The Color Change of Natural Green Sapphires by Heat Treatment
A. Mungchamnankita ,T. Kittiauchawalb , J. Kaewkhaoc , P. Limsuwand
a Department of Physics, Faculty of Science, Rangsit University, Pathumtani, 12000, Thailand b Faculty of Science and Technology, Thepsatri Rajabhat University, Lopburi, 15000, Thailand c Center of Excellence in Glass Technology and Materials Science, Faculty of Science and Technology,Nakhon Pathom Rajabhat University, Nakhon Pathom, 73000, Thailand d Thailand Center of Excellence in Physics, CHE, Ministry of Education, Bangkok, 10400, Thailand
Elsevier use only: Received 30 September 2011; Revised 10 November 2011; Accepted 25 November 2011.
Abstract
This research aimed to investigate the effects of heat treatment on color and clarity of green sapphires. The Green sapphires were heated at sequentially increasing temperature of 1300, 1400, 1500 and 1600 °C in oxygen atmosphere. At each treatment temperatures, the samples were treated for 40 hours with their color and clarity were measured before and after heat treatment at each temperature. It was found that, when green sapphires were heated at high temperatures, the color of the untreated sapphire crystal changed to yellow at higher temperatures. Whereas, there were no significant changes in the clarity of all samples found in the temperature range of 1300-1600 °C.
© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of I-SEEC2011 Keywords: Green sapphires, heat treatment
1. Introduction
The structure of Green sapphire is related to corundum (Al2O3) and involves the Al3+ ions being distributed in an ordered fashion in 2/3 of the distorted (trigonal) octahedral sites within a frame work of hexagonal close-packed O2- ions. The color of ruby arises mainly from Al3+ ions being replaced by 3d transition ions during a crystallization process. This is possible because the size of 3d transition ions is very close to that of Al3+ (0.57 Å) [1]. * Corresponding author. Tel.: +663-642-4175; fax: +663-642-4175. E-mail address: k_treedej@hotmail.com. Open access under CC BY-NC-ND license. Open access under CC BY-NC-ND license. A. Mungchamnankit et al. / Procedia Engineering 32 (2012) 950 – 955 951 Fig. 1. A polyhedral model of the structure of corundum [1] The presence of varying amounts of Cr3+ ions gives color to ruby from pink (0.01 mole %) to deep red (2 mole %) [1]. Iron usually exists in both Fe2+ as well as Fe3+ depending on the number of O2- vacancies and other point defects [2]. Fe3+ and Ni3+ in corundum yield yellow color whereas Fe2+ gives sapphire green color instead [3]. The charge transfer mechanism between 3d6 (Fe2+) and 3d0 (Ti4+) bands effectively gives sapphire its blue color. A combination of these Fe2+, Ti4+ and Cr3+ ions is clearly responsible for bluish red in ruby [4]. More detailed coloring mechanisms and impurity ions present in sapphires are listed in Table 1. Table 1. 3d Transition ions coloring gem corundum (Al2O3) [5] Sapphire Color Verneuil Synthetics Natural Coloring Ions Coloring Mechanisms Colourless Pure None None Ruby Cr3+ Cr3+ Electronic transitions & Fluorescence Pink Cr3+ Cr3+ Electronic transitions & Fluorescence(R-line) Blue Fe2+,Ti4+ Fe2+, Ti4+ Charge transfer Purple and Violet Cr3+,Fe2+,Ti4+ Cr3+, Fe2+, Ti4+ Electronic transitions , Charge transfer & Fluorescence(R-line) Yellow Ni3+ Fe3++ color centers Electronic transitions Orange Ni3+, Fe3+ Fe3+, Cr3+ +color centers Electronic transitions & Fluorescence(R-line) Green Cobalt, Vanadium + Nickel Fe2+ Electronic transitions The goal of this work was to investigate the effect of heat treatment on color of natural green sapphire measurements by spectrophotometer.
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