Oxidation of a coal particle in flow of a supercritical aqueous fluid

A study was made of the conversion of single spherical coal particles of diameter 1–5 mm in a supercritical H₂O/O₂ fluid with an oxygen mass fraction of 0–6.6% in a semibatch reactor at a pressure of 30 MPa and a temperature of 673–1023 K. A decrease in the particle mass was observed in two parallel processes: gasification of coal with water and oxidation of coal with oxygen. An activation energy 19 ± 7 kJ/mole and a pre-exponential factor 10^−2±0.4 sec⁻¹ were obtained under the assumption of zero order for the concentration H₂O and an Arrhenius dependence for the rate of gasification with water. The oxidation with oxygen at a temperature above 780 K was found to be limited by the rate of O₂ diffusion to the coal organic matter. Below 780 K, the rate of heterogeneous oxidation with oxygen is described by a first-order reaction for the concentration of O₂ and a zero-order reaction for the concentration of H₂O with an activation energy of 150 ± 27 kJ/mole and a pre-exponential factor of 10^7.6±1.9 cm³/(g · sec). __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 2, pp. 23–31, March–April, 2008.     

Properties and catalytic mechanism of γ-glutamyltranspeptidase from B.subtilis NX-2

Since γ-glutamyltranspeptidase (GGT) especially catalyses the transfer of the γ-glutamyl moiety to a variety of amino acids and short peptides, GGT has important practical value for enzymatic synthesis of γ-glutamyl compounds. In this paper, the GGT produced from Bacillus subtilis NX-2 was purified by a combination of ammonium sulfate fractionation and ion exchange chromatography, and the properties of purified GGT were investigated. At the conditions of pH 10.0, D-glutamine (D-Gln)/L-tryptophan (L-Trp) with a molar ratio of 5: 7, a temperature 40°C and a reaction time of 4 h, a higher conversion rate of 42% was obtained. According to the time course, the catalytic mechanism of enzymatic synthesis of γ-D-glutamyl-L-tryptophan (γ-D-Gln-L-Trp) was discussed. It was demonstrated that the GGT can catalyze not only the reaction of transpeptidation, but also the irreversible hydrolysis of the products which results in the decrease of the yield of the products. The affinity parameter of GGT to D-Gln (Km) was 5.08 mmol·L⁻¹ and the maximum reaction rate of transpeptidation (r _max) was determined as 0.034 mmol·min⁻¹·L⁻¹, while the affinity parameter of GGT to γ-D-Gln-L-Trp (K’_m) was 2.267 mmol·L⁻¹, and the maximum reaction rate of hydrolysis (r’_max) was 0.012 mmol·min⁻¹·L⁻¹. __________ Translated from Journal of Chemical Engineering of Chinese Universities, 2008, 22(2): 288–293 [译自: 高校化学工程学报]     

Preparing tin-doped indium oxide ceramic bulk via explosive consolidation and sintering of nanopowders

The tin-doped indium oxide (ITO) ceramic ultra-dense target is the crucial material in spurting preparation of optical-electric films. The difficulty in target fabrication is due to ITO decomposing at high temperatures. In this paper, commercial ITO nanometer powders are used and are explosively shock consolidated with two different sets and different explosives. Experiment data indicate that the shock velocity should be less than 4000 m/sec, and the shock pressure should be more than 6 and even 12 GPa for obtaining good consolidation. The samples are rapidly sintered at a high temperature after shock treatment. A suitable sinter temperature of ITO nanopowders compacted via explosive consolidation (≈1000°C) is determined by a differential thermal analysis. Scanning electron microscope images show that ITO ceramic decomposes and sublimates at a high temperature. The expansion factors of the samples from our experiments and commercial targets obtained by high isostatic pressing (HIP) are measured: α _l = 7.81 → 10⁻⁶ K⁻¹ for samples sintered at 1000°C, α _l = 8.8 → 10⁻⁶ K⁻¹ for samples sintered at 900°C, and α _l = 6.89 · 10⁻⁶ K⁻¹ for HIP samples. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 114–122, March–April, 2007.     

Kinetic Study of the Prooxidant Effect of α-Tocopherol. Hydrogen Abstraction from Lipids by α-Tocopheroxyl Radical

A kinetic study of the prooxidant effect of α-tocopherol was performed. The rates of allylic hydrogen abstraction from various unsaturated fatty acid esters (ethyl stearate 1, ethyl oleate 2, ethyl linoleate 3, ethyl linolenate 4, and ethyl arachidonate 5) by α-tocopheroxyl radical in toluene were determined, using a double-mixing stopped-flow spectrophotometer. The second-order rate constants (k _p) obtained are <1 × 10⁻² M⁻¹ s⁻¹ for 1, 1.90 × 10⁻² M⁻¹ s⁻¹ for 2, 8.33 × 10⁻² M⁻¹ s⁻¹ for 3, 1.92 × 10⁻¹ M⁻¹ s⁻¹ for 4, and 2.43 × 10⁻¹ M⁻¹ s⁻¹ for 5 at 25.0 °C. Fatty acid esters 3, 4, and 5 contain two, four, and six –CH₂– hydrogen atoms activated by two π-electron systems (–C=C–CH₂–C=C–). On the other hand, fatty acid ester 2 has four –CH₂– hydrogen atoms activated by a single π-electron system (–CH₂–C=C–CH₂–). Thus, the rate constants, k _abstr/H, given on an available hydrogen basis are k _p/4 = 4.75 × 10⁻³ M⁻¹ s⁻¹ for 2, k _p/2 = 4.16 × 10⁻² M⁻¹ s⁻¹ for 3, k _p/4 = 4.79 × 10⁻² M⁻¹ s⁻¹ for 4, and k _p/6 = 4.05 × 10⁻² M⁻¹ s⁻¹ for 5. The k _abstr/H values obtained for 3, 4, and 5 are similar to each other, and are by about one order of magnitude higher than that for 2. From these results, it is suggested that the prooxidant effect of α-tocopherol in edible oils, fats, and low-density lipoproteins may be induced by the above hydrogen abstraction reaction.     

Electrothermal parameters and useful life of carbon fibre fabricated from viscose fibre by heat treatment at 2200°C under electric load

The electrothermal behavior of carbon fibre fabricated from viscose fibre by heat treatment at 2200°C was investigated. The analytical expressions correlating the linear density with the electrical resistance, heat capacity, thermal conductivity, temperature of the surface of the fibre, predicted useful life, and electric load were obtained. A method was developed for conducting and mathematically interpreting the experiments on determination of the lifetime before combustion of conducting carbon fibre of different linear density as a function of the strength of the electrical current passed through it. The lifetime of the carbon fibre in air with no electric load was equal to 6.3·10¹⁰ sec and decreased exponentially with an increase in the current strength. The specific resistance is approximately 5.2·10⁻⁵ Ω·m at 20°C, the specific heat capacity varied from 0.64 to 0.93 J/(g·K), and thermal conductivity of 83 to 120 W/(m·K) in the 0–100°C temperature range. UVIKOM, Mytishchi. Translated fromKhimicheskie Volokna, No. 1, pp. 55–58, January–February, 2000.     

Hydrogen Production by Steam Reforming of Ethanol on Potassium-Doped 12CaO · 7Al2O3 Catalyst

The performance of hydrogen production from steam reforming of ethanol were investigated by using the K-doped 12CaO · 7Al₂ O₃ catalyst (defined as C12A7–O⁻/x%K). The conversion of ethanol and hydrogen yield over C12A7–O⁻/x%K catalyst mainly depended on the temperature, K-doping amount, steam-to-carbon ratios (S/C) and contact time (W/F). In order to identify the catalyst’s characteristic and active species on the catalyst’s surface, Brunauer-Emmett-Teller (BET) surface area, CO₂ temperature programmed desorption (CO₂TPD), X-ray diffraction (XRD), Fourier transform infrared (FT–IR) and X-ray photoelectron spectroscopy (XPS) were carried out. Based on the characterization, it was found that active oxygen species and doped potassium play important roles in steam reforming of ethanol over C12A7–O⁻/27.3%K catalyst.     

Formation and growth of dispersed carbon particles during pyrolysis of ethylene,benzene, and naphthalene in a reflected shock wave

The formation and growth rates of dispersed carbon particles were determined experimentally for pyrolysis of ethylene, benzene, and naphthalene in a reflected shock wave at temperatures of 1920–2560 K and hydrocarbon concentrations in argon of 1.8–20%. The diameter of the particles formed was estimated (30–600 Å). The maximum rate of particle formation at various temperatures [(0.7–96) · 10¹⁶ cm⁻³·sec⁻¹] and the particle growth rate (0.002–0.036 cm · sec⁻¹) were determined from results of measurements of reaction (residence) times. For pyrolysis of benzene, the activation energy of the overall process of particle formation is 410 kJ/mole, and for all hydrocarbons studied, the activation energy of the overall process of particle growth is 5–50 kJ/mole. The surface average particle diameter increases with increasing concentration of the initial hydrocarbon at a constant temperature. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 4, pp. 82–89, July–August, 2007.     

Characterizations of composite cathodes with La0.6Sr0.4Co0.2Fe0.8O3?δ and Ce0.9Gd0.1O1.95 for solid oxide fuel cells

Composite cathodes with La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and Ce_0.9Gd_0.1O_1.95 (GDC) are investigated to assess for solid oxide fuel cell (SOFC) applications at relatively low operating temperatures (650–800 °C). LSCF with a high surface area of 55 m²g⁻¹ is synthesized via a complex method involving inorganic nano-dispersants. The fuel cell performances of anode-supported SOFCs are characterized as a function of compositions of GDC with a surface area of 5 m²g⁻¹. The SOFCs consist of the following: LSCF-GDC composites as a cathode, GDC as an interlayer, yttrium stabilized zirconia (YSZ) as an electrolyte, Ni-YSZ (50: 50 wt%) as an anode functional layer, and Ni-YSZ (50: 50 wt%) for support. The cathodes are prepared for 6LSCF-4GDC (60: 40 wt%), 5LSCF-5GDC (50: 50 wt%), and 4LSCF-6GDC (40: 60 wt%). The 5LSCF-5GDC cathode shows 1.29 Wcm⁻², 0.97 Wcm⁻², and 0.47 Wcm⁻² at 780 °C, 730 °C, and 680 °C, respectively. The 6LSCF-4GDC shows 0.92 Wcm⁻², 0.71 Wcm⁻², and 0.54 Wcm⁻² at 780 °C, 730 °C, and 680 °C, respectively. At 780 °C, the highest fuel cell performance is achieved by the 5LSCF-5GDC, while at 680 °C the 6LSCF-4GDC shows the highest performance. The best composition of the porous composite cathodes with LSCF (55 m²g⁻¹) and GDC (5 m²g⁻¹) needs to be considered with a function of temperature.     

Synthesis of borosilicalite-1 with more than four boron per unit cell from fluoride containing media

More than four boron per unit cell has been introduced into the framework of silicalite-1, using fluorine containing media. The initial gels were of composition 9MF–xH₃BO₃–10SiO₂–1.25TPABr–330 H₂O with M = NH₄, Na, K and Cs and x = 0.1–10. The syntheses were carried out in hydrothermal conditions at 170°C. The amount of incorporated TPA⁺ ions remains quasi constant (3.5–3.8/u.c.) up to 4 B/u.c., while it decreases with increasing boron content. The amount of tetrahedral B/u.c. can be as high as 8 in presence of K⁺ or Cs⁺ ions in the as-synthesized samples. It is observed that the preferential countercations to framework negative charges related to the presence of tetrahedral boron, [SiOB]⁻, are TPA⁺, K⁺ and Cs⁺ ions. It is interpreted, that the −4.0 ppm versus BF₃.OEt₂ NMR line stems from tetrahedral framework boron, while the −2.9 ppm NMR line represents both extraframework and/or tetracoordinated deformed framework boron in the structure.     

Improved catalytic hydrolysis of carboxy methyl cellulose using cellulase immobilized on functionalized meso cellular foam

Cellulase from Penicillium funiculosum was immobilized on functionalized MCF (Meso Cellular Foam) silica by imine bond formation followed by reduction using NaBH₄. The specific activities of free and immobilized enzyme were measured for hydrolysis of soluble carboxymethyl cellulose (CMC). The highest activity of MCF immobilized and native enzyme was obtained at optimum pH 5 and 4.5 respectively. Kinetic parameters, Michaelis–Menten constant (Km) and maximum reaction velocity (Vmax), were calculated as Km = 0.025 × 10⁻² mg/mL, Vmax = 5.327 × 10⁻³ U/mg for the free enzyme and Km = 0.024 × 10⁻² mg/mL, Vmax = 9.794 × 10⁻³ U/mg for MCF immobilized enzyme respectively. The reusability of immobilized enzymes showed that 66% of its activity is retained even after 15 cycles. The availability of polar groups (–NH–, –OH) and large pore size of surface modified MCF could be electrostatically stabilizing the cellulase. Functionalized MCF was found to be a promising material for stabilizing cellulase with 16.4 wt% loading of enzyme.     

Kinetic characterization of Prussian Blue-modified graphite electrodes for amperometric detection of hydrogen peroxide

Prussian Blue-modified graphite electrodes (G/PB) with electrocatalytic activity toward H₂O₂ reduction were obtained by PB potentiostatic electrodeposition from a mixture containing 2.5 mm FeCl₃ + 2.5 mm K₃[Fe(CN)₆] + 0.1 m KCl + 0.1 m HCl. From cyclic voltammetric measurements, performed in KCl aqueous solutions of different concentrations (5 × 10⁻²–1 m), the rate constant for the heterogeneous electron transfer (k _s) was estimated by using the Laviron treatment. The highest k_s value (10.7 s⁻¹) was found for 1 m KCl solution. The differences between the electrochemical parameters of the voltammetric response, as well as the shift of the formal potential, observed in the presence of Cl⁻ and NO₃⁻ compared to those observed in the presence of SO₄²⁻ ions, points to the involvement of anions in the redox reactions of PB. The G/PB electrodes showed a good electrochemical stability proved by a low deactivation rate constant (0.8 × 10⁻¹² mol cm² s⁻¹). The electrocatalytic efficiency, estimated as the ratio , was found to be 3.6 (at an applied potential of 0 mV vs. SCE; Γ = 5 × 10⁻⁸ mol cm⁻²) for a H₂O₂ concentration of 5 mm, thus indicating G/PB electrodes as possible H₂O₂ sensors.     

Refractive index and compressibility of the KAlSi<Subscript>3</Subscript>O<Subscript>8</Subscript> glass at pressures up to 6.0 GPa

The refractive index of potassium aluminosilicate glass of the KAlSi₃O₈ composition in the pressure range up to 6.0 GPa has been measured using a polarizing interference microscope and an apparatus with diamond anvils. The changes in the relative density, which characterize the compressibility of the K₂O · Al₂O₃ · 6SiO₂ glass, have been estimated in the pressure range under investigation from the measured refractive indices within the framework of the theory of photoelasticity. The results have been compared with the data previously obtained for the Na₂O · Al₂O₃ · 6SiO₂ glass. Although the molar contents of Al₂O₃ and M ₂O (where M = K or Na) are identical in these glasses, the KAlSi₃O₈ glass exhibits a higher compressibility, which agrees with the lower degree of depolymerization of this glass as compared to that observed in the NaAlSi₃O₈ glass. The pressure derivative of the bulk modulus K′ _t , which is calculated from the Birch-Murnaghan equation for the KAlSi₃O₈ glass (K′ _t = 7–9), is higher than that for the NaAlSi₃O₈ glass (K′ _t = 5.5–6.0). An increase in the pressure derivative of the bulk modulus K′ _t upon replacement of the Na⁺ cations by the K⁺ cations is explained by the inhibition of compression of the large K⁺ cations, which are located in cavities and have a considerably larger orbital radius than the Na+ cations. This manifests itself in the fact that the curves describing the dependences of the change in the relative density (d − d0)/d (compressibility) on the pressure P for the KAlSi₃O₈ and NaAlSi₃O₈ glasses converge at pressures above 4.0 GPa.     

Molecular Spin Ladders Constructed from [Ni(dmit)2]− Building Blocks: Synthesis, Structure and Physical Properties

Abstract  A new ion-pair complex [1-(4-bromobenzyl)-3-methylpyridinium][Ni(dmit)₂] (1), in which dmit = 4,5-dimercapto-1,3-dithiole-2-thione, has been synthesized and structurally characterized. The anions of [Ni(dmit)₂]⁻ stack into dimers, which further construct into a two-leg ladder through lateral S···S interactions. The weak H-bonding interactions of C–H···S and van de Waals interactions between anion and cations were observed. The magnetic susceptibilities measured from 2–300 K indicate an AFM exchange interaction domination and an AFM ordering below ~8 K. The best fit to magnetic susceptibility above 40 K, using a dimer model with s = ?, gives rise to Δ/k_B = 29.8 K, zJ′ = −0.72 K, C = 3.40 × 10⁻³ emu K mol⁻¹ and χ ₀ = −5.8 × 10⁻⁶ emu mol⁻¹ with a fixed g = 2.0. Cyclovoltammetry revealed two quasi-reversible one-electron steps, which are attributed to Ni(IV/III) and Ni(III/II) redox couples. Graphical Abstract  [A novel complex [1-(4-bromobenzyl)-3-methylpyridinium][Ni(dmit)₂] has been synthesized and structurally characterized. The anions of [Ni(dmit)₂]⁻ stack into dimers, which further construct into two-leg ladder through lateral S···S interactions. The weak H–bonding interactions of C–H···S and van de Waals interactions between anion and cations were observed. Moreover, its magnetic property and electrochemical property have been investigated] .     

LiMn2O4–y Br y Nanoparticles Synthesized by a Room Temperature Solid-State Coordination Method

LiMn₂O_4–y Br _y nanoparticles were synthesized successfully for the first time by a room temperature solid-state coordination method. X-ray diffractometry patterns indicated that the LiMn₂O_4–y Br _y powders were well-crystallized pure spinel phase. Transmission electron microscopy images showed that the LiMn₂O_4–y Br _y powders consisted of small and uniform nanosized particles. Synthesis conditions such as the calcination temperature and the content of Br⁻ were investigated to optimize the ideal condition for preparing LiMn₂O_4–y Br _y with the best electrochemical performances. The optimized synthesis condition was found in this work; the calcination temperature is 800 °C and the content of Br⁻ is 0.05. The initial discharge capacity of LiMn₂O_3.95Br_0.05 obtained from the optimized synthesis condition was 134 mAh/g, which is far higher than that of pure LiMn₂O₄, indicating introduction of Br⁻ in LiMn₂O₄ is quite effective in improving the initial discharge capacity.     

Study on the properties of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript>-BaO lead-free glass using in the electronic pastes

The Sb₂O₃ doping lead-free glass in Bi₂O₃-B₂O₃-BaO ternary system were prepared in the composition of several different subsystem, and the glass powder was produced through the process of water quenching. Glass transition temperatures (T _g ), glass soften temperatures(T _s ), the volume resistivity (ρ) in the temperature range of 80–200°C, and linear thermal coefficients of expansion in the temperatures range of 25–300°C (α_25–300) were measured for subsystems along with the different ratio of Bi₂O₃, B₂O₃ and BaO. For these subsystems, T _g ranged from 458 to 481°C, and T _s ranged from 490 to 512°C, both decreasing with the increasing of Bi₂O₃/B₂O₃ ratio, and increasing with the increasing of BaO/B₂O₃ ratio. The measured α_25–300 ranged from 65.3 to 76.3 × 10⁻⁷ K⁻¹, with values increasing with increasing Bi₂O₃/B₂O₃ and BaO/B₂O₃ ratio. The volume resistivity remains at a high standards, which may caused by it’s non-alkali composition, and it fluctuated from 10¹³ to 10¹¹ Ω cm with the temperature varied from 80–200°C. The structure of Bi₂O₃-B₂O₃-BaO ternary leadfree glass system was mearsured by FT-IR. The IR studies indicate that these glasses are made up of [BiO₆], [BO₃], and [BO₄] basic structural units, and it appears that Ba²⁺ acts as a glass-modifier in this ternary system, but the Bi³⁺ has entered the glass network when it is in relative high content so as to change the α_25–300, T _s and T _g .     

Concentration dependence of electric conductivity for fluorine-containing sodium borate glasses

The influence of sodium fluoride additives on the physicochemical properties of glasses in the Na₂O-B₂O₃ systems is investigated. The introduction of sodium fluoride into the Na₂O · 2B₂O₃ and Na₂O · 3B₂O₃ glasses leads to an increase in the electric conductivity. The temperature-concentration dependence of the electric conductivity has been investigated. It is shown that, in glasses of the NaF-Na₂O · 2B₂O₃ system, an increase in the volume concentration of sodium ions from 2.4 × 10⁻² to ∼3 × 10⁻² mol/cm³ is accompanied by an insignificant decrease in the activation energy E_σ from 1.44 to 1.38 eV and a sharp (by a factor of ∼30) increase in the electric conductivity. In glasses of the NaF-Na₂O · 3B₂O₃ system, an increase in the concentration of sodium ions from 1.8 × 10⁻² to ∼2.3 × 10⁻² mol/cm³ brings about an increase in the electric conductivity by a factor of approximately 100 and an increase in E_σ from 1.6 to 1.83 eV. A further increase in the concentration of sodium ions (up to 2.5 × 10⁻² mol/cm³) virtually does not affect the electric conductivity and E_σ. At the same concentration of sodium ions (∼2.46 × 10⁻² mol/cm³) in the 9.8NaF · 90.2[Na₂O · 2B₂O₃] and 57.1NaF · 42.9[Na₂O · 3B₂O₃] glasses, the electric conductivity and the activation energy are considerably higher in the glass with a larger fluorine content. The regularities revealed are interpreted in the framework of the microinhomogeneous glass structure.     

Water vapor and oxygen permeability of wax films

The water vapor (WVP) and oxygen (O₂P) permeabilities of beeswax (BW), candelilla wax (CnW), carnauba wax (CrW) and microcrystalline wax (MW), formed as freestanding films, were determined. CnW and CrW both had small values for O₂P (0.29 and 0.26 g·m⁻¹·sec⁻¹·Pa⁻¹ × 10⁻¹⁴, respectively), which are less than half the value for high-density polyethylene and about a decade greater than the value for polyethylene terephthalate. O₂P values for BW and MW were about 6−9× greater than those of CnW and CrW. WVP of CnW was 0.18 g·m⁻¹·sec⁻¹·Pa⁻¹ × 10⁻¹², which is about one-half the value for CrW and MW and about one-third the value for BW. The WVP of CnW was somewhat less than that of polypropylene and somewhat greater than that of high-density polyethylene. Differences in permeabilities among the wax films are attributed mainly to differences in chemical composition and crystal type as determined by X-ray diffraction.     

A Novel Bimetallic Chain Based on [Mo(CN)<Subscript>8</Subscript>]<Superscript>3−</Superscript> and Yb<Superscript>3+</Superscript> Ions as Building Blocks in Which Containing Many Intriguing Structural Features

A novel Mo^V–Yb^III bimetallic chain, {[Yb^III(bpy)₂(DMF)(H₂O)][Mo^V(CN)₈]·0.5bpy·4.5H₂O}_n (1) (DMF = N,N′-dimethylformamide; bpy = 2,2′-bipyridine), has been constructed by the reaction of [Mo(CN)₈]³⁻ with Yb³⁺ and 2,2′-bipyridine. Complex 1 is confirmed as a host–guest supramolecular structure by X-ray structural analysis. The neighboring chains interact with each other with two types of hydrogen bonds and two types of π···π interactions. Thus complex 1 has a unique 3D network. Magnetic analysis of 1 indicates the presence of a strong Yb^III single-ion effect owing to spin–orbital coupling within this system.     

Permeability of silica monoliths containing micro- and nano-pores

For applications as catalyst supports in flow reactors, porous silica monoliths require a combination of connected pores of micron-scale to enable fluid flow plus nm-scale pores to enable high catalyst area and activity. We have synthesised a range porous silica monoliths, characterised their micron and nm-scale pores and measured their permeability coefficients K. K can be controlled over the range 10⁻¹⁰–10⁻¹⁴ m², primarily by adjustment of the polymer/silane concentration ratio, whilst maintaining the specific surface area and nm-scale porosity approximately constant. For the majority of the silica monolith samples, the measured permeability coefficient K is 2–5 times smaller than K for a hypothetical reference system consisting of a monolith with uniform cylindrical pores aligned in the flow direction and with the same average pore diameter and volume fraction.     

Concentration dependence of electric conductivity for fluorine-containing sodium borate glasses

The influence of sodium fluoride additives on the physicochemical properties of glasses in the Na₂O-B₂O₃ systems is investigated. The introduction of sodium fluoride into the Na₂O · 2B₂O₃ and Na₂O · 3B₂O₃ glasses leads to an increase in the electric conductivity. The temperature-concentration dependence of the electric conductivity has been investigated. It is shown that, in glasses of the NaF-Na₂O · 2B₂O₃ system, an increase in the volume concentration of sodium ions from 2.4 × 10⁻² to ∼3 × 10⁻² mol/cm³ is accompanied by an insignificant decrease in the activation energy E_σ from 1.44 to 1.38 eV and a sharp (by a factor of ∼30) increase in the electric conductivity. In glasses of the NaF-Na₂O · 3B₂O₃ system, an increase in the concentration of sodium ions from 1.8 × 10⁻² to ∼2.3 × 10⁻² mol/cm³ brings about an increase in the electric conductivity by a factor of approximately 100 and an increase in E_σ from 1.6 to 1.83 eV. A further increase in the concentration of sodium ions (up to 2.5 × 10⁻² mol/cm³) virtually does not affect the electric conductivity and E_σ. At the same concentration of sodium ions (∼2.46 × 10⁻² mol/cm³) in the 9.8NaF · 90.2[Na₂O · 2B₂O₃] and 57.1NaF · 42.9[Na₂O · 3B₂O₃] glasses, the electric conductivity and the activation energy are considerably higher in the glass with a larger fluorine content. The regularities revealed are interpreted in the framework of the microinhomogeneous glass structure.