Fresnoite Phase Research Articles

Silicate glass-ceramic fibers with nonlinear optical crystals: Evaluation of propagation loss and comparison to optical materials

Optical glass fibers can propagate light signals, while their control depends on nonlinear optical crystals. In this study, we fabricated nonlinear optical glass-ceramic (GC) fibers in which the polar fresnoite phase was crystallized based on perfect surface crystallization (PSC) in the SrO–TiO2–SiO2 system. GC fibers free from voids were obtained from precursor glass fibers with a diameter of ∼100 μm by controlling PSC. The propagation loss of a GC fiber was evaluated to be ∼0.37 dB/cm, which is comparable to that of the LiNbO3 single-crystal optical waveguide.

Structural features of the A2TiSi2O8 (A-Ba and Pb) compounds

X-ray diffraction studies of synthesized Ba2TiSi2O8 have shown that this compound has a fresnoite structure and is characterized by a polar phase [Formula: see text]4bm at room temperature. The synthesis of Pb2TiSi2O8 in the fresnoite phase under the selected synthesis conditions has led to the formation of a perovskite tetragonal phase [Formula: see text]4mm corresponding to PbTiO3.

Nanometer Scale Microstructure and Optical Properties of BaO-TiO₂-SiO₂ System-Based Glass-Ceramics.

The fresnoite phase, one of the crystals obtainable in the BaO-TiO₂-SiO₂ (BTS) system, is a material with excellent nonlinear optical properties. In this study, glass-ceramic containing nanometer scale fresnoite crystal was prepared by controlling the heat-treatment process from a BTS system, and its properties were analyzed. BTS glasses were prepared by conventional melting and quenching method. The heat treatment conditions were investigated by the non-isothermal analysis method using DTA. The formation of fresnoite crystals and the change of crystallinity according to heat treatment temperature were investigated by a XRD analysis. The glass-ceramics prepared in this study had fine crystal grains less than 100 nm in size and showed surface crystallization behavior growing from the surface to the inside. The transmittance of the prepared specimen in the range of visible light was measured up to 77%. The photoluminescence (PL) intensity of specimen was remarkably high at 470 nm and the emitted light has a blue and white light characteristic as displayed in the CIE coordinates.

Structure and Properties of Piezoelectric Strontium Fresnoite Glass-Ceramics Belonging to the Sr–Ti–Si–Al–K–O System

Crystallization of strontium fresnoite Sr2TiSi2O8 piezoelectric crystals in Sr–Ti–Si–K–Al–O parent glasses is investigated with the aim of showing the influence of composition and crystallization conditions on the microstructure and piezoelectric properties of the resulting glass-ceramic. All the investigated conditions lead to a surface crystallization mechanism that induces a preferential orientation of crystal growth in the glasses. Near the surface, all the glass-ceramics obtained exhibit (002) planes preferentially oriented parallel to their faces. Deeper in the specimens, this preferential orientation is either kept or tilted to (201) after a depth of about 300 µm. The measurement of the charge coefficient d33 of the glass-ceramic highlights that surface crystallization induces mirror symmetry in the polarization. It reaches 11 to 12 pC/N and is not significantly influenced by the preferential orientation (002) or (201). High temperature XRD shows the stability of the fresnoite phase in the glass-ceramics up to 1000 °C. Mechanical characterization of the glass-ceramics by impulse excitation technique (IET) highlights that the softening of the residual glass leads to a progressive decrease of Young’s modulus in the temperature range 600–800 °C. Damping associated to the viscoplastic transition become severe only over 800 °C.

Effect of sintering atmosphere on the microstructure and dielectric properties of barium strontium titanate glass–ceramics

The sintering of barium strontium titanate glass–ceramics in nitrogen modified their dielectric properties significantly compared to the sintering in air. The experimental results demonstrate that the glass–ceramics sintered at low temperatures contain a major phase Ba2TiSi2O8 (BTS), known as fresnoite. The fresnoite phase disappeared and the barium strontium titanate perovskite phase became the major phase when the sintering temperature was increased. In addition, the microstructure observation showed that both the proportion of crystal phase and the crystal size increase obviously with the increase of sintering temperature. Most importantly, impedance spectroscopy has been employed to study the electrical responses arising from the glass and the crystal phases in the glass–ceramics sintered at low temperatures and high temperatures. The magnitudes of impedance and modulus changed significantly for the glass–ceramics sintered at the two temperature ranges. The activation energy calculated from the complex impedance, complex modulus and dc conductivity suggests that the dielectric relaxation for the glass phase and the glass–crystal interface may be attributed to the motion of the dipole associated with oxygen vacancy. And for the barium strontium titanate perovskite glass–ceramics, the motion of the electrons from the second ionization of oxygen vacancies leads to dc electrical conduction. The mechanism for the giant dielectric properties of the glass–ceramics sintered at high temperatures in nitrogen is discussed.

SiO2/Al2O3 ratio dependence of microstructures and dielectric properties in barium strontium titanate glass ceramics

(Ba, Sr)TiO3-Al2O3-SiO2 glass ceramic system with various SiO2/Al2O3 ratios was investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), dielectric spectroscopy and impedance spectroscopy. The XRD results demonstrated that the proper SiO2/Al2O3 ratio could promote the crystallization of the major crystalline phase from the glass matrix. The dielectric property investigations showed that the dielectric constant passes through a maximum value while the dielectric breakdown strength has a minimum value with increasing SiO2/Al2O3 ratio. Meanwhile, the complex impedance analyses suggest the resistance of the glass-crystal interface rapidly decreases and the capacitance of the crystal slightly decreases with the increase of SiO2/Al2O3 ratio. The relaxation mechanisms of the (Ba, Sr)TiO3 glass ceramics changed from localized relaxation to long range conductivity as the SiO2/Al2O3 ratio was increased from 1.43 to 1.83. The variations in the dielectric response and the activation energy of the glass-crystal interface in the (Ba, Sr)TiO3 glass ceramics with the ratio of 2.40 could be attributed to the crystallization of fresnoite phase.

Phase evolution and electrical properties of BaO–SrO–TiO2–SiO2–Al2O3-based glass ceramics prepared by sol–gel process

Crystallization of BaO–SrO–TiO2–SiO2–Al2O3-based glass ceramics, prepared by sol–gel process, was evaluated in terms of the effect of sintering temperature on phase evolution and electrical properties. The characterization of the samples was performed by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) studies and impedance spectroscopy analysis. The XRD results demonstrate that fresnoite phase starts to crystallize at 700 °C and perovskite phase appears at 900 °C. The glass ceramic samples sintered at high temperatures contained three crystalline phases, including perovskite, feldspar and fresnoite. In addition, SEM observation showed that the average grain size increased and the porosity decreased with increasing sintering temperature. Furthermore, the measurement of impedance spectroscopy suggests that there is a minimum value of the activation energy associated with the sintering temperature of the glass ceramics. The possible explanation of the sintering temperature dependence was discussed.

Sol–gel preparation and structural characterization of Ba2TiSi2O8 powder

Fresnoite (Ba2TiSi2O8, BTS) nano-particles were successfully synthesized at a low temperature of 900 °C by a modified Pechini sol–gel process with starting materials of Ba(NO3)2, Si(OC2H5)4 and Ti(OC3H7)4. The structural characterization of the precursors and derived synthesized oxide powders is done by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermal analysis (TG-DTG) and electron microscopy (SEM, EDX and TEM) studies. The effect of heat-treating temperature on the crystallinity of the fresnoite was investigated. Combined the XRD data and the strong FTIR peaks assigned to Ti–O and Si–O vibration, indicate the formation of fresnoite phase at a temperature of 900 °C. Also the nano-crystals size distribution was studied and the main diameter of nanoparticles was about 47 nm.

Fabrication and Dielectric Properties of (Ba0.7Sr0.3)TiO3–Glass Composites

Bulk and thick-film (Ba0.7Sr0.3)TiO3 (BST)–glass composites with different dielectric properties were fabricated at 1000 °C from powder mixtures of a sol–gel-derived BST and a commercial glass (GP). Increasing the calcination temperature of the BST gel powder from 850 to 1000 °C resulted in an increase in the sintered density of the composites with simultaneous increases in dielectric permittivity (er) and its temperature coefficient. Similar changes were observed in composites with different GP contents. A substantial decrease in er with a broadened er–temperature curve for the 12% GP composite could be attributed to the inhibited densification and enhanced formation of the secondary fresnoite phase. The dielectric properties and dielectric tunability of the composites were examined in relation to their relative density (drel), crystallinity (Lapp), and relative fraction of the BST phase (fBST). Although changes in er, its temperature dependence, and tunability were nominally correlated with the drel change of the sintered composites, the result that the drel of the composites was basically an increasing function of Lapp and fBST suggested the substantial contributions of Lapp and fBST to those changes.

Fabrication and Dielectric Properties of (Ba0.7Sr0.3)TiO3–Glass Composites

Bulk and thick-film (Ba0.7Sr0.3)TiO3 (BST)–glass composites with different dielectric properties were fabricated at 1000 °C from powder mixtures of a sol–gel-derived BST and a commercial glass (GP). Increasing the calcination temperature of the BST gel powder from 850 to 1000 °C resulted in an increase in the sintered density of the composites with simultaneous increases in dielectric permittivity (εr) and its temperature coefficient. Similar changes were observed in composites with different GP contents. A substantial decrease in εr with a broadened εr–temperature curve for the 12% GP composite could be attributed to the inhibited densification and enhanced formation of the secondary fresnoite phase. The dielectric properties and dielectric tunability of the composites were examined in relation to their relative density (drel), crystallinity (Lapp), and relative fraction of the BST phase (fBST). Although changes in εr, its temperature dependence, and tunability were nominally correlated with the drel change of the sintered composites, the result that the drel of the composites was basically an increasing function of Lapp and fBST suggested the substantial contributions of Lapp and fBST to those changes.

Effect of Ultrasonic Surface Treatment on the Transparency and Orientation of Fresnoite Surface Crystallization

Surface crystallized glass ceramics with fresnoite (Ba2TiSi2O8) phase were prepared by conventional heat treatment of 30BaO-20TiO2-50SiO2 glass together with ultrasonic surface treatment (UST) technique. The precursor glass was fully crystallized in a bulk form without any cracks, and the optical transparency and crystallographic orientation of the crystalline layers were evaluated by UV-Vis spectroscopy and XRD diffraction analyses, respectively. These properties were both enhanced significantly by applying UST using fresnoite/water suspension before the crystallization process, which is advantage for nonlinear optical applications of bulk glass ceramics. The effects of UST on the crystallization behavior were investigated by applying UST with various conditions.

Composition and texture of barium silicate crystals in fresnoite glass-ceramics by various scanning electron microscopic techniques

Highly oriented fresnoite glass-ceramics were prepared from a non-stoichiometric melt (Ba2TiSi2O8 + 0.75 SiO2) by electrochemically induced nucleation. By using electron microscopy, cathodoluminescence (CL), electron backscatter diffraction (EBSD) and energy dispersive X-ray (EDX) spectroscopy as well as wavelength dispersive X-ray (WDX) spectroscopy, two additional barium silicate phases free of titanium were identified besides the fresnoite phase predominantly occurring throughout the sample. A barium disilicate phase (BaSi2O5) was found within a radius of approximately 300 µm from the Pt-wire where the nucleation was induced. At distances >300 µm a phase with the composition BaSiO3 was detected which occurred in between the large dendritic fresnoite crystals next to an additional glassy phase which mainly consists of SiO2. Both barium silicate phases show more intense cathodoluminescence than the fresnoite phase and the maxima in the attributed spectra were shifted to smaller wavelengths in comparison to fresnoite.

ゾルゲル法による（Ｂａ，Ｓｒ）ＴｉＯ３ナノ結晶含有複合粉末の合成とキャラクタリゼーション

Composite gel powders with a basic composition of 0.65 (Ba0.7Sr0.3) TiO3 - 0.27 SiO2 - 0.08 Al2O3 were prepared by a sol-gel method in different solvent systems. In the alcohol solvent system, the difference in gelation time caused little change in the crystallization behavior in which the perovskite phase was formed at 800°C from the gel, but a substantial change in the sintering behavior of the calcined composite powders. The TPD-MASS analysis showed that the densification of the green compacts could be closely related to CO2 gas release from the calcined powders at 700∼800°C. In the case of the acetic acid solvent system, on the other hand, the composite powders prepared were characterized by a different crystallization behavior in which the fresnoite phase was metastably produced at 700°C and by the relatively large specific surface area of the calcined powders. They also revealed that an increase in the Al2O3 ratio in the glass composition (0.27 SiO2 - 0.08 Al2O3) resulted in an increased ratio of the perovskite BST phase after sintering, leading to an enhanced dielectric permittivity of the fabricated glass-ceramic.

Preparation of crystallized glass for application in fiber-type devices

We have demonstrated that 30BaO–15TiO2–30GeO2–25SiO2 (BTGS25) glass is a candidate for fiber-type nonlinear optical devices using crystallization of glass matrix. We determined the glass composition is suitable for crystallized fiber using partial substitution of Ge in 30BaO–15TiO2–55GeO2 (BTG55) by Si. The BTGS25 satisfied both thermal stability for fiber drawing and electronic polarizability for nonlinear optical property. After crystallization, the BTGS25 bulk crystallized glass showed surface crystallization behavior with the polar c-orientation of fresnoite phase, which was favorable for large second-order optical susceptibility. Following the results of the bulk glass, we prepared the BTGS25 glass fiber without precipitation of fresnoite crystallites. The BTGS25 crystallized fiber also showed c-oriented surface crystallization of fresnoite and second harmonic generation, which shows that the crystallized fiber is a promising material for fiber-type optical active devices.

Optical Properties and Structure of BaO-TiO2-SiO2Glass Ceramics

Nanocrystallized glasses with the composition of (50-x)BaO-xTiO₂-50SiO₂ (x=10, 15, 16.7 and 20) have been prepared by heattreatment at T x (crystallization onset temperature) for 3 h, and their optical properties, photoluminescence (PL), XRD and Raman spectra have been examined. The absorption edges of the glasses were red-shifted and the absorption coefficient increased with an increase of TiO₂. The glass subjected to the heat-treatment showed a dense formation of Ba₂TiSi₂O? crystals. The XRD and Raman results show that the nanocrystallized glasses formed fresnoite phase up to TiO₂ concentrations of 15 mol%. Furthermore, blue luminescence with a peak at the wavelength of around 470 ㎚ was observed in the nanocrystallized glass, demonstrating the optical multifunctional nanocrystallized material such as non-linear optics and photo-luminescence. It is thought that the blue luminescence from the Ba₂TiSi₂O? nanocrystallized glass originated from the presence of Ti⁴? incorporated into the fresnoite-type structure.

Crystallization behavior of optical fibers with multi layered structure for nonlinear optical devices

We examined the crystallization behavior of double clad fiber composed by BaO-TiO2-GeO2-SiO2 based glasses in detail. Formation of needle-like fresnoite-type Ba2Ti(Ge,Si)2O8 were observed at the interface between the first and second clad. In addition, we employed the two-step heat-treatment (i.e., 728°C, 20 h + 749°C, 1 h) in fabrication of the crystallized fiber in order to obtain the highly-oriented fresnoite phase in the fiber structure. The first treating temperature, 728°C, which gives the maximum nucleation rate, was determined by differential thermal analysis. It was demonstrated that the two-step heat-treatment was effective for improvement of the orientation of fresnoite-type phase in the first clad of the double clad fiber.

Dual layered surface crystallization of 30BaO–15TiO2–55GeO2 glass by stepwise heat treatment

In the crystallization process of 30BaO–15TiO2–55GeO2 (BTG55), we demonstrated by an x-ray diffraction analysis that a stepwise heat treatment enables a selective crystallization of fresnoite and benitoite, two crystalline phases obtained from the BTG55 mother glass and that these two phases form a dual surface layer. The result indicates that the stepwise heat treatment generates a compositional heterogeneity inside the glass matrix, which facilitates the nucleation of the fresnoite phase. The mechanism for the generation of the dual surface layer is discussed. Due to a marked difference in optical nonlinearity of the two phases, the selective crystallization may open the possibility of an optimized heat-treatment condition to achieve a large optical nonlinearity in crystallized glass. Also, two types of fresnoite, tetragonal and orthorhombic, were observed in the crystallized phase after a stepwise heat treatment of 690∕720°C.

Selective surface crystallization of nonstoichiometric 30BaO-15TiO2-55GeO2 glass

The correlation between heat-treatment temperature for crystallization and orientation of surface crystalline phase, which should correspond to the intensity of second-order optical nonlinear susceptibility, in 30BaO-15TiO2-55GeO2 (BTG55) crystallized glass was examined by measurements of x-ray diffraction (XRD) and second harmonic generation. A highly oriented fresnoite (Ba2TiGe2O8) or benitoite (BaTiGe3O9) phase was found to nucleate from the same mother BTG55 glass depending on the heat-treatment temperature, and both the fresnoite and benitoite phases showed XRD patterns of surface crystallization. It was found that the crystallization behavior of the two phases was strongly affected by the heat-treatment temperature. Based on this finding, the nonlinear optical property has also been tailored using the heat-treatment procedures.

Large second-order optical nonlinearity in 30BaO–15TiO2–55GeO2 surface crystallized glass with strong orientation

The correlation between the orientation degree of surface crystalline phase and the second-order optical susceptibility, d coefficient for second-harmonic generation, in 30BaO–15TiO2–55GeO2 (BTG55) crystallized glass was investigated using x-ray diffraction measurements and a Maker fringe technique. The d coefficient in BTG55 crystallized glass was confirmed to depend on the orientation degree of the surface crystalline phase. An oriented fresnoite (Ba2TiGe2O8) phase was observed in transparent BTG55 crystallized glass which had been heat treated at 720°C for 3h. The orientation degree along the polar c axis in the fresnoite phase increased with increasing removal thickness by polishing from the surface up to several microns. A d coefficient in transparent BTG55 crystallized glass of 24±3pm∕V has been obtained, which is the largest value in crystallized glass ever reported.

The effect of K 2O in the glass–ceramics based on the BaO–TiO 2–SiO 2 glasses

To study the effect of K 2O in the glass–ceramics, the heat-treated glasses were characterized by using X-ray diffraction, Fourier transform infrared spectroscopy and Scanning electron microscope. The results showed that the K 2O contents were not effects on the formation of new crystals but increased the lattice constant. Combined with XRD data, the FT-IR peaks assigned to Ti–O and Si–O vibration indicate the formation of fresnoite(Ba 2TiSi 2O 8) phase. The crystal shape and crystallization tendency of Ba 2TiSi 2O 8 crystal by the K 2O contents were also discussed.