Paul A. Tick

Transparent glass-ceramics

Glass-ceramic materials may transmit visible light if either of the following conditions are operative: (i) the crystallites of all species are much smaller than the wavelength of visible light, or (ii) the optical anisotropy (birefringence) within the crystals and refractive index difference between crystals and glass are very small. These conditions are achieved in several aluminosilicate glass-ceramic systems. Solid solutions (ss) of β-quartz, spinel, and mullite are the major crystalline phases in these transparent glass-ceramics. The transparent β-quartz solid solution glass-ceramics can be divided into three areas: ultra-low expansion materials, refractory and colourless materials, and high electrical resistivity—low dielectric loss materials. These three material groups, though chemically distinct, are all characterised by high crystallinity, thermal shock resistance, excellent chemical durability, and susceptibility to mechanical strengthening by ion-exchange techniques.The transparent spinel and mullite glass-ceramics contain considerable glassy phase, but nevertheless are characterised by excellent transparency, even after thermal exposure for long periods above 1000° C.

Transparent glass ceramics for 1300 nm amplifier applications

The properties of an oxyfluoride glass ceramic that possesses high transparency after ceramming are described. Approximately 25 vol % of this material is comprised of cubic, fluoride nanocrystals and the remainder is a predominantly oxide glass. When doped with Pr+3, the fluorescence lifetime at 1300 nm is longer than in a fluorozirconate glass, suggesting that a significant fraction of the rare‐earth dopant is preferentially partitioned into the fluoride crystal phase. This material has the added advantage of being compatible with ambient air processing.

Efficient neodymium-doped glass-ceramic fiber laser and amplifier

We report an efficient glass-ceramic fiber laser and show that its slope efficiency (~30%) is not compromised by the presence of Nd-doped fluoride crystals embedded within the core of the single-mode optical fiber. In contrast, the spectroscopy (fluorescence and gain spectrum) of the Nd(3+) ions is dramatically changed by the ceramming process, an indication of strong partitioning of the rare-earth ions into the CdF(2):PbF(2):YF(3) crystal environment. The enormous potential for a new range of optical devices based on transparent glass-ceramic materials is highlighted.

ARE LOW-LOSS GLASS-CERAMIC OPTICAL WAVEGUIDES POSSIBLE?

The results of a cutback measurement of a glass-ceramic optical waveguide with single-mode fiber geometry have demonstrated that sub decibels per kilometer losses can be achieved. Difference spectra show that the limit of intrinsic scattering in these two-phase structures ought to be tens of decibels per kilometer.

Quantum efficiency of Pr3+ doped transparent glass ceramics

The quantum efficiency of the 1.3 μm transition in Pr3+ doped oxyfluoride transparent glass ceramics is measured using a self-calibrating fluorescence ratio technique. The measured value of 8%±1% is much higher than expected for Pr3+ in an oxide glass, and is consistent with the incorporation of the Pr3+ ions into the fluoride nanocrystals of the glass ceramic. Measured quantum efficiencies are compared with values calculated using the Judd–Ofelt theory, and it is found that the best agreement is obtained when fractional differences between calculated and measured oscillator strengths are minimized, rather than absolute differences. The Judd–Ofelt analysis is used along with a spectral analysis to show that excited-state absorption should not significantly reduce the gain around 1.3 μm in these materials. The fluorescence ratio technique used to measure the quantum efficiency also provides information about ion–ion interactions at higher concentrations. It is found that the addition of the monovalent dopa...

Electrical Behavior of Composite Discontinuous Films

Composite discontinuous films of gold‐rich islands grown on palladium nucleii exhibit two distinctly different regions of temperature‐dependent conductivity. At low temperatures, film properties are dominated by a process characteristic of large gold‐rich islands which are closely spaced. This results in a film with a high resistivity and a low activation energy. At higher temperatures, the process abruptly changes and the conductivity of the film begins to increase rapidly with temperature. The dominant transport process in this conductivity region is attributed to activated tunneling between the small palladium‐rich islands.

The Boson peak in alkali silicate glasses

Abstract In an effort to understand the nature of the vibrational modes contributing to the Boson peak in glasses, we have performed a low frequency Raman study of different alkali silicate glasses at 300 K (X 2 O ( g ) ·SiO 2(1 - g ) = 25%, 35% by weight; X: Na, K, Rb, Cs). The introduction of alkali ions in silica is known to break SiO bonds, resulting in non-bridging oxygens and thus reducing the connectivity of the glass. Because of their identical charge, different alkali ions are expected to modify the glass network in the same way. We find that the shape of the Boson peak can be well fitted with a Lorentzian squared function characterized by a single parameter ν 0 . Analysis of the dependence of ν 0 on the type and concentration of alkali suggests that the position of the Boson peak is essentially governed by the ratio ‘force constant’ over ‘mass’ of localized oscillators and is not determined by the connectivity of the glass. The results also show little difference between quenched and annealed glasses, indicating the absence of a strong relation to the extended structure of the glass.

Electrical relaxation in a CdF2LiFAlF3PbF2 glass and melt

Abstract Electrical impedance measurements as a function of frequency were carried out on a CdF 2 LiFAlF 3 PbF 2 (CLAP) glass both below and above the glass transition temperature. Data were analyzed in the electric modulus formalism, and the Kohlrausch-Williams-Watts β parameter characterizing the width of the distribution of electric field relaxation times determined. β was temperature-independent for the glass below the glass transition temperature, T g , but decreased with increasing temperature for the melt above T g . A similar temperature dependence of β above T g has been observed for fused nitrate, alkali silicate, alkali borosilicate and lead borate melts and for extremely concentrated electrolyte solutions, suggesting that this is a general phenomenon for ionically conducting liquids with large concentrations of mobile ions.

Water/glass reactions at elevated temperatures and pressures

Abstract Hydration behavior of two closely related glasses was studied as a function of temperature and percent saturation of steam in an autoclave atmosphere. Temperatures ranged from 225 to 350°C under steam pressures which varied from 56 to 100 percent of saturation. These glasses hydrated to give a sharp boundary layer, with H 2 O contents (wt %) up to 17.5%. The position of this boundary (penetration), weight gain per unit area, and water content of the hydrated layer, were determined as a function of time, temperature and saturation. Penetration rates in both compositions decreased with decreasing saturation and were relatively insensitive to temperature. Water contents (by LOI) increased within the hydrated layer with decreasing temperature. The implications of these findings with regard to process control are discussed.

Computational approach to equilibrium relaxation in central force systems

Abstract The equilibrium configurations of low index surfaces in materials subject to central force laws and the behavior of adatoms on such surfaces were investigated using pairwise interaction theory. A direct search optimization code utilizing trial solutions with both a one-variable-at-a-time search and a vector search to scan the variables for energy optimization was applied. It was found that the interplanar spacings adjacent to a planar free surface are larger than equivalent bulk spacings and that the degree of interplanar expansion diminishes rapidly with increasing depth below the surface. The magnitude of surface expansion depends strongly on the characteristics of the potential chosen to represent the crystal forces. Surfaces which terminate with a monatomic step show non-linear structures at the step edge. Adsorption energies for adatoms on such surfaces are large on (111) surfaces and smaller on (110) and (100) surfaces. They decrease with decreasing adatom mismatch and increase with less convergent binding potentials. The activation energy for adatom surface diffusion was found to be largest on the (110) surface and smaller on the (100) and (111) surfaces; it decreases with increasing adatom mismatch and increases with more convergent binding potentials. The clustering characteristics of adatoms on (100) surfaces were investigated and applied to the system silver on nickel. The results show that both (111) and (100) oriented silver films could grow simultaneously on (100) nickel surfaces. No special attempt is made to compare the computed results with other published data. The primary purpose is to indicate a novel computational approach to surface investigations.