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Fabrication of barium titanate nanoparticles-polymethylmethacrylate composite films and their dielectric properties.

INTRODUCTION

Composite films composed of polymer films and dielectric ceramics such as titanates are of high interest in material science fields. The incorporation of titanates is expected to allow the host polymer to have high capacitance and to make it easy to process the films owing to the increased flexibility of the composite.

Various methods for fabricating titanate-polymer composite films have been reported. In most reports, the composite films are fabricated by mixing a dielectric polymer solution and submicron- or micron-sized ferroelectric particles, and evaporating the solvent of the polymer solution (1-6). However, the use of these particles cannot be applied to production of composite films with high transparency.

In the fabrication of electric devices with high capacitance, practical composite films need to have a thin film thickness of less than one micron. Surface smoothness of the films is another requirement for the fabrication of practical dielectric devices, because films with smooth surface show stable dielectric properties. The use of nanometer-sized titanate particles can help to address these requirements and moreover could allow formation of films with high transparency. Homogeneous dispersion of particles dispersed into the film could be expected to make the films transparent. For the homogeneous dispersion, the titanate nanoparticles are required to be colloidally stable in precursor solution, and affinity between the titanate nanoparticles and host polymer has to be strengthened. Many previous researchers have applied surface modification to the dispersion. Ramesh et al. fabricated barium titanate (BT)-epoxy nanocomposites with the use of BT nanoparticles surface-modified with various silanes (2). Among the silanes examined, the silanes with epoxy, thiol, and phenyl amino functionalities resulted in production of homogeneous composite materials and high dielectric constants. Kim et al. performed surface-modification of BT nanoparticles by phosphonic acids with functional groups, and fabrication of BT-polymer (polycarbonate or poly (vinylidenefluoride-co-hexafluoropropylene)) composite films (7). The composite films fabricated with the use of surface-modified BT nanoparticles yielded uniform films with homogeneous nanoparticle dispersions. In the work of Li et al. (8), BT-polyamide (PA)-bismaleimide (BMI) composites were fabricated with the use of phthalocyanine-coated BT nanoparticles. Addition of Ni nano-particles to the composites and the combination of PA with BMI were confirmed to increase their dielectric constants and to improve the processability of matrix, respectively. Yogo and coworkers (9) fabricated transparent and self-standing BT-poly(methylmethacrylate) (PMMA) composite films with polymerization of methylmethacrylatc (MMA) in the presence of the BT particles with the C=C bonds that were prepared with hydrolysis of complex alkoxide with 2-vinyloxyethanol. They also synthesized BT-polymer nanocomposite films with the use of alkoxide modified with methacryloxyethoxy group (10).

In previous work, we developed techniques for fabricating polymer films containing barium titanate nanoparticles (11-13). BT nanoparticles prepared in the presence of poly(vinylpyrrolidone) (PVP) are suitable for fabricating BT-polymer films with smooth surface when compared with cases in the absence of PVP (13). Particle surface-modification is effective for colloidal stability, and consequently for producing composite films of high-quality.

The present work proposes a method for fabrication of BT-PMMA composite films by the application of surface-modification of BT nanoparticles. The BT nanoparticles with a perovskite crystalline structure were prepared from complex alkoxide with a sol-gel method. To increase affinity between BT particle surface and PMMA host polymer and to stabilize the BT particles colloidally, the BT particles were coated with PMMA by polymerizing MMA monomer on the BT particle surface. Before the polymerization, C=C bonds were introduced onto the BT particle surface with silane coupling agents with the C=C bonds, so that the surface was expected to react with the MMA monomer. The BT-PMMA composite films were fabricated by spin-coating precursor solution containing PMMA and the PMMA-coated BT particles on glass substrates. Measurements were performed to study the effects of PMMA-coating and BT particle size on surface roughness and dielectric properties of the films.

EXPERIMENTAL

Materials

Starting reagents for the barium titanate were metallic barium (Kanto Chemical) and tetraethyl orthotitanate (TEOT) (97%. Tokyo Kasei Kogyo). Special grade reagents (Wako Pure Chemical, Ind.) of ethanol (99.5%. dehydrated) and N-methyl-2-pyrrodinone (NMP) (dehydrated) were used as solvents for alkoxide and PMMA, respectively. 3-methacryloxypropyltrimethoxysilane (MPTMS) (Shin-Etsu Chemical), MMA (98%, Wako Pure Chemical, Ind.), and 2.2'-azobis(isobutyronitrile (AIBN) (98%, Wako Pure Chemical, Ind.) were used for surface-modification of BT nanoparticles. PMMA powder (average [M.sub.w] = 75,000, Wako Pure Chemical. Ind.) was used as a host polymer. Sodium hydrogensullite (Na[HSO.sub.3]). sodium chloride (NaCl), sodium hydroxide (NaOH), and calcium chloride (Ca[Cl.sub.2]), which were obtained from Wako Pure Chemical, Ind., were used for purification of the MMA. The MMA was washed with aqueous solution of Na[HSO.sub.3], aqueous solution containing NaCl and NaOH, and aqueous solution of NaCl in turn. To the as-washed MMA, Ca[Cl.sub.2] was added for dehydration. After the removal of Ca[Cl.sub.2] by filtration, the MMA was distilled under a reduced pressure with nitrogen atmosphere. Other chemicals were used as received. Indium tin oxide (ITO) glasses (Furuuchi Chemical) and slide glasses (Matsunami Glass Ind.) were used as substrates for dielectrical and optical measurements, respectively. Water was distilled and deionized to have an electric resistance higher than 18 M[OMEGA] cm.

Synthesis of BaTi[O.sub.3] Nanoparticles

BT crystalline nanoparticles were prepared by the hydrolysis of a BT complex alkoxide. The metallic barium (0.008 mol) and the TEOT (0.008 mol) were dissolved in ethanol to have a total volume of 40 ml at room temperature. The ethanol solution was refluxed at 73[degrees]C for 2 h, eventually forming a transparent complex alkoxide. To hydrolyze the complex alkoxide, the solution was mixed with an equal volume (40 ml) of an ethanol/water mixture, and was kept at 70[degrees]C. After 5 h, the mixed solution turned opaque, which indicated the formation of the BT particles. In hydrolysis reaction, the [H.sub.2]O concentration ranged from 10 to 30 M.

PMMA-Coating

To introduce C=C bonds on BT particle surface, MPTMS was added to the BT particle colloid, and then the mixture was kept at 70[degrees]C for 1 h. The BT particles were deposited by centrifugation, and the dispersion medium (ethanol/water) was substituted with ethanol for a succeeding PMMA-coating, in which BT concentration was adjusted to 2 wt% by the addition of ethanol.

After de-aerating the BT particle colloid for 30 min by [N.sub.2] bubbling, and raising temperature of the colloid up to 60[degrees]C, MMA was added to the colloid. AIBN was added successively to the colloid to initiate the polymerization of MMA on the particle surface. After polymerization for 3 h. the dispersion media (ethanol) of the obtained PMMA-coated BT particle colloid was substituted with NMP by repeating evaporation of the ethanol and addition of NMP for a succeeding fabrication of BT-PMMA films.

Fabrication of BT-PMMA Composite Films

The PMMA powder was mixed with the PMMA-coated BT particle colloid prepared as in the previous section, and then the mixture was stirred at 70[degrees]C for 6 h. After stirring, the mixture was sonicated at room temperature for 3 h, then spin-coated on the glass substrates at 2000 rpm for 30 s, and dried on a hot plate at 100[degrees]C for 30 min. Volume fraction of the BT particles in the composite films was adjusted to 0-40% by varying the amount of added PMMA powder.

Measurements

The BT particles were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and dynamic light scattering (DLS). TEM photographs were taken with a Zeiss LEO 912 OMEGA microscope operated at 100 kV. Samples for TEM measurements were prepared by dropping and evaporating the particle suspensions on a collodion-coated copper grid. XRD measurements (RU-200A, Rigaku) were carried out at 40 kV and 30 mA with CuK[alpha] radiation using a monochromator. For powder XRD measurement, the BTcolloid was centrifuged to remove supernatant and then the residue was dried at 50[degrees]C for 24 h in vacuum. The crystal sizes of the BT particles were calculated with the Scherrer equation from the (110) peak of the XRD pattern. DLS measurements were performed with an Otsuka Electronics ELS-8000 for obtaining average sizes of BT particles suspended in solvents.

The films were characterized with atomic force microscopy (AFM) and UV-vis spectroscopy. Surface of the film was observed with AFM (NPX200, Seiko Instruments). Thickness of the film was measured with AFM. and root mean square roughness of the film surface was estimated from data with AFM images. Transmittance of the film was measured with a Hitachi UV-3010 spectrophotometer. The films fabricated on the slide glasses were used for the transmittance measurements.

The dielectric constant of the composite film was obtained from the capacitance of the composite film fabricated on the ITO glass, the area of the Au-electrode (1.96 X [10.sup.-7] [m.sup.2]) and the film thickness. Au-electrodes were patterned as upper electrodes on the film surface by sputtering. The capacitance and dissipation factor of the film were measured with an NF Electronic Instruments 2322 LCZ meter at frequencies of [10.sup.3]-[10.sup.5] Hz. All the measurements were performed at room temperature.

RESULTS AND DISCUSSION

BT Nanoparticles

Figures 1 and 2 show TEM images and XRD patterns of the BT particles prepared at different [H.sub.2]O concentrations. Particle sizes were 7.8 [+ or -] 1.6 nm for 10 M, 11.0 [+ or -] 2.7 nm for 20 M, and 24.0 [+ or -] 13.6 nm for 30 M All the XRD peaks were attributed to the perovskite cubic phase of the BaTi[O.sub.3]. Crystal sizes of the BT particles calculated for 10, 20, and 30 M were 8.60, 8.80, and 17.7 nm, respectively. The particle size and the crystal size increased with increasing [H.sub.2]O concentrations. The increase in the [H.sub.2]O concentration probably promoted solgel reaction of the alkoxide, succeeding crystallization and particle growth. Because the particle size observed with TEM was close to the crystal size, the particles formed were considered to be mainly consist of single crystals.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

PMMA-Coated BT Particles

Figure 3 shows IR spectra of uncoated BT particles and PMMA-coated BT particles. The BT particles with the size of 7.8 nm were used. Several peaks indicated with half parentheses were clearly detected for the PMMA-coated BT particles, whereas no dominant peaks were observed for the uncoated BT particles at positions of the peaks for the PMMA-coated BT particles. Referring to the literature (14-20), the peaks at 1007 and 1175 [cm.sup.-1] could be attributed to C--O, at 1407, 1457, and 2939 [cm.sup.-1] to C--H, and at 1708 [cm.sup.-1] to C=O. PMMA could form during reaction of MMA and AIBN according to the analysis results.

[FIGURE 3 OMITTED]

Figure 4 shows TEM images of the PMMA-coated BT particles. Darker and lighter parts of particles were identified as BT particles and PMMA, respectively. The BT particles that were coated with PMMA had a thickness of about 7 nm. Some aggregates of a size of submicron order could be observed, and these possibly formed during preparation of TEM samples. DLS measurements of the PMMA-coated BT particles in the NMP solution indicated that the dispersed particles did not contain submicron size particles. The average size of the dispersed particles measured with DSL was 32.6 nm, which was significantly smaller than 54.9 nm measured for the average size of the uncoated BT particles in NMP solution.

[FIGURE 4 OMITTED]

PMMA-coating of the BT particles with the sizes of 11.0 and 24.0 nm could be also made with the same procedure. All the obtained PMMA-coated BT particles were stable in NMP for at least 1 day after substitution of the solvent with NMP. This indicates that the PMMA-coated BT particles can be expected to be applicable to fabrication of BT-PMMA composite films with smooth surfaces due to their high colloidal stability in NMP.

Effect of PMMA-Coating on BT-PMMA Composite Films

Figure 5b and c shows AFM images of BT-PMMA composite films fabricated with the use of uncoated BT particles and PMMA-coated BT particles. The BT particles with the size of 7.8 nm were used, and BT volume fractions were adjusted to about 40%. For comparison, an AFM image of a PMMA film without BT particles (pure PMMA) is also shown in Fig. 5a. The pure PMMA film had a surface roughness of 1.24 nm. For the composite films, surfaces appeared rough when compared with the pure PMMA film. Surface roughness was 42.2 nm when uncoated BT particles were used and 8.14 nm when PMMA-coated BT particles were used. The large difference in surface roughness for the uncoated and coated particles probably can be attributed to the formation of BT particle aggregates. The PMMA-coating probably increases the affinity between BT particle surfaces and PMMA and stabilizes the BT particles colloidally. Consequently, aggregation of BT particles during film fabrication was suppressed.

[FIGURE 5 OMITTED]

Figure 6 shows UV-vis transmittance spectra of pure PMMA film and BT-PMMA composite films. Transmittance of the pure PMMA film was around 90% at wavelengths longer than about 350 nm. The transmittance decreased with dispersion of BT particles into PMMA films, because of scattering of BT particles. Transmittance for the BT-PMMA film fabricated with the use of uncoated BT particles was smaller than that for the PMMA-coated BT particles. Because scattering of particles becomes strong with an increase in particle size in a range comparable to wavelengths of ultraviolet and visible rays, these results in the present work indicated that large particles, or aggregation, were probably present in the composite films for uncoated BT particles. This means that the PMMA-coating prevented aggregation of BT particles, or provided homogeneous dispersing of BT particles, which corresponds to the observation by AFM shown in Fig. 5.

[FIGURE 6 OMITTED]

Figure 7 shows dielectric constant and tan [delta] of the BT-PMMA films as a function of volume fraction of BT particles. The dielectric constant tended to increase with an increase in the volume fraction, for both the composite films containing the uncoated BT particles and the PMMA-coated BT particles. The dielectric constant achieved 14.5 at 30% for the uncoated BT particles and 14.8 at 36% for the PMMA-coated BT particles. For tan S, there was no strong dependence on the BT volume fraction for both films. The average value of tan [delta] for the uncoated was 0.100. In contrast, tan [delta] for the uncoated and the PMMA-coated BT particles was as low as 0.06. The high affinity between BT particles and PMMA provided by PMMA-coating probably homogenized the BT-PMMA films. The low tan [delta] is probably related to the homogeneity of films although the mechanism for the low tan [delta] is still unclear. On the other hand, the PMMA-coated BT particles are suitable for fabricating BT-PMMA films with smooth surfaces, homogeneous dispersibility, and low tan [delta]. In subsequent experiments, the PMMA-coated BT particles were used for fabricating BT-PMMA films.

[FIGURE 7 OMITTED]

Figure 8 shows dielectric constant and tan [delta] of the BT-PMMA films fabricated with the use of PMMA-coated BT particles as a function of measurement frequency. Both dielectric constant and tan [delta] tended to decrease with an increase in the frequency. Space-charge effects diminish at high frequencies, which provides decreases in the dielectric constant and tan [delta] of film (21), which may apply to the present work.

[FIGURE 8 OMITTED]

Effect of BT Particle Size on BT-PMMA Composite Films

Figure 5c-e shows AFM images of the BT-PMMA composite films containing the BT particles with various sizes. As the particle size increased, film surface tended to become rough. Surface roughnesses were 8.14 nm for the particle size of 7.8 nm. 9.39 nm for 11.0 nm, and 24.9 nm for 24.0 nm, demonstrating that the surface roughness was affected by particle size.

Figure 9 shows dielectric constant and tan [delta] of the BT-PMMA composite films containing the BT particles with various sizes as a function of the BT volume fraction. There was no large difference in tan [delta] among the films examined. The dielectric constant increased with an increase in the BT volume fraction for all the composite films. As BT particle size increased, the dielectric constant appeared to increase. The dielectric constant achieved 19.8 for 24 nm-BT particles at 39%.

[FIGURE 9 OMITTED]

According to Lichtenecker's mixing model (22), dielectric constant of composite film, [epsilon], can be calculated, as follows.

In [epsilon] = [phi] In [[epsilon].sub.p] + (I - [phi]) In [[epsilon].sub.m] (1)

where and [phi], [[epsilon].sub.p] and [[epsilon].sub.m] stand for volume fraction of particles, dielectric constant of particles, and dielectric constant of matrix film, respectively. Fitting the measured dielectric constants with the equation in Lichtenecker's mixing model was performed with [[epsilon].sub.p] as a fitting parameter. A value of 5.18, which was obtained for pure PMMA film or at the BT volume fraction of 0 in the Figs. 7 and 8, was used as [[epsilon].sub.m]. Figure 10 shows data fitting of the measured dielectric constant at different BT volume fractions with the Lichtenecker's mixing model. Values of [[epsilon].sub.p] estimated by the fitting were 75.3 for the particle size of 7.8 nm, 105.1 for 11.0 nm, and 166.3 for 24.0 nm showing that [[epsilon].sub.p] decreases as the BT particle size decreases. Ferroelectricity of ferroelectric fine particles decreases with decreasing particle and grain sizes (23), and this may explain some of the trends observed in this work. The present work also revealed that the decrease in [[epsilon].sub.p] is related to the decrease in BT particle size.

[FIGURE 10 OMITTED]

CONCLUSIONS

Composite films of BT nanoparticles-PMMA were spin-coated on glass substrates with NMP solvent. The BT particles with particle sizes of 7.8-24.0 nm and crystal sizes of 8.60-17.7 nm, which were synthesized via hydrolysis reaction of complex alkoxide, was successfully coated with PMMA by polymerization of MMA initiated by AIBN with the aid of coupling reagent MPTMS. The composite films fabricated with the PMMA-coated BT particles were transparent, their surface appeared smooth, and their dissipation factors were low, when compared with those for uncoated BT particles. The dielectric constant of the composite films increased with increasing BT volume fraction in the composite film and the BT particle size. The dielectric constant reached 19.8 for the 39 vol%-BT-PMMA film with a BT particle size of 24.0 nm, which was about four times larger than the dielectric constant of the pure PMMA film. Dependence of dielectric constant of the films on BT volume fraction could be well explained with Lichtenecker's mixing model, and fitting with the model resulted in dielectric constants of BT particles of 75.3, 105.1, and 166.3 for the particle sizes of 7.8, 11.0, and 24.0 nm, respectively.

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Y. Kobayashi, (1) A. Kurosawa, (2) D. Nagao, (2) M. Konno (2)

(1) Department of Biomolecular Functional Engineering, College of Engineering, Ibaraki University, 4-12-1 Nakanarusawa-cho, Hitachi, Ibaraki 316-8511, Japan

(2) Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki-aza, Aoba-ku, Sendai 980-8579, Japan

Correspondence to: M. Konno; e-mail: konno@mickey.che.tohoku.ac.jp Contract grant sponsors: The Ministry of Education. Culture, Sports, Science and Technology, a Grant-in-Aid for the COE project. Giant Molecules and Complex Systems.

DOI 10.l002/pen.21352

Published online in Wiley InterScience (www.interscience.wiley.com). [C] 2009 Society of Plastics Engineers
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Author:Kobayashi, Y.; Kurosawa, A.; Nagao, D.; Konno, M.
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1USA
Date:Jun 1, 2009
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