M. Abazari

Origin of high piezoelectric activity in ferroelectric (K0.44Na0.52Li0.04)−(Nb0.84Ta0.1Sb0.06)O3 ceramics

We study the temperature dependence of dielectric constant (K) and spontaneous polarization (Ps) in the range of −95–200°C. Cubic (C)-tetragonal (T) and T-orthorhombic (O) transitions are observed at 264 and 25°C, respectively. The Curie–Weiss temperature of C-T transition is 249°C, indicating it is first order. X-ray data indicate T-O phase coexistence at 25°C. A singularity in Ps at 25°C and a T-O phase coexistence spanning 25–31°C was observed, wherein Ps increases from 17×10−2C∕m2 at 31°Cto23×10−2C∕m2 25°C. The transition at 25°C appears diffusionless and polymorphic with martensite start and finish temperatures of 31 and 25°C, respectively. The maximum in d33 is 345pC∕N and is attributed to the instability at 25°C, where Ps and K show singularity.

Dielectric and piezoelectric properties of lead-free (Bi,Na)TiO3-based thin films

Dielectric and piezoelectric properties of morphotropic phase boundary (Bi,Na)TiO3–(Bi,K)TiO3–BaTiO3 epitaxial thin films deposited on SrRuO3 coated SrTiO3 substrates were reported. Thin films of 350 nm thickness exhibited small signal dielectric permittivity and loss tangent values of 750 and 0.15, respectively, at 1 kHz. Ferroelectric hysteresis measurements indicated a remanent polarization value of 30 μC/cm2 with a coercive field of 85–100 kV/cm. The thin film transverse piezoelectric coefficient (e31,f) of these films after poling at 600 kV/cm was found to be −2.2 C/m2. The results indicate that these BNT-based thin films are a potential candidate for lead-free piezoelectric devices.

Effect of manganese doping on remnant polarization and leakage current in (K0.44,Na0.52,Li0.04)(Nb0.84,Ta0.10,Sb0.06)O3 epitaxial thin films on SrTiO3

Single phase, epitaxial, ⟨001⟩ oriented, undoped and 1mol% Mn-doped (K0.44,Na0.52,Li0.04)(Nb0.84,Ta0.10,Sb0.06)O3 thin films of 400nm thickness were synthesized on SrRuO3 coated SrTiO3. Such films exhibit well saturated hysteresis loops and have a spontaneous polarization (Ps) of 10μC∕cm2, which is a 150% higher over the Ps of the undoped composition. The coercive field of 1mol% Mn doped films is 13kV∕cm. Mn-doping results in three orders of magnitude decrease in leakage current above 50kV∕cm electric field, which we attribute to the suppression of intrinsic p-type conductivity of undoped films by Mn donors.

Lead-free piezoelectric ceramics and thin films

Recent progress in lead-free piezoelectric ceramics and thin films with special emphasis on alkaline niobate-based and bismuth sodium titanate-based systems is reviewed concisely. Modifications of potassium sodium niobate (KNN) ceramics are presented and subsequent improvements in the electrical properties are summarized. Special attention is devoted to the phase diagram of the KNN system when a solid solution is formed with other perovskite niobates and titanates. Impact of A-site and B-site dopants on the electromechanical properties of KNN ceramics are distinguished in view of transition temperatures. It is shown that the addition of most A-site and B-site dopants reduces the transition temperatures and improves the piezoactivity at room temperature. This is attributed to the shift of polymorphic transition from tetragonal to orthorhombic phase in the vicinity of room temperature. In contrast, formation of a solid solution of KNN with 18 mol% AgNbO3 revealed a significant enhancement of properties without a notable change in the transition temperatures. Also, a bismuth sodium titanate (BNT) composition is introduced with particular emphasis on its binary and ternary derivatives. Moderate piezoelectric properties reported at the morphotropic phase boundaries, formed in BNT-based solid solutions are also represented. Advances on thin films based on these two compositions are evaluated and challenges involved with development of stoichiometric thin films with low leakage current are discussed.

Leakage current behavior in lead-free ferroelectric (K,Na)NbO3-LiTaO3-LiSbO3 thin films

Conduction mechanisms in epitaxial (001)-oriented pure and 1 mol % Mn-doped (K0.44,Na0.52,Li0.04)(Nb0.84,Ta0.1,Sb0.06)O3 (KNN-LT-LS) thin films on SrTiO3 substrate were investigated. Temperature dependence of leakage current density was measured as a function of applied electric field in the range of 200–380 K. It was shown that the different transport mechanisms dominate in pure and Mn-doped thin films. In pure (KNN-LT-LS) thin films, Poole-Frenkel emission was found to be responsible for the leakage, while Schottky emission was the dominant mechanism in Mn-doped thin films at higher electric fields. This is a remarkable yet clear indication of effect of 1 mol % Mn on the resistive behavior of such thin films.

(K 0.44 ,Na 0.52 ,Li 0.04 ) (Nb 0.84 ,Ta 0.10 ,Sb 0.06 )O 3 Ferroelectric Ceramics

Recent progress in (K0.44,Na0.52,Li0.04)O3-based ceramics (KNN) with special emphasis on (K0.44, Na0.52, Li0.04 (Nb0.84,Ta0.10,Sb0.06)O3 (KNN-LT-LS) is reviewed concisely. The base KNN and its compositional derivatives are analyzed in terms of dopant-property relationships, which are then extended to the ternary derivatives. The effects of processing conditions such as humidity, precursor purity, and oxygen partial pressure during sintering are elaborated on from a phenomenological perspective. It is also shown that the spontaneous polarization is sensitive to the processing route chosen for synthesis (mixed oxide versus perovskite routes). Special attention is devoted to the discussion of the morphotropic phase boundary (MPB) dilemma in the KNN-LT-LS system, where it is shown that the origin of high piezoelectric activity is actually due to a polymorphic transition at room temperature. It is shown that prototype transducers based on pure and 1 mol% Ba2+ doped KNN-LT-LS exhibit performance metrics comparable to those fabricated using PZT-5H. Overall, KNNLT-LS ceramics show great promise for lead-free applications, although issues such as temperature dependence of properties and strong sensitivity to processing conditions remain as the 2 major challenges.

Lead free (K,Na)NbO3-based piezoelectric ceramics and transducers

Recent advances in (K,Na)NbO3-based ferroelectrics is reviewed, with special emphasis on the (K,Na)NbO3-LiTaO3-LiSbO3 system. Processing issues are elaborated on. The importance of precursor purity, moisture sensitivity and oxygen content on the dielectric and piezoelectric properties will be shown. Properties of prototype transducers are given and it will be shown that high performance metrics reminiscent to PZT-4 based transducers can be reached. Origins of high piezoelectric activity in the (K0.5Na0.5Li0.04)(Nb0.86Ta0.04Sb0.06)O3 is discussed, where it will be shown that this particular composition has a polymorphic phase transition in the vicinity of room temperature, and does not exhibit a morphotropic phase boundary..

KNaNbO3-LiTaO3-LiSbO3 thin films by pulsed laser deposition

<001> oriented epitaxial thin films of (K,Na)NbO3-LiTaO3-LiSbO3 (KNN-LT-LS) were grown by Pulsed Laser Deposition on <100> single crystal SrTiO3 (STO) substrates, which were coated with 200 nm SrRuO3. Experimental data based on X-ray phase analysis and elemental analysis by Rutherford backscattering spectroscopy indicated that deposition under 300 mTorr oxygen partial pressure, and 1.3 J/cm2 laser fluence were found to be the optimum condition to obtain single phase epitaxial film growth. Furthermore, nanostructural analysis by field emission SEM of films, grown to 400nm thickness at 600¿750°C indicated a gradual transition from (001) textured columnar grains to a dense and crack free film. Using XRD ¿ scans, these films were found to be epitaxial with a cube-on-cube relationship with substrate. Dielectric measurements (1 kHz-1 MHz) at room temperature have revealed that the dielectric constant (K) and loss tangent (tan¿) decreased with increasing deposition temperature. Hysteresis loop measurements on 350 nm films showed saturation and spontaneous polarizations of 7.1 and 4 ¿C/cm2; respectively.

KNaNbO 3 -LiTaO 3 -LiSbO 3 Thin Films by Pulsed Laser Deposition

Morphotropic phase boundary composition KNaNbO3-LiTaO3-LiSbO3 (KNN-LT-LS) thin films were grown by pulsed laser deposition on niobium doped conductive SrTiO3 substrates. Using X-ray diffractometry, the phase evolution, stoichiometry, and growth orientation were monitored for various deposition conditions including temperature and ambient oxygen partial pressure. The results indicate that the scattering of the cations such as potassium, sodium, and lithium by the oxygen molecules is a very critical factor in the formation of the secondary phases in the films. We report thin films with a good crystallinity grown at 650degC and in ambient oxygen partial pressure. On 1 micron films, a longitudinal relative permittivity of 500, along with a loss tangent of 0.5% at 1 MHz has been obtained.