Microstructure evolution and electromechanical properties of (K,Na) NbO₃‐based thick films

Abstract

This study investigated an unconventional method of electrophoretic deposition (EPD) for the processing of environmentally benign (K 0.5 Na 0.5) 0.99 Sr 0.005 NbO 3 (KNNSr) thick films on Pt/alumina substrate. EPD allows rapid, economical, and low-waste processing of thick films and thus offers an integration advantage for electronics manufacturing. To understand the functional response of the KNNSr thick films, the effect of the sintering temperature and atmosphere on their structure, microstructure, and electromechanical properties was investigated. KNNSr thick films densify in constrained conditions in a very narrow temperature range only a few 10°C below the melting temperature of 1140°C. Up to 1100°C the relative density increases to 80%, upon further heating to 1110°C we observed only the grain growth and pore coalescence. The densification is not affected significantly by the atmosphere. The local domain structure of 25-33 μm thick KNNSr films was similar, while the dielectric and electromechanical properties increased with the increasing sintering temperature. KNNSr thick film sintered at 1100°C has a thickness-coupling factor k t of 0.4, comparable to that of bulk. The results reveal that the EPD enables the economic processing of high-performance thick films on complex-shape substrates that are difficult to fabricate using conventional thick-film methods.

Screen Printed Copper and Tantalum Modified Potassium Sodium Niobate Thick Films on Platinized Alumina Substrates

“…The obtained d 33 value is in the range of reported values for KNN-based thick films modified with Li, Mn, Sr, i.e., 60–150 pC/N. However, a direct comparison is difficult since the obtained values depend on the measurement methods, processing, sintering conditions as well as the film thickness [ 10 , 11 , 14 , 41 ].…”

“…Experimental and theoretical complex electrical impedances of the KNN-KCT-CuO_O 2 thick films as function of frequency around the fundamental resonance are collected in Figure 4 e,f. The theoretical electrical impedance was derived using the KLM model, which takes into account the acoustic properties of platinum and alumina [ 14 ]. The peaks in the real and imaginary parts of the impedance around the fundamental resonance are due to the coupling of the resonance of the thick film with the platinum electrode and the alumina substrate.…”

“…The comparison of electromechanical quantities of KNN-KCT-CuO_O 2 thick film with donor (Sr) doped KNN film, both sintered at 1100 °C [ 14 ] reveals that the dielectric constant at constant strain , the piezoelectric coefficient e 33 , elastic constant at constant displacement , and k t of KNN-KCT-CuO_O 2 film are higher, while the δ m is significantly lower compared to the values reported for Sr-doped KNN thick film, see Table 2 . Clearly the donor doping differently effects the electromechanical properties of KNN films compared to KCT-CuO, which is reflected in their properties.…”

“…In contrast to a number of studies on KNN-based bulk ceramics, the effect of processing conditions on the properties of KNN-based thick films prepared by screen printing [ 6 ], electrophoretic deposition [ 7 , 8 , 9 ], tape casting [ 10 , 11 ], hydrothermal methods [ 12 ], or pad-printing [ 13 ] has been investigated to a much lesser extent. Ways to improve the densification and thus the final functional properties of KNN-based thick films include the selection of the optimal sintering temperature, time and/or atmosphere, chemical modification of the matrix composition, or addition of sintering aids [ 6 , 14 , 15 , 16 , 17 ].…”

“…For example, some authors claim that sintering KNN-based ceramics at a low oxygen partial pressure promotes alkali evaporation [ 21 ], while others report that the evaporation of alkalis is suppressed [ 22 ]. Mercier et al [ 14 , 23 ] studied the effects of sintering temperature and atmosphere on the properties of Sr-doped KNN thick films prepared by electrophoretic deposition on platinized alumina substrates. As the sintering temperature increased from 1090 to 1110 °C, the relative density increased, which was reflected in an increase of the electromechanical properties.…”