Serpentine-Patterned Compliant Thin Films of Stiff Materials Reduce Strain During Stretching
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RESEARCH/RESEARCHERS
Grain-Size Growth Model Validated by In Situ Measurements of Nucleation and Growth Using in situ heating transmission electron microscopy (TEM) methods, a team of researchers at Yale University has devised a technique to predict the microstructure resulting from crystallization. This ability allows researchers to tailor material properties that are sensitive to microstructure and is important to a broad range of fields that includes materials science, geology, physical chemistry, and biochemistry. There are theoretical models to predict the grain size and also experiments to monitor the growth of individual crystals. However, this research is the first to check the validity of theoretical predictions by comparing them to experimental observations that can separate the effects of nucleation and growth. These methods would enable researchers to predict grain size and the associated material properties that are dependent on microstructure. In the September 19 issue of Applied Physics Letters (#124102; DOI: 10.1063/ 1.2053348), the Yale team of H.-J. Lee, H. Ni, D.T. Wu, and A.G. Ramirez reported a method of predicting grain size from the direct measurement of nucleation and growth rates. The researchers sputterdeposited amorphous 200-nm-thick equiatomic NiTi films onto silicon nitride TEM membrane samples, annealed them at various temperatures within a 200-kV field-emission TEM microscope, and took images during heating at 2-s intervals. The researchers were able to monitor the crystallization events in real time. The amorphous NiTi films underwent polymorphic crystallization during annealing and transformation times that increased drastically from a few seconds to over an hour from the highest to the lowest temperatures. The researchers determined the nucleation rate and the growth rate by measuring the number of crystals and their size as a function of time. These results were compared with values derived from the conventional Johnson– Mehl–Avrami–Kolmogorov (JMAK) method of determining the area fraction and were found to be consistent. To verify their approach, the researchers compared the activation energies of nucleation and growth with the overall activation energy determined by the JMAK method and found these values to agree. In contrast to the JMAK method, the novel contribution of this work is that the nucleation and growth rates are measured separately. This information allowed the researchers to use the mathematics of crystallization in a new way, namely, to deterMRS BULLETIN • VOLUME 30 • DECEMBER 2005
Serpentine-Patterned Compliant Compliant Thin Thin Films Films of of Stiff Stiff Materials Materials Serpentine-Patterned Reduce Strain Strain During During Stretching Stretching Reduce
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