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B L Thiel

Publications and source records attributed to B L Thiel.

10 recordsLinked to original sources
PubMed2002

On the role of electron-ion recombination in low vacuum scanning electron microscopy.

Here we demonstrate the effects of electron-ion recombination on imaging signals utilized in low vacuum scanning electron microscopes (SEMs). The presented results show that, under normal operating conditions, recombination of ionized gas molecules with secondary electrons (SEs) suppresses a significant fraction of emitted electrons. If the ion flux (and hence the spatial dependence of the SE-ion recombination rate) is laterally inhomogeneous across the imaged region of a specimen, contrast in SE images can be influenced and in some cases (under conditions of high detector field strength and long ionic mean free path) dominated by variations in the recombination rate. Consequently, SE images of features such as topographic asperities can exhibit edge-darkening, leading to inversion of some topographic contrast. Recognition of the extent and nature of electron-ion recombination is required for a correct understanding of processes occurring in variable pressure SEMs and, subsequently, for models of image formation.

Journal Article↗
PubMed2002

Consequences of positive ions upon imaging in low vacuum scanning electron microscopy.

The effects caused by an excess quantity of ionized gas molecules within the low vacuum, variable pressure and environmental scanning electron microscope (ESEM) are described with reference to mechanisms by which they can influence imaging conditions. These effects can include specimen charging, recombination and development of space charge. They are demonstrated for three different classes of sample: (1) an electrically grounded conductor, (2) an electrically floating conductor, and (3) an electrical insulator. A new device is presented that will aid excess charge removal within the ESEM and help correct for some of these effects, thereby dramatically improving imaging over a wide range of operating conditions and samples. The mechanism of image enhancement is demonstrated with reference to the three classes of sample described above.

Journal Article↗
PubMed2001

Radiation damage of water in environmental scanning electron microscopy.

Specimen damage from the electron beam poses a considerable problem with electron microscopy. This damage is particularly acute in environmental scanning electron microscopy (ESEM) for two reasons. Firstly, owing to its ability to stabilise insulating and hydrated specimens, ESEM lends itself to polymeric and biological materials that are typically highly beam-sensitive. Secondly, water acts as a source of small, highly mobile free radicals, which accelerate specimen degradation. By taking the results of single-particle simulations of electron-water interactions, we determine the concentration of reactive species in a water specimen under ESEM conditions. We consider 12 species, which are produced in a Gaussian distribution, and annihilate according to a second-order reaction scheme. Self-diffusion along the concentration gradient is also modelled. We find that the dominant reactive species is the hydroxyl (.OH) radical. Annihilation of this species is suppressed due to the lower concentration of reactants. The relatively stable hydrogen peroxide is also found at large concentrations. By comparing two beam energies, 5 and 25 keV, we find a drastic increase in the quantities of reactive species produced with beam energy. The longer range of 25 keV primary electrons spreads reactive species over a wider region, which then decay far more slowly.

Journal Article↗
PubMed2001

Topographic contrast of partially wetting water droplets in environmental scanning electron microscopy.

Partially wetting water droplets with sizes smaller than the capillary length acquire a distinct spherical cap shape controlled by the equilibrium contact angle, which is specific for different substrates and conditions. Images of such droplets in an environmental scanning electron microscope (ESEM) show strong topographic contrast. This contrast across the droplets can be analysed within a simple theoretical model, as the droplet sides are inclined smooth surfaces. Very small droplets have ESEM intensity profiles which deviate from this topographic model. Such deviations indicate that other sources of electron signal may be important for such droplets, and also demonstrate the limits of the analytical model. For droplets sufficiently large that they lie within the range of the topographic contrast model, values of contact angles on different substrates can be deduced. These are found to agree with independent direct measurements, as well as the results given in the literature. The possibilities of using this technique to analyse physical properties of different substrates are discussed.

Journal Article↗
PubMed2000

The elasticity and failure of fluid-filled cellular solids: theory and experiment.

We extend and apply theories of filled foam elasticity and failure to recently available data on foods. The predictions of elastic modulus and failure mode dependence on internal pressure and on wall integrity are borne out by photographic evidence of distortion and failure under compressive loading and under the localized stress applied by a knife blade, and by mechanical data on vegetables differing only in their turgor pressure. We calculate the dry modulus of plate-like cellular solids and the cross over between dry-like and fully fluid-filled elastic response. The bulk elastic properties of limp and aging cellular solids are calculated for model systems and compared with our mechanical data, which also show two regimes of response. The mechanics of an aged, limp beam is calculated, thus offering a practical procedure for comparing experiment and theory. This investigation also thereby offers explanations of the connection between turgor pressure and crispness and limpness of cellular materials.

Biomechanical Phenomena↗
PubMed1997

Non-periodic lattice crystals in the hierarchical microstructure of spider (major ampullate) silk.

A commonly adopted model for the microstructure of Nephila clavipes major ampullate silk (MAS) is similar to that used for Bombyx mori (silkworm) silk: a simple composite wherein discrete, essentially perfect crystals are dispersed throughout an amorphous protein matrix. However, inconsistencies arise when researchers using complementary microstructural characterisation techniques attempt to explain their results within that framework. We present here the findings of our parallel studies in x-ray diffraction, electron microscopy, and molecular modeling. These results, combined with other data gleaned from the literature, lead us to propose a revised description of the spider silk microstructure. The new model recognizes that the 70-500 nm sized ordered regions in MAS cannot be constructed from a simple motif of repeating monomers, and develops the concept of non-periodic lattice (NPL) crystals to characterize these structures. The local composition, symmetry, and perfection of order vary over distances that are small compared to the size of an NPL crystal.

Animals↗
PubMedDate not supplied

Dynamic secondary electron contrast effects in liquid systems studied by environmental scanning electron microscopy.

We report an investigation into a dynamic contrast phenomenon in water-oil emulsions imaged in the environmental scanning electron microscope. Secondary electron contrast between oil and water phases is shown to change with scan rate, even inverting in extreme cases. This effect is attributed to the fact that charge carriers in liquids have intermediate mobilities compared with those in metallic conductors and solid insulators. Thus, increasing the electron energy flux density (via slower scan rates) results in the temporary accumulation of excess charge, which in turn gives rise to increased secondary electron emission. Excess charge dissipates between frames, however, such that classical charging of the specimen is not observed. The oils used here have conductivities lower than that of water, making them more susceptible to the effect. However, the material within the primary electron interaction volume is a conductive medium. We demonstrate that charging effects are not seen in regions of the oil where the interaction volume is in contact with the more conductive continuous water phase. Secondary electron emission from these regions therefore approximates to the intrinsic yield.

Electric Conductivity↗
PubMedDate not supplied

Spider (Araneus diadematus) cocoon silk: a case of non-periodic lattice crystals with a twist?

Transmission electron microscopy was used to investigate the supramolecular structure of Araneus diadematus (garden spider) cocoon silk. Electron diffraction patterns contain features which are consistent with the presence of non-periodic lattice crystals, i.e. highly frustrated crystalline regions as identified previously in the major ampullate silk (MAS, dragline) of Nephila clavipes spiders. The diffraction patterns further suggest that crystals in A. diadematus cocoon silk may be twisted parallel to the chain direction, offering a potential explanation for the lower tensile stiffness of this fibre relative to MAS.

Amino Acid Sequence↗
PubMedDate not supplied

Interpretation of secondary electron images obtained using a low vacuum SEM.

Charging of insulators in a variable pressure environment was investigated in the context of secondary electron (SE) image formation. Sample charging and ionized gas molecules present in a low vacuum specimen chamber can give rise to SE image contrast. "Charge-induced" SE contrast reflects lateral variations in the charge state of a sample caused by electron irradiation during and prior to image acquisition. This contrast corresponds to SE emission current alterations produced by sub-surface charge deposited by the electron beam. "Ion-induced" contrast results from spatial inhomogeneities in the extent of SE signal inhibition caused by ions in the gaseous environment of a low vacuum scanning electron microscope (SEM). The inhomogeneities are caused by ion focusing onto regions of a sample that correspond to local minima in the magnitude of the surface potential (generated by sub-surface trapped charge), or topographic asperities. The two types of contrast exhibit characteristic dependencies on microscope operating parameters such as scan speed, beam current, gas pressure, detector bias and working distance. These dependencies, explained in terms of the behavior of the gaseous environment and sample charging, can serve as a basis for a correct interpretation of SE images obtained using a low vacuum SEM.

Journal Article↗