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412 SMART MATERIALS

phthalate, hydroxypropylmethylcellulose acetate, hydroxypropylmethylcellulose

succinate, diethylaminoethyl methacrylate, and butyl methacrylate.

10 LIGHT-SENSITIVE MATERIALS

There are several different material families that exhibit different types of be-

havior to a light stimulus. Electrochromism is a change in color as a function

of an electrical ﬁeld. Other types of behavior for light-sensitive materials are

thermochromism, color change with heat, photochromism, color change with

light, and photostrictism, shape changes caused by changes in electronic conﬁg-

uration due to light.

5,19,20

Electrochromic smart windows have been intensively researched over the past

few decades. There have been over 1800 patents issued for optical switching

devices with the bulk of these being issued in Japan. A typical switchable glass

is multilayered with an electrochromic device embedded inside. A window de-

vice may have glass with an interior conductive oxide layer both on the top and

bottom. Inside of the sandwich of glass and conductive oxide is the electro-

chromic device. This device consists of an electrochromic layer, an ion storage

layer, and between these two layers, an ion conductor.

An interesting light-sensitive material with both electro- and thermochromism

behaviors, Li

, was evaluated for a smart window application.

Materials have been developed to exhibit both photochromic and photographic

(irreversible behavior to light) behaviors. One such system is based upon a sub-

stituted indolinospirobenzopyran embedded in a polystyrene matrix. This system

performs as a photochromic system at low exposure in the ultraviolet range and

as a photographic system at high exposures. The image can be devisualized by

heat and can be restored with UV irradiation many times.

11 SMART POLYMERS

The term smart polymers was almost dropped as a classiﬁcation for smart ma-

terials in this treatment of the subject. It is very confusing. Each ﬁeld of science

and engineering has its own deﬁnition for a smart polymer and each can ﬁt in

another classiﬁcation, and its distinction in smartness can be confusing at times.

It is being included as a separate class due to several excellent articles written

with smart polymers in the title.

In medicine and biotechnology, smart polymer systems usually pertain to

aqueous polymer solutions, interfaces, and hydrogels. Smart gels or hydrogels

will be treated separately. Smart polymers refer to polymeric systems that are

capable of responding strongly to slight changes in the external medium: a ﬁrst-

order transition, accompanied by a sharp decrease in the speciﬁc volume of the

polymer. If the external medium is temperature, this transition is known as the

glass transition temperature of the polymer and several properties of the polymer

changes. These properties include volume, coefﬁcient of thermal expansion, spe-

ciﬁc heat, heat conductivity, modulus, and permeation. Manipulating the detec-

tion around the glass transition temperature of the polymer can develop smart

devices. There are numerous examples in product development that have resulted

in failure because of not taking into account the glass transition temperature of

the polymer. And it should be noted, as the polymer cools down from high

12 SMART (INTELLIGENT) GELS (HYDROGELS) 413

temperatures and goes below its glass transition temperature, its below glass

transition properties are returned and vice versa.

Smart polymers can respond to stimuli such as temperature, pH, chemical

species, light, UV radiation, recognition, electric ﬁelds, magnetic ﬁelds, and

other types of stimuli. The resulting response can be changes in phases, shape,

optics, mechanical strengths, electrical and thermal properties, reaction rates, and

permeation rates.

12 SMART (INTELLIGENT) GELS (HYDROGELS)

In the literature you can ﬁnd these smart materials under a variety of names as

reﬂected by the title of this section. The concept of smart gels is a combination

of the simple concept of solvent-swollen polymer networks in conjunction with

the material being able to response to other types of stimuli. A partial list of

these stimuli includes temperature, pH, chemicals, concentrations of solvents,

ionic strengths, pressure, stress, light intensity, electric ﬁelds, magnetic ﬁelds,

and different types of radiation.

35–39

The founding father of these smart gels,

Toyochi Tanaka, ﬁrst observed this phenomenon in swollen clear polyacrylamide

gels. Upon cooling, these gels would cloud up and become opaque. Upon warm-

ing, these gels regained their clarity. Upon further investigation to explain this

behavior, it was found that some gel systems could expand to hundreds of times

their original volume or could collapse to expel up to 90% of their ﬂuid content

with a stimulus of only a 1

⬚C change in temperature. Similar behavior was

observed with a change of 0.1 pH units.

These types of behaviors lead to the development of gel-based actuators,

values, sensors, controlled-release systems for drugs and other substances, arti-

ﬁcial muscles for robotic devices, chemical memories, optical shutters, molecular

separation systems, and toys. Other potential systems for the development of

products with smart (intelligent) gels (hydrogels) include paints, adhesives, re-

cyclable absorbents, bioreactors, bioassay systems, and display.

There are numerous examples of the commercialization of these smart gels

in one of the cited reference.

This chapter will only include a few examples

of smart gels. One such smart gel consists of an entangled network of two

polymers, a poly(acrylic acid) (PAA) and a triblock copolymer of poly(propylene

oxide) (PPO) and poly(ethylene oxide) (PEO) with a sequence of PEO–PPO–

PEO. The PAA portion is a bioadhesive and is pH responsive, the PPO moieties

are hydrophobic that assist solubilize lipophilic substances in medical appli-

cations, and the PEO functionalities tend to aggregate thus resulting in gelation

at body temperatures. Another smart gel system with a fairly complex compo-

sition consists of citosan, a hydrolyzed derivative of chitin (a polymer of N-

acetylglucosamine that is found in shrimp and crab shells), a copolymer of

poly(N-isopropylacrylamide) and poly(acrylic acid), and a graft copolymer of

poly(methacrylic acid) and poly(ethylene glycol). This gel system was developed

for the controlled release of insulin in diabetics. Polyampholytic smart hydrogels

swell to their maximum extent at neutral pH values. When such gels, copolymers

of methacrylic acid 2-(N,N-dimethylamino)ethyl methacrylate, are subjected to

either acidic or basic media, they undergo rapid dehydration.

One very unusual smart gel is based upon the polymerization of N-

isopropylacrylamide, a derivative of tris(2,2

⬘-bipyridyl)ruthenium(II) that has a

414 SMART MATERIALS

polymerizable vinyl group, and N,N⬘-methylenebisacrylamide. It is a self-

oscillating gel that simulates the beating of the heart with color changes.

13 SMART CATALYSTS

The development of smart catalysts is a new ﬁeld of investigation and has shown

a great deal of activity in the universities of the oil-producing states. One such

smart catalyst is rhodium-based with a poly (ethylene oxide) backbone. Smart

catalysts such as this one function opposite as a traditional catalyst, that is, as

the temperature increases, it becomes less soluble, precipitating out of the re-

action solution, thus becoming inactive. As the reaction solution cools down,

the smart catalyst redissolves and thus becomes active again.

5,23

Other smart

catalyst systems are being developed that dissociate at high temperatures (less

active) and recombine at low temperatures (more active).

5,27

14 SHAPE MEMORY ALLOYS

The shape memory effect in metals is a very interesting phenomenon. Just imag-

ine taking a piece of metal and deforming it completely and then restoring it to

its original shape with the application of heat. Taking a shape memory alloy

spring and hanging a weight on one end of the spring can easily illustrate this.

After the spring has been stretched, heat the spring with a hot air gun and watch

it return to its original length with the weight still attached.

These materials undergo a thermomechanical change as they pass from one

phase to another. The crystalline structure of such materials as nickel-titanium

alloys enters into the martensitic phase as the alloy is cooled below a critical

temperature. In this stage the material is easily manipulated through large strains

with a little change in stress. As the temperature of the material is increased

above the critical temperature, it transforms into the austentic phase. In this

phase the material regains its high strength and high modulus and it behaves

normally. The material shrinks during the change from the martensitic to the

austentic phase.

5,19,20

The nickel–titanium alloys have been the most used shape memory material.

This family of nickel–titanium alloys is known as Nitinol (Nickel Titanium Naval

Ordinance Laboratory), named in part for the laboratory where this material was

ﬁrst observed. Nitinol has been used in military, medical, safety, and robotics

applications. Speciﬁc applications include hydraulic lines used on F-14 ﬁghter

planes, medical tweezers and sutures, anchors for attaching tendons to bones,

stents for cardiac arteries, eyeglass frames, and antiscalding valves used in water

faucets and showers.

5,41,42

In addition to the family of nickel–titanium alloys there are other alloys that

exhibit the shape memory effect. These alloys are silver–cadmium, gold–

cadmium, copper–aluminum–nickel, copper–tin, copper–zinc, combinations of

copper–zinc with silicon or tin or aluminum, indium–thallium, nickel–

aluminum, iron–platinum, manganese–copper, and iron–manganese–silicon.

Not all combinations of either the two or three elements yields an alloy with the

shape memory effect thus it is recommended to review the original literature.

In the open literature there are several articles from Mitsubishi Heavy Indus-

tries, Ltd. describing the room temperature functional shape memory polyure-

thanes. To this writer, these studies only attest to the behavior of a polymer at

its glass transition temperature. Thus if you wish to describe a polymer’s be-

16 COMMENTS, CONCERNS, AND CONCLUSIONS 415

havior at its glass transition temperature and since that free volume change is

reversible, you may call it a smart polymer or a shape memory material or a

thermoresponsive material. The unique characteristic of these polyurethanes is

that their transition occurs in the vicinity of room temperature.

44,45

15 UNUSUAL BEHAVIORS OF MATERIALS

As one researches the ﬁeld of smart materials and structures, one realizes that

there are many smart materials or that there are many material behaviors that

are reversible within their lifetime. The ability to develop useful products from

smart materials is left up to one’s imagination. For example, water is a very

unique material. It expands upon freezing. And as we know, the force generated

by this expansion causes sidewalks and highways to crack. Now, what if you

surround water pipes with a heating system that consists of a heater and water

enclosed in piezoelectric polymer or elastomer container that is in a ﬁxed space.

As the temperature drops to the freezing point of water, it expands and generates

force against the piezoelectric container which in turn generates electricity, thus

powering the heater keeping the pipes from freezing.

As previously mentioned, sometimes it is difﬁcult to classify the material and

its behavior. One such case involves an assigned patent to the University of

Illinois. The title of the patent is ‘‘Magnetic gels which change volume in re-

sponse to voltage changes for magnetic resonance imaging.’’

The patent

teaches the use of a matrix that has a magnetic and preferably superparamagnetic

component and the capability of changing its volume in an electric ﬁeld.

Fullerenes are spherically caged molecules with carbon atoms at the corner

of a polyhedral structure consisting of pentagons and hexagons. The most stable

of the fullerenes is a C60 structure known as buckyball or buckminsterfullerene.

The fullerenes have been under commercial development for the past decade.

One application of the fullerenes as a smart material consists of embedding the

fullerenes into sol–gel matrices for the purpose of enhancing optical limiting

properties.

A semiconducting material with a magnetic ordering at 16.1 K was produced

from the reaction of buckyball with tetra(dimethylamino)ethylene. This organic-

based magnet had neither the coercivity nor saturation magnetization to function

totally as a ferromagnet. The replacement of buckyball with higher carbon num-

ber fullerenes in the reaction with tetra(diamethylamino)ethylene did not produce

any complexes that showed magnetic ordering.

16 COMMENTS, CONCERNS, AND CONCLUSIONS

In dealing with smart materials and structures, there is still a lot of confusion

as to the name of these materials and what makes a material or structure smart.

There are numerous products with Smart in the name that do not meet the

deﬁnition of being smart, that is, responding to its environment in a reversible

manner. The scientist/engineer should not fault the advertising professional for

doing his or her job whenever they use the term in a product description. But I

must admit I chuckle to myself whenever I see the magazine Smart Money. Does

this mean that after I spent money it will return to me because it is reversible?

The confusion also exists in the smart materials community with the term

smart polymers. The applications of most smart polymers center around the

polymer’s glass transition temperature. As more design engineers became more

416 SMART MATERIALS

familiar with that term and its signiﬁcance to a design, the term smart polymer

is some applications will be eliminated and replace with terms like working

smartly with a polymer.

We have not addressed the versatility of these smart materials. One such

example may involve the smart shock absorbers. Two literature sources discuss

the current research that is being focused on vibration suppression in automo-

biles using smart shock absorbers.

48,49

These smart shock absorbers were devel-

oped by Toyota and consisted of multilayer piezoelectric ceramics. These

multilayer stacks are positioned near each wheel. After analyzing the vibration

signals, a voltage is fed back to the actuator stack that responds by pushing on

the hydraulic system to enlarge the motion. Signal processors analyzed the ac-

celeration signals from road bumps and responded with a motion that canceled

the vibration.

3,17

Such active piezoelectric systems are used to minimize excess

vibrations in helicopter blades and in the twin tails of F-18 ﬁghter jets. One

Internet source discussed the work at the University of Rochester with smart

shock absorbers using electrorheological ﬂuids. The electrorheological ﬂuids

would sense the force of a bump and immediately send an electric signal to

precisely alter the amount to dampen the force of the bump thus providing a

smoother ride. In an engine mount, electrorheological ﬂuids damp out the vi-

brations of the engine thus reducing wear and tear on the vehicle. Another

application of electrorheological ﬂuids is in the clutches to reduce the wear

between the plates as a driver shifts gears. This can reduce the maintenance

costs of high-duty trucks.

Or one can dampen the vibrations of the road, engine

mounts, and other sources on an automobile or heavy-duty truck with magne-

torheological ﬂuids. The Lord Corporation has developed a series of trademarked

ﬂuids and systems known as Rheonetic that are commercially available for vi-

bration control as well as for noise suppression.

17 FUTURE

The future of smart materials and structures is wide open. The use of smart

materials in a product and the type of smart structures that one can design is

only limited to one’s talents, capabilities, and ability to ‘‘think out of the box.’’

In an early work

and as part of the short courses taught there were discus-

sions pertaining to future considerations. A lot of the brainstorming that resulted

from these efforts is now being explored. And there are some that were in the

conceptual stage are moving forward. Look at the advances made with the au-

tomobile and what information and comforts it provides through smart materials

and structures.

We can take automobiles to a garage for service and it is hooked up to a

diagnostic computer and the mechanic is told what is wrong with the car. Or

we have a light on the dashboard that signals ‘‘maintenance required.’’ Would

it not be better for the light to inform us as to the exact nature of the problem

and the severity of it? This approach mimics a cartoon that appeared several

years ago of an air mechanic near a plane in a hanger. The plane says ‘‘Ouch’’

and the mechanic says ‘‘Where do you hurt?’’

One application of smart materials and their application is the earlier work

mentioned of using a piezoelectric ink jet printer to serve as a chemical delivery

to print organic light-emitting polymers in a ﬁne detail on various medium. Why

not take the same application to synthesize smaller molecules? With the right

REFERENCES 417

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periments with relative easy.

A topic of research that was in the literature a few years ago was ‘‘smart

clothes’’ or ‘‘wearable computers’’ being studied at MIT. The potential of this

concept is enormous. This sounds wonderful as long as we learn how to work

smarter not longer.

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419

CHAPTER 14

OVERVIEW OF CERAMIC MATERIALS,

DESIGN, AND APPLICATION

R. Nathan Katz

Department of Mechanical Engineering

Worcester Polytechnic Institute

Worcester, Massachusetts

1 INTRODUCTION 419

2 PROCESSING OF ADVANCED

CERAMICS 420

3 BRITTLENESS AND BRITTLE

MATERIALS DESIGN 422

4 APPLICATIONS 423

4.1 Ceramics in Wear

Applications 423

4.2 Thermostructural Applications 427

4.3 Corrosion Resistance 429

4.4 Passive Electronics 430

4.5 Piezoceramics 431

4.6 Transparencies 432

5 INFORMATION SOURCES 433

5.1 Manufacturers and Suppliers 433

5.2 Data 433

5.3 Standards and Test Methods 434

5.4 Design Handbooks 435

6 FUTURE TRENDS 435

REFERENCES 436

1 INTRODUCTION

Engineering ceramics possess unique combinations of physical, chemical, elec-

trical, optical, and mechanical properties. Utilizing the gains in basic materials

science understanding and advances in processing technology accrued over the

past half century, it is now frequently possible to custom tailor the chemistry,

phase content, and microstructure to optimize, applications-speciﬁc combina-

tions of properties in ceramics (which includes glasses, single crystals, and coat-

ings technologies, in addition to bulk polycrystalline materials). This capability

in turn has led to many important, new applications of these materials over the

past few decades. Indeed, in many of these applications the new ceramics and

glasses are the key enabling technology.

Ceramics include materials that have the highest melting points, highest elas-

tic modulii, highest hardness, highest particulate erosion resistance, highest ther-

mal conductivity, highest optical transparency, lowest thermal expansion, and

lowest chemical reactivity known. Counterbalancing these beneﬁcial factors are

brittle behavior and vulnerability to thermal shock and impact. Over the past

three decades major progress has been made in learning how to design to mit-

igate the brittleness and other undesirable behaviors associated with ceramics

HandbookofMaterialsSelection.EditedbyMyerKutz

420 OVERVIEW OF CERAMIC MATERIALS

and glasses. Consequently, many exciting new applications for these materials

have emerged over the past several decades.

Among the major commercial applications for these materials are:

●

Passive electronics (capacitors and substrates)

●

Optronics/photonics (optical ﬁbers)

●

Piezoceramics (transducers)

●

Mechanical (bearings, cutting tools)

●

Biomaterials (hard tissue replacement)

●

Refractories (furnace linings, space vehicle thermal protection)

●

Electrochemical (sensors, fuel cells)

●

Transparencies (visible, radar)

This chapter will provide a brief overview of how ceramics are processed and

the ramiﬁcations of processing on properties. Next a short discussion of the

special issues that one encounters in mechanical design with brittle materials is

provided. Short reviews of several of the above engineering applications of ce-

ramics and glasses, which discuss some of the speciﬁc combinations of prop-

erties that have led design engineers to the selected material(s), follow. A section

on how to obtain information on materials sources and information is provided.

Tables listing typical properties of candidate materials for each set of applica-

tions are included throughout. Finally, some areas of future potential will be

discussed.

2 PROCESSING OF ADVANCED CERAMICS

The production of utilitarian ceramic artifacts via the particulate processing route

outlined in Fig. 1 actually commenced about 10,000 years ago.

Similarly, glass

melting technology goes back about 3500 years, and as early as 2000 years ago

optical glass was being produced.

While many of the basic unit processes for

making glasses and ceramics are still recognizable across the millennia, the level

of sophistication in equipment, process control, and raw material control have

advanced by ‘‘lightyears.’’ In addition, the past 50 years has created a fun-

damental understanding of the materials science principles that underlie the

processing–microstrucure–property relationships. Additionally, new materials

have been synthesized that possess extraordinary levels of performance for spe-

ciﬁc applications. These advances have led to the use of advanced ceramics and

glasses in roles that were unimaginable 50 or 60 years ago. For example, early

Egyptian glass ca. 2000

had an optical loss of ⬃10

dB/km, compared to an

optical loss of

⬃10

⫺

in mid-1980s glass optical ﬁbers,

a level of performance

that has facilitated the ﬁber-optic revolution in telecommunications. Similarly,

the invention of barium titanate and lead zirconate titanate ceramics, which have

much higher piezoelectric moduli and coupling coefﬁcients than do naturally

occurring materials, has enabled the existence of modern sonar and medical

ultrasound imaging.

The processing of modern ceramics via the particulate route, shown in Fig.

1, is the way that

⬃99% of all polycrystalline ceramics are manufactured. Other

techniques for producing polycrystalline ceramics, such as chemical vapor dep-

2 PROCESSING OF ADVANCED CERAMICS 421

Raw powder

Formed product

Sintered product

DRY

WET

POWDER

POWDER CONSOLIDATION

AND

SHAPING TECHNOLOGY

POWDER METAL TECHNOLOGY

PLASTIC TECHNOLOGY

CERAMIC TECHNOLOGY

GREEN CERAMIC BODY

HEAT

HOT

PRESSING

GLAZE

HEAT

FINISHED PRODUCT

MACHINING

FINISHED PRODUCT

DENSIFIED AND BONDED

CERAMIC

Fig. 1 Processing of polycrystalline ceramics via the particular route.

osition

or reaction forming

are of growing importance but still represent a very

small fraction of the ceramic industry. There are three basic sets of unit processes

in the particulate route (and each of these three sets of processes may incorporate

dozens of subprocesses). The ﬁrst set of processes involve powder synthesis and

treatment. The second set of processes involve the consolidation of the treated

powders into a shaped perform, known as a ‘‘green’’ body. The green body

typically contains about 50 vol % porosity and is extremely weak. The last set

of unit processes utilize heat, or heat and pressure combined, to bond the indi-

vidual powder particles, remove the free space and porosity in the compact via

diffusion, and create a fully dense, well-bonded ceramic with the desired mi-

crostructure.

If only heat is used, this process is called sintering. If pressure is

also applied, the process is than referred to as hot pressing (unidirectional pres-

sure) or hot isostatic pressing [(HIP), which applies uniform omnidirectional

pressure].

Each of the above steps can introduce processing ﬂaws that can diminish the

intrinsic properties of the material. For example, chemical impurities introduced

during the powder synthesis and treatment steps may adversely effect the optical,

magnetic, dielectric, or thermal properties of the material. Alternatively, the im-

purities may segregate in the grain boundary of the sintered ceramic and nega-

tively effect its melting point, high temperature strength, dielectric properties, or

optical properties. In green-body formation, platey or high aspect ratio powders

may align with a preferred orientation, leading to anisotropic properties. Simi-

larly, hot pressing may impose anisotropic properties on a material. Since ce-

ramics are not ductile materials, they can (usually) not be thermomechanically

modiﬁed after primary fabrication. Thus, the speciﬁc path by which a ceramic

component is fabricated can profoundly effect its properties. The properties en-

countered in a complex shaped ceramic part are often quite different than those