Rüdiger Nass

Synthesis of an alumina coating from chelated aluminium alkoxides

Chelating agent, such as acetylacetone (AcAcH) and ethylacetoacetate (EAA) are used to avoid the undersirable precipitation of gelatinuous aluminum hydroxides during hydrolysis of aluminum-sec-butoxides, Al(OBus). This procedure leads to alumina sols with particle size distributions in the range of 1 to 15 nm which depend on the type of chelating agent as well as on the molar Al(OBus)3/chelating agent ratio. Generally, EAA leads to particles smaller than AcAcH. Because of the small particle size, gel powders are X-ray amorphous and do not crystallize at temperature < 750°C. Crystallization of α-alumina starts between 950 and 1000°C. Complete transformation to α-alumina is achieved between 1050 and 1100°C. Coating experiments with different sols on silica glass lead to nearly crack-free dried gel films with AcAcH system. An ultrafine, dense microstructure is obtained.

Modelling of ORMOCER coatings by processing

Hydrolysis, condensation and polymerization of γ-glycidyloxipropyltrimethoxysilane was studied under different reaction conditions and yields a hard coating material for polyethyleneterephthalate substrates. The progress of hydrolysis was followed by 29Si-NMR spectroscopy and Karl Fischer filtration. Hydrolysis proceeds very slowly at pH 5.5, but can be enhanced at pH 9. Independent from submitted water and pH the solutions only one half of hydrolyzable methoxy groups can be removed. Condensation and polymerization were investigated on coated substrates and start during thermal curing. The formation of the polysiloxane network takes place by elimination of methanol. Polymerization of the epoxy groups to polethyleneoxide units is achieved above 100°C using a special catalyst. The properties of the cured coatings depend on the free water and glycol content in the solution. Minimizing the amount of both components leads to an ORMOCER coating on polyethyleneterephthalate which features a good abrasion resistance in combination with a high flexibility.

Ultrafiltration conducting membranes and coatings from redispersable, nanoscaled, crystalline SnO2:Sb particles

Inorganic membranes prepared by the sol-gel method are promising candidates for use as filters in separation processes. Conducting supported membranes and coatings have been produced from redispersable nanoscaled crystalline Sb-doped SnO 2 powders with Sb contents up to 10 mol%. The crystalline particles (ca. 4 nm) are fully redispersable in aqueous solution at pH≥8 with a solid content up to 37 vol.% and are monosized. After thermal treatment at different temperatures and times, the pore size diameter of such a powder can be adjusted from 4-20 nm with a very narrow pore size distribution (ca. ±1 nm) and a total porosity of 63%, practically independent of the sintering parameters. Uniaxial compacted substrates (unsupported membranes) present similar characteristics but with a larger pore size distribution (±5 nm) and 80% total porosity. Their electrical resistance decreases with sintering temperature and time to 4 Ω (800 °C, 8 h). Crack free transparent conducting coatings on glasses and ceramics have been obtained by spin-coating using fully dispersed aqueous solutions of the powder with volume content up to 7.8%. After thermal treatment (1 h at 550 °C) single layers 200 nm thick are still porous and exhibit specific electrical resistivity as low as ρ=2.5×10 –2 Ω cm with 90% transmission in the visible range.

Correlation of the precursor type with densification behavior and microstructure of sintered mullite ceramics

Abstract The effect of alumina component in diphasic mullite precursors containing alkoxy-derived silica on the crystallization and sintering behavior of compacts was studied. The phenomena observed were characterized using differential thermal analysis (DTA), powder X-ray diffraction (XRD), dilatometry and transmission and scanning electron microscopy (TEM, SEM). In order to change the characteristics of as-prepared gels, the alumina source was varied while keeping the silica source constant. Al(NO 3 ) 3 ·9H 2 O, γ-Al 2 O 3 and boehmite (γ-AlOOH) were used as the alumina source and TEOS as the silica source. Clear differences were found in the microstructure of sintered samples derived from the precursors with aluminum nitrate nonahydrate in comparison to the samples containing γ-Al 2 O 3 or boehmite (γ-AlOOH). The former exhibited elongated mullite grains embedded into the “equiaxial mullite matrix”. This morphology is due to the overlapping of mullite crystallization and viscous flow sintering temperatures. Transient alumina, either added as γ-Al 2 O 3 or formed in situ by decomposition of boehmite, shifts the mullite formation above the sintering temperature, and enables formation of equiaxial mullite. The smaller are the transient alumina particles, the smaller are mullite grains of sintered bodies.

SYNTHESIS OF NANOCRYSTALLINE, REDISPERSABLE ANTIMONY-DOPED SNO2 PARTICLES FOR THE PREPARATION OF CONDUCTIVE, TRANSPARENT COATINGS

Nanocrystalline, redispersable Sb-doped SnO2 with Sb contents from 0.1–10 mol% (with respect to Sn) and a primary particle size of about 5 nm was prepared from SnCl4 and SbCl3 in solution by a growth reaction. The aggregation of the particles was avoided by in situ surface modification with amino carbonic acids. The stabilizing effect of the surface modification could be maintained during the following hydrothermal crystallization step (150°C, 10 bar). The resulting nanocrystalline particles are fully redispersable in aqueous suspensions at pH ≥ 11; solid contents up to 40 wt% can be achieved.Such aqueous dispersions were used to prepare transparent, conductive coatings on glass by spin coating. After thermal densification (1 hour at 550°C) transparent coatings of 220 nm thickness were obtained. A minimum specific electrical resistance of 2.5 · 10−2 Ω cm for Sb contents between 4 and 5 mol% was measured, the transparency in the visible range against air was 90%.

Crystallization kinetics of mullite from single-phase gel determined by isothermal differential scanning calorimetry

Abstract Transformation kinetics of single-phase gel with mullite composition was studied by isothermal differential calorimetry (DSC) in temperature range from 937 to 959°C. Single exotherm was observed for annealing temperatures below 947°C, and two overlapped exothermic peaks were seen above this temperature. According to XRD analysis, mullite was the only phase crystallized either under non-isothermal or isothermal heat treatment. Johnson-Mehl-Avrami (JMA) equation for nucleation and growth could not describe mullite crystallization adequately, even below 947°C. Using bimodal JMA-type model, that proposes mullite crystallization in two steps, the fitting was remarkably good in the whole temperature range. Obtained kinetics data do not allow one to characterize the gel, either as typical single phase one (nucleation-controlled process with two rate constants and small apparent activation energies), or as hybrid gel (mullite formation via spinel and high apparent activation energies). The rate constants were an order of magnitude smaller than is proposed for single phase gel. The apparent activation energies, however (Ea1 = 1053 ± 51 kJ/mol, and Ea2 = 1028 ± 22 kJ/mmol, were in great discrepancy to those already cited for single phase gels, but they were in very good agreement with data evaluated for diphasic and hybrid gels. Mullite a-axis length and effective fraction of mullite that is formed in the first and second step of the process provided an insight in the mechanism of mullite crystallization. It is assumed that not the nucleation and crystallization limitations, rather the phase separation is the controlling process in mullite formation from single phase gel under applied experimental conditions.

Microstructural characterization of Al2O3-SiC nanocomposites

Al2O3 composite ceramics containing 10 vol% SiC nanoparticles were prepared by pressureless sintering. SiC particles < 150 nm detach from the matrix grain boundaries during grain growth and are predominantly observed in intragranular positions, whereas larger SiC particles retain intergranular positions. Thermal expansion mismatch causes local residual tensile stresses in the matrix grains, giving rise to strain contrasts in TEM imaging; occasionally microcracking around intragranular inclusions ≥ 100 nm is observed in thin TEM foils. It appears that the volume fraction of intragranular SiC particles in the size range 100 < d < 150 nm should not exceed ≈ 0.5% in high strength/high toughness Al2O3-SiC nanocomposites. The interaction between propagating cracks and internal stress fields around intragranular inclusions forces a transgranular fracture mode. Grain boundary pinning by large intergranular SiC particles, in combination with solute drag inhibits matrix grain growth, while the presence of an amorphous phase at Al2O3 grain boundaries and Al2O3-SiC phase boundaries assists in densification. Excess liquid phase is exuded from the interfaces during consolidation and accumulates in large pores, stabilized by the accidental agglomeration of large SiC particles.

Crystallization kinetics of mullite formation in diphasic gels containing different alumina components

Abstract The crystallization kinetics of mullite formation in diphasic gels containing TEOS-derived amorphous silica and alumina component in different crystalline form and particle size has been studied using quantitative X-ray diffraction (QXRD) with 3:2 mullite as a standard and transmission electron microscopy (TEM). As the source of alumina component, aluminum nitrate nonahydrate, commercial γ-Al2O3 and boehmite (γ-AlOOH) were used. The results were consistent with previous studies which indicated that mullite forms initially by nucleation and growth. During the second slow stage of transformation, mullite with approximate 2:1 composition subsequently converts further into 3:2. This process is the fastest in the precursor with the smallest particle size of alumina, and the slowest in the sample with boehmite-derived alumina. In very fine scale of morphology the conversion of 2:1 mullite started even in the first stage of transformation.

Synthesis and colloidal processing of nanocrystalline (Y2O3-stabilized)ZrO2 powders by a surface free energy controlled process

Using the controlled growth technique, nanocrystalline ZrO{sub 2} powders have been prepared from solution. By variation of the Y-content (0--8 mol%), redispersable monoclinic, tetragonal or cubic ZrO{sub 2} powders with particle sizes between 5 and 10 nm were obtained after crystallization at elevated temperature and pressure. Nanodisperse suspensions of the powders have been used for colloidal processing techniques such as tape casting, slip casting or extrusion. The resulting green bodies with densities of 55% and average pore sizes of 5 nm could be sintered at temperatures below 1,100 C leading to monoclinic, tetragonal or cubic ZrO{sub 2} ceramics.

Microstructure and mechanical properties of slip cast sol-gel derived mullite ceramics

Abstract Mullite ceramics were processed by pressureless sintering (1620 °C, 2h) of slip cast mullite derived from single-phase gel calcined at 1070 °C and attrition milled with ZrO 2 balls. The four-point bending strength was determined from room temperature up to 1400 °C. Creep behaviour in compression was determined at stresses of 20 and 100 MPa and temperatures of 1200 to 1450 °C. Microstructural and microchemical characterisation of mullite material was performed using scanning (SEM) and high resolving transmission electron microscopy (HRTEM) in conjuction with energy dispersive X-ray spectrometry (EDX). A remarkable abrasion of ZrO 2 , and the leaching of magnesium from ZrO 2 grains occurred in milling process. The bending strength and the creep behaviour is determined by residual glassy phase, observed in triple points and at the grain boundaries of mullite/mullite and mullite/ ZrO 2 grains. When ZrO 2 grain participates in the formation of triple points, the composition of the glassy phase differs from that at the junction of three mullite grains. The EDX line microanalyses across the mullite/mullite grain boundaries revealed 2 -rich glassy film. There are some grain boundaries at which only a compositional gradient of SiO 2 occurred. The creep below 1300 °C is most likely controlled by grain boundary sliding accommodated by solution-precipitation mechanism, and above by the grain boundary sliding accommodated by viscous flow of SiO 2 -rich phase.