Continuous Si3N4 fiber reinforced SiNO matrix composites (Si3N4 (f)/SiNO composites) were innovatively prepared for long-time high-temperature resistant wave-transparent materials of hypersonic aircraft. The microstructure, high-temperature mechanical and dielectric properties of Si3N4 (f)/SiNO composites were investigated in detail. The as-fabricated Si3N4 (f)/SiNO composites have homogeneous SiNO matrix distribution for the special winding process, which is beneficial for the mechanical strength and wave-transparent properties. The average tensile strength and flexural strength at room temperature is 87.8 MPa and 171.2 MPa respectively, which suggests Si3N4 (f)/SiNO composites have excellent mechanical strength. The tensile strength value decreases to 54.6 MPa after heat-treated at 1000 degrees C for the surface reactions between the SiNO matrix and Si3N4 fibers. After heat-treated at 1550 degrees C, the composites have the tensile strength value of 24.2 MPa for the high strength retention rate of Si3N4 fibers at this temperature. Si3N4 (f)/SiNO composites have excellent room temperature dielectric properties and excellent dielectric stability in different frequency bands (7-18 GHz). The dielectric constant values vary from 3.69 to 3.75 while the dielectric loss attains the order of 10(-3). The dielectric constants and dielectric loss of Si3N4 (f)/SiNO composites are relatively stable from RT to 800 degrees C. The as-fabricated Si3N4 (f)/SiNO composites that have excellent high temperature resistance and dielectric properties are the ideal high temperature wave-transparent composites.

We investigated the degradation behavior of amorphous silicon nitride (Si 3 N 4 ) fibers in low air pressure and presumed the evolution mechanism. The obtained Si 3 N 4 fibers were characterized by tensile strength, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and elemental analysis after being annealed (air pressure: 1 Pa–0.1 MPa, temperature: 1000–1600 °C, dwell time: 0–4 h). When air pressure was lower than 100 Pa or higher than 1000 Pa the strength of fibers dropped sharply. Due to the moderate partial pressure of oxygen in 100–1000 Pa, both active oxidation and passive oxidation were restrained resulting in the best mechanical property of fibers in 100–1000 Pa. Besides air pressure, annealing time also affected the thermal behavior of fibers. Firstly Si 3 N 4 fibers were passive oxidized to form SiO 2 layer on the surface, and then Si 3 N 4 decomposed into free Si and SiO 2 released gaseous SiO. Finally crystallization inside of fibers and formation of nanowires on the outer surface played the leading role in the progress of degradation.

Self-toughness porous Si3N4 ceramics are prepared by surface modification of Si3N4 particles. The asfabricated porous Si3N4 ceramics have high number of three-dimensional cage structures that can cause the crack deflection, absorb the fracture stress to produce microcrack toughening effect and increase the structural reliability of porous Si3N4 ceramics. The flexural strength is 78.0 MPa. The dielectric constant and dielectric loss is 3.47 and 0.001 respectively. This kind of porous Si3N4 ceramics with special three-dimensional cage structures have high mechanical strength, good antioxidation properties and excellent dielectric properties. (C) 2020 Elsevier B.V. All rights reserved.

The Kirkendall effect in the formation of hollow structures mainly focuses on nanoscaled metal-metal reactions; few have been performed to fabricate micrometer scale ceramic hollow structures due to the diffusion difficulty. Here, through introduction of the liquid mesophase to accelerate the diffusion rates, we identify that a micrometer scale ceramic hollowing process could be achieved via the Kirkendall effect. The formation mechanism of the Cr2O3/Al2O3 solid solution hollow fibers is analyzed. The small Kirkendall voids appear at the interface between Cr2O3 and Al2O3 via a bulk diffusion process. Then, the core material diffuses along the pore surface through the liquid mesophase, which leads to the depletion of the center matter and forms the hollow structures. The introduction of the liquid mesophase is the key factor in the formation of Cr2O3/Al2O3 hollow fibers. The as-fabricated Cr2O3/Al2O3 hollow fibers have a pore size of 8 mu m and a shell thickness of 2 mu m. The hollow structure remains well after being heat-treated at 1400 degrees C for 100 h in air, which indicates that the hollow fibers have excellent high temperature resistance. This method confirms that micrometer-scale ceramic hollow fibers can be fabricated in a simple and low-cost method using commercial raw materials without pollutant emissions. We expect that our findings could offer new perspectives in fabricating micrometer-scale ceramic hollow structures, such as hollow sphere, tube, and heterotypic structures.

The application of Si3N4 ceramics is effectively limited due to the catastrophic failure which is caused by the oxidation weight gain. Nano-modification on the surface of Si3N4 micropowders is prepared to reduce the oxidation weight gain of Si3N4 ceramics. After modification, the Si3N4 particles are encapsulated completely by spherical nano-SiO2, which can obviously reduce the oxidation weight gain of the Si3N4 ceramics for the good sealing effect of dense SiO2 coatings. The finally oxidation weight gain rate of the Si3N4 ceramics reduces about 54.3% at 1580 degrees C. The nano-SiO2 coatings can also increase the surface area of Si3N4 particles, which can improve the sintering extent of the Si3N4 ceramics through accelerating the gradual strengthening of bonding necks between Si3N4 particles. The flexural strength of the Si3N4 ceramics without sintering additives is 78 MPa. The dielectric constant is 3.47 while the dielectric loss is 0.001, which satisfies the dielectric conditions of radomes. (C) 2018 Elsevier B.V. All rights reserved.

Soluble preceramic polymer to BN ceramic,polyborazine,was synthesized via condensation of boron trichloride and hexamethydisilazane followed by a polymerization process.The average molecular weight and structure of the prepolymer during polymerization,mechanism of polymerization,pyrolysis process,and chemical composition of the obtained ceramic were investigated using gel permeation chromatography,1H NMR,infrared spectra,thermogravimetric analysis and element analysis.The results indicate that hexamethydisilazane and NH3 were released during the polymerization process.The average weight molecular and ceramic yield(1 000℃ in N2) of the polyborazine are 7 582 and 41.6 wt%,respectively.The ceramic mainly formed at the temperature range of 400 to 600℃ and white BN ceramic with low carbon content was obtained after pyrolyzed in NH3 at 800℃.

Si-Zr-C-O fibers were obtained from polyzirconocarbosilane (PZCS), which was synthesized by the reaction of polysilacarbosilane (PSCS) and Zr(AcAc)(4). The high-temperature resistance and oxidation resistance performance of Si-Zr-C-O fibers were examined by measurements of X-ray diffraction (XRD), scanning electron microscope (SEM), auger electron spectroscopy (AES) and element analysis. The surface of Si-Zr-C-O fibers is smooth, and do not exhibit obvious disfigurements. The average diameter of the fibers is 11 mu m, the average tensile strength is 2.5 GPa. Si-Zr-C-O fibers have good high-temperature-resistant performance. Processing at 1450 and 1600 C, the tensile strength retention rate is 72% and 36% respectively. The chemical formula of Si-Zr-C-O fibers is SiC(0.99)HxO(0.1)Zr(y) after processing at 1800 degrees C, the oxygen content of the fibers decreases obviously. Si-Zr-C-O fibers also have good oxidation-resistance properties. After processing for 20 h at 1000 degrees C in the air, the tensile strength retention rate is 71.2%, after processing for 100 h, the tensile strength retention rate is 50%.

In order to prepare high-performance Si-Zr-C-O fibers,polyzirconocarbosilane(PZCS)was synthesized by the reaction of zirconium acetylacetonate(Zr(AcAc)_4)(AcAc:aletylacetanate)with polysilacarbosilane(PSCS)at normal pressure.Si-Zr-C-O fibers were obtained by melting spinning of PZCS,air curing and pyrolysis at 1200℃.The composition and structure of Si-Zr-C-O fibers was examined by X-ray diffraction,nuclear magnetic resonance,scanning electron microscope,auger electron spectroscopy and ele- ment analysis.The results show that the empirical formula of Si-Zr-C-O fibers is SiC_(1.24)H_xO_(0.56)Zr_(0.0129),and the tensile strength and diameter of the fibers are 2.5 GPa and 11μm,respectively.The results of SEM indicate that the surface of Si-Zr-C-O fibers is smooth, and there are no obvious disfigurements.The Si-Zr-C-O fibers are non-crystal fibers,which contain much free C and O with the form of multiplicity phase SiC_xO_y.The multiplicity phase of Zr bonding O exists in it.

In order to improve the performance of glass fiber reinforced resin matrix composite,the surface of the glass fibers should be treated to exert their reinforcing effect.In this paper,progress on the surface of glass fiber reinforced resin matrix composite is expatiated and the main problems and development direction are also discussed.

To prepare preceramic polymers in the SiBNC system,different synthetic routes of preceramic polymers and single source precursors for SiBN,SiBC and SiBNC ceramics known to date were compiled and critically reviewed.The synthetic routes of the preceramic polymers were classified into two groups: co-condensation of different molecular species and polycondensation of single source precursors.The latter one was paid more attention these days since the single source precursors exhibit structural features that are desired to be present in the final ceramics.Current situations of the preceramic polymers in the SiBNC system at home and abroad,as well as their prospects,were also discussed in this paper.

In order to improve the properties of SIC ceramic fibers polyzirconocarbosilane (PZCS) the precursor of Si-Zr-C-O ceramic fibers, was synthesized by the reaction of zirconium acetylacetonate (Zr(AcAc)(4)) at 420 degrees C with polysilacarbosilane (PSCS), which was prepared by the thermolysis and condensation of polydimethylsilane (PDMS). By the use of liquid PSCS, Zr could be distributed uniformly, and the sublimation of Zr(AcAc)4 could also be avoided. The empirical formula of PZCS was SiC1.94 H-X O-0.066 Zr-0.0104, the number-averaged molecular weight is in the range of 1300 similar to 2500. When the reaction of PSCS with Zr(AcAc)(4) proceeded, an enormous decrease in the number of Si-H bonds in PSCS was observed. The reaction mechanism was found to be that the increase of the molecular weight of PZCS was related to the cross-linking reaction of Si-H bonds with Zr(AcAc)4 which led to the generation of Si-O-Zr. There exists SiC4 and SiC3H structure in PZCS. The ceramic yield of PZCS was 82.4 %. We got the Si-Zr-C-O ceramic fibers using PZCS, whose chemical formula was SiC1.24 H-x O-0.56 Zr-0.0129 . The average tensile strength was 2.6 GPa, the average diameter was 11 mu m. Si-Zr-C-O fibers have good high-temperature-resistant performance. Processing at 1450 degrees C and 1600 degrees C, the tensile strength retention rate was 72% and 36 % respectively. Si-Zy-C-O fibers also have good oxidation-resistance properties. After processing 20 h at 1000 degrees C in the air, the tensile strength retention rate was 71.2%, after processing 100 h, the tensile strength retention rate was 50 %.

Due to many advanced properties such as high intensity,high modulus,high-temperature resistance,erosion resistance,oxidation resistance and low density,etc.,SiC ceramic can be widely applied in the fields of high techniques.Polymer-derived method has become one of the most popular methods to develop ceramic materials for its advantages.Adding hetero-elements into SiC ceramic precursor can modify its physical and chemical properties to improve the comprehensive property of the ceramic.In this paper,we mainly introduced the recent study progress of the hetero-elements containing SiC ceramic precursors,especially emphasizing the main varieties,the manufacturing craft and the properties of the hetero-elements containing SiC ceramic precursors and the corresponding ceramics.Our prospect of the study directions in the future was also pointed out.

Using polydimethylsilane as silica source, silica aerogels were synthesized via a polymer supercritical method. The silica aerogels, with an average pore diameter of 15 nm before carbon removal, are hydrophobic, and their specific surface area and pore volume are 27.68 m2/g and 0.1037 cm3/g respectively. After carbon removal, the silica aerogels, which have an average pore diameter of 3.23 nm, become hydrophilic. Their specific surface area and pore volume are 500.6 m2/g and 0.4043 cm3/g respectively. Moreover, the reaction mechanism of silica aerogels prepared by this processing method was discussed.