Additive and pyrolysis atmosphere effects on polysiloxane-derived porous SiOC ceramics

Porous silicon oxycarbide (SiOC) is emerging as a much superior ultrahigh surface area material that can be stable up to high temperatures with great tailorability through composition and additive modifications. In this study, bulk SiOCs were fabricated from a base polysiloxane (PSO) system by using different organic additives and pyrolysis atmospheres followed by hydrofluoric acid (HF) etching. The additives modify the microstructural evolution by influencing the SiO₂ nanodomain formation. The SiOC ceramics contain significantly less SiC and more SiO₂ with Ar + H₂O atmosphere pyrolysis compared to Ar atmosphere pyrolysis. Water vapor injection during pyrolysis also causes a drastic increase in specific surface areas. The addition of 10 wt% tetraethyl orthosilicate (TEOS) with Ar + H₂O pyrolysis produces a specific surface area of 1953.94 m²/g, compared to 880.09 m²/g for the base PSO pyrolyzed in Ar. The fundamental processes for the composition and phase evolutions are discussed as a novel pathway to creating ultrahigh surface area materials. The ability to drastically increase the specific surface area through the use of pyrolysis atmosphere and organic additives presents a promising processing route for highly porous SiOC ceramics.     

Enhanced electric conductivity of polymer‐derived SiCN ceramics by microwave post‐treatment

The effect of microwave treatment on the electric conductivity and structure of a polymer‐derived SiCN ceramic is studied. It is found that the conductivity of the microwave‐treated sample is about 40 times higher than that of the conventional heat‐treated one at the same temperature and dwell time conventionally. The X‐ray diffraction patterns show that both samples are amorphous without obvious crystallization. Raman analysis reveals that the microwave‐treated sample exhibited a narrower full width at half maximum and upper‐shift of G peak. X‐ray photoelectron spectroscopy spectra show that there is a significant sp³‐to‐sp² transition of free carbon in the microwave‐treated sample. These results suggest that the microwave‐treatment can induce a distinct structure evolution of the free carbon, which contributes to the remarkable enhancement of the conductivity of the sample.     

Impedance spectroscopy study on polymer‐derived amorphous SiAlCO

The electric behaviors of both free‐carbon phase and ceramic phase in polymer‐derived amorphous silicoaluminum oxycarbide were studied by impedance spectroscopy. The results revealed that the resistance of both ceramic phase and free‐carbon phase are similar to each other and decreased with pyrolysis temperature. Meanwhile, the ceramic phase showed higher capacitance than the free‐carbon phase. With increasing pyrolysis temperature, the capacitance of the ceramic phase drastically decreased, but that of the free‐carbon phase remained similar. Each phase exhibited a relaxation process, which moved to higher frequency with increasing pyrolysis temperature. The results are correlated with the structural evolution of the two phases.     

Study on thermal enhancement mechanism of POSS‐containing hybrid nanocomposites and relationship between thermal properties and their molecular structure

In this study, a series of poly(4‐acetoxystyrene) (PAS)‐octavinyl polyhedral oligomeric silsesquioxane (POSS) blends and the polystyrene (PS)‐octavinyl POSS blends were prepared by the solution‐blending method and characterized with Fourier transform infrared (FTIR), X‐ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) techniques. The results show that the glass‐transition temperature (T_g) of the PAS‐POSS blends increases at a relatively low POSS content and then decreases at a relatively high POSS content. POSS can effectively improve the thermal stability of the PAS‐POSS blends at low POSS content, and T_g of PAS‐POSS blends decreases with the increase in POSS content at relatively high POSS content. However, the T_g of the PS‐POSS blends persistently decreases even at very low POSS content. T_g change mechanism was investigated in detail by XRD, TEM, and FTIR spectra. The influence mechanism of POSS content and dispersion in composites, and parent polymer structure on thermal properties of the blends was investigated in detail. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Influence of acid‐treated talc and Na2CO3 flux on mineralogical phase composition and porosity in steatite ceramics

The work aimed with the effect of preceramic mixtures containing natural talc (Tc) or acid‐treated talc (Tc‐ac) supplemented by montmorillonite (Mt) or kaolinite (Ka), and the flux Na₂CO₃ on the mineral phase composition, porosity and pore size distribution in the steatite ceramics prepared using solid state sintering at 1300°C. The samples were analyzed using X‐ray diffraction and porometry techniques and scanning electron microscopy. Application of Mt or Ka in preceramic mixtures with provided comparable results. The acidification of talc using hydrochloric acid resulted in partial release of Mg²⁺ from the structure. The ratio of SiO₂:MgO in Tc was 35:65 (wt%) and 70:30 (wt%) in Tc‐ac. Tc, clay minerals, and flux Na₂CO₃ in the MgO–Al₂O₃–SiO₂–Na₂O glass system resulted to the separation of forsterite and protoenstatite, while Tc‐ac provided crystallization of protoenstatite at the expense of clinoenstatite in highly porous ceramics. The preceramic mixtures were precursors of bimodal pore size distribution at prepared steatite ceramics.     

The effect of B‐doping on the electrical conductivity of polymer‐derived Si(B)OC ceramics

In this work the room temperature electrical conductivity of Si(B)OC glasses made via polymer pyrolysis at 1200°C and 1400°C (maximum temperature) and having different amount of boron was measured. When B content is increased from zero (pure SiOC glass) up to B/Si=0.5 the electrical conductivity increases in 2 orders of magnitude from 4.09±0.64×10^?5 up to 2.93±1.91×10^?3 with a corresponding decrease in the activation energy from about 1.08 to 0.51 eV. This results shows for the first time that the electrical conductivity of Si‐based polymer‐derived ceramics can be controlled by the amount of the doping element. The structure of the Si(B)OC glasses has been studied with different techniques including FT‐IR, XRD and Raman spectroscopy. The Raman study indicates that B partially substitutes C into the sp² C planes of the free carbon phase forming trigonal BC₃ units. Accordingly, the evolution of the electrical properties with the B content has been correlated with the corresponding structural evolution and a hypothesis is presented to rationalize the role of boron on the electrical conductivity of SiOBC glasses.     

Investigations of ice‐structuring agents in ice‐templated ceramics

The freeze‐cast ceramics process involves the use of an aqueous slurry in which ice acts as a fugitive template, providing control of the morphology of the resultant green body, which is then sintered. Here we report the role of various additives and procedures in tailoring the morphology of ice‐templated, freeze‐cast alumina ceramics. We show that zirconium acetate (ZrAc) in the slurry can change the structure from lamellar to columnar, whereas polyvinyl alcohol can act as both a binder and as a modifier of the ice surface, and can compete with ZrAc to provide cooperative effects and a wide range of porosities. We also show that the ceramic structure can be influenced by epitaxial growth on an oriented layer of ice.     

Effect of polyhedral oligomeric silsesquioxane on water sorption and surface property of Bis‐GMA/TEGDMa composites

2,2‐Bis[4‐(2‐hydroxy‐3‐methacryloxypropoxy)phenyl]propane (Bis‐GMA), one of the most important light‐curable dimethacrylate resins, is widely used as dental restorative material. However, one problem of Bis‐GMA is the high water sorption due to the hydrophilic hydroxyl (–OH) group, resulting in a short life in actual application. In this study, to overcome the drawback stated above, novel organic–inorganic dimethacrylate monomer containing polyhedral oligomeric silsesquioxanes (POSS), Bis‐GMA‐graft‐POSS, is synthesized via the nucleophilic addition reaction of isocyanate functionalized POSS (IPOSS) and pendent hydroxyl group of Bis‐GMA. Then the as‐synthesized Bis‐GMA‐graft‐POSS, of which hydroxyl group was substituted by hydrophobic POSS, is also introduced into the Bis‐GMA/TEGDMA matrix to prepare a series of methacrylate‐based hybrids for dental materials under visible light with camphorquinone and ethyl‐4N,N‐dimethy‐laminobenzoae (EDMAB) as initiator and coinitiator, respectively. Compared to Bis‐GMA/TEGDMA composites, water sorption of modified composites can be significantly reduced with the addition of Bis‐GMA‐graft‐POSS. Moreover, the Bis‐GMA/TEGDMA/POSS hybrids show hydrophobic surfaces, leading to much higher water contact angles than that of Bis‐GMA/TEGDMA composites. The morphology of hybrids containing POSS was furthermore studied by X‐ray diffraction (XRD) analysis and X‐ray photoelectron spectroscopy (XPS). The results show that POSS disperses in the matrix in noncrystalline form and tend to migrate to the surface of the modified composites that lead to the lower water sorption and higher water contact angles. These results are very useful for design of novel methacrylate monomers and clinical application. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synthesis and gas permeability of ABA‐type triblock copolymers derived from fluorine‐containing polyimide with polyhedral oligomeric silsesquioxane

ABA‐type triblock copolymers derived from 4,4'‐(hexafluoroisopropylidene)diphthalicanhydride‐2,3,5,6‐tetramethyl‐1,4‐phenylenediamine (6FDA‐TeMPD) and methacryl phenyl polyhedral oligomeric silsesquioxane (MPPOSS) were synthesized by atom transfer radical polymerization. The chemical structure of the synthesized ABA‐type triblock copolymer was confirmed by ¹H NMR, ¹³C NMR, ²⁹Si NMR and Fourier transform infrared analyses. The ratios of 6FDA‐TeMPD and MPPOSS determined by TGA were 94/6, 85/15, 77/23, 68/32, 57/43 and 31/69. The film density of the ABA‐type triblock copolymer films did not conform to the mixing rule because of polyimide (PI) chain aggregation. Based on contact angle and water uptake analyses, the hydrophobicity of the ABA‐type triblock copolymer film was determined to be higher than the theoretical value because of POSS cage effects and PI chain aggregation. The gas permeability coefficient of the ABA‐type triblock copolymer decreased compared with that of PI because of aggregation of PI chains and inhibition of solubility decreases by substitutes with high affinity. ABA‐type triblock copolymer CO₂/H₂ separation performance increased compared with that of PI. The ABA‐type triblock copolymer derived from PI and MPPOSS can be described as a polymer material with higher hydrophobicity and higher CO₂/H₂ selectivity than PI. © 2015 Society of Chemical Industry     

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.     

Synthesis,characterization and surface properties of star‐shaped polymeric surfactants with polyhedral oligomeric silsesquioxane core

Star‐shaped amphiphilic polymeric surfactants comprising a hydrophobic polyhedral oligomeric silsesquioxane (POSS) core and hydrophilic poly(ethylene glycol) (PEG) arms with various chain lengths are successfully synthesized using copper(I)‐catalysed azide–alkyne cycloaddition (CuAAC) click reaction. Their chemical structures and molecular characteristics are clearly confirmed using Fourier transform infrared and ¹H NMR spectroscopies and gel permeation chromatography, and no homopolymer is found after CuAAC click reaction. Aqueous solutions of these star‐shaped polymers have been investigated using atomic force and transmission electron microscopies and dynamic light scattering studies and it is found that they can self‐assemble into micelles. The sizes of the micelles can be adjusted by the length of the PEG arms, where longer chains not only lead to increased micelle sizes, but also reduce the contact angle values. Moreover, the melting points and root mean square roughness of the obtained star‐shaped polymers are slightly increased on increasing the chain length of the PEG arms. © 2017 Society of Chemical Industry     

Porosity effect on microstructure,mechanical, and fluid dynamic properties of Ti2AlC by direct foaming and gel‐casting

In this study, Ti₂AlC foams were fabricated by direct foaming and gel‐casting using agarose as gelling agent. Slurry viscosity, determined by the agarose content (at a fixed solids loading), as well as surfactant concentration and foaming time were the key parameters employed for controlling the foaming yield, and hence the foam porosity after sintering process. Fabricated foams having total porosity in the 62.5‐84.4 vol% range were systematically characterized to determine their pore size and morphology. The effect of the foam porosity on the room‐temperature compression strength and elastic modulus was also determined. Depending on the amount of porosity, the compression strength and Young's modulus were found to be in the range of 9‐91 MPa and 7‐52 GPa, respectively. Permeability to air flow at temperatures up to 700°C was investigated. Darcian (k₁) and non‐Darcian (k₂) permeability coefficients displayed values in the range 0.30‐93.44 × 10^?11 m² and 0.39‐345.54 × 10^?7 m, respectively. The amount of porosity is therefore a very useful microstructural parameter for tuning the mechanical and fluid dynamic properties of Ti₂AlC foams.     

Seed‐Free Synthesis and Characterization of Zeolite Faujasite Aluminosilicate Coating on α‐Alumina Supports

NaY zeolite was synthesized by a simple sol–gel technique. NaY zeolite membrane was formed on the alumina substrate using optimized solution composition and synthesis conditions. The formation of the NaY zeolite was ascertained by X‐ray powder diffraction, which showed that the crystallinity improved with annealing. The high‐resolution transmission electron microscopy images of samples annealed at 300°C showed the formation of cubic structure corresponding to NaY zeolite. Brunauer–Emmett–Teller analysis revealed that the synthesized zeolite is a well crystallized NaY zeolite. The SEM images revealed the formation of fine structure NaY zeolite membrane on α‐alumina substrate.     

Formation of Silicalite-1 in Channels of Novel SiO2–ZrO2 Porous Glass Tubes Used as Both a Silica Source and a Substrate

Silicalite-1 has been prepared using a novel SiO₂–ZrO₂ porous glass tube as both a silica source and a substrate. The SiO₂–ZrO₂ glass tube was found to possess sufficient physical strength to enable practical use as a separation and catalytic membrane after silicate-1 deposition within its channel. This is in contrast to porous SiO₂ glass which cannot be used to synthesize a silicalite-1 deposited membrane with enough physical strength to perform gas permeance measurements.     

Study of solubility and swelling ratio in polymer‐CO2 systems using the PC‐SAFT equation of state

We use the PC‐SAFT equation of state to model the solubility of CO₂ in various homopolymers. We also model the swelling ratio of the PP (polypropylene)‐CO₂ mixture using PC‐SAFT and then compare the results with Sanchez‐Lacombe (S‐L) and Simha‐Somcynsky (S‐S) equations. The results show that PC‐SAFT can describe the solubility of CO₂ in polymers very well. We compare two sets of parameters in the PC‐SAFT equation, Gross et al.'s and Chen et al.'s. As for the swelling ratio, PC‐SAFT using Chen et al. parameters is better than S‐L equation, which is commonly used by early researchers in studying the solubility of CO₂ in polymers. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44804.     

Thermal and morphological properties of high‐density polyethylene/ethylene–vinyl acetate copolymer composites with polyhedral oligomeric silsesquioxane nanostructure

The demand for improved properties of common polymers keeps increasing, and several new approaches have been investigated. In the study reported here, composites with a polymer matrix comprising a blend of high‐density polyethylene with ethylene–vinyl acetate copolymer (EVA), and with polyhedral oligomeric silsesquioxane (POSS) as a nanostructure, were processed and characterized in terms of their thermal and morphological properties. For the preparation of the composites, the concentrations of the blend components (0, 50 and 100 wt%) and of the POSS (0, 1 and 5 wt%) were varied. X‐ray diffraction results indicated that the presence of EVA in the composites led to the appearance of crystalline domains at lower POSS concentrations. Transmission and scanning electron microscopy showed that samples with 1 wt% of POSS had a homogeneous distribution in the polymer matrix with average dimensions of ca 150 nm. However, the formation of aggregates occurred in samples with 5 wt% of POSS. Differential scanning calorimetry and thermogravimetic analyses indicated that the POSS did not affect the melt and degradation temperatures of the polymer matrix. POSS underwent aggregation at higher concentrations during the composite processing, indicating a solubility limit of around 1 wt%. The presence of EVA in the composite favors POSS aggregation due to an increase in the polarity of the polymer matrix. Copyright © 2009 Society of Chemical Industry     

Effect of substituting CaO with BaO on the viscosity and structure of CaO‐BaO‐SiO2‐MgO‐Al2O3 slags

In this study, the effect of CaO and BaO substitution on the viscosity and structure of CaO‐BaO‐SiO₂‐MgO‐Al₂O₃ slags was investigated. The results showed that the viscosity increased with an increase in the BaO substitution concentration, which was correlated to an increase in the degree of polymerization (DOP) of the slag structural units as the activation energy increased from 207.9 to 263.8 kJ/mol for viscous flow. Deconvolution and area integration of the Raman spectrum of the slag revealed that the ratio of Q³/Q² (Qⁱ, i is the number of O⁰ in a [SiO₄]‐tetrahedral unit) increased and NBO/Si (nonbridging oxygen per unit silicon atom) decreased with higher BaO content. It was also observed from the ²⁷Al magic angles pinning nuclear magnetic resonance (²⁷Al MAS‐NMR) spectrum that the relative proportion of Al^IV increased, while that of Al^V decreased because of the decrease in the percentage of nonbridging oxygen (O^?), indicating the polymerization of the slag. O_1s X‐ray photoelectron spectroscopy (XPS) was also carried out to semi‐quantitatively analyze the various types of oxygen anions present in the slag. The XPS results correlated well with the results obtained from the analysis of the Raman and ²⁷Al MAS‐NMR spectra of the slags and its viscous behavior.     

Effect of Microwave Processing on the Crystallization and Energy Density of BaO‐Na2O‐Nb2O5‐SiO2‐B2O3 Glass‐Ceramics

Barium sodium niobate (BNN) glass‐ceramics were successfully synthesized through a controlled crystallization method, using both a conventional and a microwave hybrid heating process. The dielectric properties of glass‐ceramics devitrified at different temperatures and conditions were measured. It was found that the dielectric constant increased with higher crystallization temperature, from 750°C to 1000°C, and that growth of the crystalline phase above 900°C was essential to enhancing the relative permittivity and overall energy storage properties of the material. The highest energy storage was found for materials crystallized conventionally at 1000°C with a discharge energy density of 0.13 J/cm³ at a maximum field of 100 kV/cm. Rapid microwave heating was found to not give significant enhancement in dielectric properties, and coarsening of the ferroelectric crystals was found to be critical for higher energy storage.     

Synthesis and characterization of homo‐ and copolymers of 3‐(2‐cyano ethoxy)methyl‐ and 3‐[methoxy(triethylenoxy)]methyl‐3′‐methyl‐oxetane

Two oxetane‐derived monomers 3‐(2‐cyanoethoxy)methyl‐ and 3‐(methoxy(triethylenoxy)) methyl‐3′‐methyloxetane were prepared from the reaction of 3‐methyl‐3′‐hydroxymethyloxetane with acrylonitrile and triethylene glycol monomethyl ether, respectively. Their homo‐ and copolyethers were synthesized with BF₃· Et₂O/1,4‐butanediol and trifluoromethane sulfonic acid as initiator through cationic ring‐opening polymerization. The structure of the polymers was characterized by FTIR and¹H NMR. The ratio of two repeating units incorporated into the copolymers is well consistent with the feed ratio. Regarding glass transition temperature (T_g), the DSC data imply that the resulting copolymers have a lower T_g than pure poly(ethylene oxide). Moreover, the TGA measurements reveal that they possess in general a high heat decomposition temperature. The ion conductivity of a sample (P‐AN 20) is 1.07 × 10^?5 S cm^?1 at room temperature and 2.79 × 10^?4 S cm^?1 at 80 °C, thus presenting the potential to meet the practical requirement of lithium ion batteries for polymer electrolytes. Copyright © 2005 Society of Chemical Industry     

Effect of reaction conditions on size and morphology of SiO2 powder in a sol-gel process

We have investigated effects of the reaction conditions such as mixing of solution, reactant concentration, feeding time and molecular weight of alcohol solvents on the size and morphology of silicon oxide in a sol-gel process in Rushton type reactor. To describe the intensity of mixing of solution the power input, which means the energy dissipated in the solution, was used. The particle nucleation and growth processes of silicon oxide were varied with all the reaction conditions. However, the particle morphology of silicon oxide depended mostly on the reactant concentration and molecular weight of alcohol solvents under various reaction conditions. If the supersaturation level of silicon oxide in the solution was promoted by variation of reaction conditions, the particle nucleation and growth processes were facilitaled and it resulted in reduction of relative induction time and in increase of mean particle size of silicon oxide. To explain the principal mechanism of particle growth process of silicon oxide the two-step growth model was applied.