Next Article in Journal
Structural Effects of Anomalous Current Densities on Manganese Hexacyanoferrate for Li-Ion Batteries
Next Article in Special Issue
Charge Transport Behavior of Al-Doped ZnO Incorporated with Reduced Graphene Oxide Nanocomposite Thin Film
Previous Article in Journal
The Cluster Computation-Based Hybrid FEM–Analytical Model of Induction Motor for Fault Diagnostics
Previous Article in Special Issue
Structural and Magnetic Properties of NiZn Ferrite Nanoparticles Synthesized by a Thermal Decomposition Method
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Microstructure and Magnetic Properties of La-Ca-Co Substituted M-Type Sr-Hexaferrites with Controlled Si Diffusion

1
Department of Materials Engineering and Convergence Technology, School of Materials Science and Engineering, Gyeongsang National University, Jinju 52828, Korea
2
Department of Materials Science & Engineering, Korea National University of Transportation, Chungju 27469, Korea
*
Author to whom correspondence should be addressed.
Appl. Sci. 2020, 10(21), 7570; https://doi.org/10.3390/app10217570
Submission received: 21 September 2020 / Revised: 26 October 2020 / Accepted: 26 October 2020 / Published: 27 October 2020
(This article belongs to the Special Issue Sintering Phenomena and Microstructural Control)

Abstract

:
La-Ca-Co substituted M-type Sr-hexaferrites (Sr0.3Ca0.4La0.3Fe9.8Co0.2O19-δ) were prepared by a solid-state reaction using two different procedures, where the SiO2 additive was mixed either before calcination (pre-Si) or after calcination (post-Si). At the same sintering temperature, smaller cell volumes and reduced saturation magnetization (Ms) values were obtained for samples processed with the pre-Si method than those with the post-Si method. This implied that the pre-Si method resulted in a greater degree of Si substitution into the M-type lattice and increased Fe extrusion out of the lattice. The grain growth behavior was controlled by the SiO2 amount and sintering temperature. It was found that abnormal grains occur with a bimodal distribution in the 0.5 wt% SiO2 samples sintered at 1240 °C, due to the increased critical driving force for growth caused by an increased amount of SiO2 addition. The Ms and coercivity values were altered with the control of Si diffusion and abnormal grain growth. The control of the additive diffusion behavior is one of the important keys in the material design under same materials compositions.

1. Introduction

M-type (Ba, Sr)-hexaferrites are some of the most useful materials for permanent magnets [1]. Enhancing the magnetic and mechanical properties of these materials is economically important due to their widespread use across various global industries, such as consumer electronics and the electric automotive industry. Many studies have led to developments that have improved the intrinsic magnetic properties of Sr-hexaferrites, such as the saturation magnetization (Ms) and crystalline anisotropy, by compositional design with elemental substitutions [2,3,4,5,6,7]. Among these, La-, Ca-, and Co-substitution of the M-type Sr-hexaferrites (La-Ca-Co:SrM) is considered to be the most successful approach, leading to a significant enhancement in the crystalline anisotropy without reducing the Ms [4,8]. The resulting La-Ca-Co substituted M-type Sr-hexaferrites are thus high-grade ferrite magnets.
The coercivity (Hc) is dependent on the grain size of the La-Ca-Co:SrM permanent magnets [9,10,11]. Grain growth behavior involves the movement of interfaces, including grain boundary structures, by interface reactions. This behavior can be controlled by adding dopants to alter the diffusion of atoms into the materials [12,13,14], adjusting the sintering temperature [15], and changing the oxygen partial pressure [16,17,18]. Generally, it has been understood that dopants change the microstructure of ceramics by suppressing grain growth and increasing densification [19]. These additive elements can segregate at grain boundaries and form a secondary solid or liquid phase and suppress the migration of interfaces and grain boundaries by the solute drag and Zener effects [19,20]. In the grain growth theory, grain growth behavior can be explained by the relationship between the critical driving force for grain growth (Δgc) and the maximum driving force for grain growth (Δgmax) in the two-dimensional nucleation grain growth mechanism [20,21,22,23,24,25,26,27,28,29,30]. Phenomenologically, grain growth behavior is divided into normal grain growth (invariable distribution of relative growth sizes with sintering time) and abnormal grain growth (some large grains grow quickly) [17,20,22]. The grain growth type can be changed by the value of Δgc relative to Δgmax during sintering [17,18,20,22]. The abnormal grain growth behavior can be calculated and explained exactly by the two-dimensional nucleation grain growth mechanism [22]. To the best of our knowledge, while a number of studies have analyzed the grain growth in M-type hexaferrites [6,23,24,25,26,27], no studies on La-Ca-Co:SrM grain growth have been reported. In the present study, the effects of SiO2 additives on the grain growth in La-Ca-Co:SrM were investigated. There are many previous studies on Sr hexaferrites with addition of SiO2 [12,31,32,33,34]. However, there has been no discussion of the position of Si elements regarding which Si element is in the lattice of grains or not.
The purpose of this study is to provide experimental support for the predicted effect of adding SiO2 and to determine the conditions that suppress the diffusion of Si into the crystalline lattice of the Sr0.3Ca0.4La0.3Fe9.8Co0.2O19-δ (SCLFCO) system. The specific molar fraction of the elements in SCLFCO was selected concerning the previous works [3,7,8]. The selective diffusion of Si should be controlled in order to increase Hc without decreasing Ms.

2. Materials and Methods

SiO2-doped La-Ca-Co:SrM samples were prepared by a conventional solid-state reaction that employed two different procedures, referred to as pre-Si and post-Si. In the pre-Si method, precursor powders of Fe2O3 (99%up, Kojundo Chemical Lab., Co., Ltd., Saitama Pre, Japan), SrCO3 (99.9%, Kojundo Chemical Lab., Co., Ltd., Saitama Pre, Japan), La2O3 (99.9%, Sigma-Aldrich, St. Louis, MO, USA), CaCO3 (99.9%, Sigma-Aldrich, St. Louis, MO, USA), and CoO (98%, Kojundo Chemical Lab., Co., Ltd., Saitama Pre, Japan) were weighed according to the target cation composition of Sr0.3Ca0.4La0.3Fe9.8Co0.2 and mixed together. Additionally, SiO2 (99.9%) was added to the mixture at a dose of 1 wt% of the total precursor powder. The powder mixture was ball-milled in water for 24 h using a polypropylene jar and yttria-stabilized ZrO2 balls with 3, 5, and 10 mm diameters. The dried slurry was calcined at 1100 °C in air for 4 h. The calcined powders were crushed, ball-milled in water for 24 h, dried, and pressed in a mold with a diameter of 15 mm at a pressure of 16.6 MPa to produce a disk-shaped green compact pellet. The pelletized samples were sintered in air at a temperature range of 1200–1250 °C for 2 h. During the sintering process, the heating rate was 5 °C/min and the samples were then furnace-cooled to room temperature.
In the post-Si method, the same amount of SiO2 powder as used in the pre-Si method (1 wt% of total precursor powder) was added before the second ball-milling process after calcination, as opposed to being added before the first ball-milling process before calcination. The target composition and all the other processes were the same as in the pre-Si method.
The density of the sintered sample was calculated based on the weight and geometric dimensions of the disk-shaped sample. The phase of the sintered samples was characterized using X-ray diffraction (XRD, D8 Advance, Bruker) with a Cu-Kα radiation source (λ = 0.154056 nm). The microstructures of the sintered samples were observed via field-emission scanning electron microscopy (FE-SEM, JSM-7610F, JEOL) of the fracture surfaces of the samples. Magnetization (M-H) curves of the sintered samples were measured using a vibrating sample magnetometer (VSM, 7410-S, Lake Shore) at room temperature with a sweeping magnetic field within ± 25 kOe.

3. Results and Discussion

Figure 1a–c show the XRD patterns of undoped (no-Si), pre-Si, and post-Si SCLFCO samples sintered at various temperatures. All no-Si SCLFCO samples were single phase M-type hexaferrites at all sintering temperatures (1200–1250 °C), with a hexagonal crystal structure and a space group of P63/mmc. On the other hand, when 1 wt% SiO2 was added, a secondary phase of Fe2O3 appeared in all of the samples sintered at 1200–1250 °C. Sharp peaks of Fe2O3 were found in the pre-Si samples at lower sintering temperatures, but were not observed in the post-Si samples. The substitution of Si into an Fe site could explain the appearance of an Fe2O3 solid phase. If Si diffused into the unit cell, then Fe would diffuse out and form Fe2O3.
Table 1 shows the cell volume of the SCLFCO hexaferrites, calculated based on XRD patterns (Jade 6.0, V6.2.9200, MDI, Materials Data, Inc., USA). For each sintering temperature, the cell volume was the highest in no-Si samples and the lowest in pre-Si samples. The change in cell volume could be the result of Si substitutions in Fe sites, as the ionic size of Si4+ is smaller than that of Fe3+ [35]. In addition, the observation that cell volumes of the pre-Si samples are smaller than those of the post-Si samples at each sintering temperature implies that a larger amount of Si was substituted into the hexaferrite lattice in the pre-Si method than in the post-Si method.
The excess Fe atoms form the Fe2O3 secondary phase, which could inhibit grain growth based on the Zener effect, where the movement of the interface and grain boundary is suppressed by a secondary phase on the interface and grain boundary [19,20]. When Si is added, a liquid phase can form, the structures of the interface and grain boundary can change, and the growth rate of each grain can be altered [14,20,22,28,29,30].
Figure 2a–i display SEM micrographs of no-Si, pre-Si, and post-Si SCLFCO samples sintered at various temperatures. The effect of the sintering temperature on the average grain size of each sample type is insignificant. However, the average grain size significantly decreased as SiO2 was added to all samples. The decrease in average grain size could be explained by the effect of Fe2O3 formation, as the Fe2O3 detected in the XRD patterns can suppress the migration of grain boundaries [19,20,24].
The Hc and other extrinsic properties may increase in samples with the addition of SiO2 due to a decrease in the average grain size [31,32]. However, the intrinsic properties, such as Ms, could also be changed due to the diffusion of Si into the lattice. The unit cell volumes of the samples with SiO2 addition were changed, as shown in Table 1. This phenomenon can be explained by the diffusion of Si into unit cells which changed the intrinsic properties. As shown in Table 2, the degree of change in the Ms in the pre-Si sample is larger than that in the post-Si sample. This could be due to the larger degree of Si substitution in the hexaferrite lattice in the pre-Si samples than that in the post-Si samples.
The microstructures in Figure 3 show that the coarsening behavior changes considerably with the addition of SiO2 during sintering at 1250 °C. Here, all the samples were prepared with the pre-Si procedure but used different amounts of SiO2. The addition of 0.5 wt% SiO2 caused an exaggerated, abnormal grain growth. In the samples with the addition of 1.0 and 2.0 wt% SiO2, the grain size decreased compared to the case of the undoped and 0.5 wt% SiO2-added sample. This cannot be explained with only the solute drag or Zener effects as the grain size should decrease due to these effects. According to the two-dimensional nucleation grain growth theory, abnormal grain growth can occur when the grain growth rate or migration of the interface is non-linear with Δg and is governed by the interface reaction [14,15,20,22,36,37,38,39]. In previous studies, the abnormal grain growth phenomenon was observed in hexaferrites and the results and analyses support the theory of a non-linear grain growth rate [6,23,24,25,26,27]. During the sintering of inorganic materials, grain growth occurs due to the difference in pressure induced by interface curvature and a decrease in the total interface energy [20]. The driving force for the growth of a grain is given by [20,22,39]:
Δ g = 2 γ V m 1 r * 1 r
where γ = interface energy, Vm = molar volume, r = radius of the grain of interest, and r* = critical grain radius or the radius of a grain that does not grow or shrink, and is similar to the average grain radius. Each grain has its own Δg and the largest grain has a Δgmax as schematically shown in Figure 4. Δgc is the value that determines whether appreciable grain growth occurs in two-dimensional nucleation grain growth. The critical driving force for growth can be expressed by [22,39]
Δ g c = π σ 2 k T h ln K 1 σ 2 h
where σ = step free energy, h = height of the two-dimension nucleus, k = Boltzmann constant, T = absolute temperature, and K is a constant that accounts for the diffusion coefficient and the number of nuclei per unit area. According to Equation (2), Δgc decreases as the sintering temperature increases and can be changed by altering the step free energy. The step free energy can be explained as the energy needed to form one layer and can be changed by the interface energy and structure [36]. Δgc can be altered by controlling the interface energy and structure among grains via changing temperature [15,23], adding a dopant [14,38], or changing the sintering atmosphere [16,17].
Figure 4 depicts the grain growth rate as a function of the Δg for diffusion (dashed line) and interface reactions (solid line) according the Equations (1) and (2). In the case of the no-Si sample sintered at 1250 °C, most grains can grow since the Δg of the grains is larger than the Δgc for no-Si (Δgc0Si), as shown in Figure 4a. As the grain growth behavior changes with the addition of SiO2, the Δgc for appreciable grain growth increases when adding SiO2. This is similar to the phenomenon observed when TiO2 is added to sodium bismuth titanate [14]. As seen in Figure 4a, when 0.5 wt% SiO2 was added using the pre-Si method (pre-0.5Si), Δgc0.5Si increased but is still lower than the Δgmax. Therefore, some grains can grow rapidly and encroach on one another and Ostwald ripening can occur during the initial stage of sintering, where small grains are dissolved and absorbed into larger grains. Finally, grain growth stops in samples of pre-0.5Si. Figure 4a shows that as the amount of SiO2 increases, Δgc increases until it is greater than Δgmax, resulting in no grain growth in the initial stage of sintering. The changes in the microstructure of samples with the addition of SiO2, as shown in Figure 3, are in line with the grain growth theory and are similar to previous studies on hexaferrites [24,26].
To confirm the non-linearity of the grain growth rate with Δg and understand the microstructural changes in samples with SiO2 addition, the compacted powders of pre-Si samples with 0.5 wt% SiO2 were sintered at various temperatures. The SEM images of these samples are shown in Figure 5. As seen in Figure 5b, a bimodal grain size distribution is observed in the sample sintered at 1240 °C; aside from a few grains that grew rapidly, most grains did not grow. No grain growth occurred in the samples sintered at 1200 °C and 1220 °C, resulting in a small grain size. In the grain growth theory, the Δgc increases and the diffusion rate decreases as the sintering temperature decreases [20,22,39]. The pore and grain shape are dependent on the interface (grain boundary and surface) structure [20,21]. The anisotropy of grain and pore increases as the fraction of the atomically ordered interface increases [14,21]. The Δgc increases as the fraction of the of the atomically ordered interface increases [14,18]. The spherical pore shape can explain that the anisotropy decreases and the Δgc decreases as the sintering temperature increases.
The slope of the dashed line in Figure 4b decreases as the diffusion rate and sintering temperatures decrease [22]. As the sintering temperature decreases from 1250 °C to 1240 °C, the Δgc increases and is closer to, but still below, Δgmax. This results in an abnormal growth of a few grains with a Δg greater than Δgc1240C, while most grains cannot grow. Figure 4b indicates that as the Δgc increases and the sintering temperature decreases, Δgc1220C becomes larger than Δgmax. This results in no grain growth as none of the grains have a driving force for growth at the low sintering temperatures of 1200 °C and 1220 °C. The abnormal grain growth phenomena of SCLFCO can be explained by the grain growth theory and observed via the addition of SiO2 and the change in sintering temperature. The grain growth mechanism of SCLFCO can be explained by the migration of the facet plane, consistent with previous studies [6,23,24,25,26,27].
The M-H curves of sintered samples are plotted in Figure 6a–c. The Ms and Hc values of these samples are presented in Table 2, where Ms is the magnetization value at H = 25 kOe. Comparing samples sintered at the same temperature, the no-Si samples have the largest Ms and the pre-Si samples have the smallest Ms. These results show that a Si substitution into an Fe site in the hexaferrite matrix deteriorates the magnetism. Pre-Si samples have a lower Ms value and a greater degree of substitution of Si than post-Si samples. However, the magnitude of the difference in Ms among these samples is highly dependent on the sintering temperature. At a sintering temperature of 1200 °C, the Ms of the post-Si sample is closer to that of the no-Si sample than that of the pre-Si sample, as the driving force of Si diffusion into the interior of the grains in the post-Si sample is not high enough to produce an appreciable effect. At a sintering temperature of 1250 °C, the Ms of the post-Si sample is closer to that of the pre-Si sample than that of the no-Si sample. These data show that the Si atom has a significantly higher driving force for diffusion at 1225 °C than that at 1200 °C. The driving force for diffusion is high enough to achieve full diffusion of Si at 1250 °C. It can be assumed that there is a homogeneous distribution of Si in the pre-Si grains and the Ms values of pre-Si and post-Si samples are approximately the same. Concerning the Hc of the samples, it was found that the post-Si method is more effective than the pre-Si method, as higher Hc values were obtained for post-Si samples. However, both post-Si and pre-Si samples exhibit greatly enhanced values compared to no-Si samples. As shown in Figure 2, the average grain size between the samples prepared by pre-Si and post-Si methods is not significantly different. As previously mentioned, it was observed that Si substitution into Fe sites extrudes Fe out of the matrix and forms excess Fe2O3, which prohibits grain growth. The Hc of the samples mostly depends on the grain size.

4. Conclusions

The grain growth behaviors and magnetic properties of SCLFCO systems with SiO2 addition were investigated. Typically, when a dopant is added, the grain growth is suppressed by the solute drag and Zener effects. However, abnormal grain growth behavior of SCLFCO was observed by changing the sintering temperature and the amount of SiO2 added. Thus, the grain growth behavior cannot be explained solely based on the solute drag and Zener effects. The effects of Si addition on the intrinsic magnetic property, Ms, were studied by changing the processing procedure. It was shown that Si can diffuse into the unit cell and change the intrinsic properties. At the same sintering temperature, the cell volumes of the pre-Si samples were smaller than those of the post-Si samples, which meant that a larger amount of Si was substituted into the hexaferrite lattice in the pre-Si method. The grain growth behavior of SCLFCO with SiO2 addition can be explained by a two-dimensional nucleation grain growth mechanism with a non-linear relationship between the grain growth rate and the driving force for grain growth. As the amount of SiO2 increases, the critical driving force for grain growth increases, which results in abnormal grain growth with a bimodal distribution in the 0.5 wt% SiO2 samples sintered at 1240 °C. When more SiO2 was added, the critical driving force was larger than the driving force for growth of all grains, which resulted in no grain growth in the initial stage of sintering. The growth mechanism of SCLFCO can be explained by the migration of the facet plane with the two-dimensional nucleation grain growth mechanism. The magnetic properties of Ms and Hc can be altered through the control of the Si addition process and the abnormal grain growth in hexaferrites.

Author Contributions

Y.-M.K. and K.-S.M. designed the experiments. P.-y.Y. performed the experiments and analyzed the samples. K.-S.M. analyzed and explained the grain growth behaviors of the samples. All the authors discussed the data and wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Research Fund of the Basic Science Research Program through the National Research Foundation of Korea, funded by the Ministry of Science, ICT, and Future Planning (Grant No. 2017R1C1B2002394). This study was supported by the materials and parts technology development (Grant No. 20010938), funded by the Ministry of Trade, Industry & Energy (MoTIE, Korea).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Pullar, R.C. Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics. Prog. Mater. Sci. 2012, 57, 1191. [Google Scholar] [CrossRef]
  2. Bai, J.; Liu, X.; Xie, T.; Wei, F.; Yang, Z. The effects of La–Zn substitution on the magnetic properties of Sr-magnetoplumbite ferrite nano-particles. Mater. Sci. Eng. 2000, 68, 182–185. [Google Scholar] [CrossRef]
  3. Ogata, Y.; Takami, T.; Kubota, Y. Development of La-Co Substituted Ferrite Magnets. J. Jpn. Soc. Powder Powder Metall. 2003, 50, 636–641. [Google Scholar] [CrossRef] [Green Version]
  4. Kobayashi, Y.; Hosokawa, S.; Oda, E.; Toyota, S. Magnetic properties and composition of Ca-La-Co M-type ferrites. J. Jpn. Soc. Powder Powder Metall. 2008, 55, 541–546. [Google Scholar] [CrossRef] [Green Version]
  5. Kang, Y.-M.; Kwon, Y.-H.; Kim, M.-H.; Lee, D.-Y. Enhancement of magnetic properties in Mn–Zn substituted M-type Sr-hexaferrites. J. Magn. Magn. Mater. 2015, 382, 10–14. [Google Scholar] [CrossRef]
  6. Kang, Y.-M.; Moon, K.-S. Magnetic properties of Ce–Mn substituted M-type Sr-hexaferrites. Ceram. Int. 2015, 41, 12828–12834. [Google Scholar] [CrossRef]
  7. Kang, Y.-M. High saturation magnetization in La–Ce–Zn–doped M-type Sr-hexaferrites. Ceram. Int. 2015, 41, 4354–4359. [Google Scholar] [CrossRef]
  8. Kobayashi, Y.; Nishiuchi, T.; Oda, E.; Nakagawa, T. Cation distribution analysis of Ca-La-Co M-type ferrites by neutron diffraction, extended X-ray absorption fine structure and X-ray magnetic circular dichroism. J. Jpn. Soc. Powder Powder Metall. 2016, 63, 101–108. [Google Scholar] [CrossRef] [Green Version]
  9. Nishio, H.; Minachi, Y.; Yamamoto, H. Effect of Factors on Coercivity in Sr–La–Co Sintered Ferrite Magnets. IEEE Trans. Magn. 2009, 45, 5281–5288. [Google Scholar] [CrossRef]
  10. Sixtus, K.J.; Kronenberg, K.J.; Tenzer, R.K. Investigations on Barium Ferrite Magnets. J. Appl. Phys. 1956, 27, 1051–1057. [Google Scholar] [CrossRef]
  11. Kang, Y.-M.; Lee, J.; Kang, Y.J.; Park, J.-B.; Kim, S.I.; Lee, S.M.; Ahn, K. Understanding on coercivity behavior of M-type strontium hexaferrite through thin-film experiment and micromagnetic modeling. Appl. Phys. Lett. 2013, 103, 122407. [Google Scholar] [CrossRef]
  12. Park, C.W.; Yoon, D.Y. Effects of SiO2, CaO2, and MgO additions on the grain growth of alumina. J. Am. Ceram. Soc. 2000, 83, 2605–2609. [Google Scholar] [CrossRef]
  13. Fisher, J.G.; Kim, M.S.; Lee, H.Y.; Kang, S.-J.L. Effect of Li2O and PbO additions on abnormal grain growth in the Pb(Mg1/3Nb2/3)O3-35 mol% PbTiO3 system. J. Am. Ceram. Soc. 2004, 87, 937–942. [Google Scholar] [CrossRef]
  14. Moon, K.-S.; Rout, D.; Lee, H.-Y.; Kang, S.-J.L. Effect of TiO2 addition on grain shape and grain coarsening behavior in 95Na1/2Bi1/2O3-BaTiO3. J. Eur. Ceram. Soc. 2011, 31, 1915–1920. [Google Scholar] [CrossRef]
  15. Cho, K.; Yoon, D.Y.; Kim, B.K. Surface roughening transition and coarsening of NbC grains in liquid cobalt-rich matrix. J. Am. Ceram. Soc. 2004, 87, 443–448. [Google Scholar] [CrossRef]
  16. Lee, B.K.; Chung, S.Y.; Kang, S.-J.L. Grain boundary faceting and abnormal grain growth in BaTiO3. Acta Mater. 2000, 48, 1575–1580. [Google Scholar] [CrossRef]
  17. Jung, Y.-I.; Choi, S.Y.; Kang, S.-J.L. Effect of oxygen partial pressure on grain boundary structure and grain growth behavior in BaTiO3. Acta Mater. 2006, 54, 2849–2855. [Google Scholar] [CrossRef]
  18. Fisher, J.G.; Rout, D.; Moon, K.-S.; Kang, S.-J.L. Structural changes in potassium sodium niobate ceramics sintered in different atmospheres. J. Alloy. Compd. 2009, 479, 467–472. [Google Scholar] [CrossRef]
  19. Boutz, M.M.R.; Winnubst, A.J.A.; Burggraaf, A.J. Yttria-ceria stabilized tetragonal zirconia polycrystals: Sintering, grain growth and grain boundary segregation. J. Eur. Ceram. Soc. 1994, 13, 89–102. [Google Scholar] [CrossRef] [Green Version]
  20. Kang, S.-J.L. Sintering: Densification, Grain Growth and Microstructure; Elsevier Butterworth-Heinemann: Oxford, UK, 2005. [Google Scholar]
  21. Howe, J.M. Interfaces in Materials, 1st ed.; John Wiley & Sons: New York, NY, USA, 1997. [Google Scholar]
  22. Jung, Y.I.; Yoon, D.Y.; Kang, S.-J.L. Coarsening of polyhedral grains in a liquid matrix. J. Mater. Res. 2009, 24, 2949–2959. [Google Scholar] [CrossRef] [Green Version]
  23. Moon, K.-S.; Kang, Y.-M.; Han, I.; Lee, S.-E. Grain growth behavior of Ba1.5Sr1.5Co2Fe24O41 flakes in molten salt synthesis and the magnetic properties of flake/polymer composites. J. Appl. Phys. 2016, 120, 194102. [Google Scholar] [CrossRef]
  24. Moon, K.-S.; Kang, Y.-M. Structural and magnetic properties of Ca-Mn-Zn-substituted M-type Sr-hexaferrites. J. Eur. Ceram. Soc. 2016, 36, 3383–3389. [Google Scholar] [CrossRef]
  25. Fisher, J.G.; Sun, H.; Kook, Y.-G.; Kim, J.-S.; Le, P.G. Growth of single crystals of BaFe12O19 by solid state crystal growth. J. Magn. Magn. Mater. 2016, 416, 384–390. [Google Scholar] [CrossRef]
  26. Moon, K.-S.; Kang, Y.-M. Synthesis, structure, and magnetic properties of M-W hexaferrite composites. Ceram. Int. 2017, 43, 14309–14313. [Google Scholar] [CrossRef]
  27. Moon, K.-S.; Lim, E.-S.; Kang, Y.-M. Effect of Ca and La substitution on the structure and magnetic properties of M-type Sr-hexaferrites. J. Alloy. Compd. 2019, 771, 350–355. [Google Scholar] [CrossRef]
  28. Park, Y.J.; Hwang, N.M.; Yoon, D.Y. Abnormal growth of faceted (WC) grains in a (Co) liqiud matrix. Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 1996, 27, 2809–2819. [Google Scholar] [CrossRef]
  29. Rohrer, G.S.; Rohrer, C.L.; Mullins, W.W. Coarsening of faceted crystals. J. Am. Ceram. Soc. 2002, 85, 675–682. [Google Scholar] [CrossRef]
  30. Choi, S.Y.; Kang, S.-J.L.; Chung, S.Y. Abnormal grain growth and intergranular amorphous film formation in BaTiO3. J. Am. Ceram. Soc. 2007, 90, 645–648. [Google Scholar] [CrossRef]
  31. Töpfer, J.; Schwarzer, S.; Senz, S.; Hesse, D. Influence of SiO2 and CaO additions on the microstructure and magnetic properties of sintered Sr-hexaferrite. J. Eur. Ceram. Soc. 2005, 25, 1681–1688. [Google Scholar] [CrossRef]
  32. Taguchi, H.; Hirata, F.; Takeishi, T. Effect of SiO2-SrCO3 Addition on Magnetic Properties of Anisotropic Ba-ferrite (Ferrite Magnets Having Equal Shrinkage Ratio in A-axis and C-axis). J. Jpn. Soc. Powder Powder Metall. 1996, 43, 19–24. [Google Scholar] [CrossRef] [Green Version]
  33. Hussain, S.; Anis-ur-Rehman, M.; Maqsood, A.; Awan, M.S. The effect of SiO2 addition on structural, magnetic and electrical properties of strontium hexaferrites. J. Cryst. Growth 2006, 297, 403–410. [Google Scholar] [CrossRef]
  34. Huang, C.C.; Jiang, A.H.; Hung, Y.H.; Liou, C.H.; Wang, Y.C.; Lee, C.P.; Hung, T.Y.; Shaw, C.C.; Kuo, M.F.; Cheng, C.H. Influence of CaCO3 and SiO2 additives on magnetic properties of M-type Sr-ferrites. J. Magn. Magn. Mater. 2018, 451, 288–294. [Google Scholar] [CrossRef]
  35. Weast, R.C. (Ed.) CRC Handbook of Chemistry and Physics, 68th ed.; CRC Press: Boca Raton, FL, USA, 1987; p. F-159. [Google Scholar]
  36. Yoon, D.Y.; Park, C.W.; Koo, J.B. The Step Growth Hypothesis for Abnormal Grain Growth. In Ceramic Interfaces 2; Yoo, H.-I., Kang, S.-J.L., Eds.; Maney Publishing: London, UK, 2001; pp. 3–21. [Google Scholar]
  37. Fisher, J.G.; Choi, S.Y.; Kang, S.-J.L. Abnormal grain growth in barium titanate doped with alumina. J. Am. Ceram. Soc. 2006, 89, 2206–2212. [Google Scholar] [CrossRef]
  38. Kim, M.S.; Fisher, J.G.; Kang, S.-J.L.; Lee, H.Y. Grain growth control and solid-state crystal growth by Li2O/PbO addition and dislocation introduction in the PMN-35PT system. J. Am. Ceram. Soc. 2006, 89, 1237–1243. [Google Scholar] [CrossRef]
  39. Kang, S.-J.L.; Lee, M.G.; An, S.M. Microstructural evolution during sintering with control of the interface structure. J. Am. Ceram. Soc. 2009, 92, 1464–1471. [Google Scholar] [CrossRef]
Figure 1. XRD patterns of no-Si, pre-Si, and post-Si SCLFCO samples sintered at (a) 1200 °C, (b) 1225 °C, and (c) 1250 °C.
Figure 1. XRD patterns of no-Si, pre-Si, and post-Si SCLFCO samples sintered at (a) 1200 °C, (b) 1225 °C, and (c) 1250 °C.
Applsci 10 07570 g001
Figure 2. SEM micrographs of no-Si, pre-Si, and post-Si SCLFCO samples sintered at (a) 1200 °C, (b) 1225 °C, and (c) 1250 °C.
Figure 2. SEM micrographs of no-Si, pre-Si, and post-Si SCLFCO samples sintered at (a) 1200 °C, (b) 1225 °C, and (c) 1250 °C.
Applsci 10 07570 g002
Figure 3. SEM micrographs of SCLFCO samples sintered at 1250 °C with varying amounts of SiO2 additions using the pre-Si method: (a) No-Si, (b) 0.5 wt%, (c) 1.0 wt%, and (d) 2.0 wt%.
Figure 3. SEM micrographs of SCLFCO samples sintered at 1250 °C with varying amounts of SiO2 additions using the pre-Si method: (a) No-Si, (b) 0.5 wt%, (c) 1.0 wt%, and (d) 2.0 wt%.
Applsci 10 07570 g003
Figure 4. Schematic showing the growth rate of a crystal as a function of the driving force for diffusion (dashed lines) and a mix of diffusion and interface reaction control mechanisms (solid lines). (a) Plot of three mixed control curves for no-Si (0Si), 0.5 wt% SiO2 (0.5Si), and 1.0 wt% SiO2 (1.0Si) SCLFCO samples sintered at the same temperature. (b) Plots of three mixed control curves for 0.5 wt% SiO2 SCLFCO samples sintered at 1220 °C, 1240 °C, and 1250 °C.
Figure 4. Schematic showing the growth rate of a crystal as a function of the driving force for diffusion (dashed lines) and a mix of diffusion and interface reaction control mechanisms (solid lines). (a) Plot of three mixed control curves for no-Si (0Si), 0.5 wt% SiO2 (0.5Si), and 1.0 wt% SiO2 (1.0Si) SCLFCO samples sintered at the same temperature. (b) Plots of three mixed control curves for 0.5 wt% SiO2 SCLFCO samples sintered at 1220 °C, 1240 °C, and 1250 °C.
Applsci 10 07570 g004
Figure 5. SEM micrographs of 0.5 wt% SiO2 pre-Si SCLFCO samples sintered at (a) 1250 °C, (b) 1240 °C, (c) 1220 °C, and (d) 1200 °C.
Figure 5. SEM micrographs of 0.5 wt% SiO2 pre-Si SCLFCO samples sintered at (a) 1250 °C, (b) 1240 °C, (c) 1220 °C, and (d) 1200 °C.
Applsci 10 07570 g005
Figure 6. M-H curves of no-Si, pre-Si, and post-Si SCLFCO samples sintered at (a) 1220 °C, (b) 1225 °C, and (c) 1250 °C.
Figure 6. M-H curves of no-Si, pre-Si, and post-Si SCLFCO samples sintered at (a) 1220 °C, (b) 1225 °C, and (c) 1250 °C.
Applsci 10 07570 g006
Table 1. Densities and unit cell volumes of sintered samples.
Table 1. Densities and unit cell volumes of sintered samples.
Sintering
Temperature
1 wt% SiO2 AdditionDensity, g/cm3Unit Cell Vol., Å3
1200 °Cno-Si4.91688.481
post-Si4.55687.746
pre-Si4.78687.518
1225 °Cno-Si4.89689.005
post-Si4.72688.073
pre-Si4.89687.707
1250 °Cno-Si4.95689.025
post-Si4.82687.144
pre-Si4.93686.250
Table 2. Ms and Hc of sintered samples measured by vibrating sample magnetometer (VSM).
Table 2. Ms and Hc of sintered samples measured by vibrating sample magnetometer (VSM).
Sintering Temperature1 wt% SiO2 AdditionHc, OeMs, emu/g
1200 °Cno-Si227268.01
post-Si356067.04
pre-Si323562.48
1225 °Cno-Si221669.09
post-Si358165.37
pre-Si329363.26
1250 °Cno-Si229868.16
post-Si340064.98
pre-Si335164.85
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Moon, K.-S.; Yu, P.-y.; Kang, Y.-M. Microstructure and Magnetic Properties of La-Ca-Co Substituted M-Type Sr-Hexaferrites with Controlled Si Diffusion. Appl. Sci. 2020, 10, 7570. https://doi.org/10.3390/app10217570

AMA Style

Moon K-S, Yu P-y, Kang Y-M. Microstructure and Magnetic Properties of La-Ca-Co Substituted M-Type Sr-Hexaferrites with Controlled Si Diffusion. Applied Sciences. 2020; 10(21):7570. https://doi.org/10.3390/app10217570

Chicago/Turabian Style

Moon, Kyoung-Seok, Pyeong-yeol Yu, and Young-Min Kang. 2020. "Microstructure and Magnetic Properties of La-Ca-Co Substituted M-Type Sr-Hexaferrites with Controlled Si Diffusion" Applied Sciences 10, no. 21: 7570. https://doi.org/10.3390/app10217570

APA Style

Moon, K.-S., Yu, P.-y., & Kang, Y.-M. (2020). Microstructure and Magnetic Properties of La-Ca-Co Substituted M-Type Sr-Hexaferrites with Controlled Si Diffusion. Applied Sciences, 10(21), 7570. https://doi.org/10.3390/app10217570

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop