Calcium Nitrate Tetra-Hydrate-Doped Polyvinylidene Fluoride Thin Films as Potential Devices for Pressure Sensors.

Calcium nitrate tetra-hydrate [Ca(NO3)2·4H2O]-doped polyvinylidene fluoride (PVDF) thin films were successfully prepared by mixing 0.2-g-calcium nitrate into a PVDF solution under ultrasonification. The mixed solution was electrostatically spray coated onto aluminum foil substrates and then heat treated at 120 °C for 60 min in air. X-ray diffraction and Fourier transform infrared spectroscopy were used to confirm the crystal structure and chemical bonding conformation. A morphological characterization of the layers was performed using a scanning electron microscope. The dielectric properties of the samples were analyzed as a function of the variation of frequency. A piezoelectric characterization of the samples was performed by an electrical reaction of the layers after a single compression was applied from the impact of a pendulum. The structural and piezoelectric properties of the thin films could be related to the electroactive ß phases in the Ca-doped PVDF thin films.

Structural and Electrical Properties of Solution Casted-BaTiO3-Polyvinylidene Composite Layers.

Ceramic doped-polymer structures constitute a new area of functional materials, which are very promising because they combine hardness of ceramics with plasticity, low density, and high breaking strength of polymers. Piezoelectric composites are remarkable candidates for use in technical applications such as sensors and actuators, owing to their ability to integrate electrical and mechanical signals. Here, uniform ceramic-polymer composites (0-3 type) of tetragonal BaTiO3 powder as a ceramic filler and polyvinylidene fluoride as a polymer matrix were prepared using solution casting. X-ray diffraction analysis confirmed the existence of both ceramic and polymer crystalline phases. Field emission-scanning electron microscope was used to confirm the uniformity of the prepared composites, because for 0-3 composites it is critical to ensure a homogeneous distribution of the filler in the matrix. The layer was flexible with the total thicknesses in the 60~70 µm range. The dielectric properties of the composite layer were analyzed for frequencies ranging from 50 Hz to 1 MHz, using an LCR meter at room temperature. Dielectric permittivity measurements of the prepared ceramic-polymer composite layers revealed higher permittivity compared with a pure polyvinylidene fluoride layer.

Preparation and Optical Properties of Trivalent Erbium-Doped CaY2O4 Powders Under 980 nm Excitation.

Nanocrystalline CaY2O4:Er3+ up-conversion phosphor was prepared by the sol-gel process. A homogeneous precursor sol was heated on a hot plate and then coagulated gel was prefired at 300 °C for 4 hr in Ar, followed by final annealing at 1,200 °C for 4 h in Ar. The crystallinity of the powders after annealing was confirmed using an X-ray diffraction analysis. The shape and particle size of the powders were observed by field emission-scanning electron microscope and transmission electron microscope. Up-conversion luminescence spectra were recorded with a fluorescent spectrophotometer under a 980-nm excitation. The dependence of the up-converted intensities on pumping powers for powders was obtained by changing the excitation powers at room temperature. X-ray diffraction patterns confirmed the presence of single orthorhombic crystalline CaY2O4 and no impurity peaks were identified. Micrographical images of 3 mol%-doped CaY2O4 powder indicates that the particle size is uniform, and ranges from 100~200 nm. Up-conversion efficiency is the highest when the doping concentration of Er3+ is 3 mol%.

Up-Conversion Photoluminescence of Sol-Gel Derived CaY2O4 Powders Under 980 nm Excitation.

Up-conversion is an anti-stokes process that can convert near-infrared light into visible light. Besides the requirement of high optical conversion efficiency, the thermochemical stability of the host materials is critical for in practical applications, and a stable oxide host material is optimal. CaY2O4 follows the same ordered structure of CaFe2O4, which is composed of an (R2O4)2- (R = rare earth metal) framework of double octahedral moieties with rare earth ions residing within the framework. CaY2O4 is a promising host material due to its favorable characteristics such as high chemical and thermal stability, low-phonon energy, and environment friendliness. Rare earth ion (Er3+ and Yb3+)-doped nano-crystalline phosphors were prepared by a sol-gel process. The synthesized sample was characterized by X-ray diffraction analysis and the crystallite size was confirmed by Scherrer's formula and transmission electron microscopy. The surface morphology of the powders was determined by scanning electron microscopy. X-ray diffraction patterns confirmed their pure orthorhombic structure after annealing at 1,200 °C and the morphology of particles was found to be a nearly spherical shape with a diameter of the order of ~100 nm. Photoluminescence properties of the powders were measured by exciting the samples with a 980 nm diode laser at room temperature. Under the 980-nm laser excitation, the green and red up-conversion emissions were observed at around 520- 540 nm, 540-570 nm and 640-680 nm, which, are attributed to the transitions of ²H11/2 →4I15/2, 4S3/2 →4I15/2 and 4F9/2 →4I15/2 of Er3+ ions, respectively. The up-conversion intensity as a function of laser power shows that the up-conversion mechanism corresponding to green and red emissions occurs via a two-photon process.

Preparation of ß-Phase Poly(vinylidene fluoride) Films on Aluminum Substrate with the Addition of Hydrated Metal Salts.

Poly(vinylidene fluoride) (PVDF) is one of the most studied polymers that exhibits piezoelectric and ferroelectric properties. PVDF crystallizes into four different forms, namely α, ß, γ, and δ phases. Generally, the nonpolar α phase, as a replacement for the polar ß phase with the most superior ferroelectric and piezoelectric properties, is formed from the precursor melt and solution. Here, we report a method for the preparation of ß-phase-dominant PVDF thin films by doping PVDF solution with a metal hydrated salt, such as Co(NO3)2·6H2O, without any stretching procedure. The crystal structure of the film was analyzed using X-ray diffraction -2 scanning and Fourier transform infrared spectroscopy. The morphology of the film was observed using a field emission-scanning electron microscope. To determine the frequency dependence of dielectric properties, an LCR meter was used in the range of 50 Hz to 1 MHz. The crystalline phase and morphology of the electrostatic spray-deposited PVDF films depended on the chemical additive. Our results show that the addition of a metal hydrated salt can induce an in situ poling effect, consequently facilitating the preferred dipole orientation in the electrostatically sprayed PVDF films.

Fabrication of Up-Conversion Phosphor Films on Flexible Substrates Using a Nanostructured Organo-Silicon.

Up-conversion phosphors have attracted considerable attention because of their applications in solid-state lasers, optical communications, flat-panel displays, photovoltaic cells, and biological labels. Among them, NaYF4 is reported as one of the most efficient hosts for infrared to visible photon up-conversion of Yb3+ and Er3+ ions. However, a low-temperature method is required for industrial scale fabrication of photonic and optoelectronic devices on flexible organic substrates. In this study, hexagonal ß-NaYF4: 3 mol% Yb3+, 3 mol% Er3+ up-conversion phosphor using Ca2+ was prepared by chemical solution method. Then, we synthesized a nanostructured organo-silicon compound from methyl tri-methoxysilane and 3-glycidoxy-propyl-trimethoxy-silane. The transmittance of the organo-silicon compound was found to be over 90% in the wavelength range of 400~1500 nm. Then we prepared a fluoride-based phosphor paste by mixing the organo-silicon compound with Na(Ca)YF4:Yb3+, Er3+. Subsequently, this paste was coated on polyethylene terephthalate, followed by heat-treatment at 120 °C. The visible emission of the infrared detection card was found to be at 655 nm and 661 nm an excitation wavelength of 980 nm.

Epitaxially-Grown Europium-Doped Barium Titanate Films on Various Substrates for Red Emission.

Intense red photoluminescence under ultraviolet excitation was observed in epitaxially-grown europium-doped perovskite BaTiO3 thin films deposited on the SrTiO3 (100), MgO (100) and sapphire (0001) substrates using metal carboxylate complexes. Precursor films prepared by spin coating were pyrolyzed at 250 °C for 120 min in argon, followed by final annealing at 850 °C for 60 min in argon. Crystallinity and epitaxy of the films were analyzed by X-ray diffraction θ-2θ scan and pole-figure analysis. Photoluminescence of the thin films at room temperature under 254 nm was confirmed by a fluorescent spectrophotometer. The obtained epitaxial BaTiO3 thin films on the SrTiO3 (100) and MgO (100) substrates show an intense red-emission lines at 615 nm corresponding to the (5)D0 --> (7)F2 transitions on Eu(3+) with broad bands at 595 and 650 nm.

Hydroxyapatite forming ability of electrostatic spray pyrolysis derived calcium phosphate nano powder.

A novel fabrication technique, i.e., electrostatic spray pyrolysis (ESP), has been used in this study to prepare calcium phosphate nano powders. Final annealing was done at 400 degrees C for 30 min in air. The hydroxyapatite-forming ability of the annealed powder has been investigated in Eagle's minimum essential medium solution. X-ray diffracton, field emission scanning electron microscope, energy dispersive X-ray spectroscope, and Fourier transform infrared spectroscope were employed to characterize the annealed powders after immersion. The powder with an amorphous structure induced hydroxyapatite formation on their surfaces after immersion for 15 days.

Calcium phosphate formation on nanocrystalline ZrO2 thin film prepared by using a zirconium naphthenate.

To investigate the calcium phosphate forming ability of ZrO(2) thin film, we prepared ZrO(2)/Si structure by a chemical solution deposition with a zirconium naphthenate as a starting material. Precursor sol was spin-coated onto the cleaned Si substrate and prefired at 500 degrees C for 10 min in air, followed by final annealing at 800 degrees C for 30 min in air. Surface morphology and surface roughness of the annealed layer were characterized by field emission-scanning electron microscope and atomic force microscope. After soaking for 5 days in a simulated body fluid, formation of the calcium phosphate on nanocrystalline ZrO(2) layer annealed at 800 degrees C was observed by energy dispersive X-ray spectrometer. Fourier transform infrared spectroscopy revealed that carbonate was substituted into the calcium phosphate.

Effect of annealing titanium on in vitro bioactivity.

In order to modify titanium surfaces for various biological applications, bioactive and pure titanium oxide thin films were coated on the titanium by thermal oxidation technique. The commercially pure titanium discs after polishing were heated at 500, 550, 600, 650 and 700 degrees C, respectively, for 10 min in air or in argon. To evaluate the ability of calcium phosphate formation, samples after annealing were soaked in the Eagle's minimum essential medium solution. Surface morphology and chemical composition of the samples before or after immersion were characterized by field emission - scanning electron microscopy and energy dispersive X-ray spectrometry.

Growth of calcium phosphate on poling treated ferroelectric BaTiO3 ceramics.

Barium titanate (BaTiO3; BTO) is ferroelectric and piezoelectric after poling treatment. In this study, the bioactivity of BTO was investigated after a poling treatment by examining the formation of crystal growth on specimen surfaces in vitro. Negatively charged BTO surfaces showed calcium phosphate (Ca-P) crystal growth, while deposition of sodium chloride was observed on the positively charged BTO surfaces. After 30 days immersion in Eagle's MEM, the thickness of Ca-P crystal on negatively charged BTO surfaces increased to 0.8-0.9 microm. These data indicate that incorporating selectively polarized BTO on implant surfaces is a promising means for improving the bioactivity of implant materials.