Type & Properties of Electroceramics Dr Julie Juliewatty School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia

Types of Electroceramics Ceramic insulators High-k ceramic dielectrics Piezoelectric ceramics Ferroelectric ceramics Magnetic ceramics Superconductors Photonic ceramics

Ceramic insulators The primary function of insulation in electrical circuits is physical separation of conductors and regulation or prevention of current flow between them. Other functions are to provide mechanical support, heat dissipation, and environmental protection for conductors. Ceramic materials which in use these functions are classified as ceramic insulators. They include most glasses, porcelains, and oxide and nitride materials. The advantage of ceramics as insulators is their capability for high-temperature operation.

Ceramic insulator

Temperature Sensitive Resistor Some ceramic resistors exhibit high value of the temperature coefficient of resistance (TCR) and they may be negative (NTC) or positive (PTC). The TCR is a number used to predict how the resistance of a material changes with changes in temperature

Temperature Sensitive Resistor In a ceramic a large temperature coefficient of resistivity can arise from 3 causes: The intrinsic characteristic. A structure transition which accomponied by a change in the conduction mechanism from semiconducting to metallic. A rapid change in dielectric properties in certain ceramics which affects the electronic properties in the intergranular region to give rise to a large increase in resistivity with temperature over small temperature range. The 3rd Mechanism has led to important TCR devices.

Intrinsic - indicate a property of the material itself (i. e Intrinsic - indicate a property of the material itself (i.e. of the material substance). It is independent of how much of the material is present and is independent of the form the material is in (e.g. one large piece or a collection of smaller pieces). Intrinsic properties are dependent mainly on the chemical composition of the material. Some intrinsic properties are dependent also on the material structure A characteristic which is not essential or inherent is extrinsic.

Intrinsic properties, also sometimes called intensive properties, do not depend on the amount of the sample being examined. Some examples of intrinsic properties include temperature, melting point and density Extrinsic properties, also called extensive properties, depend on the quantity of the sample. Some examples of extrinsic properties include mass and volume.

Typical resistance-temperature response for various sensor materials

TCR Thermistor The TCR of a semiconductor is expected to be negative. In each case the resistivity depends on temperature according to where  is approximately independent of T and B is a constant related to the energy required to active the electron to conduct. Differentiating this equation leads to TCR value R:

NTC Thermistor The most NTC materials are based on solid solutions of oxides with spinel structure, e.g. Fe3O4-ZnCr2O4 and Fe3O4-MgCr2O4. A series that gives favourable combinations of low resistivity and high coefficients is based on Mn3O4 with a partial replacement of Mn by Ni, Co and Cu.

NTC Thermistor regularly used in automotive applications. For example, they monitor things like coolant temperature and/or oil temperature inside the engine and provide data to the ECU and, indirectly, to the dashboard. They can be also used to monitor temperature of an incubator modern digital thermostats and to monitor the temperature of battery packs while charging NTC thermistor, bead type, insulated wires

PTC Thermistor PTC thermistors exhibit an increase in resistance at a specified temperature. PTC resistor could be classified as critical temperature resistors because, in the case of the most widely used type, The positive coefficient is associated with the ferroelectric Curie point.

PTC Thermistor Most PTC has the negative resistivity-temperature characteristic up to about 100 oC and above about 200 oC. While between these temperatures there is an increase of several orders of magnitude in resistivity. The PTC effect is exhibited by specially doped and processed (eg. BaTiO3).

Application of PTC Thermistor The are two main groups: Applications such as temperature measurement, temperature control, temperature compensation and over- temperature protection. The second group includes applications such as over- current protection, liquid level detection and time delay.

Application of PTC Thermistor as current-limiting devices for circuit protection, as replacements for fuses. Current through the device causes a small amount of resistive heating. If the current is large enough to generate more heat than the device can lose to its surroundings, the device heats up, causing its resistance to increase, and therefore causing even more heating. This creates a self-reinforcing effect that drives the resistance upwards, reducing the current and voltage available to the device

Application of PTC Thermistor as timers in the degaussing coil circuit of CRT displays and televisions. When the unit is initially switched on, current flows through the thermistor and degauss coil. The coil and thermistor are intentionally sized so that the current flow will heat the thermistor to the point that the degauss coil shuts off in under a second Degaussing is the process of decreasing or eliminating an unwanted magnetic field

Dielectric Materials Dielectric materials can be defined as materials with high electrical resistivities, but an efficient supporter of electrostatic fields. Can store energy/charge. Able to support an electrostatic field while dissipating minimal energy in the form of heat. The lower the dielectric loss (proportion of energy lost as heat), the more effective is a dielectric material.

Dielectric materials

Dielectric Constant The capacitance, C, of a capacitor formed by two parallel plates of area A spaced d apart with the area between the plates filled with dielectric material with a relative dielectric constant of ε is: Materials Dielectric Constant Air 1 Alumina 10 Ta2O5 27 Nb2O5 42 Potassium Niobate (KN) 700 Barium Titanate (BT) 4000 Modified Barium Titanate 10000 Lead Magnesium Niobate (PMN) 20000

Q = CV Q: charge (Coulomb) C: capacitance (Farad) V: potential difference (Volt) d: separation/thickness (meter) o: permitivity of vacuum = 8.854x10-12 C2/m2 or F/m r: dielectric constant

Dielectric Loss For a lossy (imperfect) dielectric the dielectric constant can be represented by a complex relative dielectric constant: The imaginary part of this complex dielectric constant, ε at a frequency, ω is equivalent to a frequency-dependent conductivity, σ(ω), given by:

Dielectric Loss ε" is also known as the loss factor. The small difference in phase from ideal behaviour is defined by an angle δ, defined through the equation tan δ is known as the loss tangent or dissipation factor. A quality factor, Q, for the dielectric is given by the reciprocal of tan δ.

Dielectric Strength Dielectric materials are insulators. Generally, the lattice of a dielectric has sufficient strength to absorb the energy from impacting electrons that are accelerated by the applied electric field. However, under a sufficiently large electric field, some electrons present in the dielectric will have sufficient kinetic energy to ionize the lattice atoms causing an avalanching effect. As a result, the dielectric will begin to conduct a significant amount of current.

Dielectric Strength This phenomenon is called dielectric breakdown and the corresponding field intensity is referred to as the dielectric breakdown strength. So dielectric strength is electric field at which conduction occurs through the materials Dielectric strength may be defined as the maximum potential gradient to which a material can be subjected without insulating breakdown, that is where DS is the dielectric strength in kV/mm, VB the breakdown voltage, and d the thickness.

Dielectric Strength Dielectric strength depends on material homogeneity, specimen geometry, electrode shape and disposition, stress mode (ac, dc or pulsed) and ambient condition.

Effects of a dielectric materials inserted into a capacitor, with charge q

Dielectric Properties of a Matter

How does capacitance change?

How does the E field change?

How does permittivity change ?

Exercise 1 :

Answer:

Work done to charge a capacitor

Charging the capacitor in step of q=q

Exercise 2 Use the information given in exercise 1 to determine C and V

Answer

Capacitors The basic formula for the capacitance of a parallel-plate capacitor is: To increase C, one either increases , increases A, or decreases d. N is the number of stacked plates Early capacitors consisted of metal foils separated by wax ( ~ 2.5), mica ( ~ 3 - 6), steatite ( ~ 5.5 - 7.5), or glass ( ~ 5 - 10). The use of titania provided a significant increase ( ~ 170), was followed by perovskite-based, such as BaTiO3 ( ~ 1000).

Capacitors structure Disc capacitor have single dielectric layer-limits the max capacity Multilayer ceramic capacitor-monolitic (solid block) block of ceramic containing 2 sets of offsets, interleaved planar electrodes. These extend to 2 opposite surfaces of the ceramic dielectric The monolithic structure of MLCC- requires both buried electrode and ceramic dielectric be compatible with one another and with the manufacturing process Sintering process has to be suitable for both

Internal Electrode Materials MLCC Internal Electrode Materials -Basic requirement : must yield a conductive film that is continuous after firing. Does not diffuse into or react with ceramic dielectric. -BaTiO3 fired at 1300oC in air –electrode paladium. But upon cooling below 870oC its surface reoxidizes, bonding it to ceramic If ceramic contain1% Pb or Bi, paladium tend to react-so use electrode alloy cantaining gold, Pt, rhodium (to form oxide bond) - but costly -So silver with low cost often added to the electrode materials But lowers the melting point proportionally (firing temperature 1150oC)-70% paladium and 30% silver 960oC-100% silver- but ceramic do not form

Internal Electrode Materials MLCC Internal Electrode Materials Nickel –cheaper and its melting point 1453oC, enthalpy of oxidation is less negative than Co, Fe, Cr. These metals will oxidize completely when heated in air –so use inert gas N2 and Ar mix with reducing gas H2 CO, CH3 Tin lead alloy electrode-another approach to reduce cost but introduce voids and pin hole and peel off problem- cause capacitance loss

Special dielectric compositions MLCC Special dielectric compositions Other special dielectrics are: (Self reading) Strontium titanate based ceramic Lead titanate based ceramic Niobate and related relaxor dielectrics CCTO

Preparation-ceramic disc capacitors milling dispersion Extrude tape Blank disc fire terminate test finish milling dispersion Spray dry press disc fire terminate test finish

Deposit dielectric layer Preparation- MLCC milling dispersion Deposit dielectric layer Print electrode Dice into chips fire terminate test finish milling dispersion Cast tape Print electrode Stack layers Laminate stack Dice into chips fire terminate test finish Repeat sequently

Piezoelectric effect Discovered in 1880 by Jacques and Pierre Curie during studies into the effect of pressure on the generation of electrical charge by crystals (such as quartz). Piezoelectricity is defined as a change in electric polarization with a change in applied stress (direct piezoelectric effect). The converse piezoelectric effect is the change of strain or stress in a material due to an applied electric field.

Piezoelectric effect The linear relationship between stress Xik applied to a piezoelectric material and resulting charge density Di is known as the direct piezoelectric effect and may be written as where dijk (C N−1) is a third-rank tensor of piezoelectric coefficients.

Piezoelectric effect Another interesting property of piezoelectric material is they change their dimensions (contract or expand) when an electric field is applied to them. The converse piezoelectric effect describes the strain that is developed in a piezoelectric material due to the applied electric field: where t denotes the transposed matrix. The units of the converse piezoelectric coefficient are (m V−1).

Piezoelectric effect The piezoelectric coefficients, d for the direct and converse piezoelectric effects are thermodynamically identical, i.e. ddirect = dconverse. Note that the sign of the piezoelectric charge Di and strain xij depends on the direction of the mechanical and electric fields, respectively. The piezoelectric coefficient d can be either positive or negative.

Piezoelectric effect It is common to call a piezoelectric coefficient measured in the direction of applied field the longitudinal coefficient, and that measured in the direction perpendicular to the field the transverse coefficient. Other piezoelectric coefficients are known as shear coefficients.

Piezoelectricity The microscopic origin of the piezoelectric effect is the displacement of ionic charges within a crystal structure. In the absence of external strain, the charge distribution is symmetric and the net electric dipole moment is zero. However when an external stress is applied, the charges are displaced and the charge distribution is no longer symetric and a net polarization is created.

Piezoelectricity In some cases a crystal posses a unique polar axis even in the unstrained condition. This can result in a change of the electric charge due to a uniform change of temperature. This is called the pyroelectric effect. The direct piezoelectric effect is the basis for force, pressure, vibration and acceleration sensors and The converse effect for actuator and displacement devices.

Piezoelectric and subgroup The elements of symmetry that are utilized by crystallographers to define symmetry about a point in space, for example, the central point of unit cel, are a point (center) of symmetry, axes of rotation, mirror planes, and combinations of these. Utilizing these symmetry elements, all crystals can be divided into 32 different classes or point groups.

Piezoelectric and subgroup These 32 point groups are subdivisions of 7 basic crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, rhombohedral (trigonal), hexagonal, and cubic. Of the 32 point groups, 21 classes do not possess a center of symmetry (a necessary condition for piezoelectricity to exist) and 20 of these are piezoelectric. One class, although lacking a center of symmetry, is not piezoelectric because of other combined symmetry elements.

Piezoelectric and subgroup 32 Symmetry Point Groups 21 PG: Noncentrosymmetric 11 PG: Centrosymmetric 20 PG: Piezoelectric (Polarized under stress) 1 PG: Pyroelectric (Spontaneously polarized) Subgroup Ferroelectric (Spontaneously Polarized, Revesible Polarization)

Piezoelectric and subgroup As discussed in previously slide, piezoelectric coefficients must be zero and the piezoelectric effect is absent in all 11 centrosymmetric point groups. Materials that belong to other symmetries may exhibit the piezoelectric effect.

How are piezoelectric ceramics made? A traditional piezoelectric ceramic is perovskite crystal, each consisting of a small, tetravalent metal ion, usually titanium or zirconium, in a lattice of larger, divalent metal ions, usually lead or barium, and O2- ions. Under conditions that confer tetragonal or rhombohedral symmetry on the crystals, each crystal has a dipole moment.

Polarization of piezoelectric Above a critical temperature, the Curie point, each perovskite crystal exhibits a simple cubic symmetry with no dipole moment. At temperatures below the Curie point, however, each crystal has tetragonal or rhombohedral symmetry and a dipole moment. Adjoining dipoles form regions of local alignment called domains. The alignment gives a net dipole moment to the domain, and thus a net polarization. The direction of polarization among neighboring domains is random, however, so the ceramic element has no overall polarization.

Polarization of piezoelectric The domains in a ceramic element are aligned by exposing the element to a strong, direct current electric field, usually at a temperature slightly below the Curie point. Through this polarizing (poling) treatment, domains most nearly aligned with the electric field expand at the expense of domains that are not aligned with the field, and the element lengthens in the direction of the field. When the electric field is removed most of the dipoles are locked into a configuration of near alignment. The element now has a permanent polarization, the remanent polarization, and is permanently elongated.

Electric dipoles in Weiss domains; (1) unpoled ferroelectric ceramic, (2) during and (3) after poling (piezoelectric ceramic)

Piezo Materials Some examples of practical piezo materials are barium titanate, lithium niobate, polyvinyledene difluoride (PVDF), and lead zirconate titanate (PZT). There are several different formulations of the PZT compound, each with different electromechanical properties.

What can piezoelectric ceramics do? Mechanical compression or tension on a poled piezoelectric ceramic element changes the dipole moment, creating a voltage. Compression along the direction of polarization, or tension perpendicular to the direction of polarization, generates voltage of the same polarity as the poling voltage. Generator and motor actions of a piezoelectric element

Piezoelectric ceramics- applications The principle is adapted to piezoelectric motors, sound or ultrasound generating devices, and many other products. Generator action is used in fuel-igniting devices, solid state batteries, and other products; Motor action is adapted to piezoelectric motors, sound or ultrasound generating devices, and many other products.