Synthesis and pyrolysis mechanism of a novel polymeric precursor for SiBN ternary ceramic fibers

A novel polymeric precursor polyborosilazane (PBSZ) for SiBN ternary ceramic fibers was successfully synthesized from trichlorosilane (HSiCl₃), boron trichloride (BCl₃) and hexamethyldisilazane (HDMZ) by a simple one step reaction process. The chemical structures and ceramic yield of the PBSZ precursors were investigated by NMR spectroscopy, FT-IR and TGA. The preparation of PBSZ fibers was conducted in a lab-scale melt-spinning equipment at a spinning speed of 130 m/min. SiBN ternary ceramic fibers were obtained after the non-fusible treatment and pyrolysis of PBSZ fibers in an NH₃ atmosphere. The pyrolysis mechanism, high-temperature behavior and morphologies of the SiBN ternary ceramic fibers were investigated by NMR, XRD, TEM and SEM. The obtained SiBN ternary ceramic fibers had good flexibility, and possessed a tensile strength of 0.84 GPa with a diameter of ∼18 μm. Furthermore, these SiBN ceramic fibers exhibited excellent thermal stability, and maintained the amorphous state up to 1600 °C.     

Effects of boron content on the microwave-transparent property and high-temperature stability of continuous SiBN fibers

Owing to their high-temperature stability and microwave-transparent properties, SiBN fibers are promising reinforcement materials for microwave windows or radomes in harsh environments. This work investigates the high-temperature properties of continuous SiBN fibers with different boron contents. Compared with Si₃N₄ fibers, the SiBN fibers demonstrated superior high-temperature stability, retaining their original tensile strength up to 1600°C with a smooth surface and amorphous structure. The boron-containing microstructure improved the microwave-transparent properties of the SiBN fibers by decreasing their dielectric constant and loss. The values were stabilized at temperatures up to 1200°C. The high-temperature stability and microwave transparency of the SiBN fibers were likely contributed by the synergistic effect of the amorphous BN phase and the Si–N–B networks (the main existing state of the boron atoms).     

Lyocell fibers as the precursor of carbon fibers

In this work, Lyocell fibers, used as carbon fiber precursors, were investigated. Lyocell fibers used for the carbon precursors and the carbon fibers themselves were produced in our laboratory. The mechanical properties morphology and structure of the precursors and the obtained carbon fibers were studied and compared to those of rayon. The results show that Lyocell fibers have higher tenacity and modulus, and better thermal stability than rayon fibers. Scanning electron microscopy (SEM) experiments show that Lyocell precursors have round cross‐sections and fewer defects in the fibers, while rayon fiber has an oval cross‐section and many defects. Wide angle X‐ray diffraction (WAXD) results for the Lyocell precursors indicate that the degree of crystallinity of the Lyocell precursor is higher than that of a rayon precursor. They also show that Lyocell based carbon fibers have better mechanical properties than those that are rayon‐based. WAXD data of the obtained carbon fibers show that the crystallinity of Lyocell‐based carbon fiber is higher than that of rayon‐based carbon fiber. It is concluded that the Lyocell fibers are better precursors for carbon fibers than rayon. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1941–1947, 2003     

Influences of mullite fibers on mechanical and thermal properties of silica-based ceramic cores

Mullite fibers composite silica-based ceramic cores were successfully prepared by injection molding. The effects of mullite fibers on the mechanical and thermal properties of ceramic cores were investigated. The results indicated that the linear shrinkage was significantly decreased and the porosity was gradually increased with the increase of mullite fibers. In addition, the flexural strength for the room temperature and the simulated casting temperature of 1500°C was increased to a maximum value when the content of mullite fibers was about 1 wt.%, and then decreased with the increase of mullite fibers. The mullite fibers of 1 wt.% presented excellent mechanical properties with a linear shrinkage of .65%, a porosity of 6.96%, and a flexural strength of 17 MPa at room temperature and 34.83 MPa at the simulated casting temperature of 1500°C. Besides, the change in microstructure and properties in various contents of mullite fibers were analyzed.     

The formation of chemical/structural gradients in strong covalent bonded SiBN fibers under active nitrogen atmosphere

In this work, SiBN fibers with chemical/structural gradients under the radial direction were fabricated by one-step treatment at 1700 °C under active nitrogen atmosphere. The formation of chemical gradient should be contributed to the boron atoms diffusion from the fiber interior to the surface, under the driven force of BN bond-breaking in Si–N–B networks to form stable BN bonds in the BNB networks at the fiber surface. Meanwhile, the structural gradients with graded phase separation of Si–N–B networks and crystallization behavior of Si₃N₄ phase could form under the effect of graded boron content distribution. The SiBN fibers with chemical/structural gradients showed better high-temperature stability due to the possibly synergistic effect of the decomposition resistant of boron-rich surface and the optimization of graded boron distribution. We expect our finding as a universal method to fabricate chemical/structural gradients on other boron-containing multinary ceramics with various shapes.     

Pyrolysis of polyborosilazane and its conversion into SiBN ceramic

Abstract

The polyborosilazane was prepared by coammonolysis of boron trichloride and methyldichlorosilane with hexamethyldisilazane. The decarburisation process of the polyborosilazane under ammonia was studied. The results suggested carbon was effectively removed under ammonia and the removal of carbon mainly occurred during the temperature range of 400–600°C. After pyrolysed at 900°C under ammonia, the carbon content was only 0·29 wt-%. Further heat treatment at 1500°C under argon yielded SiBN ceramic. The structure and morphological properties of SiBN ceramic were studied by solid ²⁹Si nuclear magnetic resonance, X-ray diffraction and scanning electron microscopy. The results showed that the SiBN ceramic was amorphous and possessed a smooth surface. The contents of boron, silicon and nitrogen in the SiBN ceramic are 10·9, 42·6 and 38·48 wt-% respectively.     

Mechanical and electromechanical properties of piezoelectric ceramic fibers drawn by the alginate gelation method

Piezoelectric ceramic fibers are widely used in piezocomposite devices. The various methods that are used to draw ceramic fibers differ in the way the fiber form is obtained, which in return closely affects the density, uniformity and the properties of the fibers that are obtained at the end. In this study, the processing-property relationship in the piezoceramic fibers drawn using the alginate gelation method is investigated, with a focus on the mechanical and electrical properties of individual fibers. Fibers with a Weibull strength of 65.3 MPa, remanent polarization of 22 μC/cm² and a total bipolar field induced strain of 0.25% under an electric field of 2.5 kV/mm, piezoelectric coefficient of 300 pm/V and dielectric constant of 3323 were produced. 1-3 piezocomposite devices prepared from these fibers had a 6 dB higher free-field voltage sensitivity and a 50% wider bandwidth compared to a solid disk transducer of the same dimensions.     

The effects of adding nano-alumina filler on the properties of polymer-derived SiC coating

In this paper, SiC coating was prepared using the polymer-derived ceramic method; then the effect of nano-alumina as a filler material was studied. First of all, polycarbosilane(PCS) was dissolved in xylene; after that, different amounts of nano-alumina particles were added to the solution. The coating was deposited on the alumina substrate using the dip-coating method; this was followed by sintering at 1200℃. The phase content and microstructure of the samples were studied by X-ray diffraction and scanning electron microscope methods, respectively. Nanohardness, Young's modulus, and coating adhesion were investigated by a nanoindentation method. The sheet resistance was evaluated using the four-point probe technique; also, the wear resistance of the coating was studied by applying the pin-on-disk method. It was found that the addition of the nano-alumina filler up to 20 wt% drastically improved the adhesion and wear resistance of the SiC coating.     

Zirconia-alumina multiphase ceramic fibers with exceptional thermal stability by melt-spinning from solid ceramic precursor

Zirconia-alumina multiphase ceramic fibers with 80 wt% (Z80A20 fiber) and 10 wt% (Z10A90 fiber) proportions of zirconia were prepared via melt-spinning and calcination from solid ceramic precursors synthesized by controllable hydrolysis of metallorganics. The zirconia-alumina multiphase fibers had a diameter of about 10 µm and were evenly distributed with alumina and zirconia grains. The Z80A20 and Z10A90 ceramic fibers had the highest filament tensile strength of 1.78 GPa and 1.87 GPa, respectively, with a peak value of 2.62 GPa and 2.71 GPa. The Z80A20 ceramic fiber has superior thermal stability compared to the Z10A90 ceramic fiber and a higher rate of filament strength retention due to the stability in grain size. After heat treatment at 1100 °C, 1200 °C, and 1300 °C for 1 h respectively, the filament tensile strength retention rate of Z80A20 ceramic fibers was 87 %, 80 %, and 40 %. While Z10A90 ceramic fiber was fragile after being heated at 1300 °C. The results showed that the high zirconia content facilitated the fiber's thermal stability.     

Ultralight polymer-derived ceramic aerogels with wide bandwidth and effective electromagnetic absorption properties

In this work, ultralight polymer-derived ceramic aerogels (PDCA) were prepared by a facile method of hydrosilylation crosslink and freeze drying. The electromagnetic absorption properties of ultralight PDCA were investigated for the first time. The PDCA pyrolyzed at 1000 °C shows a uniform three-dimensional (3D) framework with a low bulk density of ∼0.19 g/cm³ and high surface area of 134.48 m²/g. The electromagnetic properties of PDCA were characterized by a vector network analyzer. The minimum reflection and absorption bandwidth of PDCA pyrolyzed at 1000 °C, 1200 °C and 1400 °C are −43.37 dB @ 7.6 GHz, −42.01 dB@12.5 GHz, and −31.69 dB@17.3 GHz and 3.8 GHz, 6.6 GHz and 4.2 GHz, respectively, at the frequency range of 2–18 GHz. The strong electromagnetic absorption and wide bandwidth features of PDCA could be attributed to the multiple reflections of microwaves in the 3D framework, as well as the high dielectric loss and proper conductivity.     

High-temperature phase and microstructure evolution of polymer-derived SiZrCN and SiZrBCN ceramic nanocomposites

A zirconium and a zirconium/boron containing single-source precursor were synthesized via chemical modification of a commercially available polysilazane (Durazane 1800) with tetrakis (dimethylamido) zirconium (IV) (TDMAZ) as well as with both TDMAZ and borane dimethyl sulfide complex, respectively. The polymer-to-ceramic transformation of the precursors into SiZrCN and SiZrBCN ceramics as well as the thermal evolution of their phase composition and microstructure was studied. The pyrolysis of the precursors led to the formation of amorphous SiZrCN and SiZrBCN ceramics. Interestingly, the as prepared SiZrBCN ceramic was single-phasic and fully featureless; whereas SiZrCN exhibited the presence of nano-sized ZrO₂ particles; however, only very localized in close proximity to internal surfaces. Heat treatment at higher temperatures induced crystallization processes in both prepared ceramics. Thus, at temperatures beyond 1500°C, cubic ZrC_xN_y, β-Si₃N₄ as well as β-SiC were generated. It was shown that the incorporation of B into SiZrCN suppressed the crystallization of ZrC_xN_y and, in addition, impeded the reaction of SiN_x with C, resulting in an improved thermal stability of SiZrBCN compared to SiZrCN ceramic. Moreover boron was shown to be mainly located in the sp²-hybridized “free” carbon present in SiZrBCN, forming a turbostratic BCN phase which has been unequivocally detected by means of high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS).     

聚合物先驱体转化法制备陶瓷MEMS器件

简述了聚合物先驱体转化陶瓷(PDC)法在制备陶瓷纤维、陶瓷涂层和陶瓷粘接等方面的应用,重点介绍了PDC法在微机电系统(MEMS)器件制造中的发展概况。耐高温MEMS的发展面临两大挑战:高温材料的选择和发展合适的微加工技术。指出了传统加工技术制备陶瓷MEMS的不足,着重介绍了PDC法制备陶瓷MEMS的优缺点。以国外采用PDC法制备陶瓷MEMS的几种典型器件为例,指出该方法在制备陶瓷MEMS器件中存在的问题和未来的发展方向。     

Effects of micro-sized mullite on cristobalite crystallization and properties of silica-based ceramic cores

Silica-based ceramic cores are extensively used in investment casting process, during which they must exhibit sufficient flexural strength and deformation resistance. In this study, micro-sized mullite was used as an additive to silica-based ceramic cores to optimize their high temperature properties. To investigate the effects of micro-sized mullite on cristobalite crystallization, mechanical and thermal properties of silica-based ceramic cores, ceramic cores with different amounts of micro-sized mullite were fabricated. The XRD results showed that additional micro-sized mullite diminished the crystallization of cristobalite at high temperatures, primarily caused by the mullite related compressive stresses on the surface regions of fused silica particles. Three-point bending tests and SEM results showed that micro-sized mullite had a more significant effect on the flexural strength of ceramic cores compared with conventional additives. Particularly, the fracture mechanism of silica-based ceramic cores had been changed from intergranular fracture into a mixed fracture consisting of both intergranular and transgranular fracture. The mechanical and thermal properties of ceramic cores were all reduced slightly as the mullite content exceed 4.6 wt%. Hence, to optimize the properties of silica-based ceramic cores, the micro-sized mullite content should not exceed 4.6 wt%.     

Effect of heat treatment on the structure and properties of Lyocell fibers

Lyocell fibers were heat‐treated under different conditions. The tensile strength and initial modulus of the heat‐treated Lyocell fibers increased sharply, whereas the elongation at break decreased. Moreover, applying tension to the fibers during the heat treatment further improved the tensile strength and initial modulus. In addition, the crystallinity of the heat‐treated fibers increased slightly, and there was no obvious change with an increase in the tension; the general orientation of the heat‐treated fibers increased, the crystalline orientation little changed, and the amorphous orientation improved. Also, the improved mechanical properties of the Lyocell fibers via the heat treatment could not be preserved for long. The reason may be that the crystalline structure of the Lyocell fibers was not destroyed and no new crystallites were formed during the heat‐treatment process. Therefore, the heat‐treated Lyocell fibers reverted to their original state with time because there was no crosslinking point to fix the orientation, although the cellulose molecules of the amorphous region of the Lyocell fibers were more oriented by the heat treatment with tension. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1738–1743, 2006     

Hierarchically structured polymer-derived ceramic fibers by electrospinning and catalyst-assisted pyrolysis

Hierarchically structured polymer-derived ceramic fibers were successfully produced by electrospinning a commercially available preceramic polymer to which a cobalt-based catalyst precursor was added, followed by pyrolysis in nitrogen at temperatures ranging from 1250 to 1400 °C. The nanowires formed via the vapor–liquid–solid (VLS) mechanism, involving the reaction of SiO and CO gases, generated from the decomposition of the polymer-derived-ceramic at high temperature, with the heating atmosphere assisted by the presence of nano-sized CoSi droplets. The main crystalline phase for the nanowires was Si₃N₄ below 1350 °C, and Si₂N₂O at 1400 °C, and the amount of nanowires increased with increasing heating temperature. Hierarchically structured fiber mats possessed a higher specific surface area (14.45 m²/g) than that of a sample produced without the cobalt catalyst (4.37 m²/g).     

Comparison of the structures and properties of Lyocell fibers from high hemicellulose pulp and high α‐cellulose pulp

Lyocell fibers were produced from a cheap pulp with a high hemicellulose content and from a conventional pulp with a high α‐cellulose content. The mechanical properties, supermolecular structure, fibrillation resistance, and dyeing properties as well as the fibril aggregation size of the high hemicellulose Lyocell fiber and high α‐cellulose Lyocell fiber were compared. The results showed that the high hemicellulose spinning solution could be processed at a higher concentration, which improved the mechanical properties and the efficiency of the fiber process. Compared with the high α‐cellulose Lyocell fiber, the high hemicellulose Lyocell fiber had better fibrillation resistance and dyeing properties. Therefore, it is feasible that this cheap pulp with a high hemicellulose content can be used as a raw material for producing Lyocell fibers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Structure and properties of cellulose fibers from ionic liquids

1‐Butyl‐3‐methylimidazolium chloride ([BMIM]Cl) was used as a solvent for cellulose, the rheological behavior of the cellulose/[BMIM]Cl solution was studied, and the fibers were spun with a dry‐jet–wet‐spinning process. In addition, the structure and properties of the prepared cellulose fibers were investigated and compared with those of lyocell fibers. The results showed that the cellulose/[BMIM]Cl solution was a typical shear‐thinning fluid, and the temperature had little influence on the apparent viscosity of the solution when the shear rate was higher than 100 s^?1. In addition, the prepared fibers had a cellulose II crystal structure just like that of lyocell fibers, and the orientation and crystallinity of the fibers increased with the draw ratio increasing, so the mechanical properties of the fibers improved. Fibers with a tenacity of 4.28cN/dtex and a modulus of 56.8 cN/dtex were prepared. Moreover, the fibers had a smooth surface as well as a round and compact structure, and the dyeing and antifibrillation properties of the fibers were similar to those of lyocell fibers; however, the color of these dyed fibers was brighter than that of lyocell fibers. Therefore, these fibers could be a new kind of environmentally friendly cellulose fiber following lyocell fibers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Direct ink writing of ceramic matrix composite structures

We present the development of an ink containing chopped fibers that is suitable for direct ink writing (DIW), enabling to obtain ceramic matrix composite (CMC) structures with complex shape. We take advantage of the unique formability opportunities provided by the use of a preceramic polymer as both polymeric binder and ceramic source. Inks suitable for the extrusion of fine filaments (<1 mm diameter) and containing a relatively high amount of fibers (>30 vol% for a nozzle diameter of 840 μm) were formulated. Despite some optimization of ink rheology still being needed, complex CMC structures with porosity of ~75% and compressive strength of ~4 MPa were successfully printed. The process is of particular interest for its ability to orient the fibers in the extrusion direction due to the shear stresses generated at the nozzle tip. This phenomenon was observed in the production of polymer matrix composites, but it is here employed for the first time for the production of ceramic matrix ones. The possibility to align high aspect ratio fillers using DIW opens the path to layer‐by‐layer design for optimizing the mechanical and microstructural properties within a printed object, and could potentially be extended to other types of fillers.     

Development of novel refractory ceramic continuous fibers of fly ash and comparison of mechanical properties with those of E-glass fibers using the Weibull distribution

This study aimed to develop manufacturing technology for high-strength refractory ceramic fibers (RCFs) using fly ash, which is a highly promising material for the exterior and thermal insulation industry. The technology also contributes to reducing the environmental pollution caused by landfilling fly ash after coal is burned. Fly ash discharged from a thermal power plant, which had aluminosilicate chemical compositions, was used as the main material. As auxiliary materials, basalt, anorthite, feldspar, dolomite, and calcite were used to adjust the melt flowability, and frit, silica sand, and burr stone were used to lower the melting temperature. Moreover, the development of aluminosilicate fly ash fiber has the advantages of lower cost for raw materials and processing. Fly ash and natural rocks are inexpensive, and most of all, unlike the case for glass fiber production, the high cost of B₂O₃ is not a necessary expense. Fly ash is retrieved in powder form, which is advantageous compared to the starting materials for glass; the grinding process of raw materials can be skipped. From the fibrilization index calculation, we showed that the spinnability was influenced by the chemical composition of the salt-forming oxides in the fly ash compounds. We also found a correlation between the winding speed and the fiber diameter. The mechanical properties of a series of fly ash fibers were assessed by the Weibull distribution and then compared with those of the E-glass fibers that were melt-spun under an analogous condition.     

Influence of the cross‐sectional shape on the structure and properties of polyester fibers

The effects of the fiber cross‐sectional shape on the structure and properties of polyester fibers were investigated. Fully drawn yarn (FDY) polyester fibers (167 dtex and 48 filaments) were produced under the same spinning conditions used in a spinning plant. The only difference between the fibers was their cross‐sectional shapes. Four different cross‐sectional shapes were chosen for the experimental work: round, hollow‐round, trilobal, and hollow‐trilobal. The crystallinity and values of the maximum stress, maximum strain, modulus, yield stress, shrinkage in boiling water, and unevenness of the fibers were determined. The difference in the cross‐sectional shapes influenced the modulus, maximum strain, yield stress, and shrinkage in boiling water. No effects on the crystallinity and maximum stress were observed. The results suggested that the hollow fibers had higher amorphous orientation than the full fibers. The hollow‐round fiber had the highest unevenness value. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2615–2621, 2007