The deformation of silicon at high temperatures and strain rates
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THE current widespread
interest in photovoltaic power generation has led to a renewed interest in thin silicon sheet production by inexpensive methods. The possibility of using standard hot working techniques, rolling, forging and extrusion, was investigated in the early 60's ~,2and found to be somewhat promising. In this early study both polycrystalline and single crystalline silicon samples were encased in m o l y b d e n u m jackets and subjected to various hot working schedules. It was found, for example, that silicon could be hot rolled to a thickness of 0.050 cm at temperatures above 1300 ~ (Silicon melts at 1410 ~ However, since the samples were all encased in Mo, it is not known if polycrystalline Si could be hot deformed by large amounts without such a casing. Furthermore, the forces involved in the deformation and the strains and times necessary for recrystallization could not be deduced from this study. The only other published work on the deformation of polycrystalline silicon is that of Lillie 3 in which bars were subjected to slow bend tests at temperatures up to 1200 ~ Consequently, there are no data in the literature from which the stress necessary to produce a given strain rate at a given temperature can be deduced. There is, likewise, no available information on recovery and recrystallization of this material. There is, of course, a vast literature on the deformation of single crystalline material; see Ref. 4 for a review. There have been, however, some related studies, the results of which we found useful and encouraging. Single crystalline germanium, without a casing, has been hot rolle& at very slow rolling speeds between 700 and 800 ~ in a specially constructed rolling apparatus. Recrystallization was observed above 60 pct deformation. Furthermore, [111] oriented Si single crystals have s. B. KULKARNI and C. LALL, formerly with the Department o f Materials Science and Engineering, University of Pennsylvania, are now with IBM, East Fishkill, NY 12533 and Remington Arms Co., Powder Metal Products Division, Ilion, NY 13357, respectively.D. P. POPE and C. D. GRAHAM, JR. are with the Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104. Manuscript submitted May 7, 1979.
been compressed at 1300 ~ at strain rates as high as 10 -2 s-1 using an axial stress of approximately 100 MPa. 6 Since the stress-strain curve of a single crystal loaded along a low index direction is c o m m o n l y very similar to that of a polycrystal, this result was particularly encouraging. The strain rates used in hot working operations are typically in the range of 10 -1 to 102 S -1, substantially above the 10 2 s-~ used in the study of Myshlaev e t al, 6 and also substantially above the rates possible in most standard testing machines. However, currently available servo controlled electrohydraulic testing machines are capable of strain rates at least in the 10~ to 101 s -l range. Consequently, this study was initiated to investigate the uniaxial compression of polycrystalline
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