The effects of mechanical stress on photographic emulsions

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Title: The effects of mechanical stress on photographic emulsions
Author: Weiss, Dennis A.
Abstract: The effects of mechanical stress on a photographic emulsion, namely Kodak Royal-X Panchromatic, has been investigated. The effects are related to the phenomenon of "Kink Desensitization" of the photographic emulsion. Despite the numerous experiments, results and explanations by previous authors on this phenomenon, a model relating the effects of mechanical stress on emulsions and dislocation theory of strained crystals still evades the photographic scientist. Strips of emulsion were axially stressed, using a Dillon tensiometer modified for compression. To analyze the changes in photo graphic sensitivity of the emulsion, the characteristic curves of the developed internal and external images were compared as a function of the applied stress. Both surface and deep internal developers were utilized to analyze changes in the position of latent image formation in a strained crystal. The parameter used to correlate the effect of stress on the emulsion was the change in optical density, AD, between the stressed and unstressed portion of the emulsion. Then, aging experiments were conducted on the stressed samples to investigate the effect of time on surface desensitization . Aging experiments were conducted in the temperature range between 249K and 318K. This allowed the calculation of the activation energy of the aging process which was found to be 0.42 0.03 eV. In general, it was found that the effect of stress is to produce a surface desensitization and an internal sensitization of the stressed emulsion, and that both of these phenomena were related to the applied stress. Maximum surface desensitization AD equal to 0.16, occurred at 3500 kg/cm2. The effect of stress first becomes apparent at an applied stress of 400 kg/cm2 which, in this study is assumed to be the yield stress of iodobromide crystals in a gelatin medium. Shifts in characteristic curves due to stress showed a decrease in surface sensitivity and an increase in internal sensitivity. Aging of stressed emulsions at room temperature after stressing but prior to exposure, indicated a process resulting in the decrease of surface desensitization. The value of the activation energy calcu lated for the aging process is similar to that found by previous authors to be for the diffusion of impurities to the dislocations mainly due to elastic forces. The results are discussed in terms of plastic deformation of silver halide crystals and subsequent formation of dislocations in the strained crystal. The forces associated with the dislocations and their associations with impurities were discussed in order to attempt a model explanation of the "Kink Desensitization" phenomenon.
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Date: 1982-11

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