Toshimasa Hashimoto

Temperature distribution in a thin-film chip utilized for advanced nanocalorimetry

High sensitivity and fast calorimeters based on silicon nitride thin-film technology are used to study thermal properties of sub-micron samples and transition kinetics on a millisecond time scale. A commercially available thin-film sensor was utilized in our previous works for fast-scanning calorimetric measurements. A non-adiabatic condition allows not only fast heating but also fast cooling at rates up to 10 000 K s−1. Heat transfer from the sub-micron membrane was realized through an ambient gas. In order to justify the calibration procedure utilized in non-adiabatic thin-film calorimetry, the temperature distributions in the membrane and in the ambient gas have been studied. Results from an analytical solution of the heat-transfer problem have been compared with the temperature profiles obtained by fast infrared thermographic measurements under static and oscillating heating–cooling conditions. A theoretical background for ultra-fast-cooling experiments has been formulated. Actually, the best cooling medium for the ultra-fast thin-film cooling calorimetry is a gas at a reduced pressure. The thermal conductivity of a gas is not a limiting factor for the ultra-fast-cooling experiments.

Thermal diffusivity of aromatic polyimide thin films by temperature wave analysis

The heat transport properties of aromatic polyimide thin films have become more important in the use for the electric insulation in the microelectronic devices with highly integrated circuits. The various kinds of measuring methods have been applied to obtain the anisotropic thermal conductivity and thermal diffusivity of thin films, however, if the specimens are soft and transparent, the conventional methodology requires highly advanced technology in preparing the specimens for the measurement and the results obtained vary widely. The purpose of this study is to apply the temperature wave analysis (TWA) method to measure the thermal diffusivity of thin films and spin-coated layers of aromatic polyimide in the thickness direction at various temperatures. The TWA is an absolute method to determine the thermal diffusivity by using the phase shift of temperature wave. We have performed measurements on the five different chemical structures of aromatic polyimide, including polyimide isoindoloquinazolinedione ...

Thermal conductivity/diffusivity of Nd3+ doped GdVO4, YVO4, LuVO4, and Y3Al5O12 by temperature wave analysis

Thermal diffusivity and thermal conductivity of single crystals of Nd3+ doped GdVO4, YVO4, LuVO4, and Y3Al5O12 are precisely measured over a wide range of doping concentration from 0.5to15at.% by temperature wave analysis. Thermal diffusivity serves as the most sensitive parameter to detect the effect of doping on thermal properties, where Nd3+ doped GdVO4 exhibits a decrease in thermal diffusivity (it has changed about 20% in their values in the c axis) but an increase in heat capacity (only 1.7%). It has long been understood that the thermal conductivity of YVO4 is inferior to that of Y3Al5O12; however, the thermal conductivity of YVO4 in the c axis shows the highest value in all four crystals compared at 1at.% of Nd3+ doping concentration. Thermal conductivity exhibits a decrease (∝e−1∕2, e: mass variance) with an increase of doping concentration, that is characteristic of Klemens’ point defect model for the phonon scattering. In the numerical fitting, the anisotropic decrease of thermal conductivity i...

Study of change in thermal diffusivity of amorphous polymers during glass transition

Abstract The thermal diffusivity of amorphous polymers was investigated by the newly developed a.c. joule heating technique in the practical temperature range including the melt state, glass transition and glassy state. The temperature dependence of thermal diffusivity was measured both in the glassy state and the melt state and its discontinuous change at the glass transition was clearly observed. Δα, defined as the change of thermal diffusivity during the glass transition, was closely related to the glass transition temperatures and to the change in heat capacity, ΔC p . It is confirmed that the thermal diffusivity is one of the basic thermophysical properties which can connect the chemical structure and molecular weight of polymer to the glass transition.

Activated carbon fibers and films derived from poly(vinylidene fluoride)

Abstract Activated carbons having different ranges of pore sizes from those derived by the conventional heat-treatment of organic compounds have been produced by carrying out a part of the conversion process through a liquid phase chemical treatment. Poly(vinylidene fluoride) (PVDF) was chosen as a starting polymer by taking into account the processability of this polymer into various geometries. The PVDF fibers and films were converted to activated carbons by using the combination of chemical dehydrofluorination with a strong base, high-temperature heat-treatment and activation in a carbon dioxide gas. The resulting activated carbon film exhibited superior methylene blue adsorption of 538 mg g−1. Formation of a rigid skeleton during dehydrofluorination accounted for the ability of dehydrofluorinated PVDF to maintain its macroscopic precursor geometry during high-temperature heat-treatment and the formation of pores with large sizes.

Thermal and optical properties of the femtosecond-laser-structured and stress-induced birefringent regions in sapphire.

Temperature diffusivity of laser micro-structured regions in sapphire is determined by a temperature wave method with a lateral resolution reduced to ~10 microm using a directly sputtered micro-sensor and heater. A record high reduction of the temperature diffusivity of sapphire by 12% from its (1.26+/-0.02) x 10(-5)m(2)/s in-bulk value inside the femtosecond laser-structured volumes is determined; in a BK7 glass (~4.8x10(-7) m(2)/s), a 2% decrease of the thermal diffusivity has been observed. Origin of the reduction is consistent with disorder and scattering of phonons around the laser photo-modified micro-volumes. The stress-induced birefringence is directly measured by polariscopy together with its radial distribution, and azimuthal orientation of the polarization ellipsis near the laser structured regions in sapphire. The maximum birefringence of Deltan approximately 1x10(-3) is achieved without crack formation and corresponds to a local stress of ~1.3 GPa. The stress (and birefringence) decay radially with a single-exponential constant of tau(R) = 24 microm while the azimuthal orientation of the polarization ellipsis is spiraling around the laser structured volume. Such structures are promising in waveguiding and lasing applications of optical vortices where spatial control of birefringence and optical activity are required.

Frequency dependent thermal diffusivity of polymers by temperature wave analysis

Abstract Temperature wave analysis (TWA) is an originally developed new thermal analysis that makes it possible to obtain the frequency dependent thermal diffusivity as a function of temperature. The thermal diffusivity of every materials, except for electrical conductors, can be obtained for both solid and liquid states in the frequency range from 10 Hz to 2 kHz. The technique enables studies of frequency dependent thermal diffusivity at glass transition, melting and crystallization of polymers. The experimental results for several polymers are reported in the temperature range from room temperature to above the melting point under a constant heating or cooling process.

Thermal diffusivity measurement of polymer films by the temperature wave method using joule-heating

Abstract A simple method using a temperature wave method has been developed for measuring the thermal diffusivity of thin polymer films. The variation of the temperature at the rear surface was detected by sputtered gold layar as thin-film bolometers. The thermal diffusivity of four polymers, polyimide, polypropylene, polystyrene, and polyethylene terephthalate were measured by this new method over the temperature range of 20–200 °C.

Microscale thermography of freezing biological cells in view of cryopreservation

The aim of this work is to present a device for the measurement of biological living tissues during freezing by infrared camera. Under simplified assumptions, it is shown that infrared thermography measurements and two-dimensional microscale thermal processing methods of the temperature frames allow to estimate important thermophysical fields for the cryopreservation of living tissues, such as the heating source distribution of the latent heat released from biological cells and the thermal properties during freezing. This work is related to the analysis of thermal source terms occurring during freezing of biological tissues from the processing of experimental temperature fields obtained by infrared thermography. Such information is very important in order to understand and improve the heterogeneous solidification phenomena during cryopreservation processes. A new method is proposed here in order to estimate the 2D mapping of source terms and thermal diffusivity during freezing. Such source terms (space and time distributions) are strongly related to the thermal diffusivity mapping which control the 2D in plane diffusion into the tissue.

Simultaneous measurement of heat capacity and thermal diffusivity in solid–solid and solid–liquid phase transitions of n-alkane

Abstract A new technique analyzing Fourier spectrum of higher harmonics of temperature wave is proposed, which is called as Fourier Transform Temperature Wave Analysis (FT-TWA). It enables a simultaneous measurement of thermal diffusivity, heat capacity per unit volume, and thermal conductivity at a time by temperature scanning under multiple frequencies. This method was applied to solid–solid and solid–liquid phase transitions of n -alkane, which is usually used as a model of molecular crystal. The frequency dependence of heat capacity and thermal diffusivity in the phase transitions was observed.