Robert K. Mohr

Non‐Newtonian viscous flow in glass

The viscosity of a soda‐lime silica glass was measured at high strain rates. The data show non‐Newtonian viscous flow in this inorganic oxide glass with the viscosity values below the expected Newtonian value. Following the imposition of large, steady strain rates, the observed stress increases with time to a maximum and then decreases to a time‐independent value. A comparison of the viscosity behavior of this glass with the molecular dynamics results in a ’’Lennard‐Jones’’ glass shows a number of points of correspondence and suggests the interpretation of the non‐Newtonian behavior as resulting from structural rearrangements in the material. The combined data show that the sustained, steady‐state stress asymptotically approaches a maximum at very high strain rates. This limiting stress is interpreted as the actual cohesive strength of the material and is calculated to be 1.4×108N/m2 (20,000 psi) for the glass under study.

Intrinsic and impurity infrared absorption in As2Se3 glass

Abstract A quantitative study of infrared absorption in the 250–4000 cm −1 region of As 2 Se 3 glasses doped with small amounts of As 2 O 3 or purified by various procedures has been carried out with particular attention to absorption in the wavelength regions of the CO 2 and CO lasers. The dependence of the relative intensities of the oxide impurity bands in the 650–1340 cm −1 region on the total amount of As 2 O 3 added to the glass indicates the existence of three distinct oxide-impurity species. A number of higher-frequency impurity bands which are due to the presence of hydrogen in the glass and whose intensities are highly dependent on the glass-melting conditions have been observed and classified. Intrinsic multiphonon absorption in the 400–1100 cm −1 region has been interpreted in terms of combination and overtone bands of the two highest-frequency fundamental vibrational modes. Absorption coefficients of As 2 Se 3 glass in the 920–1090 cm −1 CO 2 laser region are limited by intrinsic multiphonon absorption to values of around 10 −2 cm −1 . The lowest absorption coefficients measured in the 1700–2000 cm −1 CO laser region were around 2 × 10 −3 cm −1 and may contain contributions from hydrogen-impurity bands.

Light scattering in a number of optical grade glasses

Abstract Light scattering measurements have been made on a number of commercially available optical grade glasses. By measuring the spectrum as well as intensity of the scattered light it was found possible both to obtain the total scattered intensity and to identify the origin of the scattering. The optical attenuation coefficient due to scattering was evaluated. It was found that in all glasses studied the total scattered intensity exceeds that in fused silica by at least a factor of two.

Stress optic coefficient and stress profile in optical fibers

The stress optic coefficient and stress profile in optical fibers have been determined photoelastically using a polariscope having good reproducibility and high sensitivity. The results of the work presented in this paper indicate that the photoelastic behavior may be different in fibers and in bulk glasses. The photoelastically determined clad compression in strengthened fibers was found to correlate well with the strengthening observed in these fibers using tensile tests.

High temperature thermal conductivity of platinum microwire by 3ω method.

The 3ω method for thermal conductivity measurement has emerged as an effective technique applicable to micro/nanowires and thin films. This paper describes the adaptation of the method to temperatures as high as 725 K enabling reliable thermal conductivity measurements on such samples for which previously published methods have been found inadequate. In the technique, a sample wire is heated by applying a sinusoidal current at an angular frequency ω, which causes a temperature and resistance variation at an angular frequency, 2ω, leading to a voltage signal at 3ω. The sample is connected as a four-terminal resistor to a digital lock-in amplifier, which is used to detect the in-phase and out-of-phase 3ω voltages resulting from the applied 1ω current. The data are fitted by varying the values of the thermal resistance and diffusion time, both of which are functions of thermal conductivity. Measurements are made at steady state temperatures between 300 and 725 K. Meaningful measurements at elevated temperatures require that thermal losses be understood and minimized. Conduction losses are prevented by suspending the sample above the mounting substrate. Convection losses are minimized by maintaining a vacuum of ~10(-5) torr inside the sample chamber. To minimize radiation losses, an appropriately sized sample is shrouded with a double heat-shield, with the inner shield temperature near that of the sample. Using the 3ω method, the thermal conductivity of platinum was determined to vary between 71.8 and 80.7 Wm(-1) K(-1) over the temperature range of 300 to 725 K, in agreement with published values measured for bulk samples.

Corrosion of Inconel-690 electrodes in waste glass melts

Inconel-690, a Cr-Ni-Fe-based “superalloy,” has become the material of choice for electrodes in joule-heated waste glass melters and is currently employed in the high-level nuclear waste vitrification systems at West Valley and DWPF, as well as in GTS Durateks privatized M-Area mixed waste vitrification facility at Savannah River. Future applications of joule-heated vitrification technologies will necessitate an assessment of the limits of performance of this material under more demanding conditions than have been studied previously. In this work, Inconel 690 electrodes were tested in several simulated sodium-rich aluminosilicate waste glasses in wide ranges of AC current density, electrical waveform, temperature, and glass composition.

Strengthening of Optical Fibers by Molecular Stuffing

Even though glass has high intrinsic strength, it is a brittle material which suffers strength degradation due to static fatigue. Stringent control of preform preparation, drawing conditions and immediate coating with protective materials may limit the initial flaw population in glass fibers, but no coating or pretreatment can completely prevent strength degradation under tensile loading conditions. If the fiber surface is under compression, however, one can guarantee the preservation of the initial tensile strength for static loads less than the compression and can greatly increase the time to failure for larger loads. This paper discusses one preform fabrication technique, “Molecular Stuffing” which is especially suited to producing optical fibers having high residual surface compression. The basic aspects of molecular stuffing are discussed with particular emphasis on its applicability to strengthening.

Multiphonon Absorption in Chalcogenide Glasses

The“molecular model” of Lucovsky and coworkers for vibrational properties of chalcogenide glasses such as As2S3, As2Se3, GeS2 and GeSe2 suggests that multiphonon absorption in these materials should be analogous to overtone and combination vibrational bands in isolated molecules. A variety of experiments have been carried out whose results are in reasonable accord with this prediction. These include Raman spectra of As2S3 glass, measurement of the frequency dependence of infrared absorption in the multiphonon region for As2S3, As2Se3, and mixed As2S3-As2Se3 and As2Se3-GeSe2 glasses, and measurement of the temperature dependence of absorption coefficients in the multiphonon region for As2Se3 glass.