Cerium dioxide (CeO2, ceria) is becoming an ubiquitous constituent in catalytic systems for a variety of applications. 2016 sees the 40th anniversary since ceria was first employed by Ford Motor Company as an oxygen storage component in car converters, to become in the years since its inception an irreplaceable component in three-way catalysts (TWCs). Apart from this well-established use, ceria is looming as a catalyst component for a wide range of catalytic applications. For some of these, such as fuel cells, CeO2-based materials have almost reached the market stage, while for some other catalytic reactions, such as reforming processes, photocatalysis, water-gas shift reaction, thermochemical water splitting, and organic reactions, ceria is emerging as a unique material, holding great promise for future market breakthroughs. While much knowledge about the fundamental characteristics of CeO2-based materials has already been acquired, new characterization techniques and powerful theoretical methods are dee...

Fundamentals and Catalytic Applications of CeO2-Based Materials

Perovskite lead-free dielectrics for energy storage applications

This critical review presents an overview of the various classes of oxide materials exhibiting fast oxide-ion or proton conductivity for use as solid electrolytes in clean energy applications such as solid oxide fuel cells. Emphasis is placed on the relationship between structural and mechanistic features of the crystalline materials and their ion conduction properties. After describing well-established classes such as fluorite- and perovskite-based oxides, new materials and structure-types are presented. These include a variety of molybdate, gallate, apatite silicate/germanate and niobate systems, many of which contain flexible structural networks, and exhibit different defect properties and transport mechanisms to the conventional materials. It is concluded that the rich chemistry of these important systems provides diverse possibilities for developing superior ionic conductors for use as solid electrolytes in fuel cells and related applications. In most cases, a greater atomic-level understanding of the structures, defects and conduction mechanisms is achieved through a combination of experimental and computational techniques (217 references).

Oxide-ion and proton conducting electrolyte materials for clean energy applications: structural and mechanistic features

Antiferroelectric materials that display double ferroelectric hysteresis loops are receiving increasing attention for their superior energy storage density compared to their ferroelectric counterparts. Despite the good properties obtained in antiferroelectric La-doped Pb(Zr,Ti)O3 -based ceramics, lead-free alternatives are highly desired due to the environmental concerns, and AgNbO3 has been highlighted as a ferrielectric/antiferroelectric perovskite for energy storage applications. Enhanced energy storage performance, with recoverable energy density of 4.2 J cm-3 and high thermal stability of the energy storage density (with minimal variation of ≤±5%) over 20-120 °C, can be achieved in Ta-modified AgNbO3 ceramics. It is revealed that the incorporation of Ta to the Nb site can enhance the antiferroelectricity because of the reduced polarizability of B-site cations, which is confirmed by the polarization hysteresis, dielectric tunability, and selected-area electron diffraction measurements. Additionally, Ta addition in AgNbO3 leads to decreased grain size and increased bulk density, increasing the dielectric breakdown strength, up to 240 kV cm-1 versus 175 kV cm-1 for the pure counterpart, together with the enhanced antiferroelectricity, accounting for the high energy storage density.

Lead-Free Antiferroelectric Silver Niobate Tantalate with High Energy Storage Performance.

Structural and electrical evidence for a ferroelectric phase in yttrium doped hafnium oxide thin films is presented. A doping series ranging from 2.3 to 12.3 mol% YO1.5 in HfO2 was deposited by a thermal atomic layer deposition process. Grazing incidence X-ray diffraction of the 10 nm thick films revealed an orthorhombic phase close to the stability region of the cubic phase. The potential ferroelectricity of this orthorhombic phase was confirmed by polarization hysteresis measurements on titanium nitride based metal-insulator-metal capacitors. For 5.2 mol% YO1.5 admixture the remanent polarization peaked at 24 μC/cm2 with a coercive field of about 1.2 MV/cm. Considering the availability of conformal deposition processes and CMOS-compatibility, ferroelectric Y:HfO2 implies high scaling potential for future, ferroelectric memories.

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Ferroelectricity in yttrium-doped hafnium oxide

Crystal structure of ferroelectric silver niobate AgNbO3 was determined (Pmc21) by convergent beam electron, electron, neutron, and synchrotron diffraction techniques and first-principles calculations. The atomic displacements along the c axis in Pmc21 AgNbO3 are responsible for the spontaneous polarization, ferroelectricity, and the paraelectric−ferroelectric phase transition.

Structure of ferroelectric silver niobate AgNbO3

A polymerized complex technique was used to prepare high-purity pyrochlore Y{sub 2}Ti{sub 2}O{sub 7} powders at 750 C. Heating of a mixed solution of citric acid (CA), ethylene glycol (EG), and yttrium and titanium ions, with a molar ratio of Ca:EG:Y:Ti = 5:20:1:1, at 130 C produced a yellowish transparent polymeric gel without any precipitation; this material was used subsequently as a precursor for Y{sub 2}Ti{sub 2}O{sub 7}. Based on the results of {sup 13}C-NMR spectroscopy, it was suggested that a mixed-metal (Y,Ti)-CA{sub 3} chelate complex formed in a starting CA/EG solution. The formation of pure pyrochlore Y{sub 2}Ti{sub 2}O{sub 7} occurred when the precursor was heat-treated in a furnace set at 750 C in static air for 4 h.

Polymerized Complex Route to Synthesis of Pure Y2Ti2O7 at 750°C Using Yttrium-Titanium Mixed-Metal Citric Acid Complex

Imma perovskite-type oxynitride LaTiO2N: structure and electron density

It is of vital importance for the study of phase diagrams to investigate in situ diffusionless phase transition points at high temperatures. We have developed high-temperature techniques to measure in situ neutron and synchrotron X-ray powder diffractometry profiles. This paper reviews our recent studies obtained using these methods. The new furnace for high-temperature neutron diffraction study yields no extra peaks caused by heaters and refractory; thus, we have been able to measure very small neutron signals from the sample. Quality neutron diffraction data at high temperatures up to ∼1590°C have been analyzed using the Rietveld method for various materials, such as zirconia solid solutions and calcium titanate. High-resolution synchrotron X-ray diffractometry enables exact determination of the phase transition temperatures of perovskite-related materials.

Masatomo Yashima

Papers

Structure of ferroelectric silver niobate AgNbO3

Polymerized Complex Route to Synthesis of Pure Y2Ti2O7 at 750°C Using Yttrium-Titanium Mixed-Metal Citric Acid Complex

Imma perovskite-type oxynitride LaTiO2N: structure and electron density

In Situ observations of phase transition using high-temperature neutron and synchrotron X-ray powder diffractometry

Cubic-tetragonal phase change of yttria-doped hafnia solid solution: High-resolution X-ray diffraction and Raman scattering