Micron Thick Films Research Articles

Temperature Distribution Measurement and Internal Flow Visualization in the Lubrication Film of Non-contacting Mechanical Seals

The surface textures of non-contacting mechanical seals form a few micrometers thick lubrication film that aids in achieving high performance of the seal. Improved texture design can control internal fluid flow, improving the sealing performance and lubrication characteristics. However, film thickness is sensitive to thermal deformation, making seals vulnerable to heat. Excessive temperature increases and deviations in temperature distribution can cause performance degradation or damage to seals. Previous studies have suggested that heat transfer impacting these thermal issues may be influenced by the internal flow, however, experimental studies examining flow fields have not been conducted yet. This study presents the design of experimental equipment, measurement of temperature distributions and observation of flow fields in lubrication films with three different textures—non-textured, Rayleigh step, and spiral groove. Furthermore, the results were compared with those of numerical analysis. Temperature was found to be higher at the downstream end of the flow. In particular, the Rayleigh step seal exhibited a nonuniform temperature distribution that alternated between low and high-temperature areas. This was caused by cooling related to radial grooves that had a pumping effect. In contrast, a uniform temperature distribution was confirmed in the case of the spiral groove seal, which was formed by mixing outer and inner side fluids. The results indicate that the temperature distribution in lubrication films can be controlled via flow field by the surface texture shape or depth to protect against thermal degradation and to achieve high reliability of non-contacting mechanical seals.Graphical

(Invited) Electrochemically Induced Deposition of Thin-Film Oxides and Electronic Insulators

Electrodeposition is typically associated with the electroreduction of metal ions for the deposition of metals, alloys or semiconductors. Compounds can be electrodeposited when the metal ions form an insoluble compound upon change of its valence state at the electrode surface. A well-known example is the anodic deposition of MnO2, where aqueous solvated Mn2+ ions are oxidized to the insoluble Mn(IV) in acid sulfate solutions. Alternatively, the precipitation of a compound or oxide can be triggered by changing the local pH at the electrode by a suitable electrochemical reaction. The use of electrochemical formed base from so-called probase molecules has found applications in formation of oxides, phosphates but also organic materials such metal organic frameworks (MOFs). Nitrate was one of the first pro-bases suggested for the electrochemical precipitation of ZnO. An alternate electrochemical approach for depositing compounds and oxides is the electrochemical initiation of a sol-gel reaction first developed for the silica sol-gel process by Shacham et al. [1] During deposition, an electrode is submerged into a precursor solution followed by the application of a cathodic current. The chemical reaction is triggered by electro-generating the OH- catalyst that is required for the polycondensation of the silica precursor. Since this occurs near the surface, the method results in silica thin films deposited only on the electrode surface. Finally, also electro-polymerization can lead to thin insulating films. In this paper, several of these reaction paths will be explored. The initial stages of MnO2 electrodeposition are strongly dependent on the starting surface and determines the adhesion and attainable film thickness [2]. The relationship between (intentionally) introduced meso-porosity, growth rate and film thickness will be discussed. The poor electronic conductivity of oxides makes that the reaction is maintained by ionic conduction through the films, similar as for oxide formation by anodization. For the formation of micron thick oxide films, also good control of hydrodynamic conditions is essential. [3] The resistive nature of the layers typically allows also for good conformality over high aspect ratio substrates. Conformal deposition of oxide thin-film coatings on high aspect ratio structures is typically claimed by Atomic Layer Deposition. Inorganic-organic hybrid films such as metal cones can be similarly deposited by Molecular Layer Deposition (MLD). [4] The nature of the surface limited reactions of these vapor-phase methods allows for the formation of continuous sub-nanometer to a few tens of nanometer thin films with uniform thickness over the most complex architectures. The accuracy of the technique goes at the cost of long deposition cycles especially when very large surface areas with extreme aspect ratios (>100) are involved. The intrinsic resistive nature of the electrodeposited oxide and insulator films allows for excellent conformal coatings with growth rates much more suited for thicker films without loss in conformality or uniformity. In this paper, we will show examples where electrochemical induced deposition process are used also to coat nano-architectures such as our nanomesh with very large surface area (100 cm2 per planar cm2) and aspect ratio (100x). [1] Shacham, B. R., Avnir, D. & Mandler, D. Electrodeposition of Methylated Sol-Gel Films on Conducting Surfaces. Adv. Mater. 384–388 (1999).[2] "Electrodeposition of Adherent Submicron to Micron Thick Manganese Dioxide Films with Optimized Current Collector Interface for 3D Li-Ion Electrodes" Marina Timmermans, Nouha Labyedh, Felix Mattelaer, Stanislaw Zankowski, Stella Deheryan, Christophe Detavernier, and Philippe M. Vereecken, J. Electrochem. Soc. 164, 14, D954-D963 (2017).[3] Aggregate-Free Micrometer-Thick Mesoporous Silica Thin Films on Planar and Three-Dimensional Structured Electrodes by Hydrodynamic Diffusion Layer Control during Electrochemically Assisted Self-Assembly”, G Vanheusden, H Philipsen, SJF Herregods, PM Vereecken, Chemistry of Materials (2021).[4] “Molecular Layer Deposition of "Magnesicone", a Magnesium-based Hybrid Material" Jeroen Kint, Felix Mattelaer, Sofie S. T. Vandenbroucke, Arbresha Muriqi, Matthias M. Minjauw, Mikko Nisula, Philippe M. Vereecken, Michael Nolan, Jolien Dendooven, and Christophe Detavernier, Chem. Mater. 32, 11, 4451–4466, (2020).

Investigation of Heat Transfer of Bulk and Thin-Film PbInTe Samples by the Method of Dynamic Gratings

Measurements of the thermal diffusivity of thin and bulk indium-doped lead telluride have been taken using a modified method of dynamic gratings. Thermal gratings were recorded by a 20 ns pulsed laser radiation at a wave length of 532 nm. Dynamic gratings were recorded by a 635 nm continuous laser radiation. The analysis of the diffraction signal kinetics made it possible to determine the life-time of thermal gratings recorded in the investigated samples. It is shown that the use of an additional homodyne field coherent with respect to the diffraction signal field makes it possible to enhance and filter off the selected information component. Based on registered kinetic dependences of the diffracted signal intensity, the thermal diffusivity of bulk and thin film indium-doped lead telluride samples was determined. It has been established that for a micron-thick film, the thermal diffusivity is ten percent lower than for a bulk sample. An investigation has been made into the dependence of the heat transfer in the said samples on their temperature and it has been shown that the rise in the samples′ temperature in the range from 40 to 95°C results in a 20-percent decrease of their thermal diffusivity.

Strain induced anisotropy in liquid phase epitaxy grown nickel ferrite on magnesium gallate substrates

This work focuses on the nature of magnetic anisotropy in 2.5–16 micron thick films of nickel ferrite (NFO) grown by liquid phase epitaxy (LPE). The technique, ideal for rapid growth of epitaxial oxide films, was utilized for films on (100) and (110) substrates of magnesium gallate (MGO). The motivation was to investigate the dependence of the growth induced anisotropy field on film thickness since submicron films of NFO were reported to show a very high anisotropy. The films grown at 850–875 C and subsequently annealed at 1000 C were found to be epitaxial, with the out-of-plane lattice constant showing unanticipated decrease with increasing film thickness and the estimated in-plane lattice constant increasing with the film thickness. The uniaxial anisotropy field Hσ, estimated from X-ray diffraction data, ranged from 2.8–7.7 kOe with the films on (100) MGO having a higher Hσ value than for the films on (110) MGO. Ferromagnetic resonance (FMR) measurements for in-plane and out-of-plane static magnetic field were utilized to determine both the magnetocrystalline the anisotropy field H4 and the uniaxial anisotropy field Ha. Values of H4 range from −0.24 to −0.86 kOe. The uniaxial anisotropy field Ha was an order of magnitude smaller than Hσ and it decreased with increasing film thickness for NFO films on (100) MGO, but Ha increased with film thickness for films on (110) MGO substrates. These observations indicate that the origin of the induced anisotropy could be attributed to several factors including (i) strain due to mismatch in the film-substrate lattice constants, (ii) possible variations in the bond lengths and bond angles in NFO during the growth process, and (iii) the strain arising from mismatch in the thermal expansion coefficients of the film and the substrate due to the high growth and annealing temperatures involved in the LPE technique. The LPE films of NFO on MGO substrates studied in this work are of interest for use in high frequency devices.

3D Conformal Fabrication of Piezoceramic Films.

Piezoceramic films are an essential class of energy‐conversion materials that have been widely used in the electronics industry. Although current methods create a great freedom for fabricating high‐quality piezoceramic films, it requires well‐controlled synthesis conditions, including special high‐cost equipment and planar substrates particularly. The limited substrate selections hinder the applications of piezoceramic films in 3D conformal structures where most objects possess complex curvilinear surfaces. To overcome such limitations, a fast, energy‐efficient, and cost‐effective approach, named flame treated spray (FTS) coating, is developed for preparing piezoceramic films on free‐form surfaces. The flame treatment significantly enhances the hydrophilicity of a substrate, assisting in forming a uniform and continuous thin film. The followed spray coating deposits hundreds of nanometers to several micrometers thick films on 3D free‐form surfaces. Given the size controllability and arbitrary surface compatibility of the FTS method, a highly conformal piezoelectric tactile sensor array (4 × 4) is assembled on a spherical surface for mimicking robot fingers and an on‐site thin‐film sensor on the wing of an aircraft model to monitor the vibration in real‐time during flight. The FTS film deposition offers a highly promising methodology for the application of functional thin‐film from micro‐ to marcoscale devices, regardless of conformal problems.

Rapid Actuation of Thermo-Responsive Polymer Networks: Investigation of the Transition Kinetics.

The swelling and collapsing of thermo-responsive poly(N-isopropylacrylamide)-based polymer (pNIPAAm) networks are investigated in order to reveal the dependency on their kinetics and maximum possible actuation speed. The pNIPAAm-based network was attached as thin hydrogel film to lithographically prepared gold nanoparticle arrays to exploit their localized surface plasmon resonance (LSPR) for rapid local heating. The same substrate also served for LSPR-based monitoring of the reversible collapsing and swelling of the pNIPAAm network through its pronounced refractive index changes. The obtained data reveal signatures of multiple phases during the volume transition, which are driven by the diffusion of water molecules into and out of the network structure and by polymer chain re-arrangement. For the micrometer-thick hydrogel film in the swollen state, the layer can respond as fast as several milliseconds depending on the strength of the heating optical pulse and on the tuning of the ambient temperature with respect to the lower critical solution temperature of the polymer. Distinct differences in the time constants of swelling and collapse are observed and attributed to the dependence of the cooperative diffusion coefficient of polymer chains on polymer volume fraction. The reported results may provide guidelines for novel miniature actuator designs and micromachines that take advantages of the non-reciprocal temperature-induced volume transitions in thermo-responsive hydrogel materials.

Anodic behavior of nickel in sub-molten KOH and its relevance for the production of electroactive nickel oxides

This study investigated the electrochemical behavior of pure nickel in sub-molten KOH at 200 °C and the structure of surface films formed at different anodic polarization potentials. Potentiodynamic polarization measurements showed an active-to-passive transition followed by transpassivity. Unlike aqueous alkaline solutions, surface films created in active, far passive, and transpassive regions are few microns thick. Grazing-incidence x-ray diffraction (GI-XRD), Raman, and x-ray photoelectron spectroscopy (XPS) measurements revealed that a thick and defective layer of NiO forms during polarization in the active region. Polarization into the passive regime reduces the thickness and number of defects of NiO and promotes passivity. With the formation of +3 valent nickel at far passive region, a thick, flaky adlayer of potassium-intercalated γ-NiOOH is produced. At high transpassive potentials, the flake-like morphology deteriorated with the high rate of oxygen evolution reaction (OER). The micrometer thick electroactive anodic surface films formed during active, far passive, and transpassive region became a few micrometers thick because of the high dissolution rate of Ni and the high viscosity of the sub-molten KOH.

Multiscale Characterization of the Influence of the Organic–Inorganic Interface on the Dielectric Breakdown of Nanocomposites

Nanoscale engineered materials such as nanocomposites can display or be designed to enhance their material properties through control of the internal interfaces. Here, we unveil the nanoscale origin and important characteristics of the enhanced dielectric breakdown capabilities of gold nanoparticle/polymer nanocomposites. Our multiscale approach spans from the study of a single chemically designed organic/inorganic interface to micrometer-thick films. At the nanoscale, we relate the improved breakdown strength to the interfacial charge retention capability by combining scanning probe measurements and density functional theory calculations. At the meso- and macroscales, our findings highlight the relevance of the nanoparticle concentration and distribution in determining and enhancing the dielectric properties, as well as identifying this as a crucial limiting factor for the achievable sample size.

Micrometer thick Sm-Co films for applications on flexible systems

In the industry 4.0 eco-system, flexible electronic devices bear a huge potential for a broad range of applications due to their diverse properties, such as high stretchability, biocompatibility, portability, light weight, and low costs. In this work, Cobalt-Samarium permanent magnetic thin films on flexible polyimide substrate are studied. The influence of the sputter deposition pressure on the structural, morphological, and magnetic properties is analyzed. A method for growing flexible magnetic films is proposed by achieving a maximum coercivity of 13.86 kOe and an energy product of 16.9 MGOe. These results lay the foundations for the design and fabrication of flexible magnetic devices.

Evaluation of the magnetization dynamics in various thick YIG films using our proposed measurement technique

This paper describes the evaluation of magnetization dynamics in YIG polycrystalline films with various thicknesses using our proposed measurement technique. The saturation magnetostriction (λs) are approximately less than −1.0×10−6 regardless of the film thickness and are lower than that of bulk polycrystalline YIG, which might be attributed to the structural and/or magnetic inhomogeneities. On the other hand, the in-plane effective damping constant (αeff) decreases approximately from 0.00384±0.00029 to 0.00316±0.00013 with the increase of film thickness, and their values are much higher than those of nano-meter thick and micro-meter thick YIG films deposited on GGG substrates. The reason for this difference may be that the extrinsic damping originating from the magnetic inhomogeneities are enhanced in the film plane. In addition, αeff slightly increases as λs decreases. This tendency is opposite to those of Ni-Fe and Fe-Si polycrystalline films, and the correlation between αeff and λs does not appear. Therefore, these results suggest that magnetic inhomogeneities are mainly influenced to both αeff and λs in the film plane.

Highly Oriented Organic Ferroelectric Films with Single-Crystal-Level Properties from Restrained Crystallization

Crystallization is a key for ferroelectricity which is a collective behavior of microscopic electric dipoles. On the other hand, uncontrolled crystallization leads to uneven morphology and random crystal orientations, which undermines the application potential of ferroelectric thin films. In this work, we introduce a film fabrication method of low-temperature physical vapor deposition followed by restrained crystallization, with electrical properties monitored in real-time by in situ measurements. This method was adopted to fabricate films of 2-methylbenzimidazole (MBI), whose molecule crystals are proton-transfer type biaxial ferroelectrics and tend to grow into a hedgehog-shaped spherulites morphology. The in situ measurements confirm that the crystallization, corresponding to a clear transition of physical properties, occurs dominantly during post-deposition warming. This enables the fabrication of micron-thick films in disk-shaped morphology with one polarization axis aligned along the out-of-plane direction, while the measured spontaneous polarization and coercive field are comparable to the single-crystal values. These results mark an important advancement of film growth that is expected to benefit widely the fabrication of molecular materials films whose functional properties hinge on crystallization to achieve desirable morphology and crystallinity.

Thermal Diffusivity Measurement in Through-Thickness Direction of Dielectric Film With Metal Substrate

Thin polymer films are widely used as dielectrics in devices operating in the field of electrical engineering, and thermal diffusivity <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${D}$ </tex-math></inline-formula> in through-thickness direction is one of their crucial thermophysical parameters concerned with heat dissipation within the involved devices. However, there exist some difficulties in precisely measuring the thermal diffusivity of micron-thick dielectric film. Considering that the measuring principle of thermal pulse method (TPM) for mapping space charge profile is strongly related to the thermal diffusivity of the film sample, a novel method is proposed to retrieve the coefficient based on the data analysis of the thermal response current of thin polymer dielectric film with a thermally conductive substrate. Experimental results of presentative polymer products biaxially oriented polypropylene (BOPP) and polyimide (PI) films with thickness of several micrometers show well consistency with the data in the literature, demonstrating the feasibility of the proposed method.

Thin polyethylene (LDPE) films with controlled crystalline morphology for studying plastic weathering and microplastic generation

Plastic pollution has become one of the most visible environmental issue. Studying the aging and the fragmentation mechanism of polymers therefore raises environmental and health issues. Since it is particularly difficult to monitor all these processes in the real environment, laboratory studies are still necessary to obtain more accurate data and to identify the pathways leading to the outcome of plastic debris in the environment. In this paper we present a new method to prepare micrometer thick LDPE films with different thicknesses and controlled crystalline morphologies (i.e. spherulite size) using dip-coating process. LDPE films presenting different spherulite dimensions were submitted to an accelerated weathering test which indicates that spherulite size has only a weak influence on the rate of photo-oxidation. However, films presenting large spherulites morphologies seem to be more sensitive to surface erosion and crack initiation. We believe that the method developed in this paper can be used to study with more control and more rapidly (i.e. thin film and accelerated weathering) the influence of crystalline morphology on a wide range of semi-crystalline polymers in different aging conditions and thus help to better understand the fragment production of plastics in the environment.

Gradient magnetron co-sputtered μ m-thick Al–Si films on dielectric substrates for operation in the millimeter-wave band

Ongoing active development of modern radio frequency electronic devices operating in the millimeter (V) band, such as fifth-generation wireless communications, demands new materials to control electromagnetic interference, compatibility, and reliability of such systems. This work investigates feasibility absorptive non-reflective thin coatings deposition on dielectric substrates using simultaneous magnetron co-deposition. For this, electromagnetic waves propagation in the millimeter band through in micrometer-thick Al–Si films of varied composition was studied. The co-deposition process was controlled by the ratio of sputtered atoms fluxes. Graded segregation was observed under certain parameters of the co-deposition process, resulting in a depth gradient of an aluminum content, as confirmed by the secondary ion mass spectrometry study. A qualitative model was proposed involving aluminum-induced silicon recrystallization happening in the course of a known aluminum interlayer exchange process. The observed Al–Si segregation effect in micrometer-thick films allows for preparation of the non-reflective and absorptive material for operation in the V-band with reflection losses more than 10 dB and transmission losses around 5 dB in the bandwidth of up to 20 GHz.

High thermal conductivity and thermal boundary conductance of homoepitaxially grown gallium nitride (GaN) thin films

Gallium nitride (GaN) has emerged as a quintessential wide band-gap semiconductor for an array of high-power and high-frequency electronic devices. The phonon thermal resistances that arise in GaN thin films can result in detrimental performances in these applications. In this work, we report on the thermal conductivity of submicrometer and micrometer thick homoepitaxial GaN films grown via two different techniques (metal-organic chemical vapor deposition and molecular beam epitaxy) and measured via two different techniques (time domain thermoreflectance and steady-state thermoreflectance). When unintentionally doped, these homoepitaxial GaN films possess higher thermal conductivities than other heteroepitaxially grown GaN films of equivalent thicknesses reported in the literature. When doped, the thermal conductivities of the GaN films decrease substantially due to phonon-dopant scattering, which reveals that the major source of phonon thermal resistance in homoepitaxially grown GaN films can arise from doping. Our temperature-dependent thermal conductivity measurements reveal that below 200 K, scattering with the defects and GaN/GaN interface limits the thermal transport of the unintentionally doped homoepitaxial GaN films. Further, we demonstrate the ability to achieve the highest reported thermal boundary conductance at metal/GaN interfaces through in situ deposition of aluminum in ultrahigh vacuum during molecular beam epitaxy growth of the GaN films. Our results inform the development of low thermal resistance GaN films and interfaces by furthering the understanding of phonon scattering processes that impact the thermal transport in homoepitaxially grown GaN.

Multifunctional Metal‐oxide Integrated Monolayer Graphene Heterostructures for Planar, Flexible, and Skin‐mountable Device Applications

The adoption of nanostructured metal-oxides integrated graphene monolayers-based heterostructures appears to be a promising approach for enhancing the performance of various devices. However, precisely controlled growth of such unique heterostructures without disturbing the monolayer graphene characteristics remains a challenging task especially over a large area with good uniformity. Herein, ultrathin metal-oxide (p-Co 3 O 4 and n-ZnO) nanostructures (MONSs) integrated graphene monolayer (GML) heterostructures are carefully developed by fascinating the graphene native defects while nucleation and growth of MONSs. Metal-oxides integrated graphene monolayers with lower material densities (≤ 30 μg/cm 2 ) significantly enhanced the quality (2D/G ~5–9) and reduced the electrical resistance (11–17 Ω/sq.) of graphene layers, whereas the heterostructures developed with higher densities possess predominant water-oxidation characteristics than that of their individual components. Further, the Co 3 O 4 /GML heterostructures-based micro-supercapacitors, fabricated over 25 µm polyimide sheets, showed excellent mechanical stability and flexibility with a volumetric and specific capacitance of 7.76 F/cm 3 and 1.27 F/g, respectively. The ZnO/GML heterostructures designed over micron thick parylene film displayed exciting photoresistor characteristics with photosensitivity of ~1.54 and superb flexibility and skin-mountability. Synergistic multifunctional characteristics of these ultrathin heterostructures offer the possibility to realize various eco-friendly ultrathin as well as skin-mountable energy and health monitoring devices. Ultrathin metal-oxide(s) precisely integrated single-layer graphene heterostructures, developed by low-temperature methods, exhibit high conductivity and multifunctional characteristics. As a catalyst, the heterostructures show excellent electrocatalytic performance, and whereas, the active electrodes defined over flexible and skin-mountable surfaces exhibit significant energy storage and photoresistive characteristics. • Ultrathin metal-oxides nanostructures integrated graphene monolayers developed. • The heterostructures grown under optimum conditions possess high conductivity. • These heterostructures exhibited synergistic OER electrocatalytic performance. • Supercapacitors on flexible sheets showed good mechanical stability and durability. • The structures developed on ultrathin sheets are skin-mountable and integratable.

Properties of boron carbide thin films deposited by pulsed laser deposition

Boron carbide (B4C) is the third hardest material known after diamond or ta-C and cubic boron nitride (c-BN). Due to its super-hardness and, moreover, its stability up to high temperatures and chemical stability against ferrous materials , it is an interesting material for use as wear resistant coatings , particularly in applications where diamond or ta-C films are not possible to use. Super-hard boron carbide coatings were produced using Pulsed Laser Deposition (PLD) and their mechanical properties were investigated in dependence of the laser fluence and the substrate temperature during deposition. While substrate temperature was found to have a significant influence on the hardness of the coatings, the influence of the ablation fluence was insignificant. At all fluences used, the indentation hardness increases from some 31 GPa at room temperature up to 47 GPa at 500 °C. The Young's modulus was measured to be in the range of 470 to 600 GPa. The films show good adhesion on tungsten carbide but a poorer adhesion on cold working steel. The surface quality, however, was poor at film thicknesses sufficient for practical applications due to the incorporation of many droplets. In order to improve this, an approach was made using an alternative boron carbide target the results of which also are presented. The target was produced by depositing a several ten microns thick boron carbide film on a steel substrate using PLD. The films deposited using this target were found to have much less and much smaller droplets. • Boron carbide coatings with up to 47 GPa hardness can be prepared by Pulsed Laser Deposition. • The substrate temperature during deposition is crucial for the resulting film hardness. • Super-hard boron carbide coatings show a decrease in wear rate by a factor of 11 compared to uncoated WC hard metal. • The use of denser and more homogeneous targets provides less droplet incorporation.

Micron-Thick Hexagonal Boron Nitride Crystalline Film for Vacuum Ultraviolet Photodetection with Improved Sensitivity and Spectral Response

Micron-Thick Hexagonal Boron Nitride Crystalline Film for Vacuum Ultraviolet Photodetection with Improved Sensitivity and Spectral Response

High Strength and Low Coercivity of Cobalt with Three-Dimensional Nanoscale Stacking Faults.

Lower coercivity (HC) and magnetic anisotropy (K1) coupled with high mechanical strength are essential properties for Co-based soft magnetic thin films; however, the strength-coercivity trade-off limits their development. Co with face centered cubic structure (fcc) exhibits lower HC and K1 than its grand hexagonal close packed structure (hcp); however, metastable fcc-phase Co is hard to stabilize. Here, by using Cu (100) seed layer, we synthesized micron-thick fcc Co films with self-formed three-dimensional nanoscale stacking faults (3D-nSFs) that could achieve high strengths without sacrificing soft magnetic properties. The 3D-nSFs, induced by the Co/Cu interface, could not only stabilize the metastable fcc Co to yield lower HC but also impede dislocation motion to strengthen Co films. More importantly, we successfully tailored the density of 3D-nSFs and confirmed a large variation in magnetic coercivity (by 100%) and indentation hardness (by 25%). This work provides a new strategy for integrated performance optimization by interface design and strain engineering.

Space charge effect and application in solid insulation dielectrics

<p indent="0mm">Space charge effect and its application in solid insulation dielectrics are long-term issues with continuous development. The study of space charge effect is of great significance to high-voltage direct current (HVDC) transmission. This paper summarizes the development history and research progress of space charges in solid dielectrics, from the aspects of measurement technologies, damage, suppression, application, and simulation, etc. Currently, the most widely used space charge distribution measurement technologies can be roughly divided into two categories, acoustic effect technique and thermal effect technique, whose spatial resolution and application range are different. For the acoustic effect technique, pressure wave propagation method and pulsed electro-acoustic method are most commonly used with proven technique. The spatial resolutions of these two are similar, both can reach an average of <sc>10 μm.</sc> With extremely narrow pulses, the highest spatial resolution can reach about <sc>1 μm.</sc> The thermal effect techniques, such as thermal pulse method and laser intensity modulation method, have more advantages in spatial resolution and practicability, especially for micron-thick film detection. Future research may focus on the more widely application of space charge distribution measurement technologies to electrical equipment in operation. For the trap parameters and charge storage properties of insulation dielectrics, there is still a lack of reliable and practical measurement technologies. The results of the isothermal decay method are more reliable and credible, but the required measurement time is too long. Therefore, it has been rarely used in practice in recent years. The most commonly used methods, thermal stimulated discharge current method and photo stimulated discharge method, each has its own disadvantages. And their results for the same sample are quite different for mutual corroboration. In recent years, some researchers are more focused on piezo-stimulated current method, but this method is still in a preliminary stage of experiment and application. In general, the space charge effect has pros and cons. On one hand, space charge accumulation in solid insulation dielectrics may cause electric field distortion and discharge damage, eventually lead to breakdown and destruction of insulation. In order to prevent these damages, many effective methods have been proposed, such as surface modification of insulation dielectrics and material modification by additives. However, these methods have not yet been practically used in industrial applications. On the other hand, space charge effect also has many efficient applications. One of the most important applications is electret, a dielectric material that quasi-permanently stores real charges in surface or volume traps. A large number of significant contributions have been published on preparation process, charge characteristics and applications of electrets. The simulation research of space charges can mainly be divided into physical model research of space charge behavior and data processing of experimental results. The development of space charge simulation is relatively backward compared to other research fields as the physical model of space charge behavior has not yet been built completely. The issues of space charge effect and its application are involved with multiple factors. Although some technical and theoretical developments have been achieved in recent years, there are still some uncertain questions remained to be explored.