Nanotechnology in Mechanical Engineering

1. UEET 101 Introduction to Engineering Nanotechnology in Mechanical Engineering Presented By Pradip Majumdar Professor Department of Mechanical Engineering Northern Illinois University DeKalb, IL 60115

2. Outline of the Presentation Lecture In-class group activities Homework

3. Lecture – II: Outline Nano-Mechanics Classical Mechanics Assumptions Material mechanical properties Nanoscale Thermal Phenomena - Basics of Heat Transfer - Thermal Conductivity - Heat Transfer Coefficients

4. Nanomechanics • Classical theories • Structure – Property relations • Stress-strain relations • Mechanical properties • Issues in nanomechanics • Mechanics of nanotubes

5. 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 -give by Newton’ s law of motion • Total Energy = Internal Energy+Kinetic Energy + Potential Energy • Single phase – no phase transformation

6. 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 of 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

7. 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 defined as the ratio of the dimensional change to the original dimension. Extensional Strain

8. 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: Poisson’s Ratio Defined as the ratio of strain in x-direction to the strain in y-direction and expressed as

9. 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 Defined based on balance of external force with the internal resistance force as Where = Average stress (Internal resistance force per unit area) F = External load or force A = Cross-sectional area over which the force acts

10. 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 Where E = Modulus of Elasticity (Young’s Modulus) • Elastic modulus (E) is a measure of the stiffness of the engineering material • Greater the values of E results in a smaller elastic strains – smaller the response of the material structure to imposed load • Important parameter for the design and analysis in the estimation of allowable displacements and deflection of a component or structure

11. 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 Values of G is usually determined by torsion testing and related to E by the relation

12. 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 - undergone substantial plastic deformation - not often used for designing a component

13. Beam Deformation for Different Materials • Many materials are not strength limited, but modulus limited • In some applications, we need material of high modulus of elasticity rather than high strength • These structure may not fail if low modulus of elasticity is used • It, however, may reach too much of deflection Higher the modulus of elasticity lower is the deformation

14. 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

15. Breakdown of Continuum Concepts- Thresholds of Micromechanics Macromechanics: Force, stress balance/equilibrium Constitutive relation: Hooks law Classical thermodynamics Stress Strain Area/volume Scale: Micromechanics Force/surface energy balance Constitutive relation: nonlinear Structure property relation Adhesion & friction laws Structure Interface Adhesion Phases Scale:

16. Breakdown of Continuum Concepts- Thresholds of Micromechanics Nanomechanics: Force/energy/structure balance Constitutive relation: ?? Molecular mechanics effects Structure property relation: ?? Energies are linked Molecule Atoms Quantum energies Scale:

17. 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

18. Nano-scale Science Hierarchy Average of 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

19. 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?

20. Issues in Nanomechanics Nano-Materials Science - Nanotubes purity - Characterization of NTs - NT – polymer properties • Multifunctional composites • Approaches- Top down • Continuum models fro NTs • Strain gradient • Lattice structure

21. Models for Multiscale Effects • Development of constitutive laws fro nano-scale - modeling of nano-structural behaviors • Average nano-constitutive laws for use 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

22. 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-Difraction Neutron Scattering Electron Spin Resonance (ESR)

23. 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

24. 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

25. Nanotubes Mechanical Properties NASA Langley ResearchCenter [ ]

26. Nanotubes Density and Thermal Conductivity

27. VI: Nano-Scale Heat Transfer • Classical theories • Thermal energy transport in a solid by two primary mechanisms: - Excitation of the free electrons - Lattice vibration or phonons • Scattering phenomena in micro and nanoscale heat transfer

28. Basics of Heat Transfer Basic Modes and Transport Rate Equation Conduction Heat Transfer This mode is primarily important for heat transfer in solid and stationary fluid Conduction heat transfer is due to the activity in atomic and molecular level Heat transfer is thermal energy in transit as a result of a spatial temperature difference. Temperature at a point is defined by the energy associated with random molecular motions such as translational, rotational and vibrational motions.

29. Physical Mechanism Conduction Rate Equation: Gas: Energy transfer due to random molecular motion and collision with each other Liquid:Molecular interactions are more stronger and more frequent resulting in an enhanced energy transfer than in a gas Solid:Energy transfer due to the Lattice vibration and waves induced by the atoms. - In a electrical nonconductor, the energy transfer is entirely due to lattice waves. - In a electrical conductor it also due to the translational motion of the free electrons. Fourier’s law: Where q = Heat flow per unit area per unit time or heat flux, k is the thermal conductivity of the material defined as

30. Macroscopic Thermal Conductivity Values of Substance Type Density Thermal Conductivity Gases: 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

31. Convection Heat Transfer The convection heat transfer occurs between a moving fluid and an exposed solid surface. The fluid upstream temperature and velocity are and respectively. Convection Modes: Natural Convection: Flow induced by natural forces such as buoyancy Forced: Flow induced by mechanical means such as fan, blower or pump. Phase Change: Boiling or condensation- Bubble formations and collapses

32. Convection Rate Equation: Newton’s Law Cooling Where, is called the convection heat transfer coefficient or film coefficient. Convection heat transfer coefficients isdefined as • 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.

33. Typical Convection Heat Transfer Coefficients Convection TypesTypical Values( ) Free Convection Gases 2-30 Liquids 50-1000 Forced Convection Gases 30 – 300 Liquids 100 – 15000 Phase Change Boiling or Condensation 2500 – 100,000

34. Nano-scale Heat Transfer • Heat conduction in the micro-nanometer scale is becoming more important because of the increasing demand of cooling requirements in smaller devices with increasingly higher heat fluxes such as in electronic devices, circuits and chips • The main difficulty with the simulation of heat flow through thin films is that bulk material properties are not accurate when applied on the small scale • The understanding of the mechanism of thermal energy transfer by conduction in thin films ranging in thicknesses from micro-scale to nano-scale is becoming very important. • Thin films should be modeled at the atomic level and this entails treating the heat transfer as energy transferred by the vibrations in a crystal lattice.

35. 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.

36. Heat Conduction Dielectric Thin Films • The vibration of a crystal structure can be modeled with the concept of phonons, which is described as the quanta of lattice vibration energy. • The distribution of phonons represents the distribution of crystal energy. Heat transmission takes place as the distribution of phonons changes. • When a temperature gradient is setup in the material a steady state distribution of phonons can be kept. • Thermal conductivity therefore depends on the extent by which a distribution deviates from equilibrium for a given temperature gradient. • The distribution of phonons is modeled by the Boltzmann transport equation

37. 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

38. Nanofluids Nanofluidsare 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 Nanoparticle materials: - Metal Oxides: - Stable metals: Au, cu - Nitrides: AIN, SIN - Carbon: carbon nanotubes (SWNTs, MWNTs), diamond, graphite, fullerene, Amorphous Carbon - Polymers : Teflon Nanoparticle size: 1-100 nm

39. 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 articles. • Sedimentation over a period of time.

40. 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)

41. Enhanced Nanofluid Conductivity Shows increase in effective thermal conductivity of nanofluid with an increase in temperature and CNT concentration.

42. Possible Mechanisms for Enhanced Thermal Conductivity • Energy transport due to mixing effect of Brownian motion of nanoparticles • Formation of liquid molecule layer 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.

43. Forced Heat Convection

44. 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 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.

45. 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 [ ]

46. 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.

47. 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

48. Nanofluids as Engine Coolant • Selection potential nanofluids as coolant • Develop correlations for heat transfer coefficients and • pressure drop • Development of radiator, heat exchanger and air- • preheater using nanofluids.

49. 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

50. 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 A square chip of width 5-mm in size is mounted on a substrate that is insulated in all sides while the top surface is cooled to dissipate heat generated in the chip. From reliability point, the chip surface has to be maintained at 85 C. Determine the maximum allowable chip power for he following three cases: a) Forced convection with water at 20 C and h = 2000 b) Forced convection with nanofluid made of water and copper naoparticles at 20 C and h = 2500