共查询到20条相似文献,搜索用时 46 毫秒
1.
Ba 1−x
Sr
x
TiO 3( x = 0–0.5, BST) nanofibers with diameters of 150–210 nm were prepared by using electrospun BST/polyvinylpyrrolidone (PVP) composite
fibers by calcination for 2 h at temperatures in the range of 650–800 °C in air. The morphology and crystal structure of calcined
BST/PVP nanofibers were characterized as functions of calcination temperature and Sr content with an aid of XRD, FT-IR, and
TEM. Although several unknown XRD peaks were detected when the fibers were calcined at temperatures less than 750 °C, they
disappeared with increasing the temperature (above 750 °C) due to its thermal decomposition and complete reaction in the formation
of BST. In addition, the FT-IR studies of BST/PVP fibers revealed that the intensities of the O–H stretching vibration bands
(at 3430 and 1425 cm −1) became weaker with increasing the calcination temperature and a broad band at 540 cm −1, Ti–O vibration, appeared sharper and narrower after calcination above 750 °C due to the formation of metal oxide bonds.
However, no effect of Sr content on the crystal structure of the composites was detected. 相似文献
2.
The SrFe 12O 19/poly (vinyl pyrrolidone) (PVP) composite fiber precursors were prepared by the sol-gel assisted electrospinning with ferric
nitrate, strontium nitrate and PVP as starting reagents. Subsequently, the M-type strontium ferrite (SrFe 12O 19) nanofibers were derived from calcination of these precursors at 750–1,000 °C.The composite precursors and strontium ferrite
nanofibers were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy
and vibrating sample magnetometer. The structural evolution process of strontium ferrite consists of the thermal decomposition
and M-type strontium ferrite formation. After calcined at 750 °C for 2 h the single M-type strontium ferrite phase is formed
by reactions of iron oxide and strontium oxide produced during the precursor decomposition process. The nanofiber morphology,
diameter, crystallite size and grain morphology are mainly influenced by the calcination temperature and holding time. The
SrFe 12O 19 nanofibers characterized with diameters of around 100 nm and a necklace-like structure obtained at 900 °C for 2 h, which
is fabricated by nanosized particles about 60 nm with the plate-like morphology elongated in the preferred direction perpendicular
to the c-axis, show the optimized magnetic property with saturation magnetization 59 A m 2 kg −1 and coercivity 521 kA m −1. It is found that the single domain critical size for these M-type strontium ferrite nanofibers is around 60 nm. 相似文献
3.
Sr 0.8La 0.2Zn 0.2Fe 11.8O 19/poly(vinyl pyrrolidone) (PVP) composite fiber precursors were prepared by the sol–gel assisted electrospinning. Subsequently,
the M-type ferrite Sr 0.8La 0.2Zn 0.2Fe 11.8O 19 nanofibers with diameters about 120 nm were obtained by calcination of these precursors at different heat treatment conditions.
The precursor and resultant Sr 0.8La 0.2Zn 0.2Fe 11.8O 19 nanofibers were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive
X-ray spectrometer and vibrating sample magnetometer. With the calcination temperature increased up to 1,000 °C for 2 h or
the holding time prolonged to 12 h at 900 °C, the Sr 0.8La 0.2Zn 0.2Fe 11.8O 19 particles gradually grow into a hexagonal elongated plate-like morphology due to the dimensional control along the nanofiber
length. These elongated plate-like particles will be linked one by one to form the nanofiber with a necklace-like morphology.
The magnetic properties of the Sr 0.8La 0.2Zn 0.2Fe 11.8O 19 nanofibers are closely related to grain sizes, impurities and defects in the ferrite, which are influenced by the calcination
temperature, holding time and heating rate. After calcined at 900 °C for 12 h with a heating rate of 3 °C/min, the optimized
magnetic properties are achieved with the specific saturation magnetization 75.0 A m 2 kg −1 and coercivity 426.3 kA m −1 for the Sr 0.8La 0.2Zn 0.2Fe 11.8O 19 nanofibers. 相似文献
4.
A new electrospinning process was developed for preparing TiO2 nanofibers using a water-soluble Ti-precursor, [bis(kappa1O-hydroxo)(bis(kappa2O,O′-lactato)titanium(IV)] commonly known as titanium(IV) bis (ammonium lactato) dihydroxide (TiBALDH). The importance of the study is justified by the fact that Ti-precursors used for electrospinning, sol–gel, hydrothermal and other fiber synthesis processes are mostly non-water soluble. Accordingly, anatase TiO2 nanofibers of diameter between 20 and 140 nm were synthesized by electrospinning and annealing. Polyvinylpyrrolidone (PVP) and different concentrations of TiBALDH were dissolved in a mixture of water, ethyl alcohol and acetic acid to optimize the electrospinning conditions. The thermal decomposition and fragmentation of PVP, TiBALDH and the fibers with 50% mass fraction of TiBALDH were studied by TGA-MS measurements. The fibers were then annealed at 1 °C min?1 until 600 °C. The TiO2 fibers were characterized using SEM–EDX, FTIR and XRD 相似文献
5.
Differential scanning calorimetry data at different heating rates (5, 10, 15 and 20 °C min −1) of Se 70Te 15In 15 chalcogenide glass is reported and discussed. The crystallization mechanism is explained in terms of recent analyses developed
for use under non-isothermal conditions. The value of Avrami exponent ( n) indicates that the glassy Se 70Te 15In 15 alloy has three-dimensional growth. The average values of the activation energy for glass transition, E
g, and crystallization process, E
c, are (154.16 ± 4.1) kJ mol −1 and (98.81 ± 18.1) kJ mol −1, respectively. The ease of glass formation has also been studied. The reduced glass transition temperature ( T
rg), Hruby’ parameter ( K
gl) and fragility index ( F
i) indicate that the prepared glass is obtained from a strong glass forming liquid. 相似文献
6.
WO 3 films have been prepared onto IrO 2-coated Ti substrate by electro-deposition, and as-deposited and annealed films have been characterized by using Raman spectroscopy.
It was found that the asdeposited film consists of orthorhombic WO 3 · H 2O phase, which transforms to amorphous WO 3 by annealing at 250°C and to monoclinic phase by annealing at and above 350°C. All electrochemical experiments were carried
on Ti/IrO 2/WO 3 annealed at 450°C. The open-circuit potential could change significantly due to the hydration of the coating film. However
this process is fairly slow. Reproducible voltammograms could be obtained quickly, further revealing high electrochemical
stability of the Ti/IrO 2/WO 3 electrode. And the shapes of CV show the approximate rectangular mirror image, showing the typical characteristic of capacitive
behavior. The specific capacitance obtained at a scan rate of 50 mV s −1 is 46 F g −1. 相似文献
7.
The composite nanofibers of xSrSiO 3/(100 − x)SrFe 12O 19 ( x = 0–13 wt%) with diameters around 110 nm have been prepared by calcination of the electrospun SrSiO 3/SrFe 12O 19/poly (vinyl pyrrolidone) (PVP) composite fibers at 800–900 °C. The composite nanofibers were characterized by Fourier transform
infrared spectroscopy, X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. After calcined at
800° the M-type strontium ferrite is formed and the strontium silicate exists as an amorphous state when the calcination temperature
below about 950 °C. The addition of SrSiO 3 has an obvious suppression effect on the strontium ferrite grain growth and the ferrite grain size decreases from 66.9 to
33.5 nm corresponding SrSiO 3 content from 0 to 9 wt% in the composite. The specific saturation magnetization ( Ms) of the xSrSiO 3/(100 − x)SrFe 12O 19 composite nanofibers exhibits a continuous reduction from 58.0 to 45.6 A m 2 kg −1 with the increase of SrSiO 3 content from 0 to 13 wt%. With addition of SrSiO 3 from 0 to 13 wt%, the coercivity of the composite nanofibers obtained at 900 °C initially increases, reaching a maximum value
501.1 kA m −1 at the silicate content 7 wt%, and then shows a reduction tendency with the strontium silicate content increase further up
to 13 wt%. This influence on the coercivity by strontium silicate can be attributed mainly to the ferrite grain growth suppression
and the non-magnetic phase barrier for the domains misalignment. 相似文献
8.
The thermoanalytical curves (TA), i.e. TG, DTG and DTA for pure cephalexin and its mixtures with talc, magnesium stearate,
starch and microcrystalline cellulose, respectively, were drawn up in air and nitrogen at a heating rate of 10 °C min −1. The thermal degradation was discussed on the basis of EGA data obtained for a heating rate of 20 °C min −1. Until 250 °C, the TA curves are similar for all mixtures, up this some peculiarities depending on the additive appears.
These certify that between the pure cephalosporin and the excipients do not exists any interaction until 250 °C. A kinetic
analysis was performed using the TG/DTG data in air for the first step of cephalexin decomposition at four heating rates:
5, 7, 10 and 12 °C min −1. The data processing strategy was based on a differential method (Friedman), an integral method (Flynn–Wall–Ozawa) and a
nonparametric kinetic method (NPK). This last one allowed an intrinsic separation of the temperature, respective conversion
dependence on the reaction rate and less speculative discussions on the kinetic model. All there methods had furnished very
near values of the activation energy, this being an argument for a single thermooxidative degradation at the beginning (192–200 °C). 相似文献
9.
Tungsten carbide, WC, has shown dissimilar thermal behavior when it is heated on changeable heating rate and flow of oxidant
atmosphere. The oxidation of WC to WO 3 tends to be in a single and slow kinetic step on slow heating rate and/or low flux of air. Kinetic parameters, on non-isothermal
condition, could be evaluated to the oxidation of WC to heating rate below 15°C min −1 or low flow of air (10 mL min −1). The reaction is governed by nucleation and growth at 5 to 10°C min −1 then the tendency is to be autocatalytic, JMA and SB, respectively. 相似文献
10.
This paper describes a procedure of preparing sodium tantalite nanofibers for the first time. Sodium tantalite nanofibers
were synthesised by electrospinning a sol–gel precursor solution of poly(vinyl pyrrolidone)/sodium tantalite, followed by
careful sintering of the as-electrospun composite fibers at 550 °C for 3 h. The morphology, microstructure and crystal phase
were investigated by transmission electron microcopy and X-ray diffraction. The optical property was characterized by ultraviolet–visible
(UV–vis) spectrometer. Typical nanofibers were with diameter between 70 and 90 nm and length exceeding 0.1 mm. An unusual
phenomenon, the red-shift of optical absorption band edge happened, indicated the fabricated NaTaO 3 nanofibers were potential good candidates for photocatalytic application. The experiment photodegradation of methylene blue
by NaTaO 3 nanofibers under UV light irradiation was performed. 相似文献
11.
Isothermal and non-isothermal thermogravimetric experiments (TG) with real and synthetic (Printex U) soot were performed at
different O 2 concentrations (5–22%O 2/N 2), sample masses (0.5–10 mg), heating (5–20 °C min −1) and flow rates (80–100 mL min −1). The significance of the experimental and calculation uncertainties (i.e. experimental parameter dependencies, calculation
method and mass transfer limitations), which are related to TG for the extraction of chemical kinetics, was explored. Finally,
an intrinsic kinetic equation for soot oxidation is proposed. 相似文献
12.
The thermal decomposition paths of anilinium, 4-chloro anilinium tetrachlorocopper(II) complexes are compared to their benzilinium
derivative. All these complexes crystallize in the layered structure, typical for a A 2MX 4 family, are studied in literature for their magnetic, semiconducting properties. TG analyses of (anilinium) 2CuCl 4 ( A) and (4-chloro anilinium) 2CuCl 4 ( B) loses one molecule of organic ammonium hydrochloride along with one molecule of amine, to form (H)CuCl 3, which subsequently completely decomposes to Cu above 500 °C. On the other hand, (benzilinium) 2CuCl 4 ( C) loses two molecules of hydrochloride along with chlorine molecule first then two molecules of benzyl amine with formation
of Cu above 300 °C. DSC studies on C have shown reversible endothermic phase transition at 130.95 °C (−1.98 J g −1) while heating and exothermic phase transition at 117.07 °C (0.93 J g −1) while cooling. Thus, the observed changes in the decomposition pathway can be correlated to the order–disorder phase transition
occurred in the compound C. 相似文献
13.
A LiFePO 4/C composite was obtained by a polymer pyrolysis reduction method, using lithium polyacrylate (LiPAA) as carbon source and
fractional lithium source, and FePO 4·2H 2O as iron and phosphorus source. The structure of the LiFePO 4/C composites was investigated by X-ray diffraction (XRD). The micromorphology of the precursors and LiFePO 4/C powders was observed using scanning electron microscopy (SEM). Laser particle analyzer and BET were also used to characterize
the materials. It was found that the micromorphology, particle size distribution and specific surface area of LiFePO 4/C composites were greatly influenced by the molecular weight of LiPAA. The electrochemical properties of the LiFePO 4/C composites were evaluated by cyclic voltammograms (CVs), electrochemical impedance spectra (EIS) and constant current charge/discharge
cycling tests. The results showed that the molecular weight of LiPAA, heating rate, synthetic temperature and sintering duration
directly affected the electrochemical properties of LiFePO 4/C composites. The sample with the optimized electrochemical properties were obtained in the following conditions, i.e., LiPAA
with the molecular weight of 20,000, heating rate of 10 °C min −1, synthetic temperature of 700 °C and sintering duration of 15 h. 相似文献
14.
The thermal behaviour of ammonium nitrate (AN) and its prills coated with limestone and dolomite powder was studied on the
basis of commercial fertilizer-grade AN and six Estonian limestone and dolomite samples. Coating of AN prills was carried
out on a plate granulator and a saturated solution of AN was used as a binding agent. The mass of AN prills and coating material
was calculated based on the mole ratio of AN/(CaO + MgO) = 2:1. Thermal behaviour of AN and its coated prills was studied
using combined TG-DTA-FTIR equipment. The experiments were carried out under dynamic heating conditions up to 900 °C at the
heating rate of 10 °C min −1 and for calculation of kinetic parameters, additionally, at 2, 5 and 20 °C min −1 in a stream of dry air. A model-free kinetic analysis approach based on the differential isoconversional method of Friedman
was used to calculate the kinetic parameters. The results of TG-DTA-FTIR analyses and the variation of the value of activation
energy E along the reaction progress α indicate the complex character of the decomposition of neat AN as well as of the interactions
occurring at thermal treatment of AN prills coated with limestone and dolomite powder. 相似文献
15.
The A1, O, AlO, A1 2O, Al 2O 2, WO 2, and WO 3, partial pressures in the vapor over Al 2O 3 in a tungsten Knudsen effusion cell between 2300 and 2600 K were derived from A1 +, O +, AlO +, A1 2O +, Al 2O 2+, WO 2+, and WO 3+, ion intensities. The mass spectrometer was calibrated against the equilibrium constant of the WO 3(g) = WO 2(g) + O(g) reaction. Refined values of the ionization cross sections of AlO and A1 2O 2 were used in the partial pressure calculations. The enthalpies of atomization of aluminum suboxides were determined to be
Δ at
H
o(AlO, g, 0) = 510.7 ± 3.3 kJ mol −1, Δ at
H
o(Al 2O, g, 0) = 1067.2 ± 6.9 kJ mol −1, and Δ at
H
o(Al 2O 2, g, 0) = 1556.7 ± 9.9 kJ mol −1. 相似文献
16.
Tetragonal copper ferrite (CuFe 2O 4) nanofibers were fabricated by electrospinning method using a solution that contained poly(vinyl pyrrolidone) (PVP) and Cu and Fe nitrates as alternative metal sources. The as-spun and calcined CuFe 2O 4/PVP composite samples were characterized by TG-DTA, X-ray diffraction, FT-IR, and SEM, respectively. After calcination of the as-spun CuFe 2O 4/PVP composite nanofibers (fiber size of 89 ± 12 nm in diameter) at 500 °C in air for 2 h, CuFe 2O 4 nanofibers of 66 ± 13 nm in diameter having well-developed tetragonal structure were successfully obtained. The crystal structure and morphology of the nanofibers were influenced by the calcination temperature. After calcination at 600 and 700 °C, the nature of nanofibers changed which was possibly due to the reorganization of the CuFe 2O 4 structure at high temperature, and a fiber structure of packed particles or crystallites was prominent. Crystallite size of the nanoparticles contained in nanofibers increases from 7.9 to 23.98 nm with increasing calcination temperature between 500 and 700 °C. Room temperature magnetization results showed a ferromagnetic behavior of the calcined CuFe 2O 4 samples, having their specific saturation magnetization ( Ms) values of 17.73, 20.52, and 23.98 emu/g for the samples calcined at 500, 600, and 700 °C, respectively. 相似文献
17.
Low molecular weight endo-xylanase from Bacillus pumilus SSP-34 was purified to homogeneity using ion exchange and size exclusion chromatographies. Xylanases were isolated by novel
purification protocol which includes the use of anion exchange matrix such as DEAE Sepharose CL 6B with less affinity towards
enzyme protein. The purified B. pumilus SSP-34 have a molecular weight of 20 kDa, with optimum pH and temperature at 6.0 and 50 °C, respectively. The enzyme was
stable at 50 °C for 30 min. It showed remarkable stability at pH values ranging from 4.5 to 9 when the reaction was carried
out at 50 °C. K
m and V
max values, determined with oats spelts xylan were 6.5 mg ml −1 and 1,233 μmol min −1 mg −1 protein, respectively, and the specific activity was 1,723 U mg −1 相似文献
18.
The formation of Barium monotungstate (BaWO 4) particles in equimolar powder mixtures of BaCO 3 and WO 3 was examined under isothermal and non-isothermal conditions upon heating in air at 25–1200 °C, using thermogravimetry. Concurrence
of the observed mass loss (due to the release of CO 2) to the occurrence of the formation reaction was evidenced. Accordingly, the extent of reaction ( x) was determined as a function of time ( t) or temperature ( T). The x– t and x– T data thus obtained were processed using well established mathematical apparatus and methods, in order to characterize nature
of reaction rate-determining step, and derive isothermal and non-isothermal kinetic parameters. Moreover, the reaction mixture
quenched at various temperatures (600–1,000 °C) in the reaction course was analyzed by various spectroscopic and microscopic
techniques, for material characterization. The results obtained indicated that the reaction rate may be controlled by unidirectional
diffusion of WO 3 species across the product layer (BaWO 4), which was implied to form on the barium carbonate particles. The isothermally determined activation energy (118–125 kJ/mol)
was found to be more credible than that (245 kJ/mol) determined non-isothermally. 相似文献
19.
The kinetic characteristic of thermal decomposition of the Emulsion Explosive Base Containing Fe and Mn elements (EEBCFM)
which was used to prepare nano-MnFe 2O 4 particles via detonation method was investigated by means of non-isothermal DSC and TG methods at various heating rates of
2.5, 5 and 7.5°C min −1respectively under the atmosphere of dynamic air from room temperature to 400°C.
The results indicated that the EEBCFM was sensitive to temperature, especially to heating rate and could decompose at the
temperature up to 60°C. The maximum speed of decomposition (dα/d T) m at the heating rate of 5 and 7.5°C min −1 was more than 10 times of that at 2.5°C min −1 and nearly 10 times of that of the second-category coal mine permitted commercial emulsion explosive (SCPCEE). The plenty
of metal ions could seriously reduce the thermal stability of emulsion explosive, and the decomposition reaction in the conversion
degree range of 0.0∼0.6 was most probably controlled by nucleation and growth mechanism and the mechanism function could be
described with Avrami-Erofeev equation with n=2. When the fractional extent of reaction α>0.6, the combustion of oil phase primarily controlled the decomposition reaction. 相似文献
20.
This paper describes the synthesis of ZrW 2O 8 by the use of an aqueous citrate-gel method in order to prepare a fine, pure and homogeneous oxide mixture suitable for ceramic
processing. The thermal expansion coefficient thus obtained for α-ZrW 2O 8 is −10.6 × 10 −6 °C −1 (50–125 °C) whereas for the β-ZrW 2O 8 a value of −3.2 × 10 −6 °C −1 (200–300 °C) is obtained. The advantages of the use of a sol–gel method is expressed in the very homogeneous end-products.
The paper describes crystallographic data, morphological structure and the thermal expansion properties of the ZrW 2O 8 material. Moreover, photoluminescence and photochromic properties specific to the precursor gel are described and analyzed.
These effects support our views that the precursors show homogeneity up to nanometer level. 相似文献
|