Vibrational properties and infrared dielectric features of Gd2CoMnO6 and Y2CoMnO6 double perovskites

Gd₂CoMnO₆ and Y₂CoMnO₆ double perovskite ceramics were obtained from the polymeric precursors method and investigated by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Raman and Fourier Transform Infrared (FTIR) spectroscopies. Our results show that these samples present similar structural and vibrational characteristics that are fully compatible with a monoclinic structure belonging to the P2₁/n space group, with ordered Co²⁺ and Mn⁴⁺ cations. Infrared-reflectivity spectroscopy was employed to investigate the polar phonon features and to determine the intrinsic dielectric response of the materials. In particular, the extrapolated dielectric constants at the lower frequency infrared limit showed to be independent on the particle size, and were determined as ${\epsilon \prime }_{\mathit{intr}}\approx 17.8$ and 16.0, for Gd₂CoMnO₆ and Y₂CoMnO₆, respectively. Otherwise, it is shown that for smaller RE radius the FTIR bands become more evidenced, due to a higher octahedral rotation and lower $<$Co-O-Mn$>$ angle into the distorted monoclinic structure.     

Fabrication and thermoelectric properties of heavily rare-earth metal-doped SrO(SrTiO3)n (n = 1, 2) ceramics

To clarify the effect of substitutional electron doping on the thermoelectric figure of merit (ZT = S²σTκ⁻¹) of Ruddlesden–Popper phase SrO(SrTiO₃)_n (or Sr_n+1Ti_nO_3n+1), measurements were conducted for several thermoelectric parameters, e.g. electrical conductivity (σ), Seebeck coefficient (S) and thermal conductivity (κ), of (Sr_1−xRE_x)_n+1Ti_nO_3n+1 (n = 1 or 2, RE (rare earth): La or Nd, x = 0.05 and 0.1) dense ceramics prepared by a conventional solid-state reaction and hot-pressing technique. Crystal structures of the resultant ceramics were represented as (Sr_1−xRE_x)_n+1 Ti_nO_3n+1 evaluated by powder X-ray diffraction followed by the Rietveld analysis. All the ceramics exhibited electrical conductivity and the σ values simply depended on the dopant concentration, indicating that both La³⁺ and Nd³⁺ ions act as electron donors. The |S| values increased with temperature due to decrease in the chemical potential. Significant reduction of the κ values was observed as compared to cubic-perovskite SrTiO₃. The ZT value increased with temperature and reached 0.15 at 1000 K for (Sr_0.95La_0.05)₃Ti₂O₇.     

Effect of the Al2O3/SiO2 mass ratio on the crystallization behavior of CaO-SiO2-MgO-Al2O3 slags using confocal laser scanning microscopy

The crystallization behavior of a CaO-SiO₂-MgO-Al₂O₃ slag system with varying Al₂O₃/SiO₂ mass ratios from 0.03 to 1.10 has been investigated using a confocal laser scanning microscopy (CLSM). The resulting continuous cooling transformation (CCT) and time-temperature-transformation (TTT) curves showed that the initial crystallization temperature increased and the incubation time for crystallization slightly decreased with increasing Al₂O₃/SiO₂ ratio. The crystal growth rate first increased and then decreased with decreasing isothermal temperature. X-ray diffraction (XRD) analysis suggested that Ca₂MgSi₂O₇ or Ca₃MgSi₂O₈ precipitated as the primary phase at lower Al₂O₃/SiO₂ ratios, while the Ca₂Al₂SiO₇ phase was preferred at higher Al₂O₃/SiO₂ ratios. The observed crystalline phases correlated well with the expected thermodynamic predictions from FactSage. In addition, structural analysis using ²⁷Al magic angle spinning nuclear magnetic resonance (²⁷Al MAS-NMR) microscopy of the as-quenched slags indicated the presence of a higher ratio of tetrahedral [AlO₄]^5-structural units with increasing Al₂O₃/SiO₂ ratio, which enhanced the polymerization of tetrahedral [AlO₄]^5- and [SiO₄]^4- structural units to form Ca₂Al₂SiO₇.     

Crystal structure and electrical properties of (Li,Ce, Nd)-multidoped CaBi2Nb2O9 high temperature ceramics

(Li, Ce, and Nd)-multidoped CaBi₂Nb₂O₉ (CBN) Aurivillius phase ceramics were prepared via a conventional solid-state sintering route. The crystal structure including bond lengths and bond angles, microstructure, dielectric constant, DC resistivity, and piezoelectric properties were systematically investigated. Rietveld-refinements of X-ray results indicated that small quantity of (Li, Ce, Nd) doping (< 2.5 mol%) increases orthorhombic distortion, because of the smaller ionic radii of doping ions. However, orthorhombic distortion obviously decreased with increasing (Li, Ce, Nd) doping concentration from 5 to 25 mol%. The replacement of asymmetric A-site Bi³⁺ with 6s2 lone pair electrons by symmetric Li⁺, Ce³⁺ and Nd³⁺ ions decreased the orthorhombic distortion. The morphologies and electrical properties of sintered ceramics were tailored by the introducing (Li, Ce, Nd) multi-dopants. The improvement of piezoelectric properties of modified-CBN ceramics were attributed to decreasing grain sizes and morphotropic phase boundary (MPB). Ca_0.85(Li_0.5Ce_0.25Nd_0.25)_0.15Bi₂Nb₂O₉ (CBNLCN-15) ceramics had optimum properties, and d₃₃ and T_c values were found to be ~ 13.1 pC/N and ~ 900 °C, respectively.     

MnCo2O4 nanospheres for improved lithium storage performance

Mesoporous MnCo₂O₄ nanospheres with an average diameter of approximately 480?nm have been synthesized by a polyvinyl pyrrolidone (PVP)-assisted solvothermal method followed by thermal annealing. MnCo₂O₄ nanospheres consist of many nanoparticles having sizes in range of 20–50?nm, the specific area of the sample being 24.4?m² g^?1. When used as the anode material for lithium ion batteries, the mesoporous MnCo₂O₄ nanospheres show not only an excellent cycling stability, but also an outstanding rate capability. More specially, the discharge capacities of 749.1 and 629.6?mA?h?g^?1 can be retained at current densities of 200 and 400?mA?g^?1 after 50 cycles, respectively. In addition, the average discharge capacities of 1013.8, 827.1, 770.6, 733.3, 697.3, 651.4 and 522.4?mA?h?g^?1 could be observed at current densities of 100, 200, 400, 600, 800, 1000 and 2000?mA?g^?1, respectively. The improved cycling stability and rate capability can be ascribed to two unique structural features of mesoporous MnCo₂O₄ nanospheres: namely, the mesoporous nature of electrode materials which can help to reduce the volume variation during repeated lithiation/delithiation processes, and the nanostructure which can provide a shortened Li⁺ transmission path. The current synthesis approach can be easily spread to prepare other binary metal oxides, including Co-free anode materials.     

Effects of nitrogen pressure on microstructures and properties of Sr2Fe10/9Mo8/9O6 thin films

High quality double perovskite Sr₂Fe_10/9Mo_8/9O₆ thin films have been prepared on (111)-SrTiO₃ substrates under different N₂ partial pressures (0.1–20 Pa). The films are (111)-oriented with flat surfaces and sharp film/substrate interfaces. The epitaxy and local Fe/Mo ordering are revealed by high resolution transmission electron microscopy and selected area electron diffraction. It is established that the N₂ pressure has significant effects on structures and properties: With increasing N₂ pressure, the film thickness decreases and the strain increases, while the magnetization decreases monotonously. Interestingly, all the films show semiconductor-like behavior in the whole measuring temperature range but the resistivity increases with increasing N₂ pressure. The observed results are attributed to that N₂ pressure can affect the strain and the concentration of Fe–O–Mo, both play important roles in magnetic interaction, and thus affect the magnetic and transport behaviors.     

Effect of 2D MoS2 and Graphene interfaces with CoSb3 nanoparticles in enhancing thermoelectric properties of 2D MoS2-CoSb3 and Graphene-CoSb3 nanocomposites

The approach of incorporating a secondary phase in the bulk thermoelectric (TE) material has proved to be beneficial for enhancing the thermoelectric performance. We have investigated the effect of the presence of two-dimensional (2D) materials (MoS₂ or graphene) on the structural, electrical, and thermoelectric properties of CoSb₃ nanocomposite, in which CoSb₃ nanoparticles of sizes 20–50?nm are uniformly anchored on the surface of 2D-sheets of MoS₂ or graphene. The presence of 2D nanosheets enhances TE power factor and figure of merit (ZT). Inclusion of graphene in CoSb₃ causes large enhancement in power factor as a result of significantly high electrical conductivity and appreciable Seebeck coefficient. 2D graphene seems to work by providing extra carrier conduction channels along with a low interfacial potential barrier for charge transport. Homogeneously dispersed 2D-sheets of MoS₂ in CoSb₃ seem to cause interfacial modulation of charge carrier effective mass assisted by relatively larger interfacial barrier to result in significantly larger Seebeck coefficient and highly suppressed phonon conductivity, much more than graphene. The ZT value in both nanocomposites gets significantly enhanced in the entire studied temperature range of 300–700?K, the gain increasing with temperature over the CoSb₃. Whereas CoSb₃/graphene nanocomposites exhibit unusually high ZT at higher temperatures (550–700?K), the CoSb₃/2D-MoS₂ nanocomposites exhibit better performance (over graphene) in near room temperature range. The present study provides a possible strategy to enhance the conversion efficiency of various TE materials and has significant potential for waste heat recovery applications in various temperature ranges.     

Improving sintering behavior of MWCNT/BaTiO3 ceramic nanocomposite with Bi2O3-B2O3 addition

The present research systematically investigated the novel low-temperature fabrication of a multi-walled carbon nanotube (MWCNT)/barium titanate nanocomposite using a two-step mixing technique. The synthesis was conducted using different amounts of MWCNT (0.25%, 0.5%, 1%, 2%, 4%, and 8% wt) with different compositions of (Bi₂O₃ + B₂O₃) as a sintering aid. Scanning and transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, three-point bending strength, Vickers hardness indentation and Archimedean technique were used to characterize the as-synthesized specimens. It was found that the appropriate content of sintering aid (Bi₂O₃+B₂O₃) strongly decreased the sintering temperature from 1200 °C to 950 °C. The results also revealed that any sintering aid with the optimum composition that included 30% (mol) Bi₂O₃ was optimal for a sintering aid content of 6% (wt). Consequently, the highest values of the flexural strength and fracture toughness were achieved by applying the optimal amounts of MWCNT (1% wt) and sintering aid (6% wt).     

Effect of SiC ceramics on thermoelectric properties of SiC/SnSe composites for solid-state thermoelectric applications

Tin selenide (SnSe) based thermoelectric materials with varying amounts of embedded silicon carbide (SiC) particles were fabricated, and their thermoelectric properties were investigated. The SiC particles were evenly distributed in the SnSe matrix, thereby leading to the formation of the SiC/SnSe composite samples. The introduction of SiC into the SnSe matrix improved the power factors, owing mainly to an increase in the Seebeck coefficient, and a decrease in the thermal conductivity arising from the formation of phonon-scattering centers. Consequently, a ZT of 0.125 (at 300 K) was obtained for the SiC/SnSe composite with a SiC content of 1 wt%; this value was larger than that of the pristine SnSe. The results of this study indicate that the introduction of SiC particles into the SnSe matrix constitutes an efficient strategy for achieving thermoelectric enhancement for solid-state applications.     

Effect of surface-modified Al2O3 on the thermomechanical properties of polybutylene succinate/Al2O3 composites

This study investigates the effect of the incorporation of alumina particles on the thermomechanical properties of polybutylene succinate (PBS)/Al₂O₃ composites. The alumina surface was modified with the carboxylic groups of maleic acid through simple acid-base and in situ polymerization reactions. Scanning electron microscope (SEM) results revealed the introduction of maleic acid treated alumina significantly affect the morphology of the PBS/Al₂O₃ composites as compared to the neat PBS. The thermal conductivity of the composite (0.411?W?m^?1 K^?1) was more than twice that of neat PBS. The composite containing polymerization-modified alumina showed a 50% increase in storage modulus compared with that of neat PBS. In addition, universal testing machine (UTM) and differential scanning calorimetry (DSC) measurements indicated an increase in the tensile strength and degree of crystallinity after the incorporation of modified alumina in the PBS/Al₂O₃ composite.     

Fabrication of highly sensitive MnO2/F-MWCNT/Ta hybrid nanocomposite sensor with different MnO2 overlayer thickness for H2O2 detection

In this work, we report the development of MnO₂/F-MWCNT/Ta hybrid nanocomposite sensor with different MnO₂ overlayer thickness for the detection of H₂O₂ in real samples. A novel two-step process using e-beam evaporation and spray pyrolysis deposition was adopted for the synthesis of hybrid MnO₂/F-MWCNT/Ta electrodes. SE morphology revealed smaller-sized, compact grains of MnO₂ infiltrated on the outermost walls of MWCNTs. Raman analysis confirmed the existence of carbon nanotubes with abundant structural defects of MnO₂ in the composite. The cyclic voltammetry results displayed a high peak current and narrowed over potential towards the reduction of H₂O₂. The sensor displayed a fast response (<5?s), wide linear range (2–1510?μM) and a low limit of detection (0.04?μM) with significant anti-interfering properties, promising for the development of highly sensitive and reproducible biosensors. The three dimensional nanocomposite sensor also exhibited good recovery (> 98%), thus providing a favourable tool for analysis of H₂O₂ in milk samples.     

Effect of sintering temperature on microstructure and magnetic properties of double perovskite Y2CoMnO6

Polycrystalline double perovskite Y₂CoMnO₆ oxides ceramics sintered at four different temperatures from 1000?°C to 1300?°C have been fabricated by conventional sol-gel method. All the Y₂CoMnO₆ compounds are single phase with monoclinic structure (P21/n space group). The mean grain size grows significantly large and the shape becomes regular obviously with increasing sintering temperature. The effect of sintering temperature on magnetic properties of Y₂CoMnO₆ compounds has been studied in detail. We found that the oxygen vacancies are introduced by sintering at high temperature has a certain influence on the magnetic properties. Moreover, the magnetic entropy changes (-?S_M) as well as relative cooling power (RCP) in the double perovskite Y₂CoMnO₆ oxides ceramics around paramagnetic to ferromagnetic transition were also investigated.     

Electrospinning amorphous SiO2-TiO2 and TiO2 nanofibers using sol-gel chemistry and its thermal conversion into anatase and rutile

SiO₂-TiO₂ nanofibers were electrospun from partially hydrolyzed tetraethyl orthosilicate, and titanium isopropoxide using sol-gel chemistry. SiO₂-TiO₂ sol phase diagrams were created summarizing the role of composition on solution homogeneity and electrospinnability. Inorganic nanofiber spinnability was studied without the addition of any organic polymer, oligomer, gelator, or binder. TiO₂ concentration within SiO₂-TiO₂ fibers ranged from 25 to 100 mol%. SiO₂, SiO₂-TiO₂, and TiO₂ nanofiber structures were investigated using scanning electron microscopy and transmission electron microscopy. Inorganic fiber spinning was highly dependent on sol reaction temperature, time, and solution composition. At high TiO₂ concentrations, twisted and ribbon-like nanofibers with dumbbell-shaped cross-sections were observed. This was attributed to jet branching and splitting during electrospinning. Electrospun fibers were amorphous at room temperature, but thermally converted into crystalline anatase, which underwent additional structural changes at higher temperatures into rutile. This anatase-rutile thermal phase transformation was highly dependent upon TiO₂ concentration. Nanofiber composition, thermal stability, and crystalline structures were characterized by energy-dispersive X-ray spectroscopy; Fourier transform infrared spectroscopy, thermal gravimetric analysis, and wide-angle X-ray diffraction.     

A modified B2O3 containing Li2O-Al2O3-SiO2 glass with ZrO2 as nucleating agent - Crystallization and microstructure studied by XRD and (S)TEM-EDX

Glass-ceramics based on lithium-alumo-silicate glasses are commercially important for a wide range of applications, due to their special properties, like a vanishing thermal expansion. In order to tailor these properties, the composition of the glass and the temperature/time schedule are crucial factors. For the industrial production of most lithium-alumo-silicate glasses, high melting temperatures are required due to the high viscosities of the respective melt compositions. In this study, a simplified lithium-alumo-silicate glass composition with ZrO₂ as nucleating agent, on the basis of the commercially available Robax^® composition, is studied. Adding boron oxide leads to lower viscosities of the glass melt and notably lower melting temperatures may be supplied. The resulting glass is investigated using X-ray diffraction and transmission electron microscopy. During the crystallization process, phases such as ZrO₂ and β-quartz types are formed. The microstructure of the glass ceramics is notably coarser than that of glass-ceramics which are obtained from lithium-alumo-silicate glasses of standard compositions. EDX-analyses indicate a considerable enrichment of chemical elements in comparatively small areas of the microstructure. Especially boron oxide is found to be enriched in the residual glass of the investigated glass-ceramics.     

Enhanced magnetocaloric effect of Sr1.8Eu0.13Ba0.07FeMoO6 perovskite compound

Double perovskite (Sr_1.8Eu_0.13Ba_0.07)FeMoO₆ (SEBFMO) as well as Sr₂FeMoO₆ (SFMO) were prepared by the traditional solid reaction method. X-ray diffraction results show that all compounds are single phase, which belongs to I4/m space group. The magnetic hysteretic loops (M-H) and field-cooled magnetization (FC) curves indicate that all compounds have good ferromagnetic properties. The FC curves show a phase transition temperature at around 370 K. The Arrot plots (H/M-M²) of samples show that a second-order magnetic transition has occurred. An enhanced magnetocaloric effect was observed in SEBFMO. The maximum isothermal magnetic entropy change of SEBFMO reaches to 0.76 J kg^?1 K^?1 for ?H = 3 T, but that of SFMO only reaches to 0.50 J kg^?1 K^?1.     

Investigation on thermoelectric properties of screen-printed La1-xSrxCrO3-In2O3 thermocouples for high temperature sensing

La_1-xSr_xCrO₃-In₂O₃ thick film thermocouples were fabricated by screen-printing method for high temperature sensing and the effects of Sr²⁺ content were investigated systematically. All La_1-xSr_xCrO₃ thick films showed well crystallization with orthorhombic unit cell. Their average particle sizes showed the tendency of first increase then decrease gradually with the increase of Sr²⁺ content, and the maximum of average particle size was 1.76?μm. At the same time, the conductivities were improved with the increase of Sr²⁺ content. The thermoelectric properties of La_1-xSr_xCrO₃ depended on the Sr²⁺ content and the average Seebeck coefficients had an exponential decay tendency with the increase of Sr²⁺ content. For La_1-xSr_xCrO₃ (x?=?0.1–0.4)-In₂O₃ thermocouples, excellent repeatability and stability were observed through multi-cycles and long-time usage testing at high temperature. The La_0.7Sr_0.3CrO₃-In₂O₃ thick film thermocouple with excellent thermal stability (drift rate: 0.81?°C/h) and reliability makes it become a promising candidate high sensitivity thermal sensor.     

MgO-modified Sr0.7Ba0.3Nb2O6 ceramics for energy storage applications

We explored the phase structure, microstructure, dielectric and energy storage properties of MgO-modified strontium barium niobate Sr_0.7Ba_0.3Nb₂O₆-xwt%MgO (SBNMx; x?=?0–5) ceramics fabricated via conventional solid-state sintering precess. X-ray diffraction analysis indicates Mg²⁺ incorporates into lattice at x?=?0.5 and secondary phases come into formation for samples at x?≥?1, which results in the decrease of dielectric constant. There is also significant reduction of dielectric loss up to 0.002. Compared with pure SBN ceramics, the grain size of SBNMx ceramics becomes denser and more uniform, moreover, the dielectric breakdown strength shows increasing trend from 137?kV/cm to 226?kV/cm, which is in favor of the energy storage. SBNM0.5 ceramics presents the optimal energy storage performance: energy storage density of 0.93?J/cm³ and energy storage efficiency of 89.4% at 157?kV/cm, indicating that SBNM ceramics are prospective candidates for high voltage capacitor applications.     

Synthesis and thermoelectric performance of Ta doped Sr0.9La0.1TiO3 ceramics

Ceramics samples of Sr_0.9La_0.1Ti_1−xTa_xO₃ have been synthesized by conventional solid-state reaction method. X-ray powder diffraction characterization indicates that all samples are of single phase with cubic symmetry. The high-temperature electrical resistivity decreases with the increasing of tantalum content except for x = 0.05 sample. Negative Seebeck coefficients have been obtained for all samples, which means conduction mechanism being n-type. The absolute Seebeck coefficient decreases with the increase of tantalum concentration. The power factor decreases with increasing of tantalum substitution. Small amount tantalum doping can reduce the thermal conductivity. The lowest thermal conductivity obtained is 2.9 W/mK for x = 0.03 at 1074 K. The highest thermoelectric figure of merit still observes in Sr_0.9La_0.1TiO₃, reaches 0.29 at 1046 K, which is a relatively higher value in n-type oxide thermoelectric materials.     

Rietveld analysis of La/Al modified PbTiO3 ceramics

The polycrystalline samples of (Pb_0.70La_0.30)(Al_xTi_1−x)O₃ (PLAT) (x = 0.0, 0.05, 0.10, 0.15, 0.20) were synthesized by conventional solid-state reaction technique and annealed at two different temperatures. A single phase X-ray diffraction pattern was recorded with cubic symmetry of the space group Pm−3m.The Bragg-Brentano geometry is used for indexing, structural solution and Rietveld analysis for our powder samples. The detailed structural study was analyzed by employing Rietveld refinement techniques with the help of the Fullprof Program. The lattice parameters, cell volume, Wyckoff position of the atoms, bond lengths and angles have been calculated from the Rietveld analysis with the help of Powder Cell Program and by taking the refined parameter a stable crystal structure was suggested. It was observed that lattice parameter and cell volume decreases with increase of Al concentration. The crystallite size has been compared by three different methods: Scherrer's formula, Hall-Williamson method and Rietveld method. The scanning electron microscope (SEM) analysis of the samples shows the uniform microstructure with no abnormal grain growth and the grain size increases with annealing temperature and decreases with the increase of Al concentration. The elemental analysis was confirmed by energy dispersive spectrum analysis.     

MgTiO3/TiO2/MgTiO3: An ultrahigh-Q and temperature-stable microwave dielectric ceramic through cofired trilayer architecture

A cofired trilayer ceramic architecture showing as MgTiO₃/TiO₂/MgTiO₃ was designed to realize temperature-stable and ultrahigh-Q microwave dielectrics in the typical MgTiO₃-TiO₂ system. The effects of TiO₂ content on the microwave dielectric properties of cofired trilayer ceramics were studied. Through the design of cofired trilayer architecture, the chemical reactions between MgTiO₃ and TiO₂ were limited within a narrow region of MgTiO₃/TiO₂ interfaces (~ 15?µm in width), which could be beneficial for optimizing the microwave dielectric properties. Excellent characteristics of ε_r ~ 18.38, Q×f value ~ 169,900?GHz and τ_f ~ ??1?ppm/°C were gained for the MgTiO₃/TiO₂/MgTiO₃ ceramic architectures stacked with 1.63?vol% TiO₂. The current work could serve as new strategies to develop high-performance dielectric resonators and multilayers for 5G wireless communication applications.