Synthesis and characterization of nanocrystalline MgAl2O4 spinel by polymerized complex method

Ultrafine MgAl₂O₄ powder has been synthesized by a polymerized complex method. Heating of a precursor solution containing citric acid (CA), ethylene glycol (EG) and Mg and Al salts with a molar ratio of Mg/Al/CA/EG=1/2/8/32 at 180°C produced a transparent polymeric gel, which have been characterized by FT-IR spectroscopy and TG/DTA. The organic fraction was removed by controlled thermal treatments (350–1200°C) whereby the bimetallic oxide was formed. XRD analysis showed the presence of MgAl₂O₄ at 600°C. TEM observation showed that the spherical nanosized powders with good uniformity was obtained. Furthermore, these powders showed excellent sinterability, relative density up to 99.8% was achieved when sintered at 1550°C for 3 h in air without any sintering additive.     

Transformation characteristics in thin foils of a nickel-titanium alloy

Transformation behaviour in thin foils of a Ni-12 at.% Ti alloy was investigated by means of in situ aging and electron irradiation experiments inside a high voltage electron microscope. It is shown experimentally that the mode or the morphology of precipitation and ordering reactions in the thin foils differ somewhat from that observed in the bulk material. In the thicker part of the thin foils aged in situ at 873–973 K, a periodic modulated structure is observed to consist of a periodic array of cuboidal coherent particles along the [100] crystallographic directions. The development of ordering within the solute enriched particles appears to be much slower than in the bulk specimen In the thinner part of the foil or in the near-surface regions, no precipitation or ordering occurs and so-called precipitate-free zones (PFZs) are observed. At a higher temperature of 1073 K, precipitation takes place preferentially at the foil surfaces Electron irradiation at elevated temperatures is found to disturb the formation and growth of a metastable modulated structure. and alter the distribution and the morphology of precipitate particles initially present.

The observed transformation characteristics in the thin foil can be understood in terms of the proximity of external surfaces which act as dominant sinks for point defects or solute atoms in a thin foil. Electron irradiation affects the sink efficiency of the foil surfaces as a result of the radiation-enhanced diffusion.     

Electrochemical activities of yttrium doped spinel LiMn2O4

It was found for the first time that the catalysis of yttrium doping of spinel LiMn₂O₄ can enhance the electrochemical activities of manganese, leading to both improvement of electrochemical capacity and reactivity with the electrolyte of manganese. A proper amount of doping was 0.5%, and such yttrium-doped sample, Li(Y_0.005Mn_0.995)₂O₄, had an initial capacity of 130 mAh g⁻¹ over that of the undoped one with the capacity retention to reach 92.3% exceeding that of the undoped one at 100th cycle.     

Monochromatic blue-green and red emission of rare-earth ions in MgGa2O4 spinel

Pr³⁺-activated blue-green phosphor and Eu³⁺-activated red phosphor hosted in MgGa₂O₄ spinel have been prepared by a gel-assisted high-temperature calcination process, respectively. Both anion and cation vacancies in the host were formed by decreasing the Mg concentration in the reaction source. The induced vacancies provide possibility of the accommodation of the doped rare-earth ions with larger atomic size in the highly symmetrical spinel structure. Due to the efficient energy transfer from the spinel host to the sole 4f sub-level of the doped rare earths, monochromatic emissions with high efficiency can be obtained to allow the phosphors to find applications in solid-state laser device and other phosphors excited under low energy. The corresponding spectroscopic transition mechanism has been proposed in this work.     

Thermodynamic parameters of solids at high pressures from vibrational spectroscopy: MgO and MgAl2O4

Abstract

The effect of pressure on the optical or acoustic modes in the sideband fluorescence of Cr³⁺ doped into the MgO and MgAl₂O₄ lattices was identical to that measured for the corresponding Raman and infrared modes of MgAl₂O₄ and ultrasonic sound velocities (Chang and Barsch 1973, Jackson and Niesler 1982) for both MgO and MgAl₂O₄. This shows that the impurity cation does not measurably perturb the crystal lattice vibrations and thus may be used to probe the thermodynamic properties of such cubic materials at high pressures. The effect of pressure on the heat capacity, C_V, and entropy, S, of MgO and MgAl₂O₄ spinel was determined using statistical thermodynamics on the lattice vibrations measured to over 200 kbar. The results for MgO are identical to predictions from Debye theory to at least 10% compression.     

尖晶石结构自旋有序CaTi2O4单晶生长和磁化率特性研究

通过助溶剂熔融法并在氩气气氛炉中成功生长出高质量大尺寸的CaTi₂O₄的单晶.X射线衍射实验及能量损失谱EDS证实,制备的CaTi₂O₄单晶晶胞参数a=9.781?,b=9.966?,c=3.148?,所有样品均为单相,且符合化学计量比,样品高纯.通过直流磁化率的测量,首次给出了晶体的Van-Vleck顺磁因子为6.85×10^-5cm³/mol,Cure-Weiss温度为-0.44 K,呈弱反铁磁性.同时,通过单晶各向磁化率的测量,进一步确认了CaTi₂O₄晶体中一维有序Ti-Ti反铁磁dimer链的形成,并明确了其方向. 关键词： 2O₄')" href="#">CaTi₂O₄ 磁化率 各向异性 反铁磁二聚化     

Al掺杂的尖晶石型LiMn2O4的结构和电子性质

采用基于密度泛函理论的第一性原理方法, 在广义梯度近似(GGA)和GGA+U方法下对尖晶石型LiMn₂O₄及其Al掺杂 的尖晶石型LiAl_0.125Mn_1.875O₄晶体的结构和电子性质进行了计算. 结果表明: 采用GGA方法得到尖晶石型LiMn₂O₄是立方晶系结构, 其中的Mn离子为+3.5价, 无法解释它的Jahn-Teller 畸变. 给出的LiMn₂O₄能带结构特征也与实验结果不符. 而采用GGA+U方法得到在低温下的LiMn₂O₄和其掺杂 体系LiAl_0.125Mn_1.875O₄的晶体都是正交结构, 与实验一致. 也能明确地确定Mn的两种价态Mn³⁺/Mn⁴⁺的分布并且能够说明Mn³⁺O₆的z方向有明显的Jahn-Teller 畸变, 而Mn⁴⁺O₆则没有畸变. LiMn₂O₄的能带结构与实验比较也能够符合. 采用GGA+U方法对Al掺杂体系的LiAl_0.125Mn_1.875O₄的研究表明, 用Al替换一个Mn不会明显地改变晶体的电子性质, 但可以有效地消除Al³⁺O₆ 八面体的Jahn-Teller畸变, 从而改善正极材料LiMn₂O₄的性能, 这与电化学实验的观察结果相一致.     

Characterization and electrochemical performance of the spinel LiMn2O4 prepared from -MnO2

The preparation and characterization of the spinel LiMn₂O₄ obtained by solid state reaction from quasi-amorphous -MnO₂ is reported. A well-defined highly pure spinel was characterized from X-ray diffractograms. The average manganese valence of -MnO₂ and spinel samples was found to be 3.89±0.01 and 3.59±0.01, respectively. The electrochemical performance of the spinel was evaluated through cyclic voltammetry and chronopotentiometry. The voltammetric profiles obtained at 1 mV/s for the LiMn₂O₄ electrode in 1 M LiClO₄ dissolved in a 2:1 mixture of ethylene carbonate and dimethyl carbonate showed typical peaks for the lithium insertion/extraction reactions. The charge capacity of this electrode was found to be 110 mA h g⁻¹ for the first charge/discharge cycles.     

Electronic structure studies of the spinel CoFe2O4 by X-ray photoelectron spectroscopy

The spinel CoFe₂O₄ has been synthesized by combustion reaction technique. X-ray photoelectron spectroscopy shows that samples are near-stoichiometric, and that the specimen surface both in the powder and bulk sample is most typically represented by the formula (Co_0.4Fe_0.6)[Co_0.6Fe_1.4]O₄, where cations in parentheses occupy tetrahedral sites and those within square brackets in octahedral sites. The results demonstrate that most of the iron ions are trivalent, but some Fe²⁺ may be present in the powder sample. The Co 2p_3/2 peak in powder sample composed three peaks with relative intensity of 45%, 40% and 15%, attributes to Co²⁺ in octahedral sites, tetrahedral sites and Co³⁺ in octahedral sites. The O 1s spectrum of the bulk sample is composed of two peaks: the main lattice peak at 529.90 eV, and a component at 531.53 eV, which is believed to be intrinsic to the sample surface. However, the vanishing of the O 1s shoulder peak of the powder specimen shows significant signs of decomposition.     

Infrared active phonon modes and ionicity of single crystal MgAl2O4

Near-normal incident infrared reflectivity spectra of （100） MgAl2O4 spinel single crystal have been measured at different temperatures in the frequency region between 50 and 6000 cm^-1. Eight infrared-active phonon modes are identified, which are fitted with the factorized form of the dielectric function. The dielectric property and optical conductivity of the MgAl2O4 crystal are analysed. From TO/LO splitting, the effective Szigeti charges and Born effective charges at different temperatures are calculated for studying the ionicity and the effect of polarization. Based on the relationship between the （LO-TO）1 splitting, which represents the transverse and longitudinal frequencies splitting of the highest energy phonon band in the reflectivity spectrum, and the ionic-covalent parameter, the four main phonon modes are assigned. MgA1204 can be considered as a pure ionic crystal and its optical characters do not change with decreasing temperature, so it may be used as a suitable substrate for high-Tc superconducting thin films.     

Luminescence enhancement of Mn doped ZnS nanocrystals passivated with zinc hydroxide

Mn-doped ZnS nanocrystals prepared by solvothermal method have been successfully coated with different thicknesses of Zn(OH)₂ shells through precipitation reaction. The impact of Zn(OH)₂ shells on luminescent properties of the ZnS:Mn nanocrystals was investigated. X-ray diffraction (XRD) measurements showed that the ZnS:Mn nanocrystals have cubic zinc blende structure. The morphology of nanocrystals is spherical shape measured by transmission electron microscopy (TEM). ZnS:Mn/Zn(OH)₂ core/shell nanocrystals exhibited much improved luminescent properties than those of unpassivated ZnS:Mn nanocrystals. The luminescence enhancement was observed with the Zn(OH)₂ shell thickening by photoluminescence (PL) spectra at room temperature and the luminescence lifetime of transition from ⁴T₁ to ⁶A₁ of Mn²⁺ ions was also prolonged. This result was led by the effective, robust passivation of ZnS surface states by the Zn(OH)₂ shells, which consequently suppressed nonradiative recombination transitions.     

Synthesis and magnetic properties of Li-Mn-Zn spinel oxides

LiMn_2−XZn_XO₄ (X<0.5) compounds were prepared by sol-gel method. The specimens with a large substitution degree (X>0.2) led to symmetry reduction from Fdm to P2₁3 in the spinel oxide, while those with a small substitution degree (X<0.1) had Fdm cubic symmetry. The Zn²⁺-substitution led to the enhancement of the low-temperature magnetic susceptibility and a shift in the Weiss constant from negative to positive, indicating that the dominant exchange interaction changed from antiferromagnetic to ferromagnetic. For the compounds with X=0.5, the spontaneous magnetization was 4.48μ_B and the Curie temperature was approximately 21 K. The experimentally obtained magnetization value was close to the value calculated under the assumption that the spins of the Mn⁴⁺ ions were aligned in ferromagnetic form. In addition, the magnetic properties of Li-Mn-Zn spinel oxides were briefly discussed, and compared with those of Li-Mn-M (M=Ni, Mg) spinel oxides.     

溶胶-凝胶软石印法制备Zn2SiO4:Mn图案化薄膜及其发光性能的研究

采用溶胶-凝胶法制备了Zn2SiO4:Mn薄膜并结合毛细管微模板技术实现了薄膜的图案化,利用X射线衍射(XRD),原子力显微镜,光学显微镜,发光光谱等手段对Zn2SiO4:Mn的结晶过程、发光性质进行了研究.XRD结果表明,溶胶-凝胶法合成的样品在800℃时已开始结晶,在1000℃时可得到纯相的Zn2SiO4:Mn,这比传统的固相法的烧结温度低150℃.Zn2SiO4:Mn薄膜的激发光谱在220nm和280nm之间有一个强的吸收峰,峰值位于248nm,发射光谱的最大值位于522nm,为绿光发射.从原子力显微镜照片可知组成薄膜的粒子比较均匀,其平均直径为220nm.我们获得了四种图案化宽度,分别是5,10,20,50μm.光学显微镜的结果表明,图案薄膜烧结后相对于烧结前有10%～20%的收缩.     

Enhanced cyclability of triple-metal-doped LiMn2O4 spinel as the cathode material for rechargeable lithium batteries

To improve the cycle performance of spinel LiMn₂O₄ as the cathode of 4-V-class lithium secondary batteries, spinel phases LiM _x Mn_2 − x O₄ (M=Li, Fe, Co; x = 0, 0.05, 0.1, 0.15) and LiFe_0.05M _y Mn_{1.95 − y} O₄ (M=Li, Al, Ni, Co; y = 0.05, 0.1) were successfully prepared using the sol–gel method. The spinel materials were characterized by powder X-ray diffraction (XRD), elemental analysis, and scanning electron microscopy. All the samples exhibited a pure cubic spinel structure without any impurities in the XRD patterns. Electrochemical studies were carried out using the Li|LiM _x Mn_2 − x O₄ (M=Li, Fe, Co; x = 0, 0.05, 0.1, 0.15) and LiFe_0.05M _y Mn_{1.95 − y} O₄ (M=Li, Al, Ni, Co; y = 0.05, 0.1) cells. These cathodes were more tolerant to repeated lithium extraction and insertion than a standard LiMn₂O₄ spinel electrode in spite of a small reduction in the initial capacity. The improvement in cycling performance is attributed to the stabilization in the spinel structure by the doped metal cations.     

Cation distribution dependence of magnetic properties of sol-gel prepared MnFe2O4 spinel ferrite nanoparticles

MnFe₂O₄ nanoparticles have been synthesized with a sol-gel method. Both differential thermal and thermo-gravimetric analyses indicate that MnFe₂O₄ nanoparticles form at 400 °C. Samples treated at 450 and 500 °C exhibit superparamagnetism at room temperature as implied from vibrating sample magnetometry. Mössbauer results indicate that as Mn²⁺ ions enter into the octahedral sites, Fe³⁺ ions transfer from octahedral to tetrahedral sites. When the calcination temperature increases from 450 to 700 °C, the occupation ratio of Fe³⁺ ions at the octahedral sites decreases from 43% to 39%. Susceptibility measurements versus magnetic field are reported for various temperatures (from 450 to 700 °C) and interpreted within the Stoner-Wohlfarth model.     

Chemical synthesis of spinel nickel ferrite (NiFe2O4) nano-sheets

The present investigation is related to the deposition of single-phase nano-sheets spinel nickel ferrite (NiFe₂O₄) thin films onto glass substrates using a chemical method. Nano-sheets nickel ferrite films were deposited from an alkaline bath containing Ni²⁺ and Fe²⁺ ions. The films were characterized for their structural, surface morphological and electrical properties by means of X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and two-point probe electrical resistivity techniques. The X-ray diffraction pattern showed that NiFe₂O₄ nano-sheets are oriented along (3 1 1) plane. The FT-IR spectra of NiFe₂O₄ films showed strong absorption peaks around 600 and 400 cm⁻¹ which are typical for cubic spinel crystal structure. Microstructural study of NiFe₂O₄ film revealed nano-sheet like morphology with average sheet thickness of 30 nm. The room temperature electrical resistivity of the NiFe₂O₄ nano-sheets was 10⁷ Ω cm.     

Luminescence properties of cerium-doped di-strontium magnesium di-silicate phosphor by the solid-state reaction method

A series of Sr₂MgSi₂O₇:xCe³⁺ (x?=?1.0%, 2.0%, 3.0%, 4.0% and 5.0%) phosphors were synthesized by the solid-state reaction method. The phosphor with optimum thermoluminescence, photoluminescence and mechanoluminescence (ML) intensity was characterized by X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy and Fourier transform infrared techniques. The trapping parameters (i.e. activation energy, frequency factor and order of the kinetics) of each synthesized phosphor have been calculated using the peak shape method and the results have been discussed. Under ultraviolet excitation (325?nm), Sr₂MgSi₂O₇:xCe³⁺ phosphors were composed of a broad band peaking at 385?nm, belonging to the broad emission band which emits violet-blue color. Commission International de I’Eclairage coordinates have been calculated for each sample and their overall emission is near violet-blue light. In order to investigate the suitability of the samples for industrial uses, color purity and color rendering index were calculated. An ML intensity of optimum [Sr₂MgSi₂O₇:Ce³⁺ (3.0%)] phosphor increases linearly with increasing impact velocity of the moving piston which suggests that these phosphors can be used as fracto-ML-based devices. The time of the peak ML intensity and the decay rate did not change significantly with respect to increasing impact velocity of the moving piston.     

Luminescence properties of Ceand Tb ions codoped strontium borate phosphate phosphors

The Ce³⁺-activated, Tb³⁺-activated, and Ce³⁺ and Tb³⁺ co-activated phosphors 2SrO-nB₂O₃-(1−n)P₂O₅ were synthesized by the solid-state reaction. The structures, photoluminescent spectra and dynamics of them were systemically studied. The results demonstrate that the structure of the samples with n=0.10-0.50 belongs to the hexagonal phase. When n is beyond this range, the structures are the mixed phases of α-Sr₂P₂O₇ and Sr₂B₂O₅. The optimum composition is determined to be n=0.25 for the 2SrO-nB₂O₃-(1−n)P₂O₅ phosphors. As n varies from 0.01 to 0.50, the lifetime of Ce³⁺ ion increases gradually, while the lifetime of Tb³⁺ ion decreases, indicating that the energy transfer efficiency decreases with the increase of n. The ET efficiency between Ce³⁺ and Tb³⁺ in the optimum composition reaches to 70%. The present results demonstrate that the Ce³⁺ and Tb³⁺ co-activated hexagonal 2SrO-0.25B₂O₃-0.75P₂O₅ powders can possibly be applied as the newly developed green efficient phosphors in the field of lighting and display.     

A novel approach to synthesize lithium ion battery spinel cathode materials

Spinel LiMn₂O₄ and LiMg_0.2Mn_1.8O₄ have been synthesized by a soft chemistry method using citric acid as the chelating agent and acryl amide as the gelling agent. This technique offers better homogeneity, preferred surface morphology, reduced heat treatment conditions, sub-micron-sized particles, and better crystallinity. The synthesized spinel materials are characterized by X-ray diffraction, scanning electron microscopy, cyclic voltammetry, and charge–discharge studies.     

Thermally stimulated luminescence from alkaline earth borates

The thermally stimulated luminescence from XB₂O₄ (X=Ca,Sr,Ba) has been investigated. The results on CaB₂O₄ and SrB₂O₄ are being reported here for the first time. In both cases, the emission is found to be quite intense, contrary to the case with BaB₂O₄. The presence of two peaks in the glow curves is noted over the temperature range of 300-570 K. The emission spectra corresponding to both the peaks have been observed to be identical, consisting of two broad emissions, one in the UV and other in the blue-green regions. The emission is apparently quite different from that of BaB₂O₄ in which case only very weak emission spreading over a broad wavelength range around 410 nm is observed. This result has been understood in terms of the overlap between the excitation and the emission spectra.