Detlef Burgard

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 n 2 n 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 (800u2006°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 550u2006°C) single layers 200 nm thick are still porous and exhibit specific electrical resistivity as low as ρ=2.5×10 n –2 n Ω cm with 90% transmission in the visible range.

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%.

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.

Routes to deagglomerated nanopowder by chemical synthesis

The concept of tailored interfaces has been applied to the synthesis ofn nano-scaled Y 2 O 3 /ZrO 2 powders. Then microemulsion technique as well as the thermodynamically controlled growthn reaction have been utilized for this purpose. Both methods yieldedn agglomerate free amorphous powders with particles sizes of 8 nm and 15 nm,n respectively. Cubic zirconia was obtained by calcination between 300 and 400n °C and crystallite coarsening was not observed. The calcined powders couldn be redispersed by treating them with tertiary amines in aqueousn solutions.

Fabrication of agglomerate-free nanopowders by hydrothermal chemical processing

A chemical processing technique for the fabrication of nanopowders has been developed. The route is based on precipitation processes in solutions, either within aqueous droplets in micro-emulsions in the presence of surface modifiers like surfactants or by direct precipitation in solutions in the presence of these surface modifiers or small organic molecules directly bonded to the particle surface. In order to obtain well crystallized or densified particles, a continuous flow hydrothermal process has been developed which allows the fabrication of agglomerate-free surface modified nanopowders. The surface modification provides a full redispersibility after drying and permits a water-based processing. Nanoparticles preparation for ZrO 2 , ITO and ATO by this route are described.

Progress in powder synthesis by w/o-microemulsions

The formation of w/o-microemulsions for the synthesis of nanosized ceramic powders was investigated. Following the concept of HLB-(Hydrophilic-Lipophilic-Balance)numbers, w/o-microemulsions are formed by emulsifier mixtures with HLB-numbers between 8.9 and 10.0. The droplet size of the aqueous phase mainly depends on the emulsifier concentration and the amount of a cosurfactant. First precipitation experiments confirmed that nanocrystalline powders, such as Cr2O3 with a particle size around 20 nm, can be prepared by this technique.

Preparation of conducting ultrafiltration membranes from redispersable, nanoscaled, crystalline SNO2:Sb particles

Inorganic membranes prepared by the sol gel method are promising for use as filter in separation processes. Unsupported and supported conducting membranes have been produced from redispersable nanoscaled crystalline Sb-doped SnO2 powders with a Sb content up to 5 mole % (with respect to Sn). The crystalline particles are monosized (~4 nm) and fully redispersable in aqueous solution at pH >= 8 with a solid content up to 36.8 vol%. By thermal treatment at different temperatures and times, the pore size diameter of the material can be adjusted from 4 to 20 nm with a very narrow pore size distribution (~ +-1 nm) and a total porosity of 63%, practically independent of the sintering parameters. Uniaxial compacted unsupported membranes present similar characteristics with however larger pore size distribution (+-5 nm) and 80% total porosity. Their electrical resistance decreases with sintering temperature and time down to 4 Omega (800°C, 8 h). Fully dispersed aqueous solutions of the powder (7,8 vol%) were used to prepare transparent conducting porous coatings on glass or ceramics by spin-coating. AFter thermal treatment (1 hour at 550°C) single layers 200 nm thick have a typical specific electrical resistance p=2.5-10-² Omega cm and an optical transmission in the visible range measured against air of 90%.