Granular Grains Research Articles

Rapid solidification of undercooled monotectic alloy melts

Structural evolution of bulk undercooled Ni–11.56 at.% Pb monotectic alloy was investigated systematically by using molten glass denucleating combined with superheating cycling. Within the achieved undercooling range 10–286 K, the solidification structures were classified into three categories. When the undercooling was less than 50 K, the structures consisted of coarse dendrites and interdendritic lead phase. With the undercooling increasing into the range of 70–232 K, the dendrite clusters were refined and fine lead particles separated out from the supersaturated primary dendrite arms because of solute trapping. When the undercooling exceeded 242 K, the granular grains formed and fine lead particles homogeneously distributed in the whole sample. The phase selection of high temperature melts was analyzed by adopting steady state nucleation theory. The calculated results shown that undercooled Ni–11.56 at.% Pb monotectic alloy melts solidified in the form of α(Ni) dendrites essentially during the stage of rapid solidification and after recalescence, the interdendritic residual melts solidified in the equilibrium mode. Based on the observation of the solidification structures and the calculated results with BCT dendritic growth model, it was confirmed that the granulation mechanism of the granular grains was owing to the primary dendritic disintegration and subsequent recrystallization.

Tribological Property of CeO2 Films Prepared by Ion-Beam-Assisted Deposition

Cerium oxide (CeO2) films were prepared by ion-beam-assisted deposition (IBAD). The films were synthesized on WC–Co and Si(100) substrates by depositing CeO2 vapor under simultaneous bombardment of O2, Ar and Xe ions in the energy range of 1.0 to 5.0 keV. The elemental composition ratio (O/Ce) measured by Rutherford backscattering spectrometer (RBS) was in the range of 1.9 to 2.9, which was considerably higher than that expected from the stoichiometry. The X-ray diffractometry (XRD) patterns of CeO2 films prepared using O2, Ar and Xe ion beams exhibited a single-phase CaF2 structure, which is typical of CeO2. By increasing the transport rate ratio of the ions to CeO2 vapor, the morphology of the specimens prepared using O2 and Xe ion beams changed from a columnar grain to a granular one at transport ratios of 1.0 and 0.33, respectively. The defect density produced on the CeO2 film surface by ion irradiation was elucidated by Monte-Carlo simulation, leading to the result that the morphology change from columnar to granular is brought about at a threshold value in the defect density induced by ion bombardment. It was also been determined that the granular grain films have the very low friction coefficients of 0.15 at room temperature and 0.035–0.040 at 200°C, respectively.

傾斜冷却板を用いた６０７０および７０７５アルミニウム合金のセミソリッド連続鋳造

Semisolid casting is the most advanced technology for obtaining granular grains in aluminum-alloy cast structures. The conventional semisolid slurry is produced by electromagnetic or mechanical stirring. Both apparatuses that perform these tasks are large and costly. We developed a simple semisolid casting process for 6070 and 7075 aluminum alJoys using an inclined cooling plate. In this process, numerous granular aluminum crystals are nucleated on the inclined cooling plate while flowing downward. The semisolid slurry was held isothermally in the heat-isolating tundish and the granular grains grew inside. The semisolid slurry was then cast into a horizontal continuous casting machine. The resulting billet was 60 mm in diameter and 600 mm in length. For the 6070 aluminum alloy, the casting speed was 3 mm/s. The finest grain size of 56 μm was obtained by casting at 656°C, and by using the inclined cooling plate of 60° and 200 mm.

傾斜冷却板を用いたセミソリッドAC4CHアルミニウム合金の連続鋳造

Semisolid casting of aluminum alloy is an advanced casting technology for the foundry industry. The advantages of semisolid casting are ease of casting at low temperature and high fluidity of the semisolid slurry, which have to contain granular, fine and homogeneous crystals. Conventional semisolid slurry processing is performed by electromagnetic stirring or mechanical stirring, both of which involve large equipment and high costs. In this investigation, we developed a simple semisolid process of AC4CH aluminum casting using an inclined cooling plate. Numerous α aluminum granular crystals nucleated on the inclined cooling plate during its flow, and the semisolid slurry ran into an isolated tundish. The crystals then became granular grains in the tundish. The semisolid slurry was cast into a horizontal continuous casting machine. The billet was 60 mm in diameter and 1000 mm in length and the casting speed was between 2 and 4 mm/s. The finest grain size of 50 μm could be obtained by casting at 897 K with a casting speed of 4 mm/s, an angle of 60°, and a length of 160 mm.

セミソリッド連続鋳造で作製された６０７０および７０７５アルミニウム合金のチクソフォーミング特性

To compare the thixoformability between granular crystal structures and dendrite crystal ones of 6070 and 7075 aluminum alloy billets produced by the semisolid and conventional DC continuous casting, the thixoforming test was performed at a constant deformation speed of 4mm/s in an argon atmosphere. The test piece consisting of granular crystals of 6070 alloy reveals that the thixoforming power was less than 1 kN during three quarters of forming process and the maximum power at the final stage was 18kN. In contrast, the test piece consisting of dendrite crystal structure reveals that the power was more than 5 kN at the first stage and as the forming process proceeded, the power increased. The maximum power at the last stage was 35 kN. Granular grains of primary α aluminum coexisting with the liquid phase were possible to move easily into the die cavity, because the liquid phase played an important role of lubricant as the thixoforming was performed to the semisolid slurry. However, when dendrite network was developed throughout the billet, the liquid phase among interdendrites only moved at the first stage. Hence, the deformation resistance increased and the thixoforming power became larger at the later stages. The same results were obtained by the thixoforming of 7075 aluminum alloy.

Change of conduction mechanism by microstructural variation in Pt/(Ba,Sr)TiO3/Pt film capacitors

The effects of microstructure on the leakage current behaviors in Pt/(Ba,Sr)TiO3/Pt thin film capacitors prepared by a radio-frequency magnetron sputtering technique were studied. Both applying an epitaxial seed layer and controlling post-annealing processes enabled us to obtain (Ba,Sr)TiO3 films of three different microstructures (granular, columnar, and epitaxial) but possessing an identical interface state with the Pt electrode. The Schottky emission dominates in the film composed of granular grains. In contrast, the Fowler–Nordheim tunneling governs in the epitaxial film with much elevated leakage current levels. In the case of the columnar film, at low temperatures the tunneling dominates, while at high temperatures the Schottky emission governs with intermediate leakage levels. An energy band model accounting for the change of leakage current mechanisms by microstructure variation is presented.

Selective growth of (100)-, (110)-, and (111)-oriented MgO films on Si(100) by pulsed laser deposition

Selective growth of singly oriented (110)-, (100)-, and (111)-MgO films on Si(100) substrates were obtained by pulsed laser deposition. The effects of deposition temperature, ambient oxygen pressure, and etching of the substrate on the structural properties of the films were studied. It is found that the crystalline orientations of the MgO films are determined at the initial deposition stage by the substrate temperature only. The ambient pressure during deposition and etching of the Si substrates only effect the crystalline quality. Both (110)- and (111)-oriented films show granular grain structures. The (100)-oriented films grown on etched Si substrates display similar granular structures. Those deposited on nonetched Si substrates, however, reveal distinctive columnar grains. The observed phenomena are discussed based on the theory of crystal growth. The mechanism of the orientation selection is attributed to the energy balance between the surface and the interface energies. The varied grain structures are explained by considering the mobility of adatoms in different situations.

Novel Pb(Ti, Zr)O3(PZT) Crystallization Technique Using Flash Lamp for Ferroelectric RAM (FeRAM) Embedded LSIs and One Transistor Type FeRAM Devices

A novel method of ferroelectric capacitor formation for Ferroelectrie random access memory (FeRAM) embedded LSIs and one-transistor-type FeRAMs has been developed. Amorphous Pb(Ti, Zr)O3(PZT) films were successfully transformed to the perovskite phase by a flash lamp technique with a crystallization time of 1.2 ms at a substrate temperature of 350°C. A flash lamp energy of 27 J/cm2 was sufficient to form a ferroelectric crystal structure due to rapid thermal effects with little heat diffusion in the depth direction. This technique enabled PZT film crystallization in Pt/PZT/Pt structures on multi-Al wiring layers. Granular PZT grains were observed on Pt, Ru and RuO2 electrodes, which indicates that crystal growth begins from the film surfaces. Ferroelectric property was verified by the process at 350°C maximum temperature. PZT films were also crystallized directly on SiO2. This is useful for the fabrication of embedded FeRAM devices and 1Tr-type FeRAMs. The flash lamp process was found to have great potential for application to dielectric film formation, annealing processes and so on.

Structure of Thermoelectric Material CoSb<SUB>3</SUB> Formed by Reactive Diffusion

Thermoelectric material, CoSb 3 , has been formed in solid Co/solid Sb, solid Co/liquid Sb and solid Co/vapor Sb reactive diffusion couples in the temperature range between 723 K and 1123 K. The solid Co/vapor Sb diffusion couples were annealed in a furnace having two homogeneity temperature zones keeping Co side and Sb side temperatures independently. CoSb 3 phase formed in these Sb diffusion couples showed various structures such as granular grains, large size polygonal grains with sharp edge and a flat diffusion layer with many small voids or with Sb phase depending on the kind of diffusion couples and temperature. The formation kinetics of such structures and growth of CoSb 3 have been studied and discussed.

Generalized Sufficient Strength Criteria. Description of the Pre-Fracture Zone

A generalized sufficient discrete–integral strength criterion is proposed for opening mode cracks in structured media. The criterion is constructed for materials of two types: an elastoplastic material with restricted deformability and an elastic ideal‐plastic material. A detailed description of the pre‐fracture zone is given. The cross‐sectional dimension of the plastic zone near the tip of the initial crack is used as the cross‐sectional dimension of the pre‐fracture zone. It is assumed that the critical opening of the initial crack depends on the deformability of plastic materials. Based on the ratio of the length of the pre‐fracture zone to the length of the initial crack, the following fracture types are distinguished: brittle, quasibrittle, quasiviscous, and viscous. For the first three types, the fracture curves are described in detail. Exact and approximate equations are proposed that relate critical parameters to the theoretical strength of a granular material, grain size, and parameters characterizing the averaging interval and the damage of the initial and deformed material.

Fabrication of cordierite–mullite ceramic composites with differently shaped mullite grains

Abstract Cordierite–mullite composites were fabricated from various starting powder mixtures to study the effect of different mixing combinations of the starting materials (commercial oxide sols and/or hydroxide powders) on the resultant mullite grains in the sintered composites. The morphology of the mullite grains was substantially changed depending on the Al-source materials used. Acicular grains were produced from the alumina sol-containing mixtures, whereas the growth of angular or granular mullite grains occurred for the mixtures containing Al(OH) 3 powder. When using alumina sol, its higher reactivity combined with better dispersability in precursor powders could cause the lowering in the mullitization temperature than for those containing Al(OH) 3 powder. In addition, it could produce very fine mullite seed particles in a calcined powder, leading to the growth of the acicular mullite grains in the sintered C/M composites.

Microstructural characterization of donor-doped lead zirconate titanate films prepared by sol–gel processing

Lanthanum and niobium-doped lead zirconate titanate, Pb(Zr,Ti)O 3, (PZT) thin films were prepared on Pt/Ti/SiO 2/Si substrates by sol–gel processing. Films have a Zr/Ti ratio of 52:48 and tetragonal perovskite phase structure. The influence of donor dopants on the morphology, texture and defects of PZT films was studied using X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, breakdown field strength and fatigue. Donor doping of PZT films results in increased breakdown field strength and improved fatigue properties. These originate from (i) the reduction of texture in the [111] and increase in the texture in [100]; (ii) lower porosity, smaller grain size and smoother surfaces; (iii) granular grain structure; and (iv) reduction in the oxygen vacancy concentration in the films.

Microstructural effects on leakage current behavior of (Ba,Sr)TiO3 thin films for dram applications

We have investigated the effects of microstructure on the leakage current behavior of Pt/(Ba,Sr)TiO3/Pt film capacitors. To single out the microstructural effects only, (Ba,Sr)TiO3 films of three different microstructures (granular, columnar, possibly epitaxial), but having an identical interfacial state density, were prepared using a seed layer by sputtering. The leakage behavior depends strongly on the film microstructure. Schottky emission dominates in the film composed of granular grains, while the epi-like film shows the higher leakage current and Fowler-Nordheim tunneling mechanism. Interestingly, the film of columnar grains shows tunneling at low temperatures and Schottky emission at high temperatures. We propose a model based on energy band diagrams explaining the results.

A study of the morphological aspects of the indium electrorefining process

The cathodic deposition of In was performed under conditions suitable for electrorefining purposes. The main goal of the study was: i) to determine the possibilities for In ultrapurification through multiple electrorefining with controlled parameters and ii) to study the regularities of the morphology of the In deposit as influenced by the electrodeposition conditions. Without application of special high purity techniques, it was possible to obtain an In purity level of 99.997 %, by repeating the refining procedure in a set of modular electrolysis cells with forced electrolyte circulation and applying modified current regimes. Irregular (non planparallel) migration of indium from the anode to the cathode and vice versa, as well as dendrite formation at the edges of the electrode prevented further repetition necessary for the achievment of higher purification levels. This provoked the need to study the morphology of the In-deposits. The phenomena and some regularities of In nucleation and grain growth, as influenced by the applied overpotential and quantity of current passed through the cell, were studied. Electrodeposition was performed onto a stationary Pt-electrode from amixed sulfate-chloride solution. SEM with magnification up to 1000 x was used to identify the morphology of the deposits. Granular grains were dominant at over potentials of 85 and 110 mV, while at 160 and 185 mV, needle-like and non branched dendrites were visible. The deposit became less compact as its thickness advanced. The apparent thickness of the In layer increased from 20-25 to 320-380 ?m when the amount of charge was increased from 1 to 10 mA h cm-2.

The effect of input gas ratio on the growth behavior of chemical vapor deposited SiC films

In an effort to protect a RBSC (reaction-bonded silicon carbide) reaction tube, SiC films were chemically vapor deposited on RBSC substrates. SiC films were prepared to investigate the effect of the input gas ratios (dilute ratio, α = PH2/PMTS = QH2/QMTS) on the growth behavior using MTS (metyltrichlorosilane, CH3SiCly3) as a source in hydrogen atmosphere. The growth rate of SiC films increased and then decreased with the decrease of the input gas ratio at the deposition temperature of 1250°C. The microstructure and preferred orientation of SiC films were changed with the input gas ratio; Granular type grain structure exhibited the preferred orientation of (111) plane in the high input gas ratio region (α = 3–10). Faceted columnar grain structure showed the preferred orientation of (220) plane at the low input gas ratios (α = 1–2). The growth behavior of CVD SiC films with the input gas ratio was correlated with the change of the deposition mechanism from surface kinetics to mass transfer.

Nitridation of GaP (100) Surfaces by rf Nitrogen Radicals and by Electron Cyclotron Resonance Nitrogen Plasma

The nitridation process of a GaP (100) surface using rf nitrogen radicals and electron cyclotron resonance (ECR) N2 plasma at 400–500°C has been investigated by reflection high-energy electron diffraction (RHEED), in situ X-ray photoemission spectroscopy (XPS), atomic force microscopy (AFM) and cathodoluminescence (CL) methods. The P-stabilized (2×4) surface prepared by ultra high-vacuum (UHV) thermal cleaning was used as an initial surface. At the initial stage of nitridation, both the P–N and Ga–N components were formed on the GaP surface. The subsequent radical process led to the dominant formation of strained small granular grains of cubic GaN with diameters of 5–10 nm. Further nitridation produced much larger hemispherical GaN grains. The surface exposed to ECR N2 plasma for 60 min exhibited an anisotropic surface morphology with rectangular structures which aligned to the [011] direction. Clear CL peaks originating from the cubic GaN structure were observed on these nitrided surfaces.

Structure of Electrodeposited Zn–Mn Alloy Coatings

Structure of electrodeposited Zn–Mn alloys which are one of the corrosion-resistant zinc alloy coatings suitable for automotive body panels was investigated using X-ray diffraction, SEM and SIMS.The coatings are composed of fine grains or granular grains which contain fine particles, depending on the Mn content. The coatings have an uniform depth distribution of Mn and Zn.X-ray diffraction revealed that the coatings have either single or dual alloy phases which are the following phases, namely η phase, Γ phase, ε phase, γ-Mn phase and α-Mn phase, depending on the Mn content. The ε phase and γ-Mn phase as high temperature phases exist when the range of the Mn content is above 20 mass%. Both phases are stable when heated at 170°C. The γ-Mn phase is transformed to the β-Mnphase and the α-Mn phase by heating beyond 250°C, although the e phase is unchanged.

Occurrences of Natural Gas Hydrates beneath the Permafrost Zone in Mackenzie Delta: Visual and X‐ray CT Imagery

Abstract: The JAPEX/JNOC/GSC Mallik 2L‐38 research well was drilled to a depth of 1,150 m beneath the permafrost zone in the Mackenzie Delta, N.W.T., Canada, early in 1998. A large amount of natural gas hydrates were successfully retrieved from a variety of sandy and gravel sediments. Over 110 m of gas hydrate‐bearing sediments were found to be distributed between 897 m and 1,100 m deep. Approximately 37 meters of core were recovered in this interval with most of the recovered gas hydrates being less than 2 mm in size occurring mainly in intergranular porosity of silty to clean massive sand and conglomerate (granule to pebble). Typically, hydrate‐bearing strata were between 10 cm and more than one meter thick with an estimated porosity of 25 to 35%. The largest form of hydrate was about 2 cm in diameter, occurring as clasts and intergranular porosity within granular sands. Occurrences of natural gas hydrate have been observed visually at the drill site and in core samples preserved in pressurized storage vessels utilizing an X‐ray CT scanner technique. Quantitative assessments of gas hydrate concentrations in core samples have been made based on pressure response of dissociation vessels and direct volumetric measurements. Six types of gas hydrate have been recognized: (1) pore‐space hydrate, (2) platy hydrate, (3) layered/massive hydrate, (4) disseminated hydrate, (5) nodule hydrate, and (6) vein/dyke hydrate. The X‐ray CT images proved useful for characterizing macroscopic forms of gas hydrate. Finer grained occurrences were more difficult to study, however the distribution of gas hydrates and granular grains can be recognized. The occurrences of natural gas hydrates in the Mallik well are compared to the previous natural gas hydrate core samples obtained from ODP/DSDP programs and other field studies.

Zn-spinel in the metamorphosed Zn-Pb-Cu sulphide deposit at Mamandur, southern India

Abstract At Mamandur, southern India, zincian spinel is associated with Zn-Pb-Cu sulphide ores, metamorphosed to the granulite facies. The Zn-spinel (XZn = 0.44–0.82) occurs in assemblages of: (a) cordierite + biotite + sillimanite + sphalerite + pyrite and/or pyrrhotite; (b) garnet + biotite + sphalerite + pyrite and/or pyrrhotite; (c) hornblende + biotite + sphalerite + pyrite and/or pyrrhotite; and (d) it also occurs in highly sheared quartz veins with sphalerite + pyrite/pyrrhotite. Two texturally and compositionally distinct modes of occurrence of Zn-spinel have been recorded from the metamorphic assemblages: one shows a polygonal, granular (equilibrium) fabric with Zn-spinel grains in textural equilibrium with cordierite in the granulite facies assemblage; the other which is involved in the formation of coronas in garnet- and hornblende-bearing assemblage of a younger paragenesis (presumably belonging to a late retrograde stage), shows distinct compositional zoning with depletion of Zn and enrichment in Fe and Mg from the rim inwards. A third mode of occurrence is as perfectly euhedral grains embedded in a quartzose matrix in quartz veins, often with sphalerite moulded over the Zn-spinel grains. From textural evidence and consideration of stoichiometric balance, Zn-spinel occurring in association with cordierite is suggested to have formed by the prograde reactions: 0.46 biotite (Mg-rich) + 1.80 sillimanite + 3.01 quartz + 0.11 sphalerite = cordierite + 0.20 Zn-spinel + 0.80 K-feldspar + 0.06 pyrite + 4(OH,F) or 0.48 biotite (Mg-rich) + 2.08 sillimanite + 2.72 quartz + 0.26 sphalerite = cordierite + 0.50 Zn-spinel + 0.83 K-feldspar + 0.13 S2 + 4(OH,F) and, the Zn-spinels occurring in association with garnet and hornblende by the retrograde reactions: garnet + 4.53 Al2SiO5 + 4.17 sphalerite = 5.38 Zn (-Fe)-spinel + 0.22 Ca-feldspar + 0.92 pyrite + 7.06 quartz + 1.23 S2 and hornblende + 26.93 Al2SiO5 + 20.17 sphalerite + 0.88 pyrite = 25.93 Zn-spinel + 2 Ca-feldspar + 56.81 quartz + 11.32 S2 + 2(OH,F) Zn-spinel in the sheared quartz vein may be of (meta-) hydrothermal origin. Formation of sphalerite through sulphurization of gahnite is ruled out on the evidence of the perfectly euhedral nature of the spinel and the absence of any breakdown product from the spinel.

Pollen morphology of some selected plant species from Horton Plains, Sri Lanka

The pollen morphology of four species: Gaultheria leschenaultii (Ericaceae), Psychotria zeylanica (Rubiaceae), Rubus leucocarpus (Rosaceae) and Symplocos bractealis (Symplocaceae) from Horton Plains, Sri Lanka was investigated using LM and SEM. Psychotria zeylanica is endemic to Sri Lanka and the other three taxa are common in Horton Plains. Gaultheria leschenaultii has tetrads with 3-colporate, coarsely granular to granulo-rugulate pollen grains; in the polar region and at the colpus margin the exine is smooth with micro-rugulate structure. Psychotria zeylanica has 3- (or 4-) colporate, reticulate-spinulose to granular pollen grains. Rubus leucocarpus has 3-colporate, striate-perforate pollen grains and Symplocos bractealis has 3- (or 4-) colporate pollen grains with a verrucate-perforate exine. Pollen grains of these four species are very indicative of human impact and palaeoclimate.