Phenomena of the energetic material initiation usually associate with the energy concentration, or so-called “hot spot” generation. When generated, hot spots either disappear or sustain explosive growth leading to widespread ignition in energetic material. Understanding the initialization of the energy concentration and hot spot is essential to answer the fundamental questions about the response of energetic materials under extreme pressure and/or temperature. Additionally, it is of great interest to discover the microstructure related energy localization in the solid composite which involves thermal-mechanical conversion. This project aims to develop the in situ thermal imaging microscope (TIM) technology to study the hot spot generation under different energy input methodologies. The mid-wavelength infrared (MWIR) camera has been used to construct TIM for monitoring the initiation and growth of the hot spots, which were generated by various energy input methods such as LWIR laser, ultrasound and flyer plate impact. Below is a brief list of achieved and on-going sub-projects with this technology.

 

 

Hot spots initiated by long-wavelength infrared (LWIR) laser:

 

 

Figure 1: Experimental scheme of LWIR laser induced hot spots

 

A CO2 laser was used and synchronized with the MWIR microscope to simulate the hot spot generation by the LWIR absorption of materials (Figure 1). The hot spots were found on RDX (1,3,5-trinitroperhydro-1,3,5-triazine) generated by the CO2 laser to simulate the hot spot generation (Figure 2). This sub-project characterized the capability of the TIM and also obtained the relationship between hot spot efficiency and absorption coefficient of RDX crystals.

 

Figure 2: Hot Spots observed on RDX crystals 450ms after the CO2 laser exposure.

 

Ultrasonic hot spots:

 

Figure 3: Scheme of the experiments using TIM to trace the hot spot generation of composite materials under ultrasound irradiation.

 

 

The TIM can also be used to detect the hot spot generation under ultrasound irradiation source, which is shown in figure 3. The ultrasound wave is generated by an ultrasonic horn, which is fixed with a sample holder with four springs to provide static pressure. We have found that the surface property significantly regulated the efficiency of hot spot generation under ultrasound wave. Based on this observation, we were able to fabricate the RDX/polymer composite sample to designate the locations of hot spots on selected energetic crystals (Figure 2). This sub-project is collaborated with Dr. Kenneth S. Suslick’s research group.

 

Figure 4: The B/W picture was taken before the experiment. Three coated RDX crystals were pointed by the red arrows. False color pictures show the times after the ultrasound was applied. It is clearly that hot spots were only obtained on the coated RDX crystals.

 

Impact initiated hot spot generation:

 

Preliminary thermal images of shockwave initiated hot spots have been successfully taken with TIM apparatus. The shockwave is provided by the laser driven flyer plate system, and synchronized with the MWIR camera. Figure 5 shows the thermal image integrates the thermal emission from 0-200 ns after flyer impact on a thermite particle (8Al•MoO3), indicating the appearance of shockwave initiated hot spot on thermite particle.

 

Figure 5: (Left) Thermal image of thermite particle before impact. (Right) A hot spot is found in the false colored image of thermite particle after impact.

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