Ac Calorimetric Method Research Articles

Two-Dimensional Effects on Measurement of Thermal Diffusivity by AC Calorimetric Method: II. Effects of Heat Loss

In our previous paper [I. Hatta et al..: Jpn. J. Appl. Phys. 38 (1999) 2988.], the condition required for measuring thermal diffusivity along the surface of a thick plate-like sample using an ac calorimetric method was proposed in which the sample is subjected to an adiabatic condition. In measurement at high temperatures, we cannot avoid the effects of radiation heat loss. Therefore, it is important to extend the analysis to include a case where there is heat dissipation from the sample to surroundings. We analyze two-dimensional temperature wave propagation in a thick plate-like sample during heat loss between the plate-like sample and a thermal bath. In such a case, we propose the conditions for measuring thermal diffusivity to an accuracy of ±1%.

Two-Dimensional Effects on Measurement of Thermal Diffusivity by AC Calorimetric Method: I. Conditions for Precise Measurement

In the application of an ac calorimetric method to precise thermal diffusivity measurement of a thin material, one-dimensional temperature wave propagation is required. In a relatively thick material, one-dimensional temperature wave propagation is deformed and in the strict sense, two-dimensional temperature wave propagation should be taken into account. In the present paper, we propose the condition of the maximum thickness of a material required to measure thermal diffusivity within an accuracy of ±2% in the framework of one-dimensional temperature wave propagation, based upon the results obtained analytically for the two-dimensional temperature wave propagation.

Investigation of Grain-Size Influence on the Ferroelectric-to-Paraelectric Phase Transition in BaTiO3 Ceramics by Means of AC Calorimetry

Barium titanate ceramic samples of four different average grain sizes from 0.5 µm to 20 µm were studied by means of the ac calorimetric method. The ferroelectric-to-paraelectric transition temperature decreased with decreasing grain size. The temperature dependence of heat capacity showed hysteresis during a heating/cooling cycle. The thermal hysteresis was found to be grain-size dependent, with a possible maximum for grain sizes between 0.5 µm and 2.5 µm. The small shift of the Curie temperature detected in these ac calorimetry experiments suggests that internal stresses are the main cause of the observed grain-size effects.

Measurement of thermal diffusivity of thin films by the AC calorimetric method

Measurement of thermal diffusivity of thin films by the AC calorimetric method

Thermal conductivity reduction in GaAs-AlAs distributed Bragg reflectors

Self-heating of vertical-cavity laser diodes is strongly affected by the thermal conductivity of the distributed Bragg reflectors (DBRs). Binary GaAs-AlAs DBRs are expected to be good heat conductors, but investigations of GaAs-AlAs superlattices indicate strong interface scattering of phonons. For the first time, we present direct thermal conductivity measurements of separated GaAs-AlAs DBRs with a quarter-wave layer thickness of more than 100 nm (tuned to 1.55-μm wavelength). Using an ac calorimetric method and finite element analysis, we measure about 50% thermal conductivity reduction compared to the average bulk value. GaAs-AlAs DBR's for shorter wavelengths are expected to show an even lower thermal conductivity.

Potentiality of an ac calorimetric method in the study of phase transitions

To manifest potentiality of an ac calorimetric method, successful application of a light-irradiation ac calorimetric method is introduced in the studies of phase transitions of quartz, ferroelectric BaTiO 3, and two-dimensional antiferromagnetic K 2NiF 4-family. In quartz, the α-to-incommensurate and the incommensurate-to-β phase transitions, which lie in a narrow temperature range, were clearly separated by this method. In BaTiO 3, the heat capacity at first-order transition was measured with high precision and, therefore, it was found that the detailed behavior at the first-order transition is well-expressed in terms of the free energy expansion. As a result, in a stress-clamped crystal in which the phase transition becomes second order, the discontinuity of the heat capacity at the phase transition was estimated to be 0.19 k B This result stimulated further theoretical investigation. From this study, it was revealed that BaTiO 3 is classified as a typical displacement-type ferroelectric. In order to study crossover phenomenon from a two-dimensional Ising system to a two-dimensional Heisenberg system in the K 2NiF 4 family, the critical amplitude of the heat capacity was measured as a function of the strength of Ising-like anisotropy. Based upon the analysis for the results, the existence of an asymptotic transition temperature in a two-dimensional Heisenberg system is proposed.

Measurement of thermal diffusivities of thin metallic films using the ac calorimetric method

The thermal diffusivities of thin films of evaporated metal less than 1×10−6 m thick have been measured using the light-irradiation ac calorimetric method. The thermal diffusivities of metallic thin films become smaller than those of the bulk as the thickness of the metallic thin films decreases. It is determined, using x-ray diffraction measurement, that the decrease of thermal diffusivity of thin films of evaporated metal is caused by a decrease in crystalline size.

Thermal diffusivity of diamond/Si composite films measured by an ac calorimetric method

Thermal diffusivity of diamond/Si composite films was measured in the direction parallel to the plane of the film by an ac calorimetric method. The effect of two-dimensional heat conduction on the measured results appears to be due to the difference in the thermal diffusivities of the layers. The thermal diffusivity measured by the thermocouple attached on the diamond side was higher than that on the silicon side. This might be due to the two-dimensional effects. As a result, the estimated thermal diffusivity of the diamond layer from the result of composite film detected on the diamond side is higher than that detected on the Si side. This gives rise to an error in the estimated thermal diffusivity of the diamond layer. A criterion for the deviation of the estimated thermal diffusivity is proposed.

Thermal diffusivity of diamond/Si composite films measured by an ac calorimetric method

In measurements of thermal diffusivity, by an ac calorimetry method, on films of materials of high thermal diffusivity, attention should be paid to the effects of the sample length, i.e., the reflection of ac temperature waves at the sample edge. For such a case, the apparent thermal diffusivity of the sample having a finite length is given analytically as a function of frequency. Measurements were performed on diamond/Si composite films. The overall behavior of the frequency dependence of the apparent thermal diffusivity obtained is satisfactorily explained by the analytical expression. The true thermal diffusivity is obtained by fitting a theoretical curve to the experimental results. Especially when the apparent thermal diffusivity saturates in a high-frequency region, the saturated value gives the true thermal diffusivity.

Thermal diffusivity measurement of single fibers by an ac calorimetric method

An ac calorimetric method is applied to the direct measurement of thermal diffusivity of single fibers. Theoretical analysis is presented, taking into account the two-dimensional ac thermal wave for cylindrical systems and the effect of time delay at the temperature detecting system. Experimental tests are performed using nickel and stainless steel wires about 20 μm in diameter. The thermal conductivity of fibers about 10 μm in diameter of the advanced materials, polyacrylonitrile-based carbon, TORAYCA, and aramide, Kevlar, is determined. The relationship between the thermal conductivity and electric conductivity and the crystalline size of these fibers is discussed. For composite materials made of carbon fibers, the observed thermal conductivity agrees with the thermal conductivity estimated using the values determined by the present method.

Analysis of ac temperature wave during the measurement of thermal diffusivity of two-layered platelike samples

A theoretical consideration of two-dimensional ac temperature waves in a two-layered platelike sample for application to thermal diffusivity measurement parallel to a sample surface using an ac calorimetric method is presented. The analysis is based on the Fourier-transformed Green’s function technique. It is concluded that this thermal system deviates from an apparent one-layered system of which the thermal diffusivity parallel to a sample surface is calculated based on steady state heat flow under the following conditions: (a) the frequency of the supplied ac thermal energy is high, (b) the difference between the thermal diffusivities of the layers is large, and (c) the layer with higher thermal diffusivity is thin. This deviation is due to the inhomogeneous ac temperature distribution normal to the sample surface. If experimental conditions are the reverse of conditions (a)–(c), an ac calorimetric method can be used to measure the thermal diffusivity of thin samples on substrates.

Effect of magnetic field on the specific heats of intercalation compounds Mx TiS2 (M=3d transition metals)

Specific heats of single crystals of intercalation compounds MxTiS2 (M=Mn, Fe, Co, and Ni) have been measured using ac calorimetric method over the temperature range T = 0.38–14 K under magnetic fields up to 4.5 T. Considering the lattice dynamical properties of these intercalates, we have analyzed the observed specific heat data C, which can be expressed by the sum of electronic, lattice, magnetic, and anomalous terms, C = γT + βT3 + Cmag + Ca, where Ca is of a Schottky type. The temperature and magnetic field dependences of γ and Cmag are discussed from various viewpoints, such as existing spin fluctuation model, which reveals that the itinerant electron picture is appropriate to understand the electronic and magnetic properties of this material system.

Temperature dependence of thermophysical properties of GaAs/AlAs periodic structure

This work investigates the temperature dependence of GaAs/AlAs thin-film structures. Based on an ac calorimetric method, the thermal diffusivity of a 700 Å/700 Å GaAs/AlAs periodic structure is measured from 190 to 450 K. Thermal conductivity of the structure is derived from the experiment. The results demonstrate that the thermal conductivity/diffusivity of the structure are lower than its corresponding bulk values. The temperature dependence of its thermophysical properties is weaker than that of typical bulk III–V materials. Interface scattering is believed as the major cause of the observed reduction in thermal conductivity.

Analysis of two-dimensional temperature response during the measurement of thermal diffusivity of platelike samples by an ac calorimetric method

A new method has been developed to estimate the two-dimensional periodic temperature response of platelike samples using the Fourier transform of the two-dimensional impulse response for the designated system. This method is applied to the case of thermal diffusivity measurements parallel to the sample surface using an ac calorimetric method. It is concluded from the theoretical consideration that the thermal system can be considered as a one-dimensional system except at the boundary of the heated and nonheated regions. The thermal system cannot be considered as a one-dimensional system near the boundary of the heated and nonheated regions since the ac temperature response changes transiently perpendicular to the sample surface.

Thermal Diffusivity Measurement of a Thin Film in the Direction across the Film by AC Calorimetric Method

A method utilizing ac calorimetry for measuring thermal diffusivity of a thin film in the direction across the film is proposed based on both theoretical analysis and experimental examination. The sample system is composed of three parts: a medium, a thin film sample and a second medium. For precise measurement, it is essential that one of the two media is made of the same material as the thin film sample. With this system, a thin polyimide film with thickness of 75 µm was experimentally examined in the frequency range from 4 to 140 Hz at room temperature. The standard deviation of the analyzed thermal diffusivity was less than 0.2%. The value of the thermal diffusivity is consistent with that obtained with a laser flash method.

AC calorimetric study of phase transitions in phosphatidylcholine-cholesterol systems

Using an ac calorimetric method, detailed behaviour of the heat capacity in dipalmitoyl-phosphatidylcholine-cholesterol system was studied in the cholesterol concentration less than 5 mol%. It was revealed that the heat capacity near the main transition was composed of at least four anomalies, i.e., multipeak took place in the heat capacity. This fact indicates that a simple theory explaining coexistence of two phases in two component systems does not work in the multipeak region. Then, relation between the multipeak heat capacity and the change of the ripple structure with the cholesterol concentration should be taken into account, when we consider thermodynamical behaviour of the systems.

The Measurement of Thermal Conductivity of Ag/Al Multilayered Films

AbstractThe in-plane thermal conductivity of Ag/Al multilayered films was measured at room temperature by ac calorimetric method as a function of the modulation wavelength and was compared with the electrical conductivity. The electrical conductivity increases with wavelength, Λ, like other metallic multilayered films. The thermal conductivity also tends to increase with Λ, but the Λ dependence is not similar to that of electrical conductivity. Obtained Lorentz number value of the multilayers are almost 10–30% larger than that given by the free electron theory. The Wiedemann-Franz law did not hold in the Ag/Al films and the phonon contribution is not negligible.

Effects of Oxygen Impurity and of Heat-Treatment on Thermal Diffusivity of Sputtered SiC Film

Thermal diffusivities of sputtered SiC thin films were measured by using ac-calorimetric method. X-ray diffractions showed that the as-sputtered samples were in amorphous state and the thermal diffusivity of the SiC thin film was 0.40 cm 2 /s. After heat-treatment at 400°C for 5 hours in argon gas, there occured recrystallization and then the thermal diffusivity increased to be 0.62 cm 2 /s. Furthermore, the effect of SiO 2 impurity on thermal diffusivity was investigated. The impurity acting as a source of oxygen supply decreased the heat conductivity.

Development of ac Calorimetric Method for Thermal Diffusivity Measurement III. Heat Loss in the Measurement of Fine Wires

The measurement of the thermal diffusivity of a fine wire is developed, using a light-irradiation-type ac calorimetric method. It is pointed out that in the analysis, the contribution of heat loss or external relaxation time should be taken into account for the measurement of a fine wire. To examine the measurement, a wire of stainless steel with a diameter of 30 µm is used as a sample and the test is carried out in the wide frequency range of 1–25 Hz. The result obtained agrees with the value reported for the bulk material.

Electrothermal measurements: A calorimetric method to examine power dissipation in photovoltaic devices

Power loss mechanisms in illuminated and nonilluminated photovoltaic cells are investigated by an ac calorimetric method. In this electrothermal (ET) method, periodic temperature changes, caused by periodic electrical excitation, are sensed as a function of external parameters, e.g., applied voltage. ET signals on crystalline silicon and thin-film CuInSe2/CdS solar cells are measured as a function of applied voltage. The results are compared to those obtained from an energy balance model. Inherent to the model is the occurrence of injected carrier cooling. The ability of ET measurements to separate different power loss mechanisms is discussed and compared to that of conventional photothermal ones (e.g., photoacoustic).