Synthesis,sintering, and thermoelectric properties of Co1-xMxO (M = Na, 0 ≤ x ≤ 0.07; M = Ag, 0 ≤ x ≤ 0.05)

The structural and thermoelectric properties of Na- and Ag-substituted CoO dense ceramics have been investigated. X-ray diffraction shows that pure phase and Ag/CoO composites have been obtained for Na-doped and Ag-doped CoO, respectively. Raman spectroscopy shows an effect of Na dopants on the lattice disorder of CoO. The chemical composition, element distribution, and valence states of the samples have been characterized by Auger electron microscopy and X-ray photoelectron spectroscopy. Substitution of Co by 5 at. % Na enhances the power factor to 250 μW m⁻¹ K^-2 at 1000 K, similar to that of Ca₃Co₄O₉. The corresponding thermal conductivity is also reduced to 3.55 W.m⁻¹ K⁻¹ at 1000 K. Consequently, Co_0.95Na_0.05O exhibits the best thermoelectric figure of merit (ZT), which is 0.07 at 1000 K. On the other hand, the substitution of Ag into CoO leads to the formation of CoO/Ag composites and deteriorates ZT values.     

Structural,magnetic and magnetocaloric properties of CoFe2−xMoxO4 (0.0≤x≤0.3) ferrites

We have investigated structural, magnetic and magnetocaloric properties of CoFe_2-xMo_xO₄ (0.0≤x≤0.3) ferrites. Polycrystalline samples were prepared by the sol gel method and characterized by the powder X-ray diffraction and scanning electron microscopy. X-ray diffraction patterns show that all samples have a cubic spinel structure and the lattice parameter, a, decreases monotonically with increase in Mo concentration. Scanning electron micrographs indicate that most of the particles are in the range of 400–850 nm size. Magnetic measurements, performed by using a cryogen free vibrating sample magnetometer, show that these samples are soft ferromagnets in the measured temperature range. The saturation magnetization, M_s, values of Mo-doped samples are larger than the parent compound with a maximum value of ~106 emu/g for x=0.2 sample. The magnetic entropy change (−ΔS) increases with increase in applied magnetic field and shows a peak in the vicinity of blocking temperature. A maximum value of 0.56 J kg⁻¹ K⁻¹ at 5 T field has been observed for x=0.2 sample.     

Influence of Ce addition on the structural,magnetic, and magnetocaloric properties in La0.7−xCexSr0.3MnO3 (0≤x≤0.3) ceramic compound

We report improvement in the magnetocaloric properties of Ce-doped lanthanum manganites. Polycrystalline La_0.7−xCe_xSr_0.30MnO₃ (0≤x≥0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transition in parent sample and as well as in doped sample with Curie temperature decreasing progressively with increasing Ce-concentration from ~370 K for x=0.0 to 310 K for x=0.30. Magnetic entropy change (ΔS_M) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. A large ΔS_M associated with the ferromagnetic–paramagnetic transition in La_0.7−xCe_xSr_0.30MnO₃ samples has been observed. The value of ΔS_M was found to increase with Ce-doping up to x=0.15 and the highest value of 2.12 J kg⁻¹ K⁻¹ (at ΔH=2 T) was observed for La_0.55Ce_0.15Sr_0.30MnO₃ sample near the Curie temperature of 356 K. Also, improved relative cooling power of ~122 J kg⁻¹ was observed for the same sample. Due to the large magnetic entropy change and high Curie temperature, the La_0.55Ce_0.15Sr_0.30MnO₃ sample is suggested to be used as potential magnetic refrigerants for magnetic refrigeration technology above room temperature.     

Sintering behavior,phase evolution and microwave dielectric properties of thermally stable Li2O-3MgO-mTiO2 ceramics (1≤m≤6)

Li₂O-3MgO-mTiO₂ (1≤m≤6) ceramics were prepared by the solid state reaction method. X-ray diffraction, energy dispersive spectrometer and scanning electron microscopy techniques were used to investigate the phase composition and crystal structure. With increasing m values, the phase structures of ceramics changed as: (Li₂Mg₃TiO₆, m=1)→(Li₂Mg₃Ti₄O₁₂ and Mg₂TiO₄, m=2,3)→(Li₂Mg₃Ti₄O₁₂, m=4)→(Li₂Mg₃Ti₄O₁₂, MgTiO₃ and Li₂MgTi₃O₈, m=5)→(Li₂Mg₃Ti₄O₁₂, MgTiO₃, Li₂MgTi₃O₈ and MgTi₂O₅, m=6). The optimized sintering temperature was lowered from 1275 °C to 1050 °C. When m=5, Li₂O-3MgO-5TiO₂ ceramics showed good microwave dielectric properties at a wide sintering temperature range of 1000–1200 °C, and the best microwave dielectric properties of Q×f=71,726 GHz, ε_r=21.9 and τ_f=−20.9 ppm/°C were obtained at a relatively low sintering temperature of 1050 °C.     

Thermoelectric properties of copper-deficient Cu2-xSe (0.05 ≤ x ≤ 0.25) binary compounds

Recently, many novel superionic thermoelectric materials have been discovered along the concept of “phonon-liquid electron-crystal” (PLEC). Among them, Cu_2-xSe-based liquid-like materials are typical examples. In this study, a series of copper-deficient Cu_2-xSe (0.05 ≤ x ≤ 0.25) materials were synthesized and used to study the role of Cu vacancies on the electrical and thermal transport properties. The X-ray photoelectron spectroscopy (XPS) measurements suggest that the valence states of Cu and Se are independent on the Cu/Se atomic ratio. With increasing the content of Cu vacancies, the hole concentration is monotonously increased, leading to the improved electrical conductivity and reduced Seebeck coefficient. Based on the single parabolic band model analysis, it is found that changing the content of Cu vacancies does not obviously modify the material's electronic band structure and effective mass. Due to the presence of highly mobile Cu ions inside the crystal structure, the lattice thermal conductivities of all Cu_2-xSe (0.05 ≤ x ≤ 0.25) materials are very low with values around 0.39 W m⁻¹ K⁻¹ at 500 K. Because of the significantly reduced Seebeck coefficient and increased electronic thermal conductivity, the thermoelectric figure of merit zTs are decreased when increasing x from 0.05 to 0.25. At 750 K, a maximum zT of 0.46 is obtained in Cu_1.95Se among all Cu_2-xSe (0.05 ≤ x ≤ 0.25) materials.     

Influence of La addition on the structural,magnetic and magnetocaloric properties in Sr2−xLaxFeMoO6 (0≤x≤0.3) double perovskite

We report the effect of carrier doping via partial substitution of La³⁺ for Sr²⁺ on the structural, magnetic and magnetocaloric properties of Sr₂FeMoO₆ double perovskite. Polycrystalline Sr_2−xLa_xFeMoO₆ (x=0.0, 0.1, 0.2, 0.3) samples were prepared using the conventional solid state reaction method. Using the X-ray diffraction (XRD) analysis it was established that all the samples crystallized in a tetragonal structure with I4/mmm space group. An increase in the La doping lead to an increase in the lattice parameter ‘a’ and the volume of the unit cell. The lattice parameter ‘c’ however remained unchanged. The temperature variation in magnetization and Arrott analysis suggested a second order of ferromagnetic phase transition in all samples with Curie temperature, T_C increasing from 358 K for x=0.0–365 K for x=0.3. A gradual increase in magnetization was also observed with the increasing La content up to x=0.2. The magnetic entropy change was calculated from the measurement of isothermal magnetization versus magnetic field at different temperatures. The tunability of magnetization and T_C simply by adjusting the concentration of La and synthesis conditions makes Sr₂FeMoO₆ an attractive material for magnetic refrigeration at desired temperature.     

Li1-xTixFePO4(O≤x≤0.02)制备表征及电化学性能研究

采用固相工艺制备了具有锂位掺杂的磷酸铁锂正极材料Li1-xTixFePO4(0≤x≤0.02),并将该材料与相同条件下制备的LiFePO4/C材料进行晶体结构、表面形貌、容量性能以及倍率性能比较.通过比较发现,Li1-xTixFePO4和LiFePOJC均具有橄榄石结构,且粒径大小均在2～4 μn.电化学性能测试结果表明,Li1-xTixFePO4比LiFePO4/C具有更加优异的容量性能和倍率性能.     

Ceramic pigments from CoxNi3-xP2O8 (0 ≤ x ≤ 3) solid solutions

Co_xNi_3-xP₂O₈ (0 ≤ x ≤ 3) solid solutions were synthesized via the chemical co-precipitation method. Variation of unit cell parameters and interatomic distances indicated that these solid solutions with the Ni₃P₂O₈ structure are stable between 800 and 1200 °C in compositions with 0 ≤ x ≤ 1.5 and between 800 and 1000 °C when (0 ≤ x ≤ 3). When (2.5 ≤ x ≤ 3.0), the solid solutions lead to the Co₃P₂O₈ structure, being stable between 800 and 1000 °C.The yellow colour of the Ni₃P₂O₈ compound changes to pink or red when Co(II) ions are incorporated in the structure as Ni₃P₂O₈ solid solutions are formed. Bands corresponding to second and third electronic transitions of the Co(II) ions in octahedral coordination appear in the 450–600 nm in the UV-V spectra, and they are responsible of the observed changes in the colour. Absorbance in the visible spectra was also obtained from enamelled samples but a new band at 650 nm with considerable absorbance when x > 1.0 increased the blue amount, and colour of the enamelled samples was yellowish brown, brown, green and blue.     

Yellow-emitting (Tb1−xCex)3Al5O12 phosphor powder and ceramic (0≤x≤0.05): Phase evolution,photoluminescence, and the process of energy transfer

(Tb_1-xCe_x)₃Al₅O₁₂ yellow phosphors (0≤x≤0.05) were calcined from their coprecipitated carbonate precursors, and the effects of the temperature and atmosphere (air and H₂) of calcination on the sequence of phase evolution and the characteristics of the powders were investigated in detail. The activation energy for the crystallite growth during calcination was estimated to be ~39 kJ/mol. The powder calcined at 1000 °C showed good reactivity and was sintered into a ceramic plate of ~97% dense (average grain size: ~1.3 µm) in a H₂/Ar gas mixture at the relatively low temperature of 1500 °C. The phosphors simultaneously exhibit the 4f⁸→4f⁷5d¹, ⁷F₆→⁵D₃ and ⁷F₆→⁵D₄ excitations of Tb³⁺ and the 4f¹→5d¹ excitation of Ce³⁺ when monitoring the yellow emission of Ce³⁺ at 560 nm, suggesting the presence of efficient Tb³⁺→Ce³⁺ energy migration. The optimal Ce³⁺ content for luminescence was found to be x=0.015 and 0.01 under the direct excitation of Ce³⁺ and through Tb³⁺→Ce³⁺ energy transfer, respectively, and concentration quenching of luminescence was analyzed to be resulted from exchange interaction. Luminescence features of the phosphors, including excitation, emission, quantum yield, fluorescence lifetime, color coordinates and color temperature, were thoroughly investigated against the processing temperature and Ce³⁺ content, with an in-depth discussion on the process of energy transfer among the optically active Tb³⁺ and Ce³⁺ ions. The materials may find application in blue-light excited white LEDs.     

Refining the phase diagram of PbZr1−x−ySnxTiyO3 ceramics with 0.40≤x≤0.54 by crystal structural,dielectric response and hysteresis loop investigations

Ceramics of PbZr_1−x−ySn_xTi_yO₃ (PZST100x/100y) with 0.40≤x≤0.54 were synthesized via conventional solid state reaction and their crystal structures, dielectric response, and hysteresis loops were systematically investigated. A more precise range of the antiferroelectric/ferroelectric phase boundary of PZST100x/100y ceramics with 0.40≤x≤0.54 was established compared with the results reported by Jaffe. It was found that the introduction of Ti⁴⁺stabilized the ferroelectric order, and Sn⁴⁺substitution enhanced the antiferroelectric order. Thus, the system properties could be tailored by altering Sn and Ti concentration. In addition, the hydrostatic pressure-induced ferroelectric-to-antiferroelectric phase transition was also studied. It has been clarified that the critical transition pressure (P_t) increased with an increasing Ti content at a fixed Sn content.     

Electrochemical performance of NixCo1-xMoO4 (0 ≤ x ≤ 1) nanowire anodes for lithium-ion batteries

Ni _x Co_1-x MoO₄ (0 ≤ x ≤ 1) nanowire electrodes for lithium-ion rechargeable batteries have been synthesized via a hydrothermal method, followed by thermal post-annealing at 500°C for 2 h. The chemical composition of the nanowires was varied, and their morphological features and crystalline structures were characterized using field-emission scanning electron microscopy and X-ray powder diffraction. The reversible capacity of NiMoO₄ and Ni_0.75Co_0.25MoO₄ nanowire electrodes was larger (≈520 mA h/g after 20 cycles at a rate of 196 mA/g) than that of the other nanowires. This enhanced electrochemical performance of Ni _x Co_1-x MoO₄ nanowires with high Ni content was ascribed to their larger surface area and efficient electron transport path facilitated by their one-dimensional nanostructure.     

Li_xCoO_2(0≤x≤1)结构稳定性和力学性质的第一性原理计算

采用基于密度泛函的第一性原理,从电子层次计算研究了锂离子电池正极材料Li_xCoO_2(0≤x≤1)的晶体结构、电子结构和力学性质。结果表明:Li_xCoO_2在脱锂过程中晶体结构会发生转变,当x=0.5时由六方结构R ■ m晶型转变为单斜结构P2/m晶型,当完全脱锂(x=0)时又转变为六方结构P ■ m1晶型。随着Li原子的脱出,Li_xCoO_2导带被部分填充,价带被完全填充,金属性质和电子导电性增强,并出现自旋极化。Li_xCoO_2中Co—O键为含有部分离子性特征的共价键,随着Li原子的脱出,Co—O键的离子性特征减弱,共价性特征增强。随着Li原子的脱出,Li_xCoO_2的体积模量(B)、剪切模量(G)和弹性模量(E)均呈现逐渐减小的趋势,而Poisson比和各向异性指数逐渐增大。G/B值变化趋势表明,LiCoO_2呈脆性,脱锂过程中向韧性转变。     

(ZrO2)n(1≤n≤5)团簇的结构及其光谱特征

应用密度泛函理论方法计算了各种可能的(ZrO2)n(1≤n≤5)团簇结构,同时模拟了对应团簇结构的红外光谱.氧化锆团簇中,锆氧原子间最大程度的交互连接是最稳定的结构.团簇的红外光谱表明:600～700 cm-1附近的峰对应着氧化锆团簇的Zr-O-Zr-O四元环振动,800 cm-1附近的峰对应着氧化锆团簇的Zr-O-Zr-O-Zr-O六元环振动,大于900 cm-1的峰对应着氧化锆团簇的自由氧原子的Zr-O振动.实验分析与理论计算结果表明:500 cm-1附近较强的特征振动峰对应着四方氧化锆的特征振动.     

高镍系正极材料LiNi1-xMxO2(M=Co,Mn,Al,x≤0.4)储存性能的研究进展

高镍系正极材料LiNi1-xMxO2(M=Co, Mn, Al, x≤0.4),因其能量密度高、成本低廉以及环境友好等优点而引起研究者的广泛关注.但是,该材料也存在一些缺陷,如循环过程中结构不稳定、高温条件下稳定性差以及储存性能不佳等,这些缺点在很大程度上限制了其在锂电领域的广泛应用.着重分析了该材料储存性能不佳的根本原因,并总结了近年来研究人员关于改善高镍系正极材料储存性能的研究进展, 最后进一步针对改善高镍系正极材料的储存性能做出了展望.     

掺杂少量Sm时La0.67-xSmxSr0.33MnO3(0.00≤x≤0.30)的磁电性质

通过测量样品的磁化强度-温度(M-T)曲线、电阻率-温度(ρ-T)曲线及磁电阻(magnetoresistance)-温度(MR-T)曲线,研究了Sm掺杂(x=0.00,0.10,0.20,0.30)对La0.67-xSrmxSr0.33MnO3磁电性质的影响.发现在铁磁相主要是单磁子散射起作用,表现为金属型导电,可用公式ρ=ρ0+AT2拟合,其中ρ为温度T时的电阻率;ρ0为0 K时的电阻率;A为常数.顺磁相的输运机制主要是小极化子跃迁起作用,可以用公式ρ=BTexp·(Ea/kBT)拟合,其中Ea为激活能;kB为Bolzman常数;B为常数.在相变温区是顺磁相的小极化子与铁磁相的单磁子2种输运性质共存.     

Li_(1-x)Ti_xFePO_4(0≤x≤0.02)制备表征及电化学性能研究

采用固相工艺制备了具有锂位掺杂的磷酸铁锂正极材料Li1-xTixFePO4(0≤x≤0.02),并将该材料与相同条件下制备的LiFePO4/C材料进行晶体结构、表面形貌、容量性能以及倍率性能比较。通过比较发现,Li1-xTixFePO4和LiFePO4/C均具有橄榄石结构,且粒径大小均在2～4μm。电化学性能测试结果表明,Li1-xTixFePO4比LiFePO4/C具有更加优异的容量性能和倍率性能。     

Crystal structure,magnetic and electric properties of highly coercive ferrites Sr1 − x Sm x Fe12 − x Zn x O19 (0 ≤ x ≤ 0.4)

The samples of highly coercive ferrites Sr_{1 ? x} Sm _x Fe_{12 ? x} Zn _x O₁₉ (0 ≤ x ≤ 0.4) with the structure of magnetoplumbite have been obtained by the solid-phase method. It has been determined that the samples Sr_{1 ? x} Sm _x Fe_{12 ? x} Zn _x O₁₉ are single-phase at x ≤ 0.2 and contain the impurity phases of α-Fe₂O₃, SmFeO₃, and ZnFe₂O₄ at x ≥ 0.3. In the magnetic fields up to 14 T, the specific magnetization has been measured and the values of coercive force (_σ H _c ) have been determined at 5 and 300 K. It has been shown that semiconductor electroconductivity of single-phase samples Sr_{1 ? x} Sm _x Fe_{12 ? x} Zn _x O₁₉ gradually decreases with the increase in the degree of substitution of x from 0 to 0.2.     

Dielectric relaxation and magnetic resonance in microwave absorption performance of CoxFe3-xO4 (0≤x≤1) nanocrystals

Cobalt ferrites (Co_xFe_3-xO₄, 0 ≤ x ≤ 1) may possess large magnetocrystalline anisotropy and coercivity at certain cobalt/iron (Co/Fe) ratios, while further explorations on their microwave absorption mechanisms are not adequate so far. In this study, a series of Co_xFe_3–xO₄ nanocrystals were synthesized by a developed oxidation-precipitation method, and a combination of dielectric relaxation and magnetic resonance was revealed in electromagnetic studies. Dielectric relaxation peaks were originated from orientation polarization and affected by oxygen vacancy densities. Magnetic resonance peaks were shifted to higher frequency due to the increased magnetocrystalline anisotropy at higher Co/Fe ratios. The excellent microwave absorption performance of as-prepared Co_xFe_3–xO₄ were also obtained, which should be attributed to the electromagnetic matching of dielectric relaxation and magnetic resonance at higher frequency ranges.     

Origin of anomalous octahedral distortions and collapse of magnetic ordering in Nd1−xSrxFeO3 (0≤x≤0.5)

The effects of strontium doping on the structural properties and magnetic ordering of Nd_1?xSr_xFeO₃ orthoferrite system have been studied by employing macroscopic and microscopic structural techniques like X-ray diffraction, scanning electron microscopy and ⁵⁷Fe Mössbauer spectroscopy. X-ray diffraction confirmed that single phase materials have been synthesized. It has been observed that orthorhombic distortion, density and porosity have decreased; whereas, grain size, tolerance factor and symmetry have increased with increase in the strontium concentration. Mössbauer results showed an increase in the Fe⁴⁺/Fe³⁺ ratio, sagging and local octahedral distortions while decrease in the magnetic ordering with increase in the strontium content. The origin behind anomalous octahedral distortions in this system has been attributed to the decrease in the oxidation state and mismatch in the ionic radii of A-site cations and increase in the concentration of Fe⁴⁺, due to Sr²⁺ doping at Nd³⁺ sites. The collapse of magnetic ordering has been ascribed to the weakening of super-exchange interactions, dilution of strong long range magnetic sub-lattice of high spin Fe³⁺ (five unpaired electrons) by relatively lower spin of high spin Fe⁴⁺ (four unpaired electrons) and increase in the spin–spin relaxation frequency.     

低温燃烧合成制备NixZn1-xFe2O4(O.3≤x≤0.8) 铁氧体粉末与特性

本报告以低温燃烧法合成NixZn1-xFe2O4铁氧体粉末,整个燃烧合成在低温下进行.所得铁氧体粉末以X光粉末绕射法进行结构分析,并以穿透式电子显微镜进行粉末颗粒大小与形状之鉴定,再以振动样品测磁仪进行磁性质之量测.实验结果显示以低温燃烧合成法所制备之NixZn1-xFe2O4铁氧体粉末为立方晶结构,但在x=0.3～0.4之间时有γ-Fe2O3相出现,而在x=0.7～0.8之间有Ni结晶出现.磁性质方面,在x=0.4时饱和磁化量最高达81.2 emu/g,矫顽磁力达117.02 Oe.而在x=0.7时可得最大矫顽磁力142.5 Oe,此时饱和磁化量约为56 emu/g.以穿透式电子显微镜分析铁氧体颗粒大小约在10～30 nm之间,显示此方法可快速并且在低温下合成NixZn1-xFe2O4铁氧体粉末.