Low-temperature preparation of hydrothermal lead zirconate titanate thin films

In preparing lead zirconate titanate thin films under hydrothermal conditions, we investigated the effects of concentrations of nutrient and mineralizer, reaction time and reaction temperature on crystallinity, grain size and shape. Experiments were performed in the ranges of 0.1-1.0M Pb(NO₃)₂, and 0-6.0M KOH with varying reaction time from 0 to 48 hours at 60-200 °C. In the experiment, lead zirconate titanate thin film of homogeneous crystalline grain was obtained through a 24 hour reaction with 0.4M Pb(NO₃)₂ and 5.0M KOH at 140 °C. The thickness of the film was 0.9-1.6 μm, and it exhibited a saturation polarization (Ps) of 18.3 μC/cm², remnant polarization (Pr) of 7.4 μC/ cm₂ and coercive field (Ec) of 0.41 kV/cm. The dielectric constant and loss (δ) measured at 1 kHz were approximately 1020 and 0.15, respectively.     

Influence of feedstock concentration on tetragonality and particle size of hydrothermally synthesized barium titanate powders

Barium titanate (BaTiO₃) powders were synthesized through hydrothermal process with Ba(OH)₂·8H₂O and TiO₂. By increasing the feedstock concentration (FC) from 0.25 to 1.50 M, BaTiO₃ powders maintain a stable average particle size (~180 nm and ~6.4441 m²/g) with an increasing tetragonality (c/a: 1.0065–1.0075). Johnson-Mehl-Avrami and standard reaction rate equations were adopted to analyze the kinetic process of BaTiO₃ formation. The reaction is governed by first-order and phase-boundary-controlled mechanism for 0.25 M and 1.50 M, respectively. Lower extent of reaction is believed to lead to the better tetragonality for BaTiO₃ powders fabricated with higher FC. On the other hand, the relative stable particle size is correlated with the unvaried nucleation frequency and grain growth rate with various FC. This work can provide a guideline to manipulate the properties of BaTiO₃ powders used in electronic industry.     

Synthesis and Characterization of Bowl-Like Single-Crystalline BaTiO3 Nanoparticles

Novel bowl-like single-crystalline BaTiO₃ nanoparticles were synthesized by a simple hydrothermal method using Ba(OH)₂·8H₂O and TiO₂ as precursors. The as-prepared products were characterized by XRD, Raman spectroscopy, SEM and TEM. The results show that the bowl-like BaTiO₃ nanoparticles are single-crystalline and have a size about 100–200 nm in diameter. Local piezoresponse force measurements indicate that the BaTiO₃ nanoparticles have switchable polarization at room temperature. The local effective piezoelectric coefficient d₃₃ ^* d_{33}^{ * } is approximately 28 pm/V.     

Hydrothermal and electrochemical growth of complex oxide thin films for electronic devices

Thin film growth of complex oxides including BaTiO₃, SrTiO₃, BaZrO₃, SrZrO₃, KTaO₃, and KNbO₃ were studied by the hydrothermal and the hydrothermal–electrochemical methods. Hydrothermal–electrochemical growth of ATiO₃ (A = Ba, Sr) thin films was investigated at temperatures from 100 to 200 °C using a three-electrode cell. Current efficiency for the film growth was in the range from ca. 0.6 to 3.0%. Tracer experiments revealed that the ATiO₃ film grows at the film/substrate interface. Thin films of AZrO₃ (A = Ba, Sr) were also prepared on Zr metal substrates by the hydrothermal–electrochemical method. By applying a potential above ca. +2 V versus Ag/AgCl to the Zr substrates, AZrO₃ thin films were formed uniformly. Thin films of KTaO₃ and KNbO₃ were prepared on Ta metal substrates by the hydrothermal method. Perovskite-type KTaO₃ thin films were formed in 2.0 M KOH at 300 °C. Pyrochlore-type K₂Ta₂O₆ thin films were formed at lower temperatures and lower KOH concentrations.     

Controlled synthesis of Mg(OH)2 thin films by chemical solution deposition and their thermal transformation to MgO thin films

The chemical solution deposition of Mg(OH)₂ thin films on glass substrates and their transformation to MgO by annealing in air is presented. The chemical solution deposition consists of a chemical reaction employing an aqueous solution composed of magnesium sulfate, triethanolamine, ammonium hydroxide, and ammonium chloride. The as-deposited films were annealed at different temperatures ranging from 325 to 500?°C to identify the Mg(OH)₂-to-MgO transition temperature, which resulted to be around 375?°C. Annealing the as-deposited Mg(OH)₂ films at 500?°C results in homogeneous MgO thin films. The properties of the Mg(OH)₂ and MgO thin films were analyzed by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, UV–Vis spectroscopy, and by circular transmission line model. Results by X-ray diffraction show that the as-deposited thin films have a brucite structure (Mg(OH)₂), that transforms into the periclase phase (MgO) after annealing at 500?°C. For the as-deposited Mg(OH)₂ thin film, a nanowall surface morphology is found; this morphology is maintained after the annealing to obtain MgO, which occurred with the evident formation of pores on the nanowall surface. The assessed chemical composition from X-ray photoelectron spectroscopy yields Mg_0.36O_0.64 (O/Mg ratio of 1.8) for the as-deposited Mg(OH)₂ film, where the expected stoichiometric composition is Mg_0.33O_0.67 (O/Mg ratio of 2.0); the same assessment yields Mg_0.60O_0.40 (O/Mg ratio of 0.7) for the annealed thin film, which indicates the obtainment of a MgO material with oxygen vacancies, given the deviation from the stoichiometric composition of Mg_0.50O_0.50 (O/Mg ratio of 1.0). These results confirm the deposition of Mg(OH)₂ films and the obtainment of MgO after the heat-treatment. The energy band gap of the films is found to be 4.64 and 5.10?eV for the as-deposited and the film annealed at 500?°C, respectively. The resistivity of both Mg(OH)₂ and MgO thin films lies around 10⁸?Ω·cm.     

The effect of the hydrothermal synthesis variables on barium titanate powders

In this work, the influence of (a) Ba excess in the starting hydrothermal mixture with TiO₂, (b) hydrothermal reaction temperature, and (c) washing cycles on the hydrothermal synthesis of barium titanate (BaTiO₃) were investigated to assess their relative contributions to the final characteristics of the sintered oxide. BaTiO₃ cake was prepared by hydrothermal synthesis at 150 °C and 180 °C using BaOH₂·8H₂O and TiO₂·xH₂O as starting hydrothermal mixture with an excess of Barium (+1 Ba mol% and +2 Ba mol%). The obtained BaTiO₃ cake was washed several times from 0 to 14 (W_n<15) using simple de-ionized water and then sintered at 1120 °C for 3 h. All considered hydrothermal syntheses variables strongly contribute to the final characteristics of the sintered BaTiO₃ powders in terms of Ba²⁺/Ti⁴⁺ molar ratio, crystalline structure and mean particle size. In particular, it is clear from these experiments that the removal of the unfavorable barium salts from BaTiO₃ cake by long washing cycles before final calcination is a critical step in the hydrothermal synthesis of BaTiO₃.     

Photocatalytic Reduction of CO2 with H2O on Ti-Containing Porous Silica Thin Film Photocatalysts

Two different Ti-containing porous silica thin films having a hexagonal and cubic pore structure were synthesized and used as photocatalysts for the reduction of CO₂ with H₂O at 323 K. UV irradiation of the Ti-containing porous silica thin films in the presence of CO₂ and H₂O led to the formation of CH₄ and CH₃OH with a high quantum yield of 0.28%. These porous silica thin film photocatalysts having a hexagonal pore structure exhibited higher reactivity than the Ti-MCM-41 powder photocatalysts with the same pore structure.     

Hydrothermal synthesis of BaTiO3 from different Ti-precursors and microstructural and electrical properties of sintered samples with submicrometric grain size

Barium titanate submicrometric particles were synthesized at 180 °C in a closed PTFE-lined stainless steel reactor with continuous stirring. Precursors used for titanium were Degussa P25 TiO₂ or titanium isopropoxide (TIP). Barium hydroxide (Ba(OH)₂) was added in a Ba/Ti = 1.1 molar ratio and KOH was used as mineralizer. The obtained powders were characterized by XRD and Raman spectroscopy. Powders were uniaxially pressed into discs and sintered at 1250 °C. The resulting microstructures were characterized by XRD, SEM, and Raman spectroscopy. Electrical measurements were carried out in order to characterize the ferroelectric behavior. The Ti precursor determined the sample density and grain size distribution and, consequently, the electrical response.     

Synthesis and surface modification of submicron BaTiO3 powders via a facile surfactant-assisted method

Due to their unique properties, well-dispersed barium titanate (BaTiO₃) ultrafine powders can be used in wide-ranging fields. In the present work, by using barium hydroxide octahydrate (Ba(OH)₂·8H₂O) and α titanic acid (H₄TiO₄) as raw materials, uniform submicron BaTiO₃ powders with tetragonal structure and high degree of crystallinity were prepared via a solid-state reaction method at relatively low temperatures. Moreover, by simply using the stearic acid (St) as the modifier to modify the surface of the aggregated BaTiO₃ powders, well-dispersed BaTiO₃ particles could be obtained, which were then examined by complementary characterizations such as XRD, TEM, HRTEM, SEM, Raman, FT-IR, XPS and EDS. The results indicated that the tetragonal BaTiO₃ particles with submicron-size, good uniformity, and high crystallinity could be prepared at 800 °C for 1 h. Moreover, the addition of St for surface modification proved to be an effective way to avoid the agglomeration of the BaTiO₃ particles to get well-dispersed products, where 1 wt % of St was found to be the optimum concentration. The demonstrated surfactant-assisted surface modification method is expected to be applicable for other ultrafine powders to get well-dispersed particles.     

In situ doping of BaTiO3 and visualization of pressure solution in flux-assisted cold sintering

Cold sintering process (CSP) has attracted great interest due to its extremely low processing temperatures, fast processing times, and simplicity to allow for the densification of ceramics and composites. Understanding the detailed mechanisms underlying low temperature densification is crucial to develop advanced materials and facilitate sustainable and cost-effective industrial implementation to come. Here, by taking BaTiO₃ powder and Sr(OH)₂·8H₂O transient chemical flux as a model system, chemical transformation at solid/flux interfaces driving the dissolution-precipitation creep mechanism were investigated. We demonstrate that Sr(OH)₂·8H₂O acts both as a sintering flux and a solid solution doping additive, resulting in the formation of BaTiO₃ - Ba_1-xSr_xTiO₃ with lower Curie temperatures. Using strontium (Sr) as a tracer chemistry, transmission electron microscopy chemical mapping with energy-dispersive X-ray analysis indicates that there is a precipitation of a Ba_1-xSr_xTiO₃ mainly at grain/grain interfaces, while grain cores remain undoped. In addition, the difference in the interfacial Sr concentration, which is influenced by the applied uniaxial pressure direction, was clearly observed. This successful visualization of compositional distribution after CSP underlines the significant role of the pressure solution creep in densification process.     

Effect of copper sulfate pentahydrate on the structure and properties of poly(vinyl alcohol)/graphene oxide composite films

Poly(vinyl alcohol) (PVA)/graphene oxide (GO)/copper sulfate pentahydrate (CuSO₄·5H₂O) composite films were prepared by the solution casting method, and the effect of CuSO₄·5H₂O on the structure and properties of the PVA/GO composites was investigated. Fourier transform infrared (FTIR) analysis proved the crosslinking interaction between CuSO₄·5H₂O and the ? OH group of PVA. The crystallinity of the composite films increased first and then decreased. For the composite films, the tensile strength, Young's modulus, and yield stress values improved with increasing CuSO₄·5H₂O, whereas the elongation at break decreased compared with that of the neat PVA/GO composite film. The thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) patterns of the PVA/GO/CuSO₄·5H₂O composite films showed that the thermal stability decreased; this was consistent with the TGA–FTIR analysis. A remarkable improvement in the oxygen‐barrier properties was achieved. The oxygen permeability coefficient was reduced by 60% compared to that of the neat PVA/GO composite film. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44135.     

Preparation and tunability properties of Ba(ZrxTi1−x)O3 thin films grown by a sol–gel process

Ba(Zr_xTi_1−x)O₃ (BZT) thin films were deposited via sol–gel process on LaNiO₃, as buffer layer, and Pt-coated silicon substrates. The BZT films were perovskite phase and showed a (1 0 0) preferred orientation dependent upon zirconium content. The grain size decreased and the microstructure became dense with increasing zirconium content. The addition of Zr to the BaTiO₃ lattice decreased the grain size of the crystallized films. The temperature dependent dielectric constant revealed that the thin films have relaxor behavior and diffuse phase transition characteristics that depend on the substitution of Zr for Ti in BaTiO₃. The dependence of electrical properties on film thickness has been studied, with the emphasis placed on dielectric nonlinear characteristics. Ba(Zr_0.35Ti₆₅)O₃ thin films with weak temperature dependence of tunability in the temperature range from 0 to 130 °C could be attractive materials for situations in which precise control of temperature would be either impossible or too expensive.     

Formation of an amorphous product phase during the solid state reaction between a vitreous SiO2 thin film and a (001) BaTiO3 substrate

The solid state chemical reaction of a thin SiO₂ film with a single-crystal (001) BaTiO₃ substrate has been investigated as a model system for processes occurring during the sintering of BaTiO₃ ceramics with the sintering additive SiO₂. Thin amorphous SiO₂ films on (001) BaTiO₃ substrates react with the substrate during annealing at 1000°C for 5 to 15 min. Transmission electron microscopy (TEM) has revealed the formation of a glassy reaction product after 5 min of heating. The composition of this glass has been analyzed by energy dispersive X-ray spectrometry (EDX), showing mainly SiO₂ and about 16 at.% of BaTiO₃. The Ba:Ti ratio of the glass is 1:1. During prolonged heating, or if the initial BaTiO₃ surface is decorated with Pt markers, the reaction product crystallizes, with the fresnoite Ba₂TiSi₂O₈ forming as the dominant crystalline phase.     

Modification of poly(vinyl alcohol) films by the addition of magnesium chloride hexahydrate

Poly(vinyl alcohol) (PVA) films modified with Magnesium chloride hexahydrate (MgCl₂·6H₂O) were prepared by casting method. The prepared films were characterized by X‐ray diffraction measurements (XRD), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and mechanical testing. It was found that the presence of MgCl₂·6H₂O had considerable effects on the crystallinity, thermal, and mechanical properties of PVA films. The crystallization of PVA film was interrupted and the degree of crystallinity of PVA film decreased with the addition of MgCl₂·6H₂O. The glass transition temperature of PVA film decreased with the addition of MgCl₂·6H₂O. After modifying with MgCl₂·6H₂O, PVA film became soft, with lower tensile strength and higher elongation at break. The presence of MgCl₂·6H₂O could significantly increase the moisture content of PVA films and this may be the cause of the plasticizing. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Preparation of Highly Crystallized Strontium Titanate Powders at Room Temperature

Highly crystallized fine powder of strontium titanate (SrTiO₃) was obtained at room temperature simply by leaving a vial containing the powder mixture of Sr(OH)₂·8H₂O and hydrous titania gel (TiO₂·nH₂O) for 10 d. Solvent and additive(s) used in the conventional low‐temperature process were not used. The crystallinity judged from the FWHM of X‐ray diffraction peak was comparable to that obtained by a solid‐state reaction. To cause the reaction at room temperature, the titania gel had to contain a considerable amount of H₂O (n > 0.97). The reaction is considered to be neutralization of titanic acid [H₄TiO₄ or Ti(OH)₄] and strontium hydroxide (base). Using similar process, highly crystallized BaTiO₃ powder was also obtained at 60°C. In comparison with the formation temperature of BaTiO₃, tolerance factor in the perovskite structure was important for the room‐temperature synthesis of SrTiO₃. SrTiO₃ was hardly obtained at room temperature by the addition of saturated strontium hydroxide solution to the hydrous TiO₂ gel (n = 1.29). Therefore, the reaction seems to proceed in the hydrous titania gel. This process is characterized by three important points; “no solvent”, “no additional reagents”, and above all, “no heating”.     

Controllable preparation of two-dimensional oriented BaTiO3 polycrystals from K0.8Ti1.73Li0.27O4 crystals by a one-step solvothermal process

Titanate precursors play an important role in obtaining oriented titanate materials via topochemical conversion. In this study, a two-dimensional (2D) K_0.8Ti_1.73Li_0.27O₄ (KTLO) crystal was successfully prepared by a one-step solvothermal process using TiO₂, LiOH?H₂O, and KOH as raw materials in ethanol/water mixed solvent for the first time. Meanwhile, the conditions and influencing factors for the solvothermal preparation of KTLO were investigated and analysed. With a ratio of ethanol/water of 20:5, better 2D KTLO can be obtained by reacting at 150 °C for 24 h. On this basis, KTLO was used directly to perform solvothermal treatment with Ba(OH)₂, and 2D BaTiO₃ (BT) was easily prepared by adjusting the reaction conditions (concentration, temperature, and time). When the solvothermal treatment was carried out in 0.5 mol/L Ba(OH)₂ solution at 80 °C for 1.5 h, 2D BT crystals could be easily obtained through the topochemical reaction mechanism. This strategy not only develops new methods for preparing precursors but also greatly simplifies the preparation process of BT and provides new research ideas for the preparation of functional materials.     

High permittivity BaTiO3 and BaTiO3-polymer nanocomposites enabled by cold sintering with a new transient chemistry: Ba(OH)2∙8H2O

Cold sintering process (CSP) offers a promising strategy for the fabrication of innovative and advanced high permittivity dielectric nanocomposite materials. Here, we introduce Ba(OH)₂?8H₂O hydrated flux as a new transient chemistry that enables the densification of BaTiO₃ in a single step at a temperature as low as 150 °C. This remarkably low temperature is near its Curie transition of 125 °C, associated with a displacive phase transition. The cold sintered BaTiO₃ shows a relative density of 95 % and a room temperature relative permittivity over 1000. This new hydrated flux permits the fabrication of a unique dense BaTiO₃-polymer nanocomposite with a high volume fraction of ceramics ((1-x) BaTiO₃ – x PTFE, with x = 0.05). The composite exhibits a relative permittivity of approximately 800, at least an order of magnitude higher than previous reports on polymer composites with BaTiO₃ nanoparticle fillers that are typically well below 100. Unique high permittivity dielectric nanocomposites with enhanced resistivities can now be designed using polymers to engineer grain boundaries and CSP as a processing method opening up new possibilities in dielectric materials design.     

Potassium-graphite intercalation compounds prepared from carbon materials of different heat-treatment temperature and their reaction with water vapor

By means of the gravimetric technique, the reaction of potassium-graphite intercalation compounds (K-GICS) with water vapor has been investigated in relation to the heat-treatment temperature (HTT) of pristine carbon materials. K-GICs with the saturated composition of KC_9.6 prepared from the material whose HTT = 1500°C was remarkably reactive as compared with other K-GICs from those of HTT = 1000, 2000 and 2600°C. Through the X-ray diffraction measurement, a resultant product with an apparent composition of (K-GIC) · (H₂O)_1.6 was found to be a mixture of KOH · H₂O and KOH · 2H₂O with a residue compound of K-GIC. Even if the residue was washed with water after the reaction, potassium still remained in the matrix; the amount varied in accordance with the HTT of the pristine material from 20% for 1000°C up to 40% for 2600°C. The washed residues give X-ray diffraction patterns different from those of the original materials. It was also found that KC₂₄ reacts more rapidly with water vapor than KC₈ does, where the steep weight increase takes place in the initial stage of the reaction, followed by a characteristic weight loss not observed for KC₈.     

Characterization of metal (Ba, Al, Si, V, and W)-incorporated TiO2 and toluene photodecomposition in the presence of H2O

This study focused on toluene photodecomposition in the presence of H₂O over metal (Ba, Al, Si, V, and W)-incorporated TiO₂. The nanometer-sized, metal-TiO₂ photocatalyst samples, including Ba²⁺, Al³⁺, Si⁴⁺, V⁵⁺, and W⁶⁺ ions, were prepared by using the solvothermal method. The X-ray photoelectron spectroscopy (XPS) results showed that the Ti-OH peak, which indicates hydrophilicity, increased with increasing Al and Si ion components but decreased with increasing Ba, V, and W ion components. The contact angles were distributed over the range of 0–10° on almost all films (200-nm thick) after irradiation for 2 h, and in particular approached 0° on the Al-TiO₂ and Si-TiO₂ nanometer-sized films after just 30 min. The toluene (100 ppm) photodecomposition in the continuous system increased in the order of Al-TiO₂>Si-TiO₂>pure TiO₂>W-TiO₂>Ba-TiO₂>V-TiO₂, and the maximum toluene conversion rate achieved was 45% over Al-TiO₂ film after 120 min. The toluene conversion remarkably increased; however, over all photocatalysts, with H₂O addition during the toluene photo-decomposed reaction, and in particular, the conversion reached up to 90% after 120 min over Al-TiO₂ and Si-TiO₂ with increased hydrophilicity. After photoreaction for 24 h, minimal carbon was deposited on the photocatalyst under both reaction conditions, with and without H₂O addition, although the deposited carbon amounts were smaller for the former. These results confirmed that the hydrophilicity of the photocatalyst had a greater effect on toluene decomposition, while the photocatalytic deactivation could be retarded by H₂O supplementation during toluene decomposition.     

Cyclic voltammetric studies of nickel hydroxide and cobalt hydroxide thin films in alkali and alkaline earth metal hydroxides

Cyclic voltammetric studies of thin films of electrosynthesized (ES)-Ni(OH)₂ and Co(OH)₂ in different alkaline electrolytes suggest that the mechanism of oxidation is different from the mechanism of reduction. While the metal ion (alkali or alkaline earth metal) intercalation-deintercalation from the electrolyte into the film provides the driving force, the reduction reaction takes place heterogeneously independent of the electrolyte concentration, whereas oxidation takes place homogeneously across a nebulously defined electrode-electrolyte interphase. ES-Ni(OH)₂ permits facile intercalationdeintercalation of alkali metal ions Li⁺, Na⁺ and K⁺, irrespective of their ionic size, while the reactions of ES-Co(OH)₂ are sensitive to ionic size, requiring larger potentials in KOH compared to LiOH. In the alkaline earth metal hydroxides, both Ni(OH)₂ and Co(OH)₂ films show greater reversible characteristics in the order Ba(OH)₂ > Sr(OH)₂ > Ca(OH)₂. This may be trivially related to the order of the solubilities of the three hydroxides.