The local and average structure of Ba(Ti, Ce)O3 perovskite solid solution: effect of cerium concentration and particle size

The amazing properties of ferroelectric perovskite BaTiO3 (BT) and its solid solutions make them indispensable for many technological applications (e.g. multilayer capacitors). Unfortunately, the so-called `size effect' limits their use. Indeed, under a certain critical particle size, these materials show a suppression of the spontaneous polarization and thus of the ferroelectric properties. In pure nanometric BaTiO3, this is related to a certain local structural disorder. However, only a few studies have explored BT solid solutions, where the doping effect, coupled to the reduced particle size, can play an important role. Therefore, in this work, the structure of BaCexTi1\u2013xO3 (x = 0.02\u20130.20) was explored by traditional Rietveld method and Pair Distribution Function. Samples present a particle size from 80\u2013160 nm to 400\u20131000 nm depending on increasing x. The carbox approach was applied, investigating the evolution of the local structure, its modifications and the structural coherent correlation length, as a function of cerium amount. Results demonstrate a cooperative effect of composition and reduced size in the ferroelectricity loss. The two, in fact, contribute to intensify the local structural disorder, decreasing the structural coherent correlation length. The local structural disorder is thus confirmed to be a relevant factor in the ferroelectric properties degradation

Colossal dielectric permittivity of BaTiO3-based nanocrystalline ceramics sintered by spark plasma sintering

In pursuit of high permittivity materials for electronic application, there has been a considerable interest recently in the dielectric properties of various perovskite oxides like calcium copper titanate or lanthanum doped barium titanate. When processed in a particular way, this later material present at ambient temperature and at f=1 kHz unusual interesting dielectric properties, a so called “colossal” permittivity value up to several 106 with relatively low dielectric losses. Moreover and contrary to what is classically expected and evidenced for this type of materials, no temperature dependence is observed. This behavior is observed in nanopowders based ceramics. An assumption to explain the observed properties is proposed. These results have important technological applications, since these nanoceramics open a new route to the fabrication of very thin dielectric films

Ferroelectric hollow particles obtained by solid-state reaction

Hollow particles of barium titanate were obtained by a two-step process combining colloidal chemistry and solid-state reaction. BaCO(3) crystals (size ≈1 µm) suspended in a peroxy-Ti(IV) aqueous solution were coated with an amorphous TiO(2) shell using a precipitation process. Calcination of the BaCO(3)@TiO(2) core-shell particles at 700 °C resulted in the formation of BaTiO(3) hollow particles (shell thickness of ≈70 nm) which retain the morphology of the BaCO(3) crystals. Formation of the cavity occurs because out-diffusion of the core phase is much faster than in-diffusion of the shell material. X-ray diffraction (XRD) and Raman spectroscopy indicate that the hollow particles possess a tetragonal ferroelectric structure with axial ratio c/a = 1.005. Piezoresponse force microscopy has shown strong piezoactivity and 180° ferroelectric domains. The process described provides a general route to fabricate hollow ferroelectric structures of several compounds

Defect chemistry and dielectric properties of Yb3+:CaTiO3 perovskite

The defect chemistry of Yb3+ :CaTiO3 solid solutions has been investigated both theoretically and experimentally. Three different incorporation mechanisms with similar solution energy were predicted for Yb3+ by atomistic simulation: i Ca site substitution with Ca vacancy compensation; ii Ti site substitution with O vacancy compensation; iii simultaneous substitution at both Ca and Ti sites with self-compensation. X-ray diffraction and scanning electron microscopy results strongly support the possibility to realize the above defect chemistries in CaTiO3 by changing the Ca/Ti ratio to force Yb3+ on the Ca site Ca/Ti 1 , on Ti site Ca/Ti 1 , or on both sites Ca/Ti=1 according to the calculations. The temperature dependence of the relative dielectric constant 102\u2013105 Hz of ceramics corresponding to predominant Yb substitution either at the Ca site or the Ti site is qualitatively similar to that of undoped CaTiO3. The Curie-Weiss temperature is shifted to more negative values in comparison to CaTiO3, suggesting that the compositions Ca1 123/2xYbxTiO3 and CaYbxTi1 12xO3 are further driven away from the ferroelectric instability. In contrast, the dielectric properties 102\u2013105 Hz of ceramics corresponding to Ca1 12x/2YbxTi1 12x/2O3 are radically different. The relative dielectric constant is increased of about one order of magnitude 2200 at 30 K , is almost independent of temperature, with a maximum variation of 20% in range of 20\u2013300 K, and shows frequency dispersion above 150 K. The loss tangent at 20\u2013300 K is 5% for frequencies 1 kHz. The possible mechanism for the observed dielectric behavior is discussed

Activated carbon monoliths from particle stabilized foams

Particle stabilized liquid foams are used as templates to obtain carbon-based porous materials with open cell structure, high specific area and hierarchical porosity. This route relies on gel casting to strengthen the structure of the liquid foams, followed by high temperature treatment to eliminate organic components and obtain solid foams. The liquid foam stabilizer was a commercial activated carbon powder, characterized by high porosity and irregular morphology of the particles, associated with a cationic surfactant. The micro-structure and the textural properties of the final solid materials have been investigated by Scanning Electronic Microscopy and Nitrogen adsorption methods. The results show that this method can be used to fabricate high specific area porous materials in the form of monoliths, with adequate consistence and mechanical resistance. The materials obtained seem promising for many practical applications such as gas adsorption, filtering and catalysis