Post on 13-Jan-2016
description
Ferroelectric ceramics
1824 Pyroelectricity in Rochelle salt NaKC4H4O6*4H2O (Brewster)
1880 Piezoelectricity in quartz and Rochelle salt (Jacques & Pierre Curie – quartz balance)
1912 Ferroelectricity proposed as a property of solids
1921 Ferroelectricity in Rochelle salt (Valasek)
1935 Ferroelectricity in KH2PO4
1941 High dielectric constant in BaTiO3
1944 Ferroelectricity in BaTiO3 (von Hippel, Wul & Goldman)
1945 BaTiO3 ceramics for piezoelectric transducers (electrical poling)
1949 Theory of ferroelectricity in BaTiO3 (Devonshire)
1949 Ferroelectricity in LiNbO3 and LiTaO3
1952 Phase diagram of Pb(Zr,Ti)O3 – PZT established
1954 PZT reported as a useful piezo transducer
1955 Ferroelectricity in alkali niobates
1961 PbMg1/3Nb2/3O3-PMN reported as ferroelectric relaxor
1969 Optical transparency achieved in hot pressed PLZT
1971 Useful electrooptical properties reported for PLZT
1980 Electrostrictive PMN devices developed
1992 New types of PZT piezo actuators developed
1993 Integration of FE films on silicon technology - FERAMs
1997 Ultrahigh piezoelectric coefficients in PMN-PT and PZN-PT
2002 High polarization and magnetoelectric coupling in BiFeO3 films
2004 High-performance lead free KNN piezoceramics (KxNa1-xNbO3)
Important events in the history of ferroelectrics
Fundamental steps in the understanding and application of ferroelectric and piezoelectric ceramics
(1) The discovery of unusually high dielectric constant of BaTiO3 ( multilayer ceramic capacitors - MLCCs).
(2) The discovery that the origin of the high dielectric constant in BaTiO3 is its ferroelectric nature, thus disclosing an entire new class of piezoelectric/ferroelectric materials –ABO3 perovskites with BO6 octahedra. Ferroelectricity no longer related to hydrogen bonds.
(3) The discovery of the electrical poling process to align the electrical dipoles of the grains/domains within the ceramics obtaining properties similar to those of single crystals (large scale production and application of piezoelectric transducers and actuators).
Important events in the history of ferroelectrics
Piezoelectricity and ferroelectricity in solids
Enantiomorphism
Optical activity (Circular Dichroism)
PiezoelectricitySecond-Harmonic Generation (SHG)
Polar crystals (Pyroelectricity)
3
1
2
1
5
1
5
3
21 noncentrosymmetric crystal classes
T
curr
ent
Piezoelectricity and ferroelectricity in solids
7 Crystal systems - 32 Symmetry Point Groups
21 Noncentrosymmetric 11 Centrosymmetric (Non-piezoelectric)
20 PiezoelectricPolarized under stress
10 Pyroelectric/PolarSpontaneously polarized
Subgroup FerroelectricSpontaneously polarized
Polarization reversible
Tungsten bronzePbNb2O6
Oxygen octahedraABO3
PyrochloresCd2Nb2O7
Layered structuresBi4Ti3O12
Perovskites
BaTiO3 PbTiO3
PT(Pb,La)(Zr,Ti)O3
PLZTPb(Zr,Ti)O3
PZTPbMg1/3Nb2/3O3
PMNBiFeO3 (Na,K)NbO3
Na1/2Bi1/2TiO3
Pyroelectricity and ferroelectricity
Pyroelectric or polar materials exhibit an electrical dipole even in the absence of an external electric field. The polarization associated to this electrical dipoles is called spontaneous polarization, Ps (C/m2). The variation of Ps with temperature determines a variation of the surface charge density and originates a pyroelectric current.
Ferroelectric crystals are polar crystals in which there are at least two equilibrium orientations of the spontaneous polarization vector in the absence of an external electric field, and in which the spontaneous polarization vector may be switched between those orientations by an electric field.
“Fingerprints” of ferroelectric behaviour are: - very high dielectric constant (r>100, often >1000); - sharp peak or anomaly of r around a critical temperature TC; - permittivity obeys the Curie-Weiss law above TC; - hysteresis loop for polarization;
+Ps-Ps
Not all polar crystals are ferroelectrics, examples are tourmaline and hexagonal CdS.Quartz is only piezoelectric; polarization is induced by the electric field. Antiparallel alignment of elementary dipoles can lead to antiferroelectricity.
dt
dT
dT
dP
dt
dPi SS
In some perovskites containing Ti or Nb on the B site (BaTiO3, PbTiO3, KNbO3), a phase transition from a paraelectric cubic structure to a lower symmetry phase (tetragonal) with appearance of spontaneous polarization occurs at a critical temperature TC (Curie temperature).
The paraelectric to ferroelectric phase transition
Spontaneous strain: (cT-aT)/aC cT/ac - 1
Compound TC
(°C)
PS
(μC/cm2)
Qc-t (J/mol)
cT/aT-1 Δz(Ti)
(pm)
BaTiO3 125 26 197 1% 120
PbZr0.5Ti0.5O3 (PZT) 380 40-50 - 2.5% -
PbTiO3 495 81 4815 6.5% 300
KNbO3 435 30 796 - -
Ba
Centrosymmetric T<TC, Polar, noncentrosymmetric
Δz
TC = 2x104 (Δz)2
1/’
r
0
'
TT
Cr For T>TC:
Curie-Weiss law
The paraelectric to ferroelectric phase transition
Discontinuity of Ps at Tc: 1° order transition
Continuous decrease of Ps:2° order transition
Polarization and order of transition
6 orientations
Polarization: order parameter of transition
Ferroelectric hysteresis loop and polarization switching
PS: saturation (spontaneous) polarizationPR: remanent polarizationEC: coercive fieldThe slope of the initial polarization curve gives the dielectric constant
Ideally +PR = -PR and +EC = -EC
-Pr
The Landau-Ginsburg-Devonshire thermodynamic theory
Free energy of a crystal subjected to external stresses and electric field.
The paraelectric to ferroelectric phase transition
For centrosymmetric crystals above TC, the function G can be expanded in even powers of polarization. As the polar phases can be regarded as slightly distorted variants of the non polar cubic phase, the same thermodynamic function can be used for all ferroelectric phases (BT, PZT, NaNbO3).
For a tetragonal crystal (P1 = P2 = 0; P3 >0) :
63111
4311
231 PPPG Free energy for zero stress conditions (P = PS)
353444
232
23112
23311
26
25
244413322112
23
22
2112
63111
4311
231
2
1
2
1
PXPXQPXPXQPXQ
XXXSXXXXXXSXXXS
PPPG
The relevant physical properties (Ps, P, x, ε, etc.) can be determined by minimizing G.
PdEdXxSdTdGi
ii
3
1
strain electric field
stress polarization
The paraelectric to ferroelectric phase transition
First order transition (TC > T0)
T0 TC T1 T2
In BaTiO3 : T0 = TC-12
Second order transition (TC = T0)
T0
Phase transitions in barium titanate
Lattice parameters
RO
TC
TC
C
T
O
R
Polarization orientation(001)C
(110)C
(111)C
Spontaneous polarization
TC
R/OO/T
Dielectric constant
TC
R/O
O/T
Phase transitions in ferroelectric perovskites
Polymorphic phase transition (PPT) in ferroelectrics are determined by: - temperature change; - external electric field; - external stress;
CUBICNO POLARIZATION
The paraelectric to ferroelectric phase transition
1st order transitionTC > T0
2nd order transitionTC = T0
Relaxor ferroelectricT(max, ) TC
2
'
m
rTT
C
T > Tm
0
'
TT
Cr T > Tm
Non-linear, hysteretic Non-linear, non-hysteretic
Ferroelectric domains
Non poled FE crystals spontaneously split in domains. A domain is a region with a uniform orientation of polarization. Domains are separated by domain walls. Ferroelectric domains form to minimize the electrostatic energy and the elastic energy associated with mechanical constraints to which the ferroelectric material is subjected as it is cooled through the Curie temperature.
Domains with oppositely oriented polarization (180° walls) minimize the depolarizing field (Ed) associated to the surface charges and are purely ferroelectric domains.
Domains with perpendicular orientation of polarization (90° walls) minimize the elastic energy and reduce the depolarizing field (Ed) associated to the surface charges. Formation of 90° domain walls is determined by mechanical stresses. These domain walls differ for both the orientation of polarization (ferroelectric domains) and the orientation of spontaneous strain (ferroelastic domains)
In tetragonal BaTiO3 ceramics formation of complex domain structures with both 180° and 90° walls is observed due to the distribution of stresses and electrostatic boundary conditions to which each grain is subjected.
FE domains in tetragonal BaTiO3
Ferroelectric domains
180° and 90° domain walls
<90°
c/a = 1.01
1-10nm
1-10nm
dw 10 mJ/m2
Domain walls can easily move under the influence of mechanical stresses (90° walls) and electric field (90° and 180° walls) unless pinned by electrically charged defects. Defects such as oxygen vacancies, trapped electrons and (acceptor ion-oxygen vacancy) pairs can have a strong effect on domain movement.
Ferroelectric domains
W = (1.0 ±0.3) nm
Structure of 90° ferroelastic domain walls in PbTiO3
c
a
P
(001)
(100
)P
P
(101)c
(101)C <90°
c/a = 1.06
d1
d 2
Ferroelectric domains
Domain wall configurations
90 and 180° domain walls
60, 90, 120 and 180° domain walls
71, 109 and 180° domain walls
CUBICNO POLARIZATION
The number of possible orientations of Ps as well as the number of domain wall configurations increase with decreasing the crystal symmetry.
180° domain walls
Combinations of 90° and 180° domain walls
The “no bound space charges” principle ( div P = 4Q = 0 in the bulk ) rules the formation of domain structures.
Ferroelectric domains
FE domains in single crystal BaTiO3.
FE domains in BaTiO3 ceramics
“harring bone” domain structure
90° domain walls 180° domain walls
180°
90°
FE domains in BaTiO3 ceramicsFine grained ceramics (0.5-few m) show a simpler domain structure with 90° domain walls.The domains disappear above TC
P
Heat-to-tail arrangement of domain walls
Influence of grain size on the dielectric constant of ferroelectric BaTiO3 ceramics
g
g
g
d
Model of a cubic grain of size g with 90° domain walls
Single domain
90° dws 2D arrang.
Complex domains3D arrangement
0.3-0.5 μm 5 μm
Eq
uil
ibri
um
en
erg
y d
ens
ity
Domain-wall contribution to the properties of ferroelectric materials
Movement of domain walls (vibration, bending, jump) at weak to moderate fields (subswitching fields) is one of the most important so-called extrinsic (nonlattice) contributions to the dielectric, elastic and piezoelectric properties of ferroelectric materials and may be comparable to the intrinsic effect of the lattice.• Movement of all types of DWs affect polarization and permittivity• Movement of non-180° DWs affect polarization and piezoelectric properties (strain)
2
1',
',
',
' gAlatticerdwrlatticerr
The dielectric constant of BaTiO3 ceramics decreases with decreasing grain size down to a grain size of about 1 μm. This is ascribed to the increasing density of 90° DWs with decreasing grain size. Similar behaviour observed in PZT ceramics.
2/190gkd
Thickness of domain walls as a function of grain sizeσ90: domain wall energy
g: grain size
FE domains walls as interfaces: conducting domain walls in La:BiFeO3
PFM amplitude PFM phase
BiFeO3 thin film with 109° stripe domains
C-AFM current
Origin: domain wall doping by oxygen vacanciesOxygen vacancies segregate at domain walls (strain gradient, formation of dipoles) and determine a localized increase of the electron concentration.
22
1'2 OeVO OO
3D current plot
FE domain walls as interfaces: free-electron gas at charged domain walls in insulating BaTiO3
tail-to-tail dw head-to-head dw
25 µm
Charge compensation of polarization charge by free carriers forming a q2DEG at the dw
charged dws
Influence of grain size on the dielectric constant of ferroelectric BaTiO3 ceramics
0
1000
2000
3000
4000
5000
6000
10 100 1000 10000
Grain size (nm)
Rel
ativ
e d
iele
ctri
c co
nst
ant
Arlt et al., HPSFrey & Payne, IPRandall et al., CSMRandall et al., HPSTakeuchi et al., SPSZhao et al., SPSBuscaglia et al., SPSDeng at al., SPSZhu et al., SPSWang, 2SS
Domain size and mobility effect
Dilution effect of the non ferroelectric grain boundaries (“dead” layer)
Poling of ferroelectric ceramics
If the direction of the spontaneous polarization through the ceramic is random or distributed in such a way as to lead to zero net polarization, the pyroelectric and piezoelectric effects of individual domains will cancel and such material is neither pyroelectric nor piezoelectric. Polycrystalline ferroelectric materials may be brought into a polar state by applying a strong electric field (10–100 kV/cm), usually at elevated temperatures. This process, called poling, cannot orient grains, but can reorient domains within individual grains in the direction of the field. A poled polycrystalline ferroelectric exhibits pyroelectric and piezoelectric properties, even if many domain walls are still present. Poling is only possible in FE ceramics. Ceramics of purely piezoelectric compounds do not exhibit ferroelectric properties (examples: quartz).
Due to the random orientation of the crystallites, the maximum polarization attainable in a ceramic (PR) is always smaller than in a single crystal and dependent on the number of available domain states:PR = 0.83 PS in tetragonal BTPR = 0.87 PS in rhombohedral BTPR = 0.91 PS in orthorhombic BT
In practice PR is much smaller (less than 0.5PS in tetragonal BT) because switching of 90° domain walls is hindered by the large mechanical stress exerted on each grain by the adjacent grains. Only displacement of the 90° domain walls is observed.
Before poling, PR = 0
After poling, PR 0
Ferroelectric hysteresis loop and polarization switching
PS: saturation (spontaneous) polarizationPR: remanent polarizationEC: coercive fieldThe slope of the initial polarization curve gives the dielectric constant
Ideally +PR = -PR and +EC = -EC
Variation of the hysteresis loop of BaTiO3 with temperature
-Pr
FE hysteresis loop in BaTiO3
PR = 25 μC/cm2 PR = 8 μC/cm2
Ferroelectric (polarization) fatigue
The ferroelectric fatigue is defined as the loss of the switchable remanent polarization in a ferroelectric material as a function of the number of bipolar switching cycles. It is an irreversible phenomenon of primary importance in the development of FERAMs.
Absence of significant polarization fatigue with electric field cycling in SrBi2Ta2O9 films with metal electrodes and PZT films with conducting oxide electrodes (IrO2, SrRuO3).
Fatigue mechanisms:
(i) formation of a surface layer; (ii) pinning of domain walls by defects segregatedin the wall region; (iii) clamping of polarization reversal by volume defects;(iv) suppression of nucleation of oppositely oriented
domains at the surface; (v) damage of electrode/film interface.
Oxygen vacancies have an important role in the fatigue process of ferroelectric thin films as they can segregate at electrode/ceramic interface and/or act as pinning centers for the domain walls
Ferroelectric memories (FERAMs)
Samsung 32 Mb ferroelectric random access memories
-E-PS
+E+PS
Ir/IrO2 electrodes
Ferroelectric material
Pb(Zr0.4Ti0.6)O3
Non-volatile memories, no need for refresh as opposite to DRAMs
Ferroelectric nanocapacitor
1 & 2 MBits 4 MBits
Strain-field loops in ferroelectrics
In addition to the polarization–electric field hysteresis loop, polarization switching by an electric field in ferroelectric materials leads to strain–electric field hysteresis (butterfly loops).
Real ferroelectric (PZT)Intrinsic (lattice) + extrinsic (dws)
contributionsIdeal ferroelectric with only 180° domain walls(pure piezoelectric response - intrinsic)
ABC: elongation (piezoelectric effect S=dE))CD: strain changes from positive to negativeDE: switchingEF: elongationFG: strain changes from negative to positiveGH: switching
Multidomain (90 + 180° dws) structure.Contribution (non-linear and hysteretic) to strain from movement and switching of non-180° domain walls in addition to pure piezoelectric response. Can be comparable or even greater than the pure piezoelectric response.
P
(A)
PE
PE
P E
P E
Pyroelectricity and ferroelectricity
dP/dT
T (°C)
Pyroelectric coefficient and total released charge in a PZT ceramic
Pyroelectric current
dt
dT
dT
dP
dt
dPi SS
Pyroelectric current
Pyroelectric coefficient
Hysteresis loop
Spontaneous polarization as deduced from pyroelectric data and from hysteresis loops
T
Pp Si
Electrocaloric effect
If polarization changes rapidly (under adiabatic conditions) the entropy remains unchanged and temperature changes by ΔT. The effect is maximum slightly above TC, when an electric field can induce a large polarization which goes to zero when the field is removed.
Giant Electrocaloric Effect in Thin-Film PbZr0.95Ti0.05O3 (350 nm)
TC = 220°C
T = -12°C at 480 V/cm (25 V)
2
2P
c
TT
c: specific heatρ: densityT: temperature
P: polarizationβ: coefficient from LGD theory
Electrocaloric effect in a PLZT film (450 nm)
Tc = 115°C
Nearly constant ECE at 20-120°C
Antiferroelectrics
In FEs, the off-center displacement occurs in the same direction in all unit cells and results in a macroscopic polarization. In contrast, in same compounds such as PbZrO3 and NaNbO3, the unit cell has a spontaneous electrical dipole but with opposite orientation in adjacent cells, giving a net zero polarization. Like FEs, the AFE compound show a sharp permittivity peak corresponding to transition from a cubic paraelectric phase. The transition temperature is again denoted as Curie temperature.
Dielectric constant of ceramic PbZrO3
TC
Curie-Weissbehaviour
Antipolar arrangement in the a-b plane of orthorhombic PbZrO3. The arrows denote the Pb ions displacement.
Cubic cell
Ort
ho
rho
mb
ic
ce
ll
Antiferroelectrics
Double hysteresis loops of PbZrO3 at different temperatures
Double hysteresis loop of PLZT
Stability of different FE and AFE phases in PbZrO3
TC = 230°C
Engineering the phase transitions in ferroelectrics – pressure
Phase diagram of BaTiO3First principles calculation
with quantum fluctuations
Extrapolated from low-P measurements
Experimental
R
O
TC
Engineering the phase transitions in ferroelectrics – strain effects in thin films
Room-temperature ferroelectricity in strained SrTiO3
Incipient ferroelectric
Deviation from CW law:quantum fluctuations
0
'
TT
Cr
Non-strained crystal
Strained epitaxial film
Thermodynamic prediction (LGD)
(100) SrTiO3 film with biaxial in-plane strain
LSAT: (LaAlO3)0.29 (SrAl0.5Ta0.5O3)0.71
>0: tensile<0: compressive
P1=P2=0; P3>0 P1=P2>0; P3=0
Engineering the phase transitions in ferroelectrics – strain effects in thin films
Enhancement of Ferroelectricity in Strained BaTiO3 Thin Films
(001) BaTiO3 film with biaxial in-plane strain
Thermodynamic prediction (LGD)
>0: tensile<0: compressive
P1=P2=0; P3>0
Engineering the phase transitions in ferroelectrics – chemical composition
Dopant Site Charge compensation
Ca2+, Sr2+, Pb2+ Ba -
Zr4+, Sn4+ Ti -
Na+, K+ Ba Oxygen vac.
La3+, Nd3+, Sb3+ Ba Cation vac. / e’
Mg2+, Ca2+, Al3+, Fe3+, Yb3+, Co3+, Mn3+, Cr3+
Ti Oxygen vac.
Y3+, Dy3+, Ho3+, Er3+ Ba & Ti
Depends on incorp. site
Nb5+, Sb5+, W5+ Ti Cation vac.
Ba2+
Ti4+
O2-
T/C
T/O
O/R
SrZr
BaTiO3
Possible phase superposition in solid solutions
Ca
Engineering the phase transitions in ferroelectrics – chemical composition
Ca as amphoteric dopant
OTiBa OTiCaTiOCaO 32
OOTiBa OVCaBaBaOCaO 2''
Incorporation at the Ba site
Incorporation at the Ti site with oxygen vacancy compensation
Engineering the phase transitions in ferroelectrics – chemical composition
Ferroelectric to relaxor crossover in BaZrxTi1-xO3
FerroelectricLong-range order1st order transition
Macroscopic domains:size >100nm
Diffuse FE transitionDecrease of correlation length
Broadened transition
RelaxorShort-range order
Polar nanoregions: size: 2-10 nm
Frequency dependent properties
Correlation length of Ti off-centre displacementBaTiO3
FEBaZrO3
non FE
Engineering the phase transitions in ferroelectrics – chemical composition
Ferroelectric to relaxor crossover in BaZrxTi1-xO3
Engineering the phase transitions in ferroelectrics – chemical composition
Variation of transition temperatures with composition in Ba1-xSrxTiO3
Sr1-xBaxTiO3Sr1-xBaxTiO3
TC
TOT
SrTiO3 is a quantum paraelectric or incipient ferroelectric without a ferro/para transition.
Deviation from Curie-Weiss law
BaTiO3SrTiO3
Tunability of ferroelectric ceramics
Ferroelectric and related materials (SrxBa1-xTiO3) have strongly non linear dielectric properties and their permittivity decreases with increasing dc electric field. To avoid hysteretic behaviour they are usually used in the paraelectric region. However their application in tunable MW devices (varactors) is limited by the high losses.
)0(
)()0('
''
r
rrr
En
Tunability
Multilayer ceramic capacitors
Multilayer ceramic capacitorsMLCC: n ceramic layers of thickness dseparated by metal (Ni, Ag-Pd) electrodes.Capacitance per unit volume:
Current market trends:•increase capacitance (increase n and decrease d)•miniaturization (reduce d and size)
2d
nKCV r
Ceramic
d n
Multilayer ceramic capacitors
Some data
Main dielectric material: BaTiO3. Yearly production: 11000
tons 2x1012 MLCCs (2011); Dielectric properties are modified by adding dopants (Zr, Ca,
Mg, Nb, Y, Ho, Dy, etc.); State of the art capacitors: dielectric layer thickness of 0.5
micron (Murata, Japan); Production technology: tape casting; Metal electrodes:
- Noble metals: Ag-Pd, sintering in air with addition of glass to
reduce temperature to 1100°C (Ag-30Pd);
- Ni (base metal technology, Philips, 1990s): sintering in N2-H2
atmosphere. Addition of “magic” dopants (Y, Dy, Ho) to reduce
formation of oxygen vacancies and improve lifetime. Applications: consumer electronics (mobile phones, smart
phones, PCs, laptops, TVs, etc.), automotive (cars, hybrid cars,
electric vehicles).State-of-the-art MLCC:d = 0.5 μm gs ≈100 nm
d=0.5 µm
Multilayer ceramic capacitorsSpecifications
X7RZ5U
Class 1: εr = 5-300, tanδ <<0.01, TCε: 0 to 8000 ppm
Class 1: εr = 1000-20000, tanδ: 0.01-0.03, moderate to high TCε
X7R: εr = 2000-4000, ±15% from -55 to 125°C
Z5U: εr = 5000-15000, +22/-56% from 10 to 85°C
Multilayer ceramic capacitors
MLCCs fabrication process (multilayer cofire technology)
Ceramic
Multilayer ceramic capacitors
Metal electrode
Metal electrode
Dielectric ceramic layer
at least 5-7 grains
d = 10 m
Multilayer ceramic capacitors
State-of-the-art MLCC:d = 0.5 μm gs ≈100 nm
d=0.5 µm
Miniaturization
Multilayer ceramic capacitors
Miniaturization
0.4 x 0.2 mm
2.0 x 1.2 mm
L x W
Multilayer ceramic capacitorsPure FE phases can not be used as dielectrics due to the unacceptable variation of permittivity with temperature.
Dielectric properties of FE ceramics can be tailored by forming solid solutions and by optimizing microstructure
(i) TC and other phase transitions can be shifted and even merged together(ii) The order of the phase transition can be changed: 1° 2° diffuse relaxor(iii) The dielectric constant can be increased by decreasing grain size (limit: 1 μm)
A flat temperature dependence of the permittivity can be achieved using ceramics with core-shell grains.The grain consist of a nearly pure ferroelectric BaTiO3 core ( ) with TC = 125°C and of a shell with diffuse ferroelectric of relaxor behaviour and maximum dielectric constant around RT.Most common dopants:Nb2O5, Co3O4, Y2O3, Ho2O3, Dy2O3, MgO
BaTiO3-Nb2O5-Co3O4
Influence of dopant precursor
Influence of Nb/Co ratioInfluence of sintering temperature
TS=1320°C
Multilayer ceramic capacitors
BaTiO3-Y2O3-MgO
TS=1250°C
Multilayer ceramic capacitors
dc voltage characteristics of commercial MLCCs
High εr
BaTiO3-basedceramics
Low εr
non ferroelectricceramics
Emerging applications (e.g. ac/dc inverters) require new dielectric materials
with high permittivity and low tunability able to operate in wide temperature range
(-50 – 200°C)
Multilayer ceramic capacitors
Applications in power electronicsBus capacitors act as an energy source to stabilize the dc bus voltage in power electronic circuits such as dc/ac inverters in hybrid electric systems. They possess large capacitance (100–2000 μF), and operate under a stable dc bias with a superimposed ac transient voltage.
Functions of inverter: Power the traction motors using energy stored in batteries Regenerative breaking (inverter takes power from motor and store it in batteries)
Requirements for dielectric materials:o High and constant permittivity under high dc fields o Low hysteretic losses (especially at high fields)o High energy density
Multilayer ceramic capacitors