Question

5. 20 pts (A) What are typical values of thermal conductivity and coefficient of thermal expansion for metals and polymers? Metal Ceramic Thermal Conductivity CTE (B) If you heat the metal, what happens to its thermal conductivity, and why (25 words or less) (C) You have 10.0000 cm bars of your typical metal and ceramic from part (5A) at 25.0000 deg C. You heat them both to 300.0000 deg C. What are their lengths upon heating?

Answer 1

A)

For metals : Thermal conductivity - 50 to 1000 W/mK; CTE - 5 to 25 x 10^-6 m/mK

For ceramic : Thermal conductivity - 80 to 300 W/mK; CTE - 2 to 12 x 10^-6 m/mK

(Note : polymers and ceramics are different types of materials, thermal conductivity of polymers are lower and coefficient of thermal expansion larger as compared to ceramics.)

B)

Thermal conductivity of metal remains almost constant with change in temperature (may experience slight decrease). The slight change observed is due to the opposing effects of flow of electrons and molecular vibration at higher temperature. The conductivity depends on flow on electrons which is obstructed by the vibrating molecules at higher temperature.

C)

basis - taking maximum value of CTE

For metal :

Initial length - 10 cm. Initial temp - 25°C. Final temp - 300°C

CTE - 25 x 10^-6 m/mK

Final length = 10+0.1*(300-25)*25x10^-6 = 10.000687 cm

For ceramic

Initial length - 10 cm. Initial temp - 25°C. Final temp - 300°C

CTE - 12 x 10^-6 m/mK

Final length = 10+0.1*(300-25)*12x10^-6 = 10.00033 cm