Crystalline phases and magnetic properties of Cu-Bi-Zn co-doped Co2Z ferrites

The effects of Cu, Bi and Zn substitutions on the crystalline phase and magnetic properties of hexagonal ferrites with a composition of 3(Ba_1−xBi_xO)2(Co_1−yCu_yO)12(Fe_2-(x/4)Zn_(x/4)O₃) were investigated. The results showed that the addition of Bi and Zn can significantly promote Z phase formation. The Z phase may be triggered to decompose into U and W phases resulting from the evaporation of Bi and segregation of Co-rich ferrites at high sintering temperatures for samples with large amounts of Bi and Zn. The initial permeability decreased with increasing x value because the Bi substitution promoted Z phase decomposition into U and W phases.     

Co_6W_6C化合物的热力学研究

以高纯钨、钴、碳粉为原料,在真空条件下制备获得物相纯净的Co_6W_6C化合物,对Co_6W_6C进行系列实验测定并结合计算得出了其相关的热力学参数。结果表明:在原料粉中碳含量为0.98%~1.06%(质量分数)、真空反应温度为1000℃、保温时间为1 h的条件下,可制备获得物相纯净的Co_6W_6C。结合其等压热容及1173 K下氧化反应的反应焓测定结果,通过计算获得了Co_6W_6C的标准摩尔熵(S_m~Θ)、标准摩尔生成焓(Δ_fH_m~Θ)等热力学参数以及其等压热容(C_p)、焓(H)、熵(S)和吉布斯自由能(G)等基础热力学参量随温度变化的函数关系。     

Co1.2Mn0.8B化合物的磁性能与磁热效应

采用球磨和固相烧结相结合的方法制备了Co_(1.2)Mn_(0.8)B化合物,并对其晶体结构、磁性能和磁热效应进行了研究。结果表明,Co_(1.2)Mn_(0.8)B化合物的物相为Co_2B单相结构,具有CuAl2型体心正方晶体结构,空间群为I14/mcm;在居里温度TC=175 K附近,化合物发生了铁磁(FM)-顺磁(PM)二级磁相转变,热滞为0.7 K;Co_(1.2)Mn_(0.8)B化合物在外磁场0～5 T变化下的最大磁熵变ΔSM为1.17 J·(kg·K)~(-1),其对应的温度不随外场的变化而变化。     

Improved high-Q microwave dielectric resonator using ZnO-doped La(Co1/2Ti1/2)O3 ceramics

The microwave dielectric properties and the microstructures of ZnO-doped La(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. Doped with ZnO (up to 0.75 wt%) can effectively promote the densification of La(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. At 1320 °C, La(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt% ZnO addition possesses a dielectric constant (_r) of 30.2, a Q × f value of 73,000 GHz (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −35 ppm/°C.     

High-frequency magnetic properties of FeNi3-SiO2 nanocomposite synthesized by a facile chemical method

Monodisperse core-shell structured FeNi₃-SiO₂ composite nanoparticles (NPs) were synthesized by a facile hydrazine reduction combined modified Stöber method. SEM and TEM analysis shows that FeNi₃ cores are composed of small primary nanocrystals and are coated by amorphous SiO₂ layer. As-prepared FeNi₃-SiO₂ nanocomposites exhibit typical soft magnetic properties. The permeability spectra vary with the contents of SiO₂. When the SiO₂ content is 10 wt%, the real part μ′ of the permeability reaches about 10 and is almost independent of frequency in the frequency range up to 1 GHz. And the imaginary part μ′′ remains very low. This paper presents a facile approach to fabrication of novel soft magnetic materials for high-frequency applications.     

锂离子电池正极材料LiNi0.8Co0.15Al0.05O2的制备及电化学性能

首先以AlO2-为铝源,采用三元共沉淀法制备前驱体Ni_(0.8)Co_(0.15)Al_(0.05)(OH)_2。对前驱体进行500℃高温处理,随后与过量的锂盐混合均匀,在氧气气氛下700℃煅烧12 h制得LiNi_(0.8)Co_(0.15)Al_(0.05)O_2(NCA)材料。采用X射线衍射仪(XRD)测试可知,所得的NCA材料呈典型的α-NaFeO_2层状结构,属于R-3m空间群。扫描电子显微镜(SEM)测试显示,NCA为粒径5～6μm的球状颗粒。材料在电流倍率为0.1C下首次放电容量为167.1mAh/g,循环200次以后容量保持率为96.2%。倍率测试表明,0.1、10 C下NCA的容量分别为184.0、112.7 mAh/g,到恢复到0.1 C时,容量仍可达179.7mAh/g,具有比较好的倍率性能。     

水热电沉积法生长Co9S8薄膜电极

采用水热电沉积法在泡沫镍基体上原位沉积Co_9S_8薄膜,并对其形貌、组成、结构和电化学性能进行表征和测试。结果表明,镍基Co_9S_8薄膜呈花瓣片状,并具有优异的电化学性能,其在电流密度为10mA/cm~2时,比电容可高达2538.7 F/g。即使电流密度扩大至50 mA/cm~2时,比电容依然可达1930.7 F/g。经过1000次循环(电流密度为20 mA/cm2),比电容仍可达为1825.2 F/g,电容保有率72.8%,经过1500次循环后,电容保有率61.4%。     

Novel SnO2/MnO co-doped phosphate glasses with tunable luminescence properties

In the present work, novel SnO₂/MnO co-doped phosphate glasses are prepared and their tunable luminescence properties are presented. Under excitation at 267 nm, all co-doped glasses exhibit an assembly of SnO₂ blue emission at 420 nm and MnO red emission around 600 nm. With the increasing of MnO concentration, intensity of the SnO₂ blue emission decreases while that of MnO red emission increases. With the increasing of SnO₂ concentration, both SnO₂ and MnO emissions are greatly enhanced, resulting in the white light emitting.     

Structural, magnetic and electrical properties of nanocrystalline zinc ferrite

Nanoparticles of ZnFe₂O₄ with grain size of 30 nm have been synthesized via sol-gel auto combustion method and characterized using DSC (differential scanning calorimetry), XRD (X-ray diffraction), SEM (scanning electron microscopy), EDAX (energy dispersive X-ray analysis), FTIR (Fourier transform infrared spectroscopy), VSM (vibrating sample magnetometer) and Ac-electrical conductivity measurement setup. Structural analysis using XRD and FTIR show the formation of spinel structure in nano ZnFe₂O₄ particles. The cation distribution in the sample has been estimated theoretically and the results show that as-prepared nanoparticles of ZnFe₂O₄ have partially inverted ionic distribution in comparison with that of bulk zinc ferrite. Results from EDAX indicate the ratio of Fe:Zn as close to 2:1. The presence of a weak ferrimagnetic phase for nano ZnFe₂O₄ at room temperature has been established from its hysteresis behavior. Redistribution of cations occurring at nano-regime and surface spin canting of nanoparticles of ZnFe₂O₄ are expected to be responsible for the presence of magnetic ordering in nano ZnFe₂O₄. The Curie temperature of nano ZnFe₂O₄ determined from magnetization versus temperature measurement is equal to 375 °C. The electrical conductivity of the present ZnFe₂O₄ at a frequency of 100 kHz at room temperature is observed to be 2.11 × 10⁻⁸ Ω⁻¹ cm⁻¹ which is four orders lesser in magnitude than the value of bulk ZnFe₂O₄. Overall conductivity response with respect to temperature and frequency confirm that ZnFe₂O₄ tends to become more dielectric in the nano-regime.     

Local electric field investigation of Si2N2O and its electronic structure, elastic and optical properties

This paper demonstrates that the Clausius-Mosotti equation cannot be used for Si₂N₂O since the Lorenz approximation is invalid therein. Therefore a modified definition for the Lorenz electric field is suggested which can be derived from the optical dielectric constant calculated using the plane-wave pseudopotential method. In addition, other parameters of Si₂N₂O such as the energy band gap, density of states, elastic and optical properties are also given in this paper. Based on the new expression for local electric field, the modified Clausius-Mosotti equation is suggested, and then the dielectric constant of Si₂N₂O is discussed by using the additivity rule. It is found that the result of the phenomenological analysis of the dielectric constant is basically consistent with the experimental data and the first principles results, which explain the experimental observation that the dielectric constant of Si₂N₂O is enhanced with the increase of Li content. Also, the modified Clausius-Mosotti equation and the additivity rule are suggested to be used in predicting the dielectric behaviors of the new and complex compounds.     

Co2P: A facile solid state synthesis and its applications in alkaline rechargeable batteries

The Co₂P crystals are successfully synthesized by a facile solid state reaction. Energy dispersive X-ray spectrum (EDS) results indicate a mol ratio of 0.6686-0.3314 for Co to P, confirming the stoichiometric ratio of Co₂P. Galvanostatic charge/discharge tests show that Co₂P exhibits a high maximum discharge capacity (C_max) of 223.5 mAh g⁻¹ and excellent cyclic properties with capacity retention of 97.9% (C₃₀₀/C_max) in the 300th cycle as an anode material in alkaline rechargeable batteries. Cyclic voltammetry (CV) and XRD tests during full charge/discharge processes confirm a quasi-reversible redox mechanism between Co(OH)₂ and Co resulting from the conversion of Co₂P. The simultaneously produced P plays an important role in the whole process - with the reaction and dissolution in the electrolyte, it brings many new interspaces in Co₂P body to enlarge the contact area between the active material and the electrolyte, and make the electrochemical process easier and faster.     

镁扩散制备Pr6O11掺杂的 MgB2块体临界电流密度和磁场性能研究

采用镁扩散方法制备了Pr₆O₁₁纳米颗粒添加的MgB₂超导块体,研究了Pr₆O₁₁掺杂对其临界电流密度(J_c),不可逆磁场(H_irr)和上临界磁场(H_c2)的影响。实验结果表明Pr₆O₁₁纳米颗粒掺杂明显提高了块体的J_c,H_irr和H_c2,但没有降低其超导转变温度T_c。在20 K自场条件下,质量比为1 wt.% Pr₆O₁₁掺杂的MgB₂块体的J_c较没掺杂样品提高了将近5倍, J_c=3.61×10₅A/cm₂。在10 K温度下,MgB₂块体H_c2 和H_irr较没掺杂样品分别提高了1.9 T and 2.6 T。同时讨论了Pr₆O₁₁纳米颗粒掺杂对MgB₂块体的电性能和磁通钉扎机制的影响。     

Magnetic properties of new compounds RuMn2Sn and RuMn2Si

New compounds RuMn₂Z (Z = Si, Sn) have been synthesized. X-ray diffraction measurements have confirmed that RuMn₂Z (Z = Si, Sn) crystallizes in a Heusler-like cubic structure. The lattice parameters of RuMn₂Si and RuMn₂Sn at room temperature are estimated to be 5.8260 Å and 6.2195 Å, respectively. Magnetization measurements have been carried out in fields up to 50 kOe for RuMn₂Z (Z = Si, Sn). Furthermore, the temperature dependence of initial permeability of RuMn₂Sn has been studied. RuMn₂Sn shows ferrimagnetic behavior. The spontaneous magnetic moment at 5 K and the Curie temperature of RuMn₂Sn are found to be of 1.68 μ_B/f.u. and 272.1 K, respectively. RuMn₂Si exhibits spin-glass-like behavior with a freezing temperature estimated to be about 50 K.     

Effects of K2O-Li2O doping on surface and catalytic properties of Fe2O3/Cr2O3 system

The effects of K₂O and Li₂O-doping (0.5, 0.75 and 1.5 mol%) of Fe₂O₃/Cr₂O₃ system on its surface and the catalytic properties were investigated. Pure and differently doped solids were calcined in air at 400-600 °C. The formula of the un-doped calcined solid was 0.85Fe₂O₃:0.15Cr₂O₃. The techniques employed were TGA, DTA, XRD, N₂ adsorption at −196 °C and catalytic oxidation of CO oxidation by O₂ at 200-300 °C. The results revealed that DTA curves of pure mixed solids consisted of one endothermic peak and two exothermic peaks. Pure and doped mixed solids calcined at 400 °C are amorphous in nature and turned to α-Fe₂O₃ upon heating at 500 and 600 °C. K₂O and Li₂O doping conducted at 500 or 600 °C modified the degree of crystallinity and crystallite size of all phases present which consisted of a mixture of nanocrystalline α- and γ-Fe₂O₃ together with K₂FeO₄ and LiFe₅O₈ phases. However, the heavily Li₂O-doped sample consisted only of LiFe₅O₈ phase. The specific surface area of the system investigated decreased to an extent proportional to the amount of K₂O and Li₂O added. On the other hand, the catalytic activity was found to increase by increasing the amount of K₂O and Li₂O added. The maximum increase in the catalytic activity, expressed as the reaction rate constant (k) measured at 200 °C, attained 30.8% and 26.5% for K₂O and Li₂O doping, respectively. The doping process did not modify the activation energy of the catalyzed reaction but rather increased the concentration of the active sites without changing their energetic nature.     

Structural, dielectric and magnetic properties of Nd-doped Co2Z-type hexaferrites

Z-type hexaferrites doped with Nd³⁺, Ba_3−xNd_xCo₂Fe₂₄O₄₁ (x = 0, 0.05, 0.10, 0.15, and 0.25), were prepared by solid-state reaction. The effect of the Nd³⁺ ions substitution for Ba²⁺ ions on the microstructure and electromagnetic properties of the samples was investigated. The results reveal that an important modification of microstructure, complex permeability, complex permittivity, and static magnetic properties can be obtained by introducing a relatively small amount of Nd³⁺ instead of Ba²⁺. SEM image shows that the grains of the ferrites doped with Nd³⁺ were smaller, more perfect and homogeneous than that of the pure ferrite. The real part (?′) of complex permittivity and imaginary part (?″) increase at first, and then decrease with increasing Nd content. At low frequency, the imaginary part μ″ of complex permeability decreases with Nd content and then increases when frequency is above 7.0 GHz. The magnetization (M_s) and the coercivity (H_c) are 79.38 emu g⁻¹ and 36.94 Oe for Ba_2.75Nd_0.25Co₂Fe₂₄O₄₁. The data of magnetism show that the ferrite doped with Nd³⁺ ions is a better soft magnetic material due to the higher magnetization and lower coercivity.     

Influence of MnO2 on the sintering behavior and magnetic properties of NiFe2O4 ferrite ceramics

The samples with small amounts of MnO₂ (0, 0.5, 1.0, 1.5, 2.0, and 2.5 wt%, respectively) were prepared via ball-milling process and two-step sintering process from commercial powders (i.e. Fe₂O₃, NiO and MnO₂). Microstructural features, phase transformation, sintering behavior and magnetic properties of Mn-doped NiFe₂O₄ composite ceramics have been investigated by means of scanning electron microscopy (SEM), differential thermal analyzer, X-ray diffraction (XRD), thermal dilatometer and vibrating sample magnetometer (VSM) respectively. The XRD analysis and the result of differential thermal analysis indicate that the reduction of MnO₂ into Mn₂O₃ and the following reduction of Mn₂O₃ into MnO existed in sintering process. No new phases are detected in the ceramic matrix, the crystalline structure of the ceramic matrix is still NiFe₂O₄ spinel structure. Morphology and the detecting result of thermal dilatometer show that MnO₂ can promote the sintering process, the temperature for 1 wt% MnO₂-doped samples to reach the maximum shrinkage rate is 59 °C lower than that of un-doped samples. For 1 wt% MnO₂-doped samples, the value of the saturation magnetization (M_s) and coercivity (H_c) is 15.673 emu/g and 48.316 Oe respectively.     

In situ synthesis of Co3O4/graphene nanocomposite material for lithium-ion batteries and supercapacitors with high capacity and supercapacitance

Co₃O₄/graphene nanocomposite material was prepared by an in situ solution-based method under reflux conditions. In this reaction progress, Co²⁺ salts were converted to Co₃O₄ nanoparticles which were simultaneously inserted into the graphene layers, upon the reduction of graphite oxide to graphene. The prepared material consists of uniform Co₃O₄ nanoparticles (15-25 nm), which are well dispersed on the surfaces of graphene nanosheets. This has been confirmed through observations by field emission scanning electron microscopy, transmission electron microscopy and atomic force microscopy. The prepared composite material exhibits an initial reversible lithium storage capacity of 722 mAh g⁻¹ in lithium-ion cells and a specific supercapacitance of 478 F g⁻¹ in 2 M KOH electrolyte for supercapacitors, which were higher than that of the previously reported pure graphene nanosheets and Co₃O₄ nanoparticles. Co₃O₄/graphene nanocomposite material demonstrated an excellent electrochemical performance as an anode material for reversible lithium storage in lithium ion cells and as an electrode material in supercapacitors.     

Preparation and properties of sintered molybdenum doped with La2O3/MoSi2

Sintered Mo with the addition of La₂O₃/MoSi₂ was prepared via the process of solid–solid doping + powder metallurgy. X-ray diffraction experiment, hardness test, three-point bending test and high-temperature tensile test were carried out to characterize the samples. The XRD pattern of a typical sample shows that the sintered Mo was mainly composed of Mo, La₂O₃ and Mo₅Si₃. Mo₅Si₃ was probably formed through the reaction between MoSi₂ and the Mo matrix. Densities and fracture toughnesses of both doped Mo and pure Mo were measured and contrasted. Sintered Mo with the addition of 0.2 wt% La₂O₃/MoSi₂ has the highest toughness, while more addition of La₂O₃/MoSi₂ has smaller effect on improving toughness or even embrittles Mo. The results of three-point bending test and high-temperature tensile test show that the bending strength and high-temperature tensile strength of doped Mo are both higher than those of pure Mo. The formation of Mo₅Si₃ improves the high-temperature strength. The La₂O₃/Mo₅Si₃ dispersed in the Mo matrix refined the grains, and thus strengthened the Mo matrix by dispersion strengthening and grain refinement.     

Densification and mechanical properties of fine-grained Al2O3-ZrO2 composites consolidated by spark plasma sintering

Alumina matrix composites containing 5 and 10 wt% of ZrO₂ were sintered under 100 MPa pressure by spark plasma sintering process. Alumina powder with an average particle size of 600 nm and yttria-stabilized zirconia with 16 at% of Y₂O₃ and with a particle size of 40 nm were used as starting materials. The influence of ZrO₂ content and sintering temperature on microstructures and mechanical properties of the composites were investigated. All samples could be fully densified at a temperature lower than 1400 °C. The microstructure analysis indicated that the alumina grains had no significant growth (alumina size controlled in submicron level 0.66-0.79 μm), indicating that the zirconia particles provided a hindering effect on the grain growth of alumina. Vickers hardness and fracture toughness of composites increased with increasing ZrO₂ content, and the samples containing 10 wt% of ZrO₂ had the highest Vickers hardness of 18 GPa (5 kg load) and fracture toughness of 5.1 MPa m^1/2.