Research On DC Field Induced Effect And Pyroelectric Property Of Barium Strontium Titanate

As a kind of typical ferroelectric material, barium strontium titanate (BST) is of very strong nonlinear dielectric properties, which may vary due to DC field-induced effect. It is being broadly used in voltage-controlled filter, oscillator, microwave phase shifter and DC field-induced pyroelectric infrared detector. Therefore, this paper conducted the systematic research on the DC field-induced effect of BST material and its application on the uncooled infrared detector and thus laying a basis for the development of high-performance uncooled infrared focal plane array.The doping theory of BST material's DC field-induced effect was described, and the influences of the valence of dopant and ionic radius on the microscopic properties such as lattice constant and domain structure and the macroscopic properties like dielectric tunability, DC field-induced pyroelectric coefficient etc. were analyzed. Since the microstructure of ceramic grain, e.g. dimension and pore, had a close relationship with the DC field-induced effect of the material, this paper introduced the modifying factor a into Johnson theory for describing the microstructure's control over the electric field distribution. As pointed out by the theory, the grain size decreased, the proportion of the grain boundary rose, the scattering effect of the grain boundary made the effective electric field on the ceramic grain decrease, the a value of the material became smaller, and the dielectric tunability was weakened; there were pores in the ceramics, the effective electric field distributed on round pores was larger than that on pores of irregular shapes, the a value of porous ceramics with irregular-shape pores were larger and the effective electric field on their grains were higher, so compared with porous ceramics with round pores, their dielectric tunability was better.In order to research the doping effect on the DC field-induced effect of BST material, the Mn-doping experiment was made. As more Mn was doped, the permittivity peak was depressed and broadened, the dielectric tunability was reduced, but the dielectric loss significantly declined. When 0.3 mol% Mn was doped, the dielectric tunability of BST at room temperature and under 400 V/mm DC bias field was 20%, the dielectric loss declined to 0.2%, the FoM (figure of merit) of tunability>100, the DC field-induced pyroelectric coefficient was 84×102μC/m2℃and the FoM of detectivity was 17.3×10-5 Pa-0.5. As Mn-dopant was increased, the dielectric loss became larger, but the FoMs of tunability and detectivity fell drastically. For further enhancing the DC field-induced pyroelectric properties of BST and neutralizing the negative effects of excessive doping on the dielectric loss, the Y, Mn acceptor-donor co-doping was conducted on the basis of Mn-doping. With more Y and Mn doped, the dielectric constant decreased, and the permittivity peak broadened. When the Y and Mn doping were respectively 1.2mol% and 0.6mol%, the dielectric loss was lower than 0.2%, DC field-induced pyroelectric coefficient of BST was 85×102μC/m2℃and the detectivity FoM increased to 19.0×10-5 Pa-0.5 as compared with Mn-doping.For exploring the influence of grain size on the DC field-induced effect of BST, the two-step sintering process was adopted to prepare compact BST ceramics with the grain size ranging between 0.3μm to 4μm and discussed the influences of grain size on the microscopic parameters, e.g. BST doping element, lattice constant and domain structure, and on the macroscopic parameters such as dielectric tunability and field-induced pyroelectric coefficient etc. As the grain size became smaller, the permittivity peak was broadened and the tunability decreased, but the dielectric loss changed slightly. The proper decrease of grain size was helpful for the increase of field-induced effect of BST. If the grain size was too small, the big internal stress between grains made the tetragonal lattices reduced radically, the ferroelectric properties declined and the pyroelectric properties became worse. The experiment indicated that the best grain size of BST was 1μm and now the field-induced pyroelectric coefficient of the material under 500 V/mm DC bias field was 105×102μC/m2℃and the FoM of the detectivity was 22.0×10-5 Pa-0.5.According to theory, the microstructure of porous ceramics was designed and its preparation process was explored, with polymethyl methacrylate (PMMA) as the pore former to prepare the porous ceramics of different porosities, pore shapes and apertures for verifying the theory. The experiment indicated that, for dense ceramics with grain size larger than 1μm and ceramics with the porosity smaller than 14% and aperture larger than 10μm, the microstructure factor a was close to 1; for ceramics with non-round pores, the a value was larger than.1; when the pority was larger than 22.3%, theαvalue was 0.9. This indicated that the theory after improvement reduced about 10% of the deviation of original model from the experimental results.The pores could weaken the internal stress between grain size and the peak-depressing effect of electric field on the dielectric peak and reduce the heat capacity per volume of materials, and thus being beneficial for enhancing the pyroelectric properties. Therefore, this paper proposed and prepared field-induced porous pyroelectric BST ceramics. When the porosity was 9.6% and the DC bias field was 400V/mm, the field-induced pyroelectric coefficient was 80×102μC/m2℃, the FoM of the detectivity was 27.0×10-5 Pa-0.5, and the comprehensive pyroelectric properties were better than those of dense samples. In order to further strengthen the tuning of pores on the internal stress of ceramics and the effective electric field distribution. This paper adopted the multiwall carbon nanotubes as the pore former to prepare the microporous field-induced pyroelectric BST ceramics. The field-induced pyroelectric coefficient and the FoM of detectivity of the microporous ceramics with the porosity of 9.5% could reach 95×102μc/m2℃and 32.0×10-5Pa-0.5, thus it had more ideal DC field-induced pyroelectric properties than PMMA-prepared porous ceramics.Finally, through shear thinning and cutting of porous pyroelectric BST ceramics by the micro-processing technology, the silver and nickel chromium alloy electrode were prepared on the ceramics surface by the photolithography, metal evaporation and negative photoresist stripping processes, and the detecting element of infrared detector was fabricated; aiming at the field-induced pyroelectric effect of BST, according to the micro capacitance measurement technology, the readout circuit of infrared detector unit was designed and built, enabling the detecting element to detect the black-body infrared radiation signal, and demonstrating that the BST ceramics and detecting element prepared in this paper had good performance.