Density Of Lattice Defects Research Articles

Room-temperature nanowire ultraviolet lasers: An aqueous pathway for zinc oxide nanowires with low defect density

We report the realization of a low-temperature aqueous pathway for the chemical synthesis of zinc oxide (ZnO) nanowires with low defect density and their room-temperature ultraviolet lasing behavior at low pump fluence. The concentration of solutes determined not only the size of individual nanowires, which influences their optical waveguiding behavior, but also their lattice defect density, which affects the efficiency of ultraviolet emission. The optimal synthesis conditions led to low-temperature growth of ZnO nanowires that showed room-temperature ultraviolet lasing at a low threshold of pump fluence. Based on our experimental results and optical waveguide theory, we report two important factors for realizing high-quality ZnO nanowires that show room-temperature ultraviolet lasing via a low-temperature aqueous approach: control of the density of defects generated in aqueous solutions and the optimal microstructure of the grown nanowires to produce strong optical confinement.

Granular configurations, motions, and correlations in slow uniform flows driven by an inclined conveyor belt

The present experimental study examines the behaviour of slow granular flows, focusing on the details of particle patterns and motions over the depth of a sheared layer. A conveyor belt circuit enclosed in an inclined flume is used to generate steady uniform open-channel flows of dry granules. Particle positions near the transparent sidewall are extracted from video sequences. The Voronoi diagram is then used to characterise the configurations formed by neighbouring grains and to assist particle tracking over successive frames. This allows a qualitative visualisation of the internal structure of the flowing layer, as well as quantitative measurements of lattice defect density and granular velocities at different depths. The response of the depth profiles to different conveyor belt speeds is examined. In addition to the mean and fluctuating velocities, we probe the time and space correlations of the fluctuations.

Dissolution of nanocrystalline fluorite powders: An investigation by XRD and solution chemistry

The effect of lattice disorder and mineral surface area on the reactivity of finely ground fluorite was studied on ball-milled powders. Structural information was provided by X-ray whole powder pattern modeling (WPPM). The mean size of coherent scattering domains decreases with milling time from 70 nm to ∼20 nm, whereas the density of lattice defects increases with both time and intensity of milling treatment, from 4 × 10 15 m -2 to 24 × 10 15 m -2. High resolution transmission electron microscopy (HRTEM) of ground fluorite grains shows several line defects and a general tendency of nanometric crystalline domains to agglomerate in larger grains. Solution chemistry was investigated using batch reactors with free drift of solution saturation state with respect to fluorite. Total surface area was measured by the Brunaver, Emmet and Teller (BET) method, and dissolution rates were measured at pH = 2 (HCl) and T = 295 K. In far from equilibrium conditions, dissolution rates normalized by BET area do not increase with the dislocation density. In near-equilibrium condition, however, measured stationary ionic product clearly increases with both time and intensity of milling treatment. Thermodynamic predictions of the solubility constant indicate negligible or little effect of total surface area. Consequently, the observed increase in the stationary ionic product can be related to the increasing lattice defect content. This confirms the significant role of dislocation outcrops on mineral dissolution in close to equilibrium conditions.

Charge contrast imaging of fine-scale microstructure and compositional variation in garnet using the environmental scanning electron microscope

Gaseous secondary electron (GSE) imaging of eclogite garnets under the environmental scanning electron microscope (ESEM) at low chamber gas pressures reveals detailed image contrast patterns (charge-contrast images, CCI) that are not present in back-scattered or secondary electron images. Image intensity is a function of the amount of surface charge accumulation. Successful acquisition of CCI depends on frame size and beam scan rate at a given chamber gas pressure and beam current. Images are obtained in a few seconds, and are stable and reproducible. CCI patterns do not correlate with cracks or grain boundaries, but do correspond closely to variations in major-element composition, both in the form of concentric (growth) zoning, and branching, linear features interpreted as cracks that have been healed by new garnet growth. Causes of CCI are not yet well understood, but may be related to variations in lattice defect density and their influence on charge-trapping and dissipation. These in turn influence the rate of charge build-up at or very close to the specimen surface. One interesting possibility is that CCI images detect vacancies related to non-homovalent coupled substitutions involving, for example, REE and hydroxyl, so the method offers a way of imaging the distribution of these trace species in garnets. The CCI images are rich in microstructural detail and offer the potential for rapid, high-resolution, low-noise reconnaissance mapping of intragranular microstructure and compositional variation in both natural and synthetic garnets.

Melting behavior and origin of strain in ball-milled nanocrystalline Al powders

Ball-milled aluminum powders have been investigated by differential scanning calorimetry and high resolution X-ray line profile analysis. Ball-milling was carried out for different milling times ranging from 45 min to 32 days. The milling time dependence of the average grain-size and of the density of lattice defects, mainly dislocations, were determined by the modified Williamson-Hall and Warren-Averbach procedures based on the dislocation model of mean square strain. Characteristic grain sizes of ball-milled Al-powders decreases with increasing milling time and simultaneously, the grain-size distribution becomes sharper.

Crystallographic analysis of plate martensite in Fe–28.5 at.% Ni by FE-SEM/EBSD

Crystallographic analysis of plate martensite in an Fe–28.5 at.% Ni alloy was studied by electron backscattering diffraction (EBSD) in a scanning electron microscope equipped with a field emission gun (FE-SEM). It was shown that sound orientation mapping was possible even for the martensite having a high density of lattice defects and the FE-SEM/EBSD could be a strong tool for crystallographic/microstructural analysis of martensite in steels. It was confirmed that the martensite in this alloy held the Nishiyama–Wassermann (N–W) orientation relationship. Variant analysis of every martensite crystal was successfully done from orientation mapping data. It was clarified that a certain rule of variant selection operated within local areas. The procedures of crystallographic analysis of N–W martensite were explained in detail.

Crystal Lattice Defects and Hall Mobility of Electrons in Si : Er∕ Si Layers Grown by Sublimation Molecular-Beam Epitaxy

The density of crystal lattice defects in Si: Er layers grown through sublimation molecular-beam epitaxy at temperatures ranging from 520 to 580°C is investigated by a metallographic method, and the Hall mobility of electrons in these layers is determined. It is found that the introduction of erbium at a concentration of up to ∼5 × 1018 cm−3 into silicon layers is not accompanied by an increase in the density of crystal lattice defects but leads to a considerable decrease in the electron mobility.

Nanocrystalline LaCoO 3 perovskite particles confined in SBA-15 silica as a new efficient catalyst for hydrocarbon oxidation

Highly crystalline LaCoO 3 perovskite particles were prepared by a novel microwave-assisted process with a La–Co citrate complex precursor in the host pores of mesoporous SBA-15 silica. The resulting material exhibited considerably higher catalytic activity in the complete methane oxidation than a LaCoO 3/SBA-15 sample prepared by a conventional method with a bulk LaCoO 3 perovskite. This suggests that a higher density of lattice defects achieved on the high-surface-area LaCoO 3 nanocrystals synthesized by microwave processing is essential for the superior performance of the catalyst.

Codeposition and Microstructure of Nickel–SiC Composite Coating Electrodeposited from Sulphamate Bath

Ni–SiC composite coatings were electroplated from a nickel sulphamate solution with a SiC suspension, with and without the addition of monovalent thallium ions. Without thallium ions, the incorporated SiC particles did not modify the columnar grain structure of the nickel matrix. Conversely, the nickel matrix exhibited an equiaxed grain structure; meanwhile, the amount of codeposited SiC was markedly increased by adding thallium ions to the solution. Additionally, the density of lattice defects associated with nickel grains substantially decreased when the columnar grains were replaced by equiaxed grains. These distinct grain structures associated with the various coatings were discussed in terms of the inhibition of nickel electrocrystallization by thallium ions as well as the absorption and reduction of the various ionic species absorbed on the surface of the SiC particles.

Raman Spectroscopy of Graphitic Foams

ABSTRACTTo better understand the very high thermal conductivity to weight ratio of the graphitic foams recently developed at the Oak Ridge National Laboratory, a Raman spectroscopy study was performed. It was also shown that the Raman scattering can be useful for the characterization of the graphitic foam, being able to evaluate the quality of the samples with respect to the density and location of lattice defects.

Microscopic Feature of Fractured Implant in Practical Dental Use

Patients suffer considerable pain from fracture of placed implants. In order to reduce the number of fracturing failures in future and to obtain a better understanding of fracturing, a fractured implant has been investigated by means of radiography, scanning electron microscopy and transmission electron microscopy from a metallurgical viewpoint. The blade type implant of Ti-5V alloy has a virgin microstructure of a low density of dislocation with a dispersion of fine precipitates. Further dislocations and twins are introduced into the implant due to repeated loading in use, resulting in possessing a large amount of lattice defects until fracturing. The dislocations are considered to assist calcium penetration and to bring about intergranular fracturing near the surface of the implant. Striations are extensively observed at a center portion of the implant, indicating that the fracturing is caused by fatigue. Microscopic investigations reveal that the fracturing is initiated by some elements such as calcium in saliva in accordance with high density of lattice defects and developed through the fatigue by repeated loading.

TEM Analysis of Blisters on Silicon Surface Formed by Hydrogen Ion Irradiation

It has been known that blistering occurs in a metal surface long exposed to high-energy gas molecules or ions, such as plasma facing materials. The present authors found that the blistering takes place even in hard and brittle semiconductors and ceramics by gas ion (D+ and He+) irradiation. In the present study we conducted grazing incidence and cross-sectional transmission electron microscope (GIEM and XTEM) observations of the blisters on a H+-irradiated silicon surface. The curvature of hydrogen-blister skin is smaller than that of D-blister reported in a previous study. This suggests that the displacement damage by nuclear stopping plays an important role in the large plastic deformation in the blistering. XTEM observation revealed that the blister skin contains a high density of lattice defects. With these result in mind the formation mechanism of hydrogen-irradiation-induced blistering is discussed.

Structural and magnetic inhomogeneity and the NMR of Mn55 and La139 in the magnetoresistive ceramics La0.7Ba0.3−xSnxMnO3→La0.7−xBa0.3−xMnO3+0.5xLa2Sn2O7

The effects of substitution of barium by tin on the phase composition, structural imperfection, and properties of lanthanum manganite perovskites La0.7Ba0.3−xSnxMnO3 (x=0, 0.1, 0.15, 0.2, 0.3) are established by comprehensive studies done by x-ray diffraction, resistive, and magnetic (including NMR on Mn55 and La139) methods. It is shown that the introduction of Sn leads to the formation of a pyrochloric phase La2Sn2O7, to an increase in the density of lattice defects of the manganese-enriched main lanthanum manganite phase, and to a substantial decrease of the magnetoresistive effect. The smearing of the metal–semiconductor phase transition temperature is explained by an increase in the inhomogeneity and imperfection of the perovskite structure. The low activation energy is confirmed by a high degree of inhomogeneity and imperfection of the crystal lattice of the samples studied. The broad, asymmetric NMR spectra of Mn55 and La139 attest to high-frequency electron exchange between Mn3+ and Mn4+ and nonequivalence of the environment of those ions and La3+ due both to heterovalent ions and to vacancies and clusters.

Investigation of the Correlation between Texture and Microstrucure on a Submicrometer Scale in the TEM

AbstractIn recent years the investigation of local texture and microstructure by analysis of electron backscatter diffraction patterns (EBSP) in the SEM has become a very powerful and popular method. With the introduction of SEM with field emission guns (FEG) the spatial resolution of EBSP measurements could be enhanced from 500 nm with a tungsten emitter to better than 50 nm. This evolution of SEM techniques raises the question whether transmission electron microscopy (TEM) still has fields of application in texture research. The present article answers this question with a clear “yes” and presents three examples of investigations where TEM is indispensable. The three examples comprise the investigation of the correlation between dislocation structure and deformation texture, a study on nucleation mechanisms of recrystallization in highly deformed metals and the investigation of microtexture and microstructure in nanocrystalline materials. Together with the presentation of these cases some of the necessary measurement techniques are described briefly. It is shown that TEM has to be applied when highest spatial resolution of orientation determination and imaging and high accuracy of orientation determination are to be reached, when the three‐dimensional and quantitative characterization of lattice defects is required or when materials with a high density of lattice defects are to be investigated.

High-bandgap semiconductor dosimeters for radiotherapy applications

We present a comparison between the performance of on-line radiotherapy dosimeters made with different high-bandgap semiconductor materials. We analysed the performances of a Schottky diode made with an epitaxial n-type 4H–SiC and a Chemical Vapour Deposited (CVD) diamond film with ohmic contacts. The current response of the dosimeters has been tested under exposure to a Co60 γ-source and to 6MV photons beam from a linear accelerator. The dose range covered is 0.1–10Gy with dose rates 0.1–10Gy/min. The two devices show a charge response linear with the dose when a constant dose rate is used. The SiC diode current response increase linearly with the dose rate; for diamond a quasi-linear behaviour is observed. The epitaxial SiC device shows no priming effects and a fast velocity of response, due to the low density of lattice defects in this material. The diamond performances are affected by trapping–detrapping mechanisms at defects with energy levels at ∼1eV. To de-activate these levels the diamond sample has been pre-irradiated with fast neutrons up to a fluence of 5×1014cm−2. The sensitivity of the two devices compare favourably to those of standard silicon dosimeters.

Investigation of the Correlation between Texture and Microstructure on a Submicrometer Scale in the TEM

AbstractIn recent years the investigation of local texture and microstructure by analysis of electron backscatter diffraction patterns (EBSP) in the SEM has become a very powerful and popular method. With the introduction of SEM with field emission guns (FEG) the spatial resolution of EBSP measurements could be enhanced from 500 nm with a tungsten emitter to better than 50 nm. This evolution of SEM techniques raises the question whether transmission electron microscopy (TEM) still has fields of application in texture research. The present article answers this question with a clear “yes” and presents three examples of investigations where TEM is indispensable. The three examples comprise the investigation of the correlation between dislocation structure and deformation texture, a study on nucleation mechanisms of recrystallization in highly deformed metals and the investigation of microtexture and microstructure in nanocrystalline materials. Together with the presentation of these cases some of the necessary measurement techniques are described briefly. It is shown that TEM has to be applied when highest spatial resolution of orientation determination and imaging and high accuracy of orientation determination are to be reached, when the three‐dimensional and quantitative characterization of lattice defects is required or when materials with a high density of lattice defects are to be investigated.

Thermo‐ and galvanomagnetic measurements of semiconductors at ultrahigh pressure

AbstractThe thermoelectric power, magnetoresistance, and thermomagnetic effects were measured for Te and Se micro‐samples in the vicinity of semiconductor–metal phase transitions at high pressure. From longitudinal and transverse Nernst‐Ettingshausen effects the scattering parameter r of holes was estimated and a decrease of the effective mass of holes was found during the closing of semiconductor gap. After the high pressure treatment an inversion of the sign of parameter r corresponding to a change of scattering mechanism was observed for the Te sample due to an increase of the density of lattice defects.

Supersonic jet epitaxy of gallium nitride using triethylgallium and ammonia

Gallium nitride (GaN) films were grown on GaN(0001)/AlN/6H–SiC composite substrates at 700–780 °C by supersonic jet epitaxy using triethylgallium (TEG) and NH3. TEG was seeded in He and N2 supersonic free jets to obtain kinetic energies of ∼2.1 and ∼0.5 eV, respectively, and NH3 was supplied from a variable leak valve. Higher TEG beam intensities (by about a factor of 5) were obtained by seeding in He. In situ reflection high-energy electron diffraction indicated a transition from three-dimensional to two-dimensional (2D) growth between 730 and 750 °C for films grown using TEG seeded in He and a constant NH3/TEG flux ratio. Ex situ atomic force microscopy of films grown at 730 and 750 °C revealed smooth surfaces comprised of quasi-2D islands with irregular perimeters. Cross-sectional transmission electron microscopy evidenced that the film grown at 750 °C was homoepitaxial α-GaN with a high density of planar lattice defects. Secondary ion mass spectrometry detected high residual carbon concentrations in the films. The GaN growth rate at 750 °C was found to depend on TEG flux and NH3 pressure in a manner consistent with Langmuir–Hinshelwood kinetics. Films grown under NH3-rich conditions were faceted and microscopically rough, whereas nonfaceted, basal-plane growth was observed under Ga-rich conditions. The first-order dependence of growth rate on TEG flux under NH3-rich conditions was used to estimate Ga incorporation efficiencies for high- and low-energy TEG beams. The Ga incorporation efficiency is lower for high-energy TEG beams, consistent with a decrease in the sticking coefficient for dissociative chemisorption.

High power pulsed magnetron sputtered CrNx films

Microstructure and macroscopic properties of droplet free CrN films deposited by the recently developed high power pulsed magnetron sputtering (HIPIMS) technique are presented. Magnetron glow discharges with peak power densities reaching 3000 W cm−2 were used to sputter Cr targets in both inert and reactive gas atmospheres. The flux arriving at the substrates consisted of neutrals and ions (approx. 70/30) of the sputtered metal and working gas atoms (Ar) with significantly elevated degree of ionization compared to conventional magnetron sputtering. The high-speed steel and stainless steel substrates were metal ion etched using a bias voltage of −1200 V prior to the deposition of CrN films. The film-to-substrate interfaces, observed by scanning transmission electron microscope cross-sections, were clean and contained no phases besides the film and substrate ones or recrystallized regions. CrN films were grown by reactive HIPIMS at floating potential reaching −160 V. Initial nucleation grains were large compared to conventional magnetron sputtered films, indicating a high adatom mobility in the present case. The films exhibited polycrystalline columnar growth morphology with evidence of renucleation. No intercolumnar voids were observed and the corrosion behavior of the film was superior to arc deposited CrNx. A high density of lattice defects was observed throughout the films due to the high floating potential. A residual compressive stress of 3 GPa and a hardness value of HK0.025=2600 were measured. A low friction coefficient of 0.4 and low wear rates against Al2O3 in these films are explained by the absence of droplets and voids known to contribute to extensive debris generation.

Massive transformation and the formation of the ferromagnetic L10 phase in manganese-aluminum-based alloys

Manganese-aluminum alloys in the vicinity of the equiatomic composition exhibit an attractive combination of magnetic properties for technological applications, including bulk permanent magnets and thin-film devices. The technical magnetic properties derive from the formation of a metastable L10 intermetallic phase (τ-MnAl) characterized by a high, uniaxial magnetocrystalline anisotropy with an “easy” c-axis. Carbon is generally added to stabilize the tetragonal τ phase with respect to the stable phases in the system. The magnetic hysteresis behavior of the Mn-Al-C genre of permanent magnet alloys is extremely sensitive to the microstructure and defect structure produced during the formation of the τ phase (L10) within the high-temperature e phase (hcp). In this study, modern metallographic techniques, including high-resolution electron microscopy (HREM), have been applied to elucidate the nature of the phase transformation and the evolution of the unique microstructure and defect structure characterizing the structural state of the ferromagnetic τ phase. It is concluded that the metastable τ phase is the product of a compositionally invariant, diffusional nucleation and growth process or massive transformation. The massive product nucleates preferentially at the grain boundaries of the parent e phase and is propagated by the migration of incoherent interphase interfaces. The interphase interfaces are revealed to be faceted on various length scales. It is concluded that this faceting is not a feature of the bicrystallography of the parent and product phases. The high density of lattice defects within the τ phase, generated by the phase transformation, is attributed to growth faults produced during atomic attachment at the migrating interfaces. Classical nucleation theory has been applied quantitatively to the grain-boundary nucleation process and was found to be consistent with the observed time-temperature-transformation (TTT) behavior. Analysis of the growth kinetics gives an ΔH D value of 154 kJ mol−1 for the activation energy of the transboundary diffusional process controlling boundary migration.