Norman Y. Zhou

Recent progresses on hybrid micro-nano filler systems for electrically conductive adhesives (ECAs) applications

Abstract During the last two decades, considerable efforts have been made to explore new generations of interconnecting materials and printed lines to replace the traditionally used toxic lead-based solders in electronic packaging industries. Accordingly, development of electrically conductive adhesives (ECAs) with high electrical conductivity has become an interesting and urgent research venue in this field. Recently, the incorporation of nano-sized conductive fillers inside the conventional formulation of ECAs has drawn considerable attention as an attempt to increase their electrical conductivity. In this article, we review different types of nanofillers that have been utilized inside the conventional ECAs to improve the electrical conductivity of ECAs. We focus on the synergetic effects of silver flakes and the nanofillers on electron transportation through the electrical network; the mechanisms of electrical conductivity enhancement are discussed. Special attention is given to the surface properties of the nanofillers and their corresponding influences on the filler–filler interaction, which has direct effect on the final electrical performance of the hybrid ECAs.

Photocatalytic decomposition of organic micropollutants using immobilized TiO2 having different isoelectric points

Organic micropollutants found in the environment are a diverse group of compounds that includes pharmaceuticals, personal care products, and endocrine disruptors. Their presence in the aquatic environment continues to be a concern as the risk they pose towards both the environment and human health is still inconclusive. Removal of these compounds from water and wastewater is difficult to achieve and often incomplete, but UV-TiO2 is a promising treatment approach. In this study, the efficiency of titanium dioxide (TiO2) immobilized on porous supports were tested for treatment of target pharmaceuticals and their metabolites under UV-LED exposure, a potential low energy and cost effective alternative to conventional UV lamps. Immobilization was completed using two different methods: (1) dip coating of TiO2 onto quartz fiber filters (QFT) or (2) thermal-chemical oxidation of porous titanium sheets (PTT). Comparison against experimental controls (dark QFT, dark PTT, and photolysis using UV-LED only) showed that UV-LED/PTT and UV-LED/QFT treatments have the potential to reduce the concentrations of the target compounds. However, the treatments were found to be selective, such that individual pharmaceuticals were removed well using QFT and PTT but not both. The complementary treatment behavior is likely driven by electrostatic interactions of charged compounds with the membranes. QFT membranes are negatively charged at the experimental pH (4.5-5) while PTT membranes are positively charged. As a result, cationic compounds interact more with QFT while anionic compounds with PTT. Neutral compounds, however, were found to be recalcitrant under any treatment conditions suggesting that ionic interactions were important for reactions to occur. This behavior can be advantageous if specificity is required. The behavior of pharmaceutical metabolites is similar to the parent compounds. However, isomeric metabolites of atorvastatin with functional groups in para and ortho configurations behave differently, suggesting that the positioning of functional groups can have an impact in their interaction with the immobilized TiO2. It was also apparent that PTT can be reused after cleaning by heat treatment. Overall, these newly synthesized membrane materials have potential applications for treatment of trace organic contaminants in water.

Photocatalytic decomposition of selected estrogens and their estrogenic activity by UV-LED irradiated TiO2 immobilized on porous titanium sheets via thermal-chemical oxidation

The removal of endocrine disrupting compounds (EDCs) remains a big challenge in water treatment. Risks associated with these compounds are not clearly defined and it is important that the water industry has additional options to increase the resiliency of water treatment systems. Titanium dioxide (TiO2) has potential applications for the removal of EDCs from water. TiO2 has been immobilized on supports using a variety of synthesis methods to increase its feasibility for water treatment. In this study, we immobilized TiO2 through the thermal-chemical oxidation of porous titania sheets. The efficiency of the material to degrade target EDCs under UV-LED irradiation was examined under a wide range of pH conditions. A yeast-estrogen screen assay was used to complement chemical analysis in assessing removal efficiency. All compounds but 17β-estradiol were degraded and followed a pseudo first-order kinetics at all pH conditions tested, with pH 4 and pH 11 showing the most and the least efficient treatments respectively. In addition, the total estrogenic activity was substantially reduced even with the inefficient degradation of 17β-estradiol. Additional studies will be required to optimize different treatment conditions, UV-LED configurations, and membrane fouling mitigation measures to make this technology a more viable option for water treatment.

Effects of Heat Treatment on Grain-Boundary β-Mg17Al12 and Fracture Properties of Resistance Spot-Welded AZ80 Mg Alloy

The distribution and morphology of β-Mg17Al12 intermetallic phase in resistance spot-welded AZ80 Mg alloy were investigated by means of optical microscopy, scanning electron microscopy, and X-ray diffraction. The influence of intermetallic phase on mechanical strength was studied by tensile shear testing and fractography. The results showed that continuous networks of β-Mg17Al12 formed along grain boundaries in both the nugget and heat-affected zone of the spot-welded AZ80 Mg alloy. Those continuous grain-boundary β-Mg17Al12 networks acted as effective crack propagation paths, which had negative effects on the weld strength. Post-weld solution heat treatment effectively reduced the amount of β-Mg17Al12 and broke the grain-boundary intermetallic networks in both the nugget and heat-affected zone. This significantly increased the weld strength of AZ80 Mg alloy and changed the fracture mode from nugget pull-out in the as-welded condition to through-thickness after heat treatment.

Resistive Switching Memory of TiO2 Nanowire Networks Grown on Ti Foil by a Single Hydrothermal Method

AbstractThe resistive switching characteristics of TiO2 nanowire networks directly grown on Ti foil by a single-step hydrothermal technique are discussed in this paper. The Ti foil serves as the supply of Ti atoms for growth of the TiO2 nanowires, making the preparation straightforward. It also acts as a bottom electrode for the device. A top Al electrode was fabricated by e-beam evaporation process. The Al/TiO2 nanowire networks/Ti device fabricated in this way displayed a highly repeatable and electroforming-free bipolar resistive behavior with retention for more than 104 s and an OFF/ON ratio of approximately 70. The switching mechanism of this Al/TiO2 nanowire networks/Ti device is suggested to arise from the migration of oxygen vacancies under applied electric field. This provides a facile way to obtain metal oxide nanowire-based ReRAM device in the future.

Highly repeatable kinetically-independent synthesis of one- and two-dimensional silver nanostructures by oriented attachment

A repeatable and fast synthesis of one- and two-dimensional silver nanostructures with thickness of 20–25 nm, constructed from highly stable hexagonal and triangular nanoplates has been achieved. Various useful morphologies can be constructed on the gram-scale from the reduction of aqueous solutions of silver nitrate by L-ascorbic (L-ASB) acid in the presence of poly(methacrylic)acid sodium salt, within a few minutes by simple room temperature mixing. Contrary to the literature, the assembly mechanisms governing the final morphology are not kinetically controlled, and are instead selective based on concentration of the reaction mixture. These diverse silver nanostructures meet the criteria desired for feasible industrial production and incorporation into nanoscale devices.

Porous silver nanosheets: a novel sensing material for nanoscale and microscale airflow sensors

Fabrication of nanoscale and microscale machines and devices is one of the goals of nanotechnology. For this purpose, different materials, methods, and devices should be developed. Among them, various types of miniaturized sensors are required to build the nanoscale and microscale systems. In this research, we introduce a new nanoscale sensing material, silver nanosheets, for applications such as nanoscale and microscale gas flow sensors. The silver nanosheets were synthesized through the reduction of silver ions by ascorbic acid in the presence of poly(methacrylic acid) as a capping agent, followed by the growth of silver in the shape of hexagonal and triangular nanoplates, and self-assembly and nanojoining of these structural blocks. At the end of this process, the synthesized nanosheets were floated on the solution. Then, their electrical and thermal stability was demonstrated at 120 °C, and their atmospheric corrosion resistance was clarified at the same temperature range by thermogravimetric analysis. We employed the silver nanosheets in fabricating airflow sensors by scooping out the nanosheets by means of a sensor substrate, drying them at room temperature, and then annealing them at 300 °C for one hour. The fabricated sensors were tested for their ability to measure airflow in the range of 1 to 5 ml min(-1), which resulted in a linear response to the airflow with a response and recovery time around 2 s. Moreover, continuous dynamic testing demonstrated that the response of the sensors was stable and hence the sensors can be used for a long time without detectable drift in their response.

Fundamentals on Adsorption, Membrane Filtration, and Advanced Oxidation Processes for Water Treatment

Water treatment is the processing of water to meet or achieve specified goals or standards set by regulatory agencies and end users. New water treatment technologies are being developed that need to be evaluated on a fundamental scientific and practical basis compared to traditional remediation processes. Recent advances in nanomaterial development for water treatment in the areas of filtration membranes, high surface area adsorbents, and efficient photocatalysts require approval for their effectiveness and safeness. Fundamental theories and concepts discussed in this chapter pertain to the areas of (i) adsorption and equilibrium isotherms (ii) pressure-driven membrane filtration and its rejection mechanisms for filtration and reverse osmosis processes; and (iii) advanced oxidation processes with a focus on semiconductor photocatalytic concepts.

Development of TiO2 Nanowires for Membrane Filtration Applications

TiO2 membranes are multifunctional in that they provide liquid separation of both contaminated and treated water and offer photocatalytic degradation of pollutants. The use of TiO2 nanowires for membranes exhibit greater photocatalytic efficiency compared to conventional bulk materials due to the higher surface area and size effects on the quantum scale. TiO2 nanowires can be synthesized from hydrothermal and sol-gel methods. This chapter focuses on the synthesis, characterization, and applications of TiO2 nanowire membrane. The functional features of TiO2 membranes include filtration, self-cleaning, photocatalytic degradation, and disinfection.

Influence of alloy elements on microstructure and mechanical properties of Al/steel dissimilar joint by laser welding/brazing

The problem in dissimilar material joining (e.g., Al/steel) is the degradation of joint mechanical properties by formation of the hard and brittle interfacial intermetallic compounds. In the present study, in order to improve the joint mechanical properties, alloy elements Si and Zn are added in the form of Al-Si and Zn-Al filler metals, respectively. The effects of alloy elements on the joints are investigated in terms of interfacial microstructure and mechanical properties. The results have shown that element Si is able to suppress the growth of interfacial reaction layer, which leads to the improvement in the fracture load, while element Zn is capable of reducing the brittleness of reaction layer, and it consequently enhances the fracture load.