The pressed electrodes need to be sintered. The sintering process is divided into two stages: low-temperature dehydrogenation and high-temperature sintering, both carried out in a vacuum furnace. The initial stage of sintering is the dehydrogenation stage. As the temperature rises, hydrides decompose into nascent metals, resulting in fresh and highly reactive metal surfaces. After dehydrogenation, high-temperature sintering is performed. During high-temperature sintering, these highly reactive metals easily diffuse into each other to form various solid solutions with lower melting points. This helps eliminate high-melting-point infusible metal lumps in the ingot after smelting and improves uniformity.
The vacuum level for dehydrogenation is initially set at 10⁻⁴ Torr. The furnace temperature is gradually increased, and hydrogen gas is gradually released after 200°C. The temperature is held at 500-750°C until the vacuum reaches 10⁻⁴ Torr, at which point the dehydrogenation stage ends. The temperature is then further increased to 900-1600°C and held for 1-6 hours for high-temperature sintering. The sintered electrodes have high density, low gas content, and partial solid solutions have formed between the metals.
The sintered electrodes are then melted in one or two electric arc furnaces or plasma furnaces to obtain the alloy. The process for preparing niobium-based alloys using the method of this invention is simple and has a high metal utilization rate. In particular, direct hydrogenation of niobium ingots to produce powder reduces contamination from impurities such as oxygen, iron, and silicon. Before hydrogenation, the niobium ingots only require appropriate acid washing, making the process particularly simple. From niobium ingots to finished electrodes, the niobium utilization rate can reach over 80%. Direct batching and pressing of hydrogenated powder facilitates powder preservation and reduces gas absorption.
The niobium-based alloys produced using the manufacturing process of this invention have a uniform composition and essentially eliminate infusible lumps of high-melting-point metals. In a 30 kg Nb-50Ti alloy melted in one electric arc furnace, the titanium content fluctuates within the range of 1-1.5% by weight; in alloy ingots melted in two electric arc furnaces, the titanium content fluctuates within the range of <1% by weight. Micro-area compositional analysis of the alloy shows that the titanium content fluctuates between 2.5-4.6% by weight. This alloy fully meets the requirements for producing composite superconducting wires, and is particularly suitable for producing composite superconducting wires with core diameters less than 10 micrometers and composite superconducting wires for magnets requiring high magnetic field uniformity.
The electrodes produced by the process of this invention are very convenient for furnace loading and alignment; the electrodes have low outgassing during melting, making it easy to maintain the melting vacuum, resulting in a stable arc and less metal spatter.
Niobium ingots bombarded with secondary electrons are soaked in a mixed acid-water solution (HF∶HNO3∶H2O＝1∶1∶2) for about 15 minutes, rinsed with water, then rinsed with deionized water, dehydrated with alcohol, and dried. They are then placed in a stainless steel crucible and loaded into the furnace. The furnace is pre-vacuumed to 5×10⁻⁵ mmHg, purged three times with pure hydrogen, then charged with hydrogen (1-2 kg/cm²) and heated to 500-700°C, and then cooled to room temperature. After repeating the hydrogen charging treatment three times, the hydrogen content of the niobium hydride reaches more than 1%. At this point, the dense niobium ingot spontaneously breaks into flakes. It is then pulverized to -40 mesh. The prepared niobium hydride powder is uniformly mixed with commercially available dielectric titanium powder (<20 mesh) at a ratio of 49% by weight of Nb, and pressed into electrodes with a diameter of 40-45 × 180 mm at a pressure of 2 tons/cm². The pressed electrodes need to be sintered to remove hydrogen, increase the strength of the powder electrode, and achieve partial solid solution of niobium and titanium. During sintering, a vacuum >6 × 10⁻⁴ Torr is applied, followed by heating to remove hydrogen, starting at approximately 300°C and holding at 650-700°C until the vacuum returns to 6 × 10⁻⁴ Torr. The temperature is then further increased to 1400°C and held for 2 hours for high-temperature sintering. X-ray phase analysis confirms the formation of a solid solution between niobium and titanium.
The sintered electrodes are then melted twice in an electric arc furnace to obtain 20 kg alloy ingots. Analysis indicates that the titanium content fluctuates within ±0.5% by weight. Micro-area analysis showed that the titanium content fluctuated between 2.5% and 4.6% by weight. The niobium utilization rate was over 80% from the niobium ingot to the alloy ingot. No niobium infusible blocks were found in the cross-section and longitudinal section of the alloy ingot, and the oxygen content of the alloy was <1000 ppm. A 163-core Cu-NbTi composite superconducting wire with a diameter of 0.5 mm was fabricated using the obtained alloy ingot, and its short sample performance Jc was 1.70-1.95 × 10⁵ A/cm² (H = 6 T).
