Calorimetric study of CaCu₃Ti₄O₁₂, a ceramic with giant permittivity

Abstract

We conducted an investigation into the thermodynamic properties of two stoichiometric CaCu3Ti4O12(CCTO) samples prepared by solid-state reaction and soft chemistry methods to probe the stability of the material relative to simpler oxide constituents (e.g., CaO, CuO, and TiO2) over a wide temperature range. Thermodynamic functions (i.e., heat capacity, formation enthalpies, entropies, and Gibbs free energies) have been measured from near absolute zero to 1100 K using calorimetric methods, including drop solution, low-temperature adiabatic relaxation, and differential scanning calorimetry. In addition, the thermodynamic characteristics of the magnetic-phase transition from the antiferromagnetic to the paramagnetic state are reported. It has been shown that CCTO is very stable relative to constituent oxides and calcium titanate at room temperature and higher, independent of the synthesis route. The enthalpic factor is dominant in the thermodynamics of CCTO, with the entropic factor having almost no effect on the stability of the compound relative to other oxide assemblages. The recommended values for the standard molar enthalpy of formation from constituent oxides and from elements at 298.15 K are −122.1 ± 4.5 and −4155.7 ± 5.2 kJ/mol−1, respectively. The mean of the third law entropy at 298.15 K is 368.4 ± 0.1 J/mol−1/K−1. Based on the thermodynamic data reported, the study confirms the possibility of CCTO decomposition in a reducing atmosphere or CO2 under conditions recently observed in experiments.

Control of Heteroepitaxial Growth of CaCu3Ti4O12 Films on SrTiO3 Substrates by MOCVD

“…From a thermodynamics point of view the CCTO is stable relative to the CTO phase. [35,36] Moreover, a case experiment has been performed by annealing, at 900 8C, the CCTO film obtained at 750 8C. We observed that neither structural nor morphological modifications of the CCTO film occurred.…”

“…In the literature, some studies have been carried out on CCTO ceramics in order to test their stability after post-growth treatments (mainly due to exposure to various oxygen and/or nitrogen partial pressures and temperatures) [34,35] and on phase relations in CaO-CuO/CuO 2 /TiO 2 systems. [36] It has been found that CTO, TiO 2 , CuO, Cu 3 TiO 4þd , and Cu 2 O phases can coexist, and it seems clear that they might occur either during the growth or after annealing treatments.…”

Dielectric properties and current–voltage nonlinear behavior of Ca1−xSrxCu3Ti4O12 ceramics

“…Recently, there has been considerable interest in the electrical properties of CaCu 3 Ti 4 O 12 (CCTO) [1][2][3][4][5][6][7][8][9][10], in particular, the origin of the "high permittivity" observed at radio frequencies over a wide temperature range. Impedance spectroscopy studies have revealed CCTO ceramics to be electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries.…”

High-temperature heat capacity and heat content of CaCu3Ti4O12 (CCTO)

“…Thermodynamic stability of CCTO has been explored recently [10,11]. Levchenko et al [10] measured heat capacity in the temperature range from 2 to 1100 K and formation enthalpy of CCTO using calorimetric methods.…”

“…Levchenko et al [10] measured heat capacity in the temperature range from 2 to 1100 K and formation enthalpy of CCTO using calorimetric methods. The standard molar enthalpy of formation from constituent binary oxides at 298.15 K is −122.1 (±4.5) kJ mol −1 [10]. The third-law entropy at 298.15 K was evaluated as 368.4 (±0.1) J mol −1 K −1 [10].…”