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SUMMARY:Nano-calorimetry: A centuries-old technique applied at the nano-sc
 ale
DTSTART:20170824T100000
DTSTAMP:20260609T140023Z
UID:5842630fbc036319fffc40de9166c75aec6e74cc16ad3f257e541ce8
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Joost J. Vlassak\, Harvard  \nAn IMT Talk\n\nCalorimet
 ry has long been used to study chemical reactions and phase transitions in
  materials. The technique finds its origin in the mid-18th century when Sc
 ottish physician Joseph Black discovered the notion of latent heat and Lav
 oisier developed an ice calorimeter to measure the amount of heat given of
 f during combustion of carbon or respiration of living organisms. Since th
 en calorimetry has developed into a sophisticated technique indispensable 
 in chemistry and materials science.\n\nIn this seminar\, I will show how t
 he same technique can be used to make measurements at the nano-scale. We u
 se micromachining to fabricate arrays of calorimetric sensors that can per
 form measurements on samples as thin as a few nanometers at rates varying 
 from isothermal to 105 K/s. The sensor arrays are ideally suited to explor
 e complex materials systems in a combinatorial approach based on thin-film
  composition spreads. This methodology is illustrated using high-temperatu
 re Ni-Ti-based shape memory alloys\, where the effects of composition\, sa
 mple length scale\, heat treatment\, and thermal fatigue on the shape memo
 ry properties are revealed. Because of its large dynamic range\, nano­cal
 orimetry is also ideal for studying the kinetics of solid-state and solid-
 gas reactions. We use nano-calorimetry to evaluate the kinetics of the sol
 id-state reactions in Zr/B and Zr/B4C multilayers at temperatures up to 1\
 ,400K. We demonstrate that ultra-high temperature ceramics such as ZrB2 an
 d ZrB2/ZrC alloys can be synthesized at moderate temperature using reactiv
 e multilayers.\n\nThe formation reactions typically proceed in two distinc
 t steps: inter-diffusion and amorphization\, followed by crystallization. 
 Kinetics measurements show that the activation energies for both processes
  are very different. First-principles calculations provide insight in the 
 amorphization processes in the reactive multilayers and confirm the relati
 vely low activation energies associated with the amorphization process. Fi
 nally\, I will present some results on the crystallization kinetics of Cu5
 0Zr50 metallic glass. Measurements over a wide range of scanning rates rev
 eal that crystallization upon heating does not follow Arrhenius kinetics. 
 Instead\, the behavior is well described by a fragility-based model of gro
 wth-controlled kinetics that takes into account breakdown of the Stokes-Ei
 nstein relationship between diffusivity and viscosity as a result of the h
 eterogeneous structure that develops in the undercooled liquid state.\n \
 nBio: Professor Vlassak studies the thermo-mechanical behavior of a broad 
 range of engineering materials. He has developed experimental methods for 
 the characterization of phase transitions and solid-state reactions in thi
 n films\, plastic deformation in coatings\, elastic anisotropy in indentat
 ion\, and fracture. Current projects focus on the mechanical behavior of h
 ydrogels and the use of hydrogels as ionic conductors in 3D printed device
 s. Professor Vlassak has pioneered the use of combinatorial nanocalorimetr
 y for the analysis of complex materials systems\, including metallic glass
 es\, ultra-high temperature ceramics\, and shape memory alloys. He has aut
 hored more than 140 journal publications.\n 
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