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SCHOOL OF MATERIALS AND MINERAL RESOURCES ENGINEERING UNIVERSITI SAINS MALAYSIA
FABRICATION OF STRETCHABLE ANTENNA By:
ZURAIN NURIDAYU BINTI ZAILAN Supervisor: Assoc. Prof Dr Zulkifli Bin Ahmad
Dissertation submitted in partial fulfillment
of the requirement for the degree of Bachelor of Engineering with Honors (Polymer Engineering)
UNIVERSITI SAINS MALAYSIA
JUNE 2016
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DECLARATION
I hereby declare that I conducted, completed the research work and written the dissertation entitled “Fabrication of Stretchable Antenna”. I also declare that is has not been previously submitted for the award of any degree or diploma or other similar title of this for any other examining body or University
Name of Student: Zurain Nuridayu Bt Zailan Signature:
Date:
Supervisor: Assoc. Prof Dr Zulkifli Bin Ahmad Signature:
Date:
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ACKNOWLEDGEMENTS
First and foremost, thank to Allah S.W.T. for strength given to carry out the entire task allocated for Final Year Project throughout a semester. Also for blessing me in completing my Final Year Project successfully and smoothly.
I would like to deliver my thanks to School of Materials & Mineral Resources Engineering of USM, which give me a lot of knowledge and equip me with skills and ability to finish my project. Special thanks to my supervisor, Prof. Assoc. Zulkifli Bin Ahmad for giving me a lot of help and supervision throughout this project. Without his help, it will not be possible for me to finish my work. Also not forgotten, Prof Fadzil Ain from School of Electric and Electronic, as co-supervisor which assisted me with electronic knowledges and design of the project.
Besides, I would like to dedicate my appreciation to the research group, Mr. Solihin, master student, Dr. Fahmin and Mr Ubaidullah , PhD student for their priceless support and guidance throughout the duration of this research project.
In addition, I am deeply grateful to all the staffs and technicians of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia especially Mr. Muhammad Sofi bin Jamil for their assistance and cooperation throughout the laboratory session. Finally, I would like to thank my family members and friends for all the support given. Thank you for being there when help and support is really much needed. Thank you again to everyone, either direct or indirectly involve in helping. Thanks
ZURAIN NURIDAYU BINTI ZAILAN June, 2016
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TABLE OF CONTENTS
page
DECLARATION i
ACKNOWLEDGEMENTS ii
TABLE OF CONTENTS iii
LIST OF TABLES vi
LIST OF FIGURES vii
LIST OF SCHEMES ix
LIST OF SYMBOLS x
LIST OF ABBREVIATIONS xi
ABSTRAK xii
ABSTRACT xiii
CHAPTER 1: INTRODUCTION 1
1.1 Introduction 1
1.2 Problem Statement 3
1.3 Research Objectives 4
CHAPTER 2: LITERATURE REVIEW 5
2.1 Antennae Design 5
2.2 Choice of Antenna Substrate 9
2.3 Silver Properties 11
CHAPTER 3: EXPERIMENTAL 13
3.1 Experimental Design 13
3.2 Materials 15
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3.2.1 Monomer 15
3.3 Preparation n of The PDMS Substrate 20
3.4 Preparation of the silver ink 21
3.5 Characterization 22
3.5.1 Fourier Transform Infrared Spectroscopy, FTIR 22
3.5.2 Stress strain Tensile Test 22
3.5.3 Differential Scanning Calometry, DSC 23
3.5.4 Dielectric Constant 23
3.5.5 Hardness Test 24
3.5.6 Capacitance Test 25
3.5.7 Four Point Probe test 25
3.5.8 Network Analyzer 26
3.5.9 Aging Testing 27
3.6 Apparatus 27
3.7 Equipment and Instruments 27
3.8 Simulation CST Studio Suite 28
CHAPTER 4: RESULT AND DISCUSSION 33
4.1 Reaction of PDMS susbtrate 33
4.2 Polymer characterization of PDMS substrate 34
4.2.1 Fourier Transform InfraRed (FTIR) 34
4.2.1.1 FTIR for Poly (dimetyhlsiloxane) hydroxyterminated (PDMS) 35 4.2.2 Stress strain Tensile Test of PDMS substrate 37
4.2.3 Hardness test 37
4.2.4 Aging Test 39
4.2.5 Differential Scanning Calometry , DSC 41
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4.3 Electronic Properties for Antenna 44
4.3.1 Resistivity test 44
4.3.2 Capacitance Test 48
4.3.3 Dielectric constant 50
4.3.4 Spectrum analyzer 50
CHAPTER 5: CONCLUSION 58
5.0 Introduction 58
5.1 Conclusion 58
5.2 Recommendations 59
5.3 Future works 59
REFERENCES 60
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LIST OF TABLES
page
Table 1: Chart of Common Frequency Bands 5
Table 2: Formulation of PDMS substrate 21
Table 3: Formulation for preparing the silver ink. 22
Table 4 : Description of the aging test 40
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LIST OF FIGURES
page
Figure 1: Microstrip or Patch Atennae 8
Figure 2: chemical structure of Poly (dimetyhlsiloxane) hydroxyterminated 10 Figure 3: schematic diagram in preparation and testing of antenna 14
Figure 4: Chemical structure of PDMS 16
Figure 5: chemical structure of (3-glycidyloxypropyl) trimethoxysilane 16
Figure 6: chemical structure of Dibutyltin dilaurate 17
Figure 7: Chemical structure of Triethoxyvinylsilane 17
Figure 8: Chemical structure and of 1,3 Divnyltetramethyl-disiloxane 18
Figure 9: Chemical structure of Polydimethylsiloxane 18
Figure 10: Chemical structure of Platinum-1,3-divnyl-1,1,3,3-Tetramethyl Disiloxane 19 Figure 11: Chemical structure of Silane Terminated Polydimethylsiloxane 19
Figure 12 : Instron Universal Machine 23
Figure 13: Hardness Tester shore A 25
Figure 14: Precision LCR Meter 26
Figure 15 : Spectrum Analyzer 27
Figure 16 : PMMA Jig used to assist the measurement 28
Figure 17: perpestive view of Design 1 30
Figure 18: Top view of Design 1 30
Figure 19: perspective view of Design 2 31
Figure 20: Top view of Design 2 31
Figure 21: Side view of Design 2 31
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Figure 22 : Back view of Design 2 32
Figure 23: Illustration of Design 1 33
Figure 24: illustration of Design 2 33
Figure 25: FTIR result for PDMS substrate 37
Figure 26 : Hardness value for substrate 39
Figure 27: Transition region in polymer 43
Figure 28: DSC result for PDMS susbtrate 44
Figure 29: Resistance of material when undergo stretched 46
Figure 30: Conductance value when undergo stretched 47
Figure 31: stretching of the molecular structure 48
Figure 32: capacitance result for substrate and silver ink 50
Figure 33: S-parameter for Design 1 53
Figure 34:Return loss, S11 against frequency changes of stretchable sample at zero stretched
54 Figure 35: Return loss, S11 against frequency changes of stretchable sample at 1mm 55 Figure 36: Return loss, S11 against frequency changes of stretchable sample at 2mm 56 Figure 37: Return loss, S11 against frequency changes of stretchable sample at 3mm 56
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LIST OF SCHEME
page
Scheme 1 : Reaction of PDMS substrate 34
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LIST OF SYMBOLS
% Percentage
˚ Degree
n Repeated unit n
Tg Glass transition temperature
Ώ Conductance unit, ohm
𝜌 Density
𝑅 Resistance
𝑡 thickness
𝐶
Conductancetan δ loss tangent
Cp Capacitance
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LIST OF ABBREVIATIONS
AC alternating current
RF radio-frequency
PDMS Polydimethylsiloxane
UV ultraviolet
OH hydroxyl
FTIR Fourier Transform Infrared Spectroscopy
DSC Differential Scanning Calometry
PMMA Poly(methyl methacrylate)
Hz Hertz
ASTM American Society for Testing and Materials
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FABRIKASI ANTENA YANG BOLEH DIREGANG
ABSTRAK
Fabrikasi antenna yang boleh diregang sebagai satu peranti elektronik ini menggunakan Polydimetyhlsiloxana (PDMS) sebagai substrat dan dakwat silver sebagai bahantara yang dicetak di atas substrat. Selain itu, kajian ini juga bertujuan menganalisis prestasi antena ini yang berfungsi sebagai menerima dan memancarkan isyarat dalam reka bentuk yang terbaik.
Dua reka bentuk yang telah dicadangkan dan setiap satu meberikan keputusan yang berbeza.
Rekabentuk 1 dan rekabentuk 2 berbeza daripada segi dakwat perak dicetakkan. Rekabentuk 2 telah memberikan keputusan yang sepadan dan mempunyai lebar jalur yang besar yang membuatkan rekabentuk 2 adalah sesuai sebagai antena. Frekuensi salunan adalah pada 5.17 GHz. Untuk frekuensi kecekapan bagi simulasi ialah daripada 5.12 GHz hingga 5.26 GHz.
Untuk experimen, Frekuensi salunan adalah pada bacaan 5.12 Hz dan untuk frekuensi kecekapan ialah daripada 4.4 GHz hingga 6.10 GHz. Antenna ini menunjukkan konduktiviti yang meningkat apabila diregang. Pada masa yang sama, sifat dan ciri-ciri silicon sebagai substrat dan perak dalam fabrikasi antenna yang boleh diregang ini dikenalpasti dengan mengunakan FTIR, DSC, Ujian kapasiti, dan pemalar dielektrik. FTIR menunjukkan ikatan OH sudah tidak wujud disebabkan ia telah lengkap proses pemeluwapan. Pemalar dielektrik bagi PDMS adalah daripada 2.3 hingga 2.9. PDMS juga menunjukkan nilai kapasiti yang rendah dan membuatkan PDMS sesuai sebagai subsrat.
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FABRICATION OF STRETCHABLE ANTENNA
ABSTRACT
Fabrication of stretchable antenna as an electronic device was used Polydimetyhlsiloxane (PDMS) as a substrate and silver ink acted as resonator which printed on the substrate. Other aim for this research is to analyze the performance of antenna working in transmitting and receiving the data in the best design. Two design have been proposed which giving different result for each of it. Design 1 and Design 2 different in the way of silver ink is printed. Design 1 giving no input matching signal while for Design 2 gave matching and wide bandwith result which make it suitable as an antenna. The resonant frequency is around 5.17 GHz. For efficiency frequency obtain from simulation is from 5.12 GHz to 5.26 GHz. for experimental, the resonant frequency is at 5.3 GHz and the efficiency frequency is from 4.4 GHz until 6.10 GHz. This antenna showing increase in conductivity when the length streched is increased.
At same time, the properties of silicone as substrate and silver in fabrication of strechable antenna is been characterized in polymer and electronic properties by using FTIR, DSC, Hardness Shore A, Capacitance test, dan Dielectric constant. FTIR result showing the OH bond that already not exist due to completely cured. Dielectric constant for PDMS is from 2.3 to 2.9. PDMS also shows low capacitance value which make it suitable to be a substrate.
15 CHAPTER 1
INTRODUCTION
1.1 Introduction
Also called an aeria, an antenna is a device (usually metallic) or specialized transducer or conductor that converts radio-frequency (RF) fields into alternating current (AC) or vice-versa. It can transmit, send and receive signals such as microwave, radio or satellite signals. There are two basic types of antenna which is the receiving antenna, which intercepts RF energy and delivers AC to electronic equipment, and the transmitting antenna, which is fed with AC from electronic equipment and generates an RF field. A high-gain antenna increases signal strength, while a low-gain antenna receives or transmits over a wide angle. Anything that has a radio function will need an antenna. These include systems such as radio broadcasting equipment, broadcast television equipment, radar systems, two-way radio of any type, communication receivers, cell phones, satellite communication receivers and devices such as garage door openers, wireless microphones, wireless computer networks, baby monitors,Bluetooth enabled devices (Ankur A., 2005).
There are several critical parameters affecting an antenna's performance that can be adjusted during the design process. These are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth. Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties. An antenna may be an isotropic radiator, a dipole, yagi-uda type, horn type or patch antenna.
Antenna is the transitional structure between free space and a guiding device. The guiding
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device may be a coaxial line or a waveguide and it is used to transport electromagnetic energy from the transmitting source to the antenna or from antenna to the receiver (Ankur A. 2005).
Nowaday, some design is popularly made which is flexible and strechable. It give many benefit such as roll,stretch, twisted, compatible with human skin, wearable system and flexible. This is because wearable systems can be subject to a variety of stresses as human move around. New antenna design has proven its ability to withstand the bending and stretching that garments endure, while steadily communicating via Wi-Fi. For added restorative force it should not permanently bent out of shape the antenna by building top of a polymer layer called substrate such as silicone. On top of the silicone substrate, the conductive ink is embedded in the desired pattern as a conductor. To let the conductor and polymer substrate adhere together, one type of binder from silicone will be introduced.
Stretchable electronics is a technology that builds electronic circuits on top of a stretchable substrate or by embedding them in a stretchable matrix. For application as stretchable conductors, it is critical to render nanomaterials highly stretchable and conductive.
For stretchability, a straightforward method is to deposit the nanomaterials on top of or to embed them inside elastomeric materials to form composites. Polydimethylsiloxane (PDMS) is a widely used elastomeric material. In order to enhance the adhesion between nanomaterials and a PDMS substrate, surface treatment by oxygen plasma, ultraviolet (UV) light, or chemicals has been used to modify the naturally hydrophobic PDMS surface with hydrophilic functionalities.
According to market analysis, the revenue of flexible electronics is estimated to be 30 billion USD in 2017 and over 300 billion USD in 2028. Their light weight, low-cost
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manufacturing, ease of fabrication, and the availability of inexpensive flexible substrates such as paper or textile make flexible electronics an appealing candidate for the next generation of consumer electronics. Moreover, recent developments in miniaturized and flexible energy storage and self-powered wireless components can be best choice for to commercialize such systems.
Nowadays, flexible and stretchable electronic systems require the invention of an antennas operating in specific frequency bands to provide wireless connectivity which is highly demanded by today’s information application. The efficiency of these systems primarily depends on the characteristics of the integrated antenna. The nature of flexible and stretchable wireless technologies requires the integration of flexible, stretchable, light weight, compact, and low profile antennas. At the same time, these antennas should be mechanically robust, efficient with a reasonably wide bandwidth and desirable radiation characteristics.
1.2 Problem statement
These days, most electronic circuitry comes in the form of rigid chips. Also comercial antenna cannot be incorporate into wearable system to transmit data from sensors to receiver.
In recent year, there a great deal of interest from both academic and industry in the field of stretch able electronics.
Flexible properties also have been applied onto antenna to make it more easier to bend it. But, with only flexible caharacteristic would not make the antenna in high performance. So nowaday, stretchable antenna is introduced to make it wearable system which is at high demand for medical devices. If bendable is good, strechable is even better, especially for
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high-performance conformable circuit of the sort needed for so-called smart clothes or body armor.
1.3 Research Objectives
1. To fabricate a stretchable antenna using silicone as a substrate and silver ink as a resonator.
2. To analyze the performance of antenna working in transmitting and receiving the data in the best design
3. To study the properties of silicone as substrate in fabrication of strechable antenna
19 CHAPTER 2
LITERATURE REVIEW
2.1 Antennae Design
Nowadays, stretchable electronic devices is becoming popular due to the advances application that needed the device to be more flexible and stretchable. Antennae is one of the electronic device which functioning as transmitting and receive data. An antenna is a device to transmit and/or receive electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned (matched) to the same frequency band as the radio system to which it is connected, otherwise reception and/or transmission will be impaired. It having huge way of application. This research purposedly producing strechable antenna which will be one of the strechable electronic devices. Table 1 shows the common frequency bands used commercially.
Table 1: Chart of Common Frequency Bands (www.antenna-theory.com) Frequency Band
Name Frequency Range Wavelength
(Meters) Application Extremely Low
Frequency (ELF) 3-30 Hz 10,000-100,000 km Underwater
Communication Super Low
Frequency (SLF) 30-300 Hz 1,000-10,000 km
AC Power (though not a transmitted
wave) Ultra Low Frequency
(ULF) 300-3000 Hz 100-1,000 km
Very Low Frequency
(VLF) 3-30 kHz 10-100 km Navigational
Beacons
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Low Frequency (LF) 30-300 kHz 1-10 km AM Radio
Medium Frequency
(MF) 300-3000 kHz 100-1,000 m Aviation and AM
Radio
High Frequency (HF) 3-30 MHz 10-100 m Shortwave Radio
Very High Frequency
(VHF) 30-300 MHz 1-10 m FM Radio
Ultra High
Frequency (UHF) 300-3000 MHz 10-100 cm Television, Mobile Phones, GPS Super High
Frequency (SHF) 3-30 GHz 1-10 cm
Satellite Links, Wireless Communication Extremely High
Frequency (EHF) 30-300 GHz 1-10 mm Astronomy, Remote
Sensing Visible Spectrum 400-790 THz
(4*10^14-7.9*10^14)
380-750 nm
(nanometers) Human Eye
Today’s wearable healthcare tools are complex systems, based on advanced electronics. From a user point of view, the device should preferably be comfortable and unnoticeable. An interesting approach to achieve better wearability is by transforming the flat rigid device into a flexible or stretchable electronics device that can better follow the shape of the human body. Flexible and stretchable electronic device the irregularities of the human body. For increased user comfort, it would however be preferred that the electronics device can be deformed in more than one direction simultaneously. The device would then need to be conformable or stretchable. In the past years, a technology has been developed which enables the realization of stretchable systems from flexible foils (Jeroen V., 2015).
In industry, fabrication and component assembly are done on flat rigid or ultimately flexible subsrates. It is explained that the use of flat rigid assembled will become problematic when for a given application a circuit has to be implement on a nonflat surface. Very often,
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wearable and implantable circuits for biomedical applications, sports and leisure, safety, require nonflat assemblies, because preferably the circuit must follow the irregular shapes of the body parts, garments, or other curvilinear surfaces onto or into which the circuit is integrated. One option to achieve this degree of comfort is make it strechable and flexible to make sure the devices is wearable system with human (Takao S., 2012).
A high degree of mechanical compliance, good response to bending, compressing, and tensile strain, are all desired to impact a wider array of applications. Flexible materials suffer from the inability to either expand or contract, so the electronic device cannot move freely with moving parts, e.g. attachment to the movable human body parts. Thus, this lack of function underscores the need for stretchable electronics. Even though significant advances have been made in producing flexible electronic devices, less work has been devoted to producing stretchable electronic devices, i.e. response to tensile strain. Stretchability, or preferably reversible stretchability (or elasticity), without affecting its resulting electronic functionality is highly desirable. Hence, the advent of stretchable electronics would allow for highly portable, biocompatible devices that could function while being adhered to complex surfaces (Stephanie J., 2013)
There fews fundamental antennae types can be figured out. The most simplest antennae is called Short Dipole Antennae. Antennae can be devided into fews type which is Wire Antennae, Log-Periodic Antennae, Aperture Antennae, Travelling Wave Antennae, Reflector Antennae, and Microstrip Antennae. For this research, the antenna is included in Microstrip Antenna. Microstrip antenna actually known as patch antenna. This type of antenna consist of ground plate and metal flat surface. With a conducting ground plane, a microstrip patch antenna can be designed to operate in the vicinity of metal. These advantages make them preferable for numerous military applications and with compact design they are
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also used in personal mobile communication. Figure 1 below, illustrated Microstrip or Patch Antenna.
Figure 1: Microstrip or Patch Atennae
This PDMS antenna have two parts. One part is substrate and the other one is conducting line. PDMS can be either substrate or resonator. But due to the low dielectic constant of PDMS, it mostly suitable to be a substrate rather than a resonator. The silver will act as resonator to transmit and receive data. Resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal.
For resonator materials, the dielectric constant for a resonator must be between 8 until over 100.
2.2 Choice of Antenna Substrate
Microstrip transmission line substrate
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To comply with flexible technologies, integrated components need to be highly flexible, strechable and importanyly mechanically robust, that material also have to exhibit high tolerance levels in terms of bending and stretching repeatability and thermal endurance due to it application which need to fold and stretch repeatly. Before this, paper is one of the example of substrate for antenna. However, paper based substrates are found to be not good and robust enough and introduce discontinuities when used in applications that require high levels of bending and rolling. Moreover, they have a relatively high loss factor (loss tangent (tan δ) is around 0.07 at 2.45 GHz) which compromises the antenna’s efficiency.
Poly (dimetyhlsiloxane) hydroxyterminated (PDMS) is a main material used in making of the antennae. Besides that, the conductor lining is printed onto the substrate as conducting line which used silver ink. PDMS have colorless and transparent form which can be used in many way of application. It has low Tg which is around -125˚C to -135˚C. This will make the PDMS in liquid form at room temperature. It become solidify if the liquid is been crosslink by crosslinker. The main properties of PDMS which make it suitable to be a main material of stretchable antennae is it can be deformed enough such that conformable contact can even be achieved on surfaces that are non-planar on a micrometer scale.
PDMS is the most widely used silicon-based organic polymer since it is optically clear and generally inert, non-toxic, and non-flammable. In addition, it has low cost, ease of fabrication, and biocompatibility, thus it is widely utilized in various microfluidic applications (Ich L. N.,2016).
Figure 2: chemical structure of Poly (dimetyhlsiloxane) hydroxyterminated
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Flexible electronics, the ability to bend, are desirable due to the potential roll-to-roll printing that can be employed, including inkjet printing, slot-dye coating, gravure printing, and patterning of device components. Implementation of roll-to-roll processing in large scale fabrication requires the substrates to be flexible. Most polymeric materials are intrinsically more flexible than inorganic crystalline materials. Flexible electronic devices made from organic materials are largely manufactured by deposition of materials onto flexible plastic substrates, such as polyimide or polyethylene terephthalate (PET) (Stephanie J., 2013).
Polysiloxane have found a widespread use in science and technology due to their unique physical properties, such as their thermal stability, high transparency, high UV stability, their hydrophobic character, their dielectric properties, and their chemical inertness.
Furthermore, their biological inertness make them an ideal polymers for medical and cosmetic application. Many of the properties of polysiloxane can be deduced from their unique structure, comprising an inorganic backbone of silicon and oxygen and organic group that are covalently linked to the silicone atom. The character of the inorganic backbone delivers high flexibility of the chain and therefore low Tg and elastomeric behaviour while the organic substituent induce additional properties.
In recent years new polysiloxane materials are under development containing nanoscale entities, so called nanocomposite materials that expand the well-known properties of polysiloxane into new fields. Flexibility and stretchability can endow electronics with incomparable and fascinating features to promote the development of a new generation of products in the future. Flexibility and stretchability are required performances for new generation wearable electronics and devices (Jing Y., 2015).
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Besides that, fume silica is one of the ingredient in making of the substrate. This silica acting as reinforcing agent for the substrate which will increasing the mechanical properties.
Silica particles are abundantly used as reinforcing agent for silicone elastomers, most prominently poly(dimethyl siloxane), PDMS, leading to advanced materials that are applicable even in high-precision applications such as microcontact printing. The beneficial action of precipitated or, more popularly, fumed silica on PDMS is based upon the existence of strong multiple hydrogen bonds between surface–hydroxyl (OH) groups of the silica and the PDMS main chain (Anca S., 2009).
2.3 Silver Properties
Silver lining on the substrate act as conducting line. Conductivity refers to the ability of a material to transmit energy. This line will connecting the terminal for analyzing the data received and transmitted. Silver is the best conductor of heat and electricity. Silver also has the highest thermal conductivity of any element and the highest light reflectance. Silver is used due to it flexibility, strength and conductivity. Other examples of conductive elements including graphite, carbon black, carbon fibers, and ceramic or metal particles. Silver is the best conductor because of its electrons are more free to move than those of the other elements.
This has to do with its valence and crystal structure. Most metals conduct electricity. Other elements with high electrical conductivity, are aluminum, zinc, nickel, iron and platinum.
Brass and bronze are electrically conductive alloys, rather than elements.
Silver is been used as as the printed antenna of the cell. An antenna pattern of the silver is printed on top of the apprioprate substrate. The lower the resistivity (inverse of conductivity) the better the performance. It is desirable to use a composition that has low restivity and is suitable for fabrication. (Dorfman J. R., 2010)
26 CHAPTER 3
EXPERIMENTAL
27 3.1 Experimental Design
This chapter will discuss on preparation of stretchable substrate made up of Polysiloxane as a main ingredient and preparation of silver ink as conducting path on the prepared substrate. The main purpose of this research is to study how the substrate and the conducting line will perform as an antenna. The preparation of the antenna is separated into two parts which the first part is preparing the substrate and the other part is preparing silver ink. Then the silver part will be put on the substrate and cured at certain temperature. After been cured, the sample will undergo experimental testing using Spectrum Analyzer tfor getting the result of S-Parameter graph. The result from experimental must be equivalent with the result obtain from S-Parameter graph of CST Studio Simulation. For all cases, the solvent and reagent has been utilized carefully. The summarizing of the experimental design can be seen in Figure 3.
PREPARATION OF SUBSTRATES
PREPARATION OF SILVER INK
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CHARACTERIZATION TESTING
CASTING PROCESS (80˚C- 100˚C)
ELECTRONIC PROPERTIES
CONDUCTIVITY
RESISTIVITY
DIELECTRIC CONSTANT
CAPACITANCE TEST
SPECTRUM ANALYZER
POLYMERIC PROPERTIES
CHEMICAL STRUCTURE
FTIR
MECHANICAL PROPERTIES
HARDNESS
HARDNESS
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Figure 3: schematic diagram in preparation and testing of antenna 3.2 Materials
This section will discuss more about monomer and solvent used during experiment. In order to obtain satisfactory results, the solvent and monomer need to be care more to avoid any contaminations such as oxygen. Although it important, the standard operation procedure must be followed to avoid any accident. Some of chemical may lead to injury, harm, damage or loss.
3.2.1 Monomer
Poly (dimetyhlsiloxane) hydroxyterminated, dibutyltin dilaurate, fume silica, Silane Terminated Polydimethylsiloxane, Polydimethylsiloxane, Silver Powder, Platinum-1,3- divnyl-1,1,3,3-Tetramethyl Disiloxane
THERMAL PROPERTIES
DSC
AGING TEST
STRETCHABILITY
TENSILE TEST
30 3.2.1.1 Poly (dimetyhlsiloxane) hydroxyterminated
Poly (dimetyhlsiloxane) hydroxyterminated was purchased from Sigma-Aldrich Sdn Bhd. It was used as main material to produce a stretchable substrate. The chemical structure of PDMS as shown below.
Figure 4: Chemical structure of PDMS
3.2.1.2 (3-glycidyloxypropyl) trimethoxysilane
(3-glycidyloxypropyl) trimethoxysilane was purchased from Sigma-Aldrich Sdn Bhd.
It was used as adhesives between silver ink and substrate. The chemical structure of (3- glycidyloxypropyl) trimethoxysilane as shown below.
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Figure 5: chemical structure of (3-glycidyloxypropyl) trimethoxysilane
3.2.1.3 Dibutyltin dilaurate
Dibutyltin dilaurate was purchased from Sigma-Aldrich Sdn Bhd. It was used as catalyst. The chemical structure of Dibutyltin dilaurate as shown below.
Figure 6: chemical structure of Dibutyltin dilaurate 3.2.1.4 Fume Silica
Fume Silica was purchased from Sigma-Aldrich Sdn Bhd. It was used as reinforcing agent.
3.2.1.5 Triethoxyvinylsilane (VTEOS)
Triethoxyvinylsilane (VTEOS) was purchased from Sigma-Aldrich Sdn Bhd. The chemical structure Triethoxyvinylsilane of as shown below.
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Figure 7: Chemical structure of Triethoxyvinylsilane
3.2.1.6 Divnyltetramethyl-disiloxane
1,3 Divnyltetramethyl-disiloxane was purchased from Sigma-Aldrich Sdn Bhd. The chemical structure 1,3 Divnyltetramethyl-disiloxane of as shown in below.
Figure 8: Chemical structure and of 1,3 Divnyltetramethyl-disiloxane
3.2.1.7 Polydimethylsiloxane
Polydimethylsiloxane was purchased from Sigma-Aldrich Sdn Bhd. The chemical structure Polydimethylsiloxane of as shown below.
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Figure 9: Chemical structure of Polydimethylsiloxane
3.2.1.8 Silver powder
Silver powder was purchased from Sigma-Aldrich Sdn Bhd and sponsored by Jabil Circuit. It act as conducting ink which will printed onto the substrate.
3.2.1.9 Platinum-1,3-divnyl-1,1,3,3-Tetramethyl Disiloxane
Platinum-1,3-divnyl-1,1,3,3-Tetramethyl Disiloxane was purchased from Sigma- Aldrich Sdn Bhd. The chemical structure of Platinum-1,3-divnyl-1,1,3,3-Tetramethyl Disiloxane as shown below.
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Figure 10: Chemical structure of Platinum-1,3-divnyl-1,1,3,3-Tetramethyl Disiloxane
3.2.1.10 Silane Terminated Polydimethylsiloxane
Silane Terminated Polydimethylsiloxane was synthesized in the lab. The chemical structure of Silane Terminated Polydimethylsiloxane as shown below.
Figure 11: Chemical structure of Silane Terminated Polydimethylsiloxane
Notes: Platinum-1,3-divnyl-1,1,3,3-Tetramethyl Disiloxane are considered to be primary skin irritant and should be handled with great care. All contact with these substances is therefore, to be avoided, if necessary, wash off with plenty of water. Wear laboratory coat, and rubber gloves.
3.3 Preparation of The PDMS Substrate
Weigh the PDMS and fume silica( 4% of PDMS) in beaker. Then, pour toluene (twice weight of PDMS) in the beaker which contained of PDMS and fume silica. Stir it manually using glass rod. Stir it until become homogeneous. To put dibutyltin dilaurate catalyst, wait until the bubbles disapeared or use degasing machine. Stir it for awhile then pour it into the mold. Let toluene and polymer cure at 80-100˚C. With chart below, it will briefly explain the flow of the preparation and Table 2 is formulation used for preparing the substrate.
Stir until homogeneous Put Toluene
Weigh PDMS and Fume Silica
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Table 2: Formulation of PDMS substrate
3.4 Preparation of the silver ink
Weighed the PDMS-OH and SiH-PDMS been weighed in the beaker. It stirred for a while. After that,. Then, Polydimetylsiloxane is mixed with the other and stirred it until homogeneous is achieved. For catalyst, Platinum-1,3-divnyl-1,1,3,3-Tetramethyl Disiloxane is added for agent curing. It will be exposed to heat at 80˚C around 15 minutes until cured.
Reagent Amount
PDMS-OH 7g
Fume Silica (4%) 14 ml
Toluene
Dibutyltin Dilaurate 50 μl
36
This part also recommended to degassing after putting the catalysts. Chart below briefly explain the flow of the procedure and the formulation is stated in Table 3.
Table 3: Formulation for preparing the silver ink.
3.5 Characterization
Reagent Amount
Poly (dimetyhlsiloxane) hydroxyterminated 0.1 g Silane Terminated Polydimethylsiloxane 0.3g
Polydimetylsiloxane 400μl
microsilver particles 3g
Platinum-1,3-divnyl-1,1,3,3-Tetramethyl Disiloxane 1μl Polydimetylsiloxane
And silver
Degassing process PDMS-OH and SiH-
PDMS
Platinum-1,3-divnyl- 1,1,3,3-Tetramethyl Disiloxane and Diutyltin
Dilaurate Curing process at 80˚C in 15 minutes Print on the
substrate
37
3.5.1 Fourier Transform Infrared Spectroscopy, FTIR
The FTIR was performed in order to help characterize the presences of functional groups before the structure have been confirmed. FTIR was conducted by using a Spectrum GX Perkin Elmer Model according to ASTM-E1252. Standard Practice For General Techniques for obtaining Infrared Spectra for Qualitative Analysis at a wave number 4000- 450 cm-1. The FTIR analysis was performed on Polysiloxane and silver ink. The measurements were done by scanning four times before the spectrums were confirmed.
3.5.2 Stress strain Tensile Test
Figure 12 : Instron Universal Machine
This testing was performed using universal Instrument in accordance to ASTM D882:
Standard Test Method for Tensile Properties of Thin Plastic Sheeting. The samples were prepared in strip shape according to the ASTM D882. The speed used is 5mm/min.
38 3.5.3 Differential Scanning Calometry, DSC
DSC was performed to measure the glass transition temperature (Tg) of the Polysiloxane. DSC was conducted by using DSC-6 analyser (Perkin Elmer) in accordance with ASTM D3418 at heating rate 10˚C/min in N2. For first cycle, the sample was heated from -130˚C to 80˚C, and stop at 80˚C 1 minutes. The first cycle was intended to eliminate thermal history in the samples. Then, the samples were allowed to cool from 80˚C to -130˚C with heating rate 10˚C/min.
3.5.4 Dielectric Constant
Dielectric constant was performed to study the permitvity of the material expressed the tendency of the material to be polarized when subjected to an applied field. The dielectric constant was measured by using RF Impedance analyser HP 4218 with the dielectric material test fixture probe HP 1653A in accordance to ASTM D150: Standard Test Methods for AC Loss Characteristics and Permitivity (Dielectric Constant) of Solid Electrical Insulation. The frequency used was from 1MHz to 1GHz.
3.5.5 Hardness Test
39
Figure 13: Hardness Tester shore A
Hardness test was performed to identify the limit of Polysiloxane material receiving the penetration of specific Indenter corresponding to specific force and time. The hardness test was conducted by using Techlock Durometer GS-702G type D according to ASTM D2240. The sample was placed on flat surface with the Durometer on top of it. Then vertically the Durometer was pressed with constant load until reach a maximum and hold for 1 second. The measurement was performed at 5 point on sample with environment at ambient temperature.
3.5.6 Capacitance Test
40
Figure 14: Precision LCR Meter
This capacitance value was measured by using Mprobe UV-Vis Thin Film Measurement System. The frequency used was from 20 to 1MHz. the value given known as Capacitance, Cp in unit, F.
3.5.7 Four Point Probe test
This testing was conducted at Jabil Circuit company. The instrument used is JANDEL RM3000 Resistivity Meter. The sample was put under the probe which measured the resistivity value. the value obtained also can be converted into conductivity value by Equation 1 and Equation 2 below.
𝜌 = 𝑅 × 𝑡 Equation 1
𝐶 =
𝜌1 Equation 23.5.8 Network Analyzer
41
Figure 15 : Spectrum Analyzer
This testing was performed using PNA-X Spectrum Analyzer. A spectrum analyzer is an instrument that measures the network parameters of electrical networks. This instrument will give reading of frequency of the materials. Spectrum analyzers are an invaluable item of electronic test equipment used in the design, test and maintenance of radio frequency circuitry and equipment. a spectrum analyser will display the amplitude of signals on the vertical scale, and the frequency of the signals on the horizontal scale. Spectrum analysers it is possible to make measurements of the bandwidth of signals can be checked to discover whether they fall within the required mask. Another way of using a spectrum analyzer is in checking and testing the response of filters and networks. The range frequency been set up for this project is 10 MHz to 10 GHz. To analyze how the PDMS antenna work at functioning at what frequency. During testing, jig is used for assist in stretching the antenna into desired length.
Figure 16 below, is the jig have been used during testing. It was made up by PMMA.
42
Figure 16 : PMMA Jig used to assist the measurement 3.5.9 Aging test
This testing been conducted at two temperature which is 80˚C and 100˚C. For 80˚C, it have been conducted for 30 day while for 100˚C is conducted for 2 days only. The samples have been exposed to heat in the oven for it set time. This testing is for observed the performance of the sample during it service.
3.6 Apparatus
Magnetic bar, 10ml measuring cylinder, 50ml beaker, glass rod 3.7 Equipment and Instruments
Magnetic stirrer, mass balance, vacuum pump, oven, JANDEL RM3000 Resistivity Meter, INSTRON Universal Instrument, AGILENT Spectrum Analyzer, Differential Scanning Calorimeter, FTIR Spectroscopy, Durometer Shore A, PMMA Jig.
43 3.8 Simulation CST Studio Suite
The electromagnetic simulation software CST STUDIO SUITE is the most accurate and efficient computational solutions for electromagnetic designs can be found nowadays. It consist CST’s tools for the design and optimization of devices operating in a wide range of frequencies. It can analyse thermal and mechanical effects, as well as circuit simulation.
CST STUDIO SUITE can offer considerable product to market advantages such as shorter development cycles, virtual prototyping before physical trials, and optimization instead of experimentation. In this software, it offer many option for different workflow and application area such as static frequency, MW & RF &Optical, EDA/Electronics, and EMI/EMC.
For this project, MW & RF &Optical option is choosed because one of the workflow is antennas. As we know, antenna having many types such as waveguide, wire, reflector and planar. This project design is a planar antenna. This project have been proposed two different designs which named as Design 1 and Design 2. Figure 17 and 18 is view for Design 1 and Figure 19, 20,21, and 22 for Design 2.
44
Figure 17: perpestive view of Design 1
Figure 18: Top view of Design 1
45
Figure 19: perspective view of Design 2
Figure 20: Top view of Design 2
Figure 21: Side view of Design 2
46
Figure 22 : Back view of Design 2
From this software, the design will undergo simulation for which giving many information. One of it is S-parameter. This S-parameter has a lot of information such as how the antenna reacted at certain frequency. This information is very important to know how the antennas that have been designed is working at optimum condition. This S-parameter can analyze the design which operates at high frequency.
In this projet, the design will under 4 conditions which is zero stretched, 1mm stretched, 2mm stretched and 3 mm stretched. The result must be at same range of frequency to show that the antenna is a good antenna. If not, it showed that the design was not a good design as an antennas.
47
Figure 23: Illustration of Design 1
Figure 24 : Illustration of Design 2 Silver patch &
silver lining
Silver as ground plane
PDMS Substrate PDMS
substrate
Copper ground plane
Silver lining 3 cm
2.4 cm
2.4 cm
0.8 cm 1.5 cm 3 cm
48 CHAPTER 4
RESULT AND DISCUSSION
4.0 Introduction
This chapter is focused on the fabrication of the stretchable antenna and it properties.
The first section it describes the scheme of the PDMS substrate.The second section describes the characteristic of the antenna sample produced which include the thermal properties, and mechanical properties. For final part it emphasizes on the electronic properties such as dielectric constant, resistivity, conductivity, and communication performance.
4.1 Reaction of PDMS susbtrate
The reaction involved in preparation of the PDMS substarte. The monomer is undergo self condensation by the elimination of water. This process of dehydration leads to the formation of hydroxyl-terminated poly(dimethylsiloxane). The reaction illustrated in the Scheme 1 below.
49
Scheme 1 : Reaction of PDMS substrate
4.2 Polymer characterization of PDMS substrate
4.2.1 Fourier Transform InfraRed (FTIR)
Fourier Transform InfraRed (FTIR) spectroscopy uses a Fourier Transform to convert raw data produced by the spectrometer in to a spectrum which is generally a plot of the
H
+, Heat
50
absorbance or % transmittance of the sample versus the wave number. FTIR spectroscopy has proven to be a versatile tool in analytical chemistry for quantitative and qualitative assessment of known and unknown chemical species. Many studies on the applications of FTIR spectroscopy have been reported in the literature.
4.2.1.1 FTIR for Poly (dimetyhlsiloxane) hydroxyterminated (PDMS)
Figure 25 showed that the absorbance occured at 2970 cm -2 at the early of the graph.
Theoretically, in the range of 3000 – 2950 cm-1 region, the chemical structure of the polymer which is CH2 is in strecthing modes. Due to the side group of CH2 is situated at every Si element, absorbance occured at this range.
It also showed that the absorbance occured at 1258 cm-1. It means that the CH3 is in stretching modes. CH3 is in bending position when the frequency in range of 1260 cm -1 is supplied. Other than that, it showed the absorbance is occured at 1011 cm -1. It happen when the chemical structure is containing Si-O-Si bond. In this range of frequency, the Si-O-Si bond is strecthing and bending modes.
Based on figure below, the absorbance also occured at 864 cm-1. It means the C-Si-C bond is in strecthing and bending modes. Those peaks that been mentioned above indicates that the chemical structure is similar to PDMS chemical structure which containing C-Si-C bond, Si-O-Si bond, CH3, and CH2. (Fengxiao G., 2012)
The most important thing is there no more peak at 3020 cm -1 in the result below. It showed that, the PDMS substrate is fully cured. Peak at 3020 cm -1 is an absorbance and strectching of HO-OH linkages. When PDMS already cured, the linkages would not found in the structure.
51
Figure 25: FTIR result for PDMS substrate
trial 3 trial 1 trial 2
Name Description
3500 3000 2500 2000 1500 1000 580
cm-1
247
-11 20 40 60 100 120 140 180 200
%T
212
8 20 40 60 80 100 120 140 160 180
%T
222
12 40 60 80 100 120 140 160 180
%T
787.91cm-1 1011.62cm-1
1258.19cm-1
695.30cm-1 864.83cm-1
2963.03cm-1
599.20cm-1 1539.00cm-1
585.00cm-1 1209.00cm-1
607.00cm-1
929.00cm-1
724.00cm-1 845.00cm-1
787.92cm-1 1011.60cm-1
1258.22cm-1
696.31cm-1 864.68cm-1
2962.91cm-1
599.16cm-1 1652.00cm-1
1217.00cm-1
585.00cm-1
923.00cm-1
607.00cm-1
724.00cm-1 846.00cm-1
787.89cm-1 1011.59cm-1
1258.21cm-1 695.12cm-1
864.85cm-1 2962.95cm-1
599.21cm-1
1546.00cm-1 1217.00cm-1 928.00cm-1 585.00cm-1
613.00cm-1
724.00cm-1 846.00cm-1
52 4.2.2 Stress strain Tensile Test of PDMS substrate
Figure 26: Graph Stress Strain Curve
The tensile strength of a material is the maximum amount of tensile stress that it can be subjected to before failure. This testing was conducted on PDMS substrate samples to investigate the tensile modulus of the samples. Based on literature, the pure PDMS gave tensile modulus at 1.65 MPa. (Wu C. L., 2009) for this experiment, the average tensile modulus is 5.044 MPa and elongation at break at 244.5%. This due to the existence of fume silica as a filler. Filler gave increasing in properties of the substrate which initially only 1.65 MPa in modulus. This graph showing higher in strain but low in stress. This is due to the chain than easily move when the stress is applied. As we know, the PDMS is having Si-O-Si linkages which induce high flexibility to the chain.
4.2.3 Hardness test
Hardness is measured by using durometer Shore A GS-706G model. The sample form was then tested by using this durometer Shore A GS-706G. The basic test requires applying the force in a consistent manner, to affect depth of the indentation. The maximum hardness was noted down and pressure foot during measuring must be parallel to the
-0.0005 0 0.0005 0.001 0.0015 0.002 0.0025
0 20 40 60 80 100 120 140
stress (MPa)
strain (%)
stress strain curve
53
measuring surface. That durometer was pressed at constant load until reaching a maximum and hold for 1 second. The measurement was performed at 5 point on sample at ambient environment. The sample was measured at difference length of stretched which is zero stretched, 1 cm, and 2cm.
Figure 26 : Hardness value for substrate
Figure 26 showed the value obtained by each length after been subjected to stretching. Hardness is a suface measurement about the ability of the materials to withstand force that subjected to it.
Based on the result obtained, zero stretched showed hardness at Shore A 57.4 which is higher compared with those been stretched. While for 1 cm stretched, the result is 56.2 hardness of shore A and 55.6 hardness of shore A for 2 cm stretched. PDMS have Si-O-Si
53.5 54 54.5 55 55.5 56 56.5 57 57.5 58 58.5
0 1 2
hardness Shore A
length stretched (cm)
hardness
54
linkages which makes it flexible. With this linkages, sample become more flexible and induce lower value of the hardness.
Based on measurement taken at 5 points per sample, the error is significant. The durometer showed error to be happen. In order to get a better reading, should optimise the performance of the shore (durometer) itself. The most important is should keep the durometer at non-corrosive environment (Dry place, lowest humidity level) due to durometer material is most crucial in the spring, if the spring is rusty, it will give effect to the resiliency of the spring, which means the result of the experiment also will not be precise as desire. The other way to maximize the durometer performance is by using it carefully by not pushing the durometer towards the specimen too hard and follow the instruction given.
4.2.4 Aging Test
Oven aging is used to undergo the aging testing. It functioning as accelerator to the aging process by giving the real exposure of the product during it service. It is slow and irreversible alteration of a material chemical or physical structure. This alteration has normally a permanent effect on the material properties. It could leads to gradual loss of the design function. The samples are placed in an oven at 80˚C over 1 month. Table 4 below show the performance of the conductive ink for every reading taken.
Table 4 : Description of the aging test
55
Date Description
(1st day) conductivity is good and gave bright light when used LED tester eventhought stretched or no
stretched.
(1 week) conductivity is still the same and gave bright light.
(2nd week) conductivity is still the same and gave bright light.
(3rd week) conductivity is still the same and gave bright light.
(1 month) conductivity made some changed at zero stretched which the brightness was decreased slighty. But when the sample stretched, it giving light brigth as
usual.
Table 4 shows aging over period of 1 month was not much. So, the samples is exposed to 100˚C for two days only. At this condition, the samples giving no more light at zero and at certain stretched. It shown that, the PDMS substrate is significantly effected at 100˚C for only short exposure.
Based on the result obtained, it showed the deterioration of conductivity performance on because of the silver lining which act as conducting line. Lewicki J. P. Et. al., (2009) said that the Polysiloxanes is having high thermal stability and low electrical conductivity due to it low dielectric constant. Further, the silver lining displayed deterioration of conductivity during aging. It might be because of the thermal oxidation of the PDMS substrate and Ag filler inducing the whole packaging brittle. So when it undergo high temperature, the structure becoming more brittle and easily stretching thus disconnecting to each other anymore.
Another reason could be the density between silver large. the silver ink is the formulation containing PDMS might settled down during ageing.
56 4.2.5 Differential Scanning Calometry , DSC
Differential Scanning Calometry (DSC) was performed to measure the glass transition temperature (Tg) of the Polysiloxane. Full result as in Figure 26 is obtained and has been conducted in temperature range of -130˚C until 80 ˚C with heating rate 10˚C/min.
Figure 28 below, showing 3 phase. Due to the PDMS is an amoprhous polymer, PDMS will only have glass transition temperature, Tg.. Amorphous polymers are viscous liquids when they are held at temperatures above their glass transition temperature, Tg. Below Tg, the material is solid, yet has no long range molecular order. As shown in heating phase, the graph showing sudden drop of mW reading at -12.33˚C.
For pure PDMS, the Tg is around -130˚C to -120˚C. It is in the rubber state at room temperature because it has a glass transition temperature of less than −120 °C. (Wu C. L., 2009) A polymer with a backbone that exhibits higher flexibility will have a lower Tg. As mentioned befor at FTIR result, the PDMS having Si-O-Si linkages which make the backbone become much flexible. This is because the activation energy for conformational changes is lower. Therefore, conformational changes can take place at lower temperatures. From experimental, the result showing that sudden drop at -12.33˚C which is much more higher than the literature. But it showing almost same trend in graph as Figure 27 below. It due to the other material such as silica as a filler. Cross linking reduces chain mobility, so Tg will be increased. It also affects the macroscopic viscosity of the polymer, since if there are crosslinks between the chains, then they are fixed relative to each other, so will not be able to slide past each other.
57
Figure 27: Transition region in polymer
58
Figure 28: DSC result for PDMS susbtrate
59 4.3 Electronic Properties for Antenna
4.3.1 Resistivity test
Resistivity is electrical resistance of a conductor of unit cross-sectional area and unit length. It show how the material react with the flow of electric current. It will give comparison between different material on the basis of their ability to conduct electric currents.
Good conductor will give low value of resistivity. Example good conductor is silver, copper, and aluminium. This testing was been expected to have the resistivity at low value due to the used of silver as the resonator of the antenna.
One of the investigation found that the stretching, including uni-axial and bi-axial stretching, decreases the electrical conductivity of the composites in the stretching direction and the decrease is more evident for the bi-axial stretching compared to uni-axial stretching.
Such a stretching mode is expected to reduce the anisotropy of the electrical properties of the composites in the stretching plane compared to the uni-axial stretching case, altering the electrical behavior of the composites. (Fen C., 2014)
Existing experiments have demonstrated that stretching may significantly influence the electrical behavior of conductive polymer composites. For example, under a uni-axial stretching, experimentally examined the morphology and the electrical conductivity of carbon nanofibre composites before and after stretching and showed that the mechanical stretching could lead to decrease in the electrical conductivity of the composites due to breakdown of conductive networks. (Fen C., 2014)
This testing was done at Jabil company. In this investigation, the antenna have been stretched at horizontal way. Based on Figure 29 below, the result obtained showed that when the antenna is been stretched, the value of resistivity is decreased. At zero stretched, the resistivity is in the range of 1.2 to 1.4 ohm per square. For 10% stretched is applied, the value
60
decreased sharply until the range of 0.2 to 0.4 ohm per square. Similarly at 20% stretched, the value dropped slightly to 0.2 ohm per square. For 30% stretched, the value becomes 0.11 to 0.13 ohm per square.
Figure 29: Resistance of material when undergo stretched
Based on Figure 30 below, Equation 1 and 2 proved that the conductivity is inversely to resistance value. The result obtained is conversely to each other. When the resistance value is high than the conductivity is low or vise versa.
1.292
0.259
0.157 0.12
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
1 2 3 4
resistivity (ohm per square)
length (cm)
resistance
61
Figure 30: Conductance value when undergo stretched
Based on the result, it showed that the resistivity is decrease conversely from the existing experiments. It means, the conductivity is increase with the stretch of the antenna. It might be because of the network is rearrange and align nicely to each other when the stress is applied. Those chain is arrange closely together and form ordered regions. when stretched, it effecting the alignment of Ag particle, to each other have increase conductivity. It showed at Figure 31 below.
0.541
2.7
4.454
5.8275
0 1 2 3 4 5 6 7
0 1 2 3
Conductivity (1/Ώ)
length stretched (cm)
conductance value
62
Figure 31: stretching of the molecular structure
When stretched, the structure among the molecular structure will be orderly aligned.
This affect the resistivity as mentioned before. This allows the molecule to stick to each other and make the conductivity better. With low in resistivity, the conductivity will be higher.
stretching
63 4.3.2 Capacitance Test
To make sure the PDMS substrate is suitable as the substrate for antenna, capacitance test is been conducted. Capacitance is the ability of the material to store an electrical charge.
A material which have large capacitance will hold much more electrical charge than one with low capacitance. In Figure 32 below, the difference between PDMS substrate and silver clearly shown. Substrate having very low capacitance which is -117.515F at frequency of 20Hz. While for silver, at frequency of 20 Hz, it shown 96.47F. As been mentioned before, capacitance is the ability of the material to store electrical charge. Silver is one of the best conductor which will be conduct the electrical charge. For optimum choice of conductor, the silver must absorb as much as possible the energy supply to convert it into magnetic wave.
With high absorption of energy, the signal will be much better.
64
Figure 32: capacitance result for substrate and silver ink
-117.515 -109.598
-80.1982 -73.5963
-45.6686
-4.04199 -0.4535 -0.884 0.24834 0.23023 0.88405 96.1477 99.4051 96.3676
60.1194 67.6643 75.7197
45.6656
16.2592 15.24 15.2758 16.751
-150 -100 -50 0 50 100 150
20 40 60 80 100 1000 2000 4000 6000 8000 10000
capacitance (Cp)
Frequency (Hz)
