Dielectric Properties of Microwave Processed CaCu3Ti4O12 - Based Ceramics and Related (0-3) Epoxy-composites

Abstract

CaCu3Ti4O12 (CCTO) based ceramics are technologically important due to large frequency and temperature independent dielectric constant [104-105 at room temperature (RT)]. This material has drawbacks in terms of high ‘tan δ’, low dielectric break down strength (Ebd) and low volumetric energy density (Uvol). In this present work, high ‘tan δ’ value is utilized for synthesis by adopting microwave assisted solid state reaction (MASSR) and an attempt has been initiated towards the improvement of ‘tan δ’, ‘Ebd’ and ‘Uvol’ of CCTO based ceramics by incorporating Al3+, Nb5+, and (Nb5+0.5Al3+0.5) as substitutes of Ti4+site; therefore perturbing the TiO6 octahedron. Ceramics are synthesized via microwave assisted solid state reaction [MASSR] synthesis route. Ceramics used for this work are: (1) CaCu3Ti4O12, (2) CaCu3Ti4-xAlxO12 [CCTAxO], (3) CaCu3Ti4-xNbxO12 [CCTNxO], (4) CaCu3Ti4-x(Nb0.5Al0.5)xO12 [CCT(NA)xO]; x = 0.025, 0.05, 0.075, 0.1. Starting with single phase CCTO calcined powder; effect of sintering temperature in between 1000 to 1075 °C (with a step size of 25 ° and ceramics are assigned as CCTO1, CCTO2, CCTO3, and CCTO4 respectively) was investigated. CuO melted phase increases with increasing temperature. Maximum ‘εr’ found in CCTO3 (sintered at 1050 °C) is ~ 15,000 with tan δ ~ 0.15 at 100 Hz and amount of CuO secondary phase ~ 16 % (estimated from Rietveld analysis). In addition, non-ohmic voltage exponent (‘α’), breakdown field strength (‘Ebd’), volumetric energy density (Uvol), Shottky barrier height (ϕB) and barrier width related parameter (β) of CCTO3 are ~ 3.72, 9.7 kV/cm, 0.062 J/cm3, 0.836 eV and 2.4×10-3 (eV) V-1/2 cm-1/2 respectively. In modified CCTO ceramics, CuO secondary phase also appeared during sintering at 1050 °C. Variations of CuO phase with guest atom and ‘x’ are quite anomalous. Rietveld analysis has been performed to quantify the amount of monoclinic CuO phase. Microstructure evolution reveals the presence of melted phase and its Cu-rich nature has been confirmed through selective area EDX. Distinct variations in microstructure in terms of grain size, amount of melted phase, density are observed in this work. Dielectric, modulus, impedance, ac conductivity are studied as functions of frequency and temperature. CCTA0.05O, CCTN0.075O, and CCT(NA)0.075O are showing improved dielectric properties among individual batch of modified CCTO ceramics. Amount of CuO is found in between 10-16 % for the above mentioned composition. CCTN0.075O shows highest ‘εr (100 Hz)’ ∼ 37,239 with ‘tan δ’ ∼ 0.37 among all CCTNxO ceramics as well as other batches at RT. CuO secondary phase is ~ 10.5 %. ‘εr’ rapidly drops to ∼ 23,889 with ‘tan δ’ ∼ 0.223 and ∼ 5,708 with ‘tan δ’ ∼ 0.55 at 1 kHz and 1 MHz respectively. Lower ‘Ebd’∼ 4.58 kV/cm and ‘Uvol’ ∼ 0.035 J/cm3 are found for this composition. ‘ϕB’ and ‘β’ are ∼ 0.79 eV and ∼ 2.8 ×10-3 (eV) V-1/2 cm-1/2 respectively. CCTA0.05O shows maximum ‘εr (100 Hz)’ ∼ 23,000 with tan δ ∼ 0.08 among all CCTAxO ceramics at RT. The amount of secondary CuO phase is ∼ 12 % and high ‘Ebd’∼ 24.02 kV/cm, along with ‘α’ ∼ 5.13 and Uvol ∼ 0.59 J/cm3 has been found. ‘ϕB’ and ‘β’ are 0.84 eV and 1.5×10-3 (eV) V-1/2 cm-1/2 respectively. Maximum ‘εr (100 Hz)’ ∼ 29,648 with tan δ ∼ 0.2 has been found in CCT(NA)0.075O among all CCT(NA)xO ceramics. ‘εr (1 MHz)’ ∼ 12,995 is quite high with tan δ ∼ 0.45. Amount of CuO phase is ∼ 14.7 %. This composition shows ‘εr’ ∼ 24,173 at 1 kHz with ‘tan δ’ ∼ 0.15, and ‘εr’ remains highest throughout the frequency range between 1 kHz to 1 MHz. Maximum ‘Ebd’~ 28.8 kV/cm (at 1 mA/cm2 current density), along with ‘α’∼5.51 and highest energy density Uvol ∼ 1 J/cm3 has been achieved in CCT(NA)0.075O ceramic. Formation of ‘Shottky barrier’ is verified and maximum ‘ϕB’ ∼ 0.92 eV is observed in CCT(NA)0.075O. Barrier width parameter ‘β’ is ∼ 1.8 ×10-3 (eV) V-1/2 cm-1/2. Improvement in dielectric properties is analyzed on the basis of IBLC model in presence of CuO secondary phase. AC conductivity shows correlated barrier hopping mechanism (CBH). From the above results, CCT(NA)0.075O is considered as the best in terms of ‘εr’, ‘tan δ’, ‘Ebd’, ‘Uvol’ ‘α’, ϕB etc. In addition, an attempt has been initiated to prepare Epoxy-based composites by taking CCT(NA)0.075O (abbreviated as CCTNAO in Chapter 7) and conducting Al particle as fillers. Filler volume is restricted within 40 % in all composites. Composites prepared for the purpose are 1. (1-x) Epoxy – x Al ; x = 0, 0.05,0.1, 0.2, 0.3, 0.4 ,2. (1-x) Epoxy – x CCTNAO ; x = 0, 0.05,0.1, 0.2, 0.3, 0.4, 3. (1-x) [0.8Epoxy – 0.2 CCTNAO] – x Al ; x = 0, 0.05,0.1, 0.15, 0.2 While studying (1-x) Epoxy – x Al composites, maximum ‘εeff’ ∼ 150 achieved for x = 0.4, at 1 kHz, but loss became so high (tan δ > 10). But, the ‘εeff’ ∼ 70 with low loss (∼ 0.5 at 1 kHz) has been found for x = 0.3. At RT, highest ‘εeff’ ∼ 33.37 with tan δ ∼ 0.1 at 1 kHz has been found in 0.6 Epoxy - 0.4 CCTNAO, but ‘tan δ’ reduced to ∼ 0.055 beyond 100 kHz. Maximum ‘εeff’ ∼ 77.5 with tan δ ∼ 0.154 has been found in 0.64 Epoxy – 0.16 CCTNAO – 0.2 Al composite at 1 kHz. ‘εeff’ falls off to ∼ 72.5 with tan δ ∼ 0.1 at 10 kHz. Beyond 10 kHz, ‘tan δ’ is < 0.1. At 1 kHz, ‘εeff’ is ∼ 50 with tan δ ∼ 0.09 observed in 0.68 Epoxy – 0.17 CCTNAO – 0.15 Al composite. At 10 kHz, ‘εr’ is ∼ 47, with tan δ ∼ 0.06. Experimentally measured values of ‘εeff’ is in close agreement with EMT model (η = 0.13) and Yamada Model (η = 7) in (1-x) Epoxy – x CCTNAO composites. Proposed empirical power equation, εeff = εm (1+x)n with n ∼ 10 has a considerable agreement with the experimental data of (1-x) Epoxy – x Al and (1-x) [0.8Epoxy - 0.2 CCTNAO] – x Al composites.