Fabrication and oxidation behavior of Al4SiC4 powders

Al₄SiC₄ powders with high purity were synthesized by heating the powder mixture of aluminum (Al), silicon (Si), and carbon (C) at 1800°C in argon. The microstructure is characterized as platelike single grain. Both the nonisothermal and isothermal oxidation behavior of Al₄SiC₄ was investigated at 800°C‐1500°C in air by means of thermogravimetry method. It is demonstrated that Al₄SiC₄ powder possesses good oxidation resistance up to 1200°C and is almost completely oxidized at 1400°C. At 800°C‐1100°C, the oxide scales consist of an Al₂O₃ outer layer and a transition layer. Al₄SiC₄ remains the main phase. At 1200°C, some spallation resulting from the increment of Al₂O₃ and the mismatch of thermal expansion coefficient between different product layers can be observed. Above 1300°C, the oxide layer is composed of two part, i.e., large‐scale Al₂O₃ crystals (outer layer) and mullite with less amount of SiO₂ (inner layer). The oxidation behavior changes due to the different oxide products. For the reaction kinetics, a new kind of real physical picture model is adopted and obtains a good agreement with the experimental data. The apparent activation energy is calculated to be 176.9 kJ/mol (800°C‐1100°C) and 267.1 kJ/mol (1300°C‐1400°C).     

Fabrication of superhydrophobic and self cleaning PVA-silica fiber coating on 304L SS surfaces by electrospinning

In this work superhydrophobic coating with self cleaning property is fabricated on 304L SS samples directly using a simple one step electrospinning process followed by silane treatment by using polyvinyl alcohol (PVA) and tetraethyl orthosilicate solution. A maximum water contact angle of 169.2° ± 2.1° is obtained at an electrospinning potential of 15 kV for 2 h, with a distance of 18 cm between the collector and needle. The hierarchical nanostructures thus formed on 304L SS composed of poly(vinyl alcohol)-silica microbeads and nanofibers. The surface morphologies are optimized by varying the electrospinning voltage, time, distance between needle and the collector and aging duration of the precursors. Attenuated total reflectance-infrared spectroscopy studies at different stages of preparation confirmed the presence of PVA/SiO₂ composite nanofibers deposited on the 304L SS surface. The reaction of SiO₂ nanofibers with hexamethyl disilazane resulted in the formation of Si O Si bonds that provided water repellent property. The developed SHP surface coating on 304L SS sample showed dynamic bouncing of water droplets and excellent self cleaning performance. The sample retained the SHP behavior in chloride solutions with different ionic strengths and pH.     

Thermal insulation and antioxidation of vertical graphene oxide-grafted carbon nanofiber aerogels with ceramic coating

In this work, composite aerogels with low thermal conductivity and antioxidation were prepared by introducing graphene oxide (GO) on the carbon nanofibers (CNFs) surface, followed by ceramic coating. CNFs surface-grafted vertical morphology of GO was used as an enhancer to hinder the heat radiation transmission. The introduced shrinkage-resistant ceramic coating was used as an antioxidant barrier for carbon materials. The anti-shrinking ceramic coating was obtained by heat treatment of coating with wrinkle morphology to complete the ceramic transformation. The wrinkle morphology of the coating originated from the gradient crosslinking of the coating before its ceramic transformation. Among them, the gradient crosslinking of the coating could be done by irradiation crosslinking and crosslinker impregnation crosslinking. The thermal insulation and antioxidant of the composite aerogel were jointly investigated. Moreover, the mechanism of CNFs surface grafting GO and the anti-shrinkage mechanism of the coating were systematically discussed. The experimental results showed that the aerogel exhibited excellent thermal insulation and oxidation resistance in a wide range of temperatures. The structural design of the aerogel not only weakened the thermal radiation transmission to the CNFs aerogel but also significantly enhanced the antioxidant.     

Influence of surface properties on the dip coating behavior of hollow fiber membranes

Coating processes have become an important fabrication step in membrane production, either to form a separation layer on a porous substrate or to tune specific properties. The coating procedure depends to a large extent on the membrane properties which substantially impedes a prediction of the coating thickness. To give an insight into the coating properties of various hollow fiber membranes, a selection of membranes with different pore sizes was coated with aqueous poly(vinyl alcohol) solutions at various coating velocities. It was found that material properties and pore sizes of the membranes have great influence on coating thicknesses. An intrusion of coating material into the membrane structure was determined with increasing pore size. Pure intrusion without formation of a dense surface layer took place when using a membrane with a mean pore size of ca. 500 nm. Coating results were correlated with the theoretical LLD law and for some membranes the coating thickness can be predicted quite well by the LLD law and its enhancements. When a significant amount of coating material penetrated into the membrane structure the LLD law loses its validity. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46163.     

Ablation resistance of SiC‐modified ZrC coating prepared by SAPS for SiC‐coated carbon/carbon composites

This study evaluated the ablation resistance of ZrC/SiC coating for carbon/carbon (C/C) composites at different temperatures and heat fluxes, which improved the researches on ultra‐high temperature oxidation of ZrC/SiC system. Results showed that the protection of coating depended on temperature and heat flux. Ablation test for 120 seconds under heat flux of 2.4 MW/m² at 2270°C revealed a good ablation resistance, with the linear ablation rate reduced by 96.4% and the mass gain rate increased by 383.3% compared with those of pure ZrC coating. The good ablation resistance was attributed to the formation of dense oxide scale surface. SiC could improve the compactness of the oxide scale at this temperature by forming SiO₂. A dense scale could not form at 2105°C after ablation for 120 seconds, resulting in a dissatisfactory ablation resistance of the coating. After ablation for 120 seconds at 1738°C, the coating was integrated due to the protection of glassy SiO₂ encapsulated ZrO₂. The coating could not resist the strong shear force from the flame at heat flux of 4.2 MW/m² and was severely damaged after ablation for 60 seconds.     

Combustion synthesis of single-phase Al4SiC4 powder with assistance of induction heating

Ternary aluminum silicon carbide Al₄SiC₄ is a promising material not only in refractory applications, but also in electronics and photocatalysis. However, synthesis of Al₄SiC₄ requires high-temperature heat treatment for several hours. In this work, we describe a new, fast and effective method to synthesize single-phase Al₄SiC₄ powder. Our method is based on combustion synthesis reaction of Al, Si and C initiated under high-frequency induction heating. Heating program was optimized to prevent separation of melted aluminum and provide uniform initiation, propagation and completion of the reaction. As a result, pure-phase Al₄SiC₄ powder was synthesized in few minutes. Temperature profile of a sample measured during heating process showed appearance of two exothermic peaks, thus confirming propagation of a two-step combustion reaction. Induction heating provided capability to finely and readily control external heating rate, which is considered to be one of the key factors to reach high-purity Al₄SiC₄.     

Synthesis and performance characteristics of a water‐based polyacrylate microemulsion for UHMWPE fiber adhesive coating

The water‐based polyacrylate microemulsion for ultrahigh molecular weight polyethylene (UHMWPE) fibers adhesive coating was synthesized by the emulsion polymerization of methyl methacrylate (MMA), butyl acrylate (BA), acrylic acid (AA), and hydroxyethyl acrylate (HEA) in the presence of a composite of sodium lauryl sulfate (SLS), OP‐10, and n‐octyl alcohol (NOA) as the emulsifier. The effects of the mass fraction of emulsifier and the reaction time on the properties of emulsion and its membrane were investigated. When m(BA) : m(MMA) : m(AA) : m(HEA) was 50 : 50 : 3 : 10 (wt ratio) and the mass fraction of emulsifier was 13 wt % and the reaction time was 3 h at 80°C, the latex particle diameter was 30 nm tested by transmission electron microscope (TEM). The Fourier transform infrared (FTIR) spectrometer and differential scanning calorimeter (DSC) were used to characterize the chemical structure and the glass transition temperature (T_g) of microemulsion membrane. The application results showed that this microemulsion was an ideal adhesive coating for UHMWPE fibers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Electronic Structure and Bonding in Crystalline Y10[SiO4]6N2

The electronic structure and bonding of the complex ceramic crystal Y₁₀[SiO₄]₆N₂ is studied by a first-principles method. It is shown that this crystal is an insulator with a direct band gap of 1.3 eV. It has some unique properties related to the one-dimensional chain structure in the c -direction and the planar N-Y bonding in the x - y plane.     

Preparation of durable antibacterial and electrically conductive polyacrylonitrile fibers by copper sulfide coating

In this study, antibacterial and electrically conductive polyacrylonitrile (PAN) fibers were prepared by one‐step reaction between Cu²⁺ and S^2– ions without extra reducing agents. The copper sulfide layer was firmly adhered to PAN fibers with the help of nitrile groups from PAN fibers and chelating agent of ethylenediaminetetraacetic acid disodium salt. The copper sulfide layer was characterized by scanning electron microscopy, transmission electron microscopy, energy‐dispersive spectrometry, X‐ray diffraction, Raman and X‐ray photoelectron spectroscopy. The results illustrated that a compact and uniform copper sulfide layer was coated on PAN fibers with 108 nm thickness, and it was proved to be a pure hexagonal phase of covellite structure. The average resistance of the resultant fibers was about 360 Ω/cm and the mass content of copper sulfide was about 13.3%. The antibacterial tests demonstrated that the resultant fibers exhibited excellent antibacterial properties with antibacterial efficiency of 99.99% against Staphylococcus aureus and 96.80% against Escherichia coli. Furthermore, the antibacterial activities and electrical conductivity still remain good after 30 cycles of standard washing, indicating their good durability when in use. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45496.     

Formation of Ti3SiC2 interphase coating on SiCf/SiC composite by electrophoretic deposition

To improve the oxidation resistance of SiC composites at high temperature, the feasibility of using Ti₃SiC₂ coated via electrophoretic deposition (EPD) as a SiC fiber reinforced SiC composite interphase material was studied. Through fiber pullout, Ti₃SiC₂, due to its lamellar structure, has the possibility of improving the fracture toughness of SiC_f/SiC composites. In this study, Ti₃SiC₂ coating was produced by EPD on SiC fiber; using Ti₃SiC₂‐coated SiC fabric, SiC_f/SiC composite was fabricated by hot pressing. Platelet Ti₃SiC₂ powder pulverized into nanoparticles through high‐energy wet ball milling was uniformly coated on the SiC fiber in a direction in which the basal plane of the particles was parallel to the fiber. In a 3‐point bending test of the SiC_f/SiC composite using Ti₃SiC₂‐coated SiC fabric, the SiC_f/SiC composite exhibited brittle fracture behavior, but an abrupt slope change in the strength‐displacement curve was observed during loading due to the Ti₃SiC₂ interphase. On the fracture surface, delamination between each layer of SiC fabric was observed.     

Preparation of TiC/Ti2AlC coating on carbon fiber and investigation of the oxidation resistance properties

MAX phases were proposed as the interphase materials for carbon fiber reinforced ceramic matrix composites toward the applications in high‐dose irradiation and oxidation environments. A thickness‐controllable TiC/Ti₂AlC coating was fabricated on carbon fiber using an in situ reaction in a molten salt bath. The coating showed a multilayered structure, in which the inner layer was TiC and the outer layer was Ti₂AlC. The influence of the reaction conditions on the morphology, composition, and thickness of the coating was investigated. The oxidation resistance properties of the as‐prepared TiC/Ti₂AlC‐coated carbon fiber in static air and water vapor flow at elevated temperatures were investigated. The results showed that the as‐prepared TiC/Ti₂AlC coating could provide good protection to the carbon fiber in both static air and water vapor flow up to 800°C. As these TiC and Ti₂AlC materials have good irradiation resistance, the present work provides a potential way to develop an irradiation‐resistant interphase of carbon‐fiber‐reinforced ceramic matrix composites for nuclear applications. Furthermore, this work also provides a feasible way to prepare carbide/MAX phase coating on other carbon materials.     

耐火纤维喷涂在加热炉锅炉保温中的应用

本文简要介绍了耐火纤维喷涂工艺的优点和特点，在炼油装置加热炉、锅炉保温领域的实际应用效果。     

Model for SiC fiber strength after oxidation in dry and wet air

The strengths of oxidized SiC fibers were modeled from the effects of SiO₂ scale residual stress on fracture. Surface tractions from scale residual stress were determined for SiC surface flaws. The residual stress was the sum of the growth stress from oxidation volume expansion, thermal stress from SiO₂-SiC thermal expansion mismatch, and stress from phase transformations in crystallized scale. The partial relaxation of tensile residual stress from scale cracking was also calculated. Scale thicknesses were determined using Deal-Grove oxidation kinetics for glass and crystalline scales. Kolmogorov-Johnson-Mehl-Avrami (KJMA) kinetics was used to determine scale crystallization rates. Strengths of fibers with glass and with crystalline scales formed by oxidation in dry and wet air between 600° and 1400°C were modeled. The effects of partially crystallized scales were calculated using Weibull statistical methods. Modeled strengths were compared with measurements. Slight strength increases after glass scale formation, large decreases that accompany scale crystallization, and some differences between dry and wet air oxidation were accurately modeled. This suggests that under some conditions the scale residual stress dominates the changes in strength after SiC fiber oxidation. However, modeled strengths were significantly higher than those measured for some fibers oxidized in wet air, which suggests another degradation mechanism is active for these conditions. Modeling assumptions and implications for SiC fiber strength after oxidation for long times are discussed.     

The improvement in oxidation resistance of carbon by a graded SiC/SiO2 coating

A dense functionally gradient SiC/SiO₂ coating has been developed to improve the oxidation resistance of carbon at elevated temperatures. SiC was coated on the surface of a graphite substrate by a reaction between thermally evaporated silicon and carbon at 1400 °C. The SiO₂ layer was deposited by exposing the SiC coated specimens next to a bed of Si powder in a flowing H₂–H₂O gas (P_H2O=2.6×10⁻² atm) at 1400 °C. The formed SiC/SiO₂ layers were dense and had gradient compositions with good adhesion to the carbon substrate. However, as the coating thickness increased, the coating layer became cracked and delaminated from the substrate due to thermal stress. The specimens with the continuous SiC/SiO₂ layer showed a remarkably improved oxidation resistance up to 1200 °C.     

Viscosity-structure relationship of alkaline earth silicate melts containing manganese oxide and calcium fluoride

The effect of CaF₂ on the viscosities of the MO-SiO₂-xMnO-yCaF₂ melts (M[=Ca or Ba]O/SiO₂ = 0.4-0.5, x = 10[±1] or 40[±2] mol%, y = 0-15 mol%) have been studied at high temperatures. At MnO = 40[±2] mol%, the viscosity of Ba-Mn-silicate melts is lower than that of Ca-Mn-silicate melts because Mn²⁺ is a strong network modifier in the former system. However, the viscosities of both alkaline earth silicate melts with MnO = 10[±1] mol% are not affected by cation type because MnO activity is too low to affect the viscosity of the melts. The CaF₂ addition decreases the viscosities of the Ba-Mn-silicate melts with low-MnO contents, while it has less impact on the viscosities of the melts with high-MnO contents. The effect of CaF₂ on the relationship between viscosity and the structure of the silicate melts is quantitatively analyzed using the micro-Raman spectra of quenched glass samples. The NBO/Si ratio of alkaline earth silicate melts, which indicates the degree of polymerization of the silicate networks, has a linear relationship with the activation energy for the viscous flow of the melts.     

Insulating Properties of Lanthanum-Doped BaTiO3 Ceramics Prepared by Low-Temperature Synthesis

Samples of composition Ba_{1− x} La _x Ti_{1− x /4}O₃, x = 0, 0.003, 0.03, and 0.10, were prepared by an alkoxide sol–gel route with final firing of ceramics at 1100°C, 2 h in air. All samples showed bulk insulating behavior with no evidence of semiconductivity caused by either direct donor doping or oxygen loss.     

固相反应合成Al4SiC4材料

采用粒度14μm的磨料级碳化硅,粒度10 μm的工业级金属铝粉和粒度5 μm的工业级炭黑粉为原料,按SiCAlC质量比为225919配料制成试样,在氩气保护下,分别在1200℃8 h、1600℃2 h和1650℃2 h烧成,研究了通过固相反应合成Al4SiC4材料的条件和动力学过程.通过X射线衍射仪进行物相分析,扫描电子显微镜和透射电子显微镜的形貌分析以及能谱分析确定成分.结果表明反应体系在1200℃以下,铝和炭黑反应首先生成中间相Al4C3;从1200℃开始通过SiC+Al4C3=Al4SiC4固相反应生成Al4SiC4;当合成温度达到1650℃时,获得Al4SiC4材料;制备的Al4SiC4材料的颗粒均匀,尺寸在几百纳米到几微米之间.     

Conductive and electroactive composite paper reinforced by coating of polyaniline on lignocelluloses fibers

Direct use of lignocelluloses fibers as substrate for fabrication of conductive, electroactive, biodegradable, and low‐cost electrode materials are in demand for high‐tech applications of ion‐exchange and energy storage devices. This article presents the preparation and characterizations of conductive and electroactive lignocelluloses‐polyaniline (cellulose/PANI) composite paper. Lignocelluloses fibers were directly collected from the stem of self‐growing plant, Typha Angusitfolia, and subsequently coated with the conductive and electroactive layer of PANI through chemical synthesis. Individual PANI‐coated lignocelluloses fibers were converted into sheet and further characterized with Scanning Electron Microscopy, Fourier Transform Infrared, Thermogravimetric Analysis, electronic conductivity, and Cyclic Voltammetry. Cellulose/PANI composite paper revealed superior thermal characteristics and used as a working electrode in three different electrolytes for ion‐exchange properties. Conductive composite paper (CCP) showed the charge storage capacity of ～52 C/g at scan rate of 5 mV/s in 2M HCl solution. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42293.     

Na2SO4 deposit-induced hot corrosion of SiC fibers relevant for SiC CMCs

Hi Nicalon, Hi Nicalon S, Sylramic, and Sylramic iBN SiC fibers were exposed to ~60 μg/cm² of Na₂SO₄ in a 0.1% SO₂/O₂ gaseous environment for times between 0.75 and 24 h at 1000°C. After exposure, the corrosion products were characterized using SEM, EDS, ICP-OES, TEM, and EFTEM to determine their high-temperature resistance to Na₂SO₄ and key reaction mechanisms. All SiC fiber types tested in this work exhibited little resistance to Na₂SO₄ deposit-induced attack relative to their behavior in dry O₂ environments. It was found that Hi-Nicalon displayed the least resistance to Na₂SO₄ deposit-induced attack due to excess carbon content resulting in the formation of a highly porous crystalline oxide and promotion of basic corrosion conditions. All fiber types formed a crystalline SiO₂ reaction product, either cristobalite or tridymite. Sylramic and Sylramic iBN formed a crystalline SiO₂ reaction layer containing TiO₂ needles due oxidation of TiB₂ particles. Additionally, Na₂SO₄ deposits resulted in pitting of all fiber surfaces.     

Subcritical crack growth models for static fatigue of Hi-NicalonTM-S SiC fiber in air and steam

Three subcritical crack growth (SCG) laws were used to model strain-rates and failure times for static fatigue of Hi-Nicalon^TM-S SiC fiber tows in air and Si(OH)₄(g)-saturated steam. Models were fit to tow failure times ( t_f ) and steady-state strain rates ( ἑ ) for brittle creep measured at 700 to 1100°C under initial applied stresses ( σ_A ) of 260 to 1260 MPa. A power law, a reaction-rate law, and a bond-energy law were used to describe SCG that caused sequential filament failure, and ultimately tow failure. Two versions of each model were developed. One allowed access of chemisorbed species to flaws throughout the fiber (mode 1) and another only allowed access to flaws at the SiC-SiO₂ interface (mode 2). The stress increase on intact filaments as others fractured and as filaments oxidized, and the increase in stress intensity geometric factors ( Y ) as crack size increased were incorporated in the models. The fits to data were compared for the different models by using both simple regression analysis and orthogonal distance regression (ODR) analysis. Faster convergence and more consistent results were achieved using ODR analysis. Regression analyses found parameters for all models with similar error in data fits, so validity of a model could not be distinguished by regression analysis alone. For all models, the stress dependence of SCG rates was much stronger in steam than in air, and for most models activation energies were between 300 and 420 kJ/mol, regardless of environment. For the steam environment, the bond-length parameter ( δ ) for the bond-energy model was very close to the lattice parameter of β-SiC (.436 nm), but in air it was significantly lower at 0.25-0.26 nm, but still larger than the Si-C bond length of 0.189 nm. Other factors suggest that either a bond-energy based law or a modified version of a reaction-rate law are the best choices for a SCG law. Filament strength distributions initially described by Weibull distributions could not be described by such distributions after application of the models. SCG mechanisms are discussed.