In the last 30 days

• Open/close Contact electrification across metal-dielectric and dielectric-dielectric interfaces

  G S Rose and S G Ward 1957 Br. J. Appl. Phys. 8 121 Tag this article

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  An apparatus is described by means of which a spherical dielectric surface can be compressed under various loads against either a plane metal disk or a plane dielectric disk. A transfer of electric charge across the interface is recorded when the surfaces are separated. The polarity of charging between any two materials depends upon the relative positions of those materials in an electrostatic series; a metal surface does not always assume a positive charge as found for impact electrification. For two given materials the amount of charge is proportional to the contact area between the surfaces and provided that one of the surfaces has a resistivity above a critical value (5 × 10 ¹¹ Ωcm) then the charge density for various combinations of materials is approximately proportional to the lower dielectric constant in each combination. This result suggests that the charging may be limited by electrical back-discharge between the separating surfaces. It is considered to be unlikely that the electrification originates from a difference in the contact potentials of the two surfaces involved.

• Open/close The elasticity and strength of paper and other fibrous materials

  H L Cox 1952 Br. J. Appl. Phys. 3 72 Tag this article

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  An analysis is made of the effect of orientation of the fibres on the stiffness and strength of paper and other fibrous materials. It is shown that these effects may be represented completely by the first few coefficients of the distribution function for the fibres in respect of orientation, the first three Fourier coefficients for a planar matrix and the first fifteen spherical harmonics for a solid medium. For the planar case it is shown that all possible types of elastic behaviour may be represented by composition of four sets of parallel fibres in appropriate ratios. The means of transfer of load from fibre to fibre are considered and it is concluded that the effect of short fibres may be represented merely by use of a reduced value for their modulus of elasticity. The results of the analysis are applied to certain samples of resin bonded fibrous filled materials and moderately good agreement with experimental results is found.

• Open/close The practical determination of lattice parameters using the centroid method

  B W Delf 1963 Br. J. Appl. Phys. 14 345 Tag this article

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  A method of determining centroid positions which is based on the method of Pike and Wilson is described. Lattice parameters are determined by correcting observed centroid positions for the effects of all known systematic errors and assuming a value for the wavelength of the radiation used. The lattice parameters thus determined are found to be independent of Bragg angle within the limits of experimental error. The method is applied to specimens of aluminium, tungsten and diamond and lattice parameters with standard deviations of about 1 part in 80 000 are obtained. The advantages and disadvantages of the centroid method are discussed and the need for accurate wavelength data is emphasized.

• Open/close The size dependence of the melting point of small particles of tin

  C R M Wronski 1967 Br. J. Appl. Phys. 18 1731 Tag this article

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  The size dependence of the melting point of tin has been studied by means of transmission electron diffraction and microscopy. In accordance with the observations of other workers, it has been found that the melting point of a spherical particle of tin decreases as the radius R _m of the particle decreases. However, the decrease does not depend linearly on 1/ R _m. The experimental results are explained in terms of a thermodynamic treatment which allows for the fact that a liquid cannot be subdivided indefinitely. The theory ceases to give correct results for the melting point of particles with radii less than about 50 Å, the calculated temperatures being less than those observed. For particles to which the theory is applicable it is found that the surface tension at the interface between solid and liquid tin is σ ₁ = 62·2 ± 10 dyn cm ^-1.

• Open/close The viscosity of suspensions of rigid spheres

  R Roscoe 1952 Br. J. Appl. Phys. 3 267 Tag this article

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  An explanation is given of the dependence of the relative viscosity on the size distribution of the suspended spheres, an effect recently observed by Ward and Whitmore. ⁽¹⁾ It is shown theoretically that if the spheres are of very diverse sizes, the relative viscosity is (1 - c) ^-2.5 for all values of the volume concentration c. For spheres of equal size, the validity of the Einstein expression for the relative viscosity (1 + 2.5 c) is restricted to concentrations well below c = 0.05; while for medium and high concentrations the relative viscosity is given by the theoretical expression (1 - 1.35 c) ^-2.5. The use of the latter formula in interpreting measurements on the viscosity of solutions is briefly indicated.

• Open/close The resistivity and structure of chromium thin films

  P A Gould 1965 Br. J. Appl. Phys. 16 1481 Tag this article

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  The resistive properties of vacuum-deposited chromium films have been studied and related to their structure.

  Films deposited at a pressure of 10 ^-9 torr on substrates at temperatures of 300°C and 20°C have resistivities of approximately 1.5 times and 4 times bulk value respectively. The corresponding values for films deposited at a pressure of 10 ^-5 torr are 2 times and greater than 6 times bulk value.

  Investigation of film structure by electron microscopy, and measurement of film density, film impurities and vacuum-chamber residual gases during film deposition, show that the films consist of grains of relatively pure chromium with impurities at their boundaries and it is concluded that the grain size and impurity content - both of which depend upon deposition conditions - are the important factors that determine the resistive properties of the films.

• Open/close The performance of bismuth telluride thermojunctions

  H J Goldsmid et al 1958 Br. J. Appl. Phys. 9 365 Tag this article

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  The thermoelectric properties of n-type and p-type bismuth telluride between 150 and 300° K have been measured and the figure of merit for thermoelectric applications has been calculated. This figure of merit has been shown to be highest for material with an electrical conductivity of about 1000 Ω ^-1 cm ^-1 with current flow parallel to the cleavage planes. Its value at 290° K corresponds to a maximum cooling by means of the Peltier effect of about 65° C, and an efficiency of thermoelectric generation of 1% for a 25° C temperature difference.

• Open/close Tables of the Brillouin function and of the related function for the spontaneous magnetization

  M I Darby 1967 Br. J. Appl. Phys. 18 1415 Tag this article

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  The Brillouin function and the spontaneous magnetization derived from it are tabulated for spin values ½ to 7/2 in half-integral steps and for J = ∞.

• Open/close Kramers-Kronig analysis of reflection data

  D M Roessler 1965 Br. J. Appl. Phys. 16 1119 Tag this article

  Article References Full text PDF (324 KB)

  A method is given for using the Kramers-Kronig dispersion relations to evaluate optical and dielectric parameters of materials from normal incidence reflectance data when data over only a limited spectral range is available. No assumptions are needed concerning the behaviour of the reflectance data outside the experimental range, and no extrapolations of data are required. As an example, the extreme ultra-violet reflectance data of potassium iodide is analysed by this method.

• Open/close Hysteresis loss and dielectric constant in barium titanate

  C A Miller 1967 Br. J. Appl. Phys. 18 1689 Tag this article

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  A qualitative description is presented for the behaviour of the hysteresis loss and the dielectric constant in ceramic barium titanate in terms of the properties of single-crystal barium titanate. When the effects of imperfections and interaction between crystallites are introduced, this description explains the similarity between the dielectric constants of single crystal and ceramic when the hysteresis losses differ by orders of magnitude. Measurements of the dependence of loss and dielectric constant on grain size, as a function of temperature, are described, and the results interpreted. A new explanation of the anomalously high dielectric constant of very fine-grain ceramic barium titanate is given.