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Oxidation of Nickel and Ni-Cr and Ni-Na Alloys at High Temperatures

机译：镍，Ni-Cr和Ni-Na合金在高温下的氧化

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Oxidation kinetics of high purity nickel, as well as the nonstoichiometry and chemical diffusion in nickel oxide, have been studied as a function of temperature (1373-1673 K) and oxygen pressure (10-10~5 Pa) using modern microthermogravimetric techniques. In order to eliminate the possible participation of grain boundary diffusion in scale growth at lower temperatures, the oxidation rate measurements have always been started at highest temperature (1673 K) when coarse-grained scale was formed and the temperature and pressure dependence of the oxidation rate was determined by step-wise lowering the temperature of such pre-oxidized sample. Nonstoichiometry and chemical diffusion coefficient in Ni_(1-y)O have also been determined on coarse-grained oxide samples, obtained by complete oxidation of nickel at highest temperature (1673 K). It has been found that under such conditions the parabolic rate constant of nickel oxidation is the following function of temperature and oxygen pressure: k_p = 0.142 · p_(O_2)~(1/6) · exp(-239kJ/mol/RT). The results of nonstoichiometry and chemical diffusion measurements, in turn, may be described by the following relationships: y=0.153 · p_(O_2)~(1/6) · exp(80kJ/mol/RT) and D=0.186 · exp(-152kJ/mol/RT). The parabolic rate constants of nickel oxidation, calculated from nonstoichiometry and chemical diffusion data, are in excellent agreement with experimentally determined k_p values, clearly indicating that the predominant defects in nonstoichiometric nickel oxide are double ionized cation vacancies and electron holes and the oxide scale on nickel grows by the outward volume diffusion of cations. It has been shown that the oxidation of Ni-Cr and Ni-Na alloys, like that of pure nickel, follows also parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni_(1-y)O it has been found that the oxidation rate of Ni-Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of doubly ionized cation vacancies in Ni_(1-y)O-Cr_2O_3 solid solution is fixed on the constant level by trivalent chromium ions, substitutionally incorporated into cation sublattice of this oxide. In the case of Ni-Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of doping effect, by assuming that univalent sodium ions dissolve substitutionally in cation sublattice of nickel oxide.
机译：使用现代微热重技术研究了高纯度镍的氧化动力学，以及氧化镍的非化学计量和化学扩散与温度（1373-1673 K）和氧气压力（10-10〜5 Pa）的关系。为了消除晶界扩散可能在较低温度下引起的氧化皮生长，当形成粗粒度氧化皮时，氧化速率测量始终在最高温度（1673 K）上开始，并且氧化速率与温度和压力有关通过逐步降低这种预氧化样品的温度来确定温度。 Ni_（1-y）O中的非化学计量和化学扩散系数也已通过在最高温度（1673 K）下镍完全氧化而获得的粗晶粒氧化物样品上确定。已经发现，在这种条件下，镍氧化的抛物线速度常数是温度和氧气压力的以下函数：k_p = 0.142·p_（O_2）〜（1/6）·exp（-239kJ / mol / RT）。非化学计量和化学扩散测量的结果又可以由以下关系描述：y = 0.153·p_（O_2）〜（1/6）·exp（80kJ / mol / RT）和D = 0.186·exp（ -152kJ / mol / RT）。由非化学计量和化学扩散数据计算出的镍氧化的抛物线速率常数与实验确定的k_p值非常吻合，清楚地表明非化学计量的氧化镍的主要缺陷是双离子化的阳离子空位和电子空穴以及镍上的氧化物垢通过阳离子的向外体积扩散而生长。已经表明，与纯镍一样，Ni-Cr和Ni-Na合金的氧化也遵循抛物线速率定律，因此受到扩散控制。与Ni_（1-y）O的缺陷模型相吻合，发现Ni-Cr合金的氧化速率高于纯镍，反应速率不受压力的影响，且该过程的活化能较低。 。这暗示着Ni_（1-y）O-Cr_2O_3固溶体中双离子阳离子空位的浓度通过三价铬离子固定在恒定水平上，该三价铬离子取代地结合到该氧化物的阳离子亚晶格中。另一方面，在Ni-Na合金的情况下，氧化速率低于纯镍的氧化速率，活化能较高，并且氧化速率随着氧气压力而更快地增加。通过假设一价钠离子可替代地溶解在氧化镍的阳离子亚晶格中，可以用掺杂效应来解释这些结果。

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