Vlassak Group
Joost J. Vlassak
Abbott and James Lawrence Professor of Materials Engineering
Education
Burgerlijk Ingenieur, 1989, Metallurgical Engineering, University of Leuven
M.S., 1990, Materials Science, Stanford University
Ph.D., 1994, Materials Science, Stanford University
Academic Interests
Materials Engineering
Professor Vlassak studies the thermo-mechanical behavior of a broad range of engineering materials. He has developed experimental methods to characterize phase transitions and solid-state reactions in thin films, plastic deformation of coatings, elastic anisotropy in indentation, and fracture of thin films. Current experimental research projects focus on the mechanical behavior of hydrogels (Figure 1, 2) and the use of hydrogels as ionic conductors in 3D printed devices (Figure 3). Recently, Professor Vlassak pioneered the use of combinatorial nanocalorimetry (Figure 4) for the development and analysis of complex materials systems, including metallic glasses, ultra-high temperature ceramics (Figure 5), and high-temperature shape memory alloys. Combinatorial nanocalorimetry provides a convenient way of mapping the energetics and kinetics of phase transitions and solid-state reactions as a function of composition, making it an ideal experimental tool for the Materials Genome Project (Figure 6). The technique also lends itself for the characterization of thermal fatigue and stability of functional materials, as well as for the optimization of heat treatments.
Past research projects focused on the mechanical degradation of electrodes in lithium ion batteries as a result of lithium insertion, on the effects of microstructural length scales on the mechanical behavior of thin metal films, and on the effect of chemical species on the adhesion and delamination of multilayered structures containing low-k dielectrics.
Theoretical work includes modeling of chemical mechanical polishing (CMP) based on contact mechanics, channel cracking in films on substrates of finite thickness, and various analyses of the effects of substrate properties and film porosity on nanoindentation of thin films.
Links:
A Heat Sensor for Living Cells
Photophoresis for near-space exploration
Thermodynamics of active matter
Figure 1. (a) Self-healing hydrogel specimen with a 4 mm gauge length for tensile testing; specimen before (b) and after (c) stretching. Representative nominal (d) and true (e) stress–stretch curves of various hydrogel specimens.
Double network hydrogels are tough
Figure 2. Movie shows a ball bearing bouncing off of a thin hydrogel membrane that consists of ~ 93% water.
Figure 3. Fracture and fatigue of fibrous hydrogels
Figure 4. A single hydrogel-based strain gauge is mounted onto the index finger and held in various positions while the resistance is recorded. (a) Illustrates the mounting of the strain gauge using VHB tape and leads connecting the gauge. (b) Illustrates the various positions alongside the time-trace of the gauge’s resistance.
Figure 5. (a) Deposition of a library of thin-film samples on a nanocalorimetry substrate; substrate with micromachined nanocalorimetry sensors for combinatorial analysis of materials systems; (c) schematic of a single sensor; (d) optical micrograph of a sample.
Figure 6. Enthalpy produced during the reaction of B and Zr to form ZrB2 in a reactive multilayer. The first peak corresponds to a solid-state amorphization reaction, the second peak to a crystallization reaction.
Figure 7. Use of a combined theoretical and experimental approach to identify new glass forming materials systems for the Materials Genome Project.
Photophoretic Levitation
An article about our work: https://seas.harvard.edu/news/new-window-earths-upper-atmosphere
We have built thin, centimeter-scale flyers that consist of 100 nm thick alumina, with a chromium-based multilayer on the bottom to absorb sunlight. When light hits this structure, the temperature difference between the top and bottom surfaces causes a photophoretic lifting force that exceeds the weight of the structure.
Demonstration of photophoretic levitation under one-sun illumination
Research Projects
- Hydrogels: Solid Water at Room Temperature
- Micromachined Devices for Combinatorial Materials Analysis
- Mechanical Behavior and Microstructure of Metallic Thin Films etc.
- Mechanical Behavior of Hybrid Organic/Inorganic Systems
- Low-k Dielectric Materials
- Size Effects in NiTi Shape Memory Alloy Films
- Ferromagnetic Shape Memory Thin Films
- Nanoindentation
- Chemical Mechanical Polishing as a Contact Problem
- Mechanics of Li Ion Batteries