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Reducing Agglomeration of Explosively Disseminated Aerosol Powders

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: The objective of this effort is to develop a model capable of describing the explosive dissemination of a compacted powder. It should be appropriate for small-scale devices: for instance, a hand grenade. Understanding the underlying physical and chemical principles controlling explosive dissemination of powders is a primary knowledge gap. Of equal importance is determining how the powder’s material properties and packaging affect dissemination. Discovering the mechanisms responsible for agglomeration and deagglomeration of powders is necessity to tailor the dissemination process for effective screening of the electromagnetic spectrum for future obscuration missions. 

DESCRIPTION: Part of the mission of the Smoke and Target Defeat Branch is to develop new materials for enhanced obscuration. Typically, materials identified to have excellent theoretical screening properties for a given region of the electromagnetic spectrum, often had reduced extinction properties when tested in their native state and even worst, sometimes orders of magnitude less, when disseminated from a weaponized device. It is assumed that much of the lost performance was due to agglomeration of particles. The effectiveness of a material to screen at a certain wavelength is partially controlled by the particle size. Agglomeration increases the particle size, causing a decrease in screening ability across the wavelength range of interest. Over the past 50 years, many studies have been conducted at or for ECBC to better understand explosive dissemination. Unfortunately most of these studies relied on trial and error to develop effective dissemination devices. It was also discovered that what worked for one type of material usually was not as effective for different material, and the devices typically were not scalable. With emerging technologies and new threats on the battlefield the need for spectral dominance by using obscurants is critical to countering these threats, providing protection, and increasing survivability. Research of unique materials capable of obscuring throughout the electromagnetic spectrum against threat systems has shown promising results for spectral dominance. A major area of research in the spectrum for certain materials is the explosive dissemination of particles from military devices. During dissemination agglomeration of particles decreases the effectiveness of an obscurant cloud. The challenge is to overcome this characteristic when looking at specific particle sizes of materials for obscuring desired wavelengths of the electromagnetic spectrum. Typically spherical particles are visual obscurants (e.g. titanium dioxide) and flakes (e.g. brass) are infrared or ‘bispectral’ obscurants. Propagation of the blast wave, van der Waals forces, surface tension, thermal effects, packing pressures, as well as other parameters are currently being researched to reduce or eliminate agglomeration and provide higher performance of specific materials. The U.S. Air Force, Department of Energy and University of Florida have begun computer modeling of explosive dissemination, however their focus is on particles in the 100 µm range, which is one or more orders of magnitude larger than the particle sizes of interest for obscuration. The explosive dissemination of powders represents an extremely complex environment, with numerous parameters that can affect the resulting aerosol cloud. A partial list of parameters that could affect dissemination efficiency is as follows: • Grenade body geometry, • Grenade body material strength (fiberboard, plastic, metal, etc.) • Burster geometry • Powder fill to burster ratios • Optimal location of burster (central, implosive, one end) • Powder properties: hardness, malleability, surface energy, shear forces, van der Waals forces, frictional forces, etc. • Coatings and surface treatment of powders, can they adjust the powder’s material properties and thus enhance dissemination? • Powder packing fill fraction and pressure • Effects of explosive shock waves and reflected shock waves on dissemination and agglomeration of powders. Do shock waves reflected off of the grenade wall contribute to the agglomeration? Can the shock waves be reflected/propagated in such a way, or can the geometry be designed in such a way, that the shock waves are either negligible and do not agglomerate the powder or perhaps deagglomerate the powder in a purposeful way. • Is implosive or explosive or a combination of the two a desirable way to disseminate a cloud and reduce agglomeration? • Are there free radicals that are generated during the explosive process? Do these free radicals cause the particles to agglomerate during the dissemination? Is it wise to have free radical acceptors or charge sinks to enhance dissemination? 

PHASE I: The list of possible parameters will need to be selectively reduced to a reasonable number, with the focus being on those parameters that have the most effect on the explosive dissemination of micron and submicron obscuration materials from a hand grenade type device. This information will then be used to construct a one dimensional model, e.g., expansion of particles in the radial direction, based on fundamental physical and chemical principles that control the explosive dissemination of aerosols. The model should be constructed with the goal of expansion into 2 and 3 dimensions in Phase 2. Due to the limited scope of Phase 1, the emphasis will be placed on identifying the parameters that have the greatest effect on aerosol dissemination, in order to reduce the number experiments needed for validation and verification of the model in Phase 2. 

PHASE II: In Phase 2 the one dimensional model constructed in Phase 1 will be expanded into a 3 dimensional model of the explosive dissemination process. Ultimately it is desired to be able to model the entire dissemination process, from burster ignition through grenade body breakup to initial powder dissemination and cloud formation just prior to the environmental conditions dominating cloud transport. Experiments will be performed to validate/verify the Phase 2, 3-D model predictions. The modeled devices will be fabricated and functioned in an aerosol test chamber and the particle size distribution will be measured and compared to the predicted particle size distribution. Design parameters will be varied to prove out that the model accurately predicts the underlying chemistry and physics of the explosive dissemination. 

PHASE III: The models developed in this program can be used in the design of future explosive type dissemination systems. Understanding of how the obscuration material properties effect dissemination will be extremely beneficial and will allow more efficient and effective obscuration devices that will result the logistic burden of the soldier. Also, this model can be applied in the development of other explosively disseminated materials such as riot and crowd control devices. 

REFERENCES: 

1: MacLean, R. L.

2:  "Improved Dissemination Study for Nonlethal Agents"

3:  AFATL-TR-69-11, Jan 1969

4:  Allan, C. R. and Roth, C. W.

5:  "E-139 Bomblet Dissemination Trials"

6:  EATM 142-1, Oct 1968

7:  Bourland, Orley R., Jr. and Willard, Robert L.

8:  "The Explosive Characteristics of Dry Biological and Chemical Powders", BWL Technical Memorandum 2-39, Feb 1960

9:  Evans, K. L.

10:  "Dissemination of Solids and Liquid Smoke Agents", Smoke Symposium IX, Apr 1985, AD-B095 593, pp. 419-430.

11:  Balachandar, S, "Physics-based Simulations to Enable Game-Changing Novel Explosive and Protective Technologies", AFRL-OSR-VA-TR-2103-0518, Air Force Research Laboratory, September 2013.

KEYWORDS: Agglomeration, Particle Separation, Obscuration 

CONTACT(S): 

Zachary Zander 

(410) 436-3509 

zachary.b.zander.civ@mail.mil 

Daniel Weber 

(410) 436-2158 

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