title: Microstructure development and properties of alumina–Ti aluminide interpenetrating composites

authors: Han, Ching Zen

subjects: pressureless sintering

abstract: Interpenetrating phase Al₂O₃–Ti₃Al composites have been fabricated by reaction sintering of 2 and 4-h discus milled composite powders containing Al and TiO₂ using controlled heat-treatment in non-oxidizing atmospheres. Pressureless sintering and hot pressing of the powders has been carried out under vacuum and argon at temperatures between 1278 °C and 1500 °C and a heating rate of 5 °C/min. Densification of the composites was investigated as a function of temperature. It was shown that lower porosity was obtained by hot pressing compared to pressureless sintering.

The microstructure development and mechanical properties of dense Al₂O₃–Ti₃Al composites fabricated by hot pressing of Al–TiO₂ powder blends were examined. Fully dense microstructures could be achieved by hot pressing at 1387 °C under vacuum. An increase of Vickers hardness was observed with increasing temperature, which can be attributed to the increase of density and the decrease in the level of porosity of the composites. The mechanical behaviour exhibited by the composites can be related to properties such as Al₂O₃ particle size and porosity level. Fracture toughness of 6.3 ± 0.8 MPa m1/2 and Vickers hardness of 13.6 ± 0.4 GPa were achieved for the fully dense composite produced by hot pressing discus milled powder.

text: Microstructure development and properties of alumina–Ti aluminide interpenetrating composites Interpenetrating phase Al₂O₃–Ti₃Al composites have been fabricated by reaction sintering of 2 and 4-h discus milled composite powders containing Al and TiO₂ using controlled heat-treatment in non-oxidizing atmospheres. Pressureless sintering and hot pressing of the powders has been carried out under vacuum and argon at temperatures between 1278 °C and 1500 °C and a heating rate of 5 °C/min. Densification of the composites was investigated as a function of temperature. It was shown that lower porosity was obtained by hot pressing compared to pressureless sintering.

The microstructure development and mechanical properties of dense Al₂O₃–Ti₃Al composites fabricated by hot pressing of Al–TiO₂ powder blends were examined. Fully dense microstructures could be achieved by hot pressing at 1387 °C under vacuum. An increase of Vickers hardness was observed with increasing temperature, which can be attributed to the increase of density and the decrease in the level of porosity of the composites. The mechanical behaviour exhibited by the composites can be related to properties such as Al₂O₃ particle size and porosity level. Fracture toughness of 6.3 ± 0.8 MPa m1/2 and Vickers hardness of 13.6 ± 0.4 GPa were achieved for the fully dense composite produced by hot pressing discus milled powder.

year: 2006

title: Studies of catalysis of the hydrolysis of phenyl glycinate and some related esters

authors: Lee, Lim Eng

abstract: This thesis reports the results and conclusions from the first kinetic studies undertaken of the hydrolysis of phenyl ester of an α-amino acid (glycine), previous studies having been limited to 4-nitrophenyl esters. Some data on 4-methoxyphenyl glycinate are also reported, together with some new data on 4-nitrophenyl esters required for comparison purposes. 
Studies have concentrated on catalysts previously reported as effective in the hydrolysis of 4-nitrophenyl esters of amino acids, the aim being to determine if results for a phenyl ester with a relatively poor leaving group might help in determining mechanisms involved in catalysis. 

(i) HCO₃⁻. Unlike the case for 4-nitrophenyl esters, phenyl glycinate hydrolysis is catalysed in bicarbonate solutions not by CO₂ as previously suggested but by HCO₃⁻, the kinetic form is given by 
Rate= k[neutral ester][HCO₃⁻] 
Kinetically equivalent forms involving CO₂ and CO₃²⁻ terms have been excluded. Comparison of the catalytic effect of HCO₃⁻ with that of imidazole and that of HPO₄²⁻ excludes the possibilities of general base and nucleophilic catalysis by HCO₃⁻ and a mechanism involving initial nucleophilic addition of amine to HCO₃⁻ followed by intramolecular nucleophilic substitution is suggested as most likely.

(ii) 4-nitrobenzaldehyde. Phenyl and 4-methoxyphenyl glycinate show the same kinetic form as 4-nitrophenyl esters but the catalytic effects are much smaller and consistent only with rate-determining attack on ester carbonyl. This supports the concept of rate-determining decomposition of a carbinolamine species, which is also consistent with an observed small effect of temperature on the overall catalytic rate constant.

(iii) Imidazole. Unlike the case for 4-nitrophenyl esters, the kinetics are dominated for both neutral and protonated phenyl glycinate species by a term second order in imidazole, which is consistent with general base catalysis of nucleophilic substitution by imidazole in the formation of an acylimidazole intermediate. A term first order in imidazole makes a minor contribution and probably represents general base catalysis as indicated through comparison of the magnitude of the catalytic effect of imidazole with that of HPO₄²⁻, which is also reported on for the first time in this thesis. 

(iv) N-ethylmorpholine. An unusual non-linear dependence of the observed first order rate constants on catalyst concentration is shown to be consistent with complexing of N-ethylmorpholine and ester prior to reaction. Equilibrium constants for complex formation and rate constants for decomposition are evaluated. However, the effects of model compounds as tests for medium effects of the amine and its cation throw some doubt on the validity of this interpretation. In particular, dioxane, as a model for N-ethylmorpholine, has an inhibitory effect on the hydrolysis of unprecedented magnitude.

(v) Hydroxide. Alkaline hydrolysis of phenyl glycinate  has been studied so as to obtain rate constants for both the neutral and cationic ester species. The latter is about 300 times more reactive.

Studies on 4-nitrophenyl esters have also been advanced in tandem with those of the phenyl and 4-methoxyphenyl esters. The kinetic form for catalysis in bicarbonate solutions has been confirmed as involving CO₂ and not HCO₃⁻, as is consistent with carbamate formation, but whether formation or decomposition of carbamate is rate-determining is still uncertain. The small effect of temperature on reaction rate together with the lack of base catalysis in the reaction support rate-limiting decomposition of carbamate, but the overall rate is close to that predicted by interpolation from other studies for carbamate formation. The possibility of hydrolysis at the zwitterion stage is considered. New data are also reported for 4-nitrobenzaldehyde catalysis for comparison with those of the phenyl and 4-methoxyphenyl esters.

text: Studies of catalysis of the hydrolysis of phenyl glycinate and some related esters This thesis reports the results and conclusions from the first kinetic studies undertaken of the hydrolysis of phenyl ester of an α-amino acid (glycine), previous studies having been limited to 4-nitrophenyl esters. Some data on 4-methoxyphenyl glycinate are also reported, together with some new data on 4-nitrophenyl esters required for comparison purposes. 
Studies have concentrated on catalysts previously reported as effective in the hydrolysis of 4-nitrophenyl esters of amino acids, the aim being to determine if results for a phenyl ester with a relatively poor leaving group might help in determining mechanisms involved in catalysis. 

(i) HCO₃⁻. Unlike the case for 4-nitrophenyl esters, phenyl glycinate hydrolysis is catalysed in bicarbonate solutions not by CO₂ as previously suggested but by HCO₃⁻, the kinetic form is given by 
Rate= k[neutral ester][HCO₃⁻] 
Kinetically equivalent forms involving CO₂ and CO₃²⁻ terms have been excluded. Comparison of the catalytic effect of HCO₃⁻ with that of imidazole and that of HPO₄²⁻ excludes the possibilities of general base and nucleophilic catalysis by HCO₃⁻ and a mechanism involving initial nucleophilic addition of amine to HCO₃⁻ followed by intramolecular nucleophilic substitution is suggested as most likely.

(ii) 4-nitrobenzaldehyde. Phenyl and 4-methoxyphenyl glycinate show the same kinetic form as 4-nitrophenyl esters but the catalytic effects are much smaller and consistent only with rate-determining attack on ester carbonyl. This supports the concept of rate-determining decomposition of a carbinolamine species, which is also consistent with an observed small effect of temperature on the overall catalytic rate constant.

(iii) Imidazole. Unlike the case for 4-nitrophenyl esters, the kinetics are dominated for both neutral and protonated phenyl glycinate species by a term second order in imidazole, which is consistent with general base catalysis of nucleophilic substitution by imidazole in the formation of an acylimidazole intermediate. A term first order in imidazole makes a minor contribution and probably represents general base catalysis as indicated through comparison of the magnitude of the catalytic effect of imidazole with that of HPO₄²⁻, which is also reported on for the first time in this thesis. 

(iv) N-ethylmorpholine. An unusual non-linear dependence of the observed first order rate constants on catalyst concentration is shown to be consistent with complexing of N-ethylmorpholine and ester prior to reaction. Equilibrium constants for complex formation and rate constants for decomposition are evaluated. However, the effects of model compounds as tests for medium effects of the amine and its cation throw some doubt on the validity of this interpretation. In particular, dioxane, as a model for N-ethylmorpholine, has an inhibitory effect on the hydrolysis of unprecedented magnitude.

(v) Hydroxide. Alkaline hydrolysis of phenyl glycinate  has been studied so as to obtain rate constants for both the neutral and cationic ester species. The latter is about 300 times more reactive.

Studies on 4-nitrophenyl esters have also been advanced in tandem with those of the phenyl and 4-methoxyphenyl esters. The kinetic form for catalysis in bicarbonate solutions has been confirmed as involving CO₂ and not HCO₃⁻, as is consistent with carbamate formation, but whether formation or decomposition of carbamate is rate-determining is still uncertain. The small effect of temperature on reaction rate together with the lack of base catalysis in the reaction support rate-limiting decomposition of carbamate, but the overall rate is close to that predicted by interpolation from other studies for carbamate formation. The possibility of hydrolysis at the zwitterion stage is considered. New data are also reported for 4-nitrobenzaldehyde catalysis for comparison with those of the phenyl and 4-methoxyphenyl esters.

year: 1979

title: How Maui captured the sun: using a MUD for educational simulation

authors: Cunningham, Sally Jo

subjects: computer science

abstract: MUDs (Multi-User Dungeons) are text-based, multi-user communication and modelling programs. This paper investigates the potential of a popular extensible MUD, the LambdaMOO system, as a tool for second language training and for educational simulation gaming.

text: How Maui captured the sun: using a MUD for educational simulation MUDs (Multi-User Dungeons) are text-based, multi-user communication and modelling programs. This paper investigates the potential of a popular extensible MUD, the LambdaMOO system, as a tool for second language training and for educational simulation gaming.

year: 1995-12