Methods for thermal conductivity measurements

The two most common methods for thermal conductivity measurements for natural gas hydrates are the transient method and the steady-state method. Afanaseva 

A conventional steady-state guarded hot-plate method for thermal conductivity measurement was used by Cook and Leaist (1983). Their apparatus was used to perform an exploratory measurement of methane hydrate to within 12%. A sample of methane hydrate was made externally, pressed, and placed in the hot-plate cell at the Sample Disc. The lower sample heater had thermocouples contacting the top and the bottom of the sample to determine the temperature gradient. 

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Rahman, M.S., Evaluation of the prediction of the modified Fitch method for thermal conductivity measurements of foods, 7. Food Eng., 14(l) 71-82,1991. 

Several techniques are available for thermal conductivity measurements, in the steady state technique a steady state thermal gradient is established with a known heat source and efficient heat sink. Since heat losses accompany this non-equilibrium measurement the thermal gradient is kept small and thus carefully calibrated thermometers and heat source must be used. A differential thermocouple technique and ac methods have been used. Wire connections to the sample can represent a perturbation to the measurement. Techniques with pulsed heat sources (including laser pulses) have been used in these cases the dynamic response interpretation is more complicated. 

InN single crystals of a size suitable for thermal conductivity measurements have not been obtained. The only measurement of the thermal conductivity has been made using InN ceramics [20], InN microcrystals obtained by microwave plasma were sintered under a pressure of 70 kbar at 700°C. The room temperature thermal conductivity was measured by the laser-flash method giving k = 0.45 W/(cm K) [20], This value is much below the estimate by Slack which gives k = 0.8 W/(cm K). This result indicates that the InN ceramic has a high impurity content and consists of small size grains. 

Measuring thermal parameters of Li-ion cells is crucial for optimizing the thermal design of battery systems with respect to lifetime and safety issues. The thermal parameters of interest are heat capacity, thermal conductivity, and heat exchange between the cell s surface and the environment due to radiation and convection. Traditionally, heat capacity is obtained by calorimeter measurements and thermal conductivity is obtained by heat flux or Xenon-Flash measurements [1], Disadvantages of these methods are the requirement of expensive measurement devices and the destruction of the cell for thermal conductivity measurements. 

In order to choose a refractory or heat insulation material for a specific purpose, it is necessary to have at least two or three values of the thermal conductivity at different temperatures (e.g., at 300 and 800 °C) and to know the measurement method. The thermal conductivity measurements of the lining materials for one furnace or thermal unit preferably should be made in one laboratory by the same method. 

Many methods of thermal conductivity measurements are based on the principle of nonstationary heat flow. The methods of hot wire measurement, double-hot wire measurement, hot disc measurement, hot strip, and laser flash measurement are widely used for different materials. 

The thermal resistance between the ends of the sample and the copper blocks must be negligible compared with the thermal resistance of the sample. This assumption must be verified especially for short samples at low temperature where the contact resistance is higher. For this reason, a second measurement of the thermal conductivity of Torlon in the 4.2-25 K range was carried out. The second sample had a different length (L = 24.51 mm) and the same section A. This additional measurement gave the same value of k within 2%. Moreover, we see from Fig. 11.15 that data of thermal conductivity at 4.2 K well join data at lower temperatures (within 3%) obtained on a sample of much smaller geometrical factor and with a different method (integrated thermal conductivity method) and a different apparatus [38], Finally, at room temperature, we find k = 0.26 W/mK, which is the data sheet value. 

Thermal conductivty can be determined using either equilibrium or dynamic methods. Equilibrium methods involve a heated surface, a thin layer of sample, and a cooled surface. The energy required to maintain a steady state for a given temperature difference is measured and used in the calculations. Dynamic methods are based on thermal dif-fusivity, which is obtained from the curvatures of heating or cooling plots at various depths within the product. Procedures and applications of thermal conductivity measurements to foods have been reviewed (Peeples 1962 Reidy 1968 Woodams and Nowrey 1968). 

Jobs has carried out an extensive series of liquid thermal conductivity measurements over a wide range of temperature on a variety ot compounds. Data on butyraldehyde have been cororlaled with temperature Data ut individual temperature point have been compiled for acetaldehyde, pm-pionaldehyde. and hutyru)deh df > The Jain for formaldehyde were estimated by the method of Robbins and Kmgtea... 

Ammonia is readily detectable in air in the range of a few parts per million by its characteristic odor and alkaline reaction. Specific indicators, such as Nessler s reagent (Hgk in KOH), can detect ammonia in a concentration of 1 ppm. For the quantitative determination of ammonia in air, synthesis gas and aqueous solutions, these methods can be used74 Acidimetry and Volumetric Analysis By Absorption, Gas Chromatography, Infrared Absorption, Thermal Conductivity Measurement, Electrical Conductivity Measurement, Measurement of Heat of Neutralization, and Density Measurement (for aqueous ammonia). 

The calorimeter method is an older technique which is a direct measurement of Fourier s law. It is one of the ASTM [2] standard tests for thermal conductivity, designation C201. The experimental configuration is shown in Figure 9.3. A SiC slab... 

Thermal conductivity measurements offer a further possible method for detecting unstable reaction intermediates. Such studies have been made by Senftleben et al. to detect atomic hydrogen in the mercury sensitised photolysis of molecular... 

The phase composition of the resulted specimens was identified by X-ray diffraction (XRD). Rod-like pieces (3x3xl5mm) and disk-shaped pieces (2mm thickness and 10mm diameter) were cut out for the electrical conductivity measurement and the thermal conductivity measurement, respectively. Microstmcture and phase distribution were observed by a scanning electron microscopy equipped with EPMA (JEOL JXA-8621MX). Electrical conductivity was measured using a D.C. four-probe method. Thermal conductivity was measured using a laser-flash technique. All the measurements were performed in the temperature range of 300 to 1200 K. 

The resistivity, that could be also regarded as an apparent quantity, was also calculated in the manner that the electrical resistance (V/1) was multiplied by the cross-sectional area and divided by the length of the current pass. Such a method as the above mentioned may not be always verified right for thermoelectric measurements, but would be a simpler and easier manner to make a preliminary evaluation of FGM samples to be placed in large temperature gradients. As for the thermal conductivities, however, no data were obtained because of difficulty originated in our equipment of thermal conductivity measurement (the Laser Flash... 

The transient hot wire (THW) method has been well developed and widely used for measurements of the thermal conductivities and, in some cases, the thermal diffusivities of fluids with a high degree of accuracy [6, 42]. More than 80% of the thermal conductivity measurements on nanofluids were performed by transient hot wire method [6, 8, 18, 19, 45-47]. 

Finally, recent works on thermal conductivity measurements using the 3(0 method have reported [16, 17]. This method is very accurate and fast will be explained fully in the next section. We used this method which has also the advantage of requiring small amounts of liquids for the measurement. 

Although direct measurement of reactant temperatures have enabled more quantitative assessment of such reactions, precise tests of thermal explosion theory require a reaction for which the mechanism and Arrhenius parameters are sufficiently well established to give accurate estimates of rates under explosive conditions. Typically the reaction rates involved will be around ten times those determined by static kinetic methods. In addition the thermal conductivity of each gas mixture used and the stoicheiometry and heat of reaction must be known. Pritchard and Tyler suggest the thermal isomerization of methyl isocyanide as a suitable candidate. They report temperature-time records for diluted mixtures in which temperature excesses of 70—80 K occur without explosion. However, the roll-call of missing data—improved heats of formation, isothermal kinetic data at higher temperatures, thermal conductivity measurements up to 670 K, and the recognition and elucidation of side reactions (if any) indicate the extent of further investigations necessary if their proposal is to be fully realized. 

The effective thermal conductivity can be determined using the methods presented in Table 4.5, which includes the relevant references. Measurement techniques for thermal conductivity can be grouped into steady-state and transient-state methods. Transient methods are more popular because they can be run for as short as 10 s, during which time the moisture migration and other property changes are kept minimal. 

The determination of the thermal conductivity of grain is based on the comparison of the temperature history data obtained by using the line heat source probe with the approximate analytical and numerical methods [35,54]. The analytical method has the advantage of being quick in calculating thermal conductivity. This method, however, requires a perfect line source and a small diameter tube holding the line heat source. In reality, this requirement is difficult to meet. Therefore, a time-correction procedure has been introduced [52,54,56]. Another objection to the analytical method is that it cannot easily be used to calculate the temperature distribution in the heated grain and to compare it with the measured one. Such a comparison can be easily accomplished by a numerical method, where the estimated accuracy for thermal conductivity is determined and the thermal conductivity of the device is taken into account [54]. 

Early thermal-conductivity measurements on CVD diamond were performed on relatively thin films (less than 30 pm) with average lateral grain sizes of only a few micrometers, grown by MWP-CVD [71] or HF-CVD [72]. For both growth methods the in-plane thermal conductivity [ decreased from about 10 to less than 2Wcm K , if the methane concentration in the source gas was increased to several percent, accompanied by a deterioration of the film quality as to be judged from Raman spectra and (in the case of the HF-CVD samples) an increase of the H-content from 0.1 to 1.0%. 

METHOD FOR THE SIMULTANEOUS MEASUREMENTS OF THE THERMAL CONDUCTIVITY OF INSULATING MATERIALS AT DIFFERENT MEAN TEMPERATURES. 

THERMAL COMPARATOR METHODS FOR THE RAPID MEASUREMENT OF THERMAL CONDUCTIVITIES. 

Thermal conductivity measurements by TMDSC [9] involve two experiments on the same sample material, where only the sample thickness varies. Preparing a sample of uniform and known geometry is essential for accurate thermal conductivity measurements by this method. The sample is again in direct contact with the sample holder, so this method is also reserved for experienced users. Selecting a thin sample, the experimental conditions are chosen so that the heat capacity of the sample can be measured by the methods outlined earlier (Section 5.6). The second experiment is performed on a thick sample in such a way that controlled temperature modulation occurs only in that part of the sample in contact with the sample holder. The remainder of the thick sample functions as a heat sink. 

In this section, we construct a model of heat transport in the DMFC MEA, which takes into account the thermal effect of crossover. We derive the exact analytical solution to model equations. The solution is greatly simplified under open-circuit conditions. As in a PEFC, the respective relations suggest a method for in situ measurements of the thermal conductivities of the catalyst layers and membrane. 

The system of equations (3.83)-(3.85) and (3.78)-(3.80) is not complete it should be supplemented by the boundary conditions at the outer sides of the MEA. We will consider the cases when one side is thermally insulated while the other side is kept at a fixed temperature. These conditions suggest a simple method for in situ measurements of thermal conductivities of the catalyst layers and membrane in a DMFC. 

The effective thermal conductivity can be determined using the methods presented in Table 4.5, which includes the relevant references. Measurement techniques for thermal conductivity can be grouped into... 

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