Translucent Yttria‐ and Silica‐Doped Mullite Ceramics with Anisotropic Grains Produced by Spark Plasma Sintering

Abstract

Translucent, high‐performance, mullite ceramics with anisotropic grains were prepared by the spark plasma sintering (SPS) of a powder mixture consisting of commercial mullite powder, which already contained small amounts of alumina (θ and α) and silica (cristobalite) (≤3 wt% in total), to which 2 and 1 wt% of yttria and amorphous silica was admixed, respectively. The combination of low‐viscosity Y2O3–Al2O3–SiO2 transient liquid formation and SPS sintering provided enhanced densification, also provoking anisotropic grain growth (which became exaggerated after 20 min of SPS dwell time), at a relatively low sintering temperature of 1370°C. In this way, it was possible to meet the conflicting demands for obtaining a dense mullite ceramic with anisotropic grains, ensuring good mechanical properties, while preserving a noticeable light transmittance. In terms of mechanical and optical properties, the best results were obtained when SPS dwell times of 5 and 10 min were employed. The as‐sintered samples possessed densities in the range 3.16–3.18 g/cm3, anisotropic grains with an aspect ratio (AR) of 7 and a grain thickness of approximately 0.45 μm, a flexural strength between 350 and 420 MPa, a Vickers indentation toughness and a hardness of approximately 2.45 MPa·m1/2 and 15 GPa, respectively, and an optical transmittance of between 30% and almost 50% in the IR range.

Ultra‐low temperature preparation of mullite glass‐ceramics with high transparency sintered from EMT‐type zeolite

“…Surprisingly, g‐800 in our work reveals an obvious mullite crystalline phase at as low as 800°C, which is 180°C lower than that adopted in heating gels. What's more, the derived glass‐ceramics present the highest transmittance in the visible and infrared region among the mullite glass‐ceramics ever reported 14,38 …”

“…What's more, the derived glass-ceramics present the highest transmittance in the visible and infrared region among the mullite glass-ceramics ever reported. 14,38 The surface SEM image of g-800 shows a characteristic morphology of acicular mullite (Figure 3A). With the sintering temperature increasing from 800°C to 1000°C the size of mullite crystal become larger, as shown in the SEM images of g-800, g-900 and g-1000 (Figure 3A-C).…”

“…TiO 2 , Fe 2 O 3 , alkali/alkaline earth), significantly influencing the properties of vitreous ceramics 12,13 . In addition, the mullite glass‐ceramics prepared by the conventional methods are only transparent in the infrared region but have low transmittance in the visible region 14 . Worse yet, the high sintering temperature could lead to the decomposition and volatilization of the dopants in the glass‐ceramics and cause the energy waste, which limit the application scope of mullite glass‐ceramics.…”

“…12,13 In addition, the mullite glass-ceramics prepared by the conventional methods are only transparent in the infrared region but have low transmittance in the visible region. 14 Worse yet, the high sintering temperature could lead to the decomposition and volatilization of the dopants in the glass-ceramics and cause the energy waste, which limit the application scope of mullite glass-ceramics. Therefore, the development of novel technologies to prepare high-quality mullite glass-ceramics with an excellent transparency in both visible and infrared region at a low temperature is highly desired.…”

Sintering and characterization of in situ formed porous cordierite ceramics with nano boehmite additions

“…Mullite has a significantly higher melting point, ranging between 1810 and 1830°C, and can be considered a promising candidate for porous ceramics. It has superior high‐temperature mechanical properties such as strength and elastic modulus; though its thermal shock resistance is lower than that of cordierite 23–25 . Coefficients of thermal expansion (CTE) values for mullite and cordierite are 6 × 10 −6 and ∼1.7 × 10 −6 K −1 , respectively 26,27 .…”