《材料科学与工程基础》英文影印版习题及思考(21)

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17.6 (a) Briefly explain why Cv rises with increasing temperature at temperatures

near 0 K.

(b) Briefly explain why Cv becomes virtually independent of temperature at temperatures far removed from 0 K.

17.10 A 0.1 m rod of a metal elongates 0.2 mm on heating from 20 to 100_C .

Determine the value of the linear coefficient of thermal expansion for this

material.

17.11 Briefly explain thermal expansion using the potential energy-versus-interatomic

spacing curve.

17.12 When a metal is heated its density decreases. There are two sources that give

rise to this diminishment of _: (1) the thermal expansion of the solid, and (2) the formation of vacancies (Section 5.2). Consider a specimen of copper at room temperature (20_C) that has a density of 8.940 g/cm3. (a) Determine its density upon heating to 1000_C when only thermal expansion is considered. And (b) repeat the calculation when the introduction of vacancies is taken into account. Assume that the energy of vacancy formation is 0.90 eV/atom, and that the

volume coefficient of thermal expansion, _v, is equal to 3_l .

17.16 (a) Calculate the heat flux through a sheet of steel 10mm thick if the

temperatures at the two faces are 300 and 100℃; assume steady-state heat flow (b) What is the heat loss per hour if the area of the sheet is 0.25 m2? (c) What will be the heat loss per hour if soda–lime glass instead of steel is used? (d) Calculate the heat loss per hour if steel is used and the thickness is increased to 20 mm

17.18 (a) The thermal conductivity of a single-crystal specimen is slightly greater

than a polycrystalline one of the same material. Why is this so? (b) The thermal conductivity of a plain carbon steel is greater than for a stainless steel. Why is this so?

17.19 Briefly explain why the thermal conductivities are higher for crystalline than

noncrystalline ceramics.

17.20 Briefly explain why metals are typically better thermal conductors than ceramic

materials.

17.21 (a) Briefly explain why porosity decreases the thermal conductivity of ceramic

and polymeric materials, rendering them more thermally insulative. (b) Briefly explain how the degree of crystallinity affects the thermal conductivity of

polymeric materials and why.

17.22 For some ceramic materials, why does the thermal conductivity first decrease

and then increase with rising temperature?

17.23 For each of the following pairs of materials, decide which has the larger thermal

conductivity. Justify your choices.

(a) Pure silver; sterling silver (92.5 wt%Ag– 7.5 wt% Cu).

(b) Fused silica; polycrystalline silica.

(c) Linear polyethylene (Mn _ 450,000 g/mol); lightly branched polyethylene (Mn _ 650,000 g/mol).

(d) Atactic polypropylene (Mw _ 106 g/mol); isotactic polypropylene (Mw _ 5 _ 105 g/mol).