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ceramic composites
Physica B 406 (2011) 4312–4316
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
The effect of filler on the temperature coefficient of the relative permittivity of PTFE/ceramic composites S. Rajesh a,b, K.P. Murali a, H. Jantunen b, R. Ratheesh a,n a Microwave Materials Division, Centre for Materials for Electronics Technology (C-MET), Department of Information Technology, Government of India, Athani P.O. Thrissur, Kerala 680 771, India b Microelectronics and Materials Physics Laboratories, University of Oulu P.O. Box 4500, FI 90014 Oulu, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 May 2011 Accepted 7 August 2011 Available online 26 August 2011
High permittivity and low-loss ceramic fillers have been prepared by means of the solid state ceramic route. Ceramic-filled composites were prepared by the Sigma Mixing, Extrusion, Calendering, which was followed by the Hot pressing (SMECH) process. The microwave dielectric properties of the composites were studied using X-band waveguide cavity perturbation technique. The temperature coefficient of the relative permittivity of the composites was investigated in the 0–100 1C temperature range using a hot and cold chamber coupled with an impedance analyzer. The temperature coefficient of the relative permittivity of the composites showed strong dependence on the temperature coefficient of the relative permittivity of the filler material. In the present study, a high-permittivity polymer/ ceramic composite, having ter 63 ppm/K, has been realized. This composite is suitable for outdoor wireless applications. & 2011 Elsevier B.V. All rights reserved.
Keywords: Dielectric properties Polymer/ceramic composites Temperature stability PTFE
1. Introduction The focus of the high-frequency electronics industry is changing from conventional, military-based applications to a more entertainment-oriented consumer market. Polymer/ceramic composites offer a wide range of possibilities for making flexible materials with tailored electrical and mechanical properties, which is done to cater to the rapidly advancing requirements of the modern microelectronics industry [1]. Moderate relative permittivity (er), a low loss tangent (tand) and a low temperature coefficient of permittivity ðter Þ are the major parameters that need to be satisfied by the materials used for various microelectronic applications. Whereas a high relative permittivity can reduce the circuit size, a low loss tangent is needed for signal integrity. Besides the relative permittivity and loss tangent, the temperature coefficient of permittivity ðter Þ is the most critical parameter, because it affects high frequency devices when used in outdoor applications [2,3]. The variation of relative permittivity with respect to temperature will shift the operating frequency of the device during its operation, because they are related by tf ¼ 1/2ter a , where ‘‘tf’’ is the temperature coefficient of resonant frequency and ‘‘a’’ is the linear coefficient of thermal expansion [4]. It is reported
n
Corresponding author. E-mail addresses: [email protected] (S. Rajesh), [email protected] (R. Ratheesh). 0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.08.080
that if ter has a value of10 ppm/K, then there will be a 0.11% shift in the resonant frequency within a temperature range from 30 to þ80 1C [3]. In order to compensate for the changes caused by a large temperature coefficient of permittivity, additional circuitries or mechanical structures are required, but, again, this type of addition increases the size and cost of the devices. An alternative technique that may be used to circumvent this problem is to use temperature-stable, passive components in high-frequency integrated circuits [3,5]. Polymer/ceramic composites are the least exploited material systems to realize temperature-stable dielectric properties for high-frequency communication applications [6]. A glance at the commercially available flexible materials for highfrequency packages reveals that there are very few available materials that have both high relative permittivity and a low temperature coefficient of relative permittivity. Table 1 shows some of the flexible packaging materials that are most commonly used for high-frequency applications. Among these, only TMM10s has a very low temperature coefficient of relative permittivity. TMMs is a ceramic-filled thermoset material, and it suffers from high moisture absorption and low chemical resistance [7,8]. On the other hand, thermoplastic matrix composites, which are ideal for most high-frequency applications, have a relatively high dependence of relative permittivity with respect to temperature. Although considerable amount of work has been carried out to achieve temperature stability in ceramics, less attention has been
S. Rajesh et al. / Physica B 406 (2011) 4312–4316
Table 1 The electrical properties of certain high-frequency packaging materials. Trade name
e’r
tand
ter (ppm/K)
Reference
FR-4TM RT/duroid 6006TM RT/duroid 6010TM TMM10TM
4.4 6.15 10.2 9.2
0.05 0.0027 0.0023 0.0022
160 410 425 38
[3] [7] [7] [8]
Table 2 The density and dielectric properties of sintered BPT, BST and Rutile ceramics. Ceramic
Bulk density (g/cc)
e’r
Qxf (GHz)
t ’ (ppm/K)
BaPr2Ti4O12 BaSm2Ti4O12 Rutile[14]
5.84 5.79 4.23
88 76 100
1800 2400 10000
300 þ 30 750
er
4313
ground in an agate mortar again, and the phase formation was studied using the powder X-ray diffraction technique (CuKa, Bruker 5005, Germany). To preclude moisture absorption, the ceramic powders were treated with a silane coupling agent, as explained elsewhere [16]. In order to study the microwave dielectric properties of the filler materials, cylindrical pellets were made out of the ceramic powder by uniaxial pressing under a pressure of 250 MPa. The green pellets thus obtained were sintered at a temperature in the range between 1400 and 1450 1C for 4 h. The particle size distribution of the fillers was studied using a Microtrack X100, USA, particle size analyzer. The microwave dielectric properties of the cylindrical compacts were studied by the Hakki and Coleman post resonator technique using a Vector Network Analyzer (Agilent PNA E8362B, USA) [17]. The temperature coefficient of permittivity was measured in the temperature range between 30 and 100 1C using a hot and cold chamber (Clitech, India) coupled with an Impedance Analyzer (Hewlett-Packard 4192 A, USA). The temperature coefficient of relative permittivity was calculated from Eq. 1:
paid to controlling the temperature-induced frequency drift in polymer ceramic composites [9,10]. Walpita et al. studied the temperature stability of polyphenylenesulfide (PPS)/SrTiO3 composite substrates by evaluating the performance of a patch antenna, which was fabricated using this substrate. In this study, the temperature coefficient of the permittivity of PPS/SrTiO3 composites was controlled by adding secondary ceramic fillers, such as alumina and mica [11]. The use of ceramic fillers with different morphologies and physical properties may lead to uneven filler distribution, and use of such fillers can also cause anisotropic dielectric properties. In another study, Nisa et al. reported temperature-stable PEEK composites by controlling the SrTiO3 filler content [12]. However, temperature-stable composites were obtained far below the optimum filler loading and, hence, the composites exhibited degraded thermal properties and a low order of dielectric isotropy. Recently, Xiang et al. studied the temperature dependence of the relative permittivity of polyolefin elastomer/SrTiO3 composites, and they found this property to be relatively high ( 1480 ppm/K) [13]. In the present study, different ceramic fillers, viz., rutile, BaPr2Ti4O12 and BaSm2Ti4O12, were loaded in the PTFE matrix, near optimum filler fractions, in order to reduce the temperature dependence of the relative permittivity of the composites. Because of its excellent electrical and mechanical properties [14,15], poly(tetrafluroethylene) (PTFE) was selected as the polymer matrix. PTFE has a relative permittivity of 2.1 and a loss tangento0.0003, and it also has a relatively high service temperature ( 250 1C) [14,15]. In the present work, fillers having different temperature coefficient of permittivity ðter Þ were incorporated in the PTFE matrix near to the optimum filler loading, and we studied the effect such fillers have on the ter values of the resulting composites.
where er30 is the relative permittivity at room temperature and Der/DT is the change in relative permittivity with respect to temperature. The PTFE/ceramic composites were prepared through Sigma Mixing, Extrusion and Calendering, followed by the Hot pressing (SMECH) process, which has been explained elsewhere [18]. H71 grade PTFE (HFC, India) powder with a size of 50 mm was used for the preparation of the composites. An appropriate amount of the starting materials, i.e., the filler and the matrix, were weighed and mixed using a sigma mixer by adding dipropylene glycol (Sigma Aldrich, USA) as a lubricant. Thus, the dough-like mass that was obtained was extruded to make a preform of the desired shape. It was then calendered to a thickness of 100 mm using a calendering machine. The differential speed of the rollers of the calendering machine ensured that there was a uniform distribution of the ceramic filler in the PTFE matrix. The calendered green tapes were stacked on top of each other and were hot pressed at a temperature of 350 1C and a pressure of 210 kg/cm2 [18]. In the present case, the initial thickness was chosen so as to obtain a final thickness of 625 mm after hot pressing. Using a Vector Network Analyzer (Agilent PNA E8362B, USA), the microwave dielectric properties of the samples were measured at X-band frequency region by the waveguide cavity perturbation technique [19]. The temperature coefficient of relative permittivity of the composite samples was calculated using Eq. 1 in the 0– 100 1C temperature range. The morphology of the composites samples was studied by Scanning Electron Microscopy (SEM) (Philips XL-30, The Netherlands). The samples were freeze fractured in liquid nitrogen for cross-sectional SEM pictures.
2. Experimental
3. Results and discussion
Rutile, BaPr2Ti4O12 and BaSm2Ti4O12 were used as the filler materials. Whereas rutile-grade TiO2 (4 99%) was purchased from M/s Sigma Aldrich, USA. The BaPr2Ti4O12 and BaSm2Ti4O12 filler materials (which are referred to as BPT and BST, respectively, in the rest of this paper) were prepared through the solid-state ceramic method. BaCO3 (499%, Sigma Aldrich, USA), Sm2O3 ( 499% IRE, India), Pr6O11 ( 499%, IRE, India) and TiO2 ( 499%, Sigma Aldrich, USA) were weighed according to their stoichiometric proportions and mixed in an agate mortar. The slurry was dried using a hot air oven ( 80 oC) and calcined at 1200 1C for 4 h with a heating rate of 10 1C/min. The calcined powders were then
The powder X-ray diffraction patterns of BPT and BST powders that were calcined at 1200 1C for 4 h are shown in Fig. 1. The patterns are indexed on the basis of ICDD file no. 43-235, which confirms that the phase formation was completed during calcination. Both BaPr2Ti4O12 (BPT) and BaSm2Ti4O12 (BST) are composed of perovskite-like blocks joined to one another in the tetragonal tungsten bronze pattern [20]. The relative permittivity and quality factors of BPT and BST, which were measured using the Hakki and Colemann Post resonator technique, are shown in Table 2. Both materials have high relative permittivity values and moderate quality factors, which has already been shown [20,21]. Compared
ter ¼
1
er30
Der DT
ð1Þ
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S. Rajesh et al. / Physica B 406 (2011) 4312–4316
Fig. 1. Powder X-ray diffraction patterns of (a) BaPr2Ti4O12 and (b) BaSm2Ti4O12 calcined at 1200 1C/4 h.
Microscopy (SEM), and representative pictures are shown in Fig. 2. A very smooth surface texture without any irregularity is clearly visible in Fig. 2a, which was taken at a lower magnification. A regular surface morphology is very important for these materials, because the particulate fillers flow/rearrange during the hot pressing process. The freeze-fractured surface (Fig. 2b) shows uniform distribution of the ceramic particulates throughout the PTFE matrix. This demonstrates the efficacy of the processing methodology that was employed for the fabrication of planar composite laminates. The SEM studies show that the composites have minimal porosity. This result agrees with the experimental density values, which predict o10% porosity in the composite samples under study (Fig. 3). The relative permittivity of filled PTFE composites was studied as a function of filler loading. Because the main objective of the present investigation is to study the effect of the filler on the temperature coefficient of the relative permittivity of the PTFE composites, the fillers are selected judiciously. Rutile has high negative temperature coefficient of relative permittivity ðter Þ, BST has a nearly temperaturestable relative permittivity, although it is slightly on the positive side. BPT is a high-permittivity and low-loss ceramic having a ðter Þ value that is in between the corresponding values for rutile and BST. Previous reports show that nearly isotropic and dimensionally stable composite substrates having reproducible microwave dielectric properties necessitates an optimum filler loading in the PTFE matrix [16,18]. The effect of rutile filler loading on the microwave dielectric properties has been studied in detail and reported by us [18]. For a rutile filler having a particle size of 5 mm, the optimum filler loading was around 67 wt%. In the present case, the filler content in the PTFE matrix is varied, but it is near to the optimum filler loading (52–72 wt%) because all three of the fillers have very similar particle sizes and size distributions. The dielectric properties of the planar composite samples are measured in the X-band frequency region, and the results are shown in Fig. 3. The relative permittivity shows an increasing trend with filler loading. The main reasons for this are the higher relative permittivity of the filler, compared to that of the matrix, and the increasing amount of the interface region. For both BST and BPT-loaded composites, 67 wt% of the filler loading is observed to be the optimum filler condition, with relative permittivity values of 8.2 and 7.6, respectively. Among the three composite samples, the rutile-loaded composite showed the highest relative permittivity ðe0r 10:2Þ. The decrease in relative permittivity beyond the optimum filler loading is mainly attributed to the increase in porosity and the deterioration of the 0–3 connectivity of the composite system. The reduction in density beyond the optimum
Fig. 2. (a) A planar SEM picture of 67 wt% BPT and (b) a cross sectional SEM picture of 67 wt% BST-loaded PTFE composites.
to the near-zero positive temperature coefficient of relative permittivity of BST, BPT had a high negative temperature coefficient of relative permittivity. On the other hand, rutile has a very high negative temperature coefficient of relative permittivity. The particle size analysis shows that all three fillers have a very similar particle size distribution, with d50 value between 3 and 5 mm. The morphology of the composites and the distribution of the filler in the PTFE matrix are studied using Scanning Electron
Fig. 3. Variation of experimental density and relative permittivity (in the X-band region) of composites with respect to the filler’s content.
S. Rajesh et al. / Physica B 406 (2011) 4312–4316
filler loading is also evident from Fig. 3. A filler loading beyond 72 wt% in the PTFE matrix can be considered as an over-loaded condition for these composites. The loss tangents of the composites are measured in the X-band frequency region as a function of the filler loading (Fig. 4). All the composites have low loss tangent values up to the optimum loading level, and, beyond that, the composites exhibited relatively high loss tangents. The increase in porosity beyond the optimum filler loading may be the reason for this increase. Rutile-filled composites show the lowest loss tangent, which may be due to the high quality factor of rutile compared to that of the other two fillers. The temperature coefficient of relative permittivity is a critical parameter for high-frequency packaging materials, which are used in outdoor applications. A slight variation in the permittivity relative to the temperature will shift the operating frequency of any devices which were fabricated using such materials. The relative permittivity of the composites is measured in the 0–100 1C temperature range in intervals of 10 1C. The ter values of the composites are calculated using Eq. 1, and the results are plotted as a function of the filler loading (Fig. 5). Because of the inferior density values of the composites, the ter at 72 wt% filler loading is not taken into consideration. All three of the composites showed similar trends in the variation of ter with respect to the filler loading. The ter increased positively with the increase of
Fig. 4. Variation of the loss tangent (tand) with respect to the filler loading.
Fig. 5. Variation of the temperature coefficient of permittivity with respect to the filler loading.
4315
the filler content in the PTFE matrix. PTFE has a high negative temperature coefficient of relative permittivity, viz., ter 400 ppm=K [18]. When it is loaded with rutile having a higher negative ter , the effective values of ter of the composites vary from 630 to 425 ppm/K for filler loadings of 52 to 67 wt%. When PTFE is loaded with BST, which has a low positive temperature dependence, the value of ter of the composites increases in the positive direction with respect to the filler addition. BPT has a ter 2300 ppm=K, which is in between that of the other two fillers (rutile and BST). Also, the PTFE/BPT composite resulted in a ter of þ63 ppm/K at 52 wt%, which increased with additional filler loading (Fig. 5). e0 1 @a 3aL te0 r ¼ r ð2Þ 3 a @T The most important parameters that control the temperature coefficient of relative permittivity are the change in the polarizability of the material with respect to its temperature and its linear coefficient of thermal expansion (CTE) (Eq. 2) [4]. In this equation, the first term represents the change in polarizability of the system, and the second term represents the linear coefficient of thermal expansion. In general, particulate fillers have different relative permittivities and, hence, the percentages of effective polarizable ions in the composites are also different. The parameters that control the change in polarizability of the composites with respect to temperature are not completely understood. Previous studies show that the CTE of filled PTFE composites decreases with the increasing filler loading [22]. At optimum filler loading, it is reported to be around 20 ppm/K, irrespective of the nature of the filler [22,23]. While the filler content is increased from 52 wt% to 67 wt%, the change in CTE should be marginal for all three of the composites. The ter of the composites measured at 67 wt% filler loading is shown in Fig. 6. It is clear from the figure that the ter of the filler has a significant influence on the ter of the filled PTFE composites. When the filler had a high negative ter , the composite exhibited a high negative ter . But for a temperature-stable filler, viz., BST, the filled composite showed a high positive ter . Interestingly, because BPT had a slightly higher negative ter , the filled PTFE composites had a temperature variation of relative permittivity that was close to zero. These results show that, in order to arrive at a temperature-stable composite system, the temperature coefficient of relative permittivity of PTFE should be compensated for with the ter of the ceramic filler. In other words, the particulate filler must have an appropriate temperature variation of relative
Fig. 6. Variation of the temperature coefficient of permittivity for different fillers at 67 wt% loading.
4316
S. Rajesh et al. / Physica B 406 (2011) 4312–4316
permittivity that can negate the changes in the relative permittivity of the polymer matrix with temperature.
4. Conclusions High-permittivity ceramic fillers were prepared through the solidstate ceramic preparation method The phase purity of the powders was studied using the powder X-ray diffraction technique. The Hakki and Coleman technique was employed to study the microwave dielectric properties of the well-sintered ceramic compacts. Filled PTFE planar composites were prepared through the SMECH process, and the distribution of ceramic particulates in the PTFE matrix was evaluated using SEM studies. All three of the composites showed an optimum loading of around 67 wt% of the filler content. For all of the composites that were studied, the temperature coefficient of relative permittivity showed a similar trend with respect to the ceramic loading. BPT-filled PTFE composites showed a ter þ130 ppm=K at optimum loading (67 wt%), which decreased to þ63 ppm/K as the amount of filler in the PTFE matrix was reduced to 52 wt%. The present study shows that temperature-stable composites could be prepared by judiciously selecting filler materials so that they have a suitable temperature coefficient of relative permittivity, which can compensate for the temperature dependence of the relative permittivity of the polymer matrix.
Acknowledgment The authors are grateful to Dr. K. R. Dayas, Director, C-MET, Thrissur, India, for extending the facilities to carry out this work.
References [1] M. Arbatti, X. Shan, Z. Cheng, Adv. Mater. 19 (10) (2007) 1369. [2] M.T. Sebastian, H. Jantunen, Int. J. Appl. Ceram. Technol. 7 (4) (2010) 415. [3] M.T. Sebastian, Dielectric materials for wireless communication, First ed., Elsevier, Oxford, 2009. [4] A.J. Moulson, J.M. Herbert, Electroceramics, Second ed., Chapman and Hall, 2003 p.230. [5] H. Jantunen, A. Turunen, US Patent 5302924, 1994. [6] D.N. Light, J.R. Wilcox, IEEE Trans. Compon. Packag. Technol. Pt. A 18 (1) (1995) 118. [7] Data sheet of Rogers corporation /http://www.rogerscorp.com/documents/ 612/acm/RT-duroid-6006-6010-laminate-data-sheet.aspxS (23-05-2011). [8] /http://www.rogerscorp.com/documents/728/acm/TMM-Thermoset-lamina te-data-sheet-TMM3-TMM4-TMM6-TMM10-TMM10i.aspxS (23-05-2011). [9] N.D. Kataria, I.N. Jawahar, M.T. Sebastian, Mater. Lett. 60 (2006) 2642. [10] S. Astrong, S. Marinal, Mater. Res. Bull. 43 (2008) 2569. [11] L.M. Walpita, M.R. Ahern, P. Chen, H. Goldberg, S. Hanley, W.M. Pleban, S. Weinberg, C. Zipp, G. Adams, Y.H. Wong, IEEE Trans. Microwave Theory Technol. 47 (8) (1999) 1577. [12] V.S. Nisa, S. Rajesh, K.P. Murali, V. Priyadarsini, S.N. Potty, R. Ratheesh, Comp. Sci. Technol. 68 (2008) 106. [13] F. Xiang, H. Wong, H. Yang, Z. Shen, X. Yao, J. Electroceram. 24 (2010) 20. [14] Y.-C. Chen, H.-C. Lin, Y.-D. Lee, J. Polym Res. 10 (2003) 247. [15] G. Subodh, M. Joseph, P. Mohanan, M.T. Sebastian, J. Am. Ceram. Soc. 90 (11) (2007) 3507. [16] S. Rajesh, K.P. Murali, R. Ratheesh, Polym. Comp. 30 (10) (2009) 1480. [17] B.W Hakki, P.D. Coleman, IRE Trans. Microwave Theory Tech. MTT-8 (1960) 402. [18] S. Rajesh, K.P. Murali, V. Priyadarsini, S.N. Potty, R. Ratheesh, Mater. Sci. Eng. B 163 (2009) 1. [19] D.C. Dube, M.T. Lanagan, J.H. Kim, S.J. Jang, J. Appl. Phys. 63 (7) (1988) 2466. [20] K. Fukuda, R. Kitoh, I. Awai, J. Mater. Res. 10 (2) (1995) 312. [21] S. Solomon, N. Santha, I.N. Jawahar, H. Sreemoolanathan, M.T. Sebastian, P. Mohanan, J. Mater. Mater. Electron. 11 (2000) 595. [22] P.S. Anjana, S. Uma, J. Philip, M.T. Sebastian, J. Appl. Polym. Sci. 118 (2010) 751. [23] C.P. Wong, R.S. Bollampally, J. Appl. Polym. Sci. 74 (1999) 3396.
Contents lists available at SciVerse ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
The effect of filler on the temperature coefficient of the relative permittivity of PTFE/ceramic composites S. Rajesh a,b, K.P. Murali a, H. Jantunen b, R. Ratheesh a,n a Microwave Materials Division, Centre for Materials for Electronics Technology (C-MET), Department of Information Technology, Government of India, Athani P.O. Thrissur, Kerala 680 771, India b Microelectronics and Materials Physics Laboratories, University of Oulu P.O. Box 4500, FI 90014 Oulu, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 May 2011 Accepted 7 August 2011 Available online 26 August 2011
High permittivity and low-loss ceramic fillers have been prepared by means of the solid state ceramic route. Ceramic-filled composites were prepared by the Sigma Mixing, Extrusion, Calendering, which was followed by the Hot pressing (SMECH) process. The microwave dielectric properties of the composites were studied using X-band waveguide cavity perturbation technique. The temperature coefficient of the relative permittivity of the composites was investigated in the 0–100 1C temperature range using a hot and cold chamber coupled with an impedance analyzer. The temperature coefficient of the relative permittivity of the composites showed strong dependence on the temperature coefficient of the relative permittivity of the filler material. In the present study, a high-permittivity polymer/ ceramic composite, having ter 63 ppm/K, has been realized. This composite is suitable for outdoor wireless applications. & 2011 Elsevier B.V. All rights reserved.
Keywords: Dielectric properties Polymer/ceramic composites Temperature stability PTFE
1. Introduction The focus of the high-frequency electronics industry is changing from conventional, military-based applications to a more entertainment-oriented consumer market. Polymer/ceramic composites offer a wide range of possibilities for making flexible materials with tailored electrical and mechanical properties, which is done to cater to the rapidly advancing requirements of the modern microelectronics industry [1]. Moderate relative permittivity (er), a low loss tangent (tand) and a low temperature coefficient of permittivity ðter Þ are the major parameters that need to be satisfied by the materials used for various microelectronic applications. Whereas a high relative permittivity can reduce the circuit size, a low loss tangent is needed for signal integrity. Besides the relative permittivity and loss tangent, the temperature coefficient of permittivity ðter Þ is the most critical parameter, because it affects high frequency devices when used in outdoor applications [2,3]. The variation of relative permittivity with respect to temperature will shift the operating frequency of the device during its operation, because they are related by tf ¼ 1/2ter a , where ‘‘tf’’ is the temperature coefficient of resonant frequency and ‘‘a’’ is the linear coefficient of thermal expansion [4]. It is reported
n
Corresponding author. E-mail addresses: [email protected] (S. Rajesh), [email protected] (R. Ratheesh). 0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.08.080
that if ter has a value of10 ppm/K, then there will be a 0.11% shift in the resonant frequency within a temperature range from 30 to þ80 1C [3]. In order to compensate for the changes caused by a large temperature coefficient of permittivity, additional circuitries or mechanical structures are required, but, again, this type of addition increases the size and cost of the devices. An alternative technique that may be used to circumvent this problem is to use temperature-stable, passive components in high-frequency integrated circuits [3,5]. Polymer/ceramic composites are the least exploited material systems to realize temperature-stable dielectric properties for high-frequency communication applications [6]. A glance at the commercially available flexible materials for highfrequency packages reveals that there are very few available materials that have both high relative permittivity and a low temperature coefficient of relative permittivity. Table 1 shows some of the flexible packaging materials that are most commonly used for high-frequency applications. Among these, only TMM10s has a very low temperature coefficient of relative permittivity. TMMs is a ceramic-filled thermoset material, and it suffers from high moisture absorption and low chemical resistance [7,8]. On the other hand, thermoplastic matrix composites, which are ideal for most high-frequency applications, have a relatively high dependence of relative permittivity with respect to temperature. Although considerable amount of work has been carried out to achieve temperature stability in ceramics, less attention has been
S. Rajesh et al. / Physica B 406 (2011) 4312–4316
Table 1 The electrical properties of certain high-frequency packaging materials. Trade name
e’r
tand
ter (ppm/K)
Reference
FR-4TM RT/duroid 6006TM RT/duroid 6010TM TMM10TM
4.4 6.15 10.2 9.2
0.05 0.0027 0.0023 0.0022
160 410 425 38
[3] [7] [7] [8]
Table 2 The density and dielectric properties of sintered BPT, BST and Rutile ceramics. Ceramic
Bulk density (g/cc)
e’r
Qxf (GHz)
t ’ (ppm/K)
BaPr2Ti4O12 BaSm2Ti4O12 Rutile[14]
5.84 5.79 4.23
88 76 100
1800 2400 10000
300 þ 30 750
er
4313
ground in an agate mortar again, and the phase formation was studied using the powder X-ray diffraction technique (CuKa, Bruker 5005, Germany). To preclude moisture absorption, the ceramic powders were treated with a silane coupling agent, as explained elsewhere [16]. In order to study the microwave dielectric properties of the filler materials, cylindrical pellets were made out of the ceramic powder by uniaxial pressing under a pressure of 250 MPa. The green pellets thus obtained were sintered at a temperature in the range between 1400 and 1450 1C for 4 h. The particle size distribution of the fillers was studied using a Microtrack X100, USA, particle size analyzer. The microwave dielectric properties of the cylindrical compacts were studied by the Hakki and Coleman post resonator technique using a Vector Network Analyzer (Agilent PNA E8362B, USA) [17]. The temperature coefficient of permittivity was measured in the temperature range between 30 and 100 1C using a hot and cold chamber (Clitech, India) coupled with an Impedance Analyzer (Hewlett-Packard 4192 A, USA). The temperature coefficient of relative permittivity was calculated from Eq. 1:
paid to controlling the temperature-induced frequency drift in polymer ceramic composites [9,10]. Walpita et al. studied the temperature stability of polyphenylenesulfide (PPS)/SrTiO3 composite substrates by evaluating the performance of a patch antenna, which was fabricated using this substrate. In this study, the temperature coefficient of the permittivity of PPS/SrTiO3 composites was controlled by adding secondary ceramic fillers, such as alumina and mica [11]. The use of ceramic fillers with different morphologies and physical properties may lead to uneven filler distribution, and use of such fillers can also cause anisotropic dielectric properties. In another study, Nisa et al. reported temperature-stable PEEK composites by controlling the SrTiO3 filler content [12]. However, temperature-stable composites were obtained far below the optimum filler loading and, hence, the composites exhibited degraded thermal properties and a low order of dielectric isotropy. Recently, Xiang et al. studied the temperature dependence of the relative permittivity of polyolefin elastomer/SrTiO3 composites, and they found this property to be relatively high ( 1480 ppm/K) [13]. In the present study, different ceramic fillers, viz., rutile, BaPr2Ti4O12 and BaSm2Ti4O12, were loaded in the PTFE matrix, near optimum filler fractions, in order to reduce the temperature dependence of the relative permittivity of the composites. Because of its excellent electrical and mechanical properties [14,15], poly(tetrafluroethylene) (PTFE) was selected as the polymer matrix. PTFE has a relative permittivity of 2.1 and a loss tangento0.0003, and it also has a relatively high service temperature ( 250 1C) [14,15]. In the present work, fillers having different temperature coefficient of permittivity ðter Þ were incorporated in the PTFE matrix near to the optimum filler loading, and we studied the effect such fillers have on the ter values of the resulting composites.
where er30 is the relative permittivity at room temperature and Der/DT is the change in relative permittivity with respect to temperature. The PTFE/ceramic composites were prepared through Sigma Mixing, Extrusion and Calendering, followed by the Hot pressing (SMECH) process, which has been explained elsewhere [18]. H71 grade PTFE (HFC, India) powder with a size of 50 mm was used for the preparation of the composites. An appropriate amount of the starting materials, i.e., the filler and the matrix, were weighed and mixed using a sigma mixer by adding dipropylene glycol (Sigma Aldrich, USA) as a lubricant. Thus, the dough-like mass that was obtained was extruded to make a preform of the desired shape. It was then calendered to a thickness of 100 mm using a calendering machine. The differential speed of the rollers of the calendering machine ensured that there was a uniform distribution of the ceramic filler in the PTFE matrix. The calendered green tapes were stacked on top of each other and were hot pressed at a temperature of 350 1C and a pressure of 210 kg/cm2 [18]. In the present case, the initial thickness was chosen so as to obtain a final thickness of 625 mm after hot pressing. Using a Vector Network Analyzer (Agilent PNA E8362B, USA), the microwave dielectric properties of the samples were measured at X-band frequency region by the waveguide cavity perturbation technique [19]. The temperature coefficient of relative permittivity of the composite samples was calculated using Eq. 1 in the 0– 100 1C temperature range. The morphology of the composites samples was studied by Scanning Electron Microscopy (SEM) (Philips XL-30, The Netherlands). The samples were freeze fractured in liquid nitrogen for cross-sectional SEM pictures.
2. Experimental
3. Results and discussion
Rutile, BaPr2Ti4O12 and BaSm2Ti4O12 were used as the filler materials. Whereas rutile-grade TiO2 (4 99%) was purchased from M/s Sigma Aldrich, USA. The BaPr2Ti4O12 and BaSm2Ti4O12 filler materials (which are referred to as BPT and BST, respectively, in the rest of this paper) were prepared through the solid-state ceramic method. BaCO3 (499%, Sigma Aldrich, USA), Sm2O3 ( 499% IRE, India), Pr6O11 ( 499%, IRE, India) and TiO2 ( 499%, Sigma Aldrich, USA) were weighed according to their stoichiometric proportions and mixed in an agate mortar. The slurry was dried using a hot air oven ( 80 oC) and calcined at 1200 1C for 4 h with a heating rate of 10 1C/min. The calcined powders were then
The powder X-ray diffraction patterns of BPT and BST powders that were calcined at 1200 1C for 4 h are shown in Fig. 1. The patterns are indexed on the basis of ICDD file no. 43-235, which confirms that the phase formation was completed during calcination. Both BaPr2Ti4O12 (BPT) and BaSm2Ti4O12 (BST) are composed of perovskite-like blocks joined to one another in the tetragonal tungsten bronze pattern [20]. The relative permittivity and quality factors of BPT and BST, which were measured using the Hakki and Colemann Post resonator technique, are shown in Table 2. Both materials have high relative permittivity values and moderate quality factors, which has already been shown [20,21]. Compared
ter ¼
1
er30
Der DT
ð1Þ
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Fig. 1. Powder X-ray diffraction patterns of (a) BaPr2Ti4O12 and (b) BaSm2Ti4O12 calcined at 1200 1C/4 h.
Microscopy (SEM), and representative pictures are shown in Fig. 2. A very smooth surface texture without any irregularity is clearly visible in Fig. 2a, which was taken at a lower magnification. A regular surface morphology is very important for these materials, because the particulate fillers flow/rearrange during the hot pressing process. The freeze-fractured surface (Fig. 2b) shows uniform distribution of the ceramic particulates throughout the PTFE matrix. This demonstrates the efficacy of the processing methodology that was employed for the fabrication of planar composite laminates. The SEM studies show that the composites have minimal porosity. This result agrees with the experimental density values, which predict o10% porosity in the composite samples under study (Fig. 3). The relative permittivity of filled PTFE composites was studied as a function of filler loading. Because the main objective of the present investigation is to study the effect of the filler on the temperature coefficient of the relative permittivity of the PTFE composites, the fillers are selected judiciously. Rutile has high negative temperature coefficient of relative permittivity ðter Þ, BST has a nearly temperaturestable relative permittivity, although it is slightly on the positive side. BPT is a high-permittivity and low-loss ceramic having a ðter Þ value that is in between the corresponding values for rutile and BST. Previous reports show that nearly isotropic and dimensionally stable composite substrates having reproducible microwave dielectric properties necessitates an optimum filler loading in the PTFE matrix [16,18]. The effect of rutile filler loading on the microwave dielectric properties has been studied in detail and reported by us [18]. For a rutile filler having a particle size of 5 mm, the optimum filler loading was around 67 wt%. In the present case, the filler content in the PTFE matrix is varied, but it is near to the optimum filler loading (52–72 wt%) because all three of the fillers have very similar particle sizes and size distributions. The dielectric properties of the planar composite samples are measured in the X-band frequency region, and the results are shown in Fig. 3. The relative permittivity shows an increasing trend with filler loading. The main reasons for this are the higher relative permittivity of the filler, compared to that of the matrix, and the increasing amount of the interface region. For both BST and BPT-loaded composites, 67 wt% of the filler loading is observed to be the optimum filler condition, with relative permittivity values of 8.2 and 7.6, respectively. Among the three composite samples, the rutile-loaded composite showed the highest relative permittivity ðe0r 10:2Þ. The decrease in relative permittivity beyond the optimum filler loading is mainly attributed to the increase in porosity and the deterioration of the 0–3 connectivity of the composite system. The reduction in density beyond the optimum
Fig. 2. (a) A planar SEM picture of 67 wt% BPT and (b) a cross sectional SEM picture of 67 wt% BST-loaded PTFE composites.
to the near-zero positive temperature coefficient of relative permittivity of BST, BPT had a high negative temperature coefficient of relative permittivity. On the other hand, rutile has a very high negative temperature coefficient of relative permittivity. The particle size analysis shows that all three fillers have a very similar particle size distribution, with d50 value between 3 and 5 mm. The morphology of the composites and the distribution of the filler in the PTFE matrix are studied using Scanning Electron
Fig. 3. Variation of experimental density and relative permittivity (in the X-band region) of composites with respect to the filler’s content.
S. Rajesh et al. / Physica B 406 (2011) 4312–4316
filler loading is also evident from Fig. 3. A filler loading beyond 72 wt% in the PTFE matrix can be considered as an over-loaded condition for these composites. The loss tangents of the composites are measured in the X-band frequency region as a function of the filler loading (Fig. 4). All the composites have low loss tangent values up to the optimum loading level, and, beyond that, the composites exhibited relatively high loss tangents. The increase in porosity beyond the optimum filler loading may be the reason for this increase. Rutile-filled composites show the lowest loss tangent, which may be due to the high quality factor of rutile compared to that of the other two fillers. The temperature coefficient of relative permittivity is a critical parameter for high-frequency packaging materials, which are used in outdoor applications. A slight variation in the permittivity relative to the temperature will shift the operating frequency of any devices which were fabricated using such materials. The relative permittivity of the composites is measured in the 0–100 1C temperature range in intervals of 10 1C. The ter values of the composites are calculated using Eq. 1, and the results are plotted as a function of the filler loading (Fig. 5). Because of the inferior density values of the composites, the ter at 72 wt% filler loading is not taken into consideration. All three of the composites showed similar trends in the variation of ter with respect to the filler loading. The ter increased positively with the increase of
Fig. 4. Variation of the loss tangent (tand) with respect to the filler loading.
Fig. 5. Variation of the temperature coefficient of permittivity with respect to the filler loading.
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the filler content in the PTFE matrix. PTFE has a high negative temperature coefficient of relative permittivity, viz., ter 400 ppm=K [18]. When it is loaded with rutile having a higher negative ter , the effective values of ter of the composites vary from 630 to 425 ppm/K for filler loadings of 52 to 67 wt%. When PTFE is loaded with BST, which has a low positive temperature dependence, the value of ter of the composites increases in the positive direction with respect to the filler addition. BPT has a ter 2300 ppm=K, which is in between that of the other two fillers (rutile and BST). Also, the PTFE/BPT composite resulted in a ter of þ63 ppm/K at 52 wt%, which increased with additional filler loading (Fig. 5). e0 1 @a 3aL te0 r ¼ r ð2Þ 3 a @T The most important parameters that control the temperature coefficient of relative permittivity are the change in the polarizability of the material with respect to its temperature and its linear coefficient of thermal expansion (CTE) (Eq. 2) [4]. In this equation, the first term represents the change in polarizability of the system, and the second term represents the linear coefficient of thermal expansion. In general, particulate fillers have different relative permittivities and, hence, the percentages of effective polarizable ions in the composites are also different. The parameters that control the change in polarizability of the composites with respect to temperature are not completely understood. Previous studies show that the CTE of filled PTFE composites decreases with the increasing filler loading [22]. At optimum filler loading, it is reported to be around 20 ppm/K, irrespective of the nature of the filler [22,23]. While the filler content is increased from 52 wt% to 67 wt%, the change in CTE should be marginal for all three of the composites. The ter of the composites measured at 67 wt% filler loading is shown in Fig. 6. It is clear from the figure that the ter of the filler has a significant influence on the ter of the filled PTFE composites. When the filler had a high negative ter , the composite exhibited a high negative ter . But for a temperature-stable filler, viz., BST, the filled composite showed a high positive ter . Interestingly, because BPT had a slightly higher negative ter , the filled PTFE composites had a temperature variation of relative permittivity that was close to zero. These results show that, in order to arrive at a temperature-stable composite system, the temperature coefficient of relative permittivity of PTFE should be compensated for with the ter of the ceramic filler. In other words, the particulate filler must have an appropriate temperature variation of relative
Fig. 6. Variation of the temperature coefficient of permittivity for different fillers at 67 wt% loading.
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permittivity that can negate the changes in the relative permittivity of the polymer matrix with temperature.
4. Conclusions High-permittivity ceramic fillers were prepared through the solidstate ceramic preparation method The phase purity of the powders was studied using the powder X-ray diffraction technique. The Hakki and Coleman technique was employed to study the microwave dielectric properties of the well-sintered ceramic compacts. Filled PTFE planar composites were prepared through the SMECH process, and the distribution of ceramic particulates in the PTFE matrix was evaluated using SEM studies. All three of the composites showed an optimum loading of around 67 wt% of the filler content. For all of the composites that were studied, the temperature coefficient of relative permittivity showed a similar trend with respect to the ceramic loading. BPT-filled PTFE composites showed a ter þ130 ppm=K at optimum loading (67 wt%), which decreased to þ63 ppm/K as the amount of filler in the PTFE matrix was reduced to 52 wt%. The present study shows that temperature-stable composites could be prepared by judiciously selecting filler materials so that they have a suitable temperature coefficient of relative permittivity, which can compensate for the temperature dependence of the relative permittivity of the polymer matrix.
Acknowledgment The authors are grateful to Dr. K. R. Dayas, Director, C-MET, Thrissur, India, for extending the facilities to carry out this work.
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