Papers by Author: Günter Borchardt

Abstract: Aluminium is a key element in geological and man-made materials which has only one stable isotope and no radionuclides with half-life times suitable for standard experimental diffusion studies. Here we report on our method using the radioisotope ²⁶Al (t_1/2 = 7.4×10⁵ a) as a quasi-stable tracer for aluminium in combination with SIMS depth profiling. First, our data for the aluminium bulk diffusivity in a-alumina are discussed jointly with published oxygen bulk diffusion coefficients. They clearly show that the relation D_Al>>D₀ is valid in the temperature range 1200 °C ≤ T ≤ 1800 °C. In an analogous manner, the two rare stable isotopes ¹⁸O and ³⁰Si are used together with ²⁶Al in diffusion studies of generic examples of materials which either consist of aluminium, silicon and oxygen only, or where these three elements are key constituents of the structure. For the crystalline aluminium silicate mullite our diffusivity data for aluminium, oxygen and silicon are used to explain the kinetics of the solid state formation reaction of mullite and the segregation kinetics of alumina from mullite. Finally, the diffusivities of oxygen and aluminium in model aluminosilicate glasses are presented as a function of temperature for different Al³⁺/Na⁺ ratios. For the aluminium silicate mullite and for the aluminosilicate glasses the relation D₀>D_Al>D_Si is valid regardless of the exact composition. For the glass system the activation enthalpies of aluminium and oxygen diffusion decrease with decreasing Al³⁺/Na⁺ ratio.

Abstract: Simultaneous ¹⁸O and ²⁶Al tracer diffusion experiments were performed in nominally undoped single crystalline α-Al₂O₃. The results clearly show that the bulk diffusivity of aluminium is much higher than the bulk diffusivity of oxygen in nominally undoped alumina. Comparing the ²⁶Al tracer diffusivities of Ti doped (300-400 wt. ppm Ti) and nominally undoped single crystalline α-Al₂O₃ one finds that the aluminium bulk diffusivity is insensitive to the Ti doping.

Abstract: . In the present work we used the sol-gel process to prepare Y2SiO5 precursor sols suitable for electrophoretic deposition (EPD). The sol synthesis was performed through the controlled hydrolysis of alkoxide solutions of tetraethoxysilane and yttriumoxoisopropoxide. During sol development emphasis was put on characterization of particles size and zeta potential of the formed aggregates. We succeeded in synthesizing a clear sol containing polymeric aggregates with acceptable particle charge. The electrophoretic deposition on glassy carbon or C/C-SiC slabs led to homogenous layers. At low sol concentrations micro cracks in the deposited layers were observed whereas higher concentrations led to thin and dense layers. During constant current EPD a constant voltage was recorded indicating that the deposited layer does not lead to an increase in resistivity in this kind of EPD system.

Abstract: The paper describes an examination of the effect of the addition of zirconium as a third element on the heat-resisting properties and explains the high temperature oxidation mechanism of Fe3Al intermetallic compounds. The Fe3Al and Fe3Al-0,05Zr specimens have been isothermally oxidized in the temperature range of 1173-1473 K in synthetic air for 100 hrs. The formed oxide layer, about 1,5-2 μm thick, was Al2O3. An examination of the cross-sectioned scales by SEM-EDS showed that the alumina layer consisted of a small inner columnar layer and an outer equiaxed grain layer. Additionally, very fine (50-150 nm) oxide grains rich in Zr, further identified as ZrO2, were found across the alumina scales. To understand the role of Zr on the growth mechanism of α–Al2O3 oxide scale on Fe3Al materials, two-stage oxidation experiments were performed (16O2/18O2), followed by SIMS and TEM-SAD observations. Particular attention was paid to the use of TEM in order to precisely characterize the products on samples prepared using the FIB (Focused Ion Beam) method. A combination of analytical techniques revealed that ZrO2 particles, most of which were formed along alumina grain boundaries, enhanced oxygen diffusion along grain boundaries due to oxygen-deficient composition of zirconium oxide (ZrO2-y).

Abstract: Literature data on (self-)diffusion in transition metal borides are extremely sparse due to the low atomic mobility of the constituents and due to the fact that for B there exist no suitable radioactive tracers and only two stable isotopes with a high natural abundance of 19 % (10B) and 81 % (11B), respectively. The present paper reviews our experiments on the tracer diffusion of transition metals and boron in TiB2, WB2+x, and (TixWyCrz)B2 which were carried out using stable isotopes and secondary ion mass spectrometry (SIMS). For tracer deposition, ion implantation and magnetron sputtering were used. In order to measure boron diffusion, a specially designed experiment was build up where a TiB2 layer was sputtered on an isotope-enriched Ti11B2 bulk ceramic sample. In addition, first results on chemical interdiffusion in the system (TixWyCrz)B2 will be presented. Here, a method based on magnetron sputtered layers and secondary neutral mass spectrometry (SNMS) was used which allows to determine much lower diffusivities (down to 10-19 m2/s) than the conventional EDX line-scan method on cross-sectional samples.

Abstract: In this work we investigated the mobility of hydrogen in amorphous ceramics with the composition Si13B13C60N13 (AM26C). The material was derived from a pre-ceramic polymer and thermolyzed at 1000 °C. After thermolysis the AM26C ceramics are assumed to be separated in an amorphous SiC phase and an amorphous C(BN)x phase. To measure the diffusivities we used deuterium as a tracer, which was introduced via isotope exchange from the gas phase at temperatures between 700 °C and 1100 °C. Depth profiling was done with secondary ion mass spectrometry (SIMS). The profiles could be fitted with complementary error functions. The diffusivities obey an Arrhenius law. The activation enthalpy is 0.8 eV, the pre-exponential factor is 5×10-12 m2 s-1. These values are close to those found for glassy carbon and thin amorphous C-B-N films as reported in the literature. We therefore conclude that the amorphous C(BN)x phase is the transport path for hydrogen in AM26C ceramics. A direct interstitial diffusion mechanism can account for the activation enthalpy of 0.8 eV. The low value for the pre-exponential factor is attributed to an entropy factor arising from the temperature dependence of the chemical potential of hydrogen.

Abstract: The preparation of mullite coatings for the oxidation protection of carbon fibre reinforced composites using a combination of sol-gel synthesis and electrophoretic deposition (EPD) has been investigated. Mullite precursor sols were synthesised by controlled hydrolysis and condensation reactions of the metal alkoxides TEOS (tetraethoxysilane) and Al(OBus)3 (aluminiumtri-sec-butylate). The structure and properties of the mullite precursor were strongly influenced by the synthesis parameters, especially by the water to TEOS ratio (rw/Si) and the pH value of the water. A variety of synthesis conditions was tested for optimising the mullite precursor sols regarding their suitability for the electrophoretic deposition. The electrokinetic behaviour of the sols and the charging of the sol particles which is necessary for a successful EPD were investigated by measurements of the Electrokinetic Sonic Amplitude (ESA signal). 29Si CP/MAS NMR measurements were used to get information about the coordination of the silicon and the homogeneity of the Al/Si distribution in the precursors. Heat-treated samples were characterised by X-ray diffraction for investigating the mullite formation. Coatings prepared by EPD and sintering at 1300 °C in Ar enabled an effective oxidation protection in the temperature range 1200 °C ≤ T ≤ 1550 °C.

Abstract: Combining sol-gel synthesis of 3/2 mullite through hydrolysis and condensation of tetraethoxysilane and aluminum-tri-sec-butylate with electrophoretic deposition (EPD) yields sufficiently thick and homogeneous layers which transform into mullite at T ≥ 1000 °C. The characterisation of the mullite precursor during synthesis was performed through electroacustic measurements. The protectiveness of the deposited mullite layers was tested in air in the temperature range 1200 °C ≤ T ≤ 1550 °C by means of isothermal thermogravimetric analysis for up to 200 hours. Comparing the oxidation rate of mullite coated C/C-Si-SiC samples to that of uncoated reference samples clearly demonstrated that mullite offers a significant improvement to the oxidation resistance of the uncoated material. At temperatures above 1600 °C the protectiveness of the deposited layer is reduced due to the existence of a liquid phase and the formation of CO bubbles above the cracks in the SiC layer. In order to prolong the protectiveness of our mullite layers at higher temperatures we deposited an additional layer from a suspension of mullite precursor with 5 wt. % of Al2O3 powder. The protectiveness of so obtained mullite and mullite/ Al2O3 layers was also tested under cyclic conditions at 1500 °C and 1550 °C. These experiments clearly demonstrated that all samples withstood at least for 4-10 cycles which were performed subsequently in different time intervals (from 2-3 days to 1 h).

Abstract: Solid oxide fuel cells with an electrode supported thin film electrolyte are a promising alternative to electrolyte supported single cells because of decreased electrolyte resistance. The electrophoretic deposition (EPD) of the electrolyte was performed on A-site deficient La0.75Sr0.2MnO3-δ (ULSM) from three different suspensions: (Y2O3)0.08(ZrO2)0.92 (YSZ), (Ce0.9Gd0.1)O1.955 (GDC) and La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) in acetylacetone and isopropanol. The thickness of the deposits was controlled by varying the conditions of the electrophoretic deposition. Because of porosity reduction in the cathode layer during sintering of the electrolytes we deposited porous cathodes (La0.8Sr0.2MnO3-δ (LSM) and La0.6Sr0.4Fe0.8Co0.2O3-δ (LSCF) ) as well as anodes (NiO/YSZ and NiO/GDC) on dense YSZ foils by applying a conductive layer on top of the surface part to be coated.

Abstract: The protectiveness of mullite layers electrophoretically deposited on C/C-Si-SiC composites, against isothermal oxidation in air in the temperature range from 1200 to 1550 °C, was investigated by means of thermogravimetry (TG). The experimental results are interpreted with the help of a phenomenological model. At lower temperatures or short oxidation times the overall oxidation kinetics is determined by transport processes in the EPD mullite layer, which leads to a linear growth law. At higher temperatures or longer times of oxidation the oxidation rate is controlled by solid-state diffusion processes in the growing silica layer, which leads to a parabolic growth law. Comparison of experimental parabolic and linear rate constants with calculated ones suggests, in the framework of the model, the conclusion that carbon monoxide (CO) diffusion in the oxide layers is the rate determining step for the overall oxidation of the C/C-Si-SiC substrates.