Bonding in Aluminum Phosphate Ceramics Prepared at Low Temperatures
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BONDING IN ALUMINUM PHOSPHATE CERAMICS PREPARED AT LOWTEMPERATURES MICHAEL R. SILSBEE AND D.M. ROY* Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802 *also Department of Materials Science and Engineering ABSTRACT Phosphates have a long history of use as binding agents in ceramic systems. In particular the phosphate bonded alumina refractories and dental cements that employ the chemical reaction between a complex alumino-silicate glass and a buffered phosphoric acid to produce binding are among the most well known. The work described here has been undertaken in an effort to achieve a more complete understanding of the nature of the chemical binding in these materials. The results of a series of characterization studies on materials from the A1203-P205-H20 and Si02-A1203-P205-H20 systems will be discussed with particular emphasis on the nature of the binding produced in these materials. INTRODUCTION Aluminum ions in an aqueous phosphate solution are in the form of hydroxy-aluminum polymers. It has been proposed [1] that hydroxy-aluminum 3 polymers are composed of two closely packed planes of hydroxyl ion, with Al + residing in the octahedral holes. The Al 3 + ions are distributed as multiples or some fraction of six-member rings, with the A13 + ions at the edge not being fully satisfied by OH- ions. Thus as the size of the polymers increases the net positive charge per aluminum ion decreases (Table I). Table I.
Solubility constants and net positive charge per aluminum atom for hydroxy-al umi num polymers. Species
Al(OH)3 + 3H+ - A13+ + 3H20 Al3+ + H20 AlOH 2+ + H+ 2Al 33 + + 2H20 ÷ AI2(0H) + + 2H+ 3Al + + 40 Al 3 (OH)+ +_4H+ 13Al÷ + 2 H2 0 - A 1130 4 ( H)•
log (Ksp) 8.5 -4.97 -7.7 -13.9 -98.7
Net charge per aluminum ion 3 2 2 5/3 7/13
The formation of these hydroxy-aluminum polymers provides the basis for the development of a type of 2-dimensional binding in the paste. The phosphate ions because of their strong affinity for aluminum tend to be held tightly by the edge aluminum in its first coordination sphere (a Stern layer is developed in terms of the double-layer concept). In an idealized system where all the polymers have equal net positive charges the phosphate ions are likely to be attracted to two or more polymers. Steric considerations cause the phosphate ions to stay between the polymers and twist them to varying orientations, leading to the development of an amorphous product. As the phosphate is taken up by the hydroxy-aluminum polymers the net positive charge per aluminum atom decreases until the isoelectric point is reached where the aluminum phosphate polymers clump together and precipitate. Figure 1 shows an idealized schematic diagram of this amorphous material (after Van Wazer's aggregation polymers [2]). Mat. Res. Soc. Symp. Proc. Vol. 114. '1988 Materials Research Society
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