Nanocrystallization in Fluorochlorozirconate Glass‐Ceramics

Heat treating fluorochlorozirconate (FCZ) glasses nucleates nanocrystals in the glass matrix, resulting in a nanocomposite glass‐ceramic that has optical properties suitable for use as a medical imaging plate. Understanding the way in which the nanocrystal nucleation proceeds is critical to controlling the optical behavior. The nucleation and growth of nanocrystals in FCZ glass‐ceramics was investigated with in situ transmission electron microscopy heating experiments. The experiments showed the nucleation and growth of previously unreported BaF₂ nanocrystals in addition to the expected BaCl₂ nanocrystals. Chemical analysis of the BaF₂ nanocrystals shows an association with the optically active dopant previously thought only to interact with BaCl₂ nanocrystals. The association of the dopant with BaF₂ crystals suggests that it plays a role in the photoluminescent (PL) properties of FCZ glass‐ceramics.     

BaCl2:Er3+—A High Efficient Upconversion Phosphor for Broadband Near‐Infrared Photoresponsive Devices

Utilization of photons with subband‐gap energy, mostly near‐infrared (NIR) photons, is highly desirable for photovoltaic cells; which can be achieved by adding an upconversion layer at the rear face of photovoltaic cells. Here, we study the upconversion luminescence properties of BaCl₂:Er³⁺ phosphors and hexagonal NaYF₄:Er³⁺ phosphors upon excitation of incoherent NIR sunlight with wavelength λ > 800 nm. Higher efficacious upconversion emissions of BaCl₂:Er³⁺ phosphors have been observed in comparison with the well‐known hexagonal NaYF₄:Er³⁺ phosphors. We demonstrate that the photocurrent response from the thin‐film‐hydrogenated amorphous silicon solar cell attached with the BaCl₂:Er³⁺ phosphor is notably enhanced under irradiation of incoherent NIR sunlight with wavelength λ > 800 nm. This judicious design may be envisioned to shorten the distance for the remarkable improvement of the power conversion efficiency of the next‐generation photovoltaic cells and suggests a promising application for other NIR photoresponsive devices.     

Structural,magnetic, electrical,and electrochemical properties of Sr–Co–Ru–O: A hybrid‐capacitor application

In this study, we report the synthesis of SrCo_1?xRu_xO_3?δ nominal compositions, where x = 0.0‐1.0, using solid‐state reaction technique. XRD analysis confirms the structure of x = 0 sample as hexagonal Sr₆Co₅O₁₅. As the Co ions are substituted by Ru, a two‐phase structure (hexagonal R32 and orthorhombic Pbnm) emerges up to x ≤ 0.5. As the Ru content is increased further, the hexagonal R32 phase disappears completely and an orthorhombic Pbnm phase becomes the main phase. SEM images show that grain size of the samples decreases with increasing Ru content. Temperature‐dependent electrical conductivity studies indicate upon Ru substitution in the nominal SrCo_1?xRu_xO_3?δ compounds, resistivity decreases due to appearance of metallic SrRuO₃ phase. The cyclic voltammogram (CV) of the samples show capacitive properties upon Ru substitution. The cycle measurements of the capacitors yield promising results for potential supercapacitor applications.     

Hydrothermal formation and up‐conversion luminescence of Er3+‐doped GdNbO4

The effect of concentration of Er³⁺ on the up‐conversion and photoluminescence properties of Gd_1.00?xEr_xNbO₄, x=0‐0.50 which has monoclinic fergusonite‐type structure as a main phase has been investigated, using a processing technique based on hydrothermal method. Under weakly basic hydrothermal condition at 240°C for 5 hours, a single phase of fergusonite‐type Gd_1.00?xEr_xNbO₄ solid solution was directly formed as nanocrystals by the substitutional incorporation of Er³⁺ into GdNbO₄ because of the gradual and linear decrease in the lattice parameters of the monoclinic phase corresponding to the Vegard's Law. The gadolinium niobate doped with 2 mol% Er³⁺, Gd_0.98Er_0.02NbO₄ after heating at 1300°C for 1 hour, which has nanocrystalline structure whose crystallite size is around 29 nm, exhibits the highest photoluminescence intensity in the green spectral region, 515‐560 nm under excitation at wavelength of 254 nm. On the other hand, the up‐converted luminescence intensity of the niobate nanocrystals becomes the maximum at the concentration of 20 mol% Er³⁺, Gd_0.80Er_0.20NbO₄ under excitation at 980 nm. These results demonstrate that the material, Er³⁺‐doped GdNbO₄ nanocrystals prepared through hydrothermal route and postheating has potential for up‐converting phosphor.     

Relationship Between the Orthorhombic and Hexagonal Phases in Dy2TiO5

The thermal expansion of Dy₂TiO₅ in the hexagonal phase was evaluated and compared with the orthorhombic phase using in situ high‐temperature X‐ray diffraction. The crystal structure, volume changes before and after the transformation process, as well as the mechanism behind the thermal expansion behavior was determined and proposed. It was found that in the hexagonal phase, the thermal expansion was caused by the oxygen anions in the axial positions of the trigonal bipyramidal structure moving toward the central Ti atom. While expanding, the movement of these oxygen anions slows the expansion along the c‐axis resulting in a decrease in α₃₃ with temperature. Furthermore, a structural relationship between the orthorhombic and the hexagonal phases was proposed.     

Phase Formation,Microstructure, and Densification of Yttrium‐Doped Barium Zirconate Prepared by the Sonochemical Method

Spherical monodispersed yttrium‐doped barium zirconate (BaY_0.1Zr_0.9O₃; BYZ) particles were successfully prepared in single step without the requirement of calcination process by the sonochemical method in highly basic aqueous solution. The stoichiometric solution of BaCl₂.2H₂O, ZrOCl₂.8H₂O, and YCl₄.6H₂O was precipitated in a 20 M NaOH under high‐intensity ultrasonic irradiation (20 kHz, 150 W/cm²) for 15, 30, and 60 min. As‐prepared powders were identified by XRD, FT‐IR, and Raman spectroscopy as cubic perovskite BYZ. The microstructure examined by SEM and TEM showed spherical‐shaped BYZ particles formed by aggregation of primary nanocrystals, and this unique morphology was induced by the effects of the ultrasonication and the strong alkaline environment. BYZ powders prepared under ultrasonication for 60 min had narrow‐sized distribution with the average particle size of 267 ± 26 nm and the specific surface area of 40.2 m²/g. BYZ ceramics sintered in the air at 1550°C for 20 h showed good densification (95%) and consisted of large grain size (7.67 ± 2.79 μm).     

Room-temperature multiferroic characteristics and unique vortex domain structures of h-Yb1−xInxFeO3 solid solutions

Hexagonal rare-earth ferrites (h-RFeO₃) have attracted much scientific attention due to their room-temperature multiferroicity. However, it is still a hard job to obtain h-RFeO₃ bulk materials because of the meta-stability of such hexagonal phase, and the evaluation of room-temperature ferroelectric and magnetoelectric characteristics in such materials is also a challengeable issue. In the present work, Yb_1−xIn_xFeO₃ ceramics with the stable hexagonal structure were obtained by introducing chemical pressure, where the unique ferroelectric domain structures of sixfold vortex combined with tenfold vortex with a typical size of ~400 nm were determined. Symmetry of the present system evolved from centrosymmetric orthorhombic Pbnm (x = 0–0.4) to non-centrosymmetric hexagonal P6₃cm (x = 0.5 and 0.6) with a ferroelectric polarization up to 3.2 μC/cm², and finally to centrosymmetric hexagonal P6₃/mmc (x = 0.7 and 0.8). The Curie point decreased monotonically from 723 K to a temperature below room temperature with increasing x, and the antiferromagnetic phase transition above room temperature was determined for all compositions. Meanwhile, a large linear magnetoelectric coefficient (α_ME) up to 0.96 mV/cm Oe was obtained at room temperature, and this indicated the great application potential for magnetoelectric devices.     

Dielectric and Raman Spectroscopic Studies of Na0.5Bi0.5TiO3–BaSnO3 Ferroelectric System

A series of lead‐free perovskite solid solutions of (1 ? x) Na_0.5Bi_0.5TiO₃(NBT)—x BaSnO₃(BSN), for 0.0 ≤ x ≤ 0.15 have been synthesized using a high‐temperature solid‐state reaction route. The phase transition behaviors are studied using dielectric and Raman spectroscopic techniques. The ferroelectric to relaxor phase transition temperature (T_FR) and the temperature corresponding to maximum dielectric permittivity (T_m) are estimated from the temperature‐dependent dielectric data. Dielectric studies show diffuse phase transition around ~335°C in pure NBT and this transition temperature decreases with increase in x. The disappearance of x‐dependence of A₁ mode frequency at ~134 cm^?1 for x ≥ 0.1 is consistent with rhombohedral‐orthorhombic transition. In situ temperature dependence Raman spectroscopic studies show disappearance and discontinuous changes in the phonon mode frequencies across rhombohedral (x < 0.1)/orthorhombic (x ≥ 0.1) to tetragonal transition.     

Facile synthesis of nanocrystalline hexagonal tungsten trioxide from metallic tungsten powder and hydrogen peroxide

A hexagonal form of tungsten trioxide (h‐WO₃, particle size: 15.9‐57.1 nm) was found to be formed by a direct reaction between metallic tungsten powder (W, particle size: 0.45‐0.59 μm) and 15%‐30% hydrogen peroxide (H₂O₂) aq solution. Oxide film on the powder surface having the similar crystal structure as h‐WO₃ was essential for the formation, and the surface oxide film was formed by aging the powder in air at 45°C, a relative humidity of 100% (P_H2O 96 hPa) for 3‐28 days or in ambient atmosphere at room temperature for 12 years. The Rietveld analysis performed in the space group P6₃/mcm (Z = 6) indicated the crystal structures were the same as those of the reported h‐WO₃ and that the crystallographic characteristic was as follows: a = 0.74219 nm, c = 0.77198 nm for h‐WO₃ from the 28‐day aged powder, and a = 0.74538 nm, c = 0.77194 nm for h‐WO₃ from the 12‐year aged powder.     

Effect of lanthanide doping on crystal phase and near-infrared to near-infrared upconversion emission of Tm doped KF–YbF3 nanocrystals

Tm³⁺ doped KF–YbF₃ nanocrystals were synthesized by a hydrothermal method using oleic acid as a stabilizing agent at 190 °C. The influence of Gd³⁺ and Sm³⁺ content on the phase structure and upconversion (UC) emission of the final products was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UC spectra. XRD analyses and TEM observations evidence that the phase and size of the as prepared Tm³⁺ doped KF–YbF₃ nanocrystals are closely related to the Gd³⁺ doping content. Without Gd³⁺ impurity, the undoped nanocrystals crystallize in orthorhombic KYb₂F₇ with an average diameter of 42 nm. When the Gd³⁺ doping is below 10 mol%, the orthorhombic KYb₂F₇ nanocrystals grow up. However, with Gd³⁺ addition beyond about 30 mol%, the complete phase transformation from orthorhombic KYb₂F₇ to cubic KGdF₄ occurs in the final products. Under the excitation of a 980 nm laser diode, the as prepared Tm³⁺ doped nanocrystals exhibit strong near-infrared UC emission at 800 nm. Particularly, the intensity of high energy UV and blue UC emissions of Tm³⁺ ions in Tm³⁺ doped KYb₂F₇ nanocrystals are selectively reduced compared to the NIR emission at 800 nm by co-doping a small amount of Sm³⁺ ions into the host matrix. Possible dynamic processes for UC emissions in Tm³⁺ doped nanocrystals are discussed in detail.     

Spark Plasma Sintering of Hexagonal Structure Yb3+‐Doped Sr5(PO4)3F Transparent Ceramics

Ytterbium‐doped Sr₅(PO₄)₃F transparent ceramics have been developed through spark plasma sintering (SPS) with a low sintering temperature and short dwelling time. The XRD patterns show a polycrystalline hexagonal phase, and the TEM microstructure characterization indicates that the ceramics have a narrow grain size distribution which ranges from 40 to 200 nm, with an average grain size around 150 nm. The transmittance of a 2 mm thick ceramic sample is measured to be 74% at 1000 nm by a UV–Vis–NIR spectrophotometer. Furthermore, there is a strong emission peak around 1040 nm which has a lifetime of 1.06 ms and is exhibited by a PL spectrometer with the 980 nm laser diode excitation.     

Direct Formation and Luminescence Properties of Yttrium Niobate YNbO4 Nanocrystals via Hydrothermal Method

Yttrium niobate YNbO₄ nanocrystals with ellipsoidal morphology were directly formed from the precursor solution mixtures of YCl₃ and NbCl₅ under weakly basic conditions in the presence of aqueous ammonia by hydrothermal method. The hydrothermal treatment at 180°C for 5 h was necessary to obtain nanocrystals (18 nm) with sufficient crystallinity. The optical band gap of the as‐prepared samples was 3.6 eV. The as‐prepared YNbO₄ nanocrystals showed UV‐blue and broadband emission centered at 405 nm under excitation at 235 nm, which was due to the blue recombination luminescence, associated with charge‐transfer transitions involving the tetrahedral NbO₄ group. The emission intensity increased with increased hydrothermal treatment temperature. The photoluminescence intensity of the YNbO₄ was extremely improved via heating above 1000°C in air, which was accompanied by the increase in the optical band gap from 3.6 to 4.0 eV. By heat treatment at 1300°C, the intensity of the UV‐blue and broadband emission (with maximum at 400 nm) for the YNbO₄ became more than 22.5 times as strong as that before heat treatment.     

In Situ Domain Structure Observation and Giant Magnetoelectric Coupling for PMN‐PT/Terfenol‐D Multiferroic Composites

In situ observations of ferroelectric domain structure evolution, and magnetoelectric (ME) coupling are investigated for PMN‐28PT/Terfenol‐D (abbreviation of Pb(Mg_1/3Nb_2/3)O₃‐28PbTiO₃/Tb_0.3Dy_0.7Fe₂) and PMN‐33PT/Terfenol‐D composites under the magnetic loadings. The composite of PMN‐33PT/Terfenol‐D shows stronger ME coupling than that in PMN‐28PT/Terfenol‐D. At a thickness of 0.10–0.12 mm for the single crystal plate, a giant magnetoelectric coefficient (α_ME) up to 2 V/cm·Oe is obtained for PMN‐33PT/Terfenol‐D at a static magnetic field of 200 Oe and 1 kHz of the alternating magnetic field. In situ domain structure observations reveal the domain morphology change during the applied magnetic loadings. In PMN‐28PT, the domains are of predominantly rhombohedral (R) phase and they change into monoclinic M_A phase upon the magnetic loading via the strain transferred between Terfenol‐D plate and PMN‐PT single crystal. In PMN‐33PT, domains of orthorhombic (O), R, and monoclinic M_C coexist and phase transitions from O to M_C and further to R phase occur upon the magnetic loading. The undulation and diversity of the domain structure makes the domains more susceptible to the magnetic loading via strain transferred between Terfenol‐D plate and PMN‐PT single crystal, and consequently, a strong ME coupling in the composites.     

Instant precipitation of KMgF3:Ni2+ nanocrystals with broad emission (1.3‐2.2 μm) for potential combustion gas sensors

Optical gas sensors present fundamental and industrial importance considering their broad applications. Challenges remain to obtain new photonic materials with broadband emission covering the absorption spectrum of typical combustion gases. Here, broadband near‐infrared (NIR) photoluminescence (PL) spanning the wide absorption spectrum of typical combustion products is realized through instant precipitation of stable cubic perovskite KMgF₃:Ni²⁺ nanocrystals inside an aluminosilicate glass matrix after melt‐quenching. Excited by an 808 nm laser diode, NIR luminescence with a peak centered at ~1624 nm and a bandwidth (FWHM) greater than 315 nm is observed, originating from ³T_2g(³F) → ³A_2g(³F) electronic transition of octahedral coordinated Ni²⁺ in KMgF₃ GC. Controlled precipitation of these perovskite crystals from a supercooled aluminosilicate melt enables immediate encapsulation and, hence, stabilization in an inorganic glass phase. While the precipitation temperature has only a small effect on crystallite size, it controls the redox state of the melt and the degree of dopant incorporation into the crystalline phase so that PL performance can be optimized. Spontaneous crystallization of perovskite nanocrystals inside glass may offer a new way to stabilize these novel nanocrystals. Moreover, spontaneous crystallization can be attractive in the control of activator partitioning and in the fabrication of composite fiber devices with high transparency and emission gain. In the present case, this offers a potential platform for broadly tunable gain media, for example, for combustion gas sensing.     

Spectral Properties of Pr3+‐Doped Transparent‐Glass‐Ceramic Containing Ca5(PO4)3F Nanocrystals

A Pr³⁺‐doped transparent oxyfluoride glass‐ceramic containing Ca₅(PO₄)₃F nanocrystals was prepared by melt quenching and subsequent thermal treatment. The crystallization phase and morphology of the Ca₅(PO₄)₃F nanocrystals were investigated by X‐ray diffraction and transmission electron microscope, respectively. The volume fraction of the Ca₅(PO₄)₃F nanocrystals in the glass‐ceramic is about 10% and the fraction of Pr³⁺ ions incorporated into the Ca₅(PO₄)₃F nanocrystals is about 22%. The peak absorption cross sections at 435 and 574 nm increase up to 128% and 132% after crystallization, respectively. The peak stimulated emission cross sections of the ³P₀→³H₄ blue laser channel and ³P₀→³F₂ red laser channel for the glass‐ceramic are 4.95 × 10^?20 and 29.8 × 10^?20 cm², respectively. The spectral properties indicate that the glass‐ceramic is a potential visible laser material.     

Thermal,Structural, and Enhanced Photoluminescence Properties of Eu3+‐doped Transparent Willemite Glass–Ceramic Nanocomposites

The precursor glass in the ZnO–Al₂O₃–B₂O₃–SiO₂ (ZABS) system doped with Eu₂O₃ was prepared by the melt‐quench technique. The transparent willemite, Zn₂SiO₄ (ZS) glass–ceramic nanocomposites were derived from this precursor glass by a controlled crystallization process. The formation of willemite crystal phase, size, and morphology with increase in heat‐treatment time was examined by X‐ray diffraction (XRD) and field‐emission scanning electron microscopy (FESEM) techniques. The average calculated crystallite size obtained from XRD is found to be in the range 18–70 nm whereas the grain size observed in FESEM is 50–250 nm. The refractive index value is decreased with increase in heat‐treatment time which is caused by the partial replacement of ZnO₄ units of ZS nanocrystals by AlO₄ units due to generation of vacancies. Fourier transform infrared (FTIR) reflection spectroscopy was used to evaluate its structural evolution. Vickers hardness study indicates marked improvement of hardness in the resultant glass‐ceramics compared with its precursor glass. The photoluminescence spectra of Eu³⁺ ions exhibit emission transitions of ⁵D₀→⁷F_j (j = 0, 1, 2, 3, and 4) and its excitation spectra show an intense absorption band at 395 nm. These spectra reveal that the luminescence performance of the glass–ceramic nanocomposites is enhanced up to 17‐fold with the process of heat treatment. This enhancement is caused by partitioning of Eu³⁺ ions into glassy phase instead of into the willemite crystals with progress of heat treatment. Such luminescent glass–ceramic nanocomposites are expected to find potential applications in solid‐state red lasers, phosphors, and optical display systems.     

Adsorbed O2 on the Graphite‐Induced Growth of Ultra‐Long Single‐Crystalline 6H–SiC Nanowires

Large‐scale ultra‐long 6H–SiC nanowires were in situ synthesized on the as‐prepared SiC–Si ceramic substrate using graphite as the carbon source and substrate as the silicon source via improving the adsorbed O₂ content on the graphite precursors using milling technology. The as‐grown nanowires were typical single crystal of hexagonal 6H–SiC with diameters of 50–100 nm and lengths of up to several millimeters (or even centimeters). Vapor‐solid mechanism was proposed for the growth mode of the as‐grown ultra‐long 6H–SiC nanowires. This study not only provided new insight into the growth mode for in situ synthesizing 6H–SiC nanowires on silicon‐based ceramics, but also suggested a new design methodology for synthesizing ultra‐long nanowires.     

Microwave‐assisted Hydrothermal Synthesis of Single‐crystal Nanorods of Rhabdophane‐type Sr‐doped LaPO4·nH2O

A novel, simple, soft, and fast microwave‐assisted hydrothermal method was used for the preparation of single‐crystal nanorods of hexagonal rhabdophane‐type La_1?xSr_xPO_4?x/2·nH₂O (x = 0 or 0.02) from commercially available La(NO₃)₃·6H₂O, Sr(NO₃)₂, and H₃PO₄. The synthesis was conducted at 130°C for 20 min in a sealed‐vessel microwave reactor specifically designed for synthetic applications, and the resulting products were characterized using a wide battery of analytical techniques. Highly uniform, well‐shaped nanorods of LaPO₄·nH₂O and La_0.98Sr_0.02PO_3.99·nH₂O were readily obtained, with average length of 213 ± 41 nm and 102 ± 25 nm, average aspect ratio (ratio between length and diameter) of 21 ± 9 and 12 ± 5, and specific surface area of 45 ± 2 and 51 ± 1 m²/g, respectively. In both cases, the single‐crystal nanorods grew anisotropically along their c crystallographic‐axis direction. At 700°C, the hexagonal rhabdophane‐type phase has already transformed into the monoclinic monazite‐type structure, although the undoped and Sr‐doped nanorods retain their morphological features and specific surface area during calcination.     

Gd-based oxyfluoride glass ceramics: Phase transformation,optical spectroscopy and upconverting temperature sensing

A series of precursor glasses with compositions of SiO₂-Al₂O₃-AlF₃-Na₂O- NaF-Gd₂O₃/GdF₃-YbF₃-ErF₃ were prepared and their crystallization behaviors were investigated. For the samples with high F content, meta-stable hexagonal GdF₃ nanocrystals were preferentially precipitated from glass matrix and decreasing F/O ratio induced phase transformation to cubic NaGdF₄ and finally to hexagonal NaGdF₄. Benefited from its multiple active sites, significant enhanced upconversion luminescence was achieved for Yb/Er co-doped glass ceramic containing hexagonal NaGdF₄ nanocrystals. Importantly, significant temperature-sensitive upconversion fluorescence intensity ratio between Er³⁺: ²H_11/2 → ⁴I_15/2 transition (520 nm) and ⁴S_3/2 → ⁴I_15/2 one (540 nm) was detected owing to the competitive radiation transitions from these two thermally coupled emitting-states. Furthermore, linear temperature-dependent fluorescence intensity ratio between Er³⁺: ⁴F_9/2 → ⁴I_15/2 transition (650 nm) to ⁴S_3/2 → ⁴I_15/2 one (540 nm) was achieved, showing the advantages of high sensitivity, superior signal discriminability as well as excellent thermal stability for temperature determination.     

New Lead‐Free (1 − x)(K0.5Na0.5)NbO3–x(Bi0.5Na0.5)ZrO3 Ceramics with High Piezoelectricity

For enhancing the piezoelectric properties of ceramics (Bi_0.5Na_0.5)ZrO₃ (BNZ) was used to partially substitute (K_0.5Na_0.5)NbO₃ (KNN). The addition of BNZ changes the symmetry of KNN ceramics from orthorhombic to tetragonal, and finally to rhombohedral phase. A new phase boundary with both rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions near room temperature is identified for KNN–0.050BNZ ceramics, where optimum electrical properties were obtained: d₃₃ = 360 pC/N, k_p = 32.1%, ε_r = 1429, tanδ = 3.5%, and T_C = 329°C. The results indicated a new method for designing high‐performance lead‐free piezoelectric materials.