Electrode Performance of Oxide-Containing Tungsten
Materials in Direct Current Arcs
直流アークにおける酸化物入りタングステン電極の性能
Yasuki MIYAZAKI

Seiichiro SHIMIZU

宮崎 寧記

清水 誠一郎Keyword; direct current arcs, CeO2-W, Nd2O3-W, La2O3-W, plasma torches
キーワード；直流アーク，CeO2-W, Nd2O3-W, La2O3-W，プラズマトーチ

Abstract
We developed cerium-oxide-containing tungsten (CeO2-W) as an alternative for thoriumoxide-containing tungsten (ThO2-W) used as cathode of plasma torches. We evaluated theperformance of neodymium-oxide-containing tungsten (Nd2O3-W), lanthanum-oxide-containingtungsten (La2O3-W) and CeO2-W as cathode electrodes for DC arcs under a nitrogen atmosphereand analyzed the consumption mechanism. Under a nitrogen atmosphere, CeO2-W showed lower
erosion than ThO2-W. This is because the surface temperature of cathode of CeO2-W was lower
than that of ThO2-W. On the other hand, the erosion rate using Nd2O3-W and La2O3-W as thecathode was higher than that using ThO2-W, unlike the results under argon atmosphere reported
previously [1]. We inferred that this is caused by the decomposition of these oxides (Nd2O3, La2O3)
by nitrogen radicals.
We compared the erosion rate using CeO2-W as a cathode under an argon atmosphere with theerosion rate of the cathode (Pure-W, Nd2O3-W, ThO2-W, La2O3-W) under an argon atmospherein a previous report [1]. The erosion rate using CeO2-W as the cathode was also the lowest under
an argon atmosphere.
The factors that cause consumption of electrodes are different in argon and nitrogen
atmospheres. Among the tungsten materials currently used industrially, CeO2-W is the mosteffective material in terms of erosion resistance under both atmospheres.

Nippon Tungsten Review Vol.42 (2024)- 14 -プラズマトーチなどの陰極として使用されている，ThO2-W の代替材として，CeO2-W，Nd2O3-W，および La2O3-W を直流アーク用の陰極電極として使用し，窒素雰囲気下における性能評価及び，消耗のメカニズム解析を行った．窒素雰囲気下にて，CeO2-W は ThO2-Wよりも低い消耗量を示した．CeO2-W は ThO2-W より陰極電極の表面温度が低下しており，この効果により電極消耗量が減少したと推察される．
一方，Nd2O3-W と La2O3-W は前回報告［1］したアルゴン雰囲気下の結果とは異なり，ThO2W と比較して消耗量が増加した．これは窒素ラジカルによる酸化物（Nd2O3、La2O3-W）の分解が起因していると推察される．
さらに，アルゴン雰囲気下における CeO2-W の陰極電極の消耗量についても同様の測定を行い，前回報告［1］したアルゴン雰囲気下におけるその他の陰極電極 (Pure-W, Nd2O3-W, ThO2W, La2O3-W) の消耗量と比較した．その結果，CeO2-W の消耗量は最も低くなった．
使用する雰囲気下によって電極消耗要因が異なるため，現在工業的に使用されているタン
グステン材料の中で，どちらの雰囲気下においても耐消耗性に有効な材料は，Ce2O3-W と考えられる．

1. Introduction
In industrial fields, coating technologies such as plasma spraying are used to reduce damage andloss of functionality of components. ThO2-W was generally used for the cathode electrode in plasma
spraying. Since ThO2-W contains radioactive material (thorium), it was necessary to switch to a nonradioactive material. Furthermore, a material with lower erosion rate than ThO2-W was required [2].
In this study, CeO2-W was developed as alternatives to ThO2-W for the cathode of the direct current(DC) arc of plasma torches.

2. Experimental
A schematic of the DC arc generator in Watanabe laboratory of Kyushu University used in theexperiments is shown in Fig. 1. Five types of cathode materials were used: Pure tungsten (Pure-W),
Nd2O3-W, CeO2-W, ThO2-W and La2O3-W. All cathode electrodes had a diameter of 6 mm with a tipangle of 60°. Molybdenum was used for the anode.

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Fig. 1 Schematic of the DC arc generator
These experiments were performed under an argon-nitrogen 50% atmosphere or an argon 100%atmosphere. An arc was generated between the cathode and the anode by applying an arc current of
100 A. The distance between the electrodes was 10 mm and the flow rate of the shielding gas was 10L/mm. The cathode surface temperature was measured using BPFs at wavelengths of 785 ± 2.5 nm
and 880 ± 5.0 nm. This system using BPFs enables cathode radiation measurements without
interference from other emissions in the plasma. The cathode surface temperature was estimated basedon the two-color pyrometry, as shown in Fig. 2.
The tungsten erosion rate was estimated by measuring the cathode weight before and after theexperiment.

Fig. 2 Conceptual diagram of the cathode temperature measurement
by a high-speed camera using BPFs.

Nippon Tungsten Review Vol.42 (2024)- 16 -3. Results and Discussion
The erosion rates of the five cathode materials (Pure-W, ThO2-W, Nd2O3-W, CeO2-W, La2O3-W) in
an argon-nitrogen 50% atmosphere are shown in Fig. 3. The erosion rate using CeO2-W was lower
than that using ThO2-W. This is due to the difference in electrode surface temperature shown in Fig.4.
As shown in Table 1, the melting point of CeO2 is lower than that of ThO2, so the area of molten Ceoxide covering the cathode surface is large. It is thought that as the area where plasma was generated
increases, the current density decreased and thus the electrode surface temperature decreased.
On the other hand, the erosion rate using Nd2O3-W and La2O3-W as the cathode was higher thanthat using ThO2-W. The erosion rate of the La2O3-W electrode could not be measured because its
plasma was unstable, and the electrode was considerably eroded. This result was different from theerosion rate under an argon atmosphere reported in the previous report [1]. It is inferred that this iscaused by the decomposition of oxides (Nd2O3, La2O3) by nitrogen radicals. In the future, it isnecessary to confirm the decomposition reaction of oxides by nitrogen radicals.

Fig. 3 Comparison of the erosion rates of the five cathode materials
under an argon-nitrogen 50% atmosphere- 17 -Nippon Tungsten Review Vol.42 (2024)

Fig. 4 Comparison of surface temperature of electrode under an argon-nitrogen 50% atmosphere

Table 1 Work functions and melting points of the cathode materials

Furthermore, we conducted the erosion test under an argon 100% atmosphere five cathode materials.
We compared the present results with the erosion rates of other materials under an argon 100%
atmosphere reported in the previous report [1]. The results are shown in Fig. 5 (a). Fig. 5 (b) shows anenlarged view of the area other than the Pure-W data. The erosion rate using CeO2-W as the cathodewas lowest under an argon 100% atmosphere.
(a)

(b)

Fig. 5 Comparison of the erosion rates of the five cathode materials
under an argon 100% atmosphere

Nippon Tungsten Review Vol.42 (2024)- 18 -4. Conclusion
Under a nitrogen atmosphere, only the DC arc cathodes of CeO2-W showed lower erosion rates thanthat of the ThO2-W. CeO2-W had the lowest erosion rate even under an argon atmosphere.
The erosion rates of the oxide-containing tungsten (Nd2O3-W, La2O3-W) cathodes were different inan argon atmosphere and a nitrogen atmosphere. CeO2-W is the most effective material for the DC arc
material in both atmospheres.

References[1] S.Shimizu, T.Mouri, K.Fujii, A.Mishma, T.Watanabe Nippon Tungsten Review Vol 41 2017[2] M. Heißl, C. Mitterer, T. Granzer, J. Schröder, M. Kathrein, 18 th Plansee Seminar,Austria, 2013.

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