TAILIEUCHUNG - Ferroelectrics Characterization and Modeling Part 5

Tham khảo tài liệu 'ferroelectrics characterization and modeling part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 130 Ferroelectrics - Characterization and Modeling using the Nye notation in which elastic constants and elastic moduli are labeled by replacing the pairs of letters xx yy zz yz zx and xy by the number 1 2 3 4 5 and 6 respectively. This means that the external electric field generates electric displacement . electric polarization and strain through the converse piezoelectric effect. However ceramic materials are multicrystalline structures made up of large numbers of randomly orientated crystal grains. The random orientation of the grains results in a net cancelation of the piezoelectric effect. Thus the ceramic material must be poled - a dc bias electric field is applied usually the fired ceramic piece is cooled through the Curie point under the influence of the field which aligns the ferroelectric domains resulting in a net piezoelectric effect. As the electrical conductivity of percolative composites strongly increases on approaching the percolation threshold the feasibility of poling the PZT- samples has been checked. PZT- system has been chosen as its electrical conductivity is much lower than in the system or in the KNN-RuO2 samples which are not treated under vacuum. After poling the samples with a high dc bias electric field the piezoelectric coefficient dss strain in the direction of the applied measuring field has been measured using a small ac voltage. It should be noted that while various piezoelectric coefficients are usually determined and thus the indication is absolutely necessary the dielectric constant is almost without exception determined in the direction of the applied field . without indices in fact denotes the dielectric constant 33. Results of piezoelectric characterization are shown in Fig. 12. While in samples which are very close to the percolation threshold the breakdown electric field is below 5 kV cm samples with lower content can be poled with the dc bias .

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