Thermal Diffusivity of Film/Substrate Structures Characterized by Transient Thermal Grating Method

Transient thermal grating method is used to measure the thermal diffusivity of absorbing films deposited on transparent substrates. According to periodically modulated dielectric constant variations and thermoelastic deformations of the thin films caused by the transient thermal gratings, an improved optical diffraction theory is presented. In the experiment, the probing laser beam reflectively diffracted by the thermal grating is measured by a photomultiplier at different grating fringe spaces. The thermal diffusivity of the film can be evaluated by fitting the theoretical calculations of diffraction signals to the experimental measured data. The validity of the method is tested by measuring the thermal diffusivities of absorbing ZnO films deposited on glass substrates.     

Measurements of electron-phonon coupling factor and interfacial thermal resistance of metallic nano-films using a transient thermoreflectance technique

Using a transient thermoreflectance (TTR) technique,several Au films with different thicknesses on glass and SiC substrates are measured for thermal characterization of metallic nano-films,including the electron-phonon coupling factor G,interfacial thermal resistance R,and thermal conductivity K s of the substrate. The rear heating-front detecting (RF) method is used to ensure the femtosecond temporal resolution. An intense laser beam is focused on the rear surface to heat the film,and another weak laser beam is focused on the very spot of the front surface to detect the change in the electron temperature. By varying the optical path delay between the two beams,a complete electron temperature profile can be scanned. Different from the normally used single-layer model,the double-layer model involving interfacial thermal resistance is studied here. The electron temperature cooling profile can be affected by the electron energy transfer into the substrate or the electron-phonon interactions in the metallic films. For multiple-target optimization,the genetic algorithm (GA) is used to obtain both G and R. The experimental result gives a deep understanding of the mechanism of ultra-fast heat transfer in metals.     

The effective thermal conductivity of porous media based on statistical self-similarity

A fractal model is presented based on the thermal-electrical analogy technique and statistical self-similarity of fractal saturated porous media. A dimensionless effective thermal conductivity of saturated fractal porous media is studied by the relationship between the dimensionless effective thermal conductivity and the geometrical parameters of porous media with no empirical constant. Through this study, it is shown that the dimensionless effective thermal conductivity decreases with the increase of porosity (?) and pore area fractal dimension (Df) when ks/kg>1. The opposite trends is observed when ks/kg<1. In addition, the dimensionless effective thermal conductivity decreases with increasing tortuous fractal dimension (Dt). The model predictions are compared with existing experimental data and the results show that they are in good agreement with existing experimental data.     

Thermal conductivity and interface thermal resistance of Si film on Si substrate determined by photothermal displacement interferometry

An in situ, noncontact, photothermal displacement interferometer for performing thermal diffusivity measurements on bulk and thin-film materials has been developed. Localized transient surface motion is generated through photothermoelastic coupling of a pulsed, heating laser beam to the sample under investigation. The maximum surface displacement is found to be linearly dependent on the laser power while the proportionality is a function of the thermal diffusivity. Both thin-film conductivity and film/substrate interface thermal resistance are derived from the measured, effective thermal conductivity by employing simple heat-flow analysis. Wedge-shaped Si films, vacuum deposited on single crystal Si wafers are studied with this technique. A sample with oxide layer removed by ion bombardment of the wafer surface prior to film deposition shows the same film conductivity as a sample film deposited on an as-cast wafer, while the uncleaned sample exhibits higher interface thermal resistance. It is found that the thin-film thermal conductivity is somewhat smaller than the bulk value. However, the existence of an interface thermal resistance, when combined with film thermal conductivity, can result in an effective thermal conductivity as low as two orders of magnitude lower than the bulk value.Currently supported by the LLE fellowship     

SiNx薄膜热物性参数实验测量与分析研究

采用一种新的实验测量方案，将金属加热单元与温度探测单元合二为一，间接获得了在半导体和微电子学MEMS领域内有重要用途的SiN_x薄膜的导热系数、发射率、比热容和热扩散系数，并对实验结果进行了不确定度分析，为微电子电路设计和掩模成型工艺等提供了可靠的热物性数据. 实验结果表明，薄膜的导热系数、发射率、热扩散系数远比相应体材质低，而且还与温度、厚度有关，尺寸效应显著，而比热容则与体材质相差不大. 关键词： 微尺度传热 热物性参数 x薄膜')" href="#">SiN_x薄膜 测量技术     

Thermal wave methods in investigation of thermal properties of solids

A short review of thermal wave measuring methods is presented in the paper. Based on fundamental laws of heat transport, experimental methods for determination of thermal properties of solids are divided into two groups – steady flux techniques and variable flux ones. Special attention is paid to the wave methods belonging to the second group and methods used by the author in his experiments. The idea of ?ngstr?m's method for determination of the thermal diffusivity of metals is reminded. Then different modifications of this classical technique using in investigations of bulk materials and thin films are described. Examples of a few thermal wave measurements are also presented.     

Photoacoustic study of thermal properties of biological tissues detected by PVDF film transducer

A PVDF piezoelectric film transducer is used in a photoacoustic piezoelectric technique for studying thermal diffusivities of biological tissues. The experimental precision may be improved by using PVDF films because the acoustic impedance of PVDF films is close to that of biological tissues. Thermal diffusivities of several fresh porcine tissues in vitro are studied and the results are compared with previously obtained ones.     

Thermal diffusivities and molar volumes of ternary Al20 AsxTe80−x alloy glasses: evidence of self-organization

Thermal diffusivities (α) and molar volumes (V_m) of bulk Al₂₀As_xTe_80−x alloy glasses in the composition range 0≤x≤35 are examined. The results reveal a broad maximum in alpha and a broad minimum in V_m in the 15<x<25 range. Earlier temperature modulated differential scanning calorimetric measurements on these glasses have revealed the non-reversing heat flow to show a global minimum (reversibility window) in the same composition range. Taken together these results suggest that present glasses in the 15<x<25 range are self-organized. The thermal diffusivity maximum observed near x=20 is consistent with a minimal scattering of thermal waves in the homogeneous and stress-free glass composition residing in the center of the self-organized phase.     

Measurement of thermal conductivity in laser-heated diamond anvil cell using radial temperature distribution

ABSTRACT

Thermal conductivities of planetary materials under extreme conditions are important input parameters for modeling planetary dynamics such as accretion, geodynamo and magnetic field evolution, plate tectonics, volcanism-related processes etc. However, direct experimental measurements of thermal conductivity at extreme conditions remain challenging and controversial. Here we propose a new technique of thermal conductivity measurement in laser-heated diamond anvil cell (LH-DAC) based on radial temperature distribution around laser focal spot, mapped by imaging tandem acousto-optical tunable filter (TAOTF). The new technique provides much more information about heat fluxes in the laser-heated sample than existing static heating setups, and does not require dynamic numerical modeling using heat capacities in contrast to dynamic pulsed heating setups. In the test experiment, thermal conductivity of γ-Fe at conditions relevant to cores of terrestrial planets was measured.     

钽掺杂对钛酸钡导热性能影响的研究

利用光热检测技术测量了钛酸钡材料的导热性能，得到了不同成型压力、烧结温度以及不同 掺杂量下的钛酸钡材料的热扩散率.研究了钽掺杂对钛酸钡材料导热性能的影响，发现了钽 元素掺杂量小于1.5mol%时，钛酸钡材料的热扩散率随掺杂量的增加而增大，当钽元素掺杂 量大于1.5mol%时，热扩散率随掺杂量的增加而减少.对钛酸钡材料的导热性能做了进一步的 分析. 关键词： 光热检测 钛酸钡 导热性能 钽掺杂     

Structural,elastic, electronic and thermal properties of M2SbP (M = Ti,Zr and Hf)

The structural, elastic, electronic and thermal properties of M₂SbP (M = Ti, Zr and Hf) were studied by means of a pseudo-potential plane-wave method based on the density functional theory within both the local density approximation and the generalised gradient approximation. The optimised zero-pressure geometrical parameters, i.e. the two unit cell lengths (a, c) and the internal coordinate (z), were in good agreement with available experimental and theoretical data. The effect of high pressure, up to 20 GPa, on the lattice constants shows that the contractions along the a-axis were higher than along c-axis. The anisotropic independent elastic constants were calculated using the static finite strain technique. Numerical estimations of the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, average sound velocity and Debye temperature for ideal polycrystalline M₂SbP aggregates were performed in the framework of the Voigt–Reuss–Hill approximation. The calculated band structures show that all studied materials are electrical conductors. Analysis of the atomic site projected densities showed that the bonding is of covalent–ionic nature with the presence of metallic character. The density of states at the Fermi level is dictated by the transition metal d–d bands; the Sb element has little effect. Thermal effects on some macroscopic properties of M₂SbP were predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the volume expansion coefficient, heat capacity and Debye temperature with pressure and temperature in the ranges 0–50 GPa and 0–2000 K were obtained successfully.     

Photothermal displacement spectroscopy: An optical probe for solids and surfaces

We present a sensitive technique for determining the optical and thermal properties of solids, surfaces and thin films. This technique, photothermal displacement Spectroscopy, is based on the detection of the thermal expansion of a sample upon absorption of electromagnetic radiation. The technique is well suited for in situ ultrahigh vacuum studies and for experiments where wide temperature ranges are required. We show that surface and bulk optical absorption can be distinguished and that surface absorptions of αL=10^?6/W of incident power can be measured. The theoretical basis of the signal generation is given, and excellent experimental and theoretical agreement is demonstrated. The implications of our findings to imaging and microscopy are discussed.     

Mechanical stability and thermodynamic properties of OsC from first-principles calculations

The elastic and thermodynamic characteristics of OsC crystal have been predicted through a method of density functional theory within the generalized gradient approximation (GGA). Compared with WC-type OsC, NaCl-type OsC is not only energy unfavorable but also mechanics unstable. The five independent elastic constants (C_ij), bulk modulus (B₀), the dependence of bulk modulus on temperature and pressure as well as the thermal expansion coefficient (α_V) at various temperatures for WC-type OsC are discussed. According to our calculations, WC-type OsC should be an ultra-incompressible material with high bulk modulus about 381 GPa. In addition, the bulk modulus will increase with increasing pressure while decrease with increasing temperature. The researches on the thermal expansion coefficient indicate that there will be a knee point during the process of thermal expansion coefficient variation versus increasing temperature. Our results may provide useful information for theoretical and experimental investigation of OsC.     

Transient thermal characterization of micro/submicroscale polyacrylonitrile wires

In this work, the thermal diffusivity of single polyacrylonitrile (PAN) wires with diameters from 4.62 μm down to 324 nm is measured by using our recently developed transient electro-thermal technique. The wires span from 23 μm to 126.2 μm in our measurement. Since PAN wires are dielectric, a thin Au film is coated on the surface of the wires to make them conductive. In the experiment, a step current (with ∼2 μs rising time) is fed to the sample. The sample is heated and takes a certain time to reach its steady thermal state. The temperature rising response of the sample is sensed by measuring the resistance change of the thin Au coating. From the average temperature evolution of the sample, the thermal diffusivity can be extracted. Three PAN wires with different diameters are synthesized using the electro-spinning technique and are measured to obtain their thermal diffusivities (around 1.53×10^-7 m²/s), which are slightly smaller than the bulk value. PACS 65.80.+n; 66.30.Xj; 44.10.+i     

Piezoelectric coefficients of multilayer Pb(Zr,Ti)O<Subscript>3</Subscript> thin films

Sol–gel techniques were used to prepare thin films of Pb(Zr _x ,Ti_1−x )O₃ (PZT) with three different Zr/Ti ratios and a graded PZT film with three different compositional layers. A Michelson interferometer was used to measure the thickness strains due to an applied ac electric field. Effective d ₃₃ piezoelectric strain coefficients were computed from the experimental data. Interfacial pinning caused these coefficients to differ from the true ones. They were corrected for the pinning using both an analytical model and finite-element analysis. The corrected coefficients of the PZT (52/48) sample were in excellent agreement with values of bulk materials. The coefficients of the multilayer sample were very low, probably due to insufficient poling or domain switching.     

Spontaneous thermal expansion of nematic elastomers

We study the monodomain (single-crystal) nematic elastomer materials, all side-chain siloxane polymers with the same mesogenic groups and crosslinking density, but differing in the type of crosslinking. Increasing the proportion of long di-functional segments of main-chain nematic polymer, acting as network crosslinking, results in dramatic changes in the uniaxial equilibrium thermal expansion on cooling from the isotropic phase. At higher concentration of main chains their behaviour dominates the elastomer properties. At low concentration of main-chain material, we detect two distinct transitions at different temperatures, one attributed to the main-chain, the other to the side-chain component. The effective uniaxial anisotropy of nematic rubber, r(T) = / proportional to the effective nematic order parameter Q(T), is given by an average of the two components and thus reflects the two-transition nature of thermal expansion. The experimental data is compared with the theoretical model of ideal nematic elastomers; applications in high-amplitude thermal actuators are discussed in the end. Received 25 June 2001 and Received in final form 29 September 2001     

Thermal diffusivity of light-emitting diode packaging material determined by photoacoustic piezoelectric technique

Thermal property is one of the most important properties of light-emitting diode (LED).Thermal property of LED packaging material determines the heat dissipations of the phosphor and the chip surface,accordingly having an influence on the light-emitting efficiency and the life-span of the device.In this paper,photoacoustic piezoelectric (PAPE) technique has been employed to investigate the thermal properties of polyvinyl alcohol (PVA) and silicon dioxide,which are the new and the traditional packaging materials in white LED,respectively.Firstly,the theory of PAPE technique has been developed for two-layer model in order to investigate soft materials;secondly,the experimental system has been set up and adjusted by measuring the reference sample;thirdly,the thermal diffusivities of PVA and silicon dioxide are measured and analysed.The experimental results show that PVA has a higher thermal diffusivity than silicon dioxide and is a better packaging material in the sense of thermal diffusivity for white LED.     

Ho3+掺杂对ZnO薄膜的微观结构和磁性的影响

研究Ho³⁺掺杂对氧化锌半导体材料的微结构和磁学性质影响. 利用热蒸发技术制备了一系列沉积在Si(100)衬底的Zn_1-xHo_xO(x=0.0、0.04、0.05）薄膜. X射线光谱、表面形貌以及磁性的实验结果表明,Ho³⁺掺杂对ZnO薄膜材料的性能影响很大. X射线衍射图显示峰位出现高角度转变并且趋向于(101)取向,在ZnO晶格显示Ho³⁺置换. 扫描电子显微镜和能谱仪对薄膜的表面形貌以及化学     

Mass production of graphene-like single-crystalline NbSe2 (004) nanosheets via intercalant-assisted thermal cleavage

The author reports the mass production of dense inorganic graphene-like exfoliated single-crystalline niobium diselenide (NbSe₂) (004) multilayered nanosheets standing on quartz substrate. Similar to the most-used micromechanical cleavage to exfoliated NbSe₂ nanosheets, the synthesis was performed starting directly from bulk powdered counterparts, but using a novel type of substrate-irrelevant vapor-phase exfoliation. A possible intercalant-assisted thermal cleavage growth mechanism for the NbSe₂-derived nanosheets, including a combination of intercalant injection-atomic delamination-thermal exfoliation-lateral growth, is discussed. This two-component nanosheet-forming technique appears to potentially work on homologous series of sandwich-type inorganic layered 2H transition-metal dichalcogenide compounds under proper experimental conditions as well.