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3 PROPERTIES OF COMPOSITE MATERIALS 391

Fig. 9 Speciﬁc thermal conductivity (thermal conductivity divided by speciﬁc gravity) for

selected monolithic materials and composites used in electronic packaging.

392 COMPOSITE MATERIALS

Physical Properties of Polymer Matrix Composites

Table 13 presents physical properties of the polymer matrix composites dis-

cussed in Section 3.1. A ﬁber volume fraction of 60% is assumed. The densities

of all of the materials are considerably lower than that of aluminum, and some

are lower than that of magnesium. This reﬂects the low densities of both ﬁbers

and matrix materials. The low densities of most polymers give PMCs a signif-

icant advantage over most MMCs and CMCs, all other things being equal.

We observe that all of the composites have relatively low-axial CTEs. This

results from the combination of low-ﬁber-axial CTE, high ﬁber stiffness, and

low matrix stiffness. Note that the axial CTEs of PMCs reinforced with aramid

ﬁbers and some carbon ﬁbers are negative. This means that, contrary to the

general behavior of most monolithic materials, they contract when heated. The

transverse CTEs of the composites are all positive, and much larger than the

magnitudes of the corresponding axial CTEs. This results from the high CTE of

the matrix and a Poisson effect caused by constraint of the matrix in the axial

direction and lack of constraint in the transverse direction. The transverse CTE

of aramid composites is particularly high because the ﬁbers have a relatively

high positive radial CTE.

The axial thermal conductivities of composites reinforced with glass, aramid,

boron, and a number of the carbon ﬁbers are relatively low. In fact, E-glass and

aramid PMCs are often used as thermal insulators. As Table 13 shows, most

PMCs have relatively high thermal resistivities in the transverse direction, as a

result of the low thermal conductivities of the matrix and the ﬁbers in the radial

direction. Through-thickness conductivities of laminates tend to be similar to the

transverse thermal conductivities of unidirectional composites.

Table 14 shows the inplane thermal conductivities and CTEs of quasi-isotropic

laminates made from the same material as in Table 13. Here again, a ﬁber

volume fraction of 60% is assumed.

We observe that the CTEs of the quasi-isotropic composites are higher than

the axial values of corresponding unidirectional composites. Note, however, that

the CTEs of quasi-isotropic composites reinforced with aramid and carbon ﬁbers

are very small. By appropriate selection of ﬁber, matrix, and ﬁber volume frac-

tion, it is possible to obtain quasi-isotropic materials with CTEs very close to

zero. Note that through-thickness CTEs for these laminates typically will be

positive and relatively large. However, this is not a signiﬁcant issue for many

applications.

Turning to thermal conductivity, we ﬁnd that quasi-isotropic laminates rein-

forced with UHM pitch carbon ﬁbers have an inplane thermal conductivity sim-

ilar to that of aluminum alloys, while UHK pitch carbon ﬁbers provide laminates

with a conductivity more than 50% higher. Both materials have densities 35%

lower than aluminum.

As mentioned above, through-thickness thermal conductivities of laminates

tend to be similar to the transverse thermal conductivities of unidirectional com-

posites, which are relatively low. However, if laminate thickness is small, this

may not be a signiﬁcant limitation.

Physical Properties of Metal Matrix Composites

In this section, we consider physical properties of selected unidirectional ﬁber-

reinforced MMCs and of silicon carbide particle-reinforced aluminum MMCs.

393

Table 13 Physical Properties of Selected Unidirectional Polymer Matrix Composites

Fiber

Density

[g/cm

(Pci)]

Axial CTE

[10

⫺6

/K (10

⫺6

/⬚F)]

Transverse CTE

[10

⫺6

/K (10

⫺6

/⬚F)]

Axial Thermal

Conductivity

[W/m 䡠 K (Btu / h 䡠 ft 䡠 ⬚F)]

Transverse Thermal

Conductivity

[W/m 䡠 K (Btu / h 䡠 ft 䡠 ⬚F)]

E-glass 2.1 (0.075) 6.3 (3.5) 22 (12) 1.2 (0.7) 0.6 (0.3)

Aramid 1.38 (0.050)

⫺4.0 (⫺2.2) 58 (32) 1.7 (1.0) 0.1 (0.08)

Boron 2.0 (0.073) 4.5 (2.5) 23 (13) 2.2 (1.3) 0.7 (0.4)

SM carbon (PAN) 1.58 (0.057) 0.9 (0.5) 27 (15) 5 (3) 0.5 (0.3)

UHS carbon (PAN) 1.61 (0.058) 0.5 (0.3) 27 (15) 10 (6) 0.5 (0.3)

UHM carbon (PAN) 1.66 (0.060)

⫺0.9 (⫺0.5) 40 (22) 45 (26) 0.5 (0.3)

UHM carbon (pitch) 1.80 (0.065)

⫺1.1 (⫺0.6) 27 (15) 380 (220) 10 (6)

UHK carbon (pitch) 1.80 (0.065)

⫺1.1 (⫺0.6) 27 (15) 660 (380) 10 (6)

394

Table 14 Physical Properties of Selected Quasi-Isotropic Polymer Matrix Composites

Fiber

Density

[g/cm

(Pci)]

Axial CTE

[10

⫺6

/K (10

⫺6

/⬚F)]

Transverse CTE

[10

⫺6

/K (10

⫺6

/⬚F)]

Axial Thermal

Conductivity

[W/m 䡠 K (Btu / h 䡠 ft 䡠 ⬚F)]

Transverse Thermal

Conductivity

[W/m 䡠 K (Btu / h 䡠 ft 䡠 ⬚F)]

E-glass 2.1 (0.075) 10 (5.6) 10 (5.6) 0.9 (0.5) 0.9 (0.5)

Aramid 1.38 (0.050) 1.4 (0.8) 1.4 (0.8) 0.9 (0.5) 0.9 (0.5)

Boron 2.0 (0.073) 6.5 (3.6) 6.5 (3.6) 1.4 (0.8) 1.4 (0.8)

SM carbon (PAN) 1.58 (0.057) 3.1 (1.7) 3.1 (1.7) 2.8 (1.6) 2.8 (1.6)

UHS carbon (PAN) 1.61 (0.058) 2.3 (1.3) 2.3 (1.3) 6 (3) 6 (3)

UHM carbon (PAN) 1.66 (0.060) 0.4 (0.2) 0.4 (0.2) 23 (13) 23 (13)

UHM carbon (pitch) 1.80 (0.065)

⫺0.4 (⫺0.2) ⫺0.4 (⫺0.2) 195 (113) 195 (113)

UHK carbon (pitch) 1.80 (0.065)

⫺0.4 (⫺0.2) ⫺0.4 (⫺0.2) 335 (195) 335 (195)

3 PROPERTIES OF COMPOSITE MATERIALS 395

Physical Properties of Continuous Fiber-Reinforced Metal Matrix Compos-

ites. Table 11 presents physical properties of unidirectional composites con-

sisting of UHK pitch carbon ﬁbers in aluminum and copper matrices. These

materials both have very low, slightly negative axial CTEs for the assumed ﬁber

volume fraction of 50%. As the table shows, the axial thermal conductivities for

MMCs with aluminum and copper matrices are substantially greater than that

of monolithic copper. A major advantage of having thermally conductive matrix

materials is that the resulting composite transverse and through-thickness ther-

mal conductivities are more than an order of magnitude higher than those of an

epoxy–matrix composite.

Table 12 presents the properties of quasi-isotropic composites composed of

aluminum and copper matrices reinforced with UHK pitch carbon ﬁbers. Here,

the ﬁber volume fraction of about 26% has been chosen to achieve an inplane

similar to that of aluminum oxide, 6.5 ppm/K (3.6 ppm/

⬚F). The inplane thermal

conductivities of the aluminum- and copper-matrix composites are 290 W/m

䡠

K (168 BTU/h 䡠 ft 䡠 ⬚F) and 400 W/m 䡠 K (230 BTU/h 䡠 ft 䡠 ⬚F), respectively.

These values are considerably greater than those of any other material with a

similar CTE, with the exception of diamond particle-reinforced copper, which

is discussed later. Because of the lower ﬁber volume fractions, the through-

thickness thermal conductivities of these composites will be somewhat higher

than those of the unidirectional composites presented in Table 11.

Physical Properties of Particle-Reinforced Metal Matrix Composites. In

this section, we consider the physical properties of silicon carbide particle-

reinforced aluminum and diamond particle-reinforced copper.

Physical Properties of Silicon Carbide Particle-Reinforced Metal Matrix

Composites. The physical properties of particle-reinforced composites tend to

be isotropic (in three dimensions). As for all composites, the physical properties

of silicon carbide particle-reinforced aluminum depend on constituent properties

and reinforcement volume fraction. Figure 4 shows how the CTE of (SiC)p/Al

varies with particle volume fraction for typical commercial materials. Table 15

presents density and CTE for several speciﬁc volume fractions, along with data

for monolithic aluminum, titanium, and steel.

Thermal conductivity depends strongly on the corresponding properties of the

matrix, reinforcement, and particle volume fraction. The thermal conductivity of

very pure silicon carbide is slightly higher than that of copper. However, those

of commercial particles are much lower. The thermal conductivities of silicon

carbide particle-reinforced aluminum used in electronic packaging applications

tend to be in the range of monolithic aluminum alloys, about 160–218 W/m

䡠

K (92–126 Btu/h 䡠 ft 䡠 ⬚F).

Physical Properties of Diamond Particle-Reinforced Copper Metal Matrix

Composites. Table 12 presents the physical properties of diamond particle-

reinforced copper composites, which are developmental materials. As for other

particle-reinforced composites, the properties can be expected to be relatively

isotropic. This material has a thermal conductivity somewhat higher than that of

monolithic copper, a much lower density, and a CTE in the range of semicon-

ductors and ceramic substrates. This unique combination of properties makes

this composite an attractive candidate for electronic packaging applications.

396

Table 15 Physical Properties of Silicon Carbide Particle-Reinforced Aluminum

Property

Aluminum

(6061-T6)

Titanium

(6Al–4V)

Steel

(4340)

Composite Particle Volume Fraction

25 55 70

CTE, 10

⫺

/K (10

⫺

/⬚F) 23 (13) 9.5 (5.3) 12 (6.6) 16.4 (9.1) 10.4 (5.8) 6.2 (3.4)

Thermal Conductivity,

W/m

䡠 K (Btu / h 䡠 ft 䡠 ⬚F)

218 (126) 16 (9.5) 17 (9.4) 160–220 (92–126) 160–220 (92–126) 160–220 (92–126)

Density, g / cm

(Pci) 2.77 (0.10) 4.43 (0.16) 7.76 (0.28) 2.88 (0.104) 2.96 (0.107) 3.00 (0.108)

BIBLIOGRAPHY 397

Physical Properties of Ceramic Matrix Composites

As discussed in Section 1, there are many CMCs and they are at various stages

of development. One of the more mature systems is silicon carbide ﬁber-

reinforced silicon carbide (SiC/SiC). For a fabric-reinforced composite with a

ﬁber volume fraction of 40%, the density is 2.5 g/cm

(0.090 Pci), the CTE is

3 ppm/K (1.7 ppm/

⬚F), the inplane thermal conductivity is 19 W/m 䡠 K (11

Btu/h

䡠 ft 䡠 ⬚F), and the through-thickness value is 9.5 W/m 䡠 K (5.5 Btu/h 䡠 ft

䡠 ⬚F).

Physical Properties of Carbon/Carbon Composites

The CTE of CCCs depends on ﬁber type, volume fraction, and geometry and

matrix characteristics. In the ﬁber direction, CTE tends to be negative with a

small absolute value. Perpendicular to the ﬁber direction, composite CTE is

dominated by matrix properties. As a rule, the magnitude of transverse CTE is

small. Both positive and negative values have been reported.

It is well known that in some forms carbon has exceptionally high thermal

conductivities. For example, pyrolitic graphite can have a thermal conductivity

as high as 2000 W/m

䡠 K (1160 Btu/h 䡠 ft 䡠 ⬚F), ﬁve times that of copper. The

conductivity of some types of diamond is much higher. Some CCCs also have

very high thermal conductivities. Values have been reported as high as 400 W/

䡠 K (230 Btu/h 䡠 ft 䡠 ⬚F) for quasi-isotropic composites and 700 W/m 䡠 K

(400 Btu/h

䡠 ft 䡠 ⬚ F) for unidirectional materials.

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401

CHAPTER 13

SMART MATERIALS

James A. Harvey

Under the Bridge Consulting

Corvallis, Oregon

1 INTRODUCTION 401

2 PIEZOELECTRIC MATERIALS 403

3 ELECTROSTRICTIVE

MATERIALS 407

4 MAGNETOSTRICTIVE

MATERIALS 408

5 ELASTORESTRICTIVE

MATERIALS 409

6 ELECTRORHEOLOGICAL

MATERIALS 409

7 MAGNETORHEOLOGICAL

MATERIALS 410

8 THERMORESPONSIVE

MATERIALS 411

9 pH-SENSITIVE MATERIALS 411

10 LIGHT-SENSITIVE MATERIALS 412

11 SMART POLYMERS 412

12 SMART (INTELLIGENT) GELS

(HYDROGELS) 413

13 SMART CATALYSTS 414

14 SHAPE MEMORY ALLOYS 414

15 UNUSUAL BEHAVIORS OF

MATERIALS 415

16 COMMENTS, CONCERNS, AND

CONCLUSIONS 415

17 FUTURE 416

REFERENCES 417

1 INTRODUCTION

The world has recently undergone two materials ages, the plastics age and the

composite age. In the midst of these two ages a new era has developed. This is

the smart materials era. According to early deﬁnitions, smart materials are ma-

terials that respond to their environments in a timely manner.

1–4

The deﬁnition of smart materials has been expanded to materials that receive,

transmit, or process a stimulus and respond by producing a useful effect that

may include a signal that the materials are acting upon it. Some of the stimuli

that may act upon these materials are strain, stress, temperature, chemicals (in-

cluding pH stimuli), electric ﬁeld, magnetic ﬁeld, hydrostatic pressure, different

types of radiation, and other forms of stimuli.

The effect can be caused by an absorption of a proton, of a chemical reaction,

of an integration of a series of events, of a translation or rotation of segments

HandbookofMaterialsSelection.EditedbyMyerKutz