19 Plasma and spray pyrolysis technique

S.S. islam

1.   Plasma Synthesis of Nanomaterials

Its novel and distinct properties make plasma an important state of matter, which includes positive as well as negative charge particles (e.g., electrons, ions, etc.) such that the aggregate of all electric charges is zero. The charged species are mixed with neutral gas atoms or molecules, also present in plasma. The ionization degree, defined by the ratio of charged particles to uncharged particles, may be pretty small. “Ionized” implies the existence of one or more free electrons, which are not associated with any atom/molecule. Plasma is electrically conducting and strongly responds to electromagnetic fields owing to the presence of free electronic charges. A plasma containing particle is termed as dusty plasma and the dusty plasma is associated with the synthesis.

Among the large variety of plasma based nanoparticle processings, electrical systems have attracted much interest. There are a variety of plasma techniques, which can be distinguished on the basis of numerous conditions, as follow:

• Depending upon the process pressure, plasma can be of two types, namely – low pressure and atmospheric pressure plasma
• On the basis of thermodynamic equilibrium, plasma are of two types: (a) equilibrium or thermal plasma, where temperature of all species, i.e., electrons, ions, and gas, are equal; and (b) non-equilibrium or non-thermal plasma, wherein, the ions and gas are at around same temperatures, but the temperature of electrons is much higher than these two.
• On the basis of operating temperature, plasma can be divided into two categories: (a) low temperature (less than 2000 K) plasma, and (b) high temperature (more than 2000 K) plasma.
• Plasma can be divided into several types on the basis of the principle used to generate plasma: such as microwave, radio frequency, AC or DC, electron beam, plasma torch, corona, and electric arc discharge plasma, and so on.
• On the basis of the type of coupling used, plasma can again be of two types: (a) inductive and (b) capacitive coupling plasma.
• Depending on the precursors, plasma can be of three types: (a) gaseous, (b) solid, or (c) solution based plasma.

We will discuss the most common types of plasma synthesis systems in the following subsections.

a. Microwave Systems: In these systems, microwave frequency is used to create the plasma of the material required to be deposited. The material needs to be heated by the microwaves in order to create the plasma. Therefore, it is important to consider the energy transferred (E) to the charged particle of mass m. Let us suppose, f is the frequency of the oscillating electric field, and Q be the charge on the particle. It is well recognized that E is scales linearly with Q and varies inversely with m and the square of the frequency f. The magnitude of E decides the temperature of the charged species. Besides possessing ions and free electrons, the microwave plasma also comprises neutral gases, dissociated gas species along with the precursor molecules to carry out desired chemical reactions. Energy transferred to particles is affected by the collisions between charged (electrons, ions) and uncharged species (molecules, atoms, or particles). Let us assume z to represent the collisions frequency. The collision frequency scales directly with the gas pressure. Therefore, z should also be considered in evaluating the energy transferred, as follows:

Since z increases the gas pressure, E also varies with the gas pressure. Owing to a large difference in temperature of the electrons and ions, and also between electrons and neutral gas species, the microwave induced plasma is considered as non-equilibrium or non-thermal plasma. Such plasma is also termed as “cold plasma”. Because of the overall low temperature in microwave plasma reduce the propensity of particle clustering during synthesis process.

The schematic representation of calculated relation between z and E is shown in Figure 1, on an elementary species. This is achieved by using two most popular microwave frequencies, i.e., 0.915 GHz, and 2.45 GHz, and, also one more frequency (5.85 GHz). The figure also shows three different zones for relationship between z and the frequency of the microwave. The following conclusion can be drawn from the figure:

• E decreases with a corresponding increase in the microwave frequency. This is due to the particle formation in the plasma.
• If microwave frequency is decreased, the reaction temperature increases.
• To decrease the synthesis temperature, microwave frequency should be increased.
• An increase in gas pressure (or as z increases) leads to increase in E for z << f; reaches the maximum value at f = z; and then starts decreasing for z >> f.

Figure 1 Variation of energy transferred with microwave frequency.

Since microwave plasma is a gas phase approach, vaporized precursors are required. The presence of free electrons interferes with the chemical reactions, thus microwave plasma offers highly reactive conditions for particle synthesis. High chemical dissociation as well as ionization of the components taking part in the chemical reactions result in rapid synthesis of the particles. Reactions which are not chemically feasible (but thermodynamically feasible), can take place at moderate temperatures.

The synthesis process involves evaporating a volatile precursor and introducing a suitable reactive gas inside the reaction chamber. The chemical reactions between the compounds are initiated by applying thermal energy. This causes collisions between the molecules, leading to the formation of small crystalline particles by homogenous nucleation. The size of the particles initially increases due to condensation of molecules on the nucleation site. Further increase in the particle size is caused by coagulation, followed by coalescence. Eventually the particles agglomerate, thereby forming big clusters. The agglomeration can be prevented by rapid quenching.

In microwave plasma, particle formation is enhanced by ionization, particle charging and dissociation of the compounds. Nucleation is critical towards particle synthesis as the particle size variation is influenced by the number of nucleation sites present in plasma. The nucleation process differs from the classical homogeneous nucleation from super-saturated vapors; rather it is described as a chemical clustering mechanism, determined by the chemistries of the reacting species. These species are different for different material system, thus, a general nucleation mechanism is difficult to predict. Syntheses conditions can be selected so as to produce particles carrying like charges (i.e., all particles carrying either positive or negative charge). This prevents particle growth via coagulation/coalescence/agglomeration due to Columbic repulsions among likely charged particles.

Microwave plasma may produce highly uniform sized particles in comparison to the conventional gas phase techniques (if all particles are made to be evenly charged). Thus, non-thermal microwave plasma can be expected to produce nanoparticles with narrow size distributions.

Parameters influencing the growth of particles include microwave frequency, cavity design, system temperature and pressure, microwave power, residence time, type of precursors and their concentrations. Some of these parameters are interdependent. For example, any increase in the flow of gas does not only increase the pressure of the gas, but also causes an increase in the temperature. Temperature affects the collisions and the residence time as well. Additionally, microwave power can directly influence the system temperature. These interdependencies are further complicated by the effect of pressure on particle charging and selective particle heating. Therefore, any discussion of influencing parameters is highly complex.

b. AC and DC Plasma Systems: Elevated temperature plasma techniques are the most popular and were the earliest to be used. They operate at ambient pressures and work on electric power produced by DC, AC, or RF sources. Energy distribution in these plasma systems is typically close to thermal equilibrium. Figure 2 displays two types of assemblies used for powder synthesis. They can operate on both AC and DC power. The plasma is created between two co-axial electrodes, and a powerful gas stream blows it out of the system. The gas stream is used for two purposes: (a) It works as the reactive gas for the plasma, and (b) avoids overheating of electrodes. Higher power systems may have an integrated water-cooling system. Suitable reactive gases required to produce the desired particles may be added. These arrangements differ in the manner, precursor is supplied. The precursors used may be a solid powder, or liquid precursor in a solution. The solution based precursors strongly affect the energy of the plasma flame.

The axially fed precursor is injected in the chamber by the co-axially introduced gas stream, thereby avoiding the need for any pump for the precursor. Any unplanned deposition of product in the system is prevented by a circumferential sheath gas introduced in the nozzle system. Powder precursors are generally fed directly into the plasma flame from sides. The temperature in the plasma is extremely high (~4000 K), such that the metals and the majority of oxides are already melt or evaporated. Thus, as the precursor evaporates, there is no problem of particle or droplet size. Nozzle design is the crucial parameter influencing the synthesis. Nozzle designs vary depending upon the gas velocity required for the particular process from simple to highly sophisticated having hypersonic gas velocities. The particles, in plasma, move randomly in the direction of the gas stream. Therefore, there is high probability of particle collisions, which may result in clustering/agglomeration. To avoid particle clustering, rapid quenching is often used. The quenching zone should be properly designed for ensuring high quality product formation. The quenching gas can be introduced into the system either radially or axially against the flow direction. Nonetheless, AC and DC plasma systems suffer from producing highly clustered forms of powders.

Figure 2 Two basic schemes of plasma systems for nanoparticle synthesis. These systems only differ in the supply of the precursor.

c. RF Systems: In synthesizing nanoparticles via RF plasma method, the plasma is produced by RF coils. The precursor is kept in a pestle and the pestle is placed inside the evacuated chamber. The high frequency RF coils heat the precursor above its evaporation temperature. Helium gas is introduced inside the chamber to form a high temperature plasma around the coils. Helium gas atoms act as nucleation sites for the precursor vapors. These vapors then diffuse up to a cold collector rod, to be collected as nanoparticles. And lastly, the produced particles are passivated by introducing suitable gas (such as oxygen). A schematic of the RF plasma system is shown in Figure 3. The process gas and precursors are introduced into the system analogous to the AC and DC systems. The frequencies applied in this case vary between 50 kHz and 10 MHz.

The difference between AC/DC and RF systems is that the latter works without electrodes. Thus, contamination from electrodes is not an issue. Additionally, RF systems do not have wearable parts with limited service life. Further, consumable electrodes cannot be employed as precursors. The quality of the product can be improved by quenching in inductively coupled RF systems.

Figure 3 Setup of inductively coupled plasma device for nanoparticle synthesis. The precursor may be supplied either axially or radially.

2.   Spray Pyrolysis

It is a simple and economic procedure to deposit a wide range of thin films suitable for use in different devices including sensors, solar cells, etc. It can be employed to produce thin as well as thick films, also ceramic coatings, or powders. Films of almost any composition can be produced via this method. A standard spray pyrolysis instrument includes an atomizer, the precursor solution, a substrate heater, and a temperature controller. Atomizer used can be of any of the following types – (a) air blast (exposing liquid to an air stream); (b) ultrasonic (using ultrasonic frequencies to produce short wavelengths required for sufficient atomization); or (c) electrostatic (exposing the liquid by high electric field).

Figure 4 shows the schematics of the equipment used in spray pyrolysis technique. Typically spray pyrolysis includes spraying of the metal salt solution on a preheated substrate. Drops sprayed onto the substrate, stretch as disks and are thermally decomposed. The size as well as structure of these disks depends on momentum and size of the droplet along with the substrate temperature. Accordingly, the film is mostly formed of the overlapping metal salt disks being transformed to metal oxides on heated surfaces.

Figure 4 Schematics of spray pyrolysis instrumentation.

The following parameters affect the structure and properties of films deposited via spray pyrolysis:

a. Temperature: This technique includes several processes taking place either at once or in succession. These include – aerosol formation and transportation, solvent vaporization, droplet impact together with successive spreading, and decomposition of precursor. Deposition temperature influences all these processes barring the aerosol formation and transportation. Therefore, temperature of substrate is the most important parameter determining the structure and properties of deposited films. With rising temperature, film morphology transforms from cracked into porous.

b. Precursor solution: Precursor is next most critical process variable. The solvent, concentration and type of salt, and additional additives affect the chemical and physical properties of precursor. Consequently, the properties and structure of the deposited films may be controlled by altering the composition of the precursor solution.

References

1. Kortshagen, U. Nonthermal plasma synthesis of semiconductor nanocrystals. J. Phys. D 2009, 42, 113001.
2. Vollath, D. Estimation of particle size distributions obtained by gas phase processes. J. Nanopart. Res. 2011, 13, 3899–3909.
3. Szabó, Dorothée Vinga, and Sabine Schlabach. “Microwave plasma synthesis of materials—From physics and chemistry to nanoparticles: A materials scientist’s viewpoint.” Inorganics 2.3 (2014): 468-507.
4. https://www.jstage.jst.go.jp/article/kona/25/0/25_2007007/_pdf.

   Review Questions

1. In atomization

a. passing molten metal through orifice and cooling it by dropping into water

   b. molten metal is forced through a small orifice and broken up by a stream of compressed air

   c. powder of metal is made by ball milling

   d. the hydrogen reduces the oxide to metallic powder

2. Which method is used to make powder of metals having low melting point?

a. Mechanical pulverization

    b. Electrolytic process

    c. Chemical reduction

    d. Atomization

3. Differentiate between the microwave and RF assisted plasma synthesis technique for nanoparticles.
4. How does particle agglomeration affect the properties of produced nanoparticles?