Xiaoyang Liu

Enhanced high temperature thermoelectric characteristics of transition metals doped Ca3Co4O9+δ by cold high-pressure fabrication

A series of Fe, Mn, and Cu doped Ca(3)Co(4)O(9+delta) samples, Ca(3)(Co,M)(4)O(9+delta) (M=Fe, Mn, and Cu), were fabricated by cold high-pressure compacting technique. Their thermoelectric properties were investigated from room temperature up to 1000 K. The cold high-pressure compacting method is advantageous to increasing density and texture, in favor of the improvement of thermoelectric performance. The electrical transport measurements indicate that Fe/Mn substitutes for Co mainly in [CoO(2)] layers whereas the substitution of Cu for Co takes place in [Ca(2)CoO(3)] layers. The thermoelectric properties as well as electronic correlations depend not only on the substitution ion but also the Co site that is replaced. Thermopower can be well calculated by the carrier effective mass according to Boltzmann transport model, indicating that the electronic correlation plays a crucial role in the unusual thermoelectric characteristics of this system. From the changes in thermopower, resistivity, and thermal conductivity, thermoelectric performance of Ca(3)Co(4)O(9+delta) is efficiently improved by these transition metals doping. Fe doped samples possess the highest ZT values. Combining cold high-pressure technique, ZT of Ca(3)Co(3.9)Fe(0.1)O(9+delta) can reach similar to 0.4 at 1000 K, which is quite large among ceramic oxides, suggesting that Fe doped Ca(3)Co(4)O(9+delta) could be a promising candidate for thermoelectric applications at elevated temperatures.

Large reversible magnetocaloric effect in HoTiO3 single crystal

Large magnetocaloric effect has been observed in perovskite-type HoTiO3 single crystal accompanied by a second-order magnetic phase transition at TC ≈ 53 K. The values of maximum magnetic entropy change are about 5.96 and 11.56 J kg−1 K−1 under the magnetic field change of 2 and 5 T, respectively, without any detectable thermal and magnetic hysteresis loss. The large magnetic entropy change is attributed to the sharp magnetization jump, related to anomalies of the lattice parameters just at the Curie temperature. Such a large reversible magnetocaloric effect makes the perovskite HoTiO3 attractive for pursuing new materials for magnetic refrigeration.

Correlation of structural distortion with magnetic properties in electron-doped Ca0.9R0.1MnO3 perovskites (R=rare-earth)

A series of electron-doped orthorhombic-perovskite manganites Ca0.9R0.1MnO3 (R=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) are synthesized for a systematic study of their crystal structure and magnetic properties. The structural distortions, in terms of the average Mn–O–Mn bond angle θMn–O–Mn and Mn–O bond length dMn–O, are characterized as a function of the A-site ionic size. Two degenerate vibration modes Q2 and Q3 are used for describing the bond length splitting and the evolution of the octahedral-site distortion. With R3+ doping, the magnetization increases markedly at low temperatures, which can be attributed to the formation of ferromagnetic clusters in the antiferromagnetic matrix. Both low temperature magnetization and paramagnetic susceptibility vary with the radius of R3+ ion and enhanced ferromagnetic domain is found in Ca0.9Ho0.1MnO3. The Neel temperature TN, varying from 100 to 116 K, is strongly dependent on the crystal structural distortions and can be well described as functions of th...

Evidence of two distinct dynamical freezing processes in single-layered perovskite La0.7Sr1.3CoO4

The static and dynamic features of magnetization have been investigated in terms of dc magnetization, ac susceptibility, and memory effects on single-layered perovskite La(0.7)Sr(1.3)CoO(4). The results indicate that short-range ferromagnetic clusters coexist with the glassy magnetic state, i.e., a cluster-glass phase between T(f) and T(G) and a spin-glass-like phase below T(f). The clear evidence of memory effects in the dc magnetization is also observed below T(f) and T(G), respectively. These two glassy states with cooperative relaxation processes may derive from the different interaction processes among nanoscale ferromagnetic clusters mediated by the matrix, which is influenced by changing the spin state of Co(3+) ions in spontaneously phase-separated cobaltite.