Electroceramic Oxide Research Articles

Unravelling the La dopant induced physicochemical changes and the electrical behaviour on BaFeO3 under oxidising and reducing atmospheres

Electroceramic oxides like La-doped BaFeO3 garner greater interest owing to their tunability of mixed ionic conduction. However, reports on the effect of La on the microstructural changes, the type of conduction mechanism under different atmospheres and the negative charge transfer influenced by the Fe-O hybridization in Ba1−xLaxFeO3−δ are hardly available. Doping La induces cation vacancies and Fe reduction, creating nanorod decorated surface after the high-temperature treatment. Conductivity studies expose the rate-limiting steps for hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) as the ohmic resistances and Ba segregation effects. Among the samples studied, Ba0.9La0.1FeO3−δ performs better at 800 °C with an activation energy of ∼0.35 eV under both oxidizing and reducing atmospheres. Surface analysis after EIS reveals Fe and O with different oxidation states that enhance the total conductivity. The amount of hydroxyl species retained (23.66% for Ba0.9La0.1FeO3−δ ) and the ratio of the adsorbed to the lattice oxygen (15.33% for Ba0.9La0.1FeO3−δ ) gives insight on HOR and ORR activities. Also, computational studies validated the negative charge transfer mechanism of the samples under thermally assisted oxidation. The results indicate Ba0.9La0.1FeO3−δ as a tuneable electrode for solid oxide cells.

How space-charge behaviour at grain boundaries in electroceramic oxides is modified by two restricted equilibria.

Determining the space-charge potential at grain boundaries in oxides by various experimental methods bears the promise of providing a comprehensive, quantitative description of interfacial defect chemistry. In this study, we draw attention to the problem of unifying data measured in different temperature ranges. We focus on unifying data from elevated-temperature electrical methods, such as impedance spectroscopy and current-voltage measurements, with data from room-temperature imaging techniques, such as Scanning Probe Microscopy (SPM), Transmission Electron Microscopy (TEM), and Atom Probe Tomography (APT). By means of continuum simulations, we calculate the space-charge potential Φ0 at grain boundaries in the model electroceramic oxide acceptor-doped SrTiO3, taking into account, first, a restricted equilibrium that leads to frozen-in acceptor-dopant profiles, and subsequently, a restricted equilibrium that leads to frozen-in bulk oxygen-vacancy concentrations. Our results indicate non-trivial differences between experimental values of Φ0 obtained from electrical and from imaging methods, differences that arise from the different measurement temperatures and that are aggravated by the restricted equilibria. We also show that grain-boundary widths determined from elemental acceptor-cation profiles will not, on principle, agree with the electrical width extracted from impedance spectroscopy data.

Assessment of MgHf4P6O24 Electroceramic Oxide Electrolyte in High-Temperature Electrochemical Sensor for Sensing Mg in Non-Ferrous Alloys

The chemical synthesis of MgHf4P6O24 multicomponent nanostructured ceramic oxide electrolyte was achieved using a modified sol-gel chemical process by substituting the tetravalent Zr4+ B-site in MgZr4P6O24 solid-state electrolyte [1, 2] with tetravalent Hf4+ ceramic oxide resulting in a considerably low activation energy (Ea = 0.74 ± 0.04 eV), promising ionic conductivity (4.52 x 10-4 Scm-1 at 747oC), electroceramic MgHf4P6O24 solid-state electrolyte.MgHf4P6O24 solid-state electroceramic oxide electrolyte was calcined at a relatively low temperature range of 800-900oC, the calcined nanopowders were pressed into pellets of 13mm diameter (Ø) and 3.8mm thickness with a uniaxial steel die at 5kN compressive pressure and sintered at 1000 ≤ T/oC ≤ 1550 temperature range. However, 1300oC was adopted as the sintering temperature in this study haven achieved optimum density and stable sample composition at that temperature. X-ray diffraction reveals a good crystallinity of the electroceramic oxide with an average crystallite size of 20±2 nm and 42±2 nm at 800oC and 900oC, respectively, indicating that crystallite size increases as a function of calcination temperature, which is very consistent with the simultaneous TGA-DSC thermal analysis profiles and confirmed using HR-TEM. The refined crystallographic data and ionic conductivity properties of the novel MgHf4P6O24 ceramic oxide solid-state electrolyte was reported for the first time in this study. SEM-EDS characterisation technique was used for determining both structural and compositional homogeneity. The sintered MgHf4P6O24 electroceramic oxide electrolyte was characterised for their electrical and thermodynamic properties thereby identifying reliable ionic and transport properties of the electroceramic oxide; The average transport number for Mg2+-cations in MgHf4P6O24 electroceramic oxide electrolyte measured as a function of Mg concentration in molten Al is 0.84±0.03. The ionic conductivity and thermodynamic analysis of the novel solid-state electroceramic oxide electrolyte in this study was compared with those of MgZr4P6O24 electrolyte [3], showing novel improvement in the trend of the ionic conductivity and thermodynamic properties of both electroceramic oxide electrolytes.Relying on the structural, chemical stability and ionic conductivity data characterised in this study, solid-state electrochemical Mg-sensor was designed and fabricated, then testing of the high-temperature Mg-sensor in molten Al alloys by the electrochemical method was achieved as shown in Figure 1 [3]. The novel high-temperature solid-state Mg-sensor was fabricated using the novel high conducting Mg2+-cation solid-state electroceramic oxide electrolyte characterised in this study by incorporating a biphasic powder mixture of MgCr2O4+Cr2O3 solid-state ceramic reference electrode in air, showing promising trend after successfully sensing Mg dissolved in molten Al alloys at 700±5oC. A linear dependence of sensor voltage on the logarithm of Mg concentration was obtained. The thermodynamic activity of Mg in molten Al alloy shows a rather negative deviation from Raoult's law. Solid-state MgHf4P6O24 electroceramic oxide electrolyte has useful potential applications in solid-state Mg-sensors during refining, virgin metals alloying and scrap metal recycling for the benefit of our environment and depleting climate.

A Versatile and Low‐Toxicity Route for the Production of Electroceramic Oxide Nanopowders

AbstractAn alternative route to acrylamide gel polymerisation is presented for the preparation of oxide nanopowders. Within this method, cations first dissolved in an aqueous solution are subsequently trapped inside a 3D chelating gel beforefast‐drying, which leads to a stable precursor. In this study, we have prepared compositions corresponding to a wide range of state‐of‐the‐art fuel cells materials such as: Zr0.85Y0.15O1.875, Ce0.9Gd0.1O1.95, Nd1.95NiO4+x, GdBaCo2O5+x, Ba0.5Sr0.5Co0.8Fe0.2O3–x and La0.75Sr0.25Cr0.5Mn0.5O3. Thermogravimetry and differential thermal analysis studies have been performed to determine the best calcination conditions. After calcination, the resulting powders have been characterised by X‐ray diffraction, FEG‐scanning electron microscopy and TEM. The synthesised powders are of nanometric crystallite size and have flake‐like aggregates; pure phases are obtained at low temperature. We made this method more attractive by using a nontoxic polymer and a nonexplosive initiator. This route allows the easy preparation of more than tens of grams of nanometric powders in one batch. Finally, since this process is largely independent of the formula of the compound, it can easily be extended to other kinds of oxides such as ferroelectric, CMR (colossal magnetoresistance) or gas sensor compounds.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Enhanced dielectric tunability in barium titanate thin films with boron additions

Boron incorporation in 650 nm thick polycrystalline BaTiO3 films results in grain growth and dramatic dielectric response improvements. For levels between 0% and 3%, average grain size is increased from 150 to 200 nm and the maximum permittivity values are nearly doubled, with tunabilities increased to greater than 90% while maintaining high field loss tangents below 0.03. These results demonstrate a pathway for microstructure engineering and property optimization in refractory electroceramic oxide thin films without increasing the thermal budget.

Some recent developments in the chemical vapour deposition of electroceramic oxides

Metalorganic chemical vapour deposition (MOCVD) and atomic layer deposition (ALD) are highly promising techniques for the deposition of dielectric and ferroelectric oxide thin films, which have a variety of applications in electronic devices. A key requirement for both MOCVD and ALD is the availability of precursors with appropriate physical properties and decomposition characteristics, but there are problems associated with many of the existing precursors. Therefore, it is sometimes necessary to `tailor' the properties of the precursor in order to optimize MOCVD and ALD process parameters, such as evaporation temperature, deposition temperature, layer purity and uniformity. In this paper, the design of a number of new, improved oxide precursors is described, with emphasis on the influence of molecular structure on precursor properties and on the growth dynamics of the MOCVD and ALD processes.

Molecular design of improved precursors for the MOCVD of electroceramic oxides

Metalorganic chemical vapour deposition (MOCVD) is a highly promising technique for the deposition of dielectric and ferroelectric oxide thin films such as TiO2, ZrO2, HfO2, Pb(Zr,Ti)O3, SrBi2Ta2O9 and Pb(Sc0.5Ta0.5)O3 which have a variety of applications in electronic devices. A key requirement in this technology is the availability of precursors with appropriate physical properties and decomposition characteristics, but there are problems associated with many of the existing precursors. Therefore, in order to exploit the full potential of MOCVD it is sometimes necessary to “tailor” the properties of the precursor in order to optimise process parameters such as evaporation temperature, deposition temperature, layer purity and uniformity. In this paper the molecular design of a number of new oxide precursors is described, illustrating how the replacement of simple alkoxide groups by other groups, such as donor-functionalised alkoxide or β-diketonate ligands, can result in precursors with improved physical properties and enhanced MOCVD performance.

Synthesis and characterisation of La 1−xMnO 3± δ nanopowders prepared by acrylamide polymerisation

La 1−xMnO 3± δ (x=−0.02 to 0.35) nanocrystalline powders were prepared by a new sol–gel method. It is used the acrylamide gelification to form an organic 3D tangled network where a solution of the respective cations is soaked. This method was adapted to cover a broad range of high impact electro–ceramic oxides, which a particular example is the CMR nanopowders reported in this work. The acrylamide sol–gel process is a fast, cheaper and easy to scale-up method for obtaining fine powders of complex oxides. This synthesis method allows performing 100 g of highly pure nanopowders in one run with simple laboratory scale. The sponge like powder obtained consists of thin sheets composed of nanocrystallites whose size varies from 66 nm to 30 nm, depending on composition. The oxygen content of the manganite powder is shown to decrease with vacancy-doping on lanthanum site. Such a evolution can be explained for La/Mn<0.9 by considering a demixtion of the powder into La 0.9MnO 3 and Mn 3O 4 phases, while for La/Mn>0.9, the high oxygen excess leads to consider vacancies on both lanthanum and manganese sites. Both hypotheses are supported by magnetic measurements, which show a constant Curie temperature of 295 K for La/Mn<0.9, while for La/Mn>0.9, the occurrence of vacancies on manganese sites progressively impedes the ferromagnetic interactions, leading to a cluster–glass behaviour in the case of the highly manganese-deficient La 0.94Mn 0.92O 3 compound.

Structural and Property Evolution of Aqueous‐Based Lead Zirconate Titanate Tape‐Cast Layers

We have developed aqueous suspensions of lead zirconate titanate (PZT) and an acrylic latex emulsion binder for tape casting. Rheological measurements were conducted to optimize their flow behavior at high solids volume fraction (φsolids). Concentrated suspensions (φsolids &gt; 0.45) were tape cast onto a silicone‐coated mylar carrier film, and the effect of cellulose (a viscosifier) and surfactant additions on wetting behavior was studied. Drying stress measurements were performed on tape‐cast layers using a substrate deflection technique. The initial period of stress rise was analogous to that observed for binder‐free ceramic films, whereas the stress decay was influenced by latex coalescence. Tape‐cast layers exhibited a constant PZT volume fraction of 0.49 and uniform binder distribution independent of the PZT/latex volume ratio. The dielectric and piezoelectric properties of sintered multilayer samples were comparable to those obtained for bulk, isopressed samples. Our work provides guidelines for developing aqueous formulations for tape casting complex electroceramic oxide layers.

Gelation by Acrylamide, a Quasi-Universal Medium for the Synthesis of Fine Oxide Powders for Electroceramic Applications

The synthesis of fine powders of electroceramic oxides (e.g., superconductors, dielectrics, and materials exhibiting colossal magnetoresistance) is most commonly carried out by solid-state reaction. Wet chemical methods, which overcome some disadvantages of the solid-state route, are often difficult to scale up. The improved acrylamide sol-gel process presented here is shown to be a fast, cheap, reproducible, and easily scaled-up route to fine oxide powders. The synthesis of La2CuNiO6, which cannot be obtained by solid-state reaction at ambient pressure, demonstrates the potential of this method in the search for new materials.

MOCVD of electroceramic oxides : A precursor manufacturer's perspective

Thin films of metal oxides are finding an increasing application in electronic devices. They can be deposited by a number of methods, however metalorganic chemical vapour deposition (MOCVD) offers the most flexible approach. Key to the success of this technology is the manufacture and supply of suitable precursors with sufficient volatility and stability, as well as adequate purity. In this article a number of manufacturing issues are discussed, as well as the development of new precursors with improved physical properties and enhanced MOCVD performance.

Cation Coordination At Σ Grain Boundaries in TiO2 and SrTiO3, and its Effect on the Local Electronic Properties

Grain boundaries have long been known to have a dominant effect on the electronic properties of polycrystalline materials. In the case of electroceramic oxides, the thermodynamics of defect formation (vacancies or interstitials, cations or anions) are usually invoked to predict the presence of a space charge potential at the grain boundaries. The relative energetics for the formation of each type of defect determines the size and sign of this potential barrier and thus, the effect that boundaries have on the overall electronic properties of the materials. However, a limitation to this continuum thermodynamics approach is that it does not consider the effect of the grain boundary structure.To investigate whether the grain boundary atomic structure can have an effect on the energetics of defect formation and hence the electronic properties, here we examine the structure of Σ5 boundaries in two systems, SrTiO3 (perovskite) and TiO2(rutile).

Transmission electron spectroscopy and interferometry of electroceramic oxides

Research on ferroelectricity and related phenomena has come a long way since the discovery of hysteretic nonlinearity of polarization in "Rochelle Salt" by J. Valasek in 1920's. Ever since the discovery, microstructural issues have dominated the field. With the current trends in using ferroelectric and related oxides in nanocrystalline and thin film forms, techniques for microstructural analysis for such specimens are becoming more sophisticated.We are currently investigating nanocrystalline TiO2 as a model system to examine size dependency of electronic structure in nanocrystals, as reflected in their energy loss spectra. Electron beam evaporated nanocrystals of TiO2 (size ranging from 2 - 10 nm) have been examined with transmission EELS with a ~ 0.8 nm electron probe of our HF-2000 FE TEM. The particles were dispersed on a holey carbon grid and those protruding into the holes were analyzed for changes in valence and core loss spectra as a function of nominal particle size. The plasmon loss energies were observed to increase with decrease in particle size.

Thermodynamic and transport properties of electroceramic oxide systems

Ceramic oxides are finding a wide range of application as materials for electronic systems in the modern world. They are used as electrodes and electrolytes in fuel cells and oxygen sensors, and as magnetic and dielectric materials in control devices, to mention only a few examples. Because of their nature, the fabrication of components incorporating ceramic oxides is usually carried out at high temperatures. The transport properties and redox properties of the constituent ions in these materials therefore play a significant role in determining the ease of fabrication as well as the scope of application of devices. Examples of the limitations placed on the use of these materials as a result of the chemistry of the systems are summarized for a number of current uses. The possibility of using an assembly of contemporary information to develop new and more useful materials is examined in terms of the oxygen and temperature sensitivity of the desired physical properties of electroceramics. It appears that the field is still open to many new developments, largely because of the wide range of cationic mixtures which can be sustained in the structural and stability properties of these materials.