1 / 55

PRODUCTION OF SILICON CARBIDE NANOWIRES BY INDUCTION HEATING

PRODUCTION OF SILICON CARBIDE NANOWIRES BY INDUCTION HEATING. Kendra L. Wallis June 2006. Overview. Introduction SiC and Chemical Kinetics Induction Heating Testing and Use of Equipment Reaction Kinetics of SiC Nanowires Elimination of Excess Reactants Conclusions. Introduction.

jillian-horne
Download Presentation

PRODUCTION OF SILICON CARBIDE NANOWIRES BY INDUCTION HEATING

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. PRODUCTION OF SILICON CARBIDE NANOWIRES BY INDUCTION HEATING Kendra L. Wallis June 2006

  2. Overview • Introduction • SiC and Chemical Kinetics • Induction Heating • Testing and Use of Equipment • Reaction Kinetics of SiC Nanowires • Elimination of Excess Reactants • Conclusions

  3. Introduction

  4. Nano-structure Research • Hot topic today—nanostructured materials research • Improved physical properties • Flexibility of designing materials from nanoblocks • Nanocomposites—combination of 2 or more phases, 1 or more is nano-size

  5. Motivation • Need for low-cost, hard materials for use at high temperature • SiC: ceramic composite • Nanostructured SiC demonstrates improved high temperature mechanical properties • Carbon MWNTs demonstrate high hardness and fracture toughness

  6. What to do? • Make SiC nanowires • Study reaction • Study structure • Measure hardness and toughness • Correlate properties with structure • New uses may include hard fibers for armor

  7. SiC and Chemical Kinetics

  8. SiC • Moissanite—found in meteorites, rare • Synthetic SiC • Many ways to make it • Many uses • High melting point (2700°C) and highly inert • High thermal conductivity • High E-field breakdown and max current density • Hardness 9.25 (diamond is 10)

  9. Reaction Kinetics in Solids • Product forms between reactants • One reactant passes through product barrier phase • Product layer grows, diffusion takes longer • Reaction at interface • Diffusion controlled • Nucleation controlled Si SiC CNT Si diffuses through SiC product barrier phase

  10. Reaction Rate • Chemical reaction • Rate of increase of product • Measure reaction rate for several temperatures • Fit to theoretical model to find reaction mechanism

  11. General Rate Law  = fractional remains of reactant; k = rate constant Summary of ModelsExpected Values of n

  12. Activation Energy • Energy required to initiate process • Arrhenius equation Rate constant k at temperature T R = universal gas constant E = activation energy A = constant • Plot ln k vs. 1/T to find E

  13. Parameters in Reaction Kinetics of Solids • Reactants: Si and C MWNT Various molar ratios • Particle sizes: Si APS 30 nm (98%) C MWNT (95%) OD 60-100 nm, L 5-15 m • Mixing – ultrasonic mix in acetone Consider other methods • Products: SiC nanowires Look for formation of anything else • Temperature – effects on all parameters

  14. Nano-particle Reactants • Particle size affects: • Reaction rate • Physical properties • Mechanical properties • Decrease particle size—increases surface area, which may explain enhanced hardness • Create product with small grain size

  15. Carbon MWNT • One-dimensional system • Carbon (1s22s22p2) has 4 valence electrons • In 2-D, sp2 hybridization forms graphite • Nanotubes exhibit sp2 hybridization – but cylindrical not planar • Graphene sheet of 6-member C rings in honeycomb lattice • Multiple concentric cylindrical shells with common axis • Each shell is cylindrical graphene sheet, d = 1 to 10 nm

  16. Induction Heating

  17. Induction Heating Faraday’s law Joule’s law

  18. Inductoheat Statipower BSP12 • 480 V, 60 Hz, 3f AC current • Solid state inverter • Converts current to DC • Then to high frequency AC (30 kHz) • Variable ratio isolation transformer—feedback loop to adjust V and P for set I • Tuning capacitor—impedance matching • Coil

  19. Induction Furnace

  20. Current through a coil produces nearly uniform magnetic field down the center

  21. Alternating current  Changing magnetic field Current flows around cylindrical shellSame frequency, opposite direction  = 30 kHz max

  22. End View of Cylindrical Shell • R = inner radius • d = wall thickness •  = skin depth • d0 = screening depth

  23. Skin Effect Faraday’s law Current flowing in a conductor flows only near the surface Ampere-Maxwell law Electromagnetic wave equation for E-field

  24. Complex wave number k Substitute solution into wave equation

  25. For a good conductor Plane wave includes periodicity in time and space plus damping term in space attenuation factor

  26. Skin depth For a wave traveling in the z-direction: e-folding distance skin depth

  27. Cylindrical Shell inside Coil • External magnetic field B0 along z-axis • Frequency  • Faraday’s law • Current around shell induces magnetic field BC, screening inside of shell • Field inside BI = B0 + BC

  28. Screening Depth • Derived by Fahy, et al.1 • Screening factor – ratio of field at inner wall to applied field • Induced current falls off toward center as function of wall thickness • Interior screened when d > d0 where 1Fahy S., Kittel, C., Louie, S., Am. J. Phys. 56 (11) 1998 989

  29. Screening of external field Bout by cylindrical shell, radius R, wall thickness d in units of 2 / R, where  is skin depth d = d0 Bin = 0.7 Bout d =2 d0 Bin < ½ Bout

  30. Heat Generated by Resistive Losses Joule’s law Current density Current flows around shell,  area element dr dz Total current

  31. Resistive Heat Generated • RE electrical resistance • RE =  L / A, L = 2R, A = d L • Resistivity varies with temperature • Conductivity  = 1 /  • Heat per unit length of cylindrical shell • Q proportional to R2 d 

  32. Testing andUse of Equipment

  33. Testing and Use of Equipment • Induction furnace • 25 kW maximum power • 30 kHz frequency • Repeatable and consistent heating pattern • Heats quickly—measure accurate reaction time • Safe and efficient • Non-polluting, environmentally friendly • Non-conducting material not affected

  34. Graphite Crucible at 1400C

  35. Equilibrium Temperature • 2 min to equilibrium • Increases with input power

  36. Graphite Crucible • Graphite aged with repeated use • Possible explanations graphitization oxidation

  37. Atmosphere • Heated in nitrogen • Change in equilibrium temperature reduced-not eliminated • Rate of heating reduced

  38. Stainless Steel Crucible • Heated in N2 • no graphitization • no oxidation • Repeatable • Temperature increases with input power • Heats faster

  39. 3 Stainless Steel Crucibles STn – small radius, thin wall LTk – large radius, thick wall STk – small radius, thick wall Q = R2dQ0 • Skin effect insignificant • Screening may be related to unexpected temperature of STk

  40. Reaction Kineticsof SiC Nanowires

  41. Reaction Kinetics • Requires knowledge of quantity of product and/or quantity of reactant remaining as function of time • Determine mass concentration of SiC product to remaining Si + SiC • Correlation between XRD peak intensity and mass concentration determined experimentally by Pantea and confirmed here

  42. y = 0.36x2 + 0.64x

  43. Reaction TimeFast heating and cooling reduce error uncertainty (20s) uncertainty (10s) Reaction time

  44. Reaction Rate • General Rate Law • Find rate constant k and parameter n for different temperatures • Calculate SiC concentration  from measured XRD peak intensities • Measure sintering time

  45. Concentration v TimeFit to General Reaction Rate Law= 1 – exp [ - (kt)n]

  46. k and n • k = 7.6 x 10-6+/- 5 x 10-6 • n = 0.46 +/- 0.05 • Refer to Table of Rate Laws • Suggests diffusion-controlled 1-dimensional growth with decelerating nucleation rate • Data at more temperatures will give better understanding of reaction mechanism

  47. Elimination of Excess Reactants

  48. X-ray Diffraction – Mixture

  49. XRD Characteristic Peaks Identity 2  • C MWNT 26.28 • Si (111) 28.44 • SiC (111) 35.74 • Si (220) 47.35 • SiC (220) 60.02

  50. X-ray Diffraction – Sintered 2 min at 1200°C

More Related