Well-defined Grain Research Articles

Microstructural Evolution on Annealing and its Correlation to Structural and Magnetic Properties of Melt-Spun Ni50Mn29Ga21 Ribbons

Polycrystalline Ni50Mn29Ga21 thin ribbons were prepared by melt-spinning and the effect of annealing temperatures on the microstructural evolution, phase transformations, crystal structure, and the magnetic properties of the ribbons was systematically investigated. Different annealing temperatures from 473 to 1100 K were used to alter the degree of quenched-in disorder gradually, which led to evolution of microstructure from cellular to well-defined grain structure. The crystal structure was found to be 7 M monoclinic in ribbons annealed at 773 K or below, while a minority phase with 5 M tetragonal structure is found to co-exist on annealing at 1100 K. The as-spun ribbon exhibit superparamagnetic behavior whereas the sample annealed at 1100 K did not exhibit the same. The variations in the saturation magnetization with annealing temperatures were systematically correlated. The saturation magnetization and the martensitic transformation temperature show a correlation to the increase in the degree of Mn ordering.

Frequency dependent studies of dielectric and impedance properties of NaNb0.92V0.08O3 ceramics

Lead-free NaNb0.92V0.08O3 polycrystalline ceramics (NV) is successfully synthesized via classical solid-state reaction technique sintered at 1150 °C temperature for 4 h. The structural information has been carried out by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). XRD analysis of NV ceramics confirms an orthorhombic structure and belongs to the Pbma space group. The FE-SEM of NV sample exhibits well-defined grains with clear grain boundaries. The dielectric properties of the prepared samples are performed over a wide frequency range from 100 Hz to 5 MHz at various temperatures from 313 K to 373 K. The maximum value of dielectric constant 250 is obtained at 313 K temperature. The detailed study of dielectric constant (ε') and dielectric loss (tan δ) reveal their strong dependence on applied frequency, irrespective temperature. The impedance analysis confirms the typical negative temperature coefficient of resistance (NTCR) behavior present in the NV ceramics. Such type of characteristics of the material at low temperature can be used in antenna applications.

The impact of heat-treatment protocol on the grain size and ionic conductivity of NASICON glass-ceramics

Na2AlTi(PO4)3 (NATP) and Na1.8Al0.8Ge1.2(PO4)3 (NAGP) NASICON (Na-Superionic Conductor) glass-ceramics are obtained by applying different single (SHT) and double (DHT) heat treatments on the respective precursor glass to evaluate its effect on the microstructure and in turn, on the total ionic conductivity. The grain and grain boundary contributions are also analyzed in the NATP composition. SHT with longer crystallization times favors the development of well-defined grains in both compositions and also promotes the grain growth in NAGP samples. This behavior causes a decrease in the activation energy of the grain boundary, which enhances the total ionic conductivity. Regarding DHT samples, microstructure with larger grains and higher ionic conductivity were obtained with shorter nucleation times for NATP and NAGP compositions. Finally, the microstructural variation generated by the different thermal treatments causes the total conductivity to increase up to two times.

Undoped ZnO thin films deposited by ultrasonic spray technique: effect of the water content in the starting solution on the physical properties

Undoped ZnO thin films were deposited on sodocalcic glass substrates by the spray pyrolysis technique. Atomization of the starting solution was made by ultrasonic excitation. The effect of the water content in the starting solution on the structural, optical, and morphological properties of the films was studied. All films showed mirror-like finish with good adherence to the substrate. A preferential (002) growth was observed in all the films, although the increase in the water content leads to a slight increase in the (101), (102), (103) peaks intensity. Surface morphology changes slightly with water content variation in the starting solution, since films deposited with low water content show hexagonal shaped grains with round edges and a wide distribution in the average size; whereas well-defined rod-shaped grains are present in films manufactured from a starting solution with high water content. From photoluminescence, PL, measurements at room temperature it was found that, as the water content in the starting solution increases, both the UV and the green emission increase at a constant deposition temperature of 425 °C. PL measurements as a function of the substrate temperature for the optimal water content show that, at a low temperature the green PL peak intensity shows an increase with the temperature, reaching a maximum at 425 °C; however, at higher temperatures the PL response decreases.

SPESSARTINE-GROSSULAR GARNETS IN METAVOLCANICS FROM THE CENTRAL BUNDELKHAND GREENSTONE COMPLEX OF THE BUNDELKHAND CRATON, INDIAN SHIELD: PARAGENESES, ZONING, STRINGERS, AND INCLUSIONS

Archean metavolcanics from the Mauranipur Structure of the Central Bundelkhand greenstone complex have been subjected to three stages of metamorphism and metasomatism: 1) amphibolite-facies metamorphism (~2.7 Ga), 2) retrograde metamorphism and associated hydrothermal metasomatism (~2,68 Ga), including propylitization, and 3) prehnite-pumpellyite-facies metamorphism presumably associated with the formation of hydrothermal quartz veins (~1,8 Ga). Garnet is stable at all multiple metamorphic stages, varying in composition from almandine-spessartine to grossular and being associated with amphibole (from pargasite and tschermakite to grunerite-cummingtonite and actinolite in composition), clinopyroxene, chlorite, zoisite-clinozoisite, K-feldspar, etc. At the final stages of blastesis it is equiponderous with calcite, albite, prehnite and pumpellyite. Complex, contrasting chemical zoning with well-defined grain boundaries and zonal variations in the mineral composition of inclusions is formed in some of the garnet grains. The number of zones (three) is comparable with the number of epigenetic stages. As the BSE-images of the garnet are contrasting (because of the inverse relationship of FeO and CaO concentrations), the zoning pattern was examined and stringers, “interseasonal anomalies” and inclusions of grossular garnet in spessartine-almandine were revealed. Garnet grains are not always a closed system; recrystallization may occur in them.

Optical and electrical properties of crystalline indium tin oxide thin film deposited by vacuum evaporation technique

Indium tin oxide (ITO) thin film was deposited on glass substrate by means of vacuum evaporation technique and annealed at 200 °C, 300 °C and 400 °C in air for 1 h. The characterization and properties of the deposited film samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-VIS-NIR spectroscopy techniques. From the XRD patterns, it was found that the deposited thin film was of crystalline at an annealing temperature of 400 °C. The crystalline phase was indexed as cubic structure with lattice constant and crystallite size of 0.511 nm and 40 nm, respectively. The SEM images showed that the films exhibited uniform surface morphology with well-defined spherical grains. The optical transmittance of ITO thin film annealed at 400 °C was improved from 44% to 84% in the wavelength range from 250 nm to 2 100 nm and an optical band gap was measured as 3.86 eV. Hall effect measurement was used to measure the resistivity and conductivity of the prepared film.

Structural, optical and impedance spectroscopic studies of lead-free 0.2(K0.5Bi0.5TiO3)–0.8(NaNbO3) solid solution

Polycrystalline, lead free ceramic, 0.2(K0.5Bi0.5TiO3)–0.8(NaNbO3) is prepared by high temperature mixed oxide method. Experimental studies reveal the structural and vibrational and ferroelectric properties of the ceramic. The micro structural study exhibits that small grains are distributed uniformly throughout the surface with well-defined grain boundaries and few pores. The structural analysis illustrates that the compound is formed exhibiting the structure as orthorhombic at room temperature with space group Pmc21. While adding the NaNbO3(NN) on K0.5Bi0.5TiO3 (KBT), the vibrational phonon modes undergoes a notable change, as envisaged from the FTIR spectra. From the diffuse absorbance spectra, the optical band gap is found as 3.14 eV and useful for photo catalytic application. Raman spectra exhibit different vibrational modes in the frequency ranging from 130 to 900 cm−1. Polarization study confirms the presence of ferroelectricity. Dielectric analysis infers that phase transition temperature from ferroelectric to paraelectric is observed around 500 °C. The electrical parameters related to microstructure are reflected from impedance analysis. The frequency dependent ac conductivity depends upon frequency validating Jonscher’s power law. While increasing temperature, ac conductivity increases which represents the semiconducting nature of the sample. The bulk conductivity is found to be driven by thermally activated process following Arrhenius relation.

Thermal response of dielectric, impedance and modulus spectroscopy study of NCZA bulk ceramics

ABSTRACTThe Ni0.27Cu0.10Zn0.63Fe1.96Al0.04O4 (NCZA) magnetic bulk ceramics is prepared via conventional solid state reaction technique. X-ray diffraction study has confirmed the single phase spinel crystal structure. The surface morphology has been studied and found small grain size distributions that are separated by well-defined grain boundaries. Besides, to understand the thermal and frequency variation of the electrical and dielectric properties, the investigation has done at different selected temperatures (Tm = 300, 323, 373, 423, 473, 523 and 573) over a range of frequency (20 Hz to 2 MHz) throughout a non-destructive impedance spectroscopy technique. It is observed that the dielectric constant has increased with the increase of Tm as well as frequency. The impedance plot showed two semicircular arcs due to the distinguishing relaxation time constant of charge carrier of grain and grain boundaries. Modulus spectroscopy has been carried out to analyze the mechanism of electrical transport process in the ceramics.

Enhanced electrical properties of Ba6K2(Ca4Nb4)O21 complex perovskite oxide through modified sintering techniques

In this study the oxygen deficient perovskite Ba0.75K0.25(Ca0.5Nb0.5)O2.75(BKCN 25) is synthesized by solid state method via ball milling. Improvement in total conductivity through reducing grain boundary blocking effect is attempted by performing densification using conventional, microwave and spark plasma sintering. Physical characterizations of the pellets reveal structural stability and variation in morphology and porosity among the pellets sintered using different techniques. XRD analysis confirm cubic double perovskite structure in all sintered pellets but each having different grain sizes. SEM images show densified pellets with well-defined grains separated with grain boundary. AC impedance studies reveal that oxide ionic conductivity of BKCN 25 is ∼1.08 × 10−3 S.cm−1 at 700 °C in air atmosphere similar to reported GDC15, BaCe0.5Zr0.3Y0.2O3-δand BaCe0.45Zr0.45Sc0.1O3-δSOFC electrolytes. Significant enhancement in the total conductivity can be observed for spark plasma sintered BKCN 25 sample with lowest grain boundary blocking contribution.

Nanoscale crystallographic characterization of nanoporous catalyst by TKD

To better understand the structure-reactivity relationships of self-supported nanoporous catalysts, a nanoscale characterization of the materials is required. Transmission Kikuchi Diffraction (TKD), or transmission electron backscatter diffraction (t-EBSD), is a recent technique used to analyze the grain crystallography and morphology of thin films and nanometer-scale materials. Using a conventional EBSD detector in a FIB-SEM instrument, we investigate nanoporous gold (npAu) samples with ligament size in the 50–70 nm range. Grains with diameters below 50 nm can be clearly resolved, but also well-defined grain boundaries extending over 3 to 4 pixels, proving the high resolution of the method. The presence of contaminations during sample preparation by FIB milling is also discussed. The results provide deeper insight than conventional EBSD technique can alone.

Dielectric studies on Sm-modified two-layered BLSF ceramics

\(\hbox {Sm}_{x}\hbox {Bi}_{3-{x}}\hbox {TiNbO}_{9}\) (SBTN) with \(x=0, 0.2, 0.4, 0.6, 0.8\) and 1.0, a novel type of ceramic was prepared by a conventional solid-state reaction method. X-ray diffraction patterns confirm the formation of a single-phase, and lattice parameters were calculated based on the parent compound \(\hbox {Bi}_{3}\hbox {TiNbO}_{9}\) (BTN). Well-defined grains were seen in the scanning electron microscopy pictures. A kink observed near \(400{^{\circ }}\hbox {C}\) for all the samples in the dielectric plot as well as in the stretched exponential parameter (\(\beta \)) vs. temperature plot reveals the dielectric relaxation of the samples. The tolerance factor (t) was found to increase with increase in the size of the A-site (Bi-site) modified ion (Sm). Impedance and modulus spectroscopic studies were conducted to understand the plausible reason for the dielectric relaxation. The relaxation was found to be well-fit with Arrhenius equation. SBTN samples showed lower dielectric loss compared to BTN. The remnant polarization, obtained from experimental, was found to be less than that of theoretically calculated polarization, using Shimakawa’s formula. The proposed defect mechanism was interpreted using FTIR and Raman spectroscopic studies.

High-Quality Perovskite CH₃NH₃PbI₃ Thin Films for Solar Cells Prepared by Single-Source Thermal Evaporation Combined with Solvent Treatment.

In this work, solvent annealing process for CH3NH3PbI3 thin film prepared by single source evaporation was reported. Characterized by the scanning electron microscope (SEM), X-ray diffractometer (XRD), energy dispersive spectroscope (EDS), ultraviolet-visible (UV) spectrophotometer, and the photoluminescence (PL) spectrometer, our method ensured higher quality film with crystallinity, composition, well-defined grain structure, and reproducibility. The optimized solar cell device based on the structure of ITO/PEDOT:PSS/CH3NH3PbI3/PCBM/Ag achieved better performance in power conversion efficiency from 2.64% to 9.92%, providing an effective method to optimize the quality of perovskite film for solar cell application.

Highly Ordered Good Crystalline ZnO-Doped WO3 Thin Films Suitable for Optoelectronic Applications

Highly ordered ZnO-doped WO3 thin films with good crystalline quality are prepared using radio frequency magnetron sputtering technique, and its morphological and structural properties are studied using various characterization tools such as field emission scanning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction technique, micro-Raman spectroscopy, and x-ray photoelectron spectroscopy. Morphological analysis shows a smooth surface for pure film, whereas the ZnO-doped films presents a dense distribution of grains of larger sizes with well-defined grain boundary. X-ray diffraction studies reveal the enhancement of crystalline quality of the films with increase in ZnO doping concentration up to 5 wt.%, beyond which the crystalline quality gets deteriorated. A phase modification from a single monoclinic WO3 phase to mixed monoclinic WO3 and W18O49 phases is observed for films with higher ZnO doping concentrations.

Effect of Deposition Potential on Synthesis, Structural, Morphological and Photoconductivity Response of Cu2O Thin Films by Electrodeposition Technique

The present work describes the effect of deposition potentials on structural, morphological, optical, electrical and photoconductivity responses of cuprous oxide (Cu2O) thin films deposited on fluorine-doped tin oxide glass substrate by employing electrodeposition technique. X-ray diffraction patterns reveal that the deposited films have a cubic structure grown along the preferential (111) growth orientation and crystallinity of the film deposited at − 0.4 V is improved compared to the films deposited at − 0.2, − 0.3 and − 0.5 V. Scanning electron microscopy displays that surface morphology of Cu2O film has a well-defined three-sided pyramid-shaped grains which are uniformly distributed over the surface of the substrates and are significantly changed as a function of deposition potential. Raman and photoluminescence spectra manifest that the film deposited at − 0.4 V has a good crystal quality with higher acceptor concentration compared to other films. UV–visible analysis illustrates that the absorption of Cu2O thin film deposited at − 0.4 V is notably higher compared to other films and the band gap of Cu2O thin films decreases from 2.1 to 2.04 eV with an increase in deposition potential from − 0.2 to − 0.5 V. The frequency–temperature dependence of impedance analysis shows that the film deposited at − 0.4 V has a high conductivity. I–V measurements elucidate that the film deposited at − 0.4 V exhibits a good photoconductivity response compared to films deposited in other deposition potentials.

Synthesis and mechanical testing of grain boundaries at the micro and sub-micro scale

Abstract The important role of grain boundaries for the mechanical properties of polycrystalline materials has been recognized for many decades. Up to now, the underlying deformation mechanisms at the nano- and micro scale are not understood quantitatively. An overview of the synthesis and subsequent mechanical testing of specific grain boundaries at the micro and sub-micro scale is discussed in the present contribution, including various methods for producing one or multiple specific, crystallographically well-defined grain boundaries. Furthermore, established micromachining methods for isolating and measuring local dislocation-grain boundary interactions are portrayed. Examples of the techniques described are shown with to the aid of copper grain boundaries.

Hydrophilic Poly(vinylidene Fluoride) Film with Enhanced Inner Channels for Both Water- and Ionic Liquid-Driven Ion-Exchange Polymer Metal Composite Actuators.

This study presents a novel and facile strategy to fabricate a hydrophilic poly(vinylidene fluoride) (PVDF) electrolyte film with enhanced inner channels for a high-performance and cost-effective ion-exchange polymer metal composite (IPMC) actuator. The resultant PVDF composite film is composed of hierarchical micro/nanoscale structures: well-defined polymer grains with a diameter of ∼20 μm and much finer particles with a diameter of ∼390 nm, producing three-dimensional interconnected, hierarchical inner channels to facilitate ion migration of IPMC. Interestingly, the electrolyte matrix film has a high porosity of 15.8% and yields a high water uptake of 44.2% and an ionic liquid (IL, [EMIm]·[BF4]) uptake of 38.1% to make both water-driven and IL-driven IPMC actuators because of the introduction of polar polyvinyl pyrrolidone. Compared to the conventional PVDF/IL-based IPMC, both water-driven and IL-driven PVDF-based IPMCs exhibit high ion migration rates, thus effectively improving the actuation frequency and producing remarkably higher levels of actuation force and displacement. Specifically, the force outputs are increased by 13.4 and 3.0 folds, and the displacement outputs are increased by 2.2 and 1.9 folds. Using an identical electrolyte matrix, water-driven IPMC exhibits stronger electromechanical performance, benefiting to make IPMC actuator with high levels of force and power outputs, whereas IL-driven IPMC exhibits a more stable electromechanical performance, benefiting to make long lifetime IPMC actuator in air. Thus, the resultant IPMCs are promising in the design of artificial muscles with tunable electromechanical performance for flexible actuators or displacement/vibration sensors at low cost.

Investigations on physical properties of Mg ferrite nanoparticles for microwave applications

Nanocrystalline Mg-ferrites have been synthesized by sol-gel auto combustion method using citric acid as fuel agent at different Calcination temperatures (200 °C, 400 °C, 600 °C, and 800 °C). The physical properties of nano Mg-ferrite were investigated using X-ray diffraction (XRD) analysis, scanning electron microscopy, Fourier-transform infrared spectroscopy, and LCR-Hitester for structural, surface morphology, functional groups and dielectric properties respectively. The single-phase cubic spinel structure of all the samples has been confirmed by XRD. Surface morphology studies reveal that the grains are clear with well-defined grain boundaries and also found that the average grain size is maximum at 600 °C calcination temperature. The dielectric studies reveal that the maximum dielectric constant is attained a value of 46 with the minimum dielectric loss at 600 °C calcination temperature.

Synthesis of sub-millimeter single-crystal grains of aligned hexagonal boron nitride on an epitaxial Ni film.

Hexagonal boron nitride (h-BN), an insulating two-dimensional (2D) layered material, has attracted increasing interest due to its electrical screening effect, high-temperature-resistant gas barrier properties, and other unique applications. However, the presence of grain boundaries (GBs) in h-BN is a hindrance to obtain these properties. Here, we demonstrate the epitaxial growth of monolayer h-BN by chemical vapor deposition (CVD) on Ni(111) thin films deposited on c-plane sapphire. The Ni(111) films showed higher thermal stability than Cu(111) and Cu-Ni(111) alloy films, allowing us to perform CVD growth at a high temperature of 1100 °C. This resulted in an increase of the h-BN grain sizes to up to 0.5 millimeter, among the highest reported so far, and in a well-defined triangular grain shape. Low-energy electron microscopy (LEEM) revealed the epitaxial relationship between h-BN and the underlying Ni(111) lattice, leading to a preferential alignment of the h-BN grains. Both the large grain size and the alignment are expected to facilitate the synthesis of h-BN with a low density of GBs. We also found that the addition of N2 gas during the CVD improves the crystalline shape of the h-BN grains, changing from an irregular, truncated to a sharp triangle. The growth behavior of monolayer h-BN is further discussed in terms of the dependences on growth temperature and pressure, as well as on the structural evolution of the Ni metal catalyst. Our findings not only help understand the h-BN growth mechanism but also offer a new route to grow high-quality, monolayer h-BN films.

Structure and electrical conductivity of Ta doped La2Mo2O9 oxide ion conductors

In this work, we have investigated the crystal structure, thermal, vibrational, microstructural, and electrical properties of Ta doped La2Mo2O9 oxide ion conductors. The Rietveld refinements on the X-ray diffraction patterns confirm the α phase similar to that of undoped La2Mo2O9 at room temperature. α → β phase transition is confirmed by the differential scanning calorimetry analysis. The microcrystalline samples have grain sizes ∼10 μm along with well-defined grain boundaries. The infrared and Raman spectra mainly consist of MoO4 bands. The electrical conductivity of the highly dense pellets was analyzed by the impedance spectroscopy genetic programming method coupled with distribution function of relaxation times (DFRTs). Each DFRT consists of three main peaks conferring bulk, ionic, and electronic contributions of the grain boundary. The higher oxide ion conductivity in the β phase is identified due to the electronic contribution of the grain boundary for the samples. The capacitance for bulk and ionic parts of the grain boundary are almost temperature independent, whereas the electronic contribution varies turbulently. The incorporation reaction suggests that the sample with 5 wt. % Ta has the maximum oxide ion conductivity, which is further verified by the resistances obtained from the DFRT analysis.

Strong improvement of the transport characteristics of YBa2Cu3O7\u2212x grain boundaries using ionic liquid gating

For more than 30 years, the remarkable superconducting properties of REBa2Cu3O7−x (RE = rare earth) compounds have triggered research studies across the world. Accordingly, significant progresses have been made both from a basic understanding and a fabrication processes perspective. Yet, today, the major technological bottleneck towards the spread of their practical uses remains the exponential decay of their critical current with grain misorientation in polycrystalline samples. In this work, we used an ionic liquid to apply extremely high transverse electric fields to YBa2Cu3O7−x thin films containing a single well-defined low-angle grain boundary. Our study shows that this technique is very effective to tune the IV characteristics of these weak-links. In-magnetic field measurements allow us to discuss the type of the vortices present at the grain boundary and to unveil a large variation of the local depairing current density with gating. Comparing our results with the ones obtained on chemically-doped grain boundaries, we discuss routes to evaluate the role of local strain in the loss of transparency at cuprates low-angle grain boundaries. In short, this study offers a new opportunity to discuss scenarios leading to the reduced transport capabilities of grain boundaries in cuprates.