Spark‐Plasma‐Sintering Condition Optimization for Producing Transparent MgAl₂O₄ Spinel Polycrystal

Abstract

By controlling the heating rate at o101C/min during sparkplasma-sintering (SPS) processing, transparent polycrystalline spinel with an in-line transmission of 50% and 70% in the visible-and infrared-wavelengths, respectively, can be successfully fabricated for only a 20-min soak at 13001C. The high transmission can be attained by reducing the residual porosity and pore size, which was achieved by the low-heating rate. At high heating rates, many closed pores are formed due to the high densification rate during the heating process and remain as large pores around grain junctions. At temperatures 413001C, the coalescence of the residual pores and the precipitation of second phases, which are caused by rapid grain growth, degrade the transparency. The present study demonstrates that although the high heating rates have been regarded as a primary advantage for the SPS processing, the low heating rate is highly effective in attaining a high transparency in the spinel even at low temperatures and for short sintering times.

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

“…The high heating rate has been regarded as the primary advantage of the SPS technique [1][2][3][4]. Regarding the transmission of the a-Al 2 O 3 and spinel, however, a higher T in value can be achieved by decreasing the heating rate a [6,14]; for the visible wavelength of k = 550 nm, T in of the spinel exhibits $50% for a = 10°C min À1 though 0% for a = 100°C min À1 [14][15][16].…”

“…Fig. 1 shows an example of the in-line transmission T in of a spinel, which was obtained by the SPS technique at the low and high heating rates of a = 10 and 100°C min À1 [14][15][16]. The high heating rate has been regarded as the primary advantage of the SPS technique [1][2][3][4].…”

“…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.…”

Field‐Assisted Sintering Technology/Spark Plasma Sintering: Mechanisms, Materials, and Technology Developments

“…[139] Compromise between required pore closure and undesirable grain growth determines the holding time at elevated temperature. In some cases, no holding time at all might be necessary.…”

“…Transparent ceramics synthesized by FAST/SPS include a large number of optically isotropic materials [132] like yttrium-aluminum-garnet (YAG), pure or doped with rare earths, [133][134][135][136] yttria, [137] magnesia, [138] magnesium aluminum spinel, [139,140] fully stabilized zirconia, [141] and lutetium based oxides. [142] Optically anisotropic ceramics are even more challenging, as the transparency decreases drastically as grain size increases, and alumina, [143][144][145][146] tetragonal zirconia, [147] or hydroxyapatite [148] have been synthesized with improving success.…”