Ultrasonic hot spots in solids

Mechanical action can produce dramatic physical and mechanochemical effects when the energy is spatially or temporally concentrated. An important example of such phenomena in solids is the mechanical initiation of explosions, which has long been speculated to result from ‘hot spot’ generation at localized microstructures in the energetic material (EM). Direct experimental evidence of such hot spots, however, is exceptionally limited; mechanisms for their generation are poorly understood and methods to control their locations remain elusive. Typical solid explosives are composites of EM crystals with polymer binders. Multiple mechanisms have been proposed for hot spot formation in these structures, such as shear band formation in explosive crystals, void collapses, interfacial friction and triboelectric discharge. Although model calculations have suggested that interstitial regions between crystals are critical locations for hot spot generation, it is still difficult to identify specific contributions from multiple potential mechanisms. Furthermore, attempts to observe hot spots in solids are trapped in a dilemma: a mechanical impact strong enough to create intense energy concentration also crushes and destroys the microstructure of the material. The whole process occurs in microseconds, which is too fast to be captured by typical temporally and spatially resolved thermal imaging techniques (~100 frames per second). To improve our understanding of mechanochemical processes in solid structures, a new method that allows the both imaging and control of thermo-mechanical energy deposition in solids is clearly needed. This thesis explores the generation and control of thermal hot spots in solid structures using ultrasound. With frequencies ranging from 20 kHz to 20 MHz, ultrasound can apply thousands to millions of small mechanical impacts repetitively into the material instead of a one-time powerful shock from conventional mechanical impact. The energy deposition process is therefore less destructive and also extends over a longer time period. The thermal response of the materials under ultrasonic impact was quantitatively studied by a thermal imaging microscope. Results in this thesis demonstrate the generation of intense, localized microscale hot spots in solid composites during mild ultrasonic irradiation. Composite models investigated include polymer matrix composites and polymer bonded composites. These ultrasonic hot spots, with heating rates reaching ~22,000 K s-1, are closely correlates with specific local structures in the composites, and the modification of these structures leads to rational and precise control of the microscale hot spots. This control, in turn, produces spatially and temporally definable thermal reactions in energetic materials, including fuel/oxidizer type explosives