SiO2/TiO2 multilayer films grown on cotton fibers surface at low temperature by a novel two-step process

A novel two-step process was developed to synthesize and deposit SiO₂/TiO₂ multilayer films onto the cotton fibers. In the first step, SiO₂ particles on cotton fiber surface were synthesized via tetraethoxysilane hydrolysis in the presence of cotton fibers, in order to protect the fibers against photo-catalytic decomposition by TiO₂ nanoparticles. In the second step, the growth of TiO₂ nanoparticles into the modified cotton fiber surface was carried out via a sol-gel method at the temperature as low as 100 °C. The as-obtained SiO₂/TiO₂ multilayer films coated on cotton fibers were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy and X-ray diffraction, respectively.     

Sub-micrometer sized yttrium oxide fibers prepared through hydrothermal reaction

Yttrium oxide fibers have been synthesized via hydrothermal reaction and subsequent thermal treatment using yttrium chloride as precursor. The products before and after the thermal treatment were characterized by powder X-ray diffractions (XRD), scanning electron microscopy (SEM), ion-chromatograph analysis, and thermogravimetry and differential thermal analysis (TG-DTA). The fiber diameter ranged from 100 to 300 nm, while the length was up to tens of microns. It was found that the chemical composition and morphology of the products were closely related to the pH value of reaction solution, and fibrous products could be obtained at pH 9.5-10.25. These oxide fibers exhibited outstanding high-temperature stability, which maintained their morphology at temperature up to 1400 °C.     

Fabrication of titania-coated silica fibers and effect of substrate shape on coating growth rate

Titania coating was fabricated on fused silica glass fibers of 4-6 μm in diameter by the hydrolysis of Ti-tetraethoxide in ethanol at 20 °C. Changes in the coating thickness with the deposition time were examined by scanning electron microscopy and compared with those on flat soda-lime glass substrate examined by atomic force microscopy. Uniform titania coating was obtained, but severe cracking occurred on coatings thicker than 150 nm due to isotropic shrinkage during the drying process. The faster growth rate of coating on fiber than that on flat substrate could be explained by employing an ‘adhesion growth model.’ X-ray diffractometry showed that titania coating on fused silica glass plate was transformed to anatase-phase TiO₂ by annealing in air at 300 °C. This indicates that the titania coating on a fused silica glass substrate becomes crystalline after annealing at that temperature.     

Effect of heat treatment on phase stability, microstructure, and thermal conductivity of plasma-sprayed YSZ

The effects of 50-hour heat treatments at 1000°C, 1200°C, and 1400°C on air plasma-sprayed coatings of 7 wt% Y₂O₃-ZrO₂ (YSZ) have been investigated. Changes in the phase stability and microstructure were investigated using x-ray diffraction and transmission electron microscopy, respectively. Changes in the thermal conductivity of the coating that occurred during heat treatment were interpreted with respect to microstructural evolution. A metastable tetragonal zirconia phase, with a non-equilibrium amount of Y₂O₃ stabilizer, was the predominant phase in the as-sprayed coating. Upon heating to 1000°C for 50 hours, the concentration of the Y₂O₃ in the t-zirconia began to decrease as predicted by the Y₂O₃-ZrO₂ phase diagram. The c-ZrO₂ phase was first observed after the 50-hour heat treatment at 1200°C; monoclinic zirconia was observed after the 50-hour heat treatment at 1400°C. TEM analysis revealed closure of intralamellar microcracks after the 50-hour/1000°C heat treatment; however, the lamellar morphology was retained. After the 50-hour/1200°C heat treatment, a distinct change was observed in the interlamellar pores; equiaxed grains replaced the long, columnar grains, with some remnant lamellae still observed. No lamellae were observed after the 50-hour/1400°C heat treatment. Rather, the microstructure was equivalent when viewed in either plan view or cross-section, revealing large grains with regions of monoclinic zirconia. Thermal conductivity increased after every heat treatment. It is believed that changes in the intralamellar microcracks and/or interlamellar pores are responsible for the increase in thermal conductivity after the 1000°C and 1200°C heat treatments. The increase in thermal conductivity that occurs after the 50-hour/1400°C heat treatment is proposed to be due to the formation of m-ZrO₂, which has a higher thermal conductivity than tetragonal or cubic zirconia.     

Mechanical behavior of self-assembled carbon nanotube reinforced nylon 6,6 fibers

The versatile electrospinning technique was used to successfully align and disperse multiwalled carbon nanotubes (MWCNT) in nylon 6,6 matrix to obtain composite fibers. The morphology of the composite fibers and the dispersion of the CNTs within the fibers were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. TEM analysis revealed that the CNTs were well-dispersed, separated and aligned along the fiber axis. The thermal and mechanical properties of the composite fibers were characterized as a function of weight fraction of the CNTs. Incorporation of the CNTs in the fibers resulted in an increase in glass-transition temperature (T_g) by ∼7 °C, indicating that the addition of CNTs has restricted the mobility of the polymer chains and provided confinement to neighboring molecular chains. Tensile and nanoindentation experiments were performed to investigate the mechanical deformation behavior of the composite fibers. The results suggested that incorporation of high strength and high aspect ratio CNTs into the fiber matrix enhanced significantly the stiffness and strength of nylon 6,6 fibers. An understanding of the structure–property relationships can provide fruitful insights to develop electrospun fibers with superior properties for miniaturized and load-bearing applications.     

Structure-property relations for silicon nitride matrix composites reinforced with pyrolytic carbon pre-coated Hi-Nicalon fibers

Si₃N₄ matrix composites reinforced with pyrolytic carbon pre-coated Hi-Nicalon (SiC) fibers, were studied using tensile testing and transmission electron microscopy. Three types of samples were evaluated all with a nominal coating thickness of 200 nm. The composites were densified by hot pressing at 1550 °C (type I and II) and at 1600 °C (type III). The fibers were coated with pyrolytic carbon via CVD with identical (sample I) and opposite (samples II and III) directions of the gas flow and of the fiber movement through the reactor. Tensile testing indicated for the three sample types respectively: brittle behaviour with huge pull out of the fibers, pseudo-plastic behaviour and brittle behaviour with little pull out. TEM indicated for the three sample types debonding typically at the fiber/coating interface, at the coating/matrix interface and in the coating, respectively. The relation between processing, structure, particularly of the coating and its interfaces with the matrix and the fibers and mechanical properties is addressed.     

Experimental characterization of thermo-oxidation-induced shrinkage and damage in polymer-matrix composites

This paper focuses on the experimental characterization of thermo-oxidation in carbon-fiber-reinforced polymers (CFRPs) exposed to “high” temperatures (up to 150 °C) and “high” oxygen pressures (up to 5 bars), at the microscopic scale. Unidirectional IM7/977-2 composite specimens were aged at 150 °C under atmospheric air and under oxygen pressure (1.7 bars and 5 bars): periodic tests were carried out to characterize degradation phenomena after different aging times. The thermo-oxidation-induced resin shrinkage and fiber/matrix debonding were measured on the CFRP sample surface by confocal interferometric microscopy (CIM) and scanning electron microscopy (SEM). The results show that thermo-oxidation-induced degradation strongly depends on aging time, distance between fibers and partial oxygen pressure.     

Bonding mechanism between silicon carbide and thin foils of reactive metals

Pressureless-sintered SiC pieces and SiC single crystals were joined with foils of reactive metals at 1500° C (1773 K) for titanium and zirconium foils or at 1000° C (1273 K) for Al/Ti/Al foils. Bend testing at various temperatures up to 1400° C (1673 K), optical and electron microscopy, and electron-probe X-ray microanalysis studies were carried out on the specimens. From the results, it was concluded that the fairly high bond strength of titanium-foil joined SiC specimens might be attributed to the formation of a Ti₃SiC₂ compound, since good lattice matching between SiC and Ti₃SiC₂ was obtained in the SiC single crystals. Also in the Al/Ti/Al-foil joined SiC, high bond strength was obtained, but it decreased steeply at 600° C (873 K) because of a retained aluminium phase. The bond strength in the zirconium-foil joined SiC was low.     

Nano-precipitation in hot-pressed silicon carbide

Heat treatments at 1300°C, 1400°C, 1500°C, and 1600°C in Ar were found to produce nanoscale precipitates in hot-pressed silicon carbide containing aluminum, boron, and carbon sintering additives (ABC-SiC). The precipitates were studied by transmission electron microscopy (TEM) and nano-probe energy-dispersive X-ray spectroscopy (nEDS). The precipitates were plate-like in shape, with a thickness, length and separation of only a few nanometers, and their size coarsened with increasing annealing temperature, accompanied by reduced number density. The distribution of the precipitates was uniform inside the SiC grains, but depleted zones were observed in the vicinity of the SiC grain boundaries. A coherent orientation relationship between the precipitates and the SiC matrix was found. Combined high-resolution electron microscopy, computer simulation, and nEDS identified an Al₄C₃-based structure and composition for the nano-precipitates. Most Al ions in SiC lattice exsolved as precipitates during the annealing at 1400 to 1500°C. Formation mechanism and possible influences of the nanoscale precipitates on mechanical properties are discussed.     

Nanocomposites based on rutile-TiO2 and polyaniline

A polyaniline/rutile-TiO₂ nanocomposite, with a conductivity of 0.5 S cm^− 1 at 25 °C, was prepared by in-situ polymerization of aniline in the presence of rutile-TiO₂ nanoparticles, and was characterized by Fourier-transform infrared spectra, wide-angle X-ray diffraction, transmission electron microscopy, conductivity and cyclic voltammetry, as well as thermogravimetric analysis. The introduction of conducting polyaniline (PANI), not only improved the conductivity of rutile-TiO₂ nanoparticles, but also improved the dispersibility of rutile-TiO₂ nanoparticles. It can potentially be used in commercial applications as fillers for antistatic and anticorrosion coatings.     

Preparation of poly (vinyl alcohol)/silica composite nanofibers membrane functionalized with mercapto groups by electrospinning

Membranes of poly vinyl alcohol (PVA)/silica functionalized with mercapto groups are synthesized by electrospinning. Scanning electron microscopy (SEM) studies showed that the fiber diameters are in the range of 200-300 nm. The thickness of nanofiber decreases with an increase in calcination temperature. The results of Fourier transform infrared (FTIR) indicated that PVA/silica nanofibers are functionalized by mercapto groups via the hydrolysis poly-condensation method. N₂ adsorption-desorption showed that organic molecules can be removed completely when the PVA/silica composite fibers are calcinated at 800 °C. The fibers calcinated at 800 °C were pure inorganic silica species with a mesoporous structure. These mercapto groups functionalized PVA/silica nanofibers have a great potential application in the field of adsorption of heavy metal ions.     

Oxidation study of Cr-Ru hard coatings

Cr-Ru alloy coatings with Cr content ranging from 47 to 83 at.% were deposited at 400 °C by direct current magnetron co-sputtering with a Ti interlayer on silicon substrates. With a total input power of 300 W, the Cr content in the Cr-Ru coatings increased linearly with the increasing input power of Cr. The intermetallic compound phase Cr₂Ru with columnar structure was identified for the as-deposited Cr₅₆Ru₄₄ and Cr₆₅Ru₃₅ coatings, resulting in an increase of hardness up to 15-16 GPa. To evaluate the performance of Cr-Ru coatings as a protective coating on glass molding dies, the annealing treatment was conducted at 600 °C in a 50 ppm O₂-N₂ atmosphere. The outward diffusion and preferential oxidization of Cr in the Cr-Ru coatings resulted in the variations of the crystalline structure, chemical composition distribution, and surface hardness after annealing. X-ray diffraction and transmission electron microscopy (TEM) proved that an oxide scale consisting of Cr₂O₃ formed on the free surface. Scanning electron microscopy and TEM observed the surface morphology and structural variation. The chemical composition depth profiles were analyzed by Auger electron microscopy, verifying the presence of a Cr-depleted zone beneath the oxide scale. The hardness of Cr₅₆Ru₄₄ and Cr₆₅Ru₃₅ coatings decreased to 11-12 GPa after annealing, accompanied by the replacement of the Cr₂Ru phase by the Ru phase.     

Mechanical behavior of SiCf/SiC composites with alternating PyC/SiC multilayer interphases

In order to tailor the fiber–matrix interface of continuous silicon carbide fiber reinforced silicon carbide (SiC_f/SiC) composites for improved fracture toughness, alternating pyrolytic carbon/silicon carbide (PyC/SiC) multilayer coatings were applied to the KD-I SiC fibers using chemical vapor deposition (CVD) method. Three dimensional (3D) KD-I SiC_f/SiC composites reinforced by these coated fibers were fabricated using a precursor infiltration and pyrolysis (PIP) process. The interfacial characteristics were determined by the fiber push-out test and microstructural examination using scanning electron microscopy (SEM). The effect of interface coatings on composite mechanical properties was evaluated by single-edge notched beam (SENB) test and three-point bending test. The results indicate that the PyC/SiC multilayer coatings led to an optimum interfacial bonding between fibers and matrix and greatly improved the fracture toughness of the composites.     

Al–O–N and Al–O–B–N thin films applied on Si–O–C fibers

Protective coatings (Al–O–N and Al–O–B–N) on Si–O–C fibers (Tyranno ZMI) were applied in order to enhance oxidation resistance under severe thermo-mechanical conditions in the 400–600 °C temperature range. The coating process consisted in three steps: (i) the transformation of the Si–O–C fiber surface into microporous carbon; (ii) the impregnation of these carbon microporous layers by an aluminium trichloride (AlCl₃) solution and then, (iii) a final heat treatment under ammonia. Processing parameters were studied in order to select the best conditions. Using these conditions, obtained results have shown that coatings were present around each fiber, with a controlled thickness, and that the mechanical properties of the fibers were preserved. Although, these coatings did not entirely stop the oxygen ingress, it has been shown that they strongly reduced the oxidation of the fiber.     

Effect of BN Coating on the Strength of a Mullite Type Fiber

Nextel 480 is a polycrystalline essentially mullite fiber (70 wt.-% Al₂O₃+28 wt.-% SiO₂+2 wt.-% B₂O₃). Different thicknesses of BN were applied as coatings on this fiber. Optical, scanning electron, and transmission electron microscopy were used to characterize the microstructure of the coatings and fibers. The effects of coating and high temperature exposure on the fiber strength were investigated using two-parameter Weibull distribution. TEM examination showed that the BN coating has a turbostratic structure, with the basal planes lying predominantly parallel to the fiber surface. Such an orientation of coating is desirable for easy crack deflection and subsequent fiber pullout in a composite. The BN coated Nextel 480 fiber showed that Weibull mean strength increased first and then decreased with increasing coating thickness. This was due to the surface flaw healing effect of the coating (up to 0.3 μm) while in the case of thick BN coating (1 μm), the soft nature of the coating material had a more dominant effect and resulted in a decrease of the fiber strength. High temperature exposure of Nextel 480 resulted in grain growth, which led to a strength loss.     

Structure, hardness and thermal stability of nanolayered TiN/CrN multilayer coatings

Nanolayered TiN/CrN multilayer coatings were deposited on silicon substrates using a reactive DC magnetron sputtering process at various modulation wavelengths (Λ), substrate biases (V_B) and substrate temperatures (T_S). X-ray diffraction (XRD), nanoindentation and atomic force microscopy (AFM) were used to characterize the coatings. The XRD confirmed the formation of superlattice structure at low modulation wavelengths. The maximum hardness of the TiN/CrN multilayers was 3800 kg/mm² at Λ=80  Å, V_B=−150 V and T_S=400°C. Thermal stability of TiN, CrN and TiN/CrN multilayer coatings was studied by heating the coatings in air in the temperature range (T_A) of 400-800°C. The XRD data revealed that TiN/CrN multilayers retained superlattice structure even up to 700°C and oxides were detected only after T_A?750°C, whereas for single layer TiN and CrN coatings oxides were detected even at 550°C and 600°C, respectively. Nanoindentation measurements showed that TiN/CrN multilayers retained a hardness of 2800 kg/mm² upon annealing at 700°C, and this decrease in the hardness was attributed to interdiffusion at the interfaces.     

The stability of sapphire whiskers in nickel at elevated temperatures

Composites of 1 to 20 vol % sapphire whiskers contained in a nickel matrix were produced by roll-bonding and by hot-pressing. The composites were examined using transmission electron microscopy, X-ray and electron diffraction, optical microscopy and mass spectrographic analysis. Composites were annealed in vacuum (1.3×10^–3 N m^–2), in low-pressure air (13.3 N m^–2) and in dried hydrogen (101.3 N m^–2) in the temperature range 1100 to 1400° C for times up to 3800 h. Whiskers in situ and the whisker/matrix interface were observed by transmission electron microscopy; matrix dislocations were associated with whiskers and were stable after annealing at 1400° C. Whiskers extracted from annealed composites showed significant morphological changes. These were attributed to: (i) ovulation from the tips of whiskers by interfacial diffusion, (ii) waisting from surface undulations, (iii) Ostwald ripening and (iv) constant-volume shape changes.     

Surface morphological properties of sol-gel derived SiO2 fiber

In this study, sol-gel derived SiO₂ fibers were prepared by mixing tetraethyl orthosilicate, ethanol, water and hydrochloric acid. Fibrous SiO₂ was drawn by using a viscous solution. Dried gel fibers were final heat-treated at 1000, 1100, 1200 and 1300°C for 1 h in air. Crystallinity of the SiO₂ fiber after annealing was measured by X-ray diffraction analysis. A field emission—scanning electron microscope and an atomic force microscope were used to evaluate surface properties. SiO₂ fiber heat-treated at high temperature, i.e., 1300°C, exhibited inhomogeneous surface structure.     

Fabrication of LiNi0.5 + δMn0.5 − δO2 nanofibers by electrospinning

Polycrystalline tetranary LiNi_0.5 + δMn_0.5 − δO₂ nanofibers have been successfully fabricated by a sol-gel assisted electrospinning technique. The structures and properties of fabricated nanofibers were characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and thermal gravimetric analysis (TGA). After heat treatment of the electrospun fibers at a temperature of 800 °C, the LiNi_0.5 + δMn_0.5 − δO₂ phase was found without other trace phases. Multilayered nanoparticles with a grain size of 50 nm or less within a single fiber are notable from TEM. In this study, it was shown that the sol-gel assisted electrospun LiNi_0.5 + δMn_0.5 − δO₂ fibers could be formed with the α-NaFeO₂ type crystal structure at a temperature lower than that in a typical solid-state or sole sol-gel process and possess good thermal stability as high as 800 °C.     

Ferromagnetic composite coatings on carbon fibers

Alumina- and titania-based composite coatings containing ferromagnetic nanoparticles have been produced on UKN-5000P carbon fibers by an in situ organo-inorganic hybrid sol-gel process using appropriate aqueous metal chloride solutions containing Fe(III) and Co(II). The morphology, phase composition, and elemental composition of the coatings have been studied by high-resolution electron microscopy, X-ray diffraction, and energy-dispersive X-ray microanalysis. The results demonstrate that the oxide coatings containing ferromagnetic nanoparticles are uniform in thickness along and across the fibers and adhere well to the fiber surface. The coatings range in thickness from 100 to 500 nm. In all of the systems studied, the coatings produced on carbon fibers differ in phase composition from powders prepared from identical hydrosols.