The Micro-FUEL CELL™ is a spherical particle composed of layers of polymers (plastics) which are organic materials and high energy fuels and oxygen sources (oxidizers) which can be either organic or inorganic (salt-like) chemicals. The only way to make an MFC™ is by "growing" it, layer by layer, in a solvent, as shown in the diagram on the left, Figure 1. But the MFC™ is composed of a variety of chemicals, some of which are very similar, and others of which are very different.

Conventional solvents strictly fall into two classes, those (such as alcohol or paint thinner) in which organic chemicals are very soluble, and those (such as water) in which inorganic chemicals are very soluble. On the other hand, each class of solvent is not very specific. For instance, water will solubilize many different inorganic chemicals simultaneously, but there is no way to selectively choose one of these solubilized chemicals, and specifically make that one chemical insoluble, leaving all the rest in solution.

The MFC™ particle is made of both kinds of chemicals - organic and inorganic - each of which are soluble in very different kinds solvents. Furthermore, each layer of a MFC™ may contain chemically similar chemical ingredients to an underlying layer. Once a layer is formed on the particle, the act of "growing" the next successive layer cannot solubilize chemical components of the layer previously "grown" on the particle. From the requirements of manufacturing MFC™s, it is clear that conventional solvents are not suitable, they are not selective enough in what they solubilize. Yet an MFC™ must be "grown" in a solvent or solvent system in the manner illustrated in the above illustration.

One further limitation for using conventional solvents in the manufacture of MFC™s is related to their inability to make the ultra-thin (1 micron or less), defect-free barrier layer which is required to separate the high energy fuel layer in the MFC™ from the oxygen source (oxidizer) core of the MFC™. This is due to the inalterable fact that you cannot remove conventional solvent from a conventional solvent deposited layer without tearing the very thin layer apart, thereby destroying the layer. Thus, conventional solvents cannot be used to make "protective" defect-free layers less than 15 microns in thickness - yet the MFC™ propellant particle needs a defect-free inner plastic barrier layer only 1 micron thick!

SPS therefore needed a "new" process technology for manufacture of the MFC™s, one that could be tuned to "grow" layered particles with highly specific and tailored chemical compositions for each layer. This new technology must also be suitable for production of the ultra-thin, defect-free plastic barrier layers required in the MFC™s. This process technology is further suitable for manufacture of large volumes of propellant at production rates consistent with industry standards. The process exhibits the ability to meet the exacting standards required for production of MFC™s with consistent performance in use as solid rocket propellants.



Matter commonly exists in any of three distinct states. Matter can be in the form of a liquid, a solid, or a gas. Supercritical fluids (SCF) are materials that exist in a different state, a fourth state of matter. In this fourth, supercritical, state, materials simultaneously take on the properties of both a liquid and a gas. It is not one, nor the other, but both - in a way it is similar to a dense fog. (See illustration at right)

Both organic and inorganic materials are highly soluble in supercritical fluids. However, in supercritical fluids, a small change in temperature or pressure (the process variables) can result in these solubilized materials becoming instantaneously and totally insoluble. This feature of SCF processing is illustrated in an animation sequence in Figure 2.

Further, a supercritical solution containing many different solubilized materials can be made to completely desolubilize specific ingredients, or even combinations of ingredients, by a small change in temperature or pressure. This feature of SCF was discussed in figure 2, above. In this illustration, three different materials are made soluble in SCF, by changing pressure and temperature until all three become soluble. All three components can similarly be made insoluble, or two of the three can be made insoluble, by similar changes in pressure and temperature. This high degree of control of the solubility of chemicals in SCF makes the 'growth' of multi-layered MFC™s with specific layer compositions possible, without damaging the underlying layers in any way.

Particle "growth" in SCF is also uniquely suited for the production of MFC™s. With a slight change in pressure and temperature, the solubilized material instantaneously becomes totally insoluble. This rapid change in solubility results in the formation of a uniform "fog" of super small nanoparticles. Mutual attraction between the particles results in the formation of centers where the nanoparticles collect, forming larger particles. These particles can be "grown" to any desired size by controlling of amount of chemical originally solubilized in the SCF. When the concentration of nanoparticles is exhausted, the particle stops "growing".

Particle "growth" is extremely fast, and millions of particles can be simultaneously "grown", all of which are the same size, and made of the same materials.

Introduction of a second solubilized material under conditions where the previously "grown" particles are insoluble, followed by a change in conditions to a state where the new material is made insoluble, allows formation of a uniform layer of the new material on the previously "grown" particle. This process is continued until the multilayer MFC™ propellant particle is completely formed.

In a similar manner, two or more different materials can be "grown" into a particle and/or deposited as layers, all without re-solubilizing any components of the previously "grown" particle. Since the particles are "grown" from a uniform fog of nanoparticles, the natural shape of the MFC™ is spherical. This process is demonstrated in the accompanying animation, Figure 3.

Since SCF solvents simultaneously possesses the properties of a gas and a liquid, extremely thin layers of material, such as those required for the ultra-thin inner plastic barrier of the MFC™, can be deposited without incorporating defects. This was discussed in Figure 2.


The perfection of these SCF processed barrier layers can be seen in the electron micrograph immediately left (Figure 4), and the atomic force micrograph immediately below (Figure 5). The barrier coating is less than 1 micron (0.00004 inches) in thickness. Overview of the whole particle in Figure 4 shows no visible defects, while Figure 5, below, uses atomic force microscopy (AFM) to highly magnify (zoom in on) an extremely minute surface region of this coating. This magnification is so great that individual pieces of the plastic barrier molecules can be seen in the image. Only at this high magnification can any surface variations be seen.

The right hand portion of the image below (Figure 5b, with the x-axis labelled "Lateral Force") more clearly shows the molecular fragments, while the left hand portion of this image (titled "Topography") measures the variation in surface height (unevenness) of the coating. We can see from this image that the variation in surface height is only about 17 nanometers, or about 2% of the thickness of the 1 micron barrier layer.

 Both these graphics clearly demonstrate the high uniformity and defect-free nature of the layers formed using SPS's "Fourth State of Matter"™ MFC™ production process. Chemical analysis of this barrier layer, which is composed of three different ingredients simultaneously layered on the core particle, demonstrates that complex multi-component defect-free layers of uniform thickness can be "grown" on core particles to a precise thickness using this innovative production technology.

The patent pending Supercritical Fluids based "Fourth State of Matter"™ MFC™ Propellant Particle production technology has been seen to possess all the required features for successful, cost effective, manufacture of SPS Corporation's patented MFC™ Solid Rocket Propellants, and in high enough volume to meet all projected industry demands for solid rocket propellants within the foreseeable future.