Fabrication and characterization of optically transparent holmium doped yttrium oxide ceramics

• Main Author: Cheong, Debbie Hui Ting
• Other Authors: School of Materials Science and Engineering
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/58464
• Institution: Nanyang Technological University

Transparent Polycrystalline ceramics are excellent materials for hosting solid-state lasers due their advantages in fabrication and flexibility in designing dopant concentrations. The search for the most optimal fabrication route and conditions is widely observed in current research in the field of advanced materials. In this work, holmium doped yttrium oxide precursor powder was synthesized via a co-precipitation route with yttrium nitrate hexahydrate (Y(NO3)3.6H2O), sintering additives (zirconium nitrate, lanthanum nitrate, holmium nitrate), dispersant ammonium sulfate (NH4)2SO4 as starting materials and with ammonium hydro-carbonate (NH4HCO3) working as the precipitant. Characterisation of both transparent ceramic powders produced and transparent ceramics fabricated was also conducted. Calcined transparent ceramic nanopowders obtained via the chemical co-precipitation method first underwent phase identification by X-ray Diffraction and particle size and morphology analysis by Transmission Electron Microscopy (TEM). Additionally, measurement of specific surface area was also done using the Brunauer-Emmett-Teller (BET) method for calculation of average particle size. Grain size analysis by Scanning Electron microscope (SEM) and optical transmittance analysis of the transparent ceramics fabricated was also conducted using Ultraviolet-Visible spectroscopy. Precursor powders were found to be transformed completely into well-crystallized (Y2O3 phase) and no other phases were detected. Calcined powders appeared spherical in shape and had a narrow particle size distribution. Average grain size of ceramics obtained by transparent ceramic powders calcined at 1100°C, 1200°C, 1300°C, 1400°C had very close average grain sizes. The addition of holmium ions was found to be successful in promotion of the densification process during sintering and increased the ease of achieving optical transparency at moderate sintering temperature of 1760°C in vacuum. Optimal calcining temperature for optimising optical transmittance in final transparent ceramic fabricated was found to be 1300°C, all the Ho: Y2O3 ceramic samples showed relatively good transmittance with sample fabricated from powders calcined at 1300°C exhibiting the highest transmittance. The optimal sintering condition was concluded to be at 1760 °C/13h as the sintered sample possessed a very dense structure and no pores were observed via SEM. Powders and final ceramics fabricated via solid state reaction and chemical co-precipitation of identical chemical composition and under same calcinations condition were also compared. The average particle size of powders fabricated by solid state reaction was found to be larger as compared to the average particle size of powders fabricated by chemical co-precipitation. Ceramics fabricated by both routes were sintered at 1760°C; by visual inspection, the transparency of final ceramic obtained via co-precipitation method was observed to be higher. Implementation of freeze-drying of precursor powders and further research on the effect of heating rate and holding time during sintering was also suggested for future research.