Effect of Bi2O3–B2O3 as a sintering aid in microstructure and dielectric properties of Fe2Mo3O12 electroceramic

Molybdates from A2Mo3O12 family have been widely investigated due to its low sintering temperature, low thermal expansion coefficient, and low dielectric loss. Fe2Mo3O12 (FMO) is an oxide from this family and widely used in the catalytic field. The aim of this work is to evaluate the influence of the Bi2O3–B2O3 as a sintering aid in the microstructure and dielectric properties of FMO. The diffraction results showed that the FMO with the monoclinic structure phase was obtained after the calcination process (650 °C). Mössbauer spectroscopy showed the formation of Fe2O3 after the sintering process at 800 °C. The scanning electron-microscopic image demonstrates an increase of the grain as a function of sintering aid concentration. The samples were analyzed using the impedance spectroscopy at radiofrequency with temperature variation. The Nyquist diagram obtained in this temperature range was fitted from an equivalent circuit with three R-CPE associations, corresponding to the morphology of the electroceramics. For dielectric properties in the microwave, all the samples showed values of εr lower than 10. Values of Q x f above 14,132.35 GHz were achieved. The thermal stability was evaluated by the temperature coefficient of resonant frequency (τf). The lowest τf values of − 6.55 ppm/°C and − 4.35 ppm/°C (near-zero) were measured to FMO and FMO mixed with 7.5 wt % Bi2O3–B2O3, respectively. Based on these results, FMO can be used to low-permittivity ceramic for low-temperature co-fired ceramics (LTCC) applications, antenna substrate, and millimeter-wave range.


Introduction
The electroceramics emerged as an excellent option for integration and miniaturization of electronic circuits, making smaller and more efficient systems [1]. Due to the advances in telecommunications, it is required that the materials have appropriated dielectric constant (ε r ), low dielectric loss (tan δ), near-zero temperature coefficient of resonant frequency (τ f ), high-quality factor (Q x f), temperature stable and versatile [2,3]. Electroceramics are widely used in the manufacturing of microwave electronic devices, dielectric substrates, resonators owing to their easy integration in chips [4]. Most of these ceramics require high sintering temperature, which can be a factor that hinders the integration of them into an 2 Methodology FMO electroceramic was synthesized by solid-sate route using Fe 2 O 3 (99%, Sigma Aldrich) and MoO 3 (99.5%, Sigma Aldrich). These starting materials were ground and mixed in agate mortar following the stoichiometry, as described in Eq. 1.
The mixture was pressed into pellets and calcinated at 650 °C for 16 h. The Bi 2 O 3 -B 2 O 3 sintering aid was prepared using Bi 2 O 3 (99.9%, Sigma Aldrich) and B 2 O 3 (98%, Sigma Aldrich) and was added in molar ratio 1:1 and mixed in Fritsch Pulverisette 6 planetary mill for 10 h with 370 rpm as rotation speed. Milling was performed in sealed stainlesssteel vials under air with stainless-steel balls. A ball mass ratio of 1:9 (Bi 2 O 3 -B 2 O 3 powder mass: ball mass) was used.
For dielectric characterization, the samples were ground and mixed with a binder: polyvinyl alcohol (PVA) 10% v/v solution. Bi 2 O 3 -B 2 O 3 was added as sintering aid and the samples were labeled as FMO-X (X = 2.5, 5.0 and 7.5 wt% Bi 2 O 3 -B 2 O 3 ). Then, each composition was pressed into cylindrical pellets and sintered at 800 °C for 6 h. The volume of these pellets was determined by Archimedes' method to calculate their density. These pellets were polished on both faces and a silver paste (Joint Metal-PC200) was applied on them to ensure the electrical contact.
Powder X-ray diffraction (PXRD) patterns were obtained using a PANalytical diffractometer (Xpert Pro MPD) operating at 40 kV and 40 mA in the Bragg-Brentano geometry, with a Co tube (Ka 1 : 1.7889 Å). The diffraction patterns were collected at room temperature from pulverized FMO calcinated and sintered samples. The diffractograms were obtained from 10° to 60° at a step size of 0.013°, with an analysis time of 70 s at each step, in a graphite monochromator in the plane geometry for diffracted beam.
Mössbauer spectra were obtained using transmission mode at room temperature. The 57 Co radioactive source in Rh matrix was used mounted in speed controller scanning from − 10 mm.s −1 to + 10 mm.s −1 [34]. Spectra were also obtained for FMO calcinated and sintered samples.
Raman spectra were obtained using LabRAM Jobin-Yvon HORIBA spectometer, equipped with liquid nitrogen cooled CCD. A Laser beam was focused in the sample surface using OLYMPUS microscope lens and numeric opening of 0.75, forming a spot of 4 μm 2 under the sample surface. The slits were adjusted to obtain a resolution of 2 cm −1 .
The surface's microstructure of the FMO based electroceramics was studied by scanning electron microscopy (SEM). Micrograph images were collected at room temperature using a FEG Quanta 450 with Energy-dispersive X-ray spectroscopy (EDS) in the SEM chamber, with an incident electron beam of 20 kV. The average grain size was determined using ImageJ software [35], from at least 50 particles measurements.
For dielectric measurements at the RF range, the samples were evaluated using a Solartron 1260 computercontrolled impedance analyzer as a function of frequency (1 Hz-10 MHz) at different temperatures (220-260 °C). On the other hand, the dielectric measurements for the (1) Fe 2 O 3(s) + 3 MoO 3(s) → Fe 2 Mo 3 O 12(s) microwave range were evaluated with an Agilent Model PNA N5230A using the Hakki-Coleman method [36]. The Agilent PNA N5230A was also used to measure the τ f of the FMO as described by Silva and his coworkers [37].

Results and discussion
To confirm the complete calcination of FMO and achieve the crystalline phase, all the samples were analyzed by PXRD technique and compared to Fe 2 Mo 3 O 12 (ICSD: 100,606, monoclinic, P2 1 /a 1 ) crystalline phase. Figure 1a shows the diffraction pattern of the Fe 2 Mo 3 O 12 based ceramics. The  molybdates has a thermic decomposition in temperatures higher than 700 °C, forming initial oxides used in the reaction [38]. Therefore, all sintered samples were also analyzed by PXRD to examine the crystalline structure after the sintering process. Diffractograms of FMO, FMO-2.5, FMO-5.0 and FMO-7.5 presented the formation of Fe 2 O 3 (ICSD: 154,190, Rhombohedral, R-3c) phase, confirming the thermal decomposition of Fe 2 Mo 3 O 12 after the sintering process as described in Eq. 2.
In the literature, Bi 2 O 3 -B 2 O 3 mixture acts as a sintering aid formed an amorphous compound after thermal process [31,39]. In this work, no peaks relative to these phases (Bi 2 O 3 or B 2 O 3 ) were found in diffractograms.
Rietveld refinement parameters are presented in Table 1. Low values of agreement indices for all samples shows that the Rietveld refinements were satisfactory, with weighted profile R-factor (R WP ) below 20% or between 20 and 30% for samples with two or more phases and χ 2 near to unity [40]. Another factor is the similarity between lattice parameters of samples and the pattern of database, which indicates that structure did not change with addition of Bi 2 O 3 and B 2 O 3 , since Bi 3+ can substitute Fe 3+ in octahedral site [41].
The Mössbauer spectra of the samples are shown in Fig. 1b. All samples showed a doublet with isomer shift (δ) near to 0.41 mm/s, related to paramagnetic Fe 3+ ion in high spin state and quadrupole splitting (Δ) at 0.18 mm/s, which is assigned to [FeO 6 ] sites in Fe 2 Mo 3 O 12 monoclinic phase [42]. Spectra of sintered samples showed a low intensity simple sextet, indicating a magnetically ordered state. The high value of hyperfine field (B HF ) (above 51 T), and the values of δ near 0.37 mm/s indicates the presence of α-Fe 2 O 3 phase [43][44][45], which were also found in PXRD analysis. The experimental data of hyperfine parameters are presented in Table 2.
As discussed in PXRD, Fe 2 O 3 content rise as sintering aid content rise according to area (%) values presented in Table 1. No Fe 2 O 3 was observed in C-FMO spectrum, since it was not sintered, while FMO-7.5 presented 18.4% of Fe 2 O 3. However, in the 7.5 wt % of Bi 2 O 3 -B 2 O 3 a drop in iron oxide concentration is observed in PXRD when compared to Mössbauer spectra. This phenomenon can   (1) 18.2226 (4) 2157.68 (7) 90  (7) 5.4308 (7) 5.4308 (7) 100.73 (2) 55.241 (6) 55.241 (6) 55.241 (6) FMO-2.5  (1) 100.70 (3) 55.313 (9) 55.313 (9) 55.313 (9) FMO-5.0  (1) 18.2226 (5) 2158.96 (9) 90  (1) 100.71 (3) 55.266 (9) 55.266 (9) 55.266 (9) FMO-7.5  (1) 18.2272 (5) 2159.36 (9) 90.0 125.230 (2) 90.0  (6) 5.4293 (6) 5.4293 (6) 100.74 (2) 55.276 (5) 55.276 (5) 55.276 (5) be explained by a ternary diagram for Fe 2 O 3 -Bi 2 O 3 -MoO 3 ( Fig. 2). At region 1 of ternary diagram, the samples start to decompose above 700 °C in sintering process and producing this molar ratio for all samples. As temperature increases, MoO 3 volatizes, and the Fe 2 O 3 and Bi 2 O 3 molar fraction increases for all samples, until reach equilibrium at region 2. When FMO-7.5 decomposes, the molar ratio of Fe 2 O 3 -Bi 2 O 3 -MoO 3 system promote the formation of a vitreous phase, as described by Iardanova [46]. PXRD analysis depends on the crystallinity of the sample, but this technique is not able to quantify entire Fe 2 O 3 , since it is forming an amorphous vitreous phase, but Mössbauer identify the Fe environment. Therefore, Mössbauer spectroscopy is able to quantify the entire Fe quantity. Figure 3 shows Raman spectra for FMO samples. The Fe 2 Mo 3 O 12 monoclinic phase belongs to P2 1 /a 1 space group with eight molecular formula for unit cell. According to group theory, Fe 2 Mo 3 O 12 has 408 modes in Brillouin zone, three of which are acoustic modes and the rest are optical modes which may be described as irreducible representation Γ P2 1 /a 1 = 102 A g + 102 B g + 101 A u + 100 B u. Only A g e B g are Raman active [27]. For FMO sample, main modes of Fe 2 Mo 3 O 12 structure are observed, which reinforces the data obtained from PXRD analysis. Raman spectra of sintered sample FMO is similar to C-FMO. Fe 2 O 3 , Bi 2 O 3 and B 2 O 3 phases were not assigned due to its bands being overlapped by Fe 2 Mo 3 O 12 's spectrum. Regions between 700-900 cm −1 and 900-1200 cm −1 show that some bands keep their shape, intensity and width as it can be seen in Fig. 3. According to Moura et al.   All bands and its correlated vibrational modes are presented in Table S1 in Electronic Supplementary Material (ESM). After sintering process, the pellets were analyzed by SEM at 1000X magnification. Figure 4a shows the superficial micrograph for FMO. It presents grain size from 2 to 14 μm of diameter and average size of 5 μm, while for FMO-2.5, FMO-5.0 and FMO-7.5 (Fig. 4b-d) are between 6 and 61 μm. The average sizes are 17, 24 and 24 μm for FMO-2.5, FMO-5.0 and FMO-7.5 respectively, as can be seen in the inset of these micrographs. According Tohidifar [31], the sintering aid increases the grain size due the liquid phase flow which promotes the dissolution and diffusion of Fe 2 Mo 3 O 12 grains. Although all pellets showed relative density near to 85% ± 2%, which implies that the addition of Bi 2 O 3 -B 2 O 3 as sintering aid does not change the relative density of material in 800 °C sintering process. It was observed in past studies that relative density is dependent of Bi 2 O 3 content in sintering aid, as sintering aid content rises in mixture, Bi 2 O 3 concentration in sintering aid must decrease to preserve a high relative density [31]. Therefore, a slightly decrease in relative density was expected to be observed since Bi 2 O 3 content was kept constant. According to the scanning utilizing EDS technique ( Fig. 4e and Fig. S1 in ESM), the surface of all sintered samples are rich in Fe and poor in Mo, which reinforces that thermal decomposition during sintering process volatilizing MoO 3 , leaving the Fe 2 O 3 on the surface of pellets evidenced by mass loss of 5-10% [38,47].
The broken pellets were also analyzed to investigate inner composition (inside the pellet) and compare to the surface. Fig. S2 of ESM presents micrograph for all the samples and they showed average grain size near to 20 μm for FMO and 38 μm for the others, higher than those on pellet surface. The scanning using EDS maps are presented in Fig. S3 (ESM) and shows that the concentration of Mo is higher than the ones on surface, which indicates that the pellet's interior does not suffer critical changes in its composition. Table S2 presents the concentration of each element for each sample, for both surface and inside of pellet. Due to the difference on surface and inner composition, EDS analysis of cross section for the FMO sample was made to investigate the inner/surface interface of this material after sintering process and is presented in Fig. 5. Figure 5a shows the EDS spectrum for pellet surface, while Fig. 5b, c presents the EDS spectra for interface and inner, respectively. As can be seen, the amount of Mo is lower on surface than inside the pellet indicating the thermal decomposition of Fe 2 Mo 3 O 12 in Fe 2 O 3 and MoO 3 and then the MoO 3 volatilization at 700 °C on the surface of the pellet [39,[47][48][49].
The results of all structural and morphological characterization allow us to affirm that small part of Fe 2 Mo 3 O 12 decomposes thermally in MoO 3 [46]. Figure 6 shows the imaginary part of impedance (Z") spectra and temperature dependency (220-260 °C) for the samples. For FMO (Fig. 6a), FMO-2.5 (Fig. 6b) and FMO-5.0 (Fig. 6c) there is a peak about 1 MHz, while for FMO-7.5 (Fig. 6d) this peak appears near to 100 kHz. For the maximum of peak observed in Z" spectrum is observed a shift for higher values with increasing of the temperature what can be related to the relaxation process thermally activated [50]. As can be seen in insets of Fig. 6, the normalized peaks shift to regions of higher frequency.
The shift is thermally activated and may be described by Arrhenius' equation: where f Max is the frequency when Z" has a maximum value, f o is the pre-exponential term, E a is the activation energy of the process, K b is the Boltzmann constant and T is temperature [50]. The E a are determined by the angular coefficient of linear curves in Fig. 7.
The addition of sintering aid has a direct influence on this process. The E a increases with rising in sintering aid addition, except for FMO-7.5. This increase is probably due to the higher concentration of Bi 2 O 3 and Fe 2 O 3 spurious phase [51], as seen in PXRD and Mössbauer spectroscopy. FMO presented the lowest E a value (0.502 eV), since it has the lowest Fe 2 O 3 and Bi 2 O 3 concentration, while FMO-5.0 has higher quantity of crystalline Fe 2 O 3 and Bi 2 O 3 and presented the highest value (0.810 eV). However, However, FMO-7.5 has low quantity of crystalline Fe 2 O 3 and its E a is lower than FMO-2.5 and FMO-5.0.
Nyquist's plots (Z" versus Z') were also analyzed. The spectra presented deformed semicircles in relation to Z" axis, corresponding to non-Debye model [52]. The behavior of the ceramic material may be described as resistor-capacitor (R-C), equivalent circuit describing the electric response of grain, grain boundary and electrode effect. As the capacitive contribution does not represent an ideal behavior, the capacitor is substituted by a constant phase element (CPE). The CPE impedance (Z CPE ) is described as the following equation [50]: where Q is a proportional factor, ω is the angular frequency and n is an empirical exponent with values between 0 (ideal resistor) and 1 (ideal capacitor). All samples were fitted by a non-linear function and are presented in Fig. 8. R g , R gb and R e are grain resistance, grain boundary resistance and  Tables S3, S4, S5 and S6 of ESM, respectively.
As described previously in Z" spectra, the resistive contribution of grain, grain boundary and electrode effect decreases with temperature increasing. Grain and electrode are predominantly capacitive due to the fitted parameter n (deviation of ideal capacitance) for CPE element with increasing of the temperature [53].
The addition of sintering aid makes the electrode contribution purely resistive, eliminating the capacitive. Moreover, for all samples, the increasing of temperature leads to the decreasing of n value for grain boundary contribution, indicating an increasing in resistive contribution of this element. Thermal stability of capacitance (TCC ) was also determined for all samples using the relation described in Eq. 5 [54].
where C 100 and C 25 are the capacitance in 100 and 25 °C respectively. Table 3 presents the TCC in 100 Hz, 1 kHz, 10 kHz and 1 MHz frequencies. 5 are potential to be applied as class 1 capacitor at 10 kHz. These ceramics were also analyzed in a microwave frequency range. The temperature coefficient of resonant frequency (τ f ) was determined using the relation described in Eq. 6 [54].
where f 80 and f 25 are the resonant frequency at 80 °C and 25 °C, respectively. Table 4 presents the microwave dielectric properties for all samples. Table 4 presents the microwave dielectric properties for all samples. The addition of sintering aid does not imply many changes in microwave dielectric properties of bulks. FMO-2.5 presented the higher ε r value (9.70) among the samples. Furthermore, FMObased ceramics have ε r between 9.3 and 9.7, near of Al 2 O 3 (ε r = 9.8), one of the most important oxides used in industry [55]. All compositions showed low values of tan δ, and FMO-2.5 presented the smallest value (6.52 × 10 −4 ). This phenomenon can be related to the liquid phase of sintering .10 6 , Fig. 5 Cylindrical pellet scheme and EDS spectra of different areas. a Surface, b surface/inner interface and c inner aid between grain interstice [6]. The greatest dielectric loss of FMO can be related to the extrinsic loss due to the smaller grains on the surface of the bulk, as can be seen by SEM images [56]. Although all compositions showed a low dielectric loss, which is required for some applications [5]. Moreover, all the composition presented high values of Q x f, and the FMO-2.5 is the one that presented the highest value of 16,054.24 GHz, which shows its potential use in microwave devices. τ f values increase in modulus with increasing sintering aid content, except for FMO-7.5 bulk. FMO and FMO-7.5 showed the lower values of τ f , probably due to the lower quantity of Fe 2 O 3 as a spurious phase which presents a τ f of -66.64 ppm.°C −1 [51]. There is a smaller amount of the iron oxide in crystalline form in these samples. As previously discussed in PXRD, FMO-7.5 presents iron oxide in vitreous phase and must probably the τ f is different from that presented in the literature. However, FMO-2.5 and FMO-5.0 presented the highest values of τ f. This phenomenon is probably related to the high concentration of Fe 2 O 3 in crystalline form, as it was quantified by PXRD analysis [51,57].
Near-zero τ f is a very desirable property for resonators and substrates. Recently some researches were taking original strategies to achieve this goal. Castro and coworkers investigated (Ba 2 CoNbO 6 ) 1-x -(CaTiO 3 ) x composite and reached near-zero τ f at x = 0.37 [58]. Oliveira investigated the effect of CaTiO 3 addition in thermal stability of the BiVO 4 matrix [59] and Paiva investigated the Y 3 Fe 5 O 12 -CaTiO 3 composite and determined that nearzero τ f composition [60]. The FMO and FMO-7.5 showed τ f values of − 6.55 and − 4.35 ppm.°C −1 , respectively. Also, they are characterized as near-zero τ f since their values are  smaller than ± 10 ppm.°C −1 . Even though, the FMO presents a τ f near zero, the formation of a vitreous phase improves the τ f of the FMO.

Conclusion
The purpose of this study was to add sintering aid to improve the dielectric properties of the FMO (ε', tan δ, τ f ), relative density, etc. FMO monoclinic phase has been successfully synthesized via the solid-state route and characterized by PXRD, Mössbauer, and Raman spectroscopy. The sintering process at 800 °C was effective in producing dense solids. The SEM images associated with EDS spectra show that the surface of pellets is rich in Fe because of the thermal decomposition of and Q x f values above 14,000 GHz. This implies that they all have the potential to be applied as microwave devices, such as microstrip antenna substrate or dielectric resonator antennas. Moreover, FMO and FMO-7.5 have near-zero τ f (smaller than ± 10 ppm/°C), making them suitable candidates for dielectric resonator antenna.