Large Plate-like Grains Research Articles

Phenomenal Improvement of Piezoelectric Voltage Coefficient in [00l] Textured K0.5Bi0.5TiO3 Ceramics

[00l] textured K0.5Bi0.5TiO3 ceramics were fabricated via the reactive template grain growth technique and tape-casting. Plate-like morphology of perovskite K0.5Bi0.5TiO3 powder was achieved via the topochemical conversion of K0.5Bi4.5Ti4O15 (4-layers Aurivillius oxide) template. A Lotgering factor of 71% and large plate-like grains with an aspect ratio of ∼ 12 were observed in textured K0.5Bi0.5TiO3 (KBT-T) ceramics. Poled KBT-T ceramics exhibited a much lower RT dielectric constant of ∼160 and significantly improved d33 ∼ 135 pC/N as compared to KBT ceramics with randomly oriented grains (KBT-R). Impressively, the obtained value of piezoelectric voltage coefficient (g33 ∼ 95×10−3 Vm/N) for KBT-T ceramics is the highest ever reported for lead-free piezoceramics, which is an almost 500% increase over the KBT-R sample (g33 ∼ 16×10−3 Vm/N). The piezoelectric energy harvesting capability of the KBT-T sample was demonstrated by the observed peak-to-peak output voltage ∼ 7.5 V and current ∼ 2 μA under a finger tapping motion. The remanent polarization value (Pr ≈ 20 μC/cm2 at an applied electric field of 80 kV/cm) of the KBT-T ceramic was significantly higher than that of the KBT-R sample (Pr ≈ 12 μC/cm2).

Microstructural evolution during high-temperature tensile creep at 1,500°C of a MoSiBTiC alloy

AbstractMicrostructural evolution in the TiC-reinforced Mo–Si–B-based alloy during tensile creep deformation at 1,500°C and 137 MPa was investigated via scanning electron microscope-backscattered electron diffraction (SEM-EBSD) observations. The creep curve of this alloy displayed no clear steady state but was dominated by the tertiary creep regime. The grain size of the Moss phase increased in the primary creep regime. However, the grain size of the Moss phase was found to remarkably decrease to <10 µm with increasing creep strain in the tertiary creep regime. The EBSD observations revealed that the refinement of the Moss phase occurred by continuous dynamic recrystallization including the transformation of low-angle grain boundaries to high-angle grain boundaries. Accordingly, the deformation of this alloy is most likely to be governed by the grain boundary sliding and the rearrangement of Moss grains such as superplasticity in the tertiary creep regime. In addition, the refinement of the Moss grains surrounding large plate-like T2 grains caused the rotation of their surfaces parallel to the loading axis and consequently the cavitation preferentially occurred at the interphases between the end of the rotated T2 grains and the Moss grains.

The chemical composition of a new "mica sandwich" foraminiferal species from the East Coast of Korea: Capsammina crassa sp. nov.

We describe a new agglutinated monothalamous foraminiferal species, Capsammina crassa sp. nov., based on integrated observations of the test morphology and the chemical characteristics of materials composing the test. The new species was found at a depth of <60 m on the East coast of Korea. The test morphology is typical of the genus Capsammina, comprising two or more mica plates with a ring of finely agglutinated mineral grains sandwiched between them and surrounding the cell body. There is no distinct test aperture. Elemental analyses of the agglutinated grains revealed 15 different types of mineral grains of which quartz is the most abundant. The surface areas of grains exposed on fractured surfaces ranged from 1.6 to 7,700 μm2 and the large plate-like grains forming the upper and lower surfaces measured about 420–2,350 μm in maximum width. The new species is morphologically similar to C. patelliformis, however, the differences in size, distribution area and depth support that these two species are distinct. This discovery is the first record of the genus Capsammina from the North Pacific. Therefore, it extends the biodiversity and geographical distribution of the genus Capsammina, which has been reported only from the bathyal NE Atlantic. Our finding also suggests the possibility of additional discovery of monothalamous foraminifera from around Korea.

Effects of aluminum, boron and carbon additives on microstructure of porous silicon carbide ceramics and their properties

Abstract Porous SiC ceramics with in-situ plate-like grain growth were fabricated by pressureless-sintering using Al, B and C additives, and the effects of Al, B and C additives on the microstructure of porous SiC ceramics and their properties were investigated. Whereas the porous SiC ceramics sintered with B-C additives mostly consisted of fine SiC grains without grain growth, Al additives with B-C system enhanced plate-like grain growth of SiC in the porous SiC ceramics. Notably, the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives consisted of plate-like SiC grains of around 10-20 μm, and the plate-like SiC grains stuck out of the walls of pores and the sintered surface in the porous SiC ceramics. XRD results suggested that the plate-like SiC grains formed in the porous SiC ceramics sintered with Al-B-C additives mainly consisted from 6H (α-SiC) phase. Compressive strength and thermal conductivity of the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives at room temperature were 50 MPa and 10 W/m·K, respectively. From these results, it is concluded that high performance porous SiC ceramics with unique morphology of entangled large plate-like SiC grains were successfully achieved based on in-situ grain growth.

Phase-microstructure evolution and microwave dielectric characteristic of (1−x)(Sr0.5Ce0.5)TiO3+δ−xNdAlO3 solid solution

A perovskite solid solution (1−x)(Sr0.5Ce0.5)TiO3+δ−xNdAlO3, x=0.1 to 0.4 was prepared by conventional solid state method. X-ray diffraction spectra revealed a single phase with tetragonal structure, indicating that doping of NdAlO3 significantly stabilized the perovskite-like structure. The addition of NdAlO3 facilitated the formation of large plate-like grains with porous microstructure. The dielectric constant (εr) decreased with increasing x because of the small ionic polarizability of NdAlO3. The Q×f value was strongly dependent on the microstructure of these ceramics. The temperature coefficient of resonant frequency (τf) gradually shifted to near zero with a rise of x, which resulted from the decrease in tolerance factor (t). The solid solution with x=0.4 sintered at 1550°C for 4h showed a good combination of dielectric properties: εr=72, Q×f=12052GHz and τf=+5pmm/°C.

Tuning pore structure of corrosion resistant solid-state-sintered SiC porous ceramics by particle size distribution and phase transformation

Aggressive process environment like hot corrosive water treatment caused corrosion in ceramic membrane and strength degradation of the membrane support, resulting in the invalidation of membrane filtration. Therefore, new SiC membrane support based on its intrinsic corrosion resistance was prepared by the combination of bi-modal particle size distribution and solid-state-sintering with 1–3wt.% of B4C additive at 2150°C and 2200°C. By tuning the contents of coarse and fine powder and without introducing any pore-creating agent, the porosity of SiC supports varied in the range of 36–39%, which was suitable for the membrane filtration. The phase transformation of 6H–SiC to 4H–SiC induced by the addition of B4C at 2200°C, accompanied by the formation of large plate-like grains, caused the pore size increased from 11.8μm to 20.9μm. The corrosion resistance of solid-state-sintered SiC supports was evaluated by exposing in 90°C 20wt.% H2SO4 and NaOH solutions for 200h. The high residual flexural strength (≥92.5%) and unchanged microstructure of the solid-state-sintered SiC supports after corrosion demonstrated that the supports exhibited excellent corrosion resistance and would be promising candidates to substitute the traditional ceramic supports.

Growth Behavior of WC Grains in Ni&lt;sub&gt;3&lt;/sub&gt;Al Matrix during Liquid Phase Sintering

Growth behavior of WC grains with two different sizes of 10 μm and 4 μm in the same liquid matrix has been studied in the WC–40vol.%Ni3Al system. The samples were sintered under a carbon saturated condition. Surfaces of the coarse WC grains are strongly micro-faceted and stepped. With the increase of sintering time, the average size of the fine WC grain increases continuously and very large plate-like triangular prismatic WC grains appear. The present investigation thus shows that the growth behavior of fine WC grain is affected by lower solubility of W in Ni3Al matrix and the abnormal growth of WC grains is suppressed. However, the growth behavior of coarse WC grain is governed only by the difference in energy between the facets.

Phase-field simulation of platelike grain growth during sintering of alumina

Abstract The toughness of alumina can be improved by utilizing the in situ formation of platelike anisotropic grains during sintering, that is, abnormal grain growth (AGG). Computer simulations of AGG may be effective to realize the conditions for obtaining the desired self-composite microstructure. In the first part of this study, sintering experiments of high-purity alumina powders were conducted to confirm the effects of powder size distribution as well as the amounts of additives. In the second part, a phase-field method for simulating the platelike grain growth was proposed. The large platelike grains were reproduced when the critical driving force of coarsening was set up. The incubation time of AGG was also observed in the case of the narrow size distribution. Although the morphology of the platelike grains did not exactly agree with the experimental observations, a possibility of the present method as a computational tool for simulating platelike AGG was verified.

Grain size, texture, and crystallinity in lanthanum monosulfide thin films grown by pulsed laser deposition

We report a detailed investigation of the growth of lanthanum monosulfide (LaS) thin films by pulsed laser deposition on (001) magnesium oxide (MgO) substrates in a background of H2S for the purpose of optimizing their crystallinity, texture, and grain size. A variety of films were grown while varying the laser repetition rate, the temperature of the substrate, and the partial pressure of H2S. The thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), Raman spectroscopy, and high resolution transmission electron microscopy. Films grown at 500°C with a H2S background pressure of 3.4×10−4Pa and a laser repetition rate of 8Hz produced the LaS film with the largest grains whose size averaged 293nm. The XRD pattern of these films revealed that their orientation was predominantly (200). AFM images of the surface of these films showed large plate-like grains. This contrasts with the fine grain structure observed in LaS films grown at a lower substrate temperature and lower H2S pressure.

Effect of annealing on the electrodeposited Cu 2O films for photoelectrochemical hydrogen generation

Cu 2O films were electrodeposited on stainless steel substrates followed by Ar annealing for photoelectrochemical hydrogen generation. Plating variables including time and pH for the plating bath were explored to obtain desirable film qualities. X-ray diffraction (XRD) patterns indicated that the as-deposited Cu 2O films exhibited preferred orientations in (200) and (111) planes from the plating bath of pH 9 and pH 11, respectively. Images from scanning electron microscope (SEM) revealed pyramid-like grains in 1 µm size for the Cu 2O films from pH 9 plating bath and large plate-like grains in 3–8 µm size from pH 11 plating bath. Identical results from SEM and XRD were obtained from the Cu 2O films at longer plating time. After annealing at 350 °C for 30 and 60 min, the Cu 2O phase was nicely maintained but SEM images demonstrated coarser grains. Photoelectrochemical activity for H 2 generation was obtained on the Cu 2O films before and after annealing by recording relevant photoelectrochemical currents at − 0.3 V in 0.5 M aqueous Na 2SO 4 solution. For the Cu 2O films from both baths, substantial increments in photoelectrochemical current were observed for the annealed samples as opposed to as-deposited ones. The largest photoelectrochemical current was obtained at 0.143 mA/cm 2 from the Cu 2O film of pH 9 plating bath with 60 min annealing, which exhibited a 560% increase over the as-deposited sample. We attributed the enhanced photoelectrochemical current to the improved crystallinity and reduced defects for the annealed Cu 2O films.

Microstructure development in textured BaBi4Ti4O15 made by templated grain growth method

The effect of template and matrix particle sizes on microstructure development was examined for BaBi4Ti4O15 textured by the templated grain growth method. Microstructure development was characterized by (1) the shape change of matrix particles from equiaxed to platelike, which resulted in texture development in the matrix phase, and (2) the formation of groups of large platelike grains with parallel alignment. The template particle size determined the size of grains in the final microstructure which was formed by process (2), and the matrix particle size influenced the rate of process (1).

Pentacene Thin-film Transistors Fabricated with a Simple Solution Process

ABSTRACTWe have succeeded in developing a simple solution process of pentacene thin films without particular precursor materials. High crystallinity and large plate-like grains of the solution-processed thin films were observed with several analyses. The solution-processed pentacene thin-film transistors (TFTs) were also fabricated and exhibited good transfer characteristics with maximum carrier mobility above 1 cm2/Vs. The solution-processed TFTs also indicated a steep subthreshold swing and high stability of the threshold voltage against the storage in the atmosphere. The trap states and the bulk carrier density in the films were evaluated from the transfer characteristics by using the analytical model. We considered that these good properties could be attributed to the high crystallinity and the large grains of the solution-processed thin films.

Synthesis and thermoelectric properties of Bi2.5Ca2.5Co2Ox layered cobaltites

Bi–Ca–Co–O polycrystalline materials with a layered structure were prepared. The synthesis of the sintered specimens from two starting compositions, Bi1.8Ca2Co2Oxand Bi2.5Ca2.5Co2Ox, revealed the latter is preferentially formed at high temperature(850 °C). The increase in sintering time was shown by growth of large platelike grains (up to 50 μm in diameter and several micrometers in thickness). The reaction mechanisms during the heat treatment and the preferential formation of the Bi2.5Ca2.5Co2Ox phase were observed by x-ray diffraction, thermogravimetry–differential thermal analysis, and scanning electron microscopy. These techniques supposed the presence of a liquid phase at high temperature, origin of a highly kinetic phase formation, and the growth of large grains. Interestingly the liquid phase reaction promotes an efficient stacking and sliding of grains during hot-forging treatment, and highly (00l) oriented materials were prepared. A relationship between thermoelectric performance, texture strength, and microstructure is clarified.

E– J properties of Bi 2Sr 2CaCu 2O 8 thick films

In order to evaluate the effect of the microstructure on E– J properties, the E– J properties for Bi2212 thick films prepared under three different conditions were measured. The measured E– J properties were analyzed based on the percolation model, and the local J c ( J cl) distribution was evaluated. The distribution of the local J c was much affected by its microstructures. For the well aligned sample with large plate-like grains, which is similar to Ag-sheathed Bi2212 tape, the distribution of the local J c at percolation transition field B GL was found to be asymmetric and reached the high J cl region.

Effects of isothermal annealing on the microstructure and mechanical properties of SiC ceramics hot-pressed with Y2O3 and Al2O3 additions

Abstract Silicon carbide compacts containing various α contents and 7 wt.% of Y2O3 and Al2O3 (weight ratio 60:40) were hot-pressed and isothermally annealed at 1950 °C to study the coarsening behavior of SiC grains and the crystallization of the grain-boundary glass phase. The samples which were prepared from the powder compacts containing small amounts of the α phase contained well-developed large plate-like grains in fine matrix grains and showed enhanced fracture toughness. A subsequent isothermal annealing was effective in controlling the grain boundary phase and resulted in the enhancement of mechanical properties. When the crystallization of the grain boundary phase leads to an aluminum-rich phase (Y–Al garnet) during the intermediate stage of annealing, flexural strength is improved. On the other hand, the fracture toughness can be increased when crystallization of the grain boundary glass is directed to an yttrium-rich phase during extended annealing. These results were explained in terms of the thermal mismatch between the SiC grain and the remaining glassy phase.

Hexagonal-phase retention in pressureless-sintered barium titanate

Abstract Undoped and MgO-doped TiO2- and BaO-excess non-stoichiometric barium titanate (BaTiO3) compositions were pressureless sintered using a conventional furnace and CO2 laser. High-temperature hexagonal BaTiO3 was metastably retained to room temperature as revealed by both X-ray diffractometry and transmission electron microscopy. The sintered microstructure of the TiO2-excess composition is characterized by a bimodal grain size distribution containing large plate-like grains. The microstructure of BaO-excess BaTiO3 with MgO doping is characterized by large spherulitic grains with petals 1–2 mm long. Hexagonal phase retention in the undoped material is only obtained when quenching the laser sintered BaO-excess powder. However, it occurs in both non-stoichiometric BaTiO3 compositions with MgO doping, regardless of the sintering techniques. A possible mechanism for the hexagonal phase stabilization associated with oxygen vacancies is discussed.

Hexagonal-phase retention in pressureless-sintered barium titanate

Abstract Undoped and MgO-doped TiO2- and BaO-excess non-stoichiometric barium titanate (BaTiO3) compositions were pressureless sintered using a conventional furnace and CO2 laser. High-temperature hexagonal BaTiO3 was metastably retained to room temperature as revealed by both X-ray diffractometry and transmission electron microscopy. The sintered microstructure of the TiO2-excess composition is characterized by a bimodal grain size distribution containing large plate-like grains. The microstructure of BaO-excess BaTiO3 with MgO doping is characterized by large spherulitic grains with petals 1–2 mm long. Hexagonal phase retention in the undoped material is only obtained when quenching the laser sintered BaO-excess powder. However, it occurs in both non-stoichiometric BaTiO3 compositions with MgO doping, regardless of the sintering techniques. A possible mechanism for the hexagonal phase stabilization associated with oxygen vacancies is discussed.

Anisotropic Abnormal Grain Growth in TiO2/SiO2‐Doped Alumina

The effect of TiO2/SiO2 addition on the grain growth of alumina was reinvestigated. TiO2 promoted the grain growth, but there was no abnormal grain growth. However, codoping of TiO2 and SiO2 resulted in a duplex microstructure consisting of large platelike grains, ∼800 μm long and ∼100 μm thick, and fine matrix grains. The observed anisotropic abnormal grain growth was explained in terms of liquid formation during heat treatment.

Effect of Cr 2O 3 addition on microstructural evolution and mechanical properties of Al 2O 3

The effects of chromia (Cr 2O 3) additions on the microstructural evolution and the mechanical properties of alumina (Al 2O 3) were investigated. When small amounts (<5 mol%) of Cr 2O 3 were added in samples hot pressed at 1500°C, the grain size distribution became bimodal; large platelike grains were dispersed in a relatively small grained matrix. The large grains were composed of a core region that is free of Cr and a surrounding shell region rich in Cr. The interface between the core and the shell was composed of misfit dislocations. The high diffusion rate of Cr ions through the surface of alumina was attributed to this microstructural evolution. The mechanical properties of the specimens were strongly influenced by this microstructural change. The fracture toughness and the flaw tolerance (R-curve behavior) of Al 2O 3 were improved markedly by the formation of the large platelike grains. The hardness and the elastic modulus also increased, however, the fracture strength decreased by the addition of Cr 2O 3.

Effect of Magnesia Addition on Microstructure and Mechanical Properties of Low Soda Alumina.

The effect of magnesia addition on the microstructural development and the mechanical properties of commercial low-soda alumina was investigated in the context of sintered bodies highly toughened by plate-like abnormal grains. Magnesia addition did not affect sinterability and final density. The alumina bodies, with and without magnesia, sintered at just below the temperature for which fully dense materials are obtained with high bending strength (i.e., exceeding 600MPa). Very large plate-like grains were formed at higher sintering temperature in the sample without magnesia. These large plate-like grains are the reason of both a lowered bending strength and an extremely high increase in fracture toughness. Fracture toughness of more than 7MPa·m1/2 and bending strength of 200MPa were achieved in a sample sintered at 1800°C without magnesia addition. The small amount of magnesia effectively inhibited abnormal grain growth in low soda alumina and even the sample sintered at 1800°C showed a bending strength higher than 300MPa and 5MPa·m1/2 fracture toughness. The high fracture toughness obtained in these samples is attributed to crack bridging mechanism.