| Pb-based ceramics, such as PZT and PMN-PT, and their composites has been widely studied and used for their good pyroelectric performance. However, as a high environmental-toxicity element, Pb is being restricted now and may be forbidden in the future with the worldwide attention on environment protection. Thus, lead- free materials’ development becomes a trend in pyroelectric research. Pyroelectric composites based on ceramics and polymers have been intensively studied because they can combine the excellent properties of both ceramic and polymer phases, such as high pyroelectric activities of ceramic and the low dielectric constant and flexibility of polymer. Compared with other lead-free pyroelectric ceramics, the KNN based ceramics show relatively larger pyroelectric coefficient, higher Curie temperature and lower dielectric constant, which make it become a potential candidate for lead-free composites. As a new kind of lead- free pyroelectric materials, KNN ceramic based composites show a promising future. However, it is difficult to obtain high-quality KNN ceramic powders because of the active chemical property of Na and K. What’s worse, the structure of ceramic composites is more complex than traditional ceramics and its properties can be influenced by more parameters that limit its progress. To solve the above problems, the influence of calcined temperature and mass content of KNN powders on the properties of composites was studied and a new process was adopted to enhance the pyroelectric activities of composites based on KNN/P(VDF- TrFE). The relationship between the structure of sensitive elements and the performance of the sensors was also studied. Four-element dual-channel infrared gas sensor was designed and prepared, which verified that composites can be used in the gas sensors1. KNN ceramic powders were synthesized by solid-state reaction technique with presintering temperature ranging from 750 °C to 900 °C。The KNN/P(VDF- TrFE) composites thick films were fabricated by casting. The results confirmed the perovskite crystal structure of KNN powders and the good quality of composite thick films.2. The relationship between presintering temperature of KNN powders and the properties of composite thick films was studied. The composite thick films made up of KNN powders presintered at 850 °C got the highest detectivity figure-of- merit which was 4.94 μPa-1/2. Thick film processing was optimized and a 0.5 h, 160 °C heat treatment was adopted which decreased the volume heat capacity and dielectric loss of the composite thick films obviously.3. With the optimized process, the relationship between the mass content of KNN powders and the properties of composite thick films was studied and the optimal KNN mass content was confirmed which was 30%. It was found that the composite thick films made up of 30 w.% KNN presintered at 850 °C show optimal properties for pyroelectric appliance. The highest pyroelectric coefficient was 63 μC·m-2K-1 and the highest detectivity figure-of-merit was 8.88 μPa1/2.4. Dual-element infrared sensors were prepared and the effect of two different thermal insulation structure on sensors were compared with each other. Compared with PI substrate structure, the "free-standing" structure showed better performance at relatively lower frequency while worse at higher. The performances of those two structure were almost the same when the frequency was between 20 Hz and 30 Hz. "free-standing" structure got the highest specific detectivity 6.22×107 cmHz1/2W-1 at 14.3 Hz. The voltage responsivity of "free-standing" structure was 389.24 V/W at 5.3 Hz, which was 42% higher than PI substrate structure’s that indicates the "free-standing" structure is suitable for low frequency infrared application.5. The relationship between electrode structure and the performance of sensors was studied. Depending on the characteristics of composite thick films and "free-standing" structure, the circuit diagram and the graphic electrodes were redesigned. Sensitive elements with high quality graphic electrodes was prepared by UV laser micro-etching and high-precision mask plate. Compared with none graphic electrodes elements, the sensor got 50.4% high in ?, 16.7% lower in VN, 44.5% lower in NEP and 17.3% higher in D* with a more stable signal.6. Four-element dual-channel infrared gas sensor was designed and fabricated. The testing results of "free-standing" composite gas sensors showed that the signal ratio of test channel and reference channel decreased significantly while the target gas concentration increased. It indicated that the composite thick film can be applied in the infrared gas sensors. |
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