Effect of ultrarapid quenching on the structure and mechanical properties of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> and Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>

We have studied the detailed structure and mechanical properties of the Nb₂O₅ and Ta₂O₅ pentoxides after ultrarapid quenching in comparison with the properties of Nb₂O₅ and Ta₂O₅ ceramics prepared by a conventional ceramic processing technique and using high-intensity light (HIL) in an optical furnace. The results demonstrate that high-energy processing (HIL and ultrarapid quenching) improves the hardness and strength of Nb₂O₅ and Ta₂O₅. At the same time, HIL processing and quenching lead to structural disordering of the Nb₂O₅ and Ta₂O₅ pentoxides.     

Influence of the concentration of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> on the electrical properties of SnO<Subscript>2</Subscript> varistors

Varistors are electronic materials with nonohmic behavior. In traditional SnO₂ varistors, CoO acts as a densifying agent, Nb₂O₅ increases the electrical conductivity of SnO₂ grains, and Cr₂O₃ produces a more uniform microstructure and acts as an oxygen retaining agent at the grain boundaries. The present work involved a systematic study of the substitution of Nb₂O₅ for Sb₂O₃ in the composition of a ternary varistor system. The compositions were prepared by conventional wet ceramic processing using deionized water, and the resulting slips were dried by spray-drying. Pellets were produced under a pressure of 330 MPa and sintered at 1,350 °C for 2 h. Similar to the behavior of Nb₂O₅, increasing the concentration of Sb₂O₃ reduced the nonlinear behavior of the ceramic and its breakdown electric field while increasing its leakage current. The samples’ microstructure showed greater porosity, suggesting that higher concentrations of Sb₂O₃ reduce the sintering rate, probably in response to the higher concentration of tin vacancies in the structure.     

Dielectric,thermal and mechanical properties of Sr<Subscript>2</Subscript>ZnSi<Subscript>2</Subscript>O<Subscript>7</Subscript> based polymer/ceramic composites

Polymer/Sr₂ZnSi₂O₇ (SZS) ceramic composites suitable for substrate applications have been developed using the polymers polystyrene (PS), high density polyethylene (HDPE) and Di-Glycidyl Ether of Bisphenol A (DGEBA). The dielectric, thermal and mechanical properties of the composites are investigated as a function of various concentrations of the ceramic filler. The obtained values of relative permittivity, dielectric loss tangent, thermal conductivity and coefficient of thermal expansion of the composites are compared with the corresponding theoretical predictions. The relative permittivity of the polymer/ceramic composites increases with filler loading. The dielectric loss tangent also shows the same trend except for DGEBA/SZS composites. The major advantages of the ceramic loading are improvement in thermal conductivity and a decrease in the coefficient of thermal expansion. The tensile strength of the composites decreases with increase in filler content, whereas an improvement is observed in microhardness. The variation of relative permittivity (at 1 MHz) of the composites is also studied as a function of temperature.     

Physical properties of lead-free BaFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics obtained from mechanochemically synthesized powders

Modern electronics expect functional materials that are eco-friendly and are obtained with lower energy consumption technological processes. The multiferroic lead-free BaFe_1/2Nb_1/2O₃ (BFN) ceramic powder has been prepared by mechanochemical synthesis from simple oxides at room temperature. The development of the synthesis has been monitored by XRD and SEM investigations, after different milling periods. The obtained powders contain large agglomerates built by crystals with an estimated size about 12–20 nm depending on the period of milling. From this powder, the multiferroic BFN ceramic samples have been prepared by uniaxial pressing and subsequent sintering pressureless method. The morphology of the BFN ceramic samples strongly depends on high-energy milling duration. The properties of the ceramic samples have been investigated by dielectric spectroscopy, in broad temperature and frequency ranges. The high-energy milling of the powders has strongly affected the dielectric permittivity and dielectric loss of the BaFe_1/2Nb_1/2O₃ ceramic samples. The usage of the mechanochemical synthesis to obtain the multiferroic lead-free BFN materials reduces the required thermal treatment and simultaneously improves the parameters of the BFN ceramics.     

Effects of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> doping on the microstructure and the dielectric temperature characteristics of barium titanate ceramics

In this work, the effects of Nb₂O₅ addition on the dielectric properties and phase formation of BaTiO₃ were investigated. A core–shell structure was formed for Nb-doped BaTiO₃ resulted from a low diffusivity of Nb⁵⁺ ions into BaTiO₃ when grain growth was inhibited. In the case of 0.3–4.8 mol% Nb₂O₅ additions, two dielectric constant peaks were observed. The Curie dielectric peak was determined by the ferroelectric-paraelectric transition of grain core, whereas the secondary broad peak at lower temperature was due to strong chemical inhomogeneity in Nb-doped BaTiO₃ ceramics. The dielectric constant peak at Curie temperature was markedly depressed with the addition of Nb₂O₅. On the other hand, the secondary dielectric constant peak was enhanced when sintered above 1280 °C for higher Nb₂O₅ concentrations (≥1.2 mol%). The Curie temperature was shifted to higher temperatures, whereas the transition temperature corresponding to the secondary peak moved to lower temperatures as increasing the amount of Nb₂O₅ more than 1.2 mol%. The decrease of this lower transition temperature was assumed to be closely related with the secondary phase formation when Nb concentration greater than 1.2 mol%. From XRD analyses, a large amount of secondary phases was observed when Nb₂O₅ amount exceeded 1.2 mol%. The coefficients of thermal expansion of Nb-doped BaTiO₃ were increased with increasing Nb₂O₅ contents, resulting in large internal stress between cores and shells. Therefore, the shift of Curie temperature to higher temperatures was attributed to internal stress resulting from the formation of a core–shell structure and a large amount of secondary phase grains.     

Dielectric and piezoelectric properties of MnO<Subscript>2</Subscript>-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>Nb<Subscript>0.92</Subscript>Sb<Subscript>0.08</Subscript>O<Subscript>3</Subscript> lead-free ceramics

Lead-free MnO₂-doped K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramics have been fabricated by a conventional ceramic technique and their dielectric and piezoelectric properties have been studied. Our results show that a small amount of MnO₂ (0.5–1.0 mol%) is enough to improve the densification of the ceramics and decrease the sintering temperature of the ceramics. The co-effects of MnO₂ doping and Sb-substitution lead to significant improvements in the ferroelectric and piezoelectric properties. The K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramic with 0.5 mol%MnO₂ doping possesses optimum propeties: d ₃₃ = 187 pC/N, k _P = 47.2%, ε _r = 980, tanδ = 2.71% and T _c = 287 °C. Due to high tetragonal-orthorhombic phase transition temperature (T _O-T ~ 150 °C), the K_0.5Na_0.5Nb_0.92Sb_0.08O₃ ceramic with 0.5 mol%MnO₂ doping exhibits a good thermal stability of piezoelectric properties.     

Nanocrystalline PbIn<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> ceramics and its properties

A ferroelectric relaxor PbIn_0.5Nb_0.5O₃ (PIN) ceramics has been obtained using a modified ceramic technology, with the sintering stage preceded by compression and shear straining of the synthesized charge in Bridgman anvils. The dimensions of ceramic grains after this pretreatment are spread over a range from 100 to 1250 nm. A comparative investigation of the properties of PIN ceramics obtained using the standard and modified technology showed that the proposed mechanical action at the charge preparation stage can be used for controlled modification of the properties of ferroelectric ceramics.     

Effects of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> addition on the microstructure,electrical, and mechanical properties of PZT/ZnO nanowhisker piezoelectric composites

Nb₂O₅-modified PZT/ZnO nanowhisker (denoted as PZT/ZnO_w–Nb₂O₅) piezoelectric composites were prepared by a solid state sintering technique. Effects of Nb₂O₅ addition on the microstructure, electrical, and mechanical properties of the PZT/ZnO_w composites were investigated. With increasing Nb₂O₅ content, the grain size of the composites was reduced and the fracture mode changed from intergranular to intragranular gradually. Compared with the PZT/ZnO_w composites, the dielectric, piezoelectric, and ferroelectric properties of the PZT/ZnO_w–Nb₂O₅ composites were improved significantly, while mechanical properties were enhanced slightly. The optimum electrical and mechanical properties were achieved for the PZT/ZnO_w composites modified with 0.75 wt% Nb₂O₅ sintered at 1150 °C, with dielectric permittivity ε_r, piezoelectric coefficient d ₃₃, planar electromechanical coupling k _p, remnant polarization P _r, fracture toughness K _IC, and flexural strength σ_f being on the order of 4930, 600 pC/N, 0.63, 29.2 μC/cm², 1.56 MPa m^1/2 and 130 MPa, respectively. The Nb₂O₅-modified PZT/ZnO_w piezoelectric composites, with comparable electrical properties and improved mechanical properties than those of commercial PZT-5H ceramics, are promising candidates for further applications.     

PTCR effect in BaBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>-doped BaTiO<Subscript>3</Subscript> ceramics

The influences of BaBi₂Nb₂O₉ content on the electrical property and the microstructure of BaTiO₃-based materials have been studied. With an increase in BaBi₂Nb₂O₉ content the grain size decreases. All the prepared BaBi₂Nb₂O₉ doping BaTiO₃-based thermistors show typical PTC effect. As the amount of BaBi₂Nb₂O₉ added in BaTiO₃-based ceramics increases, resistivity appears to exhibit a minimum value. At high BaBi₂Nb₂O₉ content (≥0.0875), the resistivity increased again with increasing BaBi₂Nb₂O₉ content. At a given content of BaBi₂Nb₂O₉, the influence of sintering temperature on the electrical properties of samples has been investigated. A minimum of room temperature resistivity is obtained at the sintering temperature equal to 1,290 °C at a given content of BaBi₂Nb₂O₉.     

Fabrication of thin films by sputtering deposition using (Ba<Subscript>0.3</Subscript>Sr<Subscript>0.7</Subscript>)(Zn<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> ceramic as target

Dielectric ceramic thin films were fabricated on SiO₂ (110) substrates by the radio frequency (RF) magnetron sputtering method using (Ba_0.3Sr_0.7)(Zn_1/3Nb_2/3)O₃ microwave dielectric ceramic as target. The microstructure, components, and morphology of the thin films were investigated thoroughly. The results reveal that the experimental conditions can affect the growth of the thin films significantly. The main phases of the thin films are Ba_0.5Sr_0.5Nb₂O₆ and Ba_0.27Sr_0.75Nb₂O_5.78, which are of different composition from that of the ceramic target due to Zn loss. The thin films are polycrystalline with high-quality crystalline and are made up of dense rod-like structures. The growth mechanism of the thin films is discussed in particular.     

Ag-decorated octahedral K<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> nanoparticles with enhanced photocatalytic performance

The Ag-K₂Nb₂O₆ nanocomposites are synthesized by a two-step method where the octahedral K₂Nb₂O₆ is initially prepared by solvothermal reaction and then the Ag particles are anchored onto the surface of K₂Nb₂O₆ through the photoreduction of AgNO₃. The XRD, SEM, TEM, XPS, DRS are applied to characterize the structure, morphology and optical properties, which confirm that the Ag particles are successfully deposited on the surface of K₂Nb₂O₆. Compared with the pure K₂Nb₂O₆, the Ag modified K₂Nb₂O₆ catalysts show an obvious enhancement catalysis under UV–Vis light, because that could efficiently promote the light absorption and the separation of photoelectrons and holes.     

Synthesis of anisometric KSr<Subscript>2</Subscript>Nb<Subscript>5</Subscript>O<Subscript>15</Subscript> particles in the SrNb<Subscript>2</Subscript>O<Subscript>6</Subscript>–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–KCl system

Anisometric and agglomerate-free template particles are important for fabrication of grain-oriented ceramics. In the present work, preparation of acicular KSr₂Nb₅O₁₅ (KSN) particles was firstly explored in the SrNb₂O₆–Nb₂O₅–KCl system by molten salt synthesis (MSS) method. It was found that the molar ratio of SrNb₂O₆ to Nb₂O₅, the amount of KCl salt and synthesis time could significantly affect the phase structure and morphology of KSN particles. When calcined at 1,150 °C for 6 h with the molar ratio of SrNb₂O₆ to Nb₂O₅ was 1 and the weight ratio of salt to oxide source was 1.50, pure KSN particles with well-developed acicular morphology were successfully obtained in this system. They were agglomerate-free and with proper scale in the size range of 5–30 μm, which made them the ideal templates for fabricating textured ceramics. In addition, some new reaction and growth mechanisms were proposed in this work.     

Niobium pentoxide as promoter in the mixed MgH<Subscript>2</Subscript>/Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> system for hydrogen storage: a multitechnique investigation of the H<Subscript>2</Subscript> uptake

Nb₂O₅ is known as good promoter for adsorption/desorption kinetics of hydrogen in magnesium hydride. In this article the interaction with hydrogen of bare Nb₂O₅, the oxidic component of the mixed MgH₂/Nb₂O₅ system, is investigated in various conditions (i.e. employing atomic, molecular and nascent hydrogen). The state of the hydrogen-Nb₂O₅ system was monitored using various techniques including: X-ray Diffraction, Electron Paramagnetic Resonance, Diffuse Reflectance UV-Vis Spectroscopy, Thermal Desorption Spectroscopy -Mass Spectrometry, Differential Scanning Calorimetry and Thermal Programmed Desorption. Niobium (V) oxide is not at all inert while interacting with hydrogen. This oxide is partially reduced by hydrogen, which is incorporated in the solid and released as both molecular hydrogen and water. This peculiar behaviour, reminiscent of some properties of the bronze family, suggests an active chemical role played by Nb₂O₅ in the mixed MgH₂/Nb₂O₅ system.     

The Development of Microstructure and Ferroelectric Properties of Bi<Subscript>4</Subscript>Ti<Subscript>3.96</Subscript>Nb<Subscript>0.04</Subscript>O<Subscript>12</Subscript> Thin Films

Bi₄Ti_3.96Nb_0.04O₁₂ thin films were successfully deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a sol–gel method and rapid thermal annealing. The effects of Nb-substitution and annealing temperature (500–800°C) on the microstructure and ferroelectric properties of bismuth titanate thin films were investigated. X-ray diffraction analysis reveals that the intensities of (117) peaks are relatively broad and weak at annealing temperatures smaller than 700°C. With the increase of annealing temperature from 500°C to 800°C, the grain size of Bi₄Ti_3.96Nb_0.04O₁₂ thin films increases. The Bi₄Ti_3.96Nb_0.04O₁₂ thin films annealed at 700°C exhibit the highest remanent polarization (2P _r), 36 μC/cm² and lowest coercive field (2E _c), 110 kV/cm. The improved ferroelectric properties can be attributed to the substitution of Nb⁵⁺ to Ti⁴⁺ in Bi₄Ti₃O₁₂ assisting the elimination of defects such as oxygen vacancy and vacancy complexes.     

Densification Peculiarities of Transparent MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> Ceramics—Effect of LiF Sintering Additive

A new method of doping of spinel nanopowders with LiF sintering additive has been proposed, consisting in its introduction via solution technology, which leads to improved sinterability of the ceramic. The effect of LiF doping on densification behavior, microstructure, and optical and mechanical properties of hot-pressed MgAl₂O₄ ceramics has been studied. Samples of MgAl₂O₄ optical ceramics with density close to that theoretically achievable have been produced by hot pressing of 0.5 wt % LiF-doped nanopowders at 1600°C for 1 h. It has been shown that addition of LiF changes crack propagation in spinel ceramic from transgranular to intergranular.     

Diamond electromagnetic band gap structure based on Bi(Nb<Subscript>0.992</Subscript>V<Subscript>0.008</Subscript>)O<Subscript>4</Subscript> ceramic

Three-dimensional (3D) diamond structure electromagnetic band gap (EBG) structures containing high-K Bi(Nb_0.992V_0.008)O₄ (BVN) ceramic, fabricated by rapid-prototyping (RP) and gel casting methods, were investigated. The simulations based on finite element method (FEM) were employed to model the band structures. High-K Bi(Nb_0.992V_0.008)O₄ ceramic was made into gel to cast into the diamond structure molds fabricated by rapid-prototyping method. Then the green bodies were sintered at 900 °C to obtain well densified EBG samples. The transmission characteristics of the EBG structures were measured by transmission/reflection (T/R) methods using a vector network analyzer. Wide complete band gap was observed in the transmission characteristics from 10.08 to 12.59 GHz and it agreed well with the simulation results, which was from 10 to 12.19 GHz.     

Solid-state reactions in the system Li<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>

The formation mechanisms of Li _x Na_{1 ?x} Ta _y Nb_{1 ? y} O₃ perovskite solid solutions in the Li₂CO₃-Na₂CO₃-Nb₂O₅-Ta₂O₅ system have been studied by x-ray diffraction, differential thermal analysis, thermogravimetry, IR spectroscopy, and mass spectrometry at temperatures from 300 to 1100°C. The results indicate that the synthesis of Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions involves a complex sequence of consecutive and parallel solid-state reactions. An optimized synthesis procedure for Li _x Na_{1 ? x} Ta _y Nb_{1 ? y} O₃ solid solutions is proposed.     

Structure and microwave dielectric properties of ZnTa<Subscript>2</Subscript>O<Subscript>6</Subscript> ceramics with TiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript> additions

Ceramic samples based on ZnTa2O₆ and ZnTa₂O₆–MO₂ (M = Ti, Zr) systems have been obtained by the solid state ceramic route. The phase composition and microstructure of samples were investigated. The effect of the aliovalent substitution of ions Zn²⁺ and Ta⁵⁺ by M⁴⁺ (M = Ti, Zr) in the structure of ZnTa₂O₆ on microwave dielectric properties of ceramics was studied. The way of the compensation of the positive temperature coefficient of resonant frequency of dielectric resonators based on ZnTa₂O₆ ceramics with using the aliovalent substitution of cations was proposed. Dielectric resonators with the high temperature stability of the resonant frequency and high dielectric properties in the microwave range based on the ZnTa₂O₆–ZrO₂ system were obtained for application in electronics.     

Preparation and phase formation of perovskite Pb(Ni<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> by a reaction-sintering process

Pyrochlore-free Pb(Ni_1/3Nb_2/3)O₃ perovskite ceramics produced by a simple and effective reaction-sintering process were investigated. Without any calcination, the mixture of PbO, Ni(NO₃)₂ and Nb₂O₅ was pressed and sintered directly into PNN ceramics. Density of 98.5% of theoretical value was obtained after sintered at 1230 °C for 2 h in air. 99.3% of theoretical density was obtained after sintered at 1,200 °C for 2 h in PbO compensated atmosphere. PNN ceramic with dielectric constant 1,680 at 25 °C and 1 kHz has been obtained.     

Synthesis and thermal expansion hysteresis of Ca<Subscript>1-x</Subscript>Sr<Subscript>x</Subscript>Zr<Subscript>4</Subscript>P<Subscript>6</Subscript>O<Subscript>24</Subscript>

The low thermal expansion ceramic system, Ca_1-xSr{x}Zr₄P₆O₂₄, for the compositions with x = 0, 0.25, 0.50, 0.75 and 1 was synthesized by solid-state reaction. The sintering characteristics were ascertained by bulk density measurements. The fracture surface microstructure examined by scanning electron microscopy showed the average grain size of 2.47 μm for all the compositions. The thermal expansion data for these ceramic systems over the temperature range 25–800°C is reported. The sinterability of various solid solutions and the hysteresis in dilatometric behaviour are shown to be related to the crystallographic thermal expansion anisotropy. A steady increase in the amount of porosity and critical grain size with increase in x is suggested to explain the observed decrease in the hysteresis.