Design and Development of an Acoustic Levitation System. for Use in CVD Growth of Carbon Nanotubes

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2 Design and Development of an Acoustic Levitation System for Use in CVD Growth of Carbon Nanotubes A thesis submitted to the Graduate School of the University of Cincinnati in partial fulfillment of the requirements for the degree of Master of Science in the Department of Physics of the McMicken College of Arts and Sciences by Amal Ali Qasem B.S. Jazan University, Jazan, Saudi Arabia November 2016 Committee Chair: David B. Mast, Ph.D.

3 Abstract The most widely used methods for growth of carbon nanotubes (CNTs) arc discharge, laser ablation, and chemical vapor deposition (CVD). Some of these methods have difficulties, such as controlling the quality and straightness of the nanotube in the synthesis of CNTs from substrates. Also, the enhanced plasma chemical vapor deposition method with the catalyst on a substrate produces straighter, larger diameter nanotubes by the tip growth method, but they are short. The difficulty in the floating catalyst method is that the nanotubes stay in the growth furnace for short times limiting growth to about one mm length; this method also leaves many catalyst impurities. One factor that limits CNT growth in these methods is the difficulty of getting enough carbon atoms to the growth catalyst to grow long nanotubes. The motivation of this work is that longer, higher quality nanotubes could be grown by increasing growth time and by increasing carbon atom movement to catalyst. The goal of this project is to use acoustic levitation to assist chemical vapor deposition growth by trapping and vibrating the growing CNTs for better properties. Our levitation system consists of a piezoelectric transducer attached to an aluminum horn and quartz rod extending into the growth furnace. The most important elements of our methods to achieve the acoustic levitation are as follows. 1- Using COMSOL Multi-physic Simulation software to determine the length of quartz rod needed to excite standing waves for levitation in the tube furnace. 2 -Determining the resonance frequency of different transducers and horns. ii

4 3- Using ultrasound measurement to determine the time of flight, velocity of sound and sound wavelength of different horns. 4 - Making Aluminum horns with the appropriate lengths. 5- Using ultrasound measurement to determine the changing of quartz rod velocity of sound and length in the furnace. 6 -Mounting the transducer to booster horn and aluminum cylindrical horn above a reflector to produce the standing waves. The levitation of small Styrofoam balls was successful by using this system and verified wavelengths of standing wave and position of levitation. We could not levitate powders, most likely due to electrostatic charging, air currents, but most importantly insufficient power to drive transducer. In addition, we built a CVD growth furnace with ultrasound transducer- horn- quartz rod and reflector. The reflector support also included a sense piezoelectric element for determining standing wave strength. This reflector support was mounted on a linear translation stage to control the quartz rod-reflector separation to produce standing waves. To remove the contaminated unwanted CNTs, we built a separate tube furnace tube filled with a molecular sieve to burn the CNT s in air. Finally, we made catalyst-coated, ceramic micro-particles for levitation and used these to verify CNT growth. Future efforts research would be to levitate these micro particles at room temperature then in the high-temperature furnace for growth of carbon nanotubes. iii

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6 Acknowledgements I acknowledge that this work would not be a success without the immense support I got from numerous individuals. First and foremost, I want to appreciate my advisor, Dr. David B. Mast who encouraged and helped me in various occasions. Also, he played a huge role in making this work a success due to is assistance and guidance. I also want to appreciate Professor Donglu Shi who offered me his lab for my experiments. I am honored to thank professors; Howard Jackson, Frank Pinski, and Hans-Peter Wagner for agreeing to be my committee members. There are no words that can express my heartfelt thanks to my spouse, Abdullah. Without his help and the believing in me, I would not have endured the stress this work would sometimes subject me to; hence, I thank him for being in my life and supporting me. I appreciate my parents for the encouragement they offered me which helped me finish my Masters. Also, I immensely appreciate my kids; Ghaday and Farah, however, they sometimes negatively support me. I am also grateful to my home college, Jazan University for supporting me financially and offering me a full scholarship which enabled me to finish my Masters degree at the University of Cincinnati.. v

7 Table of Contents List of Figures... vii List of Tables...x Chapter 1 Introduction Background Discovery of Carbon Nanotubes (CNTs) Nanotube Geometry CNT Growth Methods Growth Mechanisms of Carbon Nanotubes Applications Motivation...15 Chapter 2 Acoustic levitation and simulation Acoustic Levitation Simulation...17 Chapter 3 Acoustic Transducer Tuning Determining the resonance frequency Ultrasound time of flight measurement (TOF)...36 Chapter 4 Levitation Trials Levitation inside the quartz tube Levitation in a chamber Levitation in air...46 Chapter 5 CNT Growth on Ceramic Microparticles CVD furnace Catalyst preparation Summary Future Work...58 References...59 vi

8 List of Figures Figure 1-1 Types of Carbon Nanotubes, from reference [2]... 4 Figure 1-2 Categories of Carbon Nanotubes (CNTs), from reference [2]... 5 Figure 1-3 Schematic of Arc Discharge Method, from reference [2]... 8 Figure 1-4 Laser Ablation Method, from reference [1]... 9 Figure 1-5 Schematic of thermal CVD, from reference [1] Figure 1-6 Growth Mechanisms of Carbon Nanotubes (a) tip-growth model, (b) base-growth model, from reference [4] Figure 2-a How acoustic levitation works, from reference [15] Figure 2-1 Quartz rod s geometries, image from D.Mast Figure 2-2 The quartz rod inside a vacuum container Figure 2-3 COMSOL acoustic levitation model of the total acoustic pressure and the end point of particles trajectories in the space between the transducer at the bottom and the concave reflector at the top, from reference [13] Figure 2-4 The quartz rod s acoustic pressure at different length from cm Figure 2-5 COMSOL image shows the temperature (a), acoustic pressure (b), and speed of sound (c) for the quartz rod inside the furnace Figure 2-6 Particle s positions at 0.5 and 0.3 seconds Figure 3-1 Equipment of determining the resonance frequency Figure 3-2 Block diagram describing the connection of equipment used to determine the resonance frequency Figure 3-3 Sketch of the voltage divider circuit Figure 3-4 Voltage vs. resonance frequency of the piezoelectric transducer vii

9 Figure 3-5 Voltage vs. resonance frequency of the piezoelectric transducer connected to the horn and quartz rod Figure 3-6 Voltage vs. resonance frequency of the piezoelectric transducer attached to the booster horn Figure 3-7 The resonance frequency for the transducer attached to the booster horn and the 5.75 inch length aluminum horn Figure 3-8 The resonance frequency of the transducer connected to the booster horn and the inch length aluminum horn Figure 3-9 The resonance frequency for the transducer attached to the booster horn and the inch length aluminum horn Figure 3-10 The resonance frequency for the transducer attached to the booster horn and the inch length aluminum horn Figure 3-11 The resonance frequency for the transducer attached the booster horn and the inch length aluminum horn Figure 3-12 Voltage divider circuit connected to the Boonton to measure the voltage across R3+Rt Figure 3-13 The connection of the equipment and the voltage divider circuit to measure the voltage across R3+Rt Figure 3-14 Graph of the horn length versus resonant frequency. The size of error bars for the frequency (±0.01 khz) and horn length (±0.005 ) values are approximately the size of the data points. 36 Figure 3-15 Image of the equipment used to achieve the ultrasound thickness measurement viii

10 Figure 3-16 A block diagram for the equipment connection used to achieve the ultrasound thickness measurement Figure 3-17 Guartz rod glued connected to ½ wavelength horn and transducer Figure inch diameter quartz tube for ultrasonic acoustic levitated growth of Carbon Nanotubes Figure 4-2 Equipment connection to generate standing wave and levitate particles Figure 4-3 Chamber and components used for these levitation trials Figure 4-4 Block diagram Equipment connection to create standing wave inside the chamber.. 46 Figure 4-5 Transducer and the reflector on the optical table Figure 4-6 Transducer and reflector connection with the equipment to create the standing wave in air Figure 4-7 Three Styrofoam balls levitate at a distance between the transducer and the reflector equal to 3/2 λ Figure 4-8 Particles suspended at 2/2 λ, 4/2 λ, and 5/2 λ Figure 4-9 Levitate Nanoparticles Figure 5-1 Block diagram of the expected CVD growth furnace connection Figure 5-2 CNT growth on CM shown by arrows, 500x, photo from D. Mast Fiqure5-3 CNT growth on CM, shown by arrows, 1000x, photo from D. Mast Figure 5-4 Raman spectra of CNT growth shown in Figure 5-3, from D. Mast Figure 5-5 Catalyst coated micro particle samples before and after the growth Figure 5-6 Raman spectra of carbon material in the center dish in Figure 5-5, from D. Mast Figure 5-7 Raman spectra of material in the bottom dish in Figure 5-5, from D. Mast ix

11 List of Tables Table 1-1 Differences between SWNT and MWNT, from reference [1]... 5 Table 1-2 A Summary of the main methods for growth of CNT, from reference [1]...6 Table 3-1 Resonance frequency at different lengths of the cylindrical aluminum horn Table 3-2 The transit time and the velocity of sound of differing horns Table 5-1 Gas flow rate for the growth of CNT from UC Nanoworld x

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13 Chapter 1 Introduction 1.1 Background Carbon Nanotubes (CNTs) are made up of tubes of carbon with diameters from 1 nm to 50 nm and lengths of a several microns to centimeters. The term nanotube comes from their small diameter. CNTs have many outstanding electrical mechanical, chemical and thermal properties. For example, strength 100 times greater than steel, thermal conductivity similar to that of diamond, and electrical current density of more than 10-9 A/cm2. [1-5] These properties make carbon nanotubes (CNTs) useful in many applications including fuel cells, conductive films, super capacitors, purification systems, separation membranes, and filters. [1,2] Origins of Fullerenes For many years, researchers have carried out extensive investigations into the properties of carbon. Before 1985, carbon was mainly studied in many complex and simple molecular compounds. However, pure carbon was found only in two naturally occurring forms; graphite and diamond. In 1985, this changed as scientists discovered a third form of solid carbon, the buckminsterfullerene (Buckyballs). A Buckyball is a symmetrical, soccer ball shaped molecule comprised of sixty carbon atoms. [1] In August 1985, Harry Kroto and Richard Smalley of Rice University, Houston Texas, discovered Buckyballs by accident. They did this by evaporating a sample of graphite by the use of a laser light, and a stream of helium gas which helped to transport the evaporated carbon into a spectrometer. Peaks which correspond to groups of carbon atoms are read by the mass 1

14 spectrometer, with one particularly powerful peak corresponding to particles containing sixty carbon atoms (C60). Other molecules made up of carbon atoms (C36, C70, C76, and C8) were soon discovered. Buckyballs are now considered as a closed-cage allotrope of carbon, called fullerenes, with unique properties not present in the previous compounds. [1] 1.2 Discovery of Carbon Nanotubes (CNTs) CNTs were first discovered in 1991 by Iijima and are comprised of thin layers of carbons Benzene rings; these layers are rolled up to make seamless cylinders. These cylinders have stable chemical compositions, have a heat transfer rate greater than diamond, and are one of the strongest materials that have ever existed. The CNTs properties have interested researchers in almost all areas of science and have provided scientists with a new tool which can be exploited in many applications. [1,2] CNTs are hollow carbon cylinders with a single wall or with multiple-walls. Carbon nanotubes have sp2 bonding, with every atom attached to 3 neighbors, just like a single sheet of graphite. This structure is considered to be stronger than the sp3 bonds in diamond, which give the CNTs their distinctive strength. 1.3 Nanotube Geometry Carbon nanotubes can be categorized into five main structural types: Multi-Wall Nanotubes (MWNTs), Single-wall carbon nanotubes (SWNTs), polymerized SWNT, Nanotorus, and Nanobuds. 2

15 1-Single-Wall Carbon Nanotubes (SWNTs): SWNTs were discovered in 1993 and marked the beginning of the nanoworld. SWNTs contain a single layer of carbon rings bound into cylinder with a 1-2 nm diameter with lengths ranging from 50 nm to 1 cm. SWNTs are found in curved and curled strands as opposed to straight lines. Changes in angle of the carbon-carbon bonds are categorized by a chiral vector (n, m). [1,2] The relationship between m and n represents three classes of carbon nanotubes (CNTs) as presented in figure 1-2. The first category is Zigzag nanotubes where the chiral angle, equals 0 degrees (n=0 or m=0). The zigzag name was as a result of the fact that the roll-up angle of graphene sheet makes a shape like the zigzag bonds of graphene. [1,2,4] The second kind is "Armchair" nanotubes with chiral angle, equaling 30 degrees (n=m). The Armchair name originated since graphene layers look like an arm chair. [1,2] The last category, chiral nanotubes, have their sheets aligned along the cylinder in a chiral angle different from that in the armchair or zigzag (m n) where chiral angles are between (0 and 30) degrees. SWNTs with different chiral vectors possess different features including mechanical strength, electrical conductivity and optical activity. 2-Multi-Wall Nanotubes (MWNTs) MWNTs are comprised of many sheets of graphite rolled together to form seamless tubes or cylinders. A brief summary explaining the variations between SWNTs and MWNTs is shown in the table 1-1. [1,2] 3

16 3-Polymerized SWNT This is when many single-walled nanotubes bond together to create linked networks, with other materials and compounds. [1,2] 4- Nanotorus This is a carbon nanotube joined end to end into a donut or bagel shape. [1,2] 5- Nanobuds Nanobuds, have been recently discovered and involves combining of two original discoveries of carbon: Fullerenes and carbon nanotubes. This material has C60-like buds attached to the outside wall of a CNT. [1,2] Figure 1-1 Types of Carbon Nanotubes, from reference [2]. 4

17 Figure 1-2 Categories of Carbon Nanotubes (CNTs), from reference [2]. Table 1-1 Differences between SWNT and MWNT, from reference [1]. 5

18 1.4 CNT Growth Methods There are numerous techniques that can be used to grow CNTs. Three major techniques used are Chemical Vapor Deposition (CVD), arc-discharge, laser ablation. Current is used as the energy source in the arc-discharge process. [6-11] In laser ablation, the energy source is intense laser light, while in a CVD, the energy source used is heat from a furnace. A summary of the main growth methods is shown in Table 1.2. [2] Table 1-2 A Summary of the main methods for growth of CNT, from reference [1]. 6

19 1.4.1 Arc Discharge Method The arc-discharge method was one of the first methods used to create CNTs. Temperatures above 17000C are applied in the arc discharge method for CNT fusion, which typically produces CNTs with some mechanical flaws in contrast with other processes. The setup of the arc discharge consists of graphite electrodes, a furnace, vacuum chamber, low temperature trap and a high voltage power supply. The procedure followed in arc discharge technique is depicted in figure 1-3. This process involves the creation of nanotubes by vaporization of graphite electrodes separated by about a 1mm in a chamber filled of argon or helium at a minimum pressure between 50 to 700 mbar. A direct current of 50 to 100 amps which will produce heat between two carbon electrodes to evaporate the graphite rods. Due to this high temperature, the anode evaporates, and the formed tubes will be dropped on the cathode. In the fabrication of SWNTs, the anode is immersed in a metal catalyst such as Co, Fe, or Ni, which produces SWNTs having a diameter ranging from 1.2 to 1.4 nm. MWNTs are created by the use of a purer graphite, and have an internal diameter of 1-3nm and an external diameter of 10nm. Since there is no catalyst used in this process, heavy acidic cleaning is not essential. This ensures that MWNTs can be created with fewer defects. Various other techniques use magnetic field synthesis, liquid nitrogen and plasma rotating arc discharge. [1,2] 7

20 Figure 1-3 Schematic of Arc Discharge Method, from reference [2] Laser Ablation Method In 1996, by the use of laser ablation of graphite, Smalley was able to produce higher yields exceeding 70 percent of SWNTs with few amounts of Co and Ni. A graphite board is vaporized in an oven by a pulsed laser, in an inert gas filled the oven to regulate the pressure at 500 torr. Because the background gas and the catalyst mix, this process is similar to the arc discharge technique. However, this method is costly; hence, it is not commonly used for SWNTs. Laser ablation produces more SWNTs with smaller size distribution as opposed to those created by the arc discharge method.[9] 8

21 Figure 1-4 Laser Ablation Method, from reference [1] Chemical Vapor Deposition Method (CVD) This is a method used to grow CNTs. In this procedure, the surface of the catalyst offers a surface for the disintegration of the carbon precursor. VD methods utilized in synthesis of CNTs are Plasma enhanced CVD (PECVD) and thermal CVD. CVD is usually carried out in a high temperature furnace to vaporize carbon gasses. Methane, carbon monoxide, ethylene or acetylene are normally used carbon sources. The CNT CVD synthesis is a two-step method. The first phase involves preparation of the catalyst, followed by growth of the nanotube. The catalyst is usually made by dip coating and cracking physical vapor deposition (PVD). Then the substrate is exposed to heat in a surrounding rich in carbon gasses. The temperature often used in this process ranges from 500 to 10000C this results in the production of CNTs if appropriate parameters are well regulated. [1,2,8,9] 9

22 This process is simple and a cost effective method for producing carbon nanotubes at a low ambient pressure and temperature. It is a multipurpose process since it provides hydrocarbons in many forms, different substrates can be used, and it allows CNT growth in different ways such as thick films or powder. [1,2,8,9] Typical set ups used for CVD CNT growth is shown schematically in figure 1-5. The method requires passing a hydrocarbon gas over catalyst particles on substrates, for nearly minutes at high temperature of about C in a tube furnace to break down the hydrocarbons. CNTs grow from a carbon saturated catalyst particle (base growth or tip growth). [1,2,8,9] The CNT growth rate is controlled by parameters such as amount of catalyst, type of hydrocarbon gas, and temperature. Essentially, CVD at a low temperature of about C creates MWNTs, while a higher temperature of C improves the SWNT growth. The catalyst size determines the diameter of the nanotube. Also, the morphology, material and features on the substrate control the amount and quality of the CNTs produced. [1,2,8,9] There are various factors that affect the quality of carbon nanotubes created by CVD; they include background pressure, growth temperature, catalyst particle size, and gas flow rate. Recently, there have been improvements in various methods used in nanotube synthesis by CVD. These include aero gel-supported CVD, catalytic alcohol-cvd, laser assisted CVD, plasma enhanced CVD and thermal chemical CVD. [1,2,8,9] 10

23 Figure1-5 Schematic of thermal CVD, from reference [1]. 1.5 Growth Mechanisms of Carbon Nanotubes Since their discovery, CNT growth mechanism is not well understood. Based on post-deposition analysis of reaction conditions, numerous groups have raised many possibilities which usually are contradicting. As a result, there is no single CNT growth mechanism that has been considered as being universal. However, fundamental mechanisms can be identified as follows. A hydrocarbon is vaporized after it comes into contact with the hot catalyst nanoparticles. First, hydrogen gas is emitted by decomposition of the hydrocarbon into hydrogen and carbon. The carbon then dissolves into the metal catalyst particle, however the catalyst particle can dissolve only a given amount of carbon at any given temperature. When this limit is reached, carbon precipitates on the catalyst surface and forms tubular crystals. The hydrocarbon decomposition emits heat into the catalyst particle, and the carbon precipitation removes heat from the catalyst particle. A thermal gradient formed inside the catalyst particles ensures that the procedure is maintained. [5] 11

24 There are two major cases as depicted in figure 1-6. The first involves limited substrate-catalyst interactions. The hydrocarbon decomposes and spreads out on top of the catalyst particle, and CNT precipitates from the bottom of the metal catalyst, pushing the catalyst particle from the substrate. As long as the catalyst particle is kept free for a new hydrocarbon breakdown, the CNT continues to grow. When the catalyst particle is covered entirely with carbon, the growth of CNT is stopped. This growth is referred to as a tip-growth model. [5] The second case, the CNT growth was not able to detach the catalyst particle from the substrate, and CNT grows up from the attached catalyst particles in a process known as base-growth. [5] Figure 1-6 Growth Mechanisms of Carbon Nanotubes (a) tip-growth model, (b) base-growth model, from reference [5]. 12

25 1.6 Carbon Nanotube Properties Carbon nanotubes can be considered as one of the hardest and strongest materials known, concerning elastic modulus and tensile strength respectively Mechanical properties 1. Multi-walled carbon nanotubes have a tensile strength around 65 GPa); the tensile strength of high carbon steel is 1.2 GPa. 2. CNTs possess a large elastic moduli of 1 TPa; the elastic moduli of aluminum is around 70 GPa. 3. Carbon nanotubes have density of gm/cm3; the density of high-carbon steel is around 8.5 gm/cm3. 4. When exposed to immense tensile strain, CNT s undergo plastic-like flow. They will bond together when subjected to torsional, compressive or bending stress Electrical properties The unusual electronic properties of how the graphene sheet is rolled determine the carbon nanotube electrical properties. Therefore, some carbon nanotubes are semiconductors while others are metallic. Metallic nanotubes possess an electrical current density which is a thousand times that of copper. 13

26 1.6.3 Thermal properties Nanotubes are among the best thermal conductors, due to the strong carbon-carbon bond Defects Defects greatly affect the material s properties. Defects in CNTs have structures similar to that of atomic vacancies. For example, a single monoatomic vacancy may produce magnetic properties. When these defects are present in high numbers, the CNT s tensile strength is reduced. StoneWales defect is another defect that can be present in carbon nanotubes and consists of a heptagon and pentagon pair created by a realignment of C-C bonds. The electrical characteristics of nanotubes and the thermal properties of the tubes are also greatly impacted by the existence of defects. [1,2] 1.7 Applications Current Applications CNTs play a large role in increasing our capacity to produce materials like pipes, wires, bearings, pumps, gears, and springs. [1,2] Potential Applications 1) Chemical: offers storage for hydrogen and plays a role in air pollution filtration. 2) Structural: Sports equipment, combat jackets, clothes 3) Electrical Circuits: can be used as an alternative in the reduction of the scale from MicroElectro-Mechanical Systems to Nano level. 4) Mechanical: slick surface, oscillator. [1,2] 14

27 1.8 Motivation A summary of nanotube growth method used at University of Cincinnati will provide the motivation for this project. At UC, researchers have investigated the synthesis of carbon nanotubes from substrate and found that it is hard to control the quality and straightness of the nanotubes. Substrate anchoring and nanotube levitation produces good nanotubes, but the process has not been scalable to size production. Another synthesis used is a plasma enhanced chemical vapor deposition with catalyst on substrates. This method provides better and larger diameter nanotubes by tip growth, but the nanotubes are short. The floating catalyst method produces nanotubes which are limited to about 1 mm length, and with catalyst impurities in the nanotubes. Based on previous work, the goal of this project is to produce long, high-quality nanotubes via the use of acoustic levitation technique that assisted the chemical vapor deposition (CVD) growth [12] In chapter two of this thesis, I describe a model used to get acoustic waves from an acoustic transducer at room temperature to quartz rod and into the high temperature region of a CNT growth furnace by using COMSOL MultiPhysics. [13] In chapter three, I describe a series of measurement of resonance frequencies of different acoustic transducers and to use ultrasound time of flight measurements to find the time of flight, velocity of sound and sound wavelength for various horns. The result of levitating particles at the room temperature will be shown in chapter four. Then, a brief summary explains how the CVD furnace was built and how the catalyst coated micro particles were prepared and CNTs were grown using CVD and a short summary will be outlined in chapter five. 15

28 Chapter 2 Acoustic levitation and simulation 2.1 Acoustic Levitation Acoustic levitation can be defined as a method of suspending objects in a medium against the force of gravity by the help of acoustic pressure obtained from intense sound waves in the medium. This has the effect of floating objects in the air, and object can be kept steady, where they cannot move or drift. [14-18] How Acoustic Levitation Works The elementary acoustic levitator is comprised of two major parts; a transducer and a reflector surface. The transducer makes the sound while the reflector surface receives and reflects back towards the transducer the. Normally, the reflector and transducer possess concave surfaces which aid in centering of the sound wave. Since the sound generated by the transducer is a longitudinal wave, standing wave are created when the transducer - reflector distance is a half integer numbers of wavelengths. Standing waves have regions of maximum pressure (antinodes) and regions of minimum pressure (nodes). Acoustic levitation of particles occurs just below the nodes where the acoustic pressure balances the pull of gravity, as shown in the figure 2-a. [15] 16

29 Figure 2-a How acoustic levitation works, from reference [15] 2.2 Simulation The acoustic levitation is established when an ultrasound wave of an adequate pressure reflects from a surface located at an integer number of half wavelengths from the ultrasound source, producing a stable standing wave. This standing wave has positions where particles at these points experience returning force, which restores these particles back to these trapping regions. The reflecting surface used to create the standing wave can be a planar or concave surface (concave lens). We used a concave surface because the concave surface reflector produces some areas that are deeper traps than others. Particles are suspended at regions close to nodes in the acoustic standing wave. [13] In this project, we set up a system which contains a piezoelectric transducer attached to an aluminum horn with an aluminum piece glued to a quartz rod. The transducer must be kept at room temperature but the ultrasound must be brought into the high temperature region of the 17

30 growth furnace by a quartz rod. To levitate the catalyst-coated particles, which will be used to grow the carbon nanotubes (CNTs) in a high-temperature furnace, we need a high intensity of the sound wave. To get the greatest intensity of sound wave from the transducer we have to make the length of the whole system to be a half-integral numbers of wavelengths, so the end of quartz rod vibrates as much as possible (antinode). Most of the system is at room temperature, but the quartz rod will be placed in the high-temperature region. Since the quartz rod is in the furnace, that means we have a temperature gradient, and the velocity of sound will vary along this rod. Therefore we do not know how long we should cut the rod to get the correct overall length. So instead of cutting the rod at different lengths and trying that at the high-temperature furnace to see how the particles will levitate, we used COMSOL Multi-physic Simulation software to calculate the right length. We started the simulation by building a quartz rod with 1 cm diameter and 30 cm length as shown in figure 2-1. We then applied a 20 khz acoustic wave to one end, fixed the temperature difference from room temperature to 900 degrees Celsius, and used an empirical expression for the velocity of sound as a function of temperature. With this system we measured the amplitude of the 20 khz acoustic wave at the high temperature end. Finally, we measured the quartz rod length change, due to the thermal expansion. 18

31 Figure 2-1 quartz rod s geometries, image from D.Mast. Next, we placed the quartz rod inside a container to allow for radiation and convection in the sealed container as shown in figure 2-2 to simulate the CNT growth furnace. Figure 2-2 The quartz rod inside a vacuum container, image from D.Mast. Finally, we attached the end of the quartz rod to a COMSOL model for acoustic levitation simulation. This simulation model shows a surface plot of the acoustic pressure and sound level 19

32 and particles trajectories in the space between the transducer at the bottom and the concave reflector at the top, as shown in figure 2-3. Figure 2-3 COMSOL acoustic levitation model of the total acoustic pressure and the end point of particles and end points of particle trajectories in the space between the transducer at the bottom and the concave reflector at the top. COMSOL Model taken from reference [13]. Simulation results Using COMSOL Multi-Physics we studied the relationship between the quartz rod's length and the acoustic pressure when the rod was placed in the simulated growth furnace. We found that the quartz rod's highest acoustic pressure was when the quartz rod s length was 30 cm as shown in figure 2-4. We know that the transducer and the booster horn lengths are each 1/2 wavelength; that means we have to place the quartz rod at ¾ or 5/4 wavelength (at the node) to be stable. We know that to create standing waves, which we need to levitate the particles, the total system length must be an integer number of half wavelengths (n)1/2 λ=l. L for quartz rod in the simulation model with ½ λ is 30 cm (½ λ = 30 cm). 20

33 According to these results, the best length for the quartz rod is 3/4 λ or 45cm, we then glued the rod into a 1/4 λ aluminum mounting block to attach to the ½ λ transducer and 1/2 λ horn to generate the standing wave region, where the catalyst particles will levitate to grow CNTs. Figures 2-5 and 2-6 present how to levitate the particles by using COMSOL Multi-physic acoustic model. LENGTH VS ACOUSTIC PRESSURE 3.50E+08 PRESSURE 3.00E E E E E E E l[cm] Figure 2-4 The quartz rod s acoustic pressure at different length from cm

34 Figure 2-5 COMSOL image shows the temperature (a), acoustic pressure (b), and speed of sound (c) for the quartz rod inside the furnace. Figure 2-6 Particle s positions at 0.5 and 0.3 seconds. 22

35 Chapter 3 Acoustic Transducer Tuning 3.1 Determining the resonance frequency When an AC electric field is applied to piezoelectric ceramic elements, the ceramic component changes shortens and expands cyclically with the AC field. The resonance frequency of a piezoelectric actuator or transducer is the frequency where the transducer generates the greatest amount mechanical energy from the applied electrical energy. [18] The purpose of operating at the resonance frequency is to generate the maximum sound pressure to achieve the acoustic levitation. Accordingly, we need to match the resonance frequency of the transducer with the horn and quartz rod attached to the resonance frequency of the transducer by itself. The equipment used to determine the resonant frequency is shown in figure 3-1. The equipment used are a BOONTON 1121 AUDIO ANALYZER, to generate and detect the signal, a voltage divider circuit, a 7090A MEASUREMENT PLOTTING SYSTEM, to make a plot of the voltage as a function of frequency. We need to do this for the 20 khz transducer connected to horn and quartz rod, and 20 khz transducer attached to the booster and cylindrical aluminum horns. The booster is a one-half wavelength length which, is placed between the transducer and the horn to increase the vibration amplitude. 23

36 Figure 3-1 Equipment of determining the resonance frequency. Procedure for determining resonance frequency 1-The system was set up as shown in figure 3-2; the transducer was connected to the horn and quartz rod at first measurement and then at the second measurement was with booster horn and cylindrical horn. 2- The Boonton 1121 Audio Analyzer was adjusted to give a 5-volt voltage signal, and the frequencies were adjusted to a range between khz because we determined that the resonance frequency of the transducer was at 20 KHz. 3-The voltage divider circuit will transform the large voltage from the signal generator into a smaller voltage across the transducer, as shown in Figure The transducer transfers the electrical energy from the signal generator into 20 khz mechanical vibration. 24

37 5-The ultrasonic booster horn, which in contact with the transducer, transfers the mechanical vibratory energy from the transducer to the workpiece (cylindrical aluminum horn), and the horn vibrates at its resonant frequency. 6-The strip chart recorder records the graph showing the transducer voltage vs. frequency for frequencies between 15 khz and 25 khz. Figure 3-2 Block diagram describing the connection of equipment used to determine the resonance frequency. 25

38 Figure 3-3 Sketch of the voltage divider circuit. Results Figure 3-4 shows that the resonance frequency of the piezoelectric transducer was 20.5 khz, with a maximum voltage of Volt, and the anti-resonance frequency was 21.5 khz, with a minimum voltage of 43 millivolts. The resonance frequency of the transducer connected to the horn (and quartz rod) was at khz ( khz) and the anti-resonance frequency was at khz (22.9 khz) as shown in figure

39 Figure 3-4 Voltage vs. resonance frequency of the piezoelectric transducer. 27

40 Figure 3-5 Voltage vs. resonance frequency of the piezoelectric transducer connected to the horn and quartz rod. Figure 3-6 Voltage vs. resonance frequency of the piezoelectric transducer attached to the booster horn. 28

41 Figure 3-6 shows that the resonance frequency of the piezoelectric transducer with the booster horn was khz, with a maximum voltage of Volt, and the anti-resonance frequency was 20.9 khz, with a minimum voltage of 43 millivolts. For the 5.75 inches long cylindrical horn which attached to the booster horn and the transducer, the resonance frequency was at khz with the maximum voltage at volts as shown in figure 3-7. This value is different from calculating the resonance frequency mathematically. For a 5.75 length aluminum horn at 20 khz frequency with 6320 m/s speed of sound is; Resonance frequncy= speed of sound /2* length. Resonance frequncy=63802m/s/0.2921m = 21.5 khz. The difference in these two results is most likely due to presence of a threaded steel rod needed to connect the booster and horns to the transducer in the resonance frequencies measurements, but absent in the velocity of sound measurements, Section 3.2 below. However, this difference shows the importance of experimentially determining the resonance frequency of each transducer, booster, and horm configuration used in this work. Figure 3-7 The resonance frequency for the transducer attached to the booster horn and the 5.75 inch length aluminum horn. 29

42 To match the resonance frequency of the horn with that of the transducer at 20 khz, we measured the resonance frequency while decreasing the length of the horn to 5.5 inches. As a result, the resonance frequency changed to be at khz with the maximum voltage at volts and the antiresonance frequency was at khz with a minimum voltage at millivolts as presented in figure 3-8. By decreasing the horn's length to inches, an khz resonance frequency was obtained, with a maximum voltage of volts, and an anti-resonance frequency was khz, with a minimum voltage of 43 millivolts, as shown in figure 3-9. Figure 3-8 The resonance frequency of the transducer connected to the booster horn and the 5. 5-inch length aluminum horn. 30

43 Figure 3-9 The resonance frequency for the transducer attached to the booster horn and the inch length aluminum horn. We continued cutting the horn to determine the resonance frequency of the transducer with the booster and cylindrical horn. As shown in Figures 3-10 and 3-11, for a inch horn length, the resonance frequency was khz, with a maximum voltage of volts, and the antiresonance frequency was khz, with a minimum voltage of.43 millivolts.the resonance frequency for a inch horn was khz, with a maximum voltage of volts, and the antiresonance frequency was khz, with a minimum voltage at 36.9 millivolts. 31

44 Figure 3-10 The resonance frequency for the transducer attached to the booster horn and the inch length aluminum horn. Figure 3-11 The resonance frequency for the transducer attached the booster horn and the inch length aluminum horn. 32

45 We used the circuit shown in figure 3-12 to detrmine the impedence of the transducer at resonance and at antiresonance. We treated the transducer in figure 3-12 as a resistor and called it Rt. When the transducer s resonance frequency was set at khz, the voltage amplitude across R3 (225 Ohms) was Volt. Using Ohm's law, the current through R3 is 2.56 x 10-3 A. Since R3, and Rt are in series at the circuit, they have the same current. We can measure the amplitude of the voltage across R3 plus the voltage across Rt as shown in figure 3-12 by change the connection of the circuit with the equipment as showing in figure As a result, the voltage across R3 plus the voltage across Rt was volts, so the voltage across Rt is volt. So by applying Ohm's law, we determined the amplitude of Rt, Rt= Ohms. By doing the previous measurement, the Rt for the transducer s anti-resonance frequency at khz with the minimum voltage at 0.39 millivolt is 5445 Ohm. These values are consistent with those determined by the transducers manufacturer. Since the resistance in a minimum (maximum) at resonance (antiresonance), when a constant voltage applied to the transducer, the current in the drive circuit should be maximum (minimum) at resonance (antiresonance). We use the measurement of transducer voltage and current to tune the frequency exactly to resonance for maximum standing wave amplitude for levitation. 33

46 Figure 3-12 Voltage divider circuit connected to the Boonton to measure the voltage across R3+Rt. Figure 3-13 The connection of the equipment and the voltage divider circuit to measure the voltage across R3+Rt. 34

47 In brief, by decreasing the horn's length, the resonance frequency of the transducer connected to the booster and cylindrical horns increased. That means there is an inverse relationship between the horn's length and the resonance frequency of the transducer connected to the booster and cylindrical horns as shown in Table 3-1. The expected horn's length to have a resonance frequency at 20 khz is presented in figure Resonance frequncy KHz length inch Table 3-1 Resonance frequency at different length of the cylindrical aluminum horn. 35

48 FREQUNCY VS HORN LENQTH 6.5 y = x R² = VOLTAGE VOLT RESONANCE FREQUNCY KHZ Figure 3-14 Graph of the horn length versus resonant frequency. The size of error bars for the frequency (±0.01 khz) and horn length (±0.005 ) values are approximately the size of the data points. 3.2 Ultrasound time of flight measurement (TOF) Ultrasound TOF measurement is a non-destructive measurement for measuring the length of a solid based on the time it takes for the ultrasound wave to travel through the solid and back again. [19-20] The goal of doing the Ultrasound TOF measurements is to calculate the speed of sound (C) for different horns (knowing their lengths). By finding the speed of sound will be able to calculate the wavelength of the horns to compare with the resonance frequency measurements. To growth carbon nanotubes (CNTs) using acoustic levitation assisted Chemical Vapor Deposition (CVD), we need to place the quartz rod at the end of the transducer to transfer the sound wave to the reflector generating the standing wave which will suspend the catalyst 36

49 particle. Therefore, we should attach the quartz rod at the location where the vibration is small, that means we should place it at a node. By measuring the speed of sound of the horns, we can find where the node is for each horn. The needed equipment and materials to obtain the Ultrasound length measurement shown in figure3-2-1 as follows. 1-Panametrics pulser receiver (generate and receive signal); 2- Piezoelectric transducer (generate ultrasound wave); 3-waverunner oscilloscope operators (displays the time which the ultrasound wave take to return to the surface); 4- Different samples (horns). Figure 3-15 Image of the equipment used to achieve the ultrasound thickness measurement. 37

50 Figure 3-16 A block diagram for the equipment connection used to achieve the ultrasound thickness measurement. Procedure of ultrasound length measurement 1- connect the equipment as shown in figure 3-16, the signal comes from the generator to the ultrasound transducer which converting the electrical energy to sound wave. 2- The sound wave is coupled into the horn and travels along it until it reflects off the open end of the horn. 3-waverunner oscilloscope operator display data of reflections wave (round trip transit time and frequency). 4-the first recorded return will normally be the head of the transmitted wave moving at the shortest distance which is equal to the length of the sample. All other returns signals are from multiple reflections. 5-calculate length using the simple mathematical relationship 38

51 L=c*t/2 Where L = horn length. C = the velocity of sound of the horn material. t = the round trip transit time In order to achieve good sound transmission a drop of water was placed between the transducer and horn, because MHz sound wave do not go through air. Results. Table 3-2 shows the transit time and the velocity of sound of different horns. For the 20 khz horn, we found that it is a half wavelength long, so the quartz rod must be attached at a node region, or ¼ wavelength of the horn length, so we attached the rod to a short aluminum adapter that is ¼ wavelength long, as shown in figure

52 Figure 3-17 Guartz rod glued connected to ½ wavelength horn and transducer. Table3-2 The transit time and the velocity of sound of differing horns 40

53 Chapter 4 Levitation Trials 4.1 Levitation inside the quartz tube After determining the resonance frequency of the 20 khz transducer horn and 45-centimeter quartz rod, we investigated setting up standing waves and levitating particles at the room temperature, before we attempted to levitate the catalyst particle in the tube furnace to grow the carbon nanotubes. Equipment and elements We tried to set up standing waves between the quartz rod and reflector inside the 2-inch diameter quartz tube which will be put inside the tube furnace for CNT growth. The system inside the tube includes two parts. The first one was a stationary part with the 20 khz transducer joined to the quartz rod. The second part was a movable part which contained a concave lens cemented on a separate quartz rod, connected to a piezo-sensor and translation stage, as presented in Figure 4-1. A Boonton 1121 Audio Analyzer was used to generate the signal, two amplifiers were used; one to amplify the signal from the generator to the transducer, PZD350, and the other, SRS SR560, one to increase the signal detected from the piezo-sensor. An oscilloscope was used to display the voltage trace of the signal going to the transducer and the voltage of the pressure force of the standing wave on the sensor. 41

54 Procedure The equipment was connected as shown in figure 4-2. We adjust the Boonton to give a signal with a voltage at 5-volt and frequency to match the resonance frequency of the transducer hornquartz discussed in previous chapter. The signal is amplified, PZD350, to be a hundred times the signal that came from the Boonton and is connected to the transducer. The transducer produced the sound wave which coupled to the quartz rod and is reflected by the concave lens, to create the standing wave region where the particles will levitate. The standing wave should be created when the distance between the quartz rod and the reflector is adjusted to an integer number of half wavelengths. The sensor on the end of the lower quartz rod would detect if there were standing waves which would be displayed as a maximum voltage trace on the oscilloscope. Figure inch diameter quartz tube for ultrasonic acoustic levitated growth of Carbon Nanotubes, image from D Mast. 42

55 Figure 4-2 Equipment connection to generate standing wave and levitate particles. Result The voltage trace on the oscilloscope which represents the force of the standing wave on the concave lens had the same amplitude at different distances between the quartz rod and the reflector; that means there were no standing waves created. The lack of the standing waves may due to the quartz rod not being placed correctly on the horn, or the reflector did not work as it should, or the amplitude of the signal was not enough to generate the standing waves. 43

56 4.2 Levitation in a chamber We investigated standing waves between the 20 khz transducer attached to the booster and cylindrical horn and the reflector inside a chamber to prevent air currents from disrupting particle levitation. Equipment and materials Figure 4-3 show the equipment as the following, 20 khz transducer which connected to the booster horn and cylindrical horn, Boonton 1121 audio analyzer, two amplifiers, and one oscilloscope. Procedure The equipment was connected as shown in figure 4-4. The transducer was hung up on the top of the chamber and was placed on a motion stage to adjust the distance between the end of the horn and the reflector. We adjusted the Boonton to give a signal with a voltage of 5-volt and frequency match the resonance frequency of the transducer which attached to the booster horn and aluminum cylindrical horn, as measured as described in the previous chapter. The signal travels from the Boonton to the amplifier, and is displayed on the oscilloscope channel 1. The oscilloscope channel 3 is connected to the voltage monitor on the amplifier, PZD350, and the current monitor on the amplifier was connected to oscilloscope channel 2. [The signal was amplified hundred times the signal that came from the Boonton and traveled to the transducer]. The transducer produced a sound wave which reflects off the surface to create a standing wave region in which the particles will be suspend. The standing waves will be created when the distance between the transducer and the reflector at an integer numbers of half wavelengths. The 44

57 receiver transducer was connected to the amplifier 2 to amplify the signal of the force of the standing wave on the reflector. The amplitude of this signal will display as a maximum voltage trace on oscilloscope 2 and 3 when a standing wave in present. Results The voltage trace at the oscilloscope 2 and 3 had the same amplitude along the different distance between the transducer and the reflector which means there are not standing wave generated and no particles could be levitated. We studied the reasons for the lack of the standing wave and found a problem at the transducer connection wires and fixed it as I will explain in the next part of this theses. Figure 4-3 Chamber and components used for these levitation trials. 45

58 Figure 4-4 Block diagram Equipment connection to create standing wave inside the chamber 4.3 Levitation in air There was a problem with the electric connection of the driver transducer, because of that, no standing waves were generated between the transducer and the reflector inside the chamber. We corrected the transducer's wire connection, and placed the transducer and the reflector on an optical table as shown in figure 4-5 and investigated the standing wave levitation. Equipment and material The components used were khz transducer which contacted to the booster and cylindrical aluminum horn, which its resonance frequency is found in chapter Receiver transducer (reflector). 3- A signal generator (Boonton). 4-2 amplifiers, the first one, Titan MAC-01SH, to amplify the signal which comes from the Boonton and goes to the driver transducer, and the second one, SRS SR560, to amplify the signal which comes from receiver transducer goes to channel two on the oscilloscope. 46

59 5- A voltmeter to display the transducer voltage. 6- An oscilloscope to display the voltage trace of the signal from the Boonton at ch1, and the voltage trace of the pressure force of the standing wave on the reflector after being amplified via amplifier 2. Procedure We connected the equipment, transducer, and reflector as shown in figure 4-6. We adjusted the Boonton to give a signal with a frequency to match to the resonance frequency of the 20 khz transducer, booster and cylindrical horn at khz and a voltage of 510 mv. This signal is shown on the oscilloscope channel 1. The signal goes from the Boonton to the amplifier, and the amplified signal goes to the 20 khz transducer, the voltmeter will read its amplitude. Figure 4-5 Transducer and the reflector on the optical table, photo from D.Mast. 47

60 Figure 4-6 Transducer and reflector connection with the equipment to create the standing wave in air. Results The greatest amplitude of the voltage trace from the piezo-sensor on the oscilloscope, which represents the pressure force of the standing wave on the reflector, occurred at the distance 0.87cm, 1.77cm, 2.67cm, and 3.57cm between the reflector and the transducer. The difference between these distances, is 0.9cm which, means that 1/2 wavelength (λ) of sound in air is 0.9 cm. We know to have a standing wave L= n (1/2 λ), so when n=1, L=0.9 cm, when n=2, L=1.8 cm, when n=3, L=2.7 cm, when n=4, L=3.6cm, and finally when n=5, L =4.5 cm. At these standing wave distances, we levitated small particles as shown in figures below. These pictures confirm our understanding of the acoustic levitation theory, which assumes the particles 48

61 would be suspended just below the nodes. The particle will levitate near the point where the sound pressure is at a minimum, and the pressure differential overcomes the pull of gravity. Figure 4-7 Three Styrofoam balls levitate at a distance between the transducer and the reflector equal to 3/2 λ. Figure 4-8 Particles suspended at 2/2 λ, 4/2 λ, and 5/2 λ. 49

62 The image in Figure 4-7 shows how the particles are suspended at the 3/2 λ =2.7cm. From the image in fiqure 4-7, we estimated that the distance between one particle and another is a half wave length = 0.9 cm.experimentally, we measured the distance between the transducer and the reflector in the image of fiqure 4-7 to be 5.83 cm, and the distance between one particle and another was 1.8 cm. So the scale which we will use is 0.46 cm,so by multiply the distance between the particles by the scales, we had the same result of the theoretically estimated distance. Figure 4-8 shows the suspended particles when L was 2/2 λ, 4/2 λ, and 5/2 λ, ±0.1 mm. Dr.Mast has levitated some nanoparticles as shown in figure 4-9, but here we want to levitate larger, 10 micron diameter ceramic particles coated with CNT growth catalyst because it is easier to levitate larger particles and analyze the CNT growth. We tried but could not levitate powders with the set up shown in figure 4-8, most likely due to electrostatic charging, air currents, but most importantly insufficient power to drive the transducer (were only able to get half power ~ 500 w instead of 1 kw) Figure 4-9 Levitate Nanoparticles, photo from D.Mast. 50

63 Chapter 5 CNTs growth on ceramic micro particles 5.1 CVD furnace We built the CVD growth furnace and measured the temperature of the furnace, specifically in the middle where the growth will take place. To measure the temperature inside the furnace, we drilled a hole from the top on the furnace and added a thermocouple through the middle of the furnace. Also, to observe the growth, we drilled a hole from the front in the middle of the furnace. We know that when we grow CNTs, the gas that comes out of the CVD reactor may contain unwanted CNTs, which might contaminate the lab. To remove these unwanted CNTs, we need to burn (high temperature oxidation to CO2) in a separate furnace tube filled with a molecular sieve, heated to 800 C in air. A block diagram of the CVD growth furnace system is shown in figure 51. This system was originally located in D. Shi s lab, 406 Rhodes Hall. Figure 5-1 Block diagram of the expected CVD growth furnace connection. 51

64 5.2 Catalyst preparation Sam Wagers made catalyst coated micro particle samples and grew CNTs during the summer First, he used 10-micron diameter Zeospheres (ZPs), which are a Si- Al mixed oxide ceramic, as a substrate for the catalyst. Next, he dispersed the ZPs in a Ferrocene/Hexane mixture. He then, evaporated the ZPs Ferrocene/Hexane mixture in the fume hood to produce the ZPs/Ferrocene powder. Finally, the ZPs/Ferrocene powder was placed on a Si chip, and placed in Blue CNT Growth Furnace (Nanoworld, UC) to grow CNTs. The images of CM- CNT growth, shown by arrows, are shown in Figures 5-2 and 5-3, taken by Dr. Mast using a Keyence microscope. Raman spectra, Figure 5-4, of the material in Figure 5-3, showed that these tubular materials are carbon nanotubes but of moderate quality, with G/D ratio of about 1.5. Figure 5-2 CNT growth on CM shown by arrows, 500x, photo from D. Mast. 52

65 Fiqure5-3 CNT growth on CM, shown by arrows, 1000x, photo from D. Mast. Figure 5-4 Raman spectra of CNT growth shown in Figure 5-3, from D. Mast. 53

66 Using Sam s procedure, two similar samples were made for additional CNT growth studies, and for future levitated CVD growth. For the first one, we used 10-micron diameter Zeospheres (ZPs) dispersed in Ferrocene Methanol/Hexane mixture and evaporated the ZPs Ferrocene mixture in a vacuum oven at 100 C for at least 6 hours; the samples were then placed in 750 C furnace for at least 8 hours in the air, to pre-treat the Ferrocene. The second sample is Ferrocene/iron oxide NP coated ceramic micro particles treated the same as the first sample. Table 5-1 gas flow rate for the growth of CNT, from UC Nanoworld. We used the same basic CVD method with the gas flow rate as shown in Table 5-1 to grow CNTs. After the growth, we used Raman spectroscopy and the Keyence microscope to exam the samples. Photos of the ZP ferrocene materials before and after CNT growth are shown in figure 5-5. Raman spectra, shown in figures 5-6 and 5-7, showed G/D peak ratios much less than one and small G peak, indicative of the presence of more disordered rather than pure CNT. 54

67 Figure 5-5 Catalyst coated micro particle samples before and after the growth, photo from D. Mast. Figure 5-6 Raman spectra of carbon material in the center dish in Figure 5-5, from D. Mast. 55