Thrust areas of NSF Center for Advanced Materials and Smart Structures(CAMSS)

Advanced Ceramics

Structural ceramics have drawn considerable attention because of high-temperature applications. The poor thermal shock resistance and low fracture toughness associated with these materials are the primary limiting factor in these applications. It is well established that the low toughness of the ceramics is a result of the surface cracks produced during various processing steps and surface deterioration at high temperatures in oxidizing and other environments. The object of this task is to modify the surface of the ceramics by applying high temperature intermetallic aluminide coatings of titanium , nickel and cobalt. Techniques involving physical vapor deposition, laser physical evaporation, and magnetron sputtering deposition will be used to develop the optimized coatings. Further, the coatings will be mixed with the ceramic substrate using different nonequilibrium processes such as laser induced melting/and rapid thermal annealing. Analytical and high resolution transmission electron microscopy and scanning electron microscopy will be used to characterize the samples. Various physical and mechanical properties of the surface modified ceramics will be determined to establish their suitability for high temperature service life. The coated ceramics will be evaluated trough fractographic observations, and fracture mechanisms will be proposed with the help of microstructural evolution of the coated ceramics.

Thick thermal barrier (protective) coatings will be formed using plasma spray and laser cladding methods. We propose to form thermal barrier coatings (YSZ, W, Co) by these methods and investigate microstructure (porosity, homogeneity of chemical composition, stresses and strains) and properties of these thermal barrier coatings on ceramic and metallic substrates with special emphasis on comustion and emission improvements of direct injection diesel engine components.

Joining of cramics represents a major obstacle in the increased application of advanced ceramics. This research program addresses selective laser melting of interfacial regions between metal layers and ceramic substrates to induce chemical reaction and bonding. By choosing a laser wavelength which is transparent to ceramics, it is possible to create energy absorption at the metal-ceramic interface and cause melting and joining. Analytical electron microscopy will be used to study the microstructure and chemical composition of various phases and interfaces. These features will be correlated with mechanical properties.

Advanced Composites

Ceramics constitute a very useflul class of materials owing to their high melting point strength and stability at elevated temperatures. However, there are disadvantages associated with these materials in terms of poor toughness, ductility, electrical and thermal properties. To alleviate these problems, in the first task, we propose to exploit some of the useful properties of metal precipitates and incorporate them into the ceramic oxide matrices. These metallic precipitates can provide a continuous source of ductility by generating dislocations and improve mechanical properties and modify optical, electrical and thermal properties in a useful way.

In the second task, we propose to study processing of novel composite materials (tungsten carbide and aluminide composites, titanium carbide and aluminide composites) by advanced processing methods which include high-temperature sintering, pressureless melt infiltration and rapid reaction synthesis. The rapid reaction synthesis utilizes reaction enthalpy for sintering and thus provides environmentally safe and energy efficient processing. By replacing conventional cobalt binder with aluminides, we avoid toxicity associated with cobalt dust, and at the same time achieve high-temperature strength needed for high-speed machining. One of the important characteristics of aluminides is that the strength either increases (to e.g. Ni3Al) or remains contact (NiAl, CoAl) with temperature, unlike cobalt where the strength decreases with temperature. In addition to improvements in high temperature strengths of these composites, we also propose to study diamond deposition and adhesion of diamond films on aluminide composites. Under chemical vapor deposition of diamond, we discovered that Fe, Co and Ni with partially filled 3-d shell stabilize sp2 bonding and catalyze graphite formation. We have shown that by alloying with electron donating elements such as Al, the graphitization was suppressed and the diamond film adhesion was improved consequently. Thus, we focus on high temperature strengths of these novel aluminide composites and improvements in high speed machining over conventional WC-Co composites.

The third task is related to smart materials structures based upon piezofibers such as SiC and carbon coated with high-temperature piezoelectric material such as AlN for enhancing properties and process control and monitoring. The SiC and carbon fibers will be tested for their mechanical properties as a function of temperature using a "hot-grip" method. The fibers will be coated with AlN using our pulsed laser deposition method, and fibers will be tested for integrity and adhesion. We envisage a variety of applications of coated fibers in advanced materials processing and process monitoring at high temperatures. In a related task, PZT coated patches of MgO and Ag will be used as a part of smart structures for vibration control and flaw detection.

The composites fabricated by these methods will be investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy to study the microstructure and chemical composition of various phases and interfaces. The microstructural and chemical features will be correlated with reaction synthesis modeling based upon heat generation and melting of one of the binder composites. The optimized specimens will be subjected to high speed machining tests and results correlated with microstructure and reaction synthesis modeling. The modeling of reaction synthesis based upon heat generation (thermodynamical reaction) and flow will be carried out to predict quenching rates and resulting microstructure.

Electronic Ceramic Devices, Sensors and Smart Structures

The research program addresses the fabrication of epitaxial thin-film membrane structures based on thin film high-critical temperature superconductors (HTSC) and ferroelectric perovskites and devices thereof. There are three basic challenges for the next-generation thin-film sensors: (i) fabrication of free-standing thin film membranes instead of thick solid substrates which can enhance the performance of many electronic devices, such as bolometers, piezo- and pyroelectric sensors; (ii) growth of single crystalline epitaxial films providing the most efficient active layers; and (iii) integration of epitaxial thin film deposition techniques with silicon circuit technology. Other critical issues include: stress control of constituent layers; monitoring of defects and interfaces, particularly grain boundaries and domain walls which control the properties of the active layers; physical parameters of the structures and their stability against thermal, electrical and mechanical cycling; role of dopants; development and fabrication of test structures for bolometers and piezoelectric sensors, and performance and reliability. The research program consists of two parts. The first part focuses on epitaxial multilayer superconductor heterostructures fabricated in the form of thin membranes for applications such as radiation sensors - bolometers. The proposed research includes: (1) advanced processing - pulsed laser deposition - with an emphasis on lattice matching and domain matching epitaxial growth of multilayer superconductor heterostructures and device fabrication using sacrificial NaCl substrates: YBCO/(MgO or YSZ)/(NaCl or Si), YBCO/SrTiO3/MgO/(NaCl or Si), and YBCO/Ag/NaCl, and YBCO/Ag/MgO/NaCl; (2) structure, chemistry and properties of interfaces and grain boundaries; (3) correlations among (1), (2) and transport properties (critical temperature, transition width and critical current density) and device characteristics. A simple proposed test device is a bolometer (radiation detector with sacrificial NaCl substrates to obtain free-standing membrane structure. The second part of the proposal focuses on epitaxial ferroelectric composite structures with the special emphasis on the growth of thin film structures via domain epitaxy on sacrificial substrates such as NaCl. The research program involves four critical components: (1) advanced processing including pulsed laser deposition and electron beam evaporation techniques for multilayer metal-ferroelectric structures Me/PZT/Me (Me = Ag or Pt) on NaCl, MgO/NaCl, MgO/(NaCl or Si), and SrTiO3/MgO/(NaCl or Si); (2) structure and chemistry of grain boundaries, ferroelectric domain walls, ferroelectric-metal interfaces. Mechanism of stress relaxation during growth and post-growth annealing is of primary importance for thin free-standing ferroelectric structures and will be systematically investigated as a function of substrate, growth conditions and nature of epitaxy; (3) Correlation of processing conditions and structure of defects and interfaces with the basic parameters of the ferroelectric composite structures: remnant polarization and its stability, fatigue, piezoelectric and pyroelectric characteristics achievable in thin epitaxial structures, leakage current, residual stress, mechanical strength, and the stability and compatibility of the films and electrodes with device fabrication procedures and operating conditions; (4) Fabrication of a simple proposed test device - an acoustic piezosensor consisting of (Ag or Pt)/PZT/(Ag/MgO or Ag)/NaCl composite structure, and evaluation of its characteristics.

Wide-Band-Gap III-V Semiconductors, Ohmic Contacts and Devices

The research program addresses materials processing methods based upon pulsed laser deposition and plasma source molecular beam epitaxy of hexagonal (equilibrium phase) and cubic (noneqilibrium) III-V nitrides such as AlGaN and InGaN and processing of alloyed and non-alloyed ohmic contacts to these nitrides. The III-V nitrides and their alloys in the hexagonal form will be grown on 6H-SiC substrate via lattice-matching epitaxy, and on a-Al203 with and without TiN buffer layer via domain matching epitaxy where integral multiples of lattice match across the film-substrate interface. In the cubic form these materials can be grown on MgO(100) and TiN(100) substrates via lattice-matching epitaxy. The primary focus of this proposal is on formation of device-quality III-V heterostrucures, electrical properties of active layers and formation of ohmic contacts. This will be accomplished by understanding the role of growth and substrate parameters on defect generation, and devising methodologies to control and minimize their harmful effects. Special attention is focused on defect reduction, doping characteristics, the nature of epitaxy (two-dimensional vs. three-dimensional) and on the nature of resulting defects (dislocations and domain boundaries) and interfaces. The program systematically investigates the factors (low temperature grown and low mismatch buffer layers, surfactants) that provide smooth nitride surface morphology and efficient incorporation of n- and p-type dopants. The atomic structure and chemistry of interfaces will be investigated using high-resolution and STEM-Z contrast transmission electron microscopy. Defect densities in the films, particularly threading dislocations and mismatch domain boundaries are too high 10 9 cm-2 for applications such as lasers and high power devices. We will adopt the novel approaches to reduce the number of threading dislocations and boundaries: (1) complete relaxation of the film just above the critical thickness; and (2) dislocation coiling mechanism to reduce the number of threading dislocations through the film. We investigate systematically microstructure, interface structure and chemistry, and electrical properties of three types of contact schemes: (i) based on ordered Cu3 Ge compound, (ii) TiN epitaxial layers, and (iii) based on small bandgap semiconductors (GaSb, InAs)

Copyright © 1997 NSF Center for Advanced Materials and Smart Structures. All rights reserved. Reproduction in whole or in part in any form without express written permission of the CAMSS is prohibited.

Last modified 7-aug-02