Barium Holmium Zirconate, A New Complex Perovskite Oxide: I, Synthesis, Characterization, and Potential Use as a Substrate for High-Critical-Temperature Superconductors

Barium holmium zirconate, a new complex perovskite ceramic oxide, has been synthesized through liquid-phase sintering for the first time. The conventional solid-state reaction method using constituent oxides and carbonates was found to be inadequate for the synthesis of Ba₂HoZrO_5.5 material. During high-temperature annealing, the development of stable BaZrO₃ and BaHoO_2.5 phases prevented the formation of Ba₂HoZrO_5.5 as a single-phase material, even at 1650°C. However, an addition of a small amount of CuO (1 wt%) in the reaction mixture has resulted in the formation of an ordered complex perovskite Ba₂HoZrO_5.5 phase during the heating process. The structure of Ba₂HoZrO_5.5 was studied by X-ray diffraction and found to have a cubic perovskite structure with a lattice constant of a = 8.482 Å. Dielectric constant and loss factor values of Ba₂HoZrO_5.5 are also in the range suitable for use as a substrate for microwave applications. The X-ray diffraction and resistivity measurements have shown that there is no detectable chemical reaction in YBa₂Cu₃O_7−δ–Ba₂HoZrO_5.5 and Bi(2212)–Ba₂HoZrO_5.5 composites, even under extreme processing conditions. Dip-coated and melt-textured YBa₂Cu₃O_7−δ and Bi(2212) thick films developed on polycrystalline Ba₂HoZrO_5.5 gave zero-resistivity transition temperatures of T _c(0) = 92 and 85 K, respectively.     

Crystallization of Sol–Gel-Derived Barium Strontium Titanate Thin Films

Microstructural development of thin-film barium strontium titanate (Ba _x Sr_{1– x} TiO₃) as a function of strontium concentration and thermal treatment were studied, using transmission electron microscopy (TEM) and X-ray diffractometry (XRD). Thin films, ∼250 nm thick, were spin-coated onto Pt/Ti/SiO₂/Si substrates, using methoxypropoxide alkoxide precursors, and crystallized by heat-treating at 700°C. All films had the cubic perovskite structure, and their lattice parameters varied linearly with strontium content. Films with higher strontium concentrations had a larger average grain size. In situ TEM heating experiments, combined with differential thermal analysis/thermogravimetric analysis results, suggest that the gel films crystallize as an intermediate carbonate phase, Ba _x Sr_{1– x} TiO₂CO₃ (with a solid solution range from x = 1 to x = 0). Before decomposition at 600°C, this carbonate phase inhibits the formation of the desired perovskite phase.     

Synthesis of Highly Dispersed Barium Titanate Nanoparticles by a Novel Solvothermal Method

A novel solvothermal route has been developed to synthesize highly dispersed nanocrystalline barium titanate (BaTiO₃), using a mixture of ethylenediamine and ethanolamine as a solvent. The as-synthesized BaTiO₃ nanoparticles were characterized by X-ray powder diffraction, transmission electron microscopy (TEM), high-resolution TEM, Fourier transform infrared spectroscopy, and thermal analysis. Based on the results of characterizations, the organic solvent was found to influence strongly the crystal growth and dispersibility of BaTiO₃. The BaTiO₃ nanoparticles obtained were highly dispersed and crystalline with a cubic perovskite structure. The particle size derived from the TEM ranged from 5 to 20 nm.     

Nanosized Barium Titanate Powder by Mechanical Activation

Mechanical activation, without any additional heat treatment, is used to trigger the formation of a perovskite BaTiO₃ phase in an oxide matrix that consists of BaO and TiO₂ in a nitrogen atmosphere. The resulting BaTiO₃ powder exhibits a well-established nanocrystalline structure, as indicated by phase analysis using X-ray diffractometry. A crystallite size of ∼14 nm is calculated, based on the half-width of the BaTiO₃ (110) peak, using the Scherrer equation, and an average particle size of 20–30 nm is observed using transmission electron microscopy for the activation-derived BaTiO₃ powder.     

Barium Rare-Earth Hafnates: Synthesis, Characterization, and Potential Use as Substrates for YBa2Cu3O7-delta Superconductor

A new group of complex perovskites Ba₂REHfO_5.5 (where RE = La, Pr, Nd, and Eu) has been synthesized and sintered as single-phase materials with high sintered density and stability using a solid-state reaction method for the first time. The structure of Ba₂REHfO_5.5 has been studied by X-ray diffactometry (XRD) and all of the perovskites are isostructural and have a cubic structure. The dielectric constant and loss factor values of these materials are in a range suitable for their use as substrates for YBa₂Cu₃O_7-delta superconductors. XRD and resistivity measurements show that there is no detectable reaction between YBa₂Cu₃O_7-delta and Ba₂REHfO_5.5, even when the two substances are mixed thoroughly and sintered at 950°C for 15 h. The addition of Ba₂REHfO_5.5 up to 20 vol% in YBa₂Cu₃O_7-delta-Ba₂REHfO_5.5 composite shows no detrimental effect on the superconducting transition temperature of YBa₂Cu₃O_7-delta. Thick films of YBa₂Cu₃O_7-delta fabricated on polycrystalline Ba₂REHfO_5.5 substrate have a superconducting zero resistivity transition of 92 K, indicating the suitability of these new materials as substrates for YBa₂Cu₃O_7-delta films.     

Microstructural Changes in Lanthanum-Doped Barium Magnesium Niobate

Observations of microstructural changes in (Ba_0.95La_0.05)-(Mg_0.35Nb_0.65)O₃ and (Ba_0.925La_0.075)(Mg_0.36Nb_0.64)O₃ (BLMN) were carried out using high-resolution transmission electron microscopy (HRTEM) and synchrotron powder X-ray diffractometry (XRD). In both samples, not only 1:1 and 1:2 ordered domains coexisted in a single grain, but also the intermediate phase, whose structure had a superlattice modulation of 1.42 nm, which was equivalent to 6 times the unit cube of disordered perovskite found on the nanoscale. The ordered 1:2 domains gradually transformed to 1:1 ordered structure through the formation of an intermediate superlattice structure that comprised 6 × 6 × 6 cubic unit cells with different chemical orderings of B-site ions in B-site lattices. Also, the features of thin plates could be detected by XRD patterns and HRTEM. When the thicknesses were very thin, about several atomic distances, stacking faults occurred on (111) planes. However, when their thicknesses were >50 nm, the thin plates existed as a transition phase with their own structure. They were coherent with the matrix and continuously decomposed into the matrix phase by the lateral migration of the interfaces.     

ZrO2 Nanopowders Prepared by Low-Temperature Vapor-Phase Hydrolysis

ZrO₂ powder is prepared by low-temperature vapor-phase hydrolysis of ZrCl₄. TG-DTA, XRD, Raman, BET, and TEM methods are used to investigate the particle size, phase composition, and agglomeration before and after heat treatment. The results show that the as-prepared ZrO₂ powder is characterized by large surface area (150 m²/g), fine grain size (5.8 nm), and weak agglomeration. Additionally, the as-prepared ZrO₂ powder shows predominantly tetragonal phase attributed to a grain size effect. This route is free of powder drying and calcination processes that are essential for wet chemical preparation, contributing to less agglomeration.     

Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by pyrolysis of complex compounds of aluminum and magnesium with triethanolamine (TEA). The soluble metal ion–TEA complexes formed the precursor material on complete dehydration of the complexes of aluminum–TEA and magnesium–TEA. Single-phase MgAl₂O₄ spinel powder resulted after heat treatment of the precursor material at 675°C. The precursor and the heat-treated powders were characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis, and transmission electron microscopy (TEM). The average crystallite size as measured from the X-ray line broadening was around 14 nm and the average particle size from TEM studies was around 20 nm.     

Structural and Dielectric Characterization of Nanocrystalline (Ba, Pb)ZrO3 Developed by Reverse Micellar Synthesis

Nanocrystalline zirconates of barium and lead have been synthesized using a modified reverse micellar route (avoiding alkoxides). The entire solid solution of Ba_{1− x} Pb _x ZrO₃ (0≤x≤1) has been synthesized for the first time. Powder X-ray diffraction studies show the monophasic nature of the powders after heating at 800°C except minor impurities of ZrO₂ (2%–3%) at a higher lead content ( x =0.50 and 0.75). The oxides crystallize in the cubic structure till x =0.25; for higher values, they crystallize in the orthorhombic structure. The particle size obtained from X-ray line-broadening studies and transmission electron microscopic studies is found to be in the range of 20–60 nm for all the oxides obtained after heating at 800^oC. The grain size of the solid solution of Ba_{1− x} Pb _x ZrO₃ (0≤ x ≤1) was found to increase with the lead content. The dielectric constant of the solids corresponding to Ba_{1− x} Pb _x ZrO₃ (0≤ x ≤1) was found to be a maximum at x =0.50. Note that the cubic to orthorhombic transition is also observed between x =0.25 and 0.5. Dielectric properties with respect to variation in frequency and temperature are reported for these nanocrystalline oxides for the first time.     

Hydrothermal Synthesis of Nanosize alpha-Al2O3 from Seeded Aluminum Hydroxide

alpha-Alumina and boehmite particles were synthesized by coprecipitation followed by a hydrothermal treatment. X-ray diffraction (XRD) indicated that alpha-Al₂O₃ was the major phase and coexisted with 4% of boehmite in the presence of the alpha-Al₂O₃ seeds. On the other hand, a single boehmite phase was obtained in the absence of the alpha-Al₂O₃ seed particles. The powder densified in the temperature range from 1050° to 1350°C. High-resolution transmission electron microscopy (HRTEM) showed that the particle size of the synthesized alpha-Al₂O₃ was 60 nm. The surface area was 245 m²/g.     

Synthesis of Nanosize Tin Dioxide by a Novel Liquid-Phase Process

Well-dispersed tin(IV) oxide (SnO₂) nanosize crystallites were prepared by a novel liquid-phase process using Na₂SnO₃ precursor of several micrometers in size. The product was characterized using powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and BET. The nanosize SnO₂ powder formed has a narrow particle size distribution ranging between 1 and 7 nm, a very high surface area, and crystallizes in the tetragonal structure. A possible mechanism for the liquid-phase process, which is a combined effect of coprecipitation, hydrolization, and molecular decomposition, is proposed.     

Finite Size Effect on the Sinterability and Dielectric Properties of ZnNb2O6–ZBS Glass Composites

ZnNb₂O₆ (ZN) is a columbite-structured niobate compound showing excellent dielectric properties and comparatively low sintering temperatures (∼1200°C). Hence it is a good candidate for possible low-temperature cofired ceramics (LTCC) applications. In the present investigation, ZnNb₂O₆ was synthesized in the form of micrometer-sized powder using a conventional solid-state ceramic synthesis route as well as in the form of nanosized powder by a polymer complex method. The finite size effect of ZN particles on sinterability and microwave dielectric properties of sintered pellets was evaluated. The phase formation was confirmed from the X-ray diffraction (XRD) analysis. The particle size distribution of the nanoparticles was found to be of the order of 18–20 nm by using high-resolution transmission electron microscopy analysis and 30 nm by analyzing the XRD patterns using Debye Scherrer's formula, after correcting for the instrument broadening effects. A ZN–60ZnO–30B₂O₃–10SiO₂ (ZBS) composite was made by adding predetermined amounts of glasses. The microstructures of the sintered pellets of ZN and ZN–ZBS composites were examined using scanning electron microscopy and analyzed using image analysis. The nano-ZN–ZBS composites were sintered to 93% of the reported density at 925°C/2 h, with microwave dielectric properties of ɛ_r=22.5, Q × f ∼12 800 GHz, and τ_f=−69.6 ppm/°C, emerging as a potential material for possible LTCC applications.     

Microwave Plasma Synthesis of Nanostructured γ-Al2O3 Powders

Nanostructured Al₂O₃ powders have been synthesized by combustion of aluminum powder in a microwave oxygen plasma, and characterized by X-ray diffraction and electron microscopy. The main phase is γ-Al₂O₃, with a small amount of δ-Al₂O₃. The particles are truncated octahedral in shape, with mean particle sizes of 21–24 nm. The effect of reaction chamber pressure on the phase composition and the particle size was studied. The γ-alumina content increases and the mean particle size decreases with decreasing pressure. No α-Al₂O₃ appears in the final particles. Electron microscopy studies find that a particle may contain more than one phase.     

Nano α-Al2O3 Powder Preparation by Calcining an Emulsion Precursor

An experimental study has been conducted to evaluate the formation of nano α-Al₂O₃ under various conditions, such as different calcining temperatures and emulsion ratios of aqueous aluminum nitrate solutions and oleic acid with a high-speed stirring mixer. Four batches of the precursor powders were calcined at three different temperatures of 1000°, 1050°, and 1100°C for 2 h and a terminal product of nano α-Al₂O₃ powders was obtained. The products have been identified by X-ray diffraction (XRD), specific surface area measurement scanning electron microscope, and transmission electron microscope (TEM). The XRD results show that the phase of powders is determined to be α-Al₂O₃, indicating that the overall process has been effective. The optimum calcination temperature of the precursor powder for crystallization of nano α-Al₂O₃ was found to be 1000°C for 2 h. The TEM image indicates that the particle grains have a sub-spherical shape with a mean size of 50–100 nm.     

Novel Low-Temperature Synthesis of Ferroelectric Neodymium-Doped Bismuth Titanate Nanoparticles

In this study, we report on the synthesis of nanopowders of ferroelectric Bi_3.5Nd_0.5Ti₃O₁₂ ceramic at temperatures below 500°C via a simple chemical method using citric acid as a solvent. The calcined powders were characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). Heating the as-dried powders in air first leads to crystallization of the Bi₂Ti₂O₇ phase at ∼310°C, followed by crystallization of the perovskite Nd-doped Bi₄Ti₃O₁₂ phase at ∼490°C as suggested by the peaks in the DSC analysis and confirmed by the evolution of phases in XRD patterns of the powders calcined at various temperatures. TEM of particles calcined at 550°C for 1 h in air showed an average particle size of 50–60 nm. The temperature dependence of capacitance of nanopowders calcined at 700°C for 1 h in air showed a Curie temperature of ∼615°C evincing a ferroelectric transition.     

Er-Doped Y2O3 Nanoparticles: A Comparison of Different Synthesis Methods

Nanoparticles of erbium-doped yttria (Er:Y₂O₃) are important precursors to transparent ceramics for high-power solid-state lasers systems. As structure influences properties and, subsequently, performance the purpose of this work is to compare the morphological and chemical nature of the nanoparticles synthesized using two common methods: solution precipitation and combustion synthesis. A thorough characterization of as-prepared and calcined powders was performed using Fourier transform infrared spectroscopy, X-ray diffraction, conventional and high-resolution transmission electron microscopy, and Brunauer–Emmet–Teller methods. Solution precipitation was found to lead to two different precursor compositions (yttrium carbonate or yttrium hydroxide) depending on the precipitating reagent whereas combustion synthesis yielded only phase-pure, cubic Er:Y₂O₃. The hydroxide precipitation and combustion synthesis methods exhibited agglomerated particles with low surface area after calcining the precursors at 900°C. The addition of a small amount of ammonium sulfate during combustion synthesis was found to reduce the level of agglomeration, resultant particle size, and degree of crystallinity of the calcined Er:Y₂O₃ nanoparticles. The amount of carbon dioxide (CO₂) and water (H₂O) on the surface of the Er:Y₂O₃ powders is dependent on the powder surface area, however, increasing levels of gas absorption on the particle surfaces do not have a detrimental effect on the sinterability. The sintered density increases with increasing surface area and decreasing agglomeration.     

Synthesizing Nanocrystalline Pb(Zn1/3Nb2/3)O3 Powders from Mixed Oxides

The attempt to synthesize a Pb(Zn_1/3Nb_2/3)O₃ (PZN) powder of perovskite structure via both traditional ceramic and chemistry-based novel processing routes over the last three decades has failed. Difficult-to-synthesize nanocrystallite PZN powders have, for the first time, been successfully prepared via a mechanochemical reaction either among PbO, ZnO, and Nb₂O₅ or between PbO and pre-reacted ZnNb₂O₆ from ZnO and Nb₂O₅ for more than 15 h in a high-energy mechanochemical reaction chamber. The resulting PZN powders exhibit a well-established perovskite structure and their crystallite sizes are in the range of 10 to 15 nm, as has been indicated from the peak broadening of X-ray diffraction and direct observation using a high-resolution transmission electron microscope.     

Nanosized PbZrO3 Powder from Oxalate Precursor: Microwave-Aided Synthesis and Thermal Characterization

Nanosized lead zirconate (PbZrO₃) powder was synthesized from its oxalate precursor, namely lead zirconyl oxalate (LZO). LZO heated in a microwave heating system for 1 h yielded the PbZrO₃ at 600°C. The same precursor (LZO), when heated in a resistance-heated furnace at 850°C for 3 h, does not give a pure product. Thermogravimetry, differential thermal analysis, and X-ray diffraction techniques were used to characterize the precursor and optimize the conditions for microwave processing. The particle size of PbZrO₃ powder prepared at 600°C using microwave heating was measured using transmission electron microscopy (TEM). The TEM images show that the particles of PbZrO₃ are spherical in shape and that the particle size varies between 20 and 22 nm.     

Effect of Tween-80 on the Control of Particle Size and Shrinkage Properties of Nanoscale α-Alumina Synthesized by Sol–Gel Processing

The concept of tailored interfaces has been applied to the synthesis of nanoscale α-Al₂O₃. Tween-80 (poly-oxyethylene(20) sorbitan monooleate, T-80) was used as a surface modifier in the sol–gel process for this purpose. High-resolution transmission electron microscopy study of the powder obtained with T-80 confirmed the particle size of α-Al₂O₃ (∼55 nm) and morphology (spherical). The exothermic peak temperature in the differential thermal analysis was shifted to a lower temperature (∼917°C) when the powder was derived from a T-80 modifier content of 10 wt%. X-ray diffraction showed that the α-Al₂O₃ phase was the major phase that existed in modifier-derived powder that was sintered at 1000°C. The experiments, based on linear shrinkage, indicated that the powder with T-80 (10 wt%) could be densified at a low temperature.     

Mechano-Synthesis of Lead–Magnesium–Niobate Ceramics

The synthesis of Pb(Mg_1/3Nb_2/3)O₃ (PMN) with high-energy milling was studied by X-ray powder diffraction (XRD) using the Rietveld-refinement method. The results are discussed in terms of the qualitative and quantitative composition of the crystalline and amorphous phases as a function of milling time. The mechano-synthesis of PbO, Nb₂O₅, and MgO leads to the formation of perovskite PMN. In the initial stage of milling, particle size reduction and a high degree of amorphization were observed, together with the simultaneous formation of perovskite and pyrochlore-type structures. A mechanism for the formation of PMN by the mechano-synthesis route is proposed.