BT composites

RARE METALS Vd.25. Spec. Issue, Lkc2006, p.250

Dependence of dielectric properties on BT particle size in EPBT composites YANG Xiaojun , YANG Zhimin , MA0 Changhui ,

and

DU Jun

Advanced Electronic Materials Institute, General Research Institute for Nonferrous Metals, Beijing 100088, China (Received zooS-08-17)

Abstntct: The polymer-ceramic composites of epoxy resin (EP) and barium titanate (BT) were prepared. BT powders of different BT particle sizes from 100 nm to 1 pm were used in the preparation. The dielectric properties, such as dielectric constant, dielectric loss and electrical breakdown strength, of the EP/BT composites were studied. The morphology of the composites was characterized by the means of scanning electron micmscopy (SEM). The results show that the dielectric constant of the composites is much higher than the epoxy matrix at frequency range from 1 kHz to 10 MHz, and it is also obviously dependent on the size of BT particles. The electrical breakdown strength of the composites decreases with the increase of the BT content. The dependence of electrical breakdown strength on BT particle sizes was also discussed.

Key words : dielectric property ; barium titanate ( BT) ; EP/BT composites

1. Introduction Ferroelectric ceramics such as barium titanate (BT) possess high dielectric permittivity , high pyroelectric coefficient, with poor processability and mechanical properties due to high sintering temperature and porosity. Polymers have excellent mechanical properties and processability , with low dielectric permittivity. By combining ceramic powder and polymer with 0 - 3 connectivity, polymer-ceramic composites with advantageous properties of both the polymer and the ceramic are obtained. They are easy, low cost process and have compatibility with printed wiring boards. Polymer-ceramic composites can be used in forming capacitors on organic substrates because they combine the low processing temperature of polymer and the high dielectric constant of ceramics. High-dielectricconstant polymer composites are known to be very useful materials for variety of electronic applications, such as embedded PCB, ultracapacitors for energy storage, transducers, piezosensors, hydrophones, etc . [ 1-83 , Correspwdiag author: DU Jun

E-moil: [email protected]

In this study, BT/EP composites with different ceramic powder volume fractions were prepared. BT powder of different BT particle sizes from 100 nm to 1 p was used in the preparation. The dielectric properties, such as dielectric constant, dielectric loss and electrical breakdown strength, and morphology of the composites were studied.

2. Experimental 2.1.

Materials

Epoxy resin used in this study was diglycidyl ether of bisphenol A (Epon-828 from Shell C o . ) . The curing agent was polyoxopropyleneamine D-230 from JEFFAMINE Co. High purity monodisperse barium titanate powders GT-BT-01 , GT-BT-03, and GT-BT-07 were purchased from Shandong GuoTeng Functional Ceramic Material Co. Ltd . High purity BaTi03 powder BT-10 was purchased from Beijing Research Institute of Chemical Engineering and Metallurgy. Details about the BaTi03 powders are shown in Table 1. Byk-W9010 (phosphate ester, BYK Chemie) was used as surfactant.

Yung X .J .et d . , Dependence of dielectric properties on BT in EPlBT

Acetone of A . R . grade from Beijing Yili Fine Chemicals Co . was used as the solvent. Details of BaTiOBpowders used in this

Table 1. study

Particle size/pm

Powders

'

Cubic Tetragod Tetragod Tetragonal

1.85

-

The mixture of BT powder, Byk-W9010 and acetone was ultrasonic treated to make the aggregated BT particles disagpgated prior to the use. The weight ratio of Byk-W9010 and BT powder is 3 :100. After the ultrasonic treatment, the mixture was dried to eliminate the residue acetone. Then the BT/BYK particles were obtained. The preparation procedure of BT/JP composite is shown in Fig. 1. First, epoxy resin was dissolved in acetone to form a homogenous solution, and then the BT/ BYK powder was added into the above solution. The mixture was subjected to ultrasonic agitation to evenly dispeme the powder. Suhsequently, the solvent was allowed to vaporize by stirring at Polymer resin, acctone, BT/BYK powder mixing i

IUltrasonic agitation I (Hertingtoeliminate acetone(

Preparation procedure of EPlBT cornpot&.

Table 2.

added and the slurry was cast onto a metal substrate or poured into a round mould, then cured at 70-120 T for 3 h. The fiestanding films of 0.10.2 mm in thickness or disks of 2 mm in thickness can be prepared by the above process.

characterization

2.3.

2.2. Preparation of B T m COmPOSifeS

Fig.1.

60 T . After that, the curing agent D-230 was

Gystalliity

(m2.g - )

10.9 3.60

0.1 0.3 0.7 1

GT-BT-01 GT-BT-03 GT-BT-07 BT- 10

specific surfacearea/

251

Dielectric properties of the composites were measured at mom temperaby employing HP 4194A LCR meter in the frequencies of 100 MIZ to 40 M H z . The electrical breakdown voltage of the composites was measured by applying high voltage pulses to the Composites, which is supplied by a MODEL W20 high-voltage power amplifier adjusted by AFG3021 signal generator. Then the electrical breakdown strength can be obtained. Scanning electron microscopy (SEM) was performed on an XL30 =EM-FEG autoemission scanning electron microscope operated at 10 kV. The h h fracture surface of the BT/EP composite samples was sputtered using gold as the electrode.

3. Resultsanddiscussion 3.1. Diele&icpmpertiesofBT/E3pcoapoaites BTEP composites with different ceramic powder weight ratios were prepared. For example, BT/EP( 1/1) means that the weight ratio of BT and EP is 1:1 in the composite. The corresponding BT powder volume fractions of the composites are summarized in Table 2. Frequency dependence of the dielectric constant and dielectric loss tangent of the BT/EP composites are shown in Fig. 2. The BT particle size is 700 nm. It shows that dielectric constant and tan8 of the BT/EF' composites are quite stable at the frequency range from 1 kHz to 10 MHz. The dielectric constant of the BT/EP composite increases with the increase of the BT content, and the dielectric loss tangent of the composites varied slightly from 0.015 to 0.025.

BT volume fiactions of BTEP commsites

~~~~~

Fractions

BT/( vol . % )

BT/EP(O.5/1) BT/EP(1/1) 6.1

12

BT/EP(2/1)

BT/EP(3/1)

BT/EP(5/1)

21

28

37

RARG METALS , Vol . 2 5 , Spec. Issue , Dec 2006

252

V

0.5

EP BT EP/BT7(1/0.5), 6.8~01% E P / B n ( l / l ) , 13~01%BT EP/BT7(1/2). 23~01%BT EP/Bl7(1/3), 30~01%BT EP/BT7(1/5). 42~01% BT

- (b)

EP BT EP/BT7(1/0.5), 6.8~01% A EPIBli'(lI1). 13~01% BT v EPIBT7(1/2), 23~01%BT O EPIBT7(1/3), 30~01% BT EPIBT7(1/5), 42~01%BT

0.4-

3

0.3-

VJ

0

0.2-

f-

0.1:

o,&:8B 8 9 8

Q

e e 6 G e o o 0 e 8 e e 8 6B

: -O.lt

Frquency/Hz

Frequency/Hz

MelecMc constant ( a ) and dielectric loss tangent (b) of BT/EP compdtes as function of frequency (room temperature).

Fig.2.

Fig. 3 shows the dependence of the dielectric constant and dielectric loss tangent of the BT/EP composite containing 37 vol .% BT powder on frequency r a n g e h m 1 lrHz to 1OMHz. The BTparticle size differs hq 100 nm to 1 pm. It shows that dielectric constant of the BT/ EP composite increases with the increase of the BT particle size, which is consistent with result of sintered BT ceramics. The dielectric loss tangent of the composite varied slightly from 0.02 to 0.04. As shown in Table 3, the electrical breakdown strength of the BT/EP composites is dependent on the particle size of BT powder. It decreases with the increase of BT particle size .

3.2. Dependence of dielectric properties on BT particle sizes of BT/EP composites BaTi03shows paraelectric properties in cubic structure but ferroelectric properties in tetragonal structure. High dielectric constant and ferroelectric properties of BaTi03 come from the tetragonal structure. Dielectric constant of BaTiOs strongly depends on particle size. According to some reports, when grain size is appmximately I pm, dielectric constant shows maximum around 5000-6000 and decreases drastically as grain size reduces. The origin of the decrease of dielectric constant is attributed to the reduction of tetragonality with the decrease of particle size[ 9-11] . - 0 -

-

-. 0-

- 7 -

-

0.5 (b)

100nmBTpowder 300 nm BT powder

1

- 700 nm BT powder I u m BTpowder

*

c n

0.3 0.4

2

0.2

g

0.1

-5

-a100 nm BT powder - - 300 nm BT powdcr -A700 nm BT powder - - - 1 umBTpowder

0.0

0

10'

10'

10'

Frcquency/Hz

I0 '

I 0s

I 0'

Frequency/}lz

Dielectricconstent ( a ) amlD€eiectrielosstangent(b) ofBTlEPcompositesasafunctiondtrequeocy (room tempemtme), BT pnrtkle size: 100,300,700 m, 1 pm.

Fig.3.

Yung X .J .et al . , Dependence of dielectric properties on BT in EP/BT

Table 3.

253

Electrical breakdown strength of EP/BT composites prepared by different BT powder Electrical breakdown strength/(kV .mm- ')

BT/EP( 1/1), 12 vol. % BT BT/EP (3/1), 28 vol. % BT BT/EP (5/1), 37 vol. % BT

BT size: 100 nm

BT size: 300 nm

BT size: 700 nm

BT size: 1 pm

145 93 88

140 84 47

83 56 43

90 45 40

It can be seen that the electrical breakdown strength of the composites decreases with the increase of BT volume fraction in the composite. For polymer-ceramic composites of 0-3 connectivity, the electrical breakdown is attributed to the interior defects of the material, such as pore, dust, residual solvent, etc . [ 121 The BT particles dispersed in the composites can be regarded as impurity which demolish the uniformity, and decrease the electrical breakdown strength of the materials. The decrease of electrical breakdown strength with the increase of BT particle size can be explained by the fact that the larger BT particles have larger effects on inhomogeneous degree of the composites. The decrease of electrical breakdown strength with the increase of BT volume fraction in the composite can be explained by the same reason. The electrical breakdown strength of the composites is also strongly dependent on specimen thickness. The thickness of the composite for electrical breakdown strength measurement is controlled to 0.1-0.2 mm.

3.3.

Microstructure of BTEP composites Fig.4 shows the SEM micmgl-ph for the

composite BT/EP ( 5 / 1 ) . It shows that the BT particles were well dispersed in the BT/EP composite. And the BT particles were embedded in the epoxy resin. The excellent combination of the two components is favourable to the dielectric properties of the composites.

4.

Conclusions

The dependence of dielectric properties, such as dielectric constant, dielectric loss and electrical breakdown strength, of the EP/BT composites on BT paiticle sizes was discussed The dielectric constant of the BT/EP composites increases with the increase of the BT particle size, but the electrical breakdown strength of the composites decreases with the increase of BT particle size. The BT/EP composite with dielectric constant larger than 100 and dielectric loss tangent less than 0.04 can be obtained. So it is a successful combination of polymer and ceramic, which has desirable dielectric properties for variety of electronic applications.

References : Kuo Dong-Hau, and Chang Chien-Chih . MateriaLs Chemistry and Physics, 2004, 85: 201. Tmutman T., Bhattacharya S., Tummala R . , et d ., Development of low viscosity, high dielectric constant polymers for integral passive, application. Proceeding of IMAPS , Braselton, GA ; 1999: 169. Rao Yang, and Wong C P. , Jollrnal of Applied Polymer Science, 2004, 92: 2228. Rao Yang, OgitaN S, Paul Kohl, et al. , Jownal of Applied Polymer science, 2002, 83: 1084. Adikary S.U., Chan H.L. W . , Choy C.L., et d ., CompositeJ Science and Technology, 2002, 62: 2161. Chiang C K. , and Popielarz R , Ferroelectrics , 2002, 275: 1. Bhattacharya S. K . , and Tummala R. R . , J .

.

SEM micrograph €or composite BT/EP(Sl 1 > , containing 37 vol.% BT.

Fig.4.

.

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