Synthesis of Nanocrystalline Enstatite Fiber Via Alkoxide Sol–Gel Process

An alkoxide sol–gel route to precursors to transparent enstatite fibers is described. The fibrous gels were drawn from acetic acid-modified viscous solutions. The amount of acetic acid was found to be the crucial factor in the spinnability. Infrared studies indicated that the acetate anion behaved as a ligand and modified the molecular structure of the enstatite gel. The fibrous gel was amorphous after drying by X-ray, and began to convert to enstatite at temperature as low as ∼600°C. The enstatite fibers were nanocrystalline up to 1100°C. At 1300°C, transparent and dense enstatite fibers were produced, with the room temperature dielectric constants of around 5.2–5.8 (at 1 MHz).     

Alkoxide Sol-Gel-Processed Cordierite Fiber

An alkoxide sol-gel route was developed to prepare stoichometric cordierite fibers. The influences of the aging treatment and heating rate on the sinterability of the gel fibers were also examined. X-ray diffraction analysis revealed that the unaged and aged fibrous gels all remained amorphous <800&, but began crystallization into μ-cordierite and α-cordierite at ∼900°C and 1050°C, respectively; single-phase α-cordierite fibers were obtained at 1300°C. Heating the unaged fibers yielded denser microstructures, with fine grain sizes of ∼0.2–0.4 μm, whereas the aged fibers exhibited porous microstructures following heating at 1300°C. A higher heating rate and aging treatment resulted in a higher open porosity of the fired fiber.     

Sinterability of Forsterite Prepared via Solid‐State Reaction

In this work, the sinterability of forsterite powder synthesized via solid‐state reaction was investigated. X‐ray diffraction (XRD) analyses indicate that the synthesized powder possessed peaks that correspond to stoichiometric forsterite. Scanning electron micrographs revealed that the powders were formed agglomerates, which were made up of loosely packed fine particles. Subsequently, the forsterite powders were cold isostatically pressed into a disk shape under 200 MPa and sintered in air at temperature ranging from 1200°C to 1500°C (interval of 50°C) with ramp rate of 10°C/min and dwelling time of 2 h. The sinterability of each sintered samples was examined in terms of phase stability, relative density, Vickers hardness, fracture toughness, and microstructural examination. XRD examination on all the sintered samples exhibited pure forsterite, in which the generated peaks were found to be in a good agreement with JCPDS card no. 34‐0189. The densification of forsterite progressed to reach a maximum relative density of ~91% at 1500°C. This study also revealed that high‐strength forsterite ceramic can be synthesized via solid‐state reaction as forsterite attained favorable mechanical properties, having fracture toughness of 4.88 MPam^1/2 and hardness of 7.11 GPa at 1400°C.     

Time,Temperature, and Concentration Dependence of Ricinelaidic Acid–Canola Oil Organogelation

Canola oil (CO) gels were formed using ricinelaidic acid (REA), a hydroxylated fatty acid, and the time, temperature, and concentration dependence of the resulting gel structure was studied using small-deformation rheology, light microscopy, and powder X-ray diffraction (XRD). Between 5 °C and 30 °C, REA concentration had a significant influence on gel elasticity (P < 0.05), whereas temperature had a relatively lesser influence on gel rheology. Differences were observed in the scaling exponent of G′_LVR with concentration above 20 °C, which were also correlated with significant differences in gelation time at 20 °C. However, the 5% gels at 5 °C, 20 °C, and 35 °C displayed similar microstructures and behaved like weak gels stabilized by junction zones. Most of the gels studied (i.e., the 2%, 3%, 4%, and 5% gels at 15 °C, 20 °C, and 25 °C) consisted of long, thin, fibrous REA strands, although at 25 °C, the 2% gel was characterized by more transient and circular entities. During 28 days’ storage, there were no apparent changes detected in gel microstructure by microscopy or XRD, despite increases in the gel’s opacity.     

Characterization of Nanocrystalline Forsterite Fiber Synthesized via the Sol–Gel Process

An alkoxide sol–gel route was developed to prepare forsterite fibers. Transparent precursor gel fibers were converted to the crystalline phase of forsterite by heating above 550°C. Fibers heated at 1100°C for 2 h showed a porous microstructure with nanocrystalline sizes of about 50–100 nm. The structural evolution of fibrous gel was characterized. Effects of water and ethanol content on microstructures and strengths of fired fibers are also discussed. Some properties of the fired fibers were investigated.     

Evolution of nanodroplets and fractionated crystallization in thermally annealed electrospun blend fibers of poly(vinylidene fluoride) and polysulfone

In this study, electrospun immiscible blend fibers of poly(vinylidene fluoride) (PVDF)/polysulfone (PSF) were prepared. Due to the strong shearing during electrospinning, cocontinuous fibers of PVDF in the PSF matrix were obtained despite the minor composition of PVDF (5–30 wt.%) in the blend. After annealing these electrospun blend fibers above the melting temperature of PVDF (170 °C) and the glass transition temperature of PSF (185 °C), nanosized droplets (primarily 200–300 nm) of PVDF were developed inside the PSF matrix from the breaking up of PVDF nanofibers because of the Plateau–Rayleigh instability. Fractionated crystallization occurred in these PVDF nanodroplets with the heterogeneously nucleated crystallization in the range of 105–135 °C and the homogeneous nucleation at 55–60 °C. The mechanism of homogeneous nucleation was confirmed by the study of crystallization kinetics using differential scanning calorimetry. Only the nonpolar α-phase was observed by wide-angle X-ray diffraction despite of the homogeneously nucleated crystallization at a high supercooling in these PVDF nanodroplets. This study leads to a conclusion that nanoconfined crystallization at a moderate crystallization rate is less important than the local electric field to induce the ferroelectric phases of PVDF.     

Low-temperature Ti-containing 3:2 and 2:1 mullite nanocrystals from single-phase gels

TiO₂-containig single-phase gels with (Al₂O₃ + TiO₂)/(SiO₂) molar ratios 3/2 and 2/1 were prepared by gelling mixtures of aluminium nitrate, tetraethylorthosilicate and titanium isopropoxide. Gels were fast heated at several temperatures up to 1100 °C. Dried and heated gels were characterized by differential thermal analysis (DTA), magic angle spinning nuclear magnetic resonance (MAS-NMR), X-ray powder diffraction (XRD), and scanning and transmission electron microscopies (SEM and TEM). Coupled DTA and XRD results of gels fast heated at 900 °C showed the crystallisation of two mullites as well as a small amount of alumina-silica spinel. ²⁷Al NMR spectra showed the formation of pentacoordinated aluminium before mullite crystallization. The increase of lattice parameters of single-phase mullites heated at 1100 °C indicated that the amount of TiO₂ incorporated into the mullite structure increased on raising the amount of nominal TiO₂ in both series. SEM and TEM images of heated gels at 1100 °C displayed the formation of well-shaped parallelepiped of titanium-doped mullite nanocrystals with crystalline anisotropy.     

Microstructure and reactivity evolution of colloidal silica binder in different systems at elevated temperatures

Colloidal silica as nanostructured binder for refractory castables has attracted many attentions in recent years. In the present study, phase composition, microstructure and reactivity evolution of silica gel at different heating conditions were investigated to find suitable system for colloidal silica application. The results showed that atmosphere and carbon slightly affected phase composition of the silica gel at elevated temperatures, and the crystalline phases were composed of major α-cristobalite and minor α-tridymite. The morphology and particle size of the silica gel were greatly affected by atmosphere and carbon during heating. The spherical nano-silica particles with sizes of 40–50 nm rapidly grew into macroscale rod-like particles with temperature increasing from 800-1000 °C to above 1200 °C in air, and sintering of silica particles was observed. However, the size and morphology of the spherical nano-silica particles retained at high temperature in a reducing atmosphere, and many well developed columnar mullite crystals and some SiC whiskers formed on heating silica gel, alumina fines and carbon at 1500 °C, which was due to carbon inclusions retarding the growth of nano-silica particles and the nano silica remained high reactivity at high temperature. Thus, colloidal silica was suitable for application in carbon-containing refractory castables.     

Phase evolution of YAG powders obtained by gel combustion combined with field-assisted rapid synthesis technique

Yttrium aluminum garnet (YAG) nanopowders were synthesized by a novel method combining gel combustion and field-assisted rapid synthesis technique. It is noted that hexagonal YAlO₃ (YAH) is formed first at 800–830 °C by crystallization from the amorphous, and completely transforms to YAG at about 925 °C. The grain size of YAG powders calcined at 925 °C for 3 min is about 60 nm. Moreover, the phase evolution due to different heat treatment methods was also investigated. The results indicate that the crystallization pathways are related to the atmosphere and heating rate. In air, YAG is directly crystallized from amorphous precursors without any intermediate phases. In an anoxic atmosphere, phase formation follows an amorphous-YAH-YAG route at a rapid heating rate, while the amorphous-YAH+YAG-YAG route is observed in the case of slow heating.     

Effects of precursor chemistry and thermal treatment conditions on obtaining phase pure bismuth ferrite from aqueous gel precursors

Phase pure BiFeO₃ powders are synthesized by an entirely aqueous solution–gel route, starting from water soluble Fe(III) nitrate or citrate, and Bi(III) citrate as precursors. In order to obtain stable solutions, which transform to homogeneous gels upon drying, the pH is adjusted to 7 and a citric acid content equimolar to the metal ions is selected.The presence of nitrate strongly accelerates the thermo-oxidative decomposition step of the precursor gel around 200 °C, and the decomposition is finished at a lower temperature for the nitrate containing precursor (460 °C) than without nitrates (500 °C) in dynamic dry air. An oxidative ambient is required to fully decompose the precursor.The presented synthesis allows very low temperature (400 °C) crystallization of BiFeO₃ together with a secondary phase, as shown by high temperature XRD. This parasitic phase remains up to high temperatures, where decomposition of BiFeO₃ is observed from 750 °C onwards, and Bi₂Fe₄O₉ is formed. However, optimization of the furnace treatment, considering anneal temperatures and heating rates showed that phase pure BiFeO₃ can be obtained, with the heating rate being the crucial factor (5 °C/min). The chemical purity of the powders is confirmed by FTIR, and the antiferromagnetic to paramagnetic phase transition is demonstrated by DSC measurements.     

Formation and destruction of physical crosslinks by mild treatments in chemically crosslinked poly(vinyl alcohol) gels

We report the formation, destruction, and re-formation of the microcrystallites in a chemical PVA gel, crosslinked slightly by glutaraldehyde. The dried gel, in which the microcrystallites were formed, was immersed in a poor solvent of a mixture of DMSO and water at 8 °C, where the gel stayed in its collapsed state. The gels swelled on heating to 50 °C, but did not return to the initial state on cooling to 8 °C. After washing the gel completely in water, a drying process caused the microcrystallites to be re-formed and the gel could return to the initial dried state at 8 °C. By using XRD and FT-IR measurements, it was concluded that the formation and destruction of microcrystallites in chemically crosslinked PVA gels could be controlled by the mild treatments of initial drying, temperature change, and washing and drying.     

The effect of TiO2 additions on CaO–MgO–Al2O3–SiO2 (CMAS) crystallization behavior from the melt

Titania (TiO₂) was introduced into a model calcium-magnesium aluminosilicate (CMAS) glass in additions of 5-20 wt%. The crystallization behavior of the mixtures was characterized over a series of temperature profiles and compared to that of CMAS alone. X-ray diffraction, differential scanning calorimetry, light and scanning electron microscopy, and energy dispersive spectroscopy were used to characterize glass and crystalline products. Titania additions in the amount of approximately 12.5-20 wt% aided in the formation of CaTiO₃ from melts equilibrated at either 1300 or 1500°C and cooled at 10°C/min. Holding CMAS + TiO₂ (TiO₂ ≥ 10 wt%) at 900°C after cooling from 1300/1500°C resulted in the formation of additional crystalline phases including melilite, paqueite, and diopside. Implications for CMAS interactions with thermal and environmental barrier coatings are discussed.     

High strength,low thermal conductivity and collapsible of Y2O3-stablized HfO2 crystalline fibrous membranes

HfO₂ is an important high temperature resistant material, which has a high melting point (2810 °C), good chemical stability and high thermal radiation. Fibrous material has some advantages for heavy refractory matter, such as large aspect ratio, light weight, more energy saving, flexibility and further can be processed into a variety of product forms. In this paper, high strength HfO₂ crystalline nanofibers were prepared through self-synthesis polyacetylcatonahafnium (PAHf) with electrospinning method. Phase variation process was discussed in detail, and the HfO₂ fibers that use Y₂O₃ as stabilizer with phase stability were prepared finally. The fibers were composed of nanocrystals with good flexibility even folded in half, tensile strength of 1.6 MPa @ 1000 °C and can lifting up 1288 times its own weight, low density around 35 mg/cm³. The thermal conductivity was stable around 26 mW/(m·K) @ 1200 °C and the temperature can be cooled from 1200 °C to 396 °C after 180 s through 7 mm fibrous membranes. And after treated at 1200 °C for 5 h, the fibrous membranes also maintain high strength and good flexibility. By this kind of precursor method, high strength oxide fibers can be obtained, which has a wide application prospect in field of high temperature thermal protection.     

Crystallization behavior and mechanical properties of high strength mica-containing glass-ceramics from granite wastes

The high-strength mica-containing glass-ceramics were prepared from granite wastes by bulk crystallization. The influences of SiO₂/Al₂O₃ molar ratio (S/A = 7.72, 9.62, 12.58, 17.82 and 29.67) on the crystallization behavior, microstructure, mechanical properties and machinability of glass-ceramics were investigated. The results demonstrated that the polymerization degree of the glass network decreased with the S/A ratio increasing, which further caused the decrease in glass transition temperature and crystallization temperatures. The increase in the S/A ratio promoted the precipitation of diopside, hectorite, kalsilite and tainiolite in glass-ceramics when the samples were heated at 750 °C, while inhibiting the precipitation of forsterite. For the glass-ceramics crystallized at 800 and 900 °C, the main crystalline phases transformed from diopside, forsterite, and nepheline to diopside, kalsilite, and tainiolite, with the S/A ratio increasing. As the SiO₂ gradually replaced Al₂O₃, the morphology of crystals changed from lamellar to granular, while the mean size of crystals reduced. The Vickers-Hardness values of glass-ceramics crystallized at 800 and 900 °C ascended with S/A ratio rising, and the values were above 6.30 GPa. The bending strength of most glass-ceramics is stable between 90 and 140 MPa, among which the maximum bending strength is 133.28 ± 14.81 MPa. The fracture toughness of the glass-ceramic crystallized at 800 and 900 °C declined, while that at 700 °C increased with a larger S/A ratio. Glass-ceramics after heat-treated at 900 °C with S/A ratio of 9.62 had the largest fracture toughness of 3.28 ± 0.15 MPa m^1/2. In preliminary tests of machinability, glass-ceramic after heat-treated at 900 °C with S/A ratio of 9.62 showed better results.     

Preparation of Nitrogen‐Doped Mullite Fiber by Nitridation of Silica–Alumina Gel Fiber in Ammonia

Nitrogen‐doped mullite fibers were first synthesized through the nitridation of Al₂O₃–SiO₂ gel fibers in NH₃. The results showed that nitrogen take‐up began at 800°C, reached the maximum at 900°C, and then decreased with increasing temperature. The ceramic fibers nitridated at 900°C were essentially amorphous, but contained a small amount of nano‐sized Al–Si spinel crystals. Mullite was formed after nitridation at 1200°C, accompanied by crystallization of χ‐SiAlON and δ‐Al₂O₃. The incorporation of nitrogen resulted in the formation of a variety of nitrogen‐containing crystalline phases. The grain size of the mullite fibers can be adjusted by changing of the nitrogen content.     

Dispersion and Setting of Powder Suspensions in Concentrated Aqueous Urea Solutions for the Preparation of Porous Alumina Ceramics with Aligned Pores

Highly concentrated alumina powder suspensions have been prepared in aqueous urea solutions of concentrations in the range 200–360 g/100 mL using an ammonium poly(acrylate) dispersant at 80°C. The dispersant concentration for the suspension viscosity minimum in the urea solutions is higher than that in water due to the higher processing temperature. The urea solutions having higher dielectric constant than that of water offer higher interparticle potential that resulted in better dispersion of the powder as evidenced from the lower viscosity and yield stress of the suspensions. The decrease in temperature increased the suspension viscosity and the suspension formed a strong gel when cooled to room temperature due to the crystallization of urea. The minimum urea solution concentration for a 55 vol% alumina suspension to form a dimensionally stable gel is 240 g/100 mL. The compressive strength and Young's modulus of the gels increased with the increase in urea solution concentration. The alumina ceramics prepared by the urea removal followed by sintering at 1500°C had porosity in the range 28–36 vol% with the rectangular rod‐shaped aligned pores.     

Controlled‐Release LCST‐Type Nonwoven Depots via Squeezing‐Out Thermal Response

A novel thermoresponsive fibrous matrix as controlled release depots upon heating is described. The matrix is composed of electrospun fibers of a lower critical solution temperature (LCST)‐type poly(methacrylamide‐co‐N‐tert‐butylacrylamide‐co‐4‐acryloylbenzophenone) P(MAAm‐NtbAAm‐ABP) copolymer. Spherical particles, simulating depots of drugs, are embedded with liquid‐filled inter‐fiber spaces (pores). On heating above 25 °C up to 45 °C, the nanofibers undergo a contraction of about 40%. This solid deformation is attributed to the LCST transition. Fibrous matrix contraction drives expulsion of depots and water solution stored in the pores of the matrix, as evidenced by in situ observations. The liquid flow in the deformable porous medium demonstrates liquid drainage from the matrix as a function of temperature. Experimental results reveal that 70% of the particles are expelled from the matrix upon heating to 45 °C from room temperature. The presented particles encapsulation and release model system using LCST‐type fibrous matrix can be used as a transdermal patch.     

Continuous aluminum oxide-mullite-hafnium oxide composite ceramic fibers with high strength and thermal stability by melt-spinning from polymer precursor

Continuous aluminum oxide-mullite-hafnium oxide (AMH) composite ceramic fibers were obtained by melt-spinning and calcination from polymer precursor that synthesized by hydrolysis of the aluminum isopropoxide, dimethoxydimethylsilane and hafnium alkoxide. Due to the fine diameter of 8–9 µm, small grain size of less than 50 nm and the composite crystal texture, the highest tensile strength of AMH ceramic fibers was 2.01 GPa. And the AMH ceramic fibers presented good thermal stability. The tensile strength retention was 75.48% and 71.49% after heat treatment at 1100 °C and 1200 °C for 0.5 h respectively, and was 61.57% after heat treatment at 1100 °C for 5 h. And the grain size of AMH ceramic fibers after heat treatment was much smaller than that of commercial alumina fibers even when the heat treatment temperature was elevated to 1500 °C, benefited by the grain size inhibition of monoclinic-HfO₂ (m-HfO₂) grains distributed on the boundary of alumina and mullite grains.     

Chromium-promoted preparation of forsterite refractory materials from ferronickel slag by microwave sintering

The chromium-promoted preparation of forsterite refractory materials from ferronickel slag was investigated by microwave sintering of the slag with the additions of sintered magnesia and 0–10 wt% chromium oxide (Cr₂O₃). The thermodynamic calculations revealed that the addition of Cr₂O₃ can promote the formations of spinel and liquid phase and maintain high content of forsterite below 1500 °C. The experimental results showed that there existed a stronger promoting effect of Cr₂O₃ additive on the properties of refractory materials in the microwave field than that in conventional sintering. It was attributed to the preferential formation and growth of spinel with stronger microwave absorption than other phases (e.g., enstatite), the existence of more forsterite, and the enhanced densification in association with the presence of more liquid phase at the same temperature. By microwave sintering of the mixture of ferronickel slag, 25 wt% sintered magnesia, and 4 wt% Cr₂O₃ at 1350 °C for 20 min, a superior refractory material with refractoriness of 1801 °C, thermal shock resistance of 6 times, bulk density of 2.97 g/cm³, apparent porosity of 1.4%, and compressive strength of 197 MPa was obtained. Compared with that prepared by conventional sintering at 1350 °C for 2 h, the refractoriness and thermal shock resistance were increased by 175 °C and 100%, respectively. The present study provided a novel method for preparing high-quality refractory materials from ferronickel slag and relevant industrial wastes.     

Fabrication and Piezoelectric Property of BaTiO3 Nanofibers

BaTiO₃ (BTO) nanofibers were fabricated by sol–gel combined with electrospinning method. The effects of the concentration of acetic acid and sintering temperatures on the crystal phase and microstructure of the samples were investigated by scanning electron microscopy, X‐ray diffraction, and transmission electron microscopy (TEM). BTO nanofibers with improved surface morphology were obtained as the ethanol to acetic acid ratio (E/A) was 8:3. The fibers calcined at 750°C for 2 h exhibited good morphology and crystallization. TEM studies revealed that the BTO nanofibers were polycrystalline, with diameters being on the order of hundreds nanometer, where the existence of domains offered proof of ferroelectric structure. The ferroelectric domains and piezoresponse of BTO nanofibers were characterized by piezoresponse force microscopy. The calculated d₃₃ was 20 pm/V at maximum strain amplitude.