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論文題目「Arc Temperature Measurement by Combination of Line and Continuum Emissions from Nitrogen DC Arc」

Shen Zhengtong

1．Introduction
DC arc plasma is increasingly regarded as a powerful tool for synthesis of nanoparticles due to many advantages, such as high enthalpy, rapid quenching, and high chemical reactivity. Reducing cathode erosion is important for avoiding the pollution of products from cathode material. Kinds of rare earth oxides with low work function are usually doped into tungsten cathodes in order to improve arc stability and reduce cathode erosion.
Nitrogen is a prospective gas with good protection, cheap cost, and easily available in industry. Many researches about temperature measurement of N2 DC arc have been published in the past few decades. There are many methods for arc temperature measurement, such as the Boltzmann plot method, line intensity ratio method, and the Fowler-Milne method. The continuum emission had been neglected due to relatively low emission intensity in most of measured regions in previous studies. In contrast, the continuum emission intensity is no longer negligible at the near-cathode arc with high temperature above 17,000 K. The continuum emission should be considered due to higher arc temperature.
The understanding about the effect of rare earth metal oxide on arc temperature in N2 atmosphere is insufficient. Therefore, the purpose of present work is to measure the arc temperatures in N2-Ar DC arc with different cathodes by combination of line and continuum emissions from the arc. Meanwhile, the electrode surface temperatures were measured by two-color pyrometry.

2．Experimental
The experimental apparatus of DC arc system consists of a power supply, an arc chamber, a particle collector, and a gas circulator. Plasma gas is introduced to the system after evacuating the chamber. Plasma gas is circulated in DC arc system by gas circulation pump.
In present study, the water-cooled copper plate was set as the anode. Tungsten-based cathodes doped with rare earth oxides of Ce2O3, ThO2, La2O3, and Y2O3 with concentration of 2-5wt% were used and compared. The cathode diameter was 6 mm. The electrode gap distance was fixed at 10 mm. Arc currents ranged from 100 A to 200 A.
The emissions from thermal plasma can be classified into line emissions and continuous emissions. A high-speed camera combined with suitable band pass filters (BPFs) was used for observing emissions from the DC arc. Employed BPFs’ wavelength ranges are 480±5 nm and 500±5 nm where the main emissions from arc above 17,000 K are line emission of single-charged argon ion and line emission of single-charged nitrogen ion, respectively. The intensity ratios of combination of line and continuum emissions in dual wavelength ranges were obtained. The arc temperatures were calculated based on the relationship between the intensity ratio and electron temperature.
The BPFs with wavelength ranges of 785±2.5 nm and 882±5 nm were used for observing the thermal radiation from cathode surface. The cathode surface temperature was measured based on the radiation intensity ratio in dual wavelength ranges by two-color pyrometry.

3．Results and Discussion
The temperature distribution maps in Ar and N2 50vol%-Ar atmospheres with W-2wt%Ce2O3 cathode shows that nitrogen arc becomes constricted and has higher arc temperature compared to the Ar arc.
The maximum arc temperatures with arc current for the W-2wt%Ce2O3 cathode in pure Ar atmosphere and N2 50vol%-Ar atmosphere presents that the maximum arc temperatures in N2 50vol%-Ar atmosphere are higher than those in Ar atmosphere clearly. The arc temperature increases with arc current increase because higher arc current leads to higher current density in the arc. The significant effect of N2 gas on arc temperature distribution should be attributed to the dissociation energy of N2. Since, nitrogen dissociation requires a large amount of heat, which leads to temperature decrease. Then, the arc is constricted, and the diameter of the current channel is decreased. The current density in arc center increases subsequently and higher maximum arc temperature can be obtained in comparison with pure argon case.
The arc temperature distributions for W-2wt%Ce2O3, W-2wt%La2O3, W-2wt%Y2O3 and W-2wt%ThO2 cathodes in N2 50vol%-Ar atmosphere illustrate that W-2wt%Y2O3 has the highest arc center temperature. The W-2wt%ThO2 has a higher arc center temperature than those of W-2wt%Ce2O3 and W-2wt%La2O3 cathodes. Here, the reasons are explained.
The cathode tip temperature (3,000-4,500 K) can reach the melting point of the rare earth metal oxide after arc ignition for rare earth oxide doped W-based cathodes. The melted rare earth oxide then diffuses along the tungsten grain boundaries from cathode inside to the cathode tip surface due to the temperature gradients. Cathode tip surface is covered by melted rare earth oxide. Therefore, the low work function of rare earth oxide results in the low effective work function of cathode.
The current density with the W-2wt%Y2O3 cathode was highest due to the lowest work function of Y2O3 (2.0 eV) than those of ThO2 (2.6 eV), Ce2O3 (3.2 eV), and La2O3 (3.1 eV) based on the Richardson-Dushman equation. Hence, the arc with W-2wt%Y2O3 cathode had the highest arc center temperature.
Small area of melted layer of ThO2 at covered W surface causes by higher melting point of ThO2 (3,323 K) than those of Ce2O3 (2,523 K) and La2O3 (2,577 K). Constricted current attachment at the cathode tip leads to the higher arc center temperature (20000 K) and steeper temperature gradient of W-2wt%ThO2 than W-2wt% Ce2O3 and W-2wt% La2O3.

4. Conclusion
Arc temperatures of nitrogen DC arc with different cathodes were measured successfully by combination of line and continuum emissions. The effect of different rare earth oxides doped in tungsten-based cathodes on arc temperature distribution was investigated. The effect of melting point of doped oxide, and the work function of doped oxide on current density led to the difference arc temperature between cathodes dope with different kinds of rare earth oxide.

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