Alan J. Slavin

A new model for the effective thermal conductivity of packed beds of solid spheroids : alumina in helium between 100 and 500°C

Abstract An analytical model is given for the thermal conductivity of a bed of solid spheroidal particles in static gas, when the conductivity of the solid is substantially greater than that of the gas. It has two fitting parameters, the width and average radius of the narrow gaps that exist between the irregularly shaped particles and which contribute significantly to the thermal conductivity. Since both parameters are physically measurable, the model holds the potential for calculating the thermal conductivity without any adjustable parameters. Agreement is excellent with measurements on alumina particles in helium at 100–500°C up to 100 kPa pressure.

Canadian ceramic breeder sphere-pac technology: capability and recent results

Abstract Sphere-pac ceramic breeders have been under development in Canada for several years. The goal is to fabricate and characterize these materials for use in engineering test reactors and subsequent fusion power reactors. Practical application of sphere-pac beds requires close consideration of both properties and fabrication. The present emphasis of the program is on 1–3 mm diameter Li 2 ZrO 3 spheres, with the future development of binary beds planned. Litre quantities have been produced by methods that are applicable to high production rates. These spheres are being tested for measurement of bulk properties (e.g., thermal conductivity, gas permeability, packing density, tritium release, specific heat) and long-term irradiation exposure. This paper summarizes the status of the work.

Adsorption kinetics and absolute coverages for C2H2 and C2H4 on a tantalum (110) surface held at 465 and 680 K

Abstract Auger spectroscopy has been used to measure the adsorption kinetics of acetylene and ethylene gases at 300 K on a clean tantalum (110) surface held at either 465 or 680 K, at gas pressures near 10−6 Pa. Adsorption occurs irreversibly with identical initial sticking probabilities for both gases leading to the conclusion that its value is unity. This assumption permits a calculation of absolute coverage of approximately 2 carbon atoms for each tantalum atom in both cases. The fact that this value is the same for both gases suggests that ethylene may dehydrogenate to an acetylenic species as claimed for W(110). Heating a sample previously saturated at 465 K results in desorption of hydrogen accompanied by an increase in the carbon Auger signal. This is interpreted as evidence that at least some of the hydrogen atoms lie further from the surface than do the carbon.

Oxygen uptake by thin lead deposits on polycrystalline gold

The oxidation of thin lead overlayers on a predominantly (111) polycrystalline gold surface at room temperature and oxygen pressure of 8×10 −6 Torr was investigated using Auger electron spectroscopy (AES). Oxidation was monitored using the oxide lead peak obtained by spectral decomposition. This peak experiences a shift from pure lead of 2.3±0.1 eV for deposits up to 3 monolayer-equivalents (ME) and 2.7±0.1 eV for deposits greater than about 4 ME. The oxygen signal for a 1 ME deposit at saturation exposure is about 20% of the value expected for a complete oxygen monolayer. This is explained in terms of clusters of about 5 Pb atoms for each oxygen atom. This differs from the PbO stoichiometry which has been observed under similar conditions on bulk lead and also claimed for the oxidation of thin lead deposits on Ag and Cu. Also contrary to results on Ag and Cu is the rate of oxygen uptake on Au which increases with lead coverage in the submonolayer region; this is explained by the absence of adsorption and surface diffusion of oxygen on gold, and the need for clusters of about 5 lead atoms for oxidation to occur. Deposition of more than 1 ME of lead results in lead-gold compound formation as observed by previous authors. Exposure of this compound to oxygen causes oxidation of the surface lead with inward migration of gold from the surface. The oxide layer is about 2 monolayers thick and greatly reduces the rate of further oxidation. The large decrease in the gold substrate signal on oxidation is a sensitive method for differentiating between compound formation and altermative growth modes for the bimetallic surface.

The adsorption kinetics of C2H2, and C2H4 on W(110) at 1100 K

The adsorption kinetics of C2H2 and C2H4 gases on W(110) have been studied using Auger electron spectroscopy and qualitative LEED. Below 1100 K, adsorption of either C2H2; or C2H4 does not follow any simple kinetic model to saturation. At 1100 K adsorption is identical for both gases and follows first order monolayer kinetics with unity sticking coefficient and a carbon-to-tungsten atomic ratio of 0.64 ± 0.05. This carbon is present as a surface carbide which starts to in-diffuse about 1500 K and has completely dissolved after a few seconds at 2400 K.

Thermal decomposition of the (15 × 3)R14° two-dimensional carbide on the W(110) surface

Abstract The dissolution of carbon from the two-dimensional (15 × 3)R14° carbide which forms on the W(110) surface was monitored by Auger electron spectroscopy as a function of temperature and time. The results show an unexpected increase in the rate of in-diffusion of carbon after a certain annealing time, which has been explained by assuming that the carbide decomposition is the rate-limiting step and occurs only at the edges of holes in the carbide. This appears to be the first observation of such dissolution kinetics. Analysis yields an activation energy of carbide decomposition of 3.1 ± 0.4 eV/atom.

Angular dependence of the secondary electron emission crystal current: Effects of surface modification

The variation of the angle between the incident electron beam and sample surface contributes in two ways to the secondary electron emission from single crystals. These contributions are a monotonically varying background due to the changing number of secondaries produced near the surface, and an oscillatory component previously explained as a bulk effect related to Kikuchi patterns. This work shows that for emission from a Au(111) surface, the background can be described well by a simple model based on the semiempirical theory of Kanaya and Kawakatsu. It also provides additional support for the explanation of the oscillations as a bulk effect by showing that the oscillation amplitude remains essentially unchanged either by surface sputtering or by the deposition of a thin lead layer. The presence of these oscillations requires some care in the use of the secondary electron crystal current as a measure of the thickness of thin metal films.

An inexpensive alternate to level‐sensing automatic filling systems for liquid nitrogen

This note describes an overflow sensor used as the level controller for automatic filling systems for liquid nitrogen. It serves as an inexpensive and reliable alternative to level‐sensing devices in applications where the container is to be refilled to the top at regular intervals.

Stabilization of sample temperature in a surface-science vacuum chamber to 0.03 K and quartz-crystal microbalance frequency to 0.06 Hz over 0.5 h

Improvements have been made to a high-stability quartz-crystal microbalance for use in a typical surface-science, ultrahigh vacuum chamber, with a frequency stability of one part in 10(8) (0.06 Hz) over 0.5 h. This gives a resolution equivalent to 2% of an atomic monolayer of oxygen over 0.5 h. The quartz-crystal microbalance (QCM) crystal can be rotated to different surface-analysis positions in the chamber. These characteristics open up the combination of surface and bulk adsorption studies on the same sample without transferring the sample to another chamber. To accomplish this, it was necessary to stabilize the sample temperature to ±0.03 K over several hours. The oscillator performance is illustrated by the uptake of oxygen by a gold-plated QCM crystal.

Test of a new model for the temperature and pressure dependence of the thermal conductivity of a packed pebble bed in gas: lithium zirconate in helium

This paper tests a recently developed theory for the thermal conductivity of a packed pebble bed in the presence of a gas, at substantially higher temperatures than previously used to test the theory, by fitting previous measurements on lithium zirconate in helium. The ultimate goal of the theory is the prediction of the thermal conductivity of a packed bed using only parameters that can be measured directly from the particles. The main innovations in the theory are its description of the conduction across the narrow gaps between irregularly shaped particles having several points of contact, in terms of the average gap width and area, and the introduction of a new expression for bridging the change in gas conductivity between the low and high pressure limits. In the case of the lithium zirconate beds, the theoretical fits require a third fitting parameter to take account of the oblateness of these pebbles. The fitted curves then describe very well both the temperature and pressure dependence of the experimental data, and provide reasonable values for all the fitting parameters. The model shows that conduction through the points of contact can be ignored, at least for the small test beds modeled.