British Journal of Applied Physics

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.

Experience with the scanning electron microscope has shown that there are fields of application where this instrument has distinct advantages over the conventional transmission microscope (with or without replicas) and the reflexion electron microscope. For example, there are specimens which are too thick to be viewed by direct transmission and which nevertheless do not lend themselves readily to the construction of replicas, either because they are too fragile, because their surfaces are undercut so that the replica would be keyed to them or because observation of the specimen at high temperatures is desired. The paper gives examples of the use of the scanning microscope in such cases and shows that the images produced are very similar in character to those obtained with optical microscopes. In particular, a pronounced three-dimensional effect is observed.

In the work described the specimens have been mounted so that the mean surface under observation is at a glancing angle of about 25° to the incident electron beam. Contrast is provided by the variation of the local angle of incidence as the scanning electron beam moves over the surface, and does not depend on secondary emission. With the present apparatus a resolution of about 200 Å has been attained, but there appears to be no reason why this performance should not be substantially improved.

It is shown that, with the scanning microscope, bombardment of the specimen is much less severe than with other electron microscopes.

A table of water densities over the range 0-40°C, expressed in SI units (kg m^-3, i.e. 10^-3 g cm^-3), is presented. The table has been obtained by applying the relationship 1 litre (1901) = 1.000 028 cm³ to new values derived from an improved combination of the observations of Chappuis and Thiesen on the dilatation of water, no modern determinations being, or likely soon to be, available. The table relates, as is usual, to air-free water, but the effect of air, which is small, is indicated.

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.

The dielectric constant of a colloidal zinc oxide has been measured by the method of mixtures, using cyclohexanol and ethyl acetate as the suspending liquids. The dielectric constant has been found to be 10.4 over the frequency range 105 kc/s to 10 Mc/s, falling to 9.4 at 25 Mc/s, measurements being made at 25° C with an accuracy of 2.3%. The dielectric constant of an acicular zinc oxide has been found to have an apparent value of about 40, over the frequency range 100 kc/s to 10 Mc/s. Results are given of dielectric constant measurements on mixtures of cyclohexanol and ethyl acetate, which appear to disobey Lichteneker's empirical logarithmic mixing rule.

In the past the possibility of thermoelectric refrigeration has been considered, but all attempts to produce a practical refrigerator have failed owing to lack of suitable thermocouple materials. In this paper it is proposed that semiconductors should be used and the factors governing their selection are discussed. It is concluded that the semiconductors should be chosen with high mean atomic weights and that they should be prepared with thermoelectric powers lying between 200 and 300 μVC^-1. Preliminary experiments have led to the production of a thermocouple consisting of bismuth telluride, Bi₂Te₃, and bismuth, capable of maintaining 26 C of cooling.

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 + 2.5c) 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 - 1.35c)^-2.5. The use of the latter formula in interpreting measurements on the viscosity of solutions is briefly indicated.

The stability of a thin layer of liquid moving in still air is studied theoretically with the object of throwing light on the break-up of films during atomization. It is found that instability occurs if W = T/ρ₁U²h < 1 and that the wavelength for maximum growth factor, for W « 1, is λ = (4πT/ρ₂U²) where ρ₁ is the liquid density, ρ₂ is the air density, U is the film velocity, 2h is the film thickness and T is the surface tension of the liquid. Comparison with experimental data shows fair agreement with the observed wavelengths.

The paper opens with a brief review of the original discovery of the radiation and the physical principles of the process that gives rise to it. This is followed by an elementary account of the theory of the effect. A general survey of the practical applications to cosmic-ray and high-energy physics is then presented, with two examples of modern detectors discussed in greater detail.

The article concludes with an account of recent experiments carried out on light pulses from the night-sky associated with cosmic-ray showers, found to be due to Cerenkov radiation in the atmosphere.

The index matching technique of Giordmaine and Maker has been extended to include the more general three-wave interaction. Two fundamentally different experimental arrangements may be considered; one has the two low frequency waves in parallel polarizations and the other has these waves in orthogonal polarizations. The equations relating the phase matching angle in uniaxial crystals to the crystal birefringence, dispersion, and the frequencies of the three waves are given. These equations allow a cone of phase matched directions to be generated about the crystal optic axis. The effect of the symmetry properties of the second-order polarizability tensor on the strength of the phase matched output signal at particular azimuthal angles is discussed for all non-centro-symmetrical uniaxial crystal classes. The particular form of the second-order polarizability tensor for each of the uniaxial crystals is tabulated assuming that Kleinman's symmetry conjecture is valid.

The heat capacity of potassium cobalt trifluoride was recorded over the range 80-300°K. A peak at 109·5°K is attributed to an antiferromagnetic-paramagnetic transition. The values of heat capacity, entropy, enthalpy and free energy at 298·15°K are 28·84 cal mole^-1 degK^-1, 39·40 cal mole^-1 degK^-1, 5669 cal mole^-1 and - 20·26 cal mole^-1 degK^-1, respectively.

It is known that current oscillations can occur in semiconductors in constant electric fields if the collision cross sections for carrier scattering are strongly energy dependent. The growth rate of these oscillations is calculated and a new criterion for their existence is developed.

Whereas in the past the main source of ionization of cold and hot gases in electric fields has been ascribed to collisions between electrons and ordinary gas atoms or molecules, it has recently become apparent that the presence of excited particles cannot always be neglected. In this review experimental evidence is presented, together with elementary theoretical considerations, emphasizing the importance of collisions in which the excitation energy of the particles causes emission of light, chemical changes, or speeding up of electrons. Ionization processes in gases subject to an electric field are contrasted with ionization in combustion flames, and recent advances in the understanding of the mechanism are reported. The question of the deviation from thermodynamic equilibrium of the state of a hot gas is discussed quantitatively. The presence of a large number of excited molecules in their parent gas is shown to reduce considerably the strength of a gas. New methods of finding the spatial distribution of excited molecules, as well as their concentration and potential energy, are included.

A new mechanical action method for the determination of the principal dielectric constants of synthetic sapphire is described. It is based on measurements of the couple experienced by a sphere of the material suspended in a uniform electric field. The values were found to be _{[perpendicular]} = 7·9 ± 0·5 and _[parallel] = 6·5 ± 0·4 when measured at a frequency of 50 Hz, and these values compare favourably with those obtained by other methods. The method may be applied to anisotropic crystals in general, and it is suggested that with slight modification it may be used for the corresponding electrical conductivity determinations.

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.

Whereas in the past the main source of ionization of cold and hot gases in electric fields has been ascribed to collisions between electrons and ordinary gas atoms or molecules, it has recently become apparent that the presence of excited particles cannot always be neglected. In this review experimental evidence is presented, together with elementary theoretical considerations, emphasizing the importance of collisions in which the excitation energy of the particles causes emission of light, chemical changes, or speeding up of electrons. Ionization processes in gases subject to an electric field are contrasted with ionization in combustion flames, and recent advances in the understanding of the mechanism are reported. The question of the deviation from thermodynamic equilibrium of the state of a hot gas is discussed quantitatively. The presence of a large number of excited molecules in their parent gas is shown to reduce considerably the strength of a gas. New methods of finding the spatial distribution of excited molecules, as well as their concentration and potential energy, are included.

The preparation and properties of single-crystal films of silicon on insulating substrates are reviewed. Attempts have been made to prepare single-crystal films by epitaxial deposition on insulating substrates such as Al₂O₃, SiO₂, MgAl₂O₄, SiC and BeO, and to a lesser extent by recrystallization and controlled-nucleation experiments on amorphous substrates. Evaporation, sublimation, sputtering, vapour-liquid-solid processes, and vapour techniques, including silane, silicon tetrachloride and other silicon halides, have been used to deposit silicon films. The differences between these processes and the factors affecting the perfection of films are discussed.

A good deal of research has been devoted to silicon films on single-crystal sapphire substrates. Although diode, field-effect and bipolar devices have been fabricated the perfection of the films is still inadequate for some purposes. Factors affecting the perfection include interaction of the substrate and film during deposition, the perfection of the substrate surface, the nucleation and growth processes, and the presence of impurities and stresses in the film. Particular attention is paid to the orientation relationships between the silicon and the sapphire.

A review is given of most of the recent high-pressure work above about 20 000 atmospheres. The work is divided into the following categories: static pressure generation and measurement, measurement of pressure and temperature, shock-wave methods, x-ray studies, phase transformations, spectroscopy, electrical phenomena, diffusion and plastic flow, geological studies, and miscellaneous. An extensive bibliography is included.

This review discusses systematically the nature of the cleavage crack and the structural features and mechanisms which contribute to the nucleation of cracks and which assist or retard their propagation in a wide variety of materials.

This article describes the progress that has been made during the last decade in our understanding of the processes involved in friction, lubrication and wear. It does not pretend to be an encyclopedic survey - that would be a bore - but summarizes work that seems to us to be of greatest significance. The first section deals primarily with the broader issues underlying the friction of unlubricated surfaces. The second section deals with specific items of research, many of which have opened up new lines of attack. The third, for completeness, deals with lubrication and wear, but this part is brief; it merits a full article in its own right.