Applied Project Report Transparent and Conducting

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Nov 22, 2013 - Figure – 1 Chemwiki: The Dynamic Chemistry e-textbook/Polymers and Plastics. Figure –12 http://www.sono-tek.com/accumist/. Figure –13 A.
Applied Project Report

Transparent and Conducting Polymer Nanocomposites Aruna Manoharan Project Advisor: Dr. Jingyue Liu November 22nd 2013 Arizona State University Tempe, Arizona, USA, 85287 Phone: (732) 662-8768 Email: [email protected]

Abstract— The downsizing of electronic devices, with the increase in the number density of nanoscale components, requires novel materials to remove excess heat. In this project, lightweight, highly moldable transparent materials with high thermal conductivity are of particular interest. Polymer composites, consisting of transparent base polymer poly(methyl methacrylate), also known as PMMA and a thermally conductive ZnO nanowires network, may provide a new approach for heat management in a variety of devices and systems. This project explores the feasibility of developing electrically conductive polymer nanocomposites and finds the relevant parameters to develop electrically conductive films of polymer nanocomposites.

Tacticity is the configuration of monomeric units in a polymer molecule. Tacticity of the polymer is to be considered, since the fillers tend to occupy the free volume in the polymer.

I. INTRODUCTION Any polymer which conducts electricity or heat is called a conducting polymer. Conduction of electricity or heat may be due to unsaturation of the polymer chains or due to the presence of externally added ingredients in them. Use of nanomaterials such as nanotubes, nanorods, nanocrystals as polymer fillers, to enhance the polymer property or its applications, has been an emerging trend [1]. Conductive polymer nanocomposites have been of significance lately due to their intermediate conductivity between those of plastics and metals. Transparent thermally conductive polymers have wide use in heat dissipation in microelectronics such as thermal interface material or as underfills for packaging, for flexible electronics and many more other electronic devices where heat dissipation has been a major issue. 1-D metal oxide nanostructures have now been widely used in many areas, such as transparent conductive films, ceramics, catalysis, sensors, electro-optical and electro-chromic devices [2]. Use of 1-Dimensional nanostructures such as nanowires could provide an excellent alternative for conventional nanofillers. Nanowires have higher aspect ratio, which allows percolations even at very low volume fractions. The percolations among nanowires can be achieved by properly dispersing the nanowires into the polymer matrix to form connected networks, thereby increasing the conductivity of the polymer nanocomposite of interest. (i). POLYMER: A polymer is a large molecule with a number of repeating units, known as monomers. The process of forming a chain of monomers to form polymers is called Polymerization.

Table-1 Examples of Polymers

Figure-1 Tacticity in Polymers - (a) Isotactic: The functional groups are aligned on the same side of the carbon skeleton. (b) Sydiotactic: The functional groups are aligned in alternate fashion of the main carbon skeleton (c) Atactic: The functional groups are aligned in a random manner around the carbon skeleton. (ii).ADDITIVES: Fillers: Fillers are usually solid additives. They are added to the base polymer with certain objective:  Improve polymer’s chemical or physical properties.  Simply to reduce the material cost by acting as inert loading agents. Nano fillers: Use of nano-additives/ nanofillers has been a recent trend and has had high significance in the polymer industries. Nanofillers are basically solid form filler, which generally comprises of inorganic and rarely organic materials [3]. They are completely different from polymer matrix, in terms of their structure and composition. Nanofillers are used in order to achieve certain mechanical or physical properties in the base polymer. (iii). POLYMER NANOCOMPOSITES: A polymer matrix with nano additives is called a polymer nanocomposite. The transition from micro- to nano-sizes leads to change in a material’s physical and chemical properties. Two of the major factors in utilizing nanofillers are the increase in the ratio of the surface area to volume and the size of the particle. The increase in surface area-to-volume ratio, which increases as the particles get smaller, leads to an increasing dominance of the behaviour of atoms on the surface area of a particle over that of the interior of the particle. Because of the higher surface area of nano-particles, the interaction with the other particles or the polymer chains within the mixture may be enhanced and thus enhances the performance and properties of the polymer nanocomposites. Researchers have been researching on polymer nanocomposites due to its broad potential applications.[4] A nanocomposite generally involves the association of an insulating polymer with a conductive filler that provides the

required thermal and electrical conductivity. [5] The combination of a polymer and a nanofiller that we chose was PMMA and ZnO Nanowire. The fields of commercial application for these materials are broad including batteries [1], electrodes [6], anti-static materials [8] and corrosion-resistant materials [9]. Transparent conductive thin films are widely used in electronic devices, such as touch screen panels, thin film transistors, electrostatic dissipation transparent electrodes for optoelectronic devices and field emission displays [10]. Interest has also arisen concerning their use as sensors because of the resistivity variation with thermal, mechanical or chemical requirements [11]. Many of the devices in the microelectronics industry requires greater protection from static electricity[12], which is generally achieved through electrostatic discharge (ESD) coatings [1]. The high content of particles in these nanocomposites may modify the polymerization kinetics and can often lead to advantageous changes in other mechanical properties like modulus and viscosity. (iv). GENERAL POLYMER PROCESSING TECHNIQUES: The basic steps in processing a polymer, whatever the technique might be, are causing deformation and then allowing it set in defined confinement. General techniques adopted are, 1) Deformation by dissolution: The polymer is made into a solution by using a suitable solvent and then shaped into the desired form and the solvent is removed subsequently by letting it evaporate. 2) Deformation in the rubbery state: Every polymer, when subjected to heat, goes through a rubbery state before becoming a viscous liquid. The deformed polymer is then casted. 3) Deformation through melting: The polymer is subject to melting and then molding the polymer out to shape. In case of thermoplastics, it is allowed to set by cooling whereas the thermosets are set by further heating them. 4) Deformation by dispersion: The polymer is dispersed in an incompatible medium and made into a suspension or emulsion. The dispersion is further used for casting or coating. Rubber latex is one of the polymers used in this manner. (v). MATERIALS AND PROPERTIES: Poly (Methyl methacrylate): Structure and Properties:

Figure – 2 PMMA Structure PMMA is obtained by the polymerization of methyl methacrylate. It is an acrylic polymer. Solubility of commercial PMMA is consistent with that expected of an amorphous thermoplastic, with a solubility parameter of 18.8 MPa½. Difficulties occur in dissolving cast PMMA sheet because of its high molecular weight. Cast material is stated to have number average molecular weight of about 106, Tg= 104 ͦ C, the molecular entanglements are so extensive that the material is incapable of flow below its decomposition temperature ~170 ͦC. There is thus a reasonably wide rubbery range and it is in this phase that such material is normally shaped. PMMA has extremely good weathering resistance compared to other thermoplastics. Table – 2 Properties - Thermoplastics - Acrylic

Optical Tmelt

Acrylic Transparent 160 °C

Tg H2O Absorption Oxidation resistance UV Resistance

75 – 105 °C 0.01 – 0.03 % (24 h) Good Fair

Optical Properties: An outstanding property of PMMA is its clarity. PMMA absorbs very little light, but there is about 4% reflection at each polymer-air interface for normal incident light. Thus the light transmission of normal incident light through a sheet of acrylic material, free of surface defects, is 92%. General solvents: Acetone, Ethanol, Ethyl Acetate, Ethylene dichloride, Trichlororethylene, chloroform and toluene. lowers peak temperature 45-90°C. ZnO Nanowires: ZnO is attractive due to its remarkable optical properties and environmentally friendly nature. Nanowires are promising materials for advanced optoelectronics. Among the galore of metal oxide nanostructures, the nanostructure of ZnO is considered as one of the best candidates for different applications due to its own merits and properties. Among diverse morphologies of ZnO, the 1D nanowires are the ideal system for thermal dissipation in confined objects, which is important for a variety of applications, where dissipation of heat is a major issue. ZnO is a wide bandgap (3.37eV) semiconductor. ZnO nanowires are considered to be an interesting system to examine and ascertain their optical properties as a function of size and dimensionality [13].

Figure – 3 SEM images of synthesized ZnO nanowires in different magnifications (Courtesy of Jia Xu) A significant number of studies have been reported on the preparation of ZnO-Polymer nanocomposites [14]. The ZnO nanowires synthesized in our laboratory are of 100-250 nm with lengths upto 10s of microns. Long nanowires may be advantageous for percolation purpose but they pose significant challenges to uniform dispersion in polymers. The dispersity is a measure of the heterogeneity of sizes of molecules or particles in a mixture. A collection of objects is called uniform if the objects have the same size, shape, or mass. A sample of objects that have an inconsistent size, shape and mass distribution is called non-uniform. It can also be calculated according to degree of polymerization, where, ĐX = Xw/Xn where Xw is the weight-average degree of polymerization and Xn is the number-average degree of polymerization. II. EXPERIMENTAL SECTION (i).DESIGN OF EXPERIMENT: Process Selection: The process depends on several interrelated factors:  Establishing project goals and the required end product properties and possible applications.  Selection of material that meets the average end property requirements.  Process requirements must be decided upon.  Methods for dispersing ZnO NWs for better percolation must be looked on.

 

Possible testing methods for thermal/electrical conductivity. Listing out necessary lab wares and equipments needed to carry out the process.  Purchasing lab wares, equipments and materials and warehousing the materials. The most important processing requirements are based on the polymer to be processed, the quantity and final product dimensions. Process selection is a critical step in product design. Failure to select a viable process during the initial design stages can result in material loss and time loss. The process can have a significant effect on the performance of the finished product.

Parameters Time taken to dissolve Color

(ii). MATERIALS USED: Table – 3 Specifications of Materials used. Material Poly methyl methacrylate (PMMA) ZnO Nanowires Acetone Ethanol Methanol

Specification Acrylic sheet 36”x30”x.093” 100-250nm C3H6O, 99.5+% C2H6O, 94-96% CH3OH, 100%

Vendor Optix In-lab Alfa Aesar Alfa Aesar BDH

(iii). DEFORMATION BY DISSOLUTION: The polymer is made into a solution by using a suitable solvent and then shaped into the desired form and the solvent is removed subsequently by letting it evaporate. The polymer is chosen based to satisfy certain end property requirement. Here, the final requirements being material transparency and enhanced material conductivity. The base polymer was chosen such that it satisfies certain category requirements as well, such as  Engineering polymer  Amorphous thermoplastic  Highly transparent Moderate to High glass transition temperature and melt temperature. Based on these properties, the base polymer was shortlisted to Poly metha methacrylate (PMMA) and Polycarbonate (PC). Solvents were chosen dissolving the polymer material chosen. Flowchart- 1, represents the material and its suitable solvent selection process.

Residue

:

:

Acetone 6-7 hrs

PMMA turns yellow with prolonged stirring (Doesn’t affect the base polymer property) : None

PMMA+ Ethanol 7-8 hrs

Fully transparent even with prolonged stirring

None

Methanol Did not dissolve --

--

In order to optimize the process of dissolution, taking the rate of residual solvent evaporation, time taken to dissolve PMMA and the transparency achieved on film casting into consideration, PMMA was tried to dissolve in combinations of solvents in 2:1 ratio of acetone and ethanol or methanol respectively, (i.e) 1g of PMMA was dissolved in 2ml acetone+ 1ml ethanol or methanol combination, in a sealed flask, while being subjected to constant stirring on magnetic stirrer at 200rpm. The amount of solvent per wt% of PMMA was reduced, to get highly viscous polymer to support the experimental process. The solution is brought down to 1/3rd of its initial volume. Table – 4 (ii). Observations made from dissolving PMMA in acetone + ethanol; acetone + methanol Parameters Time taken to dissolve Color

:

PMMA + Acetone + Ethanol Acetone + Methanol 3-5 hrs 4-6 hrs

:

PMMA and the film Opaque- White casted, remained transparent Residue : None Leaves white residue Use of methanol for dissolving PMMA, resulted in formation of opaque white polymer films and leftover residues, even when used in conjunction with acetone. This showed a visible loss in transparency of PMMA. (iv). MIXING: ZnO NWs dispersion in various solvents such as ethanol, methanol, their mixture even with water, prior to mixing, was performed. ZnO NWs were dispersed in different solvents in vials and sonicated for 15-20 mins and was left untouched for over a week, to see for agglomeration and sedimentation.

Flowchart – 1 Material selection and choice of solvents Trials were carried out by trying to dissolve PMMA in acetone; ethanol and methanol in a 1: 10 ratio (i.e.) 1 g of PMMA in 10ml of each solvent in a sealed flask which was constantly stirred in a magnetic stirrer at 200rpm. Table – 4 (i). Observations from dissolving PMMA in acetone; ethanol and methanol.

Nanowires must be dispersed onto substrates of interest with precise control over spatial position and orientation. Manipulating nanomaterials in this way is challenging owing to their small dimensions, mechanical fragility and tendency to aggregate and bond irreversibly to surfaces [15]. The percolation theory must be taken into consideration, to form perfect connections between the nanowires, without any open chains. In a perfect polymer nanocomposite film, with directed percolations through contact process, when voltage is

applied, the electrons move thorugh the nanowire connections to the next in an orderly fashion. [16]

as beaker solution. ZnO NWs were taken in a range of 0.1-5 wt% of PMMA. ZnO NWs were suspended in ethanol and sonicated in an ultrasonicator water bath for 20 minutes. Sonication disperses the number of particles found per grid array and makes it homogenous. To obtain homogenous and stable suspensions and a sufficient number of particles per grid surface sonication is carried out for longer time ~15 20 minutes. The suspension is left to constantly stir in a magnetic stirrer. It is then added drop-by-drop to the viscous PMMA, which is constantly left stirring in another magnetic stirrer. The process takes almost 45mins to 2hrs based on the ZnO NW solution needed to be mixed into the PMMA solution.

Figure – 5 ZnO nanowire percolations in the polymer matrix Three conceptually different types of approaches have been explored extensively.  Bulk Mixing  Drop-by-drop addition mixing  Embedding ZnO NWs onto the polymer In the first two methods, the nanowires are in fluid suspensions before or during their delivery to a receiving substrate.

Figure – 8 Visible ZnO NW agglomerations on Polymer nanocomposite film. The PMMA+ZnO NW form a white emulsion, when maintained at 1/3rd of its initial solution viscosity. The mixture becomes too dilute if excess amount of solvent is added. The ZnO NWs attracts each other and forms visible agglomerations in the thin polymer nanocomposite films. The agglomerations are formed in a pattern based on the angle of inclination of the mold form, which is in this case, a glass slide.

Figure - 6 schematic representation of (a) Bulk mixing (b) Dropby-drop addition mixing. In bulk mixing process, (figure- 6 a) the ZnO NWs were taken in a range of 0.1- 5 wt% of PMMA. ZnO NW is taken in a medium of 2ml ethanol and is ultrasonicated in a water bath for over 15mins. The sonicated batch of ZnO NW is dumped into the PMMA solution, which is left stirring in the magnetic stirrer at 200rpm. Stirring continuously makes the ZnO NWs disperse well in the polymer matrix. But, when film was casted, use of different solvents caused irregular drying of the polymer nanocomposite film, which resulted in opaqueness or white patterns formed on the surface of the film. Prolonged stirring of PMMA also caused yellowing of the samples.

Impregnating ZnO NWs onto the Polymer: PMMA is dissolved in acetone-ethanol mixture at 2:1 ratio. Usually, 1g PMMA is dissolved in 2ml of acetone and 1ml of ethanol mixture and is sealed and left stirring at 200 rpm, until it gets reduced to a required viscosity. The PMMA suspension is maintained at a viscoelastic state to allow impregnation of ZnO NW. This restricts the mobility of nanowires inside the polymer matrix, thereby physically impregnating it in the matrix.

Figure – 9 ZnO NWs impregnation onto the polymer matrix. (v). FILM CASTING: Method- I: The PMMA+ ZnO NW solution is poured out onto a petri dish and the polymer nanocomposite is left to dry at room temperature until the residual solvents gets evaporated. The thin film is then removed, labelled and sealed in bags for testing.

Figure – 7 Polymer nanocomposite samples - ZnO NW loadings 0.25- 3 wt% of PMMA. (A) 2.5 wt% ZnO NW loading (B) 0.5 wt% ZnO NW loading (C) 0.25 wt% ZnO NW loading (D) 0.75 wt% ZnO NW of PMMA (1g). In Drop-by-drop addition mixing, (figure- 6 b) the base polymer is dissolved in acetone and is brought down to 1/3rd of its initial viscosity

Figure – 10 (i) Method- I A). Polymer nanocomposite is poured onto a petri dish. B). The Polymer Nanocomposite is left to dry and form a thin film, at room temperature.

Method- II: An experimental setup using 4 glass slides is made (as represented in Figure – 10 (ii)). The polymer emulsion is spread out onto a glass slide 20mm x 20mm. The ZnO NWs were impregnated onto the polymer on the glass slide. The PMMA/ZnO NW nanocomposite is subjected to shear force between the glass slides. The nanocomposite film is allowed to dry at room temperature until the residual solvent gets evaporated.

reference measurement. The interaction between the nanofillers and the polymer chain results in reduced mobility and thus a higher surface area of the nanofillers would result in a reduction in the overall permittivity of the samples. The variations in the real permittivity values may be independent of frequency, which could be due to the presence of dc conductivity. However for dc conductivity, the imaginary permittivity value is given by, ɛ” = σdc/ (ɛoω) Since, σ dc is a constant, the imaginary permittivity is inversely proportional to the frequency. The Logarthmic- Ritchker rule is used to estimate the dielectric response in heterogeneous materials, and is given by equation:

Figure – 10 (ii) Method-II A). 4 Glass slides needed B).Glass slide expriment setup C). Polymer emulsion is spread out onto a glass slide 20mm x 20mm with a snow of ZnO NW embedded on it. The glass slide 4 is placed over it for compression D). PMMA+ZnO NW is subjected to shear stress by sliding off the glass slide 4 over it.

log ɛc = y1 log ɛ1 + y2 log ɛ2 Where, y1 and y2 are the volume fractions, ε1 and ε2 are the permittivities of the base resin and the filler respectively, while εc is the permittivity of the composite. However, this calculation do not take into account the filler shape, size and the possible change of matrix properties at the interface [23].

Polymer nanocomposites were formed by subjecting it compression force in addition to shear force (Figure – 10 (iii)) by rolling a glass rod over it. The polymer nanocomposite film is allowed to dry at room temperature until the residual solvent gets evaporated.

Figure- 10 (iii) Method-III A). Polymer emulsion is spread out onto a glass slide 1, 20mm x 20mm with a ZnO NWs embedded on it. The glass slide 2 is placed over it for compression. B). PMMA+ZnO NW is subjected to shear stress by sliding off the glass slide 4 over it. C). A glass rod is rolled over the PMMA+ ZnO NW emulsion on the glass slide 1. Physical contact with a receiving substrate while applying a controlled shear force results in the transfer of horizontally aligned arrays. The principle disadvantage of such dry assembly process is that it involves NWs whose electronic properties, dimensions and orientation are poorly controlled. [17] III. TESTING Dielectric spectroscopy measures the dielectric permittivity as a function of frequency and temperature. It can be applied to all non-conducting materials. The frequency range extends over nearly 18 orders in magnitude: from the µHz to the THz range close to the infrared region. Dielectric spectroscopy is sensitive to dipolar species as well as localized charges in a material [18][19]. The lower values of permititivity and tanδ of nanofilled samples over microfilled samples, is due to their high surface-area to volume ratio which results in large interfacial areas of nanocomposites as compared to the microcomposites. This large interaction zone can have a major impact on the permittivity values of nanocomposite materials as compared to microcomposite materials [20]. The setup involves a Solartron 1260 impedance analyzer, a Novocontrol Quatro temperature controller and a sample holder. No difference was observed in the sample measurements, when done with and without using guard rings [21]. A voltage of 50 V was applied to the samples and their permittivity values were measured over a frequency range of 1 mHz to 56 kHz. As a benchmark, a Teflon sheet with the same dimensions as the sample was run in the setup. Teflon has a frequency invariant relative permittivity of 2.04 [22]. All the samples were measured using the same

Figure – 11 Dielectric spectroscopy- Dielectric loss spectra for PMMA; Dielectric constant vs Frequency plot for Electrical conductivity measurement (Curtsey of David Swanson and Amanda Young). From the dielectric spectroscopy measurement for testing the conductivity of the polymer nanocomposite, the conductivity achieved here was measured to be 6.23 x 10-13(1/Ωm). The samples made using the solution mixing process showed conductivity of 10-16(1/Ωm), which is slightly higher than the conductivity of PMMA by itself, which is of order 10-19(1/Ωm). The sample made by method-II was tested using dielectric spectroscopy and were tested to be in the magnitude of 10 -13(1/Ωm), which is very low conductivity. The best achieved so far was 10 -6, however the conductivity aimed for is of order 10-2(1/Ωm). Conflicting results on the performance of nanocomposite fillers have been reported and the underlying mechanisms are not adequately understood [24-26]. This is due to the fact that it is not easy to prepare samples containing nano-sized fillers. Uniform dispersion of the nanofillers in the polymer matrix is essential to realize the percolation benefits. The simple addition of nanoparticles polymer matrix might be expected to lower conductivity by providing bridges between particles, but does not in fact improve conductance, due to mean free path restrictions and added interface resistances [27]. The hypothesis is that the same principles might limit the thermal conductivity of the polymer nanocomposite films. [28]

IV. FUTURE PROSPECTS Perfect dispersion of ZnOs in many organic solvents indicates that there might be some organic functional groups on their surfaces. These promote the interfacial interactions between PMMA and ZnOs. Use of ultrasonic spray for spraying out ZnO NW onto the polymer at >20 KHz, could greatly improve the ZnO NW dispersion on the polymer matrix. The ZnO nanowires suspended in the solution literally ripped apart the agglomerations. This could lead to higher interactions of nanowires, which could directly influence the conductivity of polymer nanocomposite. Ultrasonic spray is used to spray nanomaterials to create homogenous, uniform layers in energy, electronics, or other nanotechnology applications. Since ZnO NWs are prone to agglomerate in solution, as the ultrasonic vibrations of the spray nozzle continuously disperse agglomerates in suspension during the mixing process.

microelectronics, where thermal dissipation is a key challenge. Transparent thermally conductive polymer nanocomposites have wide use in heat dissipation in microelectronics such as thermal interface material or as underfills for packaging, for flexible electronics and many more electronic devices where heat dissipation has been a major issue. Thermal conductivity to be achieved is in the range of 110W/Mk for filled polymers.

Figure – 13 Flipchip BGA package To have higher transparency gives an aesthetic appeal to it, and makes it useful for a wider range of applications, as in flexible electronics, micro-electronics where light emission is a critical aspect.

Figure – 12 Ultrasonic spray For further work, the free volume of the polymer can be reduced by careful addition of ZnO NW, such that they could occupy the free volume. Addition of nanowires onto an already polymerized MMA would lead to reduction in the freedom of chain movement in PMMA. The fillers generally act as toughening agents rather than supporting the tacticity of the host polymer. Addition of ZnO NW during the polymerization of MMA, could avoid ZnO NW agglomeration. The ZnO NW thus gets attached to the functional groups and improves the performance of the polymer. Also, use of PMMA pellets and melt blending ZnO NW onto the polymer matrix would make an interesting study, however the interaction between the nanowires and polymer matrix cannot be controlled. On the thermal aspects, increase in free volume of polymer reduces the glass transition temperature (Tg). To maintain the polymer in its viscoelastic state as much as possible will help in maintaining its Tg. Use of Polycarbonate (PC) for future experiments would improve results to a much greater extent, since the base properties of PC itself proves to be exceptional when compared with base properties of PMMA. The best solvent for dissolution would be cyclopentanone. The direct evidence of enhanced percolations is a variation in glasstransition temperature (Tg) curves for PMMA and ZnO NW composites. The precise load of ZnO NW in the prepared composites must be determined to completely understand the property changes due to change in ZnO NW loadings. Also, other characterizations to analyse the nanowire interactions, to better understand the conductivity in polymer nanocomposite. Once higher conductivity is achieved, the transparent thin film polymer nanocomposites have wide spread applications in microelectronics, from nano- to micro-devices, where thermal dissipation is a key challenge. To have higher transparency gives an aesthetic appeal to it, and makes it useful for a wider range of applications, as in flexible electronics, microelectronics where light emission is a critical aspect. (i). POSSIBLE APPLICATIONS: Once higher conductivity is achieved, the transparent thin film polymer nanocomposites have wide spread applications in

Thermal Interface Materials: The focus on thermal interface materials has been taking off in the smartphone and tablet industries. The TIMs market is projected to triple to more than $300 million by the year 2020. The everincreasing power output and heat generation by smartphones and tablets with greater computing power (according to Moore’s Law) has called for better cooling solutions. Thermal interface materials (TIMs) reduce thermal contact resistance between heat-generating portions (die) and heat-sinking units by filling the voids of the non-smooth surfaces. These may be different materials or the same, depending on the desired properties .In selecting a thermal interface material, the most important properties for performance include: thermal conductivity, thermal resistance, and strength. Some other considerations for TIM properties/characteristics include: coefficient of thermal expansion (CTE), stress during thermal and mechanical cycling, bond-line thickness, dielectric properties, and volatile content.[29] Thermal resistances much lower than the current state of the art TIMs with 8 mm2 K/W could be achieved. The fillers used in TIMs are of utmost importance to maintain the desired thermal (low resistance) and mechanical (resistance to fatigue) properties. Based on the information researched above, it is likely the proprietary blends used in today’s smartphones and tablet devices utilize a combination of metal fillers, nanowires, nanotubes, and/or graphene as additives in their TIMs. V. SUMMARY Driven by the thrust of fabricating smaller devices to create integrated circuits with improved performance, 1-D metal oxide nanostructures have been exploited as potential building blocks for future nanoelectronics. Nanowires (NWs) and nanobelts (NBs) of organic or inorganic materials are the forefront in today’s nanotechnology research. Scaling of microelectronic devices has led to an interest in utilizing nanomaterials for electrical interconnects and thermal management approaches. ZnO nanowires are promising for electrical conductivity and heat removal for microelectronics packaging. The processing method and the solvents for dissolving PMMA were chosen. Experiments were carried out to finalize the solvents both for PMMA and ZnO NW. Conductive transparent films of PMMA+ZnO NW nanocomposites were prepared by different solution mixing techniques and film casting methods. In contrast, the sample results in inhomogeneous and usually opaque materials resulting from nanoparticle agglomeration and phase separation. Methods aiming to improve the dispersion of ZnO nanowires in the polymer matrix are critical to fabricate practical transparent materials.

Deagglomeration and dispersion stabilization methods lead to an increase in process complexity and processing steps [30]. Electrical measurements were carried out to determine the conductivity of the polymer nanocomposite. A comparatively low electrical conductivity was observed. The samples tested using dielectric spectroscopy and were tested to be in the magnitude of 10-13(1/Ωm), which is very low conductivity. The best achieved so far was 10 -6(1/Ωm), however the conductivity aimed for is of order 10-2(1/Ωm). This could be the resultant of uneven distribution of ZnO NWs on the polymer matrix or the conductivity of the ZnO NW itself. Uniform dispersion of the nanofillers in the polymer matrix is essential to realize the percolation benefits. The simple addition of nanoparticles polymer matrix might be expected to lower conductivity by providing bridges between particles, but does not in fact improve conductance, due to mean free path restrictions and added interface resistances [27]. The conductivity of the fillers used doesn’t actually mean that the end product conductivity would be higher. The polymer-filler interaction determines the conductivity of the polymer nanocomposite film formed. VI. ACKNOWLEDGEMENTS I would like to express my deepest gratitude to my advisor, Dr. Jingyue Liu, for his excellent guidance, patience, and providing me with an excellent research facility and group to work with for doing research. I would also like to thank Dr. John Venables for his guidance, encouragement and insightful comments. Special thanks to Dr. Amaneh Tasooji, who was willing to participate in my final defense committee at the last moment. I would like to thank David Swanson, who as a good friend was always willing to help and give his best suggestions. Thanks to Amanda Young and Subarna Samantha for conducting dielectric spectroscopy on our samples for electrical conductivity measurements. Many thanks to my fellow researchers Jia Xu, Spencer Staples, Botao Qiao, Datong Yuchi, Jiaxin Liu, Min Zhu and other workers in the laboratory of Dr. Jingyue Liu. My research would not have been possible without their help. Last but not least, I would like to thank my family, Manoharan Gopalakrishnan, Chandrakala Ratnasamy and Jency Vennila, and friends for always being there cheering me on through all phases of my life. VII. REFERENCES [1] I. Moreno, M. Arruebo, N. Navascues, S. Irusta and J. Santamaria, 2013, 07, iopscience- Nanotechnology, 1-11, Available at stacks.iop.org/Nano/24/275603. [2] G. Shen, P. Chen, K. Ryu and C. Zhou, J. Mate. Chem., 2008, 20, 828 – 839. [3] Nanocomposites and Polymers with Analytical Methods, D. M. Marquis, É. Guillaume and C. Chivas-Joly, 2011, 09, 261-284. [4] P. Prosini and S. Passerini, Eur. Polym. J. 37, 2001, 65–9. [5] Liang X, Wen Z, Liu Y, Zhang H, Jin J, Wu M and Wu X, J. Power Sources, 206, 2012 409–13 [6] Yu Z, Zhang Q, Li L, Chen Q, Niu X, Liu J and Pei Q, Adv. Mater., 23, 2011, 664–8. [7] Sorel S, Khan U and Coleman J N, Appl. Phys. Lett., 101, 2012, 103-106. [8] Schmitt C and Lebienvenu M, J. Mater. Processing Tech., 134, 2003, 9-303. [9] Thongruang W, Spontak R J and Balik C M, 43, 2002, 253717. [10] Lee S-H, Teng C-C, Ma C-C M and Wang I, J. Colloid Interface Sci. 364, 2011, 1–9. [11] T. Nardi, M. Sangermano, Y. Leterrier, P. Allia, P. Tiberto, J.E. Månson, J. Polymer, Volume 54, Issue 17, 2013, 2, 4472–4479. [12] “Vapor–Liquid–Solid Growth of Semiconductor Nanowires”, by Heon-Jin Choi.

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