Papers by Keyword: Thermodynamic Equilibrium

Abstract: The carbon vacancy (V_C) is a major limiting-defect of minority carrier lifetime in n-type 4H-SiC epitaxial layers and it is readily formed during high temperature processing. In this study, a kinetics model is put forward to address the thermodynamic equilibration of V_C, elucidating the possible atomistic mechanisms that control the V_C equilibration under C-rich conditions. Frenkel pair generation, injection of carbon interstitials (C_i’s) from the C-rich surface, followed by recombination with V_C’s, and diffusion of V_C’s towards the surface appear to be the major mechanisms involved. The modelling results show a close agreement with experimental deep-level transient spectroscopy (DLTS) depth profiles of V_C after annealing at different temperatures.

Abstract: The carbon vacancy (V_C) is a major point defect in high-purity 4H-SiC epitaxial layers limiting the minority charge carrier lifetime. In layers grown by chemical vapor deposition techniques, the V_C concentration is typically in the range of 10¹² cm^-3 and after device processing at temperatures approaching 2000 °C, it can be enhanced by several orders of magnitude. In the present contribution, we show that the cooling rate after high-temperature processing has a profound influence on the resulting V_C concentration where a slow rate promotes elimination of V_C. Further, isochronal annealing of as-grown and as-oxidized epi-layers protected by a carbon-cap was undertaken between 800 °C and 1600 °C. The results reveal that thermodynamic equilibrium of V_C is established rather rapidly at moderate temperatures, reaching a V_C concentration of only a few times 10¹¹ cm^-3 after 40 min at 1500 °C. Hence, the concept of eliminating V_C’s by annealing at moderate temperatures under C-rich equilibrium conditions shows great promise and enables re-annealing of high-temperature processed wafers, in contrast to the procedures commonly used today to eliminate V_C. In-diffusion of carbon interstitials and out-diffusion of V_C’s are discussed as the kinetics processes establishing the thermodynamic equilibrium

Abstract: Thermodynamic equilibrium is proposed to analyze the distribution of SO₃ in different flue gas circulation modes to study the migration law of sulfur for oxy-fuel combustion. Results indicate that the concentration of SO₂ increases dramatically when flue gas circulation is adopted with the decline of flue gas volume and the increase of oxygen concentration in oxy-fuel combustion, which results the increase of SO₃ concentration and formation rate. At 1200°C, thermodynamic equilibrium concentration of SO₂ and SO₃ are 5.3 times and 6.1 times respectively compared with conventional combustion. In Oxy1 circulation mode, SO₃ is the most sensitive to excess air coefficient and sulfur content, followed by Oxy2 circulation mode, while the conventional combustion has the lowest sensitivity. Therefore, SO₃ is greatly influenced by different flue gas circulation modes during oxy-fuel combustion.

Abstract: To study migration and transformation of sulfur species in oxyfuel combustion, the study attempts to analyze distribution of sulfur compounds with thermodynamic equilibrium. Results show that sulfur-containing gases predominantly are SO₂ and SO₃, the maximum thermodynamic equilibrium concentration of those in oxyfuel combustion respectively increase by 3.4 and 4.5 times compared with the conventional combustion. Furthermore, SO₂ gas formation rate decreases while SO₃ increases under oxyfuel combustion. Sulfur-containing gases are generally more sensitive to temperature and excess air coefficient. The amount of sulfur compounds significantly increases in oxyfuel combustion.

Abstract: The chemical and electrochemical equilibrium of oxygen-chlorine-titanium system in the presence of gaseous phase were investigated. Many species, which consisted of oxygen, chlorine and titanium, were considered. Various thermodynamic equilibriums were calculated in the different pressures and temperatures. Calculation results were shown as E-T diagram. This diagram will be used as important tools for corrosion study and titanium production, and it is also used to thermodynamically determine the existence areas of various species and so on.

Abstract: In this work we give heed especially to the dominating process which is the solid metal A dissolving in the melt B. During the dissolving, the melt B saturates with the metal A and the process is influenced by convections which are characteristic for the given experimental configuration. A theoretical description of the kinetics of the solid phase dissolving in melt will be presented for the case of planar and cylindrical dissolving. The aim is to derive a relation for the interface boundary (t) movement in dependence on time and a time course of growth of the element A concentration in the melt B. There are problems with an accurate determination of the interface boundary movement after certain heating times of specimens, when it is observed experimentally, since intermetallic phases create in the original A metal at both the diffusion and cooling and some phases segregate at the solidifying melt cooling. The rate constant is a fundamental parameter characterizing the dissolving rate at a certain configuration. We present a theoretical description of dissolving of a long metallic cylinder submerged into a melt column and relations for the rate constant determination from the time of the whole metallic cylinder dissolution are derived. In our experiments were performed in which Cu was dissolving in the Sn melt for a Cu cylinder (wire) diameters 0.8÷2.5 mm and the rate constant K (T = 350°C) was determined. Relationships between the solid phase dissolving rate, i.e. the solid phase interface boundary movement (t) in the melt and rates of growth of intermetallic phases in the metal A will be observed. This procedure enables to create surface and subsurface layers of regulated thickness in metallic materials by means of reactive diffusion.

Abstract: Ni-base single crystal (SC) superalloys containing high concentrations of refractory elements are prone to generate a diffusion layer called Secondary Reaction Zone (SRZ) beneath their bond coating during exposure at high temperatures. SRZ causes a reduction of the load bearing cross section and is detrimental to the creep properties of thin-wall turbine airfoils. In this study, a new coating system – “EQ coating”, which is in thermodynamic equilibrium with the substrate, has been proposed and the formation behavior of SRZ beneath bond coat materials was investigated on the 5th generation Ni-base SC superalloy developed by NIMS. Diffusion couples of several alloys were made and were heat treated at 1100°C for 300 h, 1000 h. The concentration profiles were analyzed by EPMA. Also, cyclic oxidation tests were carried out at 1100°C in air.