Boron particles

In certain respects, the combustion of boron is different from that of carbon because, under normal temperature and pressure conditions, the product oxide, B203, is not a gas. Thus, a boron particle normally has an oxide coat that thickens as the particle is heated in an oxidizing atmosphere. This condition is characteristic of most metals, even those that will bum in the vapor phase. For the efficient combustion of the boron particle, the oxide coat must be removed. The practical means for removing the coat is to undertake the oxidation at temperatures greater than the saturation temperature of the boron oxide B203. This temperature is about 2300 K at 1 atm. 

However, unlike gas-phase reactions, the boron particles are oxidized at their surfaces, so that some unreacted boron remains after combustion. A significant difference between B and A1 is that the amount of oxygen gas (O2) needed to produce boron oxide is 2.22 kg [02]/kg [B] whereas the amount required to produce aluminum oxide is just 0.89 kg [02]/kg [Al]. Since the amount of oxygen needed to burn boron particles is approximately 2.5 times larger than that needed to burn alum-... 

B-AP pyrolants made with CTPB are cured with epoxy resin as in the case of conventional AP-CTPB composite propellants. The mixture ratio of large-sized AP particles (200 pm in diameter) and small-sized particles (20 pm in diameter) is 0.30/ 0.70. The mass fraction of boron is variously 0.010, 0.050, 0.075, or 0.150, and the diameter of the boron particles, d, is either 0.5 pm, 2.7 pm, or 9 pm. 

Fig. n.l3 Temperature profiles in the combustion waves of AP pyrolants with and without boron particles. 

Fig. n.15 Burning rate augmentation as a function of pressure for B-AP pyrolants containing different mass fractions of boron particles. 

Fig. 11.16 Burning rate augmentation as a function of the total surface area of the boron particles in B-AP pyrolants composed of boron particles of different sizes.

Fig. 11.17 shows burning rate augmentation, Eb, as a function of the adiabatic flame temperatures of B-AP and Al-AP pyrolants. The incorporahon of aluminum particles into a base matrix composed of AP-CTPB pyrolant increases Ej. However, the effect of the addihon becomes saturated for adiabahc flame temperatures higher than about 2500 K. On the other hand, the incorporahon of boron particles into the same base matrix increases Eg more effectively, even though the adiabahc... 

As shown in Fig. 11.13, the temperature in the gas phase increases rapidly when boron particles are added. This is due to the higher burning rate of the boron py-rolant compared with that of the pyrolant without boron. Though the burning surface temperature is approximately 700 K for both pyrolants, the temperature in the gas phase just above the burning surface increases more rapidly when boron is added. The observed results indicate that the boron is oxidized just above the burning surface, within a distance of 0.1 mm. The heat flux transferred back from the gas phase to the burning surface is increased by the increased temperature in the gas phase. 

The boron particles are thermally inert in the solid phase beneath the burning surface of the pyrolant The oxidation of the boron particles occurs just above the burning surface. This implies that the temperature gradient in the gas phase, (j), increases and hence the burning rate is increased accordingly. 

Thus, the burning rate is increased by the addition of boron particles. Though the heat flux increases with increasing pressure for both pyrolants with and without boron particles, the heat flux at constant pressure is increased when boron particles are added. Furthermore, the heat flux also increases with decreasing at constant... 

Ib and with increasing b at constant d. Since the oxidation of the boron particles proceeds from their surfaces, the surface area of the boron particles plays a dominant role in determining the reachon rate. In fact, the total surface area of the boron particles in unit volume of pyrolant is seen to correlate with the burning rate, as shown in Fig. 11.16. 

Boron particles are incorporated into GAP pyrolants in order to increase their specific impulse.[8-i2] xhe adiabatic flame temperature and specific impulse of GAP pyrolants are shown as a function of air-to-fuel ratio in Fig. 15.10 and Fig. 15.11, respectively. In the performance calculation, a mixture of the combustion products of the pyrolant with air is assumed as the reactant. The enthalpy of the air varies according to the velocity of the vehicle (or the relative velocity of the air) and the flight altitude. The flight conditions are assumed to be a velocity of Mach 2.0 at sea level. An air enthalpy of 218.2 kj kg is then assumed. 

Data for the combustion temperature in a gas generator are shown in Table 15.5. The GAP pyrolant without boron particles, b(0-0), burns incompletely. The... 

The temperature of the boron particles is raised by the heat generated by the decomposition of GAP. However, no combustion reaction occurs between the boron particles and the gaseous decomposition products of the GAP pyrolant. Thus, the temperature in the gas generator remains low enough to protect the attached nozzle from adverse heat... 

Fig. 15.12 Effect of boron particle size on the burning rates of an AP composite pyrolant.

Boron is one of the essential materials for obtaining high specific impulse of a ducted rocket However, the combustion efficiency of boron-containing gas-generating pyrolants is low due to incomplete combustion of the boron particles in the ramburner.[i3-i l Fig. 15.20 shows the combustion temperature ofa boron-containing pyrolant with and without boron combustion as a function of air-to-fuel ratio, 8. A typical boron-containing pyrolant is composed of mass fractions of boron particles b(0-30), ammonium perchlorate ap(0.40), and carboxy-terminated polybutadiene ctpb(0-30). If the boron particles burn completely in the ramburner, the maximum combustion temperature reaches 2310 K at 8 = 6.5 and v = Mach 2 p =... 

MPa) under conditions of sea-level flight However, if no boron combustion occurs, the temperature decreases to 1550 K under the same flight conditions. The combustion efficiency of boron particles is an important parameter for obtaining high specific impulse of ducted rockets. 

Fig. 15.22 Combustion efiflciency of boron particles when two single-port intakes or two multi-port intakes are used.

Atmospheric boron is in the form of particulates or aerosols of borides, boron oxide, borates, boranes, organoboron compounds, trihalide boron compounds, or borazines. The half-time persistence of airborne boron particles is short, usually on the order of days (USPHS 1991). 

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