CHENG Wenbang ¹, QIU Riliang ¹, XIAO Qiankun ¹, HUANG Xinhua ¹, XIAO Zhuohao ^{1, 2,}

DONG Hongbo ³, LI Xiuying ¹, KONG Lingbing ¹

(1. Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China; 2. Jiangxi Bangqi Ceramic Co., Ltd., Fuzhou 344600, Jiangxi, China; 3. Linyi University, Linyi 276005, Shandong, China)

Extended Abstract:[Background and Purpose] Cordierite ceramics are widely used in electronic packaging, catalytic supports, high-temperature filtration and aerospace, due to their low thermal expansion coefficient, high mechanical strength, excellent chemical stability and low dielectric constant. With advancements in 5G communication, new energy technologies and aerospace engineering, the potential of cordierite ceramics in high-temperature electronic devices, thermal barrier coatings and precision optical components is further expanding. However, Fe₂O₃ impurities are inevitably present in the raw materials used for synthesizing cordierite ceramics, making it essential to evaluate the influence of Fe₂O₃ on structure and properties of cordierite ceramics. Although the effects of trace Fe₂O₃ on cordierite ceramics have been reported, most works were conducted according to the empirical formulations, often deviating significantly from the theoretical composition of cordierite. In this study, the conventional solid-phase sintering method was employed, using the theoretical composition of cordierite as the base formula, to explore the effects of Fe₂O₃ (in a wide range) and sintering temperature profiles on phase composition, microstructure, thermal expansion behavior, densification and mechanical properties of cordierite ceramics.[Methods]The base formula was designed according to the theoretical composition of cordierite, with mass fractions of MgO, Al₂O₃ and SiO₂ at 13.78%, 34.86%, and 51.36%, respectively. On this basis, 1.0–7.0% Fe₂O₃ was added, forming eight experimental formulations. High-purity chemical reagents were used as raw materials to fabricate cordierite ceramics through solid-phase sintering. The mixed powders were ball-milled and sieved through a 150-mesh standard sieve. Rectangular samples were pressed at 20 MPa for 1 min. The green bodies were dried at 90 ℃ for 12 h and then sintered in a muffle furnace under pressureless conditions. The sintering process involved heating at a rate of 3 ℃·min–1 to 450 ℃, holding for 2 h for binder burnout, followed by heating at 5 ℃·min–1 to 1410–1450 ℃ and holding for 1 h for sintering. Thermal expansion coefficient (30–800 ℃) of the sintered samples was measured using a PCY-Ⅲ-1400 thermal dilatometer. Water absorption, apparent porosity and bulk density were determined via the Archimedes method. The three-point flexural strength was tested using a WDW-20 microcomputer-controlled universal testing machine. Phase composition was analyzed using a D8 Advance X-ray diffractometer (XRD). Fracture surface was observed using a SU-8010 field-emission scanning electron microscope (FE-SEM, Hitachi, Japan).[Results] The addition and concentration of Fe₂O₃ significantly influenced the phase composition of cordierite ceramics. For the theoretically stoichiometric cordierite sintered at 1410 ℃, impurity phases such as quartz, cristobalite and magnesium aluminate spinel were observed, resulting in a high thermal expansion coefficient. The incorporation of Fe₂O₃ promoted the transformation of these impurity phases into the cordierite phase. At 2.0 wt.% Fe₂O₃, the quartz phase completely disappeared. When Fe₂O₃ content reached 6.0 wt.%, cristobalite and spinel phases were fully eliminated. Further increasing Fe₂O₃ to 7.0 wt.% led to the formation of a mullite phase.[Conclusions] Near the theoretical composition of cordierite, high-temperature sintered ceramic samples contained high-thermal-expansion phases, such as quartz, cristobalite and magnesium aluminate spinel, preventing the fabrication of low-thermal-expansion cordierite ceramics. The addition of Fe₂O₃ facilitated the conversion of high-thermal-expansion cristobalite and spinel phases into low-thermal-expansion α-cordierite. When 6.0 wt.% Fe₂O₃ was added and sintered at 1410 ℃, only cordierite phase was present. Within a certain range, Fe₂O₃ addition effectively reduced the thermal expansion coefficient of cordierite ceramics. Under optimal conditions (6.0 wt.% Fe₂O₃, sintering at 1410 ℃), the prepared cordierite ceramics exhibited a thermal expansion coefficient of 1.40×10–6 ℃^–1, bulk density of 1.79 g·cm^–3, water absorption of 17.29%, apparent porosity of 30.95% and flexural strength of 32.65 MPa.

Key words: cordierite ceramics; Fe₂O₃; coefficient of thermal expansion; sintering temperature