Bashir M. Suleiman

Thermal conductivity and diffusivity of wood

Summary Transient simultaneous measurements of thermal conductivity and diffusivity of Swedish wood have been performed with the plane source technique on oven-dry hardwood (birch) samples at room temperature and at 100 °C. The influences of temperature, density, porosity and anisotropy on thermal conduction were investigated. The measurements were done in longitudinal (parallel to the grain) and transverse (intermediate between radial and tangential) directions. As the temperature increased from 20 to 100 °C, the thermal conductivity of each sample increased slightly for both longitudinal and transverse directions. The effect of density and porosity on the thermal conductivity may be related to the presence of other scattering mechanisms such as voids and cell boundaries. It seems that the dominant mechanism of heat transfer across the cell lumina in these types of wood is the heat conduction through the voids. An attempt was made to explain the behaviour of the effective thermal conductivity by adopting a model based on the ratio between heat conduction in parallel and serial layers of gas, liquid, and solid phases.

Dynamic plane source technique for simultaneous determination of specific heat, thermal conductivity and thermal diffusivity of metallic samples

A technique is developed for the simultaneous determination of specific heat, thermal conductivity and thermal diffusivity of metals. The method is of a transient heat-flow type where the heating element serves both as a heat source and the temperature detector. The experiment is arranged in such a way that the temperature development in the sample is close to adiabatic. The method has been tested on samples of pure iron (99.95% of purity), cast iron (SIS 2140) and austenitic stainless steel (SIS 2343). The results obtained were in an excellent agreement with the literature data for these materials. The accuracy of determination of specific heat has been additionally tested on an Al sample.

An extension to the dynamic plane source technique for measuring thermal conductivity, thermal diffusivity, and specific heat of dielectric solids

The recently developed dynamic plane source (DPS) technique for simultaneous determination of the thermal properties of fast thermally conducting materials with thermal conductivities between 200 and 2 W/mK has now been extended for studying relatively slow conducting materials with thermal conductivities equal or below 2 W/mK. The method is self‐checking since the thermal conductivity, thermal diffusivity specific heat, and effusivity of the material are obtained independently from each other. The theory of the technique and the experimental arrangement are given in detail. The data evaluation procedure is simple and makes it possible to reveal the distortions due to the nonideal experimental conditions. The extension to the DPS technique has been implemented at room temperature to study the samples of cordierite‐based ceramic Cecorite 130P (thermal conductivity equal to 1.48 W/mK), rubber (0.403 W/mK), and polycarbonate (0.245 W/mK). The accuracy of the method is within ±5%.

A practical cryogenic resistive sensor for thermal conductivity measurements

Abstract The line heat-source is one among many classes of transient methods for measuring thermal properties, such as thermal conductivity, diffusivity and specific heat, which are directly related to heat conduction. Belonging to this category is the transient plane source (TPS) technique for simultaneous measurements of thermal conductivity and diffusivity of solids. The technique uses a ‘resistive element’ made of nickel foil in the form of a bifilar spiral (TPS element/sensor) covered on both sides with an insulating layer of Kapton. For the first time, the calibration procedures of such a sensor have been extended down to 35 K. By fitting the calibration data to a fourth-order polynomial, a reliable equation for the measured resistance of the TPS element covering the whole temperature range of interest (35–300 K) was obtained. A moderate decrease in the sensitivity of this sensor was observed around 60 K; it is attributed to a nearly temperature-independent resistivity coefficient (TCR) behaviour in this temperature range. The sensitivity decline is linked to a design factor that is related to the resistance behaviour of the material used in the construction of the sensor. To improve the practicality of using the sensor in this temperature range,a comparison was made between the behaviour of the TCR values of nickel and silver. It is found that a sensor made of silver foil will improve the accuracy and extend the measurements to lower temperatures.

Heat transfer and ion migration in the system Li2SO4–Na2SO4

Abstract The transient plane source technique has been used for simultaneous measurements of thermal conductivity and diffusivity in the lithium–sodium sulphate system starting near 300 K. Samples with 100, 77.5, or 50% Li2SO4 behave differently from pure Na2SO4. In the first case both the thermal conductivity and the diffusivity start to increase rapidly at about 640 K, while the crystal structure remains monoclinic for Li2SO4 and trigonal for LiNaSO4. Concerning pure Li2SO4, there is an additional discontinuous increase of the two thermal parameters at the structural transition from monoclinic to cubic (fcc) at 850 K, while the temperature gradients become significantly smaller in the cubic phase than in the monoclinic one. In contrast, both the thermal conductivity and the diffusivity of Na2SO4 decrease over the whole studied temperature range, which includes the phase transitions at 474 K and 520 K. Furthermore, a corresponding study was performed for silver iodide over the range 295–640 K, i.e. on both sides of the phase transition at 420 K. There is an indication of a small decrease of the thermal conductivity and the diffusivity at the phase transition. The high temperature phases fcc Li2SO4, bcc LiNaSO4 and bcc AgI are solid electrolytes, but it is characteristic for the two sulphate phases that a coupled rotation-like motion of the sulphate ions enhances the cation migration. Obviously, such motion is also of importance for heat transport.

Moisture effect on thermal conductivity of some major elements of a typical Libyan house envelope

The thermal conductivity and the assessment of moisture effect on building materials are essential for the calculation of the thermal loads on houses. Building materials such as simple units e.g. bricks, tiles, cement plasters, mortar and ground soils are investigated in this work. In the eastern coastal province of Libya, old buildings have thick walls (more than 50 cm thick made of mixed clay and stones) and consequently have good capacitive insulation. On the other hand, the relatively new houses have thin walls and need the addition of insulating materials. Unfortunately, these new houses were constructed without having enough technical data on the thermal properties of building materials and thermal loads were not considered. This leads to uncomfortable living conditions during hot and humid summers and cold and wet winters. This article reports the thermal conductivity values of three types of locally produced building materials used in the construction of a typical Libyan house envelope and gives suggestions to improve the thermal performance of such envelopes. The transient plane source technique (TPS) is used to measure the thermal conductivity of these materials at an average room temperature of 25 °C. The TPS technique uses a resistive heater pattern (TPS element) that is cut from a thin sheet of metal and covered on both sides with thin layers of an insulating material. The TPS element/sensor is used both as a heat source and as a temperature sensor. This technique has the dual advantage of short measuring time and low temperature rise (around 1 K) across the sample. This will prevent a non-uniform moisture distribution that may arise when the temperature difference across the wet samples is maintained for a long time. In addition, the flat thin shape of the TPS element substantially reduces the contact resistance between the sample and the sensor. More details about the TPS technique are included.

Thermal conductivity and diffusivity of sodium orthophosphate

The transient hot-disc method is used to measure the thermal conductivity and thermal diffusivity simultaneously for polycrystalline sodium orthophosphate (Na3PO4) in the temperature range 300–800 K. The thermal conductivity and diffusivity showed a gradual decrease up to the transition temperature Tc = 598 K and then rose rapidly, reaching a peak at 617 K, followed by a decreasing tendency identified with lower peaks up to 800 K. These data were compared with previous thermal conductivity and diffusivity measurements of pure Li2SO4. In the pure Li2SO4 case, the possibility of linking the heat conduction to the paddle-wheel mechanism has been reported. However, in the present case, the link was not so obvious, in spite of the fact that, in other non-thermal studies, it has been well justified. Possible reasons are given to explain the lack of sensitivity in the thermal behaviour of Na3PO4 for it to be linked to this mechanism. These reasons where attributed to the detailed microstructure and to the effect of replacing the anions (PO4)3− by (SO4)2− and/or replacing the cations Na+ by Li+ in the two cases.

Thermal conductivity of the ceramic Cecorite 130P between 88 and 280 K measured using the transient plane source technique

The thermal conductivity of a Cordierite-based ceramic is investigated within the temperature range 88 to 280 K using the recently developed transient plane source (TPS) technique. Measurements on fused quartz samples were performed to demonstrate that this experimental technique can be used down to liquid nitrogen temperature. The measurements on Cecorite 130P showed that the thermal conductivity is influenced by the thermal history of the sample. The results are neatly fitted to a third-order polynomial with coefficients which depend on the direction of approach to a particular temperature. However, the thermal conductivity of this material still shows a slight dependence on temperature, with a maximum value around 170 K.

Measurements of Thermal Conduction in Partially Saturated Specimens Using the Transient Hot-Disk Technique

The hot-disk, also known as the transient plane source technique, is used to measure the thermal conductivity of construction materials. The measurements were done at room temperature and under both dry and partially wet conditions. The technique is a transient one, which uses a flat thin sensor (hot-disk), and it has a dual advantage of short measuring time and low temperature rise (around 1°) across the specimen. This advantage prevents a non-uniform moisture distribution that may arise when the temperature difference across a wet test specimen is maintained for a relatively long time. In addition, the flat thin shape of the hot-disk substantially reduces the contact resistance between the specimen and the sensor. More details about this technique will be discussed. The materials consisted of one specimen of common (sand and cement) plaster, one specimen of ground soft soil, and two specimens of soft and hard bricks. A remarkable increase in the mean conductivity values due to the presence of water has been observed, particularly in the soft specimens. The increase is attributed to the increase in water content within the micro-voids (cavities) of the specimens. In other words, the higher the moisture content is, the greater the thermal conductivity will be.

Thermal properties of multiphase polymeric materials and dynamic methods for their characterization

The thermal conductivity, diffusivity and specific heat of multi phase polymeric materials such as polystyrene and polymethylmethacrylate are reported. With emphasis on the thermal properties of such materials, a variety of implementation and classification regarding heat conduction will be highlighted. These properties were measured using dynamic methods namely; the transient plane source named also Gustafsson probe (TPS), the dynamic plane source (DPS) and the pulse transient technique (PTT), for measurements of thermal effusivity, thermal conductivity, thermal diffusivity and specific heat of solids One of the advantages considering these techniques is the possibility to extract all thermophysical parameters, the thermal conductivity, thermal diffusivity and specific heat from one single transient recording. Dynamic methods use a probe that is technically a resistive element, as the heat source. The first two TPS and DPS employ this probe as both heat source and temperature sensor. The later method PTT, for temperature sensing, uses a thermocouple placed apart of the heat source. A description of the main features and the principles on which these methods are based will be highlighted.