Gel synthesis and crystallization of lithium-germanate glass powders

Lithium germanate gels, whose compositions are expressed by the general formula Li₂O·xGeO₂ with x=3; 4; 7, were synthesized by hydrolytic polycondensation of germanium ethoxide with lithium methoxide (x=3 and 7) or lithium hydroxide monohydrate (x=4) in alcoholic medium. The values of glass transition temperature of the gels exhibit a maximum at x=4. Crystallization behaviour of the gels, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. The terms x=4 and 7 crystallize in two steps. Microcrystallites of the same composition of the gel are initially formed in an amorphous matrix and are then converted at higher temperatures into well shaped crystals. In term x=3, Li₂GeO₃ and Li₂Ge₄O₉ crystals are directly formed. The values of activation energy for each crystallization step are consistent with the crystallization mechanisms and comparable with those reported with conventional melt glasses from oxides.     

Crystallization and second harmonic generation in potassium-sodium niobiosilicate glasses

Transparent glasses having molar composition (23−x)K₂O·xNa₂O·27Nb₂O₅·50SiO₂ (x=0, 5, 10, 15 and 23) have been synthesized by the melt-quenching technique and their devitrification behaviour has been investigated by DTA and XRD. Depending on the composition, the glasses showed a glass transition temperature in the range 660-680 °C and devitrified in several steps. XRD measurements showed that the replacement of K₂O by Na₂O strongly affects the crystallization behaviour. Particularly, in the glasses with only potassium or low sodium content the first devitrification step is related to the crystallization of an unidentified phase, while in the glass containing only sodium, NaNbO₃ crystallizes. For an intermediate sodium content (x=10 and 15) a potassium sodium niobate crystalline phase, belonging to the tungsten-bronze family, is formed by bulk nucleation. This system looks promising to produce active nanostructured glasses as the tungsten-bronze type crystals have ferroelectric, electro-optical and non-linear optical properties. Preliminary measurements evidenced SHG activity in the crystallized glasses containing this phase.     

The effect of TiO2 on the structure and devitrification behavior of potassium titanium germanate glass

The effect of replacing 20 mol% of GeO₂ by TiO₂ on the properties of potassium germanate glass was investigated. The structure and devitrification behaviour of glasses were studied by Fourier transform infrared spectroscopy (FTIR), differential thermal analysis (DTA) and X-ray diffraction (XRD). It was observed that potassium titanium germanate has a higher glass transition temperature and a higher thermal stability vs. crystallization. The presence of two exothermic peaks on the DTA curve of potassium germanate glass indicates the complex crystallization process. The XRD pattern of this glass heated at the temperature of the first crystallization peak indicated that the GeO₂ and K₂Ge₇O₁₅ were formed. Only the K₂TiGe₃O₉ phase was identified, in a case when potassium titanium germanate glass was heated at the temperature of the crystallization peak.     

Electronic polarizability and crystallization of K2O-TiO2-GeO2 glasses with high TiO2 contents

Some K₂O-TiO₂-GeO₂ glasses with a large amount of TiO₂ contents (15-25 mol%) such as 25K₂O-25TiO₂-50GeO₂ have been prepared, and their electronic polarizability, Raman scattering spectra, and crystallization behavior are examined to clarify thermal properties and structure of the glasses and to develop new nonlinear optical crystallized glasses. It is proposed that the glasses consist of the network of TiO₆ and GeO₄ polyhedra. The glasses show large optical basicities of Λ=0.88-0.92, indicating the high polarizabity of TiO_n (n=4-6) polyhedra in the glasses. K₂TiGe₃O₉ crystals are formed through crystallization in all glasses prepared in the present study. In particular, 20K₂O-20TiO₂-60GeO₂ glass shows bulk crystallization and 18K₂O-18TiO₂-64GeO₂ glass exhibits surface crystallization giving the c-axis orientation. The crystallized glasses show second harmonic generations (SHGs), and it is suggested that the distortion of TiO₆ octahedra in K₂TiGe₃O₉ crystals induces SHGs.     

Synthesis of nanocrystals in KNb(Ge,Si)O5 glasses and chemical etching of nanocrystallized glass fibers

The nanocrystallization behavior of 25K₂O−25Nb₂O₅-(50−x)GeO₂-xSiO₂ glasses with x=0,25,and50 (i.e., KNb(Ge,Si)O₅ glasses) and the chemical etching behavior of transparent nanocrystallized glass fibers have been examined. All glasses show nanocrystallization, and the degree of transparency of the glasses studied depends on the heat treatment temperature. Transparent nanocrystallized glasses can be obtained if the glasses are heat treated at the first crystallization peak temperature. Transparent nanocrystallized glass fibers with a diameter of about 100 μm in 25K₂O-25Nb₂O₅-50GeO₂ are fabricated, and fibers with sharpened tips (e.g., the taper length is about 450 μm and the tip angle is about 12°) are obtained using a meniscus chemical etching method, in which etching solutions of 10 wt%-HF/hexane and 10 M-NaOH/hexane are used. Although the tip (aperture size) has not a nanoscaled size, the present study suggests that KNb(Ge,Si)O₅ nanocrystallized glass fibers have a potential for new near-field optical fiber probes with high refractive indices of around n=1.8 and high dielectric constants of around ε=58 (1 kHz, room temperature).     

The Non-Isothermal Devitrification of Potassium Germanate Glasses

The devitrification behaviour of the glasses K₂O·xGeO₂ with x=4, 7 or 8 was examined by means of differential thermal analysis (DTA), the Fourier transformation infrared (FTIR) transmittance spectra and X-ray diffraction (XRD). The glass transition temperatures were related to the molar ratio GeO₄/GeO₆. For the glass with x=4, metastable K₄Ge₉O₂₀> crystals are initially formed and then converted at higher temperatures into stable K₂Ge₄O₉ crystals. The glasses with x=7 or 8 both devitrify into K₂Ge₇O₁₅> crystals. The effects of the specific surface area of the samples on the devitrification mechanisms were established. Bulk nucleation predominates in the glass with x=4, while the glasses with x=7 or 8 crystallize from the surface. The activation energies for crystal growth were evaluated from the DTA curves. This revised version was published online in July 2006 with corrections to the Cover Date.     

Infrared,Raman and XPS spectroscopic studies of Bi2O3–B2O3–GeO2 glasses

Glasses with composition 50Bi₂O₃–(50 ? x) B₂O₃–xGeO₂ (x = 0, 5, 10, 15 mol%) were prepared by conventional melting method. The thermal properties were investigated by differential thermal analysis (DTA) and the structures of the glasses were probed by Infrared, Raman and X-ray photoelectron spectroscopy (XPS). The results show that density, refractive index and optical basicity increase with the increase of GeO₂. The glass transition temperature (T_g), onset crystallization temperature (T_x) and ΔT (T_x ? T_g) increase as well. The cut-off edges in ultraviolet and infrared shift to longer wavelength by the addition of GeO₂. Infrared, Raman and XPS results indicate that the glass network consists of [Bi–O₆] octahedron, [BO₃] triangle, [BO₄] tetrahedron and [GeO₄] tetrahedron and borate oxide mainly exists in [BO₃] units. XPS result indicates Ge⁴⁺ ions form steady [GeO₄] tetrahedra units in the glass network and the number of non-bridging oxygens decreases with the addition of GeO₂.     

Thermal crystallization studies on Ga−Te glasses

Semiconducting Ga_xTe_100?x (17≤x≤25) glasses have been prepared by melt quenching method and thermal crystallization studies carried out using differential scanning calorimetry. On heating, virgin Ga_xTe_100?x glasses exhibit one glass transition and two crystallization reactions. The first crystallization reaction corresponds to the precipitation of hexagonal Te and the second one to the crystallization of the matrix into zinc blende Ga₂Te₃ phase. If Ga_xTe_100?x glasses are quenched to ambient temperature fromT _crl and reheated, they exhibit the phenomenon of double glass transition.     

Devitrification Behavior of Calcium Phosphate Glasses Containing Al2O3 and SiO2 Additives

The effects of Al₂O₃ and SiO₂ additives on the crystallization of calcium phosphate glasses were studied. When the Al₂O₃ content was higher than 7 mol%, surface devitrification occurred in the glasses. However, for glasses with Al₂O₃ contents higher than 10 mol%, bulk devitrification predominanted. For the glasses with SiO₂, a surface devitrification mechanism predominanted. Non-isothermal DTA techniques were applied in order to establish the devitrification mechanism, and the kinetic parameters of crystal growth were obtained. The parameter m depends on the mechanism and morphology of devitrification of calcium phosphate, glass containing SiO₂ as additive, the values of m being lower than 1.2. These results indicate that the devitrification is controlled by the reaction at the glass-crystal interface, or occurs from surface nuclei. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of lithium-sodium mixed-alkali on phase transformation kinetics in Er/Yb co-doped aluminophosphate glasses

Er³⁺ doped aluminophosphate glasses with various Na₂O/Li₂O ratios were prepared at 1250 °C using a silica crucible to study mixed alkali effect (MAE). The effect of relative alkali content on glass transition temperature, crystallization temperature and thermal stability were investigated using differential scanning calorimetry (DSC). In addition, apparent activation energies for crystallization, E_c, were determined employing the Kissinger equation. The effect of Al₂O₃ content on the magnitude of MAE was also discussed. No mixed-alkali effect is observed on crystallization temperature.     

Optical absorption and EPR spectroscopic studies of (30 − x)Li2O–xK2O–10CdO–59B2O3–1Fe2O3: An evidence for mixed alkali effect

Optical absorption and EPR spectroscopic studies were carried on (30 ? x)Li₂O–xK₂O–10CdO–59B₂O₃–1Fe₂O₃ (x = 0–30) glass system to understand the effect of progressive doping of Li⁺ ion with K⁺ ion. Optical absorption results show typical spectra of Fe³⁺ ions and the various optical parameters such as, optical band gap, Urbach energy, oxide ion polarizability, optical basicity and interaction parameter were evaluated from the experimental data. The observed optical band gap and Urbach energy values show large deviation from the linearity where as the other parameters show small deviation from the linearity with the progressive substitution of Li⁺ ions with K⁺ ions. The observed EPR spectra are representative of Fe³⁺ ion in octahedral and axial fields in the glass network. The number of paramagnetic centers and paramagnetic susceptibility values were evaluated at different resonance lines for all the specimens and these parameters show non-additive nature with the progressive substitution of Li⁺ ions with K⁺ ions in the glass network. This is first ever observation of mixed alkali effect (MAE) in EPR and optical parameters of mixed alkali borate glasses.     

Effect of TiO2 and SrO additions on some physical properties of 33Na2O–xSrO–xTiO2–(50 − 2x)B2O3–17P2O5 glasses

By means of the conventional quenching route, the glass series 33Na₂O–xSrO–xTiO₂–(50 ? 2x)B₂O₃–17P₂O₅ (x = 0–12.5 mol%) were prepared. The amorphous state of samples was verified by X-ray diffraction (XRD). Density, molar volume, micro-hardness, glass transition temperature (T _g), and crystallization temperature (T _c) parameters are determined for each glass. The results show that they depend strongly on the chemical compositions. The structure approach of the glasses is determined by Infrared spectroscopy (IR). This investigation highlights that the glassy-matrix contains various phosphate and borate structural units. The crystallization of the high-TiO₂ glasses by heat-treatments favors the formation of titanate phosphate Na₄TiO(PO₄)₂ or Sr_0.5Ti₂(PO₄)₃ along with Sr₃(PO₄)₂ inside the glass-matrix.     

Thermal properties and stability of lithium titanophosphate glasses

DTA was used to study thermal properties and thermal stability of (50-x)Li₂O-xTiO₂-50P₂O₅ (x=0–10 mol%) and 45Li₂Ot-yTiO₂-(55-y)P₂O₅ (y=5–20 mol%) glasses. The addition of TiO₂ to lithium phosphate glasses results in a non-linear increase of glass transition temperature. All prepared glasses crystallize under heating within the temperature range of 400–540°C. The lowest tendency towards crystallization have the glasses with x=7.5 and y=10 mol% TiO₂. X-ray diffraction analysis showed that major compounds formed by annealing of the glasses were LiPO₃, Li₄ P₂O₇, TiP₂O₇ and NASICON-type LiTi₂(PO₄)₃. DTA results also indicated that the maximum of nucleation rate for 45Li₂O-5TiO₂-50P₂O₅ glass is close to the glass transition temperature.     

Influence of thermal stability on dielectric properties of SiO<Subscript>2</Subscript>–K<Subscript>2</Subscript>O–CaO–MgO glasses

Compositions of 55SiO₂–10K₂O–(35–x)CaO–xMgO are prepared by melt and quench technique. Thermal parameters of the as-prepared glasses are studied using the differential thermal analyzer under non-isothermal conditions. Kissinger, Augis–Bennett and Lasocka models are employed to investigate the kinetics of crystallization and thermal stability of these glasses. Based on this, it is concluded that CM-15 glass exhibits highest thermal stability. Raman spectroscopy is used to reveal the structural units of the glasses. Dielectric properties are observed through impedance spectroscopy. All the glasses are phase separated. The ratio of CaO/MgO influences the thermal stability, which leads to affect the dielectric properties. The highest dielectric permittivity is observed ~22 at room temperature and 100 Hz for CM-15 glass, where CaO/MgO ratio is ~1.33.     

Lithium-cationic conductivity in the Li4 − 2x Cd x GeO4 system

The lithium-conducting solid electrolytes in the Li_{4 ? 2x} Cd _x GeO₄ (0 ≤ x ≤ 0.6) system are synthesized. Their crystal structure and temperature and concentration dependences of conductivity are studied. The specimens with the highest conductivity have a γ-Li₃PO₄-derivative structure. The solid solutions with x = 0.15–0.25 are stable at the room temperature, whereas the specimens with x ≥ 0.3 decompose yielding Li₂CdGeO₄ below 310 ± 10°C. Li_3.6Cd_0.2GeO₄ solid solution exhibits the highest conductivity (5.25 × 10^?2 S cm^?1 at 300°C). The factors, which affect the conductivity of synthesized solid electrolytes, are considered.     

Chemical state analysis of fluorine in various compounds using X-ray fluorescence spectra

F Kα X-ray fluorescence (XRF) spectra of various fluorides were measured and to investigate the chemical state of fluorine in xHoF₃-20BaF₂-10AlF₃-(70 − x)GeO₂ (mol%, x = 10-50) glass. The main peak (Kα_1,2) position and the relative intensity of the satellite peak attributable to Kα_3,4 were discussed relating to the M-F bond covalency. Results showed that the fluorine in xHoF₃-20BaF₂-10AlF₃-(70 − x)GeO₂ (mol%, x = 10-50) glass had a completely different chemical state from that of starting materials. Quantitative analyses of the glasses were also undertaken using XRF measurements for each component. These results suggest that 30% fluorine in the 50HoF₃-20BaF₂-10AlF₃-20GeO₂ system is substituted by oxygen through pyrohydrolysis of the fluorides.     

Thermal and electrical relaxation studies in Li(4+x)TixNb1−xP3O12 (0.0 ≤ x ≤ 1.0) phosphate glasses

A new superionic conducting transparent phosphate glasses with composition Li_(4+x)Ti_xNb_1?xP₃O₁₂ (0 ≤ x ≤ 1.0) were prepared by rapid melt quenching. As quenched samples were characterized by X-ray powder diffraction, differential scanning calorimetric and Fourier transform infrared spectroscopy studies. These glasses were found to have high thermal stability parameter and Li₄NbP₃O₁₂ has been found to have high glass forming ability. Electrical properties of the present glasses were studied by impedance and dielectric spectroscopy in the frequency range 10 Hz–3 MHz in the temperature range 323–523 K. Arrhenius behavior has been observed for all the glass in conductivity, dielectric loss and conductivity relaxation and their activation energies are explained and reported.     

Vibrational spectra and structure of mixed alkali borotungstate glasses: Evidence of mixed alkali effect

Mixed alkali borotungstate glasses with xLi₂O–(30 − x)Na₂O–10WO₃–60B₂O₃ (0 ≤ x ≤ 30) composition were prepared by melt quench technique. FT-IR and Raman spectroscopic studies were employed to investigate the structure of all the prepared glasses. Acting as complementary techniques, both IR and Raman measurements revealed that the network structure of the present glasses mainly based on BO₃ and BO₄ units placed in different structural groups. Raman spectra confirm the IR results regarding the presence of tungsten ions mainly as WO₆ groups. In the present work, the mixed alkali effect (MAE) has been investigated in the above glass system using FTIR and Raman studies.     

Preparation and characterization of lithium ion-conducting glass-ceramics in the Li1 + xCrxGe2 − x(PO4)3 system

Li₂O–Cr₂O₃–GeO₂–P₂O₅ based glasses were synthesized by a conventional melt-quenching method and successfully converted into glass-ceramics through heat treatment. Experimental results of DTA, XRD, ac impedance techniques and FESEM indicated that Li_1.4Cr_0.4Ge_1.6(PO₄)₃ glass-ceramics treated at 900 °C for 12 h in the Li_1 + xCr_xGe_2 − x(PO₄)₃ (x = 0–0.8) system exhibited the best glass stability against crystallization and the highest ambient conductivity value of 6.81 × 10⁻⁴ S/cm with an activation energy as low as 26.9 kJ/mol. In addition, the Li_1.4Cr_0.4Ge_1.6(PO₄)₃ glass-ceramics displayed good chemical stability against lithium metal at room temperature. The good thermal and chemical stability, excellent conducting property, easy preparation and low cost make it promising to be used as solid-state electrolytes for all-solid-state lithium batteries.     

The local structure of bismuth germanate glasses and glass ceramics doped with europium ions evidenced by FT-IR spectroscopy

Glasses of the xEu₂O₃·(100 − x)[4Bi₂O₃·GeO₂] system, with 0 ≤ x ≤ 30 mol%, have been characterized by FT-IR spectroscopy measurements in order to obtain information about the influence of Eu₂O₃ on the local structure of the 4Bi₂O₃·GeO₂ glass matrix. FT-IR spectroscopy data suggest that the europium ions play the network modifier role in the studied glasses and both Bi₂O₃ and GeO₂ play the role of network formers. Melting at 1100 °C and the rapid cooling at room temperature permitted to obtain glass samples. In order to improve the local order and to develop crystalline phases the glass samples were kept at 700 °C for 17 h. Both the influences of the europium ions as well as of the heat treatment on the local order of 4Bi₂O₃·GeO₂ glass matrix have been discussed.