In plasma spraying, hydrogen is widely used as a secondary working gas besides argon. In particular under low pressure, there are strong effects on the plasma jet characteristics even by small hydrogen percentages. Under such conditions, fundamental mechanisms like diffusion and recombination are affected while this is less relevant under atmospheric conditions. This was investigated for argon–hydrogen mixtures by optical emission spectroscopy (OES). The small electron densities under the investigated low pressure conditions implied specific difficulties in the application of several OES-based methods which are discussed in detail. Adding hydrogen to the plasma gas effected an increased plasma enthalpy. Moreover, the jet expanded radially as the reactive part of the thermal conductivity was enhanced by recombination of atomic hydrogen so that the shock waves were less reflected at the cold jet rims. In the jet cores, the lowest temperatures were found for the highest hydrogen admixture because the energy consumption due to the dissociation of molecular hydrogen outbalanced the increase of the plasma enthalpy. Variations in the radial temperature profiles were related to the jet structure and radial thermal conductivity. The local hydrogen–argon concentration ratios revealed an accumulation of hydrogen atoms at the jet rims. Clear indications were found, that higher hydrogen contents promoted the fast recombination of electrons and ions. However, it is assumed that the transport properties of the plasma were hardly affected by this, since the electron densities and thus the ionization degrees were generally small due to the low pressure conditions.

中文翻译：

---

氢气混合物如何在低压下改变喷涂工艺中的等离子体射流特性

在等离子喷涂中，氢气被广泛用作除氩气之外的二次工作气体。特别是在低压下，即使是很小的氢百分比，也会对等离子体射流特性产生强烈影响。在这种条件下，扩散和复合等基本机制会受到影响，而在大气条件下则不太相关。这是通过光学发射光谱 (OES) 对氩氢混合物进行研究的。所研究的低压条件下的小电子密度意味着在详细讨论的几种基于 OES 的方法的应用中存在特殊困难。向等离子气体中加入氢气会增加等离子焓。而且，由于氢原子的复合提高了热导率的反应部分，因此射流径向膨胀，因此激波在冷射流边缘反射较少。在喷射核心中，发现最高氢混合物的最低温度，因为分子氢解离引起的能量消耗超过了等​​离子体焓的增加。径向温度分布的变化与射流结构和径向热导率有关。局部氢-氩浓度比揭示了喷射边缘处氢原子的积累。发现明确的迹象，较高的氢含量促进电子和离子的快速复合。然而，假设等离子体的传输特性几乎不受此影响，