Papers by Keyword: Thermal Diffusivity

Abstract: The silicone rubber composites filled with various contents of alumina (Al2O3) microparticles were prepared by the solution mixing process. The fractured surface morphology, thermal diffusivity and dielectric property of the composites were investigated by scanning electron microscopy (SEM), the laser flash thermal diffusivity analyzer, a LCR digital meter and a resistor tester, respectively. The results show that with Al2O3 content increasing from 8 vol. % to 35 vol. %, the thermal diffusivity increases from 0.13 mm2/s to 0.31 mm2/s, while the dielectric property reduces slightly. At a given Al2O3 content, the thermal diffusivity of the composites exhibits a reduction tendency with the increasing temperature. The Nielsen model is generally suitable to predict the thermal diffusivity of the composites.

Abstract: The use of dimension stones in architecture and civil engineering implies the knowledge of several mechanical, physical, and chemical properties. Even though it has been usual practice to measure physical and mechanical properties of dimension stones the same is not true for thermal properties such as thermal conductivity, thermal diffusivity, specific heat capacity, and heat production. These properties are particularly important when processes related with heating and cooling of buildings must be considered. Thermal conductivity, thermal diffusivity, and specific heat capacity are related with the way thermal energy is transmitted and accumulated in stones; heat production has to do with the amount of radioactive elements in the rocks and so with the environmental impact of radioactivity and public health problems. It is important to start to measure on a routine basis those four thermal properties in rocks and, in particular, in dimension rocks so that their application can be improved and optimized. With this is mind three sets of different rock types (granites, limestones, and marbles) were collected to measure the thermal conductivity, the thermal diffusivity, and the specific heat capacity with the objective of characterizing them in terms of those properties. Since the same set of rocks has also been studied for other physical properties, a correlation amongst all the measured properties is attempted. For each rock type several samples were used to measure the thermal conductivity, the thermal diffusivity, and the specific heat capacity, and average values were obtained and are presented. As an example, for granites the thermal conductivity varies between 2.87 and 3.75 W/mK; for limestones varies between 2.82 and 3.17 W/mK; and for marbles varies between 2.86 and 3.02 W/mK. It is hoped that measuring thermal properties on dimension stones will help to better adequate them to their use in civil engineering as well as to adequate their use in terms of a CE product.

Abstract: Alloys of the Fe-Al system are interesting due to occurrence of long-range order and many thermal vacancies at high temperature, which lead to not only significant hardening, but also cause changes of physical properties. High temperature diffusion is conditioned by structural defects in solids, such as vacancies, foreign atoms and dislocations influencing thermal characteristics of a solid solution, among others the thermal diffusivity coefficient. Measurement of thermal diffusivity was performed at room temperature using the laser flash method. For characterization of the defect structure, positron annihilation lifetime spectroscopy was used. The data were presented for alloys with 28 and 38 at.% aluminium without chromium and containing 5 at. % Cr addition. The results showed that thermal diffusivity decreased with aluminium content and deviation from stoichiometry. In the studies, different structural defects in the alloys were observed.

Abstract: This research studied the feasibility of using natural fibers extracted from natural wastes as a thermal-insulating material lined in a Kraft paper box packaging. The natural fibers were extracted from natural waste of rice straws using NaOH solutions. The extracted fibers were then formed as a porous thermal-insulating pad by a spray lay-up method using natural rubbers as binders. The thermal conductivities, specific heat capacities and temperature-rise time of the natural fiber insulation and other thermal-insulating materials including polystyrene foam, a polyethylene foam, and a glass fiber insulation were studied and compared. The glass fiber insulation showed the highest thermal conductivity, while the thermal conductivities of the other studied insulating materials were found to be similar. Moreover, the polymeric and natural-fiber insulations show better temperature-rise resistance than the glass fiber insulation. The temperature rises for different insulating materials were estimated using the analytical analysis of heat transfer. The calculated temperature-rise times were compared with the empirical results; both results are in the same order of magnitude. Consequently, a Kraft paper box lined with natural-fiber pads was constructed and compared with a Kraft paper box (without insulation lining) and a polystyrene box of equal sizes. The boxes were packed with an equal amount of ice and left under room temperature for 24 hours. The results show that, after 24 hours, the temperatures inside the natural-fiber lined box and the polystyrene box were contained below 15 °C, while the temperature inside the Kraft paper box increase to room temperature only after 16 hours. The observation shows that a natural fiber pad can potentially be used as an alternative insulating material in packaging industries, which can enhance environmental-friendly packaging products.

Abstract: Suspensions of oleic acid with carbon nanotubes were prepared. Using a thermal lens experimental setup, the thermal diffusivity of oleic acid in presence of multi-walled carbon nanoparticles with different concentrations was measured. The results show that, the thermal diffusivity increases with the increase of nanotubes concentration, enhancing the thermal diffusivity in the solution. Modification in the thermal diffusivity as function of quantity of carbon nanotubes was investigated, for a constant volume of oleic acid 10 mL. The diffusivities were obtained by using the thermal lens spectrometry technique. The characteristic time constant of the transient thermal lens was obtained by fitting the theoretical expression to the experimental data.

Abstract: In this paper, the application of the photo-acoustic method to study the thermal properties of modified 9Cr-1Mo Steel is described. The photo-acoustic measurements are carried out for the thermal properties of modified 9Cr-1Mo steel samples of various thicknesses. The theoretical basis for quantitative measurements is discussed, together with the advantages and limitations of these methods as compared with conventional measurements.

Abstract: Thermal properties of intumescent fire-retardant materials are important parameters as input data in modeling combustion behavior of these materials in fire. However, Data regarding thermal properties especially thermal diffusivity of these composites are sparse. In this paper, intumescent fire-retardant polypropylene (IFR-PP) and char formed after the combustion of IFR-PP materials by the cone calorimeter at heat flux level of 50 kW/m2 are studied to obtain the thermal diffusivity data of them. Study results show the value of thermal diffusivity for IFR-PP tends to decrease when the temperature increase especially in higher temperatures. Thermal diffusivity of char formed after the combustion of IFR-PP material decreases gradually with the rise of temperature. Thermal diffusivity value measured at room temperature could not be used as input parameters in a combustion model when it is established to simulate the burning behavior of IFR-PP composites in fire.

Abstract: High quality, transparent 1.0at% Nd:YAG ceramics were fabricated successfully by a solid-state reaction and vacuum sintering, and the characteristics were also identified in different methods. The lowest transmittance is 44.85% and the highest is 71.98%. 9 samples were used for thermal diffusivity detecting using laser flash method. Thermal diffusivity decreases as temperature increases, and The diffusivity increases as average grain size grows. However, when average grain size is smaller than 8μm, the diffusivity decreases much faster. Mechanical advantages be ensured, the average grain size should be above 8μm.

Abstract: The first measurement of thermal transport properties on the polycrystalline D-Er₂Si₂O₇ have been made in the temperature range 77-300K. Both the thermal conductivity and the thermal diffusivity follow modified Euckens law in this temperature region. The Transient Plane Source technique (TPS) has been used to measure thermal conductivity and thermal diffusivity simultaneously.

Abstract: In the present study, the thermal diffusivity of four sand casting magnesium alloys: Mg-9Al-1Zn, Mg-6Al-2Sr, Mg-9Al-1.5Ca-0.3Sr and Mg-9Al-2.2Ca-0.8Sr were studied. Sand casting was performed at 730-780°C temperatures. Thermal diffusivity was measured by a LFA 427 Netzsch apparatus. The thermal diffusivity of the investigated alloys was chemical composition and temperature dependent and increased with increasing temperature. The thermal diffusivity of Mg-Al-Ca-Sr alloys was higher than that of Mg-Al alloy, because the total volume fraction of intermetallic phases in alloys containing calcium and strontium is larger than that in Mg-Al alloy. The formation of intermetallic phases caused the consumption of the solute element in the α-Mg matrix, and improved the thermal diffusivity of the Mg-Al-Ca-Sr magnesium alloy.