Proton Conduction in Dense and Porous Nanocrystalline Ceria Thin Films

The electrical conductivity of ceria thin films (epitaxial as well as dense and porous nanocrystalline) is investigated in dry and wet atmosphere at temperatures below 500 °C. For the epitaxial and the fully dense nanocrystalline samples, no significant differences can be observed between dry and wet conditions. In marked contrast, the nanocrystalline porous films obtained via spin coating exhibit a considerable enhancement of the protonic conductivity below 300 °C in wet atmosphere. This outcome reveals that the residual open mesoporosity plays the key role for the enhancement of the proton transport at low temperatures and not the high density of grain boundaries. The quantitative analysis of the various pathways, along which the proton transport can take place, indicates that the observed proton conduction can arise not only from bulk water adsorbed in the open pores but also from the space charge zones on the water side of the water/oxide interface.     

Crystallization and Microstructure of Yttria‐Stabilized‐Zirconia Thin Films Deposited by Spray Pyrolysis

The crystallization and microstuctural evolution upon thermal treatment of yttria‐stabilized zirconia (YSZ, Zr_0.85Y_0.15O_1‐δ) thin films deposited by spray pyrolysis at 370 °C are investigated. The as‐deposited YSZ films are mainly amorphous with a few crystallites of 3 nm in diameter and crystallize in the temperature range from 400 °C to 900 °C. Fully crystalline YSZ thin films are obtained after heating to 900 °C or by isothermal dwells for at least 17 h at a temperature as low as 600 °C. Three exothermic heat releasing processes with activation energies are assigned to the crystallization and the oxidation of residuals from the precursor. Microporosity develops during crystallization and mass loss. During crystallization the microstrain decreases from 4% to less than 1%. Simultaneously, the average grain size increases from 3 nm to 10 nm. The tetragonal phase content of the YSZ thin film increases with increasing temperature and isothermal dwell time. Based on these data, gentle processing conditions can be designed for zirconia based thin films, which meet the requirements for Si‐based microfabrication of miniaturized electrochemical devices such as micro‐solid oxide fuel cells or sensors.     

Low‐Temperature Superionic Conductivity in Strained Yttria‐Stabilized Zirconia

Very high lateral ionic conductivities in epitaxial cubic yttria‐stabilized zirconia (YSZ) synthesized on single‐crystal SrTiO₃ and MgO substrates by reactive direct current magnetron sputtering are reported. Superionic conductivities (i.e., ionic conductivities of the order ～1 Ω^?1cm^?1) are observed at 500 °C for 58‐nm‐thick films on MgO. The results indicate a superposition of two parallel contributions – one due to bulk conductivity and one attributable to conduction along the film–substrate interface. Interfacial effects dominate the conductivity at low temperatures (<350 °C), showing more than three orders of magnitude enhancement compared to bulk YSZ. At higher temperatures, a more bulk‐like conductivity is observed. The films have a negligible grain‐boundary network, thus ruling out grain boundaries as a pathway for ionic conduction. The observed enhancement in lateral ionic conductivity is caused by a combination of misfit dislocation density and elastic strain in the interface. These very high ionic conductivities in the temperature range 150–500 °C are of great fundamental importance but may also be technologically relevant for low‐temperature applications.     

Microstructures of YSZ and CGO Thin Films Deposited by Spray Pyrolysis: Influence of Processing Parameters on the Porosity

Microstructures of yttria‐stabilized zirconia (YSZ) thin films deposited by spray pyrolysis at 370 °C on sapphire are investigated. The as‐deposited films are predominantly amorphous and crystallize upon heating at temperatures above 370 °C, developing grains in the range of 5 nm to several 100 nm. During post‐deposition heat treatment up to 800 °C, ～ 50 vol% porosity develops in the center of the films with gradients towards almost dense interfaces to the air and substrate. The reason for this porosity is the decomposition of residues from the precursor and the free volume liberated due to crystallization. Dense YSZ thin films consisting of one monolayer of grains are obtained with annealing temperatures exceeding 1200 °C. In gadolinium‐doped‐ceria (CGO) thin films similar microstructures and porosity are found after low‐temperature heat treatments indicating that the precursor residues due to the deposition method are the main cause of the porosity. Grain growth stagnation in annealed thin films is observed in both the YSZ and in CGO thin films. Stagnating grain growth in the thin films is rather caused by reduced grain boundary mobility, here predominately due to a “secondary phase”, i.e., pores, than to other effects. The stagnation ceases at higher annealing temperatures after densification has taken place.     

Microscopic and Nanoscopic Three‐Phase‐Boundaries of Platinum Thin‐Film Electrodes on YSZ Electrolyte

Agglomerated Pt thin films have been proposed as electrodes for electrochemical devices like micro‐solid oxide fuel cells (μ‐SOFCs) operating at low temperatures. However, comprehensive studies elucidating the interplay between agglomeration state and electrochemical properties are lacking. In this contribution the electrochemical performance of agglomerated and “dense” Pt thin film electrodes on yttria‐stabilized‐zirconia (YSZ) is correlated with their microstructural characteristics. Besides the microscopically measurable triple‐phase‐boundary (tpb) where Pt, YSZ and air are in contact, a considerable contribution of “nanoscopic” tpbs to the electrode conductivity resulting from oxygen permeable grain boundaries is identified. It is demonstrated that “dense” Pt thin films are excellent electrodes provided their grain size and thickness are in the nanometer range. The results disprove the prevailing idea that the performance of Pt thin film electrodes results from microscopic and geometrically measurable tpbs only.     

Thin‐Film Electrodes: Microscopic and Nanoscopic Three‐Phase‐Boundaries of Platinum Thin Film Electrodes on YSZ Electrolyte (Adv. Funct. Mater. 3/2011)

Agglomerated Pt thin films have been proposed as electrodes for electrochemical devices like micro‐solid oxide fuel cells (μ‐SOFCs) operating at low temperatures. However, comprehensive studies elucidating the interplay between agglomeration state and electrochemical properties are lacking. In this contribution the electrochemical performance of agglomerated and “dense” Pt thin film electrodes on yttria‐stabilized‐zirconia (YSZ) is correlated with their microstructural characteristics. Besides the microscopically measurable triple‐phase‐boundary (tpb) where Pt, YSZ and air are in contact, a considerable contribution of “nanoscopic” tpbs to the electrode conductivity resulting from oxygen permeable grain boundaries is identified. It is demonstrated that “dense” Pt thin films are excellent electrodes provided their grain size and thickness are in the nanometer range. The results disprove the prevailing idea that the performance of Pt thin film electrodes results from microscopic and geometrically measurable tpbs only.     

Properties of Flame Sprayed Ce0.8Gd0.2O1.9‐δ Electrolyte Thin Films

Thin films of Ce_0.8Gd_0.2O_1.9‐δ (CGO) are deposited by flame spray deposition with a deposition rate of about 30 nm min^?1. The films (deposited at 200 °C) are dense, smooth, and particle‐free and show a biphasic amorphous/nanocrystalline microstructure. Isothermal grain growth and microstrain are determined as a function of dwell time and temperature and correlated to the electrical conductivity. CGO films annealed for 10 h at 600 °C present the best electrical conductivity of 0.46 S m^?1 measured at 550 °C. Reasons for the superior performance of films annealed at low temperature over higher‐temperature‐treated samples are discussed and include grain‐size evolution, microstrain relaxation, and chemical decomposition. Nanoindentation measurements are conducted on the CGO thin films as a function of annealing temperature to determine the hardness and elastic modulus of the films for potential application as free‐standing electrolyte membranes in low‐temperature micro‐SOFCs (solid oxide fuel cells).     

Bi_2Cu_(0.1)V_(0.9)O_(5.35-δ)固态电解质化学溶液法的合成与性能

用化学溶液法合成了Bi2Cu0.1V0.9O5.35-δ(BICUVOX.10)材料,研究了材料的物相、表面形貌和电学特性.BICUVOX.10薄膜具有室温稳定的高电导γ相.在LaNiO3/Si衬底上,BICUVOX.10薄膜具有(001)择优取向,平均晶粒大小约为200nm.低频范围的介电损耗来源于氧空位的短程扩散,BICUVOX.10薄膜主要表现为晶粒电导特性.BICUVOX.10薄膜中氧离子电导激活能约为0.3 eV,氧离子电导率约为5×10-2S.cm-1.     

Surface‐Modified Low‐Temperature Solid Oxide Fuel Cell

This paper reports both experimental and theoretical results of the role of surface modification on the oxygen reduction reaction in low‐temperature solid oxide fuel cells (LT‐SOFC). Epitaxial ultrathin films of yttria‐doped ceria (YDC) cathode interlayers (<10–130 nm) are grown by pulsed laser deposition (PLD) on single‐crystalline YSZ(100). Fuel cell current–voltage measurements and electrochemical impedance spectroscopy are performed in the temperature range of 350 °C ≈ 450 °C. Quantum mechanical simulations of oxygen incorporation energetics support the experimental results and indicate a low activation energy of only 0.07 eV for YDC, while the incorporation reaction on YSZ is activated by a significantly higher energy barrier of 0.38 eV. Due to enhanced oxygen incorporation at the modified Pt/YDC interface, the cathodic interface resistance is reduced by two‐fold, while fuel cell performance shows more than a two‐fold enhancement with the addition of an ultrathin YDC interlayer at the cathode side of an SOFC element. The results of this study open up opportunities for improving cell performance, particularly of LT‐SOFCs by adopting surface modification of YSZ surface with catalytically superior, ultrathin cathodic interlayers.     

Careful Choices in Low Temperature Ceramic Processing and Slow Hydration Kinetics Can Affect Proton Conduction in Ceria

Low-temperature ceramic proton conductors such as ceria are important for applications ranging from sensors and resistive switches to new devices like implantable solid-oxide glucose fuel cells. Spray pyrolysis offers a promising fabrication route for proton-conducting ceria, with direct liquid-to-solid synthesis and control over crystallinity and grain size. To date, there are conflicting reports on ceria's proton conduction mechanism, particularly whether the interior contributes to proton conduction or transport occurs exclusively along a surface water layer. In this work, proton conductivity is observed in sprayed ceria thin films at 125 °C and below. Post-annealed films exhibit higher conductivity than as-deposited films of 3.3 × 10⁻⁵ S cm⁻¹ at 25 °C, which is comparable to previous reports and ascribed to the increase in crystallinity and grain size by post-annealing. This indicates that the interior of ceria in fact contributes to proton conduction. Remarkably slow hydration kinetics of ceria are observed, with time-dependent conductivity equilibrating to 9.53 × 10⁻⁶ S cm⁻¹ after up to 76 h. This implies kinetics may have suppressed proton conduction in previous studies, explaining the strong fluctuations in reports to date. Slow protonation kinetics must be considered when designing functional ceria ceramics, for example, in electrochemical bio-energy conversion, sensing or neuromorphic computing.     

Tailoring of LaxSr1‐xCoyFe1‐yO3‐δ Nanostructure by Pulsed Laser Deposition

Pulsed Laser Deposition (PLD) was used to prepare thin films with the nominal composition La_0.58Sr_0.4Co_0.2Fe_0.8O_3‐δ (LSCF). The thin film microstructure was investigated as a function of PLD deposition parameters such as: substrate temperature, ambient gas pressure, target‐to‐substrate distance, laser fluence and frequency. It was found that the ambient gas pressure and the substrate temperature are the key PLD process parameters determining the thin film micro‐ and nanostructure. A map of the LSCF film nanostructures is presented as a function of substrate temperature (25–700 °C) and oxygen background pressure (0.013–0.4 mbar), with film structures ranging from fully dense to highly porous. Fully crystalline, dense, and crack‐free LSCF films with a thickness of 300 nm were obtained at an oxygen pressure lower than 0.13 mbar at a temperature of 600 °C. The obtained knowledge on the structure allows for tailoring of perovskite thin film nanostructure, e.g., for solid oxide fuel cell cathodes. A simple geometrical model is proposed, allowing estimation of the catalytic active surface area of the prepared thin films. It is shown that voids at columnar grain boundaries can result in an increase of the surface area by approximately 25 times, when compared to dense flat films.     

Mechanism of Low‐Temperature Protonic Conductivity in Bulk,High‐Density,Nanometric Titanium Oxide

Uncovering the mechanism of low‐temperature protonic conduction in highly dense nanostructured metal oxides opens the possibility to exploit the application of simple ceramic electrolytes in proton exchange fuel cells, overcoming the drawbacks related to the use of polymeric membranes. High proton conducting, highly dense (relative density 94 vol%) TiO₂ samples are prepared by a fast pressure‐assisted sintering method, which allows leaving behind an interconnected network of open nanoporosity. Solid‐state ¹H NMR is used to characterize the presence of strongly associated water confined in the nanopores and hydroxyl moieties bonded to the pores walls, providing a model to explain the unusually high protonic conductivity. At the lowest temperatures (T < 55 °C) protons hop between confined water molecules, according to a Grotthuss mechanism. The resulting conductivity values are however much higher than those of liquid water, indicating a significant increase in the charge carriers concentration. At higher temperatures (up to 450 °C) unexpected proton conduction is still present, thanks to the persistence of hydroxyl groups, derived from water chemisorption, which still produce protons by ionization. The phenomenon is strongly dependent on grain size, and not explicable by simple geometric brick‐layer models, suggesting that the enhanced ionization could rely on space charge region effects.     

Semiconductor behavior of nanocrystalline carbon

The study of the characteristics of nanocrystalline carbon films produced by deposition onto single-crystal silicon substrates from the vapor phase during the pyrolysis of methane in an electric field shows that there exists a reversible semimetal-semiconductor transition at temperatures higher than 560°C. The material contains a large number of tetrahedral nanoclusters in a pyrolytic carbon matrix. The mass density of the material is higher than 2.7 g cm^?3. The manyfold increase in the conductivity of the composite upon heating is caused mainly by the transition of the clusters to the region of intrinsic conduction. Estimation of the thermal activation energy of the conductivity of the composite gives 0.78 eV.     

Synthesis and characterization of Pb(Zr0.54Ti0.46)O3 thin films on (100)Si using textured YBa2Cu3O7−δ and yttria-stabilized zirconia buffer layers by laser physical vapor deposition technique

We have fabricated high-quality <001> textured Pb(Zr_0.54Ti_0.46)O₃ (PZT) thin films on (00l)Si with interposing <001> textured YBa₂Cu₃O_7−δ (YBCO) and yttria-stabilized zirconia (YSZ) buffer layers using pulsed laser deposition (KrF excimer laser, λ, = 248 nm, τ = 20 nanosecs). The YBCO layer provides a seed for PZT growth and can also act as an electrode for the PZT films, whereas YSZ provides a diffusion barrier as well as a seed for the growth of YBCO films on (001)Si. These heterostructures were characterized using x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering techniques. The YSZ films were deposited in oxygen ambient (∼9 × 10⁻⁴ Torr) at 775°C on (001)Si substrate having <001>YSZ // <001>Si texture. The YBCO thin films were deposited in-situ in oxygen ambient (200 mTorr) at 650°C. The temperature and oxygen ambient for the PZT deposition were optimized to be 530°C and 0.4-0.6 Torr, respectively. The laser fluence to deposit this multilayer structure was 2.5-5.0 J/cm². The <001> textured perovskite PZT films showed a dielectric constant of 800-1000, a saturation polarization of 37.81 μC/cm², remnant polarization of 24.38 μC/cm² and a coercive field of 125 kV/cm. The effects of processing parameters on microstructure and ferroelectric properties of PZT films and device implications of these structures are discussed.     

Effect of annealing on structural,optical and electrical properties of pulse electrodeposited tin sulfide films

Polycrystalline tin sulfide (SnS) thin films were grown on conducting glass substrates by pulse electrodeposition. The effect of annealing on the physical properties such as structure, morphology, optical, and opto-electronic properties were evaluated to understand the effect of post-deposition treatment for SnS films. Annealing at temperatures higher than 250 °°C resulted in the formation of SnS₂ as a second phase, however, no significant grain growth or morphological changes were observed for films after annealing at 350 °C. A small change in band gap of 0.1 eV observed for films annealed at 350 °C was interpreted as due to the formation of SnS₂ rather than due to morphological changes. This interpretation was supported by X-ray diffractometry, scanning electron microscopy, and Raman spectral data. The electric conduction in the films is controlled by three shallow trap levels with activation energies 0.1, 0.05, and 0.03 eV. The trap with energy 0.03 eV disappeared after annealing at higher temperature, however, the other two traps were unaffected by annealing.     

Effect of oxidation on the conductivity of nanocrystalline PbTe:In films in an alternating electric field

Temperature and frequency dependences of components of complex impedance for nanocrystalline PbTe:In films in the temperature range of 4.2–300 K and the frequency range from 20 Hz to 1 MHz have been studied. The films were deposited onto a cooled glass substrate and then annealed in an oxygen atmosphere at temperatures of 300 and 350°C. The charge-carrier transport in the studied films is controlled by charge transport over inversion channels at the surface of grains and by transitions through barriers at the grain boundaries. Parameters (resistance and capacitance) corresponding to each of above-mentioned mechanisms were determined. Dominant contribution to conductance of the film annealed at 350°C is made by inversion channels. It is shown that the transport of charge carriers over inversion channels in the region of low temperatures is realized by hopping conductivity.

     

Optical and electrical measurement of FeSe2 thin films obtained at low temperature

FeSe₂ thin films were prepared at low temperature by thermal annealing at 350 °C during 6 h of sequentially evaporated iron and selenium films under selenium atmosphere. The structural, optical and electrical characteristics were investigated. The roughness of films (~76 nm) was confirmed by AFM images. Moreover, optical band gap of FeSe₂, which was evaluated as nearly 1.11 eV and confirmed by the electrical study which yielded a value in the order of 1.08 eV. The electrical conductivity, conduction mechanism, dielectric properties and relaxation model of theses thin films were studied using impedance spectroscopy technique in the frequency range 5 Hz–13 MHz under various temperatures (180–300 °C). Besides, complex impedance and AC conductivity have been investigated on the basis of frequency and temperature dependence.     

Study on dielectric properties of lanthanum copper tantalate ceramics by impedance spectroscopy

A new perovskite ceramic material with the composition La_2/3CuTa₄O₁₂ was synthesized by solid state reaction method. SEM observations revealed dense and fine-grained microstructure and not uniform grain size distribution in the ceramics sintered in the temperature range 1180–1220 °C. Impedance spectroscopy studies were carried out in the temperature range from −55 to 700 °C at frequencies 10 Hz–2 MHz. Three types of dielectric responses were recognized, attributed to grains, grain boundaries and the electrode-sample interface. The relaxation times determined from the impedance data follow the Arrhenius law with the activation energies of 0.25 eV and 0.94 eV for grains and grain boundaries, respectively. DC electrical conductivity of the investigated ceramics was investigated in the temperature range 20–700 °C. The determined activation energy of electrical conduction was about 0.5 eV. The dielectric properties of La_2/3CuTa₄O₁₂ ceramics were compared with those of other A_2/3CuTa₄O₁₂ (A=Y, Nd, Sm, Gd, Dy, Bi) materials, being the spontaneously formed internal barrier layer capacitors, and some essential differences were found.     

Annealing behavior of electroplated permalloy thin films. II

The isochronal and isothermal annealing behavior of electroplated Ni-Fe thin films in the temperature range 373°–773°K has been investigated through measurements of electrical resistivity, coercivity, and relative permeability. Analysis of the experimental data indicates that the resistivity decrease is characterized by an activation energy of about 0.70 eV for temperatures up to 550 ° K, and by an activation energy of about 1.82 eV above this temperature. The coercivity initially decreases with an activation energy of about 0.71 eV, and then increases with activation energies up to 1.81 eV. The relative permeability decreases with an activation energy of 1.01 eV. The significance of these activation energies is discussed in terms of structural changes, and the results are compared with the annealing behavior of vapor deposited Ni-Fe thin films.