Quartz ceramics (SiO2) possess unique properties such as low thermal expansion, excellent chemical stability, and outstanding dielectric performance, making them widely used in semiconductor manufacturing, optoelectronic devices, and high-frequency electronic components. Traditional sintering of quartz ceramics typically requires temperatures above 1000 ℃, which inevitably induces polymorphic transformations from α-quartz to β-quartz or cristobalite, hindering the preparation of single-phase α-quartz ceramics. Recently, the cold sintering process (CSP) has emerged as a promising low-temperature densification route for ceramics, utilizing transient liquid phases to induce “dissolution–precipitation” or interfacial reaction mechanisms. However, for low-solubility ceramics such as quartz, CSP often fails to achieve full densification and crystallization due to insufficient dissolution kinetics and weak interfacial reactivity. The critical scientific problem addressed in this work is how to effectively trigger the amorphous-to-crystalline phase transition of low-solubility quartz at low temperature, thereby enabling the preparation of dense α-quartz ceramics.
This study systematically investigates the cooperative effects of transient solvent alkalinity, sintering temperature, and uniaxial pressure on the amorphous-to-α-quartz phase transition during CSP. A transient alkaline liquid phase is introduced to regulate interfacial reactions and crystallization kinetics, aiming to provide a theoretical basis and technical strategy for the low-temperature processing of low-solubility ceramics.
Amorphous mesoporous silica (SBA-15) powders were used as the starting material. The powders were homogeneously mixed with different transient solvent phases: deionized water (neutral), 5 mol∙L–1 NH3·H2O (weak alkaline), and NaOH solutions of varying concentrations (0.5–10.0 mol∙L–1, strong alkaline). Approximately 0.4 g of powder was thoroughly ground with the liquid phase in a mortar and then loaded into a 10 mm diameter steel die. Uniaxial pressures ranging from 200 MPa to 600 MPa were applied, while the sintering temperature was varied between 200 ℃ and 350 ℃. Heating was conducted at a rate of 15 ℃∙min–1 and held for 40 min at the target temperature. After natural cooling, the sintered pellets were mechanically polished for further characterization.
The density of the cold-sintered ceramics was calculated by dimensional and weight measurements using a vernier caliper and electronic balance, and relative density was determined based on the theoretical density of quartz (2.2 g·cm–3). Phase composition and structural evolution were analyzed using X-ray diffraction (XRD, Cu Kα radiation, λ = 0.15406 nm, 50 kV, 100 mA, scanning range 10 °–90 °, step size 0.01). Fourier-transform infrared spectroscopy (FTIR-ATR) was used to identify bonding characteristics and confirm phase transitions. Microstructural evolution and fracture features were observed by field-emission scanning electron microscopy (FE-SEM). The mechanical properties of the sintered ceramics were evaluated by Vickers hardness tests (3 kgf load, 10 s dwell), flexural strength using the modified small punch (MSP) method, and fracture toughness (KIC) calculated by the Anstis equation. Poisson’s ratio and Young’s modulus were determined by ultrasonic measurements.
This experimental design allows for a systematic investigation of how transient solvent alkalinity, temperature, and pressure cooperatively affect densification and amorphous-to-crystalline transformation during CSP of quartz.
The phase composition of the sintered bodies was strongly influenced by the type and concentration of transient solvent. Without a liquid phase or with neutral water, the sintered samples remained largely amorphous and exhibited low relative density (~80%). Weak alkaline NH3·H2O increased compaction and relative density (~92%) but failed to trigger phase transformation. In contrast, strong alkaline NaOH solutions (≥3 mol∙L–1) effectively promoted the dissolution of Si–OH surface species, forming soluble silicate intermediates. These intermediates subsequently underwent reprecipitation and recrystallization under external pressure and temperature, leading to complete transformation into α-quartz at 300 ℃ and 500 MPa. XRD and FTIR confirmed the disappearance of the amorphous broad peak and the emergence of α-quartz characteristic double peaks at 798 cm–1 and 778 cm–1, indicating a complete amorphous-to-α-quartz transition.
A clear alkalinity-dependent phase transition sequence was identified: amorphous → keatite (0.5 mol∙L–1 NaOH) → keatite +stishovite (1 mol∙L–1) → keatite + α-quartz (3 mol∙L–1) → α-quartz (≥5 mol∙L–1). Simultaneously, relative density increased from 71% (no solvent) to 95.7% (10 mol∙L–1 NaOH). SEM revealed that strong alkalinity produced well-defined grain boundaries and uniform microstructures, while weak or neutral conditions resulted in porous, poorly bonded networks.
The phase transition was also sensitive to sintering temperature and pressure. At 200 ℃, no crystallization occurred even at 600 MPa. Crystallization initiated at 250 ℃ and 300 MPa, and complete α-quartz formation occurred at ≥300 ℃ and ≥400 MPa. Increasing pressure facilitated particle rearrangement, pore elimination, and enhanced atomic diffusion at the interface, accelerating phase transition. A comprehensive temperature–pressure–phase diagram was established, clearly delineating the non-crystalline, partially crystalline, and fully crystalline regions.
Mechanical properties were strongly correlated with microstructure and phase composition. The α-quartz ceramics cold-sintered with 5 mol∙L–1 NaOH exhibited a Vickers hardness of 5.1 GPa, Young’s modulus of 67.8 GPa, fracture toughness of 0.98 MPa·m1/2, and flexural strength of (58 ± 7) MPa. These values represent increases of 30%, 40%, and 110% in hardness, modulus, and toughness, respectively, compared to the amorphous samples. The enhanced mechanical properties are attributed to the formation of well-bonded crystalline interfaces that enable efficient stress transfer and crack deflection, unlike the disordered amorphous structure.
This work demonstrates a controllable strategy to induce amorphous-to-α-quartz transformation in low-solubility silica ceramics through the regulation of transient solvent alkalinity during cold sintering. By introducing strong alkaline NaOH solutions (≥3 mol∙L–1), the activation energy for crystallization can be significantly reduced, enabling complete transformation at 300 ℃ under 500 MPa. The critical crystallization threshold was identified at 250 ℃ and 200 MPa, and a detailed temperature–pressure–phase diagram was established to illustrate the transition pathways. The resulting ceramics achieved a relative density above 95% and exhibited excellent mechanical performance, including a Vickers hardness of 5.1 GPa, Young’s modulus of 67.8 GPa, fracture toughness of 0.98 MPa∙m1/2, and flexural strength of (58 ± 7) MPa. These results clearly indicate that alkaline regulation during CSP not only enables precise control of phase structure but also produces dense, mechanically robust α-quartz ceramics at dramatically reduced sintering temperatures. This approach provides both fundamental insights and practical guidance for the low-energy fabrication of advanced low-solubility ceramic components.
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