Magnetic and magnetoresistive properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Sr<Subscript>2</Subscript>FeMoO<Subscript>6–δ</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoheterostructures

A method has been developed for fabricating nanoporous matrices based on anodic aluminum oxide for the deposition of ferromagnetic nanoparticles in them. The modes of deposition of strontium ferromolybdate thin films prepared by the ion-plasma method have been worked out, and the magnetic and magnetoresistive properties, structure, and composition of the films have been investigated. It has been revealed that the microstructure and properties of the strontium ferromolybdate films deposited by ionplasma sputtering depend on the deposition rate and the temperature of the substrate. Based on the measurement of the electrical resistivity of nanoheterostructures in a magnetic field, it has been found that the magnetoresistance reaches 14% at T = 15 K and B = 8 T, which is due to the manifestation of tunneling magnetoresistance.     

Sol–gel electrophoretic deposition and optical properties of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> nanowire arrays

Ordered Fe₂O₃ nanowire arrays embedded in anodic alumina membranes have been fabricated by Sol–gel electrophoretic deposition. After annealing at 600 °C, the Fe₂O₃ nanowire arrays were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and X-ray diffraction (XRD). SEM and TEM images show that these nanowires are dense, continuous and arranged roughly parallel to one another. XRD and SAED analysis together indicate that these Fe₂O₃ nanowires crystallize with a polycrystalline corundum structure. The optical absorption band edge of Fe₂O₃ nanowire arrays exhibits a blue shift with respect of that of the bulk Fe₂O₃ owing to the quantum size effect. PACS 78.67.Lt; 81.05.Je; 81.07.Vb     

Synthesis of cubic Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Zr<Subscript>2</Subscript>O<Subscript>12</Subscript> by modified sol–gel process

Polycrystalline cubic Li₇La₃Zr₂O₁₂ (LLZ) with garnet-related type structure has been synthesized at 700 °C by modified sol–gel processes using citric acid as organic complexing agent and butan-1-ol or propan-2-ol as surface active agent. Thermal analysis (thermogravimetric/differential thermal analysis) indicated that the gel must be annealed at around 700 °C to completely remove the organic solvent. X-ray powder diffraction, X-ray fluorescence, and scanning electron microscopic investigations revealed that Al may not be essential to form cubic-phase LLZ; however, the addition of Al₂O₃ led to enhanced sintering of LLZ.     

Study of the KNO<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system by differential scanning calorimetry

The structural and the thermodynamic properties of potassium nitrate KNO₃ and its composites with nanosized aluminum oxide Al₂O₃ have been studied by differential scanning calorimetry. It has been found that an amorphous phase forms in composites (1–x)KNO_3–xAl₂O₃. The thermal effect corresponding to this phase has been observed at 316°C. It has been found that the phase transition heats of potassium nitrate decreased as the aluminum oxide fraction increased.     

On the Coexistence of Chemically Similar Stable and Metastable Phases in the BeO–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> System

The joint crystallization of the stable phase (a number of solid solutions of chromium-containing beryllian indialite) and metastable phases (crystalline modifications) of the compound with β-quartz structure (Si_0.64Al_0.28Mg_0.21Be_0.09)^IVO₂ ~ Mg_1.89Be_0.81Al_2.52Si_5.76O₁₈ admixed with Cr₂O₃ phases of khmaralite Mg_1.21Cr_0.01Be_0.46Al_1.78Si_3.38O₂₀ and spinel {(Mg_0.95Be_0.045Si_0.005)^IV(Al_1.31Cr_0.67Mg_0.02)^VI}O₄ is performed using melted chromium–beryllian indialite preliminarily obtained by solid-phase synthesis as a precursor. Simultaneously, the residual X-ray amorphous melt of composition Mg_1.83Cr_0.01Be_1.04Al_2.64Si_5.57O₁₈ is hardened. Thus, a reconstructive transition from the beryllian indialite melt to a phase with the β-quartz structure is implemented, and the chemical similarity of these compounds is demonstrated. The rate of change in the crystallization isotherm of 2°С/h and increased heat outflow through the highly heat-conducting walls of the Pt–Rh crucible (taper) contribute to this process.     

Microwave dielectric properties of the 5.5Li<Subscript>2</Subscript>O–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–7TiO<Subscript>2</Subscript> ceramics

A new Li₂O–Nb₂O₅–TiO₂ (LNT) ceramic with the Li₂O:Nb₂O₅:TiO₂ mole ratio of 5.5:1:7 was prepared by solid state reaction route. The phase and structure of the ceramic were characterized by X-ray diffraction and scanning electron microscopy (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. The microwave dielectric ceramic has low sintering temperature (∼1075°C) and good microwave dielectric properties of ε _r=42, Q×f=16900 GHz (5.75 GHz), and τ _f =63.7 ppm/°C. The addition of B₂O₃ can effectively lower the sintering temperature from 1075 to 875°C and does not induce degradation of the microwave dielectric properties. Obviously, the LNT ceramics can be applied to microwave low temperature-cofired ceramics (LTCC) devices.     

XRD,impedance, and Mössbauer spectroscopy study of the Li<Subscript>3</Subscript>Fe<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> + Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> composite for Li ion batteries

The synthesis procedure of the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite is presented. The monoclinic (A type) and hematite phases were detected by X-ray diffraction after the synthesis of the composite. The structural α–β (at a temperature of 460 K) and β–γ (at a temperature of 523 K) phase transitions in the composite were indicated by the anomalies of the electrical conductivity, dielectric permittivity, and changes of activation energies of conductivity. Two phase transitions have been detected in the Li₃Fe₂(PO₄)₃?+?Fe₂O₃ composite by ⁵⁷Fe Mössbauer spectroscopy: the phase transition in Li₃Fe₂(PO₄)₃ from the paramagnetic to antiferromagnetic phase at temperature T _N?=?29.5 K and the Morin phase transition in Fe₂O₃ at temperature T _M?=?235 K.     

Structural properties of composite cathode material LiFePO<Subscript>4</Subscript>–Li<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>

Composite cathode material LiFePO₄–Li₃V₂(PO₄)₃ is synthesized through a chemical reduction and lithiation using FeVO₄·xH₂O as both iron and vanadium sources. The structural properties of LiFePO₄–Li₃V₂(PO₄)₃ are investigated. X-ray diffraction results show the composite material containing olivine type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ phases. High-resolution transmission electron microscopy and energy-dispersive X-ray spectrometry results indicate that mutual doping effects take place between the LiFePO₄ and Li₃V₂(PO₄)₃ particles with V³⁺ doping the LiFePO₄ while Fe²⁺ dopes the Li₃V₂(PO₄)₃. LiFePO₄–Li₃V₂(PO₄)₃ nanocomposites are formed in the carbon webs. There is no structural compatibility between monoclinic (Li₃V₂(PO₄)₃) and olivine (LiFePO₄) domains in composite material LiFePO₄–Li₃V₂(PO₄)₃.     

A new composite material Li<Subscript>4</Subscript>Ti<Subscript>5</Subscript>O<Subscript>12</Subscript>–SnO<Subscript>2</Subscript> for lithium-ion batteries

A new Li₄Ti₅O₁₂–SnO₂ composite anode material for lithium-ion batteries has been prepared by loading SnO₂ on Li₄Ti₅O₁₂ to obtain composite material with improved electrochemical performance relative to Li₄Ti₅O₁₂ and SnO₂. The composite material was characterized by X-ray diffraction and scanning electron microscopy. The results indicated that SnO₂ particles have encapsulated on the surface of the Li₄Ti₅O₁₂ uniformly and tightly. Electrochemical results indicated that the Li₄Ti₅O₁₂–SnO₂ composite material increases the reversible capacity of Li₄Ti₅O₁₂ and has good cycling reliability. At a current rate of 0.5 mA/cm², the material delivered a discharge capacity of 236 mAh/g after 16 cycles. It suggests the existence of synergistic interaction between Li₄Ti₅O₁₂ and SnO₂ and that the capacity of the composite is not a simple weighted sum of the capacities of the individual components. In the composite material, SnO₂ can act as a bridge between the spinel particles to reduce the interparticle resistance and as a good material for the Li intercalation/deintercalation. Thus, electrochemical performance of the Li₄Ti₅O₁₂ spinel can be improved by the surface modification with SnO₂, and the stability of Li₄Ti₅O₁₂ also serves to buffer the internal stress caused by the volume changes in lithium insertion and extraction reactions.     

Deep metastable eutectic nanometer-scale particles in the MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> system

Laboratory vapor phase condensation experiments systematically yield amorphous, homogeneous, nanoparticles with unique deep metastable eutectic compositions. They formed during the nucleation stage in rapidly cooling vapor systems. These nanoparticles evidence the complexity of the nucleation stage. Similar complex behavior may occur during the nucleation stage in quenched-melt laboratory experiments. Because of the bulk size of the quenched system many of such deep metastable eutectic nanodomains will anneal and adjust to local equilibrium but some will persist metastably depending on the time–temperature regime and melt/glass transformation.     

Nanocrystalline CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> powder by PVA sol–gel route: synthesis,characterization and its giant dielectric constant

Nanocrystalline CaCu₃Ti₄O₁₂ powders were synthesized by a simple PVA sol–gel route and calcined at 700 and 800°C in air for 8 h. The diameter of the powders ranges from 40–100 nm. The calcined CaCu₃Ti₄O₁₂ powders were characterized by TG-DTA, XRD, FTIR, SEM, and TEM. Sintering of the powders was conducted in air at 1100°C for 16 h. The XRD results indicated that all sintered samples had a typical perovskite CaCu₃Ti₄O₁₂ structure although the sintered samples contained some amount of CaTiO₃. SEM of the sintered CaCu₃Ti₄O₁₂ ceramics showed the average grain sizes of 13–15 μm. The samples exhibit a giant dielectric constant, ε′∼10⁵ at 150 to 200°C with weak temperature dependence below 1 kHz in the sample sintered using the powders calcined at 700°C. The Maxwell–Wagner polarization mechanism is used to explain the high permittivity in these ceramics. It is also found that all sintered samples have the same activation energy of grains, which is ∼0.122 eV.     

Elastic properties of TeO<Subscript>2</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Ag<Subscript>2</Subscript>O glasses

A series of glasses [(TeO₂) _x (B₂O₃)_1−x ]_1−y [Ag₂O] _y with x = 70 and y = 10, 15, 20, 25 and 30 mol% were synthesised by rapid quenching. Longitudinal and shear ultrasonic velocity were measured at room temperature and at 5 MHz frequency. Elastic properties, Poisson's ratio, microhardness, softening temperature and Debye temperature have been calculated from the measured density and ultrasonic velocity at room temperature. The experimental results indicate that the elastic constants depend upon the composition of the glasses and the role of the Ag₂O inside the glass network is discussed. Estimated parameters based on Makishima–Mackenzie theory and bond compression model were calculated in order to analyse the experimental elastic moduli. Comparison between the experimental elastic moduli data obtained in the study and the calculated theoretically by the mentioned above models has been discussed.     

Electrical switching in the MoO<Subscript>3</Subscript>–WO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> and MoO<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

High field electrical switching on blown films of MoO₃(60%)–P₂O₅(40%), MoO₃(50%)–WO₃(10%)–P₂O₅(40%), and MoO₃(45%)–WO₃(15%)–P₂O₅(40%) having different thicknesses was studied and compared. Switching was observed using two terminal samples. S-type current–voltage characteristic (current-controlled negative resistance—CCNR) with memory was observed in molybdenum–phosphate glasses, but N-type characteristic (voltage-controlled negative resistance—VCNR) with threshold in tungsten–molybdenum–phosphate glasses was observed. The important observation was that with the addition of WO₃ to binary MoO₃–P₂O5 led to a change of I–V characteristic from CCNR with memory to VCNR with threshold. The measurements of density and molar volume showed linear relation between MoO₃ content and density which decreased with the increase of MoO₃ content. The samples’ thickness had no significant effect on threshold voltage. The attained results also indicated that the electrode material had no effect on switching property of devices. The switching behavior of the devices did not show any dependence on the polarity of the applied voltage. In terms of the effect of heat on the switching behavior of molybdenum–phosphate glasses, it was found that threshold voltage decreases with increasing of temperature. Finally, the switching phenomenon was explained by thermal (formation of crystalline filaments) and electronic models.     

Phase formation in the BaCO<Subscript>3</Subscript>–PbO–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> system

Features of the formation of lead-ferroniobate compounds in the xBaCO₃–(1 – x)PbO–Fe₂O₃–Nb₂O₅ system by solid-phase synthesis are investigated. For perovskite-type lead-ferroniobate solid solution, a single-phase concentration region is revealed at 1233 K. The crystalline structures of the synthesized compounds are refined using Rietveld analysis and the Pm3?m and R3m space groups. Ceramic samples of lead ferroniobate are studied by scanning electron microscopy.     

Controlled flame synthesis of αFe<Subscript>2</Subscript>O<Subscript>3</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles: effect of flame configuration,flame temperature,and additive loading

Superparamagnetic iron oxide nanoparticles are used in diverse applications, including optical magnetic recording, catalysts, gas sensors, targeted drug delivery, magnetic resonance imaging, and hyperthermic malignant cell therapy. Combustion synthesis of nanoparticles has significant advantages, including improved nanoparticle property control and commercial production rate capability with minimal post-processing. In the current study, superparamagnetic iron oxide nanoparticles were produced by flame synthesis using a coflow flame. The effect of flame configuration (diffusion and inverse diffusion), flame temperature, and additive loading on the final iron oxide nanoparticle morphology, elemental composition, and particle size were analyzed by transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), energy dispersive spectroscopy (EDS), and Raman spectroscopy. The synthesized nanoparticles were primarily composed of two well known forms of iron oxide, namely hematite αFe₂O₃ and magnetite Fe₃O₄. We found that the synthesized nanoparticles were smaller (6–12 nm) for an inverse diffusion flame as compared to a diffusion flame configuration (50–60 nm) when CH₄, O₂, Ar, and N₂ gas flow rates were kept constant. In order to investigate the effect of flame temperature, CH₄, O₂, Ar gas flow rates were kept constant, and N₂ gas was added as a coolant to the system. TEM analysis of iron oxide nanoparticles synthesized using an inverse diffusion flame configuration with N₂ cooling demonstrated that particles no larger than 50–60 nm in diameter can be grown, indicating that nanoparticles did not coalesce in the cooler flame. Raman spectroscopy showed that these nanoparticles were primarily magnetite, as opposed to the primarily hematite nanoparticles produced in the hot flame configuration. In order to understand the effect of additive loading on iron oxide nanoparticle morphology, an Ar stream carrying titanium-tetra-isopropoxide (TTIP) was flowed through the outer annulus along with the CH₄ in the inverse diffusion flame configuration. When particles were synthesized in the presence of the TTIP additive, larger monodispersed individual particles (50–90 nm) were synthesized as observed by TEM. In this article, we show that iron oxide nanoparticles of varied morphology, composition, and size can be synthesized and controlled by varying flame configuration, flame temperature, and additive loading.     

Dielectric response of SrTiO<Subscript>3</Subscript>–SrMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> solid solutions in the terahertz–infrared range

The reflection and transmission spectra of ceramic samples of SrTiO₃–SrMg_1/3Nb_2/3O₃ solid solutions have been measured in the frequency range of 5–5000 cm^–1 and in the temperature range of 5–370 K. Based on these spectra, the spectra of the real ε'(ν) and imaginary ε''(ν) parts of the complex permittivity ε*(ν) have been simulated by the method of dispersion analysis. It has been found that the temperature evolution of the dielectric constant is entirely determined by the behavior of the soft mode.     

Dielectric relaxation in magnetoelectric composite 0.85BiFeO<Subscript>3</Subscript>–0.15MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>

Dielectric properties of 0.85BiFeO₃–0.15MgFe₂O₄ composite fabricated with conventional ceramic technology are studied in the temperature interval of 20–250°C at frequencies of 25 Hz to 1 MHz. Dielectric relaxation is observed, the nature of which is discussed in the context of a mechanism of interaction between ferroelectric domain boundaries and point defects.     

Synthesis and characterization of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>–Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> magnetoelectric composites with dielectric and magnetic properties

Composite structures consisting of (001)-oriented SrTiO₃ (STO)/La_0.7Sr_0.3MnO₃ (LSMO) films of 30 nm thickness, grown on an (001) Pb(Mg_1/3Nb_2/3)TiO₃– 28 mol.% PbTiO₃ piezoelectric relaxor-ferroelectric single-crystalline wafer were investigated by means of Wide-Angle X-ray Diffraction (WAXRD) in situ under influence of a d.c. electric field with strength E up to ±18 kV/cm. The WAXRD measurements of the films and substrate reflection profiles resulted in a determination of the strain s in the films and the substrate separately. The strained state of the STO/LSMO films is effectively controlled by a huge converse piezoelectric effect of the PMN-PT substrate. The coefficients of coupling between electric-field-induced out-of-plane strain in the films and in the substrate for the composite system STO/LSMO/PMN-PT are obtained.