Hiromichi Ohta

High-temperature thermal expansion of six metallic elements measured by dilatation method and X-ray diffraction

The thermal expansion of silver, gold, copper, nickel, molybdenum and tungsten has been measured by the dilatation method. The thermal expansion values of silver, gold, copper and nickel have also been evaluated from lattice parameter measurements by X-ray diffraction. These six metals were found to exhibit a nearly uniform expansion over the temperature ranges covered. The thermal expansion values obtained by the dilatation method are in good agreement with those determined by X-ray diffraction. The present results appear also to agree well with those reported previously in the literature.

Thermoreflectance technique to measure thermal effusivity distribution with high spatial resolution

We have developed an apparatus to measure thermal effusivity distribution in solid materials with a high spatial resolution better than 10μm by the thermoreflectance technique and the periodic heating method. A metal film sputtered on the surface of a sample is periodically heated by a modulated laser beam. The temperature response is measured by using another thin laser beam as a thermoreflectance signal. The thermal effusivity of the sample is derived from the phase lag of the temperature response from the periodic heating. Measurements of a functionally graded material and a fiber composite material are presented as application examples of this thermal effusivity distribution measurement technique.

Thermal diffusivity measurements of molten salts using a three‐layered cell by the laser flash method

A simple cell and easy data processing are described for measuring the thermal diffusivity of a liquid sample at high temperatures using the laser flash method. A cell consists of a liquid sample sandwiched by two metallic plates. The front surface of one metallic plate is exposed to a single pulse of beam laser and the resulting temperature rise of the back surface of the other metallic plate is measured. The logarithmic analysis proposed by James using the initial time region of the temperature response curve of a two layered cell system has been extended to apply to the present three layered cell system in order to estimate the thermal diffusivity value of a liquid sample. Measurements of distilled water and methanol were made first and the results were found to be in good agreement with the reference data. Then, the thermal diffusivities of molten NaNO3 at 593–660 K and of molten KNO3 at 621–694 K were determined and the results also appear to agree reasonably well with those reported in the literature.

Novel laser flash technique to measure thermal effusivity of highly viscous liquids at high temperature

A simple but useful experimental technique featuring a novel configuration for a sample cell, high-speed infrared detector and a heating pulse laser has been developed to obtain reliable thermal effusivity of a highly viscous liquid at high temperature. The sample is melted in a platinum crucible and heated to the desired temperature. A single laser pulse is flashed on the bottom of the platinum crucible and the infrared ray irradiated from the same bottom surface is measured to obtain the temperature decay, from which the thermal effusivity of the liquid sample can be estimated. The usefulness and validity of this new laser flash technique were successfully confirmed by measuring the thermal effusivity of silicate glass in the temperature range between 773 and 1673 K.

A high-temperature system based on the laser flash method to measure the thermal diffusivity of melts

A high-temperature measuring system has been developed to undertake measurements of thennal difusivity and specific heat up to 1900 K. The overall design allows measurements on solids to be undertaken using the accepted standard techniques and analytical procedures. The specific design for molten materials and especially slags is based on the differential threelayer technique utilizing a special cell which can be accomodated in the system. In this method, the liquid specimen is sandwiched between an upper inner platinum crucible and a lower outer platinum crucible, to provide a three-layered sandwich. A laser pulse irradiates the surface of the upper platinum crucible and the temperature response of the surface of the lower platinum crucible is observed. For the purpose of accurate measurement of specimen thickness at the measuring temperature, two runs are performed in which the thicknesses arel andl+°Dl, wherel is unknown butΔl can be set accurately with a built-in micrometer. The thermal difusivity is obtained through a curve-fitting method by a personal computer using a three-layer analysis with a correction for the radiative component based on the transparent body assumption. Following verification of the basic performance, using solids of known properties and water and ethanol, a continuous casting mixture has been evaluated. The initial results on the fluids are in good agreement with those in the literature.

New attempt for measuring thermal diffusivity of thin films by means of a laser flash method

A new facility and its requirements have been developed for the measurement of the thermal diffusivity of thin films or ribbons which are less than about 100 μm in thickness, using a laser flash method. In this method, a pulsed laser beam which has a narrow line‐shaped cross section compared with the length of the sample is flashed onto the front surface of the sample. The temperature response at the back surface at a certain distance away from the line‐shaped illuminated area of the laser beam is measured by an infrared detector. The thermal diffusivity value can be estimated from the time required for the temperature to reach the half of the maximum temperature rise finally acquired. The usefulness of the present facility has been demonstrated by measuring successfully thermal diffusivities of thin foils of platinum (25 μm) and of copper (18 μm). This method can be applied to the samples having various thermal diffusivities, particularly in cases where the conventional method using a thermocouple is found to be technically difficult. For example, a thin fragile sample for which it is almost impossible to weld the temperature detecting device onto it.

Thermal effusivity distribution measurements using a thermoreflectance technique

We have developed a thermal effusivity measurement system that can measure distribution of local thermal effusivity of inhomogeneous materials. A specimen is uniformly coated with a molybdenum thin film of about 100 nm thick before the measurements. A modulated beam from an argon-ion laser sinusoidally heats a small spot of the specimen surface. Absorbed heat in the molybdenum film diffuses into the specimen and the local temperature of the heated spot changes proportionally to the sinusoidal heating with a phase lag. The temperature change of the molybdenum film surface at the same spot is measured with the thermoreflectance technique. The phase responses of the 50 nm, 100 nm, 200 nm and 500 nm molybdenum thin films deposited on the Pyrex 7740 glass substrates were measured to verify the validity of the measurement technique.

A novel laser flash method for measuring thermal diffusivity of molten metals

Abstract A novel laser flash method has been developed to measure the thermal diffusivity value of molten metals at 700–1800 K. The upper surface of molten metal is instantaneously irradiated by a Nd glass laser. Then, the temperature response of the same (upper) surface is measured by using an InSb infrared detector. The thermal diffusivity value can be estimated from the measured temperature response. This new laser flash method was applied to molten tin, copper and Inconel 601 at high temperature and its validity and usefulness was confirmed by showing good agreement with the values obtained by an ordinary laser flash method and with the reference values as well.

Recent Development in the Investigation on Thermal Conductivity of Silicate Melts

Abstract Accurate values of thermal conductivity of the silicate melts systematically measured as a function of chemical composition are necessary to understand a mechanism of heat transfer in the silicate melts. Hot wire method and laser flash methods have been used to measure thermal conductivity or thermal diffusivity of oxides melts at high temperatures. Laser flash method has been improved to measure thermal diffusivity of oxides melts with high accuracy. However the effects of radiative heat transfer and low electrical resistivity of samples have been made it difficult to derive precise values. To overcome these difficulties, a front-heating front-detection laser flash method with use of high time resolution detector has been proposed. The temperature response at the bottom surface of thin platinum cell containing sample irradiated by pulse laser is measured. The measurement techniques used for measurement oxide melts are compared. Then, thermal conductivity of Al2O3-Na2O-CaO-SiO2 silicate melts was measured at temperature up to 1830 K. Thermal conductivity of the molten silicate shows insignificant temperature dependence for all investigated melts. A fairly good correlation has been found between the thermal conductivity and the value of NBO/T (Non-Bridging Oxygen ions/Tetrahedrally coordinated cation) calculated from the chemical composition. The thermal conductivity increases with decrease of NBO/T for small NBO/T value and becomes constant for larger NBO/T value.

Thermal Conductivity of R-Na2O-SiO2 (R = Al2O3, CaO) Melts

Abstract Reliable values of thermal conductivity of molten silicates are strongly requested to establish a model which can describe the relationship between thermal conductivity and structure of the molten silicates. Thermal conductivity of R-Na2O-SiO2 (R = Al2O3, CaO) molten slags has been measured by using the novel laser flash method. A thin platinum crucible was filled with silicate melt and the bottom surface of the platinum crucible at high temperature was irradiated by a pulsed laser. After irradiation, a temperature decay of the bottom surface of the platinum crucible was measured with an InSb infrared detector. The value of the thermal conductivity was determined by fitting the measured temperature response curve to theoretical one with a least square method. Measured thermal conductivity slightly depends on temperature in molten state with the chemical compositions of the slags presently investigated. On the other hand, the thermal conductivity was found to decrease with increasing NBO/T (Non-Bridging Oxygen ions / Tetrahedrally coordinated cation) values.