3. Grain size evolution and multiferroic properties of holmium and yttrium manganite

Figure 2(a) and (b) shows the micrograph of the holmium and yttrium manganite powder samples milled for 12 h, which have been measured by using transmission electron microscopy (TEM) for particle size confirmation. The particle size was measured by taking diameters of 200 particles, and it was found to have inhomogeneity variation from 7 to 70 nm with an average around 27 nm (a) and 11–87 nm with average particle sizes around 30 nm (b) for holmium and yttrium manganite, respectively. The wide range of particle size distribution was due to the nonuniformity of the force between the balls and the vial during the milling process. In order to obtain a uniform grain size, particle size is an important quantity that needs to be considered. The agglomerations of the particles also occurred due to the large surface area and being subjected to repeated cold welding and fracturing during the HEBM process. This particle size is an average value considering that the particles are spherical. The particles are known to have higher reactivity due to the larger surface area. The surface atoms are more unstable (and reactive). This instability is related to their position on the lattice that forces them to unbind to their neighboring atoms or molecules. For the case of nanoparticles, as the ratio of surface/bulk atoms increases, the instability (and reactivity) also increases. Higher

Figure 2. TEM micrograph of resulting particles after 12 h of milling time for (a) HMO (b) YMO.

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline… DOI: http://dx.doi.org/10.5772/intechopen.81753

reactivity in the starting powder is required in order to prepare a series of samples in which the observation of nanosized starting particles followed by nano-to-micron size grains is required in this study.

A control of the crystal orientation is significantly important, because the ferroelectric polarization of HoMnO3 and YMnO3 appears along the hexagonal c-axis. Figure 3(a) and (b) reveals the XRD spectra of HMO and YMO samples, respectively. At 12 h high-energy ball milling, broad diffraction peaks belonging to

Figure 3.

(a) XRD patterns of for as milled continued with sintering from 600 to 1250<sup>o</sup> C for HMO. (b) XRD patterns of for as milled continued with sintering from 600 to 1250o C for YMO.

of a material. The purpose of the sintering process is to complete the interdiffusion of the component metal ions into the desired crystal lattices and to develop the polycrystalline microstructure at as low a temperature as possible, first to ensure a high grade of microstructural homogeneity, second to avoid cannibal grain growth, and third to save energy cost. At this stage, atoms are closely in contact with each other, hence reducing the space between them. Therefore, shrinkage occurs as a consequence of aggregation of the entire substance. As required by commercial

Since the aim of this research is to study in detail the evolution of the microstructure and the subsequent effect of the ferroelectric processes, sintering was used as an agent to control such desired changes that would be observed in the microstructure. The sintering temperature used for the purpose of detecting the sintering effects on the P-E hysteresis loops was from 600 to 1250°C with the increase of 50°C giving a total of 14 samples for each batch. They were sintered separately for 10 h in an electric furnace. The average dimensions of the final product with the pellet shape were about 10 mm diameter and 1–2 mm thickness.

3. Grain size evolution and multiferroic properties of holmium and

respectively. The wide range of particle size distribution was due to the

TEM micrograph of resulting particles after 12 h of milling time for (a) HMO (b) YMO.

nonuniformity of the force between the balls and the vial during the milling process. In order to obtain a uniform grain size, particle size is an important quantity that needs to be considered. The agglomerations of the particles also occurred due to the large surface area and being subjected to repeated cold welding and fracturing during the HEBM process. This particle size is an average value considering that the particles are spherical. The particles are known to have higher reactivity due to the larger surface area. The surface atoms are more unstable (and reactive). This instability is related to their position on the lattice that forces them to unbind to their neighboring atoms or molecules. For the case of nanoparticles, as the ratio of surface/bulk atoms increases, the instability (and reactivity) also increases. Higher

Figure 2(a) and (b) shows the micrograph of the holmium and yttrium manganite powder samples milled for 12 h, which have been measured by using transmission electron microscopy (TEM) for particle size confirmation. The particle size was measured by taking diameters of 200 particles, and it was found to have inhomogeneity variation from 7 to 70 nm with an average around 27 nm (a) and 11–87 nm with average particle sizes around 30 nm (b) for holmium and yttrium manganite,

yttrium manganite

Functional Materials

Figure 2.

102

production practice, the desired shrinkage is between 10 and 20%.

orthorhombic HoMn2O5 and YMn2O5 were observed to exist, which means that fully amorphous powders were formed. Hexagonal HoMnO3 and YMnO3 peaks were not observed in the as-milled powder due to insufficient thermal energy to form the structure. Impurity phases other than raw powders were not detected in the as-prepared sample. However, high-energy ball milling facilitates the reaction between the raw materials where the sintering temperature could be reduced to lower than that normally required in the conventional method due to higher surface reactivity [14]. The energy is transferred from the milling media to the powder particles by the continuous fracture and cold welding process. As the grain size started to increase, small peaks of hexagonal HoMnO3 and YMnO3 were detected even with fairly low intensity. Starting from 600°C sintering temperature up to 1000°C, both peaks consisting of orthorhombic HoMn2O5/ YMn2O5 and hexagonal HoMnO3/YMnO3 intensity were observed to decrease and increase, respectively. As the sintering temperature continued to increase, the secondary phase reduced in intensity. As can be seen in the figure, the main peak in each sintering temperature from 600 to 1000°C exists a splitting which is the evidence of the formation of a new structure from orthorhombic to hexagonal. The transformation from orthorhombic to a single-phase hexagonal structure started to occur at 1050°C for the HMO sample and at 1000°C for the YMO sample. Further increasing the sintering temperature from 1050 to 1250°C raised the intensity of the major peaks belonging to hexagonal holmium and yttrium manganese oxide (HoMnO3 and YMnO3), which demonstrated the improvement in the degree of crystallinity of the sintered samples besides releasing the strain induced by milling. The results demonstrate that the hexagonal phase of HoMnO3 and YMnO3 could not be formed only by milling since the energy imparted by the collision of the milling media to the starting powders is not sufficient to increase the reactivity between the particles. Besides, without the sintering process, insufficient energy will be provided to stimulate the reaction between particles. As the sintering temperature increases, a larger grain size would be formed due to the combination of grains, thus inducing the formation of a more crystalline phase with a hexagonal structure. Furthermore, a ferroelectric phase would be formed because of the increasing electric dipole moment of the crystalline hexagonal structure, whereas it is known that ferroelectricity is geometrically driven by the displacement between RE3+ and O<sup>2</sup> as a result of a structural phase transition [8–11]. Therefore, a higher sintering temperature, say 1250°C, would lead to a higher value of dielectric constant and ferroelectric order. At lower sintering temperature, the sample exhibited a smaller grain size that will contribute a larger amount of amorphous grains compared to samples with a larger grain size. This amount was significant in view of the fact that the grain boundary volume was not negligible for the nanometer grain size [15].

The microstructural images of the sintered pellets were obtained using a Nova NanoSEM 50 scanning electron microscope. The SEM micrographs of the HMO and YMO samples are shown in Figures 4(a)–(n) and 5(a)–(n). In the HMO samples, the sintering temperature from 600 to 900°C with a grain size of 30 to 65 nm determines the slow rate of grain growth, while at 950, 1000, and 1050°C (from 108 to 296 nm), it indicates moderate increment. The grain growth was observed to be increased at 1100 up to 1250°C (854 nm–2.9 μm) due to creation of a pure single hexagonal HMO phase. As for the YMO samples, grain growth occurred at 600–900°C (from 49 to 75 nm) and continued by adequate increment at 950, 1000, and 1050°C (from 82 to 314 nm). The grain growth was observed to be increased at 1100°C up to 1250°C (543 nm–2.1 μm). The results demonstrate that multi-sample sintering involved a transition from the slow-moderate-rapid grain growth process. The grains grew at the expense of others and were created from movement of grain boundaries and grains, respectively. The pores will form an interconnected channel along grain edges when

Figure 4.

105

(a–n) FESEM micrograph from 600 to 1250<sup>o</sup>

C for HMO.

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline…

DOI: http://dx.doi.org/10.5772/intechopen.81753

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline… DOI: http://dx.doi.org/10.5772/intechopen.81753

Figure 4. (a–n) FESEM micrograph from 600 to 1250<sup>o</sup> C for HMO.

orthorhombic HoMn2O5 and YMn2O5 were observed to exist, which means that fully amorphous powders were formed. Hexagonal HoMnO3 and YMnO3 peaks were not observed in the as-milled powder due to insufficient thermal energy to form the structure. Impurity phases other than raw powders were not detected in the as-prepared sample. However, high-energy ball milling facilitates the reaction between the raw materials where the sintering temperature could be reduced to lower than that normally required in the conventional method due to higher surface reactivity [14]. The energy is transferred from the milling media to the powder particles by the continuous fracture and cold welding process. As the grain size started to increase, small peaks of hexagonal HoMnO3 and YMnO3 were detected even with fairly low intensity. Starting from 600°C sintering temperature up to 1000°C, both peaks consisting of orthorhombic HoMn2O5/ YMn2O5 and hexagonal HoMnO3/YMnO3 intensity were observed to decrease and increase, respectively. As the sintering temperature continued to increase, the secondary phase reduced in intensity. As can be seen in the figure, the main peak in each sintering temperature from 600 to 1000°C exists a splitting which is the evidence of the formation of a new structure from orthorhombic to hexagonal. The transformation from orthorhombic to a single-phase hexagonal structure started to occur at 1050°C for the HMO sample and at 1000°C for the YMO sample. Further increasing the sintering temperature from 1050 to 1250°C raised the intensity of the major peaks belonging to hexagonal holmium and yttrium manganese oxide (HoMnO3 and YMnO3), which demonstrated the improvement in the degree of crystallinity of the sintered samples besides releasing the strain induced by milling. The results demonstrate that the hexagonal phase of HoMnO3 and YMnO3 could not be formed only by milling since the energy imparted by the collision of the milling media to the starting powders is not sufficient to increase the reactivity between the particles. Besides, without the sintering process, insufficient energy will be provided to stimulate the reaction between particles. As the sintering temperature increases, a larger grain size would be formed due to the combination of grains, thus inducing the formation of a more crystalline phase with a hexagonal structure. Furthermore, a ferroelectric phase would be formed because of the increasing electric dipole moment of the crystalline hexagonal structure, whereas it is known that ferroelectricity is geometrically driven by the displacement between RE3+ and O<sup>2</sup> as a result of a structural phase transition [8–11]. Therefore, a higher sintering temperature, say 1250°C, would lead to a higher value of dielectric constant and ferroelectric order. At lower sintering temperature, the sample exhibited a smaller grain size that will contribute a larger amount of amorphous grains compared to samples with a larger grain size. This amount was significant in view of the fact that the grain

Functional Materials

boundary volume was not negligible for the nanometer grain size [15].

104

The microstructural images of the sintered pellets were obtained using a Nova NanoSEM 50 scanning electron microscope. The SEM micrographs of the HMO and YMO samples are shown in Figures 4(a)–(n) and 5(a)–(n). In the HMO samples, the sintering temperature from 600 to 900°C with a grain size of 30 to 65 nm determines the slow rate of grain growth, while at 950, 1000, and 1050°C (from 108 to 296 nm), it indicates moderate increment. The grain growth was observed to be increased at 1100 up to 1250°C (854 nm–2.9 μm) due to creation of a pure single hexagonal HMO phase. As for the YMO samples, grain growth occurred at 600–900°C (from 49 to 75 nm) and continued by adequate increment at 950, 1000, and 1050°C (from 82 to 314 nm). The grain growth was observed to be increased at 1100°C up to 1250°C (543 nm–2.1 μm). The results demonstrate that multi-sample sintering involved a transition from the slow-moderate-rapid grain growth process. The grains grew at the expense of others and were created from movement of grain boundaries and grains, respectively. The pores will form an interconnected channel along grain edges when

Figure 6.

107

for HMO samples sintered at 1100–1250°C.

(a) P-E for HMO samples sintered at 600–800°C. (b) P-E for HMO samples sintered at 850–1050°C. (c) P-E

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline…

DOI: http://dx.doi.org/10.5772/intechopen.81753

Figure 5. (a–n) FESEM micrograph of YMO sintered from 600 to 1250<sup>o</sup> C.

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline… DOI: http://dx.doi.org/10.5772/intechopen.81753

(a) P-E for HMO samples sintered at 600–800°C. (b) P-E for HMO samples sintered at 850–1050°C. (c) P-E for HMO samples sintered at 1100–1250°C.

Figure 5.

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106

(a–n) FESEM micrograph of YMO sintered from 600 to 1250<sup>o</sup>

C.

there is formation of a necking process between particles in the powder compacts. However, the pore channels were disconnected and isolated when the sintering process was introduced. Microstructural interpretations at higher sintering temperatures from 1100 to 1250°C in Figures 4(k)–(n) and 5(k)–(n) correspond to the final stage of sintering. The diffusion process of vacancies from the pores along grain boundaries will cause the pores grew to be closed and have been slowly eradicated with a slight densification. The grains with a hexagonal structure could be perceived

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline…

Figures 6(a)–(c) and 7(a)–(c) show the polarization induced by applying an

HoMnO3 and YMnO3 multiferroics were successfully synthesized via highenergy ball milling, and the parallel evolution of ferroelectric with microstructural properties has been studied. High-energy ball milling was able to produce high-

electric field for all sintered samples of HMO and YMO, respectively. Lower sintering temperatures, 600–800°C for the HMO PE loop (Figure 6a) and 600– 700°C for YMO (Figure 7a), reveal a lossy capacitor response with low Pr, indicating the major paraelectric behavior contributed by a large amount of orthorhombic phase with very little contribution of the hexagonal phase. It is speculated that the presence of the second phase (shown in the XRD spectra) makes the electron to be detached and free as free charges which contributed to cause the loop to leak. As the sintering temperature increased from 850 to 1050°C for HMO (Figure 6b) and 750 to 1000° C for YMO (Figure 7b), an ideal resistor response curve with higher values of Pr and Ec was obtained. This is due to the existence of the geometric ferroelectric derived from the hexagonal phase, which started to dominate as the grain size increased with the increase of sintering temperature. The grain boundary volume in a smaller grain size is higher compared to a larger grain size, which was the reason why the Pr got better as the grain size increased. The effect of grain boundary on polarization includes two factors. The first one is, the grain boundary is a lowpermittivity region, which means that the grain boundary has poor ferroelectricity with little or even no polarization. Secondly, space charges in the grain boundary exclude polarization charge on the grain surface, which would form a depletion layer on the grain surface, which results in polarization discontinuity on the grain surface to form a depolarization field and the polarization decreases. At higher sintering temperature as seen in Figure 6c for HMO and Figure 7c for YMO, it was confirmed that the contribution of the majority hexagonal phase has improved the geometric ferroelectric behavior. Sintering at higher temperature shows the increase of the average grain size, which gave a good response in the polarization hysteresis loop. The contribution of larger grains reduced the secondary peaks while having high crystallinity and enhancing the response of the electric field. As shown in the polarization hysteresis (Figures 6 and 7), applying the electric field increases the polarization values but still showed that the P-E hysteresis loops are not saturated. HMO and YMO are known to behave as a leaky ferroelectric as mentioned in previous reports [16, 17]. As can be seen in Figures 6 and 7, the shapes of the hysteresis loop are mainly related to the combination of an ideal resistor and lossy capacitor response. The mobility of free charges will also contribute to higher conductivity and cause higher leaking current, which would be the main reasons for the Ec values to be unpredictable. Even though the hysteresis loops are seen to be leaky in nature, they are still significant enough to definitely confirm the ferroelec-

in this stage as the sintering temperature and grain size increased.

DOI: http://dx.doi.org/10.5772/intechopen.81753

tricity of the HMO and YMO samples.

4. Conclusion

109

Figure 7. (a) P-E for YMO samples sintered at 600–700°C. (b) P-E for YMO samples sintered at 750–1000°C. (c) P-E for YMO samples sintered at 1050–1250°C

Evolution of Nanometer-to-Micrometer Grain Size in Multiferroic Properties of Polycrystalline… DOI: http://dx.doi.org/10.5772/intechopen.81753

there is formation of a necking process between particles in the powder compacts. However, the pore channels were disconnected and isolated when the sintering process was introduced. Microstructural interpretations at higher sintering temperatures from 1100 to 1250°C in Figures 4(k)–(n) and 5(k)–(n) correspond to the final stage of sintering. The diffusion process of vacancies from the pores along grain boundaries will cause the pores grew to be closed and have been slowly eradicated with a slight densification. The grains with a hexagonal structure could be perceived in this stage as the sintering temperature and grain size increased.

Figures 6(a)–(c) and 7(a)–(c) show the polarization induced by applying an electric field for all sintered samples of HMO and YMO, respectively. Lower sintering temperatures, 600–800°C for the HMO PE loop (Figure 6a) and 600– 700°C for YMO (Figure 7a), reveal a lossy capacitor response with low Pr, indicating the major paraelectric behavior contributed by a large amount of orthorhombic phase with very little contribution of the hexagonal phase. It is speculated that the presence of the second phase (shown in the XRD spectra) makes the electron to be detached and free as free charges which contributed to cause the loop to leak. As the sintering temperature increased from 850 to 1050°C for HMO (Figure 6b) and 750 to 1000° C for YMO (Figure 7b), an ideal resistor response curve with higher values of Pr and Ec was obtained. This is due to the existence of the geometric ferroelectric derived from the hexagonal phase, which started to dominate as the grain size increased with the increase of sintering temperature. The grain boundary volume in a smaller grain size is higher compared to a larger grain size, which was the reason why the Pr got better as the grain size increased. The effect of grain boundary on polarization includes two factors. The first one is, the grain boundary is a lowpermittivity region, which means that the grain boundary has poor ferroelectricity with little or even no polarization. Secondly, space charges in the grain boundary exclude polarization charge on the grain surface, which would form a depletion layer on the grain surface, which results in polarization discontinuity on the grain surface to form a depolarization field and the polarization decreases. At higher sintering temperature as seen in Figure 6c for HMO and Figure 7c for YMO, it was confirmed that the contribution of the majority hexagonal phase has improved the geometric ferroelectric behavior. Sintering at higher temperature shows the increase of the average grain size, which gave a good response in the polarization hysteresis loop. The contribution of larger grains reduced the secondary peaks while having high crystallinity and enhancing the response of the electric field. As shown in the polarization hysteresis (Figures 6 and 7), applying the electric field increases the polarization values but still showed that the P-E hysteresis loops are not saturated. HMO and YMO are known to behave as a leaky ferroelectric as mentioned in previous reports [16, 17]. As can be seen in Figures 6 and 7, the shapes of the hysteresis loop are mainly related to the combination of an ideal resistor and lossy capacitor response. The mobility of free charges will also contribute to higher conductivity and cause higher leaking current, which would be the main reasons for the Ec values to be unpredictable. Even though the hysteresis loops are seen to be leaky in nature, they are still significant enough to definitely confirm the ferroelectricity of the HMO and YMO samples.

#### 4. Conclusion

HoMnO3 and YMnO3 multiferroics were successfully synthesized via highenergy ball milling, and the parallel evolution of ferroelectric with microstructural properties has been studied. High-energy ball milling was able to produce high-

Figure 7.

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108

for YMO samples sintered at 1050–1250°C

(a) P-E for YMO samples sintered at 600–700°C. (b) P-E for YMO samples sintered at 750–1000°C. (c) P-E

degree crystallinity and some trends of electrical properties due to the high surface reactivity of starting materials. It must be noted that this clear understanding has been made possible only through a progressive sintering scheme of nanometer-particle compacts from an unusually low sintering temperature (600°C) to a somewhat high sintering temperature (1250°C). From the discussion presented, the evolution of a nanometer-to-micron grain size regime has been presented and the changing patterns of ferroelectrics are now clearly understood. It was found that the ferroelectric and magnetic properties generally correlated with intrinsic and extrinsic properties. The intrinsic contribution came from the contribution of the crystal structure in which orthorhombic, hexagonal, and more or less orthorhombic + hexagonal phases affected the phase that will contribute to the proper behavior of the ferroelectric. The effects of grain size of the two series of manganites have been observed by the scheme of nanosized starting particles followed by nano-to-micron grain sized regime data. Microstructural changes revealed a revolution of the crystal structure from orthorhombic to hexagonal at a larger grain size regime. The ferroelectric behavior was also observed to change with the change of microstructure along with the structural transformation from orthorhombic to hexagonal. For a general conclusion, the intrinsic effect occurred in the low sintering temperature region (600–1000°C for HoMnO3) and (600–900°C for YMnO3). The property changes at this region are due to crystal structure transformation. The extrinsic effect was more obvious at higher sintering temperature that is 1050–1250°C for HoMnO3 and 950–1250°C for YMnO3 in the hexagonal structure. The optimum condition to obtain a sample with very fine properties could be obtained by performing high-energy ball milling for 12 h followed by sintering at 1250°C with 10 h holding time. These steps were required in order to reach a very stable hexagonal structure for most advantageous ferroelectric and magnetic properties. The study of the evolution work has resulted in greater appreciation of the theoretical and experimental difficulties involved, if not in new knowledge of the behavior of multiferroic studies in evolution. In fact, there were no reported studies regarding these evolution works in the multiferroic field.
