Synthesis and Densification of Transparent Magnesium Aluminate Spinel by SPS Processing

Abstract

Mixtures of elementary oxides, MgO–Al2O3, were used to fabricate transparent polycrystalline magnesium aluminate spinel specimens by means of the spark plasma sintering technique. A sintering aid, 1 wt% of LiF, was added to the mixed powder. The presence of the additive promotes the synthesis of spinel that starts at 900°C and is completed at 1100°C. The LiF additive wets spinel on its melting and promotes densification, which is completed at 1600°C. LiF vapor plays a cardinal role in eliminating residual carbon contamination and in the fully dense state, allows attaining a 78% level of optical transmittance. The optimal conditions for achieving adequate transparency were determined and the role of the LiF addition in the various stages of the process is discussed.

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

“…In previous studies, the discoloration in the SPSed samples has often been related to the trace carbon impurities [18][19][20]39] and to the point defects (oxygen vacancies) [7,22,27,38]. For the present sample, it was found based on the spectroscopic analysis that the discoloration can also be explained by the combination of both the carbon contamination and the formation of the F + -type defects, which are introduced in the spinel polycrystals depending on the SPS conditions.…”

“…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 .…”

“…1. According to the previous studies by Krell et al [34], Meir et al [18], Bernard-Granger et al [19], Casolco et al [23] and Zhang et al [27], the discoloration has been regarded as a possible source of the limited light transmission. Such a discoloration is known to be a typical phenomenon in the SPS processed oxide ceramics, such as Al 2 O 3 [7], ZrO 2 [22] and spinels [18,19], and appears to be more pronounced in the SPS processing than in the HIPing [19].…”

“…Such a discoloration is known to be a typical phenomenon in the SPS processed oxide ceramics, such as Al 2 O 3 [7], ZrO 2 [22] and spinels [18,19], and appears to be more pronounced in the SPS processing than in the HIPing [19]. Since the trace carbon and the point defects (oxygen vacancies) act as a possible light scattering and/or absorption source due to the formation of the color centers [19,38], the discoloration has often been related to the trace carbon contamination [18][19][20]39] and the oxygen vacancies generated under the vacuum conditions [7,22,27,38].…”

“…Recently, instead of the well-known hot pressing or hot isostatic pressing (HIP) techniques, the spark-plasma-sintering (SPS) technique [1][2][3][4] has been widely utilized for the densification of various types of transparent oxide ceramics [5], such as a-Al 2 O 3 [6][7][8][9][10][11][12], MgAl 2 O 4 spinel [13][14][15][16][17][18][19][20][21], cubic (c-) and tetragonal (t-) ZrO 2 [22][23][24][25][26][27], MgO [28], Y 2 O 3 [29][30][31] and yttrium aluminum garnet [32,33]. Fig.…”