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Explosives Explosives

Measuring the Thermal Conductivity of Explosive Materials

trident instrument 3 methods

Testing the thermal conductivity of energetic materials is necessary for understanding the time to ignition through the self accelerating decomposition temperature (SADT). However, explosives pose a challenge with traditional steady-state techniques, as the required large volumes of material pose undesirable safety risks or the necessary sample geometries are impractical. This often leads to estimation of thermal conductivity in predictive models rather than true measurement. C-Therm’s Trident Thermal Conductivity Instrument can be configured with multiple sensors, including the Modified Transient Plane Source (MTPS) sensor. This sensor was designed to be able to test explosives safely and quickly. The MTPS employs a small, rapid heat pulse, and only requires volumes of 1.5 mL, ensuring a safe testing atmosphere compared to other methods.

  • The MTPS sensor equipped with the powder cell, ideal for testing the thermal conductivity of energetic powders, emulsions, or liquids.

    The MTPS sensor equipped with the powder cell, ideal for testing the thermal conductivity of energetic powders, emulsions, or liquids.

  • The Kamenetskii Theory is a model commonly used in the explosives industry as a way to predict heat generation within explosive materials. Thermal conductivity is a key physical property in this theory.

    The Kamenetskii Theory is a model commonly used in the explosives industry as a way to predict heat generation within explosive materials. Thermal conductivity is a key physical property in this theory.

  • Understanding the thermal conductivity of an explosive material can help predict the conditions that will cause ignition, which helps prevent premature explosions

    Understanding the thermal conductivity of an explosive material can help predict the conditions that will cause ignition, which helps prevent premature explosions

Why is Thermal Conductivity Important for Energetic Materials?

Understanding the thermal conductivity of energetic materials is crucial both for safety and performance evaluations. Knowing the thermal conductivity and diffusivity of an energetic material can forecast how it will behave in the event of accidental exposure to heat, such as a fire. The faster an energetic material gains heat, the quicker the reaction rate; the speed of this temperature increase can be determined with the thermal properties of the material. Likewise, the thermal conductivity of a material undergoing an exothermic reaction is dependent on the local temperature of the material; this has a significant effect on the critical conditions for thermal ignition. The theory of thermal ignition – whether or not consumption and diffusion of a reactant is taken into account – has been commonly analyzed using the Frank-Kamentskii theory, for which thermal conductivity is a key parameter. Likewise, thermal conductivity is a necessary parameter for the determination of SADT which is crucial both for the performance and safety of explosives.

  • National Aeronautics and Space Administration (NASA)

    We purchased the C-Therm Thermal Conductivity Analyzer after seeing a demonstration of how fast and easy it is to operate. The instrument provides unequivocal results and provides the flexibility to test powders and liquids. In terms of our satisfaction with the purchase, I’d give it a 10 out of 10 - extremely satisfied.”

    Dr. Enrique Jackson,
    NASA (Sector: Aerospace)

    National Aeronautics and Space Administration (NASA) More Testimonials
  • Haydale Composites Solutions Ltd.

    The C-Therm Thermal Conductivity Instrument has been a key piece of testing equipment at Haydale, providing fast and accurate thermal conductivity measurements for our product development of nanocomposites. Having this capability has allowed a better understanding of the dispersion of nanomaterials in polymer matrices through thermal mapping sample surfaces. The support and customer service from C-Therm has been excellent over the years, we look forward to dealing with them again in the near future.”

    Stuart Sykes

    Haydale Composites Solutions Ltd. More Testimonials

Case Highlights

Canadian Explosives Research Laboratory: Thermal Conductivity of Ammonium Nitrate Emulsion

This following is a case highlight from work done by the Canadian Explosives Research Laboratory. Read it here.

The use of ammonium nitrate emulsion (ANE) in mines, quarries and construction is well established and ANE’s are, in fact, very insensitive and stable products at standard temperature and pressure of normal manufacturing, especially compared to previously used nitroglycerin-based products. The purpose for studying thermal decomposition behaviour of ANE explosives is due to the continued worldwide occurrence of tragic incidents involving the production, processing and handling of ANE’s.

In order to test the ANE, a small volume test kit was used in order to minimize required material. For the sample testing, about 2.5mL was used which equates to roughly 4 grams of ANE. The sensor was placed inside an oven and a weighted cap was used to ensure that there was complete coverage of the sensor during testing.

The table below lists the measured thermal conductivity values for various batches of emulsion with increasing amounts of aluminum and microballoons.

Additive blended with X3153 (%) Measurement Temperature (°C) Thermal Conductivity (W/mK)
No additive 23 0.422
0.2% Aluminum 19 0.422
5% Aluminum 20 0.454
10% Aluminum 20 0.431
1% glass microballoons 21 0.414
4% glass microballoons 21 0.318
6% glass microballoons 23 0.309
100% glass microballoons 23 0.021
Preventing Accidental Ignition of Military Explosives

The following is a case highlight of work done by the French-German Research Institute of Saint-Louis. Read it here.

In an effort to improve military safety, a study was performed on ways to reduce the accidental initiation of explosives. Ignition occurs due to the movement of heat, particularly the buildup of heat within the material. Heat can often build up in heterogeneous areas of a material containing voids, defects, or air pockets often called “hot-spots.”

The study focused on the development of nano-β-HMX, a high density and high detonation velocity explosive. On the nanoscale, most of the void spaces within this structure are taken up by air; air has a low thermal conductivity and thus allows heat to build up in the surrounding areas of the void. To combat this, a filler of Clariant Licowax BJ was used, since it was both inert and thermally conductive. The thermal effusivity of the explosive was measured to determine how fast the HMX will exchange heat with the environment – in this case, the void space. Results can be seen below.

Table 1: The thermal conductivities of various combinations of HMX and Licowax BJ

The HMX containing the Licowax exhibits a much higher thermal effusivity than without. This indicates that the heat will more easily spread to the void areas with the addition of wax, thereby reducing hotspots and thus, accidental ignition.

Dust Explosions: Preventing the Risk with Inert Solid Materials

The following is a case highlight from a paper done by the French National Institute for Industrial Enviornmental and Risks. Read it here.

Areas with high concentrations of combustible dust and powder are notoriously an explosive hazard. An explosive atmosphere can be caused from mining, or the grain processing industry, where the large number of fine particulates are at risk of explosion due to their combustibility and high surface area. One strategy to reduce this is by mixing inert solid materials with the combustible powders to reduce the chance for ignition, and the severity if one does occur.

For this study, multiple inerts were tested, using a variety of volume percentages for a variety of combustible organic powders. The inert tested with the lowest thermal conductivity was a rock powder known as Kieselguhr, with a thermal conductivity of just 0.049 W/mK. The results for various organic dusts can be seen below:

Figure 1: A graph showing the minimum ignition energy as a function of volume concentration of Kieselguhr. A higher minimum ignition energy indicates a safer atmosphere.

As can be seen, the inclusion of solid inerts had little to no effect on the minimum ignition energy, until a threshold was crossed, after which the energy increased dramatically. Further research from this work is to compare other inerts with different thermal conductivities to see how this effects the threshold point.

SIMPLIFYING THERMAL CONDUCTIVITY

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