Ralph Nonninger

Wet chemical deposition of ATO and ITO coatings using crystalline nanoparticles redispersable in solutions

The deposition of SnO2:Sb (ATO) and In2O3:Sn (ITO) transparent conducting coatings on glass substrate has been demonstrated by many techniques such as CVD, sputtering, vacuum deposition and sol-gel process. This paper presents an alternative process for the deposition of such coatings at room temperature by spin, dip and spray coating techniques using solutions prepared with crystalline nanoparticles fully redispersed in water (for ATO, ITO) or alcohol (for ITO) with solid contents up to 10‐15 Vol.%, respectively. The deposited green coatings have been sintered at temperature as high as 9008C. In2O3:Sn coatings have a resistivity of 1.5 £ 10 22 V cm as sintered and 3.4 £ 10 23 V cm after annealing in nitrogen atmosphere. The resistivity of ATO single coatings shows a minimum ra 1:7 £ 10 22 V cm after annealing at 5508C. SnO2:Sb coatings present long term stability but the resistivity of annealed ITO coatings steadily increases with time to a value three times higher. All coatings have a high optical quality with transmission in the visible range larger than 90%. q 1999 Elsevier Science S.A. All rights reserved.

Sol-gel-derived hydroxyapatite powders and coatings

Hydroxyapatite (HAP) and tri-calcium phosphate (TCP) powders and coatings with a Ca/P molar ratio from 1.56 to 1.77 were prepared by the sol-gel technique using calcium 2-ethylhexanoate (Ca(O2C8H15)2) and 2-ethyl-hexyl-phosphate as calcium and phosphorus precursors, respectively. The structural evolution and phase formation mechanisms of HAP and tri-calcium phosphate in calcined powders and coatings on Si wafer and Ti-alloy substrates (Ti-30Nb-3Al and Ti-5Al-2.5Fe) were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The elimination of organics was studied by differential thermal analysis (DTA) and thermogravimetry (TGA). Two different formation mechanisms of crystallization are proposed. In sols with Ca/P ≤ 1.67, β-tricalcium phosphate is formed as the major phase and hydroxyapatite as a minor phase by calcination at 700°C. At 900°C these phases react to form AB-type carbonated hydroxyapatite (Ca10−2x/3[(PO4)6−x(CO3)x][(OH)2−x/3−2y(CO3)y]). A release of CO2 substituting PO43− occurs between 900°C and 1100°C yielding carbonate apatite, Ca10(PO4)6[(OH)2−2y(CO3)y], whereas CO2 substituting OH− groups in the apatite structure is released above 1200°C. In sols with Ca/P ≥ 1.70, rather than carbonate apatite, B-carbonated hydroxyapatite Ca10−2x/3[(PO4)6−x(CO3)x](OH)2 is formed, which subsequently decomposes into HAP and CaO above 1200°C. The optimum sintering conditions for coatings on Ti-alloys are found to be 600°C for 10 minutes, since, at higher temperature, oxidation of titanium and the formation of rutile (TiO2) occur. Dip coating and sintering in two cycles yielded a homogeneous and dense coated film with a thickness of 250 nm.

Wet Chemical Deposition of Crystalline, Redispersable ATO and ITO Nanoparticles

A new wet chemical concept to produce coatings by dip, spin or spray processes is presented. It is based on the preparation of solutions made of crystalline nanoparticles fully redispersable in a solvent. It is exemplified for the preparation of SnO2 : Sb (ATO) and In2O3 : Sn (ITO) transparent conducting coatings. The process combines the advantages of using particles having already a low resistivity and the possibility to sinter the coatings at low temperature. The particles are prepared using an in-situ monitoring of the surface energy to control the growth of the particles and to avoid their agglomeration. The dried powders can be fully redispersed in alcohol (ITO) or water (ATO). Single layers with thickness up to 200 nm (ATO) and 400 nm (ITO) have been fabricated. The sheet resistance of the coatings decreases with the sintering temperature. Typical values are 430 Ω□ for ATO (550°C) and 380 Ω□ for ITO (550°C). Sols made by redispersing the powders in organosilanes allow to produce coatings at low temperature with antistatic (R□ > 100 kΩ□) and anti-glare properties (R□ > 100 kΩ□, 60 to 80 gloss units).

Production of Si3N4/TiN nano composites

Silicon nitride is used in different applications because of its resistance against thermal shock, high temperature and mechanical load. An appropriate second phase like TiN could make Si 3 N 4 attractive for additional applications. This work deals with the introduction of a TiN phase from a nanosize TiN powder into the Si 3 N 4 matrix by a single-step wet chemical process.

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.

Organic-Inorganic Hybrid Materials Processing And Applications

Hybrid materials as inorganic-organic nanostructured composites require tailored surface chemistry in order to obtain a homogeneous distribution of the nanoparticles in the matrix. For this reason, nanoparticles with organic functions have been synthesized, first, to provide the desired ae-potential at a given pH value, second, to avoid irreversible agglomeration due to the spacing effect, and third, to provide the appropriate surface chemistry. I could be shown that using this approach, it is possible to switch the ζ-potential of SiO2 nanoparticles from a negative to a positive potential at neutral and to bind DNA fragments to the particles for an effective transfection into cells. Other examples show that nanoparticles (TiO2, SiO2) coated with epoxy and methacryloxy groupings can be used as coating sol for the fabrication of thin films with green densities up to 67% by volume only by photochemical crosslinking of the polymerizable groupings. Using this approach, interference layers have been fabricated on transparent plastics. In soft matrices, these particles permit to establish appropriate ζ-potentials and in electric fields by electrophoresis, it was possible to up-concentrate them to form gradient index optics. The investigations show that surface chemistry-tailored nanoparticles are a useful tool for the fabrication of nanocomposite hybrids.

Herstellung von nanoskaligen TiN Pulvern und deren kolloidale Verarbeitung zu Al2O3/TiN-Nanokompositkeramiken

Die Synthese der nanoskaligen Titannitrid-Pulver (n-TiN) mit spezifischen Oberflachen von 20 bis 80 m²/g erfolgte uber den Chemical-Vapour-Reaction-Prozes (CVR). Die Pulver wurden nach einer chemischen Oberflachemodifizierung redispergiert und zu stabilen, wassrigen n-TiN-Suspensionen mit bis zu 40 Gew.-% Feststoffgehalt und dynamischen Viskositaten unter 10 mPa.s (Scherrate 200 min-1) verarbeitet. Zusammen mit wassrigen Al2O3-Suspensionen wurden hieraus Al2O3/TiN-Kompositsuspensionen hergestellt, bei denen die oberflachenchemischen Eigenschaften von Al2O3 und TiN so aufeinander abgestimmt wurden, das die nanodisperse Verteilung des TiN aufgrund elektrostatischer Beschichtung erhalten blieb. Kompositschlicker mit TiN-Gehalten bis zu 5 Vol.-% wurden anschliesend mittels Gefriertrockung zu Prespulvern verarbeitet und uber Heispressen verdichtet. Gefugeuntersuchungen an verdichteten Kompositproben ergaben, das eine nanodisperse TiN-Verteilung vorlag. Die TiN-Partikel bewirkten in der Al2O3-Matrix neben der Anderung des Bruchmodus (interkristalliner Bruch - interkristalliner/intrakristalliner Bruch) eine signifikante Kornfeinung. Die mittlere Korngrose der Kompositproben mit 5 Vol.-% n-TiN betrug 0.6 µm (Vergleichsproben ohne TiN 1.4 µm). Die Biegebruchfestigkeit von Al2O3 konnte durch 2.5 Vol.-% n- TiN geringfugig von 285 MPa auf 337 MPa verbessert werden. Die Bruchzahigkeit blieb unverandert bei 2.4 Mpa.m0.5.

Relations between microstructure and mechanical properties of pressureless sintered SiC

SiC green bodies with B{sub 4}C and nano-scale carbon black as sintering aids were prepared by slip casting. After presintering at 2160{degrees}C for 10 min the samples were annealed at the same temperature for different times in order to obtain different microstructures. By this procedure the initial relative density of 0.981 was increased to 0.989 accompanied by grain growth and morphological changes. Samples annealed for 30 min possessed a fracture strength around 440 MPa, as determined by the double ring test. The maximum fracture toughness was 3.5 MPa.m{sup 0.5}. Examination of the grain growth behaviour showed that the microstructure is up to relative densities of 0.985 stable against the accelerated grain growth.

New processing techniques for the production of pressureless sintered SiC parts

For the preparation of SiC parts by casting techniques two different processes for the precipitation of sintering aids onto the surface of sub-micron SiC powder have been developed. In the first case, SiC particles were electrostatically coated with nano scaled carbon. In the second case, SiC has been surface modified to provide coupling groups for the subsequent chemical bonding of carbon. The results show that aqueous slips could be prepared which show no segregation for carbon during slip casting. Green parts (p > 58 %) with a homogeneous distribution of carbon additive have been obtained. As a consequence, samples could be pressureless sintered to high densities (> 98.5 %), exhibiting a high fracture strength (> 450 MPa) and a high Weibull modulus (> 10).