Electrical Properties of Polymers, Ceramics, Dielectrics, and Amorphous...
Transcript of Electrical Properties of Polymers, Ceramics, Dielectrics, and Amorphous...
Electrical Properties of Polymers, Ceramics, Dielectrics, and
Amorphous Materials
Dae Yong JEONG
Inha University
Review & Introduction
We learned about the electronic transfer in Metal and
Semiconductor.
In general, Polymer and ceramic materials are
insulating. But, some polymers and ceramics show
S.C and conducting properties.
Beside electron, Any other charges?
Ionic conduction
For insulating materials, what happens under E-field?
Capacitor (condenser)
* Same Materials for different naming • For used for insulation (Generally for DC) Insulator
• For used for AC current For materials, Dielectrics
• For circuit, Capacitor (condenser) : Dielectric materials for capacitor application
Materials vs Conductivity
Conductor Semiconductor
Insulator (utilize R ~ ∞)
Dielectric for AC
Capacitor (C) in circuit
Metal Most Si ?
Ceramic
Electron conductor
ITO
IrO2, RuO2,
SrRuO3 ZnO, SnO2, TiO2
etc (some) Most
Ionic conductor
ZrO2 (O2-),
Na3Zr2Si2PO12
(NASICON: Na+)
Polymer Conducting
polymer (rare)
Conducting
polymer (rare) Most
Charge Transport in Materials
Electronic Conductor Ionic Conductor
Materials
Metal
Semiconductor
Al, Cu
Si, GaAs
SnO2, ZnO, TiO2
LiMnO2
Semiconducting polymer
Electrolyte (solid, liquid)
ZrO2
Li-polymer electrolyte
Applications
Electric connection (metal)
Semiconductor device
Opto-electronic device
(LED, LASER, Solar Cell)
Battery electrode
Fuel cell
Battery
Electrochemical sensor
O2- transport
Light metal ion (Li+, Na+ …)
9.1. Conducting Polymers and Organic Metals
About “Polymer”
Basically Carbon (C: 1S2 2S2 2P2) based materials
Depending on bond, different hybridization
The simple repeating unit of a polymer is the monomer.
Copolymer is a polymer made up of two or more monomers
Styrene-butadiene rubber
(CH CH2 CH2 CH CH CH2)n
Homopolymer is a polymer made up of only one type of monomer
( CF2 CF2 )n
Teflon
( CH2 CH2)n
Polyethylene
(CH2 CH)n
Cl
PVC
• % Crystallinity: % of material that is crystalline. -- Tensile stress and Young’s modulus often increase with % crystallinity.
-- Annealing causes crystalline regions to grow. % crystallinity increases.
crystalline region
amorphous region
Microstructure & Crystallinity
Molecular Orbital Theory
Similar to Band Theory
Powerful to explain the electronic properties in polymer Different material different suitable theory
Energy level can be calculated as,
A bonding molecular orbital has lower energy and greater
stability than the atomic orbitals from which it was formed.
An antibonding (*) molecular orbital has higher energy and
lower stability than the atomic orbitals from which it was formed.
9.1. (S.C) Conducting Polymers and Organic Metals
Conduction polymer
Peculiar property in polymer
OLED, capacitor, sensor, photovoltic devices, antistatic…
crystalline region
amorphous region
Distinct band structure
(From Molecular orbital theory)
Periodic structure
(High conductivity)
9.1. (S.C) Conducting Polymers and Organic Metals
For example,
Polyacetylene Conjugated organic polymer: alternating single and double bond
High conductivity from a high degree of crystallinity.
Conductivity ~ comparable to Si
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
Trans Polyacetylene
Cis Polyacetylene
Different structure
Different conductivity
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
Calculation (a)/observation of Band Structure
Trans Polyacetylene
Same length
Continuous band
Metallic
In reality, no same length
Little difference/ large difference
Band gap
S.C/insulating
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
Where are electrons in conduction band from?
Double bond weak bond, easy breakage easily disassociated by thermal energy easily accelerated by E-field
Electron properties The effective mass: m* = 0.6 mo at k = 0, 0.1 mo at k = π/a
For τ 10-14s μ ~ 200 cm2/Vs
LUMO
Lowest Unoccupied Molecular Orbital
(similar to conduction band)
HOMO
Highest Occupied Molecular Orbital
(similar to valence band)
Band width: 10- 14 eV
Band gap: 1.5 eV
How to increase the conductivity?
Same length bw carbon (Fig. 9.5 (a))
Doping (20 ~ 40%, lot of amount!!)
Arsenic-pentafluoride ~ x 107
undoped trans- comparable to that
of metal
Oxidant p-type
Alkali metal n-type
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
How to explain the conduction in polymer?
In fact, the conduction in polymer is different from metal and S.C such as Si.
In metal and S.C, conduction was explained with electron movement. Where, electrons are from ionization.
But, polymer conduction is from breakage of pi-bonding.
Let’s introduce “SOLITONs” for the conduction in polymer.
Double-single-double-single- Double-single-single- double-single-
Locally negative (soliton)
Seem like “additional energy level in forbidden band.”
Same length at the center of a soliton
Many soliton over lap metallic
Soliton movement (soliton wave) ~ moving electron
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
9.1. Conducting Polymers and Organic Metals
Conducting polymer Polyanilines
Polypyrroles
Polythiophenes
Polyphenylenes
Poly(p-phenylene vinylene)
……
PEDOT : poly(3,4-ethylenedioxythiophene) water soluble
With thin layer transparent
OLED, sensor, flexible device, wearable device..…
Toxic, poor stability, sensitive to environmental condition
9.2. Ionic conduction
In general, ionic X-tal large band gap insulating
Ion movement (hop from lattice site to lattice site under E-field)
ionionion eN
Question
Which conditions are required for ion to move?
Sufficient energy to overcome energy barrier
Empty site (vacancy) around movable ions (defects)
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
Defect in Ceramic (charge carrier)
Δ E
Δ T
Δ C
conduction
Electrical
chemical
Why defects inside crystal?
Thermodynamical explanation
Naturally with Defect increase randomness (mixture)
lower E
STHG
Defect in Ceramic (charge carrier)
What kind of defects (intrinsic)?
Mi
X
M VMM
Metal move to another interstitial place
Metal gone vacancy generation, effective charge (-2)
At the same time
Insertion of Metal ion effective change (+2)
MO VVnull
(Metal ion & Oxygen ion) gone
Oxygen ion gone vacancy generation, effective charge (+2)
At the same time
Metal ion gone vacancy generation effective change (-2)
Frenkel defect
Schottky defect
9.2. Ionic conduction
Nion : ~ # of defect
μion ~ ΔC (concentration of defects) Diffusion
ionionion eN
Diffusion: movement due to the gradient of concentration
Fick’s first law for Diffusion
Tk
De
B
ion
mxm
mols
Dsm
mol
dx
dDJ
distance:,ion concentrat :,mtcoefficiendiffusion : ,flux] [diffusion: J
3
2
3
Mobility is related with diffusion coefficient. [Einstein Relation]
Larger Diffusion coefficient fast movement
High Temp scattering increases
larger diffusion coefficient (D is function of T.)
9.2. Ionic conduction
Energy. activation is Q ,exp
Tk
QDD
B
o
Temp effect on Diffusion coefficient:
Arrhenius Eq.
Tk
Q
Tk
Q
Tk
DeN
Tk
Q
Tk
Q
Tk
DeN
B
oion
BB
oiono
B
o
BB
oionion
1lnln
exp
expexp
2
2
ionionion eN Conductivity of ionic materials
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
9.2. Ionic conduction
From experimental
data (beside) slope
activation energy
extrinsic region
Temp increase easy movement
High temp: intrinsic region
Generate additional defects
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
9.2. Ionic conduction
Extrinsic Disorder : generated by the addition of
impurities or solutes
Example: CaO doped ZrO2-δ Nonstoichiometric
compound
OZr VaCCaO
As Ca2+ substitutes Zr4+, to maintain charge neutrality Oxygen
vacancy is formed resulting in the non-stoichiometric compound.
As there is Oxygen vacancies, O2- can move easily. (O2-
electrolyte for fuel cell.)
9.3. Electron conduction in Metal Oxides
Examples
ZnO Zn : 4S2 Zn2+ : 4S0
O : 2P4 O 2- (2 electrons from Zn): 2P6
ZnO: stoichiometric compound insulating or wide band gap S.C
ZnO1-δ (Oxygen vacancy) + 2e- n-type S.C
Sensor, varistor etc…
SnO2 In2O3 doped SnO2 ITO: transparent electrode with high conductivity
NiO Ni : 4S2 Ni2+ : 4S0
O : 2P4 O 2- (2 electrons from Zn): 2P6
Stoichiometric compound insulating
Nonstoichiometric : Ni1-δO (Ni vacancy) + 2P+ p-type S.C
9.4. Amorphous Materials (Metallic Glasses)
Most metal crystalline
Amorphous phase (metallic glasses or glassy metals) Rapid solidification ~ 105K/s
Short range ordering
Nondirectional bonding
Bloch theorem (periodic potential) for band calculation can not be utilized.
Nano-size grain: Unusual electrical, mechanical, optical, magnetic, corrosion properties.
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
9.4. Amorphous Materials (examples)
Amorphous metallic
Example: Cu-Zn
Partially filled band
But, small Z(E) at near EF
Poor conductivity (5 x 103 1/Ω cm)
Amorphous S.C Strong binding force localized energy
level, discrete energy level band gap
Small Z(E)
Small conductivity (10-7 1/Ω cm at RT)
H-doped a-Si for photovoltic device
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
9.4.1. Xerography (electrophotography) An important application of a-S.C (ex: a-Se, a-Si)
Laser printer or copy machine
a-S.C deposited
High voltage eletrostatic
charge on insulating a-S.C
Scanning light electron/hole pair generation
discharge the affected parts patterning
Charged toner with
magnetic particle
Transfer the charged
toner particles with
E-field
Adhere the polymer
toner particle onto
paper
From Electronic properties of materials, Fourth Edition, Hummel (© Springer, 2010)
Materials Scientists
PLS, be curious about the devices. How to operate?
What is the basic principle?
And, make your own ideas!! Environmentally friendly materials
Energy (materials) saving
Save the world and make money!!
Electronic Materials
Electron conductivity
Conduction (charge flow) V=iR (Ohm’s law) 전기장을 가하면, 전하가 이동 직선적으로 이동 Energy conversion: heat (i2R)
Conduction (charge flow) 전하의 농도를 조절 (Extrinsic S.C) “전하의 농도차에 의한 diffusion을 조절” Ex: P-N junction : Non-linear I-V behavior
Energy band…. Energy conversion (ex: light, E=hv)
Capacitive (charge storage) Energy conversion {ex:
Piezoelectric (size change)} …