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1.
This work focuses on silicon oxycarbide thin film preparation and characterization. The Taguchi method of experimental design was used to optimize the process of film deposition. The prepared ceramic thin films with a thickness of c. 500 nm were characterized concerning their morphology, composition, and electrical properties. The molecular structure of the preceramic polymers used for the preparation of the ceramic thin films as well as the thermomechanical properties of the resulting SiOC significantly influenced the quality of the ceramic films. Thus, an increase in the content of carbon was found beneficial for the preparation of crack-free thin films. The obtained ceramic films exhibited increased electrical conductivity as compared to monolithic SiOC of similar chemical composition. This was shown to correlate with the unique hierarchical microstructure of the SiOC films, which contain large oxygen-depleted particles, mainly consisting of highly graphitized carbon and SiC, homogeneously dispersed in an oxygen-containing amorphous matrix. The matrix was shown to also contain free carbon and to contribute to charge carrier transport between the highly conductive large particles. The ceramic thin films possess electrical conductivities in the range from 5.4 to 8.8 S/cm and may be suitable for implementation in miniaturized piezoresistive strain gauges.  相似文献   

2.
Polymer-derived ceramic materials exhibit adjustable composition, microstructure, and dielectric properties and are widely used as high temperature microwave absorption materials in the aerospace field. In this study, polymer-derived SiCN ceramic aerogels with excellent electromagnetic wave absorption (EMA) properties were successfully fabricated through a combination of the sol-gel, freeze- drying, and polymer precursor conversion methods. The hydrosilylation reaction between the Si-H bond in polysilazane (a polymer ceramic precursor) and the C = C bond in divinylbenzene (a cross-linking agent) occurs to form a wet gel. The effects of the molar ratios of the two bonds (C = C/Si-H) on the microstructure and EMA properties of the SiCN ceramic aerogels were systematically studied. At a C = C/Si–H molar ratio of 1.25:1, the minimum reflection loss of the SiCN ceramic aerogels is −37.57 dB at 10.88 GHz. Moreover, the corresponding effective absorption bandwidth covers almost the entire X-band, showing excellent EMA properties.  相似文献   

3.
In the present work, a high surface area SiC(O)-based ceramic powder was synthesized upon thermal transformation of a polymer-derived macromolecular precursor, which was obtained by the chemical modification of a allylhyldrido polycarbosilane with poly(ethylene glycol) methaacrylate under argon environment. The pyrolysis of developed precursor led to the formation of amorphous and high surface area SiC(O)-based ceramic powder with in situ generated micro/meso-porosity. The specific surface area of the obtained powders depends on the processing temperature. It decreases from 363 to 122 m2/g as the pyrolysis temperature increases from 600 to 1200°C, respectively. Furthermore the promising samples were fabricated using pressing technique, which led to crack-free SiC(O) monoliths on subsquent heat treatment. The present study also emphasizes the potential of produced SiC(O) ceramic powder to support NiO catalyst. The impregnation method were used to produce high surface area NiO@SiC(O) ceramic powder (NiO as a catalyst; SiC(O) as a catalyst support) for further catalytic applications. Interestingly, the distribution of the NiO was shown to strongly depend on the oxygen content present in the SiC(O) matrix. Thus, larger oxygen contents induce homogeneously distributed flower-like NiO catalyst onto SiC(O).  相似文献   

4.
A new type of high-temperature-resistant SiZrBOC ceramics was prepared by sol-gel method using polymethyl-hydro siloxane (PMHS), boric acid (B(OH)3), and n-propyl zirconate (Zr(OPr)4) as raw materials. After high-temperature pyrolysis, the SiZrBOC precursor was transformed into a crystalline ceramic material with a yield of 89.5 wt%. Fourier infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) were applied to characterize the polymer-ceramic conversion process and thermal behavior of ceramic precursors. According to the results, the addition of boron elements led to the formation of Si-O-B links in the system. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to study the phase composition and microstructure of SiZrBOC ceramics. Finally, the oxidation test at 1200 °C revealed that SiZrBOC ceramics with a boron/zirconium molar ratio of 2.5:1 exhibited the best oxidation resistance at a weight gain of 0.4 wt% only.  相似文献   

5.
The electrical and dielectric properties of boron nitride nanotubes (BNNTs) reinforced ceramic composites using the polymer-derived ceramic (PDC) processing route were investigated in this work. The electrical resistivity of the pristine PDC increases from 106 to 108 Ω m after the addition of BNNTs. When the BNNT loading was increased to 5 wt%, the average real relative permittivity of the PDC decreased from 2.94 to 2.80, while the quality factor (Q) of the PDC increased from 134.40 to 176.77. The BNNTs can increase the Q factor of the PDC due to the reduction in the porosity cause by the introduction of the BNNTs. Further increasing the BNNT content decreases the real relative permittivity of the nanocomposites and increases the Q factor at high frequency. The average real relative permittivity decreases to 2.29, while the average Q factor increases to 208.60 when the BNNT content is increased to 30 wt%. The dielectric loss after the addition of high fraction of BNNTs can be explained by the Lorentz resonance relaxation process. Results of this work showed that PDC-BNNT nanocomposites are satisfactory electromagnetic transparent materials when the BNNT fraction is less than 10 wt%.  相似文献   

6.
《Ceramics International》2021,47(22):31561-31566
In the present work, polymer-derived SiCN ceramic aerogels (PDCA-SiCN) were fabricated via a combined sol-gel/freeze drying/polymer precursor conversion method. The microstructure of PDCA-SiCN was studied by regulating the synthesis temperature and time during the sol-gel process. PDCA-SiCN showed a unique three-dimensional network structure, and the specific surface area and pore size of PDCA-SiCN prepared at 150 °C for 20 h were 134 g/m2 and 18 nm, respectively. To assess the electromagnetic wave absorption (EMA) properties of PDCA-SiCN, the materials were uniformly blended with common paraffin, and the influence of PDCA-SiCN/paraffin ratio on the EMA properties was also investigated. The sample with a PDCA-SiCN/paraffin ratio of 20:80 exhibited the best EMA performance, with a minimum reflection loss (RL) of −43.37 dB at 7.6 GHz and electromagnetic absorption bandwidth of 3.8 GHz, which correspond to an absorption of 99.99 % of the electromagnetic waves. The excellent EMA properties of PDCA-SiCN could be attributed to a synergistic effects of good impedance matching, multiple reflections and high dielectric loss.  相似文献   

7.
This paper presents the pressureless preparation of fully dense and crack-free SiOC ceramics via direct photo-crosslinking and pyrolysis of a polysiloxane. Elemental analysis revealed the presence of high levels of carbon in the SiOC ceramics. Thus, the samples showed the highest content (78-86 mol%) of segregated “free” carbon reported so far. XRD investigations indicated that the materials prepared at 1100 °C were X-ray amorphous, whereas the sample prepared at 1400 °C contained a turbostratic graphite-like phase and silicon carbide as crystalline phases, as additionally confirmed by TEM and Raman spectroscopy. Vickers hardness was measured to be 5.5-8.6 GPa. The dc resistivity of the prepared material at 1100 °C was 0.35 Ω m, whereas the ceramic pyrolyzed at 1400 °C showed a value of 0.14 Ω m; both values are much lower than those of other known SiOC materials. This latter feature was attributed to the presence of a percolating carbon network in the ceramic.  相似文献   

8.
The greatest challenge in developing polymer/graphene nanocomposites is to prevent graphene layers stacking; in this respect, we found effective solution-mixing polymers with cost-effective graphene of hydrophobic surface. Since graphene oxide is hydrophilic and in need of reduction, highly conducing graphene platelets (GnPs) of ∼3 nm in thickness were selected to solution-mix with a commonly used elastomer – styrene–butadiene rubber (SBR). A percolation threshold of electrical conductivity was observed at 5.3 vol% of GnPs, and the SBR thermal conductivity enhanced three times at 24 vol%. Tensile strength, Young's modulus and tear strength were improved by 413%, 782% and 709%, respectively, at 16.7 vol%. Payne effect, an important design criteria for elastomers used in dynamic loading environment, was also investigated. The comparison of solution mixing with melt compounding, where the same starting materials were used, demonstrated that solution mixing is more effective in promoting the reinforcing effect of GnPs, since it provides more interlayer spacing for elastomer molecules intercalating and retains the high aspect ratio of GnPs leading to filler–filler network at a low volume fraction. We also compared the reinforcing effect of GnPs with those of carbon black and carbon nanotubes.  相似文献   

9.
Polymer nanocomposites consisting of electrically conductive nanofillers with high aspect ratios are widely utilized for high-performance applications such as sensors and electronics. Silver nanowires (AgNWs) synthesized through polyol reduction have been reported to show excellent electrical conductivity, hydrophilicity, and high aspect ratios. In this study, the influence of the aspect ratios of the AgNWs on the rheological and electrical properties of the fabricated polystyrene (PS)/AgNW nanocomposites was chiefly investigated. The nanocomposites were made by combining a dispersion of AgNWs with a suspension of PS particles, followed by freeze-drying the PS/AgNW mixture harnessing the latex technology. Scanning electron microscopy, UV–Vis spectroscopy, and thermogravimetric analysis were performed on the nanocomposites to investigate the morphological, optical, and thermal properties, respectively; in addition, X-ray photoelectron spectroscopy was performed to examine the hydrophilic polymer poly(vinylpyrrolidone)-capped AgNW surfaces. The rheological behavior of the nanocomposites changed from liquid-like to solid-like after the addition of AgNWs with high aspect ratios. The electrical percolation threshold of the AgNWs in the nanocomposites was determined by the aspect ratio of the nanofiller rather than by its length. Thus, the various properties of the PS/AgNW nanocomposites could be tuned by tailoring the aspect ratios of the AgNWs. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47927.  相似文献   

10.
We present here the single-source-precursor synthesis of Fe3Si and Fe5Si3-containing SiOC ceramic nanocomposites and investigation of their magnetic properties. The materials were prepared upon chemical modification of a hydroxy- and ethoxy-substituted polymethylsilsesquioxane with iron (III) acetylacetonate (Fe(acac)3) in different amounts (5, 15, 30 and 50 wt%), followed by cross-linking at 180 °C and pyrolysis in argon at temperatures ranging from 1000 °C to 1500 °C. The polymer-to-ceramic transformation of the iron-modified polysilsesquioxane and the evolution at high temperatures of the synthesized SiFeOC-based nanocomposite were studied by means of thermogravimetric analysis (TGA) coupled with evolved gas analysis (EGA) as well as X-ray diffraction (XRD). Upon pyrolysis at 1100 °C, the non-modified polysilsesquioxane converts into an amorphous SiOC ceramic; whereas the iron-modified precursors lead to Fe3Si/SiOC nanocomposites. Annealing of Fe3Si/SiOC at temperatures exceeding 1300 °C induced the crystallization of Fe5Si3 and β-SiC. The crystallization of the different iron-containing phases at different temperatures is considered to be a consequence of the in situ generation of a Fe–C–Si alloy within the materials during pyrolysis. Depending on the Fe and Si content in the alloy, either Fe3Si and graphitic carbon (at 1000–1200 °C) or Fe5Si3 and β-SiC (at T > 1300 °C) crystallize. All SiFeOC-based ceramic samples were found to exhibit soft magnetic properties. Magnetization versus applied field measurements of the samples show a saturation magnetization up to 26.0 emu/g, depending on the Fe content within the SiFeOC-based samples as well as on the crystalline iron silicide phases formed during pyrolysis.  相似文献   

11.
Mechanical properties of polymer-derived ceramics are usually measured on samples pyrolyzed in inert atmosphere. Here, we report the hardness and elastic modulus of SiOC and SiCN pyrolyzed in both inert (Ar) and reactive (CO2) atmosphere. The external surface of the specimens exposed to the pyrolysis gas was characterized by Vickers microhardness measurements and infrared spectroscopy. The elastic modulus was evaluated by three-point bending tests on thin (150-200 µm) and dense specimens. Polished sections of the SiOC samples were prepared to study, by energy-dispersive X-ray spectroscopy (EDXS) and nanoindentation, how the elemental composition, hardness, and elastic modulus vary from the surface toward the bulk. For both compositions, pyrolysis in CO2 leads to a strong decrease in the hardness and elastic modulus. The hardness of both the samples pyrolyzed in CO2 approaches the typical value for fused silica, suggesting that CO2 selectively breaks the Si–C and Si–N bonds and leads to the formation of a silica-like network. EDXS and nanoindentation reveal that the modification induced by the CO2 flow extends below the surface at least for a thickness of about 30 µm.  相似文献   

12.
The present paper reports the results of an experimental study of the flash pyrolysis of cellulose pellets (prepared from microgranular cellulose powder) subjected to concentrated radiation delivered by an imaging furnace. The masses and chemical compositions of the gaseous, liquid and solid products are studied as functions of the heat flux density and of the time of exposure to the radiation (between 0.05 and several seconds). The mass balances are very good despite the very low quantities involved (10−6-10−7 kg). All experiments confirm the primary formation, on the surface of the pellets, of a short lifetime (around 20 ms) intermediate liquid compound (ILC) whose thickness reaches rapidly a steady state value. The condensable vapors and the gases appear to be formed from ILC, as well as char in the case of low flux densities. The HPLC and HPLC/MS analysis of ILC and of the vapors show the unique presence of anhydrooligosaccharides containing only small amounts of levoglucosan and cellobiosan.  相似文献   

13.
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.  相似文献   

14.
A novel PVDF dielectric nanocomposite was achieved by controlling phase morphology and incorporating conductive fillers simultaneously, and the mechanical, thermal, dielectric properties of the resultant dielectric nanocomposites were investigated. Mechanical analysis showed that incorporation of modified MWCNTs (MWCNTs-COOH) in the PVDF nanocomposites resulted in significant improvements on the tensile strength (Ts) and elasticity modulus (Em). When the filler content was 12 wt%, the Ts of MWCNTs-COOH/PVDF could reach 64.6 MPa. XRD test showed that the addition of MWCNTs-COOH and MWCNTs promoted the formation of β-phase of PVDF. DMA analysis showed that the glass-transition temperature of the PVDF nanocomposites slightly increases on loading of original MWCNTs and this effect was more pronounced on loading MWCNTs-COOH. The dielectric property analysis showed that the original MWCNTs were more likely to form local conductive networks in the PVDF matrix, promoting the electron displacement polarization, and improving the dielectric constant. When the contents of MWCNTs was 12 wt%, the percolation threshold was obtained and the dielectric constant (ε′) reached 286, which was 36 times of pure PVDF. Our work provides a simple way to fabricate polymer blends with excellent dielectric performances, good mechanical properties as well as good processing capability but low cost. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48463.  相似文献   

15.
This study focuses on the early stage of polymer-derived SiOC ceramic conversion. We demonstrate that the perceived SiOC phase separation is nonexistent. Instead, SiO2 and free carbon clusters form first and then carbothermal reduction sets in to induce SiOC formation. Such fundamental understanding is supported by both synchrotron X-ray diffraction study and reactive force field simulation. This work for the first time unifies the understanding of atomic evolution process of polysiloxane-based polymer to ceramic conversion.  相似文献   

16.
For the first time, the present work reports the dielectric properties and electromagnetic wave (EMW) absorbing performance of polymer-derived carbon-rich NbC-SiC-C nanocomposites. In our previous work, NbC-SiC-C nanocomposites with the ultra-high temperature ceramic phase NbC as the main phase were synthesized with the allylhydridopolycarbosilane (AHPCS) and niobium pentachloride (NbCl5) as starting materials. On this basis, divinyl benzene was chosen as carbon-rich source and introduced into the AHPCS and NbCl5 to form a single-source-precursor. Finally, carbon-rich NbC-SiC-C nanocomposites were successfully synthesized by polymer-derived ceramic approach. Compared with ceramic samples without Nb and with lower carbon content, the carbon-rich NbC-SiC-C nanocomposites show extremely enhanced EMW absorbing performance with minimum reflection coefficient of −51.1 dB at 6.88 GHz for the thickness of 2.27 mm. As a consequence, the resultant carbon-rich NbC-SiC-C nanocomposite has to be considered as structure&function integrated material with excellent EMW absorption performance, which can be applied in hostile environment.  相似文献   

17.
Purification of montmorillonite rich bentonite followed by surface modification using organic salts was performed. The bentonite was purified by sedimentation and then surface modified by ion exchange using alkyl‐ and aryl‐based phosphonium salts. The thermal stability, morphology, melt flow, and mechanical properties of the poly(ethylene terephthalate) (PET) nanocomposites prepared with these organoclays were studied with and without using a reactive elastomeric compatibilizer. TEM results showed that the alkyl based organoclay exhibited better dispersion and thus, higher tensile strength and elongation at break in the PET/organoclay/elastomer ternary nanocomposites than the aryl‐based organoclay did. The notched Charpy impact strength of PET increased from 2.9 to 4.7 kJ m?2 and 3.4 kJ m?2 for alkyl and aryl phosphonium organoclay‐based ternary nanocomposites, respectively. Upon compounding PET with alkyl and aryl phosphonium organoclays, the onset decomposition temperature of PET increased from 413°C to 420°C and 424°C, respectively. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013  相似文献   

18.
This paper presents the pressureless preparation of dense and crack-free near stoichiometric SiC monoliths via cross-linking and pyrolysis of a polycarbosilane, followed by polymer-infiltration-pyrolysis cycles. The composition and the porosity of the samples strongly depend on the processing temperature. Thus, at 1050–1100 °C, the SiC monoliths are X-ray amorphous and exhibit low amounts of oxygen and excess carbon; their porosity was rather high (>10%). Higher processing temperatures induced the crystallization of β-SiC. The removal of oxygen and excess carbon due to CO release allowed for obtaining near-stoichiometric compositions at 1700 °C. However, the residual porosity of the samples increased. The use of the PIP technique led already after six cycles to dense monoliths (residual porosity ca. 0.5%).The present study emphasizes the potential of the polymer processing technique for the fabrication of near stoichiometric and dense SiC monoliths, which might be used for structural applications in harsh conditions.  相似文献   

19.
Highly electrically conductive polypropylene (PP) composites were prepared by melt mixing using tin–lead (Sn‐Pb) alloy as conducting fillers. Two Sn‐Pb alloys, Sn‐Pb (20/80) and Sn‐Pb (40/60), were used for comparison. The effects of Sn‐Pb loading and molding temperature on electrical conductivity and mechanical properties of composites were investigated. It was found that both factors had significant effect on electrical conductivity of the final Sn‐Pb/PP composite. It was interesting to observe that the molding temperature could control the formation of long‐range conducting paths. The mechanism has been explored through the nonequilibrium phase behavior of Sn‐Pb alloy. The influence of Sn‐Pb loading on mechanical properties of Sn‐Pb/PP composites was also investigated. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011  相似文献   

20.
A series of highly soluble aromatic polyimides with excellent thermal properties were fabricated by traditional two‐step polycondensation reaction of dianhydride monomer 4,4′‐(4,4′‐isopropylidenediphenoxy)bis(phthalic anhydride) or 4,4‐(hexafluoroisopropylidene)diphthalic anhydride with diamine monomer 1,3‐bis(4‐aminophenoxy)benzene or 1,3‐bis(3‐aminopropyl) tetramethyldisiloxane in N,N‐dimethylacetylamide solvent. Results revealed that copolyimide of PI‐4 containing trifluoromethyl and tetramethyldisiloxane possessed excellent solubility and remarkable thermal properties. PI‐4 could dissolve well in common low boiling point solvents such as THF of up to 80 mg/mL and acetone of 40 mg/mL. Moreover, the 10% weight loss temperature of the PI‐4 was 539°C and the Tg value of the PI‐4 was 311°C. PI‐4 might be easily cast into flexible and tough films applied in optoelectronic devices. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41713.  相似文献   

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