Effect of nanocrystallization on the electrical conductivity enhancement and Mössbauer hyperfine parameters of iron based glasses

Selected glasses of Fe₂O₃-PbO₂-Bi₂O₃ system have been transformed into nanomaterials by annealing at temperature close to crystallization temperature (T_c) for 1 h. The effects of the annealing of the present samples on its structural and electrical properties were studied by Mössbauer spectroscopy, transmission electron micrograph (TEM), differential scanning calorimeter (DSC) and dc conductivity (σ). Mössbauer spectroscopy was used in order to determine the states of iron and its hyperfine structure. The effect of nanocrystalization on the Mössbauer hyperfine parameters did not exhibit significant modifications in present glasses. However, in case of glass ceramic nanocrystals show a distinct decrease in the quadrupole splitting (Δ) is observed, reflecting an evident decrease in the distortion of structural units like FeO₄ units. In general, the Mössbauer parameters of the nano-crystalline phase exhibit tendency to increase with PbO₂ content. TEM of as-quenched glasses confirm the homogeneous and essentially featureless morphology. TEM of the corresponding glass ceramic nanocrystals indicates nanocrystals embedded in the glassy matrix with average particle size of about 32 nm. The crystallization temperature (T_c) was observed to decrease with PbO₂ content. The glass ceramic nanocrystals obtained by annealing at T_c exhibit improvement of electrical conductivity up to four orders of magnitude than the starting glasses. This considerable improvement of electrical conductivity after nanocrystallization is attributed to formation of defective, well-conducting phases “easy conduction paths” along the glass-crystallites interfaces.     

The electrical conductivity of fluorozirconate and chloro-fluorozirconate glasses

The electrical conductivities of glasses in the ZrF₄-BaF₂ and ZrF₄-BaF₂-BaCl₂ systems have been measured as a function of temperature. The conductivies of binary fluorozirconate glasses had values comparable to those previously measured for ternary systems. The introduction of chlorine in the glass network markedly decreased the electrical conductivity and increased the activation energy for conduction. The decrease in conductivity could not be explained exclusively on the basis of zero mobility for the Cl atoms; it is proposed that these atoms effectively hinder the motion of the conducting fluorine ions. The activation energy for electrical conduction in fluorozirconate glasses was analysed via a modification of the Anderson and Stuart model for anionically conducting glasses, in which the electrostatic energy was found to predominate. In chlorofluorozirconate glasses, this term appears to be less predominant.     

DC electrical conductivity of some Cd-Ge-As glasses

Results of measurement of d.c. electrical conductivity σ from 85 to 550 K are reported for eleven glass compositions of the Cd-Ge-As system. Three regions are seen in theσ-T data of all these glasses. In region I (85 to 140 K),σ is essentially constant and independent of temperature. In region II (200 to 430 K),σ is thermally activated with a single activation energy. In region III (>430 K) a sudden increase is seen inσ with temperature. From an analysis of the results, it has been possible to identify the mechanism of hopping conductivity among localised ‘defect’ states with region I, normal band type conductivity due to carriers excited to the extended states with region II and thermally assisted memory switching with region III. In the composition dependence ofσ and ΔE for the CdGe _x As₂ glasses, special features are seen at the composition Cd_28·57 Ge_14·28 As_57·15, which has equal mol fractions of CdAs₂ and CdGeAs₂. For the Cd₂Ge-As glasses also, a maximum in ΔE and a minimum inσ are seen at this composition.     

Electron spin resonance and electrical conductivity of vanadate glasses

Electron spin resonance (ESR) and d.c. conductivity were measured for a series of vanadium borophosphate glasses before and after heat treatment. The ESR spectra showed the presence of vanadium in the V⁴⁺ state in all untreated and heat-treated samples free from iron. The variable temperature ESR and d.c. conductivity results obtained on the sample free from iron showed an inflexion at about 140°C. The electrical conductivity was found to decrease on substitution of 1 mol.% V₂O₅ by 1 mol.% Fe₂O₃ which may be due to a decrease in the V⁴⁺/V ratio. However, the electrical conductivity was found to increase on addition of more than 1 mol.% Fe₂O₃ which may be due to possible hopping conduction between Fe²⁺−Fe³⁺, V⁴⁺−Fe³⁺ and Fe²⁺−V⁵⁺. The increase in conductivity in the sample heat treated at 350°C relative to those heat treated at 300°C and 400°C may be due to the variation in the V⁴⁺/V total ratio. The activation energy values for untreated and heat-treated samples were calculated and were found to depend on the variation in the V⁴⁺/V ratio and the microstructure.     

Preparation and electrical conductivity of some chalcogenide glasses at high temperatures

Chalcogenide glasses have been synthesized rapidly by heating the constituents at temperatures in the range 1000 to 1500° C under high pressure argon in internally heated pressure vessels. Some glasses of molecular constitution As₂Se₃Mn_x, and glasses of the Ovshinsky type, e.g. As₃₀Te₄₈Ge₁₀Si₁₂, have been made by this method. The conductivities of the glasses prepared have been measured at temperatures up to about 1200° C, an atmosphere of high pressure argon being employed to suppress volatilization. The liquid glasses were found to change from semiconductors to metallic-type conductors as the temperature was raised. The technique of conductivity measurement has been applied to liquid tellurium to obtain data up to temperatures beyond the normal boiling point. The conductivity was found to increase with temperature to a maximum at around 900° C, and thereafter to diminish. At the highest temperatures, where the liquid can be regarded as virtually metallic, the resistivity was found to show approximately a positive linear dependence of temperature.     

Influence of structural relaxation on the low-temperature thermal conductivity of ancient natural glasses

The thermal conductivities of two natural glasses, amber and obsidian, were measured over the temperature range 0.06–10 K both in the as-received condition and following heat-treatments near and above the glass-transition temperature. The of amber decreased 6% with heat treatment revealing that this glass had undergone configurational relaxation during the 10⁷ years prior to measurement, but with a relaxation time greater than seven years. The of the obsidian did not change with heat treatment, a null result possibly produced by the ratio of oxides comprising the glass.     

Diffusion and electrical conductivity studies in some heterophase glasses containing bismuth

Self-diffusion coefficients of sodium in the temperature range 300 to 375° C have been measured for glasses in the system Na₂O-B₂O₃-Bi₂O₃-SiO₂, by using a radioactive tracer technique. All these glasses have a two-phase structure. The d.c. resistivities of these glasses were also measured over the temperature range 30 to 375° C. The correlation factors,f, of all the samples have been calculated by using the diffusion coefficients and d.c. resistivity values for temperatures above 300° C. The anomalously largef values in the case of Bi₂O₃-containing glasses are thought to be due to a distribution of sodium ions in the dispersed as well as the continuous phases in these systems.     

Control of electrical conductivity with the admixture of chlorine in copper tellurite glasses

Copper tellurite glasses containing. CuCI₂ with composition 65TeO₂-(35–x)CuO-xCuCl₂ (mol%) with x=0, 1, 2, 3, 4, 5 were prepared by quenching the melt. An increase in density with the addition of CuCl₂ and with a corresponding decrease in molar volume, has been observed. The d.c. conductivity of copper tellurite glasses is found to be very sensitive to the reduced valency rationC=[Cu⁺]/[Cu_total] and depends on the relative concentrations of Cu⁺ (reduced valency state) and Cu²⁺ (higher valency state) ions. It is found that by adding cupric chloride to the melt when the glass is formed, the chlorine in the salt which acts as an oxidizing agent alters the ratio of the concentrations of Cu⁺ and Cu²⁺ ions in the glass and hence the conductivity. It is found that more than 2 mol% of cupric chloride reduces the conductivity very sharply due to the formation of chlorine clusters in the form of local TeCl₂₊ whereas less than 2 mol% of CuCl₂ leads to an increase in conductivity due to the Cu⁺Cu²⁺ transition which is negligibly affected by the chlorine due to the formation of TeCl₂ which is amorphous in nature. The increase and decrease of electrical conductivity of glasses containing less or more than 2 mol% CuCl₂ is also interpreted in terms of the electronic transitions between the orbitals of tellurium 3d electrons, their binding energies and peak widths on the basis of XPS (X-ray Photoelectron Spectroscopy) study, and it was found that the increase in bandwidth supports the idea of clustering of chlorine above 2 mol% CuCl₂ and causing a decrease in the conductivity. Overall the conductivity is found to be somewhat uncontrollable in these glasses because it is not simply dependent on the concentration of Cu²⁺ ions.     

Chemical analysis and electrical conductivity of tellurium phosphate glasses doped with bismuth oxide

The chemical composition and the temperature dependence of d.c. electrical conductivity are presented for TeO₂-P₂O₅ and Bi₂O₃-TeO₂-P₂O₅ glass systems. The results have shown that the network former ion has a substantial effect on the electrical conductivity of oxide glasses. Log and activation energy values were found to be sensitive to the addition of TeO₂ and Bi₂O₃. They showed an anomalous behaviour.     

Influence of structural ordering on electrical conductivity of nanostructured manganese zinc ferrite

In the present study, nanostructured manganese zinc ferrite of 11 nm grain size was synthesized by co-precipitation technique and subsequently suitably heat treated to obtain higher grain sizes. The plot of temperature dependence of dc conductivity shows the semiconducting nature of samples. The observed changes in the electrical conductivity have been attributed with the influence of structural ordering upon annealing. The observed decrease in conductivity when the grain size is increased from 11 to 69 nm upon annealing is clearly due to the structural ordering which is evident from FESEM.     

Effect of MnO2 on the electrical conductivity of ZrO2

Electrical conductivity of ZrO₂ doped with MnO₂ has been measured at various temperatures for different molar ratios. The conductivity increases due to hopping of oxygen into neighboring vacancies, created by doping. Increase in temperature increases the rate of hopping, which results in the rise in conductivity and after attaining a maximum the conductivity, decrease due to collapse of the fluorite framework. All compositions show phase transition in ZrO₂ from monoclinic to tetragonal at 746 K. The XRD, DTA, and FT-IR studies were carried out for confirming the doping effect and transition in ZrO₂. The text was submitted by the authors in English.     

Effect of iron on the electrical properties of lead–bismuth glasses

Semiconducting oxide glasses of the system (80 − x)Bi₂O₃–20PbO–xFe₂O₃, where x = 5, 10 and 15 mol.%, were prepared and investigated for dielectric properties in the frequency range 120–100 KHz and temperature range 300–550 K. 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. The ac conductivity results suggest that the correlated barrier hopping (CBH) is dominant in ac conductivity.     

A study of the structural,electrical and optical properties of copper tellurium oxide glasses

The results of an investigation of some properties of CuO-TeO₂ glasses are reported. X-ray diffraction, differential scanning calorimetry and optical absorption measurements are discussed with respect to the compositions. D.c. conductivity is measured in the temperature range 293 to 453 K and discussed in terms of small polaron theory.     

Some structural,electrical and optical properties of copper phosphate glasses containing the rare-earth europium

Various properties of copper phosphate glass containing small amounts of the rare-earth europium have been investigated in the two glass systems Eu₂O₃-CuO-P₂O₅ and Eu₂O₃-CaO-CuO-P₂O₅. Density and molar volumes showed an increase as Eu₂O₃ replaced CuO and CaO in the glass compositions. Enhanced reduction from Cu²⁺ to Cu⁺ valency states of the ions in the glasses due to the presence of europium has been inferred from the results of electron spectroscopy for chemical analysis, and confirmed by results of electron spin resonance measurements. Corresponding electrical and optical band-gap characteristics, however, showed little systematic variation. The tetrahedral structure of P₂O₅ appeared to dominate the structural characteristics and infrared behaviour of the glass. The results are interpreted in terms of molecular structure, electronic configuration and small polaron hopping models. The role of europium in the glass is discussed.