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.     

Synthesis of Pb(Mg1/3Nb2/3) O3 perovskite by an alkoxide method

Pb(Mg_1/3Nb_2/3)O₃ materials have been synthesized using sol-gel, freeze-drying or spray-pyrolysis techniques. The as-prepared powders were of an amorphous form which could be converted into a crystalline form by calcination. The pyrochlore phase was inevitably formed with an accompanying perovskite phase. As the calcining temperature increased, greater proportions of the desired perovskite phase occurred. The residual pyrochlore phase could be completely transformed into the perovskite phase when the powders were prepared via freeze-drying or by a spray-pyrolysis method. The maximum proportion of the pyrochlore phase was, however, only 92% when the powders were synthesized by a sol-gel route. Thermal gravimetric analysis/differential thermal analysis (TGA/DTA) and infrared transmission spectroscopy (FTIR) indicated that Mg(OEt)₂ and Nb(OEt)₅ formed a double alkoxide but Pb(OAc)₂ formed separate clusters during the hydrolysis of the solution in the sol-gel process. Inhomogeneous mixing meant that the intermediate phase formed was rather difficult to eliminate completely. Homogeneous mixing was preserved when the solution was directly freeze dried or spray pyrolysed. The size of the preferentially formed pyrochlore phase was very fine and further transformation was feasible. Pb(Mg_1/3Nb_2/3)O₃ materials, free of the pyrochlore phase, could therefore be obtained.     

Preparation of Mg-modified Pb[Zn1/3(Ta0.4Nb0.6)2/3]O3 ceramics and dielectric characteristics

Mg (full composition range) and Nb (60 at %) were simultaneously substituted into Pb(Zn_1/3Ta_2/3)O₃, in an attempt to stabilize the perovskite structure. System powders were prepared using a B-site precursor method in order to enhance perovskite formation. The developed structures were examined by X-ray diffraction, from which perovskite phase yields as well as lattice parameters of the pyrochlore and perovskite were determined. Low-frequency dielectric responses of the ceramics were investigated. Phase transition modes (reflected in the dielectric constant spectra) were further analyzed in terms of diffuseness parameters.     

Effect of surfactants on preparation of nanoscale α-Al2O3 powders by oil-in-water microemulsion

The nanoscale α-Al₂O₃ powders have been synthesized by the O/W microemulsion system using cyclohexane as the oil phase, ultrapure water as the water phase, OP-10 and alcohol as the surfactant and co-surfactant. It has been found that the nature of surfactants played an important role to regulate the size and morphologies of the α-Al₂O₃ nanoparticles. Three different nonionic surfactants (OP-10, Triton X-100 and Tween-80) have been used for the preparation of microemulsions. The microemulsions were characterized by Zeta Potential Analyzer. The results showed that the OP-10 system possesses wide and stable microemulsion phase regions. The synthesized α- Al₂O₃ powders have been comprehensively characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD). The powders calcined at 900–1150 °C showed the presence of alumina phase with crystal structure, The SEM images showed that the powders was an average diameter of 30–100 nm.     

Mechanochemical synthesis and characterization of nanocrystalline 0.4Bi(Zn1/2Ti1/2)O3-0.6PbTiO3 powders

Perovskite 0.4Bi(Zn_1/2Ti_1/2)O₃-0.6PbTiO₃ (BZT-PT) powders were successfully synthesized from precursor oxides using a high-energy planetary ball milling. The phase development of the powders during milling was studied by means of X-ray diffraction and Raman scattering techniques. The microstructure of the powders was characterized using transmission electron microscopy, and the thermal behavior was studied as well. The results reveal that after 15 h of milling the formation of BZT-PT phase can be completed and submicron agglomerates of small crystallite sized ~ 12 nm are present in the powders. However, further prolonging the milling time to 25 h leads to the amorphization of the BZT-PT phase.     

Synthesis of MgNb2O6 by coprecipitation

A simple coprecipitation technique was used successfully to synthesize fine powders of MgNb₂O₆ (MN) phase. An aqueous mixture of ammonium carbonate and ammonium hydroxide was used to precipitate Mg²⁺ and Nb⁵⁺ cations as carbonate and hydroxide respectively under basic conditions. This precipitate on heating at 750 °C produced MN powders. For comparison MN powders were prepared by the traditional solid state method. The phase content and the lattice parameters were studied by powder X-ray diffraction (XRD). Particle size and morphology of the particles were studied by scanning electron microscopy (SEM).     

Crystalline – Amorphous AA4048 – Al60Nb40 composites processed by mechanical alloying and powder compaction

Commercial crystalline AA4048 powders and mechanically alloyed amorphous Al₆₀Nb₄₀ (at.%) powders were used for fabrication of crystalline-amorphous composites containing 10, 20 and 30?vol% of amorphous phase. High pressure high temperature technique was used for powders compaction. The applied pressure was 7.7?GPa and temperature was in the range 600–1000?°C. The powders and bulk samples were characterized by structural investigations (X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, X-ray fluorescence spectroscopy), hardness and microhardness tests and measurements of density. The obtained sinters revealed relative density above 98.7%. The amorphous component was observed as agglomerates or as single particles surrounding the grains of crystalline AA4048 phase. The crystallization of amorphous phase was not observed. Simultaneously, up to 2.5?wt% of Fe was detected as impurity from milling media. Significant increase of hardness was observed, from 226 to 288HB, resulting from the presence of amorphous component which prevent from cracks propagation during deformation.     

A coprecipitation technique to prepare ZnNb2O6 powders

A simple coprecipitation technique was successfully applied for the preparation of pure ultrafine single phase, ZnNb₂O₆ (ZN). Ammonium hydroxide was used to precipitate Zn²⁺ and Nb⁵⁺ cations as hydroxides simultaneously. This precursor on heating at 750°, produced ZN powders. For comparison, ZN powders were also prepared by the traditional solid state method. The phase contents and lattice parameters were studied by the powder X-ray diffraction (XRD). Particle size and morphology were studied by transmission electron spectroscopy (TEM).     

Pb(Zr0.95Ti0.05)O3 powders prepared by aqueous Pechini method using one-step pyrolysis process: characterization and porous ceramics

Zr-riched lead zirconate titanate, Pb(Zr_0.95Ti_0.05)O₃ (PZT 95/5) powders were prepared using lead acetate, zirconium oxynitrate, and titanium sulfate by aqueous Pechini method. The chelation behaviors of metallic ions and citric acid were investigated and the development of the phase formation of perovskite structure was detected. PZT 95/5 powders were obtained directly from the as-synthesized gels by one-step pyrolysis process at 450 °C for 10 h. Perovskite phase was formed at about 450 °C and no distinct intermediates were obtained. There were some carbonates as impurities but they did not affect the formation of the complete perovskite phase of PZT 95/5 ceramics after sintering at 1,100–1,150 °C for 2 h. The decomposition of few organic residues among the one-step pyrolyzed powders could form uniform porous structure and the formation mechanism of porous ceramics was also presented.     

Sol-gel synthesis and properties of doped lanthanum chromite nanopowders

La_{1 ? x} Sr _x CrO₃ (0.2 ≤ x ≤ 0.3) powders have been prepared by a sol-gel process and dried in a microwave oven. Synthesis conditions were optimized using differential scanning calorimetry and mass spectrometry data. The phase composition of the synthesis products obtained at temperatures of 800, 850, 900, and 950°C was determined by X-ray diffraction. Scanning electron microscopy was used to determine the particle size of the powders and examine their morphology.     

Rapid formation of lead magnesium niobate-based ferroelectric ceramics via a high-energy ball milling process

Pyrochlore-free nano-sized 0.90Pb(Mg_1/3Nb_2/3)O₃(PMN)-0.10PbTiO₃(PT) and 0.65PMN-0.35PT powders were synthesized from oxides via a high-energy ball milling process. Single perovskite phase PMN-PT were readily formed from the oxide mixture after milling for only 2 h. The grain size calculated from X-ray diffraction (XRD) patterns of all samples is about 20 nm, which is in agreement with the observation from scanning electron microscopy (SEM) (20-50 nm). PMN-PT ceramics were obtained by sintering the milled powders at temperature from 1000 to 1100°C for 2 h. The dielectric, ferroelectric properties of the PMN-PT ceramics derived from the synthesized powders were comparable with the reported results in the literature.     

Phase developments in Pb(Mg<Subscript>1/2</Subscript>W<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> and Pb(Zn<Subscript>1/2</Subscript>W<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> via B-site precursor route

Phase formation stages of MgWO₄ and ZnWO₄ (precursor compositions for following steps) were investigated by monitoring the reactions of oxide chemicals at various temperatures. Developed phases were examined by using X-ray diffraction (XRD). Successive attempts were also conducted for Pb(Mg_1/2W_1/2)O₃ (PMW) and Pb(Zn_1/2W_1/2)O₃ (PZW) by reacting PbO with the precursor compounds. Stages of phase development in the two compositions were also analyzed. The results are compared with those of another tungsten-containing perovskite Pb(Fe_2/3W_1/3)O₃ (PFW) and its B-site precursor Fe₂WO₆. After PbO addition to the precursor powders, a perovskite phase formed directly (i.e., without any intermediate phases) in the case of PMW. For PbO + ½ZnWO₄, in contrast, the decomposition of ZnWO₄ and preferential reaction with PbO resulted in Pb₂WO₅ and ZnO, instead of the perovskite PZW.     

Perovskite formation and dielectric responses in Pb(Zn1/3Nb2/3)O3-modified Pb(Zn1/3Ta2/3)O3-PbTiO3 ceramics

Pb(Zn_1/3Ta_2/3)O₃-PbTiO₃ ceramic compositions were modified by the introduction of Nb to the octahedral lattice sites. Resultant tendencies in the perovskite formation and dielectric properties were examined. System powders were prepared using a B-site precursor method. Developed structures and lattice parameters of the system compositions were investigated by powder X-ray diffractometry, from which the parameter of a hypothetical perovskite Pb(Zn_1/3Ta_2/3)O₃ is proposed. Weak-field low-frequency dielectric responses of the system ceramics were measured.     

Characterization and electrical properties of sol-gel synthesized (Sr, Pb)TiO3 powders

Y-doped (Sr, Pb)TiO₃ powders were prepared by a sol-gel route as well as the calcination of gel precursors. The results of DTA/TG showed that the thermal decomposition of dry precursors mainly occurred below 600°C. Meanwhile, infrared ray (IR) spectrum meter, X-ray diffraction (XRD) meter and transmission electron microscope (TEM) were used to characterize the synthesized powders, respectively. Using the synthesized powders as starting materials, Sr_0.5Pb_0.5TiO₃ semiconducting ceramics were fabricated at 1050°C. Sample's room temperature resistivity is 1.51 × 10² · cm, its resistivity jumps more than 5 orders of magnitude above the Curie temperature (T _c). With increasing the soaking time, the room temperature resistivity and the negative temperature coefficient of resistance (NTCR) effect below T _c increased, showing the electrical properties of (Sr, Pb)TiO₃ thermistors are obviously affected by PbO loss.     

Na0.5K0.5NbO3 and 0.9Na0.5K0.5NbO3–0.1Bi0.5Na0.5TiO3 nanocrystalline powders synthesized by low-temperature solid-state reaction

Nanocrystalline powders of K_0.5Na_0.5NbO₃ (KNN) and 0.9Na_0.5K_0.5NbO₃–0.1Bi_0.5Na_0.5TiO₃ (KNN–BNT) have been prepared using a low-temperature solid-state reaction. Phase development of the powders incurred during various calcination temperatures was examined by X-ray diffraction (XRD). Crystallite size and particle morphology of KNN powders were examined by XRD and transmission electron microscopy, respectively. Perovskite phase was formed at the temperature as low as 500 °C, and the average crystallite size of KNN powders depended on calcination temperature. In addition, the crystalline structure of KNN powders tended to change from tetragonal symmetry to orthorhombic symmetry with increase in crystallite size. Similar results were obtained in KNN–BNT system. The developed method is well suited for the mass production of niobate nanocrystalline powders due to its simplicity and low cost.     

A novel route for the low temperature synthesis of p-type transparent semiconducting CuAlO2

CuAlO₂ is an excellent p-type semiconductor material which usually needs very high temperature for its preparation. In this letter we report a novel synthesis procedure of p-type CuAlO₂ powders through a very convenient chemical process at temperature much lower than needed for conventional solid state reaction method. CuAlO₂ powders were prepared using Cu₂O/CuO and Al₂O₃ powders of appropriate amounts as Cu and Al sources in molten NaOH at 360 °C. The prepared CuAlO₂ powder was phase pure and was characterized by studying X-ray diffraction, Fourier transformed infrared spectroscopy, field emission scanning electron microscopy and X-ray photoelectron spectroscopy.     

Studies of Combustion Mechanochemical Reactions in Off-Stoichiometric Powder Mixtures of Fe1 − x S1 + x

The reaction between iron and sulfur powders induced by high-energy ball milling was studied as a function of milling time and the composition of starting powders Fe(1 – x) + S(1 + x), where x = –0.03, 0, 0.03, 0.05, 0.10. The reaction products were studied by using the Mössbauer spectra and X-ray diffraction analysis. The analyzed process is self-sustaining and runs for a period of time of 2–5 hours of milling. The iron powder used in this case was characterized by a particle size of 1600 mesh. The Mössbauer spectra obtained at room temperature after 2 hours of milling revealed partial sulfiding missed in the course of X-ray diffraction analysis. In all other samples, we observed a sextet (for an internal magnetic field of 21 MA/m) and a doublet (with insignificant deviations from the corresponding parameters for FeS₂). This confirms the results of X-ray phase diffraction analysis according to which FeS is the sole significant phase.     

Al–Cu–Fe coatings manufactured by the flame spraying process

Two-hour milled (activated) Al–20Cu–15Fe (at%) powders were subjected to annealing at 600°C for 1?h. The phase and microstructural evolutions were characterized by X-ray diffractometry and scanning electron microscopy. Al₇Cu₂Fe and Al₆₀Cu₃₀Fe₁₀ phases were formed after annealing. Both annealed and milled powders were consolidated using the flame spraying process. In both cases, FeAl(Cu) was the major phase while some oxides and α-Fe(Al,Cu) were also found. The coating produced from the milled powder was severely cracked and showed higher oxide content. The coating prepared from the annealed powder showed better quality. It was annealed at 600°C for 1?h to investigate the thermal stability of the various phases. All phases persevered after annealing while the Fe₂Al₅ phase was formed as a new phase.     

Production of Fe3Al–TiC nanocomposite by mechanical alloying of FeTi2–Al–C powders

Abstract

The aim of the present work was to produce Fe₃Al/TiC nanocomposite by mechanical alloying of the FeTi₂30Al10C60 (in at-%) powder mixture. The morphology and the phase transformations in the powder during milling were examined as a function of milling time. The phase constituents of the product were evaluated by X-ray diffraction (XRD). The morphological evolution during mechanical alloying was analysed using scanning electron microscopy (SEM). The results obtained show that high energy ball milling, as performed in the present work, leads to the formation of a bcc phase identified as Fe(Al) solid solution and an fcc phase identified as TiC and that both phases are nanocrystalline. Subsequently, the milled powders were sintered at 873 K. The XRD investigations of the powders revealed that after sintering, the material remained nanocrystalline and that there were no phase changes, except for the ordering of Fe(Al), i.e. formation of Fe₃Al intermetallic compound, during the sintering process.     

Structural investigations on TiO2 and Fe-doped TiO2 nanoparticles synthesized by laser pyrolysis

Undoped and Fe-doped TiO₂ nanopowders with Fe/Ti (atomic ratio) precursor concentration ranging from 7% up to 25% have been prepared by the IR laser pyrolysis technique. A sensitized mixture of TiCl₄ and Fe (CO)₅ was used as titanium and iron precursor, respectively. Reference undoped titania samples with a major concentration of anatase phase (about 90%) were obtained by the same technique by using very high flows of the oxidizing agent (air). The effects of the iron-dopant concentration on the essential structural properties of the resultant powders such as the phase formation, the crystallinity, the average particle size and distributions were systematically investigated by X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The decrease of the TiO₂-anatase crystalline phase, the simultaneous increase of the amorphous phase and the decrease in size of particle mean diameter appear as main effects induced by the Fe-dopant concentration.