Influence of Normal-to-High Anodizing Voltage on AAO Surface Hardness from 1050 Aluminum Alloy in Oxalic Acid.

Anodic aluminum oxide (AAO) has been widely applied for the surface protection of electronic component packaging through a pore-sealing process, with the enhanced hardness value reaching around 400 Vickers hardness (HV). However, the traditional AAO fabrication at 0~10 °C for surface protection takes at least 3-6 h for the reaction or other complicated methods used for the pore-sealing process, including boiling-water sealing, oil sealing, or salt-compound sealing. With the increasing development of nanostructured AAO, there is a growing interest in improving hardness without pore sealing, in order to leverage the characteristics of porous AAO and surface protection properties simultaneously. Here, we investigate the effect of voltage on hardness under the same AAO thickness conditions in oxalic acid at room temperature from a normal level of 40 V to a high level of 100 V and found a positive correlation between surface hardness and voltage. The surface hardness values of AAO formed at 100 V reach about 423 HV without pore sealing in 30 min. By employing a hybrid pulse anodization (HPA) method, we are able to prevent the high-voltage burning effect and complete the anodization process at room temperature. The mechanism behind this can be explained by the porosity and photoluminescence (PL) intensity of AAO. For the same thickness of AAO from 40~100 V, increasing the anodizing voltage decreases both the porosity and PL intensity, indicating a reduction in pores, as well as anion and oxygen vacancy defects, due to rapid AAO growth. This reduction in defects in the AAO film leads to an increase in hardness, allowing us to significantly enhance AAO hardness without a pore-sealing process. This offers an effective hardness enhancement in AAO under economically feasible conditions for the application of hard coatings and protective films.

Advances in the Fabrication of Nanoporous Anodic Aluminum Oxide and Its Applications to Sensors: A Review.

Nanoporous anodic aluminum oxide (AAO) is an important template for 1D nanomaterial synthesis. It is used as an etching template for nanopattern transfer in a variety of contexts, including nanostructured material synthesis, electrical sensors, optical sensors, photonic and electronic devices, photocatalysis, and hardness and anticorrosion improvement. In this review, we focus on various fabrication methods, pore geometry modification, and recent advances of AAO, as well as sensor applications linked to our environment, daily life, and safety. Pore geometry is concerned with the material composition, applied voltage mold, electrolyte type, temperature, and anodizing time during the fabrication of AAOs and for adjusting their pore size and profile. The applied voltage can be divided into four types: direct current anodization (DCA), reverse pulse anodization, pulse anodization (PA), and hybrid pulse anodization (HPA). Conventional AAOs are fabricated using DCA and mild anodization (MA) at a relatively low temperature (-5~15 °C) to reduce the Joule heating effect. Moreover, the issues of costly high-purity aluminum and a long processing time can be improved using HPA to diminish the Joule heating effect at relatively high temperatures of 20-30 °C with cheap low-purity (≤99%) aluminum. The AAO-based sensors discussed here are primarily divided into electrical sensors and optical sensors; the performance of both sensors is affected by the sensing material and pore geometry. The electrical sensor is usually used for humidity or gas measurement applications and has a thin metal film on the surface as an electrode. On the contrary, the AAO optical sensor is a well-known sensor for detecting various substances with four kinds of mechanisms: interference, photoluminescence, surface plasma resonance, and surface-enhanced Raman scattering (SERS). Especially for SERS mechanisms, AAO can be used either as a solid support for coating metal nanoparticles or a template for depositing the metal content through the nanopores to form the nanodots or nanowires for detecting substances. High-performance sensors will play a crucial role in our living environments and promote our quality of life in the future.

Design and Fabrication of Polymer Triboelectric Nanogenerators for Self-Powered Insole Applications.

Triboelectric nanogenerators (TENGs) are a kind of mechanical energy harvester with a larger force sensing range and good energy conversion, which is often applied to human kinetic energy collection and motion sensing devices. Polymer materials are the most commonly used materials in TENGs' triboelectric layers due to their high plasticity and good performance. Regarding the application of TENGs in insoles, research has often used brittle Teflon for high output performance together with hard materials, such as springs, for the mechanism to maintain its stability. However, these combined materials increase the weight and hardness of the insoles. Here, we propose a polyethylene terephthalate (PET)-based TENG with a micro-needle polydimethylsiloxane (PDMS) elastomer, referred to as MN-PDMS-TENG, to enhance performance and maintain comfort flexibility, and structural stability. Compared with a flat PDMS, the TENG with a microstructure enhances the output open-circuit voltage (Voc) from 54.6 V to 129.2 V, short-circuit current (Isc) from 26.16 µA to 64.00 µA, power from 684 µW to 4.1 mW, and ability to light up from 70 to 120 LEDs. A special three-layer TENG insole mechanism fabricated with the MN-PDMS-TENG and elastic materials gives the TENG insole high stability and the ability to maintain sufficient flexibility to fit in a shoe. The three-layer TENG insole transforms human stepping force into electric energy of 87.2 V, which is used as a self-powered force sensor. Moreover, with the calibration curve between voltage and force, it has a sensitivity of 0.07734 V/N with a coefficient of determination of R2 = 0.91 and the function between force and output voltage is derived as F = 12.93 V - 92.10 under human stepping force (300~550 N). Combined with a micro-control unit (MCU), the three-layer TENG insole distinguishes the user's motion force at different parts of the foot and triggers a corresponding device, which can potentially be applied in sports and on rehabilitation fields to record information or prevent injury.

An Effective Resistive-Type Alcohol Vapor Sensor Using One-Step Facile Nanoporous Anodic Alumina.

With the increases in work environment regulations restricting alcohol to 1000 ppm, and in drink-driving laws, testing for alcohol with a simple method is a crucial issue. Conventional alcohol sensors based on sulfide, metal oxide, boron nitride or graphene oxide have a detection limit in the range of 50-1000 ppm but have disadvantages of complicated manufacture and longer processing times. A recent portable alcohol meter based on semiconductor material using conductivity or chemistry measurements still has the problem of a complex and lengthy manufacturing process. In this paper, a simple and effective resistive-type alcohol vapor sensor using one-step anodic aluminum oxide (AAO) is proposed. The nanoporous AAO was produced in one-step by anodizing low-purity AA1050 at room temperature of 25 °C, which overcame the traditional high-cost and lengthy process at low temperature of anodization and etching from high-purity aluminum. The highly specific surface area of AAO has benefits for good sensing performance, especially as a humidity or alcohol vapor sensor. With the resistance measurement method, alcohol vapor concentration of 0, 100, 300, 500, 700 and 1000 ppm correspond to mean resistances of 8524 Ω, 8672 Ω, 9121 Ω, 9568 Ω, 10,243 Ω, and 11,045 Ω, respectively, in a linear relationship. Compared with other materials for detecting alcohol vapor, the AAO resistive sensor has advantages of fast and simple manufacturing with good detection limits for practical applications. The resistive-type alcohol vapor-sensing mechanism is described with respect to the resistivity of the test substance and the pore morphology of AAO. In a human breath test, the AAO sensor can quickly distinguish whether the subject is drinking, with normal breath response of -30% to -40% and -20% to -30% response after drinking 50 mL of wine of 25% alcohol.

Design, Fabrication, and Applications of SERS Substrates for Food Safety Detection: Review.

Sustainable and safe food is an important issue worldwide, and it depends on cost-effective analysis tools with good sensitivity and reality. However, traditional standard chemical methods of food safety detection, such as high-performance liquid chromatography (HPLC), gas chromatography (GC), and tandem mass spectrometry (MS), have the disadvantages of high cost and long testing time. Those disadvantages have prevented people from obtaining sufficient risk information to confirm the safety of their products. In addition, food safety testing, such as the bioassay method, often results in false positives or false negatives due to little rigor preprocessing of samples. So far, food safety analysis currently relies on the enzyme-linked immunosorbent assay (ELISA), polymerase chain reaction (PCR), HPLC, GC, UV-visible spectrophotometry, and MS, all of which require significant time to train qualified food safety testing laboratory operators. These factors have hindered the development of rapid food safety monitoring systems, especially in remote areas or areas with a relative lack of testing resources. Surface-enhanced Raman spectroscopy (SERS) has emerged as one of the tools of choice for food safety testing that can overcome these dilemmas over the past decades. SERS offers advantages over chromatographic mass spectrometry analysis due to its portability, non-destructive nature, and lower cost implications. However, as it currently stands, Raman spectroscopy is a supplemental tool in chemical analysis, reinforcing and enhancing the completeness and coverage of the food safety analysis system. SERS combines portability with non-destructive and cheaper detection costs to gain an advantage over chromatographic mass spectrometry analysis. SERS has encountered many challenges in moving toward regulatory applications in food safety, such as quantitative accuracy, poor reproducibility, and instability of large molecule detection. As a result, the reality of SERS, as a screening tool for regulatory announcements worldwide, is still uncommon. In this review article, we have compiled the current designs and fabrications of SERS substrates for food safety detection to unify all the requirements and the opportunities to overcome these challenges. This review is expected to improve the interest in the sensing field of SERS and facilitate the SERS applications in food safety detection in the future.

Advances in Humidity Nanosensors and Their Application: Review.

As the technology revolution and industrialization have flourished in the last few decades, the development of humidity nanosensors has become more important for the detection and control of humidity in the industry production line, food preservation, chemistry, agriculture and environmental monitoring. The new nanostructured materials and fabrication in nanosensors are linked to better sensor performance, especially for superior humidity sensing, following the intensive research into the design and synthesis of nanomaterials in the last few years. Various nanomaterials, such as ceramics, polymers, semiconductor and sulfide, carbon-based, triboelectrical nanogenerator (TENG), and MXene, have been studied for their potential ability to sense humidity with structures of nanowires, nanotubes, nanopores, and monolayers. These nanosensors have been synthesized via a wide range of processes, including solution synthesis, anodization, physical vapor deposition (PVD), or chemical vapor deposition (CVD). The sensing mechanism, process improvement and nanostructure modulation of different types of materials are mostly inexhaustible, but they are all inseparable from the goals of the effective response, high sensitivity and low response-recovery time of humidity sensors. In this review, we focus on the sensing mechanism of direct and indirect sensing, various fabrication methods, nanomaterial geometry and recent advances in humidity nanosensors. Various types of capacitive, resistive and optical humidity nanosensors are introduced, alongside illustration of the properties and nanostructures of various materials. The similarities and differences of the humidity-sensitive mechanisms of different types of materials are summarized. Applications such as IoT, and the environmental and human-body monitoring of nanosensors are the development trends for futures advancements.

Fiber-Based Triboelectric Nanogenerator for Mechanical Energy Harvesting and Its Application to a Human-Machine Interface.

Mechanical energy harvesters including piezoelectric nanogenerators, electromagnetic generators and triboelectric nanogenerators (TENG) used to convert the mechanical motion into electricity are more and more important in the recent decades. Specifically, the fiber-based TENG (FTENG) has gained considerable favors due to its flexibility, light weight, and high environmental tolerance for the wearable devices. The traditional FTENGs made of Teflon result in better performance but are not suitable for long-term wear in person. Here, we propose a novel FTENG using a flexible micro-needle-structured polydimethylsiloxane (MN-PDMS) together with the comfortable commercially available 2D-polyester fibers, and electroless nickel-plated cotton cloth of which two are widely used in human daily life. The MN-PDMS is formed by a laser engraved mold for improving its output performance of FTENG compared to the flat-PDMS. The open-circuit voltage (Voc) and the short-circuit current (Isc) of MN-FTENG increased to 73.6 V and 36 µA, respectively, which are 34% and 37% higher than the flat-FTENG. In terms of power, the performance of MN-FTENG reaches 1.296 mW which is 89% higher than that of flat-TENG and it can also light up 90 LEDs. For application, human motion at the joints can be detected and collected with various signals that are used for the human-machine interface (HMI) through the cooperation of components for the Internet of Things (IoT). It can light up the LED bulb through MN-FTENG to potentially develop IoT HMI systems for human motion control of robot in the future.

PDMS Microfabrication and Design for Microfluidics and Sustainable Energy Application: Review.

The polydimethylsiloxane (PDMS) is popular for wide application in various fields of microfluidics, microneedles, biology, medicine, chemistry, optics, electronics, architecture, and emerging sustainable energy due to the intrinsic non-toxic, transparent, flexible, stretchable, biocompatible, hydrophobic, insulating, and negative triboelectric properties that meet different requirements. For example, the flexibility, biocompatibility, non-toxicity, good stability, and high transparency make PDMS a good candidate for the material selection of microfluidics, microneedles, biomedical, and chemistry microchips as well as for optical examination and wearable electronics. However, the hydrophobic surface and post-surface-treatment hydrophobic recovery impede the development of self-driven capillary microchips. How to develop a long-term hydrophilicity treatment for PDMS is crucial for capillary-driven microfluidics-based application. The dual-tone PDMS-to-PDMS casting for concave-and-convex microstructure without stiction is important for simplifying the process integration. The emerging triboelectric nanogenerator (TENG) uses the transparent flexible PDMS as the high negative triboelectric material to make friction with metals or other positive-triboelectric material for harvesting sustainably mechanical energy. The morphology of PDMS is related to TENG performance. This review will address the above issues in terms of PDMS microfabrication and design for the efficient micromixer, microreactor, capillary pump, microneedles, and TENG for more practical applications in the future.

High-Performance Al/PDMS TENG with Novel Complex Morphology of Two-Height Microneedles Array for High-Sensitivity Force-Sensor and Self-Powered Application.

Triboelectric nanogenerators (TENGs) are widely applied to self-powered devices and force sensors. TENGs consist of the electrode-layer frequently made of high-cost conductors (Ag, Au, ITO) and the tribo-layer of rigid negative-triboelectricity fluoropolymers (PTFE, FEP). The surface morpholoy is studied for enhancing performance. Here, a high-performance Al/PDMS-TENG is proposed with a complex morphology of overlapped deep two-height microneedles (OL-DTH-MN) fabricated by the integrated process of low-cost CO2 laser ablation and PDMS casting for self-powered devices and high-sensitivity force/pressure sensors. The high open-circuit voltage and short-circuit current of the OL-DTH-MN-TENG are 167 V and 129.3 µA. Also, the sensitivity of the force/pressure sensor of the OL-DTH-MN-TENG is very high, 1.03 V N-1 and about 3.11 V kPa-1 , at an area of 30 cm2 that is much higher than the sensitivity of about 0.18-0.414 V N-1 and 0.013-0.29 V kPa-1 of conventional TENG sensors. Meanwhile, the high-performance OL-DTH-MN-TENG not only exhibits the energy storage capability of charging a 0.1 µF capacitor to 2.75 V at 1.19 s, to maximum 3.22 V, but also activates various self-powered devices including lighting colorful 226 LEDs connected in series, the "2020-ME-NCKU" advertising board, a calculator and a temperature sensor. Numerical simulation is also performed to support the experiments.

A Facile Method and Novel Mechanism Using Microneedle-Structured PDMS for Triboelectric Generator Applications.

The triboelectric generator (TEG) is a cost-effective, multi-fabricated, friendly mechanical-energy-harvesting device. The traditional TEG, generally formed by two triboelectric materials in multilayers or a simple pattern, generated triboelectricity as it worked in the cycling contact-separation operation. This paper demonstrates a novel, high-aspect-ratio, microneedle (MN)-structured polydimethylsiloxane (PDMS)-based triboelectric generator (MN-TEG) by means of a low-cost, simple fabrication using CO2 laser ablation on the polymethyl methacrylate substrate and a molding process. The MN-TEG, consisting of an aluminum foil and a microneedle-structured PDMS (MN-PDMS) film, generates an output performance with an open-circuit voltage up to 102.8 V, and a short-circuit current of 43.1 µA, corresponding to the current density of 1.5 µA cm-2 . With introducing MN-PDMS into the MN-TEG, a great increase of randomly closed bending-friction-deformation (BFD) behavior of MNs leads to highly enhanced triboelectric performance of the MN-TEG. The BFD keeps increasingly on in-contact between MN with Al that results in enhancement of electrical capacitance of PDMS. The effect of aspect ratio and density of MN morphology on the output performance of MN-PDMS TEG is studied further. The MN-TEG can rapidly charge electric energy on a 0.1 µF capacitor up to 2.1 V in about 0.56 s. The MN-TEG source under tapping can light up 53 light-emitting diodes with different colors, connected in series.

Microfluidic chip for plasma separation from undiluted human whole blood samples using low voltage contactless dielectrophoresis and capillary force.

A plasma separating biochip is demonstrated using a capillary-driven contactless dielectrophoresis method with low voltage (~1 V) and high frequency induced electrostatics between red blood cells. The polarized red blood cells were aggregated and separated from plasma with a 69.8% volume separation and an 89.4% removal rate of red blood cells.

Anodization of nanoporous alumina on impurity-induced hemisphere curved surface of aluminum at room temperature.

Nanoporous alumina which was produced by a conventional direct current anodization [DCA] process at low temperatures has received much attention in various applications such as nanomaterial synthesis, sensors, and photonics. In this article, we employed a newly developed hybrid pulse anodization [HPA] method to fabricate the nanoporous alumina on a flat and curved surface of an aluminum [Al] foil at room temperature [RT]. We fabricate the nanopores to grow on a hemisphere curved surface and characterize their behavior along the normal vectors of the hemisphere curve. In a conventional DCA approach, the structures of branched nanopores were grown on a photolithography-and-etched low-curvature curved surface with large interpore distances. However, a high-curvature hemisphere curved surface can be obtained by the HPA technique. Such a curved surface by HPA is intrinsically induced by the high-resistivity impurities in the aluminum foil and leads to branching and bending of nanopore growth via the electric field mechanism rather than the interpore distance in conventional approaches. It is noted that by the HPA technique, the Joule heat during the RT process has been significantly suppressed globally on the material, and nanopores have been grown along the normal vectors of a hemisphere curve. The curvature is much larger than that in other literatures due to different fabrication methods. In theory, the number of nanopores on the hemisphere surface is two times of the conventional flat plane, which is potentially useful for photocatalyst or other applications.PACS: 81.05.Rm; 81.07.-b; 82.45.Cc.

Theoretical modeling developed to evaluate the hardness and reduced modulus for the C/a-Si composite film using nanoindentation tests.

A general mechanical model, which is composed of the mechanical models employed to describe the contact behaviors and deformations arising in all layers (including the substrate), is successfully developed in the present study for multilayer specimens in order to evaluate the contact projected area by a theoretical model, and thus the hardness and reduced modulus, using nanoindentation tests. The governing differential equations for the depth solutions of the indenter tip formed at all layers of the specimen under their contact load are developed individually. The influence of the material properties of the substrate on a multilayer specimen's hardness and reduced modulus at various indentation depths can thus be evaluated. Transition and pop-in occurred at depths near, but still before, the C (top layer)/a-Si (buffer layer) interface and the a-Si/Si (substrate) interface, respectively. Using the present analysis, the depths corresponding to the transition and pop-in behaviors can be predicted effectively.