Novel Optical Ceramics: α-Sialons

The transparency of hot-pressed α-sialon samples containing rare earth elements in visible and infrared bands was observed, and the luminescence effect of Dy-α-sialon sample excitated by 350nm light was also detected. The results showed that all thinned α-sialon samples were translucency in the visible and near-middle (NM) infrared band in different extents, depending on the compositions and doping elements. Some of them had a very high transmittance around 70% in the range 2 to 4. 5μm wavelength. Some thick translucent α-sialon samples can also be fabricated by a controlled processing. Dy-α-sialon can launch obvious visible light associated with 4 F9/2 6 H15/2 and 4 F9/2 6 H13/2 emissions of the Dy 3+ . Therefore, α-sialon ceramics have potential for making components with combined mechanical and optical properties. Introduction α-Sialon (α′, MxSi12-(m+n)Alm+nOnN16-n), as one of the most attractive high temperature structural materials, has some unique characteristics, such as high hardness (Hv ~ 22GPa), low thermal expansion coefficients (~ 3.7-4.0×10/°C) and potential of cleaning up grain boundaries[1]. Studies in the last twenty years were mainly carried out for their properties as structural materials, with less attention on their functional properties. Developments in modern medicine, optical communication, space flying vehicles and high-energy physics etc. have increased the demands for the transparent and luminescent materials that combine good mechanical properties and thermal or chemical stabilities [2] as well as industrial processing ability. In comparison with glass and crystals and with most oxides ceramics, sialon ceramics have much better thermal and chemical stability due to their low linear thermal expansion coefficients and strong covalent bonds. In the present work, the optical properties of α-sialon ceramics have been reported. Transparency of hot-pressed α-sialon samples containing rare earth elements in visible and infrared bands was observed, and their luminescence effect was also detected using 350nm exciting light. Experimental Based on the phase relationships in Ln-Si-Al-O-N systems [3], all samples in the present work were designed to have an overall composition lie in the edge of the single-phase α-sialon area, as listed in Table1. The starting powders were α-Si3N4, Al2O3, AlN and R2O3 (where R=Y, Dy, Sm, Nd, Er and Gd). Powder mixtures were ball milled with absolute alcohol in a plastic pot for 24h and dried under an infrared lamp. The mixed powder was hot-pressed in nitrogen at 1800°C under 25MPa for 1h in a graphite furnace. Phase characterization was performed with X-ray diffraction (XRD) using an RAX-10 diffractometer. Transmission in the 250-2500nm wavelength region was measured by a UV-3101PC Spectrophotometer (Shimadzu, Japan) and transmission in 2500nm-25μm wavelength region was detected by FTS-185 (Bio-Rad USA). The luminescence Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 905-908 doi:10.4028/www.scientific. net/KEM.264-268.905 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-13/03/20,18:14:03) effect was observed using LS50 (Perkin Elmer UK). Results and discussion The phase compositions of sintered samples are given in Table1. α-Sialon was the only crystalline phase in most samples, except a few melilite phase was detected in the Nd-α-sialon. The as-hot-pressed samples were not translucency in visible band. But, when they were thinned to about 0.3mm, all of them became translucency to some extent, and some of them are colorful, as shown in Fig.1. This result is similar to the work of Karunaratne [4], Shen [5] and Hanson [6]. However, the translucency of the samples appeared very different. For samples A and D, although containing the same doping element (yttrium), the former one is much more translucency due to their difference in compositions. On the other hand, for samples D and E, or samples B and A, the only difference between them was the different doping elements used, but the latter compositions appeared much more translucency. It is interesting that in the Nd-doped sample, some areas showed dark color and opaque, and some areas showed similar translucency to samples A and E. When comparing these two kinds area on the surface of the Nd-sample using an optical microscope, more pores were found in the opaque area, as shown in Fig. 2. The porosity is thought to be responsible for the opacity. Table.1 Compositions of starting powder and phases in as-sintered samples Samples Nominal composition Actual phase in as-sintered* Thickness (mm) A Y0.33Si9. 3Al2.7O1. 7N14.3 α-sialon (s) 0.3 B Er0.33Si9. 3Al2.7O1. 7N14.3 α-sialon (s) 0.3 C Nd0.33Si9. 8Al2.2O1. 2N14.8 α-sialon (s), Melilite (w) 0.3 D Y0.67Si9. 3Al2.7O1. 7N14.3 α-sialon (s) 0.3 E Dy0.67Si9. 3Al2.7O1. 7N14.3 α-sialon (s) 0.3