Peeling stress analysis for an inhomogeneous high-Tc superconductor with a discontinuous interface at the substrate

This paper presents an analysis of a superconductor–substrate system to calculate the peeling stress of a high temperature superconductor (HTS) when the temperature decreases from ambient to operating conditions (cryogenic temperatures). Firstly, the values for the material properties of the inhomogeneous high temperature superconductor (HTS) were obtained by fitting a second order polynomial to the experimental data. It is assumed that the material properties of the inhomogeneous HTS vary with varying height coordinate and temperature. Then, through the proposed graded finite element method, the coupled thermo-mechanical equations were solved numerically. The numerical results show that the thermal stress generated in the inhomogeneous HTS is larger on a SiTiO₃ substrate than on a MgO substrate. The maximum thermal stresses, i.e., the peeling stresses, occur near the bottom corner of the inhomogeneous HTS and may induce fracture behavior at the bi-material interface. The inhomogeneous HTS cools at a slower pace than the homogeneous HTS from the room temperature to the operating temperature. It is also shown that the magnitude of the peeling stress for a homogeneous HTS is larger than that for an inhomogeneous HTS. It is intended that the model presented here be useful to researchers who are interested in the mechanical properties of an inhomogeneous HTS.     

Temperature profile evolution in quenching high-T c superconducting composite tape

Irreversible normal zones leading to quench is an important aspect of high-temperature superconductors (HTS) in all practical applications. As a consequence of quench, transport current gets diverted to the matrix stabilizer material of the high-T _c composite and causes Joule heating till the original conditions are restored. The nature of growth of the resistive zone in the superconductor greatly influences the temperature evolution of the quenched zone. In this investigation, a complete mathematical analysis of the temperature profile evolution following a quench in a HTS has been carried out. Such prediction in temperature profile would aid the design of HTS tape-based practical applications in limiting the thermal stress-induced damages in off-normal scenarios.     

YBCO线圈在低温下的温度仿真分析

近几年,高温超导线材的制造技术已日趋成熟。作为第二代高温超导带材的代表,YBCO在低温下具有的优点使其在超导磁体领域的应用越来越突出。但是在高温超导磁体的运行中会因为受到扰动而失超。为了探讨这个问题,模拟了一个磁体在低温下(20K左右)有加热点扰动时的稳定性情况,即对YBCO线圈在低温下的工作情况进行了模拟仿真。文中根据实验设计的YBCO单饼线圈参数,利用热传导方程和基本电路方程,采用有限元计算方法,模拟出线圈内部的温度分布。分析仿真结果可以为后续实验设计提供参考。     

Influence of magnesium oxide nanoparticles embedded in Bi-2212/Ag tapes on their superconducting properties

A MgO-doped Bi₂Sr₂CaCu₂O _y (Bi-2212) high-temperature superconductor is investigated experimentally. The influence of the MgO particle concentration and size on the high-temperature superconductivity parameters is studied. It is shown that, when incorporated into the superconductor, MgO particles do not change their structure. The optimal parameters of the particles exerting the strongest effect on the material are found. The optimal size of the particle is in the range 40–50 nm for a cubic structure. Measures should be taken against clustering. Ways of particle preparation and incorporation into the superconductor are considered. The superconductivity of the material with embedded MgO particles is investigated. At temperatures close to 20 K, the magnetic field of doped tapes exceeds that of undoped ones by a factor of 2.5–10.0 depending on the MgO concentration. The temperature dependences of the superconducting properties of the doped tapes at different magnetic fields are presented.     

Laser cutting of steel and thermal stress development

In laser cutting of sheet metals, thermal stresses are developed in the region of the cutting section. Depending on the cutting conditions and substrate material properties, the thermal stress levels can attain high values. In the present study, thermal stress developed in the region of the laser cut edges is modeled and temperature as well as stress fields are predicted. Temperature predictions are validated through the experimental results. It was found that the temporal variation of the maximum temperature along y-axis follows the laser heating source. However, temporal variation of von-Mises stress deviates slightly from the temporal variation of temperature along the cutting direction. Increase in scanning speed enhances the von-Mises stress levels due to the attainment of high temperature gradients in the substrate material.     

利用电子顺磁共振和紫外/可见吸收光谱研究熔融KNO3-NaNO2-NaNO3盐分解过程中的超氧负离子

On account of excellent thermal physical properties, molten nitrates/nitrites salt has been widely employed in heat transfer and thermal storage industry, especially in concentrated solar power system. The thermal stability study of molten nitrate/nitrite salt is of great importance for this system, and the decomposition mechanism is the most complicated part of it. The oxide species O₂^2- and O₂^- were considered as intermediates in molten KNO₃-NaNO₃ while hard to been detected in high temperature molten salt due to their trace concentration and low stability. In this work, the homemade in situ high temperature UVVis instrument and a commercial electron paramagnetic resonance were utilized to supply evidence for the formation of superoxide during a slow decomposition process of heat transfer salt (HTS, 53 wt% KNO₃/40 wt% NaNO₂/7 wt% NaNO₃). It is found that the superoxide is more easily generated from molten NaNO₂ compared to NaNO₃, and it has an absorption band at 420-440 nm in HTS which red shifts as temperature increases. The band is assigned to charge-transfer transition in NaO₂ or KO₂, responsible for the yellow color of the molten nitrate/nitrite salt. Furthermore, the UV absorption bands of molten NaNO₂ and NaNO₃ are also obtained and compared with that of HTS.     

Fracture behaviors induced by thermal stress in an anisotropic half plane superconductor

This Letter presents a theoretical analysis to the fracture behavior of a large single domain YBCO superconductor under thermal stress based on the two-dimensional theory of anisotropic thermoelasticity, the coupled finite element and infinite element numerical method. The thermal stress intensity factors are obtained due to a uniform heat flux by a line crack in a generally half plane superconductor. It is found that the thermal stress intensity factors decrease with the decrease of temperature, and while the longer the crack length is, the larger the stress intensity factors. Additionally, the J-integral at the crack tip is also investigated, a similar behavior to the thermal stress intensity factors is found. These results are benefit for us to understand the fracture mechanism of superconductor both in theory and application.     

Substrate-induced stress in silicon nanocrystal/SiO2 multilayer structure

A Raman frequency upshift of nc-Si phonon mode is observed at room temperature, which is attributed to a strong compressive stress in Si nanocrystals. The 10-period amorphous-Si(3 nm)/amorphous-SiO₂ (3 nm) layers are deposited by high vacuum radio-frequency magnetron sputtering on quartz and sapphire substrates at different temperatures. The samples are then annealed in N₂ atmosphere at 1100 ℃ for 1 h for Si crystallization. It is demonstrated that the presence of a supporting substrate at the high grown temperature can induce different types of stresses in the Si nanocrystal layers. The strain is attributed to the difference in thermal expansion coefficient between the substrate and the Si/SiO₂ SL film. Such a substrate-induced stress indicates a new method to tune the optical and the electronic properties of Si nanocrystals for strained engineering.     

WEAK QUANTUM FLUX CREEP AND STRONG PINNING IN THE NEW SUPERCONDUCTOR MgB2

The newly discovered superconductor MgB₂ has a transition temperature T_c of about 40 K, which is touching the upper line of that predicted by the phonon-mediated Bardeen--Cooper--Schrieffer (BCS) theory. It is interesting to investigate the flux creep in MgB₂ and compare it with other superconductors. We have measured the magnetization relaxation of MgB₂ sintered at high temperature and high pressure. It is found that the quantum tunnelling and the thermally activated flux creep are very weak, implying a strong pinning in this material.     

Superconducting behavior of amorphous Zr70Cu30

Measurement of the Meissner penetration depth, λ(T) were made in amorphous Zr₇₀Cu₃₀ samples. The results indicate that this amorphous alloy behaves as a BCS superconductor with 2Δ(0)?kT_c = 3.8, where Δ(0) is the superconducting energy gap at T=0 and T_c the critical temperature. It is also concluded that the low energy excitation, TLS, characteristics of amorphous material does not contribute to T_c.     

Optothermal effects of laser modes in laser materials processing

All laser materials processing involves two fundamental phenomena: the conversion of optical energy into thermal energy, and the generation of thermal stresses in the laserprocessed materials. The distributions of the thermal energy and stresses in the substrate depend on the laser mode used for materials processing. A proper understanding of the temperature and stress distributions is essential for producing high-quality products using the laser technology. The solution of the complete thermoelastic problem is very complex and requires numerical methods. This paper presents simple analytic expressions for the temperature and thermal stress distributions in the substrate processed with TEM₀₀ and TEM _inf01 ^sup* mode laser beams. The temperature is found to be maximum at the point that encounters the maximum laser intensity. The thermal stress distributions are found to be different owing to the differences in the laser modes used for this study.     

Relevancy of phase separation between electrical and thermal properties in La0.8Sr0.2MnO3 thin films

For the purpose of finding the relevancy of phase separation between electrical and thermal properties, La_0.8Sr_0.2MnO₃ thin films, deposited on quartz glass substrates, were studied for temperature dependent resistance and thermal emittance. Based on the phase separation concept, metal phase volume fraction f was calculated from the temperature dependence of resistance and emittance properties by a phenomenological model and two-energy-level Boltzmann distribution, respectively. The two sets of f coincide with each other very well. The results show f plays an important role in the electrical transportation and thermal properties, and the two properties are essentially correlated by f.     

基于波导口的新型Ka频段超导滤波器设计

传统的Ka频段高温超导滤波器设计都是基于同轴接口．本文首次提出了一种新型的波导接口的Ka频段高温超导滤波器,滤波器与波导微带转换集成设计．设计中利用参数扫描技术克服了由于介质非均匀性而引起的频率偏移问题．测试结果表明本文提出的方案可以显著改善滤波器的性能．相对传统的基于同轴接口的设计方案,本文提出的方案可以显著改善滤波...     

The effect of sintering temperature on the intergranular properties of Bi2223 superconductors

A systematic study of the intergranular properties of (Bi,Pb)₂Sr₂Ca₂Cu₃O_y (Bi2223) polycrystalline samples has been done using the electrical resistivity and Ac susceptibility techniques. In this project, we have prepared a series of Bi2223 samples with different sintering temperature. The XRD results show that by increasing sintering temperature up to 865 °C the Bi2212 phase fraction decrease. It was found that the Bi2212 phase on the grain boundaries is likely to play the role of weak links and consequently reduces the intergranular critical current densities. Analysis of the temperature dependence of the Ac susceptibility near the transition temperature (T_c) has been done employing Bean's Critical State Model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the weak links are changed from superconductor–normal metal–superconductor (SNS) for well-coupled sample to superconductor–insulator–superconductor (SIS) type of junctions for the sample with high Bi2212 phase fraction.     

Enhancement of the upper critical field of MgB2 by carbon-doping

We have measured the temperature dependence of the upper critical field, H_c2(T), of carbon-doped MgB₂. H_c2(T) does not follow the well-known Werthamer-Helfand-Hohenberg (WHH) result for a one-gap dirty superconductor but can be described well by the result of a recent theoretical calculation for a two-gap dirty superconductor. H_c2(0) of the carbon-doped material is determined to be between 29 and 38 T, substantially higher than that of pure MgB₂ (15-23 T).     

Finite element simulation of stress distribution and development in 8YSZ and double-ceramic-layer La2Zr2O7/8YSZ thermal barrier coatings during thermal shock

In this paper, the thermal stress of the double-ceramic-layer (DCL) La₂Zr₂O₇/8YSZ thermal barrier coatings (TBCs) fabricated by atmospheric plasma spraying (APS) during thermal shock has been calculated. The residual stress of the coating after being sprayed has been regarded as the initial condition of the first thermal cycle. The characteristic of the stress development during the thermal cycle has been discussed, and the influence of the defects on the failure mode during the thermal cycle has also been discussed systematically. Finite element simulation results show that there exist higher radial thermal shock stresses on the ceramic layer surface of these two coatings. There also exist higher thermal stress gradient at the interface between the ceramic layer and the metallic layer. Higher thermal stress in 8YSZ/NiCoCrAlY coating lead to the decrease of thermal shock property as compared to that of LZ/8YSZ/NiCoCrAlY coating. The addition of LZ ceramic layer can increase the insulation temperature, impede the oxygen transferring to the bond coating and can also reduce the thermal stress. Considering from the aspects of thermal insulation ability and the thermal shock resistance ability, DCL type LZ/8YSZ TBCs is a more promising coating material compared with the single-ceramic-layer (SCL) type 8YSZ TBCs for the application.     

Electronic structure and Fermi surface of new K intercalated iron selenide superconductor KxFe2Se2

Using the ab initio FLAPW-GGA method we examine the electronic band structure, densities of states, and the Fermi surface topology for a very recently synthesized ThCr₂Si₂-type potassium intercalated iron selenide superconductor K_xFe₂Se₂. We found that the electronic state of the stoichiometric KFe₂Se₂ is far from that of the isostructural iron pnictide superconductors. Thus the main factor responsible for experimentally observed superconductivity for this material is the deficiency of potassium, i.e. the hole doping effect. On the other hand, based on the results obtained, we conclude that the tuning of the electronic system of the new K_xFe₂Se₂ superconductor in the presence of K vacancies is achieved by joint effect owing to structural relaxations and hole doping, where the structural factor is responsible for the modification of the band topology, whereas the doping level determines their filling.     

Phase transitions in hybrid SFS structures with thin superconducting layers

Calculations of critical temperature T _c of the phase transition to superconducting state of a superconductor/ ferromagnet/superconductor (SFS) hybrid structure with proximity effect is performed on the base of linearized Usadel equations. It is shown that the proximity effect between S and F metals and the exchange interaction can induce an inhomogeneous superconducting state with longitudinal to layers Δ _∝ exp(ipz) modulation of the superconductivity order parameter, which is characterized by nonzero value of the wave number p, describing the Larkin–Ovchinnikov–Fulde–Ferrell instability. Influence of this instability on transitions between 0- and π-states of the SFS structure is studied. It is shown that the 0–π transition is accompanied by a nonmonotonic dependence of both the critical temperature T _c and the effective penetration depth Λ of the magnetic field into the hybrid structure on the characteristic size of the ferromagnetic region.     

Surface electronic structure of the organic superconductor κ-(ET)2Cu(NCS)2 studied via photoemission microscopy

The surface electronic structure of cleaved single crystals of the organic superconductor κ-(ET)₂Cu(NCS)₂ has been studied using photoemission microscopy. Two types of cleaved surfaces were observed, displaying different valence band photoemission spectra and different spectral behavior near the Fermi level, E_F. In particular, spectra from one surface type display relatively broad spectral features in the valence band and finite spectral intensity at E_F, while spectra from the other surface type show well-defined valence band emission features and zero photoemission intensity at E_F. We propose that the spectral differences are due to a very short electron mean free path in this material, and our results are used to explain the differences between previously published photoemission spectra from this superconductor. We also report the results of an investigation of the electronic structure of defects in this material.     

Structural and spectroscopic studies of Lu2O3/Eu nanocrystallites embedded in SiO2 sol-gel ceramics

Preparation and optical properties of Lu₂O₃/Eu³⁺ nanocrystals embedded in silica gel glass are described. The structure of the obtained ceramics was determined by XRD and transmission electron microscopy. It was found that with increasing sintering temperature the Lu₂O₃/Eu³⁺ nanocrystallites underwent transformation into monoclinic Lu₂Si₂O₇ nanocrystallites. The fluorescence properties were investigated as a function of thermal treatment of the samples. It was found that after sintering at 1250 °C the fluorescence lifetime increased to 1.5 ms at room temperature. The material studied in this work exhibits a very high intensity of fluorescence with potentials for display application.