The development of dielectric elements suitable for telecommunication systems performing at microwave band demands for very stringent criteria. In order to be applied in devices such as amplifiers, filters and resonators the ceramic materials must display a combination of planned properties. A high dielectric permittivity promotes the intended device miniaturization since the wavelength in dielectrics is inversely proportional to the squared root of their relative dielectric permittivities. This produces impacts positively on the signal propagation velocity and cross-coupling effect minimization. Additionally, a low dielectric loss or equivalently high quality factor lead to prominent frequency selectivity and operational stability. Often pointed out as determinant practical requisite, the thermostability at different working temperatures is ensured by a small temperature of the resonant frequency. There is a significant amount of effort devoted to this research field continuously. In part, this is due to the difficulty to attain simultaneously the three requested features highlighted [1] - [3] .

CaTiO₃-based ceramics, are attractive candidates for use as dielectric resonators in wireless telecommunication systems, can exhibit dielectric properties of ε_r~162, Q × f~12,000 GHz and τ_f ~850 ppm∙˚C⁻¹ [4] . Its temperature coefficient can effectively restrict its application in a microwave device. In general, an effective way to achieve near-zero or zero coefficient and boost the material quality factor is by compensating its large temperature coefficient values using compounds having temperature coefficient opposite signal. Cr_3/4Fe_5/4O₃ is a polycrystalline solid solution and member of the hematite (α-Fe₂O₃) and eskolaite (Cr₂O₃) system (CrYFe₂-YO₃, Y = 3/4) [5] - [7] , in additional exhibited negative τ_f [8] [9] , which makes candidate to balance the temperature coefficient as composite together with CaTiO₃.

This work deals with the investigation of the structural and microwave dielectric properties of a two-phase system derived from CaTiO₃ and Cr_3/4Fe_5/4O₃ oxide ceramics. Their synthesis involved mechanical activation followed by thermal treatments of the corresponding raw materials, and their composites were produced via thermal processing by considering determined composition of each original synthesized phase. Along the compositional process of the proposed system, a better combination of microwave dielectric properties was achieved.

CaTiO₃ and Cr_3/4Fe_5/4O₃ compounds (hereafter referred as CTO and CRFO, in this order) were individually synthesized by the solid-state reaction using high-purity oxide powders: CaCO₃ (Aldrich, 99%) and TiO₂ (Aldrich, 99.9%) for CTO, Cr₂O₃ (Reagen, 99.8%) and Fe₂O₃ (Aldrich, 99%) for CRFO production, weighed according to their specific stoichiometry. The starting materials were hand-grounded in an agate mortar. Prior to the first heat treatment, high-energy grinding of the homogeneous powder mixture was carried out in a planetary mill (Pulverisette 5, Fritsch GmbH, Germany) using stainless steel vials (volume approx. 110 mL) with 20 stainless steel spheres (4 g and 10 mm diameter) experiencing 370 rpm at room temperature. The time of the mechanical milling operation was set at 180 min for the CTO and 60 min for the CRFO. Subsequently, the reagents were calcined in conventional controlled furnaces (EDG1800, EDG Equipamentos, Brazil) at 1100˚C for 3 h to synthesize CTO, and at 1300˚C for 5 h to synthesize CRFO, both in atmospheric air. The calcined powders were mixed according to the molar fraction (x = 0.1, 0.5, 0.9) to produce the (1 − x)CTO:(x)CRFO composite materials. For dielectric microwave properties characterization, the polycrystalline powders were uniaxially pressed (100 MPa) into pellets in a steel die. It was added about 3 wt.% of an organic binder (epoxy resin). The resin (contributing to the pellets preparation) was eliminated along the subsequent thermal stage. In conclusion, all palletized compounds (whose typical dimensions were in diameter and in thickness) were sintered at 1200˚C for 3 h in atmospheric air.

The X-ray powder diffraction experiments were performed in a powder diffractometer (DMAXB, Rigaku, Japan) using the Brag-Brentano geometry in a continuous mode with a scan speed of 0.5˚ min⁻¹. A CuKα radiation tube with the line focus was operated at 40 kV and 25 mA at room temperature (294 K). The X-ray powder diffractions (XRPD) were taken in the range of 20˚ - 60˚ (2θ) in step sizes of 0.02˚. The diffracted X-ray beam coming from the sample is focused into the detector slit with a curved graphite monochromator. Rietveld refinement procedures [10] were applied to diffraction patterns using the interface DBWS9807-Tools [11] , as described by Young and coworkers [12] . The peak shape was assumed to be pseudo-Voigt (pV) function with asymmetry. The background of each pattern was fitted by a polynomial function of degree 5. The fitting of the model to the data was carried out by observing parameters such as the R indexes (R_P, R_WP), goodness-of-fit (S_GoF) and Durbin-Watson d-statistic (d_DW). The theoretical densities of the polycrystalline samples calculated (from crystal cell and contents, ρ_th) in the structural refinement were compared to the bulk (experimental, ρ_exp) densities of the sintered specimen determined by Archimedes’s method. In a complementary manner, the Fourier-infrared (FT-IR) spectra were obtained using a FT-IR infrared spectrometer (IR-prestige, Shimadzu, Japan), applying standard KBr pellet as dispersant agent (1:100 wt./wt.) in the mid range from 400 up to 1000 cm⁻¹.

Microwave relative dielectric permittivity (ε_r) and unloaded quality factor (Q_u) values at microwave frequencies were measured using the TE_01δ resonance mode obtained by the Hakki-Coleman [13] method as modified and improved by Courtney [14] and Kobayashi and Tanaka [15] . The cylindrical pellets were inserted in a cavity connected to a network analyzer (HP 8719ET, Agilent, USA) ranging from 1 to 12 GHz in the transmission setup. The electromagnetic field pattern obtained in this method is widely used in characterization of properties of materials due to the nonexistence of the current crossing both the dielectric and the conducting plates. Hence, possible air gaps between the dielectric and the conducting plates have no effects on resonance properties of the TE_01δ mode [16] [17] . By measuring the resonant frequency in the temperature ranging from 25˚C (T_i) to 100˚C (T_f) at a step of 10˚C via temperature controlled thermocouple, the temperature coefficient of the resonant frequency (T_f) was evaluated according to the Equation (1):

where f₂₅ is the resonant frequency at arbitrary standard temperature (25˚C) and is the difference between the final and initial measured frequencies. Usually, is scaled up by 10⁶ and reported in parts per million per Celsius degree (ppm∙˚C⁻¹).

The XRPD observed (y_o) and calculated (y_c) profiles of CTO (x = 0.0) and CRFO (x = 1.0) are shown in Figure 1(a). The XRPD polycrystalline patterns for the composite members (x = 0.1, 0.5, 0.9 mol%) of the series are shown in Figure 1(b). The red lines (positioned bellow diffractograms) represent the relative difference between the experimentally observed and the computed intensities by the refinement method. According to the XRD results, there are well defined two-phased ceramic composites combining CaTiO₃ (perovskite structure) and a Cr₃/ 4Fe₅/4O₃ (hematite structure), both indexed in the Inorganic Crystal Structure Database (ICSD) [18] under the codes 163,528 and 163,943, respectively. The Rietveld refinement of CTO was carried out based on a reference model [19] . The CTO present, at room temperature, orthorhombic structure belonging to the spatial group ( 62), with four molecules per unity cell (Z = 4). In this structure, the calcium is localized in the Wyckoff position 4c, where the oxygen ions occupies the sites 4c and 8d and titanium occupies the site 4b. A crystallographic model based on iron oxide and on chromium oxide was proposed [7] to study the CRFO phase. It was suggested that CRFO presents (at room temperature) a trigonal structure belonging to ( 167) spatial group, with six molecules per unity cell (Z = 6). In this proposition the iron and chromium are localized both in the 12c Wyckoff Position, and the oxygen ion occupies the 18e site.

The numerical criteria of fit considered here are the weighted profile R-factor (R_WP), the value of the Durbin- Watson d-statistic (d_DW) and the goodness of fit (S_GoF). From a purely mathematical point of view, R_WP is the most meaningful of the R’s because the numerator is the residual being minimized, scaled by the weighted intensities [12] . Based on this criterion, the refinement associated to the phase CRFO presented the best result. The value of the Durbin-Watson d-statistic shows the serial correlation of the refinement, where an ideal value should be around 2, indicative of insignificant serial correlation in the refinement process [20] . According to the results, it is suggestive a significant serial correlation, in principle associated with the presence of a slight amount of extra phases present and/or overlapping. Graphical representations of the Rietveld model fit to the measured XRD diffraction patterns for CTO and CRFO are shown in Figure 1(a) & Figure 1(b). The residual exhibits satisfactory matching (minimization of the difference) between observed peaks of the crystalline phases and the employed model. Also, the obtained values for S_GoF around 1.0 indicate the adequacy of the proposed models, which justify the effectiveness of structural elucidation for the materials analyzed and validates a good purity grade achieved by them. The crystallographic data, refinement indices and density of the materials under investigation are summarized in the Table 1. The results for the composites are reported in Table 2.

Figure 2 shows the room temperature infrared transmission spectra observed for CTO, CRFO and derived

composites. The characteristic vibrational modes, ranging from 400 cm⁻¹ to 600 cm⁻¹, are summarized in the Table 3. The values for CTO are slightly different, but close to previously published results [21] [22] . Evidencing the solid-state reaction, the CTO and CRFO present displacements in relation to that characteristic observed for their reagent oxides: the Fe₂O₃ transmission spectrum occurs in 575 cm⁻¹ and 480 cm⁻¹, in 580 cm⁻¹ and 445 cm⁻¹ for eskolaite (Cr₂O₃) and in 423 cm⁻¹ for rutile (TiO₂). As suggested by the XRPD patterns, the composites seem to reflect the quantities of each phase therein.

Table 4 presents the microwave dielectric properties of the samples investigated considering the measurement

methodology described in the literature [13] - [15] . All materials resonated in the range from 2 GHz to 7.94 GHz. The CTO (x = 0.0) and CRFO (x = 1.0) base compounds limited the ε_r of the series: 140.1 and 8.3, respectively. The decrease in relative dielectric permittivity values accompanies the resonant frequency raising. The dielectric loss (tan(δ)) for the materials is around 1 × 10⁻³. The behavior of the quality factor is not explained considering only the proportion of CTO/CRFO present in each composite, as seen in the case for (x = 0.1), with the lower result. It seems that additions of CRFO improved this parameter along the system: the Q_u × f_r increases from 1499 GHz (x = 0.0) to 6853 GHz (x = 1.0). Alteration in the production route, and hence in the resulting microstructure, can be an indicative of the differences observed for literature reports of the same material, presently exemplified by CaTiO₃. The temperature coefficient of resonant frequency values verified ranges from high positive (+921 ppm∙˚C⁻¹ for x = 0.0) to moderately negative (−56 ppm∙˚C⁻¹ for x = 1.0). In detriment of other characteristics, relatively better combination of microwave dielectric properties was reached for (x = 0.9) member: Q_uxf_r = 6291 GH, a relatively good factor [23] , and τ_f = −47 ppm∙˚C⁻¹. The potentiality of these composites would be significantly increased by further improving the quality factor values.

The results for microwave dielectric properties measurements are graphically illustrated in Figure 3. There was employed an exponential PDF based with offset (fit) function to predict the response of τ_f (Figure 3(a)) between x = 0.0 and x = 1.0 extreme compositions as modeled by the Equations (2)-(3):

This mathematical model probably is restricted to the results observed, not general and neither linked to physical parameters. According to it, approximately for a composition of x = 0.72 it is possible to get a thermostable response (τ_f = 0 ppm∙˚C⁻¹).

The structural and microwave dielectric properties of the (1−x)CaTiO₃:(x)Cr_3/4Fe_5/4O₃ (CTO:CRFO) ceramic composites were investigated in this paper. The XRPD patterns and infrared spectra indicated the formation of the extreme phases as well as their preservation in composites, suggesting that, in the thermal processes, it was suppressed the occurrence of unwanted solid-state reactions and/or persistence of reagents. Reportedly, the microwave dielectric properties are related to the composition of the material. The proposed composite (x = 0.9) exhibits a satisfactory combination of e_r = 8.5, Q_uxf_r = 6291 GHz, and t_f = −47 ppm∙˚C ⁻¹. With compositional adjustments, the operational and thermal stability of this system can be optimized, fully enabling its application in devices operating at microwave bands. Theoretically, the fitting model (Equations (2)-(3)) provides that the composition with x = 0.72 would have the temperature coefficient near zero.

This work was jointly supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) under project No. 407725/2013-0 (Edital MCTI/CNPq/CT-Energ No. 49/2013) grant No. 385487/2013-4, Coordenação de Aperfeioamento de Pessoal de Nível Superior (CAPES), and Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico (FUNCAP), Brazilianfunding agencies. The XRD measurements were performed in the Laboratório multiusuário de caracterização de nanopartículas por difração de raios-X atthe Universidade Federal do Ceará undergrant CNPq 402561/2007-4 (Edital MCT/CNPq No. 10/2007).

Francisco Nivaldo Aguiar Freire,Manoel Roberval Pimentel Santos,Hélio Henrique Barbosa Rocha,Ana Fabiola Leite Almeida,Selma Elaine Mazzetto,Antonio Jefferson Mangueira Sales,Antonio Sergio Bezerra Sombra, (2016) The Thermal Stability of (CaTiO₃)1-x (Cr_3/4Fe_5/4O₃)_x Ceramic Composites in the Microwave Region. Materials Sciences and Applications,07,202-209. doi: 10.4236/msa.2016.74020