Hard and tough novel high-pressure γ-Si_(3)N_(4)/Hf_(3)N_(4) ceramic nanocomposites

Cubic silicon nitride(-Si_(3)N_(4))is superhard and one of the hardest materials after diamond and cubic boron nitride(cBN),but has higher thermal stability in an oxidizing environment than diamond,making it a competitive candidate for technological applications in harsh conditions(e.g.,drill head and abrasives).Here,we report the high-pressure synthesis and characterization of the structural and mechanical properties of a γ-Si_(3)N_(4)/Hf_(3)N_(4) ceramic nanocomposite derived from single-phase amorphous silicon(Si)-hafnium(Hf)-nitrogen(N)precursor.The synthesis of the-Si_(3)N_(4)/Hf_(3)N_(4) nanocomposite is performed at~20 GPa and ca.1500 ℃ in a large volume multi anvil press.The structural evolution of the amorphous precursor and its crystallization to-Si_(3)N_(4)/Hf_(3)N_(4) nanocomposites under high pressures is assessed by the in situ synchrotron energy-dispersive X-ray diffraction(ED-XRD)measurements at~19.5 GPa in the temperature range of ca.1000-1900℃.The fracture toughness(K_(IC))of the two-phase nanocomposite amounts~6/6.9 MPa·m^(1/2) and is about 2 times that of single-phaseγ-Si_(3)N_(4),while its hardness of ca.30 GPa remains high.This work provides a reliable and feasible route for the synthesis of advanced hard and tough-Si_(3)N_(4)-based nanocomposites with excellent thermal stabililty.

Microstructural evolution and electromagnetic wave absorbing performance of single-source-precursor-synthesized SiCuCN-based ceramic nanocomposites

Copper(Cu)-containing single-source precursors(SSPs)for the preparation of SiCuCN-based ceramic nanocomposites were successfully synthesized for the first time using polysilazane(PSZ),copper(II)acetate monohydrate(CuAc),and 2-aminoethanol via nucleophilic substitution reactions at silicon(Si)centers of PSZ.The synthesis process,polymer-to-ceramic transformation,and high-temperature microstructural evolution of the prepared ceramics were characterized.Dielectric properties and electromagnetic wave(EMW)absorbing performance of the ceramics were investigated as well.The results show that the polymer-to-ceramic transformation finishes at ca.900 ℃,and Cu nanoparticles are homogeneously distributed in a SiCN matrix,forming a SiCN/Cu nanocomposite.After annealing at 1200 ℃,the Cu nanoparticles completely transform into copper silicide(CusSi).Interestingly,the thermal stability of the Cu nanoparticles can be strongly improved by increasing the free carbon content,so that a part of metallic Cu nanoparticles can be detected in the ceramics annealed even at 1300 ℃,forming a SiCN/Cu/Cu_(3)Si/C nanocomposite.Compared with SiCN,the SiCuCN-based nanocomposites exhibit strongly enhanced dielectric properties,which results in outstanding EMW absorbing performance.The minimum reflection coefficient(RC_(min))of the SiCN/Cu/Cu_(3)Si/C nanocomposites annealed at 1300 ℃ achieves-59.85 dB with a sample thickness of 1.55 mm,and the effective absorption bandwidth(EAB)broadens to 5.55 GHz at 1.45 mm.The enhanced EMW absorbing performance can be attributed to an in situ formed unique network,which was constructed with Cu and Cu_(3)Si nanoparticles connected by ring-like carbon ribbons within the SiCN matrix.

Si-based polymer-derived ceramics for energy conversion and storage

Since the 1960s,a new class of Si-based advanced ceramics called polymer-derived ceramics(PDCs)has been widely reported because of their unique capabilities to produce various ceramic materials(e.g.,ceramic fibers,ceramic matrix composites,foams,films,and coatings)and their versatile applications.Particularly,due to their promising structural and functional properties for energy conversion and storage,the applications of PDCs in these fields have attracted much attention in recent years.This review highlights the recent progress in the PDC field with the focus on energy conversion and storage applications.Firstly,a brief introduction of the Si-based polymer-derived ceramics in terms of synthesis,processing,and microstructure characterization is provided,followed by a summary of PDCs used in energy conversion systems(mainly in gas turbine engines),including fundamentals and material issues,ceramic matrix composites,ceramic fibers,thermal and environmental barrier coatings,as well as high-temperature sensors.Subsequently,applications of PDCs in the field of energy storage are reviewed with a strong focus on anode materials for lithium and sodium ion batteries.The possible applications of the PDCs in Li–S batteries,supercapacitors,and fuel cells are discussed as well.Finally,a summary of the reported applications and perspectives for future research with PDCs are presented.

Single-source-precursor synthesis and phase evolution of SiC-TaC-C ceramic nanocomposites containing core-shell structured TaC@C nanoparticles

A novel single-source-precursor for SiC-TaC-C nanocomposites was successfully synthesized by the chemical reaction between a polycarbosilane(allylhydridopolycarbosilane,AHPCS)and tantalum(V)chloride(TaCls),which was confirmed by Fourier transform infrared spectra(FTIR)measurement.After pyrolysis of the resultant single-source-precursors at 900"C,amorphous ceramic powders were obtained.The 900 C ceramics were anncaled at different temperatures in the range of 1200-1600℃ to gain SiC-TaC-C nanocomposites.The phase evolution of ceramic nanocomposites was investigated by X-ray diffraction(XRD)and transmission electron microscopy(TEM).The results indicate that the TaC starts to crystallize at lower temperature than theβ-SiC.It is particularly worth pointing out that the unique core-shell structured TaC-C nanoparticles were in-situ formed and homogeneously distributed in the ceramic matrix after annealing at 1400 C.Even at a high temperature of 1600 C,the grain sizes ofβ-SiC and TaC are smaller than 30 nm,flilling the definition of nanocomposites.The present study related to SiC-TaC C nanocomposites paves a new road for enriching ultra-high temperature ceramic family suitable for structural/functional applications in harsh environment.

Elucidating the role of preferential oxidation during ablation:Insights on the design and optimization of multicomponent ultra-high temperature ceramics

Multicomponent ultra-high temperature ceramics(UHTCs)are promising candidates for thermal protection materials(TPMs)used in aerospace field.However,finding out desirable compositions from an enormous number of possible compositions remains challenging.Here,through elucidating the role of preferential oxidation in ablation behavior of multicomponent UHTCs via the thermodynamic analysis and experimental verification,the correlation between the composition and ablation performance of multicomponent UHTCs was revealed from the aspect of thermodynamics.We found that the metal components in UHTCs can be thermodynamically divided into preferentially oxidized component(denoted as MP),which builds up a skeleton in oxide layer,and laggingly oxidized component(denoted as ML),which fills the oxide skeleton.Meanwhile,a thermodynamically driven gradient in the concentration of MP and ML forms in the oxide layer.Based on these findings,a strategy for pre-evaluating the ablation performance of multicomponent UHTCs was developed,which provides a preliminary basis for the composition design of multicomponent UHTCs.

First-principles calculations on the structure and electronic properties of boron doping zigzag single-walled carbon nanotubes

Calculations have been made for single-walled zigzag(n,0) carbon nanotubes containing substitutional boron impurity atoms using ab initio density functional theory.It is found that the formation energies of these nanotubes depend on the tube diameter,as do the electronic properties,and show periodic fea-ture that results from their different π bonding structures compared to those of perfect zigzag carbon nanotubes.When more boron atoms are incorporated into a single-walled zigzag carbon nanotube,the substitutional boron atoms tend to come together to form structure of BC3 nanodomains,and B-doped tubes have striking acceptor states above the top of the valence bands.For the structure of BC3,there are two kinds of configurations with different electronic structures.

Erratum to:Si-based polymer-derived ceramics for energy conversion and storage

Besides the original acknowledgements,the authors Ralf Riedel and Magdalena Graczyk-Zajac would like to also acknowledge EU support in the frame of H2020 project SIMBA under grant agreement number 963542.

Single-source-precursor synthesis and air-plasma ablation behavior of(Ti,Zr,Hf)C/SiC ceramic nanocomposites at 2200℃

Dense monolithic(Ti,Zr,Hf)C/SiC ceramic nanocomposites with four different molar ratios of metallic elements in the(Ti,Zr,Hf)C phase(i.e.,Ti:Zr:Hf=1:1:1,2:3:5,2:3:3,and 1:2:1)were prepared upon pyrolysis of novel(Ti,Zr,Hf)-containing single-source precursors(SSPs),followed by spark plasma sintering(SPS).A thorough characterization was conducted to elucidate the synthesis of the SSPs,polymer-to-ceramic transformation,chemical/phase compositions,and microstructure of the SiTiZrHfC-based ceramics.The results revealed the feasibility of synthesizing nanocomposites with high(Ti,Zr,Hf)C contents using the SSP method.These nanocomposites were characterized by a unique microstructure with in situ generated(Ti,Zr,Hf)C@C core-shell nanoparticles homogeneously mixed withβ-SiC.The ablation behavior of the nanocomposites was evaluated on an air-plasma device for 60 s.Impressively,the nanocomposites exhibited excellent ablation resistance,and the lowest linear ablation rate reached−0.58μm/s at 2200°C.Notably,the ablation resistance can be dramatically improved by precisely tailoring the atomic ratios of metal elements within the(Ti,Zr,Hf)C phase via the molecular design of the SSPs.The formation of a multiple-oxide layer with both a high-meltingpoint phase((Ti,Zr,Hf)O_(2))and low-melting-point phases((Zr,Hf)TiO4)and glassy SiO_(2),as well as their structure,played a critical role in the enhanced ablation resistance.The uniform distribution of the high-melting-point(Ti,Zr,Hf)O_(2)nano/microparticles throughout the glassy SiO_(2)matrix significantly enhanced the viscosity and stability of the oxide layer by the pinning effect,offering superior protection against the ingress of oxygen atoms and excellent resistance to mechanical erosion.