Today s Objectives Dielectrics and Ferroelectrics

Today s Objectives Dielectrics and Ferroelectrics PowerPoint PPT Presentation


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Capacitance. Two electrodes separated by a gap define a capacitor.When a bias is applied across the capacitor plates, one charges positively, the other negatively.The amount of charge that the capacitor can store (Q) is proportional to the bias (V) times how good the capacitor is, the

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Today s Objectives Dielectrics and Ferroelectrics

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1. Today’s Objectives Dielectrics and Ferroelectrics What happens to a material when an electric field is applied across it (i.e., in a capacitor)? How does the electric field change, and how does the charge/area change? What are the 3 primary contributions to the dielectric constant. How fast would a dielectric respond? Describe the 4 primary dielectric breakdown mechanisms. How can the breakdown strength be improved? What are ferroelectrics? What are other applications of ferroelectrics?

2. Capacitance Two electrodes separated by a gap define a capacitor. When a bias is applied across the capacitor plates, one charges positively, the other negatively. The amount of charge that the capacitor can store (Q) is proportional to the bias (V) times how good the capacitor is, the ‘capacitance’ (C). The capacitance is related to the area of the plates (A), their separation (d), and the Dielectric Constant (eeo) of the dielectric between the plates Dielectric constant of vacuum; eo = 8.85x10-12 F/m=55.2 Me/(V*m)

3. Why does charge built up? There is generally not a built-in electric field between the plates of an unbiased capacitor. When an electric field is applied, any charged carriers or species within the material will respond. For a conductor or semiconductor, e- will flow to the + plate, and possibly also holes will flow to the - plate. Current is carried=no charge buildup. For an insulator, there aren’t a significant number of free carriers. There are highly ionic species, however, but they aren’t very mobile at low temperatures. No appreciable current is carried=charge buildup.

4. Polarization in Insulators

5. Frequency Response (Switching Time)

6. Microwave Ovens A microwave oven generates electromagnetic radiation at about 2.5 GHz. This energy is pretty good at causing H2O molecules to oscillate their orientation (orientational dielectric constant changes greatly). 5 GHz - 100 GHz would be ideal, but then most of the energy would be absorbed by the outermost layer of the food, defeating the purpose. Ice has a low dielectric constant, so not much energy is absorbed by it. Once there is a bit of melted ice, though, then you are really cooking.

7. Relative Dielectric Constants Generally, the less conducting and more polar a material is, the greater will be its dielectric constant.

8. A Materials/Design Problem

9. Breakdown Strength You cannot charge a capacitor infinitely. Eventually, the capacitor will fail, usually catastrophically. The so-called breakdown strength of a dielectric is the electric field greater than which the material breaks down. The breakdown strength is separate from the dielectric properties of the material. High purity, low defect densities, and low temperature are important.

10. Dielectric Breakdown Mechanisms Thermal (heat=defects=ionic conduction=more heat=…) Avalanche (accelerated electrons free more electrons that accelerate and free more electrons…) Discharge (fields grow enough to arc across pores, leading to erosion, leading to more arcing, …) Electrolytic (conduction paths created over time due to ionic and/or environmental conduction)

11. Slight Ionic Asymmetry - Ferroelectrics

12. Ferroelectrics

13. Spontaneous Polarization

14. Electrostriction and Piezoelectricity

15. Pyroelectricity

25. Summary What happens to a material when an electric field is applied across it (i.e., in a capacitor)? How does the electric field change, and how does the charge/area change? What are the 3 primary contributions to the dielectric constant. How fast would a dielectric respond? Describe the 4 primary dielectric breakdown mechanisms. How can the breakdown strength be improved? What are ferroelectrics? What are other applications of ferroelectrics?

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