Microstructural engineering for temperature-stable piezoresponse in KNN-based lead-free piezoceramics: a comprehensive review

Diffuse phase transition and multiphase coexistence

Stabilizing the PPT region over a broad temperature range through diffuse phase transitions and multiphase coexistence is a widely adopted strategy^[54,59]. By constructing a diffuse phase transition region near room temperature, the temperature sensitivity of the PPT can be reduced, resulting in more stable piezoelectric performance over an extended temperature range^[23,60]. Liu et al. designed the composition 0.93(Li_xNa_0.52K_0.48-x)(Nb_1-y, Sb_y)O₃-0.05BaZrO₃-0.02(Bi_0.5Na_0.5)HfO₃-0.01MnO₂ (abbreviated as LxKNNSy-5BZ-2BNH-1Mn) to construct a diffusion phase transition at room temperature [Figure 3A]^[29]. Figure 3B presents the PPT regions of the LxKNNSy-5BZ-2BNZ-1Mn compositions. The PPT regions exhibit a decreasing trend as the Li and Sb content increases. Figure 3C illustrates the room-temperature d₃₃ of LxKNNSy-BZ-BNH-1Mn ceramics. The d₃₃ increases with increasing Li and Sb content, but excessive doping of either dopant leads to a decline in performance. A maximum d₃₃ of approximately 510 pC N^-1 is obtained at x = y = 0.025, positioning it among the highest d₃₃ reported for KNN systems.

Figure 3. Diffuse phase transition and composition-induced structural flexibility to enhance the temperature stability. (A) Dielectric-temperature curves of different compositions. (B) The PPT point of LxKNNSy-BZ-BNH-1Mn compositions. (C) The d₃₃ of the LxKNNSy-BZ-BNH-1Mn composition at room temperature. (D) Variation of d₃₃ with temperature. (E) In situ temperature neutron diffraction of the (002)_PC reflection peak of the x = y = 0 and x = y = 0.025 samples. (F) PFM images of the x = y = 0.025 sample in both virgin state and local poling states at different temperatures. This figure quoted with permission^[29]. Copyright 2020, Oxford University Press.

The in situ temperature stability of d₃₃ for the compositions was assessed, as shown in Figure 3D^{[6,28,55,61-67]}. As anticipated, all compositions reached their maximum d₃₃ near the phase transition temperatures, followed by a monotonic decline as the temperature deviated from the phase transition region. In addition to the diffusion of the PPT, the author identified another key factor: composition-induced structural flexibility. Temperature stability is closely linked to the evolution of the phase structure^[67]. In situ temperature neutron diffraction measurements were performed on compositions with x = y = 0 and x = y = 0.025. Figure 3E illustrates the (002)_PC reflections across the temperature range of 20-160 °C. For the x = y = 0 sample, the (002)_PC reflections exhibited noticeable changes with increasing temperature, whereas the x = y = 0.025 compositions showed minimal variation, remaining nearly unchanged between 20 and 100 °C. This phenomenon demonstrates the significant role of structural flexibility in enhancing temperature stability. It should be noted that, although structural flexibility may contribute to the improved temperature stability of KNN ceramics, its effect has not been quantitatively evaluated. The contribution of structural flexibility may be relatively limited, and the excellent temperature stability observed in the composition with x = y = 0.025 is likely still primarily attributed to the construction of a diffuse phase transition.

The piezoelectric properties are closely related to domain morphology^[68]. Micro-scale domain structures were examined using the piezoelectric force microscope (PFM). In situ measurements of the domain morphology in both the virgin and locally poled states of the x = y = 0.025 composition were conducted at various temperatures. Figure 3F presents magnified domain images, revealing no significant changes within the temperature range of 25-100 °C.

Enabling PPT to exhibit MPB-like features as a new phase boundary, making it temperature-insensitive, is an important objective. Texture engineering is widely employed to enhance the piezoelectric properties of ceramics, and the oriented structure contributes to the stability of the domain structure^[69,70]. Therefore, the joint effect of temperature-insensitive PPT phase transition and texturing has been demonstrated by Xu et al. to effectively improve the temperature stability of KNN ceramics^[30].

As shown in Figure 4A and B, in the composition (0.97-x)(Na_0.56K_0.44)NbO₃-0.03Bi_0.5Na_0.5ZrO₃-x(Bi_0.5K_0.5)HfO₃ (KNN-BNZ-3BKH, x = 0.03), the (001)/(100) and (002)/(200) diffraction peaks split into two peaks with nearly equal intensity, indicating that KNN-BNZ-3BKH ceramics possess O-T phase coexistence^[71]. As the temperature increases, the O-T phase coexistence gradually transforms into the T phase. This behavior is similarly demonstrated in Figure 4C, where KNN-BNZ-xBKH becomes more relaxed with the addition of BKH.

Figure 4. Diffuse PPT phase transition and texturing engineering to enhance the piezoelectricity and temperature stability. (A and B) In situ temperature XRD of KNN-BNZ-3BKH. (C) Dielectric-temperature curves of different compositions. (D) Schematic diagram of the phase boundary. (E) In situ electric field XRD of KNN-BNZ-3BKH. (F) Schematic diagram of crystal structure transformation with and without an electric field. Temperature dependence of (G) random and (H) textured KNN-BNZ-3BKH ceramics. This figure quoted with permission^[30]. Copyright 2024, Springer Nature.

As shown in Figure 4D, by adjusting the appropriate content of BKH, a phase transition region similar to MPB can be constructed in KNN^[72]. To evaluate the phase transition under an electric field, in situ electric field X-ray diffraction (XRD) was performed on the KNN-BNZ-3BKH composition [Figure 4E]. Upon application of an electric field, the (002)/(200) diffraction peak is gradually shifted to a smaller angle, and KNN-BNZ-3BKH is gradually transformed from O-T coexistence to the O phase. The crystal structure with and without an electric field is shown in Figure 4F. The lattice distortion is reversible upon removal of the electric field, and this reversible lattice deformation is the origin of the superior piezoelectric properties^[73].

Subsequently, the piezoelectric properties and temperature stability of KNN-BNZ-3BKH were further improved by texturing engineering. As shown in Figure 4G and H, texturing increased the d₃₃ of KNN-BNZ-3BKH from 340 to 550 pC N^-1 at room temperature. The stability of the textured ceramics was also significantly enhanced, with piezoelectricity decreasing by only 10% over the temperature range of 25-250 °C. These results demonstrate that the combined effects of temperature-insensitive PPT boundary and texturing engineering enhance both the piezoelectric properties and temperature stability of KNN ceramics. However, whether an MPB-like phase transition also occurs in textured ceramics requires further verification, as it remains unclear whether the grain orientation influences the phase transition behavior of KNN-BNZ-3BKH.

Polar nanoregions

The electrical properties of piezoelectric materials can be enhanced through doping, which leads to the formation of polar nanoregions (PNRs). These localized nanoregions, with randomly oriented polarization, exhibit low free energy states^[74,75]. The presence of PNRs promotes a more relaxed ferroelectric state, which can also lead to a diffuse phase transition and improve temperature stability, although this may occasionally slightly reduce piezoelectric performance^[76]. It is important to note that PNRs are different from the diffuse PPT in the intrinsic mechanisms. While a diffuse PPT arises from the broadening and diffusion of the phase transition region, the formation of PNRs is primarily driven by local polarization vector disorder.

Hua et al. improved both the piezoelectric properties and temperature stability of KNN ceramics by combining PNRs with texturing engineering^[56]. In their study, the base composition was 0.99[(K0.5Na0.5)(Nb0.98Ta0.02)O3]-0.01[Bi(Ni0.67Nb0.33)O3]^[77]. They then introduced (Bi_0.5K_0.5)HfO₃ (BKH) into the KNN-based system to induce relaxation states and influence the phase transition temperature^[78,79]. Additionally, texturing engineering was employed to enhance anisotropy and preserve piezoelectric performance. As shown in Figure 5A-C, the results indicate that the novel multi-scale structure enabled the 0.96((K_0.5Na_0.5)(Nb_0.98Ta_0.02)O₃)-0.01(Bi(Ni_0.67Nb_0.33)O₃)-0.03(Bi_0.5K_0.5)HfO₃ ceramics to achieve both excellent piezoelectric performance and exceptional temperature stability.

Figure 5. Improvement of piezoelectric performance and thermal stability in KNN ceramics through PNRs and texturing. (A-C) Strategy for simultaneously enhancing piezoelectricity and thermal stability in KNN ceramics by constructing a single-phase structure and embedding PNRs. (D) In situ temperature dependence of d₃₃ for random and textured xBKH-KNN ceramics. (E) In situ variable-temperature k_p for 3BKH-KNN ceramics. (F) Electrostrains at different electric fields for T-3BKH-KNN ceramics. (G) In situ variable-temperature PFM phase images for T-3BKH-KNN ceramic. This figure quoted with permission^[56]. Copyright 2024, Wiley-VCH.

The temperature stability of the piezoelectric response was further compared between textured (T-xBKH) and random (R-xBKH) ceramics, as shown in Figure 5D. By constructing a single T-phase to prevent temperature-induced phase transitions, the temperature stability of R-xBKH ceramics was successfully achieved when x ≥ 3, with further improvement through texturing. Specifically, the d₃₃ of T-3BKH ceramics fluctuated by only 2.0% from 25 to 100 °C and by 10.0% up to 200 °C. More importantly, the k_p of the T-3BKH textured ceramics fluctuated by just 9.4% over the temperature range from 25 to 200 °C [Figure 5E]. As shown in Figure 5F, the large signal d₃₃* of T-3BKH ceramics remains stable from 25 to 200 °C, indicating good temperature stability.

To investigate the mechanism of thermal stability from the domain morphology, PFM was employed to examine the temperature-dependent changes in the domains of poled T-3BKH ceramics, as shown in Figure 5G. As the temperature increased, the domains did not exhibit significant depolarization. Although the temperature-dependent PFM images did not fully correspond to the observed temperature stability of T-3BKH, this was attributed to the higher depolarization energy associated with the larger domain structures in textured ceramics, which may enhance the thermal stability of the piezoelectric performance. However, when the temperature was higher than 150 °C, only a few smaller-sized domains with low domain wall energies were recovered due to thermal stimulation, resulting in a slight reduction in the domain phase response.

Similarly, Liu et al. constructed a rhombohedral-orthorhombic-tetragonal (R-O-T) phase transition to enhance thermal stability^[57]. A diffuse phase transition with excellent temperature stability was achieved by introducing relaxation states via PNRs. Figure 6A presents the temperature-dependent d₃₃* values measured across a range of temperatures (20-110 °C) for the compositions 0.93(Na_0.52,K_0.48)(Nb_1-x,Sb_x)O₃-0.05BaZrO₃-0.02(Bi_0.5,Na_0.5)HfO₃-1 wt%MnO₂ (x = 0, 0.01, 0.02, 0.025, 0.03). A small substitution of Sb significantly increased the room-temperature d₃₃* to over 600 pm V^-1 with minimal fluctuation from room temperature to 100 °C (x = 0.025). However, as the Sb content increased to 0.03, d₃₃* decreased, likely due to a reduction in spontaneous polarization caused by a decrease in tetragonal phase content. For all samples, d₃₃* initially increased with temperature, reaching its highest value between 50-90 °C. The negative strain (S_neg) decreased, while the positive strain (S_pos) increased with increasing temperature, as shown in Figure 6B. Figure 6C shows the temperature dependence of the unipolar strain (S_uni), remanent polarization (P_r), maximum polarization (P_max), and the term $$P_{\max }^{2}-P_{r}^{2}$$ at E_max = 15 kV cm^-1. The temperature dependence of S_uni aligns with the ($$P_{\max }^{2}-P_{r}^{2}$$) trends, as shown in Figure 6C. A similar temperature-stable behavior for d₃₃* under varying electric fields is illustrated in Figure 6D. Figure 6E displays the transmission electron microscope (TEM) images of the x = 0.025 composition, revealing intricate domain structures, including nano-strip-like domains and tweed-like striations within a single grain. The broad range of domain sizes suggests a wide distribution of domain mobility^[80-82]. Interestingly, strip-like Moiré fringes are observed in Figure 6F and G, and the authors suggest that these mismatched regions, with sizes ranging from 5 to 10 nm, correspond to PNRs rather than nanodomains^[83,84]. Moreover, the large signal d₃₃* will possess better temperature due to the electric field compensation effect, while the small signal d₃₃ is more directly influenced by intrinsic PPT and temperature depolarization effect, making it more sensitive and challenging to stabilize^[85]. Therefore, the temperature stability of small signal d₃₃ requires further discussion.

Figure 6. Temperature stability of large-signal inverse piezoelectric coefficient through PNRs. (A) Temperature stability of d₃₃*. (B) Bipolar electrostrain of the x = 0.025 composition. (C) Temperature dependence of S_uni, P_r, P_max and $$P_{\max }^{2}-P_{r}^{2}$$. (D) Temperature dependence of d₃₃* under different electric fields. (E) Representative bright-field TEM images. (F and G) TEM images of Moiré patterns. This figure quoted with permission^[57]. Copyright 2018, Royal Society of Chemistry.

Domain engineering

Electrostrain in piezoceramics primarily originates from the domain activity, making the temperature stability of electrostrain closely dependent on their ferroelectric domain structures^[86]. This effect is particularly pronounced near the phase transition region, where enhanced domain dynamics contribute significantly to the macroscopic strain response^[87,88]. Consequently, understanding the temperature-dependent evolution of domain structures and their corresponding piezoresponse is crucial for achieving stable piezoelectric performance.

To address this, Zhao et al. proposed a strategy to achieve stable strain performance by leveraging the contributions of domain dynamics and polarization evolution at phase boundaries^[89]. As shown in Figure 7A, the (1-x-y)K_0.4Na_0.6Nb_0.955Sb_0.045O₃-xBiFeO₃-yBi_0.5Na_0.5ZrO₃ (KN_0.6NS_0.045-xBF-yBNZ) composition (x = 0.006) with an R-T phase boundary exhibited relatively stable strain performance over a broad temperature range of 23-130 °C. For compositions with O-T, R-O-T, and R-O phase boundaries, prominent fluctuations were demonstrated, as shown in Figure 7B and C.

Figure 7. Enhancing temperature stability through domain engineering. (A) Temperature-dependent S_neg for the composition with x = 0.006. (B) Temperature-dependent d₃₃* for four compositions with different phase boundaries. (C) Normalized temperature-dependent d₃₃* against the room-temperature values. Temperature-dependent (D) P-E loops and (E) Bipolar S-E curves of the x = 0.006 composition. (F) Phase angle loops and (G) piezo-response curves of the selected region. (H and I) Domain structures of KN_0.6NS_0.045-0.006BF-0.03BNZ ceramics. Schematic diagram illustrating domain switching at (J) room temperature and (K) high temperature. This figure quoted with permission^[89]. Copyright 2018, Royal Society of Chemistry.

The domain structure plays a crucial role in determining the piezoelectric and ferroelectric properties, as well as the temperature stability of piezoelectrics^[90,91]. Domain switching and domain wall motion serve as the mesoscopic mechanisms underlying the P-E loops and S-E curves in these materials^[92,93]. As temperature increases, thermal disturbances inevitably destabilize domains^[94]. For instance, in the KN_0.6NS_0.045-0.006BF-0.03BNZ ceramic, both P_max, P_r [Figure 7D] and electrostrain [Figure 7E] exhibit a decrease as temperature rises. Additionally, the reduction in the coercive field (E_c) [Figure 7D] and bias voltage required for polarization switching [Figure 7F] indicate easier domain switching, resulting in a decline in the piezoresponse [Figure 7G]. Furthermore, some domains gradually deteriorate and disappear, as evident when comparing the A-B and A′-B′ regions in Figure 7H and I. Unfortunately, the authors only compared the piezoresponse at room temperature and 170 °C, without providing data for other temperatures, which makes it challenging to accurately assess its temperature stability. Under applied electric fields, the reduction in polarization and domain activity ultimately leads to a decline in positive strain (S_pos) at elevated temperatures [Figure 7J and K]. Thus, the temperature stability of electrostrain is intrinsically linked to domain evolution under temperature fluctuations, particularly in phase boundary regions where phase transitions and polarization switching are more pronounced.

Multilayer structures and compositional gradient design

In a homogeneous system (one-component ceramic), the phase transition temperature is typically well-defined, which can limit temperature stability. However, by stacking homogeneous single layers into a multilayer structure, the overall system can exhibit a diffuse phase transition region, significantly enhancing temperature stability. Multilayer structure with varying compositions holds great potential for achieving excellent temperature stability in bulk ceramics^[32,34]. Zhao et al. demonstrated that the multilayer structural design in lead-free systems could maintain excellent temperature stability performance in the range of room temperature to 130 °C^[95]. However, this strategy requires careful matching of single-layer compositions, making the rational design of multilayer ceramics for temperature stability both demanding and challenging. On the other hand, chemical diffusion between the single layers during high-temperature sintering will cause the composition to deviate from the designed stoichiometric ratio.

Building on this strategy, Zheng et al. further improved KNN ceramics by combining a multilayer structure with composition grading, enabling continuous phase transitions across layers and achieving enhanced temperature stability [Figure 8A and B]^[33]. The system (1-x)(K_0.48Na_0.52)(Nb_0.955Sb_0.045)O₃-xBi_0.5Na_0.5ZrO₃-0.2%Fe₂O₃ [(1-x)KNNS-xBNZ-Fe], which features varying phase transition temperatures through chemical composition modification [Figure 8A], was prepared using the tape casting method [Figure 8B]. Figure 8C illustrates the dielectric permittivity ε/ε_max at different temperatures of each layer. By adjusting the BNZ content, different O-T transition temperatures are obtained, accompanied by a broadened O-T phase transition region. An increase in piezoelectric performance for the four compositions is also observed near the O-T transition, as demonstrated by the in situ temperature dependence of d₃₃ [Figure 8D]. Figure 8E displays the dielectric-temperature curves of the multilayer structure, where diffuse O-T phase transitions occur over the range of 25-112 °C (indicated by the shaded region), resulting in minimal piezoelectric variation of less than 6%.

Figure 8. Multilayer structures and compositional gradient design. (A and B) Schematic diagrams for the mechanism of multilayer compositional gradients. Temperature dependence of (C) ε/ε_max (D) d_33T/d_33RT, and (E) ε_r of each layer. (F) PFM images of the single layer near the O-T transition. (G) Phase-field simulation of the multilayer composite. (H) Temperature-dependent d₃₃ of the single and multilayer composite, calculated from the simulation results. This figure quoted with permission^[33]. Copyright 2022, Wiley-VCH.

The domain structure of ferroelectrics is highly sensitive to temperature, leading to performance variations under thermal treatment^[6]. Given the critical role of domains in thermal stability, the evolution of the domain structure in each layer is shown in Figure 8F. In layer I, the oriented domains exhibit almost no back-switching due to the higher O-T transition temperature (≈ 112 °C), ensuring stable domain evolution. In contrast, the other three layers display significant back-switching when the temperature exceeds their respective O-T transition temperature. Consequently, in the multilayer composite, integrating layers with different phase transition temperatures allows the oriented domains to remain stable as long as the measurement temperature stays below the O-T transition temperature of each layer.

To gain deeper insight into this phenomenon, phase-field simulations were conducted to compare the temperature stability of d₃₃ between the multilayer composite and the individual layers. Figure 8G presents the multivariate surface of composition, temperature, and d₃₃, illustrating the variation in simulated d₃₃ along the composition gradient. Figure 8H compares the temperature stability of d₃₃ between the multilayer structure and the single-layer composition. Notably, in contrast to the abrupt changes in the piezoelectric coefficient observed in single-layer constituents, the phase-field simulation demonstrates stable performance across the temperature range of 25-100 °C for the multilayer composite. This result aligns well with the experimental data shown in Figure 8C-E.

As previously discussed, the piezoelectric performance significantly deteriorates as the temperature deviates from the PPT region [Figure 9A]. Song et al. designed a dopant-graded ceramic (DGC) by combining three ceramic layers, labeled as KNN1, KNN2, and KNN3, each with distinct PPT temperatures [Figure 9B]. The DGC structure demonstrated a wide operating temperature range and achieved a high d₃₃ value of 450-515 pC N^-1[31]. First, the distinct PPTs of the KNN1, KNN2, and KNN3 layers were leveraged to optimize piezoelectric performance. Second, the composition (Li_xNa_0.52K_0.48-x)(Nb_1-ySb_y)O₃-BaZrO₃-(Bi_0.5Na_0.5)HfO₃-0.01MnO₂ was chosen to create a layered distribution structure of Li/Sb to tailor the PPT temperatures across the layers. Third, similar compositions of the monomer units were carefully controlled to ensure excellent co-firing compatibility. With slight variations in Li/Sb doping, the DGC structure was successfully fabricated [Figure 9C], validating the effectiveness of the proposed strategy.

Figure 9. Dopant-graded strategy to enhance temperature stability. (A) Schematic diagram illustrating the temperature-dependent piezoelectricity of KNN ceramics. (B) Schematic diagram of composition grading with diffuse PPT. (C) SEM image of the DGC with a multilayer structure. (D) Dielectric-temperature curves of the DGC and each individual layer. (E) Calculated γ using the modified Curie-Weiss law. (F and G) Temperature dependence of d₃₃ for the DGC and each individual layer. This figure quoted with permission^[31]. Copyright 2022, Wiley-VCH.

The dielectric-temperature (ε_r-T) curves of the single-layer ceramics and the DGC were analyzed to understand their phase structures. The curves revealed two distinct peaks [Figure 9D], corresponding to the T_O-T and the T_c, respectively. The phase transition temperatures for the KNN1, KNN2, and KNN3 layers were 50, 75, and 100 °C, respectively, which aligns with the initial design. In contrast, the DGC exhibited a broader and more diffuse PPT peak, spanning 25-150 °C [Figure 9D]. Additionally, the tetragonal-cubic phase transition in the ε_r-T curve of the DGC exhibited significant diffusion, with a dispersion coefficient γ of 1.80 [Figure 9D and E1]^[16]. This γ value surpassed the average values for the individual ceramics, which were 1.74, 1.72, and 1.81, respectively [Figure 9E2-E4]. The room-temperature d₃₃ of the DGC is 463 pC N^-1, exceeding the values for each individual layer (434, 394, and 344 pC N^-1), as shown in Figure 9F. Notably, the d₃₃ of the DGC reached 508 ± 10 pC N^-1 at 50 °C and maintained 402 pC N^-1 at 150 °C, demonstrating enhanced thermal stability [Figure 9F]. In contrast, the d₃₃ of the single layer dropped sharply near their respective PPT temperatures with increasing temperature, indicating pronounced temperature instability. When the temperature deviated from the PPT region, d₃₃ decreased significantly [Figure 9F]. To quantify the temperature stability, Figure 9G presents the calculated ∆d₃₃/d₃₃ for both the DGC and the individual layers. The DGC achieved a value of 13%, significantly lower than those for the single layers.

Atomic-scale structure design

In addition to the pronounced temperature dependence associated with PPT, the polarization configuration also plays a crucial role in determining the piezoelectric response and temperature stability of KNN ceramics. Zeng et al. proposed that embedding PNRs with large-angle polarization into long-range ordered tetragonal phases will significantly improve temperature stability^[96]. This improvement is attributed to the dynamic equilibrium between the polarization configuration and the permittivity throughout the temperature variation. Therefore, both the average phase structure and microstructural features, such as polarization configurations, must be considered concurrently when evaluating the temperature stability of KNN-based ceramics. Multi-element doping not only influences the phase structure of solid solutions but also alters the local atomic structure^[58]. These modifications impact the system energy barriers and polarization rotation, thereby affecting the performance of the piezoceramics^[97,98]. As a result, the atomic-scale design of local structures and the ferroelectric state plays a crucial role in optimizing both the performance and stability of piezoceramics^[99,100].

Zou et al. demonstrate a synergistic enhancement of piezoelectric properties and temperature stability in 0.945KNN-0.055(Bi_0.5K_0.5)_1-xBa_xZrO₃ ceramics through atomic-scale ferroelectric structure design^[40]. By modulating the localized Landau energy barrier at the doping site, temperature-induced fluctuations in d₃₃ were effectively suppressed. The Local Ferroelectric Distortion (LFD) at dopant sites was defined by the difference between the two Nb-O bond lengths along the direction of spontaneous polarization. As stated above, the temperature instability of KNN ceramics arises from the PPT^[101-103]. The authors aim to mitigate this instability by minimizing A-site LFD, thereby constructing a PPT region with a lower energy barrier at room temperature. First, comparisons between PbTiO₃ and BaTiO₃ have shown that a reduction in A-site ferroelectric distortion decreases the stability of the tetragonal phase^[104]. Thus, by decreasing A-site LFD, the O-T phase transition temperature can be shifted upward, resulting in a more diffuse PPT region. Second, reducing A-site LFD creates numerous lower-energy-barrier regions, which promote polarization rotation [Figure 10A]. These regions help compensate for the reduction in ε_r [Figure 10B], thereby simultaneously enhancing both d₃₃ and temperature stability.

Figure 10. Strategy schematic for synergistically enhancing d₃₃ and temperature stability. (A) Reduced LFD in the matrix to promote polarization rotation. (B) Construction of a diffused PPT phase boundary by elevating the O-T transition temperature. (C) Temperature dependence of d₃₃ for different KNN ceramic compositions. (D) Dielectric-temperature curves for different KNN ceramic compositions. (E) Schematic diagrams of the PEC at room temperature, the O-T transition, and above the O-T transition. This figure quoted with permission^[40]. Copyright 2024, Springer Nature.

To validate this approach, the system 0.945KNN-0.055(Bi_0.5K_0.5)_1-xBa_xZrO₃ was chosen as a model. This system demonstrates an impressive d₃₃ of approximately 360 pC N^-1[105,106]. A room-temperature R-(O)-T phase boundary was established by adjusting the parameter y. At x = 0, a high d₃₃ of about 360 pC N^-1 was achieved. However, similar to other compositions with R-T phase boundaries at room temperature^{[55,61,107-110]}, a significant fluctuation of approximately 50% was observed within the range of 25-100 °C, with d₃₃ decreasing to around 180 pC N^-1 at 100 °C [Figure 10C]. To improve stability, the Ba element was introduced into the system to reduce the A-site LFD. As shown in Figure 10D, increasing the Ba content shifted the O-T transition temperature to a higher temperature while simultaneously enhancing the room-temperature d₃₃. At x = 0.27, a stable d₃₃ of approximately 400 pC N^-1 was achieved at room temperature, with only ~13% fluctuation from 25 to 100 °C. Further increasing x to 0.36 resulted in a higher room-temperature d₃₃ of ~430 pC N^-1, with fluctuations reduced to ~7%, maintaining a consistently high d₃₃ above 400 pC N^-1 in the range of 25-100 °C. However, as the x value increases to 0.45, both the room-temperature ε_r and d₃₃ decreased.

The potential energy curves (PEC) at different temperatures were analyzed to understand the effect of Ba doping on the phase boundary region. At room temperature, increasing the Ba content sharpens the PEC in the phase boundary region [Figure 10E1]. As the system approaches the O-T transition [Figure 10E2], the PEC becomes more moderate, which enhances d₃₃. Beyond the O-T transition, but still in the vicinity [Figure 10E3], the phase boundary gradually disappears, leading to a sharper PEC. However, the reduction in the local energy barrier compensates for this sharpening, keeping the overall PEC relatively moderate and maintaining a high d₃₃ of over 400 pC N^-1. It should be noted that the variation in PEC at different temperatures provides a potential explanation for the excellent temperature stability; however, the underlying cause may still be the diffusion of the phase transition resulting from the reduced A-site LFD.

Defect engineering

The external contribution to large-signal piezoelectric coefficients arises mainly from domain switching^[111]. Therefore, a key strategy to enhance the temperature stability of electrostrain performance is to minimize temperature-induced domain structure evolution. Defects and defect dipoles play a significant role in stabilizing ferroelectric domains by exerting a pinning effect on domain walls, thereby inhibiting domain wall motion. This stabilization helps maintain the integrity of ferroelectric domains, significantly improving the temperature stability of piezoelectric materials at elevated temperatures^[112,113].

Li et al. introduced ($$F e_{A}^{\cdot\cdot }-{V}^{\prime}_{A}$$) and ($$Fe_{Nb}^{\prime \prime}-{V^{\cdot \cdot}_{O}}$$) defect dipoles by A-site or B-site doping of Fe³⁺ into the KNN system^[35]. The pinning effect of these defect dipoles stabilizes ferroelectric domains, contributes to the temperature stability of 0.95(K_0.45Na_0.55)NbO₃-0.05Bi_0.5Na_0.5HfO₃ (KNN-BNH) ceramics. As shown in Figure 11A, the bipolar strain remains stabilized in the temperature range of 25-140 °C. To evaluate the pinning effect of defect dipoles, the internal bias electric fields (E_i) for different doping conditions are shown in Figure 11B. B-site doping with 5mol% Fe³⁺ results in the highest E_i, which is attributed to the space charge characteristics and the presence of oxygen vacancies that are less mobile^[114]. Furthermore, to examine the impact of space charges, the temperature-dependent resistance of different compositions is plotted in Figure 11C. The B-site-0.05 composition exhibits the lowest resistance within the 480 to 600 °C range compared to other compositions, indicating a strong correlation between space charge effects and defect stabilization.

Figure 11. Effect of defect dipoles and space charges on the temperature stability of KNN ceramics. (A) Temperature-dependent bipolar strain of KNN-BNH-0.05Fe ceramics. (B) Variation of E_i for different dopants at different temperatures. (C) Temperature-dependent resistance comparison for different compositions at high temperatures. (D) Schematic diagrams of ferroelectric domain alignment and defect dipole behavior under an external electric field. This figure quoted with permission^[35]. Copyright 2021, Elsevier Ltd.

Beyond space charge effects, the pinning effect of defect dipoles significantly influences temperature stability^[37]. As shown in Figure 11D, when an external electric field is applied at high temperatures, ferroelectric domains and defect dipoles within the ceramics align with the field direction^[115]. To maintain the system’s energy minimum state, these defect dipoles remain aligned with the ferroelectric domains, restricting their switching^[116]. This alignment stabilizes the domain structure even after the removal of the external field, thereby contributing to the temperature stability of the ceramics.

To maintain a strong pinning effect, Tian et al. proposed a poling-aging-repoling strategy to realign defect dipoles with the poling direction in the (K_0.48Na_0.52)_0.94Li_0.06Nb_0.94Ta_0.06O₃-2%wt Cu (KNNLT-2Cu) ceramics^[37]. As shown in Figure 12A, fresh piezoceramics contain almost no defect dipoles. After pre-poling, the ferroelectric domains align with the poling direction. However, during the subsequent aging process, some domains gradually deviate from the poling direction, accompanied by the formation of a large number of defect dipoles^[112]. To counteract this, a repoling process is introduced to realign the ferroelectric domains and defect dipoles that have deviated from the poling direction. This realignment enhances the pinning effect, thereby improving temperature stability.

Figure 12. Poling-aging-repoing strategy to achieve strong pinning of defect dipoles. (A) Schematic illustration of the repoling process. (B) Impedance spectra of KNNLT-2Cu ceramics measured at different temperatures. (C) Calculated d₃₃, Q_m, and FOM at different temperatures. (D) Electrostrain behavior of KNNLT-2Cu after the repoling process, measured from 30 to 240 °C. This figure quoted with permission^[37]. Copyright 2024, Elsevier Ltd.

Figure 12B shows the temperature dependence of the impedance spectra of KNNLT-2Cu ceramics. As the temperature increases, the phase angle remains almost constant, with only slight changes in the resonance and anti-resonance frequencies. The d₃₃, Q_m, and Figure of Merit (FOM = Q_m*d₃₃) at different temperatures are calculated and plotted in Figure 12C. The d₃₃, Q_m, and FOM remain virtually unchanged between room temperature and 100 °C, demonstrating excellent temperature stability. The high Q_m values further indicate the potential of KNNLT-2Cu ceramics for high-temperature and high-power applications.

The temperature stability of electrostrain is also evaluated [Figure 12D]. Due to the strong pinning on the ferroelectric domains, asymmetric switching under different electric field directions leads to asymmetric strain curves at room temperature. While an increase in temperature does not significantly affect the strain values, it alters the symmetry of the strain curve, reflecting changes in the switching characteristics of the ferroelectric domains. Unfortunately, although KNNLT-2Cu has good temperature stability, it demonstrates a low large signal strain of 0.04%. Moreover, due to the asymmetry of the electrostrain curves, the direction of the electric field must be considered in practical applications.