Microwave sintering of Yb:YAG transparent laser ceramics Laura Esposito a,⇑ , Andreana Piancastelli a , Yury Bykov b , Sergei Egorov b , Anatolii Eremeev b a ISTEC CNR, Institute of Science and Technology for Ceramics, Via Granarolo 64, 48018 Faenza, Italy b Institute of Applied Physics RAN, 46 Ulyanov Str., 603950 Nizhny Novgorod, Russia article info Article history: Available online 16 September 2012 Keywords: Transparent ceramics Microwave sintering Laser application Yb:YAG abstract Reactive sintering of YAG based ceramics is generally performed under high vacuum in graphite-free fur- naces in order to guarantee the elimination of pores and absence of any contamination. An alternative densification technique is the field assisted process such as spark plasma sintering and microwave sinter- ing. Both of these methods are characterized by very fast heating rates, low sintering temperatures and short sintering times. The microwave sintering process is different from electric resistance heating since heat is generated in the bulk of the powder compact through electromagnetic radiation absorption and creates within its body uniform temperature distribution. Microwave sintering of laser ceramics is advan- tageously distinguished by the absence of any elements having high temperature such as electric heaters or dies which materials can contaminate the sintered parts. In addition, the inverse temperature distri- bution that exists within the body under volumetric microwave heating is favorable for elimination of porosity. Microwave sintering of Yb:YAG samples were tested and the obtained results are presented. The sam- ples were sintered on a gyrotron-based system operating at a frequency of 24 GHz with microwave power up to 6 kW. Reactive sintering of YAG doped with 1.0, 5.0, and 9.8 at.% Yb 2 O 3 was performed in different temperature–time regimes. The microstructure and the optical transmittance of the obtained samples were compared to those of samples obtained by conventional high vacuum sintering. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Laser ceramics are proposed as a powerful laser source alterna- tive to single crystals [1,2]. The ceramic technology enables the fabrication of transparent, pore-free polycrystalline materials (specimens or articles), with complicated geometries, as for exam- ple graded or layered structures. The flexibility of the ceramic tech- nology permits to improve the thermal management and to produce ceramic parts of large size with high concentration of laser active ions. However, the optical quality of laser ceramics can be limited by the residual porosity, grain boundaries impurities and dopant segregation acting as optical scattering centres. The sinter- ing technique plays a key role in the microstructure optimization. Laser ceramic are commonly sintered under high vacuum, but microwave sintering is an attractive alternative because it has many significant advantages against conventional sintering meth- ods [3,4]. Microwave sintering is widely investigated for various constructive and functional ceramics. The dielectric properties of many ceramic materials allow the microwave energy to be ab- sorbed directly within the bulk of the specimen. The energy depo- sition does not rely on the thermal conductivity of the material, resulting in a faster heating and a more uniform temperature dis- tribution within the body of the specimen compared to conven- tional sintering techniques. This distinctive feature of microwave heating is beneficial for saving the processing time and energy, but it also gains greatly in importance in case of optical ceramics sintering. Microwave sintering of optical ceramics is appealing be- cause this method is free of high temperature resistive heaters. Chemical elements evaporated from heaters in the furnaces used for vacuum sintering at temperatures as high as 1750–1800 °C is one of the chief causes of contaminations in sintered materials which give rise to the optical scattering. Conversely, since the in- verse temperature distribution that exists in a specimen under vol- umetric microwave heating is favorable for elimination of porosity [5], the high vacuum conventionally used during sintering to pro- mote the pore closure should not be necessary, as well as the sub- sequent sintering in air. Densification starts up in the central part of the specimen where the temperature is maximal, and the periphery parts remain open up to higher temperature, thus facil- itating the pore exit. Few studies on microwave sintering of optical ceramics have been undertaken to this date. Authors of [6] reported on sintering Nd:YAG and Nd:Y 2 O 3 ceramics using the millimeter-wave power at a frequency of 83 GHz. The sintered samples were translucent, their density was more than 99% of the theoretical value. Samples of 1–3 at.% Nd:Y 2 O 3 with a thickness of 1.1 mm were sintered by the millimeter-wave power in [7] exhibiting a transmittance of 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.07.014 ⇑ Corresponding author. E-mail address: laura.esposito@istec.cnr.it (L. Esposito). Optical Materials 35 (2013) 761–765 Contents lists available at SciVerse ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat