Conyers Herring

Diffusional Viscosity of a Polycrystalline Solid

According to a suggestion of Nabarro, any crystal can change its shape by self‐diffusion in such way as to yield to an applied shearing stress, and this can cause the macroscopic behavior of a polycrystalline solid to be like that of a viscous fluid. It is possible that this phenomenon is the predominant cause of creep at very high temperatures and very low stresses, though not under more usual conditions. The theory underlying it is developed quantitatively, and calculations of rate of creep, or equivalently of effective viscosity, are given for aggregates of quasi‐spherical grains and for wires composed of cylindrical grains. Allowance is made for the effect of tangential stress relaxation at the grain boundaries. It is suggested that mosaic boundaries and boundaries between grains of nearly the same orientation may be unable to serve as sources or sinks of the diffusion currents, in which case the creep rate will depend only on the configuration of grain boundaries having a sizable orientation differen...

Effect of Change of Scale on Sintering Phenomena

It is shown that when certain plausible assumptions are fulfilled simple scaling laws govern the times required to produce, by sintering at a given temperature, geometrically similar changes in two or more systems of solid particles which are identical geometrically except for a difference of scale. It is suggested that experimental studies of the effect of such a change of scale may prove valuable in identifying the predominant mechanism responsible for sintering under any particular set of conditions, and may also help to decide certain fundamental questions in fields such as creep and crystal growth.

Stress generation by electromigration

Stresses in aluminum thin films on TiN were studied in situ by transmission x‐ray topography. Stress gradients were seen to build up in thin aluminum films during passage of electrical currents. The stresses are more compressive in the anode regions. These stress gradients seem to be a concomitant of the backflow responsible for the reported threshold in electromigration, and can probably be correlated quantitatively with it.

Effect of Random Inhomogeneities on Electrical and Galvanomagnetic Measurements

This paper is designed to supplement the existing extensive literature on the conductivity of a randomly inhomogeneous medium, by treating the effects of inhomogeneities on piezoelectric, galvanomagnetic, and thermoelectric measurements. The scale of the inhomogeneities is supposed small compared with the dimensions of the specimen being measured, but large compared with mean free path, Debye length, etc. Formulas for all the effects are derived which are asymptotically exact in the limit of small fractional fluctuations in the local conductivity, etc. Comparison with other approximations and application to various exactly soluble cases show that these formulas are often roughly valid for quite sizable fluctuations. For material which, if uniform, would show a high field saturation of transverse magnetoresistance, the presence of appreciable inhomogeneities in the Hall constant will cause the magnetoresistance to increase indefinitely with field. This effect is due to the current distortions arising from ...

Surface Tension as a Motivation for Sintering

The sintering together of powder particles into a dense solid mass at temperatures below the melting point of the particles is a process whose rate and end result are known to be influenced by many factors, e.g., particle size, distribution of particle sizes, compacting pressure, temperature of sintering, surrounding atmosphere, gas dissolved in the particles, etc. Because of the many factors involved, it is difficult to draw from practical metallurgical results any reliable conclusions regarding the detailed laws governing the processes occurring. Recently, however, interest has been growing in attempts to isolate the physical processes likely to be important and to study them in experiments designed for unambiguous interpretation. Such experiments have a twofold interest, in that they not only provide clues toward the elucidation of more complicated metallurgical phenomena but also throw light on some fundamental fields of solid-state physics. This chapter undertakes to provide a broad and logically precise formulation of certain physical laws which underly the interpretation of some of the simplest experiments of this type.

The State of d Electrons in Transition Metals

This paper gives a brief critique, in elementary language, of the principal types of theoretical pictures which have been advanced concerning the electronic states of transition metals, especially those of the iron group. It also calls attention to the possibility that some of the properties of these metals can be correlated by the use of concepts which have an exact, not just approximate, meaning for a many-electron system. The Fermi surface is probably a concept of this type. Major conclusions are that in the iron group metals the 3d electrons ought not to differ radically from those in the free atoms either in number or in spatial distribution, and that in most, though perhaps not all, of these metals the 3d electrons, magnetic or nonmagnetic, have an itinerant behavior.

High‐Field Susceptibilities of Iron and Nickel

The dependence of magnetization M on field H has been measured in high fields (15–150 kOe) for single crystals of Ni and Fe+4% Si in their directions of easy magnetization. The specimen, at liquid‐nitrogen or liquid‐helium temperature, was pulled from one to the other of two balanced coils connected in a series opposition to an integrating digital voltmeter. Over the range 15 to 50 kOe, where the most accurate data were obtained, the mean volume susceptibility κ=∂M/∂H is a little over 0.5×10−4 emu/cm3 for Ni, 1.1×10−4 for Fe, with an error probably <0.1×10−4. Although there is a small contribution to each of these figures from orbital paramagnetism and, at the higher temperature, from the dependence of spin‐wave amplitudes on field, the major contribution is almost certainly from the exchange‐enhanced Pauli paramagnetism of itinerant d‐band electrons. For this contribution to be as large as is observed for Ni, it seems necessary that the commonly accepted band structure of Ni be replaced by one with holes...

Nature of Exchange Coupling in Transition Metals

It is now fairly well accepted that the magnetic electrons of the d‐shell transition metals must be viewed as itinerant. It appears, moreover, that they are only moderately correlated: although they occupy moderately narrow bands and have a spatial distribution similar to that of atomic d electrons, they fluctuate from atom to atom almost as randomly as the electrons of nontransition metals, thanks to the operation of various screening effects. In the exchange coupling of these electrons, their itinerancy plays a central role, and the concepts of the Heisenberg model are quite inapplicable. Significant contributions to the exchange coupling can arise both from polarity energy (the energy cost of having too many or too few electrons on an atom) and from the Hunds‐rule coupling of electrons on the same atom. Correlation effects, though not strong enough to prevent sizable polar fluctuations, are very important in that they give rise to magnon modes, critical fluctuations, etc. Details of band structure are...