Shijia Zhang

This study elucidates the role of carbothermal reduction in tailoring the microstructure, oxygen concentration, and thermal conductivity of Si₃N₄ ceramics synthesized via two‐step gas pressure sintering. Carbon incorporation promoted the evolution of elongated β‐Si₃N₄ grains and altered the nature of intergranular phases. Electron energy loss spectroscopy (EELS) revealed substantial lattice oxygen depletion in C‐doped specimens, alongside a pronounced diffusion gradient from grain interiors to boundaries in undoped samples—indicative of oxygen redistribution driven by concentration gradients. These modifications, combined with thinner grain boundary films and a reduced volume fraction of secondary phases, substantially boosted heat conduction. As a result, the thermal conductivity of the C‐doped ceramic reached 97 W·m⁻¹·K⁻¹, representing a 54% enhancement over its undoped counterpart. This work underscores the efficacy of carbothermal reduction as a microstructural and chemical engineering approach to advance the thermal performance of Si₃N₄ ceramics for next‐generation thermal management technologies.

Si ₃ N ₄ ceramics with enhanced thermal conductivity and mechanical properties were prepared by using nitride sintering additives of YN and MgSiN ₂ . In addition to the conventional oxide sintering additives such as Y ₂ O ₃ and MgO, the use of nitride sintering additives offered more possibilities for the optimization of the microstructure and properties of Si ₃ N ₄ ceramics. The Si ₃ N ₄ sample with binary nitride additives of YN‒MgSiN ₂ showed a coarse‐grained microstructure and the maximum thermal conductivity of 112 W m ⁻¹ K ⁻¹ after heat treatment. However, the binary nitride additives were insufficient to supply sufficient liquid phase during the sintering of Si ₃ N ₄ ceramics, which caused porosity and impaired the mechanical properties. A good balance between thermal conductivity and mechanical properties was achieved by using nitride‐oxide hybrid sintering additives of YN‒MgO, where the Si ₃ N ₄ sample showed both high thermal conductivity of 109 W m⁻ ¹ K⁻ ¹ and good mechanical properties with flexural strength of 707 ± 7 MPa, fracture toughness of 9.5 ± 0.2 MPa m ^1/2 , and Vickers hardness of 14.9 ± 1 GPa.

Si ₃ N ₄ ceramics with uniform grain size distribution were fabricated using spark plasma sintering method and post‐heat treatment. The effect of sintering additive content on the grain growth and room‐temperature and high‐temperature mechanical properties of Si ₃ N ₄ ceramics were studied. By modulating the competitive growth of β‐Si ₃ N ₄ grains along the basal and the prism planes, the grains in Si ₃ N ₄ ceramics were homogeneous. The room‐temperature and high‐temperature flexural strength of the samples after post‐heat treatment were tested. At temperature above 1300°C, the samples with lower sintering additive content exhibit higher strength.

Boron nitride (BN) powders with near‐spherical particles of more than 10 µm were synthesized by templated carbothermal reduction and nitridation. The effects of processing factors, such as raw material mixing methods, C/B₂O₃ molar ratios, and carbon templates, were systematically studied, and the reaction mechanism was discussed. The experimental results showed that the integrity and morphology of the carbon templates was usually destroyed during ball milling or grinding, but preserved during magnetic stirring. The morphology of synthesized BN particles depended on the C/B₂O₃ molar ratio in starting reactant mixtures and gradually changed from sheet‐like to near‐spherical shape with decreasing C/B₂O₃ molar ratio. The morphology and size of synthesized BN particles agreed with that of the carbon templates, which means that the morphology and size of synthesized BN particles can be tailored by controlling that of the carbon template. The BN near‐spherical particles synthesized in this work may be used as thermally conductive fillers for electronic packaging.

When a small atom is induced into a larger lattice, new phonon scattering modes are aroused and thermal conductivity is significantly reduced by increased anharmonicity. This effect, called “rattling,” has been reported in pyrochlores and some cage compounds, but not in α‐SiAlON. In this study, we reveal that in (Yb+Ca) co‐doped α‐SiAlON ceramics (Yb_aCa_bSi_7.5Al_4.5O_1.5N_14.5, x = a/[a+b]), the rattling effect plays a crucial role in reducing thermal conductivity. Samples with different Yb/Ca ratios (x = 0, 0.1, 0.2, 0.3…, 1.0) were sintered by spark plasma sintering (SPS) at 1600°C and their thermal diffusivity/conductivity were measured by the laser‐flash method. We found that substituting Ca²⁺ with smaller Yb³⁺ cations in Ca‐α‐SiAlON caused significant reduction in thermal conductivity. On the contrary, substituting Yb³⁺ with larger Ca²⁺ cations in Yb‐α‐SiAlON caused only slight reduction in thermal conductivity. A lowest thermal conductivity of 3.3 W/(m·K) was achieved, when x = 0.3 rather than x = 0.5 or 0.7. Further analysis of phonon scattering intensity confirmed that both intrinsic and extrinsic scatterings are enhanced in the sample of x = 0.3. This study demonstrates that by utilization of the rattling effect, α‐SiAlON, well‐known for its excellent mechanical properties, can be tuned to exhibit low thermal conductivity comparable to that of La₂Zr₂O₇ and 8YSZ.

Si3N4 ceramics with improved thermal conductivity and mechanical properties were prepared by carbothermal reduction and two‐step sintering. The addition of 2 wt% C led to the development of an intergranular phase YNSiO2 with reduced oxygen content. Notably, setting the pre‐sintering temperature at 1450°C promoted the formation of a rough bimodal morphology within the Si3N4 ceramics. Furthermore, an extended holding period at 1850°C resulted in additional grain growth and the elimination of intergranular phases. The Si3N4 specimens produced at 1450°C for 3 h followed by 1850°C for 8 h, incorporating 2 wt% C, exhibited a notable continuity between Si3N4 grains. The thickness of the grain boundary layer was only about 1 nm, and the N/O atomic ratio of the intergranular phase was measured to be 0.42. Consequently, the resulting thermal conductivity, flexural strength, and fracture toughness of Si3N4 specimens were found to be 101 W·m⁻¹·K⁻¹, 712 ± 29 MPa, and 7.7 ± 0.2 MPa·m^1/2, respectively. Compared with samples without C additive and directly sintered at 1850°C for 8 h, the Si3N4 specimens produced in this work showed a significant increase of more than 50% in thermal conductivity and about 17% in fracture toughness.

Highly dense α ‐silicon nitride (Si ₃ N ₄ ) ceramics were densified at a lower temperature of 1450°C under a moderate mechanical pressure of 50 MPa from commercial α ‐Si ₃ N ₄ powder and with conventional Al ₂ O ₃ –Y ₂ O ₃ sintering additives. The fabricated α ‐Si ₃ N ₄ ceramics exhibited a hardness of 19 GPa, which is higher than that of the β ‐Si ₃ N ₄ ceramics usually obtained. The effect of sintering temperature and sintering pressure in the liquid phase sintering of the α ‐Si ₃ N ₄ ceramics is studied. The role of the dissolution–reprecipitation process and the particle rearrangement during densification of the α‐Si ₃ N ₄ ceramics is discussed.

Silicon nitride (Si ₃ N ₄ ) ceramics with initial β‐Si ₃ N ₄ content from 3% to 88% were prepared by spark plasma sintering from 1450 to 1700°C. The effect of initial β‐Si ₃ N ₄ content on the phase transition, microstructure evolution, and mechanical properties of the Si ₃ N ₄ ceramics was studied. The existence of initial β‐Si ₃ N ₄ accelerated the α–β phase transition. When the initial β‐Si ₃ N ₄ content was 12% and 46%, the aspect ratio of the grains was increased. When the initial β‐Si ₃ N ₄ content was 88%, the grains were mainly composed of short rod‐like grains. As the initial β‐Si ₃ N ₄ content increased, the densification process was slowed down. The hardness and fracture toughness of samples reached about 18 GPa and 6 MPa·m ^1/2 . It is proposed that the mechanical properties of samples are connected with the densification process and β‐Si ₃ N ₄ content.

β‐Si₃N₄ powder with well‐dispersed spherical particles was synthesized by liquid‐phase modified carbothermal reduction and nitridation. The influence of the type and content of additives on the particle morphology was investigated. The liquid phase formed at the reaction temperature modifies the morphology of the Si₃N₄ powder. The Si₃N₄ products precipitated along the liquid‐solid interface, thus forming a spherical morphology and promoting the transformation from α‐Si₃N₄ to β‐Si₃N₄. The type of additives has a strong influence on the formation temperature and viscosity of the liquid phase and hence affects the product morphology. In the samples with CaF₂ and CaO additives, sufficient liquid phase occurred and spherical particles were obtained. In the sample with Y₂O₃, however, the liquid phase was insufficient due to the higher formation temperature, thus resulting in an irregular particle morphology.

Si₃N₄ ceramics with tailored gradient in color and microstructure were prepared by a rapid cost‐effective one‐step approach. The gradient microstructure was obtained by the manipulation of the dissolution‐reprecipitation process, by controlling the sintering temperature and sintering additive content. In the Si₃N₄ ceramics, the β‐phase content gradually changed from 84% to 11%. The Si₃N₄ ceramics exhibited white color on one side and showed a hardness of 19 GPa and fracture toughness of 7 MPa·m^1/2 and may be suitable for bio‐implantation applications.