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Abstract

he thesis describes a study of aluminum nitride (AlN) powder hydrolysis in the temperature range between 5 °C and 90 °C. The analysis of the temperature dependence of the course of the hydrolysis and the reaction kinetics, along with a mechanistic model set up for the hydrolysis reactions, gives a unique perspective of the hydrolysis, in a form that has not been presented in the literature so far. Knowledge of the degradation phenomena of AlN powder in water makes it possible to employ hydrolysis reactions for the synthesis of nanostructured aluminum oxide (Al2O3) coatings as well as the preparation of porous ceramics based on the Hydrolysis Assisted Solidification (HAS) process.
The first part of my research was dedicated to an investigation of the AlN powder reaction with water and of the influence of the temperature and pH of diluted AlN powder suspensions on the course of the hydrolysis. The hydrolysis process was divided into three temperature-dependent stages. The first stage includes the induction period, when the degradation of the AlN powder in water is slow. During the induction period a dense, amorphous aluminum hydroxide layer is formed on the surface of the AlN particles, although it is also dissolving at the same time. The second, faster, stage represents the formation of poorly crystalline boehmite on the surface of the AlN particles. The third stage is accompanied by the bayerite formation, until the AlN completely degrades. With even longer ageing times of the suspension, bayertite is formed by the dissolution of boehmite and a subsequent crystallization. The reaction kinetics was described using an un-reacted core model, and the rate-controlling step was found to be the chemical reaction at the product-layer/un-reacted core interface for the second stage of the hydrolysis in the temperature range between 22 °C and 70 °C. The calculated activation energy for the reaction is 101.4 kJ/mol. At 90 °C, the rate-controlling step for the second stage of the hydrolysis became diffusion through the product layer (boehmite). While at 5 °C the hydrolysis of AlN powder stopped after a few days of contact with water. During this period a 3-nm-thick amorphous aluminum hydroxide layer was formed on the surface of the AlN particles. The reaction kinetics was independent of the starting pH value of the suspension between 5.5 and 10; however, the pH influenced the induction period, which practically disappeared at pH values of 10.
In second part of my work, the hydrolysis of a 3 wt.% AlN powder water suspension was exploited during the synthesis of nanostructured Al2O3 coatings. The synthesis was feasible, since a sufficiently high concentration of dissolved aluminum ions was obtained during the hydrolysis, which provoked the heterogeneous nucleation of boehmite lamellas on the ceramic substrate. Irrespective of the starting temperature, the thickness of a single lamella was 3.3 mn. On the other hand, the lamellas length, from 135 nm to 259 nm, the lamellas height, from 159 nm to 237 nm, and their number per unit area on the substrate, were all temperature dependent. The calculated specific surface area of the coatings was in between 216 m2/g and 220 m2/g.
In the third part some examples of the exploitation of AlN powder hydrolysis in practice are presented. The synthesis of the nanostructured Al2O3 coatings is an effective preparative method for yttria-stabilized tetragonal zirconia (Y-TZP) dental restorations prior to cementation with dental cements. The method resulted in a patent being granted to the Engineering Ceramics Department. This nanostructured Al2O3 coating significantly improves the adhesion between the ceramic surface and the dental cement. Similar coatings, additionally modified with low-surface-energy chemicals, exhibit a similar structure and a sufficiently high specific surface, so that they can be used as superhydrophobic and self-cleanning surfaces. The HAS process, which exploits the AlN powder hydrolysis, can also be used for the preparation of porous Al2O3 ceramics, exhibiting higher flexural strength values compared to those prepared by conventional methods. α-Al2O3 crystallites, which nucleate within θ-Al2O3 lamellas during sintering, greatly enlarge the surface area of the necks formed between the main particles of the specimen and are the strength-determining factor in porous ceramics prepared in this way.