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Sintering-dependent mechanical and magnetic properties of spinel cobalt ferrite (CoFe2O4) ceramics prepared via sol-gel synthesis
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Sintering-dependent mechanical and magnetic properties of spinel cobalt ferrite (CoFe2O4) ceramics prepared via sol-gel synthesis Luis Eduardo Caldeiraa,∗, Waleska C. Guaglianonia, Janio Venturinia,b, Sabrina Arcaroc, Carlos Pérez Bergmanna,b, Saulo Roca Bragançaa a
Graduate Program in Mining, Metallurgical and Materials Engineering, Laboratory of Ceramics (LACER), Federal University of Rio Grande Do Sul, Av. Osvaldo Aranha 99, Porto Alegre, 90035-190, Brazil b Department of Industrial Engineering, Federal University of Rio Grande Do Sul, Av. Osvaldo Aranha 99, Porto Alegre, 90035-190, Brazil c Graduate Program in Materials Science and Engineering (PPGCEM), Laboratory of Technical Ceramics (CerTec), Universidade Do Extremo Sul Catarinense, Av. Universitaria 1105, P.O. Box 3167, Criciuma, 88806-000, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Ferrites CoFe2O4 Sol-gel Conventional sintering Mechanical hardness
Despite increasing interest in cobalt ferrites as advanced ceramics, there is still much uncertainty regarding the relationship between sintering temperature and the magnetic and mechanical properties of CoFe2O4. Precursors prepared via sol-gel synthesis were treated at 750, 800, 850 and 900 °C, pressed into pellets and fired at 1150 and 1200 °C. Samples were analyzed via XRD, Raman spectroscopy, SEM, VSM and Vickers microhardness. Pure CoFe2O4 ceramics were produced from hematite-containing precursors. While magnetic parameters remained unaffected, a significant influence was observed for CoFe2O4 grain size and morphology. The highest hardness, obtained for the 1150 °C samples, was directly associated to smaller grain sizes. Mechanical behavior diverged from the classical Hall-Petch mechanism, likely due to lower grain size homogeneity. Our study achieved a thorough understanding of the magnetic and mechanical behavior of CoFe2O4 ceramics, enabling the optimized selection of processing according to the desired final properties of these materials.
1. Introduction There is a growing body of literature recognizing that electromagnetic pollution has become a severe problem worldwide [1,2]. It occurs due to the advancement of electronic technology, not only causing interference to electronic equipment but also being harmful to human health [3]. Several studies have reported advances in controlling and reducing electromagnetic pollution through microwave absorbing materials [4–6]. These materials are capable of absorbing electromagnetic waves, converting this energy into other forms of energy or acting as interference agents, dissipating electromagnetic waves [7]. Some rare earth materials have interesting applications as microwave absorbers [8,9], but due to the uneven distribution of economically-viable rare-earth deposits around the world, it has become necessary to find substitutes for this function. Among the materials that have the potential for microwave absorption, ferrites have been actively studied [10–12] for their remarkable structural [13,14], dielectric [15] and magnetic properties [16], as well as being cost-effective [17] and lightweight [18], with relatively easy processing and high stability
∗
[19]. Ferrites can be classified into groups, some of which are the spinel, garnet and hexaferrites, according to the crystallization of their structures [20]. Among this classification, spinel ferrites (MFe2O4) are currently some of the most important and versatile materials in terms of advanced ceramics. Their vast field of its application and optimal responses in terms of magnetic properties account for the interest of the scientific and industrial community in this family of materials [21,22]. Cobalt ferrites (CoFe2O4), in particular, have shown interesting magnetization results [23,24], such as high coercivity and remanence, also presenting cost-effectiveness, shape versatility and high chemical and structural stability when processed at high temperatures [25,26]. CoFe2O4 crystallizes in a spinel cubic structure, with a fcc packing of oxygen anions and the cations distributed among the tetrahedral (A) and octahedral (B) sites of the framework [26]. More specifically, this ferrite usually exhibits an inverse spinel structure, with trivalent metal ions (Fe3+) occupying A sites while the B sites are filled by both divalent (Co2+) and trivalent ions (Fe3+). The ferrimagnetism presented by cobalt ferrites is a consequence of the antiparallel spin alignment between the cations present in the A and B sites [27]. The methods used for the synthesis of magnetic cobalt ferrite include sol-gel,
Corresponding author. E-mail address: [email protected] (L.E. Caldeira).
https://doi.org/10.1016/j.ceramint.2019.09.240 Received 26 August 2019; Received in revised form 23 September 2019; Accepted 24 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Luis Eduardo Caldeira, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.240
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hydrothermal synthesis, combustion, thermolysis, wet chemical coprecipitation, flow injection synthesis and microwave plasma [28,29]. Among these processes, the sol-gel synthesis stands out due the ability to obtain materials with homogeneous, crystalline, and high-purity structures [30]. A wide field of application – such as permanent magnets, electronic components and devices, microwave devices and high-density recording storage – can be assigned to cobalt ferrite due to its outstanding electronic and magnetic properties [31,32]. Despite such properties being widely studied, the mechanical behavior of these materials is also of paramount importance for their full realization as an advanced magnetic material. It is well established that mechanical resistance is required in magnetic materials due to their fragile character. Nevertheless, there is still a wide gap in the literature when it comes to studies focused on the mechanical properties of CoFe2O4 ceramics and their correlation with the structural properties of these materials. This work presents, for the first time, a detailed study on the mechanical, magnetic and structural properties of CoFe2O4 spinel ferrites prepared via sol-gel. This work aimed to find an optimized route for the production of ferrite ceramics while defining a relationship between their properties and the firing temperature. Complete characterizations of different cobalt ferrite powders and sintered pellets were performed. Our results show that the mechanical properties of CoFe2O4 can be improved by varying sintering temperatures, seemingly without affecting the magnetic behavior of these ceramics. We also show that the purity of the precursors does not influence the phases present in the final ceramics, which could result in a decrease in costs involved with raw materials. Furthermore, we also show that the Hall-Petch relation does not hold for the produced materials, which was related to a large fraction of smaller particles in the sintered materials. The findings expand the application field of this material, further enabling the utilization of cobalt ferrite as an advanced rare-earth magnet substitute.
The second part of the characterization refers to the analysis of the CoFe2O4 fired samples. The apparent densities (ρAp) were determined by the Archimedes method (ASTM C373-88) at 25 °C, using a Shimadzu equipment (AX200, Japan) equipped with a device for density measurement. The structure was characterized by XRD and Raman. Likewise, the pellets were also subjected to VSM analysis. The microstructure of each fired sample was observed on a fracture surface using a SEM. The linear intercept method (ASTM E112-13) was used to estimate the grain size distribution from the micrographs using an image analysis software (ImageJ). Vickers hardness tests were performed, with ten measurements for each sample, using a Buehler Micromet 2000 with a load of 3 N. 3. Results and discussion 3.1. Powders In order to select the thermal processing temperatures for the production of the CoFe2O4 powders, the thermal behavior of the xerogel from room temperature up to 1300 °C was investigated. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) data are shown in Fig. 1. At first, a mass loss of 4% from room temperature up to 140 °C is observed. This event can be associated to the evaporation of residual water from the gel structure and also from the coordination spheres of the cations. A second mass loss of 20% occurs in the temperature range between 140 and 280 °C. This mass loss is usually associated to the decomposition of uncoordinated hydroxyl or carboxyl functional groups from the citric acid [33]. Subsequently, a third and most pronounced mass loss event (280–360 °C) takes place, accompanied by a strongly exothermic DSC peak centered at 360 °C. This reaction corresponds to the rapid decomposition of residual organic matter in the presence of nitrate. This distinct mass loss of 35% relates to the violent autocombustion of nitrates with citric acid, resulting in an intense heat release. The total weight loss is of approximately 59% for the precursor of CoFe2O4; only a small weight loss of ~ 2% can be observed above 360 °C. A single exothermic peak at 360 °C occurred along the heating of the xerogel, confirming that no other reaction took place; above this temperature, the material is thermally stable. Nevertheless, temperatures slightly above the onset of autoignition have previously shown to lead to particles of nanometric dimensions, which would hinder the proper pressing of the green pellets for further firing [34]. Thence, a temperature range 750–900 °C was selected. The crystalline structures of CoFe2O4 samples prepared via sol-gel technique and thermally treated at 750, 800, 850 and 900 °C were
2. Materials and methods Cobalt ferrite (CoFe2O4) powders were prepared via sol-gel synthesis using citric acid as a complexant. A mixture of Co(NO3)2·6H2O (Dinamica, 98%), Fe(NO3)3·9H2O (Synth, 98%) and citric acid (Exodo Cientifica, 99.5%) were used in a molar ratio of 1:2:3 and dissolved in 20 mL of Ultra-pure Type I water for the production of 10 mmol of product. The solution was heated to 85 °C for 1 h under magnetic stirring. The resulting gel was dried at 110 °C for 24 h. The xerogel was transferred to an alumina crucible and heat treated for 6 h at different temperatures (750, 800, 850 and 900 °C), previously selected by the thermal analysis of the precursor gel. Powder samples were uniaxially pressed into pellets (10 mm in diameter and 5 mm thick) using a cylindrical steel die through a hydraulic press (ST Bovenau P10, Brazil) at 100 MPa. The produced pellets were fired at 1150 and 1200 °C for 6 h at a heating rate of 10 °C.min−1. In order to select the treatment temperatures for the synthesis of CoFe2O4 powders, simultaneous thermogravimetric and differential scanning calorimetry analyses (TG/DSC, SDT Q600, TA Instruments) were performed. The experiment examined the thermal decomposition behavior of the gels in synthetic air at a heating rate of 10 °C.min−1. The crystalline structure of the precursors was characterized in an X-ray diffractometer (XRD, Philips, X'pert MPD) equipped with a Cu-Kα source. Samples were analyzed in a 2θ range of 10 to 75°, using a step size of 0.05° and dwell time of 1 s per step. The resulting crystalline phases were identified by ICSD data banks. The microstructure of the powders was observed using a scanning electron microscope (SEM, EVO MA10, Carl Zeiss), equipped with a secondary electron detector. Raman studies were performed in an inVia Renishaw Raman spectrometer equipped with a 532 nm laser, in a range of 100–1000 cm−1. Magnetic hysteresis curves were acquired in a vibrating sample magnetometer (VSM, Model EZ9, Microsense).
Fig. 1. TG (black line) and DSC (blue line) curves of the CoFe2O4 xerogel. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
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Fig. 2. X-ray diffractograms of the CoFe2O4 powders treated thermally at 750, 800, 850 and 900 °C. Indexed reflections are related to spinel CoFe2O4. Reflections identified with a rhombus are related to the presence of α-Fe2O3 (hematite).
Fig. 3. SEM images of the CoFe2O4 powder precursors treated thermally at varying temperatures: (a) 750, (b) 800, (c) 850 and (d) 900 °C.
analyzed via X-ray diffraction. The results are shown in Fig. 2. The main reflections are all related to spinel cobalt ferrite (CoFe2O4, JCPDS 00001-1121, Fd 3 m , nº 227), demonstrating that a successful synthesis of cobalt ferrite was achieved under all selected temperatures to heat treatment. Five reflections – identified in Fig. 2 by rhombuses – indicate the presence of a secondary phase of hematite (α-Fe2O3, JCPDS 01-0840310, R 3 c , nº. 167). The intensities of the reflections related to hematite increase with an increase in treatment temperature until 850 °C, above which the intensities decrease. The powder obtained through an 850 °C treatment showed the highest intensity of the reflections related to the hematite phase. This phenomenon does not necessarily mean a higher quantity of hematite; it could instead be related to a higher crystallinity of the sample. Above 850 °C, the decrease of the secondary phase can also arise from its solubilization into the structure of magnetite; this phenomenon is expected when the mixture is submitted to higher temperatures, according to Fe–O phase diagrams [35]. Analyses of the microstructure of the precursors heat-treated at different temperatures were also performed. SEM micrographs of the CoFe2O4 powders (Fig. 3) show the expected microstructure of magnetic cobalt ferrite prepared via sol-gel. The sample treated at 750 °C (Fig. 3a) showed a morphology composed of smaller and agglomerated grains, when compared to the images of the other samples. As previously seen in the XRD results, a possible second phase also appears in the SEM images. The image of the sample treated at 800 °C (Fig. 3b) exhibits some scattered lighter grains, i.e., phases in which the electronic conductivity might differ from the other regions, thus possibly indicating the presence of a secondary phase. Although still presenting agglomerates, this image shows that the temperature input contributed to the modification of the sample morphology; the ferrite grains show a marked growth along the increase in treatment temperature. This variation becomes even more evident in the other samples (Fig. 3c and d). The rise in treatment temperature also contributed to the increase in the volume fraction of the secondary phase, as expected from their diffractograms (Fig. 2). The composition of the synthesized cobalt ferrite was also evaluated via Raman spectroscopy. Fig. 4 shows the Raman spectra of the powders treated at different temperatures. All signals are related to cobalt ferrite, with no remaining signals indicating secondary phases. According to factor group analysis, five modes are Raman-active in spinels: three T2g, one Eg and one A1g mode [36]. One T2g mode corresponds to the translational movement of metal ions at the tetrahedral
Fig. 4. Raman spectra of the CoFe2O4 powders treated thermally at 750, 800, 850 and 900 °C.
site, while the Eg signal is attributed to the symmetric bending of the metal-oxygen bond. The remaining T2g modes are an anti-symmetric stretch and anti-symmetric bending of the metal-oxygen bond, respectively. The last mode, A1g, corresponds to the symmetric stretching of oxygen atoms along the Me–O bond in the tetrahedral sites. The five Raman active modes T2g (196 cm−1), Eg (304 cm−1), T2g (464 cm−1), T2g (560 cm−1) and A1g (685 cm−1) are identified in the spectra of the prepared powders. Furthermore, a splitting of the A1g mode is observed, with a secondary signal at approximately 613 cm−1. According to the literature, this band can be attributed to Co–O vibrations at tetrahedral sites [37]. The presence of Co2+ ions in these positions leads to a reduction in symmetry of the original spinel structure of the ferrite [38]. This splitting, therefore, indicates a higher inversion degree of the spinel structure. Besides, treatment temperature also influenced the position of the modes. A slight shift of the Raman modes can be observed for the sample treated at 900 °C. As reported by Ref. [39], the mode shift associated with the increase of temperature occurs due to 3
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Fig. 5. Hysteresis curves of the cobalt ferrite powders treated thermally at varying temperatures.
Fig. 6. Densities of the CoFe2O4 pellets fired at 1150 and 1200 °C, prepared from powders at different treatment temperatures.
the increase in particle size and to the redistribution of cations among tetrahedral and octahedral sites. The influence of the treatment temperature on the magnetic properties of the precursors was verified via VSM analysis. The hysteresis curves of the powder samples are shown in Fig. 5. All hysteresis curves display a classic ferrimagnetic behavior, as would be expected of spinel CoFe2O4, albeit with different remanence and coercivity values. This variation shows that the thermal treatment within the selected range has indeed affected the magnetism of the precursors. The magnetic parameters obtained from the hysteresis curves are displayed in Table 1. Both the remanent (Mr) and the saturation (Ms) magnetization of the precursors decrease along increases in the treatment temperature. This factor is likely related to the increase in the mass fraction of hematite present in the sample. Since the magnetization of this material is virtually negligible when compared to cobalt ferrite [40], the presence of the secondary phase dilutes the specific magnetism displayed by the material. The maximum value of Mr among the samples, which is the condition that the material continues magnetic even in the absence of a magnetic field, belongs to the powders treated at 750 °C (22.68 emu.g−1), which is also the material with the lowest fraction of hematite. This parameter decreases linearly along increasing temperatures, reaching a value of 14.20 emu.g−1 for the powder prepared at 900 °C. The same decrease tendency occurs for Ms, as seen in Table 1. Coercivity (Hc) follows a similar pattern of Mr and Ms, with decreasing values along increases in treatment temperature. The sample treated at 750 °C reached a coercivity of 715 Oe, while the value of 498.1 Oe was observed for the 900 °C sample. The increase in particle size – i.e., a tendency away from single-domain particles – would incur in slightly decreasing coercivity [41], as observed in Table 1. The grain growth observed in SEM images of the samples (Fig. 3) corroborates the mentioned decrease, since the particle sizes increase with the increase of treatment temperature.
3.2. Fired Samples The previously characterized powders – thermally treated at 750, 800, 850 and 900 °C – were used as precursors for the production of the pellets, obtained at two firing temperatures, 1150 and 1200 °C. These conditions were selected based on literature describing the densification of cobalt ferrite ceramics [42,43]. Fig. 6 shows the apparent densities of the fired pellets. The density results did not present any significant variation. For instance, a difference of only 0.03 g cm−3 was observed between the pellets thermally treated at 750 °C and fired at 1150 and 1200 °C (5.08 and 5.11 g cm−3, respectively). Such minor variation was observed repeatedly for all samples. The minimum apparent density was of 5.04 g cm−3 (for pellets obtained by precursors treated at 850 °C and later fired at 1150 °C) and the maximum was of 5.12 g cm−3 (treatment temperature of 850 °C and fired at 1200 °C). Nevertheless, the standard deviation of the measurements does not indicate a meaningful variation between samples. Compared to the theoretical density of CoFe2O4 (5.31 g cm−3), it was observed that the relative densities are all found in a range between 95% and 96.4%. The increase in synthesis temperature of the precursors, as well as the increase of the firing temperature of the pellets, had no influence on the relative density of the fired pellets. The density values prove their densification – thus agreeing with literature data – and also indicate a rather low porosity for the fired materials. Therefore, one can assume that other factors had a more pronounced effect over the final density displayed by the samples. The fired pellets were also analyzed with regard to their crystalline structure, as shown in the X-ray diffractograms in Fig. 7. The hematite (Fe2O3) phase present in the powders is not visible on the diffractograms of the fired pellets; a single phase of CoFe2O4 was achieved under both firing conditions for all precursors. The phenomenon observed in the diffractograms is quite striking, since it shows that is possible to obtain a pure final material with the adopted method of firing, regardless of the purity of the precursors. The complete disappearance of the second phase of Fe2O3 in the fired ferrites can be directly indexed to the incorporation of this secondary phase into the crystalline structure of the spinels. As stated earlier about the X-ray diffractograms of the powders, the hematite phase solubilizes in the spinel structure at higher temperatures. A small shift is noticed on the angles of the reflections, which is likely related to the fact that the X-ray diffractograms were obtained from solid samples; the reflection angle shifts arise due to the fact that solid pellets can have directionality and much less anisotropy than a powder sample.
Table 1 Magnetic parameters of the CoFe2O4 powders prepared by different thermal treatments. Temperature (°C)
Hc (Oe)
Mr (emu.g−1)
Ms (emu.g−1)
Mr/Ms
750 800 850 900
715.0 584.2 538.5 498.1
22.68 20.13 15.27 14.20
75.22 62.35 49.23 50.70
0.302 0.323 0.310 0.280
Note: Hc – coercivity; Mr – remanence; Ms – saturation; Mr/Ms – squareness. 4
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to verify the influence of firing temperature on the magnetic properties of CoFe2O4. The fired pellets were subjected to VSM analysis, as were the precursors. The hysteresis curves of the samples treated at 750, 800, 850 and 900 °C and later fired at 1150 and 1200 °C are shown in Fig. 9. A classic ferrimagnetic behavior is displayed once again, as occurred with precursors, thus showing that the firing process maintained the spin alignment of the structure. Furthermore, no significant difference in the values of coercivity and remanent magnetization was observed between the samples. The effect observed in Fig. 9 strongly indicates that the cobalt ferrite materials have a unique magnetic behavior when fired at higher temperatures, regardless of the previous thermal treatment to which they were subjected for the obtaining of the precursors. Such invariance can be readily related to the similar Raman spectra of the samples; since they indicated very similar cationic arrangements, it follows that the net spin alignment should not vary amongst the samples, and neither should their magnetic properties. Magnetic parameters, such as coercivity (Hc), remanence (Mr), saturation magnetization (Ms) and squareness, are displayed in Table 2. The Hc and Mr values of Table 2 are similar to the ones obtained from the precursor thermally treated at 900 °C (Table 1). Considering the treatment temperatures and the firing temperatures adopted in this work, it was observed that the coercivity and remanence tend to decrease with increasing treatment temperature, saturating at temperatures above 900 °C. Thus, this phenomenon indicates a stabilization of magnetic properties at treatment temperatures above 900 °C, stating that the magnetic behavior of CoFe2O4 materials is not susceptible to changes caused by firing at high temperatures. SEM micrographs were obtained by analyzing the fresh fractures of all fired samples (Fig. 10). Microstructural changes, especially regarding grain dimensions, can be noted both along increases in synthesis temperature of the powders and along increases in firing temperature. Initially, the micrograph of the precursor heat treated at 750 °C and fired at 1150 °C shows individualized grains with rounded and hexagonal morphology. These observations indicate that the applied temperatures are already high enough to allow the sintering process to take place, which intensifies as the temperature increases. As it is possible to observe in the micrographs, the grains show considerable growth and become ever more rounded. As expected, with the increase in treatment temperature from 1150 to 1200 °C, an increase in grain size is observed. The main phenomenon observed in the thermal treatment was grain growth, which can be easily noticed by observing the grain size of the 750 °C samples sintered at 1150 °C and 1200 °C (1.36 μm and 2.56 μm, respectively). The appearance of grains with more than six sides and different boundary curvature was noticed in samples sintered at 1200 °C. Furthermore, the neck morphologies observed for the samples sintered at 1150 °C (treated at 800–900 °C) disappear when the samples are subjected to 1200 °C, an indication of improved sintering of the ferrite grains. In general, the increase of both treatment and sintering temperature leads to an inhomogeneous size distribution. These materials display a large fraction of smaller particles distributed along the surface. With a higher heat input, these smaller grains are dissolved into the large particles, disappearing entirely in the samples treated at 1200 °C, with the appearance of pores. A similar growth effect happens when considering the synthesis temperature. A marked grain growth was noticed in all samples as the treatment temperature increased from 750 to 900 °C – circa 1.36, 2.49, 2.86 and 3.31 μm, for the pellets sintered at 1150 °C, respectively. The pellets sintered at 1200 °C showed grain sizes circa 2.59, 2.85 and 3.79 (for the 750, 800 and 850 °C, respectively). A wide grain size distribution was displayed by the 900/1200 °C sample. The coercivity (Hc) results obtained in the VSM analyses of the CoFe2O4 pellets can be explained by the grain growth noticed in the SEM images of the sintered samples (Table 2). The lower values of Hc caused by sintering of the green samples was discussed by Ref. [44]. It was reported that the increase in sintering temperature tends to elevate the crystallinity and grain size of the material. This event, associated to
Fig. 7. X-ray diffractograms of the pellets fired at (a) 1150 and (b) 1200 °C. Indexed reflections are related to spinel CoFe2O4.
Fig. 8. Raman spectra of the samples fired at (a) 1150 and (b) 1200 °C.
Raman spectra of the samples fired at 1150 and 1200 °C are displayed in Fig. 8a and b, respectively. The vibrational modes of spinel cobalt ferrite (3T2g + Eg + A1g [36]) are present in all pellets. No additional modes were observed, thus demonstrating the purity of the samples after firing. In addition, the Raman spectra are remarkably similar for all treatment temperatures of the precursors and for both firing conditions, suggesting that the inversion degree is rather similar for all obtained samples, regardless of their thermal history. However, differences from the Raman spectra of precursors (Fig. 4) are noticed. As can be seen in Fig. 8, the Raman modes of the fired pellets present a shift towards higher wavenumbers. The position of the modes changed to 200 (T2g), 310 (Eg), 467 (T2g), 570 (T2g) and 691 cm−1 (A1g). Furthermore, an increase of intensities and of signal-to-noise ratio was observed, indicating a higher degree of crystallinity after the firing process. Hysteresis curves and magnetic parameters were obtained in order 5
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Fig. 9. Hysteresis curves of the cobalt ferrite pellets fired at (a) 1150 and (b) 1200 °C.
the growth of the magnetic domains, decreases the displacement resistance of domain walls, increasing the uniformity and weakening the magnetic anisotropy of the cobalt ferrites, thus resulting in a decrease of coercivity (Hc). The grain growth and its morphology identified in the SEM images had a remarkable influence on the mechanical properties of the materials. Vickers hardness measurements (Fig. 11) showed that the samples produced at 1150 °C yielded larger hardness values at all studied treatment temperatures when compared to the 1200 °C pellets. In all cases, the increase in temperature – i.e., grain growth – resulted in a decrease in Vickers hardness. For instance, a Vickers hardness of 133.9 HV was obtained by the sample thermally treated at 750 °C and sintered at 1150 °C, which was the highest value found in all the analyzed pellets. The similar precursor treated at 750 °C yet sintered at 1200 °C achieved a Vickers hardness of 78.4 HV. The minimum value found was of 33.8 HV associated with the sample treated at 900 °C and sintered at 1200 °C. In Fig. 10, it is possible to verify that the sample thermally treated at 750 °C and sintered at 1150 °C presented the smallest grain size amongst those studied. This sample also presented the highest Vickers hardness, which is an extremely important property when it comes to magnetic applications of materials. The small standard deviation of the results shows that the samples are remarkably homogeneous, a very positive result to achieve regarding mechanical properties. The results also clearly showcase the improved mechanical performance of the samples sintered at the lower temperature. All
Table 2 Magnetic parameters of the fired pellets. Firing Temp. (°C)
Treat. Temp. (°C)
Hc (Oe)
Mr (emu.g−1)
Ms (emu.g−1)
Mr/Ms
1150
750 800 850 900
438.06 477.92 457.20 447.40
12.95 15.29 15.79 13.38
76.18 75.59 74.80 75.67
0.170 0.202 0.211 0.177
1200
750 800 850 900
367.48 485.96 447.57 410.76
10.44 14.75 11.00 10.85
76.08 75.11 72.51 75.01
0.137 0.196 0.152 0.145
Note: Hc – coercivity; Mr – remanence; Ms – saturation; Mr/Ms – squareness.
Fig. 10. SEM images of the CoFe2O4 pellets obtained by precursors thermally treated at 750, 800, 850 and 900 °C and fired at 1150 and 1200 °C.
Fig. 11. Vickers Hardness of the thermally treated samples fired at two different temperatures, 1150 °C and 1200 °C. 6
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4. Conclusions The proposed sol-gel method has shown to be successful in the production of CoFe2O4 powder precursors at all studied temperatures. Furthermore, both analyzed firing conditions have led to an improved compositional homogeneity of the samples; the secondary phase was dissolved into the spinel structure, as shown by their diffractograms. Raman spectra of the pellets show a very similar cationic arrangement in all fired samples. This phenomenon was reflected in the final magnetic properties of the samples, which varied neither with synthesis nor with firing temperature. The saturation magnetization of all pellets was of approximately 75 emu.g−1. Nevertheless, the mechanical properties of the ferrite ceramics were deeply affected by their thermal history. A lower sintering temperature incurred in an improved hardness for all studied precursors. A maximum in Vickers Hardness of 133.9 HV was obtained for the sample prepared from precursors treated at 750 °C and sintered at 1150 °C, which was related to the lower grain size of this sample, as seen in SEM images. The samples did not follow the linear Hall-Petch relation. The presence of scatter smaller particles in the samples sintered at 1150 °C accounted for such deviation from linearity. Our results are a further step towards allowing the application of CoFe2O4 as an advanced industrial magnetic material, especially in the field of microwave absorbers for the electromagnetic pollution reduction.
Fig. 12. Hall-Petch plot of the sintered samples fired at 1150 °C and 1200 °C.
samples treated at 1150 °C display larger hardness than their 1200 °C analogue. This phenomenon indicates that the enhancement of the mechanical properties of the sintered CoFe2O4 does not require an energy-intensive process. Grain size can be directly related to mechanical resistance, since materials with smaller grain sizes have higher mechanical strength, as dictated by the Hall-Petch relation [45]. Therefore, in order to assess the validity of said relation, a Hall-Petch plot of the sintered samples was analyzed (Fig. 12). Average diameters were obtained from SEM images (Fig. 10). The data clearly does not follow a linear relationship. Nevertheless, a trend towards larger hardness with decreasing grain size can be observed. This deviation from linearity is likely related to the lack of size homogeneity in the sintered pellets and also the appearance of pores, cracks and local melt formation (for the samples at 900/1200 °C). As one can observe in the SEM images, CoFe2O4 samples sintered at 1150 °C display a large fraction of smaller particles. These smaller grains disappear when the sample is submitted to treatment at 1200 °C. The smaller particles tend to increase the hardness displayed macroscopically by the pellet – as seen in samples sintered at 1150 °C (treated at 750, 800 and 850 °C). Therefore, samples with particles with a large volume fraction of small particles – albeit with similar average grain size – would display improved mechanical strength, as seen in the Hall-Petch plot of the samples. Our work presented, for the first time, a thorough study regarding the influence of the sol-gel precursors on the mechanical and magnetic properties of CoFe2O4 sintered ceramics. We have shown that, regardless of the purity of the precursors utilized in the conformation of pellets, the final product is composed of pristine CoFe2O4, which in turn opens up the possibility of the utilization of impure raw materials in the fabrication of cobalt ferrite devices. Furthermore, despite not influencing the magnetic properties of the final product, both synthesis and sintering temperatures have shown to affect the hardness of the sintered samples profoundly. This behavior is paramount when considering applications that might require mechanical resistance, such as would be expected of permanent magnets, considering the practical requirements of these materials. The effectiveness of the studied sintering process is noticeable, excelling in the production of pure ferrite pellets, despite the initial impurities in the precursors. Our results open up possibilities in the field of ferrite manufacturing, further allowing the application of CoFe2O4 as an advanced rare-earth magnet substitute.
Declaration of completin interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank the National Council for Scientific and Technological Development (CNPq), the Coordination for the Improvement of Higher Education Personnel (CAPES) and the Support Program for Excellence Centers of the Foundation for Research Support of the State of Rio Grande do Sul (PRONEX/FAPERGS) for their financial support. References [1] W. Shen, B. Ren, K. Cai, Y. fei Song, W. Wang, Synthesis of nonstoichiometric Co0.8Fe2.2O4/reduced graphene oxide (rGO) nanocomposites and their excellent electromagnetic wave absorption property, J. Alloy. Comp. 774 (2019) 997–1008, https://doi.org/10.1016/j.jallcom.2018.09.361. [2] L. Liu, H. Zhang, Y. Mu, Y. Bai, Y. Wang, Binary cobalt ferrite nanomesh arrays as the advanced binder-free electrode for applications in oxygen evolution reaction and supercapacitors, J. Power Sources 327 (2016) 599–609, https://doi.org/10. 1016/j.jpowsour.2016.07.104. [3] J. Breckenkamp, G. Berg, M. Blettner, Biological effects on human health due to radiofrequency/microwave exposure: a synopsis of cohort studies, Radiat. Environ. Biophys. 42 (2003) 141–154, https://doi.org/10.1007/s00411-003-0203-x. [4] N. Gill, A.L. Sharma, V. Gupta, M. Tomar, O.P. Pandey, D.P. Singh, Enhanced microwave absorption and suppressed reflection of polypyrrole-cobalt ferrite-graphene nanocomposite in X-band, J. Alloy. Comp. 797 (2019) 1190–1197, https:// doi.org/10.1016/j.jallcom.2019.05.176. [5] J. Zhao, L. Liu, R. Jiang, H. Zhang, Dielectric relaxation and magnetic resonance in microwave absorption performance of CoxFe3-xO4 (0≤x≤1) nanocrystals, Ceram. Int. 45 (2019) 18347–18355, https://doi.org/10.1016/j.ceramint.2019.06.049. [6] M. Zong, Y. Huang, N. Zhang, H. Wu, Influence of (RGO)/(ferrite) ratios and graphene reduction degree on microwave absorption properties of graphene composites, J. Alloy. Comp. 644 (2015) 491–501, https://doi.org/10.1016/j.jallcom. 2015.05.073. [7] K. Kombaiah, J.J. Vijaya, L.J. Kennedy, M. Bououdina, B. Al Najar, Self heating efficiency of CoFe2O4 nanoparticles: a comparative investigation on the conventional and microwave combustion method, J. Alloy. Comp. 735 (2018) 1536–1545, https://doi.org/10.1016/j.jallcom.2017.11.279. [8] G. Bulai, L. Diamandescu, I. Dumitru, S. Gurlui, M. Feder, O.F. Caltun, Effect of rare earth substitution in cobalt ferrite bulk materials, J. Magn. Magn. Mater. 390 (2015) 123–131, https://doi.org/10.1016/j.jmmm.2015.04.089. [9] T. Shang, Q. Lu, L. Chao, Y. Qin, Y. Yun, G. Yun, Effects of ordered mesoporous structure and La-doping on the microwave absorbing properties of CoFe2O4, Appl.
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Sintering-dependent mechanical and magnetic properties of spinel cobalt ferrite (CoFe2O4) ceramics prepared via sol-gel synthesis Luis Eduardo Caldeiraa,∗, Waleska C. Guaglianonia, Janio Venturinia,b, Sabrina Arcaroc, Carlos Pérez Bergmanna,b, Saulo Roca Bragançaa a
Graduate Program in Mining, Metallurgical and Materials Engineering, Laboratory of Ceramics (LACER), Federal University of Rio Grande Do Sul, Av. Osvaldo Aranha 99, Porto Alegre, 90035-190, Brazil b Department of Industrial Engineering, Federal University of Rio Grande Do Sul, Av. Osvaldo Aranha 99, Porto Alegre, 90035-190, Brazil c Graduate Program in Materials Science and Engineering (PPGCEM), Laboratory of Technical Ceramics (CerTec), Universidade Do Extremo Sul Catarinense, Av. Universitaria 1105, P.O. Box 3167, Criciuma, 88806-000, Brazil
ARTICLE INFO
ABSTRACT
Keywords: Ferrites CoFe2O4 Sol-gel Conventional sintering Mechanical hardness
Despite increasing interest in cobalt ferrites as advanced ceramics, there is still much uncertainty regarding the relationship between sintering temperature and the magnetic and mechanical properties of CoFe2O4. Precursors prepared via sol-gel synthesis were treated at 750, 800, 850 and 900 °C, pressed into pellets and fired at 1150 and 1200 °C. Samples were analyzed via XRD, Raman spectroscopy, SEM, VSM and Vickers microhardness. Pure CoFe2O4 ceramics were produced from hematite-containing precursors. While magnetic parameters remained unaffected, a significant influence was observed for CoFe2O4 grain size and morphology. The highest hardness, obtained for the 1150 °C samples, was directly associated to smaller grain sizes. Mechanical behavior diverged from the classical Hall-Petch mechanism, likely due to lower grain size homogeneity. Our study achieved a thorough understanding of the magnetic and mechanical behavior of CoFe2O4 ceramics, enabling the optimized selection of processing according to the desired final properties of these materials.
1. Introduction There is a growing body of literature recognizing that electromagnetic pollution has become a severe problem worldwide [1,2]. It occurs due to the advancement of electronic technology, not only causing interference to electronic equipment but also being harmful to human health [3]. Several studies have reported advances in controlling and reducing electromagnetic pollution through microwave absorbing materials [4–6]. These materials are capable of absorbing electromagnetic waves, converting this energy into other forms of energy or acting as interference agents, dissipating electromagnetic waves [7]. Some rare earth materials have interesting applications as microwave absorbers [8,9], but due to the uneven distribution of economically-viable rare-earth deposits around the world, it has become necessary to find substitutes for this function. Among the materials that have the potential for microwave absorption, ferrites have been actively studied [10–12] for their remarkable structural [13,14], dielectric [15] and magnetic properties [16], as well as being cost-effective [17] and lightweight [18], with relatively easy processing and high stability
∗
[19]. Ferrites can be classified into groups, some of which are the spinel, garnet and hexaferrites, according to the crystallization of their structures [20]. Among this classification, spinel ferrites (MFe2O4) are currently some of the most important and versatile materials in terms of advanced ceramics. Their vast field of its application and optimal responses in terms of magnetic properties account for the interest of the scientific and industrial community in this family of materials [21,22]. Cobalt ferrites (CoFe2O4), in particular, have shown interesting magnetization results [23,24], such as high coercivity and remanence, also presenting cost-effectiveness, shape versatility and high chemical and structural stability when processed at high temperatures [25,26]. CoFe2O4 crystallizes in a spinel cubic structure, with a fcc packing of oxygen anions and the cations distributed among the tetrahedral (A) and octahedral (B) sites of the framework [26]. More specifically, this ferrite usually exhibits an inverse spinel structure, with trivalent metal ions (Fe3+) occupying A sites while the B sites are filled by both divalent (Co2+) and trivalent ions (Fe3+). The ferrimagnetism presented by cobalt ferrites is a consequence of the antiparallel spin alignment between the cations present in the A and B sites [27]. The methods used for the synthesis of magnetic cobalt ferrite include sol-gel,
Corresponding author. E-mail address: [email protected] (L.E. Caldeira).
https://doi.org/10.1016/j.ceramint.2019.09.240 Received 26 August 2019; Received in revised form 23 September 2019; Accepted 24 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Luis Eduardo Caldeira, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.240
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hydrothermal synthesis, combustion, thermolysis, wet chemical coprecipitation, flow injection synthesis and microwave plasma [28,29]. Among these processes, the sol-gel synthesis stands out due the ability to obtain materials with homogeneous, crystalline, and high-purity structures [30]. A wide field of application – such as permanent magnets, electronic components and devices, microwave devices and high-density recording storage – can be assigned to cobalt ferrite due to its outstanding electronic and magnetic properties [31,32]. Despite such properties being widely studied, the mechanical behavior of these materials is also of paramount importance for their full realization as an advanced magnetic material. It is well established that mechanical resistance is required in magnetic materials due to their fragile character. Nevertheless, there is still a wide gap in the literature when it comes to studies focused on the mechanical properties of CoFe2O4 ceramics and their correlation with the structural properties of these materials. This work presents, for the first time, a detailed study on the mechanical, magnetic and structural properties of CoFe2O4 spinel ferrites prepared via sol-gel. This work aimed to find an optimized route for the production of ferrite ceramics while defining a relationship between their properties and the firing temperature. Complete characterizations of different cobalt ferrite powders and sintered pellets were performed. Our results show that the mechanical properties of CoFe2O4 can be improved by varying sintering temperatures, seemingly without affecting the magnetic behavior of these ceramics. We also show that the purity of the precursors does not influence the phases present in the final ceramics, which could result in a decrease in costs involved with raw materials. Furthermore, we also show that the Hall-Petch relation does not hold for the produced materials, which was related to a large fraction of smaller particles in the sintered materials. The findings expand the application field of this material, further enabling the utilization of cobalt ferrite as an advanced rare-earth magnet substitute.
The second part of the characterization refers to the analysis of the CoFe2O4 fired samples. The apparent densities (ρAp) were determined by the Archimedes method (ASTM C373-88) at 25 °C, using a Shimadzu equipment (AX200, Japan) equipped with a device for density measurement. The structure was characterized by XRD and Raman. Likewise, the pellets were also subjected to VSM analysis. The microstructure of each fired sample was observed on a fracture surface using a SEM. The linear intercept method (ASTM E112-13) was used to estimate the grain size distribution from the micrographs using an image analysis software (ImageJ). Vickers hardness tests were performed, with ten measurements for each sample, using a Buehler Micromet 2000 with a load of 3 N. 3. Results and discussion 3.1. Powders In order to select the thermal processing temperatures for the production of the CoFe2O4 powders, the thermal behavior of the xerogel from room temperature up to 1300 °C was investigated. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) data are shown in Fig. 1. At first, a mass loss of 4% from room temperature up to 140 °C is observed. This event can be associated to the evaporation of residual water from the gel structure and also from the coordination spheres of the cations. A second mass loss of 20% occurs in the temperature range between 140 and 280 °C. This mass loss is usually associated to the decomposition of uncoordinated hydroxyl or carboxyl functional groups from the citric acid [33]. Subsequently, a third and most pronounced mass loss event (280–360 °C) takes place, accompanied by a strongly exothermic DSC peak centered at 360 °C. This reaction corresponds to the rapid decomposition of residual organic matter in the presence of nitrate. This distinct mass loss of 35% relates to the violent autocombustion of nitrates with citric acid, resulting in an intense heat release. The total weight loss is of approximately 59% for the precursor of CoFe2O4; only a small weight loss of ~ 2% can be observed above 360 °C. A single exothermic peak at 360 °C occurred along the heating of the xerogel, confirming that no other reaction took place; above this temperature, the material is thermally stable. Nevertheless, temperatures slightly above the onset of autoignition have previously shown to lead to particles of nanometric dimensions, which would hinder the proper pressing of the green pellets for further firing [34]. Thence, a temperature range 750–900 °C was selected. The crystalline structures of CoFe2O4 samples prepared via sol-gel technique and thermally treated at 750, 800, 850 and 900 °C were
2. Materials and methods Cobalt ferrite (CoFe2O4) powders were prepared via sol-gel synthesis using citric acid as a complexant. A mixture of Co(NO3)2·6H2O (Dinamica, 98%), Fe(NO3)3·9H2O (Synth, 98%) and citric acid (Exodo Cientifica, 99.5%) were used in a molar ratio of 1:2:3 and dissolved in 20 mL of Ultra-pure Type I water for the production of 10 mmol of product. The solution was heated to 85 °C for 1 h under magnetic stirring. The resulting gel was dried at 110 °C for 24 h. The xerogel was transferred to an alumina crucible and heat treated for 6 h at different temperatures (750, 800, 850 and 900 °C), previously selected by the thermal analysis of the precursor gel. Powder samples were uniaxially pressed into pellets (10 mm in diameter and 5 mm thick) using a cylindrical steel die through a hydraulic press (ST Bovenau P10, Brazil) at 100 MPa. The produced pellets were fired at 1150 and 1200 °C for 6 h at a heating rate of 10 °C.min−1. In order to select the treatment temperatures for the synthesis of CoFe2O4 powders, simultaneous thermogravimetric and differential scanning calorimetry analyses (TG/DSC, SDT Q600, TA Instruments) were performed. The experiment examined the thermal decomposition behavior of the gels in synthetic air at a heating rate of 10 °C.min−1. The crystalline structure of the precursors was characterized in an X-ray diffractometer (XRD, Philips, X'pert MPD) equipped with a Cu-Kα source. Samples were analyzed in a 2θ range of 10 to 75°, using a step size of 0.05° and dwell time of 1 s per step. The resulting crystalline phases were identified by ICSD data banks. The microstructure of the powders was observed using a scanning electron microscope (SEM, EVO MA10, Carl Zeiss), equipped with a secondary electron detector. Raman studies were performed in an inVia Renishaw Raman spectrometer equipped with a 532 nm laser, in a range of 100–1000 cm−1. Magnetic hysteresis curves were acquired in a vibrating sample magnetometer (VSM, Model EZ9, Microsense).
Fig. 1. TG (black line) and DSC (blue line) curves of the CoFe2O4 xerogel. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 2
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Fig. 2. X-ray diffractograms of the CoFe2O4 powders treated thermally at 750, 800, 850 and 900 °C. Indexed reflections are related to spinel CoFe2O4. Reflections identified with a rhombus are related to the presence of α-Fe2O3 (hematite).
Fig. 3. SEM images of the CoFe2O4 powder precursors treated thermally at varying temperatures: (a) 750, (b) 800, (c) 850 and (d) 900 °C.
analyzed via X-ray diffraction. The results are shown in Fig. 2. The main reflections are all related to spinel cobalt ferrite (CoFe2O4, JCPDS 00001-1121, Fd 3 m , nº 227), demonstrating that a successful synthesis of cobalt ferrite was achieved under all selected temperatures to heat treatment. Five reflections – identified in Fig. 2 by rhombuses – indicate the presence of a secondary phase of hematite (α-Fe2O3, JCPDS 01-0840310, R 3 c , nº. 167). The intensities of the reflections related to hematite increase with an increase in treatment temperature until 850 °C, above which the intensities decrease. The powder obtained through an 850 °C treatment showed the highest intensity of the reflections related to the hematite phase. This phenomenon does not necessarily mean a higher quantity of hematite; it could instead be related to a higher crystallinity of the sample. Above 850 °C, the decrease of the secondary phase can also arise from its solubilization into the structure of magnetite; this phenomenon is expected when the mixture is submitted to higher temperatures, according to Fe–O phase diagrams [35]. Analyses of the microstructure of the precursors heat-treated at different temperatures were also performed. SEM micrographs of the CoFe2O4 powders (Fig. 3) show the expected microstructure of magnetic cobalt ferrite prepared via sol-gel. The sample treated at 750 °C (Fig. 3a) showed a morphology composed of smaller and agglomerated grains, when compared to the images of the other samples. As previously seen in the XRD results, a possible second phase also appears in the SEM images. The image of the sample treated at 800 °C (Fig. 3b) exhibits some scattered lighter grains, i.e., phases in which the electronic conductivity might differ from the other regions, thus possibly indicating the presence of a secondary phase. Although still presenting agglomerates, this image shows that the temperature input contributed to the modification of the sample morphology; the ferrite grains show a marked growth along the increase in treatment temperature. This variation becomes even more evident in the other samples (Fig. 3c and d). The rise in treatment temperature also contributed to the increase in the volume fraction of the secondary phase, as expected from their diffractograms (Fig. 2). The composition of the synthesized cobalt ferrite was also evaluated via Raman spectroscopy. Fig. 4 shows the Raman spectra of the powders treated at different temperatures. All signals are related to cobalt ferrite, with no remaining signals indicating secondary phases. According to factor group analysis, five modes are Raman-active in spinels: three T2g, one Eg and one A1g mode [36]. One T2g mode corresponds to the translational movement of metal ions at the tetrahedral
Fig. 4. Raman spectra of the CoFe2O4 powders treated thermally at 750, 800, 850 and 900 °C.
site, while the Eg signal is attributed to the symmetric bending of the metal-oxygen bond. The remaining T2g modes are an anti-symmetric stretch and anti-symmetric bending of the metal-oxygen bond, respectively. The last mode, A1g, corresponds to the symmetric stretching of oxygen atoms along the Me–O bond in the tetrahedral sites. The five Raman active modes T2g (196 cm−1), Eg (304 cm−1), T2g (464 cm−1), T2g (560 cm−1) and A1g (685 cm−1) are identified in the spectra of the prepared powders. Furthermore, a splitting of the A1g mode is observed, with a secondary signal at approximately 613 cm−1. According to the literature, this band can be attributed to Co–O vibrations at tetrahedral sites [37]. The presence of Co2+ ions in these positions leads to a reduction in symmetry of the original spinel structure of the ferrite [38]. This splitting, therefore, indicates a higher inversion degree of the spinel structure. Besides, treatment temperature also influenced the position of the modes. A slight shift of the Raman modes can be observed for the sample treated at 900 °C. As reported by Ref. [39], the mode shift associated with the increase of temperature occurs due to 3
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Fig. 5. Hysteresis curves of the cobalt ferrite powders treated thermally at varying temperatures.
Fig. 6. Densities of the CoFe2O4 pellets fired at 1150 and 1200 °C, prepared from powders at different treatment temperatures.
the increase in particle size and to the redistribution of cations among tetrahedral and octahedral sites. The influence of the treatment temperature on the magnetic properties of the precursors was verified via VSM analysis. The hysteresis curves of the powder samples are shown in Fig. 5. All hysteresis curves display a classic ferrimagnetic behavior, as would be expected of spinel CoFe2O4, albeit with different remanence and coercivity values. This variation shows that the thermal treatment within the selected range has indeed affected the magnetism of the precursors. The magnetic parameters obtained from the hysteresis curves are displayed in Table 1. Both the remanent (Mr) and the saturation (Ms) magnetization of the precursors decrease along increases in the treatment temperature. This factor is likely related to the increase in the mass fraction of hematite present in the sample. Since the magnetization of this material is virtually negligible when compared to cobalt ferrite [40], the presence of the secondary phase dilutes the specific magnetism displayed by the material. The maximum value of Mr among the samples, which is the condition that the material continues magnetic even in the absence of a magnetic field, belongs to the powders treated at 750 °C (22.68 emu.g−1), which is also the material with the lowest fraction of hematite. This parameter decreases linearly along increasing temperatures, reaching a value of 14.20 emu.g−1 for the powder prepared at 900 °C. The same decrease tendency occurs for Ms, as seen in Table 1. Coercivity (Hc) follows a similar pattern of Mr and Ms, with decreasing values along increases in treatment temperature. The sample treated at 750 °C reached a coercivity of 715 Oe, while the value of 498.1 Oe was observed for the 900 °C sample. The increase in particle size – i.e., a tendency away from single-domain particles – would incur in slightly decreasing coercivity [41], as observed in Table 1. The grain growth observed in SEM images of the samples (Fig. 3) corroborates the mentioned decrease, since the particle sizes increase with the increase of treatment temperature.
3.2. Fired Samples The previously characterized powders – thermally treated at 750, 800, 850 and 900 °C – were used as precursors for the production of the pellets, obtained at two firing temperatures, 1150 and 1200 °C. These conditions were selected based on literature describing the densification of cobalt ferrite ceramics [42,43]. Fig. 6 shows the apparent densities of the fired pellets. The density results did not present any significant variation. For instance, a difference of only 0.03 g cm−3 was observed between the pellets thermally treated at 750 °C and fired at 1150 and 1200 °C (5.08 and 5.11 g cm−3, respectively). Such minor variation was observed repeatedly for all samples. The minimum apparent density was of 5.04 g cm−3 (for pellets obtained by precursors treated at 850 °C and later fired at 1150 °C) and the maximum was of 5.12 g cm−3 (treatment temperature of 850 °C and fired at 1200 °C). Nevertheless, the standard deviation of the measurements does not indicate a meaningful variation between samples. Compared to the theoretical density of CoFe2O4 (5.31 g cm−3), it was observed that the relative densities are all found in a range between 95% and 96.4%. The increase in synthesis temperature of the precursors, as well as the increase of the firing temperature of the pellets, had no influence on the relative density of the fired pellets. The density values prove their densification – thus agreeing with literature data – and also indicate a rather low porosity for the fired materials. Therefore, one can assume that other factors had a more pronounced effect over the final density displayed by the samples. The fired pellets were also analyzed with regard to their crystalline structure, as shown in the X-ray diffractograms in Fig. 7. The hematite (Fe2O3) phase present in the powders is not visible on the diffractograms of the fired pellets; a single phase of CoFe2O4 was achieved under both firing conditions for all precursors. The phenomenon observed in the diffractograms is quite striking, since it shows that is possible to obtain a pure final material with the adopted method of firing, regardless of the purity of the precursors. The complete disappearance of the second phase of Fe2O3 in the fired ferrites can be directly indexed to the incorporation of this secondary phase into the crystalline structure of the spinels. As stated earlier about the X-ray diffractograms of the powders, the hematite phase solubilizes in the spinel structure at higher temperatures. A small shift is noticed on the angles of the reflections, which is likely related to the fact that the X-ray diffractograms were obtained from solid samples; the reflection angle shifts arise due to the fact that solid pellets can have directionality and much less anisotropy than a powder sample.
Table 1 Magnetic parameters of the CoFe2O4 powders prepared by different thermal treatments. Temperature (°C)
Hc (Oe)
Mr (emu.g−1)
Ms (emu.g−1)
Mr/Ms
750 800 850 900
715.0 584.2 538.5 498.1
22.68 20.13 15.27 14.20
75.22 62.35 49.23 50.70
0.302 0.323 0.310 0.280
Note: Hc – coercivity; Mr – remanence; Ms – saturation; Mr/Ms – squareness. 4
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to verify the influence of firing temperature on the magnetic properties of CoFe2O4. The fired pellets were subjected to VSM analysis, as were the precursors. The hysteresis curves of the samples treated at 750, 800, 850 and 900 °C and later fired at 1150 and 1200 °C are shown in Fig. 9. A classic ferrimagnetic behavior is displayed once again, as occurred with precursors, thus showing that the firing process maintained the spin alignment of the structure. Furthermore, no significant difference in the values of coercivity and remanent magnetization was observed between the samples. The effect observed in Fig. 9 strongly indicates that the cobalt ferrite materials have a unique magnetic behavior when fired at higher temperatures, regardless of the previous thermal treatment to which they were subjected for the obtaining of the precursors. Such invariance can be readily related to the similar Raman spectra of the samples; since they indicated very similar cationic arrangements, it follows that the net spin alignment should not vary amongst the samples, and neither should their magnetic properties. Magnetic parameters, such as coercivity (Hc), remanence (Mr), saturation magnetization (Ms) and squareness, are displayed in Table 2. The Hc and Mr values of Table 2 are similar to the ones obtained from the precursor thermally treated at 900 °C (Table 1). Considering the treatment temperatures and the firing temperatures adopted in this work, it was observed that the coercivity and remanence tend to decrease with increasing treatment temperature, saturating at temperatures above 900 °C. Thus, this phenomenon indicates a stabilization of magnetic properties at treatment temperatures above 900 °C, stating that the magnetic behavior of CoFe2O4 materials is not susceptible to changes caused by firing at high temperatures. SEM micrographs were obtained by analyzing the fresh fractures of all fired samples (Fig. 10). Microstructural changes, especially regarding grain dimensions, can be noted both along increases in synthesis temperature of the powders and along increases in firing temperature. Initially, the micrograph of the precursor heat treated at 750 °C and fired at 1150 °C shows individualized grains with rounded and hexagonal morphology. These observations indicate that the applied temperatures are already high enough to allow the sintering process to take place, which intensifies as the temperature increases. As it is possible to observe in the micrographs, the grains show considerable growth and become ever more rounded. As expected, with the increase in treatment temperature from 1150 to 1200 °C, an increase in grain size is observed. The main phenomenon observed in the thermal treatment was grain growth, which can be easily noticed by observing the grain size of the 750 °C samples sintered at 1150 °C and 1200 °C (1.36 μm and 2.56 μm, respectively). The appearance of grains with more than six sides and different boundary curvature was noticed in samples sintered at 1200 °C. Furthermore, the neck morphologies observed for the samples sintered at 1150 °C (treated at 800–900 °C) disappear when the samples are subjected to 1200 °C, an indication of improved sintering of the ferrite grains. In general, the increase of both treatment and sintering temperature leads to an inhomogeneous size distribution. These materials display a large fraction of smaller particles distributed along the surface. With a higher heat input, these smaller grains are dissolved into the large particles, disappearing entirely in the samples treated at 1200 °C, with the appearance of pores. A similar growth effect happens when considering the synthesis temperature. A marked grain growth was noticed in all samples as the treatment temperature increased from 750 to 900 °C – circa 1.36, 2.49, 2.86 and 3.31 μm, for the pellets sintered at 1150 °C, respectively. The pellets sintered at 1200 °C showed grain sizes circa 2.59, 2.85 and 3.79 (for the 750, 800 and 850 °C, respectively). A wide grain size distribution was displayed by the 900/1200 °C sample. The coercivity (Hc) results obtained in the VSM analyses of the CoFe2O4 pellets can be explained by the grain growth noticed in the SEM images of the sintered samples (Table 2). The lower values of Hc caused by sintering of the green samples was discussed by Ref. [44]. It was reported that the increase in sintering temperature tends to elevate the crystallinity and grain size of the material. This event, associated to
Fig. 7. X-ray diffractograms of the pellets fired at (a) 1150 and (b) 1200 °C. Indexed reflections are related to spinel CoFe2O4.
Fig. 8. Raman spectra of the samples fired at (a) 1150 and (b) 1200 °C.
Raman spectra of the samples fired at 1150 and 1200 °C are displayed in Fig. 8a and b, respectively. The vibrational modes of spinel cobalt ferrite (3T2g + Eg + A1g [36]) are present in all pellets. No additional modes were observed, thus demonstrating the purity of the samples after firing. In addition, the Raman spectra are remarkably similar for all treatment temperatures of the precursors and for both firing conditions, suggesting that the inversion degree is rather similar for all obtained samples, regardless of their thermal history. However, differences from the Raman spectra of precursors (Fig. 4) are noticed. As can be seen in Fig. 8, the Raman modes of the fired pellets present a shift towards higher wavenumbers. The position of the modes changed to 200 (T2g), 310 (Eg), 467 (T2g), 570 (T2g) and 691 cm−1 (A1g). Furthermore, an increase of intensities and of signal-to-noise ratio was observed, indicating a higher degree of crystallinity after the firing process. Hysteresis curves and magnetic parameters were obtained in order 5
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Fig. 9. Hysteresis curves of the cobalt ferrite pellets fired at (a) 1150 and (b) 1200 °C.
the growth of the magnetic domains, decreases the displacement resistance of domain walls, increasing the uniformity and weakening the magnetic anisotropy of the cobalt ferrites, thus resulting in a decrease of coercivity (Hc). The grain growth and its morphology identified in the SEM images had a remarkable influence on the mechanical properties of the materials. Vickers hardness measurements (Fig. 11) showed that the samples produced at 1150 °C yielded larger hardness values at all studied treatment temperatures when compared to the 1200 °C pellets. In all cases, the increase in temperature – i.e., grain growth – resulted in a decrease in Vickers hardness. For instance, a Vickers hardness of 133.9 HV was obtained by the sample thermally treated at 750 °C and sintered at 1150 °C, which was the highest value found in all the analyzed pellets. The similar precursor treated at 750 °C yet sintered at 1200 °C achieved a Vickers hardness of 78.4 HV. The minimum value found was of 33.8 HV associated with the sample treated at 900 °C and sintered at 1200 °C. In Fig. 10, it is possible to verify that the sample thermally treated at 750 °C and sintered at 1150 °C presented the smallest grain size amongst those studied. This sample also presented the highest Vickers hardness, which is an extremely important property when it comes to magnetic applications of materials. The small standard deviation of the results shows that the samples are remarkably homogeneous, a very positive result to achieve regarding mechanical properties. The results also clearly showcase the improved mechanical performance of the samples sintered at the lower temperature. All
Table 2 Magnetic parameters of the fired pellets. Firing Temp. (°C)
Treat. Temp. (°C)
Hc (Oe)
Mr (emu.g−1)
Ms (emu.g−1)
Mr/Ms
1150
750 800 850 900
438.06 477.92 457.20 447.40
12.95 15.29 15.79 13.38
76.18 75.59 74.80 75.67
0.170 0.202 0.211 0.177
1200
750 800 850 900
367.48 485.96 447.57 410.76
10.44 14.75 11.00 10.85
76.08 75.11 72.51 75.01
0.137 0.196 0.152 0.145
Note: Hc – coercivity; Mr – remanence; Ms – saturation; Mr/Ms – squareness.
Fig. 10. SEM images of the CoFe2O4 pellets obtained by precursors thermally treated at 750, 800, 850 and 900 °C and fired at 1150 and 1200 °C.
Fig. 11. Vickers Hardness of the thermally treated samples fired at two different temperatures, 1150 °C and 1200 °C. 6
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4. Conclusions The proposed sol-gel method has shown to be successful in the production of CoFe2O4 powder precursors at all studied temperatures. Furthermore, both analyzed firing conditions have led to an improved compositional homogeneity of the samples; the secondary phase was dissolved into the spinel structure, as shown by their diffractograms. Raman spectra of the pellets show a very similar cationic arrangement in all fired samples. This phenomenon was reflected in the final magnetic properties of the samples, which varied neither with synthesis nor with firing temperature. The saturation magnetization of all pellets was of approximately 75 emu.g−1. Nevertheless, the mechanical properties of the ferrite ceramics were deeply affected by their thermal history. A lower sintering temperature incurred in an improved hardness for all studied precursors. A maximum in Vickers Hardness of 133.9 HV was obtained for the sample prepared from precursors treated at 750 °C and sintered at 1150 °C, which was related to the lower grain size of this sample, as seen in SEM images. The samples did not follow the linear Hall-Petch relation. The presence of scatter smaller particles in the samples sintered at 1150 °C accounted for such deviation from linearity. Our results are a further step towards allowing the application of CoFe2O4 as an advanced industrial magnetic material, especially in the field of microwave absorbers for the electromagnetic pollution reduction.
Fig. 12. Hall-Petch plot of the sintered samples fired at 1150 °C and 1200 °C.
samples treated at 1150 °C display larger hardness than their 1200 °C analogue. This phenomenon indicates that the enhancement of the mechanical properties of the sintered CoFe2O4 does not require an energy-intensive process. Grain size can be directly related to mechanical resistance, since materials with smaller grain sizes have higher mechanical strength, as dictated by the Hall-Petch relation [45]. Therefore, in order to assess the validity of said relation, a Hall-Petch plot of the sintered samples was analyzed (Fig. 12). Average diameters were obtained from SEM images (Fig. 10). The data clearly does not follow a linear relationship. Nevertheless, a trend towards larger hardness with decreasing grain size can be observed. This deviation from linearity is likely related to the lack of size homogeneity in the sintered pellets and also the appearance of pores, cracks and local melt formation (for the samples at 900/1200 °C). As one can observe in the SEM images, CoFe2O4 samples sintered at 1150 °C display a large fraction of smaller particles. These smaller grains disappear when the sample is submitted to treatment at 1200 °C. The smaller particles tend to increase the hardness displayed macroscopically by the pellet – as seen in samples sintered at 1150 °C (treated at 750, 800 and 850 °C). Therefore, samples with particles with a large volume fraction of small particles – albeit with similar average grain size – would display improved mechanical strength, as seen in the Hall-Petch plot of the samples. Our work presented, for the first time, a thorough study regarding the influence of the sol-gel precursors on the mechanical and magnetic properties of CoFe2O4 sintered ceramics. We have shown that, regardless of the purity of the precursors utilized in the conformation of pellets, the final product is composed of pristine CoFe2O4, which in turn opens up the possibility of the utilization of impure raw materials in the fabrication of cobalt ferrite devices. Furthermore, despite not influencing the magnetic properties of the final product, both synthesis and sintering temperatures have shown to affect the hardness of the sintered samples profoundly. This behavior is paramount when considering applications that might require mechanical resistance, such as would be expected of permanent magnets, considering the practical requirements of these materials. The effectiveness of the studied sintering process is noticeable, excelling in the production of pure ferrite pellets, despite the initial impurities in the precursors. Our results open up possibilities in the field of ferrite manufacturing, further allowing the application of CoFe2O4 as an advanced rare-earth magnet substitute.
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