Stress analysis of ceramic insulation coating on Cu/MgB2 wires for W&R MgB2 coils

Ceramic insulation coatings were produced on Cu/MgB₂ wires, which were fabricated by Hyper Tech Research Inc., using Continuous Tube Forming and Filling (CTFF) process, from the solution of Zr, and Y based organometalic compounds, solvent and chelating agent using reel-to-reel sol–gel technique for MgB₂ coils. Y₂O₃–ZrO₂/Cu/MgB₂ wires were annealed at 700 °C for 30 min with 5.8 °C/min heating rate under 4% H₂–Ar gas flow. Residual stresses were examined for Cu/MgB₂ wire and YSZ coatings with varying thicknesses. It was observed that displacement values are independent from YSZ thicknesses and the maximum effective stress value is in the Cu region. The surface morphologies and microstructure of samples were characterized using SEM. SEM micrographs of the insulation coatings revealed cracks, pinholes and mosaic structure.     

EDX and ion beam treatment studies of filamentary in situ MgB2 wires with Ti barrier

In situ SiC-doped filamentary MgB₂ wires (with the diameter of 0.860 and 0.375 mm) with Cu stabilization separated by Ti barrier layers supported by outer SS sheath and annealed at 800 °C/0.5 h have been studied by combination of EDX analysis and ion beam selective etching. It was found that several Ti-Cu inter-metallic compounds were created by Cu-Ti interdiffusion and thus the barrier protection against Cu penetration into the superconducting filaments is limited. We showed an advantage of Ti use as the barrier material in our wires. Ti getters silicon out from the superconducting filament, what purges superconducting MgB₂ from Si and creates an additional Si-rich layer in inner part of Ti barrier which prevents Cu diffusion more effectively.     

Superplastic deformation and crystallization behavior of Cu54Ni6Zr22Ti18 metallic-glass sheet

Superplastic deformation and crystallization behavior of a Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass were investigated. A maximum elongation of 650% was obtained at 733 K at 1 × 10⁻² s⁻¹ from the sheet fabricated by squeeze copper-mold casting method. At low strain rates, the strain-rate-sensitivity exponent value was close to 1, suggesting that Newtonian-like behavior governed the plastic flow. At a high strain rate around 10⁻² s⁻¹, a transition from Newtonian to non-Newtonian behavior took place with decrease in m value. Large strain hardening by crystallization occurred during the course of deformation. The strain hardening was found to be caused by crystallization according to the analyses of the relation of true stress vs. testing time, T-T-T diagram and DSC characteristics. The time periods up to the strain before strain hardening at 733 K for the Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass were similar to that of the Zr₆₅Al₁₀Ni₁₀Cu₁₅ metallic glass at 696 K as 180–300 s (3–5 min). This coincidence could be explained by comparison of their T-T-T diagrams showing that the incubation times for crystallization of the Cu BMG at 733 K and for Zr BMG at 696 K are similar.     

Application of controlled and electrical discharge assisted mechanical alloying for the synthesis of nanocrystalline MgB2 superconducting compound

In this paper we present the results of our efforts to synthesize the nanocrystalline MgB₂ superconducting compound from elemental Mg and B powders by combination of controlled mechanical pre-alloying in a magneto-mill Uni-Ball-Mill 5 under shearing mode followed by electrical discharge (ED) assisted mechanical alloying (MA). There is no conclusive evidence of MgB₂ formation in the Mg-2B mixture using crystalline boron after controlled mechanical alloying (CMA) under protective argon or helium atmosphere as well as subsequent ED assisted alloying. There seems to be some XRD evidence of the strongest (1 0 1) MgB₂ peak presence in the Mg-2B mixture processed using both crystalline and amorphous boron after CMA under hydrogen as well as subsequent ED assisted alloying but this evidence is rather ambiguous. We postulate here that it is highly likely that a certain critical Mg nanograin size must be achieved before a successful reaction to form nanocrystalline MgB₂ is going to be completed. Following recent report by Gümbel et al. [Appl. Phys. Lett. 80 (2002) 2725] this critical value can be roughly estimated at 15 nm or less. Calculations of the Mg nanograin size in the present work show that only three Mg-2B powders ball milled under hydrogen meet this critical nanograin size criterion for the Mg phase. However, a massive formation of the β-MgH₂ hydride in these powders consumes the available Mg in the reaction with hydrogen which may leave inadequate concentration of Mg to form MgB₂ even though the nanograin size of Mg is sufficiently refined, say below 15 nm.     

Effect of CrB2 addition on densification, properties and oxidation resistance of TiB2

This paper presents the results of detailed studies carried out on the densification of TiB₂ with CrB₂ as sinter additive by hot pressing. The dense compacts were characterized by measurement of hardness, indentation fracture toughness, flexural strength, coefficient of thermal expansion and electrical resistivity. Oxidation characteristics were investigated between 600 °C and 1000 °C and isothermal oxidation kinetics at 850 °C. Phase identification and surface morphology analysis of hot pressed and oxidized samples were done using XRD and SEM. A high density of 96.61% Τ.D was obtained with the addition of 2.5% CrB₂ by hot pressing at 1750 °C under 35 MPa pressure. Hardness values of composites with 2.5–10% CrB₂ were close to 24 GPa and fracture toughness in the range of 3–5 MPa m^1/2. Coefficient of thermal expansion of the composite with 10% CrB₂ was measured in the range of 6.21–7.43 × 10⁻⁶/K from room temperature to 1000 °C. Electrical resistivity of TiB₂ + 10%CrB₂ was measured as 32.83, 75.97 and 120 μΩ cm at 25 °C, 500 °C and 900 °C, respectively. Observed nature of oxidation was parabolic for all composites. Formation of continuous and thick glassy film was observed with increased CrB₂ content in the composite. TiO₂ and CrBO₃ phases were identified on the oxidized surface which are responsible for the improved oxidation resistance of this composite.     

Bi2Sr2CaCu2Ox bulk superconductor with MgO particles embedded

MgO nano-size particles in various concentrations were added to the Bi-2212 superconductor precursor before partial melting. The resulting ceramic superconductors had a transition sharpness and volume fraction which increased with MgO addition up to 9 wt.%. Compositional variations in the superconducting matrix with varying MgO addition were found, with possible important consequences for the pinning mechanism, since compositional variations are expected to be accompanied by variations in T_c. By TEM it was directly documented that MgO particles could be embedded within the superconducting Bi-2212 grains. Hence wetting occurs. Thus, the requirements for MgO to play an effective role in pinning are satisfied. It is suggested that two distinct pinning mechanisms, both related to MgO addition, are likely to be present. Magneto-optic images clearly confirm the improvement of pinning, and screening, with increasing addition of MgO to Bi-2212.     

Formation of zirconium diboride (ZrB2) by room temperature mechanochemical reaction between ZrO2, B2O3 and Mg

A mixture of magnesium, boric oxide and zirconium dioxide were mechanically milled under argon for up to 15 h in a laboratory scale ball mill. X-ray diffraction showed that there was an increasing conversion of ZrO₂ to ZrB₂ with milling time with >98% reaction after 15 h. Differential thermal analysis revealed there were multiple, overlapping reactions all of which seemed to be formation of ZrB₂. The energy evolved decreased with milling time and the sample after 15 h milling showed no thermal reaction. After milling, separation of the ZrB₂ from the coproduct MgO was easily achieved by a mild acid leaching leaving essentially pure ZrB₂ with a crystallite size of 75 nm.     

Achieving tensile superplasticity in 8 mol% Y2O3 cubic stabilized ZrO2 through the addition of intergranular silica

The addition of 5 wt.% SiO₂, a viscous second phase, to 8 mol% Y₂O₃ cubic stabilized ZrO₂ (8Y-CSZ) made superplastic 8Y-CSZ. This material had a fine grain size of 0.4 μm and exhibited deformations in tension as large as 520% at 1430 °C with a strain rate of 1.0 × 10⁻⁴ s⁻¹.     

Micro-indentation study of Mg-addition MgB2 superconducting wires

The Mg-added (0%, 5%, and 10%) MgB₂/Cu superconducting wires were prepared by the powder-in-tube (PIT) method. Vickers micro-indentation tests were performed on the samples with different peak loads at room temperature. The loading-unloading (P-h) curves were analyzed by the displacement approach to indentation. It was found that hardness (H) and the effective elastic modulus (E) values increased with Mg-added. In addition, these values showed peak load dependence (i.e. indentation size effect (ISE)).     

Cu/Y2O3掺杂MgB2的组织控制及动力学分析

观察并研究了不同Y2O3含量的Cu/Y2O3共掺杂MgB2的体系,样品通过传统的固相烧结法制备。结合扫描电镜和X射线衍射技术的分析发现,只有在5%Y2O3掺杂的样品中获得了层状组织。选择5%Y2O3含量的体系结合热分析曲线对固相反应阶段进行了动力学分析,得出最概然机理函数为Avrami-Erofeev,n=2,表达式为2[-ln(1-α)]1/2。其意义是随机形核和随后的瞬时生长。最后对动力学参数进行了计算,结果表明反应的活化能和指前因子均先趋于稳定后减小。     

Ce(Au,Sb)2 with UHg2 structure type, a new member of the AlB2 family

A new ternary compound Ce(Au,Sb)₂, with a homogeneity range has been observed from X-ray powder diffraction of as cast alloys, a = 4.743–4.712 Å, c = 3.567–3.768 Å. Its crystal structure was investigated by X-ray diffraction from Ce(Au_1−xSb_x)₂ (x = 0.266) single crystal: CAD-4 automatic diffractometer, Mo K radiation, a = 4.7256(6) Å, c = 3.6711(6) Å, P6/mmm space group, V = 70.997(17) Å³, Z = 1, ρ = 10.732 Mg/m³, μ = 76.369 mm⁻¹, R₁ = 0.0415, wR₂ = 0.0793 for 99 reflections with I > 2σ(I₀). The coordination polyhedron of X (X = 0.734Au + 0.266Sb) atom is a full-capped trigonal prism [XCe₆X₃X₂]. Ce atom is coordinated by 14 atoms: [CeX₁₂Ce₂]. The compound is isotypic with UHg₂ structure, a deformation derivative of AlB₂ structure type. It forms isostructural compounds with La and Pr.     

层状ZrB2基陶瓷的增韧机制分析

将传统陶瓷中的轧膜工艺应用到超高温陶瓷的制备上,制备ZrB2基层状复合陶瓷。根据层间应力需求设计材料组分。基体层组分为ZrB2+10vol%SiCp+10volSiCw;中间层组分为ZrB2+44.1vol%SiCp+37vol%MoSi2。复合陶瓷在1950℃,25MPa条件下热压烧结制成并进行机械性能测试及微结构观察。结果显示,复合陶瓷有较高的致密度,力学性能较纯ZrB2陶瓷有较大的提升。微观结构观察表明,由于层状结构的存在,裂纹在扩展过程中反复偏转,吸收了大部分能量,有效的提高了复合陶瓷的韧性。复合陶瓷的增韧机制为弱界面层对裂纹的偏折、裂纹分支、基体片层的破坏以及基体片层内部增韧的协同增韧。     

Solid-state properties of hot-pressed TiB2 ceramics

Due to the increasing industrial interest in TiB₂ the present work was accomplished to establish various solid-state properties of TiB₂ and to study the influence of sintering aids and different powder conditioning methods on the densification behaviour of TiB₂.

The powders were hot-pressed in graphite dies with various loads up to 45 MPa and the vertex temperature of 1800 °C was held for 1 h. For the pure, non-activated powders theoretical densities between 97.4% and 99.5% were obtained at a pressure of 45 MPa. Hot-pressed activated powders at this pressure led to densities of up to 99.9%. The addition of 0.5 wt.% of various sintering aids also increased the densities. It is shown that pre-alloying TiB₂ with CrB₂ is to favour over mixing the powders. If TiB₂ and the sintering aid were mixed the best result was obtained with Cr₂N. Ceramics with high Young’s moduli and hardnesses were obtained. The Poisson’s ratios of two samples were 0.08 and 0.09, respectively, which are the lowest known values for a ceramic hard material. The heat conductivity of pure TiB₂ is approximately 1/4 of that of copper and the electrical resistivity is only 6 times higher than that of copper.     

Microwave-assisted combustion synthesis in a mechanically activated Al-TiO2-H3BO3 system

The Al₂O₃-TiB₂ in-situ composite has been fabricated by different techniques. In this work, the mechanical activation process has been used to aid microwave-assisted combustion synthesis (MACS) to produce the Al₂O₃-TiB₂ in-situ composite. For this purpose, the thermite mixture of Al, TiO₂ and boric acid (H₃BO₃) powders was used as the raw materials, and was mechanically activated at different milling speeds. The results of X-ray phase analysis of the mechanically activated samples after combustion synthesis showed that the Al₂O₃-TiB₂ in-situ composite has been successfully fabricated by thermal explosion mode of combustion synthesis in microwave, while no combustion synthesis occurred for the unmilled sample. Also, it was found that by increasing the milling speed from 250 to 400 rpm, the purity of the final products has been increased; while further milling speed up to 550 rpm reduced the purity of the final products. The effects of milling speed were also studied by means of differential scanning calorimetry (DSC) measurements. It was shown that by increasing the energy level of the reactants via milling speed, the ignition temperature and the intensity of exothermic peaks in the DSC curves have been changed. Finally, in order to have a good understanding about the in-situ formation of such ceramic composites, a reaction mechanism was proposed based on the experimental results. The synthesized composite exhibited high microhardness value of about 1950 Hv in dense parts.     

MgB2/Mo多层膜的制备与超导性质的研究

本文采用磁控溅射技术(MS)和混合物理化学气相沉积法(HPCVD)在单晶Al₂O₃基底上制备MgB₂/Mo多层膜。通过扫描电子显微镜(SEM)、X射线衍射(XRD)和标准四线法对样品的表面形貌、晶体结构和超导特性进行了测量研究。实验结果表明随着后续MgB₂沉积温度的增加各膜层结晶程度进一步提高,晶粒尺寸不断增大,各自保持着良好的物质稳定性。在730℃温度下生长的MgB₂薄膜的超导转变温度T_con和零电阻温度T_c0分别为39.73K~39.53K,剩余电阻率~0.77μΩcm,表明样品处于干净极限。     

The crossovers of the resistance behaviors in the top and bottom surface layers of 2H-NbSe2

The critical currents and the resistances in the top and the bottom surface layers have been studied in 2H-NbSe₂single crystal using the flux transformer geometry. In the peak effect region, the critical currents and the resistances in the top and bottom surface layers have distinctly different values. These results are tentatively understood in terms of an order–disorder transition and have implications for the correlations of the vortex lattice throughout the sample.     

Characteristic and synthesis mechanism of MgB2 nanoparticles in solid-state reactive sintering

MgB₂ nanoparticles were synthesized by a one-step reactive sintering method. The sample was heated from room temperature to 994 K, and then directly cooled down at a rate of 40 K/min. The results of X-ray diffraction indicate that MgB₂ superconductors with high quality were successfully prepared, and only a few impurities were detected on it. The sample consisted of two distinguishable structures by the TEM observation: nanoparticles and single crystals, which is a result of nonequilibrium conversion under the direct cooling condition. The nanoparticles with a diameter of about 10–20 nm are considered to be at the onset for the formation of MgB₂ phase. Transition temperature (T_c) and critical current density (J_c) of the compound structures were determined to be 38.5 K and 1.8 × 10⁵ A cm⁻² by means of SQUID measurement, which indicate good superconducting properties. The inter-diffusion and dislocation incorporation mechanisms for the formation and growth of MgB₂ nanoparticles are also proposed, and the further growth will be accelerated due to an adequate holding time at 994 K.     

Microstructural modifications induced by hydrogen absorption in Mg5Ga2 and Mg6Pd

We have recently proposed a new method to design one-dimensional structures of MgH₂ in the nano- and micrometer ranges by hydrogen-induced disproportionation of bulk Mg₂₄Y₅. The present study confirms the same behavior in hydrogenated Mg₅Ga₂ and Mg₆Pd. Single-crystalline one-dimensional structures and microparticles of MgH₂ are formed by hydrogen absorption and subsequent partial disproportionation of Mg₅Ga₂ and Mg₆Pd. The MgH₂ whiskers and particles grow with different morphologies for different alloying partners. Growth mechanisms are proposed in relation to the morphology and the chemical surface composition of original compounds.     

Production of MgB2 superconducting coatings by electrophoresis on metal substrates

Production of MgB₂ coatings on various metallic substrates was achieved by means of the direct electrophoretic deposition technique. An inexpensive simple heat treatment in evacuated quartz tubes was developed as an alternative to inert gas flow during the process. The films were characterized by XRD, SEM and SQUID. It resulted that the procedure led to the production of uniform, dense and well-adhesive superconducting films. Stainless steel proved to be the best substrate among the investigated metals.