Papers by Keyword: Chemical Potential

Abstract: Expansion of single Shockley stacking faults (SSFs) during forward current operation is an important issue, because it decreases the reliability of 4H-SiC bipolar devices. In this paper, we propose a method for analyzing SSF dynamics based on free energy under current conduction, temperature, and resolved shear stress conditions. The driving force for dislocation dissociation reactions and formation of SSFs is incorporated into the free energy function, including chemical potential, stacking fault energy, crystallographic energy, gradient energy and elastic strain energy. The net energy gain of the chemical potential was calculated as a function of temperature and current conduction through use of the a TCAD device simulator based on the Boltzmann equation, Poisson equation and the current continuity equation concerning electron and hole distributions with self-consistency. It was confirmed that SSF dynamics can be simulated by the proposed method. It was also found that SSF formation can be attributed to quantum well variation in which electrons in n-type 4H–SiC enter SSF-induced quantum well states to lower the energy of the dislocation system.

Abstract: A mathematical model for analysis of features of the drug release , previously introduced into a polymer implant, into a biological tissue is proposed. A carbon nanolayer obtained as a result of plasma-immersion ion implantation was created to improve biocompatibility with biological tissue on the surface of the implant. The medicine can go through micro-ruptures in this layer. Calculations show that the carbon layer allows a uniform release of the drug.

Abstract: The work is devoted to the discussion of hypotheses that are put forward to explain the processes occurring during ion-plasma treatment of polyurethane. A carbonized layer forms on the surface of the polymer as a result of ion-plasma treatment. However this layer is not even. Wavy relief, the geometric features of which depend on the fluence (the number of ions entering the unit surface of the sample) and the energy of ions, is formed. It is shown that a simple explanation related to material heating and subsequent shrinkage does not allow explaining the cause of the phenomenon. The second hypothesis can be the pressure of the ion flow on the surface of the sample. It causes deformation and subsequent changes in the stress-strain state after the irradiation is stopped. Calculations show that this mechanism cannot explain the formation of the folded relief of the layer. A hypothesis, based on information about a significant material change, is expressed in the article. Polymer chains under ion-plasma treatment are broken into atoms. After striking ions move deep into the material causing the polymer to swell in the near-surface layer. This swelling can cause material to move close to the sample boundary and leads to the formation of a wavy surface.

Abstract: The interaction between interstitially diffusing atoms and substitutional solute atoms, acting as trapping sites, causes a non-negligible influence on the diffusion process itself and, consequently, on many aspects of alloys, such as phase transformations, solubility, precipitation of carbides and nitrides etc. The most important quantity in this treatment is the so-called trapping enthalpy (depth of trap), which has been used in several approaches in literature over the last century. However, the determination of the trapping enthalpy so far relies on approximations or assumptions on the one hand (statistical approaches, quasi chemical approach) or is significantly limited due to high complexity (ab initio approaches) on the other hand. The model introduced in this paper illustrates a rigorous and efficient thermodynamically-based concept utilizing only the dependence of the chemical potential of the interstitial component on the chemical composition of the alloy. Such a dependency is available in a very precise form from CALPHAD thermodynamic databases. Using the most recent databases available, the trapping enthalpies of carbon and nitrogen at various solute atoms (trapping sites) are evaluated for austenitic and ferritic steels. Good agreement with previous literature results is observed. The flexibility of the concept allows also for the treatment of trapping in a multi-component system, where different types of solute atoms are responsible for different depths of traps.

Abstract: The generalized thermoelastic diffusion problem with temperature-dependent properties is investigated in the context of the theory of generalized thermoelastic diffusion. The results show that all the considered variables have a non-zero value only in a bounded region and vanish identically beyond this region, and the temperature-dependent properties act to reduce all the considered variables.

Abstract: The generalized thermoelastic diffusion problem with temperature-dependent properties is investigated in the context of the theory of generalized thermoelastic diffusion. The problem is solved by means of finite element method and the derived finite element equations are solved directly in time domain. The effects of diffusion and temperature-dependent properties on generalized thermoelastic wave and mass diffusion wave are studied in detail. The results show that all the considered variables have a non-zero value only in a bounded region and vanish identically beyond this region, the temperature-dependent properties act to reduce all the considered variables and the diffusion barely influences the considered variables.

Abstract: The structural and electronic properties of TiC(111) surfaces are calculated using the first-principles total-energy plane-wave pseudopotential method based on density functional theory. As a polar surface, (111) surface shows large charge depletion in the upper part of the atoms, while charge accumulation happens in the inferior part of the atoms, interlayer Ti-C chemical bonds are reinforced and the outermost interlayer distances are largely reduced. Meanwhile, the charge accumulation and depletion for Ti-terminated surface is more than that for C-terminated surface on the same position of the two slabs after full relaxation. The surface energy of C-terminated surface is in the range from 7.61 to 9.83 J/m2, much larger than that of Ti-terminated surface from 3.13 to 1.35 J/m2, and the Ti-terminated surface is thermodynamically more favorable over all of the range of (chemical potential of TiC slab). This present work makes a beneficial attempt at exploring TiC surface as an ab initio method for studying possible nucleation mechanism of Aluminum on it.

Abstract: In the present work, a thermodynamic study was carried out in order to analyze the thermodynamic stability of the and phases in equilibrium with the phase using the calculation of phase diagrams (Calphad) formalism. The two phases and are modelled as substitutional and interstitial solid solutions of boron. The expressions of the chemical potentials of B and Fe are derived in both phases to perform the thermodynamic calculations. A comparison is made between the results provided by the substitutional and interstitial models and good agreement is observed between these two models.

Abstract: We report the new structures of aluminum hydrides derived from the Al4 tetrahedral cages. We perform ab initio quantum chemical calculation for these new aluminum hydrides. Our calculation of binding energies of the new aluminum hydrides reveal that stability of these hydrides increases as more hydrogen atoms are adsorbed, while stability of Al-H bonds decreases. We also calculate electronic stress tensor to evaluate the chemical bonds of these hydrides. As a result, we find that the bonds of the Al4 tetrahedral cage are strengthened as more hydrogen atoms are adsorbed on the aluminum hydrides. Our calculation of the potential energy surfaces and the regional chemical potential show that hydrogen atoms are likely to adsorb on bridge site at first.

Abstract: Strength of nanostructured Cr-B and Cr-Ni alloys prepared by sputtering was related with grain size. The alloying of B or Ni to Cr reduces the grain size of the alloys. The increase in volume of grain boundaries or amorphous phase induced by alloying elements causes the departure of strength of nanostructured Cr alloys from the values estimated by the Hall-Petch relation. The formation and microhardness of Cr or Ti base amorphous alloys could be discussed by the chemical bonding between elements. The strength of dissimilar joints was dominated by the thickness and structure of the interfacial reaction zone between SiC and metal. The formation of interfacial reaction zone is discussed by the change of chemical potentials of elements between ceramics and metal.