Effect Of Crystallite Size Research Articles

Crystallite size effects on the Raman spectra of Mn 3O 4

The Raman spectra of nanometer Mn 3O 4 with different sizes at room temperature are reported. The Mn 3O 4 powder was prepared by γ-irradiation then annealed at different temperatures to obtain different crystal sizes. The result of experiment shows that the shifting and broadening of Raman spectrum was mainly due to grain size effect. In principle, Raman spectroscopy is relatively simple and easier to use for determination of crystallite size of some materials.

The coercivities of nanophase melt-spun PrFeB alloys

The effects of the mean Pr2Fe14B crystallite size (dg) and alloy composition on the magnetic properties of a series of nanocrystalline melt-spun PrxFe94−xB6 alloys (with 6≤x≤20 at%) have been investigated. In all cases, the remanent polarisation Jr increases with decreasing dg up to a maximum, at which vitrification is initiated. Except for the single phase near-stoichiometric alloy, the intrinsic coercivity also rises with decreasing dg. The composition dependencies of Jr and jHc for dg generally in the range 20–30 nm are compared with those for corresponding NdFeB alloys. Significantly better combinations of jHc and maximum energy product (BH)max are obtained for the PrFeB than for the NdFeB resulting from the higher anisotropy field of the Pr2Fe14B phase.

Effect of solvent, host precursor, dopant concentration and crystallite size on the fluorescence properties of Eu(III) doped yttria

A systematic spectroscopic study of Eu 3+ doped yttria powder phosphors are reported. Fluorescence study of this system has been investigated under the different solvent systems, e.g ethanol and water. In this study, the effect of the precursor of yttria, e.g. acetate, nitrate and chloride, on the fluorescence behaviour of Eu 3+ have been observed in particular. Attention was paid to their effect on the emission intensity of the 5 D 0→ 7 F 2 transition which is characteristic of the `red fluorescence' of the Eu 3+ ion. Results show that the CH3COO − ion has a tremendous effect on the enhancement of emission intensity, greater than NO 3 − and Cl − ions. The Fourier transformed infrared (FTIR) spectroscopy was used for chemical and structural analysis. It has also been stated that the concentration quenching of dopant was more pronounced in the powders derived from yttrium chloride than from yttria precursor. In the case of Y(CH 3COO) 3 as a precursor, the concentration quenching was totally absent and emission intensity increased with an increase in the concentration of dopant. This paper also includes some interesting results on the effect of modifiers, which are a mixture of Tween-80 with ε-caprolactum/ β-alanine/emulsogen-OG, on the 5 D 0→ 7 F 2 transition of Eu 3+ doped yttria in terms of crystallite size in brief.

Crystallite Size and Microstrain of Thermally Aged Low‐Ceria‐ and Low‐Yttria‐Doped Zirconia

The final phases of zirconia powders depend on the synthesis method employed, and the amounts of stabilizer present. In this study, ceria‐ and yttria‐doped zirconia powders were prepared by urea hydrolysis and subsequent hydrothermal treatment. The amount of tetragonal (t) vs monoclinic (m) phase in the powders increased with increasing stabilizer content, while the tetragonal phase size decreased and the microstrain of t crystals remained unchanged. The thermal degradation behavior of the metastable t phase in zirconia containing a low CeO2 or YO1.5 doping level was explored during aging treatment by means of X‐ray line profile broadening analysis. Both ultrafine yttria‐ and ceria‐doped zirconia powder pellets exhibit isothermal t→m transformation after aging at 900°C for various times. It is argued that a crystallite size effect, rather than the dopant valence, dictates the occurrence of the t→m transformation in ultrafine zirconia powders. The change in crystallite sizes of both t and m phases during aging depends significantly on the amount of stabilizer, aging time, and mechanism of t→m phase transformation. However, the change of microstrain in both t and m phases is related to the amount of stabilizers present and the matrix constraints.

Effect of crystallite size on the catalysis of alumina-supported cobalt catalyst for steam reforming of ethanol

The selectivity for steam reforming of ethanol on alumina-supported cobalt catalysts is closely related to the crystallite size of cobalt, which varies the strength of ethanol adsorption.

X-ray diffractometry and high-resolution electron microscopy of neutron-irradiated SiC to a fluence of 1.9×10 27 n/m 2

Neutron-induced damage in SiC up to a fluence of 1.9×10 27 n/m 2 ( E>0.1 MeV) was examined by means of X-ray diffractometry and high-resolution electron microscopy. Specimens of β-SiC were irradiated in fast breeder reactors at 370 to 650°C. The lattice parameters of all specimens were increased by the irradiation, but above 2×10 26 n/m 2 lattice expansion decreased and was accompanied by significant peak broadening. Electron microscopy revealed a high density of interstitial loops on {111}, where X-ray diffraction peaks showed marked broadening. Peak broadening could be attributed mainly to the crystallite size effect at lower fluences, but a strain contribution was significant above 2×10 26 n/m 2. Electron diffraction patterns and high-resolution images indicated preservation of crystallinity up to the highest fluence observed. Thermal annealing up to 1000°C did not affect the peak broadening and average loop diameter. Above 1400°C, decrease in lattice strain and increase in crystallite size with increasing annealing temperature were observed.

Temperature Dependence of Cation Distribution and Oxidation State in Magnetic Mn−Fe Ferrite Nanocrystals

The magnetic properties of spinel nanoparticles are determined by crystal chemistry issues such as cation distribution and oxidation states. The cation distribution and oxidation state of Mn -Fe spinel nanoparticles have been systematically studied at various temperatures by using neutron diffraction and electron energy loss spectroscopy, respectively. The Mn-Fe spinel nanoparticles prepared by coprecipitation have a high degree of inversion with 61% of the tetrahedral sites occupied by Fe 3+ cations. The degree of inversion correlates with the distribution expected from random occupancy of cations consisting of Fe (60%) and Mn (40%). After heat treatment in a vacuum, the cation distribution reaches an equilibrium state with a 29% inversion. Initially, one-half of the Mn cations are in the +3 oxidation state and the other half are in the +2 oxidation state. Mn 3+ cations are slowly and irreversibly reduced to Mn 2+ with increasing temperature. When the temperature approaches 600 °C, all Mn cations are in the +2 state. These results provide direct evidence for the temperature-dependent change of crystal chemistry in Mn-Fe spinel nanoparticles, which has been closely related with the controversy on attributing the changes in the magnetic properties of the nanoparticles to crystallite size effect. These results will also provide an understanding of how to control crystal chemistry in order to control the properties of these magnetic nanoparticles.

Modification of Visible Light Emission from Silicon Nanocrystals as a Function of Size, Electronic Structure, and Surface Passivation

AbstractThe effect of surface passivation and crystallite size on the photoluminescence of porous silicon is reported. Oxygen-free porous silicon samples with medium to ultra high porosities have been prepared by using electrochemical etching followed by photoassisted stain etching. As long as the samples were hydrogen-passivated the PL could be tuned from the red (750nm) to the blue (400nm) by increasing the porosity. We show that when surface oxidation occurred, the photoluminescence was red-shifted. For sizes smaller than 2.8nm, the red shift can be as large as 1eV but for larger sizes no shift has been observed. Comparing the experimental results with theoretical calculations, we suggest that the decrease in PL energy upon exposure to oxygen is related to recombination involving an electron or an exciton trapped in Si=O double bonds. This result clarifies the recombination mechanisms in porous silicon.

Microcrystalline Silicon n-i-p Solar Cells Deposited Entirely by the Hot-Wire Chemical Vapor Deposition Technique

ABSTRACTWe describe a series of microcrystalline (μc) silicon n-i-p solar cell devices fabricated entirely by the hot-wire chemical vapor deposition technique. These devices are deposited on flat stainless-steel at a substrate temperature below 250°C, and are evaluated using solar-cell performance and quantum-efficiency (QE) measurements. We explore the effect of crystallite size, as examined by X-ray diffraction, by varying the hydrogen-to-silane ratio from 5 to 40, while keeping the μc-n and the μc-p layers the same. We find a significant blue shift of the QE peak and an enhancement of red response compared with a standard a-Si:H solar cell. The blue shift increases with increasing hydrogen-to-silane ratio. We attribute this shift to the i-layer becoming more n-type with increasing hydrogen dilution. We also use a hydrogen gas purifier and find a large improvement in device performance.

Laser crystallization and structuring of amorphous germanium

The short-pulse laser crystallization and interference structuring of amorphous germanium films were investigated by time resolved reflection measurements and Raman spectroscopy. We demonstrate that submicrometer crystalline structures with very sharp lateral interfaces can be produced by laser interference crystallization of nonhydrogenated samples. In hydrogenated films, on the other hand, the film surface disrupts upon laser exposure leading to the formation of a free-standing crystalline membrane. The Raman spectra of laser crystallized germanium display effects of finite crystallite size and stress.

Kinetics of Oxygen Reduction on Pt(hkl) Electrodes: Implications for the Crystallite Size Effect with Supported Pt Electrocatalysts

The kinetics of the oxygen reduction reaction (ORR) on Pt(hkl) surfaces is found to vary with crystal face in a different manner depending on the electrolyte. In perchloric acid, the variation in activity at 0.8 to 0.9 V is relatively small between the three low index faces, with activity increasing in the order (100) < (110) ≈ (111). A similar structure sensitivity was observed in KOH, increasing in the order (100) < (110) < (111), but with larger differences. In sulfuric acid, the variations in activity with crystal face were much larger, with the difference between the most active and the least active being about two orders of magnitude, and increased in the opposite order (111) << (100) < (110). The variations in activity with crystal face in perchloric acid and KOH arise from the structure sensitive inhibiting effect of , i.e., a strongly inhibiting effect on (100) and smaller effects on (110) and (111). The variations in activity with crystal face in sulfuric acid arise from highly structure specific adsorption of sulfate/bisulfate anions in this electrolyte, which has a strongly inhibiting effect on the (111) surface. The crystallite size effect for the ORR reported for supported Pt catalysts in sulfuric acid at ambient temperature is fully explained by applying our single crystal results to classical models of the variation in particle shape with size.

Rietveld refinement of nanocrystalline phases

Nanocrystalline phases give rise to x-ray diffraction patterns similar to those obtained for the corresponding microcrystalline system, but broadened by effect of crystallite size. Because of this similarity, for the analysis of the diffraction patterns of nanostructures, the x-ray diffraction techniques developed for microcrystalline powders can be applied. One of these techniques is the refinement of the crystalline structure by the Rietveld method. From the refinement, lattice parameters, atom positions in the unit cell, their occupation numbers, their root-mean-square thermal displacements, average crystallite size, average strain, and the concentration of each of the nanophases in the sample can be obtained. We describe the refinement of the crystalline structure of nanophases of MgO, TiO 2, ZrO 2, and KMn 8O 16

The reactivity of carbon-containing species on nickel. Effects of crystallite size and related phenomena

A pronounced variation of the reactivity of sub-monolayers of carbon-containing species deposited from synthesis gas with the nickel crystallite size is observed. Nickel crystallites of about 4 nm display the presence of a carbon-containing ad-layer of maximum reactivity. The rate of methanation is discussed in terms of the reactivity of the carbonaceous fragments, and the catalyst preparation procedure.

Preparation and properties of Y2O3-doped ZrO2 thin films by the sol-gel process

Y2O3-doped ZrO2 (YZ) thin films were prepared on alumina substrates by the dip-coating process. The dip-coating solution consisted of a homogeneous sol, and was prepared by using the respective chlorides as raw materials, with ethylene glycol, 2-butanol and distilled water as solvents. The thin films containing 0–20 mol % Y2O3 were successfully produced by thermal treatment above 600 °C. The characterization for the film preparation was performed by means of thermogravimetric–differential thermal analysis for the thermal analysis, and scanning electron microscopy for the morphological analysis and thickness measurements. The properties of the films were characterized in terms of a study of the crystalline phase, the crystallite size, the microstructure and the electrical conductivity by using X-ray diffraction, scanning electron microscopy and the complex impedance techniques. In all YZ thin films, the tetragonal phase was stable at low temperatures as a result of the crystallite size effect. However, at higher temperatures, the tetragonal phase was transformed into either the monoclinic phase or the cubic phase, depending on the doping concentration. The YZ thin film of 8 mol% Y2O3 content was stabilized to almost cubic phase at 1000 °C. Resonable conductivity behaviour at YZ was observed for the YZ thin films. The electrical conductivity of YZ thin films was similar to the values of the sintered body.

Size and shape characterization of TiO 2 aerogel nanocrystals

TiO 2 aerogels were obtained by supercritical evacuation of solvent from gels prepared through HCl controlled hydrolysis condensation reactions of titanium isopropoxide in isopropanol. XRD line broadening analysis was used to characterize powders resulting from the calcination of aerogels. Because of diffraction lines overlapping at high angles, the separation between crystallite size and microstrain effects could not be satisfactorily achieved by pattern decomposition. Whole pattern fitting was then used to demonstrate that TiO 2 nanocrystals are practically free of microstrain. According to this result, the analysis of integral widths based on peak by peak fitting was performed to analyze the observed anisotropic size effect of prismatic shaped titanium oxide crystals, approximately 30 by 15 by 15 nanometers in size. These results are in good agreement with High Resolution Transmission Electron Microscopy observations.

Modeling of Extended Defects in the Vanadium Phosphate Catalyst for Butane Oxidation, (VO)2P2O7

X-ray diffraction patterns of catalysts based on (VO)2P2O7show peak broadening effects which cannot be explained by strain and crystallite size effects. We show that the extensive broadening of peaks where thehorlindices are odd is due to extended defects. These defects can be modeled as stacking faults perpendicular to the a and c axes. The proposed defects also explain the streaking effects observed by single crystal X-ray diffraction and by electron diffraction. One of the proposed defects can explain the presence of some V5+in this phase. The amount of V5+found both by titration and by31P NMR spin echo mapping correlates well with defect concentration. The NMR studies also confirm that the V5+present is associated with (VO)2P2O7rather than a second phase. The defects are most prominent in samples prepared to be optimum catalysts for the oxidation ofn-butane to maleic anhydride. Our work suggests a new explanation for the fact that good VPO catalysts must be prepared by dehydration of a V2P2O9·xH2O precursor.

Effects of Nanocrystalline Structure and Passivation on the Photoluminescent Properties of Porous Silicon Carbide

AbstractWe present the results of an investigation of the dependence of the photoluminescence (PL) spectra on preparation conditions, the resulting microstructure, and post-anodization treatment of porous silicon carbide films which were formed from both p and n-type 6H-SiC substrates. Porous samples were prepared by anodic dissolution under different galvanostatic conditions, resulting in different porosities and crystallite sizes. Selected-area electron diffraction patterns taken on similarly prepared porous silicon carbide (PSC) samples confirmed that the films were monocrystalline. Transmission electron microscopy of as-anodized films revealed an isotropie porous network; a dependence of porosity and nanocrystallite size on porous layer formation current density was established. Some PSC samples were passivated using a short, thermal oxidation treatment. The effects of porosity and crystallite size, and of oxide passivation in these PSC films, on PL spectra and intensity were studied using a 365 nm Kr-ion laser as excitation. Under certain conditions, the spectrally integrated PL intensity of a passivated film is more than 450x that for the original bulk SiC substrate. PL spectra are presented, and possible mechanisms are discussed.

Reforming of Methane with Carbon Dioxide to Synthesis Gas over Supported Rhodium Catalysts: I. Effects of Support and Metal Crystallite Size on Reaction Activity and Deactivation Characteristics

The effects of carrier and metal particle size on the catalytic performance (initial intrinsic activity and deactivation characteristics) of Rh in the reaction of reforming of methane with carbon dioxide were investigated. The specific activity of Rh catalysts was found to strongly depend on the carrier employed to disperse the metal, decreasing in the order yttria-stabilized zirconia (YSZ) > Al2O3> TiO2> SiO2> La2O3> MgO, a result which correlates directly with the acidity characteristics of the carrier. The initial intrinsic activity and rate of deactivation of Rh were also found to be sensitive to the metal particle size, in the range 1 ∼ 6 nm. Both activity and rate of deactivation were found to decrease with increasing metal particle size. However, the degree of these dependences was found to be largely affected by the nature of the carrier, suggesting that the dependence of activity and rate of deactivation on metal particle size is likely to be related to metal–support interactions. While relatively high deactivation rates were observed over Rh supported on TiO2and MgO, lower deactivation rates were observed over Rh supported on YSZ, Al2O3, La2O3, and SiO2. Evidence was found that at least three kinds of factors contribute to catalyst deactivation, namely, carbon deposition, metal sintering, and poisoning of surface Rh sites by species originating from the carrier. The importance of each of these factors was found to be determined by the nature of the carrier

Crystallite Size Effects of Carbon‐Supported Platinum on Oxygen Reduction in Liquid Acids

Pt/C catalysts with two different Pt crystallite sizes [as‐received (20 weight percent (w/o) Pt/C powder from E‐TEK) and heat‐treated (900°C in inert atmosphere)] were analyzed for the reduction of oxygen in half‐cells using rotating disk electrodes. Measurements in HF (1.17 M) show that the specific activity of the catalysts increases with the Pt crystallite size. In contrast, measurements in (1.17 M) indicate that the specific activity is nearly independent of the Pt crystallite size. One of the reasons for the difference in behavior observed between these acids is the difference in specific adsorption of anions on the Pt sites from (adsorbing) and HF (nonadsorbing) solutions. This study indicates that both the oxygen reduction reaction and anion adsorption on the Pt sites are crystallite‐size sensitive in solution. In solid polymer fuel cells (SPFCs), the anions are anchored on the backbone of the polymer matrix, and hence only a minimal anion adsorption is expected on the Pt sites of gas diffusion electrodes. From results of the present study, it is believed that the Pt/membrane interface in SPFCs could be better mimicked by the Pt/HF interface rather than the interface.

On the electrical and optical properties of CdS films thermally deposited by a modified source

CdS films were prepared by a physical vacuum deposition technique on an amorphous (glass) substrate using a modified evaporation source. The effect of the processing conditions (the film thickness, substrate temperature and the deposition rate) were considered in comparison with the normal evaporation source. A van der Pauw four-probe arrangement was used for measuring the electrical resistivity, which varies between 0.1 × 104 and 8.18 × 104Ωcm. It decreases with increasing film thickness, and decreasing deposition rate and substrate temperature. This was attributed to the effect of crystallite size, the degree of preferred orientation, internal microstrain and the stoichiometry. From spectrophotometric measuring and using an approximate formula, the optical parameters (n,k, α) were evaluated. The values of the refractive indices and the energy gaps were evaluated and correlated with the preparation parameters.