Synthesis and analysis of lead-free tungsten bronze ferroelectrics

The polycrystalline Ba₂Sr₃GdTi₃V₇O₃₀, a member of tungsten bronze structural family, was prepared by a solid-state reaction method at high temperature (calcination and sintering temperatures at 950 and 1,000 °C, respectively). Preliminary structural study showed that the compound has orthorhombic crystal structure at room temperature. Study of surface morphology of the compound by scanning electron microscopy exhibits the uniform grain distribution on the surface of the sample with less number of voids. The dielectric anomaly observed at 313 °C is considered as a ferroelectric-paraelectric phase transition temperature which has been confirmed by appearance of hysteresis loop at room temperature. The trend of variation of ac conductivity with inverse of absolute temperature provides the nature of conduction mechanism in the material. The different value of activation energy in different temperature regions suggests that the conduction process in the material is of mixed-type (i.e., ionic–polaronic and space charge due to the oxygen ion vacancies).     

Preparation of hexagonal potassium tungsten bronze thin films by thermal vacuum evaporation

$\text{K}{}_{\mathrm{x}}\text{WO}{}_{\mathrm{<mtext>3+</mtext><mtext>x</mtext><mtext>2</mtext>}}\text{(}\text{x}\text{= 0.3)}$ thin films have been prepared by the thermal vacuum evaporation technique. Depending on the substrate temperature during deposition, either amorphous or crystalline films could be obtained. X-ray diffraction patterns and pole-figure information revealed that fresh crystalline films deposited on a hot (about 400°C) substrate of indium-tin oxide on glass microscope slides had a preferred orientation, with the c-axis perpendicular to the surface of the film. SIMS depth profiles and RBS experiments were used to confirm the stoichiometry and uniformity of the chemical components in the thermally deposited films.     

Synthesis of tungsten bronze powder and determination of its composition

Sodium tungsten bronze powders were synthesized by thermal reduction of a gas/melt system at high temperature. Samples having a cubic structure with different compositions were prepared. The initial melt included Na₂WO₄, WO₃ and 10–40% mol. NaCl while the reducing gas was hydrogen at 750 °C. An original mechanism of controlling the powders size and distribution was suggested and discussed. A quantitative novel and simple method to determine the bronze composition based on TGA data was developed. An increase in the NaCl content led to a decrease of the crystals size and improved the powder uniformity. Fine powders, in the 2–5 μm size range, were synthesized from melt with 40% mol of NaCl. The stoichiometry parameter x of the obtained bronzes ranged from 0.8 to 0.92. An excellent agreement between x values determined by the classical XRD route and the proposed TGA method was demonstrated.     

Phenomenological analysis of tetragonal tungsten bronze ferroelectrics

A simple Devonshire form has been derived for the phenomenological elastic Gibbs function to describe the elasto-dielectric parameters of simple proper ferroelectrics in the tungsten bronze structure family which has 4/mmm prototypic point symmetry. For the assumption that all temperature dependence is carried by the Curie-Weiss behaviour implicit in the quadratic term and that the expansion may be terminated at the first sixth-order term, reasonable agreement between calculated and derived P _s against T curves in the ferroelectric phase can be obtained for a wide range of bronze compositions. From the fitting it is clear that second and sixth rank terms are remarkably constant over a very wide range of bronze compositions. Variation in the negative fourth rank term is larger, but this is to be expected since it contains large contributions from electrostrictive and elastic terms which will depend on boundary conditions. These initial studies suggest that the phenomenological method may be used to derive expectation values for tensor parameters across the whole family of ferroelectric bronzes. The study also points up the need for more careful detailed studies of lattice strain, birefringence and permittivity as a function of temperature in model bronze compounds to provide more detailed checks of the method.     

Synthesis of highly crystalline hexagonal cesium tungsten bronze nanoparticles by flame-assisted spray pyrolysis

Highly crystalline and hexagonal single-phase cesium tungsten bronze (Cs_0.32WO₃) nanoparticles were successfully synthesized by a flame-assisted spray pyrolysis followed by annealing under a reducing gas atmosphere. The resulting Cs_0.32WO₃ nanoparticles featured a pure hexagonal Cs_0.32WO₃ phase with a high crystallinity and homogeneous chemical composition. Unlike conventional methods, the proposed process in this paper has several advantages, including a short reaction time and the ability to yield products with high purity and good energy efficiency. Furthermore, the Cs_0.32WO₃ nanoparticles produced in this research showed a remarkable near-infrared shielding ability with a 97.7% cut-off at 1500?nm.     

Synthesis and NO2-sensing properties of one-dimensional tungsten oxide nanowire bundles

Tungsten oxide nanowires were synthesized by solvothermal method with tungsten hexachloride (WCl6) as precursor. X-ray diffraction, field emission scanning electron microscope and transmission electron microscope characterizations indicated that the as-synthesized nanowires are single phase monoclinic W18O49. With WCl6 concentration increasing, the bundled nanowire became shorter and thicker. The gas-sensing properties of W18O49 nanowire towards NO2 gas were evaluated and the results showed that the optimal gas sensitivity is achieved at 150 degrees C and the thinner nanowire exhibits the higher sensitivity. The results indicate that tungsten oxide nanowire is a promising gas-sensing material for high performance and low power cost NO2 gas sensor.     

Mechanochemical synthesis of sodium tungsten bronze nanocrystalline powders

Nanocrystalline powders of sodium tungsten bronze Na_xWO₃ (x∼0.88) have been prepared by mechanochemical process using starting materials of Na pieces and WO₃ powders in a planetary ball mill. The synthesis reactions proceed with increasing milling time and are almost completed after 44 h. Phase-pure nanoparticles of Na_xWO₃ with average size of 17 nm were directly obtained after a simple washing process to remove the by-product of Na₂WO₄. The resistivity was measured in the temperature range from 77 to 300 K. The sample displays semiconducting behavior and can be characterized by three-dimensional variable-range hopping. The mechanochemical process seems to be an attractive route to fabricate tungsten bronzes because of several advantages such as easy preparation, less cost, operating at low temperature and suitability for a large-scale production.     

A structural study of the hexagonal potassium tungsten bronze,K0.26WO3

The crystal structure of the hexagonal potassium tungsten bronze, K_XWO₃, has been investigated by single crystal X-ray and powder neutron diffraction techniques. The space group is P6₃22. The new structural feature found in this study is the puckering of the ring of 6 oxygen atoms forming the “windows” in the hexagonal tunnels. The potassium ions appear to be disordered over two sites in the tunnel, towards one of the two sets of window oxygen atoms.     

Synthesis and property of short tungsten fibre/Zr-based metallic glass composite

The short tungsten fibre reinforced Zr_41.2Ti_13.8Ni_10.0Cu_12.5Be_22.5 bulk metallic glass composites with macroscopic isotropic mechanical properties were prepared by infiltration and rapid solidification. The diameters of the tungsten fibres are 300, 500 and 700?µm with a length of 1000?µm and the fibre volume fraction in three kinds of composites is between 60 and 65%. The room temperature compressive deformation behaviours of these composites were investigated systematically. The results show that the strength and plasticity of the composites increase with the decrease in the tungsten fibre diameter. The maximum compressive strength and plastic strain of the composite with 300?µm fibre, respectively, reach 3079?MPa and 37%. The fracture modes of all the composites are shear fracture. The superior compressive property of the composites with short tungsten fibre is due to the competition among different fracture modes and the inhibition effect of the interface on the shear band extension.     

Resistive switching phenomena of tungsten nitride thin films with excellent CMOS compatibility

We report the resistive switching (RS) characteristics of tungsten nitride (WN_x) thin films with excellent complementary metal-oxide-semiconductor (CMOS) compatibility. A Ti/WN_x/Pt memory cell clearly shows bipolar RS behaviors at a low voltage of approximately ±2.2 V. The dominant conduction mechanisms at low and high resistance states were verified by Ohmic behavior and trap-controlled space-charge-limited conduction, respectively. A conducting filament model by a redox reaction explains the RS behavior in WN_x films. We also demonstrate the memory characteristics during pulse operation, including a high endurance over >10⁵ cycles and a long retention time of >10⁵ s.     

Structural properties of inorganic materials: tungsten bronze SBN ceramics

Rare-earth doped strontium barium niobates were synthesized using usual ceramic technique. The dopants are La, Ce, Gd, Sm and Nd. The materials were characterized by XRD and density measurements. The grain sizes were determined from SEM analysis. Lattice parameters changed uniformly with rare-earth dopants in unfilled structures. Density measurements and SEM analysis confirmed only minute changes in the densities of the ceramics.     

Vaporisation of tungsten bronzes II Sodium and potassium tungsten bronzes

Thermodynamic calculations indicate that the disproportionation of sodium tungsten bronze is expected to occur at a lower temperature than that required for direct decomposition via congruent vaporisation. This expectation has been tested for cubic sodium bronzes and also for cubic europium and cerium bronzes, a tetragonal II potassium bronze and hexagonal bronzes of potassium, indium and thallium. Knudsen weight loss and Knudsen mass spectrometry results confirm that disproportionation is the dominant reaction for cubic and tetragonal bronzes. The hexagonal bronzes are resistant to disproportionation and vaporisation of K_0.22WO₃ occurs by the following K_0.22WO₃(s) → 0.22K(g) + 0.14 O₂(g + 0.056 W₁₈O₄₉(s) with an enthalpy of vaporisation of 405kJmol^?1.     

Fabrication of organic shell-covered gold nanospheres with near-infrared absorption

A simple protocol is developed for the fabrication of stable organic shell-covered gold nanospheres with near-infrared absorption characteristics. The designed strategy mainly concentrates on two steps: (i) the polymerization of gold nanoparticles (AuNPs) induced by 1,2-ethanedithiol in water/ethanol; (ii) conjugation of the polymerized AuNPs with marocyclic compound for the formation of the organic shell. In the first step, the 1,2-ethanedithiol containing two thiols towards both ends of the chain enables the successful polymerization of AuNPs due to the forceful appetency of thiols to AuNPs. In the second step, the polymerized AuNPs are covered by the marocyclic compound attributing to the hydrogel-bonding effect between NH and SH. Because of the organic shell, the stability and dispersibility of the obtained nanospheres are improved. The optical properties of the fabricated gold nanospheres (400 nm average diameter) are well investigated by the UV-vis absorption spectroscopy which exhibit intense near-infrared absorption at 972 nm in acetone. The strategy developed in this study is promising in that excellent stability and dispersibility of gold nanospheres can be achieved in a facile and economic way.     

Irreversible anomalies in the electrical properties of oxides with the tetragonal tungsten bronze structure

The anomalies in the electrical properties of (Pb,Sr,Ba)₄Li₂Nb₁₀O₃₀ solid solutions were studied by measuring dielectric permittivity and electrical conductivity as functions of temperature and time (during isothermal holding). The anomalies were shown to occur in defect-rich ferroelectrics. Both the dielectric permittivity and conductivity vary exponentially with time during isothermal holding. The effects of heating rate and UV irradiation time are analyzed. The results can be accounted for in the framework of a fluctuation-cluster model.     

Synthesis and optical absorption property of ordered macroporous titania film doped with Ag nanoparticles

Ordered macroporous anatase titania films doped with Ag nanoparticles (NPs) were synthesized by infiltrating titania sol into the interstitial voids of polystyrene colloidal sphere templates, removing the templates, immersing the films in a silver nitrate solution, and subsequently reducing in hydrogen atmosphere. Ag NPs were distributed uniformly within the macropores of the anatase titania film. Both the macroporous titania films doped with Ag NPs and those without doping have a blue-shift compared with that of the sol–gel derived titania film. Except for the plasma resonance absorption of Ag NPs, the macroporous titania film doped with Ag NPs also has a red-shift compared with the undoped sample. The macroporous titania film doped with Ag NPs have potential applications in the fields of optics, gas sensors, and catalysis.     

Dielectric and pyroelectric properties of niobium based complex tungsten bronze ferroelectrics

This paper highlights the dielectric and pyroelectric properties of two new complex tungsten bonze ceramics (K₂Pb₂Eu₂W₂Ti₄Nb₄O₃₀ and K₂Pb₂Nd₂W₂Ti₄Nb₄O₃₀) which were prepared by a high temperature mixed oxide method. Room temperature X-ray structural analysis confirms the formation of single phase compounds. The SEM micrographs show uniform distribution of densely packed rod like grains. Variation of dielectric parameters with temperature (27–500 °C) and frequency (1–5 MHz) shows the phase transition at 315 and 299 °C for the above mentioned respective samples. The temperature dependence of hysteresis loops confirms the existence of ferroelectricity in the materials below transition temperature. The current variation with voltage at different temperatures shows the semiconducting behaviour of the materials. The nature of temperature dependent dc conductivity follows the Arrhenius equation, and reveals the negative temperature coefficient of resistance behaviour of the materials. The current (for a fixed voltage) variation with temperature shows that the materials have high pyroelectric co-efficient and figure of merit, thus making them useful for pyroelectric sensors.     

Structural,dielectric and electrical properties of a new tungsten bronze ferroelectric ceramics

The polycrystalline sample of Li₂Pb₂Nd₂W₂Ti₄Ta₄O₃₀ was prepared by a solid-state reaction technique. Room temperature X-ray structural analysis confirms the formation of a single phase compound. The morphology of the sintered sample recorded by scanning electron microscope exhibits a uniform grain distribution. Detailed studies of the nature of variation of dielectric constant, tangent loss and polarization with temperature and frequency confirmed the existence of ferroelectricity in the material. The temperature and frequency dependence of impedance parameters (impedance, modulus, etc.) of the material exhibits a strong correlation of its micro-structure (i.e., bulk, grain boundary, etc.). The nature of variation of pyroelectric-coefficient and current with temperature suggests that material has good pyroelectric properties useful for pyroelectric detector.