Problems in Obtaining High-Density, Pure-Phase BiFeO3 Ceramics

Abstract

Multiferroic materials exhibit at least two of the so-called ferroic properties (ferroelectric, (anti)ferromagnetic and ferroelastic) in the same time. They are very interesting from the theoretical point of view because of a different nature of those properties, but coupling between the properties opens up huge possibilities for application as magnetoresistors, memory devices, sensors and many other devices [1]. Being ferroelectric up to 830 °C and antiferromagnetic (weakly ferromagnetic) up to 370 °C, bismuth ferrite (BiFeO3) is one of the very few room-temperature singlephase multiferroic materials and one of the most studied ceramic materials in the last two decades. BiFeO3 has also good potential to be used as a pigment, catalyst, photocatalyst or solar cell material [1,2]. However, because of specific obstacles in obtaining pure, dense and highly resistive ceramics, harnessing of those properties is still far from being achieved and the possibility of its application as a multiferroic material is arguable. High volatility of bismuth above 800 °C and thermodynamic instability of BiFeO3 between 447 °C and 767 °C make the densification of BiFeO3 ceramics very difficult, especially by conventional methods. High leakage currents in BiFeO3 (originating mostly from oxygen and bismuth vacancies) disable ceramic samples to be polarized and to exhibit ferroelectric properties. Spiral structure of magnetic moments lowers the coupling between ferroelectric and magnetic orders [3,4]. BiFeO3 ceramic materials presented in this study were synthesized by autocombustion method with idea to lower the temperature needed for effective sintering in order to prevent volatilisation and improve the density and phase composition. Auto-combustion is a type of sol-gel route which enables high homogeneity in solutions stabilized by organic compounds, which oxidize vigorously producing the ash powders as a wanted product. Because of such fast reaction, the defects are incorporated into structure enabling solid state sintering to take place at lower temperatures and more quickly. Presented microstructures are illustrating the usual problems that occur during the synthesis of powders and processing of ceramic materials. Powders tend to agglomerate, and although the agglomerates can be destroyed by milling (Figure 1), this process often disturbs the phase composition of ceramics synthesized from the milled powders. Because of a wide range of temperatures at which bismuth evaporates and secondary phases form, it is important to conduct heating and cooling of samples very fast (quenching), but even this way some secondary phases are formed (Figure 2) and densification is not complete (Figure 3). The study presents the evolution of the mentioned problems during attempts to overcome them by modification of the synthesis conditions, by using different treatments of the powders and by modification of the sintering process.