Effect of sheath material and reaction overpressure on Ag extrusions into the TiO2 insulation coating of Bi-2212 round wire.
Effect of sheath material and reaction overpressure on Ag extrusions into the TiO2 insulation coating of Bi-2212 round wire.
复制标题
护套材料和反应超压对 Ag 挤出到 Bi-2212 圆线 TiO2 绝缘涂层中的影响。
DOI：
10.1088/1757-899x/279/1/012021
复制
发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Hellstrom,E
中科院分区：
文献类型：
--
作者：
Hossain,I;Jiang,J;Matras,M;Trociewitz,UP;Lu,J;Kametani,F;Larbalestier,D;Hellstrom,E
为了在线圈中形成高电流密度，Bi-2212线必须在紧密的绕组包中是电离散的。至关重要的是使用薄的绝缘层，满足介电要求，并且可以经受住氧气中最高温度达到890 C的热处理。与铝硅酸盐编织纤维绝缘层相比，薄（20-30 µm）陶瓷涂层作为绝缘层可能更好，铝硅酸盐编织纤维绝缘层约150 μm厚，在热处理期间与银护套Bi-2212导线发生反应。目前，TiO 2似乎是用于这种薄绝缘体的最可行的陶瓷材料，因为它与Ag和Bi-2212化学相容，并且其烧结温度低于用于Bi-2212热处理的最高温度。然而，最近的一个大的Bi-2212线圈绝缘只与TiO 2的测试表明，严重的电线之间的电短路后，超压热处理（OPHT）。短路的起源是频繁的银突起到多孔TiO 2层中，该多孔TiO 2层电连接相邻的Bi-2212线。为了理解这种意想不到的行为的机制，我们研究了鞘材料和静水压力对Ag突起的影响。我们发现，只有当TiO 2绝缘的Ag-0.2% Mg护套线（Ag（Mg）线）在50 bar下进行OPHT时，才会出现Ag突起。当在1巴下加工TiO 2绝缘的Ag（Mg）线时，没有观察到Ag突起。经过50巴OPHT的纯Ag包覆的TiO 2绝缘导线也没有Ag突起。一个关键的发现是，从Ag（Mg）鞘的Ag突起实际上不含MgO，这表明MgO的局部耗尽有利于局部的，非均匀变形的鞘下静水压力。我们的研究还表明，预致密的Ag（Mg）线之前，绝缘与TiO 2和做最后的OPHT可以潜在地限制银突起。
In order to develop a high current density in coils, Bi-2212 wires must be electrically discrete in tight winding packs. It is vital to use an insulating layer that is thin, fulfils the dielectric requirements, and can survive the heat treatment whose maximum temperature reaches 890 C in oxygen. A thin (20-30 µm) ceramic coating could be better as the insulating layer compared to alumino-silicate braided fiber insulation, which is about 150 μm thick and reacts with the Ag sheathed Bi-2212 wire during heat treatment. At present, TiO 2 seems to be the most viable ceramic material for such a thin insulation because it is chemically compatible with Ag and Bi-2212 and its sintering temperature is lower than the maximum temperature used for the Bi-2212 heat treatment. However, recent tests of a large Bi-2212 coil insulated only with TiO 2 showed severe electrical shorting between the wires after over pressure heat treatment (OPHT). The origin of the shorting was frequent silver protrusions into the porous TiO 2 layer that electrically connected adjacent Bi-2212 wires. To understand the mechanism of this unexpected behaviour, we investigated the effect of sheath material and hydrostatic pressure on Ag protrusions. We found that Ag protrusions occur only when TiO 2-insulated Ag-0.2% Mg sheathed wire (Ag (Mg) wire) undergoes OPHT at 50 bar. No Ag protrusions were observed when the TiO 2-insulated Ag (Mg) wire was processed at 1 bar. The TiO 2-insulated wires sheathed with pure Ag that underwent 50 bar OPHT were also free from Ag protrusions. A key finding is that the Ag protrusions from the Ag (Mg) sheath actually contain no MgO, suggesting that local depletion of MgO facilitates local, heterogeneous deformation of the sheath under hydrostatic overpressure. Our study also suggests that predensifying the Ag (Mg) wire before insulating it with TiO 2 and doing the final OPHT can potentially limit Ag protrusions.