Preparation,thermal stability and magnetic properties of new AgY1−xGdx(WO4)2 ceramic materials

Polycrystalline samples of α-AgY_1−xGd_x(WO₄)₂ with x=0, 0.005, 0.01, 0.025, 0.05, 0.1, 0.2, and 1 have been prepared by a solid state reaction method and the influence of Gd³⁺ substitution for Y³⁺ on microstructure, thermal and magnetic properties was investigated. The X-ray diffraction analysis showed the phases to crystallize in the monoclinic symmetry, space group C2/m. A reversible monoclinic to tetragonal phase transition occurs in AgY_1−xGd_x(WO₄)₂ and strongly depends on Gd³⁺ ion concentration. Electron paramagnetic resonance (EPR) spectra of Gd³⁺ ions showed non-monotonous dependence of interaction strength on gadolinium concentration. Magnetic measurements showed paramagnetic behavior and strong increase of magnetic moment as the yttrium content decreases.     

High-temperature mechanical and thermal properties of Ca1−xSrxZrO3 solid solutions

Perovskite oxides are promising thermal barrier coatings (TBCs) materials but their thermophysical properties still need to be further improved before commercial applications. In this work, mechanical/thermal properties of calcium-strontium zirconate solid solutions (Ca_1−xSr_xZrO₃) are investigated. Comparing to the end-compounds CaZrO₃ and SrZrO₃, the solid solutions achieve the enhanced thermal expansion coefficient, decreased thermal conductivity as well as good high-temperature mechanical properties. The experimental thermal conductivities of Ca_1−xSr_xZrO₃ (x = 0.2, 0.4, 0.6, 0.8) are in the range of 1.76-1.94 W·(m·K)⁻¹ at 1073 K, being lower than that of the yttria-stabilized zirconia (YSZ). At the same time, their thermal expansion coefficients (10.75 × 10⁻⁶-11.23 × 10⁻⁶/K at 1473 K) are comparable to that of YSZ. Moreover, the Young's moduli of Ca_0.8Sr_0.2ZrO₃, Ca_0.6Sr_0.4ZrO₃, Ca_0.4Sr_0.6ZrO₃, and Ca_0.2Sr_0.8ZrO₃ at 1473 K are 70.7%, 69.4%, 68.8%, and 71.1% of the corresponding values at room temperature, respectively. The good high-temperature mechanical and thermal properties ensure the potential applications of Ca_1−xSr_xZrO₃ solid solutions as high-temperature thermal insulation materials including TBCs.     

Oxygen losses and electrical conductivity of SrTi1−yNbyO3+δ materials

Porous samples were used to evaluate the changes in electrical conductivity of SrTi_1−yNb_yO_3−δ materials with the oxygen partial pressure; this is needed to avoid slow responses to changes in the atmosphere. The power law dependence on the oxygen partial pressure corresponds to a predominant n-type conductivity. Deviations in strongly reducing conditions were interpreted on assuming that the charge imbalance caused by niobium additions may change with the working conditions. This defect chemistry model was confirmed by varying the oxygen partial pressure in the atmosphere. A coulometric titration technique was used to evaluate the changes in oxygen stoichiometry.     

High-temperature dielectric properties and microwave absorption abilities of Bi1−xMgxFeO3 nanoparticles

BiFeO₃ (BFO) multiferroic nanoparticles have attracted increasing attention owing to the coexistence of ferroelectric and ferromagnetic properties. In this work, Bi_1−xMg_xFeO₃ (x = 0.05, 0.1, 0.15) multiferroic nanoparticles were synthesized by the sol-gel method. The electromagnetic properties and microwave absorption performance in the temperature range of 323–723 K at X-band were investigated. The qualified bandwidth (absorption intensity < −10 dB) of the Mg-doped BFO materials covers the whole X-band at 673 K, suggesting promising candidates as high-temperature electromagnetic absorbers.     

Preparation and thermoelectric properties of Bi2SexTe3−x bulk materials

Bi₂Te₃ and Bi₂Se₃ nanoplates were synthesized by a microwave-assisted wet chemical method, and Bi₂Se_xTe_3−x (x=1, 2, 3) bulk nanocomposites were then prepared by hot pressing the Bi₂Te₃ and Bi₂Se₃ nanoplates at 80 MPa and 723 K in vacuum. The phase composition and microstructures of the bulk samples were characterized by powder X-ray diffraction and field-emission scanning electron microscopy, respectively. The electrical conductivity of the Bi₂Se_xTe_3−x bulk nanocomposites increases with increasing Se content, and the Seebeck coefficient value is negative, showing n-type conduction. The absolute Seebeck coefficient value decreases with increasing Se content. A highest power factor, 24.5 µWcm⁻¹ K⁻², is achieved from the sample of x=1 at 369 K among the studied samples.     

Structure, thermal stability and electrical conductivity of CaMoO4+δ

Single-phase tetragonal scheelite CaMoO₄ (space group, Pmmm) was prepared via the conventional solid state reaction method. The oxidation state of the transition metal species for the as-sintered CaMoO₄ was analyzed by X-ray photoelectron spectroscopy (XPS). The average thermal expansion coefficient was measured as about 11 × 10⁻⁶ K⁻¹ over the temperature range of 303-1373 K. From the thermodynamic point of view, the phase diagram for CaMoO₄ was constructed by computing the equilibrium phase boundaries. Finally, the electrical conductivity of CaMoO₄ was also investigated by an AC impedance analyzer. The activation energy of bulk conductivity for CaMoO₄ was 2.1 eV.     

Thermal and magnetic properties of new scheelite type Cd1−3x□xGd2xMoO4 ceramic materials

The limited scheelite type Cd_1−3x□_xGd_2xMoO₄ solid solution, where 0 < x ≤ 0.25 and □ are cationic vacancies have been successfully synthesized by high-temperature annealing of CdMoO₄/Gd₂(MoO₄)₃ mixtures composed of 50.00 mol.% and less of Gd₂(MoO₄)₃. The obtained materials as well as CdMoO₄ and Gd₂(MoO₄)₃ were characterized by powder XRD, DTA–TG, DSC, and SEM techniques. A phase diagram of the pseudobinary CdMoO₄–Gd₂(MoO₄)₃ system was constructed. The eutectic point corresponds to 1350 ± 5 K and ∼70.00 mol.% of Gd₂(MoO₄)₃ in an initial CdMoO₄/Gd₂(MoO₄)₃ mixture. With decreasing of Gd³⁺ amount in the crystal lattice of CdMoO₄, a melting point of the Cd_1−3x□_xGd_2xMoO₄ solid solution increases from 1351 (x = 0.25) to 1408 K (x = 0). EPR method was used to identify the paramagnetic Gd³⁺centers in Cd_1−3x□_xGd_2xMoO₄ for different values of x parameter as well as to select biphasic samples containing both Cd_0.2500□_0.2500Gd_0.5000MoO₄ and Gd₂(MoO₄)₃.     

Compatibility and conductivity of La2Ni1−xFexO4+δ and LaNi0·6Fe0·4O3−δ with GDC electrolyte

The compatibilities and conductivities of K₂NiF₄ typed La₂Ni_0·9Fe_0·1O_4+δ (L₂NF91) and LaNi_0·6Fe_0·4O_3?δ (LNF64) perovskites, promising cathode materials for solid oxide fuel cell, with Gd_0·1Ce_0·9O_1·95 (GDC) electrolyte were investigated. L₂NF91 and LNF64 were synthesised using citrate and modified citrate methods with the calcination temperature of 1000°C for 5 h. The single phased oxides with the average particle sizes of L₂NF91 and LNF64 ～0·2 μm were obtained. The thermal expansion coefficients of L₂NF91 and LNF64 were 12·7×10^?6 and 13·2×10^?6 K^?1 respectively. The mixtures of cathode materials and the electrolytes were heated between 800 and 1200°C to observe the formation of secondary phases at the operation temperatures of solid oxide fuel cell. The X-ray diffraction and scanning electron microscopy–energy dispersive X-ray results indicated that L₂NF91 and LNF64 had good chemical compatibility with GDC from room temperature up to 900°C. Both L₂NF91 and LNF64 showed higher conductivities when in contact with GDC electrolyte than with Zr_0·92Y_0·08O_1·96 electrolyte.     

Thermal properties and elastic constants of ζ-Ta4C3−x

Thermal and elastic properties were measured for ceramics that contained as much as 96 wt% zeta phase tantalum carbide (ζ-Ta₄C_3−x). The ceramics were produced from tantalum hydride that was milled to reduce particle size and then blended with carbon. Powders were reaction hot-pressed at 1800°C for 2 hours under a flowing He environment, which resulted in ζ-Ta₄C_3−x that was about 99% dense. The main secondary phases present in the reacted ceramic were TaC and Ta₂O₅. ζ-Ta₄C_3−x had a thermal conductivity of 9.6 W/m·K and an electrical resistivity of 160 ± 4.2 μΩ-cm, which are lower and higher than those of TaC, respectively. The Young's modulus was 379 ± 5 GPa and the hardness was 5.1 ± 0.7 GPa, which are also both lower than TaC. This study is the first to report the thermal properties and elastic moduli of high-purity ζ-Ta₄C_3−x.     

Structural,vibrational, electrical and magnetic properties of Bi1−xPrxFeO3

Crystalline solid solution of Bi_1−xPr_xFeO₃ (x=0.05, 0.1, and 0.15) ceramics has been successfully synthesized by a low temperature assisted co-precipitation method. Rietveld-refinement of the X-ray diffraction data reveals rhombohedral structure for Bi_1−xPr_xFeO₃ (x=0.05, 0.10) and triclinic for Bi_1−xPr_xFeO₃ (x=0.15). The crystallite sizes of the Bi_1−xPr_xFeO₃ (x=0.05, 0.1 and 0.15) are found to be approximately 33, 27 and 22 nm respectively calculated using Debye–Scherrer equation. The SEM images of Bi_1−xPr_xFeO₃ (x=0.05, 0.10 and 0.15) ceramics show grains with almost spherical morphology. 4A1 and 7E Raman modes have been observed in the range 100–650 cm⁻¹ and two phonon modes centered around 1150–1450 cm⁻¹ have also been observed corresponding to 2A₄ (LO), 2E₈ (TO) and 2E₉ (TO) modes of Bi_1−xPr_xFeO₃ (x=0.05, 0.1 and 0.15). The changes in Raman modes such as prominent frequency shift, line broadening and intensity have been noticed with the increase of Pr concentration in BiFeO₃ (BFO) suggesting a structural transformation as revealed by the Rietveld refinement. An anomaly in the temperature dependent dielectric studies has been noticed in all the samples at the vicinity of Neel temperature (T_N) indicating a magnetic ordering and an increase in magnetization with increase of Pr concentration is noticed from the room temperature magnetic studies. Further, the leakage current density is found to be reduced with increasing Pr concentration.     

Preparation and structural properties of Fe3O4–polyimide hybrid nanocomposites

Polyimide films in which magnetic Fe₃O₄ nanoparticles are uniformly distributed are prepared. Before the preparation of the Fe₃O₄–polyimide composites, pure magnetite nanoparticles (Fe₃O₄) have been synthesized in water by co-precipitation (from ferric chlorides). Its surface was firstly modified with the 3-aminopropyl triethoxysilane. The prepared polyimide–Fe₃O₄ nanocomposite films were characterized for their structure, morphology, and thermal behavior employing Fourier transform infrared spectroscopy, scanning electron micrograph, X-ray diffraction, and thermal analysis (DTA/TGA/DSC) techniques.     

Bond characteristics,mechanical properties,and high-temperature thermal conductivity of (Hf1−xTax)C composites

Bond characteristics, mechanical properties, and high-temperature thermal conductivity of ultrahigh-temperature ceramics (UHTCs), hafnium carbide (HfC), tantalum carbide (TaC), and their solid solution composites, were investigated using first-principles calculations. Mulliken analyses revealed that Ta formed stronger covalent bonds with C than did Hf. Bond overlap analyses indicated that the Hf–C bond possessed mixed covalent and ionic bond characteristics, compared with the more covalent character of the Ta–C bond. Consequently, the overall elastic properties were enhanced with increasing number of Ta–C bonds in the composites. The overall metallicity of the composites also increased with increasing TaC content; thus, the mechanical properties did not improve monotonically. Our results indicate that adding a small amount of TaC to HfC or vice versa to produce a composite would create a new UHTC with greatly improved elastic and mechanical properties as well as high-temperature thermal conductivity.     

Redox stability and electrical conductivity of Fe2.3Mg0.7O4±δ spinel prepared by mechanochemical activation

This paper addresses the potential of mechanochemical activation of MgO and α-Fe₂O₃ precursor powders to obtain Fe_2.3Mg_0.7O₄ ceramics with enhanced redox stability and electrical conductivity. X-ray diffraction (XRD) and Mössbauer spectroscopy suggest the initial formation of the spinel phase after 5 h of high-energy milling in inert gas, but after 10 h of mechanoactivation, the precursor still comprised hematite as a major phase with minor amounts of magnesiowustite as by-product. The activated mixtures can be nearly completely converted to spinel solid solution by heating to 1173 K, whereas single-phase, dense spinel ceramics can be prepared by sintering at 1773 K in inert atmosphere. These ceramics demonstrated redox stability under mildly reducing conditions (p(O₂) ～ 10 Pa), as confirmed by XRD, thermogravimetry and electrical measurements. The electrical conductivity of Fe_2.3Mg_0.7O₄ at this oxygen partial pressure is lower compared to magnetite, but it is still as high as 60 S/cm at 1073 K and 15 S/cm at room temperature. Cooling below 1473 K in air results in a drop of conductivity due to segregation of hematite phase at the grain boundaries. However, the phase separation is kinetically stagnated at 1073 K, and, after slight initial degradation, the retained electrical conductivity is more than 3 orders of magnitude higher compared to hematite and MgFe₂O₄ spinel.     

Dielectric and electrical conductivity properties of multi-stage spark plasma sintered HA–CaTiO3 composites and comparison with conventionally sintered materials

One of the different issues limiting the wider application of monolithic hydroxyapatite (HA) as an ideal bone replacement material is the lack of reasonably good electrical transport properties. The comprehensive electrical property characterization to evaluate the efficacy of processing parameters in achieving the desired combination of electroactive properties is considered as an important aspect in the development of HA-based bioactive material. In this perspective, the present work reports the temperature (RT-200 °C) and frequency (100 Hz–1 MHz) dependent dielectric properties and AC conductivity for a range of HA–CaTiO₃ (HA–CT) composites, densified using both conventional pressureless sintering in air as well as spark plasma sintering in vacuum. Importantly, the AC conductivity of spark plasma sintered ceramics [～upto 10^?5 (Ω cm)^?1] are found to be considerably higher than the corresponding pressureless sintered ceramics [～upto 10^?8 (Ω cm)^?1]. Overall, the results indicate the processing route dependent functional properties of HA–CaTiO₃ composites as well as related advantages of spark plasma sintering route.     

Low thermal conductivity of SrTiO3−LaTiO3 and SrTiO3−SrNbO3 thermoelectric oxide solid solutions

Electron-doped SrTiO₃ has been attracting attention as oxide thermoelectric materials, which can convert wasted heat into electricity. The power factor of the electron-doped SrTiO₃, including SrTiO₃-LaTiO₃ and SrTiO₃-SrNbO₃ solid solutions, has been clarified. However, their thermal conductivity (κ) has not been clearly identified thus far. Only a high κ (>12 W m⁻¹ K⁻¹) has been assumed from the electron contribution based on Wiedemann–Franz law. Here, we show that the κ of the electron-doped SrTiO₃ is lower than the assumed κ, and its highest ZT exceeded 0.1 at room temperature. The κ slightly decreased with the carrier concentration (n) when n is below 4 × 10²¹ cm⁻³. In the case of SrTiO₃-SrNbO₃ solid solutions, an upturn in κ was observed when n exceeds 4 × 10²¹ cm⁻³ due to the contribution of conduction electron to the κ. On the other hand, κ decreased in the case of SrTiO₃-LaTiO₃ solid solutions probably due to the lattice distortion, which scatters both electrons and phonons. The highest ZT was 0.11 around n = 1 × 10²¹ cm⁻³. These findings would be useful for the future design of electron-doped SrTiO₃-based thermoelectric materials.     

Structural and magnetic properties of Yb1−xSrxMnO3

The structural and magnetic properties of YbMnO₃ and Yb_0.82Sr_0.18MnO₃ multiferroics were studied by neutron powder diffraction (NPD), magnetometry and electron spin resonance (ESR) technics in a wide temperature range. Neutron diffraction measurements showed that the substitution of ytterbium ions with strontium ions in hexagonal h - YbMnO₃ (space group P6₃cm) leads to the destabilization of the crystal structure of the last compound and appearance of the mixture of three phases with different structure: hexagonal phase h - Yb_0.95Sr_0.05MnO₃ (space group P6₃cm), orthorhombic phase o - Yb_0.69Sr_0.31MnO₃ (space group Pbnm), hexagonal phase SrMnO₃ (space group P6₃cm). This fact was proved by the ESR measurements in which a several signals due to the phases of different structure were observed. NPD measurements showed that the magnetic structure of h - Yb_0.95Sr_0.05MnO₃ phase is similar to the magnetic structure of the pure h - YbMnO₃ and demonstrate the presence of the antiferromagnetic ordering in the samples. ESR and magnetization measurements of h - YbMnO₃ sample proved the presence of the antiferromagnetic correlations and also they showed the appearance of the ferromagnetically correlated nanoregions.     

Influence of processing on stability,microstructure and thermoelectric properties of Ca3Co4 − xO9 + δ

Due to high figure of merit, Ca₃Co_4 ? xO_9 + δ (CCO) has potential as p-type material for high-temperature thermoelectrics. Here, the influence of processing including solid state sintering, spark plasma sintering and post-calcination on stability, microstructure and thermoelectric properties is reported. By a new post-calcination approach, single-phase materials were obtained from precursors to final dense ceramics in one step. The highest zT of 0.11 was recorded at 800 °C for CCO with 98 and 72% relative densities. In situ high-temperature X-ray diffraction in air and oxygen revealed a higher stability of CCO in oxygen (～970 °C) than in air (～930 °C), with formation of Ca₃Co₂O₆ which also showed high stability in oxygen, even at 1125 °C. Since achievement of phase pure high density CCO by post-calcination method in air is challenging, the phase stability of CCO in oxygen is important for understanding and further improvement of the method.     

Synthesis,characterization and catalytic properties of La2−x Sr x NiO4−δ

Various compositionsx in the catalyst system La_2–x Sr _x NiO_4– have been prepared by conventional techniques and characterized by X-ray powder diffraction, electron microscopy and BET surface measurements. The catalytic properties of these catalysts have been tested in the propylene oxidation reaction. The catalytic activity can be correlated with the oxygen content and with the strontium substitution.     

Study of structural,dielectric, electric,magnetic and magnetoelectric properties of K0.5Na0.5NbO3 − Ni0.2Co0.8Fe2O4 composites

Magnetoelectric composites with general formula (x) Ni_0.2Co_0.8Fe₂O₄- (1- x) (K_0.5Na_0.5)NbO₃ (x = 0, 0.10, 0.20, 0.30, 0.40, 0.50 and 1.0) have been synthesized by solid state reaction method. X-ray diffraction pattern asserts the existence of both the constituent phases in the synthesized composites. FESEM micrographs are used to investigate the microstructure and for calculation of average grain size of the composites. Temperature dependent dielectric properties are investigated as a function of temperature and found to enhance with addition of ferrite in the composites. P-E hysteresis loops obtained for individual (K_0.5Na_0.5)NbO₃ (KNN) phase and composites indicate the ferroelectric ordering in the composites. Saturation and remnant magnetization show increasing trend with increase in NCFO concentration. FC-ZFC magnetization curves indicate charge ordering, metal-insulator transition (Verwey transition) in NCFO and composites. Impedance spectroscopy shows that bulk resistance reduces with increase in temperature, thereby indicating negative temperature coefficient of resistance (NTCR) behaviour of the composites. The magnetoelectric effect is confirmed by measuring magnetoelectric voltage coefficient, α_ME and the maximum value of α_ME is 5.389 mV/cm-Oe for 20% NCFO-80% KNN composite.     

Synthesis and electrochemical characterization of La0.75Sr0.25Mn0.5Cr0.5−xAlxO3, for IT- and HT-SOFCs

The main emphasis of this work is to create a new perovskite material with three different compositions (La_0.75Sr_0.25Mn_0.5Cr_0.5−xAlxO₃, x = 0.1, 0.2, 0.3) applied in both Intermediate- and High-temperature Solid Oxide Fuel Cells (IT- and HT-SOFCs). Perovskite-type polycrystalline La_0.75Sr_0.25Mn_0.5Cr_0.5−xAl_xO_3−δ (x = 0.1, 0.2, 0.3) powders were synthesized and formed in a single phase structure by a dry chemistry route (standard solid-state reaction method). The effect of Al doping on physicochemical and surface properties has been discovered. The compounds were crystallized in single phase rhombohedral symmetry (R-3C Space. Group). Total conductivity of Al doping in wet 5% H₂ was higher than both dry 5% H₂ and air. The obtained results enhance the electro-catalytic performance and the material conductivity as well, which will be good for anode materials in IT- and HT-SOFCs and the optimum doping is 10%.