Defects and transport in Gd-doped BaPrO3

The electrical conductivity of BaPr_1−x Gd_xO_3−δ has been characterized by means of the four-point van der Pauw technique at 200–1100 °C as a function of pO₂ and pH₂O. The contributions from ionic charge carriers were investigated by the EMF of concentration cells and the H⁺/D⁺ isotope effect on the total conductivity. BaPr_1−x Gd _x O_3−δ is predominately a p-type electronic conductor under oxidizing conditions, while ionic conduction is barely measurable. Gd(III) substituted for Pr(IV) is charge compensated mainly by electron holes, with protons and oxygen vacancies contributing significantly but as minority defects only at low temperatures (wet conditions) and at high temperatures, respectively. The conductivity behaviour has been modelled under these assumptions to extract thermodynamic parameters for the defect reactions at play. The practical use of this material is limited by its poor chemical stability.     

Temperature dependence of the dielectric dispersion and ferroelectric properties of neodymium doped bismuth titanate ceramics

Nd-doped bismuth titanate Bi_{4 − x} Nd _x Ti₃O₁₂ ceramics (x = 0–1.0) were prepared by the solid state reaction method. The temperature dependence of the dielectric dispersion and ferroelectric properties were investigated. With the increase of the Nd substitution for Bi ion, the Curie temperature decreased and the corresponding dielectric constant peak broadened. In addition, the strong low-frequency dielectric dispersions were exhibited. The Nd doping decreases the temperature dependence of the ac conductivity and increases the temperature dependence of the remanent polarization, which is caused by the induced polarization by defects, such as bismuth and oxygen vacancies.     

Synthesis and sintering properties of (La0.8Ca0.2−x Sr x )CrO3 perovskite materials for SOFC interconnect

Synthesis and sintering properties of the (La_0.8Ca_0.2−x Sr _x )CrO₃ samples doped by two alkaline earth metals in comparison to the doped only by one alkaline earth metal were evaluated by phase analysis, sintering properties, thermal expansion behaviors, and electrical conductivity. The sintered (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) and (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0.2) were found to have orthorhombic and rhombohedral symmetries, respectively. Relative density of the (La_0.8Sr_0.2)CrO₃ sample sintered at 1500^∘C for 5 h was lower than that of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) sample. TECs of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, 0.1, and 0.2) in air were 11 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, and 11.3 × 10⁻⁶/^∘C, respectively. The electric conductivity of the (La_0.8Ca_0.2−x Sr _x )CrO₃ sample was determined.     

The electronic conductivity of Ba0.5Sr0.5CoxFe1?xO3?δ (BSCF: x?=?0?～?1.0) under different oxygen partial pressures

The electronic conductivity of sintered BSCF ceramics (Ba_0.5Sr_0.5Co _x Fe_1−x O_3−δ, 0 ≤  x  ≤ 1) was measured as a function of temperature up to 1273 K in air. The conductivity of BSC is thermally activated over 298–1273 K with an activation energy of 0.21 eV. The conductivity of BSF and BSCF (0.2 ≤  x  ≤ 0.8) is thermally activated below ∼673 K with activation energies of 0.21 eV–0.40 eV. Above 673 K, the formation of oxygen vacancies results in a decrease in p-type carrier concentration and a decrease in electronic conductivity. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF5582) was also measured under 10⁻⁵ atm ≤ pO₂ ≤ 1 atm. Below ∼673 K, the electronic conductivity of BSCF 5582 shows no dependence on pO₂. Above 673 K, the conductivity of BSCF5582 increases with increasing pO₂ for pO₂ ≥ 0.01 (p-type conduction) and decreases slightly with increasing pO₂ for pO₂  ≤ 0.01 atm. The activation energy for conduction above ∼673 K and at pO₂ ≥ 0.1 is ∼0.07 eV. Above ∼823K and at pO₂ ≥ 0.01 atm, the activation energy for conduction is ∼0.2 eV.     

Properties of Cu,Ni, and V doped-LaCrO <Subscript>3</Subscript> interconnect materials prepared by pechini,ultrasonic spray pyrolysis and glycine nitrate processes for SOFC

The ceramic interconnect,La_0.8Sr_0.05Ca_0.15(Cr_{1 −x} , B_x)O₃ (B = Cu, Ni, V, x = 0.02, 0.1, 0.5) (LSCCB) powders were prepared by Pechini method, Ultrasonic Spray Pyrolysis (USP) and Glycine Nitrate Process (GNP). Nano sized powders were synthesized by GNP and their chemical compositions were confirmed by ICP analysis. The electrical conductivities of LSCCCu, LSCCNi, and LSCCV samples were 34 S/cm, 48 S/cm, and 22 S/cm at 800^∘C in air, respectively. Among the LSCCB powders, the LSCCNi sample shows highest relative density and electrical conductivity. In a low oxygen partial pressure, however, LSCCV sample was more stable. The perovskite phase of the composition LSCCV sample is of large practical interest for interconnects in SOFC because of the stability in low oxygen partial pressure.     

Defects and Transport in Langasite II: Donor-doped (La<Subscript>3</Subscript>Ga<Subscript>4.75</Subscript>Nb<Subscript>0.25</Subscript>SiO<Subscript>14</Subscript>)

The electrical conductivity of Nb doped langasite (La₃Ga_4.75Nb_0.25SiO₁₄) was examined as a function of temperature and oxygen partial pressure by complex impedance spectroscopy. A pO₂-independent regime was found at high pO₂ followed by a pO₂^{− 1/6} dependent regime at low pO₂. A defect model consistent with these results was derived in which the electron density n is fixed by the density of ionized Nb donors at high pO₂ and by the generation of oxygen vacancies at low pO₂. The temperature and oxygen partial pressure dependence of the electron density was obtained independently by thermoelectric power measurements. The Nb donor ionization energy was determined to be 1.52 ± 0.06 eV, confirming Nb to be a deep donor in langasite. By combining conductivity and thermoelectric power data, an expression for the electron mobility given by μ_e = 1.1× 10^{− 2}exp( ) was obtained. After evaluating the temperature dependent conductivity data under reducing conditions, in light of the defect model, a value for the reduction enthalpy (E_r = 6.57 ± 0.24 eV) was derived.     

Defect structure of oxide ferroelectrics—valence state,site of incorporation,mechanisms of charge compensation and internal bias fields

The defect structure of aliovalent transition-metal and rare-earth functional centers in ferroelectric perovskite oxides is characterized by means of multifrequency electron paramagnetic resonance spectroscopy, assisted by density-functional theory calculations. The review is mainly focused on lead zirconate titanate (Pb[Zr _x Ti_1−x ]O₃, PZT) compounds. However, where available also results on ferroelectric ’lead-free’ compounds are discussed. The results include the formation of charged defect dipoles, causing internal bias fields, multivalence manganese centers, acceptor-type copper functional centers creating isolated oxygen vacancies that promote ionic conductivity, as well as donor-type centers. Moreover, the impact of the defect structure on macroscopic material properties is discussed.     

The effect of alumina addition on the electrical conductivity of Gd-doped ceria

Acceptor doped-ceria is a possible electrolyte material for the IT-SOFC (intermediate temperature solid oxide fuel cell) due to its high oxygen-ion conductivity. However, its use has been limited by its mechanical weakness and the appearance of electronic conductivity in reducing condition. In this study, alumina was selected as an additive in the doped-ceria to see if it increases the oxygen-ion conductivity and mechanical strength. Effects of alumina addition in doped ceria were studied as a function of alumina content and acceptor (Gd) content. The electrical conductivity of (Ce_1−x Gd _x O_2−δ)_1−y + (Al₂O₃) _y (x = 0–0.35, y = 0–0.10) was measured by using impedance spectroscopy. The grain conductivity of Ce_0.8Gd_0.2O_2-δ (GDC20) with 5 mol% alumina increased ∼3 times from that of GDC20 at 300^∘C. The grain conductivity was even ∼2 times higher than that of Ce_0.9Gd_0.1O_2−δ (GDC10) at 300^∘C. The electrical conductivity of GDC20 without alumina addition, measured at 500^∘C in air, rapidly decreased after exposure to reducing condition (Po₂∼10⁻²² atm) at 800^∘C. However, the decrease was much slower in GDC20 with alumina addition, indicating the improved mechanical strength. Among the examined compositions, (Ce_0.75Gd_0.25 O_2-δ)_0.95 + (Al₂O₃)_0.05 (GDC25A5) showed the highest conductivity at most temperatures.     

Study of bulk and grain-boundary conductivity of Ln<Subscript>2+x</Subscript>Hf<Subscript>2−x</Subscript>O<Subscript>7−δ</Subscript> (Ln = Sm-Gd; x = 0, 0.096) pyrochlores

The electrical conductivity of new solid electrolytes Eu_2.096Hf_1.904O_6.952 and Gd₂Hf₂O₇ have been compared with those for different pyrochlores including titanates and zirconates Ln_2+xМ_2−xO_7−δ (Ln = Sm-Lu; M = Ti, Zr; x = 0−0.81). Impedance spectroscopy data demonstrate that Eu_2.096Hf_1.904O_6.952 and Gd₂Hf₂O₇ synthesized from mechanically activated oxides have high ionic conductivity, comparable to that of their zirconate analogues. The bulk and grain-boundary components of conductivity in Sm_2.096Hf_1.904O_6.952 (Т_synth = 1600oС), Eu_2.096Hf_1.904O_6.952 and Gd₂Hf₂O₇ (Т_synth = 1670oС) have been determined. The highest bulk conductivity is offered by the disordered pyrochlores prepared at 1600oC and 1670oC: ~1.5 × 10⁻⁴ S/cm for Sm_2.096Hf_1.904O_6.952, 5 × 10⁻³ S/cm for Eu_2.096Hf_1.904O_6.952 and 3 × 10⁻³ S/cm for Gd₂Hf₂O₇ at 780oС, respectively. The conductivity of the fluorite-like phases at the phase boundaries of the Ln_2+xМ_2−xO_7−δ (Ln = Eu, Gd; M = Zr, Hf; x ~ 0.286) solid solutions, as well as that of the high-temperature fluorite-like phases Ln_2+xМ_2−xO_7−δ (Ln = Eu, Gd; M = Zr, Hf; x = 0−0.286), is lower than the conductivity of the disordered pyrochlores Ln_2+xМ_2−xO_7−δ (Ln = Eu, Gd; M = Zr, Hf; x = 0−0.096).     

Electrical conductivity of the Al-stabilized La<Subscript>2/3</Subscript>TiO<Subscript>3</Subscript>

A-site deficient lanthanum titanate (La_2/3TiO₃) materials with perovskite structure are attractive due to their electrical applications such as ion conductors and dielectrics. However, its stability at room temperature in air is obtained only if Na or Li etc. is incorporated into La site or Al into Ti site. In this study, the electrical conductivities of La_0.683(Ti_0.95Al_0.05)O₃ have been measured in oxygen partial pressure (Po₂) between 1 and 10⁻¹⁸ atm at 1000~1400°C. The electrical conductivity exhibited −1/4, −1/6 and −1/5 dependence (log σ ∝ log , n = −1/4, −1/6, −1/5) depending upon temperature and Po₂. The defect model explaining the observation was proposed and discussed. The chemical diffusion coefficient was estimated from the electrical conductivity relaxation.     

Praseodymium-cerium oxide thin film cathodes: Study of oxygen reduction reaction kinetics

Praseodymium-Cerium Oxide (Pr_xCe_1-xO_2−δ; PCO), a potential three way catalyst oxygen storage material and solid oxide fuel cell (SOFC) cathode, exhibits surprisingly high levels of oxygen nonstoichiometry, even under oxidizing (e.g. air) conditions, resulting in mixed ionic electronic conductivity (MIEC). In this study we examine the redox kinetics of dense PCO thin films using impedance spectroscopy, for x = 0.01, 0.10 and 0.20, over the temperature range of 550 to 670°C, and the oxygen partial pressure range of 10⁻⁴ to 1 atm O₂. The electrode impedance was observed to be independent of electrode thickness and inversely proportional to electrode area, pointing to surface exchange rather than bulk diffusion limited kinetics. The large electrode capacitance (10⁻²F) was found to be consistent with an expected large electrochemically induced change in stoichiometry for x = 0.1 and x = 0.2 PCO. The PCO films showed surprisingly rapid oxygen exchange kinetics, comparable to other high performance SOFC cathode materials, from which values for the surface exchange coefficient, k ^q , were calculated. This study confirms the suitability of PCO as a model MIEC cathode material compatible with both zirconia and ceria based solid oxide electrolytes.     

Phase transition and electrical studies of wolframium doped SrBi2Ta2O9 ferroelectric ceramics

In this study, crystalline structure, dielectric and impedance properties of SrBi₂Ta₂O₉ (SBT) - based ferroelectric ceramics have been investigated with the substitution of wolframium/tungsten (W) onto the tantalum site. Wolframium doped SrBi₂(W _x Ta_{1 − x} )₂O₉ (0.0 ≤ x ≤ 0.20) ceramics were synthesized by solid state reaction method. The X-ray diffractogram analysis revealed that the substitution formed a single phase layered perovskite structure for the doping content up to x ≤ 0.05. The dielectric measurements as a function of temperature show an increase in Curie temperature (T _c ) over the composition range of x = 0.05 to 0.20. The W^{6 +} substitution in perovskite-like units results in a sharp dielectric anomaly at the ferroelectric phase transition. Furthermore, the dielectric constant at their respective Curie temperature increases with wolframium doping. Both enhanced Curie temperatures and dielectric constants at the Curie points indicate an increase in polarizability, which could be attributed to the increased “rattling space” due to the incorporation of the smaller tungsten cations. The dielectric loss reduces significantly with tungsten addition. AC impedance properties vis-à-vis wolframium content has also been studied.     

The effect of Co addition on the electrical conductivity of Sr- and Mg-doped LaAlO3

Co was added to see its effect on the electrical conductivity of Sr- and Mg-doped LaAlO₃ (La_0.9Sr_0.1 Al_0.9Mg_0.1O₃, LSAM). Electrical conductivities of La_0.9Sr_0.1(Al_0.9Mg_0.1)_{1− x}Co_xO₃ (LSAMC) for x = 0–0.20 were measured using 2-probe a.c. and 4-probe d.c. method at temperature between 300 and 1300^∘C in air, and as a function of Po ₂ (1–10⁻¹⁵ atm) at 1200^∘C. Electrical conductivities in air increased with increasing Co content, while their activation energy decreased. From the impedance spectroscopy analysis, it was found that both the grain and the grain boundary conductivities of LSAMC samples increased rapidly with Co-addition. LSAMC samples were oxygen ion conductors in low Po₂ and mixed conductors in high Po ₂ up to x = 0.1 just like LSAM. With Co-doping, p-type conductivities increased, however, ionic conductivities remained nearly constant.     

Ni–Mn–Fe–Cr–O negative temperature coefficient thermistor compositions: Correlation between processing conditions and electrical characteristics

A study has been carried out to correlate the effect of sintering temperature on the microstructural, electrical and reliability aspects of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ (x = 0 to 0.3 and y = 0 to 0.3) negative temperature coefficient thermistor compositions prepared by solid-state route. The calcined and sintered compositions were characterized by X-ray diffraction and Scanning Electron Microscopy. The existence of cubic spinel single-phase region was determined by sintering Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ samples in air at temperatures 1150 to 1250 °C. X-ray diffraction patterns of samples sintered above 1200 °C shows additional Bragg reflections of a rock salt structured NiO phase besides normal cubic spinel. A maximum B-value of 4044 K was obtained for Ni_0.75Mn_1.95Cr_0.25Fe_0.05O₄ composition at a sintering temperature 1250 °C/3 h. The reliability of the thermistor compositions were evaluated by performing accelerated ageing based on thermal cycling test. We found that chromium enhances the reliability of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ (x = 0 to 0.3 and y = 0 to 0.3) based NTC thermistor compositions. A maximum reliability of +0.25% resistance drift was observed at sintering temperature 1200 °C for 0.25 mol% chromium content. Excellent reliability of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ NTC thermistor compositions makes it ideal candidates for high-performance thermal sensor applications.     

Electrical conductivity and thermoelectric power of a-Ge20 Se80 − x Bi x thin films

The dc electrical conductivity and thermoelectric power of a-Ge₂₀Se_{80 − x} Bi _x (x = 0, 4, 6, 8, 10, 12) thin films are reported in the present work. The thin films were deposited by flash evaporation at 10^{− 5} Torr pressure and were well-characterized taking XRD, XRF, DSC and EPMA measurements of the system. The dc conductivity was measured over a temperature range 77 to 476 K. Conduction type and activation energies of electrical conductivity have been determined. The electrical transport takes place via two modes extended state conduction at higher temperatures and variable range hopping at lower temperatures. The conductivity was found to change by few orders of magnitude with Bi doping and the electrical activation energies (Δ E _σ) were found to decrease with increasing Bi content. The density of localized states and pre-exponential factor were determined. The thermopower measurements carried out using differential dc method in the temperature range 4.2 to 300 K and the activation energy (Δ E _s ) for TEP determined. The change in band gap with increasing Bi content is due to increased band tailing and increase of Bi–Se bonds and decrease of Se–Se bonds thus leading to the modification of the network structure of Ge₂₀Se₈₀ system.     

Structural evolution and microwave dielectric properties of (1 − x)BaZn1/3Nb2/3O3 + xBaMg1/2W1/2O3 ceramics

Structural evolution and microwave dielectric properties of (1 − x)BaZn_1/3Nb_2/3O₃ + xBaMg_1/2W_1/2O₃ (0 ≤ x ≤ 1) system have been investigated in this work. All samples exhibit single perovskite phase except for the samples with x = 0 and x ≥ 0.8 in which barium niobate and BaWO₄ second phase existed, respectively. 1:1 cations ordering existed in the samples with x ≥ 0.1, and the ordering degree increases with the increase of x. Liquid phase sintering was observed in the sample with x ≥ 0.8. Dielectric constant decreases almost linearly from 40.8 to 17.4 with increasing x. Q × f value monotonically increases from 26,162 GHz to 64,705 GHz with increasing x. The τ_f value changes from +30 ppm/°C to −27.8ppm/°C. Near zero τ_f value of −1.4 ppm/°C could be obtained at x = 0.4 composition.     

Aging of nickel manganite NTC ceramics

Effect of thermal history and chemical composition on aging of Ni _x Mn_3 − x O_{4 +  δ} (0.56 ≤ x ≤ 1.0) ceramics was investigated. It was found that all the Ni _x Mn_3 − x O_{4 +  δ} ceramic samples metallized by co-firing at 1050°C showed significant electrical stability with an aging coefficient less than 1.0%, while aging of those metallized by annealing at 850°C was increasingly serious with a rise in Ni content x, the aging coefficient ranging from 0.2% to 3.8%. However, the ceramic samples with Ni content x ≤ 0.70, whether metallized by co-firing or by annealing, exhibited extraordinarily high electrical stability with an aging coefficient less than 0.5%. The composition dependence of aging of the ceramic samples was explained qualitatively, based on the electrical conduction mechanism of small polaron hopping and on the aging mechanism of the cationic vacancy-assisted migration of cations to their thermodynamically preferable sites under thermal stress.     

Tunability and leakage currents of (Ba,Sr)TiO3 ferroelectric ceramics with various additives

The Ba_xSr_{1 − x} TiO₃ ferroelectric ceramics with magnesium (BSM) and neodymium (BSN) additives were studied. Measurements were made of tunability, dielectric losses (tan δ), leakage currents, the correlations between current-voltage I(U) and capacitance-voltage C(U) characteristics. I(U) characteristics of high quality BSM ceramics have four regions: Ohmic, where the conductivity is linear; the horizontal region (or negative differential resistivity); the exponential dependence; and the vertical current enhancement. These BSM samples (∼20% Mg additives) were distinguished by highest breakdown strength (more than 1000 V), low tan δ (less than 10^{− 3} at 1 MHz) and high tunability (up to 10% at E _max∼2 V/μm).     

Dielectric properties and morphotropic phase boundaries in the xPb(Zn1/3Nb2/3)O3−(1−x)Pb(Zr0.5Ti0.5O3 pseudo-binary system

Ceramics in the xPb(Zn_1/3Nb_2/3)O₃−(1−x)Pb(Zr_0.5Ti_0.5)O₃ [xPZN–(1−x)PZT] solid solution system are expected to display excellent dielectric, piezoelectric, and ferroelectric properties in compositions close to the morphotropic phase boundary (MPB). The dielectric behavior of ceramics with x = 0.1−0.6 has been characterized in order to identify the MPB compositions in this system. Combined with X-ray diffraction results, ferroelectric hysteresis measurements, and Raman reflectivity analysis, it was consistently shown that an MPB exists between x = 0.2 and x = 0.3 in this binary system. When x ≤ 0.2, the tetragonal phase dominates at ambient temperatures. In the range of x ≥ 0.3, the rhombohedral phase dominates. For this rhombohedral phase, electrical measurements reveal a profound frequency dispersion in the dielectric response when x ≥ 0.6, suggesting a transition from normal ferroelectric to relaxor ferroelectric between 0.5 ≤ x ≤ 0.6. Excellent piezoelectric properties were found in 0.3PZN–0.7PZT, the composition closest to the MPB with a rhombohedral structure. The results are summarized in a PZN–PZT binary phase diagram.     

Synthesis of (Ba<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>)(Ti<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> ceramics and effect of Sr content on room temperature dielectric properties

The compositions in the system (Ba_1−x Sr _x )(Ti_0.5Zr_0.5)O₃ with different Sr (x) content, were synthesized through solid oxide reaction route. The phase formation behaviors in the system were investigated by XRD. The room temperature dielectric properties of the compositions were investigated in the frequency range 10 Hz to 13 MHz. The solid solution system Ba_1−x Sr _x Ti_0.5Zr_0.5O₃ remains as cubic perovskite up to x < 0.6 and transforms into the tetragonal structure above x > 0.6. Composition with x = 0.6 contains a mixture of cubic and tetragonal phases with broadened diffraction pattern. It is observed that the increasing of Sr substitution results in the decreasing of bulk density, average grain size and dielectric constants etc. in the composition system. The AC dielectric conductivity of the ceramics also decreases with the increase in Sr-substitution due to decrease in loss as well as grain size with that substitution.