Development Status of Textured Piezoelectric Ceramics and Preparation Processes

Abstract

There are many factors that determine the performance parameters of piezoelectric ceramics, but the process of preparing ceramics plays a decisive role in their properties. By improving the preparation process, significant improvements in ceramic properties can be achieved. In order to meet the growing demand for improvement in the performance of piezoelectric ceramics, various crystal preparation methods such as Template Crystal Growth (TGG), Reflected Template Growth (RTGG), and Oriented Curding (OCAP) techniques have been used for the preparation of piezoelectric ceramics. This article first introduces the performance parameters of piezoelectric ceramics, then summarizes the influence of various preparation processes on the performance of piezoelectric ceramics, and finally summarizes the parameters of high-tech ceramics reported this year, and predicts the future of the preparation of high-tech ceramic materials.

1. Introduction

The demands on the performance of piezoelectric materials are increasing, particularly in the fields of electronics, sensors, and medical devices, and traditional piezoelectric ceramics often fail to meet the requirements of high-performance materials due to the limitations of grain arrangement [1]. In this context, textured ceramics have emerged as a solution. Textured piezoelectric ceramics are ceramics in which the grains or crystals are oriented in a specific direction through certain processing techniques, thus achieving superior electromechanical properties [2,3]. Compared to traditional ceramics with random grain arrangements, textured piezoelectric ceramics can significantly improve the material’s piezoelectric performance, mechanical strength, and other physical properties [4,5]. The fabrication of textured ceramics not only addresses the performance bottlenecks of traditional materials under high-frequency, high-voltage, and high-temperature conditions but also provides higher efficiency and broader application prospects [6,7,8,9].

In the past, lead-based ceramics such as PZT [10,11,12], PMN-PT [13,14,15], and PLZT [16,17] were widely used. Common lead-free ceramics, such as BNT-BT [18,19,20,21], BKT [22,23], and KNN [24,25], generally cannot match the performance of lead-based ceramics, which is why lead-based ceramics remain the mainstream material for most commercial MLA. However, environmental awareness is growing; thus, the environmental impact of lead-based ceramics has become increasingly apparent, and the development of lead-free ceramics has become a hot topic in the current research on functional ceramics [26,27,28,29,30,31].

Currently, the most common methods used for the development of high-performance piezoelectric ceramics include ion doping modification [32,33,34,35,36,37], group element optimization [38,39], and texturing techniques [40,41,42,43,44,45,46,47,48]. Ion doping refers to the doping of different valence ions in ceramics to replace the A-site or B-site ions in the original crystal lattice [33,49], which can effectively regulate the physical structure and electrical properties of the ceramics [32,50]. The group element optimization involves optimizing the crystal structure and properties of piezoelectric ceramics by adjusting their chemical compositions and ratios.

Techniques such as introducing appropriate doping elements, adjusting the lattice structure, optimizing crystal defects, etc., improve the piezoelectric properties. The texturing technique controls the orientation and arrangement of the grains of the ceramic material, allowing the optimal orientation of the grains to be achieved, thus improving the piezoelectric properties of the material; this is mainly achieved using methods such as hot pressing texturing, tensile texturing, and electric field polarization [40,44]. Although there are many methods that can be used to improve the properties of piezoelectric ceramics, using texturing techniques to prepare textured ceramics with optimal grain orientation is still an effective way to improve the properties of piezoelectric ceramics [38,51].

In this paper, we will provide a detailed introduction of the process flow of the texturing technique and the development of the application of the texturing process in the field of high-performance piezoelectric ceramics in recent years.

2. Piezoelectric Material Texturing Process and Main Parameters

2.1. Main Parameters Characterizing the Properties of Piezoelectric Materials

(1)

Curie temperature (T_c): When it comes to piezoelectric materials, the Curie temperature (T_c) refers to the temperature at which the ferroelectric phase transforms into the cis-phase. It is generally believed that the higher the T_c, the higher its use or working temperature.

(2)

Saturation polarization (P_s): Saturation polarization refers to the maximum polarization intensity that a piezoelectric material can achieve under the maximum electric field. A higher-saturation polarization means that the material has a higher output charge and greater displacement capacity [52]. When the electric field is raised to a certain value, the spontaneous polarization vectors of all the cells rotate in the direction closest to the external electric field, and the interior of the material approximates the state of a single domain. The voltage continues to increase at this point, and the increase in the polarization vector can be entirely attributed to the contribution of the non-ferroelectric part, so that the segment of the electric hysteresis loop in Figure 1a that is close to the maximum polarization vector (P_max) is linear. Extrapolating this segment to E = 0, the intersection with the y-axis then represents the saturation polarization strength (P_s) [53].

(3)

Piezoelectric coefficient (d): The piezoelectric coefficient is the proportionality between the amount of charge generated by a piezoelectric material when it is subjected to a mechanical stress or an electric field excitation and the intensity of the stress or electric field. Usually, d₃₃, d₃₁, d₁₅, etc., are used to indicate the piezoelectric coefficient under different orientations. A higher piezoelectric coefficient indicates that the material is capable of generating a greater charge output or displacement. When pressure T₃ is applied along the polarization direction of a piezoelectric material, a surface charge is generated through the inverse piezoelectric effect, where the surface charge density δ₃ satisfies the relation.

$${\sigma }_3=d_{33}{T}_3$$

(1)

And in the MKS system of units, the potential shift D₃ is equal to the surface density of the surface charge δ₃ and therefore satisfies the following relation:

$$D_3=d_{33}{T}_3$$

(2)

$$S_3=d_{33}{E}_3$$

(3)

In the formula, d₃₃ is the piezoelectric constant in pC/N, T is the stress, and S is the strain produced by the inverse piezoelectric effect of the piezoelectric material. In the positive/inverse piezoelectric effect, d₃₃ embodies the piezoelectric material’s ability to generate charge under applied pressure and the ratio of S to E under the action of E, respectively.

(4)

Electromechanical coupling coefficient (k): As shown in Figure 1b, the electromechanical coupling coefficient is a parameter used to respond to the conversion efficiency of the piezoelectric material in the conversion of mechanical and electrical energy. k’s anisotropic behavior is mainly determined by ε_T [54], which can be defined as follows:

$$k={\displaystyle \frac{U_I}{\sqrt{U_M{U}_E}}}$$

(4)

where U_I, U_M, and U_E stand for mechanical energy–electrical energy interaction density, mechanical energy density, and electrical energy density, respectively. k values can have different physical meanings and values depending on the vibration modes of the piezoelectric oscillator. The most commonly used value of k in practical studies is the planar electromechanical coupling coefficient k_p.

(5)

Dielectric constant (ε): as shown in Figure 1c, the dielectric constant is the degree of polarization of a piezoelectric material under an applied electric field [54]. It reflects the ability of a material to store and release energy in response to an electric field. The dielectric constant usually includes the electrostatic dielectric constant (ε₀) and dielectric loss (tanδ). The dielectric properties of ferroelectric materials are directly related to the temperature, and the curve of its dielectric constant ε with the temperature T initially exhibits a rise and then a fall with a peak, and the temperature corresponding to this peak is the value of T_c. At temperatures higher than the Curie temperature T_c, the dielectric constant of ordinary ferroelectric materials generally follows the Curie–Weiss rule:

$$\mathsf{\epsilon }={\displaystyle \frac{C}{T-T_C}}$$

(5)

where ε is the dielectric constant; C is the Curie constant; T is the temperature; and T_c is the Curie temperature.

However, for relaxing ferroelectrics, the temperature dependence of the dielectric constant follows the modified Curie–Weiss law in the temperature T_m interval above the temperature corresponding to the maximum value of the dielectric constant:

$${\displaystyle \frac{1}{\mathsf{\epsilon }}}-{\displaystyle \frac{1}{{\mathsf{\epsilon }}_m}}={\displaystyle \frac{\mathsf{\gamma }\left(T-T_m\right)}{C_1}}$$

(6)

where ε is the sample dielectric constant; ε_m is the maximum value of the dielectric constant; T_m is the temperature corresponding to the maximum value of the dielectric constant; T is the temperature; C₁ is the Curie constant; and γ is a constant indicating the degree of dispersion, with a range of 1~2. The larger the value, the larger the degree of dispersion.

2.2. Definition, Classification, and Main Processes of Texturing Processes

The enhanced piezoelectricity should be attributed to several combined effects, mainly including the R-O-T phase boundary that promotes polarization rotation, crystal orientation-induced intrinsic piezoelectric anisotropy, electric field-induced lattice distortion, and phase transition [55]. The strain output of randomly oriented ceramics can be suppressed due to limited polarization reversal induced by grain confinement [56]. Strongly anisotropic ferroelectric single crystals have a much higher piezoelectric response compared to polycrystals, and the texturing technique is a technique that is used to control the orientation and alignment of grains in ceramic materials, which essentially takes advantage of the anisotropy of piezoelectric properties [57]. The process comparison of the texturing technique with ion doping modification and group element optimization techniques is shown in Figure 2. In contrast to both, woven ceramics tend to exhibit single-crystal-like properties in the direction of grain orientation growth, which provides an opportunity to achieve superior strain output [56]. The optimal grain orientation is achieved by tuning the preparation parameters to allow such materials to grow optimally along the crystallographic direction with greater piezoelectric properties [40,58,59]. Compared with unoriented polycrystalline materials, after texturing, the highly optimal grains enhance the piezoelectric coefficient, d₃₃, by a factor of 1.4 to 2.1 [44]. The following methods are used to prepare textured ceramic materials: thermal processing [60,61], template grain growth (TGG) [62,63,64], reactive template grain growth (RTGG) [47,48,65], and electroscopes and strong magnetic field-assisted method [66], oriented curing (OCAP) technique [67], and Rolling-assisted Biaxially Textured Substrates [68]. Figure 2 illustrates the main process flows of these techniques, and in this paper, we will focus on the template grain growth (TGG) and reactive template grain growth (RTGG) methods.

As shown in Figure 3, heat treatment texturing techniques include hot pressing, hot forging, and hot rolling [70], which are used to achieve texturing by applying an external force under high-temperature conditions to produce dislocation motion and grain boundary slip, etc., within the grains, which results in the directional arrangement of the grains [60]. Template grain growth (TGG) is a method that involves guiding the growth of other crystals through the use of a stationary crystal template [71]. This usually involves preparing a template crystal with the desired crystal structure, contacting the template crystal with a suitable solution or melt so that substances in the solution are deposited on the surface of the template crystal, controlling the growth conditions of the deposits (e.g., temperature, pressure, etc.) so that new crystals are formed on top of the template crystal, and, ultimately, removing the template crystal, leaving behind a new template-based crystal [36,69]. Reactive template growth (RTGG) is a method that utilizes a chemical reaction process to generate new crystals in a template. The steps of the method are as follows: prepare an appropriate template, e.g., porous material, nanoparticles, etc. Place the template in a suitable reactant solution or gas phase. When the chemical reaction occurs, the reactant reacts with the template to generate new crystals. Reaction conditions, such as temperature, concentration, and reaction time, are regulated to control the growth process of the new crystals. The reaction template growth method can be used to synthesize a wide variety of crystalline materials and can be used to control the shape, size, and structure of the crystals by adjusting the reaction conditions. The advantage of this method is that the structure, shape, and orientation of the new crystals can be controlled by the choice of template crystals. Therefore, it is widely used in materials science to synthesize materials with specific crystal structures, such as nanomaterials and polycrystalline materials [72,73].

The TGG (template grain growth) technique improves upon the original OCAP (oriented curing) technique by introducing a template material as a guiding agent for grain growth to facilitate the formation and control of crystal orientation. This innovation is important for improving the crystal orientation and properties of ceramic materials and lays the foundation for subsequent research and applications [39]. The reactive template grain growth technique RTGG is a texturing ceramic preparation technique developed based on TGG, and based on the idea of preparing texturing ceramics by template grain growth (TGG), the piezoelectric ceramic properties have been substantially improved [69,71,72,73].

Important structural properties of textile ceramics include the degree of orientation as well as the sintering density and grain size [45]. There are many methods for evaluating the degree of preferred orientation of ceramics, such as the Lotgering method and the backscattered electron diffraction (EBSD) technique [74,75,76]. Among them, the Lotgering method is used to establish the relationship between the relative intensities of X-ray diffraction peaks and grain orientation and calculate the ceramic degree of texturing (F_hkl) by measuring the relative intensities of X-diffraction peaks, which has the advantages of being fast, simple, and convenient, and it is the most commonly used method. Taking 001 orientation as an example, the specific method is as follows:

$$F_{001}={\displaystyle \frac{P-P_0}{1-P_0}}{\displaystyle \frac{P-P_0}{1-P_0}}$$

(7)

$$P={\displaystyle \frac{\sum I_{\left(001\right)}}{\sum I_{\left(hkl\right)}}},P_0={\displaystyle \frac{\sum I_{\left(001\right)}}{\sum I_{0\left(hkl\right)}}}{\displaystyle \frac{\sum I_{\left(001\right)}}{\sum I_{0\left(hkl\right)}}}$$

(8)

where P and P₀ correspond to the ratio of the (00l) diffraction peak intensity to the sum of all diffraction peak intensities for woven and randomly oriented ceramics, respectively, and I and I₀ are the X-ray diffraction intensities for woven and random ceramic samples, respectively.

3. Development and Application of Texturing Processes

3.1. Hot-Pressing Method

The detailed process of the hot-pressing method is shown in Figure 4a. Back in the 1960s, Jaeger [77] et al. developed a method for preparing potassium sodium niobate (KNN) piezoelectric ceramics by hot pressing, which showed that the density of piezoelectric ceramics could be significantly increased by hot pressing, thus enhancing their piezoelectric and dielectric properties. As shown in Figure 4b, Liu [60,78] et al. prepared dense lead-free ceramics K₀.₅Na₀.₅NbO₃ (KNN) via a hot-pressing sintering method in a wide temperature range. The microstructure, dielectric, ferroelectric and piezoelectric properties of the ceramics were systematically investigated. The experimental results show that all ceramics exhibit a single orthorhombic phase and a dense microstructure with high relative density (>96.8%). The grain size is positively correlated with the domain size. Ceramics with average grain sizes of 490 and 720 nm showed submicron domain sizes of 100 and 150 nm, respectively; while ceramics with an average grain size of 160 nm had nanoscale host crystals, and fine-grained ceramics with nanoscale domains exhibited excellent electrical properties. Yu et al. [79] significantly enhanced the piezoelectric properties of the resulting ceramics by hot press sintering and by adding 0.5 mol% Al₂O₃ to K₀.₅Na₀.₅NbO₃. The ceramics exhibit good temperature stability, with ceramic d₃₃ reaching 137 pC/N and d₃₃ remaining as high as 97 pC/N with increasing annealing temperature. As shown in Figure 4c, Li et al. [80] applied the OPAHP technique to the sintering process of KNN piezoelectric ceramics. KNN ceramics with fine and uniform grain distribution were prepared by OPAHP technology. The specimens sintered at the lowest temperature of 860 °C exhibited the highest relative density of 99.6%, with d₃₃ reaching 142 pC/N. Compared with ceramics prepared by other methods, the OPAHP-prepared samples possessed enhanced dielectric and piezoelectric properties, as well as ultra-high hardness and bending strength due to the presence of oscillating pressure.

3.2. Template Method

Thermal processing methods are unable to amend the texture of isotropic materials [1]. Template grain growth (TGG) and reaction template grain growth (RTGG) solve this problem. Both TGG and RTGG utilize the shape and structure of the template to control the growth process of the ceramic material to obtain the desired shape and microstructure [81,82]. Therefore, they can both be categorized as template methods. The use of piezoceramic template methods for the preparation of materials with specific texture configurations can introduce an orderly arrangement of crystal orientations into the microstructure of the material, which can significantly improve the properties of the material. The texture structure can affect the piezoelectric properties, thermo-mechanical stability, and piezoelectric coupling properties of piezoelectric ceramic materials [83].

One of the most important techniques for the preparation of ceramics with a high degree of texturing is the preparation of template particles, which have morphologies categorizes as plate-like or fibrous. Examples include BaTiO₃ (BT) and NaNbO₃ (NN). To date, BT and NN templates have been preferentially used for the fabrication of highly doped PbTiO₃ and BaTiO₃ as well as (K, Na)NbO₃ texturing ceramics, respectively [84]. Saito [1] et al. developed a new technique for synthesizing template particles which allowed them to completely transform BiNN₅ particles into NaNbO₃ particles with a plate-like, regular chalcocite structure, and successfully synthesized them via the RTGG method. High-performance KNN-based piezoelectric ceramic with a d₃₃ of 416 pC/N and a k_p of 0.61 was successfully synthesized by RTGG with a textured <001> orientation, and this discovery was like opening the door to lead-free piezoelectric ceramic systems. However, poor temperature stability is the main drawback of KNN-based ceramics [85]. Zheng et al. [86] proposed to design a new ceramic system (1−x)(K_1−yNa_y)(Nb_1−zSb_z)O_3−xBi₀.₅(Na_1−wK_w)0.5HfO₃(KNNS-BNKH), where the R-T phase boundary can be constructed by optimizing x, y, z, and w. The piezoelectric properties can be improved by controlling the phase boundary. T_C decreases rapidly with as Xandz increases. In addition, their TR-O and T_O-T increase and decrease, respectively, with the increase in both. By changing the composition of the ceramics, high piezoelectric constants can be obtained in the ceramics. Using this method, the piezoelectricity and fatigue resistance properties of the material were enhanced simultaneously, further promoting the development of lead-free ceramics. Priya et al. [87] prepared (K, Na) NbO₃-based piezoelectric ceramics with a high degree of texturing along the (001) direction using a NaNbO₃ (NN) stencil in the process flow shown in Figure 5a. As shown in Figure 5b, the degree of texturing reached 94%, and experiments showed that the textured assemblage (KNN-5NN) with 5 wt% NN template showed a significant improvement in the electromechanical coefficients at room temperature, with d₃₃ up to 590 pC/N, k₃₁ up to 0.46 and d₃₁ up to 215 × 10⁻¹² C/N. Gao et al. [86] used a template grain growth (TGG) method to prepare Ba²⁺ doped 0.97 (K₀.₄₈Na₀.₅₂)(Nb₀.₉₆Sb₀.₀₄)O₃−0.03Bax(Bi₀.₅Ag₀.5)_1−xZrO₃(KNNS-BxBAZ) woven ceramics. As shown in Figure 5c, the monopole strain of the x^¼0.2 sample reached about 0.17% at 40 kV/cm and is insensitive to temperature changes. The strain value ratios between the strain values at different temperatures and room temperature were calculated from the variation in the sample’s monopole strain, as shown in Figure 5c, the strain value of the x = 0.2 sample fluctuated around 0.17% as the temperature increased, and the variation in the strain was less than 5% in the temperature range from room temperature to 165 °C. The temperature stability of KNN-based crystals was effectively improved by the texturing technique [86,87,88].

4. The Effect of Stencils on the Properties of Textured Ceramics

4.1. BT/ST Stencil Texture Ceramics

The template content, the size of the template particle morphology, and the sintering temperature can directly affect the growth of crystals, etc. [89,90], and BaTiO₃ (BT) or SrTiO₃ (ST) templates can greatly reduce the sintering temperature [63]. Zhao [90] et al. used the reaction template growth method (RTGG), which used Bi₄Ti₃O₁₂ (BT) as a crystalline species based on the reaction depicted in formula (5). This method produced woven ceramics with ternary BNT-BKT-BT in <001> orientation, which resulted in a texturing degree of 81%, and the piezoelectric properties were improved by a factor of 1.4 compared with random orientation.

0.12Bi₄Ti₃O₁₂ + 0.2125Na₂CO₃ + 0.0275K₂CO₃ + 0.6TiO²⁺ +0.04BaTiO₃→0.85BNT + 0.11BKT + 0.04BT + 0.24CO₂

(9)

Yang [63] used a BT template in combination with the TGG method to prepare <001> texture-structured 0.19Pb(In₀.₅Nb₀.₅)O₃-xPb(Sc₀.₅Nb₀.₅)O₃-(0.8_1−x)PbTiO₃(PIN-PSN-PT) ceramics (x = 0.44~0.49) with <001> orientation via two-step topological chemical reaction synthesis with the following reaction equation:

2Bi₂O₃ + 3TiO₂→Bi₄Ti₃O₁₂

(10)

Bi₄Ti₃O₁₂ + 3BaCO₃→3BaTiO₃ + 2Bi₂O₃ + 3CO₂

(11)

As shown in Figure 6a, due to its high melting point, the template-induced PIN-PSN-PT grains are very small at high temperatures, but the temperature leads to the volatilization of PbO, thus greatly reducing the densification and properties of the final samples. To solve this problem, experiments were carried out that involved adding small amounts of CuO and B₂O₃ (0.2~0.5 wt%). The results show that this method can effectively promote the growth of PIN-PSN-PT single crystals on BT templates. The performance parameters of all the woven ceramics created using varying BT templates are shown in

Table 1. Electromechanical properties of textured 0.19PIN–0.445PSN–0.365PT ceramics.

Samples	εr	tanδ (%)	Tc (°C)	d33 (pC N−1)	kp (%)
T-3BT	2120 ± 60	0.8	261	850 ± 30	82.2 ± 0.7
T-5BT	2310 ± 90	1.2	247	1090 ± 50	83.4 ± 0.7
T-7BT	2250 ± 110	1.3	241	1060 ± 50	80.5 ± 0.5

. Compared with the nonwoven samples, Figure 6b shows that all the samples have a pure chalcogenide structure, whereas the intensity of the peaks of the woven ceramics at <001> is enhanced, and most of the grains are well orientated along the <001> direction in the woven PIN-PSN-PT ceramics [63,91]. The problem of the low-ferroelectric phase transition temperature in the relaxation phase of lead titanate bismuth niobate lead titanite ferroelectric materials, which has an impact on the design freedom of high-performance piezoelectric devices, was solved. Bian et al. [92] used a BaTiO₃ template as in Figure 6c(i) to prepare a woven 0.42PNN-0.21PZ-0.37PT ceramic using the TGG method. The total capacitance of the material was equal to the substrate and the fillers in series and parallel connections [34]. Therefore, the curve of dielectric constant versus temperature can be flattened effectively by introducing stencils with appropriate dielectric constants as fillers. Moreover, the texturing volume fraction increases as the BT template concentration increases because a high concentration of BT templates introduces more grain orientation growth [93]. As shown in Figure 6c, the peak corresponding to the R-T phase transition diminishes with the increase in BT template volume fraction and disappears at 3% of BT template volume. On the other hand, the T-C phase transition temperature T exhibits very few changes due to the addition of the BT template [92]. In summary, the temperature stability of piezoelectric ceramics was significantly improved by ceramic texturing [94,95].

4.2. Texturing Ceramics Using PT Stencils

The piezoelectric properties of Pb-based ceramics can be improved by two methods, namely the formation of metamorphic phase boundaries (MPBs) and the doping of rare earth elements [96,97,98,99]. Moriana et al. [100] developed chalcogenide Pb(Sc_1/2Nb_1/2)O₃-PbZrO₃-PbTiO₃(PSN-PZ-PT) systems with different compositions using the TGG method, and the ferroelectric properties are shown in Figure 7a. Comparing the degree of texturing of the templates with different contents reveals that the PT content is positively correlated with the degree of texturing, which is due to the suppression of TGG due to, for example, the higher melting points of Sc₂O₃ and ZrO₂. As shown in Figure 7b, PSN-PZ-PT has a high Curie temperature and polarization stability while maintaining comparable piezoelectric properties to binary Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT); achieving synergistic effects using the two mechanisms in the same material has been difficult because the microstructural design is affected by the compositionally induced local structural inhomogeneities. As shown in Figure 7c, Yan [101] et al. successfully addressed this challenge for the first time by proposing a synergistic design of local structural heterogeneity and grain texturing under image-only modeling and phase-field simulation, where local structural heterogeneity is introduced by doping rare earth elements (including relaxation ferroelectrics), and the d₃₃ of the PMN-PTs is enhanced with the increase in the volume fraction of heterogeneity [101], and then PMN-PT piezoelectric ceramics are prepared using the TGG process. Figure 7d,e show that the Eu³⁺-doped woven PMN-PT piezoelectric ceramics are highly [001] oriented compared to the others. Li et al. [102] found that Eu³⁺ doping has a great influence on the morphology of the domains and the size of the domains of the PMN-0.25PT ceramics due to the fact that a high Eu³⁺ doping level helps to enhance the local structural inhomogeneity, disrupting the long-range ordered ferroelectric states and producing more disordered and smaller domain microstructures [102,103,104]. However, PMN-PT and PZN-PT single crystals show very low phase transition temperatures and low Curie temperatures (T_c~130–170 °C), resulting in poor temperature stability and a relatively low mechanical quality factor (Q_m), both of which limit their applications [105].

4.3. Textured Ceramics Using NN Templates

For KNN-based ceramics, it is very difficult to improve their thermal stability while maintaining good piezoelectric properties [106]. Both phase boundaries and engineered domain structures have an impact on the piezoelectricity of woven KNN-based ceramics [55]. Although a multiphase coexistence (MPC) strategy at room temperature improves the piezoelectric coefficient (d₃₃) of potassium sodium niobate ((K, Na) NbO₃, KNN) ceramics, it is still affected by the compositional and temperature dependence [107]. Polycrystalline phase transition (PPT) in KNN ceramics is considered to have the intrinsic drawback of poor temperature stability, and apart from the ionic substitution method shown in Figure 8a,b, which could improve the temperature stability, adjusting the crystal orientation can also lead to effective but improved temperature-stabilized strain characteristics of KNN-based piezoelectric ceramics [108,109]. Wang et al. [110] introduced V′_K/Na-V_O... defect dipole and constructed grain orientation to improve the strain performance and temperature stability of KNN-based piezoelectric ceramics. To prepare the woven ceramics, NN plate templates were synthesized by a two-step molten salt method, and the reaction equation for the template synthesis is as follows:

1.25Bi₂O₃ + 1.75Na₂CO₃ + 2.5Nb₂O₅ → BiNN₅ +1.75CO₂

(12)

BiNN₅+ 0.75Na₂CO₃ → 5NaNbO₃ + 1.25Bi₂O₃+ 0.75CO₂

(13)

This KNN-based ceramic exhibits predominantly orthorhombic symmetry at RT, with four equivalent domains at 45° angles in the [001] direction. Due to the V′_K/Na-V_O defect dipole synergies and good grain orientation, normalized monopolar and bipolar strains are higher than most lead-free piezoelectric ceramics and even some single crystals [111,112]. Extensive experimental characterization has shown that more equivalent domains and smaller angles improve the longitudinal piezoelectric response [113]. As shown in Figure 8d, Yu et al. [114] proposed a strategy that involved building multilayer composites by band casting to achieve temperature stability where the strain is insensitive to temperature and electric field. As shown in Figure 8c, by selecting components with different strain responses under electric field and temperature and utilizing the strain coupling and complementary effects of each component, superior temperature stability with almost no electric field dependence of strain can be achieved [114,115].

Figure 8. (a) Appropriately replace (Bi₀.₅Na₀.₅) ²⁺ MPC with Ba²⁺ at room temperature. Reprinted with permission from Ref. [107], Copyright 2020, ACS. (b) Schematic diagram showing the effects of the interreplacement between (Bi₀.₅Na₀.₅)²⁺ and Ba²⁺ on the ionicity and covalency of the A−O bond; variations of P_bs, ε_rP_r, and θ with x. Reprinted with permission from Ref. [107], Copyright 2020, ACS. (c) Temperature−dependent strain of the multilayer composite ceramic (I−II−III−IV) under different electric fields. ΔX of the multilayer composite ceramic (I−II−III−IV) and other KNN−based ceramics. Reprinted with permission from Ref. [112], Copyright 2020, ACS. (d) Design schematic diagrams of a multilayer composite ceramic. Reprinted with permission from Ref. [112], Copyright 2020, ACS. (e) Suni(T)/Suni(RT) of different ceramics under different electric fields at a high temperature. Reprinted with permission from Ref. [112], Copyright 2020, ACS.

Figure 8. (a) Appropriately replace (Bi₀.₅Na₀.₅) ²⁺ MPC with Ba²⁺ at room temperature. Reprinted with permission from Ref. [107], Copyright 2020, ACS. (b) Schematic diagram showing the effects of the interreplacement between (Bi₀.₅Na₀.₅)²⁺ and Ba²⁺ on the ionicity and covalency of the A−O bond; variations of P_bs, ε_rP_r, and θ with x. Reprinted with permission from Ref. [107], Copyright 2020, ACS. (c) Temperature−dependent strain of the multilayer composite ceramic (I−II−III−IV) under different electric fields. ΔX of the multilayer composite ceramic (I−II−III−IV) and other KNN−based ceramics. Reprinted with permission from Ref. [112], Copyright 2020, ACS. (d) Design schematic diagrams of a multilayer composite ceramic. Reprinted with permission from Ref. [112], Copyright 2020, ACS. (e) Suni(T)/Suni(RT) of different ceramics under different electric fields at a high temperature. Reprinted with permission from Ref. [112], Copyright 2020, ACS.

Template grain growth techniques have been successfully used to improve piezoelectric properties, and the development of heterostructured templates and the construction of novel piezoelectric composites are also helpful for enhancing piezoelectric properties [48,84,115]. Heterostructured BT (h−BT) templates are prepared by nucleating and growing undoped KNN nanoparticles on bare BT. h−BT can reduce the mechanical stress at the interface of the woven ceramics (h−BT−KNN), leading to larger grain growth and a higher degree of weaving than that of the woven ceramics (BT−KNN) prepared using bare BT. As shown in Figure 9a, Park et al. [84] used a BaTiO₃ (BT) template and Ba, Mn, and Bi-doped (K, Na) NbO₃ (BMBKNN) matrix powder to prepare heterostructured BT templates during the solvothermal reaction by heterogeneous nucleation growth of (K, Na) NbO₃ (KNN) nanoparticles deposited onto BT templates by heterogeneous nucleation (h-) BT, as shown in Figure 9a. The process flow is shown in Figure 9b, and three different samples—nonwoven random ceramics, BT-KNN, and h-BT-KNN woven ceramics—were prepared using h-BT, BT template, and the same KNN substrate powders as shown in Figure 9(aiii). h-BT-KNN ceramics, compared to the BT-KNN ceramics prepared using the BT template via the conventional TGG process, showed that the dielectric and piezoelectric properties of BT-KNN ceramics were further improved. In particular, the piezoelectric constant (d₃₃) in the woven h-BT-KNN ceramics was improved by about 170% compared to BT-KNN. Lin et al. [116] used a combination of the template grain growth (TGG) technique and composite design strategy to develop novel KNNTa- BNN piezoelectric composites. As shown in Figure 9c, Xu et al. [117] prepared 0.96(K₀.₄₄Na₀.₅₆) (Nb₀.₉₅Sb₀.₀₅) O₃-0.04Bi₀.₅(Na₀.₁₈K₀.₈₂)₀.₅ZrO₃ ceramics, with d₃₃ ≈ 420–490 pC/N, in 2014, using a conventional solid-state method. Higher piezoelectricity was subsequently achieved by constructing ternary ceramics in KNN-based (1−x−y) K_1−wNa_wNb_1−zSb_zO₃-yBaZrO₃-xBi₀.₅K₀.₅HfO₃ (KNNS-BZ-BKH) ternary ceramics using a combination of Sb⁵⁺, BaZrO₃, and Bi₀.₅K₀.₅HfO₃ as additives to transfer their TR-O and TO-T to room temperature or near room temperature. By adjusting the x, y, z and w values to design the composition of the materials, an R-T phase boundary can be constructed, which can realize the multiphase coexistence in ternary ceramics [118], and the experimental results show that d₃₃ reaches 570 ± 10 pC/N, which surpasses the results from the previous reports, and the piezoelectric properties are close to those of Pb-based ceramics.

5. Template Method and Ion Doping

(Bi₀.₅Na₀.₅) TiO₃ (BNT)-based lead-free piezoelectric ceramics have excellent electric field-induced strain (electrical strain) properties but tend to suffer from large hysteresis and poor fatigue resistance, which severely limits their applications. Zhou et al. [56] successfully used a combination of compositional and structural modulation using a slab NaNbO₃ (NN) template-induced grain orientation growth to synthesize the woven (1-x) (0.76Bi₀.₅Na₀.₅TiO₃-0.24SrTiO₃)-xNaNbO₃ (BNT-ST-xNN, x = 0.00–0.03). Doping Na⁺ at the A-site and Nb⁵⁺ at the B-site increases chemical disorder and enhances the random field, realizing BNT-based ceramics with a highly <001> woven configuration in the pure ER state. Plate-like NN templates were prepared by applying shear stress through strip casting, and a schematic of the strip casting process is shown in Figure 10a. The reaction equation for the synthesis of NN templates by the two-step lava method is as follows:

5Bi₂O₃ + 7Na₂CO₃ +10Nb₂O₅→4Bi₂.₅Na₃.₅Nb₅O₁₈ + 7CO₂↑

(14)

4Bi₂.₅Na₃.₅Nb₅O₁₈ + 3Na₂CO₃→ 20NaNbO₃ + 5Bi₂O₃ + 3CO₂↑

(15)

Figure 10b shows that the woven BNT-ST-0.03NN ceramics exhibit the best strain stability in the temperature range studied. This is mainly because the constituent BNT-ST-0.03NN deviates from the NR/ER phase boundary (located around the constituent BNT-ST-0.01NN), and temperature changes do not lead to significant changes in the phase structure. Second, despite the presence of ER states, the highly <001> crystal structure and more ferroelectric R⁻3cPNR can provide an additional driving force for the relaxation-ferroelectric transition and resist the effect of temperature [56]. As shown in Figure 10(cii), Lai et al. [119] used Bi₄Ti₃O₁₂ as a template and prepared Nb⁵⁺-doped 0.8(Bi₀.₅Na₀.₅₎ TiO₃-0.2(Bi₀.₅K₀.₅) TiO₃ with a high degree of weaving (~80%) using the reaction template grain growth (RTGG) method (0.8BNT-0.2BKT) ceramics. Using a combination of B-site donor doping and the RTGG method, the electrical strain performance was significantly improved and an ultra-high strain of 0.65% was obtained at 6 kV/mm, which was attributed to the fact that adding Nb⁵⁺ to the B-site not only reduces the concentration of the oxygen vacancies, but also forms a defective dipole with A-site vacancies, which would be favorable for the electrical strain response [120]. As shown in Figure 10(ci), after 105 cycles of unipolar electric field loading, the shape of the S-E curve did not change and the electrical strain performance did not decrease significantly, showing excellent fatigue resistance. Liu et al. [121] used a low-εT₃₃/ε₀ (~168) BT template to develop a weave with ultra-high power generation performance based on the TGG technology and excellent fatigue resistance of woven BCTZ ceramic piezoelectric energy harvester, as shown in Figure 10e, where the electron backscatter diffraction (EBSD) mapping image confirms the strong [001] optimal crystal orientation of the grains in the woven ceramic. The piezoelectric cantilever test system is shown in Figure 10d, where the BCTZ ceramic piezoelectric energy harvester maintains high robustness for up to 106.5 vibration cycles. This excellent mechanical durability is attributed to the use of grain-oriented ceramics, whose fatigue resistance is significantly improved. Figure 10f shows the output voltage and current of the randomly oriented and woven BCTZ energy harvester over time. The output voltage and current of the woven energy harvester are significantly improved.

Figure 10. (a) Schematic diagram of tape−casting process for preparation of textured ceramics. Reprinted with permission from Ref. [56], Copyright 2022, Journal of Advanced Ceramics. (b) S–E curves of the non-textured BNT–ST, non-textured BNT–ST–0.03NN, and textured BNT–ST–xNN(x = 0.01–0.03) ceramics measured within the temperature range of 25~100 °C. Reprinted with permission from Ref. [56], Copyright 2022, Journal of Advanced Ceramics. (c) (i) Unipolar S–E curves recorded at 1 Hz after 105 electric cycles, and unipolar S−E curves for temperatures in the range of 20–120 for textured ceramics. (ii) Cross−section SEM image of textured BNKT₁₅Nb ceramics. Reprinted with permission from Ref. [119], Copyright 2023, ACS. (d) Schematic illustration of the test system of a piezoelectric cantilever. Reprinted with permission from Ref. [121], Copyright 2018, ACS. (e) EBSD inverse pole figure maps and (001) pole figures of randomly oriented and textured BCTZ ceramics. Reprinted with permission from Ref. [121], Copyright 2018, ACS. (f) The variations in output voltage and current with time for randomly oriented and textured BCTZ energy harvesters under cyclic sine−wave loading at different accelerations. Reprinted with permission from Ref. [121], Copyright 2018, ACS.

Figure 10. (a) Schematic diagram of tape−casting process for preparation of textured ceramics. Reprinted with permission from Ref. [56], Copyright 2022, Journal of Advanced Ceramics. (b) S–E curves of the non-textured BNT–ST, non-textured BNT–ST–0.03NN, and textured BNT–ST–xNN(x = 0.01–0.03) ceramics measured within the temperature range of 25~100 °C. Reprinted with permission from Ref. [56], Copyright 2022, Journal of Advanced Ceramics. (c) (i) Unipolar S–E curves recorded at 1 Hz after 105 electric cycles, and unipolar S−E curves for temperatures in the range of 20–120 for textured ceramics. (ii) Cross−section SEM image of textured BNKT₁₅Nb ceramics. Reprinted with permission from Ref. [119], Copyright 2023, ACS. (d) Schematic illustration of the test system of a piezoelectric cantilever. Reprinted with permission from Ref. [121], Copyright 2018, ACS. (e) EBSD inverse pole figure maps and (001) pole figures of randomly oriented and textured BCTZ ceramics. Reprinted with permission from Ref. [121], Copyright 2018, ACS. (f) The variations in output voltage and current with time for randomly oriented and textured BCTZ energy harvesters under cyclic sine−wave loading at different accelerations. Reprinted with permission from Ref. [121], Copyright 2018, ACS.

Specific amounts of BaZrO₃ (BZ) modification also led to a significant improvement in the electromechanical properties of BNT ceramics [122,123]. As shown in Figure 11b, Hussain et al. [124] used plate templates with a single-phase chalcogenide structure synthesized by a conventional solid-state reaction (CSSR) method, and then used the RTGG method to prepare the woven 0.94Bi_1/2Na_1/2TiO₃−0.06BaZrO₃(BNT−BZ) ceramics, as shown in Figure 11a, where the woven samples exhibit a high level of lattice anisotropy/aberration compared to the nonwoven and composite BNT−BZ samples. This slight distortion (90o-αR = 0.031o) may originate from the lattice mismatch between cubic and rhombic symmetry, with an increase in the phase fraction (%) of the rhombic phase, which improves the piezoelectric properties of the woven ceramics [124]. The effect of plate-like BNT template particles on the structural phases and electromechanical properties of nonwoven and woven BNT−BZ ceramics is shown. Figure 11c shows the field-induced bipolar and unipolar strain responses of nonwoven, composite, and woven samples of BNT−BZ ceramics under an applied electric field of 6.0 kV/mm. The development of weaving plays an important role in improving the electromechanical properties of BNT−BZ ceramics compared to composites. Since BNT has a highly symmetric chalcogenide structure and usually grows isospinally, it is difficult to prepare BNT slabs by one-step molten salt synthesis. In this case, as shown in Figure 11g, Cha et al. [125] found that using plate-like BNT₁₅ or BiT precursors and incorporating the TMC method is an effective way to obtain BNT slabs, and the reaction equation is shown below:

Bi₄Ti₃O₁₂ + 3/4Na₂CO₃ → 3Bi₀.₅Na₀.₅TiO₃ + 5/4Bi₂O₃ + 3/4CO₂

(16)

5/4Bi₂O₃ + 5/4Na₂CO₃ + 5TiO₂ → 5Bi₀.₅Na₀.₅TiO₃ + 5/4CO₂

(17)

Texturized Bi₀.₅(Na₀.₈K₀.₂)₀.₅TiO₃ (BNKT) ceramics were prepared using plain BNT templates and BNT₁₅, respectively, as shown in Figure 11e. The best texturization effect was achieved by the addition of the 3 mol BNT₁₅ template as compared to the non-texturized and plain BNT templates, and the experiments revealed that the texturized BNT ceramics were synthesized by using the TMC method via RTGG. The lack of plate-like BNT particles in the texturized BNKT ceramics prepared by the process is due to the unavoidable presence of equiaxed BNT particles. Therefore, more BNT templates need to be added to the matrix to achieve better texturization. These results can be applied to the preparation of other BNT-based textured piezoelectric ceramics prepared by the RTGG process, such as BNT−BT, BNT−BZ, and BNKT−BT. As shown in Figure 11d, Gio et al. [126] used the conventional solid-phase reaction method and the template grain growth method to synthesize (1−x) [Bi₀.₅ (Na₀.₄ K₀.₁)TiO₃]−xBaTiO₃ (BNKT−BT) lead-free woven ceramics. Figure 11f shows the SEM and grain size distribution of BNKT−BT ceramics with different BaTiO₃ contents. It is easy to see that the addition of BaTiO₃ can improve the microstructure and the grain size increases with the increase in BaTiO₃ content. When the amount of BaTiO₃ added is greater than 0.02 moles, a large discrepancy between the experimental data and the theoretical calculations occurs. This is due to the fact that the diffusivity of solute atoms is usually different from that of most atoms, so the addition of BaTiO₃ beyond the solubility limit in BNKT ceramics leads to a solute drag effect and inhibits grain growth. In summary, the increase in BaTiO₃ content in BNKT ceramics may contribute to the improvement of ferroelectric properties, leading to a significant improvement in the energy storage performance of the materials used for dielectric capacitor applications.

Figure 11. (a) XRD peaks fitted with Retvield refinement for space group of R3c and Pm3 m for non−textured, textured BNT−BZ, composite ceramics, and lattice constants (aC, and aR), unit cell volume, and Rhombohedral distortions (90°–αo). Reprinted with permission from Ref. [124], Copyright 2023, Ceramics International. (b) Scanning electron micrographs showing cross−sections of the non−textured and textured BNT−BZ ceramics measured at room temperature. Reprinted with permission from Ref. [124], Copyright 2023, Ceramics International. (c) Room temperature field−induced strain curves of the non−textured, composite, and textured BNT−BZ ceramics’ bipolar field−induced strain; Reprinted with permission from Ref. [124], Copyright 2023, Ceramics International. (d) The process of fabrication technology for textured ceramics. Reprinted with permission from Ref. [126], Copyright 2021, Journal of Alloys and Compounds. (e) SEM of non−textured BNKT and BNKT textured using 3 mol of BNT₁₅, 12 mol of BNT, and 32 mol of BNT templates. Reprinted with permission from Ref. [125], Copyright 2018, Journal of the European Ceramic Society. (f) Typical SEM images of the BNKT−BT ceramics with different amounts of BaTiO₃ content. Reprinted with permission from Ref. [126], Copyright 2021, Journal of the European Ceramic Society. Journal of Alloys and Compounds. (g) Crystal structures of BT, BNT15, and BNT. Reprinted with permission from Ref. [125], Copyright 2018, Journal of the European Ceramic Society.

Figure 11. (a) XRD peaks fitted with Retvield refinement for space group of R3c and Pm3 m for non−textured, textured BNT−BZ, composite ceramics, and lattice constants (aC, and aR), unit cell volume, and Rhombohedral distortions (90°–αo). Reprinted with permission from Ref. [124], Copyright 2023, Ceramics International. (b) Scanning electron micrographs showing cross−sections of the non−textured and textured BNT−BZ ceramics measured at room temperature. Reprinted with permission from Ref. [124], Copyright 2023, Ceramics International. (c) Room temperature field−induced strain curves of the non−textured, composite, and textured BNT−BZ ceramics’ bipolar field−induced strain; Reprinted with permission from Ref. [124], Copyright 2023, Ceramics International. (d) The process of fabrication technology for textured ceramics. Reprinted with permission from Ref. [126], Copyright 2021, Journal of Alloys and Compounds. (e) SEM of non−textured BNKT and BNKT textured using 3 mol of BNT₁₅, 12 mol of BNT, and 32 mol of BNT templates. Reprinted with permission from Ref. [125], Copyright 2018, Journal of the European Ceramic Society. (f) Typical SEM images of the BNKT−BT ceramics with different amounts of BaTiO₃ content. Reprinted with permission from Ref. [126], Copyright 2021, Journal of the European Ceramic Society. Journal of Alloys and Compounds. (g) Crystal structures of BT, BNT15, and BNT. Reprinted with permission from Ref. [125], Copyright 2018, Journal of the European Ceramic Society.

6. Challenges and Prospects of Ceramic Fabrication Processes

This paper summarizes the progress of research on the piezoelectric ceramic-based weaving process and the factors that affect the performance of weaving ceramics. As more in-depth research has become available, researchers have gained a comprehensive understanding of woven high-performance piezoelectric ceramics and have rationalized the reaction mechanisms used to prepare woven ceramics through various characterization methods, and various high-performance piezoelectric ceramics have been prepared by the texture process. However, in the field of lead-free piezoelectric ceramics, including the existing KNN system, the piezoelectric performance has never been able to catch up with lead-based ceramics. The development of environmentally friendly piezoelectric ceramics with excellent performance remains an ongoing challenge.

Table 2 summarizes data from the literature pertaining to the performance parameters of piezoelectric ceramics prepared by the weaving process, from which we can easily ascertain that the performance of lead-free ceramics has never been able to exceed that of lead-based ceramics, especially with regard to piezoelectricity. Compared with the high-performance lead-free ceramics that have been developed previously, the performance of lead-based ceramics is up to three to four times higher. Regarding the degree of texture, most of the lead-based ceramics exhibit performance improvements of more than 95%, and there is a clear discrepancy between the performances of lead-free ceramics and lead-based ceramics. The performance of the lead-free ceramics is not as good as that of the lead-based ceramics. Regarding the Curie temperature of lead-free ceramics and lead-based ceramics, their performances are fairly equal, and in a few cases, the Curie temperature of lead-free ceramics has exceeded that of lead-based ceramics. When it comes to woven ceramics, follow-up research can consider the following topics:

Material improvement: Continue to improve the performance of lead-free piezoelectric ceramics, including their piezoelectric properties, stability and reliability. This may involve the study of new ceramic material synthesis and processing methods.

Preparation process optimization: Optimize the preparation process to improve efficiency and reduce the cost of material preparation. This may include optimization of the sintering temperature and time, and the material formulation.

Expansion of application areas: Study the potential applications of lead-free piezoelectric ceramics in different fields, such as sensor technology, medical devices, acoustic devices, and communication devices.

Environmental impact assessment: Evaluating the environmental impact of the production and use of lead-free piezoelectric ceramics to ensure that they are truly environmentally friendly.

Table 2. Piezoelectric ceramic texturing process and performance parameters.

	Textured Materials	Lotgering Factor	Tc (°C)	d33 (pC/N)	kp	Refs.
	KNN KNLN-BZ-BNT	-	-	319	42%	[27]
TGG	BCZT	94%	120	-	-	[47]
TGG	KNN-CZ-xBKH	-	256	550	72%	[55]
HPS	KNN	-	396	151	-	[60]
RTGG	NK(NS)— 0.02BZ—0.02BAZ	97%	178	805	0.68	[62]
TGG	0.19PIN–0.445PSN–0.365PT	99%	247	1090	0.83	[63]
TGG	0. 16PYN-0.52PMN-0.32 PT	99%	205	1087	0.9	[64]
RTGG	KNN	96%	162	535	0.57	[65]
TGG	PIN–PMN–PT	95%	225	780	0.59	[69]
RTGG	BT	84.6%	126	788	-	[72]
	PIN-PHT-xSb	-	300	706	0.68	[74]
	PIN-PHT	-	360	490	0.56
	PIN-PHT-CuF2	-	348	488	0.60
	PIN-PMN-PT-Mn	-	209	347	0.57
TGG	KNN-BCZT	74%	388	190	0.54	[82]
TGG	BT-KNN	91%	390	180	-	[84]
TGG	PMN-PT	95%	158	770	-	[85]
TGG	KNNS-BxBAZ		274	406	0.63	[86]
TGG	KNN-XNN	94%	240	590	0.8	[87]
TGG	KNN	-	276	380	0.54	[88]
TGG	BNT–BKT–BT	73%	-	-	-	[89]
TGG	0.42PNN-0.21PZ-0.37PT	-	167	910	0.74	[92]
TGG	PNN-PZT	93%	103	1210		[93]
	LKNNS-EuNZ-0.03BZ	98%	-	331	0.53	[94]
	NBT-SBT	91%	-	-	-	[95]
	PMN-PT	97%	-	800	-	[99]
	PMN-PT	-	-	1950	-	[101]
	0.42PMN–0.26PIN–0.32PT	-	180	426	-	[105]
RTGG	KNLN-BZ-BNT	90%	247	319	0.48	[106]
	KNNS-BNBZ	-	-	505	-	[107]
TGG	KNN-CZ	89.7%	192	330	0.46	[112]
	KNNTa–BNN	94%	360	435	0.71	[116]
RTGG	KNLN-BZ-BNT-Mn	-	205	292	0.47	[117]
RTGG	KNLN-BZ-BNT-Mn	66%	-	320	-	[118]
RTGG	0.8BNT–0.2BKT	80%	-	-	-	[119]
TGG	BCTZ	99%	-	728	-	[121]

Table 2. Piezoelectric ceramic texturing process and performance parameters.

	Textured Materials	Lotgering Factor	Tc (°C)	d33 (pC/N)	kp	Refs.
	KNN KNLN-BZ-BNT	-	-	319	42%	[27]
TGG	BCZT	94%	120	-	-	[47]
TGG	KNN-CZ-xBKH	-	256	550	72%	[55]
HPS	KNN	-	396	151	-	[60]
RTGG	NK(NS)— 0.02BZ—0.02BAZ	97%	178	805	0.68	[62]
TGG	0.19PIN–0.445PSN–0.365PT	99%	247	1090	0.83	[63]
TGG	0. 16PYN-0.52PMN-0.32 PT	99%	205	1087	0.9	[64]
RTGG	KNN	96%	162	535	0.57	[65]
TGG	PIN–PMN–PT	95%	225	780	0.59	[69]
RTGG	BT	84.6%	126	788	-	[72]
	PIN-PHT-xSb	-	300	706	0.68	[74]
	PIN-PHT	-	360	490	0.56
	PIN-PHT-CuF2	-	348	488	0.60
	PIN-PMN-PT-Mn	-	209	347	0.57
TGG	KNN-BCZT	74%	388	190	0.54	[82]
TGG	BT-KNN	91%	390	180	-	[84]
TGG	PMN-PT	95%	158	770	-	[85]
TGG	KNNS-BxBAZ		274	406	0.63	[86]
TGG	KNN-XNN	94%	240	590	0.8	[87]
TGG	KNN	-	276	380	0.54	[88]
TGG	BNT–BKT–BT	73%	-	-	-	[89]
TGG	0.42PNN-0.21PZ-0.37PT	-	167	910	0.74	[92]
TGG	PNN-PZT	93%	103	1210		[93]
	LKNNS-EuNZ-0.03BZ	98%	-	331	0.53	[94]
	NBT-SBT	91%	-	-	-	[95]
	PMN-PT	97%	-	800	-	[99]
	PMN-PT	-	-	1950	-	[101]
	0.42PMN–0.26PIN–0.32PT	-	180	426	-	[105]
RTGG	KNLN-BZ-BNT	90%	247	319	0.48	[106]
	KNNS-BNBZ	-	-	505	-	[107]
TGG	KNN-CZ	89.7%	192	330	0.46	[112]
	KNNTa–BNN	94%	360	435	0.71	[116]
RTGG	KNLN-BZ-BNT-Mn	-	205	292	0.47	[117]
RTGG	KNLN-BZ-BNT-Mn	66%	-	320	-	[118]
RTGG	0.8BNT–0.2BKT	80%	-	-	-	[119]
TGG	BCTZ	99%	-	728	-	[121]

7. Conclusions

This paper summarizes various approaches for achieving high-performance piezoelectric ceramics based on the piezoelectric ceramic weaving process, preparation methods, and conditions. The main points are as follows:

Preparation consistency: Ensuring that the properties and quality of mass-produced woven ceramics remain consistent is challenging. Small variations in process parameters can lead to instability in terms of product performance.

Control of material composition: The composition of woven ceramics may affect their properties, including their mechanical strength, thermal stability, and electrical characteristics. Achieving precise control of material composition is an important issue.

Molding method: Choosing the appropriate molding method to obtain the desired textile structure is critical. Different molding methods may require different process parameters and equipment.

Sintering conditions: The sintering process is critical to the final properties of the woven ceramics. Controlling the sintering temperature and time and the atmosphere is key to the preparation process.

Interfacial treatments: Woven ceramics often contain multiple materials or layers, so interfacial treatments need to be precisely controlled to ensure good bonding and performance.