Front. Chem. Eng. China 2008, 2(4): 417–421 DOI 10.1007/s11705-008-0078-0

RESEARCH ARTICLE

Dielectric property of polyimide/barium titanate composites and its influence factors (II) Weidong LIU1,2, Baoku ZHU (*), Shuhui XIE1, Zhikang XU1 1 Institute of Polymer Science, Key Laboratory of Macromolecular Synthesis and Functionalization, Ministry of Education, Zhejiang University, Hangzhou 310027, China 2 Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Zhejiang Normal University, Jinhua 321004, China

E

Higher Education Press and Springer-Verlag 2008

Abstract Using poly(amic acid) (PAA) as a precursor followed by thermal imidization, the polyimide/barium titanate composite films were successfully prepared by a direct mixing method and in situ process. The influence of processing factors, such as particle size, distribution mode and polymerization method on dielectric properties was studied. Results revealed that the dielectric constant (e) of the composite film increased by using bigger fillers or employing in situ polymerization and bimodal distribution. When the composite film containing 50 Vol-% of BaTiO3 with size in 100 nm was prepared via in situ process, its dielectric constant reached 45 at 10 kHz.

versatile performances such as good resistance to organic solvents, high tensile modulus and mechanical strength, low thermal expansion and excellent dielectric properties etc [6], aromatic polyimides are chosen as the polymer matrix in this study. On the basis of previous studies about the dielectric properties of polyimide/BaTiO3 composites [7–9], the effect of processing factors, such as particle size, distribution mode and polymerization method on the dielectric properties of polyimide/BaTiO3 composite films is studied in this work.

Keywords polyimide, barium titanate, composite, dielectric property

2.1

1

Introduction

With the development of electronic technology, miniaturization and low energy consumption, etc., have become the trend of electronic components and products. The high capacitance density capacitors of high capability electronic components need flexible and processable high dielectric constant films [1]. Barium titanate is a ferroelectric with widely applications in the electronics and optical industry because of its high dielectric constant and low dielectric loss [2,3]. However, it is a fragile material that needs to be sintered at high temperature. So the preparation of polymer/BaTiO3 composite materials by dispersing BaTiO3 particles into a polymer matrix provides a new route to overcome the shortcoming [4,5]. Owing to their

Translated from Journal of Functional Materials, 2008, 39(2): 264– 267 [译自: 功能材料] E-mail: [email protected]

2

Experiments Materials

Pyromellitic dianhydride (PMDA) and 4,49-oxydianiline (ODA) were purchased from Aldrich and were purified by vacuum sublimation before use. N, N-dimethyl acetamide (DMAc, Shanghai Chemical Reagent Company, China) was distilled over powered calcium hydrate under reduced pressure. 3-Amino-propyl-triethoxysilane (APTS) were obtained from Acros Organics (Morris Plains, NJ) and used as received. Barium titanate (BaTiO3) with different diameters were purchased from the Heibei Xiongwei Ceramic Material Company (China) and used as received. 2.2 2.2.1

Preparation of polyimide/BaTiO3 composites Modification of barium titanate powders

Barium titanate (BaTiO3) particles and the coupling agent of APTS were added into a water/ethanol mixture (5 mL/ 95 mL). At 70uC, under vigorous stirring, ultrasonication was applied to this suspension for 2 hours. By filtrating and drying in vacuum, the modified barium titanate powders were obtained.

418

Weidong LIU, et al.

2.2.2 Preparation of polyimide/BaTiO3 composite films by direct mixing method

3 Results and discussion 3.1.

The modified BaTiO3 particles were added into DMAc, and the mixture was vigorously stirred for 2 hrs under ultrasonication at room temperature. Then, the poly(amic acid) (PAA, the precursor of polyimide, synthesis method see Ref. [10]) solution was added, and the mixture was stirred for 24 hrs to yield a homogenous suspension. The precursor films were obtained by casting the aforementioned suspension onto glass plates followed by drying at 50uC for 10 hrs to evaporate the solvent. By heating the precursor film at 100uC, 200uC, and 300uC consecutively (for 1 hour at each temperature) as the imidization step, the final polyimide/BaTiO3 composite film was obtained. 2.2.3 Preparation of polyimide/BaTiO3 composite films by in situ process The modified BaTiO3 particles were added into a 10 mL DMAc and the mixture was vigorously stirred for 2 hours under ultrasonication. After 0.4786 g ODA had been added and the suspension had been stirred for 0.5 h, 0.5214 g PMDA was added in batches. Then, the mixture was stirred for 24 hrs at room temperature to yield a homogenous suspension. The procedure for preparation of polyimide/BaTiO3 composite film was the same as the direct mixing method. 2.3

Characterizations

The phase morphology of the composite films was imaged on a Cambridge S260 scanning electron microscope (SEM). X-ray diffraction (XRD) of the samples was carried out by a Mac Science M18. The dielectric constant (e) was measured on a HP4276A LCR bridge meter using the coated silver layer on the two face of the composite film as electrodes.

Effect of polymerization method

A series of polyimide/barium titanate composite films were successfully prepared by direct mixing method and in situ process with the BaTiO3 size in 100 nm. As shown in Fig. 1, compared to the direct mixing method via the in situ process, the BaTiO3 particles dispersed in a more compact and homogeneous way in the polyimide matrix. It was caused by two factors. The first was that when the reaction began, the viscosity of the mixture was low and the monomer was absorbed on the surface of the BaTiO3 particles, so the BaTiO3 particles were dispersed homogeneously in the suspension. Second, with polymerization proceeding, the viscosity of the system increased, and the Brownian motion and sedimentation of the particles were confined [11]. The BaTiO3 particles could be dispersed homogeneously in the precursor of polymer until the final composite films formed. However, using the direct mixing method, when the reaction began, the viscosity of the system was high and it was adverse to the uniformity of the BaTiO3 particles in the suspension and in the final composite films. BaTiO3 particles in the composite films prepared via the in situ process dispersed in the polyimide matrix in a more homogeneous state, so the dielectric constants of the resulting composite films were larger than that of the composite films prepared via the directly mixing method (Fig. 2). 3.2 Connection between BaTiO3 particle size and dielectric property of composite films Apart from the size of 100 nm (signed as BT-01), BaTiO3 particles with size of 300 nm (BT-03), 400 nm (BT-04) and 500 nm (BT-05) were also used to prepare the composite films by the direct mixing method and in situ process, respectively. The influence of BaTiO3 particle size on the dielectric properties and structure of composite films was studied.

Fig. 1 SEM photos of PI/BaTiO3 composite films prepared by (a) directly mixing method and (b) in situ polymerization (VolBaTiO3 5 50 Vol-%)

Dielectric property of polyimide/barium titanate composites and its influence factors

Fig. 2 Dielectric constant of PI/BaTiO3 composites prepared by different methods (VolBaTiO3 5 50 Vol-%)

Fig. 3

419

Figure 3 shows SEM images and XRD patterns of BaTiO3 with four different grain sizes. As the particle size increased to 300 nm, the tetragonal phase was best identified by the splitting of the peak with 2h around 45u into two peaks, which correspond to the (002) reflections, indicating that the metastable cubic phase crystallized gradually transformed to a tetragonal phase and dielectric constant of BaTiO3 particle increased with the particle size [12]. SEM pictures and dielectric constant of polyimide/ BaTiO3 composite films prepared with four different grain size particles are shown as Figs. 4 and 5. First, in the composite films prepared by the direct mixing method and in situ process, all the four different grain size particles were dispersed in the polyimide matrix in a homogeneous state. Therefore, the smaller the grain size was, the denser the packing of the particles grew. As the big grain size particles had a high dielectric constant, with increasing the BaTiO3 particle grain size, the dielectric

SEM images (a) and XRD patterns (b) of BaTiO3 particles with different size

Fig. 4 SEM pictures of polyimide/BaTiO3 composite films prepared by (a) direct mixing method and (b) in situ polymerization (VolBaTiO3 5 50 Vol-%)

420

Weidong LIU, et al.

Fig. 5 Dielectric constant of polyimide/BaTiO3 composite films prepared with different particles (VolBaTiO3 5 50 Vol-%)

constant of the films increased. At the same time, homogenization of the composite film microstructure was affected with the increase of grain size. Since this homogenization is in request in the field of high dielectric constant materials, both the increase of dielectric constant and the microstructure of material need to be taken into consideration. Second, because BaTiO3 particles dispersed in the polyimide matrix in a more homogeneous and compact state in the composite films prepared via the in situ process, the dielectric constant of these films was obviously higher than that of the composite films prepared by the direct mixing method. 3.3

volume is filled in closed-packed arrangement of unitary size particles. So, the contents of BaTiO3 particles could be improved by the bimodal method using two different grain size particles [13]. In the bimodal distribution, the small grain size particles fit into the interspace between the big particles, so the contents of BaTiO3 particles increased. Using bimodal distribution BT-05 and BT-01 (weight ratio 3: 1) were used to prepare the polyimide/ barium titanate composite films by the in situ process. The SEM pictures of polyimide/BaTiO3, composite films prepared bimodally (BT-05 42 Vol-% and BT-01 14 Vol%) and unimodally (volume fraction of BT-05 is 42%) are shown in Fig. 6. As can be seen, the BaTiO3 particles in composite films prepared bimodally were denser than that in composite films prepared unimodally and in bimodal distribution the pores (corresponding to the dielectric constant of air clearance) in the materials were reduced, so the dielectric constant was enhanced. Dielectric constant of polyimide/BaTiO3 composite films prepared by two different distribution modals are shown in Fig. 7. The dielectric constant of composite films was enhanced effectively by bimodal distribution.

Distribution modal

The dielectric constant of the composite films increases gradually with the increase of BaTiO3 contents and grain size of BaTiO3 particles [9]. Also, the BaTiO3 particle contents and the distribution modal affect on the dielectric constant of composite films. As shown in Fig. 4, in the composite films prepared with big grain size particles, only 74% of the available

Fig. 7 Dielectric constant of polyimide/BaTiO3 composites films prepared by different modal

Fig. 6 SEM pictures of polyimide/BaTiO3 composite films prepared by different modal (VolBT-05 5 42 Vol%). (a) Bimodal (BT-05 : BT-01 5 3 : 1); (b) unimodal(BT-05)

Dielectric property of polyimide/barium titanate composites and its influence factors

4

Conclusions

Using poly(amic acid) as a precursor followed by thermal imidization, the polyimide/barium titanate composite films were successfully prepared by a direct mixing method and in situ process. The dielectric properties are affected by many factors. The study revealed that the dielectric constant (e) of the composite film increased by using bigger fillers or employing in situ polymerization and a bimodal distribution. When the composite film containing 50 Vol-% of BaTiO3 with a size of 100 nm was prepared via the in situ process, its dielectric constant reached 45 at 10 kHz. Acknowledgements The authors greatly acknowledge the National Natural Science Foundation of China (Grant No. 50103010) and the Education Department of Zhejiang Province (No. 20071157) for supporting this work.

References 1. Dang Z M, Wang H Y, Peng B, Lei Q Q. Polymer-based nanocomposite dielectric materials with high dieclectric constant. Proceedings of CSEE, 2006, 26: 100–104 (in Chinese) 2. Zhou J, Li L, Gui Z, Zhang X, Barber D J. Sol-gel derived BaTiO3 thin films with embedded silver nanoparticles: preparation and dielectric properties. Nanostructured Materials, 1997, 8: 321–328 3. Maison W, Kleeberg R, Heimann R B, Phanichphant S. Phase content, tetragonality, and crystallite size of nanoscaled barium titanate synthesis by the catecholate process: effect of

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

421

calcinations temperature. J Euro Ceram Soc, 2003, 23: 127– 132 Gao N K, Peng Z R. Study of the dielectric properties of polymer/barium titanate composites. Insulating Materials, 1997, 5: 28–31 (in Chinese) Devaraju N G, Kim E S, Lee B I. The synthesis and dielectric study of BaTiO3/polyimide nanocomposite films. Microelectronic Engineering, 2005, 82: 71–83 Ding M X, He T B. Polyimide: chemistry, relationship between structure and properties and materials. Beijing: Science Press, 2006: 224–245 (in Chinese) Zhu B K, Xie S H, Xu Y Y, Xu Z K. Preparation and properties of polyimide/barium titanate composite films with high dielectric constant. J of Functional Materials, 2005, 36: 546– 548 (in Chinese) Xie S H, Zhu B K, Wei X Z, Xu Z K, Xu Y Y. Polyimide/ BaTiO3 composites with controllable dielectric properties. Composites: Part A, 2005, 36: 1152–1157 Liu W D, Liu X F, Zhu B K, Xie S H, Xu Z K. Study of the dielectric properties of polyimide/barium titanate composites and its influence factors. J of Functional Materials, 2007, 38: 1106–1109 (in Chinese) Hsiao S H, Chen Y J. Structure-property study of polyimides derived from PMDA and BPDA dianhydrides with structurally different diamines. Eur Polym J, 2002, 38: 815–828 Charlier Y, Hedrick J L, Russell T P, Jones A, Volksen W. High temperature polymer nanofoams based on amorphous, high Tg polyimides. Polymer, 1995, 36: 987–1002 Ma Y, Vileno E, Suib S L, Dutta P K. Synthesis of tetragonal BaTiO3 by microwave heating and conventional heating. Chem Mater, 1997, 6: 3023–3031 Cho S D, Paik K W. Relationship between suspension formulations and properties of BaTiO3/epoxy composite films for integral capacitors. IEEE Electronic Components & Technology Conference, 2001, 1418–1422