Chemical and sol–gel processing of tellurite glasses for optoelectronics

Recent developments in the application of sol–gel processing technology for tellurite glass systems are reviewed and reported. The processing of telurite glasses via sol–gel entails some difficulties, mainly due to the anomalously high reactivity of Te(IV) alkoxides toward hydrolysis. Although conventional approaches to steric stabilisation of the alkoxides is not successful for these compounds, various successful approaches have been developed which allow the fabrication of transparent films from these precursors. In particular, diol complexation, chemical liberation of water from esterification processes and peptisation methods have been demonstrated. Other successful approaches involve the use of Te(VI) alkoxides and acids as precursors, with TeO₂ based glasses being formed via TeO₃ intermediates which liberate oxygen during heat treatment. One drawback with all these methods is the inherent thermal instability of the sol–gel derived material, which leads to both the liberation of free tellurium and devitrification of the glass on heat treatment. However this problem is less significant when Te(VI) precursors are used. The fabrication of multicomponent tellurite glasses by sol–gel approaches is very successful. Systems such as TeO₂–TiO₂ and TeO₂–PbO–TiO₂ have been successfully fabricated, and exhibit much greater resistance to devitrification allowing fully dense, transparent glasses to be produced.     

Glass formation range in the SeO2-TeO2-V2O5-MoO3 system

The glass forming region in the quaternary system under increased oxygen pressure and at a slow melt cooling rate (2 to 2.5° C min^-1) has been determined. The stable glasses are located in the central part of the system but nearer to the SeO₂-TeO₂ side. The structural units of these two glass formers are of decisive importance in building up the glass lattice. Infrared spectra of selected compositions from the glass forming region are taken. From the data obtained for the binary glasses in the TeO₂-V₂O₅, TeO₂-SeO₂, TeO₂-MoO₃, V₂O₅-MoO₃ systems and the spectra of the four component compositions, it is shown that the basic structural units participating in the glass lattice formation are the SeO₃, VO₅, TeO₄ and TeO₃ groups. Structural models are proposed: glasses in the SeO₂ direction possess laminar and chain structure, while with increase of TeO₂ concentration, a three-dimensional structure is built up.     

A study of optical absorption in tellurite and tungsten-tellurite glasses

A series of glass specimens was prepared from TeO₂ glass and from the binary tungsten tellurite glasses (TeO₂-WO₃) and their densities, optical absorption edges and infrared absorption spectra were measured. It was found that the fundamental absorption edge is a function of glass composition, and absorption in this region is due to indirect electronic transitions in k-space. The main infrared absorption bands in the TeO₂-WO₃ glasses are related to those characteristics of the TeO₂ component.     

Optical absorption studies for TeO2-P2O5 and Bi2O3-TeO2-P2O5 glasses

A range of TeO₂-P₂O₅ and Bi₂O₃-TeO₂-P₂O₅ glass systems were prepared. The optical absorption spectra were measured in the spectral range 300–800 nm and it was found that the fundamental absorption of these glasses is dependent on the glass composition. The optical energy gap of binary glasses increases with increasing TeO₂ content while the addition of Bi₂O₃ to TeO₂-P₂O₅ decreases the optical energy gap. The absorption edges of these glasses arise from direct forbidden transitions and occur at photon energies in the range of 2.17 to 2.97 eV for TeO₂-P₂O₅ glasses and 2.63 to 2.32 eV for Bi₂O₃-TeO₂-P₂O₅ glasses depending on their composition.     

Viscosity properties of tellurite-based glasses

The viscosity behavior of glasses with the composition (90-x)TeO₂-10Bi₂O₃-xZnO with x = 15, 17.5, and 20 (TBZ glasses) and 80TeO₂-(20-y)Na₂O-yZnO system with y = 0, 5, and 10 (TNZ glasses) have been measured as a function of temperature using a beam-bending (BBV) and a parallel-plate (PPV) viscometer. The structure of the glass’ network has been characterized using Raman spectroscopy and has been related to the viscosity temperature behavior and the fragility parameter (m) of the glasses. As the concentration of ZnO in the TBZ system (x) increases, the fragility parameter of the glass increases, whereas it decreases with an increase of the ZnO concentration (y) in the TNZ system. In both glasses, these variations in m have been related to the partial depolymerization of the tellurite network associated with the level of modifier content. The depolymerization of the tellurite network is believed to be the result of a reduction in the number of [TeO₄] units and the formation of [TeO₃] and [TeO₃₊₁] units that occurs with a change in TeO₂ content in the TBZ system and modifier content in the TNZ system.     

Nanoparticles in low melting amorphous selenite materials

The aim of the present investigation is to establish the appropriate routе of nanoparticles formation during heat treatment of selected selenite glasses. Multicomponent compositions containing SeO₂, V₂O₅, TeO₂, MoO₃, ZnO and Ag₂O have been selected. Different preparation methods of the initial glass samples have been combined with heat treatment to influence the glass microstructure and formation of different types of microheterogeneites. TEM and SEM have been used to prove the formation of nanosized particles, randomly distributed in the amorphous matrix volume. Samples containing above 50 wt% Ag₂O show the formation of elementary silver with an average particle size of 50–100 nm. Glass-ceramic materials have been obtained after a long thermal treatment. The main crystal phases detected are Ag₂SeO₃, Ag₂TeO₃ and TeO₂.     

Preparation and structural studies in the (70 − x)TeO2–20WO3–10Li2O–xLn2O3 glasses

Quaternary tellurite glass systems (70 ? x)TeO₂–20WO₃–10Li₂O–xLn₂O₃ where x = 0, 1, 3 and 5 mol% and Ln are La, Pr, Nd, Sm, Er and Yb, respectively, have been prepared by the melt quenching technique. Densities of the obtained glasses were measured and the molar volume was calculated. IR absorption spectra of the present glass systems were determined at room temperature over the range of wavenumbers from 400–1,600 cm^?1. Raman spectra of the present glass samples were measured in the range of 30–1,030 cm^?1. Density, molar volume, IR and Raman spectra of the glasses were discussed by calculating average cross-link density, packing density, theoretically calculated Poisson’s ratio and number of bonds per unit volume of the studied glasses. Also, the quantitative interpretations were based on concentration of ions per unit volume of Te, Ln and O, short distance in nanometre between ions for (Te–O) of TeO₄ and TeO₃ groups, (W–O) of WO₄, WO₆ groups and calculated wavenumber, $ \bar{\upsilon } $ , for TeO₄ and TeO₃, respectively. The average stretching force constant that present in these quaternary glasses has been calculated in order to interpret the data obtained.     

New phospho-tellurite glasses with optimization of transition temperature and refractive index for hybrid microstructured optical fibers

The glass formation and compositional dependences of glass thermal properties and optical properties were investigated in TeO₂–ZnO–Na₂O–P₂O₅ system. The refractive index at 1.55 μm and glass transition temperature varied in a wide range from 1.513 to 2.036 and from 265 °C to 376 °C by controlling of the TeO₂/P₂O₅ and ZnO/Na₂O content, respectively. These properties enable phospho-tellurite glasses with large freedom in designing and fabrication of hybrid microstructured optical fiber. The structures of glasses were investigated by Raman spectra to understand their dependence of structure on composition. Using the present glasses, some hybrid microstructured optical fibers with various dispersion profiles were designed.     

A mathematical model for analysis of sequentially coupled crystallization–melting differential scanning calorimetry peaks and the use of the model for assessing the crystallization resistance of tellurite glasses

Differential scanning calorimetry (DSC) characterization of tellurite glasses doped with lanthanum oxide, which improves their crystallization resistance, has revealed a phase transformation specific to such glasses, in which partial crystallization of a sample is followed by melting of the crystals formed. The experimentally observed dependence of the decrease of crystallization–melting peaks across a series of disperse samples of (TeO₂)_0.72(WO₃)_0.24(La₂O₃)_0.04 glass with increasing particle size upon extrapolation to the size of a bulk sample has been used to assess the crystallization resistance of tellurite glasses for optical applications. The assessment technique comprises DSC characterization of particle-size-classified glass samples and the use of a mathematical model for obtaining the degree of crystallization as a function of temperature and time, α(T, t) through analysis of nonisothermal DSC peaks representing a partial glass crystallization process passing into melting. The crystallization resistance of glass is estimated by extrapolating the maximum α values as a function of particle size to a preform size. Tested for (TeO₂)_0.72(WO₃)_0.24(La₂O₃)_0.04 glass, the technique offers the possibility of selecting preforms for producing fibers from compositionally new, chemically pure tellurite glasses at a given phase purity level.     

Chemical analysis and electrical conductivity of tellurium phosphate glasses doped with bismuth oxide

The chemical composition and the temperature dependence of d.c. electrical conductivity are presented for TeO₂-P₂O₅ and Bi₂O₃-TeO₂-P₂O₅ glass systems. The results have shown that the network former ion has a substantial effect on the electrical conductivity of oxide glasses. Log and activation energy values were found to be sensitive to the addition of TeO₂ and Bi₂O₃. They showed an anomalous behaviour.     

Glass transition temperature-structural studies of tungstate tellurite glasses

Raman, IR and DSC studies have been carried on the (100 − x)TeO₂–xWO₃ (TW) glasses with 10 ≤ x ≤ 40 mol%. The Raman, IR spectra of these samples show that glass network consists of [TeO₃]/[TeO₃₊₁], [TeO₄], [WO₄] and [WO₆] groups as basic structural units. The W ion coordination state changes from 4 to 6 when WO₃ concentration increases beyond 30 mol%. Addition of WO₃ oxide to the TW glasses increases the amount of lower coordination of [TeO₃]/[TeO₃₊₁] units and decreases the higher coordination [TeO₄] units, Te–O–Te chains. From DSC thermogram, thermal properties such as the glass transition temperature (T_g), onset crystallization (T_o) of the glass systems were calculated. The compositional variation of glass transition temperature (T_g) is found to linear with an increase in WO₃ content.     

Comprehensive comparison on optical properties of samarium oxide (micro/nano) particles doped tellurite glass for optoelectronics applications

Rare-earth oxides microparticles doped tellurite-based glass have been studied extensively to improve the capability of optoelectronic devices. We report a detailed comparison between two sets of glass series containing samarium microparticles and nanoparticles denoted as ZBTSm-MPs and ZBTSm-NPs, respectively. The two sets of glass have been successfully fabricated via melt-quenching technique with chemical formula {[(TeO₂)_0.70 (B₂O₃)_0.30]_0.7 (ZnO)_0.3}_1?y (Sm₂O₃ (MPs/NPs))_y with y?=?0.005, 0.01, 0.02, 0.03, 0.04 and 0.05 mol fraction. The TEM analysis confirmed the existence and formation of nanoparticles in ZBTSm-NPs glasses. The density of ZBTSm-NPs glasses was found higher than ZBTSm-MPs glasses due to the distributions of nano-scale particles in tellurite glass network. There was a linear trend of increment in the refractive index in both sets of glass series along with the concentrations of dopants. The refractive index of ZBTSm-NPs glasses was found higher than ZBTSm-MPs glasses due to the shift in compactness of glass structure with nano-scale particles. In comparison, the absorption peaks of ZBTSm-MPs glasses were greater than ZBTSm-NPs glasses which were mainly due to the restriction of electrons mobility in glass network with nano-scale particles. The optical band gap energy in ZBTSm-NPs glasses was found greater than ZBTSm-MPs glasses which correspond to the widening of forbidden gap with nano-scale particles. The polarizability of ZBTSm-NPs and ZBTSm-MPs was found in non-linear trends along with dopant concentrations. Based on these findings, the improvement of optical properties has been made by introducing samarium oxide nanoparticles in tellurite glass which is beneficial for optoelectronic devices.

     

Determination of impurities in TeO2-WO3 fiber-optic glasses by chemical atomic emission spectrometry

We describe a procedure for arc source chemical atomic emission analysis of high-purity TeO₂-WO₃ tellurite glasses using preconcentration of nonvolatile impurities via reactive vaporization of the major glass constituents by fluorination with xenon difluoride in an autoclave. The detection limits of impurities are 10⁻⁸ to 10⁻⁶ wt %.     

Rare-earth ion doped TeO2 and GeO2 glasses as laser materials

Germanium oxide (GeO₂) and tellurium oxide (TeO₂) based glasses are classed as the heavy metal oxide glasses, with phonon energies ranging between 740 cm^?1 and 880 cm^?1. These two types of glasses exhibit unique combinations of optical and spectroscopic properties, together with their attractive environmental resistance and mechanical properties. Engineering such a combination of structural, optical and spectroscopic properties is only feasible as a result of structural variability in these two types of glasses, since more than one structural units (TeO₄ bi-pyramid, TeO₃ trigonal pyramid, and TeO_3+δ polyhedra) in tellurite and (GeO₄ tetrahedron, GeO₃ octahedron) in GeO₂ based glasses may exist, depending on composition. The presence of multiple structural moities creates a range of dipole environments which is ideal for engineering broad spectral bandwidth rare-earth ion doped photonic device materials, suitable for laser and amplifier devices. Tellurite glasses were discovered in 1952, but remained virtually unknown to materials and device engineers until 1994 when unusual spectroscopic, nonlinear and dispersion properties of alkali and alkaline earth modified tellurite glasses and fibres were reported. Detailed spectroscopic analysis of Pr³⁺, Nd³⁺, Er³⁺, and Tm³⁺ doped tellurite glasses revealed its potential for laser and amplifier devices for optical communication wavelengths. This review summarises the thermal and viscosity properties of tellurite and germanate glasses for fibre fabrication and compares the linear loss for near and mid-IR device engineering. The aspects of glass preform fabrication for fibre engineering is discussed by emphasising the raw materials processing with casting of preforms and fibre fabrication. The spectroscopic properties of tellurite and germanate glasses have been analysed with special emphasis on oscillator strength and radiative rate characteristics for visible, near IR and mid-IR emission. The review also compares the latest results in the engineering of lasers and amplifiers, based on fibres for optical communication and mid-IR. The achievements in the areas of near-IR waveguide and mid-IR bulk glass, fibre, and waveguide lasers are discussed. The latest landmark results in mode-locked 2 μm bulk glass lasers sets the precedence for engineering nonlinear and other laser devices for accessing the inaccessible parts of the mid-IR spectrum and discovering new applications for the future.     

Crystallization kinetics and structure of (TeO2-TiO2-Fe2O3) Glasses

Crystallization kinetics and structure of (85TeO₂ + 15TiO₂) and (85TeO₂ + 10TiO₃ + 5Fe₂O₃) glasses are studied using differential scanning calorimetry DSC, IR spectroscopy and XRD. DSC curves in the temperature range from 50 to 525°C with different heating rates from 10 to 40°C/min are used to study the crystallization behavior of the glasses and effects of different heating rates on the glass transition and crystallization temperatures (T_g and T_p). The activation energies of the glass transition and crystallization processes were determined from the shift of T_g and T_p with the heating rates using Kissinger's formula. Effects of the polymorphic nature of TeO₂ on the crystallization mechanisms are discussed and the phases crystallized during the DSC process were identified by XRD. IR spectra in the frequency range (500–4000 cm^-1 ) are measured and possible coordination states of the constituent oxides are discussed for heat-treated and untreated glasses.     

Synthesis and characterization of Cu2O·TeO2 and CuI·Cu2O·TeO2 glasses

Cu₂O·TeO₂ and CuI·Cu₂O·TeO₂ glasses were synthesized and characterized by complex impedance measurement, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy techniques. Samples of the binary and the ternary systems are found to have both Cu⁺ and Cu²⁺ with their relative concentration being composition dependent. Bonds like -O-Cu²⁺-O-, leading to the formation of bridging oxygen are found to form in the binary system. Structural units like (Te₃O₈ ^4–)_n are also found to form when Cu₂O content is high in the binary system. Phase separation is observed in the ternary system. The glass structure and hence the ionic conduction behavior are found to depend upon chemical composition. When CuI content exceeds 60 mol%, the crystalline phase of -Cul gets stabilized at room temperature, thus causing the enhancement in conductivity.     

Magnetic Properties of Some Tellurite Glasses

The AC magnetic susceptibility in the range 5–130 K of the tellurite glass systems: TeO₂–MnO₂–ZnO–PbO and TeO₂–MnO₂–V₂O₅–Fe₂O₃ was measured and analyzed. The investigations of the AC magnetic susceptibility facilitated the determination of the molar susceptibility, paramagnetic magnetic susceptibility, paramagnetic Curie temperature, and magnetic entropy changes of the tellurite glasses. The results clarified that the temperature dependence of the magnetic susceptibility deviated from the Curie law and the increase of the small negative values of Curie temperature indicated negative interchange interactions between the antiferromagnetically coupled manganese ions within the present glass network. The magnetic moments evaluated from susceptibility measurements of the glasses show the predominance of the Mn²⁺ valence state than Mn³⁺ valence state of MnO₂.     

Conducting glasses

Ion-conducting tellurite glasses are built from trigonal bipyramidal TeO₄ units. Neutron diffraction as well as Raman and IR spectroscopic studies have shown that there is a continuous transition from TeO₄ → TeO₃₊₁ → TeO₃ as the alkali oxide content is progressively increased, non-bridging oxygens being created in the process. Electrical conduction in both single and mixed alkali tellurite glasses is satisfactorily explained by the interchange transport mechanism, based on the site-memory effect exhibited by the glass network.     

Glass formation and immiscibility in the TeO2-B2O3-Fe2O3-MnO system

The glass formation in the quaternary TeO₂-B₂O₃-MnO-Fe₂O₃ system and in its ternary systems was investigated. A range of liquid immiscible phases, located near to the binary TeO₂-B₂O₃ and B₂O₃-MnO systems was established. Using transmission electron microscopy, a trend to metastable liquid-phase separation in the single-phase glasses, located near to the boundary of immiscibility was observed. With an increase in the Fe₂O₃ and MnO content still in the process of cooling of the melts, it was possible for a fine glassy crystalline structure to be formed in them. It was shown that by changing the upper limit of the melting temperature and the cooling rate, the glassy crystalline structure and the Fe₃O₄ content could be modified.     

Incorporation of LiNbO3 crystals into tellurite glasses

Transparent tellurite glasses containing 5–10 m diameter LiNbO₃ crystals (3–7 wt%) have been successfully prepared using an incorporation method in which LiNbO₃ crystals are directly dispersed into the 80TeO₂-15Li₂O-5Nb₂O₅ glass. The dissolution behaviour of the LiNbO₃ crystals greatly depends on the Li₂O: Nb₂O₅ ratio in the matrix glasses. In the 80TeO₂-10Li₂O-10Nb₂O₅ matrix glass, the crystals remaining after incorporation have the composition LiNb₃O₈. A small difference in the refractive indices, n, between the TeO₂-based glasses (n=2.07) and the incorporated LiNbO₃ crystals (n=2.296) is a significant reason for the transparency. It is feasible to prepare the highly transparent TeO₂-based glasses containing a large amount of LiNbO₃ crystals by controlling the incorporation process.