Phosphate Glasses
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Until recently, phosphate glasses were regarded as somewhat of a scientific curiosity—materials limited in technological application but nevertheless exhibiting interesting fundamental properties. Although the basic properties of many phosphate glasses have been studied on a laboratory scale during the past 50 years, very few of these materials proved to be of commercial interest. This lack of extensive practical application was primarily related to their poor chemical durability and a tendency to crystallize during processing. The pace of modern technological development continues to result in an increased demand for new materials with specific characteristics, and this demand has led to renewed interest in a number of previously neglected materials — including phosphate glasses. The combined physical and chemical characteristics of phosphate glasses make them relatively unique among the families of inorganic glasses. These characteristics include: large thermal expansion coefficients, low preparation and softening temperatures, low melt viscosities, chemical compatibility with living bone, and a polymeric structure similar to that in organic polymers. Most recent research on phosphate glasses has emphasized one or more of these characteristics. The major scientific questions pertinent to technological applications of phosphate glasses are related to utilizing the desirable characteristics of these materials while maximizing their corrosion resistance and thermal stability. In order to tailor a phosphate glass for a particular application, the corrosion mechanism of the glass must often be understood in detail, along with the relationship between the microscopic structure of the glass and its macroscopic properties.
connected (see Figure 1). In pure P 2 0 5 , the P0 4 tetrahedra are connected to form a three-dimensional random network structure. As metal is added to the glass structure, nonbridging oxygens are created that tend to break up the P 0 4 network. If enough metal cations are added to the glass so that there is, on the average, one nonbridging oxygen per P0 4 tetrahedron, the structure of the phosphate network consists primarily of long chains of P0 4 tetrahedra that wind through the glass. These chains are interconnected via bonds to the metal cations. With higher concentrations of metal, the average length of the
polymeric phosphate chains becomes shorter and shorter until the phosphate "glass network" consists primarily of isolated pyrophosphate dimers. Phosphate glasses with even larger metal-tophosphorus ratios are difficult to prepare because of the tendency of the glass melts to crystallize during cooling. In the more familiar silicate glasses the building blocks of the glass network are Si04 tetrahedra, also corner-linked to form the glass network. In spite of the basic structural similarity between phosphate and silicate glasses, the chemical properContinued
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Basic Chemistry and Structure of Phosphate Glasses: Areas of Current Scientific Interest
In phosphate glasses the basic
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