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The Journal of Physical Chemistry, Vol. 87, No. 21, 1983
one should consider the buildup of the dislocation substructure as the source of the decrease observed when specimens are deformed throughout a t a particular pressure. Jones: A t the moment I think that there is a slight decrease in creep rate in the "one sample" experiment, but I have not done enough of these tests to be certain. If there is a significant difference between the two types of test than I agree that dislocation substructure may be the source of the decrease in strain rate.
Rate-ControllingProcesses in the Creep of Polycrystalline Ice (P.Duval) E. M . Schulson: In view of the behavior of metals, it is rather surprising that in ice recrystallization begins after a plastic strain of only 0.01 a t temperatures around -5 "C. What is the evidence for this recrystallization? Do you think that hydrostatic pressure retards the rate of recrystallization? Duual: The evidence of recrystallization after a strain of 0.01 has been shown by observing the modification of the ice texture. An indirect evidence of recrystallization is that tertinary creep is observed at low stresses (CO.1 Pa). The formation of cracks cannot be involved. The small strain necessary for driving recrystallization is probably due to the importance of internal stresses, as discussed in the paper. The effect of hydrostatic pressure on creep rate has been verified (see the paper of S. Jones and A. Chew). But, it cannot be concluded that hydrostatic pressure has an effect on the critical strain for recrystallization. S. H . Kirby: You mentioned the reduction of total grain boundary energy as a driving force for dynamic grain growth. How does this driving force compare to that resulting from reduction in dislocation line energy in ice? Duual: Grain growth is driven by the grain boundary energy (=3/27GB/d).With d = m, the driving force is about 90J/m3. The driving force associated with dislocations is pGb2/2, where p is the dislocation density and G is the shear modules. With the relation ug= Gbp1I2/3and for a typical stress of 0.5 MPa3, this driving force is too small to drive dynamic recrystallization. But, if certain badly oriented grains deform at a stress comparable to that for nonbasal glide, the driving force associated with dislocation is about lo5 J/m3 compared to lo2 J/m3 associated with the grain boundary energy.
The Velocity of Dislocations in Ice on [OOOl] and [lOiO] (R. W.Whitworth) T. Hondoh: To observe extended dislocations by HVEM, the separation width of the two particles has to be much larger than its resolution limit. What width, or stacking fault energy, are you expecting? Whitworth: I do not know what value to assume for the stacking fault energy and am looking forward to hearing your paper about this. My attitude to the electron microscopy experiment is that dissociation has been observed in many materials with related structures, and it is therefore worth having a look at ice. Whatever is seen still provide evidence about the stacking fault energy even if only a lower limit.
Frequency Dependence of the Surface Conductivity of Ice (A. J. Illingworth) J . Ocampo: Have you seen any aging effect on cleared surfaces? Zllingworth: No-we have only done aging studies on rime.
J. Glen: Does the great increase in roughening the rimed crystal not have any serious effect? Zllingworth: Both the riming and rubbing should be isotropic with no preferred direction along the surface. The most convincing evidence, however, is when the surface is melted and rapidly frozen with a blast of cold air; the surface is reasonably smooth yet the dc surface conductivity falls. T. Takahashi: Can the surface conductivity of rimed ice be modified by the increase of surface potentials? Zllingworth: I think its more likely that the change in potential and the change in surface conductivity both result from the rapid freezing.
Time-Induced Changes in the Dielectric Properties of Ice Ih (P. R. Camp) G. P. Johari: I wonder if the effect of aging you observed is not due to a continuous shear of ice during the course of aging. Camp: I doubt it for the following reasons. First, the magnitude of the stresses are very small. The weight of the top electrode is only a few times that of the sample itself. Indeed, weightproduced stresses on the ice in storage may well exceed those during measurement. Long ago (Nature (London) 1957, 169, 623-4) I did find a pressure effect but it was small at pressures 500 times as large as those exerted here. Second, if continuous shear were responsible, I would have expected all observers t o have found an increase in low-frequency dispersion with time regardless of the electrode material used. Third, it would not seem possible in this way to explain the change in direction of the process Gbserved when the temperature was lowered from -2.5 to -14 "C.
J. Perez: In the case of your work you have concluded in terms of extrinsic defects. But we have done experiments of aging by putting ice single crystals in organic liquid for more than 1000 h a t -2 "C (J.Phys. Appl. 1981). We have observed (i) an increase of the dislocations density (X-ray topography) and (ii) an increase of the point defect concentration (mechanicalspectroscopy). Thus it seems difficult to discard in all cases the possibility of the apparition of intrinsic defects during the aging of ice. Camp: This work addresses directly only the cause of the low-frequency dielectric dispersions. Whether or not the appearance of point defects can be influenced by the nature of the ice surroundinginterface, I do not know. It might be an interesting subject of study. If the kind of increase in dislocations and defects which you have found is interface independent, then apparently these do not affect the low-frequency dispersion. Otherwise our experiments should have been substantially the same as those of others who used different electrodes. Moreover the reversal of the direction of change upon cooling the sample would imply that cooling reduced the concentration of such defects. V. F. Petrenko: I did not quite catch whether there is some influence of aging on high-frequency conductivity? If there is some diffusion of carriers from the electrodes into the volume of ice one can expect an increase of the high-frequency conductivity as well as the low-frequency one. Camp: The increase in conductivity is almost frequency dependent (rising slightly with frequency) so the high-frequency conductivity increases as well. However this is dwarfed by the effect of the Debye dispersion.
Relevance of Ice Studies to Bioenergetics (J. F. Nagle) F. H. Stillinger: I presume that Halobacterium in which the bacteriorhodopsin resides can be acclimated to pure D20. It would be valuable to know if the resulting deuteron pumping and ATP synthesis rates track the corresponding shift in ice mobility for D+ vs. H+.If so, your mechanistic suggestions would seem to be supported. Any major discrepancy would constitute an invitation to further thought. Nagle: The photocycle of bacteriorhodopsin has been studied in D20 (Lozier, R. H.; Niederberger, W. Fed. Proc. Fed. Am. SOC. Exp. €301. 1977,36,,1805). Although this complicated photocycle has not been fully deciphered, it is encouraging that two of the decay rates are strongly isotope dependent, being slower by ratios of 2 and 5, respectively. This compares to an isotope effect of 2 for Bjerrum defects and 2.7 for ions (Kunst and Warman, this conference). Unfortunately, even though this is moderately encouraging, one must remember that the rate-limiting steps for the photocycle may be due to the active site and not the proton channels. The experimental finding (Keszthelyi, L.; Ormos, P. FEBS Lett. 1980, 109, 189) that the electrical response of bacteriorhodopsin closely follows the photocycle is consistent with this and with our prediction that the transport along the channels is very fast. Therefore, at the present time the D/H isotope effect offers neither support nor refutation of the proton channel mechanism.
P. Deulin: (1)Along that line (of possibly too fast predicted
