Showing papers by "Marija Kosec published in 2012"

Structure and the Electrical Properties of Pb(Zr,Ti)O3 – Zirconia Composites

Abstract: In this work, the effect of introducing tetragonal yttria-stabilized zirconia (TZ) particles in soft [Pb0.98Ba0.01][(Zr0.53Ti0.47)(0.98)Nb-0.02]O-3 (PZT) was investigated. Both microstructure and electrical properties of the PZT-xTZ (x = 0, 2, 5, 10, 20 vol%) composites were studied and correlated. The addition of zirconia hinders the matrix grain growth, resulting in smaller grains. According to X-ray diffraction analysis, zirconia containing composites have a higher rhombohedral-to-tetragonal phase ratio, in addition to lower dielectric and piezoelectric properties, in comparison to pure PZT. Electrical properties, in terms of strain- and polarization-electric field hysteresis curves, are presented and correlated with the observed phase compositions and microstructures. The extrinsic contribution to the piezoelectric properties in PZT and PZT-xTZ was studied by measuring the frequency and the stress field amplitude dependences of the piezoelectric d(33) coefficient.

Lattice dynamics and broad-band dielectric properties of the KTaO3 ceramics

Abstract: High-density KTaO3 ceramics were synthesized and studied by means of microwave, terahertz, infrared, and Raman spectroscopies. The results were analyzed together with recently published radio-frequency data. [S. Glinsek et al., J. Am. Ceram. Soc. 94, 1368 (2011)] Three polar modes expected for the cubic structure were observed. As in single crystals, the lowest-frequency TO1 mode (soft mode) strongly softens on cooling, while the TO2 and TO4 mode frequencies do not change with temperature. The permittivity does not show any significant dispersion below the soft mode frequency and its value in the kHz and GHz range is mainly given by the intrinsic polar lattice modes contribution. The soft mode frequency agrees with the values found in single crystals; this indicates a negligible influence of the grain boundaries on the dielectric response in KTaO3 unlike in other ferroelectric or incipient ferroelectric perovskite ceramics.

Dielectric Response of BaTiO3 Thin Film with Grain Size at Nanometer Scale

Abstract: In ceramic BaTiO3 thin film prepared by chemical solution deposition, the influence of small grain size (10–150 nm) and grain boundaries on ferroelectric behavior is investigated by the studies of low-frequency dielectric response. The apparent permittivity is suggested to result from an ensemble of grains possessing different properties and volume fractions, and having a nonferroelectric boundary layer. The effective permittivity of the boundary layer is found to be close to that of an interfacial layer in epitaxial thin-film ferroelectrics, indicating possible fundamental resemblance of these layers. The maximum effective permittivity of the grain interiors is estimated to be about 1500. The observed small intrinsic grain permittivity, small Curie constant, broad dielectric peaks, and high-temperature dielectric hysteresis are discussed in terms of size-induced changes of the phase diagram.

Using Infrared Spectroscopy to Identify New Amorphous Phases - A Case Study of Carbonato Complex Formed by Mechanochemical Processing

Abstract: Since the first laboratory experiments of M. Carey Lea and the original definition by F. W. Ostwald at the end of the 19th century, mechanochemistry, a field treating chemical changes induced in substances as a result of applied mechanical stress, has been evolved as an important area of chemistry from the viewpoint of both the fundamental research and applications (Takacs, 2004; Boldyrev & Tkacova, 2000). Whereas the fundamentals of mechanochemistry are still being extensively explored, the mechanical alloying, a powder metallurgy process involving ball milling of particles under high-energy impact conditions, met the commercial ground as early as in 1966 and was used to produce improved nickeland iron-based alloys for aerospace industry (Suryanarayana et al., 2001). In addition to metallurgy, the science and technology of mechanochemical processes are continuously developing within various other fields, including ceramics processing, processing of minerals, catalysis, pharmaceutics, and many others.

Top-down processing of NaNbO 3 nanopowder

Abstract: We report the processing of NaNbO3 nanopowder by combining the solid-state synthesis and subsequent milling in the agitator bead mill. The effect of different rotation speeds of the agitator shaft on the comminution process was followed by laser granulometry. The morphology and specific surface area of the powders were investigated by scanning electron microscopy and the N2 adsorption method, respectively. With the optimized milling parameters, we obtained NaNbO3 nanoparticles with an average size of 25nm and a narrow particle size distribution. The result is comparable to other processing techniques, such as solutionbased chemical routes or mechanochemical synthesis; however, the presentedmethod does not require any complicated processing and it can be easily upscaled to yield large quantities of the NaNbO3 nanopowder. Furthermore, the compaction behaviour of the obtained nanopowder was investigated, and a compaction-response diagram was constructed revealing good compactability of the powder. The green compacts, isostatically pressed at 740MPa, had a relative density of 70% and a narrow pore size distribution with an average pore radius of 4 nm.

Combined Structural and Quantitative Texture Analysis of Morphotropic Phase Boundary Pb(Mg1/3Nb2/3)O3‐PbTiO3 Ceramics

Abstract: The combination of the quantitative texture analysis and Rietveld methods is used to study the preferential orientation of morphotropic phase boundary (MPB) Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics obtained by homogeneous Templated Grain Growth. Their complex structure, with the coexistence of several polymorphs, and the presence of texture that is required to take advantage of their excellent piezoelectric properties, raise serious problems for a precise characterization of these materials. Conventional methods of texture analysis, although simple, are limited, and the information obtained, if not studied with care, may be misleading. The use of linear detectors and χ-scan are more appropriate for the study of diffraction peaks with several contributions as it is the case for MPB compositions. This work shows the simultaneous determination of structural and texture parameters through the application of the so-called combined method, already used for the analysis of similar problems in a range of polycrystalline materials, to this specific case.

Ferroelectric domain configurations in 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 thick films determined by piezoresponse force microscopy

Abstract: The spatial distribution of the out-of-plane and in-plane components of ferroelectric polarization in (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 thick films with x = 0.35 on Al2O3 substrates has been studied by piezoresponse force microscopy. Complex non-180° domain configurations with strong variations in characteristic length (from micrometre size down to the nanoscale) and morphology are found. This is most probably the result of the slowing down of the kinetics of the relaxor to ferroelectric transition on approaching a grain size around the micrometre, an effect previously reported for bulk ceramics and that results in the stabilization of submicrometre and nanoscale intermediate domain configurations. Nevertheless, fluctuations of the in-plane compressive stress exerted by the substrate must exist and play a role, as indicated by in-plane polarization regions next to grain boundaries. Film stress might also be responsible for conventional 180° domains not being found, and for the appearance of 180° superdomain bands formed from bundles of submicrometre-size non-180° domains instead, as previously described in Pb(Zr,Ti)O3 films. Also, the presence of polar nanodomains and the possibility of them being observed are addressed. Finally, the mobility of non-180° domain walls under electric fields is clearly demonstrated, which is the key to relatively high electromechanical response in films.

Structural evolution of the KTa0.6Nb0.4O3 alkoxide-based solutions: probing the transition metals local environment by X-ray absorption spectroscopy

Abstract: The KTa0.6Nb0.4O3 sols for chemical solution deposition of thin films were prepared from potassium acetate and transition metal ethoxides by the 2-methoxyethanol based route. The local environment of both transition metals after reflux times 1, 4, 24, and 48 h, whereby the crystallization behavior of the films was strongly affected, was monitored by extended X-ray absorption fine structure spectroscopy, at Ta L3 and Nb K edges. The Ta species existed in the sols as monomers, remaining stable even with prolonged reflux time. The Ta–O–K correlations were confirmed in all cases. In contrast, the Nb-alkoxide formed dimers, with a gradual formation of oligomeric species with prolonged refluxing. The Nb–O–K correlations were present after all reflux times. The number of K neighbours around Nb increased upon refluxing, saturating at 24 h.

Synthesis of 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 by Controlled Agglomeration of Precursor Particles

Abstract: 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 powder was prepared by a single-step, solid-state synthesis at 850°C. This was enabled by the controlled agglomeration of precursor particles by the change of pH in water suspensions. With the design of the contacts between the particles in the agglomerates, a phase-pure perovskite powder was synthesized from a suspension with a pH = 12.5. At the inherent pH = 11.4, the agglomerates that promote the pyrochlore formation were formed, resulting in a mixture of perovskite and pyrochlore phases after the calcination. The ceramics prepared from the suspension with pH = 12.5 were sintered to 96% of theoretical density at only 950°C, and exhibited electrical properties comparable to ceramics prepared by the columbite method, sintered at much higher temperatures. In contrast, the properties of the ceramics from the pH = 11.4 suspension were heavily deteriorated due to the presence of the pyrochlore phase and the high degree of porosity.

Deconvolving Ferroelastic and Phase Transformation Toughening in Pb(Zr1−xTix)O3 and Pb1−yLay(Zr1−xTix)O3

Abstract: Ferroelastic toughening in Pb(Zr1−xTix)O3 (PZT) ceramics can be augmented by phase transformation toughening through the inclusion of partially stabilized TZ (PZT–ZrO2). In order to deconvolve both these toughening modes, cubic Pb1−yLay(Zr1−xTix)O3 (PLZT) with and without zirconia (PLZT–ZrO2) was characterized. R curve and stress–strain behavior yielded a quantification of strong toughening and a lack for switching propensity that could be contrasted to PZT and zirconia-toughened PZT. A clear path for toughening of PZT for future application is therefore provided.

PZT thick films obtained by electrophoretic deposition (EPD) process for high frequency focused transducers

Abstract: Piezoelectric lead zirconate titanate (PZT) thick films were prepared on flat and curved substrates by electrophoretic deposition (EPD) process. PZT and PbO particles were dispersed in ethanol to obtain a stable suspension and deposited on flat alumina at constant current density of 1.56 mA/cm2 for 90 seconds and on curved unpoled porous PZT substrates at constant current density of 3.57 mA/cm2 for 60 seconds. The deposits were sintered at 950°C for 2 hours in a PbO-controlled atmosphere. The thickness of the PZT film on curved substrate is 30 μm and the porosity is 17 %. An effective thickness coupling factor of around 50 % and a dielectric constant at constant strain of 320 were measured on flat substrate. An operating high frequency single element transducer with a center frequency close to 30 MHz was fabricated and characterized. The transducer was integrated into an echographic imaging system. In-vivo skin images were illustrated to show the great potential of EPD method for the fabrication of high frequency transducers for medical applications.

Direct measurements of the electrocaloric effect in lead-free K 0.5 Na 0.5 NbO 3 -SrTiO 3 ceramics sintered in air

Abstract: By utilizing direct measurements of the electrocaloric effect (ECE) temperature change via high-resolution calorimeter, the ECE has been studied in bulk lead-free relaxor K 0.5 Na 0.5 NbO 3 -SrTiO 3 (KNN-STO) ceramics. Observed magnitude of the ECE confirms existence of the large EC effect in this system near the ferroelectric phase transition.

The Effect of Phase Composition on the Mechanical and Thermal Properties of LTCC Material

Abstract: Low-temperature co-fired ceramic (LTCC) is an important material in the production of ceramic multilayer structures. Large and complex multilayer structures are usually fired at higher temperatures and/or longer firing times compared to the relatively thin LTCC tapes. The firing conditions of LTCC determine the phase composition and the microstructure, which both influence the physical characteristics, such as the mechanical and thermal properties. In this work the effect of the phase composition on the biaxial flexural strength and the temperature coefficient of expansion of the DuPont 951 LTCC is presented. The samples were fired at different temperatures and times to obtain different phase compositions. The phase composition, especially the mass fraction of anorthite, was correlated with the biaxial flexural strength and the thermal coefficient of expansion (TCE). A very long firing time, i.e., 100h at 800 °C, yields, apart from crystalline anorthite, the cristobalite phase. The anorthite that crystali...