CoO3 Samples Research Articles

Structure, magnetic and electrical properties of Tb0.5Nd0.5CoO3 perovskite solid solutions with carbon material flakes (CMF)

In this paper, we study the ceramics of cobaltite [Tb0.5Nd0.5CoO3]1−yCy with equiatomic contents of terbium and neodymium and with the addition of a carbon material flakes (СMF) with the content of carbon in the range of weight fraction 0 < y < 0.01. The ceramics were obtained by the method of solid-phase reactions according to the standard ceramic technology in air. Using X-ray diffraction and energy-dispersive analysis, it was established that the initial (undoped) solid solution is single-phase with perovskite structure. It was shown that Co3+ ions in undoped ceramics under study are in a non-magnetic low-spin state over the entire temperature range under consideration (2–320 K), and the entire contribution to the magnetization is due to Tb3+ and Nd3+ ions. An analysis of the temperature dependences of the electrical resistance R(T) shows that the initial solid solution behaves like a dielectric with a reduced activation energy of about 0.3–0.35 eV in the temperature range of 280–320 K, due to hopping conduction with a constant activation energy (constant range hopping) by defects in Tb0.5Nd0.5CoO3 matrix. At lower temperatures (200 – 275 K) in the initial (undoped) Tb0.5Nd0.5CoO3 samples, the Mott-type hopping conductivity mechanism with a variable range hopping and high values of the characteristic temperature To ∼ 109 K was observed. Simultaneously, at 300 K, we observed hopping conductivity on alternating current according to the law σ(ω) ∼ ω-α(ω), where the exponent α, which determines a hopping probability, depends on the frequency. In [Tb0.5Nd0.5CoO3]1−yCy samples, heavily doped with СMF with y = 0.01, carrier transport in the entire studied temperature range of 2–320 K mainly occurs along highly conductive carbon-based channels, formed inside of Tb0.5Nd0.5CoO3 matrix by СMF. At temperatures below 15–20 K, these samples show hopping behavior of R(T) with a variable range hopping over localized states, described by the Mott–Kirkpatrick law. In the temperature range 20–100 K electrical resistance versus temperature R(T) along carbon-based channels obey the law R(T) ∼ Ln T. In this case, the relative magnetoresistance MR(T, B) is characterized by a negative sign at temperatures below 10 K in magnetic fields less than 1 T. This behavior of the R(T, B) dependences were described on the basis of the theory of quantum corrections to the Drude conductivity for two-dimensional samples under conditions of weak localization, which is consistent with the behavior of layered carbon-based materials known from the literature. At T > 150 K curves R(B) follow to Lorentz-like positive magnetoresistive effect in Tb0.5Nd0.5CoO3 matrix in the whole magnetic field range.

Simulation of magnetocaloric properties in La1-xBaxCoO3

The magnetocaloric effect-(MCE) of La1-xBaxCoO3 near the magnetic phase transition was simulated at levels of doping x = 0.3 and 0.5. It is shown that Ba doping allows MCE in La1-xBaxCoO3 to be adjusted, which gives La1-xBaxCoO3 more significant for construction magnetic refrigeration. The range of temperatures that the simulated MCE of La1-xBaxCoO3 can function is nearly between 90 and 220 K. The simulated MCE of La0.7Ba0.3CoO3 has more impact than the MCE of La0.5Ba0.5CoO3. Furthermore, the simulated results demonstrate that the MC properties of the La0.7Ba0.3CoO3 sample are considerably larger than some of the MC ones of many MC materials in published works.

Ba-doped vs. Sr-doped LaCoO3 perovskites as base catalyst in diesel exhaust purification

Perovskites (ABO3) have emerged in recent years as an economic alternative to noble metals for oxidation-reduction reactions in diesel engines aftertreatment systems. Sr-doped La1-xSrxCoO3 perovskites have shown high efficiency in NO oxidation and limited NOx reduction efficiency. Alternatively, in this work, the effects of barium doping on NOx removal efficiency of LaCoO3 perovskite are explored with the aim of developing a more promising perovskite base catalyst. Bulk perovskites were prepared by substituting La3+ with increasing Ba2+ doping levels, i.e. La0.9Ba0.1CoO3, La0.8Ba0.2CoO3, La0.7Ba0.3CoO3, La0.6Ba0.4CoO3 and La0.5Ba0.5CoO3. The prepared catalysts were characterized in terms of crystalline structures identification (XRD), specific surface area (N2 adsorption-desorption at -196 °C), reducibility and oxidation state of Co ions (H2-TPR), concentration and strength of adsorbed oxygen species (O2-TPD) and surface basicity (CO2-TPD). The characterization results suggest that charge imbalance associated to Ba2+ accommodation in the perovskite lattice in substitution of La3+ leads to a preferential formation of oxygen vacancies. As a result, Ba-doped perovskites improve NO-to-NO2 conversion with respect to LaCoO3 perovskite. La0.7Ba0.3CoO3 perovskite shows the best NO oxidation efficiency (66 % at 350 °C). This fact is associated to the higher oxygen vacancies concentration, which favors the exchange capacity between oxygen in the lattice and in the gas phase. This sample also shows the best NOx storage and reduction efficiency (maximum NOx-to-N2 reduction of 40 % at 350 °C). On the one hand, NOx adsorption is promoted due to the best balance between NO oxidation capacity and NOx adsorption sites accessibility. On the other hand, the higher strength of NOx adsorption sites promotes a slower nitrates decomposition, which favors NOx reduction during rich period. The obtained results improve the NOx removal efficiency of reference La0.7Sr0.3CoO3 sample (maximum NOx-to-N2 reduction of 10 % at 350 °C). Thus, La0.7Ba0.3CoO3 catalyst is considered as a more promising base material for automotive applications.

Critical Behavior Near the Paramagnetic-Ferromagnetic Phase Transition in La1−x Sr x CoO3 (x = 0.3 and x = 0.5) Cobaltites

In this work, we present the study of structural, magnetic and critical behavior of La1−x Sr x CoO3 (x = 0.3 and 0.5) cobaltites. X-ray diffraction analysis using the Rietveld refinement shows that the sample La0.7Sr0.3CoO3 crystallizes in a rhombohedral distorted perovskite structure with $R\overline {3} c$ space group, while La0.5Sr0.5CoO3 exhibits both phases rhombohedric and cubic, with $R\overline {3} c$ and $Pm\overline {3} m$ space groups. Our samples exhibit thermo-magnetic irreversibility behavior in the magnetization data under a magnetic applied field of 0.05 T. The temperature dependence of magnetic susceptibility at higher temperatures for La0.7Sr0.3CoO3 sample reveals the presence of the Griffiths’ phase above the Curie temperature, while it follows the Curie–Weiss law perfectly for La0.5Sr0.5CoO3. Different techniques including the modified Arrott plot, the Kouvel-Fisher method and the critical isotherm analysis have been used to analyze the paramagnetic-ferromagnetic phase transition. The values of the critical exponents associated to this transition were close to those expected in the mean-field model. The validity of the calculated critical exponents has been confirmed by the scaling equation of state. The determined critical exponents using various methods are found to follow the scaling equation with the magnetization data scaled into two different curves below and above the critical temperature.

Key factors in Sr-doped LaBO3 (B = Co or Mn) perovskites for NO oxidation in efficient diesel exhaust purification

Perovskites have attracted attention in recent years as an economic alternative to noble metals in oxidation processes. Synthesis conditions of LaCoO3 and LaMnO3 perovskites have been studied varying citrate to nitrate molar ratio in the starting solution, pH and calcination protocol, with the aim of obtaining high purity perovskites, absence of impurities, and with enhanced textural properties. Once synthesis conditions were established, strontium was incorporated in the perovskite lattice as a textural and structural promoter, by substituting lanthanum with different doping levels, i.e. La0.9Sr0.1BO3, La0.8Sr0.2BO3, La0.7Sr0.3BO3, La0.6Sr0.4BO3 and La0.5Sr0.5BO3 with B = Co or Mn. The prepared solids were characterized in terms of crystalline phase identification (XRD), specific surface area (N2 adsorption–desorption at −196 °C), reducibility and oxidation state of transition metal ions (H2-TPR), quantification of adsorbed oxygen species (O2-TPD) and surface elemental composition (XPS). Charge imbalance associated to strontium (Sr2+) incorporation in the perovskite lattice in substitution of lanthanum (La3+) was preferentially balanced by Mn4+ promotion in La1−xSrxMnO3 perovskites, whereas formation of oxygen vacancies seems to be the mechanism for charge compensation in La1−xSrxCoO3 perovskites, where Co ions remained as Co3+ ions. Strontium doped perovskites further improved NO conversion compared to the non-substituted formulations. The best NO oxidation performance was obtained with La0.7Sr0.3CoO3 and La0.9Sr0.1MnO3 samples, achieving maximum NO conversion of 83 and 65% at 300 and 325 °C, respectively. Higher oxidation capacity of La0.7Sr0.3CoO3 sample was associated to the higher oxygen mobility and exchange capacity between oxygen in the lattice and gas phase oxygen. It is worth noting that prepared perovskites presented far higher NO oxidation capacity than platinum-based NSR model catalysts, confirming perovskites as an economic alternative to catalyze NO oxidation reactions in automotive catalysis.

Glassy ferromagnetism in La0.75Sr0.25-xKxCoO3 polycrystalline perovskite cobaltites

In this paper, the structural and magnetic properties of La0.75Sr0.25-xKxCoO3 (0≤x≤0.15) powder samples are reported. X-ray diffraction analysis using Rietveld refinement show that all our samples crystallize in the rhombohedral structure with $$ R\overline{3}c $$ space group. At low temperatures, with increasing K amount, the samples change from ferromagnetic like behavior (x = 0, 0.05 and 0.1) to spin glass one (x = 0.15). The ferromagnetic-paramagnetic transition temperature decreases with increasing K amount from 235K (x = 0) to 200K (x = 0.1). In the paramagnetic phase, all our synthesized samples obey the Curie-Weiss law. In the vicinity of TC, the magnetic entropy change |ΔS M | for La0.75Sr0.2K0.05CoO3 sample reached maximum values of 0.47Jkg/K and 2.27Jkg/K under magnetic field variation in the range 1T and 7T, respectively. The magnetization hysteresis loops demonstrate a weakening the ferromagnetism with increasing K content.

Investigation of the origin of glassiness in La0.5Sr0.5CoO3

A comprehensive magnetic study has been carried out on the two sets of La0.5Sr0.5CoO3 samples with a view to understand the origin of low temperature glassiness in the ferromagnetic state. The samples prepared by the conventional solid-state synthesis method show a low temperature shoulder in both dc magnetization as well as in the ac susceptibility measurements, which exhibit characteristics of glassiness such as the frequency dependence and memory effect. These observations suggest the existence of a distinct low temperature cluster-glass like phase within dominant ferromagnetic phase. But, once the same sample is properly homogenized by repeated grinding and annealing process, the low temperature glassy phase disappears, and it shows a pure ferromagnetic behavior. Our comparative study clearly reveals that the reentrant spin-glass like nature is not intrinsic to La0.5Sr0.5CoO3 system, in fact this is an outcome of the compositional inhomogeneity.

Infrared emissivity and microwave absorption property of Sm0.5Sr0.5CoO3 perovskites decorated with carbon nanotubes

Pure perovskite-type Sm0.5Sr0.5CoO3 (SSC, in short) oxide and SSC/carbon nanotubes composites were sintered under nitrogen atmosphere to study the in-service performance. The phase stability, optical reflection, infrared emissivity and microwave absorption property were systematically investigated. The pure Sm0.5Sr0.5CoO3 sample maintained perovskite structure and low infrared emissivity in nitrogen atmosphere below 1000°C. For the composite specimens, the emergence of A2BO4, Sm2O3, CoO and cobalt metal phases was inevitable due to the flowing nitrogen atmosphere, the excess temperature and the carbon nanotubes content, which in consequence played a key role in increasing the infrared emissivity. However, the composites displayed remarkable enhanced microwave absorption property compared with the pure perovskites.

Effect of Y-doping on the transport and magnetic properties of La0.5Sr0.5CoO3 and La0.7Sr0.3CoO3

The temperature variation of magnetization, resistivity, and thermoelectric power of undoped and Y-doped La0.7Sr0.3CoO3 and La0.5Sr0.5CoO3 samples have been investigated. Y-doping decreases the magnetization possibly due to the spin-state transition of Co ions. The low temperature conduction in (La1−y Y y )0.7Sr0.3CoO3 is consistent with the variable range hopping. With Y-doping, value of the Seebeck coefficient increases as Y-doping decreases bandwidth and increases distortion. Seebeck coefficient value also reflects that the orbital stability increases with Sr concentration.

Thermoelectric and magnetic properties of nanocrystalline La0.7Sr0.3CoO3

This work reports on the effect of grain size on the electrical, thermal, and magnetic properties of La0.7Sr0.3CoO3 samples obtained by solid-state reaction combined with ball milling. Electrical characterization made for samples with grain sizes ranging from 26 nm to 0.57 μm shows a size-induced metal-to-insulator transition for the sample with smallest grain size. As a consequence of the grain size reduction, there is a change of the thermopower sign and the thermal conductivity displays extremely low values. Interestingly, the thermoelectric figure of merit displays an enhancement as a consequence of the grain size reduction. The Curie temperature was found to be constant whereas the saturation magnetization decreases as grain size decreases. Thereby electrical and magnetic properties of nanocrystalline samples are interpreted in terms of a core-shell structure in which grain boundaries display an insulating behavior while the core region of each grain exhibits the bulk properties.

The incongruous observation of magnetic phase separation in La0.85Sr0.15CoO3 spin glass system

Phase separation (PS) in hole-doped cobaltites (La1−xSrxCoxO3) is drawing renewed interest recently. In particular, the magnetic behavior of La0.85Sr0.15CoO3 has been subjected to a controversial debate for the past several years; while some groups show evidence for magnetic PS, others show spin glass (SG) behavior. Here, an attempt is made to resolve the controversy related to “PS versus SG” behavior in this compound. We present the results of a comprehensive investigation of the dc magnetization, ac susceptibility, and the magnetotransport properties of La0.85Sr0.15CoO3 samples. We contemplate that the magnetic PS in La0.85Sr0.15CoO3 is neither intrinsic nor inherent, but it is a consequence of the preparation conditions. It is realized that a low temperature annealed (LTA) sample shows PS whereas the high temperature annealed (HTA) sample shows SG behavior. The Brillouin-like behavior of field cooled dc magnetization and apparently no frequency dependent peak shift in ac susceptibility for the LTA sample characterize it to be of ferromagneticlike whereas a kink in field cooled dc magnetization and a considerable amount (∼3 K) of frequency dependent peak shift in the ac susceptibility for the HTA sample characterize it to be of SG state. The magnetotransport properties show that the HTA sample is more semiconducting as compared to the LTA sample. This is interpreted in terms of the presence of isolated as well as coalescing metallic ferromagnetic clusters in the case of LTA sample. The magnetoresistance (MR) at 10 K for the HTA sample exhibits a huge value (∼65%) as compared to the LTA sample, and it monotonically decreases with the rise in temperature. Such a high value of MR in the case of HTA sample is strongly believed to be due to the spin dependent part of random potential distribution. Further, the slow decay of remnant magnetization with progress of time and the existence of hysteresis at higher temperatures (up to 200 K) in the case of LTA sample as compared to the HTA sample clearly unveil different magnetic states associated with them.

Phase separation versus spin glass behavior in La0.85Sr0.15CoO3

We present comprehensive studies of dc magnetization, ac susceptibility, and magnetotransport of two sets of La0.85Sr0.15CoO3 samples, one exhibits phase separation and the other exhibits spin glass behavior. Our study reveals that the phase separation in La0.85Sr0.15CoO3 is neither inherent nor ubiquitous; rather, it is a consequence of preparation condition. It is realized that the low temperature annealed sample exhibits phase separation while the high temperature annealed one shows the characteristic of spin glass behavior. This study shows that the most probable magnetic state of La0.85Sr0.15CoO3 is spin glass.

Sensing Properties of Nanostructured Sm1-xBaxCoO3 (x = 0, 0.1) obtained by Solution Method

AbstractSome cobaltites with perovskite-type structure exhibit outstanding transport properties and high chemical activity, which make these materials suitable for applications in areas of gas sensors, heterogeneous catalysis, gas separation membranes and cathodes for solid oxide fuel cells. In this work, polycrystalline samples of Sm1-xBaxCoO3 (x = 0, 0.1) were prepared by an aqueous solution method using the corresponding nitrates. X-ray diffraction patterns of calcined samples showed that single-phase SmCoO3 was obtained at 900°C, whereas Sm0.9Ba0.1CoO3 was formed at 700°C. Electron microscopy images revealed that micron-sized particles were obtained for SmCoO3, whereas a nanostructured and nanoporous material wasobserved for Sm0.9Ba0.1CoO3. Electrical measurements made on thick films of the oxides revealed a semiconductor behavior in both phases, however Sm0.9Ba0.1CoO3 samples showed a larger conductivity compared with SmCoO3; dynamic response of resistance experiments made in air and CO2 revealed that Sm0.9Ba0.1CoO3 is selective to CO2.