Thermal Diffusivity Values Research Articles

THERMAL DIFFUSIVITY STUDIES OF ZnO-CuO AT HIGH TEMPERATURES

An n-type semiconducting oxide such as Zinc Oxide (ZnO) has been exploited for their well-known gas sensing properties. Previous studies on these applications have mainly focused on electrical properties. Only limited reports were available on thermophysical properties of ZnO-based ceramics gas sensor. Therefore in this work, we report on the thermal diffusivity of Zinc Oxide-Copper Oxide (ZnO-CuO) ceramic composites by solid-state method using a laser flash technique. Thermal diffusivity of samples was measured at temperatures between 27 °C to 400 °C. The role of CuO was observed to enhance the thermal diffusivity of ZnO system with respect to the temperatures. ZnO-CuO samples played a significant role in improvement of thermal diffusivity value at temperature of 200 °C and above. Subsequently, sample of higher thermal diffusivity will exhibit lower initialization time for gas sensor to activate. Hence, the enhanced thermal diffusivity suggested that ZnO-CuO composite samples hold a promising possibility in gas sensor application.

Sublimation and oxidation zone ablation behavior of carbon/carbon composites

Three-dimensional carbon/carbon (C/C) composites comprising four reinforcement directions (4D) were fabricated using intermediate modulus carbon fibers and densified using a hybrid process. This consists of a pre-densification step using a thermal-gradient chemical vapor infiltration process followed by a high-pressure pitch impregnation and carbonization process. The specimens machined along Z-axis of the preform architecture were tested in an arc plasma heater for studying its ablation behavior at different temperatures. Regimes from ultra-high temperature (4750K) sublimation to high-temperature (2467K) oxidation zones were created by varying the mass flow rate of secondary air in the heater. The ablation rate showed a progressive increase as the environment changed from oxygen-lean sublimation to oxygen-rich oxidation conditions while the back-face temperature showed a similar temperature profile during the plasma exposure period. The thermal diffusivity value decreased with the rise in temperature till 1173K and later on became fairly flat till 1523K and onwards. In the compression test, 4750K exposed specimen showed toughening in the plasma-affected zone and crushed with shear mode from the opposite face while the 2467K exposed specimen showed end brushing in the plasma heat-affected face with a lower residual strength and Young’s modulus.

Assessing the accuracy of 1‐D analytical heat tracing for estimating near‐surface sediment thermal diffusivity and water flux under transient conditions

AbstractAmplitude decay and phase delay of oscillating temperature records measured at two vertical locations in near‐surface sediments can be used to infer water fluxes, thermal diffusivity, and sediment scour/deposition. While methods that rely on the harmonics‐based analytical heat transport solution assume a steady state water flux, many applications have reported transient fluxes but ignored the possible violation of this assumption in the method. Here we use natural heat tracing as an example to investigate the extent to which changes in the water flux, and associated temperature signal nonstationarity, can be separated from other influences. We systematically scrutinize the assumption of steady state flow in analytical heat tracing and test the capabilities of the method to detect the timing and magnitude of flux transients. A numerical model was used to synthesize the temperature response to different step and ramp changes in advective thermal velocity magnitude and direction for both a single‐frequency and multifrequency temperature boundary. Time‐variable temperature amplitude and phase information were extracted from the model output with different signal‐processing methods. We show that a worst‐case transient flux induces a temperature nonstationarity, the duration of which is less than 1 cycle for realistic sediment thermal diffusivities between 0.02 and 0.13 m2/d. However, common signal‐processing methods introduce erroneous temporal spreading of advective thermal velocities and significant anomalies in thermal diffusivities or sensor spacing, which is used as an analogue for streambed scour/deposition. The most time‐variant spectral filter can introduce errors of up to 57% in velocity and 33% in thermal diffusivity values with artifacts spanning ±2 days around the occurrence of rapid changes in flux. Further, our results show that analytical heat tracing is unable to accurately resolve highly time‐variant fluxes and thermal diffusivities and does not allow for the inference of scour/depositional processes due to the limitations of signal processing in disentangling flux‐related signal nonstationarities from those stemming from other sources. To prevent erroneous interpretations, hydrometric data should always be acquired in combination with temperature records.

Study the Electrochemical Corrosion Behavior, Microstructure and Some Physical Properties of Lead- Calcium Rapidly Solidified Alloys

Electrochemical corrosion behavior, microstructure, elastic modulus, Vickers hardness, internal friction and electrical resistivity of lead- calcium rapidly solidified alloys have been investigated. Elastic modulus of Pb99.5xCa0.5+x (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5 wt. %) alloys increased with increasing Ca content. Also internal friction values of Pb99.5-xCa0.5+x alloys variable decreased with increasing Ca content. But Vickers hardness and thermal diffusivity values of Pb99.5-xCa0.5+x alloys variable increased with increasing Ca content. Electrical resistivity values of Pb99.5-xCa0.5+x alloys decreased with increasing Ca content. The corrosion potential of Pb and Pb99.5-xCa0.5+x (x= 0.2 and 0.5 wt. %) alloys exhibited a negative potential. Also the cathodic and the anodic polarization curves also showed similar corrosion trends. The corrosion resistance of Pb99.5-xCa0.5+x alloy in 0.25 M HCl increased with increasing Ca content ratio. The Pb99Ca1 alloy has best properties for storage battery grids.

Effective Thermal Diffusivity Study of Powder Biocomposites via Photoacoustic Method

The effective thermal diffusivity for biocomposites of hydroxyapatite (HAp), and niobium pentoxide (Nb2O5) on powder form was studied via photoacoustic method adapted for porous materials. The concentration of each element was accompanied with the results of X-ray diffractometer (XRD) and scanning electron microscopy (SEM). A theoretical model for the thermal coupling of a three layered sample, designed to contain the powder material is proposed. The method for mixtures obeyed the formula [(1 − x)H A p + (x)N b 2O5] for 0.0 ≤ x ≤ 1.0. Experimental results for effective thermal diffusivity ranged between (6.4 ± 0.3) × 10−6 m2 s−1 and (9.8 ± 0.4) × 10−6 m2 s−1 for x ≤ 0.7. Values of the effective thermal diffusivity have decreased sharply to (0.7 ± 0.03) ×10−6 m2 s−1 for x > 0.7. SEM micrographs showed a coating of HAp over the particles of Nb2O5 for some mixtures.

Thermophysical properties of Gd2Zr2O7 powders prepared by mechanical milling: Effect of homovalent Gd3+ substitution

This contribution analyzes the thermophysical properties of Gd1.6Ln0.4Zr2O7 (Ln = La3+, Nd3+, Sm3+, Dy3+ and Er3+) ceramics synthesized at room temperature, by mechanically milling stoichiometric mixtures of high purity oxides. Regardless of chemical composition, powders milled for 27 h show XRD patterns similar to fluorite-type materials. Post-milling thermal treatments at 1500 °C, facilitates the evolution to the ordered pyrochlore derivative for Gd2Zr2O7, and the La3+-, Nd3+-, and Sm3+-containing materials. By contrast, samples containing the smaller lanthanides (Dy3+ or Er3+), maintain the fluorite structure. Thermal conductivity of the as-prepared samples was obtained as a function of temperature, from thermal diffusivity, heat capacity and density values, using sintered pellets. We found that doping has an important effect in lowering Gd2Zr2O7 thermal conductivity, with final values ranging from 1.22 to 1.94 W m−1 K−1; Nd3+- and Er3+-containing samples represent an optimum combination of defects and disordering of oxygen vacancies that generate the lowest conductivity values of all samples tested.

Shape dependent heat transport through green synthesized gold nanofluids

Nanofluids hold promise as a more efficient coolant for thermoelectric devices. Despite the capability of tailoring the thermo physical properties of nanofluids, by tuning the particle parameters such as shape, size and concentration, the toxicity of chemicals used for the preparation of nanoparticles is a serious concern. Green synthesis of nanoparticles is emerging as an alternative to the conventional chemical and physical methods for the preparation of nanoparticles. In this work, the results of the preparation of gold nanoparticles using plant extracts as reducing agents are presented. The green synthesis route employed for the present study provides particles of similar size, but the shape of the particles is found to vary depending upon the source of the natural reducing agents. The thermal diffusivity values of the gold nanofluid measured using laser based dual beam thermal lens technique elucidate the role of shape and concentration of the green synthesized nanoparticles on the effective thermal diffusivity values of the nanofluids.

Photothermal and morphological characterization of PLA/PCL polymer blends

Nowadays, some synthetic polymers have been replaced by biodegradable polymers in order to avoid environmental contamination. Among these biodegradables polymers, aliphatic polyesters such as polylactic acid (PLA) and polycaprolactone (PCL) have been widely used. In the present study, solvent-casting films of PLA, PCL and polymer blends with and without compatibilizer (PLA grafted with maleic anhydride) were prepared. The thermal diffusivity (α) of each sample was obtained by using the open photoacoustic cell technique. Morphology and thermal properties were determined by using scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry (DSC), respectively. The blends showed lower thermal diffusivity compared to pure polymers. However, when the compatibilizer was used, the highest value of thermal diffusivity was obtained. Also, cold crystallization with the highest value of enthalpy of fusion was observed for the compatibilized sample, which was revealed by DSC. To our knowledge, this is the first time that the thermal diffusivity of these biodegradable polymer blends is reported.

On the optimization of experimental parameters in photopyroelectric investigation of thermal diffusivity of solids

In this paper, the possibility of optimizing the experimental conditions for a correct photopyroelectric evaluation of the thermal diffusivity of solid samples is studied. For this purpose, a glassy carbon sample, with known thermal properties, was selected as test material and two types of techniques were applied in order to get the value of its thermal diffusivity: (i) the photopyroelectric calorimetry in back detection configuration and (ii) the infrared thermography. Assuming that the values of thermal diffusivity obtained by thermography are correct (a non-contact technique), we studied how to eliminate the underestimation (due to the presence of the coupling fluid) of the results in the back photopyroelectric calorimetry investigations. Experiments with different types of coupling fluids and numerical simulations were performed in order to evaluate the influence of the coupling fluid on the value of the thermal diffusivity. The conclusion is that a proper choice of the type of coupling fluid and some improvements performed in the experimental design of the photopyroelectric calorimetry detection cell (with the purpose of reducing the coupling fluid’s thickness), can eliminate the difference between the results obtained with the two photothermal (contact and non-contact) techniques.

Precipitation dependence of thermal properties for Al–Si–Mg–Cu–(Ti) alloy with various heat treatment

The effect of precipitation in Al–6.5Si–0.45Mg–0.9Cu–(Ti) alloys (in wt.%) after various heat treatment has been studied by means of laser flash apparatus (LFA) and differential scanning calorimetry (DSC). The solution treatment was performed at 535 °C for 6 h followed by water quenching and the samples were aged in air at different temperatures from 170 °C to 220 °C for 5 h. Al–6.5Si–0.45Mg–0.9Cu, the titanium free alloy, had higher thermal diffusivity than Al–6.5Si–0.45Mg–0.9Cu–0.2Ti alloy over all temperature range. In temperature range below 200 °C and above 300 °C, the value of thermal diffusivity (α) decreased with increasing temperature, which is temperature-dependent. The solid solubility in aluminum matrix rapidly changes at precipitation temperature of the phases and then the thermal diffusivity was increased, which is called precipitation-dependent. The precipitation of phases occurred as increasing temperature of samples between 200 °C and 400 °C, and it is confirmed by DSC and X-ray analysis.

Influence of Grain Size on the Thermal Diffusivity of Pt-SnO<sub>2</sub> Ceramic

Pt-SnO2 ceramics were prepared via solid-state route from a mixture of powders of (100-x) SnO2.x Pt (0≤ x wt %≤ 1). The samples were then sintered at 600, 800 and 1000 °C for 3 hours. The resulting samples were then characterized using Laser Flash Apparatus (LFA) for determining thermal diffusivity (α ) value. The measurements of α were carried out at room temperature up to 400 °C with the intervals of 50 °C. Experimental results showed that thermal diffusivity value is in the range of 5.0×10-7m2s-1 - 3.0×10-6m2s-1. We also show that larger grain size increases the thermal diffusivity of the ceramic. Data concerning the effects of additive amount, pore content, and temperature were also reported.

Thermal conductivity of Al–Gd–TM glass-forming alloys

Thermal diffusivity, specific heat capacity and thermal conductivity of Al86Gd6TM8 (TM = Cu, Ni, Co, Fe, Mn, Cr, Ti, Zr, Mo, Ta) glass-forming alloys in the temperature range of 300–880 K were determined by laser flash method. The temperatures of endothermic and exothermic reactions of the alloys were determined by differential scanning calorimetry method. The alloys were prepared by conventional arc-melting technique under helium atmosphere. All the alloys studied exhibit strong supercooling of the liquidus temperatures up to 80 K, which indicates their good glass-forming ability. The specific heat capacity of the alloys achieves the Dulong–Petit's value in the temperature range of 350–550 K except Al86Cr8Gd6 and Al86Zr8Gd6 compositions. The values of both thermal diffusivity and thermal conductivity of the alloy studied are significantly lower than those for pure aluminum. It is found that embedding 14% (mole fraction) of transition elements (Gd+TM) in the aluminum matrix leads to significant decrease in the absolute magnitudes of both thermal diffusivity and thermal conductivity in crystalline state. The thermal conductivity of glass-forming Al86Gd6TM8 alloys is strongly affected by directed chemical bonding between alloy components.

An analysis of Granier sap flow method, its sensitivity to heat storage and a new approach to improve its time dynamics

Granier sap flow method is a simple and easily applicable method to monitor sap flow in trees in field conditions, and is thus in wide use. However, it has been suggested that the method is slow to capture transient changes in actual sap flux density due to heat storage and release within the stem. We show here how this may lead to biases in the estimation of the dynamics of sap flux density especially at low flow rates when thermal diffusivity is low. We also demonstrate how the traditional Granier sap flow method could be modified to improve the temporal precision of the sap flow measurement. In the new system, the temperature difference between the heated and the reference needle is kept constant by varying the heating power and the sap flux density is calculated from the power consumption. This leads to reduced changes in the heat content of stem. These modifications also make the method more robust in terms of stability of power supply and reduce power consumption during low flow conditions. The time dynamics of the Granier method and the new “steady temperature method” are simulated with a previously published numerical model of xylem heat balance and tested in a laboratory experiment with cut pieces of stem. The numerical model is also used to demonstrate that the relation between parameter K, calculated from instantaneous sensor temperature and maximum sensor temperature, and actual sap flux density is not constant for either the traditional Granier or the new modified sensor, but is dependent on the range of sapflow rates examined and on the value of thermal diffusivity. Continuous measurements of thermal diffusivity of the sapwood along with the needle temperature/power consumption could help to improve the accuracy of the sap flow measurements.

The study of frequency-scan photothermal reflectance technique for thermal diffusivity measurement.

A frequency scan photothermal reflectance technique to measure thermal diffusivity of bulk samples is studied in this manuscript. Similar to general photothermal reflectance methods, an intensity-modulated heating laser and a constant intensity probe laser are used to determine the surface temperature response under sinusoidal heating. The approach involves fixing the distance between the heating and probe laser spots, recording the phase lag of reflected probe laser intensity with respect to the heating laser frequency modulation, and extracting thermal diffusivity using the phase lag-(frequency)(1/2) relation. The experimental validation is performed on three samples (SiO2, CaF2, and Ge), which have a wide range of thermal diffusivities. The measured thermal diffusivity values agree closely with the literature values. Compared to the commonly used spatial scan method, the experimental setup and operation of the frequency scan method are simplified, and the uncertainty level is equal to or smaller than that of the spatial scan method.

Photothermal study of RF-plasma polymerized hexamethyldisilazane thin films

Plasma polymerization by RF excited low pressure plasmas is a powerful technique for synthesizing thin films for a wide range of applications such in microelectronics, biomaterials and aeronautical industries. Among these applications plasma polymerized hexamethyldisilazane (HMDSN here in) is a promising material due its biocompatibility, optical and electrical properties. In the present paper, we reported the thermal diffusivity measurements of plasma polymerized HMDSN thin films using the probe beam deflection technique (PBD). The samples are thermally thin and optically transparent, with thicknesses ranging from 170 to 600nm, difficulting their thermal characterization by other photo-thermal techniques (photoacoustic and pyroelectric). PBD measurements were carried out with a diode (20mW, λ=640nm) and a HeNe (0.9mW, λ=632nm) laser, used as pump source and probe beam, respectively. The probe beam was focused close to the sample surface and its direction was perpendicular to the pump beam. The transverse PBD in skimming configuration is employed and the deflection signal is analyzed using the phase method for the determination of thermal diffusivity. The results show the decreasing of thermal diffusivity with the decreasing of film thickness. HMDSN film׳s thermal diffusivity values are reported in this paper for the first time being in the range of typical polymeric samples.

Thermal Properties of Metallic Films at Room Conditions by the Heating Slope

An analytical model and a methodology for determining the thermal diffusivity and the thermal conductivity of metallic thin films at room conditions are proposed. The analytical model is based on the one-dimensional heat transfer equation, which allows estimation of the thermal conductivity and thermal diffusivity through the initial heating slope value. The thermal conductivity and thermal diffusivity values of gold and aluminum films from 20 to 200 nm thicknesses, determined from the initial slope of the measured heating profiles, and their comparison with the analytical ones are discussed. Results show that thermal diffusivity increases from to for gold films, and it increases from to for aluminum films; although thermal conductivity increases from () to for gold films and from () to for aluminum films, when the film thickness increases.

Thermophysical properties of americium-containing barium plutonate

Polycrystalline specimens of americium-containing barium plutonate have been prepared by mixing the appropriate amounts of (Pu0.91Am0.09)O2 and BaCO3 powders followed by reacting and sintering at 1600 K under the flowing gas atmosphere of dry-air. The sintered specimens had a single phase of orthorhombic perovskite structure and were crack-free. Elastic moduli were determined from longitudinal and shear sound velocities. Debye temperature was also determined from sound velocities and lattice parameter measurements. Thermal conductivity was calculated from measured density at room temperature, literature values of heat capacity and thermal diffusivity measured by laser flash method in vacuum. Thermal conductivity of americium-containing barium plutonate was roughly independent of temperature and registered almost the same magnitude as that of BaPuO3 and BaUO3.

Prediction of porosity and thermal diffusivity in a porous fin using differential evolution algorithm

In this paper, simultaneous inverse prediction of two parameters such as the porosity and thermal diffusivity of the fluid in a porous fin is done for satisfying a given temperature distribution. Only three temperature measurements are assumed to be available on the surface of the fin and prediction of the parameters is accomplished by using the differential evolution (DE)-based optimization technique. It is shown that the present problem is inherently ill-posed in terms of the retrieval of the value of fluid thermal diffusivity for which many possible solutions exist, which is expected to adapt the fin under different conditions. In the present work, two numerical examples provide engineering insight into the problem of designing porous fins using good thermal conductors like aluminum and copper along with the working of DE. Finally, the efficacy of DE for the present problem is also shown by comparing its performance with few other optimization methods.

Structural, optical, opto-thermal and thermal properties of ZnS-PVA nanofluids synthesized through a radiolytic approach.

This work describes a fast, clean and low-cost approach to synthesize ZnS–PVA nanofluids consisting of ZnS nanoparticles homogeneously distributed in a PVA solution. The ZnS nanoparticles were formed by the electrostatic force between zinc and sulfur ions induced by gamma irradiation at a dose range from 10 to 50 kGy. Several experimental characterizations were conducted to investigate the physical and chemical properties of the samples. Fourier transform infrared spectroscopy (FTIR) was used to determine the chemical structure and bonding conditions of the final products, transmission electron microscopy (TEM) for determining the shape morphology and average particle size, powder X-ray diffraction (XRD) for confirming the formation and crystalline structure of ZnS nanoparticles, UV–visible spectroscopy for measuring the electronic absorption characteristics, transient hot wire (THW) and photoacoustic measurements for measuring the thermal conductivity and thermal effusivity of the samples, from which, for the first time, the values of specific heat and thermal diffusivity of the samples were then calculated.

Influence of the Window Thermal Diffusivity on the Silicon Wafer Temperature in a Rapid Thermal System

The heating of a silicon wafer in a rapid thermal process is studied by numerical simulation. In the model, the equations of conservation of mass and energy are solved with the finite-volume method and the determination of the solutions of the radiative transfer equation is based on the Monte-Carlo method. The results of numerical simulations, without optimization and in steady state, show a close relationship between the thermal profiles of the silicon wafer and the ones of the quartz window. By introducing a high thermal diffusivity value for the window, the homogeneity of the wafer temperature is improved by 54%. The effect of heat storage by the quartz window on the temperature profile of the silicon substrate is hence well appreciated. Finally, a selection of materials is proposed for the implementation of the high diffusivity infrared window.