Dominique Goeuriot

How the trapping of charges can explain the dielectric breakdown performance of alumina ceramics

The aim of this work is to bring new elements to the understanding of breakdown phenomena. Different alumina materials are studied in breakdown tests and by absorbed current measurement. The association of these two characterisation methods displays correlations between breakdown strength, the ability to trap or diffuse electrical charges and microstructural parameters. Different materials have been studied with a growing quantity of defects: structural defects in the single crystal, grain boundaries in very pure polycrystalline alumina (99.99%), impurities in less pure polycrystalline alumina (99.8%), interfaces in the case of alumina with a dispersion of zirconia and finally the adding of sintering aids. It appears that materials capable of diffusing injected charges have higher breakdown values than those trapping charges locally. Two other favourable behaviours are characterized: trapping charges at the injection point in order to limit the injection process, and trapping followed by reemission which relaxes the insulator and delays breakdown.

Enstatite Based Ceramics for Machinable Prosthesis Applications

Abstract Fabrication and properties of polycrystalline ceramics from talcs are described. Two types of talcs, lamellar and equiaxe, were dehydrated, attrition milled (0·5 to 4xa0h), pressed and sintered. Dilatometric studies and XRD analyses were used to determine phase transformations into enstatite during heat treatment. The soaking temperature leading to the highest densities must be chosen to avoid clinoenstatite formation, that gives porosity. A CaO–P2O5–AlPO4–Al2O3 glass addition (up to 15 wt%) lowers the sintering temperature of 50°C. Flexural strength and toughness are higher than those of dental commercial glass ceramics, and cutting grooves do not chip. Biocompatibility is also studied: those enstatite ceramics enhance the deposit of mineralized bone tissue. These enstatite ceramics could be good candidates for dental (implants or crowns) or bone restorations.

Microstructural and mechanical characterization of SiC-submicron TiB2 composites

Abstract The aim of this work is to characterize ceramic composites SiC–TiB 2 . After preparation of dense composites αSiC–TiB 2 (5, 10 and 15xa0vol% TiB 2 ) by reactive pressureless sintering, the materials have been characterized by their microstructure and their mechanical properties. The dispersion of TiB 2 particles is quite homogeneous, observed both by optical and scanning electron microscopies. Image analysis has revealed a majority of submicronic particles. Atomic force and transmission electron microscopies have shown the presence of nanometric TiB 2 particles. Concerning mechanical properties, toughness increases with the TiB 2 content, whereas hardness decreases when the TiB 2 content increases.

SiC-reinforcement of an Al2O3-γAlON composite

Abstract Three phase Al 2 O 3 –A1ON–SiC composites can be obtained by hot-pressing Al 2 O 3 –AlN–SiC mixtures under 40 MPa at temperatures around 1700°C. Silicon carbide can be added in the form of powders or platelets. In the case of powder, densification and aluminum oxynitride formation occur at higher temperatures than in the case of platelets, because of a steric and chemical hindrance by SiC on both Al 2 O 3 densification and the Al 2 O 3 –AlN reaction. Composites containing powders present the highest values for rupture moduli of up to 800xa0MPa and toughnesses close to those of alumina (4 MPa m ). For platelets, a composite containing 20 vol.% SiC with a size of d 50 =8 μm produces a reinforcement effect ( K 1C =6–7 MPa m ).

Alumina-γ-aluminum oxynitride composites from alumina nitrided by ammonia

Abstract By reaction between transition (γ) aluminas from boehmite and ammonia (T ≈ 1000–1100°C), up to 1%wt nitrogen can be fixed in the form of a γ-aluminum oxynitride layer on γ-Al 2 O 3 surface grains. This layer delays the polymorphic transformation of γ- into α-Al 2 O 3 . For higher nitrogen contents, aluminum nitride appears. An application can be the preparation of partially nitrided Al 2 O 3 powder in γ-AlON and /or AlN, that can lead to γ-AlON single phase or Al 2 O 3 -γ-AlON composites. Such composites, obtained by γ-Al 2 O 3 partially nitrided in AlON, present γ-AlON along grain boundaries; that microstructure is efficient in increasing the composite wear resistance.

Nitridation of γ alumina with ammonia

Abstract The action of ammonia on boehmite was studied in the temperature range 900–1200°C. This reaction begins at about 900°C and leads to the formation of a γ aluminium oxynitride (γ AlON) layer on the γ alumina grains. This layer retards the allotropic transformation of γ into αAl 2 O 3 . Starting from boehmite, it is not possible to get more than 1% wt N in the form of γ AlON. For higher nitrogen contents, aluminium nitride appears. An application can be the preparation of partially nitrided alumina powders containing γ AlON and/or AlN that can lead to either Al 2 O 3 ue5f8AlN composites or single phase γ AlON ceramics.