Thin Substrate Research Articles

Post-processing of additively manufactured IN625 thin-walled structures using laser remelting in directed energy deposition

Blown-powder directed energy deposition (DED) is especially useful for depositing on an existing part for the purpose of repair or adding features. Thin substrate deposition, wherein the width of the substrate is smaller than the laser diameter, is an important configuration for building and repairing complex components in various geometries. The present study seeks to understand the efficacy of in-line post-processing methods based on laser remelting (LR) for reducing internal (e.g., porosity) and external (e.g., surface roughness) for such thin-walled repair configurations. In this work, different levels of LR energy density and substrate width were evaluated using the LR process. Post processing outcomes in terms of deposit geometry as well as spatial distribution of porosity, pore density, pore volume fraction, pore diameter, and surface roughness were evaluated using X-ray microcomputed tomography and optical profilometry. The results showed that LR reduced pore density up to 88 % and reduced areal surface roughness to 4.159 μm. At some threshold, increasing LR energy density increased surface waviness and modified deposition geometry, this indicative of limits to the post processing method. Strategies for using LR to reduce total porosity, near-surface porosity, and improving surface roughness are briefly discussed.

High-performance and space-qualified hybrid integrated circuit based on multi-group homogeneous gradient eutectic in quality conformance inspection

The quality and reliability of high integration electronic components has become one of the main issues restricting the advancement of aerospace products. During the development of a military-standard microwave hybrid integrated circuit, the penetration rate of the eutectic welding of the thin film substrates, thick film substrates, and carrier plates in the cavity directly affects the significant performance characteristics, such as gain and working current. By using of traditional process methods, the hybrid integrated circuit cannot pass the experiments (group A, group B, group C and group D) which presents the different types of grouping categories, test parameters, along with acceptance criteria that the quality conformance inspection (QCI) stipulated in the GJB 2438B General Specification for Hybrid Integrated Circuits, which is due to the low rate of finished products. We propose and experimentally demonstrate a multi-group homogeneous gradient eutectic-based method with penetration rate of >92 %, which ensured the grounding performance and reliability inspection requirements of the individual circuit unit. When the hybrid integrated circuit works under the temperature range −55~+125 °C and other environmental test conditions, the delta limits of working current and gain are kept in the extreme range of ±10 mA and ±2 dB which shows high stability and reliability. Then we finally achieved that the aerospace product successfully passed the QCI. Our research progress can expand the process applications of Class K(Space-Qualified) hybrid integrated circuits with the maximum level of assurance for use in extreme environment applications such as space where reliability is paramount.

Refractive axicon for X-ray microscopy applications: design, optimization, and experiment.

In a full-field transmission X-ray microscopy (TXM) setup, a condenser X-ray optical element is used to illuminate the sample by condensing the X-ray beam delivered by the synchrotron storage ring. On-going and future upgrades of synchrotron facilities to diffraction-limited storage rings will pose new challenges to these TXM setups, such as much smaller X-ray beams on the condenser. Here, we demonstrate that a refractive axicon can be used as an X-ray beam shaper to match the ring-shaped aperture of the condenser. Aiming at more efficient use of the incoming X-ray intensity, we explore several axicon designs both analytically and with numerical simulations. The axicons were produced by two-photon polymerization 3D printing on thin silicon nitride membrane substrates. The first characterization of the axicon was carried out at the TOMCAT beamline of the Swiss Light Source (Switzerland).

Analytical solutions for film stress and bending deformation of coated optical lenses

Axisymmetric film/substrate systems are irreplaceable for many applications, owing to special structural requirements. Stoney's formula is widely used to analyze the thin plate film/substrate system. Many researches have been presented to relax limitations of Stoney's formula, which considered the substrate as a thin plate without any curvature. For the optical lens, the axisymmetric thin shell substrate with different curvatures on its upper and lower surfaces is common due to the need of optical focusing. The film stress of the optical lens needs to be optimized and measured due to its effects on system deformations and optical performances. However, the optical lens cannot be solved by Stoney's formula and its extensions due to the curvatures. In this paper, analytical solutions for the thin shell optical lens with different curvatures on upper and lower substrate surfaces are developed by the thin shell theory with the axisymmetric distribution film stress resultant (FSR) in consideration. The FSR is modeled by thermal stress equivalent method, which allows the calculation for the axisymmetric distribution FSR induced by many factors. The forward and inverse solutions between the FSR and the bending deformation are provided in the form of simple formulas with correction coefficient vectors depending on the global substrate thickness, which enables its feasibility in practical applications. Moreover, the two solutions are verified by the finite element method considering the deformation normal to the substrate surface measured by the interferometer in inverse solution, and small errors are found when the optical lens conforms to the assumptions of the method in this paper. The proposed method is efficient to predict the optical lens’ bending deformations, and extends the usage of curvature-based techniques from plates to shells with different curvatures on upper and lower surfaces in the film stress measurement, which makes great significance to the thin film/substrate application.

Elastic Mismatch Influence on Modes I and II Ratio during Buckling-induced Delamination

In the field of modern electronic, medical and aero-space industry there has been an increasing demand on the investigation and development of new materials and their usage as thin films and coatings. To ensure a proper and long-life reliability of the device composed of several micro- and nano-scale thin layers, not only the reliability of used materials has to be engineered, but also the interface stability between the layers must be ensured. Modern thin film applications use the combination of metallic conductive thin films on compliant substrates or hard coatings, connecting two, highly dissimilar materials. Therefore, in order to correctly assess the interface adhesion and fracture criteria, the elastic mismatch between the thin film and substrate must be properly accounted for. A widely used method to measure the adhesion between thin film and substrate is the buckling-induced delamination by Hutchinson and Suo (1992) where the adhesion energy of the thin film is evaluated as a function of the mode-mixity (for mixed-mode I+II) angle. However, a common practice in such measurements is to disregard the elastic mismatch of film and substrate. Recent research showed that the elastic mismatch has significant impact on the value of the mode-mixity angle. Since the dependence of the adhesion energy on the ratio between modes I and II is used for the interface fracture criteria, the errors introduced by disregarding the elastic mismatch influence may lead to errors in estimation of the critical crack driving force. In this study, we apply our model of the influence of the elastic mismatch on several fracture criteria in combination with experimental measurements of three film-substrate systems. Application of a more precise approach with use of the elastic mismatch influence show change in obtained mode I critical crack driving force. Therefore, the presented work highlights the need for precise assessment of the elastic mismatch influence on the mode-mixity when measuring the adhesion of thin films.

ZnO/CuO nanostructures anchored over Ni/Cu tubular films via pulse electrodeposition for photocatalytic and antibacterial applications

In this work, we report the fabrication of Ni and Cu tubular substrates and the synthesis of ZnO/CuO nanocomposite on them through the process of pulse electrodeposition. The systematic variation in CuO incorporation in the ZnO matrix and the processing technique were noticed to affect the structural, optical, photocatalytic, and anti-bacterial properties, which are well in accordance with the Field Emission-Scanning Electron Microscope, X-ray Diffraction, Fourier transform Infrared Spectroscopy and UV-Differential reflectance spectroscopy results. The remediation capabilities of the photocatalytic substrates were assessed through the degradation of methylene blue (MB) dye under solar irradiation. Optimized CuO incorporation within the ZnO nanorods resulted in the degradation of a 20 ppm of MB dye solution within 40 min and a higher concentration of 50 ppm within 95 min. The Ni and Cu electroformed tubes as substrates provided not only a reusable supporting frame but also a large surface-area for the growth of ZnO/CuO nanocomposite. The current study also dealt with the anti-bacterial efficacy of the above-mentioned substrates against E.coli. Hence, the Ni and Cu tubular thin film substrates with nanorods of ZnO/CuO composite were explored for the removal of organic as well as biological contaminants from waste water.

Probing buried interfaces in SiOxNy thin films via ultrafast acoustics: The role transducing layer thickness

Probing buried interfaces in thin films is a crucial task in many fields, including optical coating. Ultrafast acoustics provide a means to characterize the interfaces by using an acoustic wave localized on the nanometer scale. We provide a brief overview of our thorough study of the interface between SiOxNy thin films and Si substrate by using both single-color and broadband picosecond acoustics. The experiment allows us to track the effect of stoichiometry on the acoustics wave propagation and transition over the layer-substrate interface. To optimize the experiment, we also created simulations to study the effect of optoacoustic layer thickness. We show that the used Ti layer features an optimum thickness between 5-10 nm to reveal details of the interface properties.

High-resolution imaging and analysis of the intestinal bacterial load of Caenorhabditis elegans during early adulthood.

We study the presence within the worm Caenorhabditis elegans (C. elegans) of a fluorescent strain of the worm's bacterial food (Escherichia coli (E. coli) OP50) during early adulthood. Use of a microfluidic chip based on a thin glass coverslip substrate allows investigation of the intestinal bacterial load using a Spinning Disk Confocal Microscope (SDCM) equipped with a high-resolution objective (60×). High-resolution z-stack fluorescence images of the gut bacteria in adult worms, which were loaded in the microfluidic chip and subsequently fixed, were analyzed using IMARIS software and 3D reconstructions of the intestinal bacterial load in the worms were obtained. We present an automated bivariate histogram analysis of the volumes and intensities of the bacterial spots for each worm and find that, as the worms age, the bacterial load in their hindguts increases. We show the advantage of single-worm resolution automated analysis for bacterial load studies and anticipate that the methods described in our work can be easily implemented in existing microfluidic solutions to enable thorough studies of bacterial proliferation.

A High Angular Stability, Single-Layer Transmission Linear-to-Circular Polarization Converter for Dual ISM-Band Operation

This paper presents a planar metasurface structure for linear to circular polarization (CP) conversion at the industrial, scientific, and medical bands of 2.4 GHz and 5.8 GHz. The new unit-cell structure consists of two loaded symmetric cross-shaped stubs with a size of 0.26 λ × 0.26 λ in a single thin substrate with a thickness of 0.0064 λ. The design process and parametric study are presented to clarify the operating mechanism for each frequency. The unit-cell element is analyzed and simulated, which exhibits an axial ratio (AR) bandwidth of 15.8% and 12.6% at a 3-dB criterion with the same polarization mode at 2.4 GHz and 5.8 GHz, respectively. The proposed LP-to-CP converter achieves a stable AR bandwidth with an oblique incident angle up to 45°. Moreover, a dual-band linear polarization (LP) antenna is combined with an array of 4 × 4 unit cells to validate the polarization converting ability of the proposed metasurface. The LP-to-CP converter and the antenna are fabricated and measured. The measurement results show that the antenna obtains a good impedance matching and CP waves radiation at 2.33–2.51 GHz and 5.55–5.89 GHz. The integrated antenna with dual-band CP waves radiation is suitable for improving the transmission efficiency in communication and wireless power transfer applications.

Perturbing the spin state and conduction of Fe (II) spin crossover complexes with TCNQ

We investigated modifications driven by 7,7,8,8-tetracyanoquinodimethane (TCNQ) to the spin state configuration of [Fe(3-bpp)2](TCNQ)2 co-crystal and both spin state and electric conductivity of [Fe{H2B(pz)2}2(bipy)] and TCNQ mixtures. The Fe2+ site in the [Fe(3-bpp)2](TCNQ)2 co-crystal has a sizable orbital moment. During X-ray absorption measurements, the iron ion is partially excited to the high spin state and strong surface effects are indicated. Mixing TCNQ with the [Fe{H2B(pz)2}2(bipy)] spin crossover complex leads to a molecular combination with increased conductivity and drift carrier lifetimes. [Fe{H2B(pz)2}2(bipy)] thin films with TCNQ, grown using dimethylformamide (DMF), are to great extent locked mainly in the low spin (LS) state across a broad temperature range and exhibit drift carrier lifetimes approaching 0.5 s. When deposited onto a ferroelectric polyvinylidenefluoride-hexafluoropropylene thin film substrate, [Fe{H2B(pz)2}2(bipy)], shows enhanced transistor carrier mobility, likely associated with the increasing cationic character of [Fe{H2B(pz)2}2(bipy)] thin films with TCNQ.

A flexible neural implant with ultrathin substrate for low-invasive brain–computer interface applications

Implantable brain–computer interface (BCI) devices are an effective tool to decipher fundamental brain mechanisms and treat neural diseases. However, traditional neural implants with rigid or bulky cross-sections cause trauma and decrease the quality of the neuronal signal. Here, we propose a MEMS-fabricated flexible interface device for BCI applications. The microdevice with a thin film substrate can be readily reduced to submicron scale for low-invasive implantation. An elaborate silicon shuttle with an improved structure is designed to reliably implant the flexible device into brain tissue. The flexible substrate is temporarily bonded to the silicon shuttle by polyethylene glycol. On the flexible substrate, eight electrodes with different diameters are distributed evenly for local field potential and neural spike recording, both of which are modified by Pt-black to enhance the charge storage capacity and reduce the impedance. The mechanical and electrochemical characteristics of this interface were investigated in vitro. In vivo, the small cross-section of the device promises reduced trauma, and the neuronal signals can still be recorded one month after implantation, demonstrating the promise of this kind of flexible BCI device as a low-invasive tool for brain–computer communication.

Removal of bisphenol A from synthetic and treated sewage wastewater using magnetron sputtered CuxO/PVDF thin film photocatalytic hollow fiber membrane

Flexible hollow fiber polymeric membrane based thin film substrate sputtered with thin layer photocatalyst is a novel configuration for photocatalytic applications. This article focuses on the fabrication of copper oxide/polyvinylidene fluoride thin film hollow fiber membrane (CuxO/PVDF TF HFM) by using radio frequency (RF) magnetron sputtering of CuxO over PVDF hollow fiber membrane (HFM). Different phase of copper oxide (CuxO) detected on the layer of PVDF HFM by X-ray diffraction (XRD) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) and confirmed by X-ray photoelectron spectroscopy (XPS), depending on oxygen flow rate. 10–20 mL/min O2 flow rate produces Cu2O phase while 25 and 30 mL/min produces CuO phase. Sputtered Cu2O/PVDF TF HFM at 20 mL/min O2 shows bigger grain size of 2.1 nm with 0.35 nm of dislocation density and 0.019 nm of lattice strain. The higher the O2 flow rate, the thicker and the more uniform the sputtered CuxO/PVDF TF HFM was produced as indicated by field emission scanning electron microscopy (FESEM). The atomic force microscopy (AFM) analysis showed the increasing surface roughness of the sputtered 10 mL/min CuxO/PVDF TF HFM at 8.26 nm, as compared to the neat PVDF HFM at 6.15 nm. About 74 % of 2 mg/L bisphenol A (BPA) in synthetic wastewater was successfully photodegraded and filtered while outstanding 91 % removal of BPA in real treated sewage wastewater achieved using 20 mL/min Cu2O/PVDF TF HFM after 360 min light irradiation. About 71.33 % of regeneration efficiency was achieved after 3 consecutive cycles that this technique promises the good reusability with strong adhesion capability.

Flexible Nanoarchitectonics for Biosensing and Physiological Monitoring Applications.

Flexible and implantable electronics hold tremendous promises for advanced healthcare applications, especially for physiological neural recording and modulations. Key requirements in neural interfaces include miniature dimensions for spatial physiological mapping and low impedance for recognizing small biopotential signals. Herein, a bottom-up mesoporous formation technique and a top-down microlithography process are integrated to create flexible and low-impedance mesoporous gold (Au) electrodes for biosensing and bioimplant applications. The mesoporous architectures developed on a thin and soft polymeric substrate provide excellent mechanical flexibility and stable electrical characteristics capable of sustaining multiple bending cycles. The large surface areas formed within the mesoporous network allow for high current density transfer in standard electrolytes, highly suitable for biological sensing applications as demonstrated in glucose sensors with an excellent detection limit of 1.95µm and high sensitivity of 6.1mA cm-2 µM-1 , which is approximately six times higher than that of benchmarking flat/non-porous films. The low impedance of less than 1 kΩ at 1kHz in the as-synthesized mesoporous electrodes, along with their mechanical flexibility and durability, offer peripheral nerve recording functionalities that are successfully demonstrated in vivo. These features highlight the new possibilities ofournovel flexible nanoarchitectonics for neuronal recording and modulation applications.

Extensive reduction of graphene oxide on thin polymer substrates by ultrafast laser for robust flexible sensor applications

Low-cost, flexible wearable graphene-based sensors demand a high-quality and highly conductive graphene. The scalable production of graphene is challenged by complexity, cost, and energy requirements resulting in no direct facile method for volume production. Reduced graphene oxide (rGO) is a promising graphene derivative that could serve as an alternative with similar properties of graphene. Graphene oxide (GO) is an electrical insulator due to distorted sp2 bonding and is transformed to rGO by eliminating oxygen through a reduction process. We report an extensive reduction of oxygen in GO films with producing high quality rGO on flexible polyimide (PI) and polyethylene terephthalate (PET) substrates using ultrashort laser pulses. After laser reduction, the sheet resistance of GO significantly reduces from MΩSq−1 to few hundreds ΩSq−1 where the C/O ratio enhances from 1.8 to 10.0 obtained from XPS analysis. Raman spectroscopy revealed highly conductive rGO thin film where the ID/IG ratio decreases from 0.94 to 0.27, and I2D/IG ratio increases from 0.12 to 0.63 confirming the restoration of sp2 carbon as a result of laser reduction. The fabrication of LrGO based multifunction sensor and pH sensor with high sensitivity is demonstrated which provide a great potential for use in healthcare and flexible sensor applications.

Drying stresses in cellulose nanocrystal coatings: Impact of molecular and macromolecular additives

The industrial implementation of cellulose nanocrystals (CNCs) in films and coatings requires thorough evaluation of the internal stresses post-consolidation, as they cause fracturing and peeling. Characterizing the impact of plasticizing additives on stress is therefore critical. Herein, we use the deflection of thin glass substrates to measure drying stresses in consolidating CNC films, and benchmark the impact of five additives (glucose, glycerol, poly(ethylene glycol) (PEG), poly(vinyl alcohol) (PVA) and bovine serum albumin). Glycerol and PEG reduced drying stresses effectively, while PEG of increased molecular weight (from 0.2 to 10 kDa), PVA, and BSA were less effective. We analyzed the temporal aspects of the process, where stress relaxation of up to 30 % was observed 2 years after coating formation. Finally, we provide a framework to evaluate the impact of CNC morphology on residual stresses. The introduced approach is expected to fast-track the optimization and implementation of coatings based on biocolloids.

The curious case of the second/end peak in the heat release rate of wood: A cone calorimeter investigation

AbstractThe reasons behind the occurrence of a second/end peak heat release rate (PHRR) during wood combustion under radiative heating were determined. Effects of the type of rear material, wood thickness, char progression, and its microstructure, as well as moisture content/transport in spruce wood, were studied. Rear materials used were insulating Kaowool, conducting steel, and the same wood but physically separated from test specimen by aluminium foil. The intensity of the second/end PHRR with Kaowool was almost 50% more than that of the sample with steel. Thus, the second/end peak is governed by the boundary condition defined by the rear material, which determines the heat losses at the rear side of the specimen and consequently the temperature of the specimen. Higher specimen temperature enhances the pyrolysis rate, thereby causing the second/end PHRR. The appearance times and values of the second/end PHRR for 30, 20, and 10 mm wood were 1740 s/78 kWm−2, 685 s/134 kWm−2, and 450 s/160 kWm−2, respectively. Char progressed to the rear of the samples even with a thin (8 mm) conductive steel substrate. Cracks in char grew almost three times wider during the second/end PHRR compared to the sample with no second/end peak. Char cracking had no significance on the time of occurrence of the second/end PHRR but affected the overall heat release. High moisture content reduced the charring rate and delayed the time of occurrence of the second/end PHRR as more water was needed to undergo a phase change, requiring a higher amount of energy.

High-stability flexible radio frequency transistor and mixer based on ultrathin indium tin oxide channel

In this work, a high-performance flexible radio frequency transistor using an ultrathin indium tin oxide film channel based on a solution-cast thin polyimide substrate has been demonstrated. The 60 nm short channel transistor shows a record high cut-off frequency of 5 GHz and a maximum oscillation frequency of 11 GHz with high uniformity among 40 devices. The radio frequency characteristics under various bending conditions have been systematically studied under a bending radius of 5 mm for 10 000 times and a bending radius of 1 mm for 1000 times, showing excellent stability with only 20% decrease in the cut-off frequency. Furthermore, a flexible frequency mixer has also been demonstrated at 2.4 GHz with decent conversion gains.

Bundled carbon nanofiber arrays grown on Cu-Ni tube textile boosting superior sodium-ion storage kinetics

Achieving high energy and power density in micro sodium-ion storage device is imperative and challenging due to the large radius of sodium ions and induced sluggish sodiation kinetics. Herein, we develop a vertically aligned carbon nanofiber arrays with a graphite dome (G-CNFAs/Textile) using the unique Cu-Ni solid solution textile as catalyst and thin substrate. As a flexible electrode with a maximum thickness of 4.0 µm, the G-CNFAs/Textile combines the merits of the thinner intertwined/ interconnected carbon nanofiber bundles, high porosity and abundant defects, thereby deriving enhanced sodium-ion storage kinetics. As a result, the flexible micrometer-scale G-CNFAs/Textile not only delivers an impressive gravimetric performance of 332.6 mA h g−1 at 1000 mA g−1 after 500 cycle numbers, but also exhibits outstanding volumetric capacity of ca. 1387.6 mA h cm−3 in half-type SIBs. Meanwhile, as the submicron-scale self-supporting symmetric electrode of SICs, the G-CNFAs//G-CNFAs capacitor also presents an impressive gravimetric capacitance of 66.5 F g−1 at 0.1 A g−1, superior energy-power density (80.9 W h kg−1 and 7771.8 W kg−1), as well as outstanding cyclic stability. Thus, we hope that this flexible micro-submicron-scale electrode structures could provide inspirations for thin, light-weight and miniaturized sodium-ion storage devices.

Copper laser patterning on a flexible substrate using a cost-effective 3D printer

We studied the cost effective direct laser patterning of copper (Cu) on thin polyimide substrates (PI thickness: 12.5–50 µm) using a 405 nm laser module attached to an inexpensive 3D printer. The focal length of the laser was intentionally controlled to reduce defects on patterned Cu and surface damage of PI under predetermined process conditions. The appropriate focal length was examined at various focal distances. Focal distances of − 2.4 mm and 3 mm were found for the shorter focal length (SFL) and longer focal length (LFL), respectively, compared to the actual focal length. This resulted in clean Cu line patterns without line defects. Interestingly, the SFL case had a different Cu growth pattern to that of LFL, indicating that the small difference in the laser incident angle could affect Cu precursor sintering. Cu square patterns had a lower resistivity of 70 μΩ·cm for an LFL after three or four laser scans, while the SFL showed a resistivity below 48 μΩ·cm for a one-time laser scan. The residues of the Cu precursor on PI were easily removed with flowing water and normal surfactants. However, the resistivity of the patterns decreased after cleaning. Among the scan gaps, the Cu square pattern formed at a 70 μm scan gap had the lowest sheet resistance and the least change in resistance from around 4 to 4.4 Ω/ϒ after cleaning. This result implies that the adhesion of the patterned Cu could be improved if the coated Cu precursor was well sintered under the proper process conditions. For the application of this method to bioelectronics, including biosensors, LEDs were connected to the Cu patterns on PI attached to the arm skin and worked well, even when the substrate PI was bent during power connecting.

Comprehensive characterization of thermal and mechanical properties in thin metal film-glass substrate system by ultrafast laser pump-probe method.

Picosecond ultrasonics (PU), time-domain Brillouin scattering (TDBS), and time-domain thermo-reflectance (TDTR) are all in-situ, non-destructive, and non-contact experimental techniques based on the ultrafast laser pump-probe method, which can generate and detect coherent acoustic phonons (CAP) and thermal transport in thin metal film-glass substrate system. However, these techniques are generally considered different experimental methods to characterize the thermal or mechanical properties of metal nano-objects or transparent materials. Here we present a comprehensive characterization of the generation, propagation, and attenuation of high-frequency CAP and cross-plane thermal transport in the thin Cr film-glass substrate system by PU, TDBS, and TDTR. To investigate the key factors of characterizations, two kinds of thin Cr film-glass substrate systems were measured on the film side and substrate side. The measured thermal and mechanical properties show that boundary conditions and film thickness have significantly affected the characterization.