Showing papers by "National Physical Laboratory published in 1983"

Automatic fringe analysis with a computer image-processing system

Abstract: The application of a general-purpose image-processing computer system to automatic fringe analysis is presented. Three areas of application have been examined where the use of a system based on a random access frame store has enabled a processing algorithm to be developed to suit a specific problem. Furthermore, it has enabled automatic analysis to be performed with complex and noisy data. The applications considered are strain measurement by speckle interferometry, position location in three axes, and fault detection in holographic nondestructive testing. A brief description of each problem is presented, followed by a description of the processing algorithm, results, and timings.

Characterization of a high depth-resolution tantalum pentoxide sputter profiling reference material

Abstract: A major use of surface analysis instruments concerns composition depth profiling. In many instances poor depth resolution is obtained, one cause of which is poor ion beam alignment and uniformity. To enable instrument users to optimise instrument settings and know (1) the instrument contribution to depth resolution and (2) the ion etch rate, tantalum pentoxide reference materials have been developed with thicknesses of 30 nm and 100 nm and interface widths, measured by AES, of 1.5 and 1.7 nm respectively. The effects of argon ion beam energy, electron beam current density, sputtering geometry and gas purity effects on etch rate and interface width are measured and interpreted in terms of basic parameters, showing that the material is a well-behaved model system that is robust and easily used. The precise shape of the interface and preferential sputtering effects are also analysed.

The depth dependence of the depth resolution in composition–depth profiling with Auger Electron Spectroscopy

Abstract: The depth dependence of the depth resolution in composition–depth profiles using Auger electron spectroscopy is briefly appraised to emphasize some of the different mechanisms operating for different classes of material and to show how the use of a characterized Ta2O5 on Ta reference material can help to optimize the depth resolution. In addition it is shown that, even for flat uniformly-layered samples sputtered under ideal conditions where the ion beam crater is flat and impurities may be ignored, five factors still limit the depth resolution: information depth, statistical effects and cascade mixing, electron stimulated desorption and sample roughening. These may each be observed to dominate at the appropriate depth for amorphous materials but, for evaporated polycrystalline metal films, the last term dominates for depths greater than 10 nm.

Applying the RSA Digital Signature to Electronic Mail

Abstract: Because of the increased cost-effectiveness of computer technology and its subsequent acceptance into the business world, computer-based message systems are likely to become the principal carriers of business correspondence. Unfortunately with the efficiency of these systems come new possibilities for crime based on interference with digital messages. But the same technology that poses the threat can be used to resist and perhaps entirely frustrate potential crimes. For some messages, a degree of privacy or secrecy is needed, which is possible with encryption. However, predicting the extent encryption will be used in electronic mail is difficult, since much depends on the cost and convenience of its applications. For nearly all messages, authenticity is a prime requirement. Authenticity implies that the message is genuine in two respects: its text has not changed since it left the sender and the identity of the sender is correctly represented in the text header or in the signature attached to the message. Neither of these authenticity indicators is sufficient by itself because an altered message from sender A is in no way different from a message appearing to come from A but in fact coming from an enemy. The technique of authentication, which is closely related to cryptography, normally uses the symmmetric type of cipher, typified by the Data Encryption Standard, or DES, algorithm. This kind of authentication is seriously deficient because both the sender and receiver must know a secret key. The sender uses the key to generate an authenticator, and the receiver uses it to check the authenticator. With this key, the receiver can also generate authenticators and can therefore forge messages appearing to come from the sender. In other words, authentication can protect both sender and receiver against thirdparty enemies, but it cannot protect one against fraud committed by the other. If A sends a message to B, for example, B might fraudulently claim to have received a different message. Supposing B takes some action in response to a genuine received message, A can still claim that B in fact forged the message. For these reasons, authentication by the customary methods using symmetric ciphers can do nothing to resolve disputes arising from the dishonesty of either sender or receiver. As a solution to the dispute problem, Diffie and Hellmant proposed the use of a digital signature based on certain public-key cryptosystems (Figure 1). The sender of the message is responsible for generating the

Some Regular Properties of the ‘Data Encryption Standard’ Algorithm

Abstract: A cipher function y = E(k,x) should appear to be a random function of both the key k and the plaintext x Any regular behaviour is of interest to the users In the extreme case regular properties might point to a weakness of the cipher Precautions are needed in the use of a cipher that has regular features This note describes five regular properties of the ‘Data Encryption Standard’ or DES, two of which have been described elsewhere, are included for completeness

Segregation to interphase boundaries in liquid-phase sintered tungsten alloys

Abstract: Scanning Auger electron spectroscopy has been used to examine the distribution of impurity elements on the fracture surfaces of liquid-phase sintered W-Ni-Cu and W-Ni-Fe alloys. On the interphase boundaries between the fcc Ni-based matrix phase and the tungsten particles, segregation levels of ~0.4 and ~0.2 monolayers of phosphorus have been observed in as-sintered, furnace-cooled specimens of W-Ni-Cu and W-Ni-Fe, respectively. The phosphorus is homogeneously distributed but at fracture adheres preferentially to the matrix phase. High temperature heat treatment (1350 °C) followed by water quenching reduces significantly the phosphorus segregation and improves the degree of cohesion across these boundaries. Segregated sulfur is detected on both sides of the interphase boundaries after fracture. The sulfur is much less uniformly distributed than the phosphorus, and its segregation level increases in the heat treated specimens. Copper also segregates to the interphase boundaries during the heat treatment of W-Ni-Cu specimens, but no equivalent segregation of iron was observed in the W-Ni-Fe system. The boundaries developed between adjacent tungsten particles are free of impurity contamination in both alloy systems but have a segregated layer of nickel.

Bi-directional fringe counting interference refractometer

Abstract: A bi-directional fringe counting interference refractometer has been developed primarily to measure the refractive index of air so that the accuracy of laser length measuring interferometer systems used in the free atmosphere can be maintained. Two signals suitable for electronic counting in phase-quadrature and sinusoidally related to path difference are produced by a technique that does not require any form of modulation and yet has the advantages of a modulated system. This refractometer is cheap to produce and easy to set up and align. Measurement sensitivity of 2.5 parts in 10 7 for the refractive index of air can be obtained directly from the counting signals and this has been further improved by fringe sub-division. Comparisons with calculated values indicate that the refractometer can measure ambient refractive index in a laboratory without temperature control with an uncertainty approaching 1 part in 10 7

On some transport properties of strontium-doped lanthanum chromite ceramics

Abstract: D.C. Electrical conductivity, Hall mobility and magnetic susceptibility measurements on La1−xSrxCrO3 (0⩽x⩽0.25) perovskite ceramic system, and their temperature dependence, have been carried out to understand the nature of the transport mechanism in them. The electrical conductivity and Hall mobility displayed thermally activated temperature dependence with activation energies that varied from 0.13 to 0.23 ev. The variation of the d.c. conductivity of the polycrystalline La1−xSrxCrO3 with strontium content (X) has been found to be strongly affected by the changes in microstructure. Magnetic susceptibility data indicate that the electronic transport is due to the presence of Cr4+ ions in the lattice, and that the localized level of hopping is associated with the chromium 3d band.

Helium Vapour Pressure Equations on the EPT-76

Abstract: Equations are presented relating the vapour pressures of 3He and 4He to temperature using the EPT-76 scale. Values of vapour pressure, and its derivative, dP/dT76, are tabulated at 0.01 K intervals, and the differences with respect to the 1958 4He scale and the 1962 3He scale are given.

New definition of the metre

Abstract: Some of the physical assumptions which are implicit in the proposed new definition of the metre are discussed, including the mass of the photon and the constancy of the fine structure constant.

Goniophotometric and polarization properties of white reflection standard materials

Abstract: The angular distribution of luminance factor has been determined for all plane polarized states for nine types of reference white material, namely: different preparations of MgO, BaSO4 and the opal...

Viscoelastic behavior of amorphous polymers in the glass‐rubber transition region: Birefringence studies

Abstract: Viscoelastic processes associated with the glass-rubber transition of an amorphous polymer are considered to involve hindered rotations around main-chain bonds, which give rise to a decrease in molecular distortion in the stressed material and to an orientation or disorientation of chain segments. The local distortional changes are associated with variations in energy-elastic stress. The orientational rearrangements yield changes in conformational entropy and energy and may be described in the long-time region by the normal mode motions of Gaussian submolecules. Assuming that the net time-dependent stress and birefringence may each be expressed as the sum of distortional and orientational contributions, methods are proposed on the basis of stress-optical data for resolving the distortional and orientational stresses or moduli during stress-relaxation, creep, and dynamic mechanical tests. An analysis of dynamic birefringence data for different amorphous polymers indicates that the orientational processes become important in the long relaxation-time region and are consistent with predictions of the submolecular theory. The distortional changes dominate the shorter relaxation-time behavior and are characterized by somewhat narrower distributions of relaxation times than are apparent from the unresolved data.

Sensitization of photoconductivity in tetragonal lead monoxide

Abstract: The thermal sensitization process for the enhancement of photoconductivity in tetragonal lead monoxide has been investigated. At a critical annealing temperature the photosensitivity has been found to have a maximum value (≅5×102) while the conductivity is observed to have a minimum value (≅10−10Ω cm). The transient response of the photocurrent exhibits an overshoot, the height of which is strongly dependent on the annealing temperature. These and other findings have been explained on the basis of sensitization due to the creation of two types of centres in the material.

Deformation Characteristics and Mechanical Properties of Hardmetals

Abstract: Fundamental deformation characteristics of transition metal interstitial-compounds are briefly reviewed, and reference is made to experimental and proven metallic cermet systems using WC/Co hardmetals as a basis for comparison. Emphasis is given to the need for a critical approach to mechanical testing of hardmetals and for caution in interpreting results for empirical and intrinsic property measurements. Deformation mechanisms and microstructuralmechanical property correlations are examined for elastic and plastic properties, strength, fracture, fracture toughness and fatigue.

Growth kinetics of monofilamentary Nb3Sn and V3Ga synthesized by solid-state diffusion

Abstract: Studies of growth kinetics of Nb3Sn and V3Ga formation have been carried for mono-filamentary composites of niobium and vandium filaments embedded in bronze wires containing varying concentrations of tin and gallium, respectively. The samples are diffusion reacted at different temperatures and for different lengths of time and the thickness and the microstructure of the resulting A-15 layer are investigated using optical and scanning electron microscopy techniques. The results are discussed in the light of the analytical model previously proposed by the present authors and it is shown that while the rate controlling step for the formation of Nb3Sn is diffusion of tin through the bronze matrix, for V3Ga it is the diffusion of gallium through the grain boundaries of the compound layer. The data are used to calculate the activation energies for Nb3Sn and V3Ga formation.