Direct measurements of quasi -zero grain boundary energies in ceramics
- PDF / 444,791 Bytes
- 8 Pages / 584.957 x 782.986 pts Page_size
- 62 Downloads / 212 Views
Nanocrystalline bulk materials (also called nanograined materials) are intrinsically unstable due to the excess grain boundary (GB) free energies. Dopants designed to segregate to boundaries have been proposed to lower excess GB energies, increasing stability against coarsening and enabling nanostructure features to survive high temperature processing and operational environments. It has been theoretically proposed that the GB energy of a material can eventually become zero as a function of dopant concentration, signifying negligible driving force for growth—an infinitely stable nanomaterial. In this work we use ultrasensitive microcalorimetry to experimentally measure the absolute GB energy of gadolinium-doped nanocrystalline zirconia as a function of grain size and show that the energy can indeed reach a quasi-zero energy state (;0.05 J/m2) when a critical GB dopant enrichment is achieved. This thermodynamic condition leads to unprecedented coarsening resistance, but is a temperature dependent function; since increasing temperatures deplete the GB as the dopant dissolves back in the crystalline bulk. Professor Ricardo Castro is an associate professor in the Department of Materials Sciences and Engineering at University of California, Davis. He joined UC Davis in 2009 and is the lead of the Nanoceramics Thermochemistry Laboratory (NTL), a laboratory dedicated to provide fundamental understanding, using experimental thermodynamics (based on microcalorimetric data), on ceramic nanomaterials and their behavior under processing conditions and operation at extreme environments, such as high temperatures and radiation. Castro has a Ph.D. in Metallurgical and Materials Engineering from the University of São Paulo, Brazil, and a B.Sc. in Molecular Sciences. He received the 2014 Robert L. Coble Award by the American Ceramic Society, and the 2015 Global Young Investigator Award by the Engineering Ceramic Division among other important awards.
Ricardo H.R. Castro
I. INTRODUCTION
Thermal stability of nanocrystalline materials has been the topic of extensive scientific and technological research in the past decades.1–3 This relates to the need to control undesirable grain coarsening that nanosamples incur when exposed to high temperatures during either processing or operation conditions, compromising the unprecedented properties that this class of materials can present in terms of hardness and yield strength, for instance.4 Grain growth inhibition can be engineered from both kinetics and thermodynamics perspectives.5,6 While the kinetic approach focuses on inputting ‘pinning’ agents along a grain boundary (GB), slowing migration by creating dragging forces,7 the thermodynamic strategy targets a decrease in GB energy, the direct driving force for coarsening.8 Several Contributing Editor: Gary L. Messing a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.282
works have modeled the energetic stabilization of GBs,2,8–10 quantitatively describing the effect of a solute enrichment on
Data Loading...
