Influence of secondary phases on ferroelectric properties of Bi0.5Na0.5TiO3 ceramics

The effects of secondary phases on ferroelectric properties of Bi_0.5Na_0.5TiO₃ (BNT) have been studied. Ceramic powders were prepared by solid state reaction employing different sintering temperatures and characterized by X-ray diffraction (XRD), Scanning Electron Microscopy and impedance spectroscopy. The perovskite structure was detected by XRD; together with small peaks corresponding to a secondary phase assigned to the Na₂Ti₆O₁₃-based phase in calcined powders. In addition, morphology and the content of the secondary phase were modified by the sintering temperatures, affecting the ferroelectric properties, and ac and dc conductivities. We believe that our results can benefit not only the understanding of BNT ceramics, but also expand the range of applications.     

Thermodynamic and experimental approach of the effect of Si on the sintering of Y3NbO7

The effect of silicon on the natural sintering of Y₃NbO₇ powders is studied. Characterizations shows that Si increases densification rate and grain growth but it also leads to the apparition of a secondary silicate phase. The silicate precipitation results from the segregation of silicon favored by matter transport through a liquid phase. The distribution of the secondary phase have been studied by electronic microscopy and mapped by micro-luminescence. Thermodynamic calculations of Gibbs energy of the different phase as well as binary and ternary phase diagrams have been conducted to support interpretation of the sintering experimental results.     

Preparation of Giant Dielectric CaCu3Ti4O12 Ceramics via the Molten Salt Method from NaCl Flux

CaCu₃Ti₄O₁₂ (CCTO) powders have been synthesized by the molten salt method using NaCl as the flux. The effects of the synthesis temperature on their crystal structures and micromorphologies were investigated in this study. A new secondary phase, Na₂Ti₆O₁₃, was found in the as‐synthesized powders when the temperature exceeds 800°C. Na₂Ti₆O₁₃ and CuO secondary phases result in a significant increase in the dielectric constant and a certain increase in the dielectric loss. 800°C is a suitable synthesis temperature to prepare CCTO ceramics with relatively good dielectric performance using NaCl as the molten salt.     

In situ synthesis,fabrication and Rietveld refinement of the hydroxyapatite/titania composite coatings on 316 L SS

The development of Hydroxyapatite (HAP)/Titania (TiO₂) composite coatings on metallic implants have received a great deal of attention during the recent years owing to their superior advantages in biomedical applications. The present study has focused on the in situ formation of HAP/TiO₂ composite powders through aqueous precipitation technique. Five different HAP/TiO₂ composite powders of varied HAP to TiO₂ ratios has been synthesized in the present study and the results were compared with the stoichiometric HAP, Rutile TiO₂ and Anatase TiO₂ which also have been synthesized by adapting a similar synthetic procedure. All the synthesized powders have been analyzed using X-ray diffraction (XRD) and X-ray fluorescence (XRF) techniques. Rietveld Refinement technique has been employed to generate quantitative information about the structural characteristics and phase content in all the powder samples. Further, the electrophoretic deposition (EPD) method has been employed to fabricate HAP/TiO₂ composite coatings on 316 L SS and the resultant coatings were analyzed for its quantitative structural characteristics. The results from the present investigation has confirmed that concentration of TiO₂ in the HAP/TiO₂ composites and heat treatment temperatures have played a major role in the degradation of HAP to β-Tricalcium phosphate and also in the conversion of Anatase to Rutile TiO₂ phase.     

The indirect synthesized method induced relaxor state to ferroelectric characteristics for Pb0.95La0.05(Fe2/3W1/3)0.65Ti0.35O3 ceramics

Pb_0.95La_0.05(Fe_2/3W_1/3)_0.65Ti_0.35O₃ ceramics are synthesized by using the conventional direct oxide synthesized (DS) method and the indirect synthesized (IS) method. The low-field dielectric response is investigated by using the empirical law, two ordering models and the space charge polarization. The temperature-dependent dielectric peak of the samples prepared using the IS synthesized method is obviously increased; the temperature-dielectric constant is changed to sharp and the diffused phase characteristic is decreased. It is concluded that the IS synthesized method can change the dielectric natures from a relaxor like behavior to a sharp ferroelectric state. The space charge polarization is also changed by using the IS synthesized method. According to these results, it is suggested that the 1:1 ordered domain is enhanced based on the random-site type by using the IS synthesized method for Pb_0.95La_0.05(Fe_2/3W_1/3)_0.65Ti_0.35O₃ ceramics. However, if the sintering temperature is not high enough for some calcined powders, the random-site type can be changed to the space charge type.     

Magnesiothermal synthesis of W-WB powders in ionic melts

Conditions for synthesis of W-W₂B and W-WB powders by simultaneous reduction of the metal oxide- and boron-containing compounds with magnesium in molten salts of the NaF-NaCl system (1: 1) have been considered. It is shown that synthesis involves successive redox reactions yielding tungsten powders with variable concentrations of boride phases. The powder fineness characteristics have been determined in experiments. The volume particle size distribution of the powders has been calculated. It is found that the specific surface area decreases from 1.18 × 10⁶ to 0.63 × 10⁶ as the concentration of the boride phase in the powders rises to ～40%.     

Relationships between sintering conditions,microstructure and dielectric properties of lead magnesium niobate

Lead magnesium niobate, (Pb(Mg_1/3Nb_2/3)O₃; PMN) ceramics have been produced by sintering PMN powders synthesized from lead oxide (PbO) and magnesium niobate (MgNb₂O₆). As these PMN powders could be prepared in a reproducible manner, attention has been focused on relationships between sintering conditions, phase formation, density, microstructural development and dielectric properties. A method has been developed whereby volatilisation of PbO can be minimised, thereby avoiding the formation of pyrochlore phases and maximising the perovskite yield. The optimum sintering conditions have been identified as 1275 °C for 2 h.     

Gas-to-Particle Conversion in the Particle Precipitation–Aided Chemical Vapor Deposition Process II. Synthesis of the Perovskite Oxide Yttrium Chromite

In the particle precipitation-aided chemical vapor deposition process, an aerosol is formed in the gas phase at elevated temperatures. The particles are deposited on a cooled substrate. Coherent layers with a controlled porosity can be obtained by a simultaneous heterogeneous reaction, which interconnects the deposited particles. The synthesis of submicrometer powder of the perovskite oxide yttrium chromite (YCrO₃) by gas to particle conversion, which is the first step of the PP-CVD process, has been investigated, and preliminary results are shown. The powders have been synthesized using yttrium trichloride vapor (YCl₃), chromium trichloride vapor (CrCl₃), and steam and oxygen as reactants. The influence of the input molar ratio of the elements on the composition and characteristics of the powders has been investigated. Phase composition has been determined by X-ray diffraction (XRD). The powders have been characterized by transmission electron microscopy (TEM) and sedimentation field flow fractionation (SF³). At a reaction temperature of 1283 K the powders consist of chromium sesquioxide (Cr₂O₃), or a mixture of Cr₂O₃ and YCrO₃. At stoichiometric input amounts of metal chlorides and steam the formation of YCrO₃ seems to be favored. Two typical particle size distributions have been observed. The primary particle size ranges from 5 to 30 nm for small particles, and from 40 to 250 nm for large particles, depending on the process conditions. The particles tend to be agglomerated. The weight of the agglomerates is independent of the primary particle diameter.     

Gram scale synthesis of monoclinic VO2 microcrystals by hydrothermal and argon annealing treatment

We report gram scale synthesis of 100% phase fraction of VO₂ (M) monoclinic in powder form with reversible phase transition by combining hydrothermal method and Ar annealing at high temperature. Optimization of single phase VO₂ (M) growth and its phase transition characteristics have been analysed systematically by varying growth parameters such as time, synthesis temperature and post growth annealing conditions. Argon annealing of hydrothermally grown VO₂ powders at 800?°C found to play key role in obtaining VO₂ (M) phase in gram scale with characteristic phase transition temperature of 68?°C. In-situ TEM has been performed to investigate the microstructure and phase change across the annealing temperature. Detailed characterizations have been carried out to correlate the phases, microstructure and transition temperature of VO₂ with respect to growth parameters.     

The properties of magnesia powders for the hot welding process

A number of powders derived from MgO calcined at various temperatures within the range 770–1770 K was submitted to the treatment usually used in preparation of the batches to welding process (hot-pressing in the presence of liquid salt phase).Changes in powders characteristics during this treatment have been discussed based on results of sedimentation analysis and specific surface area measurements (BET method).The samples made of these powders with and without salt addition (CaCl₂LiCl eutectic mixture) were examined using mercury porosimetry and SEM. It has been shown that powder characteristics and the presence of liquid salt phase essentially influence the results of densification process. Sharp differences have been discovered in apparent densities and total amounts of open pores between the samples hot-pressed with and without salt additions. The former reached densities of about 85% theoretical and the latter were usually lower than 50%.     

Solution combustion synthesis of MnFeCoNiCu and (MnFeCoNiCu)3O4 high entropy materials and sintering thereof

Single-phase, fine-grained high entropy MnFeCoNiCu alloy and (MnFeCoNiCu)₃O₄ oxide powders have been pioneered by an in-situ solution combustion synthesis without post-treatment, utilizing alanine and glycine as organic reducers and ammonium nitrate as an auxiliary oxidizing agent. In order to regulate the porosity, homogeneity, phase composition and morphology of the obtained powders, multivariate studies on the influence of the following parameters were performed: type and quantity of reducers, pH of solution, ignition temperature, presence of oxidizing agent, etc.. It has been shown that the amount of the reducer holds the key for governing combustion temperature and speculation of lower and upper combustion limits in the systems under study. In turn, according to the temperature-time profiles of combustion, product microstructure, as well as phase composition of the product, the combustion area turned out to be divided into two sub-domains: volume combustion (with vigorous flaming) and self-propagation combustion. After thorough characterization, high entropy MnFeCoNiCu alloy and high entropy (MnFeCoNiCu)₃O₄ oxide powders were subjected to consolidation by spark plasma sintering and mechanical properties were evaluated. The influence of sintering temperature on the phase transformation of the products was revealed.     

Optical,thermal and dielectric properties of Bi4(TiO4)3 ceramic powders

Bismuth titanate (Bi₄(TiO₄)₃) ceramic powders have been synthesized by using a solid state reaction method. Prominently intense blue emission at 480 nm has been measured with an excitation at 418 nm. The reason for the observance of such a blue emission from this ceramic powder has been explained. The phase formation has been investigated by X-ray diffraction analysis (XRD). The morphology and composition of the ceramic powders have been studied from the measurement of SEM and EDS profiles. FTIR and Raman spectra have also been recorded to analyze the presence of functional groups and Raman active modes in the Bi₄(TiO₄)₃ ceramic powders. The sintering temperature has been optimized to be 1100 °C based on the measured TG–DTA profiles of the as prepared material. Besides these, dielectric properties of ceramic powder in the frequency range of 200 Hz–3 MHz at 300 K have also been carried out.     

New KNbTeO6 transparent tellurate ceramics

New transparent defect pyrochlore KNbTeO₆ ceramics were successfully prepared by Spark Plasma Sintering (SPS) of same composition polycrystalline powders elaborated by classic solid-state reaction from oxide precursors (K₂CO₃, Nb₂O₅, TeO₂) and followed by high energy milling powders. As such precursors are not available as commercial nanopowders, a suitable process has been developed by combining solid-state reactions and high energy milling. The determination of appropriate consolidation conditions and sintering parameters of the green body such prepared, are described in this paper. The resulting ceramic is transparent in both the visible and near infrared range (up to 5.5 μm). The maximum of transmittance is reached in the near infrared region around 2500 nm with a value of 78 % (1 mm thick sample), close to the maximum theoretical value of transmittance. This transparent KNbTeO₆ ceramic demonstrates a homogeneous and dense microstructure with an average grain size less than 500 nm. A small content of secondary phase has been detected by nanoscale observations without drastic effects on transparency. This ceramic exhibits very good mechanical properties similar to the Y₂O₃ transparent ceramic, as well as interesting dielectric properties in the microwave range. This innovative method should drive the development of new transparent materials with technologically relevant applications.     

Synthesis of nanosized alumina powders by pulsed wire discharge in air flow atmosphere

Pulsed wire discharge (PWD) is a method to form nanosized powders which is based on evaporation of thin metal wires by applied large electric current. Nanosized oxide powders can be synthesized by PWD in oxygen atmosphere. Since only small amount of powders can be synthesized by single discharge to a piece of wire, the number of discharges has to be increased for mass production of nanosized powders. Then, PWD equipments with wire feeding and gas flow atmosphere systems were developed for mass production. However, effects of gas flow on formed powders have never been investigated. In this study, PWD in air flow atmospheres were tried to synthesize alumina powders from industrial viewpoints. The effects of air flow rate on the oxidation phenomena and the synthesized powders have been investigated. PWD of pure Al wires with diameters of ? 0.3–0.5 mm were examined in air flow rates of 15 L/min and 33 L/min as well as 0 L/min (closed atmosphere condition). The synthesized powders at all experimental conditions were dominantly composed of γ-Al₂O₃ and δ-Al₂O₃ phases. The δ/γ ratios of the Al₂O₃ phases increased with increasing air flow rate and wire diameter. Besides them, small amount of metallic Al phase was remained in the powders synthesized by PWD in the air flow atmosphere of 33 L/min. It is concluded that appropriate air flow rate should be adopted for mass production of alumina powders in order to achieve enough collection of particles and avoid mingling of metallic Al phase.     

Fine TiAl and NiAl powders by SHS with a reduction stage

Fine TiAl and NiAl powders have been synthesized by SHS with a reduction stage. Explored was the effect of green composition, stoichiometric ratio, and additives on the particle size, morphology, and phase composition of resultant powders. Addition of NaCl and excessive Mg was found to suppress crystallization. In the presence of heat-generating additives, such as Mg(ClO₄)₂ and CaO₂, the synthesized powders had a particle size of below 3 μm. The morphology and phase composition of synthesized NiAl powders were investigated upon variation in the Al content of green mixture.      

Formation of nanostructured particles of cerium-activated yttrium aluminum garnet by the combustion method

The synthesis of ultradispersed powders of the yttrium aluminum garnet (activated by ions of cerium, lithium, silicon, and manganese) by the combustion method in the presence of hexamethylenetetramine has been optimized. The structural, physicochemical, and spectral-luminescence properties of synthesized finely dispersed samples have been studied. It has been found that exclusively the phase of garnet Y₃Al₅O₁₂ of the cubic modification is identified in the products of the synthesis performed under optimum condition.     

Electrochemical energy storage of Co powders in alkaline electrolyte

The electrochemical charge and discharge behavior of Co powders has been investigated by using X-ray diffraction (XRD), charge and discharge testing, and electrochemical impedance spectroscopy (EIS) at room temperature. Mechanical milling (MM) has been used to treat the Co powders for a comparative experiment. Mechanical milling induces a phase transition of fcc phase to hcp phase, an increase in particle size, and a decrease in grain size. The results of the XRD indicate a reversible reaction between Co and Co(OH)₂. The non-milled Co has a higher discharge capacity at a current density of 60 mA g⁻¹ as compared to the milled one. However, the milled Co presents a better HRD, in spite of the discrepancy in particle size. The results of EIS show that the electrochemical reaction process of Co powders consists of three steps, that is the charge-transfer of Co/Co(OH)₂ or Co/CoH_x, the mass-transfer of HCoO₂⁻, and hydrogen diffusion within Co, depending on the depth of discharge.     

Influence of Microwave‐Assisted Pechini Method on La0.80Sr0.20Ga0.83Mg0.17O3–δ Ionic Conductivity

With the aim of investigating the microwave influence on the electrolyte material properties, La_0.80Sr_0.20Ga_0.83Mg_0.17O_2.815 was prepared by both a conventional and a microwave‐assisted sol–gel Pechini method. With respect to the conventional Pechini method (hereafter SGP), the microwave assisted process (hereafter MWA‐SGP) guaranteed a faster procedure, reducing the time needed to remove the excess solvents to complete the polyesterification reaction from some days to a few hours. In fact, when a MWA‐SGP method was used, powders having higher phase purity were obtained. The sintering process at 1,450 °C of the powders prepared by both methods yielded pellets with similar density values (≥92% of theoretical). Nevertheless, only by microwave‐assisted process single‐phase products were obtained and no secondary phases such as tetragonal LaSrGaO₄ and LaSrGa₃O₇ were detected. These by‐products have been demonstrated to be detrimental for conductivity. Indeed, pellets obtained by MWA‐SGP method showed oxygen ionic conductivity values higher (about 30–40%) than those checked for SGP samples, thus demonstrating the important role of the microwave process on reducing time and costs and on improving the electrolyte properties.     

A modified two-Stage mixed oxide synthetic route to lead magnesium niobate and lead iron niobate

A modified mixed oxide synthetic route has been developed for the synthesis of lead magnesium niobate [Pb(Mg_1/3Nb_2/3)O₃; PMN] and lead iron niobate [Pb(Fe_1/2Nb_1/2)O₃; PFN] powders. The formation of perovskite and pyrochlore phases in the calcined PMN and PFN powders has been investigated as a function of calcination temperature and time by XRD and DTA techniques. The particle size distribution of calcined powders was determined by laser diffraction, with the morphology, phase composition and crystal structure determined via SEM, TEM and EDX techniques. In both cases it has been found that cubic pyrochlore phases in the PbO–Nb₂O₅ system tend to form, as well as the perovskite phase. However, pyrochlore-free PMN and PFN powders were successfully obtained for a calcination temperature of 800°C for 4 and 3 h, respectively, without the introduction of excess PbO and/or MgO.     

Nanoscaled BaTiO3 powders with a large surface area synthesized by precipitation from aqueous solutions: Preparation,characterization and sintering

Nanosized BaTiO₃ powders with a specific surface area of 60–75 m²/g have been prepared by precipitation of a titanium ester with Ba(OH)₂ solution at temperatures less than 100 °C. The effects of the Ba(OH)₂ concentration, isopropanol mixing with water as a solvent, the Ba:Ti ratio and surface modifiers on the surface area, the particle size, the crystalline phase, the agglomeration and aggregation degree of the synthesized powders as well as dielectric properties of sintered pellets have been investigated. The properties of the obtained powders have been characterized with XRD, BET, TG-DTA, ICP-AES, HRTEM and dilatometer. A high concentration of Ba(OH)₂ can increase the agglomeration and aggregation degree of the particles while the addition of isopropanol in water is beneficial for lowering it. To obtain stoichiometrical barium titanate, the ratio of Ba:Ti should be 1.1. The leaching of barium ions during processing can be limited by washing the powder with ammonia solution at pH10.2. A BaTiO₃ ceramic (95.8% of the theoretic density) has been fabricated by sintering the powders at 1250 °C for 2 h.