Influence of cobalt addition and calcination temperature on the physical properties of BaFe12O19 hexaferrites nanoparticles

Nano-scale particles of pure Barium hexaferrite ‘BaFe12O19’ and Cobalt added Barium hexaferrite ‘CoxBaFe12O19’, with x = 0.04, 0.06 and 0.1 wt%, were successfully synthesized by the chemical co-precipitation method. The synthesized powder was subjected to different calcination temperatures (T = 850 °C, 900 °C, 950 °C and 1050 °C). X-ray powder diffraction (XRD) clarified that nearly single phase of BaFe12O19 with tiny traces of Fe2O3 phase were obtained when the precursor was calcined at 1050 °C for 2 h. The lattice parameters and unit cell volume were almost unchanged with either Cobalt addition or calcination temperatures. From Debye–Scherrer equation, the crystallite size (D) was found to gradually increase with increasing calcination temperature to reach its maximum values for samples calcined at 1050 °C. The formation of Barium hexaferrite phase was also confirmed from Fourier transform infrared (FTIR) spectra through the existence of strong absorption peaks that appeared between 581 cm−1 and 435 cm−1. The morphology and grain size of the samples were examined using transmission electron microscopy (TEM) technique. Optical properties of the samples were studied through ultraviolet ‘UV’ visible spectroscopy. The optical band gap (Eg) of the samples was obtained from Tauc relation as function of Cobalt addition (x) and calcination temperature (T). Finally, the mechanical properties were examined using Vickers microhardness. The microhardness data revealed that the samples exhibited reverse indentation size effect (RISE). The Elastic modulus (E) and yield strength (Y) for the prepared samples were calculated, in accordance with Vickers microhardness, as function of Cobalt addition. Furthermore, the indentation size effect ISE was analyzed using indentation induced cracked model (IIC). The IIC model was found to be a suitable model for describing the microhardness results of the prepared samples. Time dependent Vickers microhardness was done through indentation creep test at different dwell time (t = 10, 20, 30, 40 and 50 s) and constant applied loads (F = 0.98, 4.90 and 9.80 N). Results clarified that the specimens revealed grain boundary sliding together with dislocation climbs at small loads and a dislocation creep in the operating creep process for greater loads.


Introduction
Hexaferrites are wide category of ferromagnetic oxide materials that crystallize in complex hexagonal structure with space group P63/mmc [1]. Six types of hexaferrites are classified and symbolized as M, Y, W, Z, X and U denoting BaFe 12 60 , respectively. In the chemical formulae of hexaferrites Me is a divalent metal ion (such as: Co, Ni, ZnK etc) and Ba can be replaced by Pb or Sr [2]. The unit cell of Barium-based hexaferrites consists of specific stacking sequence of close-packed oxygen layers with Barium ions partially replacing Oxygen ions at particular positions, and the small metallic ions occupying interstitial positions [3]. Among all Barium hexaferrites, the M-type Barium hexaferrite 'BaFe 12 O 19 ' is found to be scientifically and technologically promising due to its amazing properties and its numerous applications. BaFe 12 O 19 characterizes by its high magnetic anisotropy field, high coercive force and high Curie temperature. Also, it has an excellent chemical stability and corrosion resistivity [4,5]. On the technological side, it has been used to manufacture permanent magnets, magnetic recording media, microwave devices and circuits, magneto-optic media, signal processing devices, telecommunications devices, supercapacitors and microwave filters [6][7][8][9][10][11][12][13]. Synthesis of nano-scale particles of BaFe 12 O 19 can be achieved through different methods including: co-precipitation [14,15], sol-gel [16,17], carbon combustion [18], citrate auto-combustion [19] and hydrothermal [20].
The effect of substitution/addition on the physical properties of BaFe 12 O 19 has been recorded in many literatures. Substitutions can be done by the partial replacement of Fe 3+ ions with other cations such as Mn-Sn, Co-Sn and Mn-Co-Sn [21], Co [22,23] and Ni [24]. Moreover, substitutions can be done by the partial replacement of Ba 2+ ions with other cations as reported by Taufeeq et al [25] for studying the structural and thermal properties of pure BaFe 12 O 19 and Sr doped Ba 0.9 Sr 0.1 Fe 12 O 19 nanostructure. They concluded that Srdoping sustains the hexagonal crystal structure of Ba hexaferrites. Also, they observed a decrement in the crystallite size and an increment in the lattice parameters. The thermal property studied by TGA showed weight loss as a function of both temperature and Sr doping. Rusianto et al [26] studied the magnetic and mechanical properties of Sr x Ba 1−x Fe 12 O 19 (with x=0.00, 0.25, 0.50, 0.75 and 1.00) synthesized with conventional method. They found that at sintering temperature 1100°C the saturation magnetization M s and the retentivity M r decreased with x, while the coercivity H c increased. The flexural strength and the Vickers microhardness are found to increase with increasing sintering temperature. The addition of Bismuth [27] increased the density and improved the synthesis rate of BaFe 12 O 19 . Also, it is found that the addition of Si reduces the grain size and increases the coercivity [28]  The magnetic properties of BaFe 12 O 19 were extensively studied, since their discovery, in many literatures [26][27][28][29]. On the contrary few data are available on their mechanical properties. One of the drawbacks of hexaferrites is their ceramic nature that makes them not flexible and moldable. However, for some applications for instance; in magnetomechanical transducers and in surface-mounted devices (SMD) such as inductors and transformer cores [30], materials must possess appropriate mechanical strength requiring precise control of the microstructure. Therefore, it is of major interest to study the mechanical properties of hexaferrites, which can give a broad insight into their microstructure. Vickers microhardness regarded as a quick and non-destructive test to extract information on the mechanical behavior of materials such as; resistance to plastic deformation, modulus of elasticity, brittleness index, yields strength, ductility, fractures toughness and cracking temperature.
In this study, the influence of Cobalt addition and the calcination temperatures on the mechanical and optical properties of Barium hexaferrites was investigated.

Characterization and measurements
Room temperature x-ray powder diffraction was performed using advance powder diffractometer with Cu target (λ=1.540 56 Å) in 2θ range of 10°to 80°. FTIR analysis was carried out using FTIR Nicolet iS5-Thermoscientific. JEOL transmission microscope JEM-100CX is used to provide morphology information for the synthesized samples. At room temperature, UV-visible measurements have been taken using spectrophotometer V-670 ultraviolet-near infrared (NIR). Vickers microhardness was measured with a digital microhardness tester model MHVD-1000IS. Vickers indenter is in the form of square-pyramid diamond. On a clean sample surface, the indenter was applied under test loads varying from 0.49 N to 9.80 N for dwell time 60 s. To assure accuracy of microhardness data, the average value of 3 indentations at different locations on the sample surface was taken for each specific load. Vickers indentation marks left on the sample surface after offloading were nearly square in shape with two diagonals. A calibrated micrometer eye-piece mounted to the microscope was employed to estimate the length of the two diagonals after offloading. The diagonal lengths of indentation were estimated with precision of ±0.1 μm. At different dwell times (t=10, 20, 30, 40 and 50 s) and for constant loads of 0.98, 4.90 and 9.80 N, indentation creep test was carried out for the prepared samples.    [33] reported about the fact that the appearance of Fe 2 O 3 with orthorhombic hexahedron structure prevents the formation of BaFe 12 O 19 hexagonal structure at low temperature below 750°C.
Lattice parameters (a and c) and unit cell volume (V) are calculated according to the following equations [34,35] where d is the interplanar distance and (h k l) are the Miller indices. The average crystallite size of the samples was estimated using Debye-Scherrer equation; where D is the average crystallite size in nanometers, S is the shape factor and it is equal to 0.9, λ is the wavelength of the x-ray radiation, β hkl is the peak width at half maximum intensity and θ is the peak position. The values of a, c, V and D for Co x BaFe 12 O 19 samples prepared at different calcination temperatures (T=850°C, 900°C, 950°C and 1050°C) are listed in table 1. It is clear, from table 1, that the lattice parameters and unit cell volume are almost unchanged with either Cobalt addition or calcination temperature. This indicates that Cobalt occupies only interstitial places in Barium hexaferrite matrix without entering the structure and that the calcination temperature has no effect in changing the crystal structure of the phase. The crystallite size D varies directly with increasing the Co-content (x) for samples prepared at T=850°C and 1050°C while it has unsystematic variation with x for samples prepared at T=900°C and 950°C. Also, it can be seen from table 1 that the crystallite size (D) is gradually increased with increasing calcination temperature to reach its maximum values for samples calcined at 1050°C. This result is consistent with the XRD peaks width which becomes narrower with increasing calcination temperature, indicating that the average crystalline size of synthesized ferrites is gradually increased. Table 1. Phase percentage (BaFe 12 O 19% ), lattice parameters (a and c), unit cell volume (V), crystalline size (D) and grain size (D TEM ) of Co x BaFe 12 O 19 nanoparticles with (0x0.1) at different calcination temperatures.

T(°C)
x (wt%) 3.4. Optical measurements (UV-visible spectroscopy) Figure 4(a) displays the UV-visible absorbance spectra of Co x BaFe 12 O 19 as a function of Co addition (x) calcined at T=850°C. It is clear that, the absorbance peak is slightly shifted to higher wavelengths with increasing Co addition. Agrawal et al [43] reported similar results for Ca and Ni doped Barium hexaferrite. This slight shift could be explained in terms of the s, p-d spin-exchange interactions between delocalized s-or p-type band electrons of Fe and O atoms, respectively and localized d-electrons of transition metal Co [44]. Figure 4(b) shows the UV-visible absorbance spectra of Co x BaFe 12 O 19 with x=0.1 wt% as function of calcination temperatures T=850°C, 900°C, 950°C and 1050°C. The absorbance peak around wavelength of 352 nm for Co x BaFe 12 O 19 with x=0.1 wt% is slightly shifted to lower wavelengths with increasing the calcination temperature from 850 to 1050°C. In general, the absorbance shift depends mainly on various factors such as band gap, grain size, impurities, oxygen deficiency and surface effects. In the optical region, when electromagnetic waves fall on the samples it causes the valance band electrons to absorb energy and to rise to higher energy level. This absorbed band gap energy is calculated from Tauc relation [45]: where α is the absorption coefficient, B is a constant, hν is the energy of incident photons, and n=1/2 for allowed direct transitions. E g is the optical band gap. Figure 5 shows the plot of (αhν) 2 versus photon energy hν, according to Tauc relation, for pure BaFe 12 O 19 calcined at T=850°C. The optical band gap E g for pure and Co added BaFe 12 O 19 samples are obtained from the extrapolation of (αhν) 2 to zero and tabulated in table 2. It is found that the band gap of pure BaFe 12 O 19 is 4.25 eV, 4.05 eV, 3.83 eV and 3.71 eV calcined at 850°C, 900°C, 950°C and 1050°C, respectively. Karmakar et al [45] estimated that the band gap for Barium hexaferrite is around 3.18 eV which is lower than those recorded for our pure BaFe 12 O 19 calcined at different temperatures. It is clear that, the values of the energy bandgap decreases with Co addition from 4.25 ev to 3.95 eV as x increases from 0 wt% to 0.1 wt% at T=850°C. Same results were obtained for the other temperatures (T=900°C, T=950°C and T=1050°C) where the E g decreases with Co addition. As a result, both Co addition and calcination temperatures decrease the band gap of barium hexaferrite nanoparticles. The reduction in optical band gap E g values could be attributed to the enhancement in the samples grain size [45].

where F is the applied indentation test load in Newton and d is the average diagonal length of indentation in μm.
The dependence of Vickers microhardness (H v ) on the applied indentation test load is known as indentation size effect (ISE). Usually two types of ISE could be observed for ceramics materials: (I) normal ISE (NISE) which involves a decrease in the apparent microhardness with increasing the applied test load and (II) reverse indentation size effect (RISE) where the apparent microhardness increases with increasing the applied test load. It can be seen, from figures 6(a)-(c), that Vickers microhardness H v for all the prepared samples increases with increasing the applied test load F. Therefore, the samples undergo reverse indentation size effect (RISE). Therefore, Cobalt addition has the ability to increase grain connectivity, reduce pores, enhance relative volume fraction of BaFe 12 O 19 and to reduce resistance to crack propagation among grains. This result is consistent with XRD results. Figure 7( Figure 8(a) and b clarifies that E and Y increase as the Co-content increases. Therefore, they follow the same H v -Co % variation trend. Zewn et al [47] examine the relation of elastic modulus to determine the critical grain size above which the micro-cracking will initiate due to anisotropic thermal stresses that appear during processing. The greater elastic modulus (E) and yield strength (Y) of a sample, the stronger connection between atoms or molecules in materials.

Indentation-inducedcracking model (IIC)
In this model, Li and Bradt [48] try to explain the reverse ISE by considering that at maximum penetration, the applied indentation test load is balanced by the total specimen resistance which results from the following four factors: (1) friction at the indenter/specimen facet interface (frictional component), (2) elastic deformation, (3) plastic deformation and (4) specimen cracking. According to this model the indentation cracking is responsible for the reverse ISE, while frictional and elastic effects give rise to the normal ISE. In the case of indentation cracking, the Vickers microhardness measured is given by [49] where d is the indentation diagonal length, and λ 1 , k 1 and k 2 are constants. The constant k 2 depends on the applied load F, while k 1 is a constant depends on the indenter geometry. For an ideally perfect plastic body        The theoretical microhardness values according to the indentation induced cracking model H IIC were calculated as a function of the applied test load F according to equation (7) and their values are listed in table 3 together with the experimental microhardness values (H v ) exp. estimated in the plateau region. It is clear that the theoretical data, calculated according to IIC model, is well matched the experimental measured data. Therefore, it can be concluded that the indentation induced cracked model IIC is a suitable model that fits our experimental microhardness data and explains the reverse indentation size effect (RISE) nature of the prepared samples.

Time dependent Vickers microhardness (Indentation creep)
Time dependent movement of a hard indenter into a solid, under fixed applied load, is known as indentation creep. The measurements of indentation creep can be seen as a fast, clear and anti-destructive technique for gathering information on the mechanical performance of materials [51]. Indentation creep test was performed on our samples to examine the power law indentation creep performance of Cobalt added Barium hexaferrite nanoparticles calcined at 900°C, 950°C and 1050°C. Figure 10 displays the variation of H v with the dwell time (t) at fixed applied load F=0.98 N, for Co x BaFe 12 O 19 with x=0.00, 0.04, 0.06 and 0.10 wt% calcined at 950°C. It is evident from the figure that with rising dwell time, microhardness declines and there are two stages of transitions. The first stage is characterized by a rapid reduction in H V -t curves for the duration of a dwell time t<40 s, followed by a second stage that  occurs at dwell-time t greater than 40 s where microhardness is gradually reduced at a low rate as the dwell-time increases. The H v -t trend, demonstrating the nature of the indenter's penetration depth and illustrates the typical behavior between H v and t [52]. This is also a predictor of the material bearing creep deformation [53].
Sargent-Ashby model [54] can be used to test the indentation creep nature of the prepared samples. The time dependent microhardness is calculated by the following relationship, according to this model; where e 0 is the strain rate at reference stress r , 0 c is a constant and m is the stress exponent. Figure 11 demonstrates the experimental data fitted to Sargent-Ashby model for Co x BaFe 12 O 19 with x=0.00, 0.04, 0.06 and 0.10 wt% calcined at 900°C at constant load F=4.90 N. Straight lines with slopes equal to the negative reverse stress exponent -1/μ are obtained from ln(H v ) versus ln (t) plot. The fitted lines can be seen nearly parallel. The parallel lines reveal that cobalt additions do not affect the stress exponents significantly [55]. The calculated stress exponent values for the applied test loads (F=0.98, 4.90 and 9.80 N) for samples calcined at 900°C and 950°C are listed in table 4. It is clear, from table 4, that the stress exponents m for all samples calcined at 900°C have values that range from 3.1 to 7.7 while samples calcined at 950°C have the stress exponents m ranges from 2.9 to 8.0. In addition, with respect to the power law creep; the higher the stress exponent, the higher the yield strength [56,57]. The specimens are therefore more resistant to indentation creeps for greater loads. Several details on the deformation mechanism can be shown from the value of the stress exponent μ. When μ has a value nearly one, it means a diffusion creep in the sample [58], a grain boundary sliding [55,56] will occur at a value of μ around two. Dislocations climbs arise for μ values between 4 and 6 [59]. Dislocation creep, however, is governed when μ has values between 3 and 10 [60,61]. The numerical values of μ range from 3.1 to 8.0 for the prepared samples. Therefore, the samples investigated exhibit grain boundary sliding, accompanied by dislocation climbs at small loads followed by dislocation creep for great loads throughout the operative creep mechanism.

Conclusion
Chemical co-precipitation method is successfully utilized to prepare nano-scale particles of pure Barium hexaferrite BaFe 12 O 19 and Cobalt added Barium hexaferrite Co x BaFe 12 O 19 , with x=0.04, 0.06 and 0.1 wt%. The effect of Cobalt addition and calcination temperature on the microstructure, morphology, optical and mechanical properties of the synthesized samples was investigated. XRD results revealed the hexagonal structure of the prepared samples and indicated enhancement in the phase formation of BaFe 12 O 19 and reduction of Fe 2 O 3 phase with increasing the calcination temperature. Also, it was concluded from XRD that the optimum calcination temperature for nearly single phase of BaFe 12 O 19 is at T=1050°C. The lattice parameters (a and c) and unit cell volume (V) were almost unchanged with either Cobalt addition or calcination temperatures. By the use of Debye-Scherrer equation, the crystallite size was calculated to be in the range of 51-142 nm. FTIR spectra confirmed the formation of hexaferrite phase through the existence of strong absorption peaks that appeared between 581 cm −1 and 435 cm −1 . TEM micrographs clarified that the samples composed of agglomerated grains that form clusters of different sizes and shapes. Moreover, the hexagonal shaped particles become clearly defined as the Cobalt content and calcination temperature were increased to 0.1 wt% and 1050°C, respectively. An absorbance peak was observed from the UV-visible spectroscopy at wavelength of 352 nm for Co x BaFe 12 O 19 with x=0.1 wt% was slightly shifted to lower wavelengths with increasing the calcination temperature. It was concluded that the absorbance shift depends mainly on various factors such as band gap, grain size, impurities, oxygen deficiency and surface effects. Moreover, the optical band gap E g for the samples was estimated from Tauc relation as function of both calcination temperature (T) and Cobalt addition (x). It was found that both Co addition and calcination temperatures decrease the band gap of barium hexaferrite nanoparticles. This reduction was attributed to the enhancement in the samples grain size which is consistent with TEM results. Vickers microhardness was employed to investigate the mechanical performance of the prepared samples. Microhardness results revealed that the samples experience reverse indentation size effect (RISE) attitude. It was concluded that Vickers microhardness, elastic modulus and yield strength of the prepared samples were enhanced with increasing Co-addition and calcination temperature. Thus, Cobalt in Ba 12 Fe 19 O matrix and elevated calcination temperatures have the ability to enhance the mechanical properties and microstructure of the prepared samples. In addition, indentation induced cracked model (IIC) was found to be an adequate model for the description of the microhardness data. Indentation creep results showed that the samples investigated exhibit grain boundary sliding, accompanied by dislocation climbs at small loads followed by dislocation creep for great loads throughout the operative creep mechanism.