Flame Spray Technology

The nanoscience and nanotechnology have been arising as an important field of science to change the industrial production and economics for the next decades. These sciences have been considered a powerful tool that can bring benefits to a wide range of scientific areas, attracting fast and increasing investments by the government and companies from different places around the world.

The flame spray (FS) technique has been practiced in a non-intentional way since the prehistoric age, according to paintings observed in the walls of caves in China. At that time, its principles were not understood, but people had the knowledge to produce the pigments used in the paint by the FS process. The first contemporary reactors for nanoparticles flame synthesis started in the 1940s, by the production of fumed silica. Only in 1971, G.D. Ulrich pioneered the first principles of the FS method. Since this day, several laboratories and companies have developed different apparatus and alternative methods, aiming to obtain materials with improved properties. This is the main driving force for the evolution of the FS process during the last decades, where more materials and equipments are reported in literature.

The technology involving the flame spraying of a precursor solution is based on the formation of small particles from the gas or vapor phase in a flame. Flame spray (FS) is regarded as a consolidated industrial technique, as discussed in the previous chapter. However, the principles and fundamentals of the particles synthesis in the flame are not completely understood. One of the main reasons for the absence of a complete description of FS synthesis is the fact that chemical reactions and particle formation occurs in a short time and high temperature during the process. The basic principle of FS technique is the formation of vapor or aerosol of the precursor compounds, which reacts in the high temperature of the flame leading to the formation of a ceramic compound. There are two basic types of process based on the formation of an aerosol and further spraying in a flame. The former is the conversion to a particle from the vapor phase, due to the presence of a flame, and named vapor synthesis in a flame. The second process comprises the conversion of a droplet to a particle, also assisted by a flame, and named FS pyrolysis.

One of the most important requisites concerning the FS process is the equipment. All fundamental principles are based on the flame configuration, atomization device, flame temperature, powder collection system, and burner. The wide range of apparatus configurations contribute to the several materials that have been produced by the technique described in this book. As it has a strong influence in the final properties of the product, the description of the different equipments reported in the literature is an important feature. Most of the devices reported in this chapter were built in laboratory facilities, but several companies such as DuPont, Cabot, Degussa, Kemira, Tioxide, Corning Glass, and General Electric have been using their own equipments. The flame spray (FS) equipment can be basically divided in three sub-components: the atomization device, the group of flames, and finally the powder collection system. Each of these devices has its importance in the process of producing powders using the FS method, and all of them have different designs and different sub-components, according to the industry or institution that developed/fabricated the equipment. Thus, in this chapter, the main features of the FS apparatus are discussed, and a special attention was given to the flame device, which is the most important device of the equipment.

The nanomaterials era has enormously contributed to the development of new materials, and the flame spray (FS) method has arrived as a potential technique for its development. One of the great features of the FS method is the wide range of ceramic commodities produced by this technique, as well the wide range of morphologies available for these nanomaterials. Consequently, several types of application for them are possible. In addition, more and more laboratories and companies around the world have been developed and upgraded different FS apparatus. Recent ceramic materials, which years ago were not imagined to be produced by the FS process, are nowadays obtained in a simple step process. Because the FS is a versatile technique, it allows the production of single and mixed oxides, since black carbon to more complex oxides as hydroxyapatite (HA) and spinels. Thus, this chapter presents numerous examples of ceramic nanomaterials produced in different equipments, either commercial or academic, as well their morphology and main applications. One example is black carbon, which is the first material produced by the FS technique and has a large industrial production rate, and even today, is widely used in different industries and products. Recent nanomaterials, such as Ca₁₀(PO₄)₆(OH)₂, ZnO, TiO₂, Al₂O₃, Y₂O₃, GeO₂, MgO-Al₂O₃, CoMo/Al₂O₃, and SnO₂, are also describe in this chapter.

This chapter introduces the main future trends associated with the use and development of the flame spray (FS) technology. The FS process has been considered by many researchers as a novel and powerful method for the production of nanoparticles and nanostructured particles. However, if we consider that its principles and fundamentals are not fully understood, and considering that only few commodities (black carbon, titania, and silica for example) are commercially available up to now, there is a great demand for the development of the FS process. Today, there are several areas where FS process can be developed, which justify the strong efforts made by many scientists. In addition, we must consider that the increasing use and interest in the FS process makes it a promising technique for the production of a wide range of commodities, especially if we take in mind that most of the products obtained with FS are in the nanosize scale. Another important point concerning the future of the FS comprises the synthesis of unconventional oxides and can be also considered as a great development done for the improvement of this method. It is clear to many scientists that a deep understanding of the particle formation mechanisms in the flame is an important step in the development of the FS process. The better understanding of what happens in the flame basically comprises the acquisition of experimental data, leading to a further modeling and simulation of particle formation mechanisms and their kinetics in flames. The use of the principles of this technology but aiming the production of thin films and coatings is another point and has emerged as an important area inside this technology. These are just two short examples of the potential areas inside the FS technology. This chapter intends to present to reader the main future trends concerning the use of a flame to obtain ceramic products, as well as the development and improvement of new devices to be used as flame, powder collector, and atomization are discussed. Another important point of this chapter is the perspective of the production of nanoparticles aiming its application in several industrial concerns as biomaterials, electronics, catalysis, etc.