PROF. M. BARSOUM WINTER 1996

FUNDAMENTALS OF CERAMICS

First Exam (1 hr.)

Carefully read the following statements which may or may not be true. First note whether statement is true or false. If the statement is correct elaborate on why you think it is correct. If you think statement is incorrect change it so as make it correct and explain why you made the changes. 10 each.

i) The propensity of a cation to be tetrahedrally coordinated increases as the atomic number of the cation increases.

ii) Nonstoichmetric oxides are more easily oxidized and reduced than stoichiometric oxides.

iii) The configurational entropy of an oxide is not a function of temperature but is a function of oxygen partial pressure.

iv) The Madelung constant has the dimensions of m-1.

v) Covalent bonds are more likely between atoms that are similar to each other in electronegativity.

vi) MgO has a higher melting point than KCl.

vii) The melting point of a ceramic is a direct measure of the bond energy.

viii) Na₂Si₂O₅ is comprised of infinite double chains linked together by covalent bonds.

ix) Mg₂SiO₄ is comprised of infinite single chains linked together by covalent bonds.

x) Life would be so much easier if __________________________________________

(bonus points for making me laugh out loud)

__________________________________________________________________________

2) a) Starting with an expression for the energy of attraction between a cation and an anion and one for the energy of repulsion, derive an expression for the bond energy between the two. 16 pts.

b) Sketch a curve for the net energy between a cation and an anion. Using that sketch explain what information can in principle be obtained from such a curve. Be specific and brief.

18 pts.

c) It is usually claimed that the theoretical strength of ceramics is ˜ Y/10. Prove it.

16 pts.

3) The O/Fe ratio for FeO in equilibrium with Fe is quite insensitive to temperature over the temperature range 750-1250 °C and is fixed at about 1.06. When this oxide is subjected to various oxygen partial pressures in a thermobalance at 1150 °C, the results obtained are shown below

i) Explain in your own words, using equations if necessary, why the higher oxygen partial pressure resulted in a greater weight gain. 25 pts.

ii) From these results determine the O/Fe ratio for FeO at PO2 = 10-11. 25 pts.