ABSTRACTInstead of aiming to prepare homogeneous gels and xerogels, this paper reports on work done to prepare deliberately diphasic materials. This has been achieved by three different paths: (1) mixing 2 sols; (2) mixing 1 sol with 1 solution; and (3) post formation diffusion of either one or two solutions.By the last named process we have made SiO2, mullite and alumina based composites, with silver halides, BaSO4, CdS, etc., as the dispersed phase. The crystal size can be confined to the initial pores by rapid diffusion giving rise to extremely fine second phases in the submicron range. Subsequent reduction of appropriate metallic salts can be used to give finely dispersed metals (e.g. Cu, Ni) in essentially any xerogel matrix. The open porosity makes these metal atoms very accessible.By the first two processes we have made both single phase and di-phasic gels of the same composition (prototype: mullite) and shown that though they cannot be distinguished by XRD, SEM, and TEM, by DTA and thermal processing, they are radically different. Such di-phasic gels store more metastable energy than any other solids.
Collective phenomena in Dy-doped silver halides in the near- and mid-IR