Optical properties and structure of R2O–Ga2O3–SiO2 and RO–Ga2O3–SiO2 glasses

Refractive index and molar refraction of Li₂O–, Na₂O–, CaO–, and BaO–Ga₂O₃–SiO₂ glasses have been used to test the validity of a structural model of silicate glasses containing Ga₂O₃ glasses. Ga₂O₃ enters these types of glass in a similar manner as Al₂O₃. It is assumed that, for (SiO₂/Ga₂O₃) >1 and (Ga₂O₃/R₂O) ≤1, Ga₂O₃ associates primarily with modifier oxides to form GaO₄ units. The rest of modifier oxide forms silicate units with non-bridging oxygen ions. Silicate structural units have the same factors as found for binary alkali- and alkaline earth silicate glasses. Differences between experimental and model values suggest another structure for (Ga₂O₃/SiO₂) ≥1.     

Densification and dielectric properties of SrO–Al2O3–B2O3 ceramic bodies

The influence of SrO (0·0–5·0 wt%) on partial substitution of alpha alumina (corundum) in ceramic composition (95 Al₂O₃–5B₂O₃) have been studied by co-precipitated process and their phase composition, microstructure, microchemistry and microwave dielectric properties were studied. Phase composition was revealed by XRD, while microstructure and microchemistry were investigated by electron-probe microanalysis (EPMA). The dielectric properties by means of dielectric constant (ε _r ), quality factor (Q × f) and temperature coefficient of resonant frequency (τ _f ) were measured in the microwave frequency region using a network analyser by the resonance method. The addition of B₂O₃ and SrO significantly reduced the sintering temperature of alumina ceramic bodies to 1600 °C with optimum density (∼ 4g/cm³) as compared with pure alumina powders recycled from Al dross (3·55g/cm³ sintered at 1700 °C).     

Structure and electrical conductivity relaxation studies of Nb2O5-doped B2O3–Bi2O3–LiF glasses

Glasses with composition (70 − x) B₂O₃·15Bi₂O₃·15LiF·xNb₂O₅ with x = 0–1.0 mol% were prepared by conventional glass-melting technique. The molar volume V _m values decrease and the glass transition temperatures T _g increase with increase of Nb₂O₅ content up to 0.2 mol%, which indicates that Nb⁵⁺ ions act as a glass former. Beyond 0.2 mol% Nb₂O₅ the V _m increases and the T _g decreases, which suggests that Nb⁵⁺ ions act as a glass modifier. The FTIR spectra suggest that Nb⁵⁺ ions are incorporated into the glass network as NbO₆ octahedra, substituting BO₄ groups. The temperature dependence of the dc conductivity follows the Greaves variable range hopping model below 454 K, and follows the small polaron hopping model at temperatures >454 K. σ _dc, σ _ac conductivity and dielectric constant (ε) decrease and activation energy for dc conduction ΔE _dc which increases with increasing Nb₂O₅ content up to 0.2 mol%, whereas σ _dc, σ _ac and (ε) increase and ΔE _dc decreases with increasing Nb₂O₅ content beyond 0.2 mol%. The impedance spectroscopy shows a single semicircle or arcs which indicate only the ionic conduction mechanism. The electric modulus formalism indicates that the conductivity relaxation is occurring at different frequencies exhibit temperature-independent dynamical process. The (FWHM) of the normalized modulus increases with increase in Nb₂O₅ content suggesting that the distribution of relaxation times is associated with the charge carriers Li⁺ or F⁻ ions in the glass network.     

Synthesis and properties of magnetic solid superacid: SO4/ZrO2–B2O3–Fe3O4

A magnetic SO₄²⁻/ZrO₂–B₂O₃–Fe₃O₄ solid superacid catalyst is prepared via a simple chemical co-precipitation approach. The obtained materials were characterized in detailed by X-ray powder diffraction, thermogravimetric analysis–different scanning calorimetry, Fourier transform infrared spectroscopy (FTIR), electron microscopy (SEM and TEM), and Mossbauer spectra. Powder X-ray diffraction patterns show that in this composite oxide the transformation temperature of ZrO₂ from tetragonal to monoclinic phase is higher compared to the pristine SO₄²⁻/ZrO₂ material. The introduction of Fe₃O₄ endows the superacid with a super-paramagnetic property while in a ferromagnetic state after calcination. The superacid exhibits high catalytic activity in forming ethyl acetate by esterification.     

Crystallization and magnetic properties of a 10Li2O–9MnO2–16Fe2O3–25CaO–5P2O5–35SiO2 glass

The crystallization behavior and magnetic properties of 10Li₂O–9MnO₂–16Fe₂O₃–25CaO–5P₂O₅–35SiO₂ (10LFS) glass have been studied using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) to observe the crystallization behavior and a superconducting quantum interference device (SQUID) for measurements of the magnetic properties. The DTA shows that the 10LFS glass has one broad exothermic peak at approximately 674 °C and one sharp (the highest) exothermic peak at 764 °C. When the 10LFS glass crystallized at 850 °C for 4 h, the crystalline phases identified by XRD were lithium silicate (Li₂SiO₃), β-wollastonite (β-CaSiO₃), lithium orthophosphate (Li₃PO₄), magnetite (FeFe₂O₄) and triphylite (Li(Mn_0.5Fe_0.5)PO₄). The SEM surface analysis revealed that the β-wollastonite and lithium silicate have a lath morphology. The TEM microstructure examination showed that the largest FeFe₂O₃ particles have a size of approximately 0.3 μm. When the 10LFS glass was heat treated at 850 °C for 16 h and a magnetic field of 1000 Oe was applied, a very small remnant magnetic induction of 0.01 emu g⁻¹ and a coercive force of 50 Oe were obtained, which revealed an inverse spinel structure.     

Effects of copper and Al2O3 particles on characteristics of Cu–Al2O3 composites

High-energy milling was used for production of Cu–Al₂O₃ composites. The inert gas-atomized prealloyed copper powder containing 2 wt.%Al and the mixture of the different sized electrolytic copper powders with 4 wt.% commercial Al₂O₃ powders served as starting materials. Milling of prealloyed copper powders promotes formation of nano-sized Al₂O₃ particles by internal oxidation with oxygen from air. Hot-pressed compacts of composites obtained from 5 and 20 h milled powders were additionally subjected to the high-temperature exposure in argon at 800 °C for 1 and 5 h. Characterization of processed material was performed by optical and scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness, as well as density and electrical conductivity measurements. Due to nano-sized Al₂O₃ particles microhardness and thermal stability of composite processed from milled prealloyed powders are higher than corresponding properties of composites processed from the milled powder mixtures. The results were discussed in terms of the effects of different size of starting copper powders and Al₂O₃ particles on the structure, strengthening of copper matrix, thermal stability and electrical conductivity of Cu–Al₂O₃ composites.     

Preparation and characterization of Y2O3–Sm2O3 co-doped ceria electrolyte for IT-SOFCs

Y₂O₃–Sm₂O₃ co-doped ceria (YSDC) powder was synthesized by a gel-casting method using Ce(NO₃)₃·6H₂O, Sm₂O₃ and Y₂O₃ as raw materials. Phase structure of the synthesized powders was characterized by X-Ray diffraction analysis. Sinterability of the powders was investigated by testing the relative density and observing the microstructure of the sintered YSDC samples. Electrical conductivity of the sintered YSDC samples was measured using impedance spectra method. Single solid oxide fuel cells based on the YSDC electrolyte were also assembled and tested. The results showed that YSDC powders with single-phase fluorite structure can be obtained by calcining the dried gelcasts at temperature above 800 °C. Average particle size of the YSDC powder is 50–100 nm. Relative density of more than 95% of the theoretical can be achieved by sintering the YSDC compacts at temperature above 1400 °C. The sintered YSDC sample has an ionic conductivity of 4.74 × 10⁻² S cm⁻¹ at 800 °C in air. Single fuel cells based on the YSDC electrolyte with 50 μm in thickness were tested using humidified hydrogen as fuel and air as oxidant, and maximum power densities of about 190 and 112 mW cm⁻² were achieved at 700 and 600 °C, respectively.     

Low-temperature sintering of ZrW2O8–SiO2 by spark plasma sintering

Amorphous ZrW₂O₈ powder and amorphous SiO₂ powder were prepared by a sol–gel process as raw materials, and high-density ZrW₂O₈–SiO₂ were successfully prepared at a much lower temperature of 923 K for a much shorter holding time of 10 min by spark plasma sintering (SPS) method rather than by conventional melt-quenching method. The relative densities of 0.85ZrW₂O₈–0.15SiO₂ and 0.70ZrW₂O₈–0.30SiO₂ were 99.4% and 96.6%, respectively. The combined technique of a sol–gel process and SPS should enable us to prepare the varied types of high-density composites of ZrW₂O₈ without severe thermal cracking caused by melt-quenching. The thermal expansion properties and dielectric properties of ZrW₂O₈–SiO₂ were also investigated.     

Thermal properties and crystallization of PbO–MoO3–P2O5 glasses

Thermal properties and crystallization of glasses from PbO–MoO₃–P₂O₅ ternary system were studied in three compositional series (100 − x)[0.5PbO–0.5P₂O₅]–xMoO₃ (A), 50PbO–yMoO₃–(50 − y)P₂O₅ (B), and (50 − z)PbO–zMoO₃–50P₂O₅ (C). Glass transition temperature, crystallization temperature, coefficient of thermal expansion, and dilatation softening temperature of the studied glasses were determined by differential thermal analysis and dilatometry. Crystallization products of annealed glass samples were investigated by X-ray diffraction and Raman spectroscopy. X-ray diffraction analysis of crystallized glasses revealed the formation of PbP₂O₆, Pb₃P₄O₁₃, and PbMoO₄ in the samples of the B series. In the series A and C in the samples with a high MoO₃ content, crystalline compounds of Pb(MoO₂)₂(PO₄)₂ and (MoO₂)(PO₃)₂, respectively, were identified. Raman spectra of crystalline samples confirmed the results of X-ray diffraction measurements and provided also information on thermal stability of glasses and formation of glass-crystalline phases in the studied glass series.     

Microwave dielectric properties of MgAl2O4–CoAl2O4 spinel compounds prepared by reaction-sintering process

MgAl₂O₄ spinel exhibits fascinating microwave dielectric properties, but the synthesis of dense MgAl₂O₄ ceramics requires high firing temperatures. In this study, Co is introduced into MgAl₂O₄ ceramics to improve their sinterability and microwave dielectric properties. An Mg_1−xCo_xAl₂O₄ solid solution of a spinel structure was observed in the MgAl₂O₄–CoAl₂O₄ system, and dense Mg_1−xCo_xAl₂O₄ ceramics were obtained by sintering at 1475–1500 °C in air for 2–6 h. Co addition is effective in lowering the sintering temperature to 1475 °C. Q × f of Mg_1−xCo_xAl₂O₄ ceramics was increased to 49,300 GHz with an increase in Co content to 0.2, but degraded with a further increase in Co content. The temperature coefficient of resonant frequency of Mg_1−xCo_xAl₂O₄ ceramics was sustained at between −73 and −23 ppm/°C to the variation of Co content.     

Relaxation process in PbI2–Ag2O–V2O5–B2O3 system: Dielectric,AC conductivity and modulus studies

Silver ion conducting super-ionic glass system xPbI₂–(100 − x) [Ag₂O–2(V₂O₅–B₂O₃)], where, 5 ≤ x ≤ 25, were prepared via melt quenching route and -characterized by XRD and DSC. Their electrical properties were measured by impedance spectroscopy in the frequency range of 2 MHz to 20 Hz from 30 to 120 °C. The electrical relaxation mechanism has been studied using AC conductivity, dielectric modulus function and frequency dependent dielectric permittivity over a wide range of frequency and temperature. Two different scaling approaches for AC conductivity as well as dielectric permittivity spectra were used to understand the nature of relaxation processes.     

Synthesis and properties of glasses in the K2O–SiO2–Bi2O3–TiO2 system and bismuth titanate (Bi4Ti3O12) nano glass–ceramics thereof

Glasses were prepared by the melt-quench technique in the K₂O–SiO₂–Bi₂O₃–TiO₂ (KSBT) system and crystallized bismuth titanate, BiT (Bi₄Ti₃O₁₂) phase in it by controlled heat-treatment at various temperature and duration. Different physical, thermal, optical, and third-order susceptibility (χ³) of the glasses were evaluated and correlated with their composition. Systematic increase in refractive index (n) and χ³ with increase in BiT content is attributed to the combined effects of high polarization and ionic refraction of bismuth and titanium ions. Microstructural evaluation by FESEM shows the formation of polycrystalline spherical particles of 70–90 nm along with nano-rods of average diameter of 85–90 nm after prolonged heat treatment. A minor increase in dielectric constants (ε_r) has been observed with increase in polarizable components of BiT in the glasses, whereas a sharp increase in ε_r in glass–ceramics is found to be caused by the formation of non-centrosymmetric and ferroelectric BiT nanocrystals in the glass matrix.     

Giant electrical conductivity enhancement in BaO–V2O5–Bi2O3 glass by nanocrystallization

The effects of the annealing of 20BaO–30V₂O₅–50Bi₂O₃ glass on the structural and electrical properties were studied by scanning electron micrographs (SEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC) density (d) and dc conductivity (σ). The XRD and SEM observations have shown that the sample under study undergoes structural changes: from amorphous at the beginning, to partly crystalline after nanocrystallization at crystallization temperature (T_c) for 1 h and to colossal crystallization after the annealing at the same temperature for 24 h. The average size of these grains after nanocrystallization at T_c for 1 h was estimated to be about 25–35 nm. However, the glass heat treated at T_c = 580 °C for 24 h the microstructure changes considerably. The nanomaterials obtained by nanocrystallization at T_c for 1 h exhibit giant improvement of electrical conductivity up to four order of magnitude and better thermal stability than the as-received glass. The major role in the conductivity enhancement of this nanomaterial is played by the developed interfacial regions “conduction tissue” between crystalline and amorphous phases, in which the concentration of V⁴⁺–V⁵⁺ pairs responsible for electron hopping is higher than inside the glassy matrix. The annealing at T_c for 24 h leads to decrease of the electronic conductivity. This phenomena lead to disappearance of the abovementioned “conduction tissue” for electrons and substantial reduction of electronic conductivity. The high temperature (above θ/2) dependence of conductivity could be qualitatively explained by the small polaron hopping (SPH) model. The physical parameters obtained from the best fits of this model are found reasonable and consistent with the glass compositions.     

Investigation on the phase stability,sintering and thermal conductivity of Sc2O3–Y2O3–ZrO2 for thermal barrier coating application

Rare earth oxides co-doped zirconia have been developed for application in thermal barrier coating systems to promote the performance and durability of gas turbines. 8 mol%Sc₂O₃, 0.6 mol%Y₂O₃–stabilized ZrO₂ (ScYSZ) powder was synthesized by chemical co-precipitation method. The phase stability, sintering resistance and thermo-physical properties of ScYSZ and 8 wt%Y₂O₃ stabilized ZrO₂ (8YSZ) were investigated. The results indicated that both ScYSZ and 8YSZ show single tetragonal phase before heat treatment. After heat treating at 1500 °C for 300 h, ScYSZ exhibits excellent phase stability with 100% metastable tetragonal (t′) phase, whereas the content of monoclinic phase in 8YSZ reached 49.4 mol%. ScYSZ also exhibits higher sintering resistance and lower thermal conductivity than 8YSZ. ScYSZ can be considered to be explored as candidate material for TBC application.     

Microstructures formed during devitrification of Na2O·Al2O3·B2O3·SiO2·Fe2O3 glasses

Soda alumina borosilicate glasses of composition (24-y)Na₂O·yAl₂O₃·14B₂O₃·37SiO₂·25Fe₂O₃, y = 8, 12, 14, 16 mol%, were melted using Fe₂O₃ as raw material. Besides, samples with y = 12 and Fe₂O₃ concentrations of 14.32, 17.8, and 25.0 mol% were prepared from FeC₂O₄·2H₂O as raw material. The X-ray diffraction analyses showed the presence of magnetite for the samples from all the investigated compositions. Transmission electron microscopy (TEM) evidenced that all the samples are phase separated and droplets in the diameter range 100–1000 nm, enriched in iron, are formed. Inside these droplets, numerous small magnetite particles, with size in the 25–40 nm interval, are crystallized.     

Structural and dielectric properties of Li2O–ZnO–BaO–B2O3–CuO glasses

Glass samples of the system (15Li₂O–30ZnO–10BaO–(45 − x)B₂O₃–xCuO where x = 0, 5, 10 and 15 mol%) were prepared by using the melt quenching technique. A number of studies, viz. density, differential thermal analysis, FT-IR spectra, a.c. conductivity and dielectric properties (constant εφ, loss tan δ, a.c. conductivity, σ_ac, over a wide range of frequency and temperature) of these glasses were carried out as a function of copper ion concentration. The analysis of the results indicate that the density increases while molar volume decreases with increasing of copper content indicates structural changes of the glass matrix. The glass transition temperature, T _g, and crystallization temperature, T _c, increase with the variation of concentration of CuO referred to the growth in the network connectivity in this concentration range, while glass-forming ability parameter (T _c − T _g) decreases with increasing CuO content, indicates an increasing concentration of copper ions that take part in the network-modifying positions. The FT-IR spectra evidenced that the main structural units are BO₃, BO₄, and ZnO₄. The structural changes observed by varying the CuO content in these glasses and evidenced by FTIR investigation suggest that the CuO plays a network modifier role in these glasses while ZnO plays the role of network formers. The dielectric constant decreased with increase in temperature and CuO content. The variation of a.c. conductivity with the concentration of CuO passes through a maximum at 5 mol%. In the high temperature region, the a.c. conduction seems to be connected with the mixed conduction viz., electronic conduction and ionic conduction.     

Glass formation and third-order optical nonlinear characteristics of bismuthate glasses within Bi2O3–GeO2–TiO2 pseudo-ternary system

Glass-forming region of Bi₂O₃–GeO₂–TiO₂ (BGT) pseudo-ternary system was determined by using melt-quench method. A series of high transparent glass samples were selected and their structural characteristics were investigated by FT-IR and Raman spectra. By employing Z-scan and optical Kerr shutter techniques with femtosecond laser pulses as excitation source, third-order optical nonlinearities (TON) of the BGT glasses as well as the TON response time were investigated at wavelength of 800 nm. The ultrafast nonlinear response and high figure of merit suggest great potentials of BGT glasses in applications of all-optical switching or related optical devices.