We survey the magnetocaloric effect in perovskite-type oxides （including doped ABO3-type manganese oxides, A3B2OT-type two-layered perovskite oxides, and A2B＇B＇＇O6-type ordered double-perovskite oxides）. Magnetic entropy changes larger than those of gadolinium can be observed in polycrystalline La1-xCaxMnO3 and alkali-metal （Na or K） doped La0.8Ca0.2MnO3 perovskite-type manganese oxides. The large magnetic entropy change produced by an abrupt reduction of magnetization is attributed to the anomalous thermal expansion at the Curie temperature. Considerable mag- netic entropy changes can also be observed in two-layered perovskites Lal.6Cal.4Mn207 and La2.5-xK0.5＋xMn2O7＋6 （0 〈 x 〈 0.5）, and double-perovskite Ba2Fe1＋xMol-xO6 （0 〈 x 〈 0.3） near their respective Curie temperatures. Com- pared with rare earth metals and their alloys, the perovskite-type oxides are lower in cost, and they exhibit higher chemical stability and higher electrical resistivity, which together favor lower eddy-current heating. They are potential magnetic refrigerants at high temperatures, especially near room temperature.
We present a detailed study on the magnetic coercivity of Co/CoO-MgO core-shell systems, which exhibits a large exchange bias due to an increase of the uncompensated spin density at the interface between the CoO shell and the metallic Co core by replacing Co by Mg within the CoO shell. We find a large magnetic coercivity of 7120 Oe around the electrical percolation threshold of the Co/CoO core/shell particles, while samples with a smaller or larger Co metal volume fraction show a considerably smaller coercivity. Thus, this study may lead to a route to improving the magnetic properties of artificial magnetic material in view of potential applications.
In this work,we put forward a scheme to exquisitely design and selectively synthesize the core@shell structured MSe_(2)/FeSe_(2)@MoSe_(2)(M=Co,Ni)flower-like multicomponent nanocomposites(MCNCs)through a simple two-step hydrothermal reaction on the surfaces of MFe_(2)O_4 nanospheres with the certain amounts of Mo and Se sources.With increasing the amounts of Mo and Se sources,the obtained core@shell structured MSe_(2)/FeSe_(2)@MoSe_(2)(M=Co,Ni)MCNCs with the enhanced content of MoSe_(2)and improved flower-like geometry morphology could be produced on a large scale.The obtained results revealed that the as-prepared samples displayed improved comprehensive microwave absorption properties(CMAPs)with the increased amounts of Mo and Se sources.The as-prepared CoSe_(2)/FeSe_(2)@MoSe_(2)and NiSe_(2)/FeSe_(2)@MoSe_(2)MCNCs with the well-defined flower-like morphology could simultaneously present the outstanding CMAPs in terms of strong absorption capability,wide absorption bandwidth,and thin matching thicknesses,which mainly originated from the conduction loss and flower-like geometry morphology.Therefore,the findings not only develop the very desirable candidates for high-performance microwave absorption materials but also pave a new way for optimizing the CMAPs through tailoring morphology engineering.
Ferromagnetic shape memory alloys, which undergo the martensitic transformation, are famous multifunctional materials. They exhibit many interesting magnetic properties around the martensitic transformation temperature due to the strong coupling between magnetism and structure. Tuning magnetic phase transition and optimizing the magnetic effects in these alloys are of great importance. In this paper, the regulation of martensitic transformation and the investigation of some related magnetic effects in Ni–Mn-based alloys are reviewed based on our recent research results.
The crystal structure,magnetic and magnetostrictive properties of high-pressure synthesized Prx Nd1-xFe1.9（0≤x≤1.0） alloys were studied.The alloys exhibit single cubic Laves phase with MgCu 2-type structure.The initial magnetization curve reveals that Pr0.2Nd0.8Fe1.9 has a minimum magnetocrystalline anisotropy at 5 K.The magnetostriction curve at 5 K shows that Pr0.2Nd0.8Fe1.9 has a very good low-field magnetostrictive property,and the magnetostriction of the PrxNd1-xFe1.9 alloy in high magnetic field is attributable mainly to Pr.The temperature dependence of the magnetostriction（λ ｜｜） at the field of 5 kOe shows that the substitution of Nd reduces the K 1 remarkably,and the values of λ｜｜ of Pr0.2Nd0.8Fe1.9 and Pr0.8Nd0.2Fe1.9 alloys are nearly five times larger than that of the PrFe 1.9 alloy below 50 K;the λ｜｜ of Pr0.8Nd0.2Fe1.9 reaches up to 1082 ppm at 100 K,which makes it a potential candidate for application in this temperature range.
In order to effectively utilize the magnetic-dielectric synergy and interfacial engineering,in this paper,yolk–shell structured magnetic multicomponent nanocomposites(MCNCs)including CoNi@void@C and CoNi@void@C@MoS_(2) were produced in large scale by in-situ pyrolysis of cubic CoNi Prussian blue analogs(PBAs)followed by the hydrothermal process,respectively.Because of their unique structures,excellent synergistic effect between dielectric and magnetic loss,the as-prepared CoNi@void@C and CoNi@void@C@MoS_(2) MCNCs displayed very outstanding electromagnetic wave absorption performances(EMWAPs)including strong absorption capabilities,broad absorption bandwidth and thin matching thicknesses.Furthermore,the as-prepared CoNi@void@C and CoNi@void@C@MoS_(2) MCNCs well maintained the cubic configuration of CoNi PBAs even after the thermal treatment and hydrothermal processes.The unique structure and formed carbon layers effectively prevented the corrosion of internal CoNi alloy during the formation of MoS_(2),and CoNi@void@C@MoS_(2) MCNCs with different MoS_(2) contents could be synthesized by controlling the hydrothermal temperature.The obtained results revealed that the EM parameters,dielectric and magnetic loss capabilities of CoNi@void@C@MoS_(2) MCNCs could be tuned by controlling hydrothermal temperature and filler loading,which made their outstanding EMWAPs could be achieved in different frequency regions.Taking account of simple process,low density and high chemical stability,our findings provided a new and effective pathway to develop the strong wideband microwave absorbers.
Based on the first-principles plane wave calculations, we show that Li adsorbed on monolayer and bilayer MoS2 forming a uniform and stable coverage can serve as a high-capacity hydrogen storage medium, and Li-coated MoS2 can be recycled by operations at room temperature due to Li having strength binding, big separation and is stable against clustering. The full Li coverage MoS2 system（2 ＊ 2 hexagonal MoS2 supercell） can reach up to eight H2 molecules on every side, corresponding to the gravimetric density of hydrogen storage up to 4.8 wt% and 2.5 wt% in monolayer and bilayer MoS2, respectively. The adsorption energies of hydrogen molecules are in the range of 0.10 e V/H2–0.25 e V/H2,which are acceptable for reversible H2 adsorption/desorption near ambient temperature. In addition, compared with light metals decorated low dimension carbon-based materials, the sandwiched structure of MoS2 exhibits the greatly enhanced binding stability of Li atoms as well as slightly decreased Li-Li interaction and thus avoids the problem of metal clustering.It is interesting to note that the Li atom apart from the electrostatic interaction, acts as a bridge of hybridization between the S atoms of MoS2 and adsorbed H2 molecules. The encouraging results show that such light metals decorated with MoS2 have great potential in developing high performance hydrogen storage materials.
Defect and interface engineering are efficient approaches to adjust the physical and chemical properties of nanomaterials.In order to effectively utilize these strategies for the improvement of microwave absorption properties(MAPs),in this study,we reported the synthesis of hollow carbon shells and hollow carbon@MoS_(2)nanocomposites by the template-etching and templateetching-hydrothermal methods,respectively.The obtained results indicated that the degree of defect for hollow carbon shells and hollow carbon@MoS_(2)could be modulated by the thickness of hollow carbon shell,which effectively fulfilled the optimization of electromagnetic parameters and improvement of MAPs.Furthermore,the microstructure investigations revealed that the precursor of hollow carbon shells was encapsulated by the sheet-like MoS_(2)in high efficiency.And the introduction of MoS_(2)nanosheets acting as the shell effectively improved the interfacial effects and boosted the polarization loss capabilities,which resulted in the improvement of comprehensive MAPs.The elaborately designed hollow carbon@MoS_(2)samples displayed very outstanding MAPs including strong absorption capabilities,broad absorption bandwidth,and thin matching thicknesses.Therefore,this work provided a viable strategy to improve the MAPs of microwave absorbers by taking full advantage of their defect and interface engineering.
