Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl₂O₄) Spinel

Abstract

The need for materials for demanding optical applications has engendered a resurgent interest in transparent ceramics. Transparent polycrystalline magnesium aluminate spinel is one especially promising and rapidly maturing technology that can fill this niche. Although it has been studied for over 50 yr, it is only recently that highly transparent components with acceptable mechanical properties have been reliably fabricated at reasonable cost. Development has been hindered by the inherent difficulty in sintering spinel to the near-theoretical density required for transparency, a high sensitivity to powder and processing parameters, variable stoichiometry, and a lack of understanding of the synthesis-processing-property relationships. The driver of recent success is an emerging understanding of complex, multiscale, multivariable interactions that occur during green-body formation and sintering. In particular, certain key variables play a decisive role in determining compact properties and their evolution must be controlled from synthesis to the finished product. This article features the interactions between these key variables during processing and gives an expos e of the state of the art in transparent polycrystalline spinel fabrication.

Spectroscopic study of the discoloration of transparent MgAl2O4 spinel fabricated by spark-plasma-sintering (SPS) processing

“…In addition, the formation has been expected to be enhanced by sintering in graphite due to the reaction of carbon and oxygen; C + O x O ! V O + CO(g) [59]. However, this is unlikely to fit with the ESR measurements.…”

“…The LiF doped into spinel acts also as carbon cleanser by reacting with C (LiF + C ! Li + (CF) gas ") and can remove the CF phases as gas [18,59,70]. Second, Bonnefont et al [21] used higher purity starting spinel powder (S30CR, Baikowski Chimie, France) and utilized a very slow heating rate of a = 1°C min À1 .…”

“…For the MgAl 2 O 4 spinel, it is known that several factors, such as stoichiometry, cation disorder structures, impurities, mechanical treatment and sintering atmosphere, lead to the formation of a wide variety of complicated point defects [57][58][59][60][61]. Since there is a possibility that the large number of defects serve as trapping centers, it difficult to explain the signals with ESR data.…”

Microstructure evolution of SiO2, ZrO2-doped Y3Al5O12:Nd3+ ceramics obtained by reactive sintering

“…Anisotropic Ca 5 (PO 4 ) 3 F (FAP) laser ceramics doped by neodymium or ytterbium ions were synthesized by the orientation control of micro-domains by formation of textured compacts under magnetic field followed by hot isostatic pressing (HIP) [8,9]. Other examples include engineering of MgAl 2 O 4 [10] and (AlN) x ⋅(Al 2 O 3 ) 1-x (ALON) ceramics [11,12] for both military and civil optical applications.…”

Structure and optical properties of Sn4+ doped Ba(Mg1/3Nb2/3)O3 transparent ceramics

“…Transparent ceramics, such as Al 2 O 3 [1], MgAl 2 O 4 [2], Nd: YAG [3] and PLZT [4], have been broadly investigated for their broad range of applications, varying from optical windows to laser and optoelectronic switches. However, the miniaturization of optical functional elements, such as lens for optical information storage, waveguides for flat optical components and substrates for fluorescent components, still presents a challenge for optical materials with high refractive index.…”