Direct Current Bias Field Research Articles

Effect of direct current bias field and alternating current excitation field on vibration amplitudes and resonance frequencies of a magnetostrictively actuated bimorph microresonator

The dynamic behavior of magnetomechanical bimorph microresonators is analyzed. These resonators are composed of a sputter-deposited TbDy(CoFe)2 thin film on silicon cantilevers. This film has giant magnetostrictive properties. It is made anisotropic by postdeposition annealing under magnetic field. We prepared two samples with different orientations of the easy axis: along the cantilever width axis and with an orientation of 22°, respectively. Both bending and torsion modes of vibration can be excited by proper combination of external magnetic field and the direction of the easy axis. Vibration amplitudes and resonance frequencies are studied in those modes. Several experimental conditions are investigated. The dependence of vibration amplitudes on a magnetic dc bias field applied either along the hard axis or along the easy axis is studied. The bending/torsion vibration amplitude ratio was shown to be tunable over a very wide range, offering interesting practical applications. In addition, the dependence of the resonance frequencies on both dc and ac fields applied along the hard axis is studied. For both vibration amplitudes and resonance frequencies, the experimental data are compared with a theoretical model and good agreement is found. The magnetomechanical resonator may be considered as a building block for more complex microelectromechanical systems using magnetostrictive actuation and as an alternative to more classical solutions based on electrostatic or electrothermal actuation.

Effect of stoichiometry on dielectric and field-induced piezoelectric properties of 0.75Pb(Ni1/3Nb2/3)O3-0.25PbTiO3 ceramics

The influences of stoichiometry on the dielectric and field-induced piezoelectric properties of 0.75Pb(Ni1/3Nb2/3)O3-0.25PbTiO3 are examined. The addition of excess Ni(Ac)2 was found to restrain the generation of pyrochlore phase and resulted in pure perovskite phase, while the addition of lead titanate can not enhance the yield of the perovskite phase. With increasing excess nickel content, the maximum dielectric constant decreases gradually and the maximum Curie temperature does not shift. This decrease was attributed to the increase in minor phases with lower dielectric constant. The piezoelectric effect induced by a direct current bias field was also investigated. The evaluation showed that the induced electromechanical thickness coupling coefficients measured at room temperature decreased as the content of excess nickel oxide increased.

Large Piezoelectric Effect Induced by Direct Current Bias in PMN:PT Relaxor Ferroelectric Ceramics

The longitudinal piezoelectric effect induced by a direct current bias field was investigated in PMN:PT family of relaxor ferroelectric ceramics in the frequency range from 0. 1 Hz to 50 kHz. The piezoelectric d33 coefficient was studied by measuring the strain induced by the applied electric field. Under the 10 kHz driving field, the large piezoelectric strain coefficient, d33, of 800 pC/N is obtained. The piezoelectric strain coefficient related dielectric and electrostrictive parameters were also studied under direct current bias and the d33 coefficient was calculated by the equation d33=2Q11P3ε33. The results are in good agreement with the directly measured ones. The dispersion of the piezoelectric effect is mainly due to the dielectric dispersion.

Electromechanical Response and Dielectric Loss of Prepolarized Barium Titanate Ceramics under Maintained Electric Bias

The effect of a maintained aiding direct current bias field on the performance of prepolarized barium titanate ceramics has been investigated with respect to both loss factor and electromechanical response over the temperature range from 25° to 120°C as a function of bias field strength. In addition the effect on the temperature variation of the reversible dielectric constant was studied. The dielectric losses under high ac fields are considerably reduced by the application of bias, and the magnitude of this effect shows a pronounced temperature dependence. Aiding bias also proves beneficial by substantially increasing the electromechanical coupling, and it raises the Curie point by up to about 4°C. Dielectric loss factors were determined semiquantitatively by bridge measurements at moderate ac fields and by means of hysteresis loop areas at higher fields. Both the electrostrictive coefficient u31 relating transversal strain to the square of the total charge density and the linear piezoelectric coefficient g31 relating the increment of strain to the increment of charge density in prepolarized ceramics have been measured directly. From these results, approximate values of retained polarization referring to various physical conditions have been determined; they range to about 11 microcoulombs per square centimeter.