Soft chemistry synthesis of the perovskite CaCu3Ti4O12

The perovskite CaCu3Ti4O12 (CCT) has been obtained after calcination of oxalate precursors at 900–1000 °C in air. Those precursors are prepared using a soft chemistry method, the coprecipitation. The oxalate powders consist of disk-like particles of 2–3 μm diameter and 300–400 nm thickness. By varying the ratio of the initial amounts of metal chlorides, additional phases (CaTiO3, TiO2 and CuO) could be obtained besides CCT. The corresponding multiphased ceramics present improved dielectric properties

Dielectric properties of CaCu3Ti4O12 based multiphased ceramics

A “soft chemistry” method, the coprecipitation, has been used to synthesize the perovskite CaCu3Ti4O12 (CCT). Three main types of materials were obtained for both powders and sintered ceramics: a monophased consisting of the pure CCT phase, a biphased (CCT + CaTiO3), and a three-phased (CCT + CaTiO3 + copper oxide (CuO or Cu2O)). These ceramics, sintered at low temperature, 1050 °C, present original dielectric properties. The relative permittivity determined in the temperature range (−150 < T < 250 °C) is significantly higher than the one reported in the literature. Internal barrier layer capacitor is the probable mechanism to explain the particular behaviour. Moreover, the presence of a copper oxide phase beside the perovksite CCT plays an important role for enhancing the dielectric properties

Mixed manganese spinel oxides: optical properties in the infrared range

Spinel oxides in manganite family are studied in terms of optical properties in the infrared range (3–12 lm). The reflectivity is measured on sintered pellets. The complex refractive index is estimated by fitting hemispherical directional reflectance in both polarizations, perpendicular and parallel. The influence of different metallic cations (Ni, Co, Fe, Cu) is compared. In particular, in the case of manganese nickel copper oxides, the impact of variations in copper and nickel contents is evaluated. Cationic distribution is determined and correlated to the optical characteristics. These materials, usually used for NTC thermistor applications, are investigated for IR charges in coating

Magnetic properties of cobalt and manganese oxide spinel ceramics

Magnetic susceptibility measurements, magnetization and neutron diffraction results at low temperature for cobalt and manganese oxide spinel ceramics are presented. The Curie temperature varies similarly with the sample composition in ceramics and powders. The experimental molar Curie constant variation is explained by the presence of Co2+, CoIII, Mn3+ and Mn4+, and possibly Co3+ in the octahedral sites for the cobalt rich phases. The magnetic moments of the cations in tetrahedral and octahedral sites are not collinear and the global magnetization is oriented in a third direction

Investigation of nucleation and crystal growth kinetics of nickel manganese oxalates

The nucleation and the crystal growth rates of mixed nickel manganese oxalates have been determined from the changes of the ionic concentration of the solution and the crystal size distribution during the precipitation process within a supersaturation range 0–0.1 M. Thermodynamic solubility calculations have been used to identify the different species contributing the precipitation reaction and for estimation of the thermodynamic constant. Experimental data show that the nucleation rate of mixed nickel manganese oxalate in this supersaturation range is consistent with a primary heterogeneous mechanism and was found to obey to an exponential law. The crystal growth rates indicate a surface-integration-controlled mechanism with a first-order law with respect to the supersaturation

BaTiO3 thick fi lms obtained by tape casting from powders prepared by the oxalate route

BaTiO3 powders were prepared by co-precipitation via oxalate route. The size, morphology and particle size distribution of the oxalate powders have been optimized by the control of different synthesis parameters during the precipitation reaction (nature of salts, concentration of different solutions, aging time). The single phase BaTiO3 oxide particles were obtained after a thermal decomposition of the as-synthesized powders at 850°C for 4 hours under air atmosphere. Oxide powders with a suitable specifi c surface area were selected in order to obtain thick fi lms by the tape casting technique. The microstructure and dielectric properties of the thick films varied obviously depending on the deposition-calcination-sintering cycle used. A double depositioncalcination cycle followed by sintering, as well as a two step deposition-calcination-sintering procedure was used in order to improve the compactness and therefore, the dielectric behaviour. A higher dielectric constant value (~ 750) and lower dielectric losses (~ 2 %) were achieved at room temperature and at 1 kHz frequency for the dense, double-deposited fi lm obtained after two deposition-calcination-sintering cycles. For this film, a superior value of the dielectric constant (~ 1100), almost frequency independent in the frequency range of 100 Hz – 10 kHz was gained also at the ferroelectric-paraelectric phase transition temperature of 130°C

Thermochemistry of iron manganese oxide spinels

Oxide melt solution calorimetry has been performed on iron manganese oxide spinels prepared at high temperature. The enthalpy of formation of (MnxFe1−x)3O4 at 298 K from the oxides, tetragonal Mn3O4 (hausmannite) and cubic Fe3O4 (magnetite), is negative from x=0 to x=0.67 and becomes slightly positive for 0.670.6) spinels of intermediate compositions. The enthalpies of formation are discussed in terms of three factors: oxidation–reduction relative to the end-members, cation distribution, and tetragonality. A combination of measured enthalpies and Gibbs free energies of formation in the literature provides entropies of mixing. ΔSmix, consistent with a cation distribution in which all trivalent manganese is octahedral and all other ions are randomly distributed for x>0.5, but the entropy of mixing appears to be smaller than these predicted values for x<0.4

Hydrothermal synthesis of nanosized BaTiO3 powders and dielectric properties of corresponding ceramics

BaTiO3 fine powders were synthesized by hydrothermal method at 150 °C or 250 °C for 7 h, starting from a mixture of TiCl3 + BaCl2 or TiO2 + BaCl2. The size of the crystallites is close to 20 nm whatever the starting mixture and the reaction temperature. These powders are well crystallized and constituted of a mixture of the metastable cubic and stable tetragonal phases. The ceramics obtained after uniaxial pressing and sintering at 1250 °C for 10 h or 20 h present high densification (up to 99.8%). The Curie temperature (Tc) and the electrical permittivity ( r) of the ceramics strongly depend on the type of titanium source that has been used for preparing the powder and on the sintering dwell time. Particularly, Tc is shifted towards lower temperature when TiCl3 is used. The permittivity value at Tc of BaTiO3 sintered at 1250 °C for 10 h reaches 7000 and 11,000 with respectively TiCl3 and TiO2 used as titanium source

Internal friction investigation of phase transformation in nearly stoichiometric LaMnO3+δ

Rhombohedral LaMnO3+δ powders, prepared by two different soft chemistry routes (co-precipitation and hydrothermal synthesis), are sintered at 1400 °C for 2 h in air. Measurements of internal friction Q−1(T) and shear modulus G(T), at low frequencies from −180 to 700 °C under vacuum, evidence three structural transitions of nearly stoichiometric orthorhombic LaMnO3+δ. The first one, at 250 or 290 °C, depending on the processing followed, is associated to either a Jahn–Teller structural transition or a phase transformation from orthorhombic to pseudo-cubic. The second one at 610 or 630 °C is related to a phase transformation from pseudo-cubic or orthorhombic to rhombohedral. Below the Neel temperature, around −170 °C, a relaxation peak could be associated, for samples prepared according to both processing routes, to the motion of Weiss domains

Grain Growth-Controlled Giant Permittivity in Soft Chemistry CaCu3Ti4O12 Ceramics

We report a dielectric constant of up to 5.4105 at room temperature and 1 kHz for CaCu3Ti4O12 (CCTO) ceramics, derived from multiphase powders (coprecipitation products), made by a ‘‘chimie douce’’ (coprecipitation) method, and then sintered in air. The sintered products are pure-phase CCTO ceramics. The high dielectric constant is achieved by tuning the size of grains and the thickness of grain boundaries. The grain growth is controlled by varying the concentration of excess CuO in the initial powder (calcined coprecipitation products) between 1 and 3.1 wt%. The dielectric constant of pure CCTO ceramics increases with the initial CuO concentration, reaching its maximum at 2.4 wt% of CuO. A further increase of excess CuO in powders results in a permittivity decrease, accompanied by the formation of CuO as a separate phase in the sintered products. The unusual grain growth behavior is attributed to a eutectic reaction between CuO and TiO2 present in the initial powder