Mechanical Properties and Fatigue Life Analysis of Motion Cables in Sensors under Cyclic Loading.

Motion cables, which are widely used in aero-engine sensors, are critical components that determine sensor stability. Because motion cables have unique motion characteristics, the study of their mechanical properties and reliability is very important. In addition, motion cables are complex in structure and cannot be applied to conventional fixed cable research methods. In this study, a new approach is proposed to introduce the theory of anisotropic composites into a simplified cable model, so that the cable is both physically conditioned and has good mechanical properties. While applying the theory of anisotropic composites, the forces of tension and torsion are considered in a motion cable under the combined action. In this context, the reliability of the structure is the fatigue life of the cable. In this paper, the mechanical properties and fatigue life of motion cables are investigated using the finite element method at different inclination angles and fixation points. The simulation results show that there is a positive correlation between the inclination angle and the extreme stress in the motion cables, and the optimal inclination angle of 0° is determined. The number of fixing points should be reduced to minimize the additional moments generated during the movement and to ensure proper movement of the cables. The optimal configuration is a 0° inclination angle and two fixing points. Subsequently, the fatigue life under these optimal conditions is analyzed. The results show that the high-stress zone corresponds to the location of the short-fatigue life, which is the middle of the motion cables. Therefore, minimizing the inclination angle and the number of fixing points of the motion cables may increase their fatigue life and thus provide recommendations for optimizing their reliability.

Ultralight and Highly Conductive Silver Nanowire Aerogels for High-Performance Electromagnetic Interference Shielding.

Metal-based materials possess superior electromagnetic interference (EMI) shielding performance because of their extraordinary electrical conductivity. Nevertheless, the high density and structural rigidity of metals seriously limit their applicability in portable and wearable electronic equipment. A common method for reducing the density of metal-based materials is to prepare metal nanowire aerogels by freeze-drying, but the weak connection among the nanowires results in poor mechanical and electrical properties. Herein, a facile approach is developed for the one-step synthesis of silver nanowire (AgNW) aerogels with ultralow density, good flexibility, high electrical conductivity, and a robust structure. The gel is directly formed by in situ assembly of AgNWs. The end-to-end nanojoining of AgNWs contributes to constructing an interconnected three-dimensional (3D) network, resulting in improved mechanical and electrical properties. The AgNW aerogel with an ultralow density of 4.87 mg cm-3 demonstrates a high electrical conductivity of 4584 S m-1. Moreover, the porous structure of the AgNW aerogel provides numerous interfaces for multiple reflections and scattering of EM waves, allowing them to be continuously absorbed and dissipated within the aerogel. Thus, the AgNW aerogel exhibits a superb EMI shielding effectiveness (SE) of 109.3 dB and a normalized surface specific SE (SSE/t, calculated as the SE divided by the density and thickness) of 353 183 dB cm2 g-1, significantly above that of previously known shielding materials. This work provides a new route for preparing high-performance metal nanowire aerogels and their great potential in EMI shielding.

High-Efficient Vacuum Ultraviolet-Ozone Assist-Deposited Polydopamine for Poly(lactic-co-glycolic acid)-Coated Pure Zn toward Biodegradable Cardiovascular Stent Applications.

Zinc is a prospective metal for biodegradable cardiovascular stent applications, but the excessively released Zn2+ during degradation remains a huge challenge in biocompatibility. Considerable efforts have been made to develop a high-efficient surface modification method, while maintaining adhesion strength, mechanical support, and vascular compatibility. Biomimetic polydopamine (PDA) can adhere to Zn tightly, subsequently achieving robust chemical bonds with poly(lactic-co-glycolic acid) (PLGA) coating. However, the deposition of PDA on Zn depends on the controlled conditions such as a sensitive pH and a long period of time. Herein, we introduce vacuum ultraviolet-ozone (VUV/O3) assist-deposition technology to accelerate the polymerization of PDA on pure Zn, which shortens the process to 40 min at a moderate pH of 8.5 and improves the deposition rate by 1-2 orders of magnitude under sufficient active oxygen species (ROS). Additionally, PLGA/PDA coating enhances the corrosion resistance, and their effective protection maintains the mechanical properties after long-term corrosion. Moreover, the controlled Zn2+ release contributes to the superior in vitro biocompatibility, which inhibits the hemolysis rate and smooth muscle cell (SMC) proliferation. The enhanced endothelial cell (EC) proliferation is promising to promote the re-endothelialization, avoiding in-stent restenosis and neointimal hyperplasia. Such modified Zn might be a viable candidate for the treatment of cardiovascular diseases.

A Multilayer Paste Based on Ag Nanoparticles with Cu@Sn for Die Attachment in Power Device Packaging.

A 3-5 µm Cu@Sn core-shell powder was prepared by chemical plating. Based on the mixture of this Cu@Sn and Ag NPs (nanoparticles), a soldering material for third-generation semiconductors was prepared. The joints prepared with this soldering material had a shear strength of over 40 MPa at 25 °C. This joint did not fail after more than 600 thermal cycles from -40 °C to 140 °C. The special feature of this joint is that the energy potential difference between nanoparticles and micron particles generated in the surface force field during reflow promoted the surface pre-melting of the particles by releasing the excess energy. By this mechanism, it was possible to reduce the porosity of the sintered layer. At the same time, due to the high surface activity energy of nano-silver, the diffusion of the Sn atoms was promoted, further enhancing the bond strength.

Ultrafast Parallel Micro-Gap Resistance Welding of an AuNi9 Microwire and Au Microlayer.

Welding between an AuNi9 microwire and Au microlayer is of great importance for fabricating electrical contact structures for high precision inertial devices, satellite slip ring brushes, robots, etc. In this paper, the achievement of parallel micro-gap resistance welding (PMRW) with 200-µm AuNi9 microwires on a 3-µm Au layer was presented. The study on the orthogonal design of the experiment was carried out. The effect of the process parameters (welding current, welding time, and welding pressure) was discussed in reference to the morphologies and tensile force of the joint using range analysis. It is shown that too much or too little heat input will decrease the welding performance. A group of optimized process parameters (0.275 kA welding current, 3 ms welding time, and 28.7 N welding pressure) was obtained. During the welding process, the dynamic resistance of the whole welding system was measured, which can reflect the welding quality. Finite element simulation is utilized to calculate the welding temperature. The highest temperature was located in the center area of the AuNi9 microwire, reaching 1397.2 °C, which is higher than the melting point of AuNi9. By contrast, the highest temperature for the pad was 826.47 °C (lower than the melting point of Au). Hence, under optimized process parameters, a transient interfacial reaction between the liquid AuNi9 microwire and solid Au pad occurred, and the strength of the welded joint reached 5.54 N.

Fabrication and characterization of silk fibroin coating on APTES pretreated Mg-Zn-Ca alloy.

To delay the degradation of magnesium alloys, silk fibroin as a natural organic polymer coating was fabricated on a 3-amino-propyltriethoxysilane (APTES) pretreated Mg-Zn-Ca alloy. APTES pretreatment coated the surface of magnesium alloys with amino groups, which can bond with functional groups in silk fibroin to form a compact coating/substrate interface. The influences of the APTES concentration and drying temperature on the coating adhesion and interface were investigated to explore the optimal parameters in the fabrication process. The nanoporous silk fibroin films completely covered the APTES pretreated Mg-Zn-Ca surface, which reached a thickness of ~7 µm. The chemical states for the coated Mg-Zn-Ca alloy were compared to those of the bare Mg-Zn-Ca alloy and the APTES pretreated Mg-Zn-Ca alloy to illustrate the coating mechanism. During in vitro degradation and electrochemical measurements in simulated body fluid (SBF), the samples with the silk fibroin coating showed remarkably improved corrosion resistance and a slower degradation rate compared to those of the bare samples, suggesting that the silk fibroin coating was an effective protection coating for the substrates and can delay the degradation of magnesium alloys. Moreover, a model for the in vitro degradation was proposed. In vitro cell experiments confirmed the excellent biocompatibility of silk fibroin coated Mg-Zn-Ca structure.

High-Performance Magnesium-Carbon Nanofiber Hygroelectric Generator Based on Interface-Mediation-Enhanced Capacitive Discharging Effect.

This study reports the concept of a water/moisture-induced hygroelectric generator based on the direct contact between magnesium (Mg) alloy and oxidized carbon nanofibers (CNFs). This device generates an open-circuit voltage up to 2.65 V within only 10 ms when the unit is placed in contact with liquid water, which is higher than the reduction potential of magnesium. The average peak short-circuit current density is ∼6 mA/cm2, which is among the highest values yet reported for water-induced electricity generators. Our results indicate that galvanic corrosion occurs at the interface between the CNF and Mg electrode, but the device can still generate electricity because of the high contact resistance caused by the work function difference between Mg and CNF and the surface oxidation. The oxidized CNF is shown to absorb water/moisture and get reduced, leading to a capacitive discharging effect to provide enhanced signal amplitude and sensitivity. These devices are found to be highly sensitive to small quantities of water, and their high output voltage and current make them useful for the detection of water vapor in the human breath as well as changes in ambient humidity. The Mg/CNF systems thus provide a new technology for use in the fabrication of self-powered water/moisture sensors and the development of portable electric power generators.

One-Step Fabrication of Copper Nanopillar Array-Filled AAO Films by Pulse Electrodeposition for Anisotropic Thermal Conductive Interconnectors.

By using pulse electrodeposition, a copper nanopillar array (CuNPA) was filled into porous anodized aluminum oxide (AAO) films to achieve a highly thermal conductive interconnector with anisotropic property. After 120 min pulse deposition, CuNPA uniformly filled the pores of AAO with a pore-filling percentage of 99.4%, as the ion concentration in AAO pores can re-equilibrate to electrolyte concentration during the current-off period. The CuNPA-filled AAO film showed a high thermal conductivity of 153.12 W/(m·K) in the vertical direction and a low thermal conductivity of 3.43 W/(m·K) in the horizontal direction. Hence, the anisotropic ratio of the thermal conductivity reached 44.64. Moreover, the fabrication process was facile and cost-effective, showing a potential application prospect in the field of high-density packages and power electronic devices.

Self-Limited Nanosoldering of Silver Nanowires for High-Performance Flexible Transparent Heaters.

Silver nanowires (Ag NWs) are key materials to fabricate next-generation flexible transparent electrodes (FTEs). Currently, the applications of Ag NWs are impeded by the large wire-wire contact resistance. Herein, a self-limited nanosoldering method is proposed to reduce the contact resistance by epitaxially depositing silver nanosolders at the Ag NW junctions, which have a negligible effect on the optical transparency, while decreasing the sheet resistance of the Ag NW film from 18.6 to 7.7 Ω/sq at a transmittance of 90%. In addition, the deposited nanosolders at the junctions remarkably improve the electrical and mechanical stabilities of the Ag NW electrodes. Notably, this simple nanosoldering process can be rapidly conducted under room temperature and ambient conditions and is free of any technical support or specific equipment. This technique is easily applied to the nanosoldering of 210 × 297 mm FTEs. Based on these FTEs, a high-performance flexible transparent heater with a sheet resistance 3.7 Ω/sq at a transmittance of 82.5% is constructed. Because of the high heating rate (4.8 °C/s), the heater can produce uniform heating (145 °C) at a short response time (30 s) and low input voltage (6 V).

Fabrication of Novel Printable Electrically Conductive Adhesives (ECAs) with Excellent Conductivity and Stability Enhanced by the Addition of Polyaniline Nanoparticles.

Electrically conductive adhesives (ECAs) are one of the low temperature bonding materials. It can be used to replace toxic Sn-Pb solder. The key issue for the application of ECAs is how to improve their electrical properties. In the present study, we develop an effective method to promote the electrical properties of ECAs by addition of polyaniline (PANI) nanoparticles. PANIs were synthesized via a facile one-step chemical oxidative polymerization method. After adding 0.5 wt% PANI nanoparticles, the conductivity of ECAs increased dramatically by an order of magnitude. The bulk resistivity of 8.8 × 10-5 Ω·cm is achieved for 65 wt% silver fillers with 0.5 wt% PANIs loaded ECAs. Besides, this improvement has no negative effect on the shear strength and the aging life of ECAs. Moreover, the use of PANIs not only lowers the percolation threshold of ECAs, but also reduces the cost and improves the bonding reliability. Finally, PANIs enhanced ECAs patterns were successfully printed by a stencil printing method, which proved their potential applications in replacing conventional solder pastes and printing functional circuits.

Silk fibroin film-coated MgZnCa alloy with enhanced in vitro and in vivo performance prepared using surface activation.

Magnesium and its alloys have generated considerable interest as one of the most promising biodegradable metals for biomedical bone implants. However, the enormous challenges are to improve their rapid corrosion excessively as well as to endow them with biocompatibility and biosafety. Herein, we introduce a natural silk fibroin protein coating to control the corrosion resistance and enhance the biocompatibility of MgZnCa alloy. To obtain a robust and reliable coated structure, different surface-activation processes are employed to increase the available functional groups on MgZnCa surfaces before coating. Compared to oxygen plasma activation, our unique vacuum ultraviolet-ozone (VUV/O3) activation method is effective in realizing uniform silk fibroin films as a protective barrier on MgZnCa alloy surfaces, and the nanoscratch test verified the superior adhesion strength of the silk fibroin-coated magnesium alloy structure. Long-term immersion results combined with electrochemical tests showed the preferable in vitro anticorrosion behavior and a low degradation rate of coated Mg alloy (1/8 times that of uncoated Mg alloy). Cell adhesion and cytotoxicity tests demonstrated that silk fibroin-coated MgZnCa presented improved biocompatibility with bone marrow mesenchymal stem cells. An animal study involving silk fibroin-coated MgZnCa implanted on one side of a rabbit spine for 180 days showed remarkably improved in vivo corrosion resistance, with 1/18 times the degradation rate of uncoated MgZnCa. These results not only comprehensively confirmed the validity of the VUV/O3-activation method as a coating strategy but also implied the tremendous potential of the modified Mg alloy for application as a degradable biomedical implant material. STATEMENT OF SIGNIFICANCE: MgZnCa alloy is a promising material in clinical implantation. Silk fibroin (SF) is a natural organic material with biocompatibility and biodegradability. To date, the combination of SF and MgZnCa alloy has exhibited considerable prospects for orthopedic applications. The realization of a direct coating is an enormous challenge because strong chemical bonds cannot be easily formed between organic and inorganic materials. To solve this bottleneck, we proposed a unique vacuum ultraviolet-ozone (VUV/O3) surface-activation method for the first time to modify the Mg alloy surface before SF coating, which significantly enhanced both in vitro and in vivo performance, such as superior biocompatibility and remarkably improved corrosion resistance of magnesium alloys (∼1/18 the in vivo degradation rate of uncoated MgZnCa).

Cohesively enhanced electrical conductivity and thermal stability of silver nanowire networks by nickel ion bridge joining.

A facile method for producing high-performance nickel enhanced silver nanowire (Ag NW) transparent electrodes on a flexible substrate is reported. The modified electroplating method called enhanced nickel ion bridge joining of Ag NWs, which provides a new route for improving the loose junctions in bare Ag NW networks. The sheet resistance of Ag NW electrode drops from over 2000 Ω sq-1 to 9.4 Ω sq-1 with excellent thermal uniformity after the electroplating process within 10 s. Nickel enhanced Ag NW transparent films are applied on flexible heaters with good thermal stability (165 °C for 2 h) and mechanical flexibility (3500 cycles under 2.5 mm bending radius) after mechanical bending process. Moreover, the mechanism of nickel growth is also confirmed that the nickel electroplating of the Ag NWs obeyed Faraday's Laws.

Low Temperature Bonding by Infiltrating Sn3.5Ag Solder into Porous Ag Sheet for High Temperature Die Attachment in Power Device Packaging.

We have proposed a high temperature die attach method with porous Ag sheet as an interlayer for power device packaging. Sn-3.5Ag solder paste can infiltrate into the porous Ag sheet through capillary forces and Sn can react with the porous Ag sheet and Ag metallizations at the interfaces to form Ag3Sn after reflow at 260 °C for 10 min. The large specific surface area and the high diffusion rates between Ag and Sn accelerate the Sn consumption in the porous Ag structure, thus significantly reducing the processing time. The difference of the melting points of the die attach material before and after reflow could be expanded as large as 259 °C. The bondlines show good electrical and thermal conductivities. Furthermore, the average shear strength of the bondlines at 300 °C is higher than 20 MPa. The porous Ag skeleton remained in the bondline would contribute greatly to the heat dissipation and the electrical signal transmission in power devices.

Growth kinetics of Cu6Sn5 intermetallic compound in Cu-liquid Sn interfacial reaction enhanced by electric current.

In this paper, electric currents with the densities of 1.0 × 102 A/cm2 and 2.0 × 102 A/cm2 were imposed to the Cu-liquid Sn interfacial reaction at 260 °C and 300 °C with the bonding times from 15 min to 960 min. Unlike the symmetrical growth following a cubic root dependence on time during reflowing, the Cu6Sn5 growth enhanced by solid-liquid electromigration followed a linear relationship with time. The elevated electric current density and reaction temperature could greatly accelerate the growth of Cu6Sn5, and could induce the formation of cellular structures on the surfaces because of the constitutional supercooling effect. A growth kinetics model of Cu6Sn5 based on Cu concentration gradient was presented, in which the dissolution of cathode was proved to be the controlling step. This model indicates that higher current density, higher temperature and larger joint width were in favor of the dissolution of Cu. Finally, the shear strengths of joints consisted of different intermetallic compound microstructures were evaluated. The results showed that the Cu6Sn5-based joint could achieve comparable shear strength with Sn-based joint.