The effect of dopants on crystal growth kinetics of lithium disilicate: surface versus bulk crystallization

Glasses with the compositions 2 R_yO_z·98 Li₂Si₂O₅ (R = Al, Ti, La, Ce, or Nb) and x ZrO₂·(100 − x) Li₂Si₂O₅ (x = 1, 2, 3, or 5 mol%) were studied with respect to their crystallization behavior using differential scanning calorimetry, X-ray diffraction, as well as optical and scanning electron microscopy. During thermal treatment, in all samples lithium disilicate was crystallized. From the differential scanning calorimetry profiles recorded using different heating rates in the range from 5 to 20 K/min, activation energies and Avrami parameters were calculated. The activation energies decreased for coarser powders and bulk samples. The Avrami parameters were smallest for fine powders and largest for the bulk samples, which stand for sole surface and predominant bulk crystallization, respectively. The crystal growth velocities determined from optical and scanning electron micrographs were largest for the undoped sample. For most samples, the crystal growth velocities were notably larger (up to two times) for the bulk crystallization than for surface crystallization. This should be due to the formation of concentration gradients formed by the shoved-away components, which are not incorporated into the lithium disilicate crystals. In the case of bulk crystallization, radial diffusion plays an important part which enhances diffusion and hence leads to higher growth velocities.