Synthesis and characterization of holmium doped lithium niobate powders

Holmium doped lithium niobate (LN:Ho) powders with initial concentration of holmium in the range 0–7 mol% were synthesized by the ceramic powder processing method. Niobium penta-oxide Nb₂O₅, lithium carboxide LiCO₃ and holmium oxide Ho₂O₃ with a purity of 99.99% were the starting materials. The phase content and lattice parameters of powders and ceramic pellets were characterized by X-ray diffraction (XRD). To further investigate the quality of the synthesized LN:Ho powders, the scanning electron microscopy (SEM) was used to determine the particle size and the morphology. The main results of this work point out the fact that the ceramic powder processing method is a well adapt method for obtaining high quality LN:Ho ceramics in the holmium concentration range analysed as the LiNbO₃ phase is lonely present in the ceramics at the end of the synthesized process and as their grain sizes are regular, with a maximum for the sample doped with 7 mol% of holmium.     

Composition dependence of the electro-optic properties of iron-doped lithium niobate crystals mounted as bulk modulator

In this contribution, we report a complete set of electro-optic coefficients of iron-doped lithium niobate as function of the composition varying from sub-congruent to near-stoichiometric in a series of four crystals. The concentration of iron is constant for all crystals of the series. The electro-optic measurements of the r₂₂ and r_c coefficients were performed on a Sènarmont ellipsometric setup while those of r₁₃ and r₃₃ were performed on a Mach-Zehnder interferometric setup, by adaptation of the same frequency doubling electro-optic modulation and the MDM methods implemented in both electro-optical arrangements. The corresponding dielectric permittivities ε₁₁ and ε₃₃ as a function of composition were also determined. All electro-optic coefficients reveal a small change with composition, but significant above the experimental error. The nonmonotonous dependences of all electro-optic coefficients are found to be closely linked to their corresponding dielectric permittivities.     

Nanometer-sized etching of lithium niobate domain wall devices

Ferroelectric LiNbO₃ single crystals have piezoelectric, pyroelectric and electro-optic properties and are widely used in nonvolatile memories, sensors, and electro-optic modulators. The material is hard and relatively inert for dying etching using thick Cr/Ni mask layers which are incompatible with the CMOS manufacturing technologies. Moreover, the etching angles are generally below 80° that could result in poor polarization retention and a large imprint effect. Here we developed an etching technique on the basis of the solid-state reaction and wet solution corrosion together that used CMOS-compatible SiO₂ masks to fabricate large arrays of LiNbO₃ nanodevices in high precision. The LiNbO₃ nanopatterns at the regions beyond the SiO₂ masks can react with the deposited Al metal below 500 °C in formation of amorphous layers in targeted thicknesses that vary with the annealing temperature and time at a reduced atmosphere. The amorphous layers can be dissolved in mixed solutions of H₂O, NH₄OH, and H₂O₂ at 85 °C, and the etching angles of memory cells in lateral sizes of 20–220 nm can approach 90° after the passivation of the LiNbO₃ surface layer. The transmission electron microscope study shows the oxygen deficiency at the surface. After the connection of the cell to two side electrodes, domain switching occurs under an applied horizontal electrode field along the polar Z-axis accompanying the formation of conducting domain walls against the peripheral unswitched domain at the substrate. The read current can highly reach the level of 2–20 μA in high reliability. This etching technique promotes the large-scale fabrication of LiNbO₃ nanodevices on the Si platform.     

Optical properties of ferroelectric lanthanum lithium niobate

Fine structures (ferroelectric domains), ferroelectricity and Second Harmonic Generation results were found and studied as a function of laser linearly and circularly polarization dependent polarized excitations in ferroelectric stoichiometric La_0.05Li_0.85NbO₃ nanocrystals ceramic material. Scanning Electron Microscope images are taken as a comparison to Second Harmonic Generation intensity profiles revealed fine structures. By using laser polar measured response we are able to find the angle orientation from 0° to 90° angles of ferroelectric domains with highly good definition contrast obtained in blue/gray colors. It shows ferroelectric hysteresis loops at room temperature with a polarization saturation of (0.247 μC/cm²), remnant polarization of (0.15 μC/cm²) and coercitivity field of (1.31 kV/cm). X-ray Diffraction, Atomic Force Microscope, Raman spectroscopy and X-ray Photoelectron Spectroscopy, revealed well formed of ferroelectric ABO₃ perovskite crystal structure, piezoelectric image response indicate ferroelectric pattern domain structure, new vibrations modes on LaO₆, LiO₆ and NbO₆ octahedral sites and binding energies of electronic structure of La⁵⁷, Nb⁴¹, O⁸, Li³ from the surface of the ferroelectric stoichiometric La_0.05Li_0.85NbO₃ nanocrystals ceramic material, respectively.     

Effect of point defects on Curie temperature of lithium niobate

The effect of point defects on the Curie temperature (T_c) of LiNbO₃ (LN) was investigated by combining T_c measurements with an analysis of the defect structures of LN doped with impurities having various valences. The data show that T_c of congruent LN increases with the impurity concentration up to around 3 and 2 mol% for divalent and trivalent impurities, respectively, whereas it decreases continuously with increased concentrations of tetravalent impurities. These T_c variations were examined with respect to the defect structures of impurity‐doped LN, which are expressed in the form chemical formulae using Kröger‐Vink notation. The defect structures of divalent and trivalent impurity‐doped LN are and , respectively (Nb_Li: Nb at Li sites; V_Li: vacancies at Li sites, M_Li: impurities at Li sites, and Li/Nb = the congruent ratio). The defect structure in the case of tetravalent impurities is . Analyses of the defect structures indicated that the Nb_Li concentration decreases with divalent or trivalent impurity doping, which increases T_c. In contrast, the Nb_Li concentration increases with tetravalent impurity doping, which decreases T_c. In addition, the divalent or trivalent impurity concentrations at which the Nb_Li concentration becomes zero were found to correspond to the concentrations at which T_c is maximized, suggesting that T_c of LN depends on the Nb_Li concentration.     

Trap-induced self-recoverable photochromism of rare-earth doped sodium niobate translucent ceramics

Most photochromic ceramics can back up post-irradiation to achieve color state transition. Unfortunately, color recovery usually requires exposure of material to external physical fields (such as light and heat), which severely limits the application as a convenient energy building material such as smart windows. Here, we report a kind of sodium niobate translucent ceramics whose photochromic effect can be completely self-recoverable without any external physical field stimulation. Based on the analysis of time-response transmittance, up-conversion emission spectra and thermoluminescence spectra, the behavior of carrier migration and trap capture/release in the process of self-recovery photochromic reaction has been proposed. It is believed that the emergence of intermediate trap level is an important factor to produce this spontaneous behavior. Moreover, by controlling the amount of rare earth doping and designing two types of materials with different carrier behaviors, Sm/Er-codoped NaNbO₃ ceramics are regarded as an appropriate candidate for tracing the time of light irradiation and intelligent regulation of smart building materials. This work can promote the fundamental understanding and practical applications of self-recoverable photochromic bulk materials.     

Electro‐Optic Property of Mg2+/Er3+‐Codoped LiNbO3 Crystal: Mg2+ Concentration Threshold Effect

A series of Mg²⁺/Er³⁺‐codoped congruent LiNbO₃ crystals were grown by Czochralski method from the growth melts containing 0.5 mol% Er₂O₃ while varied MgO content from 0.0 to 7.0 mol%. The unclamped electro‐optic coefficients γ₁₃ and γ₃₃ of these crystals were measured by Mach–Zehnder interferometry. Two different voltage‐applying schemes were adopted: one is the DC voltage applied to the crystal via Al films coated onto crystal surfaces and another is via a pair of external Cu slab electrodes. The coefficients measured by the two schemes show similar strong dependence on Mg²⁺ concentration. The dependence is non‐monotonous, dramatic, and unusual, and reveals the features of two Mg²⁺ concentration thresholds of optical damage: one in the Mg²⁺ concentration range of 1.2–2.0 mol% (in crystal) and another in 4.5–5.0 mol%. Around the threshold the electro‐optic coefficient decreases abruptly at first and then recovers quickly, and the coefficient drops by >20% (12%) at the first (second) threshold, which exceeds the error 3% considerably. The dramatic behavior is qualitatively explained on the basis of the EO coefficient model of LiNbO₃ and the defect structure model for Mg²⁺‐doped LiNbO₃.     

硒掺杂改性新能源汽车锂电池正极材料的结构与电化学性能

为了开发出具有能量密度大、循环性能优越等特性的高能量密度电池材料,研究了硒掺杂量对富锂锰基正极材料显微组织和电化学性能的影响。结果表明,硒掺杂量增加有助于减小正极材料中颗粒粒径,但是硒掺杂量过高（x=0.21）会出现严重颗粒团聚现象,锂电池正极材料中适宜的硒掺杂量为x=0.14,此时正极材料可以获得粒径细小、均匀的颗粒;x=0.14的正极材料由于具有最佳的抑制氧损失的作用而具有最高的库伦效率（77.1%）;当倍率为0.1C~10C时,正极材料的放电比容量会随着硒含量升高而先增大后减小,在x=0.14时取得最大值,即x=0.14的正极材料的倍率性能最优;x=0.14的正极材料的循环性能明显高于x=0的正极材料。     

Electric properties of alkali metal doped potassium niobate crystals

Alkali metal (Na, Rb or Cs) doped KNbO₃ single crystals are grown using an original pulling down method by means of co-doping with Na (small ionic size), Rb (large ionic sizes) or Cs (large ionic sizes) into KNbO₃. Single-phase crystals are successfully grown with orthorhombic system at room temperature, for all the pure and doped KNbO₃. Their electric properties, such as the dielectric constant and the impedance, are found to be changed according to the co-doping elements.     

Structure and conductivity of yttria and scandia‐doped zirconia crystals grown by skull melting

In this paper a detailed study of the (ZrO₂)_1‐x(Y₂O₃)_x (x=0.025–0.15), (ZrO₂)_1‐x(Sc₂O₃)_x (x = 0.06 – 0.11) and (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y (x=0.07 – 0.11; y=0.01 – 0.04) solid solution crystals grown by skull melting technique is presented. The structure, phase composition, and ion conductivity of the obtained crystals were investigated by X‐ray diffraction, transmission electron microscopy, Raman scattering spectroscopy, and impedance spectroscopy. Maximum conductivity as (ZrO₂)_1‐x(Y₂O₃)_x and (ZrO₂)_1‐x(Sc₂O₃)_x solid solution crystals is observed for the compositions containing 10 mol% stabilizing oxide, and the conductivity of 10ScSZ is ~3 times higher than for 10YSZ. Experiments on crystal growth (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y solid solutions showed that uniform, transparent crystals 7Sc3YSZ, 7Sc4YSZ, 8Sc2YSZ, 8Sc3YSZ, 9Sc2YSZ, 9Sc3YSZ, 10Sc1YSZ, and 10Sc2YSZ are single phase crystal containing t″ phase. It is established that a necessary condition of melt growth of (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y single‐phase crystals is the total concentration of the stabilizing oxides from 10 to 12 mol%. The addition of Y₂O₃ affects the (ZrO₂)_1‐x‐y(Sc₂O₃)_x(Y₂O₃)_y solid solution conductivity different ways and depends on the Sc₂O₃ content in the starting composition. The effects of structure, phase composition, concentration, and type of stabilizing oxides on the electrical characteristics of obtained crystals are discussed.     

Thermoelectric properties and phase transition of doped single crystals and polycrystals of Bi2Te3

The temperature dependences of the electrical conductivity , Seebeck coefficient , and heat capacity C_p(T) of polycrystalline samples of Bi₂Te₃, Bi₂Te₃+1%CuI, and Bi₂Te₃+1%(CuI+1/2Pb) are investigated in the temperature range below room temperature. Based on the temperature dependences of all investigated physical properties, it is discovered that phase transition occurs at 120–200 K. Investigation of single crystals shows that anomalies in the electrical resistivity occur only across the crystal growth axis (across the well-conducting Bi–Te plane). Investigation of the low-temperature dependence of electrical conductivity shows that all polycrystalline samples exhibit quasi-two-dimensional electron transport. Additionally, quasi-two-dimensional transport is detected in single crystals based on anisotropy analysis (where is the resistivity along the crystal growth axis, and is resistivity across the crystal growth axis) and temperature dependence below 50 K. The Fermi energy is estimated using the temperature dependence of . It is discovered that an increase in at T > 200 K is associated with the phase transition. For single-crystal samples, the maximum thermoelectric figure of merit ZT, as observed along the crystal growth axis, increases with doping. A maximum ZT value of ∼1.1 is observed for the Bi₂Te₃+1%(CuI+1/2Pb) sample at room temperature ().     

Synthesis of nanocrystalline lithium niobate powders via a fast chemical route

Lithnium niobate (LiNbO₃) can be obtained by mixing lithium nitrate (LiNO₃), ammonium niobate oxalate hydrate (C₄H₄NNbO₉) and glycine and then calcining at 600 °C for 1 h. The thermal analysis, structure, and morphology of the as-prepared LiNbO₃ were characterized by thermogravimetric and differential thermal analyses (TG/DTA), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The crystallization temperature of LiNbO₃ precursor is 580 °C based on the TG/DTA results. After being calcined at 600 °C, the structure of the LiNbO₃ synthesized using various ratios of glycine to metal nitrates (Ψ-value) was formed with a particle size of about 29-38 nm, as found by XRD analysis. The crystal size has the lowest value at Ψ = 2, and the highest level of crystallization is at Ψ = 3.     

Dielectric and thermal expansion properties of LHPG grown potassium lithium niobate single crystals

Compositions of potassium lithium niobate K₆Li_4+xNb₁₀O₃₀ (x for excess mole % of Li₂O) with tetragonal tungsten bronze structure were prepared as crystal fibers by the Laser Heated Pedestal Growth technique. The lattice constants of KLN were measured by using X-ray diffraction and typical values for a = 12.5703 ± 0.0018 and c = 3.9485 ± 0.0012 were measured. This paper reports the dielectric permittivity measurements on single crystal along the a- and c- axis in the temperature range of 90-450K and frequency range of 10²-10⁵ Hz. The dielectric permittivity at room temperature along a- and c- axis was 430 and 330 respectively. Mechanism for the low temperature dielectric relaxation will be discussed. Thermal expansion behavier in the a- and the c-direction will also be reported.     

Ultrathin N‐doped carbon‐coated TiO2 coaxial nanofibers as anodes for lithium ion batteries

The structural optimization of TiO₂ materials has a significance for improving the electrochemical performance since TiO₂ suffers from poor electronic conductivity. For this purpose, ultrathin N‐doped carbon‐coated TiO₂ coaxial nanofibers have been designed and synthesized by a facile electrospinning approach. Microstructure analysis indicates that the TiO₂ nanofibers can be coated by the ultrathin carbon layers. Electrochemical tests reveal that the rate performance and cycling ability of TiO₂@C nanofibers have been enhanced obviously. The TiO₂@C6 nanofibers carbonized at 600°C exhibit superior features with a specific discharge capacity of 284 mAh g^?1 at a current density of 100 mA g^?1 after 100 cycles. Besides improved rate performance of 117 mAh g^?1 at a high current density of 2000 mA g^?1 and excellent cycling stability with only about 0.008% capacity loss per cycle were also obtained in the sample TiO₂@C6 after 500 cycles at the current density of 1000 mA g^?1. Such remarkable performance may be ascribed to the unique one‐dimensional nanofibers as flexible carbon matrix.     

Dielectric and magnetic properties,NEXAFS spectroscopy of Co-doped of multicomponent bismuth niobate pyrochlore

Two series of Bi₂Mg_1-xCo_xNb₂O₉ and Bi₂MgNb_2-2xCo_2xO_9-δ preparations with pyrochlore structure were synthesized using solid phase method. It was found that solid solutions are formed in Bi₂MgNb_2-2xCo_2xO_9-δ series, foreign phases of MgNb₂O₆ and Bi₅Nb₃O₁₅ are fixed in Bi₂Mg_1-xCo_xNb₂O₉ samples, no impurity cobalt-containing phases were detected. The method of magnetic susceptibility and NEXAFS-spectroscopy investigated the electronic state and character of exchange interactions of cobalt atoms in solid solutions with the pyrochlore structure. According to X-ray spectroscopy and magnetic susceptibility of cobalt atoms in solid solutions in a dominant amount are in the charge state of Co(II) in the form of monomers and exchange-bound clusters mainly with antiferromagnetic type of exchange. Ceramics Co-doping of multicomponent bismuth niobate pyrochlore has at room temperature a high dielectric permeability of 84 and small dielectric losses of 0.001. The energy of activation of conductivity is equal to 0.75–1.03 eV. This type of ceramics with excellent dielectric properties can potentially be used as multilayer ceramic capacitors.     

Spheroidization of lithium niobate powder by radio-frequency inductively coupled plasma

To solve the problem that lithium niobate is difficult to process into microspheres, a method of spheroidizing lithium niobate powder twice by radio-frequency inductively coupled plasma (RF-ICP) was proposed. The morphology, phase composition and particle size of primary and secondary spheroidized lithium niobate powders were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and laser particle size analysis, respectively. The results show that the powder with a larger particle size is difficult to spheroidize and easily cracks, while the powder with a smaller particle size is elliptical without cracks after primary spheroidization. Nearly all spherical lithium niobate powder can be obtained after secondary spheroidization. In addition, the crystal steps grow during the solidification of the powder. The smaller the curvature of the powder surface, the more obvious the crystal growth. Lithium niobate loses part of lithium after spheroidization, and the loss of lithium increases with the decrease of particle size.     

Kinetics of {001} Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 Single Crystals Grown by Seeded Polycrystal Conversion

The kinetics of {001}-oriented Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ (PMN–35PT) single crystals grown by seeded polycrystal conversion were systematically quantified as a function of excess PbO liquid phase. The coarsening behavior of the corresponding matrix grains was similarly quantified. Single-crystal seed plates were embedded in a matrix of PMN-35PT with varying amounts of liquid phase (PbO) content in the range of 0 to 5 vol% and annealed at 1150°C for 0–10 h. Apparent maxima in the growth rates were observed at a PbO content of ∼3 vol% for both the single crystal and matrix grains. In both cases, the growth data were found to most closely follow cubic growth kinetics. Implications regarding the effect of PbO volume fraction on the matrix and single-crystal growth mechanisms are discussed.     

Preparation of lithium niobate optical thin film and patterned microstructure via chemical modification

In this paper, a stable lithium niobate (LN) sol was prepared using niobium pentachloride, niobium ethoxide, lithium acetate, anhydrous ethanol, and benzoylacetone (BzAcH) as a chemical modifier. The effects of different starting materials and organic solvents on the obtained gel film quality of the sols were studied and the optimization mechanism of BzAcH on the film quality was analyzed in detail. The effects of heat treatment temperature on the structure and performance of the LN thin films were also studied. X-ray diffraction (XRD) spectra showed that LN was more highly crystalline in the heat treatment temperature range of 600–800 °C. The refractive index of the LN thin films first increased and then decreased with increasing heat treatment temperature, and the LN thin film prepared at 700 °C had the largest refractive index. Light transmittance tests showed that as the heat treatment temperature increased, light transmittance decreased. Finally, it was determined that LN gel films modified with BzAcH were more highly sensitive to UV light and that after UV irradiation, it was difficult to dissolve LN gel films in organic solvent. The UV-sensitive property was utilized to realize microstructure processing of the LN thin films. The results indicated that a high-quality, non-destructive, and photoresist-free micro-fabrication method was realized. This method was used to fabricate microscale LN fine patterns with smooth sidewalls while avoiding lateral corrosion. These results are of great significance for LN thin film applications and the development of LN-based optoelectronic devices prepared by chemical methods.     

Influence of Excess PbO Additions on {111} Single-Crystal Growth of Pb(Mg1/3Nb2/3)O3–35 mol% PbTiO3 by Seeded Polycrystal Conversion

The influence of additions of excess PbO to Pb(Mg_1/3Nb_2/3)O₃–35 mol% PbTiO₃ (PMN–35PT) on {111} single-crystal growth by seeded polycrystal conversion was studied in the range of 0–5 vol% PbO. PbO volatilization and hence weight loss during annealing was controlled effectively by a double-crucible type of arrangement. PbO additions increased boundary mobility significantly in PMN–35PT, thus facilitating single-crystal growth by seeded polycrystal conversion (SPC). This is attributed to the formation of a boundary wetting PbO-based liquid phase. The growth process occurs very rapidly initially, after which it slows down. This is presumably due to both a decrease in the driving force for boundary migration because of an increase in matrix grain size, and a transition to lower mobility facets. It is also shown that for a given annealing time, the size of the grown crystal scales with the lateral dimensions of the seed crystal.     

锂离子电池材料钛酸锂钠的研究进展

Na₂Li₂Ti₆O₁₄电池具有较低的电位平台（1.3 V）以及经济成本低的特点,对便携式电子设备、能源汽车、生态环境等领域具有重大意义。由于钛酸锂钠电池固有离子电导率低的特点,因此提高钛酸锂钠电池锂离子扩散系数是目前研究中的主流方向,为此综述了钛酸锂钠的结构特点以及合成方法对钛酸锂钠材料粒径、形貌及电池电化学性能的影响;对比了不同掺杂离子和表面包覆改性对钛酸锂钠电池的放电比容量、循环性能及离子扩散系数的影响。掺入适量元素铌具有更高的锂离子扩散系数;包覆碳纳米管有更大的容量保持率,更有助于进一步提高钛酸锂钠电池电化学性能。