J. F. Fernandez

Controlled precipitation methods: formation mechanism of ZnO nanoparticles

Abstract Zinc oxide nano-particles with different morphologies have been obtained by controlling different parameters of the precipitation process: solution concentration, pH and washing medium. The reactions leading to the formation of zinc metastable complexes were followed by potentiometric and conductimetric titrations. During the ethanol washing step the intermediate zinc complexes have been modified by weathering and solution processes which drived both the phase transformation reactions and the particles recrystallization. The nucleation of zinc alkoxides during this process is an “organic matrix-mediated” process, where the matrix is formed on the intermediate zinc complexes surface during the NH 4 OH addition to the solution of the zinc precursor. The existence of this matrix has been supported by FTIR. The transformation of the zinc metastable complexes during the washing with ethanol was followed with XRD. Through this process it is possible to obtain agglomerates of zinc oxide nanoparticles from a random network, as was revealed by SEM and TEM.

Skutterudites as thermoelectric materials: revisited

The research on skutterudites in the last few years has contributed to a better understanding of the physical processes which play an important role in enhancing their thermoelectric performance and to the discovery of novel filled compounds, with one of the most promising zT values at intermediate temperatures. Skutterudites are still an ongoing field of research, and an improvement of their efficiencies, stabilities, contacts, industrial scalable fabrication processes and other factors are expected in the near future in order to develop viable modules for intermediate temperature range applications, such as in the automobile industry, factories or incinerators. This paper gives a review on the status of research in the field of skutterudites.

High spatial resolution structure of (K,Na)NbO3 lead-free ferroelectric domains

In this paper, we highlight some practical aspects in the study of ferroelectric domains in lead-free (K,Na)NbO3 piezoceramics. This work presents and discusses the spatial resolved structure of the ferroelectric domain existing in (K,Na)NbO3 based ceramics studied by confocal Raman microscopy (CRM) coupled with atomic force microscopy. In addition to the domain identification, CRM allows a determination of the nature of domain walls and correlation between the structure and piezoelectric properties. The tetragonal constraint for orthorhombic domain formations is demonstrated by this technique. Alternating polarization differences in 180° domain templates are resolved by 60° and 120° domains. The internal stress results in the appearance of an unusual polarization relaxation at the 90° domain wall in (K,Na)NbO3 ceramics. Finally, the implications of the present findings introduce a fruitful discussion in the search for effective structures to improve the piezoelectric response and represent the key point to design new domains that allow engineering the piezoelectric response.

Low-temperature sintering of submicronic randomly oriented Bi4Ti3O12 materials

Ultrafine powders of Bi4Ti3O12 with a narrow size distribution were prepared by the oxalate coprecipitation method. Compacts of the calcined powders were pressureless sintered at 850–1100 °C in air, and the densification process was studied by non-isothermal and dilatometric experiments. A rapid densification (~98% of theoretical density) below 875 °C took place, which was attributed to both an uniform pore-size distribution in the green starting compact and a rearrangement of particles. The development of platelike grains, the platelike colony formation, and a rapid grain growth decreased densification above 900 °C. Microstructural development was studied and dielectric and electrical preliminary results are also given.

ZnO-doped BaTiO3: Microstructure and electrical properties

In the present work, ZnO additions ranging from 0.1 to 10 wt% to ceramic BaTiO3 have been studied. By means of a controlled processing, two different compounds were tested as a source of ZnO. The dopant distribution plays a key role in the microstructural development of the ZnO-doped BaTiO3. When zinc stearate was used, homogeneous finegrained microstructure was obtained for 0.1 wt% of ZnO. However, high density was avoided by burn out of the long organic chain. When solid ZnO was used as dopant, homogeneous fine-grained microstructure and density values around 99% Dt (Dt = 6.017 g cm−3) were obtained for ZnO compositions starting from 0.5 wt%. The sintering temperature was 100 °C lower than for undoped BaTiO3. Control of grain growth in a wide temperature range led to ceramics with permittivity values close to 3000 and dielectric losses well below 1%. The incorporation of Zn2+ cations into the BaTiO3 lattice takes place in Ba2+ sites, i.e. as an isovalent dopant. The solid solubility of ZnO in BaTiO3 was stated to fall below 0.5 wt% in the range of sintering temperatures studied. Additions of large amounts of ZnO led to BaTiO3-ZnO composite materials.

Compositional fluctuations and properties of fine-grained acceptor-doped PZT ceramics

Fine-grained acceptor-doped lead zirconate titanate (PZT) ceramics were sintered in a wide range of temperatures and times. The differences in dielectric and piezoelectric properties were relevant and quite independent of grain size. The incorporation of acceptor dopant into the solid solution of PZT was responsible for the change of properties, the evolution of secondary phases and the weight loss behaviour. The PbO rich phase located between grains displaced from the PbO-PZT system to PbO-PZ system when iron cations entered the PZT lattice. At this time, precipitated monoclinic grains of zirconia among with tetragonal perovskite grains revealed the compositional fluctuations. After the iron cations entered into solid solution the liquid formed dissolved new PZT grains again and displaced to PbO-PZT system with increasing of weight loss. Decomposition experiments revealed also the nature and the evolution of the second phases that are in accordance with TEM observations.

Non-ohmic behaviour and switching phenomena in YMnO3-Based ceramic materials

Abstract Ceramic materials based on the ferroelectric compound YMnO 3 have been prepared by solid state reaction of the corresponding oxides to obtain sinterable powders, which were isostatically pressed and sintered at temperatures ranging from 1350 to 1525°C. The samples were characterised by X-ray diffraction, (XRD), scanning electron microscopy, (SEM), apparent density measurements, and their electrical properties were established at several temperatures. The ceramic materials showed a semiconducting behaviour when they were subject to low electrical fields. The activation energy of conductivity was calculated for a narrow range of temperatures. Applying higher electrical fields the ceramics showed non-ohmic behaviour and switching phenomena from low-to-high conductivity state with a threshold voltage which depended on the testing temperature. Values of current density as high as 80 A/cm 2 were measured in the high conductivity state. The process is reversible and the ceramics returned to low conductivity mode lowering or leaving the applied field. Analysis of the I–characteristic curves have allowed to advance a model based on the Poole–Frenkel effect.

Extrinsic contribution and non-linear response in lead-free KNN-modified piezoceramics

Finding lead-free ceramics with good piezoelectric properties is nowadays one of the most important challenges in materials science. The (K, Na, Li)(Nb, Ta, Sb)O3 system is one of the most promising candidates as a lead-free ceramic for transducer applications and is currently the object of important research work. In this paper, (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3 was prepared by a conventional ceramic processing route. For this composition, orthorhombic-to-tetragonal phase transition was observed at temperatures very close to room temperature. As a consequence, good room temperature electromechanical properties were observed, displaying good thermal stability. We show that the most important contribution to dielectric, piezoelectric and elastic response comes from extrinsic effects, as was observed in other perovskite based materials. Nonlinearities in electromechanical properties induced by high electric field or mechanical stress were studied. Non-linear dielectric response was found to be less important than for soft PZT ceramics and was analysed within the Rayleigh framework. The results reveal that the non-linear response at room temperature in this material is mainly due to the irreversible wall domain movement.

Towards materials with enhanced electro-mechanical response: CaCu3Ti4O12-polydimethylsiloxane composites

We describe a straightforward production pathway of polymer matrix composites with increased dielectric constant for dielectric elastomer actuators (DEAs). Up to date, the approach of using composites made of high dielectric constant ceramics and insulating polymers has not evidenced any improvement in the performance of DEA devices, mainly as a consequence of the ferroelectric nature of the employed ceramics. We propose here an unexplored alternative to these traditional fillers, introducing calcium copper titanate (CCTO) CaCu3Ti4O12, which has a giant dielectric constant making it very suitable for capacitive applications. All CCTO–polydimethylsiloxane (PDMS) composites developed display an improved electro-mechanical performance. The largest actuation improvement was achieved for the composite with 5.1 vol% of CCTO, having an increment in the actuation strain of about 100% together with a reduction of 25% in the electric field compared to the raw PDMS matrix.

Phase transition and electrical conductivity in the system YMnO3-CaMnO3

Solid solution formation between the ferroelectric hexagonal compound YMnO3 and the semiconducting perovskite compound CaMnO3 has been studied in the Y-rich region of the pseudobinary system. The materials were prepared by reaction of the corresponding oxides and the calcium carbonate for obtaining the corresponding phases and subsequent sintering of the pressed powders in the 1400–1525 °C-temperature range. Their ceramic characteristics were established. Samples with relative density >98% Dth were obtained. The hexagonal symmetry of the pure yttrium manganite changes to an orthorhombic one, isostructural with that of the GdMnO3 compound, for ∼32 mol % CaMnO3. The incorporation of the Ca cation causes an increasing in the compactness of the lattice cell, promoting a rise in the apparent density, despite of the smaller atomic weight of the Ca cation against that of the Y one, until the transition to the orthorhombic lattice occurs. Subsequent increase in the Ca amount lead to a decrease in the theoretical density.The electrical conductivity of the solid solutions increases strongly from 100 to the 90/10 Y/Ca composition, and more moderately for higher Ca amounts. Conductivity values as high as 10−2 S · cm−1 were measured on the denser samples. The activation energy for conduction suffers a corresponding decrease when the Ca amount is increased.