Chemical Heterogeneity in PMN–35PT Ceramics and Effects on Dielectric and Piezoelectric Properties

Pb[(Mg_1/3Nb_2/3)_0.65Ti_0.35]O₃ (PMN–35PT) powder was prepared using the columbite precursor method. Fully dense compacts were formed by hot-pressing the powder at 950°C, and then the compacts were annealed at 1150°C for 5 and 10 h, respectively. Dielectric and piezoelectric properties of the as-hot-pressed and annealed samples were measured and correlated with microstructure. The as-hot-pressed material exhibited relaxor–ferroelectric-like behavior, with a relatively low dielectric constant maximum measured at 1 kHz ( K _m@1kHz) of 8160. Annealing resulted in a transition to weak normal-ferroelectric behavior, a shift in the dielectric maximum temperature from 190°C to 169°C, and a dramatic increase of K _m@1kHz to a maximum value of 41 720 for the longer anneal. The as-hot-pressed microstructure was chemically heterogeneous, characterized by submicrometer-sized regions of varying magnesium, niobium, and titanium content that likely originated from chemical heterogeneities that were present in the as-prepared PMN-PT powder. The as-hot-pressed properties have been explained as being the integrated response of many discrete ferroelectric responses as dictated for each of these regions by the local chemistry. The transition on annealing has been explained in terms of chemical homogenization to a near-morphotropic phase-boundary composition that is intrinsically weak normal-ferroelectric. Differences in polarization-versus-electric-field and strain-versus-electric-field behavior between the hot-pressed and annealed materials have been discussed in terms of differences in domain mobility.     

Water absorption characteristics of hybrid materials incorporating tetrabutyl titanate with acrylic polymer via sol–gel process

The sol–gel process was used to prepare organic/inorganic hybrids utilizing a terpolymer of n‐butyl methacrylate, methyl methacrylate, and methacrylic acid as an organic phase and titanium tetrabutoxide as a precursor of the inorganic phase. Such hybrid materials may be of interest as primers for corrosion protection of metal substrates. In the absence of electrochemical influence, which is usually provided by anticorrosive pigments in current protective coatings, the barrier mechanism becomes one of the basic elements for protecting metals against atmospheric corrosion. In that context, we report the water uptake characteristics of hybrid films with titania content up to 16.3 wt %. The slow hydrolysis of butoxy, acetate, and carboxylate ligands, which becomes more and more obvious with an increase in titanium oxide cluster content, is found to produce a chemically driven force for water uptake in hybrids. The 4.6 wt % TiO₂ hybrid results in the best balance of properties with a far lower diffusion coefficient and nearly the same amount of water absorbed at saturation compared to the polymer matrix. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 92–102, 2005     

Chemical and Structural Properties of Nickel Hydroxide Xerogels Obtained by the Sol-Gel Procedure in the Presence of Acetic Acid

The sol-gel route to glasses and ceramics has attracted an increasing amount of scientific and technological interest recently. In this process, sols with different concentrations are used as precursors for xerogels and to produce materials that consist of fine oxide particles. In the present work, nickel hydroxide gels have been obtained via the hydrolysis of a molecular precursor in the presence of acetic acid. The chemical aspects of the material transformation have been studied by using Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC) for different acetic acid contents and several heat-treatment temperatures. The carboxylic acid acts as a ligand at a molecular level in the precursor, therefore modifying the entire hydrolysis and condensation process. Small-angle X-ray scattering (SAXS) studies and density measurements have been performed for the structural characterization of the xerogels. A denser, final oxide material is obtained when a higher acetic acid concentration is used. The porosity of the dry gels coarsens when they are heat-treated up to a temperature of ∼400°C and its density decreases. The material that has been heat-treated up to a temperature of 800°C densifies and exhibits a finer porosity. The chemical properties at a molecular level satisfactorily explain and are well correlated with the structural characteristics of the studied material.     

Lead Zirconate Titanate Prepared from Different Zirconium and Titanium Precursors by Sol-Gel

Modified sol-gel processes have been developed for the preparation of lead zirconate titanate (PZT) (52/48) powders. These processes use different starting sources to introduce the titanium and zirconium components, namely tetraethyl orthotitanate, titanium isopropoxide, or titanium diisopropoxide bis(2,4-pentanedionate) for titanium and zirconium propoxide or zirconium acetylacetonate for zirconium. To achieve stable and homogeneous precursor systems, several solvents (acetic acid, 1,2-propanediol, propanol, and distilled water) and chemical modifying additives, such as acetylacetone and nitric acid, were also introduced for the preparation processes. The influence of the different precursors on the crystallization behavior of the sol-gel-derived powders was studied. Well-crystallized single-phase PZT powders were obtained after heat treatment at 600°C for 1 h. The powders obtained sintered well at 1000°C/2 h and a homogeneous microstructure with small grain sizes was obtained.     

A Novel Hybrid Method of Sol–Gel and Ultrasonic Atomization Synthesis and Piezoelectric Properties of SrBi4Ti4O15 Ceramics

SrBi₄Ti₄O₁₅(SBTi) powders were synthesized by a novel hybrid method of sol–gel and ultrasonic atomization. TiO₂ particle was used as a starting material to replace other expensive soluble titanium salts. X-ray diffraction results showed that the pure-phase SBTi powders were obtained at 700°C for 2 h, which is much lower than the calcination temperature (800°–850°C) required in solid-state reactions. The ceramics sintered at 1100°C for 1 h exhibited 94.5% of relative density and a piezoelectric coefficient of 21 pC/N. The results showed that this hybrid method could lead to an attractive method for the industrial fabrication of SBTi materials.     

Deformation of Poly crystalline Transition Metal Carbides

Plastic deformation was studied in hot-pressed poly crystalline specimens of titanium carbide, vanadium carbide, niobium carbide, tantalum carbide, and tungsten carbide tested in four-point bending and under compression to 2100°C. Transition from brittle to ductile behavior occurred at approximately half the absolute melting temperature for each material. Above this temperature the yield strengths decreased exponentially with increasing temperature. Between the ductile-brittle transition temperature and about 1900°C the apparent activation energy for flow in the carbides with the rock-salt structure was near O.S ev/atom. Above 1900°C the value was approximately 2 ev/atom; the value for tungsten carbide was apparently 0.3 ev/atom to 2070°C. Vanadium carbide and titanium carbide recovered extensively on annealing for short times near 2000°C. No such recovery occurred in tungsten carbide under these conditions. The results for polycrystalline carbides were compared with available data for single crystals of titanium carbide.     

From Chelating Precursor to Perovskite Oxides and Hollow Fiber Membranes

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) is a promising mixed-conducting ceramic membrane material in addition to being a good electrode catalyst for solid oxide fuel cells. In this study, BSCF powder was synthesized via a chelated water-soluble complex method at relatively low temperatures. The combined ethylenediaminetetraacetic acid and citric acid was used for the synthesis of a complex-based precursor, followed by thermal decomposition of the precursor at high temperatures. Thermal behavior, crystal phases, and structures of the prepared powders were characterized by thermogravimetric analysis/differential scanning calorimetry, XRD, and scanning electron microscopic (SEM) techniques, respectively. Pure and single-phase perovskite could be obtained after sintering at a temperature higher than 800°C for 5 h. The soft precursor powder synthesized at lower temperatures, i.e., 600°C, is water insoluble and more appropriate for use as a membrane material to prepare gas-tight tubular or hollow fiber ceramic membranes. By contrast, the hollow fibers prepared via the traditional techniques where the perovskite powder is used as the starting membrane materials display gas leakage. The fibers were characterized by SEM, XRD, and tested for air separation at ambient pressure and temperatures between 700° and 950°C. The oxygen flux measured in this work reached 3.90 mL·(min·cm²)⁻¹ and compares favorably with any experimental values reported in the open literature.     

Inorganic–organic hybrid materials based on functionalized silica and carbon: A comprehensive understanding toward the structural property and catalytic activity difference over mesoporous silica and carbon supports

Inorganic–organic hybrid materials based on functionalized silica and carbon were synthesized by anchoring molybdovanadophosphoric acid (H₅[PMo₁₀V₂O₄₀] · 32.5H₂O) onto amine-functionalized SBA-15, ethane-bridged SBA-15 and mesoporous carbon, respectively. Small angle X-ray diffraction, N₂ sorption analysis, HRTEM, SEM, FT-IR, CP-MAS NMR were used to diagnose the mesoporous structure of inorganic–organic hybrid materials. The structural integrity of molybdovanadophosphoric acid has been found to be retained after immobilization over mesoporous materials. These inorganic–organic hybrid materials were tested in the environmentally friendly oxidation of 2-methylnaphthalene (2MN) with 30% aqueous hydrogen peroxide. Molybdovanadophosphoric acid containing mesoporous organosilica hybrid material (ethane-bridged SBA-15) exhibited higher catalytic activities in the oxidation of 2MN to give a clean product 2-methy-1,4-naphthoquinone (menadione vitamin K3 precursor), because of the improved hydrophobicity of the material. The correlation between structural properties and catalytic activities of these hybrid materials has been well addressed in our present studies.     

Synthesis of Metastable Tetragonal (t') ZrO2-12 mol% YO1.5 by the Organic Polymerized Complex Method

Zirconia-yttria (ZrO₂-12 mol% YO_1.5) powders were prepared via an organic polymerized complex method that was based on the Pechini-type reaction route. A mixed solution of citric acid, ethylene glycol, and zirconium and yttrium ions were polymerized to form a transparent resin, which then became a black powder after charring at ∼450°C. This product was used as a precursor for ZrO₂-12 mol% YO_1.5. The formation of a metastable tetragonal ( t ´) phase with compositional homogeneity occurred when the precursor was heated at the rate of 10°C/min up to a temperature >560°C. X-ray diffraction peaks became sharp with an increase in the heat-treatment temperature. A t ´-form with cell parameters of a = 5.1265(6) Å and c = 5.1559(9) Å were obtained via heat treatment up to 1400°C. This study shows that the organic polymerized complex method is effective for obtaining the compositionally homogeneous metastable t ´-form in the ZrO₂-YO_1.5system for short annealing times at relatively lower temperatures.     

Thermal and optical properties of PMMA–titania hybrid materials prepared by sol‐gel approach with HEMA as coupling agent

A series of sol‐gel derived organic–inorganic hybrid materials consisting of organic poly(methyl methacrylate) (PMMA) and inorganic titania (TiO₂) were successfully synthesized by using 2‐hydroxyethyl methacrylate (HEMA) as coupling agent. In this work, HEMA is first copolymerized with methyl methacrylate monomer at specific feeding ratios by using benzoyl peroxide (BPO) as initiator. Subsequently, the as‐prepared copolymer (i.e., sol‐gel precursor) is then cohydrolyzed with various contents of titanium butoxide to afford chemical bondings to the forming titania networks to give a series of hybrid materials. Transparent organic–inorganic hybrid materials with different contents of titania are always achieved. Effects of the material composition on the thermal stability, optical properties, and morphology of neat copolymer and a series of hybrid materials, in the form of both coating and free‐standing film, are also studied by differential scanning calorimetry, thermogravimetric analysis, UV–Vis transmission spectra, refractometer, and atomic force microscopy, respectively. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 400–405, 2004     

Ambient-Temperature Mechanochemical Formation of Titanium Nitride-Alumina Composites from TiO2 and FeTiO3

The fabrication of a homogeneous submicrometer-sized powder composed of nanocrystalline (<10 nm) alumina and titanium nitride during high-energy ball-milling is reported in this paper. The starting materials were rutile (TiO₂) and aluminum powder. A similar composite with iron was also produced using the mineral ilmenite (FeTiO₃) as the starting material. The powders were ball-milled together under a nitrogen atmosphere for 100 h in a laboratory-scale mill and subjected to thermal analysis and isothermal annealing at up to 1200°C. X-ray diffraction showed that all of the phases formed within the milling step and underwent grain growth on annealing. Differential thermal analysis indicated no residual elemental aluminum, confirming that the reaction was completed during the milling operation.     

Direct Formation of Zirconia-Doped Titania with Stable Anatase-Type Structure by Thermal Hydrolysis

Nanoparticles of anatase-type titania (TiO₂) doped with zirconia (ZrO₂) were directly synthesized from acidic precursor solutions of TiOSO₄ and Zr(SO₄)₂ by simultaneous hydrolysis, under mild hydrothermal conditions, at 200° and 240°C. Doping ZrO₂ into TiO₂ suppressed the crystal growth of anatase and shifted the phase transformation from anatase-type to rutile-type structure to a high temperature. The presence of an anatase-type structure with high crystallinity and high phase stability, even after annealing at 1000°C for 1 h, was fully achieved by both the doping of ZrO₂ into TiO₂ through direct precipitation and the simultaneous hydrolysis of the sulfate solutions.     

Preparation of an Organic-Inorganic Hybrid Ionic Conductive Material with Thermal and Chemical Stability

An organic–inorganic hybrid ionic conductive material was prepared through oxidation of thiol groups in a surface-modified porous glass. Surface-modified pores in the hybrid materials acted as permeation paths for protons. Proton conductivity of the hybrid material was 4.2 × 10⁻² S/cm at 120°C under 100% relative humidity. This hybrid material was thermally stable at high temperature (above 100°C) due to the temperature-tolerant inorganic frameworks.     

Microstructure Development of Oxycarbide Composites during Active-Filler-Controlled Polymer Pyrolysis

The microstructure development of a ceramic composite material fabricated by active-filler-controlled polymer pyrolysis (AFCOP) was investigated. During heating of a polysiloxane precursor mixed with titanium powder in argon atmosphere up to 1400°C, thermally induced decomposition of the polymer phase is combined with simultaneous carburization of the transition metal filler. Precipitation of nanocrystalline titanium carbide at the filler particle surface starts above 400°C, and larger, faceted carbide particles have growth above 800°C. A skeleton of turbostratic carbon is formed above 800°C in the polymer-derived silicon oxycarbide matrix from which b-silicon carbide and cristobalite crystallize above 1000°C. A pronounced reduction in linear shrinkage involved in polymer–ceramic conversion is observed. The shrinkage reduction ranges from more than 25% for the filler-free precursor to less than 10% in the presence of 30 vol% of the titanium filler. Thus, active-filler-controlled pyrolysis offers the possibility of controlling shrinkage and porosity formation during polymer–ceramic conversion in order to fabricate bulk components from organometallic polymer precursor systems.     

Influence of Precursor Densification on Strength Retention of Zirconia-Coated Nextel™ 610 Fibers

Strength degradation of Nextel^™ 610 fibers by continuous liquid phase coating was investigated for four different zirconia precursors. The precursors differed regarding their chemical composition (with or without yttrium), phase composition (amorphous or crystalline), and decomposition behavior. Phase transformation and densification of the films were characterized and found to depend on the kind of precursor. Single fiber Weibull's strength was measured for calcination temperatures between 250° and 1150°C for all precursors. Each precursor had an individual degradation behavior. For an annealing temperature of 1150°C highly damaged (∼1600 MPa) and undamaged (>3300 MPa) fibers were obtained depending on the kind of precursor. Fiber degradation could be correlated to mechanical stresses. Stress concentration due to inhomogeneous film thickness distribution is proposed as the cause of fiber strength degradation. Full strength could be retained for porous coatings or coatings where stresses were reduced by phase transformation.     

DNA-inorganic hybrid material as selective absorbent for harmful compounds

Double-stranded DNA is one of functional polymers, but the large amounts of DNA sources, such as salmon milt and shellfish gonads, have been discarded as industrial wastes. Therefore, conversion of this discarded DNA to be a useful material would be beneficial to utilize the unique property of DNA. These materials including DNA have been prepared by mixing with the organic polymers, such as alginic acid, collagen, and chitosan. However, since these materials have consisted from entirely organic components, these do not have the mechanical strength for a material. So, we prepared the organic-inorganic hybrid materials by mixing DNA with silane coupling reagents bis(trimethoxysilylpropyl)amine or bis[(3-trimethoxysilyl)propyl]ethylenediamine. These hybrid materials with the flexibility were water-insoluble and resistant to hydrolysis by nuclease. In addition, the mechanical strength of this hybrid material was approximately twice as high as that of DNA without mixing with silane coupling reagents. Furthermore, the double-stranded DNA in the hybrid materials has been maintained in a B-form structure in aqueous solution. Thus, we demonstrated the utilization of DNA as a functional material. As a result, this material could selectively accumulate harmful DNA-intercalating compounds with the planar structure, such as dibenzo-p-dioxin, dibenzofuran, and ethidium bromide. Organic-inorganic hybrid material including double-stranded DNA has potential to serve as a useful biomaterial for medical, engineering, and environmental applications.     

Preparation and characterization of sol–gel derived UV-curable organo-silica–titania hybrid coatings

In this work, UV-curable organic–inorganic hybrid coatings based on cycloaliphatic epoxyacrylate were prepared by sol–gel technique. Acid catalyzed solutions of tetraethylorthosilane (TEOS) containing Ti:acac complex were used as an inorganic precursors. UV-curable, transparent hybrid coating materials were applied on plexiglass substrates and their coating performance was investigated by the analyses of various tests such as hardness, gloss, cross-cut adhesion tests, stress–strain test and optical transmission. The mechanical measurements showed that, the tensile properties of coatings underwent an abrupt change from a brittle to a tough material when the inorganic part was incorporated into the cycloaliphatic epoxy acrylate based organic network. UV–vis transmission spectroscopy results indicated that the hybrid materials with high titanium content have good transparences. The thermal behaviour of the coatings was also evaluated. It is observed that the thermal stability of the hybrids is enhanced with incorporation of sol–gel precursor.     

Preparation and corrosion protective properties of nanostructured titania-containing hybrid sol–gel coatings on AA2024

Titania-containing organic–inorganic hybrid sol–gel films have been developed as an alternative to chromate-based coatings for surface pretreatment of aluminium alloys. Stable hybrid sols were prepared by hydrolysis of 3-glycidoxypropyltrimethoxysilane and different titanium organic compounds in 2-propanol solution in the presence of small amounts of acidified water. Different diketones were used as complexing agents in this synthesis for controllable hydrolysis of titanium organics. The properties of the obtained coatings were compared with those of zirconia-containing films. Electrochemical impedance spectroscopy (EIS) measurements and standard salt spray tests were performed to investigate the corrosion protection performance of the hybrid coatings. It was revealed that their protective properties depend significantly on the nature of metalorganic precursors and complexing agents used in the process of sol preparation. The best anticorrosive protection of AA2024 in chloride solutions is provided by the titania-containing sol–gel films prepared with titanium(IV) tetrapropoxide and acetylacetone as starting materials. In the case of zirconia-containing films, better protective properties were found when applying ethylacetoacetate as a complexing agent.     

Polymerized Complex Route to Synthesis of Pure Y2Ti2O7 at 750°C Using Yttrium-Titanium Mixed-Metal Citric Acid Complex

A polymerized complex technique was used to prepare high-purity pyrochlore Y₂Ti₂O₇ powders at 750°C. Heating of a mixed solution of citric acid (CA), ethylene glycol (EG), and yttrium and titanium ions, with a molar ratio of CA:EG:Y:Ti = 5:20:1:1, at 130°C produced a yellowish transparent polymeric gel without any precipitation; this material was used subsequently as a precursor for Y₂Ti₂O₇. Based on the results of ¹³C-NMR spectroscopy, it was suggested that a mixed-metal (Y,Ti)-CA₃ chelate complex formed in a starting CA/EG solution. The formation of pure pyrochlore Y₂Ti₂O₇ occurred when the precursor was heat-treated in a furnace set at 750°C in static air for 4 h.     

Porous Titania Ceramic Prepared by Mimicking Silicified Wood

A porous titania ceramic with a woodlike microstructure that was analogous to silicified wood was prepared. The production of "titanified wood" was performed using the following process: (i) introduction of titanium tetraisopropoxide into wood materials via vacuum infiltration, (ii) hydrolysis of the titanium tetraisopropoxide in the cell structure to form a titania gel, and (iii) firing at a temperature of 600°–1400°C in air. The resulting titania ceramic had the same external and internal forms of the original wood.