Towards good quality Bi2ZnB2O7 fibers grown by the micro-pulling down technique

•Colorless and transparent crystal fibers can be obtained by the micro-pulling down technique.•Bi2ZnB2O7 crystal fibers are obtained only through an incongruent crystallization.•Obtaining long fibers requires a very stable growth regime over the long time period necessary.

Bi2ZnB2O7 (BZBO) fibers grown by the micro-pulling down technique (μ-PD) usually present a more or less pronounced color ranging from yellow to red and a microstructure showing glassy clear parts (more concentrated in bismuth) dispersed in a darker matrix. In a previous paper, we assumed a reaction between the platinum crucible and the melt to explain both their color and microstructure. To confirm or invalidate this hypothesis, BZBO fibers were pulled under different conditions by the μ-PD or laser heated pedestal growth (LHPG) techniques. Various physical characterizations methods such as: SEM, EDS microprobe, Raman micro-spectroscopy, DTA and X-ray diffraction were performed to evaluate their crystal quality. Finally, it appears that the origin of the observed features of the fibers grown lies in the evaporation of a substantial amount of boron oxide from the melt. This leads to a shift of its composition in the ZnO-B2O3-Bi2O3 ternary system and an incongruent melting behavior. Therefore, the obtainment of colorless and transparent fibers requires very low pulling rates.