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Ferroelectric Ceramics

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Title: Ferroelectric Ceramics


1
Ferroelectric Ceramics
  • EBB 443 Technical Ceramics
  • Dr. Sabar D. Hutagalung
  • School of Materials and Mineral Resources
    Engineering
  • Universiti Sains Malaysia

2
You may say anything you like but we all are made
up of ferroelectrics (B.T. Matthias)
3
Ferroelectricity
  • Ferroelectricity is an electrical phenomenon
    whereby certain materials may exhibit a
    spontaneous dipole moment, the direction of which
    can be switched between equivalent states by the
    application of an external electric field.
  • The internal electric dipoles of a ferroelectric
    material are physically tied to the material
    lattice so anything that changes the physical
    lattice will change the strength of the dipoles
    and cause a current to flow into or out of the
    capacitor even without the presence of an
    external voltage across the capacitor.

4
Ferroelectricity
  • Two stimuli that will change the lattice
    dimensions of a material are force and
    temperature.
  • The generation of a current in response to the
    application of a force to a capacitor is called
    piezoelectricity.
  • The generation of current in response to a change
    in temperature is called pyroelectricity.

5
Ferroelectricity
  • Placing a ferroelectric material between two
    conductive plates creates a ferroelectric
    capacitor.
  • Ferroelectric capacitors exhibit nonlinear
    properties and usually have very high dielectric
    constants.
  • The fact that the internal electric dipoles can
    be forced to change their direction by the
    application of an external voltage gives rise to
    hysteresis in the "polarization vs voltage"
    property of the capacitor.
  • Polarization is defined as the total charge
    stored on the plates of the capacitor divided by
    the area of the plates.
  • Hysteresis means memory and ferroelectric
    capacitors are used to make ferroelectric RAM for
    computers and RFID cards.

6
Ferroelectricity
  • The combined properties of memory,
    piezoelectricity, and pyroelectricity make
    ferroelectric capacitors some of the most useful
    technological devices in modern society.
  • Ferroelectric capacitors are at the heart of
    medical ultrasound machines, high quality
    infrared cameras, fire sensors, sonar, vibration
    sensors, and even fuel injectors on diesel
    engines.
  • The high dielectric constants of ferroelectric
    materials used to concentrate large values of
    capacitance into small volumes, resulting in the
    very tiny surface mount capacitor.
  • The electrooptic modulators that form the
    backbone of the Internet are made with
    ferroelectric materials.

7
Ferroelectric properties
  • Most ferroelectric materials undergo a structural
    phase transition from a high-temperature
    nonferroelectric (or paraelectric) phase into a
    low-temperature ferroelectric phase.
  • The paraelectric phase may be piezoelectric or
    nonpiezoelectric and is rarely polar.
  • The symmetry of the ferroelectric phase is always
    lower than the symmetry of the paraelectric
    phase.

8
Ferroelectric properties
  • The temperature of the phase transition is called
    the Curie point, TC.
  • Above the Curie point the dielectric permittivity
    falls off with temperature according to the
    CurieWeiss law
  • where C is the Curie constant, T0 (T0 TC) is the
    CurieWeiss temperature.
  • Some ferroelectrics, such as BaTiO3, undergo
    several phase transitions into successive
    ferroelectric phases.

9
BaTiO3
  • BaTiO3 has a paraelectric cubic phase above its
    Curie point of about 130C.
  • In the T of 130C to 0C, the ferroelectric
    tetragonal phase with a c/a ratio of 1.01 is
    stable.
  • The spontaneous polarization is along one of the
    001 directions in the original cubic structure.
  • Between 0C and -90C, the ferroelectric
    orthorhombic phase is stable with the
    polarization along one of the 110 directions in
    the original cubic structure.
  • On decreasing T below -90C the phase transition
    from the orthorhombic to ferroelectric
    rhombohedral phase leads to polarization along
    one of the 111 cubic directions.

10
001 directions
111 directions
110 directions
The phase transition sequence in perovskites
11
Phase diagram of BaTiO3 (a) bulk single crystal
and (b) epitaxial (001) single domain thin films
grown on cubic substrates of high temperatures as
a function of the misfit strain. The second- and
first-order phase transitions are shown by thin
and thick lines, respectively.
12
Curie Point Phase Transitions
  • All ferroelectric materials have a transition
    temperature called the Curie point (Tc).
  • At T gt Tc the crystal does not exhibit
    ferroelectricity, while for T lt Tc it is
    ferroelectric.
  • On decreasing the temperature through the Curie
    point, a ferroelectric crystal undergoes a phase
    transition from a non-ferroelectric phase to a
    ferroelectric phase.
  • If there are more than one ferroelectric phases,
    the T at which the crystal transforms from one
    phase to another is called the transition
    temperature.

13
Curie Point Phase Transitions
  • For example, the variation of the relative
    permittivity ?r with temperature as a BaTiO3
    crystal is cooled from its paraelectric cubic
    phase to the ferroelectric tetragonal,
    orthorhombic, and rhombohedral phases.
  • Near the Curie point or transition temperatures,
    thermodynamic properties including dielectric,
    elastic, optical, and thermal constants show an
    anomalous behavior.
  • This is due to a distortion in the crystal as the
    phase structure changes.

14
Curie Point Phase Transitions
Variation of dielectric constants (a and c axis)
with temperature for BaTiO3
15
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16
The perovskite structure ABO3 shown here for
PbTiO3 which has a cubic structure in the
paraelectric phase and tetragonal structure in
the ferroelectric phase.
17
Ferroelectric Domains
  • As described above, pyroelectric crystals show a
    spontaneous polarization Ps in a certain
    temperature range.
  • If the magnitude and direction of Ps can be
    reversed by an external electric field, then such
    crystals are said to show ferroelectric behavior.
  • Hence, all single crystals and successfully poled
    ceramics which show ferroelectric behavior are
    pyroelectric, but not vice versa.
  • For example tourmaline shows pyroelectricity but
    is not ferroelectric.

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19
Domain Wall Movement
20
Ferroelectric hysteresis loop
  • The most important characteristic of
    ferroelectric materials is polarization reversal
    (or switching) by an electric field.
  • One consequence of the domain-wall switching in
    ferroelectric materials is the occurrence of the
    ferroelectric hysteresis loop.
  • The hysteresis loop can be observed
    experimentally by using a SawyerTower circuit.

21
Ferroelectric hysteresis loop
  • As the field is increased the polarization of
    domains with an unfavourable direction of
    polarization will start to switch in the
    direction of the field, rapidly increasing the
    measured charge density (segment BC).

22
Ferroelectric hysteresis loop
  • The polarization response in this region is
    strongly nonlinear.
  • Once all the domains are aligned (point C) the
    ferroelectricity again behaves linearly (segment
    CD).
  • If the field strength starts to decrease, some
    domains will back-switch, but at zero field the
    polarization is nonzero (point E).
  • The value of polarization at zero field (point E)
    is called the remanent polarization, PR.

23
  • To reach a zero polarization state the field must
    be reversed (point F).
  • The field necessary to bring the polarization to
    zero is called the coercive field, EC.
  • It should be mentioned that the coercive field EC
    that is determined from the intercept of the
    hysteresis loop with the field axis is not an
    absolute threshold field.
  • The spontaneous polarization PS is usually taken
    as the intercept of the polarization axis with
    the extrapolated linear segment CD.
  • Further increase of the field in the negative
    direction will cause a new alignment of dipoles
    and saturation (point G).

24
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28
Perovskites
  • Perovskite is a family name of a group of
    materials and the mineral name of calcium
    titanate (CaTiO3) having a structure of the type
    ABO3.
  • Many piezoelectric (including ferroelectric)
    ceramics such as Barium Titanate (BaTiO3), Lead
    Titanate (PbTiO3), Lead Zirconate Titanate (PZT),
    Lead Lanthanum Zirconate Titanate (PLZT), Lead
    Magnesium Niobate (PMN), Potassium Niobate
    (KNbO3), Potassium Sodium Niobate (KxNa1-xNbO3),
    and Potassium Tantalate Niobate (K(TaxNb1-x)O3)
    have a perovskite type structure.

29
Size effect
  • The dielectric properties of BaTiO3 are found to
    be dependent on the grain size.
  • Large grained BaTiO3 (³ 1 m m) shows an extremely
    high dielectric constant at the Curie point.
  • This is because of the formation of multiple
    domains in a single grain, the motion of whose
    walls increases the dielectric constant at the
    Curie point.
  • For a BaTiO3 ceramic with fine grains ( 1 m m),
    a single domain forms inside each grain.
  • The movement of domain walls are restricted by
    the grain boundaries, thus leading to a low
    dielectric constant at the Curie point as
    compared to coarse grained BaTiO3.

30
The variation of the relative permittivity (er)
with temperature for BaTiO3 ceramics with (a) 1
mm grain size and (b) 50 mm grain size.
31
PLZT
  • The electro-optic applications of PLZT ceramics
    depends on the composition.
  • PLZT ceramic compositions in the tetragonal
    ferroelectric (FT) region show hysteresis loops
    with a very high coercive field (EC).
  • Materials with this composition exhibit linear
    electro-optic behavior for E lt EC.
  • PLZT ceramic compositions in the rhombohedral
    ferroelectric (FR) region of the PLZT phase
    diagram have loops with a low coercive field.
  • These PLZT ceramics are useful for optical memory
    applications.

32
Representative hysteresis loops obtained for
different ferroelectric compositions (a) FT (b)
FR (c) FC and (d) AO regions of the PLZT phase
diagram.
33
Interest in Ferroelectric
  • Interest in ferroelectric properties, materials
    and devices has been considerable over the last
    10 years.
  • This interest has been driven by the exciting
    possibility of using ferroelectric thin films for
    nonvolatile memory applications and new
    microelectromechanical systems (MEMS).
  • The main interest is in polycrystalline (ceramic)
    ferroelectrics and thin films, which are easier
    to make and which offer a larger variety of
    easily achievable compositional modifications
    than single crystals.

34
MFS-FET Operation
35
Problem in Ferroelectric
  • Problems associated with applications of
    ferroelectric materials, such as
  • polarization fatigue,
  • ageing and field and frequency dependence of the
    piezoelectric,
  • elastic and dielectric properties.

36
Problem in Ferroelectric
  • The disadvantage of polycrystalline
    ferroelectrics and films is that their properties
    are often controlled by contributions from
    domain-wall displacements and other so-called
    extrinsic contributions, which are responsible
    for most of the frequency and field dependence of
    the properties, and whose theoretical treatment
    presents a considerable challenge.
  • In addition, geometry of thin films imposes
    boundary conditions which sometimes lead to very
    different properties of films with respect to
    bulk materials and which must be taken into
    account when modelling devices.

37
MFS Structure Problems
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