Current Research Projects

 

Understanding Cracking and Defect Formation during AlN Crystal Growth

Sponsor: NSF, CMMI-0928556, GOALI/Collaborative Research

Participants: Antoinette Maniatty (PI), Matthew Miller (PI, Cornell), Crystal IS, Inc.

Students: Payman Karvanirabori, Kristen Lee

Summary:
The objectives of this research are to gain a better understanding of the thermo-mechanical behavior, and associated underlying micromechanical phenomena, of aluminum nitride (AlN) at temperatures experienced during crystal growth and to develop models and simulations tools based on this understanding that can accurately predict cracking and crystal quality during the growth process. AlN is a wide band gap material that is particularly well-suited for high power, high frequency, energy efficient electronic and photonic devices. However, the use of AlN in many potential applications is currently limited by the high cost of AlN wafers, which results from the challenges in manufacturing high quality bulk substrates. Crystal IS, the industrial partner on this proposal, has developed new technologies that allows the growth of low defect, 50 mm diameter substrates, but yield may be adversely affected by high thermal stresses that induce cracking as the crystal is cooled from the growth temperature. While the electrical properties of AlN associated with its potential applications are well-known, the mechanical properties associated with deformation and failure during the growth process, particularly at the high growth temperatures, are not known.

An integrated effort involving experimentation, modeling, and simulation is being used to address the above objectives. The experimental work focuses on in situ x-ray diffraction of single crystals under loading conditions over a broad range of temperatures in order to discover the mechanisms and temperature dependent properties associated with elastic-plastic deformation at the very high growth temperatures and fracture at the lower temperatures. This effort is closely coupled with the modeling and simulation work that is focused on developing a dislocation based crystal plasticity model that captures the plastic behavior at high temperatures associated with dislocation glide, and implementation of this model into an advanced finite element framework. The crystal plasticity model will be based on observed phenomena in the x-ray diffraction experiments, and predictions of lattice strains from the finite element simulations of the loaded crystal will be matched to the experimental measurements to back out key temperature-dependent mechanical properties, including elastic moduli, critical resolved shear stresses, and fracture toughness. The finite element simulation tool will then be used to predict cracking and dislocation densities in crystals cooled from growth temperatures, and the results will be validated against crystal growth experiments performed at Crystal IS for different processing conditions.

This work will be integrated into educational programs at the participating institutions, and the industrial collaboration will provide excellent learning opportunities for participating students as well as the PIs. If successful, this work will guide crystal growth process designers so that larger crystals with a higher yield, while maintaining high quality, can be achieved. The knowledge gained and the tools developed will also be applicable to other III-nitrides.

 

3D Prostate Cancer Imaging Based on "Crawling Wave" Excitation

Sponsor: NIH, 5R01AG029804-03

Participants: Deborah Rubens (PI, University of Rochester), Kevin Parker (co-PI, University of Rochester), Joyce McLaughlin (co-PI, Rensselaer Polytechnic Institute), Assad Oberai (co-PI, Rensselaer Polytechnic Institute), Antoinette Maniatty (co-PI), Kai Thomenius (co-PI, GE Global research)

Student: Ben Szajewski

Summary:
This project combines expertise from three institutions, the University of Rochester (UR), General Electric Global Research (GE), and Renssalear Polytechnic Institute (RPI), to create and assess a novel three dimensional (3D) imaging scanner applied to prostate cancer. The imaging is based on the newly discovered phenomenon of crawling waves (UR), which are a set of interfering shear waves that can be visualized in real time using Doppler spectral techniques. The local behavior of the crawling waves is strongly linked to the biomechanical properties of the local tissue, which, in turn, depend on the tissue structure. Previous studies have demonstrated that there is a large biomechanical contrast between many prostate cancers and surrounding non-cancerous tissue; and that this contrast can be imaged by a variety of elastographic imaging techniques. The phenomenon of crawling waves along with in situ excitation and advanced estimator techniques now makes it possible to take real-time imaging of biomechanical properties to a new level of quantitative accuracy, at better resolution than was previously possible. Researchers at RPI are developing algorithms for modeling the resulting shear waves induced by acoustic radiation forces and the interference patterns that are set up in soft tissue and for reconstructing spatial maps of tissue properties based on crawling wave images.

 

Recent Prior Projects

 

Strength and Formability of Fine Grain Size Al-Mg Alloys

Sponsor: NSF, CMMI-0502891

Participants: Antoinette Maniatty (PI), Catalin Picu (co-PI, Rensselaer Polytechnic Institute), Frederic Barlat (co-PI, Pohang University of Science and Technology, Korea and University of Aveiro, Portugal; previously at Alcoa, Inc.), J. J. Gracio (collaborator, University of Aveiro, Portugal)

Student: Frank Xu

Summary:
The objective of this research program is to explore ways to improve the strain rate sensitivity and hardening rates in Al-Mg (5xxx) alloys by controlling the grain size. Grain sizes on the order of 1 um are considered. The goal is to gain a better understanding of the relationship between micro and nano-structure and formability and to develop a predictive capability to determine the optimal grain size and texture that can be produced which will lead to improved formability.

The formability of aluminum alloys is largely dictated by a combination of its strain rate sensitivity and hardening rate. Most aluminum alloys, in sheet form, exhibit a negative strain rate sensitivity at room temperature leading to unstable yielding phenomena, rapid localization, and failure. This behavior is associated with a process called "dynamic strain aging" (DSA), which refers to fast diffusing solute atoms interacting with dislocations causing unsteady, collective motion of the dislocations. The high hardening rates exhibited by these materials, to some degree, act to counter the effect of the negative strain rate sensitivity improving deformation stability. The strain rate sensitivity of many metals has been found to increase when the grain size is very small Thus, reducing the grain size may compensate for the reduced strain rate sensitivity resulting from DSA.

Grain size reduction may be achieved by various methods, of which the most popular are ECAP extrusion and asymmetric rolling. As the second procedure is viewed as more amenable to implementation in an industrial setting, we have studied primarily microstructures produced by this method.

The objectives of this research were:
1. Investigate by means of nano and mesoscale modeling and experimentation the mechanisms that control strain rate sensitivity in Al alloys. Develop models applicable at the macroscopic scale and which contain information about the dominant smaller scale deformation mechanisms.
2. Model and predict the evolution of microstructure (grain structure, texture, etc) during asymmetric rolling and the material response of the resulting fine-grained material. Identify differences between the mechanical behavior of the alloy processed by normal and asymmetric rolling.
3. Use this international and diverse collaboration to integrate what we learn from this research and from our discussions about education in the US, Portugal and France, into our respective educational programs. Provide new and exciting opportunities for both graduate and undergraduate students.

 

X-ray Microbeam Studies of Electromigration

Sponsor: NSF, DMR-0312189

Participants: G. Slade Cargill III (Lehigh University, PI), Antoinette Maniatty (co-PI)

Students: Chia-Ju Yang, Linda Ge

Summary:
In this research, x-ray microbeam diffraction and fluorescence and other experimental techniques, together with microstructure-dependent modeling and simulation was used to investigate the effect of microstructure on electromigration and thermally induced elastic strains in aluminum and copper conductor lines. The effect of microstructure on thermal stress variability was examined using a combination of x-ray microbeam experiments and a polycrystal finite element model that accounted for the grain orientations and dislocation interactions. The effect of microstructure on diffusion paths was studied by comparing x-ray microbeam data from lines with different microstructures with finite element simulations of electromigration with different diffusion paths considered.

 

Fatigue Modeling of Polycrystalline Metal Alloys Based on Microstructural Phenomena

Sponsor: Northrop-Grumman Corporation/DARPA Structural Integrity Prognosis System (SIPS)

Modeling and Simulation Group Collaborators: Elias Anagnostou (NG), John Papazian (NG), Anthony Rollett (Carnegie-Mellon), Anthony Ingraffea (Cornell), Robert Wei (Lehigh), and Gary Harlow (Lehigh)

Post-doc and Student:David Littlewood, Devin Pyle

Summary:
The overall response of polycrystalline metals is strongly tied to grain-scale phenomena. Material heterogeneities, particularly constituent particles in AA7075 and AA7050, are sources of localized plastic deformation, damage accumulation, and crack nucleation. The local grain orientations and structure surrounding constituent particles strongly affects the heterogeneous stress and strain states that develop, which, in turn, affects crack nucleation and microstructurally short crack propagation. The formulation and implementation of crystal plasticity models and damage metrics for the SIPS project are centered on the relationship between heterogeneous response at the grain scale and the accumulation of fatigue damage leading to crack nucleation. My group developed and implemented into a parallel finite element framework a crystal plasticity model and damage metrics for predicting fatigue crack nucleation.

 

Modeling and Control of Bulk Growth of AlN Crystals

Sponsor: New York State Energy Research and Development Authority (NYSERDA)

Participants: Leo Schowalter (PI, Crystal IS), Antoinette Maniatty (co-PI), Sheppard Salon (co-PI, Rensselaer), John Wen (co-PI, Rensselaer)

Post-doc: David Littlewood

Summary:
We developed both analytic and finite element thermo-elastic models to predict the stress state in the AlN boule during cooling from the growth temperature, and, we examined the effect of certain process design parameters on the normal stress on the cleavage planes.

 

Polycrystal Microstructure Reconstruction and Meshing

Sponsor: Simmetrix, Inc./Army Reseaerch Lab SBIR

Participants: O. Klaas (PI, Simmetrix, Inc.), A. Maniatty (co-PI), A. Ingraffea (co-PI, Cornell), A. Rollett (co-PI, Carnegie Mellon)

Post-doc: David Littlewood

 

Methods for High Resolution Imaging of Shear Stiffness

Sponsor: NIH

Participants: J. McLaughlin (PI), A. Maniatty (co-PI)

Student: Eunyoung Park

 

Modeling Microstructure Evolution during Hot Bulk Forming of Al-Mg-Si Alloys

Sponsor: NSF

Participants: A. Maniatty (PI), W. Misiolek (co-PI, Lehigh), D. Williams (co-PI, Lehigh)

Student: Jing Lu