Comparative Study on Electrical Conductivity of CeO2-Doped AlN Ceramics Sintered by Hot-Pressing and Spark Plasma Sintering.

Aluminum nitride (AlN) ceramics were prepared by both Hot-pressing (HP) and Spark-Plasma-Sintering (SPS) using cerium oxide as the sintering aid. The characterization of AlN raw powder denoted the presence of an amorphous layer that led to the formation of aluminum oxide. During the sintering process, CeO2 introduced as a sintering aid was reduced into Ce2O3. The latter reacted with aluminum oxide to form a transient liquid phase that promotes sintering by both HP and SPS. A reactional path leading to the formation of secondary phases, such as CeAlO3 and CeAl11O18, has been proposed according to the pseudo-binary Al2O3 - Ce2O3. Ceramics obtained from HP and SPS are presented as similar, except for the secondary-phase distribution. The influences of secondary phase composition and distribution on electrical conductivity were evaluated by leakage current measurements. The mechanism of DC conduction and the global conductivity of ceramics were discussed according to the sintering process and the number of secondary phases.

Modeling the Electroelastic Moduli of Porous Textured Piezoceramics.

A new model for piezoelectric textured ceramics was developed that considers the presence of porosity, which can appear during heat treatment (ceramic sintering). In the long wavelength approximation, a matrix method, which has already been applied to piezoelectric composites, was extended to textured ceramics for three phases [porosity (air), piezoelectric single-crystal (related to the texturation degree), and ceramic] to calculate the effective electroelastic modulus. This method was first compared and validated with finite-element calculations. A computation was applied to two systems with lead-based (PMN-PT) and lead-free (KNN) compositions. The results showed that the introduction of porosity in the whole material promotes electromechanical performance, particularly the electromechanical coupling factor kt , while limiting the degree of texturation. As an example, for the chosen PMN-PT system, an equivalent kt factor of 60% can be obtained with 1% porosity and an 85% single-crystal volume fraction or with 16% porosity and a 40% single-crystal volume fraction. According to the database used, this tradeoff is different. With the chosen lead-free composition, the degree of texture is less important than in the lead-based composition. Consequently, the porosity content is of primary importance for significantly improving the electromechanical coupling factor kt .

Ferroelectric domain wall motion induced by polarized light.

Ferroelectric materials exhibit spontaneous and stable polarization, which can usually be reoriented by an applied external electric field. The electrically switchable nature of this polarization is at the core of various ferroelectric devices. The motion of the associated domain walls provides the basis for ferroelectric memory, in which the storage of data bits is achieved by driving domain walls that separate regions with different polarization directions. Here we show the surprising ability to move ferroelectric domain walls of a BaTiO3 single crystal by varying the polarization angle of a coherent light source. This unexpected coupling between polarized light and ferroelectric polarization modifies the stress induced in the BaTiO3 at the domain wall, which is observed using in situ confocal Raman spectroscopy. This effect potentially leads to the non-contact remote control of ferroelectric domain walls by light.

Correlation between the structure and the piezoelectric properties of lead-free (K,Na,Li)(Nb,Ta,Sb)O3 ceramics studied by XRD and Raman spectroscopy.

This article reviews on the use of Raman spectroscopy for the study of (K,Na,Li)(Nb,Ta,Sb)O(3) lead-free piezoceramics. Currently, this material appears to be one of the most interesting and promising alternatives to the well-known PZT piezoelectric materials. In this work, we prepare piezoceramics with different stoichiometries and study their structural, ferroelectric, and piezoelectric properties. By using both Raman spectroscopy and X-ray diffraction, we establish a direct correlation between the structure and the properties. The results demonstrate that the wavenumber of the A(1g) vibration is proportional to the tetragonality, the remnant polarization, and the piezoelectric coefficients of these materials. Thus, Raman spectroscopy appears as a very useful technique for a fast evaluation of the crystalline structure and the ferroelectric/ piezoelectric properties.

Piezoceramics properties as a function of the structure in the system (K,Na,Li)(Nb,Ta,Sb)O3.

The influence of the sintering conditions and stoichiometry on the crystalline symmetry and electrical properties of the system (K(0.44+x)Na(0.52)Li(0.04))(Nb(0.86)Ta(0.10)Sb(0.04)) O(3+x/2), with x = -0.06, 0.00, and 0.04, has been evaluated. By lowering the concentration of K cations, a faster stabilization of the tetragonal phase is reached. Increasing the sintering time also leads to the stabilization of the tetragonal phase at room temperature, as evidenced by Raman spectroscopy, thus improving the piezoelectric properties of these materials. The Raman spectra versus temperature showed 2 anomalies at temperatures of approximately 70 and approximately 270 to 330 degrees C, associated with polymorphism and the ferro-paraelectric phase transitions, respectively. Active Raman scattering modes are observed above Curie temperature, indicating that the symmetry is not cubic but pseudocubic. This pseudocubic phase is associated with relaxor diffuseness, attributed to composition fluctuations. In addition, the piezoelectric properties were correlated with the tetragonality of the system, showing a linear dependence between the piezoelectric properties and the tetragonality ratio. The ceramics with c/a = 1.011 ratio exhibit enhanced electrical properties, d(33) approximately 255 pC/N and k(p) approximately 0.47.