Fabrication of Transparent Grain‐Oriented Polycrystalline Alumina by Colloidal Processing

Abstract

Grain‐oriented polycrystalline alumina (PCA) ceramics with very high, nearly single‐crystal transparency were successfully fabricated through colloidal processing in a high magnetic field followed by hot isostatic pressing. The c‐axis of the produced material was oriented along the direction parallel to the magnetic field. The transmittance of the specimen with a thickness of 0.8 mm was 78% at a wavelength of 650 nm, close to the reported value of 84.5% for sapphire and more than 10% higher than the largest transmittance magnitude ever reported for PCA ceramics. In addition, the Na salt of 4,5‐dihydroxy‐1,3‐benzenedisulfonic acid (named “Tiron”) was found to be a highly effective dispersant for processing high‐purity materials.

Hot‐pressing platelet alumina to transparency

“…The visible spectrum was measured from 200‐800 nm using a photomultiplier tube detector, and the near‐infrared (IR) spectrum was measured from 1000‐2500 nm using a lead sulfide (PbS) detector. A wavelength of 645 nm was chosen as the representative value for optical properties in this study, which is a similar wavelength used in the literature . Total transmission, in‐line transmission, reflection, and absorption can be measured using the spectrophotometer, and the forward and backward scattering can be derived from them.…”

“…Grinding and polishing the hot‐pressed samples to a consistent thickness was challenging, so it was necessary to normalize the optical properties of the samples to the same thickness. A modified version of Krell et al's equation was used to normalize the optical properties of the hot‐pressed samples to a thickness of 0.8 mm, which is the thickness most commonly reported in the literature . A detailed description of these equations are given in the .…”

“…Heuer et al claimed that by applying the pressure at the maximum temperature, substantial primary recrystallization occurs in the powder compact, which assists in densification. However, they used an equiaxed‐morphology alumina powder in their studies, as have all other studies aiming to produce transparent alumina . To the extent of the author's knowledge, there have been no reported attempts to hot‐press platelet‐morphology alumina powder to transparency.…”

Transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite and Al2O3 corundum

“…10 Elemental mappings were performed for all the samples in scanning TEM (STEM) mode using an EDX detector (Si/Li, EDAX). 9 The sample synthesized at 1600°C is opaque for visible light. The starting glass is transparent and the contained pores disappeared during the high-PT synthesis.…”

“…Transparent spinel (MgAl 2 O 4 ) ceramic 2 is the most popular example and other transparent ceramics with spinel structure are, e.g., c-AlONs 3 and cubic silicon nitride. 7 The visible in-line transmission is higher than 80% of the theoretical maximum [6][7][8][9] attributed to full densification (>99.99%), small grain sizes (submicrometer), and small refractive index difference (Dn max = 0.008) for different polarizations. Commonly, polycrystalline ceramics made of birefringent crystals are opaque due to successive refractive index mismatch.…”