Outstanding Piezoelectric Performance in Lead‐Free 0.95(K,Na)(Sb,Nb)O₃‐0.05(Bi,Na,K)ZrO₃ Thick Films with Oriented Nanophase Coexistence

Abstract

human health concerns, thus increasing efforts are made to develop high performance lead-free piezoelectric thin films as replacements. [6][7][8][9][10][11][12][13][14][15][16] Among lead-free piezoelectric ceramics, potassium sodium niobate, (K, Na)NbO 3 (KNN), is one of the most promising candidates with competitive piezoelectric properties over a broad temperature range, as obtained in KNN-based bulk ceramics by constructing phase coexistence with complex chemical compositions. [17][18][19][20][21][22] Compared to the bulk counterparts, it is much more challenging to achieve excellent piezoelectric properties in KNN-based thin films, because of the difficulties in controlling composition and phase coexistence with much more severe volatility issues, and more complicated stress and thickness effects. [6,13,16,[23][24][25] Our previous work provided an important technical solution to the above problems and achieved a very high effective piezoelectric strain coefficient, 33 eff d of 184 pm V −1 in solution-derived KNN-based thin films, with a complex leadfree composition of 0.95(K, Na)(Sb, Nb)O 3 -0.05(Bi, Na, K)ZrO 3 (KNSN-BNKZ0.05). [16] However, there was still a gap between this value and the highest 33 eff d (200-340 pm V −1 ) as reported in [100]-oriented polycrystalline PZT thin films on silicon. [26] The multiphase coexistence structure was not observed directly, although numerical simulation and X-ray diffraction results indicated an overall presence of different phases in the films. This work aims to demonstrate outstanding piezoelectric Lead-free 0.95(K 0.48 Na 0.52 )(Nb 0.95 Sb 0.05 )O 3 -0.05Bi 0.5 (Na 0.82 K 0.18 ) 0.5 ZrO 3 (KNSN-BNKZ0.05) piezoelectric films with preferred crystal orientation and enhanced thickness are fabricated on silicon substrates from a chemical solution approach. Adequate K excess is introduced to obtain a single perovskite phase in the resulting thicker films. The effects of thickness, crystal orientation, and structure of the films on the performance are investigated. Outstandingly large effective piezoelectric strain coefficient up to 250 pm V −1 is demonstrated over a macroscopic scale using a laser scanning vibrometer in the [100]-KNSN-BNKZ0.05 film with an enhanced thickness of 2.7 µm, competitive to the benchmark oriented lead zirconate titanate films on silicon. Atomically resolved electron microscopy reveals the coexistence of oriented ferroelectric rhombohedral (R) and tetragonal (T) phases at the nanometer scale with gradual polarization rotation, which can lower the domain wall energy and facilitate the large piezoelectric response. The increased film thickness reduces the in-plane mechanical clamping to enable more free deformation in the thickness direction and improve domain wall mobility, both further contributing to enhanced piezoelectric response.

Emerging new phase boundary in potassium sodium-niobate based ceramics

“…Then, they further prepared the h100i-oriented KNNS-xBNKZ thin films. 231 The results showed that h100i-oriented KNNS-0.05BNKZ ceramics exhibited a higher d eff 33 value of 250 pm V À1 , which was the highest value in lead-free piezoelectric thin films and was even higher than that of the h100i-oriented PZT film (see Fig. 36(d)).…”

“…Encouragingly, the NPB, which was firstly proposed in KNNbased bulk ceramics, is also suitable for KNN-based thin films. 230,231 It is well-known that KNNS-xBNKZ ceramics are the typical materials for constructing the NPB. 44 Yao et al fabricated the KNNS-xBNKZ thin films by using the solution deposition method and measured their effective piezoelectric strain coefficient (d eff 33 )…”

Alkali-deficiency driven charged out-of-phase boundaries for giant electromechanical response

“…The main strategy has been to construct multiphase boundaries by tuning the chemical compositions so that nanoscale domains with local structural and polar heterogeneity are formed 16 – 19 . As piezoelectric devices continue to be miniaturized, thin films with high piezoelectric response are in demand for micro- and nano-electromechanical systems (MEMS and NMES) applications 20 . In contrast to the rapid progress in lead-free bulk ceramics, the development of lead-free piezoelectric films lags far behind.…”

Piezoelectric Materials for Controlling Electro-Chemical Processes

“…as a green initiative [134][135][136][137], and due to the need for environmental protection and socially sustainable materials, research related to alternative lead-free materials has become a significant research activity in recent decades [130,138,139]. The general piezoelectric/ferroelectric perovskites (ABO 3 -type) [140,141], in particular barium titanate [142][143][144][145] and bismuth ferrite [146][147][148][149], have been utilized to process functional advanced devices [150][151][152][153].…”

“…However, these lead-based piezoelectric materials contain the element Pb, which is one of the major heavy metal pollutants and can endanger human health [ 130 – 133 ]. Lead-free piezoelectric materials are being examined intensively as a green initiative [ 134 – 137 ], and due to the need for environmental protection and socially sustainable materials, research related to alternative lead-free materials has become a significant research activity in recent decades [ 130 , 138 , 139 ]. The general piezoelectric/ferroelectric perovskites (ABO 3 -type) [ 140 , 141 ], in particular barium titanate [ 142 – 145 ] and bismuth ferrite [ 146 – 149 ], have been utilized to process functional advanced devices [ 150 – 153 ].…”