Thermal conductivity of high-Tc superconductors

This paper reviews existing data on the thermal conductivity of high-T _c superconductors. Included are discussions of pristine polycrystalline high-T _c ceramics, single crystal specimens, and high-T _c materials structurally modified by substitution or by radiation damage. The thermal conductivity of high-T _c superconductors is compared with that of conventional superconductors, and dramatic differences are found between the two families. Mechanisms of thermal conductivity applicable to high-T _c perovskites are discussed and implications for theories of high-T _c superconductivity are noted.     

Thermal conductivity of high-T c crystals: Dependence on magnitude and orientation of magnetic field,and effect of Zn doping

We present precise measurements of in-plane thermal conductivity for superconducting single crystals of YBa₂Cu₃O_7?x (YBCO) withT _c =92 and 60 K, Bi₂Sr₂CaCu₂O₈ (BSCCO), and of Zn-doped YBCO. Magnetization and thermal conductivity data obtained with the same 90-K YBCO crystal demonstrate a close relationship between the magnetic thermal resistivity and the internal magnetic fieldB in a superconductor in the mixed state. For all superconductors studied here, the magnetic thermal resistivity is a sublinear function of magnetic field. The origins of the nonlinearity are discussed. Angular dependences of the magnetic thermal resistivity have been shown to depart from the anisotropic 3D superconductor model and are in quantitative agreement with a quasi-2D model. Implications for spatial modulation of the order parameter are made.     

Magnetic field enhancement of high-Tc superconducting pancake coils by Mu metal sheets

High permeability ring-shaped mu metal sheets have been used to enhance magnetic fields in the bore of Bi-based high-T _c superconducting coils. The central magnetic fieldB ₀, generated by pancake coils placed between mu metal sheets, was measured at liquid nitrogen temperature 77 K. The increasing rate (IR) of the central magnetic fieldB ₀ for single pancake coils was from 15 to 63% by using a 0.1 mm thick mu metal sheet. IR was decreased when the originalB ₀ value increased. The thickness of mu metal sheets also affected IR. It was found that IR increased as the number of mu metal sheets was increased. In a test double-pancake coil IR reached 101%,B ₀ was enhanced from 902 to 1815 G, when the total thickness of mu metal sheets on the top and bottom surface of the coil was 2.0 mm. These results indicate that the well-designed high-permeability materials can significantly enhance the magnetic fields generated by high-T _c superconducting coils and magnets.     

磁场对聚苯胺结构、掺杂行为及热稳定性的影响

在恒定磁场(0.8T) 和无磁场环境下,采用化学氧化聚合法合成了分别用磺基水杨酸(SSA)、对甲基苯磺酸(TSA)以及用十二烷基苯磺钠/盐酸(SDBS HCl)掺杂的聚苯胺盐,并采用X射线光电子能谱(XPS)、傅立叶红外光谱(FT-IR)、X粉末衍射(XRD),热重分析(TGA)等手段研究了磁场对聚苯胺结构、掺杂行为及热稳定性的影响.结果表明在合成聚苯胺的过程中,作为表面活性剂的十二烷基苯磺酸钠在无机酸存在条件下转变成十二烷基苯磺酸并与无机酸共同掺杂进入聚苯胺链;磁场环境下合成的聚苯胺盐比无磁场下合成的聚苯胺盐的链更加规整,热稳定性更好,掺杂程度以及结晶性能也有所提高;从红外光谱分析中可知,磁场取向作用使分子链更加伸展,分子链离域程度更大,但磁场没有明显地改变聚苯胺的基本单元结构.     

Disorder effects and critical temperature for the diffusive model of high-Tc superconductors

The effect of disorder and magnetic correlations is considered in connection with the occurrence of the superconducting state. The effect of disorder can reduce the pair-weakening parameter. The Coulomb pseudopotential is strongly dependent on disorder and decreases drastically the critical temperature. The competition between disorder and magnetic correlation effects is analyzed.     

Influence of long-range and strong correlation effects on the electron-phonon coupling in high-Tc oxides

Calculations of the Eliashberg function ²F and the corresponding transport function _tr ² F for high-T _c oxides are presented using a screened ionic model (rigidly shifted ionic potentials screened by charge carriers in the CuO₂ planes within the RPA) for the electron-phonon coupling. It is shown that this model yields a large difference between the transport and the superconducting electron-phonon interaction due to imperfect screening and contributions beyond nearest-neighbor interactions. Using these results, the electron and the lattice heat conductivities are calculated both in the normal and the superconducting state and compared with experiment. Finally, effects due to a strong on-site electron-electron repulsion are included in leading order in an 1/N expansion, whereN is the number of spin degrees of freedom. In particular, it is shown for the infiniteU, one-band Hubbard model that correlations tend to suppress _tr ² F strongly and ² F somewhat.The author thanks M. Kulic for discussions and cooperation on the topic discussed in Section 4.     

Intrinsic surface depression of the order parameter under mixed (s+id)-wave pair symmetry and its effect on the critical current of high-Tc SIS Josephson junctions

An intrinsic gap depression at the superconductor-insulator interface due to the very short value of the coherence length in high-T _c superconductors [HTS] is considered in the framework of a mixed (s+id)-wave pair symmetry for the order parameter ranging from pures to pured-wave. This gap depression acts as the main physical agent causing the relevant reduction ofI _c (T)R _n (T) values with respect to BCS expectations in HTS SIS Josephson junctions. Good agreement with various experimental data is obtained with both pures-wave and pured-wave symmetries of the order parameter, but with amounts of gap depression depending on the pair symmetry adopted. Regardless of the apir symmetry considered, these results prove the importance of the surface order-parameter depression in the correct interpretation of theI _c (T)R _n (T) data in HTS SIS junctions. In the case of a planar YBCO-based junction the use of the de Gennes condition allowed us to tentatively obtain an upper limit for the amount ofd-wave present in the gap of YBCO.     

磁场对Ni/硅橡胶复合材料交流电导率及介电性质的影响

采用硅橡胶（110型）与金属（镍粉）按质量比为1∶2.7进行配料, 应用室温二次固化合成Ni/硅橡胶压敏复合材料样品。室温下测量了样品的压阻效应, 比较了外加0.024 T磁场前后样品的介电性质。结果表明, 当压强从3.75kPa到312.5kPa时, 样品直流电阻率下降了8个数量级。与加磁场前相比, 0.024T的磁场使得低频(40~104Hz)交流电导率提高了2.46倍, 介电常数提高了20%, 介电损耗提高了2倍, 这主要是由复合材料中铁磁绝缘体铁磁颗粒膜的隧道磁电阻效应以及磁电耦合引起的。撤去磁场后交流电导率、介电常数和介电损耗均不能回到加磁场前的初始值, 这与Ni粉的铁磁性有关。Ni/硅橡胶压敏复合材料的压阻、 磁电阻效应及磁电耦合等物理性质在磁传感器件、 信息储存等领域有潜在的应用价值。      

异丙醇铝溶胶-凝胶掺杂的SiO2-Al2O3/PEG导热增强型定形相变储能材料

以异丙醇铝(AIP)与正硅酸乙酯(TEOS)为溶胶前驱体、 聚乙二醇(PEG)为相变组分, 经超声辅助溶胶-凝胶过程制备了SiO₂-Al₂O₃/PEG 导热增强型定形相变储能材料。利用FTIR和XRD对SiO₂-Al₂O₃/PEG进行结构表征及结晶性能测定, 利用DSC与TGA研究其热性能与热稳定性, 利用显微热台与数码拍照技术测试材料的定形效果; 利用热导率仪测定了材料的导热系数。结果表明, SiO₂-Al₂O₃/PEG的相变焓值可以达到100 J/g以上, 低于300 ℃时具有良好的热稳定性, 当SiO₂-Al₂O₃/PEG中的铝硅质量比为0.2:1时, 其导热系数可达0.414 W/(m·K), 导热性能比纯PEG提高39.3%, 并具有良好的定形效果。     

Comparative study on the doping effect of 3d elements in Bi1.5Pb0.2Sr2Ca2Cu2.8M0.2Oy (M=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, or Zn)

With XRD,R-T, and ac measurements a comparative study on the doping effects of 3d elements in Bi_1.5Pb_0.2Sr₂Ca₂Cu_2.8M_0.8O _y (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, or Zn) has been carried out. The effects of the former five members are significantly different, both on phase formed and onT _c , from the latter four. It seems that the effect on phase stabilization correlates with the valency of the doped cation. In connection with the instability of the 2223 phase, the correlation has been discussed.     

Ab initio, crystal field and experimental spectroscopic studies of pure and Ni-doped KZnF3 crystals

Pure and Ni²⁺ doped KZnF₃ single crystals were studied using the combination of the DFT-based ab initio methods, crystal field theory and experimental spectroscopic techniques. The electronic, optical and elastic properties have been calculated and compared with available experimental data and good agreement was achieved. Elastic anisotropy of pure KZnF₃ was modeled; calculations of the sound velocity, Debye temperature, Grüneisen parameter and specific heat capacity were performed. Comparison of the calculated results for the pure and doped material, which is reported for the first time for the considered material, enabled to identify the changes in the optical and electronic properties, which are due to the introduced nickel impurity ions. In particular, it was shown that the lowest Ni 3d states appear in the host's band gap at about 1.0 eV above the valence band. The changes of the electron density distribution after doping were also shown. Microscopic analysis of the crystal field effects based on the performed ab initio calculations of the Ni²⁺ density of states at different external pressures enabled to estimate the constants of the electron–vibrational interaction, Huang-Rhys factor, Stokes shift and local bulk modulus around impurity ions. The crystal field calculations of the Ni²⁺ energy levels were performed to analyze and assign the experimental absorption spectrum. Such a combination of the ab initio and semi-empirical calculating techniques leads to a complementary picture of the physical properties of KZnF₃:Ni²⁺ and can be applied to other doped crystals.     

磁场辅助下ZrO2纳米颗粒质量浓度对镀层耐磨性能的影响研究

为了进一步延长45钢工件的耐磨性能,减少企业维修和更换45钢类零件的成本,在传统材料45钢表面,开展了稳恒磁场辅助的电喷镀试验研究,通过配制不同ZrO₂纳米颗粒质量浓度的镀液,制备了Ni-P-ZrO₂镀层,利用扫描电子显微镜(SEM)、能谱分析仪(EDS)、显微硬度检测仪和共聚焦显微镜等仪器对镀层表面形貌、Zr元素含量、显微硬度以及耐磨性能进行了分析。研究发现:ZrO₂纳米颗粒和稳恒磁场的加入改善了镀层表面形貌,提高了显微硬度和耐磨性能;当ZrO₂纳米颗粒质量浓度达到15 g/L时,ZrO₂纳米颗粒易出现团聚现象,不利于镀层表面性能的进一步提升。磁场辅助下,镀液中ZrO₂纳米颗粒质量浓度为10 g/L时,得到的镀层表面更加平整、显微硬度最高达到了739.30HV,且磨痕宽度、深度以及截面面积达到最小值,分别为367.617、5.673 μm和1 288.155 μm²,镀层耐磨性能最佳。     

Numerical study on effect of external magnetic field, current profile, arc current and arc column radius on helical instabilities of arcs

This model differs from previous works in that the electrical conductivity (and the current profile) is approximated in a more realistic way (parabolic instead of flat profile). The magnetohydrodynamic equations in an electrostatic approximation serve as the starting point of the theory. The effects of the external axial magnetic field, current profile, arc current and arc column radius on the helical instabilities of arcs were numerically studied. Numerical results show that the short wavelength perturbation can be stabilized by positive direction magnetic field, whereas the long wavelength perturbation can be stabilized by reverse magnetic field when the current profile is the uniform distribution. In the short wavelength perturbation case, the effect of positive direction magnetic field on the arc stability is very small when the current profile has a parabolic distribution. However, its stabilizing effect is enhanced for the long wavelength perturbation. The intermediate and long wavelength perturbation can also be stabilized by reverse magnetic field. The low current is beneficial for stabilizing arcs.     

Effect of high magnetic field on the morphology of (Cu, Ni)6Sn5 at Sn0.3Ni/Cu interface

The effect of high magnetic field on the microstructure of (Cu, Ni)₆Sn₅ intermetallic compound layer in Sn0.3Ni/Cu couples at 250 °C was examined. The applied magnetic field changed the morphology of outer (Cu, Ni)₆Sn₅ crystals on the Sn side from faceted shape to stick shape. The high magnetic field affected the crystal orientations of (Cu, Ni)₆Sn₅ and reduced the Ni content in the outer layer. The morphology evolution of (Cu, Ni)₆Sn₅ is attributed to the content of Ni solute decreased by magnetic field. The effects of high magnetic field on the liquid convection and phase diagram are considered to be responsible for the Ni content.     

Thermal, spectroscopic and magnetic properties of the CoxNi1−x(SeO3)·2H2O (x = 0, 0.4, 1) phases: Crystal structure of Co0.4Ni0.6(SeO3)·2H2O

The Co_xNi_1−x(SeO₃)·2H₂O (x = 0, 0.4, 1) family of compounds has been hydrothermally synthesized under autogeneous pressure and characterized by elemental analysis, infrared and UV-vis spectroscopies and thermogravimetric and thermodiffractometric techniques. The crystal structure of Co_0.4Ni_0.6(SeO₃)·2H₂O has been solved from single-crystal X-ray diffraction data. This phase is isostructural with the M(SeO₃)·2H₂O (M = Co and Ni) minerals and crystallizes in the P2₁/n space group, with a = 6.4681(7), b = 8.7816(7), c = 7.5668(7) Å, β = 98.927(9)° and Z = 4. The crystal structure of this series of compounds consists of a three-dimensional framework formed by (SeO₃)²⁻ selenite oxoanions and edge-sharing M₂O₁₀ dimeric octahedra in which the metallic cations are coordinated by the oxygens belonging to both the selenite groups and water molecules. The diffuse reflectance spectra show the essential characteristics of Co(II) and Ni(II) cations in slightly distorted octahedral environments. The calculated values of the Dq and Racah (B and C) parameters are those habitually found for the 3d⁷ and 3d⁸ cations in octahedral coordination. The magnetic measurements indicate the existence of antiferromagnetic interactions in all the compounds. The magnetic exchange pathways involve the metal orbitals from edge-sharing dimeric octahedra and the (SeO₃)²⁻ anions which are linked to the M₂O₁₀ polyhedra in three dimensions.     

Effect of Sintering Duration on the Thermal Conductivity of (Bi, Pb)-2223 Superconducting Pellets Between 10 and 150 K

The influence of sintering duration on the electrical resistivities and thermal conductivities of (Bi_0.8Pb_0.2)₂Sr₂Ca₂Cu₃O_9.8+ pellets with 0.11 < < 0.54 is reported between 10 and 150 K. The results indicate a gradual transformation of the 2212 phase to the 2223 phase and this transformation starts within 5 h of sintering in air at 840°C. The thermal conductivity of the pellets sintered for shorter durations display two maxima at T _c0 and around 110 K, respectively. The shape and magnitude of these maxima depend on the relative amount of the 2212 and 2223 phases present in the pellets. While the magnitude of the total thermal conductivity over the measured temperature range is strongly influenced by the duration of sintering, the phonon-dominant nature of heat transport is retained. The relative contribution of the electronic part (_E) to the total thermal conductivity () remains small and does not change appreciably with sintering time.