Evaporation characteristics of metallic materials for vacuum electron devices

Evaporation properties of Monel and stainless-steel alloy materials that are widely applied in microwave vacuum electron devices have been studied by time-of-flight mass spectrometry (ToFMS) as their evaporants has an immediate impact on their lifetime and reliability. The composition and sizes of evaporants of Monel and stainless steel have been determined. Minor evaporation of Mn, Cu, and Cr in Monel and stainless steel occurred at ∼800°C, which is much lower than the melting points of Mn, Cu, and Cr, and evaporation of Mn, Cu, and Cr increased with increasing temperature. Experimental results show that ToFMS is a very fast testing method for researching metallic material evaporation. At 900°C, substantial changes occurred in the surface microstructures of Monel and stainless steel as high porosity and many grain boundaries were observed. With increasing treatment time, grain boundaries gradually became larger, decreasing material strength and leading to outgassing or even air leakage. Such phenomena have been analyzed based on relevant theory, and appropriate feasible methods for use of metallic materials have been discussed from the perspective of vacuum device design and process control.