Electrical properties and internal friction in vanadium phosphate glasses

The electrical properties and internal friction in the glasses of (50–80)V₂O₅ · (50-20)P₂O₅ and 70V₂O₅ ·xB₂O₃ · (30–x)P₂O₅ were measured. In both system glasses, one peak on internal friction was observed in the temperature range of –100 to 200° C at a frequency of about 1 Hz. The area of the internal friction peak increased with increasing valence ratioR, similar to those in the d.c. conductivity and dielectric increment. Frequency temperature dependence of dielectric loss peak and peak on internal friction showed a parallel line, and its activation energy was 0.33 eV. From the above results, the peak observed at internal friction was due to the hopping of localized electrons in small polarons.     

Non-adiabatic polaron hopping conduction in semiconducting V2O5-Bi2O3 oxide glasses doped with BaTiO3

BaTiO₃-doped (5–40 wt %) 90V₂O₅-10Bi₂O₃ (VB) glasses have been prepared by a quick quenching technique. The d.c. electrical conductivities, _d.c., of these glasses have been reported in the temperature range 80–450 K. The electrical conductivity of these glasses, which arises due to the presence of V⁴⁺ and V⁵⁺ ions, has been analysed in the light of the small-polaron hopping conduction mechanism. The adiabatic hopping conduction valid for the undoped VB glasses (with 80–95 mol % V₂O₅), in the high-temperature region, is changed to a non-adiabatic hopping mechanism in the BaTiO₃-doped VB glasses. At lower temperatures, however, a variable range hopping (VRH) mechanism dominates the conduction mechanism in both the glass systems. Such a change-over from adiabatic to non-adiabatic conduction mechanism is a new feature in transition metal oxide glasses. Various parameters, such as density of states at the Fermi level N(E_F), electron wave-function decay constant, , polaron radius, r _p, and its effective mass, m _p ^* , etc., have been obtained for all the glass samples from a critical analysis of the electrical conductivity data satisfying the theory of polaron hopping conduction.     

Electrical properties and infrared spectra of TeO2-Fe2O3 glasses

The d.c. and a.c. electrical properties on TeO₂-Fe₂O₃ glasses with various compositions of PbO, B₂O₃ and SiO₂ were studied over a frequency range of 10²–10⁵ Hz and in the temperature range 300–500 K. The a.c. conductivity is proportional tow ⁿ, and the conduction mechanism is due to an electronic hopping process. The effects of composition and temperature on the dielectric constant and loss factor (tan ) were studied. The infrared absorption spectra of these glasses reveal that the addition of PbO, B₂O₃ and SiO₂ of these glasses does not introduce any new absorption band in the infrared spectrum of TeO₂-Fe₂O₃ glasses. These results prove the distribution of TeO₄ polyhedra which determines the network and basic oscillation of the building units in the tellurite glasses.     

Electrical conductivity and electron paramagnetic resonance investigations in manganese-doped polycrystalline Na2Ti3O7

D.c. electrical conductivity studies have been reported in Na₂Ti₃O₇ in the temperature range 370–750 K. Three distinct regions have been identified in the log(T) versus 1000/T plots, the lowest temperature region being proposed to be due to electronic hopping conduction involving loose electrons of Ti₃O ₇ ^2– groups, the intermediate region to associated extrinsic ionic conduction through dilated interlayer space and the highest temperature region to modified interlayer ionic conduction, the modification being affected by loose oxygens from Ti₃O ₇ ^2– groups. The room-temperature electron paramagnetic resonance (EPR) investigations have been interpreted in terms of manganese substitution in the lattice. For a lower percentage of doping the substitution of manganese ions occur as Mn³⁺ at Ti⁴⁺ sites, whereas for a higher percentage of doping, Mn²⁺ ions predominantly occupy the interlayer Na⁺ sites. The dilation of interlayer space has further been proposed to occur due to a Ti⁴⁺ substitution of manganese ions.     

EPR study of Cu2+ in Na2O-NaF-B2O3-Bi2O3 glasses

Electron paramagnetic resonance (EPR) studies have been performed on the glass system (30–x) NaF-xNa₂O-50B₂O₃-20Bi₂O₃ doped with CuO as a paramagnetic probe. The calculated g values indicate that Cu²⁺ is in a tetragonally elongated octahedral co-ordination, and the Jahn-Teller character gives rise to anisotropic hyperfine structure. The spin-Hamiltonian parameters and -bonding parameter, ², are calculated. Varying the fluoride ion concentration and its effect on these parameters is also discussed.     

Infra-red spectra of crystalline phases and related glasses in the TeO2-V2O5-Me2O system

The crystal line phases Me₂O·V₂O₅·2TeO₂ (Me=Li, Na, K, Cs, Ag) and their glasses are studied with the aid of infra-red spectroscopy. The radial distribution function (RDF) curves of two glasses, Na₂O·V₂O₅·2TeO₂ and Cs₂O·V₂O₅·2TeO₂, are obtained by an X-ray diffraction study. An attempt is made to identify the main bands in the infra-red spectra of the crystalline compounds and the glasses. The absorption bands in the 970 to 880 cm^–1 range are assigned to the stretching modes of the VO₂ isolated groups. A trend is observed towards a shift of the high-frequency band by the replacement of an alkaline ion with another in the order Ag⁺, Cu⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, which is explained by their different polarizing ability. With the aid of X-ray diffraction studies it is shown that the basic structure units in the glasses studied are the VO₄ and TeO₄ groups.     

Seebeck coefficient of V2O5-R2O3-TeO2 (R = Sb or Bi) glasses

The thermoelectric power of glasses in the systems V₂O₅-Sb₂O₃-TeO₂ and V₂O₅-Bi₂O₃-TeO₂ was measured at temperatures in the range 373–473 K. The glasses in both systems were found to be n-type semiconductors. The Seebeck coefficient, Q, at 473 K was determined as –192 to –151 VK^–1 for V₂O₅-Sb₂O₃-TeO₂ glasses, and –391 to –202 VK^–1 for V₂O₅-Bi₂O₃-TeO₂ glasses. For these glasses in both systems, Heikes' formula was satisfied adequately for the relationship between Q and In [C _v/(1-C_v)] (C _v = V⁴⁺/V_total, C _v is the ratio of the concentration of reduced vanadium ions), and discussions confirmed small polaron hopping conduction of the glasses in both systems. Mackenzie's formula relating to Q and V⁵⁺/V⁴⁺ was also applicable to the glasses in both systems, and it was concluded that the dominant factor determining Q was C _v.     

Direct current electrical conduction in lead tungsten phosphate glasses

Measurements have been made on d.c. electrical conductivity of semiconducting lead tungsten phosphate glasses (X mol% WO₃–(60–x) mol% PbO–40 mol% P₂O₅;x= 10, 20, 30, 40, 50 and 60) of six different compositions over a temperature range of 240 to 500 K. It is shown that the conduction can be described by a small polaron hopping model. When the WO₃ content is 30 mol% the d.c. conductivity _dc decreases and the activation energyW increases with increasing WO₃ content and the glass samples exhibit predominant ionic conduction, however, when the WO₃ content is > 30 mol%, _dc increases andW decreases with the increase of tungsten ion concentration and the glass samples exhibit electronic conduction. The electronic conduction in these glasses havingx > 30 mol% seems to be adiabatic. Greaves variable range hopping has been found to be valid. The value of the electron wavefunction decay constant a is of the order of 16 nm^–1. A minimum in the d.c. conductivity has been observed when the PbO concentration is 30 mol%.     

Hydrothermal formation of Ni-Zn ferrite from heavy metal co-precipitates

The hydrothermal synthesis of Ni-Zn ferrite from simulated wastewater containing Ni²⁺ and Zn²⁺ ions has been studied. The influence of co-precipitation order, the existence of Na⁺ in suspension, the hydrothermal reaction time and temperature on the composition, morphology and saturation magnetization (_s) of the hydrothermal products is reported. Adding the simulated wastewater to the NaOH solution can prevent the formation of -Fe₂O₃ in the Ni-Zn ferrite. Increasing the hydrothermal reaction time improved the magnetization of the Ni-Zn ferrite, while the influence of temperature, stirring intensity and Na⁺ in suspension on the hydrothermal formation of ferrite were not obvious. Thermodynamic calculation indicated that under hydrothermal conditions (180–240°C), the order of chemical stability is as follows: NiFe₂O₄ > Fe₂O₃ > Na₂Fe₂O₄. The high Gibbs formation energy of Na₂Fe₂O₄ prevented the incorporation of Na⁺ into the ferrite lattice.     

Rare-gas mobility in some anisotropic ceramic oxides: Al2O3, Cr2O3, Fe2O3, TiO2, U3O8

The mobility of the inert gases xenon or radon in five anisotropic oxides (hexagonal corundum Al₂O₃, Cr₂O₃, Fe₂O₃, tetragonal rutile TiO₂, and orthorhombic U₃O₈) was studied. The gases were introduced by ion bombardment. The oxides were in the form of powders, sinters, or single crystals. Normal volume diffusion was found at low gas concentration, the activation energies in kilocalories per mole being 85 for Al₂O₃, 73±5 for Cr₂O₃, 68±5 for Fe₂O₃, 78±5 for TiO₂, and 85±8 for U₃O₈, and the pre-exponential termsD ₀ falling into the ideal range of about 3×10^–1±1 cm²/sec. Structural radiation damage, the annealing of which coincided with gas release at low temperatures, and, in some cases, retardation of the gas release were found at higher gas concentrations. Some evidence is presented that grain boundaries, pre-existing vacancy clusters, and dislocation loops may act as trapping sites for gas atoms (or bubbles) and may be stabilised after trapping of the gas.     

Chemical and structural properties of the system Fe2O3-Cr2O3

Chemical and structural properties of the mixed metal oxides (1–x)Fe₂O₃+xCr₂O₃ were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe_1–x Cr _x )₂O₃, over the whole concentration region, 0x1. The gradual replacement of Fe³⁺ with Cr³⁺ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx0.3–0.9. The samples having the fraction of Cr₂O₃ in the region from 0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.⁵⁷Fe Mössbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr₂O₃ content. The sample having the fraction of Cr₂O₃ of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr₂O₃ content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for -Fe₂O₃ to the spectrum typical for Cr₂O₃ was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and⁵⁷Fe Mössbauer spectroscopic results.     

Generalized Maxwell-Wagner response in dispersive silver borophosphate glasses

The dielectric response of a glass-forming system (Ag₂OB₂O₃P₂O₅) has been measured in the frequency range from 10^–3–10⁵ Hz and over temperatures in the range 150–400 K for three different compositions. The dynamic behaviour of the conductance and capacitance in these glasses has been observed to follow fractional power-law dependencies on frequency which obey the generalized Maxwell-Wagner relationships. The power-law dispersions for the bulk and the surface layer of the non-ideal solid electrolyte 0.6Ag₂OxB₂O₃(0.4–x) P₂O₅ have been modelled mathematically using frequency-dependent resistive and capacitive elements in a conventional equivalent network. It is shown that controlled substitution of B₂O₃ in the glassy network influences the response and introduces an imperfect charge transport, the quasi-d.c. process of limited charge transport in place of bulk conduction, at higher frequencies, and affects the diffusion barrier at the electrodes to make them, weakly, more conductive at the lowest frequencies. The magnitudes of the activation energies of conduction indicate thermally activated localized hopping of silver ions between neighbouring sites in a structure that is modified by the addition of boron oxide.     

Preparation and characterization of two new dielectric ceramics Ba4NdTiNb3O15 and Ba3Nd2Ti2Nb2O15

Two new cation-deficient hexagonal perovskites Ba₄NdTiNb₃O₁₅ and Ba₃Nd₂Ti₂Nb₂O₁₅ were prepared in the BaO–Nd₂O₃–TiO₂–Nb₂O₅ system by high temperature solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction and scanning electron microscope (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. Ba₄NdTiNb₃O₁₅ has a dielectric constant of 38.15, a high-quality factors (Q _u ×f >18 700 at 5.4422 GHz), and a small temperature coefficient of resonant frequency ( _f )+12 ppm °C^–1 at room temperature; Ba₃Nd₂Ti₂Nb₂O₁₅ has a higher dielectric constant of 46.83 with high-quality factors Q _u ×f >19 500 at 5.0980 GHz, and _f +28 ppm °C^–1.     

The dielectric response of doped semiconducting glasses

Measurements are reported of the dielectric properties of two semiconducting glasses of composition SiO₂ · Na₂O with admixtures of Sb_{2O 5} and SnO₂, covering a wide range of temperatures and a range of six decades of frequency 10^–2 to 10⁴ Hz. Both glasses show a gradual evolution of growing low frequency dispersion (LFD) with increasing temperature, with both the real and imaginary components of the complex susceptibility following a power law of the type ^n–1, with n 1 at the lowest frequencies. There are also clear indications of reversible phase transitions taking place which manifest themselves by changes in the spectral shape of the dielectric response.     

EPR and optical absorption of Cr3+ in binary Na2O-B2O3 glasses

The optical absorption and EPR spectra of Cr³⁺ in binary sodium borate glasses have been studied as functions of chromium concentration and Na₂O/B₂O₃ ratio in the glass; the ligand field and EPR parameters have been calculated and were found to be independent of Cr₃₊ concentration in any particular glass. In low-alkali borate glasses (Na₂O = 11 or 14 mol %) a single symmetrical EPR line was observed with g _¦=g =1.984±0.001 corresponding to perfect octahedral symmetry of the Cr³⁺ ion in these glasses. With increasing Na₂O content of the glass, the EPR line becomes more asymmetric (characteristic two-peaked pattern); this has been explained as being due to axial elongation of the six co-ordinated Cr³⁺-complex in these glasses.     

A.c. conduction mechanism and dielectric properties of co-evaporated SiO/B2O3 amorphous thin films

The a.c. electrical properties on Al-SiO/B₂O₃-Al sandwich devices with various compositions of SiO/B₂O₃ were studied over a frequency range of 2×10² to 1×10⁶ Hz and in the temperature range 158 to 463 K. The a.c. conductance G varies with frequency according to the relation G ^s, where the exponent s was found to be 0.92 (at 158 K) in the frequency range 8×10² to 2×10⁴ Hz and above 10⁵ Hz the conductance shows a square law dependence on frequency. These results suggest that the a.c. conduction mechanism in SiO/B₂O₃ thin films is due to an electronic hopping process. The numbers of localized sites were calculated using the conductivity relations given by Elliott and by Pollak and the results are compared. The effects of composition, temperature and annealing on the dielectric constant and loss factor were studied. The relative dielectric constant and loss factor were found to decrease with the increase of B₂O₃ content in SiO. Annealing of the samples reduces the values of the dielectric constant, loss factor tan , temperature coefficient of capacitance and a.c. conductivity. The variation in capacitance with the composition of SiO/B₂O₃ was investigated at room temperature, the results being normalized to 10 kHz. The dispersion was found to decrease with the addition of B₂O₃ into SiO.     

Optical and esr spectra of titanium (III) in Na2O-B2O3 and Na2O-P2O5 glasses

The optical absorption and the esr spectra of titanium(III) in binary Na₂O-B₂O₃ and Na₂O-P₂O₅ glasses have been studied. Titanium(III) produces two optical absorption bands around 20 000 and 14 000 cm^–1 which are assigned to the²B_2g ²B_1g and²B_2g ²A_1g transitions respectively of Ti³⁺ in a tetragonally distorted octahedral environment. The absorption bands in phosphate glasses are narrower and absorption coefficients higher than those in borate glasses. The esr spectrum of titanium(III) in all the glasses consists of a broad asymmetric line withg 1.94 in borate glasses, andg 1.92 in phosphate glasses; no hyperfine structure has been observed.     

Glass formation and crystallization of barium ferrite in the Na2O-BaO-Fe2O3-SiO2 system

Amorphous ribbons of approximate composition 0.13Na₂O-0.30BaO-0.30Fe₂O₃-0.27SiO₂ were successfully fabricated using roller-quenching, after melting at 1400C. Devitrification of the amorphous phase by annealing at 710C for 2 h formed a uniform distribution of barium ferrite (BaFe₁₂O₁₉) particles with a mean diameter of 55.88 nm. Other crystalline phases formed on heat treatment were BaFe₂O₄ and Na₂Ba₂Si₂O₇. Devitrified ribbons showed a saturation magnetization (M _S) of 24.41 emu g^–1 and a coercivity (H _C) of 3.06 kOe.     

Effect of heat treatment and irradiation on electrical conductivity and crystallization in the BaO-B2O3-Fe2O3 glass system

A glass system was prepared according to the formula 75 mol % B₂O₃-(25 –x) mol % BaO –x mol % Fe₂O₃, wherex = 0, 1, 2.5, 5, 7.5 and 10. The glasses were subjected to heat treatment at 550° C for 2, 6, 12, 18 and 24 h. The glasses were also irradiated using-rays at a dose of 4.805 × 10⁴ rad h^–1 for 12, 18 and 24 h. An X-ray diffraction technique was used to identify the separated crystalline phases. The electrical conductivity and activation energy of untreated, heat-treated and irradiated samples were measured and calculated. The rate and the dimensions of crystallization were also calculated by using the Avrami equation. It was found that-Fe₂O₃ is the separated phase when a sample containing 7.5 mol% Fe₂O₃ is heat treated for 24h;-Fe₂O₃ and Fe₂O₃ are the separated phases when the sample containing 10 mol% Fe₂O₃ is heat treated for 6, 12 and 18 h, with the addition of BaO when the sample is heat treated for 24 h. A miminum value for the electrical conductivity of glass samples was found to occur around an Fe₂O₃/BaO ratio of 0.425. The rate of crystallization in the sample containing 10 mol% Fe₂O₃ is 1.30607 × 10^–3 and the geometry of crystallizationn is 1.2238, which indicates that the crystallization was in one dimension.     

Solid state reactions in the Fe2O3-CaCO3-In2O3 system

The kinetics of solid-state reactions of powdered reactants were investigated by X-ray and by differential thermogravimetry in a magnetic field. Measurements revealed mutual diffusion of the Fe³⁺ and In³⁺ ions in the Fe₂O₃-In₂O₃ system heat treated for 3 h at 700 to 1400° C. Diffusion of indium into the Fe₂O₃ lattice caused a shift of the Curie temperature of the antiferromagnetic iron oxide towards lower temperatures. Only Caln₂O₄ was found between CaCO₃ and In₂O₃ up to 1400° C. Also, in the Fe₂O₃-CaCO₃-In₂O₃in system, the reaction started with the mutual diffusion of iron and indium and the forming of CaFe₂O₄. End-products were the magnetic -Ca₄Fe₁₄O₂₅ and CaFe₄O₇, and the non-magnetic CaFe₅O₇, depending on the In³⁺ concentration. Indium stabilized the magnetic calcium-iron oxide structures, shifting their Curie temperatures towards lower values.