Joost Vlassak's blog

Applications are invited for a postdoctoral position in the Joost Vlassak group at Harvard University with an expected start date of October 1, 2018. This goal of this position is to develop new calorimetric sensing technologies and use these to study cellular bioenergetics and thermodynamics of materials. This highly interdisciplinary project includes the development of new devices and sensing systems, materials characterization, and cellular biology.

HARVARD SCHOOL OF ENGINEERING AND APPLIED SCIENCES

Tenure-track Faculty Position in Materials Science and Engineering

School of Engineering and Applied Sciences, Harvard University

The Harvard School of Engineering and Applied Sciences seeks highly qualified applicants for a post-doctoral research position in experimental mechanics.  The successful candidate will participate in a project to make ultra-high-temperature calorimetry measurements on nanoscale quantities of materials. This apparatus will be used to study phase transformations in complex ternary and quaternary materials systems.

POST-DOCTORAL RESEARCH POSITION IN ENVIRONMENTAL ENGINEERING AND MATERIALS SCIENCE

School of Engineering and Applied Sciences, Harvard University

POST-DOCTORAL RESEARCH POSITION IN EXPERIMENTAL MATERIALS SCIENCE

School of Engineering and Applied Sciences, Harvard University

The Harvard School of Engineering and Applied Sciences seeks highly qualified applicants for a post-doctoral research position in experimental materials science.  The successful candidate will participate in a project to design an apparatus for high-temperature calorimetry on nanoscale quantities of materials. This apparatus will be used to study phase transformations in complex ternary and quaternary materials systems.

This is a paper we recently published in JMPS on a study of the mechanical properties on thin films comparing experimental results with discrete dislocation simulations. It provides insight in the strengthening that occurs in thin metal films when surface or interface effects become important.

The abstract is below; the full paper can be downloaded from here

Abstract - Experimental measurements and computational results for the evolution of plastic deformation in freestanding thin films are compared. In the experiments, the stress–strain response of two sets of Cu films is determined in the plane-strain bulge test. One set of samples consists of electroplated Cu films, while the other set is sputter-deposited. Unpassivated films, films passivated on one side and films passivated on both sides are considered. The calculations are carried out within a two-dimensional plane strain framework with the dislocations modeled as line singularities in an isotropic elastic solid. The film is modeled by a unit cell consisting of eight grains, each of which has three slip systems. The film is initially free of dislocations which then nucleate from a specified distribution of Frank–Read sources. The grain boundaries and any film-passivation layer interfaces are taken to be impenetrable to dislocations. Both the experiments and the computations show: (i) a flow strength for the passivated films that is greater than for the unpassivated films and (ii) hysteresis and a Bauschinger effect that increases with increasing pre-strain for passivated films, while for unpassivated films hysteresis and a Bauschinger effect are small or absent. Furthermore, the experimental measurements and computational results for the 0.2% offset yield strength stress, and the evolution of hysteresis and of the Bauschinger effect are in good quantitative agreement.

Ninth U.S. National Congress on computational mechanics
July 22 -26, 2007. San Francisco, California

A mini-symposium on

COMPUTATIONAL METHODS FOR MICRO AND NANO SYSTEMS

Call for Papers
Micro and Nano Electro Mechanical Systems have recently attracted much attention from the industry and from the scientific community. MEMS are nowadays routinely met in various fields like in the automotive, aerospace and large consumer applications.
It can be said that for various micro systems the pioneering phase has been substituted by a phase of industrial applications. Hence, new challenges concerning reliability, optimization and increasing miniaturizations must be tackled by the designers. All these issues need a multi-disciplinary approach and must be supported by multi-physics numerical and experimental analyses able to contribute to the definition of a unified design and analysis methodology of MEMS and NEMS.

Ting Y. Tsui, Andrew J. McKerrow, and Joost J. Vlassak

Published in the Journal of The Mechanics and Physics of Solids, 54 (5), 887-903 (2006)

Abstract – Organosilicate glass (OSG) is a material that is used as a dielectric in advanced integrated circuits. It has a network structure similar to that of amorphous silica where a fraction of the Si-O bonds has been replaced by organic groups. It is well known from prior work that OSG is sensitive to subcritical crack growth as water molecules in the environment are transported to the crack tip and assist in rupturing Si-O bonds at the crack tip. In this study, we demonstrate that exposure of an OSG containing film stack to water prior to fracture results in degradation of the adhesion of the film stack. This degradation is the result of the diffusion of water into the film stack. We propose a quantitative model to predict adhesion degradation as a function of exposure time by coupling the results of independent subcritical crack growth measurements with diffusion concentration profiles. The model agrees well with experimental data and provides a novel method for measuring the water diffusion coefficient in film stacks that contain OSG. This study has important implications for the reliability of advanced integrated circuits.