Topochemical Synthesis of a High-Aspect-Ratio Platelet NaNbO3 Template

A high-aspect-ratio platelet sodium niobate (NaNbO₃) was synthesized by a topochemical reaction. Plate-like layered-perovskite Bi_2.5Na_3.5Nb₅O₁₈ (BNN5) as a precursor was first prepared, and then Bi³⁺ in the precursor phase was replaced by Na⁺ through topotactic conversion and perovskite NaNbO₃ formed at 950°C in NaCl molten salt. NaNbO₃ crystals had an average length of 15 μm and a thickness of 0.5 μm. Scanning electron microscope and high-resolution transmission electron microscope analyses indicated that the retro-synthesis process of the desired perovskite NaNbO₃ retained the morphological and structural features of the BNN5 precursor. It may be possible to synthesize other perovskite-structured templates using the same method.     

Low-Temperature Synthesis of Ba(Mg1/3Ta2/3)O3 Ceramics from Ba-Mg-Ta Alkoxide Precursor

BMT perovskite ceramics were synthesized at low temperature (∼100°C) by chemical processing comprised of the hydrolysis of Ba-Mg-Ta alkoxide precursor. FT-IR and Raman spectroscopic studies of the Ba-Mg-Ta ethoxide precursor reveal that barium, magnesium, and tantalum ethoxides are bonded with each other via alkoxy bridging to form a trimetallic alkoxide. The hydrolysis of the Ba-Mg-Ta ethoxide with a large amount of water under refluxing results in the direct formation of a crystalline BMT phase with the pseudocubic perovskite structure at a low temperature of ∼100°C. The alkoxide-derived BMT powder has high sinterability to provide a high-density sintered body with a density of 94.0–98.0% at a low firing temperature of 1400°C.     

Multiferroic BiFeO3 Thin Films Buffered by a SrRuO3 Layer

Multiferroic BiFeO₃ thin films of huge polarization have been successfully realized by using SrRuO₃ as a buffer layer on a Pt/TiO₂/SiO₂/Si substrate. They consist of a single perovskite phase and are nearly randomly orientated, where the SrRuO₃ buffer layer lowers the crystallization temperature and improves the crystallinity of BiFeO₃. With increasing deposition temperature during magnetron sputtering, they undergo an apparent grain growth and reduction in surface roughness. The multiferroic thin films deposited on the SrRuO₃-buffered Pt/TiO₂/SiO₂/Si substrate at higher temperatures show much improved polarization and reduced coercive field, together with a lowered leakage current. A huge remnant polarization (2 P _r) of 150 μC/cm² and a coercive field (2 E _c) of 780 kV/cm were measured for the BiFeO₃ film deposited at 650°C.     

Synthesis of Oriented Ba2NaNb5O15 (BNN) Thin Films from an Alkoxy-derived Precursor

Ba₂NaNb₅O₁₅ (BNN) thin films with a tungsten bronze structure were fabricated using a precursor solution that was synthesized from barium, sodium, and niobium alkoxides. Highly (002)-oriented BNN films were prepared successfully on Pt(100)/MgO(100) substrates at 700°C, using a BNN underlayer. X-ray pole-figure measurement showed that the BNN films that crystallized on Pt(100)/MgO(100) substrates had two in-plane orientations. The remanent polarization and coercive field of the BNN film (thickness of 1.0 µm) that was crystallized at 700°C were 12.3 µC/cm² and 101 kV/cm, respectively, at –150°C (123 K). BNN films on fused silica substrates exhibited second harmonic generation upon irradiation with 1064 nm light.     

The Mechanochemical Synthesis of NaNbO3 Using Different Ball-Impact Energies

We studied the mechanochemical synthesis of NaNbO₃ from a starting-powder mixture of sodium carbonate and Nb₂O₅ using ball-impact energies of 15 and 370 mJ/hit. Based on the results of a quantitative phase analysis we propose a mechanism for the mechanochemical synthesis of NaNbO₃. During milling a condition is established, where the amounts of perovskite, X-ray diffraction -amorphous phase and residual carbonate in the mixture do not change with increasing milling time. Similarly, the NaNbO₃ crystallite size and the quantity of microstrains reach final values that are independent of the applied ball-impact energy.     

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 Crystallization of Sol-Gel Processed Pb0.5Ba0.5TiO3: Powders and Oriented Thin Films

A novel sol-gel process suitable for depositing thin-film lead barium titanate has been developed. X-ray diffraction analysis showed perovskite phase crystallization to occur at a temperature as low as 400°C with single-phase Pb_0.5Ba_0.5TiO₃ (PBT) resulting at a temperature as low as 500°C. Small concentrations of barium carbonate were evident by X-ray diffraction at 400°C, and indications of minor, carbonate-containing phases were evident by FTIR at 600°C. Deposition of the sol by spin coating on single-crystal and thin-film MgO on silicon resulted in highly oriented PBT films after calcination at 600°C. Mixed (100)/(001) films were obtained on single-crystal MgO, whereas entirely (100) films were obtained on thin-film MgO.     

Low-Temperature Synthesis of CaZrO3 Powder from Molten Salts

Calcium zirconate (CaZrO₃) powder was synthesized using calcium chloride (CaCl₂), sodium carbonate (Na₂CO₃), and zirconia (ZrO₂) powders. On heating, CaCl₂ reacted with Na₂CO₃ to form NaCl and CaCO₃. NaCl–Na₂CO₃ molten salts provided a liquid reaction medium for the formation of CaZrO₃ from in situ -formed CaCO₃ (or CaO) and ZrO₂. CaZrO₃ started to form at about 700°C, increasing in amount with increasing temperature and reaction time, with a concomitant decrease in CaCO₃ (or CaO) and ZrO₂ contents. After washing with hot-distilled water, the samples heated for 5 h at 1050°C were single-phase CaZrO₃ with 0.5–1.0 μm grain size.     

Microcontact Printed BaTiO3 and LaNiO3 Thin Films for Capacitors

There is an ongoing need to develop new technologies to enable further down-scaling of layer thicknesses in multilayer ceramic devices, for example, in multilayer capacitors (MLC). Microcontact printing of chemical solutions of both the dielectric and electrode layers was explored as an economical means of preparing patterned thin films for MLC without requiring photolithography. For this purpose, methanol/acetic acid-based BaTiO₃ solutions were spun onto polydimethylsiloxane stamps, printed onto substrates, pyrolyzed, and crystallized. LaNiO₃ was used as a prototype electrode that could also be microcontact printed. The line edge roughness produced this way was on the order of a tenth of a micrometer, which should enable very small margins. The printed layer thickness was also very uniform. Microcontact printed capacitors with a single dielectric layer were fabricated and found to have dielectric constants >800 with loss tangents <2%. Alignment between subsequent layers is readily achieved. Multilayer dielectric/electrode stacks could be fabricated without cracking or delaminations. Consequently, microcontact printing appears to be a viable potential means of preparing MLC with layer thicknesses in the range of ≤0.2 μm.     

Mechanochemical Activation-Assisted Low-Temperature Synthesis of CaZrO3

Calcium zirconate (CaZrO₃, CZ) was prepared using a solid-state reaction with mechanochemical activation through vibro-milling, aiming at completing the reaction CaO+ZrO₂=CaZrO₃ at relatively low calcination temperatures. Changes in the crystallite size and homogeneity of the mixed components CaO and ZrO₂ in the starting mixtures were observed with different milling times. The influence of milling on the incipient temperature of CZ formation and completion of CZ formation was investigated. It is concluded that milling of the reactants for 20 h lowered the incipient temperature of CZ formation from 800° to 600°C, and the temperature of complete CZ formation from above 1100° to 800°C.     

Optical and Electrical Properties of Amorphous and Nanocrystalline (La0.8Sr0.2)0.9MnO3 Thin Films Prepared from Low-Temperature Processing Technique

The results of a study on the optical and electrical properties of (La_0.8Sr_0.2)_0.9MnO₃ (LSM) thin films obtained by a polymeric precursor spin coating technique were presented. This method allowed preparation of optical quality thin films at annealing temperatures around 800°C. Amorphous and crystalline LSM thin films were studied by optical and electrical conductivity measurements. The energy-dependent absorption coefficients for the crystalline specimen were calculated from optical spectra and extra absorption was observed in the range of 1.8–2.5 eV with the exchange-gap excitation behavior in the 3–5 eV range. In comparison to the amorphous specimens, the electrical conductivity of the nanocrystalline specimen increased two to three orders of magnitude with decreasing activation energy. The charge carrier absorption model suggested an increase of the carrier concentration in the nanocrystalline specimen which may be a reason for the change in the electrical conductivity.     

Hydrothermal Processing of PbTiO3 and PbTiO3/Polymer Thin Films from Titanium Metallo-organic Precursors

PbTiO₃ and PbTiO₃/polymer thin films were synthesized from metallo-organic precursors on metallized quartz substrates. Titanium dimethoxy dineodecanoate (TDD) was spin-cast onto the substrates and converted to polycrystalline TiO₂ via hydrolysis in deionized water for 5 h at 80°C. Polycrystalline PbTiO₃ films were then formed by reacting the TiO₂ films for 4 h at 200°C in aqueous solutions of KOH and Pb(CH₃COO)₂·3H₂O. Low KOH concentrations suppressed film coarsening, thereby facilitating the formation of fine-grain continuous PbTiO₃ films. PbTiO₃/polymer thin films were processed as above after first dissolving TDD and a polystyrene/polybutadiene block copolymer in p -xylene. PbTiO₃ and PbTiO₃/polymer films had relative permittivities of ≈56 and 34 and dielectric strengths of ≈250 and 850 kV/cm, respectively.     

TIOx/Al2O3 Multilayer Ceramic Thin Films

Titanium oxide/aluminum oxide films have been deposited using molecular beam epitaxy methods and characterized by reflection high-energy electron diffraction and transmission electron microscopy techniques. Growth on silicon substrates below 973 K resulted in primarily amorphous multilayers. At 1323 K, the deposition of titanium in an oxygen atmosphere on (0001) Al₂O₃ substrates resulted in films of Ti₂O₃. These films consisted of small domains, up to 60 nm, slightly misoriented from a [1120] ∥ [1120] orientation relationship. Two variants of Ti₂O₃ were observed due to multiple positioning during growth. Closing the titanium shutter during growth resulted in an oriented TiO₂ film.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

Novel Fabrication of High-Quality ZrO2 Ceramic Thin Films from Aqueous Solution

We report on the direct deposition of high-quality ZrO₂ thin films on various kinds of substrates by the liquid phase deposition method. After reaction for 24 h, thin films formed on various kinds of substrates, and the obtained thin film was comprised of densely packed nano-sized particles. The film annealed at 500°C showed a tetragonal phase at room temperature and this phenomenon has been discussed from the viewpoint of crystallite size effect. The result of optical transmittance measurement revealed that high transparency, more than 70% transmittance, has been achieved for the film after annealing at 900°C.     

Control of Microstructure and Orientation in Solution-Deposited BaTiO3 and SrTiO3 Thin Films

Columnar and highly oriented (100) BaTiO₃ and SrTiO₃ thin films were prepared by a chelate-type chemical solution deposition (CSD) process by manipulation of film deposition conditions and seeded growth techniques. Randomly oriented columnar films were prepared on platinum-coated Si substrates by a multilayering process in which nucleation of the perovskite phase was restricted to the substrate or underlying layers by control of layer thickness. The columnar films displayed improvements in dielectric constant and dielectric loss compared to the fine-grain equiaxed films that typically result from CSD methods. Highly oriented BaTiO₃ and SrTiO₃ thin films were fabricated on LaAlO₃ by a seeded growth process that appeared to follow a standard "two-step" growth mechanism that has been previously reported. The film transformation process involved the bulk nucleation of BaTiO₃ throughout the film, followed by the consumption of this matrix by an epitaxial overgrowth process originating at the seed layer. Both BaTiO₃ and PbTiO₃ seed layers were effective in promoting the growth of highly oriented (100) BaTiO₃ films. Based on the various processing factors that can influence thin film microstructure, the decomposition pathway involving the formation of BaCO₃ and TiO₂ appeared to dictate thin film microstructural evolution.     

Low-Temperature Synthesis of Bismuth Titanate Niobate (Bi7Ti4NbO21) Nanoparticles from a Metal-organic Polymeric Precursor

This paper describes the preparation of homogeneous Bi₇Ti₄NbO₂₁ single-phase ceramic powders of ∼55 nm crystallite size, at temperatures as low as 400°–500°C using a metal citrate complex method based on the Pechini-type reaction route. The thermal decomposition/oxidation of the polymerized resin, as investigated by TG/DTA, XRD, and SEM, led to the formation of a well-defined orthorhombic Bi₇Ti₄NbO₂₁ compound with lattice parameters a = 0.544, b = 0.540, and c = 2.905 ± 0.0005 nm. Reaction takes place through an intermediate binary phase with a stoichiometry close to Bi₂₀TiO₃₂ which forms between 300° and 375°C. The metal-organic precursor synthesis method, where Bi, Ti, and Nb ions are first chelated to form metal complexes and then polymerized to give a gel, allows control of the Bi/Ti/Nb stoichiometric ratio leading to the rapid formation of nanosized bismuth titanate niobate (Bi₇Ti₄NbO₂₁) ceramic powders, at temperatures much lower than usually needed by conventional processing of mixed-oxide powders.     

Low-Temperature Synthesis of YVO4 Nanoparticles and their Photocatalytic Activity

Yttrium orthovanadate (YVO₄) nanoparticles with nearly spherical shape were successfully synthesized via a molten salt method at a low temperature of 200°C. The as-prepared powders were characterized by X-ray diffraction, transmission electron microscopy and UV-Vis spectroscopy, respectively. The results show that increasing salt amount and/or elevating calcining temperature can greatly promote the crystallization and growth of YVO₄ phase. UV-Vis absorption spectra suggested that YVO₄ nanoparticles with the smaller particle size have the stronger UV absorption, and the sequent photocatalytic degradation data also confirmed their higher photocatalytic activity.