Creation of DIPOLE (two poles) (distortion of crystal structure by the small displacement of the ion in direction of electric field)

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Evelyn Alexander
5 years ago
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1 Dielectricity Dielectric materials: Materials which is generally insulators. Under applied electric field, there is a tiny movement of charge inside the material. Electric field Creation of DIPOLE (two poles) The phenomena of formation of dipole is called Polarization (Analogous to magnetization of magnetic dipole) Polarization can occur in 4 ways 1. Electronic Polarization Cloud of electrons moves in direction of electric field. 2. Orientation Polarization Dipole molecule (HCl, H 2 O, HF, etc) moves in direction of electric field 3. Space Charge Polarization Accumulation of charge in some area of material. 4. Atomic หร อ Ionic Polarization Distortion or change of position of ion in crystal structure. Electrode Electrode Dielectric E (in polar molecule, H + 2O - ) (movement of charged particles in direction of electric field) (distortion of crystal structure by the small displacement of the ion in direction of electric field) Fixed charge Mobile charge E Grain boundary or interface (c) Accumulated charge Space Charge Polarization no electric field positive ion has high mobility (c) accumulation at grain boundary, phase boundary, or defects. (the ions which are more mobile will move to available space inside the solid, such as grain boundaries, and other defects)

2 Ionic polarization p + p Cl Na + p' + p' x Electric field: volt per meter. The voltage of the electrodes divided by distance between the electrodes. The larger the voltage or the nearer the two electrodes, the stronger the electric field. Capacitance (C) C = Q/V farad = coulomb/volt Capacitance = amount of accumulated charge under 1 V applied voltage. E V Voltage from battery Fig. 7.8: A NaCl chain in the NaCl crystal without an applied field. Average or net dipole moment per ion is zero. In the presence of an applied field the ions become slightly displaced which leads to a net average dipole moment per ion. Q is amount of charge that is accumulated inside the materials Dielectric Constant is a material property, showing the capacity of a material to accumulate charges, under applied electric field. C = εa / l farad = (farad/cm)(cm 2 )/(cm) Capacitance depends on size (A), distance between plates (l), and materials in the middle. value ofεis a property of material called Permittivity εof vacuum = ε 0 = 8.85x10-14 farad/cm ε = ε/ε 0 = relative permittivity = dielectric constant some textbooks use K and K instead of ε and ε C = ε ε 0 (A/l) Analogous to B = µ r µ 0 H in magnetism.

3 Which one will be your selection? To increase C, one can increase ε, A, and decrease l. Increase ε by selection of proper dielectric materials with high ε value Increase A by using multilayer configuration Effect of frequency to dielectric constant at constant temperature Dielectric constant depends on frequency of the electric field (how fast the switching of + and electrode) The charge movement inside the materials cannot keep up with the changing of the charge of the electrodes Dielectric constant Interfacial and space charge ε r '' ε r ' Why does the decrease is in steps? Orientational, Dipolar ƒ Radio Ionic Infrared Electronic Ultraviolet light ε r ' = 1 Frequency Fig. 7.14: The frequency dependence of the real and imaginary parts of the dielectric constant in the presence of interfacial, orientational, ionic and electronic polarization mechanisms.

4 Effect of temperature to dielectric constant at constant frequency Effect of both frequency and temperature to dielectric constant Effect of both frequency and temperature to dielectric constant Dielectric Strength Maximum voltage that can be applied per thickness of material. Unit is V/mil or V/cm (mil = inch) Keep in mind that short circuit will occur at large voltage.

5 Dielectric Loss Amount of energy loss from movement of ions under applied alternate voltage. Why these materials have high dielectric constant? Movement of large positive charge (Ti 4+ ) inside the unit cell of perovskite structure

6 Cubic (Perovskite) random orientation of Ti 4+ random polarization Tetragonal Ti 4+ move to near O 2- at face center permanent dipole Phase transformation Cubic Tetragonal is called Curie temperature Doping will change the shape of the graph and position of maximum K BaTiO 3 +CaZrO 3 +MgZrO 3 broadening of peak BaTiO 3 +PbTiO 3 increase curie temperature BaTiO 3 +SrTiO 3 +SrSnO 3 +CaSnO 3 +BaSnO 3 decrease curie temperature read Strontium stannate

7 Single Layer Ceramic Mica Film Multilayer Ceramic Paper and Plastic Film Solid Electrolytic Al, Ta Electrolytic Al, Ta 1 pf 1 nf 1 µf 1 mf Capacitance Fig. 7.27: Examples of dielectrics that can be used for various capacitance values. Al Electrolytic Ta Electrolytic High Permittivity Ceramic Low-loss ceramic and glass Polymer Film Mica Film 1 Hz 1 khz 1 MHz Frequency 1 GHz Fig. 7.28: Examples of dielectrics that can be used in various requency ranges. Al metallization Polymer film Metal termination Ceramic Epoxy Leads Metal electrode Single layer ceramic capacitor (e.g. disk capacitors) Multilayer ceramic capacitor (stacked ceramic layers) Fig. 7.29: Single and multilayer dielectric capacitors Fig Two polymer tapes in each with a metallized film electrode on the surface (offset from each other) can be rolled together (like a Swiss roll-cake) to obtain a polymer film capacitor as in. As the two separate metal films are lined at oppose edges, electroding is done over the whole side surface.

8 Al foils Al 2 O 3 Anode Electrolyte Cathode Epoxy Silver paint Ta 2 O 5 Graphite Ta MnO 2 Silver paste Al Al Ta Al case Lads Fig. 7.31: Al electrolytic capacitor. Electrolyte: Sodium Borate Fig. 7.32: Solid electrolyte tantalum capacitor. A cross section without fine detail. An enlarged section through the Ta capacitor. Piezoelectricity: Pressure Electricity When material is under stress, it generates Dipole in the material. Amount of Dipole is linearly proportional of applied stress. On the other hand, when applying voltage, it generate stress in the material, and may cause change of shape of material. Application: ultrasonic device, microphone, phonograph cartridge, accelerometer, strain gauges, sonar devices, actuator With appropriate applied force in a direction, it will generate Dipole in the unit cell of Piezoelectric ceramic, causing voltage in the material. On the other hand, applied voltage causing distortion of unit cell.

9 Unit cell without Center Symmetry has Piezoelectric property, under applied force in an appropriate direction. Ex: Quartz (a crystal structure of SiO 2 ) in direction [100] cause Polarization but direction [001] does not cause Polarization: Your homework! Voltage signal from amplifier cause contraction and expansion of the piezoelectric ceramic. That causes vibration of the diaphragm of the speaker of the headphone. Air pressure from wave of sound is store as roughness on the surface of the wax. Fig. Edison cylinder phonograph: 1899 Roughness causes vibration at stylus (pen) and causes stress to the piezoelectric ceramic. Source:

38: Piezoelectric transducers are widely used to generate ultrasonic waves in solids and also to detect such mechanical waves.

10 Hydrophone is a device for listening to sound under water. Accelerometer is a device for measuring mass movement or acceleration. Mechanical vibrations Piezoelectric transducer A Oscillator Elastic waves in the solid Piezoelectric detector B Oscilloscope Fig. 7.38: Piezoelectric transducers are widely used to generate ultrasonic waves in solids and also to detect such mechanical waves. The transducer on the left is excited from an ac source and vibrates mechanically. These vibrations are coupled to the solid and generate elastic waves. When the waves reach the other end they mechanically vibrate the transducer on the right which converts the vibrations to an electrical signal.

Configuration is similar to a capacitor.

Range up to 2 mm Fast Response ( 10 msec) Nanometer Scale Resolution Low

[above] works when a current bends a piezoelectric crystal, forcing the fluid

11 Configuration is similar to a capacitor. The input is voltage, the output is small movement due to distortion of crystal. Source: Source: Inkjet head Piezoelectric Multilayer Bender Actuator Positioning Range up to 2 mm Fast Response ( 10 msec) Nanometer Scale Resolution Low Operating Voltage (0 to 60 V) Low Temperature Compatible A piezoelectric nozzle [above] works when a current bends a piezoelectric crystal, forcing the fluid down and out of the nozzle. To heat and expand the fluid, instead of a crystal. Piezoelectric nozzles create very smaller droplets. Source:

Driven by dual orthogonal vibration modes with a phase shift of 90, the contact point between two surfaces vibrates in an elliptical path, producing a frictional force between the surfaces.

Using the resonance effect, a much lower voltage can be used to produce a high vibration amplitude.

Ex: Wurtzite (hexagonal ZnS), Tourmaline, Rochelle salt, Triglycine salfate, BaTiO 3, Pb(Zr,Ti)O 3, Lithium sulfate, LiTaO 3 Usage: measuring of temperature at high sensitivity: 10-6 o C Some of the

1 2 3 RF u 0 1: Single crystal LiTaO 3 2&3: Electrode u 0 : output voltage RF: radiant flux (heat) Ferroelectricity It is a Subclass of Pyroelectricity It has Hysteresis loop which is change of

12 Driven by dual orthogonal vibration modes with a phase shift of 90, the contact point between two surfaces vibrates in an elliptical path, producing a frictional force between the surfaces. Usually, one surface is fixed causing the other to move. In most piezoelectric motors the piezoelectric crystal is excited by a sine wave signal at the resonant frequency of the motor. Using the resonance effect, a much lower voltage can be used to produce a high vibration amplitude. Pyroelectricity: Fire + Electricity It is a subclass of Piezoelectricity Under heat, material is distorted and expanded, causing formation of Dipole, and generate voltage in the material. Ex: Wurtzite (hexagonal ZnS), Tourmaline, Rochelle salt, Triglycine salfate, BaTiO 3, Pb(Zr,Ti)O 3, Lithium sulfate, LiTaO 3 Usage: measuring of temperature at high sensitivity: 10-6 o C Some of the common applications for piezoelectric motors includes camera focus systems, computer disk drives, material handling, robotics, and semiconductor testing and production systems RF u 0 1: Single crystal LiTaO 3 2&3: Electrode u 0 : output voltage RF: radiant flux (heat) Ferroelectricity It is a Subclass of Pyroelectricity It has Hysteresis loop which is change of Polarization under applied voltage. P = Polarization: µc/cm 2 E = Electric field: V/cm P s Spontaneous polarization P r Remanent polarization B Saturation E c Coercive field p + p Cl Na + p' + p' E Fig. 7.8: A NaCl chain in the NaCl crystal without an applied field. Average or net dipole moment per ion is zero. In the presence of an applied field the ions become slightly displaced which leads to a net average dipole moment per ion. x

13 +Q E -Q Bound polarization charges on the surfaces -Q P +Q P V Area = A p total P -Q P +Q P d (c) P = p total / volume = (Q p d) / (A d) = Q p /A = coulomb/m 2 Fig. 7.4: When a dilectric is placed in an electric field, bound polarization charges appear on the opposite surfaces. The origin of these polarization charges is the polarization of the molecules of the medium. (c) We can represent the whole dielectric in terms of its surface polarization charges +QP and -QP. Domain บร เวณท ม ท ศทางของ Dipole ไปในทางเด ยวกน

Classification of Dielectrics & Applications

Classification of Dielectrics & Applications DIELECTRICS Non-Centro- Symmetric Piezoelectric Centro- Symmetric Pyroelectric Non- Pyroelectric Ferroelectrics Non-Ferroelectric Piezoelectric Effect When