Polymer precursor‐derived HfC–SiC ultrahigh‐temperature ceramic nanocomposites

Due to poor mechanical properties and antioxidation properties, etc of single phase ultrahigh‐temperature ceramics (UHTCs), the second phase such as SiC was usually introduced for improving those properties. Herein, a novel stratagem for synthesis of binary HfC–SiC ceramics has been presented. A Hf–O–Hf polymer as a HfO₂ precursor has been synthesized for preparing soluble HfC–SiC precursors with high solid content and low viscosity solutions without additional organic solvents. The structure of PHO was characterized by FTIR and ¹H‐NMR, the crystalline behavior and morphologies of polymer‐derived ceramics were identified by XRD, SEM‐EDS, and TEM. It was shown that PHO firstly transformed into HfO₂, and then reacted with in situ carbon derived from DVB and PCS thus producing cubic HfC through carbothermal reduction. In addition, the obtained HfC–SiC nanopowders exhibited spherical morphology with a diameter less than 100 nm, while the Hf, Si, and C are homogeneously distributed.     

Role of single‐source‐precursor structure on microstructure and electromagnetic properties of CNTs‐SiCN nanocomposites

Novel single‐source‐precursors (SSPs), namely carbon nanotube modified poly (methylvinyl) silazane (CNTs‐HTT 1800), were synthesized via amidation reaction of poly (methylvinyl) silazane (HTT 1800) with carboxylic acid functionalized carbon nanotubes (CNTs‐COOH) at the assistance of ZnCl₂ catalyst, which was confirmed by means of Fourier transform infrared spectra (FT IR) and transmission electron microscopy (TEM). Besides, the TEM results unambiguously show the homogeneous distribution of the CNTs in the matrix of SSPs while serious aggregation of the CNTs in the matrix of physically‐blended‐precursor. Crack‐free monolithic silicon carbonitride modified by carbon nanotubes ceramic nanocomposites (CNTs‐SiCN) were prepared through pyrolysis of the obtained SSP green bodies at 1000°C. Due to the strong influence of polymer structure on the microstructure of final ceramics, the SSP‐derived CNTs‐SiCN nanocomposites clearly show the homogeneous distribution of the CNTs in the SiCN matrix while the physically‐blended‐precursor derived CNTs‐SiCN nanocomposites exhibit serious aggregation and entangling of the CNTs in the SiCN matrix. With the same CNT content in the feed, the SSP‐derived CNTs‐SiCN nanocomposites possess significant improvements of electromagnetic (EM) absorbing properties compared to those from physically‐blended‐precursors, due to the quality of the dispersion of CNTs in the ceramic matrices.     

Preparation and high-temperature performance of HfC-based nanocomposites derived from precursor with Hf-(O,N) bonds

A novel precursor was synthesized by reacting hafnium chloride with dicyandiamide and dimethylformamide. The precursor was characterized via FT-IR and NMR, as well as TG. Subsequently, the precursor was annealed in Ar over a range of temperatures from 1000 °C to 2000 °C, and the microstructural evolution of the ceramics was investigated by XRD, XPS, and TEM. The results show that the carbothermal reduction of the precursor starts at 1150 °C and the ceramic yields at 1500 °C reach 44.6 wt%. The obtained powders exhibit a uniform distribution and are composed of N-doped HfC and graphite. The N-doped structure postponed the oxidation of the HfC(N) ceramics. The HfC(N) ceramics were first oxidized to yield HfO₂, carbon, and nitrogen, and then the carbon was oxidized with the evolution of CO₂. The presented synthesis method is believed to be applicable to the preparation of other high-performance ceramics.     

Fabrication of nearly stoichiometric polycrystalline SiC fibers with excellent high‐temperature stability up to 1900°C

Due to the extensive applications of SiC fiber‐reinforced composite materials in the fields of aviation, aerospace, and nuclear power, there are increasing demands for SiC fibers with both excellent mechanical performance and high‐temperature stability. In this work, nearly stoichiometric polycrystalline SiC fibers were fabricated using amorphous Si–C–Al–O fibers with excess carbon and oxygen (C/Si = 1.34, O content: 7.74 wt%). The nearly stoichiometric composition (C/Si = 1.05) of the product fibers was achieved by thermal decomposition of the starting fibers. The fibers were well‐crystallized with grain sizes of ~200 nm due to sintering at a high temperature of 1900°C. The fibers exhibited a high tensile strength and a high elastic modulus and were composed of SiC grains with twins and stacking‐faults, exhibiting intragranular fracture behavior. Furthermore, the fibers maintained their original tensile strength after being maintained at 1800°C for 5 hour or at 1900°C for 1 hour under an inert atmosphere, and they exhibited a high strength retention (97%) after exposure at 1300°C for 1 hour under air. The high‐temperature stability and creep resistance of the fibers were comparable to that of commercial Hi‐Nicalon S and Tyranno SA fibers.     

High‐temperature mechanical behavior of ZrB2‐based composites with micrometer‐ and nano‐sized SiC particles

Using micrometer‐ and nano‐sized SiC particles as reinforcement phase, two ZrB₂‐SiC composites with high strength up to 1600°C were prepared using high‐energy ball milling, followed by hot pressing. The composite microstructure comprised finer equiaxed ZrB₂ and SiC grains and intergranular amorphous phase. The temperature dependency of flexure strength related to the initial particle size of SiC. In the case of micrometer‐sized SiC, the high‐temperature strength was improved up to 1500°C compared to room‐temperature strength, but the strength degraded at 1600°C, with strength values of 600‐770 MPa. In the case of nano‐sized SiC, the enhanced high‐temperature strength was observed up to 1600°C, with strength values of 680‐840 MPa.     

Oxygen diffusion mechanisms during high‐temperature oxidation of ZrB2‐SiC

Oxygen diffusion mechanisms during oxidation of ZrB₂‐30 vol% SiC were explored at temperatures of 1500°C and 1650°C using an ¹⁸O tracer technique. Double oxidation experiments in ¹⁶O₂ and ¹⁸O₂ were performed using a modified resistive heating system. A combination of scanning electron microscopy, energy‐dispersive spectroscopy, and time‐of‐flight secondary ion mass spectrometry was used to characterize the borosilicate and ZrO₂ oxidation products. Oxygen exchange with the borosilicate network was observed to occur quickly at the oxygen‐borosilicate surface at both 1500°C and 1650°C, while evidence of oxygen permeation was only observed at 1650°C for short time (<1 min) exposures. At longer times, >5‐9 min, complete oxygen exchange throughout both the borosilicate glass and ZrO₂ was observed at both temperatures preventing identification of the oxygen transport mechanisms, but demonstrating that oxygen transport is rapid in both oxide phases.     

Preparation of carbon/carbon‐ultra high temperature ceramics composites with ultra high temperature ceramics coating

In order to increase the oxidation resistance of carbon/carbon (C/C) composites at long‐term high temperature, C/C‐Ultra High Temperature Ceramics composites (UHTCs) with a dual‐layer UHTCs oxidation coating was successfully designed and fabricated. The microstructure and ablation resistance were investigated and discussed. After ablation in arc‐heated wind tunnel with temperature being 2200°C for 1000s, the mass ablation rate and linear ablation rate were ?1.9 × 10^?2 mg/cm²s and 2.9 × 10^?5 mm/s, respectively. The formation of thermodynamically compatible oxide scale including ZrO₂ skeleton and SiO₂ or Zr–Si–O glass on the surface were mainly contributed to the excellent ablation resistance of the composite.     

Study on the high‐temperature behavior and rehydration characteristics of hardened cement paste

The high‐temperature behavior and rehydration characteristics of the hardened cement paste and their mechanisms have been studied in this paper. X‐ray diffraction and thermogravimetry are used to establish the effect of elevated temperatures on the mineralogical changes that occurred in the hardened cement paste. The change of microstructure was characterized by scanning electron microscopy. The results showed that with the temperature increased, the compressive strength of hardened cement paste first increased and then decreased. According to micromeasurements, at 400°C, the porosity and average pore diameter of hardened cement paste increased slightly, while at 800°C, the porosity and average pore diameter of hardened cement paste increased sharply. When hardened cement paste was cured after exposing to 400°C, its pore structure and phase composition had no change, while when hardened cement paste was cured after exposing to 800°C, there are new hydration products, and its pore structure may be finer, but it cannot fully recover to the original state. Copyright © 2014 John Wiley & Sons, Ltd.     

Preparation and magnetic properties of Fe/Si/C/N ceramics derived from a polymeric precursor

Iron‐containing polysilazanes (PSZI) were prepared by the amine displacement reaction along with heat‐induced vinyl crosslinking reactions between Fe[N(SiMe₂Vi)₂]₃ (Vi = ? CH?CH₂) and polysilazane containing ? Si? Vi (PVSZ). The PSZIs were converted into magnetic ceramics by the pyrolysis in N₂. The ceramics produced were investigated by X‐ray diffraction, transmission electron microscope and vibrating sample magnetometer at room temperature. It was indicated that α‐Fe is the only magnetic crystalline embedded in the amorphous Si/C/N‐based matrix from 500 to 900°C. Moreover, the sample prepared at 500°C showed few hysteresis at room temperature, consistent with the behavior of superparamagnetic particles, which was confirmed by the zero‐field‐cooled and field‐cooled magnetization measurement. Additionally, the results indicated that the magnetic properties of the ceramics could be tuned by controlling the content of iron and the pyrolysis temperature. This flexibility may be advantageous for some particular magnetic materials applications. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Laser ablation behavior of SiHfC-based ceramics prepared from a single-source precursor: Effects of Hf-incorporation into SiC

Laser ablation test of SiHfC-based ceramic nanocomposites as well as ceramic matrix composites (CMCs) was conducted by exposure to a CO₂ laser beam in air. Laser ablation behavior and possible degradation mechanisms of dense monolithic HfC/SiC ceramic nanocomposites as well as of C_f/SiHfC CMCs were investigated. Dense SiC monoliths and C_f/SiC CMCs were exposed to same laser ablation conditions and considered as reference materials. The evolution of microstructure and chemical/phase composition of the studied ceramics was addressed by scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX) as well as by X-ray diffraction. The results reveal that from the center to the edge of the damaged region of the materials three sections with different surface morphologies and ablation mechanisms are identified. The comparation between the SiC-based monoliths and CMCs with and without Hf demonstrates the positive effects of Hf-incorporation on their laser ablation resistance.     

Fabrication of dense polymer‐derived silicon carbonitride ceramic bulks by precursor infiltration and pyrolysis processes without losing piezoresistivity

Dense polymer‐derived silicon carbonitride (SiCN) ceramic bulks were fabricated by powder consolidation following precursor infiltration and pyrolysis (PIP) densification. The density and open porosity of the ceramics varied from 1.42 g/cm³ and 32.75% before the PIP to 2.29 g/cm³ and 3.64% after the PIP, respectively. The electrical conductivity of the ceramics sharply increased from 6.26 × 10^?10 S/cm before the PIP process to 3.20 × 10^?7 S/cm after the 1st cycle of PIP and then gradually increased to 6.89 × 10^?6 S/cm after four cycles of PIP. However, the piezoresistive coefficient did not change with the PIP. The Raman and electron paramagnetic resonance results show that the graphitization level of free carbon in ceramics derived from PIP was higher than the ceramics derived from powder consolidation. The high graphitization level of free carbon leads to a high conductivity, and thus the conductivity of ceramics increased significantly after the PIP process. The carbon cluster size, which is related to the gauge factor of piezoresistivity, did not change significantly after the PIP process; thus, the gauge factor did not change significantly. Dense, large‐scale polymer‐derived ceramics were fabricated by combined conventional powder consolidation and PIP without the loss of piezoresistivity. These ceramics have potential application as both structural and functional components that can bear loads as well as monitor variations in external stress.     

Facile one‐step high‐temperature spray pyrolysis route toward metal carbide nanopowders

Fine ultrahigh‐temperature ceramic (UHTC) powders have found very important applications in many fields. In this work, a facile high‐temperature spray pyrolysis (HTSP) approach is implemented for the synthesis of HfC and TaC UHTC nanopowders starting from organic solvent (e.g., ethanol or 1‐pentanol) solutions of metal precursors (HfCl₄ or TaCl₅). It is proposed that, during HTSP, the precursor solution droplets would continuously undergo rapid drying, thermolysis (i.e., removal of low molecular weight species such as H₂, H₂O, and CO), and finally in situ carbothermal reduction (CTR) process to give rise to metal carbide nanopowders. The as‐obtained materials are shown by SEM as uniform and separated nanoparticles (~90 nm), whereas TEM reveals the carbide (e.g., HfC) nanoparticles are actually even smaller (~10‐20 nm) and embedded in amorphous carbon from excess solvent decomposition. It is found that among different processing parameters, the organic solvent used and the metal precursor concentration could largely influence the formation of metal carbide. In addition, lower HTSP temperatures (≤~1500°C for HfC) only lead to oxide‐carbon mixtures while higher temperatures (≥~1650°C) promote carbide formation. The HTSP method developed in this work is simple, low‐cost and efficient, and could potentially be optimized further for future large‐scale manufacturing of ultrafine UHTC nanopowders.     

Phase control during synthesis of nanocrystalline ultrahigh temperature tantalum‐hafnium diboride powders

Ta_1?xHf_xB₂ material is attractive for various aerospace applications. In this study, 2 low‐cost approaches were adopted to synthesize nanocrystalline Ta_0.5Hf_0.5B₂ solid solution and related composite powders. The first was based on carbothermal reduction reaction (CTR) of intimately mixed tantalum‐hafnium‐boron oxide(s) and carbon obtained from aqueous solution processing of TaCl₅, HfCl₄, B₂O₃, and sucrose as precursors. It was found that when using this method, due to the low solubility of each other for Ta₂O₅ and HfO₂ and the difference in reactivity of those 2 oxides with carbon (as well as B₂O₃), individual TaB₂ (‐rich) and HfB₂ phases always form separately. Those borides tend to remain phase separated due to the slow inter‐diffusion between them. However, it was observed that addition of copper “catalyst” noticeably accelerates the inter‐diffusion and the solid solution formation. The second approach was based on alkali metal reduction reaction, in which TaCl₅ and HfCl₄ are directly reacted with sodium borohydride (NaBH₄). This method yields a single phase Ta_0.5Hf_0.5B₂ solid solution nanopowders in one step at much lower temperatures (e.g., 700°C) by avoiding the oxides formation and the associated phase separation of individual borides as observed in the CTR‐based process.     

Synthesis of SiC nanowires via catalyst-free pyrolysis of silicon-containing carbon materials derived from a hybrid precursor

SiC nanowires were successfully synthesized without catalyst by pyrolysis of silicon-containing pitch-derived carbon materials in a closed graphite crucible. These silicon-containing carbon materials were obtained by homogenization and co-carbonization of a hybrid precursor consisting of the toluene soluble fraction of coal tar pitch with polycarbosilane (PCS). The composition, morphology and structure of the nanowires were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The influence of pyrolysis temperature on the growth of the nanowires was investigated by Fourier transform infrared spectroscopy (FTIR) and thermo-gravimetry coupled with mass spectroscopy (TG-MS) analysis. The results indicate that the growth of the SiC nanowires starts at around 1200 °C. As the pyrolysis temperature increases to 1300–1500 °C, a large quantity of nanowires are formed on the top surface of the pitch-derived carbon substrate. In addition, increasing the pyrolysis temperature leads to an increase in the average diameter and a change in the typical morphology produced. The synthesized SiC nanowires have single-crystalline structure and are grown along the [111] direction with numerous stacking faults and twins. The vapor-solid (VS) mechanism may be responsible for the growth process of the SiC nanowires.     

Improvement of the tribological behavior of ultra‐high‐molecular‐weight polyethylene by incorporation of poly (phenyl p‐hydroxyzoate)

Ultra‐high‐molecular‐weight polyethylene/poly (phenyl p‐hydroxyzoate) composites (coded as UHMWPE/PPHZ) were prepared by compression molding. The effects of the poly (phenyl p‐hydroxyzoate) on the tribological properties of the UHMWPE/PPHZ composites were investigated, based on the evaluations of the tribological properties of the composites with various compositions and the examinations of the worn steel surfaces and composites structures by means of scanning electron microscopy and transmission electron microscopy. It was found that the incorporation of the PPHZ led to a significant decrease in the wear rate of the composites. The composites with the volume fraction of the PPHZ particulates within 45% ～ 75% showed the best wear resistance. The friction coefficient of the UHMWPE/PPHZ composites decreased with increasing load and sliding velocity, while the wear rates increased with increasing load. This was attributed to the enhanced softening and plastic deformation of the composites at elevated load or sliding velocity. The UHMWPE/PPHZ composites of different compositions had differences in the microstructures and the transfer film characteristics on the counterpart steel surface as well. This accounted for their different friction and wear behaviors. The transfer film of the UHMWPE/PPHZ composites appeared to be thinner and more coherent, which was largely responsible for their better wear resistance of t composite than the UHMWPE matrix. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2336–2343, 2005     

Phase evolution and microstructure characteristics during the carbothermal reduction of Hf(C,N,O) ceramics derived from a precursor

In this study, nanosized Hf(C,N,O) ceramics were successfully prepared from a novel precursor synthesised by combining HfCl₄ with ethylenediamine and dimethylformamide. Subsequently, the carbothermal reduction of these Hf(C,N,O) ceramics into hafnium carbide was investigated. The Hf(C,N,O) ceramics comprised Hf₂ON₂ and HfO₂ nanocrystals and amorphous carbon. Upon carbothermal reduction, conversion began at 1300 °C, when HfC first appeared, and continued to completion at 1500 °C, resulting in irregularly shaped crystallites measuring 50–150 nm. Upon increasing the dwelling time, the oxides were completely converted into carbides at 1400 °C. Furthermore, nitrogen was introduced into the reaction to catalyse the conversion of oxides into carbides considering the beneficial gas–solid reaction between CO and Hf₂ON₂. We expect that the ceramics prepared in this study will be suitable for the fabrication of high-performance composite ceramics, with properties superior to those of current materials.     

Evaluation of poly(methylsilane‐carbosilane) synthesized from methyl‐dichlorosilane and chloromethyldichloromethylsilane as a precursor for SiC

A new poly(methylsilane‐carbosilane) (PMSCS) for silicon carbide precursor was synthesized by Wurtz‐type copolycondensation of methyldichlorosilane (MeHSiCl₂) with chloromethyldichloromethylsilane (ClCH₂MeSiCl₂) and terminated with vinylmagnesium chloride (ViMgCl). The use of insufficient sodium made the reaction more economic and safe. By changing the ratios of two monomers and the end‐block agent, the properties of the obtained PMSCS and the C/Si ratio of its derived ceramic could be tuned. Upon pyrolysis at 1000 °C under argon, silicon carbide with nearly stoichiometric C/Si ratio and low oxygen content was obtained in 64% of ceramic yield. PMSCS showed high potential as an economical SiC ceramic precursor for the fabrication of SiC matrix, coating, and adhesives. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46610.     

Solidification of welded SiC–ZrB2–ZrC ceramics

A silicon carbide‐based ceramic, containing 50 vol% SiC, 35 vol% ZrB₂, and 15 vol% ZrC was plasma arc welded to produce continuous fusion joints with varying penetration depth. The parent material was preheated to 1450°C and arc welding was successfully implemented for joining of the parent material. A current of 138 A, plasma flow rate of ~1 L/min or ~0.5 L/min, and welding speed of ~8 cm/min were utilized for repeated joining, with full penetration fusion zones along the entire length of the joints. Solidification was determined to occur through the crystallization of β‐SiC (3C), then the simultaneous solidification of SiC and ZrB₂, and lastly through the simultaneous solidification of SiC, ZrB₂, and ZrC through a ternary eutectic reaction. The ternary eutectic composition was determined to be 35.3 ± 2.2 vol% SiC, 39.3 ± 3.8 vol% ZrB₂, and 25.4 ± 3.0 vol% ZrC. A dual fusion zone microstructure was always observed due to convective melt pool mixing. The SiC content at the edge of the fusion zone was 57 vol%, while SiC content at the center of the fusion zone was 42 vol% although the overall SiC content was still nominally 50 vol% throughout the entire fusion zone.     

High‐temperature and high‐pressure ethylene polymerization using a cationic activated metallocene catalytic system

BACKGROUND: Normally, olefin polymerization via metallocene‐based catalysis occurs under mild conditions. However, most technology developed for polyolefin production is designed for more severe temperature and pressure processes. Attaining more thermally stable metallocene systems for industrial applications is an important challenge for researchers. RESULTS: A systematic study of ethylene homopolymerization at higher temperatures and pressures, employing the ternary system Ph₂C(Cp)(Flu)ZrCl₂/PhNHMe₂B(C₆F₅)₄/(i‐Bu)₃Al, is presented The optimal activity for this system is achieved with a Zr/B/Al molar ratio of 1/6/250 and a temperatures of around 130 °C. However, the amount of activator strongly affects the molecular weight and the polydispersity of the polymers produced. Polyethylene produced with Zr/B/Al molar ratios between 1/2/250 and 1/6/250 show no significant difference in their temperature of fusion (T_m) and their crystallinity (X_c). In contrast, in the presence of activator amounts higher than 1/6/250, both the temperature of fusion and polymer crystallinity undergo a steep decrease. All polymers presented lamellar morphology when the activator was present, and an amorphous aspect when the activator was not employed. CONCLUSION: The presence of the activator is essential for thermal stabilization of the catalytic system. Variation of the Zr/B/Al ratio leads to modifications of the catalytic activity as well as to the properties of the polymers synthesized. Copyright © 2008 Society of Chemical Industry     

Preparation and performance of high‐impact polystyrene (HIPS)/nano‐TiO2 nanocomposites

High‐impact polystyrene (HIPS)/nano‐TiO₂ nanocomposites were prepared by surface pretreatment of nano‐TiO₂ with special structure dispersing agent (TAS) and master batch manufacturing technology. The results show that when the nano‐TiO₂ content is 2%, the notched impact strength, tensile strength, and elastic modulus of HIPS/nano‐TiO₂ nanocomposites increased to a maximum. This result indicates that nano‐TiO₂ has both toughening and reinforcing effects on HIPS. The heat‐deflection temperature and flame‐retardance of HIPS/nano‐TiO₂ nanocomposites are also obviously improved as the nano‐TiO₂ content is increased. The nanocomposites manufactured by the two‐step method have better mechanical properties than that made by a one‐step method. HIPS/nano‐TiO₂ nanocomposites are also non‐Newtonian and pseudoplastic fluids. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 381–385, 2003