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THERMAL CONDUCTIVITY: A perspective from Nanotechnology

THERMAL CONDUCTIVITY: A perspective from Nanotechnology

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THERMAL CONDUCTIVITY: A perspective from Nanotechnology

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  1. THERMAL CONDUCTIVITY:A perspective from Nanotechnology Diego A Gomez-Gualdron Seminar II Nanotechnology CHEN 689-601 Texas A&M University April 13th 2010


  3. The thermal conductivity relates to the ability of a material to transfer heat Definition Fourier’s law

  4. Unsuitable values of thermal conductivity might render a new material useless for an application. Relevance POWER DISSIPATION THERMO ELECTRICITY INSULATION HEAT EXCHANGE FLUIDS

  5. Overview: Power Dissipation • Decrease in size of electronic devices requires ingenuous ways to dissipate heat and protect the device components structure and performance THINGS TO LOOK FOR: • Good thermal contact between components and heat sink • Materials with high thermal conductivity and low coefficient of thermal expansion

  6. Overview: Insulation • The basic principle is the protection of a system from the harsh (hot or cold) conditions in a neighboring region, while fulfilling additional requirements A MATTER OF COMPROMISE • Space suits require insulating materials, while being light enough to be handled by the astronaut • Skylights require insulating characteristics, while allowing light to pass through

  7. Overview: Thermoelectricity • In many technologies a vast quantity of heat is eliminated as waste. Nonetheless, the efficiency of the process would be much higher if some of the heat were transformed into electricity THE FIGURE OF MERIT • Materials with a high Seebeck coefficient (S=∆V/∆T) are needed • Also a low thermaland a high electrical conductivity would be ideal

  8. Overview: Heat-Exchange Fluids • Conventional heat-transfer fluids have inherently poor thermal conductivity compared to solids. Several industries would benefit from increasing their thermal conductivity to reduce heat exchanger sizes and pumping needs TO HAVE IN MIND • High thermal conductivity • Low friction coefficient • Clogging of microchannels is undesired • Lubricating behavior is a plus

  9. Preliminary Approaches: INSULATION • Evolution of new materials from ceramics to modern composites bricks asbestos fiber glass

  10. Preliminary Approaches POWER DISSIPATION • Changes in the electronics technology rather than in cooling methods CMOS technology vacuum tube BJT transistor www.noveltyradiocom

  11. Preliminary Approaches THERMOELECTRICS • Not much interest until the 90’s, because of conflicting characteristics of materials (figure of merit) Radioactive heating Thermoelectric Module


  13. Preliminary Approaches HEAT EXCHANGE • Playing with the design equation Q=UA (Ti-To) and making heat integration Microchannel heat exchanger Helically baffled heat exchanger

  14. The intelligent design of the nanostructure of a material can provide all the desired properties, including the thermal conductivity Contextualization Nanotechnology-based revolution!!! REQUIREMENTS • Understanding the heat transfer phenomena at the molecular level • Modification of the structure of the material accordingly • Computational and experimental resources to determine k at the nanolevel

  15. Current Research: Nanotechnology Nanofluids/Heat Exchange Aerogels/Insulation Reduce k Increase k

  16. Current Research: Nanotechnology Thin Film/Thermoelectrics MEMS/Power Dissipation Reduce k Nature Materials (2008) Vol 7, 105 Nature Nanotechnology(2008) Vol 3, 275

  17. Emphasis: Polymer Industry

  18. One of the most pervasive materials in modern society Motivation: Polymer Industry Bayern chemical Plant, Baytown, Texas • Ease of processing and versatility • Attractive for the development of new materials • Integral part of high-tech applications Nature Materials (2008) Vol 7, 261

  19. Structural Reinforcement • Increase of Electrical Conductivity • Increase of Thermal Conductivity Research Status: Polymer Industry


  21. Mechanism: Electron Heat Transport • Characteristic of metallic compounds Free Electrons High Kinetic Energy Electrons Metal Atoms Interaction between energetic electron and atom Strong vibration HOT REGION Increased vibration

  22. Very effective heat transport mechanism • Characterized by electron mean free path • Not so sensitive to lattice defects • Typically 20-400 W/m.K Mechanism: Electron Heat Transport

  23. Mechanism: Phonon Heat Transport • Characteristic of most compounds Vibrational excitation being transmitted Strong vibration HOT REGION A Diamond lattice

  24. Mechanism: Phonon Heat Transport • Heat is transferred through lattice vibrations

  25. Mechanism: Phonon Heat Transport • Phonons are quantized analogous to the vibrations of a guitar string Phonon velocity (sound speed) L k=1/3(CVv l) Mean free path length Heat capacity

  26. Imperfections in the structure enhance phonon scattering and decrease k Mechanism: Phonon Heat Transport Scattering point

  27. Not as efficient as electron heat transport • Characterized by phonon free path and velocity • Very sensitive to defects (e.g. amorphous structure of polymers) • Typical values range from 0.01-50 W/m.K Mechanism: Phonon Heat Transport

  28. The Green-Kubo expression for thermal conductivity is widely used Molecular Simulation k= V ∫dt <JQ(t)JQ(0)> kBT2 • Force Field defining potential energy • Instantaneous velocities related to kinetic energy • Sometimes and external field

  29. Analogy with electric circuits with R ~ 1/k Thermal Conductivity Design Aerogel structure Serial Resistances

  30. Analogy with electric circuits with R ~ 1/k Thermal Conductivity Design Parallel Resistances

  31. Altering the value of the resistances… Thermal Conductivity Design Improving crystallinity Adding defects decrease resistance Increase resistance Nature Materials (2008) Vol 7, 105

  32. PART III CASE STUDY: A Polymer more conductive than metal

  33. Embedding thermally conductive nanostructures in a polymeric matrix Alternative work: Polymer Composites TEM image of a composite Nature (2007), Vol 447, p. 1066

  34. Molecular simulations reveal a thermal conductivity of ~ 104 W/m.K Alternative work:Carbon Nanotube Conductivity Nanotube (10,10) Green-Kubo relation Phys. Rev. Let. (2000) Vol 84, p. 4663

  35. An effort to conduct through the nanotube network instead of the polymer matrix Alternative Work: Nanotube-Polymer Composites TEM side view Ideal structure model Adv. Mat. (2005) Vol 17, p. 1562

  36. Even the most promising results only enhance 6.5 W/m.K Alternative Work:Nanotube-Polymer Composites Adv. Mat. (2005) Vol 17, p. 1562

  37. Experimental work shows ultrafast thermal transport in self-assembled molecules Preliminary Work: Conduction in Molecular Chains Set-up schematics Self-assembly Summary • Sample is heated with a pulsed laser • Sum Frequency Generation (SFG) spectroscopy is performed Science (2007) Vol 317, p. 787

  38. Heat is transferred in a time frame of picoseconds Preliminary Work: Conduction in Molecular Chains Molecular excitations Heat transfer Science (2007) Vol 317, p. 787

  39. Molecular Dynamics results provide further inside Preliminary Work: Conduction in Molecular Chains Thermal Disorder after 10 ps Science (2007) Vol 317, p. 787

  40. Polyethylene chains were shown to have k in the order of 103 W/m.K Preliminary work:Thermal Conductivity of Polymer Chains Thermal conductivity for different domain sizes Polyethylene chain Phys. Rev. Let. (2008) Vol 101, p. 235502

  41. Modification of thermal properties in polymers composites not as good Motivation • Molecular simulations and experiments suggest high thermal conduction in hydrocarbon chains • Thermal conductivity enhancement done on microfibers

  42. Featured Paper:Synthesis Procedure Fiber Drawing Schematics a) Polyethylene gel preparation b) Gel sample heating c) Tungsten tip contact wit gel d) Tungsten tip withdrawing e) Microscope inspection f) Secondary heating activated Nature nanotechnology (2010), Vol. 5, p. 251

  43. Featured Paper:Nanostructure Changes • Molecular chains are expected to align, thus approaching the ideal case of a thermal transport on a single chain nanostructure in gel sample nanostructure in nanofiber Nature Nanotechnology (2010), Vol. 5, p. 251

  44. Featured Paper:Nanostructure Changes • The structure achieves crystallinity as confirmed by diffraction measurements TEM image of the fiber Diffraction pattern of the fiber Orthorhombic Structure Nature Nanotechnology (2010), Vol. 5, p. 251

  45. Featured Paper:Thermal Conductivity Measurements Measurement Setup a) Cantilever holds the fiber b) Fiber cut at 300µm from the tip c) Loose end joined to thermocouple d) Thermocouple heated up e) Cantilever is stimulated f) Laser picks up the signal

  46. A thermal conductivity around 110 W/m.K was achieved. This is higher than for most pure metals!!! Featured Paper:Thermal Conductivity Results

  47. Improve uncertainty in measurements • Understand mechanism in nanostructures • Trade-off in design of material properties General Challenges

  48. Structure uniformity along the nanofiber • Adapt process for future scaling up • Vanish thermal resistance among fibers Particular Challenges