Papers by Keyword: Surface Reaction

Abstract: We investigate the possibility to use silicon, titanium and tungsten as bonding materials between a SiC substrate and a SiC layer, a novel substrate for application in high-power electronics. By using transmission electron microscopy, scanning transmission electron microscopy and X-ray scattering techniques, we address the high temperature-induced phase and morphology changes in thin layers composed of these materials and at their interfaces with SiC. For all three materials, we show that the homogenous continuous film created after low temperature deposition transforms into a discontinuous structure following high temperature annealing. All layer’s structures tend to reach an epitaxial relation with the SiC substrates. In contrast to Si layer which preserves its composition, both Ti and W layers are transformed into new phases which were identified. We evidence that these peculiar structural and compositional changes in the layers, which were studied as a function of annealing temperature and time, are related to mechanisms of SiC dissolution and transport of C, Si, Ti, W atoms at the interface. Potential chemical and structural reactions during interface reconstructions are discussed in relation to the experimental findings.

Abstract: The platinum (Pt) degradation, poisoning and carbon corrosion in acidic fuel cell has led to explore the research in alkaline fuel cell. However, the high cost of Pt has brought a lot of studies to find replacement for Pt catalyst. Due to that, silver metal is selected as non-Pt catalyst and supported by the nitrogen and phosphorus-doped on graphene for oxygen reduction reaction in alkaline medium. The adsorption energy and mechanism of the oxygen reduction reaction is studied by using density functional theory (DFT) calculation. The support catalyst of graphene is doped with three atom nitrogen and phosphorus namely as N3 and P3, respectively. The Ag supported on N3 and P3 are tested on O₂, OOH, O and OH species. There are two types adsorption of O2 on N3 and P3 which is side and end-on adsorption configuration. The N3-Ag has similar adsorption energy for both configurations, but P3-Ag has low adsorption energy by end-on adsorption configuration. The effect of doped atoms on graphene also have been tested on O₂, OOH, O and OH species. The result shows that increasing nitrogen doping atom has decreased the adsorption energy of O2 and vice versa on phosphorus atoms. A single phosphorus doping atom on graphene has shown the lowest adsorption energy, but the end-on configuration of P3-Ag has shown most stable adsorption. The schematic free energy profile shows that both N3-Ag and P3-Ag have high possibilities to be followed in oxygen reduction reaction mechanism but P3-Ag has advantage due to stable adsorption as non-Pt catalyst. The Ag metal supported on nitrogen and phosphorus-doped graphene show promising result to be a catalyst in alkaline fuel cell.

Abstract: Oxygen diffusivity in strontium titanate films with intentional non-stoichiometry, Sr_1+xTi_1+yO_δ with (1+x)/(1+y) = 0.71.5, was studied to reveal degradation behavior of Sr_1+xTi_1+yO_δ films. In order to utilize isotope tracer diffusion measurement, so-called gas/solid exchange method using ¹⁸O₂ gas, at relatively low temperature, very thin YSZ layer deposited on top of Sr_1+xTi_1+yO_δ film was used as catalyst for enhanced ¹⁸O[gas]/¹⁶O[solid] exchange at lower temperature. As a result, very high oxygen diffusivity at 673 K in Sr_1+xTi_1+yO_δ films with (1+x)/(1+y)>>1 was observed.

Abstract: Bioceramics used as coatings show different biocompatibility and bioactive behavior in relation to their chemical and morphological behavior. Bioactive ceramics such as β-tricalcium phosphate (β-TCP) promote and enhance biological fixation. Stable coatings require an optimum between resorption rate, flexural strength and adhesive strength of the coating. Therefore new bioceramic coating materials that ensure the balance between loss of substances and osteointegration need to be designed and investigated. By modifying the high velocity suspension flame spraying (HVSFS) process parameters, five coatings with different materials were obtained. The in vitro cytotoxicity was determined by the microculture tetrazolium (WST) assay after 24, 48 and 72 h. Cells were grown on the materials for 3, 7, 14, and 21 days and counted. Cell morphology, cell attachment, and cell spreading were investigated using fluorescence microscopy and raster scanning electron microscopy. All substrates supported sufficient cellular growth for 19 days and showed no cytotoxicity. On each material an identical cell colonisation of well communicating, polygonal, vital cells was verified.

Abstract: For resistive oxygen sensor elements of Ce1-xZrxO2 (x = 0.0, 0.05, 0.1, 0.2), grain diameter was varied in the range of 86 – 300 nm by changing sintering temperature or changing Zr content. The grain diameter decreased with increasing Zr content. The response time was approximately proportional to the square of the grain diameter. In the relationship between the amplitude of sensor output, An and the frequency, f of sine wave of variation in oxygen partial pressure, the gradient in the high-frequency region of a plot of log An vs. log f in was approximately –0.5. From these results, it was concluded that the sensor response was determined by the oxygen vacancy diffusion rate. The grain diameter of Ce0.8Zr0.2O2 element was 86 nm and the response time at 1073 K was 9 ms, which result opens the door to the technological development of independent control of engine cylinders.

Abstract: Bioactive ceramics such as β-tricalcium phosphate (β-TCP) promote and enhance biological fixation. Ceramics with a porous interconnected structure are suited for facilitation of bony ingrowth. An interconnected pore system with pore diameters in excess of 100 µm is required for cell penetration, tissue ingrowth, vascularization and nutrient delivery to the centre of the regenerating tissue. Human osteoblasts were cultured on the surface of a ceramic. In an in-vivo study, β-TCP samples with a porous interconnected structure were implanted into the femur of sheep and then investigated 6 weeks after operation. Histological analysis was performed on the area surrounding the implant. An indentation test was performed to complete failure of the bone/ceramic compound. Linear load, peak load and stiffness were recorded. All cylinders were found to be biocompatible and osteoconductive. Bone was more abundant in the outer ring than in the rest of the cylinder. The ceramic/bone compound was of low mechanical grade.

Abstract: Bioactive ceramics such as hydroxypatite (HA) promote and enhance biological fixation. There is still a discussion on the desired longevity of the coating. Stable coatings require an optimum between resorption rate, flexural strength and adhesive strength of the coating. Ceramic coatings containing fluorapatite (FA, Ca5(PO4)3F) and calcium zirconium phosphate (CZP, CaZr4(PO4)6) promise lower resorption rates than conventional HA coatings in the biological milieu. It is hoped that they can improve the long-term stability of implants by eliminating the detrimental resorption of coating material. For the in vivo studies plasma sprayed coatings were generated. The materials were implanted into the distal femur epiphysis of rabbits and investigated after 2, 4, 6, 12, and 24 weeks postoperatively. Histological analysis was preformed on the areas surrounding the implant. The amount of osseointegration was determined by using the automatically image analysis. The bonding strengths were compared with HA coating and uncoated titanium alloy. According to available data, there is inhibition of mineralization of bone at the interface of calcium zirconium phosphate ceramics of the described composition.

Abstract: The origin of the polytypes of SiC has been investigated from the viewpoint of surface reactions by the density functional theory (DFT) within the Projector Augmented Wave-Generalized Gradient Approximation. Three radicals were considered here as the major species in the crystal growth process: Si, Si2C and SiC2. We supposed that these radicals contribute to the crystal growth directly through the adsorption on the 4H-SiC (000-1) C-face surface. The DFT calculations showed that the Si2C, which relatively has a similar structure with the SiC crystal, had no activation barrier to be adsorbed chemically to the 4H-SiC C-face surface. On the other hand, SiC2 with Si showed an activation barrier of 0.79eV to form the 4H-SiC crystal. In order to investigate the arrangements to decide polytypism in SiC, we compared the adsorption energies between the different sites, which correspond to the 4H-SiC crystal and a disordered arrangement. The activation energies had almost no difference. Our calculations indicated that these radicals do not contribute to the origin of the polytypes of SiC.

Abstract: High temperature chemical vapor deposition (HTCVD) simulations of silicon carbide (SiC) were demonstrated with experimental results. A vertical cylindrical reactor was used in an RF inductive heating furnace and the temperature was more than 2200
. SiH4 and C3H8 were used as source gases and H2 as carrier gas. A gas phase reaction model from the literature was used on the condition that the gas phase reaction is a quasi-equilibrium state. It was found that the major species were Si, Si2C, SiC2 and C2H2 in the gas phase reaction model as well as in the thermodynamic equilibrium calculation. Sublimation etching was considered in the surface reaction rates by modifying partial pressures of species with equilibrium vapor pressures. CFD-ACE+ and MALT2 software packages were used in the present calculation. The sticking coefficients were determined by fitting the calculated growth rates to the experimental ones. The simulated growth rate in a different reactor is in good agreement with the experimental value, using the same sticking coefficients. The present simulation could be useful to design a new reactor and to find optimum conditions.

Abstract: The reaction of animal bone to Ti6Al4V pins coated with ceramics of Ca2KNa(PO4)2 or Ca10[K/Na](PO4)7 as main crystalline phases were tested. It is shown that the new materials possess a very high resorption in comparison with hydroxyapatite (HA).