Ebru Mensur Alkoy

Effects of Ce, Cr and Er Doping and Annealing Conditions on the Microstructural Features and Electrical Properties of PbZrO3 Thin Films Prepared by Sol–Gel Process

Lead zirconate (PbZrO3) thin films doped with Ce, Cr and Er were grown on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel spin coating process. Polycrystalline films with a phase-pure perovskite structure and a random orientation were obtained regardless of doping materials. Microstructural examinations of the films revealed a two-phase microstructure consisting of large rosette-type perovskite regions with a nanocrystalline phase located in between. Perovskite rosettes were found to be of polycrystalline in nature with grain size ranging between 50–250 nm. Increasing number of annealing cycles and temperature were found to improve the microstructure, crystallinity and electrical properties. Doping elements were also found to increase electrical properties with a saturation polarization (Psat) reaching 65 ×10-6 C/cm2 in Ce- and Cr-doped coatings compared to 39 ×10-6 C/cm2 for undoped PZ films.

Electrical properties and impedance spectroscopy of pure and copper-oxide-added potassium sodium niobate ceramics

Pure and 1 mol% CuO-added lead-free potassium sodium niobate K0.5Na0.5NbO3 (KNN) ceramics were prepared by the conventional solid-state calcination method. Copper oxide was mainly used as a sintering aid in the KNN structure. Microstructural analyses clearly showed that the CuO formed a secondary phase at the grain boundaries. Impedance spectroscopy was used as a tool to analyze the electrical behavior of KNN ceramics as a function of frequency from 100 Hz to 10 MHz at various temperatures. The impedance studies proved that CuO led to the formation of a secondary grain boundary phase, as well as creation of highly mobile point defects. The relaxation time of copper-added samples was less than that of pure KNN. This shorter time indicated a higher space charge mobility for CuO-added samples. The thermal activation energy for relaxation of charge carriers (Eg) was calculated as 0.73 eV for CuO-added samples.

Electric field-induced strain behavior in lithium- and copper-added potassium sodium niobate piezoceramics and 1-3 piezocomposites

Potassium sodium niobate (KNN)-based leadfree materials were prepared and their field-induced strain behaviors were investigated. Ceramic lead-free piezoelectric materials were prepared in bulk and fiber forms with 1 mol% CuO-added potassium sodium niobate K0.5Na0.5NbO3 and x = 7 mol% lithium-modified (K0.5-x/2 Na0.5-x/2Lix)NbO3 compositions. Fibers were drawn using a novel alginate gelation technique. Piezocomposites were prepared from these fibers with 1-3 connectivity and an epoxy matrix. A fully recoverable electrostrain of up to approximately 0.11% was observed in the CuO-added sample, whereas the Li-modified sample yielded up to 0.10% at 50 kV/cm electric field. A strain value of up to approximately 0.03% at 50 kV/cm electric field was obtained for piezocomposites prepared from lithium-modified fibers. The high-field converse piezoelectric coefficient was calculated from the strain-electric field (x-E) graph for all samples. Strain characteristics of the bulk and piezocomposite samples were analyzed based on the variation of strain with respect to square of the polarization (x-P2) to determine the electrostrictive contribution to the strain.

Microstructure and Crystallographic Orientation Dependence of Electrical Properties in Lead Zirconate Thin Films Prepared by Sol–Gel Process

Lead zirconate (PbZrO3) thin films are grown on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel using precursor solutions with stoichiometric and 20 mol % excess Pb. Films with no preferred orientation and [111] pseudocubic texture (denoted as [111]pc) are obtained by changing the drying temperature at the pyrolysis stage. Randomly oriented films were found to have a two-phase microstructure consisting of large rosette-type perovskite regions with a nanocrystalline phase located in between. The ratio of the rosettes increase to cover the entire surface of the films with the increase of Pb content. The films with [111]pc orientation have a uniform microstructure with micron size grains. The electrical properties of the films were influenced markedly by the microstructure and orientation of the films. The [111]pc oriented films exhibit a square-like double hysteresis loop with maximum polarization (Pmax) reaching 61×10-6 C/cm2 under 550 kV/cm, whereas stoichiometric films with no preferred orientation have a Pmax of 36×10-6 C/cm2 with slimmer hysteresis curves.

Field-induced recoverable strain behavior of CuO-added K0.5Na0.5NbO3 ceramics and 1–3 fiber/epoxy piezocomposites

Lead-free, dense, 1 mol % CuO-added potassium sodium niobate K0.5Na0.5NbO3 (KNN) fibers were successfully drawn using a novel alginate gelation technique. Piezocomposites with 1–3 connectivity were prepared with an epoxy matrix with various fiber volume fractions (Vf). Electrical properties of the piezocomposites were investigated and compared with bulk KNN ceramics of the same composition. The dielectric constant of the piezocomposites were found to increase from 67 to 191 with the addition of CuO into KNN and increasing Vf in the piezocomposite. A constricted, antiferroelectriclike double hysteresis polarization versus electric field (P-E) loop was observed in bulk KNN. A large and fully recoverable electrostrain of 0.07% was measured for bulk KNN and 0.03% for the piezocomposites under an electric field of 50 kV/cm, respectively. The double hysteresis P-E loop and the recoverable nature of the electrostrain was due to the CuNb‴–VO•• defect dipoles creating an internal field that restores the original d...

Improving fatigue resistance of Pb(Zr,Ti)O3 thin films by using PbZrO3 buffer layers

Ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) thin films with PbZrO3 (PZ) buffer layers were prepared on Pt(111)∕Ti∕SiO2∕Si(100) substrates using a hybrid rf magnetron sputtering and sol-gel process. Texture of PZT films was found to depend on Pb content of PZ buffer layers. Buffered PZT films displayed comparable ferroelectric properties (2Pr=38–53μC∕cm2,2Ec=136–170kV∕cm) with unbuffered PZT. Asymmetric leakage current and fatigue behavior with superior fatigue resistance was observed in PZ buffered PZT compared to unbuffered films. PZ buffer layers were found to affect crystallization and texture of PZT, and act as a capacitive interface layer possibly blocking charge injection from electrodes.

Investigation of the Electrical Properties of [111] Oriented PbZrO3 Thin Films Obtained by Sol–Gel Process

Lead zirconate–PbZrO3 (PZ) thin films were produced on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel spin coating using precursor solutions of various molarities and with various thicknesses. Increasing molarity and thickness led to further crystallization and grain growth in the films. Double polarization vs electric field hysteresis loops with square-like shapes were obtained due to fully [111] pseudo-cubic orientation. The maximum polarization of well-crystallized PZ films (200–400 nm) reaches 48 µC/cm2 under 400 kV/cm applied field, with switching fields between 270–300 kV/cm in the forward direction and 150–180 kV/cm in the reverse direction at room temperature. Increasing crystallinity of the films led to lower leakage current densities on the order of 10-6 A/cm2 or less. The room temperature dielectric constant of the PZ films was measured to be 240 with a Curie temperature of 240 °C. Temperature dependent P–E hysteresis measurements indicated a shift in the forward and reverse switching fields towards lower field levels as the temperature of measurement increased.

Fatigue Behaviour of Pb(Zr,Ti)O3/PbZrO3 Multilayer Ferroelectric Thin Films

Fatigue behaviour of Pb(Zr0.45Ti0.55)O3 (PZT) thin films on Pt(111)/Ti/SiO2/Si(100) substrates with PbZrO3 (PZ) buffer layers were investigated. Two different buffer layer configurations were studied: bottom- and top-only buffer layers. The PZ buffer layers were obtained by RF magnetron sputtering of a loose powder target containing a mixture of PbZrO3 and ZrO2 powders. The PZT thin films were prepared by sol–gel spin coating method. All of the films had well-crystallized, uniform and dense microstructure with full (111) orientation. The bottom buffer layers influenced the nucleation and grain growth of the upper PZT layers creating a bimodal grain size distribution. Secondary ion mass spectrometry (SIMS) elemental depth profiles indicated that a PZ buffer layer led to well-defined and sharp composition profiles. The P–E hysteresis measurements indicated comparable remnant polarization values (2Pr=40–50 µC/cm2) and coercive field levels (2Ec=150–180 kV/cm) with un-buffered film. The fatigue endurance of PZT thin films with PZ buffer layers was superior than the un-buffered counterpart. A fatigue-free behaviour was observed up to 109 switching cycles.

Preperation of Lead-Free Potassium Sodium Niobate Based Piezoelectrics and their Electromechanical Characteristics

In this study, %7 Li modified and 0.67 % copper oxide added potassium sodium niobate (KNN) ceramics were investigated. Copper oxide was used as a sintering aid. The ceramics were prepared with conventional solid state calcination technique. All samples were crystallized in pure perovskite phase with no additional peak. The density of the samples increased with copper addition and lithium modification. The Curie temperature of KNN ceramics was found to shift to lower temperatures by CuO addition. The Curie temperature was measured as 414°C and 504°C for copper oxide added and lithium modified KNN samples, respectively. The maximum strain of copper oxide added sample was 0.12%, whereas Li modified KL ceramics yielded up to 0.10 %.

Increased Fatigue Endurance in Pb(Zr,Ti)O3 Thin Films through Use of PbZrO3 Buffer Layers

Thin PbZrO3 (PZ) films were investigated as a buffer layer to improve fatigue endurance of Pb(Zr0.45Ti0.55)O3 (PZT) thin films. The PZ thin films were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by RF magnetron sputtering from a loose powder target containing a mixture of PbZrO3 and ZrO2 powders. The PZT thin films on the PZ buffer layer were obtained by sol–gel spin coating. The PZ buffered PZT films had well crystallized, uniform and dense microstructure with partial (111) orientation. The P–E hysteresis measurements indicated comparable or slightly lower remnant polarization values (2Pr=50–70 µC/cm2) and higher coercive field levels compared to unbuffered film (2Pr=66 µC/cm2). The PZ buffered films displayed an asymmetric leakage current. The fatigue endurance of PZT thin films with PZ buffer layers was superior compared to the unbuffered counterpart with fatigue-free behavior observed up to 109 switching cycles.