Effect of calcination conditions on phase formation and particle size of nickel niobate powders synthesized by solid-state reaction

The solid-state mixed oxide method via a rapid vibro-milling technique is explored in the preparation of single-phase nickel niobate (NiNb₂O₆) powders. The formation of the NiNb₂O₆ phase in the calcined powders has been investigated as a function of calcination conditions by TG-DTA and XRD techniques. Morphology, particle size and chemical composition have been determined via a combination of SEM and EDX techniques. It has been found that the minor phases of unreacted NiO and Nb₂O₅ precursors and the Ni₄Nb₂O₉ phase tend to form together with the columbite NiNb₂O₆ phase, depending on calcination conditions. More importantly, it is seen that optimization of calcination conditions can lead to a single-phase NiNb₂O₆ in an orthorhombic phase.     

Effect of calcination conditions on phase formation and particle size of Zn3Nb2O8 powders synthesized by solid-state reaction

The solid-state mixed oxide method via a rapid vibro-milling technique is explored in the preparation of single-phase Zn₃Nb₂O₈ powders. The formation of the Zn₃Nb₂O₈ phase in the calcined powders has been investigated as a function of calcination conditions by TG-DTA and XRD techniques. Morphology, particle size and chemical composition have been determined via a combination of SEM and EDX techniques. It has been found that the minor phases of unreacted ZnO and Nb₂O₅ precursors and the columbite ZnNb₂O₆ phase tend to form together with the Zn₃Nb₂O₈ phase, depending on calcination conditions. It is seen that optimization of calcination conditions can lead to a single-phase Zn₃Nb₂O₈ in a monoclinic phase.     

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.     

Phase and morphology evolution of magnesium niobate powders synthesized by solid-state reaction

Magnesium niobate (MgNb₂O₆; MN) powders have been prepared and characterized by TG-DTA, XRD, SEM and EDX techniques. The effect of calcination temperature, dwell time and heating/cooling rates on phase formation, morphology and chemical composition of the powders are examined. The calcination temperature and dwell time have been found to have a pronounced effect on the phase formation of the calcined magnesium niobate powders. It has been found that the minor phases of unreacted MgO and Nb₂O₅ phases tend to form together with the columbite-type MgNb₂O₆ phase, depending on calcination conditions. It is seen that optimisation of calcination conditions can lead to a single-phase MgNb₂O₆ in an orthorhombic phase. Higher calcination times and heating/cooling rates clearly favoured particle growth and the formation of large and hard agglomerates.     

Effects of PNN/PZT ratios on phase structure,electric properties and relaxation behavior of PZN–PNN–PZT ceramics

Pb(Zn_1/3Ni_2/3)_c(Ni_1/3Nb_2/3)_a(Zr_xTi_y)_bO₃ (PZN–PNN–PZT, the ratios of PNN/PZT a/b were 0.88, 1 and 1.136) piezoelectric ceramics were prepared by a traditional solid-state reaction method. The effects of PNN/PZT ratio on phase structure, microstructure and electric properties as well as the relaxation behaviors of PZN–PNN–PZT ceramics were investigated. The XRD patterns showed that all ceramics samples had a pure perovskite phase structure. Meanwhile, it was found that the phase structure undergoes a tetragonal, tetragonal-rhombohedral to rhombohedral transition as ratios of PNN/PZT increased. With the increasing of a/b from 0.88 to 1.136, the dielectric constant and diffusive phase coefficient decreases, it was indicated that relaxation behaviors also decreased. When ceramics with a/b was 1.136, the dielectric relaxation γ reached the minimum and electrical properties were poor. The electric properties of ceramics with a/b was 1.00 have an excellent properties, it was indicated that ceramics reached an optimization at the MPB structure.     

A modified two-stage mixed oxide synthetic route to lead zirconate titanate powders

An approach to synthesis lead zirconate titanate [Pb(Zr_1−xTi_x)O₃; PZT] powders with a modified two-stage mixed oxide synthetic route has been developed. To ensure a single-phase perovskite formation, an intermediate phase of zirconium titanate (ZrTiO₄) was employed as starting precursor. The formation of perovskite phase in the calcined PZT powder has been investigated as a function of calcination temperature, soaking time and heating/cooling rates by differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques. The morphology evolution was determined by scanning electron microscopy (SEM) technique. It has been found that the unreacted PbO and ZrTiO₄ phases tend to form together with PZT, with the latter appearing in both tetragonal and rhombohedral phases, depending on calcination conditions. It is seen that optimisation of calcination conditions can lead to a 100% yield of PZT in a tetragonal phase.     

Ga- and Ti-containing Ni4Nb2O9-based solid solutions for Ni-Selective electrodes

Gallium- and titanium-containing niobates with the compositions Ni_{4 ? x} Ga_2/3x Nb₂O₉ and Ni_{4 ? x} Ti _x/2Nb₂O₉ have been synthesized and their physicochemical properties have been studied. Their unitcell parameters have been determined by X-ray diffraction and their electrical conductivity has been measured as a function of temperature. According to thermomechanical analysis data, the niobates undergo no phase transitions. Some of the solid solutions have been used to fabricate Ni-selective electrodes, which have been tested in ion-selective measurements. In a number of cases, satisfactory results have been obtained.     

Effects of vibro-milling time on phase formation and particle size of Zn3Nb2O8 nanopowders

A monoclinic phase of zinc niobate, Zn₃Nb₂O₈, nanopowder was synthesized by a solid-state reaction via a rapid vibro-milling technique. The effect of milling time on the phase formation and particle size of Zn₃Nb₂O₈ powder was investigated. The formation of the Zn₃Nb₂O₈ phase in the calcined powders has been investigated using DTA and XRD techniques. Morphology, particle size and chemical composition have been determined via a combination of SEM and laser diffraction techniques. It was found that an average particle size was achieved at 20 h of vibro-milling after which a higher degree of particle agglomeration was observed on continuation of milling to 30 h. In addition, by employing an appropriate choice of the milling time, a narrow particle size distribution curve was also observed.     

Dielectric and impedance spectroscopic studies of (Sr1−x Pb x )TiO2 glass ceramics with addition of Nb2O5

Glasses were made by melt-quench method in the system [(Sr_1?x Pb _x )O·TiO₂]-[2SiO₂·B₂O₃]-5[K₂O-BaO] (0·0 ≤ x ≤ 0·4) with addition of 1 mol% Nb₂O₅. Perovskite strontium lead titanate in solid solution phase has been crystallized in borosilicate glassy matrix with suitable choice of composition and heat treatment schedule. Addition of 1 mol% of Nb₂O₅ enhances the crystallization of lead strontium titanate phase in the glassy matrix. Scanning electron microscopy (SEM) is performed to study the surface morphology of the crystallites and crystalline interface to the glass. Dielectric properties of these glass ceramics were studied by measuring capacitance and dissipation factor as a function of temperature at a few selected frequencies. Nb₂O₅ doped strontium lead titanate glass ceramic shows a high value of dielectric constant. It is of the order of 10,000 while the dielectric constant of undoped glass ceramic sample is of the order of 500. Complex impedance and modulus spectroscopic techniques were used to find out the contributions of polarization of crystallites and glass crystal interfaces to the resulting dielectric behaviour.     

Effect of excess Pb on formation of perovskite-type 0.67Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-0.33PbTiO<Subscript>3</Subscript> powders synthesized through a sol–gel process

Perovskite-type 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMNT) powders were fabricated by using a sol–gel process. Excess Pb(CH₃COO)₂·3H₂O (0, 2, 5, 10 or 15 mol%) was added to starting materials to compensate PbO loss from volatilization during heat treatment. X-ray diffraction (XRD) was employed to investigate the effect of excess Pb on the perovksite phase formation of the PMNT powders. It was found that the optimal level of the excess Pb content is 5 mol%. When the raw materials contained 5 mol% excess Pb, the PMNT powders of purest perovskite form was obtained at the calcination temperature of 850 °C. In the PMNT powders, most part of the intermediate phase was Pb-rich pyrochlore Pb₂Nb₂O₇ which was transformed into perovskite phase after calcination at 650 °C, while the residual pyrochlore phase was Pb-deficient Pb₃Nb₄O₁₃ which required calcination at a higher temperature (650–850 °C) to transform into perovskite phase. Compared with the conventional solid-state reaction methods and the solution-based methods reported previously, the present sol–gel route is better at synthesizing PMNT powders of perovskite phase at a low temperature.     

Perovskite phase formation in the PbMg1/3Nb2/3 O3-PbZrO3-PbTiO3 system by the columbite route

The reaction mechanism of PbMg_1/3Nb_2/3O₃-PbZrO₃-PbTiO₃ (PMN-PZT) perovskite phase prepared by the columbite route has been studied in the temperature range from 600 to 800 °C. The effects of heating and cooling rate during the calcination of 3PbO +MgNb₂O₆+PZT powder mixtures have also been investigated. Nearly pure perovskite phase, 0.9 PMN-0.1 PZTsolid solution with no pyrochlore phase, as determined by X-ray diffraction, could be prepared at 800 °C for 2 H. From DTA/TGA, dilatometry and XRD data the reaction mechanism of PMN-PZT solid solution formation could be divided into three steps: (i) decomposition of columbite (MgNb₂O₆) by reacting with PbO at 350 to 600 °C (ii) the formation of a B-site-deficient pyrochlore phase Pb₂Nb_1.33Mg_0.17O_5.50 at close to 650 °C, and (iii) the formation of perovskite phase PMN-PZT solid solution from the reaction of Pb₂Nb_1.33Mg_0.17O_5.50 pyrochlore phase with MgO and PZT above 650 °C.     

Influence of laser surface boronizing on the high-temperature oxidation of nickel

Laser nickel boronizing was performed by simultaneous melting of a predeposited boron powder and the nickel substrate and was shown to lead to the formation of a Ni₃B-rich coating. During oxidation, after a period during which B₂O₃ forms and evaporates, a protective scale of Ni₃B₂O₆ appears, inhibiting the formation of NiO.     

The structural transitions during leaching of Ni2Al3 phase in a Raney Ni-Al alloy

The microstructure transitions during leaching of a rapidly solidified Ni-Al alloy have been investigated by means of X-ray diffraction, transmission electron microscopic (TEM) and high resolution electron microscopic (HREM) techniques. Ni₂Al₃ was the main phase in the starting Ni-Al alloy. The microstructure of the Raney nickel catalyst consists of nano-scale nickel crystallites, residue of source phases surrounded by nano-scale boundary regions. The transformation during leaching of Ni₂Al₃ phase was an advancing interface type process. Clusters of AuCu-structure type face-centrered tetragonal Ni₃Al₂ as an intermediate phase seems to appear in the reaction front. Based on an analysis of the atomic configurations of phases Ni₂Al₃, Ni₃Al₂ and nickel, a reasonable explanation for the transition mechanism during leaching of Ni₂Al₃ phase and the arc characteristic of diffraction spots was proposed. The nickel crystallites generated during leaching obey an orientation relationship with the source Ni₂Al₃ phase, which is consistent with the Delannay's orientation relationship proposed for nickel and NiAl phases. The nano-scale structural characteristic of the Raney nickel catalyst, especially its porous structure at the boundary regions, provides an excellent hydrogenation catalytic activity and selectivity of the catalyst.     

New families of cobalt substituted nickel oxyhydroxides and hydroxides obtained by soft chemistry

New cobalt substituted nickel oxyhydroxides and hydroxides have been obtained from NaCo_xNi_1−xO₂ sheet oxides by soft chemistry reactions. These families were studied from the crystalchemical point of view. The Jahn-Teller effect which characterizes nickel (III) in NaNiO₂, disappears for x ≥ 0.20 in the NaCo_xNi_1−xO₂ solid solutions. Hydrolyses in oxidizing conditions allow obtaining γ-type oxyhydroxides in which the average oxidation state of nickel and cobalt is close to 3.5. Reduction reactions lead to α1-type hydroxides, where cobalt is trivalent. The excess of positive charge is compensated by CO²⁻₃ anions which are intercalated in the intersheet space.     

Low-temperature fabrication of BaBi2Nb2O9 ceramics by reaction controlled sintering

Reaction controlled sintering was applied to the fabrication of BaBi₂Nb₂O₉ (BBN) ceramics at lower temperature. A powder mixture of BaCO₃ and Nb₂O₅ was heated at 600 °C in a 1st step calcination to produce a binary precursor of BaNb₂O₆. The pre-heated powder was then mixed with a fixed amount of Bi₂O₃, which was subsequently pressed into a disk pellet. After a powder compact of the mixture was subjected to heating at 950 °C for 4 h, a BBN bulk sample with a relative density of 92% was successfully obtained. The low-temperature fabrication of dense BBN ceramics could be attributed to the inhibited formation of an intermediate phase of Ba₅ Nb₄O₁₅ and the production of submicron powder with an appropriate reactivity during a 1st step calcination.     

Phase transformation and grain growth of doped nanosized titania

The influence of altervalent cation doping of TiO₂ on its phase transition and grain growth has been investigated. It is shown that dopants like Fe³⁺, Si⁴⁺, V⁵⁺, Ru³⁺ and Ni²⁺ affect the phase transition temperature of the titania host, and that significant variation is observed for silicon doping. Iron titanium oxide and nickel titanium oxide phases were found in the iron-doped and nickel-doped system, respectively, at elevated calcination temperatures, while other doped systems only show the modifications of anatase and rutile at the observed range of calcination temperature and dopant content. Compared with the pure TiO₂, grain growth is arrested for the iron-doped and silicon-doped systems, and this tendency is especially distinct in the silicon-doped system.     

Low-temperature sintering and piezoelectric properties of Pb(Fe2/3W1/3)O3–added Pb(Zn1/3Nb2/3)O3–Pb(Ni1/3Nb2/3)O3–Pb(Zr,Ti)O3 ceramics

Low-temperature sintering of (a–x)Pb(Zr_0.48Ti_0.52)O₃–bPb(Ni_1/3Nb_2/3) O₃–cPb(Zn_1/3Nb_2/3)O₃–xPb(Fe_2/3W_1/3)O₃ (a + b + c + x = 1, 0.06 ≤ x ≤ 0.10) ceramics were prepared through two-step synthesis process using perovskites-structured ferroelectric materials Pb(Fe_2/3W_1/3)O₃ (PFW) as a sintering aid. The effects of PFW content on the densification, microstructure, phase structure, dielectric and piezoelectric properties of the ceramics were investigated. The sintering temperature was reduced from 1,180 °C (without PFW addition) to 940 °C when the material was PFW-doped. PFW-doping increased the sintered density and the average grain size of PFW–PNN–PZN–lead zirconate titanate ceramics. The ceramics sintered at 940 °C for 4 h with x = 0.08 exhibited favorable properties, which were listed as follows: d₃₃ = 496pC/N, εT 33/ε₀ = 3,119, tanδ = 2.1 % and Curie temperature = 242 °C. These values indicated that the newly developed composition might be suitable for multilayer piezoelectric devices application.     

Stoichiometric Pb(Mg1/3Nb2/3)O3 perovskite ceramics produced by reaction-sintering process

Stoichiometric lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), perovskite ceramics produced by reaction-sintering process were investigated. Without calcination, a mixture of PbO, Nb₂O₅, and Mg(NO₃)₂ was pressed and sintered directly. Stoichiometric PMN ceramics of 100% perovskite phase were obtained for 1, 2, and 4 h sintering at 1250 and 1270 °C. PMN ceramics with density 8.09 g/cm³ (99.5% of theoretical density 8.13 g/cm³) and K_max 19,900 under 1 kHz were obtained.     

Effects of milling time and calcination condition on phase formation and particle size of lead titanate nanopowders prepared by vibro-milling

Effect of calcination conditions on phase formation and particle size of lead titanate (PbTiO₃) powders synthesized by a solid-state reaction with different vibro-milling times was investigated. Powder samples were characterized using XRD, SEM, TEM and EDX techniques. A combination of the milling time and calcination conditions was found to have a pronounced effect on the phase formation and particle size of the calcined PbTiO₃ powders. The calcination temperature for the formation of single-phase perovskite lead titanate was lower when longer milling times were applied. More importantly, by employing an appropriate choice of the milling time and calcination conditions, perovskite lead titanate (PbTiO₃) nanopowders have been successfully prepared with a simple solid-state reaction method.     

Composition dependent phase connectivity, dielectric and magnetoelectric properties of magnetoelectric composites with Pb(Mg1/3Nb2/3)0.67Ti0.33O3 as piezoelectric phase

Ferrite (Ni_0.6Co_0.4Fe₂O₄) phase, ferroelectric (Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃) phase and magnetoelectric composites of (x)Ni_0.6Co_0.4Fe₂O₄ + (1 − x)Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ with x = 0.15, 0.30 and 0.45 were prepared using solid-state reaction technique. Presence of Ni_0.6Co_0.4Fe₂O₄ and Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ was confirmed using X-ray diffraction technique. The scanning electron microscopic images were used to study the microstructure of the composites. Connectivity scheme present in the magnetoelectric (ME) composites are discussed from the microscopic images. Variation of dielectric constant and dielectric loss with temperature for all the composites was studied. Here we report the effect of Ni_0.6Co_0.4Fe₂O₄ mole fraction on connectivity schemes between Ni_0.6Co_0.4Fe₂O₄ and Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ composite. The variation of magnetoelectric voltage coefficient with dc magnetic field shows peak behaviour. The maximum value of magnetoelectric voltage coefficient of 9.47 mV/cm Oe was obtained for 0.15Ni_0.6Co_0.4Fe₂O₄ + 0.85Pb(Mg_1/3Nb_2/3)_0.67Ti_0.33O₃ composites. Finally we have co-related the effect of Ni_0.6Co_0.4Fe₂O₄ content and dielectric properties on magnetoelectric voltage coefficient.