Herbert Giesche

Synthesis of monodispersed silica powders. II: Controlled growth reaction and continuous production process

Abstract The present paper will describe the preparation of monodispersed silica particles by a controlled growth process. This procedure has several advantages compared with the original Stober (batch) process. A simple calculation allows seeds of a given size to be grown exactly to a pre-set final particle size. Spherical and exceptional monodispersed particles of up to 3·6 μm in diameter and a very high (particle) mass fraction of up to 10 vol.% silica could be prepared by this process. Moreover, a simple tube reactor is shown, which allows those particles to be produced on a continuous basis.

Synthesis of Monodispersed Silica Powders I. Particle Properties and Reaction Kinetics

Abstract Currently there are several models discussed to describe the formation of monodispersed silica particles by the controlled hydrolysis and condensation of tetraethylorthosilicate (TEOS) in mixtures of water/ammonia and ethanol. The present study will show results on an extended range of experimental conditions. Particle growth followed a first-order reaction kinetic with respect to TEOS concentration. Reaction orders for the ammonia and water content were 0·97 and 1·18, respectively. Only monomeric (≈ 70–90%) and dimeric (≈ 10–25%) silicate units were detected during the growth reaction and an activation energy of 27 kJ mol−1 (6·5 kcal mol−1) was indicated. Particles revealed an ultramicroporous internal structure. A narrower size distribution and a more spherical shape were observed at higher water concentrations. The results suggested an aggregation mechanism of nanometer-sized primary nuclei during the first stage of the reaction followed by a monomer addition growth mechanism thereafter.

Liquid intrusion and alternative methods for the characterization of macroporous materials (IUPAC Technical Report)

This document deals with the characterization of porous materials having pore widths in the macropore range of 50 nm to 500 μm. In recent years, the development of advanced adsorbents and catalysts (e.g., monoliths having hierarchical pore networks) has brought about a renewed interest in macropore structures. Mercury intrusion–extrusion porosimetry is a well-established method, which is at present the most widely used for determining the macropore size distribution. However, because of the reservations raised by the use of mercury, it is now evident that the principles involved in the application of mercury porosimetry require reappraisal and that alternative methods are worth being listed and evaluated. The reliability of mercury porosimetry is discussed in the first part of the report along with the conditions required for its safe use. Other procedures for macropore size analysis, which are critically examined, include the intrusion of other non-wetting liquids and certain wetting liquids, capillary condensation, liquid permeation, imaging, and image analysis. The statistical reconstruction of porous materials and the use of macroporous reference materials (RMs) are also examined. Finally, the future of macropore analysis is discussed.

PREPARATION and APPLICATIONS of COATED POWDERS in CERAMICS and RELATED FIELDS

Abstract Coated powders offer a variety of different applications. A general description of heterocoagulation and heteronucleation processes and how they are used to prepare well defined coaled participates will be given as an introduction, Following that part will be a number of practical examples. Essential properties of the powder or the final product can be influenced by a specific coaling layer. Examples are the homogeneous distribution of a second component, the protection of the core material towards a reactive environment, the intentional arrangement of electrically conductive or non conductive outer-layers, hydrophilic or hydrophobic behavior, color effects, or the design of controlled microstructures. The present review article will demonstrate how coaled powders can be used to achieve an improved control of numerous properties in composite material systems.

Characterization of Porous AlN Separators for Batteries

A ceramic membrane of porous, sintered aluminum nitride (AlN) is under consideration for use as a separator in a Lithium-Metal Sulfide battery due to the corrosion resistance and thermal stability of AlN. The pore structure and electrolytic permeability of the membrane must be appropriate if AlN is to be used. Commercialization of these membranes can occur only after repeatability in the processing conditions can be assured. In this study, extensive permeability and porosity testing was performed on membranes prepared under various processing conditions. Attempts to correlate processing conditions with permeability were made in order to determine the optimum method for fabrication of the membranes. It was determined that membranes produced with a lower molecular weight binder will have a higher permeability, higher porosity, and a larger pore structure than membranes produced with a higher molecular weight binder.