Dielectric Dispersion at Microwave Frequencies of Some Low Loss Mixed Oxide Perovskites

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1 Ferroelectric Letters, 35:79 85, 2008 Copyright C Taylor & Francis Group, LLC ISSN: print / online DOI: / Dielectric Dispersion at Microwave Frequencies of Some Low Loss Mixed Oxide Perovskites JYH SHEEN, 1 RUNYAN GUO, 2,3, A. S. BHALLA, 2,3 and L. E. CROSS 3 1 Department of Electronic Engineering, National Formosa University, Hu-Wei, Yun-Lin 632, Taiwan 2 Department of Electrical and Computer Engineering, University of Texas at San Antonio, San Antonio, Texas 78249, USA 3 Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA Communicated by Dr. George W. Taylor (Received March 21, 2008) This paper reports the dielectric permittivities of several low loss ceramics of A(B1 1/2 B2 1/2 )O 3 mixed oxide perovskites, Ba(Mg 1/3 Ta 2/3 )O 3 (BMT), Sr(Al 1/2 Nb 1/2 )O 3 (SAN), Sr(Al 1/2 Ta 1/2 )O 3 (SAT), Sr(Ga 1/2 Ta 1/2 )O 3 (SGT), and their modified compositions 0.7Sr(Al 1/2 Nb 1/2 )O 3-0.3NdGaO 3 (SAN-NG) and 0.7Sr(Al 1/2 Ta 1/2 )O 3-0.3NdGaO 3 (SAT- NG), measured at microwave frequencies. Measurements of the frequency-dependent dielectric constant and loss tangent are reported. Their loss tangent values are on the order of 10 4 to 10 5, which can not be evaluated by the traditional reflection/transmission spectra or impedance measurement techniques. A parallel-plate dielectric resonance method was adopted for dielectric measurements for such low loss materials. The frequency spectra of dielectric constants and loss tangents of were investigated by the well known classical oscillator model. Keywords: Perovskite; dielectric properties; dielectric constant; loss tangent; microwave; dielectric dispersion I. INTRODUCTION The dielectric properties of perovskites have been the subect of many investigations [1]. A(B1 1/2 B2 1/2 )O 3 mixed oxide perovskites of Ba(Mg 1/3 Ta 2/3 )O 3 (BMT), Sr(Al 1/2 Nb 1/2 )O 3 (SAN), Sr(Al 1/2 Ta 1/2 )O 3 (SAT), Sr(Ga 1/2 Ta 1/2 )O 3 (SGT), and their modified compositions *Corresponding author. ryguo@psu.edu 79

2 80 J. SHEEN et al. 0.7Sr(Al 1/2 Nb 1/2 )O 3-0.3NdGaO 3 (SAN-NG) and 0.7Sr(Al 1/2 Ta 1/2 )O 3-0.3NdGaO 3 (SAT-NG) were synthesized and explored as promising substrates of high Tc superconducting thin films for applications in microwave frequencies [2 5]. It is a very important issue to understand the dielectric dispersion properties in microwave frequencies. Many efforts have been made to understand the frequency dependent of complex permittivity, ε ε [6 20]. Far infrared reflectivity spectra were usually obtained and transformed to the complex dielectric data by the Kramers-Kronig procedure and the classical oscillator model [6 16]. However, loss tangent values of these perovskites are on the order of 10 4 to 10 5, which can not be obtained by the traditional reflection/transmission spectra or impedance measurement techniques. Therefore, a parallel-plate dielectric resonance method was adopted for dielectric measurements for such low loss materials [21, 22]. Unlike other dielectric resonance methods where only a single frequency point can usually be measured, the parallel-plate setup can measure various frequency values within a certain frequency range by adusting the distance between the two conducting plates. II. THEORIES According to the classical oscillator model, complex dielectric permittivity ε and ε can be expressed in the following equations [6 16]: ε = ε + ε = =1 =1 ( ω 2 ω 2) S ( ω 2 ω 2) 2 + ω2 γ 2 ωγ S ( ω 2 ω 2) 2 + ω2 γ 2 (1) (2) where ε is the optical frequency dielectric constant caused by the electronic polarizations at higher frequencies (ε n 2, n is the refractive index), ω is the resonant frequency of the damped lattice oscillator (usually ω >10 12 ), γ and S are the damping constant and intensity of resonant peaks, respectively. For ω ω, the equations (1) and (2) can be simplified to have the

3 MICROWAVE DIELECTRIC DISPERSION 81 following forms: It can be seen that at ω ω, ε ε + ε =1 S ω 2 =1 ω γ S ω 4 1. From equation (3), the dielectric constant ε is independent of the frequency thus, (3) (4) ε constant (5) 2. From equation (4), the loss tangent (tan δ = ε /ε ) is proportional to the frequency which means, f/tan δ constant. (6) The general validity of the equations (3) and (4) for various materials at microwave frequencies is still a subect of research. In this work, measurements on and of the above perovskites are conducted and the results obtained are compared with equations (3) and (4). The classical oscillator model is adopted in the analysis of the low loss tangent materials studied in this research. III. EXPERIMENTS AND DISCUSSIONS The dielectric properties of the mixed oxide perovskites were measured by the parallel-plate dielectric resonance method [21, 22]. The measurement results are summarized in Tables I and II. The plots for the trend of dielectric constant and the ratio of frequency vs. loss tangent as functions of frequency are shown in Figs. 1 and 2, respectively. From the experimental results, we can find that, 1. From Table I, the dielectric constant ε is independent of the frequency with a margin of error estimated to be within 3%.

4 82 J. SHEEN et al. TABLE I Measurement results of dielectric constants Materials Parameters Measurement Data Deviation from eq. (6) BMT Freq. (GHz) ±1.3% ε SAN Frequency ±2.9% ε SAT Frequency ±2.8% ε SGT Frequency ±1.7% ε SAN-NG Frequency ±1.6% ε SAT-NG Frequency ±1.7% ε From Table II, the deviation of f/tan δ = constant varies from 4% to 15%. Therefore, based on the classical oscillator model and the measurement results summarized in Table I and Figure 1, the independence of dielectric constant with frequency over the measured frequency range is concluded. The expected relation that f/tan δ = constant is found however not quite accurate. The fitted lines in Figure 2 are indicative of the slightly increasing trend of the quantity with frequency. From Table II the deviations from the constants are up to +15% in the case of SAN and relatively small TABLE II Measurement results of loss tangents Materials Parameters Measurement Data Deviation from eq. (6) BMT Freq. (GHz) f/tan δ ±4.5% SAN Frequency f/tan δ ±14.5% SAT Frequency g f/tan δ ± 3.9% SGT Frequency g f/tan δ ± 8.0% SAN-NG Frequency g f/tan δ ± 11.6% SAT-NG Frequency g f/tan δ ± 6.8%

5 MICROWAVE DIELECTRIC DISPERSION 83 Figure 1. Dielectric constant measured at microwave frequencies. The linear fitted lines are indicative of the frequency independence of the dielectric properties. Figure 2. The (frequency/tanδ) values measured at microwave frequencies. The fitted lines are indicative of the slightly increasing trend of the quantity with frequency.

6 84 J. SHEEN et al. (+4%) in SAT. Lastly, since only single frequency point can usually be measured [23] using the widely adopted dielectric resonance techniques on microwave dielectric properties measurements, the above relationship of dielectric constant and loss tangent with frequencies becomes very useful in forecasting the dielectric properties at microwave frequencies removed from the measurement point. ACKNOWLEDGMENT This work was partially supported by the National Science Council of Taiwan under Contract No. NSC E REFERENCES [1] A. S. Bhalla, Ruyan Guo, and Rustum Roy, The perovskite structure a review of its role in ceramic science and technology, Mat. Res. Innovat. 4, 3 26 (2000). [2] Ruyan Guo, P. Ravindranathan, U. Selvara, A. S. Bhalla, L. E. Cross, and Rustum Roy, Modified mixed oxide perovskites 0.7Sr(Al 1/2 B 1/2 )O 3 D0.3LaAlO 3 and 0.7Sr(Al 1/2 B 1/2 )O 3 D0.3NdGaO 3 (B=Ta 5+ or Nb 5+ ) for high Tc superconductor substrate applications, Journal of Materials Science. 29, (1994). [3] Ruyan Guo, A. S. Bhalla, Jyh Sheen, F. W. Ainger, E. C. Subbarao, and L. E. Cross, Strontium aluminum tantalum oxide and strontium aluminum niobium oxide as potential substrates for HTSC thin films, Journal of Materials Research. 10(1), (1995). [4] Ruyan Guo, A. S. Bhalla, Rustum Roy, and L. E. Cross, Candidate HTSC film substrates of complex oxide perovskite compositions, Proceedings of Materials Research Society Symposium. 341, (1994). [5] Ruyan Guo, A. S. Bhalla, and L. E. Cross, Ba(Mg1/3Ta2/3)O3 single crystal fiber grown by the laser heated pedestal growth technique, J. Appl. Phys. 75, (1994). [6] N. Qin, X. C. Fan, S. Y. Wu, and X. M. Chen, Infrared reflection spectra of Ba 6 3x Sm 8+2x Ti1 8 O 54 (x=0.5, 0.67, and 0.75) microwave dielectric ceramics, Journal of Applied Physics. 101, (2007). [7] J. Lou, T. A. Hatton, and P. E. Laibinis, Effective dielectric properties of solvent mixtures at microwave frequencies, (J. Phys. Chem. A). 101, (1997). [8] Y. Park and K. M. Knowles, Dielectric dispersion at microwave frequencies in PbCo 1/2 W 1/2 O 3 single crystal, Solid State Communications. 117, (2001). [9] M. C. Wu, S. Kamba, V. Bovtun, and W. F. Su, Comparison of microwave dielectric behavior between Bi 1.5 Zn 0.92 Nb 1.5 O 6.92 and Bi 1.5 ZnNb 1.5 O 7, Journal of the European Ceramic Society. 26, (2006). [10] K. Wakino, M. Murata, and H. Tamura, Far infrared reflection spectra of Ba(Zn,Ta)O 3 - BaZrO 3 dielectric resonator material, J. Am. Ceram. Soc. 69, (1986).

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Temperature dependence of microwave and THz dielectric response

The 10 th European Meeting on Ferroelectricity, August 2003, Cambridge, UK Ferroelectrics, in press. Temperature dependence of microwave and THz dielectric response in Sr n+1 Ti n O 3n+1 (n=1-4) D. Noujni