# Magnetoelectric Composites: Engineering for Tunable Filters and Energy Harvesting Applications
## 3 Results  
Figure 1 depicts SEM images of microstructures of the P-F multiferroic composite samples created using two image capture techniques, i.e., SB (Figure 1a-f) and BSE (Figure 1a'-f'), for the same analysis area of the surface composite samples. BSE enables convenient visualization of the distribution of individual components on the surface of the composite sample, i.e., areas with magnetic (dark regions) and ferroelectric (bright regions) phases. By filtering out low-energy secondary electrons, the BSE analysis reveals bright areas with a predominance of grains with a higher atomic number. In comparison, the dark grains indicate areas with a predominance of elements with a lower atomic number. The assignment of grains to the appropriate phase of the composite was confirmed via spot EDS analysis. The presented photos show how the appearance of the microstructure of the analyzed series of multiferroic composite samples with the change in the ferrite (ferroelectric) phase evolves. The PZT-type material (ferroelectric phase) shows a high degree of solidification; the grains are fine and firmly solidified with each other, with precise edges. On the other hand, magnetic grains are usually larger and are also correctly crystallized with clear grain boundaries. SEM images show that ferroelectric and magnetic grains are evenly distributed in the composite samples' microstructure of the cross-section. However, not all magnetoelectric materials and composites can obtain a homogeneous microstructure in the technological process, e.g., [41; 42; 43]. In the case of compositions with a dominant ferroelectric phase (constituting the matrix of the multiferroic composite), fine grains of the ferroelectric component surround the larger magnetic grains (P90-F10 sample). In many cases, the magnetic grains tend to aggregate into larger clusters of the magnetic fraction. When the amount of a magnetic phase in composite composition increases, the situation reverses. For the P20-F80 composition, numerous magnetic grains surrounding small amounts of the ferroelectric component distributed randomly. Figure 2 shows the grain size distribution of the P-F multiferroic composite materials. In general, magnetic materials have lower sintering temperatures than PZT materials; hence, the growth of magnetic grains starts at a lower temperature. In composite compositions with a significant predominance of the ferroelectric component, the magnetic grains have more space suitable for their expansion, which results in a larger average grain size. Ferroelectric grains grow similarly in these compositions. In the case of composite compositions with a dominant magnetic phase, the overwhelming number of magnetic grains hinders their excessive growth and promotes the improvement in grain size homogeneity. However,  
As it is known, the physical (dielectric) properties of multiferroic materials depend on their microstructure, including homogeneity, grain size, and density. The microstructure, in turn, is influenced by an adequately conducted technological process of obtaining multiferroic materials (including mixing powders, the appropriate selection of the method, and conditions of synthesis and sintering). In general, the homogeneity of the microstructure of ceramic materials has a positive effect on their final properties, and in the case of dielectric properties, on the degree of diffusion of the phase transition (ferroelectric/paraelectric). Ceramic materials with high uniformity of microstructure show a sharp phase transition. Grain size also affects the dielectric properties of ceramic materials. Materials with smaller grains show a stronger effect of mechanical stress and an external effect of grain boundaries, which result in increased blurring of the phase transition, lower Curie temperature, and lower permittivity [44; 45]. The dielectric constant of the materials is linearly proportional to the average value of grain size and inversely proportional to the grain boundary thickness (as grain size increases, permittivity increases) [46].