Clusters Of Grains Research Articles

The influence of hydrogen gas in the ambient gas mixture on the properties of indium tin oxide films deposited on glass and acrylic substrates by dc magnetron sputtering

In this paper we report the results of our studies on the influence of adding hydrogen to the ambient gas mixture of argon and oxygen on the properties of indium tin oxide films deposited by the dc magnetron sputtering method. The substrates used are glass and acrylic. The hydrogen flow rate has been varied keeping other deposition parameters fixed. The electrical, optical, structural and morphological properties are studied. The sheet resistance is seen to attain a minimum at a certain flow rate of hydrogen while it increases at both lower and higher flow rates. The transmission characteristics attain a maximum at the highest hydrogen gas flow rate. X-ray analysis reveals a crystalline nature with peaks at the (222), (440) and (431) directions. Glass substrates show an additional peak at (541). SEM studies show a cluster of grains and in the case of glass a more regular pattern is observed.

Free volume distributions and compactivity measurement in a bidimensional granularpacking

We investigate experimentally the distribution of the free volume inside a bidimensionalgranular packing associated either with single grains or with clusters of grains. This is donefor two different types of grains and two levels of compaction. The logarithm of thefree volume distribution scales in a non-extensive way with the cluster size forcluster sizes up to a few hundred grains. Having factorized this size dependence,we characterize the distributions by two intensive parameters. We discuss theinterpretation of these parameters and their possible relation to Edwards’ compactivity.

Point-contact spectroscopy on RuSr2GdCu2O8

We present Point-Contact experiments on polycrystalline RuSr$_2$GdCu$_2$O$_8$ samples. The majority of tunneling curves shows a zero-bias conductance peak, which is modeled by assuming a d-wave pairing symmetry of the superconducting order parameter.The magnetic field dependence of the conductance spectra has been measured in very stable junctions. In some cases, due to the granularity of the samples, clusters of grains in series introduce peculiar features in the conductance spectra.

Numerical tests of constitutive laws for dense granular flows

We numerically and theoretically study the macroscopic properties of dense, sheared granular materials. In this process we first consider an invariance in Newton's equations, explain how it leads to Bagnold's scaling, and discuss how it relates to the dynamics of granular temperature. Next we implement numerical simulations of granular materials in two different geometries--simple shear and flow down an incline--and show that measurements can be extrapolated from one geometry to the other. Then we observe nonaffine rearrangements of clusters of grains in response to shear strain and show that fundamental observations, which served as a basis for the shear transformation zone (STZ) theory of amorphous solids [M. L. Falk and J. S. Langer, Phys. Rev. E. 57, 7192 (1998); M.R.S. Bull 25, 40 (2000)], can be reproduced in granular materials. Finally we present constitutive equations for granular materials as proposed by Lemaître [Phys. Rev. Lett. 89, 064303 (2002)], based on the dynamics of granular temperature and STZ theory, and show that they match remarkably well with our numerical data from both geometries.

Crystal-plasticity analysis of ridging in ferritic stainless steel sheets

The occurrence of ridging in ferritic stainless steel (FSS) sheets is caused by the collective deformation of bandlike clusters of grains with a similar crystallographic orientation. In this article, largescale (1.8×3.6 mm) orientation maps obtained by electron backscattered diffraction (EBSD) are input into a viscoplastic self-consistent polycrystal plasticity model to analyze the strain anisotropy caused by the topographic arrangement of the recrystallization-texture orientations. Two versions of the ridging model were devised: (1) the local dispersion in strain-rate components is analyzed for the full EBSD map, and (2) narrow bands in the EBSD maps aligned parallel to the ridges on the sheet surface are considered, and the variation in macroscopic strain response from band to band is derived. The effects caused by spatial variations in through-thickness strains and in out-of-plane shears are compared and related to ridging. The model is applied to two sheets distinguished by different levels of ridging.

Dependence of electrical and optical properties of sol–gel prepared undoped cadmium oxide thin films on annealing temperature

The effect of the annealing temperature (Ta) on the optical, electrical and structural properties of the undoped cadmium oxide (CdO) thin films obtained by the sol–gel method, using a simple precursor solution, was studied. All the CdO films annealed in the range from 200 to 450 °C are polycrystalline with (111) preferential orientation and present high optical transmission >85% for wavelengths above 500 nm. The resistivity decreases as Ta increases until it reaches a value of 6×10−4 Ω cm for Ta=350 °C. For higher temperatures the resistivity experiences a slight increase. Images obtained by atomic force microscopy show an evident incremental change of the aggregate size (clusters of grains) as Ta increases. The grain size also increases when Ta increases as observed in data calculated from X-ray measurements.

Developing realistic grain boundary networks for use in molecular dynamics simulations

A Voronoi/Delaunay technique which allows for the construction of three-dimensional grain boundary networks with special boundaries for use in molecular dynamics simulations is presented. Using this technique, we make a series of samples containing clusters of grains with given configurations of CSL, low-angle tilt, and general high-angle boundaries and demonstrate the importance of including special grain boundaries in three-dimensional grain boundary networks in the study of the mechanical properties of nanocrystalline systems.

Investigations on chemically deposited Cd1−xZnxS thin films with low Zn content

Thin films of Cd1−xZnxS (0≤x≤0.8) were deposited on glass substrates by the chemical bath deposition technique. The as-deposited films were exposed to a stream of slowly flowing nitrogen at annealing temperature ranging 100–500 °C. XRD patterns of as-deposited Cd1−xZnxS over all mixture ratio values showed the cubic zincblende structure of which reflection peaks associated to (111), (220) and (311) planes were clearly identified. However, XRD characteristics of hexagonal wurtzite structure appeared at annealing temperature beyond 400 °C. From scanning electron microscope, the as-deposited films were formed by different sizes of clusters of grains of which average grain size smaller than 50 nm was observed. The increase in mixture ratio value in the starting solution was found to produce a decrease in the grain size. Energy gap value of annealed films, which is obtained from optical transmission measurements, seems to shift to lower energy as the annealing temperature increases. Electrical sheet resistance of the films, performed in the darkness and under illumination conditions by using a halogen lamp, as a function of annealing temperature was also investigated. The variation of energy gap of thin films with annealing temperature was discussed by employing quantum size effect phenomena.

Microstructure Evolution During Electric Current Induced Thermomechanical Fatigue of Interconnects

AbstractWe demonstrate the evolution of microstructure and deformation associated with the use of electrical methods for evaluating mechanical reliability of patterned interconnects on rigid substrates. Thermomechanical fatigue in aluminum and copper interconnects was induced by means of low frequency (100 Hz), high density (> 10 MA/cm2) alternating currents, which caused cyclic Joule heating and associated thermal expansion strains between the metal lines and oxidized silicon substrate. The failure mechanism involved formation of localized plasticity, which caused topography changes on the free surfaces of the metal, leading to open circuit eventually taking place by melting at a region of severely reduced cross-sectional area. Both aluminum and copper responded to power cycling by deforming in a manner highly dependent upon variations in grain size and orientation. Isolated patches of damage appeared early within individual grains or clusters of grains, as determined by a quasi in situ scanning electron microscopy and automated electron backscatter diffraction measurement. With increased cycling, the extent of damage became more severe and widespread. We document some examples of the types of damage that mechanically confined interconnects exhibited when subjected to thousands of thermal cycles, including growth and re-orientation of grains in a systematic manner. We observed in the case of Al-1Si certain grains increasing by nearly an order of magnitude in size, and reorienting by greater than 30°. The suitability of electrical methods for accelerated testing of mechanical reliability is also discussed.

Microscopic aspects of the region-by-region polarization reversal kinetics of polycrystalline ferroelectric Pb(Zr,Ti)O3 films

The region-by-region polarization switching in ferroelectric Pb(Zr,Ti)O3 thin films sandwiched between Pt electrodes has been directly observed using piezoelectric scanning probe microscopy. A resolution improved by one order-of-magnitude compared to the standard piezoelectric response imaging technique for ferroelectric capacitors was achieved by reducing the top electrode thickness to 10–15nm through polishing. It was demonstrated that the individually switched regions correspond to single grains or clusters of grains where the grain boundaries act as frontiers limiting the propagation of the switched state. The study of the propagation of the reversed polarization state as a function of voltage applied shows a rather discontinuous growth of the switched areas, the movement of the domain walls being triggered abruptly by different threshold voltages. This result agrees with the earlier proposed nucleation-limited switching model. The observation of the frozen regions that do not switch even at higher voltages provides significant insight into the “bits-failure” problem in submicron ferroelectric capacitors used for nonvolatile memory applications.

Thermal-induced normal grain growth mechanism in LPCVD polysilicon film

Normal grain growth mechanism due to high-temperature annealing of the polysilicon film has been shown experimentally while observing the limiting grain size to the order of the polysilicon film thickness. Polycrystalline silicon film shows grain size variation under the influence of thermal annealing. Low-voltage contact mode atomic force microscopy (AFM) has been employed to analyze the morphology of the annealed polysilicon film at four relevant different temperatures. A systematic grain growth leads to a stabilized grain size, due to clustering of grains under the influence of associated surface energy.

Effects of Water Gas Shift Gases on Pd−Cu Alloy Membrane Surface Morphology and Separation Properties

Palladium−copper alloy membranes (2−4 μm thick), which were deposited on porous ceramic tubular supports by electroless plating, separated H2 from a water gas shift (WGS) mixture at 623−723 K. However, within 120 min at 623 K, the total permeance increased and membrane selectivity decreased. H2, N2, or He did not change the transport properties of the membrane before WGS gas exposure. Annealing in CO2 and CO at 523−723 K increased the height of micron-scale conical hillocks and defect sizes on the membrane surface by at least a factor of 3. Annealing in H2 and He after CO and CO2 exposure decreased the hillock heights by half. The hillocks were clusters of grains with defined valleys. The membrane defects allowed gases other than H2 to permeate through the membrane. Surface topology changes are partially due to the removal of C impurities by CO2 to form CO. Hillock heights on 25-μm-thick cast and rolled Pd−Cu alloy foils, which had no C impurities, increased by a factor of 4 at 723 K in the presence of CO...

The effect of microstructure

Abstract The influence of microstructural parameters on low cycle fatigue damage in some single phase and biphasic stainless steels has been studied. In a single phase austenitic steel, the fatigue life increase with decreasing grain size is attributed to the positive effect of microstructural barriers in the bulk. In a metastable austenitic steel, the grain size of the initial microstructure has a combined influence on the volume fraction of martensite formed during cyclic loading and on the fatigue life. In this case, the beneficial effect of grain refinement is indirect because all cracks grow in the martensite. Possible damage patterns in a cluster of grains with different properties are discussed using the example of a duplex austenitic-ferritic steel. It is demonstrated that the propagation behaviour of short cracks related to the resistance of microstructural barriers in the bulk has a decisive influence on the fatigue life. The macroscopic fatigue resistance is thus controlled by damage events at the microstructure size scale.

Porous a/nc-Si:H films produced by HW-CVD as ethanol vapour detector and primary fuel cell

This work reports the use of undoped porous amorphous/nanocrystalline hydrogenated silicon (a/nc-Si:H) thin films produced by hot wire chemical vapour deposition (HW-CVD) as ethanol detector above 50ppm and as a primary fuel cell where a power of 4μW/cm2 was obtained in structures of the type glass/ITO/i-a-nc-Si:H/Al. The porous silicon looks like a sponge constituted by grains and cluster of grains that determines the type of surface morphology and the behaviour of the structure under the presence of vapour moisture. Apart from that, the detector/device performances will also depend on the type of interlayer and interfaces with the metal contacts. The sponge like structure adsorbs the OH groups in uncompensated bonds, which behave as donor-like carriers, leading to an increase in the current flowing through the material, directly dependent on the ethanol vapour pressure. The corresponding role of the components of the microstructure on this detector was investigated by spectroscopic impedance. The response time of the current of the sensor and its recovery time are in the range of 10–50s at room temperature.

A preliminary study of PGE in the Late Caledonian Loch Borralan and Loch Ailsh alkaline pyroxenite-syenite complexes, north-west Scotland

The Siluro-Ordovician Loch Borralan (c. 430 Ma) and Loch Ailsh Complexes (c. 439 Ma) comprise a suite of intrusive rocks ranging in composition from clinopyroxenites, through potassic melasyenites to quartz syenites. The rock suite at Loch Borralan also includes nepheline syenite. Geochemical data in the literature indicate that the intrusions are alkaline, with pronounced enrichments in LILE and LREE relative to contemporaneous calc-alkaline magmatic centres further to the south-east, although they share similar high LILE/Nb, subduction-related characteristics. The Loch Borralan Complex is associated with marginal gravity and magnetic anomalies which can be interpreted in terms of a shallow body less than 400 m thick. Analysis of rocks and drill core revealed widespread elevated Pt and Pd values in pyroxenites and syenites in both complexes. The highest concentrations, up to about 900 ppb Pt+Pd, occur in pyroxenites in the Loch Borralan Complex. Extensive archived drill core provides an excellent section through the marginal pyroxenites, which host the PGE at Loch Borralan. The pyroxenites show unusual petrological features; early clinopyroxene is followed by biotite, apatite, magnetite, sphene and plagioclase with later garnet, which might be metasomatic in origin. Sulphides occur in minor amounts. High-temperature shearing produced local granulation and mylonitisation. Later brittle deformation caused extensive micro-fracturing and the introduction of minor carbonate veining. Platinum-group minerals (PGM) were located in a number of samples with high PGE assay values. These occur as clusters of grains around 1–2 μm in diameter, and are difficult to identify uniquely. A few grains appear to be magmatic; these are Pt and PtPd sulphides that occur enclosed in clinopyroxene or garnet. All other grains occur in late fractures or along grain boundaries. These include sperrylite (PtAs2), Pd antimonides and PdBi tellurides, along with hessite (Ag2Te). They occur in microfractures accompanied by carbonate and barite and fine-grained sulphides and are probably of secondary origin. The PGE in the pyroxenites may have been introduced during the magmatic phase of the intrusion but the observed distribution of PGM results from low-temperature, hydrothermal remobilisation following brittle deformation and introduction of fluids.

Multiscale clustering in granular surface flows.

We investigate steady granular surface flows in a rotating drum and demonstrate the existence of rigid clusters of grains embedded in the flowing layer. These clusters appear to be fractal and their size is power law distributed from the grain size scale up to the thickness of the flowing layer. The implications of the absence of a characteristic length scale on available theoretical models of dense granular flows are discussed. Finally, we suggest a possible explanation of the difference between velocity profiles observed in surface flows and in flows down a rough inclined plane.

Using OIM to investigate the microstructural evolution of Ti-6Ai-4V

Orientation imaging microscopy provides both microstructural and crystallographic-texture information that allows a coupled analysis of changes in texture and microstructure. This capability has been used to determine how microstructure develops by transformation from Β to α in Ti-6Al-4V and by subsequent hot working to develop a fine-grain microstructure that is technologically desirable for secondary processes such as rolling and superplastic forming. With this technique, the mechanisms that account for the retention of large clusters of grains with similar grain orientation, heterogeneous deformation phenomena such as shear banding, and cavity nucleation and growth, can be elucidated. From such examinations, the development of physically based constitutive models and new forming strategies to improve the homogeneity of the microstructure are possible.

Results from the nuclear microprobe PIXE analysis of selected rare earth fluor compounds

Most previous research measures fluorescence properties over the macroscopic regime. Properties of individual microscopic grains could be significantly different than those measured over the macroscopic scale. Until recently, it was difficult to measure properties of individual fluor grains. Existing characterization techniques like scanning electron microscopy are not practical, since the resulting fluorescence masks the electron surface profile. Starting in September 2000, a research program was initiated at the Acadiana Research Laboratory to determine microscopic fluorescence properties for selected inorganic rare earth compounds. The initial phase of this program utilized microscopic proton induced X-ray emission (μPIXE) to characterize the elemental composition of individual fluor grains. Results show that both individual grains and small clusters of grains could be seen using μPIXE. Maps of this type can be used to estimate grain dimensions for the selected rare earth fluor. This technique is a new and innovative method to characterize a fluor material.

Hysteresis and thermal relaxation in random anisotropy systems

Recent experimental studies shown that relaxation and hysteresis phenomena at elevated temperatures measured on soft nanocrystalline materials (Finemet-type) with low crystalline fraction, cannot be described by a single activation volume. To clarify the nature of the magnetization processes in these materials we studied the behavior of clusters of grains with random anisotropy in which a direct correspondence between zero-temperature switching fields h c, energy barriers Δ F and activation moments Δ m, is involved, namely: Δ F∼Δ m 1/2; h c∼Δ m −1/2. We show that some aspects of the dynamics of field-rate-dependent hysteresis loops observed in real systems can be reproduced by choosing simple distributions of cluster size.

Dielectric and electrical properties of preferentially (111) oriented Zr-rich 0.1Pb(Mg 1/3Nb 2/3)O 3–0.9Pb(Zr xTi 1− x)O 3 thin films by chemical solution deposition

Ferroelectric thin films with rhombohedral composition of 0.1Pb(Mg 1/3Nb 2/3)O 3–0.9Pb(Zr x Ti 1− x )O 3 (0.6≤ x≤0.9) have been prepared by a chemical solution deposition on the (111) Pt/Ti/SiO 2/Si substrates. The films were consisted of nearly pure perovskite phases with the (111) preferred orientation. Fine grains below approximately 200 nm in average size tended to be the clusters of grains when x was higher than 0.8. The dielectric constant of the films, which gradually decreased with x, showed a minimum (∼650) at the composition of x=0.9. Meanwhile the values of remanent polarization ( P r) of the films increased with x up to 0.8 and then decreased for x=0.9. The increasing tendency is thought to be due to the preferred (111) orientation and distortion of perovskite unit cell. While the decreased polarization properties of x=0.9 implies the role of antiferroelectric properties of the lead zirconate phase, which can be possibly explained by DC bias-dependent capacitance behavior. The storage charge densities of the films decreased with x, but significantly increased upon the field-induced ferroelectric phase transition of the films.