Non-isothermal crystallization kinetics of niobium-doped BGO70 glasses

Niobium-doped BGO glass of composition 85[3Bi₂O₃–7GeO₂ (BGO70)]–Nb15 was investigated for its kinetic parameters n and activation energies E _i : i = 1, 2, g of transformations via available models. Two crystallization exothermal peaks were observed for all heating histograms exploiting 5, 10, 15, and 20 K/min heating rates following the glass transition in non-isothermal DTA curves. An exothermal shoulder prior to first crystallization peak P₁ was identified as thermally favorable crystallizing metastable phase Bi₂GeO₅. P₁ was assigned to thermally stable crystal phase Bi₄Ge₃O₁₂. T _g lied between 729 and 731 K ± 4 K; T _p1 varied from 835–855 K ± 2 K, and T _p2 had values 965–986 K ± 2 K. Independent of the model exploited, activation energies E _g, E _p1, and E _p2 were 32.89 ± 3.1, 47.33 ± 0.72, and 62.74 ± 0.72 kcal/mol, respectively. P₁ corresponds to crystalline disks of Bi₄Ge₃O₁₂, and P₂ identified stable phase BGO crystal rods growing inward from the surface imbedded in glass matrix. Niobium doping modified the transformation kinetic parameters n and E _a of the composition by providing a control on growth mechanism for BGO platelets or rods.