Nano Multilayers Research Articles

Preparing Al2O3/TiO2 nano-multilayers on the surface of FeAl/Al2O3 coating as tritium permeation barrier

Abstract Preparing tritium permeation barrier (TPB) coating on the surface of structural materials can effectively alleviate tritium permeation in fusion reactor system. However, due to the existence of inevitable certain number of defects in TPB, there is still a gap between the commonly prepared TPB coatings and the requirements proposed by fusion reactors. In this work, composite coatings with Al2O3/TiO2 nano-multilayers (NMLs) covering on the surface of FeAl/Al2O3 coating were prepared as TPB. The FeAl/Al2O3 TPB coating on the surface of 316L stainless steel was prepared by electrochemical deposition of Al, aluminization and in-situ oxidation. Due to the aggregation of Cr, there are micro sized defects in the Al2O3 layer. The presence of defects seriously reduces its hydrogen isotopes permeation resistance. The atomic layer deposition was then adopted to prepare Al2O3/TiO2 NMLs with period thickness of 15, 7.5 and 3 nm on the surface of Al2O3 layer, which not only cover the defects, but also utilize interfaces to suppress hydrogen isotopes permeation. The composite coating sample with the period thickness of 3 nm showed great enhancement of two orders of magnitude in deuterium permeation resistance property compared to the original FeAl/Al2O3 TPB coating at 500 °C, and the permeation reduction factor reaches a high value of 5 × 105, which far above the requirements of future fusion reactors.

Controlled Directional Cu Outflow in Cu/W Nanomultilayers

AbstractIn this study, we have investigated the feasibility of localized, focused ion beam (FIB)-stimulated Cu outflow in Cu/W nanomultilayers (NMLs) for manufacturing of heterogeneous micro-/nanojoints. Sub-micron-sized trenches were created on the nanomultilayer surface prior to heat treatment with the aim of directing the diffusion of Cu to locally defined NML surface regions. Cu outflow was triggered by annealing at 500 °C in a reducing atmosphere and lead to formation of (sub-)micron-sized Cu particles that are firmly joined to the W-terminated Cu/W NML. The results show that not only the depth of trenches (i.e., the parameters of the FIB treatment), but also the stress and the microstructure of the NMLs influence the Cu directional transport. The Cu outflow was found to be much more pronounced when the multilayer has a disordered microstructure with pores and open grain boundaries, as observed for NMLs with a tensile stress. We have thus demonstrated that FIB surface patterning enables the localized generation of (sub-)micron-sized Cu particles that can be used for manufacturing of micro-/nanojoints.

Characterization of Cr/CrN nano multilayers obtained through reactive sputtering with different degrees of unbalance in the magnetron

Nanoscale hard coatings of bilayers of Cr/CrN were obtained and characterized through unbalanced magnetron sputtering (UBMS) in an atmosphere of Ar and Ar + N2 on silicon and hardened and tempered H13 steel, with a degree of unbalanced K0 between 0.87 and 1.37. The deposition parameters were as follows: a power of 160 watts, Ar and N2 flow rates of 9 and 3 sccm, and a substrate target distance of 5 cm. A Gen coa VT 100 magnetron with varying degrees of unbalance, obtained by displacing its central magnet, was used. The electrochemical behavior of these coatings was characterized using the technique of potentiodynamic anodic polarization. The coatings' microstructure and hardness were analyzed via X-ray diffraction (XRD) and nanoindentation tests; Cr, CrN, and Cr2N phases were obtained, with a hardness that increases with the degree of unbalance between 18.90 and 24.97 GPa.

Microstructure and thermal stability of crystalline/amorphous Fe/FeW nanomultilayers

The thermal stability of crystalline-amorphous interfaces was investigated in Fe/FeW nanomultilayers (NMs), where the alloy layers were amorphous in the as-sputtered state with concentrations of Fe-38 at.% W or Fe-67 at.% W. Compositionally driven devitrification, layer breakdown, and recrystallization were compared using both single-layer and multilayer configurations at temperatures ranging from 250 °C to 750 °C. Annealing of the NMs to 500 °C revealed destabilization in the Fe-67 W layers with the formation of crystalline-crystalline interfaces (CCIs) whereas the Fe-38 W layers remained intact with stable crystalline-amorphous interfaces (CAIs). Further annealing to 750 °C resulted in multilayer evolution and recrystallization, where breakdown of the CAIs was attributed to layer intermixing while the CCIs experienced intermetallic grooving and pinch-off. The influence of amorphous stability, composition, and intermetallic formation are discussed with respect to the NM breakdown mechanisms. This work highlights a promising strategy for exploring compositionally driven stability at the nanoscale in crystalline-amorphous alloys.

Ag Surface Outflow Kinetics in Ag/AlN Nanomultilayers

The heat‐induced microstructure evolution of nanomultilayers (NMLs) with metallic components is widely studied due to its critical importance in ensuring the reliable application of these nanomaterials. The thermal degradation of NMLs is controlled by mass transport of one of the NML components from the NML volume to its surface. The outflow occurs at temperatures well below the melting point of metal, offering new opportunities for the engineering of innovative NML‐based brazing fillers. A combined experimental‐modeling approach for the quantitative analysis of silver (Ag) outflow kinetics in Ag/AlN NMLs is presented. The method is based on in situ monitoring of the X‐ray diffraction intensity evolution of Ag acquired from the near‐surface region of NML (grazing incidence geometry) upon annealing in the temperature range of 230–425 °C (atmosphere of N2). The experimental data are compared to model predictions of Ag diffusion. It is assumed that Ag diffusion is driven by the relaxation of residual stresses in Ag nanolayers. Due to the multilayer geometry, the surface outflow kinetics are mainly defined by the kinetics of Ag diffusion along Ag/AlN interphase boundaries. The parameters defining the Ag surface outflow kinetics, i.e., diffusion coefficients and activation energy for diffusion, are derived.

Influence of multilayer nanoarchitecture on phase transformations in the Ti-Cr-Zr system

Nanocrystalline thin films find extensive applications due to their high mechanical strength and wear resistance. However, the challenge is to prevent unwanted grain growth during operation, especially at high temperatures, due to the increased grain boundary volume, which elevates the system's overall energy. Addressing this issue necessitates identifying optimal heat treatment conditions and selecting appropriate chemistry to stabilize grain boundaries. This study investigates the grain growth and phase evolution within Ti-Cr-Zr magnetron sputtered nano multilayers with 5 and 10 nm individual layer thickness, subjected to vacuum heat treatment at 1100 °C for 5 and 10 min. Characterization involved X-ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD), and Transmission Electron Microscopy (TEM). During deposition, evidence of diffusion was observed as the multilayers formed Cr4TiZr, TiCr2, and Cr2Zr, phases. The Ti-Cr-Zr nanoscale system multilayers with 10 nm individual layer thickness also exhibited the emergence of Ti0.3Zr0.7 phase. During heat treatment grain growth occurred, after 10 min an average grain size of 173.7 nm and 119.1 nm was noted for the Ti-Cr-Zr nanoscale system multilayer with 5 nm and 10 nm individual layer thickness, respectively. The different growth rates are attributed to their distinct interfacial volumes, influencing the reaction velocity due to the variations in nano-layer thickness. No new phases were formed after heat treatment for the case of 10 nm individual layer thickness coating. However, 5 nm individual layer coatings showed the emergence of Ti0.3Zr0.7 phase, not present after deposition. In conclusion, this research achieved the desired nano grain stabilization of the Ti-Cr-Zr system nanoscale multilayers after heat treatment at 1100 °C for 10 min. This process led to the complete decomposition of the multilayer structure, resulting in the formation of grains smaller than 200 nm, marking a significant step toward the effective control of grain growth in nanocrystalline materials.

The mechanical performance of optically tuned ceramic nanomultilayers

Multifunctional optical nanomultilayers (NMs) are needed for a wide array of applications, ranging from optical windows to electronic screens. Since the individual material constituents in optical coatings are often selected only for their desirable optical properties, mechanical characterization of the optical nanomultilayers is typically limited. Here, AlN/SiO2, TiO2/SiO2, and AlN/Al2O3 nanomultilayers synthesized in non-optimized and optimized optical performance layer configurations are tested using microtensile, nanoindentation, and pillar splitting techniques to highlight the effects of optical optimization on mechanical performance. Trends within each type of test and across all deformation modes reveal that layer thickness, volume fraction, and interfacial crystallinity can be viewed as controlling features for optical and mechanical performance, although their correlation may vary. It was observed that both configurations of the AlN/Al2O3 NMs exhibit the highest mechanical performance across all three testing techniques with an average experimental transmittance of 93.8% for the optimized layer configuration. Overall, the results from this expanded view of mechanical properties in ceramic optical nanomultilayers suggest the possibility of tuning film characteristics for joint optimization of opto-mechanical nanomultilayered coatings.

Atomistic Assessment of Melting Point Depression and Enhanced Interfacial Diffusion of Cu in Confinement with AlN.

The continuing trend in heterogeneous integration (i.e., miniaturization and diversification of devices and components) requires a fundamental understanding of the phase stability and diffusivity of nanoconfined metals in functional nanoarchitectures, such as nanomultilayers (NMLs). Nanoconfinement effects, such as interfacial melting and anomalous fast interfacial diffusion, offer promising engineering tools to enhance the reaction kinetics at low temperatures for targeted applications in the fields of joining, solid-state batteries, and low-temperature sintering technologies. In the present study, the phase stability and atomic mobility of confined metals in Cu/AlN NMLs were investigated by molecular dynamics, with the interatomic potential compared to the ab initio calculations of the Cu/AlN interface adhesion energy. Simulations of the structural evolution of Cu/AlN nanomultilayers upon heating in dependence on the Cu nanolayer thickness demonstrate the occurrence of interfacial premelting, a melting point depression, as well as extraordinary fast solid-state diffusion of confined Cu atoms along the defective heterogeneous interfaces. The model predictions rationalize recent experimental observations of premelting and anomalous fast interface diffusion of nanoconfined metals in nanostructured Cu/AlN brazing fillers at strikingly low temperatures.

Microstructural evolution of Cu/W nano-multilayers filler metal during thermal treatment

Copper/Tungsten (Cu/W) nano-multilayers show potential for application as novel low-temperature brazing filler metals. Therefore, researchers are interested in understanding phase stability and microstructural evolution of the nano-multilayers during thermal treatment. A repetition of 50 alternating nanolayers of Cu and W with individual thicknesses of 10 nm were prepared by magnetron-sputtering on silicon substrates. The structural evolution of Cu/W nano-multilayers (NMLs) within the temperature range 400 °C–800 °C was monitored using real-time in-situ XRD, SEM, TEM, SAXS, DSC and in-house XRD system. The results showed that the melting point of Cu/W nano-multilayers determined using DSC was 793.694 °C was remarkably lower than the melting point of bulk Cu(1083 °C) and W (3140 °C). After annealing at 400 °C for 30 min, the surface of the NMLs exhibited more copper grains, with significant coarsening of the copper grains. The layered structure of the Cu/W NMLs was unaffected after annealed at 400 °C. When annealed at 600 °C for 30 min, some Cu particles migrated into the W layers along the internal interface leading to cracks which partially collapsed the original stratified structure. The nano-multilayered structure was completely destroyed when annealed at 800 °C. Further, the in-situ XRD results showed that the copper grains grew substantially, while the tungsten size remained unchanged with increasing temperature.

An Overview of Nano Multilayers as Model Systems for Developing Nanoscale Microstructures.

The microstructural transformations of binary nanometallic multilayers (NMMs) to equiaxed nanostructured materials were explored by characterizing a variety of nanoscale multilayer films. Four material systems of multilayer films, Hf-Ti, Ta-Hf, W-Cr, and Mo-Au, were synthesized by magnetron sputtering, heat treated at 1000 °C, and subsequently characterized by transmission electron microscopy. Binary systems were selected based on thermodynamic models predicting stable nanograin formation with similar global compositions around 20–30 at.%. All NMMs maintained nanocrystalline grain sizes after evolution into an equiaxed structure, where the systems with highly mobile incoherent interfaces or higher energy interfaces showed a more significant increase in grain size. Furthermore, varying segregation behaviors were observed, including grain boundary (GB) segregation, precipitation, and intermetallic formation depending on the material system selected. The pathway to tailored microstructures was found to be governed by key mechanisms and factors as determined by a film’s initial characteristics, including global and local composition, interface energy, layer structure, and material selection. This work presents a global evaluation of NMM systems and demonstrates their utility as foundation materials to promote tailored nanomaterials.

The effect of the graded bilayer design on the strain depth profiles and microstructure of Cu/W nano-multilayers

The properties and thermal stability of thin films and nano-multilayers (NMLs) are generally governed by the in-depth stress (strain) gradients rather than the average stress state. The effect of strain gradient variation in Cu/W NMLs on the thermal stability between 400 and 800 °C was investigated. The strain distribution in the NML stacks was varied by combining Cu/W bilayers with different Cu and W thicknesses of either 3 or 10 nm. A recently developed method based on in-plane grazing X-ray diffraction was adopted to extract the strain depth profiles. In addition, the evolution of the average stress in the Cu/W NMLs during growth was monitored by an in-situ wafer curvature technique. The mean residual stresses in Cu and W were found to be independent of the disposition of the different Cu/W bilayer substacks. On the contrary, the strain depth profile of the W nanolayers was found to strongly depend on the disposition of Cu/W bilayer substacks in the Cu/W NML, which resulted in different Cu outflow characteristics upon annealing. Moreover, application of different Cu/W bilayer units within the NML stack also provides an innovative pathway for producing Cu/W nanocomposites with graded thermal and mechanical properties.

Electric transmission behavior of self-assembled Cu–W nano multilayers

Metallic nano multilayers were usually prepared by dual targets alternating deposition method. In this paper, a series of self-assembled Cu–W nano multilayers with different modulation periods were deposited on single crystal silicon substrate by dual targets confocal magnetron sputtering technique. The self-assembled film presented an alternation of W-rich layer and Cu-rich layer. The degree of coherence of the layered interface can be adjusted by controlling both the solid solubility of W-rich and Cu-rich layers. The film resistance increment of the self-assembled Cu–W multilayers is only 14% when the modulation period decreases from 68.2 nm to 5.3 nm, having less size effect compared to the film prepared by alternating deposition method. It noticed that the film resistance even decreased slightly when the modulation period decreased to below 5.3 nm. These results suggested that the coherence could weak the interface scattering ability to electrons, so the self-assembled Cu–W multilayers have lower resistance than the multilayer prepared by alternating deposition technique. This study presented a new pathway to enhance the conductivity of the multilayers.

Strengthening mechanism of Al matrix composites reinforced by nickel-coated graphene: Insights from molecular dynamics simulation

The inadequate bonding strength of graphene and metal matrix is a major challenge to improve the mechanical properties of graphene metal-matrix composites. Here, molecular dynamics simulation is performed to investigate the effect of layer thickness on the mechanical properties of the nickel-coated graphene-reinforced aluminum (NGR-Al) matrix nano-multilayers (NMs) under uniaxial tension and compression load. The results show that the Ni coating on the surface of graphene is an effective method to ameliorate the load transfer ability between graphene and metal matrix. There is a critical layer thickness above which the tensile yield strength and the layer thickness obey the Hall-Petch (HP) relation, and under which the inverse HP relation is followed. The results indicate that compared with pure Al, the introduction of Ni-coated graphene makes the sample have a significant plastic strain strengthening effect under compression load, and the smaller the layer thickness is, the better the strengthening effect is.

Synthesis and characterization of optically transparent ceramic crystalline/amorphous and amorphous/amorphous multilayers

Optical nano multilayers (NMs) are promising candidates for durable transparent multifunctional coatings, where the contributions of interfaces and high transparency configurations have yet to be explored. To address this, high transparency NMs (%T380-1100nm≈94–99%) were synthesized by magnetron sputtering, and the effects of crystalline/amorphous (AlN/SiO2, AlN/Al2O3) and amorphous/amorphous (TiO2/SiO2) interfaces were characterized by spectrophotometry, transmission electron microscopy, and nanoindentation. We demonstrate that tuning layer configurations for improved transmittance resulted in substantial variations in microstructure and multifunctional film properties. Overall, this work presents a methodology for evaluating the structure-properties relationship in optical NMs in the context of interface character and layer microstructure.

Synthesis and Characterization of Optically Transparent Ceramic Crystalline/Amorphous and Amorphous/Amorphous Multilayers

Optical nano multilayers (NMs) are promising candidates for durable transparent multifunctional coatings, although the contribution of interfaces and high transparency configurations has yet to be explored. To address this, high transparency NMs (%T380-1100nm≈94-99%) were synthesized by magnetron sputtering, and the effects of crystalline/amorphous (AlN/SiO2, AlN/Al2O3) and amorphous/amorphous (TiO2/SiO2) interfaces were characterized by spectrophotometry, transmission electron microscopy, and nanoindentation. We demonstrate that tuning layer configurations for improved transmittance resulted in substantial variations in microstructure and multifunctional film properties. Overall, this work presents a methodology for evaluating the structure-properties relationship in optical NMs in the context of interface character and layer microstructure.

Exploring microstructural variations in highly transparent AlN/SiO2 nano multilayers

The microstructure of optically optimized transparent AlN/SiO2 nano multilayers were investigated and compared with baseline repeated bilayer configurations. The multilayered films were synthesized by magnetron sputtering and characterized by transmission electron microscopy and spectrophotometry with multifunctional behavior evaluated by nanoindentation and residual stress analysis. The optically optimized AlN/SiO2 multilayers exhibit higher transmittance (%T300-800nm≈95%), distinct crystalline/amorphous interfaces, and changes in the grain morphology as compared to the periodic baseline samples (%T300-800nm≈70-80%). Varying both layer thickness and layer ratio to maximize transparency showed a significant impact on microstructure and interface character.

Effect of internal stress on short-circuit diffusion in thin films and nanolaminates: Application to Cu/W nano-multilayers

The stability, reactivity and functionality of modern nanostructured and nanoarchitectured materials, like nano-multilayers (NMLs) and nanocomposites (NCs), are generally ruled by fast short-circuit diffusion of atoms along internal interfaces, such as phase and grain boundaries (GBs), at relatively low temperatures. Residual stresses can have a significant effect on the kinetics of short-circuit diffusion in these nanomaterials, which has been largely neglected up to date. We present a combined experimental-modelling approach for deriving stress-free activation energies (and related diffusion coefficients) for the grain boundary diffusion of a given element across an inert barrier nanolayer, which can be utilized to perform fundamental investigations and model predictions of GB diffusion kinetics across inert barrier nanolayers in the presence of a varying stress field. The method was applied to investigate the diffusion of Cu along W grain boundaries in Cu/W nano-multilayers in the temperature range from 400 °C to 600 °C. Firstly, the GB diffusion kinetics as function of the temperature and stress state were determined from experiment by combining insitu heating AES analysis and exsitu XRD stress analysis. The experimental data were compared to model predictions from a modified Hwang–Balluffi model, which accounts for a varying stress level within the barrier during the annealing. The extracted stress-free activation energy for grain boundary diffusion of Cu in W (~0.29 eV) is substantially lower than the respective experimental value for a high compressive stress within the W barrier nanolayer. The corresponding stress-free Cu GB diffusion coefficient in W equals Db∗=3.2±0.6×10-15exp-0.29±0.02eV/kTcm2/s.

Improving External Quantum Efficiency by Subwavelength Nano Multi-Layered Structures for Optoelectronic Devices

Based on thin film optics (TFO) and finite element method (FEM), we have theoretically investigated improving external quantum efficiency (EQE) by anti-reflection (AR) films constructed from subwavelength nano multi-layers (NML) of low and high index materials, where the low and high index materials are MgF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and Ta <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> , respectively. This kind of NML dielectric structures have the advantages of low-cost and flexible. Three kinds of substrates have been studied here, which are glass, polyethylene naphthalate (PEN), and polyethylene terephthalate (PET), respectively. TFO theory has been used to obtain the transmittance and reflectance, which agrees well with FEM results. For NML structure, TFO is more efficient than FEM in terms of calculation time and accuracy. The average AR effects (AAREs) are about 3.69%, 3.46% and 3.07% on glass substrate, about 6.15%, 5.56% and 5.02% on PEN substrate, and about 5.06%, 4.62% and 4.17% on PET substrate, for AR bands (ARBs) 350~800 nm, 350~1100 nm and 350~1500 nm, respectively. The results also reflect that wider AR bandwidth needs more NML layers. In practice, this kind of AR films can be widely applicable to enhance the EQE of the optoelectronic devices (OEDs).

Maskless Patterning of Metal Outflow in Alternating Metal/Ceramic Multiple Nanolayers by Femtosecond Laser Irradiation

In this work, solid-state metal transport from internal metal nanolayers onto the surface of metal/ceramic nano-multi-layers (NMLs) has been directed in a controlled way by femtosecond (fs) laser irradiation and subsequent low temperature thermal annealing. Laser irradiation induced modifications of the NML microstructures and stress states can be limited within the first few top nanolayers due to the focused laser energy input at the metal/ceramic interface by exploiting the local plasmonic effect. Accompanied laser peening can further refine the crystallites and introduce compressive stress at the laser-irradiated region, which reduces the activation energies for vacancy formation and migration of metal atoms in the nano-confinement. Patterned Cu surface nanostructures (outflow) appear selectively along the laser path after air annealing at temperatures down to 360 ℃. For the solid-state diffusion of Cu in confinement, in-plane metal transport along the Cu-AlN interfaces is much faster than the outward ...

Effect of the individual layer thickness on the transformation of Cu/W nano-multilayers into nanocomposites

Thermal treatment of nano-multilayers (NMLs) constituted of alternating immiscible metals, like W and Cu, can evolve in a nanocomposite (NC) with tailored mechanical, electrical and/or thermal properties. The NML-to-NC transformation can result in unique material properties of the resulting NC, which can hardly be achieved by conventional composite fabrication. In this paper, we systematically study the role of the individual Cu and W layer thicknesses in Cu/W NMLs on the diffusion, internal stress evolution and resulting NC microstructure upon heating by X-ray diffraction, high-resolution scanning electron microcopy and Auger electron spectroscopy. In the as-deposited state, a strong compressive stress state of Cu and W is observed, which is governed by a dominating interface stress contribution of the Cu{111}/W{110} interfaces of 11.25 ± 0.56 J/m2. Isothermal annealing of the as-deposited NMLs with different Cu and W layer thicknesses yields to the formation of nets of Cu protrusions on the NML surface, acting as a stress relaxation mechanism, which becomes thermally activated at 400 °C. The kinetics of Cu surface outflow are governed by the initial Cu/W stress state and the individual layer thickness ratio. During annealing at 700–800 °C, the Cu surface particles diffuse back into the volume of the near stress-free NML (dissolution), which defines the onset of accelerated degradation of the nanolaminated structure by thermal grooving, eventually forming a NC. The thickness ratio of Cu and W layers is shown to strongly affect the onset of the NML-to-NC transformation and the final NC microstructure.