Y₃NbO₇-YNbO₄ composite ceramic materials were synthesized by in-situ synthesis and solid-state reaction methods. The effect of YNbO₄ content (i.e., 0 to 70% in volume fraction) on the two-phase structure, distribution, and properties of the composite ceramics were investigated. W The results show that the hardness and strength of the composite ceramics decrease with the increase in YNbO₄ content. At YNbO₄ content of 30%, the composite ceramic can maintain a high Vickers hardness of 8.99 GPa and a strength of 121.89 MPa, while the fracture energy is increased by approximately 18%, showing good comprehensive strength and toughness. Also, YNbO₄ in C1 can be diffusely distributed around Y₃NbO₇, preventing the formation of high thermal conductive pathways. Meanwhile, the interface between the two phases and the refractive index difference enhances the scattering of sound/photons, thereby increasing the infrared radiation emissivity in the range of 2.5-14.0 μm at high temperatures and suppressing an increase in thermal conductivity at high temperatures. At 1000 ℃, the thermal conductivity is only 1.29 W·m^-1·K^-1.

In this study, Y₃NbO₇-YNbO₄ series ceramic materials were prepared by in-situ synthesis and solid-state reaction methods. The phase of Y₃NbO₇-YNbO₄ composite ceramics was characterized by X-ray diffraction (XRD). The microstructure morphology of the ceramic samples was analyzed by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The grain size of each sample was statistically analyzed bya software named ImageJ, and the size distribution was fitted by the Gaussian peak. The Vickers hardness (H_V) of the samples was determined by indentation method using a micro-Vickers hardness tester (HVT-1000A), and the indentation and crack were prefabricated. The three-point bending test was carried out on the composite ceramic samples by an electronic universal testing machine to obtain the stress-strain curve and the flexural strength of the material. The thermal diffusivity (α) was measured at 25-1000 ℃ by a laser flash diffusivity analyzer. The infrared emissivity in the range of 2.5-14.0 μm was characterized by Fourier transform infrared spectroscopy (FTIR).

For the mechanical properties, the Vickers hardness (H_V) of the pure phases of Y₃NbO₇ and YNbO₄ are 11.19 GPa and 4.82 GPa, respectively. The hardness of the Y₃NbO₇-YNbO₄ composite ceramics lies between the two pure phases, and its hardness has a good stability due to the uniform dispersion of the two phases in the composite material. At YNbO₄ content of 30%, the fracture surface of the composite ceramics shows a tortuous and undulating feature, accompanied by the pull-out phenomenon of the second phase grains, and the layered fracture morphology within the YNbO₄ grains also occurs. This fracture feature is closely related to the layered ferroelastic domain skeleton structure within the YNbO₄ grains. Under the driving of the stress field at the crack tip, the domain structure can adjust the atomic arrangement and orientation, thereby changing the crack propagation path and ultimately forming a layered fracture morphology. Regarding thermal properties, the thermal conductivity upturn phenomenon of sample C1 is significantly mitigated, and its high-temperature thermal conductivity is lower than that of the pure phase of Y₃NbO₇, with a thermal conductivity of only 1.29 W·m^-1·K^-1 at 1000 ℃. The microstructural analysis indicates that a small amount of YNbO₄ second phase in sample C1 is dispersed around the Y₃NbO₇ grains, and this distribution effectively blocks the formation of high thermal conductivity pathways, resulting in the lowest overall thermal conductivity performance in this series of materials. At the interface between the Y₃NbO₇ and YNbO₄ phases, due to the differences in lattice constants and crystal orientations, more significant lattice distortions occur compared to single-phase grain boundaries, forming strong phonon and photon scattering centers. The Y₃NbO₇-YNbO₄ composite material exhibits superior high-temperature thermal radiation shielding performance due to the high intrinsic emissivity of the YNbO₄ phase and the special optical properties formed after the two-phase composite.

A series of Y₃NbO₇-YNbO₄ composite ceramics with composition gradients were in-situ synthesized. The content and distribution state of the second phase YNbO₄ had a decisive influence on the comprehensive performance of the composite ceramics, especially the composite ceramic with 30% YNbO₄ content (sample C1) showed the optimum comprehensive performance. In terms of mechanical properties, the sample C1 maintained a relatively high bending strength (i.e., 121.89 MPa) and its fracture energy was 18% greater than that of the pure Y₃NbO₇ phase, achieving an optimization of toughness. Regarding thermal insulation performance, the YNbO₄ phase in the sample C1 was dispersed in a spherical form at the grain boundaries of the Y₃NbO₇ phase, and the Y₃NbO₇ phase blocked the interconnection between the YNbO₄ phases. This unique distribution could effectively block the formation of high thermal conductivity pathways. The second phase effectively suppressed the upward trend of the high-temperature thermal conductivity of the Y₃NbO₇ phase. At 1000 ℃, the thermal conductivity was only 1.29 W·m^-1·K^-1, which was 20% lower than that of the pure Y₃NbO₇ phase (i.e., 1.59 W·m^-1·K^-1), significantly reducing the upward trend. This optimization effect could be attributed to the increase in the high-temperature infrared emissivity of the material after the introduction of the YNbO₄ phase, thereby weakening the heat flow transmission dominated by infrared thermal radiation. In summary, the Y₃NbO₇-YNbO₄ composite ceramics achieved the performance synergy of low thermal conductivity and high strength and toughness through reasonable composition design and in-situ synthesis strategy. The sample C1 with excellent comprehensive performance could have a great application potential in the field of high-temperature thermal protection.