Transient Hot-strip Method Research Articles

Limits and potentials of the transient hot-strip method for identifying thermal transport properties of layered composites

Transient methods are widely used to determine the thermal transport properties of dielectrics and other materials. We investigated the limits and potentials of the transient hot-strip (THS) method for the simultaneous determination of the individual thermal conductivity and the thermal diffusivity of layered composites. The experimental data are analysed by solving the related inverse problem of parameter identification which is formulated as an output least-squares problem. The forward problem is governed by the finite-element solution of the heat conduction equation. As expected, classical difficulties of inverse problems such as ill-posedness and, especially, the verified existence of several local minima complicate a unique solution. The proposed identification algorithm considers two strategies to achieve success. First, the transport parameters of the individual layers are determined successively, beginning with the layer next to the heater. And second-which is surprising-the measurement time has to be sufficiently long so that the THS signal contains information about the sample surface. Otherwise, the thermal properties cannot be uniquely determined and the iteration may lead to a local minimum. The identification procedure is widely verified using simulated data. Results for experimental data are also given.

Transient Hot Strip (THS) Method: Uncertainty Assessment

The standard method for measuring thermal transport properties of dielectric solids such as ceramics and refractories is the transient hot wire (THW) technique. In its simplest arrangement, a thin wire is embedded between two sample halves, where it acts simultaneously as a resistive heat source and a thermometer. From its temperature signal, the thermal conductivity and the thermal diffusivity of the dielectric can be derived. Up to now, there is no uncertainty assessment for this technique strictly following the ISO Guide to the Expression of Uncertainty in Measurement. Here we analyze the ISO standard uncertainty of the THW technique in the same way as in a previous paper on the uncertainty of the closely related transient hot strip (THS) technique. The two papers provide a comprehensive comparison of the most important advantages and disadvantages of these two transient techniques. The results obtained here for the uncertainty (5.8% for the thermal conductivity and 30% for the thermal diffusivity) are nearly the same as those for the THS method. Experiments on a Pyrex standard-reference sample confirm the results.

Uniaxial pressure dependence of the Debye temperature in metals

The electrical resistance and its temperature coefficient have been measured for a platinum foil as a function of uniaxial pressure over the pressure range 0 to 60 MPa. The measurements were performed at room temperature using the transient hot-strip method. The data are analyzed using the electrical resistivity formula within the Block-Gruneisen approximation. The pressure dependence of the Debye temperature was directly obtained from an expansion of this formula and using the basic definition of the temperature resistivity coefficient. The reliability of the experimental data was then verified using the basic definition of Gruneisen constant. Within the investigated pressure range, the analysis supports the interpretation that the change in resistance of platinum under pressure is mainly due to the change in the amplitude of the atomic vibrations that are directly related to the change in Debye temperature. The pressure dependence of resistance and the Debye temperature of the platinum were reasonably good in spite of the approximations involved.

Thermal transport properties of D-Er2Si22O7 at room temperature

The compound Er2Si2O7 exists in three modifications-a triclinic low-temperature phase (type B), a monoclinic modification (type C) and a high-temperature monoclinic modification (type D). The thermal transport properties of single crystals of D-Er2Si2O7 have been measured by the transient hot strip method. The average thermal conductivity was found to be about 1 W m-1 K-1. By using the calorimetric method the heat capacity per unit mass of D-Er2Si2O7 at room temperature was found to be 0.52+or-0.04 J g-1 K-1, corresponding to the molecular heat capacity of 31.4 R. The calculated molecular heat capacity at room temperature is 33 R.

A new method for thermal diffusivity and thermal conductivity evaluation from transient hot-strip measurements

Abstract The transient hot-strip method is a technique for the simultaneous measurement of thermal conductivity, thermal diffusivity and volumetric heat capacity. The basic idea of the method is similar to that of the transient hot-wire technique. A short-time quadratic approximation is available for the temperature rise of the electrically heated strip. However the information recorded in the short-time range is not sufficient for the properties evaluation in case of granular materials. In the present contribution a new technique is proposed for data evaluation within the whole time domain. As an example the thermal conductivity and diffusivity of some granular materials, quartz sand, Mg-powder and LaNi 4.7 Al 0.3 -powder, are determined.

Transient hot‐strip (THS) method for measuring thermal conductivity of thermally insulating materials

AbstractThe fast and precise transient hot‐strip (THS) method is well suited for thermal conductivity measurements on solid materials. The THS method may, however, give large experimental errors when applied to thermally insulating materials of low heat capacity per unit volume. Models to deal with those potential error sources and some indications about the precautions to be taken in order to minimize them are described in the present work. Measurements of thermal conductivity of a styrofoam insulating material (thermal conductivity 0.036 W m−1K−1, density 25.4 kg m−3) was performed to verify the models. The result obtained is in good agreement with the standard hot plate method, indicating that the THS method is also well suited for thermal conductivity measurements of thermal insulators.

Thermal conductivity measurements using a short transient hot-strip method

The transient hot-strip (THS) method has been modified to measure thermal conductivity of small test specimens. The conventional metal strip used simultaneously as the heater and the temperature sensor was substituted with a 50-Ω commercial temperature sensor (grid 5.85 mm×6.35 mm). The short transient hot-strip (STHS) method was tested on Pyrex 7740, stainless steel (NIST 1462 standard reference material), and aluminum to cover more than two orders of magnitude in thermal conductivity. The STHS method agreed with conventional measurement techniques within 3% in the thermal conductivity range 1–200 W m−1 K−1.

Measurements of liquid thermal conductivity and diffusivity by the transient hot-strip method

The basic idea of the transient hot-strip method is similar to that of the transient hot-wire method. A thin strip of platinum serves as both a heating element and a resistance thermometer. The time history of the heat source temperature allows for evaluating both the thermal conductivity (within at least an accuracy of 1 or 2% with either method) and the thermal diffusivity. The latter may be obtained from hot-strip experiments within an accuracy of 3 or 4% compared to about 10% for hot-wire. In the present contribution the hot-strip method is applied to various liquids. Thermal conductivities and diffusivities obtained for water, glycerol, toluene, n-heptene, acetone, CFC 113 at ambient pressure and temperature agree with “standard reference data” within 2% (conductivity) and 3% (diffusivity). Finally the experimental set-up is used to measure conductivity and diffusivity of HFC 152a which is thought to become an alternative for CFC 12. Results are presented for saturated liquid up to p = 60 bar.

Simultaneous measurement of thermal conductivity and thermal diffusivity of rock-marbles using transient plane source (TPS) technique

The recently developed transient plane source (TPS) technique has been employed for the simultaneous measurement of thermal conductivity and thermal diffusivity of five marble samples. In this paper we discuss briefly the theory of the transient plane source method. The source of heat also serves as a sensor of temperature increase in the sample. The measured values of the thermal conductivity and diffusivity of these samples are in close agreement with the values of other marble samples reported earlier using the transient hot strip method. A comparison of these two techniques has also been given in the light of the experimental results.

Effective thermal conductivity of copper powders

Effective thermal conductivity of copper powders of different particle size has been measured experimentally at room temperature and normal pressure using the transient hot strip method. A variation in the values of the effective thermal conductivity of these powders with their porosities is observed. To explain this variation a model for a two-phase system (powders) is developed. Theoretical calculations of the effective thermal conductivity of powders using this model and a model developed by Hadley (1986) (volume-averaging formalism) have been carried out. It has been found on comparison that the theoretical results are in good agreement with the experiment.

Temperature dependence of thermal conductivities and thermal diffusivities of composites using transient hot-strip method

Simultaneous measurements of the thermal conductivity and thermal diffusivity of four composite building materials, namely black marble stone, normal building stone, red sand brick and cement brick have been made at room temperature using the transient hot-strip method. The power supplied to these samples is quite large compared with that for loose building materials owing to their composite (brick) nature. The temperature dependence of thermal conductivity and thermal diffusivity of these materials has also been investigated in a temperature range from 10 to 60°C. Temperature stability is assured within 0.1°C by using a platinum resistance thermometer in the chamber containing samples and a digital voltmeter to record any variation in resistance. The results of these experiments show that there is a very slight variation in thermal conductivity and thermal diffusivity of these materials in this temperature range.

Interstitial Air Pressure Dependence of Effective Thermal Conductivity and Diffusivity of Rajasthan Desert Sand Using Transient Hot-Strip Method

The transient hot strip (T H S) method used for the simultaneous measurement of effective thermal conductivity and diffusivity of loose granular two-phase materials at normal pressure and room temperature is now used for the measurement of effective thermal conductivity and diffusivity of Rajasthan Desert sand at different interstitial air pressure and room temperature. Experimental values of effective thermal conductivities and diffusivities at different interstitial air pressures have been compared with the estimated values using empirical models for the variation of thermal conductivity and diffusivity with interstitial air pressures. Estimated values are in good agreement with the experimental results in the interstitial air pressure range from 1 mm to 760 mm of mercury, whereas in the low pressure range (0.1 mm to 1 mm Hg) deviations from the experimental results have been observed.

Determination of kinetics and thermal conductivity of hydrates simultaneously by using thermogravimetry and transient hot strip method

In a previous paper [1], a discussion has been given of the necessity for knowledge of the kinetic properties of a solid-gas reaction in order to be used in a chemical heat pump. However, a more complete assessment of the suitability of sorbents requires the knowledge of thermal properties and, in particular, of thermal conductivity. As the greatest contribution to the heat transfer in solid particles and between particles is the conduction of heat. In this communication, we want to present a combination of two different experimental techniques: Thermogravimetry and the Transient Hot Strip in order to study kinetic and thermal conductivity simultaneously. The experimental conditions in which the experiments can be carried out are the same as those prevailing in chemical heat pump application, i.e., a constant temperature and vapour pressure of the working fluid. The sodium sulphide system was selected to test this new technique because it has recently been investigated by us [2,3] from the point of view of kinetics and mechanisms as well as used in the Tepidus chemical heat pump system [4].

Thermal transport studies of electrically conducting materials using the transient hot-strip technique

The transient hot-strip (THS) method has been used for thermal conductivity and thermal diffusivity studies of electrically conducting materials by introducing a thin electrically insulating layer between the hot strip and the metallic material under study. The insulating layer introduces a certain thermal contact resistance between the hot strip (heat source cum temperature sensor) and the surface of the sample to be studied. To account for this thermal resistance a theory has been developed which indicates how measurements on these kind of materials should be performed and how the reduction of data from transient recordings should be carried out to give reliable results. The new experimental approach, which should be applied whenever a thermal contact resistance is suspected, has been demonstrated by two series of measurements on a stainless steel at room temperature.

Thermal conductivity ofn-Tetracosan around the liquid-liquid transition temperatureT u

Forn-Tetracosan the temperature dependence of the thermal conductivity has been determined between 52 °C and 150 °C by the transient hot strip method. A decreasing thermal conductivity is followed by a constant one forT≳100 °C. The change in slope can be well estimated by using data of Brillouin scattering and specific volume measurements thus confirming the existence of theT u -phenomenon.

Simultaneous measurement of thermal conductivity and thermal diffusivity of some building materials using the transient hot strip method

A transient hot strip method developed by Gustafsson (1979), for simultaneous measurement of thermal conductivity and thermal diffusivity of solids and fluids with low electrical conductivity, is used for the simultaneous measurements of thermal conductivity and thermal diffusivity of some building materials such as dry dune sand, dry brick powder, dry cement, dry mud powder and dry asbestos powder at room temperature and normal pressure. The hot strip works both as a plane heat source and a temperature sensing device. The experimental values of thermal conductivity and thermal diffusivity of these materials are compared with those obtained from thermal probe and hot-wire methods. A good agreement in the values indicates that the transient hot strip (ThS) method can also be used for loose porous materials for obtaining these thermal parameters.

Thermal conductivity of a microporous particulate medium: moist silica gel

A systematic study of the thermal conductivity of beds of moist silica gel is presented. The influence of porosity, water content, total gas pressure and temperature is determined through measurements under transient conditions with the transient hot-strip (THS) method and under static conditions in a bench-scale reactor. The predictions of the effective thermal conductivity of the beds from four different simple models (Russell, geometric mean value, unit-cell model and stochastic model) agree reasonably well with the experimental results. The unit-cell model is extended in order to account for the water sorbed in the micropores and describes satisfactorily the dependency of the effective thermal conductivity on the water content.

Circuit design for transient measurements of electrical properties of thin metal films and thermal properties of insulating solids or liquids

A general circuit design is described for the transient hot-strip method which is presently used for studies of electrical properties of thin metal films and for measurements of thermal properties of insulating solids and liquids. It is demonstrated that a low driving voltage can be used for thin films having comparably high resistance, and that the output of power in the strip source (being the nonlinear component of the circuit) can be kept constant throughout the transient recording. The experimental arrangement for constant power conforms with the assumptions of the theory for the method and extends its validity to substantially larger temperature increases of the nonlinear resistance.

Transient hot-strip probe for measuring thermal properties of insulating solids and liquids

The transient hot-strip (THS) method has been developed for experimental situations with two different materials kept against the sides of the strip. Based on this development THS probes, with known or separately measured thermal properties, have been designed and used successfully in a series of reported experiments. If the thermal diffusivity of the probe is less than that of the material under study and good contact is established between the probe and the sample, the accuracy of the measurements turns out to be as good as with the ordinary transient hot-strip method.

Simultaneous measurement of resistivity and temperature coefficient of resistivity of metallic thin films with the transient hot-strip method

The recently described transient hot-strip method for measuring thermal transport properties of solid insulators has been developed for simultaneously measuring the resistivity ϱ F and the temperature coefficient of resistivity (TCR) α F of metallic thin films. The experimental arrangement is similar to the well-known four-probe technique but the constant current pulse passed through the thin film is sufficiently high to increase its mean temperature by about 1 K. By measuring the time-varying increase or decrease in the resistance, information is obtained not only about the resistivity but also about the TCR. The main advantage of the method is the possibility to measure ϱ F and α F simultaneously without increasing or decreasing the temperature of the film by more than 1 K, i.e. without measurably changing the properties of the film. The method has been tested on unannealed copper films at room temperature.