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