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Mar 31, 2023Nature Communications 13권, 기사 번호: 3565(2022) 이 기사 인용
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압전 재료의 전기기계적 결합 계수 k는 기계 에너지를 전기 에너지로, 전기 에너지를 기계 에너지로 변환하는 효율을 결정합니다. 여기에서는 입자 방향 또는 질감이 있는 세라믹 제조를 통해 전기결정질 이방성을 활용하여 거의 이상적인 크기의 k를 제공하는 압전 재료를 설계하는 근본적인 접근 방식을 제공합니다. <001> 조직화된 Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 세라믹에 대한 결합 위상 필드 시뮬레이션 및 실험적 조사는 k가 단결정의 경우와 동일한 크기에 도달할 수 있음을 보여줍니다. 전통 도자기. 우리의 접근법에 대한 원자 규모의 이해를 제공하기 위해 우리는 페로브스카이트 강유전체에서 k의 물리적 기원을 결정하는 이론적 모델을 사용하고 dp 혼성화를 통한 B-사이트 양이온과 산소 사이의 강한 공유 결합이 k의 크기에 가장 크게 기여한다는 것을 발견했습니다. 질감이 있는 세라믹에서 거의 이상적인 k 값을 입증하는 것은 초광대역폭, 고효율, 고출력 밀도 및 안정성이 높은 압전 장치의 설계에 엄청난 영향을 미칠 것입니다.
압전 재료는 전기 에너지와 기계 에너지 사이의 전기 기계적 변환을 가능하게 하며 그 반대도 가능합니다. 이는 센서, 액추에이터, 변환기, 이미징 장치 및 에너지 수확기1,2에 널리 활용됩니다. 전기기계적 결합 계수 k는 전기 에너지와 기계 에너지 사이 또는 그 반대로 변환을 제공하는 압전 재료의 효율성을 정량화합니다. 매개변수 k2는 입력 전기 에너지에 대한 저장된 기계 에너지의 비율, 또는 입력 기계 에너지에 대한 저장된 전기 에너지의 비율을 반영합니다. k2 = 저장된 기계 에너지/입력 전기 에너지, 또는 k2 = 저장된 전기 에너지/ 입력 기계적 에너지3. k가 높은 압전 재료는 높은 최대 달성 가능 대역폭과 최대 분말 밀도를 고효율로 제공하므로 k는 압전 변환 장치의 가장 중요한 매개변수 중 하나입니다3.
또 다른 특별한 이점으로, 높은 k를 갖는 압전 재료의 설계는 다음 관계식에 따라 압전 계수 d를 증가시키는 대체 접근 방식을 제공할 수 있습니다. \(d=k\sqrt{s\cdot \varepsilon }\), 여기서 s는 다음과 같습니다. 탄성 컴플라이언스이고 ε은 유전율3입니다. 이 접근 방식은 구성 디자인을 통해 기존 접근 방식의 여러 병목 현상을 극복합니다. 전통적인 접근 방식을 사용하면 d = 2QPsε 식을 기반으로 유전 유전율 ε을 증가시켜 압전 응답 d를 향상시킬 수 있습니다. 여기서 Ps는 자발 분극, Q는 전기 변형 계수, ε은 유전 유전율입니다. ε의 증가는 특히 조성 유도 다상 공존(예: 이형성 상 경계 및 다형성 상전이)을 설계하여 분극에 대한 자유 에너지 환경을 평탄화(강유전성 분극 회전에 대한 에너지 장벽을 낮춤)함으로써 실현됩니다. 5,6 또는 국소 구성 구조 엔지니어링(예: 나노규모 단거리 정렬7 및 국소 구조 이질성8,9). 이 전통적인 접근 방식과 관련된 병목 현상은 다음과 같습니다. (1) 향상된 압전 특성 d는 온도 안정성(낮은 탈분극 온도 Td 또는 낮은 퀴리 온도 Tc)을 희생하여 얻어지며 다음과 같은 경향을 따릅니다. d ∝ 1/T1, 10,11; (2) ε의 증가는 압전 전압 계수 g를 감소시켜 압전 센서로서 감도가 감소합니다. (3) 이 접근법은 d·g13을 특징으로 하는 압전재료의 에너지 밀도를 향상시키는 데 효과적이지 않다. 예를 들어, Sm 도핑된 Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT로 약칭) 랜덤 세라믹의 d33이 ε33을 13,000으로 증가시켜 1500pC N−1에 도달할 수 있더라도 d33·g33은 19.8 × 10−12 m2 N−1로 제한되며, 이는 질감 있는 Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3(PMN-PZT로 약칭) 세라믹 값의 1/3에 불과합니다9,13 ; (4) d의 증가는 위에서 언급한 압전소자의 가장 중요한 요소 중 하나인 k의 큰 증가를 가져올 수 없다. (5) 자유 에너지 지형을 평탄화하여 유전율 ε을 높이면 일반적으로 보자력장 Ec가 감소합니다. 이는 전기장 안정성(디폴링)을 약화시키고 고전력 응용 분야에서 재료의 사용을 제한합니다. 여기서 우리는 높은 k를 갖는 압전 재료를 설계함으로써 이러한 과제를 극복할 수 있음을 보여줍니다.
oriented textured piezoelectrics can have higher k than those of their random counterparts13,18,19. This provides us initial direction towards addressing several questions: what is the maximum value of k that can be achieved in textured ceramic and is it possible to obtain the same or even higher k in textured ceramics than the values in their single-crystal counterparts? Does the neighboring grain correlation in textured ceramic limit the maximum achievable k? Does this approach of increasing k by microstructure texturing show the advantages over the traditional approach of increasing dielectric permittivity ε by composition design as mentioned above? In order to answer these questions, we carried out phase-field simulation to investigate the effects of crystallographic orientation and grain boundary in textured ceramics on the electromechanical coupling factor k. Results from the simulation were experimentally verified to confirm the increase in k in highly <001> textured PMN-PZT ceramics. A theoretical model is developed to gain an understanding of the physical origin of electromechanical coupling in perovskite ferroelectrics and determine the key correlations./p> 0.9) was observed for relaxor-PT single crystals, and the magnitude of k is orientation-dependent20. Figure 1c shows the simulated k for the rhombohedral PMN-PT single crystal under the electric field with various orientations. The corresponding simulated permittivity εT and εS are shown in Supplementary Fig. 1. Based on the microscopic model, the magnitude of k depends on the competition between chemical energy and elastic energy. Since both energies are polarization-dependent and anisotropic, k must be also anisotropic and thus depend on the electric field orientation. Because of the strong strain constraint, the change of εS is almost negligible in comparison with that of εT. Therefore, the anisotropic behavior of k will be mainly determined by εT. Since the polarization of PMN-PT behaves as a rotator rather than an extender21, the permittivity and thus k will be higher if the angle between electric field and polarization is larger. As shown in Fig. 1c, the highest k occurs when the electric field is perpendicular to polarization, while the lowest k occurs when the electric field is parallel with polarization, which suggests that k15 mode possesses the largest value of k in rhombohedral relaxor-PT single crystals22. However, upon electrical poling, only the values of k with θ ∈ [0,90°] are allowed, indicated by the solid blue line in Fig. 1c. Among those values, the highest k occurs when the electric field is along [001] direction, i.e., [001]-poled single crystal can exhibit the highest k33. In polycrystal ceramics, however, since the grain orientations are randomly distributed, the highest k33 can't be obtained as in single crystals. Nevertheless, the [001]-textured polycrystal can help to realize the highest k33 in ceramics. For a [001]-textured polycrystal, all grains are oriented in [001] crystallographic axis while the other crystallographic axes are completely random. Even though the [001]-textured polycrystal can possess the highest k33 in ceramics, can it be as large as that of a single crystal?/p>-textured ceramic can possess a large k that is comparable to the value of the single crystal, highly <001>-textured PMN-PZT ceramic was prepared by templated grain growth technique with a different volume percentage of BaTiO3 templates. Figure 3a shows the X-ray diffraction patterns for random PMN-PZT ceramic with 0 vol% BaTiO3 templates and textured PMN-PZT ceramic with 3 vol% BaTiO3 templates, respectively. Both samples exhibit a perovskite phase, while textured ceramic shows a remarkable enhancement in the intensities of the {001} diffraction peaks compared to random ceramic. The Lotgering factor of the textured sample is over 98%, indicating a strong [001] preferred grain orientation. Figure 3b shows the grain orientation of random and textured PMN-PZT ceramics via inverse pole figure (IPF) maps measured by the SEM-EBSD technique along thickness (Z) direction (The IPF-X and IPF-Y maps are shown in Supplementary Fig. 3). In order to evaluate the electromechanical coupling, longitudinal 33 mode and transverse 31 mode samples with dimensions according to IEEE standards were prepared, and their impedance spectra are shown in Fig. 3c, d. The k33 and k31 of <001> textured PMN-PZT are surprisingly as high as 0.93 and 0.65, respectively. Figure 3e lists the k33 of representative [001] oriented single crystals and polycrystalline random ceramics. The k33 of <001> single crystals are in the range of 0.90–0.94, while the k33 of random ceramics are limited to below 0.80. The k33 of <001> textured PMN-PZT is the same as the value of PMN-PZT single crystal counterpart. Figure 3f lists the k31 of representative [001] oriented single crystals and polycrystalline random ceramics. In general, the k31 of random ceramics are in the range of 0.30–0.40. The k31 and k31(45o) of [001] oriented single crystals are about 0.43 and 0.80, respectively. Here k31(45o) is the k31 of [001] oriented single crystal sample with 45o cut, where the orientations of the sides are [110], [\(\bar{1}10\)] and [001], respectively. The k31 in textured PMN-PZT ceramic is 0.65, which is slightly higher than the average value of k31 and k31(45o) of [001] oriented single crystals. This characteristic is due to the distribution of grain orientation in <001> textured ceramics, where the [001]-orientation of grains in textured samples are well aligned along the thickness direction (z, out of casting plane) but the [100] and [010] orientations of grains in textured samples are randomly distributed in the casting plane, which is related to the fact that unidirectional shear force was used for aligning the templates. Based on these observations, it can be suggested that the k, either k31 or k33, in textured ceramic is solely dependent on the texture direction and texture degree, and is not limited by the existence of the grain boundary. The <001>-textured ceramic can possess a large k33 that is comparable to those of the <001>-oriented single crystals, which verifies the prediction in Figs. 1 and 2./p> textured PMN-PZT ceramics. c Impedance spectra of <001> textured PMN-PZT ceramics in longitudinal 33 mode, the electromechanical coupling factor, k33, is as high as 0.93. d Impedance spectra of <001> textured PMN-PZT ceramics in transverse 31 mode, the electromechanical coupling factor, k31, is as high as 0.65. e Comparison of k33 among [001] oriented single crystals, random ceramics and <001> textured ceramics. f Comparison of k31 among [001] oriented single crystals, random ceramics and <001> textured ceramics. Here k31(45o) is the k31 of [001] oriented single crystal sample with 45o cut, where the orientations of the sides are [110], [\(\bar{1}10\)] and [001], respectively. The references for k33 and k31 data for single crystals and ceramics in e, f can be found in Supplementary Note 1./p>-textured ceramics suggests that they may possess similar domain configurations. It is well known that the ability of domains to switch in ferroelectric polycrystals depends critically on the crystallographic symmetry of the ferroelectric phase23. The domains in random polycrystal ceramics that are either tetragonal or rhombohedral are difficult to switch due to the constrain by the differently oriented neighboring grains. <001> texture is requisite for non-180o domain switching in tetragonal phase23. Figure 4 shows the in-situ electric field XRD patterns of <001>-textured PMN-PZT-3BT ceramics. It can be observed that the unpoled textured sample has MPB composition with the coexistence of rhombohedral and tetragonal phases (Fig. 4a), characterized by peak splitting near 44o. With increasing the electric field (1st up), the high angle peak (a-axis) is merged into the low angle peak (c-axis) and becomes a single peak, indicating that the a-domains in the tetragonal phase can be fully switched to c-domains. This new domain structure is very stable during the removal of the electric field (1st down, Fig. 4b) and application of electric field (2nd up, Fig. 4c). Based on these observations, it can be suggested that the non-180o domains in <001> textured ceramics are switchable, further confirming that <001>-textured ceramics could have the same electromechanical coupling factor k as [001]-oriented single crystal./p>0.9) prefers the strong covalent bonding effect rather than the ionic effect. These findings suggest an optimal condition for perovskite ferroelectrics that simultaneously possess a large permittivity, large piezoelectricity, and a high electromechanical coupling factor: strong B-O covalent bonding and close to the phase transition boundary. Similar studies based on experimental measurements to analyze the effects of A-site or B-site ions on the electromechanical coupling factor have been reported by Yamashita27,28,29,30./p> textured PMN-PZT is measured to be 0.93, which closes the gap between the ceramic and single crystal. In addition, increasing k via texturing provides an alternative approach of increasing the piezoelectricity, which overcomes the bottlenecks of the traditional approach via composition design. Further, we employed a theoretical model to understand the physical origin of k in perovskite ferroelectrics and found that strong covalent bonding between B-cation and oxygen via d-p hybridization contributes most to k. These findings provide a novel design strategy to develop the next generation of high-performance piezoelectric materials with ultrahigh piezoelectricity and low cost, to fulfill the demands for ultra-wide bandwidth, high efficiency, high power density, and high stability piezoelectric devices./p>
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