High‐Performance Strain of Lead‐Free Relaxor‐Ferroelectric Piezoceramics by the Morphotropic Phase Boundary Modification

Abstract

Bismuth sodium titanate (BNT)‐based lead‐free piezoceramics are promising for replacing lead‐based piezoceramics in piezoelectric actuators due to their large strains. However, achieving low‐hysteresis large‐strain BNT‐based ceramics over a broad temperature range is challenging, owing to the complexity of the composition design and phase transformation. Herein, a lead‐free relaxor‐ferroelectric (1−x)Bi0.47Na0.47Ba0.06TiO3‐xK0.47Na0.47Li0.06Nb0.99Sb0.01O2.99 system (BNBT‐KNLNS) near the morphotropic phase boundary (MPB), achieved by phase‐field simulations and rational composition design (i.e., BNBT with the MPB as the base and the ferroelectric phase of KNLNS as the dopant) is reported. This ceramic exhibits large strains (0.32–0.51%) and low strain hysteresis (11.1–59.9%) over a wide temperature range (25–125 °C), outperforming many state‐of‐the‐art lead‐free piezoceramics. A small fraction of ferroelectric states embedded in the relaxor matrix is experimentally observed, where these states act as seeds, facilitating the reversible relaxor‐to‐ferroelectric transition. In addition, the MPB composition with low energy barriers yields large strain responses, owing to the easy polarization reversal and extension. Consequently, low‐hysteresis large strains are obtained over a broad temperature range. This work provides a novel design route for discovering high‐performance piezoceramics for actuator applications.

Lead‐Free Relaxor Ferroelectric Ceramics with Ultrahigh Energy Storage Densities via Polymorphic Polar Nanoregions Design

“…up-and-coming candidates due to their enormous polarization responses under electric field. [18] For instance, both the 0.55NBT-0.45SBT bulk ceramics and the corresponding multilayer ceramic capacitors (MLCCs) were investigated, exhibiting large recoverable energy densities (W rec ) of 2.5 and 9.5 J cm −3 , respectively, and accompanied by energy efficiencies (η) >90%. [19] And Yan et al achieved a high W rec of 6.3 J cm −3 in a BNT-based ceramic with a macroscopic layered structure.…”

“…[4,21] Aiming at this principle, the polymorphic polar nanoregions (PNRs) design strategy shown in Figure 1 has been adopted to develop a BNT-based ceramic dielectric with high energy storage performances. Previous studies have proved that the introduction of dopants such as Bi 0.5 K 0.5 TiO 3 , [22][23][24] BaTiO 3 [18,25,26] or SrTiO 3 [27,28] into BNT ceramics can construct the morphotropic phase boundary (MPB) and coexistence structures of micrometersize domains and polymorphic nanodomains, which effectively promoted the polarization reversal and extension owing to low energy barriers, [18] thus achieving greater electric field-induced polarization behaviors (Figure 1b). Then the long-range ferroelectric (FE) order of the matrix compositions was interrupted and relaxation behavior was enhanced via adding modifiers, decreasing the domain size to the nanoscale, the formation of polymorphic PNRs results in a small P r (Figure 1c).…”

Enhanced Energy Storage Properties in Lead-Free (Na0.5Bi0.5)0.7Sr0.3TiO3-Based Relaxor Ferroelectric Ceramics through a Cooperative Optimization Strategy

“…A two-dimensional (2D) single crystal with a cubic ( C ) → tetragonal ( T ) RFE transition upon cooling is assumed. The total free energy F of the system is defined as − F = ∫ V ( f bulk + f grad + f fr LF ) d V + ∫ V ( f elas + f elec ) d V where f bulk represents the bulk free energy density, given as f bulk = α 1 false( P 1 2 + P 2 2 + P 3 2 false) − α 11 ( P 1 2 + P 2 2 + P 3 2 ) 2 + α 12 false( P…”

“…A two-dimensional (2D) single crystal with a cubic ( C ) → tetragonal ( T ) RFE transition upon cooling is assumed. The total free energy F of the system is defined as − where f bulk represents the bulk free energy density, given as where P i ( i = 1, 2, 3) is the spontaneous polarization, a ij is a coefficient that depends on concentration c ( r ), and T is the temperature. a 1 = 0.004124­( T – 1300 c ( r ) – 350), a 11 = 4.0, a 12 = −8 + 3­( c ( r ) – 0.3), a 111 = 12.94, a 112 = 51.76, and a 123 = −12.94­(6 + 3­( c ( r ) – 0.3)), all which are obtained according to our previous work and experimental reported phase diagram of NBT-based samples. , …”

High piezoelectricity and low strain hysteresis in PMN–PT-based piezoelectric ceramics

“…Most ceramics or crystals possess large H in strain-electric field curves. Although a large strain (~over 0.2%) can be achieved, there is > 25% strain hysteresis in Bi0.5Na0.5TiO3 based ceramics [18][19][20], and also >15% in K0.5Na0.5NbO3 based ceramics [21][22][23], which restricting their applications in precisely controlled devices and system [24,25]. Compared with the lead-free ceramics, lead-based ceramics has a much smaller strain hysteresis.…”