Non-isothermal crystallization kinetics of niobium-doped BGO70 glasses

Niobium-doped BGO glass of composition 85[3Bi(2)O(3)-7GeO(2) (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 P-1 was identified as thermally favorable crystallizing metastable phase Bi2GeO5. P-1 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. P-1 corresponds to crystalline disks of Bi4Ge3O12, and P-2 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.