A method to create a super-pyroelectric material that combines large pyroelectric coefficient and relatively low dielectric constant has been discovered by U.S. and Israeli researchers. The ferroelectric material is barium titanate (BaTiO3).
Ferrorelectric nano-grain films with super-pyroelectric properties have been manufactured by Weizmann Institute of Science Professor Prof. Igor Lubomirsky (Petach Tikva, IL), University of Maryland (College Park, MA) Materials Engineering Professor Alexander L. Roytburd and researcher Vera Lyhovitsky (Rehovot, Israel). Using temperature increase to polarize and change the alignment of ferroelectric nanocrystalline grains in infrared sensors produces pyroelectric current which is 10 to 100-fold larger than can be obtained in today's ordinary infrared sensors. The super-pyroelectric films are detailed in U.S. Patent Application 20100059724 and are available for licensing from Yeda Research and Development Company Ltd.
The film comprises nano-sized grains being in a ferroelectric phase and having at least three different crystallographic variants defining at least two polycrystalline macro-domains. The film is shaped to define at least one film region with the macro-domains of a predetermined shape and different orientations of crystallographic axes with respect to the film's surface, thereby enabling to apply a temperature change to the film to induce movement of the polycrystalline macro-domains boundaries enabling super-pyroelectric properties
The film exhibits a super pyroelectric effect, the origin of which is fundamentally different from regular pyroelectricity. The latter is due to changes in the absolute value of the polarization. The former is due to reversible 90 degree polarization switching at the grains at the boundary between the linear and the wedge-oriented regions.
The stress arising due to a change in temperature is concentrated at the boundary between the linear and the wedge-oriented regions and facilitates 90 degree polarization switching. Such switching results in changes in the total polarization in the out-of-plane direction which are much larger than those observed for either primary or secondary pyroelectricity. Similar phenomenon can be observed in a polydomain single crystal, where mechanical stress in response to temperature variation may also move 90 degree domain walls in constrained single crystalline ferroelectric films.
However, displacement of domain boundaries in polycrystalline macro-domains would be expected to occur much more rapidly than in single crystals, because the thickness of the domain walls between them are of the order of magnitude of a few grains, rather than one unit cell as in crystals. In buckled films, the super-pyroelectric current is generated in a small fraction of the film volume but its contribution to the total effect is large. One may anticipate existence of film configurations with a large density of macro-domain boundaries and, therefore, yet larger super-pyroelectric effect. One has to emphasize that the super-pyroelectric effect appears only if the macro-domains can follow temperature change. Remarkably, in our case, the macro-domains rearrange within a few .mu.sec. This indicates that they can rapidly reach an equilibrium state, which is consistent with the observed periodicity of the wedge-ordered regions.
The invention may be used to produce infrared detectors in a form of single units and detector arrays (focal plates). Detectors using the films with polycrystalline domains may effectively operate till the frequency of 105 Hz and withstand large accelerations (few g's), which may be particularly useful for mobile systems. The detectors can be constructed to operate within large temperature range (-10+50 degrees Celsius) without temperature stabilization.
The invention is based on the recently discovered phenomenon of self-organization of ferroelectric nanocrystalline grains into polycrystalline macro-domains. As a result, a thin film comprising of ferroelectric grains splits spontaneously into the regions, within which the directions of the polar axes of the grains become spontaneously correlated. In response to temperature variations, some of the ferroelectric grains undergo so-called 90-degrees polarization switching and therefore produce pyroelectric current, which is 10-100 times larger than the current that could be generated in a single crystal of the same material in similar conditions.
The technology is available for licensing from the Yeda Research and Development Company Ltd. which is the Technology Transfer Company of the Weizmann Institute of Science, reference Project Number: 1399.
The most important practical consequence of the rapid and reversible rearrangement of the macro-domains is that it gives the films the ability to adapt to external mechanical constraints. The films with macro-domains do not accumulate mechanical stress in response to small deformations. Therefore, systems with polycrystalline macro-domains open a wide range of new opportunities for creating materials with exceptional mechanical stability.
The process for forming a super-pyroelectric effect in polycrystalline macro-domains organized into linear and wedge-ordered regions in a buckled nanocrystalline ferroelectric film is as follows: in a forming step a buckled film, such as film, is formed. The buckled films spontaneously form macro-domains. Thereafter in a contact forming step, contacts are formed on the upper and lower surfaces of the buckled region, typically by sputtering. To induce a super-piezoelectric effect, a heat or energy source can be used to heat the film. Alternatively, sound wave pressure energy is applied to the film.
It should be noted that this invention is also directed to devices comprising one or more buckled film, exhibiting super-pyroelectricity. Devices employing the super-pyroelectric effect include, but are not limited to, motion sensors and uncooled radiation detectors and arrays made of them.
The process for forming a super-pyroelectric effect in polycrystalline macro-domains organized into linear and wedge-ordered regions in a buckled nanocrystalline ferroelectric film is as follows: in a forming step a buckled film, such as film, is formed. The buckled films spontaneously form macro-domains. Thereafter in a contact forming step, contacts are formed on the upper and lower surfaces of the buckled region, typically by sputtering. To induce a super-piezoelectric effect, a heat or energy source can be used to heat the film. Alternatively, sound wave pressure energy is applied to the film.
It should be noted that this invention is also directed to devices comprising one or more buckled film, exhibiting super-pyroelectricity. Devices employing the super-pyroelectric effect include, but are not limited to, motion sensors and uncooled radiation detectors and arrays made of them.
Films for which the difference in linear dimensions is approximately 3% exhibit a strongly enhanced pyroelectric coefficient 1 .mu.Cl/(cm.sup.2K), which is attributed to the contribution of 90 degree polarization switching in grains located at the macro-domain boundaries. The characteristic time for macro-domain rearrangement was found to be <0.1 ms. Due to the mobility of macro-domain boundaries, the self-supported films do not accumulate mechanical stress in response to small deformation. Instead, they reversibly adapt to external mechanical constraints. Systems with polycrystalline macro-domains may open a wide range of new opportunities for creating materials with exceptional mechanical stability.
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