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ion Energetics of Pore Elimination

Ricardo H. R. Castro, University of California-Davis, DMR 1055504Outcome: Researchers at University of California, Davis have shown that the densification phenomenon in ceramics, known to be irreversible, are actually reversible at the nanoscale due to a thermodynamic effect. Impact: Such discovery will potentially enable the control of pore elimination during processing of ceramics by targeting the thermodynamics of the system rather than usage of kinetic approaches typically adopted by manufacturers.Explanation: The consolidation of ceramic powders into parts involves heating at high temperatures to allow atomic mobility and then porosity elimination by joining particles together via the formation of a particle boundary (grain boundary).

The driving force for this phenomena is the excess energy due to the surface. Therefore, as surface is eliminated, it could not be reversed as this would bring more energy to the system. At the nanoscale, the amount of interfaces is so much higher, that a change on their composition may cause the stabilization of the surface with relation to the newly formed boundary, reversing the atomic flow, leading to particle separation.Castro is the recipient of an NSF Faculty Early Career Development (CAREER) award on Thermochemistry of Nanoceramics.

Castro running a differential scanning calorimeter used to determine the energetics of nano-densification.

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Pure tin dioxide shows a strong resistance to pore elimination during processing. UC Davis researchers showed that by adding 3% of Mn, fast densification can be induced within seconds, dramatically reducing processing costs. Now, graduate student Chi-Hsiu Chang showed that de-densification is also achievable in this same system by using different processing conditions. That is, after severe pore shrinkage at 1300 °C for 30 seconds, one can re-inflate the pores by lowering the temperature to 800 °C. This phenomenon cannot be explained by conventional diffusion theories, and Castro is proving by calorimetry it to be a thermodynamic feature. The results show the feasibility of thermodynamic control of densification.

Reversing Densification of CeramicsRicardo H. R. Castro, University of California-Davis, DMR 1055504

Unprecedented decrease in the density of Mn-doped tin dioxide compacts upon sintering after previous fast densification at higher temperature. The microstructure in the microscopy image shows a clear increase in porosity for 20h.(courtesy of R. Castro)

Furnace inside view

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ion Materials & You: Bringing Materials Concepts to Your Life

Ricardo H. R. Castro, University of California-Davis, DMR 1055504

At Midtown Middle School, Woodland, CA, students are exposed to concepts of materials by building in-class an incandescent light filament among many other “cool” materials demonstrations.(courtesy of R. Castro)

http://www.chms.ucdavis.edu/research/web/castro/materials&you.html

Prof. Castro developed a program called “Materials & You” to show middle and high school students how “cool” materials are by performing in-class demonstrations of Strong materials, Smart materials and Nano-supermaterials. He has designed educational kits for these activities (to be made available on-line), and used them to successfully inspire students from two local schools with a majority of underrepresented students. This program is a successful partnership with the Yolo County Office of Education, and has the participation of undergraduate and graduate students from UC Davis.