Porosities of porous silicon layers formed on different types of substrates and under different experimental conditions are compared with and related to the pore size distribution determined by gas adsorption experiments. Results show that porous layers formed on lightly P-doped silicon exhibit a network of very narrow pores, of radii less than 2 nm. Porous films formed on heavily doped silicon present larger radii, ranging between 2 and 9 nm according to the experimental conditions. Larger porosities and larger pore sizes are obtained by increasing the forming current density or by decreasing the HF concentration. Heavily P-doped porous silicon layers are homogeneous in depth and generally present a quite sharp pore size distribution. With heavily N-doped silicon, an increase in porosity with increasing thickness is found, which corresponds to an increase in pore size, leading to a broadening of size distributions. This porosity gradient is attributed to a chemical dissolution of the layer occurring during anodization. In addition, a strong dependence of porosity with small variations in doping level is found.

381ChemInform Abstract have been investigated as function of substrate used and experimental conditions. Results show that porous layers formed on lightly P-doped silicon exhibit a network of very narrow pore sizes (r 2 nm), which increases to radii of up to 9 nm on heavily P-doped Si substrates. A layer of a desired porosity having a narrow pore size range can be obtained by proper choice of the acid concentration and current density for the anodic oxidation. A good correlation between porosity and pore size is observed. The low porosity values correspond to the smallest pores and greater porosity to larger pores. Porous layers prepared on heavily N-doped substrates exhibit similar characteristics, but show a critical dependence of porosity on the thickness of the layer due to chemical dissolution phenomena. In addition, a strong sensitivity of porosity to small variations of the doping level of the Si substrate is observed.

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RESEARCHprosthodontics have a laser in their laboratory. However, looking at the available literature, since 1970 many experiments have been conducted into non precious and precious alloys and have shown contradictory results, probably because the experimental procedures used were different. 15,16,17,18,19 Therefore no comparison could be made. In this study, the weldability of three non precious alloys was investigated. The procedure consisted of determining the appropriate welding parameters which could achieve a deep joining in depth without cracks or gas entrapments in the welding area. The quality of the joining was checked by metallographic observations on polished sections. Wires of different diameters were then laser-welded and tested in tension. In order to understand the different behaviours between the three alloys (2 Ni-Cr and 1 Co-Cr based alloys), the composition of the welded area was determined by a X-Ray microprobe. Particular attention was given to the role played by the impurities. Methods Chemical compositionsTwo Ni-Cr-Mo alloys and 1 Co-Cr-Mo alloy, manufactured by Dentaurum (Pforzheim, Germany) were chosen for their applications in the dental field (Table 1) : 1 G mou is a low cost alloy used for crowns and bridges. 2 Remanium Cs is used for ceramic fixed partial dentures. 3 GM 800 is a cobalt chrominium alloy used for removable and fixed partial dentures Laser parameters and welding conditionsIn order to determine the welding conditions against the metal thickness and the ability of the laser device, two series of cast wires (0.6 to 2 mm thick), and five specimen DIN 50125 (3mm thick) were prepared by investment casting. The samples were then cut, joined by hand, edge to edge and welded without fillers or fluxes, using a commercial Nd-Yag laser (DL 2002 manufactured by Dentaurum) whose technical characteristics are given in table 2. In order to avoid the laser beam reflection and to improve the weld penetration, all the wires were sand-blasted with alumina powders and welded under Argon shielding atmosphere to decrease the oxidation contamination. All the samples were welded using the same procedure. The Laser conditions depend on the thickness and the volume of material to be welded (Table 2).Each wire was first assembled by the 4-cardinal points and thenThe laser welding technique applied to the non precious dental alloys procedure and results Aim The laser welding technique was chosen for its versatility in the repair of dental metal prosthesis. The aim of this research is to assess the accuracy, quality and reproducibility of this technique as applied to Ni-Cr-Mo and Cr-Co-Mo alloys often used to make prosthesis Method The alloy's ability to weld was evaluated with a pulsed Nd-Yag Laser equipment. In order to evaluate the joining, various cast wires with different diameters were used. The efficiency of the joining was measured with tensile tests. In order to understand this difference, metallographic examinations and XRay microprobe analysis were performed through the welded area and c...