The following points highlight the top five applications and uses of ferroelectric ceramics in electrical engineering. The applications are: 1. Capacitors 2. Ferroelectric Memories 3. Ferroelectric Thin Film Waveguides 4. Ferroelectric Thin Film Optical Memory Displays 5. Pyroelectric Detectors.
Application # 1. Capacitors:
A capacitor consists of a dielectric material sandwiched between two electrodes. The total capacitance for this device is given by-
C = ԑ0ԑrA/t
where ‘C’ is the capacitance, ԑ0 is the permittivity of free space, ԑr is the relative dielectric permittivity ‘t’ is the distance between the electrodes, and ‘A’ is the area of the electrodes.
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To get a high volumetric efficiency (capacitance per unit volume) the dielectric material between the electrodes have a large dielectric constant, a large area and a small thickness. BaTiO3 based ceramics having a perovskite type structure show dielectric constant values as high as 15,000 as compared to 5 to 10 for common ceramic and polymer materials. The high dielectric constant BaTiO3 ceramics based disk capacitors are simple to make and have captured more than 50% of the ceramic capacitor market.
The volumetric efficiency can be further enhanced by using multilayer ceramic (MLC) capacitors. As shown in Fig. 6.20, the MLC capacitor structure consists of alternate layers of dielectric and electrode material. Each individual dielectric layer contributes capacitance to the MLC capacitor as the electrodes terminate in a parallel configuration. Hence the effective equation for capacitance becomes,
C = nԑ0ԑr A/t
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where n represents number of dielectric layers. The advances in tape casting technology have made it possible to make dielectric layers <20 μm thick. This, combined with the use of a high dielectric constant ceramic like BaTiO3, allows large capacitance values to be achieved in relatively small volume capacitor devices.
Application # 2. Ferroelectric Memories:
Semiconductor memories such as dynamic random access memories (DRAM’s) and static random access memories (SRAM’s) currently dominate the market. However, the disadvantage of these memories is that they are volatile, i.e., the stored information is lost when the power fails. The non-volatile memories available at this time include complementary metal oxide semiconductors (CMOS) with battery backup and electrically erasable read only memories (EEPROM’s).
These non-volatile memories are very expensive. The main advantage offered by ferroelectric random access memories (FRAM’s) include non-volatile and radiation hardened compatibility with CMOS and GaAs circuitry, high speed (30 ns cycle time for read/erase/rewrite) and high density (4(μm)2 cell size).
Ferroelectric materials spontaneously polarize on cooling below the Tc. The magnitude and direction of polarization can be reversed by the application of an external electric field. The FRAM’s made from ferroelectric thin films make use of this phenomena to store data.
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Data is stored by localized polarization switching in the microscopic regions of ferroelectric thin films. The FRAM’s are non-volatile because the polarization remains in the same state after the voltage is removed (as ferroelectric have a non-linear hysteresis curve). The radiation hardness of FRAM’s allow for the use of device containing these memories in harsh environments such an outer space.
Application # 3. Ferroelectric Thin Film Waveguides:
An optical waveguide controls the propagation of light in a transparent material (ferroelectric thin film) along a certain path. For the waveguide to work properly, the refractive index of the film should be higher than that of the substrate. For light to propagate in the waveguide, the thin film should be optically transparent. This can be achieved by fabricating the film under clean conditions and aiming for a fine grain size with ultra-phase purity and high density.
A great deal of work has been done on making ferroelectric thin film waveguides from LiNbO3 and Li(Nb, Ta)O3, PZT and PLZT thin films are even better candidates for optical waveguide applications because of their large electrotopic coefficients. PLZT thin films are even better candidates for optical waveguide applications because of their large electro-optic coefficients.
Application # 4. Ferroelectric Thin Film Optical Memory Displays:
Ferroelectric thin films may replace the use of PLZT bulk ceramics for optical memory and display applications. The advantages offered by thin films for display applications include a simplification of the display device design and lower operating voltages as compared to PLZT ceramic devices. Optical memories using PLZT thin films will also need lower operating voltages.
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The material requirements for thin film optical memory and displays include large electro-optic coefficients and/or strong photo-sensitivities for the film. PZT and PLZT thin films show a lot of promise for these optical applications.
Application # 5. Pyroelectric Detectors:
Pyroelectricity is the polarization produced due to small change in temperature. Single crystals of triglycine sulfate (TGS), LiTaO3, and (Sr, Ba) Nb2O6 are widely used for heat sensing applications.
The use of ferroelectric thin films for pyroelectric devices is advantageous because of the high cost of growing single crystals and also the thin film geometry is convenient for device design. PbTiO3 (Pb, La) TiO3 and PZT have been widely studies for thin film pyroelectric sensing applications.