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In this paper, the effects of hot isostatic pressing (HIP) temperature on the crystal structure, optical, dielectric and magnetic properties of GdMnO3 ceramics were studied. All samples form a single-phase structure without structural transformation, while HIP temperature induces the changes in lattice parameters. HIP causes the change in Mn ions valence state, oxygen vacancy concentration, Raman vibration modes and microscopic morphology of GdMnO3. Vacancy concentration of the samples prepared by HIP at 800 °C increases compared with that of the samples without HIP, then remains unchanged when the HIP temperature is from 800 to 900 °C, and finally decreases with the further increase of HIP temperature. Appropriate HIP temperature can increase the dielectric constant and decrease the dielectric loss. The transition temperature of paramagnetism to antiferromagnetism and magnetization can be significantly affected by HIP temperature. Magnetic transition temperature and magnetization are closely related to Mn2+ ions concentration and cation vacancy concentration, respectively.

NiCo2S4 powders were synthesized using a hydrothermal method and oxidized in various environments. We have carried out the density functional theory (DFT) calculation of NiCo2S4 and its oxidation structure. Both theory and experiments show that unoxidized NiCo2S4 was found to be paramagnetic, but, the magnetic states of the oxidized NiCo2S4 powders were changed significantly. It is found by the DFT calculation that the octahedrons in the partially oxidized NiCo2S4 spinel lattice were distorted, leading to changes in the orbital ordering of the Co3+ t2g level. In particular, one electron of the dyz orbital jumps into the d ( x 2 − y 2 ) orbital of the eg level, and the nonmagnetic low spin Co3+ transforms to an intermediate spin state. The magnetic measurements show the ferromagnetism in the partially oxidized NiCo2S4 powders, which originated from the exchange interaction between intermediate spin Co3+ and Ni ions mediating through intervening O ions. When the NiCo2S4 powders were heavily oxidized, Co3+ ions were gradually reduced to Co2+. Due to a strong covalent effect, Co2+ makes little contribution to the magnetic properties of the sample, but antiferromagnetic exchange interactions occur between Ni3+ ions in the heavily oxidized sample. All the NiCo2S4 powders with different degrees of oxidation were found to retain metallic conductivity.

To investigate the evolution of the structural and enhanced magnetic properties of GdMnO3 systems induced by the substitution of Mn with Cr, polycrystalline GdMn1-xCrxO3 samples were synthesized via solid-state reactions. XRD characterization shows that all GdMn1-xCrxO3 compounds with single-phase structures crystallize well and that Cr3+ ions entering the lattice sites of GdMnO3 induce structural distortion. SEM results indicate that the grain size of the synthesized samples (a few microns) decreases as the Cr substitution concentration increases. Positron annihilation lifetime spectroscopy reveals that vacancy-type defects occur in GdMn1-xCrxO3 ceramics and that the vacancy size and concentration clearly change with the Cr content. The temperature and field dependence of the magnetization curves show that Cr substitution significantly influences the magnetic ordering of the gadolinium sublattice, improving the weak ferromagnetic transition temperature and magnetization of GdMn1-xCrxO3. The enhanced magnetization of GdMn1-xCrxO3 is closely related to the vacancy defect concentration.

A series of FeCo2O4 powders was initially synthesized using a hydrothermal method and subsequently calcined at various temperatures to produce the final product. Pure phase FeCo2O4 powders can only be formed in the temperature range of 950–1050 °C. In this work, we study the cation occupation, cation valence, bond length and bond angle changes of the pure phase FeCo2O4 powders formed in such a narrow temperature range. Octahedral lattice distortion in the pure phase FeCo2O4 samples has been observed. More tetrahedral Fe3+ and octahedral Co2+ are excited and exchanged their sites as the calcination temperature increases from 950 °C to 1000 °C, and part of Co3+ ions are reduced to Co2+ in the sample calcined at 1050 °C. The structure of the sample calcined at 1000 °C is close to that of the ideal FeCo2O4 spinel. Magnetic measurements show that ferrimagnetism and anti-ferromagnetism coexist in the pure phase FeCo2O4 samples. Interaction changes between ferrimagnetism and antiferromagnetism caused by the structural changes of the samples have been studied. Due to the pinning of the local anti-ferromagnetism to ferrimagnetism in the sample, the sample shows a Barkhausen jump below 150 K. As the measurement temperature increases further, the system enters into a reentrant spin glass state.

Polycrystalline CuFeO2 (CFO) and CuFe0.99R0.01O2 (R–CFO, R = La, Sm, Eu, Gd, Dy, and Ho) were prepared via solid-phase synthesis to investigate the effects of rare-earth (R) ion addition on the microstructures and magnetism. Positron annihilation lifetime spectroscopy (PALS) experimental results demonstrated that the size, concentration, and defect types were altered by R substitution due to the competition between the agglomeration of small-sized defects and the decomposition of vacancy clusters, which resulted in the redistribution of electrons around annihilation sites. The magnetic measurements revealed that the antiferromagnetic stability was influenced by R substitution, which was ascribed to the effect of magnetic moments, variations in exchange interactions, and the partial reduction of spin frustration. The magnetization behaviors of the R–CFO samples were also influenced by doping ions, which was mainly explained by the different magnitudes of the R ions’ magnetic moments. Ferromagnetism and antiferromagnetism were present in all of the samples. The local electron density ne was associated with the maximum magnetization (Mm). The correlative physical mechanisms are discussed in detail. The results demonstrated that the ne plays an important role in regulating the magnetic properties of CFO ceramics.

In solid oxide fuel cells (SOFCs), it is important that the sealing glass has adequate mechanical strength (rigidity) to withstand stack loads while simultaneously having appropriate resiliency to release the internal stress and thus resist cracking. However, at present, sealing glass is either resilient or rigid. Animal teeth exhibit a bilayer structure that combines remarkable surface hardness to withstand mastication loads with the high resiliency of the inner layer to resist crack propagation. Drawing from this, in this study, we selected a heat-treatment temperature of 700 °C to simulate the operating temperature of the sealing glass and SOFCs, and carried out experimental and theoretical investigation of a typical borosilicate sealing-glass system. The results revealed that surface strengthening occurs within 200 nm depth as the heat-treatment time increases; that is, there is a gradient change in the mechanical property from the surface to the interior of glass, indicating that the surface has greater hardness and elastic modulus to bear stack loads, while the interior has better resiliency and may help resist crack initiation and propagation. A possible mechanism for this strengthening is proposed. The 4-coordinated B and Al centred BO4 and AlO4 can form a glass network structure with SiO4 (i.e., B–O–Si, Al–O–Si), which may lead to surface strengthening due to the increase in the degree of network crosslinking.

Ho:Y 2 O 3 ceramics were prepared using co-precipitated powders, with ammonium sulfate as dispersant. Y 3+ was co-precipitated together with Ho 3+ and Zr 4+ to produce precursors, which were calcined at 1100–1400 °C to produce yttria-based powders. At calcination temperatures of ≤1300 °C, agglomeration of powders was not observed. When the temperature was increased to 1400 °C, severe agglomeration was detected. Densification was closely related to the calcination temperature: a lower calcination temperature resulted in a faster densification of ceramics to the relative density of 99.7%. The ultimate densification to ~100% was also closely related to powders' impurity level and agglomeration. Grain growth was mainly determined by sintering temperature, but not by the initial crystallite size of powders. The optimal calcination temperature was 1300 °C, at which the obtained Ho:Y 2 O 3 powder was free from agglomeration. Using this powder, the resultant Ho:Y 2 O 3 ceramics showed pore-free microstructure and good optical transparency.

Effects of (Lu, Ni) co-doping on spin entropy of Ca 3 Co 4 O 9+ δ have been investigated. Results of this study show that (Lu, Ni) co-doping can be more effective than single Lu doping for optimizing the spin entropy of Ca 3 Co 4 O 9+ δ . X-ray photo-emission spectroscopy confirms a reduction in Co 4+ concentration, which leads to enhanced spin entropy from Co sites. The magnetic ions of Ni with unpaired 3d electrons can produce extra spin entropy through a new hopping model, thereby contributing to an increase in total spin entropy. Incorporating magnetic ions is an effective way to optimize the spin entropy of thermoelectric materials. This study opens a new way to broaden and design prospective thermoelectric materials.

We report a detailed investigation of the thermopower and magnetic properties of La 0.95 Ba 0.05 CoO 3 . The large negative magnetothermopower is found to scale with both magnetic field and temperature, indicating a spin-entropy contribution to the thermpower. We formulate a new general expression to describe the thermopower from spin entropy with spin-interaction considered. The scaling behaviour is analyzed in a theoretical model, which provides a satisfactory description of the observed data. The magnetic results further support our analysis. This investigation shows that spin entropy plays a substantial role in the thermopower.

The effect of V-doping on the magnetic and electrical transport properties for La 0.8 Sr 0.2 Co 1− x V x O 3 ( x =0.00, 0.02, 0.04, and 0.06) has been investigated through X-ray diffraction, magnetization and resistivity measurements. The ferromagnetic order is dramatically suppressed by the V substitution. The magnetic results suggest that the Co 3+ and Co 4+ ions are both in the intermediate-spin state, indicating that a strong-coupling double exchange between Co 3+ and Co 4+ intermediate-spin state via t 2 g -orbitals is the driving force for the ferromagnetic order in the present compound. The resistivity increases as the V doping level increases, in agreement with the magnetic results. Moreover, the transport mechanism at the low temperature is dominated by the variable-range-hopping conduction.

The effect of Cu doping on the spin entropy of Ca3Co4O9+δ system has been investigated through the measurements of magnetothermopower and magnetic properties. It is found that the thermopower decreases with increasing Cu doping. Magnetothermopower results reveal that Cu doping suppresses the spin entropy of Ca3Co4O9+δ system. The magnetic results demonstrate that the Co4+ concentration increases with increasing Cu doping level. This investigation suggests that the decrease in thermopower is attributed to the spin entropy suppression, which originates from the increase of the Co4+ concentration induced by Cu doping.

The influence of NiO-doping on the microstructure and dielectric properties of CaCu3Ti4O12-xNiO (x=0, 0.003, 0.006, 0.010, 0.015, and 0.020) ceramics has been investigated using SEM, Raman spectra and dielectric spectrum measurements. The positron annihilation lifetime spectra (PALS) are used to investigate the influence of defects on the dielectric properties. The SEM results show that the grain morphology varies significantly with increasing NiO content. An appropriate small amount of NiO can promote the grain growth, which is beneficial to improve the dielectric properties in a CaCu3Ti4O12 (CCTO) system. Positron results show that there are vacancy-type defects in the experimental samples, and the concentration of the defect and the defect type both change with increasing NiO content. The effects of microstructure including the grain morphology and the vacancy defects on the mechanism of the dielectric properties by adding NiO are discussed. The results demonstrate the importance of the grain morphology and the characteristic of defects in controlling the electrical properties of CCTO ceramics. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Ni doped In 2 O 3 nanocrystals were prepared by a coprecipitation method, and calcined in the absence of a magnetic field or in 12 T magnetic field. X-ray diffraction and transmission electron microscopy images reveal that the nanocrystals were highly crystalline and no secondary phase was detected. It is found that growth behavior of the nanocrystals was affected by the external magnetic field treatment. Room temperature ferromagnetism was observed in (In 1− x Ni x ) 2 O 3 ( x =0.05 and 0.12) nanocrystals. The ferromagnetism of the nanocrystals was controlled by Ni doping concentration and the external magnetic field treatment. The photoluminescence properties of the Ni doped In 2 O 3 nanocrystals were also investigated.

The effect of spin states on the spin entropy has been studied in La 0.7− x Y x Ca 0.3 CoO 3 ( x =0.0, 0.05, 0.1 and 0.15). Magnetothermopower results reveal that Y-doping improves the spin entropy of La 0.7 Ca 0.3 CoO 3 . The magnetic results indicate that the Co 3+ ions undergo a transition from an intermediate spin state (t 5 2g e 1 g ; S =1) to a low spin state (t 6 2g e 0 g ; S =0), which results in the enhancement of the spin entropy. A suitable theoretical model is suggested to explain well the enhancement of the spin entropy induced by spin state transition. This investigation gives strong evidence that the enhanced spin entropy contribution to the thermopower is tuned by the spin state transition.

We report the effects of rare earth Gd doping on the spin entropy in Ca 3 Co 4 O 9+ δ . Magnetothermopower is found to be strongly magnetic-field dependent and increases upon Gd doping, revealing that Gd doping can improve the spin entropy of Ca 3 Co 4 O 9+ δ . X-ray photoemission spectroscopy (XPS) results allow us to determine the decrease in Co 4+ concentration induced by Gd doping. Our results confirm that the spin-entropy enhancement originates from the decrease of Co 4+ concentration. The present study suggests that we can improve the spin entropy by suppressing Co 4+ concentration in layered cobalt oxides.