Preparation and characterization of TiO2-BaO-ZnO-B2O3 glass systems for optical devices

We study the TiO₂-BaO-ZnO-B₂O₃ glass system, where the ZnO and B₂O₃ compositions were constant and the ratio TiO₂/BaO was varied from 0.87 to 1.76. A super kanthal resistance furnace was used to melt the compounds inside an alumina crucible, at 1200 °C, for 10 min. After melting, the glasses were poured out into steel moulds and rapidly cooled by quenching. The glasses obtained were homogeneous, bubble free and transparent. They were characterized by X-ray diffractometry, Fourier transform infrared spectroscopy (FTIR), UV-VIS spectroscopy, dilatometry, density and linear refractive index. An infrared cut off caused by the composition influence was found in both IR and UV-vis spectra. From dilatometry T _d and T _g were verified as being anomalous. The linear thermal expansion coefficient presented an anomalous behaviour in relation to TiO₂ concentrations. The density and linear refractive index increased with increasing TiO₂/BaO ratio arriving at their peak value of TiO₂/BaO=1.5 and then decreasing. The dependence of softening point T _d on the ratio TiO₂/BaO exhibited the same behaviour. It is suggested that Ti⁴⁺ plays a dual part in the glass system, assuming a predominantly tetrahedral coordination in the low titania region and a predominantly octahedral coordination in the high titania region. With a heat treatment of the glass around 600 °C, we observed a rapid change of refractive index with increasing temperature.     

Electrical properties and infrared spectra of TeO2-Fe2O3 glasses

The d.c. and a.c. electrical properties on TeO₂-Fe₂O₃ glasses with various compositions of PbO, B₂O₃ and SiO₂ were studied over a frequency range of 10²–10⁵ Hz and in the temperature range 300–500 K. The a.c. conductivity is proportional tow ⁿ, and the conduction mechanism is due to an electronic hopping process. The effects of composition and temperature on the dielectric constant and loss factor (tan ) were studied. The infrared absorption spectra of these glasses reveal that the addition of PbO, B₂O₃ and SiO₂ of these glasses does not introduce any new absorption band in the infrared spectrum of TeO₂-Fe₂O₃ glasses. These results prove the distribution of TeO₄ polyhedra which determines the network and basic oscillation of the building units in the tellurite glasses.     

Fluorescence properties of Eu3+-doped alumino silicate glasses

Alumino silicate glasses of a very broad range of molar compositions doped with 1 ⋅ 10²⁰ Eu³⁺ cm⁻³ (about 0.2 mol% Eu₂O₃) were prepared. As network modifier oxides Li₂O, Na₂O, K₂O, MgO, CaO, SrO, BaO, ZnO, PbO, Y₂O₃ and La₂O₃ have been used. All glasses show relatively broad fluorescence excitation and emission spectra. For most glasses only a weak effect of the glass composition on the excitation and emission spectra is observed. Although the glasses should be structurally similar, notable differences are found for the fluorescence lifetimes. These increase steadily with decreasing mean atomic weight, decreasing refractive index and decreasing optical basicity of the glasses, which may be explained by local field effects. An exception from this rule are the strontium, barium and potassium containing glasses, which show significantly increased fluorescence lifetimes despite of their high refractive index, optical basicity and molecular weight. The non mono-exponential fluorescence decay curves as well as the fluorescence spectra indicate a massive change in the local surroundings of the doped rare earth ions for these glasses.     

Glasses and glass-ceramics in the SrO–TiO2–Al2O3–SiO2–B2O3 system and the effect of P2O5 additions

The glass formation abilities of various compositions in SrO–TiO₂–Al₂O₃–SiO₂, SrO–TiO₂–B₂O₃–SiO₂, SrO–TiO₂–Al₂O₃–B₂O₃, and SrO–TiO₂–Al₂O₃–SiO₂–B₂O₃ systems were studied. Many new compositions were found to be suitable for the casting of crack-free, optically clear glasses of different color and with glass transition temperatures ranging from 595 to 775 °C. The crystallization behavior, structure, and thermal expansion behavior of selected glasses were analyzed by DTA, XRD, dilatometry, and heat treatment. The effect of P₂O₅ on the glass structure and crystallization behavior was also studied. P₂O₅ played a dual role depending on composition. In some glasses it acted as a nucleating agent while in others it suppressed crystallization. Heat treatment of borate and borosilicate glasses transformed them into glass-ceramics while comparable SrO–TiO₂–Al₂O₃–SiO₂ glasses showed a lower tendency to crystallize and form glass-ceramics under the same conditions.     

Effect of compositional changes on the structure and properties of alkali-alumino borosilicate glasses

The effect of replacing BaO by SrO or Al₂O₃ by MgO on the structure and some physical properties of the glasses of the system Li₂O-B₂O₃-(SrO)BaO-(MgO)Al₂O₃-SiO₂—containing TiO₂ have been investigated. Fourier transform infrared (FTIR) spectroscopy revealed that the addition of SrO at the expense of BaO gives no changes in the main structural building units. The addition of MgO instead of Al₂O₃ decreases the fraction of BO₃ and increases the fraction of BO₄ groups. Dilatometric measurements showed that the thermal expansion coefficients (α-values) were increased by gradual addition of SrO or MgO instead of BaO or Al₂O₃, respectively, however the transformation (T_g) and softening (T_s) temperature values of the glasses were decreased. The density was found to decrease as SrO/BaO or MgO/Al₂O₃ replacements increased. The conductivity, dielectric constant and dielectric loss (dielectric constant × loss tangent) of the glasses were investigated using the frequency response in the interval 200 Hz-100 KHz and the effect of compositional change on the measured properties was investigated. Measurements showed that the electric responses of samples were different and complex. The addition of SrO instead of BaO generally, increases the conductivity, dielectric constant and dielectric losss of the glasses. Increasing the MgO at the expense of Al₂O₃, the conductivity and dielectric constant of the glasses were decreased. However, the dielectric losss was increased. The electrical properties were found to be factors that are able to distinguish the various electrical parameters as a result of the change in composition. The obtained data were correlated to the internal structure of the glasses and the nature and role played by glass forming cations.     

Sol-gel preparation and properties of TiO2-P2O5 bulk glasses

Monolithic transparent and colorless, or Ti³⁺-free TiO₂-P₂O₅ glasses containing very large amounts of TiO₂ (up to 93 mol%) were successfully prepared by heat-treating the xerogels, which were made from titanium tetraisopropoxide and triethyl phosphate, through the sol-gel reaction. The density and refractive index n_632.8 nm of the sol-gel-derived glasses were higher than the melt-derived glasses of the corresponding compositions. The glasses of TiO₂ content of larger than 80 mol% seemed somewhat porous, but n_632.8 nm of these glasses was very high as 2.2-2.3. Higher density and higher n_632.8 nm than the melt-derived glasses were considered to be due to more abundance of six-fold coordinated Ti⁴⁺ ions.     

The Raman and infrared spectra of the glasses in the system BaO-TiO2-B2O3

Infrared and Raman spectra were measured for various glasses in the system BaO-TiO₂-B₂O₃, and interpreted on the basis of structure. Emphasis was placed on the spectral effects due to changes in coordination for boron along with BaO and TiO₂ addition. Various vibrational bands and splitting in the spectra indicated Ti-O bonds and Ti-O-B linkages. The resulting structural model was used to interpret compositional effects upon properties, such as glass transformation temperature, d.c. conductivity and coefficient of thermal expansion.     

Influence of calcination,sintering and composition upon microwave properties of the Ba6−xSm8+2x/3Ti18O54-type oxide

Dielectric ceramic compositions for microwave applications belonging to the (BaO) (Sm₂O₃) (TiO₂) ternary phase diagram were studied. Calcination, sintering, microwave properties and influence of secondary phases were investigated. By varying composition and/or sintering process, a high dielectric constant with low dielectric losses and modulable negative or positive temperature coefficient of the resonant frequency can be reach.     

Dielectric characteristics of composite ceramics in the Ba(Mg1/3Ta2/3)O3-BaO · Nd2O3 · 5TiO2 system

Dielectric characteristics of composite ceramics in the system Ba(Mg_1/3Ta_2/3)O₃-BaO · Nd₂O₃ · 5TiO₂ were investigated to search for a new candidate system for microwave dielectric ceramics with modifiable dielectric constant, low dielectric loss and small temperature dependence. The dielectric constant could be adjusted in the range 25–81 by controlling the concentration of BaO · Nd₂O₃ · 5TiO₂, while the dielectric loss remained of the order of 10^–4 for some compositions. Moreover, the dielectric properties in the present system could be significantly improved by post-densification thermal treatment.     

Processing, sintering characteristics and dielectric properties of barium-substituted Pb(Mg1/3Nb2/3)O3–PbTiO3 solid solution

Solid solutions in the lead-based relaxor system Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ were modified by minor substitutions of Ba in the Pb-site of the perovskite lattice. The modified compositions were calcined at 830 °C for 3 h to yield fine-grained, single-phase perovskite materials. A small amount of excess MgO(0.05 wt %), which mostly served as a sintering aid, was added to the calcined batches and the resulting mixtures were sintered at temperatures between 1150 and 1250 °C for periods ranging from 3 to 5 h. The substitutions of BaO for PbO in the perovskite solid solution lattice caused a progressive lowering of the Curie point with increasing BaO content. On average, the Curie point decreased by about 10 °C for each mole of BaO substituted for PbO. Among the various Ba-substituted solid solutions studied, the one with a nominal composition Pb_0.99Ba_0.01[(Mg_1/3Nb_2/3)_0.9Ti_0.1]O₃ which has a Curie point located near 28 °C, exhibited excellent dielectric properties. On sintering at 1250 °C for 3 h, this composition yielded a density near 96% of the theoretical density. The peak dielectric constant of the composition at 1 KHz was slightly higher than 22000, and the corresponding tan δ value was 1.5% with a specific resistivity of 2.5 × 10¹² Ω cm^-1. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructural and dielectric behaviour of glass ceramics in the system PbO-BaO-TiO2-B2O3-SiO2

Glasses in the system PbO-BaO-TiO₂-B₂O₃-SiO₂ with and without P₂O₅ as nucleant have been prepared. The glass samples were ceramized based ondta studies. The ferroelectric phase crystallizing out has been found to be BaTiO₃ fromxrd. The optical and scanning electron micrographs show the presence of BaTiO₃ as major phase. In these glass ceramic samples, dielectric constant and dissipation factor are approximately constant with temperature and frequency upto the glass transition temperatureT _g and thereafter increase sharply with temperature and finally level off. The addition of P₂O₅ as nucleant and molar ratio of (PbO + BaO) to TiO₂ has marked influence on the dielectric behaviour and composition of ferroelectric phase crystallizing out.     

The role of TiO2 and composition in the devitrification of near-stoichiometric cordierite

Devitrification behavior and thermal expansion of glasses and glass-ceramics, doped with TiO₂, near the stoichiometric cordierite composition were investigated. The activation energy for growth of surface nucleated crystals was shown to be approximately 433 kJ/mol. and was independent of TiO₂ content in the glass. Volume nucleation was achieved by the addition of approximately 8 wt% TiO₂, but the mechanism of volume nucleation was different depending upon cordierite composition. Regions in the MgO-Al₂O₃-SiO₂ phase diagram near the stochiometric cordierite compound were investigated using thermal analysis, X-ray powder diffraction and dilatometry. Glass in glass phase separation was postulated for MgO-lean glasses, whereas precipitation of mullite and Al₂TiO₅ preceded devitrification in other compositions. In each case, the formation of a silica-rich glass is believed to initiate the devitrification. Coefficients of thermal expansion for the glass-ceramics increased with increasing TiO₂ content resulting from increasing levels of uncrystallized glass and the formation of mullite and rutile during crystallization.     

The vibrational spectra of MgO-Al2O3-SiO2 glasses containing TiO2

The infrared spectra from a series of MgO-Al₂O₃-SiO₂ glasses containing TiO₂ are consistent with the existence of a phase-separated structure consisting of a high silica phase and a high modifier phase of metasilicate composition. The invariance of the spectra throughout the range of pre-crystallization heat treatments and compositions precludes the possibility of significant changes in the average number of non-bridged oxygen ions per silica tetrahedron either during nucleation treatments or upon the addition of TiO₂ to the base glass. The Raman spectra from the same series of glasses consist of two main high-frequency bands, at 1000 and 910 cm^–1 which change markedly in relative intensity as TiO₂ is added to the base glass, and several subsidiary bands at lower frequencies. It is suggested that the high-frequency bands arise from two dissimilar metasilicate-type structures which are the pre-cursors of the major crystalline phases which precipitate upon devitrification of the glasses, namely cordierite in the low titania glasses (rings of [SiO₄]^2– tetrahedra), and a pyroxene similar to enstatite in the high titania glasses (chains of [SiO₄]^2– tetrahedra).     

The role of bulk nucleation in the formation of crystalline cordierite coatings produced by air plasma spraying

Near stoichiometric cordierite (2MgO–2Al₂O₃–5SiO₂) glasses doped with 6.3, 8.3 and 10.1 weight percent (wt.%) TiO₂ were plasma sprayed onto SiO₂-based refractory concrete substrates. Substrate temperatures were controlled such that the maximum surface temperature exceeded the crystallization temperature during plasma spraying. Only the cordierite powders containing 8.3 and 10.1 wt.% TiO₂ produced crystalline coatings. For compositions with less than 8 wt.% TiO₂ the crystallization is believed to be controlled by surface nucleation at imperfections. Surface nucleation became more difficult with increasing preheat temperature and is related to improved wetting between coating splats. Bulk nucleation-controlled crystallization was observed at compositions above 8 wt.% TiO₂ and the presence of Al₂TiO₅ was detected in these compositions.     

Liquid phase epitaxy of hexagonal ferrites and spinel ferrites on non-magnetic spinel substrates

Using a PbOBaOB₂O₃ fluxed melt which had been employed previously to grow hexagonal ferrite Zn₂Y films on hexagonal ferrite M substrates, isothermal dipping liquid phase epitaxy produced epitaxial films on non-magnetic spinel substrates. The substrates studied were single crystals of three different compositions: MgAl₂O₄, MgGa₂O₄ and Mg(In,Ga)₂O₄. Two different film phases were identified: a lightly Zn-substituted M-type hexagonal ferrite and a heavily Zn-substituted magnetite.     

Structural and other physical properties of barium vanadate glasses

XRD, IR spectra, DTA, density, oxygen molar volume and dc conductivity of barium vanadate glasses of compositions xBaO(100 − x)V₂O₅, where x = 30, 35, 40, 45 and 50 mol%, are reported. The IR studies of the glasses suggest the glass network is built up of mainly VO₄ polyhedra. The glass transition temperatures are observed to increase with an increase of BaO content in the compositions. The cross-linking density decrease with increasing BaO content in the compositions. Introduction of BaO into the V₂O₅ matrix changes the 2D layer structure of the crystalline V₂O₅ into a more complicated 3D structure. Analysis of the electrical properties has been made in the light of small polaron hopping model. The parameters obtained from the fits of the experimental data to this model are reasonable and consistent with glass composition. The conduction is attributed to non-adiabatic hopping of small polaron.     

Photoconduction of rapidly quenched films in the PbOTiO2 system

The PbOTiO₂ films were prepared by a rapid quenching technique using a twin-roller type equipment. The quenched films containing from 10 to 30 mol% TiO₂ precipitated tetragonal PbO solid solution as single phase. The PbTiO₃ crystal precipitated from the melt of equimolar composition of PbO and TiO₂ had a denser structure and a lower tetragonality compared to that obtained by solid phase reaction. In the films containing 60 mol% TiO₂ or above, the formation of amorphous phase was predominant. Crystalline films, as well as PbTiO₃ film, showed photoconductivity.     

IR study of Pb-Sr titanate borosilicate glasses

The infrared spectra (IR) of various glass compositions in the glass system, [(Pb _x Sr_1−x )O·TiO₂]-[2SiO₂·B₂O₃]-[BaO·K₂O]-[La₂O₃], were recorded over a continuous spectral range (400–4000 cm⁻¹) to study their structure systematically. IR spectrum of each glass composition shows a number of absorption bands. These bands are strongly influenced by the increasing substitution of SrO for PbO. Various bands shift with composition. Absorption peaks occur due to the vibrational mode of the borate network in these glasses. The vibrational modes of the borate network are seen to be mainly due to the asymmetric stretching relaxation of the B-O bond of trigonal BO₃ units. More splitting is observed in strontium-rich composition.     

Preparation and properties of rapidly quenched glasses in the V2O5-PbO system

Rapidly quenched glassy films in the V₂O₅-PbO system were prepared using a twin-roller type apparatus. The glass formation region was 0–60 mol% PbO. The first crystallization temperature increased from 210 C for the V₂O₅ glass to 292 C for the glass of 60 mol% PbO. The metastable unknown phase was first crystallized in the glasses around 50 mol% PbO. Infrared absorption spectra of the glasses were measured. Electrical conductivity of the glasses decreased with increasing of PbO content. The electrical behaviour of the glasses is discussed based on the glass structure estimated from the infrared spectral data.     

Preparation and properties of glass-ceramic materials obtained by recycling goethite industrial waste

The recycling of toxic goethite waste, originated in the hydrometallurgy of zinc ores, in glass-ceramic matrices has been studied. Oxide compositions suitable to form glasses were prepared by mixing the goethite waste with granite scraps and glass cullet, yielding the following oxide composition (wt%): SiO₂, 44.6; Al₂O₃, 3.3; Fe₂O₃, 25.5; MgO, 1.6; CaO, 4.5; Na₂O, 5.9; PbO, 3.1; ZnO, 6.5; K₂O, 1.0; TiO₂, 2.0; other 2.0. By proper addition of carbon powder, the initial Fe³⁺/Fe²⁺ ratio (12) of glasses melted in air at 1450 °C was approximated to the stoichiometric value of magnetite (2) to obtain high nucleation and crystallization rates. The heat treatment of iron supersaturated goethite glasses above 630 °C led to the formation of magnetite nuclei with a high tendency to grow and coalesce with time. The crystallization of pyroxene, occurring on the magnetite crystals above 800 °C, was found to be influenced by the nucleation period, so that the highest crystalline volume fraction, V _f (0.80–0.85), was obtained for 90–120 min nucleation time at 670 °C and 120 min crystallization at 860 °C. This revised version was published online in November 2006 with corrections to the Cover Date.