Reaction engineering, nanopowders, explosives and detonations, analysis of chemical plants
Vladimir Hlavacek, C.C. Furnas Eminent Professor
Research conducted in the Laboratory for Ceramic and Reaction Engineering is directed in a number of areas.
Stored energy in solid materials and increased reactivity
During the Manhattan Project it was observed that fast-fission neutrons with energy close to 2MeV displace about 60 carbon atoms. Many of the dislocated atoms do not anneal during the irradiation process and the net result is an increase of internal energy of the graphite. This increase is usually referred to as a “stored energy”, in the older literature this phenomenon is sometimes called as the “total Wigner energy”.
This radiation damage alters many properties of the material. In heavily irradiated material the stored energy can amount to as much as 600 cal/gram. If suddenly released this energy can result in adiabatic temperature increase by several hundred degrees. The spontaneous release of the stored energy can be triggered off by an external heating process. Such energy release can take place since lattice defects can move inside the crystal and take part in various annealing processes in which the cluster rearranges in a more stable form.
Many metals behave in a similar way as graphite. After deformation which can be also of mechanical nature a number of dislocations and other imperfections increase It is obvious that the effect of “stored energy” represents thermodynamically a highly non-equilibrium state. Usually the relaxation time is sufficiently long, sometimes expressed in years or months scale. Therefore, because of the energetically stressed medium these systems can be extremely reactive. Experimental data indicate that the stored energy can change the reactivity of solid materials by several orders of magnitudes.
The group is investigating systematically the effect of stored energy on the reactivity of chemical reactions occurring in the solid phase. In our experiments we observed that reactivity of certain metals, as for example aluminum, can increase by several order of magnitudes. For instance commercially available aluminum powder would initiate reaction with oxygen at temperatures around 6000 C; aluminum powder that contains stored energy would react with oxygen already at 2000C. Commercially available aluminum powder would not react with water; aluminum powder containing stored energy reacts with water in the violent way. For more details on stored energy in aluminum powder see the paper by Pranda P., Prandová K., and Hlavacek V. (2000), “Particle Size and Reactivity of Aluminum Powders”, Combustion Science and Technology, 156, 81.
Carbon as promising rocket fuel
Carbon is an ideal material for solid rocket fuel. It may have a high density, its ignition temperature can be tailored by addition of small amounts of catalyst, and it can store energy. In addition carbon can chemisorb high amount of fluorine that can effectively oxidize other fuels in the solid matrix. Carbon is not pyroforic, can be produced in a cheap way, and the final product of oxidation is gas.
Currently used solid fuel for different type of missiles is based on using fine aluminum powder. Aluminum powder turned out to be a reliable oxidizer, however features different disadvantages as price, high reactivity (pyroforicity) if very fine submicron particles are used, generating of solid alumina particles instead of gas and high affinity to water vapor. In the past NASA tried to test fine carbon particles as an alternative solid fuel, however because of low reactivity of carbon particles with ammonium perchlorate the experiments have been abandoned. Carbon is an ideal fuel since it can be produced in a very cheap way, does not produce pyroforicity effects, the ignition point can be precisely tuned up and the results of combustion is only gas.
To our surprise the heat of the combustion for different carbon types were never systematically investigated, however the energy release from matrices as anthracite is higher than from aluminum; adiabatic temperature rise is over 4,0000C. In our opinion the most energetic is carbine and carbon containing stored energy.
It should be noted that carbon black and polymer carbon are extreme examples of nano-crystalline carbon. Irradiation is a very expensive way to store the energy in carbon materials. The group has developed a different way of storing energy in materials that makes it possible to produce the material on the large scale. The method is based on mechanical stress-related rather than radiation displacement of carbon atoms in the carbon lattice.
It is well known that carbonaceous materials as graphite, soot, etc. feature very high ignition point and consequently the carbon material cannot be burned completely even in large excess of oxidizer. As an example we present the incomplete combustion of fine coal particles in industrial combustors producing synthesis gas or ejection of soot particles from Diesel engine. Addition of small amount of catalyst lowered the ignition temperature by 200-5000C. This catalyst is ideally suited to tune up the ignition point of carbon in matrix of solid fuel; the price of the catalyst is negligible.
Carbon materials under certain conditions eagerly “react” with fluorine compounds to produce strongly chemisorbed fluorine. The fluorine will desorb at high temperatures. This fact will also qualify carbon as a promising rocket fuel since the fluorine released in the process of combustion of carbon may oxidize, for example, boron carbide that can be added to carbon fuel in small to medium amounts.
Deflagration and detonation in solid-solid reaction systems
Solid-solid reaction systems involve a variety of intricate physicochemical processes and mechanisms, which are poorly understood. In order to understand the fundamental interactions occurring during the solid-solid reactions, it is necessary to model the process. These models, if properly formulated, can provide an insight in process characteristics as, for example, temperature and conversion in the reaction front, velocity of the front propagation etc. If the solid-solid reaction is strongly exothermic a rich spectrum of completely new phenomena can occur. Many exothermic solid-solid reactions as, for example, aluminothermic operations and intermetallic processes are far more energetic than energetic substances which can detonate, as for instance TNT or RDX Therefore, some time ago we asked an interesting question: Is it possible to find conditions under which the solid-solid reactions listed above will detonate?
The rapid and violent form of energy release, called detonation, is caused by a shock wave propagating into the energetic material. This shock heats the material by compressing it and thus triggering a chemical reaction. Eventually a balance is attained such that the chemical reaction supports the shock. In this process material is consumed several order of magnitude (103-108) faster than in a flame, making detonation easily distinguishable from other energy release processes. For example, a good solid explosive converts energy at a rate 1010 W/cm2 in the detonation front. For perspective, this is hundred times higher energy flux than regular laser or energy beam.
Traditionally, conduction of heat was considered as the main form of pre-heating and propagation velocities were observed in the order of few mm to few cm per second. In the past nobody observed fast phenomena of propagation if thermal initiators have been used.
Our analysis of the problem reveals that four different regimes of operation can exist: kinetic, slow SHS deflagration, fast SHS deflagration and SHS detonation. The kinetic regime is extremely slow and is important only in corrosion science. If mixture of ultra-fine particles is thermally initiated, the velocity of propagation is in the range of 10-800m/s and we refer to this regime as a fast deflagration SHS. However, if such a mixture of powders is initiated by a shock wave a fast detonation regime can result with velocities 4-10 km/s. We call this regime a SHS detonation regime.
Solid Oxygen and Applications
Medical applications of oxygen include use of oxygen tents, inhalators and pediatric incubators. Oxygen-enriched gaseous anesthetic insures life support during general anesthesia. Oxygen generation from oxygen-containing compounds has been used for respiratory support in submarines, aircraft, spacecraft, and bomb shelters, as well as in breathing apparatus.
The chlorates and perchlorates of lithium, sodium, and potassium evolve oxygen when heated. These salts can be compounded with a fuel to form a chlorate-based candle that produces oxygen by a continuous self-propagating reaction. Components include the oxygen-producing material, fuel, material that fixes traces of chlorine, and usually an inert binder. Once the reaction begins, oxygen is released from the hot salt by thermal decomposition. A portion of the oxygen reacts with the fuel to produce heat, namely the thermal decomposition of chlorates is a weakly exothermic reaction and the heat released by decomposition alone would not suffice to guarantee a self-propagation reaction.
Chlorate candles are quite stable. While normally hermetically sealed, these candles have been stored uncontained for as long as 20 years, and then operated successfully with no loss of oxygen output. Thus, they are well suited as emergency oxygen generation systems. The superoxides are ionic solids containing the superoxide, O2--. Superoxides of all the alkali metals have been prepared. Metal superoxides are yellow-to-orange solids. One of the prominent properties is the evolution of oxygen by reaction with water.
4 KO2 + 2 H2O → 4 KOH + 3 O2
Ozonides are the most promising materials for oxygen generation. A major contraversy regarding their thermal stability. The more stable KO3 contains 45.9% of active oxygen. Accordingly 1 kg of KO3 (or 0.502 litre) can produce oxygen for 11.6 man hours.
The technology of production of KO3 is based on contact a rich ozon stream at room or lower temperature (-10°C) with solid KOH. The process of production potassium ozonide has an advantage that the consumed material can be easily regenerated. After the hydrolysis is over the ozonide is fully converted to KOH and water. The recycled material will be evaporated/dried (the technology is 100 years old) and ozonized again. If by mishandling in houshold operation-oxygen generation part of KOH is converted to K2CO3 the resulting mixture KOH/K2CO3 will be mixed with equimolar amount of Ca(OH)2 and the resulting CaCO3 removed by filtration.
Thermal cutting of steel and concrete
Current military operations require the ability of armed forces to defeat different targets. Among the group of potential targets is a thin steel barrier surface of 0.25-0.5 inch which can be placed both horizontally or vertically. In recent military operations the US Marines, Seals and other Special Units and the US Army identified defeating these targets as a very critical military operation. In particular the US Navy is regularly exposed to the problem of checking closed welded spaces on ships carrying various cargos. A non-explosive rapid method of entry to welded metallic structures is of prime interest.
The technology we advocate here is a new highly- efficient approach of cutting metallic or ceramic materials in the flow of very pure oxygen. The oxidizer, flow of very pure oxygen, is produced from ma solid source of oxygen. Sodium or potassium chlorates are mixed with a small amount of fine iron powder, after ignition a self-propagating reaction takes place and a continuous flow of very clean oxygen is produced. Once the reaction begins, oxygen is produced from the hot salt by thermal decomposition. A small portion of the oxygen reacts with the iron powder to produce heat, namely the thermal decomposition of chlorates is a weakly exothermic reaction and the heat released by decomposition alone would not suffice to guarantee a self-propagating reaction. The chlorate candles are quite stable. When hermetically sealed, these candles can be stored for as long as 20 years and then operated successfully with no loss in oxygen output.
As a fuel we have developed a lance Sharp-Fire Snake™; this lance is a multistrand steel wire tow having a hollow core with an outer sheet of material that is capable of efficient focusing of the torch flame. The steel wire is preferably high-tensile steel, as distinct from soft iron lances which burn away quickly. The external sheet of our proprietary material that has been patented recently provides an extremely efficient focus of the torch and consequently this torch will outperform any torch currently commercially available. One end of the flexible lance is provided with an end fitting for connecting the oxygen gas supply. The flexibility of the lance enables it to be made in appreciable length to avoid interruption of the operation as with known limited-length lances, as well as making it convenient to use and suitable for operations in tight spaces not possible with rigid lances.
