陶瓷热障涂层的热导率和热扩散率测量

提出了应用3ω法进行等离子喷涂热障涂层材料的热导率和热扩散率测量的方法。测试了室温下2种典型的热障涂层材料Y2SiO5和La2Zr2O7的热导率和热扩散率,测试结果与文献中的结果吻合良好。实验中对不同孔隙率的样品的热导率在室温附近的温度区间内进行测试,结果表明,孔隙率的变化对热导率有明显的影响。另外,孔隙率对热扩散率有双向的影响,即存在某一孔隙率值使得涂层样品的热扩散率最大。     

Temperature oscillation techniques for simultaneous measurement of thermal diffusivity and conductivity

Simple temperature ocillation techniques are described for the last measurement of thermal dill'usivity and conductivity of liquids. The liquid specimen is a slab bounded above and below by a reference material. Two Peltier elements mounted on the outer Surfaces of the reference layers generate temperature ocillationS of these surfaces. Temperature waves propagate tluough the reference layers into the specimen. The thermal dilhusivity of the specimen is deduced by measuring all evaluating the amplitude attenuation and or the phase shift between the fundamental temperature oscillations at the surface of the liquid specimen and at a well-defined position inside the specimen. If the thermal diffusivity of the specimen is known. the thermal conductivity is determined by the measured amplitude attenuation and or the phase shill between the fundamental temperature oscillations at the surface of the reference layer and at the surface of the specimen. Slab and semi-infinite body geometries are considered. Measurement cells are designed and experiments are carried out with water, ethanol. heptane. monane. and glycerine. The results of the measurements of thermal dilhusivity asree very well, and those of thermal conductivity reasonably well, with the data obtained from the literature.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19-24, 1994, Boulder, Colorado, U.S.A.Author to whom correspondence should be addressed.     

Optical measurements of thermal diffusivity of a material

The measurement of thermal diffusivity of a material (in particular, a thin film) is important for various reasons, e.g., to predict the heat transfer in the solid subjected to a thermal process, to monitor surface composition or morphology, or to detect invisible subsurface defects like delaminations. This measurement can be done in a noncontact manner using various photothermal methods. Such methods typically involve pulsed heating of the surface by small amounts using a laser source; the decay of the surface temperature after this pulsed photothermal heating is then probed to provide the thermal diffusivity. Various probing methods have been developed in the literature, including the probing of reflection, refraction, and diffraction from the pulsed heated area, infrared thermal radiometry, and surface deformation. This paper provides an overview of such techniques and some examples of their applications.     

Numerical simulation and error analysis for thermal diffusivity measurements using laser-induced thermal grating technique

The laser-induced thermal grating technique has been used to determine the thermal diffusivity of liquids and liquid mixtures. But the dynamic behaviour of the transient thermal grating has not yet been thoroughly investigated, and the systematic errors, which result from the departures from one-dimensional heat conduction, have scarcely been studied quantitatively. In this paper. a three-dimensional numerical simulation and results of the transient thermal grating technique are presented, which enable a good understanding of the dynamic behaviour of the transient thermal grating. The results of this simulation are important for the proper design of the experimental setup to keep the systematic errors for the diffusivity measurement small. Based on the simulation method, the systematic errors were analyzed quantitatively. Here, the following effects were studied: (I) sample thickness, (2) intersection angle, (3) absorption, (4) Gaussian beam intensity distribution and focusing of heating laser beam, and (5) heating pulse duration and laser power. This error analysis makes it possible to specify the criteria for optimum measuring conditions, to correct the measured thermal-diffusivity values for systematic errors, and to estimate the accuracy of the measurements.     

Simultaneous measurements of thermal conductivity and thermal diffusivity of liquids under microgravity conditions

A transient short-hot-wire technique is proposed and used to measure the thermal conductivity and thermal diffusivity of liquids simultaneously. The method is based on the numerical evaluation of unsteady heat conduction from a wire with the same length diameter ratio and boundary conditions as those in the experiments. To confirm the applicability and accuracy of this method. Measurements were made for five sample liquids with known thermophysical properties and were performed under both normal gravity and microgravity conditions. The results reveal that the present method determines both the thermal conductivity and the diffusivity within 2 and 5%. respectively. The microgravity experiments clearly indicate that even under normal gravity conditions, natural-convection effects are negligible for at least l s after the start of heating. This method would be particularly suitable for a valuable and expensive liquid, and has a potential for application to electrically conducting and or corrosive liquids when the probe is effectively coated with an insulating and anticorrosive material. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

Effect of radiation heat transfer on thermal diffusivity measurements

Experimental data on thermal conductivity and thermal diffusivity of a semitransparent material generally include an error due to the radiation heat transfer. This error varies in accordance with the experimental conditions such as the temperature level of the sample and the measuring method. In this paper, research on the influence of radiation heat transfer on thermal diffusivity are reviewed, and as an example, the method to correct the radiation component in the apparent thermal diffusivity measured by the stepwise heating technique is presented. The transient heat transfer by simultaneous thermal conduction and radiation in a semitransparent material is analyzed when the front surface is subjected to stepwise heating. The apparent thermal diffusivity, which includes the radiation component, is calculated for various parameters.Paper presented at the Second U.S.-Japan Joint Seminar on Thermophysical Properties, June 23, 1988, Gaithersburg, Maryland, U.S.A.     

Determination of thermal conductivity and diffusivity from measurements of unsteady temperatures

Explicit formulas are presented for determining the thermal conductivity and diffusivity from measurements of unsteady temperatures in various shaped samples.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 35, No. 2, pp. 250–256, August, 1978.     

Spatially resolved thermal diffusivity measurements on functionally graded materials

Photothermal generation of thermal waves was used in combination with the probe beam deflection technique to study the thermal diffusivity of functionally graded materials (FGMs) quantitatively. An amplitude modulated Ar ion laser was used as a heat source and the HeNe probe laser was reflected from the specimen surface at almost normal incidence. It is demonstrated that this measuring technique can be used for a precise determination of the thermal diffusivity for a wide variety of materials if appropriate measuring conditions are chosen. The precision of the thermal diffusivity measurement was better than 5% for all materials studied. The achieved spatial resolution of the thermal diffusivity measurement was about 100 m, but higher spatial resolutions can be achieved if necessary. In a graded Al₂O₃/Al composite local fluctuations of the thermal diffusivity were observed due to the coarseness of the microstructure, but the overall behaviour of the thermal conductivity could be described well by the Maxwell-Eucken relationship. In a functionally graded AlCu alloy, a smooth thermal diffusivity profile was observed in the region where the alloy consisted of a solid solution of Cu in Al.     

Determination of thermal conductivity from specific heat and thermal diffusivity measurements of plasma-sprayed cermets

The thermal conductivities of three plasma-sprayed cermets have been determined over the temperature range 23–630°C from the measurement of the specific heat, thermal diffusivity, and density. These cermets are mixtures of Al and SiC prepared by plasma spray deposition and are being considered for various applications in magnetic confinement fusion devices. The samples consisted of three compositions: 61 vol% Al/39 vol% SiC, 74vol% Al/26vol% SiC, and 83 vol% Al/17 vol% SiC. The specific heat was determined by differential scanning calorimetry through the Al melt transition up to 720°C, while the thermal diffusivity was determined using the laser flash technique up to 630°C. The linear thermal expansion was measured and used to correct the diffusivity and density values. The thermal diffusivity showed a significant increase after thermal cycling due to a reduction in the intergrain contact resistance, increasing from 0.4 to 0.6 cm²·^–1 at 160°C. However, effective medium theory calculations indicated that the thermal conductivities of both the Al and the SiC were below the ideal defect-free limit even after high-temperature cycling. The specific heat measurements showed suppressed melting points in the plasmasprayed cermets. The 39 vol% SiC began a melt endotherm at 577°C, which peaked in the 640–650°C range depending on the sample thermal history. Chemical and X-ray diffraction analysis indicated the presence of free silicon in the cermet and in the SiC powder, which resulted in a eutectic Al/Si alloy.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

A new method for the evaluation of thermal conductivity and thermal diffusivity from transient hot strip measurements

The standard straight-line fit to data of a transient hot strip (THS) experiment to determine the thermal conductivity and thermal diffusivitya suffers from two major drawbacks: First, due to the statistical nature of the estimation procedure, there is no relation between the uncertainty of the measured value on one hand and the transport properties obtained on the other. Second, in order to account for he heat capacity of the strip and outer boundary conditions, two intervals of the plot must he rejected before analyzing it. So far, these intervals are selected arbitrarily. We now treat the THS working equation as a function of the four parameters concerned. a.U ₀ (initial voltage), andt ₀ (time delay). Chi-square fittings. following the Levenberg-Marquardt algorithm. are performed separately for several overlapping time intervals of the entire plot to find and a with minimal standard deviation. In the course of subsequent iterations an individual weighting factor is applied to each point to account for systematic errors. This procedure yields the "best" values of anda along with their individual errors. comprising the systematic and the statistical errors. Experimental results on Pyrex glass 7740 were taken to verify the new procedure.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder. Colorado, U.S.A.     

Theoretical fundamentals of the method for thermal diffusivity measurements from auto-oscillation parameters in a system with a thermal feedback

Theoretical fundamentals of the method to determine thermal diffusivity from auto-oscillation parameters in a control system, CS, with thermal feedback through the test specimen, are developed. The equation of a CS with a flat specimen and proportional controller (nonlinear boundary value problem of nonstationary heat conduction) is considered. Periodic solutions of the boundary value problem, which is linearized in the vicinity of the stationary solution, are analyzed. It is proved that, with a certain value of CS gain factor, excitation of auto-oscillations occurs. Their frequency _c is related to the thermal diffusivity as = C_c, where C is constant. By nonlinear analysis, it is revealed that the auto-oscillation excitation mode is soft and the frequency depends on the gain factor to a very weak degree. Formulas for calculation of the thermal diffusivity and the specimen temperature field are obtained.Nomenclature A Generalized gain factor - A _c Critical value of A - a Thermal diffusivity - b ₂, c₂ Lyapunov coefficients - K Controller gain - k Wavenumber - q Heat flux - R Heater resistance - S Heater surface area - T(x, t) Difference between specimen and thermostat temperatures - t Real time - u ₀ Reference voltage - u ₁,u₂ Voltage - x Coordinate - x ₀ Thermocouple coordinate - Thermo emf factor - Specimen thickness - Relative deflection of A from A _c - Thermocouple normalized coordinate - Thermal conductivity - v Temperature wave phase delay through the whole specimen - Auto-oscillation amplitude - Heaviside step function - Normalized time - ¢ Spatial part of phase - Frequency     

Effects of infrared detector nonlinearity on thermal diffusivity measurements using the flash method

When using an infrared detector to measure temperature changes as in the case of the flash technique, the effects of detector nonlinearity can have drastic effects on the experimental data. In the flash technique, the detector nonlinearity tends to shift the calculated half-time to larger values, resulting in underpredicted values of thermal diffusivity especially in experiments performed at room temperature. In order to predict the error in the diffusivity calculation, the nonlinear relationship between the detector signal and the temperature change was developed into a Taylor series expansion used in the flash technique's mathematical model. The nonlinear detector model proves to yield accurate correction factors for the presently calculated values of diffusivity. In order to utilize the model, it is necessary to estimate the maximum temperature rise of the back surface and the degree of detector nonlinearity.     

A contact method of determining the thermal effusivity of solids

A contact method of measuring the thermal effusivity of solids, based on periodic heating of the surface of a standard plate, in contact with the solid being investigated, and measuring the temperature oscillations in the plate at various distances from the source, is described.     

Heat capacity and thermal conductivity measurements down to 25 mk on samples of poor thermal diffusivity and small specific heat

A heat pulse technique is described with a permanent heat link to the cold sink used for the measurement of small heat capacities, such as vitreous dielectrics, down to 25 mK. Total heat capacities of the order of 5μJ K^?1 are measured below 50 mK with an accuracy of 5%. Two carbon resistor thermometers are used to avoid any systematic error.Thermal conductivity measurements are also performed in the same temperature range by the steady flow method. The accuracy of the carbon resistor thermometer calibration enables the measurement of 2 or 3 mK temperature gradients at 25 mK.     

Temperature-dependent effective thermal conductivity and thermal diffusivity of catalysts and their supports in various gas media and in vacuum. 1. Thermal conductivity and thermal diffusivity of granular porous supports and catalysts

The basic characteristics of catalysis and their supports and the results of experimental studies of the thermal conductivity and thermal diffusivity of some supports and catalysts in the temperature range of 293–1073 K in vacuum (P=1.07 Pa) and in an atmosphere of air, argon, nitrogen, helium, hydrogen and in a gas mixture (0.2NH₃+0.3N₂+0.5H₂) are reported. It is established that as the temperature rises in the all gas media and in vacuum the thermal conductivity and thermal diffusivity of a burden of the catalysts and their supports increase according to a linear law and depend on the size and shape of the granules rather than on their specific surface.K. Dzhuraev Pedagogical University, Dyushambe, Uzbekistan. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 5, pp. 799–809, September–October, 1995.     

A noncontact method for measuring thermal conductivity and thermal diffusivity of anisotropic materials

A noncontact method for measuring the thermal conductivity and thermal diffusivity of anisotropic materials is proposed. This method is based on the fact that the surface temperature variation with time depends on the thermal properties of the material when its surface is heated locally. The three-dimensional transient heat conduction equation in the material is solved numerically. The dimensionless average surface temperature variations are obtained along each principal axis: that is, thex andy axes. The relation between the dimensionless temperature and the Fourier number is expressed by a polynomial equation and used as a master plot, which is a basic relation to be compared with measured temperature variation. In the experiments, the material surface is heated with a laser beam and the surface temperature profiles are measured by an infrared thermometer. The measured temperature variations with time are compared with the master plots to yield the thermal conductivity λ _x and thermal diffusivityx _v in thex direction and the thermal conductivity ratioE _xy (=λ _y λ _x ) simultaneously. To confirm the applicability and the accuracy of the present method, measurements were performed on multilayered kent-paper, vinyl chloride, and polyethylene resin film, whose thermal properties are known. From numerical simulations, it is found that the present method can measure the thermophysical properties λ _x , α _x andE _xy within errors of ±6, ±22, and ±5%, respectively, when the measuring errors of the peak heat flux, the heating radius, and the surface temperature rise are assumed to be within ±2, ±3%, and ±0.2 K, respectively. This method could be applied to the measurement of thermophysical properties of biological materials.     

Pulse determination of thermal diffusivity and thermal conductivity for hemispherical specimens: nickel

A method is described for determining the thermal diffusivity, specific heat, and thermal conductivity in a hemispherical volume on the basis of duration of the reference signal.Notation r radius - R radius - r dimensionless coordinate - dimensionless temperature - time - _i duration of heat pulse - _1/2 time for temperature signal at r to attain half the maximum value - q_o amount of heat - a thermal diffusivity - thermal conductivity - density - c_p heat capacity Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 40, No. 5, pp. 864–869, May, 1981.     

Photothermal self-phase-modulation technique for absorption measurements on high-reflective coatings

We propose and demonstrate a new measurement technique for the optical absorption of high-reflection coatings. Our technique is based on photothermal self-phase modulation and exploits the deformation of cavity Airy peaks that occurs due to coating absorption of intracavity light. The mirror whose coating is under investigation needs to be the input mirror of a high-finesse cavity. Our example measurements were performed on a high-reflection SiO2-Ta2O5 coating in a three-mirror ring-cavity setup at a wavelength of 1064 nm. The optical absorption of the coating was determined to be α=(23.9±2.0)·10(-6) per coating. Our result is in excellent agreement with an independently performed laser calorimetry measurement that gave a value of α=(24.4±3.2)·10(-6) per coating. Since the self-phase modulation in our coating-absorption measurement affects mainly the propagation through the cavity input mirror, our measurement result is practically uninfluenced by the optical absorption of the other cavity mirrors.     

Thermal-wave resonant-cavity measurements of the thermal diffusivity of air: A comparison between cavity-length and modulation-frequency scans

The application of a thermal-wave resonant cavity to thermal-diffusivity measurements of gases has been investigated. The cavity was constructed using a thin aluminum foil wall as the intensitv-modulated laser-beam oscillator source opposite a pyroclectric polyvilidene fluoride wall acting as a signal transducer. Theoretically, cavity-length and modulation-frequency scans both produce resonance-like extrema in lock-in in-phase and quadrature curses. These extrema can be used to measure the thermal diffusivity of the gas within the cavity. It was found experimentally that one can obtain. very accurate and reproducible measurements of the thermal diffusivity of the gas by using simple cavity-length scanning without any signal normalization procedure. rather than traditional modulation-frequency scanning; normalized by the frequency-dependent transfer function of the instrumentation. By scanning the cavity length, the thermal diffisivity of room air at 299 K was measured with three-significant figure precision as 0.216±11.001 cm²·s^–1, with a standard deviation 0.5%. Only two significant figure accuracy could be obtained by scanning the frequency: 0.22±0.03 cm²·s^–1, with a standard deviation of 14%. Cavity-length scanning consistently exhibited a much higher signal-to-noise ratio.