Reason for Adding Rare Earth Oxides to Tungsten Electrodes

Metal tungsten, due to its high melting point, strong electron emission capability, and low vapor pressure, has become the preferred choice for thermionic emission materials. However, its high electron work function, elevated electrode tip temperature, and tendency for grain growth lead to unstable arc beams, difficult arc initiation, and a short service life in pure tungsten electrodes, limiting their use to alternating current welding. To overcome these drawbacks, manufacturers often add rare earth oxides with low electron work functions, such as lanthanum oxide, cerium oxide, and yttrium oxide, to pure tungsten materials. This not only increases the recrystallization temperature of the product but also enhances electron emission.

Research indicates that the properties of rare earth oxides during combustion are the most critical factors affecting electrode performance, electrode temperature, work function, and thermal stability.

Generally, rare earth metal oxides react with tungsten to form tungstates or tungsten oxysalts. Since the melting points of these salts are lower than those of the reactants, they melt first during arc burning and migrate along axial boundaries from low-temperature to high-temperature regions. The reaction behavior of different rare earth oxides and the melting points of the resulting tungstates or tungsten oxysalts vary.

Taking cerium oxide, lanthanum oxide, and yttrium oxide as examples, cerium tungstate has the lowest melting point and the highest migration rate of cerium oxide; the higher melting point of lanthanum tungstate is associated with its stability, and its migration provides a reasonable and stable compensation for evaporation; yttrium tungstate has a higher melting point, and the migration of yttrium oxide along grain boundaries to the ends and surface requires a longer time.

In summary, adding small amounts of rare earth oxides to a tungsten base is an effective approach to improve the thermal stability, service life, and emission capability of electrode materials, while enhancing their arc performance.

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