Developing the dielectric mechanisms of polyetherimide/multiwalled carbon nanotube/(Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 composites
© Su et al; licensee Springer. 2012
Received: 29 November 2011
Accepted: 16 February 2012
Published: 16 February 2012
Various amounts of multiwalled carbon nanotubes [MWNTs] were embedded into polyetherimide [PEI] to form PEI/MWNT composites, and their dielectric properties were measured at 1 MHz. The Lichtenecker mixing rule was used to find a reasonable dielectric constant for the MWNTs used in this study. The dielectric constants of the developed composites were significantly increased, and the loss tangents were significantly decreased as 2.0 wt.% (Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 ceramic powder [BSTZ] was added to the PEI/MWNTs to form PEI/MWNT/BSTZ composites. The Lichtenecker and Yamada mixing rules were used to predict the dielectric constants of the PEI/MWNT and PEI/MWNT/BSTZ composites. Equivalent electrical conduction models of both composites were established using the two mixing rules. In addition, the theoretical bases of the two mixing rules were used to explain the measured results for the PEI/MWNT and PEI/BSTZ/MWNT composites.
Keywordscomposites mixing rule dielectric properties electrical conduction mechanism
Discovered accidentally by Sumio Iijima in 1991 , carbon nanotubes [CNTs] were a new form of carbon with unique physical, electrical, and mechanical properties. The CNTs can behave either as a semiconductor or as a metal and may have a number of practical applications. CNTs have also been embedded into polymers to fabricate composites with good electrical properties, including dielectric constants with higher values and good thermal stability . In the present work, we investigate polymer/matrix composites with high dielectric constants using multiwalled carbon nanotubes [MWNTs] as fillers.
Polymer/ceramic composites with high dielectric constants have attracted much attention due to their simple, low-temperature processing and their flexibility. High-tech electronic devices require new materials with high dielectric constants, suitable dielectric properties, mechanical strength, and easy fabrication processes. Recently, polymer/ceramic composites have been studied in various applications, including integrated capacitors, acoustic emission sensors, and microwave substrates [3, 4]. When BaTiO3 was used as a dielectric material, although it had a relative high dielectric constant (above 1,000), the effective dielectric constants of composites with high BaTiO3 content still remained relatively low due to the lower dielectric constant of the epoxy matrix. Bai et al.  reported a high dielectric constant for a polymer matrix composite containing a large amount of ferroelectric ceramic particles, which made the composite lose its flexibility. On the other hand, using metal particles as a filler yielded polymer/metal composites with high dielectric constants as only a small weight percentage of conductive particles was added, but the thermal stability of the dielectric constants was not good . In previous reports, when the MWNTs were added to the polyetherimide [PEI] matrix and polyvinylidene fluoride/BaTiO3 composites, it enhanced the dielectric, thermal, and tensile properties of composites [7, 8].
The ratio of the passive elements to active components in mobile communication, computer, and consumer electronic devices is over 20, and nearly 70% of the circuit board area is occupied by discrete capacitors. Because of that, the cost and size of an electronic device will apparently increase. To solve these problems, embedded capacitor technology, which incorporates capacitors into one of the inner layers of a multilayer substrate, has been investigated. The important requirements for embedded capacitor materials are high dielectric constant, low capacitance tolerance, and low cost. In the present study, the dielectric properties of PEI/MWNT composites were developed first for the possible applications in embedded capacitors. The imide groups provide strength at high temperatures, while the flexible ether group linkages support a relatively easy processing. The properties of the MWNTs were similar to those of metals, and high dielectric constants were obtainable for the polymer/MWNTs with just a small weight percentage of MWNTs. The Lichtenecker mixing rule was used to find a reasonable dielectric constant for the MWNTs used in this study. (Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 [BSTZ] has a higher dielectric constant, lower dielectric loss, and broader dielectric peak , so BSTZ was added to the PEI/MWNT (MWNTs = 2.0 wt.%) composites to increase the dielectric constants and decrease the loss tangents of the PEI/MWNT composites. Finally, the Lichtenecker and Yamada mixing rules were used to predict the dielectric constants of the PEI/MWNT and PEI/MWNT/BSTZ composites.
A 125-ml round-bottom flask equipped with a condenser and a stirrer was charged with MWNTs, sulfuric acid (98%), and nitric acid (63%). The flask was sonicated for 30 min using an ultrasonic apparatus, and chemical oxidation was carried out at 60°C for 48 h. The diameter distribution of functionalized CNTs was 20 to 50 nm, and the length distribution was 2 to 15 μm. The MWNTs were functionalized with carboxylic acid groups (COOH) on their surfaces. BaCO3, SrCO3, TiO2, and ZrO2 were mixed to achieve the BSTZ ceramic. The powder was calcined at 1,100°C for 2 h; the calcined powder was uniaxially pressed into pellets, and then the pellets were sintered at 1,450°C for 2 h. Next, the ceramic was ground into a fine powder; the particle size distribution was 1 to 5 μm, and the average particle was 3 μm. Using an ultrasonic cleaner, the neat PEI was dissolved in dichloromethane [CH2Cl2] solvent, and the MWNTs were mixed with a solution of PEI and CH2Cl2 to form the PEI/MWNT composites. The PEI/MWNT/BSTZ composites were prepared using a special methylene chloride solvent mixing method, and commercial KD1 dispersant was added. The MWNTs and BSTZ ceramic powder in PEI matrix solutions were cast in a rotation mold at 60°C, and the residual solvent was vaporized in a vacuum at 60°C for 24 h. Fourier transform infrared [FTIR] spectra were used to identify the functional groups responsible for the chemical modification of the MWNTs. The morphologies of the PEI/MWNT/BSTZ composites were observed from scanning electronic micrographs [SEM]. The dielectric constants (εr) and loss tangents (tanδ) of the PEI/MWNT and PEI/MWNT/BSTZ composites were measured at 1 MHz using an LCR meter HP 4294 (Agilent Technologies Inc., Santa Clara, CA, USA).
Results and discussion
where n = 1/η is the morphology factor, which depends on the shape of the particles and their orientation relative to the composite surfaces. The morphology factor is any number between 0 and 1, with 0 representing all connections in parallel and 1 representing all connections in series.
First, the Lichtenecker mixing rule was used to find a reasonable dielectric constant of the MWNTs from the measured dielectric constants of the PEI/MWNT composites. The reasonable dielectric constant of the MWNTs was approximately 1015, and the conductivity was conjectured to be similar to that of a metal. The measured dielectric constants of the PEI/MWNT composites were compared with the predicted results from the two mixing rules, with εPEI = 4.1 and εMWNTs = 1015. According to the measurements, the η value of the Yamada equation was not constant. Therefore, an attempt was made to evaluate this parameter from the measured dielectric constants of the PEI/MWNT composites. As we know, the morphology factor of the PEI/MWNT composites changed from 0.473 to 0.271 as the MWNT content increased. Figure 3 also shows that the measured dielectric constants of the PEI/MWNT composites with higher MWNT content are different from the Lichtenecker-predicted results but agree closely with the Yamada-predicted results.
In this investigation, a conductivity material in the form of MWNTs and a ferroelectric material in the form of BSTZ ceramic powder were added to a PEI matrix to form PEI/MWNT and PEI/MWNT/BSTZ composites. The dielectric constants of the PEI/MWNT composites increased from 3.9 to 9.7 as the MWNT content increased from 0 to 2.5 wt.%. The dielectric constants of the PEI/2 wt.% MWNT/BSTZ composites increased from 14.2 to 35.8 as the BSTZ content increased from 10 to 70 wt.%. The loss tangents of all the PEI/2 wt.% MWNT/BSTZ composites measured at 1 MHz were less than 0.05. Using the Lichtenecker and Yamada mixing rules, equivalent electrical conduction models of the PEI/MWNT and PEI/2 wt.% MWNT/BSTZ composites were established. The results indicate that these PEI/MWNT and PEI/2 wt.% MWNT/BSTZ composites are attractive materials for applications in electrical devices.
The authors acknowledge the financial support of NSC 99-2221-E-390-013-MY2, NSC 99-2221-E-390-006-, and NSC 100-3113-S-244-001-.
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