Design and characterization of novel manganite perovskites Ba1-xBixTi1-xMnxO3 (0≤x≤0.2)

Polycrystalline manganite powders of Ba_1-xBi_xTi_1-xMn_xO₃ (x = 0, x = 0.1 and x = 0.2) were synthesized by the conventional solid-state reaction process. Their crystal structure, morphological, optical, dielectric and electrical properties were investigated. X-ray diffraction of the prepared samples was made at room temperature and confirmed the formation of a perovskite phase. Structural refinement, using the Rietveld method, revealed a tetragonal P4mm phase of pure BTO and a tetragonal P4/mmm phase with the presence of vacancies for both doped samples (x = 0.1 and x = 0.2). Scanning electron microscopy indicated that the perovskite samples had a grain size smaller than 1 μm. From UV–vis–NIR spectra, we found that the band gap reduces from 3.29 eV to 1.48 eV with the increase of Bi and Mn amounts, resulting in a shift of the absorption wavelength region toward the visible range. Dielectric analysis was conducted in a wide range of temperatures at different frequencies. Phase transitions were identified from thermal dielectric results, showing that the samples exhibited a non-relaxor behavior. The structural transformation from tetragonal to cubic structure corresponding to the transition from ferroelectric phase to paraelectric phase was observed in the dielectric properties investigation. The complex impedance spectroscopy indicated the presence of grain and grain boundary effects in the conduction mechanism. Electrical analysis showed that doping with Bi and Mn enhanced the DC conductivity. Furthermore, the DC conductivity temperature dependence confirmed that the studied samples present a semiconductor behavior. The activation energies of grain and grain boundaries depended on the amount of incorporated Bi and Mn. The activation energy of grain varied between 0.54 and 0.87 eV suggesting that the DC electrical conductivity is governed by ionized oxygen vacancies. The activation energy of grain boundaries varied between 0.85 and 0.58 eV.     

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.     

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.     

LaCr1-xFexO3 (0≤x≤0.7): A novel NTC ceramic with high stability

Perovskite-phase LaCr_1-xFe_xO₃ (0 ≤ x ≤ 0.7) ceramics were fabricated by two-step sintering of powders synthesized by conventional solid-state reaction. XRD patterns reveals that the ceramics are single-phased LaCr_1-xFe_xO₃ solid solution at lower Fe contents, but form a biphasic mixture of LaCrO₃-LaFeO₃ at higher Fe contents. SEM analysis indicates that Fe doping causes a reduction in porosity, and a corresponding increase in density. The effects of Fe doping on electrical properties have also been investigated. The electrical resistivity ρ_25°C decreases initially and then increases within a wide range of 634.4–83915.9 Ω·cm with increasing Fe content, whereas B varies within a relatively narrow range of 3651−4301 K. Such combination enables it to be a potential candidate for NTC thermistors. An investigation of elevated-temperature aging behavior shows that the resistance shifts at 25 °C were less than 0.65 % after annealing at 125 °C in air for 1000 h, revealing that the material exhibits excellent stability.     

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.     

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。     

Preparation and characterization of powellite ceramics Ca1-xLix/2Cex/2MoO4 (0 ≤ x ≤ 1) for Mo-rich HLW condition

Powellite ceramic represent one candidate to immobilize minor actinides and Mo from reprocessed UMo nuclear fuel. In this work, the Ca_1-xLi_x/2Ce_x/2MoO₄ (0 ≤ x ≤ 1) series is prepared via a solid-state reaction using Ce³⁺ as trivalent minor actinide (Am³⁺) surrogate, with structure/microstructure characterized by XRD, XPS, HRTEM, and SEM as well. The Ca_1-xLi_x/2Ce_x/2MoO₄ (0 ≤ x ≤ 1) compositions crystallize in tetragonal and are isostructural with scheelite. Rietveld refinements show that Ce³⁺ and Li⁺ simultaneously enter into the eight-fold coordinated Ca site of powellite crystal. The chemical durability of powellite phases is evaluated by the ASTM C1285-14 product consistency test method B. The leaching behaviours of Ce and Mo are accordance with the interfacial dissolution-reprecipitation mechanism. For all the Ca_1-xLi_x/2Ce_x/2MoO₄ (0 ≤ x ≤ 1) ceramics, 7-days NL_Ce and NL_Mo are found to be in the order of 10⁻³-10⁻⁵ g·m⁻² and 10⁻²-10⁻⁴ g·m⁻² respectively, which exhibit the great retention of Ce and Mo. Interestingly, the values of 7-days NL_Ce and NL_Mo are predominantly controlled by the distortion of MoO₄ tetrahedra and disordered arrangements of Ce³⁺ and Li⁺. Thus, our initial understanding on the structure and chemical durability relation will provide insight to design new waste forms for the Mo-rich HLW condition.     

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.     

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.     

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.     

Sr1-xLaxFe12O19 (0.0≤x≤0.5) hexaferrites: Synthesis,characterizations, hyperfine interactions and magneto-optical properties

A simple sol-gel auto combustion process was used to synthesize La³⁺ substituted M-type strontium hexaferrite, Sr_1-xLa_xFe_12-xO₁₉ (0.0≤x≤ 0.5). Structural, magnetic, and optical behavior as a function of La³⁺ substitutions were investigated by Fourier transformed infrared spectroscopy (FT-IR). X-ray powder diffraction (XRD). Scanning electron microscopy (SEM), Mössbauer spectroscopy, vibrating sample magnetometer (VSM), and Diffuse reflectance spectroscopy (DRS). XRD data showed single phase magnetoplumbite structure and Rietveld analysis confirmed P6₃/mmc space group for all the series. The average crystallite size was found to be in the range of 43.2–48.4 nm. The variation in line width, isomer shift, quadrupole splitting, relative area and hyperfine magnetic field values have been determined from ⁵⁷Fe Mössbauer spectroscopy data. The fittings accounted for the Fe²⁺/Fe³⁺ charge compensation mechanism at the 2a site due to replacement of Sr²⁺ by La³⁺. The saturation magnetization (σ_s) decreases from 57,21 to 63,23 emu/g and remnant magnetization (σ_r) decreases from 35.6 emu/g to 28.7 emu/g with increasing La substitution. The decrement is sharper at coercive field (H_c) from maximum value of 5325 to minimum 1825 Oe. Demagnetizing factor (N) is 3 times more for the x=0.3, 0.4, and 0.5. However all samples exhibit ferromagnetic behavior at room temperature. Magnetic anisotropy of Hexaferrites was detected as uniaxial and effective anisotropy constants (K_eff) were between 5.93×10⁵ and 4.76×10⁵ Ergs/g. The high magnitudes of anisotropy fields (H_a) in the range of 13863–15574 Oe reveal that all hexaferrites are magnetic hard materials. Tauc plots were applied to extrapolate the direct optical energy band gap (E_g) of hexaferrites. The E_g values decreased from 1.83 to 1.34 eV with increasing La content.     

Enhanced magnetic,ferroelectric, and photocatalytic properties of (1-x)BiFeO3-xBaTiO3 (0.0 ≤ x ≤ 0.4) powders

The magnetic, ferroelectric, and photocatalytic properties of (1-x)BiFeO₃-xBaTiO₃ (0.0 ≤ x ≤ 0.4) powders synthesized by sol-gel method have been investigated. X-ray diffractometry confirms that the phase of the samples changed from rhombohedral to cubic with the increase in BaTiO₃ content. The grain size decreases and the particle shape becomes homogeneous with the introduction of BaTiO₃. BaTiO₃ substitution enhances the multiferroic properties of the ceramics and the maximum remnant magnetization (0.261 emu/g) and remnant polarization (20 μC/cm²) have acquired in 0.8BiFeO₃-0.2BaTiO₃ and 0.7BiFeO₃-0.3BaTiO₃, respectively. The absorbance in ultraviolet and visible light regions is improved obviously for powder with x = 0.3. The energy band gap of the samples decreases from 2.06 eV to 1.57 eV with the introduction of BaTiO₃, indicating that the excitation rate of photogenerated electron-hole pairs is improved. The highest methylene blue degradation efficiency of ~62% within 3 h under the visible light is achieved in the 0.7BiFeO₃-0.3BaTiO₃ which can be attributed to its suitable energy band gap and large remnant polarization.     

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.     

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.     

Studies on NO2 gas sensing properties of sprayed Co1−xMnxFe2O4 (0≤x≤0.5) spinel ferrite thin films

The Co_1−xMn_xFe₂O₄ (0≤x≤0.5) spinel ferrite thin films were deposited on quartz substrates by chemical spray pyrolysis technique. The effect of Mn substitution on to the structural, electrical, dielectric and NO₂ gas sensing properties of cobalt ferrite thin films was studied. The X-ray diffraction analysis reveals that deposited films exhibit spinel cubic crystal structure. The lattice constant increases with the increase in Mn²⁺ content. The decrease in resistivity with increase in temperature suggests that the films have a semiconducting nature. The room temperature dielectric properties such as dielectric constant (ε′), loss tangent (tanδ), dielectric loss (ε′′) and AC conductivity have been studied in the frequency range 20 Hz–1 MHz. The film shows the highest sensor response at moderately low (150 °C) operating temperature. The effect of operating temperature, gas concentration, film selectivity and substitution of Mn on to gas response is carefully studied. The manganese substituted cobalt ferrite films are extremely selective towards NO₂ with a 20 times gas response compared with other gases. The gas response achieved nearly 92% of its initial value after 150 days, indicating good stability of the films.     

LaMn1-xTixO3-NiMn2O4(0≤x≤0.7): A composite NTC ceramic with controllable electrical property and high stability

A series of NTC thermistor ceramics based on LaMn_1-xTi_xO₃-NiMn₂O₄(0≤x≤0.7) composite system have been fabricated by solid-state method. X-ray diffraction analysis indicates the composite ceramics mainly consisting of a rhombohedral perovskite LaMn_1-xTi_xO₃ phase and a cubic spinel NiMn₂O₄ phase. SEM images show high density of the as-prepared composite ceramics. The effects of the Ti doping and the weight ratio of LaMn_1-xTi_xO₃/NiMn₂O₄ on electrical property have been studied. The electrical resistivity ρ increases significantly with Ti addition increasing and decreases obviously with LaMn_1-xTi_xO₃ concentration increasing. The ρ_25°C and B values are in the range of 3.2–53,200.0Ω·cm and 1300–4008 K, respectively, and could be adjusted to desired values and applied in various fields. The conductive mechanism may be related to the ion migration and the percolation theory. After annealing at 125°C for 1000h, the resistance shifts are less than 0.52%, suggesting good stability of the composite and high potential for NTC thermistor applications.     

Physicochemical and electrochemical performance of LiFe1−xNixPO4 (0≤x≤1.0) solid solution as potential cathode material for rechargeable lithium-ion battery

In this work, olivine-type LiFePO₄ (LFP) and nickel (Ni) substituted LiFe_1-xNi_xPO₄ (0≤ x≤1.0) solid solutions were synthesized using the wet-milling route. Synthesized samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). XRD results show that all the samples were crystallized in the orthorhombic phase with Pnma space group and no secondary phase was detected even at higher Ni-concentration. Rietveld refinement results reveal that average <Li-O> distance increases up to x=0.03 (LFNP3) and decrease thereafter monotonically on further increase in Ni-concentration. Moreover, the LFNP3 sample shows highest electrical conductivity as compared to other solid solutions. Among the synthesized samples for x≤0.1, LFNP3 shows highest discharge capacity at all C-rates. LFNP3 exhibits a discharge capacity of 158 (±5) mAh g⁻¹ at 0.1 C (almost 93% of the theoretical capacity) and displays the high and stable discharge capacity of 145 (±5) mAh g⁻¹ at 1 C rate for 150 charge/discharge cycles. Among high Ni content (0.1< x≤1.0) samples, LiFe_0.7Ni_0.3PO₄ (LFNP30) delivers best charge/discharge capacity at all C-rates. LFNP30 exhibits a discharge capacity of 90 (±5) mAh g⁻¹ at 0.1 C rate.