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 todaynanostructured materials research
• Improved physical properties
• Flexibility of designing materials from
  nanoblocks
• Nanocompositescombination 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

• Moissanitefound in meteorites, rare
• Synthetic SiC
• Many ways to make it
• Many uses
• High melting point (2700C) 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 sizeincreases 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

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Induction Heating
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Induction Heating
Faradays law
Joules law
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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 transformerfeedback
  loop to adjust V and P for set I
• Tuning capacitorimpedance matching
• Coil

19
Induction Furnace
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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
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End View of Cylindrical Shell

• R inner radius
• d wall thickness
• ? skin depth
• d0 screening depth

23
Skin Effect

• Current flowing in a conductor flows only near
  the surface

Faradays law
Ampere-Maxwell law
Electromagnetic wave equation for E-field
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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 ?
• Faradays 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 gt d0 where

1Fahy S., Kittel, C., Louie, S., Am. J. Phys. 56
(11) 1998 989
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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 lt ½ Bout
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Heat Generated by Resistive Losses
Joules 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
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Testing and Use of Equipment

• Induction furnace
• 25 kW maximum power
• 30 kHz frequency
• Repeatable and consistent heating pattern
• Heats quicklymeasure accurate reaction time
• Safe and efficient
• Non-polluting, environmentally friendly
• Non-conducting material not affected

34
Graphite Crucible at 1400?C
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Equilibrium Temperature

• 2 min to equilibrium
• Increases with input power

36
Graphite Crucible

• Graphite aged with repeated use
• Possible explanations graphitization oxidation

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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

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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
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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
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Reaction Time Fast 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
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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
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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

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X-ray Diffraction Sintered 2 min at 1200C
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X-ray Diffraction Burned 2 hr at 700C
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X-ray Diffraction Washed in KOH
53
Conclusions
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Conclusions

• Induction heatingsafe and efficient method of
  producing nanostructured SiC
• Oxygen-free environment preferred
• Material and geometry of crucible should be
  considered
• Reaction rate constant at 1040 C suggests
  diffusion-controlled 1-dimensional growth with
  decelerating nucleation rate
• Burning in air, washing with KOHsafe and
  efficient method of purification

55
Future Work

• Activation energyexplain reaction mechanism
• Nanostructure of SiC nanowires
• X-ray (grain size and strain)
• Raman (grain size and strain)
• TEM (nanowires)
• Mechanical properties
• Correlation between mechanical properties and
  structure
• SiC nanograss