Oxidation behavior of amorphous and nanocrystalline SiBCN ceramics–Kinetic consideration and microstructure

In this study,the structural evolution of SiBCN ceramics during crystallization and its effects on oxidation behavior involving different atomic units or formed phases in amorphous or crystalline SiBCN ceramics were analyzed.The amorphous structure has exceptionally high oxidation activity but presents much better oxidation resistance due to its synchronous oxidation of atomic units and homogeneous composition in the generated oxide layer.However,the oxidation resistance of SiBCN ceramic will degrade during the continual crystallization process,especially for the formation of the nanocapsule-like structure,due to heterogeneous oxidation caused by the phase separation.Besides,the activation energy and rate-controlling mechanism of the atomic units and phases in SiBCN ceramics were obtained.The BNCx(Ea=145 kJ/mol)and SiC(2-x)(Ea=364 kJ/mol)atomic units in amorphous SiBCN structure can be oxidized at relatively lower temperatures with much lower activation energy than the corresponding BN(C)(Ea=209 kJ/mol)and SiC(Ea=533 kJ/mol)phases in crystalline structure,and the synchronous oxidation of the SiC(2-x)and BNCx units above 750C changes the oxidation activation energy of BNCx(Ea=332 kJ/mol)to that similar to SiC(2-x).The heterogeneous oxide layer formed from the nanocapsule-like structure will decrease the activation energy SiC(Ea=445 kJ/mol)and t-BN(Ea=198 kJ/mol).

飞行器的电气化的实现

当前航天航空工业领域中有两项能勻够对未来发展有着重要影响的电气化举措。第一项举措是MEA多电飞机的发展趋势。MEA多电飞机的方案减轻了飞行器的重量,因为越来越多的机械制造和液压系统被电力驱动装置取代了。第二项举措是越来越多的用电力驱动或者综合型驱动系统取代了传统的机械驱动系统。电力驱动系统保证了飞行器更加高效、静音的飞行,也更加环保,满足了新的排放监管要求。

Fabrication of dense SiBCN monolith at a lower temperature and its high-temperature performance

In this study,a crack-free pyrolysis process of partially cured precursor powder compacts was developed to prepare dense silicon boron carbonitride(SiBCN)monoliths at much lower temperatures(1300℃),thereby circumventing the challenges of sintering densification(>1800℃).Unlike the elastic fracture in over-cured precursors or the viscoelastic deformation in under-cured precursors,the partially cured precursor,exhibiting elastic-plastic deformation behavior,facilitates limited nanoscale pore formation in a dense structure,achieving a balance between crack-free pyrolysis and densification.Compared to SiBCN derived from the over-cured precursor(σ=~159 MPa,K_(IC)=1.9 MPa:m^(1/2),Vickers hardness(HV)=7.8 GPa,and E=122 GPa),the resulting SiBCN monolith exhibited significantly improved mechanical properties(σ=~304 MPa,K_(IC)=3.7 MPa-m12,HV=10.6 GPa,and E=161 GPa)and oxidation resistance.In addition,this study investigated the high-temperature performance of SiBCN monoliths,including crystallization and oxidation,and determined the oxidation kinetics induced by pore structure healing and the different oxidation mechanisms of Si-C-N and B-C-N clusters in the amorphous structure.Due to its unique composition and structure,the SiBCN ceramic oxide layer exhibits exceptional self-healing effects on repairing the nanoporous system in the initial stage and shows outstanding high-temperature stability during prolonged oxidation,mitigating adverse effects from bubble formation and crystallization.Due to the nanoporous structure,the oxidation rate is initially controlled by gas diffusion following a linear law before transitioning to oxide layer diffusion characterized by a parabolic law.Finally,due to different valence bond configurations,Si-C-N transforms into an amorphous SiCNO structure after phase separation,unlike the nucleation and growth of residual B-N-C.

3D-printed Lunar regolith simulant-based geopolymer composites with bio-inspired sandwich architectures

Over time,natural materials have evolved to be lightweight,high-strength,tough,and damage-tolerant due to their unique biological structures.Therefore,combining biological inspiration and structural design would provide traditional materials with a broader range of performance and applications.Here,the application of an ink-based three-dimensional(3D)printing strategy to the structural design of a Lunar regolith simulant-based geopolymer(HIT-LRS-1 GP)was first reported,and high-precision carbon fiber/quartz sand-reinforced biomimetic patterns inspired by the cellular sandwich structure of plant stems were fabricated.This study demonstrated how different cellular sandwich structures can balance the structure–property relationship and how to achieve unprecedented damage tolerance for a geopolymer composite.The results presented that components based on these biomimetic architectures exhibited stable non-catastrophic fracture characteristics regardless of the compression direction,and each structure possessed effective damage tolerance and anisotropy of mechanical properties.The results showed that the compressive strengths of honeycomb sandwich patterns,triangular sandwich patterns,wave sandwich patterns,and rectangular sandwich patterns in the Y-axis(Z-axis)direction were 15.6,17.9,11.3,and 20.1 MPa(46.7,26.5,23.8,and 34.4 MPa),respectively,and the maximum fracture strain corresponding to the above four structures could reach 10.2%,6.7%,5.8%,and 5.9%(12.1%,13.7%,13.6%,and 13.9%),respectively.

Open-cell mullite ceramic foams derived from porous geopolymer precursors with tailored porosity

Porous geopolymer precursors were firstly prepared by the direct foaming method using bauxite,fly ash(FA),and metakaolin(MK)as raw materials,and porous mullite ceramics were prepared after ammonium ion exchange and then high-temperature sintering.The effects of chemical foaming agent concentration,ion-exchange time,and sintering temperature on porous geopolymerderived mullite ceramics were studied,and the optimal preparation parameters were found.Studies have shown that the concentration of blowing agent had great influence on open porosity(q)and porosity and cell size distributions of geopolymer samples,which in turn affected their compressive strength(σ).Duration of the ion exchange had no obvious effect on the sintered samples,and the amount of mullite phase increased with the increase in the sintering temperature.Mullite foams,possessing an open-celled porous structure,closely resembling that of the starting porous geopolymers produced by directly foaming,were obtained by firing at high temperatures.Stable mullite(3Al_(2)O_(3)·2SiO_(2))ceramic foams with total porosity(ε)of 83.52 vol%,high open porosity of 83.23 vol%,and compressive strength of 1.72 MPa were produced after sintering at 1400 for 2 h in℃ air without adding any sintering additives using commercial MK,bauxite,and FA as raw materials.

Additive Manufacturing(AM)of Advanced Ceramics:From Materials,Structural Designing,AM Technologies,to Performance of Printed Components

Advanced ceramic materials have been widely used in a range of high-end technical fields due to their high mechanical performance,chemical stability,and impressive acoustic,optical,electrical,magnetic,and thermal properties.With the continuous improvement of the current level of science and technology,especially in cutting-edge application scenarios,the structural and functional requirements of advanced ce-ramic parts are becoming increasingly higher.

Additive Manufacturing of Ceramics from Liquid Feedstocks

In this review,we summarize the research activities carried out by our research group at the University of Padova on the additive manufacturing of ceramics from liquid feedstocks.Particularly,we evaluate the use of preceramic polymers,geopolymers,and sol-gel solutions.We mainly focus on processing with liquid feedstocks because they have some advantages with respect to slurry-based feedstocks in which powders are present.Particularly,lower viscosity,enhanced transparency,and lack of scattering and sedimentation are advantageous features for vat photopolymerization processes,whereas the absence of particulates reduces clogging problems at the nozzle for extrusion-based processes.Simultaneously,preceramic polymers and geopolymers have some limitations in terms of the range of ceramic compositions that can be obtained;sol-gel solutions are intrinsically unstable,whereas printed objects suffer from drying issues.Nevertheless,we successfully produced high-quality parts using a variety of additive manufacturing techniques,some of which(e.g.,volumetric additive manufacturing)have been proposed for the fabrication of ceramic components for the first time.