Journal of the American Ceramic Society最新文献

The selection of binders in ceramic additive manufacturing plays a high role in determining the feasibility and quality of printed components. This study investigates the performance of sodium carboxymethylcellulose (CMC) and hydroxypropylmethylcellulose (HPMC) as bio-based binders in aqueous alumina suspensions for direct ink writing (DIW). CMC, characterized by its polyelectrolyte nature, demonstrated rapid dissolution, exceptional dispersion stability, and consistent rheological properties, facilitating smooth extrusion and the formation of high-quality surfaces. These characteristics are critical for DIW, where ink homogeneity and stability directly impact printing resolution and part integrity. HPMC, by contrast, exhibited slow dissolution, thermally induced gelation, and printing inconsistencies, likely due to its lack of electrostatic stabilization. Microstructural and mechanical evaluations of sintered parts confirmed that CMC-based systems achieved higher density, homogeneity, and microhardness. This study highlights the significance of CMC as a binder, providing a pathway to overcome common challenges in DIW, such as agglomeration, foaming, and poor sintered properties, while enabling the production of high-performance ceramic components.

Glass lubricants with suitable viscosity are essential in the hot extrusion forming of Ti-6Al-4 V alloy. In this work, the effect of CeO₂ content on viscosity and structure of (25.5-0.255x)B₂O₃-(39.95-0.400x)SiO₂-(5.95-0.060x)Al₂O₃-(11.05-0.111x)Na₂O-(14.55-0.146x)BaO-(3-0.030x)TiO₂-xCeO₂ glass lubricant was investigated. At 950°C, the viscosity decreased by 42.1%, from 46.6 to 27.0 Pa·s, with an increased CeO₂ content from 0 to 12 wt.%. Using X-ray photoelectron spectroscopy and molecular dynamics simulation, it was found that the percentages of BO, Q⁴, Q³, Q², [AlO₄], [TiO₄], and [BO₃] in the melt were reduced with an increased CeO₂ content, while those of NBO, Q¹, Q⁰, [BO₄], [AlO₅], [AlO₆], [TiO₅], and [TiO₆] were increased. Meanwhile, the Ce oxidation state shifted from Ce⁴⁺ to Ce³⁺ when CeO₂ content was increased, accompanied by a change in the coordination of Ce cations from predominantly 4-coordination to 5- and 6-coordination. The structural analysis showed that increasing CeO₂ content gradually depolymerized the glass network, forming unstable units that weakened the network and lowered viscosity. Furthermore, a comparative analysis of TiO₂, CeO₂, and BaO revealed that, at the same mass fraction, CeO₂ exhibited a weaker viscosity-reducing capability but enhanced the thermal stability of the melt.

We investigated structures, sintering behaviors, and microwave dielectric properties of (A_(1-x)Ca_x)₃(PO₄)₂ (A═Ba, Sr; 0≤x≤0.03). XRD and BSEM investigations confirmed the formation of limited solid solutions, accompanied by cell volume contraction as the Ca²⁺-doping concentration increases. A trace amount of (Sr,Ca)₂P₂O₇ secondary phase appears in the x = 0.03 composition of (Sr_(1-x)Ca_x)₃(PO₄)₂. The under-bonded P–O bond valences increase, while those of the over-bonded A-O bonds decrease with increasing the Ca²⁺-doping concentration for both solid solutions. The Ca²⁺-doping induces a phase transition accompanied by volume expansion around 1200°C for the (Ba_(1-x)Ca_x)₃(PO₄)₂, leading to a decrease in relative density with increasing doping concentration when the sintering temperature is ≥ 1200°C/2 h. The (Sr_(1-x)Ca_x)₃(PO₄)₂ and (Ba_(1-x)Ca_x)₃(PO₄)₂ sintered at 1250°C/2 h have relative dielectric permittivities (ε_r) of ∼13.5 and ∼11.8, respectively, changing little with Ca²⁺-doping concentration. The Q × f value decreases for the (Ba_(1-x)Ca_x)₃(PO₄)₂, while increases for the (Sr_(1-x)Ca_x)₃(PO₄)₂ solid solutions with increasing the Ca²⁺-doping level. The temperature coefficient of resonant frequency τ_f of the (Ba_(1-x)Ca_x)₃(PO₄)₂ could be tuned to a near-zero value of ∼1 ppm/°C coupled with a high Q × f value of ∼ 60 000 GHz at the x = 0.025 composition after sintering at 1250 C/2 h.

The CaAl₂O₄–CaAl₄O₇ (CA–CA₂) tailing slag was adopted to replace CaCO₃, synthesizing calcium hexaluminate (CA₆) ceramics through reactive sintering with Al₂O₃. Digital image correlation (DIC) technology was employed to monitor volumetric effects in real time, while phases and morphology were combined to elucidate the densification mechanism. The results demonstrate that reducing intermediate reactions while modifying the crystal structure of CA₆ through doping can significantly enhance densification. Thermal expansion, in situ CA, CA₂, and CA₆ formation sequentially dominated the volumetric expansion. Using CA–CA₂ tailing slag as the calcium source reduced the expansion caused by CA and CA₂ formation. The trace TiO₂ and MgO impurities inherently present in the CA–CA₂ tailing slag dissolved into CA₆ lattice, promoting a morphological transition from plate-like to equiaxed grains, thereby further enhancing densification. Dense CA₆ ceramics with an apparent porosity of 2.4% and bulk density of 3.39 g·cm⁻³ were successfully prepared via one-step sintering at 1700°C and performed good alkali corrosion resistance.

Dual-phase carbide-boride ceramics in different vanadium-Me (Me = Cr, Hf, Ti, and Zr) binary systems were synthesized by boro/carbothermal reduction under stoichiometric and carbon-deficient conditions and densified by spark plasma sintering. Thermodynamic analysis was used to evaluate the influence of composition on the phase stability and metal segregation between phases, along with the resulting effects on microstructure and hardness. Pairing vanadium with Group IV elements (Hf, Ti, and Zr) consistently formed one boride and one carbide phase, while the Cr-V system formed a monoboride phase and a carbon-deficient carbide. Metal segregation trends depended on composition. Vanadium preferentially segregated to the carbide phase in the Ti-V and Cr-V systems, while it was enriched in the boride phase in Hf-V and Zr-V systems. Hardness measurements showed a higher hardness for the Ti-V sub-stoichiometric system, reaching 27.7 ± 0.9 GPa at 9.8 N, while the Cr-V sub-stoichiometric system presented the lowest hardness at 18.8 ± 0.3 GPa at the same load. Notably, in the Zr-V system, thermodynamic predictions based solely on standard Gibbs energy deviated from experimental observations. However, when combined with first-principles calculations that accounted for non-stoichiometry in vanadium carbide, the predictions aligned more closely with experimental observations, qualitatively indicating a preference for vanadium segregation to the boride phase and zirconium to the carbide phase. These results suggest that elemental distribution between phases results from complex interactions beyond simple Gibbs free energy minimization, especially in systems like Zr-V with strong carbide-to-boride ratio dependence. The optimized systems were characterized with nominal compositions of (Ti_0.61,V_0.39)B₂-(Ti_0.39,V_0.61)C_0.9, (Hf_0.12,V_0.88)B₂-(Hf_0.88,V_0.12)C_0.9, (Zr_0.29,V_0.71)B₂-(Ti_0.71,V_0.29)C_0.9 and (Cr_0.60,V_0.40)B-(Cr_0.40,V_0.60)C_0.8. Their thermodynamic interactions provided insights into metal segregation in dual-phase ceramics, demonstrating its strong composition dependence.

Ultrafine equiaxed α-Si₃N₄ powder is a promising raw material for next-generation high-performance substrates. However, conventional industrial methods are often hampered by high energy consumption, prolonged production cycle, and the use of polluting additives. Thus, the low-cost production of high-purity α-Si₃N₄ powder remains a persistent technical challenge. In this study, equiaxed α- Si₃N₄ particles were synthesized via an additive-free combustion synthesis approach. The influences of N₂ pressure and Si/Si₃N₄ molar ratio on the combustion process, phase composition, and microstructure of the synthesized powders were systematically investigated. Under optimized conditions—specifically, a N₂ pressure of 3 MPa and a Si to Si₃N₄ molar ratio of 2.5:1—equiaxed α-Si₃N₄ particles with an average size of 0.35 µm were obtained. Furthermore, the growth mechanism of α-Si₃N₄ particles was elucidated through heat absorption–assisted quenching and thermo-kinetic analysis. The resulting sintered Si₃N₄ ceramic exhibited a bending strength of 767 MPa and a thermal conductivity of 94.5 W·m⁻¹·K⁻¹, respectively. This work provides a cost-effective and efficient strategy for synthesizing high-quality α-Si₃N₄ powder, demonstrating its potential application as a substrate raw powder for microelectronic applications.