G.S. Rohrer: Research Interests

The Structure and Properties of Grain boundaries and Surfaces
Polycrystalline materials, which are dense aggregates of single crystals joined together by a network of internal interfaces called grain boundaries, are ubiquitous in engineered systems: integrated circuits, aircraft and automotive components, communications devices, machine tools, power generation systems, and medical implants all contain polycrystalline materials. It is widely recognized that the grain boundary networks in these materials influence their properties. However, fundamental challenges must be overcome before we will be able to predictably design polycrystalline materials with desired properties. To overcome these challenges, our group is working to define structural metrics for polycrystals, to understand the origins of polycrystalline structure during materials processing, and develop methods to predict the relationship polycrystalline structure and materials properties. Further information can be found here.

Photochemical Properties of Ceramic Surfaces
Certain transition metal oxide ceramics can catalyze water photolysis and produce hydrogen from water and sunlight. While electrical power generated from photolytic hydrogen is potentially sustainable and generates no toxic or radioactive byproducts, it is also more expensive than the power generated by conventional systems that use fossil or nuclear fuels. The objective of this research is to develop a photolysis catalsyt capable of producing hydrogen at a lower cost. Further information can be found here.

Structure-Property relationships for Hard Materials
Cemented carbides (or sintered carbides) are common hard materials which have outstanding mechanical properties that make them commercially useful in machining, mining, metal cutting, metal forming, construction, wear parts, and other applications. Like many other engineering materials, the mechanical properties of cemented carbides are influenced by their microstructures. This work focuses on a comprehensive analysis of relationships between microstructural and mechanical properties of cemented carbides and their coatings. Further information can be found here.

Jason Gruber (Doctoral Student): Simulation of the Evolution of the Five Parameter Grain Boundary Distribution during Grain Growth. Co-advised with Rollett

Herb Miller (Doctoral Student): Experimental Assessment of the Evolution of the Grain Boundary Distribution during Grain Growth and Grain Boundary Engineering

Abhilasha Bhardwaj (Doctoral Student): The Ferroelectric Field Effect in Catalysis

Nina Burbure (Doctoral Student): Dipolar Field Effect in Photochemical Reactions

Lisa Chan (Doctoral Student): Relationship between grain boundary structure and corrosion in 2000 series aluminum alloys. Co-advised with Rollett

Lizza D. McGregor: (Bachelors Student): The Orientation Dependence of Oxidation Reactions on Titania.

Francine Papillon: (Research Staff): Influence of Composition on the Grain Boundary Character Distribution in Metals and Ceramics.

Michael Gao: (Post-Doctoral Researcher): Microstructural-Property Relations for Multilayer Hard Coatings.

Tomoko Sano, Interface Anisotropy and its Effect on Microstructural Evolution During Coarsening

Chang-Soo Kim, Microstructural-Mechanical Property Relationships in WC-Co composites

Jennifer Giocondi, Effect of Dipolar Fields, Surface Termination, and Surface Orientation on Photochemical Reactions on Transition Metal Oxides

David Saylor, The Character Dependence of Interfacial Energies in Magnesia

Jennifer Lowekamp, The Anisotropy of the Surface Energy and Photochemical Activity of Rutile

Richard Smith, The Structural Evolution of the MoO₃(010) Surface during Reduction and Oxidation Reactions.


Research Interests
The Structure and Properties of Grain boundaries and Surfaces Polycrystalline materials, which are dense aggregates of single crystals joined together by a network of internal interfaces called grain boundaries, are ubiquitous in engineered systems: integrated circuits, aircraft and automotive components, communications devices, machine tools, power generation systems, and medical implants all contain polycrystalline materials. It is widely recognized that the grain boundary networks in these materials influence their properties. However, fundamental challenges must be overcome before we will be able to predictably design polycrystalline materials with desired properties. To overcome these challenges, our group is working to define structural metrics for polycrystals, to understand the origins of polycrystalline structure during materials processing, and develop methods to predict the relationship polycrystalline structure and materials properties. Further information can be found here. Photochemical Properties of Ceramic Surfaces Certain transition metal oxide ceramics can catalyze water photolysis and produce hydrogen from water and sunlight. While electrical power generated from photolytic hydrogen is potentially sustainable and generates no toxic or radioactive byproducts, it is also more expensive than the power generated by conventional systems that use fossil or nuclear fuels. The objective of this research is to develop a photolysis catalsyt capable of producing hydrogen at a lower cost. Further information can be found here. Structure-Property relationships for Hard Materials Cemented carbides (or sintered carbides) are common hard materials which have outstanding mechanical properties that make them commercially useful in machining, mining, metal cutting, metal forming, construction, wear parts, and other applications. Like many other engineering materials, the mechanical properties of cemented carbides are influenced by their microstructures. This work focuses on a comprehensive analysis of relationships between microstructural and mechanical properties of cemented carbides and their coatings. Further information can be found here.
Current projects
Jason Gruber (Doctoral Student): Simulation of the Evolution of the Five Parameter Grain Boundary Distribution during Grain Growth. Co-advised with Rollett Herb Miller (Doctoral Student): Experimental Assessment of the Evolution of the Grain Boundary Distribution during Grain Growth and Grain Boundary Engineering Abhilasha Bhardwaj (Doctoral Student): The Ferroelectric Field Effect in Catalysis Nina Burbure (Doctoral Student): Dipolar Field Effect in Photochemical Reactions Lisa Chan (Doctoral Student): Relationship between grain boundary structure and corrosion in 2000 series aluminum alloys. Co-advised with Rollett Lizza D. McGregor: (Bachelors Student): The Orientation Dependence of Oxidation Reactions on Titania. Francine Papillon: (Research Staff): Influence of Composition on the Grain Boundary Character Distribution in Metals and Ceramics. Michael Gao: (Post-Doctoral Researcher): Microstructural-Property Relations for Multilayer Hard Coatings.
Recent Doctoral Theses
Tomoko Sano, Interface Anisotropy and its Effect on Microstructural Evolution During Coarsening Chang-Soo Kim, Microstructural-Mechanical Property Relationships in WC-Co composites Jennifer Giocondi, Effect of Dipolar Fields, Surface Termination, and Surface Orientation on Photochemical Reactions on Transition Metal Oxides David Saylor, The Character Dependence of Interfacial Energies in Magnesia Jennifer Lowekamp, The Anisotropy of the Surface Energy and Photochemical Activity of Rutile Richard Smith, The Structural Evolution of the MoO₃(010) Surface during Reduction and Oxidation Reactions.
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The Structure and Properties of Grain boundaries and Surfaces
Towards the objective of characterizing the structures of polycrystals, our group has developed techniques to measure the distribution and properties of grain boundaries as a function of the five macroscopically observable parameters (three of these parameters specify the lattice misorientation, and two specify the boundary plane orientation). Using these techniques, we have found that the boundaries of individual grains in dense polycrystals prefer certain crystallographic habit planes, almost as if they were independent of the neighboring crystals. Furhtermore, the specific habit planes within the polycrystal correspond to the same planes that dominate the external growth forms and equilibrium shapes of isolated crystals of the same phase. The observations reduce the apparent complexity of interfacial networks and suggest that the mechanisms of solid state grain growth may be analogous to conventional crystal growth. The results also indicate that a model for grain boundary energy and structure based on grain surface relationships is more appropriate than the widely accepted models based on lattice orientation relationships. The grain boundary character distribution is a five dimensional quantity. One way to look at it is to plot the distribution of planes for fixed misorientations, as shown at the right. These are the distribution of grain boundary planes in WC for grain boundaries with misorientations of 30 deg. around [0001] (upper) and 90 deg. around [10-10] (lower). The units are in multiples of a random distribution and values greater than one indicate that these planes are observed more than would be expected. Our research in this area covers ceramics, metals, and composites. Data is collected by automated electron back-scattered diffraction mapping, or orientation imaging microscopy (OIM). Recently, we have begun to collect data in all three spatial dimensions using OIM in a dual beam focussed ion beam scanning electron microscope. Some recent publications (complete list): • C.-S. Kim, A.D. Rollett, and G.S. Rohrer, "Grain Boundary Planes: New dimensions in the Grain Boundary Character Distribution," Scripta Materialia, 54 (2006) 1005-1009. • D.M. Saylor, B.S. El-Dasher, Y. Pang, H.M. Miller, P. Wynblatt, A.D. Rollett, and G.S. Rohrer, "Habits of Grains in Dense Polycrystalline Solids," J. Amer. Ceram. Soc., 87 (2004) 724-726. • D.M. Saylor, A. Morawiec, and G.S. Rohrer, "Distribution of Grain boundaries in Magnesia as a Function of Five Macroscopic Parameters," Acta Mater., 51 (2003) 3663-74.

Photochemical Properties of Ceramic Surfaces
Heterogeneous photochemical reactions can occur on ceramic surfaces when the absorption of light with an energy greater than the band gap creates electrons and holes that, instead of recombining, become trapped on the surface and react with adsorbed surface species. There are a number of potential applications of ceramics with high photochemical activity. For example, solar activated devices to photochemically purify air and water have been patented and building materials with photochemically active self-cleaning surface treatments are being marketed. However, the application that has the greatest potential to benefit society is the use of photochemically active ceramics as catalysts for the dissociation of water (photolysis) to produce H₂ and O₂. We are studying the photochemical reactivity of different phases, the orientation dependence of the photochemical activity, and the influence of internal dipolar fields on the photochemical activity (the dipolar field effect). The atomic force microscope image to the right shows the preferential oxidation of Pb at certain facets on the rutile surface. Some recent publications (complete list): • J.L. Giocondi and G.S. Rohrer, "The Influence of the Dipolar Field Effect on the Photochemical Reactivity of Sr₂Nb₂O₇ and BaTiO₃ Microcrystals," Topics in Catalysis, (2006) in press. • J.L. Giocondi and G.S. Rohrer, "Structure Sensitivity of Photochemical Oxidation and Reduction Reactions on SrTiO₃ Surfaces," J. Amer. Ceram. Soc., 86 (2003) 1182-89. • J.L. Giocondi and G.S. Rohrer, "Spatially Selective Photochemical Reduction of Silver on the Surface of Ferroelectric Barium Titanate," Chemistry of Materials, 13 (2001) 241-2.

Structure-Property relationships for Hard Materials
The purpose of this work is to understand how the microstructural characteristics of WC-Co composites (contiguity, angularity, aspect ratio, and grain size distribution) affect their responses to mechanical and/or thermal loads. The work is divided in two parts: the quantitative and comprehensive assessment of microstructural characteristics, and the simulation of the microstructure's response to loads. The images to the right show an orientation imaging micrograph of a WC-Co composite (upper) and plot showing the distribution of hydrostatic stresses after cooling from the process temperature. Simulations of real and hypothetical microstructures allow the independent variation of particular microstructural characteristics (the contiguity, angularity, aspect ratio, or grain size distribution) while keeping the others as constant as possible, so that each parameter's influence on the strength could be independently determined. Some recent publications (complete list): • C.-S. Kim, T.R. Massa, G.S. Rohrer, "Modeling the Relationship between Microstructural Features and the Strength of WC-Co Composites," International Journal of Refractory Metals and Hard Materials, 24 (2006) 89-100. • C.-S. Kim and G.S. Rohrer, "Geometric and Crystallographic Characterization of WC Surfaces and Grain Boundaries in WC-Co Composites," Interface Science, 12 (2004) 19-27.