Defect and Diffusion Forum Vols. 297-301

Abstract: Transport of hydrogen through oxide layers formed on zirconium alloys has been the focus of studies associated with hydrogen ingress into nuclear reactor core components. The studies have shown that microstructure and microchemistry of the underlying alloy can affect the characteristics of the oxide and in turn the transport of hydrogen through the oxide and into the underlying metal. In certain cases the oxide layer can be a homogeneous medium for hydrogen diffusion, while in most cases it is found to be heterogeneous and comprised of homogeneous, nonporous and fully oxidized, cells surrounded by a network of fast diffusion paths for hydrogen. Since in such heterogeneous systems the derived diffusion parameter and the cell size are interdependent, an absolute diffusion parameter could only be derived if the cell size were known. However, no such information is available from the microstructural studies on the oxides grown on these alloys. Another alternative is to carry out the measurements in a material, which is homogeneous at least within the dimensions of a few μm - the range of the measurements. With this in mind, hydrogen diffusion measurements were carried out in a set of single crystal zirconia specimens with the prospect that the single crystals would provide a non-porous and homogeneous medium to study the diffusivity of hydrogen. These measurements show that the single crystal specimens, contrary to the initial thinking, are not entirely homogeneous and the results do not yield an absolute diffusion coefficient for hydrogen in zirconium oxides. The details and analyses of the results from the single crystal zirconia specimens are discussed in this paper.

Abstract: Widely used in mechanics is the method of surface parameters investigation by indentation of some kind of penetrator into test surfaces and registration of the indentation/load dependence through the whole test duration. A small ball made of hardened steel or diamond pyramid, so-called Berkovich pyramid, is used for penetration. An actuator performing the indentation process is used for displacement enforcement as well as for load measurement. Mainly two types of devices are in use: based on piezoelectric effect or purely electromagnetic LVDT. Knowledge about the penetrator tip is essential for the proper evaluation of the measured parameters and knowledge about the actuation method is also desired for better identification of the whole system parameters when using DSP type algorithms at the stage of experimental data elaboration. In this paper we deal with some kind of DSP elaboration of the experimental data. Our attitude is based on successive digital filtration applied to the experimental data as well as to the intermediate stages of calculation, especially for compliance estimation. Different digital filters have been developed and applied to the experimental data. Some conclusions have been taken out due to the precision of the layer thickness estimation based on data obtained by indentation of the investigated surface with a Berkovich pyramid driven by a piezoelectric actuator. The Boussinesq/Sneddon theory has been used as the basis of our analysis. Titanium azide layers imposed on a magnesium alloy have been tested using PVD method. Obtained results especially due to the hardness/indentation plot allow evaluating the layer thickness, which should be also compared with thickness values evaluated by other methods.

Abstract: For this research it was considered that the heat exchanger was affected by leakage in the head across the partition plate and the wall between the tube passes. Leakage was a problem in the plastic shell and tube heat exchanger, because it was difficult to seal the partition plate to the head of the exchanger. The material used for manufacturing the heat exchanger was polyvinylidene fluoride, PVDF. In order to predict the amount of flow leaking through the clearances of the tube passes, a numerical simulation was carried out using the computational Fluid Dynamics CFD Fluent Software. To obtain the percentage of the heat loss across the 4 tube passes, different clearance sizes between the partition plate and the wall of the head of the exchanger were analysed. For the smaller clearance size of 0.2 mm the heat transfer coefficient was reduced up to 15%. These results suggest that the flow mass bypassing the head between tube passes affect the results of the heat transfer coefficient and confirm the experimental observation, that its performance was affected by leakage between tube passes. This research served as an extension of the preliminary plastic heat exchanger design.

Abstract: The side effect of electrosurgery includes tissue charring, smoke generation and the adhesion of tissue to electrodes. These effects prolong surgery and interfere with effective coagulation. In this paper, CrWNx, CrOx and ZrOx coating were prepared by an unbalanced magnetron sputtering. The microstructure of films was characterized using XRD, XPS, TEM and AFM. The hydrophobicity and surface energy of coatings were calculated by contact angle measurement and Wu harmonic mean approach. Anti-sticking in vitro test was performed by monopolar electrosurgery using pork liver tissue. The hardness of CrWNx , ZrOx and CrOx coatings were 44 GPa, 26.3 GPa and 20.7 GPa, respectively. The CrOx coating had the lowest surface energy 33.5 mN/m and the highest contact angle of water as high as 103°. The high surface O-H bonds density of CrOx coating and N-H bonds density of CrWNx coating could explain about their lower polar component of surface energy. All the three PVD coatings remarkably reduced the quantity of tissue adhesion on the electrode from about 2 times (ZrOx and CrWNx coatings) to 4.88 times (CrOx coating) than uncoated SUS304 electrode.

Abstract: A two-dimensional diffusion mathematical model with moving boundary conditions was developed to evaluate the diffusion kinetics of bovine serum albumin (BSA) through the network of poly(N-isopropylacrylamide) hydrogel (poly(NIPAAm)). These thermosensitive hydrogels were experimentally tested for their response to BSA by exposing the hydrogel disc-shaped geometry to different temperatures and varied protein concentration. The BSA release, which is coupled with hydrogel shrinking when reaching the low critical solution temperature (LCST) of poly(NIPAAm), could be satisfactory described by the model. During the early course of hydrogel shrinking, the hydrogel outermost surface layer collapses to form a dense layer in comparison to the interior bulk matrix. Due to the hydrophobic interaction between polymer chains and polymer protein, the formed layer is thick and dense enough to restrict the outward permeation of entrapped BSA molecules from the hydrogel interior, which greatly slows down the release rate. A good agreement between experimental and calculated data was achieved.

Abstract: This paper addresses the key issue of calculating fluxes at the control-volume interfaces when finite-volumes are employed for the solution of partial differential equations. This calculation becomes even more significant when unstructured grids are used, since the flux approximation involving only two grid points is no longer correct. Two finite volume methods with the ability in dealing with unstructured grids, the EbFVM-Element-based Finite Volume Method and the MPFA-Multi-Point Flux Approximation are presented, pointing out the way the fluxes are numerically evaluated. The methods are applied to a porous media flow with full permeability tensors and non-orthogonal grids and the results are compared with analytical solutions. The results can be extended to any diffusion operator, like heat and mass diffusion, in anisotropic media.

Abstract: A mathematical model was developed to describe the coupled heat and mass transfers in membrane processes. Equations for the heat and mass transfer resistances were derived and the coupling effects of the heat and mass transfer were analyzed. With taking the membrane separation process of moist air as an example, the effects of air temperature and water vapor concentration on the heat and moisture transfer process were investigated. The results show that neither the thermal resistance nor the moisture resistance are constant, they are affected by not only the membrane parameters but also the air state. As the temperature difference between the two airstreams separated by the membrane increases, both the thermal and moisture resistances decrease, causing an improved heat and mass transfer. As the average temperature of the two airstreams increases, the thermal resistance remains almost constant while the moisture resistance decreases significantly. Further, as the water vapor concentration difference between the two airstreams increases, both the thermal and moisture resistances increase. As the average water vapor concentration of the two airstreams increases, the thermal resistance remains unchanged while the moisture resistance decreases.

Abstract: A method that combines the experimental measurements and numerical simulations to determine the moisture diffusivity in a membrane has been developed. An experimental set-up was designed and constructed to measure the total moisture resistance. The test section consists of an airflow channel, a membrane, and a water tank, which form a sandwich structure. The process of moisture transport from the water surface to the airstream in the channel is numerically simulated to obtain the variation of the total moisture resistance with the moisture diffusivity in the membrane, which is then determined by comparing the experimental and numerical total moisture resistances. There are three features with the present method, i.e., simple structure of the test section, combination of the experiment and simulation, and consideration of the boundary layer resistances on both sides of the membrane. Tests were conducted on two PVDF membranes with 0.22 and 0.45 μm mean pore diameters. The results show that the moisture diffusivities in both membranes are in the order of 10-6 kgm-1s-1, with a larger pore size tending to yield a larger diffusivity. The moisture diffusivities in both membranes are insensitive to the airflow rate.

Abstract: Oxygen out-diffusion from CZ-Si or in-diffusion into FZ Si was studied with Secondary Ion Mass Spectrometry. For anneals up to 1200oC , the value of 16O concentration Csurf that develops at the semiconductor surface in bare wafers with ~2 nm thick natural oxide was found to be comparable to that in thermally oxidized wafers with 260 nm thick SiO2. Beyond this temperature, at 1280oC, Csurf in non-oxidised sample appears 5 times lower that in the oxidised one. This means that the spontaneous oxide becomes permeable to the oxygen species and no longer constitutes a barrier preventing the oxygen atoms from direct out-diffusion into the ambient. The effect becomes more pronounced when the specimens were heated to 1150oC in a chamber evacuated to ~ 10-7 Torr. The resultant bare Cz-Si and, to even greater extent, FZ Si with previously in-diffused oxygen exhibited deep oxygen depletion, 10x below the respective values for annealing under atmospheric pressure. Undoubtedly, it is the removal of residual oxides by sublimation that enables oxygen atoms to freely out-diffuse into vacuum.

Abstract: In order to investigate the high temperature exposure effect on Nb Ti/Cu superconducting strands, as might be encountered in joining by soldering and in cabling annealing, X-ray diffraction and resistometry measurements were performed in situ during heat treatment, and complemented by conventional metallography, mechanical tests and superconducting properties measurements. Changes of the Nb Ti nanostructure at temperatures above 300°C are manifested in the degradation of critical current in an applied external magnetic field, although degradation at self field was insignificant up to 400°C for several minutes. Above 500°C, the formation of various Cu Ti intermetallic compounds, due to Ti diffusion from Nb Ti into Cu, is detected by in situ XRD albeit not resolvable by SEM-EDS. There is a ductile to brittle transition near 600°C, and liquid formation is observed below 900°C. The formation of Cu Ti causes a delayed reduction of the residual resistivity ratio (RRR) and adversely affects the deformation behaviour of the strands.