Submicrometer Grain Research Articles

Two‐Stage Sintering of Alumina with Submicrometer Grain Size

This work verifies the applicability of two‐stage sintering as a means of suppressing the final stage grain growth of submicrometer alumina. The first heating step should be short at a relatively high‐temperature (1400°–1450°C) in order to close porosity without significant grain growth. The second step at temperatures around 1150°C facilitates further densification with limited grain growth. Fine‐grained alumina with a relative density of 98.8% and a grain size of 0.9 μm was prepared by two‐stage sintering. A standard sintering process resulted in ceramics with identical relative density and a grain size of 1.6 μm.

Novel Au/La 1− xSr xMnO 3 and Au/La 1− xSr xCrO 3 composites: Catalytic activity for propane partial oxidation and reforming

This work describes the low temperature synthesis of novel nanosized Au/La 1− x Sr x MnO 3 ( x ≈ 0.4) and Au/La 0.85Sr 0.15CrO 3 composites and investigates their potential as anodic catalysts for propane reforming. The pristine La 1− x Sr x MnO 3 ( x ≈ 0.4) and La 0.85Sr 0.15CrO 3 were prepared by a modified nitrate–citrate Pechini method and the process was optimized in order to obtain high surface area perovskite powders. These materials with nanometric or sub-micrometric grain size have been isolated as a single-phase and used for the preparation of the composite. A homogenous distribution of nanosized gold particle (< 5 nm) was deposited on the perovskite powders surface by an ultrasound-driven synthesis. The catalytic behavior of these inorganic composites was investigated for the following propane reforming processes, i.e. steam reforming, partial oxidation and autothermal reforming. Good propane conversions in the partial oxidation conditions were detected for sub-hundred nanometer La 1− x Sr x MnO 3 perovskite and the role of gold as a promoter for this process was assessed. In particular, nanosized gold on La 1− x Sr x MnO 3 resulted in an increase of the perovskite stability under all catalysis conditions. The influence of water steam in the feeding gas stream was also tested. The low influence of gold presence in catalytic tests of Au/La 0.85Sr 0.15CrO 3 suggested poor gold catalytic activity in the tested conditions but a presumable role of gold on the stabilization of the Mn mixed oxidation state in the case of the manganites.

Cryogenic graphitization of submicrometer grains embedded in nanostructured tetrahedral amorphous carbon films

A growth mechanism for submicrometer grains in diamondlike carbon films has been investigated. Films were formed by laser deposition on quartz substrates at 20–573K. Atomic force microscopy shows that films deposited at 20–100K consist of a matrix assembled from 40nm nanoparticles that further aggregate into 400–500nm grains which become embedded in the matrix. Nanohardness measurements show that these embedded grains possess a lower hardness than that of the matrix. Electronic structure obtained from resonant Raman spectra shows a remarkable clustering of sp2 bonds in the sp3 bonded matrix. It is believed that the sp2 bonded phase is concentrated in these submicrometer grains. X-ray photoelectron spectra show that this segregated structure has an enhanced affinity for oxygen.

Characterization of ZnO-degraded varistors used in high-tension devices

The effects of the degradation process on the structural, microstructural and electrical properties of ZnO-based varistors were analyzed. Rietveld refinement showed that the BiO 2− x phase is affected by the degradation process. Besides the changes in the spinel phase, the degradation process also affects the lattice microstrain in the ZnO phase. Scanning electron microscopy analysis showed electrode-melting failure, while wavelength dispersive X-ray spectroscopy qualitative analysis showed deficiency of oxygen species at the grain boundaries in the degraded samples. Atomic force microscopy using electrostatic mode force illustrated a decrease in the charge density at the grain boundaries of the degraded sample. Transmission electron microscopy showed submicrometric spinel grains embedded in a ZnO matrix, but their average grain size is smaller in the degraded sample than in the standard one. Long pulses appeared to be more harmful for the varistors’ properties than short ones, causing higher leakage current values. The electrical characteristics of the degraded sample are partially restored after heat treatment in an oxygen-rich atmosphere.

A new view of the onset of plasticity during the nanoindentation of aluminium

In nanoscale contact experiments, it is generally believed that the shear stress at the onset of plasticity can approach the theoretical shear strength of an ideal, defect-free lattice, a trend also observed in idealized molecular dynamics simulations. Here we report direct evidence that plasticity in a dislocation-free volume of polycrystalline aluminium can begin at very small forces, remarkably, even before the first sustained rise in repulsive force. However, the shear stresses associated with these very small forces do approach the theoretical shear strength of aluminium (approximately 2.2 GPa). Our observations entail correlating quantitative load-displacement measurements with individual video frames acquired during in situ nanoindentation experiments in a transmission electron microscope. We also report direct evidence that a submicrometre grain of aluminium plastically deformed by nanoindentation to a dislocation density of approximately 10(14) m(-2) is also capable of supporting shear stresses close to the theoretical shear strength. This result is contrary to earlier assumptions that a dislocation-free volume is necessary to achieve shear stresses near the theoretical shear strength of the material. Moreover, our results in entirety are at odds with the prevalent notion that the first obvious displacement excursion in a nanoindentation test is indicative of the onset of plastic deformation.

Effect of Alumina Additions on Mechanical and Electrical Properties of 8mol% Yttria-Stabilized Zirconia Prepared by Spark Plasma Sintering

Dense 8mol% yttria-stabilized zirconia (8YSZ) consisting of submicrometer-sized grains was prepared using spark plasma sintering (SPS) along with Al2O3 additives. The starting powder with average particle size of 50nm was densified to 98% of the relative density with short sintering time (5min) at 1200 while preserving a submicrometer grain size. The fracture toughness and bending strength showed maximum values of 2.54MPam1/2 and 380MPa at 2vol% alumina-added 8YSZ, due mainly to the higher relative density and small grain size. The electrical conductivity of 2vol% alumina-added 8YSZ was 0.0278 S/cm at 700 in airThus, alumina additions in 8YSZ using the SPS method are an effective process to improve the mechanical strength and electrical conductivity.

A self-heating gas sensor with integrated NiO thin-film for formaldehyde detection

This study develops a MEMS-based formaldehyde gas sensor based on a suspended silicon nitride microstructure with an integrated micro Pt heater, a thin-film NiO sensing layer and Pt interdigitated electrodes (IDEs) to measure the resistance changes of the NiO layer in the presence of formaldehyde. A specific orientation of the NiO layer is observed as the substrate temperature in the sputtering process is increased. The increase in substrate temperature assists in the formation of a NiO layer with the correct stoichiometric ratio (1:1). When formaldehyde is present in the atmosphere, oxidation occurs near the heated NiO sensing layer. This oxidization causes a change in the electrical conductivity of the NiO film, and hence changes the measured resistance between the interdigitated electrodes. The formaldehyde concentration is then determined from the change in the measured resistance. The application of a voltage to the Pt heaters causes the temperature of the micro-hotplate to increase, which in turn enhances the sensitivity of the sensor. The current experimental results show that the sub-micrometer grain sizes of the sputtered oxide thin film yield a high degree of sensitivity (0.33 Ω ppm −1), a low hysteresis value (0.7 ppm), a detection capability of less than 0.8 ppm, a quick response time (13.2 s), a quick recovery time (40.0 s) and a high selectivity over a wide range of formaldehyde concentrations in the presence of interfering species, such as acetone, ethanol and methanol. The novel micro formaldehyde gas sensor developed in this study is ideal for applications aimed at preventing and controlling sick building syndrome (SBS).

Improved Conductivity of Ce1 − xSmxO2 − δ Ceramics with Submicrometer Grain Sizes

Freeze-drying was used to prepare nanocrystalline powders of , , with high-purity and crystallite sizes in the range . These powders were used to obtain ceramic samples with in average diameter by firing at , without sintering additive, and for samples sintered at , with previous addition of cobalt nitrate solution to the freeze-dried powders. Impedance spectroscopy was used to deconvolute components of the spectra related to grain interiors and grain boundaries. For samples with grain sizes in the range , the bulk conductivity increases with decreasing content of Sm up to 10%, whereas the grain boundary conductivity shows the opposite trend. However, samples with relatively low content of Sm and submicrometer grain sizes, obtained with addition of Co, retain the highest conductivity for both the bulk and grain boundary conductivities. The differences in grain boundary conductivities between samples with small and large grain sizes were about 1 order of magnitude for 20% Sm and 2 orders of magnitude for 5% Sm and 10% Sm. Differences in grain boundary behavior were interpreted on the basis of a space-charge layer adjacent to the boundary core. The estimated values of grain boundary thickness were in the order of for samples prepared with and without addition of cobalt nitrate.

An analysis of superplastic flow after processing by ECAP

Processing by equal-channel angular pressing (ECAP), when carried out optimally, produces submicrometer grain sizes, arrays of equiaxed grains and a significant fraction of high-angle grain boundaries. These conditions are appropriate for achieving high strain rate and/or low temperature superplasticity. This paper reviews experimental results describing superplastic flow in an Al–Mg–Sc alloy after ECAP. It is shown that the experimental data are in good agreement with a model for superplasticity developed for conventional superplastic alloys and based on grain boundary sliding accommodated by the intragranular flow of dislocations. In addition, an analysis shows the region of superplastic flow lies very close to, or even exceeds, the predicted transition between the viscous glide of dislocations and the breakaway of dislocations from their solute atmospheres.

Texture of Al-Cu Alloys Deformed by ECAP

Materials of ultra-fine grained microstructure (sub-micrometer grain size) exhibit large strength, hardness and ductility and also the increased toughness in comparison with conventional coarse-grained ones. In these materials also the super-plastic flow at lower temperatures is observed. This behaviour may be interesting when aluminium alloys like AlCuZr, used in superplastic forming, are considered. In the paper, the methods of preparing such materials by equal-channel angular pressing (ECAP) is proposed and the texture analysis, based on neutron diffraction pole figure measurements and calculated orientation distribution function of two alloys AlCu4SiMn and AlCu5AgMgZr is discussed. The influence of short time recrystallization is discussed in relation with TEM and SEM observations.

Spark plasma sintering of ceramics: understanding temperature distribution enables more realistic comparison with conventional processing

Spark plasma sintering (SPS) is a consolidation technique that can combine high heating and cooling rates with a uniaxial applied pressure resulting in short processing time. SPS has been successful in producing ceramics with novel microstructures, which are often reported to have been produced at temperatures lower than would be possible using conventional densification techniques. This has led many authors to infer the presence of additional densification mechanisms intrinsic to the SPS process. The present study considers the relationship between temperature measured in SPS and that experienced by the sample. A simple analytical model enables calculation of the equilibrium temperature distribution in a cylinder of known thermal conductivity and emissivity with constant heat generation per unit volume. Application of this model to the die assembly used in SPS allows estimation of the equilibrium sample temperature. Optimised processing conditions were identified for a fully dense submicrometre grain size alumina, and a SiN–40 vol.-%TiB2 composite. Both materials can only be processed to full density with optimised microstructures in a narrow temperature window. Using the analytical model, the temperature seen by the sample was calculated, and compared with the temperature required using conventional processing techniques. Observed microstructures were consistent with those obtained using conventional processing techniques when processed at the calculated temperatures, suggesting that when SPS is applied to ceramics, additional densification mechanisms intrinsic to the SPS process are not required to explain the observed microstructures. The dynamics of the temperature distribution and problems associated with producing large samples using SPS are also considered.

Simulation of the electrical properties of polycrystalline ceramic semiconductors with submicrometer grain sizes

A numerical model for the calculation of the electrical characteristics of polycrystalline ceramic semiconductors is suggested. The model is applicable in a situation where the average grain size is comparable with the depletion region width near a grain boundary and the double Schottky barriers of neighboring boundaries overlap. The two-dimensional calculation is carried out in the diffusion-drift approximation using the Voronoi grid method. The effect of crystalline grain size on the current-voltage characteristic and specific capacitance of the material is analyzed usingp-SrTiO3 as an example.

EBSD FEG-SEM, TEM and XRD techniques applied to grain study of a commercially pure 1200 aluminum subjected to equal-channel angular-pressing

Microstructural evolution due to equal-channel angular-pressing (ECAP) with increasingly severe deformation was investigated in a commercially pure 1200 aluminum alloy. A true strain of eight produced sub-micrometer scale grains and very fine subgrains in the grain interior. The deformation process was documented and described using field-emission (FEG) gun scanning and transmission electron microscopy techniques. After eight ECAP passes, the high-angle grain boundaries accounted for ∼70% of all boundaries. The fine spacing resolution of FEG scanning electron microscopy allowed detailed grain and subgrain statistical evaluation in the deformed microstructure; transmission electron microscopic inspection afforded appreciation of the role of very low-angle misorientation boundaries in the microstructure-refining process. ECAP results were compared with those produced by cold rolling. The material's texture evolved in a decreasing trend of Cube {001}〈100〉 intensities in favor of Cube rotated toward the normal-to-pressing direction {001}〈120〉, while Goss {110}〈001〉 and {111}〈110〉, {111}〈112〉 directions slightly increased with strain.

Elastic moduli of grain boundaries in nanocrystalline MgO ceramics

Dense MgO ceramics with nanometer to submicrometer grain size were fabricated by high-temperature hot-isostatic pressing, low-temperature hot-pressing, and spark plasma sintering. The elastic properties were determined by sound wave velocity measurements. Young's and shear moduli of nanocrystalline MgO were lower by 13% than those with submicrometer grain size. Softening of the elastic properties was analyzed and related to the lower density and lower elastic moduli of the grain boundaries compared to the crystal interior. Young's and shear moduli of the grain boundaries were evaluated as 90 and 34 GPa, respectively. This leads to a more than 3-fold decrease in the effective elastic moduli with the decrease of grain size into the nanometer range.

Grain size effects on oxygen permeation in submicrometric CaTi 0.8Fe 0.2O 3− δ ceramics obtained by mechanical activation

Ceramic samples of CaTi 0.8Fe 0.2O 3− δ were obtained from mechanically activated mixtures of TiO 2, Fe 2O 3 and CaCO 3 sintered at 1150 °C for 2 h. The ceramics are dense with submicrometric grains with sizes in the order of 100–200 nm and 400–600 nm. X-ray diffraction analysis revealed a single perovskite phase structure indicating a high level of homogeneity, which was confirmed by transmission electron microscopy. The oxygen permeability through these samples, measured in the range 700–950 °C, is about 50% lower than for ceramics with considerably larger grains, close to 10 μm. The permeability increases linearly with the increase of the grain size in the range from ≈200 nm to ≈10 μm. Observations by results suggest that the grain boundaries have a negative impact on the high temperature ionic transport properties of CaTi 0.8Fe 0.2O 3− δ ceramics.

Developing a Superplastic Forming Capability in Nanometals

Exceptionally high superplastic ductilities may be achieved in metallic alloys when the microstructure consists of very small and reasonably equiaxed arrays of grains separated by grain boundaries having high angles of misorientation. It is now well established that processing through the application of severe plastic deformation provides the potential for achieving submicrometer or nanometer grain sizes in bulk solids, where these grain sizes are generally significantly lower than those attained using conventional thermomechanical processing. This paper describes the development of a superplastic forming capability in three different aluminum-based alloys by processing using the technique of equal-channel angular pressing (ECAP) in which a material is subjected to a high imposed strain by pressing repetitively through a die. Processing by ECAP can be used to achieve superplastic ductilities in these alloys because precipitates are present within the matrix and they serve to retain the ultrafine grain sizes at elevated temperatures.

Influence of boundary characters on the tensile behavior of sub-micron-grained aluminum

Abstract Pure aluminum having similar sub-micrometer grain size but quite different grain boundary characters was prepared by the use of severe plastic deformation with different strain paths. Different boundary characters were found to have a strong influence on tensile deformation behaviors at low temperature.

An early extrasolar planetary system revealed by planetesimal belts in beta Pictoris.

beta Pictoris (beta Pic) is a main-sequence star with an edge-on dust disk that might represent a state of the early Solar System. The dust does not seem to be a remnant from the original protoplanetary disk, but rather is thought to have been generated from large bodies like planetesimals and/or comets. The history and composition of the parent bodies can therefore be revealed by determining the spatial distribution, grain size, composition and crystallinity of the dust through high-resolution mid-infrared observations. Here we report that the sub-micrometre amorphous silicate grains around beta Pic have peaks in their distribution around 6, 16 and 30 au (1 au is the Sun-Earth distance), whereas the crystalline and micrometre-sized amorphous silicate grains are concentrated in the disk centre. As sub-micrometre grains are blown quickly out from the system by radiation pressure from the central star, the peaks indicate the locations of ongoing dust replenishment, which originates from ring-like distributions of planetesimals or 'planetesimal belts'.

Evolution of the Mechanical Properties of Aluminum Containing Al8Fe2Si Precipitates During Equal Channel Angular Pressing

Mechanical behaviour of submicrometer grain sized aluminium (99.1%), containing a low volume fraction of Al8Fe2Si precipitates, processed by Equal Channel Angular (ECA) pressing was investigated using Vickers microhardness, compression and tensile tests. In compression, samples machined along three orthogonal directions exhibited different yield strengths and cylindrical samples became elliptic after pressing. Tensile results showed an increase of the yield strength without loss of ductility. After ECA pressing for one pass, annealing at 200°C leads to a significant strengthening. A dependence of the ductility with strain rate was also observed.

Aging Effect of 8 mol % YSZ Ceramics with Different Microstructures

Aging effect of dense 8 mol % yttria-stabilized zirconia (8YSZ) ceramics with different microstructures has been studied. Spark-plasma sintered (SPS) ceramics consisting of submicrometer grains showed relatively slow conductivity degradation with aging time compared with conventionally sintered ceramics. Conductivity degradation was attributed to the increase in the bulk resistivity, which would be due to subtle structural changes in the 8YSZ unit cells. SPS ceramics consisted of relatively small crystallites, which might be responsible for the relatively slow conductivity degradation. © 2004 The Electrochemical Society. All rights reserved.