Microstructure and electrical properties of Nb2O5 doped titanium dioxide

The present work attempts to investigate the sintering characteristics, grain boundary morphology and electric conductivity of Nb₂O₅ doped TiO₂ semiconductor ceramics. X-ray diffraction results showed evidence of a second phase beside the rutile TiO₂ when Nb₂O₅ exceeds 0.7 mol%. SEM images showed that Nb₂O₅ doping can lowers the sintering temperature of TiO₂, although not significantly. Lattice images of the grain boundary morphology obtained by high resolution transmission electron microscopy revealed defects introduced from the doping. Grain boundaries vary from amorphous to a faceted structure. Finally, electrical conductivity measurements showed that the grain boundary resistance is greatly reduced at high temperature.     

Effect of sintering temperature on structural and electrical properties of gadolinium doped ceria (Ce<Subscript>0·9</Subscript>Gd<Subscript>0·1</Subscript>O<Subscript>1·95</Subscript>)

Gadolinium doped ceria oxide is one of the promising materials as an electrolyte for IT-SOFCs. Ce_0·9Gd_0·1O_1·95 (GDC10) powder was prepared by solid state reaction and sintered at 1473 K, 1573 K, 1673 K and 1773 K. All samples were studied using X-ray diffraction, scanning electron micrograph and d.c. conductivity measurement. The crystallinity and surface morphology of the samples improved with sintering temperature. Further, the electrical conductivity measurement indicated that the conduction mechanism is mainly ionic. The conductivity of samples sintered at 1673 K and 1773 K at 800°C are of the order of 0·1 S-cm⁻¹. The activation energies decreased from 1·25–0·82 eV with increase in sintering temperature.     

Microstructure and properties of the sintered composite prepared by hot pressing of TiN-coated alumina powder

Alumina powders (average grain size: 50 m) coated with TiN film of thickness 0.5 and 1.2 m were prepared by rotary powder-bed chemical vapour deposition for 15 and 90 min, respectively. These Al₂O₃-TiN composite powders were hot-pressed at 1800 °C and 40 MPa for 30 min. The microstructure of the Al₂O₃-TiN sintered composite was composed of a TiN network homogeneously distributed on the grain boundaries of alumina. The mechanical properties (hardness, bending strength and fractured toughness) and thermal conductivity of the sintered composite were found to depend on the composition and microstructure of the sintered composite, even with a small content (3–7 wt%) of TiN. The resistivity of the sintered composite was 10^–1-10^–3 cm. The relatively high electrical conductivity of the Al₂O₃-TiN composite was caused by the grain boundary conduction of TiN.     

Microstructure and thermoelectric properties of tungsten trioxide ceramics doped with a low amount of terbium dioxide

The effect of the nonstoichiometric compound terbium dioxide (Tb₄O₇) on the thermoelectric properties of tungsten trioxide (WO₃) ceramics was investigated. Among the sintered ceramics, the sample doped with 0.1 mol% Tb₄O₇ showed the maximum grain size and density. Doping with Tb₄O₇ also increased the electrical conductivity (σ) of the ceramics by about two orders of magnitude, and the sample doped with 0.1 mol% Tb₄O₇ showed the highest electrical conductivity. The absolute value of Seebeck coefficient (|S|) of the doped samples increased as well. Consequently, the power factor (σs ²) markedly increased. The sample doped with 2.0 mol% Tb₄O₇ demonstrated the maximum σs ² of 2.88 μW m^?1 K^?2 at 973 K, which was larger than the highest recorded σs ² for WO₃ ceramics (2.71 μW m^?1 K^?2 at 1,023 K). In addition, the low-doped sample (0.1 mol%) exhibited excellent thermoelectric properties.     

Effect of preparative parameters on the electrical conductivity of Li2SO4-Al2O3 composites

The effect of various preparative parameters, such as the size and form of alumina and also the time of sintering, on the electrical conductivity of the Li₂SO₄-Al₂O₃ composite system has been investigated. The sintering time appears to be an insignificant preparative parameter. The role of different phases of Al₂O₃ on the electrical conductivity of the composite clearly establishes that the maximum enhancement is achieved for γ-Al₂O₃. The 50 m/o Al₂O₃ composition was found to exhibit the highest conductivity, an enhancement of three orders of magnitude at 500°C. The experimental data indicates higher conduction in the space charge layer near the surface to be the possible mechanism of conductivity enhancement.     

Cluster formation in Ag2O-P2O5-CdCl2 glass system

Ag₂O-P₂O₅ and Ag₂O-P₂O₅-20 wt% CdCl₂ glasses were prepared by melt quenching method and characterized with the help of several experimental techniques. Powder X-ray diffraction study indicated that the glasses are amorphous in nature. DSC studies showed that CdCl₂ doped glass is chemically more durable. Electrical conductivity and ionic transference number measurements have shown that both the glasses are ionic conductors with Ag⁺ ions as the charge carriers. The electrical conductivity of the doped glass is found to be higher than the undoped one. Structures of the glasses have been proposed on the basis of IR spectral analysis. From SEM studies it has been inferred that addition of 20 wt% CdCl₂ modifies the morphology of Ag₂O-P₂O₅ glass and in its presence formation of clusters composed of nanofibers occur.     

Modification of the physical properties of semiconducting MgAl2O4 by doping with a binary mixture of Co and Zn ions

The effects of doping of MgAl₂O₄ by a binary mixture of Co and Zn ions on the absorbance, electrical resistivity, capacitance, thermal conductivity, heat capacity and thermal diffusivity are reported in this paper. The materials with the nominal composition Mg_1−2x(Co,Zn)_xAl₂O₄ (x = 0.0-0.5) are synthesized by solution combustion synthesis assisted by microwave irradiation. The substituted spinels are produced with a Scherrer crystallite size of 18-23 nm, as opposed to 45 nm for undoped samples, indicated by X-ray diffraction and confirmed by transmission electron microscopy. These materials also show better thermal stability in the temperature range of 298-1773 K. Three strong absorption bands at 536, 577 and 630 nm are observed for the doped samples which are attributed to the three spin allowed (⁴A₂ (F) → ⁴T₁ (P)) electronic transitions of Co²⁺ at tetrahedral lattice sites while pure magnesium aluminate remains transparent in the whole spectral range. The semiconducting behavior of the materials is evident from the temperature dependence of the electrical resistivity. Resistivity and activation energy are higher for the substituted samples. Fitting of the resistivity data is achieved according to the hopping polaron model of solids. Both dielectric constant and loss increase on account of doping. The dielectric data are explained on the basis of space charge polarization. The thermal conductivity and diffusivity are lowered and the heat capacity is increased in the doped materials. Wiedemann-Franz's law is used to compute the electronic and lattice contributions towards the total thermal conductivity.     

Multiphonon-based comparison of erbium-doped yttria in large, fine grain polycrystalline ceramics and precursor forms

We have studied the phonon-induced non-radiative decay in erbium doped yttria (Y₂O₃). The technique employed allows for the evaluation of potential ceramic and crystalline laser materials. The frequency of the dominant phonon that deactivates the fluorescing levels and an approximate prediction of 0 K lifetime can be determined. Results show no significant quantitative difference between very large grain polycrystalline (with grain size ∼200-500 μm) ceramic, fine grain polycrystalline (with grain size ∼0.3 μm) ceramic and the precursor powder (with ∼30 nm particle size) of Er³⁺ doped Y₂O₃, when it comes to the dominant phonon frequency and the phonon occupancy number.The results show that a correct evaluation of the final product can be made in the precursor stage of the process eliminating the need to proceed to crystalline or fully sintered ceramic form to evaluate the spectroscopic properties of the material. It should be noted that the powders must be carefully prepared and handled. Adsorbed species such as water can drastically change the effective lifetimes observed in powder samples.     

High oxygen ion conduction in some Bi2O3-Y2O3 (Er2O3) solid solutions

Highly densified bodies (~ 98 % theoretical density) of the Bi₂O₃-Y₂O₃(Er₂O₃) systems containing 30 moles % Y₂O₃ and 20 moles % Er₂O₃ respectively were prepared from both mixed oxides and coprecipitated oxalates. DC ionic conductivity and impedance complex plane analysis on sintered samples were performed. Under oxygen partial pressure ranging from 1 to 10 Pa was found that, samples containing 30 moles % yttria showed a pure ionic conductivity up to an oxygen partial pressure of 10⁻²⁰ Pa. Samples containing 20 moles % erbia, extended its ionic conductivity up to an oxygen partial pressure of 10⁻²² Pa. The impedance analysis shows the presence of only one semicircle at low tempertures and it is attributed to bulk conductivity contribution. The conductivity was higher for the Bi₂O₃-Er₂O₃ sintered solid electrolytes, in such a case, a conductivity as high as 1.38 ohm cm at 700ºC was obtained. The activation enthalpy for the conduction process was calculated in the temperature range from 200º to 700ºC in all the cases. Microstructural development in the sintered sample was also studied.     

Dielectric properties of NaF-B2O3 glasses doped with certain transition metal ions

Dielectric constant ε, loss tan δ, a.c. conductivity Σ and dielectric breakdown strength of NaF-B₂O₃ glasses doped with certain transition metal ions (viz. Cu²⁺, VO²⁺, Ti⁴⁺ and Mn⁴⁺) are studied in the frequency range 10²-10⁷ Hz and in the temperature range 30–250°C. The values of ε, tan δ, Σ_a.c. are found to be the highest for Cu²⁺ doped glasses and the lowest for Mn⁴⁺ doped glasses. Activation energy for a.c. conduction and the value of dielectric breakdown strength are found to be the lowest for Cu²⁺ doped glasses and the highest for Mn⁴⁺ doped glasses. With the help of infrared spectra, increase in the values of ε and tan δ of these glasses with frequency and temperature are identified with space charge polarization. An attempt has been made to explain a.c. conduction phenomenon on the basis of quantum mechanical tunneling model (QMT)/carrier barrier hopping model.     

Influence of the additives and processing conditions on the characteristics of dense SnO2-based ceramics

Three different SnO₂-based powder mixtures, containing 2 wt% CuO as sintering aid and Sb₂O₃ in amounts from 0 to 4 wt% as activator of the electrical conductivity, were sintered to high density at temperatures in the range 1000–1400°C and soaking times from 1 to 6 h. Densification behaviour and microstructure development are strongly dependent on the presence of CuO, that gives rise to a liquid phase, and on Sb₂O₃ that retards the liquid phase formation and hinders grain growth. Cu and Sb cations can enter s.s. in the SnO₂ network with different oxidation states and in different positions, depending on the sintering conditions. The characteristics of the grain boundary phase, of the SnO₂ solid solutions and their modification depending on thermal treatments were analyzed. The electrical resistivity values varied in a wide range from 10^–1 to 10⁴ cm, depending on starting composition and processing conditions: in terms of the final density and of the electrical conductivity, the optimal sintering conditions were found to be 1200°C, for 1–3 h. The electrical resistivity was related to the microstructural features, particularly to the characteristics of the resulting SnO₂-based solid solutions.     

Study on the electrical conductivity and oxygen diffusion of oxide-ion conductors La2Mo2−xTxO9−δ (T = Al, Fe, Mn, Nb, V)

In this paper, the influence of cationic substitutions at Mo site with Al³⁺, Fe³⁺, Mn⁴⁺, Nb⁵⁺ and V⁵⁺ ions on the structure, oxygen ion diffusion and electrical properties in La₂Mo₂O₉ oxide-ion conductors have been investigated by X-ray diffraction method, dielectric relaxation technique and direct current conductivity measurement. Except for V⁵⁺ substitution all of these substitutions up to 5% cannot suppress the phase transition in La₂Mo₂O₉. In the dielectric measurement, one prominent relaxation peak is observed in temperature spectrum as well as in frequency spectrum, which is associated with the short-distance diffusion of oxygen vacancies. The activation energy for oxygen ion diffusion is deduced as in the range of 1-1.1 eV for Al, Fe, Mn and Nb doped samples and 1.4-1.5 eV for V doped samples. All substituted samples have a higher conductivity than the un-doped compound. In the Al, Fe, Mn and Nb substituted materials the phase transition is not suppressed; however, K substitution at the La site can completely suppress the transition and maintains high conductivity at low temperature.     

Electrical properties and microstructure of glass—ceramic materials from CaO-MgO-Al2O3-SiO2 system

The electrical (volume conductivity) and dielectric (loss factor and dielectric constant) properties of glass-ceramics belonging to the CaO-MgO-Al₂O₃-SiO₂ system have been studied, as a function of microstructure, in their glassy and ceramized forms on samples obtained as bulk materials or sintered powders. A possible application of these materials as substrates for electronic devices can be envisaged, on account of their low conductivities (<10^–14S cm^–1 up to 250°C), loss factor and permittivity values.     

Properties of barium-strontium titanate PTCR ceramics sintered on different powder beds

Barium-strontium titanate (BST) ceramics, co-doped with Sb and Mn oxides, were sintered on different powder beds: Sb-doped BST; Al₂O₃; or Sb,Mn-codoped BST powder. Phase formation, microstructure and the electrical properties of the samples were analysed. The PTCR behavior depended significantly on the type of powder bed used. The BST ceramic pellet sintered on the Sb-BST powder displayed the largest PTCR effect, with a ρ_max/ρ_RT ratio of ∼10⁶. This was an order of magnitude greater than for samples sintered on the other two powders. Complex impedance analysis confirmed that this was due to a large increase in grain boundary resistance at 250 °C.     

Phase transformation and low temperature sintering of manganese oxide and scandia co-doped zirconia

The powders of 89 mol% ZrO₂-11 mol% Sc₂O₃ (11ScSZ) doped with various Mn₂O₃ contents were prepared by a co-precipitation method combined with a SCFD (supercritical fluid drying) route. Inhibition of the cubic-rhombohedral phase transformation in both oxidation and reduction atmospheres is achieved for 11ScSZ by the addition of 2.0 mol% Mn₂O₃. The Mn₂O₃ addition can lower the sintering temperature of 11ScSZ ceramics, and the 11ScSZ-2Mn₂O₃ compact can be sintered to nearly full density at 850 °C. 11ScSZ-2Mn₂O₃ ceramic with the cubic structure sintered at 900 °C possesses the conductivity of ∼ 0.10 S cm^− 1 at 800 °C, which is very promising for intermediate temperature solid oxide fuel cell (SOFC) applications.     

Oxide ion conductivity in La0.8Sr0.2Ga0.8Mg0.2−XNiXO3 perovskite oxide and application for the electrolyte of solid oxide fuel cells

Although hole conduction was present, it was found that doping with Ni was effective in improving the oxide ion conductivity in La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ based perovskite oxides. Considering the ionic transport number and the electrical conductivity, the optimized composition for Ni doped samples was La_0.8Sr_0.2Ga_0.8Mg_0.13Ni_0.07O₃ (LSGMN). In this composition, electrical conductivity was found to be virtually independent of the oxygen partial pressure from 1 to 10^–21 atm. Consequently, the oxide ion conductivity was still dominant in this optimized composition. In agreement with the improved oxide ionic conductivity, the power density of the solid oxide fuel cell using LaGaO₃ as an electrolyte increased by doping with a small amount of Ni on the Ga site. In particular, the power density of 224 mW/cm² at 873 K, which is the maximum power density in the cells using LaGaO₃ based oxide as the electrolyte, was attained using LSGMN in spite of the use of electrolyte plates with a thickness of 0.5 mm. Therefore, LSGMN is highly attractive for the electrolyte material of low temperature operating SOFCs.     

Grain boundary wetting in ceramic cuprates

Grain boundary (GB) wetting by a chemically compatible liquid in ceramic cuprates and its influence on transport properties of ceramics are considered, using model Bi₂CuO₄-Bi₂O₃ system as an example. It is shown that the GB wetting transition enhances significantly ionic conductivity and diffusional permeability of the ceramics. The experimental data on grain boundary wetting transition, electrical conductivity, diffusivity, and high temperature corrosion of copper are discussed.     

Electrical conductivity of tetragonal stabilized zirconia

The electrical conductivity change on annealing for tetragonal stabilized zirconia (TZP) was studied with the help of a.c. impedance dispersion analysis techniques. The dependences of the conductivity on annealing time at 1000 ° C and on temperature cycling between room temperature and 1000 ° C were investigated. A decrease in conductivity of about 30% at 1000 ° C of TZP with 3 mol% Y₂O₃ was observed during the first 200 h of annealing at 1000 ° C, and no change was observed during further annealing. A similar result was observed for TZP with 2.9 mol% Sc₂O₃. For TZP with 3.0mol% Yb₂O₃, the conductivity decreased gradually during an annealing time of over 2000 h. The impedance dispersion analysis at lower temperature suggested that the decrease in electrical conductivity by annealing at 1000 ° C could be attributed to the increases of both grain boundary and intragrain resistance. No monoclinic phase was observed for the samples annealed at 1000 ° C for 2000 h. On the other hand, a trace of a monoclinic phase was found for TZP with 3mol% Y₂O₃ after the 50th temperature cycling, but no significant decrease in conductivity was observed with the cycling.     

Low-temperature sintering and electrical properties of Co-doped ZnO varistors

ZnO–Bi₂O₃–B₂O₃-based varistors doped with each kind of cobalt oxides were prepared by conventional ceramic processing. The effects of CoO, Co₂O₃ and Co₃O₄ on the microstructure and the electrical characteristics of varistor samples sintered at 880 °C were investigated separately. Analysis of microstructure indicated the cobalt cations were distributed both in grain regions and grain boundary regions and no crystalline phases containing cobalt were detected in XRD patterns for the samples with various cobalt oxides. All these cobalt oxides could effectively enhance the varistor performance by effectively increasing the nonlinear coefficient and lowing the leakage current, while the breakdown voltage fields increased slightly. Capacitance–voltage characteristics showed the potential barriers of varistor samples increased with the addition of each cobalt oxide. It was found that the addition of same amount of cobalt cations in various cobalt oxides had a different effect on the varistor samples. Best electrical properties were obtained for the varistor sample containing Co₃O₄, in which the nonlinearity coefficient is 28.5, the leakage current density is 3.4 μA and the breakdown voltage field is as low as 260 V/mm.     

Thermoelectric performance of SPS sintered [(Ca0.95M0.05)2CoO3][CoO2]1.61

The doped [(Ca_0.95M_0.05)₂CoO₃][CoO₂]_1.61 ceramics were sintered by the spark plasma sintering (SPS). The electrical conductivity, Seebeck coefficient and thermal conductivity of all samples were measured from 373 K to 973 K. The doped ions improve thermoelectric performance due to increase of Seebeck coefficient and decrease of thermal conductivity. Since the electrical resistivity of the Y-doped sample has no enhancement, the thermoelectric performance of the Y-doped sample is superior to that of the Gd-doped sample. The reason of the different effects on the thermoelectric performance comes from magnetic action.