation of chemical compounds, which is explained by an insignificant impurity content, although a significant amount of chemical compounds is formed according to state diagrams. It was also established that impurities affect the kinetics of phase growth in the junction zone.

Abstract: The improvement of the thermal insulating material thermophysical characteristics of the thermal protection elements by studying the porous structure is a promising direction of research. The article describes the effects of the porosity and coupling of the porous structure on the thermophysical characteristics of thermal insulating materials. The article uses standard systematized techniques and instruments of scientific research applied in thermophysics. The research methodology of highly-porous material thermophysical properties is based on performance of empirical laboratory investigations of the samples obtained. It was found that for the pore structure effect on the material characteristics it is rational to use the following complex indices: porosity, number of pores, pore position in space, the pore form, pore formation energy. The article shows the effect of the porous structure on the thermophysical characteristics of the material. The complex parameters of the porous structure, which will allow to develop a new method of control of the porous structure, are proposed. As a result of the experiment planning method, the regression equation of an effective coefficient of thermal conductivity for porous thermal protection structures was developed. It was established that for a more even distribution of the mixture in a volume it is necessary to minimize the size of the dispersed components, thereby increasing the area of their contacts. The experimental method revealed that the moisture evaporation caused the formation of pores inside the clay. The shape of the pores was determined using electron microscope MMP-2P, both on the sample section and surface. The clearest clay has the greatest porosity (no iron oxide and calcium oxide). The pores have a spherical shape in it. The presence of impurities reduces the material porosity due to the increased clay viscosity.