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.
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).
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.
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.
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