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.     

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.     

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.     

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.     

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.     

Evaluation of the electrochemical behavior and structural reversibility of Li3−xNaxV2(PO4)3/C (0≤x≤3) as anode materials for Na-ion batteries

A series of Li_3−xNa_xV₂(PO₄)₃/C (0≤x≤3) materials are successfully prepared by a simple solid-state reaction method and used for the first time as anode materials for Na-ion batteries. Powder X-ray diffraction (XRD) results show that the phase structures of Li_3−xNa_xV₂(PO₄)₃/C evolve along with the change of Li/Na atomic ratio (0≤x≤3). With increasing x in Li_3−xNa_xV₂(PO₄)₃/C from 0.0 to 3.0, the main phase in as-prepared sample transforms from monoclinic Li₃V₂(PO₄)₃ to rhombohedral Li₃V₂(PO₄)₃, and finally to rhombohedral Na₃V₂(PO₄)₃, which results in different sodium storage behavior and performance between Li_3−xNa_xV₂(PO₄)₃/C (0≤x≤3) materials. Electrochemical results show that Li_3−xNa_xV₂(PO₄)₃/C (x=0.0, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0) can deliver the initial charge capacities of 21.1, 35.9, 33.8, 41.7, 43.3, 43.9 and 47.7 mAh g⁻¹ at a current density of 10 mA g⁻¹, respectively. After 45 cycles, the reversible capacities can be kept at 16.9, 45.1, 32.6, 44.6, 43.7, 37.8 and 27.3 mAh g⁻¹ for Li₃V₂(PO₄)₃/C, Li_2.5Na_0.5V₂(PO₄)₃/C, Li₂NaV₂(PO₄)₃/C, Li_1.5Na_1.5V₂(PO₄)₃/C, LiNa₂V₂(PO₄)₃/C, Li_0.5Na_2.5V₂(PO₄)₃/C and Na₃V₂(PO₄)₃/C, respectively. Furthermore, the structural reversibility of Li_3−xNa_xV₂(PO₄)₃/C (x=1.0, 2.0, and 3.0) is also observed by in-situ XRD observation during sodiation/de-sodiation process. All these observed evidences indicate that only some of Li_3−xNa_xV₂(PO₄)₃/C (0≤x≤3) can be used as possible sodium storage materials.     

Transport properties of p-type Ca3-xLnxCo4O9-Ag (Ln = Lu,Yb; 0.1≤x≤0.2) oxides

Thermoelectric transport properties of p-type Ca_3-xLn_xCo₄O₉/yAg oxides (Ln = Lu & Yb; 0.1 ≤ x ≤ 0.2; 0.05 ≤ y ≤ 0.1) synthesized by sol-gel methodology were investigated in this paper. The structural analyses (SEM, XRD and TEM) confirmed the presence of two phases, viz, Ca_3-xLn_xCo₄O₉ and Ag-metallic phases. The contribution of rare earth doping in one hand and presence of Ag as secondary phase on the other hand were studied. The resistivity measurements indicated the reduction of electrical resistance at the grain boundary leading to an overall decrease in electrical resistivity with increasing Ag-concentration. The enhancement of Seebeck coefficient is attributed to the substitution of Ln³⁺ at Ca²⁺ sites that in turn reduces hole concentration through formation Co³⁺ for charge concentration counter balance in Ca_3-xLn_xCo₄O₉/yAg matrix. The tuning of electrical transport properties through Ca_3-xLn_xCo₄O₉ and Ag-metallic bi-phasic formation resulted high power factor of 582 μW m⁻¹ K⁻² for Ca_2.8Ln_0.2Co₄O₉/0.05Ag and 548 μW m⁻¹ K⁻² for Ca_2.8Yb_0.2Co₄O₉/0.05Ag at 950 K highlighting its potential application on small scale energy harvesting to power sensor and wireless sensor network where requirement of power is in the milliwatt range.     

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.     

Ca3–xMgxYb2Ge3O12(0≤x≤3)石榴石的A位Rattling效应与微波介电性能

微波介质陶瓷是5G/6G通讯技术的关键基础材料，具有高品质因数(Q×f)、低介电常数(ε_r)以及近零谐振频率温度系数(τ_f)的材料已逐渐成为研究与开发的重点。通过固相反应法制备了系列Ca_3–xMg_xYb₂Ge₃O₁₂ (0≤x≤3)石榴石陶瓷。当0≤x≤2时，样品为正石榴石结构，相对介电常数ε_r逐渐从10.3增加至11.8，品质因数Q×f值逐渐从98 000 GHz降低到78 000 GHz，谐振频率温度系数τ_f在(-40～-56)×10^–6/℃之间波动。当22+进入A位，ε_r迅速增大(～13.5),Q×f值显著降低(～19 800 GHz)，τ_f值则从负急剧转变为正(+70.5×10^–6/℃)，可归之于A位Mg²⁺的Rattling效应以...     

Finite size effect on Sm3+ doped Mn0.5Zn0.5SmxFe2−xO4 (0≤x≤0.5) ferrite nanoparticles

Samarium doped Mn–Zn ferrite nanoparticles of composition Mn_0.5Zn_0.5Sm_xFe_2−xO₄ (0≤x≤0.5) have been synthesized by a chemical co-precipitation method for developing low Curie temperature stable ferrofluid. These samples were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Electron Paramagnetic Resonance (EPR) spectroscopy and search coil method analytical techniques for their structural, morphological and magnetic properties. X-ray diffraction patterns confirmed the formation of crystalline single spinel phase of as grown nanoparticles. Lattice parameter and lattice strain increases with the increase in Sm³⁺ content. SEM images revealed the presence of ultrafine particles and their agglomerated structures in higher Sm³⁺ ions concentration analogues. The stoichiometry of the final product agreed well with the initial substitution composition as evidenced by EDS data. Electron paramagnetic resonance (EPR) spectra proved the ferromagnetic nature of nanoparticles. The magnetic measurements by search coil method showed superparamagnetism for x=0, 0.1 the samples with saturation magnetization of 23.95 emu/g for Mn_0.5Zn_0.5Fe₂O₄ sample which increases with rise in Sm³⁺ ions content. The results are explained and correlated with the structural, morphological and magnetic properties for developing stable kerosene based ferrofluid by using these nanoparticles.     

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.     

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.     

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.     

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.     

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.