Laboratory for Parallel Computational Mechanics - 408 EES
Model-Based Simulations to Engineer Nanoporous Thin Films
This project, which is sponsored by AFOSR through Grant No. F49620-02-1-0106, will create a model-based simulation (MBS) capability for tailoring the fabrication processes and the properties ofwide band gap semiconductors with engineered nanoscale porosity via the:
• computational prediction of the morphology of a thin film
as it derives from a given set of deposition parameters
• theoretical determination of the deposition conditions leading
to the creation of a given target nanostructure;
• computational prediction of mechanical properties such as the
porosity distribution throughout the film, assessment of its mechanical
stability along with the spatial distributions of the intrinsic stresses,
interfacial fracture toughness, and (heterogeneous) elastic moduli as
well as their symmetry properties;
• computational measure of the constitutive parameters required
for use in advanced bending theories of thin films. This will contribute
to future developments in which in situ measurements of the spatial
distribution of thin film curvature are used for the real-time estimation
of the intrinsic stresses in nanoporous thin films.
Porous thin films made from wide band gap semiconductors, which include
SiC and the group III nitrides, are being explored for a wide range
of applications. Nanosculptured thin films, in particular, are of interest
for short wavelength optical devices. For example, porous SiC is of
interest for chemical sensing. Much work has been conducted on porous
Si sensors, with which a wide variety of gases have been sensed (e.g.,
benzene, toluene, trichloroethylene, chloroform, methanol, and water
vapor). The advantage of using SiC over Si is that the wider band gap
of SiC permits operation of the semiconductor at much higher temperatures
(> 300C) than is 1 possible for Si. Researchers are currently studying
porous SiC and GaN onto which transition metals, transition metal oxides,
and metallic sulfides have been deposited for use as catalysts, with
experiments focusing on the reaction of reagent molecules such as methane,
methanol, and toluene. Unique porous nanostructures of the group III
nitride semiconductors will be realized in this project using in-situ
sculpturing.
Dynamic Fracture in Temperature Sensitive Materials with Cohesive Zones: A Discontinuous Galerkin Approach
This project, which is sponsored by AFOSR through Grant No. F49620-02-1-0318, intends to expand the understanding of the role of temperature in controlling the dynamic failure behavior of advanced materials subject to combined thermo-mechanical loading. These objectives will be achieved by improving the continuum-based modeling of the fracture properties of materials in conjunction with the formulation and development of a corresponding numerical approach for the solution of the resulting governing equations.
The goal of the proposed modeling effort is the inclusion of temperature dependence in the description of the fracture behavior of the material. In addition to modeling the bulk material as a fully coupled linear thermo-elastic solid, this goal will be achieved by using rate and temperature dependent cohesive zone (CZ) models to account for the highly localized, nonlinear, and softening behavior of the material ahead of the propagating crack. In this project, the cohesive stresses will be assumed to be physically-based constitutive functions of the opening displacement, opening displacement rate and temperature. Finally, the CZ will be assumed to fail in two basic ways: (i) by achieving a critical crack opening displacement; and/or (ii) by experiencing a critical value of cohesive stress. Both the critical crack opening displacement and the critical cohesive stress will be assumed to be functions of temperature.
The primary objective of the proposed numerical development is the
formulation of a high accuracy and unconditionally stable solution scheme
for the combined parabolic/hyperbolic problem describing dynamic fracture
in a linear thermo-elastic solid. Adaptivity will be a primary feature
of the proposed numerical scheme as it will be is crucial for the accurate
quantification of the microscopic features that are nucleated during
propagation as these features are a means of comparison between theory
and experiments. Said numerical approach will be based on the discontinuous
Galerkin finite element method (DG FEM). The DG FEM has been shown to
be extremely effective in the solution of both parabolic and hyperbolic
problems in the presence of moving discontinuities.
Modeling the Dynamics Response of Nanowire Structures for Integrated Nanomechanical Biosensor Arrays
The purpose of this research, which is sponsored via a Penn State MRI seed grant, is to analyze the dynamics of nanowire biosensors. In particular, this work is intended to be used as an aid in the design of sensors being developed here at Penn State by the research groups of Prof. Theresa Mayer (Electrical Engineering) and of Prof. Christine Keating (Chemistry).
The basic concept is to design a sensor that can detect particles on
molecular scales. Such a sensor would be of great use in biology and
medicine for the detection of DNA strands of viruses. The nucleotides
that comprise the DNA strands have molecular weights on the order of
100 amu, and a 12-mer DNA sequence typical of the targets sought would
have a mass on the order of 10-24 kg. Detection of such small targets
suggests that a small sensor must be used to achieve the necessary sensitivity.
Therefore, the idea for this sensor is to construct a nanoscale beam,
or nanowire, in such a way that the target DNA strands can attach to
the beam. The attached DNA strands will add mass to the beam and cause
a lowering of the natural resonance frequency of the beam. Measuring
a shift in resonance frequency indicates that DNA strands are attached
to the beam, and hence, are present in the test environment.
Our efforts have been directed at determining the dynamic response of the nanowire as a function of the mass and position along the length of the sensor of the target DNA; of the geometry of the nanowire and its supporting structure; of the material properties of the nanowire; and of the dissipation that affects the quality factor of the response.
A New Approach to Introductory Engineering Mechanics Education
This project, which is sponsored by the NSF through Grant No. DUE-0127511,
aims to develop a prototype package of educational materials to facilitate
the adoption of Problem-Based Learning (PBL) into sophomore-level mechanics
courses. The objective of this 18-month proof of concept (POC) project
is to create PBL-based materials sufficient to cover those topics in
undergraduate dynamics typically covered in one
semester. These new materials include
• the relevant sections of a dynamics textbook; and
• enough homework and example problems, whose emphasis will be
motion over
intervals of space and time, to get through a semester.
A subset of these materials was tested in the spring of 2004 in concert
with the textbook we are currently using. Baseline assessment was done
during the spring and fall 2003 semesters. In addition, through a contract
with McGraw-Hill, the materials are being continually reviewed by faculty
throughout the United States and will be commercially published in 2005.
Now that the POC period is complete, we intend
to submit a proposal to complete the development of the full package,
including substantial web- and team-based components.