Thermal Conductivity Of Small Samples Research Articles

Evaluation of methods to accurately characterize the thermal conductivity of micro-and nanocellular polymers based on poly(methyl-methacrylate) (PMMA) produced at lab-scale

The characterization of the thermal conductivity of new and enhanced thermal insulators developed at lab-scale is a challenge. The small dimensions of the prototypes make impossible the use of the conventional techniques because steady-state methods require large samples. Furthermore, the accuracy of transient methods to measure the thermal conductivity is not clear. In this work, we compare four different approaches to measure the thermal conductivity of small prototypes of nanocellular poly(methyl-methacrylate) (PMMA). Both steady-state and transient techniques are used. Results show that the transient plane source method is not suitable for the characterization of these materials (the deviation from the steady-state methods is on average higher than 15%). In addition, two different approaches for measuring the thermal conductivity of small samples via a steady-state technique are proposed and validated.

New Methodology for Measuring the Thermal Conductivity of Small Samples Using the Boxes Method with Reduced Sensors

The reduction of device components is of a major interest for the conception of low-cost devices with a simple design. Generally, the boxes method measures the effective thermal conductivity of relatively large-sized samples due to its industrial purposes. However, to determine the thermal conductivity of small samples, additional sensors are required. In this work, a new experimental procedure is developed to characterize the thermal conductivity of small samples without any additional sensor. A detailed uncertainty analysis in the thermal conductivity measurements is conducted. The improvement of the instrument uncertainty was also considered. Three different scenarios were examined, from ideal to worst-case, in order to assess the performance of the proposed methodology. The values of the measured thermal conductivity were compared with those obtained by a reference method (hot disk). The comparison shows a good agreement and proves the effectiveness of the used technique. The overall study of the propagation of measurement uncertainties shows that the instrument’s uncertainty was found to be less than 6.1 %.

Extension and optimization of a three-layer method for the estimation of thermal conductivity of super-insulating materials

The three-layer method is a characterization method dedicated to the measurement of the thermal conductivity of small samples made out of insulating or super-insulating materials. It is based on the transient recording of the mean temperatures of two brass plates placed, respectively, above and under the sample, one of them being heated. In case of a super insulating material, the rate of heat flow from the heated to the unheated plate through the surrounding air is not negligible with respect to the rate of heat flow through the sample. It is shown that introduction of a simple parallel thermal resistance allows taking this flow of heat into account. An analytical model, based on a new quadrupolar approach, was developed to estimate this resistance considered further as a known parameter in the estimation process. An experimental study was carried out to characterize samples whose thermal conductivities have been measured previously with another method. The deviation of the estimated thermal conductivities from these reference values with respect to the three-layer method results reaches 29% without considering parallel heat flow but is lower than 2% using the new model.

Development of Thermal Conductivity Measurement Test Rig for Engineering Material

This paper describe the development of a hot rod method by using water as a coolant medium which measure the heat loss through the rod for the steady state measurement of thermal conductivity of small samples. The heat flow through the test sample was essentially one dimensional and heat loss through engineering material is made to use heated guard to block the flow of heat from the hot rod to the surroundings. Since large correction factors must be applied to account for guard imperfection and not maintained ideal condition. So that it may be preferable to simply measure and correct for the heat that flows from the heater disc to directions other than into the sample. Experimental measurements taken in a prototype apparatus combined with extensive computational modeling of the heat transfer in the apparatus show that sufficiently accurate measurements can be obtained to allow determination of thermal conductivity of engineering material. Suggestions are made for further improvements in the method based on results from regression analysis of the generated data. I. Introduction Thermal conductivity is the physical property of the material denoting the ease with a particular substance can accomplish the transmission of thermal energy by molecular motion. Thermal conductivity of a material depends on the chemical composition of the substance or substances of which it is composed, the phase (i.e. gas, liquid or solid) in which it exists, its crystalline structure of a solid, the temperature and pressure to which it is subjected. It also depends on whether the material is homogeneous or not. In all solids, energy may be transferred by means of elastic vibrations of the lattice moving through the crystal in the form of waves. In some solids, notably metals, free electrons moving through the lattice also carry energy in a manner similar to thermal conduction by a true gas phase. At ordinary temperature all solids store thermal energy as vibratory motion of their atoms and potential energy in the bonding between atoms. Therefore the thermal conductivity is related to the heat capacity.Because the selection of measuring method for thermal conductivity had significant role in this work, a brief review of thermal conductivity measurement methods was warranted and had to be included in this thesis. Two general classifications of thermal conductivity measurement methods were reported in literature: (i) steady-state and (ii) transient state methods. In either case the measurement may be absolute or comparative. The difference in thermal conductivity values for absolute or comparative cases is because comparative methods require the property of another material in the calculation of a test sample thermal conductivity, typically a distinct disadvantage. Jensen et al. included a more complete overview of common methods.

Thermal characterisation of μL volumes using a thin film thermocouple based sensor

A microfabricated sensor capable of measuring the thermal conductivity of small samples is presented. Thin film thermocouples and a resistive heater were fabricated on a polyimide substrate, to form a hot-wire-like sensor. Microfabrication allowed the manufacture of small sensors, with current devices formed with 20 μm line widths and 75 μm separations between the heater and sensors. A thin layer of polyimide is used to encapsulate the device, without degrading thermal sensitivity. This gives the devices the advantages of flexibility, robustness, electrical isolation, chemical resistance and biocompatibility. The operation of the devices was demonstrated by measuring the thermal conductivity of 100 μL droplets of glycerin/water mixtures. Additionally, 2-propanol/water mixtures were tested to derive the dependence of the thermal conductivity on the concentration of the mixture, as this is not available in the literature. The devices were subsequently used to monitor the time varying thermal properties of smaller volumes of non-liquid samples. Wetted paper was used, due to its thickness (∼100 μm) and relatively slow change in thermal conductivity as it dries. It was found that the thermal conductivity decreases as the paper dries out and that this change was detected by the devices. Furthermore, it was demonstrated that the theoretical and experimental drying times of the sheet of paper investigated exhibit a similar dependence on the air temperature. This provided clear evidence for the device's ability to monitor the thermal properties of small volumes.

A Simple Transient Method for Measurement of Thermal Conductivity of Rigid Polyurethane Foams

Rigid polyurethane foams (PU) are widely used as thermal insulators in various applications. The thermal conductivity of the foam is the key parameter that governs the efficiency of thermal insulation provided by the foam. The usual technique employed to measure thermal conductivity is based on the rate of steady state heat transfer across a known thickness, induced by two different known temperatures at two opposite surfaces of the foam. We introduce a technique based on the transient measurement of heat transfer measured by an embedded needle probe. This technique is not only rapid but the instrumentation required for such a measurement is simple and the cost is only a fraction of the steady state counterpart. The values of thermal conductivity obtained by both methods are compared and found to agree within 4% over the range of 0.02—0.03 W/mK, which is the usual range of thermal conductivity for commercial rigid PU foams. The sensitivity of the needle probe technique is demonstrated by measuring the thermal conductivity values of foams made with various concentrations of chemical blowing agent (water). The present technique is also shown to be effective for measuring the thermal conductivity of small samples, especially, free rise cup foams for which the steady state technique can not be used.

Description of the parallel thermal conductance technique for the measurement of the thermal conductivity of small diameter samples

In order to measure the thermal conductivity of small needlelike samples (2.0×0.05×0.1 mm3) such as pentatellurides and single carbon fibers, we have developed a new technique called the parallel thermal conductance (PTC) technique. In the more typical steady state method that is used for measuring thermal conductivity, thermocouples are attached to the sample in order to measure the temperature gradient and a heater in order to supply this gradient. However, attaching thermocouples and heaters directly to small samples may be relatively difficult, and cause large heat losses and errors. Thus, the measurement of the thermal conductivity of small samples and thin films has been a formidable challenge, with only few successes, due, among other factors, to the heat loss. It is also difficult for the small samples to support the heaters and thermocouples without causing damage to the sample. In this paper we describe the recently developed PTC method providing measurements on standards as well as single carbon fibers, in addition to preliminary pentatelluride crystals measurements.

Application of DSC for the measurement of the thermal conductivity of elastomeric materials

A rapid method for measuring the thermal conductivity of small samples (cylinders of diameter 5 mm and length 5–8 mm) using an attachment to commercial DSC instrumentation is described. The precision of the technique enables comparison between polymeric materials and assessment of the varying degrees of crystallinity in polyurethane/polyethylene adipate-based elastomers.

Thermal radiation effects in thermal conductivity measurements: analysis and remedies

The authors have studied the effect of thermal radiation on measurements of the thermal conductivity of small samples below room temperature. By using the different thermal conductivity technique they were able to measure the importance of these effects for temperatures greater than about 100K and to determine the principal source of error of the double flux correction, i.e. the different heater temperature with each different heat flux. They thus propose a better configuration for performing such measurements.

A Method for Measuring the Thermal Conductivity of Small Samples of Poorly Conducting Materials such as Optical Crystals

Thermal conductivities of from 2 to 500×10−4 c.g.s. units are measured by comparing the flow of heat through a given sample with that through a standard material. On the basis of repeat runs and checks with literature values, an accuracy of about five percent is estimated for this method. The dimensions of the apparatus limit the face size of both the unknown and standard samples to a maximum of 10 millimeters square; thicknesses of 2 to 10 millimeters are suitable. Values at average temperatures between 30°C and 75°C are reported for nine materials:—sodium chloride, potassium chloride, potassium bromide, calcium fluoride, lithium fluoride (both air-grown and vacuum-grown), silver chloride, thallium bromide-iodide, thallium bromide-chloride, and fused silica.