Anisotropy of Hc2 in Rb0.2WO3

The T_c and H_c2 of hexagonal single crystals Rb_0.2WO₃ were measured. Upper critical field rotation data in the plane perpendicular to the c-axis showed a large anisotropy with a 60° periodicity. Rotation in a plane containing the c-axis showed an even larger anisotropy having 180° symmetry.     

Determination of proton diffusion anisotropy by thermal decay of fixed holograms with K-vector perpendicular to the c-axis in LiNbO3:Fe

In this paper the thermal fixing of holographic gratings with K-vectors perpendicular to the crystallographic c-axis of LiNbO₃ is considered in order to obtain information about anisotropy of the proton thermal diffusion in this crystal. Specifically, thermal decays of fixed holograms in particular crystallographic directions are measured and related with proton diffusion. The values obtained are compared with previous data of decays of fixed holograms with K-vector parallel to the c-axis. The results show a high anisotropy of the thermal diffusion of protons in lithium niobate crystals.     

室温制备低电阻率高透过率H,W共掺杂ZnO薄膜

采用脉冲直流磁控溅射法,以WO3:ZnO陶瓷靶为溅射靶材,通过在溅射气氛中引入H2的方式,在室温条件下制备了低电阻率、高可见和近红外光区透过率的H,W共掺杂ZnO (HWZO)薄膜.系统地研究了H2流量对所制备的HWZO薄膜的结构、组分、元素价态、光电特性的影响.结果表明:掺入的H可促进Zn的氧化,改善薄膜的结晶质量,提高薄膜透过率.H引入之后薄膜的载流子浓度增加,电阻率降低.在H2流量为6mL/min时制备的HWZO薄膜性能最优,电阻率为7.71×10-4 Ω·m,光学带隙为3.58 eV,400-1100 nm的平均透过率为82.4％.     

Correlation between lithium storage and diffusion properties and electrochromic characteristics of WO3 thin films

As essential electrochromic(EC) materials are related to energy savings in fenestration technology,tungsten oxide(WO3) films have been intensively studied recently.In order to achieve better understanding of the mechanism of EC properties,and thus facilitate optimization of device performance,clarification of the correlation between cation storage and transfer properties and the coloration performance is needed.In this study,transparent polycrystalline and amorphous WO3 thin films were deposited on SnO2:F-coated glass substrates by the pulsed laser deposition technique.Investigation into optical transmittance in a wavelength range of 400-800 nm measured at a current density of 130 μA·cm-2 with the applied potential ranging from 3.2 to 2.2 V indicates that polycrystalline films have a larger optical modulation of ～ 30% at 600 nm and a larger coloration switch time of 95 s in the whole wavelength range compared with amorphous films(～ 24% and 50 s).Meanwhile,under the same conditions,polycrystalline films show a larger lithium storage capacity corresponding to a Li/W ratio of 0.5,a smaller lithium diffusion coefficient(2×10-12cm2·s-1 for Li/W=0.24) compared with the amorphous ones,which have a Li/W ratio of 0.29 and a coefficient of ～2.5×10-11cm2·s-1 as Li/W=0.24.These results demonstrate that the large optical modulation relates to the large lithium storage capacity,and the fast coloration transition is associated with fast lithium diffusion.     

Investigation of electrochromic tungsten trioxide thin films prepared by the ILGAR method

An ion layer gas reaction dip coating process for the deposition of tungsten trioxide has been developed. Thin films of electrochromic tungsten trioxide with thicknesses of up to 150 nm were prepared. The films were found to be microcrystalline by X-ray diffraction analysis. The growth rate of the films was measured by profilometry. The chemical diffusion coefficient of lithium was investigated as a function of the concentration of lithium by the electrochemical galvanostatic intermittent titration technique. The chemical diffusion coefficient was found to increase slightly from 7×10⁻¹² to 3×10⁻¹ cm²/s, with x increasing from 0.2 to 0.8 in Li _x WO₃.     

Magnetic properties of strontium hexaferrite films prepared by pulsed laser deposition

The magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films fabricated by pulsed laser deposition on the Si(100) substrate with Pt(111) underlayer have been studied as a function of film thickness (50–700 nm). X-ray diffraction patterns confirm that the films have c-axis perpendicular orientation. The coercivities in perpendicular direction are higher than those for in-plane direction which indicates the films have perpendicular magnetic anisotropy. The coercivity was found to decrease with increasing of thickness, due to the increasing of the grain size and relaxation in lattice strain. The 200 nm thick film exhibits hexagonal shape grains of 150 nm and optimum magnetic properties of M_s=298 emu/cm³ and H_c=2540 Oe.     

Structure and electrical conduction of WO3 nanorods epitaxially grown on mica

Tunsten oxide nanorods are grown on mica substrate in air from WO₃ vapor at 590 °C. They are epitaxially oriented on the substrate in three directions according to the hexagonal symmetry of the mica. Their morphology was investigated by Atomic Force Microscopy (AFM), their structure by Electron Diffraction (ED) and High Resolution Electron Microscopy (HRTEM). The energy dispersive X-ray analysis (EDX) associated with the TEM revealed the presence of potassium in the first step of the nanostructures growth suggesting the formation of a tungsten bronze. According to their thickness the nanorods have a structure either hexagonal or monoclinic. The structural investigations showed that numerous kinds of planar defects parallel to the growth direction are formed. The electrical conduction was analyzed with a Conductive Atomic Force Microscopy (CAFM) method which allows to obtain either an image of the resistance variations along the nanorods or a current-voltage response. The result demonstrates that the WO₃ nanorods form electrically networks suitable for gas sensing experiments.     

Growth, structure and electrical conduction of WO3 nanorods

We present a very simple method to obtain tungsten trioxide nanorods. The nanorods are epitaxially grown on a mica substrate in low supersaturation conditions. Investigations of morphology, crystallographic structure and chemical composition of the nanorods allow us to propose a growth model in which the potassium ions of the substrate play a major role inducing the one-dimensional structure. The nanorod growth is initiated by the formation of a hexagonal tungsten bronze (HTB) epitaxially oriented on the mica. By using a conductive atomic force microscopy technique, we characterise the electrical conduction of WO₃ networks.     

Microstructural and electrochromic properties of tungsten oxide thin films produced by surfactant mediated electrodeposition

By electrochemically controlling the structure of the surface aggregates, the grain microstructure has been optimized to yield mesoporous thin films of tungsten oxide (WO₃) at the electrode-electrolyte interface in a peroxotungstate sol in the presence of a structure-directing agent (Triton) at room temperature. Apart from the dominant ultrafine nanocrystallites and pores (5-10 nm), well-developed abutting grains (25-100 nm) and nanofibrils also constitute an integral part of the film matrix. X-ray photoemission spectra reveal the as-deposited film (WO_3−x) to be constituted by a high proportion of W⁶⁺ states with a low oxygen deficiency (x = 0.02). A relatively high W⁵⁺ content in the film, upon intercalation of 18 mC cm⁻² charge translates into a large coloring efficiency (η_VIS ∼ 70 cm² C⁻¹) and transmission modulation. At a lithium intercalation level of 22 mC cm⁻², in addition to W⁵⁺ and W⁶⁺ states, the film also comprises of W⁴⁺ states. The extremely fast color-bleach kinetics (3 and 2 s, respectively, for a 50% change in transmittance) shown by the as-deposited WO₃ film are repercussions of the mesopore morphology, the multiple nanostructures and the sixfold channels of its hexagonal modification. The film shows a high cycling stability as the switching times do not show any significant decline even after 3500 repetitive cycles. Coloration efficiency over the solar and photopic regions and current density for lithium intercalation for the as-deposited film are superior to that observed for the films annealed at 100, 250 and 500 °C. The abysmal electrochromic response of the annealed films is a consequence of surface defects like cracks and uncontrolled densification and pore shrinkage.     

Effect of oxygen pressure on microstructure and magnetic properties of strontium hexaferrite (SrFe12O19) film prepared by pulsed laser deposition

The effects of oxygen pressure during deposition on microstructure and magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films grown on Si (100) substrate with Pt (111) underlayer by pulsed laser deposition have been investigated. X-ray diffraction pattern confirms that the films have c-axis perpendicular orientation. The c-axis dispersion (Δθ₅₀) increases and c-axis lattice parameter decreases with increasing oxygen pressure. The films have hexagonal shape grains with diameter of 150-250 nm as determined by atomic force microscopy. The coercivities in perpendicular direction are higher than those in in-plane direction, which shows the films have perpendicular magnetic anisotropy. The saturation magnetization and anisotropy field for the film deposited in oxygen pressure of 0.13 mbar are comparable to those of the bulk strontium hexaferrite. Higher oxygen pressure leads to the films having higher coercivity and squareness. The coercivity in perpendicular and in-plane directions of the film deposited in oxygen pressure of 0.13 mbar are 2520 Oe and 870 Oe, respectively.     

Annealing induced microstructural evolution of electrodeposited electrochromic tungsten oxide films

A significant influence of microstructure on the electrochromic and electrochemical performance characteristics of tungsten oxide (WO₃) films potentiostatically electrodeposited from a peroxopolytungstic acid (PPTA) sol has been evaluated as a function of annealing temperature. Powerful probes like X-ray diffractometry (XRD), transmission electron microscopy (TEM), UV-vis spectrophotometry, multiple step chronoamperometry and cyclic voltammetry have been employed for the thin film characterization. The as-deposited and the film annealed at 60 °C are composed of nanosized grains with a dominant amorphous phase, as well as open structure which ensues from a nanoporous matrix. This ensures a greater number of electroactive sites and a higher reaction area thereby manifesting in electrochromic responses superior to that of the films annealed at higher temperatures. The films annealed at temperatures ≥250 °C are characterized by a prominent triclinic crystalline structure and a hexagonal phase co-exists at temperatures ≥400 °C. The deleterious effect on the electrochromic properties of the film with annealing is ascribed to the loss of porosity, densification and the increasing crystallinity and grain size. Amongst all films under investigation, the film annealed at 60 °C exhibits a high transmission modulation (ΔT ∼ 68%) and coloration efficiency (η ∼ 77.6 cm² C⁻¹) at λ = 632.8 nm, charge storage capacity (Q_ins ∼ 21 mC cm⁻²), diffusion coefficient (6.08 × 10⁻¹⁰ cm² s⁻¹), fast color-bleach kinetics (t_c ∼ 275 s and t_b ∼ 12.5 s) and good electrochemical activity, as well as reversibility for the lithium insertion-extraction process upon cycling. The remarkable potential, which the film annealed at 60 °C has, for practical “smart window” applications has been demonstrated.     

Metamagnetism in ErGa2 studied on a single crystal

Magnetization measurements and neutron difrraction experiments were performed on a single crystal of ErGa₂. Because of the strong crystal field effects a huge uniaxial anisotropy imposes the Er moments to be parallel to the c-axis of the hexagonal structure. We have shown that the transition from the antiferromagnetic structure to the ferromagnetic one, when the field is applied along c, occurs through an intermediate phase. Such a behaviour is very well accounted for using a simple Ising-like model which takes into account first and second nearest neighbours exchange interactions in the hexagonal layers. A quantitative determination of the crystalline electric field effects is also presented.     

X-ray study of the thermal expansion anisotropy in the quarternary semiconductor CuGaSn□Se4

Accurate lattice parameters a and c of the tetragonal chalcopyrite quaternary semiconductor CuGaSn□Se₄ have been determined as a function of temperature by the X-ray powder diffraction method in the temperature range 300 K to about 900 K. The data have been used to evaluate the axial expansion coefficients α_a and α_c at various temperatures. The thermal expansion studies revealed the anisotropy between the axial expansion coefficients having a larger coefficient of expansion along the a-axis than that along the c-axis (α_a > α_c). The mean values α_a and α_c, in the temperature range 300–900 K, are found to be 14.02 × 10^-6K^-1 and 5.02 × 10^-6K^-1 respectively, and the axial ratio, c/a, changes with a coefficient of -8.96 × 10^-6K^-1. This result indicates an increase in the tetragonal distortion, δ = 2 - c/a with temperature. An attempt is made to explain the increase in tetragonal distortion with temperature and the anisotropic thermal expansion of CuGaSn□Se₄ in terms of the thermal expansion of the A>−;Se (where A is Cu and Ga randomly distributed) and B>−;Se (where B is Sn and vacancy randomly distributed) bonds. The results are also discussed in terms of the principal Grüneisen parameters of chalcopyrite structure compounds.     

Growth,spectral and laser properties of Nd3+:Na2La4(WO4)7 crystal

A Nd³⁺:Na₂La₄(WO₄)₇ crystal with dimensions of ? 17 × 30 mm³ was grown by the Czochralski method. The thermal expansion coefficients of Nd³⁺:Na₂La₄(WO₄)₇ crystal are 1.32 × 10^?5 K^?1 along c-axis and 1.23 × 10^?5 K^?1 along a-axis, respectively. The spectroscopic characteristics of Nd³⁺:Na₂La₄(WO₄)₇ crystal were investigated. The Judd–Ofelt theory was applied to calculate the spectral parameters. The absorption cross sections at 805 nm are 2.17 × 10^?20 cm² with a full width at half maximum (FWHM) of 15 nm for π-polarization, and 2.29 × 10^?20 cm² with a FWHM of 14 nm for σ-polarization. The emission cross sections are 3.19 × 10^?20 cm² for σ-polarization and 2.67 × 10^?20 cm² for π-polarization at 1,064 nm. The fluorescence quantum efficiency is 67 %. The quasi-cw laser of Nd³⁺:Na₂La₄(WO₄)₇ crystal was performed. The maximum output power is 80 mW. The slope efficiency is 7.12 %. The results suggest Nd³⁺:Na₂La₄(WO₄)₇ crystal as a promising laser crystal fit for laser diode pumping.     

Field-emission properties of transparent tungsten oxide nano-urchins

The field-emission properties of transparent tungsten oxide nano-urchin (NU) films deposited on conducting glass substrates were examined. The novel crystalline tungsten oxide NUs consisted of nanowires added to a spherical shell. The WO_2.72 NUs showed better field-emission properties than the WO₃ NUs with a low turn-on field of approximately 5.8 V/μm and a current density as high as 1.3 mA/cm² at 7.2 V/mm. The WO _x NUs films could be used in FE applications using a large-area glass substrate without the need for a catalyst and a mechanical rubbing or lift-up process. These results have implications for the enhancement of FE properties by further tuning the WO _x phases.     

Dependence of magnetic anisotropy of the La0.67Ca0.33MnO3 films on substrate and film thickness

Strain in the La_0.67Ca_0.33MnO₃ films has been tuned by varying substrate and film thickness, and its effects on magnetic anisotropy are studied based on the measurements of isothermal magnetization. Measuring the strain in the films by the out-of-plane lattice parameter (c), we found a strong dependence of the magnetic anisotropy constant (Ku) on strain. Ku decreases linearly from ∼−1.1×10⁶ erg/cm³ for c=0.763 nm to 1.2×10⁶ erg/cm³ for c=0.776 nm, corresponding to a change from tensile strain to compressive strain. Positive Ku signifies a uniaxial anisotropy with the easy axis perpendicular to the film plane, while negative Ku demonstrates an anisotropy of the easy plane character. Smaller or larger c leads a decrease or increase in Ku, which indicates the presence of other effects in addition to those associated with strain. Three distinctive processes for the magnetization are observed along the hard magnetic axis of the films on (001)SrTiO₃, suggesting a possibility of strain relaxation even in ultra-thin films.     

Anisotropy of the magnetocaloric effect in DyNiAl

We present study of the anisotropic magnetocaloric effect in DyNiAl. This compound crystallizes in the hexagonal ZrNiAl-type structure, orders magnetically below and undergoes a further magnetic phase transition at . The Dy-moments are aligned ferromagnetically along the hexagonal c-axis below T_C, the additional antiferromagnetic component develops within the basal plane below T₁. The magnetocaloric effect was evaluated from the magnetization measurements with field applied along the c-axis and perpendicular to it. Our data reveal a strong anisotropy of the magnetocaloric effect. The large effect occurs for field applied along the c-axis whereas the entropy change is small for the perpendicular field direction.     

Highly uniform electrochromic tungsten oxide thin films deposited by e-beam evaporation for energy saving systems

In the last few decades, there has been a surge of interest in using tungsten oxide thin films as an active layer of electrochromic device. These devices have several practical applications such as smart window of buildings and automobile glazing for energy saving. The main objective of this work was to construct highly homogeneous and uniform e-beam evaporated amorphous WO_3-x based films into electrochromic devices, which were fully characterized for switching speed, coloration efficiencies and cycling voltammetry responses. Fabricated devices contain indium doped transparent oxide coated glass as the transparent conductive electrode, ～200?nm thickness of WO_3-x as the cathodically coloring material and a lithium perchlorate based conducting gel electrolyte. X-ray diffraction patterns indicate that all as-deposited films are amorphous. Experimental results showed that both solid and liquid electrolyte electrochromic devices are initially very transparent that exhibit perfect optical modulation and coloration efficiency (up to 68.7?cm²/C and 52.6?cm²/C at 630?nm, respectively) due to easier intercalation of the Li⁺ within their structure. One of the more significant findings to emerge from this study is that e-beam coated electrochromic devices based on tungsten oxide thin films showed superior performance among to other coating methods. Therefore, excellent reversibility of color change behavior is attractive for pertinent use in electrochromic energy storage devices.     

Collective excitations in the 1-D-ferromagnet CsFeCl3 with singlet ground state

By means of inalastic neutron scattering we have determined the dispersion relation of the magnetic excitations in CsFeCl₃ at different temperatures.The dispersion in c-direction, along the Fe-chains is typically ferromagnetic and in the hexagonal plane antiferromagnetic. Due to the lack of an applicable theory the data were parametrized by the simple heuristic formula:?ω = [2J[1 - cos πq_c] [A + 2J(1 - cos π)q_c)] + [C + J' (1.5 + γ(q))]²]^{$\text{1}\text{2}$}The gap was found to be C = 0.148 THz, the easy plane anisotropy A = 0.308 THz, the ferromagnetic interaction J = 0.148 THz and the antiferromagnetic interaction J' = -0.04 THz. At 1.25 K all excitations had a width smaller than the instrumental resolution ΔE = 0.025 THz. These results can be interpreted as follows: CsFeCl₃ is a singlet ground state system with strong ferromagnetic interaction J along the crystallographic c-axis and weak antiferromagnetic interaction J' in the plane perpendicular to c.In addition we have measured the influence of a magnetic field along the hexagonal c-axis. The splitting found agrees with the assumed level scheme yielding g = 2.5 for the first excited level.     

Thermochemistry of the cubic and hexagonal lithium tungsten bronze phases LixWO3

The enthalpies of formation of hexagonal WO₃, hexagonal and cubic Li_xWO₃ have been determined by solution calorimetry. The ambient-temperature materials h-Li_0.44WO₃ and c-Li_xWO₃ (x=0.51, 0.71) were prepared by n-butyl lithiation in hexane. The thermochemistries of the lithium insertion compounds were examined and related to structural features.