Functional solid state materials Materials History the secret for transmuting base metals into precious gold Electrical properties Optical properties MEchanical properties Alchemist Material Chemist Development of Materials vs Human Society Historical Perspective began to make tools The history of human society can be marked with inorganic NAtural matene ended abe ut 000 ey, ns ago with intron.o of the Stone Age ah Historical Perspecti ron->steel- Advanced materials ammered or cast into a variety of shapes er by alloying, corrode only slowly after a surface oxid The Iron Age began about 3000 years ago an se of iron and steel, a stronger and cheaper a common perso Materials: throughout the Iron Age many new telligent design of new mato g, and performance of materialeng composites. ). Understanding of the relationship among Structure- Composition Properties Types of Materials Let us classify materials according to the way the atoms are Metals: valence electrons the ions togethe A better understanding of heat well, are shiny if miconductors: the bonding is covalent (electrons are shar rogress in the strength to tensity ratio of materials that resulted in a wide variety ucts. from dents materials to tennis racquets. r Waals forces, and usually based on C and H. They lightweight. Examples plastic, rubes(100-1000C), and ar mpose at moder
1 Functional Solid State Materials Electrical properties Optical properties Magnetic properties Mechanical properties “the secret for transmuting base metals into precious gold” Materials & History Stone age Bronze age Iron age Silicon age Alchemist Material Chemist Development of Materials vs Human Society The history of human society can be marked with inorganic materials. Historical Perspective Stone ® Bronze ® Iron ®steel ® Advanced materials Beginning of the Material Science ¾ People began to make tools from stone ¾ Start of the Stone Age about two million years ago. Natural materials: stone, wood, clay, skins, etc. The Stone Age ended about 5000 years ago with introduction of Bronze in the East Asia. Bronze is an alloy (copper + < 25% of tin + other elements). Bronze: can be hammered or cast into a variety of shapes, can be made harder by alloying, corrode only slowly after a surface oxide film forms. The Iron Age began about 3000 years ago and continues today. Use of iron and steel, a stronger and cheaper material changed drastically daily life of a common person. Age of Advanced Materials: throughout the Iron Age many new types of materials have been introduced (ceramic, semiconductors, polymers, composites… ). Understanding of the relationship among structure, properties, processing, and performance of materials. Intelligent design of new materials. Historical Perspective A better understanding of structure-compositionproperties relations has lead to a remarkable progress in properties of materials. Example is the dramatic progress in the strength to density ratio of materials, that resulted in a wide variety of new products, from dental materials to tennis racquets. Structure-Composition-Properties Types of Materials Let us classify materials according to the way the atoms are bound together. •Metals: valence electrons are detached from atoms, and spread in an “electron sea”that “glues”the ions together. Strong, ductile, conduct electricity and heat well, are shiny if polished. •Semiconductors: the bonding is covalent (electrons are shared between atoms). Their electrical properties depend strongly on minute proportions of contaminants. Examples: Si, Ge, GaAs. •Ceramics: atoms behave like either positive or negative ions, and are bound by Coulomb forces. They are usually combinations of metals or semiconductors with oxygen, nitrogen or carbon (oxides, nitrides, and carbides). Hard, brittle, insulators. Examples: glass, porcelain. •Polymers: are bound by covalent forces and also by weak van der Waals forces, and usually based on C and H. They decompose at moderate temperatures (100-400°C), and are lightweight. Examples: plastic, rubber
Materials Tetrahedron Life Cycle of Materials Performance engineering materials materials Synthesis cle/r roduct Design Structure and [chemistryl Composition Properties Electric Properties of Crystals pRoperties are the way the material responds to the PMechanical properties- response to mechanical forces, Crystals can be classified by electric properties strength. ete COnductors and magnetic fields, conductivity, et -response electrical E Dielectric crystals Thermal properties are related to transmission of heat Semiconductors SUperconductors aChemical Stability in contact with the environment- corrosion resistance Dielectric Material Dielectric Properties A dielectric material is an insulator in which electric dipoles can be induced by the electric The biggest difference between dielectric field (or permanent dipoles can exist even materials and conductors is that the transfer without electric field), that is where positive ways of electrons are totally different nd negative charge are separated on an pielectric--in manner of induced polarization conductor manner of conduction
2 Materials Tetrahedron Performance Properties Structure and Composition Synthesis and Processing [chemistry & engineering] [chemistry & physics] [engineering] [chemistry] Life Cycle of Materials Synthesis and Processing Engineered Materials Product Design Manufacture Assembly Waste Applications Recycle/Reuse Raw Materials Properties are the way the material responds to the environment and external forces. Mechanical properties ¾ response to mechanical forces, strength, etc. Electricaland Magnetic properties ¾ response electrical and magnetic fields, conductivity, etc. Thermal properties are related to transmission of heat and heat capacity. Opticalproperties include to absorption, transmission and scattering of light. Chemical Stability in contact with the environment ¾ corrosion resistance. Properties Electric Properties of Crystals Crystals can be classified by electric properties: Conductors Dielectric crystals Semiconductors Superconductors Dielectric Properties The biggest difference between dielectric materials and conductors is that the transfer ways of electrons are totally different: Dielectric¾¾in manner of induced polarization conductor ¾¾in manner of conduction Dielectric Material A dielectric material is an insulator in which electric dipoles can be induced by the electric field (or permanent dipoles can exist even without electric field), that is where positive and negative charge are separated on an atomic or molecular level + _ + _ + _
Dielectric Materials Mechanisms of Dipole formation and/or orientation along the external Polarization redistribution so that the surface nearest to the positive apacitor plate is negatively charged and vice versa at the surface, Q ⊙⊙ Q net positive charge at ionic polarization The process of dipole formation/alignment in electric field is called polarization and is described by P=Q7A molecular (orientation) polarization Dipole moments Basic Conception Orientation of dipole moments a Dielectric- material that is electrically insulating or can be made to exhibit an electrie dipole. Permittivity -ratio of the electric displacement in a Capacitance-The ratio of charge to potential on an electrically charged, isolated conductor Voong Dielectric strength-magnitude of the electrie field necessary to produce breakdown Relative Permittivity Dielectric Strength a Very high electric fields(>10 V/m)can excite s The resultant capacitance can then be electrons to the conduction band and accelerate measured due to the dielectric them to such high energies that they can, in turn, free other electrons, in an avalanche process(or C=EAld electrical discharge). The field necessary to start the dielectric constant E=E/ Eo the avalanche process is called dielectric strength the dielectric constant, or relative a The dielectric strength is a measure of how much rmittivity, is the ratio of the permittivity of be applied to a dielectric before electric he material to the permittivity of free space current begins to are across the dielectric (1=8854x1012Fm) ross the dielectric is known as dielectric a Dielectric strength has the units of vim
3 Dipole formation and/or orientation along the external electric field in the capacitor causes a charge redistribution so that the surface nearest to the positive capacitor plate is negatively charged and vice versa. + + + + + + + + - - - - - - - - - - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + - + Q0+Q’ -Q0 -Q’ region of no net charge net negative charge at the surface, -Q’ net positive charge at the surface, Q’= PA P Dielectric Materials The process of dipole formation/alignment in electric field is called polarization and is described by P = Q’/A electronic polarization ionic polarization molecular (orientation) polarization Mechanisms of Polarization Dipole Moments Orientation of dipole moments Basic Conception Dielectric¾ material that is electrically insulating or can be made to exhibit an electric dipole. ß Permittivity ¾ ratio of the electric displacement in a medium to the intensity of the electrical field producing it. ß Capacitance ¾ The ratio of charge to potential on an electrically charged, isolated conductor ß Dielectric strength ¾ magnitude of the electric field necessary to produce breakdown . Relative Permittivity The resultant capacitance can then be measured due to the dielectric: C = erA/d ß the dielectric constant er= e/ e0 ß the dielectric constant, or relative permittivity, is the ratio of the permittivity of the material to the permittivity of free space (e0=8.854x10-12 F·m-1) Dielectric Strength Very high electric fields (>108 V/m) can excite electrons to the conduction band and accelerate them to such high energies that they can, in turn, free other electrons, in an avalanche process (or electrical discharge). The field necessary to start the avalanche process is called dielectric strength or breakdown strength. The dielectric strength is a measure of how much voltage can be applied to a dielectric before electric current begins to arc across the dielectric Arcing across the dielectric is known as dielectric breakdown. Dielectric strength has the units of V/m
Dielectric material The Relations of Dielectric Crystals na dielectric material is a material that is nonmetallic and exhibits or may be made to exhibit an electric dipole sThe dielectric crystals can d as aA dielectric material is characterized and selected according be to its dielectric constant. Zr. often called the relati permittivity. There are many ceramics and polymers that exhibit dielectric behavio lectric Applications for dielectric materia ferroelectric Dielectric materials to insulate electrical conductors tThe ber in the Dielectrie materials used in capacitor parentheses is the point Communications (radio, radar and microwave groups that the crystal Microelectronics possibly exist Piezoelectricity Piezoelectricity fThe piezoelectric effect was first mentioned in 1817 by the materials, application of external forces produces an electric(polarization) field and vice demonstrated by Pierre and Jacques Curie in 1880 versa tThe direct piezoelectric effect consists of the ability of certain piezoelectric effect and converse piezoelectric effect Some dielectrics have a crystal structure with one polar on of an externally applied force is. mechanical deformation of the crystal lattice APplications of piezoeled causes electric displacement On the other hand, the I polar axis causes a deformation of the crystal lattice gauges et al)The direct piezoelectric effect has been widely when electric charges are being displaced. This is sed in transducers design (accelerometers, force and pressure called converse piezoelectric effect. pIezoelectric materials include barium titanate eccording to the inverse piezoelectric effect, an electric field BaTiOa, lead zirconate PbZrO3 quartz piezoelectric effect has been applied in actuators design. Piezoelectric Effect basics Piezoelectric Effect basic trie charge How to produce piezoelectric effect ly electric field Mechanical deformation produced a Dipole each molecule has a polarization, one end is more negatively charged and the other end is positively a Monocrystal the polar axes of all of the dipoles lie in one a Polycrystal there are different regions within the material that have a different polar axis. a) Material without stress/ charge b) Compress same polarity e)Stretched= opposite polarity d)Opposite voltage= expand tttttttttt e)Same voltage =c DAC signal=vibrate 每y
4 Dielectric Material A dielectric material is a material that is nonmetallic and exhibits or may be made to exhibit an electric dipole structure. A dielectric material is characterized and selected according to its dielectric constant, Σr, often called the relative permittivity. There are many ceramics and polymers that exhibit dielectric behavior. Applications for dielectric materials –Dielectric materials to insulate electrical conductors –Dielectric materials used in capacitors –Communications (radio, radar and microwave) –Microelectronics The Relations of Dielectric Crystals The dielectric crystals can be classified as: dielectric piezoelectric pyroelectric ferroelectric The number in the parentheses is the point groups that the crystal possibly exist Piezoelectricity In some ceramic materials, application of external forces produces an electric (polarization) field and viceversa piezoelectric effect and converse piezoelectric effect: Some dielectrics have a crystal structure with one polar axis. mechanical deformation of the crystal lattice causes electric displacement. On the other hand, the polar axis causes a deformation of the crystal lattice when electric charges are being displaced. This is called converse piezoelectric effect. Piezoelectric materials include barium titanate BaTiO3 , lead zirconate PbZrO3 , quartz. Piezoelectricity The piezoelectric effect was first mentioned in 1817 by the French mineralogist Rene Just Hauy. It was first demonstrated by Pierre and Jacques Curie in 1880. The direct piezoelectric effect consists of the ability of certain crystalline materials (i.e. ceramics) to generate an electrical charge in proportion of an externally applied force. Applications of piezoelectric materials is based on conversion of mechanical strain into electricity (microphones, strain gauges et al.)The direct piezoelectric effect has been widely used in transducers design (accelerometers, force and pressure transducers ...). According to the inverse piezoelectric effect, an electric field induces a deformation of the piezoelectric material. The inverse piezoelectric effect has been applied in actuators design. Piezoelectric Effect Basics Apply mechanical stress Þ Electric charge produced Apply electric field Þ Mechanical deformation produced Dipole: each molecule has a polarization, one end is more negatively charged and the other end is positively charged. Monocrystal: the polar axes of all of the dipoles lie in one direction. ¾¾ Symmetrical Polycrystal: there are different regions within the material that have a different polar axis. ¾¾ Asymmetrical Piezoelectric Effect Basics ß How to produce piezoelectric effect a) Material without stress / charge b) Compress Þ same polarity c) Stretched Þ opposite polarity d) Opposite voltage Þ expand e) Same voltage Þ compress f) AC signal Þ vibrate
Rfall tetrahedra have the sam Piezoelectricity The Piezoelectric orientation or some other Effect vs Crystal mutual orientation that does no o( Greek: piezo"to press") Structure the action of all dipoles adds up 9 Some ionic crystals with polar axis d the whole crvstal becomes show a piezoelectric effect ryor opposite faces of the external pressure causes ACrystals can only be deformation and results in piezoelectric if they are non- centrosymmetric SPhalerite, tourmaline, ammonium chloride and quartz Piezoelectricity AApplication of pressure to a piezoelectric crystal Piezoelectric materials have crvstal structures that lack displaces the crystal ions with respect to each other When the crystal is stressed however it develops a NE variation AThis change of polarization can be detected as a voltage across the crystal and this effect is referred to as piezoelectricity Piezoelectric crys mechanical pulses are to be concerted to electrical pplication of stress to AThe opposite effect to piezoelectricity is ystal, the net polarization is the crystal gives rise to a electroconstriction which is an effect in which an electric field is used to produce a change in the agnitude of the dipole moments dimensions of a piezoelectric crystal: e.g. Mechanical lding the three symmetry vibrations are induced in the quartz with the aid of electric pulses Piezoelectricity Applications of Piezoelectric Crystals . Crystals where electrical polarization generated by mechanical stress -in general, they are Mechanical to Electrical Conversio centrosymmetrIc Phonograph cartridges eStrain shifts the relative positions of the positive and negative charges, giving ri a net electric Microphones Vibration sensors +In 32 crystallographic point groups, 21 Accelerometers possess inversion symmetry elements, plu Photoflash actuators oic has a combination of symmetries 20 groups can be piezoelectric Gas igniter Many crystals with tetrahedral structure units (SiO, ZnO etc ).shearing stress causes distortional train of tetrahedry
5 Piezoelectricity (Greek: piezo "to press") Some ionic crystals with polar axis show a piezoelectric effect. The Piezoelectric Effect vs Crystal Structure If all tetrahedra have the same orientation or some other mutual orientation that does not allow for a compensation, then the action of all dipoles adds up and the whole crystal becomes a dipole. Two opposite faces of the crystal develop opposite electric charges. Crystals can only be piezoelectric if they are noncentrosymmetric. Sphalerite, tourmaline, ammonium chloride and quartz are examples. external pressure causes deformation and results in electric dipole Piezoelectricity Piezoelectric materials have crystal structures that lack inversion symmetry but show NO spontaneous polarization Þ When the crystal is stressed however it develops a NET polarization in an unstressed piezoelectric crystal, the net polarization is equal to zero (arrows indicate the magnitude of the dipole moments along the three symmetry directions of the crystal) application of stress to the crystal gives rise to a net polarization p STRESS P Application of pressure to a piezoelectric crystal displaces the crystal ions with respect to each other and so causes a change in polarization. This change of polarization can be detected as a voltage across the crystal and this effect is referred to as piezoelectricity. Piezoelectric crystal serve whenever mechanical pulses are to be concerted to electrical signals, e.g. in microphones. The opposite effect to piezoelectricity is electroconstriction which is an effect in which an electric field is used to produce a change in the dimensions of a piezoelectric crystal: e.g. Mechanical vibrations are induced in the quartz with the aid of electric pulses. Piezoelectricity Crystals where electrical polarization generated by mechanical stress ¾¾ in general, they are noncentrosymmetric. Strain shifts the relative positions of the positive and negative charges, giving rise to a net electric dipole. In 32 crystallographic point groups, 21 do not possess inversion symmetry elements, plus one cubic has a combination of symmetries, thus, only 20 groups can be piezoelectric. Many crystals with tetrahedral structure units (SiO2 , ZnO etc.),shearing stress causes distortional strain of tetrahedra. Applications of Piezoelectric Crystals ß Mechanical to Electrical Conversion –Phonograph cartridges –Microphones –Vibration sensors –Accelerometers –Photoflash actuators –Gas igniters –Fuses
Applications of Piezoelectric Crystals Applications of Piezoelectric Crystals Electrical to mechanical conversion -Valves Electrical- Mechanical-Electrical Conversion Surface acoustic wave devices earphones and speakers Filters Ultrasonic cleaners Oscillators Emulsifiers Transformer Sonic transducers Applications of Piezoelectricity Quartz Crystal .Electroconstriction gnals into sound in earphones ploited to convert electrical .Another important application is quartz resonators enerates an electrical charge when aning that it hich may be used as frequency selective elements .Piezoelectric materials such as PZT (PbZrTiO3) are d to control the motion of the scanning tip in the scanning tunneling microscope(STM) qUalities, mechanical strength and durability, quartblore:Gc Cady also concluded that the crystal could be cut in specif Quartz crystals were first used as a time standard by Warren Marrison, who invented the first quartz clock in 927. Juergen Staudte invented a method for mass- STM of a corrals of atoms arranged using an STM producing quartz crystals for watches in the early 1970s. TThe frequeney of the quartz oscillator is determined by the cut and shape of the Scanning Tunneling Microscope come in various shapes and frequencies The most common crystals are miniature 32, 768 times per second. Other types of than 50 million mes per surface]at wheel the atomic level rovide the time standard in mechanical atches. the balance wheel oscillated first at 2.5, then at 3, and finally at 5 cycles per
6 Applications of Piezoelectric Crystals ß Electrical to Mechanical Conversion –Valves –Micropumps –Earphones and speakers –Ultrasonic cleaners –Emulsifiers –Sonic transducers Applications of Piezoelectric Crystals ß Electrical-Mechanical-Electrical Conversion –Surface acoustic wave devices –Filters –Oscillators –Transformers Electroconstriction is exploited to convert electrical signals into sound in earphones Another important application is quartz resonators which may be used as frequency selective elements Piezoelectric materials such as PZT (PbZrTiO3 ) are used to control the motion of the scanning tip in the scanning tunneling microscope (STM) STM image of a corrals of atoms arranged using an STM Applications of Piezoelectricity Quartz Crystal Quartz is a piezoelectric material, meaning that it generates an electrical charge when mechanical pressure is applied. These crystals also vibrate when a voltage from an outside source, such as a battery, is applied. In the early 1920s W.G. Cady recognized that, due to their elastic qualities, mechanical strength and durability, quartz crystals could be used to fabricate very stable resonators. Cady also concluded that the crystal could be cut in specific ways that would create resonators of almost any frequency that were practically independent of temperature variations. Quartz crystals were first used as a time standard by Warren Marrison, who invented the first quartz clock in 1927. Juergen Staudte invented a method for massproducing quartz crystals for watches in the early 1970s. The frequency of the quartz oscillator is determined by the cut and shape of the quartz crystal. The quartz crystals inside watches today come in various shapes and frequencies. The most common crystals are miniature encapsulated tuning forks which vibrate 32,768 times per second. Other types of crystals vibrate at more than 50 million times per second. The oscillations of the balance wheel provide the time standard in mechanical timepieces. In contrast, in the history of mechanical watches, the balance wheel oscillated first at 2.5, then at 3, and finally at 5 cycles per second. Scanning Tunneling Microscope Ability to probe the geometric and electronic structure of a surface in-situ at the atomic level in real space
Piezoelectricity another area of application for dielectrie materials is in cro"electro mechanical systems (MEMS) Dielectrics are used to make mic Ie gear structures Example: phonograph for a range of applications dn the transducer stress exerted on some cantilever in turn generates an electrie current strueture is converted into an electrical signal via the Bumps in the piezoelectric effect eelectric back and nerating sound 最餐 formed in PMMA (size mm and length 140 m scale is 100 mm!) silicon substrate Applications of Piezoelectric Crystals aSensor technology Pyroelectrics force sensors .Typical applications for piezoelectricity can be found lin every household. One example are lighters. In this e a subset where spontaneous polariza caused by intrinsic internal strain se a tiny prestressed hammer hits accompanied by a lowering in symme material thus igniting a spark. a different crystal structure s Pyroelectricity results from the temperature dependence of the spontaneous polarization of polar materials Gas and cigarette lighters Pyroelectricity Examples for Application s ome piezoelectric ionic crystals additionally show a lectric effect sensor PYroelectric materials possess a temperature dependent macroscopic electrical polarization pYroelectric materials are very sensitive! For instance thermal radiation of an human being is sufficient to create measurable electric voltag This is widely used for commercial motion Temperature dependence of the spontaneous Motion detector
7 Piezoelectricity Amplifier Bumps in the record groove bounce the stylus up and down Stylus motion squeezes a piezoelectric that in turn generates an electric current Amplified current flexes a second piezoelectric back and forth, generating sound Example: phonograph Another area of application for dielectric materials is in micro-electro-mechanical systems (MEMS) Dielectrics are used to make microscopic gear structures for a range of applications In the transducer, stress exerted on some cantilever structure is converted into an electrical signal via the piezoelectric effect microscopic gear wheels formed in PMMA (size scale is 100 mm!) cantilever structure of width 10 mm and length 140 mm patterned in a silicon substrate Sensor technology force sensors Typical applications for piezoelectricity can be found in every household. One example are lighters. In this case a tiny prestressed hammer hits a piezoelectric material thus igniting a spark. Gas and cigarette lighters Applications of Piezoelectric Crystals Pyroelectrics a subset where spontaneous polarization is caused by intrinsic internal strain accompanied by a lowering in symmetry to a different crystal structure Pyroelectricity results from the temperature dependence of the spontaneous polarization of polar materials Pyroelectricity Greek: pyro "to burn" Some piezoelectric ionic crystals additionally show a pyroelectric effect. Pyroelectric materials possess a temperature dependent macroscopic electrical polarization. Temperature dependence of the spontaneous polarization of triglycerin sulfate. Examples for Application Sensor technology motion detectors Pyroelectric materials are very sensitive! For instance thermal radiation of an human being is sufficient to create measurable electric voltage. This is widely used for commercial motion detectors. Motion detector
The Pyroelectric Effect Heat sensors Tourmaline(Calcium Carbide Calcium Carbide trigonal crystal, C3, point fold rotation axis. The differential change in the remnant happens on the direction of 3 -Remnant polarization is sensitive to V xa ouring the mixture of sulfur powder .Pyroelectric heat sensor a on a heated calcium carbide(CaCy)crystal.Due T=AH'AA hpg 6 to the o charge n cover the two. ns have negativ to the friction effect of the sieve. the pbo hav AH: the change in heat energy in J rge, they will A.' area of the slab inm ps along the 3- h.: thickness of the slab in m makes the two tops of CaC, crystal have different he density of the ferroelectric slab in kg/m charges along the 3-fold axi s,' the specific heat of the ferroeleetric slab in d/kg-K Ferroelectrics Ferroelectricity sa subset of pyroelectric electrical polarization can be reversed b in each primitive unit cell. e. g. Zno is pyroelectric he application of external electric field tal is also ferroelectric if moment, all point in the same direction.s a net dipole the direction of the spontaneous Ferroelectric is a pyroelectric solid in which the polarization can be reversed under an spontaneous electrical polarization in a unit cell car applied electric field be reversibly changed between tP by application of Ferroelectricity Ferroelectricity u Ferroelectricity is a phenomena which was e Ferroelectricity derives its name from discovered in 1921 ferromagnetic en called seignette aA magnetization can be observed that is electricity, as Seignette or Rochelle Salt(RS)wa reversible by applying a certain magnetic field the first material found to show ferroelectric Ferroelectrics show a reversibility, but dealing with applied electric fields to reverse a materials A huge leap in the research on ferroelectric polarization. naterials came in the 1950s, leading to the widespread use of barium titanate(BaTiO based annies in capacitor applications and piezoelectric
8 The Pyroelectric Effect of Tourmaline (Calcium Carbide) Calcium Carbide: trigonal crystal, C3n point group, have only 3-fold rotation axis. The Pyroelectric effect happens on the direction of 3- fold rotation axis. Examples: Pouring the mixture of sulfur powders (yellow) and PbO powders (red) through a sieve on a heated calcium carbide (CaC2 ) crystal. Due to the friction effect of the sieve, the PbO have positive charge and sulfur powders have negative charge, they will cover the two tops along the 3- fold rotation axis of CaC2 , indicating that heating makes the two tops of CaC2 crystal have different charges along the 3-fold axis. Heat Sensors ßTemperature change Þ differential change in the remnant polarization –Remnant polarization is sensitive to temperature change ßPyroelectric heat sensor s :the specificheat of the ferroelectric slab in J/kg K : the density of the ferroelectric slab in kg/m h :thickness of the slab in m A : area of the slab in m H: the change in heat energy in J T H /( A h s ) p 3 0 1 2 s s 1 0 p * × r D D = D r Ferroelectrics a subset of pyroelectrics electrical polarization can be reversed by the application of external electric field A pyroelectric crystal is also ferroelectric if the direction of the spontaneous polarization can be reversed under an applied electric field. Ferroelectricity Pyroelectricity have a permanent net electric dipole in each primitive unit cell. e.g. ZnO is pyroelectric because ZnO4 tetrahedra, each possessing a net dipole moment, all point in the same direction. Ferroelectric is a pyroelectric solid in which the spontaneous electrical polarization in a unit cell can be reversibly changed between ±Ps , by application of and E field of suitable polarity. Ferroelectricity Ferroelectricity derives its name from ferromagnetic. A magnetization can be observed that is reversible by applying a certain magnetic field. Ferroelectrics show a reversibility, but dealing with applied electric fields to reverse a material’s polarization. Ferroelectricity Ferroelectricity is a phenomena which was discovered in 1921. Ferroelectricity has also been called Seignette electricity, as Seignette or Rochelle Salt (RS) was the first material found to show ferroelectric properties. A huge leap in the research on ferroelectric materials came in the 1950's, leading to the widespread use of barium titanate (BaTiO3 ) based ceramics in capacitor applications and piezoelectric transducer devices
Atomic Arrangement and Ferroelectricit Ferroelectricity Ferroelectric materials exhibit spontaneous The arrangement of the atoms in all polarization. This polarization can be aligned by an ferroelectric crystals result in an equally stable electric field and will rema state but with reoriented ps field is removed. It occurs from the nonsymmetry A simple example is BaTiOa for which the shape of the complex ferroelectrics unit cell rroelectrics are principally used to improve the prototype is cubic performance of capacitors The paraelectric to ferroelectric transformation may be viewed in terms of a lowfrequency mperature dependent mode of the cryst lattice, observable by optical or neutron pectroscopy Normal Capacitor Ferroelectric Capacitor Ferroelectric Transitions Ferroelectricity previously we saw that dielectric cUrie Temperature, Te: transition from tals develop a net polarization ndomized paraelectric and ordered ferroelectri necte presence or an app ein certain crystals known as MATERIAL TC(OT rroelectrics however the polarization can persist when the applied electric field is removed! Tmperature known as the Curie At temperatures above this ormal dielectrie behavior is ained Ferroelectric Domains and Hysteresis Loop fThe characteristic signature of ferroelectric crystals is the observation of hysteresis in their Polarization vs. Electric field roelectric crystals I PRior to applying the electric field, the crystal is unpolarized possess regions with uniform polarization HOwever, with a strong applied electric field, the permanent called ferroelectric wWhen the field is then lowered back to zero a remnant plarization then remains pOlarization ys. Electric Field (P-E)hysteresis vAriation of polarization with eleetrie field for a op for a typical m zero polarization at zero field, the ferroelectric crystal is shown on the right. larization does not return emnant polarization p
9 Atomic Arrangement and Ferroelectricity ß The arrangement of the atoms in all ferroelectric crystals result in an equally stable state but with reoriented Ps. ß A simple example is BaTiO3 for which the prototype is cubic. ß The paraelectric to ferroelectric transformation at Tc may be viewed in terms of a low-frequency temperature-dependent mode of the crystal lattice, observable by optical or neutron spectroscopy. Ferroelectricity + + + + - - - - Normal Capacitor + + + + - - - - ++ ++++++++ ++ - - - - - - - - - -- - - - - - - - - - - - ++ + ++ +++ ++ - - - Ferroelectric Capacitor Ferroelectric materials exhibit spontaneous polarization. This polarization can be aligned by an electric field, and will remain aligned even after the field is removed. It occurs from the nonsymmetric shape of the complex ferroelectric’s unit cell. Ferroelectrics are principally used to improve the performance of capacitors. Ferroelectric Transitions Curie Temperature, Tc : transition from randomized paraelectric and ordered ferroelectric phase. Ferroelectric Paraelectric Ferroelectricity Previously we saw that dielectric crystals develop a net polarization in the presence of an applied electric field. In certain crystals known as ferroelectrics however the polarization can persist when the applied electric field is removed! Ferroelectric behavior is only observed below a critical temperature known as the Curie temperature TC Þ At temperatures above this normal dielectric behavior is obtained MATERIAL TC (K) Ps (mCcm-2 ) BaTiO3 408 26.0 SrTiO3 110 KNbO3 708 30.0 PbTiO3 765 >50 LaTaO3 938 50 LiNbO3 1480 71 GeTe 670 Ferroelectric Domains and Hysteresis Loop Ferroelectric crystals possess regions with uniform polarization calledferroelectric domains. Polarization vs. Electric Field (P-E) hysteresis loop for a typical ferroelectric crystal is shown on the right. The characteristic signature of ferroelectric crystals is the observation of hysteresis in their Polarization vs. Electric field curves. Prior to applying the electric field, the crystal is unpolarized since the permanent dipoles in the crystal are randomly oriented. However, with a strong applied electric field, the permanent dipoles polarize and saturation of the polarization is observed. When the field is then lowered back to zero a remnant polarization then remains. Variation of polarization with electric field for a ferroelectric crystal below the curie temperature Starting from zero polarization at zero field, the electric field is increased and the polarization eventually saturates at the value ps When the field is lowered back to zero the polarization does not return to zero but exhibits a remnant polarization p r Pr P 1 E Ps 2
With the ferroelectric crystal now polarized at zero field, it necessary to apply a finite coercive field in the opposite abole e s: when E is large enough, the Hysteresis Cf the reversed electric is then further increased beyond the larization P remains, i. e the crystal e field, saturation of the polarization will occur again. Is an electret however, now the pol gwhen a coercive field F-E)is opposite direction to that obtained originally the polarization vs. electric field curve is therefore said to hibit hysteresis o external E the dipole moments of ferroelectric crystals typically shows nt domains compensate each we indicate the coercive electric field acts on the sample, those indication of the energy dissipated by full cycle of the hysteresis ctric field will grow at the expense of loop has been achieved he remaining domains). Crystal structure of barium nat titanate at temperatures 7>T above the Curie temperature turn ooO Cin this crystal configuration ooO there is no net polarization the unit cell and the crysta (BaTiOj) behaves as a normal charged ions and one negatively temperatures above the a\bove the Curie temperature these curle temperature electric materials are haves as a normal dielectric wi said to be paraelectric no spontaneous polarization. dAs the temperature is lowered curie temperature Barium Titanate mber of dipoles align to ferroelectric domains that are typically randomly oriente eln electric field may be used to align the domains with . Barium-Titanate(BaTiO) -the first material to be ●oo● eavailable in single crystal form The absence of center symmetry in crystal structure gives rise to spontaneous polarization Cubic above Curie temperature; tetragonal as it cools different domains in a ferroelectri down c/a=1.04 crystal with no applied electrie field
10 With the ferroelectric crystal now polarized at zero field, it is necessary to apply a finite coercive field in the opposite direction to return the polarization to zero. If the reversed electric is then further increased beyond the coercive field, saturation of the polarization will occur again. Þ however, now the polarization will be oriented in the opposite direction to that obtained originally Þ the polarization vs. electric field curve is therefore said to exhibit hysteresis P E Ec the polarization-field curve of ferroelectric crystals typically shows hysteresis on the figure we indicate the coercive field ec required to remove a net positive polarization in the crystal the area enclosed by the curve provides an indication of the energy dissipated by the field once a full cycle of the hysteresis loop has been achieved curve s: when E is large enough, the whole crystal is one large domain when E is removed, a remnant polarization Pr remains, i.e. the crystal now is an electret. when a coercive field (-Ec ) is applied, Pr can be removed. Ps is the spontaneous polarization corresponding to the polarization within a domain. No external E: the dipole moments of different domains compensate each other curve j: the total polarization of the crystal (if an increasing external electric field acts on the sample, those domains whose polarization corresponds to the direction of the electric field will grow at the expense of the remaining domains). Hysteresis Loop At a microscopic level, ferroelectrics can be understood to be those materials whose crystal structures contain charged ions that are displaced from high-symmetry points. This displacement in turn gives rise to a net polarization of the crystal unit cell. A good example of a ferroelectric crystal is barium titanate (BaTiO3 ) which features two positivelycharged ions and one negativelycharged ion. Above the Curie temperature these ions are distributed in a perovskite crystal structure and the crystal behaves as a normal dielectric with no spontaneous polarization. crystal structure of barium titanate at temperatures above the Curie temperature in this crystal configuration there is no net polarization of the unit cell and the crystal behaves as a normal dielectric at temperatures above the curie temperature ferroelectric materials are said to be paraelectric Barium Titanate lBarium –Titanate (BaTiO3 ) ¾ the first material to be developed as a Ferroceramic lavailable in single crystal form lThe absence of center symmetry in crystal structure gives rise to spontaneous polarization lCubic above Curie temperature; tetragonal as it cools down As the temperature is lowered below the curie temperature, the crystal structure deforms and the unit cell develops a net dipole moment along the vertical axis of the unit cell. In the ferroelectric state, a large number of dipoles align to form ferroelectric domains that are typically randomly oriented at zero field. An electric field may be used to align the domains with respect to each other. random alignment of the dipoles of different domains in aferroelectric c/a=1.04 crystal with no applied electric field