Dispersion of barium gallogermanate glass

Gallogermanate glasses are the subject of intense study as a result of their unique combination of physical and optical properties, including transmission from 0.4 to beyond 5.0 microm. These glasses can be easily made into large optics with high-index homogeneity for numerous U.S. Department of Defense and commercial visible-IR window applications such as reconnaissance, missile domes, IR countermeasures, avionics, and collision avoidance on automobiles. These applications require a knowledge of the refractive index of glass throughout the region of transmission. Consequently, we have measured the refractive index of BaO-Ga2O3-GeO2 glass from 0.4 to 5.0 microm and calculated the Sellmeier coefficients required for optical device design.     

The positive temperature coefficient of resistivity in barium titanate

Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above ∼ 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.     

Sintering of glass bonded ceramic barium hexaferrite magnetic powders

Mixtures of low-melting glasses (glazes) and barium hexaferrite magnetic powders sinter at relatively low temperatures (800° C) and produce dense items in which excessive grain growth has been inhibited.     

Elastic properties of lanthanum gallogermanate glasses

Room temperature ultrasonic velocities of eight lanthanum gallogermanate glasses were determined by pulse-echo technique. The results indicate that both longitudinal and transverse velocities of these glasses are composition dependent. The density and index of refraction of the samples were also studied. The experimental results were used to obtain elastic constants. The measured values of Young's modulus and bulk modulus for our glasses show good agreement with the theoretically calculated results based on the model of Makishima and Mackenzie.     

Dielectrics of lead zirconate bonded with barium borate glass

Dielectric constant variation with temperature and frequency is reported for barium-borate glass-bonded lead zirconate. Lowering of the relative permittivity of the ceramic is attributed to the presence of the glass.     

EPR in a barium borosilicate glass containing titanium ions

EPR spectra were studied in an equimolar barium borosilicate glass containing 0 to 12 mol% titanium ions. The valence concentration ratio, defined as the ratio of the concentration of the lower valence state transition metal ion Ti³⁺ to the total concentration of the transition metal ion Ti³⁺ + Ti⁴⁺, was varied between 0.15 and 0.85 at each concentration. The spectra were studied as a function of temperature down to 15 K. A broad asymmetric absorption peak is observed with effectiveg ~ 1.938 at room temperature. This absorption is attributed to a tetragonally distorted octahedral ligand field around Ti³⁺ ion. Theg value decreases as the temperature decreases, with a distinct transition between 60 and 100 K. Accompanied by this decrease ing is a distinct broadening of the curves with a transition in the same transition range. These and other observations are explained in terms of a lengthening of the spin-lattice relaxation time in this transition range and this transition is analogous to the NMR motional narrowing phenomena in solids. Many observations at low temperatures suggest a non-random distribution of titanium ions in these glasses.     

Effect of barium on diffusion of sodium in borosilicate glass

Diffusion coefficients of sodium in barium borosilicate glasses having varying concentration of barium were determined by heterogeneous isotopic exchange method using (24)Na as the radiotracer for sodium. The measurements were carried out at various temperatures (748-798 K) to obtain the activation energy (E(a)) of diffusion. The E(a) values were found to increase with increasing barium content of the glass, indicating that introduction of barium in the borosilicate glass hinders the diffusion of alkali metal ions from the glass matrix. The results have been explained in terms of the electrostatic and structural factors, with the increasing barium concentration resulting in population of low energy sites by Na(+) ions and, plausibly, formation of more tight glass network. The leach rate measurements on the glass samples show similar trend.     

Measurements of the thermo-optic coefficient of a barium fluoride single crystal

The thermo-optic coefficient dn/dT of single-crystal barium fluoride is measured with a precision spectrometer. The values over the visible region in the 30-90 °C and 90-150 °C ranges are -18.8 and -19.7 × 10(-6)/°C, respectively.     

Devitrification and microstructural coarsening of a fluoride-containing barium aluminosilicate glass

Barium aluminosilicate (BAS) glass-ceramics have the potential to be used in the production of cast prostheses for biomedical applications because of their radiopacity and increased strength compared with traditional feldspathic porcelains. It is essential to understand the crystallization kinetics of these materials in order to fabricate products with increased fracture resistance rapidly. It was hypothesized that the addition of fluoride (F) to the composition of BAS glass would reduce the necessary processing time and temperatures by obviating the need for a separate crystal nucleation treatment. BASF glass was subjected to both linear non-isothermal and one-stage isothermal crystallization treatments, and the resulting glass-ceramics were characterized using x-ray diffraction, differential thermal analysis, and stereology. BASF glass had a low energy barrier to crystallization (397 kJ/mol) and transformed to 76 ± 2% crystallinity within 30 min at 975°C. A fine-grained microstructure was produced by bulk crystallization without the need for a separate crystal nucleation stage. After the initial crystal precipitation, the mean crystal size and mean free path between crystals increased over time at elevated temperature by a diffusion rate-limited coarsening mechanism.     

The crystallization behavior and kinetics of a barium fluorozirconate type glass

An investigation of the crystallization behavior of a Zr-Ba-La-Al-F glass which is subjected to isothermal heat treatments is presented. The number and nature of the crystalline phases which form and their nucleation sites are determined at three temperatures. Also, the growth rate and volume fraction of crystals which nucleate internally in the glass, when heated at 320°C, are determined as a function of time.     

Impact of mechanical stress on barium titanate-based positive temperature coefficient resistive material

The sensitivity toward mechanical stress of barium titanate-based positive temperature coefficient resistor material was investigated by determining the resistance change with application of uniaxial stress from room temperature to 200 °C, which is well above the Curie temperature T_C. Using the Landau–Ginsburg–Devonshire theory the resistance increases in the paraelectric state, the negligible impact of stress close to T_C and the observed increase in T_C with increasing stress could be rationalized. For the ferroelectric state, the stress-related resistance increase was attributed to ferroelasticity, a change in bulk permittivity and interfacial stress inducing a piezoelectric potential. The obtained results are also discussed with respect to recent endeavors to tune properties of potential barriers in piezoelectric semiconductors by mechanical stress.     

Optical properties of down-shifting barium borate glass for CdTe solar cells

CdTe thin film solar cells have a poor response in the ultraviolet and blue spectral range, mainly due to absorption and thermalization losses in the CdS buffer layer. To overcome this efficiency drop in the short wavelength range trivalent rare-earth doped barium borate glass is investigated for its potential as frequency down-shifting cover glass on top of the cell. The glass is doped with either Tb³⁺ or Eu³⁺ up to a level of 2.5 at.% leading to strong absorption in the ultraviolet/blue spectral range. Tb³⁺ shows intense emission bands in the green spectral range while Eu³⁺ emits in the orange/red spectral range. Based on rare-earth absorption and luminescence quantum efficiency the possible gain in short-circuit current density is calculated.     

Temperature dependence of elastic moduli of lanthanum gallogermanate glasses

Temperature dependence of sound velocities of a series of Lanthanum Gallogermanate glasses have been determined from ultrasonic pulse-echo and Brillouin scattering measurements ranging from room temperature up to and through the glass transition temperature. Both longitudinal and transverse velocities of these glasses are composition dependent. The density and index of refraction of the samples were also studied. The experimental results are used to obtain elastic moduli. The correlation of elastic stiffness, the crosslink density, and the fractal bond connectivity of the glass are discussed. The normal behavior of negative temperature dependence of elastic properties is observed in these glasses. A possible explanation of the observed discrepancy of high temperature sound velocity of these glasses from two different measurements is given.     

Thermo-optic dispersion formula for AgGaS2

AgGaS(2) has been found to be 90 degrees phase matchable at 192 degrees C for sum-frequency mixing between the output of the KTP parametric oscillator at 3.2627 mum and its pump source at 1.0642 mum. The thermo-optic dispersion formula that can be used for good reproduction of the temperature-dependent phase-matching conditions thus far reported in the literature as well as our new data for second-harmonic generation of a CO(2) laser and its harmonics at 1.7652-5.2955 mum is presented.     

Near infrared absorption coefficient of molten glass by emission spectroscopy

Emission spectroscopy is applied in the determination of the near infrared spectral absorption coefficient of molten glass. The glass is held in a small horizontal platinum alloy crucible, within an electrically heated cell, optically coupled to a Fourier transform spectrometer. A formula is derived which relates emissivity to absorption coefficient, thickness, and reflectivities for the glass-air and glass-metal interfaces. The reflectivity parameters are determined, in effect, by varying the thickness. Spectral absorption coefficient results are compared with results of transmission spectroscopy. The emission technique is advantageous in that it eliminates the problem of chemical reactions with window materials used in the transmission method, and sample preparation and interfacing to commercially available spectrometers is simplified.     

The effect of glass addition on the dielectric properties of barium strontium titanate

Ba_0.7Sr_0.3TiO₃ (BST) ceramics were prepared by the conventional solid state ceramic route. Different weight percentages of twelve different glasses were added to the calcined BST ceramics and sintered. The structure and microstructure of the sintered materials were investigated by X-ray diffraction and Scanning Electron Microscopic techniques. The low frequency dielectric properties of the glass-ceramic composites were measured using LCR meter. Some of the glasses improved the dielectric properties and considerably lowered the sintering temperature. The glasses were prepared and characterized under identical conditions. Among the different glasses, 1.5 wt% addition of 71ZnO–29B₂O₃ lowered the sintering temperature of BST to 975 °C with a dielectric loss of 9 × 10⁻³ and dielectric constant of 875 at 1 MHz. The curie temperature of BST ceramics was found to shift towards lower temperature with glass addition.