Low sintering temperature effect on crystal structure and ferroelectric properties lead-free piezoelectric BNT-FNT

Description

Bi0.5Na0.5TiO3 (BNT) emerges as a promising ferroelectric and piezoelectric lead-free candidate to substitute the contaminant Pb[TixZr1-x]O3 (PZT). However, to obtain optimal ferroelectric and pi-ezoelectric properties, BNT must be sintered at high temperatures. In this work, the reduction of sintering temperature by using Fe added to BNT is demonstrated, without significative detriment in the electric properties. BNT-xFe with Fe in the range from x= 0 to 0.1 mol (x=0.025) were synthe-sized by high-energy ball milling followed by sintering at 900 °C. XRD analysis confirmed the presence of rhombohedral BNT together with a new phase of NaFeTiO4 (NFT), also corroborated by means of optical and electronic microscopy. The relative permittivity, in the range of 400 to 500 across all the frequencies demonstrated the stabilization effect of the iron in BNT. Additionally, the presence of iron elevates the Curie temperature, increasing it from 330°C in the Fe free sample to 370 °C in the sample with the maximum Fe concentration (0.1 mol). The dielectric losses maintain a constant values lower than 0.1. In this case, low dielectric loss values are ideal for ferroelectric and piezoelectric materials, as they ensure minimal energy dissipation. Likewise, the electrical conduc-tivity maintains a semiconductor behavior across range of 50 Hz to 1x106 Hz, indicate the potential of these materials for applications at different frequencies. Additionally, it was observed that the introduction of iron in BNT in low concentration promotes a flattening of the hysteresis loops by improving the alignment of electric dipoles, displaying values of 6 and 12 µC/cm2. Similarly, the piezoelectric constant (d33) values undergo a slight decrease when low concentrations of iron are added, maintaining values between 30 and 48 pC/N for BNT-0.025Fe and BNT-0.05Fe respectively. un

Steps to reproduce

A specific stoichiometry was selected to compensate the charges, according to the composition Bi0.5Na0.5Ti1-xFexO3-0.5x where x = 0.025, 0.05, 0.075 and 0.1 mol of Fe. High-purity oxide powders Fe2O3 (Sigma-Aldrich, 99.8% purity), TiO2 (Sigma-Aldrich, 99.6% purity), Bi2O3 (Sigma-Aldrich, 99.8% purity), and carbonates Na2CO3 (Sigma-Aldrich, 99.6 % purity), Ba2CO3 (Sigma-Aldrich, 99 % purity) were used as precursors. Prior to the milling process, the precursors were dried at 150 ◦C for 1 h. Afterward, the precursors were mixed according to the stoichiometric reaction shown in following equation: x"Fe2O3"+0.5"Bi2O3"+2(1-x)"TiO2 "+0.5"Na2CO3"→ 2"Bi0.5Na0.5Ti(1-x)FexO3-0.5x"+0.5"CO2" (1) A total of 5 g of powder mixture, along with steel balls of 12.7 mm whit a ball-to-powder weight ratio of 10:1, was loaded into a steel vial of 50 cm3 in volume at room temperature in air atmosphere. Subsequently, the mixture was high-energy ball milled by means of a shaker mixer mill (SPEX model 8000D) for 5 h. After that, the milled powder was pressed using a hydraulic press, applying 1400 MPa to obtain pellets of 10 mm in diameter. The pellets were sintered at 900 ◦C for 4 h using a tubular muffle furnace (Lindberg/Blue M model STF54459C) in an air atmosphere. The phases and crystal structures were determined by X-Ray Diffraction (XRD) using a diffractometer Equinox 2000 (INEL) with CoKα1 radiation (λ = 1.7018 Å). XRD patterns were measured in a 2ϴ range from 20 to 80, followed by Rietveld refinement using free-software Material Analysis Using Diffraction (MAUD), to study the evolution of the cell parameters, crystallite size, microstrain, and phase quantification as the Fe3+ concentration (x) was increased. The crystallographic PDF data was obtained from Crystallography Open Database (COD). The pellets were painted on both surfaces with silver paste for electric characterization. It was, determined the relative permittivity (r), loss tangent (tan) and AC conductivity (AC) as a function of temperature from 25 °C to 400 °C by means of LCR-meter (HIOKI 3532-50) at the frequency range from 50 to 5×106 Hz. Ferroelectric hysteresis loops of the pellets were obtained at room temperature using a ferroelectric Premier II ferroelectric test system (Radiant Technologies). The surface morphology and microstructure of the samples were observed using both, Digital-optical (KEYENCE VHX-7000) and scanning electron microscopy (SEM, JEOL-100-CXII), equipped with Energy Disperse Spectroscopy (EDS) analysis to obtain elemental chemical analyses.

Institutions

• Universidad Autonoma del Estado de Hidalgo

  Pachuca
• Instituto Politecnico Nacional

  Distrito Federal, Ciudad de Mexico

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