Lead Free Ceramic Polymer Composites for Embedded Capacitor and Piezoelectric Applications

Abstract

In recent years, composite materials are on the frontier row of research field due to their greater technological importance. Modern composite materials constitute a significant proportion of engineered materials market ranging from everyday product to sophisticated niche applications. These wonder materials are extensively used in space vehicle to home building, electronic packaging to medical equipments, etc. The essence of a composite material is to combine the properties of two or more chemically and physically different phases resulting in a new material with different properties. In particular, composites based on ferroelectric ceramic as fillers in the polymer matrix have received great attention. The properties of these ceramic-polymer composites can be tailored by using different ferroelectric ceramics as fillers and different polymers as matrix for achieving the better electrical properties compared to the individual phases. The most successful ferroelectric ceramics are based on lead zirconates and lead titanates, therefore environmental concerns have been raised to find alternative ways to substitute with “lead-free” ferroelectrics, particularly those with properties comparable with their lead-based counterparts. Furthermore, the necessity for using transducers with lead-free materials in therapeutic and monitoring ultrasound devices, which requires the embedded system in the human body, is another driving force for lead-free ferroelectric investigations. Among the lead free ferroelectric materials, Ba(Zr0.2Ti0.8)O3-(Ba0.7Ca0.3)TiO3/(BZT-BCT) is a lead free ferroelectric system having perovskite structure exhibiting better piezoelectric, ferroelectric and electromechanical properties compared with lead based ferroelectric materials near a morphotropic phase boundary (MPB) 50BZT-50BCT composition at room temperature (RT).Recently miniaturization of electronic devices requires high dielectric constant materials with good temperature and frequency stability. Calcium copper titanate, CaCu3Ti4O12 (CCTO) has attracted much interest due to its extraordinary high dielectric constant. CCTO is a oxide based cubic perovskite like ceramics (AA’3B4O12), exhibiting gigantic dielectric permittivity of ~104- 5, which is frequency independent up to 106 Hz and is almost constant in the temperature range 100-400K . Therefore, CCTO is a promising high dielectric constant non-ferroelectric material which can be added within the composites for enhancing the dielectric properties without decreasing the resistivity substantially. There have been very limited studies reported on the lead free ferroelectric ceramic-polymer composites. PMMA, polyaniline (PANI), polyurethane (PU), etc. are used as polymer matrices in the ceramic-polymer composites. Recently, the focus of research is on PVDF and epoxy based polymers. This is due to the high mechanical strength of epoxy and high dielectric constant of PVDF polymers. In our knowledge, 0-3 composites of PVDF and epoxy and (BZTBCT) ferroelectric ceramics are not reported in the literature. Therefore, in this thesis work, various structural, dielectric and piezoelectric studies of (BZT-BCT) and PVDF & (BZT-BCT) and epoxy ceramic polymer composites are reported. With the addition of polymer to ceramics and hence forming the ceramic polymer composite, the dielectric constant of ceramics decreases drastically. In order to increase the dielectric constant and temperature stability of these (BZT-BCT) and PVDF and (BZT-BCT) and epoxy ceramic polymer composites, CCTO ceramics, having high dielectric constants and good temperature stability are further added.Structural, Optical & Electrical Properties of (BZT-BCT) & CCTO Ceramics The thermal analysis on (BZT-BCT) and CCTO systems hinted the phase formation of these systems to be ~ 1000 and 900oC, respectively. XRD studies confirmed the formation of single perovskite phase in the (BZT-BCT) and CCTO systems at 1300 and 1050oC for 4 h, respectively. The relative densities of the 0.48BZT-0.52BCT, 0.52BZT-0.48BCT and 0.50BZT-0.50BCT ceramics were found to be ~ 95, 96 and 97 %, respectively. Whereas, the relative densities of the CCTO ceramics sintered at 1050 and 1100oC were found to be ~ 91 and 94 %, respectively. The value of band gap energy Eg ~ 3.12 eV is found for the 0.50BZT- 0.50BCT ceramic sample, sintered at 1400oC. The RT value and the temperature stability of the dielectric constant (εr) of the CCTO system were found to be better compared to the (BZTBCT) MPB compositions. From dielectric measurements of the (BZT-BCT) MPB compositions, sintered at different temperatures, the Tc was found to vary between 96 to 115oC. Maximum value of εr at RT was obtained in (0.50BZT-0.50BCT) ceramics samples. The RT values of er and tand at 1 kHz frequency of the CCTO ceramic samples sintered at 1100oC were found to be ~ 11,537 and 0.21, respectively. Maximum value of piezoelectric coefficient (d33) ~ 281 pC/N was found in the 0.50BZT-0.50BCT system. The 0.50BZT-0.50BCT system sintered at 1400oC showed better microstructural, dielectric, ferroelectric and piezoelectric properties, whereas CCTO ceramics sintered at 1400oC showed better dielectric properties.