Effects Of P₂O₅ On The Structure And Properties Of Na₂O-Al₂O₃-SiO₂ System Glass

The silicate glasses containing P₂O₅ have very important applications in high-tech glasses and the immobilization of nuclear waste.However,the effect of high content of P₂O₅ on the glass structure of Na₂O-Al₂O₃-SiO₂ system remains to be studied.In addition,in the immobilization of nuclear waste,high-level radioactive waste has a very complex composition,which makes that direct research is very difficult.Therefore,we can design a simplified Na₂O-Al₂O₃-SiO₂ system glass which is located in nepheline formation area as a simulated HLW glass,and study the effect of P₂O₅ on the phase crystallization and separation behavior of the system.In this paper,SiO₂ and Al₂O₃ in glasses composed of 25Na₂O-25Al₂O₃-50SiO₂were replaced by P₂O₅.The microstructure of the samples was characterized by MAS NMR or Raman to study the effect of high content of P₂O₅ on the glass structure of Na₂O-Al₂O₃-SiO₂ system.At the same time,the quenched glass samples were heat treated by isothermal heat treatment.The immiscibility of the heat treated samples was studied by XRD and SEM.Therefore,the effect of P2O5 on the crystallization and phase separation of Na₂O-Al₂O₃-SiO₂ system was clarified.The results are as follows:（1）In 25Na₂O-25Al₂O₃-（50-x）SiO₂-xP₂O₅ system,with the addition of P₂O₅,the degree of polymerization of silicate structure decreases,the degree of polymerization of phosphate structure increases,and the degree of polymerization of the overall network of glass decreases,which is consistent with the change of glass transition temperature Tg.When the amount of P₂O₅ is 35 mol%,the degree of polymerization of the overall network of the glass increases,and the Tg also increases.When P2O5 was not added,Na⁺were all used to balance the Al（OSi）₄ and NaAlO₂ structural units.With the addition of P₂O₅,Al（OSi）₄ were destroyed.Therefore,Na⁺were released from Al（OSi）₄ and act as network modifiers.In the silicate structure,Na⁺destroy its structure by providing free oxygen,while in the phosphate structure,Na⁺act as charge compensators in[PO₄]^3-tetrahedrons by providing free oxygen.With the addition of P₂O₅,the coordination number of Al changes from Al^IV to Al^IV and a small amount of Al^V,while the coordination polyhedron of Al changes from Al（OSi）₄ to Al（OP）₄ and a small amount of Al（OP）₅.With the addition of P₂O₅,the NaAlO₂ structural units in the glass aren’t destroyed.（2）In 25Na₂O-25Al₂O₃-（50-x）SiO₂-xP₂O₅ system,with the addition of P₂O₅,the formation of nepheline is inhibited mainly by the change of chemical composition and the ability of P2O5 to induce phase separation.（3）In 25Na₂O-（25-x）Al₂O₃-50SiO₂-xP₂O₅ system,with the addition of P₂O₅,the degree of polymerization of silicate structure decreases,the degree of polymerization of phosphate structure increases,and the degree of polymerization of the overall network of glass decreases,which is consistent with the change of glass transition temperature Tg.When P₂O₅ was not added,Na⁺were all used to balance the Al（OSi）₄and NaAlO₂ structural units.With the addition of P₂O₅,Na⁺were released and act as network modifiers.In the silicate structure,Na⁺destroy its structure by providing free oxygen,while in the phosphate structure,Na⁺act as charge compensators in[PO₄]^3-tetrahedrons by providing free oxygen.However,the information about coordination number and coordination polyhedron of Al can’t be obtained,which needs to be further judged by ²⁷Al MAS NMR test.And whether the structural units of NaAlO₂ have been destroyed needs to be further judged by ³¹P MAS NMR test.（4）In 25Na₂O-（25-x）Al₂O₃-50SiO₂-xP₂O₅ system,with the addition of P₂O₅ and when Al/P<1,stable phase separation occurs,and the mechanism of phase separation changes from nucleation-growth to spinodal decomposition,which is consistent with the"double Tg"phenomenon.The coexistence of phase separation and crystallization after heat treatment indicates that phase separation promotes crystallization.With the addition of P2O5,the formation of nepheline is inhibited mainly by the change of chemical composition and the ability of P₂O₅ to induce phase separation.