Research into Nanopowder Production Methods

Nanosized Powders are known to have very useful applications in the areas of medical, structural, chemical and other various fields because of their enhanced physical, optical, and magnetic properties. Several Synthesis methods have been developed for the production of nanopowders. This paper will discuss different methods used for the production of nanosized powders and their advantages and disadvantages. The methods include Combustion Flame-Chemical Condensation, Sol-Gel Process, Microwave Plasma Processing, Laser Ablation, Precipitation by Liquids and Rapid Expansion of Supercritical Solution. In the RESS process, solute is dissolved in a supercritical solvent and this solution is then sent through a capillary nozzle to atmospheric conditions where expansion takes place. During this expansion supersaturation increases and results in high-speed nucleation producing the nanosized powders. In this paper, it is proposed by Fisher, Mansoori and co-workers that by using the Redlich-Kwong Equation of State and mixing rules developed by Kwak and Mansoori will accurately predict the radius of the particles formed during nucleation. Radius of the nanoparticles formed can be predicted by mathematically modeling the capillary inlet, capillary and free jet, where the formation of nanoparticles begin. This paper discusses the steps involved in mathematically modeling the capillary nozzle so the radius of the particles could be predicted. Introduction Production of nanosized powders, particles with diameters ranging in the nanoscale, has been studied intensively for the past several years. The unique properties of nanopowders when compared to their bulk materials have sparked the interest in this particular field [1]. Nanopowders have very large surface area, which enables scientists to control the way these particles interact with other materials. They also exhibit properties like low temperature sinterability [2,3], improved UV scattering [2], high degree of metastability [3], superior magnetic and dielectric strength, and enhanced optoelectronic properties [2,4]. Because of their unique characteristics nanopowders have many applications in various fields, such as high surface area catalysts, nanolithography, and the food and pharmaceutical industries [2]. Several methods have been developed for the synthesis of nanopowders. Some of these methods include, Sol-Gel Process [1,7-9], Combustion Flame-Chemical Vapor Condensation (CF-CVC)[2,3,5,6], Laser Ablation [4], Microwave Plasma Processing [10], Precipitation from Liquids [11] and Rapid Expansion of Supercritical Solution [12,13]. The Sol-Gel method includes mixing of two precursors together to form a gel. This gel is then dried under pre-determined temperatures to obtain the nanosized powders [1,7-9]. CF-CVC method pryolzes the precursor gas and subsequently condenses the chemical precursor at a reduced pressure environment, thus producing nanopowders [2,3,5,6]. The third method is Laser Ablation. A high-energy laser beam transforms the material from a solid phase into a gaseous phase, and this condenses into the desired nanoparticles [9]. The next method is Microwave Plasma Processing, where microwaves generate heat by ionization of gases; the heat produced during this process will evaporate, dissociate, and recondense the material into nanosized powders [10]. The next method is called Precipitation by Liquids. This process involves mixing of two or more liquid precursors through violent and continuous stirring. During this process a precipitate begins to form and this is dried to form the nanosized powders [11]. The last method discussed is the Rapid Expansion of Supercritical Solution, in which a solute is dissolved in a supercritical solvent in an extraction vessel. This supercritical solution is then send through a capillary nozzle where it expands to atmospheric conditions. During this expansion process superstauration increases resulting in high-speed nucleation, thus producing the desired nanopowders [12-13]. When selecting a method for nanopowder production, there are several important issues that need to be taken into consideration. They are: cost, particle size, distribution, purity, and agglomeration. Most methods described here are highly uneconomical, so it is important to take cost of the process into account. Secondly, the smaller the particle the better advantages it provides. The next issue is distribution; the particles should maintain a narrow size distribution throughout to be highly effective. Purity is important; because the particles are so small any amount of contamination could decrease the quality of the powder. Finally agglomeration, the tendency for the particles to clump together to form a jumbled mass, is a major issue when it comes to production of nanosized powders. An agglomerated mass of powder has little use because it eliminates the benefits of a narrow size distribution and also it is difficult to disperse agglomerated mass into a solution [2]. This paper will further explain the methods, their advantages and their disadvantages. Sol-Gel Process Sol-Gel method of nanopowder production involves mixing of two precursors to form a gel and this gel is dried to produce the nanosized powders. This process is known to be very useful and simple. Firstly, the two precursors were dissolved in hot nitric acid. The desired stoichiometry of the two precursors was achieved by using EDTA (ethylenedamine-tetraacetic acid) complex titration and this solution was mixed vigorously to ensure a homogenous solution. Afterwards the solution was dropped slowly into a predetermined amount of EDTA under strong and continuous stirring. Extra care should be taken at this point to avoid precipitation by adding ammonium hydroxide to keep the pH below 6. Predetermined amount of PEG (polyethylene glycol), a polymerization agent was added once the solution was clear and transparent. Polymerization agent was added to ensure that the two precursors combine together. Heating the solution then evaporated the unnecessary water and this decreased the volume of the solution and increased its viscosity. After all the unnecessary water was removed, the solution resembled a highly viscous and transparent gel. This gel was then placed in an oven and was heated for 5 hours at 180C to char and it formed a dry black sponge. After the charring, the volume increased as the precursors became loose. This solid precursors were then divided into 3 parts and calcined, heated till all its moisture was lost, in a muffle furnace to obtain the nanosized powders [1,7-9]. Y2O3:Eu nanosized powders were produced using the Sol-Gel method. Y2O3:Eu is an efficient red-emission phosphor and is used in fluorescent lights and cathode ray tube. The char was calcined at different temperatures at 600C the particles are nearly spherical with a diameter of 30nm. At 1000C the particles have an increased diameter of 70nm and have regular shape. Figure 3 shows SEM photographs of Y2O3:Eu particles at 600C and 1000C [5]. Figure 3. SEM photographs of Y2O3:Eu particles (a) 600C and (b) 1000C Combustion FlameChemical Vapor Condensation (CF-CVC) CF-CVC process combines rapid thermal decomposition of a precursor/carrier gas stream in a reduced pressure environment and condenses from the gas phase to form the nanosized powders. The experimental schematic is shown in figure 1. The entire apparatus is contained in a water-cooled chamber where the pressure is maintained using a roughing pump and pressure controller. The chamber holds a 2.5” diameter flat flame burner that is pre-determined distance away from the cold substrate. The carrier gas, usually He, passes through a bubbling unit that contains the precursor. The He entrains the precursor vapor and combines with the flame fuel, methane is used in most cases, and O2. This mixture moves upstream in a highly one-dimensional flow. The precursor pyrolyzes, undergoes transformation into the gas phase in the flame and condenses to nanosized powders as the flame gas cools upon approaching the cold substrate. The high degree of uniformity in the flat flame burner insures that all the particles experienced similar time/temperature resulting in narrow size distribution as desired [2,3,5,6]. Figure 1. A schematic of the experimental setup for combustion flame chemical vapor condensation (CF-CVC). Combustion Flame-Chemical Vapor Condensation is one of the methods that offer both non-agglomeration and scalability. The similar time and temperature experienced by the gas phase precursor ensures pyrolysis and the low pressure helps to minimize the inter-particle collision thus decreasing agglomeration to its minimum [2,3,5,6]. Another advantage in using this method is that process is taking place in the vacuum chamber thus eliminating any process related contamination to take place during the production [3]. This process was used to produce nanopowders of SiO2 and TiO2. The precursor used in this method to produce each oxide nanopowders were Hexamethyldisalazane (HMDS) for nanoSiO2 and Titanium ethoxide fro nanoTiO2. Surface area measurements were made using nitrogen adsorption BET and nanoTiO2 powder has a surface are of 70-80m/g and anoSiO2 has a surface area of 270-300m/g. Figure 2 shows a TEM micrograph of synthesized nanoTiO2 powders [2]. Figure 2. TEM micrograph of synthesized nanoTiO2 powder. Laser Ablation Laser Ablation is where high-energy laser beam evaporates the compressed ceramic powder, then recondenses to form the nanosized powders. For this method, the following parameters were used: a laser pulse width of .5ms, pulse repetition rate 34Hz and laser light pulse power density of 5600 W/mm. The apparatus setup is schematically shown in Figure 4. The laser beam with an high intensity is focused on the target inorganic material through a quartz window and the beam evaporates the material and transforms it into a gaseous phase. The evaporated materials then condense into nanoparticles by collision and coalescence or nucleation and growth. Figure 5 shows the formation of particles in the nanopowder. When the laser beam hits the target, part of the energy of the laser is absorbed into the material due to the interaction between the laser’s photons and the target’s electrons. This creates a plasma at the surface, and the absorbed energy creates a temperature difference within the target; which depends on the heat conductivity of the target and the intensity of the laser beam used. The target is heated, melted, and evaporated due to the temperature difference. These evaporated materials condense to form the nanosized powders [4]. Figure 4. Schematic diagram of the laser ablation apparatus. Figure 5. A simple model of particle formation. One of the advantages of laser ablation is that any inorganic materials can be easily converted into nanopowder using this process because all inorganic materials can be evaporated by a high intensity laser beam and recondensed into nanosized powders. Another advantage is that nanopowders created using laser ablation have little tendency to agglomerate. The disadvantages to this method include low production rate, high cost and deactivation of any organic material due to high energy laser used in the process [4]. Microwave Plasma Processing Microwave plasma processing is one of the methods where production rate is high compared to other methods. During this process, microwaves generate heat by ionization and the heat produced will evaporate, dissociate, and recondense the materials into nanosized powders. Firstly, the precursor is placed into a conical flask in a glove box filled with carrier gas, Ar. This flask was then heated close to the precursor’s boiling point ensuring that the vapors were carried by the argon gas into the plasma region at a desired feed rate. The plasma gas, also argon, undergoes dissociation, ionization and recombination resulting in a plasma because of the high microwave energy focused on the gas. The heat released from the recombination process of the plasma is used to breakdown the chemical precursor. The precursor undergoes rapid heating, evaporation, thermal dissociation, and recondensation to form fine nanosized powders [10]. Nanosized Iron powders were produced by the microwave plasma processing method. The precursor used for the production of nanoIron was Iron pentacarbonyl Fe(CO)5, with argon as the carrier and plasma gas. The surface area of the Iron nanopowders was measured to be 42m/g and the average particle size was approximately 10nm. Vapors of iron pentacarbonyl decomposed into iron powders by the following reaction [10]: Fe(CO)5 ! Fe(s) + 5 CO↑ Precipitation Method Precipitation method is simple process but not highly recommended. This method involves mixing two precursors under strong and continuous stirring. During this process, a precipitate begins to form. This precipitate is then filtered, washed with warm acetic acid and then with distilled water. The precipitate is then dried under predetermined temperature for several hours to form the nanosized powders [11] An advantage in using precipitation method is that the process is done in low temperatures, T < 100C. Two disadvantages make this method highly unreliable to produce nanopowders, low production rate, and large degree of agglomeration of powder from using the liquid as the chemical precursor [10]. Rapid Expansion of Supercritical Solution One of the major applications of RESS is producing nanosized powders for the drug-delivery system [12]. In the RESS process, solute is dissolved in a supercritical solvent and this solution is then sent through a capillary nozzle to atmospheric conditions where expansion takes place, which increases supersaturation and results in high-speed nucleation producing the nanosized powders. Figure 6 shows schematically the apparatus used for the RESS process. Firstly, the gaseous solvent is sent through a column to be cleaned of all impurities and then to a liquefier where it is condensed from a gas to a liquid. Liquefied solvent is then sent through a pump and pressurized to supercritical pressure. The supercritical solvent is preheated in a heater to extraction temperature and passed through an extraction column packed with the solute. This supercritical solution is then sent through a heated tube where the pre-expansion temperature is achieved and the velocity of the solution is reduced. The supercritical mixture is then expanded through a capillary nozzle to atmospheric conditions. This expansion process causes supersaturation to increase and high-speed nucleation to begin. During this nucleation process, nanosized powders of the desired solute begin to form [12,13]. Figure 6. Schematic diagram of the apparatus used in the RESS process. The solvent used for the RESS process is CO2, because it is inexpensive, very abundant, and environmentally safe. It also has a low critical temperature and pressure, and yields to a solvent-free product. The solute under consideration is Ibuprofen shown in Figure 7. Ibuprofen is a non-steroidal, anti-inflammatory drug (NSAID) usually prescribed for arthritis, fever and pain. Since Ibuprofen is not soluble in water, which is the major transport fluid in the body, by using the RESS process size of the particles can be decreased thus increasing the surface area of the particles [12]. Figure7. Ibuprofen (p-isobutylhydratropic acid) In any industrial application, it is important to predict the product formed. In addition to the experimental investigations, the radius of the nanosized particles can be predicted by numerically modeling the capillary-inlet, capillary and free jet where the formation of nanosized powders begins [13]. Figure 8 shows an enlarged picture of the capillary nozzle [12]. Research has been done by Turk and co-workers on numerically modeling the capillary flow field by using the Bender equation of state and the PengRobinson equation of state with the van der Waals mixing rules to determine the radius of the particles formed [13]. In this paper, Fisher, Mansoori and co-workers propose that by using the Redlich-Kwong equation of state and mixing rules developed by T.A. Kwak and G.A. Mansoori, will accurately predict the radius of the nanoparticles formed [12]. Since particle formation starts within the expansion unit, which is the capillary nozzle, it is important to model pressure, temperature, density and velocity throughout the nozzle to calculate the size of the particles formed [12,13]. Figure 8. An enlarged picture of the capillary nozzle. The mass, momentum and energy balance equations are used, equations (1) (3), along with an appropriate equation of state (4) to model pressure, temperature, density and velocity along the nozzle [12,13].