Preform fabrication and drawing of KNbO3 modified tellurite glass fibers

For fiberization of tellurite glasses the 70TeO₂–25ZnO–5Na₂O composition is selected based on its good thermal stability and refractive index compatibility with most of the ferroelectric oxides. A modified built-in casting method is used to fabricate preforms. The fiber drawn by the rod-in-tube technique consists of two layers of cladding glass and a core with 5 mol% ferroelectric KNbO₃. In this study we also address the low mechanical strength problem with tellurite glass fibers, by subjecting the preforms to polishing and wet chemical etching. Apart from the fabrication methods, we also report here a selective core heat-treatment and ferroelectric phase crystallization in a low dimensional system.     

Conductivity stabilization by metal and oxide additives in ceramics on the basis of VO2 and glass V2O5–P2O5

Conductivity behavior during the temperature cycling through the phase transition temperature of VO₂ (T_t = 68 °C) was investigated in glass-ceramics based on VO₂ and vanadium phosphate glass (VPG) for compositions without and with Cu and SnO₂ additives. Copper and SnO₂ additives stabilize the conductivity of glass-ceramics at temperature cycling. For ceramics (wt%) (80 − y)VO₂–5Cu–15VPG–ySnO₂ the best stabilizing effect takes place when SnO₂ content is in the interval 35 wt% < y < 50 wt%. Ceramics with such SnO₂ content keeps a stable value of the conductivity change (∼10²) in the vicinity of VO₂ phase transition temperature and shows the conductivity decrease no more than of 2.5 times after 3000 thermal cycles. The reasons of conductivity stabilizing in VO₂-based glass-ceramics with additives of Cu and SnO₂ are being discussed. The analysis resting on the percolation theory has shown the increase of conductivity stability in VO₂-based glass-ceramics when the VO₂ volume fraction and the average size of VO₂ crystallites decrease and the ceramics surface tension increases.     

Broad blue-green-red emissions of SnO2/Pr3+ co-doped phosphate glasses

The photoluminescence properties of SnO₂/Pr^3 + co-doped strontium phosphate glasses (75P₂O₅-25SrO) are studied. An ultrabroad emission band covering blue, green and red is observed in co-doped glasses. In co-doped samples, three downward peaks appear in blue emission region, these coincide with Pr^3 + excitation peaks, indicating the energy transfer through cross-relaxation between SnO₂ and Pr^3 +. The mechanism has been detailed based on the energy level diagrams of SnO₂ and Pr^3 +. The chromaticity coordinates of the co-doped samples with varying doping ratio of SnO₂ to Pr^3 + are calculated. The result demonstrates the possibility of generating white light in the SnO₂/Pr^3 + co-doped phosphate glasses.     

Radiation response behavior of high phosphorous doped step-index multimode optical fibers under low dose gamma irradiation

This paper revealed the role of various parameters on the radiation response behavior of P₂O₅ doped step-index multimode (SIMM) fibers having different content of P₂O₅ (12–6 mol%) at room temperature. Their suitability for use as a radiation sensitive fiber in fiber optic dosimeter was studied under ⁶⁰Co-gamma irradiation at different dose rates 0.01–0 Gy/h. The suitable wavelength region is found to be 500–600 nm where the fibers shows the maximum radiation sensitivity due to formation of phosphorous–oxygen hole centers identified from their radiation induced absorption spectra. The influence of each parameter like the doping levels of P₂O₅, dose rates, doping region and energy of the radiation source on their sensitivities was examined. The radiation sensitivities recorded at 502, 540 and 560 nm depends strongly on P₂O₅ content of the fiber at low dose rates within 0.01–1 Gy/h. However, the fibers shows almost dose rate independent behavior at high dose rates (>1.0 Gy/h) under ⁶⁰Co-gamma radiation source of energy 1.25 MeV with respect to all the three wavelengths. The fiber shows almost linear relation to the total dose up to saturation levels of 4.0 Gy at all dose rates. Their sensitivities could be explained through analyses of such type of glass as well as behavior of the radiation induced P-related defects generated in the light guiding core region. At low dose rates the fibers becomes more radiation sensitive compared to the high dose rates due to conversion of POHC centers to P₁ (phosphorous E′) defect centers as observed from their induced loss curves during gamma irradiation. At low dose rates 0.1–1 Gy/h under 502 nm transmission wavelength the fibers shows an excellent linear relation with respect to Cs-137 radiation source of energy 0.662 MeV having average sensitivity of 1.0452 ± 0.0346 dB/m/Gy and very low fading behavior at room temperature. The results suggest that P-doped SIMM fiber (40 μm core diameter) containing 16 mol% P₂O₅ exhibit an excellent linear radiation response property of high sensitivity around 0.89 ± 0.09 dB/m/Gy at 502 nm wavelength with very low recovery and little dose rate dependence within 0.5–10 Gy/h region that makes them a very promising candidate as radiation sensor for use in fiber optic personal dosimeter to detect low dose gamma radiation of 0.002 Gy for human safety purpose.     

Orthorhombic Pbcn SnO2 nanowires for gas sensing applications

Pure monocrystalline orthorhombic SnO₂ nanowires decorated and non-decorated with cassiterite SnO₂ nanoclusters are analyzed and compared with pure monocrystalline cassiterite SnO₂. We corroborate the coexistence of both, cassiterite and orthorhombic phases, having a higher growth speed for the cassiterite one, in the obtained nanowires by the evaporation/condensation technique. For both phases, the building blocks are the [SnO₆]^8? octahedron which are forming chains of edge-sharing octahedral along the [0 0 1] direction for the cassiterite phase, while in the orthorhombic phase, chains run in a zigzag fashion and contains four octahedra on each unit of chain instead of two previously reported for orthorhombic material obtained at high pressure conditions as Pbcn SnO₂ orthorhombic structure. Results obtained reveal singular structural characteristics of these synthesized orthorhombic nanowires.     

Fabrication of Yb-doped silica glass through the modification of MCVD process

A method to deliver Yb and Al compounds in vapor phase to the reaction/deposition zone has been devised for modified chemical vapor deposition (MCVD) process. By using this MCVD setup, we succeeded to prepare silica glass preforms presenting uniform dopants concentrations in the radial and longitudinal directions with good reproducibility. Preforms with a core diameter larger than 5 mm were easily prepared by depositing around 40 layers. By changing some parameters in the deposition step, such as carrier gases flow and temperatures of Yb(DPM)₃ and AlCl₃ furnaces, different concentrations of Yb and Al were incorporated into the core region of the silica glass preforms. By adjusting the Yb and Al concentrations, we succeeded to prepare preforms for large mode area (LMA) fiber with a change less than 10% of nominal refractive index.     

2 μm spectroscopic investigation of Tm3+-doped tellurite glass fiber

TmF₃ doped TeO₂–ZnO–La₂O₃ (TZL) glasses and fibers have been prepared by the conventional melt-quenching and suction casting methods, respectively. 2 μm emission properties and energy transfer mechanisms of the TZL glasses and fibers have been analyzed and discussed. The oscillator strength, Judd–Ofelt parameters, radiative transition probability and radiative lifetime of Tm³⁺ have been calculated based on the absorption spectra and Judd–Ofelt theory. The maximum emission cross-section of Tm³⁺ is 6.9 × 10^?21 cm² near 2 μm. Emission spectra have been obtained from both TZL fibers and bulk glass when excited with a 794 nm pump. The results of 2 μm emission spectra indicate that the line width of Tm³⁺ measured in fibers is narrower than that in the bulk glass sample. The peak position of the emission spectra shifts to longer wavelength with increment of the fiber length.     

Tm-doped aluminate glass fibers for S-band optical amplification

Thulium-doped fiber amplifiers (TDFAs) have been proposed as practical devices for the amplification of light signals in the so-called S-band (1460–1530 nm) of the transparency window of standard telecommunications fiber. As the quantum efficiency of the desired ³H₄ → ³F₄ luminescence of Tm³⁺ is adversely affected by non-radiative decay when high maximum phonon energy (MPE) host glasses are used, a practical TDFA requires an active fiber made from a glass with intermediate to low MPE. We have explored the possibility of using aluminate fibers for this application, as bulk samples of Tm-doped alkaline earth aluminate glass are characterized by a MPE of 780 cm⁻¹ and a quantum efficiency for the 1460 nm fluorescence of ∼35%. Despite the high devitrification tendency of aluminate glass, pure aluminate core fibers with minimum losses of ∼0.5 dB/m have been successfully fabricated by the rod-in-tube technique using viscosity- and expansion-matched alkaline earth aluminosilicate cladding glasses.     

Fabrication and characterization of Er3+-doped GeO2–PbO and GeO2–PbO–Bi2O3 glass fibers

Glass fibers were drawn from GeO₂–PbO–Bi₂O₃ and GeO₂–PbO melts previously doped with Er³⁺. From the differential thermal analysis curve, the glass transition temperature was determined to be 420 °C, and no crystallization peak was observed in the temperature range of that analysis, indicating stability with regard to devitrification. Raman spectroscopy was performed to characterize the structure of the glasses, which exhibited large transmission windows (0.5–5.0 μm) and large refractive indices (∼2.0). Infrared to visible upconversion of Er³⁺ was observed in the fibers. The visible emissions were related to the upconverted green emissions at about 530 nm (²H_11/2 → ⁴I_15/2) and 550 nm (⁴S_3/2 → ⁴I_15/2), and red emission at 668 nm (⁴F_9/2 → ⁴I_15/2) under 980 nm excitation. The infrared transition (⁴I_13/2 → ⁴I_15/2) was peaked at 1.53 μm. The results obtained suggest that the fibers exhibit the same structures as the parent glasses and can be used in upconversion fiber optical devices.     

Phase transformation and crystal growth behaviors of La3+/Ce3+-doped TiO2–20 wt% SnO2 gels

The transformation behaviors of La³⁺/Ce³⁺-doped TiO₂–SnO₂ gels were studied by using differential thermal analysis and X-ray diffraction methods so as to improve the phase transformation and decrease the granularity of crystals. Experimental results show that, anatase, rutile and SnO₂ nanocrystals can exist in the sintering products by varying La³⁺/Ce³⁺ contents and sintering temperatures. 0.8–1.1 wt% of La₂O₃ or CeO₂ doping greatly depresses the growth of anatase and rutile crystals, obtaining nanosized crystals when sintered up to 600 °C for 2 h. With La³⁺/Ce³⁺-doping and increasing their contents, the transformations of gel to anatase and anatase to rutile, as well as the growth of anatase and rutile crystals can be depressed, while the transformation temperature of anatase to rutile receives much less affect. Moreover, the La³⁺-doping has stronger effects on them than Ce³⁺ doping, but has a weaker inhibiting effect on precipitation and growth of SnO₂ crystals.     

Spectroscopic characterization of signal gain and pump ESA in short-lengths of RE-doped tellurite fibers

We report the results of emission and amplification in Tm³⁺- and Er³⁺-fibers for signal gain in the 1460–1600 nm region, which covers a large part of S-, C- and L-bands of silica fiber optical communication networks. The paper explains the mechanism for alleviating the pump-excited absorption in Er³⁺-doped and Tm³⁺-doped tellurite fibers for maximizing the pump inversion efficiency at 980 nm using the co-dopants and via the structural modification of TeO₂ glass by incorporating a high phonon energy oxide namely, B₂O₃. The spectroscopic data and gain bandwidth of Er-doped fibers are reported in the C- and L-bands. To date the measured maximum relative gain in short fibers of 5–10 cm in length in C- and L-bands are: 30 dB and 15 dB, respectively. By comparison the internal gain in a 20 cm long Tm/Yb ion co-doped fiber pumped with a 980 nm source was 7 dB.     

An alumina mat with a nano microstructure prepared by centrifugal spinning method

Ceramic fiber products specially alumina mat because of low thermal conductivity and high melting point are used as high temperature insulating materials. Alumina has so high melting point (T_m > 2040 °C) that its mat can be produced through sol–gel method. In this research alumina mat has been manufactured by sol–gel spinning method using our laboratory-designed centrifugal spinneret. The desired viscosity of sol for spinning is 150 P. Phase transformation of the product begins at 600 °C and there is not any amorphous phase at 800 °C and theta alumina (θ-Al₂O₃) is the main phase. In this work, transformation of transitional phase to alpha alumina (α-Al₂O₃) takes place from 1000 °C to 1200 °C. The optimum percent of silica in alumina mat is 4 wt%. Fibers constitute network structure that their average diameter is about 10 μm and contains very fine grains (100 nm). The silica percent concerning the limits of this study (<10 wt%) does not effect on fiber diameter, but grain size decreases from about 200 nm to less than 100 nm while increasing silica percent.     

Bulk-quantity SnO2 nanorods synthesized from simple calcining process based on annealing precursor powders

Tin dioxide (SnO₂) nanorods have been successfully synthesized in bulk quantity by a calcining process based on annealing precursor powders in which sodium chloride, sodium carbonate, and stannic chloride were homogeneously mixed. Transmission electron microscopy shows that the as-prepared nanorods are structurally perfect and uniform, with widths of 10–25 nm, and lengths of several hundreds nanometers to a few micrometers. X-ray diffraction and energy-dispersive X-ray spectroscopy analysis indicate that the as-prepared nanorods have the same crystal structure and chemical composition found in the tetragonal rutile form of SnO₂. Selected area electron diffraction and high-resolution transmission electron microscopy reveal that the as-prepared nanorods grow along the [1 1 0] crystal direction. We found that the calcined temperature has a strong influence on the size and morphology of SnO₂ nanorods. The growth process of SnO₂ nanorods is suggested to follow an Ostwald ripening mechanism. Our findings indicate that other nanorods or nanowires may be manipulated by using this technique, and might provide insight into the new opportunities to control materials fabrication.     

Structural study of undoped and (Mn, In)-doped SnO2 thin films grown by RF sputtering

Undoped and 5%(Mn, In)-doped SnO₂ thin films were deposited on Si(1 0 0) and Al₂O₃ (R-cut) by RF magnetron sputtering at different deposition power, sputtering gas mixture and substrate temperature. X-ray reflectivity was used to determine the films thickness (10–130 nm) and roughness (～1 nm). The combination of X-ray diffraction and Mössbauer techniques evidenced the presence of Sn⁴⁺ in an amorphous environment, for as-grown films obtained at low power and temperature, and the formation of crystalline SnO₂ for annealed films. As the deposition power, substrate temperature or O₂ proportion are increased, SnO₂ nanocrystals are formed. Epitaxial SnO₂ films are obtained on Al₂O₃ at 550 °C. The amorphous films are quite uniform but a more columnar growth is detected for increasing deposition power. No secondary phases or segregation of dopants were detected.     

Synthesis and characterization of hafnium oxide and hafnium aluminate ultra-thin films by a sol–gel spin coating process for microelectronic applications

Currently there are intense industry-wide efforts in searching for new high dielectric constant (high-k) materials for use in future generations of ultra-large scale integrated circuits (ULSI). There are number of requirements for the new high-k materials, such as high dielectric constant, thermal stability (400 °C or higher), high mechanical strength, and good adhesion to neighboring layers. Oxide spinels comprise a very large group of structurally related compounds many of which are of considerable technological significance. Spinels exhibit a wide range of electronic and magnetic properties in particular nickel, hafnium, cobalt, containing spinels. In the present investigation, crack free, dense polycrystalline monoclinic structure of pure HfO₂, and Al₂HfO₅ ultra-thin films have been prepared by a simple and cost effective sol–gel spin coating method. The formation of the monoclinic HfO₂ phase at 600 °C and complete formation of the single phase Al₂HfO₅ at 800 °C has been reported. The composition of the annealed films has been measured and found to be 70 at.% of O, 30 at.% of Hf for HfO₂ and 22 at.% of Al, 12 at.% of Hf and 66 at.% of O for Al₂HfO₅ films, which are close to the stoichiometry of the HfO₂ and Al₂HfO₅ thin films.     

Structural evolution of SnO2 nanostructure from core–shell faceted pyramids to nanorods and its gas-sensing properties

Tin oxide (SnO₂) nanorods were synthesized through an aqueous hexamethylenetetramine (HMTA) assisted synthesis route and their structural evolution from core–shell type faceted pyramidal assembly was investigated. Structural analysis revealed that the as-synthesized faceted SnO₂ structures were made of randomly arranged nanocrystals with diameter of 2–5 nm. The shell thickness (0–80 nm) was dependent on the molar concentration of HMTA (1–10 mM) in aqueous solution. It was revealed that the self-assembly was possible only with tin (II) chloride solution as precursor and not with tin (IV) chloride solution. At longer synthesis hours, the pyramidal nanostructures were gradually disintegrated into single crystalline nanorods with diameter of about 5–10 nm and length of about 100–200 nm. The SnO₂ nanorods showed high sensitivity towards acetone, but they were relatively less sensitive to methane, butane, sulfur dioxide, carbon monoxide and carbon dioxide. Possible mechanisms for the growth and sensing properties of the nanostructures were discussed.     

Microstructure and stored energy evolutions during rolling of Cu60Zr20Ti20 bulk metallic glass

The microstructure and stored energy of Cu₆₀Zr₂₀Ti₂₀ bulk metallic glass rolled at cryogenic temperature in a wide strain rate range 1.0 × 10^?4 ? 5.0 × 10^?1 s^?1 have been investigated. As the specimen is rolled to be thinner, the stored energy first increases linearly, and then saturates above a critical thickness reduction at lower strain rates, or decreases at high strain rates. At the initial stage of rolling, no phase transformation except shear bands appears in the glass. Phase transformation occurs only when the specimen is severely deformed at strain rates higher than 1.0 × 10^?4 s^?1. As strain rate increases, the critical strain for the stored energy to saturate increases, but the critical strain for phase separation to occur decreases, and meanwhile the type of the phase transformation changes from phase separation to nanocrystallization. The stored energy does not change with the occurrence of phase separation, but decreases due to nanocrystallization. It is proposed that coalescence of more free volume in shear bands into nano-voids should be principally responsible for the saturation of the stored energy, which balances the results from the increase in shear band number at higher strains.     

Optical activity of Sn-variants of oxygen deficient centers in fluorine-modified silica

Photoluminescence in fluorine-modified Sn-doped silica has been analyzed by means of synchrotron radiation in the UV and vacuum-UV, from 120 to 330 nm, looking at the optical activity of oxygen-deficient-centers ODC(II) in Sn-substituted cationic sites. The comparison between F-modified Sn-doped samples and previous data on F-free Sn-doped material evidences differences in the intensity of the 3.2 eV emission band excited at 3.7 eV, and in the thermal dependence of the intensity of this emission excited via intersystem crossing. The role of fluorine in modifying the optical activity of ODC(II) and in the SnO₂ clustering is discussed, showing that an efficient excitation transfer may be activated from SnO₂ to the Sn-variant of ODC(II).     

Tellurite photonic crystal fiber made by a stack-and-draw technique

We report a method for producing, from the raw materials, high optical and geometrical quality glass tubes and photonic crystal fiber (PCF) preforms, without using extrusion or drilling at any stage. A thermal glass study was carried out in order to choose the appropriate glass composition to avoid crystallization problems during the tube, preform and fiber fabrication. A two period PCF was fabricated in addition to a co-doped Erbium and Thulium photonic crystal fiber. In the latter, a 187 nm wide amplified spontaneous emission (ASE) spectrum was obtained when pumping a 15 cm long fiber at a wavelength of 790 nm.     

Preparation and ionic conductivity of transparent glass−ceramics containing a large quantity of lithium−mica

In order to crystallize a large quantity of the lithium?mica in glass?ceramics, 5.1 mass% MgF₂ was added to the starting materials of the parent glasses having chemical compositions of Li_(1+x)Mg₃AlSi_3(1+x)O_10+6.5xF₂ (x = 0.5 and 1.0). Transparent glass?ceramics, in which a large quantity of lithium?mica with particle size of <50 nm was separated, could be prepared from the MgF₂-added parent glass with x = 0.5. While the parent glass, which had a binodal phase separation structure, did not exhibit electrical conductivity, the transparent glass–ceramic was given conductivity by the formation of an interlocking structure of mica. As the separated mica formed a tighter interlocking structure, the conductivity increased and reached a value of 2.0 × 10^?3 S/cm at 600 °C. The MgF₂-added parent glass with x = 1.0 was not transparent because of coarse spinodal phase separation. The conductivity was 4.3 × 10^?4 S/cm at 600 °C but was significantly decreased by the separation of mica.