Xianghong Liu

Fabrication and Superconducting Properties of Rod‐In‐Tube Multifilamentary Nb 3 Al Wire With Rapid Heating and Quenching Heat‐Treatment

In this paper, we reported the recent progress in developing practical kilometer-length rod-in-tube (RIT) multifilamentary Nb₃Al superconducting wires at WST, which included the preparation of Cu/Nb-Al precursor wires, design and manufacture of rapid heating and quenching (RHQ) equipment, and RHQ heat-treatment process of Nb₃Al wires. Superconducting properties and microstructure of the RIT Nb₃Al wires have been discussed. By measuring magnetization and transport critical current properties of Nb₃Al wires, it is found that, for all the samples, the onset superconducting transition temperature T_c reaches 16.8-17.3 K, and at 4.2 K and 15 T, the critical current density J_c of 830 A/mm² at the Nb₃Al wires have been achieved. This paper suggests that it is possible to develop the practical Nb₃Al superconducting wires by using the RIT Nb₃Al precursor wires, in combination with RHQ heat-treatment process.

Optimization of Bronze Processed

Operated under high magnetic field (>;11 T), the toroidal field (TF) system consists of complex conductors using Nb3Sn as superconducting material. According to the strand requirements for the ITER TF conductor, bronze processed Nb3Sn strands with high superconducting performance have been manufactured using high tin content bronze at WST. The studies on filament diameter, diffusion barrier, sample heat treatment and annealing temperature have been carried out to enhance Nb3Sn strand properties. The results of these investigations have been presented. The result indicates that the different filament diameter has a weak influence on the critical current (Ic) value for the different reaction degree. In the manufacturing process the annealing temperature should be kept below 500°C to realize a high n value. The continuous Nb3Sn layer formation on the internal surface of the barrier causes a considerable increase of hysteresis loss when single Nb barrier is used. There is almost no difference of Ic, hysteresis loss and grain morphology for two heat treatments: ITER heat treatment cycle B and WST heat treatment cycle.

{\hbox {Nb}}_{3}{\hbox {Sn}}

The NbTi superconducting wires are used for the Poloidal Field (PF) coil of International Thermonuclear Experimental Reactor (ITER). Western Superconducting Technologies Co., Ltd. (WST) has produced conventional NbTi superconducting strand by single stacking for ITER. The final strands has been heat-treated with different duration. The properties of the NbTi strands meet the requirements to ITER PF strand specification. The diameter of the strand is 0.73 mm and filament diameter is 7.8 . The multifilament composite strand is made up of Nb-Ti filaments embedded in a high purity Cu matrix. This paper presents the results of comparative investigations on critical currents(Ic), critical current density(Jc), AC losses of the strands.

Strand for ITER at WST

The multifilamentary Nb3Sn strands with a diameter of 0.82 mm and different local area ratios of Nb to copper (LAR) have been successfully fabricated by an internal tin process in Western Superconducting Technologies Co., Ltd. (WST). The heat treatments have been completed in a vacuum condition. The Critical Current (Ic) and hysteresis loss of the strands are measured after different heat treatments according to ITER requirement. The maximum Ic is about 285 A (4.22 K, 12 T, 0.1 ¿V/cm) and the hysteresis loss is 290 ~ 410 mJ/cm3 (4.22 K, ±3 T) per strand unit volume. In this paper, the Ic and the hysteresis loss of the strands with different LARs and different heat treatments are investigated. It is a reference for fabricating the qualified Nb3Sn strands for ITER project in large scale in WST.

Development of Fine Filament NbTi Superconducting Strands for ITER

As one of low temperature superconducting materials, multifilamentary Nb₃Sn strand is an ideal choice for high-field superconducting magnets (>10 T). The International Thermonuclear Experimental Reactor (ITER) has special requirements for the Nb₃Sn strands: high critical current density and low hysteresis loss. Multifilamentary Nb₃Sn strand with high performance has been developed by bronze process. The highest non-Cu J_cn(12 T, 4.22 K, 0.1 ¿V/cm) value of 812 A/mm² has been obtained. The hysteresis loss of the strand is 380 mJ/cm³ with 650°C ×100 h. The properties of the Nb₃Sn strand can meet the specifications proposed by the ITER project.

Investigation of Superconducting Properties of

The recrystallization behavior of deformed Ti40 alloy during a heat-treatment process was studied using electron backscatter diffraction and optical microscopy. The results show that the microstructural evolution of Ti40 alloy is controlled by the growth behavior of grain-boundary small grains during the heating process. These small grains at the grain boundaries mostly originate during the forging process because of the alloy’s inhomogeneous deformation. During forging, the deformation first occurs in the grain-boundary region. New small recrystallized grains are separated from the parent grains when the orientation between deformation zones and parent grains exceeds a certain threshold. During the heating process, the growth of these small recrystallized grains results in a uniform grain size and a decrease in the average grain size. The special recrystallization behavior of Ti40 alloy is mainly a consequence of the alloy’s high β-stabilized elemental content and high solution strength of the β-grains, which partially explains the poor hot working ability of Ti–V–Cr-type burn-resistant titanium alloys. Notably, this study on Ti40 burn-resistant titanium alloy yields important information related to the optimization of the microstructures and mechanical properties.

{\rm Nb}_{3}{\rm Sn}

ITER toroidal field (TF) systems consist of 18 independent coils that are around the torus, whose primary function is to confine the plasma particles. The TF coil conductor is a cable-in-conduit conductor (CICC) made up of superconducting, Nb3Sn-based strands mixed with pure copper strands. As the only supplier in China, Western Superconducting Technologies Company, Ltd. (WST) will supply TF Nb3Sn strands using internal tin route for ITER, and over 6,000 km of Nb3Sn strands have been produced in the past four years. Main performance of Nb3Sn strands, including critical current, n value, wire diameter, Cu/non-Cu ratio, hysteresis loss and RRR are reported and analyzed in this paper.

Strands by Internal Tin Process for ITER

We report on the study of (Bi,Pb)2Sr2Ca2Cu3Ox(Bi-2223) precursor powder prepared by oxalate coprecipitation, which mainly focus on the process parameters and heat treatment for large-quantity preparation. Three kilograms of powder with a high homogeneity and an ideal composition can be prepared in one batch by controlling the pH value (2.9) and the aging time (2 days) of mixed solutions. Meanwhile, the powder has a particle size of less than 3 μm and a narrow size distribution. The different phase composition and Bi-2212 structure can be obtained with different heat treatment temperatures and atmospheres. To illuminate the properties of the precursor powder, a 19-filament tape was fabricated using a precursor powder calcined at 800 °C/20 h and 820 °C/20 h in air. A superconducting transition temperature Tc of up to 110 K was obtained using a thermomechanical treatment based on two heat treatments and one intermediate rolling. Our results show that it can be applied in a large-quantity preparation of Bi-2223 precursor powder.

Investigation of

Three kinds of Nb3Sn strands with different designs were made by bronze route. Studies were carried out on bronzeto-NbTa-volume ratio, filament diameter, annealing, hot isostatic pressure process, diffusion barrier, and heat treatment. It is important for strand designs to achieve a good performance. Moreover, bronze-to-NbTa-volume ratios of 2.4 and 2.8 were applied in the strand designs. Strands with lower bronze ratio means that more Nb3Sn layer fraction formed, while, at the same time, it is provided with a high critical current density (Jc). Jc increases with the reduction of filament diameter, when there is the same bronze ratio. In addition, Nb and Ta were used as diffusion barrier materials. A strand that used Nb barrier has several times hysteresis loss to a strand that used Ta barrier. When it comes to the intermediate processes, such as annealing and hot isostatic pressure, they could affect the homogeneous deformation and properties of strands. It is important to keep a low annealing temperature to realize a high Jc and n-value. As for the hot isostatic pressure process, it is helpful for uniform cross section. Furthermore, different heat treatments were also carried out, while analysis was also conducted on the microstructure.microstructure.

{\rm Nb}_{3}{\rm Sn}

Multistep heat treatments are required to produce the superconducting Nb3Sn in the International Thermonuclear Experimental Reactor toroidal field coils; however, deviations in the temperature and dwell time during heat treatment of the big conductors are unavoidable, and these could affect the performance of the Nb3Sn strands. To investigate the influence of heat treatment tolerances, both internal-Sn- and bronze-process-type Nb3Sn strands were heat treated with different cycles. For the internal-Sn process strands, the critical current density Jcn increases as the temperature increases from 630 °C to 650 °C and remains unchanged at 670 °C for 100 h. The Sn content in the filament increases with increasing temperature, and the grain sizes significantly increase from an average of 130-202 nm from 630 °C to 670 °C. For both the internal-Sn process strands and bronze route strands, Jcn seldom changes when the duration at 650 °C is increased from 100 to 200 h. Despite these changes, this study shows that Nb3Sn strands are not very sensitive to small heat treatment variations at 650 °C, and a variance of ±5 °C is acceptable for both types of Nb3Sn strands.