MAT2840 SECTION A
Hello, dear friend, you can consult us at any time if you have any questions, add WeChat: daixieit
MAT2840
SECTION A – Compulsory – Answer ALL of Section A
1. (a) Briefly explain the observation that good electrical conductors are usually good heat conductors.
(b) One side of a copper block [k = 386 W m-1 K-1] 4 cm thick is maintained at
175˚C. The other side is covered with a layer of fibreglass [k = 0.038 W m-1 K-1] 1.5 cm thick. The outside of the fibreglass is maintained at 80˚C, and the total heat flow through the composite slab is 300 W . What is the area of
the slab?
(c) Briefly explain what is meant by the following terms: natural convection and forced convection.
(d) A large concrete slab is at initially at a uniform temperature of 5°C. The surface of the slab is now subjected to convective cooling by air at −5°C with a convective heat transfer coefficient of h = 8 W m−2 K−1 . Given that the density of the concrete is 2.3 ×103 kg m−3 , the thermal conductivity of the concrete is 1 W m−1 K−1 and the specific heat capacity is 800 J kg−1 K−1 , what is the surface temperature of the slab after 40 minutes?
(d) Briefly explain the difference in the two quantities: thermal conductivity and thermal diffusivity.
(e) Air at 20˚C blows across a hot ceramic tile 50 cm wide maintained at 55˚C.
The flow velocity is 12 m s-1 . Calculate the convective heat transfer coefficient, h for this situation.
The properties of air at 20˚C are:
ρ = 1.1614 kg m-3
μ = 1.846 × 10-5 kg m-1 s-1
k = 0.0263 W m-1 K-1
cp = 1.007 × 103 J kg-1 K-1
2. (a) Explain what is the mean square displacement (MSD) in diffusion and why we use it.
(b) i. |
Carbon defects form interstitials in bcc iron (Fe) . Discuss what you think is the most likely atomic diffusion mechanism for the carbon in the Fe, explain your reasoning. |
ii. Would you expect the same atomic diffusion process you have described for carbon for interstitial vanadium (V) in bcc Fe? Explain your answer.
(c) Discuss an experiment that you could perform to determine the diffusion constant and diffusion activation energy of an atomic species. Provide details of advantages and drawbacks of the method.
(d) A new processing method is being tried for a lithium battery cathode material
– LiCoO2 . The process leaves the ceramic oxygen deficient, which is causing problems with the maximum capacity and cyclability of the material. To counter this problem the LiCoO2 is annealed at a low temperature (400 K) in an O2 atmosphere to encourage oxygen take up. This generates a localised atmosphere of oxygen at the surface of 1.16x105 mol m-3 .
i. The initial concentration of oxygen in the cathode material is 9.64x104 mol m-3 . The diffusion constant is 4.92x10-7 cm2s-1 for the oxygen in the cathode material. Calculate how long the material must be annealed for it to reach the desired concentration of 1.04x105 mol m-3 across the whole sample (which is 5 mm in depth).
ii. The Oxygen is thought to primarily diffuse into the material via a grain boundary diffusion mechanism. Discuss why and how grain boundaries would affect the diffusion. Draw a diagram showing how the oxygen concentration would vary across the material and around the grain boundaries.
(e) It is suggested that doping the material with a substitution onto the Co might
be able to alter the initial retention of oxygen. Two different cations, La3+ and Mg2+ , have been suggested. Using the information below in the table and your own reasoning explain what influence you think these defects might
have on the oxygen retention.
Cation |
Ionic Radius (pm) |
Co3+ |
75 |
Mg2+ |
86 |
La3+ |
117.2 |
SECTION B – Answer ONE Question Only
3. (a) Explain what is meant by the following terms: black body radiation and grey body approximation.
(b) Define the term: radiation shape factor
(c) Two identical plates which are 1.0 m by 2.0 m in size face each other and are 2.0 m apart. Given that these plates have an emissivity of 0.25 at 200 ˚C and 0.35 at 500 ˚C and that the emissivity varies linearly between these two temperatures, calculate the net radiative heat transfer per unit area from plate 1 to plate 2 when plate 1 is at 500˚C and plate 2 is at 200˚C.
(d) Calculate the percentage reduction in the net radiative heat transfer if the temperature of plate 1 is reduced to 350˚C.
(e) A truncated cone (conical frustrum – see Figure below)
has top and bottom diameters of 10 and 20 cm and a height of 10 cm. Calculate the shape factor between the top surface and the side and also the
shape factor between the side and itself.
Note: the area of a conical frustrum is given by:
,
where L is the height of the frustrum.
4. You are studying the diffusion of Cr impurities in an FCC Ni lattice. The activation energy for the native Ni atoms in the pure FCC Ni has been recorded as 1.32 eV. The lattice parameter of the FCC Ni is 3.52 Å . The Debye frequency has been recorded as 6.84x1012 s-1 . The Cr atoms are diffused into the Ni lattice by pressing an extremely thin sheet of Cr onto the Ni sample. A set of values for the different atomic hops that can occur in the lattice have been recorded as (at 375 K) as:
ω0 = 6.45x10 11 s-1
ω1 = 3.12x1012 s-1
ω2 = 5.73x1012 s-1
ω3 = 4.32x1011 s-1
ω4 = 9.73x1011 s-1
7F = 4.6
(a) Calculate the value of the diffusion constant, D, for the native Ni atoms (via a
vacancy mechanism) using the values listed above assuming the
temperature of 375 K.
(b) The Cr atoms are recorded with a diffusion constant of 4.70x10-7 cm2s-1 .
Calculate the concentration of Cr at a depth of 12.0 mm into the Ni sample
after 12.0 hours. The Cr concentration is 1.55x105 molm-2 .
(c) Explain the meaning of correlation in diffusion?
(d) Calculate the correlation factor for the Cr diffusion in the FCC Ni lattice.
(e) Explain what each ω value listed at the beginning of the question refers to
with reference to different hops within the crystal. You may find a diagram
aids your explanation.
(f) Eventually, so much Cr diffuses into the Ni sample that it reaches a mole
fraction of ~0.2 and it is found that the standard diffusion equations are failing to accurately reproduce the diffusion profile. Discuss why you think this could be the case and suggest what process could be included to
improve the model of the diffusion.
(g) During a different manufacturing route the Ni system is annealed several
times before applying the Cr sheet. When this process is used it is found that Cr diffusion into the Ni system is significantly slowed. Explain why you
think this might occur.
2022-08-15