Growth and spectroscopic properties of Nd-doped Na2Gd4(MoO4)7 crystal

This paper reports the growth and spectral properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystals. An Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal with dimensions of Ø20 × 25 mm³ has been grown by the Czochralski method. The spectroscopic properties of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal were investigated. The Judd-Ofelt theory was applied to calculate the spectral parameters. The polarized absorption cross-sections of Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal are 4.25 × 10⁻²⁰ cm² with full width at half maximum (FWHM) of 14.6 nm for the π-polarization and 2.87 × 10⁻²⁰ cm² with FWHM of 16.2 nm for the σ-polarization, respectively. The emission cross-sections are 10.0 × 10⁻²⁰ cm² at 1060 nm for π-polarization and 13.6 × 10⁻²⁰ cm² at 1067 nm for σ-polarization, respectively. The fluorescence quantum efficiency has been estimated to be 90.0%. Nd³⁺:Na₂Gd₄(MoO₄)₇ crystal may be considered as a potential laser gain medium for the diode laser pumping.     

α-Fe2O3 nanoplates: PEG-600 assisted hydrothermal synthesis and formation mechanism

Quasi-hexagonal α-Fe₂O₃ nanoplates with lateral sizes of 40-60 nm and thickness of ca. 10 nm were fabricated by a facile poly(ethylene glycol 600) (PEG-600) assisted hydrothermal technique in combination with calcination method. The final α-Fe₂O₃ nanoplates inherited perfectly the morphology of the preliminarily hydrothermal products with phases of dominant α-Fe₂O₃ and minor α-FeOOH. The platelets could be tailored from nano- to meso- and to micro-scale via adjusting PEG-600 quantities. An adsorption-extension-attachment model was proposed to explain the formation and growth mechanism of the platelets. The as-obtained α-Fe₂O₃ nanoplates exhibited a specific Langmuir surface area of 59 m²/g and a maximum N₂ adsorption of 137.3 cm³/g at 1 atm. UV-vis measurement showed a strong absorption in a wide range from UV to visible light with a blue-shifting band gap of 2.33 eV due to the nanosize effect.     

Spectroscopic investigation of Dy-doped Li2Gd4(MoO4)7 crystal for potential application in solid-state yellow laser

Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal with dimensions of ∅20 × 25 mm³ has been grown by the Czochralski method. The spectroscopic properties of Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal have been investigated. Based on the analysis of polarized absorption spectra in the framework of the Judd-Ofelt theory, the main spectroscopic characteristics, including the phenomenological intensity parameters, spontaneous transition probabilities, fluorescence branching ratios and radiative lifetimes of Dy³⁺ in the crystal, have been determined. The emission cross-sections for the ⁴F_9/2 → ⁶H_13/2 transition of special interest for laser application have been calculated using the Füchtbauer-Ladenburg (F-L) equation. The fluorescence and radiative lifetimes of ⁴F_9/2 manifold are equal to 126.5 μs and 201.1 μs, respectively, which mean the quantum efficiency is 62.9%. The results propose the possibility of Dy³⁺:Li₂Gd₄(MoO₄)₇ crystal for solid-state yellow laser pumped by commercially available blue laser diodes.     

Effects of K2O-Li2O doping on surface and catalytic properties of Fe2O3/Cr2O3 system

The effects of K₂O and Li₂O-doping (0.5, 0.75 and 1.5 mol%) of Fe₂O₃/Cr₂O₃ system on its surface and the catalytic properties were investigated. Pure and differently doped solids were calcined in air at 400-600 °C. The formula of the un-doped calcined solid was 0.85Fe₂O₃:0.15Cr₂O₃. The techniques employed were TGA, DTA, XRD, N₂ adsorption at −196 °C and catalytic oxidation of CO oxidation by O₂ at 200-300 °C. The results revealed that DTA curves of pure mixed solids consisted of one endothermic peak and two exothermic peaks. Pure and doped mixed solids calcined at 400 °C are amorphous in nature and turned to α-Fe₂O₃ upon heating at 500 and 600 °C. K₂O and Li₂O doping conducted at 500 or 600 °C modified the degree of crystallinity and crystallite size of all phases present which consisted of a mixture of nanocrystalline α- and γ-Fe₂O₃ together with K₂FeO₄ and LiFe₅O₈ phases. However, the heavily Li₂O-doped sample consisted only of LiFe₅O₈ phase. The specific surface area of the system investigated decreased to an extent proportional to the amount of K₂O and Li₂O added. On the other hand, the catalytic activity was found to increase by increasing the amount of K₂O and Li₂O added. The maximum increase in the catalytic activity, expressed as the reaction rate constant (k) measured at 200 °C, attained 30.8% and 26.5% for K₂O and Li₂O doping, respectively. The doping process did not modify the activation energy of the catalyzed reaction but rather increased the concentration of the active sites without changing their energetic nature.     

Synthesis, structure, and properties of Li2Pb2CuB4O10 with isolated [CuB4O10] units

The copper borate Li₂Pb₂CuB₄O₁₀ has been synthesized in air by the standard solid-state reaction at temperature in the range 550-650 °C and the structure of Li₂Pb₂CuB₄O₁₀ was determined by single-crystal X-ray diffraction. Li₂Pb₂CuB₄O₁₀ crystallizes in the monoclinic space group C2/c (no. 15) with a = 16.8419(12), b = 4.7895(4), c = 13.8976(10) Å, and β = 125.3620(10)°, V = 914.22(12) Å³, and Z = 4, as determined by single-crystal X-ray diffraction. The Li₂Pb₂CuB₄O₁₀ structure exhibits isolated units of stoichiometry [CuB₄O₁₀]⁶⁻ that are built from CuO₄ distorted square planes and triangular BO₃ groups. The IR spectroscopy and thermal analysis investigations of Li₂Pb₂CuB₄O₁₀ are also presented.     

Microstructure and magnetic properties of HVOF thermally sprayed Fe75Si15B10 coatings

Partially amorphous Fe₇₅Si₁₅B₁₀ coatings were prepared from nanostructured feedstock powders by using high velocity oxy-fuel spraying. Scanning electron microscopy, X-ray diffraction, Vickers indenter and magnetic measurements were used to investigate microstructural, structural, microhardness and magnetic properties of the coatings. The Rietveld refinement of the X-ray diffraction patterns reveals the presence of an amorphous phase, nanocrystalline α-Fe(Si,B) structure having a lattice parameter close to 0.2841 nm and an average crystallite size of about 78-83 nm in addition to small amounts of Fe₃O₄ oxide (104 nm) and Fe₂B boride (151 nm), which disappear completely with increasing coating thickness. Coercivity and microhardness values are 15.5 Oe and 478 Hv, respectively, for 84 μm thickness.     

Cemented carbide surface modifications using laser treatment and its effects on hard coating adhesion

A pulsed HyBrID copper laser (510 nm, 30 ns, 13.8 kHz) was used for the treatment of cemented carbide substrate before deposition of TiCN/Al₂O₃/TiN coating by the MT-CVD process. The influence of the laser treatment on the surface morphology, surface structure and coating adhesion was investigated based on the laser irradiation dynamics used here. The experimental results showed that a large variety of cemented carbide surface textures could be obtained, depending on the laser intensity and number of applied laser pulses. Moreover, this laser process was found to produce some less carbon non-stoichiometric WC phases such as β-WC_1 − x and α-W₂C. Finally, using the Rockwell C adhesion test as output criteria, two sets of laser parameters were identified that produced a surface with adhesion strength comparable to that of commercial tools pretreated by micro-sandblasting.     

Energetics of compounds related to Mg2Si as an anode material for lithium-ion batteries using first principle calculations

Electronic energy calculations of (1) Li-intercalated Mg₂Si assuming 4b sites occupancy by Li and (2) the formation of MgSiLi₂ with the assumed structures by Wengert et al. and Herbst and Meyer have been performed by a density-functional theory. The calculated energy changes for intercalation reactions of Mg₈Si₄ + nLi → Mg₈Si₄Li_n are +0.349 eV, +0.822 eV, +1.178 eV, and +1.741 eV for n = 1-4, respectively, and the energy change for Mg₈Si₄ + 8Li → Mg₄Si₄Li₈ + 4Mg is −1.95 eV when Mg is in the metallic state, while +4.12 eV when Mg is in the state of an isolated atom. If we can retard the growth of metallic Mg from Mg₂Si by some methods, undesirable structural change of the Mg₂Si into MgSiLi₂ during charge-discharge cycles would be prevented and intercalation/disintercalation reaction of Li into/from Mg₂SiLi_n (n = 0-1.0) would proceed reversibly by applied electric field.     

Enhancement of upconversion luminescence of Y2O3:Er nanocrystals by codoping Li-Zn

The upconversion (UC) luminescence in sol-gel synthesized Li⁺, Zn²⁺, or Li⁺-Zn²⁺ codoped Y₂O₃:Er³⁺ nanocrystals were investigated under the excitation of a 970 nm diode laser. Compared to undoped Y₂O₃:Er³⁺ samples, proper doping of Li⁺-Zn²⁺ leads to an drastic increase of the UC luminescence centered at 560 nm by a factor of 28. The UC luminescence enhancement is a result of the increased lifetime of the intermediate state ⁴I_11/2 (Er). The intensity ratio of the green over red emissions (green/red) is also affected by the codoping of Zn²⁺, Li⁺ and Li⁺-Zn²⁺ ions. Our results demonstrated that the Li⁺-Zn²⁺ codoping in Y₂O₃:Er³⁺ phosphors produced remarkable enhancement of the UC luminescence and green/red ratio, making this nanocrystal a promising candidate for photonic and biological applications.     

Synthesis of chromium carbide (Cr3C2) nanopowders by the carbonization of the precursor

The solution-derived precursor method was used to synthesize chromium carbide (Cr₃C₂) nanopowders, ammonium dichromate ((NH₄)₂Cr₂O₇) and nanometer carbon black were used as raw materials. The products were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the single phase Cr₃C₂ can be synthesized under the conditions of 21 wt.% C, 1100 °C and 30 min, and the average crystallite size is 27.2 nm. The powders show good dispersion and are mainly composed of spherical or near- spherical particles with a mean diameter of ~ 30 nm. The surface of the specimen mainly consists of Cr, C and O three species elements. The XPS spectrum of Cr2p consists of two peaks with the binding energies of 577.5 eV and 575.3 eV, which are assigned to the Cr2p_3/2 species of Cr₂O₃ and Cr₃C_2 − x (0 ≤ x ≤ 0.5), respectively. The XPS spectrum of O1s energy region for chromium carbide contains three peaks (Oa, Oh and Od), which are considered to be due to O⁻, OH⁻ and Cr₂O₃, respectively.     

Moderate temperature and high-speed synthesis of α-Al2O3 films by laser chemical vapor deposition using Nd:YAG laser

Al₂O₃ films were prepared at deposition temperatures (T_dep) from 980 to 1230 K by laser chemical vapor deposition (LCVD) using the continuous wave of a Nd:YAG laser with laser power (P_L) up to 260 W. γ-Al₂O₃ films were obtained at T_dep < 1100 K, whereas α-Al₂O₃ films were obtained at T_dep > 1100 K. γ-Al₂O₃ films were morphologically characterized by a cone-like structure, while α-Al₂O₃ films had hexagonal faceted grains. The highest deposition rate (R_dep) of γ-Al₂O₃ film was 570 μm h^− 1, while that of α-Al₂O₃ film was 250 μm h^− 1. α-Al₂O₃ films in a single phase were obtained at 170 K lower in T_dep and 100 times higher in R_dep than those by conventional thermal CVD.     

Optical diffraction of second harmonic generation in SrBi2(Nb0.7V0.3)2O9 in the SrO–Bi2O3–0.7Nb2O5–0.3V2O5–Li2B4O7 glass system

Transparent glasses in the system (100 − 3x)(Li₂O–4B₂O₃)–x(SrO–Bi₂O₃–0.7Nb₂O₅–0.3V₂O₅) (where x = 10, 30 and 50, in molar ratio) embedded with nanocrystallites of SrBi₂(Nb_0.7V_0.3)₂O₉ exhibited intense second harmonic signals in transmission mode when exposed to IR laser light at λ = 1064 nm. The second harmonic waves were found to undergo optical diffraction. The origin of optical diffraction in these samples was attributed to the self organised structures of fine crystallites of submicrometer size that were inscribed in-situ by the IR laser radiation. Laser Raman studies confirmed these crystallites to be vanadium doped strontium bismuth niobate.     

Epitaxial stress and texture in thin oxide layers grown on Fe-Al alloys

The development of internal stress and texture in the oxide scales grown on Fe-15 at.% Al (1 0 0) and (1 1 1) single crystals at 700 °C was determined in situ using synchrotron X-ray diffraction. At this low oxidation temperature oxide scale thickness as revealed by X-ray photoelectron spectroscopy and transmission electron microscopy was situated between 100 and 150 nm after 650 min oxidation. Trigonal α-Fe₂O₃ was found to act as a crystallographic template for the nucleation of isostructural α-Al₂O₃. Strain behavior during oxidation was found to be influenced by the epitaxial relationship between α-Al₂O₃ and α-Fe₂O₃.     

Microwave dielectric properties and its compatibility with silver electrode of Li2MgTi3O8 ceramics

The effects of BaCu(B₂O₅) (BCB) additions on the sintering temperature and microwave dielectric properties of Li₂MgTi₃O₈ ceramic have been investigated. The pure Li₂MgTi₃O₈ ceramic shows a relative high sintering temperature (∼1000 °C) and good microwave dielectric properties as Q × f of 40,000 GHz, ?_r of 27.2, τ_f of 2.6 ppm/°C. It was found that the addition of a small amount of BCB can effectively lower the sintering temperature of Li₂MgTi₃O₈ ceramics from 1025 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 0.5 wt% BCB added Li₂MgTi₃O₈ ceramic sintered at 900 °C for 2 h exhibited good microwave dielectric properties of Q × f = 36,200 GHz (f = 7.31 GHz), ?_r = 26 and τ_f = −2 ppm/°C. Compatibility with Ag electrode indicates this material can be applied to low temperature-cofired ceramics (LTCC) devices.     

Upconversion luminescence in Er/Yb codoped Y2CaGe4O12

Calcium yttrium tetrametagermanates Y₂CaGe₄O₁₂ doped with Er³⁺ and Er³⁺/Yb³⁺ reveal upconversion emission in visible spectral range under near-infrared excitation, λ_ex = 980 nm. For the solid solution Er_xY_2−xCaGe₄O₁₂ concentration dependencies for the green and red lines of the visible emission around 526 nm (²H_11/2 → ⁴I_15/2), 545 nm (⁴S_3/2 → ⁴I_15/2) and 670 nm (⁴F_9/2 → ⁴I_15/2) show the optimal value for the sample x = 0.2. The power dependence of the visible luminescence measured at room temperature in the low-power limit indicates two-photon upconversion process. Direct intensification of the upconversion emission signals has been achieved by ytterbium sensitizing. The other upconversion excitation mechanism in Y₂CaGe₄O₁₂:Er³⁺ is discussed for an 808 nm incident laser irradiation. A scheme of excitation and emission routes involving ground/excited state absorption, energy transfer upconversion, nonradiative multiphonon relaxation processes in trivalent lanthanide ions in Y₂CaGe₄O₁₂:Er³⁺ and Y₂CaGe₄O₁₂:Er³⁺, Yb³⁺ has been proposed. Conditions for visible emission occurrence under quasi-resonance λ_ex = 1064 nm excitation depending on pump power values are considered. In the low-power regime only near-infrared emission caused by the transition ⁴I_13/2 → ⁴I_15/2 in erbium ions has been detected.     

One-step synthesis and properties of monolithic photoluminescent ruby colored cuprous oxide antimony oxide glass nanocomposites

Cuprous oxide (Cu₂O) antimony glass (K₂O-B₂O₃-Sb₂O₃) monolithic nanocomposites having brilliant yellow to ruby red color have been synthesized by a single-step melt-quench technique involving in situ thermochemical reduction of Cu²⁺ (CuO) by the reducing glass matrix without using any external reducing agent. The X-ray diffraction (XRD), infrared transmission and reflection spectra, and selected area electron diffraction analysis support the reduction of Cu²⁺ to Cu⁺ with the formation of Cu₂O nanoclusters along with Cu_ySb_2−x(O,OH)_6-7 (y ≤ 2, x ≤ 1) nanocrystalline phases while Cu⁰ nanoclusters are formed at very high Cu concentration. The UV-vis spectra of the yellow and orange colored nanocomposites show size-controlled band gap shift of the semiconductor (Cu₂O) nanocrystallites embedded in the glasses while the red nanocomposite exhibits surface plasmon resonance band at 529 nm due to metallic Cu. Transmission electron microscopic image advocates the formation of nanocystallites (5-42 nm). Photoluminescence emission studies show broad red emission band around 626 nm under various excitation wavelengths from 210 to 270 nm.     

Sol-gel synthesis and electrochemical performance of Li4Ti5O12/graphene composite anode for lithium-ion batteries

Li₄Ti₅O₁₂/graphene composite was prepared by a facile sol-gel method. The lattice structure and morphology of the composite were investigated by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The electrochemical performances of the electrodes have been investigated compared with the pristine Li₄Ti₅O₁₂ synthesized by a similar route. The Li₄Ti₅O₁₂/graphene composite presents a higher capacity and better cycling performance than Li₄Ti₅O₁₂ at the cutoff of 2.5-1.0 V, especially at high current rate. The excellent electrochemical performance of Li₄Ti₅O₁₂/graphene electrode could be attributed to the improvement of electronic conductivity from the graphene sheets. When discharged to 0 V, the Li₄Ti₅O₁₂/graphene composite exhibited a quite high capacity over 274 mAh g⁻¹ below 1.0 V, which was quite beneficial for not only the high energy density but also the safety characteristic of lithium-ion batteries.     

Reaction path of the synthesis of α-Al2O3-TiC-TiB2 in an Al-TiO2-B4C system

Differential thermal analysis was undertaken to determine the reaction path of the synthesis of α-Al₂O₃-TiC-TiB₂ in an Al-TiO₂-B₄C system under argon. The Al content plays a significant role in controlling the reaction path and product. When the Al content is no more than 26.7 wt.%, TiO₂ first reacts with B₄C to yield TiB₂ with TiBO₃ being the intermediate phase, and then increasing temperature leads to the subsequent reactions between Al and TiO₂ or its sub-oxides to yield α-Al₂O₃ and Al₃Ti, and the resultant Al₃Ti then reacts with B₄C to produce TiC and TiB₂. When the Al content is high (e.g. ≥ 34 wt.%), the reaction between Al and TiO₂ for the formation of α-Al₂O₃ and Al₃Ti occurs initially, and then the Al₃Ti reacts with B₄C. With the increasing Al content, the onset of the exothermic reaction in the Al-TiO₂-B₄C system shifts to lower temperature and the degree of reaction conversion is enhanced.     

Study on the crystal structure of the rare earth oxyborate Yb26B12O57 from powder X-ray and neutron diffraction

Yb₂₆B₁₂O₅₇, a rare earth oxyborate previously assigned as Yb₃BO₆, has been studied by various techniques including neutron and X-ray diffraction, ¹¹B NMR spectroscopy, electron diffraction and high angle angular dark field-scanning transmission electron microscopy (HADDF-STEM). It crystallizes in the space group C2/m with cell parameters of a = 24.5780(4) Å, b = 3.58372(5) Å, c = 14.3128(3) Å and β = 115.079(1)°. The structure consists of slabs of rare earth sesquioxide and borate groups. The sesquioxide part is identified from the structural refinement, and is observed from HADDF-STEM image, while elucidation of borate groups is not straightforward. An additional oxygen atom (O31), which links two B₂O₅ groups into a B₄O₁₁ polyanion, is identified from the analysis of neutron diffraction data. The occupancy of this oxygen site is only quarter, which results in a random distribution of B₄O₁₁ and B₂O₅ groups along the b-direction. The chemical formula of ytterbium oxyborate is Yb₂₆(BO₃)₄(B₂O₅)₂(B₄O₁₁)O_24, instead of the simple stoichiometric formula Yb₃BO₆. This compound is paramagnetic but its susceptibility deviated from the Curie-Weiss law at low temperature.     

Internal structure of clusters of partially coherent nanocrystallites in Cr-Al-N and Cr-Al-Si-N coatings

Nano-sized clusters consisting of strongly preferentially oriented, partially coherent nanocrystallites were observed in Cr-Al-N and Cr-Al-Si-N coatings deposited using cathodic arc evaporation. Microstructure analysis of the coatings, which was done using the combination of X-ray diffraction (XRD) and transmission electron microscopy with high resolution (HRTEM), revealed furthermore stress-free lattice parameters, size and local disorientation of crystallites within the nano-sized clusters in dependence on the aluminium and silicon contents, mean size of these clusters and the kind of structure defects. Within the face-centred cubic (fcc) Cr_{1 − x − y}Al_xSi_yN phase, the stress-free lattice parameter was described by the equation a = (0.41486 − 0.00827 · x + 0.034 · y) nm. The size of individual crystallites decreased from ∼ 11 nm in Cr_0.92Al_0.08N to ∼ 4 nm in Cr_0.24Al_0.65Si_0.10N. These nanocrystallites formed clusters with the mean size between 36 and 56 nm. The mutual disorientation of the partially coherent nanocrystallites forming the clusters increased with increasing aluminium and silicon contents from 0.5° to several degrees. The disorientation of neighbouring nanocrystallites was explained by the presence of screw dislocations and by presence of phase interfaces in coatings containing a single fcc phase and several phases, respectively.