Measurement of water content distribution in catalyst coated membranes under water permeation conditions by magnetic resonance imaging

The through-plane water content distribution in a polymer electrolyte membrane (PEM) and in catalyst coated membranes (CCMs) and the flux of water through these membranes were determined under water permeation conditions by means of magnetic resonance imaging and dew point measurements. The PEM or CCM was placed in an experimental cell and subjected to water permeation conditions with various water activities across the membrane. A comparison of the water content distribution and the water flux in the various samples showed that the PEM and CCMs have different water transport properties. These differences suggested that diffusion through the membrane and interfacial transport across the membrane-gas interface, which are the rate determining processes for water transport through the membrane, change depending on whether the membrane has a catalyst coating.     

Effects of operating and design parameters on PEFC cold start

During startup from subzero temperatures the water produced in a polymer electrolyte fuel cell (PEFC) forms ice/frost in the cathode catalyst layer (CL), blocking the oxygen transport and causing cell shutdown once all CL pores are plugged with ice. This paper describes an experimental study on the effects of operating and design parameters on PEFC cold-start capability. The amount of total product water in mg cm⁻² during startup is used as an index to quantify the cold-start capability. The newly developed isothermal cold-start protocol is used to explore the basic physics of cold start, and the effects of purge methods prior to cold start, startup temperature and current density, and the membrane thickness are shown. The experimental data also confirm the current density effect predicted earlier by a multiphase model of PEFC cold start.     

Improved bioabsorbability of synthetic hydroxyapatite through partial dissolution-precipitation of its surface

Even though synthetic hydroxyapatite (HAp) has a chemical composition similar to the mineral phase of bone, it is minimally absorbed and replaced by bone tissue. This could be because HAp is composed of compactly arranged apatite crystals with homogenously large grains. In this study, the surface and non-stoichiometry of the synthetic HAp crystals was modified by partial dissolution and precipitation (PDP) to improve bioabsorbability of HAp. In vitro cell culture demonstrated that more osteoclasts were activated on PDP-HAp compared with HAp. In vivo implantation using a rabbit bone defect model revealed that PDP-HAp was gradually degraded and was replaced by bone tissue. Consistent with the in vitro results, more osteoclasts were activated in PDP-HAp than in HAp, indicating that the former was absorbed through the stimulation of osteoclastic activity. These results suggest that the PDP technique may have clinical utility for modifying synthetic HAp for use as superior bone graft substitutes.     

Heparinization of a biomimetic bone matrix: integration of heparin during matrix synthesis versus adsorptive post surface modification

This study intended to evaluate a contemporary concept of scaffolding in bone tissue engineering in order to mimic functions of the extracellular matrix. The investigated approach considered the effect of the glycosaminoglycan heparin on structural and biological properties of a synthetic biomimetic bone graft material consisting of mineralized collagen. Two strategies for heparin functionalization were explored in order to receive a three-component bone substitute material. Heparin was either incorporated during matrix synthesis by mixing with collagen prior to simultaneous fibril reassembly and mineralization (in situ) or added to the matrix after fabrication (a posteriori). Both methods resulted in an incorporation of comparable amounts of heparin, though its distribution in the matrix varied as indicated by TOF-SIMS analyses, and a similar modulation of their protein binding properties. Differential scanning calorimetry revealed that the thermal stability and thereby the degree of crosslinking of the heparinized matrices was increased. However, in contrast to the a posteriori modification, the in situ integration of heparin led to considerable changes of morphology and composition of the matrix: a more open network of collagen fibers yielding a more porous surface and a reduced mineral content were observed. Cell culture experiments with human mesenchymal stem cells (hMSC) revealed a strong influence of the mode of heparin functionalization on cellular processes, as demonstrated for proliferation and osteogenic differentiation of hMSC. Our results indicate that not only heparin per se but also the way of its incorporation into a collagenous matrix determines the cell response. In conclusion, the a posteriori modification was beneficial to support adhesion, proliferation and differentiation of hMSC.     

A new approach to cutting state monitoring in end-mill machining

A new cutting state monitoring approach is proposed for the real-time predicting of the machining trouble and the surface quality of the machined products. In this approach, the relationships among the mechanical model of cutting process and its corresponding time series model, the surface roughness of the machined workpiece are evaluated through theoretical analysis and experimental investigation. It is therefore revealed that there is the linear relationships among the AR parameter a₁, the stiffness k₃ of cutting model and surface roughness P_z, and consequently the cutting process state can be estimated by only monitoring time series parameter a₁ of vibration signal measured during machining operation. In particular, it was found that the variation in the surface roughness of P_z=3–5 μm can be fully monitored.     

Atomic-scale theory on current-assisted thermochemical degradation mode and its field acceleration via charge trapping of O vacancy in HfSiO4

An atomic-scale model for current-assisted field-accelerated thermochemical degradation via charge capturing by O vacancy (V_O) is proposed based on first-principles calculations. We found that the electron current enhances the V_O formation around the V_O itself and the hole current makes the V_O transform into the puckered structure. The calculated activation enthalpy for breakdown and the effective dipole moment agree well with the experimental values. We also found that the subordinate carriers in the generated subordinate carrier injection (GSCI) model play critical roles through a disproportion reaction of positively charged V_Os.     

An experimental study on rewetting phenomena in transient conditions of bwrs

It is known that rod temperature rise after boiling transition (BT) is not excursive and that the peak cladding temperature (PCT) is suppressed by rewetting to return to nucleate boiling, even if BT occurs under severe conditions exceeding abnormal operational transients for a BWR. The purpose of this study is to develop and verify the rewetting correlation. The rewetting correlation was developed based on single rod data, as a function of quality, mass flux, pressure and heat flux. The transient thermal-hydraulic code used in the BWR design analysis (SCAT) with this rewetting correlation was compared with transient rod temperature result after the occurence of BT obtianed by the 8×8 and 4×4 rod bundle. It is concluded that the transient code with the developed rewetting correlation predicts the PCT conservatively, and the rewetting time well.     

Analytical study of effects of finger-growth velocity on reaction characteristics of reactive miscible viscous fingering by using a convection-diffusion-reaction model

Reactive miscible viscous fingering occurs when a reactive and miscible less-viscous liquid displaces a more-viscous liquid. Effects of bulk finger-growth velocity on reaction characteristics of miscible viscous fingering with a chemical reaction were studied analytically by using a convection-diffusion-reaction model. The model assumes the existence of a distinct interface between both liquids, assumes the existence of a two-dimensional, steady, stagnated flow field in the less-viscous liquid, and assumes an infinite chemical reaction rate. The model was then used to determine the reaction characteristics, such as the location of the reaction surface and the profile of the product, as functions of the velocity and initial reactant concentrations. The results reveal that the effects of the velocity on the reaction characteristics can be divided into low-, intermediate-, and high-velocity regions. In the low-velocity region, the reaction characteristics strongly depend on the reactant concentrations. In the intermediate-velocity region, the dependence of the reaction characteristics on the reactant concentrations decreases with increasing velocity. In the high-velocity region, the reaction characteristics are nearly independent of the reactant concentrations. Experiments confirm the existence of these three velocity regions predicted by the model.     

The Surface Density of States of Superfluid 3He for Different Surface Boundary Conditions

We measured the complex transverse acoustic impedance of superfluid ³He in the B phase by coating a surface with ⁴He to study the boundary condition dependence of the surface bound states. Observed features in the temperature dependence were similar to those without ⁴He coating. However, temperatures showing singularities became higher and variations in the dependence became smaller. The small variations indicated that a specular surface was realized by the ⁴He coating. Increase in the singularity temperature indicated that the bandwidth of the surface Andreev bound states became broader by the coating.