Dielectric–spectroscopic and AC conductivity studies in iron doped layered Na2Ti3O7 ceramics

The results of AC electrical conductivity studies in Na₂Ti₃O₇ doped with 0.1 and 1.0 mol% of iron have been reported in the temperature range of 373–773 K. The corresponding log(σ_ACT) versus 1000/T plots have been divided into four distinct regions, namely I, II, III and IV. Lowest temperature region I is attributed to electronic hopping with exchangeable interlayer ionic conduction, in region II associated interlayer ionic conduction is dominated over electronic hopping conduction. Moreover, the conduction in the next region III is due to co-existence of hindered interlayer ionic and suppressed electronic hopping conduction, while conduction in highest temperature region IV is modified interlayer ionic conduction. Dielectric loss (tan δ) and dielectric constant (ɛ) versus frequency plots at different temperatures have also been described. Furthermore, the results of tan δ and ɛ versus temperature at various frequencies have been reported for iron doped Na₂Ti₃O₇ samples. The losses are the characteristic of dipole mechanism in both the samples. The possibility of ferroelectric phase transition is proposed in this paper.     

Dramatic effect of multiwalled carbon nanotubes on the electrical properties of alumina based ceramic nanocomposites

We report recent work on electrical properties of multiwalled carbon nanotubes (MWNTs)/alumina composites. The composites with different contents of MWNTs were consolidated by spark plasma sintering and their temperature dependence dc conductivity was scrutinized in the temperature range from 5 to 300 K. The analysis of the temperature dependence of the conductivity suggests that for temperatures higher than 50 K, conduction can be ascribed to thermal fluctuation induced tunneling of the charge carriers through insulating barriers between MWNTs, while at temperatures below 50 K, the conduction can be attributed to three dimensional variable range hopping through MWNTs network in the alumina matrix. The frequency dependence of the conductivity was studied from 5 to 1.3 × 10⁷ Hz. The universality of the ac conduction in MWNT/alumina composites was examined by construction of master curve.     

Hopping conductivity of metal-dielectric nanocomposites produced by means of magnetron sputtering with the application of oxygen and argon ions

Testing of electrical properties of nanocomposites (Co_0,45Fe_0,45Zr_0,1)_x + (Al₂O₃)_1+x within the concentration range of 0.30 < x < 0.65, produced by means of magnetron sputtering of a target composed of stripes of metallic alloy and dielectric, has been carried out. It has been found that the studied materials contain metallic nanoparticles of a diameter ranging from 6 to 10 nm. Alternating current conduction at x < 0.50 is realized by hopping mechanism while at x > 0.50 metallic conductivity is observed.The obtained results have been analyzed using a model of hopping conductivity in the egime developed earlier. This analysis allowed to extract dependences of activation energy ΔE_τ and times τ in a hopping regime after isochronous (15 min) thermal annealings within the range from 293 K to 673 K.     

Effect of reaction temperature on crystallite size and sensing response of chromium oxide nanoparticles

Nanoparticles of chromium oxide have been synthesized following a precipitation technique at reaction temperatures 5, 27 and 65 °C. Synthesized powders were characterized using X-ray diffraction, transmission electron microscope and Brunauer–Emmett–Teller techniques to carry out structural and morphological analysis. The reaction temperature has been found to be playing a crucial role in controlling particle size. It has been observed that Cr₂O₃ nanoparticles synthesized at 27 °C were smaller as compared to those synthesized at 5 and 65 °C. Chromium oxide samples thus prepared were deposited as thick films on alumina substrates to act as gas sensors and their sensing response to ethanol vapour was investigated at different operable temperatures. It has been observed that all the sensors exhibited optimum response at 250 °C. The investigations revealed that sensing response of Cr₂O₃ nanoparticles synthesized at 27 °C was exceptionally higher than that of Cr₂O₃ nanoparticles synthesized at 5 and 65 °C.     

Electrical properties of nanostructures (CoFeZr)x + (Al2O3)1−x with use of alternating current

CoFeZr–Al₂O₃ nanocomposite films of 3–5 μm thickness, containing metallic alloy nanoparticles embedded into the dielectric alumina matrix, have been deposited on a glass ceramic substrate using magnetron sputtering of composite target in Ar gas ambient. Measurements of AC conductance and lagging have been performed within the frequency range of 50 Hz–1 MHz at the temperatures from 79 K to 373 K in the initial (as-deposited) samples as well as directly after their isochronous (15 min) annealings within the temperature range from 398 K to 648 K with 25 K step.The observed variations of real part AC electrical conductivity with temperature and frequency σ_real(T, f) in the as-deposited films display transition from dielectric to metallic behaviour when crossing the percolation threshold x_C in the studied nanocomposites. After annealing of the samples below the x_C the σ_real(T, f) progress follows the hopping law of electron conductivity with sigmoidal frequency dependence. The samples being far beyond the percolation threshold revealed transition from metallic to activational σ_real(T) law after high-temperature annealing attributed to the internal oxidation of metallic nanoparticle by excess of oxygen presented in the as-deposited samples.     

Synthesis and electrical conductivity of La0.6Sr0.4Ru1−xMgxO3−δ (x = 0–0.6) perovskite solid solution

Sr and Mg were doped at La- and Ru-sites of perovskite oxide LaRuO₃, respectively, to enhance electrical conductivity and catalytic property as a cathode material for a low temperature solid oxide fuel cell. Crystal structure and particle morphology of La_0.6Sr_0.4Ru_1?xMg_xO_3?δ powders (shorten as LSRM) and electrical conductivity of sintered LSRM were studied. LSRM powders (x = 0–0.6) were prepared by co-precipitation method using metal nitrate solutions and ammonium carbonate solution. The freeze-dried powders were heated at 1273 K in air to form LSRM solid solution of orthorhombic structure. The true densities and particle sizes of LSRM solid solution, where valence of Ru was estimated to be 3+, decreased with increasing Mg content. The electrical conductivity of LSRM at x = 0–0.3 was almost independent of temperature and was in a range of 19–360 S cm^?1 at 1073 K. Hole conduction contributed to the high electrical conductivities. LSRM at x = 0.4 and 0.5 was a mixed conductor of oxide ions and holes, and showed a conductivity of 11 S cm^?1 at 1073 K in air. This conductivity decreased at a lower oxygen pressure and reached a constant value below 10 Pa of oxygen pressure.     

Relaxations in Ba2BiTaO6 ceramics investigated by impedance and electric modulus spectroscopies

Impedance spectroscopy analysis of the dielectric properties of a Ba₂BiTaO₆ ceramic was performed in the temperature range from room temperature to 500 K. The sample was prepared using conventional solid state synthesis under air and the X-ray diffraction shows the presence of Ba₅Ta₄O₁₅ as a minor secondary phase (0.09%). The impedance data clearly show contributions of the grain and grain boundary. The results indicate that the conduction in Ba₂BiTaO₆ is due to hopping of oxygen vacancies and that the impurities not influence the conduction mechanism.     

Studies on the structural,electrical and magnetic properties of LaCrO3, LaCr0.5Cu0.5O3 and LaCr0.5Fe0.5O3 by sol–gel method

The structural, electrical and magnetic properties of LaCr_0.5M_0.5O₃ (M = Cr³⁺, Cu²⁺ and Fe³⁺) synthesized by a sol–gel technique were studied. The X-ray diffraction pattern shows the structure to be orthorhombic and the size of the particles is around 100 nm as seen from the TEM images. The effects of Cu²⁺ and Fe³⁺ on the electrical properties of LaCrO₃ were studied using impedance spectroscopy at room temperature (RT). The properties of LaCr_0.5Cu_0.5O₃ were studied over a wide range of temperature from RT to 533 K. A maximum conductivity of 1.7 × 10^?3 S cm^?1 was observed for LaCr_0.5Cu_0.5O₃ at a measured temperature of 533 K. The impedance spectra indicate a negative temperature coefficient of resistance (NTCR) and also imply the conduction is through bulk of the material. The magnetic studies performed using a SQUID magnetometer interpret the antiferromagnetically ordered LaCrO₃ to behave ferromagnetically on the addition of Cu²⁺ and Fe³⁺, and the magnetization was found to be enhanced in the LaCr_0.5Fe_0.5O₃.     

Transport properties of ZnO substituted lead vanadate glass system at eutectic composition

Glasses of the system x(ZnO)(50 − x)PbO:50V₂O₅ were prepared by melt-quench process in the range x = 0–15 mol%. Measurements are reported for dc electrical conductivity as well as thermoelectrical power for the above compositions in the temperature range 27–227 °C. The experimental results are analyzed with reference to various theoretical models proposed for dc electrical conduction in amorphous semi-conductors. The analysis shows that at high-temperatures the temperature dependence of dc conductivity is consistent with Mott's model of phonon-assisted hopping conduction, variable range hopping mechanism and Schnakenberg's model mechanism. The high-temperature thermoelectric power (TEP) was satisfactorily explained by Heikes’ relation and the data also showed evidence of small polaron formation in these glasses. Thermo EMF measurements indicate that all the glass samples including unsubstituted lead metavanadate are n-type at room temperature. As temperature is increased ZnO substituted samples change from n-type to p-type. When Heikes’ formula is applied to all the systems at room temperature the amount of disorder was found to be the same in all the systems. When Emin's formula is used for the estimation of W_D, the activation energy due to disorder, in ZnO substituted samples, unusually large values of W_D (∼0.6 eV) are obtained. The present results indicate that the Emin's formula cannot be directly used to estimate the disorder energy in ZnO substituted lead vanadates. The temperature dependent change of sign of the thermoelectric power S, in these systems may arise due to change in V⁴⁺/V⁵⁺ ratio with the change of temperature or due to the onset of band type of conduction as in MnO or other extrinsic compensated semi-conductors.     

Low temperature charge transport and microwave absorption of carbon coated iron nanoparticles–polymer composite films

In this paper, the low temperature electrical conductivity and microwave absorption properties of carbon coated iron nanoparticles–polyvinyl chloride composite films are investigated for different filler fractions. The filler particles are prepared by the pyrolysis of ferrocene at 980 °C and embedded in polyvinyl chloride matrix. The high resolution transmission electron micrographs of the filler material have shown a 5 nm thin layer graphitic carbon covering over iron particles. The room temperature electrical conductivity of the composite film changes by 10 orders of magnitude with the increase of filler concentration. A percolation threshold of 2.2 and an electromagnetic interference shielding efficiency (EMI SE) of ～18.6 dB in 26.5–40 GHz range are observed for 50 wt% loading. The charge transport follows three dimensional variable range hopping conduction.     

Evidence of additional phase transitions at lower temperatures in the flux grown Pb (Zn1/3 Nb2/3)0.91 Ti0.09 O3 single crystal

Evidence of phase transitions in low temperature ferroelectric region of Pb (Zn_1/3 Nb_2/3)_0.91 Ti_0.09 O₃ single crystals has been found in the dielectric and ac conductivity measurements in the temperature range (? 150 to 200 °C) at ? 10, ? 49 and ? 132 °C. The total activation energies E_t (in eV) below and above these temperatures are found to be 0.332 and 0.326, 0.326 and 0.354 and 0.292 and 0.354, respectively. The variation of activation energy and conduction behavior has been studied in the vicinity of transition temperatures in the ac conductivity measurements. On the basis of these findings a modified phase diagram has been proposed.     

Structural and other physical properties of barium vanadate glasses

XRD, IR spectra, DTA, density, oxygen molar volume and dc conductivity of barium vanadate glasses of compositions xBaO(100 − x)V₂O₅, where x = 30, 35, 40, 45 and 50 mol%, are reported. The IR studies of the glasses suggest the glass network is built up of mainly VO₄ polyhedra. The glass transition temperatures are observed to increase with an increase of BaO content in the compositions. The cross-linking density decrease with increasing BaO content in the compositions. Introduction of BaO into the V₂O₅ matrix changes the 2D layer structure of the crystalline V₂O₅ into a more complicated 3D structure. 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.     

Thermoelectric properties of Re6GaxSeyTe15−y (0 ≤ x ≤ 2; 0 ≤ y ≤ 7.5)

Thermoelectric properties of Re₆Ga_xSe_yTe_15?y (0 ≤ x ≤ 2; 0 ≤ y ≤ 7.5) were studied in the temperature range 90–320 K. The measurements revealed p-type semi-conductivity in all samples. Relatively high values of the Seebeck coefficients, α, were obtained in all samples. The electrical resistivities and room temperature Seebeck coefficients increased as selenium concentrations increased, for each value of x. The room temperature Seebeck coefficients and resistivities decreased as gallium content increased, for each value of y. Low carrier concentrations were found at room temperatures, in agreement with large Seebeck coefficient values. Measurements suggested hopping conduction between 150 K and 280 K for all samples. Temperature dependences of the Seebeck coefficient below 150 K were accounted for by phonon drag effect. The power factors for the samples were calculated. Theoretical discussions of dependences of the measured quantities on temperature and composition are given. Usefulness of these materials as thermoelectrics is also discussed.     

Effects of the intercalation time on the dielectric properties of Na2xMn1−xPS3

Manganese thiophosphate powders have been intercalated with sodium ions at two different intercalation times (150 min and 180 min) in order to evaluate the influence of this parameter on the dielectric properties of the obtained compounds labeled like Na_2xMn_1?xPS₃. In particular, dielectric permittivity measures have been carried out as a function of temperature (80 K–350 K) and frequency (20 Hz to 1 MHz) and compared with each other and with those of the pure matrix and of the Na_2xMn_1?xPS₃ compound corresponding to a 120 min intercalation time. Both investigated compounds show a dielectric response characterized by a loss peak at low temperatures, by a strong dispersion at low frequency and at not very high temperatures and by a crossover frequency showing an Arrhenius temperature dependence in agreement with what already observed in Na_2xMn_1?xPS₃ (120 min). The results have been also discussed in terms of ac conductivity, Maxwell–Wagner–Sillar (MWS) polarization, electrode polarization and dc conductivity and all the above cited features have been attributed to the Na⁺ ions which are, by their nature, hopping charge carriers that behave like jumping dipoles in their alternate motions and simultaneously show conducting characteristics resulting from their extended hopping over many sites. This attribution allows us to classify the analyzed sodium compounds like hopping charge carrier systems in which a greater Na intercalation time translates into a decrease in the loss peak intensity and an increase in the activation energy associated to the crossover frequency.     

Alumina shunt for precooling a cryogen-free 4He or 3He refrigerator

In this technical report a cryogen-free 1 K cryostat is described where the pot of the ⁴He refrigeration unit is precooled by the 2nd stage of a pulse tube cryocooler (PTC) from room temperature to T ∼ 3 K via a shunt made from sintered alumina (SA); the total mass of the 1 K stage is 3.5 kg. SA has high thermal conductivity at high temperatures; but below ∼50 K the thermal conductivity drops rapidly, almost following a T³-law. This makes SA an interesting candidate for the construction of a thermal shunt, especially as the heat capacity of metals drops by several orders of magnitude in the temperature range from 300 K to 3 K. At the base temperature of the PTC, the heat conduction of the shunt is so small that the heat leak into the 1 K stage is negligible.     

Metal-organic framework derived hexagonal layered cobalt oxides with {1 1 2} facets and rich oxygen vacancies: High efficiency catalysts for total oxidation of propane

A cobalt-based metal–organic framework was used as a precursor to synthesize Co₃O₄ catalysts exhibiting a hexagonal layered morphology by calcination at varying temperatures. Various characterization techniques, such as XRD, SEM, Raman, H₂-TPR, O₂-TPD and N₂ adsorption–desorption, were used to study the effects of calcination temperature on the grain size, surface area, and pore volume of the catalysts. The Co₃O₄ catalyst obtained by calcination at 350 °C (Co₃O₄-350) exhibited the highest catalytic activity for the total oxidation of propane. Furthermore, the small grain size and layered structure of Co₃O₄-350 allowed it to possess a high specific surface area, a highly exposed {1 1 2} facets, and abundant oxygen defects that facilitated a favorable low-temperature reducibility and oxygen mobility, thereby improving catalytic activity. This research offers a simple strategy for synthesis of Co₃O₄ with layered structure, highly exposed {1 1 2} facets and rich oxygen defects.     

(TiZrHf)P2O7: An equimolar multicomponent or high entropy ceramic with good thermal stability and low thermal conductivity

ZrP₂O₇ is a promising material for high temperature insulating applications. However, decomposition above 1400 °C is the bottleneck that limiting its application at high temperatures. To improve the thermal stability, a novel multicomponent equimolar solid solution (TiZrHf)P₂O₇ was designed and successfully synthesized in this work inspired by high-entropy ceramic (HEC) concept. The as-synthesized (TiZrHf)P₂O₇ exhibits good thermal stability, which is not decomposed after heating at 1550 °C for 3 h. It also shows lower thermal conductivity (0.78 W m⁻¹ K⁻¹) compared to the constituting metal pyrophosphates TiP₂O₇, ZrP₂O₇ and HfP₂O₇. The combination of high thermal stability and low thermal conductivity renders (TiZrHf)P₂O₇ promising for high temperature thermal insulating applications.     

Failure analysis of a fluidisation nozzle in biomass boiler and the long-term high temperatures oxidation behaviour of 304 stainless steel

The long-term (> 10 years) oxidation behaviour of stainless steels (SS) at high temperatures was previously unknown. The behaviour was studied through a case study of failure analysis. A fluidisation nozzle made of 304 austenitic SS. After over 100,000 h of service at temperatures of 790–820 °C in a biomass boiler, the nozzle fractured. Failure analysis pinpoints that the nozzle wall temperature fluctuation caused the oxide scale cracking, which intensified the oxidation. The brittle fracture was due to fully oxidization.The long-term oxidation behaviours are distinct from the short-term oxidation behaviours. XRD analysis indicates that the scale was mainly Fe^+ 2Cr₂O₄ and (Fe_0.6 Cr_0.4)₂O₃. ESEM/EDS analysis indicates internal oxidation and sulfidation along the grain boundaries. The different diffusion rates of the Fe, Cr and Ni atoms formed a Ni-rich (48%) layer underneath the scale, and a Cr-rich (35%) core in the remained SS. A schematic is proposed to describe the diffusion mechanism of the internal oxidization and sulfidation behaviour.     

Optical,electrical properties,characterization and synthesis of Ca2Co2O5 by sucrose assisted sol gel combustion method

Sol–gel combustion method has been used as an efficient and simple method to synthesize pure Ca₂Co₂O₅ (CCO-225) ceramic powder using sucrose which play a dual role as the gelling agent and combustion fuel. The advantage of this method is simple low cost and environmental friendly. The synthesized sample is sintered at various temperatures the products were characterized by powder X-ray diffraction (XRD), Thermogravimetric and differential thermal analysis (TGA-DTA), Fourier transformer infrared spectroscopy (FTIR), Scanning electron microscope (SEM) and UV–Visible diffuse reflectance spectroscopy (DRS). X-ray diffraction pattern of sintered sample at 800 °C confirmed the formation of single phase Ca₂Co₂O₅ and also it is proved in thermal analysis. SEM image indicates the obtained samples are diffused platelet like morphology and its grain size will be in the range of 150–300 nm. CCO-225 ceramic material has a wide range of optical and electronic applications due to its wide band gap energy of 3.50 eV. The dielectric constant, dielectric loss and AC conductivity were analyzed at different temperatures and frequencies of the applied field. The AC conductivity studies carried out in the frequency range of 50 Hz to 5 MHz at various temperatures from 30 °C to 400 °C. The result reveals that the space charge polarization leads for conduction mechanism.     

Effect of Fe and Co doping on electrical and thermal properties of La0.5Ce0.5Mn1−x(Fe,Co)xO3 manganites

The effect of Fe and Co doping on structural, electrical and thermal properties of half doped La_0.5Ce_0.5Mn_1−x(Fe, Co)_xO₃ is investigated. The structure of these crystallizes in to orthorhombically distorted perovskite structure. The electrical resistivity of La_0.5Ce_0.5MnO₃ exhibits metal-semiconductor transition (T_MS at ∼225 K). However, La_0.5Ce_0.5Mn_1−xTM_xO₃ (TM = Fe, Co; 0.0 ≤ x ≤ 0.1) manganites show semiconducting behavior. The thermopower measurements infer hole as charge carriers and electron–magnon as well spin wave fluctuation mechanism are effective at low temperature domain and SPC model fits the observed data at high temperature. The magnetic susceptibility measurement confirms a transition from paramagnetic to ferromagnetic phase. The observed peaks in the specific heat measurements, shifts to lower temperatures and becomes progressively broader with doping of transition metals on Mn-site. The thermal conductivity is measured in the temperature range of 10–350 K with a magnitude in between 10 and 80 mW/cm K.