HAO Weimin 1, 2, SUN Shihao 2, HAO Wenjun 2, LI Zikang 2, XU Yichen 2, YU Dechuan 1
(1. School of Textile and Material Engineering, Dalian Polytechnic University, Dalian 116034, Liaoning, China; 2. Institute of Structured Materials Technology, Liaoning Materials Laboratory, Shenyang 110167, Liaoning, China)
Extended abstract:
[Background and purposes] Porous SiO2 ceramics are widely utilized in applications of high-temperature thermal insulation, catalyst supports, filtration membranes and lightweight structural components, owing to their excellent thermal stability, high chemical inertness, favorable dielectric properties, low density, large specific surface area and tunable permeability. However, their broader industrial applications face two major challenges: low mechanical strength caused by the formation of weak cristobalite sintering necks and the difficulty in fabricating complex fine‑scale architectures. To improve mechanical properties, reinforcing phases were incorporated. However, the disordered powder distribution in the green body unavoidably leads to direct connection between SiO2 particles. The resulting formation of weak cristobalite necks during sintering imposes a ceiling on the improvement of mechanical properties.To address these issues, a novel strategy that combines kaolin reinforcement with direct ink writing (DIW) was proposed to fabricate hierarchical porous SiO2 structures. A "house-of-cards" microstructure was designed through pH-regulated assembly, ensuring the compatibility with DIW. Moreover, kaolin provides a coating and isolation effect for SiO2 particles, preventing the growth of weak cristobalite necks during sintering and effectively enhance the mechanical properties.
[Methods] Spherical SiO2 and plate-like kaolin powders were employed as starting materials. Surface charge of the powders, which governs their interaction and assembly behavior, was modulated by adjusting pH value (3.515.45) of the aqueous SiO2-kaolin suspension with a solid loading of 70 vol.%. Zeta potentials of the samples and the isoelectric point of the kaolin edge (IEPedge) were monitored. Rheological properties of both the kaolin and composite slurries, including viscosity, storage modulus and thixotropy, were characterized to evaluate their printability for DIW. Green bodies with rod and grid structures were fabricated by extrusion and DIW, respectively. After drying, the samples were sintered in air at 1200–1425 ℃. Phase composition was examined by using X-ray diffraction (XRD). Microstructures of the green bodies and sintered ceramics were observed by using scanning electron microscopy (SEM). Open porosity and bulk density were measured via the Archimedes method. Mechanical properties were evaluated, including bending strength of the rod-shaped specimens and compressive strength of the grid‑like structures.
[Results] Both kaolin and silica exhibited negatively charged across the entire range of pH value, with no isoelectric point observed. Sedimentation experiments, however, revealed an isoelectric point of 5.87 for the edge of kaolin powders. At pH values below the IEPedge, the positively charged edges of the kaolin platelets adhered electrostatically to the negatively charged surfaces of the SiO2 particles. This interaction facilitated the formation of a three-dimensional, interlocking "house-of-cards" network that effectively coated and isolated individual SiO2 particles, as further confirmed by SEM observations of the dried green bodies. This distinctive powder arrangement endowed the composite slurry with exceptional rheological properties for DIW. Specifically, at pH=4.86, the slurry displayed a high zero-shear viscosity of 4922 Pa·s and a storage modulus of 40,803 Pa, ensuring excellent shape retention after deposition. Thixotropy tests further revealed that the disrupted "house-of-cards" structure could recover rapidly within 10 s following high-shear extrusion, a characteristic crucial for reliable layer-by-layer printing. The sintering behavior and phase evolution were highly sensitive to temperature. XRD results confirmed the formation of cristobalite and mullite above 1200 ℃. A competitive interplay was observed between matrix densification and cracking induced by the cristobalitephasetransition. At 1275 ℃, densification was optimized while microcracking was effectively suppressed, resulting in a microstructure where SiO2 particles were primarily interconnected through robust mullite sintering necks rather than fragile cristobalite bridges. This specific architecture yielded rod‑shaped ceramics with a peak bending strength of (33.561.51) MPa and a bending modulus of (19.10.7) GPa. In contrast, sintering at temperatures of ≥1350 ℃ promoted the formation of excessive cristobalite, which triggered extensive microcracking during cooling and consequently deteriorated the mechanical properties. Grid-like hierarchical porous ceramics were successfully fabricated by using DIW. The macro-scale pore size and total porosity were precisely controlled by the horizontal distance. The total porosity [ranging from (59.341.41)% to (75.541.72)%] exhibited a clear inverse correlation with compressive strength, which spanned from (5.330.57) MPa to (0.970.13) MPa. Under compression, the grid-like structures displayed a characteristic layer-by-layer failure mode, indicative of robust structural integrity and efficient energy dissipation capacity. The coexistence of intentional millimeter-scale lattice pores and inherent micrometer-scale interparticle pores, thereby achieving a well-defined hierarchical porous architecture.
[Conclusions] A pH-regulated "house-of-cards" slurry design was successfully developed to fabricate high-performance hierarchical porous SiO2 structures by using DIW. This design enables precise control over powder arrangement, yielding slurries with high solid loading, excellent shape retention and rapid thixotropic recovery for reliable DIW processing. The optimal sintering temperature was 1275 ℃ to achieve densification and avoid obvious cracks, thereby maximizing mechanical properties. Furthermore, by adjusting printing parameters, intentional millimeter-scale lattice pores and inherent micron-scale interparticle pores can be coordinately engineered. This enables wide-range modulation of total porosity [(59.341.41)%(75.541.72)%], resulting in compressive strengths varying from (0.970.13) MPa to (5.330.57) MPa. This study provides a viable pathway for manufacturing lightweight high-strength porous SiO2 ceramics with complex geometries and tailored hierarchical architectures, promising for advanced functional and structural applications.
Key words: direct ink writing; powders packing structure; hierarchical porous SiO2; mechanical properties