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

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Overview
Overview

  • Introduction

  • SiC and Chemical Kinetics

  • Induction Heating

  • Testing and Use of Equipment

  • Reaction Kinetics of SiC Nanowires

  • Elimination of Excess Reactants

  • Conclusions



Nano structure research
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


Motivation
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


What to do
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



Production of silicon carbide nanowires by induction heating
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)


Reaction kinetics in solids
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


Reaction rate
Reaction Rate

  • Chemical reaction

  • Rate of increase of product

  • Measure reaction rate for several temperatures

  • Fit to theoretical model to find reaction mechanism


General rate law
General Rate Law

 = fractional remains of reactant; k = rate constant

Summary of ModelsExpected Values of n


Activation energy
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


Parameters in reaction kinetics of solids
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


Nano particle reactants
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


Carbon mwnt
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



Induction heating1
Induction Heating

Faraday’s law

Joule’s law


Inductoheat statipower bsp12
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




Production of silicon carbide nanowires by induction heating
Alternating current field down the center Changing magnetic field Current flows around cylindrical shellSame frequency, opposite direction

 = 30 kHz max


End view of cylindrical shell
End View of Cylindrical Shell field down the center

  • R = inner radius

  • d = wall thickness

  •  = skin depth

  • d0 = screening depth


Skin effect
Skin Effect field down the center

Faraday’s law

Current flowing in a conductor flows only near the surface

Ampere-Maxwell law

Electromagnetic wave equation for E-field


Complex wave number k
Complex wave number k field down the center

Substitute solution into wave equation


For a good conductor
For a good conductor field down the center

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

attenuation factor


Skin depth
Skin depth field down the center

For a wave traveling in the z-direction:

e-folding distance

skin depth


Cylindrical shell inside coil
Cylindrical Shell inside Coil field down the center

  • 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


Screening depth
Screening Depth field down the center

  • 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


Production of silicon carbide nanowires by induction heating
Screening of external field B field down the centerout 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


Heat generated by resistive losses
Heat Generated by Resistive Losses field down the center

Joule’s law

Current density

Current flows around shell,  area element dr dz

Total current


Resistive heat generated
Resistive Heat Generated field down the center

  • 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 


Testing and use of equipment
Testing and field down the centerUse of Equipment


Testing and use of equipment1
Testing and Use of Equipment field down the center

  • 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


Graphite crucible at 1400 c
Graphite Crucible at 1400 field down the centerC


Equilibrium temperature
Equilibrium Temperature field down the center

  • 2 min to equilibrium

  • Increases with input power


Graphite crucible
Graphite Crucible field down the center

  • Graphite aged with repeated use

  • Possible explanations graphitization oxidation


Atmosphere
Atmosphere field down the center

  • Heated in nitrogen

  • Change in equilibrium temperature reduced-not eliminated

  • Rate of heating reduced


Stainless steel crucible
Stainless Steel Crucible field down the center

  • Heated in N2

    • no graphitization

    • no oxidation

  • Repeatable

  • Temperature increases with input power

  • Heats faster


3 stainless steel crucibles
3 Stainless Steel Crucibles field down the center

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


Reaction kinetics of sic nanowires
Reaction Kinetics field down the centerof SiC Nanowires


Reaction kinetics
Reaction Kinetics field down the center

  • 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


Y 0 36 x 2 0 64 x
y field down the center = 0.36x2 + 0.64x


Reaction time fast heating and cooling reduce error
Reaction Time field down the centerFast heating and cooling reduce error

uncertainty (20s)

uncertainty (10s)

Reaction time


Reaction rate1
Reaction Rate field down the center

  • General Rate Law

  • Find rate constant k and parameter n for different temperatures

  • Calculate SiC concentration  from measured XRD peak intensities

  • Measure sintering time


Concentration v time fit to general reaction rate law 1 exp kt n
Concentration v Time field down the centerFit to General Reaction Rate Law= 1 – exp [ - (kt)n]


K and n
k field down the center 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


Elimination of excess reactants
Elimination of field down the centerExcess Reactants


X ray diffraction mixture
X-ray Diffraction – Mixture field down the center


Xrd characteristic peaks
XRD Characteristic Peaks field down the center

Identity 2 

  • C MWNT 26.28

  • Si (111) 28.44

  • SiC (111) 35.74

  • Si (220) 47.35

  • SiC (220) 60.02





Conclusions
Conclusions field down the center


Conclusions1
Conclusions field down the center

  • 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


Future work
Future Work field down the center

  • 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