Professor

Electrochemical materials science and engineering

Porous anodic oxide films for functional electronic devices. Surface oxide films are formed on metals such as aluminum and titanium by oxidation in electrochemical cells, or “anodizing.”  Under the right conditions, the films consist of highly regular arrangements of pores with submicron diameters, and can be used to create a wide range of electronic devices. These devices exploit the easy fabrication of the porous films, as well as their high internal surface area, uniform pore size, and the ability to control the pore dimensions for a given application through the electrochemical conditions of anodizing. A particularly promising example of such a device is the dye-sensitized solar cell based on porous semiconducting titanium oxide.  Until now the mechanisms of porous oxide growth have not been well understood.  However, our modeling and experimental studies have led to significant insight into this process. We have developed criteria that specify the conditions needed for formation of ordered porous films on a wide variety of materials.

Associated publications

• J.E. Houser and K.R. Hebert, “The role of viscous flow of oxide in the growth of self-ordered porous anodic alumina films,” Nature Mater. 8, 415 (2009)
• K.R. Hebert, S.P. Albu, I. Paramasivam and P. Schumki, “Morphological instability leading to the formation of porous anodic oxide films,” Nature Mater., published online December 4, 2011. doi:10.1038/nmat3185. Also see “Understanding porous oxide films,”
  (http://nanotechweb.org/cws/article/tech/48118), technology update in Nanotechweb.org
• Ӧ. Ӧ. Çapraz, P. Shrotriya and K.R. Hebert, “Measurement of Stress Changes During Growth and Dissolution of Anodic Oxide Films on Aluminum,” J. Electrochem. Soc., 161, D256-D262 (2014).
• Ӧ. Ӧ. Çapraz, P. Shrotriya, P. Skeldon, G.E. Thompson and K.R. Hebert, “Factors Controlling Stress Generation during the Initial Growth of Porous Anodic Aluminum Oxide,” Electrochim. Acta, 159, 16-22 (2015).
• • Ö. Ö. Çapraz, P. Shrotriya, P. Skeldon, G. E. Thompson and K R. Hebert, “Role of Oxide Stress in the Initial Growth of Self-Organized Porous Anodic Aluminum Oxide,” Electrochimica Acta 167, 404-411 (2015).

Electrochemical mechanisms leading to initiation of materials degradation. Certain environmental conditions rapidly accelerate failure of metal structures by promoting stress-corrosion cracking (SCC). SCC is a critical problem that limits the development of new energy technologies, and affects the safety and reliability of many large-scale systems, including oil and gas pipelines, aircraft and nuclear reactors. However, the chemical pathways by which corrosion enhances crack initiation are not yet understood. In studies on aluminum and steel, we have shown that corrosion can result in the rapid buildup of stress, apparently because the metal dissolution reaction forms stress-inducing defects. Such corrosion-generated stress can significantly decrease the threshold external stress at which cracks form, and thus help explain the SCC phenomenon. We are developing methods for detection of corrosion-induced subsurface defects in metals, as a valuable early indication of degradation prior to the catastrophic stage of rapid crack propagation.

Associated Publications

• K. R. Hebert, “Trapping of Hydrogen Absorbed in Aluminum during Corrosion,”Electrochimica Acta 168, 199-205 (2015).
• J. W. Shin, G. R. Stafford and K. R. Hebert, “Stress in Aluminum Induced by Hydrogen Absorption During Cathodic Polarization,” Corros. Sci. 98, 366-371 (2015).
• K.R. Hebert, J. H. Ai, G.R. Stafford, K.M. Ho and C.Z. Wang, “Vacancy defects in aluminum formed during aqueous dissolution,” Electrochim. Acta, 56, 1806 (2011).
• M. Ji, C.Z. Wang, K.M. Ho, S. Adhikari, and K.R. Hebert, “Statistical model of defects in Al-H system,” Phys. Rev. B, 81, 024105 (2010).
• S. Adhikari, J.H. Ai, K.R. Hebert, K.M. Ho, and C.Z. Wang, “Hydrogen in Aluminum During Alkaline Corrosion,” Electrochim. Acta 55, 5326-5331 (2010).
• S. Adhikari, L.S. Chumbley, H. Chen, Y.C. Jean, A.C. Geiculescu, A.C. Hillier and K.R. Hebert, “Interfacial Voids in Aluminum Created by Aqueous Dissolution,” Electrochim. Acta., 55, 6093-6100 (2010).