A. Moure

Sodium niobate ceramics prepared by mechanical activation assisted methods

Abstract Sodium niobate is an interesting material that is nowadays considered to be used as a lead-free piezoelectric ceramic. However, it is very difficult to process fully dense ceramics of this composition starting from crystalline powdered precursors. It seems a priority to develop a new synthesis route for the processing of high quality ceramics. A new synthesis method based on high-energy milling, not yet explored for this type of materials, has been applied to obtain NaNbO 3 . Stoichiometric mixtures of analytical grade Na 2 CO 3 and Nb 2 O 5 were mechanically activated in a vibrating-type mill. A progressive broadening of the different X-ray diffraction peaks was observed when the milling time was increased. The starting products did not become amorphous. The thermal behaviour of the milled samples was studied by high temperature in situ X-ray powder diffraction and thermal analysis techniques. Ceramic materials were processed by natural sintering and hot-pressing or a combination of both. Ferro-piezoelectric properties were measured to assess their practical use as piezoceramics.

Piezoelectric ceramics based on Bi3TiNbO9 from mechanochemically activated precursors

Abstract Ceramics based on the Aurivillius type structure compound Bi 3 TiNbO 9 were processed by natural sintering and hot pressing of amorphous precursors obtained by mechanochemical activation of oxides and carbonates mixtures. Synthesis, grain growth and sintering take place in a single thermal treatment at moderate temperatures in comparison with ceramics processed from crystalline precursors. The influence of the processing parameters on the ceramic texture and microstructure at mesoscopic scale were studied by XRD and quantitative optical microscopy. It was possible to obtain both isotropic and textured ceramics. The occurrence of abnormal grain growth was observed under some conditions. An ample electrical characterisation of the ceramics was carried out comprising dielectric, ferroelectric and piezoelectric properties. The influence of the microstructure on the properties and the interest of the materials as high temperature piezoelectrics are discussed.

Temperature dependence of piezoelectric, elastic and dielectric coefficients at radial resonance of piezoceramics with an Aurivillius-type structure

Aurivillius-type structure compounds are considered good candidates for piezoelectric materials at high temperature, due to their high ferro-paraelectric phase transition temperature. Despite this fact, very few papers have been published on the study of piezoelectric properties at the expected working temperatures. An iterative automatic method has been used in this work to characterize the piezoelectric, electromechanical, and elastic properties at radial resonance of thin ceramic disks with composition (SrBi/sub 2/Nb/sub 2/O/sub 9/)/sub 0.35/(Bi/sub 3/TiNbO/sub 9/)/sub 0.65/[SBN/BTN 35/65], from room temperature up to the ferro-paraelectric phase transition. Ceramics were prepared by sintering or by recrystallization after hot-pressing of mechanically activated precursors. By this new method, ceramics with controlled texture and microstructure are obtained. The influence of the processing route in the properties of the ceramics, over the whole temperature range of piezoelectric activity, is discussed. Values of d/sub 31/=2.1 pC/N and k/sub p/=2.9% at 500/spl deg/C are achieved.

Microcharacterisation of grain-oriented ceramics based on Bi3TiNbO9 obtained from mechanochemically activated precursors

Abstract Hot pressing was applied to a novel powder, synthesised by the mechanochemical activation of starting oxides, in order to obtain dense ceramics of Bi 3 TiNbO 9 for use as piezoelectric material at high temperatures. Since these compositions belong to the family of layered perovskites, hot pressing produces a preferential orientation of the grains. An assessment of the degree of orientation achieved was carried out by quantitative texture analysis using experimental X-ray pole figures. Although texture could be considered as the most influential factor on the final properties, other microstructural features were studied by transmission electron microscopy including grain boundaries and ferroelectric domains. The results of the microcharacterisation of these ceramics are discussed in order to understand the process involved in the development of preferential orientation in these ceramics.

Texture and Microstucture Control in (SrBi 2 Nb 2 O 9 ) 1−x (Bi 3 TiNbO 9 ) x Ceramics

Texture and microstructure are determinant factors of the physical properties of piezoelectric ceramics. Among them we have those based on compositions (SrBi 2 Nb 2 O 9 ) 1 m x (Bi 3 TiNbO 9 ) x , with an Aurivillius-type structure. It has been shown that from mechanochemically activated precursors it is possible to obtain isotropic and highly densified (>99%) ceramics by hot uniaxial pressing at temperatures as low as 700°C. The ceramics obtained are difficult to pole due to the submicron grain size. In order to promote grain growth without affecting the high density achieved, a combination of hot pressing and natural sintering is tested. The isotropic character of the ceramics, i.e., the absence of texture, is monitored by X-ray diffraction and pole figures. Dielectric strength and piezoelectric response are measured and correlated to the porosity content and grain size.

Electric Anomalies and Their Relation with the Thermal Evolution of the Planar Resonance in Aurivillius-Type Structure Piezoceramics

Aurivillius-type structure ceramics are considered to be good candidates for use as high-temperature piezoelectrics. To this aim, it is essential to know the resonance characteristics in the working temperature range, rather than at room temperature. In this paper, an automatic iterative procedure has been used to characterize the planar resonance mode of ceramics with composition (SrBi 2 Nb 2 O 9 ) 0 . 3 5 (Bi 3 TiNbO 9 ) 0 . 6 5 (n = 2), processed by recrystallization after hot-pressing of mechanically activated precursors. The appearance of additional resonances at high temperatures is related to the anomalies in the electric characteristics of the ceramics (dielectric permittivity and losses and dc conductivity). These features are most probably associated with the presence of defects. The role of microstructure is discussed. The range of temperature in which these ceramics can be used as high-temperature piezoelectrics is established.

Synthesis and Characterization of Sr 2 [Sr n−1 Ti n O 3n+1 ] Series by Mechanochemical Activation

A new synthesis method, based on high-energy milling, has been applied to obtain the n=1 to 3 members of Sr 2 [Sr n-1 Ti n O 3n+1 ] Ruddlesden-Popper series, producing a very important decrease in the temperature and time reactions as compared with the traditional ceramic method. The mechanosynthesis of SrTiO 3 was observed during ball milling processes. Similar results were obtained with vibrating and planetarium milling systems, although the time scales involved are shorter when last one was used. The milled samples were annealed at different temperatures up to the formation of the layered perovskites in one single step. Final products as well as milled powders were studied by thermal analysis, infrared absorption spectroscopy and X-ray diffraction. The microstructure of ceramic samples with Sr 2 TiO 4 and Sr 3 Ti 2 O 7 compositions was investigated.

Piezoceramics from the solid solution SBN/BTN: Microstructure and electromechanical properties

Aurivillius-type compounds are good candidates for their use as high temperature piezoelectrics, due to their high ferro-paraelectric phase transition temperature. This is the case of Bi3TiNbO9 (BTN), with Tc > 900°C. However there are difficulties to pole ceramics with this composition, due to its high coercive field. To reduce it, solid solutions with SrBi2Nb2O9 (SBN, Tc ∼ 420°C) are used. It results in ceramics with higher piezoelectric activity. The study of the structure, texture and composition of the solid solution is determinant to understand and to improve the piezoelectric properties. In this work, ceramics of (SBN)1 − x(BTN)x composition, with x = 0.65 (Tc ∼ 780°C) have been obtained from mechanochemically activated precursors. The ceramics were obtained by natural sintering, hot-pressing or a combination of both. XRD and EDS were used to characterise the degree of solubility obtained with the different processing routes. Mechanical properties were studied by the Vickers indentation method, and an iterative automatic procedure was used to characterise the piezoelectric and elastic properties from impedance measurements at resonance of thin disks. The relationship between microstructure and electromechanical properties is discussed.