Calculating the Thermal Conductivity of Heated Powder Compacts

This work is devoted to the theoretical understanding of the microstructure and thermal conductivity relationships of compacted ceramic powders in the initial, nondensifying stage of sintering. A model based on surface diffusion of vacancies for the growth of the neck between particles is combined with numerical fully three-dimensional code calculations, which solve for the effective heat conductivity coefficient of lightly sintered particles. The predictions of the model are in agreement with experimental observations. The approach presented can be applied to solve a series of related problems, like dielectric properties, with arbitrary microstructure and multicomponent composite of the powders.     

Modeling the Heat Wave Generated during Microwave Heating of Powdered Zinc Oxide in a Nitrogen Atmosphere

A theoretical investigation of previous experiments on the microwave heating of cylindrical samples of powdered ZnO in nitrogen and air environments has been presented. Unlike heating in air, which showed an exponentially saturating temperature behavior, a large discontinuity in the measured temperature was observed in nitrogen. Energy considerations suggested that the microwave power was mostly absorbed in a narrow, localized region that moved radially outward as a propagating heat wave. It was assumed that this behavior was caused by an anomaly in the temperature dependence of the permittivity, and the present simulations verify this assumption. The phenomenon has been investigated using a newly developed two-dimensional code, and the results are in good agreement with the experimental observations. The calculated width of the microwave absorbing region was ∼1 mm, and the radial velocity of the electromagnetically driven heat wave was inversely proportional to the square of the distance from the center of the sample, with a maximum speed of ∼20 mm/min.     

On the automatic generation of optimal internal schemata

A Data Base Design, problem using the most modern techniques is usually divided into Conceptual Schema Design, External Schemata Design and Internal Schema Design. Currently, each of these design steps is done manually with few computerised aids.A method is described for deriving automatically the Internal Schema from the Conceptual and External Schemata. The method is proved to generate the optimal Internal Schema and to be easily amendable to updates in the Conceptual and External Schemata.     

Simulation of Microwave Sintering of Ceramic Bodies with Complex Geometry

Microwave sintering, an emerging technology in which the energy is applied directly to the material, enabling rapid sintering, shows potential for the synthesis of advanced ceramic materials with superior properties. The process is complex, combining the propagation and absorption of electromagnetic waves in the ceramic material, heat transport within the geometric body, and densification. The densification changes both macroscopic shape and microstructural morphology. A dynamic balance between the rate of electromagnetic energy absorbed within the bulk of the sample and the rate of energy loss from its surface generally results in temperature gradients. These temperature gradients may be especially important during the microwave sintering of bodies with a complex geometry, because neither the diffusion distance nor the electromagnetic penetration depth scale with sample dimensions. The gradients generated in a ZnO green body of a complex geometry were studied theoretically using various microwave-sintering approaches, and it was found that (1) dual-frequency (2.45 and 30 GHz) microwave processing leads to a decrease in the duration of the temperature gradients, and (2) an increase in the heating rate from 5°C/min to 1400°C/min at 2.45 GHz decreases the total required microwave energy by a factor of 55, while at the same time the internal temperature gradients are maintained over a substantially shorter time.     

Mutagenicity and cytotoxicity of reactive oxygen and nitrogen species in the MN-11 murine tumor cell line

The concept of uniform reporting of data in resuscitation has demonstrated its potential value in pre-hospital and in-hospital cardiac arrest in adults, infants and children, in laboratory research, in disaster research and, hopefully, also in trauma care and research.     

Large saturation of absorption of CdSSeTe nanocrystallites in glass measured by femtosecond Z-scan technique

The optical non-linearity of CdSSeTe nanocrystallite-doped glasses has been studied using a single-beam, femtosecond Z-scan technique. We have observed very large absorption saturation in a composite of CdSSeTe nanocrystals in a glass host compared with CdS or CdSe, although we have not reached the saturation limit. The wavelength and power dependence of absorption saturation in these composites are reported. The experimental data are interpreted by two-level model system. A comparison is also made with other nanocrystallite composites with different stoichiometric compositions. Copyright © 2000 John Wiley & Sons, Ltd.     

Enhanced geometrical superresolved imaging with moving binary random mask

In this paper, we address the geometrical resolution limitation of an imaging sensor caused by the size of its pixels yielding insufficient spatial sampling of the image. The spatial blurring that is caused due to inadequate sampling can be resolved by placing a two-dimensional binary random mask in an intermediate image plane and shifting it along one direction while keeping the sensor as well as all other optical components fixed. Out of the set of images that are captured, a high resolution image can be decoded. In addition, this approach allows improved robustness to spatial noise.