2004. 5.12
1.Ti(NMe2)4 has been used as a precursor in a chemical vapour deposition(CVD)
process to prepare thin films of TiN.
(i)Assess critically the suitability of this compound as a precursor for
CVD of TiN.
(ii)When this precursor is used in conjunction with gaseous ammonia, Me2NH
is detected in the exhaust gases and purer TiN films are produced.
Suggest a reason for these observations.
(iii)Give two techniques that could be used for the characterization of the
TiN film, and discuss the information that they would provide.
(15%)
2.Explain the following terms. Please write a short essay on the following
subjects:
(i)Divergent approach of dendrimer growth, use real example to discuss.
(ii)Joblonski Diagram (internal conversion, intersystem crossing, S1 and
T1 states etc)
(iii)Molten salt flux method in solid state synthesis
(iv)Ostwald Rippening
(v)RAHB, Resonance Assisted Hydrogen Bonding
(40%)
3.In an Alkoxide-Hydroxide Synthesis of Sr2AlTaO6, the following procedures
were followed:
a.Reflux a mixture of Ta(OC2H5)5 and Al(OC2H5)3 overnight in a solution of
ethanol. This results in the formation of polymeric (Ta,Al) ethoxide
species
b.Add a stoichiometric quantity of Sr(OH)2‧8H2O in acetone, mix well and
reflux overnight.
c.Filter off the solution and heat at 120度C to drive off remaining solvent
. Heat to 1200-1400度C to form highly crystalline Sr2AlTaO6 or heat to
800-1000度C to form high surface area Sr2AlTaO6
Question:
(i)In step a, can we use an aqueous solution? Why?
(ii)Sketch a likely structure for the polymeric (Ta,Al) ethoxide species.
(iii)In step b, the hydroxide ions and water of hydration are sufficient to
trigger a slow precipitation, why?
(iv)Why would one get a high surface area product when heating at a lower
temperature?
(20%)
4.What does it mean if a compound is said to melt congruently? Draw a phase
diagram. What approach would you take to grow single crystals of a
congruently melting compound?
(10%)
5.In the theory of homogeneous nucleation, it is found the activation free
energy of formation of critical nuclei is
W(r*) ≡ W* = 16π(σ^3) / 3(ΔGcryst(T))^2
(i)Please derive the above equation and explain the meaning of each terms.
(ii)Derive a formula to give the size of critical nuclei.
(iii)Why the activation free energy for heterogeneous nucleation is lower
than homogeneous nucleation?
(15%)
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