Papers by Author: Günter Borchardt

Abstract: Due to favorable chemical and mechanical properties of yttrium silicate coatings (low Young’s modulus, low thermal expansion coefficient, low evaporation rate and oxygen permeability, good erosion resistance), this material is a promising complement to SiC coatings for protecting C/C-Si-SiC composites. For the preparation of silicate powders, the Pechini method was used. As a coating method, electrophoretic deposition from stable suspensions based on isopropanol was chosen. Under controlled deposition voltage and duration, coatings of various thicknesses were deposited. The deposited layers were characterized by SEM and EDX analysis. The protectiveness of these coatings was tested by isothermal thermogravimetry.

Abstract: Exact mathematical solutions of the grain boundary diffusion equation in thin films have generally a complicated form, which is too cumbersome for the evaluation of experimental average concentration depth profiles obtained by sectioning techniques. On the other hand the accuracy of the exact solutions is not necessary for practical purposes so that it is useful to derive sufficiently accurate approximate solutions. We propose a method to derive such solutions for a thin film if the grain boundary diffusion is in the B2 regime. These solutions are derived for different diffusion sources.

Abstract: The tracer diffusion of ion implanted deuterium is studied in amorphous and polycrystalline magnetron-sputtered Si3N4:H films with secondary ion mass spectrometry (SIMS). The experimentally obtained diffusion profiles are numerically simulated by computer calculations based on the concept of trap-limited diffusion where the tracer atoms form immobile complexes with: (a) intrinsic film defects like dangling bonds and (b) extrinsic defects caused by the implantation damage. For amorphous Si3N4:H films a moderately high dissociation rate of intrinsic complexes (dangling bonds) is present and time independent effective diffusivities are observed, which obey an Arrhenius law with an activation energy of DE = 3.4 eV and a pre-exponential factor of D0 = 4 x 10-4 m2/s. For polycrystalline Si3N4 films non-Gaussian depth profiles and strongly time dependent diffusivities are observed, which have their reason in the presence of intrinsic traps with negligible dissociation, presumably located at the grain boundaries.

Abstract: The self-diffusion of nitrogen is investigated in polycrystalline thin silicon nitride films using a gas-exchange method (14N2/Si3 15N4) in comparison to Si3 14N4/Si3 15N4/Si3 14N4 isotope heterostructures. The films are produced by reactive r. f. magnetron sputtering. Depth profile analysis is carried out with secondary ion mass spectrometry (SIMS), secondary neutral mass spectrometry (SNMS), and nuclear resonant reaction analysis (NRRA). The nitrogen diffusivities determined with the use of isotope heterostructures follow an Arrhenius law in the temperature range between 1200 and 1700 °C with an activation enthalpy of DH = 4.9 eV and a pre-exponential factor of D0 = 1 x 10-6 m2/s, indicating a conventional diffusion mechanism via localized point defects. Using the gas-exchange method, the nitrogen diffusivities could be obtained only in the temperature range between 1600 and 1700 °C. This is due to the fact that at temperatures below 1600 °C the surface exchange process with its high activation enthalpy (about 10 eV) is rate limiting, leading to non detectable diffusion profiles. The application of the different methods of depth profiling leads to the same diffusivities within estimated errors.

Abstract: In order to bring a contribution to cationic diffusion in alpha-alumina, results on diffusion of 14 lanthanides and Y are presented. Samples are either single crystals with a known orientation or polycrystals with various grain-sizes elaborated from Y-doped alumina powders (100 ppm or 1000 ppm) or from pure alumina powder. After pre-heating in air at the diffusion temperature, small drops of a solution containing these elements are deposited at the surface of the sample. After the diffusion treatment (1200°C, 48 h or 1300°C, 21h), the diffusion profiles are obtained by a CAMECA IMS 4F SIMS device. The isotopes which are taken in account are: 45Sc, 89Y, 139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 158Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 174Yb and 175Lu which are the most suitable for this analysis. At first, it can be observed that the diffusion profiles are quite the same for all lanthanides. Diffusion profiles obtained for single crystals allow to calculate the lattice diffusion coefficients, which are necessary to determine the diffusion coefficients values in grain boundaries. Lattice and grain-boundary diffusion coefficients values are compared with cationic diffusion coefficients determined earlier.