Dielectric and Mechanical Properties of CaCu3Ti4O12 and La, Nb, Sn and Zr doped CaCu3Ti4O12 / Poly(vinylidene fluoride) Composites

Abstract

As electronics industry is progressing towards higher functionality, miniaturization of electronics devices has become a main concern of engineers. As capacitor attracts special attention due to variety of function it performs, lot of research work is going on to create a high dielectric permittivity material which can reduce the size of capacitor. Ceramics have high εˈ, but their brittleness and challenging processing conditions impede their use as high-k materials. On the other hand, polymers have the advantage of easy processing, mechanical flexibility and low cost. But low dielectric permittivity in the range of 2–5 is impeding their use for high-k applications. To overcome these drawbacks, new composites associated with high dielectric permittivity can be fabricated by combining the merits of polymers and ceramics. Recently, CaCu3Ti4O12 (CCTO) powders has gained considerable attention to be used as a filler in the polymer matrix to develop high dielectric permittivity composites for potential capacitor applications due to its large dielectric permittivity (ε~104–5) which is nearly independent of frequency and temperature (100–600 K range). The present thesis is divided into following nine chapters. Chapter 1: High dielectric permittivity Polymer Ceramic Composites (PMC) - An Introduction and Literature Review The present chapter focuses on the overview of high dielectric permittivity Polymer Ceramic Composites along with literature review. Continuous demand for miniaturization of electronic devices such as capacitors has created a need for material having high dielectric permittivity. High dielectric permittivity (High ε') polymerceramic composites have become potential materials for this purpose. A high permittivity material can store more electric energy than a one with low permittivity. Lot of work has been done using ferroelectric materials such as BaTiO3, Pb (Zr,Ti)O3 etc having high dielectric permittivity as fillers in these composites. In ferroelectric oxides such as BaTiO3 or relaxors such as (Bi,Sr) TiO3, temperature dependence of the permittivity near transition temperature is very large. It is not a desirable feature. xi CaCu3Ti4O12 (CCTO) has attracted increasing scientific and technological interest because being lead-free, it is environment friendly, a high dielectric permittivity ceramic and its dielectric permittivity is nearly temperature (T)-independent in the temperature range 100-600K. Among various polymers viz polyethersulfone, epoxy, cyanate ester, and poly(vinylidene fluoride) (PVDF), PVDF is widely used as a matrix in these composites due to its ferroelectric and thermoplastic properties. Therefore, in the present thesis, investigations have been done on CaCu3Ti4O12 and La, Nb, Sn, Zr doped CaCu3Ti4O12 dispersed Poly(vinylidene fluoride) composites. Chapter 2: Aims and Objectives of Present Work From literature survey it was found that lot of work has been done on BaTiO3 and undoped CaCu3Ti4O12 dispersed polymer composites. But there was a problem with these composites. In case of BaTiO3 dispersed polymer composites dielectric permittivity could not be increased beyond 50 even at a very high content of loading. Whereas in case of CCTO dispersed polymer composites high dielectric permittivity has been achieved, only at a very high content of dispersion. Drawbacks with these composites are deterioration of mechanical strength, agglomeration and porosity at higher content of the ceramic dispersion. Therefore, the aim and objective of the present investigation is to firstly enhance the dielectric permittivity of CCTO by various dopants such as La, Nb, Sn, Zr on suitable sites and then to disperse these ceramics in the PVDF matrix. So that high dielectric permittivity polymer ceramic composites can be developed at a low content of ceramic, without affecting the mechanical strength and avoiding the above mentioned problems. The fabrication and characterization of these composite specimens is carried out in the following manner:  Synthesis of CCTO and La, Nb, Sn, Zr doped CCTO by solid state method and semi wet method.  Preparation of CCTO and La, Nb, Sn, Zr doped CCTO dispersed PVDF composites by extrusion method.  Determination of the phases using X-Ray Diffractometer.  Study of microstructure using Scanning Electron Microscope.  Study of thermal behaviour of composites using Thermogravimetric analysis (TGA). xii  Study of mechanical behaviour of composites by performing tensile test.  Dielectric measurements of the samples in the frequency range 10-2 - 106 Hz using two probe Novocontrol set up (ZG4) from room temperature (40oC) to 120°C.  Study of Temperature-dependent dielectric relaxation by Havriliak-Negami (H-N) function using Win fit software. Chapter 3: Experimental Work This chapter focuses on (i) synthesis of the CCTO and La, Nb, Sn, Zr doped CCTO by solid state method and semi wet method, (ii) preparation of CCTO and La, Nb, Sn, Zr doped CCTO dispersed PVDF composites by extrusion method, (iii) details of different characterization techniques such as XRD, SEM, TGA, Tensile Test and Measurement and Analysis of dielectric properties. To synthesise CCTO and La, Nb, Sn, Zr doped CCTO powders of CaCO3 (99.98%), CuO (99.5%), TiO2 (99.55), SnO2 (99.98%), Nb2O5 (99.97%) and ZrO2 (99.98%) were used as starting materials. Powders taken in stoichiometric amount were mixed and ground for 12 hours. These were calcined in air at 10000 C for 12 hours with intermittent grinding. Formation of single phase solid solution was confirmed by powder X-ray diffraction (XRD) using CuKα radiation. After completion of calcination process, powder was ground and mixed with 2% PVA (Molecular weight 37000) solution to make pellets of 15 mm diameter and 2 mm thickness under a load of 6 tons using a hydraulic press. The pressed pellets were sintered at 10000C for 6 hours. Sintered pallets were again ground to make fine powder using an agate mortar and pestle. Composite preparation: Melt extrusion process was used for making composites of PVDF- CCTO and La, Nb, Sn, Zr doped CCTO. Twin-screw extruder (Hakke Mini Lab) was used for extrusion. Mixing was done at 200°C for ~15 minutes under a speed of 70 rpm. During melt mixing ceramic particles mix uniformly with the polymer chains. Approximately 100 μm thick films of these composites were made using compression molding machine at 200oC under a load of 5 tons. Phase analysis: X-ray diffraction (XRD) patterns were recorded using Rigaku Desktop Miniflex II X-Ray diffractometer employing Cu-Kα radiation (wavelength, xiii λ= 1.5418 Å) and Ni-filter. PVDF and composites were scanned in the 2θ angle range 10 - 90° at a 3°/min. Microstructure: SEM images were recorded using INSPECT S 50 FP 2017/12 Scanning Electron Microscope. Samples were coated with gold to make surface conducting. Thermal analysis: Thermogravimetric analysis (TGA) of PVDF and composites was done from 30 to 700°C at a heating rate of 10°C/min in air using Perkin-Elmer, USA TGA/DTA Analyser. Mechanical Properties: Tensile tests were performed on the microinjected dog bone shaped samples at room temperature using Instron 3369 Tensile Machine. A constant crosshead speed of 5 mm/min was selected and the stress–strain data were recorded till the samples broke. Three samples of each composition were tested. Dielectric measurements: Dielectric measurements were performed on disc-shaped films having 12 mm diameter. These were silver coated on opposite faces and measurements were made between 10-2 - 106 Hz using four probe Novocontrol set up (ZG4) from room temperature (40oC) to 120°C. H-N function: Temperature-dependent dielectric relaxation has been explained by Havriliak-Negami (H-N) function using Win fit software. Chapter 4: Dielectric and Mechanical properties of CCTO/ PVDF Composites The present chapter describes the structural, dielectric and mechanical behavior of CCTO / PVDF composites. 10, 20 and 50 wt% CCTO dispersed PVDF composites were prepared by melt extrusion process. X-ray diffraction (XRD) patterns confirm the successful formation of CCTO as well as composites. SEM micrographs of pure PVDF and composites show that spherulitic morphology of PVDF gets severely affected by the dispersion of CCTO in composites. Addition of CCTO ceramic fillers influences the thermal decomposition behaviour of PVDF. There is considerable increase in the value of Young’s modulus calculated from the slope of the linear region of the plots. This increase in Young’s modulus with increase in weight percent of filler can be attributed to interaction between stiffer ceramic and PVDF matrix making composites stiffer as compared to pure PVDF. xiv Dielectric permittivity increases with increase in CCTO content in PVDF. Dielectric permittivity increases with decreasing frequency and increasing temperature. Dielectric loss slightly increases with increase in the CCTO content in comparison to pure PVDF. Dielectric loss increases slightly with increasing temperature and decreases with increasing frequency. Chapter 5: Dielectric and Mechanical properties of La doped CCTO/ PVDF Composites This chapter explains the effect of La doping on CCTO (LaCCTO) and studies the structural, dielectric and mechanical properties of PVDF dispersed with 10, 20 50 wt % LaCCTO. Composites were prepared my melt extrusion process. X-ray diffraction (XRD) patterns confirm the formation of CCTO and LaCCTO as well as composites. No secondary phase was present. SEM micrographs of PVDF and composites indicate that there is homogeneous distribution of ceramic in PVDF matrix. Spherulitic morphology of PVDF changes completely with ceramic dispersion. Addition of LaCCTO ceramic fillers influences the thermal decomposition behaviour of PVDF polymer. Young’s modulus increases considerably with the increase in LaCCTO content in PVDF. Dielectric permittivity increases with the increase in La doped CCTO content in PVDF. Dielectric permittivity increases with decreasing frequency and increasing temperature. This is always observed in the composites due to interfacial polarization. Dielectric loss slightly increases with the increase in CCTO content in comparison to pure PVDF. Dielectric loss increases slightly with increasing temperature and decreases with increasing frequency. Chapter 6: Dielectric and Mechanical properties of Nb doped CCTO/ PVDF Composites Results of investigations of the structural, dielectric and mechanical behavior of Nb doped CCTO / PVDF composites are presented in this chapter. PVDF with 10, 20 and 50 wt% Nb doped CCTO composites have been prepared by melt extrusion method. X ray diffraction peaks confirms the formation of single phase compound. There is no evidence of presence of any secondary phase in NbCCTO. SEM micrographs of PVDF and composites indicate that homogeneous distribution of ceramic has taken xv place in PVDF matrix. There is considerable increase in the value of Young’s modulus. On doping with Nb, there is an increase in the value of dielectric permittivity. Dielectric permittivity increases with decreasing frequency and vice versa. Dielectric permittivity increases with increasing content of NbCCTO in PVDF. It is important to note that dielectric permittivity does not change much over the frequency range 102-106 Hz. It is a desirable feature for use in device. In composites, dielectric loss is slightly more than that in pure PVDF. Chapter 7: Dielectric and Mechanical properties of Sn doped CCTO/ PVDF Composites The present chapter reports the structural, dielectric and mechanical properties of Sn doped CCTO / PVDF composites. Sn doped CaCu3Ti4O12 (CCTO) was prepared by solid state ceramic method. PVDF/SnCCTO composites were prepared by melt extrusion method. Formation of single phase solid solution in SnCCTO was confirmed by powder X-ray diffraction (XRD) using CuKα radiation. Pure PVDF exhibits spherulitic morphology. It is observed that the spherulitic morphology of pure PVDF is significantly changed by dispersion of SnCCTO powder. Composites have higher value of Young’s modulus than that of PVDF. Dielectric permittivity of PVDF increases with increasing content of SnCCTO. Dielectric loss increases slightly with increasing temperature and decreases with increasing frequency. Chapter 8: Dielectric and Mechanical properties of Zr doped CCTO/ PVDF Composites This chapter describes the structural, dielectric and mechanical behavior of Zr doped CCTO / PVDF composites. Zr doped CaCu3Ti4O12 (CCTZO) was prepared by solid state synthesis method. CCTZO dispersed PVDF (PVDF-ZrC) composites have been prepared by melt extrusion method. X ray diffraction patterns confirm the successful formation of single phase Zr doped CCTTO and composites. Microstructural, dielectric and mechanical properties have been investigated. With CCTZO dispersion morphology of PVDF completely changes. This indicates that there is a homogeneous dispersion of CCTZO. Composites exhibit higher Young’s modulus than that of PVDF. Dielectric permittivity increases with the increase in CCTZO content. Dielectric loss increases slightly with increasing temperature and decreases with increasing frequency. Chapter 9: Conclusion and Scope for Further Research Work This chapter outlines the conclusions of work done on CaCu3Ti4O12 and La, Nb, Sn and Zr doped CaCu3Ti4O12 / Poly(vinylidene fluoride) composites prepared by melt extrusion method. Future work which can be done on the high dielectric permittivity polymer ceramic composites is also proposed in the present chapter. CCTO and La, Nb, Sn, Zr doped CCTO have been successfully prepared.CCTO and La, Nb, Sn, Zr doped CCTO dispersed PVDF composites were made by extrusion method. X-Ray Diffraction patterns confirmed the formation of single phase desired ceramics as well as composites. Scanning Electron Microscopy shows the homogeneous distribution of ceramic in the PVDF matrix.Thermogravimetric analysis (TGA) shows that composites exhibits better thermal stability than pure PVDF. Results of tensile test indicate that composites have better mechanical strength.Dielectric measurements show a considerable enhancement in the dielectric permittivity of PVDF with the ceramic dispersion. It is expected that the results of the present investigations will be helpful in developing high dielectric permittivity polymer ceramic composites for embedded capacitor applications. Dielectric strength is an important parameter from the application point of view. This needs to be studied. Effect of the particle size of the ceramic on the dielectric as well as mechanical behavior can also be studied.