Akira Higashi

Determination of diffusion coefficients of self-interstitials in ice with a new method of observing climb of dislocations by X-ray topography

The diffusion coefficients of self-interstitials in ice were determined from the growth processes of interstitial-type dislocation loops and dipoles introduced by cooling on the basis of the fact that the growth takes place by the diffusion of excess interstitials to dislocations. The time variation of their size was obtained by in situ X-ray topographic observations using a high-power X-ray source. By combining the obtained diffusion coefficients and the equilibrium concentration of interstitials previously obtained, it is possible to determine the self-diffusion coefficient Ds in ice. The value of Ds obtained by this method and its activation energy E coincide well with those determined by tracer experiments. This result confirms the interstitial mechanism for self-diffusion in ice.

Plastic Yielding in Ice Single Crystals

Stress-strain relations in single crystals were investigated using ice cylinders in tension. The cylinders were deformed at strain rates varying from 8×10-6 min-1 to 4×10-4 min-1 and at temperatures between -15 and -40°C. The stress-strain curves obtained showed large yield drops and the amount of the drop increased with an increase in the deformation rate or with decreasing temperature. The stress-strain curves of ice single crystals from those of materials such as LiF, Ge, InSb etc., which also show large yield drops in that the slope of initial, linear portion of the curves varies with the strain rate and with temperature. Another characteristic feature of the curves is that past the yield point the curves do not show minimums. This indicates that ice crystals do not work-harden. The maximum stress τmax, and the slope of the initial, linear portion M=(dτ/de)i are taken as characteristic parameters of the curves. Their dependence on temperature and strain rate are given as follows: M=(M0+D\dote exp (E1/RT) and τmax∝\dote1/m exp (E2/RT) where m=1.53 and E1=8.4 Kcal/mole and E2=10.4 Kcal/mole Values for the activation energy E2 and m indicate that the results are in good agreement with those expected from interpretations of creep experiments based on Johnstons theory of movement and multiplication of dislocations. Specific features of the stress-strain curves are also discussed in terms of a special characteristic of ice crystals.

X-ray Diffraction Topographic Studies of Dislocations in Natural Large Ice Single Crystals

The method of X-ray diffraction topography was adopted to reveal the dislocation structure in natural, large ice single crystals which has been hitherto used for the extensive experiments of plastic deformation. The topographs show clear images of curved and straight dislocation lines lying on the basal planes of the crystal. Dislocation density is in the order of 104 cm-2 and Burgers vectors are (a/3) or c[0001]. In many cases, edge dislocations compose the small angle tilt boundaries of which tilt angle is less than 1° of arc. Size of the subgrains bordered with such boundaries is in the order of 1 mm or more and it differs very much from the size of mosaic blocks evaluated from primary and secondary extinctions of X-rays. Another characteristic feature is that screw dislocations of one Burgers vector are much less than those of other two equivalent Burgers vectors of (a/3) system.

Observation of Etch Channels on the (0001) Plane of Ice Crystal Produced by Nonbasal Glide

A method developed by one of the authors for producing minute etch pits on the (0001) surface of ice crystal in coordination with electron microscope observations was used to reveal the behaviors of dislocations in ice crvstal when it was deformed. Etch channels which run in the crystallographic direction, either or , were found. The origins of these etch channels were considered and a mechanism was presented of their formation, constituted of two stages: formation of vacancy trails behind the moving screw or mixed dislocations on glide systems of {10\bar10} , {10\bar10} and {11\bar22} and precipitation of impurities in the vacancy trails. In the case of inclined trails on the glide plane of (10\bar10), deposition of impurities on the (0001) plane was assumed to motivate etching. This mechanism is believed to be plausible by reason of various features of the etch channels.

Concentric dislocation loops with [0001] burgers vectors in ice single crystals doped with NH3

Abstract Ice single crystals, both pure and doped with NH3, were grown from the melt by a modified Bridgman method. The low dislocation density obtained (approximately 102 cm 2) made it possible to take x-ray diffraction topographs showing many concentric dislocation loops in the NH3-doped ice as well as occasional loops in the pure ice. The origin of these loops is attributed to the bunching effect of the steps produced by non-uniform distribution of impurities.

Void formation by non-basal glide in ice single crystals

Abstract When ice single crystals are plastically deformed by tension parallel to the basal plane, hexagonal or circular thin plate-shaped microscopic voids are, found to develop in the basal plane. These voids increase in size and number with continuing deformation and eventually join together producing large cavities which lead to crystal fracture. Clusters of these voids form bands along the directions which are parallel t o the non-basal active slip planes. Tentative explanation of the void formation is presented.

Growth and perfection of ice crystals

Abstract Attempts were made to grow perfect, or nearly perfect, ice single crystals for solid state studies. Dislocation densities as low as 10 2 cm -2 were found in crystals grown by the modified Bridgman method whereas using the Czochralski method the lowest densities found were of the order of 10 4 cm -2 . The configurations and structures of dislocations in the crystals grown by the different techniques and under various conditions were examined by X-ray diffraction topographs. Characteristic features of dislocation images in topographs are interpreted as showing that the Peierls trough is shallow for dislocations having a 1 3 〈11 2 0〉 Burgers vector on the basal plane. Thus, dislocations in the basal plane extend perpendicular to the growth interface due to the preferential line tension when the crystal was grown in the direction perpendicular to the c -axis. The generation of dislocation densities of the order of 10 4 cm -2 in Czochralski grown crystals is mainly attributed to those inherited from the large area of seed crystals and in addition to the thermal stresses caused by steep temperature gradients in the crystal. Reduction of dislocation densities to the order of 10 2 cm -2 in Bridgman grown crystals is achieved by limiting the inheritance of dislocations or by the selection of a single mosaic from the seed crystal through a neck of the growth cell. Dislocation loops of [0001] Burgers vector and stacking faults found both in NH 3 -doped ice are described.

Growth of ice single crystals from the melt, with special reference to dislocation structure

Abstract X-ray diffraction topographic studies were carried out on artifical ice single crystals grown from water in various ways, including large ones by a modified Czochralskis method and very thin ones in capillary tubes. The characters and structures of dislocations in the crystals were investigated in topographs taken with different conditions of diffracting planes and scanning planes. Since no dislocation was found in topographs of a thin ice cylinder grown in the direction of the c -axis, it was concluded that layer growth with two-dimensional nuclei took place in the case of growth parallel to the c -axis. Many dislocations run parallel to the direction of growth when it was perpendicular to the c -axis in large crystals. This fact indicates the mechanism of spiral growth around screw dislocations should be valid for the case of growth perpendicular to the c -axis. These growth mechanisms coincide well with those predicted by growth-kinetic experiments by Hillig and Turnbull [J. Chem. Phys. 24 (1956) 914]. A mechanism of introducing dislocations in large single crystals grown in the direction of the c -axis was proposed and better methods to obtain ice single crystals which are dislocation-free or of low dislocation density were suggested.

Anisotopy of migration and faceting of large-angle grain boundaries in ice bicrystals

Abstract Migration of large-angle grain boundaries in ice bicrystals has been investigated by the method of capillary force developed by Sun and Bauer (1970 a, b). Strong anisotropy of the migration rate was observed between two cases when the boundaries moved parallel and perpendicular to the direction of rotation axis ω. X-ray diffraction topographic observations of grain boundaries (〈1010>/34〉) during migration by the capillary force revealed that they had a strong tendency to be parallel to low-energy facets and that the boundaries that were not parallel to any low-energy facets moved faster than those that were parallel. From observations in three specific directions of the normals of these facets, it was concluded that the facets were the high-density planes of the coincidence site lattice composed of two adjacent crystals. The anisotropy of the migration rate was interpreted in terms of the difference in the density of steps among facets: the non-faceted boundary with many steps moves faster than t...

Formation of Stacking Faults in Pure Ice Single Crystals by Cooling

Fault vectors of various stacking faults formed in pure ice single crystals by cooling were determined from combinations of X-ray diffraction topographs taken with various diffracting planes. It was found that the formation of stacking faults is accompanied by the dissociation of perfect dislocations with Burgers vector b= a+ c. The concentration of excess self-interstitials which precipitated to form faulted dislocation loops during cooling by several tens of degrees was estimated to be 1016 cm-3 from the density of loops observed. Such a concentration of point defects in ice crystals, higher than previous estimates, casts a new light on the understanding of structure-sensitive properties of ice.