Fabrication of joint Bi-2223/Ag superconducting tapes with BSCCO superconducting powders by diffusion bonding

61-Filaments Bi-2223/Ag superconducting tapes have been successfully joined with BSCCO superconducting powder interlayer by diffusion bonding. The electrical properties of the diffusion bonding joints were tested by standard four probe method and the microstructures of the joints were also examined by SEM. Additionally, the phase constituents of the superconducting powders between the tapes before and after bonding process were evaluated by XRD analysis. The result shows that the diffusion bonding joints are superconductive. The microstructures of the joint display a good bonding with no cracks and discontinuities. The joining zones are mainly composed of Bi-2223 phase, Bi-2212 phase and a small amount of CuO, Ca₂PbO₄. At last, the phase transformations of the superconducting powders in the bonding process are discussed.     

Diffusion of nickel in single- and polycrystalline Cr2O3

Chromia protective layers are formed on many industrial alloys to prevent corrosion by oxidation. Their role is to limit the inward diffusion of oxygen and the outward diffusion of cations. A number of chromia-forming alloys contain nickel as a component, such as steels, FeNiCr and NiCr alloys. To ascertain if chromia is a barrier to outward diffusion, nickel diffusion in chromia was studied in both single crystals and polycrystals in the temperature range 900–1100°C at an oxygen pressure of 10^?4 atm (argon + 100 ppm O₂). A nickel film of ～35 nm thick was deposited on the chromia surface and, after diffusing treatment, nickel penetration profiles were established by secondary ion mass spectrometry (SIMS). Two diffusion domains appear in polycrystals, the first domain is assigned to bulk diffusion and the second is due to diffusion along grain boundaries. For the bulk diffusion domain and diffusion in single crystals, using a solution of Fick's second law for diffusion from a thick film, bulk diffusion coefficients were determined at 900 and 1000°C. At the higher temperature, a solution of Fick's second law for diffusion from a thin film could be used. For the second domain in polycrystals, Le Claire's model allowed the grain boundary diffusion parameter (αD _gb δ) to be established. Nickel bulk diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain boundary diffusion is slightly greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Nickel diffusion was compared with cationic self-diffusion and with literature data on Fe and Mn heterodiffusion in the bulk and along grain boundaries. All results were analyzed in relation to the oxidation process of stainless steel.     

Rapid nickel diffusion in cold-worked carbon steel at 320–450 °C

The diffusion coefficient of nickel in cold-worked carbon steel was determined with the diffusion couple method in the temperature range between 320 and 450 °C. Diffusion couple was prepared by electro-less nickel plating on the surface of a 20% cold-worked carbon steel. The growth in width of the interdiffusion zone was proportional to the square root of diffusion time to 12,000 h. The diffusion coefficient (D_Ni) of nickel in cold-worked carbon steel was determined by extrapolating the concentration-dependent interdiffusion coefficient to 0% of nickel. The temperature dependence of D_Ni is expressed by D_Ni = (4.5 + 5.7/?2.5) × 10^?11 exp (?146 ± 4 kJ mol^?1/RT) m²s^?1. The value of D_Ni at 320 °C is four orders of magnitude higher than the lattice diffusion coefficient of nickel in iron. The activation energy 146 kJ mol^?1 is 54% of the activation energy 270.4 kJ mol^?1 for lattice diffusion of nickel in the ferromagnetic state iron.     

Interface microstructure and mechanical properties of laser welding copper–steel dissimilar joint

Relatively the high reflectivity of copper to CO₂ laser led to the difficulty in joining copper to steel using laser welding. In this paper, a new method was proposed to complete the copper–steel laser butt welding. The scarf joint geometry was used, i.e., the sides of the copper and steel were in obtuse and acute angles, respectively. During the welding process, the laser beam was fixed on the steel side and the dilution ratio of copper to steel was controlled by properly selecting the deviation of the laser beam. The offset of laser beam depended on the scarf angle between the copper and steel, the thickness of plate and the processing parameters used in the laser welding. The microstructure near the interface between Cu plate and the intermixing zone was investigated. Experimental results showed that for the welded joint with high dilution ratio of copper, there was a transition zone with numerous filler particles near the interface. However, if the dilution ratio of copper is low, the transition zone is only generated near the upper side of the interface. At the lower side of the interface, the turbulent bursting behavior in the welding pool led to the penetration of liquid metal into Cu. The welded joint with lower dilution ratio of copper in the fusion zone exhibited higher tensile strength. On the bases of the microstructural evaluation at the interface of the welded joint, a physical model was proposed to describe the formation mechanism of the dissimilar joint with low dilution ratio of copper.     

Synthesis,structure, and magnetic properties of iron and nickel nanoparticles encapsulated into carbon

Nanocomposites based on iron and nickel particles encapsulated into carbon (Fe@C and Ni@C), with an average size of the metal core in the range from 5 to 20 nm and a carbon shell thickness of approximately 2 nm, have been prepared by the gas-phase synthesis method in a mixture of argon and butane. It has been found using X-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy that iron nanocomposites prepared in butane, apart from the carbon shell, contain the following phases: iron carbide (cementite), α-Fe, and γ-Fe. The phase composition of the Fe@C nanocomposite correlates with the magnetization of approximately 100 emu/g at room temperature. The replacement of butane by methane as a carbon source leads to another state of nanoparticles: no carbon coating is formed, and upon subsequent contact with air, the Fe₃O₄ oxide shell is formed on the surface of nanoparticles. Nickel-based nanocomposites prepared in butane, apart from pure nickel in the metal core, contain the supersaturated metastable solid solution Ni(C) and carbon coating. The Ni(C) solid solution can decompose both during the synthesis and upon the subsequent annealing. The completeness and degree of decomposition depend on the synthesis regime and the size of nickel nanoparticles: the smaller is the size of nanoparticles, the higher is the degree of decomposition into pure nickel and carbon. The magnetization of the Ni@C nanocomposites is determined by several contributions, for example, the contribution of the magnetic solid solution Ni(C) and the contribution of the nonmagnetic carbon coating; moreover, some contribution to the magnetization can be caused by the superparamagnetic behavior of nanoparticles.     

Phases and reactive-diffusion kinetics for mixed Al-Ni powders

Reactive diffusion has been examined for mixtures of nickel and aluminum powders and also Ni-Al bimetal. An intermediate layer is formed at the boundary between particles, and the phase sequence in this in the direction from nickel to aluminum is NiAl-Ni₂Al₃-NiAl₃. The growth of this intermediate layer under isothermal conditions obeys the law xⁿ=gk(t–t₀), with n=3 for a mixture of the powders and n=2 for the bimetal. The energies of formation for the compounds NiAl, Ni₂Al₃, NiAl₃ are large, so the reactive diffusion in the powder mixtures is accompanied by considerable exothermic effects.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 34–40, November, 1973.     

Influence of high-energy ball milling on the solid-phase sintering kinetics of ultrafine-grained heavy tungsten alloy

The mechanisms for the sintering of ultrafine-grained 95%W–3.5%Ni–1.5%Fe heavy tungsten alloy powders have been investigated. It has been established that a decrease in the activation energy of grain boundary diffusion and the formation of a nonequilibrium solid solution of nickel and iron in the surface layer of tungsten particles upon high-energy ball milling are responsible for the decrease in optimal sintering temperature.     

Evaluation of Nanostructured coating layers formed on Ni balls during mechanical alloying of Cu powder

A novel coating method utilizing mechanical alloying was used to deposit a nickel-copper solid solution on the surface of nickel balls in a planetary ball mill. Mechanically milled copper powders were deposited by cold welding on the activated surface of the nickel balls. The impact of the nickel balls causes surface wear, leading to formation of mixed layered particles of copper-nickel and rewelding. In addition, diffusion takes place through the coating layer to form a Ni-Cu solid solution. The hardness of the coating reached a threefold increase (HV_0.01594) in comparison with the substrate hardness. Microstructural characterization of the coating surface performed using an optical microscope, SEM, and EPMA indicates that, with appropriate processing conditions, a thick, fully dense coating can be metallurgically bonded to the nickel balls. XRD results revealed the presence of a solid solution and nanocrystalline structure.     

Residual stresses in ceramic-to-metal joints: diffraction measurements and finite element method analysis

The residual stress (RS) field in ceramic–metal diffusion bonds has been studied by spatial-resolved neutron strain scanning. Strain profiles were directly determined by neutron diffraction along selected lines perpendicular to the bonding interface of cube-shaped Ni/Si₃N₄ specimens. Finite element method (FEM) calculations were carried out to simulate the joining process and residual strains have been obtained among the whole body of specimens. The simulations were validated by comparison with the experimental strain data obtained by neutron and (previous) X-ray diffraction along some particular line of the specimen. Finally, the RS field across the whole sample was obtained from the FEM-calculated strain field, showing that neutron strain scanning combined with FEM analysis is a very useful technique to study the RS map in silicon nitride–metal diffusion bonds at both sides of the joining interface. Maxima of the axial stress were found at the lateral surface and close to the joining interface, being tensile for both ceramic and nickel. On the other hand, the largest radial stress at the joining interface was found at the centre of the specimen on the ceramic side. From the point of view of FEM analysis, it is shown that in order to simulate the joining process of nickel and silicon nitride, nickel must be considered as a ductile material having strain hardening and Si₃N₄ must be considered as purely elastic material having a nearly temperature-independent elastic modulus.     

Ultrasonic-accelerated metallurgical reaction of Sn/Ni composite solder: Principle,kinetics, microstructure,and joint properties

The high-melting-point joints by transient-liquid-phase are increasingly playing a crucial role in the die bonding for the high temperature electronic components. In this study, three kinds of Sn/Ni composite solder pastes composed of different sizes of Ni particles were synthesized to accelerate metallurgical reaction among Sn/Ni interfaces under the ultrasonic-assisted transient liquid phase (U-TLP) soldering. The temperature evolution, microstructure and mechanical property in joints composed by these composite solder pastes with or without ultrasonic energy were systemically investigated. The intermetallic joint consisted of high-melting-point sole Ni₃Sn₄ intermetallic compound with a little residual Ni was obtained under the conditions of no pressure and lower power (200 W) in a high-temperature duration of only 10 s, its shear strength was up to 45.3 MPa. Ultrasonic effects significantly accelerated the reaction among the interfaces of liquid Sn and solid Ni, which attributed to the temperature rise caused by acoustic cavitation because of large number of liquid/solid interfaces during U-TLP, resulting in accelerated solid/liquid interfacial diffusion and growth of intermetallic compounds. This intermetallic joint formed by U-TLP soldering has a promising potential for applications in high-power device packaging.     

Ba2CaWO6-Sr2CaWO6赝二元系的研究

用差热分析、X射线物相分析和点阵常数的精确测定,研究了Ba₂CaWO₆-Sr₂CaWO₆赝二元系的相平衡和相变。发现在高温(860℃以上)时,形成连续固溶体。在室温时,在富Ba₂CaWO₆一侧,形成以Ba₂CaWO₆为基的固溶体;在富Sr₂CaWO₆一侧,形成以Sr₂CaWO₆为基的固溶体,相变点在成分为Ba₂CaWO:Sr₂CaWO₆=25:75(克分子比)处。同时研究了作为高压钠灯灯丝涂层材料的Ba_xSr_2-xCaWO₆的几个重要性质:电子发射和多次启动时的灯压升高等。发现成分为BaSrCaWO₆时,性质最好。 关键词：     

Acceleration of nickel diffusion by high tensile stress in cold-worked type 316 stainless steel at 450°C

The effect of tensile stress on diffusion was studied by the diffusion couple method. A diffusion couple was prepared by electroless plating a nickel thin layer on the round notch surface of a compact tension-type specimen of 20% cold-worked Type 316 stainless steel. The couple was diffused at 450°C for 4003?h under the maximum tensile stress of 553?MPa in the load direction. A rapid diffusion coefficient of nickel in the Type 316 stainless steel was observed at the high tensile stress zone that was 6.5 times faster than that at the low-stress zone.     

Reaction diffusion and interdiffusion in some binary metallic systems

This paper reviews studies from the Münster laboratory on multiphase and interdiffusion performed on several binary metallic systems. Optical microscopy and electron microprobe analysis were used to study interdiffusion and growth of the intermetallic layer(s) in infinite diffusion couples. Both methods have resolutions at the micron level. Thus, the results summarised in this paper concern the diffusion-controlled regime of the reaction diffusion process. We first remind the reader of some of the basics of multiphase diffusion and interdiffusion. Then we discuss results reported for the following systems: cobalt–niobium, nickel–niobium, nickel–aluminium and magnesium–aluminium. In the case of Co₂Nb we also compare interdiffusion and tracer diffusion of the components. Multiphase diffusion experiments also contain information about solid–solid equilibria of the phase diagram(s).     

Superconducting joint of Bi-2223/Ag superconducting tapes by diffusion bonding

61-Filaments Bi-2223/Ag superconducting tapes have been joined by diffusion bonding. The critical currents (I_Cs) of the joints are obtained by using standard four probe method under no magnetic field in the liquid nitrogen. The microstructures of the joints are evaluated by the electron microscope in electron backscatter diffraction mode and the phase compositions of the superconducting cores of the joint and the original tape are determined by X-ray diffraction (XRD). The results show diffusion bonding is effective bonding technique for HTS tapes, and the bonding time is reduced greatly from hundreds of hours to a few hours, and the bonding pressure also changes from 140–4000 MPa to 3 MPa. Furthermore, the diffusion bonding joints sustain superconducting properties, and the critical current ratios (CCR_O) of the joints are in the range of 35%–80%. Microstructures of the typical joint display a good bonding and some defects existed in traditional method are avoided. XRD results show that the phase compositions of the superconducting cores have no obvious changes before and after diffusion bonding, which offers physical and material bases for high superconducting property of the joints.     

Formation of MgO nanorods in the reaction zone of a Mg–CuO powder mixture by in-situ reaction

We report the in-situ formation of MgO nanorods during sintering of a Mg–20?wt%?CuO powder mixture at 450°C. After sintering, we identified three regions with distinct microstructures in the reaction zone between the Mg grain and the newly formed Cu grain. Region I contained MgO nanorods and Cu nanoparticles, region II was composed of MgO nanorods, while larger nanometre-sized MgO crystals were found in region III. The MgO nanorods were single crystals with a diameter of about 20?nm and a length of about 100?nm. The growth of these nanorods was controlled by the vapour–solid mechanism. The progressive change in morphology of the MgO phase had induced a hardness gradient across the reaction zone. As a result, the interfacial bonding between the major phases in the sintered product was enhanced.     

Nanocrystalline FeAl intermetallics obtained in mechanically alloyed Fe50Al40Ni10 powder

B2-Fe₄₇Al₅₃ intermetallics has been produced by mechanical alloying in a planetary ball mill, using elemental Fe, Al and Ni powder mixture. The microstructural and magnetic properties of the mechanically alloyed Fe₅₀Al₄₀Ni₁₀ powdered samples were investigated by X-ray diffraction and ⁵⁷Fe Mössbauer spectrometry at 300 and 77 K. As resulted from the X-ray diffraction studies, the ordered B2 structure was formed in the Fe₅₀Al₄₀Ni₁₀ powder, together with the bcc α_i-Fe(Al, Ni) (i = 1, 2) solid solutions. Further milling led to a partial disordering of B2-Fe₄₇Al₅₃; it has undergone an order–disorder transition which is characterized by an expansion of the volume Δa₀ (lattice disorder) and a magnetic transition from the paramagnetic to ferromagnetic state which is characterized by strong ferromagnetic interactions in the alloy. The nanocrystalline bcc α_i-Fe(Al, Ni) solid solution was ferromagnetic with a mean crystallite size of 6 nm.     

The influence of aliovalent impurities on the oxidation kinetics of nickel at high temperatures

The influence of chromium and sodium on the nickel oxidation kinetics has been studied as a function of temperature (1373-1673 K) and oxygen activity (10−10⁵ Pa O₂), using microthermogravimetric techniques. It has been shown that the oxidation of Ni-Cr and Ni-Na alloys, like that of pure nickel, follows strictly the parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni_1−yO, it has been found that the oxidation rate of the Ni-Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of double ionized cation vacancies in a Ni_1−yO-Cr₂O₃ solid solution is fixed on a constant level by trivalent chromium ions, substitutionally incorporated into the cation sublattice of this oxide. In the case of the Ni-Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of the doping effect, by assuming that univalent sodium ions dissolve substitutionally in the cation sublattice of nickel oxide.     

Factors affecting the microstructure and mechanical properties of Ti-Al3Ti core-shell-structured particle-reinforced Al matrix composites

This work studied the effects of matrix powder and sintering temperature on the microstructure and mechanical properties of in situ formed Ti–Al₃Ti core–shell-structured particle-reinforced pure Al-based composites. It has been shown that both factors have significant effects on the morphology of the reinforcements and densification behaviour of the composites. Due to the strong interfacial bonding and the limitation of the crack propagation in the intermetallic shell during deformation by soft Al matrix and Ti core, the composite fabricated using fine spherical-shaped Al powder and sintered at 570 °C for 5 h has the optimal combination of the overall mechanical properties. The study provides a direction for the optimum combination of high strength and ductility of the composites by adjusting the fabrication parameters.     

Synthesis and structure refinement studies of LiNiVO4 electrode material for lithium rechargeable batteries

The lithium nickel vanadate (LiNiVO₄) cathode material has been synthesized by using sol-gel method. The thermal behavior of the material has been examined by thermogravimetric and differential thermal analysis (TG/DTA). The structure of LiNiVO₄ compound has been studied by the Rietveld refined x-ray diffraction (XRD) technique. The Brunauer–Emmett–Teller (BET) surface area of 0.79 m² g^?1 was estimated with N₂ absorption characteristics. The synthesized powder morphology was observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) studies of synthesized LiNiVO₄ powder indicate that the oxidation states of nickel and vanadate are +2 and +5, respectively. The electrochemical properties were monitored using 2032 coin cells by cyclic voltammetry and EIS, which showed that the microscopic structural features were deeply related with the electrochemical performance.     

Metal ion release and surface composition of the Cu-18Ni-20Zn nickel-silver during 30 days immersion in artificial sweat

In order to study nickel ion release associated with nickel allergy, Cu-18Ni-20Zn nickel-silver alloy was immersed in artificial sweat and Ringer physiological solution for 30 days. Dissolution of metal ions was measured as a function of time, and the characteristics of the solid surface layer formed after 30 days were studied by SEM/EDS and XPS. The dissolution of nickel prevails over dissolution of copper and zinc. Nickel release in artificial sweat is approximately 10 times higher than in Ringer physiological solution and in both solutions the nickel release exceeds 0.5 μg cm⁻² week⁻¹, the threshold above which the allergy is triggered. Evidence of selective nickel dissolution is reported. The composition of the surface layer formed in artificial sweat and in Ringer physiological solution differs in the content of nickel and chlorine. In artificial sweat, the major constituents of the surface layer are dominantly oxides, Cu₂O and ZnO, with traces of chlorine. In Ringer physiological solution, the composition of the surface layer changes to a mixture of oxides, chlorides and/or oxychlorides. Two components peaks were detected in the Cl 2p_3/2 peak; however, it was not possible to distinguish the exact nature of the chloride compound formed. The mechanism of nickel release is discussed as a function of the composition of the solution.