Magnetothermal Transport of Oriented Graphite at Low Temperatures

We have studied the magnetic field dependence of the thermal conductivity κ(T,B) of highly oriented pyrolytic graphite samples at temperatures 0.2K≤T < 10K and fields 0T≤ B ≤ 9T. The samples show clear deviations from the Wiedemann-Franz law with a kink behavior at fields B ～ 0.1 T near the metal-insulator transition observed in electrical resistivity measurements. We further show that the oscillations in the thermal conductivity at the quantum limit B > 1 T, which are correlated with the Landau quantization observed in Hall measurements, increase in amplitude with temperature following a ～ T³ law at T > 0.2 K and show a maximum at T ～ 6 K, suggesting that they are phonons mediated. PACS numbers: 72.20.My, 72.15.Eb, 73.43.-f     

Mechanism of Superconductivity in Graphite Intercalation Compounds Including CaC6

On the basis of the model that was successfully applied to KC₈, RbC₈, and CsC₈ about a quarter century ago, we have implemented a quantitative calculation of the superconducting transition temperature T _c to find that the observed values for T _c in both CaC₆ and YbC₆ are also reproduced in the model, indicating that the model is a unified one for superconductivity in the graphite intercalation compounds with T _c ranging three orders of magnitudes.     

Thermal Conductivity of Superconductor Sr2RuO4 at Low Temperatures and Magnetic Fields

The temperature dependence of thermal conductivity of Sr₂RuO₄ is investigated by the Boltzmann equation approach. We consider both the isotropic and anisotropic gap energy in superconducting state in the absence and presence of low magnetic field, and obtain the temperature dependence of the components of thermal conductivity in both cases. In the case of isotropic gap, it is proportional to T ^1/2, whereas in the case of anisotropic gap, K _xx ∝T ^?1,K _yy ∝T, and K _xy =K _yx =constant. Furthermore, we show that at a low magnetic field K behaves as in the case of the zero magnetic field.     

Electrical Spin Injection from Ferromagnetic Nanocontacts into Nondegenerated Silicon at Low Temperatures

We present results on the magnetoresistance of the system Ni/Al₂O₃/Si/Al₂O₃/Ni fabricated in lateral nanostructures. The substrate n-type Si is a nondegenerated semiconductor with a doping level of 10¹⁵ cm^?3. The results are presented between 11 and 30 K, where the electrical resistivity of the semiconductor varies about 4 orders of magnitude. The reduction of magnetoresistance at 30 K is consistent with the standard theory for spin injection between a metal and a semiconductor. By fitting the data with $e^{ - t_{N} / L_{\mathrm{SD}}}$ , the diffusion condition, as a function of the channel length t _N , where the magnetoresistance takes place, we deduced the values of spin diffusion length L _SD and spin lifetime τ _s .     

X-ray Diffraction Study of MgB2 at Low Temperatures

We have performed powder X-ray diffraction of MgB₂ superconductor between 10 and 300 K. Temperature dependence of integrated intensities of both (002) and (110) X-ray reflections shows a peak at around T_C, superconducting transition temperature. The integrated intensity of the X-ray reflection is related to the phonon frequency through a Debye--Waller factor. Lattice parameters a and c show negative thermal expansions at low temperatures. The negative thermal expansion might be due to an electronic origin and not directly related to the superconducting transition.     

Possible Phase Transition at Low mK Temperatures in Liquid Helium Mixture Films Adsorbed on Graphite

We report heat capacity and magnetisation measurements of ³He adsorbed on the surface of graphite plated with three atomic layers of ⁴He. For ³He coverages n ₃>4 nm^?2 the heat capacity corresponds to a 2D Fermi fluid. The inferred hydrodynamic mass of the ³He quasiparticles is 1.38±0.05 m₃. The ³He effective mass ratio increases with coverage to 2.4 at n ₃=4 nm^?2, due to Fermi liquid interactions. The heat capacity isotherm exhibits a steplike increase centred on n ₃=4.5 nm^?2, similar to that previously observed on four layers of ⁴He. This is associated with the population of the first excited Andreev surface bound state. However, in the present case, as n ₃ is increased through the step a pronounced anomalous feature develops in the temperature dependence of the heat capacity, around 10 mK. Below 5 mK the heat capacity is approximately linear in temperature. With n ₃=7 nm^?2, we find that this behaviour is very sensitive to small changes in the ⁴He third layer coverage, around the completed third layer. Measurements of the ³He magnetisation,, by continuous wave NMR methods, find a significant increase with decreasing temperature below around 20 mK. Together the data suggest that a phase transition takes place in the film at low mK temperatures.     

Coherent Spin Precession in Superfluid 3He-B Excited in a Field Minimum at Low Temperatures

No Heading The persistent precessing domain (PPD) is an isolated region of coherent spin precession which is observed in the B phase of superfluid ³He at the lowest achievable temperatures. It has many unusual properties and its free decay can exceed 1000s at the lowest temperatures. Previous observations of the PPD were highly irreproducible but we now find the PPD to be very reproducible when there is a field minimum along the cell axis. Here we discuss measurements of the PPD as we control the magnetic field profile, allowing the depth of the minimum to be adjusted.PACS numbers: 67.57.Lm, 67.57.Jj, 67.57.Fg     

X-Ray Diffraction Study of Correlated Electron System at Low Temperatures

By using an ultra-low temperature X-ray diffractometer, we studied low temperature phase transitions of several materials. These phase transitions are classified into a ferro quadrupole ordered phase, that is, Jahn–Teller distortion (TmVO, ZnCrO), an antiferro quadrupole ordered phase (CeB, PrPb), and a superconducrivity (MgB). We also investigated unknown phases, such as the low temperature phase in PrPtBi and the 4th phase in CeLaB. Our low temperature X-ray measurement gives rise to a fruitful information about the phase transition. Not only a whole profile of the reflection peaks, which can be analyzed by the Rietveld method, we also made the precise measurement of the temperature dependence of the integrated intensity of the reflection peak, the full width at half maximum, and the lattice spacing for some fixed reflections. The temperature dependence of the integrated intensity (I.I.) can be expressed by the Debye–Waller expression. At low temperatures, the change of the phonon frequency can be obtained from the I.I. through the Debye–Waller factor. In MgB we observed the hardening of the phonon and the softening of phonon above and below the superconducting transition temperature, respsctively. This result suggests the relevant mechanisim of superconductivity in MgB is the electron–phonon interaction. From the precise measurement of the lattice constant, we found the negative thermal expansion at low temperatures in many correlated electron system. This negative thermal expansion can be explained by the Fermi liquid theory by Misawa.     

Cryogenic Processing: A Study of Materials at Low Temperatures

Cryogenics is an exciting, important and inexpensive technique that already has led to main discoveries and holds much future assurance. Cryogenic processing is the treatment of the materials at very low temperature around 77 K. This technique has been proven to be efficient in improving the physical and mechanical properties of the materials such as metals, alloys, plastics and composites. It improves the wear, abrasion, erosion and corrosion resistivity, durability and stabilizes the strength characteristics of various materials. Cryogenic refines and stabilizes the crystal lattice structure and distribute carbon particles throughout the material resulting a stronger and hence more durable material. In present paper, we have reviewed the effect of cryogenic treatment on some metals, alloys, plastics and composites.     

SQUID-based Technique to Study the Elastic Properties of Solids at Very Low Temperatures

Mechanical oscillators are widely used for measurements of the elastic properties of solids at low temperatures. In most of such experiments mechanical displacements are detected via capacitive changes. In order to improve the sensitivity we have developed a novel detection technique relying on the high sensitivity of a commercial dc SQUID. To demonstrate the performance of the new setup we have measured the variation of the sound velocity and the internal friction of vitreous silica and of the borosilicate glass BK7 in the temperature range from 5 mK to 1 K. Our results agree favorably with former experiments where the displacement was detected capacitively. Furthermore, our results confirm that deviations of the elastic behavior of amorphous solids from the predictions of the standard tunneling model exist at very low temperatures.      

Matrix Isolation of H Atoms at Low Temperatures

The recent history of the matrix isolation of atomic free radicals at low temperatures started with a research program at the US National Bureau of Standards and continued with the important breakthrough at Chernogolovka in Russia where a jet containing atomic free radicals was directed onto the surface of superfluid ⁴He. The samples collected consisted of gel-like substances made up of molecular nanoclusters, allowing the atomic free radicals to be isolated from one another and studied at 1.3 K. More recently, techniques were developed at Turku University which have been made the region T<1 K accessible for studies of H atoms entrapped in H₂ films. Very high concentrations of H atomic free radicals (??10¹⁸?C10¹⁹ cm^?3) have been attained using both the Turku and Chernogolovka methods. A discussion of the most recent experiments at Cornell and Turku will be given. Microwave and mm wave electron paramagnetic resonance techniques have been employed in these experiments. These techniques permitted studies of the exchange tunneling chemical reaction D+HD??H+D₂. Diffusion of H atoms through solid H₂ proceeds via the reaction H+H₂??H₂+H, leading to recombination (H+H??H₂). Quantum overlap of H atoms is thought to be responsible for exotic behavior of H atoms in solid H₂ films below 1 K, including a significant departure from the Boltzmann distribution of the relative populations of the two lowest hyperfine levels of atomic H.     

低温制备纳米α-Al_2O_3粉体

研究了以 Ａｌ（ＮＯ３）３·９Ｈ２Ｏ和ＮＨ３·Ｈ２Ｏ溶液为原料,采用高纯氧化铝球磨原位引入晶种和添加 ＺｎＦ２、 ＡｌＦ３的制备工艺．通过提高凝胶中晶种的均匀性和加速相转变时物质的扩散和传输,可降低过渡型氧化铝向α型氧化铝相转变温度,所制铝凝胶经９００～９２０℃煅烧可获得平均粒径＜５０ｎｍ的α型纳米氧化铝．     

Models of Ballistic Propagation of Heat at Low Temperatures

Heat conduction at low temperatures shows several effects that cannot be described by the Fourier law. In this paper, the performance of various theories is compared in case of wave-like and ballistic propagation of heat pulses in NaF.     

Measurements of the Dielectric Constant of Solid Helium at Low Temperatures

A careful measurement of the dielectric constant of solid ⁴He was made to search for signatures associated with the reported onset of nonclassical rotational inertia and shear modulus stiffening. The samples studied include zero-pressure liquid, solid-liquid mixtures on the melting-curve and solid helium. It was found that the dielectric constant of solid ⁴He decreases smoothly with temperature below 300?mK, showing no measurable anomalies down to 50?mK. The new measurements do not show the anomalous upturn effect that was reported in our previous measurements.     

Long-Time Heat Release in the Quasi-1D Conductor (TMTSF)2PF6 at Low Temperatures

We have measured the heat capacity and the heat release of the quasi-1D conductor (TMTSF)₂PF₆ in its spin-density wave ground state. Below 1K the presence of low-energy excitations is at the origin of slow heat release over time scales from about 1s up to 10⁵s. In a first attempt, we analyzed this behaviour with a modified version of the standard tunneling model used in the case of amorphous solids, introducing a short-time cut-off in the distribution of the relaxation times of tunneling states. A remarkable feature of the heat release in (TMTSF)₂PF₆ is its high absolute value, which is 3–4 orders of magnitude larger than in structural glasses. The heat capacity contains a Schottky and a quasi-linear term.     

全氟己酮灭火剂高温热裂解性能研究

基于GC、GC-MS和KM900手持式烟气分析仪测试手段,研究了全氟己酮在管式反应器中滞留时间为2s和5 s、裂解温度为500~750℃时的热裂解规律。结果表明:全氟己酮在550℃时开始分解,超过650℃裂解剧烈;主要裂解气体产物为十氟丁烷、六氟丙烯和全氟异戊烷;随着裂解温度和滞留时间的增加,全氟己酮裂解程度加剧,十氟丁烷和六氟丙烯生成量增加,全氟异戊烷的生成量先增加后减少;值得重视的是全氟己酮高温热裂解时有剧毒气体全氟异丁烯和毒性气体一氧化碳产生。     

石墨的添加对NiCr-W-Ti自润滑复合材料高温摩擦学性能的影响

本文以Ni20Cr合金为基体添加稀有金属Ti、W粉末及石墨后,充分混合,采用机械合金化及热压烧结工艺制备了NiCr金属基复合材料,研究了石墨含量对NiCr金属基复合材料的组织结构和摩擦学性能的影响。在UMT-3高温摩擦试验机上进行了该复合材料同Al_2O_3陶瓷球的滑动摩擦磨损实验,利用扫描电子显微镜(SEM)、X射线衍射仪(XRD)对复合材料及其摩擦测试后的形貌与结构进行观察分析,结果表明:当所添加石墨的质量分数为3wt.%时,复合材料具有相对较好的力学性能和摩擦学性能;当测试温度低于300℃时,摩擦表面未形成有效的摩擦膜,故磨损率较高;当温度高于500℃时,摩擦层中含有摩擦氧化物,摩擦表面被光滑氧化物所组成的润滑膜覆盖,对磨面具有很好的保护作用,因而磨损率降低。