Nanocrystalline Character Research Articles

Characterization of Al/SiC Nanocomposite Prepared by Mechanical Alloying Method

In this research work, a high-energy ball mill has been applied to prepare an Al/SiC nanocomposite. The formation mechanism of the nanocomposite was investigated. This nanocomposite contained the nanocrystalline characteristics. Crystallite size, lattice strain and particle size of the nanocomposite as a function of milling time were determined. SEM micrographs showed that the nanocomposite powders agglomerated after milling. The particle size analysis confirmed the agglomeration of the nanocomposite particles. TEM observations showed that the SiC particles were in the nanometer size and these particles embedded in the Al matrix, and the nanocomposite produced in the final stage of mechanical alloying. In addition, a simple model checked for the refinement of the crystallite and the particle size of nanocomposite.

Mesoporous Anatase TiO2Films: Use of Ti K XANES for the Quantification of the Nanocrystalline Character and Substrate Effects in the Photocatalysis Behavior

Mesoporous titania thin films with accessible porosity and anatase structure were synthesized on conductive glass or silicon substrates. Ti K-edge XANES was used to assess Ti local coordination. Analysis of the pre-edge region permitted accurate quantification of the degree of crystalline nature of the inorganic walls upon thermal treatment. The substrate has a marked effect: film crystallization takes places at temperatures 100 °C lower in the crystalline Si, with respect to conductive glass. Accordingly, remarkable photocatalytic properties are found in well-crystallized mesoporous titania deposited onto conductive silicon.

A comparative research on physical and mechanical properties of (Ti, Al)N and (Cr, Al)N PVD coatings with high Al content

Abstract Arc ion plating (Ti 0.34 , Al 0.66 )N and (Cr 0.28 , Al 0.72 )N coatings are deposited onto cemented carbide substrates at 350 °C. The crystal structure and microstructure of the deposited coatings are characterized by means of X-ray diffraction (XRD), electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). XRD examination indicates that both (Ti 0.34 , Al 0.66 )N and (Cr 0.28 , Al 0.72 )N coatings are of fcc structure. The atomic ratios of Al against Ti (Cr) for the two coatings are approximately close to those of the alloy targets according to EPMA measurement. In accordance with SEM observation, the fractured cross-section of the (Cr 0.28 , Al 0.72 )N coating is composed of columnar crystallites repeatedly interrupted by a renucleation process. Based on SEM and TEM methods, the cross-sectional fracture of (Ti 0.34 , Al 0.66 )N coating is found to show nano-crystalline feature without appearance of the columnar structure. (Cr 0.28 , Al 0.72 )N coating exhibits better wear resistance, approximately equal adhesion with the substrate, but lower nano-hardness compared with (Ti 0.55 , Al 0.45 )N coating.

Synthesis and reaction kinetics for monodispersive Y 2O 3:Tb 3+ spherical phosphor particles

Monodispersive Y 2O 3:Tb 3+ spherical phosphor was synthesized by homogenous precipitation process. By using X-ray diffractometer, thermogravimetric analysis (TGA), transmission and scanning electron microscopes, and inductively coupled plasma (ICP)-optical emission spectrometry (OES), the precipitation behaviors were analyzed. The results indicated that the kinetics of the hydrolysis reaction of Y 3+ and Tb 3+ was a zero-order reaction, controlled by the decomposition of urea with an activation energy of 131 kJ/mol. Besides, the effect of urea concentration, reaction temperature, and aging time for the control of the precipitate size were also investigated. The dopant Tb 3+ in the solution did not significantly influence the precipitation of the yttrium component. After 800 °C calcination, the Y 2O 3:Tb 3+ powder showed nano-crystalline characteristics and emitted strong green light by a UV excitation light source.

Role of Nanocrystalline Spinel Additive on the Properties of Low Cement Castable Refractories

Hydrated magnesium aluminate gel prepared via a cost-competent sol-gel route has been found to retain nanocrystals after optimum thermal treatment. The reactive gel-derived powder incorporated to the matrix of a high alumina based refractory castable upgraded its physical properties as revealed from the laboratory scale results. The performance of sol-gel spinel is better than coprecipitated spinel and almost comparable to presintered spinel powders popularly utilized by the refractory manufacturers. Attempts have been made to evaluate and analyze the better slag corrosion and thermal shock resistance properties of spinel-bonded castable by the nanocrystalline feature of the sol gel spinel. SEM, TEM, SAED, XRD, IR, and the physical properties of selected samples have been studied to understand the characteristics of the respective refractory materials.

Annealing of ZnS nanocrystals grown by colloidal synthesis

ZnS nanocrystals (NCs) capped with tetramethylammonium (TMAH) were synthesized from ZnCl2·2H2O and thiourea using a wet chemical process. Further treatments of the nanocrystals such as aging, and annealing have been conducted to examine the stability of the grown samples. The X-ray diffraction spectra show that the crystal has a zinc blende structure with particle size of about 2nm. The evidence of nanocrystalline character is also clear in the UV–Vis absorption that shows an excitonic peak at about 236nm (5.2eV) arising from band edge transitions. A photoluminescence emission peak centered at about 450nm (2.7eV) is attributed to transitions between shallow donors and Zn+ vacancies. Both absorption and photoluminescence spectra show that sample aging does not affect the characteristics of the sample, possibly due to protection by TMAH capping. Annealing at 700°C and 900°C results in the red shift of the photoluminescence.

Nano-Crystalline Grained Silicon Formed by Rapid Thermal Chemical Vapor Deposition for Advanced Gate Electrode Application

With continuous down scaling of semiconductor devices, poly depletion has become a significant portion of gate dielectric inversion thickness. In an effort to reduce poly depletion, nano-crystalline grained (NCG) silicon has been developed using a rapid thermal chemical vapor deposition process. The grain structures were controlled through varying deposition temperature, pressure and flow rate of gases. NCG silicon formation has been engineered through a new two step process. The structure demonstrated retention of nano-crystalline characteristics with limited grain growth after boron and phosphorous doping and high temperature dopant activation annealing. Higher boron concentration at gate and dielectric interface provides a potential poly depletion reduction of about 1.2 Aå, compared with conventional columnar grained poly- silicon.

A Methodology for Investigating New Nonprecious Metal Catalysts for PEM Fuel Cells

This paper reports an approach to investigate metal-chalcogen materials as catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells. The methodology is illustrated with reference to Co-Se thin films prepared by magnetron sputtering onto a glassy-carbon substrate. Scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) have been used, in parallel with electrochemical activity and stability measurements, to assess how the electrochemical performance relates to chemical composition. It is shown that Co-Se thin films with varying Se are active for oxygen reduction, although the open circuit potential (OCP) is lower than for Pt. A kinetically controlled process is observed in the potential range 0.5-0.7 V (vs reversible hydrogen electrode) for the thin-film catalysts studied. An initial exposure of the thin-film samples to an acid environment served as a pretreatment, which modified surface composition prior to activity measurements with the rotating disk electrode (RDE) method. Based on the SAM characterization before and after electrochemical tests, all surfaces demonstrating activity are dominated by chalcogen. XRD shows that the thin films have nanocrystalline character that is based on a Co(1-x)Se phase. Parallel studies on Co-Se powder supported on XC72R carbon show comparable OCP, Tafel region, and structural phase as for the thin-film model catalysts. A comparison for ORR activity has also been made between this Co-Se powder and a commercial Pt catalyst.

Influence of uranium content on magnetostriction of [formula omitted] alloys

The aim of this work was to study the influence of annealing treatment on structure, magnetic properties and magnetostriction of the Fe 73.5 Cu 1 Nb 3 - x U x Si 13.5 B 9 nanocrystalline alloys. The results confirmed the nanocrystalline character of these alloys in the temperature range 550– 650 ∘ C . The influence of the uranium content on the structural stability has been observed for annealing treatment at higher temperature, i.e. at about 700 ∘ C . The coercivity and the magnetostriction strongly depended both on the annealing temperature and on the uranium content.

Field emission measurements from carbon films of a predominant nano-crystalline diamond character grown by energetic species

Here we report for the first time on the electron field emission (EFE) properties of carbon films of a predominant nano-crystalline diamond character deposited by the glow discharge direct current (GDDC) technique in our laboratory. By this method, film growth occurs by a subsurface implantation process of energetic carbon species without any pretreatment. These Nano-diamond (ND) films comprise an amorphous layer a few nanometer thick on their surface. The presence of this surface layer inhibits EFE of the as-deposited films, which only comes to electrical discharge (ED). After removal of the amorphous layer by short hydrogen plasma etch (leaving the layer to be H-terminated ND), the film emits at a turn on field (TOF) of 15.4 ± 2 and 14 ± 3 V/μm for different samples. Thermal treatment of this film at 1030 °C removes the hydrogen from the surface layer and results in surface reconstruction creating a partially graphitic surface, which leads to degraded EFE properties. Surface composition, structure and morphology of the films are correlated to the EFE properties by using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS).

Microstructure and Mechanical Properties of Magnetron Sputtered (Ti, Al)N Coatings with fcc Structure

Magnetron sputtered (Ti0.55, Al0.45)N and (Ti0.34, Al0.66)N coatings are deposited onto cemented carbide substrates at 350°C. The crystal structure and microstructure of the deposited coatings are characterized by means of X-ray diffraction (XRD), electron probe microanalysis (EPMA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Nanoindentation is employed to determine the microhardness of the coatings. The adhesion between the coating and substrate is investigated using scratch test. Measurement of flank wear is used to evaluate the wear resistance of the coatings. XRD examination indicates that both (Ti0.55, Al0.45)N and (Ti0.34, Al0.66)N coatings are of fcc structure. The atomic ratios of Al against Ti for the two coatings are approximately close to those of the alloy targets according to EPMA measurement. In accordance with SEM observation, the fractured cross-section of (Ti0.55, Al0.45)N coating is characterized by dense columnar grain structure with most of grains extending from the interface of coating and substrate to the surface. SEM and TEM investigations show that the (Ti0.34, Al0.66)N coating indicates the nano-crystalline feature without appearance of the columnar structure. (Ti0.34, Al0.66)N coating exhibits higher microhardness and wear resistance, but lower adhesion with the substrate than (Ti0.55, Al0.45)N coating.

Interaction effects in magnetic oxide nanoparticle systems

The interaction effects in magnetic nanoparticle system were studied through a Monte Carlo simulation. The results of simulations were compared with two different magnetic systems, namely, iron oxide polymer nanocomposites prepared by polymerization over core and nanocrystalline cobalt ferrite thin films prepared by sol-gel process. The size of the particles in the nanocomposites were estimated to be ∼15 nm with very little agglomeration. The low values of the coercivity obtained from the hysteresis measurements performed confirm that the system is superparamagnetic. SEM studies showed the cobalt ferrite films to have a nanocrystalline character, with particle sizes in the nanometer range. Hysteresis measurements performed on the thin films coated on silicon do not give evidence of the superparamagnetic transition up to room temperature and the coercivity is found to increase with decreasing film thickness. Comparison with simulations indicate that the nanocomposites behave like a strongly interacting array where exchange interactions lead to high blocking temperatures, whereas the films are representative of a semi-infinite array of magnetic clusters with weak interactions and thickness-dependent magnetic properties.

Solvothermal synthesis and electrochemical behavior of nanocrystalline cubic Li–Ti–O oxides with cationic disorder

Electrochemically active Li–Ti–O oxides of approximate composition LiTi 2O 4 + δ were prepared by solvothermal reaction between TiO 2 and lithium hydroxide in water or ethanol at temperatures below 200 °C. This reaction proceeds via dissolution–precipitation mechanism and its course is not sensitive to crystal structure of TiO 2 used in synthesis. The product of the hydrothermal process has cubic structure derived from that of spinel ( Fd3m), with O atoms in 32e sites, Li in 8a sites and Ti atoms randomly distributed between 16c and 16d positions. The materials prepared in ethanol contain Ti in the 16d position only; on the other hand, Li is distributed between both types of available tetrahedral sites 8a and 8b. All prepared materials are active for Li insertion. Li entering the material during reduction process is preferentially located in 8b position. The presence of Li tetrahedrally coordinated positions indicates that Li insertion leads to a formation of a metastable structure stabilized probably by the nanocrystalline character of the material.

Engineering nanocrystals of silicon

In this paper we describe the production of nanocrystals of silicon embedded in an amorphous silicon matrix. These were then modified by partially removing the amorphous phase. The band gap that defines the optoelectronic properties of silicon changes with crystallite size. Silicon nanocrystals exhibit a significant blue shift in the band gap with decreasing size. The contribution of the nanocrystals to the properties of the amorphous silicon matrix depends on their size, shape, orientation, distribution and volume fraction. These are properties that we are investigating and modifying. Such engineering of the optical and electronic properties could lead to new and improved optoelectronic devices. Thin films of silicon ranging from fully amorphous to polycrystalline have been deposited from pure silane by the Hot Wire method. The effect of deposition conditions: filament temperature, substrate temperature, silane flow rate, pressure, and time have been investigated. The conditions under which isolated nanocrystals embedded in an amorphous matrix form have been mapped. Selected films have been anisotropically etched to enhance the nanocrystalline features. The films have been investigated using UV–visible spectroscopy, Raman Spectroscopy, Fourier transform infra-red spectroscopy, transmission electron microscopy and selected area electron diffraction.

Deposition of nanocrystalline diamond films on silicon nitride ceramic substrates using pulsed microwave discharges in Ar/H 2/CH 4 gas mixture

Silicon nitride (Si 3N 4) ceramics coated with nanocrystalline diamond (NCD) films are promising materials for tribological applications. In this work, NCD films are deposited on Si 3N 4 ceramic substrates in pulsed Ar/H 2/CH 4 microwave discharges. Comparisons with films elaborated in continuous mode on Si 3N 4 and Si substrates, and on Si substrates in pulsed mode are achieved. The results give evidence for the growth of diamond films with a relatively high growth rate above 1 μm/h. Good nanocrystalline features are obtained, with a small grain size below 20 nm and a surface roughness lower than 40 nm. The paper emphasizes that the great interest of the pulsed mode is to enable the deposition of good-quality NCD films on ball-shaped Si 3N 4 ceramics, by preventing the graphite formation observed in continuous regime due to a high plasma heating. Pulsed microwave discharges are then hopeful processes for treating complex-shape Si 3N 4 ceramics appropriate for specific tribological applications.

Fabrication and characterizations of new nanocomposite WC/Al 2O 3 materials by room temperature ball milling and subsequent consolidation

Reactant material powders of pure WO 3, Al and graphite have been milled at room temperature using a high-energy ball mill. After a few kiloseconds of milling (11 ks), numerous fresh surfaces of the reactant materials are created as a result of the repeated impact and shear forces which are generated by the balls. After 43 ks of milling, a mechanical solid state reduction is successfully achieved between the fresh Al and WO 3 particles to form a product of nanocrystalline mixture of Al 2O 3 and W powders. A typical mechanical solid state reaction takes place between the W particles and graphite powders to obtain fine grains of nanocrystalline WC. Towards the end-stage of ball milling (360 ks), the nanocrystalline WC grains (5 nm) are embedded into the fine matrix of Al 2O 3 to form fine nanocomposite powders (less than 1 μm in diameter) of WC–32 at.% Al 2O 3 material with spherical-like morphology. This composite powder was then consolidated under vacuum at 1963 K, with a pressure ranging from 19.6 to 38.2 MPa for 0.3 ks, using a plasma activated sintering method. The consolidation step does not lead to a dramatic grain growth and the compacted samples that are fully dense still maintain their unique nanocrystalline characteristics. The elastic properties and the hardness of both the consolidated samples have been investigated. A model for fabrication of new refractory nanocomposite WC/Al 2O 3 materials at room temperature is proposed.

Influence of gel-derived nanocrystalline spinel in a high alumina castable: Part 1

A newer class of spinel-alumina castable was fabricated by incorporating 8.0 wt.% of modified spinel additives synthesized via sol–gel and coprecipitation methods. These additives prepared by cheaper ingredients after optimum grinding and calcination, were characterized in terms of particle size distribution, infrared spectra and X-ray diffraction patterns. The properties of those spinel-alumina castables, with another prepared by the same amount of commercial preformed spinel fine, were studied and compared. Cold crushing strength, apparent porosity, hot modulus of rupture, pore size distribution, scanning electron microscopy, energy dispersive spectral analysis, thermal shock and slag corrosion resistance tests of selected samples were done to identify the immense prospect of sol–gel additive. Transmission electron microscope (with SAED facility) was also utilized to establish the nanocrystalline feature of the sol–gel additive.

Preparation of boron carbide thin films from reactive sputtering of boron

AbstractBoron carbide thin films were prepared by RF magnetron sputtering. The films were deposited onto CoCr/Cr/SiO2/Si(100) substrates with an RF power of 25, 50 and 75 W in the range of 200 °C. The reactive gas was introduced up to 1.5 vol% (CH4/(Ar + CH4)) during deposition, and the resulting composition of the films matched these ratios, as observed by Auger electron spectroscopy. In X‐ray photoelectron spectroscopy, two well‐resolved peaks in C (1s) spectra were observed around 284 eV, indicating C–C bond at 284.5 eV and B–C bond at 283.5 eV. The X‐ray diffraction of these films revealed the amorphous or nanocrystalline characteristics. At the deposition conditions of 75 W, 100 °C and 1.0 vol%, the nano‐indented films were hardened to 16 GPa and the scratched surfaces endured load of 50 N. In the wear test, the coefficient of friction was also measured down to 0.15. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Key role of nanocrystalline feature in porous polycrystalline silicon diodes for efficient ballistic electron emission

The cold electron emission characteristics of nanocrystalline porous polysilicon (PPS) diodes have been investigated for two PPS diodes prepared under different conditions. The origin of the difference in the emission efficiency and electron energy distribution between the two samples is analyzed in relation to the respective photoluminescence properties. The photoluminescence spectrum in the efficient electron emitter consists of red and blue emission bands corresponding to nanocrystalline silicon and interfacial oxide, respectively, while that in the low-efficiency emitter consists only of a blue band. Too much oxidation results in a reduction of the emission efficiency due to increased electron scattering. It is evident that both the formation of nanocrystalline silicon and its appropriate surface oxidation are key issues for obtaining efficient ballistic emission. These results are consistent with the emission model that electrons are accelerated in the PPS layer by multiple tunneling through interfacial oxide barriers between interconnected silicon nanocrystallites.

Structural properties of CaCu 3Ti 4O 12 obtained by mechanical alloying

Mechanical alloying has been used successfully to produce nanocrystalline powders of CaCu 3Ti 4O 12 (CCTO), for the first time, using two different experimental procedures. The milled CCTO were studied by X-ray powder diffraction, infrared and Raman scattering spectroscopy. For two different milling procedures, CCTO was obtained after a couple of hours of milling (in average 30 h of milling, depending on the reaction procedure). The X-ray diffraction (XRD) patterns indicate that the crystallite size is within the range of 20–35 nm. After 100 h of milling the formation of CCTO was confirmed by X-ray powder diffraction in both procedures, with good stability. We also prepare the CCTO ceramic using the traditional procedure described in the literature and compared the physical properties of these samples with those ones obtained by milling process and good agreement was observed. The infrared and Raman scattering spectroscopy results suggest that the increase of the milling time leads to the formation of nanocrystalline CCTO, as seen by XRD analysis. These materials are attractive for capacitor applications and certainly for microelectronics, microwave devices (cell mobile phones for example), where the decrease of the size of the devices are crucial. This milling process presents the advantage that melting is not necessary and the powder obtained is nanocrystalline with extraordinary mechanical properties. The material can be compacted and transformed in solid ceramic samples or used in others procedures of film preparation. The high efficiency of the process opens a way to produce commercial amount of nanocrystalline powders. Due to the nanocrystalline character of this powder, their mechanical properties have changed and for this reason a pressure of 1 GPa is enough to shape the sample into any geometry.