Fundamental Research On Pyroelectric Composite Materials And Its Arrays

The pyroelectric composite materials which are based on ceramic and polymer and have the merit of ceramic and polymer, is the research hotspot in pyroelectric materials. Pyroelectric composite materials have low pyroelectric property, poor interface bonding and low polarizability of ceramic phase. In order to solve the problem, this thesis conducted a systematic research on choice of ceramic phase, improving of PVDF, modifying of interface, and the polarization of ceramic phase in composite materials. Then a new type of uncooled infrared detectors with polyimide substrate as the thermal isolation structure was prepared. The arrays were made by pyroelectric composite materials which have a low preparation temperature, which layed a basis for the development of uncooled infrared detectors.Concerning on the problem that pyroelectric composite materials have poor properties, the ceramic phase was studied. By theoretical analysis, we found that the ceramic used in composite materials should have lower dielectric constant, larger pyroelectric coefficient and smaller piezoelectric coefficient. On the basic of laboratory study, The ceramic of Pb[(Mn1/3Nb2/3)1/2(Mn1/3Sb2/3)1/2]o.04(ZrxTi1-x)o.9603 and 0.95(KxNa1-x)NbO3-0.05LiSbO3 was investigated and compared. The results showed that the PZT ceramics have lower dielectric constant, larger pyroelectric coefficient and smaller piezoelectric coefficient. So in the study the PZT ceramic was used as the ceramic phase in pyroelectric composite materials when x was 0.95.For the solution of the problem that PVDF has lower dielectric constant and conductivity, the montmorillonite (MMT) and graphite nanosheets (GNS) was added. The impact of MMT and GNS content on the phase transformation, conductivity, pyroelectric property formation mechanisms were discussed. The results showed that with the addition of MMT, the beta phase was induced, the crystallinity and dielectric constant was increased. When the content of MMT was 2wt%, the composite materials have large crystallinity and dielectric constant,67% and 9.54, respectively. The pyroelectric coefficient and figure of merit of detectivity of the composites were reached to 51.0μC/m2℃and 0.8×10-5Pa-0.5 respectively at 36℃. The GNS was made by solution casting method with the thickness of 25nm. With the addition of GNS, the beta phase was induced, the crystallinity, dielectric constant and conductivity was increased. When the content of GNS was 6mL being preparation, the composite materials have a large crystallinity, dielectric constant and conductivity,75%,10 and 1.24×1013S×cm-1, respectively. The pyroelectric coefficient and figure of merit of detectivity of the composites was reached to 54.0μC/m2 and 0.64×10-5Pa-0.5 respectively at 36℃.With a view to solve the problem that the interface bonding between ceramic phase and matrix was poor, the silane coupling agent KH570, titanate coupling agent T1-9 and chelating agent EDTA were added to improve the interface and property. The impacts of interfacial modifier content on the interface, crystallinity, breakdown strength, dielectric, piezoelectric and pyroelectric properties were discussed. The results revealed that with the addition of interfacial modifier, the interface was improved, the beta-PVDF was induced, the crystallinity and dielectric properties was increased. With the proper content of interfacial modifier, the piezoelectric and pyroelectric properties were increased. When the content of titanate coupling agent T1-9 was 0.3wt%, the composite materials have a large pyroelectric coefficient and figure of merit of detectivity,95.0μC/m2℃and 1.24×10-5Pa-0.5, respectively at 36℃.In order to solve the problem that the ceramic in composite materials was difficult to be polarized, the nano carbon and carbon nanotube was added. The impacts of conductive phase on the conductivity, specific heat, dielectric property, piezoelectric property and pyroelectric property were discussed.Experiments indicated that the conductivity and dielectric properties was increased and the specific heat was decreased with the increase of conductive phase, and finally the conductivity reached percolation threshold. The piezoelectric and pyroelectric properties increased with the increase of conductive phase, and then dropped with further increase. These results indicated that when the content of nano carbon was 0.9wt%, the composite materials possessed enhanced pyroelectric properties, with the average of pyroelectric coefficient was exceed 130μC/m2℃, and the pyroelectric coefficient and the figure of merit of detectivity was 138.68μC/n2℃, 1.63 x 10-5Pa-0.5 respectively at 36℃. Finally, a new type of uncooled infrared detectors with polyimide substrate as the thermal isolation structure was prepared. The arrays were made by pyroelectric composite materials which have a low preparation temperature. The bottom electrode and back electrode leader were prepared on the polyimide substrate by direct-writing. Then the bottom electrode and back electrode leader was connected via hole by laser micro cladding technology. Finally, the pyroelectric composite materials and top electrode were prepared by screen-printing technology on the bottom electrode. The new uncooled infrared detectors structure which made the bottom electrode and back electrode leader at different plane of polyimide, could increase the figure of merit of detectivity and the number of array in unit area, and thus laying a basis for small size and multi-elements based uncooled infrared detectors.