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