David C. Cranmer

Transient creep behaviour of hot isostatically pressed silicon nitride

Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y2O3 as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of longterm transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330–1430°C for times exceeding approximately 1100 h, an apparent activation energy of ≈ 1260 kJ mol−1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of − 1.12 was determined.

Effects of the structure and composition of lead glasses on the thermal lensing of pulsed laser radiation

The thermal lensing characteristics of several silicate, germanate, phosphate, and borate glasses were studied using a laser with a 7 ns pulse at 457 nm in a tight focus geometry. A geometric model was developed to describe the quadratic radial profile of the refractive index resulting from the laser‐induced temperature profile. This model was utilized to interpret the effects of some of the relevant experimental parameters on the fluence transmission experiments. The influence of material properties such as different types of network former and modifier ions on the nonlinear optical properties of these materials were also studied. It was found that: (i) the greatest influence of the network modifier ions was due to their effect on the absorption coefficient of the glasses; (ii) in lead glasses, the thermo‐optic coefficients dn/dT of the germanates and silicates with random network structures were greater than those of the borate and phosphate glasses with ring and chain structures; and (iii) the main con...

Refractive index gratings in rare‐earth‐doped alkaline earth glasses

Four‐wave‐mixing techniques were used to produce permanent laser‐induced refractive index gratings in Eu3+ ‐doped silicate glasses. These gratings are associated with a thermally induced change in the local glass structure at the site of the Eu3+ ions, leading to a double‐minimum potential well for the electronic energy levels of the Eu3+ ions. The effects on the characteristics of the permanent laser‐induced gratings produced by changing the divalent modifier ions of the glass host are reported and a theoretical model is presented to explain the physical origin of the change in the refractive index of the material.Four‐wave‐mixing techniques were used to produce permanent laser‐induced refractive index gratings in Eu3+ ‐doped silicate glasses. These gratings are associated with a thermally induced change in the local glass structure at the site of the Eu3+ ions, leading to a double‐minimum potential well for the electronic energy levels of the Eu3+ ions. The effects on the characteristics of the permanent laser‐induced gratings produced by changing the divalent modifier ions of the glass host are reported and a theoretical model is presented to explain the physical origin of the change in the refractive index of the material.

6Li-doped silicate glass for thermal neutron shielding

Abstract Glass formulations are described that contain high concentrations of 6 Li and are suitable for use as thermal neutron shielding. One formulation contained 31 mol% of 6 Li 2 O and 69 mol% of SiO 2 . Studies were performed on a second formulation that contained as much as 37 mol% of 6 Li 2 O and 59 mol% of SiO 2 , with 4 mol% Al 2 O 3 added to prevent crystallization at such high 6 Li 2 O concentrations. These lithium silicate glasses can be formed into a variety of shapes using conventional glass fabrication techniques. Examples include flat plates, disks, hollow cylinders, and other more complex geometries. Both in-beam and in-core experiments have been performed to study the use and durability of Li silicate glasses. In-core experiments show the glass can withstand the intense radiation fields near the core of a reactor. The neutron attenuation of the glasses used in these studies was 90%/mm. In-beam studies show that the glass is effective for reducing the gamma-ray and neutron fields near experiments.

The dependence of laser‐induced refractive index changes in glass on the chemical composition

The properties of permanent, laser induced refractive index gratings in rare‐earth doped glasses have been characterized in a variety of materials including lithium borates, lead and magnesium silicates, lead borate‐germanates and lead germanates. This work is combined with the previous results obtained on phosphate and silicate glasses to develop an understanding of the effects of glass composition (both network former and modifier ions) on the ability of a material to produce gratings with high scattering efficiencies. The grating efficiencies and the writing/erasing dynamics were studied, and the two‐level‐system‐model developed previously was employed to explain the results. The results show the importance of having flexible glass networks with low polarizing power of the network former ions, and a high content of nonbridging oxygen ions. In addition, it is important to have light, highly polarizable modifier ions with weak chemical bond strengths to the oxygen ions. This type of glass combined with r...

Moisture- and water-induced crack growth in optical materials

A number of optically important materials such as ZnS, SiO2, SiO2-TiO2, GaAs, and heavy metal fluoride (e.g., ZBLAN) glasses are subject to moisture- and/or liquid water-induced crack growth. A notable exception to this behavior appears to be Si. Such environmentally enhanced crack growth can lead to ultimate failure in service at stresses well below those expected from normal strength tests. The sensitivity of a material to water can be obtained by determining a crack growth parameter, N. This parameter can be combined with other easily obtainable fracture information which include measures of the strength and strength distribution to create a lifetime design diagram using fracture mechanics concepts. Methods for determining these fracture parameters including direct crack growth measurements and dynamic fatigue are reviewed, and the influence of environmental water on the materials is discussed. Crack growth mechanisms including physical (dielectric) and chemical reaction mechanisms are discussed, and lifetime design diagrams which can be used to determine stress levels in service are presented.

Fibre-matrix interface testing of ceramic composites

ABSTRACT An instrumented indenter technique has been used to measure fiber/matrix bonding in several ceramic composites including SiC/LAS-III, chemically vapor infiltrated (CVI) SiC/SiC, and SiC monofilament/Borosilicate Glass. The indenter measures the load on and displacement of the fiber directly, thus avoiding the necessity of measuring small indent sizes. Values of fiber/matrix debond strength ( T d ) and interfacial frictional stress ( T f ) have been measured in each of these systems. In several instances, no movement of the fiber was observed. Differences in T have been ascribed to differences in bonding between the fibers, coatings, and matrices. Variability in r within each sample has been observed and will be discussed.