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Structural properties of 0.8CaZrO3–0.2CaFe2O4 composite
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 8688–8695 www.elsevier.com/locate/ceramint
Structural properties of 0.8CaZrO3–0.2CaFe2O4 composite Edyta Śnieżeka,n, Paweł Stocha, Jacek Szczerbaa, Dominika Madeja, Ryszard Proroka, Agata Stochb a
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. A. Mickiewicza 30, 30-059 Krakow, Poland b Institute of Electron Technology Krakow Division, Zablocie 39, 30-701 Krakow, Poland Received 28 January 2015; accepted 14 March 2015 Available online 23 March 2015
Abstract A composite with the 0.8CaZrO3–0.2CaFe2O4 general formula was prepared by means of a double sintering process. Temperature of 1400 1C was found suitable to obtain the composite based on scanning electron microscopy and X-ray diffraction analysis. Phase composition, crystal structure, Mössbauer effect, microstructure and dielectric properties were studied. The influence of Fe ions on the CaZrO3 structure has been reported. It was found that Fe substituted Zr in CaZrO3 and vice versa in CaFe2O4 in the amount about 1.5 at% in both cases. The approximated formula of the obtained composite was 0.8Ca(Zr0.9Fe0.1)O3–0.2Ca(Fe0.8Zr0.2)2O4. The SEM observations revealed that liquid CaFe2O4 occurred as a filling phase between CaZrO3 grains which significantly enhanced densification of the material and influence on its properties. The dielectric constant reached the value of 170 at 1 MHz and the accompanying dissipation factor was very low – 0.005. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Composites; C. Dielectric properties; D. Ferrites; D. Perovskites; Mössbauer spectroscopy
1. Introduction Calcium zirconate belongs to a large group of compounds, called perovskites. In general, their formula can be written as ABO3. CaZrO3 prefers the orthorhombic Pcmn space group where Ca2 þ ions are situated between slightly deformed ZrO6 octahedrals. In the CaZrO3 structure, a topological distortion of ZrO6 octahedrals can be observed [1,2]. The reaction of CaZrO3 synthesis proceeds in the solid state. The dominating mass transfer mechanism consist in Ca2 þ ions diffusion through a newly created compound [3]. Moreover, kinetics around the temperature of 1170 1C is controlled by diffusion and nucleation, which is associated with the ZrO2 polymorphic transformation [4]. Calcium zirconate synthesis from ZrO2 and CaO or a natural raw material, such as dolomite (CaMg (CO3)2) starts at 900 1C and is completed at about 1500 1C. The intensity of the synthesis is noticeable within the temperature range from about 1100–1400 1C and is assisted by volume expansion. The linear expansion can reach even 11% at 1500 1C [5,6].
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Corresponding author. Tel.: þ48 126175139; fax: þ 48 126334630. E-mail address: [email protected] (E. Śnieżek).
http://dx.doi.org/10.1016/j.ceramint.2015.03.085 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
CaZrO3 has appeared interesting in many engineering fields because of its unique properties, such as high melting point (2345 1C), dielectric constant ( 30 at 1 GHz), high thermal and chemical stability and refractory properties [7–11]. Stoichiometric, undoped calcium zirconate is a p-type semiconducting material in air at low temperatures and an oxygen ionic conductor at high temperatures. Besides, nonstoichiometric CaZrO3 and doped with some oxides, such as Al2O3, Y2O3, MgO is an oxygen-ion conductor [12–16]. CaZrO3 incorporates actinides relatively easily into its crystal structure and because of its high chemical durability is considered to be a host material for spent nuclear fuel immobilization [17]. In the last years, materials based on various ferrites have been interesting due to their versatile application, such as transmitting microwaves, telecommunication and computer devices. CaFe2O4 is a ferrite representative of a spinel-type group with the AB2O4 general formula. Calcium ferrite exhibits the orthorhombic crystal structure with the Pbnm space group. Two distinct iron sites are composed by the corner and the edge-shared FeO6 octahedral. It is built of eight-fold-coordinated Ca atoms and a distorted FeO6 octahedral. The network of CaFe2O4 is similar to the one formed in perovskite compounds [18]. This characteristic structure may have interesting physical properties, both magnetic and electric.
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Ferrite type compounds, due to their magnetic properties, are widely used for electrical and radio devices. Magnetic properties of superstoichiometric CaFe2O4, which was prepared during thermobaric synthesis, were reported in [19] and, based on the results obtained, the antiferromagnetic character of magnetic interactions was suggested. The temperature of magnetic ordering for CaFe2O4, which was obtained by different researchers based on magnetic measurements, neutron diffraction study, and γ resonance spectroscopy, significantly differ (from 180 to 200 K) [19,20]. CaFe2O4 shows electrical polarization Tc=160 K. Moreover, a substitution of Fe by Ti suppresses antiferromagnetic ordering [21]. It is also known that CaFe2O4 is a p-type semiconductor and it is stable up to 1223 1C – melting point [22]. It is worth mentioning, that in the research system CaO–Fe2O3– ZrO2, Fe2O3 and ZrO2 create solid solution in the whole range up to 1434 1C [23]. Moreover, CaO and Fe2O3 create two calcium ferrites, CaFe2O4 and Ca2Fe2O5 (brownmillerite type) [24]. A combination of CaZrO3 and CaFe2O4 properties may lead to obtaining a material with unique magnetic and electric properties. Furthermore, the melting points of both compounds are significantly different, consequently a synthesis will occur in the presence of the ferrite liquid phase. Therefore, it is interesting to study the influence of ferrite liquid phase on synthesis process, microstructure, structure of CaZrO3 and dielectric properties of the obtained composite. 2. Material and methods 2.1. Preparation of 0.8CaZrO3–0.2CaFe2O4 composite In this paper 0.8CaZrO3–0.2CaFe2O4 composite, based on calcium zirconate with the perovskite structure and calcium ferrite with the spinel structure, was synthesized using the twostep heat treatment at 1200 1C and 1300 1C, 1400 1C, 1500 1C. Starting chemicals used in this study comprised calcium carbonate CaCO3 (Chempur; 98.81% purity), zirconium dioxide ZrO2 (Merck; 98.08% purity) and ferric oxide Fe2O3 (POCH; 96% purity). A composition containing 5, 4 and 1 mol% of CaO, ZrO2 and Fe2O3, respectively, was prepared. The CaCO3, ZrO2 and Fe2O3 reagents in appropriate amount, were mixed together during 2 h in the vibratory zirconium ball mill. The pellets (20 mm diameter, 10 mm thickness) were obtained from homogenized mixtures pressed uniaxially in the hardened steel die at a pressure of 30 MPa. The green pellets were initially calcined at 1200 1C in air for 10 h. After cooling to the room temperature, the pellets were ground to the grain size lower than 0.063 mm.The calcined powder was then used to prepare the final pellets (the same way as above), which were fired at 1300 1C, 1400 1C, 1500 1C under air atmosphere with 10 h soaking time at the maximum temperature. The specimens were cooled together with the furnace. 2.2. Characterization techniques The microstructure of samples was discussed according to SEM observations (FEI NovaNanoSem 200), accompanied by the EDS chemical analysis in micro areas.
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The phase composition of samples was determined by the powder X-ray diffraction (XRD) technique at a room temperature, using Cu-Kα radiation in the 2Θ angel range 10–901 with a step of 0.021 and average time of 10 s/step (Philips Panalytical X'Pert-Pro MPD diffractometer). The obtained data were analyzed using the X'Pert Pro Highscore Plus software. Basing on X-ray diffraction and SEM/EDS results a sample fired at 1400 1C was chosen to further investigations. The crystal structure parameters were obtained using a full-profile Rietveld method [25], implemented in the FullProf software package [26]. Structural parameters, including scale factor, zero shift, background function, lattice parameters, atomic coordinates and fractions, isotropic temperature fraction, peak profile and texture parameters, were taken into account in the course of refinement. The 57Fe Mössbauer effect measurements were performed using the standard technique at the liquid nitrogen temperature ( 196 1C) in a transmission mode, using a conventional constantacceleration spectrometer and a 25 m Ci 57Co source in Rh matrix. The velocity scale was calibrated by means of α-Fe foil. The low temperature spectrum was collected with the sample immersed in liquid N2 using a bath cryostat. Spectra were fitted to Lorentzian lines using the non-linear least square method. The hyperfine interaction parameters, such as isomer shifts (IS), relative to metallic iron at a room temperature, quadrupole splitting Δ and hyperfine magnetic field m0Hhf, were obtained. Dielectric measurements were carried out with an LCR Quad Techmeter. The specimen was prepared from a sintered pellet by polishing the face flat. Silver electrodes were deposited on both sides of the ceramic pellets and fired at 800 1C to ensure good electrical contacts. The variations of dielectric constant and loss tangent at a room temperature were studied by recording these parameters at different frequencies. The real part of dielectric constant was calculated according to the following equation. ε0 ¼ Cd=ε0 A
ð1Þ
where C ¼ capacitance, d ¼ thickness of the pellet, A ¼ area of the surface of the pellet and ε0 ¼ permittivity of the free space (ε0 ¼ 8.854 10 12 F/m). 3. Results 3.1. SEM/EDS observation of 0.8CaZrO3–0.2CaFe2O4 composite formation In this study the two-step heating process at 1200 1C, and then, after grinding and pressing, at 1300 1C, 1400 1C and 1500 1C, tending to obtain the 0.8CaZrO3–0.2CaFe2O4 composite, was performed. The SEM was used to investigate the morphology of the composite prepared in three different temperatures. The SEM images of microstructure evolution and creation of 0.8CaZrO3– 0.2CaFe2O4 composite are shown in Figs. 1–3. In all cases the ferrite phase (dark gray – point 2) fills the space between calcium zirconate grains (point 1 – light gray). The average diameter of the calcium zirconate grains changes from about 1–5 mm (Fig. 1)
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Fig. 1. SEM micrograph of the sample sintered at 1300 1C.
Fig. 3. SEM micrograph of the sample sintered at 1500 1C.
3.2. Phase composition The prepared materials were defined in terms of phase composition by the XRD method. Fig. 4(a) shows the XRD patterns of the samples heat treated at 1200 1C. It is clear that the samples calcined at 1200 1C for 10 h have CaZrO3 and CaFe2O4 phases as main compounds. There are some impurity components, such as Fe3O4 and ZrO2, in the sample. ZrO2 is still present in a sample heated at 1300 1C, Fig. 4(b). It is clear from Fig. 4(c) and (d) when the heat treatment temperature is raised up to 1400 1C and 1500 1C, the crystal phase is completely transformed into the CaZrO3 and CaFe2O4 phases without any impurity component. Based on the SEM and XRD analysis the sample sintered at 1400 1C was selected for further research. 3.3. Crystal structure parameters
Fig. 2. SEM micrograph of the sample sintered at 1400 1C.
through 3–8 mm (Fig. 2) to 3–10 mm (Fig. 3). There are some obvious pores distributed in the samples. The SEM results presented in Fig. 1 demonstrate transition microstructure, where the synthesis has been not complete. There are areas (point 3), where the phase creation is still in progress. The microstructure presented in Fig. 1 and the shape of calcium zirconate grains indicates that the diffusion process proceed intensively at 1300 1C. Figs. 2 and 3 show the SEM images of microstructure of the samples sintered at 1400 and 1500 1C, respectively. A homogeneous microstructure is made up by two well-distributed phases: CaZrO3 and CaFe2O4. Using the energy dispersive spectroscopy analysis (EDS), calcium zirconate (light gray grains – point 1) and calcium ferrite (dark gray phase – point 2) were identified. This microstructure showed calcium zirconate grains surrounded by the ferrite groundmass.
The sample was analyzed using the Rietveld method. The crystal structure parameters of CaZrO3 and CaFe2O4 according to JCPDS: 76-2401 and 72-1199, respectively were taken as starting parameters to the fitting procedure. The measured diffraction pattern and theoretical fit are presented in Fig. 5. CaZrO3 and CaFe2O4 crystallize in the orthorhombic crystal structures (space group no 62). The obtained lattice parameters are summarized and compared with the literature data in Table 1 [27]. 3.4. Mössbauer spectroscopy The Mössbauer spectra of the sample at a liquid nitrogen temperature are presented in Fig. 6. As magnetic ordering in CaFe2O4 occurs approximately at a temperature below 73 1C, a magnetically split spectrum is observed. This spectrum was fitted by two magnetic sextets and a quadrupole doublet which are corresponding to different crystallographic sites, occupied by iron. The obtained hyperfine interaction parameters are presented in Table 2.
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Fig. 4. X-ray diffraction patterns of the samples prepared from solid state reaction of CaCO3, ZrO2 and Fe2O3 at (a) 1200 1C, (b) 1300 1C, (c) 1400 1C and (d) 1500 1C.
Table 1 Crystal structure parameters of the CaZrO3 and CaFe2O4 phases.
a [Å] b [Å] c [Å] V [Å3]
CaZrO3
CaZrO3 [27] CaFe2O4
CaFe2O4 (JCPDS: 72-1199)
5.5851(2) 8.0098(4) 5.7532(2) 257.372
5.5890(1) 8.0140(3) 5.7586(2) 257.929
9.2300 10.7050 3.024 298.79
9.2550(13) 10.7496(16) 3.0251(4) 300.969
Fig. 5. The Rietveld analysis of the 0.8CaZrO3–0.2CaFe2O4 composite prepared at 1400 1C. At the bottom of the figure are presented the differences between the observed and the calculated patterns.
3.4.1. Microstructure analysis The microstructure of the composite sintered at 1400 1C is presented in Fig. 7(a). An average chemical composition of the grains and filling phase according to EDS (Fig. 7(b) and (c)) analysis is presented in Table 3. It can be established that about 1.5 at% of Fe is observed in CaZrO3 grains, which together with Zr gives about 19.6 at%, which is very close to stoichiometric 20
Fig. 6. 57Fe Mössbauer effect spectra of the studied compound at liquid nitrogen temperature.
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at%. The similar amount, such as Fe (1.5 at%), of Zr is also present in the CaFe2O4 phase. Thus the SEM/EDS analysis confirmed the XRD and Mössbauer measurements. 3.4.2. Dielectric measurement The variation of dielectric constant with frequency is presented in Fig. 8. It can be observed that the sample reveals dielectric dispersion. The dielectric constant decreases rapidly, Table 2 The hyperfine interaction parameters (A–area of the subspectra, IS–isomer shift, μ0Hhf–magnetic hyperfine filed, QS–quadrupole split, Γ/2–half width ) for the sample prepered at 1400 1C.
from about 1050–170, with the increasing frequency, and then it reaches a constant value at higher frequencies, indicating an usual dielectric dispersion. Fig. 9 presents variations of the dissipation factor with frequency. The curve corresponds to the curve in Fig. 8. It is observed rapid decrease in the dissipation factor value with frequency increase. At higher frequencies it reaches a low value of about 0.005. Table 3 An average chemical composition (EDS) of the grains and filling phase according to Fig. 7(a). at%
No
A [%]
IS [mm/s]
m0Hhf [T]
QS [mm/s]
Γ/2 [mm/s]
1 2 3
21.0 37.9 41.1
0.156(8) 0.426(8) 0.408
– 43.50(18) 45.03(10)
0.312(30) 0.046(9) 0.004(7)
0.307(17) 0.273(18) 0.236(17)
The filling phase The oval grains
O
Zr
Ca
Fe
60.1 60.1
1.3 18.1
15.1 20.3
23.5 1.5
Fig. 7. (a) SEM micrograph and (b, c) X-ray emission spectra in the point 1 and 2, respectively of the prepared sample at 1400 1C.
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Fig. 8. Variation of dielectric constant (ε') with frequency at a room temperature.
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specific microstructure. Melting point of CaFe2O4 (1223 1C) is much lower than of CaZrO3 (2345 1C). Consequently, according to Figs. 1–3, when heat treatment temperature is increasing densification of the material and creation of the characteristic microstructure is more intense. The synthesis of calcium zirconate was completed at 1400 1C which corresponds to [5,6]. The composite consisted of calcium zirconate where calcium ferrite filled space between CaZrO3 grains. Microstructure in Fig. 3 with cracks indicates 1500 1C as an excessive temperature. The present of cracks can be the result of grain growth and thermal expansion. Figs. 1–3 demonstrate the proposed synthesis method as an appropriate technique for manufacturing the dense 0.8CaZrO3–0.2CaFe2O4 composite. After the first heating step at 1200 1C (calcination process), the XRD analysis confirmed that the reaction has not been completed yet (Fig. 4(a)). The reaction proceeded according to Eq. (2). Besides, the calcination step is necessary to remove CO2, when CaCO3 is used as a raw material. During heating, calcium carbonate decomposes giving solid calcium oxide, CaO and carbon dioxide, CO2 (gas) according to Eq. (3). The formed CaO reacts with ZrO2, creating CaZrO 3 and with Fe2O3, forming CaFe2O4. ZrO2 þ CaCO3 þ Fe2O3-CaZrO3 þ CaFe2O4 þ ZrO2 þ CO2 (2) CaCO3-CaO þ CO2↑
Fig. 9. Variation of dissipation factor with frequency at a room temperature.
4. Discussion The results presented in the paper were focused on optimization of synthesis conditions and examined an influence of combination solid and liquid phase on composite properties. Special attention was paid to the role of iron in the obtained composite, which in greatly way influence on final properties of the composite. A combination of solid and liquid phase during synthesis may provide important knowledge to control final properties. Analysis of the literature of the past 30 years clearly indicates that the synthesis of electroceramics using powders is the critical step. Obtaining dense materials with homogeneous grain size is an actual challenge. Final properties of components, besides individual features of compounds, depend on microstructure. Thus, a big effort has been made to synthesis conditions and microstructure of ceramics dielectric materials. From this point of view the control of chemical parameters and the physical properties play significance role in obtaining materials with required properties. The synthesis presented in the paper proceeded in the solidstate in the presence of liquid ferrite phase allowed to obtain
(3)
At temperature 1400 and higher (Fig. 4(c) and (d)) the only crystalline phases are CaZrO3 and CaFe2O4. Below this temperature the synthesis reaction has not been completed. In Fig. 4(b) unreacted compounds such as ZrO2 and Fe3O4 are presented in the sample. Thus, the heat treatment temperature for the preparation of 0.8CaZrO3–0.2CaFe2O4 composite is chosen to be 1400 1C. The XRD analysis confirms the SEM investigation. According to the phase composition and microstructure observation, CaZrO3 and CaFe2O4 was identified as two main phases. Basing on the obtained results, the sample sintered at 1400 1C were subjected to further testing. The calculated crystal structure parameters (the Rietveld method – Fig. 5; Table 1) for CaZrO3 phase are lower in comparison to [27]. Nevertheless, in both cases the samples were prepared in a similar way. An effective ionic radius of Fe3 þ (0.55 Å) is considerably lower than Zr4 þ (0.72 Å) [28]. Thus if part of Zr is substituted by Fe, it should decrease the crystal structure parameters. Therefore, we can assume that a part of Zr ions is replaced by Fe in CaZrO3. The opposite effect should be and is observed in the case of CaFe2O4. The Mössbauer spectroscopy (Fig. 6) was conducted to check and confirm the position of iron ions in the studied composite The magnetically split subspectra (No 2 and 3, Table 2) could be assigned to iron in CaFe2O4, which was previously reported [29]. The obtained isomer shifts and hyperfine magnetic fields are lower in comparison to [29], due to different temperatures of measurements. The hyperfine interaction parameters confirmed the high-spin Fe3 þ state in octahedral coordination. Besides the magnetically split components, there is also a paramagnetic doublet (No 1, Table 2) on the measured spectra. The isomer shift value is too low for Fe3 þ and agreed to Fe4 þ , thus this doublet could be only assigned to iron ions substituting Zr4 þ ,
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which are present in CaZrO3. Taking into account the area of the doublet (Table 2), about 20% of total iron ions replaced zircon in the CaZrO3 structure. This leads to the lack of iron in CaFe2O4. Thus, zircon ions incorporate into the ferrite structure. The diffusion and incorporation process are thermally activated. The microstructure (Fig. 7(a)) indicates that composite was obtained by sintering with the liquid phase which wetted the grains of CaZrO3. A new phase crystallized from the liquid. Hence it follows an oval shape of calcium zirconate. Moreover, the presence of the liquid facilitated sintering and compaction of the material. Consequently, the composite is characterized by low porosity. Thus the SEM/EDS analysis confirmed the XRD and Mössbauer measurements. Methods of preparing in the greatest degree influence on the microstructure and consequently on the electrical properties. This is especially important in the case of diphase systems without any reactions at the interfaces [30,31]. The Rietveld method, Mössbauer spectroscopy and SEM/ EDS analysis indicated that the substitution of Fe3 þ /Zr4 þ in CaFe2O4 causes an imbalance of a cell charge. Parameters of both the obtained crystalline phases are slightly different, in compare to the reference data This effect causes the appearance of the larger electric field gradient acting on the Fe3 þ ions. According to the structural analysis results, we can determine an approximated formula of the composite to be 0.8Ca(Zr0.9Fe0.1)O3–0.2Ca(Fe0.8Zr0.2)2O4. On the other hand incorporation of Fe3 þ cations, which replaced bigger Zr4 þ cations in the perovskite CaZrO3 structure and vice versa in the CaFe2O4 structure was observed. Therefore, centrosymmetric space group of CaZrO3 was distorted. It was established that about 1.5 at% of Fe is observed in CaZrO3 grains. The similar amount, such as Fe (1.5 at%), of Zr is also present in the CaFe2O4 phase. These substitutions, especially in ferrite phase, unbalance the charge of the unit cell what must lead to small distortion of Zr or Fe octahedra in the both phases. Dielectric constant of pure CaZrO3 measured at a room temperature at 1 MHz is about 30 [32]. In compare to [32], the dielectric constant about 170 of 0.8Ca(Zr0.9Fe0.1)O3–0.2Ca (Fe0.8Zr0.2)2O4 measured at high frequencies reached the high value. At lower frequencies the sample reveals dielectric dispersion It is due to the effect of heterogeneity of the composite microstructure (Fig. 7). The dielectric behavior of the composites can be explained on the basis of a polarization mechanism in ferrites, which can be described as the conduction process because beyond the percolation limit of the ferrite phase in composites, the conduction is mainly due to ferrite phase [33,34]. 5. Conclusions In this study, the composite consisting of two main phases, i.e. CaZrO3 and CaFe2O4, was prepared by the conventional ceramic method. The XRD and SEM/EDS results suggest that a two-step heat treatment at 1200 1C and 1400 1C is needed to obtain the dense 0.8CaZrO3–0.2CaFe2O4 composite. During firing the ferrite liquid phase appeared, which facilitated the consolidation of the composite. The XRD results showed that the composite was crystalline in its nature and revealed the formation of orthorhombic CaZrO3 and also CaFe2O4 phase. The Mössbauer
spectroscopy confirmed the predominant oxidation state of iron cations is 3 þ in the ferrite but also a part of the iron substituted Zr4 þ in CaZrO3 crystals and contrary in case of CaFe2O4. The approximated formula of the obtained composite is 0.8Ca (Zr0.9Fe0.1)O3–0.2Ca(Fe0.8Zr0.2)2O4. The variation of the dielectric constant with frequency was measured. The sample reveals a characteristic dielectric dispersion. The dielectric constant reached a constant value of about 170 at higher frequencies.
Acknowledgments The work was partially supported by the grant no. INNOTECH K2/IN2/16/181920/NCBR/13
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 8688–8695 www.elsevier.com/locate/ceramint
Structural properties of 0.8CaZrO3–0.2CaFe2O4 composite Edyta Śnieżeka,n, Paweł Stocha, Jacek Szczerbaa, Dominika Madeja, Ryszard Proroka, Agata Stochb a
AGH University of Science and Technology, Faculty of Materials Science and Ceramics, al. A. Mickiewicza 30, 30-059 Krakow, Poland b Institute of Electron Technology Krakow Division, Zablocie 39, 30-701 Krakow, Poland Received 28 January 2015; accepted 14 March 2015 Available online 23 March 2015
Abstract A composite with the 0.8CaZrO3–0.2CaFe2O4 general formula was prepared by means of a double sintering process. Temperature of 1400 1C was found suitable to obtain the composite based on scanning electron microscopy and X-ray diffraction analysis. Phase composition, crystal structure, Mössbauer effect, microstructure and dielectric properties were studied. The influence of Fe ions on the CaZrO3 structure has been reported. It was found that Fe substituted Zr in CaZrO3 and vice versa in CaFe2O4 in the amount about 1.5 at% in both cases. The approximated formula of the obtained composite was 0.8Ca(Zr0.9Fe0.1)O3–0.2Ca(Fe0.8Zr0.2)2O4. The SEM observations revealed that liquid CaFe2O4 occurred as a filling phase between CaZrO3 grains which significantly enhanced densification of the material and influence on its properties. The dielectric constant reached the value of 170 at 1 MHz and the accompanying dissipation factor was very low – 0.005. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: B. Composites; C. Dielectric properties; D. Ferrites; D. Perovskites; Mössbauer spectroscopy
1. Introduction Calcium zirconate belongs to a large group of compounds, called perovskites. In general, their formula can be written as ABO3. CaZrO3 prefers the orthorhombic Pcmn space group where Ca2 þ ions are situated between slightly deformed ZrO6 octahedrals. In the CaZrO3 structure, a topological distortion of ZrO6 octahedrals can be observed [1,2]. The reaction of CaZrO3 synthesis proceeds in the solid state. The dominating mass transfer mechanism consist in Ca2 þ ions diffusion through a newly created compound [3]. Moreover, kinetics around the temperature of 1170 1C is controlled by diffusion and nucleation, which is associated with the ZrO2 polymorphic transformation [4]. Calcium zirconate synthesis from ZrO2 and CaO or a natural raw material, such as dolomite (CaMg (CO3)2) starts at 900 1C and is completed at about 1500 1C. The intensity of the synthesis is noticeable within the temperature range from about 1100–1400 1C and is assisted by volume expansion. The linear expansion can reach even 11% at 1500 1C [5,6].
n
Corresponding author. Tel.: þ48 126175139; fax: þ 48 126334630. E-mail address: [email protected] (E. Śnieżek).
http://dx.doi.org/10.1016/j.ceramint.2015.03.085 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
CaZrO3 has appeared interesting in many engineering fields because of its unique properties, such as high melting point (2345 1C), dielectric constant ( 30 at 1 GHz), high thermal and chemical stability and refractory properties [7–11]. Stoichiometric, undoped calcium zirconate is a p-type semiconducting material in air at low temperatures and an oxygen ionic conductor at high temperatures. Besides, nonstoichiometric CaZrO3 and doped with some oxides, such as Al2O3, Y2O3, MgO is an oxygen-ion conductor [12–16]. CaZrO3 incorporates actinides relatively easily into its crystal structure and because of its high chemical durability is considered to be a host material for spent nuclear fuel immobilization [17]. In the last years, materials based on various ferrites have been interesting due to their versatile application, such as transmitting microwaves, telecommunication and computer devices. CaFe2O4 is a ferrite representative of a spinel-type group with the AB2O4 general formula. Calcium ferrite exhibits the orthorhombic crystal structure with the Pbnm space group. Two distinct iron sites are composed by the corner and the edge-shared FeO6 octahedral. It is built of eight-fold-coordinated Ca atoms and a distorted FeO6 octahedral. The network of CaFe2O4 is similar to the one formed in perovskite compounds [18]. This characteristic structure may have interesting physical properties, both magnetic and electric.
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Ferrite type compounds, due to their magnetic properties, are widely used for electrical and radio devices. Magnetic properties of superstoichiometric CaFe2O4, which was prepared during thermobaric synthesis, were reported in [19] and, based on the results obtained, the antiferromagnetic character of magnetic interactions was suggested. The temperature of magnetic ordering for CaFe2O4, which was obtained by different researchers based on magnetic measurements, neutron diffraction study, and γ resonance spectroscopy, significantly differ (from 180 to 200 K) [19,20]. CaFe2O4 shows electrical polarization Tc=160 K. Moreover, a substitution of Fe by Ti suppresses antiferromagnetic ordering [21]. It is also known that CaFe2O4 is a p-type semiconductor and it is stable up to 1223 1C – melting point [22]. It is worth mentioning, that in the research system CaO–Fe2O3– ZrO2, Fe2O3 and ZrO2 create solid solution in the whole range up to 1434 1C [23]. Moreover, CaO and Fe2O3 create two calcium ferrites, CaFe2O4 and Ca2Fe2O5 (brownmillerite type) [24]. A combination of CaZrO3 and CaFe2O4 properties may lead to obtaining a material with unique magnetic and electric properties. Furthermore, the melting points of both compounds are significantly different, consequently a synthesis will occur in the presence of the ferrite liquid phase. Therefore, it is interesting to study the influence of ferrite liquid phase on synthesis process, microstructure, structure of CaZrO3 and dielectric properties of the obtained composite. 2. Material and methods 2.1. Preparation of 0.8CaZrO3–0.2CaFe2O4 composite In this paper 0.8CaZrO3–0.2CaFe2O4 composite, based on calcium zirconate with the perovskite structure and calcium ferrite with the spinel structure, was synthesized using the twostep heat treatment at 1200 1C and 1300 1C, 1400 1C, 1500 1C. Starting chemicals used in this study comprised calcium carbonate CaCO3 (Chempur; 98.81% purity), zirconium dioxide ZrO2 (Merck; 98.08% purity) and ferric oxide Fe2O3 (POCH; 96% purity). A composition containing 5, 4 and 1 mol% of CaO, ZrO2 and Fe2O3, respectively, was prepared. The CaCO3, ZrO2 and Fe2O3 reagents in appropriate amount, were mixed together during 2 h in the vibratory zirconium ball mill. The pellets (20 mm diameter, 10 mm thickness) were obtained from homogenized mixtures pressed uniaxially in the hardened steel die at a pressure of 30 MPa. The green pellets were initially calcined at 1200 1C in air for 10 h. After cooling to the room temperature, the pellets were ground to the grain size lower than 0.063 mm.The calcined powder was then used to prepare the final pellets (the same way as above), which were fired at 1300 1C, 1400 1C, 1500 1C under air atmosphere with 10 h soaking time at the maximum temperature. The specimens were cooled together with the furnace. 2.2. Characterization techniques The microstructure of samples was discussed according to SEM observations (FEI NovaNanoSem 200), accompanied by the EDS chemical analysis in micro areas.
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The phase composition of samples was determined by the powder X-ray diffraction (XRD) technique at a room temperature, using Cu-Kα radiation in the 2Θ angel range 10–901 with a step of 0.021 and average time of 10 s/step (Philips Panalytical X'Pert-Pro MPD diffractometer). The obtained data were analyzed using the X'Pert Pro Highscore Plus software. Basing on X-ray diffraction and SEM/EDS results a sample fired at 1400 1C was chosen to further investigations. The crystal structure parameters were obtained using a full-profile Rietveld method [25], implemented in the FullProf software package [26]. Structural parameters, including scale factor, zero shift, background function, lattice parameters, atomic coordinates and fractions, isotropic temperature fraction, peak profile and texture parameters, were taken into account in the course of refinement. The 57Fe Mössbauer effect measurements were performed using the standard technique at the liquid nitrogen temperature ( 196 1C) in a transmission mode, using a conventional constantacceleration spectrometer and a 25 m Ci 57Co source in Rh matrix. The velocity scale was calibrated by means of α-Fe foil. The low temperature spectrum was collected with the sample immersed in liquid N2 using a bath cryostat. Spectra were fitted to Lorentzian lines using the non-linear least square method. The hyperfine interaction parameters, such as isomer shifts (IS), relative to metallic iron at a room temperature, quadrupole splitting Δ and hyperfine magnetic field m0Hhf, were obtained. Dielectric measurements were carried out with an LCR Quad Techmeter. The specimen was prepared from a sintered pellet by polishing the face flat. Silver electrodes were deposited on both sides of the ceramic pellets and fired at 800 1C to ensure good electrical contacts. The variations of dielectric constant and loss tangent at a room temperature were studied by recording these parameters at different frequencies. The real part of dielectric constant was calculated according to the following equation. ε0 ¼ Cd=ε0 A
ð1Þ
where C ¼ capacitance, d ¼ thickness of the pellet, A ¼ area of the surface of the pellet and ε0 ¼ permittivity of the free space (ε0 ¼ 8.854 10 12 F/m). 3. Results 3.1. SEM/EDS observation of 0.8CaZrO3–0.2CaFe2O4 composite formation In this study the two-step heating process at 1200 1C, and then, after grinding and pressing, at 1300 1C, 1400 1C and 1500 1C, tending to obtain the 0.8CaZrO3–0.2CaFe2O4 composite, was performed. The SEM was used to investigate the morphology of the composite prepared in three different temperatures. The SEM images of microstructure evolution and creation of 0.8CaZrO3– 0.2CaFe2O4 composite are shown in Figs. 1–3. In all cases the ferrite phase (dark gray – point 2) fills the space between calcium zirconate grains (point 1 – light gray). The average diameter of the calcium zirconate grains changes from about 1–5 mm (Fig. 1)
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Fig. 1. SEM micrograph of the sample sintered at 1300 1C.
Fig. 3. SEM micrograph of the sample sintered at 1500 1C.
3.2. Phase composition The prepared materials were defined in terms of phase composition by the XRD method. Fig. 4(a) shows the XRD patterns of the samples heat treated at 1200 1C. It is clear that the samples calcined at 1200 1C for 10 h have CaZrO3 and CaFe2O4 phases as main compounds. There are some impurity components, such as Fe3O4 and ZrO2, in the sample. ZrO2 is still present in a sample heated at 1300 1C, Fig. 4(b). It is clear from Fig. 4(c) and (d) when the heat treatment temperature is raised up to 1400 1C and 1500 1C, the crystal phase is completely transformed into the CaZrO3 and CaFe2O4 phases without any impurity component. Based on the SEM and XRD analysis the sample sintered at 1400 1C was selected for further research. 3.3. Crystal structure parameters
Fig. 2. SEM micrograph of the sample sintered at 1400 1C.
through 3–8 mm (Fig. 2) to 3–10 mm (Fig. 3). There are some obvious pores distributed in the samples. The SEM results presented in Fig. 1 demonstrate transition microstructure, where the synthesis has been not complete. There are areas (point 3), where the phase creation is still in progress. The microstructure presented in Fig. 1 and the shape of calcium zirconate grains indicates that the diffusion process proceed intensively at 1300 1C. Figs. 2 and 3 show the SEM images of microstructure of the samples sintered at 1400 and 1500 1C, respectively. A homogeneous microstructure is made up by two well-distributed phases: CaZrO3 and CaFe2O4. Using the energy dispersive spectroscopy analysis (EDS), calcium zirconate (light gray grains – point 1) and calcium ferrite (dark gray phase – point 2) were identified. This microstructure showed calcium zirconate grains surrounded by the ferrite groundmass.
The sample was analyzed using the Rietveld method. The crystal structure parameters of CaZrO3 and CaFe2O4 according to JCPDS: 76-2401 and 72-1199, respectively were taken as starting parameters to the fitting procedure. The measured diffraction pattern and theoretical fit are presented in Fig. 5. CaZrO3 and CaFe2O4 crystallize in the orthorhombic crystal structures (space group no 62). The obtained lattice parameters are summarized and compared with the literature data in Table 1 [27]. 3.4. Mössbauer spectroscopy The Mössbauer spectra of the sample at a liquid nitrogen temperature are presented in Fig. 6. As magnetic ordering in CaFe2O4 occurs approximately at a temperature below 73 1C, a magnetically split spectrum is observed. This spectrum was fitted by two magnetic sextets and a quadrupole doublet which are corresponding to different crystallographic sites, occupied by iron. The obtained hyperfine interaction parameters are presented in Table 2.
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Fig. 4. X-ray diffraction patterns of the samples prepared from solid state reaction of CaCO3, ZrO2 and Fe2O3 at (a) 1200 1C, (b) 1300 1C, (c) 1400 1C and (d) 1500 1C.
Table 1 Crystal structure parameters of the CaZrO3 and CaFe2O4 phases.
a [Å] b [Å] c [Å] V [Å3]
CaZrO3
CaZrO3 [27] CaFe2O4
CaFe2O4 (JCPDS: 72-1199)
5.5851(2) 8.0098(4) 5.7532(2) 257.372
5.5890(1) 8.0140(3) 5.7586(2) 257.929
9.2300 10.7050 3.024 298.79
9.2550(13) 10.7496(16) 3.0251(4) 300.969
Fig. 5. The Rietveld analysis of the 0.8CaZrO3–0.2CaFe2O4 composite prepared at 1400 1C. At the bottom of the figure are presented the differences between the observed and the calculated patterns.
3.4.1. Microstructure analysis The microstructure of the composite sintered at 1400 1C is presented in Fig. 7(a). An average chemical composition of the grains and filling phase according to EDS (Fig. 7(b) and (c)) analysis is presented in Table 3. It can be established that about 1.5 at% of Fe is observed in CaZrO3 grains, which together with Zr gives about 19.6 at%, which is very close to stoichiometric 20
Fig. 6. 57Fe Mössbauer effect spectra of the studied compound at liquid nitrogen temperature.
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at%. The similar amount, such as Fe (1.5 at%), of Zr is also present in the CaFe2O4 phase. Thus the SEM/EDS analysis confirmed the XRD and Mössbauer measurements. 3.4.2. Dielectric measurement The variation of dielectric constant with frequency is presented in Fig. 8. It can be observed that the sample reveals dielectric dispersion. The dielectric constant decreases rapidly, Table 2 The hyperfine interaction parameters (A–area of the subspectra, IS–isomer shift, μ0Hhf–magnetic hyperfine filed, QS–quadrupole split, Γ/2–half width ) for the sample prepered at 1400 1C.
from about 1050–170, with the increasing frequency, and then it reaches a constant value at higher frequencies, indicating an usual dielectric dispersion. Fig. 9 presents variations of the dissipation factor with frequency. The curve corresponds to the curve in Fig. 8. It is observed rapid decrease in the dissipation factor value with frequency increase. At higher frequencies it reaches a low value of about 0.005. Table 3 An average chemical composition (EDS) of the grains and filling phase according to Fig. 7(a). at%
No
A [%]
IS [mm/s]
m0Hhf [T]
QS [mm/s]
Γ/2 [mm/s]
1 2 3
21.0 37.9 41.1
0.156(8) 0.426(8) 0.408
– 43.50(18) 45.03(10)
0.312(30) 0.046(9) 0.004(7)
0.307(17) 0.273(18) 0.236(17)
The filling phase The oval grains
O
Zr
Ca
Fe
60.1 60.1
1.3 18.1
15.1 20.3
23.5 1.5
Fig. 7. (a) SEM micrograph and (b, c) X-ray emission spectra in the point 1 and 2, respectively of the prepared sample at 1400 1C.
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Fig. 8. Variation of dielectric constant (ε') with frequency at a room temperature.
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specific microstructure. Melting point of CaFe2O4 (1223 1C) is much lower than of CaZrO3 (2345 1C). Consequently, according to Figs. 1–3, when heat treatment temperature is increasing densification of the material and creation of the characteristic microstructure is more intense. The synthesis of calcium zirconate was completed at 1400 1C which corresponds to [5,6]. The composite consisted of calcium zirconate where calcium ferrite filled space between CaZrO3 grains. Microstructure in Fig. 3 with cracks indicates 1500 1C as an excessive temperature. The present of cracks can be the result of grain growth and thermal expansion. Figs. 1–3 demonstrate the proposed synthesis method as an appropriate technique for manufacturing the dense 0.8CaZrO3–0.2CaFe2O4 composite. After the first heating step at 1200 1C (calcination process), the XRD analysis confirmed that the reaction has not been completed yet (Fig. 4(a)). The reaction proceeded according to Eq. (2). Besides, the calcination step is necessary to remove CO2, when CaCO3 is used as a raw material. During heating, calcium carbonate decomposes giving solid calcium oxide, CaO and carbon dioxide, CO2 (gas) according to Eq. (3). The formed CaO reacts with ZrO2, creating CaZrO 3 and with Fe2O3, forming CaFe2O4. ZrO2 þ CaCO3 þ Fe2O3-CaZrO3 þ CaFe2O4 þ ZrO2 þ CO2 (2) CaCO3-CaO þ CO2↑
Fig. 9. Variation of dissipation factor with frequency at a room temperature.
4. Discussion The results presented in the paper were focused on optimization of synthesis conditions and examined an influence of combination solid and liquid phase on composite properties. Special attention was paid to the role of iron in the obtained composite, which in greatly way influence on final properties of the composite. A combination of solid and liquid phase during synthesis may provide important knowledge to control final properties. Analysis of the literature of the past 30 years clearly indicates that the synthesis of electroceramics using powders is the critical step. Obtaining dense materials with homogeneous grain size is an actual challenge. Final properties of components, besides individual features of compounds, depend on microstructure. Thus, a big effort has been made to synthesis conditions and microstructure of ceramics dielectric materials. From this point of view the control of chemical parameters and the physical properties play significance role in obtaining materials with required properties. The synthesis presented in the paper proceeded in the solidstate in the presence of liquid ferrite phase allowed to obtain
(3)
At temperature 1400 and higher (Fig. 4(c) and (d)) the only crystalline phases are CaZrO3 and CaFe2O4. Below this temperature the synthesis reaction has not been completed. In Fig. 4(b) unreacted compounds such as ZrO2 and Fe3O4 are presented in the sample. Thus, the heat treatment temperature for the preparation of 0.8CaZrO3–0.2CaFe2O4 composite is chosen to be 1400 1C. The XRD analysis confirms the SEM investigation. According to the phase composition and microstructure observation, CaZrO3 and CaFe2O4 was identified as two main phases. Basing on the obtained results, the sample sintered at 1400 1C were subjected to further testing. The calculated crystal structure parameters (the Rietveld method – Fig. 5; Table 1) for CaZrO3 phase are lower in comparison to [27]. Nevertheless, in both cases the samples were prepared in a similar way. An effective ionic radius of Fe3 þ (0.55 Å) is considerably lower than Zr4 þ (0.72 Å) [28]. Thus if part of Zr is substituted by Fe, it should decrease the crystal structure parameters. Therefore, we can assume that a part of Zr ions is replaced by Fe in CaZrO3. The opposite effect should be and is observed in the case of CaFe2O4. The Mössbauer spectroscopy (Fig. 6) was conducted to check and confirm the position of iron ions in the studied composite The magnetically split subspectra (No 2 and 3, Table 2) could be assigned to iron in CaFe2O4, which was previously reported [29]. The obtained isomer shifts and hyperfine magnetic fields are lower in comparison to [29], due to different temperatures of measurements. The hyperfine interaction parameters confirmed the high-spin Fe3 þ state in octahedral coordination. Besides the magnetically split components, there is also a paramagnetic doublet (No 1, Table 2) on the measured spectra. The isomer shift value is too low for Fe3 þ and agreed to Fe4 þ , thus this doublet could be only assigned to iron ions substituting Zr4 þ ,
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which are present in CaZrO3. Taking into account the area of the doublet (Table 2), about 20% of total iron ions replaced zircon in the CaZrO3 structure. This leads to the lack of iron in CaFe2O4. Thus, zircon ions incorporate into the ferrite structure. The diffusion and incorporation process are thermally activated. The microstructure (Fig. 7(a)) indicates that composite was obtained by sintering with the liquid phase which wetted the grains of CaZrO3. A new phase crystallized from the liquid. Hence it follows an oval shape of calcium zirconate. Moreover, the presence of the liquid facilitated sintering and compaction of the material. Consequently, the composite is characterized by low porosity. Thus the SEM/EDS analysis confirmed the XRD and Mössbauer measurements. Methods of preparing in the greatest degree influence on the microstructure and consequently on the electrical properties. This is especially important in the case of diphase systems without any reactions at the interfaces [30,31]. The Rietveld method, Mössbauer spectroscopy and SEM/ EDS analysis indicated that the substitution of Fe3 þ /Zr4 þ in CaFe2O4 causes an imbalance of a cell charge. Parameters of both the obtained crystalline phases are slightly different, in compare to the reference data This effect causes the appearance of the larger electric field gradient acting on the Fe3 þ ions. According to the structural analysis results, we can determine an approximated formula of the composite to be 0.8Ca(Zr0.9Fe0.1)O3–0.2Ca(Fe0.8Zr0.2)2O4. On the other hand incorporation of Fe3 þ cations, which replaced bigger Zr4 þ cations in the perovskite CaZrO3 structure and vice versa in the CaFe2O4 structure was observed. Therefore, centrosymmetric space group of CaZrO3 was distorted. It was established that about 1.5 at% of Fe is observed in CaZrO3 grains. The similar amount, such as Fe (1.5 at%), of Zr is also present in the CaFe2O4 phase. These substitutions, especially in ferrite phase, unbalance the charge of the unit cell what must lead to small distortion of Zr or Fe octahedra in the both phases. Dielectric constant of pure CaZrO3 measured at a room temperature at 1 MHz is about 30 [32]. In compare to [32], the dielectric constant about 170 of 0.8Ca(Zr0.9Fe0.1)O3–0.2Ca (Fe0.8Zr0.2)2O4 measured at high frequencies reached the high value. At lower frequencies the sample reveals dielectric dispersion It is due to the effect of heterogeneity of the composite microstructure (Fig. 7). The dielectric behavior of the composites can be explained on the basis of a polarization mechanism in ferrites, which can be described as the conduction process because beyond the percolation limit of the ferrite phase in composites, the conduction is mainly due to ferrite phase [33,34]. 5. Conclusions In this study, the composite consisting of two main phases, i.e. CaZrO3 and CaFe2O4, was prepared by the conventional ceramic method. The XRD and SEM/EDS results suggest that a two-step heat treatment at 1200 1C and 1400 1C is needed to obtain the dense 0.8CaZrO3–0.2CaFe2O4 composite. During firing the ferrite liquid phase appeared, which facilitated the consolidation of the composite. The XRD results showed that the composite was crystalline in its nature and revealed the formation of orthorhombic CaZrO3 and also CaFe2O4 phase. The Mössbauer
spectroscopy confirmed the predominant oxidation state of iron cations is 3 þ in the ferrite but also a part of the iron substituted Zr4 þ in CaZrO3 crystals and contrary in case of CaFe2O4. The approximated formula of the obtained composite is 0.8Ca (Zr0.9Fe0.1)O3–0.2Ca(Fe0.8Zr0.2)2O4. The variation of the dielectric constant with frequency was measured. The sample reveals a characteristic dielectric dispersion. The dielectric constant reached a constant value of about 170 at higher frequencies.
Acknowledgments The work was partially supported by the grant no. INNOTECH K2/IN2/16/181920/NCBR/13
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