Thermoelectric phase diagram of the SrTiO3-SrNbO3 solid solution system

Thermoelectric energy conversion - the exploitation of the Seebeck effect to convert waste heat into electricity - has attracted an increasing amount of research attention for energy harvesting technology. Niobium-doped strontium titanate (SrTi1-xNbxO3) is one of the most promising thermoelectric material candidates, particularly as it poses a much lesser environmental risk in comparison to materials based on heavy metal elements. Two-dimensional electron confinement, e.g. through the formation of superlattices or two-dimensional electron gases, is recognized as an effective strategy to improve the thermoelectric performance of SrTi1-xNbxO3. Although electron confinement is closely related to the electronic structure, the fundamental electronic phase behavior of the SrTi1-xNbxO3 solid solution system has yet to be comprehensively investigated. Here, we present a thermoelectric phase diagram for the SrTi1-xNbxO3 (0.05 =< x =< 1) solid solution system, which we derived from the characterization of epitaxial films. We observed two thermoelectric phase boundaries in the system, which originate from the step-like decrease in carrier effective mass at x ~ 0.3, and from a local minimum in carrier relaxation time at x ~ 0.5. The origins of these phase boundaries are considered to be related to isovalent/heterovalent B-site substitution: parabolic Ti 3d orbitals dominate electron conduction for compositions with x < 0.3, whereas the Nb 4d orbital dominates when x > 0.3. At x ~ 0.5, a tetragonal distortion of the lattice, in which the B-site is composed of Ti4+ and Nb4+ ions, leads to the formation of tail-like impurity bands, which maximizes the electron scattering. These results provide a foundation for further research into improving the thermoelectric performance of SrTi1-xNbxO3.Comment: 20 pages, 6 figure

ダイイチ ゲンリ ケイサン ニ ヨル フクゴウ サンカブツ ノ フォノン ト カンレン ブッセイ

京都大学0048新制・課程博士博士(工学)甲第12279号工博第2608号新制||工||1368(附属図書館)24115UT51-2006-J272京都大学大学院工学研究科材料工学専攻(主査)教授 田中 功, 教授 乾 晴行, 教授 田村 剛三郎学位規則第4条第1項該当Doctor of EngineeringKyoto UniversityDA

Nanowire of hexagonal gallium oxynitride: Direct observation of its stacking disorder and its long nanowire growth

The crystal structure of gallium oxynitride nanowire was investigated by using scanning transmission electron microscopy. Gallium oxynitride nanowire was directly observed to have a biphasic wurtzite and zinc-blende structure. There was a stacking disorder of several atomic layers between the two phases. The new biphasic nanowire formed due to the presence of Ni in starting material because its nitride has a zinc-blende structure whereas gallium oxynitride has the wurtzite structure. Crystal growth of gallium oxynitride nanowires was studied using seed crystals. Seed crystals and amorphous gallium oxide precursors were annealed under different ammonia flow rates to grow gallium oxynitride nanowires. The nanowires grew to length of 150 μm but they did not grow laterally when the ammonia flow rate was 50 mL/min

Geometric ferroelectricity in rare-earth compounds RGaO3 and RInO3

We have studied the stability and ferroelectric properties of hexagonal RGaO3 and RInO3 (R: rare-earth elements) by first-principles calculations. Computed spontaneous polarization in the series shows a systematic increase with the rare-earth elements, with values being larger in RInO3 than in the corresponding RGaO3. The largest polarization found is about 10 μC/cm2 for ErInO3, which is about twice as large as those observed in hexagonal RMnO3. The polarization can be further increased by applying in-plane compressive stress. The Born effective charges of constituent ions in the compounds are found to be similar to their formal values, implying that the ferroelectric displacements are merely driven by the ionic size effect. A transition to the high-symmetry phase at around 1500 K was confirmed in GdInO3 and DyInO3 by in situ high-temperature powder x-ray diffractometry. The present systems should belong to the family of geometric ferroelectrics

High-temperature operation of gallium oxide memristors up to 600 K

Abstract Memristors have attracted much attention for application in neuromorphic devices and brain-inspired computing hardware. Their performance at high temperatures is required to be sufficiently reliable in neuromorphic computing, potential application to power electronics, and the aerospace industry. This work focuses on reduced gallium oxide (GaO x ) as a wide bandgap memristive material that is reported to exhibit highly reliable resistive switching operation. We prepared amorphous GaO x films to fabricate Pt/GaO x /indium tin oxide memristors using pulsed laser deposition. Stable resistive switching phenomena were observed in current–voltage properties measured between 300 and 600 K. The conduction mechanism analysis revealed that the resistive switching is caused by the transition between ohmic and space charge limiting current conductions. We elucidated the importance of appropriate control of the density of oxygen vacancies to obtain a high on/off resistance ratio and distinct resistive switching at high temperatures. These results indicate that GaO x is a promising memristor material that can be stably operated even at the record-high temperature of 600 K