Interconnected Grains Research Articles

Sintering of porous alumina obtained by biotemplate fibers for low thermal conductivity applications

Abstract In this research report, a sintering process of porous ceramic materials based on Al 2 O 3 was employed using a method where a cation precursor solution is embedded in an organic fibrous cotton matrix. For porous green bodies, the precursor solution and cotton were annealed at temperatures in the range of 100–1600 °C using scanning electron microscopy (SEM) and thermogravimetric (TG) analysis to obtain a porous body formation and disposal process containing organic fibers and precursor solution. In a structure consisting of open pores and interconnected nanometric grains, despite the low porosity of around 40% (calculated geometrically), nitrogen physisorption determined a specific surface area of 14 m 2 /g, which shows much sintering of porous bodies. Energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analytical methods revealed a predominant amount of α-Al 2 O 3 in the sintered samples. Thermal properties of the sintered Al 2 O 3 fibers were obtained by using the Laser Flash which resulted in the lower thermal conductivity obtained by α-Al 2 O 3 and therefore improved its potential use as an insulating material.

A new approach for improving global critical current density in Fe(Se0.5Te0.5) polycrystalline materials

A novel method to prepare bulk Fe(Se0.5Te0.5) samples is presented, based on a melting process and a subsequent annealing treatment. With respect to the standard sintering technique, it produces much more homogeneous and denser samples, characterized by large and well interconnected grains. The resulting samples exhibit optimal critical temperature values, sharp resistive and magnetic transitions, large magnetic hysteresis loops and high upper critical fields. Interestingly, the global critical current density is much enhanced as compared to the values reported in the literature for bulk samples of the same 11 family, reaching about 103 A cm−2 at zero field at 4.2 K, as assessed by magnetic, transport and magneto-optical techniques. Even more importantly, its field dependence turns out to be very weak, such that at μ0H = 7 T it is suppressed only by a factor ∼2.

Microstructural Evolution of AZ91D-1.5%Er during Semi-Solid Isothermal Treatment

In this paper, chip recycling technology combined with SIMA method which is called CR-SIMA method was adopted to prepare semi-solid billets. AZ91D magnesium alloy was refined by Er and its microstructural evolution was investigated during semi-solid isothermal treatment. The results show that Er can improve the feature of cast structure and decrease the grain size. Moreover, the γ-Mg17Al12 phase is well refined and disperses in the α-Mg matrix. A semisolid microstructure with small and spheroidal primary particles can be obtained after partially remelting. With increasing heating temperature, the dissolution of eutectic Mg17Al12 phase first took place, resulting in the primary dendritic grains coarsening into interconnected non-dendritic grains. With heating continuously, the residual interdendritic γ-Mg17Al12 at the edges of the primary grains melted in succession and the primary grains separated into small polygon grains. During the semi-solid isothermal treatment, the amount of liquid increased until the solid-liquid system reached its equilibrium state. At the same time, owing to the decreasing of interfacial energy, the grains gradually spheroidized and began to grow with a further increasing of the holding time.

Formation of porous SiC ceramics via recrystallization

In this study, porous SiC ceramics with interconnected huge plate-like grains were fabricated from oxidized β-SiC powder with 1 wt% B4C. When the β–α SiC phase transformation occurred at 2100 °C, rapid grain growth of α-SiC consumed the unstable β-SiC matrix resulting in an interconnected network structure with huge plate-like grains. The oxidation of β-SiC powder and the addition of B4C are necessary conditions for rapid grain growth. The observed results are discussed based on thermodynamic considerations. The measured porosity of the specimens sintered at 2100 °C for 30 min was 47% and the mean pore size was 6–7 μm. The strength of the sintered specimen was 45 ± 5 MPa.

Correlation between solid fraction and tensile properties of semisolid RAP processed aluminum alloys

Recrystallization and remelting (RAP), a promising route for thixoforming process, was used to process three aluminum alloys, AA2024, AA6061, and A356, under experimental conditions designed to yield various solid fractions (liquid fractions) within the microstructure. The thixotropic microstructures obtained were characterized in detail and linked to the corresponding tensile properties. When the liquid fraction increased (the solid fraction decreased), an initial reduction in the tensile properties was perceived. The lowest tensile properties were obtained at liquid fractions of 35% (610 °C), 25% (623 °C), and 40% (585 °C) for AA2024, AA6061, and A356 alloys, respectively. For the AA6061 and A356 alloys, a further increase in the liquid fraction resulted in improvement of the tensile properties. However, for the AA2024 alloy, the samples could not withstand their own weight at higher liquid fractions that further tensile tests were rendered. It was concluded that for alloys below the liquid fraction corresponding to the minimum tensile properties, the structural properties of interconnected grains (skeletons), and the quenched liquid eutectic phase located at the grain boundaries (liquid film) controlled the fracture behavior. Nevertheless, the nature of the cohesion between the grains and the liquid film was the determining factor in the improvement in the tensile properties that resulted when the liquid fraction was increased to 46% and 60% for the AA6061 and A356 alloys, respectively.

Dye‐Sensitised Solar Cells Based on Large‐Pore Mesoporous TiO2 with Controllable Pore Diameters

AbstractMesoporous TiO2 (meso‐TiO2) is a promising photoelectrical nanomaterial for high dye‐sensitised solar cell (DSSC) performance, because its nanochannels offer a large internal surface area to allow for more dye adsorption. The interconnected grains facilitate rapid electron transport both within the meso‐TiO2 film and at the film electrode dye/redox shuttle electrolyte interfaces. In this work, a series of DSSCs has been fabricated on the basis of meso‐TiO2 with large controllable pore sizes (6.5, 8.2 and 11.0 nm). It was found that the DSSC with the 8.2 nm meso‐TiO2 photoelectrode has the highest photoelectrical conversion efficiency, which is higher than that of the conventional P25 nanoparticulate DSSC under the same conditions. The measurements of the N2 sorption isotherms and the photovoltage transient demonstrates that such improved efficiency can be ascribed to the larger surface area and the fastest interfacial charge transfer. Meanwhile, the effect of different pore sizes on the photoelectrical conversion efficiency has been systematically investigated, and a possible model for the efficient electrolyte percolation in the dye‐sensitised mesoporous photoelectrode has been proposed.

Geophysical signatures of fluids in a reactivated Precambrian collisional suture in central India

The Central India Tectonic Zone (CITZ) marks the trace of a major suture zone along which the south Indian and the north Indian continental blocks were assembled through subduction-accretion-collision tectonics in the Mesoproterozoic. The CITZ also witnessed the major, plume-related, late Cretaceous Deccan volcanic activity, covering substantial parts of the region with continental flood basalts and associated magmatic provinces. A number of major fault zones dissect the region, some of which are seismically active. Here we present results from gravity modeling along five regional profiles in the CITZ, and combine these results with magnetotelluric (MT) modeling results to explain the crustal architecture. The models show a resistive (more than 2000 Ω·m) and a normal density (2.70 g/cm 3) upper crust suggesting dominant tonalite–trondhjemite–granodiorite (TTG) composition. There is a marked correlation between both high-density (2.95 g/cm 3) and low-density (2.65 g/cm 3) regions with high conductive zones (<80 Ω·m) in the deep crust. We infer the presence of an interconnected grain boundary network of fluids or fluid-hosted structures, where the conductors are associated with gravity lows. Based on the conductive nature, we propose that the lower crustal rocks are fluid reservoirs, where the fluids occur as trapped phase within minerals, fluid-filled porosity, or as fluid-rich structural conduits. We envisage that substantial volume of fluids were transferred from mantle into the lower crust through the younger plume-related Deccan volcanism, as well as the reactivation, fracturing and expulsion of fluids transported to depth during the Mesoproterozoic subduction tectonics. Migration of the fluids into brittle fault zones such as the Narmada North Fault and the Narmada South Fault resulted in generating high pore pressures and weakening of the faults, as reflected in the seismicity. This inference is also supported by the presence of broad gravity lows near these faults, as well as the low velocity in the lower crust beneath regions of recent major earthquakes within the CITZ.

Evolution of nanostructure, defect-free photoluminescence and enhanced photoconductivity of oxidized Zn films

Nanostructured zinc oxide thin films were prepared by the oxidation of nanostructured zinc films deposited on glass substrates by thermal evaporation of metallic zinc in an atmosphere of nitrogen. The films were oxidized at different temperatures. X-ray diffraction was used to study the structural transformations of the films with oxidation temperature. Atomic force microscopy images of surface morphology of the films revealed the kinetics of aggregation of the grains as the oxidation temperature was increased. Raman spectra of the samples contained a surface mode at ∼482 cm−1 showing characteristics of fine grain size. UV-Visible absorption spectra of the samples showed a blueshift of bandgap in comparison with that of bulk ZnO crystals. The photoluminescence emission spectra of the samples were free from defect related emissions. The increase in oxidation temperature of these samples caused a narrowing of the photoluminescent emission band in the UV region and an increase in the UV photocurrent. The ratio of maximum photocurrent to dark current was found to increase from 100 to 2000 as the oxidation temperature of the films increased from 350 to 500 showing an enhanced photoconductivity. The increased photocurrent was attributed to the interconnected grain structure of the ZnO samples oxidized at higher temperature.

Acicular Mullite–Cordierite Composites with Controllable CTE Values

A new, two‐step method for the synthesis of porous mullite–cordierite composites with controllable coefficients of thermal expansion (CTE) values is reported. In the first step, a porous structure of interconnected acicular mullite grains with high aspect ratios is formed by using SiF4 gas at elevated temperatures. This intermediate contains particles of MgF2 and cristobalite that are attached to the mullite grains. In the second step, MgF2 and cristobalite react with parts of the acicular mullite structure, and they form cordierite, while maintaining the acicular nature of the resulting mullite–cordierite composite. CTE of mullite–cordierite composites synthesized using this method decrease about linearly with increasing cordierite content, and porosities range from 49% in cordierite‐rich composite to 54% in a mullite‐rich composite. The CTE value of a desired porous composite can be easily controlled by choosing the appropriate ratios of reactants Al2O3, SiO2, and MgO, which result in the desired mullite‐to‐cordierite ratio with its characteristic CTE value.

Photoreductive Dissolution of Iron Oxides Trapped in Ice and Its Environmental Implications

The availability of iron has been thought to be a main limiting factor for the productivity of phytoplankton and related with the uptake of atmospheric CO(2) and algal blooms in fresh and sea waters. In this work, the formation of bioavailable iron (Fe(II)(aq)) from the dissolution of iron oxide particles was investigated in the ice phase under both UV and visible light irradiation. The photoreductive dissolution of iron oxides proceeded slowly in aqueous solution (pH 3.5) but was significantly accelerated in polycrystalline ice, subsequently releasing more bioavailable ferrous iron upon thawing. The enhanced photogeneration of Fe(II)(aq) in ice was confirmed regardless of the type of iron oxides [hematite, maghemite (gamma-Fe(2)O(3)), goethite (alpha-FeOOH)] and the kind of electron donors. The ice-enhanced dissolution of iron oxides was also observed under visible light irradiation, although the dissolution rate was much slower compared with the case of UV radiation. The iron oxide particles and organic electron donors (if any) in ice are concentrated and aggregated in the liquid-like grain boundary region (freeze concentration effect) where protons are also highly concentrated (lower pH). The enhanced photodissolution of iron oxides should occur in this confined boundary region. We hypothesized that electron hopping through the interconnected grain boundaries of iron oxide particles facilitates the separation of photoinduced charge pairs. The outdoor experiments carried out under ambient solar radiation of Ny-Alesund (Svalbard, 78 degrees 55'N) also showed that the generation of dissolved Fe(II)(aq) via photoreductive dissolution is enhanced when iron oxides are trapped in ice. Our results imply that the ice(snow)-covered surfaces and ice-cloud particles containing iron-rich mineral dusts in the polar and cold environments provide a source of bioavailable iron when they thaw.

Magnetotransport effects of ultrathin Ni80Fe20 films probed in situ

We investigated the magnetoresistance of Permalloy (Ni80Fe20) films with thicknesses ranging from a single monolayer to 12 nm, grown on Al2O3, MgO and SiO2 substrates. Growth and transport measurements were carried out at 80 K in UHV. Applying in-plane magnetic vector fields up to 100 mT, the magnetotransport properties were ascertained during growth. With increasing thickness the films exhibited a gradual transition from tunnelling magnetoresistance to anisotropic magnetoresistance. This corresponds to the evolution of the film structure from separated small islands to a network of interconnected grains, as well as the film's transition from superparamagnetic to ferromagnetic behaviour. Using an analysis based on a theoretical model of island growth, we found that the observed evolution of the magnetoresistance in the tunnelling regime originated from changes in the island size distribution during growth. Depending on the substrate material, significant differences in the magnetoresistance response in the transition regime between tunnelling magnetoresistance and anisotropic magnetoresistance were found. We attributed this to an increasingly pronounced island growth, and to a slower percolation process of Permalloy when comparing growth on SiO2, MgO and Al2O3 substrates. The different growth characteristics resulted in a markedly earlier onset of both tunnelling magnetoresistance and anisotropic magnetoresistance for SiO2. For Al2O3 in particular the growth mode results in a structure of the film containing two different contributions to ferromagnetism, which lead to two distinct coercive fields in the high thickness regime.

Synthesis and characterization of nanostructured nickel oxide thin films prepared with chemical spray pyrolysis

Nickel oxide thin films have been deposited in an open atmosphere onto glass substrates by chemical spray pyrolysis using aqueous nickel acetate solutions and air as driving gas. The films show a strong variation in the surface morphology depending on the substrate temperature and the precursor solution flux. At 350 °C substrate temperature, a reticular tissue-like film morphology is obtained, becoming the reticular nickel oxide fibres of the film thicker with increasing precursor solution flux. At 450 °C substrate temperature, the film growth rate is 4 times slower and a highly symmetric self-ordering of the material at nanometer length scale occurs. These films consist of interconnected grains separated by pores, both of about 100 nm in size. XRD and TEM revealed that the films are cubic NiO, being the crystallite size around 10 nm. The optical band gap of the films decreases strongly for increasing film thickness from 4.3 eV to 3.65 eV.

Morphology evolution and magnetic properties improvement in FePt epitaxial films by in situ annealing after growth

This paper presents a detailed study of structure, morphology, and magnetic properties in FePt thin films epitaxially grown on MgO(100) at intermediate temperatures (≅380°C). It focuses on the effects obtained by in situ annealing the FePt films after deposition. We have demonstrated that the annealing allows one to complete the ordering, thus obtaining an epitaxial L10-FePt film with large perpendicular anisotropy (up to 3×107erg∕cm3) and high perpendicular squareness and remanence ratio (both higher than 0.85). At the same time, we have found a peculiar morphology evolution by increasing the annealing time: the interconnected mazelike grains evolve towards interconnected dotlike grains of reduced size (down to 10nm). Due to the interconnection at the grains basis, the increase in the film perpendicular coercivity with the annealing process is moderate (up to 4.1kOe), leading to an increasing anisotropy/coercivity ratio with the annealing time. As a further step towards the understanding of the properties evolution with annealing, we have investigated the magnetic domain pattern and analyzed the domain-period dependence on the anisotropy constant in comparison with the behavior expected by the Kooy–Enz theory [C. Kooy and U. Enz, Philips Res. Rep. 15, 7 (1960)]. The main magnetic properties and reduced grain size obtained after the in situ annealing are promising to realize perpendicular recording media with reduced bit size, high thermal stability, and suitable switching field.

Preparation of Porous Tin Oxide Nanobelts Using the Electrospinning Technique

The electrospinning method is adopted to prepare utralong PEO/stannic hydroxide composite nanofibers. Tin‐oxide nanobelts can be obtained by calcination of the composite nanofibers in an open atmosphere. The nanobelts were characterized by a field emission scanning electron microscope, X‐ray diffraction, a transmission electron microscope, a Raman spectromicroscope, and Fourier transform infrared spectroscopy. Microstructural analysis has shown that the nanobelts prepared consist of a continuous network of interconnected SnO2 grains. As a result, the SnO2 nanobelts possess a high surface area and continuous porosity, which may be applied for the fabrication of sensitive gas sensors.

Nonclassical Rotational Inertia in Helium Crystals

It has been proposed that the observed nonclassical rotational inertia (NCRI) in solid helium results from the superflow of thin liquid films along interconnected grain boundaries within the sample. We have observed NCRI in large (4)He crystals grown at constant temperature and pressure, demonstrating that the superfluid grain boundary model cannot explain the phenomenon.

Scanning tunneling microscopy/magnetic force microscopy study of ultrathin Fe film on MgO(001) in ultrahigh vacuum

Using ultrahigh vacuum high resolution scanning probe microscopy, we investigated the growth and magnetic properties of epitaxial Fe films grown on MgO(001) at different substrate deposition temperatures. On room temperature substrates, Fe forms interconnected grains and for 14nm films produce very large magnetic domains. The domain walls are comprised of a rich variety of Néel, Bloch, and cross ties with interesting characteristics. On high temperature substrates, Fe grows by pyramidal island formation, with well-defined single atomic step terraces. The magnetic properties are dependent on the film thickness, which determines the amount of exchange coupling between the islands. The magnetic structure varies from complex patterns at low coverage to well ordered magnetic ripples and domain walls at higher coverage. The detailed explanation will be shown in this paper.

Properties of NbC–Co cermets obtained by spark plasma sintering

Sub-micrometer sized NbC–Co powder mixtures with 8, 12, 18 or 24.5 wt.% Co were consolidated by spark plasma sintering (SPS) for 2 min at 1200–1280 °C and 30–60 MPa. The optimum densification conditions were determined by analysing the dimensional change of the NbC–12 wt.% Co powder compact. SPS for 2 min at 1280 °C under a pressure of 60 MPa allowed full densification of the NbC–Co cermets with limited NbC grain growth. The microstructure is characterized as a highly interconnected NbC grain network with an inhomogeneously distributed Co binder. The Vickers hardness increased from 11.70 to 15.40 GPa whereas the fracture toughness decreased from 9.0 to 5.5 MPa m 1 / 2 with decreasing Co content from 24.5 to 8 wt.%.

Organic light-emitting transistors using concentric source/drain electrodes on a molecular adhesion layer

Bottom-contact tetracene light-emitting transistors employing a mercaptosilane derivative self-assembled monolayer as adhesive between gold concentric interdigitated source/drain electrodes and SiO2 gate dielectric are described. Devices that employ the mercaptosilane adhesive have a higher mobility and electroluminescence compared to those employing a standard metallic adhesive. This is rationalized in terms of the large, well interconnected grains found in tetracene films deposited on substrates using the mercaptosilane adhesive. Our work represents a step forward in the understanding of physical processes at semiconductor/metal and semiconductor/dielectric interfaces in organic devices.

Semisolid microstructure evolution of the predeformed AZ91D alloy during heat treatment

The semisolid microstructure evolution process of the predeformed AZ91D alloy during heat treatment was studied. After being compressed by a ratio of 20.6% in height at 220 °C, all the dendrites oriented in the same way. With increasing heat-treatment temperature, the dissolution of eutectic β-Mg 17Al 12 first took place, resulting in the primary dendritic grains coarsening into interconnected non-dendritic grains. With heating continuously, a large amount of polygon recrystallization grains occur in the microstructure. Subsequently, the residual interdendritic β-Mg 17Al 12 at the edges of the primary grains and the new recrystallization grains melted in succession at higher heat-treatment temperature and the primary grains separated into small polygon grains. After being heat-treated at the semisolid temperature, the amount of liquid increased until the solid–liquid system reached its equilibrium state, and the grains spheroidized.

The microstructural and constitutional evolution of cast dendritic ZA27 alloy during partial remelting

The microstructural and constitutional evolution process of cast dendritic ZA27 alloy during heating into the semi-solid state was studied by scanning electron microscopy and X-ray diffraction techniques. Dissolution of the interdendritic eutectic η-phase first took place towards the insides of the primary dendrites, resulting in the primary dendritic grains coarsening into interconnected non-dendritic grains. Subsequently, the residual interdendritic η-phase and the edges of the primary grains melted in succession, making the microstructure of the alloy change into a solid skeleton structure penetrated by liquid. Correspondingly, the phase transformations of α+η+ε→β, η+β→L, β→α′+L and α′→L occurred in turn during this process. In addition, special attention was paid to the formation of the liquid pools entrapped within the solid grains.