Niobium porous anodizing with glycerin addition

ABSTRACT Niobium is a metal that forms adherent protective oxide in contact with the air, which has the property of self-protection. The method used to accelerate the growth of the oxide is known as anodizing, in which the oxide formed may be barrier or porous. Porous oxide is widely studied and can be used in solar cells, gas sensors, as biomaterial, among others. However, porous oxides in niobium have been obtained with the use of F-, which causes an intense dissolution process of the oxide. In order to minimize this reaction, electrolytes containing glycerine have favored the anodizing process forming uniform porous surfaces. In this sense, the objective of this study is to obtain nanoporous structures in niobium by anodization from known parameters. From (12.73 mA.cm-2, 100 V), times (5 and 30 minutes), HF concentration (2.24%) and platinum cathode use, already used in a previous study, was initially tested the influence of the cathode of stainless steel in substitution to the platinum, aiming to promote a future technological transference. From this, in order to minimize the dissolution process that occurred using HF (2.24%), only HF (1%) was tested. Finally, in order to obtain a more uniform oxide structure, glycerin was added to the HF electrolyte (1%). The use of the stainless steel cathode, although influencing the results of the anodizing transients, due to its probable chemical dissolution, proved effective in the process. Anodized samples with lower concentrations of HF and with addition of glycerine, as expected, presented lower oxide dissolution during oxide formation, originating nanoporous structures, besides microcones.

摘要：铌（Niobium）是一种在暴露于空气时可形成附着紧密防护氧化层的金属，具备自保护性能。用于加速该氧化层生长的工艺称为阳极氧化（anodizing），所生成的氧化层可分为阻挡型与多孔型两类。多孔氧化层已得到广泛研究，可应用于太阳能电池（solar cells）、气体传感器（gas sensors）、生物材料（biomaterial）等诸多领域。但当前制备铌基多孔氧化层时多采用氟离子（F⁻）作为电解质组分，该体系会引发氧化层的剧烈溶解反应。为缓解这一问题，含甘油的电解质（electrolytes）被用于优化阳极氧化工艺，以获得均匀的多孔表面。本研究旨在基于已知工艺参数，通过阳极氧化法在铌基底上制备纳米多孔结构（nanoporous structures）。研究初始采用前人研究中已验证的参数：电流密度12.73 mA·cm⁻²、直流电压100 V、氧化时长5分钟与30分钟、氢氟酸（HF）浓度2.24%以及铂阴极（platinum cathode），旨在测试以不锈钢阴极（stainless steel cathode）替代铂阴极的影响，为后续的技术转化（technological transference）奠定基础。在此基础上，为缓解2.24%浓度氢氟酸体系中出现的溶解问题，本研究后续测试了仅采用1%浓度氢氟酸的工艺方案。最后，为获得更为均匀的氧化层结构，向1%浓度的氢氟酸电解质中添加了甘油。尽管不锈钢阴极可能因自身化学溶解而对阳极氧化瞬态过程产生影响，但实验结果证明其在该工艺中具备实用性。如预期所示，采用低浓度氢氟酸并添加甘油的阳极氧化样品，在氧化层形成过程中的溶解程度更低，最终不仅制备得到纳米多孔结构，同时还形成了微锥（microcones）结构。