SiO2–CaO–Na2O–P2O5–ZrO2 based bioactive glasses with different compositions of SiO2 and yttrium stabilized ZrO2 were prepared by the conventional melt quenching technique. The effects on the chemical–mechanical properties of bioactive glasses due to the addition of ZrO2 by replacing SiO2 were investigated. Microstructure and phase behavior were studied by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction analysis. Compressive strength, porosity, Vickers hardness, and Young’s modulus were measured as mechanical properties. Bioactivity and cell viability were investigated by immersion in simulated body fluid and MTT assay analysis. Osteosarcoma cell proliferation on the specimen surfaces was examined by confocal laser scanning microscopy. The results showed that replacing SiO2 with ZrO2 helps the bioactive glass to be completely vitrified at comparatively lower sintering temperature than conventional Bioglass®. The mechanical properties were also improved without compromising biocompatibility. Bioactive glass containing 10 wt% ZrO2 and 35 wt% SiO2 showed compressive strength of 399.71 MPa, Young's modulus of 22.3 GPa, Vicker’s hardness of 502.54 HV, and porosity of 26 vol%. 相似文献
Guayule natural rubber (GNR) is an alternative resource of Hevea natural rubber (HNR) with 99.9% cis content in its 1,4-polyisoprene chemical backbone. In this study, compounds were formulated independently with four different reinforcing fillers such as carbon black (HAF), precipitated silica (VN3), fume silica (FUM) and nanofly ash (NFA) for the advancement of GNR based products. The cure characteristic, dynamic-mechanical performance and mechanical properties of GNR composite were studied with the reinforcing effect of different fillers on GNR. The cure characteristic results demonstrated that HAF and FUM silica filled compounds had more processing safety than VN3 and NFA filled compounds. Viscoelastic parameters of the vulcanizates were studied by dynamic mechanical analysis to estimate the glass transition characteristics and dynamic behavior. The higher storage modulus of FUM silica vulcanizate was an indication of superior filler reinforcing nature and improved rolling resistance than other filled systems. Additionally, HRTEM analysis also proved the better filler dispersion ability of FUM silica in GNR matrix. The mechanical properties were studied with a variation of each filler loading of 8, 16, and 32 phr in GNR vulcanizates. The tensile strength of each filled system increased with an increase of filler content from 8 to 32 phr. In comparison, FUM silica GNR vulcanizates exhibited better mechanical properties, therefore, it was considered as a better structure-performance composite than those of HAF, VN3 and NFA filled composites.
Animal studies suggest that perfluorocarbons (PFCs) may alter sexual maturation. Relationships of human PFC exposure with puberty are not clear. We conducted a cross-sectional study to investigate whether perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) were associated with indicators of sexual maturation in a 2005-2006 survey of residents with PFOA water contamination from the Mid-Ohio Valley. Participants were 3076 boys and 2931 girls aged 8-18 years. They were classified as having reached puberty based on either hormone levels (total >50 ng/dL and free >5 pg/mL testosterone in boys and estradiol >20 pg/mL in girls) or onset of menarche. We estimated the odds of having reached puberty classified by these criteria and the fitted median age of reaching puberty in relation to serum PFOA and PFOS concentrations measured when puberty status was assigned. For boys, there was a relationship of reduced odds of reached puberty (raised testosterone) with increasing PFOS (delay of 190 days between the highest and lowest quartile). For girls, higher concentrations of PFOA or PFOS were associated with reduced odds of postmenarche (130 and 138 days of delay, respectively). In conclusion, our study showed a later age of puberty in this population correlated with PFC concentrations. 相似文献
The oxy-CO2 methane reforming reaction (OCRM) has been investigated over CoOx supported on a MgO precoated highly macroporous silica–alumina catalyst carrier (SA-5205) at different reaction temperatures (700–900 °C), O2/CH4 ratios (0.3–0.45) and space velocites (20,000–100,000 cc/g/h). The reaction temperature had a profound influence on the OCRM performance over the CoO/MgO/SA-5205 catalyst; the methane conversion, CO2 conversion and H2 selectivity increased while the H2/CO ratio decreased markedly with increasing reaction temperature. While the O2/CH4 ratio did not strongly affect the CH4 and CO2 conversion and H2 selectivity, it had an intense influence on the H2/CO ratio. The CH4 and CO2 conversion and the H2 selectivity decreased while the H2/CO increased with increasing space velocity. The O2/CH4 ratio and the reaction temperature could be used to manipulate the heat of the reaction for the OCRM process. Depending on the O2/CH4 ratio and temperature the OCRM process could be operated in a mildly exothermic, thermal neutral or mildly endothermic mode. The OCRM reaction became almost thermoneutral at an OCRM reaction temperature of 850 °C, O2/CH4 ratio of 0.45 and space velocity of 46,000 cc/g/h. The CH4 conversion and H2 selectivity over the CoO/MgO/SA-5205 catalyst corresponding to thermoneutral conditions were excellent: 95% and 97%, respectively with a H2/CO ratio of 1.8. 相似文献
Human walking influences indoor air quality mainly by resuspending dust particles settled on the floor. This study characterized walking‐induced particle resuspension as a function of flooring type, relative humidity (RH), surface dust loading, and particle size using a consistent resuspension mechanism. Five types of flooring, including hardwood, vinyl, high‐density cut pile carpet, low‐density cut pile carpet, and high‐density loop carpet, were tested with two levels of RH (40% and 70%) and surface dust loading (2 and 8 g/m2), respectively. Resuspension fraction ra (fraction of surface dust resuspended per step) for house dust was found to be varied from 10?7 to 10?4 (particle size: 0.4–10 µm). Results showed that for particles at 0.4–3.0 µm, the difference in resuspension fraction between carpets and hard floorings was not significant. For particles at 3.0–10.0 µm, carpets exhibited higher resuspension fractions compared with hard floorings. Increased RH level enhanced resuspension on high‐density cut pile carpet, whereas the opposite effect was observed on hard floorings. Higher surface dust loading was associated with lower resuspension fractions on carpets, while on hard floorings the effect of surface dust loading varied with different RH levels. 相似文献
This paper develops an integrated production-recycling system over a finite time horizon. Here, the dynamic demand is satisfied
by production and recycling. The used units are bought back and then either recycled or disposed of which are not repairable.
The used units are collected continuously from the customers. Recycling products can be used as new products which are sold
again. The rate of production and disposal are assumed to be function of time. The setup cost is reduced over time due to
“Learning curve” effect. The optimum results are presented both in tabular form and graphically. 相似文献
Abstract. A standard assumption while deriving the asymptotic distribution of the quasi maximum likelihood estimator in ARCH models is that all ARCH parameters must be strictly positive. This assumption is also crucial in deriving the limit distribution of appropriate linear estimators (LE). We propose a weighted linear estimator (WLE) of the ARCH parameters in the classical ARCH model and show that its limit distribution is multivariate normal even when some of the ARCH coefficients are zero. The asymptotic dispersion matrix involves unknown quantities. We consider appropriate bootstrapped version of this WLE and prove that it is asymptotically valid in the sense that the bootstrapped distribution (given the data) is a consistent estimate (in probability) of the distribution of the WLE. Although we do not show theoretically that the bootstrap outperforms the normal approximation, our simulations demonstrate that it yields better approximations than the limiting normal. 相似文献