Introduction

The piezoelectric community has been interested in developing lead-free piezoelectric ceramics having high-performance as an alternative to the market-dominating lead-based piezoelectric ceramics for more than two decades in response to increasingly stringent environmental requirements.^1-2 The discovery of a Morphotropic Phase Boundary (MPB) in (K,Na)NbO³-system (KNN) (between rhombohedral phase and tetragonal phase)³ that appears to compete with PZTs⁴ and of incipient piezostrains in Bi-based relaxor ferroelectric ceramics, the strain performance of which are almost twice as large as those of typical piezo-ceramics^5-6, are two potentially notable outcomes from more than 20 years of research. The former is progressing steadily in the community, while the latter is still in need of a breakthrough due to the relatively high electric field level necessary to activate the aforementioned big strain behavior. In this work, we describe solely how to enhance strain value of incipient piezoelectric materials in lead-free relaxor ferroelectrics.

The phenomenology of incipient piezoelectricity, i.e., the piezoelectricity grows out of a macroscopic paraelectricity beyond a certain level of electric field, was the inspiration for its name.⁵ The necessity for this new nomenclature was supported by the prevalent assumption that the existence of an as known

‘double hysteresis loop’ or ‘pinched hysteresis loop’ is sufficient for anti-ferroelectricity, despite the fact that it is only one of several essential conditions. In fact, the constriction of polarization hysteresis loop means that the system is a paraelectric state macroscopically without an external electric field, whereas the system is able to matained in a ferroelectric state as long as an external electric field are applied. To put it another way, the ferroelectricity of a specific material system is only stable when a certain degree of electric field is applied, which might be the case of relaxor ferroelectrics, standard ferroelectrics slightly over their curie point, anti-ferroelectrics, and so on.

From a practical standpoint, relaxor ferroelectrics are the most important of all the known incipient piezo-ceramics. Due to a considerable volume change which is reversible during each driving cycle, anti-ferroelectric materials are mechanically unstable, while standard ferroelectrics slightly over their curie point have a very restricted temperature range for usage. Since the incipient piezo-ceramics show relatively stable electrostrictive coefficient for temperature change, incipient piezoelectricity in relaxor ferroelectrics arises as long as they are above the so-called freezing point and continues up to quite high temperatures. Furthermore, the high strain is due to a continuous poling process that is free of fatigue during each cycle.^7-8 Because of the cooling phenomenon from the electrocaloric effect at each cycle, relaxor-based incipient piezo-ceramics are predicted to have thermal stability against self-heating, as recently reported.⁹

Since their mechanism was fully established in 2009, a lot of research have been conducted to make the

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concept of incipient piezostrains of lead-free relaxor ferroelectrics realistically practicable, thanks to those excellent features.¹⁰ Early investigations in diverse systems focused on changing the freezing temperature to generate incipient piezostrains.^11-16 However, despite the fact that numerous compositions including most promising system (e.g., KNN-based, BNT-based, BT-based) had been exhaustively scanned, all of the developed incipient lead-free system did not surmounted the disadvantage of a quite large activating (E^R-F: required electric field to transform to ferroelectric phase) electric field for arising the incipient piezostrain. At roughly 2.3 kV/mm, a (Bi^1/2Na^1/2)TiO³-SrTiO³ (BNT-ST) ceramics sample set a new record for the smallest E^R-F.¹⁹ Because maximum driving field of actuator is 2 kV/mm, the ER-F requires placement near 1 kV/mm for stable and repeatable cycling.

In 2012, the alkaline niobite system having a core-shell structure was claimed to be the first to reduce E^R-F.²³ When CaZrO³ was added to a KNN-based system, the strain reached up to 0.4 percent at 4 kV/mm (i.e., 1,000 pm/V) with ER-F less than 0.5 kV/mm. Since the applied external electric field must be concentrated to the ferroelectric core among the core-shell structure with a reduced dielectric permittivity, the community quickly adopted this core-shell technique as the "ultimate" answer to the E^R-F reduction. Unfortunately, no subsequent reports of a comparable effect were made, which was attributed to a lack of repeatability.

However, there was not some time, a 0-3 composite strategy came out to replace the core-shell approach.

25-26 The concept was simple: a ferroelectric composition was added as a '0' component into the '3' component which is a relaxor matrix, simulating the core-shell technique. Despite the fact that the operating concept was identical to that of core-shell structured systems, it was thought to be significantly easier to implement and reproduce than the core-shell method. This is why the 0-3 composite technique is currently being researched thoroughly.^27-29 Despite the fact that the 0-3 composite technique is technically accurate, the effect does not appear to be important in terms of lowering EF-R below the formal expectation.

Previous attempts had all been aimed at inhomogenizing electric field distribution in order to expose a specific section of the system to a stronger electric field than was really delivered. Given the importance of electric field concentration, it's logical to assume that an electrode with pattern, similar to a lightning rod, would have the same impact. In terms of the concept that electric fields tend to concentrate on edges of the object, we initially designed 5 different sorts of patterns to investigate which shape of electrode had the best performance on electric field concentration. Of course, the higher electric field concentration, the better; nevertheless, too much concentration might result in the dielectric breakdown of piezoelectric ceramics. As a result, we modified the inter electrode spacing at three different levels in order to determine the best patterning technique.

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Figure 5 - 1 The schematic diagram of designed electrode patterns for electric field concentration.

Figure 5-1 shows the complete 15 patterns examined in this study, which were applied to a well-known incipient lead-free system, 0.97Bi1/2(Na0.78K0.22)1/2TiO3-0.03BiAlO3 (BNKT-BA).³⁰ As will be demonstrated and explained, electrode patterning has a beneficial impact on improving incipient piezostrains, with plenty of potential for improvement.

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