Metallized pyrolants

The pressure generated by the combustion of metallized pyrolants is sometimes lower than that generated by the combustion of non-metallized energetic materials. 

The combustion temperature of metallized pyrolants is high due to the high heat of combushon of metal parhcles, and the molecular mass of the combustion pro-... 

When a metallized energetic material is burned as a propellant igniter in a rocket chamber, a consequence of the aforementioned production of metal oxides as hot condensed particles is that there is very tittle associated pressure increase. However, the surface of the propellant grain in the chamber is ignited by the hot particles and a stable burning pressure is established. Typical metallized pyrolants used as igniters are shown in Table 11.1. 

Table 11.1 Chemical compositions of metallized pyrolants used as igniters.

As it follows from Table 5, many catalysts contain metallic platinum. We have developed bi-layer porous hydrophobic air electrodes, which do not contain platinum metals, are active and can be cycled [24, 25] (Figures 4-6). These bifunctional catalysts are pyrolized Co - macrocyclic compounds. Said catalyst has high catalytic activity for the oxygen reduction and also features acceptable stability, however its activity for the oxygen evolution is not high enough. 

When a pyrolant is composed of metallic particles and an oxidizer component, both gaseous molecules and metal oxides are formed as combustion products. Since the metal oxides are produced in the form of condensed-phase particles, the equation of state shown in Eq. (10.1) is no longer valid to evaluate the pressure in the cham-... 

There are various types of pyrolants that generate gasless combustion products. The pyrolant composed of aluminum powder and iron oxide powder generates aluminum oxide and metallic iron as combushon products. This reaction represented by... 

Since pyrolants are mixtures of various chemicals, such as crystalline particles, metal particles, metal oxide particles, and/or polymeric materials, the physico-... 

The chosen combinations of these chemicals and metals depend on the requirements of the specific application. Gasless combustion prevents pressure increase in a closed combustion chamber. Some combinations of metal particles and metal oxide particles or of metal particles and crystalline oxidizers are chosen as chemical ingredients of gasless pyrolants. On the other hand, hydrocarbon polymers are used to obtain combustion products of low molecular mass, such as H2O, CO, CO2, and H2. High pressure is thus obtained by the combustion of hydrocarbon polymers. Table 10.6 shows the chemical ingredients used to formulate various types of pyrolants. 

Several types of metaUic oxides are used as oxidizers for pyrolants. A metallic oxide can oxidize a metal if the chemical potential of the resulting metal oxide is lower than that of the starting oxide. No gaseous reaction products are formed when metal particles are oxidized by metallic oxides in a closed system and very Uttle pressure increase occurs since the reactants and products are aU solids. Typical metallic oxides are MnOj, FcjOj, CoO, CuO, ZnO, and Pb304. 

The selechon of fuel components to be mixed with oxidizer components is also an important issue in the development of pyrolants for various applications. Metal particles are used as fuel components to develop small-scale pyrolant charges as deployed in igniters, flares, and fireworks. Non-metal particles such as boron and carbon are used to formulate energetic pyrolants. Polymeric materials are commonly used as fuel components to develop relatively large-scale pyrolant charges, such as gas generators and fuel-rich propellants. 

Aluminum (Al) is a silver-colored light and soft metal used as a major component of aluminum alloys, which are used to construct aircraft and vehicles, similar to Mg alloys. However, Al is known as a readily combustible metal. Thus, Al particles are used as major fuel components of pyrolants. Al particles are mixed with ammonium perchlorate particles and polymeric materials to form solid propellants and underwater explosives. The reaction between aluminum powder and iron oxide is known as a high-temperature gasless reaction and is represented by ... 

As shown in Table 10.5, non-metaUic fuels used as ingredients of pyrolants are boron, carbon, silicon, phosphorus, and sulfur. Similarly to metal particles, non-metal particles are oxidized at their surfaces. The processes of diffusion of oxidizer fragments to the surface of a particle and the removal of oxidized fragments therefrom are the rate-controlling steps for combustion. 

Hydrocarbon polymers (HCP) are used not only as fuel components but also as binders of crystalline oxidizers and metal powders in the formulation of pyrolants, similar to composite propellants and plastic-bonded explosives. There are many types of HCP, the physicochemical properties of which are dependent on their molecular structures. The viscosity, molecular mass, and functionality of the poly-... 

Sodium azide is not as sensitive as lead azide or silver azide to friction or mechanical shock. Since sodium azide reacts with metal oxides to generate nitrogen gas, mixtures of sodium azide and metal oxides are used as pyrolants in gas generators. However, sodium azide reacts with copper and silver to form the corresponding azides, both of which are detonable pyrolants. 

The heat produced by the reaction of a pyrolant is dependent on various physicochemical properties, such as the chemical nature of the fuel and oxidizer, the fractions in which they are mixed, and their physical shapes and sizes. Metal particles are commonly used as fuel components of pyrolants. When a metal particle is oxidized by gaseous oxidizer fragments, an oxide layer is formed that coats the particle. If the melting point of the oxide layer is higher than that of the metal particle, the metal oxide layer prevents further supply of the oxidizer fragments to the metal, and so the oxidation remains incomplete. If, however, the melting point of the oxide layer is lower than that of the metal particle, the oxide layer is easily removed and the oxidation reaction can continue. 

Table 11.3 Maximum flame temperatures of metal-GAP pyrolants.

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