Influence of gamma-irradiation sterilization on the structural defects of sapphire single crystals (alpha-Al(2)O(3)).

The influence of sterilization by gamma rays on the structure and the electrical behaviour of sapphire single crystal (alpha-Al(2)O(3)) was studied successively by thermoluminescence, by cathodoluminescence and by observation of the scanning electron microscope mirror effect. The mirror method allowed us to measure the capacity of an insulating material to trap electrons. The structural analysis of the alpha-Al(2)O(3) showed that there were oxygen vacancies, as well as chromium and titanium impurities. It was possible to demonstrate that these defects, especially the oxygen vacancies, are in a different state after a 30 kilogray irradiation. The valency state changes of these defects and the presence of trapped charges are accompanied by a deformation of the crystalline lattice which results in a modification of its electrical properties. At room temperature, the irradiated alpha-Al(2)O(3), unlike non irradiated alpha-Al(2)O(3), is capable of trapping electrons. It can be concluded that gamma-ray sterilization modifies the cohesive energy of alpha-Al(2)O(3), which could lead to mechanical changes (surface charge, friction, wear, fracture strength, em leader) in this material.

Aluminas with dispersoids. Tribologic properties and in vivo aging.

Three base alumina ceramics with dispersoids: monoclinic zirconia alumina (A5Z), tetragonal zirconia alumina (A20Z) and aluminalon (Aa20) have been investigated because of their improved mechanical properties with reference to pure alumina (AI203). Bending strength and fracture toughness are twice higher than alumina for the most toughened. These two parameters were measured by four-points bending tests after 1 week, 1, 2, 3, 6, 12 months of non loaded in vivo aging in wistar rats. They showed a little decrease for AI203 and Aa20 and a total stability for A5Z and A20Z even after one year. For tribologic study of the ceramic/polyethylene combination, cylinder against plane and pin on flat tests were conducted on an alternative movement with hip parameters of speed and load in presence of ringer solution. For the most representative test (pin on disk), the friction coefficients were nearly the same for all materials but UHMWPE wear volume was twice lower against A20Z than against AI203. A20Z/UHMWPE combination would be very interesting as new friction couple for total hip arthroplasty. Further studies are currently conducted.

[Methods of determination of fluoride in trace amounts. Application to the study of the diffusion of fluoride from an amalgam in dental tissues]. / Les méthodes de dosage du fluor à l'état de trace. Application à l'étude de la diffusion du fluor apporté par un amalgame dans les tissus dentaries

The study of fluoride diffusion in dental tissues has been performed by microprobe analysis. The fluoride supply is carried out in vivo by various fluorides added to conventional amalgams. This method differs then from previous work (topical applications, electrophoresis or fluoride solutions) by the study of fluoride penetration from amalgams either in dentine or in deep enamel, which are more susceptible to caries than superficial enamel. These experiments give evidence of a fast penetration of fluoride into dental tissues, which may be characterized by an apparent diffusion coefficient of about 10(-10) cm2/s.