Electronic structures of new tunnel barrier spinel MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript>: first-principles calculations

The electronic structures of spinel MgAl2O4 and MgO tunnel barrier materials were investigated using first-principles density functional theory calculations. Our results show that similar electronic structures are found for the MgAl2O4 and MgO tunneling barriers. The calculated direct energy gaps at the Γ-point are about 5. 10 eV for MgAl2O4 and 4. 81 eV for MgO, respectively. Because of the similar feature in band structures from Γ high-symmetry point to F point (△ band), the coherent tunneling effect might be expected to appear in MgAl2O4-based MTJs like in MgO-based MTJs. The small difference of the surface free energies of Fe (2. 9 J. m-2) and MgAl2O4 (2. 27 J·m-2) on the {100}orientation, and the smaller lattice mismatch between MgAl2O4 and ferromagnetic electrodes than that between MgO and ferromagnetic electrodes, the spinel MgAl2O4 can substitute MgO to fabricate the coherent tunneling and chemically stable magnetic tunnel junction structures, which will be applied in the next generation read heads or spintronic devices.     

聚丙烯酸络合法制备掺铝锰酸锂纳米粉体

采用溶胶-凝胶法并结合热处理工艺制备掺铝锰酸锂粉体,利用热重-差热分析、X射线衍射、透射电镜等方法对前驱体的热分解行为、粉体的结构及形貌进行表征.结果表明:直接以聚丙烯酸(PAA)为螫合剂合成了稳定的溶胶和凝胶,经750℃热处理后获得了粒径分布均匀、无团聚的尖晶石LiAlgMn2-xO4纳米粉体.随热处理温度升高,LiAl2Mn2-xO4纳米粉体的品粒尺寸不断增大,尖晶石型结构愈趋完整.掺杂少量铝并没有改变锰酸锂的尖晶石结构,但明显增强了结构稳定性,有助于改善电化学性能.     

Electrochemical properties of spinel compound LiNi_（0.5）Mn_（1.2）Ti_（0.3）O_4 as cathode materials for lithium ion batteries

The electrochemical properties of spinel compound LiNi0.5Mn1.2Ti0.3O4 were investigated in this study.The chemicals LiAc·2H2O,Mn(Ac)2·2H2O,Ni(Ac)2·4H2O,and Ti(OCH3)4 were used to synthesize LiNi0.5Mn1.2Ti0.3O4 by a simple sol-gel method.The discharge capacity of the sample reached 134 mAh/g at a current rate of 0.1C.The first and fifth cycle voltammogram almost overlapped,which showed that the prepared sample LiNi0.5Mn1.2Ti0.3O4 had excellent good cycle performance.There were two oxidation peaks at 4.21 V and 4.86 V,and two reduction peaks at 4.55 V and 3.88 V in the cycle voltammogram,respectively.By electrochemical impedance spectroscopy and its fitted result,the lithium ion diffusion coefficient was measured to be approximately 7.76 × 10?11 cm2/s.     

低温熔盐燃烧法制备LiMn_2O_4

采用一种新的低温熔盐燃烧法制备尖晶石型LiMn_2O_4物质.研究了在550℃时草酸和柠檬酸为燃料对燃烧产物物相组成及其电性能的影响.结果表明,燃烧产物的主晶相为LiMn_2O_4,含有Mn_2O_3杂质.在Li:Mn:草酸=1:2:0～1.5(摩尔比)时,草酸对燃烧产物纯度影响不大,Li:Mn:草酸=1:2:2.0时,产物纯度降低,柠檬酸对产物纯度影响较大,燃烧产物随柠檬酸用量增加先升高后降低;在Li:Mn:柠檬酸=1:2:0.5时所得产物纯度最高,其放电容量达到121 mAh/g,但循环性能不是很理想.     

Electrochemical properties of carbon-coated LiFePO<Subscript>4</Subscript> and LiFe<Subscript>0.98</Subscript>Mn<Subscript>0.02</Subscript>PO<Subscript>4</Subscript> cathode materials synthesized by solid-state reaction

Olivine structured LiFePO4/C (lithium iron phosphate) and Mn2+-doped LiFe0. 98Mn0. 024/C powders were synthesized by the solid-state reaction. The effects of manganese partial substitution and different carbon content coating on the surface of LiFePO4 were considered. The structures and electrochemical properties of the samples were measured by X-ray diffraction (XRD), cyclic voltammetry (CV), charge/discharge tests at different current densities, and electrochemical impedance spectroscopy (EIS). The electrochemical properties of LiFePO4 cathodes with x wt. ％ carbon coating (x=3, 7, 11, 15) at γ=0. 2C, 2C (1C=170 mAh·g-1) between 2. 5 and 4. 3 V were investigated. The measured results mean that the LiFePO4 with 7 wt. ％ carbon coating shows the best rate performance. The discharge capacity of LiFe0. 98Mn0. 02PO4/C composite is found to be 165 mAh·g 1 at a discharge rate, γ=0. 2C, and 105 mAh·g-1 at γ=2C, respectively. After 10cycles, the discharge capacity has rarely fallen, while that of the pristine LiFePO4/C cathode is 150 mAh·g-1 and 98 mAh·g-1 at γ=0. 2 and 2C, respectively. Compared to the discharge capacities of both electrodes above, the evident improvement of the electrochemical performance is observed, which is ascribed to the enhancement of the electronic conductivity and diffusion kinetics by carbon coating and Mn2+-substitution.     

LiMn2L(Ac)2热分解制备的尖晶石LiMn2O4及其电化学性能

通过XRD、SEM及电化学测试等手段研究了前驱体LiMn2L(Ac)2(L为柠檬酸根)的焙烧工艺条件对尖晶石LiMn2O4产物的结构、形貌及电化学性能的影响.结果表明:提高前驱体的焙烧温度有利于获得晶相结构、微观形貌及电化学性能均较好的LiMn2O4样品.在500℃焙烧2 h再于750℃下保温8～16 h的分段焙烧工艺所得样品的初始容量达到126.0 mAh/g,循环50次后容量衰减了14.5%.     

Synthesis of nano-sized Co<Subscript>3</Subscript>O<Subscript>4</Subscript> powder by spray conversion method for anode material of lithium battery

A nano-sized Co₃O₄ powder was prepared using a spray conversion method that could be applied for mass production. The spray-conversion process consisted of spray drying of a metallic liquid solution, a calcination treatment, and a ball milling process. The calcined Co₃O₄ powder consisted of agglomerated spherical clusters with nano-sized particles. After milling for 24 h, agglomerated powders were fragmented into fine powders sized below 60 nm. The lithium/cobalt oxide cell was charge-discharged at a constant current density of 0.2 mAcm⁻² and showed a first discharge capacity of 1100 mAhg⁻¹. The discharge capacity of the Li/Co₃O₄ cell drastically decreased with cycle number. By increasing the carbon content of the anode, the cycle life was improved. For a Co₃O₄ electrode containing 40 wt.% carbon, the discharge capacity was over 400 mAhg⁻¹ after 50 cycles. The spray conversion method might be a useful method to prepare nano-sized Co₃O₄ powder for the anode material of lithium batteries.     

Preparation and properties of a nano TiO<Subscript>2</Subscript>/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> composite superparamagnetic photocatalyst

Nano TiO₂/Fe₃O₄ composite particles with different molar ratios of TiO₂ to Fe₃O₄ were prepared via sol-gel method. X-ray diffraction, transmission electron microscopy, and vibration sample magnetometry were used to characterize the TiO₂/Fe₃O₄ particles. The photocatalytic activity of the particles was tested by degrading methyl blue solution under UV illumination (254 nm). The results indicate that with the content of TiO₂ increasing, the photocatalytic activity of the composite particles enhances, while the magnetism of the particles decreases. When the molar ratio of TiO₂ to Fe₃O₄ is about 8, both the photocatalytic activity and magnetism of the TiO₂/Fe₃O₄ particles are relatively high, and their photocatalytic activity remains well after repeated use.     

溶胶-凝胶法制备LiCoxMn2-xO4及电化学性能研究

采用溶胶-凝胶(sol-gel)法制备了颗粒较小(100～300nm)、分布均匀的尖晶石LiCoxMn2-xO4粉体,研究了不同掺杂水平对其结构及电化学性能的影响.结果表明,掺杂少量Co于LiMn2O4中并不改变材料的尖晶石结构;随着Co掺杂量增加,材料结构稳定性提高,极化降低,首次放电比容量逐渐减小,但充放电循环性能却明显改善;在低温(500℃)条件下退火6h后,LiCoxMn2-xO4粉体的放电比容量稍有增加,但对循环性能影响不大;在电流密度0.1mA/cm2和截止电压3.5～4.4V时,LiCo0.1Mn1.9O4粉体首次放电比容量达123mAh/g,20次循环后的稳定放电比容量为106mAh/g,具有较好的电化学性能.     

Cold spray of SiC and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with soft metal incorporation: A technical contribution

Al + SiC, Al + Al₂O₃ composites as well as pure Al, SiC, and Al₂O₃ coatings were prepared on Si substrates by the cold gas dynamic spray process (CGDS or cold spray). The powder composition of metal (Al) and ceramic (SiC, Al₂O₃) was varied into 1:1 and 10:1 wt.%, respectively. The propellant gas was air heated up to 330 °C and the gas pressure was fixed at 0.7 MPa. SiC and Al₂O₃ have been successfully sprayed producing coatings with more than 50 μm in thickness with the incorporation of Al as a binder. Also, hard ceramic particles showed peening effects on the coating surfaces. In the case of pure Al metal coating, there was no crater formation on hard Si substrates. However, when Al mixed with SiC and Al₂O₃, craters were observed and their quantities and sizes depended on the composition, aggregation and size of raw materials.     

Performance and capacity fading reason of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript>/graphite batteries after storing at high temperature

Spinel LiMn₂O₄ was synthesized by a solid-state method. A 204468-size battery was fabricated and stored at 55°C. The structure and morphology of the LiMn₂O₄ cathode were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) technique. Energy dispersive spectroscopy (EDS) was used to analyze the surface component of the carbon anode. The discharge capacities of LiMn₂O₄ stored for 0, 24, 48, and 96 h are 106, 98, 96, and 92 mAh·g⁻¹, respectively. The cyclic performance is improved after storage. The capacity retentions of LiMn₂O₄ stored for 0, 24, 48, and 96 h are 83.8%, 85.8%, 86.9%, and 88.6% after 180 cycles. The intensity of all the LiMn₂O₄ diffraction peaks is weakened. Mn is detected from the carbon electrode when the battery is stored for 96 h. Cyclic voltammograms and electrochemical impedance spectroscopy (EIS) were used to examine the surface state of the electrode after storage. The results show that the resistance and polarization of LiMn₂O₄/electrolyte is increased after storage, which is responsible for the fading of capacity.     

Magnetic properties and crystallization kinetics of Zn<Subscript>0.5</Subscript>Ni<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript>

Precursor of nanocrystalline Zn0.5Ni0.5Fe2O4 was obtained by grinding mixture of ZnSO4·7H2O,NiSO4·6H2O,FeSO4·7H2O,and Na2CO3·10H2O under the condition of surfactant polyethylene glycol(PEG)-400 being present at room temperature,washing the mixture with water to remove soluble inorganic salts and drying it at 373 K.The spinel Zn0.5Ni0.5Fe2O4 was obtained via calcining precursor above 773 K.The precursor and its calcined products were characterized by differential scanning calorimetry(DSC) ,Fourier transform infrared(FT-IR) ,X-ray diffraction(XRD) ,and vibrating sample magnetometer(VSM) .The result showed that Zn0.5Ni0.5Fe2O4 obtained at 1073 K had a saturation magnetization of 74 A·m2·kg-1.Kinetics of the crystallization process of Zn0.5Ni0.5Fe2O4 was studied using DSC technique,and kinetic parameters were determined by Kissinger equation and Moynihan et al.equation.The value of the activation energy associated with the crystallization process of Zn0.5Ni0.5Fe2O4 is 220.89 kJ·mol-1.The average value of the Avrami exponent,n,is equal to 1.59±0.13,which suggests that crystallization process of Zn0.5Ni0.5Fe2O4 is the random nucleation and growth of nuclei reaction.     

Synthesis and electrochemical performance of YF<Subscript>3</Subscript>-coated LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> cathode materials for Li-ion batteries

Spinel LiMn₂O₄ cathodes were coated with 1 mol% YF₃. X-ray diffraction (XRD) analyses showed that Y and/or F did not enter the lattice of the LiMn₂O₄ crystal. Transmission electron microscopy (TEM) showed that a compact YF₃ layer of 5–20 nm in thickness was coated onto the surface of LiMn₂O₄ particles. Scanning electron microscopy (SEM) observation showed that the YF₃ coating caused the agglomeration of LiMn₂O₄ particles. The cycling test demonstrated that the YF₃ coating can improve the electrochemical performance of LiMn₂O₄ at both 20 and 55°C. Moreover, YF₃-coated LiMn₂O₄ exhibited an improved rate capability compared with the uncoated one at high rates over 5C. The immersion test in electrolytes showed that YF₃-coated LiMn₂O₄ is more erosion resistant than the uncoated one.     

Effect of a surface treatment for LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> cathode material in lithium secondary batteries

LiNi_1/3Co_1/3Mn_1/3O₂ cathode material was surface-treated to improve its electrochemical performance. Al₂O₃ nanoparticles were coated onto the surface of LiNi_1/3Co_1/3Mn_1/3O₂ powder using a sol-gel method. The as-prepared Al₂O₃ nano-particle was identified as the cubic structure of Al₂O₃. XRD showed that the LiNi_1/3Co_1/3Mn_1/3O₂ structure was not affected by the Al₂O₃ coating. With a coating of 3 wt.% Al₂O₃ on LiNi_1/3Co_1/3Mn_1/3O₂, the cyclic-life performance and rate capability were improved. However, heavier coatings (5 wt.%) on LiNi_1/3Co_1/3Mn_1/3O₂ resulted in a considerable decrease of the discharge capacity and rate capability. The thermal stability of LiNi_1/3Co_1/3Mn_1/3O₂ materials was greatly improved by the 3 wt.% Al₂O₃ coating.     

Influence of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> doping on the properties of KBT-NBT-BT lead-free piezoelectric ceramics

0.144(K0.5Bi0.5)TiO3-0.85(Na0.5Bi0.5)TiO3-0.006BaTiO3(KBT-NBT-BT) lead-free piezoelectric ceramics were prepared using a conventional solid state method.The influence of Sb2O3 doping on the crystal phase,surface microstructure and properties of the KBT-NBT-BT lead-free piezoelectric ceramics were investigated using X-ray diffraction(XRD),scanning electron microscope(SEM) and other analytical methods.The results show that all compositions are of pure perovskite structure solid states.Sb2O3 doping does not influence the microstructure of KBT-NBT-BT lead-free piezoelectric ceramics obviously in the Sb2O3 doping range of 0.1-0.5 wt.%.Sb2O3 functions as a donor when doped small amount,while functions as a acceptor when doped large amount.The piezoelectric strain constant(d33) increases first and then decreases;the dielectric constant(ε3T3/ε0) and the dielectric loss(tanδ) decrease continuously when the amount of Sb2O3 dopant increases.When the doping amount of Sb2O3 is 0.1 wt.%,the KBT-NBT-BT piezoelectric ceramics with good comprehensive properties are obtained,whose d33,ε3T3/ε0 and tanδ are 147 pC/N,1510 and 4.2%,respectively.     

Influence of CaF2 on the structure and dielectric properties of Ag（Nb0.8Ta0.2）O3 ceramics

Ag(Nb0.8Ta0.2)O3 ceramics were prepared by the traditional solid-state reaction method. The effect of CaF2 addition on the structure and di-electric properties of Ag(Nb0.8Ta0.2)O3 ceramics was investigated. The addition of CaF2 led the ceramics to a larger grain size and distortion of lattice. With the addition of 4.5 wt.% CaF2, the permittivity of the ceramics increased from 442 to 1028, the dielectric loss decreased sharply from 6.12 × 10-3 to 8.6 × 10-4, and the temperature coefficient of capacitance decreased from 1834 ppm/°C to-50 ppm/°C (at 1 MHz). These results indicated that the high permittivity was related with a large grain size, a low grain boundary density, and the weak Ta-O or Nb-O bond strength caused by the addition of CaF2.     

Effects of dopant content on optical and electrical properties of In<Subscript>2</Subscript>O<Subscript>3</Subscript>: W transparent conductive films

The In2O3:W (IWO) films with different W content were deposited on glass substrate using direct current sputtering method. The structure, surface morphology, and optical and electrical properties were investigated. Results showed that both the carrier concentration and carrier mobility were increased with the doping of W. The IWO film with the lowest resistivity of 1. 0× 10-3 Ω· cm, highest carrier mobility of 43. 7 cm2. W-1. s-1 and carrier concentration of 1. 4× 1020 cm-3 was obtained at the content of 2. 8 wt. ％. The average optical transmittance from 300 nm to 900 nm reached 87. 6％.     

The effect of boron oxide on the crystallization behavior of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel phase during the cooling of the CaO-SiO<Subscript>2</Subscript>-10 mass.% MgO-30 mass.%Al<Subscript>2</Subscript>O<Subscript>3</Subscript> systems

The microstructural characteristics of the CaO-SiO₂-B₂O₃-10 mass.% MgO-30 mass.% Al₂O₃ systems solidified during slow cooling from 1600 °C were investigated using SEM-EDS and a thermochemical computation package. The effect of boron oxide on the crystallization behavior of the spinel in the aluminosilicate system was observed because boron oxide is believed to become a potential flux to reduce the melting point of the liquid oxides. The primary crystalline phase was spinel, mainly MgAl₂O₄, irrespective of the boron content. The liquidus temperature T _L continuously decreased as the boron oxide content increased, indicating that the boron oxide decreased the activity of the MgAl₂O₄ spinel phase in liquid melts at high temperatures. The size of the spinel crystals increased as the temperature range for the solid + liquid coexisting region, viz. the mushy zone, increased. In the present systems, because the T _L continuously decreased with the increase in the boron oxide content, the viscosity of the liquid oxide may have affected the crystallization behavior of the spinel during cooling. Based on these results, an injection of a small amount of B₂O₃ flux into molten steel containing liquid aluminosilicate inclusions is not recommended because large spinel crystals can originate from the changes in the thermophysical properties of the liquid inclusions due to the incorporation of boron oxide into the aluminosilicate networks.     

Influences of NiCO<Subscript>3</Subscript> content on the microstructure and magnetic properties of Ni<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> powders prepared by SHS

Stoichiometric Ni_0.5Zn_0.5Fe₂O₄ powders were produced by self-propagating high temperature synthesis (SHS). The effects of NiCO₃ content in the raw materials on the microstructure and magnetic properties of Ni-Zn ferrite powders were systematically studied. The Ni_0.5Zn_0.5Fe₂O₄ powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The magnetic properties of the powders were evaluated by vibrating sample magnetometry (VSM). The results show that the introduction of NiCO₃ into reactants improves the conversion percentage and refines the Ni_0.5Zn_0.5Fe₂O₄ particles. The increase of NiCO₃ content enhances the magnetic properties of Ni_0.5Zn_0.5Fe₂O₄. Particularly, the saturation magnetization reaches the maximum when the NiCO₃ content is 3 at.%.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> content on the oxidation behavior of Fe-Cr-Al-based ODS alloys

A study was conducted to investigate the cyclic oxidation behavior of two oxide dispersion strengthened (ODS) Fe-Cr-Al based alloys containing 0.17 wt.% and 0.7 wt.% Y₂O₃. The alloys were oxidized in air for 100 h at 1200°C based on a 24 h cycle period. X-ray diffraction (XRD) and analytical transmission electron microscopy (TEM) were used to characterize the structure, morphology, and composition of the oxide scales. Both alloys formed highly adherent and continuous layers of α-Al₂O₃ exhibiting a morphology indicative of inward scale growth. The role of Y₂O₃ was to promote adherence by segregating to the grain boundaries within the oxide. Concurrently, Y₂O₃ generated micro-porosity resulting in a scale of comparatively higher thickness in the alloy with 0.7 wt.% Y₂O₃.