Adolph L. Micheli

Origin of the “up,” “down” hysteresis offsets observed from polarization-graded ferroelectric materials

“Up” and “down” hysteresis offsets were observed in polarization-graded ferroelectrics. The polarization gradients were achieved by imposing temperature gradients across a bulk ferroelectric material near its Curie temperature. In the absence of temperature gradients, no hysteresis offsets were observed, ruling out extraneous sources as the cause of the aberrant behavior, thereby establishing the origin of the offsets in the class of polarization-graded ferroelectric devices.

Stress-induced polarization-graded ferroelectrics

Polarization-graded ferroelectrics and their electrically active embodiments, graded ferroelectric devices and transpacitors, have been formed from a variety of material systems, both by grading the composition of the ferroelectric and by imposing temperature gradients normal to the electrode surfaces. In this letter, we show how these same devices can be formed from homogeneous ferroelectric films of lead strontium titanate by imposing stress gradients on the material normal to their electrode surfaces.

Magnetization-graded ferromagnets: The magnetic analogs of semiconductor junction elements

Compositionally graded ferrites are formed as the magnetic analogs of semiconductor junction devices. The internal, or the “built-in,” magnetic field is intrinsic to the structure and is determined from ferromagnetic resonance microscopy. Magnetic analysis of a nickel–zinc–ferrite system in terms of its spatially dependent order parameter, the magnetization, yields a value for the internal magnetic field consistent with experimental observations. Our results are extended to the general class of ferroic and other “smart” materials via a spatially dependent free-energy potential.

Internal magnetostatic potentials of magnetization-graded ferromagnetic materials

The authors investigated the internal magnetic field induced by a spatially varying magnetization in a compositionally graded ferromagnet. The authors discuss results on a hexagonal ferrite sample, with a variation in saturation magnetization of 30emu∕g over a sample thickness of 2.5mm. The direct current magnetization shows a small anisotropy depending on the direction of the external magnetic field relative to the grading direction. This contribution from a grading induced magnetic field is more pronounced in alternating current susceptibility measurements. The authors find a shift in magnetic properties corresponding to an internal magnetic field of 30Oe, which is significantly lower than the predicted value of approximately 1900Oe. The authors discuss reasons for this discrepancy.

Polarization-graded ferroelectrics: Transpacitor push-pull amplifier

Functionally graded ferroelectric devices have been configured as transcapacitive push–pull charge amplifiers, whereby the net charge gain is a function of the difference in thermal environments of the individual device elements.

Polarization-graded ferroelectrics: Transpacitor energy gain

The energy gain of polarization-graded ferroelectric devices, configured as active transcapacitive elements, have been measured. Gain factors in excess of 1000 were obtained for small signal, static inputs; yielding charge gain amplifiers with gain factors of ∼150, remarkably similar to what is typically found for transistor current gain devices.

Graded ferroelectrics: A new class of steady-state thermal/electrical/mechanical energy interchange devices

Abstract The diffuse-phase-transition class of ferroelectric materials allows the fabrication of films graded in composition: the consequence of which is an attendant gradient in electric dipole moment density. Alternating electric-field excitation of these graded structures reveal an asymmetrical hysteresis characteristic, together with a static displacement of the hysteresis trace. This displacement bias is observed to be a function of excitation level and temperature, which leads to a giant effective pyroelectric coefficient. Said displacement, for example, provides a continuous open-circuit voltage, or short-circuit current, provided that the structure is held in an isothermal condition. All other combinations of heat, electrical and strain energies suggest a whole new class of devices, and invite ingenious concepts and/or contributions. Graded ferroelectrics, which operate on principles of asymmetrical bound-charge, are broadly the dielectric analogue of conductive asymmetrical free-charge devices. F...

Enhanced pyroelectric sensitivity using ferroelectric active mode detection

Active pyroelectric detection (APD), using ferroelectrics as the sensing materials, is described and compared to traditional-passive pyroelectric modes of operation. The active approach yields a number of distinct advantages over its passive counterparts, including greater effective pyroelectric coefficient and improved signal to noise ratio. Thin film test structures formed from strontium bismuth tantalate (SrBi2Ta2O9) are used to demonstrate the APD principle.