Xiaoming Duan

Progress of a novel non-oxide Si-B-C-N ceramic and its matrix composites

In the past twenty years, Si-B-C-N ceramic has attracted wide attention due to its special structure and outstanding properties. The ceramic generally has an amorphous or a nano-crystalline structure, and has excellent structural stability, oxidation resistance, creep resistance and high-temperature mechanical properties, etc. Thus, Si-B-C-N ceramic attracts many researchers and finds potential applications in transportation, aerocraft, energy, information, microelectronics and environment, etc. Much work has been carried out on its raw materials, preparation processes, structural evolution, phase equilibrium and high-temperature properties. In recent years, many researchers focus on its new preparation methods, the preparation of dense ceramic sample with large dimensions, ceramic matrix composites reinforced by carbon fiber or SiC whisker, or components with various applications. Research on Si-B-C-N ceramic will develop our insight into the relationship between structures and properties of ceramics, and will be helpful to the development of novel high-performance ceramics. This paper reviews the preparation processes, general microstructures, mechanical, chemical, electrical and optical properties, and potential applications of Si-B-C-N ceramic, as well as its matrix composites.

TEM observation of dynamic distortion in 2Y-PSZ/steel composites

Abstract 2Y-PSZ/steel composites have been sintered by vacuum hot-pressing. The microstructure was studied by TEM after the composites were impacted at the average strain of 1.0×103 S−1 by split Hopkinson method. The results showed that the phase transformation from γ to α′ occurred in the matrix of TRIP steel under dynamic loading. Dynamic mechanical properties of 2Y-PSZ/steel composites were improved because of the transformation of distortion induced Martensite. The morphology of Martensite induced by distortion was mainly twins. The sub-structure of Martensite was mainly twins. But there was no mid-ridge and distortion twins comptetely expanded to the whole Martensite. Moreover, there was dislocation Martensite at the same sample. The inhomogeneous spread of stress wave in 2Y-PSZ/steel composites led to the different dimension of twins Martensite. When dynamic distortion was relatively efficient, Martensitic dimension and twins distance were increased and the second twins would occur. ZrO2 grains are finely distributed in matrix TRIP steel, which improved the dynamic flow stress of 2Y-PSZ/steel composites. Moreover, ZrO2 could undergo tetragonal→monoclinic phase transformation, which would improve the dynamic yield strength of the composites. But when the content of ZrO2 exceeded 20 vol.%, it would restrain the transformation of dynamic distortion induced Martensite in TRIP steel and led to the decrease of the dynamic mechanical properties of 2Y-PSZ/steel composites.

Microstructure and mechanical properties of SiC f /SiBCN ceramic matrix composites

SiC fiber reinforced SiBCN ceramic matrix composites (CMCs) have been prepared by mechanical alloying and consolidated by hot pressing. During the sintering process, amorphous SiC fibers crystallized seriously and transformed into β-SiC. Meanwhile, the interfacial carbothermal reactions caused the strong bonding between the matrix and fibers. As a result, SiCf/SiBCN fractured in a typical catastrophic manner. Room-temperature mechanical properties reached the maximums for the CMC samples sintered at 1900 °C/60 MPa/30 min. The density, flexural strength, Young’s modulus and fracture toughness are 2.56±0.02 g/cm3, 284.3±17.9 MPa, 183.5±11.1 GPa and 2.78±0.14 MPa·m1/2, respectively.

Microstructures, mechanical properties and oxidation resistance of SiBCN ceramics with the addition of MgO, ZrO2 and SiO2 (MZS) as sintering additives

Nano-crystalline SiBCN ceramics were prepared by mechanical alloying (MA) plus hot pressing (HP) with the addition of MZS1 (MgO, ZrO2 and SiO2) and MZS2 (ZrSiO4 and SiO2) as the sintering additives. The effects of the two additives on the microstructure, mechanical properties and oxidation resistance of SiBCN ceramics were carefully evaluated. The addition of MZS1 and MZS2 additives in the SiBCN matrix can boost elemental diffusion and matrix densification. Owing to the effective densification caused by the sintering additives, the mechanical properties of SM1 and SM2 samples are far superior to monolithic SiBCN. The oxide layers of SM1 and SM2 samples remain relatively dense and continuous, and have a strong binding capacity with the matrix after oxidation testing at 1500 °C for 20 h. The residual excess carbon in the SiBCN matrix should be responsible for the formation of ZrC and the outermost oxide layer structure is comprised of SiO2 and ZrC after the oxidation test. The flexural strength, Young’s modulus, fracture toughness and Vicker’s hardness of the SM2 sample are much higher than monolithic SiBCN ceramics, reaching to 394.2 ± 41.7 MPa, 152.9 ± 16.0 GPa, 5.86 ± 0.86 MPa m1/2 and 8.3 ± 0.6 GPa, respectively.

Structure evolution, amorphization and nucleation studies of carbon-lean to -rich SiBCN powder blends prepared by mechanical alloying

A number of carbon-lean and -rich SiBCN powder blends were subjected to mechanical alloying by high-energy ball milling generating a composite microstructure with varying proportions of amorphous and crystalline phases. Microstructural characterization at different milling stages and the evolution of free carbon and nanocrystalline SiC are discussed by using X-ray diffraction, Raman spectroscopy, transmission electron microscopy and nucleation magnetic resonance. Furthermore, the chemical bonding states of various SiBCN powder blends at different stages of milling were studied by FT-IR. The amorphization of carbon for carbon-lean powder blends was somewhat quicker than for carbon-rich analogues, which retained t-carbon and multiple graphene structures after being subjected to 40 h of milling. The chemical bonding state changes are similar for all investigated powder blends, while detailed microstructure changes are evident and consist of a considerable amorphous nature and a small amount of nanocrystallites after 40 h of milling. The forming nanograins are assigned to Si and SiC for carbon-lean and -rich powder blends, respectively. Ball milling leads to alloying, complete or partial solid state amorphization, accompanied by strain-induced heterogeneous or homogeneous nucleation of nanocrystalline phases from an amorphous matrix.

Synthesis of Novel Cobalt-Containing Polysilazane Nanofibers with Fluorescence by Electrospinning

Emission in the nanostructured materials is important in micro/nanoelectronic devices. We report here a strategy for the processing of micron and submicron fibers from a cobalt-containing hyperbranched polysilazane by electrospinning. The electrospun nanofibers have uniform average diameters of ~600 nm and lengths of ~10 μm. The photophysical properties of polycobaltsilazane (PCSN) are studied using UV-VIS and photoluminescence spectroscopies. PCSN fibers display a series of emission peaks between 490 and 615 nm. The Co(II) doping into polysilazane leads to the emission from 465 to 415 nm. The emission wavelength shift of Co(III)-containing polysilazane is specific under 340 and 470 nm excitation wavelengths, respectively, while it is not observed with metal-free polysilazane. Thermogravimetric analysis-Differentical thermal analysis (TGA-DTA) profiles also show good thermostability of the PCSN fibers at 800 °C under Ar atmosphere. The use of PCSN offers both enhanced ceramic yields against ~5 wt % starting material and the fluorescence intensity of polymeric fibers.

Mechanical Properties and Plasma Erosion Resistance of ZrO2p(3Y)/BN-SiO2 Ceramic Composites under Different Sintering Temperature

ZrO2p(3Y)/BN-SiO2 ceramic composites were hot pressed under different sintering temperature. The ceramic composites were composed by BN, m-ZrO2, t-ZrO2 and SiO2. The relative density, bending strength, elastic modulus and fracture toughness increase with the sintering temperature increasing, the maximum value of which at the sintering temperature of 1800°C are 97.5%, 229.9MPa, 60.8GPa and 3.55MPam1/2, respectively. The erosion resistance ability of ZrO2p(3Y)/BN-SiO2 ceramic composites rise gradually with the sintering temperature increasing, and the erosion rate of the ceramic composite sintered at 1800°C is 8.03×10−3mm/h.

Effect of the BN content on the thermal shock resistance and properties of BN/SiO2 composites fabricated from mechanically alloyed SiBON powders

Wave-transparent composites of BN/SiO2 were prepared via hot pressure sintering at 1650 °C of mechanically alloyed amorphous SiBON powders. The mechanical, dielectric and thermal properties and thermal shock resistance of the composites were carefully investigated with different BN contents. With increasing BN content, the flexural strength and fracture toughness of the composites increased first and then decreased. The sample with the composition of SiO2–5BN exhibited the highest flexural strength of 256.3 MPa and a fracture toughness of 3.15 MPa m1/2. The relative density decreased with the increase of BN content, which would influence the thermal properties and the thermal shock resistance of the composites. The formation of borosilicate glass and crystallization of fused silica in the surface at the high temperature greatly improved thermal shock resistance of the samples. The SiO2–6BN sample, possessing low relative density, exhibited the best thermal shock resistance, and the retained strength after a thermal shock of 1100 °C was 98.6% of the original strength. Besides excellent mechanical and thermal properties and thermal shock resistance, the as-sintered composites exhibited low dielectric constant (e < 4.62) and loss tangent (tan δ < 0.002), meeting the required values for high temperature wave-transparent materials.

Microstructure and erosion resistance of in-situ SiAlON reinforced BN-SiO2 composite ceramics

BN-SiO2-SiAlON composite ceramics were successfully prepared by the means of hot pressed sintering. Xe plasma flow generated by Hall Thruster was used for sputtering the surface of the samples in order to evaluate the plasma erosion resistance. XRD, TEM, SEM, and LSCM were used to characterize the phase composition and morphologies of as-made composite ceramics before and after Xe plasma erosion. The ceramics were composed of h-BN, fused silica, and SiAlON, which maintained structural stability during the process of Xe plasma sputtering. In conclusion, comparing with BN-SiO2 composite ceramics, the plasma erosion rate of BN-SiO2-SiAlON composite ceramics decreases significantly at first then rises with the increase of AlN addition. Erosion pits can be observed by using SEM on the surface after plasma sputtering, which demonstrates that the BN grains have dropped off the surface. In addition, mechanical denudation by high-speed Xe ions is recognized as the injury mechanism for the BN-matrix composite materials.

Effects of graphene oxide on the geopolymerization mechanism determined by quenching the reaction at intermediate states

The effects of graphene oxide on the geopolymerization reaction products at different times were investigated by quenching the reaction. The phase composition and valence bond structure evolution were investigated systematically. The results show that the ethanol/acetone mixture helps isolate reaction products early in the process (0–24 h). RGO bonded well with the geopolymer matrix during the geopolymerization. The degree of densification increased and the amorphous nature of the material decreased with reaction time. The addition of rGO accelerated the conversion of five and six coordinate Al–O sites into four coordinates and Si atoms forming Q4(3Al) network structure.