Defect and Diffusion Forum Vols. 297-301

Abstract: Evolution of interdiffused Gaussian-shape nanolayer of Au-Si, formed due to diffusion of Au into Si(111) substrate at ambient conditions, depends strongly on the Si surface pretreatment/passivation conditions. Negligible diffusion in the Au-OSi(111) sample, confirms the strong barrier action of the oxide-layer against diffusion, while large diffusion in the Au-HSi(111) sample compared to that in the Au-BrSi(111) sample suggests that the H-passivated Si(111) surface is more stable. This nature of the Au-Si(111) system is qualitatively similar to that of the Au-Si(001) system but it differs quantitatively. The size, electronegativity and bond-energy of the passivating elements and the number of dangling bonds on the Si surface influence the instability of the Si surface. This instability, parameterized by growth-time of oxide layer alone, can be utilized to tune the amount of diffusion into the sub-surface Si region. The distribution of growth-time and fractional passivated area, which are related to the improper Si surface passivation, are against such control and needs perfection.

Abstract: Gd2O3 doped CeO2 nanopowders were prepared under high temperature and pressure conditions by precipitation from metal nitrates with aqueous ammonium hydroxide. Spherical shape Gd2O3 doped CeO2 nanopowders were obtained By a reaction at the temperature range of 200°C – 230°C for 6 h. The average size and size distribution of the synthesized particles were below 10 nm and narrow, respectively. The XRD diffraction pattern shows that the synthesized powders were crystalline. This study has shown that the synthesis of Gd2O3 doped CeO2 nanopowders is possible under glycothermal conditions in ethylene glycol solution. The effects of synthesis parameters, such as the solvent ratio of starting solution, pH of starting solution, reaction temperature and time, are discussed.

Abstract: This paper presents the comparison of performance characteristics for the several natural gas liquefaction cycles. The liquefaction cycle with the staged compression was designed and simulated for improving the cycle efficiency using HYSYS software. This includes a cascade cycle with a two-stage intercooler which is consisted of a Propane, Ethylene and Methane cycle. In addition, these cycles are compared with a modified staged compression process. The key parameters of the above cascade cycles were compared and analyzed. The COP (Coefficient of Performance) of the cascade cycle with a two-stage intercooler and a modified staged compression process is 13.7% and 29.7% higher than that of basic cycle. Also, the yield efficiency of LNG (Liquefied Natural Gas) improved compared with the basic cycle by 28.5%.

Abstract: We present phase-field simulations of isothermal phase transformations in the peritectic system below and above the peritectic temperature TP , and in the monotectic system below the monotectic temperature TM. We focus particularly on the Liquid-Film-Migration (LFM) mechanism, which appears to be the generic process for phase transformations above TP . Below TP , we obtain an assymetric LFM, suggesting the existence of a doublon structure in free space. In the monotectic system, the transformation from a liquid L1 to a solid+liquid L2 mixture proceeds via the migration of a L2 film, which is the analogous of the LFM process. When the metastable state consists of a liquid-liquid mixture, a dendritic-like solidification is obtained.

Abstract: Laser gas nitriding of Ti-6Al-4V alloy was carried out using a Nd:YAG pulsed laser under pure nitrogen environment at a flow rate of 30 l/min. The microstructure and corrosion behavior of the nitrided samples were examined using scanning electron microscopy, XRD, XPS, and anodic polarization tests in 2M HCl solution. For comparison, untreated samples were tested under the same conditions. After laser treatment, samples showed a relative flat surface with no problems of cracks or delamination of the alloyed tracks. Laser nitriding produced dendritic structures. The microstructure of the laser melted zone consisted of a thin continuous layer followed by a nearly perpendicular growth of dendrites. Below this a mixture of small dendrites and large needles with random orientation was produced. X-ray spectrum and XPS analyses from the surface of the laser nitrided specimen at different depth, confirmed that the thin top layer and large dendrites close to the surface corresponded to TiN. It can be also seen that the strong TiN peaks on the top surface gradually decrease with depth which suggests that the structure beneath the top surface is likely TiN0.3 and ά-Ti mixtures. In general, the corrosion potential of laser gas nitrided specimens was relatively nobler than the untreated sample. Furthermore, the proper laser nitrided specimen exhibited less corrosion current density, passivated more readily and also maintained a lower current density over the duration of the experiment. This was correlated with the formation of very thin, continuous TiNxOy film in an oxidation state that was confirmed by XPS analyses of the passive layers.

Abstract: This work investigates the characteristics of oxide films formed on Ti and the Ti alloy through a plasma electrolyte oxidation (PEO) process in highly alkaline medium in the presence or absence of phosphate ions. The obtained coatings showed different characteristics when they formed in high alkaline phosphate solutions particularly was anatase for Ti and rutile/titanium phosphate for the Ti alloy layer. Films formed in aqueous solution without KOH caused a reduction in current density and also reduced the number of microarcs, while in electrolytes with high OH- concentration, the current was low and the density of sparks was significant.

Abstract: As is well known, the explosion welding is considered for a specific type of cold pressure welding. Welded joints fabricated by this technology show a typical undulated boundary, which characterises both, the welding process and bond quality. This technology allows to weld a relatively wide assortment of combinations of various metals and their alloys [1,2]. This contribution presents the technological procedure for preparation of composite materials based on three metals (trimetals) by use of industrial explosive Semtex S. It concerns the trimetals as structural carbon steel-Cu-Al and structural carbon steel-Ti-Al. For the anodes and cathodes of electrolysers, used for manufacture of aluminium, mostly bimetals were used up to now, as for example the structural carbon steel – Al. However, the mentioned trimetals exert certain priorities over bimetals, regarding mainly the structural stability at elevated temperatures. This also results in higher reliability at breakdown situations and also in longer life of weldments proper. The mentioned trimetals were prepared at Explosia Inc., Research Institute of Industrial Chemistry Pardubice-Semtín. The bond quality was assessed by optical microscopy, microhardness measurement across the bond boundary and also by X-ray microanalysis.

Abstract: The aim of the present work was to solve the technology for welding Mg alloy type AZ 63 with aluminium and to assess the quality of the fabricated joints (bimetals-composites). In solving the welding technology, the high affinity of Mg to oxygen has to be considered and therefore as suitable technology either metallurgical joining in vacuum (electron beam etc.) or high-speed solid state welding seem to be feasible. Explosion welding with Semtex S 30 explosive was approved experimentally. Parallel arrangement of welded materials was applied. The more plastic Al material was accelerated. Quality of bonds (bimetals) was assessed by defectoscopy (ultrasound), optical microscopy, microhardness measurement and X-ray microanalysis. It can be generally stated that the basic requirement on joint quality (namely the undulated boundary) was met. The structural composition and absence of inhomogeneities in fabricated bimetals suggest that the desired quality was achieved.

Abstract: Hard coatings show various tribological behaviors against different contact materials (counter-faces) during dry sliding depended on their microstructure, surface morphology and encountered tribological systems and condition. In this work, the tribological and wear mechanisms of vanadium carbide (VC) and vanadium nitrocarbide (V(N,C)) layers were examined against WC/Co cemented tungsten carbide pin during pin-on-disk sliding wear testing. The V(N,C) layer was produced by a duplex surface treatment involving the gas pre-nitrocarburising followed by thermo-reactive diffusion (TRD) vanadizing technique. The coating layers were characterized by a cross sectional and morphological examination methods and X-ray diffraction analysis to identify damages of the coating’s surface. Wear mechanisms were determined by SEM microscope in BSI and SE mode accompanied by EDS analysis. Results revealed that the surface morphology of the V(N,C) coating consist of dense and smooth layer in comparison with the VC coating surface which reveals a non-uniform structure with chasms. It was determined that the activation of tribo-chemical system and oxidizing of the coating layer together with minor plastic deformation are the dominant wear mechanism in the V(N,C) coated steel. In the case of the VC coating, combination of abrasive wear and adhesion of pin material to coating and vice versa are the major impairing mechanisms.

Abstract: A novel Metal Micro-Textured Thermal Interface Material (MMT-TIM) has been developed to address a number of shortcomings in conventional TIMs. This material consists of a thin metal foil with raised micro-scale features that plastically deform under an applied pressure thereby creating a continuous, thermally conductive, path between the mating surfaces. One of the difficulties in experimentally characterizing MMT-TIMs however, is distinguishing the bulk thermal resistance of the MMT-TIM from the thermal contact resistance that exists where it contacts the test apparatus. Since these materials are highly electrically conductive, this study attempts to employ electrical contact resistance measurements to estimate their thermal contact resistance. Tests using flat silver and gold specimens of known bulk thermal conductivity were used to develop a correlation between electrical and thermal contact resistance. This relationship was then employed to estimate the thermal contact resistance of a prototype silver MMT-TIM and indicates the thermal contact resistance accounts for approximately 10% of the measured thermal contact resistance. A number of issues related to this technique are discussed as well as its future outlook.