Plate-like Grains Research Articles

Sintering, Structural and Optical Properties of Aurivillius Bi4LaTi3TMO15 (TM = Co, Cr, Fe, Mn and Ni) Ceramics

Aurivillius Bi4LaTi3TMO15 (TM = Co, Cr, Fe, Mn and Ni) ceramics were prepared by the conventional solid state reaction. Crystalline structures were determined by X-ray diffraction technique. Microstructures were characterized using field emission electron scanning microscopy, and typical plate-like grains but different grain morphologies are found in the Aurivillius compounds. Optical behaviors were measured by a UV-near-IR ellipsometer, and optical parameters are further extracted. Electronic structures with a direct inter-band transition are determined for all the Aurivillius ceramics. Finally, the values of the optical energy band gap (Egopt) of the Bi4LaTi3TMO15 (TM = Co, Cr, Fe, Mn and Ni) samples are respectively calculated to be about 3.883, 3.948, 3.759, 3.891 and 3.967 eV through the Tauc's power law.

Effects of WO3 and Ta2O5 Dopants on the Structure, Microstructure, and Microwave Dielectric Properties of Ca5Nb4TiO17 Ceramics

Ca5Nb4TiO17 ceramics were doped with WO3 and Ta2O5 to improve their microwave dielectric properties. The substitution of W6+ into Nb5+/Ti4+ sites resulted in the reduction of the sintering temperatures of the Ca5Nb4−1.2xWxTiO17 ceramics to 1450°C for x > 0.3 due to the formation of a second phase, CaWO4. In addition, the densification temperatures of the Ca5Nb4−xTaxTiO17 ceramics increased with Ta5+ content. Some irregular grains of CaWO4 were observed in the microstructures with plate-like grains, which increased with increasing W6+ content in the Ca5Nb4−1.2xWxTiO17 ceramics. All the Ca5Nb4−xTaxTiO17 samples exhibited dense microstructures with closely packed plate-like grains and a few pores. The dielectric constant (er) of the Ca5Nb4−1.2xWxTiO17 ceramics decreased with increasing W6+ content from 45.0 for x = 0 to 36.4 for x = 0.9. This decrease occurred because the more highly polarizable Nb5+ ions were replaced by less polarizable W6+ ions at B-sites, and the formation of the CaWO4 second phase diluted er. The quality factor (Q × f) reached a maximum of 26,478 GHz for x = 0.3 because of the cation distribution and decrease in the volume of cation sites as well as the increase in the average grain size. The CaWO4 second phase caused the temperature coefficient of the resonant frequency (τf) of the Ca5Nb4−1.2xWx TiO17 ceramics to move in the positive direction. For the Ca5Nb4−xTaxTiO17 ceramics, er decreased almost linearly with increasing Ta5+ content from 45.2 for x = 0 to 36.2 for x = 2.5 because of the dampening of the ionic mobility and decrease in the molecular polarizability. The Q × f and τf values decreased with increasing x value.

Synthesis of single-phase Ca3Co4O9 ceramics and their processing for a microstructure-enhanced thermoelectric performance

Single-phase Ca3Co4O9 with a high porosity, having a ZT of 0.09 at 627°C, was successfully prepared by a simple solid-state reaction using fine powders of CaCO3 and Co3O4 after a calcination at 760°C for 12 hours. It was excellent either for the preparation of the starting powder for the processing of the Ca3Co4O9 ceramics or for direct processing. The influence of already-reported processing methods (classic sintering, hot pressing, free-edge spark-plasma sintering (SPS) of a sintered pellet) and new methods, such as free-edge SPS of a pellet from just-compacted powder, cold pressing of a sintered pellet without post annealing, and free-edge cold pressing of a sintered pellet with post-annealing, on the preparation of Ca3Co4O9 ceramics was studied. The results showed how the density, the grain morphology and the microstructural anisotropy can all influence the thermoelectric characteristics of Ca3Co4O9 ceramics measured in directions parallel and perpendicular to the applied pressure. While the fully dense (99%) and perfectly textured ceramics prepared by the free-edge SPS of a pellet from just-compacted powder had a ZT of 0.17, the highest ZT of 0.31 was obtained for the free-edge cold-pressed and annealed ceramics with modest texturing and low density (65%), having a very low κ of 0.47W/mK. The results also showed that thin and irregular-shaped plate-like grains with sharp edges are preferred, while their thickening accompanied by rounding of their shape resulted in a reduced thermoelectric performance. The study revealed both the possibilities and the limitations for enhancing the TE characteristics of Ca3Co4O9 ceramics just through microstructure optimisation.

Microstructure and thermoelectric characteristics of (ZnO)k×In2O3: Based ceramics (k = 5 and 11)

Oxide thermoelectric ceramics are needed for high-temperature applications that involve harvesting the waste heat by direct transformation into electricity. In the case of n-type materials, the homologous (ZnO)kIn2O3-type phases are very promising for tailoring the TE properties. In this investigation the thermoelectric (ZnO)kIn2O3-based ceramics for k=5 and 11 were prepared using a classic ceramics procedure and sintering at 1500oC. During their preparation, either a fresh powder mixture of ZnO and In2O3 or a mixture of ZnO and In2O3 also containing plate-like grains of the pre-reacted (ZnO)kIn2O3 phases was used to induce possible microstructure texturing. It was demonstrated that the starting composition in terms of k value and the type of starting powder mixture affect the processes, leading to the formation of the equilibrium (ZnO)kIn2O3 homologous phase and, consequently, also the microstructure development. Nevertheless, all the samples had similar final microstructures and no texturing was observed. Regardless of the use of different starting-powder mixtures, the samples with the same composition had a similar ZT: at 627oC it was 0.11 for the samples with k=5 and 0.08 for k=11, which is not so inferior if we take account of the significantly lower content of In2O3 in these samples.

Production and characterization of CuCrO2 nanofibers

Composite nanofibers were prepared from homogeneously mixed copper chloride, chromium nitrate and polyvinylpyrrolidone solution by electrospinning and were subsequently heat treated under isochronal and isothermal annealing conditions. In the isochronal experiments, CuCrO2 formation began at 773 K. Annealing at 973 K yielded single-phase, highly porous CuCrO2 nanofibers accompanied by the formation of large quasi-hexagonal grains. In contrast, isothermal annealing at 973 K resulted in the formation of phase-pure CuCrO2 nanofibers with belt-like morphology and relatively higher density. A substantial decrease was observed in the size and number of plate-like grains in these nanofibers. When the fibers were annealed isothermally at 973 K for 1 h, the average width and specific surface area of the synthesized fibers were 150 nm and 7.85 m2 g−1, respectively, and the direct bandgap was 2.82 eV. The CuCrO2 behaved like a semiconductor, and the thermally activated energy was approximately 0.157 eV. A simple formation mechanism was proposed for the quasi-hexagonal grains.

The effect of ZrO2 dispersion on the thermoelectric power factor of Ca3Co4O9

In the present study, Ca3Co4O9/ZrO2 composites of various compositions have been synthesized by dispersing the ZrO2 particles in the Ca3Co4O9 matrix and their thermoelectric properties are investigated as a function of temperature from room temperature to 553K. For the prepared composite samples, phase purity and microstructure are analyzed. X-ray diffraction studies show that no unwanted reaction has occurred between Ca3Co4O9 and ZrO2 particles during the final sintering process. From scanning electron micrographs, it is observed that all samples show randomly oriented plate-like grains. Furthermore, the electrical resistivity measurement showed that all composite samples exhibit lower electrical resistivity than the pure Ca3Co4O9. The maximum Seebeck coefficient of 177.35μV/K at 553K is achieved for the ZrO2 dispersed Ca3Co4O9 sample. A significant improvement of the power factor (S2σ) has been realized in the prepared composite sample containing 8wt% ZrO2 which is approximately 40% higher than the pure Ca3Co4O9 at 553K. The improved power factor achieved for Ca3Co4O9-8wt% ZrO2 composite sample is mainly due to the obvious decrease in electrical resistivity.

Microstructural evolutions, elastic properties and mechanical behaviors of W/Cr Co-doped Bi4Ti3O12 ceramics

For Aurivillius phase Bi4Ti3−xWxO12+x+0.2wt.% Cr2O3 (BTWx-C, x=0–0.1) ceramics prepared by a conventional ceramic process, plate-like grains with random orientations are formed there. The aspect ratio of grains increases after the doping of W for ceramics, at the same time the bulk density of ceramics decreases. The change of Poisson's ratio may be related to the change of bulk density, whilst the change of bond strength could be responsible for the change of Young's modulus. The large-grained samples tend to possess a lower frequency constant but a higher elastic compliance, as well as a lower hardness. Especially, a larger grain size provides a higher fracture toughness for the sample at x=0.05 due to the mechanism of crack deflection. The stress–strain curves of BTWx-C ceramics under a uniaxial compression load are characterized as three stages: linear elastic deformation, ferroelastic domain switch and microcrack propagation. The microcrack propagation along grain boundaries results in the final axial flerry of ceramics. Additionally, the fracture strength of ceramics may be largely influenced by the internal stress resulted from the paraelectric–ferroelectric phase transformation during sintering.

Phase, microstructure and microwave dielectric properties of Nb and Ga doped Ca0.6La0.267TiO3 ceramics

Nb and Nb0.5Ga0.5 doped A-site deficient Ca0.6La0.267TiO3 perovskites were prepared via a mixed oxide solid state sintering route. Analysis of X-ray diffraction patterns of Nb-doped Ca0.6La0.267TiO3 samples sintered at 1300–1500 °C revealed the formation of single phase ceramics with orthorhombic symmetry. Ca0.6La0.267Ti1−x (Nb0.5Ga0.5) x O3 compositions crystallized into single phase ceramics at x = 0.3; however, the formation of a secondary phase (Ca2Nb2O7) was also observed at x > 0.3. The microstructure of Nb-doped samples comprised randomly oriented sharp-edged rectangular and/or square plate-like grains along with some voids while the microstructure of Nb/Ga-doped samples comprised relatively larger densely packed irregular-shaped grains with deformed edges. The substitution of Nb increased the relative permittivity (e r) from 114 to 136 but at the cost of both the quality factor (Q × f o) and temperature coefficient of resonant frequency (τ f). On the other hand, Nb and Ga co-doping was effective in minimizing τ f accompanied by an increase in Q × f o. The optimum microwave dielectric properties (i.e. e r = 41, Q × f o = 16,052 GHz and τ f = −13 ppm/ °C) were achieved for Ca0.6La0.267Ti0.5(Nb0.5Ga0.5)0.5O3.

In-situ synthesis of porous calcium hexa-aluminate ceramics and growth mechanism of the plate-like grains

Porous calcium hexa-aluminate (CA6) ceramics have been in-situ synthesized by heating calcium hydroxide and alumina powder/aluminum hydroxide powder. The influence of aluminum source and sintering temperature on calcium hexa-aluminate grain growth and porosity was studied. A significant feature was noticed that the CA6 grains maintained the original skeleton of aluminum source as a whole, but individual CA6 grain formed a plate-like shape. Moreover, we proposed a three-step growth mechanism for the formation and further development of the plate-like CA6 grains.

Effects of excess Bi2O3 on grain orientation and electrical properties of CaBi4Ti4O15 ceramics

A CaBi4Ti4O15 (CBTO) ceramic in which the Bi2O3 concentration was controlled from 0 to 10 wt% was fabricated using a solid-state reaction method. Structural analysis by field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) indicated differences in the preferred grain orientation and size of the plate-like grains according to the Bi2O3 concentration. The orientation of plate-like grains was also found to vary with the Bi2O3 concentration. There was no noticeable change trend of dielectric properties with different Bi2O3 concentrations. Relatively low dielectric constants (about 135) were exhibited by the CBTO ceramic with 1 wt% Bi2O3 and CBTO ceramic with 10 wt% Bi2O3 only, and similar values (about 150) were exhibited by the other ceramics. The dielectric loss exhibited a low value in the range of 0.01–0.09 for all samples (frequency range of 1–100 kHz). Regarding the ratio changes of the piezoelectric coefficient (d33) and the ratio of a-axis orientation of plate-like grains, the trends of these two values were shown to be similar. These results suggest that the addition of Bi2O3 greatly influences the microstructure of CBTO ceramics, including the grain size and orientation of plate-like grains. In particular, the change in the preferred grain orientation is closely related to the change in the piezoelectric properties.

Effects of Nd2O3 on the mechanical properties and oxidation behavior of Ti/Al2O3 composites by vacuum hot pressing sintering

A range of Ti/Al2O3 composites with different Nd2O3 content (0–5 vol.%) were fabricated by vacuum hot pressing sintering at 1420 °C for 1.5 h under the pressure of 30 MPa. The sintered samples with Nd2O3 additives exhibited more superior performances than those without additives. When 3.0 vol.% Nd2O3 was added, Ti/Al2O3 composite showed optimal density (relative density of 98.79%), hardness (Vickers hardness of 15.87 GPa), strength (flexural strength of 412.45 MPa), toughness (fracture toughness of 8.77 MPa m1/2) and oxidation resistance (oxidation depth of ∼244 μm). However, excessive additive (＞3.0 vol.%) would weaken these positive effects. SEM results revealed that the superior performances could be attributed to the grain refinement and microstructure densification. Moreover, plate-like grains were found at the interface and additional experiments demonstrated that their formation was caused by enrichment of Nd element, which is beneficial to the microstructure densification.

Comparison Study of <inline-formula> <tex-math notation="TeX">$\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-{\rm x}}$</tex-math></inline-formula> Films Deposited by Using Various Carboxylate Solutions

In this work, four fluorine-free solutions based on various carboxylates (propionates, butyrates, valerates, and hexanoates, respectively) were prepared and used for deposition of YBCO films on $\hbox{LaAlO}_{3}$ single-crystal substrates, in order to further understand the role of precursors on properties of the final films. After the same pyrolysis and sintering processes, the films from propionates and butyrates show pure YBCO phase and were epitaxially grown, as determined by X-ray diffractometer analysis. There are clear differences in morphologies of these four films, i.e., the films from propionates and butyrates exhibit platelike grains without distinct grain boundaries, while almost randomly orientated grains were observed on the surface of the other two films. As a result, we noticed significant discrepancies on their superconducting performance. The films from propionates and butyrates give $J_{c}$ values (at 77 K, self-field) of 2.1 $\hbox{MA/cm}^{2}$ and 1.3 $\hbox{MA/cm}^{2}$ , respectively, while no superconducting transition above 70 K was observed in the other two films. A relatively larger amount of carbonaceous residue left as a by-product after decomposition of carboxylates with longer ligands, particularly under fast pyrolysis and sintering processes, could be one of the explanations of the poor superconductivity in the films from the valerate- and hexanoate-based precursors.

Synthesis of Bulk FeTe1−x Se x (x = 0.1−0.5) at Ambient Pressure

In this study, polycrystalline samples with nominal composition FeTe1−xSe x (x=0.1−0.5) were synthesized by solid-state reaction method at ambient pressure. In order to minimize oxidation, argon gas flow was maintained throughout the heat treatment. The phase formation of FeTe1−xSe x samples was checked by X-ray diffraction (XRD). The polycrystalline FeTe1−xSe x samples were indexed to a tetragonal structure with space group of P4/nmm. The lattice parameters a- and c-axes shrink significantly with the substitution of Se. As shown by the scanning electron microscope (SEM) images, the samples developed a plate-like grain structure gradually with the increase of Se concentration. Measurements of temperature dependence of magnetic moment showed that the onset of superconducting transition temperature, T c, increased with Se concentration. The T c is about 10.6 and 13.5 K for x= 0.1 and 0.5, respectively. All the samples exhibit ferromagnetic behavior asshown by the field-dependent magnetization measured at room temperature.

Structure, Ferroelectric and Photoluminescence Properties of Eu-Doped CaBi4Ti4O15 Multifunctional Ceramics

Lead-free multifunctional ceramics of CaBi4−xEuxTi4O15 were prepared by a traditional solid state method, and the effects of Eu doping on structure, ferroelectric, piezoelectric and photoluminescence properties of the ceramics were investigated. All the ceramics possessed typical bismuth layer structure with m = 4, belonging to orthorhombic symmetry, and no secondary phase was detected. The plate-like grains were observed in all the ceramics, and the grain growth was slightly inhibited after the addition of Eu. Saturated polarization hysteresis loops were observed for all the ceramics, and the remanent polarization Pr decreased with Eu doping. The Eu-doped ceramics had a sharp excitation band centered at 465 nm due to the f–f transition from the 7F0 ground state to the 5D2 excited state of Eu3+. Under the excitation by 465 nm light, the main emission peaks located at 595 nm and 616 nm were obtained, which corresponds to the transition of 5D0–7F1, 5D0–7F2 levels in Eu3+, respectively. Our study shows that CaBi4Ti4O15 ceramics may be a promising multifunctional material for potential applications in optical-electro integration devices.

The effect of Mn doping contents on the structural, dielectric and magnetic properties of multiferroic Bi5Ti3FeO15 Aurivillius ceramics

Aurivillius Bi5Ti3FeO15 (BTFO) multiferroic ceramics with different Mn doping contents were synthesized through the conventional solid state reaction. The effect of Mn doping on the structural, magnetic and dielectric behaviors of BTFO ceramics is investigated in detail. All these Mn-doped BTFO ceramics are determined to be layered Aurivillius structure by X-ray diffraction. No microstructure variation of plate-like grains is obviously confirmed due to Mn doping through field emission scanning electron microscopy. Similar evolution of ε and tanδ with frequency is found both in BTFO and 10% Mn-doped BTFO ceramics. However, obvious dielectric dispersion phenomena are presented as the Mn-doping content is beyond 0.1, and this dispersion behavior becomes strong with increasing Mn-doping content, clearly indicated by the appearance of dielectric loss relaxation peaks. Finally, the same magnetic orders of 10% Mn-doped BTFO ceramics as those of the BTFO one are unambiguously found while weak ferromagnetism is observed in BTFO ceramics with Mn-doping content beyond 0.2.

Study on the Magnetic Properties and Microstructure of Single-Stage Hot-Deformed Nd-Fe-B Magnets

The Nd-Fe-B powder was prepared to bulk by cold pressing, and then was hot-deformed by the vacuum hot-pressing machine. Effects of hot-deformation temperature and rate on the density, anisotropy, performance and microstructure were studied. The results showed that the grain size was small, but the density and orientation degree was low, thus leading to poor performance when the deformation temperature was too low or deformation rate was too large. When the temperature was higher than 750 °C or the rate was lower than 0.1mm/s, the anisotropic magnets with good density and texture could be obtained. However, the magnetic properties of hot-deformed magnets deteriorated due to the present of coarse grains. From the microstructure of hot-deformed magnets it could be seen that there were plenty of coarse and irregular grains, which have a higher Nd content than the plate-like grains. The optimum magnetic properties could be obtained at deformation temperature of 700 °C and rate of 0.1 mm/s.

Phase, microstructure and microwave dielectric properties of Mg0:95Ni0:05Ti0:98Zr0:02O3 ceramics

Abstract Mg0:95Ni0:05Ti0:98Zr0:02O3 ceramics was prepared via conventional solid-state mixed-oxide route. The phase, microstructure and microwave dielectric properties of the sintered samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and a vector network analyzer. The microstructure comprised of circular and elongated plate-like grains. The semi quantitative analysis (EDS) of the circular and elongated grains revealed the existence of Mg0:95Ni0:05T2O5 as a secondary phase along with the parent Mg0:95Ni0:05Ti0:98Zr0:02O3 phase, which was consistent with the XRD findings. In the present study, εr ~17.1, Qufo~195855 ± 2550 GHz and τf ~ -46 ppm/K was achieved for the synthesized Mg0:95Ni0:05Ti0:98Zr0:02O3 ceramics sintered at 1325 °C for 4 h.

Room temperature multiferroic behaviour and magnetoelectric coupling in Sm/Fe modified Bi4Ti3O12 ceramics synthesized by solid state reaction method

Lead-free and single phase Bi4−xSmxTi3−xFexO12±δ (0⩽x⩽0.3) ceramics were synthesized using solid state reaction method. Relaxation of distortion in TiO6 octahedron was confirmed by Raman spectroscopy analysis. The study of surface morphology by field emission scanning electron microscope (FE-SEM) reveals the growth of plate-like grains. Frequency dependent dielectric studies display the reduction of dispersion in both dielectric constant as well as loss tangent with substitution. It is observed that ferroelectric transition temperature (Tc) of bismuth titanate (BIT) decreases with increasing content of Sm3+ and Fe3+ ions. The increase in dc resistivity at room temperature has been found with substitution. A significant improvement in remnant polarization (2Pr) and magnetization (2Mr) has been found in the system on substitution. Magnetoelectric coupling coefficient, α=0.84mV/cm Oe is realized for x=0.3 ceramic sample at room temperature. Our results clearly demonstrates the lead-free, multiferroic nature of Sm/Fe substituted BIT, which may find useful application in designing multiple state memory elements, data storage devices and sensors.

Rheological profile of boron nitride–ethylene glycol nanofluids

The paper presents the complete rheological profile of boron nitride (BN)–ethylene glycol (EG) nanofluids. Nanofluids have been produced by two-step method on the basis of commercially available powder of plate-like grains of nanometrical thickness. Viscoelastic structure has been determined in oscillatory measurements at a constant frequency and temperature. Viscosity and flow curves for these materials have been measured. Studies have shown that the Carreau model can be used for the modeling of dynamic viscosity curves of the material. The samples were tested for the presence of thixotropy. The dependence of viscosity on temperature was also examined. The effect of temperature on the dynamic viscosity of BN-EG nanofluids can be modelled with the use of Vogel-Fulcher-Tammann expression.

Enhanced Ferroelectric Properties of Multilayer SBT-BTN Thin Films for NVRAM Applications

Ferroelectric SrBi2Ta2O9-(Bi4Ti3)1-xNbxO12 (x = 0.02) (SBT-BTN) multilayer thin films with various stacking periodicity have been synthesized on Ir/Ti/SiO2/Si substrate by metal organic chemical vapor deposition technique (MOCVD). Tributylbismuth [Bi(C4H9)3], Strontium-bis[Tantal(pentanethoxy)(2-methoxyethoxid)] [Sr[Ta(OEt)5(OC2H4OMe)]2], Titanium Bis(isopropoxy)bis(1-methoxy-2-methyl-2-propoxide) [Ti(OiPr)2(mmp)2] and Niob-ethoxide [Nb(OC2H5)5] were selected as precursors. X-ray diffraction patterns show that the multilayer films annealed at 800oC consist of fully formed perovskite phase with polycrystalline structure and plate-like grains with no crack. The remanent polarization () and coercive field (Ec) are 16.2 μC/cm2 and 230 kV/cm, respectively, which is much higher, compared to pure SBT film ( = 6.4 μC/cm2, Ec = 154 kV/cm). In the films prepared above 700oC, postannealing increased the capacitor shorting rate; this was attributed to oxidizing of the top iridium layer. In this paper, the dependence of composition variation around stoichiometric on ferroelectric properties in SBT-BTN multilayer films is studied.