Shape memory ceramics in small volumes
Author(s)Lai, Alan, Ph. D. Massachusetts Institute of Technology
Massachusetts Institute of Technology. Department of Materials Science and Engineering.
Christopher A. Schuh.
MetadataShow full item record
Shape memory ceramics rely on martensitic transformations which are similar to those found in metallic shape memory materials, but ceramics offer several advantages over metals such as higher operating temperatures and larger transformation stresses. However, polycrystalline shape memory ceramics have shown poor cycling performance which limits their use in practical applications. This is due to the inherent physical constraints of the grains that create stress concentrations and eventually leads to intergranular fracture. Here it is proposed that single crystalline and oligocrystalline ceramics-made with a single grain or very few grains-will avoid the physical constraints found in polycrystalline materials that lead to intergranular fracture and result in shape memory ceramics with enhanced cycling performance. Zirconia was chosen for study because it has the necessary martensitic transformations and has shown limited shape memory properties when in the bulk polycrystalline form. Focused ion beam milling was used to make single crystal and oligocrystal pillars of varying diameter that were compression tested using a nanomechanical testing platform to determine the mechanical properties. This work showed that removing grain constraints in micron-scale shape memory zirconia prevented cracking and fracture. It also enhanced the number of achievable repeatable cycles from five in bulk materials to at least hundreds in small structures. The transition from single- to oligo- to poly-crystal was explored and it was found that fracture is more likely in polycrystals and that the transformation stresses increase as pillar diameter is increased, the opposite of what is observed in shape memory metals. This phenomenon is attributed to the higher stiffness of ceramics making the stored elastic energy more important. The effect of crystal orientation was investigated to aid in design and optimization. Orientation maps were produced for fracture behavior, elastic modulus, transformation stress, and transformation strain. Finally four different scale-up architectures were proposed and implemented - powders, wires, foams, and thin films - and each demonstrated shape memory properties thereby paving the way for deployment in practical applications.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 133-139).
DepartmentMassachusetts Institute of Technology. Department of Materials Science and Engineering.
Massachusetts Institute of Technology
Materials Science and Engineering.