Individual Grain Size Research Articles

Surface morphology and transport studies of epitaxial graphene on SiC(0001̲)

Annealing conditions are critical to the properties of epitaxial graphene formed by thermal decomposition of silicon carbide. Here, we report the evolution of coherent electronic transport with increasing anneal temperatures, combined with low energy electron micrographs of equivalent surfaces showing corresponding structural coherence. Ultrahigh vacuum conditions and temperatures in the range of 1250--1300 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C produce granular films with a lateral grain size of $~$20 nm, while temperatures of 1400 ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C or higher result in grains with progressively larger lateral dimensions in the micron range. Transport measurements show how the electronic coherence length increases as a result of the more coherent physical structure, with a crossover from two-dimensional variable range hopping to the weak localization regime. Here, we show that while the duration of the anneal affects coverage and clustering of grains, the size of individual grains is determined by anneal temperature, with evidence of coalescence of smaller grains into larger domains, suggesting that multistage anneals at different temperatures may yield high-quality graphene.

Nanostructural Features and Optical Performance of RF Magnetron Sputtered ZnO Thin Films

Zinc oxide (ZnO) thin films were grown on silicon substrate by RF (radio frequency) magnetron sputtering. Surface topography of these films exhibited a nanostructured granular appearance with the size of individual grains between 50 to 100 nm. Corresponding cross-sectional electron micrographs revealed columnar grains in the form of aggregated nanorods/wires with length of about 500 nm, similar to the thickness of these thin films of ZnO nucleated and grown vertically on the silicon substrate. High resolution lattice scale imaging using high resolution transmission electron microscope (HRTEM) elucidated the single crystalline 10 0 planes of hexagonal-ZnO constituting the columnar grains with the individual nanorod diameter between 3 and 4 nm. The photoluminescence measurements showed the prominent emission peak at around 460 nm for the blue band, normally attributed to intrinsic defects in particular interstitial zinc (Zn). These films were further characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and electron paramagnetic resonance (EPR) to evaluate various aspects on preferred growth orientations, band structures and vibrational modes originated in such fascinating nano-grained thin films of ZnO. The present investigations inferred that these films are advantageous in various potential applications for fabricating nano-scaled devices.

Implantation-assisted Co-doped CdS thin films: Structural, optical, and vibrational properties

This paper reports on structural, optical, vibrational, and morphological properties of cobalt-doped CdS thin films, prepared by 90 keV Co+ implantation at room temperature. In this work, we have used cobalt concentration in the range of 0.34–10.8 at. %. Cobalt doping does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. However, with increasing cobalt concentration a decrease in the optical band gap, from 2.39 to 2.26 eV, is observed. This reduction is addressed on the basis of band tailing due to the creation of localized energy states in association with Urbach energy calculations. In addition, implantation gives rise to grain growth and increase in the surface roughness. Size and shape fluctuations of individual CdS grains, at higher fluences, give rise to inhomogeneity in strain. The results are discussed in the light of ion-matter interaction in the keV regime.

A three‐dimensional microstructure‐based photon‐tracking model of radiative transfer in snow

Solar radiation is a key component of the energy budget of snow‐covered landscapes. Even a thin snow cover reflects most of the incident sunlight and transmits little. An understanding of the interaction of solar radiation with snow is essential to the study of the snow thermodynamics, chemistry, hydrology, ecology, and remote sensing. To investigate this interaction, a microstructure‐based photon‐tracking algorithm is presented. The three‐dimensional snow microstructure is provided either by a discrete element model defined by shape, size, and spatial arrangement of individual ice grains or by an X‐ray microtomography image of a snowpack. The model uses refraction, Fresnel reflection, and absorption laws, and the only optical input parameters are the complex index of refraction and absorption coefficient. The model follows individual photons through the microstructure, a porous network of ice and air, applying the fundamental optics laws at the ice‐air interfaces and within the ice. By firing tens of thousands of photons a detailed examination of the spectral radiance and irradiance above, below, and within the snowpack is possible. The model was compared to results from a discrete ordinates model, and its sensitivity to the microstructural representation was studied. It was applied to investigate changes in reflected, absorbed, and transmitted light as a function of wavelength, snow depth, grain size, and snow density, used to predict the amount and direction of scattering within the snowpack, and used to explore the interaction of a collimated beam with snow.

Statistical measures of distribution patterns of silicon and calcium in marine sedimentary layers

Abstract. We analyze electron microscope X-ray spectroscopy data of recent supratidal marine sediments. Statistical measures are used to characterize the distribution of silicon and calcium in different layers of the sediments. We also use cluster analysis and symbolic dynamics to filter measurement noise and to classify different density regions. This allows to calculate characteristic patch sizes which reflect the sizes of individual clastic grains and the corresponding pore sizes. Silicon indicates the independent processes in the sedimentation history of certain grains. Calcium is capable to monitor intrinsic early diagenetic processes of biogeochemical calcium mineralization of primary organic matter as documented in more organized distributions with higher clustering.

Heterogeneous Deformation and Internal Stresses Developed in BCC Wires by Axisymmetric Elongation

BCC wires macroscopically deformed by axisymmetric elongation (wire drawing) develop an intense <011> fibre texture and exhibit a characteristic non-uniform deformation of the grains evident in transverse sections (grain curling or “Van Gogh sky structure”). The extraordinary grain morphology induced by the <011> fibre texture is also accompanied by a peculiar constant strain hardening rate in single-phase BCC wires (exponentially increasing in case of BCC containing composite wires) that allows to reach very high strengths. Here we present a calculation of the elastoplastic axial elongation of such an aggregate of BCC grains with the ideal <011> fibre texture, using a slip-gradient dependent large-strain crystal plasticity constitutive equation incorporated into a finite element method (FEM) code, i.e., with proper account of the influence of the evolving shape and size of individual grains and of the local grain interactions. The results reproduce well the observed macroscopic behaviour (linear flow stress-strain curve at large strains) and the peculiar mesoscopic structural changes (grain curling in transverse sections). The simulation is focused on the analysis of strain and dislocation density heterogeneities and on the building up of mesoscopic (inter- and intra-granular) internal stresses during deformation. The computed average transverse tensile stresses acting normal to the axially oriented {100} planes approximately parallel to the boundaries of the flattened grains is close to 0.3 times the tensile flow stress of the aggregate, in good agreement with previous calculations based on the Taylor-Bishop-Hill model or on elasticplastic self-consistent calculations and with available neutron diffraction measurements. Such a high level of internal tensile stresses explains the well-known tendency of high strength BCC wires to fail by longitudinal splitting.

Photoluminescence, open circuit voltage, and photocurrents in Cu(In,Ga)Se 2 solar cells with lateral submicron resolution

Thin film Cu(In,Ga)Se 2 solar cells with average efficiencies in the 14% regime have been analyzed by photoluminescence (PL) with lateral micrometer-resolution. PL has been evaluated according to Planck's generalized law with respect to splitting of quasi-Fermi levels under open circuit (OC) and short circuit (SC). Lateral patterns of luminescence yields signalize local fluctuations in quasi-Fermi energies in the range of some tens of milli-electron-volt. These patterns spatially extend to some microns and by far exceed the size of individual grains of 1 μm or less. The responses of PL, and thus the local V OC and short circuit currents turn out to be significantly anti-correlated. This behaviour is explained in terms of a two dimensional network coupling illuminated and dark hypothetic micrometer-sized diodes with different characteristic parameters such as series and parallel resistances, as well as reverse saturation currents.

Lateral variations of optoelectronic quality of Cu(In 1– xGa x)Se 2 in the submicron-scale

Optoelectronic properties of Cu(In 1– x Ga x )Se 2 (CIGSe) have been studied by confocal microscopic photoluminescence (PL) with lateral submicron resolution. The lateral patterns of PL-yields, varying by factors of up to 10 between regimes with low and high emission, exhibit structures in the length scale of some micrometers (3–10 μm) whereas geometrical sizes of individual grains are in the 1-μm range or below. In addition to the local PL-variation, we observe that the geometrical extension of PL-patterns depend on excitation flux, that spectral PL-shapes are varying with respect to different onset energies of the low-energy wings, signalizing different local band gaps, and that low-energy PL-wings get steeper with rise in excitation flux pointing towards potential fluctuations and their respective screening by photoexcited excess carriers.

Structural properties and quality of the photoexcited state in Cu(In 1− xGa x)Se 2 solar cell absorbers with lateral submicron resolution

We analyze the quality of the photoexcited state in Cu(In,Ga)Se 2 absorbers prepared under conditions similar to module pilot line production with different Ga-admixtures in terms of the splitting of the quasi-Fermi levels by confocal microscopic photoluminescence (PL) with submicron lateral resolution. The lateral luminescence patterns exhibit structures in the length scale of some micrometers (3–10 μm) which by far exceed the geometrical sizes of individual grains with diameters of 1 μm or below, detected simultaneously by optical reflection, and by scans of atomic force microscopy (AFM). The luminescence yields recorded at (83–250) K and at about (10 4–10 2) air mass 1.5 (AM1.5)-equivalent photon fluxes show lateral alterations by factors (2–20) depending on Ga-content, excitation level, and temperature. From temperature and excitation regimes, we are able to extrapolate the lateral variations towards 300 K and AM1.5-equivalent fluxes, and we translate PL-yields into lateral alterations of the splitting of quasi-Fermi-levels Δ( E Fn− E Fp) and of corresponding variations in maximum hypothetic open circuit voltages V oc.

Rate of dehydration of corn (Zea mays L.) pollen in the air.

The water content of corn (Zea mays L.) pollen directly affects its dispersal in the atmosphere through its effect on settling speed and viability. Therefore, the rate of water loss from pollen after being shed from the anther is an important component of a model to predict effective pollen transport distances in the atmosphere. The rate of water loss from corn pollen in air was determined using two methods: (1) by direct weighing of samples containing approximately 5 x 10(4) grains, and (2) by microscopic measurement of the change in size of individual grains. The conductance of the pollen wall to water loss was derived from the time rate of change of pollen mass or pollen grain size. The two methods gave average conductance values of 0.026 and 0.027 cm s-1, respectively. In other experiments, the water potential, psi, of corn pollen was determined at various values of relative water content (dry weight basis), either by using a thermocouple psychrometer or by allowing samples of pollen to come to vapour equilibrium with various saturated salt solutions. Non-linear regression analysis of the data yielded psi (MPa) = -3.218 theta(-1.35) (r2 = 0.94; for -298 < or = psi < or = -1 MPa). This result was incorporated into a model differential equation for the rate of water loss from pollen. The model agreed well (r2 approximately 0.98) with the observed time-course of the decrease of water content of pollen grains exposed to a range of temperature and humidity conditions.

The mechanical strength of polysilicon films: Part 2. Size effects associated with elliptical and circular perforations

A systematic study of failure initiation in small-scale specimens has been performed to assess the effect of size scale on “failure properties” by drawing on the classical analysis of elliptically perforated specimens. Limitations imposed by photolithography restricted the minimum radii of curvature of the specimen perforations to one micron. By varying the radius of curvature and the size of the ellipses, the effects of domain size and stress concentration amplitude could be assessed separately to the point where the size of individual grains (∼0.3 μm) becomes important. The measurements demonstrate a strong influence of the domain size under elevated stress on the “failure strength” of MEMS scale specimens, while the amplitude, or the variation, of the stress concentration factor is less significant. In agreement with probabilistic considerations of failure, the “local failure strength” at the root of a notch clearly increases as the radius of curvature becomes smaller. Accordingly, the statistical scatter also increases with decreasing size of the (super)stressed domain. When the notch radius becomes as small as 1 μm the failure stress increases on average by a factor of two relative to the tension values derived from unnotched specimens. This effect becomes moderate for larger radii of curvature, up to a radius of 8 μm (25 times the grain size), for which the failure stress at the notch tip closely approaches the value of the tensile strength for un-notched tensile configurations. We deduce that standard tests, performed on micron-sized, non-perforated, tension specimens, provide conservative strength values for design purposes. In addition, a Weibull analysis shows for surface-micromachined specimens a dependence of the strength on the specimen length, rather than the surface area or volume, which implies that the sidewall geometry, dimensions and surface conditions can dominate the failure process.

Atomic force microscopy of surface relief in individual grains of fatigued 316L austenitic stainless steel

Surface relief adjacent to the persistent slip bands (PSBs) in polycrystalline 316L stainless steel cycled with constant plastic strain amplitude up to 60% of fatigue life was studied using atomic force microscopy with respect to the crystallographic orientation (determined by electron backscattering diffraction—EBSD) and the size of individual grains. Surface relief is formed mostly by ribbon-like extrusions whose height in a grain was found to be proportional to the thickness of the corresponding PSB. The extrusions grow predominantly in the direction of the active Burgers vector. The height of an extrusion in the direction of the active Burgers vector is proportional to the grain size. No systematic dependence of the extrusion height in the active slip direction on the grain orientation was observed. The experimental results are discussed in terms of the recent vacancy models of surface relief evolution.

In-situ annealing experiments of octachloropropane as a rock analogue: kinetics and energetics of grain growth

Two-dimensional in-situ observation of the change of size of individual grains of octachloropropane (OCP) during post-deformation annealing revealed that grain boundary migration occurred to reduce the grain boundary energy. Within the polycrystalline aggregate studied some grains show a cyclic change in grain size, others have a stable grain size, while most grains were consumed and disappeared in order that the mean grain size could increase. During grain coarsening, some grains were dissected, then coalesced, amalgamated with others and their centers migrated across the aggregate. The average grain size (D̄) can be expressed by D ̄ =k 0 exp(−Q/RT)t n , where k 0 is a constant, Q is the activation energy, R is the gas constant, T is the temperature, t is the annealing time and n is a constant. We obtained n = 0.1–0.2 and Q = 6.2 kcal/mol. The driving force ( P) is the surface energy stored at grain boundaries. The driving force can be expressed as P∝ t m , where m is the constant, and hence the grain growth rate ( D ̄ ′ ) can be expressed as D ̄ ′∝P (n−1)/m . The value of ( n−1)/ m ranges from 4 to 10.

Dwarfing genes and cell dimensions in different organs of wheat

A field experiment was conducted under non-limiting water and nutritional conditions with three near-isogenic lines of spring wheat (dwarf, DD; semi-dwarf, SD and standard height, SH) to study the impact of the GA-insensitive alleles Rhtl and Rht2, at the cellular level, on the growth of different vegetative organs and of the pericarp of grains. Cell length and width of blades of different leaves (3, 7 and flag leaf), the flagleaf sheath and the penultimate internode as well as the pericarp of basal grains from central spikelets of the spike were evaluated. With the exception of the flag leaf, dwarfing genes produced a significant reduction in cell length in all the different vegetative organs analysed. There was no effect on the number of cells nor their width. Therefore, in vegetative organs, the effects of these alleles appeared to be exclusively due to a reduction in cell length. It would appear that dwarfing genes act on cell elongation without affecting cell division. The Rht alleles did not modify cell length nor width in the pericarp. Grain weight was different between the lines and these differences were associated with grain volume at the beginning of linear grain growth. Thus, they reduced the size of individual grains by reducing the total number of cells in the pericarp. It appears that Rht alleles reduced the final sizes of vegetative organs (such as internodes and leaves) and of tissues (pericarp) associated with reproductive structures (grains), but the modes of action in these different organs were different.

Dwarfing genes and cell dimensions in different organs of wheat

A field experiment was conducted under non-limiting water and nutritional conditions with three nearisogenic lines of spring wheat (dwarf, DD; semi-dwarf, SD and standard height, SH) to study the impact of the GA-insensitive alleles Rht1 and Rht2, at the cellular level, on the growth of different vegetative organs and of the pericarp of grains. Cell length and width of blades of different leaves (3, 7 and flag leaf), the flagleaf sheath and the penultimate internode as well as the pericarp of basal grains from central spikelets of the spike were evaluated. With the exception of the flag leaf, dwarfing genes produced a significant reduction in cell length in all the different vegetative organs analysed. There was no effect on the number of cells nor their width. Therefore, in vegetative organs, the effects of these alleles appeared to be exclusively due to a reduction in cell length. It would appear that dwarfing genes act on cell elongation without affecting cell division. The Rht alleles did not modify cell length nor width in the pericarp. Grain weight was different between the lines and these differences were associated with grain volume at the beginning of linear grain growth. Thus, they reduced the size of individual grains by reducing the total number of cells in the pericarp. It appears that Rht1 alleles reduced the final sizes of vegetative organs (such as internodes and leaves) and of tissues (pericarp) associated with reproductive structures (grains), but the modes of action in these different organs were different.

Characterizing and Modeling Plastic Strain Inhomogeneity in Thin Metallic Sheets

AbstractUsing a newly developed plastic strain measurement technique based on digital image correlation, surface plastic deformation of polycrystalline aluminum alloys in a thin sheet form has been experimentally characterized at a length scale comparable to that of the thickness of the aluminum sheets but much larger than the average size of individual grains. Both static and dynamic local straining patterns in these aluminum alloys have been observed and these strain patterns can not be simulated using the conventional plasticity models. The texture clustering of grains may contribute to the static local plastic strain patterns detected in an Al-Mg alloy. Two distinctive dynamic straining behaviors resulted from the dynamic strain aging of dislocations due to the alloying elements have been experimentally established for 5XXX and 6XXX alloys, respectively. First observation of dynamic strain inhomogeneity is also made in a sheet metal specimen deforming predominately in a plane strain state.

Vickers contact damage of micro-layered Ti 3SiC 2

The nature, evolution, and degree of deformation microfracture damage around and beneath Vickers contacts in monophase Ti 3SiC 2 (312) are studied. The 312 material exhibits a pronounced shear deformation during indentation, indicating microscale plasticity which can be associated with infragrain multiple basal-plane slip between microlamellae, intergrain sliding, lamellae or grain pushout, and microfailures at the ends of the constrained shear-slips. No contact-induced cracks are observed and the micro-damage is widely distributed within the shear-compression zone around and below the contacts. The damage process is stochastic which results from a complex interplay of statistical variation in both relative size and crystallographic orientation of individual grains. The ability of 312 to absorb energy from the loading system and to distribute damage is somewhat akin to that of ceramics with either coarse-grained or heterogeneous microstructures, and perhaps geological structures.

Refinement of Microstructures and Improvement of Mechanical Properties in Intermetallic TiAl Alloys by Rheocasting

An investigation was made on the refinement of microstructures and improvement of mechanical properties in intermetallic TiAl binary alloys containing 44, 49 and 54 at% aluminum produced by the rheocasting in which the solidifying alloy was vigorously agitated at high rotation speeds of 15 to 70 rev/s s of a stirrer immersed in the alloys in argon gas atmosphere. In the microstructure of the rheocast Ti-44at%Al alloy such a lamellar structure as seen in the alloy solidified without stirring was destroyed completely, and an extremely refined microstructure composed of dispersed agglomeration aggregates was formed. The mean sizes of agglomeration aggregates and individual crystal grains of the Ti-44%Al alloy rheocast at a rotation speed of 70 s -1 were 6.4 ± 3.7 μm and 2.2 ± 1.3 μm in major axis and 2.0 ± 1.1 μm and 1.0 ± 0.6 μm in minor axis, respectively. However, oval lamellar grains were seen in the microstructure of the rheocast Ti-49at%Al alloy and lamellar substructure was observed in the oval primary crystal grains in the microstructure of the rheocast Ti-54at%Al alloy. In the microanalysis of the Ti-44%Al alloy rheocast at 70 s -1 the uniform distribution of Al and Ti elements could be achieved by destruction of the lamellar structure with the mechanical stirring, although lamellar segregation was observed in the alloy solidified without stirring. The room temperature elongations of the Ti-44%Al alloy rheocast at 15 and 70 s -1 were 2.7% and 3.3%, respectively, being higher than the elongation of the alloy solidified without stirring, i.e. 2.0%. The room temperature tensile strengths of Ti-44%Al alloy rheocast at 15 and 70 s -1 were 359 and 468 MPa, respectively. They were also higher than the tensile strength of the alloy solidified without stirring, i.e. 245 MPa. The elevated temperature elongation and tensile strength of the rheocast Ti-44%Al alloys were improved as compared with those of the alloy solidified without stirring, although a fluctuation existed. The tensile strengths of the alloys rheocast at 70 s -1 and solidified without stirring were 538 and 363 MPa at 1173 K, and 439 and 389 MPa at 1273 K, respectively.

A palynological study of surface and suspended sediments on a tidal flat: implications for pollen transport and deposition in coastal waters

This study examines patterns of pollen deposition and resuspension on a tidal flat of the Dollard, a tidal basin near the mouth of the Ems River, at the border of Germany and The Netherlands (Fig. 1). Sediment samples were collected during three sequential low tides at the spring stage of the lunar tidal cycle when the greatest area of the flat would be exposed and, with the exception of storm events, when maximum scour and redeposition would be expected to occur. Fifteen sample stations were selected along a 5 km transect, with paired ripple crest and trough samples collected at three stations. Water samples were collected hourly during two tidal cycles, from a single location on the flat. Both numbers and condition of pollen and spore taxa were determined for all samples. Grain size and LOI of suspended matter were also determined. The same eight types of pollen and spores were important in both water and sediment samples: Ericaceae, Alnus, Betula, Chenopodiaceae, Corylus, Gramineae and Polypodiaceae. In water samples the lack of a relationship between concentrations of some pollen types and mineral fractions is assumed to be evidence that certain pollen types are preferentially incorporated into floccules suspended in local waters. Analysis of tidal flat sediments indicates that pollen concentrations increase in troughs between ripples and at lower elevations where finer grain sizes accumulate. There is minimal evidence of selective sorting of pollen types. Ordination (using Principal Components Analysis) of pollen percentages and concentrations in sediments indicates that both elevation and distance from the estuary mouth influence concentrations of all abundant pollen types. These parameters, however, have minimal influence on pollen percentages. Incorporation of pollen into fecal pellets, which have relatively high settling velocities, is assumed to be a critical control on pollen deposition in this environment. Analysis of stomach contents of locally common bivalves supports this hypothesis. Transport and deposition of individual pollen types cannot be predicted as floccules and pellets have varying densities and settling velocities. Some pollen and spores may be deposited in coarser sediments than would be predicted based on the size and density of individual grains, but in general display predicted patterns of deposition of fine-grained clastic sediments.

撹拌凝固による金属間化合物TiAl合金のミクロ組織の微細化と機械的性質の改善

An investigation was made on the refinement of microstructures and improvement of mechanical properties in intermetallic TiAl binary alloys containing 44, 49 and 54 at% aluminum produced by the rheocasting in which the solidifying alloy was vigorously agitated at high rotation speeds of 15 to 70 rev/s s of a stirrer immersed in the alloys in argon gas atmosphere. In the microstructure of the rheocast Ti-44at%Al alloy such a lamellar structure as seen in the alloy solidified without stirring was destroyed completely, and an extremely refined microstructure composed of dispersed agglomeration aggregates was formed. The mean sizes of agglomeration aggregates and individual crystal grains of the Ti-44%Al alloy rheocast at a rotation speed of 70 s -1 were 6.4 ± 3.7 μm and 2.2 ± 1.3 μm in major axis and 2.0 ± 1.1 μm and 1.0 ± 0.6 μm in minor axis, respectively. However, oval lamellar grains were seen in the microstructure of the rheocast Ti-49at%Al alloy and lamellar substructure was observed in the oval primary crystal grains in the microstructure of the rheocast Ti-54at%Al alloy. In the microanalysis of the Ti-44%Al alloy rheocast at 70 s -1 the uniform distribution of Al and Ti elements could be achieved by destruction of the lamellar structure with the mechanical stirring, although lamellar segregation was observed in the alloy solidified without stirring. The room temperature elongations of the Ti-44%Al alloy rheocast at 15 and 70 s -1 were 2.7% and 3.3%, respectively, being higher than the elongation of the alloy solidified without stirring, i.e. 2.0%. The room temperature tensile strengths of Ti-44%Al alloy rheocast at 15 and 70 s -1 were 359 and 468 MPa, respectively. They were also higher than the tensile strength of the alloy solidified without stirring, i.e. 245 MPa. The elevated temperature elongation and tensile strength of the rheocast Ti-44%Al alloys were improved as compared with those of the alloy solidified without stirring, although a fluctuation existed. The tensile strengths of the alloys rheocast at 70 s -1 and solidified without stirring were 538 and 363 MPa at 1173 K, and 439 and 389 MPa at 1273 K, respectively.