Defect structure of acceptor-doped calcium titanate at elevated temperatures

The defect structure of acceptor (Al or Cr)-doped polycrystalline calcium titanate was investigated by measuring the oxygen partial pressure dependence (at 10° to 10–18 atm) of the electrical conductivity at 1000 and 1050° C. The observed electrical conductivity data were proportional to for the oxygen pressure range < 10^–10 atm and proportional to for the oxygen pressure range ( 10^–7 atm. The conductivity values were observed to increase with the acceptor concentration in the p-type region with the shift in the conductivity minima towards lower oxygen partial pressure. The absolute value of the electrical conductivity in the acceptor-doped samples were lower in the n-type region compared to the values in the undoped CaTiO₃. Aluminium and chromium were found to be equally effective in acting as acceptor impurities in CaTiO₃. The defect chemistry of CaTiO₃ is dominated by the added acceptor impurities for the entire oxygen partial pressure range used in this investigation.     

Oxygen ion conduction in γ-Bi2O3 doped with Sb2O3

Electrical conduction in bcc-Bi₂O₃ doped with Sb₂O₃ was investigated by measuring electrical conductivity, as a function of temperature and oxygen partial pressure , and ionic transference number. The-Bi₂O₃ doped with 1 to 3 mol% Sb₂O₃ was stable up to 550° C and showed an oxygen ionic conduction in the region of 10⁵ to 10^–9 Pa. As the Sb₂O₃ content increased, ionic conductivity increased up to 2.5 mol % Sb₂O₃ (1.8×10^–3–1cm^–1 at 500° C) and then decreased. However, the activation energy for ionic conduction remained almost unchanged. It was proposed that the-Bi₂O₃ contains a lot of oxygen vacancies and incorporated Sb⁵⁺ ions at tetrahedral sites which affect the concentration of oxygen vacancy effective for conduction.     

Defect structure and charge transfer in undoped and doped lanthanum cobaltites

The results of electronic conductivity and Seebeck coefficient are presented for perovskite-type undoped cobaltite LaCoO_3−δ, doped with strontium La_0.7Sr_0.3CoO_3−δ, and doped with copper LaCo_0.7Cu_0.3O_3−δ. The electronic conductivity and the Seebeck coefficient were studied as a function of pO₂ in the range 10⁻⁶ ≤ pO₂/atm ≤ 1 at temperatures between 1073 and 1323 K. The charge transfer mechanism in the oxides studied is revealed within the framework of localized charge carriers approach on the basis of the defect structure modeling. LaCoO_3−δ and La_0.7Sr_0.3CoO_3−δ are shown to be the typical small polaron hopping conductors with p-type small polarons at Co sites, , as major carriers and n-type small polarons, Co_Co^/, as minor carriers in the temperature range investigated. The charge transfer in copper-doped cobaltite LaCo_0.7Cu_0.3O_3−δ seems to be carried out by means both hopping of n-type small polarons, Cu_Co^/ and Co_Co^/, at copper and cobalt sites, respectively, and p-type small polarons at Co sites. The mobilities and nonconfigurational entropies of charge transfer are determined for all types of charge carriers.     

RETRACTED ARTICLE: Determination of dopant of ceria system by density functional theory

Oxides with the cubic fluorite structure, e.g., ceria (CeO₂), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy–dopant interactions, represented by association (binding) energies of vacancy–dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately. An erratum to this article can be found at     

Electrical conduction of partially stabilized zirconia Zr0.94Ca0.06O1.94 as a function of temperature and oxygen partial pressure

Partially stabilized zirconia (PSZ), Zr_0.94Ca_0.06O_1.94was prepared by a hot kerosene drying method and a conventional oxide wet-mixing method. The total d.c. conductivities of these zirconia specimens were measured by the three-terminal technique as a function of temperature in the range 1088 to 1285 K and oxygen partial pressure in the range 1 to 10^–24 bar. The specimen prepared by the hot kerosene drying method showed near oxygen ion conduction with four times higher conductivity than the specimen prepared by the conventional mixing method at T=1088–1285 K and bar. The higher oxygen pressure conductivity tended approximately towards a to dependence, indicative of p-type conduction, whereas the lower oxygen pressure conductivity tended to be virtually independent of oxygen pressure, indicative of oxygenion conduction. The activation energy was found to be 130 kJ mol^–1 at T=1088–1285 K, bar (air) for pure electron-hole conduction and 153kJ mol^–1 at T=1088–1285 K for ionic conduction.     

Charge transport in CaTiO3: I. Electrical conductivity

The present paper reports the electrical conductivity of polycrystalline undoped CaTiO₃ in the temperature range 973–1323 K under controlled oxygen partial pressure (10–10⁵ Pa). The electrical conductivity data are considered in terms of defect disorder and related semiconducting properties of CaTiO₃. The values of the p(O₂) exponent of electrical conductivity at high p(O₂), that vary between 1/4.3 and 1/6.2 at 973 and 1323 K, respectively, are considered in terms of theoretical defect disorder model of p-type CaTiO₃ and increasing effect of minority charge carriers (electrons) with temperature on p-type conduction. The activation energy of the electrical conductivity, assuming 125.3 kJ/mol at 10 Pa and 94.4 kJ/mol at 72 kPa, has been considered in terms of the formation of defect and their mobility. The band gap, determined from the minimum of electrical conductivity corresponding to the n–p transition is equal to 2.77 eV.     

Oxygen nonstoichiometry, thermal expansion and high-temperature electrical properties of layered NdBaCo2O5+δ and SmBaCo2O5+δ

Layered LnBaCo₂O_5+δ (Ln = Nd, Sm) with the cation-ordered double perovskite structure were synthesized by the solid-state reaction route and characterized by X-ray diffraction, thermogravimetric analysis and dilatometry. For NdBaCo₂O_5.73 and SmBaCo₂O_5.61 equilibrated with atmospheric oxygen at low temperatures, tetragonal and orthorhombic polymorphs were found to form, respectively. The oxygen content at 300-1300 K decreases with decreasing rare-earth cation size, whilst δ variations and chemical contribution to the apparent thermal expansion in air are substantially lower compared to the disordered (Ln, A)CoO_3−δ (A = Ca, Sr) analogues. The average thermal expansion coefficients are 23.1 × 10⁻⁶ K⁻¹ for NdBaCo₂O_5+δ and 20.8 × 10⁻⁶ K⁻¹ for SmBaCo₂O_5+δ at 300-1370 K and atmospheric oxygen pressure. These values are comparable to those of Bi₂O₃-based ionic conductors, but are incompatible with common electrolytes such as stabilized zirconia or doped ceria. The oxygen partial pressure dependencies of the total conductivity and Seebeck coefficient, studied in the P(O₂) range from 10⁻¹⁰ to 1 atm, confirm predominant p-type electronic conductivity.     

Ultra-high quality surface passivation of crystalline silicon wafers in large area parallel plate reactor at 40 MHz

We use a new in-house, large area and automated deposition system: the usable deposition area is 410 × 520 mm with RF-frequency of 40 MHz. We deposit intrinsic a-Si:H layer on flat p-type or n-type c-Si wafers after performing an HF dip. The overall recombination of these double-side passivated c-Si wafers is measured with an effective lifetime measurement set-up. We pay particular attention to the uniformity of the passivation obtained on the whole deposition area.We point out a major role of hydrogen dilution on quality of c-Si passivation. Excellent uniformity is obtained on the whole area with implied open-circuit voltages (V_oc) in a ± 1.5% range. We achieve excellent passivation with overall lifetimes approaching 7 ms (at Δn ≈ 4.5·10¹⁴ cm^− 3) resulting in implied V_oc of 708 mV on p-type c-Si; and lifetimes superior to 4.7 ms resulting in implied V_oc of 726 mV on n-type c-Si (S_eff less than 2 cm/s for both). These results open the way to very high efficiency heterojunction solar cell fabrication in large area reactors.     

Computer Modeling of Ionic Conductivity in Low Temperature Doped Ceria Solid Electrolytes

Solid oxides, such as ceria (CeO₂) doped with cations of lower valance, are potential electrolytes for future solid oxide fuel cells. This is due to the theoretically high ionic conductivity at low operation temperature. This paper investigates the feasibility of two potential electrolytes which are samarium-doped ceria (SDC) and gadolinium-doped ceria (GDC) to replace the traditional yttria-stablized zirconia (YSZ). Molecular simulation techniques were employed to study the influence of different dopant concentrations at different operation temperatures on the ionic conductivity from the atomistic perspective. Simulation results show that the optimized ionic conductivity occurs at 11.11mol% concentration using both dopants of Gd₂O₃ and Sm₂O₃. The temperature effect was also examined under a fixed concentration simulation to check how low temperature they still function. The predicted ionic conductivities have been verified with published experimental results and show reasonable agreements. This simulation technique reveals a clear picture with qualitative and quantitative connection between the choice of the dopant and the improvement of the ionic conductivity of fuel cell electrolytes.     

Characterization of n-type and p-type semiconductor gas sensors based on NiOx doped TiO2 thin films

This work presents the development of n-type and p-type gas-sensitive materials from NiO_x doped TiO₂ thin films prepared by ion-assisted electron-beam evaporation. TiO₂ gas-sensing layers have been deposited over a wide range of NiO_x content (0-10 wt.%). The material analysis by atomic force microscopy, X-ray photoemission spectroscopy, and X-ray diffraction suggests that NiO_x doping does not significantly affect surface morphology and Ni element may be a substitutional dopant of the TiO₂ host material. Electrical characterization shows that NiO_x content as high as 10% wt. is needed to invert the n-type conductivity of TiO₂ into p-type conductivity. There are notable gas-sensing response differences between n-type and p-type NiO_x doped TiO₂ thin film. The responses toward all tested reducing gases tend to increase with operating temperature for the n-type TiO₂ films while the response decreases with temperature for p-type TiO₂ film. In addition, the p-type NiO_x doping results in the significant response enhancement toward tested reducing gases such as acetone and ethanol at low operating temperature of 300 °C.     

Synthesis and thermoelectric characterization of polycrystalline Ni1−x Ca x Co2O4 (x=0–0.05) spinel materials

Ca doped NiCo₂O₄ spinel materials were synthesized by conventional solid state reactions at 900 °C. Thermoelectric properties of polycrystalline products were characterized at high temperature range of 800 °C in air. d.c. conductivity of the prepared polycrystalline 5 mol % Ca doped NiCo₂O₄ was about 60 S m^–1 at 300 °C. The value of d.c. conductivity was increased with the temperature increasing. Thermoelectric voltage of polycrystalline Ni_1–x Ca _x Co₂O₄ (x=0–0.05) was positive at 300–800 °C, this showed p-type thermoelectric properties. The Seebeck coefficient of 5 mol % Ca doped NiCo₂O₄ was ca. 300 V/K at 600 °C. The value of the Seebeck coefficient of Ni_1–x Ca _x Co₂O₄ polycrystalline products decreased with the increasing temperature. Thermal conductivity of 5 mol % Ca doped NiCo₂O₄ was ca. 2.2 W m^–1 K^–1 at 600 °C. The estimated thermoelectric figure-of-merit, Z, of 5 mol % Ca doped NiCo₂O₄ spinel polycrystalline product was about 3.5×10^–5 K^–1 at 600 °C.     

Synthesis and properties of copper and samarium doped yttria-bismuth oxide powders and membranes

Powders of Bi_1.5Y_0.5–y Cu _y O_3– (BYC) and Bi_1.5Y_0.3Sm_0.2O₃ in the fluorite phase structure were prepared by the citrate method. The chemical reactions involved in the synthesis of these powders were proposed and verified experimentally. Dense BYC and BYS membranes were prepared from the powders by the press and sintering method. The optimum conditions for powder preparation as well as for membrane fabrication were determined in this study with the aid of various characterization methods. The BYC and BYS membranes were gas-tight to helium. These doped bismuth-yttrium (BY) oxide membranes show similar ionic conductivity but much higher oxygen permeability (electronic conductivity) as compared to the undoped BY membrane.     

X-ray absorption spectroscopy of Pr x Y1−x Ba2Cu3O7−y single crystals

Substituting Y in orthorhombic (Y,RE)Ba₂Cu₃O₇ by any rare-earth element RE has generally little effect on the superconducting properties. For RE = Pr, however, superconductivity is completely suppressed. To elucidate this effect we have studied the unoccupied electronic structure of Pr_xY_1–xBa₂Cu₃O_7–y (x = 0.0,0.4, 0.8) by polarization-dependent O1s x-ray absorption spectroscopy on detwinned single crystals. Along with the comparison of undoped (y 0.9) to the doped materials (y 0.1), this allows a test of the current theoretical explanations for the suppression of superconductivity. While we can rule out models involving hole filling or charge transfer from planes to chains our data is consistent with approaches based on Pr4f–02p hybridization.     

Oblique Vortex Lattice Implies Unconventional Pairing Symmetry in High Temperature Superconductors

The symmetry of order parameters of YBa₂Cu₃O_{7 –} high temperature superconductor was studied with the Ginzburg–Landau theory. The vortex lattice of a YBa₂Cu₃O₇ superconductor is oblique at a temperature well below the transition temperature T _c, where the mixed s– state is expected to have the lowest energy, whereas very close to T _c, the -wave is slightly lower in energy, and a triangular vortex lattice recovers. The coexistence and the coupling between the s- and d-waves would account for the unusual behaviors such as the upward curvature of the upper critical field curve H _C2(T).     

Conductivity measurements on ceria at high oxygen pressures

The a.c. conductivity of high purity ceria has been measured at oxygen pressures and temperatures in the ranges 0.01 to 400 atm and 700 to 1100° C respectively. Similar measurements have been made on impure ceria over the same temperature range for oxygen pressures up to 100 atm. The conductivity isotherms obtained exhibit flattish minima where the ionic contribution to the conductivity is a maximum, a region at low oxygen pressures, where n-type conduction occurs and increases with diminishing pressure, and a high pressure region where p-type conduction occurs and increases with pressure. Activation energies derived for the conductivity minima are 1.55 and 0.85 eV for the high purity and impure ceria respectively; the higher value for the high purity material implies that a significant electronic contribution is present.     

PSYCO homologues: p-type superconduct, but n-type do not

The high-temperature superconducting homologues of Pb ₂ Sr ₂ YCu ₃ O ₈ (PSYCO) and Nd _2–z Ce _z CuO ₄ are p-type. The n-type homologues of PSYCO do not superconduct. And the dopant in Nd _2–z Ce _z CuO ₄ is the p-type (Ce, O _interstitial ) pair. Hence, a proper theory of high-temperature superconductivity does not have to feature particle-hole doping symmetry.     

The structural and electrical property of CuCr1 − xNixO2 delafossite compounds

A series of CuCr_1 − xNi_xO₂ (0 ≤ x ≤ 0.06) polycrystalline samples was prepared. The electrical conductivity was measured in the temperature range of 160–300 K. It was found that the electrical conductivity (σ) increases rapidly with the doping of Ni²⁺ ions. At room temperature, the σ is 0.047 S cm^− 1 for the sample with x = 0.06, which is two orders of magnitude larger than that of the CuCrO₂ sample (9.49E− 4 S cm^− 1). The Seebeck coefficients are positive for all samples, which indicate p-type conducting of the samples. The experimental results imply that it is possible to get higher electrical conductivity p-type transparent conducting oxides (TCO) from CuMO₂ by doping with divalent ions.     

Current transport mechanism of heterojunction diodes based on the sol–gel p-type ZnO and n-type Si with H2O2 treatment

The present work reports the fabrication and detailed electrical properties of heterojunction diodes based on the sol–gel p-type ZnO and n-type Si. The p-type ZnO/n-type Si diode without H₂O₂ treatment showed a poor rectifying behavior with an ideality factor (n) of 6.4 and high leakage. n > 2 implies that the interfacial defects influence the electronic conduction through the device. However, the p-type ZnO/n-type Si diode with H₂O₂ treatment showed a good rectifying behavior with n of 1.6 and low leakage. Such an improvement indicates that a good passivation is formed at the interface as a result of the reduction of the defect density. These experimental demonstrations suggest that it may be possible to minimize the adverse effects of the interface states to obtain functional devices using H₂O₂ treatment.     

High performance Gd and Y co-doped ceria-based electrolytes for intermediate temperature solid oxide fuel cells

Co-doped ceria of Ce_1−xGd_x−yY_yO_2−0.5x, wherein x = 0.15 and 0.2, 0 ≤ y ≤ x, were prepared by glycine-nitrate method. Their structures and ionic conductivities were characterized by X-ray diffraction (XRD) and AC impedance spectroscopy (IS). All the electrolytes were found to be ceria-based solid solutions of fluorite type structures. Co-doping was found to effectively enhance the conductivity. In comparison to the singly doped ceria, the co-doped ceria showed much higher ionic conductivities at 673-973 K. At 773 K, the ionic conductivity of Ce_0.8Gd_0.05Y_0.15O_1.9 is 0.013 S cm⁻¹ which is three times as high as that of Ce_0.8Gd_0.2O_1.9. These Gd³⁺and Y³⁺ co-doped ceria are ideal electrolyte materials of intermediate temperature solid oxide fuel cells (SOFCs).     

Enhancement of ionic conduction in CaF2 and BaF2 by dispersion of Al2O3

Ionic conductivity measurements were performed on polycrystalline CaF₂, BaF₂ and those dispersed with Al₂O₃ particles. The ionic conductivity of both CaF₂ and BaF₂ increased by about 1 to 2 orders of magnitude by dispersion of Al₂O₃ particles, while X-ray diffraction measurements showed there were no other phases present other than fluoride and Al₂O₃. The conductivity of the dispersed system strongly depended on the particle size and the concentration of Al₂O₃, which suggested the high ionic-conductivity layers were formed at the interface between the ionic conductor matrix and the Al₂O₃ particles. The effective thickness and electrical conductivity of the interface layer at 500° C were calculated, using a simple mixing model, to be 0.3 to ~ 0.6m and ~ 10^–3 S cm^–1, respectively.