[Background and purposes]Si3N4 ceramics are widely regarded as one of the most promising advanced engineering ceramic materials,due to their excellent high-temperature mechanical properties,outstanding thermal stability and remarkable thermal shock resistance,with extensive applications in various fields,including electronic power devices,ceramic cutting tools,biomedical implants and aerospace components.However,conventional machining methods often suffer from low efficiency,long production cycles,high costs and difficulties in ensuring precision,when fabricating complex-shaped Si3N4 components,leading to the emergence of additive manufacturing technologies as a viable alternative.Among these,digital light processing(DLP)has attracted widespread attention for its high forming efficiency,superior dimensional accuracy and excellent surface quality,yet the inherent gray color of Si3N4 powder,coupled with its high light absorption and significant refractive index,severely restricts the curing accuracy and penetration depth during photopolymerization,a challenge that has prompted various surface modification strategies,which nevertheless remain relatively scarce in terms of systematically and simultaneously improving the rheological behavior and photopolymerization characteristics of Si3N4 slurries,along with the mechanical performance of sintered bodies.Therefore,this study was motivated to employ low-melting-point glass powder as a coating material for Si3N4 particles,combined with ball milling and sintering processes for surface modification,while utilizing DLP as the shaping technique to systematically investigate the comprehensive effects of this coating on the rheological properties,curing behavior,microstructure and mechanical and thermal properties of the resulting ceramics. [Methods]Si3N4 powder was used as the raw material coated with low-melting-point glass powder,employing MgO and Y2O3 as sintering aids,while the photosensitive resin system consisted of OPPEA and HDDA with Irgacure 819 as the photoinitiator,KMT-3331 and BYK-110 as dispersants and DOP as the plasticizer.The Si3N4 powder was blended with 0-5 wt.％glass powder and sintering aids,then ball-milled,dried,sieved and heat-treated at 700 ℃ to form a uniform coating.The coated powder was mixed with resin and additives through vacuum homogenization to prepare photosensitive slurry,which was layer-wise cured via DLP at 30 μm per layer to obtain green bodies.The samples were subsequently vacuum-debinded at 500 ℃ for 3 h and air-debinded for 4 h,followed by pressureless sintering at 1850 ℃ for 8 h.The slurry's rheology was characterized with a rotational rheometer(MCR301),while curing parameters(Ew,Sw,Ed,Sd)were determined by using single-layer curing tests based on quasi-Beer-Lambert and Beer-Lambert semi-logarithmic models.Phase identification was performed by using XRD,microstructure was examined by using SEM and bending strength of the sintered ceramics was evaluated via three-point bending,along with measurement of Vickers hardness(10 kg,10 s)and Archimedes density. [Results]As the content of low-melting-point glass powder for coating was increased from 0 wt.％to 5 wt.％,the viscosity of the Si3N4 slurry decreased from 0.594 Pa·s to 0.477 Pa·s,indicating effective improvement in rheological properties,while the curing behavior showed systematic changes with increasing depth sensitivity coefficient(Sd)but decreasing critical energy density(Ed),along with reduced width sensitivity coefficient(Sw)yet enhanced critical energy density(Ew)and curing width.XRD analysis results confirmed the negligible influence of the glass powder on phase composition of the final ceramics,due to decomposition at 1850 ℃ during sintering and low Al2O3 content(27.87 wt.％).SEM results revealed high densification and reduction in average grain size from 2.61 μm to 2.29 μm(12.26％),consistent with liquid-phase sintering.The flexural strength increased from(679.5±46.6)MPa to(776.5±54.7)MPa(14.3％improvement),due to grain refinement,while Vickers hardness decreased from(15.90±0.85)GPa to(15.300±0.265)GPa and fracture toughness displayed an initial rise followed by a decline,reflecting the critical effect of glass phase content. [Conclusions]To address the challenge of conventional surface modification methods in synergistically optimizing both the Si3N4 slurry properties and the mechanical performance of the sintered bodies,surface coating modification of Si3N4 powder was implemented by precisely controlling the content of the low-melting-point glass powder,combined with ball milling and sintering processes.This treatment significantly reduced the viscosity of the Si3N4 slurry and improved its curing performance.During sintering,an appropriate amount of glass powder partially volatilized without adversely affecting composition or densification of the ceramics.Furthermore,the glass phase formed during sintering effectively refined the grains,resulting in a maximum increase of 14.3％in flexural strength and 31.1％in fracture toughness for the Si3N4 ceramics,thereby achieving synergistic optimization of printing processability and mechanical properties.
[Background and purposes]Solid oxide electrolysis cells(SOECs)represent a highly efficient technology for energy conversion,playing a pivotal role in integrating renewable energy and achieving carbon neutrality goals.However,their performance and durability are critically limited by the microstructure of porous electrodes,where complex multiphysics processes occur.Non-uniform electric field distributions induced by heterogeneous pore structures can lead to localized current crowding,Joule heating and accelerated material degradation.Traditional design approaches often rely on empirical methods,lacking a systematic understanding of the effect of dynamic pore morphology on electric field behavior.Therefore,this study was aimed to elucidate the relationship between the three-dimensional microstructure of porous electrodes and the electric field distribution in SOECs through a combination of advanced characterization and multiscale modeling. [Methods]SOEC single cells with a NiO/GDC-SSZ-GDC/LSCF configuration were fabricated by using a multi-step ceramic processing route.Microstructural properties of the electrodes were characterized using field-emission scanning electron microscopy(FE-SEM)and X-ray computed tomography(XCT).High-resolution XCT scanning at 500 nm resolution enabled the reconstruction of 3D pore networks,which were processed using anisotropic diffusion filtering and watershed segmentation in Avizo software.Key morphological parameters,including porosity,pore size distribution,tortuosity and shape factors,were quantified.Based on the reconstructed structures,two-dimensional(2D)and three-dimensional(3D)models were developed using a finite element-based platform.These models incorporated coupled charge conservation and electrochemical kinetics to simulate the electric field and current density distributions under SOEC operating conditions.Both idealized particle-based and fiber-based electrode architectures were generated and compared to evaluate their electrochemical characteristics. [Results]The 3D microstructure analysis results revealed that the NiO fuel electrode had a porosity of 0.3214,with interconnected pores accounting for 91.54％of the total porosity.The average tortuosity was calculated to be 1.2,significantly lower than that reported for typical particle-based electrodes(1.8),indicating the presence of more efficient transport pathways.Simulations results demonstrated that pore morphology is a primary cause of local electric field and current density distortion.Both 2D models and 3D idealized models showed significant fluctuations in electric field strength and potential distribution within the porous electrodes.Crucially,simulations based on the real 3D reconstructed structure identified that the local current density at narrow pore throats was significantly higher than those in other regions,directly confirming that complex pore morphology induced current concentration.Furthermore,the fiber-structured electrode model exhibited more uniform potential and current density distributions,as compared with the particle-based structure,benefiting from continuous conductive paths that reduced contact resistance and transport tortuosity. [Conclusions]It is revealed the intrinsic relationship between the 3D microstructure of porous electrodes and the electric field distribution in SOECs was revealed in this study.The findings underscore the necessity of 3D characterization for accurately assessing electrode transport properties.The simulation results confirmed that complex pore morphology,especially narrow pore throats,acted as a hotspot for current concentration,posing a potential risk for localized degradation.Moreover,the fiber structure,with its continuous conduction paths and low tortuosity,was demonstrated to be a superior electrode architecture for achieving uniform electrochemical performance.The insights and methodologies provide a theoretical basis for the rational design of high-performance SOEC electrodes.
[Background and purposes]In extra-high voltage systems,overvoltage and protection have always been one of the key research issues for ensuring the safe and stable operation of power systems.Parallel insertion of closing resistors can convert the L-C oscillating electrical energy in the power grid into thermal energy,which is then dissipated into the external environment,thereby achieving the purpose of suppressing switching overvoltage.Although extensive research has been conducted on materials for closing resistors,the core technology for producing high-voltage closing resistor materials has not been mastered,while there remains a significant gap in key performance indicators between domestically produced resistor discs and foreign products.Carbon-ceramic resistor materials have become the mainstay of high-performance foreign closing resistor products,due to their advantages,such as excellent electrical conductivity,strong pulse resistance,superior overload capacity and efficient heat dissipation.In recent years,research on carbon-ceramic resistor materials has primarily utilized natural minerals as raw materials.However,these raw materials have complex compositions,making the reaction processes during calcination relatively difficult to control.Based on the analysis of a foreign product,a series of samples using high-purity alumina and silica as the main raw materials and graphite as the conductive filler have been designed and prepared. [Methods]Phase composition of the prepared samples was analyzed,while their microstructure is observed and their porosity,bulk density,and electrical resistivity were characterized.Combined with the analysis and test results,the effects of the alumina/silica mass ratio,type of sintering aid,graphite particle size and content of graphite on phase composition and properties of the samples are studied. [Results]When 300-mesh graphite is added at 15％and iron oxide(Fe2O3)at 5％,the alumina/silica mass ratio in the range of 2∶1 to 3∶1 has almost no effect on bulk density of the samples,but significantly influences the open porosity and electrical resistivity.This is mainly attributed to the change in the content of corundum phase with the proportion of alumina.When the alumina/silica mass ratio is fixed at 3∶1 and 15％300-mesh graphite is used as the conductive filler,and the effect of sintering aid types was studied. [Conclusions]As compared with Fe2O3,CaCO3 is more conducive to promote the transformation of quartz to cristobalite and the formation of mullite.Samples prepared with 3％CaCO3 as the sintering aid are mainly composed of corundum,mullite,cristobalite,and graphite.When the alumina/silica mass ratio is fixed at 3∶1,3％CaCO3 is used as the sintering aid,11％-15％of 800-mesh graphite are added as the conductive filler,composite resistor materials with corundum,cristobalite,mullite,and graphite as the main phases are successfully prepared.The prepared samples exhibit an open porosity of 33.2％-35.5％,a bulk density of 1.99-2.10 g·cm-3,and an electrical resistivity of 0.11-4.66 Ω·m.The research results of this paper provide data support for the study of carbon-ceramic composite resistor materials.
[Background and purposes]Blue-and-white porcelain,as a distinguished representative of Chinese ceramic art,holds a prominent place in the global history of decorative arts.Its unique cobalt-based underglaze technique and iconic blue-and-white palette not only demonstrate exceptional craftsmanship but also embody profound cultural symbolism.The color characteristics of blue-and-white porcelain have evolved over time under the combined influence of raw material composition,glaze formulation,firing conditions and prevailing aesthetic values,forming distinctive historical trajectories.However,most existing studies have focused on qualitative explorations from aesthetic,semiotic and archaeological perspectives,with limited application of systematic quantitative methods,thus constraining objective comparisons and the identification of developmental patterns across different periods.With the rapid advancement of digital imaging,computer vision and machine learning,it has become possible to analyze artifact colors with precision using non-contact,repeatable and quantifiable techniques,offering new pathways for heritage preservation and revitalization.In this study,the K-means++clustering algorithm was integrated with the Natural Color System(NCS)to digitally extract and parameterize the color features of blue-and-white porcelain from the Yuan,Ming and Qing dynasties,aiming to reveal the historical evolution of its hue,blackness and chromaticness,and hence establish a structured color database that can support heritage conservation,design innovation and cross-media application. [Methods]A total of 223 representative blue-and-white porcelain sherds from various historical periods were collected from the Tao Xi Chuan Ceramic Shard Museum in Jingdezhen and private collections.Compared with complete vessels,sherds offer flatter surfaces that minimize light distortion and facilitate accurate color capture,while their accessibility broadens the sample base for quantitative studies.Each sample was photographed in a controlled studio environment using a Canon EOS 6D camera at a resolution exceeding 300 dpi and 2000×2000 pixels.Lighting was maintained between 5000K and 6000K to simulate natural daylight,with diffusers and black light-absorbing cloth to eliminate direct glare,shadows and environmental reflections.A standard color card was included in each frame,while the images were color-corrected in Adobe Lightroom through white balance adjustment and matching to reference values,ensuring high fidelity between digital and physical color.The corrected images were processed using the K-means++clustering algorithm.Unlike standard K-means,K-means++optimizes the selection of initial centroids through probabilistic dispersion,thereby avoiding local minima and improving clustering stability.RGB values of each pixel were extracted,normalized and grouped into clusters,with the optimal cluster number determined by combining the elbow method,to identify the inflection point in the SSE curveand silhouette coefficient analysis,which measures cluster cohesion and separation.Both metrics consistently indicated K=5 as optimal for most periods.RGB values of the five dominant colors from each sample were then converted to the NCS parameters of hue(H),blackness(S)and chromaticness(C)for further statistical and comparative analysis. [Results]The analysis revealed a consistent concentration of hues within the NCS range R90B-R60B across all three dynasties,indicating a strong bias toward cool blue tones with occasional shifts toward purpleblue.In the Yuan Dynasty,colors were characterized by high blackness(50-70)and low chromaticness(10-30),producing deep blue-gray tones.These visual attributes correspond with the use of both domestic and imported cobalt sources containing higher impurity levels,resulting in muted saturation.In the Ming Dynasty,particularly during the Yongle and Xuande reigns,the refinement of imported cobalt and improved firing techniques led to increased chromaticness(20-50,occasionally exceeding 60)and reduced blackness(30-60),producing purer brighter blues.Later Ming wares incorporated domestic cobalt,expanding the tonal range to include both light azure and intense deep blues.In the Qing Dynasty,especially during the Kangxi,Yongzheng and Qianlong reigns,colors exhibited greater hue diversity,with shifts toward R60B purpleblue tones.Blackness was moderate(30-50)and chromaticness frequently ranged from 40 to 70,producing vibrant saturated blues.These patterns confirm the correlation between technological refinement,material purity and aesthetic diversity,as well as the influence of court tastes and production scale on color variability. [Conclusions]It is demonstrated that integrating K-means++clustering with the NCS system provides a precise,replicable and non-invasive approach for quantifying the color characteristics of ceramic artifacts.By revealing the historical trajectory of blue-and-white porcelain,from the Yuan Dynasty's deep,restrained palette,through the Ming Dynasty's balanced and refined blues,to the Qing Dynasty's diverse and saturated tones,a robust color reference framework was established,which is applicable to digital heritage archiving,cultural product development and cross-media design translation.Furthermore,the workflow presented here addresses a broader methodological gap in heritage studies,by bridging computational analysis with perceptual color systems.This enables not only the preservation of color data in standardized formats but also its transformation into actionable design strategies.Such strategies can inform visual identity systems,spatial design schemes and product color planning,ensuring that the cultural essence of traditional palettes is retained,while adapting to contemporary design languages.Ultimately,this approach advances both the academic study and practical application of historical color knowledge,fostering new avenues for innovation in the digital humanities and creative industries.
[Background and purposes]Recrystallized silicon carbide(R-SiC)porous ceramics are pivotal advanced refractories utilized in high-temperature applications,such as kiln furniture and thermal exchangers,due to their exceptional thermal stability,chemical inertness and mechanical strength at elevated temperatures.Recrystallization sintering process,following the evaporation-condensation mechanism without sintering aids,is favored for producing high-purity SiC ceramics.However,a significant challenge lies in balancing the material's porosity with its mechanical performance.In traditional approaches,powders with bimodal or multimodal particle size distributions were used to enhance densification,but often at the cost of complex processing and possible uncontrolled grain growth.The introduction of silicon carbide fibers(SiCf)into ceramic matrices is a well-established strategy for toughening and strengthening.However,its application and mechanistic understanding within a single-particle-size R-SiC system,where the reinforcement mechanism differs fundamentally from dense composites,remain unexplored.This study was aimed to systematically explore the influence of SiCf and sintering temperature on phase evolution,microstructural development and resultant mechanical properties of single-particle-size R-SiC porous ceramics,seeking to identify the optimal processing parameters and elucidate the underlying mechanisms. [Methods]The green bodies were prepared using a single-size 6H-SiC powder(D50=1.07 μm)as the matrix.Silicon carbide fibers(SiCf,diameter of 5-10 μm,length of 50-100 μm)were incorporated as a reinforcing phase at a concentration of 20 wt.％.The powder and fibers were homogenized via ball-milling in an ethanol suspension for 4 h.The mixed batches were subsequently dried,mixed with 2 wt.％of a binder solution(hydroxyethyl cellulose and polyvinyl alcohol)and uniaxially pre-pressed at 10 MPa,followed by cold isostatic pressing(CIP)at 180 MPa,to form green compacts with dimension of 40 mm× 8 mm×6 mm.The binder was removed at 600 ℃ for 3 h in Ar.The final recrystallization sintering was conducted in a high-temperature furnace in Ar,at temperatures ranging from 1600 ℃ to 2200 ℃ for 1 h.Phase composition was determined by using X-ray diffraction(XRD)with Rietveld refinement.Microstructure,grain size and sintering neck development were analyzed by using scanning electron microscopy(SEM).Apparent porosity and bulk density were measured according to the Archimedes principle.The cold modulus of rupture(CMOR)was evaluated by using three-point bending test. [Results]XRD analysis results confirmed that the primary phases were 6H-SiC,with additional 4H-SiC and 3C-SiC polymorphs.The introduction of SiCf,which contained a fraction of the metastable 3C-SiC polytype,influenced the phase evolution during sintering.At the optimal temperature of 2100 ℃,the SiCf-added sample(PSC2)exhibited an increase in the content of 4H-SiC and 3C-SiC,as compared with the pure SiC sample(PSC1),which was attributed to the evaporation-condensation and phase transformation of the SiCf-derived 3C-SiC.At 2100 ℃,the addition of SiCf resulted in a more uniform distribution of finer grains and a narrower size distribution of sintering necks,as quantified by a lower SPAN value(1.30 for PSC2 vs.1.67 for PSC1).This indicated that SiCf promoted homogeneous neck development by providing additional vapor sources and nucleation sites.Consequently,the mechanical properties were significantly enhanced.The sample with 20 wt.％SiCf sintered at 2100 ℃ demonstrated optimal performance,with an apparent porosity of(45.5±0.8)％,a bulk density of(1.80±0.03)g·cm-3 and a CMOR of(43.44±4.75)MPa.This represented a 35.7％increase in flexural strength over the baseline sample[PSC1:(32.02±6.89)MPa]at the same temperature.However,a critical finding was the temperature-dependent behavior of SiCf.When the sintering temperature was elevated to 2200 ℃,the beneficial effect of SiCf was negated.Excessive grain growth,particularly abnormal or exaggerated grain growth,was observed in the SiCf-containing sample.This microstructural degradation led to a precipitous drop in mechanical strength,with the CMOR of PSC2 decreasing to(23.62±1.47)MPa,a 45.6％reduction from its peak value at 2100 ℃ and lower than that of the pure SiC sample at 2200 ℃[(26.85±0.82)MPa].This suggests that,beyond a critical temperature,the enhanced mass transport facilitated by SiCf becomes detrimental,triggering unstable grain coarsening. [Conclusions]High-porosity and high-strength SiC porous ceramics were prepared using a single-particle-size SiC powder and SiC fibers(SiCf)as the raw materials,at sintering temperatures in the range of 2100-2200 ℃.At 2100 ℃,the decomposition of SiCf occurred,leading to a decrease in the content of 4H-SiC and 3C-SiC polytypes.This indicates a phase transformation process from the metastable 3C-SiC to the more stable 4H-SiC and 6H-SiC polytypes.When the sintering temperature was elevated to 2200 ℃,abnormal grain growth was observed.The addition of SiCf further promoted secondary recrystallization,which resulted in a significant degradation of mechanical properties,as evidenced by the decrease in the cold modulus of rupture from(26.85±0.82)MPa to(23.62±1.47)MPa.The optimal comprehensive performance of the recrystallized SiC porous ceramic was achieved with the addition of 20 wt.％SiCf and sintering temperature of 2100 ℃,yielding an apparent porosity of(45.5±0.8)％,a bulk density of(1.80±0.03)g·cm-3 and a superior cold modulus of rupture of(43.44±4.75)MPa.
