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This review paper summarizes early Mössbauer and DTA studies of different oxide glasses containing small amounts of iron (III) or tin (IV) as the probe. A lot of valuable information of the atomic level has been obtained about the role of nonbridging oxygen (NBO), network former (NWF), network modifier (NWM), local network structure, glass transition, structural relaxation, crystallization, etc. Introduction of alkali oxide into iron (III)-containing oxide glass causes a marked decrease in glass transition temperature ( T g ) amounting to 100 °C and a concordant decrease in quadrupole splitting ( Δ ) of Fe III , which reflects decreased distortion of NWF–oxygen polyhedra and formation of NBO. By contrast, introduction of non-alkali oxide into oxide glass causes an increase in T g amounting to more than 100 °C and a concordant increase in Δ , reflecting increased distortion of NWF–oxygen polyhedra in highly cross-linked network. These experimental results led to a discovery of “ T g - Δ rule”, which was consistent with the “ conformer model ” proposed for polymers by Matsuoka and Quan. Debye temperatures ( θ D ) obtained by low-temperature Mössbauer measurements proved to be useful to determine short- and long-range structures of glass and glass ceramics. Isothermal annealing of vanadate glasses at temperatures higher than T g or crystallization temperature ( T c ) causes a “ tunable ” decrease in DC-resistivity from the order of MΩ cm to Ω cm. Introduction of metal oxide with a narrow bandgap ( E g ) is highly effective to increase the conductivity after the annealing. It was proved that “ structural relaxation ” of NWF–oxygen polyhedra and resultant decrease in the activation energy ( E a ) for conduction are responsible for the improved conductivity. Heat treatment of IR-transmitting aluminate, gallate, and tellurite glasses at temperatures higher than T g or T c revealed that crystallization was triggered by the cleavage of NWF–oxygen bonds. These findings will contribute to the development of functional glass and glass ceramics such as smart glass and eco-friendly glass.

The carbonophosphate Na 3 FePO 4 CO 3 was synthesized by the mechanical ball milling method for the first time. The composition of the obtained sample with a higher amount of Fe 2+ was Na 2.66 Fe 2+ 0.66 Fe 3+ 0.34 PO 4 CO 3 as confirmed by Mössbauer analysis, owing to the good airtight properties of this method. The obtained samples in an organic electrolyte delivered an initial discharge capacity of 124 mAh/g at room temperature, and a larger discharge capacity of 159 mAh/g (1.66 Na + /mole) at 60 °C. With 17 m NaClO 4 aqueous electrolyte, a discharge capacity of 161 mAh/g (1.69 Na + /mole) was delivered because of the high ionic conductivity of the concentrated aqueous electrolyte. During the charge-discharge process, the formation of Fe 4+ after charging up to 4.5 V and the return of Fe 2+ after discharging down to 1.5 V were detected by ex-situ X-ray absorption near edge structure (XANES) analysis.

Heat treatment of 20BaO· x M m O n ·(10− x )Fe 2 O 3 ·70V 2 O 5 glasses containing different metal oxides (M m O n ), such as Fe 2 O 3 , Al 2 O 3 , MnO 2 , CuO, Cu 2 O, ZnO, Ga 2 O 3 , GeO 2 , MoO 3 , SnO 2 and WO 3 , was accompanied by a remarkable decrease in DC -resistivity ( ρ ) at room temperature from the order of MΩ cm to Ω cm. N -type semiconductor model could be successfully applied to discuss the conduction mechanism. Introduction of CuO into network former (NWF) sites caused a remarkable change of ρ from 2.6 × 10 5 to 3.1 Ω cm when annealed at 450 °C for 30 min. Activation energy for conduction ( E a ) decreased from 0.16 to 0.10 eV, suggesting an increased carrier density in the conduction band. Diffuse reflectance spectra of this glass showed a decrease in the bandgap energy ( E g ) from 2.41 to 2.14 eV. Introduction of ZnO into network modifier (NWM) sites resulted in a remarkable decrease in ρ from 4.0 × 10 5 to 4.8 Ω cm after annealing at 450 °C for 30 min, together with a decrease in E a from 0.23 to 0.14 eV. Diffuse reflectance spectra showed a decrease in E g from 2.39 to 2.11 eV. These results prove that heat treatment of vanadate glass causes structural relaxation and concordant decrease in E a and E g which are responsible for the remarkable increase in the high conductivity.

Lithium iron phosphovanadate glasses containing CrIII were obtained by the melt-quench method and applied as cathode active materials in lithium-ion batteries. Amorphous glass structures were produced with compositions of 15Li2O·10Fe2O3·5P2O5·xCr2O3·(70-x)V2O5 and 15Li2O·(10-x)Fe2O3·5P2O5·xCr2O3·70V2O5, where x varied from 0 to 5. Glass ceramic samples were prepared by annealing at 500 °C for 90 min. 57Fe-Mössbauer spectra of 15Li2O‧10Fe2O3‧5P2O5‧70V2O5 glass measured at room temperature before and after the annealing proved that some iron (III) atoms changed the structural role from distorted “octahedral” NWM sites to distorted “tetrahedral” NWF sites. High capacities of 250–300 mA‧h− 1 were achieved during discharge from 4.2 to 2.0 V. The substitution of V with CrIII was shown to have a considerable positive effect on the electrochemical performance of the glass material.

20BaO·5ZnO·5Fe ·70V glass annealed at 450°C for 30 min showed a marked decrease in the electric resistivity ( ) from 4.0×10 to 4.8 Ωcm, while 20BaO·5Cu O·5Fe ·70V glass from 2.0×10 to 5.0 Ωcm. As for the conduction mechanism, it proved that in conjugation with the was most probable. Since Zn and Cu have a 3d -electron configuration in the outer-most orbital, Ga - and GeO -containing vanadate glasses were explored in this study. 20BaO·5Ga ·5Fe ·70V glass showed a less remarkable decrease of from 4.5×10 to 100 Ωcm, and 20BaO·5GeO ·5Fe ·70V glass from 3.3×10 to 400 Ωcm. Activation energies for the conduction ( ) of GeO - and Ga -contaning glasses before the annealing were respectively estimated to be 0.42 and 0.41 eV. It proved that barium iron vanadate glass with a smaller value could attain the higher conductivity after the annealing at temperaures higher than the crystalization temperature.

57 Fe Mössbauer spectrum of conductive barium iron vanadate glass with a composition of 20BaO·10Fe 2 O 3 ·70V 2 O 5 (in mol%) showed paramagnetic doublet peak due to distorted Fe III O 4 tetrahedra with isomer shift ( δ ) value of 0.37 (± 0.01) mm s −1 . Mössbauer spectra of 20BaO·10Fe 2 O 3 · x MoO 3 ·(70 −  x )V 2 O 5 glasses ( x  = 20–50) showed paramagnetic doublet peaks due to distorted Fe III O 6 octahedra with δ ’s of 0.40–0.41 (± 0.01) mm s −1 . These results evidently show a composition-dependent change of the 3D -skeleton structure from “vanadate glass” phase, composed of distorted VO 4 tetrahedra and VO 5 pyramids, to “molybdate glass” composed of distorted MoO 6 octahedra. After isothermal annealing at 500 °C for 60 min, Mössbauer spectra also showed a marked decrease in the quadrupole splitting ( Δ ) of Fe III from 0.70 to 0.77 to 0.58–0.62 (± 0.02) mm s −1 , which proved “ structural relaxation ” of distorted VO 4 tetrahedra which were randomly connected to FeO 4 , VO 5 , MoO 6 , FeO 6 and MoO 4 units by sharing corner oxygen atoms or edges. DC -conductivity ( σ ) of barium iron vanadate glass ( x  = 0) measured at room temperature was 3.2 × 10 −6  S cm −1 , which increased to 3.4 × 10 −1  S cm −1 after the annealing at 500 °C for 60 min. The σ ’s of as-cast molybdovanadate glasses with x ’s of 20–50 were ca . 1.1 × 10 −7 or 1.2 × 10 −7 S cm −1 , which increased to 2.1 × 10 −2 ( x  = 20), 6.7 × 10 −3 ( x  = 35) and 1.9 × 10 −4  S cm −1 ( x  = 50) after the annealing at 500 °C for 60 min. It was concluded that the structural relaxation of distorted VO 4 tetrahedra was directly related to the marked increase in the σ , as generally observed in several vanadate glasses.

This paper reviews recent studies of highly conductive barium iron vanadate glass with a composition of 20 BaO x 10 [Fe.sub.2][O.sub.3] x 70 [V.sub.2][O.sub.5] (in mol %). Isothermal annealing of the vanadate glass for several ten minutes at a given temperature, higher than glass transition temperature or crystallization temperature, caused an increase in σ. Substitution of [Cu.sup.I] (3[d.sup.10]), [Zn.sup.II] (3[d.sup.10]) and [Cu.sup.II] (3[d.sup.9]) for [Fe.sup.III] (3[d.sup.5]) was investigated to elucidate the effect of electron configuration on the conductivity (σ). A marked decrease in the activation energy of conduction ([E.sub.a]) was also observed after the annealing. Values of [E.sub.a] were correlated to the energy gap between the donor level and the conduction band (CB) in the n-type semiconductor model. Isothermal annealing of [Zn.sup.II]-substituted vanadate glass (20 BaO x 5 ZnO x 5 [Fe.sub.2][O.sub.3] x 70 [V.sub.2][O.sub.5]) at 450°C for 30 min showed an increase in a from 2.5 x [10.sup.-6] to 2.1 x [10.sup.-1] S [cm.sup.-1], which was one order of magnitude larger than that of non-substituted vanadate glass (3.4 x [10.sup.-2] S [cm.sup.-1]). Under the same annealing condition, σ's of 2.0 x [10.sup.-1] and 3.2 x [10.sup.-1] S [cm.sup.-1] were observed for 20 BaO x 5 [Cu.sub.2]O x 5 [Fe.sub.2][O.sub.3] x 70 [V.sub.2][O.sub.5] and 20 BaO x 5 CuO x 5 [Fe.sub.2][O.sub.3] x 70 [V.sub.2][O.sub.5] glasses, respectively. These results demonstrate an increase in the carrier (electron) density in the CB, primarily composed of anti-bonding 4s-orbitals. Keywords: highly conductive vanadate glass, heat treatment, structural relaxation, small polaron hopping, n-type semiconductor.

57Fe Mössbauer spectra of 20BaO·3SnO2·7Fe2O3·70V2O5 glass measured at room temperature shows a marked decrease in quadrupole splitting (Δ) of iron (III) from 0.71 to 0.57 and 0.58 (± 0.02) mm s−1 after isothermal annealing at 500 °C for 30 and 60 min, respectively. The Δ value also shows a decrease from 0.71 to 0.65 and 0.60 (± 0.02) mm s−1 after the annealing at 450 °C for 30 and 60 min, respectively. These results reflect decreased distortion of FeO4 and VO4 tetrahedra or structural relaxation of the 3D-network (skeleton). 119Sn Mössbauer spectra of 20BaO·3SnO2·7Fe2O3·70V2O5 glass are comprised of a broad singlet due to distorted SnIVO6 octahedra, which have an identical Δ of 0.51 (± 0.02) mm s−1 irrespective of the annealing at 450 and 500 °C. These Mössbauer studies reveal that FeIII atoms occupy network former (NWF) sites by sharing corner oxygen atoms with VIV and VV atoms to constitute the 3D-network, while SnIV atoms are ionically bonded to the oxygen atoms at “interstitial” sites to play a role of network modifier (NWM). A marked decrease in the DC-resistivity (ρ) from 1.7 MΩ cm to 5.6 and 5.0 Ω cm was observed at room temperature after isothermal annealing at 500 °C for 15 and 30 min, respectively. This is attributed to an increase in the carrier mobility associated with the wide isotropic 5s orbitals of SnIV atoms occupying NWM sites in addition to the increased carrier density caused by the structural relaxation of the 3D-network.

New porous ceramics (PC) prepared by recycling waste glass bottle of soft drinks (80 mass%) and bamboo charcoal (20 mass%) without any binder was applied to the waste water purification under aeration at 25 °C. Artificial waste water (15 L) containing 10 mL of milk was examined by combining 15 mL of activated sludge and 750 g of PC. Biochemical oxygen demand (BOD) showed a marked decrease from 178 to 4.0 (±0.1) mg L −1 in 5 days and to 2.0 (±0.1) mg L −1 in 7 days, which was equal to the Environmental Standard for the river water (class A) in Japan. Similarly, chemical oxygen demand (COD) decreased from 158 to 3.6 (±0.1) mg L −1 in 5 days and to 2.2 (±0.1) mg L −1 in 9 days, which was less than the Environmental Standard for the Seawater (class B) in Japan: 3.0 mg L −1 . These results prove the high water purification ability of the PC, which will be effectively utilized for the purification of drinking water, fish preserve water, fish farm water, etc.

Water purification was examined using porous ceramics prepared by sintering a powder mixture of volcanic ash, waste glass and a small amount of wood charcoal. The porous ceramics had cross-linked 3D-channels of which the diameter ranged from several nm to several μm. Three kilograms of porous ceramics placed in 90 L of circulating artificial seawater, in which several tropical fishes were actually living under aeration, caused a decrease in COD from 23.8 to 13.1 mg L −1 in a week. The number of coliform bacteria was almost constant in a range of 52–65 mL −1 despite that a lot of excrements were discharged frequently. The number of the coliform bacteria in the seawater examined “without the tropical fishes” decreased from 900 to 1 mL −1 in 2 weeks, and COD decreased from 37.9 to 7.9 mg L −1 . It proved that several aerobic bacteria proliferating in the macropores inside the porous ceramics could effectively decompose several organic materials. Graphical abstract