Temperature compensated Sr2Al2SiO7 ceramic for microwave applications

Materials Chemistry and Physics 133 (2012) 21–23

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Temperature compensated Sr2 Al2 SiO7 ceramic for microwave applications Kurusaroor M. Manu, Tony Joseph, Mailadil T. Sebastian ∗ Materials Division, National Institute for Interdisciplinary Science and Technology, CSIR, Thiruvananthapuram 695019, India

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Article history: Received 29 April 2011 Received in revised form 18 November 2011 Accepted 23 December 2011 Keywords: A. Ceramics B. Sintering C. Electron microscopy Microwave dielectric properties X-ray diffraction

a b s t r a c t The Sr–Gehlenite (Sr2 Al2 SiO7 ) ceramic has been prepared by the conventional solid-state ceramic route. Phase pure Sr2 Al2 SiO7 (SAS) ceramic sintered at 1525 ◦ C for 4 h has εr = 7.2 and Qu × f = 33,000 GHz. The SAS showed large negative  f of −37.0 ppm/ ◦ C. A low value of  f was achieved by preparing SAS–CaTiO3 composite. The composite with 0.04 volume fractions (Vf ) CaTiO3 sintered at 1500 ◦ C for 4 h showed good microwave dielectric properties: εr = 8.6, Qu × f = 20,400 GHz and  f = +8.5 ppm/◦ C. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The remarkable progress in microelectronic industry demands development of high speed digital devices in the microwave band. Hence the identification of new low permittivity dielectric ceramics which can deliver minimum signal propagation delay is essential in this context [1–3]. The dielectric ceramic should have a low relative permittivity (εr ), high unloaded quality factor (Qu × f) and nearly zero temperature variation of resonant frequency ( f ) for substrate and packaging applications [1,4]. Moreover, it can also be used for high frequency communications [1]. Silicates are found to be attractive candidates for these applications due to their low εr and high Qu × f. The Si–O bond in SiO4 tetrahedra of silicates is predominantly covalent in nature which restricts the rattling of atoms leading to low εr and high Qu × f [1]. Several silicates have been reported as excellent candidates for microwave applications [1,5,6]. However, the continuous development of microwave devices necessitated the need for identification of new silicates with suitable dielectric properties. Over the last two decades, considerable attention has been paid to the synthesis, structural and optical studies of Melilite type ceramics [7–9]. Akermanite (A2 MgSi2 O7 ) and Gehlenite (A2 Al2 SiO7 ) are the two end members of Melilite group where A = Ca or Sr. Recently, Joseph and Sebastian reported the microwave dielectric properties of Sr–Akermanite-type ceramics [5]. However, microwave dielectric properties of Gehlenite type ceramics are not

∗ Corresponding author. Tel.: +91 471 2515294; fax: +91 471 249 1712. E-mail address: [email protected] (M.T. Sebastian). 0254-0584/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.12.067

reported in the literature. They have tetragonal crystal structure with space group P 4¯ 21 m (no. 113). In the present paper, we report for the first time, the microwave dielectric properties of Sr2 Al2 SiO7 ceramic and tailoring the properties for temperature compensation by composite formation with CaTiO3 . 2. Experimental The Sr2 Al2 SiO7 (SAS) ceramic was prepared by the conventional solid-state ceramic route. Stochiometric amounts of high purity powders of SrCO3 (99.9%, Sigma–Aldrich, Inc., Milwaukee, WI, USA), Al2 O3 (99.7%, Sigma–Aldrich) and SiO2 (99.6%, Sigma–Aldrich) were weighed and ball milled for 24 h in distilled water medium. The resultant slurry was dried and calcined at 1350 ◦ C for 4 h. CaTiO3 ceramic was prepared by the same procedure using high purity powders of CaCO3 (99+%, Sigma–Aldrich) and TiO2 (99.8%, Sigma–Aldrich). Different volume fractions of CaTiO3 ceramic were added to SAS ceramic in order to develop a SAS–CaTiO3 ceramic composite with nearly zero  f. Cylindrical pucks of dimensions 11 mm × 6 mm were made and the sintering was carried out in the temperature range 1450–1550 ◦ C for 4 h. The sintered samples were powdered and used to analyze the crystal structure and phase purity by X-ray diffraction (XRD) method (PANanalytical X’Pert PRO Diffractometer having Ni filtered CuK␣ radiation, The Netherlands). The microstructure was recorded using a scanning electron microscope (SEM) (JEOL-SEM 560lv, Tokyo, Japan). The sintered densities of the specimens were measured by the Archimedes method. The microwave dielectric properties were measured in the frequency range 8–15 GHz using a vector network analyzer (Agilent, Model No.: E8362B, USA). The relative permittivity and

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Fig. 1. XRD pattern of (a) Sr2 Al2 SiO7 (SAS) sintered at 1525 ◦ C for 4 h and (b) SAS + 0.04 Vf CaTiO3 sintered at 1500 ◦ C for 4 h.

unloaded quality factor were measured by Hakki–Coleman method modified by Courtney and by the cavity method respectively [1]. The temperature variation of resonant frequency was measured in the temperature range 25–70 ◦ C. 3. Results and discussion Fig. 1(a) shows the powder X-ray diffraction pattern of SAS sintered at 1525 ◦ C for 4 h. All the recognizable peaks were indexed according to the standard ICDD file no: 38–1333 and no extra peaks are observed. The values of lattice parameters a = 7.833 A˚ and c = 5.270 A˚ calculated using the method of graphical extrapolation with the help of Nelson–Riley function [10] is in good agreement with earlier report (ICDD file no: 38–1333). Fig. 1(b) shows XRD pattern of SAS + 0.04 Vf CaTiO3 sintered at 1500 ◦ C for 4 h. The figure clearly indicates the absence of chemical reaction of SAS with CaTiO3 . The major peak of CaTiO3 ceramic which was indexed according to the ICDD file no: 42-0423 was clearly seen from the XRD pattern. Fig. 2(a) shows microstructure of SAS ceramic sintered at 1525 ◦ C for 4 h. The relatively dense microstructure with grains of uniform contrast is a clear indication of single phase nature of SAS ceramic. Fig. 2(b) shows SEM image of SAS + 0.04 Vf CaTiO3 sintered at 1500 ◦ C for 4 h. The two types of grains with different contrast might be a clear indication of presence both SAS and CaTiO3 ceramics. Fig. 3 shows the variation of relative density and relative permittivity of SAS ceramic with sintering temperature. The relative density first increases reaching a maximum value of 97.3% at 1525 ◦ C and thereafter decreases. The reduction in relative density at elevated temperatures may be due to the entrapped porosity developed by the exaggerated grain growth [11]. The variation in εr is similar to that of relative density. The SAS ceramic sintered at 1525 ◦ C for 4 h has εr = 7.2. Further increase in sintering temperature decreases the εr to 6.9 which can be attributed to the reduction in relative density. The variation of Qu × f and  f as a function of sintering temperature is shown in Fig. 4. The observed variation in Qu × f can be attributed to the extrinsic losses such as defects, impurities, porosity etc. [12]. The variation of  f with sintering temperature is relatively small compared to that of relative density. The relatively dense (97.3%) SAS ceramic sintered at 1525 ◦ C for 4 h shows εr = 7.2, Qu × f = 33,000 GHz and  f = −37.0 ppm/ ◦ C. The large negative  f value of SAS ceramic restricts its immediate practical applications in wireless technology. Hence it is necessary to

Fig. 2. SEM images of (a) Sr2 Al2 SiO7 (SAS) sintered at 1525 ◦ C for 4 h (b) SAS + 0.04 Vf CaTiO3 sintered at 1500 ◦ C for 4 h.

Fig. 3. Variation of relative density and relative permittivity of Sr2 Al2 SiO7 ceramic as a function of sintering temperature.

Fig. 4. Variation of Qu × f and  f of Sr2 Al2 SiO7 ceramic as a function of sintering temperature.

K.M. Manu et al. / Materials Chemistry and Physics 133 (2012) 21–23

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Table 1 The relative density and the microwave dielectric properties of Sr2 Al2 SiO7 –CaTiO3 composites at their optimized sintering temperatures. Volume fraction of CaTiO3 (Vf )

Sintering temperature (◦ C)

Relative density (%)

εr

εr (porosity corrected)

0.00 0.03 0.04 0.05

1525 1475 1500 1475

97.3 98.4 97.3 97.2

7.2 8.4 8.6 9.1

7.5 8.6 9.0 9.5

tailor the  f value to nearly zero by the addition of suitable amount of materials having high positive values of  f. It is a well established fact that CaTiO3 having εr ≈ 174, Qu × f ≈ 11,260 GHz and  f ≈ +800 ppm/ ◦ C is a good candidate for tuning materials having high negative  f [13]. Table 1 gives relative density and microwave dielectric properties of SAS–CaTiO3 composites at their optimized sintering temperatures. The relative densities of all the compositions are above 95% which is essential for achieving excellent microwave dielectric properties. The εr increases with increase in volume fraction of CaTiO3 which is due to the relatively high εr of CaTiO3 [13] compared to that of pure SAS. The porosity corrected εr values calculated using the following formula derived by Penn et al. [14] are given in Table 1.



ε = εm 1 −

3P(εm − 1) 2εm + 1



Qu × f (GHz)

33,000 20,450 20,400 14,380

 f (ppm/◦ C)

−37.0 −9.5 +8.5 +24.0

negative  f value of SAS ceramic was tailored by the addition of suitable amount of CaTiO3 . SAS ceramic mixed with 0.04 Vf of CaTiO3 sintered at 1500 ◦ C for 4 h had εr = 9.0 (porosity corrected), Qu × f = 20,400 GHz and  f = +8.5 ppm/ ◦ C. The observed properties indicate that SAS + 0.04 Vf CaTiO3 may be a possible candidate for microwave substrate applications. Acknowledgments Kurusaroor M. Manu is grateful to the Department of Science and Technology, Government of India for the financial assistance. The authors are grateful to Dr. Prabhakar Rao, students from electron microscopy and Mr. M.R. Chandran of NIIST (CSIR) for extending XRD and SEM facilities.

(1)

where ε and εm are experimentally observed and porosity corrected εr values and P is the fractional porosity. It can also be seen that with increase in CaTiO3 content, the unloaded quality factor of SAS–CaTiO3 composites decreases. The relatively low Qu × f of CaTiO3 ceramic [13] may be the reason for this reduction in Qu × f. However, the  f value reaches to −9.5 ppm/ ◦ C as the volume fraction of CaTiO3 increases from 0 to 0.03 and it becomes positive with higher CaTiO3 loading. The composition SAS + 0.04 Vf CaTiO3 sintered at 1500 ◦ C for 4 h shows a relatively low  f of +8.5 ppm/ ◦ C with εr = 9.0 (porosity corrected) and Qu × f = 20,400 GHz which indicate its possibility for practical applications. 4. Conclusions SAS ceramics were prepared by the conventional solid-state ceramic route. SAS sintered at 1525 ◦ C for 4 h had εr = 7.5 (porosity corrected), Qu × f = 33,000 GHz and  f = −37.0 ppm/ ◦ C. The large

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