Effect of calcination rate on mechano-chemically activated powders for the synthesis of lead-free KNN-type ceramics

Lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 (BCZT) ceramics have demonstrated excellent dielectric, ferroelectric and piezoelectric properties in comparison to lead-based materials. The synthesis of pure and crystalline BCZT nanopowders at low temperatures of 25, 80 and 160 C was reported previously by using a sol-gel method followed by a hydrothermal route. In this study, the structural, dielectric and ferroelectric properties of sintered BCZT ceramics at 1250 {\deg}C for 10h were investigated. XRD measurements revealed the presence of a single perovskite phase at room temperature with the coexistence of the orthorhombic and tetragonal symmetries. The increase of grain size and the ceramic density in BCZT ceramics result in an enhancement of the dielectric and ferroelectric properties of BCZT ceramics. More interestingly, the synthesis temperature of BCZT powders with high dielectric and ferroelectric properties could be decreased to a low temperature of 160 {\deg}C, which is about 1200 {\deg}C lower when compared with solid-state reaction and 840 {\deg}C lower when compared with sol-gel methods. The BCZT ceramics elaborated at 160 {\deg}C revealed excellent electrical properties. Hence, the use of low-temperature hydrothermal processing can be encouraging for the synthesis of lead-free ceramics with high dielectric and ferroelectric properties.

Synthesis and characterizations of KNN ferroelectric ceramics near 50/50 MPB

Synthesis of lead-free perovskite-type ceramics by gas phase reaction and their piezoelectric properties.

Lead-free piezoelectric powders (K0.44Na0.52Li0.04)(Nb0.82Ta0.10Sb0.04)O3 were obtained by conventional and microwave-assisted reactive heating. Firstly, the synthesis of the material was carried out following the mixed oxide route and employing both traditional methods and microwave technology. Thermogravimetry, X-ray diffraction, field emission scanning electron microscopy and electrical properties analyses were evaluated. X-ray diffraction of the powders calcined by the microwave process shows the formation of perovskite structure with orthorhombic geometry, but it is possible to observe the presence of other phases. The presence of the secondary phases found can have a great influence on the heating rate during the synthesis on which the kinetics of the reaction of formation of the piezoelectric compound depend. The calcined powder was sintered at different temperatures by conventional and non-conventional processes. The microstructure of the ceramics sintered by microwave at 1050 °C for 10 min shows perovskite cubes with regular geometry, of size close to 2–5 µm. However, the observed porosity (~8%), the presence of liquid phase and secondary phases in the microstructure of the microwave sintered materials lead to a decrease of the piezoelectric constant. The highest d33 value of 146 pC/N was obtained for samples obtained by conventional at 1100 °C 2 h compared to samples sintered by microwave at 1050 °C 10 min (~15 pC/N).

Facile synthesis of high-entropy lead-free relaxor ferroelectric ceramics via high-energy ball milling

The synthesis of lead-free (Na0.5K0.5)NbO3 (KNN) ceramics for potential piezoelectric applications is reported by conventional solid-state reaction between alkaline carbonates and Nb2O5. Prior to the synthesis, the reactant powders and their corresponding stoichiometric mixture are alumina ball-milled to homogenize the particle size and as pre-activation treatment, respectively. The synthesis of the KNN-based ceramics was investigated systematically with the duration of the ball milling and calcination conditions in terms of mass change evolution at involved temperature steps. The properties of the obtained ceramics including phase structure, morphology, composition, relative density and microhardness were assessed by Field Emission Scanning Electron microscopy, Energy Dispersive Spectroscopy and X-ray diffraction. The obtained results indicate that longer ball milling duration is detrimental to the synthesis of KNN ceramics while tailoring of the KNN properties can be achieved by adjustment of calcination conditions including calcination rate, calcination temperature stage and calcination dwell duration.

The present study reports the structural aspects of BiFeO3, Bi0.9Ba0.1Fe0.9M0.1O3, (M=Co, Mn) and Bi0.80RE0.2FeO3 (RE=Nd, Gd and Dy) powders as prepared by solid state reaction route while Bi0.80Sr0.2FeO3 ceramic has been prepared using citrate sol-gel process. X-ray diffraction along with the Rietveld-refinement reveals the rhombohedral (R3c) structure for BiFeO3 and Bi0.9Ba0.1Fe0.9M0.1O3, (M=Co, Mn), whereas, tetragonal (P4/mmm) for Bi0.80Sr0.2FeO3 ceramic. In case of rare earth substitution there is an abrupt change in the crystal structure. Bi0.8Nd0.2FeO3 ceramic crystallizes in triclinic structure (P1), Bi0.8Gd0.2FeO3 compound shows a major contribution is related to orthorhombic (Pna21) symmetry and minor contributions are attributed to Pnma and R3c phase, whereas the X-ray diffraction of Bi0.8Dy0.2FeO3 confirms the biphasic (Pnma+R3c) nature of the compound. All the properties of the ceramics reflect their structure so, structural evolution is important for enhancing the physical properties at room temperature.

In the present work lead-free compositions with a stoichiometric formula (1-x)(K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Na <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> )Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> Sb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -xBaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (x=0.01, 0.02, 0.04; y=0.04, 0.07) were produced by solid state sintering method. Manganese oxide MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> was used as a dopant in order to lower the sintering temperatures. The influence of BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> on the microstructure, density, mechanical and electrical properties of Sb-substituted KNN ceramics was investigated. X-ray diffraction analysis revieled pure perovskite structure. Microstructural investigation of the ceramics samples showed homogeneous structure with a slightly rounded cubical grain shape. Phase transition temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is shifted to lower temperatures in comparison to Sb-substituted KNN ceramics. Phase transition peak at T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> becomes broader indicating the diffuse phase transition.

Synthesis of Lead-Free Ceramics of the Perovskite Type for Piezoelectric Applications by Conventional Solid-State Reaction

Glycine exhibits a little piezoelectric response when poled, while lead-free alkaline niobate-based ceramics show much higher responses. This research investigates the synthesis of a dielectric composite from a combination of glycine and (K0.45Na0.51Li0.04) (Nb0.85Ta0.1Sb0.04) O3 (KNNLST) ceramics. The mixed oxide ceramics synthesis method was used to produce the ceramics, while glycine powder was commercially procured. The composition range of the shaped and heat-treated composites is from no ceramics to 100 wt.% ceramics content. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), dielectric studies, and hysteresis measurements were used to characterize the samples. The obtained phases transformed from the monoclinic phase in glycine to a two-phase orthorhombic-tetragonal phase in the ceramics. The samples’ morphology revealed a dense microstructure with some cracks, large porosity, and smaller grain sizes. The dielectric properties showed increasing dielectric constant and loss values with increasing ceramics content, while the ac conductivity also increased with rising ceramics content. Improving the range of ceramics led to polarization hysteresis graphs indicating ferroelectricity in the samples. The properties of the composites show they can be used in electromechanical devices.