Shock-induced polarization

Shock-induced electrical polarization Mineev and Ivanov [76M01] (19, 148)... 

Fig. 4.6. Piezoelectric pulse diagrams can be used to obtain explicit representations of the time dependent electric fields in piezoelectric substances. The magnitudes and orientations of these electric fields are critical to development of shock-induced conduction. As an example, the diagram on the left shows the polarization and displacement relations for a location at the input electrode. The same functions for a location within the crystal is shown on the right (after Davison and Graham [79D01]).

The effect of shock-induced conduction is less distinct in ferroelectrics than in piezoelectrics but is nevertheless apparent from a number of studies. (See Davison and Graham [79D01] and Novitskii [79N03].) Differences in conduction with sample polarity, such as those seen in quartz but of opposite sign, are observed in ferroelectrics. 

In this chapter piezoelectric crystals and polymers ferroelectric and ferromagnetic solids resistance of metals shock-induced electrical polarization electrochemistry elastic-plastic physical properties. 

Fig. 5.19. Shock-induced volume polarizations have been observed in a wide range of solids due to a number of different physical phenomena, including piezoelectricity and ferroelectricity. The signals observed from polymers and ionic crystals are not due to established phenomena, and are described as due to shock-induced polarization effects.

Fig. 5.20. The shock-induced polarization of a range of ionic crystals is shown at a compression of about 30%. This maximum value is well correlated with cation radius, dielectric constant, and a factor thought to represent dielectric strength. A mechanically induced point defect generation and migration model is preferred for the effect (after Davison and Graham [79D01]).

Fig. 5.21. The shock-induced polarization of polymers as studied under impact loading is shown. For the current pulse shown, time increases from left to right. The increase of current in time is due to finite strain and dielectric constant change. (See Graham [79G01]).

Fig. 5JS2. Shock-induced polarization of polymers has been studied by many investigators, with data as summarized. The typical behavior indicates a threshold compression of about 10%-15% followed by a rapid increase in value. The polarizations shown vary over three orders of magnitude. The author has proposed a mechanically induced bond-scission model to describe the effects. (See Graham [79G01].)...

Several overall features of the shock-induced polarization are apparent. First, there appears to be a threshold compression below which the signals are not observed. The compression for this threshold is considerable, about 15%, such that it is not difficult to believe that the material must be considerably altered in structure before polarizations appear (shown in Fig. 5.22). Following the threshold compression, the polarizations increase extraordinarily rapidly with increasing compression, finally reaching a saturation value at compressions of perhaps 30%. 

Fig. 5.23. The shock-induced polarization of the indicated polymers containing benzene rings in their structure is shown to be strongly dependent on the number of rings (after Graham [82G02]).

These observations were the basis for the proposal that polymers, like ionic crystals, exhibit shock-induced polarization due to mechanically induced defects which are forced into polar configurations with the large acceleration forces within the loading portion of the shock pulse. Such a process was termed a mechanically induced, bond-scission model [79G01] and is somewhat supported by independent observations of the propensity of polymers to be damaged by more conventional mechanical deformation processes. As in the ionic crystals, the mechanically induced, bond-scission model is an example of a catastrophic shock compression model. 

In this chapter studies of physical effects within the elastic deformation range were extended into stress regions where there are substantial contributions to physical processes from both elastic and inelastic deformation. Those studies include the piezoelectric responses of the piezoelectric crystals, quartz and lithium niobate, similar work on the piezoelectric polymer PVDF, ferroelectric solids, and ferromagnetic alloys which exhibit second- and first-order phase transformations. The resistance of metals has been investigated along with the distinctive shock phenomenon, shock-induced polarization. 

Finally, the phenomenon of shock-induced polarization represents perhaps the most distinctive phenomenon exhibited by shock-compressed matter. The phenomenon has no counterpart under other environments. The delineation of the details of the phenomenon provides an unusual insight into shock-deformation processes in shock-loading fronts. Description of the phenomenon appears to require overt attention to a catastrophic description of shock-compressed matter. In the author s opinion, a study of shock-induced polarization represents perhaps the most intriguing phenomenon observed in the field. In polymers, the author has characterized the effect as an electrical-to-chemical investigation [82G02]. 

Shock phenomena, such as shock-induced polarization, have no known counterpart in other environments. In that regard, the distinctive behaviors present the greatest opportunity to determine details of shock-compression processes. Unexplored phenomena, such as electrochemistry [88G02], offer considerable potential for developing improved descriptions of shock-compressed matter. 

Since the discovery of piezoelectricity on certain asymmetrical crystals like quartz by J. and P. Curie in 1880, the piezoelectricity of various crystals has been extensively studied on account of its importance in science and technology (Cady, 1964 Mason, 1950). Early work on the piezoelectricity of polymeric materials is found in the paper by Brain (1924) who investigated the piezoelectricity of various dielectrics including ebonite, rubber, and celluloid. In 1965, Harris (1965), Allison (1965), and Hauver (1965) investigated both experimentally and theoretically the shock-induced polarization of plastics. 

Allison, F.E. Shock-induced polarization in plastics. I. Theory. J. Appl. Phys. 36,2111 (1965). 

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