Effect of Sintering Temperature to the Mechanical Properties of Different Filler Loading of Zirconia Powder in Hydroxyapatite Composites

Hydroxyapatite (HA)-Zirconia (ZrO2) composite with varying zirconia composition ranging from 1 to 10 wt% was investigated for biomedical applications in order to produce high compressive strength. Precipitation method was used to prepare both hydroxyapatite and zirconia powders. To find the ideal composition, mixture-containing 1, 3, 5 and 10 wt% ZrO2 powder was added. Each mixture was sintered for 4 hours at 750oC, 1050oC and 1250oC. Hardness and compressive strength test were used for evaluation. It was found that with 1 wt% of ZrO2 sintered at 1250oC showed the greatest structural strength as its volume fraction porosity is the lowest. The hardness and compressive strength of this sample were found to be 2.75 GPa and 72.0 MPa respectively. This can be useful for biomedical applications especially in promoting osteo-integration.

EFFECT OF POROSITY ON BEHAVIOURS OF PLATE STRUCTURES

In this paper, effect of porosity on nonlinear analysis of plate structures is presented. Two porous distributions are considered. Governing equations are expressed by using isogeometric analysis (IGA) and the third-order shear deformation theory (TSDT). With these approaches, it is easy to fulfil the C1-continuity requirement of the plate model. The obtained results demonstrate the significance of porosity volume fraction, porosity distributions and volume fraction exponent on nonlinear analysis of the plate structures.

Modeling microelectrode biosensors: free-flow calibration can substantially underestimate tissue concentrations

Microelectrode amperometric biosensors are widely used to measure concentrations of analytes in solution and tissue including acetylcholine, adenosine, glucose, and glutamate. A great deal of experimental and modeling effort has been directed at quantifying the response of the biosensors themselves; however, the influence that the macroscopic tissue environment has on biosensor response has not been subjected to the same level of scrutiny. Here we identify an important issue in the way microelectrode biosensors are calibrated that is likely to have led to underestimations of analyte tissue concentrations. Concentration in tissue is typically determined by comparing the biosensor signal to that measured in free-flow calibration conditions. In a free-flow environment the concentration of the analyte at the outer surface of the biosensor can be considered constant. However, in tissue the analyte reaches the biosensor surface by diffusion through the extracellular space. Because the enzymes in the biosensor break down the analyte, a density gradient is set up resulting in a significantly lower concentration of analyte near the biosensor surface. This effect is compounded by the diminished volume fraction (porosity) and reduction in the diffusion coefficient due to obstructions (tortuosity) in tissue. We demonstrate this effect through modeling and experimentally verify our predictions in diffusive environments. NEW & NOTEWORTHY Microelectrode biosensors are typically calibrated in a free-flow environment where the concentrations at the biosensor surface are constant. However, when in tissue, the analyte reaches the biosensor via diffusion and so analyte breakdown by the biosensor results in a concentration gradient and consequently a lower concentration around the biosensor. This effect means that naive free-flow calibration will underestimate tissue concentration. We develop mathematical models to better quantify the discrepancy between the calibration and tissue environment and experimentally verify our key predictions.

The void fraction of melter feed during nuclear waste glass vitrification

ABSTRACTTo efficiently vitrify Hanford waste, the melting process (i.e., melter feed turning into waste glass) must be modeled and optimized. The rate of heat transfer to the melter feed in a waste glass melter, and thus the rate of melting, is strongly affected by the melter feed porosity, especially in the final stages where the glass-forming melt produces foam that insulates the feed from the molten glass. The volume expansion test allows the determination of the melter feed porosity as a function of temperature. This test measures the profile area of the feed pellet as it turns into glass. This contribution presents the calculation of the void fraction (porosity) of the melter feed as a function of temperature, heating rate, and material parameters. The process of finding the void fraction is described as well as results from the application of this process.

Application of Homogenization Theory to Study the Mechanics of Cortical Bone

Homogenization theory is utilized to study the effect on the axial stiffness of secondary osteons in cortical bone due to the presence of micro porous features (e.g., lacunae, canaliculi clusters, and Haversian canals). Specifically, 2 geometric characteristics were used to describe these features within the secondary osteons: volume fraction (% porosity) and shape (circular- or elliptical-shaped). Such information was determined for each individual porous feature from an image segmentation methodology developed earlier by Hage and Hamade. For each feature, aspect ratio vectors (or arrays of ratios for each individual porous feature) were used to classify each pore inhomogeneity as cylindrical, elliptical or irregular shape. Two prominent homogenization theories were used: the Mori-Tanaka (MT) and the generalized self-consistent method (GSCM). Using the results of image segmentation, it was possible to calculate the respective Eshelby tensors of each porous feature. To calculate the isotropic stiffness tensors for matrix (Cm) and pores (Cp) the Young’s modulus and Poisson’s ratio for the matrix (Em, νm) were assigned as obtained from literature and as those of blood (Ep=10MPa, νp= 0.3), respectively. The effective elastic stiffness tensors (C*) for the secondary osteons were obtained from which axial Young’s modulus was obtained as function of volume fraction (% porosity) of each pore type and their individual shapes. The normalized axial Young’s modulus was found to 1) significantly decrease with increasing volume fraction (%) of porosity and 2) for the same % porosity, to slightly decrease (increase) with increasing ratio of circular-shaped to elliptical-shaped (elliptical-shaped to circular-shaped) porous features. These findings were validated using experimental micro-indentation study performed on secondary osteons.

Predicting the Nusselt Number of Heterogeneous (Porous) Enclosures Using a Generic Form of the Berkovsky–Polevikov Correlations

A well-known set of Berkovsky–Polevikov (BP) correlations have been extremely useful in predicting the wall-averaged Nusselt number of “wide” enclosures heated from the side and filled with a fluid undergoing natural convection. A generic form of these correlations, dependent on only two coefficients, is now proposed for predicting the Nusselt number of a heterogeneous (fluid–solid), porous enclosure, i.e., an enclosure filled not only with a fluid but also with uniformly distributed, disconnected and conducting, homogeneous solid particles. The final correlations, and their overall accuracies, are determined by curve fitting the numerical simulation results of the natural convection process inside the heterogeneous enclosure. Results for several Ra and Pr, and for 1, 4, 9, 16, and 36 solid particles, with the fluid volume-fraction (porosity) maintained constant, indicate the accuracy of these correlations to be detrimentally affected by the interference phenomenon caused by the solid particles onto the vertical boundary layers that develop along the hot and cold walls of the enclosure; the resulting correlations, in this case, present standard deviation varying between 6.5% and 19.7%. An analytical tool is then developed for predicting the interference phenomenon, using geometric parameters and scale analysis results. When used to identify and isolate the interference phenomenon, this tool is shown to yield correlations with much improved accuracies between 2.8% and 9.2%.

Measured temperature and pressure dependence of compressional (Vp) and shear (Vs) wave speeds in compacted, polycrystalline ice Ih

We report on laboratory measurements of compressional- and shear-wave speeds in a compacted, polycrystalline ice-Ih sample. The sample was made from triply distilled water that had been frozen into single crystal ice, ground into small grains, and sieved to extract the 180–250 µm diameter fraction. Porosity was eliminated from the sample by compacting the granular ice between a hydraulically driven piston and a fixed end plug, both containing shear-wave transducers. Based on simultaneous compressional- and shear-wave-speed measurements, we calculated Poisson's ratio and compressional-wave, bulk, and shear moduli from –20 to –5°C and 22 to 33 MPa. PACS Nos.: 91.60Lj