CARACTERIZAO E ESTUDO DAS

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HIGH DIELECTRIC PERMITTIVITY AND LOW LOSS OF SrBi4Ti4O15 WITH PbO AND V2O5 ADDITIONS FOR RF AND MICROWAVE APPLICATIONS

C.A.RodriguesJunior 1,2, J.M.S.Filho1,2, P.M.O.Silva1,2C. C. M. Junqueira3 and A. S. B. Sombra2cauby@fisica.ufc.br55-85-870913131 Departamento de Engenharia de Teleinformtica, Centro de Tecnologia, UFC , CEP: 60455-760, Fortaleza, Cear , Brasil

2Laboratrio de Telecomunicaes e Cincia e Engenharia de Materiais (LOCEM), Universidade Federal do Cear CEP: 60455-760, Fortaleza, Cear Brasil.

3Instituto de Aeronutica e Espao IAE, Praa Marechal Eduardo Gomes, 50, Vila das Accias, CEP 12228-904 - So Jos dos Campos, So Paulo, Brasil.ABSTRACT

In this paper SrBi4Ti4O15 (SBTi), a perovskite-type ceramics, with ction deficit A5B4O15, was prepared by solid state reaction method and PbO and V2O5 were added into SBTi (2, 5, 10 and 15 weight %). Samples were characterized through X-Ray Diffraction (XRD), Raman Spectroscopy and Scanning Electron Microscopy (SEM). Impedance Spectroscopy was carried out at room temperature. The analysis by X-ray diffraction (XRD) using the Rietveld renement has conrmed the formation of single-phase compound with a crystalline orthorhombic system (a = 5.4400 , b = 5.4326 and c = 41.2169 ). Raman spectra of SBTi over the frequency range of 01200 cm-1 have been investigated, at room temperature, under the excitation of 514.5nm in order to evaluate the effect of additives on the structure of the ceramic matrix. A Scanning Electron Microscopy micrograph (SEM) shows globular grains (with addition of PbO) and crystal-shape ones (with additions of V2O5), from about 1 to 2 m. The dielectric properties: dielectric permittivity (K) and dielectric loss tangent (tg() were measured at room temperature over a range of 100 Hz 40 MHz by complex impedance spectroscopy (CIS) and microwave (MW) frequency bands were studied. The study showed that these properties are strongly dependent on frequency and on the added level of the impurity. All the samples were analyzed taking into account to possible applications in radio frequency (RF) and microwave devices.Keyword: perovskite characterization of SBTi, microwave, frequency radio.

1 INTRODUCTIONSrBi4Ti4O15 (SBTi) is one of the Aurivillius family members [1], which is called bismuth layer-structured ferroelectrics (BLSF), which have a natural superlattice structure along c axis consisting of two kinds of two-dimensional nanolayers, defined as bismuth oxide (Bi2O22+) sheet and a pseudoperovskite block generally described as (Am-1BmO3m+1)2- , where m is a number of BO6 octahedra in a pseudo-perovskite block [2,3]. SBT has been extremely studied for many researchers for possible applications in piezoelectric devices and has potential use as a RF device because of the relative high Curie temperature (Tc = 520oC to 620oC), high dielectric breakdown strength, low dielectric loss, and high anisotropy [2,3].SBTi has a crystalline structure similar to Bi4Ti3O12 [BIT] which is another common ferroelectric structure [4].This crystalline structure can be considered a junction of layers of (Bi2O2)2+ and a perovskite (Am-1BmO3m-1), where A can be a mono, bi or trivalent element allowing dodecahedral coordination, B is a transition element adequate to octahedral coordinate and m is an integer that represents the number of perovskite[5]. The expectative in such materials is that they can become potential candidates for replacing the main ferroelectric material, lead zirconate titanate (PZT), where the good ferroelectric properties are counterbalanced by an excessive toxicity due to its high binding capacity[6].This work aims to studying electrical and dielectric properties of a series of SrBi4Ti4O15 (SBTi) material added with V2O5 and PbO with different percentages (2%, 5%, 10% e 15% weight). The Polyvinyl Alcohol (PVA), an organic binder was used in order to minimize the possibility of forming defects found in the pores, creating strain on the sample, enough for degradation of its dielectric permittivity [7].Several experimental techniques were employed looking for the ceramic characterization and its study of electrical and dielectric properties of produced samples. The X Ray Diffractmeter (XRD), scanning electron microscopy (SEM) and Raman Spectroscopy was used to have a complete description of the structural characteristics. A fundamental study on Impedance Spectroscopy was also carried out. The dielectric properties of the samples were investigated at room temperature: dielectric permittivity (K) and dielectric loss (tg() in view of possible applications for electrical and electronic circuitry at radio frequency (RF) and microwave frequency (MW). 2EXPERIMENTAL PROCEDURE2.1Synthesis of SrBi4Ti4O15 perovskite

The SrBi4Ti4O15 perovskite was prepared and synthesized by the solid state reaction method of thoroughly ground mixtures of Bismuth (Bi2O3) oxide [ALDRICH 99, 9%], Titanium oxide (TiO2) [VETEC 99, 9%] and Strontium Carbonate (SrCO3) [ALDRICH 99,9%] in the required stoichiometry ratio. The milling operation was carried out by a high energy planetary ball mill (planetary ball mill Fritsch Pulverisette 6). Steel crucibles and balls were used in ratio of 12 balls for each 5 grams of sample in isopropyl alcohol; mill speed was programmed to rotate 370 rpm during 5 hours at room temperature. Milled powders were calcined at 850C in a furnace (EDG 3000) for 3 hours, starting from room temperature and heating/cooling rate of 5C/min, were applied in order to find the formation of SrBi4Ti4O15. After the calcination process, PbO (2, 5, 10 and 15% wt.) and V2O5 (2, 5, 10 and 15% wt.) were added. The fine and homogeneous calcined powders were used to make cylindrical pellets of diameter ~12 mm and thickness 1.0 mm - 1.5 mm, using 3% of mass of Polyvinyl Alcohol (PVA) for each pellet as a binder. Pellets were sintered at 950C for 3 hours. Table 1 shows the summary identification of the sintered samples. The sintered pellets were subjected to X-ray diffraction, Raman spectroscopy measurements, Scanning Electron Microscopy (SEM), Impedance Spectroscopy and dielectric measurements at microwave frequency. The followings subsections will discuss these five types of characterizations and the set up tests used to achieve the experimental results. 2.2X-ray diffractionThe crystalline phase of sintered ceramics were identified by XRD using CuK (= 0.15406nm) radiation with DMAXB/Rigaku Diffract meter. The principle used to this equipment is Braggs diffraction law, and can be very useful in viewing single cells for instance. (1) According to Bragg law, diffraction will occur when the waves scattered from a set of equally spaced and periodic in the direction perpendicular to the planes and the repeat distance in this direction is equal to the interplanar distance dhkl or about the same wavelength value.The Braggs law states that diffracting will occur when the waves emitted from the x-ray interact with matter that has a repeated distance that is approximately the same as the wavelength. In this work, the diffract meter operated in a voltage of 40 kV and a current of 25 mA using Bragg Brentano geometry at a scanning rate of 0.5/min in a wide range of Bragg angle (202). The cell and structure parameters were refined by using Rietveld method for quantitatively identifying the phase successfully[8]. The refinement process is done according with the Goodness of fit (SGOF) quality factor (1.18 SGoFand continues until convergence is reached. In Table 2 can be seen the determination phase composition of the SrBi4Ti4O15 (SBTi) obtained from X-ray Rietveld Analysis.2.3Raman Spectroscopy[no precisa explicar a tcnica] This technique is based on inelastic scattering of monochromatic light, usually from a laser source. In the inelastic scattering the frequency of photons in monochromatic light changes upon interaction with a sample, and photons of the laser light are absorbed by the sample and then reemitted. Frequency of the photons is shifted up or down in comparison with original monochromatic frequency, with is called the Raman effect. This shift provides information about vibrational, rotational and other low frequency transitions in molecules.

In this work, Raman spectroscopy measurements were performed by means of a Jobin-Yvon T64000 Triple Spectrometer set up in a backscattering geometry. The 514.5 nm line of a new Coherent laser operating at 200 mW was used to excite the signal, which was collected in a N2-cooled CCD system. Low-temperature measurements were performed by using a closed-cycle He cryostat where the temperature was controlled to within 0.1 K. High-temperature measurements were performed in a furnace controlled by a PID temperature controller with an accuracy of 1 K [10].2.4Scanning Electron Microscopy (SEM)

The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time. The SEM also produces images of high resolution, which means that closely spaced features can be examined at a high magnification. Samples were prepared relatively easy since it is common and usual for a sample to be conductive. Combination between higher magnitude Preparation of the samples is relatively easy since most SEMs only require the sample to be conductive. The combination of higher magnification, larger depth of focus, greater resolution, and ease of sample observation makesthe SEM one of the most heavily used instruments in research areas today.

In this work the sintered pellets were prepared for SEM measurements polishing the pellets with fine emery paper in order to make both the surfaces flat and parallel and painting its surfaces using high-purity silver paste (Joint Metal PC-200) .The SEM equipment used was the Vega XMU/Tescan, Bruker, operating the primary group of electrons varying the energy from 12 to 20 keV.

2.5Dielectric characterization

The measurement of the response of a material to an applied a static or dynamic electric field allows the determination of the electric susceptibility. In a weak fields, the response is linear, but in a strong fields it may become nonlinear. Dielectric properties depend on frequency, homogeneity, temperature, and, in the case of ferroelectrics, applied DC bias field. A very readable overview to relaxation theory is presented in Von Hippel [11].The dielectric measurements were obtained from a HP 4194A impedance analyzer in conjunction with a HP 4194 impedance analyzer, which jointly cover the region from 100 Hz to 40 MHz at room temperature.

The dielectric measurements were carried out on polished samples with diameter of 12 mm and 1 mm of thickness. Screen printed silver electrodes were applied on both sides of the sample to ensure a good electrical contact [12].2.6 Dielectric Measurements at Microwave FrequencyThe dielectric properties at microwave frequencies were studied using the HakkiColemans method, based on the resonant TE011 mode. The measurements were done using a HP 8719ET network analyzer [13,14]. From the resonant frequency of the TE011 mode, the dielectric permittivity K and dielectric loss tg(() are obtained. The values obtained in this experiment can be used as a guide in the numerical simulation of the antennas, with the assumption that the variation of permittivity with the frequency is not high between 2-4 GHz (mean resonant frequency of the HakkiColeman experiment).3. RESULTS AND DISCUSSION

3.1 X-ray diffraction

Figure.1 shows X-ray diffraction patterns of SBTi sample, standard ICSD - Inorganic Crystal Structure Database/ Capes, number 51863 [15], obtained at room temperature. Identification of all higher peaks of diffraction indicates the presence of isolated SrBi4Ti4O15 (SBTi) phase with orthorhombic symmetry (A21am), four-layer Autryvilles family structure and ferroelectric behavior.

Quantitative phase analysis was carried out by using Rietveld refinement method confirmed crystalline structure with lattice parameters as following a = 5.4400 , b = 5.4326 and c = 41.2169 ; = = = 90o; calculated density = 7.751 gr/cm3; unit cell volume = 1218.085 . There was no new phase from V2O5 and PbO added samples.Figure 2 shows X-ray diffraction patterns of pure SBTi sample and V2O3-added and PbO-added specimens (5 e 15 wt%).

3.2 Raman

Raman spectroscopy measurement is a very effective and sensitive tool for identifying the phase purity of a multicomponent oxide material. Raman spectrum of SBTi and PbO-added specimens (SBTi5P and SBTi15P) depicted in Figure 3, which show bands approximately at 57cm-1, 269 cm-1, 560 cm-1, and 867 cm-1. Several weak peaks that have already reported by Kojima et al [16]. The two V2O3-added samples showed a band intensity increases

The V2O3-added specimens (SBTi5V and SBTi15V), there was a band intensity increase 867 cm-1 as modes at 57 cm-1, 269 cm-1 and 560 cm-1 diminished their intensity. The observed mode at 57 cm-1 is recognized and reported by Bi+3shift on Bi2O2 layers. The mode at 269 cm1 corresponds to the torsion bending of TiO6 octahedral and the modes at 840870 cm1 are related to the stretching modes of octahedral TiO6 [17].3.3Scanning Electron Microscopy (SEM)

The Figure 4 depicts the Scanning Electron Micrographics (SEM) in SBTi5P,SBTi15P, SBTi5V, and SBTi15V.PbO-added sintered pellets present an expected high density and few pores related to V2O5-added specimens. PbO-added pellets present an heterogeneous size distribution of crystallites (~1 a 2 (m), as V2O5-added specimens present a well-defined and large pores spacing structures.

3.4RF AnalysisImpedance analysis was carried out by using impedance analyzer HP 4194A controlled by a personal computer (capacitance measurement) as a function of frequency (100 Hz to 40 MHz). Dielectric permittivity value K, was calculated from the capacitance measurement (C()), pellets thickness (t) and electrodes area A were obtained from electrical impedanceZ(), and it is a complex quantity whose real and imaginary parts thoroughly corresponding to real and imaginary parts of complex permittivity:

C() = C() jC() = (A/d)[K() jK()] (1), whereK= C ().(d/A)

(2)

K= C ().(d/A)

(3)

K() corresponding to an ordinary capacitance (stored energy), as imaginary component K( ) is associated to the dielectric loss (dissipated energy), and d are the transversal area and samples thickness, j = (-1)1/2, ( = 2(f angular frequency. Relative dielectric permittivity is given by:

K*= C*()/Co

(4)

, where (geometric capacitance) = (o(A/d), and (o is the electrical permittivity in vacuum and A and d are respectively the area and pellets thickness [18,19]. The dielectric permittivity value (K) and dielectric loss (tg() of SrBi4Ti4O15 over frequencies 1 MHz, 10 MHz, and 20 MHz are shown in Table 3. Relative dielectric permittivity K of PbO-added and V2O5-added specimens as a function of frequency is shown in Figure 5. Each PbO-added and V2O5-added samples present relative permittivity increase (K) along with concentration increase. The dielectric permittivity value in pure sample (SBTi) is 21.93 comparing to176.40 and 89.30 for SBTi10P and SBTi5V at 10 MHz frequency, shown in Table 3. The dielectric loss value(0.0155) is higher in pure sample (SBTi) comparing to 0.0060 in SBTi2P and to 0.0056 in SBTi2V samples at 1 MHz frequency,(see Table 3). In Figure 7 shows variation in dielectric permittivity and loss measured at different concentrations at 20 MHz frequency, room temperature. One can observe that PbO-added samples have higher dielectric permittivity and loss than V2O5-added ones, except for permittivity of SBTi2P, however, all V2O5-added samples have lower losses compared to1 MHz PbO-added samples.The high dielectric permittivity observed in the PbO-added samples can be due to a better densification in relation to V2O5-added samples as shown in Figure 4.3.5 Dielectric function and loss studies in the MW frequency range

The variation of the dielectric permittivity (K) and dielectric loss (tg(E) for SrBi4Ti4O15 of PbO-added and V2O5-added as function of frequency is shown in Table.4. Each PbO-added and V2O5-added in the frequency range of 2 GHz to 4 GHz is reported in Figure.8.PbO-added samples have shown a higher dielectric permittivity than V2O5-added samples, we highlight the 15% PbO addition sample with a dielectric permittivity of 119.02, is the highest obtained value. The 5% V2O5 addition sample had the highest dielectric permittivity of 89.06 among the other V2O5-added samples (see Table.4). V2O5-added samples dielectric loss have shown the lower values comparing to PbO-added samples, in the spotlight, SBTi5V and SBTi15V with dielectric loss 0.0238 and 0.0232, respectively.

Those results are probably related to good densification reached by SBTi PbO-added samples comparing to V2O5-added ones as shown in Figure.4.4. CONCLUSIONS

The SrBi4Ti4O15 (SBTi), perovskite-type ceramics, with ction deficit A5B4O15, was prepared by the solid state reaction method then PbO and V2O5 were added into SBTi with following percentages 2, 5, 10 and 15 wt%. We found four Raman active modes in our SBTi reference sample. For SBTi5P, SBTi15P, shift of the Raman peaks was not observed, indicating that the stoichiometry was not signicantly changed. On adding V2O5, several peaks disappear (57cm-1, 269cm-1, 560cm-1) indicating that with the addition of vanadium oxide, some modes are degenerated. PbO-added specimens enhanced dielectric and electric properties relating V2O5-added specimens, even though V2O5-added specimens also reached excellent results if one compare them to pure specimen. We can also highlight SBTi15P specimen has achieved high permittivity (257.16) at 10 kHz frequency and lower dielectric loss SBTi2V (0.0056), at 1 MHz frequency and room temperature. Comparing the dielectric permittivity values at Radio Frequency (RF) and Microwave (MW) PbO-added samples and V2O5-added samples we can notice that PbO-added samples present higher values of permittivity compared to V2O5-added samples. Results have shown a good possibility for the use of these materials for miniaturization of electronics components such as capacitors and resonators. ACKNOWLEDGMENTThis work was partly sponsored by CNPq and CAPES (Brazilian

agencies) and the U. S. Air Force Office of Scientific Research (AFOSR) (FA9550-11-1-0095)REFERENCES

[1] B. Aurivillius, Ark. Kemi., 1 (1949) 463. [2] Lrie H, Miyayama M, Appl Phys Lett 79 (2001) 251 [3] Noguchi Y, Miyayama M, Kudo T, Appl Phys Lett 77 (2000) 3639

[4] Hirose M, Suzuki T, Oka H, Itakura K, Miyauchi Y, Tsukada T, Jpn J Appl Phys 38 (1999) 55615563 [5] Xu Y "Ferroelectric materials and their application ", North-Holland (1991) [6] Haertling, J Am Ceram Soc 1999, 82, 797 [7] A. J. Moulson and J M Herbert (Eds ) Electroceramics: Materials, Properties, Applications 2nd Edition John Wiley & Sons, Ltd (2003) [8] H.M.Rietveld Line profiles of neutron powder-diffraction peaks for structure refinement ActaCryst 22 (1967) 151-152 [9] R. A. Young, A. Sakthivel, T. S. Moss, C. O. Paiva-Santos DBWS-9411 - an upgrade of the DBW* programs for Rietveld refinement with PC and mainframe computers J Appl Cryst , 28 (1995), 366-367 [10] J. Raman Spectrosc 2009, 40, 12051210

[11] A.von Hippel: A New Method for Measuring Dielectric Constant and Loss in the Range of Centimeter Waves (1946)[12] J. Mater Sci: Mater Electron (2008) 19:627638 [13] Hakki B W and Coleman P D, IRE Trans Microwave Theory Tech MTD-8 (1960), 402410 [14] Almeida A.F.L, Silva R.R, Rocha H.H.B, Fechine P.B.A, Cavalcanti F.S.A, Valente M.A, Freire F.N.A, Moretzsohn R.S, and Sombra A.S.B, Phys B: Condens Matter 403 (2008), 586

[15] JOINT COMMITTEE ON POWDER DIFFRACTION STANDARD (JCPDS), International Center for Diffraction Data, (JCPDS 84-0757)

[16] Kojima S, Imaizumi R, Hamazaki S, Takashige M, Jpn J Appl Phys ,Part 1 33 (1994) 5559 [17] Osada M, Tada M, Kakihana M, Watanabe T, and Funakubo H, Jpn J

Appl Phys , Part 1 40, 5572 (2001)

[18] SCHMIDT, Valfredo. Materiais Eltricos Vol I, Ed. Edgard Blucher, So Paulo, 1979. [19] Silva C C and Sombra A S B Mat Sci and Appl 2(9)(2011) 1349Table Captions

Table I: Summary identification of the sintered samples.

Table II: X-ray diffraction patterns of SBTi, SBTi5P, SBTi15P, SBTi5V and SBTi15V samples, sintered at 950 C/3h.

Table III: Dielectric permittivity (K) and dielectric loss tg(() in the 1 MHz, 10 MHz,20 MHz frequency of the SBTi, SBTi2P, SBTi5P, SBTi10P, SBTi15P, SBTi2V, SBTi5V, SBTi10V, SBTi15V, sintered at 950 C/3h at room temperature.Table IV: Microwave measurements obtained from Hakki-Coleman procedure; thickness (e), diameter (D), dielectric resonant TE011 (fr), dielectric permittivity (K) end dielectric loss (tg(E).Figure Captions

Figure 1: X-ray Rietveld refinement method SrBi4Ti4O15 (SBTi) perovskite. The differences between the observed and calculated intensities are show.

Figure 2: X-ray diffraction patterns of SrBi4Ti4O15 (SBTi) perovskite, sintered at 950oC/3h.

Figure 3: The Raman spectra for SBTi, SBTi5P, SBTi15P, SBTi5V and SBTi15V samples at room temperature.

Figure 4: SEM micrographs of SrBi4Ti4O15 sintered at 950 C: (a) SBTi5P, (b) SBTi15P, (c) SBTi5V and (d) SBTi15V.

Figure 5: Dielectric permittivity as a function frequency of SrBi4Ti4O15 added with PbO and V2O5 (0; 2; 5, 10 and 15 wt%) at room temperature.

Figure 6: Dielectric loss as a function frequency of SrBi4Ti4O15 added with PbO and V2O5 (0; 2; 5; 10 and 15 wt%) at room temperature.

Figure 7: (a) relative dielectric permittivity and (b) Dielectric loss , for SBTi, SBTi2P, SBTi5P, SBTi10P, SBTi15P, SBTi2V, SBTi5V, SBTi10V, SBTi15V, samples at 20 MHz frequency, at room temperature.Figure 8:(a) Dielectric permittivity and (b) Dielectric loss, for SBTi, SBTi5P, SBTi10P, SBTi15P, SBTi5V,SBTi10V, SBTi15V, samples at microwaves frequency, at room temperature.Table ISamplesAddition (wt%)Binder (5wt.%)

SBTi-PVA

SBTi2P2%PbOPVA

SBTi5P5%PbOPVA

SBTi10P10%PbOPVA

SBTi15P15%PbOPVA

SBTi2V2%V2O5PVA

SBTi5V5%V2O5PVA

SBTi10V10%V2O5PVA

SBTi15V15%V2O5PVA

Table IIPhasesRp (%)Rexp(%)Rwp (%)SGOFDDW(%)

SBTi15.9713.4020.391.520.13

SBTi5P15.9115.7420.441.290.84

SBTi15P16.2217.5220.871.181.23

SBTi5V17.5616.8022.321.320.91

SBTi15V20.6217.9425.611.420.74

Table IIISAMPLEs1MHz10kHz20MHz

Ktg(()Ktg(()Ktg(()

SBTi22.090.051321.930.005422.000.0013

SBTi2P65.220.006060.490.009361.350.0049

SBTi5P137.500.0127125.240.0180128.260.0178

SBTi10P168.340.0710176.400.0262184.660.0268

SBTi15P232.990.0401166.200.0545170.760.0652

SBTi2V78.720.005670.760.005971.320.0014

SBTi5V102.500.009689.300.006490.040.0010

SBTi10V44.600.008141.270.011641.470.0043

SBTi15V64.230.012361.490.017861.790.0082

Table IVSamplee (mm)D (mm)fr (GHz)Ktg(()

SBTi9.0018.233.42545.470.0288

SBT5V9.2716.192.53989.060.0238

SBTi10V9,2418,553.85134.360.0284

SBTi15V9.5018.263.28846.260.0232

SBTi5P8.1615.372.430117.130.0352

SBTi10P7,7915,902.93482.230.0544

SBTi15P8.7015.532.316119.020.0524

Figure 1

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