High engineering critical current density ( J_E of 1300 A/mm ² at 4.2 K and 15 T) in Bi-2212 round wire has been achieved through a partial melt, overpressure heat treatment process. J_E varies strongly with processing conditions, particularly the maximum heat treatment temperature ( T _max ). Increasing T _max results in longer time in the melt (defined as the time between when Bi-2212 melts on heating and when Bi-2212 begins to form on cooling), more bridging between the filaments, lower J_E , and higher ac losses. A wide processing window with a large range of T _max that has a nearly constant J_E is desired for processing large coils with large thermal mass and significant thermal time constants that may make precise control over the desired temperature – time profiles uncertain. Accordingly, we wanted to explore broadening the T _max window by controlling the Bi-2212 powder melting or wire architecture design. Here we report on studies of the performance variation with T _max for two production wires with a filling factor of about 20% and 85 × 18 filaments where filament size was varied by changing the wire diameter, a process which also shortens the distance between filaments. We found that wires with smaller filament diameter (9 to 11 μm) showed a peak J_E at the low end of T _max and also a J_E that was more sensitive to T _max . A J_E – T _max plot showed a plateau J_E (4.2 K, 5 T) of ∼1100 A/mm ² between T _max of 886 and 894 °C for 1.0 and 1.2 mm wires, where J_E is less sensitive to the wire diameter and T _max . This J_E plateau range is a preferred processing window for achieving high J_E in coils.

中文翻译：

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线径和细丝尺寸对Bi-2212圆线加工窗口的影响

高工程临界电流密度（ Ĵ _é通过部分熔融，过压热处理工艺，在Bi-2212圆线中获得了1300 A / mm ^2（在4.2 K和15 T时）。Ĵ _é 随加工条件而变化很大，尤其是最高热处理温度（ Ť _最高 ）。越来越多Ť _max导致熔体时间更长（定义为Bi-2212在加热时熔化到Bi-2212在冷却时开始形成之间的时间），细丝之间的桥接更多，更低Ĵ _é ，以及更高的交流电损耗。宽广的处理范围Ť 具有几乎恒定的_最大值Ĵ _é对于加工具有大的热质量和显着的热时间常数的大型线圈是理想的，这可能使精确控制所需的温度–时间分布不确定。因此，我们想探索扩大Ť _最大窗口可通过控制Bi-2212粉末熔化或金属丝架构设计来实现。在这里，我们报告了有关性能变化的研究Ť 填充系数约为20％的两根生产线的_最大线数为85×18，其中通过改变线径来改变长丝尺寸，该工艺还缩短了长丝之间的距离。我们发现细丝直径较小（9至11μm）的导线出现一个峰值Ĵ _é 在低端 Ť _最大也是Ĵ _é 对 Ť _最大 。一种Ĵ _é – Ť _最大图显示了一个平台Ĵ _é （4.2 K，5 T）〜1100 A / mm ²之间Ť 对于1.0和1.2 mm的电线，_最高886和894°C，其中Ĵ _é 对电线直径较不敏感，并且 Ť _最大 。这Ĵ _é 高原范围是实现高水平的首选加工窗口 Ĵ _é 在线圈中。