Relationships among the physical parameters required to give a linear relation between slope and intercept of Fowler-Nordheim plots.

For field emitters with a given surface condition, but different apex radii, there exists an empirical linear relationship between the slopes and intercepts of their Fowler-Nordheim plots. This behavior is quite different from that predicted by the theory with the assumption of a smooth surface. We have already characterized the possible reasons for this discrepancy; one possibility is that the emission area rapidly increases with increase in the apex radius. The previous report (Jpn. J. Appl. Phys. Part II 32 (1993) L342), however, showed a curved relation between the slope and the intercept of Fowler-Nordheim plots. In the present article, we try to extract the relationship between the emission area and the apex radius needed in order to give a linear relationship between slope and intercept of the F-N plots. As an additional result of the present analysis, the change in work function of the field emitting surface is demonstrated.     

Microscopy and computational modelling to elucidate the enhancement factor for field electron emitters

We report on the computation of the electric field at the surface of single-tip field emitters for a variety of geometries and wide range of geometrical parameters. In conjunction with experimental work, this has allowed the determination of quantities useful for characterizing and comparing the performance of field emitters. The ratio of the field at the tip surface to field at a tip supporting base (enhancement factor) has been calculated for hemispherical tips with parallel or conical shanks, for ratios of tip length to tip radius from 1 to 3000. Enhancement factors greater than 1000 are achievable with suitable tip geometry. The threshold voltage dependence on the tip–anode separation for cylindrical tips facing a flat anode has also been calculated and reported.     

New control algorithms via brain theory.

New concepts and viewpoints are needed to develop control algorithms for large scale systems. It is hypothesized that brain functions in human beings clearly demonstrates the existence of superior techniques for controlling complex systems with multiple objectives. The theory of compacta is investigated as a basis for learning, memory, and perception.     

Rogowski coils: theory and experimental results

The theory is given of the voltage output of a Rogowski coil excited by a current pulse flowing along the axis of the coil. In this theory the Rogowski coil is considered as a delay line. The results do not differ from those obtained usually by considering the coil as a voltage source dphi/dt with an inductive output impedance. Details are also given of the design of two Rogowski coils and their working modes are fully analyzed.     

Analysis of computational EELS modelling results for MgO-based systems

The ab-initio density functional theory (DFT) code CASTEP was used to model oxygen K edges in various magnesium oxide systems. Firstly, for the bulk material the process of geometry optimisation was carried out. Predicted oxygen K edges were found for a single cell with experimental lattice parameters, and parameters obtained after geometry optimisation, both with single electron core-holes in place. After geometry optimisation, a different predicted result was obtained, although it was qualitatively similar to the result for experimental lattice parameters in some respects. For example, approximately the same sets of peaks are observed, though in different energy positions, and with different relative peak intensities within those sets. Ultimately for the single cell results the experimental lattice parameters generated the predicted result that was in the closest agreement with experiment. It was further observed that a large supercell result (based on the experimental lattice parameters, utilising a core-hole) led to a slightly improved comparison with experiment as compared to the corresponding single cell result, although the latter result, and indeed a ground state calculation also give reasonable agreement with experiment. To rationalise these observations it was necessary to investigate the density of states (DOS) for the MgO cell and its constituent atoms, and it was observed that the conduction bands were of predominantly magnesium character. Furthermore, the core-hole’s introduction had relatively little overall effect on the p DOS prediction for oxygen, though there is a significant localised change close to the Fermi level. This work also considers interface and surface results. The principal aim of the study was to explore the interface of Fe (0 0 1)/MgO (0 0 1), crucial in certain classes of magnetic tunnel junctions (MTJs), which have significant technological applications. An initial step was to consider a MgO (0 0 1) surface. It was verified that a surface could be constructed such that within that surface a theoretical result could be found that matched the bulk result. It was then valid to use this surface as part of an interface with iron. Theoretical results obtained at that interface compare well with experimental results from an epitaxially grown MTJ, and various conclusions are drawn with regard to the nature of the interface.     

Some results concerning the development of the molecular-mechanical theory of friction

This paper is dedicated to the 100th anniversary of the birth of one of the authors of the molecular-mechanical theory of friction, outstanding tribologist I.V. Kragel’skii. The nature of the mechanical and molecular components of friction is analyzed; the experimental methods of their determination and the mutual influence of these components are considered.     

Confinement of surface state electrons on Cu and Au field emitters.

Field-emission images of clean Cu and Au emitters show a peculiar halo-like ring centered at the [1 1 1] pole. Typical diameter and width of the ring are approximately 10 nm and approximately 2 nm, respectively. Since we found no geometrical features around the [1 1 1] pole that gives rise to such a ring pattern, we interpreted the ring pattern as representing enhanced emission from an annular terrace that resonantly confines surface electrons. A simple analysis shows that the observed ring pattern appears at a terrace whose width nearly matches the confinement condition.     

The shape of field emitters and the ion trajectories in three-dimensional atom probes

The lateral resolution of three-dimensional atom probes is mainly controlled by the aberrations of the ion trajectories near the specimen surface. For the first time, a simulation program has been developed to reconstruct the ion trajectories near a sharp hemispherical electrode defined at the atomic scale. Surface atoms submitted to the highest field were removed one by one. The consecutive gradual change of the surface topology was taken into account in the calculation of ion trajectories. As the tip was 'field evaporated', the initial spherical shape of the emitter was observed to transform gradually into a polygonal shape. When the tip reached its equilibrium shape, the field distribution at the tip surface was found to be much more uniform compared to the initial distribution. The calculated distribution of ion impacts on the detector exhibits the presence of depleted zones both at the centre of low index poles and along <001> zone axes. These predictions are in excellent agreement with experiments.     

Use of confocal and multiphoton microscopy for the evaluation of micro-optical components and emitters

We report on the application of confocal and multiphoton microscopic techniques for the evaluation of the latest generation of micro optical components. The optical emitting characteristics of arrays of matrix addressable GaN micrometer-sized light emitting diodes (micro-LEDs) have been measured using a commercial confocal microscope utilising the LEDs' own emission along with reflection confocal microscopy to determine the surface structure. Multiphoton induced luminescence from the 10-20-micron diameter emitters has also been used to examine the structure of the device and we compare this with electrically induced emission. In related work, the optical properties of micro lens arrays (10-100-micron diameter) fabricated in SiC, Sapphire, and Diamond have been determined using transmission confocal microscopy. Such optical microscopy techniques offer a simple, non-destructive method to determine the structure and performance of such novel devices.     

Characterization of a high voltage flat panel display unit using nanotube-based emitters.

Details are given of an experimental investigation carried out to study the field electron emission characteristics of a field emission flat panel display unit using a carbon nanotube-epoxy composite as electron emission material. These include: (i) dependence of direct emission current-voltage characteristic on vacuum gap spacing, (ii) the variation of the proportion of emission current passing through an aperture hole of a gate electrode with changing structural parameters of the device, and (iii) the uniformity and display characteristics of a typical display unit. Our findings indicate that it is very likely for one to produce a near-market prototype high voltage field emission flat panel display, if more sophisticated fabrication and assembly technique is adapted.     

Simulation and modelling in measurement

The versatility of objects controlled by information-measuring systems requires the formation of information-measuring instruments with adequately versatile functional capabilities. The aim of the paper is to name some simulation problems for multifunctional systems and to discuss the modelling of measurement errors. A new modelling algorithm for measurement errors is introduced. Furthermore, simulation methods for the investigation of complex measurement procedures are recommended.     

Comparison between experimental and computer simulations of current-voltage (I-V) characteristics of dielectric-coated photon-stimulated field emitters.

For the purpose of simulating photon-stimulated field emission by taking account of three-dimensional aspects, a transfer-matrix formulation of electronic scattering was combined with a Floquet expansion of the wave function for taking account of quanta exchanges between the electrons and the external radiation. With specific techniques to preserve numerical stability, this transfer-matrix formalism is well suited to compute the transmission of the field-emitted/photon-stimulated electrons between two electrodes. This theory is applied to the computation of Fowler-Nordheim curves describing the photon-stimulated field emission of a tungsten plane emitter (described by z< or =0), which supports a nanometric protrusion and a dielectric coating. The extraction bias ranges from 12 to 24V, for an inter-electrode distance of 4nm. The electromagnetic radiation has a wavelength of 0.67 microm and a power flux density ranging from 5.96 x 10(10) to 5.96 x 10(12) W/m2. The effects due to the protrusion and the dielectric coating are studied. These theoretical results are compared with the experimental data.     

Progress in the theory of ultrasonic flaw detection. Problems and prospects

This paper was prepared upon request from the organizers of the XVIII St. Petersburg conference Ultra-sonic Flaw Detection in Metal Structures. UZDM 2004. The progress made in the theoretical issues of ultrasonic flaw detection over the past 20 years is briefly considered, and some practical issues are discussed. Specific problems that are waiting to be solved are formulated, and the authors opinions about the prospects of developments in ultrasonic flaw detection are presented. The preparation of the paper was much facilitated by book [1], which was published recently.Translated from Defektoskopiya, Vol. 40, No. 10, 2004, pp. 13–48.Original Russian Text Copyright © 2004 by Ermolov.     

Analysis of the results of acoustic-emission inspection in an overloading test performed on the basis of the kinetic theory of fracture

The strength margin and the rupture life of a structure with a flaw detected via AE measurements are estimated during an overloading test. The kinetic theory of fracture is used. Stages of damage accumulation and crack growth have been considered. It is shown that the lifetime can be extended for a given period, according to the results of the testing, only at moderate operating temperatures.     

New results for the fretting-induced stress concentration on Hertzian and flat rounded contacts

Recent work on fretting fatigue has emphasized the role of stress concentration on fretting damage, while previous work had concentrated on empirical parameters to assess influence of fretting on fatigue life. In particular, analogies with fatigue in the presence of a crack or a notch have been noticed, suggesting that the stress field induced by frictional contact per se may explain the reduction of fatigue life due to fretting.In the paper, new analytical and numerical solutions are produced for the stress concentration induced in typical fretting contacts involving the Hertzian geometry or the flat punch with rounded corners in view of application to the dovetail joints. Normal and tangential load (in the Cattaneo–Mindlin sense) is considered with “moderate” or “large” bulk stresses.     

Macroscopic electrical field distribution and field-induced surface stresses of needle-shaped field emitters

One major concern since the development of the field ion microscope is the mechanical strength of the specimens. The macroscopic shape of the imaging tip greatly influences field-induced stresses and there is merit in further study of this phenomenon from a classical perspective. Understanding the geometrical, as opposed to localized electronic, factors that affect the stress might improve the quality and success rate of atom probe experiments. This study uses macroscopic electrostatic principles and finite element modelling to investigate field-induced stresses in relation to the shape of the tip. Three two-dimensional idealized models are considered, namely hyperbolic, parabolic and sphere-on-orthogonal-cone; the shapes of which are compared to experimental tips prepared by electro-polishing. Three dimensional morphologies of both a nano-porous and single-crystal aluminium tip are measured using electron tomography to quantitatively test the assumption of cylindrical symmetry for electro-polished tips. The porous tip was prepared and studied to demonstrate a fragile specimen for which such finite element studies could determine potential mechanical failure, prior to any exhaustive atom probe investigation.     

Studies of refractory carbides,nitrides, and borides as the thermionic emitters for electron microscopy

Thermionic emission properties of several kinds of refractory carbides, nitrides, and borides of the transition metals in the form of powder were investigated with a newly developed measuring device and evaluated by the figure of merit defined as the ratio of the effective work function to the working temperature at which the vapor pressure becomes 1 × 10^?5 Torr. There are several materials whose thermionic emission properties are better than those of tungsten or compatible to those of tungsten among the carbides and borides, such as TaC, HfC, ZrC, LaB₆, and CeB₆, as judged by the figure of merit. New preparation methods for carburization, nitriding, and boriding of the wires of matrix metals and alloys were successfully developed for using these materials as the cathode of the electron microscope. Other necessary techniques such as spot welding and electrolytic etching were also developed. From the brightness characteristics, it was found that some of carbides, carbide solid solutions, and borides such as HfC, ZrC (Ta_0.8–0.7Hf_0.2–0.3)C, TaB₂, and HfB₂ are very good emitters comparable to LaB₆. It is emphasized that the work functions of the carbide-solid solutions (Ta_0.8Hf_0.2)C and (Ta_0.7Hf_0.3)C, which have low rates of evaporation at high temperature, show no remarkable rise as compared with that of HfC, so that their figures of merit are better than that of HfC. Feasibility of providing good cathodes with HfC and (Ta_0.8Hf_0.2)C tips was demonstrated by taking high-resolution electron micrographs.     

三元流动理论在叶轮设计中的一种新方法

通过用平面圆弧加锥体圆弧的方法使其尽可能接近三元流动理论的需要，对改造叶轮进行设计，在使用中效果显著的事实证明该方法的确是一种简单易行的好方法。