首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 640 毫秒
1.
Suspension of nanocrystalline cellulose (NCC) produced from bleached cotton by controlled sulphuric acid hydrolysis was treated with low frequency ultrasound at 20 kHz and 60% amplitude for 0, 1, 2, 5 and 10 min and the effects of sonication on the properties of both the cellulose nanocrystals and their aqueous suspensions were investigated. Furthermore, a series of nanocellulose films were manufactured from the suspensions that were sonicated for different periods of time and tested. Laser diffraction analysis and transmission electron microscopy proved that sonication not only disintegrated the large NCC aggregates (Dv50 14.7 μm) to individual nanowhiskers with an average length and width of 171 ± 57 and 17 ± 4 nm, respectively, but also degraded the nanocrystals and yielded shorter and thinner particles (118 ± 45 and 13 ± 3 nm, respectively) at 10-min sonication. The ultrasound-assisted disintegration to nano-sized cellulose whiskers decreased the optical haze of suspensions from 98.4% to 52.8% with increasing time from 0 to 10 min, respectively. Sonication of the suspensions significantly contributed to the preparation of films with low haze (high transparency) and excellent tensile properties. With the increasing duration of sonication, the haze decreased and the tensile strength rose gradually. Irrespectively of sonication, however, all films had an outstanding oxygen transmission rate in a range of 5.5–6.9 cm3/m2 day, and a poor thermal stability.  相似文献   

2.
Cavitation holds the promise of a new and exciting approach to fabricate both top down and bottom up nanostructures. Cavitation bubbles are created when a liquid boils under less than atmospheric pressure. The collapse process occurs supersonically and generates a host of physical and chemical effects. We have made an attempt to fabricate natural cellulose material using hydrodynamic as well as acoustic cavitation. The cellulose material having initial size of 63 micron was used for the experiments. 1% (w/v) slurry of cellulose sample was circulated through the hydrodynamic cavitation device or devices (orifice) for 6 h. The average velocity of the fluid through the device was 10.81 m/s while average pressure applied was 7.8 kg/cm2. Cavitation number was found to be 2.61. The average particle size obtained after treatment was 1.36 micron. This hydrodynamically processed sample was sonicated for 1 h 50 min. The average size of ultrasonically processed particles was found to be 301 nm. Further, the cellulose particles were characterized with X-ray diffraction (XRD) and differential scanning calorimetry (DSC) to see the effect of cavitation on crystallinity (Xc) as well as on melting temperature (Tm). Cellulose structures consist of amorphous as well as crystalline regions. The initial raw sample was 86.56% crystalline but due to the effect of cavitation, the crystallinity reduced to 37.76%. Also the melting temperature (Tm) was found to be reduced from 101.78 °C of the original to 60.13 °C of the processed sample. SEM images for the cellulose (processed and unprocessed) shows the status and fiber–fiber alignment and its orientation with each other. Finally cavitation has proved to be very efficient tool for reduction in size from millimeter to nano scale for highly crystalline materials.  相似文献   

3.
Iron-doped nickel oxide (Fe0.01Ni0.99O, abbreviated as FNO) nanoparticles were prepared by sol–gel process using 1,3-propanediol as a solvent and also as a chelating agent, and calcined at the various temperatures (400–1000 °C) for 2 h. The phase composition and the microstructure of the calcined products were investigated by X-ray diffraction and scanning electron microscopy techniques, respectively. Magnetic properties were measured at room temperature using a vibrating sample magnetometer. All calcined samples showed the single phase of FNO cubic rock-salt structure without the presence of any impurity phases. The crystallite size from XRD and particle size from SEM increased as calcining temperature increased. The FNO powders calcined at 400?600 °C revealed the uniform and dense spherical particles in nanosize. The room-temperature ferromagnetism was observed for all samples. When the calcining temperature was increased, the saturation magnetization decreased whereas the coercivity increased, corresponding to the less dense and larger particles. The calcined sample at 400 °C had the best magnetic properties with the highest Ms of 5.34 emu/g (at 10 kOe) and the lowest Hc of 372 Oe.  相似文献   

4.
Synthesis of scorodite (FeAsO4·2H2O) using dynamic action agglomeration and the oxidation effect from ultrasound irradiation was investigated. The effect of different reaction temperatures (90, 70, 50, and 30 °C) on the size and morphology of scorodite particles synthesized under O2 gas flow and ultrasound irradiation was explored because the generation of fine bubbles depends on the solution temperature. At 90 °C, the size of scorodite particles was non-homogeneous (from fine particles (<1 μm) to large particles (>10 μm)). The oxidation–reduction potential (ORP) and yield at 90 °C showed lower values than those at 70 °C. The scorodite particles, including fine and non-homogeneous particles, were generated by a decrease in the oxidation of Fe(II) to Fe(III) and promotion of dissolution caused by the generation of radicals and jet flow from ultrasound irradiation. Using ultrasound irradiation in the synthesis of scorodite at low temperature (30 °C) resulted in the appearance of scorodite peaks in the X-ray diffraction (XRD) pattern after a reaction time of 3 h. The peaks became more intense with a reaction temperature of 50 °C and crystalline scorodite was obtained. Therefore, ultrasound irradiation can enable the synthesis of scorodite at 30 °C as well as the synthesis of large particles (>10 μm) at higher temperature. Oxide radicals and jet flow generated by ultrasound irradiation contributed significantly to the synthesis and crystal growth of scorodite.  相似文献   

5.
In this work, the effect of PZT particle size on the properties of PZT–PC composites was investigated. PZT of various median particle sizes (3.8–620 μm) were used at 50% by volume to produce the composites. The results showed that the dielectric properties of the composites increased marginally with PZT particle size where εr = 176 and 167 for composites with 620 μm and 3.8 μm PZT particle size, respectively. A noticeable increase in d33 values was also found when the particle size was increased where the composite with 620 μm PZT particles size was found to have d33 value of 26 pC/N compared to 17 pC/N for the composite with 3.8 μm PZT particle size. The enhancement in the dielectric and piezoelectric properties was contributed to lesser contacting surfaces between the cement matrix and the PZT particles.  相似文献   

6.
In order to enhance the superconductive properties of the high temperature superconductors, nanoparticles acting as pinning centers can be intentionally introduced into the structure by chemical doping. In this study, a Dy-doped YBa2Cu3O7?x (YBCO) coated conductor, prepared by a metal organic decomposition process, was investigated to determine the size, composition and 3D distribution of the nanoparticles. It was found that the addition of Dy results in the formation of a high density of secondary phase nanoparticles of composition (YsDy1?s)2Cu2O5 with s  0.6. A tomographic tilt series was acquired by using a scanning transmission electron microscope to analyze the interaction between the particles and the structural defects and to determine the 3D distribution of nanoparticles. For the investigated sample volume (0.06 μm3), 71 particles were located with a particle size distribution ranging between 13 and 135 nm with an average size of ~30 nm. The distribution uniformity, position and the size of the particles are observed to be dependent on the interaction of the particles with the twin boundaries. It is observed that the larger particles are generally located on more than one twin boundary, moreover, the particle size is smaller on the twin boundaries shared by several particles. This suggests that the growth of the particles is determined by fast twin boundary diffusion and the formation of the large particles might be prevented by altering the temperature–time parameters of the production processing to enhance the flux pinning characteristic of the superconductors by achieving a more uniform size of flux pinning centers.  相似文献   

7.
Single-domain nanoscale magnetic iron particles have been embedded uniformly in an amorphous matrix of alumina using a pulsed laser deposition technique. Structural characterization by transmission electron microscopy (TEM) reveals the presence of a crystalline iron and an amorphous alumina phase. Fine particle magnetism have been investigated by carrying out field and temperature dependence of magnetization measurements using superconducting quantum interference device magnetometer. The particle size of Fe in Al2O3 matrices prepared by changing the deposition time of Fe, have been found to be 9, 7 and 5 nm from TEM studies. At 10 K, the coercivities of these samples are found be 450, 350 and 150 Oe, respectively. At 300 K, the coercivity of Fe–Al2O3 sample decreases from 100 to 50 Oe as the particle size decreases from 9 to 7 nm and finally the sample turns superparamagnetic when the Fe particle size becomes around 5 nm. Based on the calculated value of blocking temperature, TB, (481 K), magnetic anisotropy K (4.8×105 erg/cm3) for Fe, and the Boltzmann constant kB (1.38×10−16 erg/K) from TB=KV/25kB, the mean radius of Fe particles is found to be 9.3 nm. in one of the samples. This is in good agreement with the particle size measured using TEM studies.  相似文献   

8.
Recent research has shown that high frequency ultrasound (0.4–3 MHz), can enhance milkfat separation in small scale systems able to treat only a few milliliters of sample. In this work, the effect of ultrasonic standing waves on milkfat creaming was studied in a 6 L reactor and the influence of different frequencies and transducer configurations in direct contact with the fluid was investigated. A recombined coarse milk emulsion with fat globules stained with oil-red-O dye was selected for the separation trials. Runs were performed with one or two transducers placed in vertical (parallel or perpendicular) and horizontal positions (at the reactor base) at 0.4, 1 and/or 2 MHz (specific energy 8.5 ± 0.6 kJ/kg per transducer). Creaming behavior was assessed by measuring the thickness of the separated cream layer. Other methods supporting this assessment included the measurement of fat content, backscattering, particle size distribution, and microscopy of samples taken at the bottom and top of the reactor. Most efficient creaming was found after treatment at 0.4 MHz in single and double vertical transducer configurations. Among these configurations, a higher separation rate was obtained when sonicating at 0.4 MHz in a vertical perpendicular double transducer setup. The horizontal transducer configuration promoted creaming at 2 MHz only. Fat globule size increase was observed when creaming occurred. This research highlights the potential for enhanced separation of milkfat in larger scale systems from selected transducer configurations in contact with a dairy emulsion, or emulsion splitting in general.  相似文献   

9.
LiFePO4 nanoparticles were synthesized via polyol process. The temperature of the solution was rapidly increased up to 320 °C to obtain larger particles and the temperature was maintained for 16 h in a round-bottomed flask attached to a refluxing condenser. The X-ray diffractrion (XRD), pattern was indexed on the basis of orthorhombic olivine structure. The LiFePO4 compound prepared through polyol process exhibited a high crystallinity. The particles show the various shapes with size ranging from 100 to 300 nm. The initial discharge curve of LiFePO4 capacity shows 168 mA h/g at the 0.1C rate in the voltage range of 2.5–4 V with well-formed plateau.  相似文献   

10.
A joint chemical reactor system referred to as an ultrasonic-intensified micro-impinging jetting reactor (UIJR), which possesses the feature of fast micro-mixing, was proposed and has been employed for rapid preparation of FePO4 particles that are amalgamated by nanoscale primary crystals. As one of the important precursors for the fabrication of lithium iron phosphate cathode, the properties of FePO4 nano particles significantly affect the performance of the lithium iron phosphate cathode. Thus, the effects of joint use of impinging stream and ultrasonic irradiation on the formation of mesoporous structure of FePO4 nano precursor particles and the electrochemical properties of amalgamated LiFePO4/C have been investigated. Additionally, the effects of the reactant concentration (C = 0.5, 1.0 and 1.5 mol L−1), and volumetric flow rate (V = 17.15, 51.44, and 85.74 mL min−1) on synthesis of FePO4·2H2O nucleus have been studied when the impinging jetting reactor (IJR) and UIJR are to operate in nonsubmerged mode. It was affirmed from the experiments that the FePO4 nano precursor particles prepared using UIJR have well-formed mesoporous structures with the primary crystal size of 44.6 nm, an average pore size of 15.2 nm, and a specific surface area of 134.54 m2 g−1 when the reactant concentration and volumetric flow rate are 1.0 mol L−1 and 85.74 mL min−1 respectively. The amalgamated LiFePO4/C composites can deliver good electrochemical performance with discharge capacities of 156.7 mA h g−1 at 0.1 C, and exhibit 138.0 mA h g−1 after 100 cycles at 0.5 C, which is 95.3% of the initial discharge capacity.  相似文献   

11.
Heterostructured CdS-pillared H2Ti3O7 nanohybrids were prepared by the self-assembly of exfoliated trititanate nanosheets and CdS nanosol particles under the electrostatic interactions. It was revealed that the present nanohybrids were mesoporous with specific surface areas of about 90 m2 g−1. The nanohybrids exhibited high photocatalytic activity and good recurrence stability in the H2 evolution from water splitting. When the preparation molar ratio of H2Ti3O7/CdS was 2:1, the nanohybrid reached a high H2-evolution rate of 1523 μmol h−1 g−1 under a 300 W Xe lamp irradiation, which was 13 times higher than the bare CdS. Apart from the wider spectral responsive range, the superior photocatalytic performance of the nanohybrids was predominantly attributed to the efficient photogenerated charge separation between the trititanate nanosheets and the encapsulated CdS nanoparticles.  相似文献   

12.
The catalytic performance of nanostructured Ni-Co/Al2O3-ZrO2 catalysts, prepared by ultrasound-assisted impregnation method was examined in the dry reforming of methane. The effect of irradiation power and irradiation time have been studied by changing time (0, 20, 80 min) and power of the sonication (30, 60, 90 W) during the synthesis which resulted in different physiochemical properties of the nanocatalyst. The nanocatalysts were characterized by XRD, FESEM, PSD, EDX, TEM, TPR-H2, BET, FTIR and TG analyses. Based on the characterization results, ultrasound treatment endowed the sample with more uniform and smaller nanoparticles; higher surface area, stronger metal-support interaction and more homogenous dispersion. Moreover, the analyses exhibited smaller particles with higher surface area and less population of particle aggregates at longer and highly irradiated nanocatalysts. The nanocatalyst irradiated at 90 W for 80 min (the longest irradiation time and the most intense power) showed a uniform morphology and a very narrow particles size distribution. More than 65% of particles of this nanocatalyst were in the range of 10–30 nm. Activity tests demonstrated that employing ultrasound irradiation during impregnation improves feed conversion and products yield, reaching values close to equilibrium. Among sonicated nanocatalysts, with increasing power and time of irradiation, the nanocatalyst represents higher activity. The superior performance amongst the various bimetallic catalysts tested was observed over the catalyst with 90 W and 80 min ultrasonic irradiation which is stable in 24 h time on stream test. The excellent anti-coking performance of this bimetallic catalyst, confirmed by TG and FESEM analyses of spent catalyst, is closely related to the promoting effect of sonication on the metal-support interaction, Ni dispersion and particle size; and probably, the synergy between metallic species.  相似文献   

13.
The ultrasonic fractionation of milk fat in whole milk to fractions with distinct particle size distributions was demonstrated using a stage-based ultrasound-enhanced gravity separation protocol. Firstly, a single stage ultrasound gravity separation was characterised after various sonication durations (5–20 min) with a mass balance, where defined volume partitions were removed across the height of the separation vessel to determine the fat content and size distribution of fat droplets. Subsequent trials using ultrasound-enhanced gravity separation were carried out in three consecutive stages. Each stage consisted of 5 min sonication, with single and dual transducer configurations at 1 MHz and 2 MHz, followed by aliquot collection for particle size characterisation of the formed layers located at the bottom and top of the vessel. After each sonication stage, gentle removal of the separated fat layer located at the top was performed.Results indicated that ultrasound promoted the formation of a gradient of vertically increasing fat concentration and particle size across the height of the separation vessel, which became more pronounced with extended sonication time. Ultrasound-enhanced fractionation provided fat enriched fractions located at the top of the vessel of up to 13 ± 1% (w/v) with larger globules present in the particle size distributions. In contrast, semi-skim milk fractions located at the bottom of the vessel as low as 1.2 ± 0.01% (w/v) could be produced, containing proportionally smaller sized fat globules. Particle size differentiation was enhanced at higher ultrasound energy input (up to 347 W/L). In particular, dual transducer after three-stage operation at maximum energy input provided highest mean particle size differentiation with up to 0.9 μm reduction in the semi-skim fractions. Higher frequency ultrasound at 2 MHz was more effective in manipulating smaller sized fat globules retained in the later stages of skimming than 1 MHz. While 2 MHz ultrasound removed 59 ± 2% of the fat contained in the initial sample, only 47 ± 2% was removed with 1 MHz after 3 ultrasound-assisted fractionation stages.  相似文献   

14.
An attempt to prepare a metalorganic precursor of gallium with reactivity at low temperature in chemical vapor deposition (CVD) systems was done by reacting N-methylpyrrolidine with metal gallium or gallium nitrate under mild conditions. The precursors were bubbled into a CVD assemblage and then reacted with ammonia at temperatures between 400 and 700 °C. The depositions onto silicon substrates were pyramidal particles of 100 nm width at the base and up to 55 nm in height. The rise in growth temperature increased particle density from 0.9 to 27.1 particles per square micron, but reduced the height from 50 to 10 or 2 nm. XPS spectra showed the presence of gallium and nitrogen. The intensity of the gallium spectrum decreased as the process temperature increased indicating that GaNx particles were deposited rather than the stoichiometric gallium nitride (GaN). An additional N1s band of impurities appeared whose intensity increases with the reaction temperature. The lower impurity content corresponded to the sample prepared with Ga(NO3)3 at 400 °C.  相似文献   

15.
The polymorphic forms of lactose in alcoholic suspensions have been determined by 13C CP-MAS NMR spectroscopy, employing hand-made glass inserts. Suspensions of alpha lactose monohydrate (Lα·H2O) with particle size between 2 and 200 μm were prepared by 24 h reflux or by storage for 28 d in anhydrous ethanol without agitation. These suspensions were compared to an ethanolic sub-micron lactose suspension provided by a 3 M Health Care (Loughborough). The 13C CP-MAS NMR spectra indicated that Lα·H2O dehydrated to stable anhydrous alpha lactose polymorph (LαS) whilst suspended in ethanol. In addition, strong ethanol 13C resonances were observed for some samples, indicating a liquid–solid interaction between the ethanol and lactose surface. Replacement of ethanol with anhydrous methanol, n-butanol and 3-methylbutan-2-ol implied that the solvent mediated dehydration of Lα·H2O to LαS occurs as a result of sterically controlled interactions.  相似文献   

16.
A 3C-silicon carbide (SiC) thin film grown on a Si(1 0 0) surface using an ethylene (C2H4) molecular beam has been studied by atomic force microscopy. At the center of the irradiation area of the ethylene beam, the shape of the SiC islands was rectangular, the average length of which was 74.5 nm and the average height was 13.1 nm. Each SiC island consists of the SiC particles with the average diameter of 17 nm. Just inside of the boundary region of the beam irradiation, the average size and height of the islands decreased to 50.1 and 8.2 nm, respectively. Just outside of the boundary region, the average size and height decreased to 17.7 and 5.1 nm, respectively. The average reaction probabilities at the above three points were estimated to be 0.14, 0.27 and 2.7%, respectively. New growth mode of the crystal growth is proposed (particles gathering island mode).  相似文献   

17.
NdVO4 nanoparticles are successfully synthesized by efficient sonochemical method using two different structural directing agents like CTAB and P123. The phase formation and functional group analysis are carried out using X-ray diffraction (XRD) and fourier transform infra red (FT-IR) spectra, respectively. Using Scherrer equation the calculated grain sizes are 27 nm, 24 nm and 20 nm corresponding to NdVO4 synthesized by without surfactant, with CTAB and P123, respectively. The TEM images revealed that the shape of NdVO4 particles is rice-like and rod shaped particles while using CTAB and P123 as surfactants. The growth mechanism of NdVO4 nanoparticles is elucidated with the aid of TEM analysis. From electrical analysis, the conductivity of NdVO4 nanoparticles synthesized without surfactant showed a higher conductivity of 5.5703 × 10−6 S cm−1. The conductivity of the material depends on grain size and increased with increase in grain size due to the grain size effect. The magnetic measurements indicated the paramagnetic behavior of NdVO4 nanoparticles.  相似文献   

18.
We studied the effect of TiO2 doping on flux pinning and superconducting properties of a melt-grown (Nd0.33Eu0.33Gd0.33) Ba2Cu3Oy + 35 mol% Gd2BaCuO5 (70 nm in size) composite (NEG-123) processed in Ar–1% O2 atmosphere. As indicated by similar, sharp superconducting transitions, the small quantities of TiO2 used in our experiments did not deteriorate superconducting properties of the NEG material. Transmission electron microscopy (TEM) analysis found 20–50 nm Ti-based particles in the NEG-123 matrix. However, we have not observed the clouds of <10 nm sized particles in the NEG-123 matrix, as in the case of recently reported NEG-123 composites doped by Mo and Nb nanoparticles. Nevertheless, quite a good JcB performance in the 0.1 mol% Ti-doped sample, namely 550 kA/cm2 at the self-field and at the secondary peak field (4.5 T) was achieved at 65 K, while 320 kA/cm2 was obtained at zero-field at 77 K, and 50 kA/cm2 at 90.2 K. The pinning effectiveness decreased with increasing Ti content above 0.2 mol%. The analysis of the pinning force showed that higher concentration of Ti (>0.2 mol%) increased the amount of normal pins (δl pinning), indicated by the Fp(h) peak shift from h = 0.42–0.36. The maximum pinning effect in a broad field range could be achieved by optimizing Ti content and adding sub-micron Gd-211 particles.  相似文献   

19.
The transition metal-doped spinel cathode materials, LiM0.5Mn1.5O4 (M=Ni. Co, Cr) were prepared by solid-state reaction. The structure and morphology of the samples were investigated by X-ray diffraction, Rietveld refinement and scanning electron microscopy (SEM). The diffraction peaks of all the samples corresponded to a single phase of cubic spinel structure with a space group Fd3m. Field-emission SEM shows octahedron like shapes and the primary particles size was between 500 nm and 2 μm. Oxidation states of Ni, Co and Cr were found to be 2+, 2+ and 3+ as revealed by X-ray photoelectron spectroscopy. During discharging, LiNi0.5Mn1.5O4 and LiCo0.5Mn1.5O4 sample shows more than 130 mAh/g between 3.5 and 5.2 V at a current density of 0.65 mA/cm2 and well developed plateau around 5 V, respectively.  相似文献   

20.
Low temperature solution combustion method was employed to synthesize Dy2O3 nanophosphors using two different fuels (sugar and oxalyl dihydrazine (ODH)). Powder X-ray diffraction confirm pure cubic phase and the estimated particle size from Scherrer's method in sugar and ODH fuel was found to be 26 and 78 nm, respectively, and are in close agreement with those obtained using TEM and W–H plot analysis. SEM micrographs reveal porous, irregular shaped particles with large agglomeration in both the fuels. An optical band gap of 5.24 eV and 5.46 eV was observed for Dy2O3 for sugar and ODH fuels, respectively. The blueshift observed in sugar fuel is attributed to the particles size effect. Thermoluminescence (TL) response of cubic Dy2O3 nanophosphors prepared by both fuels was examined using gamma and UV radiations. The thermoluminescence of sugar used samples shows a single glow peak at 377 °C for 1–4 kGy gamma irradiations. When dose is increased to 5 kGy, two more shouldered peaks were observed at 245 and 310 °C. However, in TL of ODH used samples, a single glow peak at 376 °C was observed. It is observed that TL intensity is found to be more in sugar used samples. In UV irradiated samples a single glow peak at 365 °C was recorded in both the fuels with a little variation in TL intensity. The trapping parameters were estimated by different methods and the results are discussed.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号