The project aims to develop materials with optimized pyroelectric properties using the polarization gradient concept and develop integral pyroelectric detectors for the near infrared (700 nm) to THz (≤100 μm) wavelengths range. These detectors have potential application also in the detection of high power or high energy laser beams (e.g. the lasers of ELI-NP project). The materials to be used in this project are ferroelectrics with a perovskite structure such as Pb(Zr,Ti)O3 (PZT) or (Ba,Sr)TiO3 (BST) due to the fact that the transition temperatures can be modified by changing the Zr or Sr content. These materials will be combined in structures of multilayers with gradient in concentration and polarization in order to increase the figure of merit M given by the ratio between the pyroelectric coefficient p and the dielectric constant ε (M=p/ε).
The present project proposes a novel way of increasing the merit figure M by increasing the pyroelectric coefficient. This can be achieved by developing materials that exhibit a concentration gradient in the direction of the polarization, which introduces a succession of phase transitions at different temperatures, leading to a more abrupt variation of the polarization with the temperature and thus to a larger pyroelectric coefficient.
Another effect to turn to account is the temperature variation of the dielectric constant which can contribute to further increase the total pyroelectric coefficient. The temperature variation of ε can contribute to the pyroelectric signal if an electric field is applied to the ferroelectric material in order to maintain a stable polarization state, thus averting possible signal variations caused by the ambient temperature conditions.
The materials with gradient in concentration and polarization will be realized in bulk form, as ceramic wafers (25 mm minimum diameter and 6 mm thickness) by using the spark plasma sintering (SPS). Alternately, the ceramic technology coupled with classical sintering, or hot press, can be used. The sintering conditions will be optimized in order to obtain the best p/ε ratio. The selected material will then be used to build the active elements for the pyroelectric detection. In this respect, metallic electrodes will be deposited and one of them will be blackened in order to ensure a better absorbtion of the incident electromagnetic radiation. A novel approach is that carbon nanotubes are to be used for the blackening. This way the absorbtion coeficient can be increased close to 1. The active element will then be used to create pyroelectric detectors, including the electronics for signal processing and the sofware needed for PC display. Beside the mentioned ceramic materials, epitaxial multilayered structures with gradient in concentration and polarization will be realized and their pyroelectric detection properties will be investigated as well during the project.
The consortium is formed by 3 partners: coordinator of the project –CO is a national institute with experience in ceramic materials and pyroelectric detection; one university –P1 with experience in preparation of ceramic powders; one company –P2 specialized in signal procesing and different types of electrical measurements. CO and P1 will develop the active element for the pyroelectric detection and P2 will develop and test the experimental model of the system for pyroelectric detection including all the electronics and the sofware needed for the different types of applications for which the pyroelectric detector is developed by CO and P2: automatizations, non-contact measurements of temperature or monitoring of the high power/energy laser beams. The ultimate goals are to obtain: a technological process for obtaining the active element of pyroelectric detection, as well as two experimental models,
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one for the Pyroelectric Detector and one for a Pyroelectric Detection System used to detect high intensity laser beams.