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6
SELECTED APPLICATION AREAS
ELECTROCERAMICS
In considering the properties of electrocerPmic composites, relevant
areas of interest are phase morphology, symmetry, and microstructural scale.
Some of the morphological aspects have been discussed and need no further
elaboration. Symmetry considerations have been extensively discussed in
papers by Newnham and coworkers (1978~. In what follows, the influence of
scale and periodicity are examined, with emphasis on the need to develop
appropriate nanoscale composite structures.
A good deal has been written about the importance of scale in magnetic
optical, and semiconductor materials. Many of the same effects occur in
ferroelectrics (critical domain sizes, resonance phenomena, electron
tunneling, and nonlinear effects). In ferromagnetic materials, there are
three kinds of magnetic structures for small particles. Multidomain
structures are common for particles larger than a critical size; magnetization
in large particles takes place through domain wall motion.
Below this
_ . _
critical size, single domain particles are observed, and switching takes place
by rotation rather than wall movement, thereby raising the coercive field.
Very small particles exhibit a superma~netic effect in which the spins rotate
in unison under thermal excitation.
to align the spins of adjacent particles.
particles has yet to be fully established,
experimental results are accumulating. In BaTiO3 ceramics,
behavior is observed in grains less than approximately 1 ~m, while dielectric
phenomena resembling those found in superparamagnetism are found in relator
ferroelectrics. The fluctuating microdomains in this superparaelectric state
are about 20 nm across. Composite materials made up of single domain and
superparaelectric particles have yet to be investigated ~ n a systematic way
with proper control of the connectivity and surrounding environment. The
~ .
Only modest magnetic fields are required
Analogous behavior in ferroelectric
but a variety of interesting
89
single domain
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controlled synthesis of submicrometer ferroelectric grains will do much to
stimulate research in this area.
Surface treatment of the ferroelectric phase allows control of the
mechanical boundary conditions. Titanyl coupling agents are effective in
bonding PZT to epoxy. Mechanical pull tests have been used to demonstrate the
strength of the ceramic-polymer bond. Improved stress transfer and large
piezoelectric coefficients in piezoelectric composites are obtained as a
result of better bonding. Polymers are about a hundred times more compliant
than ceramics. If a ceramic grain is surrounded by polymer, the mechanical
constraints are relatively small. This means that more complete poling is
possible as demonstrated in ferroelectric composites. Electrical boundary
conditions can also be controlled by adjusting the dielectric constant and
conductivity of the surrounding phase.
Periodicity and scale are important factors when composites are to be
used at high frequencies where resonance and interference effects occur. When
the wavelengths are on the same scale as the component dimensions, the
composite no longer behaves like a uniform solid. An interesting example of
unusual wave behavior occurs in composite transducers made from poled
ferroelectric fibers embedded in an epoxy matrix. When driven in thickness
resonance, the regularly spaced fibers excite resonance modes in the polymer
matrix, causing the matrix to vibrate with much larger amplitude than the
piezoelectric fibers. The difference in compliance coefficients causes the
nonpiezoelectric phase to respond far more than the stiff ceramic
piezoelectric. Composite materials are therefore capable of mechanical
amplification from prepoled PZT fibers mounted in a polymer matrix. Domain-
divided transducers operate on a similar principle. Multidomain crystals and
ceramics have been used as acoustic phase plates and high-frequency
transducers.
The extension of this thinking to phenomena associated with optical
excitations automatically focuses attention on equivalent nanoscale
structures. Specifically for wavelengths of 400 to 800 nm (optical spectra),
to avoid serious scattering the internal structures must be below the 10- to
20-nm scale. In ferroelectrics, the superparaelectric internal polar
structures of the relaxer materials offer the possibility of tailoring
optically isotropic or anisotropic behavior under external electric field
control. The recent demonstrations of ferroelectricity in some species down
to 20 nm indicate that these anisotropies can be exploited in suitable
assembled nanostructures.
A wide range of potential property modifications, including shape-induced
optical birefringence, shape-controlled optical nonlinearity, and potential
modes for inducing optical bistability, remain to be explored. It is clear
that there will be corresponding magnetic nanocomposites and that for these
materials additional versatility can be expected because of the nanoscale
interaction with the transport phenomena and the associated optical
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91
absorption. Highly transparent ferromagnets, new types of indirectly coupled
magnetoelectrics, and piezomagnets of enhanced properties are to be expected.
At these very fine nanoscales, new work to explore the possibility of
super paraelasticity could give rise to families of highly nonlinear elastic
materials with potential for tailoring the nonlinearity.
Clearly on all other electro-, magneto-, and elasto-optical systems,
regular periodicities in the nanoscale internal structures will give rise to
interesting new pass or stop bands in their interaction with electromagnetic
or acoustic waves, analogous to the Brillouin zone structures in crystalline
systems. Again, much work remains to be done to develop the technologies
necessary to assemble composites on the regular scaling necessary for these
realizations.
ULTRASTRUCTURED CERAMICS
In conventional ceramic processing the powders employed are often
characterized by uncontrolled geometry and chemistry. This results in
microstructures (irregularly shaped particles or spheres) and ultrastructures
(interphases, secondary phases, and pores between spheres) that produce levels
of performance far below the theoretical limit (Figure 26~.
,_ ~~
rho U_ U - .
___ _ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Coo
060 ~
~ O ~~
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_ _ - ~
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i_ ^~ ~ .-
_~_~_/ ~
"pea d ~~a Dreg ~
FIGURE 26 The impact of ultrastructure processing on ceramic performance.
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Representative terms from entire chapter:
permanent magnets
92
The attainment of properties that approach theoretical values in high-
temperature structural ceramics by the ultrastructure processing approach is
schematically shown in Figure 27. The complete control of raw materials
includes the design of molecules and chemistry for powders that, upon
densification, provide the compositional stoichiometry necessary for glass-
free grain boundaries. The viability of the ultras tructure concept in the
chemical processing of mullite (3Al2O3~2SiO2) has been demonstrated.
To
2.'
_ z~
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· z0
1..
1..
1.4
1.:
1.0
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~ ed a--_
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1 1 41 ' 1 ' 1 ~ 1 1 _
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U"T~I
93
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~0
~0
C 40
20
O I I ~ I I ma\
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We_, c,,,_,
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FIGURE 28 Infrared-transparent mullite from sol-gel.
PERMANENT MAGNETS
Te~ 16~.C
me
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Y.
Rapid solidification (i.e., melt spinning) of Re2Fel4B-type materials has
permitted the development of permanent magnets whose performance depends in a
very important way on submicron- scale microstruc~cures . Two different types of
microstructures have been identified in these materials, which lead to
significantly different permanent magnet behavior. The mean grain size of the
two microstructures is submicron and very similar, approximately 20 nm to 30
nm. However, another submicron structural feature controls the magnetic
performance differences. It is a 1-nary to 2-nm-thick uniform intergranular
phase. The presence of the submicron intergranular phase shuts off the short-
range intergrain ferromagnetic exchange interactions by effectively isolating
the grains from magnetic interactions with each other. When the intergranular
phase is absent, ferromagnetic exchange couples the magnetizations of the two
grains at their surfaces, leading to significant enhancement of the remanent
magnetization of all the grains. Thus the submicron microstructure acts as a
switch for enhancement of the magnetic properties in these materials.
Specifically, it was shown by Croat et al. (1984a,b) that material with
compositions in the vicinity of Re2Fe~4B stoichiometry could be melt-spun at
specific quench rates (i.e., specific wheel speeds) producing material with
coercivities in excess of 10 kOe and energy products up to 14 MGOe. [Similar
performances were produced by Koon (1980) with heat treatment of amorphous
melt spun material of similar composition.] The values by Croat et al. were
in good agreement with the predictions for remanence and energy product of
94
crystallographically isotropic permanent magnets predicted by Stoner and
Wohlfarth (1948), wherein the limits to the magnitude of the remanent
magnetization (AMMO) are less than one-half the saturation magnetization
(firms), and the energy product is less than (4~Mr/MS)2. Bright- and dark-field
transmission electron micrographs of optimum as-quenched material (Croat et
al., 1984a,b) are shown in Figure 29. It is clear that the sample consists of
a two-phase microstructure with small (approximately 30-nm diameter) grains of
the principal phase, Re2Fe:4B (Herbst et al., 1984) surrounded by a very thin
film of an amorphous phase some 1 to 2 nm in thickness. In permanent magnets
with this type of microstructure, it is believed that magnetic domain walls
are pinned in the amorphous grain boundary regions, thus generating coercity.
The submicron microstructure shown in Figure 29 has no amorphous
intergranular phase. The mean grain size for this type of material has been
determined to be between 14 and 23 nm (Keem et al., 19881. In contrast to the
I've
-
.. `d
,.- ~
,.e ~
~3
. .
~-
- ' ~3
.. ~ .-/ ~
Am'
,; -
.~
Fan
~ I_
_— ~
;_ ·~ i_
_~
~ - __
~~-~
it_
is_
.__
be
Few
T
F.-
100nm
F ._
-- earn_
MEL
-
FIGURE 29 Bright-field image and selected area diffraction of enhanced
remanence material. No evidence of intergranular phases is found in either
the image or the selected areas diffraction patterns (Keem et al., 1988~.
9s
material with the intergranular phase, Re2Pe:4B material with a slightly
smaller grain size and no intergranular phase has magnetic performance that
significantly exceeds the Stoner-Wolfarth limits, with remanences up to 10 kG
and energy products in excess of 19 MGOe. The enhancement behavior has been
attributed to the intergrain ferromagnetic exchange interaction (Keem et al.,
1988~. Remanence enhancement is limited to less than full polarization of the
adjacent grains because of an accommodation between the intergrain exchange
and intergrain anisotropy of the single-domain grains. It has been shown
(Keem et al., 1988) that the presence of any intergranular phase damps this
intergrain interaction and essentially eliminates the remanence enhancement.
POLYMER-SILICA MICROCOMPOSITES
Liquid crystalline solutions of rigid-rod macromolecules such as poly-p-
phenylenebenzobisthiazole (PBZT) can be solution-processed into high-
performance fibers and films. After coagulation into water, the
microstructure consists of a highly oriented three-dimensional interconnected
network of 20-nm-diameter microfibrils (Cohen and Thomas, 1988) (Figure 301.
50 Um ~
FIGURE 30 PBZT-sol-gel glass interpenetrating networks (Cohen and Thomas,
1988~.
This high-strength and high-stiffness polymer framework has recently been
exploited for novel polymer-glass composites. The process involves exchange
of the water phase in the wet coagulated PBZT film for an alkoxide solution,
which is then hydrolyzed to form a PBZT-sol-gel composite that can then be
further densified to yield a PBZT-silica composite in which both phases are
continuous. The liquid-crystal solution can be processed to yield a spectrum
of variously oriented PBZT networks that can then be infiltrated with a
variety of materials for specific applications. A particular target is the
improvement of compressive strength of neat PBZT fibers. The low overall
96
material density and near-zero coefficient of thermal expansion plus the
ability to form laminates by interdiffusion of the glass phase provides strong
motivation for employment of PBZT-silica microcomposites for aerospace
structural applications.
CATALYSTS
The application of new catalysts that replace current catalysts will be
based primarily on performance criteria. Preparation techniques might be
transferred from model catalyst systems, if favorable properties are
identified and the preparation can be scaled up conveniently. New applica-
tions will be based on the potential for new product schemes and the economics
for the entire process, of which the catalyst is j ust one part . The cost of
the catalyst can be a factor if the preparation scheme is particularly
complex, the raw materials are expensive, and the catalyst is used in very
high volume, as for exhaust emission control catalysts.
Broad classes of catalytic reactions that make use of submicron-sized
catalytic particles include emission control catalysis, catalytic reforming,
synthesis gas catalysis, Ziegler process for polyethylene, and oxidation
catalysis. Other processes that similarly incorporate a catalytic component
are photocatalysis and oxygen sensors. Possible future catalytic processes
include catalytic activation of fuel for energy conversion.
The performance of a catalyst is determined by measuring product yield.
Catalysts are evaluated by monitoring their activity, selectivity, and
durability under realistic conditions. Testing the activity of a catalyst for
a particular reaction is the best way to choose a catalyst. Moreover,
important discoveries are made in doing kinetic measurements, where
serendipity can play a role. Catalyst characterization that is not related to
performance is ancillary if the concern is economics.
CERMETS
Metal-ceramic (cermet) composites are routinely produced by conventional
powder-metallurgy methods. A limitation to the approach has been the
inability to obtain a microstructure in which the hard ceramic phase is less
than 1 to 5 mm in cross section. Both the chemical-synthetic approach and the
gas-condensation method provide the means for generating novel cermet
materials with submicron-scale structures. Such ultrafine structures present
the opportunity to synthesize a new class of cutting- tool materials, which
will have the ability to form and maintain a very fine cutting edge that is
resistant to chipping. For this reason, it is believed that nanophase
composite cermets, such as Co-WC, will find great utility for such high-value-
added applications as microtome blades and surgeon's scalpels. A number of
97
the other synthesis methods also discussed in Chapters 2 and 3 have the
potential for generating similar ultrafine composite structures.
MULTILAYER COATINGS
An important structural application of multilayer thin films is as
protective coatings for Co-WC cutting tools. A typical composite film
comprises three vapor-deposited layers of Tic, TiN, and A12O3, each of which
performs a specific function that contributes to the overall performance and
durability of the cutting tool. The first example of an application of a
nanophase- composite coating has inspired much recent activity in the coatings
industry to devise and exploit multilayer films for a variety of applications,
including protective coatings for mirrors, wear-resistant surfaces on
polymers, and low-friction bearings. In this context, particularly intriguing
is the low-temperature synthesis of superhard materials such as diamond and
cubic BN. An interesting feature of such materials is the inherently
nanoscale structure of the deposited films, which themselves are an integral
part of a nanoscale architecture.
For certain multilayered systems with compatible structures, the
possibility exists of generating a strained-layer superlattice that exhibits
the supermodulus effect. Exceptionally stiff coatings on stiff substrates
have been made for experimental purposes. Potential applications are being
considered for producing superstiff coated filaments for reinforcement
purposes in composite structures.
REFERENCES
Cohen, Y. ~ and E. L. Thomas. 1988. Microfibrillator network of a rigid rod
polymer: I--Visualization by electron microscopy. Macromolecules 21:433.
Croat, J. J., J. F. Herbst, R. W. Lee, and F. E. Pinkerton. 1984a. High-
energy product Nd-Fe-B permanent magnets. Appl. Phys. Lett. 44~1~:148-
149.
Croat, J. J., J. F. Herbst, R. W. Lee, and F. E. Pinkerton. 1984b. Pr-Fe and
Nd-Pe-based materials: A new class of high-performance permanent
magnets. J. Appl. Phys. 55:2078.
Herbst, J. F., J. J. Croat, F. E. Pinkerton, and W. B. Yelon. 1984.
Phys. Rev. B. 29:4176.
Keem, J. E., G. B. Clements, A. M. Kadin, and R. W. McCallum. 1988. P. 27 in
Hard and Soft Magnetic Materials With Applications. Proceedings of a
Conference Held in ASM's Materials Week 1987. Metals Park, Ohio: ASM
International.
98
Koon, N. C., C. M. Williams, and B. N. Das. 1980. 26th Annual Conference
on Magnetism and Magnetic Materials, Dallas, Texas, November 11-14.
Newnham, R. E., D. P. Skinner, and L. E. Cross. 1978. Mat. Res. Bull.
13:525.
Stoner, E. C., and E. P. Wohlfarth. 1948. Philos. Trans. R. Soc. Land. Ser.
A 240:599.
