A systematic study on electrical properties of the BaTiO3–epoxy

A systematic study on electrical properties of the BaTiO 3–epoxy composite with different sized BaTiO 3asfillersWenhu Yang a ,⇑,Shuhui Yu a ,Suibin Luo a ,Rong Sun a ,⇑,Wei-Hsin Liao b ,Ching-Ping Wong b ,caCenter for Advanced Materials,Shenzhen Institutes of Advanced Technology,Chinese Academy of Sciences,Shenzhen 518055,China bDepartment of Mechanical and Automation Engineering,The Chinese University of Hong Kong,Shatin,Hong Kong,China cSchool of Materials Science and Engineering,Georgia Institute of Technology,Atlanta,GA 30332-0245,United Statesa r t i c l e i n f o Article history:Received 9June 2014Received in revised form 15September 2014Accepted 17September 2014Available online 28September 2014Keywords:Polymer–matrix composites Embedded band-pass filter Complex permittivity Dielectric relaxation Electric modulusa b s t r a c tThe epoxy composites consisting of BaTiO 3fillers with size of 100nm and 200nm were prepared,respec-tively.The thermo-mechanical and electrical properties of BaTiO 3–epoxy composites were investigated using complex permittivity,electric modulus and ac electrical conductivity.The effects of filler size on the structure,filler–matrix interface and properties were discussed.The Havriliak–Negami model was used to analyze the dielectric relaxation of composites.The results show that larger filler tends to exhibit enhanced dynamic modulus and higher T g because of the increase of effective volume fraction.The tetragonal phase formation in the BaTiO 3is important to achieve enhanced permittivity of the composite.The relaxation responses of composites revealed by electric modulus and complex permittivity show that the Maxwell–Wagner–Sillars (MWS)polarization and tetragonal phase transition of BaTiO 3affect the dielectric responses of composites.The conduction process is not only related with the barrier hopping but also tunneling effect of electrons.Based on the electrical properties of BaTiO 3–epoxy composite with 100nm BaTiO 3at GHz,a compact band-pass filter with significant area reduction was designed and fabricated.The measured results show that the BaTiO 3–epoxy composite is promising for advanced wire-less device applications.Ó2014Elsevier B.V.All rights reserved.1.IntroductionDuring the past few years,the inorganic particles filled polymer composites have attracted considerable interest because of their good processability and tailorable characteristics by controlling the content of the filler [1–3].Various nano-particles,such as TiO 2[4],CaCu 3Ti 4O 12(CCTO)[5],SrTiO 3and BaTiO 3[1,6]have been employed as fillers for high permittivity dielectrics.Among them,BaTiO 3is one of the most popular fillers in polymer composites [7–9]for embedded capacitor application.The properties of the polymer composites depend strongly on the filler size,filler–matrix interface and loading of filler.Therefore,understanding these factors is important to achieve the desirable polymer dielec-tric material with high performance.Previous work reported that there was a relation between the permittivity and grain size of BaTiO 3ceramic [8–10].It was gener-ally recognized that the permittivity of BaTiO 3increased with the increasing size of BaTiO 3[11–16]and the permittivity of BaTiO 3strongly depends on the grain size of the tetragonal phase structure while it is almost independent of grain size of the cubic structure [17].The internal stress resulting in lattice distortion plays an important role in size effects.However,there is no clear conclusion for the BaTiO 3–polymer composites.Mao et al.[11]have reported that the remanent polarization of BaTiO 3particles increased as the size of BaTiO 3particles increased from 25to 500nm and the permittivity of the BaTiO 3–PVDF composites exhibited a maximum value in the composites with BaTiO 3nano-particle sizes of 80–100nm and decreased again when the particle size was further reduced under 50nm.However,Fan et al.[12]found that the values of permittivity for the PI matrix filled with the size of 30–50nm BaTiO 3particles were higher than that filled with 80nm BaTiO 3particles.Moreover,some reports [10]indicated that the smaller fillers would bring the serious influence to the mechanical properties of polymer composite.Thus,it is desired to clarify the issue of size effect on the dielectric characteristics and dynamic mechanical properties of the BaTiO 3polymer composites.Although significant efforts have been made for improving the permittivity of the BaTiO 3–epoxy composite,the temperature and frequency dependent dielectric responses of the composites are not fully understood.In particular,it is important to precisely/10.1016/j.jallcom.2014.09.1420925-8388/Ó2014Elsevier B.V.All rights reserved.⇑Corresponding authors.E-mail addresses:wh.yang@ (W.Yang),rong.sun@ (R.Sun).evaluate influence offiller on the relaxation processes of the poly-mer and the possible contributions from the Maxwell–Wagner–Sillars(MWS)interfacial polarization.Meanwhile,the dielectric losses of polymer composites are mainly associated with the electrical conductivity of the polymer matrix and the polymer/ nanoparticles interface effects.Therefore,understanding the con-duction mechanism in the polymer composite system is important to realize the relaxation process of polymer composite.For the practical application,the dielectric characteristics at near or over one gigahertz are important for wireless communication applications.Meanwhile,much work[18,19]has been focused on the embedded planar capacitor.However,there are few reports on the fully embedded VHF(very high frequency)band-passfilters using BaTiO3–epoxy composites.In this paper,BaTiO3–epoxy com-posites with the same BaTiO3volume concentration and different particle size were prepared.The influences of particle size,temper-ature and frequency on the dielectric properties and relaxation pro-cesses were evaluated by using complex permittivity and electrical modulus.The Havriliak–Negami model was used tofigure out the contributions from interfacial polarization and conductivity.The conduction mechanism was analyzed by overlapping large polaron tunneling model.Specifically,based on the BaTiO3–epoxy compos-itefilm,an embedded VHF band-passfilter was designed and fabri-cated,to extend the usablefields of BaTiO3–epoxy composites.2.Experiments2.1.Preparation of BaTiO3–epoxy compositefilmThe bisphenol F type epoxy resin(Bluestar New Chemical Materials Co.,China), BaTiO3(100nm,200nm,Guangdong Fenghua Advanced Technology Co.,China), dicyandiamide,imidazole,and2-butanone were used as the polymer matrix, dielectricfiller,curing agent,catalyst and solvent,respectively.Dispersing agent (Hypermer PS3)was used to improve the dispersion of BaTiO3in the epoxy matrix. The following procedure independent of the particle size was used:20g of epoxy resin was dissolved in160g2-butanone.Then,100g of barium titanate powder, and6g dispersing agent were added.The mixture was ultrasonically agitated for 2h to obtain the mixture A.Then,6g dicyandiamide and6g imidazole were dis-solved in5ml N,N-Dimethyl formamide solvent to obtain the mixture B.The mix-ture B was added in the above mixture A.The mixture was ultrasonically agitated for2h to obtain the stable and uniform slurry.The prepared slurry was coated on copper foil with a scratch coating method and heat-treated at120°C for5min to remove residual organic solvents.Two pieces were laminated face to face and cured at190°C for2h.The thickness of the dielectricfilm is ranged from8l m to22l m,which is determined by the demands.2.2.Fabrication of embedded band-passfilterIn a typical process,one side of the layer capacitor(Cu/BaTiO3–epoxy/Cu) trode was selectively etched by an identical photolithography process for the aration of capacitor laminates.The etched side of the capacitor core was laminated with a layer of prepreg dielectric material.This was followed bythe second capacitor electrode and subsequent lamination with another dielectric.Top and bottom capacitor electrodes were connected through (conductive holes)to make thefilter operational.2.3.Characterization techniquesThe crystal structure of BaTiO3was analyzed using X-ray diffraction(XRD, max-2500/PC,Rigaku Co.)with Cu K a radiation,at a scanning speed of2°/min steps of0.02°.The cross-section of the compositefilm samples was examined scanning electron microscopy(Nova Nano SEM450,FEI).The thermal stability the BaTiO3–epoxy composites were investigated by dynamic mechanical (DMA,Q800,TA Instruments)with a frequency of1Hz,a heating rate of3°and underflowing N2atmosphere in a temperature range from30to180°C.The frequency dependencies of the dielectric permittivity and loss tangent the composites were measured by employing an Impedance Analyzer4294A)in a frequency range of100–106Hz.The microwave dielectricof the composite were measured using resonant cavity method combined Vector Network Analyzer(Agilent8722ES).In the resonant cavity technique, resonant frequency and the quality factor of the empty cavity and materials different.The relative permittivity and dielectric loss of the samples can be mated from the shift in the resonant frequency and change in the Q of theusing the following equations:e0r¼/cðf cÀf sÞs sþ1ð1Þe00r¼/cs1SÀ1cð2Þtan d¼e00e0ð3Þwhere e0r is the real permittivity and e00r is imaginary permittivity.f c is the resonant frequency of the empty cavity,f s the resonant frequency of the cavity with sample, U c the volume of the cavity,U s the volume of the sample,Q c the quality factor of empty cavity,and Q s the quality factor of the sample.The fabricated band-passfilter was measured using a Network Analyzer(Agilent N5244A)and coplanar ground-sig-nal-ground probes with a500l m pitch size.3.Results and discussions3.1.Characterization of BaTiO3and BaTiO3–epoxy compositesFig.1shows the X-ray diffraction patterns of the BaTiO3with size of100nm and200nm.It indicates that the BaTiO3material is in good agreement with the conventional tetragonal structure with the P4mm space group(JCPDS No.05-0626),with no impu-rity peak appearing in the diffractogram.The200nm BaTiO3pow-ders show peak splitting at45°corresponding to the(hkl)Miller index{002}and{200},whereas100nm BaTiO3powders has one single peak at45.3°corresponding to{200}.In general,the cubic BaTiO3(JCPDS No.31-0174)has one single peak at44.9°cor-responding to{200}.Therefore,it can be concluded that the BaTiO3 powders with size of100nm and200nm show a pseudo cubic and tetragonal-dominant structure,respectively.Fig.2presents the SEM image of the fresh fractured cross sec-tion of the BaTiO3–epoxy composites.Fig.2(a)is the microstruc-ture of pure epoxy which shows large-scaled lamellar structures. Fig.2(b)is the images of the compositefilled with40vol% 100nm BaTiO3fillers.It presents some honeycomb structure because of thefillers falling from the epoxy matrix,and thefillers are homogeneously dispersed in the epoxy matrix.Fig.2(c)is the cross section of the composites with40vol%200nm BaTiO3.The BaTiO3particles are surrounded by the epoxy matrix.The observed morphologies would be used to explain mechanical properties of the composite.According to the earlier work by other researchers [20–22],thefiller size,microstructure of composite and the inter-face between thefillers and matrix would affect the mechanical properties of the composite.diffraction patterns of BaTiO3with particle size of100nmshows the{200}reflection of BaTiO3powders.316W.Yang et al./Journal of Alloys and Compounds620(2015)315–323(below T g ).The storage modulus is more associated with molecular elastic response of the composites,while the loss modulus reflects the viscous behavior in the composites.Fig.3(a)shows that the E 0of composite-1and -2is 4725MPa and 5650MPa,respectively,which is much higher than that of neat epoxy 2700MPa at room temperature.This observation can be attributed to the good BaTiO 3dispersion in the matrix and the mismatch in the coefficient of thermal expansion (CTE)between the matrix and the BaTiO 3.Ther-mal mechanical analysis have shown that the CTE of the epoxy resin is 6.2Â10À5K À1while that of BaTiO 3is about 5Â10À6K À1($200nm)[23,24].So,a residual compressive stress between the two phases could transfer from matrix to filler,and a notable stiff-ening effect is obtained.However,when the temperature further increases to the glass-transition range of the samples,an obvious decrement for all samples is observed because of the softening of the matrix and initiation of the relaxation process.Accordingly,one loss peak was observed for neat,composite-1and -2in Fig.3(b)and (c).It indicates existing interfacial interac-tions between the BaTiO 3and epoxy,and there is no phase sepa-ration in the composites.This is consistent with the microstructure of the composite in Fig.2.Generally,the temper-ature corresponding to the maximum in the loss modulus was taken as T g of the composite.As shown in Fig.3(b),the T g of com-posite-1and -2is 114.6°C and 115.9°C,respectively.Obviously,the T g of the composite is much lower than the neat epoxy (144.8°C).According to the previous literatures [25–27],the mechanism of T g decrement in polymer composites is more com-plex.The molecular weight and cross-linking density were found to have a significant effect on the T g .Some reports [25,26]sug-gested that the surface structure of the filler will determine the T g of the composite.In polymer composite,a nano-layer was formed on the fillers because of interactions between the polymer chains and fillers.These interactions in the interfacial region willfinally affect the mobility of the polymer chains and lead to the T g increase,decrease or remain constant.The results of molecular dynamics (MD)simulations [28]and experiments on polymer mobility in thin polymer films have suggested that the T g could be shifted to either higher or lower temperatures by increasing or decreasing the attraction between the polymer and nanoparti-cle,respectively.Moreover,if this interpretation is active,some of the interphase is immobilized due to interfacial binding,which results in an increased effective region for the particles.Assuming that the immobilized polymer regions do not contribute to energy loss,the ‘‘effective volume fraction’’of the particles in composite V e can be written as [24]:V e ¼1ÀE 00mE 00ð4ÞV e ¼1þD R R3V fð5Þwhere V e is the effective volume fraction of BaTiO 3in epoxy matrix.E 00and E 00m is effective volume fraction,the loss modulus of compos-ite and matrix,respectively.According to Eq.(5),the effective thick-ness of the interfacial region,D R ,can be estimated.R is the radius of the spherical particle and V f is the real volume fraction of filler load-ing.V e is usually employed to quantitatively analyze the interphase properties of particulate-filled polymers in a nanometer or microm-eter scale.Based on Eq.(4),the temperature dependence of effective volume content is shown in Fig.4.It can be seen that the value of V e increases with the increasing temperature below T g .Nevertheless,a clear tendency can be easily found that larger filler relates to higher V e and T g .These results indicate that the interfacial region plays an important role on the impact mechanical properties of polymer composites.The decrease of the T g of BaTiO 3–epoxy compositewithfractured cross-section of BaTiO 3/epoxy composites (a)neat epoxy,(b)with 40vol%100nm BaTiO 3,(c)with BaTiO 3and 40vol%200nm BaTiO 3were marked as composite-1and -2,respectively).loading can be attributed to the weak interaction between and epoxy.Dielectric properties of BaTiO 3–epoxy compositesorder to understand the dielectric response of the the real permittivity (e 0r ),imaginary permittivity loss at 30°C are shown in Fig.5.One can see Storage modulus of composites,(b)loss modulus as a function fraction,and (c)glass transition temperatures of neat epoxy,Fig.4.Temperature dependence of effective volume content.Frequency dependences of (a)real permittivity e 0r ,(b)imaginary permittivity real permittivity and loss tangent for composite -1and -2at 3-18and15at over GHz,as well as relative low loss tangent no more than0.06,respectively.This is significant for the practical applica-tion in microwave communication.At low frequencies,the molecular polarization will have more time to complete compared with that at high frequencies.Thus, the degree of polarization of composites is high and the dissipation of polarization is low at low frequencies.This reveals that the e0r decreases but the loss increases with increasing frequency.It is worth noting that e00r of composite is higher than the neat epoxy in the test frequency range because more polarization groups are introduced and only one apparent relaxation process at high fre-quencies is observed.3.4.Temperature dependence of dielectric properties of BaTiO3–epoxy compositesThe dipolar polarization relaxation process is generally charac-terized by a peak in the temperature or frequency dependence of the imaginary part of permittivity[31].In order to clarify the relax-ation process in the composite,Fig.6presents the dielectric prop-erties of the neat epoxy at different frequencies as functions of temperature.It can be seen that the e0r of epoxyfirst increases sig-nificantly with the temperature increasing fromÀ50to50°C,then decreases or increases with the temperature further increases depending on the frequency,as shown in Fig.6(a).Correspond-ingly,a loss peak between25°C and50°C is observed in Fig.6(b).It indicates a typical dielectric relaxation,which corre-T g of the compositefilms,the contribution by epoxy matrix to the permittivity is limited,as shown in Figs.6and7(a).Thus,the transformation in BaTiO3crystal phase and the interfacial effect resulted in the enhanced permittivity of BaTiO3–epoxy composite films.The e00r of all composites decreases with increasing temperature fromÀ50°C to50°C and then increases with the further increasing temperature in Fig.7(b).The possible reason for this behavior is the limited segmental mobility of epoxy resin at low temperature. At high temperature,especially at the temperature near T g,the good mobility of main polymeric chains in combination with the diploes leads to the high dielectric loss.For the frequency dependence e00r of the composite in Fig.7(b), losses do not form any peak in the frequency window of the mea-surements.In general,the electrode characteristics,the space charge injection and the absorbed impurity conduction,all will obscure the relaxation in the permittivity.To overcome these diffi-culties in evaluating various polarization mechanisms,the electric modulus approach was utilized to analyze the electrical relaxation phenomena in polymer composites[32–37].The complex electric modulus is defined in the following equations:Mü1eüM0þjM00¼e0e02þe002þje00e02þe002ð6ÞAccording to the definition of the electric modulus,the varia-tions in the large values of permittivity at low frequencies are minimized.Temperature dependences of(a)e0r and(b)e00r s(c)for neat Temperature dependences of(a)e0r and(b)e00r at103Hz forW.Yang et al./Journal of Alloys and Compounds620(2015)315–3233193.5.Size effect on the dielectric properties of BaTiO3–epoxy compositesThe size effect on the dielectric performance is complex.It is noted that the effective volume fraction of the particles in compos-ite increases with the increasing temperature below T g as shown in Fig.4.This indicates that the interface acts an important role for polymer composites,which agree with the previous reports [5,33–35].However,the results of Fig.5and Fig.1indicate that the dominant tetragonal phase structure of BaTiO3fillers is the ori-gin of the high dielectric mechanism for composite-2.It is inconsis-tent with the results of BaTiO3/PI composites[12],in which the permittivity of cubic(30–50nm)BaTiO3–PI composites is higher than that of tetragonal(50–80nm)BaTiO3–PI composites.The pos-sible reason for this discrepancy is that the dielectric properties are dominated by the interface while the size of BaTiO3becomes smal-ler than100nm.Another important factor to be considered here is the tetragonal phase transition at near the transition temperature T c($120°C).For the BaTiO3ceramics,there is a dielectric peak around T c.It is close to the T g($115°C)of the present BaTiO3–epoxy compositefilms.At high temperature near the T g of the com-positefilms,the contribution by epoxy matrix to the permittivity is conductivity of the materials.In order to analyze the contribution of each relaxation,the dielectric loss spectra are analyzed using the Havriliak–Negami(H–N)equation with a conductivity term:[36] eÃðxÞ¼e1þðe sÀe1Þ½1þðj x sÞa bþrdcj e0xð7ÞThe e0and e00here can be calculated ase0ðxÞ¼e1þðe0Àe1Þcosðb hÞð1þ2ðx sÞa cosða p=2Þþðx sÞ2aÞb=2ð8Þe0ðxÞ¼ðe0Àe1Þsinðb hÞð1þ2ðx sÞa cosða p=2Þþðx sÞ2aÞb=2þrdcj e0xð9Þwithu¼tanÀ1ðx sÞa sinðp a=2Þ1þðx sÞa cosðp a=2Þð10Þwhere e s and e1are the low and high-frequency limiting dielectric constant,s and r are the relaxation time and dc conductivity,electric modulus spectra as temperature and frequency for(a)neat epoxy,(b)composite-1and(c)composite-2;(d)M00 320W.Yang et al./Journal of Alloys and Compounds620(2015)315–323Cole–Davidson model and the interaction between dipolesbe taken into consideration.As an example of composite,Fig.9shows thedent imaginary part of dielectric constant for composite-1atand80°C.The contributions of individual relaxations to theinary part of dielectric constant calculated fromequation are presented.It can be observed that the MWStion and a relaxation as well as tetragonal phaseBaTiO3affect the dielectric response of composites.The lossuted by the DC loss at low temperature and high frequencies ignored in Fig.9(a).Meanwhile,DC conductivity and MWSzation are mainly responsible for the dielectric loss of theepoxy composite at high temperature and lowFig.9(b).The MWS polarization is associated with the largefields generated in composite because of the largepermittivity between the epoxy matrix and thefiller.As a result, a large number of space charges are trapped and accumulated at thefiller-matrix interface to give a large interfacial polarization. It can be interpreted by the behavior of conductivity.3.7.Theoretical investigation of mechanism of conductivityThe frequency dependence of AC conductivity r(x)in compos-ite at room temperature is shown in Fig.10.It can be observed that the conductivities of composite increase with BaTiO3fillers involved and stay at a low level at low frequency.The conductivity dispersion can be analyzed by equation:[38]rðxÞ¼rdc þA x sð11Þcomposite.Herein,the value of s indicates that the motion of thesecharges leads to not only conduction but also considerablepolarization.According to the Arrhenius equation,electrical conductivity isstrongly dependent on temperature,which can be written as:[40]rðTÞ¼rexpÀD E AkTð12Þwhere r0is the high temperature limit of conductivity,D E A,k and Tare activation energy,Boltzmann constant and absolute tempera-ture,respectively.Fig.11shows the plot of ln r versus1000/T forthe composites at103Hz,in which the dashed line is thefittedresults using Eq.(12).The plots of composites show two differentwith positive and negative slopes in Fig.11indicatetwo different conduction mechanisms.In contrast,one conduction mechanism in neat epoxy.It isbehaviors of dielectric loss.Frequency-dependent imaginary part of dielectric constant for and(b)80°C.Frequency-dependent AC conductivity at room temperature shown with dashed line).Conductivity data in Arrhenius format for nanocomposite are shown with dashed line).corresponding parameters from the Arrhenius equation.r0E epoxy-L7.234Â10À5E epoxy-H 4.63Â10À5E1L 2.333Â10À3E1H 4.33Â10À10E2L 1.222Â10À3E2H 1.49Â10À7W.Yang et al./Journal oflow and high temperature,respectively.This indicates that the conduction process is not only related to the barrier hopping but also tunneling effect of electrons.In order to explain the mecha-conductivity for the polymer composite,the variationexponent s as a function of temperature is depictedexponent s decreases with rising temperature value and then increases with increasing temperature.that overlapping large-polaron tunneling modelcharacterize the electric conduction mechanism[41].Application in embeddedfilterembedded microstrip band-passfilter with spiralstructure(DGS)is proposed based on composite-1dielectric constant of10.8and a thickness of11offilter consisting of one layer BaTiO3–epoxytwo layers prepreg with four metal layers is Fig.13(a).The X-ray photograph of the fabricatedfilter is shown in Fig.13(b).As shown,the top and bottom electrode graphs match well with the design and there are six symmetric test pads for Cas-cade GSG500l m coplanar probes.Fig.13(c)and(d)show the measured and simulated performances of the insertion loss and return loss offilter,respectively.The measured results of S21and S11are similar to the simulation results.Owing to a tolerance in the substrate and process,the measured center frequency is 360MHz,which is higher than the simulated260MHz.The mea-sured insertion loss is aboutÀ3.8dB at the center frequency,which is related to the dielectric loss of the composite.The bandwidth over10dB return loss ranges from284MHz to409MHz.Note that over10dB suppression is obtained from measured results at above 1.2GHz,which is important for the practical application in com-munication.The high light for the embeddedfilter has a very small size of5Â5mm2.According to the transmission line theory,the finite-frequency attenuation pole f r can be defined as:fr¼1ffiffiffiffiffiffipð13ÞThe variation for universal exponent s as a function of temperature.embedded band-passfilter based on the composite-1,(a)photograph and(b)X-ray photograph of fabricatedfilter,(c)and(d)are the S-parameters measurement.Compounds620(2015)315–323permittivity in comparison with the neat epoxy and composite with100nm BaTiO3.The introduction of nano-sized BaTiO3in epoxy results in a lower glass transition temperature,which can be interpreted by the weak interaction between the BaTiO3and epoxy matrix.The real permittivity of composite-2is higher than that of composite-1because more tetragonal phases were involved in200nm BaTiO3.The conduction behavior of the composites showed that the conduction process is not only related with barrier hopping but also tunneling effect of electrons.Based on the epoxy composite with100nm BaTiO3,a compact band-passfilter by embedding all the passive components into a printed circuit board was designed and fabricated.The measured results showed that the BaTiO3–epoxy composite it is promising to be applied to advanced wireless devices.AcknowledgementsThis research is supported by the Guangdong Innovative Research Team Program(No.2011D052),the National Natural Sci-ence Foundation of China(No.51377157),Shenzhen Peacock Plan for Innovative Reasearch Team(No.KYPT20121228160843692) and the SIAT Innovation Program for Excellent Young Researchers (201311).References[1]Z.F.Zhang,X.F.Bai,J.W.Zha,W.K.Li,Z.M.Dang,Preparation and dielectricproperties of BaTiO3/epoxy nanocomposites for embedded capacitor 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