Computer simulation of microcomposite ceramics with high transcrystalline mobility

Computer simulation is used to consider the possibility of creation of microcomposite technical ceramics with a high transcrystalline mobility and to show that allowance for the exchange interactions of atoms through nonequilibrium electrostatic fields causes formation of self-organized mesoscopic (SM) blocks, which provide microplasticity and relaxation of strain at lower temperatures. The simulation results agree with the yield stria observed in mechanical compression tests of A1₂,O₃ specimens obtained from sol-gel solutions. It is shown that the yield stria are directly connected with the sizes of the SM blocks. The high mobility of SM blocks also promotes a decrease in the temperature of the gel → A1₂O₃ transformation.     

Studies on the respiratory health of primary school children in urban communities of Hong Kong

The first stage of a 2-year survey of respiratory morbidity in primary school children was conducted in two districts of Hong Kong in April/May 1989. One group (2009) of children was from Kwai Tsing District, which had high levels of exhaust emission from factories. The other group (1837) was from Southern District where atmospheric pollution was considered to be relatively low. After adjustment for gender, age, socioeconomic factors, child smoking and exposure to parental smoking, the prevalence ratios of sore throat, evening cough, cough for more than 3 months, morning phlegm and wheezing were found to be significantly higher in Kwai Tsing. The difference between the districts is likely to be related to the environmental air quality. The study, which is continuing, will provide the basis for an evaluation of the impact of new low sulphur fuel regulations introduced in July 1990.     

在无溶剂系统中由植物油生产脂肪酸酯的一条新颖酯交换反应路线（英）

研究了长链醇和甲酯使用耐热的固定化脂肪酶生产蜡酯的技术。酯交换反应是将两种反应是将两种反应物以不同的化学计量混合,在低浓度酶制剂存在下,于５５℃－６５℃温度范围内连续搅拌完成。最适宜的操作温度为６０℃,油酸甲酯五硬脂醇的摩尔比为１：０．５。一般能够得到高酯化率。基质抑制作用由硬脂醇引起。动力学参数的研究证实了这些结论。     

A Novel Processing Route to Control Grain Growth in Submicrometer Alumina Compacts

A novel processing procedure for significantly suppressing grain growth in submicrometer alumina compacts has been developed and implemented with the intent of ultimately using the same processing route to control grain size in nanophase alumina compacts. In this study, partially sintered alumina pellets made from 0.5 µm starting powders are altered by the chemical infiltration of Si₃N₄. The control and infiltrated pellets are then heated to 1650°C for 4 h. The fully sintered pellets are approximately 97% dense. Suppressed grain growth is observed in the infiltrated pellets. The average grain size in the control pellets after densification is 4.2 and 1.2 µm in the infiltrated pellets. Depth of infiltration is measured using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Depending on the specific infiltration conditions used, the outer 15-50% of the infiltrated pellets exhibit a graded microstructure consisting of a region of abnormal grain growth and a region of suppressed grain growth. Abnormal grain growth is visible on the outer surfaces of the infiltrated pellets where a relatively high ratio of Si to N is present. Further into the pellet, after some depletion of the Si source gas has occurred, regions of suppressed grain growth are apparent. Based on these results, an infiltration profile is determined. A mathematical model is developed to describe the infiltration process and to determine optimal infiltration conditions. High-resolution electron microscopy (HREM), energy dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS) are used to study the infiltrated samples.     

Effects of MgO Stoichiometry on the Dielectric and Mechanical Response of Pb(Mg1/3Nb2/3)O3

Careful control of the magnesia stoichiometry and hydration level overcomes the problem of pyrochlore formation in the synthesis of Pb(Mg_1/3Nb_2/3)O₃ (PMN). Careful removal of volatiles from MgO showed that excess MgO is not required to form pyrochlore-free PMN using the columbite method; this clarifies the previously reported need of excess MgO. The stoichiometric PMN shows a peak dielectric constant of ~15 000 for 1 kHz. In addition, the presence of volatiles in the MgO has been linked with a change in fracture mode. The relation between the MgO stoichiometry and the microstructural and dielectric properties is presented in an integrated study using X-ray diffraction at various temperatures, dielectric response, and microscopy.     

Surface area, pore size distribution and microstructure of combustion engine deposits

The structural characteristics of combustion engine deposits produced from fuels with 22–44% aromatics by volume have been studied using Raman spectroscopy and gas sorption techniques. The lateral sizes of graphitic crystallites were found to increase slightly with the aromatic content of the fuel used. The surface areas of the deposits were evaluated using the BET and DR theories. Density functional theory (DFT) and mercury porosimetry were used to evaluate pore size distributions. The deposits have large internal surface areas (in the approximate range 100–300 m²/g) and their structures are highly porous. The average pore sizes of the largely microporous deposits do not depend on the aromatic content of the precursor fuel. Walls of pores with widths of about 0.5 nm are primarily responsible for the majority of the internal surface area found in the deposits. Heat treatment at temperatures above 573 K results in increased surface areas because the release of small hydrocarbon fragments from the deposits opens pores which were not accessible following heat treatment at lower temperatures.