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1.
Phase evolution and morphology of Fe3O4‐Si‐Al powder mixtures during ball milling from 30 min to 20 h were investigated. A 3‐h critical milling was necessary for the occurrence of mechanically activated combustion reaction. The reaction results in the formation of Fe (Si), Fe3Si, and α‐Al2O3. During ball milling from 3 to 20 h, Fe (Si) and Fe3Si were combined into disordered Fe3Si intermetallic and Fe3Si‐Al2O3 composite powder was formed. The presence of in situ formed alumina leads to a decrease in crystallite and particle sizes. The Fe3Si‐Al2O3 particles after milling for 20 h had a crystalline size of 10~12 nm.  相似文献   

2.
Nanocrystalline microstructure is regarded as a strategic approach to overcome the brittleness of alumina ceramics, and the preparation of disperse equiaxed α‐Al2O3 nanoparticles is an essential step for the preparation of nanocrystalline alumina ceramics. In this work, disperse equiaxed α‐Al2O3 nanoparticles were prepared using α‐Fe2O3 as seed and isolation phase. At first, the composite of α‐Al2O3 nanoparticles embedded in α‐Fe2O3 matrix was obtained by calcining the precursor powder containing γ‐AlOOH and Fe(OH)3 (Fe3+/Al3+ mole ratio of 5) at 770°C for 2 h. Then disperse equiaxed α‐Al2O3 nanoparticles with a mean size of 12 nm and a size distribution from 2 to 40 nm without vermicular microstructure were obtained by removal of α‐Fe2O3 and other impurities in the composite through acid corrosion.  相似文献   

3.
Disperse fine equiaxed α‐Al2O3 nanoparticles with a mean particle size of 9 nm and a narrow size distribution of 2–27 nm were synthesized using α‐Fe2O3 as seeds and isolation via homogeneous precipitation‐calcination‐selective corrosion processing. The presence of α‐Fe2O3 acting as seeds and isolation phase can reduce the formation temperature to 700°C and prevent agglomeration and growth of α‐Al2O3 nanoparticles, resulting in disperse fine equiaxed α‐Al2O3 nanoparticles. These α‐Al2O3 nanoparticles were pressed into green compacts at 500 MPa and sintered first by normal sintering to study their sintering behavior and finally by two‐step sintering (heated to 1175°C without hold and decreased to 1025°C with a 20 h hold in air) to obtain nanocrystalline α‐Al2O3 ceramics. The two‐step sintered bodies are nanocrystalline α‐Al2O3 with an average grain size of 55 nm and a relative density of 99.6%. The almost fully dense nanocrystalline α‐Al2O3 ceramic with finest grains achieved so far by pressureless sintering reveals that these α‐Al2O3 nanoparticles have an excellent sintering activity.  相似文献   

4.
A detailed structural analysis on the in situ synthesized β‐Ca3(PO4)2/α‐Fe2O3 composites is demonstrated. Compositional ratios, the influence and occupancy of iron at the β‐Ca3(PO4)2 lattice, oxidation state of iron in the composites are derived from analytical techniques involving XRD, FT‐IR, Raman, refinement of the powder X‐ray diffraction and X‐ray photoelectron spectroscopy. Iron exists in the Fe3+ state throughout the investigated systems and favors its occupancy at the Ca2+(5) site of β‐Ca3(PO4)2 until critical limit, and thereafter crystallizes as α‐Fe2O3 at ambient conditions. Fe3+ occupancy at the β‐Ca3(PO4)2 lattice yields a Ca9Fe(PO4)7 structure that is isostructural with its counterpart. A strong rise in the soft ferromagnetic behavior of β‐Ca3(PO4)2/α‐Fe2O3 composites is obvious that depends on the content of α‐Fe2O3 in the composites. Overall, the diverse level of iron inclusions at the calcium phosphate system with a Ca/P ratio of 1.5 yields a structurally stable β‐Ca3(PO4)2/α‐Fe2O3 composites with assorted compositional ratios.  相似文献   

5.
Nanoscaled cristobalite and α‐Al2O3 powders were used as the starting materials for synthesizing mullite by solid‐state reaction. The thermal reaction of the cristobalite with α‐Al2O3 during the thermal treatment was examined. Cristobalite powder with a D50 value of 430 nm was adopted to mix with α‐Al2O3 powders with a D50 values of 230, 310, and 400 nm in a stoichiometric composition of 3Al2O3?2SiO2 (71.8 wt% α‐Al2O3 and 28.2 wt% SiO2). Samples for thermal reaction were prepared using uniaxial pressed from the three mixtures that showed various particle number ratios of SiO2/Al2O3 due to the different particle sizes of α‐Al2O3. Examinations were performed by differential thermal analysis, X‐ray diffraction, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and transmission electron microscopy techniques. The results showed that cristobalite particles amorphized during the thermal treatment, and then reacted with the α‐Al2O3 particle to form mullite via nucleation and growth. The amorphization temperature can be reduced by using finer‐sized α‐Al2O3 powders, thus leading to a lower temperature for mullite formation. Mullite crystals with a multidomain structure were observed in the α‐Al2O3 particle matrixes. The crystal orientation of the mullite was controlled by the α‐Al2O3 matrix, that is, [001] α‐Al2O3 → [001] mullite. These results indicate that the amorphization of cristobalite may trigger the reaction of SiO2 with α‐Al2O3, initiating the nucleation of mullite. The α‐Al2O3 particles act as the hosts for mullite formation and determine the size of the mullite particles.  相似文献   

6.
The purpose of this study was to systematically synthesize and characterize the high surface area 10 wt% nanocomposites of α‐Fe2O3 (hematite)/silica using a simple and economically effective homogenous precipitation (HP) route via Response Surface Method combined with Central Composite Design (CCD). Accordingly, the RSM‐CCD approach including 20 experiments was designed to investigate the effects of three factors including concentration of iron chloride solution, pH and calcinations temperature on the final surface area of α‐Fe2O3/silica nanocomposites. The optimum surface area was 373 m2/g at the condition including iron chloride concentration of 0.018 mol/L, pH=8.95, and calcination temperature of 573°C.  相似文献   

7.
Fine powders of GaOOH nanocrystals are synthesized via a facile hydrolysis process through the solution–solution interface reactions of anhydrous GaCl3 and distilled water followed by subsequent solvothermal treatment at mild conditions. Well‐faceted α‐GaOOH hexagonal prism‐like nanorods are prepared through solvothermal treatment at 180°C with CTAB as the morphology controlling surfactant. Ga2O3 nanocrystals are fabricated via the pyrolysis of α‐GaOOH hexagonal prism‐like nanostructures at temperatures above 410°C. A peculiar back‐transformation from β‐Ga2O3 to α‐Ga2O3 has been observed to occur between about 557°C and 719°C, which is considered to be responsible for the coexistence of the two phases. The phase transformation mechanisms of Ga2O3 at elevated temperatures, as well as the possible transformation route, have been postulated from a thermodynamic point of view.  相似文献   

8.
Synthetic α‐Al2O3 platelets, also referred to as corundum and white sapphire, represent attractive fillers improving the mechanical properties of vinylester‐based chemical anchoring systems. Even in the absence of coupling agents, as verified by scanning electron microscopic (SEM) analyses of fracture surfaces, α‐Al2O3 platelets of 200 nm thickness and 5–10 µm size are uniformly dispersed in vinylester resins which are cured by free radical polymerization at room temperature. With increasing content of ultrahard α‐Al2O3 platelets (0–40 wt%) the Young's modulus of α‐Al2O3 platelet/vinylester composites increases from 3200 to 9000 MPa. However, 1–5 wt% 3‐methacryloyloxypropyl‐trimethoxysilane (MPS) as coupling agent, added to the vinylester resin or preferably used to functionalize α‐Al2O3 surfaces in a filler pretreatment step, improves elongation at break (+50%) without sacrificing high stiffness and strength. The X‐ray photoelectron spectroscopy (XPS) analysis confirms the successful surface‐functionalization of α‐Al2O3 platelets by using pretreatments with MPS in toluene, acidified ethanol/water or tetrahydrofuran, respectively. The MPS filler pretreatment simultaneously enhances tensile strength (+22%), elongation at break (+50%), and Young's modulus (+12%) as compared to composites containing unmodified filler. According to SEM analyses of composite fracture surfaces, MPS‐mediated functionalization affords significantly improved interfacial adhesion between α‐Al2O3 platelets and the polymer matrix.

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9.
Herein, Si3N4 powders of comparatively high α‐phase but with distinct morphologies, especially α‐Si3N4 fibers, were successfully prepared by a developed combustion synthesis (CS) strategy. Different proportions of Fe and Fe2O3 were innovatively doped in reactants as additives to control the phase constitution and their relative percentage, as well as morphologies of final microstructures. One step further, the effects of Fe‐contained impurities on the CS process were rationally proposed and verified based on a series of meticulous designed experiments. It turns out that two contradictory effects of metal Fe on the formation of α‐Si3N4 synergistically play vital roles in the CS reaction. The existence of metal Fe can accelerate the crystallization of the amorphous SiO2, which act as protection layer outside the Si powders and subsequently promote the generation of gaseous SiO. These gaseous SiO easily reacts with N2 and eventually form α‐Si3N4. On the other hand, the formation of β‐Si3N4 will be promoted by the assistance of some liquid phases, and in this case, they mainly come from the reaction between Fe and Si. For this study, when the content of doped Fe is below 2 mol%, the prior effect on promoting α‐phase content is pronounced. Otherwise, the latter dominates the CS process as the content of Fe additive is further increased above 2 mol%. In a different way, Fe2O3 mainly encourages the formation of β phase through the large amount of newly generated liquid phases, although the reduced SiO2 and Fe may still promote the α/β ratio on some extent.  相似文献   

10.
This work reports the use of acrylated fatty acid methyl ester (AFAME) as a biomonomer for the synthesis of bio‐based hybrid magnetic particles poly(styrene‐co‐AFAME)/γ‐Fe2O3 produced by miniemulsion polymerization. Poly(styrene‐co‐AFAME)/γ‐Fe2O3 can be tailored for use in various fields by varying the content of AFAME. The strategy employed is to encapsulate superparamagnetic iron oxide nanoparticles (SPIONs) as γ‐Fe2O3 into a styrene/AFAME‐based copolymer matrix. Raman spectroscopy is employed to ensure the formation of the SPIONs (γ‐Fe2O3) obtained by a co‐precipitation technique followed by oxidation of Fe3O4. The functionalization of SPIONs with oleic acid (OA) is carried out to increase the SPIONs–monomer affinity. The presence of OA on the surface of γ‐Fe2O3 is certified by identification of main absorption bands by fourier‐transform infrared spectroscopy (FTIR). Thermal analysis (differential thermogravimetry/differential thermo analysis and differential scanning calorimetry) results of poly(styrene‐co‐AFAME)/γ‐Fe2O3 show an increase in AFAME content leading to a lower copolymer glass transition temperature (T g). Dynamic light scattering (DLS) measurements result in poly(styrene‐co‐AFAME)/γ‐Fe2O3 particles with diameter in the range of 100–150 nm. It is also observed by transmission electron microscopy (TEM) and cryo‐TEM techniques that γ‐Fe2O3 particles are successfully encapsulated into the poly(styrene‐co‐AFAME) matrix.  相似文献   

11.
From rod‐climbing rheometer measurements, we systematically investigated the normal stress values of suspended particles in polymeric liquids at low shear rates using the second‐order fluid constitutional relationship. The climbing constants β of the suspended α‐Fe2O3 particles in polyisobutylene/polybutene solutions, which exhibit Boger fluid characteristics (highly elastic, but no shear‐thinning), were estimated from the rod‐climbing experiment, showing that β increased with polymer concentration and polymer molecular weight. The first normal stress difference coefficient of the suspended α‐Fe2O3 in polymeric liquids obtained from the rod‐climbing rheometer was well correlated with the rheological properties measured by rotational rheometers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1548–1552, 2004  相似文献   

12.
Polyaniline (PANI)‐α‐Fe2O3 nanocomposites (NCs) have been synthesized by chemical oxidative in situ polymerization of aniline in presence of α‐Fe2O3 nanoparticles at 5°C using (NH4)2S2O8 as an oxidant in an aqueous solution of sodium dodecylbenzene sulphonic acid (SDBS), as surfactant and dopant under N2 atmosphere. The room temperature conductivity of NCs decreases and coercive force (Hc) increases with an increase addition of α‐Fe2O3 in PANI matrix. The result of FTIR and TGA shows that the interaction between α‐Fe2O3 particles and PANI matrix could improve the thermal stability of NCs. NCs demonstrate the superparamagnetic behavior. The performance of PANI and PANI‐α‐Fe2O3 NCs as protective coating, against corrosion of 316LN stainless steel in 3.5% NaCl was assessed by potentiodynamic polarization technique. The study shows a good corrosion inhibition effect of both the coatings. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013  相似文献   

13.
The hydrogenation of trans,4-phenyl,3-buten,2-one (benzalacetone) and trans,3-phenyl, propenal (cinnamaldehyde) was carried out on Au supported on iron oxides catalysts. Commercial goethite (FeOOH), maghemite (γFe2O3) and hematite (αFe2O3) were used as supports. The catalytic activity of Au/Fe2O3 reference catalyst, supplied by the World Gold Council, was also investigated. Gold catalysts and the parent supports were characterized by BET, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of ammonia (NH3-TPD) and high resolution transmission electron microscopy (HRTEM).Among the catalysts investigated Au supported on FeOOH shows the highest activity and selectivity to UA in the hydrogenation of unsaturated carbonyl compounds whereas Au supported on αFe2O3 are the less active and selective catalysts.The catalytic activity and selectivity to unsaturated alcohols (UA) in the hydrogenation of benzalacetone and cinnamaldehyde are less influenced by the morphology of gold particles and are mainly influenced by the nature of the support.A correlation between the reducibility of the catalysts and the activity and selectivity to UA has been found. Increasing the reducibility of the catalysts both the activity and selectivity to UA increase. These results let us to argue that active and selective sites are formed by negative gold particles formed through the electron transfer from the reduced support to the metal.  相似文献   

14.
[0001] textured alumina ceramics with a fine grain size were fabricated between 1400°C and 1600°C via templated grain growth (TGG) using fine alumina platelets (~0.6 and ~3 μm diameter) aligned by tape casting in either a 50 nm α‐Al2O3 matrix powder, or in a seeded boehmite sol. The 3 μm templates could be readily aligned by tape casting in both matrices (orientation parameters r = 0.27 and 0.18, respectively), whereas 0.6 μm diameter templates were well aligned in the seeded boehmite sol only (r = 0.29). Improved alignment in boehmite sols is attributed to inorganic gelation, resulting in a strongly pseudo‐plastic rheology that preserves template alignment against the influence of Brownian motion. The in situ formation of fine α‐Al2O3 matrix after transformation in the seeded boehmite system results in a higher driving force for TGG and improves texture development. The combination of 3 μm templates with a seeded boehmite matrix results in extremely high texture qualities (texture fraction f = 0.97–0.99, r = 0.17) while maintaining a relatively fine grain size (5–10 μm in diameter and 1.5–3 μm in thickness). Although undoped samples can be fully textured at 1600°C, adding as little as ~0.25 wt% CaO/SiO2 dopant improves TGG kinetics and yields full texture at 1400°C.  相似文献   

15.
2‐Mercapto‐5‐methylpyridine‐N‐oxide (MMPNO) and its sodium salt (NaMMPNO) were synthesized. The reaction of the latter with Fe3+ generates Fe(MMPNO)3 chelate. The thermolysis of this chelate at 350 °C yielded highly pure reddish‐brown γ‐Fe2O3 nanocrystallites with an average particle size of 6.2 nm, a particle size range of 4.2 to 14.8 nm, and a specific surface area of 51.5 m2g–1. The thermolysis process was optimized using the 22 fractional design. Quantitative tests and characterization of products were carried out by UV‐vis spectroscopy, XRD, LLS, SEM, TGA, BET, TEM, FT‐IR, elemental microanalysis, and classical analytical measurements.  相似文献   

16.
Novel polyimide‐γ‐Fe2O3 hybrid nanocomposite films (PI/γ‐Fe2O3) has been developed from the poly(amic acid) salt of oxydianiline with different weight percentages (5, 10, 15 wt %) of γ‐Fe2O3 using tetrahydrofuran (THF) and N,N‐dimethylacetamide (DMAc) as aprotic solvents. The prepared polyimide‐γ‐Fe2O3 nanocomposite films were characterized for their structure, morphology, and thermal behavior employing Fourier transform infrared spectroscopy (FTIR), scanning electron micrograph (SEM), transmission electron micrograph (TEM), X‐ray diffraction (XRD), 13C‐NMR, and thermal analysis (TGA/DSC) techniques. These studies showed the homogenous dispersion of γ‐Fe2O3 in the polyimide matrix with an increase in the thermal stability of the composite films on γ‐Fe2O3 loadings. Magnetization measurements (magnetic hysteresis traces) have shown very high values of coercive force indicating their possible use in memory devices and in other related applications. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 834–840, 2007  相似文献   

17.
Iron oxide supported oxygen carrier (OC) is regarded to a promising candidate for chemical looping combustion (CLC). However, phase separation between Fe2O3 and supports often occurs resulted from the severe sintering of supports during calcination, which leads to the sintering and breakage of Fe2O3 thus the decrease of redox reactivity. In this article, La‐promoted Fe2O3/α‐Al2O3 were used as OCs for CLC of CH4 and for the first time found that the OC with the addition of 18 wt % La exhibited outstanding reactivity and redox stability during 50 cycles of CLC of CH4. Such a superior performance originated from the formation of LaAl12O19 hexaaluminate (La‐HA) phase with not only small particle size but also excellent thermal stability at CLC conditions, which worked as a binder to prevent the phase separation thereby the sintering and breakage of active species α‐Fe2O3 were avoided during reaction. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2827–2838, 2017  相似文献   

18.
Citric acid‐based sol‐gel method has been used to synthesize metal oxides widely. Iron‐based one‐dimensional nanostructured materials, including Fe2O3 nanotubes and Fe3O4 nanofibers, have been successfully prepared by directly annealing electrospun citric acid‐based precursor fibers under different atmospheres in this study. Thermo‐gravimetric and differential thermal analyses were carried out from room temperature to 800°C under air and argon atmosphere, respectively. The results reveal the formation mechanisms for Fe2O3 nanotube and Fe3O4 nanofiber. Fe2O3 tubular structures with average inner diameter about 500 nm and wall thickness about 20 nm were obtained. Fe3O4 nanoparticles were self‐assembled along the one dimensional orientation to form Fe3O4 nanofibers with average diameter around 500 nm. The reflection losses as a function of frequency for the samples with 23 and 33 wt% Fe3O4 nanofibers in paraffin were examined. The frequency dependence of reflection losses under various matching thicknesses (2, 3, 4, 6 and 8 mm) was simulated. The as‐fabricated Fe3O4 nanofibers can be believed to be promising candidates as highly effective microwave absorbers.  相似文献   

19.
A single crystal of α‐Ca2[HSiO4](OH) (α‐C2SH) was repeatedly imaged at room temperature with synchrotron mid‐infrared microscopy after heating to 310°C, 340°C, 370°C, and 400°C respectively. The mechanisms of the observed phase transformations are discussed on the basis of a modular concept of the crystal structures. All images show domains of dellaite, Ca6[Si2O7][SiO4](OH)2, which are predominantly formed in the core of the crystal. In the crystal rim area α‐C2SH persists in higher abundance. The mechanism of the phase transformation of α‐C2SH into dellaite includes the following: (1) Partial formation of killalaite (Ca3[HSi2O7](OH)) as nuclei according to the isochemical reaction 2Ca2[HSiO4](OH) → Ca3[HSi2O7](OH) + Ca(OH)2 probably induced by anisotropic thermal expansion, local chemical fluctuations, structural (proton) disorder, and different bond strengths of the OH groups in the α‐C2SH structure. (2) Further dehydration of killalaite and α‐C2SH domains results in the formation of dellaite according to Ca3[HSi2O7](OH) + Ca(OH)2 + Ca2[HSiO4](OH) – 2H2O → Ca6[Si2O7][SiO4](OH)2. The results suggest that the polymerization of two isolated [HSiO4] tetrahedra takes place without dehydration according to reaction (1) rather than through condensation with simultaneous H2O release: 2[HSiO4] → [Si2O7] + H2O. We suggest that reaction (1) cannot be completed at ambient pressure. Thus in the regions close to the rim of the crystals we expect the formation of x‐C2S, which starts along the crystal edges according to Ca2[HSiO4](OH) → Ca2SiO4 + H2O. Based on a modular concept, a structural relationship between α‐C2SH, killalaite, dellaite, and x‐C2S has been established. Similarities and differences in the thermal behavior of α‐C2SH and afwillite have been highlighted.  相似文献   

20.
An optically active poly(amide‐imide) (PAI) was synthesized from the polymerization reaction of N,N′‐(Pyromellitoyl)‐bis‐l ‐alanine diacid chloride with 2,5‐diaminotoluene. The obtained inorganic metal oxide nanocomposites composed of PAI/nanostructured hematite (α‐Fe2O3) were synthesized through ultrasonic irradiation. The resulting nanocomposites were characterized by Fourier transform infrared spectroscopy, powder X‐ray diffraction, transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The TEM results indicated that the nanoparticles were dispersed homogeneously in PAI matrix on nanoscale. TGA confirmed that the heat stability of the nanocomposites was improved in the presence of α‐Fe2O3 nanoparticles. POLYM. COMPOS., 37:1805–1811, 2016. © 2014 Society of Plastics Engineers  相似文献   

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