E. E. Donaldson

Adsorption and desorption of ammonia, hydrogen, and nitrogen on ruthenium (0001)

The adsorption of ammonia, hydrogen, and nitrogen on a Ru(0001) surface have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. The adsorption of ammonia on Ru(0001) can be divided into a low temperature mode (100 K) and a higher temperature mode (300–500 K). For a crystal temperature of 100 K the ammonia adsorbs into two weakly bound molecular γ states with s = 0.2. The ammonia desorbs as NH3 molecules with desorption energies of 0.32 and 0.46 eV. At 300–500 K adsorption occurs via an activated process with a low sticking probability (s ⩽ 2 × 10−4).This adsorption is accompanied by dissociation and formation of an apparent (2 × 2) LEED pattern. Hydrogen adsorbs readily (s = 0.4) on Ru(0001) at 100 K and desorbs with 2nd order kinetics in the temperature range 350–450 K. Nitrogen does not appreciably adsorb on Ru(0001) even at 100 K; maximum nitrogen coverage obtained was estimated to be <2% of a monolayer. Changes in the ammonia flash desorption spectra after hydrogen preadsorption at 100 K will be discussed.

The Emission of Electrons and Positive Ions from Fracture of Materials.

The emission of electrons and positive ions from materials undergoing fracture is investigated. We present a survey of charged particle emission from a number of materials including crystalline insulators, glass, graphite, polymers and composites. Particular attention is given to fibre-reinforced epoxy systems which yield unique forms of charge emission. Energy distributions of the emitted particles are given for E-glass-epoxy strands, polybutadiene filled with glass beads, and mica. Evidence is presented that interfacial failure and charge separation play important roles in the observed emission.

Effects of an electron beam on adsorption and desorption of ammonia on ruthenium (0001)

Abstract The effects of an electron beam on ammonia adsorption and desorption on Ru(0001) have been investigated by Auger electron spectroscopy, low-energy electron diffraction, and thermal flash desorption. Appreciable adsorption at room temperature occurred only on the area of the Ru crystal which had been bombarded by an electron beam during dosing. The adsorption rate was a function of beam current density and ammonia pressure, and an apparent (2 × 2) diffraction pattern appeared in the area bombarded by the electron beam. Electron bombardment of the molecular γ states of ammonia followed by flash desorption showed that less ammonia and more hydrogen and nitrogen were desorbed as the bombardment time increased. An analysis of this process based on electron-induced dissociation of the ammonia molecule yielded an effective initial dissociation cross section of 3 × 10 −16 cm 2 . Hydrogen flash desorption spectra after bombardment of the γ states showed two states obeying first order kinetics with desorption energies of 0.78 and 1.0 eV. Electron bombardment of the γ states for short times produced the same effects on the ammonia flash desorption spectra as preadsorption of hydrogen.

Interaction of hydrogen with a (100) niobium surface

Abstract Hydrogen adsorbs on the (100) face of niobium at 90 K into three states sequentially. The first state, which appears to be molecular, has a dipole moment of −0.022 Debye/molecule, an initial sticking coefficient of 0.23, and a saturation coverage of 3.2 × 10 14 molecules/cm 2 reached after an exposure of about 1 L (1 L = 10 −6 Torr sec). The first state may be a precursor for adsorption into the second state. The second state is atomic and has a dipole moment of −0.141 Debye/atom, an initial sticking coefficient of 0.056, and a saturation coverage of 9 × 10 14 atoms/cm 2 which is reached after an exposure of about 12 L. The heat of adsorption for state 2 was found to be 26.5 ± 3 kcal/mole. At an exposure of 12 L, hydrogen adsorbs into a third state and simultaneously diffuses into the bulk suggesting that the third state may be the precursor for bulk diffusion.

Production and Properties of Ejecta Released by Fracture of Materials

Abstract We have examined ejecta (particles in the size range 0.1 to 500 μm) which are released by fracture of a variety of materials. The ejecta from most non-metallic materials are electrically charged and frequently have high velocities. The amount of ejecta produced depends on the material and the conditions of fracture. For unfilled, glassy polymers the ejecta are produced in regions of fast-hackled fracture. Detailed measurements have been made on the ejecta mass and size distributions from the fracture of composites. From these measurements the total particle surface areas can be estimated and are found to be comparable to or greater than the cross-sectional area of the fractured samples. Thus, the ejecta should be a consideration in the analysis of surface energy and other parameters from fractographical analysis.

Acoustic emission and electron emission during deformation of anodized aluminium

Tribostimulated exoelectron emission provides a sensitive way to characterize anodic films on aluminium. This paper describes the detection of acoustic emission (AE) and electron emission (EE) during the tensile deformation of anodized aluminum. It is shown that the observed bursts of AE are associated with cracking of the oxide, that AE and EE count rates vs strain are closely related, and that most EE occurs very soon after AE events. Evidence is also presented for a component of EE unrelated to AE which is attributed to chemi‐emission.

Emission of electrons and positive ions upon fracture of oxide films

During tensile deformation of oxide‐coated aluminum, small cracks a few hundredths of a mm in length occur in the oxide film. During and following the appearance or elongation of these cracks, electron emission (EE) and positive ion emission (PIE) are detected in the surrounding vacuum. Crack propagation in the oxide coating can be detected with an acoustic emission (AE) transducer. Correlations between charged particle emission and film fracture can then be determined. A comparison of rates of EE, PIE, and AE, the distribution of the number of electrons or ions per burst, and the time distributions relative to crack propagation of both EE and PIE are presented. The time distributions indicate distinct differences in the rate limiting steps governing EE and PIE.

Time and Size Correlations of Photon and Radiowave Bursts from Peeling Pressure Sensitive Adhesives in Air

Abstract During separation in air of an adhesive from a polymer substrate we have observed intense bursts of photons (phE for photon emission) and long wavelength electromagnetic radiation (RE-for radiowave emission), similar to those reported earlier by Deryagin et al. In this paper we present detailed measurements of phE time distributions as well as time and size correlations between bursts of phE and RE. These results support the view that patches of electrical charge produced by charge separation between dissimilar materials lead to gaseous breakdown in and near the crack tip. We discuss the role of these discharges in producing sustained phE after the discharge has been extinguished.

Autographs from Peeling Pressure Sensitive Adhesives: Direct Recording of Fracture-induced Photon Emission

Abstract It is well known that visible light is emitted during the peeling of adhesives from various substrates. The major source of light has been identified as small gaseous discharges that result from the intense charge separation accompanying detachment of dissimilar materials. In this paper, we describe an experimental technique which produces clear images of the photons created by peeling pressure sensitive adhesives directly from the surface of a film emulsion or from the surface of a glass fiber optic face plate in contact with the film. The resulting autographs of the emitted light show in considerable detail the spatial structure of the photon emission which in turn reveals the mechanical and electrical behavior of these materials during the peeling process.

Electron and acoustic emission accompanying oxide coating fracture

Abstract The behavior of an oxide coating during the tensile deformation of its substrate depends on the physical properties of the oxide and the oxide-substrate interface. These properties strongly influence the rate of appearance and extension of cracks in the oxide layer. We investigate the relationship between the growth of cracks in anodized Al 1350 and electron and acoustic emission. We present evidence that both types of emission are strongly influenced by the energy released during crack propagation.