«Dissertation zur Erlangung des akademischen Grades Doktoringenieurin (Dr.-Ing.) von: Yashodhan Pramod Gokhale geboren am: 05. October 1981 in Pune, ...»
The goal was to contribute to the understanding of the modeling to improve the yield and quality of products and the scale up of new processes from a laboratory scale level to an industrial level. In both the cases the model needs to capture the important physico-chemical parameters of the formation of nanoparticles, their interactions with each other. This requires detailed models of the chemical reactions, the population of particles and for the mostly thermodynamics of the reaction.
There are three main reasons why sizes of nanoparticles matter so much one, they have a very high surface area, which makes nanoparticles very suitable for catalytic reaction, drug delivery and energy storage. Two have higher surface tension and local electromagnetic effects, which makes it harder and less brittle compare to larger size of material. Three, could be manipulated on the fundamental properties of materials without changing the chemical composition. Based on the fact above one could say that materials can be engineered. New properties of materials that may exist but have not been found in nature may emerge because of the manipulation. Combination between nanoparticles, its technologies, and other science leads to a revolutionary invention.
1.3 Outline of Contents
This research mainly includes two parts; the first is synthesis of silver and titanium dioxide nanoparticles by different chemical methods. In the second part, we develop population balance model for titanium dioxide nanoparticles by using different reaction parameters. The outline of the proposed research is organized as follows.
In total, there are seven chapters included in this thesis work. Chapter one will introduce the readers to the enormous and booming phenomena of nanoparticles and its technology used recently. All the basic knowledge concerning nanoparticles such as available methods of production and the chemistry behind interaction of particles will be reviewed in chapter two.
In Chapter 3 deals with the different experimental techniques that are used extensively for characterization of the oxide and noble materials. It contains information about related measurement apparatuses commonly used for characterizing nanoparticles.
Chapter four, provides information about the materials, the experimental set-up, and synthesis of silver and titanium nanoparticles by different techniques. First we give synthesis of silver nanoparticles by chemical double reduction method. The second part gives a synthesis of titanium dioxide nanoparticles by sol-gel method. It also discusses the synthesis of surface stabilized TiO2 nanoparticles with diﬀerent surfactants. The author will explain the purposes of selecting such a condition and the kind of variation made for generating nanoparticles. This is very important since it will gives a apply for all of the experiments.
Chapter 5 gives a brief overview of simulation methods for solving population balance equations. Section 5.3 particularly focuses on the mathematical model and the existing schemes for solving aggregation and disintegration equations. Further we present the different agglomeration and disintegration kernels (section 5.4) which we use as the building block for population balance model. Furthermore, the idea of solving population balance equations by using different numerical methods is discussed in section 5.5. The newly developed sectional numerical scheme as cell average technique has been summarized in section 5.5.2.
Results and discussion regarding the optimum shear rates for generating the smallest silver and titanium dioxide nanoparticles and surface stabilized TiO2 nanoparticles will be discussed in chapter 6. Numerically derived results from a population balance model that accounts for agglomeration and disintegration are in good agreement with experimental observations. At the end of this work, concluding remarks are given in chapter 7. Finally, some future developments for improving the nano process design are pointed out.
At the end of the thesis we put two Appendixes. Appendix A gives all shear rate calculations used in this work. Some more mathematical formulation for disintegration kernel is presented in Appendix B.
2 Fundamental Aspects
2.1 Nano Scale Materials N anoscale materials can be defined as those whose characteristic length scale lies within the nanometric range i.e. between one to hundred of nanometer. Within this length scale, the properties of matter are sufficiently different from individual atoms or molecules, and bulk materials. The idea of manipulating and positioning individual atoms and molecules is still new.
On 29th December 1959, Nobel laureate Prof. Richard Feynman gave an illuminating talk on nano technology. It was entitled „There‟s Plenty of Room at the Bottom‟. Prof. Feynman said, "The principles of physics, as far as I can see, do not speak against the possibility of maneuvering things atom by atom. It is not an attempt to violate any laws; it is something, in principle, that can be done; but in practice, it has not been done because we are too big."
In future, nanotechnology will help to assemble these atoms or these building blocks to give new products. We will be able to put together the fundamental building blocks of nature easily, inexpensively, and in most of the ways permitted by the laws of physics.
The properties of materials change as their size approaches to nanoscale and as the percentage of atoms at the surface of a material becomes significant. Size dependent properties of nanomaterials include quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and super-para-magnetism in magnetic materials. Materials reduced to nanoscale can suddenly show very different properties compared to what they exhibit on macroscale, enabling unique applications. For instance; inert materials become catalysts (platinum), solids turn into liquids at room temperature (gold), and insulators become conductors (silicon).
A unique aspect of nanotechnology is the vast increase in ratio of surface to volume present in many nanoscale materials which opens new possibilities in surface based science, such as catalysis. Hence they have enhanced chemical, mechanical, optical and magnetic properties, and this can be exploited for a variety of structural and non-structural applications.
Nanoparticles represent metastable clusters exhibiting the fundamental property to aggregate.
Thus, the stabilization of the nanoparticles may be accomplished by the capping of the nanoparticles with weak electrostatically bound ions (Schmid 1994), by molecular ligands (Green and OBrien 2000), micellar assemblies and different surfactants (Cole, Shull et al.
1999). The association of ligands to growing nanoparticles can control the dimensions and shape of the nanocrystals (Pileni, Gulik-Krzywicki et al. 1998). Nanotechnology uses knowledge from chemistry, physics and biology, and more specifically it is concerned with observing atoms and molecules and manipulating them through visual observations at the nanoscale level. A blend of nanoparticles, technology, and other sciences leads to a revolutionary invention.
2.2 Synthesis of Nano Materials
We can synthesize nanoparticles by two different methods: by downscaling i.e. by making things smaller, and by upscaling i.e. by constructing things from small building blocks. The first method is called “top-down”, and the second method is called “bottom-up” approach.
The top-down approach follows the general trend of the microelectronic industry towards miniaturization of integrated semi-conductor circuits. The lithographic techniques (top-down) offer the connection between structure and technical environment. Top-down approach involves typical solid-state processing of the materials. These methods are based on the reduction of bulk (micro) sized materials into the nano-scale. High energy ball milling or microfluidizers are used to break down dispersed solids to 100 nm. Coarse-grained materials (metals, ceramic, and polymers) in the form of powders are crushed mechanically in ball milling by hard materials such as steel or tungsten carbide. This repeated deformation due to applied forces can cause large reduction in grain size since energy is being continuously pumped into crystalline structures to create lattice defects. However, this approach is not suitable for preparing uniformly shaped materials, and it is very difficult to realize very small particles even with high energy consumption.
The bottom-up approach is based on molecular recognition and chemical self-assembly of molecules. Bottom-up routes are more often used for preparing most of the nano-scale materials with an ability to generate uniform size, shape, and distribution. Bottom-up routes effectively cover chemical synthesis and the precisely controlled deposition and growth of materials. In the bottom-up route, physical/aerosol and wet/chemical synthesis are widely used for nanoparticles generation. There are several processes for synthesis of nanoparticles.
Mechanochemical processing is one of the processes known in particle technology. It uses energy from dry milling to induce chemical reactions during ball powder collisions. This process is still under development and rarely used because of its high energy demands. By forming additional dilutes, the agglomeration of particles can be minimized by encapsulating the particles (Komarneni 2003). Microemulsion techniques for nanoparticles synthesis are becoming a new focus in this study of nano-scale materials. The principle of micro-emulsion process is to conduct production of nanoparticles inside nanosized reactor. The reactors are so small in a way that particles cannot grow large. A descriptive example would be interactions between water and surfactant inside hydrocarbon solvent. Surfactant is an amphiphilic molecule (has two distinct regions) with hydrophobic tail and hydrophilic head. The head tends to gather with water and leave its tails encircled by solvent. This course of action will disperse water into very small droplets, which react as nano reactors. The use of microemulsion systems is introduced to precipitate BaSO4 nanoparticles, which shows the ability for efficient control of particle properties (Qi, Ma et al. 1996; Li and Mann 2000;
Summers, Eastoe et al. 2002; Adityawarman, Voigt et al. 2005).
The term „Aerosol‟ defined as the suspension of very fine particles of solids or droplets of liquid in a gaseous medium. The medium acts to restrain the motion of random particles, and support the particles against gravity energy (Reist 1993). Aerosol processes like flame hydrolysis, spray pyrolysis and plasma synthesis which are mostly useful to produced carbon black are well established for industrial scale, despite very high energy demands (U.Schubert and Hüsing 2000).
Chemical/wet syntheses include classical crystallization, bulk or emulsion precipitation.
Sol-gel methods are widely used for fine chemical preparation. These routes involve the reaction of chemical reactants with other reactants in either an aqueous or non-aqueous solution. These chemical reactants react and self-assemble to produce a supersaturated solution with the product. This supersaturated solution at certain conditions results in particle nucleation. These initial nuclei then grow into nanometer size particles. Chemical precipitation can be held in room temperature, by simply adding reducing agent to a metal salt solution in order to precipitate out fine particles (Brinker and Scherer 1990; Yang, Zhang et al. 2006). This process is inevitable of contaminants, either from excess of reactants (caused by incomplete reaction due to low temperatures) or from reducing agent. Removal of those impurities will lead to another problem. The phenomena that usually take place are nucleation, crystal growth, agglomeration, and disintegration. These phenomena need to be controlled to get desired sizes, shapes and morphology of the particles. As a result various synthesis methods aim towards manufacturing materials for diverse products with new functionalities at the nanoscale.
2.3 Different methods for synthesis of Silver and TiO2 nanoparticles 2.3.1 Synthesis of silver nanoparticles by different processes With infinite applications in almost every field, nanotechnology is growing and becoming popular in academia and industry. Nanomaterials have attracted considerable interest due to their peculiar characteristics such as optical, mechanical, electronic and magnetic properties.
The synthesis of noble metal nanoparticles has been a subject of numerous applications.
Over the last decades silver has been engineered into nanoparticles, structures from 1 to 100 nm in size. Owing to their small size, the total surface area of the nanoparticles is maximized, leading to the highest value of the activity to weight ratio. The ancient Greek and Roman civilizations used silver vessels to keep water potable. Since the nineteenth century, silver based compounds have been used widely in bactericidal applications in healing of burns and also in wound therapy (H.Klasen 2000).
Furthermore, currently a diverse range of consumer products contain silver nanoparticles.
These products contain antibacterial/antifungal agents. Few examples of such products are air sanitizer, respirators, wet wipes, detergents, soaps, shampoos, toothpastes, air filters, coatings of refrigerators, vacuum cleaners, washing machines, food storage containers, cellular phones etc (Buzea 2007). The silver particles in nano-scale exhibit high-antibacterial activity and have no intolerable cytotoxic effects for human beings. The antibacterial effect has been tested for yeast and E. coli by (Kim, Kuk et al. 2007). The experimental results showed that the growth inhibition effect of silver nanoparticles was in a concentration-dependent manner.
They concluded that the silver nanoparticles were applicable to diverse medical devices and antimicrobial systems.
A number of methods have been used in the past decades for preparing these noble silver nanoparticles. The methods are as follows:This include condensation in vapour phase (Stabel, Eichhorst-Gerner et al. 1998), chemical vapour deposition (CVD) or electrostatic spraying on solid substrate (Okumura, Tsubota et al. 1998), ultrasound-induced reduction in solutions or reverse micelles (Ji, Chen et al. 1999), and thermal decompositions of precursors in solvents, and polymer films (Lidia Armelao, Renzo Bertoncello et al. 1997; Yanagihara, Uchida et al. 1999).