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UEET 101 Introduction to Engineering Nanotechnology in Mechanical Engineering Presented By Pradip Majumdar Professor Department of Mechanical Engineering Northern Illinois University DeKalb, IL 60115 1 Outline of the Presentation Lecture In-class group activities Homework 2 Lecture – II: Outline Nano-Mechanics Classical Mechanics Assumptions Material Mechanical Properties Nanoscale Thermal Phenomena - Basics of Heat Transfer - Thermal Conductivity - Heat Transfer Coefficients 3 Nanomechanics Classical theories Structure – Property relations Stress-strain relations Mechanical properties Issues in nanomechanics Mechanics of nanotubes 4 Classical Mechanics: Assumptions Solid is assumed as homogeneous Smallest material element has macroscopic properties Involves only mechanical forces such as inertia, gravity and friction Motion is uniquely determined by forces - given by Newton’ s law of motion Total Energy = Internal Energy+Kinetic Energy + Potential Energy Single phase – no phase transformation 5 Basics of Classical Mechanics Mechanical Behavior of Materials: Material’s response to applied and residual forces Deformation: • When a material is subjected forces, its atoms may be displaced from their equilibrium position. • Any separation or displacement from the equilibrium position requires energy, which is supplied by the force. - As a material is stretched, atoms tend to separate and brings attractive forces into play. - As a material is compressed, atoms tend to come together and causes repulsion 6 Elastic Deformation: Atoms resumes back to the original position when imposed forces are released – represents the relative resilience of the materials. Plastic Deformation: When a material exceeds the elastic capability (elastic limit) to restore back to equilibrium position as the imposed forces are released - the deformation is permanent. Engineering Strain: It is the deformation l defined as the ratio of l the dimensional change l to the original l l dimension. Extensional Strain 7 Shear Strain: This is the deformation of a material between two parallel plane through a certain angle when subjected to tangential or shear forces. - Shear strain is defined as the displacement to the distance between the planes: Shear Force x x tan h h Poisson’s Ratio Defined as the ratio of strain in x y x-direction to the strain in y- direction and expressed as y P P x 8 Stress: • Stress is the internal response or resistance that a material creates when exposed to some kind of external force. • This internal resistance is due to the inter-atomic attractive and repulsive forces. • Displacement in either direction produces an increase in the force (tensile or Compression) that oppose the deformation F F F Defined based on balance of Where = Average stress (Internal external force with the F internal resistance force as resistance force per unit area) A F = External load or force A = Cross-sectional area over which 9 the force acts Hooks’s Law ( Macroscopic Constitutive Relation or Stress-strain relation) Defines the proportional relation between the stress and strain for material below the elastic limit as E = Linear relation Where E = Modulus of Elasticity (Young’s Modulus) •Elastic modulus (E) is a measure of the stiffness of the engineering material • A higher values of E results in a smaller elastic strains – smaller the response of the material structure to imposed load • This is an important parameter for design and analysis in the estimation of allowable displacements and deflection of a component or structure 10 Modulus of Rigidity (G) The modulus of rigidity is the modulus of elasticity in shear (Relation between shear stress and shear strain) and defined as G Values of G is usually determined by torsion testing and related to E by the relation E G 2(1 ) 11 Tensile Strength Yield Strength (Point-C) Stress required to produce a small amount of plastic deformation Ultimate Strength (Point – D) Maximum stress that a material can withstand under the condition of uniaxial loading - undergoes substantial plastic deformation - not often used for designing a component 12 Beam Deformation for Different Materials •Many materials are not strength limited, but Steel modulus limited Titanium •In some applications, m we need material of high modulus of elasticity rather than Aluminum high strength •These structure may not fail if low modulus Higher the modulus of elasticity of elasticity is used lower is the deformation •It, however, may reach too much of deflection13 Typical Material Properties Material Elastic Shear Tension Possson’sratio Modulus (E) Modulus Yield (GPa) (GPa) (MPa) Aluminum Alloy 72.4 27.6 504 0.31 Steel- Low Carbon 207.0 75.9 140 0.33 SS -304 193.2 65.6 960-1450 0.28 Titanium 110.5 44.8 1035 0.31 Silcon Carbide 469.2 Polycarbonate 3.4 SWNT 0.191(TPa) 0.45 TPa 0.18 14 Breakdown of Continuum Concepts- Thresholds of Micromechanics Macromechanics: Force, stress balance/equilibrium Stress Constitutive relation: 10 23 atoms Strain Hooks law – linear Area/volume Classical thermodynamics Scale: 10 3 m Micromechanics Force/surface energy Structure balance Interface Constitutive relation: 1011 atoms nonlinear Adhesion Structure property Scale: 106 m Phases relation Adhesion & friction laws15 Breakdown of Continuum Concepts- Thresholds of Micromechanics Nanomechanics: Force/energy/structure Molecule balance Atoms Constitutive relation: ?? 10 2 atoms Quantum Molecular mechanics effects Scale: 10 9 m energies Structure property relation: ?? Energies are linked 16 Structure – Property Relations Nano Macro Inter-molecular Strength interaction Bond rotation/ Modulus angle/strength Chemical sequence Viscosity/conductivity Nanotube diameter/ density/toughness/ Nanotube l/d ratio dielectric/plasticity 17 Nano-scale Science Hierarchy Average material properties: - Surface effects vs volume average - Molecular network homgenization - Electromechanical interactions Nano-scale laws - Application of classical mechanics law - New and coupling forces - Properties/energy depend on molecular structure - Role of quantum effects 18 Nanomechanics Nanomechanics vs. molecular mechanics Structure – property relations and dependencies Scaling analysis of molecuar structures Reliability of characterization techniques at nano-scale – what are to be measured? 19 Issues in Nanomechanics Nano-Materials Science Approaches- top down - Nanotubes purity -Continuum models for NTs - Characterization of NTs -Strain gradient - NT – properties -Lattice structure - Multifunctional composites 20 Models for Multiscale Effects Development of constitutive laws for nano-scale - modeling of nano-structural behaviors Average nano-constitutive laws for use in higher scale model Models for nano-structure/force potentials to take into account of multi-scale model Nanotechnology – Modeling Methods • Quantum Mechanics • Atomistic Simulations • Molecular Mechanics and Dynamics - nanomechanics 21 Nano-scale Measurement Techniques and Tools Atomic Force Microscopy (AFM) Magnetic Force Microsopy (MFM) - Scanning Electron Microscopy (SEM) - Transmission Electron Microscopy (TEM) - Scanning Tunnel Microscopy (STM) Raman (IR) Spectroscopy Electron Nano-Diffraction Neutron Scattering Electron Spin Resonance (ESR) 22 Nano-Structured Material Properties Physical Material Mechanical Thermal Density Stiffness Optical Crystallinity Strength Electronic Crosslink density Fracture toughness Magnetic Orientation Fatigue Chemical Textures Durability Acoustic Absorption Viscoelastic 23 Mechanics of Carbon Nanotubes The structure of single wall nanotubes (SWNTs) - molecules or crystals - Effective geometry - length scales - geometric parameters Properties of Carbon nanotubes - Thermal and electrical conductivities - density - mechanical properties such as modulus, strength - effect of geometry and molecular structure - classes of NTs Deformation of NTs - Tension, compression, torsion - nonlinear elastic and plastic deformation 24 Nanotubes Mechanical Properties NASA Langley ResearchCenter [ ] 25 Nanotubes Density and Thermal Conductivity 26 VI: Nano-Scale Heat Transfer Classical theories breaks down Thermal energy transport in a solid by two primary mechanisms: - Excitation of the free electrons - Lattice vibration or phonons Scattering phenomena dominates in micro and nanoscale heat transfer 27 Basics of Heat Transfer Heat transfer is thermal energy in Basic Modes and transit as a result of a spatial Transport Rate Equation temperature difference. Conduction Heat Temperature at a point is defined by the energy associated with Transfer random molecular motions such as This mode is primarily translational, rotational and important for heat transfer in vibrational motions. solid and stationary fluid Conduction heat transfer is due to the activity in atomic TH and molecular level q TL 28 Physical Mechanism Conduction Rate Equation: Gas: Energy transfer due to random molecular motion and collision with each other Fourier’s law: Liquid: Molecular interactions are more stronger and more frequent dT resulting in an enhanced energy q kA dx transfer than in a gas Solid: Energy transfer due to the Where q = Heat flow per Lattice vibration and waves induced by the atoms. unit area per unit time or - In a electrical nonconductor, the heat flux, energy transfer is entirely due to k is the thermal conductivity lattice vibration waves. - In a electrical conductor it also of the material defined as due to the translational motion of the free electrons. dT k (q / A) / dx 29 Macroscopic Thermal Conductivity Values of Substance Type Density Thermal Conductivity W Gases: mo C Air: 0.026 Liquid Water 0.63 Ethylene Glycol 0.25 Solid Aluminum 2702 237 Copper 8930 401 Gold 19300 317 Carbon Steel 7850 60.5 SS 304 7900 14.9 Carbon Amorphous 1950 1.6 Diamond 3500 2300 Silicon Carbide 3160 490 30 Convection Heat Transfer The convection heat transfer occurs between a moving fluid and an exposed solid surface. u Convection Modes: y Natural Convection: T Flow induced by natural x Hydrodynamic TS Thermal forces such as buoyancy Boundary layer Boundary layer Forced: Flow induced by The fluid upstream mechanical means such as temperature and velocity fan, blower or pump. are T and u respectively. Phase Change: Boiling or condensation- Bubble formations and collapses 31 Convection Rate Equation: Newton’s Law Cooling qc hc A(TS T ) Where, hc is called the convection heat transfer coefficient or film coefficient. Convection heat transfer qc / A T hc kf A coefficients is defined as (TS T ) y y 0 • Convection heat transfer coefficients are influenced by the velocity field and temperature field in the boundary layers. • This depends on fluid types and properties, solid surface geometry and orientations. 32 Typical Convection Heat Transfer Coefficients W Convection Types Typical Values ( m2 o C ) Free Convection Gases 2-30 Liquids 50-1000 Forced Convection Gases 30 – 300 Liquids 100 – 15000 Phase Change Boiling or Condensation 2500 – 100,000 33 Nano-scale Heat Transfer • Heat conduction in the micro-nanometer scale is important because of the increasing demand of cooling in smaller devices with increasingly higher heat fluxes such as in electronic devices, circuits and chips • The main difficulty is that bulk material properties are not accurate when applied on the small scale • Mechanism of thermal energy transfer by conduction in nano-thin films is dominated by electron-phonon scattering process. . 34 Thermal Interactions phonon – phonon interaction electron – electron interaction phonon – electron interaction In most pure metals, the electron – electron interaction is the dominant scattering process and the conduction of heat by phonon is negligible In dielectric crystalline solid, the phonon – phonon interaction is the dominant scattering process and heat conduction by free electron is negligible. 35 Applications nanothin films and nanoparticles in Heat Transfer Used for enhanced conduction heat spreaders in electronic chips, devices and circuits. Use of dielectric thin films of diamond or nitrides Used as filler materials (SWNTs) between two material surfaces in contact -Reduces resistance to heat transfer 36 Nanofluids Nanofluids are engineered colloid formed with stable suspensions of solid nano-particles in traditional base liquids. - Thermal conductivity of solids are order of magnitude higher than liquids. - Use of macro or micro-size particle can not form stable suspensions Base fluids: Water, organic fluids, Glycol, oil, lubricants and other fluids Al 2O3 ZrO 2 SiO 2 Fe3O4 Nanoparticle materials: - Metal Oxides: - Stable metals: Au, cu - Nitrides: AIN, SIN - Carbon: carbon nanotubes (SWNTs, MWNTs), diamond, graphite, fullerene, Amorphous Carbon - Polymers : Teflon 37 Major Characteristics and Challenges Stability in dispersion of nanoparticles in base fluid - Nanoparticles can stay suspended for a longer period of time - sustained suspension is achieved by using surfactants/stabilizers Surface area per unit volume is much higher for nanoparticles Forming a homogeneous mixture of nanoparticles in base fluid Reduce agglomeration of nanoparticles and formation of bigger particles. Sedimentation over a period of time. 38 Nanofluid Heat Transfer Enhancement Thermal conductivity enhancement - Reported breakthrough in substantially increase (20- 30%) in thermal conductivity of fluid by adding very small amounts (3-4%) of suspended metallic or metallic oxides or nanotubes. Convective heat transfer enhancement Critical Heat Flux enhancement (CHF) 39 Enhanced Nanofluid Conductivity Shows increase in effective thermal conductivity of nanofluid with an increase in temperature and CNT concentration. 40 Possible Mechanisms for Enhanced Thermal Conductivity Energy transport due to mixing effect of Brownian motion of nanoparticles Formation of liquid molecule layerr around nanoaprticles, enhancing local ordering (Phonon energy transport) Balastic transport in nanoparticles – Balastic phonon initated by a nanoparticle transmits through fluid to other nanoparticles Possibility of formations of clusters of nanoparticles Micro convection and turbulence formed due to nanoparticle concentration and motion. 41 Forced Heat Convection 42 Boiling Heat transfer Boiling is considered as convection which occurs at solid-liquid interface. In the case of boiling fluid phase changes from liquid to vapor through rapid formation of bubbles and subsequent collapse in the bulk fluid. - This causes heat transfer from solid heating surface - Fluid temperature remains constant – Latent heat contributes to the heat transfer Surface roughness influences critical heat flux. - Critical heat flux can be enhanced by roughening surface. 43 Critical Heat Flux Enhancement (CHF) Pool boiling heat transfer tests with nanfluids containing alumina, zirconia and silica nanoparticles show increased critical heat flux values (Kim et al. [2006] Nanoparticles settles and forms porous layer of heat surface - Surface wettability increases - Show increased contact angle on nanofluid boiled surface compared to pure water boiled surface. Helps formation of bubbles at boiling surfaces Boiling heat transfer is increased mainly due to the formation of nanoparticle coating on heating surface. 44 Enhanced Critical Heat Flux Experiment with nanofluid (suspending alumina nanoparticles in distilled water) indicate increase in critical heat flux by 200% in comparison to pure water. The nucleate boiling heat transfer coefficients remain almost the same. Kim and You [ ] 45 Nanofluid Applications Energy conversion and energy storage system Electronics cooling techniques Thermal management of fuel cell energy systems Nuclear reactor coolants Combustion engine coolants Super conducting magnets Biological systems and biomedicine 46 Nanofluids as Engine Coolant Air Fuel Diesel Engine Diesel Air Engine Fuel Air Pre-heater Engine water Nano-fluid Cooling loop system Heat Exchanger Engine water Heat Rejection Radiator Cooling loop to Atmosphere • Select potential nanofluids as coolant • Develop correlations for heat transfer coefficients and pressure drop for nanofluids • Development of radiator, heat exchanger and air- preheater using nanofluids. 47 Group Project Engine cylinders are typically cooled by forced convection heat transfer technique by circulating water-glycol solution through the cooling jackets around the cylinder walls. • Identify new cooling techniques based on nanotechnology for improved cooling system performance. • Identify major advantages and gains • Identify major challenges and technical difficulties 48 HOME WORK Problem # 1 A load of 4000 N is suspended from three identically sized wire 1- mm diameter. Wires are made of SS-304, Aluminum and wire made of SWNTs. Determine the strain (deformation) produce in three wires. Problem #2 http://www.ceet.niu.edu/cecourse/UEET101_Fall10/ Nanotech in ME homework.doc 49 Nanotechnology – Video clips http://www.youtube.com/watch?v=sITy14zCvI8 http://www.youtube.com/watch?v=YcqvJI8J6Lc &feature=related http://www.youtube.com/watch?v=zAIUsssNK mg&feature=channel 50

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