Short answer (26 pts total) a. What material would you expect to have a higher t

Question

Short answer (26 pts total) a. What material would you expect to have a higher thermal conductivity (circle one: (2) [SiC, Al 0, or mullite] b. relative to a single crystal Thermal conductivity of a polycrystalline material is generally (circle one): (2) [higher; lower, the same] Thermal shock depends on all the following parameters EXCEPT (circle one): (2) [fracture strength; thermal expansion coefficient; heat capacity] c· d. Ceramic failure is generally (circle one: (2) [ductile brittle; elastic] than theoretical strength (circle one): (2) Average measured strength is always [higher; lower, the same] e. f. Which of the following is NOT a method for testing ceramic strength (circle one): (2) [4-point bending; ring-on-ring biaxial; Charpy impact] (circle one): (2) Glasses deform by [dislocation slip; superplasticity; viscous flow] g. h. Which of the following raw materials are NOT available as natural minerals (circle one): (2) i. Optical glass fibers and fiberglass for composite reinforcement have the same composition and are made in the same way (circle one): (2) true; false j. A glass is in thermodynamic equilibrium (circle one): (2) [true; false] k. In a glass-ceramic, quenched glass is reheated, where the desired crystals and grow by (circle one): (2) ripen; nucleate; germinate] 1. The three main types of crystal growth are solution-grown, melt-grown, and impurity pressure; vapor] grown: (2) m. The most important single crystal growth method for silicon is [Verneuil/flame-fusion; Bridgman-Stockbarger, Czochralski]

Explanation / Answer

a. thermal conductivity is a measure of a material for the ability to conduct heat. Metals are the materials widely used as conductors. However, ceramics like SiC, Al2O3, and mullite are better as insulators as they have low thermal expansion coefficient. Comparing these, mullite is an alloy, but SiC and Al2O3 have more covalent bonding. But, SiC can act as a higher conductor.

So, SiC

b. Thermal conductivity of a polycrystalline material is generally higher than single crystal, as single crystal has grain boundaries defects which decrease the conductivity.

c. Thermal shock depends on fracture strength sigmaf, thermal conductivity k, modulus of elasticity E and coefficient of thermal expansion alpha according to following

Thermal Shock = (sigmaf x k)/ (E x alpha)

So, it does not depend on HEAT CAPACITY

d. Ceramic failure is generally brittle as most ceramics fracture before they can form a plastic deformation.

(4 parts at a time)

Have a nice day!

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