UV transmitters of aluminum polyphosphates prepared by high pressure technique at room temperature

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The possibility to obtain nanostructured ceramic materials from nanometric powders has been studied over the last few years. Such interest was motivated by the promise of potential improvement in optical and mechanical properties, like transparency, ductility, hardness and strength, among other potentially interesting properties [1–4]. These enhanced properties have been related to the nanostructure of these materials [5]. However, it is very difficult to retain the nanocrystalline grain size during the sintering process in order to produce a high-density bulk nanostructured ceramic. Therefore, it is necessary to find favorable conditions to enhance densification and limit grain grow. The high pressure technique has been used successfully to obtain nanostructured ceramic or composite materials with the desired properties [6–10]. Aluminum polyphosphate nanostructured systems have been used extensively as pigment for painting [11, 12], as matrix for composite materials [13], as well as ceramic foams [14]. However, as far as we know, there is no report in the literature concerning the preparation of aluminum polyphosphate ceramic materials by using the compaction of nanometric powders. In this work, we studied the required conditions to produce bulk ceramic materials from nanometric powders of aluminum polyphosphate, using a special high pressure cell configuration with a highly hydrostatic pressure transmitting medium and at room temperature. The samples were analyzed by Vickers microhardness, density, UV-VIS spectroscopy and X-ray diffraction. Nanocrystalline aluminum polyphosphate with P:Al mol ratio = 0.60 ± 0.01 in the reaction admixture, was obtained by precipitation from solutions of sodium polyphosphate, aluminum nitrate, and ammonium hydroxide, under strong stirring and at room temperature. Particle diameters were in the 2–6 nm range. The powder obtained was dried at 600 ◦C [15]. For the high pressure experiment the aluminum polyphosphate powder was initially pre-compacted in a piston-cylinder type die to approximately 0.1 GPa.