Highly transparent spinel windows by microwave sintering

ABSTRACT Spinel ceramic exhibits excellent optical and mechanical properties, but its widespread use in high volume applications has been limited primarily due to the high cost associated with hot pressing and finishing. While, we have previously demonstrated techniques to reduce finishing costs, in this paper we report on the use of microwave sintering to make spinel ceramic at significantly lower cost than traditional hot pressing. We also identify preferred grain growth as well as an intra-granular fracture mode. Keywords: Spinel ceramic, microwave sintering, transparent. 1. INTRODUCTION (i) Spinel Background Polycrystalline magnesium aluminate spinel (MgAl 2 O 4 ) ceramic has superior mechanical properties compared with multispectral grade polycrystalline ZnS (Cleartran): 10x higher hardness, 2x higher fracture toughness and 3x higher strength [1]. While the current mechanical properties are slightly lower than for polycrystalline AlON and single crystal sapphire, spinel exhibits superior transmission at a wavelength of 5 Pm due to lower multiphonon absorption [2]. Consequently, spinel is an excellent window material for applications requiring transmission from the visible to mid-IR (see Table 1 and Figure 1).