Erbium-doped LAS glass ceramics prepared by spark plasma sintering (SPS)

https://doi.org/10.1016/j.jeurceramsoc.2005.08.003Get rights and content

Abstract

Dense nanocrystalline glass ceramics of the Li2O–Al2O3–SiO2 (LAS) system were obtained by spark plasma sintering (SPS) of powders prepared by sol–gel method. The low thermal expansion LAS glass ceramic was chosen as host matrix for erbium ions. ZrO2 was added both as a nucleating agent and as a possible good environment for the rare earth. The developed crystalline phases were analysed by X-ray diffraction (XRD) and the amorphous phase was quantified. Scanning and transmission electron microscopy (SEM, TEM) was used to investigate the microstructure. A different behaviour during the crystallisation process was observed between the sample prepared through the sintering of powders and the glass produced by the melting technique. A photoluminescence characterisation was also performed.

Introduction

Transparent glass ceramics have been investigated for optical applications such as solar collectors, up-conversion devices and laser media.1 Some recent papers2, 3, 4, 5, 6, 7 have proposed some interesting applications of transparent glass-ceramics of various chemical compositions containing nano-sized crystalline phases doped with luminescent lanthanide ions. The goal is to obtain crystal-like optical properties in a composite material with macroscopic glass properties. Depending on the glass host and the crystal phase composition, it is possible to obtain materials with improved mechanical, thermal, electrical or optical properties. Some recent studies concern the luminescent properties of Er3+-doped TiO2 or ZrO2 nanocrystals or glasses in which Er2Ti2O7 and ErPO4 nanocrystals8, 9, 10 are developed after thermal treatment.

Luminescent oxide glass ceramics in the Li2O–Al2O3–SiO2 system could be extremely interesting because of the high thermal-mechanical strength, near zero thermal expansion and transparency.11, 12 Glass-ceramics in this group are usually produced by promoting the volume nucleation in melt-derived bulk. It is possible to obtain a large number of nuclei (up to 1017 nuclei/cm3) by introducing ZrO2, TiO2, P2O5 or a mixture of them. In such a way a large number of nanocrystallites (5–20 nm), belonging to the nucleating phase (for example ZrO2 or ZrTiO4), is developed in the glassy matrix.

Alternative processes are based on the compaction and sintering of fine glass-melt or sol–gel-derived powders.13, 14, 15, 16, 17, 18, 19, 20 The sintering usually requires long treatment time and develops a crystalline coarse grain structure but it is possible to obtain full-density materials in the amorphous state if high heating rates are used.16

The glass composition studied in this work belongs to the LAS phase diagram (SiO2 73 wt.%, Al2O3 23 wt.%, Li2O 4 wt.%) known to lead to a transparent glass ceramic constituted of β-quartz solid solutions (75%) and a residual glass.

In the present work LAS glass ceramics containing small amounts of zirconium oxide are chosen as host matrix for erbium ions. ZrO2 was added not only as a nucleating agent but also because it is an excellent host for the luminescence ions thanks to its optical transparency, hardness, high chemical and photochemical stability, high refractive index and low phonon energy.21 In fact, in order to reduce non-radiative transitions generally due to multi-phonon relaxations, it is necessary to surround the active ions by a matrix that possesses low vibrational energies.

The powders obtained by the sol–gel route were sintered by spark plasma sintering (SPS)22, 23, 24 which allows to achieve higher densities at lower temperatures and in a very short time with respect to other approaches (i.e. traditional hot pressing), even with materials that are difficult to sinter uniformly (e.g. ZrO2-based materials).

In this paper, we compared Er-doped LAS glass ceramic materials obtained by SPS with those prepared by melting. To the best of our knowledge this is the first paper that verifies the applicability of SPS on LAS glass ceramic.

Section snippets

Sample preparation

The samples prepared by the sol–gel and melting techniques have the molar composition given in Table 1. The powders were prepared by the aqueous sol–gel route. The starting materials were tetraethoxy silane (TEOS) (98%, Aldrich), lithium carbonate (99%, Aldrich), zirconium oxychloride octahydrate (98%, Aldrich), aluminum nitrate nonahydrate (98%, Aldrich) and erbium chloride hexahydrate (99.9%, Aldrich). Freshly prepared aluminum and zirconium hydroxides, obtained by the addition of 30% ammonia

Results and discussion

Three sintering conditions characterized by different final temperature and pressure were used during the densification process: SPS1 850 °C 35 MPa 2 min; SPS2 840 °C 53 MPa 5 min; SPS3 900 °C 53 MPa 5 min.

Fig. 1 shows the XRD patterns of the sample prepared by sol–gel and sintered according to the conditions listed above. Only SPS3 achieved a marked crystallization while SPS1 and SPS2 are completely or almost completely amorphous. The XRD analysis, see inset in Fig. 1, shows that the crystalline

Conclusions

Monodispersed Erbium-stabilized zirconia nanoparticles (10 nm) were successfully developed from a LAS glass ceramic matrix through Spark Plasma Sintering of powders prepared by the sol–gel method. Completely amorphous samples or with 45 wt.% of crystalline phases can be obtained by choosing different conditions of temperature and pressure in the densification process. Although the sintered samples were not transparent, density and hardness of the most crystalline sample are very close to the

Acknowledgements

The financial support from MURST (COFIN-2002) and INFM is gratefully acknowledged.

References (27)

Cited by (31)

  • Lanthanide-doped oxyfluoride transparent glass-ceramics prepared by sol-gel

    2020, Sol-Gel Derived Optical and Photonic Materials
  • Enhanced upconversion emissions in Er<sup>3+</sup> doped perovskite BaTiO<inf>3</inf> glass-ceramics via electric-stimulated polarization technique

    2019, Ceramics International
    Citation Excerpt :

    But the emission intensity begins to fade down along with the continuous heating-up to 870 °C. This phenomenon is related to the concentration quenching effect since more Er3+ ions incorporates into the BaTiO3 crystallites with the increasing of crystallization temperature [31]. Further enhanced emission of the green band has been achieved, as reflected in a series of exploratory experiments in Fig. 5, by applying a high bias voltage (160 V) to the glass-ceramics subtlety.

  • Advanced Glass-Ceramic Nanocomposites for Structural, Photonic, and Optoelectronic Applications

    2016, Glass Nanocomposites: Synthesis, Properties and Applications
  • Fundamentals of Glass and Glass Nanocomposites

    2016, Glass Nanocomposites: Synthesis, Properties and Applications
View all citing articles on Scopus
View full text