Crystallization and microwave dielectric properties of ZnO-B2O3-P2O5 glass ceramics

We have successfully prepared densified glass ceramics of ZnO-B₂O₃-P₂O₅ ternaries via externally nucleated crystallization of glass powder compacts, investigated crystallization, phase assemblage, morphologies, and microwave dielectric properties of the obtained glass ceramics. The glass transition temperatures (T_g) are within ~ 480 and 510°C for all compositions. All glasses except the 30ZnO-10B₂O₃-60P₂O₅ composition crystallize after annealing at some temperatures above the T_g. BPO₄, Zn₃(PO₄)_2, and Zn₂P₂O₇ phases crystallize sequentially with increasing annealing temperature. An additional trace amount of B exists as the annealing temperature is over 700°C due to the incongruent evaporation of BPO₄. An increase in ZnO and a decrease in B₂O₃ without much change in the P₂O₅ content of the glass composition results in reducing the crystallization temperatures. The crystallization activation energies of the α-Zn₂P₂O₇ in the 30ZnO-40B₂O₃-30P₂O₅, 40ZnO-60BPO₄, and Zn₃(PO₄)₂ in the 60ZnO-10B₂O₃-30P₂O₅ glasses were estimated to be ~ 211–197, ~ 189–199, ~ 96–108 kJ/mol, respectively, by both Kissinger and Qzawa methods. The crystallization of BPO₄, Zn₃(PO₄)₂, and α-Zn₂P₂O₇ phases would increase the ε_r, Q × f value, and negative value of τ_f. The 60ZnO-10B₂O₃-30P₂O₅ glass ceramic annealed at 875°C/2h has good combined microwave dielectric properties with ε_r ~ 5.60, Q × f ~ 51,989 GHz, τ_f ~ -99.4 ppm/^oC.