Insulating Surface Layer Research Articles

Tuning enhanced dielectric properties of (Sc3+–Ta5+) substituted TiO2 via insulating surface layers

In this study, we achieved significantly enhanced giant dielectric properties (EG-DPs) in Sc3+–Ta5+ co-doped rutile-TiO2 (STTO) ceramics with a low loss tangent (tanδ ≈ 0.05) and high dielectric permittivity (ε′ ≈ 2.4 × 104 at 1 kHz). We focused on investigating the influence of insulating surface layers on the nonlinear electrical properties and the giant dielectric response. Our experimental observations revealed that these properties are not directly correlated with the grain size of the ceramics. Furthermore, first-principles calculations indicated the preferred formation of complex defects, specifically 2Ta diamond and 2ScVo triangular-shaped complexes, within the rutile structure of STTO; however, these too showed no correlation. Consequently, the non-Ohmic properties and EG-DPs of STTO ceramics cannot be predominantly attributed to the grain boundary barrier layer capacitor model or to electron-pinned defect-dipole effects. We also found that the semiconducting grains in STTO ceramics primarily arise from Ta5+, while Sc3+ plays a crucial role in forming a highly resistive outer surface layer. Notably, a significant impact of grain boundary resistance on the nonlinear electrical properties was observed only at lower co-dopant concentrations in STTO ceramics (1 at%). The combination of low tanδ values and high ε′ in these ceramics is primarily associated with a highly resistive, thin outer-surface layer, which substantially influences their non-Ohmic characteristics.

Formation and Etching of the Insulating Sr‐Rich V5+ Phase at the Metallic SrVO3 Surface Revealed by Operando XAS Spectroscopy Characterizations

AbstractIn the search of low cost and more efficient electronic devices, here the properties of SrVO3 transparent conductor oxide (TCO) thin film are investigated, both visible‐range optically transparent and highly conductive, it stands as a promising candidate to substitute the standard indium‐tin‐oxide (ITO) in applications. Its surface stability under water (both liquid and vapor) and other gaseous atmospheres is especially addressed. Through the use of spectroscopy characterizations, X‐ray photoemission and operando X‐ray absorption measurements, the formation of a thin Sr‐rich V5+ layer located at the surface of the polycrystalline SrVO3 film with aging is observed, and for the first time how it can be removed from the surface by solvating in water atmosphere. The surface recovery is associated to an etching process, here spectroscopically characterized in operando conditions, allowing to follow the stoichiometric modification under reaction. Once exposed in oxygen atmosphere, the Sr‐rich V5+ layer forms again. The findings improve the understanding of aging effects in perovskite oxides, allowing for the development of functionalized films in which it is possible to control or to avoid an insulating surface layer. This constitutes an important step towards the large‐scale use of V‐based TCOs, with possible implementations in oxide‐based electronics.

Fabrication and soft magnetic properties of FeSiB based flakes with insulating surface layer suitable for high frequency power applications

FeSiB based soft magnetic flakes (SMFs; ϕ: 63-106 µm, t: 2-5 µm) coated with optimally thick (∼10 nm) SiO2 were fabricated as the potential magnetic material for pressed cores suitable for use in power inductors operating beyond 10 MHz. Such thin SMFs were produced by mechanical ball milling of commercially available thick (∼27 µm) ribbons followed by an optimized sol-gel coating in presence of a silane coupling agent. Milling helped to reduce ribbon thickness below the skin depth to minimize eddy current loss at high frequencies. The effect of milling (medium, duration, rotation speed, and ball-to-powder ratio) and coating conditions on the soft magnetic properties of the SMFs were investigated. An optimally tiny fraction of the milled flakes was allowed to become nanocrystalline, which augments saturation flux density (BS) and permeability (µ) without incurring additional losses. The results from X-Ray photoelectron spectroscopy and magnetization measurement confirm that a stress-releasing post-annealing process reduced coercivity from ∼62 Oe to ∼6 Oe and enhanced the magnetic saturation moment from 126 emu/g to 142 emu/g without affecting the thin yet robust insulating SiO2 layer.

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films.

Vanadium dioxide (VO2) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long- and short-range elastic distortions, as well as the symmetry change and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue toward mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO2 while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO2-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO2. The tunneling barrier is formed by a very thin but persistently insulating surface layer of the VO2. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO2 properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics.

Novel Micronized Mica Modified Casein–Aluminum Hydroxide as Fire Retardant Coatings for Wood Products

Sustainable coating solutions that function as a fire retardant for wood are still a challenging topic for the academic and industrial sectors. In this study, composite coatings of casein protein mixed with mica and aluminum trihydroxide (ATH) were tested as fire retardants for wood and plywood; coating degradation and fire retardancy performance were assessed with a cone calorimeter, and a thermogravimeter was used for the thermal stability measurement. The results indicated that casein–mica composites were beneficial as coatings. The heat release rate (HRR) and the total heat released (THR) of the sample coated with casein–mica composite were reduced by 55% and 37%, respectively; the time to ignition was increased by 27% compared to the untreated sample. However, the TTI of the sample coated with the casein–mica–ATH composite was increased by 156%; the PHR and THR were reduced by 31% and 28%, respectively. This is attributed to the yielded insulating surface layer, active catalytic sites, and the crosslink from mica and endothermic decomposition of ATH and casein producing different fragments which create multiple modes of action, leading to significant roles in suppressing fire spread. The multiple modes of action involved in the prepared composites are presented in detail. Coating wear resistance was investigated using a Taber Abrader, and adhesion interaction between wood and a coated composite were investigated by applying a pull-off test. While the addition of the three filler types to casein caused a decrease in the pull-off adhesion strength by up to 38%, their abrasion resistance was greatly increased by as much as 80%.

Mechanism of Film Boiling Elimination and IQ Process Design for Hardening Steel in Low Concentration of Water Polymer Solutions

The paper considers a mechanism of the elimination of the film boiling process during intensive quenching (IQ) of steel parts in water polymer solutions of low concentration. The use of the IQ process results in improvement of material mechanical properties and steel part performance characteristics. Evaluation of ways of eliminating of the film boiling process using a modern physics point of view allows significant improvement of the IQ equipment making it less costly and more efficient. All of this cardinally simplifies the implementation of the IQ technology in heat treat practice. The paper shows how creation of a thin insulating surface layer during quenching of steel parts in low concentration of inverse solubility polymers results in eliminating of film boiling processes that makes the quench process intensive. Historically in heat treating industry, an effective heat transfer coefficient was widely used for evaluating of the nucleate boiling process. And quenching during the nucleate boiling mode of heat transfer was considered as slow cooling.

Low dielectric loss induced by annealing in (La0.5Nb0.5)0.005Ti0.995O2 colossal permittivity ceramics

Low dielectric loss (tanδ 104) (La0.5Nb0.5)0.005Ti0.995O2 ceramics. The effects of annealing on two important dielectric relaxations, electron-pinned defect-dipole (EPDD), and Maxwell–Wagner polarization were explored. An enhancement of activation energy (Ea) value and a large distribution of relaxation time τ were detected in EPDD relaxation for the ceramics annealed at 1123 K. The EPDD polarization was destroyed, accompanied by the disappearance of CP in the high annealing temperature range (> 1273 K). Impedance spectroscopy analysis suggested that grain boundary impedance could greatly enhanced as the ceramics were annealed at 1123 K. Strong electrode-material-dependent dielectric properties were detected, and a high tanδ was observed after the insulating surface layer was removed. The CP and low tanδ were ascribed to the maintenance of EPDD polarization, the enhancement of grain boundaries resistance, and the formation of an insulating surface layer through appropriate annealing.

Towards an Atomic-Level Understanding of Solid Electrolyte Interphase Formation through Molecular Simulation

Lithium ion batteries (LIBs) have been one of the greatest technological achievements in recent history, allowing for leaps forward in consumer technologies. However, growing demand for electric vehicles worldwide is fueling demand for LIBs with larger capacity, faster charging, and longer cycle lives without compromising on safety. This has prompted a resurgence in the study of lithium metal as a next-generation anode, despite challenges with stabilization of the anode-electrolyte interface and the formation of dendrites. In order to realize the large theoretical capacity offered by lithium metal as an anode material, the interface must be engineered to allow for stable cycling through the in situ formation of an ionic conducting and electrically insulating surface layer known as the solid electrolyte interphase (SEI) by reductive decomposition of the electrolyte. While formation of a stable SEI remains an issue in the advancement of next-generation batteries, a fundamental understanding of the process at the graphite electrode/electrolyte interface remains elusive despite its ubiquity in commercial LIBs. The time- and size-scales at which the SEI is formed makes its study experimentally difficult at the appropriate resolution for the underlying phenomena which are critical to determining its composition, morphology, and properties. In order to build an atomic-level understanding of the SEI formation process, we have utilized molecular dynamics (MD) simulations with both classical force fields and density functional theory (DFT) to explore the reaction and transport processes which occur at the anode/electrolyte interface including reduction of ethylene carbonate (EC), intermediate diffusion near the electrode surface, and subsequent chemical reaction of the intermediate species. We have explored the effects of acyclic carbonates on reactions kinetics and interfacial structure as well the effects of some common additives. In this talk, we will present some of our recent findings on how local structure of the electrolyte influences reaction kinetics and transport processes which lead to the aggregation and precipitation of reduced carbonate solvents to form the SEI. This improved understanding of the underlying processes which contribute to the formation of the SEI on graphite electrodes may not only improve engineering of such interfaces in commercial applications, but also generate new ideas in the pursuit of a stable lithium/electrolyte interface.

Effects of short-term variability of meteorological variables on soil temperature in permafrost regions

Abstract. Effects of the short-term temporal variability of meteorological variables on soil temperature in northern high-latitude regions have been investigated. For this, a process-oriented land surface model has been driven using an artificially manipulated climate dataset. Short-term climate variability mainly impacts snow depth, and the thermal diffusivity of lichens and bryophytes. These impacts of climate variability on insulating surface layers together substantially alter the heat exchange between atmosphere and soil. As a result, soil temperature is 0.1 to 0.8 ∘C higher when climate variability is reduced. Earth system models project warming of the Arctic region but also increasing variability of meteorological variables and more often extreme meteorological events. Therefore, our results show that projected future increases in permafrost temperature and active-layer thickness in response to climate change will be lower (i) when taking into account future changes in short-term variability of meteorological variables and (ii) when representing dynamic snow and lichen and bryophyte functions in land surface models.

Surface layer characterizations and sintering time effect on electrical and giant dielectric properties of (In0.05Nb0.05)Ti0.9O2 ceramics

In3+ and Nb5+ co-doped TiO2 (INTO) ceramics were sintered at 1400 °C for 5 and 10 h. The room temperature dielectric permittivity of the as-fired INTO ceramics significantly decreased from ε′≈ 7824 to 4669 (at 1 kHz) as sintering time increased, while the microstructure changed slightly. The corresponding loss tangent (tanδ) values were ≈ 0.040–0.047. The resistance of the insulating part and associated conduction activation energy greatly increased with increasing sintering time, corresponding to an increase in the breakdown voltage (Eb). After removing the outer surface layer, Eb was greatly reduced from 104 to 101 V/cm, corresponding to the large increase in tanδ (≈ 0.3–0.4) and ε′ (≈ 105–106). Very high ε′≈ 50,000 with low tanδ≈ 0.06 can be achieved by annealing the polished samples in air, indicating the important role of the surface layer in controlling the dielectric and electrical properties of INTO ceramics. Characterizations revealed that the formation of an insulating surface layer of the as-fired INTO ceramics may not be associated with the chemical compositional gradients of either the Ti3+ or oxygen vacancies. It was suggested that the inner and outer surface regions of INTO ceramics may consist of the same phase but with a slightly different composition.

Anode Surface Evolution in Aqueous Sodium-Ion Batteries

Aqueous sodium-ion batteries may solve the cost and safety issues associated with the energy storage systems for the fluctuating supply of electricity based on solar and wind power. Compared to their lithium counterparts, aqueous sodium-ion batteries offer multiple advantages including more earth abundant sodium, cheaper electrode materials and electrolyte solutions as well as less costly manufacturing conditions. However, poor overall performance and low electrode utilization (much of the electrode material ends up being electrochemically inactive) are the main barriers implementing them in (micro)grid systems. Here we characterize the surface reactions on NASICON-type phosphate anode material and rationalize their close associations with capacity fading upon slow cycling of aqueous sodium-ion batteries. The surface reactions result in the formation of an electrically insulating surface layer causing the failure of electrochemical performance and the precipitation of surface particles that blocks the pores thereby leading to poor electrode utilization. These findings provide insight into new possibilities of improving the electrochemical performance of aqueous sodium-ion batteries by the design of protective layers through surface modifications that prevent the formation of insulating surface layers and insoluble precipitates.

Bias voltage-dependent low field spin transport properties of Fe3O4–PEG with different particle sizes

Spin-dependent transport properties of Fe3O4 spheres with diameters from 200 nm to 900 nm have been investigated and polyethylene glycol (PEG) exists on the surface of Fe3O4 particles. The nonlinear I–V curve became obvious with the increase of Fe3O4 diameter, which indicated the tunneling barrier height decreases with the increasing diameter. The magnetoresistance (MR) can reach −13% with an applied low field of 0.2 T at room temperature. With the diameter increase, the MR decreases and the required applied field increases. Moreover, the decrease of MR with the bias voltage increase can be attributed to the spin-dependent tunneling effect through the insulating surface layer of Fe3O4 and PEG.

Sputtering-induced reemergence of the topological surface state inBi2Se3

We study the fate of the surface states of Bi$_2$Se$_3$ under disorder with strength larger than the bulk gap, caused by neon sputtering and nonmagnetic adsorbates. We find that neon sputtering introduces strong but dilute defects, which can be modeled by a unitary impurity distribution, whereas adsorbates, such as water vapor or carbon monoxide, are best described by Gaussian disorder. Remarkably, these two disorder types have a dramatically different effect on the surface states. Our soft x-ray ARPES measurements combined with numerical simulations show that unitary surface disorder pushes the Dirac state to inward quintuplet layers, burying it below an insulating surface layer. As a consequence, the surface spectral function becomes weaker, but retains its quasiparticle peak. This is in contrast to Gaussian disorder, which smears out the quasiparticle peak completely. At the surface of Bi$_2$Se$_3$, the effects of Gaussian disorder can be reduced by removing surface adsorbates using neon sputtering, which, however, introduces unitary scatterers. Since unitary disorder has a weaker effect than Gaussian disorder, the ARPES signal of the Dirac surface state becomes sharper upon sputtering.

An XPS investigation on the thermal stability of the insulating surface layer of soft magnetic composite powder

Ferromagnetic powder particles coated with electrical insulating inorganic layers constitute composite materials used as powdered iron cores in electromagnetic applications. The aforementioned surface layers of these soft magnetic composites (SMC) are responsible for their high electrical resistivity. The perseverance of the coating through annealing processing step is crucial to the performance of finalized SMC products. In the present study, the thermal stability and microstructure of an iron phosphate based surface layer from a commercially available SMC powder were investigated by coupling various analytical techniques. The material was annealed at different temperature regimes, both in oxidizing (air) and inert (N2) conditions in order to isolate the effect of temperature from the atmosphere composition on the chemical state of the coating. X‐ray photoelectron spectroscopy (XPS) was utilized to assess the composition of the surface layer at all conditions. Moreover, phosphate‐based chemical standards were processed and analyzed in a similar manner so as to facilitate the interpretation of the observed XPS spectra from the SMC powder. High resolution scanning electron microscopy, energy‐dispersive X‐ray spectrometry and X‐ray diffraction were implemented in order to fully characterize the material under question. The results indicated the transition of the insulating layer from an amorphous state to fully crystalline under annealing in inert atmospheric conditions. Moreover, it was observed that phosphates are still present in the coating in mixture of valance states. Conversely, a thick iron oxide scale was formed under treatment in air, and no phosphorus signal was detected, indicating a total decomposition of the layer. Copyright © 2016 John Wiley & Sons, Ltd.

The Influence of La Doping and Heterogeneity on the Thermoelectric Properties of Sr3Ti2O7 Ceramics

La‐doping mechanisms and thermoelectric properties of Sr3Ti2O7 Ruddlesden–Popper (RP) ceramics sintered under air and flowing 5% H2 at 1773 K for 6 h have been investigated. Changes in lattice parameters and conductivity revealed a limited interstitial anion mechanism (~1 at.%) based on La3+ + ½O2−→Sr2+, which resulted in insulating samples when processed in air. In contrast, electronic donor‐doping (La3+ + e− → Sr2+) and oxygen loss [O2− → ½ O2 (g) + 2 e−] are the dominant mechanism(s) in 5% H2‐sintered ceramics with a solution limit of ~5 at.%. The increased solubility limit is attributed to the formation of Ti3+ during reduction, which compensates for the extra positive charge associated with La on the A‐site and also to the occurrence of oxygen loss due to the reducing conditions. For 5% H2‐sintered samples, an insulating surface layer was formed associated with SrO volatilization and oxygen uptake (during cooling) from the sintering. Unless removed, the insulating layer masked the conductive nature of the ceramics. In the bulk, significantly higher power factors were obtained for ceramics that were phase mixtures containing highly conductive perovskite‐based (Sr,La)TiO3−δ (ST). This highlights the superior power factor properties of reduced perovskite‐type ST phases compared to reduced RP‐type Sr3Ti2O7 and serves as a precaution for the need to identify low levels of highly conducting perovskite phases when exploring rare‐earth doping mechanisms in RP‐type phases.

Low-energy μSR Study on the Tetradymite Topological Insulator Bi1.5Sb0.5TeSe2

A new method utilizing the low energy muon spin rotation (μSR) technique to investigate the spin structure within the surface layer of topological insulator has been proposed. In order to detect the spin polarization parallel with the surface, one applies a weak field Bpar, which, by breaking the time reversal symmetry, is expected to induce a net magnetization perpendicular the surface. This will affect the profile of muon spin depolarization spectra. Preliminary measurements on the topological insulator Bi1.5Sb0.5TeSe2 with tetradymite structure are also shown.

Effect of heat treatment in air on surface composition of iron-phosphate based soft magnetic composite components

Soft magnetic composite materials (SMC) manufactured by conventional powder metallurgical techniques for electromagnetic applications constitute individually encapsulated ferromagnetic powder particles with an insulating surface layer, bonded together into 3D finished structures. The production procedure includes compaction of the SMC base powder mixed with a lubricant substance and a post-annealing treatment that aims to relieve the stresses induced in the component during pressing. In the present study, the effect of the heat treatment process to the nature of the insulating layer was investigated under different temperature regimes using analytical techniques. Its surface chemistry was determined based on the XPS depth profiling technique, and its morphology and structure were evaluated using HR-SEM and XRD. Differences between interior and exterior areas of the samples suggested the development of an oxide scale in the outer regions that prevents its further bulk oxidation at temperatures above 500 °C, while below that temperature incomplete de-lubrication takes place.

Origin of the insulating state in exfoliatedhigh-Tctwo-dimensional atomic crystals

An optical spectroscopic study of exfoliated two-dimensional atomic crystals of Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$ reveals a pronounced suppression of optical conductivity with thickness. Through detailed analysis, the authors interpret this finding as the formation of an insulating surface layer.

(Invited) Impact of Surface Chemistry on the Electrochemical Performance of Perovskite Cathodes

Surface chemistry and reactivity of solid oxide fuel cell (SOFC) and electrolyzer (SOEC) electrodes play a key role for transport and exchange processes, such as the oxygen reduction reaction in SOFC cathodes, and drive electrochemical performance and durability. In order to identify drivers and problems, it is necessary to probe electron and ion transfer and transport processes at electrolyte/electrode interfaces and surfaces and build broad fundamental understanding. This knowledge can then be used to drive the innovation of novel, more powerful electrode materials and cell designs.We used a combination of spatially resolved scanning photoelectron microscopy and electrochemical measurements to study in situ the surface chemistry of cathode perovskite catalysts and electrolyte in electrochemical model cells during high temperature operation. The electrodes were made of screen-printed (La1-xSrx)MeO3 with Mn, Fe, Co, Ni as transition metal Me and x = 0 – 0.4 and covered a wide range in ionic and electric conductivity from pure ion to mixed conduction. The electrolyte was a thin yttrium-stabilized zirconia sheet. We studied the surface chemistry of oxide catalyst and electrolyte at temperatures between 400 and 700°C, oxygen pressure (10-7bar – 1mbar) and also in humidity and/or hydrocarbon containing environment and demonstrated systematic changes in the perovskite surface termination with oxygen chemical potential. Oxidizing environment was found to promote transition metal termination and presence of various oxygen surface species, while reducing environment drove segregation of strontium to the surface and suppressed the level of surface oxygen.We also showed that this simple segregation behavior was dramatically changed in presence of impurities. Silica segregated to the surface, forming a glassy, insulating surface layer that slowed down the oxygen incorporation reaction and produced a strong degradation of the cell performance. While the degradation of Ni/NiO electrodes could be reversed by an applied cell voltage, no such remedy was observed for perovskite electrodes.In our in-situ set up, oxygen in- or excorporation could be driven by an applied electric field across the cell. Oxygen ion flux and electric field caused dynamic changes of catalyst and electrolyte surface chemistry, including redox reaction, changes in surface segregation and long range surface diffusion at the electrode surface. The electrochemical response of the model cells was interpreted in terms of reactions and reaction steps that matched the spectroscopic observations, thus constructing new understanding of the processes in operating electrodes.

Model design on calculations of microwave permeability and permittivity of Fe/SiO2 particles with core/shell structure

Fe/SiO2 particles with core/shell structure were prepared by coating silica on the surface of a commercial spherical carbonyl iron via the hydrolysis process of tetraethyl orthosilicate (TEOS). The electromagnetic performance of commercial carbonyl iron and as-prepared Fe/SiO2 particles was studied theoretically and experimentally. As predicted by the theoretical calculation based on the Bruggeman formula and the Landau–Lifshitz–Gilbert (LLG) theory, the insulating surface layer of silica was effective to reduce the permittivity parameters of pure carbonyl iron. The measured results showed good agreement with the theoretical prediction. Although there was a little decrease in the permeability of the Fe/SiO2 core/shell particles, a better impedance match especially at higher frequency range was obtained when used as a microwave absorber. The reflection loss (RL) curves show that the lowest reflection loss of Fe/Epoxy composite (−20.5GHz) was obtained corresponding to the frequency of 8.5GHz when the thickness of the absorber was 3mm. A different trend was observed in Fe/SiO2/Epoxy composite. The reflection loss value got lower by decreasing the thickness of absorbers. At the thickness of 2.2mm, a relative low reflection loss (−17GHz) corresponding to the frequency of 13.6GHz was obtained. Compared with the Fe/Epoxy composite, the improvement on shifting the reflection loss peak to higher frequency and on reducing the optimal thickness of absorbers was made by Fe/SiO2/Epoxy composite.