Glass-Ceramics Processed by Spark Plasma Sintering (SPS) for Optical Applications

This paper presents a review on the preparation of glass-ceramics (GCs) and, in particular, transparent GCs by the advanced processing technique of spark plasma sintering (SPS). SPS is an important approach to obtain from simple to complex nanostructured transparent GCs, full densification in a short time, and highly homogeneous materials for optical applications. The influence of the different processing parameters, such as temperature, pressure, sintering dwell time on the shrinkage rate, and final densification and transparency, are discussed and how this affects the glass material properties. Normally, transparent glass-ceramics are obtained by conventional melt-quenching, followed by thermal treatment. Additionally, the GC scan is produced by sintering and crystallization from glass powders. Hot pressing techniques (HP) in which the source of heating is high-frequency induction can be also applied to enhance this process. In the case of transparent ceramics and glass-ceramics, spark plasma sintering is a promising processing tool. It is possible to enhance the material properties in terms of its compactness, porosities, crystallization, keeping the size of the crystals in the nanometric scale. Moreover, the introduction of a high concentration of active gain media into the host matrix provides functional glass-ceramics systems with enhanced luminescence intensity through reducing non-radiative transitions like multi phonon relaxation (MPR) and cross relaxations (CR), thus providing longer lifetimes. More effort is needed to better understand the sintering mechanisms by SPS in transparent GC systems and optimize their final optical performance.

Tunable upconversion emission in NaLuF4-glass-ceramic fibers doped with Er3+ and Yb3

Novel glass-ceramic optical fibers containing NaLuF4 nanocrystals doped with 0.5ErF3 and 2YbF3 (mol%) have been prepared by the rod-in-tube method and controlled crystallization. NaLuF4 nanocrystals with a size around 20 nm are obtained after heat treatment at 600 °C. Intense upconverted green and red emissions due to (2H11/2, 4S3/2) → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively, together with a blue emission due to 2H9/2 → 4I15/2 transition have been observed under excitation at 980 nm. The intensity of the green and red upconversion bands shows a nearly linear dependence on the excitation power which can be explained by saturation effects in the intermediate energy states and proves that a sensitized energy transfer upconversion process is responsible for the population of the emitting levels of Er3+ ions. The upconversion emission color changes from yellow to green by increasing the excitation power density which allows to manipulate the color output of the Er3+ emission in the glass-ceramic fibers. The tunable emission color is easily detected with the naked eye. This interesting characteristic makes these glass-ceramic fibers promising materials for photonic applications.