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J.-C. ZhaoAssociate ProfessorPh.D., Lehigh University, 1995 Tel. (614) 292-9462 Office: 286 Watts Hall
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Dr. Zhao's research passions are:
- High-throughput materials research and materials property microscopy tools
- Hydrogen storage materials and materials for energy
- Computational thermodynamics and kinetics for materials design
- Advanced alloys and coatings
High-Throughput Materials Research and Materials Property Microscopy Tools
Around the year 1999, I originated the idea of taking advantage of the compositional variations and the formation of intermetallic compounds in diffusion couples and diffusion triples to perform localized measurements of materials properties as a function of composition and phases (in addition to phase diagram mapping). Such measurements are much more effective and systematic than the one-alloy-at-a-time practice, but would require accurate, micron-scale resolution measurement/mapping tools for various materials properties. I had a dream of inventing a suite of materials property microscopy (MPM) tools with micron-scale spatial resolution for imaging key materials properties similar to the way that SEM reveals the microstructure of a specimen. The key properties would include: elastic modulus, optical properties, dielectric constants, electrical conductivity, thermal conductivity, density, hardness, yield strength, ductility, thermal expansion coefficient, magnetic properties, melting temperatures, and thermodynamic properties. Back then, nanoindentation was available to perform micron-scale measurements of hardness and elastic modulus. Optical ellipsometry was available to map optical properties. An evanescent microwave probe was developed to image dielectric constants of ceramic materials. Micro-scale, 4-point probes were used to measure electrical conductivity, but they are not very robust when the spatial resolution requirement goes down to a few microns. Robust, accurate micro-scale measurement tools for other properties on the above list were essentially unavailable (Qualitative mapping/measurement tools were developed for some of these properties by the combinatorial materials science community to show trends, but not to provide accurate data in micron-scale spatial resolution. These combinatorial screening tools were mostly for thin film samples).
Over the past several years, my collaborators (Prof. David Cahill at University of Illinois especially) and I have been developing robust MPM tools. The first success was the development of a femtosecond laser based thermal conductivity microscopy tool [Nature Mater., 3:298, 2004] with ±8% accuracy and 3 micron spatial resolution. It operates like an SEM as demonstrated on a simple Ni / Ni0.455Al0.545 diffusion couple that has three phases with compositional gradients as shown in the following figure. The quantitative thermal conductivity image revealed order-disorder transitions in Ni3Al and a strong compositional singularity of thermal conductivity in NiAl. These data are very valuable inputs to materials design, materials informatics and database development.
(Color): (a) Quantitative thermal conductivity image of a Ni / Ni0.455Al0.545 diffusion couple (of the rectangular region marked on the SEM image in (b)); (b) SEM image showing the microstructure and phases in the diffusion couple; and (c) Comparison of the data obtained from the quantitative thermal conductivity microscopy with literature values (Terada et al.) obtained from 20 individual alloys.
Built upon the quantitative thermal conductivity microscopy, David Cahill, Xuan Zheng and I progressively developed quantitative MPM tools for specific heat capacity, elastic modulus, and coefficient of thermal expansion (CTE). A very revealing example is a cross-sectional sample of a human tooth. An abnormal CTE increase in a 100-micron region of dentin adjacent to the dentin-enamel junction of a human tooth is clearly observed, which is in contrast to the normal behavior in both thermal conductivity and Young's modulus. Such a CTE abnormality would be difficult to find using other techniques. As another example, we were able to find the Invar alloy composition from a simple Fe / Ni diffusion couple using the quantitative CTE microscopy (The Invar alloy was found by Charles Edouard Guillaume and was awarded the 1920 Nobel Prize in physics).
These tools in conjunction with composition-varying and structure/phase-varying samples such as diffusion multiples and combinatorial thin-films are extremely effective in constructing composition-phase-structure-property relationships [Prog. Mater. Sci., 51:557, 2006]. For instance, in the period of several months, we collected more composition-dependent thermal conductivity data than the whole world combined ever. This amount of data would take years to collect using one alloy at a time. The ability to rapidly and efficiently establish composition-phase-structure-property relationships will have a profound impact on materials informatics, testing of materials theories and designing new materials. Included in this impact is a much faster evaluation with attendant lower cost. These MPM tools may become as widely used as SEM, thus reshaping the way experimental property measurements are performed. They can be applied to metallic, ceramic, polymeric, and some biological samples as well.
I am passionate about continuing pursuit of developing MPM tools for other properties in the above list and applying these tools for accelerated development of both structural alloys and multifunctional intermetallics. Intermetallic-based functional materials such as magnetic, magnetocaloric, magnetoelastic, spintronic, thermoelectric, and phase-changing materials are becoming an increasingly important part of our daily life for sensing, actuation, computing, energy conversion, and communication. These materials take advantage of their unique physical, chemical, and structural properties and very often involve complex interplays of structural, magnetic, and electronic transitions with external or induced stress field, magnetic field, electrical field, or thermal field. High-throughput methodology can greatly accelerate the discovery of such functional intermetallics. The combination of several MPM tools will be especially powerful, not only for materials discovery, but also for understanding the mechanisms and the structure-property relationship.
Hydrogen Storage Materials and Materials for Energy
Energy is an increasingly important issue for human society. Materials for energy generation, storage, transportation, conversion, conservation, and usage will become an extremely important area for research. These materials include hydrogen and energy storage materials, materials for energy conversion (thermoelectric materials, magnetocaloric materials, photovoltaics, etc.), and materials for nuclear energy generation, among others.
A "grand challenge" to the implementation of hydrogen-powered vehicles is the development of suitable on-board hydrogen storage systems and materials that can satisfy the performance targets proposed by the U. S. Department of Energy (DOE). As Dr. Robert F. Service, an editor of the Science magazine put it "If producing hydrogen cheaply has researchers scratching their heads, storing enough of it on board a car has them positively stymied." (Science 2004). Mr. Masatami Takimoto, Executive Vice President of Toyota also stated in 2006 "There exists the necessity for an epoch-making advance in new materials for hydrogen storage.... This is the hardest challenge."
I started hydrogen storage materials research in 2003 while working at GE. I became GE's project leader for hydrogen storage research and PI of a DOE funded program "lightweight intermetallics for hydrogen storage" as part of the DOE Metal Hydride Center of Excellence (MHCoE). Our research focus is on metal hydrides, a solid storage option that has the advantage of high volumetric density. The key is to develop reversible and high gravimetric density metal hydrides that will meet the DOE FreedomCAR 2010 hydrogen storage targets. Substantial progress has been made in synthesis, characterization and mechanistic understanding of complex metal hydrides, especially high-capacity borohydrides.
I am one of the representative US experts in the International Energy Agency (IEA) hydrogen storage Task 17 and Task 22. Strong collaborations with both U.S. and international experts are a very important part of our research. A significant part of my research group at OSU will be devoted to hydrogen storage materials research. I am actively seeking for funding for research on other materials for energy such as superconductors, thermoelectrics, and materials for nuclear energy applications.
Low (left) and high (right) temperature crystal structures of Mg(BH4)2, a candidate for hydrogen storage.
Computational Thermodynamics and Kinetics for Materials Design
Computational thermodynamics using the CALPHAD approach is one of the most important tools for computational design of multicomponent alloys. It is literally performed on a daily basis in many industrial laboratories for the development of superalloys, steels, Ti alloys, Al alloys, Mg alloys, electronic materials, and ceramics, among others. Its impact on alloy design depends heavily on the accuracy and elemental coverage of thermodynamic databases, which in turn depend on the availability and reliability of experimental data inputs. Utility of calculations quickly hits a plateau dictated by the accuracy of the database: using thermodynamic modeling for trend analysis is very different from performing reliable quantitative predictions. True predictive capability would make the databases much more valuable for alloy design by cutting down the long-exposure experiments now required to evaluate detrimental phase formation. The lack of reliable experimental data input is responsible for some of the problems in the databases.
A high quality thermodynamic database is at the heart of computational design of materials. It provides multicomponent phase equilibrium data, the driving force for precipitation modeling, the segregation and latent heat data for solidification modeling, and thermodynamic factors for constructing diffusivity matrices for multicomponent alloys. Thermodynamic databases are also linked to phase field and other models to predict microstructure evolution and properties in alloys.
Similarly, a high quality diffusivity database is essential for simulating precipitation kinetics and materials processes. The diffusion-multiple approach will provide an efficient way to obtain diffusion profiles and development of automated tools to extract diffusion coefficients from these profiles will become imperative.
We need more experimental data and better databases for wider applications and bigger impact. The diffusion-multiple approach I developed is a good start to speed experimentation. This highly efficient technique can provide crucial phase diagram data as input for quality thermodynamic modeling assessments. At the same time, we need faster, more efficient assessment techniques. Assessments of complex systems are currently too time-consuming, and the quality depends too much on the experience and patience of the individuals who are performing the assessments. This is one area I would like to explore funding opportunities to develop more robust thermodynamic assessment methodologies. It is also important to develop better and more efficient ways to measure thermodynamic quantities (heat of formation, activity, specific heat, etc.).
Computational thermodynamics and kinetics will always be one focus of my research. I am especially interested in developing reliable databases for thermodynamics, kinetics/diffusivity, and properties (such as elastic properties, lattice parameters, and thermal expansion), and to help integrate them into multi-scale modeling for microstructure and property predictions. I am also very interested in integrating first-principles results with thermodynamic and kinetic assessments.
Phase stability in superalloy Nimonic 263 based on experiment and calculation.
Advanced Alloys and Coatings
I have more than a decade of experience at GE on research of superalloys and other advanced structural alloys. I also had several years' experience on bond coat and thermal barrier coating (TBC) systems. I am a co-inventor of about 50 U.S. patents and pending patents on alloys and coatings. After joining the OSU, my focus will no longer be inventing new alloys and coatings, but to develop methods, databases, and tools, and to teach students how to use them to design advanced alloys and coatings. I believe there is a lot that can be gained from coatings to improve the temperature capability and the life of structural components. Coating design and development involves lots of knowledge on phase equilibria and diffusion behavior in multicomponent systems. I would like to explore the opportunities to: 1) study phase stability and build thermodynamic databases for advanced alloy systems, 2) study and model precipitation kinetics, and 3) study interaction of materials and coatings with various aggressive environments such as oxidation, hot corrosion, nitridation, and radiation.
Front covers of Dr. Zhao's publications on alloys and coatings.
