Charles A. Crane

Determination of the spatial temperature distribution from combustion products: A diagnostic study

Temperature measurements within the highly complex reaction field of energetic materials are complicated but existing technology enables point source measurements that identify a maximum temperature at a single location. This study presents a method to extend point source measurements to thermally map the spatial distribution of temperature over a large field of interest. The method couples point source temperature measurements from a multi-wavelength pyrometer with irradiance measurements from an infrared camera to produce a highly discretized thermal map that includes the reaction and surrounding field. This technique enables analysis of temperature gradients within the field of interest and an understanding of energy propagation beyond the point of reaction. Point source measurements of maximum temperature are within 10% of reported values. The method was illustrated for the aluminum and polytetrafluoroethylene reaction and the thermal distribution of temperature produced 30,720 temperature measurements over a field of interest corresponding to 3.5 cm × 8 cm.

Investigating the trade-offs of microwave susceptors in energetic composites: Microwave heating versus combustion performance

Recently, microwave technology has been used to ignite energetic materials when studies showed that metal powders readily absorb microwave energy. This study investigates adding a graphite susceptor to an energetic composite consisting of aluminum (Al) and iron (III) oxide (Fe2O3) and examines microwave coupling to the sample. In a companion study, the combustion of this thermite as a function of susceptor concentration was also studied to evaluate the trade-off between enhancing microwave coupling and flame propagation speed. Results show that graphite enhances microwave coupling up to 10% by mass concentration but reduces heating at higher percentages that exceed a percolation threshold. As susceptor concentrations increased greater than one mass percent, the flame propagation speed correspondingly decreased.

Spatial observation and quantification of microwave heating in materials

An electromagnetic exposure chamber was designed to safely deliver electromagnetic power in the range of microwaves between 0.8 and 4.2 GHz to a thin cylindrical materials. This instrumentation is unique because the diagnostics not only measure sample heating with a response time of 1.3 ms, but also energy transmitted and reflected. Energy absorption at different frequencies was quantified via electromagnetic heating using an infrared camera. This in situ IR imaging of the spatial distribution of temperature during microwave exposure coupled with sensors for determining transmitted and reflected energy enables novel new microwave energy experiments. Samples were exposed to a portion of both the electric and magnetic fields inside a waveguide and based on sample dimensions, the field strengths were assumed uniform across the sample. Three materials were examined: two were borosilicate, first coated with graphite paint and a second without the coating; and, the third was a compressed sample of flake graphite pressed to 69% of its bulk density. Results are in agreement with the theories of microwave heating and verify the functionality of this experimental design. This diagnostic will be important in future tests where a variety of different materials can be exposed to weak electromagnetic waves and their efficiency in coupling to the microwaves can be examined.

Utilizing Microwave Susceptors to Visualize Hot-Spots in Trinitrotoluene

Abstract In recent years, researchers have shown that the inclusion of susceptors in monomolecular explosives increases microwave absorption where the monomolecular explosive alone is nearly transparent to the microwave energy. In this study, graphite particles are used to absorb microwave energy and decompose trinitrotoluene 1, 3, 5 (TNT). Aluminum particles were also used with TNT but very little energy was absorbed. Thermal and electric properties of the susceptor affect microwave energy conversion to thermal energy in explosives. The temperature gradients across the broad face of a cylindrical sample of TNT with and without susceptors were spatially observed using an infrared camera. Hot-spots were observed during 120 seconds exposure to microwave energy at a frequency of 3.3 GHz and found to directly correlate with susceptor concentration. When using graphite, the TNT heated above its melting temperature and decomposed as a vapor.

Analyzing the Effects of Electromagnetic Exposure on Energetic Compositions

Energetic materials such as explosives, pyrotechnics, propellants, and thermites have been extensively studied for over a century and as technologies become more advanced, so do the techniques used to understand these materials. Currently, there are research plans for advancing detection methods in finding explosive materials using high frequency techniques (80 MHz to 4.2 GHz). A thorough understanding of the response of energetic materials when exposed to these frequencies must be realized before effective detection systems can be advanced. An electromagnetic exposure chamber was developed at Texas Tech University to expose energetic samples to various frequencies and power levels. After exposure, the physical and chemical changes that occur resulting from localized heating are examined. Diagnostics were developed to study the spectral energy absorption and observe hot-spot formations due to localized heating experimentally. Simulations using COMSOL 4.2a, a simulation modeling software that enables analysis of electromagnetic fields on localized heating within an energetic material will also be presented.

Quantifying Energy Transfer From a Reacting Thermite to a Target Using Infrared Diagnostics

A study was conducted to examine the energy transfer from a reacted thermite placed on a steel target substrate. A high speed infrared camera captured a temporally evolving thermal distribution on the substrate, while the thermite, which was placed in a v-notch, self propagated. The thermite investigated for this experiment was Aluminum with Iron (III) Oxide (Al-Fe2 O3 ). An energy balance model was developed to predict temperatures near the v-notch in order to quantify the amount of energy transferred into the steel. Results quantified the percent of energy available from the reaction that was conducted through the substrate and energy losses were estimated. The thermite reaction transferred 10% of the heat of reaction to the steel. The Al-Fe2O3 exhibited greater heat losses to convection and radiation upon propagation through the powder mixture. The Al-Fe2 O3 reaction produced more gas by chemistry, 10% by mass, which contributed to transporting energy away from the v-notch. Much work had been performed that examined the combustion behaviors from a reacting thermite, but there are very few studies that quantify the energy transfer from a reacting thermite to a target. This diagnostic approach and numerical analysis were the first steps toward understanding energy transferred from a thermite into a target, and lost to the environment.Copyright