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Previous in situ small-angle neutron scattering studies of nitrogen adsorption and desorption at 78 K on the mesoporous silica xerogels Gelsil 50 and Gelsil 75 revealed the formation of ramified clusters of vapor-filled pores on desorption, which is characteristic for a percolation process. In the present work, we check whether the adsorption/desorption isotherm data for a monolithic sample of Gelsil 50 can be analyzed in terms of a bond-percolation model. Three powder samples were studied too. Percolation probability data are presented and the effects of heterogeneous nucleation, finite size, and surface clusters on drainage from Gelsil 50 are addressed. The mean coordination number was derived. The results of the analysis are discussed with respect to recent theoretical work for interactions of fluids with complex pore systems. The monolithic sample and a powder sample were characterized by small-angle neutron scattering data.

Certified reference materials (CRMs) are very important for the reliability of the results of chemical analyses and, at the same time, constitute the basis of comparability of analytical data. The development of a new type of CRMs is demonstrated using the example of porous and finely dispersed solids having specific surface area, specific pore volume, pore volume distribution, and mean pore radius as their characteristic values.Many of the properties of reference materials to be certified, such as their elemental content, are established by means of measurement procedures relying on different and independent principles of measurement. With porous or finely dispersed solids, however, the characteristic values referring to pores strongly depend on the size of the probe molecules used, a fact which is due to the fractal character of these substrates. This is particularly true for the specific surface area or the pore volume. As a result, when applying the method of gas adsorption, the certified values refer to a specific probe gas, e.g. to nitrogen. The use of other gases yields different numerical values for the characteristic values to be certified.Generally, the pore system of a solid contains pores of irregular shape. Therefore, depending on the presence of e.g. cylinder or slit pores, different evaluation models must be applied. For the verification of model calculation of microporous solids, zeolites with their crystallographically defined voids are particularly well suited. They can serve as a reference point for the correctness of the model used. Zertifizierte Referenzmaterialien (ZRM's) sind fur die Sicherheit und Zuverllissigkeit von Analysenergebnissen von groder Bedeutung und gleichzeitig Voraussetzung fur die Vergleichbarkeit von Analysendaten. Am Beispiel der fiir porose und feindisperse Festkorper typischen physikalischen Kenngrohn spezifische Oberflache, spezifMhes Porenvolumen, Porenvolumenverteilung und mittlerer Porenradius wird die Entwicklung eines ZRM dargestellt. Gegenuber der Zertifizierung der meisten Eigenschaften von Referenzmaterialien, etwa der Bestimmung der Elementgehalte, die mit moglichst verschiedenen, auf unabhiingigen MeSprinzipien beruhenden Verfahren vorgenommen wird, sind die Meflwerte porenbezogener KenngroDen von por& sen bzw. feindispersen Feststoffen infolge ihres Fraktalcharakters von der G r o k des eingesetzten Sondenmolekuls abhangig, was beispielsweise fur die spezifische Oberflache oder das Porenvolumen zutrifft . Daraus resultierend beziehen sich die zertifizierten Werte bei Anwendung der Methode der Gasadsorption auf ein spezifisches Medgas, z. B. Nz. Die Verwendung anderer Gase fiihrt zu anderen Zahlenwerten fur die zu zertifizierenden Kenngrohn. Im allgemeinen weist das Porensystem eines Festkorpers Poren von unregelmaiger Gestalt auf. Es bedarf: daher der Anwendung unterschiedlicher Modelle, z. B. fiir Zylinder-oder Schlitzporen. Fur die Uberprufung von Modellrechnungen fiir mikroporose Festkorper eignen sich Zeolithe mit ihren kristallografisch definierten Hohl...

Abstract. Titania nanopowders obtained by hydrolysis of titanium alkoxides and consecutive hydrothermal treatment were compacted and sintered. The influence of preparation conditions (alkyl, pH of hydrolysis, temperature of hydrothermal treatment, compacting pressure, sintering temperature) on the porosity of powders, compacts and sintered bodies was investigated by nitrogen sorption and mercury intrusion. Optimum conditions are derived to prepare high-density ceramics at relatively low sintering temperature.Key words: titania, hydrothermal synthesis, pore volume distribution, density, compacting pressure.For industrial applications titania ceramics should be of high, homogeneous density [1]. This can be achieved at moderate sintering temperatures starting from appropiate powders.The hydrothermal treatment [21 is a suitable method to obtain titania powders with optimal properties concerning the production of ceramics for electronic components. But the properties of the titania powder and, consequently, of the compacts and ceramics depend on the physical and chemical conditions of the synthesis [3] e.g. pH, temperature during the precipitation, concentrations, dropspeed, freeze drying [4]. A controlled hydrolysis is important to get extreme small particles (submicron, nanopowders) and a close particle size distribution [5].The building of the structure of sintered ceramic compacts depends upon the properties of the primary powders used. Conventionally the green density of the used shapes seemed to be most important, but new results emphasize the significance of the influence of the * Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday ** To whom correspondence should be addressed arrangement of the initial nanopowder particles and the pore volume distribution in the green body. The pore volume distribution is an essential factor strongly influenced by the presence or absence of agglomerates.Variation in the processing parameters delivers in some cases powders with nearly the same properties concerning particle size distribution and surface area, but after compacting at identical pressure and identical sintering temperature, some have densities about 90 %, other 99% and more. The differences can be predicted by viewing the porosity of the green bodies and described with the characteristics of the porosity and the pore size distribution of the sintered pellets and shapes, respectively.The development of the porosity of titania shapes synthesized hydrothermally from titanium alkoxides was examined with regard to the dependence on pH and the temperature during synthesis, the compacting pressure of the green bodies, and the temperature at the sintering process. ExperimentalA solution of 0.15 mol titanium tetra-ethoxide in 11 ethanol was added gradually within 1 h to a boiling solution of 0.5 mol urea and 1.5mol demineralized water in 11 ethanol. Before mixing, the solutions were ultrafiltrated to exclude seeding particles. A precipitation occurs during the addition. Due to...