Modified BCZN particles filled PTFE composites with high dielectric constant and low loss for microwave substrate applications

Ceramics International xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Ceramics International journal homepage: www.elsevier.com/locate/ceramint

Modified BCZN particles filled PTFE composites with high dielectric constant and low loss for microwave substrate applications Hao Wanga, Fuming Zhoua, Jianming Guoa, Hui Yanga, Jianxi Tongb, Qilong Zhanga,∗ a b

School of Materials Science and Engineering, State Key Lab Silicon Mat, Zhejiang University, Hangzhou, 310027, PR China Jiaxing Glead Elect Co Ltd, Jiaxing, 314003, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Composites Dielectric properties Niobates Substrates

The modified 0.7Ba (Co1/3Nb2/3)O3-0.3Ba(Zn1/3Nb2/3)O3 (BCZN) powders filled PTFE composites were synthesized by hot-pressing. The influences of BCZN content on the microstructure, dielectric, thermal, mechanical properties and moisture absorption were investigated systematically. The modified BCZN powders filled PTFE composites exhibited better microstructure and dielectric properties compared with untreated powders. Various mathematic models were utilized to predict the dielectric constant of different composites and the effective medium theory (EMT) showed perfect consistency with the experimental results. The modified BCZN/PTFE composites possess the best comprehensive properties at the powders content of 50 vol% with high dielectric constant (εr) of 7.7, low loss (tanδ) of 0.0014, acceptable temperature coefficient of dielectric constant (τε) of −125.6 ppm/°C and temperature coefficient of resonant frequency (τf) of 29.4 ppm/°C at 7 GHz, low moisture absorption of 0.07% and low coefficient of thermal expansion (CTE) of 33 ppm/°C. All the results show modified BCZN/PTFE composites are the potential materials for microwave substrate applications.

1. Introduction

substrate materials for application should possess the acceptable comprehensive properties including the dielectric, thermal and mechanical properties. In the previous works about high dielectric constant PTFE composites, the Ba(Mg1/3Ta2/3)O3, Ba4.2Nd9.2Ti18O54, TeO2, Li2Mg3TiO6, (Na0.6Li0.4)0.5Nd0.5TiO3, Li2TiO3 ceramic powders had been added into the PTFE matrix to fabricate composites for microwave substrate applications [19,20,23–26]. All the works mostly concentrate on the research of dielectric properties of substrate materials and don't pay enough attention to other significant properties for the practical application to a certain extent, such as low coefficient of thermal expansion and low moisture absorption. There still exist some challenges in the fabrication of high dielectric constant PTFE-based composites with acceptable comprehensive properties, which is largely due to the difficulty in synthesis of high-quality ceramic fillers, surface modification of ceramic fillers and fabrication method. 0.7Ba(Co1/3Nb2/3)O3-0.3Ba(Zn1/3Nb2/3)O3 (BCZN) microwave dielectric ceramic had been proved to be the potential materials with perfect dielectric properties (εr = 33.5, Qf = 80000 GHz, τf = 4.2 ppm/°C) [27]. Therefore, it's very reasonable to predict that the PTFE-based composites filled with BCZN ceramic powders could exhibit outstanding dielectric properties. However, another significant problem existed in the polymer-ceramic composites is the incompatible surface between polymer matrix and fillers [28]. In order to enhance

In recent years, the 5G wireless communication systems, high integrated circuits, satellite communication systems, collision avoidance systems, radar technology and navigation systems are developing continually at an unexpectable speed [1–3], which proposes the huge demands for advanced substrate materials, especially the guarantee of high-speed and high-frequency signal transmission in the electronic equipment [4]. Currently, the composite substrate materials consisted of polymer matrix and inorganic fillers are the hot topic, due to the advantages of the excellent comprehensive properties and simple fabrication process [5–12]. Among these researches, PTFE shows the great potential in the advanced substrate composites because of its low εr, extremely low tanδ, low moisture absorption, high chemical and thermal stability. Therefore, different kinds of inorganic fillers were introduced into PTFE to fabricate the composites for microwave substrate applications. The PTFE-based composites with low εr has attracted too much attention in the past [13–18], and recently high dielectric constant composites gradually come into researchers' sight because of the application in the specific situation [8,19,20]. However, in order to ensure the normal operation of substrates in electronic circuits, just owning the excellent dielectric properties is not enough [21,22]. The appropriate ∗

Corresponding author. E-mail address: [email protected] (Q. Zhang).

https://doi.org/10.1016/j.ceramint.2019.11.252 Received 8 October 2019; Received in revised form 21 November 2019; Accepted 27 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Hao Wang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.252

Ceramics International xxx (xxxx) xxx–xxx

H. Wang, et al.

the compatibility of different components, lots of surface modification agents (such as KH550, PTMS, TEOS, TBT and F8261) had been explored by many researchers in the previous works [12,28–31]. The coupling agent of F8261 with functional groups of –OCHCH3 and –(CF2)5CF3 had attracted more and more attention in recent years because of the special “bridge” effect constructed by F8261 between the ceramic powders and PTFE polymer [15]. In this work, BCZN ceramic powders were modified by F8261. Then, different volume fractions (20 vol%-70 vol%) of BCZN powders were added into PTFE to fabricate composites using hot-pressing. The influences of BCZN content on the microstructure, dielectric, thermal and mechanical properties of the composites were expounded systematically. As a result, the high dielectric constant, low dielectric loss composites with low moisture absorption and low coefficient of thermal expansion were fabricated when the content of BCZN ceramic powders is 50 vol%. 2. Experimental procedures 2.1. Synthesis of BCZN powders Fig. 1. X-ray diffraction pattern of BCZN powders and BCZN/PTFE composites.

BCZN ceramic powders were fabricated by solid-state route. BaCO3, CoO, ZnO and Nb2O5 with 99% purity were weighed according to the stoichiometric ratio of 0.7Ba(Co1/3Nb2/3)O3-0.3Ba(Zn1/3Nb2/3)O3. Then, the powders were ball milled with ethanol medium for 24 h. After drying at 60 °C for 12 h, the powders were calcined at 1200 °C for 4 h. In order to obtain BCZN powders with uniform particle size, the sintered ceramic was ball milled again for 12 h and dried in 60 °C.

EMPYREAN, PANalytical Co., the Netherlands) analysis with Cu Kα radiation. The change of contact angle on the surface of BCZN powders was revealed by the video-based contact angle measuring device (OCA 20, Dataphysics, Germany). The dielectric properties of BCZN/PTFE composites were tested by Agilent E5071C microwave network analyzer using Hakki-Coleman method. The universal materials testing machine (CMT5205) was utilized to measure the flexure strength according to IPC-TM-650 2.4.4B. Coefficient of thermal expansion (CTE) of all the samples was characterized by TMA Q400EM according to IPCTM-650 2.4.41. The laser particle analyzer (Beckman Coulter LS13320, USA) was used to research the particle size distribution of BCZN powders. The moisture absorption was calculated according to IPC-TM-650 2.6.2 and the thermal conductivity was measured by Hot Disk TPS 2500 S in the isotropic pattern.

2.2. BCZN powders surface modification The modification procedures included two parts: the pre-hydrolysis of F8261 and surface treatment of BCZN powders. Firstly, C14H19F13O3Si (TCI Corporation, Japan) was added into the ethanol and mixed for 1 h, the content of F8261 was 2 wt% of BCZN powders, then the deionized water was mixed in the solution and the content was controlled precisely for the hydrolysis of surface modification agent. All solution was put into the water bath at 55 °C for 1 h to accomplish the process of pre-hydrolysis. Next, the untreated BCZN ceramic powders were mixed with ethanol and ultrasonic treatment for 1 h was used to improve the uniformity of powders in the solution. Then, all the hydrolyzed C14H19F13O3Si solution was added into the suspension composed of BCZN powders and ethanol and mixed at 60 °C for 6 h. At last, the modified BCZN powders were obtained by drying at 120 °C for 12 h.

3. Results and discussion 3.1. Microstructure analysis Fig. 1 shows that the peaks of BCZN ceramic match with the results in the work of Ahn [27]. The continually decreasing peaks with the volume fraction of BCZN increasing at about 17° are correspond to the PTFE phase (#JCPDS number 00-054-1595). Due to that the height of peak in XRD pattern is related to the content of every component, the density of PTFE in the composites is changing along with the content of BCNZ ceramic increasing. Meanwhile, the position of peaks of BCZN ceramic in the composites is the same with pure BCZN ceramic and there are no other peaks and second phase appearing in the XRD patterns, which reveals that no reaction occurs between BCZN ceramic powders and PTFE matrix. Therefore, the BCZN powders and PTFE form the heterogeneous mixture and show excellent chemical stability [24]. In order to characterize the modification of F8261 in BCZN, XPS full spectrum was employed to detect the F element in the modified and untreated powders, the grafted organic groups were identified by carbon peak fitting and FTIR. Meanwhile, to highlight the great change of hydrophobicity of BCZN powders caused by the modification of F8261, the contact angle between water and untreated, modified BCZN powders was investigated. As depicted in Fig. 2, the elements including zinc, barium, cobalt, oxygen, carbon and niobium are all revealed in the XPS full spectrum of both the untreated and modified BCZN powders and the positions of different peaks are nearly the same [32,33], because the BCZN ceramic was consisted of the above components. But the F element can be only detected in the modified powders and no

2.3. Synthesis of BCZN/PTFE composites The BCZN ceramic powders were weighed according to the different volume fraction (20 vol%-70 vol%), the powders and PTFE suspension (DISP30; Dupont, China) were mixed by ball milling for 3 h. Then the mixture underwent heat treatment at 100 °C for 12 h to remove the water absorbed in the composite and heated to 280 °C for 6 h to wipe off the active agent existed in PTFE suspension. Finally, the composites were fabricated by preforming and hot-pressing sintering (25 MPa, 360 °C) for 2 h. 2.4. Characterization The morphology features of raw BCZN powders, modified BCZN powders, the surface and intersecting surface of BCZN/PTFE composites were observed by a field emission scanning electron microscopy (FESEM; SU8010, Hitachi Ltd, Japan). The density and porosity of all the specimens with various volume fractions of ceramic powders were measured by Archimedes’ method. X-ray photoelectron (XPS; Al, Kα, AXIS SUPRA, England) was employed to characteristic the modification of ceramic powders. The crystal phase of BCZN powders and BCZN/ PTFE composites was characterized by X-ray powder diffraction (XRD, 2

Ceramics International xxx (xxxx) xxx–xxx

H. Wang, et al.

the full spectrum, the untreated powders show just one obvious peak, while the modified powders exhibit different carbon peaks, which indicates that there exist the changes of carbon states after the modification of F8261. In order to investigate the detailed changes of carbon states, the carbon spectrum was characterized. It's obviously shown in Fig. 3, five carbon states with different energy appear in carbon spectrum analysis, which are C–F3, C–F2, C–F, C1s and C–H (or C), with energy of 292.75eV, 290.46ev, 289.5eV, 284.6ev and 283.5eV [33], respectively. However, there is only one carbon state which can be detected and especially the C–F3 and C–F2 can't be discovered in the untreated BCZN powders. The appearance of C–F3, C–F2, C–F and C–H peaks is mainly attributed to the long chain of (CH2)2-(CF2)5-CF3 exited on the surface of BCZN powders, and the intensity of every peak is corresponding to the quantity of every organic group in F8261, which can prove that F8261 has been introduced into the BCZN. Fig. 4 shows the contact angle improves from 15.6° to 130° when the untreated BCZN powders were modified by F8261, indicating the surface nature of BCZN powders changes from hydrophilia to hydrophobicity because the functional group of –(CH2)2(CF2)5CF3 can greatly improve the hydrophobicity. FTIR is an effective method to detect functional groups in materials. Fig. 5 depicts the FTIR spectrum of untreated BCZN powders and modified BCZN powders. The characteristic peaks of 3433 cm−1, 1612 cm−1 and 611 cm−1 are detected in both untreated and modified BCZN powders. While, the new absorption band at 2917 cm−1 appear in the modified BCZN powders, which is attributed to the –CH2- in F8261. Due to that the characteristic absorption bands of functional groups including C–F, C–F2 and C–F3 are complex and readily affected by other groups, the accurate peak positions of these groups are difficult to confirm. The absorption peaks in the range of 1000 cm−1–1500 cm−1 of both FTIR spectrums are totally different and many new peaks appear in the modified BCZN powders, which could be imputed to the functional group of –(CF2)5CF3 in the surface modification agent. In brief, all the above results of XPS full spectrum, carbon peak fitting, contact angle and FTIR can draw a conclusion that the BCZN ceramic powders have been coated and modified by F8261 successfully. Because of the functional group of –(CF2)5CF3 in F8261, especially the -C-F2- component similar to PTFE, the binding force between the BCZN powders and polymer matrix are predicted to be enhanced through the modification. The properties of BCZN/PTFE composites are affected by the individual property of every component for the most part. However, the particle size of ceramic powders, morphology, dispersibility and bond ability between powders and polymer matrix are also the important factors [34,35]. Hence, the particle size distribution is investigated by laser particle analyzer, the morphology features of untreated BCZN powders, modified BCZN powders, surface and the cross section were all characterized by SEM. The SEM image of untreated BCZN powders is shown in Fig. 6(a) and the surface of powders are very rough. Meanwhile, the agglomeration of the powders is meanly due to the

Fig. 2. XPS survey scan spectrum of (a) untreated BCZN powders and (b) modified BCZN powders.

Fig. 3. Carbon peak fitting spectrum of modified BCZN powders.

peak appears at the same binding energy position in the untreated BCZN powders, which is attributed to the F element existed in the functional group of –(CF2)5CF3 in F8261. There are many organic groups containing carbon existed in F8261, which provides a new approach to confirm the modification. With respect to the carbon peaks in

Fig. 4. Contact angle of water droplet on the (a) untreated BCZN powders and (b) modified BCZN powders. 3

Ceramics International xxx (xxxx) xxx–xxx

H. Wang, et al.

Fig. 7. Particle diameter distribution of BCZN powders after ball milling again. Fig. 5. FTIR spectrum of (a) untreated BCZN powders and (b) modified BCZN powders.

which is owing to that the F8261 can improve the dispersibility and reduce the reunion of BCZN ceramic powders. Fig. 9 depicts the microstructure of cross section belongs to modified BCZN/PTFE composites containing various volume fraction of 20 vol%-60 vol% (a-e) and untreated BCZN/PTFE composites with 40 vol% (f) ceramic powders. The cross-sectional SEM graphics of all the modified BCZN/PTFE composites show the tight microstructure and the modified BCZN powders are distributed uniformly in PTFE, no agglomeration of powders or PTFE appears at the content of modified powders is in the range of 20 vol%-50 vol%. However, the composites with 60 vol% ceramic powders show the phenomenon of powders agglomeration. It's mainly attributed to the low content of PTFE matrix, which is not enough to coat the powders completely. To protrude the influence of F8261 on microstructure of cross section, the SEM image of untreated BCZN/ PTFE composites at the content of 40 vol% BCZN is used for comparing with the modified BCZN/PTFE composites. It's obvious that the untreated BCZN/PTFE composites possess more pores, agglomeration among powders and the loose microstructure, which is on account of the weak interaction between untreated BCZN powders and PTFE matrix compared with the modified BCZN/PTFE composites. The most important reason for the weak interaction is the huge discrepancy of surface energy between untreated BCZN ceramic powders (high surface energy) and PTFE (extreme low surface energy). Hence, the distribution, interaction with the polymer matrix of the ceramic powders are all improved by the modification of F8261, and the BCZN/PTFE composite substrate material with tight and homogeneous microstructure are obtained. In order to depict ulteriorly the uniform distribution of ceramic

hydrophilia nature, high surface energy, which causes the strong interaction force among the raw ceramic powders. However, the welldistribution, less agglomeration and smooth surface of the modified BCZN powders in Fig. 6(b) are mainly attributed to the coated layer of F8261, which can decrease the interaction among the BCZN ceramic powders [36]. It's depicted in Fig. 7 that the particle diameter exhibits the normal distribution in the range of 0.04 μm–1.5 μm, and the mean particle diameter of BCZN ceramic powders which are ball milling again is 0.456 μm, which indicates the particle size of BCZN powders is well controlled by ball milling. The morphology of the surface of modified BCZN/PTFE composites with 20 vol%-60 vol% ceramic powders (a-e) and the untreated BCZN/ PTFE with 40 vol% ceramic powders (f) are shown in Fig. 8. It's obvious that in the range of 20 vol%-50 vol% ceramic powders, the surface of all the samples is smooth and pyknotic because the PTFE polymer can coat the BCZN ceramic powders completely when the volume fraction of filler is low. However, many pores and defects appear on the surface, and the surface becomes to change from smoothness to toughness when the content of BCZN is 60 vol%, which could be due to that at the highvolume fraction of BCZN powders, the PTFE can't coat the ceramic. Then, the powders begin to be exposed on the surface of composites. Meanwhile, from the comparation between the modified (c) and untreated (f) BCZN powders/PTFE with the same volume fraction (40 vol %), there are many pores on untreated BCZN/PTFE composites surface, while the surface of modified BCZN/PTFE composites is more smooth,

Fig. 6. SEM micrograph of (a) untreated BCZN powders and (b) modified BCZN powders. 4

Ceramics International xxx (xxxx) xxx–xxx

H. Wang, et al.

Fig. 8. Surface SEM images of modified BCZN/PTFE composites, (a) 20 vol%, (b) 30 vol%, (c) 40 vol%, (d) 50 vol%, (e) 60 vol% and untreated BCZN/PTFE composites, (f) 40 vol%.

Fig. 9. Cross sectional SEM images of modified BCZN/PTFE composites, (a) 20 vol%, (b) 30 vol%, (c) 40 vol%, (d) 50 vol%, (e) 60 vol% and untreated BCZN/PTFE composites, (f) 40 vol%.

proportion of modified BCZN powders increases, the possibility of the contact among the powders increases, which would cause the higher porosity. While, the high porosity of the substrate materials will bring out the high moisture absorption. It's noteworthy that the porosity and moisture absorption all keep the low values at the range of 20 vol %-50 vol% ceramic powders, while when the content of BCZN powders is 60 vol%, the relative density, porosity and moisture absorption all show the great change, which is mainly attributed to that PTFE can coat powders completely at low content of BCZN powders, and the contact among powders keep in the low level. When the content of BCZN ceramic powders is 60 vol%, PTFE is not enough to coat the powders, contact among powders increases a lot, which causes the high porosity and high moisture absorption, which exhibits the same regularity with the variation of microstructure of different samples. In general, the theoretical density ( ρ ) and moisture absorption (W) are calculated by the following equations:

powders and PTFE in the cross section of BCZN/PTFE composites, the elements mapping was investigated to describe the distribution of every element in the composites, such as barium, zinc, niobium, cobalt, fluorine and oxygen. As illustrated in Fig. 10, all the elements are welldispersed which also indicates the BCZN powders and PTFE polymer are well-dispersed and no serious agglomeration exists in the composites. Meanwhile, the content of all the elements in the 40 vol% BCZN/ PTFE composites was characterized by SEM-EDS in Fig. 10, which manifests the composites are composed of O, F, Co, Zn, Nb and Ba indeed and the content of elements is consistent with the proportion of BCZN powders and PTFE. 3.2. Density, porosity and moisture absorption As for substrate materials, the moisture absorption depends on the relative density, porosity and hydrophobicity of every component greatly. Meanwhile, the dielectric properties are also affected to a large extent by the porosity and moisture absorption. The dielectric properties will become to deteriorate when moisture absorption and porosity of the composites increase. As depicted in Fig. 11, the density, porosity and moisture absorption all show an increase with the volume proportion of BCZN ceramic filler increasing. The porosity is mainly influenced by the contact among the powders. Therefore, as the volume

ρ=

∑ ρn Vn n

Vn =

5

mn ρn m ∑n ρ n n

(1)

(2)

Ceramics International xxx (xxxx) xxx–xxx

H. Wang, et al.

Fig. 10. Corresponding element mapping of Ba, Zn, Nb, Co, F, O (a–f), EDS spectra (g) and element content of BCZN/PTFE composites with 40 vol% loading.

W=

m1 − m 0 m0

60 vol%, the εr and tanδ of untreated and modified BCZN/PTFE composites all show the growth. The εr of modified BCZN/PTFE composites changes from 3.9 to 9.2, and the tanδ exhibits an increase from 0.0006 to 0.002. The increase of εr with the variation of BCZN content is mainly ascribed to the higher εr of BCZN ceramic compared to PTFE polymer. The tanδ of ceramic-polymer composites largely relies on the porosity, defects and relative density. From the results about the relative density, porosity and SEM images of microstructure about BCZN/PTFE composites, there would be more and more pores and the relative density decreases continually with the volume proportion of fillers increasing, which can account for the increase of the dielectric loss [37]. Meanwhile, it's obvious that compared with untreated BCZN/PTFE composites, the modified BCZN/PTFE composites possess the superior properties of higher εr and lower tanδ, which is also related to the variation of porosity and relative density caused by the modification of F8261. The interaction force between PTFE polymer matrix and BCZN powders can be enhanced by the modification of F8261, which can reduce the

(3)

where ρn , Vn , mn represent the density, volume fraction and weight of the ceramic filler and polymer, respectively. While, the m1 and m 0 represent the weight after absorbing water and the original weight, respectively. 3.3. Dielectric properties As for the substrate materials applied in the microwave range, the dielectric properties including the εr, tanδ and temperature stability are of great concern. The dielectric properties are mainly contingent on the individual dielectric properties of the polymer and fillers, as well as the pores and defects existed in the composites. The εr and tanδ of untreated and modified BCZN/PTFE composites with different volume proportions of BCZN ceramic powders are described in Fig. 12 (a) and (b). When the volume fractions of BCZN powders vary from 20 vol% to

Fig. 11. Experimental density, relative density, porosity and moisture absorption of modified BCZN/PTFE composites. 6

Ceramics International xxx (xxxx) xxx–xxx

H. Wang, et al.

Fig. 12. The (a) dielectric constant and (b) dielectric loss of untreated and modified BCZN/PTFE composites, (c) comparison of experimental and theoretical dielectric constant and (d) the temperature stability of dielectric constant.

numerical value of m is 0.25, which depends on the morphology and microstructure of the composites. Nevertheless, n was often fixed at 0.3 in the majority of previous researches and in this research, n is equal to 0.1. Usually, n is the fitting factor which reflects the interaction between ceramic filler and polymer, and n changes continually with the variation of different polymer-ceramic composites system. The reduction of n is probably due to the modification of F8261, which largely enhances the binding force between BCZN ceramic and PTFE. The variation of theoretical dielectric constant calculated by the above four mathematical models, and comparation between the experimental dielectric constant of modified BCZN/PTFE composites and theoretical values are depicted in Fig. 12 (c). It's obvious that the theoretical values of the EMT and modified Lichtenecker model are consistent with the experimental dielectric constant, while the results of Maxwell-Wagner model are lower than the experimental values and the dielectric constant of Lichtenecker model shows a large separation with experimental values at the high-volume fractions of BCZN ceramic powders. The Maxwell-Wagner model is only suitable for the mixture consist of similar components. While, there exists a wide discrepancy of dielectric and mechanical properties between the BCZN ceramic and PTFE polymer matrix, which could cause the mismatching of MaxwellWagner model. Meanwhile, the Lichtenecker model regards the polymer-ceramic composites as the ideal and homogeneous mixture with random and nearly spherical fillers, while regardless of the pores and contact among the fillers. Hence, at the low volume fractions of BCZN ceramic powders (< 30 vol%), the experimental εr is well consistent with theoretical value. However, when the volume proportion of BCZN ceramic exceeds 30 vol%, the deviation between the numerical values calculated by Lichtenecker model and experimental dielectric constant becomes more and more large with the volume proportion increasing, which is attributed to the increase of porosity and contact among the fillers at the high-volume fraction of BCZN powders. Among

pores in the composites. It's obvious that the surface and cross-sectional SEM images of untreated BCZN/PTFE composites possess more pores and loose microstructure. However, the pores are the important reason for the reduction of εr and the increase of tanδ. All the analyzation are confirmed by SEM images of untreated and modified BCZN/PTFE composites for comparation. Ulteriorly, four frequently-used theoretical models are also introduced to research the theoretical εr of the modified BCZN/PTFE composites, which would be utilized to compare with the experimental values. Four equations for the theoretical models are listed as follows [13]: Maxwell-Wagner equation [38]

εeff = εm

2εm + εf + 2Vf (εf − εm) 2εm + εf − Vf (εf − εm)

(4)

Effective medium theory (EMT) [39]:

Vf (εf − εm) ⎤ εeff = εm ⎡1 + ⎥ ⎢ ε m (1 V )( ε ε ) + − − m f f m ⎦ ⎣

(5)

Lichtenecker equation [40]:

lnεeff = Vf lnεf + (1 − Vf ) lnεm

(6)

Modified Lichtenecker equation [41]:

εf lnεeff = Vf (1 − n) ln ⎛ ⎞ + lnεm ⎝ εm ⎠ ⎜

⎟

(7)

where εeff , εm , εf are the theoretical dielectric constant of the BCZN/ PTFE composites calculated by the above equations, the permittivity of polymer matrix and ceramic powders filler, respectively. Vf are the volume fraction of BCZN ceramic powders, m in equation is the morphology parameter, n in equation is a fitting factor. In the research, the 7

Ceramics International xxx (xxxx) xxx–xxx

H. Wang, et al.

the four mathematical models, the EMT model are the most suitable with the experimental data of all the volume fractions of BCZN ceramic powders filled PTFE composites, which is similar with the results in the CNT/PTFE composites [8] and LMT/PTFE composites [20]. Hence, all the analysis confirms that the experiment is credible and predictable. PTFE owns the τε of about −400 ppm/°C, which limits the application in the outdoor environment with large temperature variation. The BCZN ceramic powders with relatively stable τε can compensate this shortcoming effectively. The variation of τε of all the composites with various BCZN volume fractions in the temperature range of 25°C–85 °C are shown in Fig. 12 (d). Meanwhile, according to Guo's 1 work [4], the τε shows a linear relation with τf : τf = − 2 τε + αL . Hence, the τf of the modified BCZN/PTFE composites are also measured. It's obvious that the τε shows an increase from −334.7 ppm/°C to −75.2 ppm/°C and τf decreases from 79.9 ppm/°C to 10.7 ppm/°C with the content of BCZN powders increasing. And, τε and τf of the BCZN/ PTFE substrate materials are adjusted to −125.6 ppm/°C and 29.4 ppm/°C respectively when the loading of BCZN ceramic powders are 50 vol%, which is acceptable for the circuits and instruments requiring high dielectric constant substrate materials, which are applied in the microwave region.

(

)

Fig. 14. The coefficient of thermal expansions of BCZN/PTFE composites with different BCZN content.

3.4. Thermal and mechanical properties

60 vol%, which is due to that BCZN ceramic powders possess the much lower CTE than PTFE polymer matrix. And, the modified BCZN/PTFE composites gets the acceptable CTE of 33 ppm/°C with the filler loading of 50 vol%, which can prevent the delamination effectively. Fig. 15 describes the load-deflection curve and the variation about flexure strength of modified BCZN/PTFE composites with various BCZN volume fractions. It's obvious that the maximum binding force and flexure strength all exhibit the decrease with loading of BCZN increasing, and especially at the high content of BCZN powders (60 vol %), the flexure strength declines sharply to 6.35 MPa. The mechanical properties of ceramic filled polymer composites largely lie on crosslink density of polymer, porosity and interface defects existed in the composites. Usually, the addition of BCZN ceramic powders would reduce the crosslink density of PTFE matrix to some extent [42,43]. And, as shown in previous analysis about the porosity and microstructure of the composites, the porosity and defects are continually increasing as the loading of BCZN powders increases, which would reduce the interaction between BCZN ceramic and PTFE matrix, then resulting in the decrease of flexure strength. Meanwhile, in the range of BCZN volume fraction of 20 vol% to 50 vol%, the increase of porosity is not obvious, which is the main reason for the little decrease of flexure strength from 26.1 MPa to 23.3 MPa. However, the porosity of modified BCZN/PTFE composites shows the sharp increase at the BCZN ceramic loading of 60 vol%, which can account for the sharp decline of flexure strength. Finally, when the volume fraction of modified BCZN ceramic powders in the composites is 50 vol%, the acceptable flexure strength of 23.3 MPa for ceramic-enhanced PTFE composites is obtained. In order to show the contributions of BCZN/PTFE composites, the related works about PTFE-based composites with high constant are shown in Table 1. There is no doubt that the PTFE-based composites for microwave substrate application with the high dielectric constant (7.7), low tanδ (0.0014), low coefficient of thermal expansion of 33 ppm/°C, low moisture absorption of 0.07% and acceptable temperature coefficient of dielectric constant (−125.6 ppm/°C) exhibit good comprehensive properties and competitive advantages. Meanwhile, the BCZN/ PTFE composites also exhibit acceptable thermal conductivity and bending strength. All results indicate that 0.7Ba (Co1/3Nb2/3)O3-0.3Ba (Zn1/3Nb2/3)O3 (BCZN) ceramic is an ideal filler in PTFE for the microwave substrate materials.

The thermal conductivity of modified BCZN/PTFE composites is shown in Fig. 13. The thermal conductivity increases from 0.39 W/ (m·K) to 0.48 W/(m·K), as the loading of BCZN changes from 20 vol% to 60 vol%, which is due to that ceramic powders would connect with each other and construct the path for heat transmission with the loading of BCZN increasing. Finally, when the volume fraction of BCZN ceramic reaches 50 vol%, the composites with thermal conductivity of 0.46 W/(m·K) are fabricated. The substrate materials often need to bond with copper foil to fabricate the printed circuit board (PCB), which can be applied in the electronic equipment. However, the coefficient of thermal expansion of copper foil is about 18 ppm/°C, which is much smaller than the PTFE polymer matrix (about 109 ppm/°C). The wide difference of CTE existed in the copper foil and substrate materials could cause the delamination between each other, which would damage the circuits and restrict the application of substrate materials in outdoor environment. The CTE of modified BCZN/PTFE composites is measured in the temperature region of 25–100 °C. As depicted in Fig. 14, the CTE of the composites decreases from 87.5 ppm/°C to 26.94 ppm/°C, with respect to the increase of BCZN powders volume fraction from 20 vol% to

4. Conclusion Fig. 13. The thermal conductivity of BCZN/PTFE composites with different BCZN content.

The 0.7Ba(Co1/3Nb2/3)O3-0.3Ba(Zn1/3Nb2/3)O3 (BCZN) powders 8

Ceramics International xxx (xxxx) xxx–xxx

H. Wang, et al.

Fig. 15. Variation of load-deflection curve and the flexure strength of BCZN/PTFE composites. Table 1 Comparison of comprehensive properties of this work with other high dielectric constant PTFE-based composites. Filler

Loading

Dk

tanδ

f (GHz)

τε (ppm/°C)

CTE (ppm/°C)

Moisture absorption (%)

Reference

TeO2 Ba (Mg1/3Nb2/3) O3 (Ca, Li, Sm)TiO3 Ba (Mg1/3Ta2/3) O3 ZrTi2O6 Li2Mg3TiO6 (Na0.6Li0.4)0.5Nd0.5TiO3+Glass fiber Ba4.2Nd9.2Ti18O54 Ca0.5Nd0.3TiO3 CaTiO3 Li2TiO3 0.7Ba (Co1/3Nb2/3)O3-0.3Ba(Zn1/3Nb2/3)O3

60 68 50 76 46 70 60 70 50 40 60 50

5.6 5.84 6.22 6.7 7.42 6.17 10.4 10.6 12 13 6.8 7.7

0.006 0.0015 0.0012 0.003 0.0022 0.0019 0.0026 0.004 0.00085 0.0053 0.001 0.0014

7 10 10 X-band 10 10 10 11.5 10 5 8 7

– – – – −89 123 −0.9 – – – −29.6 −125.6

32 – – – 20 – – 18 35 – 28.3 33

– – – 0.072 0.15 0.16 0.34 – 0.27 – – 0.07

[25] [44] [45] [23] [46] [20] [19] [24] [8] [47] [26] This work

vol% wt% wt% wt% vol% wt% wt% wt% vol% vol% wt% vol%

National Natural Science Foundation of China (Grant No. 51772267) and the Key R&D Program of Zhejiang Province (Grant No. 2019C05001).

were modified by F8261 coupling agent and mixed with PTFE to fabricate substrate materials by hot-pressing. The variation of dielectric, thermal, mechanical properties, microstructure and moisture absorption of different composites with respect to BCZN content was researched systematically. With the content of BCZN powders increasing, the εr and tanδ all show an increase, and the tanδ maintain relatively low values which is attributed to the low tanδ of BCZN and the low porosity of the composites. The temperature stability of dielectric constant for PTFE were compensated by BCZN ceramic and the acceptable τε and τf were also obtained. Four mathematic models were investigated to calculate the theoretical εr of modified BCZN/PTFE composites and the effective medium theory (EMT) exhibited the best consistency with the experimental values. The modified BCZN/PTFE composites with 50 vol% BCZN ceramic powders own the high dielectric constant of 7.7, low dielectric loss of 0.0014 at 7 GHz, meanwhile possess other acceptable comprehensive properties: the τε of −125.6 ppm/°C and τf of 29.4 ppm/°C, low moisture absorption of 0.07%, low CTE of 33 ppm/°C and acceptable flexure strength of 23.3 MPa, thermal conductivity of 0.46 W/(m·K). Hence, the PTFEbased composites filled with modified BCZN ceramic powders are the potential materials applied in microwave substrate.

References [1] X. Zhou, Z. Jia, A. Feng, X. Wang, J. Liu, M. Zhang, H. Cao, G. Wu, Synthesis of fish skin-derived 3D carbon foams with broadened bandwidth and excellent electromagnetic wave absorption performance, Carbon 152 (2019) 827–836. [2] A. Feng, M. Ma, Z. Jia, M. Zhang, G. Wu, Fabrication of NiFe2O4@carbon fiber coated with phytic acid-doped polyaniline composite and its application as an electromagnetic wave absorber, RSC Adv. 9 (2019) 25932–25941. [3] Z. Gao, B. Xu, M. Ma, A. Feng, Y. Zhang, X. Liu, Z. Jia, G. Wu, Electrostatic selfassembly synthesis of ZnFe2O4 quantum dots (ZnFe2O4@C) and electromagnetic microwave absorption, Compos. B Eng. (2019) 107417. [4] J. Guo, X. Zhao, T. Herisson De Beauvoir, J.H. Seo, S.S. Berbano, A.L. Baker, C. Azina, C.A. Randall, Recent progress in applications of the cold sintering process for ceramic-polymer composites, Adv. Funct. Mater. 28 (2018) 1801724. [5] W. Li, W. Huang, Y. Kang, Y. Gong, Y. Ying, J. Yu, J. Zheng, L. Qiao, S. Che, Fabrication and investigations of G-POSS/cyanate ester resin composites reinforced by silane-treated silica fibers, Compos. Sci. Technol. 173 (2019) 7–14. [6] K.P. Murali, S. Rajesh, O. Prakash, A.R. Kulkarni, R. Ratheesh, Preparation and properties of silica filled PTFE flexible laminates for microwave circuit applications, Compos. Part A Appl. Sci. Manuf. 40 (2009) 1179–1185. [7] H. Peng, H. Ren, M. Dang, Y. Zhang, Z. Gu, X. Yao, H. Lin, The dimensional effect of MgTiO3 ceramic filler on the microwave dielectric properties of PTFE/MgTiO3 composite with ultra-low dielectric loss, J. Mater. Sci. Mater. Electron. 30 (2019) 6680–6687. [8] H. Peng, H. Ren, M. Dang, Y. Zhang, X. Yao, H. Lin, Novel high dielectric constant and low loss PTFE/CNT composites, Ceram. Int. 44 (2018) 16556–16560. [9] B. Zhang, F. Ye, S. Liu, H. Zhang, Q. Liu, Y. Gao, J. Ma, Fabrication and properties of BADCy/Ni0.5Ti0.5NbO4/ZnNb2O6 composites for dielectric device application, J. Alloy. Comp. 658 (2016) 476–482. [10] C. Pan, K. Kou, Y. Zhang, Z. Li, T. Ji, G. Wu, Investigation of the dielectric and thermal conductive properties of core–shell structured HGM@hBN/PTFE composites, Mater. Sci. Eng. B 238–239 (2018) 61–70. [11] C. Pan, K. Kou, Y. Zhang, Z. Li, G. Wu, Enhanced through-plane thermal conductivity of PTFE composites with hybrid fillers of hexagonal boron nitride platelets and aluminum nitride particles, Compos. B Eng. 153 (2018) 1–8.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the financial support from 9

Ceramics International xxx (xxxx) xxx–xxx

H. Wang, et al.

[12] C. Pan, K. Kou, Q. Jia, Y. Zhang, G. Wu, T. Ji, Improved thermal conductivity and dielectric properties of hBN/PTFE composites via surface treatment by silane coupling agent, Compos. B Eng. 111 (2017) 83–90. [13] T.S. Sasikala, M.T. Sebastian, Mechanical, thermal and microwave dielectric properties of Mg2SiO4 filled polyteterafluoroethylene composites, Ceram. Int. 42 (2016) 7551–7563. [14] F. Luo, B. Tang, Y. Yuan, Z. Fang, S. Zhang, Fabrication of 0.8BaTi4O90.2BaZn2Ti4O11 filled and glassfiber reinforced polytetrafluoroethylene composites with near-zero temperature coefficient of dielectric constant, J. Alloy. Comp. 769 (2018) 1034–1041. [15] F. Luo, B. Tang, Y. Yuan, Z. Fang, S. Zhang, Evaluation of surface treatment on Li2Mg3SnO6 ceramic powders and the application of Li2Mg3SnO6 powders filled polytetrafluoroethylene composites, Appl. Surf. Sci. 456 (2018) 637–644. [16] Y. Yuan, J. Yang, B. Tan, B. Tang, E. Li, S. Zhang, Preparation, characterization and properties of FEP modified PTFE/glass fiber composites for microwave circuit application, J. Mater. Sci. Mater. Electron. 28 (2017) 6015–6021. [17] Y. Yuan, Y. Yin, D. Yu, H. Lin, J. Wang, B. Tang, E. Li, Effects of compound coupling agents on the properties of PTFE/SiO2 microwave composites, J. Mater. Sci. Mater. Electron. 28 (2016) 3356–3363. [18] Z. Li, J. Liu, Y. Yuan, E. Li, F. Wang, Effects of surface fluoride-functionalizing of glass fiber on the properties of PTFE/glass fiber microwave composites, RSC Adv. 7 (2017) 22810–22817. [19] Z. Li, Y. Yuan, M. Yao, L. Cao, B. Tang, S. Zhang, Synthesis and characterization of PTFE/(NaxLi1-x)0.5Nd0.5TiO3 composites with high dielectric constant and high temperature stability for microwave substrate applications, Ceram. Int. 45 (2019) 22015–22021. [20] F. Luo, B. Tang, Z. Fang, Y. Yuan, H. Li, S. Zhang, Effects of coupling agent on dielectric properties of PTFE based and Li2Mg3TiO6 filled composites, Ceram. Int. 45 (2019) 20458–20464. [21] Z. Jia, Z. Gao, A. Feng, Y. Zhang, C. Zhang, G. Nie, K. Wang, G. Wu, Laminated microwave absorbers of A-site cation deficiency perovskite La0.8FeO3 doped at hybrid RGO carbon, Compos. B Eng. 176 (2019) 107246. [22] X. Zhou, C. Zhang, M. Zhang, A. Feng, S. Qu, Y. Zhang, X. Liu, Z. Jia, G. Wu, Synthesis of Fe3O4/carbon foams composites with broadened bandwidth and excellent electromagnetic wave absorption performance, Compos. Part A Appl. Sci. Manuf. 127 (2019) 105627. [23] N.K. James, K.S. Jacob, K.P. Murali, R. Ratheesh, Ba(Mg1/3Ta2/3)O3 filled PTFE composites for microwave substrate applications, Mater. Chem. Phys. 122 (2010) 507–511. [24] S. Jin, X. Qiu, B. Huang, L. Wang, Q. Zhang, Z. Fu, Dielectric properties of modified BNT/PTFE composites for microwave RF antenna applications, J. Mater. Sci. Mater. Electron. 27 (2016) 8378–8383. [25] G. Subodh, M. Joseph, P. Mohanan, M.T. Sebastian, Low dielectric loss polytetrafluoroethylene/TeO2 polymer ceramic composites, J. Am. Ceram. Soc. 90 (2007) 3507–3511. [26] H. Wang, H. Yang, J. Tong, Q. Zhang, Medium dielectric constant and low-loss PTFE composites filled with MgO-LiF co-doped Li2TiO3 particles, J. Appl. Polym. Sci. 136 (2019) 47980. [27] C.W. Ahn, H.J. Jang, S. Nahm, H.M. Park, H.J. Lee, Effects of microstructure on the microwave dielectric properties of Ba(Co1/3Nb2/3)O3 and (1−x)Ba(Co1/3Nb2/3) O3–xBa(Zn1/3Nb2/3)O3 ceramics, J. Eur. Ceram. Soc. 23 (2003) 2473–2478. [28] X. Zhang, Y. Ma, C. Zhao, W. Yang, High dielectric constant and low dielectric loss hybrid nanocomposites fabricated with ferroelectric polymer matrix and BaTiO3 nanofibers modified with perfluoroalkylsilane, Appl. Surf. Sci. 305 (2014) 531–538. [29] S. Thomas, S. Raman, P. Mohanan, M.T. Sebastian, Effect of coupling agent on the

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38] [39]

[40]

[41] [42]

[43]

[44]

[45]

[46] [47]

10

thermal and dielectric properties of PTFE/Sm2Si2O7 composites, Compos. Part A Appl. Sci. Manuf. 41 (2010) 1148–1155. S.Y. Wu, Y.L. Huang, C.C.M. Ma, S.M. Yuen, C.C. Teng, S.Y. Yang, Mechanical, thermal and electrical properties of aluminum nitride/polyetherimide composites, Compos. Part A Appl. Sci. Manuf. 42 (2011) 1573–1583. Y. Shen, A. Gu, G. Liang, L. Yuan, High performance CaCu3Ti4O12/cyanate ester composites with excellent dielectric properties and thermal resistance, Compos. Part A Appl. Sci. Manuf. 41 (2010) 1668–1676. R.W. Paynter, M. Ménard, ARXPS study of a plasma-treated polymer surface: an example of Case II diffusion? J. Electron. Spectrosc. Relat. Phenom. 151 (2006) 14–18. B.H. Park, M.H. Lee, S.B. Kim, Y.M. Jo, Evaluation of the surface properties of PTFE foam coating filter media using XPS and contact angle measurements, Appl. Surf. Sci. 257 (2011) 3709–3716. J. Gu, Y. Guo, Z. Lv, W. Geng, Q. Zhang, Highly thermally conductive POSS-g-SiCp/ UHMWPE composites with excellent dielectric properties and thermal stabilities, Compos. Part A Appl. Sci. Manuf. 78 (2015) 95–101. L. Liu, F. Lv, P. Li, L. Ding, W. Tong, P.K. Chu, Y. Zhang, Preparation of ultra-low dielectric constant silica/polyimide nanofiber membranes by electrospinning, Compos. Part A Appl. Sci. Manuf. 84 (2016) 292–298. F. Luo, B. Tang, Y. Yuan, Z. Fang, S. Zhang, Microstructure and microwave dielectric properties of Na1/2Sm1/2TiO3 filled PTFE, an environmental friendly composites, Appl. Surf. Sci. 436 (2018) 900–906. G. Subodh, C. Pavithran, P. Mohanan, M.T. Sebastian, PTFE/Sr2Ce2Ti5O16 polymer ceramic composites for electronic packaging applications, J. Eur. Ceram. Soc. 27 (2007) 3039–3044. Y. Sun, Z. Zhang, C.P. Wong, Influence of interphase and moisture on the dielectric spectroscopy of epoxy/silica composites, Polymer 46 (2005) 2297–2305. Y. Rao, J. Qu, T. Marinis, C. Wong, A precise numerical prediction of effective dielectric constant for polymer-ceramic composite based on effective-medium theory, IEEE Trans. Compon. Packag. Technol. 23 (4) (2000) 680–683. J. Krupka, S.A. Gabelich, K. Derzakowski, B.M. Pierce, Comparison of split post dielectric resonator and ferrite disc resonator techniques for microwave permittivity measurements of polycrystalline yttrium iron garnet, Meas. Sci. Technol. 10 (11) (1999) 1004. A.H. Sihvola, J.A. Kong, Effective permittivity of dielectric mixtures, IEEE Trans. Geosci. Remote Sens. 26 (4) (1988) 420–429. J. Varghese, D.R. Nair, P. Mohanan, M.T. Sebastian, Dielectric, thermal and mechanical properties of zirconium silicate reinforced high density polyethylene composites for antenna applications, Phys. Chem. Chem. Phys. 17 (2015) 14943–14950. S. Thomas, J. Kavil, A. Mathew Malayil, Dielectric properties of PTFE loaded with micro and nano Sm2Si2O7 ceramics, J. Mater. Sci. Mater. Electron. 27 (2016) 9780–9788. Y. Qi, Q. Luo, J. Shen, L. Zheng, J. Zhou, W. Chen, Surface modification of BMN particles with silane coupling agent for composites with PTFE, Appl. Surf. Sci. 414 (2017) 147–152. L. Zheng, J. Zhou, J. Shen, Y. Qi, S. Li, S. Shen, TEOS surface modification of CLST ceramic particles for PTFE-based composites, J. Mater. Sci. Mater. Electron. 29 (2018) 17195–17200. K.T. Wu, Y. Yuan, S.R. Zhang, X.Y. Yan, Y.R. Cui, ZrTi2O6 filled PTFE composites for microwave substrate applications, J. Polym. Res. 20 (2013). Y. Hu, Y. Zhang, H. Liu, D. Zhou, Microwave dielectric properties of PTFE/CaTiO3 polymer ceramic composites, Ceram. Int. 37 (2011) 1609–1613.