PRODUCTION OF SILICON CARBIDE NANOWIRES BY INDUCTION HEATING

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PRODUCTION OF SILICON CARBIDE NANOWIRES BY INDUCTION HEATING

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PRODUCTION OF SILICON CARBIDE NANOWIRES BY INDUCTION HEATING

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PRODUCTION OF SILICON CARBIDE NANOWIRES BY INDUCTION HEATING

Kendra L. Wallis

June 2006

- Introduction
- SiC and Chemical Kinetics
- Induction Heating
- Testing and Use of Equipment
- Reaction Kinetics of SiC Nanowires
- Elimination of Excess Reactants
- Conclusions

- 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

- 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

- Make SiC nanowires
- Study reaction
- Study structure
- Measure hardness and toughness
- Correlate properties with structure
- New uses may include hard fibers for armor

- 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)

- 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

- Chemical reaction

- Rate of increase of product

- Measure reaction rate for several temperatures

- Fit to theoretical model to find reaction mechanism

= fractional remains of reactant; k = rate constant

Summary of ModelsExpected Values of n

- 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

- 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

- 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

- 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

Faraday’s law

Joule’s law

- 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

= 30 kHz max

- R = inner radius
- d = wall thickness
- = skin depth
- d0 = screening depth

Faraday’s law

Current flowing in a conductor flows only near the surface

Ampere-Maxwell law

Electromagnetic wave equation for E-field

Substitute solution into wave equation

Plane wave includes periodicity in time and space plus damping term in space

attenuation factor

For a wave traveling in the z-direction:

e-folding distance

skin depth

- 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

- 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

d = d0 Bin = 0.7 Bout

d =2 d0 Bin < ½ Bout

Joule’s law

Current density

Current flows around shell, area element dr dz

Total current

- RE electrical resistance
- RE = L / A, L = 2R, A = d L
- Resistivity varies with temperature
- Conductivity = 1 /
- Heat per unit length of cylindrical shell

- Q proportional to R2 d

- 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

- 2 min to equilibrium
- Increases with input power

- Graphite aged with repeated use
- Possible explanations graphitization oxidation

- Heated in nitrogen
- Change in equilibrium temperature reduced-not eliminated
- Rate of heating reduced

- Heated in N2
- no graphitization
- no oxidation

- Repeatable
- Temperature increases with input power
- Heats faster

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

- 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

uncertainty (20s)

uncertainty (10s)

Reaction time

- General Rate Law

- Find rate constant k and parameter n for different temperatures
- Calculate SiC concentration from measured XRD peak intensities
- Measure sintering time

- 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

Identity 2

- C MWNT26.28
- Si (111)28.44
- SiC (111)35.74
- Si (220)47.35
- SiC (220)60.02

- Induction heating—safe 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 KOH—safe and efficient method of purification

- Activation energy—explain 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