Plate-shaped Grains Research Articles

Texture in silicon carbide via aqueous suspension material extrusion and seeded grain growth

Textured SiC with anisotropic crystallographic texture is proposed as a material with improved mechanical properties. Textured SiC was created via alignment of platelet seed particles during aqueous suspension material extrusion, also known as direct ink writing, and subsequent pressureless liquid phase sintering and annealing. The microstructure and texture of the SiC fabricated with and without 5 vol% platelet seeds, and with and without annealing at 2050 °C and 2150 °C was explored via SEM, XRD, and EBSD. All samples fabricated had over 95% theoretical density. Annealing leads to the development of large, high aspect ratio plate-shaped grains among a matrix of many finer, low aspect ratio grains. Higher annealing temperatures and addition of platelet seeds both increased the size of the large grains. Samples were found to be textured regardless of having platelet seeds. Via XRD and EBSD, unseeded SiC was found to have texture where the crystallographic direction [0001] had a preferred orientation perpendicular to the normal direction. This occurred for both direct ink written and cast SiC, so the texture development must have occurred during sintering, though the mechanism is unknown. For seeded SiC, platelet seeds aligned in direct ink writing seeded the grain growth to develop crystallographic texture. The texture was mainly influenced by the alignment of platelet seed particles via shear stresses in the print nozzle, causing a one-dimensional texture where [0001] is perpendicular to the printing direction. However, it was found that the texture was not the expected concentric alignment of platelet particles in direct ink writing, so the shear stresses in the nozzle are not solely responsible for the texture developed.

Comparative analysis of morphology 1D and 2D particles effect in starting powders on microstructure and thermoelectric properties of grained Bi2Te2.7Se0.3 compound

Features in microstructure and thermoelectric properties of grained Bi2Te2.7Se0.3 compound, prepared by spark plasma sintering of starting powders with various morphologies, are comparatively examined. The 1D samples, prepared from the starting powder consisting of 1D bar-shaped particles, were weakly-textured (orientation factor is equal to ∼0.07) and preferentially consisted of bar-shaped grains. The 2D samples, prepared from the starting powder consisting of 2D plate-shaped particles, were highly-textured (orientation factor is equal to ∼0.28) and consisted of plate-shaped grains. Both weakly-textured 1D sample and highly-textured 2D sample demonstrated remarkable anisotropy in the behavior of their transport properties (specific electrical resistivity and total thermal conductivity). For highly-textured sample, this anisotropy is a typical phenomenon, resulting from recovering in crystal anisotropy, which is inherent for single-crystalline Bi2Te3-based material. For the weakly-textured 1D sample, the anisotropy in the transport properties is unexpected enough. In this case, anisotropy can be attributed to the anisotropic grain size effect on the transport properties. This effect is due to a different number of grain boundaries, overcome by the electrons or phonons under different measuring orientations. The highest value of the thermoelectric figure-of-merit (∼0.68) was found for the 1D and 2D samples and at perpendicular measuring orientation.

Formation of Ti2AlC-based thin films via magnetron sputtering and annealing under vacuum

High-purity Ti, Al and C targets were used to fabricate Ti2AlC-based thin films on industrial polycrystalline Al2O3 substrates via magnetron sputtering under argon, followed by vacuum annealing. The phase compositions of the Ti2AlC-based thin films were analysed, and scanning electron microscopy was performed to investigate their structures. Grain size and morphology depended on the deposition pattern. A material composed of ∼97.1% Ti2AlC and ∼2.9% TiC was obtained by sputtering Al for thirty minutes, then sputtering four alternating layers of Ti and C for ten minutes each. The thin film was subsequently annealed under vacuum at 900°C for 60 minutes. The samples contained lamellar hexagonal plate-shaped grains structures. The plate-shaped grains ranged from 200 nm to 400 nm in length depending on the sputtering mode. The long sides of the lamellar grains were predominantly oriented perpendicular to the alumina substrates. Regardless of the sputtering conditions, the coherent scattering region of Ti2AlC was ∼64 nm in size after annealing at 900°C. Changes in the size of the coherent scattering region of the TiC phase were non-linear and ranged from 9 to 60 nm.

ИССЛЕДОВАНИЕ ВЛИЯНИЯ ЧАСТИЦ TiB2 НА СТРУКТУРУ, ДЕФОРМАЦИОННОЕ ПОВЕДЕНИЕ И СВОЙСТВА АЛЮМИНИЕВОГО СПЛАВА 1550

In this work, the special master alloys containing aluminum and TiB2 powder with bimodal particle size distribution in three mixture compositions are prepared. The master alloys are infused into the melts using an external ultrasound source. The castings with particles had smaller grain sizes than the initial castings without particles. It is found that the hardness, yield strength, and ultimate tensile strength reach higher values with an increase in the relative elongation of the cast alloys with added particles. A warm rolling mode is employed for the studied alloys to obtain sheet blanks. It is shown that the staged shrinkage of the billets up to deformation of 80 % with periodic heating up to 300 °C allows one to obtain defect-free sheet products. The structure of the rolled sheet-alloys is characterized by the plate-shaped grains elongated along the rolling direction with pockets of submicron-sized grains in between. The strength properties of the studied rolled alloys exceeded those of the cast alloys. In the case of the rolled alloys, an increase in the yield strength, ultimate tensile strength, and ductility is revealed for the alloys with particles as compared to the ones with no particles added.

Habit planes of twins in a deformed Mg alloy determined from three-dimensional microstructure analysis

Serial sectioning and electron backscatter diffraction were used to image the three-dimensional microstructures of Mg alloy AZ31B deformed in tension and compression along sample directions that promote twinning. These measurements were used to statistically determine the habit planes of twin boundaries within the samples. For the sample deformed in tension, the boundary population is dominated by the so-called extension twin (lattice disorientation of 86° about 21¯1¯0with symmetric 011¯2 boundary planes) and boundaries formed by the intersection of these defects (lattice disorientation of 60° about 011¯0with 112¯1 and 011¯1 boundary planes). The sample deformed in compression, however, was different. Fewer twin variants were activated and this led to many fewer intersections and intervariant boundaries. Furthermore, while the dominant boundary also had the lattice disorientation of 86° about 21¯1¯0, consistent with an extension twin, the dominant habit planes have the asymmetric 01¯11 and 011¯3 orientations. The difference in the habit planes was attributed to the presence or absence of twin intersections. When many twins intersect, the twins form plate-shaped grains, bounded by two parallel 011¯2 interfaces. Without these intersections, the twins are wedge-shaped, with the bounding 01¯11 and 011¯3 interfaces meeting at a sharp edge.

The microstructure of composite cermet coatings produced by carbo-vibroarc surfacing

The results of investigation of the microstructure of composite coatings produced by carbo-vibroarc surfacing using cermet pastes are presented. It is shown that the composite cermet coating consists of two zones – the main and transition. The main zone of the coating contains the Fe2B hardening phase with nickel and chromium atoms implanted in the crystal lattice of the phase. The zone is situated in the matrix of the solid solution of iron and nickel Ni3Fe. The transition zone of the coating is formed by the mutual diffusion of the coating and the substrate and consists of the (Fe, Ni)2B hardening phase and the Ni3Fe matrix. It is also shown that the hardening phases form plate-shaped grains with thicknesses of 1–4 μm and more than 100 μm long, distributed in the form of colonies throughout the entire volume of the coating zone. In the direction to the transition zone, the shape of the structural components changes to circular with the diameter of up to 5 μm.

Effect of Bi3.99Ti2.97Nb0.03O12 Addition on Dielectric and Ferroelectric Properties of PZT Ceramics

In this study, bismuth niobium titanate Bi3.99Ti2.97Nb0.03O12; BNbT which is known as a bi-layer structured ferroelectric material was added into lead zirconate titanate (Pb(Zr0.52Ti0.48)O3; PZT). The ceramic with formula (1-x)PZT-xBNbT (x = 0.03, 0.05, 0,07, 0.15 and 0.20) were prepared by a solid state mixed oxide method. Phase analysis using XRD indicated that the pattern of the ceramic showed the single phase tetragonal of PZT, however, a BNbT peak appeared as BNbT content (x = 0.20) was increased. The microstructure of ceramic x = 0.03–0.15 had an equiaxed grain. With the addition of BNbT up to 0.20BNbT, the SEM micrographs also showed that plate-shaped grains started to dominate. The 0.97PZT–0.03BNbT ceramic showed the maximum dielectric constant (ϵr ∼ 679). With increasing amount of BNbT content, the dielectric constant and dielectric loss tangent gradually decreased. An addition, small amounts of BNbT content could improve normalized polarization (Pr ∼ 14.97 µC/cm2) of the ceramic.

Research on Properties of Ca<sub>3-x</sub> La <sub>x</sub>Co<sub>4</sub>O<sub>9</sub> Thermoelectric Material with Adding La

Ca3Co4O9 and Ca3-xLaxCo4O9 powders were prepared by sol-gel process. The Ca-Co-O thermoelectric material ceramics were prepared. The ceramics materials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microstructure and thermoelectric properties of ceramics were researched. The results indicate that the crystal of Ca3-xLaxCo4O9 with La adding growth ideally, the material had high density and low pore rate. The Ca3-xLaxCo4O9 ceramics contain directional array plate-shaped grains with layered structure. The thermoelectric properties of Ca3-xLaxCo4O ceramics were excellent. There were high Seebeck coefficient and high electrical conductivity for Ca3-xLaxCo4O9 ceramics. The power factor increased with temperature increasing.

Research on Properties of Ca-Co-O Thermoelectric Material with Adding Sm

Ca3Co4O9 and Ca3-xSmxCo4O9 powders were prepared by sol-gel process. The Ca-Co-O thermoelectric material ceramics were prepared. The ceramics materials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microstructure and thermoelectric properties of ceramics were researched. The results indicate that the crystal of Ca3-xSmxCo4O9 with Sm adding growth ideally, the material had high density and low pore rate. The Ca3-xSmxCo4O9 ceramics contain directional array plate-shaped grains with layered structure. The thermoelectric properties of Ca3-xSmxCo4O ceramics were excellent. There were high Seebeck coefficient and high electrical conductivity for Ca3-xSmxCo4O9 ceramics. The power factor increased with temperature increasing.

Fabrication and characterization of Ca3Co4O9 nanoparticles by sol-gel method

Ca3Co4O9 nanoparticles were fabricated by using calcium nitrate and cobalt nitrate as raw materials and citric acid as complexing agent through a citrate sol-gel method. Polyethylene glycol (PEG) 1000 was added as dispersing agent with different mass ratios. Pyrolytic decomposition of the dried gel and generative process of Ca3Co4O9 were investigated by TG-DSC, XRD. The microstructures of the particles were characterized by SEM and TEM. Ca3Co4O9 particles contained plate-shaped grains with layered structure. No second phases were found in the samples which were heated in the calcined temperature range of 750–900 °C for 4 h in air. The average size of the Ca3Co4O9 particles prepared with 3% PEG 1000 calcined at 800 °C for 4 h is 30–70 nm.

New Theory of Transformation Induced Grain Growth in Porous SiC

- and -SiC powder mixtures with 0.3 mass% of AlB2 were sintered at 2200oC. Porous materials having 43-45% porosity were obtained. Grains in the powder mixtures grew into plate-shaped grains of 6-8 m size, and -SiC transformed into -SiC. Free energy due to the polytype transformation enhanced material transport very much. The new rate theory proposed here explained very well the behavior of the grain growth.

Low-Temperature Sintering of α- and β-SiC Powders with AlB<sub>2</sub> Additive

We sintered α (6H)- and β(3C)-SiC powders using an Al-B-C additive. SiC powders were densified to > 98% of the theoretical density from 1950 to 2150oC with 0.67-2.7 mass % AlB2 and 2.0 mass % C. Sintering temperatures are 150-200 oC lower than the conventional. During sintering, 6H polytype in α-SiC powder was partly transformed to 4H. α-SiC powder grew moderately into plate-shaped grains. β-/SiC powder was completely transformed to 6H and subsequently 4H with large grain growth. Low-temperature sintering enabled us to use hot isostatic pressing resulting in pore-free SiC materials.

Microstructural, ferroelectric, and dielectric properties of Bi3.15Nd0.85Ti3O12 ceramics

Dense Bi3.15Nd0.85Ti3O12 (BNdT) ceramics of a layered perovskite structure were sintered at 1100 °C by solid state reaction. Ferroelectric domains were probably observed in plate-shaped grains. The BNdT ceramics exhibit saturated ferroelectric hysteresis loops with a remanent polarization (2Pr) of 45μC∕cm2 and a coercive field of 67.6kV∕cm. The dielectric constant and dissipation factor of the ceramics at 100 kHz are 221 and 0.0064, respectively. A broad dielectric peak was observed around 408 °C, and it might come from the oxygen-vacancy-related dielectric relaxation. The leakage current density of the ceramics is less than 7.5×10−7A∕cm2 under an applied field below 239kV∕cm. The BNdT ceramics show Schottky emission behavior under low electric field below 75kV∕cm.

Enhanced Grain Growth in Porous Materials from .ALPHA.- and .BETA.-SiC Powder Mixtures

α- and β-SiC powders and their powder mixtures with 0.3 mass% of AlB2 additive were sintered at 2200°C. Porous materials having approximately 55-57% theoretical density were obtained from the powders. Grains in the porous material prepared from α-SiC powder grew into round-shaped grains with a 4 μm size, and those prepared from β-SiC powder grew into plate-shaped grains with the same size. Grain growth was considerably enhanced in the powder mixtures forming a locking structure with plate-shaped grains. The mean grain sizes of the porous materials from the powder mixtures were 5.6-8.2 μm. β(3C)-SiC in the powder mixtures was transformed completely to α, that is, 6H and small amounts of 4H and 15R. An energy difference accompanied by β-to-α transformation enhanced grain growth. The strength of the porous materials prepared from the mixtures was 70-83 MPa.

Nonequiaxial Grain Growth and Polytype Transformation of Sintered α‐Silicon Carbide and β‐Silicon Carbide

α(6H)‐ and β(3C)‐SiC powders were sintered with the addition of AlB2 and carbon. α‐SiC powder could be densified to ∼98% of the theoretical density over a wide range of temperatures from 1900° to 2150°C and with the additives of 0.67–2.7 mass% of AlB2 and 2.0 mass% of carbon. Sintering of the β‐SiC powder required a temperature of >2000°C for densification with these additives. Grains in the α‐SiC specimens grew gradually from spherical‐shaped to plate‐shaped grains at 2000°C; the 6H polytype transformed mainly to 4H. On the other hand, grains in the β‐SiC largely grew at >2000°C; the 3C polytype transformed to 4H, 6H, and 15R. The stacking faults introduced in grains were denser in β‐SiC than in α‐SiC. The rapid grain growth in the β‐SiC specimen was attributed to polytype transformation from the unstable 3C polytype at the sintering temperature.

Transport properties of SnTe–Bi 2Te 3 alloys

Bismuth telluride based alloys have been a class of important thermoelectric materials widely used in cooling devices around room temperature. In this work, a series of polycrystalline samples of ternary compounds in the SnTe–Bi 2Te 3 system were prepared by rapid quenching of the melt followed by a sintering procedure of compacted pellets. The electrical conductivity and Seebeck coefficient of samples with various compositions were measured from room temperature up to about 600 K. The measured results show that the transport properties strongly vary with the composition and temperature. The microstructure in this system shows randomly oriented fine plate-shaped grains with multi-layered structure.

Anisotropic grain morphology, crystallographic texture and their implications for flux pinning mechanisms in MgB2 pellets, filaments and thin films

Grain morphology and crystallographic texture were investigated by electron microscopy in four different polycrystalline forms of superconducting MgB2. The materials included a hot-pressed sintered MgB2 pellet, a pellet reacted in situ from Mg and B, an in situ reacted MgB2 filament and a pulsed-laser-deposited thin film grown on a single crystalline [111] oriented SrTiO3 substrate. Thick plate-shaped grains with an aspect ratio of ∼3 and large faces parallel to (0001) planes dominated the microstructure in all four types of sample. The intermediate-sized plate-shaped grains (0.1 μm × 0.3 μm on average) in the electromagnetically most homogeneous parts of the hot-pressed pellets were strongly facetted, but not textured. Large (3–5 μm) plate-shaped grains were seen in the pellet reacted directly from stoichiometric Mg and B. A tendency for parallel alignment of the [0001] axes of the considerably larger grains (∼0.25 μm × 1 μm) in the filament was observed near its W core, but degradation of this texture away from the core was apparent. The very small grains (∼10 nm) of the thin film possessed a well-defined fibre texture with [0001] parallel to the film normal and no preferred orientation in the plane of the film. Electrical resistivity of the finest grain samples was some 103 times higher than the largest grain sample and their critical current density about one order of magnitude higher. We conclude that, in contrast to the cuprate-based high-Tc superconductors, grain boundaries do not limit the critical current density of polycrystalline MgB2 and indeed act as flux-pinning centres, which enhance the critical current density.

Recrystallization in oxide-dispersion strengthened mechanically alloyed sheet steel

Systematic annealing at temperatures between 1300 °C and 1380 °C was applied to sheets of INCOLOY MA-956, an oxide-dispersion strengthened (ODS), mechanically alloyed, iron-base steel containing (in mass percent) 20.8Cr, 5.0A1, 0.5Y2O3, and 0.5Ti. The billets, comprised of hot isostatically pressed (“hipped”), mechanically alloyed powder, were hot- and cold-rolled to produce a 0.5-mm-thick sheet with a strong (100)«110» deformation texture. Light and transmission electron microscopy established that recrystallization initiated by nucleation at the sheet centerline. Initial rapid growth of the centerline-nucleated grains, designated stage I, resulted in plate-shaped grains oriented parallel to the rolling plane at the sheet centerline. Subsequent growth, designated stage II, was developed by planar growth fronts through the sheet thickness at a slower rate. The final product was a very coarse grain structure, sometimes with only a single grain through the sheet thickness. The recrystallization kinetics were typified by an incubation time, a temperature dependance characterized by an activation energy of 506 kJ/mole, and a decreasing rate of boundary migration with increasing time at temperature. The microstructural evolution is discussed in terms of the influences of deformation texture, residual stress, dislocation substructure, and oxide dispersion on the recrystallization process.

Seeded directional solidification of YBa 2Cu 3O X superconductors

In order to utilize melt-textured YBa 2Cu 3O X superconductors in conductor applications, both the size of the oriented domains and the alignment of the grains have to be improved. A seeded directional solidification method has been developed to accomplish this goal. Due to the presence of the seed, single-domained bulk samples with preferential grain alignment have been obtained. Since the plate-shaped grains are aligned along the sample axis, the current capacity of these seeded superconductors are found to be insensitive to thermal cycling. In addition, the magnetic field dependency of critical current density in these long seeded superconductors is found to be in excellent agreement with that of well-textured short specimens.

Structural characteristics and anisotropic pinning centres of MTG YBCO

The microstructure and defects in melt-textured growth (MTG) YBa2Cu3O7-x (YBCO) bulk material have been studied systematically using high-resolution transmission electron microscopy (HREM), and the relationship between the anisotropic critical current density Jc and the microstructure have been explored. The results indicate that the well-textured dense plate-shaped grains oriented parallel to the ab plane and the numerous intergrowth-type layered defects inside the plates are the two main structural characteristics in MTG YBCO. The former reduce weak links of grain boundaries, and the latter provide effective anisotropic pinning centres. The intergrowth-type layered defects parallel to the ab plane together with the well known (110) twin boundaries are responsible for the anisotropic behaviour of Jc, depending upon the orientation of the external magnetic field.