Translucent yttrium aluminum garnet: Microwave-assisted route to synthesis and processing

M. Panneerselvam, G. N. Subanna, K. J. Rao
2001 Journal of Materials Research  
A novel and fast microwave route is described for the synthesis of yttrium aluminum garnet (YAG) and for its sintering to translucent bodies. Precursor was made by microwave decomposition (20 min) of aluminum tri-sec-butoxide and yttrium nitrate dissolved in ethyl acetate. The precursor, conventionally calcined at 1000°C (1 h), was sintered in microwave using SiC as secondary heater for just 35 min. Resulting translucent YAG has a microhardness (H V ) of 18.1 GPa and fracture toughness (K IC )
more » ... f 4.3 MPa m 1/2 . A 0.86-mm-thick sintered pellet exhibits approximately 45% transmission for 520-nm radiation. The entire microwave process requires less than 3 h. Yttrium aluminum garnet (Y 3 Al 5 O 12 , YAG) is a technologically important oxide ceramic. It is used as a host for lasers and for phosphors of high resolution and also as a coating on electronic devices. It is chemically stable at high temperatures. 1-4 Because of its proven technological potential, several chemical routes have been developed for its synthesis. 5-7 Sinterability of the YAG powders obtained from the various chemical routes has also been examined. 6, 8 Sintering requires temperatures of the order of 1600-1800°C and longer sintering times of the order of several hours. 8-10 Translucent YAG has been made by sintering at very high temperatures in vacuum and by addition of sintering aids. 6,9 Both the preparative and sintering processes now known are slow. In this communication, we report a novel microwave route that has well-known advantages. [11] [12] [13] [14] [15] [16] [17] [18] It was established by Warshaw and Roy 19 that YAG is the most stable phase in the Y 2 O 3 -Al 2 O 3 phase diagram, which has since been repeatedly confirmed. 20 De With sintered YAG to almost theoretical density and obtained a translucent product at a temperature of 1750°C using SiO 2 and MgO as sintering aids. 9 This was followed by a spate of activities in quest of methods for producing YAG powders that are sinterable at lower temperatues. Gowda 21 reported a sol-gel process where use is made of yttrium acetate and aluminum alkoxide and showed that the resulting amorphous gels decomposed at about 800°C giving rise to the fine powders of monophasic YAG. Fine powders YAG have also been made by combustion synthesis, 22 where urea and carbohydrazide are used as media for holding the reactants and for triggering the reaction. In the glycothermal method 23 1,4butanediol is used as medium for dissolving aluminum isopropoxide and yttrium acetate. In a somewhat similar procedure use is made of polyols such as glycerol. 24 Citrates, nitrates, etc., 5,7 have also been used as starting materials. The objective in all these methods has been to obtain an initial sol with atomic level mixing of the reactants, so as to drive the decomposition and crystallization processes leading to the YAG at as low a temperature as possible. Preparation of translucent YAG bodies, in the present approach, consists of three stages. In the first stage aluminum tri-sec-butoxide (Merck, 97% purity) and yttrium nitrate hexahydrate (Alrich, 99.9% purity) are dissolved in ethyl acetate to obtain a clear solution [18.3 cm 3 of aluminum tri-sec-butoxide and 16.65 g of Y(NO 3 ) 3 и 6H 2 O dissolved in 50 cm 3 of ethyl acetate separately and then mixed and stirred for 30 min]. The mixture is exposed to microwaves in a glass beaker using a domestic microwave oven (Batliboy eddy, Thrissur, India, operating at 2.45 GHz and maximum output power of 500 W, in steps of 100 W) set at its maximum power level. In under 30 s evolution of bubbles occurs, and in under 15 min the solution becomes increasingly viscous, white, and heterogeneous. Brown fumes are seen during the initial stages of drying of the viscous solution. In under 20 min a fluffy dry white mass is formed in the beaker, which is the precursor powder coded as YAG-AP. The white mass feels like talcum powder when pressed between fingers. In the second stage, YAG-AP powder
doi:10.1557/jmr.2001.0379 fatcat:nbmiiutfwbcgrb4gjnso2rhtay