The application of nano aluminum powder on solid propellant Metal incendiary agent is one of the important components of modern solid propellant, which can improve the explosion heat and density of propellant. Meanwhile, the solid metal oxide micro-particles that generated in the combustion process can inhibit oscillating burning. Metal incendiary agents mainly include lithium, beryllium, boron, magnesium, aluminum and so on. As aluminum powder has high density, low oxygen consumption and high combustion enthalpy, it will be used a lot in solid propellant, thus it can obviously improve specific impulse. Besides, due to the abundant resources and low cost of aluminum powder, it can be used as the additives of energy material and can be used in propellant and explosives. However, the wide application of normal aluminum powder and micro aluminum powder has been limited due to long ignition delay and slow combustion power. They will be condensed into large aggregate on the burning surface of propellant, thus it will extend burning time. The large aggregate (usually will be 50-20 microns) will be burnt up after 10-100ms. Meanwhile, it may lead to incomplete combustion, may increase infrared signal, may lead to nozzle two-phase flow loss as well as will generate smoke plume and will discharge. Therefore, nanoscience is expected to solve this problem.
Nanoscience is used in studying the unique phenomenon and function of materials that have the particle size in the range of 0.1-100nm. Nanotechnology mainly includes the synthesis and processing of nano particles as well as the manufacturing of materials that have nanostructure. When the size of some kinds of materials has been reduced to smaller than the critical size and can lead to the mutation of physical phenomenon, they will generate many new properties. If we try to control material from atomic level and molecular level, new acting forces and effects will generate, such as surface effect, small size effect, quantum size effect (blue shift), macroscopic quantum tunneling effect and so on. Due to the small particle size and large specific area and as the surface bonding state and electronic state are different from the internal of particles as well as the incomplete surface ligating atom, the surface activity of nano particles will increase, thus nano particles will have very high chemical reactivity. The protection methods and characterization of nano aluminum powder activity As propellant cannot be prepared and used at the same time, it may be replaced under atmospheric environment for a long time.
Therefore, we should make full use of the effect of nano aluminum powder that served as metal incendiary agent and try to find some methods of protecting the activity of nano aluminum powder. Due to the special volume and surface effect of nano particles, they will keep in highly activated state and will be very sensitive to the surroundings. Nano aluminum powder can be oxidized in the early time under the temperature of lower than the melting point of aluminum in the air. Besides, the reactions of aluminum metal powder in the air will depend on particle size, aluminum powder type and thickness of aluminum powder passivation layer. As nano particles have huge specific area (which can reach several hundred square meters or even higher), thin and uniform interface layer as well as atomic level step-shaped surface, all of the particles will react instantly at critical temperature. Besides, as it has various active sites, it will be easy to lead to phase change as well as will have high reaction speed in every active site. It can be seen that nano aluminum particles surface will be easy to oxidize in the air, thus it will lose activity. At this time, as the properties of nano material have been changed, if this kind of nano aluminum powder has been added in the propellant, the explosion heat may become lower than the propellant that contains normal aluminum powder, which may reduce propellant burning rate or will even lead to accidents. In addition, metal nanoparticles also will be easy to have interaction with surrounding gas and liquid molecules, thus it will form surface contamination layer and will lose activity. This process often can be regarded as a dynamic process, which is not only related with surrounding temperature, wetness, material variety and other external factors, but also will be influenced by the internal structure of nano particles (crystal or amorphous, defect type and density), surface state, particle size and other internal factors. Therefore, the activity control and stability of nano particles could be a complex and basic problem. Besides, according to related
international research trends, it can be found that we are still lack of a systematic study on activity control and stability of nano aluminum particles. Especially due to the importance of nano aluminum particles in military field, it will be difficult for Chinese researchers to get to know the research details about activity control and stability of nano aluminum particles from foreign countries. However, compared with foreign countries, China still has limited research about the application of nano aluminum powder on solid propellant. Therefore, in order to improve the research ability as well as develop the applications of nano aluminum particles and other metal nanoparticles in China, it will be very important and urgent to study the activity and stability problem. The activity protection of metal nanoparticles can start with the surface structure design and modification of nanoparticles. Below are the main methods of protecting the activity of nano aluminum powder. a. Use inertia gas atoms that have adsorptive action or fill inert gases, such as nitrogen and argon in the bottles or bags that have been filled with nano materials before sealing and storage. b. Use aluminum oxide shell or other oxide shells. c. Store it in the atmosphere that has been filled with binders or plasticizer, thus it can be easy to use and can protect particle surface at the same time. d. Use organic shell. For example, some organic thin-film that made by surfactant can effectively isolate air, prevent the oxidation of nano metal powder and promote the complexing and cladding between organic materials and metal powder chemical bond. e. Nano material can be combined (or compound) into compound with principle component. It can be regarded as a promising method by preparing nanocomposite particles, which can solve
activity protection problem of nanoparticles, but it will have certain difficulties. Therefore, it is very important to make some organic components (such as hydroxyl-terminated polybutadiene, plasticizer and so on) of nano aluminum powder and solid propellant into nanocomposite, improve the compatibility between nano aluminum particles and other components of propellant, maintain the activity of nano aluminum particles and simplify the manufacturing technology of solid rocket propellant that contains nano aluminum particles. In summary, it can be seen that the activity protection of nano aluminum powder will be of great research value, besides, we should also intensify the research on some basic works. For example, we can study the change rule of nano aluminum powder activity by observing the influence of surface modification, particle size, temperature, wetness and other factors on the activity and stability of nano aluminum powders. Besides, the activity of nano aluminum powder can be evaluated by observing oxidation reaction heat release enthalpy, initial temperature of oxidation reaction and reaction temperature interval width, thus it can set up multi-factor model for controlling activity and can lay a foundation for the successful application of nano aluminum powder on national defense industry in China.