Creation of Oxide functional thin film with nano Scale Structure Formed by Modulated Electrolysis Process

在热硫酸中，方波电位脉冲极化可以在奥氏体不锈钢、Fe-18Cr合金和Ni-18Cr合金上获得极厚的多孔钝化膜。材料在5 kmol / m^3 H_2S0_4溶液中以50 ~ 80‰的温度进行极化，外加电位调制为方波。透射电镜显示，薄膜由大小约为1 ~ 4nm的纳米椭圆形颗粒组成。薄膜具有大量相互连接的开放通路，容易被离子或水分子渗透。根据应用电位的不同，将成膜机理分为两个过程。并对多孔膜的电化学阻抗和光电化学性能进行了测试。对于阳极型薄膜，电容比衬底钢的电容大得多，并且几乎与薄膜的总厚度成正比。另一方面，阴极型薄膜的电容几乎等同于衬底钢的电容，并且与厚度、结构和其他性能无关。因此，电容仅在衬底/溶液界面处产生，厚膜的多孔结构与衬底没有电化学联系。测定了阳极型薄膜的光电化学响应。光电流表现为p型半导体响应，光电流谱与在硫酸中形成的普通钝化薄膜相似。光电流效率比钝化薄膜大得多。光电流随膜厚的增加而增大，在膜厚约10 nm处达到最大值，然后减小。电化学阻抗和光电化学响应表明，多孔厚膜具有不同结构的电化学性能，可通过电解条件进行排序。

Extraordinarily thick and porous passive film can be obtained on austenitic stainless steels, Fe-18Cr alloy, and Ni-18Cr alloys by square wave potential pulse polarisation in hot sulphuric acid.The material was polarised in 5 kmol / m^3 H_2S0_4 solution at 50 - 80 ﾟC with applied potential modulated as square wave.The transmission electron microscopy revealed that the film consists of nano-crystalline oval-shape grains of around 1-4 nm in its size.The film has large amount of open pathways linking with each others to be penetrated easily by ions or water molecule.The film formation mechanism is classified into two processes according to the applied potentials.Electrochemical impedance and photoelectrochemical properties of the porous films were also examined.For the anodic type film, the capacitance is fairly larger than that of the substrate steel, and is nearly proportional to the total thickness of the film.On the other hand, the capacitance for the cathodic type film was almost equivalent to that of the substrate steel, and was independent of thickness, structure, and other properties.Therefore, the capacitance arised only at the substrate / solution interface, and porous structure of the thick film has no electrochemical connection with substrate.Photoelectrochemical response was measured for the anodic type film.The photo current revealed the p-type semiconductor response, and the photo current spectrum is similar to that for ordinary thin passive film formed in sulphuric acid.The photo current efficiency is fairly larger than that for thin passive film.That is, the photo current increased with increasing film thickness, revealed a maximum at the thickness of about 10 nm, then decreased.The electrochemical impedance and the phoelectrochemical response revealed that the thick porous film has various electrochemical properties depending on the structure, which is possible to be ordered by electrolysis conditions.