Shinya Sawai

Luminescence properties of ZnS phosphor nanocrystals prepared by the laser-induced gas-evaporation method

Nanocrystalline particles of ZnS:(Ag, Al) semiconductor phosphor, whose sizes are mostly 3–5 nm in diameter, are prepared by the gas-evaporation method with cw CO2 laser heating. The Raman scattering spectrum as well as the transmission electron microscope observation demonstrates that the crystallization of the nanoparticles was caused successfully through the gas-phase condensation. Under irradiation of ultraviolet light, the nanoparticles exhibit blue luminescence, as in the case of the starting material of ZnS:(Ag, Al) bulk powder. The peak of the luminescence spectrum of the nanoparticles shifts to lower energy with increasing delay time and also with decreasing excitation intensity, showing that the luminescence originates from the donor–acceptor pair recombination. However, it is concluded that the luminescence of the nanoparticles is not ascribed to the blue Ag luminescence mechanism responsible for the luminescence of the bulk powder, by taking into account the spatial confinement of an electron ...

Evaluation of hemispherical total emissivity for thermal radiation calorimetry

An attempt to derive the hemispherical total emissivity from the normal emission spectrum is proposed for Vycor and fused silica glasses. The normal emission spectrum from a clear surface has been measured at steady state in the temperature range from 400 to 750 K. The sample is heated on one metal-backed face by thermal radiation from a heater. Temperatures inside the sample were monitored by thermocouples at two points near the surfaces. Evaluation of the hemispherical total emissivity from the normal emission spectrum is determined by means of Kramers–Krönig analysis and virtual mode equations. Assuming a linear temperature distribution within the sample, the thermal conductivities of silicate glasses were obtained at elevated temperatures. The results are comparable with those obtained by previous investigators. The effect of radiation heat transfer in a sample is also discussed.

Simultaneous measurements of specific heat capacity and dielectric constant of ferroelectric SrBi2(Nbx Ta1-x)2O9 ceramics

Abstract Simultaneous measurements of specific heat capacity and dielectric constant have been performed for ferroelectric SrBi2 (Nbx, Ta1—x)2O9 ceramics with bismuth layer-structure. An apparatus based on thermal radiation calorimetry was used in the present measurement. Following the Stefan-Boltzmann law, a disk-shaped sample with blackened surfaces was taken through heating and cooling modes in a vacuum chamber. Heating was performed using double flat heaters. The specific heat capacity is obtained only from the sample temperature, the heater radiative power, and the time rate of the sample temperature change on both modes, while the dielectric constant was measured using an inductance-capacitance-impedance meter at 1 MHz. Anomalies in the specific heat capacity were observed near the transition temperature expected from the simultaneous dielectric measurement. The Curie temperatures of SrBi2(Nb, Ta)2O9 ceramics determined from the experimental results are 10 ∼ 20° C lower than those obtained by previo...

Simultaneous measurement of thermophysical properties and dielectric properties of PZT-based ferroelectric ceramics by thermal radiation calorimetry

Simultaneous measurements of thermophysical properties and dielectric properties have been performed for PZT-based ferroelectric ceramics. An apparatus based on thermal radiation calorimetry was used in the present measurements. Anomalies in the thermophysical properties were observed near the ferroelectric-to-paraelectric phase transition temperature. The anomalous peak was at almost the same temperature as the inflection point of the dielectric constant. It was found that modification of PZT with increasing Nb, Mg, Zn, and Sr causes a decrease of the Curie temperature and an increase of the hysteresis phenomena for the phase transition, and the values of the thermal conductivity increase with temperature similar to amorphous materials.

Simultaneous Measurement of Specific Heat Capacity, Thermal Conductivity and Thermal Diffusivity of Ferroelectric Ba(Ti1-x, Snx)O3 Ceramics by Thermal Radiation Calorimetry.

Simultaneous measurements of specific heat capacity, thermal conductivity and thermal diffusivity have been performed for ferroelectric Ba(Ti1-x,Snx)O3 ceramics. An apparatus based on thermal radiation calorimetry was used in the present measurement. The measurement is performed in a vacuum chamber kept at room temperature in which a disk-shaped sample is heated and cooled slowly on one face by thermal radiation from a flat heater. The values of three thermophysical properties were obtained from the temperatures and the ramp rates of both sample surfaces, the surface emissivity of the sample and the temperature of the heater surface. In this calorimetry, these thermophysical properties were independently obtained, a self-check of the reliability of the results was performed. The measurements were performed in the temperature range from 310 to 440 K. Anomalies in the thermophysical properties were observed near the ferroelectric phase transition temperature. The Curie temperatures of Ba(Ti1-x,Snx)O3 ceramics determined from the experimental results are almost consistent with those obtained by previous authors.

Measurements of IR Spectra and Thermophysical Properties of Tetragonal Zirconia by Thermal Radiation Calorimetry

Normal emission spectra from a clear surface of tetragonal zirconia ceramic stabilized by 5.3 wt% yttria are measured in the frequency range of 200–4500 cm-1. The values of the hemispherical total emissivity are derived from a spectral analysis based on virtual mode equations and Kramers-Kronig relations. With these values, thermal conductivity data are obtained from measurements of temperature gradients in the sample at temperatures between 400 K and 850 K. Heat capacity measurements are carried out by quasi-static thermal radiation calorimetry. The present results are consistent with data in the literature obtained by other methods. Based on the comparison of the spectral reflectivity with the emission spectrum, it is suggested that the effects of radiation heat transfer in the ceramic sample may be compensated for by the scattering process of grain boundaries.

Simultaneous measurements of specific heat capacity and dielectric constant of ferroelectric Ba1-xSrxTiO3

Abstract Simultaneous measurements of the specific heat and the dielectric constant of Ba1-xSrx TiO3 (x= 0.2, 0.4,0.6,0.8) were performed in the wide temperature range from 150 to 330 K. Anomalies associated with successive phase transitions were observed in both the specific heat and the dielectric constant. It was observed that the temperature at which the peak of the specific heat occurs coincides with that of the inflection point of the temperature dependence of the dielectric constant. The phase transition temperatures were taken as the temperatures of the peak in the differential of the dielectric constant with temperature. With an increase of Sr ion content, the magnitude of the specific heat anomaly decreases and the peak temperature becomes lower.

Simultaneous Measurement of Specific Heat Capacity, Thermal Conductivity, and Thermal Diffusivity by Thermal Radiation Calorimetry

Thermal radiation calorimetry has been applied to measure the thermal diffusivity of a solid specimen, along with simultaneous measurements of specific heat capacity and thermal conductivity. In this calorimeter, a disk-shaped solid specimen whose surfaces are blackened is heated and cooled slowly on one face by irradiation in a vacuum chamber. A quasi-steady-state approximation in which a linear temperature gradient within the specimen was assumed is considered in the analysis. The validity of this approximation was confirmed by the results of computer simulation based on the control-volume method. Measurements of Pyroceram 9606 and Pyrex 7740 by use of thermocouples in the temperature range between 250 and 400°C gave values consistent with those obtained by previous authors, within experimental error, for all three thermophysical properties.

Conventional Simultaneous Measurement of Specific Heat Capacity and Thermal Conductivity by Thermal Radiation Calorimetry

Thermal radiation calorimetry has been applied to measure the thermal conductivity and the specific heat capacity of an isolated solid specimen simultaneously. The system, in which a disk-shaped specimen and a flat heater are mounted in a vacuum chamber with the specimen heated on one face by irradiation, is presented. A theoretical formulation of the simultaneous measurement at quasi-steady state is described in detail. Noncontact temperature measurement of both specimen surfaces has been performed using pyrometers and a thermocouple set in the gap between the heater and the specimen. Pyroceram 9609 specimens, whose surfaces were blackened with colloidal graphite, were used in the measurement. The largest error involved in the noncontact temperature measurement is ±2°C in the range from 450 to 650°C. The resultant values of the specific heat capacity and the thermal conductivity deviate by about 10% from the recommended values for the Pyroceram specimen.

Measurement of Specific Heat Capacity from 150 to 310 K by Thermal Radiation Calorimetry

A novel method for measurement of the specific heat capacity in the temperature range from 150 to 310 K is described. In order to achieve good temperature homogeneity in a disk-shaped specimen, a cylindrical heater was used in an apparatus based on thermal radiation calorimetry. A mixture of Bi2O3 and MgO powders was used for blackening the surfaces of the specimen, the heater, and the inside wall of the chamber. The specific heat capacities of Ni, fused quartz glass, and BaTiO3 ceramic were measured to test the performance of the calorimeter. Agreement to within 5% of the values published in the literature was obtained for these samples. Thermal hysteresis and anomalies associated with the first-order phase transition in BaTiO3 were detected in the present experiment.