Ac Calorimetric Method Research Articles

Thermophysical Properties of Metal-Insulator Transition Materials During Phase Transition for Thermal Control Devices

This study reports on the fabrication of metal-insulator transition materials and measurement of the temperature dependency of their thermophysical properties, namely, specific heat, thermal conductivity, and total hemispherical emittance to evaluate the potential of these materials as multifunctional thermal control devices. Perovskite-type manganese oxide, La0.8Sr0.2MnO3 (LSMO) and La0.8Pb0.2MnO3 (LPMO), and vanadium dioxide (VO2) are selected as candidate materials. LSMO and LPMO are prepared using the solution combustion and simple sintering methods, and VO2 is prepared using the spark plasma sintering method. The phase transition temperatures, specific heat capacities, and heat storage capabilities during the phase transition of these materials are measured via differential scanning calorimetry. The thermal conductivities are measured via AC calorimetric method. The total hemispherical emittances are measured using the steady-state calorimetric method. Results show that VO2 has the highest heat storage capability during phase transition. The large change in the total hemispherical emittances of LSMO and LPMO and the large change in the thermal conductivity of VO2 before and after phase transition are confirmed. Moreover, tungsten-doped vanadium dioxide (VWO2) is fabricated to reduce the phase transition temperature to much lower than that of VO2, and the measurement results of its thermophysical properties are also presented.

Change in Thermal Conductivity By Hydrogen Intercalation in Amorphous WO3 Film

We are developing “thermal switching materials”, whose thermal conductivity varies over a wide range. It has a wide range of applications such as temperature controls, thermal energy storages and so on. WO3 is expected as a thermal switching material because the electrical conductivity of WO3 can widely change owing to the metal-insulator transition (MIT) caused by hydrogen interactions. Moreover, hydrogens can be intercalated into and deintercalated from WO3 film reversibly and quickly by an electrochemical reaction. Therefore, the thermal conductivity is expected to change reversibly. Here, we report the thermal conductivity changes in WO3 film associated with hydrogen intercalations. WO3 film was prepared on ITO coated glass at room temperature by RF magnetron sputtering method in an Ar-O2 atmosphere. The total pressure and the oxygen content of the atmosphere were 4.0 Pa and 13%, respectively. The thickness of WO3 film was 0.7-1.4 μm. Hydrogen intercalations into WO3 film on ITO coated glass were performed in an 0.5 M H2SO4 electrolyte with a Pt counter electrode and a calomel reference electrode. The structure of the films was determined by x-ray diffraction analysis. The thermal diffusivity of the films was measured by an ac calorimetric method and the thermal conductivity was calculated by the differential method using measured thermal diffusivity. The electrical conductivity was measured by a four-point probe method. X-ray diffraction measurements showed that the WO3 films were amorphous. The value of thermal conductivity decreased with increasing hydrogen content x in H x WO3, from x = 0 to x = 0.33, and the value of thermal conductivity increased markedly with increasing x above x = 0.33. The hydrogen content of x = 0.33 is close to that of MIT. On the other hand, the electrical conductivity after MIT was 1×105 times higher than that before MIT. However, a variation of the electron thermal conductivity given from the electrical conductivity by the Wiedemann-Franz law is much smaller than a variation of the thermal conductivity. Therefore, the lattice thermal conductivity is expected to contribute to a change in the thermal conductivity. Present results indicate that the thermal conductivity of WO3 film can be controlled by hydrogen intercalations and potentialize the hydrogen-induced active control of thermal switches.

Specific heat anomaly in ferroelectric: Bis(imidazolium) pentachloroantimonate(III) (C3 N2 H5)2[SbCl5

Single crystals of ferroelectric bis(imidazolium) pentachloroantimonate(III) (C3N2H5)2[SbCl5] have been grown and the heat capacity was measured by the use of AC calorimetric method. The temperature dependence of excess heat capacity and excess entropy in the ferroelectric phase can be described in the frame of the classical Landau-Devonshire theory of phase transitions. The results of experimental studies were analyzed and the α, γ and δ values of the Landau potential coefficients were calculated. The temperature dependence of the order parameter was also evaluated from the heat capacity data.

ウェアラブル発電用 ZnO ナノ構造/綿布材料の熱電特性

For high-efficiency wearable power generator, we have investigated the ZnO nanostructures grown on cotton fabric (CF) as a novel flexible thermoelectric material. We have fabricated ZnO nanostructures on CF by two-step growth method consisting of a seed creation process and a nanostructure growth process. It has been confirmed from ultraviolet transmittance spectra that the ZnO nanostructures are strongly fixed on the CF even after laundering them ten times. In the present study, ZnO nanopillar-nanosheet composites were formed on CF by the two-step growth method. By means of an ac calorimetric method, the thermal diffusivity of the ZnO nanostructure/CF sample was evaluated to be 1.4x10-4m2/s.

Thermal Diffusivity along X-Axis of Periodically Poled Mg-Doped Near-Stoichiometric LiTaO3 Device with Domain-Inverted Period of 8.0 µm

To clarify whether the thermal diffusivity along the X-axis of a periodically poled Mg 1-mol %-doped near-stoichiometric LiTaO3 (PPMgSLT) frequency-conversion device is the same as that of bulk-MgSLT crystal, we measured the thermal diffusivity along the X-axis (the direction perpendicular to the domain-wall) of a PPMgSLT device with a domain-inverted period of 8.0 µm by using a modified AC calorimetric method (laser-heating Ångström method) at room temperature. We found that the thermal diffusivity [(1.98±0.06)×10-6 m2/s] was almost the same as that of bulk-MgSLT crystal [(2.09±0.04)×10-6 m2/s]. This means that domain-walls are not phonon scattering centers with respect to thermal diffusivity and that domain-walls do not affect thermal diffusivity at room temperature. Consequently, the thermal diffusivity of bulk-MgSLT crystal is acceptable for PPMgSLT devices.

Thermal diffusivity measurement by a light-irradiated ac calorimetric method

Thermal diffusivity measurement by a light-irradiated ac calorimetric method

Specific Heat Anomalies in Ferroelectric PyBF4

Pyridinium fluoroborate (PFB) single crystals have been grown and the heat capacity of PFB was measured by the use of AC calorimetric method. The temperature dependence of excess heat capacity and excess entropy in the ferroelectric phase can be described in the frame of the classical Landau theory of phase transitions. The linear relation between the excess entropy and the linear birefringence has been verified. The results of experimental studies were analyzed and the A, B and C values of the Landau potential coefficients were calculated. The provisional temperature dependence of the order parameter was also determined from the heat capacity data. Dedicated to Professor Julio A. Gonzalo on the occasion of his 70-th birthday

Thermal Transport Properties of Various Thin Films for MEMS Applications

The thermal properties of thin films, such as thermal conductivity and diffusivity, are important in design and analysis of MEMS (micro electro mechanical systems), particularly in microscale thermal systems and high-power electronic/optoelectronic devices. In the present study, the thermal conductivity and diffusivity of a variety of thin film materials, which are commonly used in MEMS applications, are measured. The samples include Au, Sn, Mo, Al/Ti alloy, AlN, and SiC. The Au sample is deposited by the e-beam evaporation technique while the rest of the metallic samples are deposited by sputtering processes. The AlN and SiC films are also prepared by sputtering processes. In the experiment, the thermal diffusivities of metallic thin films are measured by two independent methods — the AC calorimetric method and photothermal mirage technique. The thermal conductivities of dielectric thin films are measured by the 3 omega technique. The results show that the thermal transport properties of some of the films are significantly smaller than those of the same material in bulk form. Especially, the AlN and SiC thin films exhibit pronounced thermal conductivity reduction because of the size effect. The electrical conductivities of the metallic thin films are measured as well. The results for Au and Sn are consistent with the thermal conductivity, confirming the Wiedmann-Franz law. However, Al/Ti and Mo thin films show considerable deviation from the law. The results are analyzed based on the XRD (X-Ray diffraction) and AFM (Atomic Force Microscope) measurement.

Physical kinetics and thermodynamics of phase transitions probed by dynamic nanocalorimetry

A specific ac nanocalorimeter is presented for the study of kinetics in phase transitions. This apparatus designed by means of microfabrication technologies requires a 50-μm-thick solid or liquid sample. The use of the ac calorimetric method allows an operating frequency range around two decades combined with a very high sensitivity. The ability of this apparatus for thermal spectroscopy is illustrated by measuring the thermal behavior of a homopolymer, the polytetrafluoroethylene, which exhibits two phase transitions at room temperature. The variation of the thermal frequency allows the observation of kinetic effects directly seen on the thermodynamic properties (enthalpy and heat capacity). We demonstrate the possibility of extracting a quantitative value of the kinetic time constant occurring at a first-order phase transition and consequently the capabilities offered by highly sensitive ac calorimetry to investigate direct thermal dynamics in macromolecular samples.

In-Plane Thermal Diffusivity Measurement of Thin Films Basedon the Alternating-Current Calorimetric Method Using an OpticalReflectivity Technique

An advanced ac calorimetric method to measure thermal diffusivityof a thin sample is developed by using an optical reflectivitytechnique. A modulated infrared laser is used to heat the front surfaceof a foil specimen. The reflectivity of a continuous-wave He–Ne laserat the rear surface of the specimen is detected by a photoreceiver.According to the temperature dependence of reflectivity, the actemperature response is obtained. Thermal diffusivity is deduced from aone-dimensional heat conduction equation. A stainless-steel foil and acopper foil are chosen as the samples. The present results agree wellwith the data in the literature.

Ac nanocalorimeter for measuring heat capacity of biological macromolecules in solution

A precise calorimeter has been developed to measure the heat capacity of a small amount of liquid using a novel ac calorimetric method in which the effect of heat loss from a sample cell is corrected using the phase of the ac temperature. The sample cell is made of a fine glass tube, whose outer surface is plated with a nickel film as an ac heater. The ac temperature of the sample is detected precisely with a microbead thermistor attached to the middle of the tube. The resistance of the thermistor is measured with an ac Wheatstone bridge which is composed of resistors with a low temperature coefficient of ±1 ppm/K. The unbalance ac signal of the bridge is measured with a lock-in amplifier. To reduce the drift of measured values caused by the variation of room temperature, the amplifier and measuring instruments with temperature coefficients of >1 ppm/K are kept at constant temperature. Moreover, the gain of the amplifier is calibrated at every measuring point. Consequently, the heat capacity of 10 μl liquid can be determined with an extremely high sensitivity of ±0.001%, which corresponds to heat capacity changes of ±300 nJ/K. A test of the performance was made of the heat capacity measurement during thermal denaturation of lysozyme dissolved in buffered solution. This method is particularly useful for studying thermal properties of biological macromolecules in solution, since the heat capacity of macromolecules can be measured with a 10 μg sample.

Thermal and thermoelectric properties of III-nitride and III-oxynitride films prepared by reactive RF sputtering

ABSTRACTTowards fabricating a thermoelectric power device using III-nitrides and III-oxynitrides, we studied their thermal and thermoelectric properties and, in particular, we have focused on their thermal conductivity. AInN and AlInON were prepared by reactive radio-frequency sputtering method. We measured the thermal diffusivity and specific heat using the ac calorimetric method, and we estimated values of the thermal conductivity to be 2.2 W/mK (at 773 K) for AlInN and 3.3 W/mK (at 673 K) for AlInON. These values are much smaller than expected considering the large thermal conductivity of nitrides. As for a device composed of 20-pair Chromel-AlInN films, the maximum output power was 0.36 μW at the temperature difference of 180 K, and the open voltage was 0.10 V.

Thermophysical Properties of High-Thermal-Conductivity Graphite Sheets for Spacecraft Thermal Design

Thermophysical properties of a new material-a graphite sheet, which has characteristics of high thermal conductivity, anisotropy, lightweight and flexibility-have been measured in order to apply this sheet to a spacecraft thermal control material. The following measurements were performed: 1) The thermal diffusivities in the in-plane and out-of-plane directions were measured over the temperature range from 100 to 350 K using a laser heating ac calorimetric method. 2) The specific heat and the total hemispherical emittance were measured over the temperature range from 173 to 375 K using a transient calorimetric method. 3) The solar absorptance was measured using a spectroscopic method for angles of incidence ranging from 5 to 60 deg. Additionally, the thermal conductivities and the specific thermal conductivities were calculated using the measured results, and the high potential of this graphite sheet as a material for spacecraft thermal control was confirmed.

Measurement of the Thermal Diffusivity of an Anisotropic Graphite Sheet Using a Laser-Heating AC Calorimetric Method

The thermal diffusivity of a graphite sheet having an extremely high anisotropy has been measured by a laser heating AC calorimetric method in the temperature range from 30 to 350 K. This graphite sheet has characteristics of high thermal diffusivity and high anisotropy, and it is only 100 μm thick. Thus, it is difficult to apply the conventional AC technique. Therefore, we propose a simultaneous measurement method for the in-plane and out-of-plane thermal diffusivities, by analyzing the three-dimensional heat conduction process, which contains the effects of anisotropy and thermal wave reflections. This method was verified by checking with thermal diffusivity measurements of isotropic materials such as stainless steel and pure copper and was then applied to the anisotropic thermal diffusivity measurement of the graphite sheet.

Thermal diffusivity of graphite sheet at low temperatures

The temperature dependence of the in-plane thermal diffusivity and the out-of-plane thermal diffusivity for graphite sheet over the temperature range from 30 to 375 K, and the in-plane anisotropy at room temperature measured by a laser-heating AC calorimetric method, are reported. It was found that the value of the in-plane thermal diffusivity was significantly greater than that of the out-of-plane direction. It was also found that the in-plane thermal diffusivity did not show any directional dependence.

Two-Dimensional Effects on Measurement of Thermal Diffusivity by AC Calorimetric Method: III. Advantage of Double-Heating Method

In our previous paper [F. Takahashi, Y. Hamada and I. Hatta: Jpn. J. Appl. Phys. 38 (1999) 5278], the conditions required for precise thermal diffusivity measurement along the surface of a plate-like sample using an ac calorimetric method was proposed. It was concluded that an ac calorimetric method could be applied to a relatively thick sample. In the present paper, to perform the measurement in a much thick sample, we propose a new measuring system in which ac heat flux is applied to both surfaces of the sample, and analyze the two-dimensional temperature wave propagation. Consequently, the conditions for measuring thermal diffusivity to an accuracy of ±1% are proposed.

Calorimetric measurements of the phase transition in Rochelle salt–ammonium Rochelle salt mixed crystals

The specific heats, which are given with absolute value, of Rochelle salt--ammonium Rochelle salt mixed crystals $({\mathrm{RS}}_{1\ensuremath{-}x}\ensuremath{-}{\mathrm{ARS}}_{x})$ in a region $0.18l~xl~0.9$ have been measured between about 10 K and 300 K by an ac calorimetric method. A very small anomaly was found around the transition for any x. It becomes larger as x increases. The anomalies have been well described using the Landau-type free energy expanded to the sixth order of polarization. It was found that, as x increases, the transition point approaches the tricritical point at which a crossover from the second-order phase transition to the first-order one takes place. Transition entropy has been calculated from the anomalous specific heat. It increases from about $8%$ to about $55%$ of $R\mathrm{ln}2$ with increasing x in the measured region. Very small values for small x in this region suggest the displacivelike character in the phase transition. And the order-disorder motion contributes more and more to the phase transition with increasing x.

Thermal Diffusivity Measurement of Chemical-Vapor-Deposited Diamond by an AC Calorimetric Method

An ac calorimetric method has been extensively applied to the thermal diffusivity measurement of a variety of materials from polymer to diamond. In a sample with high thermal diffusivity, the reflection of ac temperature waves at the edge of the sample affects the thermal diffusivity measurement. In addition, in a short sample, it becomes very significant. In this paper, we propose a new method to derive the thermal diffusivity in such a sample from the decay and the phase shift of ac temperature waves. We applied the present method to the thermal diffusivity measurement of chemical-vapor-deposited (CVD) diamond plates. As a result, it was found that the obtained thermal diffusivity for the diamond sample with finite length agrees with that of the plate-like sample with infinite length.

Correction of Edge Effect in AC Calorimetric Method for Measuring Thermal Diffusivity of CVD Diamond Films1

In the thermal diffusivity measurement of a CVD diamond film using an ac calorimetric method, the reflection of an ac temperature wave at the edge of the film sample should be considered due to the limited length of the sample and its high thermal diffusivity, i.e., the edge effect. In this case, the measured thermal diffusivity is given as a function of frequency. The relation between the measured thermal diffusivity and the frequency is represented as an analytical expression. The real thermal diffusivity is obtained by correcting the edge effect by two means. One is an iterative method using the directly measured edge length of the sample to fit the analytical expression. The other is a parameter estimation method by which a simplex method is used to estimate the edge length and the real thermal diffusivity. Thermal diffusivities of two diamond films were measured, and data were analyzed using the above methods. The result shows that the parameter estimation method is relatively accurate and convenient in processing test data.

Analysis of two-dimensional effect in the measurement of thermal diffusivity of thin films with an AC calorimetric method

An ac calorimetric method for measuring the thermal diffusivity of thin-film materials has been widely applied. In the application of this method, the systematic errors caused by the heat loss effect, the edge reflection effect, etc., have been analyzed and corresponding correction methods have been developed. But when measuring films with low thermal diffusivity or with thickness comparable to the thermal diffusion length, a two-dimensional effect which will also result in a systematic error of the measurement is present. In this paper, the mechanism of two-dimensional heat conduction within a thin sample which is supplied a periodic heat flux by a chopped light beam is analyzed. A numerical analysis method is developed to study the effect of the two-dimensional heat conduction on the measured thermal diffusivity values. The relations between the measured thermal diffusivity and independent parameters such as frequency, thickness of sample, width of light spot, etc., are demonstrated to indicate the two-dimensional effect. The experimental precondition for minimizing the systematic error caused by the two-dimensional effect is determined. In addition, the analysis method presented in this paper should be useful for more difficult problems such as error estimation of the thermal diffusivity measurement of coatings or composite films.