Dielectrics and Ferroelectric Materials PDF

Dielectrics and Ferroelectric Materials 1 Course Syllabus 2  Dielectric Material A dielectric material is one that is electrically insulating (nonmetallic) and exhibits or may be made to exhibit an...

Dielectrics and Ferroelectric Materials 1 Course Syllabus 2  Dielectric Material A dielectric material is one that is electrically insulating (nonmetallic) and exhibits or may be made to exhibit an electric dipole structure (molecular or atomic level). In principle all insulators are dielectric, although the capacity to support charge varies greatly between different insulators Dielectric materials are used in many applications, from simple electrical insulation to sensors and circuit components. 3 Electric Dipole 4  Dielectric Constant Dielectric Constant: Also known as relative permittivity ε ε𝑟 = ε0 ε = Permittivity of medium ε0 Permittivity of vacuum (8.854⨯10−12 𝐹/𝑚) 5  Dielectric Strength The dielectric strength, sometimes called the breakdown strength, represents the magnitude of an electric field necessary to produce breakdown. Sometimes localized melting, burning, or vaporization produces irreversible degradation and perhaps even failure of the material 6  Electric Displacement Charge per unit area that would be displaced across a layer of conductor placed across an electric field. Also known as electric flux density or Surface charge density. Electric displacement is used in the dielectric material to find the response of the materials on the application of an electric field E. The SI unit Coulomb per meter square (C m-2). D = 𝜺𝟎 𝑬 + 𝑷 The surface charge density D, or quantity of charge per unit area of capacitor plate (C/m2), is ϵ0: Vacuum permittivity proportional to the electric field P: Polarization density E: Electric field D: Electric displacement field The electric displacement field D represents how an electric field E influences the organization of electric 7 charges in a given medium, including charge migration and electric dipole reorientation. 8 Capacitor 9  Capacitor Ceramic capacitors. (a) Section showing construction. (b) Steps in manufacture: (1) after firing ceramic disk; (2) after applying silver electrodes; (3) after soldering leads; (4) after applying dipped phenolic coating. 10 Representative formulations for some ceramic dielectric materials for capacitors 11 Parallel Plate Capacitor in the presence of dielectric Plate A Plate B D = E𝜺𝟎 + 𝑷 12 Dipole orientations for change in orientation of electric field. In reality, a dipole moment is a vector that is directed from the negative to the positive charge In the presence of an electric field E, which is also a vector quantity, a force (or torque) will come to bear on an electric dipole to orient it with the applied field. The process of dipole alignment is termed polarization. 13 14 Polarization Mechanism 15  Polarization Polarization direction -ve to +ve Polarization is dipole moment per unit volume Polarization is the alignment of permanent or induced atomic or molecular dipole moments with an externally applied electric field. Electronic polarization Results from a displacement of the center of the negatively charged electron cloud relative to the positive nucleus of an atom by the electric field This polarization type is found in all dielectric materials and, of course, exists only while an electric field is present. Ionic Polarization Ionic polarization occurs only in materials that are ionic. An applied field acts to displace cations in one direction and anions in the opposite direction, which gives rise to a net dipole moment. Orientation or dipolar Polarization It is found only in substances that possess permanent dipole moments. Polarization results from a rotation of the permanent moments into the direction of the applied field, This alignment tendency is counteracted by the thermal vibrations of the atoms, such that polarization decreases with increasing temperature. 16 HCl molecule 17 Interfacial Polarization or Space Charge Polarization It occurs when there is an accumulation of charge at an interface between two materials or between two regions within a material because of an external field. This can occur when there is a compound dielectric, or when there are two electrodes connected to a dielectric material. This type of electric polarization is different from orientational and ionic polarization because instead of affecting bound positive and negative charges i.e. ionic and covalent bonded structures, interfacial polarization also affects free charges as well. As a result interfacial polarization is usually observed in amorphous or polycrystalline solids. Free Charges The electric field will cause a charge imbalance because of the dielectric material's insulating properties. However, the mobile charges in the dielectric will migrate over maintain charge neutrality. This then causes interfacial polarization 18 Dielectric Constant 19 20 Relaxation Time A dielectric becomes polarised in an electric field. Now imagine switching the direction of the field. The direction of the polarisation will also switch in order to align with the new field. This cannot occur instantaneously: some time is needed for the movement of charges or rotation of dipoles. If the field is switched, there is a characteristic time that the orientational polarisation (or average dipole orientation) takes to adjust, called the relaxation time. Typical relaxation times are ~10-11 s. Therefore, if the electric field switches direction at a frequency higher than ~1011 Hz, the dipole orientation cannot ‘keep up’ with the alternating field, the polarisation direction is unable to remain aligned with the field, and this polarisation mechanism ceases to contribute to the polarisation of the dielectric. 21  Dielectrics Losses For each polarization type, some minimum reorientation time exists, which depends on the ease with which the particular dipoles are capable of realignment. (A) A relaxation frequency is taken as the reciprocal of this minimum reorientation time A dipole cannot keep shifting orientation direction when the frequency of the applied electric field exceeds its relaxation frequency and, therefore, will not make a contribution to the dielectric constant. The absorption of electrical energy by a dielectric material that is subjected to an alternating electric field is termed dielectric loss. A.C field A low dielectric loss is desired at the frequency of utilization. (B) 180o Oscillation of dipoles + - Animation 22  Dielectric Loss Dielectric loss is the dissipation of energy through the movement of charges in an alternating electromagnetic field as polarisation switches direction. Dielectric loss is especially high around the relaxation or resonance frequencies of the polarisation mechanisms as the polarisation lags behind the applied field, causing an interaction between the field and the dielectric’s polarisation that results in heating. Dielectric loss tends to be higher in materials with higher dielectric constants. This is the downside of using these materials in practical applications. Dielectric loss is utilized to heat food in a microwave oven Microwave Heating The frequency of the microwaves used is close to the relaxation frequency of the orientational polarisation mechanism in water, meaning that any water present absorbs a lot of energy that is then dissipated as heat. The exact frequency used is slightly away from the frequency at which maximum dielectric loss occurs in water to ensure that the microwaves are not all absorbed by the first layer of water they encounter, therefore allowing more even heating of the food. 23  Dielectric Loss Factor If the voltage used to maintain the charge on a capacitor is sinusoidal, as is generated by an alternating current, the current leads the voltage by 90 degrees when a loss-free dielectric is between the plates of a capacitor. However, when a real dielectric is used in the capacitor, the current leads the voltage by (90° - δ), where the angle δ is called the dielectric loss angle. The product of dielectric cosntant(κ) tan δ is designated the loss factor and is a measure of the electric energy lost (as heat energy) by a capacitor in an ac circuit. Note: In circuits with primarily capacitive loads, current leads the voltage. This is true because current must first flow to the two plates of the capacitor, where charge is stored. Only after charge accumulates at the plates of a capacitor is a voltage difference established. 24  Dielectric Breakdown At high electric fields, a material that is normally an electrical insulator may begin to conduct electricity – i.e. it ceases to act as a dielectric. This phenomenon is known as dielectric breakdown. If the electric field across an insulator is too high, the insulator will start to conduct a current. This phenomenon is known as dielectric breakdown. The mechanism for the breakdown is that some free carriers (e.g., caused by impurities) are accelerated in the field, so much that they can ionize other atoms and generate more free carriers. Then, the breakdown proceeds like an avalanche. The breakdown can be facilitated by operating the material close to a resonance frequency where much energy is dissipated, the material is heated, and the probability of having free carriers is increased. 25 26 27 28 29 30 31 32 33 Linear and Non-Linear Dielectrics 34 35 36 37 38 39 40 Ferroelectric Materials 41  FERROELECTRICITY The group of dielectric materials called ferroelectrics exhibit spontaneous polarization—that is, polarization in the absence of an electric field. They are the dielectric analogue of ferromagnetic materials, which may display permanent magnetic behavior. BaTiO3 42  4 3 Non-centrosymmetric structure and origin of piezoelectricity (a) centrosymmetric structure and (b) non-centrosymmetric structure Perovskite structure of oxides ABO3 ferroelectric phase with upward and downward polarization. 44  + - + - + 45 (a) Schematic illustration of electric dipoles within a piezoelectric material. (b)Compressive stresses on material cause a voltage difference to develop due to change in electric dipoles. (c) Applied voltage across ends of a sample causes a dimensional change and changes the electric dipole moment. 46  4 7 Fig. 5 Dipole configuration in ferroelectric mateirals. 48  Review of Voltage or Charge Measurement in Piezo and Pyroelectrics Electroding (Silver paste) + - = Bounded Charge + - = Free Charge - - - - + + + + Bounded Charge density changes due to the change in the Polarisation state of the sample which is further dependent on the external stimulus such as mechanical force or change in V temperature Insulating Material - - - - (Dielectric) + + + + + - 49 Depolarisation 1. Thermal depoling If the material is exposed to excessive heat, such that its temperature approaches its Curie temperature, the dipole moments regain their unaligned state. At the Curie temperature, a ferroelectric becomes entirely unaligned. In order to prevent this occurring, it is sensible to use piezoelectrics well below their Curie temperature. 2. Electrical depoling A strong electric field, when applied in the reverse direction to the already poled material, will lead to depoling. If an alternating field is used to produce ultrasound waves (see later) the field will depolarise the piezoelectric during the periods in which it is opposing the polarisation. 3. Mechanical depoling If the stress placed on a piezoelectric is too high, it is possible to immediately depolarise the piezoelectric as the atom positions are altered. This completely ruins its properties. Source: https://www.doitpoms.ac.uk/tlplib/piezoelectrics/depolarisation.php 50 A = Saturation Polarization B= Remnant Polarization C= Coercive Field Credits: Dr. Sumeet K Sharma 51 Sawyer Tower Circuit for P-E loop measurement Polarization = Dipole moment / Volume = C.m /m3 = C/m2 = Charge/ Area 52 Explanation: In this experiment, the voltage is cycled by the signal generator. Its direction is reversed at high frequency, and the voltage across the reference capacitor is measured. The charge on the capacitor must be the same as the charge over the ferroelectric capacitor, as they are in series. This means the charge on the ferroelectric can be found by: Q=C×V where C is the capacitance of the reference capacitor, and V is the voltage measured over this capacitor. We can therefore represent the polarisation of a material in an oscillating electric field, by plotting the voltage applied to the material on the x-axis of the oscilloscope, and the surface charge on the y-axis. This can be done because the capacitance of the reference capacitor is much higher than the capacitance of the ferroelectric, so most of the voltage lies over the ferroelectric. It is only possible to measure P by cycling the polarisation through cycling the voltage across the ferroelectric. We cannot measure absolute values instantaneously , but can deduce absolute values from the changes measured when cycling the polarisation. 53 Curie Temperature: The temperature at which the Ferroelectric properties or the capability to get spontaneous polarization is lost. Temperature dependence of the Hysteresis loop We have only observed it at one particular temperature, one at which the material is ferroelectric. What happens if the temperature is raised? The hysteresis loop changes with temperature, becoming sharper and thinner, and eventually disappearing As you can see, the polarisation increases at 90°C, as a result of a phase transition. Between this temperature and room temperature, the polarisation increases steadily, as a direct relation with temperature, such that: ΔP = p ΔT where p = pyroelectric coefficient (C m-2 T-1). 55  Brief History of Pyroelectricity Pyroelectricity as a phenomenon has been known for 24 centuries—the Greek philosopher Theophrastus probably wrote the earliest known account. He described a stone, called lyngourion in Greek or lyncurium in Latin, that had the property of attracting straws and bits of wood. Those attractions were no doubt the effects of electrostatic charges produced by temperature changes most probably in the mineral tourmaline. Theophrastus and other writers of the two millennia that followed were far more interested in the origin of the stone and its physical explanations. Theophrastus proposed that lyngourion was formed from the urine of a wild animal. 56 Introduction Pyroelectrics are the bridge between ferroelectrics and piezoelectrics. They possess a spontaneous polarisation which is not necessarily switchable by an electric field. If their polarisation is switchable, i.e. they are ferroelectric, then they are mainly used in situations in which ferroelectric properties are required. However, if they are not ferroelectric, then their properties as pyroelectrics are more useful. Whether a given sample possesses a net dipole moment depends on domain configurations, which in turn depend on sample history. This polarisation will change when a stress is applied to the material, as pyroelectrics are a sub-set of piezoelectrics. But it will not reverse under the application of an electric field because it will breakdown first, i.e. the coercive field exceeds the breakdown field. This is only true for a pyroelectric material which is not ferroelectric, whereas if it is ferroelectric, the coercive field is smaller than the breakdown field. In other words, ferroelectrics are a subset of pyroelectrics. 57  Pyroelectricity Pyro =fire,” “heat,” “high temperature Pyroelectricity is the property presented by certain materials that exhibit an electric polarization ∆P when a temperature variation ∆T is applied uniformly: ∆P = γ∆T where γ is the pyroelectric coefficient at constant stress. Pyroelectric crystals actually have a spontaneous polarization, but the pyroelectric effect can only be observed during a temperature change (ΔT). 58  If a spontaneous polarization is already present, a change of temperature alters it. The change in temperature modifies the positions of the atoms slightly within the crystal structure, such that the polarization of the material changes. This polarization change gives rise to an electric polarization across the crystal. If the temperature stays constant at its new value, the pyroelectric polarization gradually disappears due to leakage current (the leakage can be due to electrons moving through the crystal, ions moving through the air, current leaking through a voltmeter attached across the crystal, and so on) 59 Types of Ferroelectric Materials and Their Applications 60  Types of Ferroelectric Materials Perovskite-Type Structures Perovskite is a family name of a group of materials having the mineral name of calcium titanate (CaTiO3) exhibiting a structure of the type ABO3. Many piezoelectrics including FE ceramics such as Barium titanate (BaTiO3) Strontium titanate (STO) (SrTiO3) Barium strontium titanate (BST) Lead titanate (PbTiO3), Lead Zirconate Titanate (PZT) Lead Lanthanum Zirconate Titanate (PLZT) Lead Magnesium Niobate (PMN) Potassium niobate (KN) (KNbO3), Potassium sodium niobate (KxNal-xNbO3), and Potassium tantalate niobate (KTaxNbl-xO3) have a perovskite-type structure. 61 Barium titanate (BaTiO3) 62  Strontium Titanate (SrTiO3) Barium Strontium Titanate (BST) 63 Lead Titanate (PbTiO3) Lead Zirconate Titanate (PZT) 64  Polymers based Piezoelectrics Organic Polymers Molecular structure of ferroelectric polymer (a) Polyvinylidene fluoride or Polyvinylidene difluoride (PVDF) and (b) Polyvinylidene fluoride-trifluoroethylene P(VDF-TrFE) Ceramic Polymer Composites Ceramic polymer materials are inorganic–organic composites consisting of ceramic fillers and a matrix of organic polymers. The formation of ceramic polymer is based on thermal curing of functionalized resins being able to form ceramic-like structures as a result of heat treatment above 200 deg.C. Processed by a broad variety of plastic-forming techniques such as high-pressure injection molding or extrusion. Ceramic Polymer composites are characterized by high thermal stability (possible service temperatures above 600 deg. C), low shrinkage, high stability of shape, and high dimensional accuracy. Relevant usage properties (e.g., electrical conductivity, thermal conductivity) and processing parameters can be adjusted by the choice of appropriate functional fillers, binder systems, and plasticizing additives. The application of ceramic polymer materials could pay off if a cost-efficient, easy processing of the material including plastic-forming techniques in order to realize complex shaped parts is required and the thermal stability of standard materials such as plastics does not 65 suffice.  Phase Diagram MPB: Coexistence of Rhombohedral and Tetragonal Phases Good Piezoelectric properties due to the coexistence of two phases. 66 Piezoelectric Constants one needs to know 67 Charge Constant Voltage Constant 68 Piezoelectric Charge Constant 𝑃𝑜𝑙𝑎𝑟𝑖𝑧𝑎𝑡𝑖𝑜𝑛 d= 𝑀𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑆𝑡𝑟𝑒𝑠𝑠 69 Source: https://www.americanpiezo.com/ Piezoelectric Voltage Constant 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝐹𝑖𝑒𝑙𝑑 g= 𝑀𝑒𝑐ℎ𝑎𝑛𝑐𝑖𝑎𝑙 𝑆𝑡𝑟𝑒𝑠𝑠 70 Permittivity Constant 𝐷𝑖𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 ε= 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝐹𝑖𝑒𝑙𝑑 71 Elastic and Compliance Inverse of Modulus of Elasticity 𝑀𝑒𝑐ℎ𝑎𝑛𝑐𝑖𝑎𝑙 𝑆𝑡𝑟𝑎𝑖𝑛 s= 𝑀𝑒𝑐ℎ𝑎𝑛𝑐𝑖𝑎𝑙 𝑆𝑡𝑟𝑒𝑠𝑠 Young’s Modulus 72 Electromechanical Coupling Factor 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝐸𝑛𝑒𝑟𝑔𝑦 𝑀𝑒𝑐𝑎ℎ𝑛𝑖𝑐𝑎𝑙 𝐸𝑛𝑒𝑟𝑔𝑦 k= OR 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝐸𝑛𝑒𝑟𝑔𝑦𝑔𝑦 𝑀𝑒𝑐ℎ𝑎𝑛𝑐𝑖𝑎𝑙 𝐸𝑛𝑒𝑟𝑔𝑦 73 Applications of Ferroelectric Materials 74  Applications Use in Camera Flash Ferroelectrics are very useful for devices and are used in many different ways today. If a ferroelectric is used in its linear region, above TC it makes a very good capacitor, as its dielectric constant can be very high indeed. These are often used in cameras as a way of powering a flash. A battery slowly stores a charge on a capacitor, which when connected to a bulb, releases a burst of high current, creating the flash. First, the battery charges the ferroelectric capacitor and then, once fully charged, the ferroelectric is connected to the bulb and causes it to flash. Source: https://www.doitpoms.ac.uk/tlplib/ferroelectrics/why.php 75  Sensors and Actuators Source: Advances and challenges in impedance-based structural health monitoring (Research Gate) 76  Energy Harvesting Sidewalks Shoes Gyms and Work Places Piezoelectric Tiles, Floor mats and Carpets Mobile Keypads and Keyboards 77 Other Applications 78 79