K. Sako

The microwave effects on the properties of alumina at high frequencies of microwave sintering

Microwave sintering of materials has attracted much research interest because of its significant advantages (e.g. reduced sintering temperatures and soaking times) over the conventional heating. Most researchers compared processes that occurred during the microwave and conventional heating at the same temperature and time. The enhancements found in the former method are indicated as a `non-thermal effect` which is usually used for explaining the phenomena in microwave processing. Numerous recent studies have been focused on the effect to elucidate the microwave interaction mechanism with materials. Moreover, recent progress on microwave sources such as gyrotrons has opened the possibility for processing materials by using a higher microwave frequency. Therefore, the technology is expected to exhibit a stronger non-thermal effect. This paper presents results from a series of experiments to study the non-thermal effect on microwave sintered alumina. Sintering by using a wide rage of microwave frequencies up...

Microstructural and Thermal Properties of Nanocrystalline Silica Xerogel Powders Converted from Sago Waste Ash Material

In the present investigation, nanocrystalline silica xerogel (NSX) powders were produced from an amorphous silica xerogel (ASX) extracted from sago waste ash. The NSX powders have been calcined at 1200oC, milled and then annealed at temperatures ranging from a room temperature to 1200oC. Their properties (and most notably the size of the particles) have been characterized on the basis of the experimental data obtained using thermal analysis (DSC/TGA), X-ray diffraction (XRD), Infrared and Raman spectroscopy. For the crystalline silica xerogel powders the results show a narrow distribution of the particle sizes centered around an average value of 636  67 nm. The DSC analysis of NSX indicates that in the temperature range from a room temperature to 300oC five distinct stages of the crystallization process take place, which are delimited by the transition temperature of 38oC, 92oC, 129oC, 168oC, and 246oC, respectively. Above 300oC, the crystalline phase is similar to an amorphous silica xerogel (ASX), i.e. cristoballite-like and tridymite-like crystalline silica phases confirmed by the XRD analysis. It has been observed that the characteristic band of cristoballite is strongly dependent on the thermal history and the NSX transforms into a stable form at a temperature of 1200oC. Both the Raman and the FTIR spectra elucidate the bonding system of the constituent atoms and groups (such as Si, O and OH) and throw light on their underlying structure. The obtained results are important for optimization of the parameters of the technological processes for production of nanocrystalline silica glass ceramics used as a host matrix for luminescence materials, each of which requires a specific porosity and structure.

Development of a material heating system by using a 300 GHz gyrotron FU CW I

The effective characteristics of high frequency and strong electric field processing were observed. One of the interesting effects is so called non-thermal effect or microwave effect. In order to widely study the non-thermal effect of ceramics sintering and application of material heating, we developed a material processing system by using a 300 GHz gyrotron. This system are consisted of a 300GHz, 3.5 kW, CW gyrotron with a cryogen free 12 T superconducting magnet, a corrugated circular waveguide and an applicator. In submillimeter wave region, the distribution of electromagnetic wave shows inhomogeneity inside a simple applicator. In order to improve distribution, polygonal reflector was developed and installed into an applicator. By using a polygonal reflector, homogeneity of electromagnetic wave inside applicator is improved three times.

Submillimeter wave sintering of pure alumina ceramics

The sub-millimeter (300 GHz) wave sintering of the alumina ceramics had been performed. The sub-millimeter wave sintering curve shows the sintering slower at the same temperature, for sub-millimeter wave processing as compared to millimeter wave processing.