[Significance]With the continuous utilization of fossil fuels,environmental pollution has become increasingly severe.Various renewable energy sources,such as solar and wind energies,have been widely deployed.However,their applications are limited by geographic constraints.Solid oxide fuel cells(SOFCs)are clean energy conversion devices capable of directly converting chemical energy into electrical energy,which have garnered significant attention.Traditional SOFCs require different electrode materials to function in distinct atmospheres,necessitating specific material properties.This not only requires thermal compatibility between the electrodes and electrolytes,but also increases manufacturing costs,due to the use of different materials.In symmetrical solid oxide fuel cells(SSOFCs),the anode and cathode are made of same materials,simplifying the fabrication process and hence cutting the down production costs.However,identifying materials that maintain structural stability and exhibit excellent catalytic activity in both oxidizing and reducing atmospheres remains a significant challenge.This paper was aimed to review recent research progress on iron-based perovskites as SSOFC electrode materials from the perspectives of single perovskites,double perovskites and Ruddlesden-Popper(RP)type perovskites.In addition,the catalytic activity and stability of different Fe-based perovskite structures and their applications in SSOFCs are also analyzed and summarized,along with an outlook on future developments of Fe-based perovskite electrodes in SSOFCs. [Progress]Most SSOFC electrode materials are modified from conventional anode and cathode materials.However,they often suffer from low catalytic activity,high thermal expansion coefficients(TECs)and instability in reducing atmospheres.Additionally,many electrode materials contain cobalt,which significantly increases the fabrication cost.Among all perovskite materials,Fe-based perovskites have similar TECs to those of electrolytes.Moreover,Fe exhibits variable oxidation and spin states,which endows them with high catalytic activity.However,most Fe-based perovskites are unstable in reducing atmospheres,necessitating modifications to enhance their stability and catalytic activity.Firstly,Fe-based single perovskites follow the general formula ABO3,where the A-site is typically occupied by large-radius alkaline earth metals or lanthanide elements(such as La,Pr,Ba),while the B-site is usually occupied by transition metals(such as Fe,Mn,Ni).By doping and modifying the A-site and B-site elements,as well as adjusting their types and ratios,electrochemical performance,structural stability and conductivity of the perovskite materials can be optimized.Secondly,Fe-based double perovskites can be categorized into A-site ordered double perovskites(AA'BBO6)and B-site ordered double perovskites(AABB'O6).In A-site ordered double perovskites,the A-site elements are lanthanides with smaller ionic radii(such as La,Pr,Nd),whereas the A'site is typically composed of larger cations(such as Ba,Sr,Ca).In B-site ordered double perovskites,the B-site elements are lower-valence cations(such as Fe,Co,Ni),whereas the B'site elements are higher-valence cations(such as Nb,Mo,W).Compared with Fe-based single perovskites,double perovskites offer greater flexibility in tuning the crystal structure and optimizing the catalytic performance.Then,for Fe-based Ruddlesden-Popper(RP)perovskites,the chemical formula is expressed as An+1BnO3n+1.In RP perovskites,the AO layers contain a large number of interstitial oxygen species,while the ABO3 layers have a small number of oxygen vacancies.The stability of RP perovskites is significantly influenced by the doping of transition metal elements at the B-site.Finally,regarding the stability of Fe-based perovskites under different fuel conditions,matching the TECs of the electrode materials with electrolytes can be used to enhance the long-term stability of the material.Additionally,the exsolution of metallic nanoparticles from the material in a reducing atmosphere may further improve the long-term stability of the SSOFCs. [Conclusions and prospects]Fe-based perovskites have attracted extensive attention in recent years as promising electrode materials for SOFCs.This paper was aimed to review the progress of Fe-based perovskite materials as symmetrical electrodes in SSOFCs.Since Fe-based perovskites are mainly derived from modifications of conventional cathode materials,they exhibit relatively high polarization resistance on the anode side.Overall,by adjusting the type and proportion of A-site and B-site elements,the electrochemical performance,structural stability,and conductivity of perovskite materials can be optimized.However,despite the development of many high-performance Fe-based SSOFC materials,the cell performance and durability still do not match those of traditional SOFCs.To address this issue,reducing the thickness of the electrolyte is necessary,such as fabricating electrode-supported cells via phase inversion techniques,which could better exploit the high catalytic activity of Fe-based perovskite materials.Additionally,in current SSOFCs,La0.8Sr0.2Ga0.8Mg0.2O3-δ(LSGM)has been widely used as electrolyte.From commercial application perspective,exploring more stable scandium-stabilized zirconia(ScSZ)electrolytes is important.Lastly,Fe-based perovskites still grape with issues associated with carbon deposition and sulfur poisoning,during the ultilization of hydrocarbon fuels.Further efforts are needed to solve these problems.
[Background and purposes]Foamed ceramics,as a novel category of inorganic non-metallic materials,have found applications in building energy efficiency,due to their advantageous properties,including light weight,thermal insulation,sound-absorbing and fire resistance.However,the production of these silicate materials,to achieve porous structure through high-temperature foaming processes,relies heavily on clay-feldspar-quartz mineral systems.This dependence represents a significant consumption of high-quality natural mineral resources.Furthermore,challenges in pore structure control can detrimentally impact the overall performance profile.Consequently,strategies for performance enhancement through the synergistic combination of industrial solid waste substitution and optimized process control are of considerable importance.China,as a major consumer of industrial resources,generates vast quantities of industrial solid wastes,such as tailings and fly ash,alongside its economic development.The resource utilization of these wastes demands urgent breakthroughs.This study was aimed to address this need by utilizing representative iron tailings and fly ash from the Chengde region as the primary raw materials for fabricating lightweight high-strength foamed ceramics.The influence of fly ash content on phase composition,pore structure and key performance metrics(namely bulk density,compressive strength,and thermal conductivity)of the resulting ceramics was systematically examined,aiming to provide a robust technical pathway for the high-value utilization of these solid wastes. [Methods]Based on the fundamental composition-structure-property relationship,fly ash was incorporated as a mineral additive,which was specifically selected for its demonstrated ability to enhance mechanical performance.The approach leverages the beneficial high-temperature complementarity between the primary mineral constituents,iron ore tailings and fly ash.Single-magnetite iron ore tailings(Chengde source)and power plant fly ash served as the primary raw materials.The total solid waste content(iron tailings+fly ash)was fixed at 60 wt.％,while the foaming agent content was 1.5 wt.％.The fly ash to iron tailings ratio varied across the proportions(by weight,relative to the total solid waste)of 0∶60,10∶50,20∶40,30∶30,40∶20,50∶10.The batched raw materials were blended with 0.1 wt.％anhydrous ethanol as a process control agent(PCA).The mixtures were then ball-milled using a powder grinder operating at 1500 r·min-1 for 2 h(powder-to-ball weight ratio=1∶2).The milled powders were sieved through a 325-mesh screen(aperture～45 μm).The undersized fraction was collected and dried at 100 ℃ for 15 min.The dried powders were loaded into moulds,compacted using vibration,leading to green bodies with smooth surface.The green compacts were subject to sintering.The samples were heated from room temperature to 800 ℃ at a ramping rate of 5 ℃·min-1.Subsequently,they were heated from 800 ℃ to the target sintering temperature of 1160 ℃ at a ramping rate of 3 ℃·min-1.They were held isothermally at 1160 ℃ for 30 min.The resultant foamed ceramic samples were characterized to evaluate the influence of the fly ash to iron tailings ratio on bulk density,mechanical properties(specifically compressive strength)and thermal conductivity.Mineral phase evolution,pore structure and microstructural morphology were analyzed using X-ray diffraction(XRD)and scanning electron microscopy(SEM). [Results]Porous multiphase ceramics,comprising quartz,mullite(3Al2O3·2SiO2)and anorthite,were successfully fabricated through powder-based in-situ foaming,utilizing single-magnetite iron ore tailings(Chengde source)and coal fly ash as primary solid waste feedstocks.Fly ash content critically governs pore architecture.An optimal fly ash proportion(20-30 wt.％of total solid waste)promoted formation of uniform,closed-cell pores,thickened pore walls.This microstructural refinement enhanced mechanical integrity.Conversely,exceeding this optimal range led to pore coarsening,increased pore wall defects,which are detrimental to mechanical performance.Appropriate fly ash content optimized pore size homogeneity and closed porosity fraction,thereby improving thermal insulation by minimizing gas-phase convection.Excess fly ash induced large,interconnected pore channels,which significantly degrade both mechanical strength and thermal insulation efficacy.The addition of fly ash significantly promoted the phase formation of mullite(3Al2O3·2SiO2).This crystalline phase served as the primary mechanical reinforcement agent,substantially enhancing compressive strength of the composites.Under the formulation of 60 wt.％total solid waste(fly ash+tailings)and 1.5 wt.％foaming agent,the ceramics achieved bulk density of 430-480 kg·m 3,compressive strength of 5.94-12.96 MPa and thermal conductivity of 0.18-0.42 W·m-1·K-1. [Conclusions]High-performance foamed ceramics with excellent properties were successfully prepared using iron tailings and fly ash from the Chengde area.The fly ash provides reactive sources of Si,Al and Ca,acting as a promoter for composite silicate-aluminate mineral phases.During high-temperature processing,it facilitates the formation of mullite-anorthite networks,which serve as the primary reinforcing phase within the ceramic matrix.In terms of pore structure regulation,the proportion of fly ash was the core factor in controlling pore morphology(uniformity,closed pore rate and pore size),directly affecting the balance between mechanical and thermal insulation properties of the materials.Under the formula framework of a total solid waste content of 60 wt.％and a foaming agent content of 1.5 wt.％,adjusting the proportion of fly ash(about 20-30％)could produce foamed ceramics with a bulk density of 430-580 kg·m-3,compressive strength of 5.94-12.96 MPa and thermal conductivity of 0.18-0.42 W·m-1·K-1.Our results can be used as a reference for the resource utilization of solid waste to prepare high-value-added building materials in Chengde and similar regions rich in solid waste,with significant environmental and economic benefits.
[Background and purposes]Negative temperature coefficient(NTC)thermosensitive ceramics are critical for temperature sensing applications,whereas their high resistivity in systems like Y2O3-YCr0.5Mn0.5O3 limits practical use.Although prior doping with Zr4+enhanced densification,the resistivity was further increased,deviating from the desired scale of kΩ·cm.Adjusting the phase ratio offered limited resistivity control and compromised thermal stability.This study was aimed to explore co-doping with alkaline earth ions to synergistically optimize resistivity,B-value and aging stability of Zr4+-modified 0.6Y2O3-0.4YCr0.5Mn0.5O3 ceramics. [Methods]The samples were prepared by using a two-step sintering process at 1600 ℃.A baseline group(A0)was co-doped with 4-8 mol％CaO(A1-A3)or MgO(A1 1-A13).Phase composition was analyzed by using XRD,elemental distribution and valence states were characterized by using SEM-EDS and XPS.Electrical properties were measured after electrode coating.Aging resistance was evaluated by resistance drift(％)after 800 h at 300 ℃. [Results]All samples retained dual-phase structures.Ca2+/Mg2+dissolved primarily in the perovskite phase,forming a dense"sea-island"microstructure.Ca2+doping drastically reduced resistivity.At 8 mol％,the room-temperature resistivity(ρ)decreased by approximately 82％compared to the undoped sample(A0),while the B-value remained in the range of 2500-3000 K.In contrast,with 4 mol％Mg2+doping,ρ was lowered to 5.14×104Ω·cm,accompanied by higher B-values of 3300-3500 K.XPS revealed that the Mn4+/Mn3+and Cr4+/Cr3+ratios increased after doping,which was attributed to charge compensation:Ca2+substitution at Y3+sites generated holes and enhanced small-polaron hopping.Mg2+likely enabled partial B-site substitution,weakening its impact.Aging tests showed that doping with Ca2+/Mg2+increased the resistance drift;however,the presence of Zr4+suppressed the drift by 1-2％through densification and grain refinement,thereby impeding cation vacancy migration. [Conclusions]The doping with Ca2+effectively reduced the resistivity of Zr4+-modified NTC ceramics through charge compensation and enhanced polaron hopping,while maintaining a low B-value for wide temperature applicability.Mg2+'s inferior performance stemmed from possible B-site substitution and limited vacancy generation.Crucially,Zr4+mitigated the negative aging effects of Ca2+/Mg2+by improving microstructural stability.The Ca2+/Zr4+co-doping strategy achieved balanced electrical properties and reliability,demonstrating significant potential for the development of industrial NTC thermistors.
[Significance]Ceramic matrix composites(CMCs),especally continuous riber reinforced CMCs(CFRCMCs),overcome the brittleness of monolithic ceramics,offering exceptional strength,fracture toughness and thermal stability,for demanding aerospace,energy and automotive applications.This is reflected in a growing market,projected to rise from $7.83 billion in 2024 to $15.83 billion by 2030.However,manufacturing complex-shaped components is hindered by traditional methods,such as chemical vapor infiltration(CVI),polymer impregnation and pyrolysis(PIP)and liquid silicon infiltration(LSI),which are multi-step,time-consuming and costly.Additive manufacturing(AM)emerges as a revolutionary solution.The layer-by-layer fabrication process enables unparalleled design freedom,allowing for rapid and cost-effective production of complex CFRCMC components that cannot be achieved by using the traditional techniques,thus overcoming a critical manufacturing bottleneck. [Progress]This review was aimed to systematically chronicle the significant progress in AM of CFRCMCs,with research currently dominated by material extrusion(ME)processes,due to their relative simple in integrating continuous fibers.There are two primary ME techniques:fused deposition modeling(FDM)and direct ink writing(DIW).The FDM involves the co-extrusion of a thermoplastic filament(often blended with ceramic or precursor powders)and a continuous fiber to create a composite green body.This polymer-rich structure is subsequently transformed into a ceramic component through critical post-processing techniques,primarily PIP or LSI.Successes include the fabrication of Cf/SiC,Cf/SiOC and Cf/ZrB2-SiC composites.In DIW method,the most prevalent for CFRCMCs,employs the co-extrusion of a ceramic slurry with continuous fibers,typically through a coaxial nozzle.Innovations include optimized nozzle designs guided by finite element simulation,core-shell printing for multi-filament structures and thermal-assisted systems for high-precision printing.To overcome the limited fiber volume fractions,alternative strategies,such as separate fiber placement and even vertical fiber layup,have been developed.Beyond ME,preliminary explorations using stereolithography(SLA),powder bed fusion(PBF)and laminated object manufacturing(LOM)have demonstrated the potential for a broader AM toolkit.Substantial research has been dedicated to overcoming two core challenges:slurry rheology and printability.Successful co-extrusion hinges on tailored rheological properties.Strategies include formulating shear-thinning slurries using nanoparticles(e.g.,nano-SiO2)or adjusting binder content(e.g.,carboxymethyl cellulose)to enhance viscosity and adhesive drag forces.Concurrent optimization of nozzle geometry(diameter,insertion angle)and printing parameters(layer height,pressure)is essential for stable extrusion and dimensional accuracy.In terms of fiber-matrix interface design,the interface is critical for activating toughening mechanisms.Research has been focused on the precise deposition of nano-scale interphases,such as pyrolytic carbon(PyC)or SiC via CVI or colloidal deposition.It has been systematically shown that an optimal interphase thickness(e.g.,～110 nm for PyC)promotes crack deflection and fiber pull-out,dramatically enhancing fracture toughness and transforming the composite's failure from brittle to non-catastrophic.For densification and pore control,the inherently porous AM green bodies require extensive post-processing.PIP is widely used,with multiple cycles progressively reducing open porosity to below 10％.Alternatively,LSI has proven highly effective,infiltrating a carbonized preform to form a dense SiC matrix,achieving final porosities of as low as 10％-20％or even below 1％.Managing the pre-infiltrated carbon density is crucial for optimizing LSI outcomes.Furthermore,AM enables the precise control and study of key toughening mechanisms,interfacial debonding,fiber bridging and pull-out,crack deflection and plastic deformation of ductile phases,which collectively enable a graceful,damage-tolerant failure mode.This control has also facilitated the fabrication of multifunctional CFRCMCs,including components for ultra-high-temperature ablation protection(e.g.,Cf/ZrB2-SiC),electromagnetic wave absorption/transparency,thermal insulation,fireproofing and biocompatible bone scaffolds,showcasing the technology's vast potential beyond structural applications. [Conclusions and prospects]In summary,AM has established itself as a viable and transformative route for fabricating continuous fiber-reinforced ceramic matrix composites(CFRCMCs),enabling the production of complex high-performance components beyond the capabilities of the traditional methods.Progress in material extrusion techniques and a deeper understanding of post-processing have laid a solid foundation for future development.However,key challenges must be addressed to realize the full potential of AM.Future research needs to focus on:(1)achieving true 3D fiber reinforcement with innovative print-head designs to minimize interlayer defects,(2)advancing multi-material integration with novel precursors and systems for functionally graded components and(3)implementing intelligent manufacturing via digital twins and AI for real-time process monitoring and optimization,ensuring reliability and cost-effectiveness.By addressing these challenges through interdisciplinary efforts,AM for CFRCMCs is poised to transition from a laboratory innovation to a mainstream high-reliability manufacturing technology,enabling a new generation of engineering systems for extreme environments.
[Significance]Currently,the globe is confronted with the dual challenges of energy structure transformation and environmental pollution control.The proposal of China's"dual carbon goals"(carbon peaking and carbon neutrality)imposes higher requirements on the adjustment of energy structure and the innovation of pollution control technologies.As a green and environmentally benign technology,photocatalytic technology can directly utilize solar energy to drive energy conversion(photocatalytic water splitting for hydrogen production)and environmental purification(pollutant degradation)without secondary pollution,thus serving as one of the core approaches to address these issues.Silicon carbide(SiC),as a semiconducting material,exhibits prominent advantages,including tunable band gap(2.3-3.3 eV),excellent chemical stability and high carrier mobility.In particular,3C-SiC(with a band gap of 2.48 eV)can absorb visible light.Moreover,the positions of its conduction band(CB,-1.32 eV)and valence band(VB,+1.16 eV)are compatible with redox reactions,making it an ideal photocatalyst.However,pristine SiC suffers from intrinsic drawbacks,such as high photogenerated carrier recombination rate,low solar light utilization efficiency and small specific surface area,which seriously restrict its practical application.In contrast,constructing a heterostructure system with SiC as the substrate can significantly enhance the performance of SiC-based photocatalysts through multiple mechanisms,driving the directional separation of carriers via the interfacial built-in electric field,exposing high-active crystal facets through defect engineering and broadening the light absorption range.This strategy holds significant theoretical value and practical significance for promoting the practical application of photocatalytic technology and facilitating the achievement of the dual carbon goals. [Progress]In photocatalytic reactions,the valence band(VB)/conduction band(CB)positions of semiconductors,light absorption range,specific surface area,crystallinity and separation efficiency of photogenerated carriers are all key factors affecting photocatalytic performance.However,a single semiconductor often fails to satisfy all these criteria.By compositing different semiconductor materials to construct heterojunction systems,these issues can be addressed simultaneously.In the early research on SiC heterojunction systems,the focus was mainly on the construction of Type-Ⅱ heterojunctions.Nevertheless,the intrinsic defects in their carrier migration pathways cannot realize the maximized utilization of redox potentials.The subsequently proposed Z-scheme heterojunction and S-scheme heterojunction systems not only maintain the advantageous redox potentials of photocatalysts but also exhibit excellent charge separation efficiency,thus demonstrating strong photocatalytic performance.For SiC/metal heterojunctions,the potential difference generated at the contact interface of the two distinct materials enables the directional migration of photogenerated electrons,thereby improving the separation efficiency of photogenerated carriers.For SiC/carbon-based heterojunctions,the excellent electrical conductivity and large specific surface area of carbon-based materials facilitate the rapid migration of electrons,which in turn enhances photocatalytic performance. [Conclusion and prospects]Starting from the physical properties of SiC,construction of heterojunction systems and photocatalytic applications,SiC-based heterojunction photocatalysts were focused,as a novel and highly promising material system in the photocatalysis field.Their performance breakthroughs and charge transfer mechanisms are discussed in detail.Heterojunction construction is the core strategy to overcome the performance bottlenecks of pristine SiC.Type-Ⅱ,Z-scheme,S-scheme and metal/carbon-based heterojunctions have been constructed,to solve the problem of high photogenerated carrier recombination rate,while simultaneously broadening the light absorption range and hence improving photocatalytic performance.However,the industrial application of SiC-based heterojunction photocatalysts currently faces three major technical barriers,including the immaturity of large-scale preparation technology,the unclear charge transport kinetic mechanisms and the limitation of material systems.Looking ahead,key breakthroughs should be made in the following three aspects. (1)Construction of multi-level heterojunction systems.By designing ternary/quaternary structures and leveraging band engineering,efficient separation and directional migration of carriers can be achieved,thereby enhancing the overall catalytic efficiency. (2)Innovation of in-situ characterization technologies.It is important to develop in-situ dynamic characterization methods with atomic-scale resolution,which is combined with theoretical calculation methods to deeply analyze the microscopic mechanisms of charge transfer in various heterojunction systems,thus providing theoretical guidance for the design of high-performance heterojunctions. (3)Development of ceramic-based composite systems.The advantageous properties of SiC ceramics(e.g.,high hardness,high temperature resistance and excellent chemical stability)can be utilized to construct SiC ceramic-based composite photocatalytic systems.By doing this,it is possible to overcome the drawbacks of traditional powder photocatalysts(such as low mechanical strength and poor cyclic stability)and realize the photocatalytic application of SiC ceramic composite systems.
