Chapter 4 Permittivity, permeability, and EMI shielding effectiveness of LFO-based
4.3 Results and discussion
4.3.1 Surface micrograph and structural analysis
ferrite and Dy substituted lithium ferrite matrix. The Dy substitution is expected to promote polarization, magnetic loss, and microstructure, enhancing the EMI efficiency.
ceramics composites sheet-like structure. The flat sheet-like structure of CB provides a pathway for the first migration of electrons. With an increase in CB concentration, more sheet-like structures are observed. Also, some flexes are quite large (10 µm). The CB is agglomerated and assembled on the grain boundary of LFO. With an increase in CB content, a more disturbed grain structure is observed.
Figure 4-1. Surface morphology of (a) LFO, (b) LFO/CB (5), (c) LFO/CB (10), (d) LFO/CB (20).
Figure 4-2 shows the surface morphology of the composites of Dy substituted samples. The irregularly shaped grains with well-defined grain boundaries are observed.
The carbon sheets have an average size of 1- 4 µm. However, a few are much more prominent, within the range of 4 - 8 µm. It is observed that the average grain size decreases slightly from 1.19 µm (LFO/CB) to 0.92 µm (LD15FO/CB). This decrease in grain size with an increase in Dy3+ concentration is because of the large size of Dy3+ ions, which hinders grain growth by diffusing to the grain boundaries and forming the DyFeO3
secondary phase.[193] The obtained irregular grains with porous structure facilitate microwaves' absorption, leading to better SE.
Figure 4-2. FESEM micrograph of (a) LFO/CB, (b) LD5FO/CB, (c) LD10FO/CB (d) LD15FO/CB.
The typical XRD pattern of LFO, CB, and composite materials is depicted in Figure 4-3(a). Two prominent peaks are observed for CB at 2θ of 26.43, 54.51correspond to (002) and (004), respectively (hexagonal, P63/mmc, JCPDS # 00-060-0053), confirming the pure phase formation. All the observed peaks of LFO are appropriately matched with the ordered cubic phase (P4132, JCPDS # 82-1436)[194]. Interestingly, no additional peak for any other undesired phases has been observed in the composites apart from LFO and CB. The intensity of LFO is suppressed with an increase in CB content. The predominant peak of LFO (311) shifted towards the lower 2θ, which is attributed to the addition of CB that led to the generation of strain (Figure 4-3(b)). For further clarification about lattice parameters, Rietveld refinement is carried out by Fullprof software. The refined pattern is shown in Figure 4-3(c). For CB lattice parameters are : a= b= 2.4616 Å, c= 6.7226 Å, α = β = 90°, γ
ceramics composites
= 120° and for LFO: a= b= c= 8.3452 Å, α = β = γ = 90°. Also, the presence of two phases is examined through refinement, where we can get information regarding volumetric concentration (percentage contribution of individual phase).
Figure 4-3. (a) XRD pattern of LFO, CB, and composites, (b) Relative shift of predominant peak (311), (c) Rietveld refinement pattern of LFO/CB (10), (d) Schematic
illustration of the Unit cell of LFO, CB, and LFO/CB.
The Scherrer equation estimates the average crystallite size (D) of LFO and composites (all the intense peaks are considered).
𝐷 = 𝑘𝜆 𝛽𝑐𝑜𝑠𝜃
4.1
here, β, λ, θ, k is Full width at half maximum (FWHM), the wavelength of incident X-ray, Bragg’s angle, and shape factor (0.98), respectively. The calculated D for pristine LFO is
33.98 nm, whereas it varies from 35.85 to 35.19 nm for composites. The unit cell image of LFO, CB, and composites are shown in Figure 4-3(d). Li, Fe, O, and C atoms are represented in green, magenta, red, and blue, respectively.
The XRD patterns of pure CB, LFO/CB, and LDFO/CB composites are depicted in Figure 4-4(a). For the LFO/CB and LDFO/CB composites, intense peaks are observed for (220), (311), (400), (422), (511), and (440) planes, thereby confirming the spinel phase formation of lithium ferrite. The shift of the peaks indicates the substitution of Dy3+ cations in the matrix (Figure 4-4(b)). The most intense peak (311) gets shifted towards higher 2θ values with an increase in Dy3+ doping up to x = 0.15. For x = 0.2, the (311) peak shifted to the left concerning the peak positions of other composites. Fullprof software is used to perform Rietveld refinement of the XRD patterns (Figure 4-4(c)). The various lattice parameters are enlisted in Table 4-1.
Table 4-1. Structural parameters of LDFO.
LFO LD5FO LD10FO LD15FO LD20FO
a (Å) 8.3452 8.3415 8.3392 8.3374 8.3397
Volume (Å3) 581.17 580.40 597.92 579.55 580.03
χ2 1.24 1.32 1.26 1.44 1.47
The highest lattice constant is observed for the LFO/CB composite, whereas it is the lowest for LD15FO/CB. With an increase in Dy concentration, the lattice constant is found to be decreased up to LD15FO/CB. On doping, the larger Dy3+ (0.91 Å) ions in place of the smaller octahedral Fe3+ (0.64 Å) ions, the lattice strain, and disorder may arise due to ionic radii mismatch of the dopant and host ions, resulting in an increase or decrease of the lattice parameter.[195][196] Here, the substitution of Dy3+ cations has reduced the lattice constant initially. A similar trend has been observed in other rare-earth-doped spinel ferrites like
ceramics composites dysprosium doped cobalt-zinc ferrite,[197] Dy3+ doped nickel-cobalt ferrite,[165] Ce-Dy substituted cobalt ferrite,[198] and Gd3+ doped cobalt ferrite[195]. For a high amount of Dy-doping (x = 0.15, 0.2), extra peaks are also found around 2θ ~ 33° corresponding to DyFeO3 phase formation.[199] The large Dy3+ ions cannot completely replace all the Fe3+
ions in the octahedral positions at high doping concentrations. This results in distortion of the spinel structure, and the excess Dy-ions move to the grain boundaries resulting in the DyFeO3 impurity phase.[200][201] The composites' average crystallite sizes (D) are calculated using Equation 4.1. For the LDFO/CB composites, the D values ranged from 30.05 to 42.39 nm.
Figure 4-4. (a) XRD pattern of all the composites, (b) Relative shifting of the predominant peak, (c) Reitveld refinement pattern of LFO/CB, (d) LD10FO/CB.