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Harrinauth, Reshma K.
Sedimentation behavior of organic -inorganic composites by optical turbidometry
h [electronic resource] /
by Reshma K. Harrinauth.
[Tampa, Fla] :
b University of South Florida,
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Thesis (M.S.Ch.E.)--University of South Florida, 2008.
Includes bibliographical references.
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ABSTRACT: Sedimentation is one of many characterization tools used to test materials in nanotechnology. Characterization of settling behavior is complex as there are many variables which can affect sedimentation. In our research, we focused on sedimentation in colloidal systems with the aid of an optical turbidometer. Nanoparticles of CeO (Ceria Oxide) and TiO (Titanium Dioxide) are embedded onto a polymeric matrix of a thermally responsive microgel of poly(N-isopropylacrylamide) (PNIPAM) and interpenetrating chains of poly(acrylic acid) to create novel composites. The composites are loaded with the inorganic oxide nanoparticles at different weight percent from a low value of 10 weight % to 75 weight %. The loading of the colloidal particles affects the sedimentation rate. In this thesis a turbidomenter is used to characterize the settling rate, which is an important characteristic for application of these new composites.TiO is a key constituent in many industrial products; cosmetics, paints, ceramics and used in waste water remediation. It is a potent photocatalyst which breaks down almost any organic compound when exposed to ultraviolet light. By combining nanoparticles of TiO with microgels of a polymer, the composites can facilitate use and recovery of the catalyst. Gravity settling of these loaded composites provides an easy separation of TiO nanoparticles. In this context, characterization of settling plays an important role. CeO composites are used to polish oxide coatings in the semiconductor industry and sedimentation of the composite particles is important as it can affect the efficiency of the planarization process. Therefore, measuring sedimentation of these composites is necessary.In this study, the settling behavior is measured optically for a variety of conditions that differ in loading of inorganic nanoparticles within the microgels, temperature of the solution, and concentration of particles in solution. The overall goal is to understand the sedimentation behavior of these novel composites and facilitate their use in industrial processes.
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Advisor: Vinay K. Gupta, Ph.D.
x Chemical Engineering
t USF Electronic Theses and Dissertations.
Sedimentation of Organic Inorga nic Composites by Optical Turbidity by Reshma K. Harrinauth A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Chemical Engineering Department of Chemical a nd Biomedical Engineering College of Engineering University of South Florida Major Professor: Vinay K. Gupta, Ph.D. Scott Campbell, Ph.D. Babu Joseph, Ph.D. Date of Approval: November 4, 2008 Keywords: settling velocity, turbidometer, titania, ceria, pnipam Copyright 2008 Reshma K. Harrinauth
DEDICATION To my family
ACKNOWLDEGEMENTS I would like to expres s sincere gratitude and thanks to Dr Vinay K. Gupta who has offered invaluable assistance, guidance and s upport throughout the tim e it took for me to complete my research and thesis. I would al so like to thank my committee members, Dr Babu Joseph and Dr Scott Campbell, who have been integral in my education at the University of South Florida. I would also like to thank the members of my research lab, David Walker, Fadena Fanord, Bijith Mankidy and Chhavi Manocha, who has been a constant source of knowledge and assistance. I would like to give a special thanks to my lab mate, Cecil Coutinho w ithout his guidance and support this thesis would not be possible. My thank to the many friends I ha ve made at USF especially, Nada Elsayed and Brandon Smeltzer, they have proven our fr iendship would last a lifetime. I would like to thank my extended family, for their support, love and encouragemen t in the pursuit of my education. Finally to my parents and sister, without their support, unconditional love and sacrifice this degree would not be possible.
TABLE OF CONTENTS LIST OF FIGURES..ii ABSTRACT.iv NOMENCLATURE....vi CHAPTER ONE: INTRODUCTION... CHAPTER TWO: TURBIDITY: MEAS UREMENTS AND INTEPRETATION .5 2.1 Background...5 2.2 Experimental apparatus.....................................................................................6 2.3 Sedimentation and Stokes Law.....6 2.4 Interpretation of turbidity during sedimentation...................................................8 2.5 Results and discussion.10 CHAPTER THREE: SETTLING OF TITANIA POLYMER MICROGEL COMPOSITES............17 3.1 Introduction.17 3.2 Experimental method...18 3.3 Results and discussions........20 3.3.1 Effect of titania loading.......20 3.3.2 Effect of temperature...21 3.3.3 Effect of concentration....23 3.3.4 Optical microscopy of floccu lated composite particles...23 CHAPTER FOUR: SETTLING OF CERIAPOLYMER MICROGEL COMPOSITES........34 4.1 Introduction.34 4.2 Results and discussion.36 CHAPTER FIVE: SUMMARY AND CONCLUSION.42 REFERENCES...44 i
LIST OF FIGURES Figure 1.1 Schematic of the embedded nanoparticles with in the polymeric matrix of a crosslinked microgel ..4 Figure 2.1 Schematic of turbidometer setup for settling experiments Figure 2.2 Schematic of forces acti ng on a particle settling in liquid.....13 Figure 2.3 Schematic of normalized turbidity14 Figure 2.4 Normalized turbidity signal for the settling large silica sphere..15 Figure 2.5 Normalized turbidity signal for the settling fine silica spheres..16 Figure 3.1 Settling of composites at various weight percentage of titania in each particle...25 Figure 3.2 Schematic of microgel respons e above and below the volume phase transition temperature....26 Figure 3.3 Settling of composite with 25% titania at temperatures below and above transition temperature (T~25 C) ....27 Figure 3.4 Settling of composite with 50% titania at temperatures below and above transition temperature (T~25 C).28 Figure 3.5 Settling of composite with 50% titania and different particle concentration in solution...29 Figure 3.6 Optical images of flocs of composites with 50% titania at 4X magnification.....30 Figure 3.7 Optical images of flocs of composites with 50% titania at 10X magnification. Figure 3.8 Optical images of flocs of composites with 75% titania at 4X magnification.32 Figure 3.9 Optical mages of flocs of composites with 75% titania at 10X magnification.....33 Figure 4.1 Schematic of slurry polishing in CMP process.38 ii
Figure 4.2 Settling of pure ceria nanopart icles a ambient temperature compared against composite with 25% and 50% ceria loading Figure 4.3 Settling of composite with 50% ceri a at temperatures below and above the transition temperature (T~25 C)..40 Figure 4.4 Settling of composite with 25% ceri a at temperatures below and above the transition temperature (T~25 C)..41 iii
SEDIMENTATION BEHAVIOR OF OR GANIC INORGANIC COMPOSITES BY OPTICAL TURBIDOMETRY Reshma K. Harrinauth ABSTRACT Sedimentation is one of many characterizat ion tools used to test materials in nanotechnology. Characterization of settling behavior is complex as there are many variables which can affect sedimentation. In our research, we focused on sedimentation in colloidal systems with the aid of an optical turbidometer. Nanoparticles of CeO2 (Ceria Oxide) and TiO2 (Titanium Dioxide) are embedded onto a polymeric matrix of a thermally responsive microgel of poly( N-isopropylacrylamide) (PNIPAM) and interpenetrating chains of poly(acrylic acid) to create novel composites. The composites are loaded with the inorganic oxide nanopartic les at different weight percent from a low value of 10 weight % to 75 weight %. The loading of the colloidal particles affects the sedimentation rate. In this thes is a turbidomenter is used to characterize the settling rate, which is an important characteristic for application of these new composites. TiO2 is a key constituent in many industrial products; cosmetics, paints, ceramics and used in waste water remediation. It is a potent photocatalyst which breaks down almost any organic compound when exposed to ultraviolet light. By combining nanoparticles of TiO2 with microgels of a polymer, the composites can facilitate use and recovery of the catalyst. Gravity settling of these loaded composites provides an easy iv
separation of TiO2 nanoparticles. In this context, ch aracterization of settling plays an important role. CeO2 composites are used to polish oxide coatings in the semiconductor industry and sedimentation of the composite pa rticles is important as it can affect the efficiency of the planarization process. Th erefore, measuring se dimentation of these composites is necessary. In this study, the settling be havior is measured optically for a variety of conditions that differ in loading of inorganic nanopartic les within the microgels temperature of the solution, and concentration of particles in solution. The overall goal is to understand the sedimentation behavior of these novel compos ites and facilitate th eir use in industrial processes. v
NOMENCLATURE Af Projected frontal area of the settling particle CD Drag coefficient for a solid particle d Diameter of the particle Dp Diameter of the polymer Fg Gravitational force Fb Buoyant force Fd Drag force g Gravitational force h1 height from the top of the turbi dometer holder to top of aperture h2 height from the top of turbidimeter holder to the bottom of the aperture H difference (h2-h1), representing the he ight of the window Io Intensity of transmitted light It Intensity of incident light L Optical path length Np number concentration of particles Npi number of concentration of particle i NRe Reynolds Number t Time Vs Settling velocity Y(Vsi) Fraction of total particles in class i vi
p Density of the polymer f Density of the fluid sp Bulk density of the settling particle w Mass density of water Turbidity parameter urbidity at initial time Velocity of the object Viscosity w Viscosity of water vii
CHAPTER ONE: INTRODUCTION The main purpose of this thesis is to obt ain a better understanding of novel composite particles by characteriza tion of their sedimentation behavior Sedimentation of particles has been utilized in fields ranging from engineering to wastewater remediation to materials science15. Sedimentation has also been th e subject of many experimental and theoretical studies. In the latter case, development of a theoretical framew ork for sedimentation has been a challenge to many researchers when dealing with complex systems such as highly concentrated suspensions, polydisperse solutions, and aggreg ating or flocculating dispersions6-8. In this thesis, our interest lies in novel composite particles that combine organic polymers with inorganic metal oxides9. Composite materials are of incr easing interest a nd are made up of two or more materials that are present together but remain chemically different entities. There has been a surge in research on composite materi als as they are extremely useful in medicine, paints, and many cosmetic products10-15. Recently, novel composite materials that are co mposed of polymeric microgels and either titanium dioxide (TiO2) or cerium oxide (CeO2) nanoparticles have been developed9. Titanium dioxide is a widely recognized photocatalyst that has been used in wastewater remediation10, 16, 17. Nanoparticles of titanium dioxide, when exposed to ultraviolet light, have been found to be very efficient in the breakdown of organic matter. Cerium oxide nanoparticles are known to be useful in polishing silica wafers in the se mi-conductor industry for planarization purposes18-20. 1
Combination of these inorganic metal oxides within crosslinked and thermally responsive microgels (Figure 1.1) of poly (N-isoproprylacryla mide) provides many benefits for applications in photocatalysis or planarization. For exampl e, embedding nanometer sized titanium dioxide particles within the polymeric gels can provide a useful method for the recovery and re-use of the TiO2 photocatalyst. Cerium oxide nanoparticles em bedded within the polymer microgels can eliminate surface scratches and defects for the case of chemical mechanical polishing since the composite particle has both soft and hard characteristics. Sedimentation is known to be determined by density differences between the dispersed particles and the fluid medium as well as factor s such as porosity when dealing with permeable systems. Therefore, measuring and interpreting settling behavior is a simple approach for characterizing composite particles. One goal of this thesis is to investigate the optical technique of turbidity measurement for characterization of the settling of composite particles and in turn, establish sedimentation as a tool for characterizing the ceria-microgel and the titania-microgel composite particles. A second goal is to explor e the sedimentation behavior of titania-polymer particles as this can help in developing gravity settling approaches for separation and recovery of the photocatalyst. Finally, charac terizing the settling behavior of the ceria-polymer particles is also important as it can affect the slurry polishing process. The research performed in this thesis accomp lishes the goals above. Chapter 2 of this thesis describes the technique of optical turbidity, the experiment al apparatus, and the model for interpretation of the experimental data. Ch apter 2 also reviews background information on optical turbidity and the validation of the techni que against solid silica spheres using Stokes Law. Chapter 3 describes sett ling of titanium dioxide composite particles and the effect of titania loading, temperature, a nd concentration on the sedimentation behavior. Here, optical 2
microscopy results are also presented to gain insight into the flocculation of the composite particles. Chapter 4 details the experiments on the ceria composite particles and the effect of temperature and loading on the settling behavior. Finally, Chapter 5 provides a summary and conclusions for the project. 3
Polymeric Microgels (Titanium dioxide) Nanoparticles Composite Material Figure 1.1: Schematic of the embedded nanoparticles within the polym eric matrix of a crosslinked microgel 4
CHAPTER TWO: TURBIDITY: MEASUREMENTS AND INTERPRETATION 2.1 Background Optical techniques such as st atic light scattering, dynamic li ght scattering and turbidity measurements, due to their non-contact, non-inva sive properties are well suited to the study of colloidal and macromolecular suspensions21-26. Static and dynamic light scattering are standard methods for investigating size, shape, and diffu sion of particles and pol ymers in fluid media27,28. However, methods such as dynamic light scattering (DLS) are of limited use when characterizing systems containing particles that sediment sin ce a necessary requirement for DLS is Brownian diffusion. Turbidometeric methods, on the other ha nd, have the advantage of being simple and well-suited to sedimenting systems. Turbidity refers to light attenuation (by scat tering and absorbing) from the presence of finely suspended materials29, 30. Turbidity measurements or nephelometry involve the relative measurement of intensity for light scattered through a range of angles and its ratio to the intensity of the incident beam. The use of nephelometr y is a common procedure in environmental and water engineering where pollutant concentration or fine suspensions need to be routinely characterized31. The light attenuation by a single particle depends str ongly on its size and for a collective suspension of particles, turbidity of the solution then becomes a function of both concentration of the particles a nd their sizes. Changes in the particle concentration due to sedimentation can be manifested in the turbidity of the solution. The goal in this chapter is to 5
demonstrate that measurement of the changes in turbidity of a solution with time provides a simple and convenient method to ch aracterize colloidal suspensions. 2.2 Experimental apparatus The experimental apparatus used to measur e the settling rate of the particles was a turbidometer (model DRT 1000, HF instruments). The turbidometer work s in a simple manner wherein light is scattered at a 90o angle to the incident beam and a photo detector converts the light intensity into a voltage29. A schematic of the turbidometer is seen in figure 2.1. As shown in the figure, a standard size cylin drical test tube (12 mm x 75 mm) contains the particle solution. A water bath was connected to the turbidometer holder to circulate water and maintain a desired temperature. The photodetector signal was recorded using a computer with a program written in Hewlett Packard Visual Engi neering Environment (HP VEE). During the experiment, 1000 points were acquired at an analog-to-digital samp ling frequency of 1 kHz and the mean value of the voltage was recorded as a function of time. For samples which took much longer than a day for settling, a timer was used to swit ch the turbidometer on and off. 2.3 Sedimentation and Stokes Law Sedimentation, where particles fall under the action of gravity through a fluid in which they are suspended, is a way of separating particle s from fluids as well as classifying particles with different settling speeds1. Stokes law has been commonly used to predict the velocity of a single solid particle in an infinite fluid medium at low Reynolds number1, 4, 32. A particle settling in a liquid is acted upon by the grav itational force, buoyancy force, a nd the fluid drag force. As indicated by the figure 2.2, the gravitational force, Fg, acting in the downward direction is 6
counteracted by the buoyancy force and the drag force. The terminal velocity of the particle is achieved when drag force is equal to the gravitational force minus the buoyancy force dbgFFF (1) The net gravitational force on a sphere can be given as33 63d g FFfp bg (2) The drag force can be further written as: 22 1ffD dACF (3) The drag coefficient (CD) is a function of several parameters such as particle shape, particle aggregation, permeability, and fluid characteristics 33. For creeping flow or Stokes regime, where the inertial effects are negligible the drag coefficient for a spheri cal solid particle can be given exactly as33-35: Re24 N Cd (4) where NRe is the Reynolds number and can be expressed as: spVD N Re (5) Thus, the Stokes settling velocity for solid spherical particles is as follows9, 33-35 218p w wsp sD g V (6) 7
Settling behavior under Stokes law is valid for very dilute suspensions, where hindered settling effects are negligible, and for either a small solid spherical particle or a very viscous medium (NRe<<<1). One of the first tasks in our research was to validate the use of turbidity measurement to measure settling velocity. Towards this end, we m easured settling of solid silica spheres of two different sizes and compared the results with the prediction of Stokes law. 2.4 Interpretation of turbidity during sedimentation The established theory of photo-sedimentati on uses low volume frac tion of particles and measures the attenuation of the light beam occurri ng by the particles at vary ing settling depths as a function of time31. The attenuation can be expressed as o tI I and a single turbidity parameter is commonly used, (), which is the fractional decrease in intensity of light. The turbidity can be simply related to the number of particles per unit volume ( Np) by p t oN I I L ln 1 (7) Equation (7) suggests that a normalized turb idity signal can provide information on the evolution of particle concentration due to sedimentation36. For particles with a single settling Vs, we can use the following equation for the turbidom eter setup shown in figure 2 with H being the height of the aperture (h2-h1)36: s s sp sPV h t H V VN VtN11 ,0 (8) 8
In figure 2.3, the schematic shows the concentr ation of the particles settling in the test tube with a single velocity. At a time t1, the particles fall th rough a height of h1 and the normalized turbidity signal starts decreasing beca use the concentration in the optical aperture starts to decrease. As the particle s continue to fall with the same rate, a linear decay in the signal is observed as shown in the graph. Once the majo rity of the particles have fallen through the region where the light enters (between h1 and h2), the turbidity si gnal goes to zero. In the case of the composite particles, diffe rent settling velocities would be observed due to differences in loading of th e nanoparticles. In this case, it has been shown by Coutinho and coworkers that the evolution in the normalized turb idity signal can give a distribution of settling velocities by using the following relation36. si sipi sipi oVY VN VtN t ,0 (9) In this equation, the number of particles in class, i at intial time, is indicated by Npi (0, Vsi) where the settling velocity is Vsi. The number of particles in class i with a settling velocity of Vsi at time t in the sampling window is then Npi (t,Vsi) and Y( Vsi) is the fraction of total particles in class i. Complete details of the mathematical model are available in the paper by Coutinho, Harrinauth, and Gupta36. 2.5 Results and discussion Solid silica spherical particle s were used to validate the experimental method and the mathematical model. Commercially available, l arge silica sphere particles with an average diameter ~3.21.35 m were suspended in deionized wate r and turbidity measurements were 9
performed. Figure 2.3 shows settling data for the la rge silica spheres. We can make an easy estimate of the settling velocity us ing equation 8 from the value of t1, which is the time at which the turbidity signal starts to drop steeply in fi gure 2.3. The experimental data indicates that t1~7500 seconds. Using the value of h1=3.9 cm (Figure 2.1) of the turbidometer, we can estimate that the average settling velocity is ~5.2x10-4 cm/s. Applying Stokes law with the known properties of the particle and the fluid, the se ttling velocity can be predicted to be 5.4 x10-4 cm/s, which clearly indicates good agr eement between the settling measur ed using the turbidity setup and theoretical expectations. A more accurate an alysis of the turbidity signal that captures the small variations in terms of polydispersity eff ects can be performed using equation 9. Coutinho and coworkers have shown that th is analysis allows interpretation of the turbidity signal as a distribution of settling velocities36. In addition to the large silica particles, settli ng behavior of fine silica particles was also performed. These fine silica partic les were synthesized in the research laboratory using a sol-gel technique (materials courtesy of Shim and G upta) and had a diameter of ~450nm. Figure 2.4 shows the data for settling of these sub-micron part icles. The impact of the small size is easily observed by the long settling times. The complete settling occurred over five days. Figure 3.4 also indicates that in the case of these small particles it is more difficult to distinguish a sharp break in the turbidity signal. As a quick estim ate we can pick a time where the most noticeable change in slope is occurring and this gives t1=2.5 days. Using a similar reasoning as before, the estimated average settlin g velocity is then 1.8x10-5 cm/s. The settling velocity of the fine particles calculated via Stokes Law is 1.1x10-5 cm/s, which is in good agreement with the estimate from experiments. In the case of the fi ne particles, the more accurate analysis of the settling distribution has been performed by Coutinho and coworkers36, which shows that the 10
velocity distribution is broader than the case for the large particles and is consistent with the absence of a sharp break in the turbidity signal. In summary, the turbidity measurements for th e solid silica spheres across a size range of approximately 0.5 5 m agree very well with the expected re sults from theoretical relation such as Stokes law. This provides support for the poten tial use of turbidity measurements as a simple and useful tool to characterize composite particle s by analysis of their sedimentation behavior. The following chapters focus on the characteriza tion two novel composite particles made from titania nanoparticles in a polymeric microgel and ceria nanoparticles in a polymeric microgel. 11
Cooling water Sample Tube h2 h1 Aperture Detected Light ( = 90) Metal Sample Holder Figure 2.1 Schematic of turbidomet er set-up for settling experiments12
Fb Fd Fd Fg Figure 2.2 Schematic of forces actin g on a particle settling in liquid 13
h2 h1 t =0 t =t1 t =t2 1 / 0 0 0 t2 t1 Figure 2.3 Schematic of normalized turbidity 14
1.2 1.0 0.8 0.6 0.4 0.2 0.0Normalized Turbidity 10x103 8 6 4 2 0Time (seconds) t1 3.2 m Figure 2.4 Normalized turbidity signal for the settling lar ge silica spheres 15
1.0 0.8 0.6 0.4 0.2 0.0Normalized Turbidity 6 5 4 3 2 1 Time (days) 0.45 m t1 Figure 2.5 Normalized turbidity signal for the settling fine silica sphere 16
CHAPTER THREE: SETTLING OF TITANIUM OXIDE COMPOSITE 3.1 Introduction Titanium dioxide (TiO2) is a common metal oxide that has emerged as an excellent photocatalyst material in environmental remediation37, 38. Titanium (IV) oxide exists in nature in two tetragonal forms as rutile and anatase. A th ird form, Brookite, is a rhombic form. Anatase and rutile can be easily prepared in the laborator y and these two forms have been used in many photocatalytic studies39. Commercially available TiO2 is commonly DegussaTM P25, which contains the anatase and rutile phases in a ratio of about 3:17. Photocatalytic reaction on TiO2 surfaces has generated a great deal of interest in chemical degradation of contaminants because of its low cost, simplicity, and high efficiency10, 34, 37, 40. Organic chemicals that are found as pollutant s in wastewater from industrial or domestic sources must be removed or destroyed before being released to the environment. These pollutants can also be found in ground and surf ace waters, which also require treatment to achieve potable quality. The increase in these environmental pollutants has seen a rise in public concern for the development of novel treatment methods. Us ing nano-scale TiO2 greatly increases the surface area of titanium dioxide and permits a better reduction of organic pollutants in wastewater remediation. However, the recovery of titania nanoparticles suspended in an aqueous medium has remained a challenge. In this context, the attachment of TiO2 nanoparticles to polymeric microgels is an innovative approach to address recovery and use of TiO2 photocatalyst. Coutinho 17
and Gupta have shown that cross-linked microgels of PNIPAM containing interpenetrating chains of poly(acrylic acid) (PAAc ) allow composite particles to be prepared that can settle rapidly36. The titania retains its chemical identity and works as an excellent photocatalyst while the highly porous microgel allows light to reach the nanoparticle surface and permits easy exchange of fluid. The rapid settling of the comp osite facilitates the retrieval of the composite material in an efficient manner. In this ch apter, the settling of tit anium dioxide composite particles, the effect of titania loading, te mperature, and concentration is evaluated. 3.2 Experimental method Different stock solutions of the titania-mic rogel composite containing a fixed, average mass fraction of titania (10%-75%) were used. Prior to characterization in the turbidometer, each stock solution was diluted with the addition of deionized water to a total volume of 5cm3 To study the effect of particle concentration, the relative amounts of stock solution and water was varied. The test tube with the sample so lution was sealed from the top with Parafilm and sonicated for 5 minutes to redisperse the partic les uniformly in solution. The sample was then removed and placed in the turbidometer holder for approximately five minutes to ensure the content is equilibrated at the desired temperat ure. After thermal equilibration, the sample was then quickly removed and inverted four times in order to redisperse the particles. Acquisition of the turbidity as a photodetector voltage was performed every five seconds as described in Chapter 2. Eight runs were perf ormed for each sample and these were averaged. In the experiments that were conducted at 15C, the test tube had to be wiped after each run with kimwipe as a thin layer of condensation was formed on the outside of the te st tube. It should be 18
noted neither the polymeric microgels without any titania nor the titania nanoparticles alone settle over a time frame of days. Flocculation of the composite particles was also examined using optical microscopy of flocs deposited on a glass slide. Since the flocs were very small, in order to measure the area of the flocs it was very important that the glass slide was cleaned properly of any dust particles. The glass slide was cleaned thoroughly by initially so aking it in deionized wa ter. The glass slide was then carefully removed from the water bath and placed into a soapy water bath solution. A second clean water bath was used to rinse the soap and the glass slide was then dried with the aid of a stream of nitrogen. Each loading of the titania co mposite was measured using opt ical microscopy. A test tube containing 850 l of the titania-composite stock soluti on and 4.15ml of water was shaken and agitated in the same manner as the turbidity test ing. The clean glass slide was placed into a large Petri dish and the content of the test tube was poured gently onto the glass slide and the settled aggregates on the slide were observed in tran smission using an optical microscope. Objective lenses with 4X and 10X magnification were used in order to view the flocs. 3.3 Results and discussions 3.3.1 Effect of titania loading Understanding settling of the titania-microgel for different loadings of titania nanoparticles is an important aspect of this research. The settling rate of the composite particles will be an important piece of in formation in any process applicat ion where gravity settling of the composites will be exploited. 19
In this study, particles with various weight percentages of titania nanoparticles ranging from 10% to 75% have been characterized. As the mass fraction of the titania increases, the effective density of the particle increases and this should also impact the sedimentation of the particles. We can calculate the effective density of the dry polymer particles as pol TiO TiOpol pff f 12 2 (10) where TiO2 is the density of the TiO2 (~ 4.16 g/cm3), f is the mass fraction of the TiO2 per particle, pol is the density of the polymer (~1.07g/ml)9. From the equation given above, 10% of titania loading in each partic le gives the effective density of the composite as 1.16 g/cm3 At 75% loading of the titania particles, the effective density changes to 2.42 g/cm3 Thus, there is a substantial change in effective density of the composite particles with increase in its titania loading. Figure 3.1 shows the settling data as normalized turbidity for the various loadings of titania in composite particles at ambient temperatur e (~25C). It is clear from the data, that at the lowest loading of 10% the settling is slowest and at the highest loading of ~75%, the settling is fastest. The data in figure 3.1 shows that when f=10%, the settling time is approximately 2000 seconds. For values of loading of 25% and 50% loaded, the particles show a settling time of approximately 300 seconds and 600 seconds. In c ontrast, particles with 75% titania settle extremely rapidly in approximately 100 seconds. Coutinho and coworkers9 have shown that the mean settling velocity of the composite particles obtained from data in figure 3.1 can be correlated to the average loading of titania. A theoretical framework that accounts for both the changes in effective density and the changes in 20
permeability of the particle with increased loading of titania ha s been applied by Coutinho and coworkers. Thus, results such as those shown in figure 3.1 clearly indicate the usefulness of turbidity measurements for characterizing the nov el composite particles as the measurement of settling velocity can be rapidly in terpreted in terms of average load ing of the titania photocatalyst in the composites. 3.3.2 Effect of temperature One of the polymeric constituents in the co mposite particles is PNIPAM, which is a thermally responsive polymer and is known to exhibit a lower critical solution temperature(LCST) near 32C in aqueous solutions41, 42. Stimuli responsive polymers, where the polymer can change size in res ponse to external stimu li like temperature, pH and ionic strength, have many applications36, 41. For cross-linked polymeric microgels of PNIPAM, when the solution temperature is low, the polymeric microge ls absorb water and exhi bit a swollen state. At high temperatures, an abrupt volume shrinkage of the particle results in expulsion of free water within the polymer network42-48. The expansion and collapsing property of PNIPAM microgels has been extensively investigated in the field of drug delivery system, biosensors, tissue engineering42-47. Figure 3.2 shows schematically the swelling a nd shrinking of the polymeric microgels at a transition temperature. The microgels used in this study were approximately 750nm in the swollen state and 330nm in the collapsed state. As mentioned earlier, chai ns of a polyelectrolyte (PAAc) were were introduced within the crosslinked PNIPAM microgel to promote the loading of the TiO2 nanoparticles within th e polymeric particles36. The change in the overall volume of the microgel as it transitions with temperature can affect the effective density of the composite particle and also the permeability, which are both important factors in the settling behavior. 21
Figure 3.3 shows the settling behavior of comp osites containing 25% by weight of titania at three temperatures. The experimental data shows that as the temperature increases from 15C to 25C, the composite particles settle at earlier times. Further increase beyond the transition temperature has a significantly smaller effect. We can qualitatively understand this trend in terms of the changes in the particle properties. At the low temperature of 15C the particle is highly swollen with water and has a very high porosity (> 0.95). As a consequence, the effective density contrast between the particle and water is very low, which makes the particle settle slowly. As the temperature increases, there is a decrease in the size of the polymer microgel due to the thermally responsive nature of the PNIPAM. The incr ease in density due to smaller size appears to dominate the effects of porosity and the particle settles faster. Figure 3.4 shows the experimental data on se ttling for a composite with 50% titania loading at the three temperatures. The general e ffect of temperature here is similar to the 25% loading. However, the shift in settling between 15C and 25C is perceptibly smaller because the particles were denser to begin with and settled faster. 3.3.3 Effect of concentration Concentration of the sample is an important ch aracteristic of the se ttling behavior as the particle to particle interaction is an important contribution. It is known that for solid particles as the concentration of the sample becomes too high, hindered settling can occur and reduce the settling velocity. Therefore, the effect of concentration was st udied to ensure that all the experiments did not entail hindered settling. Figure 3.5 shows data for three different dilute concentrations of th e sample particles. Over the range of particle concentrations studied, it is clear that even a four-fold increase in 22
particle mass concentration (0.083 mg/ml to 0.33 mg/ml) does not change settling time significantly (the change of a few tens of seconds is within the error of the measurement). We can conclude that the particle concentration in all our experiments is low enough that effects of hindered settling can be discounted. 3.3.4 Optical microscopy of flo cculated composite particles Smoluchowski was one the first scientists to examine the dynamics of floc growth for suspensions subjected to shear and showed that co llision of the fine particles with interactions could lead to an increase in the size of the aggregates49. Since the titania-microgel composite particles have polymeric chains of PAAc that can interact with other pa rticles through the PAAc chains as well as titania nanopart iles in the neighboring particles, we have observed that these composites have a tendency to aggregate as th ey settle within the sample test tube. Coutinho and coworkers9 have shown the floc formation and the ensuing increase in size as well as porosity are very important in the lo w drag force and rapid settling of the composite particles. In the settli ng tube, the appearance of the flocs is very powdery. These flocs are also very delicate and under slight ag itation break apart very easily. To gain some insight into the nature and sizes of these aggr egates, analysis of the flocs was done using optical microscopy. Figures 3.6 to 3.9 show optical images of the flocs formed using composites with two different loading of titania. As shown in the figures we can observe the range of the aggregates size. Regardless of the loading of the oxide, the sizes of these aggregates were within 10-100 m and their appearance is very similar. Coutinho and coworkers9 have shown that by accounting for the fractal-like natu re of these flocs and by modeli ng the settling of these highly porous, large aggregates can explai n the rapid settling of the compos ite particles. They have 23
found that good agreement between theoretical predictions and e xperimental data can be found when the floc size of 10-100 m is used. 24
5 6 0.1 2 3 4 5 6 1 Normalized Turbidity 2500 2000 1500 1000 500 0 Time (seconds) 10% 25% 50% 75% Figure 3.1: Settling of composites at various weight percentage of titania in each particle 25
Ttransition ~ 32C Low Temp. High Temp. Figure 3.2 Schematic of microgel response a bove and below the volume phase transition temperature. 26
0.1 2 3 4 5 6 7 8 9 1Normalized Turbidity 2000 1500 1000 500 0 Time(seconds) 15C 25C 35C Figure 3.3 Settling of composite with 25% ti tania at temperatures below and above transition temperature (T~32C) 27
0.1 2 3 4 5 6 7 8 9 1Normalized Turbidity 700 600 500 400 300 200 100 0 Time(seconds) 15C 25C 35C Figure 3.4 Settling of composite with 50% ti tania at temperatures below and above transition temperature (T~32C) 28
0.1 2 3 4 5 6 7 8 9 1Normalized Turbidity 600 500 400 300 200 100 0 Time (seconds) 0.083 mg/ml 0.165 mg/ml 0.330 mg/ml Figure 3.5: Settling of composite with 50% tita nia and different partic le concentration in solution 29
Figure 3.6 Optical images of flocs of compos ites with 50% titania at 4X magnification. The image size is 2672 m x 2136 m. 30
Figure 3.7 Optical images of fl ocs of composites with 50% titan ia at 10X magnification. The image size is 1069 m x 854 m. 31
Figure 3.8 Optical images of flocs of composite s with 75% titania at 4X magnification. The image size is 2672 m x 2136 m. 32
Figure 3.9 Optical images of flocs of composite s with 75% titania at 10X magnification. The image size is 1069 m x 854 m. 33
CHAPTER FOUR: SETTLING OF CERIUM OXIDE COMPOSITE 4.1 Introduction The rapid advances in the microelectronics industry demand significant improvements in Chemical Mechanical Polishing (CMP), which is a widely used technique for the planarization of metal and dielectric films to accomplish multilevel metallization (Figure 4.1). As the microelectronic device dimensions keep on d ecreasing and the minimum feature size becomes smaller than 0.1 m, a very thin layer of material has to be removed and a flat and clean surface finish has to be achieved during the polishing of wafers. The fabrication of these small devices without imperfections requires improvements in the CMP process19, 50, 51. The CMP process generally consists of a rotating wafer pressed face down onto a rotating, resilient polishing pad while polishing slu rry containing abrasive particles and chemical reagents flows in between the wafer and the pad. A schematic of the CMP process is illustrated in figure 4.1. The combined action of polishing pad, abrasive particles and chemical reagents results in material removal and polishing of the wafer surface52. The polishing slurry provides both chemical and mechanical action where it is us ed to remove and planarize the wafer surface. The mechanical action helps achieve the required planarization and uniform ity of the silica wafer which is accomplished by the use of the abrasive particles in the slurry. The chemical action is achieved by the slurry incorpor ating oxidizing agents or addi tives content which improves degradation. The erosive action in CMP is mostly provided by the submicrometer size abrasive 34
particles as they flow between the pad and wa fer surface under pressure. The magnitude of the mechanical action is in turn determined by the size and nature of the abrasive particles. The major process variables in CMP are the platen speed, down force and the slurry18, 19, 50, 51. The use of hard inorganic particles in commerc ially available slurries can cause scratches on the surface of the wafer. It has been studied in the past that mixed or modified abrasive particles can reduce the number of imperfections on the wafer53, 54. The creation of a novel inorganicorganic composite pa rticle of the types discussed in earlier chapters has been proposed as a good candidate to be used as an abrasive slurry in the CMP process55, 56. In these novel particles, the polymeric network consists once again of the PNIPAM microgels and interpenetrating chains of PAAc. In addition, siloxane functional group was incorporated onto the network. Ceria nano particles have been shown19, 20 to be useful in CMP and therefore, these new composites are ceria-microgel rather than titania-microgel. The characteristics of the composite particle are the softness of the pol ymer network and surface hardness due to the presence of the ceria nanoparticle s. This combination allows for the use of the composite particle to be suitable for the prevention of defects a nd any aggressive scratching on the wafer. One of the main characteristic behaviors of the ceria lo aded composite particles is the settling rate, which is an important parameter during the slurry polis hing. Therefore, in this chapter the sedimentation behavior of the ceria-polymer co mposites has been explored as a function of temperature and loading. As a comparison, the sedimentation of ceria nanoparticles alone is also characterized. The experimental set-up for the settling studies was similar to the titania-microgel particles and has been described in Chapters 2 and 3. Briefly, the ceria composite was added to the turbidometer sample tube from the stock solution and diluted with deionized water. The 35
sample was placed in the turbidometer and wher e it was allowed to equilibrate to the desired temperature. It was quickly removed and inverted to ensure the particles were fully dispersed in the suspension. It was placed into the turbidomet er holding and data was acquired using the data acquisition software as described in section 2.2 4.2 Result and discussion The use of pure ceria particles was used as a reference for the settling time for the hybrid ceria particles and it is observed in figure 4.2 th at these nanoparticles settle over a long time of approximately 12000 seconds. Figure 4.2 also sh ows the settling of the polymer composites loaded with 50% and 25% ceria nanoparticles. The loading of the particles affects the sedimentation rate and similar behavior to th e titania microgels was observed. At the 50% loading there is a faster sett ling time of approximately 1500 s econds at ambient temperature (25C). This faster settling time indicates that the effective density of the hybrid particles has increased substantially from the bare ceria nanoparticles. The 25% loaded ceria composite had a settling time of 3500 seconds and agai n this is a substantial differe nce in the sedimentation rate of the pure ceria particles. The trend with ceria loading is similar to that for the titania-microgel composites where decreasing the loading caused slower settling. The effect of temperature also had an eff ect of the settling rate of the 25% and 50% ceriamicrogels. In figure 4.3 the temperature at 35C shows a settling time of approximately 1000 seconds for the 50% ceria-microgel composites. However as the temperature decreases to 15C the settling rate is longer, the particle se ttles at approximately 1800 seconds. The 25% ceria loaded particles were also tested at temperatures of 25C and 35C (F igure 4.4). The settling time was 4000 seconds and 2000 seconds, respectivel y. As discussed in Chapter 3, the changes 36
in temperature affect the porosity of the particle s, their size, and the de nsity contrast with the fluid. The results in Figure 4.3 a nd 4.4 show that as the temperat ure of the composite particle suspension decreases below the transition temperatur e, the drag force decreases. The results also show that when using the hybrid ceria composite particles, the enhanced se ttling of the particles in the slurry will require conti nuous agitation of the slurry mixtur e to maintain uniform particle distribution. 37
38 Applied Force Pad W afer Slurry solution Polisher Figure 4.1 Schematic of slurry polishing in a CMP process.
1.0 0.8 0.6 0.4 0.2Normalized Turbidity 14x103 12 10 8 6 4 2 0 Time (seconds) 50% 25% pure ceria Figure 4.2 Settling of pure ceria nanoparticles at ambient te mperature compared against composites with 25% and 50% ceria loading. 39
0.1 2 3 4 5 6 7 8 9 1Normalized Turbidity 4000 3000 2000 1000 0 Time(seconds) 35C 25C 15C Figure 4.3: Settling of composite with 50% ceria at temperatures below and above the transition temperature (T~32C) 40
0.1 2 3 4 5 6 7 8 9 1Normalized Turbidity 5000 4000 3000 2000 1000 0 Time (seconds) 25C 35C Figure 4.4: Settling of composite with 25% ceria at temperatures below and above the transition temperature (T~32C) 41
CHAPTER FIVE: SUMMARY AND CONCLUSIONS In summary, this thesis has focused on the se dimentation of two composite particles with an interpenetrating network of polymers and em bedded with inorganic oxide nanoparticles of titania or ceria. The focus has been on the characterization of these par ticles via sedimentation. Turbidity has been used as a simple technique to identify the settling rate of these colloidal particles. The settling rate of solid silica particles has been used to validate the technique by comparing theoretical predictions of settling with the experimental method. The novel titania-microgel particles have s hown rapid settling beha vior once the titania nanoparticles are embedded within the polymeric fr amework. This has been advantageous in the field of waste water remediation where the titania nanoparticles ar e used as photocatalyst and the recovery of the composite particles can be much simpler using sedimentation. It was found that the loading of the titan ia nanoparticles onto the framework of the polymeric network effectively changes the density of the part icles and the increased porosity of the flocs of the composites affects the settling rate of these particles. Te mperature is also a factor since the polymeric framework consists of a temperature sensitive polymer, PNIPAM, which causes the particles to exhibit a collapse and expansion behavior near a transition temper ature. The combined influence of changes in effective density and the porosity with temperature influence the settling behavior. The use of novel composite particles with embe dded ceria nanoparticles is effective in the CMP process and these can be used as an alternative for the commonly used commercial 42
abrasives. However, the ceria-microgels show ra pid settling rates and i ndicate that constant agitation is required for their suspension. Overall, the technique of sedimentation is ideal in the case of characterizing these composite particles as it forms the basis of a simple characterization. The experiments in this thesis have explored the settli ng rate of porous composite partic les and shown that a variety of parameters such as temperature, particle loadi ng, and concentration affect the settling behavior. Combination of experiments such as the ones described in this thesis with theoretical understanding can be valuable in the study of complex systems of polymeric microgels and inorganic nanoparticles. 43
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