PRODUCTION OF SILICON CARBIDE NANOWIRES BY INDUCTION HEATING

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