Thermal Conductivity of Small Nickel Particles

The thermal conductivity of nanoscale nickel particles due to phonon heat transfer is extrapolated from thin film results calculated using nonequilibrium molecular dynamics (NEMD). The electronic contribution to the thermal conductivity is deduced from the electrical conductivity using the Wiedemann–Franz law. Based on the relaxation time approximation, the electrical conductivity is calculated with the Kubo linear-response formalism. At the average temperature of T=300 K, which is lower than the Debye temperature Θ_D=450 K, the results show that in a particle size range of 1.408–10.56 nm, the calculated thermal conductivity decreases almost linearly with decreasing particle size, exhibiting a remarkable reduction compared with the bulk value. The phonon mean free path is estimated, and the size effect on the thermal conductivity is attributed to the reduction of the phonon mean free path according to the kinetic theory.     

Total Hemispherical Emittance of Polyimide Films for Space Use in the Temperature Range from 173 to 700 K

This paper describes the temperature dependence of the total hemispherical emittance _h in the temperature range from 173 to 700 K for three types of thermal control materials, which are based on a thin polymide film coated with aluminum on the back surface. The results obtained from the measurements are compared with calculated values from optical constants. The principle of the present measurement is based on the steady-state calorimetric method, and _h is obtained by measuring the equilibrium temperature of a sample corresponding to different heat inputs to a heater attached to the sample. On the other hand, the present calculation method is performed by using data for the optical constants of polyimide films and vapor-deposited metal in the wavelength region from 0.25 to 100 m. These results agree with each other on the whole. It has been observed that the temperature dependence of _h is remarkable, and the values have a maximum around 410 K.     

Density and Thermal Conductivity Measurements for Silicon Melt by Electromagnetic Levitation under a Static Magnetic Field

The density and thermal conductivity of a high-purity silicon melt were measured over a wide temperature range including the undercooled regime by non-contact techniques accompanied with electromagnetic levitation (EML) under a homogeneous and static magnetic field. The maximum undercooling of 320 K for silicon was controlled by the residual impurity in the specimen, not by the melt motion or by contamination of the material. The temperature dependence of the measured density showed a linear relation for temperature as: ρ(T) = 2.51 × 10³−0.271(T−T _m) kg · m⁻³ for 1367 K < T < 1767 K, where T _m is the melting point of silicon. A periodic heating method with a CO₂ laser was adopted for the thermal conductivity measurement of the silicon melt. The measured thermal conductivity of the melt agreed roughly with values estimated by a Wiedemann–Franz law.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.     

Development of an Apparatus for the Determination of Spectral Reflectivity at High Temperatures in the Visible

The paper presents a reflectometer for high temperature measurements. In this apparatus, the directional-hemispherical spectral reflectivity is measured by comparing the optical response of the sample to white light with the response of a reference material. The reflected light, collected by an integrating sphere, is dispersed in a spectrograph and detected by an ICCD camera. This procedure allows the simultaneous measurement of the reflectivity in a large, continuous wavelength range (presently 510 to 860 nm). An electrical resistance heater is used to heat the samples up to about 1200 K; for higher temperatures a flash-lamp pumped dye laser is used. To avoid laser induced plasma generation, the integrating sphere is placed inside a vacuum chamber, which also allows measurements under a controlled atmosphere. The response of the apparatus is calibrated to an absolute scale which allows the determination of the sample temperature by fitting the thermal emission spectrum with Planck's formula. To check the performance of the apparatus, measurements on Fe₂O₃ (hematite) and NiO have been carried out.     

Measurement of Specific Heat Capacity from 150 to 310 K by Thermal Radiation Calorimetry

A novel method for measurement of the specific heat capacity in the temperature range from 150 to 310 K is described. In order to achieve good temperature homogeneity in a disk-shaped specimen, a cylindrical heater was used in an apparatus based on thermal radiation calorimetry. A mixture of Bi₂O₃ and MgO powders was used for blackening the surfaces of the specimen, the heater, and the inside wall of the chamber. The specific heat capacities of Ni, fused quartz glass, and BaTiO₃ ceramic were measured to test the performance of the calorimeter. Agreement to within 5% of the values published in the literature was obtained for these samples. Thermal hysteresis and anomalies associated with the first-order phase transition in BaTiO₃ were detected in the present experiment.     

A study of the properties of the YBa2Cu3O7−δ high temperature superconductor

High temperature,T _c>90 K, superconductivity in YBa₂Cu₃O_7−δ samples was observed from resistivity and magnetic susceptibility measurements. The zero-field-cooled susceptibility of the material was 60 to 100% of the ideal diamagnetic value depending on sample preparation. Structure identification by X-ray diffraction at both 293 and 89 K exhibited the same orthorhombic structure. Scanning tunnelling microscope investigations of the sample surface revealed steep step structure with 10×10 nm² size crystallites and 5 to 100 nm high terraces. Temperature cycling between 77 and 293 K sometimes led to the appearance of white barium lumps on the surface and in the bulk of the sample causing the transition to the low resistance state at 90 K to disappear.     

Thermal-Conductivity and Thermal-Diffusivity Measurements of Nanofluids by 3<Emphasis Type="Italic">ω</Emphasis> Method and Mechanism Analysis of Heat Transport

A 3ω technique is developed for simultaneous determination of the thermal conductivity and thermal diffusivity of nanofluids. The 3ω measuring system is established, in which a conductive wire is used as both heater and sensor. At first, the system is calibrated using water with known thermophysical properties. Then, the thermal conductivity and thermal diffusivity of TiO₂/distilled water nanofluids at different temperatures and volume fractions and the thermal conductivity of SiO₂ nanofluids with different carrier fluids (water, ethanol, and EG) are determined. The results show that the working temperature and the carrier fluid play important roles in the enhancement of thermal transport in nanofluids. These results agree with the predictions for the temperature dependence effect by the Brownian motion model and the micro-convection model. For SiO₂ nanofluids, the thermal-conductance enhancement becomes strong with a decrease in the heat capacity of the carrier fluids. Finally, according to our results and mechanism analysis, a corrected term is introduced to the Brownian motion model for providing better prediction of heat transport performance in nanofluids.     

Evaluation of the Effective Sample Temperature in Thermal Diffusivity Measurements Using the Laser Flash Method

In the measurement of thermal diffusivity by the laser flash method, a temperature rise occurs in the sample as a pulsed laser hits on the sample surface. Due to the temperature dependence of thermal diffusivity of the sample, the thermal diffusivity corresponds to a temperature that is larger by T _eff than the temperature before laser irradiation is applied. This effective temperature rise, T _eff, has been investigated by using a numerical simulation. The results indicate that the effective temperature rise is almost equal to a maximum temperature rise, T _M, of the back surface of the sample in cases where both linear and nonlinear temperature variations of thermal diffusivity are considered.     

均匀共沉淀法制备荧光水滑石的结构及性能

采用均匀共沉淀法制备了荧光黄含量不同的一组荧光水滑石(MgAl-LDHs-C20H12O5),研究了其晶体结构、外观形貌、荧光性能、表面官能团及热稳定性。X射线衍射(XRD)图谱显示少量荧光黄的加入没有影响水滑石的结晶性能,荧光水滑石具有典型的水滑石特征峰;扫描电镜(SEM)图片显示采用均匀共沉淀法制备的荧光水滑石是片层状结构,荧光黄均匀地吸附在水滑石片层表面;荧光水滑石样品在470nm波长光激发下的荧光发射光谱(PL)显示荧光水滑石在500~600nm间出现了1个黄光发射峰,其荧光强度与其中荧光黄的含量有关,荧光黄的含量要适中,过高或过低均影响样品的荧光性能;热重分析(TG)曲线表明荧光类水滑石的热稳定性比纯荧光黄提高很多;红外光谱(IR)显示荧光水滑石含有水滑石和荧光黄的特征官能团。     

Luminescence from praseodymium doped AlN thin films deposited by RF magnetron sputtering and the effect of material structure and thermal annealing on the luminescence

Thin films of Praseodymium doped AlN are deposited on silicon (111) substrates at 77 K and 950 K by rf magnetron sputtering method. About 500–1000 nm thick films are grown at 100–200 watts RF power and 5–8 mTorr nitrogen, using a metal target of Al with Pr. X-rays diffraction results show that films deposited at 77 K are amorphous and those deposited at 950 K are crystalline. Cathodoluminescence studies are performed at room temperature and luminescence peaks are observed in a wide range from ultraviolet to infrared region. The most intense peak is obtained in green at 526 nm from amorphous films as a result from ³P₁→³H₅ transition. In crystalline films the intense peak was obtain in red at 648 nm as a result from ³P₀→³F₂ transition. Films are thermally activated at 1300 K for half an hour in a nitrogen atmosphere. Thermal activation enhances the intensity of luminescence. Two peaks at 488 nm and 505 nm merged after thermal activation, giving rise to a single peak at 495 nm.     

Thermal stability of cryomilled nanocrystalline aluminum containing diamantane nanoparticles

The thermal stability of nanoscale grains in cryomilled aluminum powders containing 1% diamantane was investigated. Diamantane is a diamondoid molecule consisting of 14 carbon atoms in a diamond cubic structure that is terminated by hydrogen atoms. The nanostructures of the resulting cryomilled powders were characterized using both transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The average grain size was found to be on the order of 22 nm, a value similar to that obtained for cryomilled Al without diamantane. To determine thermal stability, the powders were heated in an inert gas atmosphere at constant temperatures between 423 and 773 K (0.51T _m to 0.83T _m) for exposure times of up to 10 h. The average grain size for all powders containing diamantane was observed to remain in the nanocrystalline range (1–100 nm) for all exposures and was generally less than half of that for cryomilled pure Al heated under the same conditions. The thermal stability data were found to be consistent with a grain growth model based on drag forces exerted by dispersed particles against grain boundary migration. The present findings indicate that the presence of diamantane results in a substantial increase in the thermal stability of nanoscale grains in Al.     

Preparation and thermoelectric properties of BP films on SOI and sapphire substrates

The chemical vapor deposited (CVD) BP films on Si(100) (190 nm)/SiO _x (370 nm)/Si(100) (625 μm) (SOI) and sapphire (R-plane) (600 μm) substrates were prepared by the thermal decomposition of the B₂H₆–PH₃–H₂ system in the temperature range of 800–1050 °C for the deposition time of 1.5 h. The BP films were epitaxially grown on the SOI substrate, but a two-step growth method, i.e., a buffer layer at lower temperature and sequent CVD process at 1000 °C for 1.5 h was effective for obtaining a smooth film on the sapphire substrate. The electrical conduction types and electrical properties of these films depended on the growth temperature, gases flow rates and substrates. The thermal conductivity of the film could be replaced by the substrate, so that the calculated thermoelectric figure-of-merit (Z) for the BP films on the SOI substrate was 10⁻⁴–10⁻³/K at 700–1000 K. Those on the sapphire substrate were 10⁻⁶–10⁻⁵/K for the direct growth and 10⁻⁵–10⁻⁴/K for the two-step growth at 700–900 K, indicating that the film on a sapphire by two-step growth would reduce the defect concentrations and promote the electrical conductivity.     

Kapitza conductance and thermal conductivity of copper niobium and aluminium in the range from 1.3 to 2.1 K

The Kapitza conductance and thermal conductivity of ofhc-copper, niobium, ultra high purity aluminium, and of the aluminium alloy 6061 A1 have been measured in the temperature range from 1.3 to 2.1 K, yielding both quantities in the same steady state experiment. The temperature dependence of the Kapitza conductance, h_o, was between T^3.3 and T^4.6 for the different samples, which is higher than the most frequently observed T³ dependence. The magnitude of h_o for both ofhc-copper and aluminium agrees well at 1.9 K with an empirical prediction, but for niobium it is a factor of two to four lower than the value predicted. At 1.9 K, h_o is higher by a factor of two for an annealed and chemically polished niobium sample than for an untreated sample. The thermal conductivity measured from ofhc-copper and 6061 A1 as in good agreement with the value calculated from the resistivity of these materials and the Wiedemann-Franz law. The measured thermal conductivity obtained for an annealed niobium sample is a factor of 2.8 higher than the highest published value.     

Thermal expansion of bismuth telluride

The results of X-ray and dilatometric measurements of the thermal expansion of bismuth telluride in the temperature range of 4.2–850 K have been critically analyzed. The joint statistical processing of the experimental data has been performed by the least squares method and the most reliable temperature dependences of the linear thermal expansion coefficients along the principal crystallographic axes α _a and α _c and the average linear coefficient [`(a)] _L\bar \alpha _L , as well as the density of Bi₂Te₃, have been recommended. The results indicate that the linear thermal expansion coefficients along the directions parallel and perpendicular to the cleavage planes decrease with an increase in the temperature in a narrow temperature range. At temperatures below 298 K, the character of the temperature dependences of the linear thermal expansion coefficients of Bi₂Te₃ has been analyzed in terms of the anharmonicity of the chemical bonding forces in the layer structure and anisotropy of the elastic constants. In terms of the deviations of the composition of the Bi₂Te₃ compound from the stoichiometric one and the real (defect) structure of the compound, a model has been proposed to explain the minimum in the (α _a T) dependence of Bi₂Te₃ near the melting temperature of bismuth. A method for calculating the temperature dependence of the linear thermal expansion coefficients of the anisotropic layer crystals using the data on the specific heat has been discussed, which provides good agreement of the calculated linear thermal expansion coefficients with the experimental data within the accuracy of the measurements of the linear thermal expansion coefficients.     

Room-Temperature Photomagnetic Effect of Fe<Subscript>3</Subscript>Ga<Subscript>4</Subscript> Grown on GaAs Substrates

Ga–As–Fe composite films prepared by molecular beam epitaxy at 600°C on GaAs(100) substrates with the stacking sequence of [100-nm GaAs/50-nm Fe₃Ga_{2− x} As _x /100-nm GaAs] exhibit the distinct photo-enhanced magnetization at room temperature. Transmission electron microscopy reveals the formation of metamagnetic Fe₃Ga₄ grains on the sample surface. Illumination power dependence of the enhanced magnetization has been carefully compared with the antiferromagnetic-type magnetization–temperature (M–T) curve (Neel temperature of T _N = 340–390 K), from which we have discussed the existence of photon-mode photo-enhanced magnetization of some sort in addition with the enhancement due to the light-induced heating.     

Effect of annealing on EPR spectra of Ti-Si-C-N samples

Two nanocrystalline samples of TiC+SiC+20%C (sample 1) and Si₃N₄+Si(C,N)+Ti(C,N)+1%C (sample 2) were prepared by non-hydrolytic sol-gel method. The latter sample was produced from sample 1, by subjecting it to additional annealing at high temperature. XRD measurements showed the presence of aggregates of cubic SiC+TiC nanoparticles (10 to 30 nm in size). In both samples, a very narrow electron paramagnetic resonance (EPR) line originating from localized magnetic centers was centered at g _eff??2. At T = 130 K, we registered the linewidths ??H _pp = 1.41(2) G and ??H _pp = 2.92(2) G for the sample without and with thermal annealing, respectively. For the non-annealed sample, the resonance line was fitted by a Lorentzian line in the low temperature range, and by a Dysonian line above 70 K, which indicates a significant change in electrical conductivity. Therefore, thermal annealing can significantly improve the transport properties of samples. An analysis of the temperature dependence of the EPR parameters (g-factor, linewidth, integrated intensity) showed that thermal annealing has a significant impact on the reorientation processes of localized magnetic centers.     

Disordering and the electronic transport behaviors of NbC–Al<Subscript>4</Subscript>C<Subscript>3</Subscript>–C composite

A composite nanomaterial composed of NbC nanocrystals, Al₄C₃ nanorods and carbon nanofibers as well as amorphous carbon was fabricated by arc-discharging a Nb₃Al block as an anode in CH₄ gas. The growth process of the NbC–Al₄C₃–C composite was deduced according to the microstructures of its components and the experimental conditions. NbC nanocrystals with non-stoichiometric chemical composition were in a cubic shape. As the decomposition product of the precursor of CH₄ gas, carbon nanofibers were thought to be as templates, reacting with Al atoms, to form Al₄C₃ nanorods with a diameter of 15–40 nm. A thin layer of aluminum carbide oxide covered the surface of Al₄C₃ nanorods. The temperature dependence of the resistivity for the NbC–Al₄C₃–C composite was described by the variable-range-hopping (VRH) model between about 100 K and 300 K because of the strong localization of electrons by disorder in the carbon matrix. Below 100 K, the transport behaviors of the pellet deviated from the VRH model due to the conduction competition between the semi-conducting carbon matrix and metallic NbC nanocrystals.     

Temperature dependence of Bi4Ge3O12 photoluminescence spectra

Bi₄Ge₃O₁₂ single crystals were obtained using Czochralski growth method. Photoluminescence spectra were analyzed versus temperature from 12 to 295 K. Besides the previously observed emission bands at 610 and 820 nm, the new emission band at 475 nm was found by a careful temperature dependence measurement in the present study. The influence of basic and defect structure on the shape and position of the spectra versus temperature was discussed.     

Thermal conductivity measurements of PTFE and Al2O3 ceramic at sub-Kelvin temperatures

The design of low temperature bolometric detectors for rare event searches necessitates careful selection and characterization of structural materials based on their thermal properties. We measure the thermal conductivities of polytetrafluoroethylene (PTFE) and Al₂O₃ ceramic (alumina) in the temperature ranges of 0.17–0.43 K and 0.1–1.3 K, respectively. For the former, we observe a quadratic temperature dependence across the entire measured range. For the latter, we see a cubic dependence on temperature above 0.3 K, with a linear contribution below that temperature. This paper presents our measurement techniques, results, and theoretical discussions.