Unformatted text preview: current can be conducted by the surrounding copper, which also
effectively removes the heat. Figure 14.6–1 shows a close-up of a multiﬁlament superconducting cable.
Recently, superconducting materials containing copper, yttrium, barium, and oxygen
have been developed with critical temperatures greater than 100 K (see Sections 10.2.9
and 12.9). This will allow superconducting behavior at liquid nitrogen temperatures. The
wires made from these materials will also be composites. An unreﬁned example of such
a wire is shown in Figure 14.6–2.
Electronic package casings are another example of composites used in the electronics
industry. With the revolution in integrated chip manufacturing, more-active devices are
packaged in smaller and lighter packages (see Section 10.3.5). One factor that limits the
density of active devices is the ability to rapidly remove heat. A 10 C rise in temperature
can reduce a chip’s lifetime by a factor of 2.5 to 3. Thus, thermal conductivity of the casing | e-Text Main Menu | Textbook Table of Contents pg605 [R] G1 7-27060 / IRWIN / Schaffer iq Chapter 14 Composite Materials FIGURE 14.6–1 Close-up view of Nb-Ti multiﬁlamentary superconducting cable. The superconducting Nb-Ti
ﬁlaments are surrounded by a copper matrix. (Source: K. K. Chawla, Composites Material Science and Engineering,
1987, Springer-Verlag, New York. Reprinted with permission of Springer-Verlag, New York Publishers.) FIGURE 14.6–2 A picture of a Y-Ba-Cu-O superconducting composite wire fabricated by chemical vapor deposition
of YBa2 Cu3 O7 x (light surface layer) onto a multiﬁlament Al2 O3 ﬁber (darker core). (Source: Reprinted with permission
from W. J. Lackey). | v v material (without the associated high electrical conductivity of metals) becomes critical.
Carbon ﬁber composites can be used to increase the thermal conductivity of package
materials. An additional design option to reduce thermal fatigue problems is tailoring the
coefﬁcient of thermal expansion of the composite casing to match that of the silicon. Thus,
when silicon chips are mounted on the casing, the thermal stresses are low, extending the
life of the package. Package casings are commonly made from either an iron-copper alloy
called Kovar or a silicon carbide particulate-reinforced aluminum alloy. The composite casing has superior heat transfer characteristics and represents a signiﬁcant weight
reduction. | e-Text Main Menu | Textbook Table of Contents 13.01.98 plm QC3 rps MP 605 pg606 [V] G2 7-27060 / IRWIN / Schaffer Part III 13.01.98 plm QC2 rps MP Properties FIGURE 14.6–3 A picture of a eutectic in situ composite used as a low-voltage ﬁeld emitter array cathode.
Reprinted with permission from D. N. Hill.) (Source: Figure 14.6–3 shows a low-voltage ﬁeld emitter array cathode for potential use in
microwave ampliﬁers. It is made from a directionally solidiﬁed oxide-metal eutectic
composite consisting of parallel arrays of continuous ﬁbers of refractory metals such as
W and Mo in an oxide matrix. The ﬁbers range from 0.3 to 1 m in di...
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