Effects of electrode on resistance switching properties of ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript> films deposited by magnetron sputtering

ZnMn₂O₄ films for resistance random access memory (RRAM) were fabricated with different device structures by magnetron sputtering. The effects of electrode on I-V characteristics, resistance switching behavior, endurance and retention characteristics of ZnMn₂O₄ films were investigated. The ZnMn₂O₄ films, using p-Si and Pt as bottom electrode, exhibit bipolar resistive switching (BRS) behavior dominated by the space-charge-limited conduction (SCLC) mechanism in the high resistance state (HRS) and the filament conduction mechanism in the low resistance state (LRS), but the ZnMn₂O₄ films using n-Si as bottom electrodes exhibit both bipolar and unipolar resistive switching behaviors controlled by the Poole-Frenkel (P-F) conduction mechanism in both HRS and LRS. Ag/ZnMn₂O₄/p-Si device possesses the best endurance and retention characteristics, in which the number of stable repetition switching cycle is over 1000 and the retention time is longer than 10⁶ seconds. However, the highest R _HRS/R _LRS ratio of 10⁴ and the lowest V _ON and V _OFF of 3.0 V have been observed in Ag/ZnMn₂O₄/Pt device. Though the Ag/ZnMn₂O₄/n-Si device also possesses the highest R _HRS/R _LRS ratio of 10⁴, but the highest values of V _ON,V _OFF, R _HRS and R _LRS, as well as the poor endurance and retention characteristics.     

Enhanced Ferroelectric Polarization in Laser-ablated Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> Thin Films by Controlling Preferred Orientation

Polycrystalline Bi₄Ti₃O₁₂ thin films with various fractions of a-axis, c-axis and random orientations have been grown on Pt(111)/Ti/SiO₂/Si substrates by laser-ablation under different kinetic growth conditions. The relationship between the structure and ferroelectric property of the films was investigated, so as to explore the possibility of enhancing ferroelectric polarization by controlling the preferred orientation. The structural characterization indicated that the large growth rate and high oxygen background pressure were both favorable for the growth of non-c-axis oriented grains in the Bi₄Ti₃O₁₂ thin films. The films with high fractions of a-axis and random orientations, i e, f (a-sxis) = 28.3% and f (random) = 69.6%, could be obtained at the deposition temperature of 973 K, oxygen partial pressure of 15 Pa and laser fluence of 4.6 J/cm², respectively. It was also noted that the variation of ferroelectric polarization was in accordance with the evolution non-c-axis orientation. A large value of remanent polarization (2Pr = 35.5 μC/cm²) was obtained for the Bi₄Ti₃O₁₂ thin films with significant non-c-axis orientation, even higher than that of rare-earth-doped Bi₄Ti₃O₁₂ films.     

Effect of pH on crystallization of nanocrystalline zirconia in a microwave-hydrothermal process

Nanocrystalline zirconia (ZrO₂) was synthesized using a microwave-hydrothermal process. The effect of pH on the crystallization of the ZrO₂ powders was investigated. The phase and microstructure of ZrO₂ powders were examined using X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). Results show that pure m-ZrO₂ can be obtained at low pH (pH<2). Pure t-ZrO₂ is formed at pH = 7 and 14. The size of the ZrO₂ crystals is in the range of 8-26 nm and decreases with increasing pH. The formation of m-ZrO₂ results from the precipitation of ZrO₂ from solution. The t-ZrO₂ is formed through the in-situ structural rearrangement of amorphous Zr(OH) _x O _y . The stabilization of t-ZrO₂ is attributed to the small crystal size and the adsorption of hydroxy ions on the surfaces of the crystals.     

Modified k-ω model using kinematic vorticity for corner separation in compressor cascades

A new method of modifying the conventional k-ω turbulence model for corner separation is proposed in this paper. The production term in the ω equation is modified using kinematic vorticity considering fluid rotation and deformation in complex geometric boundary conditions. The corner separation flow in linear compressor cascades is calculated using the original k-ω model, the modified k-ω model and the Reynolds stress model(RSM). The numerical results of the modified model are compared with the available experimental data, as well as the corresponding results of the original k-ω model and RSM. In terms of accuracy, the modified model, which significantly improves the performance of the original k-ω model for predicting corner separation, is quite competitive with the RSM. However, the modified model, which has considerably lower computational cost, is more robust than the RSM.     

Preparation and magnetic properties of Mn-Zn ferrites by the Co-precipitation method

Mn-Zn ferrites (Mn_1?x Zn _x Fe₂O₄) with different compositions were prepared by the coprecipitation method, and the influences of such synthesis conditions as pH value, composition and volume ratio (R) of the mixed solution and NH₄HCO₃ solution on their microstructures and magnetic properties were discussed. The samples were characterized by X-ray diffraction (XRD) and magnetization measurement instrument. Lattice parameters and average crystalline size of the synthesized materials were calculated from the corresponding XRD patterns with the related software Jade.5. For samples of different pH values, only one phase was found when pH values were 7.0, 8.0 and 9.0. The sample with pH value of 7.0 exhibited the highest saturation magnetic induction, the lowest coercive force, and crystallized best. For samples of different R values with pH value of 7.0, only one phase was observed in all samples, and the sample with R value of 2.3 exhibited the highest saturation magnetic induction and the lowest coercive force. The composition has mainly afected the magnetic properties, and the saturation magnetic induction increases with the increase of the content of Zn (x), but decreases when x is beyond 0.6. The trend of coercive force is on the contrary. However, no magnetism is exhibited when the x value is up to 0.8.     

Morphology study of oriented SmBCO film deposited by MOCVD

c-axis-oriented SmBa_2Cu_3O_7(SmBCO) films have been deposited on(100)- LaA1O_3(LAO)substrate by metal organic chemical vapor deposition(MOCVD) technique.The effects of deposition temperature(T_(dep)) and total pressure(P_(tot)) on the orientation and microstructure of SmBCO films were investigated.The orientation of SmBCO films transformed from α-axis to c-axis with increasing of T_(dep) from 900 to 1 100℃.At T_(dep)=1 050℃,SmBCO films had c-axis orientation and tetragon surface.At P_(tot)~(dep)=400-800 Pa and T_(dep)=1 050 ℃,totally c-axis-oriented SmBCO films were obtained.The R_(dep) of SmBCO films increased firstly and then decreased with increasing P_(tot).The surface of SmBCO films exhibited tetragon morphology at 1 050 ℃ and400 Pa.Maximum thickness of SmBCO film deposited was 1.2μm at P_(tot)= 600 Pa,and the corresponding R_(dep)was 7.2 μm·h~(-1).     

Crystal growth,structure and properties of bismuth triborate BiB<Subscript>3</Subscript>O<Subscript>6</Subscript> crystal

BiB₃O₆ (BIBO) single crystals with size of 46×23×10 mm³ and weight of 26.0 g have been successfully grown by top-seeded method. Problems encountered in the growth process of this crystal have been discussed in detail, and the methods of growing high-quality large crystals have been put forward. The relationship between their structure and properties is studied. The space group of monoclinic BiB₃O₆ is C2 and the cell parameters are a=7.1203(7) Å, b=4.9948(7) Å, c=6.5077(7) Å, β=105.586(8)″, and V=222.93(5) ų. The density of BIBO is 4.8965 g/cm³. The Mohs’s scale of hardness is 5.5–6. There is no cleavage face in the crystal. The transmittance of BIBO is about 80 percent in the range from visible coherent light to near-infrared light. The ultraviolet cutoff wavelength is at 276 nm. BiB₃O₆ is a biaxial crystal and has two sets of axes, and the relative orientation of (X, Y, Z) with regard to (a, b, c) is: X//b, (Y, c)=47.2°, (Z, a)=31.6°, determined by X-ray analysis combined with polarized microscopy. Second-harmonic-generation (SHG) experiments were carried out for the first time. In type I phase-matching (PM) directions (11.1°, 90°) and (168.9°, 90°), SHG conversion efficiencies of two directions for 1.064 μm light are up to 67.7% and 58%, respectively. We have also obtained the third-harmonic-generation (THG) of 1.064 μm. The comparative experiments between BIBO and KTP were carried out on conversion efficiency, transmittance and hardness. All the above results indicate that BiB₃O₆ is a kind of excellent nonlinear optical (NLO) crystal.     

Synthesis and electrochemical behaviors of <Emphasis Type="Italic">α</Emphasis>-MoO<Subscript>3</Subscript> nanobelts/carbon nanotubes composites for lithium ion batteries

α-MoO₃ nanobelts/carbon nanotubes (CNTs) composites were synthesized by simple hydrothermal method followed by CNTs incorporating, and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Cyclic voltammogram (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge testing techniques were employed to evaluate the electrochemical behaviors of α-MoO₃ nanobelts /CNTs composites. The results exhibited that compared to bare α-MoO₃ nanobelts, the α-MoO₃ nanobelts/CNTs composites have better electrochemical performances as cathode materials for lithium ion battery, maintaining a reversible specific capacity of 222.2 mAh/g at 0.3 C after 50 cycles, and 74.1% retention of the first reversible capacity. In addition, the Rct value of the α-MoO₃ nanobelts/ CNTs is 13 Ω, much lower than 66 Ω of the bare α-MoO₃ nanobelts. The better electrochemical performances of the α-MoO₃ nanobelts /CNTs composites can be attributed to the effects of the high conductive CNTs network.     

Structural and electrical properties of high Curie temperature Aurivillius phase composite ceramics with largely enhanced piezoelectricity

High performance piezoelectric ceramics with high Curie temperatures(TC) are the bottle necks of relevant high temperature devices. In this study, the electrical performance and microstructure of Li and Mn codoped Aurivillius-type composite ceramics with a composition Ca_(0.99-x_Bi_(6.99+x)(Li Mn)_(0.01) Nb Ti_5O_(24)(x = 0–0.8) were systematically investigated. The results indicated that uniform intergrowth structure with a lattice similar to that of the end member CBT could be formed at a low x value(x 0.4). Phase separation occurred when more A-site Ca~(2+) ions were replaced by Bi~(3+) ions. Nevertheless, all composite samples showed d_(33) values about 2 to 3 times of that of the constituent phase Ca Bi_4Ti_4O_(15) and Bi_3 Ti NbO_9 with still a high depolarization temperature. The performance of the samples was found to be related to the density and larger lattice distortion along the polarization a axis. The results also demonstrated that formation of the compound system was an effective way in improving the performance of Aurivillius-type high TC piezoelectric ceramics.     

Composition dependence of phase structure and electrical properties of (1−y)Bi1−xNdxFeO3−yBiScO3 ceramics

In this work, we have studied a new lead-free ceramic of(1-y)Bi_(1-x)Nd_xFeO_(3-y)BiScO_3(0.05≤x≤0.15 and 0.05≤y≤0.15) prepared by a conventional solid-state method, and the influences of Nd and Sc content on their phase structure and electrical properties were investigated in detail. The ceramics with 0.05≤x≤0.10 and 0.05≤y≤0.15 belong to an R3 c phase, and the rhombohedral-like and orthorhombic multiphase coexistence is established in the composition range of 0.125≤x≤0.15 and y=0. The electrical properties of the ceramics can be enhanced by modifying x and y values. The highest piezoelectric coefficient(d33~51 p C/N) is obtained in the ceramics with x=0.075 and y=0.125, which is superior to that of a pure BiFeO_3 ceramic. In addition, a lowest dielectric loss(tan δ~0.095%, f=100 k Hz) is shown in the ceramics with x=0.15 and y=0 due to the involvement of low defect concentrations, and the improved thermal stability of piezoelectricity at 20–600oC is possessed in the ceramics. We believe that the ceramics can play a meaningful role in the high-temperature lead-free piezoelectric applications.     

Effects of annealing processes on Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript> thin films

The Cu _x Si_1-x thin films have been grown by pulsed laser deposition (PLD) with in situ annealing on Si (001) and Si (111), respectively. The transformation of phase was detected by X-ray diffraction (XRD). The results showed that the as-deposited films were composed of Cu on both Si (001) and Si (111). The annealed thin films consisted of Cu + η”-Cu₃Si on Si (001) while Cu + η’-Cu₃Si on Si (111), respectively, at annealed temperature (T _a) = 300-600 °C. With the further increasing of T _a, at T _a= 700 °C, there was only one main phase, η”-Cu₃Si on Si (001) while η’-Cu₃Si on Si (111), respectively. The annealed thin films transformed from continuous dense structure to scattered-grain morphology with increasing T _a detected by field emission scanning electron microscope (FESEM). It was also showed that the grain size would enlarge with increasing annealing time (t _a).     

YIG thin film for RF integrated inductor

The yttrium iron garnet(YIG) thin films prepared by the sol-gel method and rapid thermal annealing(RTA) process for integrated inductor are investigated. The X-ray diffraction(XRD) results indicate that the YIG film annealed above 650 ℃ is poly-crystalline with single-phase garnet structure. Moreover, it can be found that the initial permeability μi, saturation magnetization M_S and coercivity H_c of these YIG films increase with increasing RTA temperature. Low temperature annealing after crystallization can further improve the magnetic properties of YIG film. Thereby, a planar integrated inductor in the presence of Si substrate/SiO_2 layer/Y_(2.8)Bi_(0.2)Fe_5O_(12) thin film/Cu spiral coil structure is fabricated successfully by the standard IC processes. Due to the magnetic enhancement originated from YIG film, the inductance L and quality factor Q of the inductor with YIG film are improved in a certain frequency range.     

Electronic Structure and Optical Properties of LiBiO<Subscript>3</Subscript> Doped with V,Nb, and Ta

The crystal structure, band structure, density of states, Mulliken charge, bond population and optical properties for LiBi_1-xM_xO₃ (M=V, Nb, and Ta) were investigated using hybrid density functional theory. It was found that LiBiO₃ doped with V, Nb, and Ta presented distinctly stronger covalent interactions in M-O (M=V, Nb, and Ta) than Bi-O, thus resulting in mild distortion of the structure and facilitating the separation of photogenerated carriers. Furthermore, the hybridizations of Bi-6s, M-d (M=V, Nb, and Ta) and O-2p widened the valence and conduction bands, which promoted transmission of photogenerated carriers in the band edge and thus caused better photocatalytic performance.     

Probabilistic composition of cone-based cardinal direction relations

Composition tables play a significant role in qualitative spatial reasoning (QSR). At present, a couple of composition tables focusing on various spatial relations have been developed in a qualitative approach. However, the spatial reasoning processes are usually not purely qualitative in everyday life, where probability is one important issue that should be considered. In this paper, the probabilistic compositions of cone-based cardinal direction relations (CDR) are discussed and estimated by making some assumptions. Consequently, the form of composition result turns to be {(R ₁,P ₁), (R ₂,P ₂), ..., (R _n ,P _n )}, where P _i is the probability associated with relation R _i . Employing the area integral method, the probabilities in each composition case can be computed with the assumption that the target object is uniformly distributed in the corresponding cone regions.     

Effect of oxygen partial pressure on epitaxial growth and properties of laser-ablated AZO thin films

Al-doped ZnO (AZO) thin films were grown on c-sapphire substrates by laser ablation under different oxygen partial pressures (P _O2). The effect of PO2 on the crystal structure, preferred orientation as well as the electrical and optical properties of the films was investigated. The structure characterizations indicated that the as-grown films were single-phased with a wurtzite ZnO structure, showing a significant c-axis orientation. The films were well crystallized and exhibited better crystallinity and denser texture when deposited at higher P _O2. At the optimum oxygen partial pressures of 10 - 15 Pa, the AZO thin films were epitaxially grown on c-sapphire substrates with the (0001) plane parallel to the substrate surface, i e, the epitaxial relationship was AZO (000 1) // Al₂O₃ (000 1). With increasing P _O2, the value of Hall carrier mobility was increased remarkably while that of carrier concentration was decreased slightly, which led to an enhancement in electrical conductivity of the AZO thin films. All the films were highly transparent with an optical transmittance higher than 85 %.     

Effect of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> on structure and properties of calcium aluminate glasses

The effect of Ga₂O₃ on the structure and properties of calcium aluminate glasses fabricated by vacuum melting process was investigated by Raman spectrum, differential scanning calorimeter (DSC), and infrared spectrum methods. The results show that calcium aluminate glass network only consists of [AlO₄] tetrahedral units. With the gradual addition of Ga₂O₃, the quantity of [GaO₄] tetrahedral units increases. Substitution of Ga₂O₃ for Al₂O₃ results in a decrease in T _g T _x, and T _p, and an increase in the thermal stable index ΔT. Similarly, the absorption band around 3.0 μm obviously reduces and the transparency in 4.0-6.0 μm rapidly increases with increasing Ga₂O₃ content. However, the chemical stability of calcium aluminate glasses decreases if Ga₂O₃ is introduced due to the increasing of [GaO₄] units in the glass network.     

Effect of Cu-Ti-C reaction composition on reinforcing particles size of TiC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Cu composites

TiC _x /Cu composites were fabricated by combustion synthesis and hot press technology. Using XRD, SEM, EDS, FESEM analysis methods, the effects of various carbon sources and different Cu contents on the microstructures of TiC _x /Cu composites and the size of TiC _x particles were investigated. Results showed that TiC _x reinforcing particles size increases with decreasing Cu content in Cu-Ti-C reaction system. With carbon nanotubes (carbon black) serving as carbon source, the generated TiC _x particles size transits from nanometer to submicron when Cu content corresponding to the reaction system is reduced to 60 vol% (70 vol%); while graphite serves as carbon source, there is no clear limiting concentration. C particles with smaller size, larger specific surface area and better distribution result in finer TiC _x particles, which is more beneficial to generating nano-sized TiC _x /Cu composites.     

Spontaneous Magnetic Transitions and Corresponding Magnetoelastic Properties of Intermetallic Compounds <Emphasis Type="Italic">R</Emphasis>Mn<Subscript>2</Subscript>Ge<Subscript>2</Subscript> (<Emphasis Type="Italic">R</Emphasis>=Gd,Tb and Dy)

The spontaneous magnetic transitions and corresponding magnetoelastic properties of intermetallic compounds RMn₂Ge₂ (R=Gd, Tb and Dy) were investigated by using the X-ray diffraction method and magnetic measurement. The results showed that the compounds experience two magnetic transitions, namely the second-order paramagnetic to antiferromagnetic transition at temperature T_N (T_N=368, 423 and 443 K for GdMn₂Ge₂, TbMn₂Ge₂ and DyMn₂Ge₂, respectively) and the first-order antiferromagnetic - ferrimagnetic transition at temperature T_t (T_t=96, 80 and 40 K for GdMn₂Ge₂, TbMn₂Ge₂ and DyMn₂Ge₂, respectively) as the temperature decreases. The temperature dependence of the lattice constant a(T) displays a negative magnetoelastic anomaly at the second-order transition point T_N and, at the first-order transition T_t, a increases abruptly for GdMn₂Ge₂ and TbMn₂Ge₂, Δa/a about 10^-3. Nevertheless, the lattice constant c almost does not change at these transition points indicating that such magnetoelastic anomalies are mainly contributed by the Mn-sublattice. The transitions of the magnetoelastic properties are also evidenced on the temperature dependence of magnetic susceptibility χ. The first-order transition behavior at T_t is explained by the Kittel mode of exchange inversion.     

Preparation and piezoelectric properties of CuO-added (Ag<Subscript>0.75</Subscript>Li<Subscript>0.1</Subscript>Na<Subscript>0.1</Subscript>K<Subscript>0.05</Subscript>)NbO<Subscript>3</Subscript> lead-free ceramics

CuO-doped (Ag_0.75Li_0.1Na_0.1K_0.05)NbO₃ (ALNKN-xCuO, x = 0–2mol%) lead-free piezoelectric ceramics were prepared by the solid-state reaction method in air atmosphere. The effects of CuO addition on the phase structure, microstructure, and piezoelectric properties of the ceramics were investigated. The experimental results show that the ALNKN ceramics without doping CuO possess rhombohedral phase along with K2Nb6O16-type phase and metallic silver phase. For all of the CuO-doped ALNKN ceramics, a pure perovskite structure with the orthorhombic phase was obtained by enclosing the samples in a corundum tube. A homogeneous microstructure with the grain size of about 1 μm was formed for the ceramics with 0.5mol% CuO. The grain size increases with increasing amount of CuO. The temperature dependence of dielectric properties indicates that the ferroelectric phase of the ALNKN-xCuO ceramics becomes less stable with the addition of CuO. The ceramics with x = 1mol% exhibit relatively good electrical properties along with a high Curie temperature. These results will provide a helpful guidance to preparing other AN-based ceramics by solid-state reaction method in air atmosphere.     

Microstructure and growth kinetics of silicide coatings for TiAl alloy

In order to improve the oxidation resistance of Ti Al alloy, silicide coatings were prepared by pack cementation method at 1273, 1323, and 1373 K for 1-3 hours. Scanning electron microscopy(SEM), energy dispersive spectrometry(EDS) and X-ray diffraction(XRD) were employed to investigate the microstructures and phase constitutions of the coatings. The experimental results show that all silicon deposition coatings have multi-layer structure. The microstructure and composition of silicide coatings strongly depend on siliconizing temperatures. In order to investigate the rate controlling step of pack siliconizing on Ti Al alloy, coating growth kinetics was analyzed by measuring the mass gains per unit area of silicided samples as a function of time and temperature. The results showed that the rate controlling step was gas-phase diffusion step and the growth rate constant(k) ranged from 1.53 mg~2/(cm~4·h~2) to 2.3 mg~2/(cm~4·h~2). Activation energy(Q) for the process was calculated as 109 k J/mol, determined by Arrhenius' equation: k = k0 exp[–Q/(RT)].