引用 张静茹:
磁控管跟食物是通的呀，会有这么都的东西？

刚刚查了一下，这货不光是绝缘用的，是发射电子用的！氧化物加热后会发射电子，跟显象管阴极一样，也涂有氧化物。
“The barium oxide film advantageously has a secondary electron emission ratio as high as 4.8 among the oxide material films. Strontium oxide or calcium oxide may be used.”