Resistive Switching in Bulk Silver Nanowire–Polystyrene Composites

Traditionally, bulk nanocomposites of electrically conducting particles and insulating polymers have been categorized as either insulating or conducting when the nanoparticle concentration is below or above the percolation threshold, respectively. Meanwhile, thin‐film polymer nanocomposites can exhibit resistive switching behavior appropriate for digital memory applications. Here, we present the first report of reversible resistive switching in bulk, glassy polymer nanocomposites. At compositions close to the electrical percolation threshold measured at low voltage, silver nanowire‐polystyrene nanocomposites demonstrate reversible resistive switching with increasing voltage at room temperature. Nanocomposites with compositions outside of this range exhibit either irreversible switching, or no switching at all. We propose that resistive switching in these materials is the result of the field‐induced formation of silver filaments that bridge adjacent nanowire clusters, extending the percolation network and decreasing the sample’s bulk resistivity. These findings break from the usual dichotomy of insulating or conducting properties in polymer nanocomposites and could inspire new devices that capitalize on this responsive behavior in these versatile materials.     

Low Electrical Percolation Threshold of Silver and Copper Nanowires in Polystyrene Composites

Silver and copper nanowires have been synthesized using a scalable method of AC electrodeposition into porous aluminum oxide templates, which produces gram quantities of metal nanowires ca. 25 nm in diameter and up to 5 and 10 μm in length for Ag and Cu, respectively. The nanowires have been used to prepare polystyrene nanocomposites by solution processing. Electrical resistivity measurements performed on polymer nanocomposites containing different volume fractions of metal indicate that low percolation thresholds of nanowires are attained between compositions of 0.25 and 0.75 vol %.     

Improved Thermoelectric Performance of Eco‐Friendly β‐FeSi2–SiGe Nanocomposite via Synergistic Hierarchical Structuring,Phase Percolation,and Selective Doping

A β‐FeSi₂–SiGe nanocomposite is synthesized via a react/transform spark plasma sintering technique, in which eutectoid phase transformation, Ge alloying, selective doping, and sintering are completed in a single process, resulting in a greatly reduced process time and thermal budget. Hierarchical structuring of the SiGe secondary phase to achieve coexistence of a percolated network with isolated nanoscale inclusions effectively decouples the thermal and electrical transport. Combined with selective doping that reduces conduction band offsets, the percolation strategy produces overall electron mobilities 30 times higher than those of similar materials produced using typical powder‐processing routes. As a result, a maximum thermoelectric figure of merit ZT of ≈0.7 at 700 °C is achieved in the β‐FeSi₂–SiGe nanocomposite.     

Ultrahigh Electrical Resistance of Poly(cyclohexyl methacrylate)/Carbon Nanotube Composites Prepared Using Surface‐Initiated Polymerization

Multiwalled carbon nanotubes on which poly(cyclohexyl methacrylate)s are densely grafted (PCHMA‐CNTs), are synthesized using a modified surface‐initiated atom transfer radical polymerization technique. The electrical resistance of PCHMA‐CNT is systematically characterized under direct current (DC) and alternating current and compared to that of conventional nanocomposites prepared by blending PCHMA with the CNT (PCHMA/CNT). At a comparable volume fraction of CNT, DC volume resistivity of PCHMA‐CNT is 14 orders of magnitude higher than that of PCHMA/CNT. This is because the grafted polymer with a combination of the high molecular weight and the high grafting density isolates individual CNTs at a long distance in the PCHMA‐CNT system. In addition, impedance analysis reveals that the highly insulated PCHMA‐CNT has the same electrical nature as neat PCHMA, i.e., it is a dielectric. Furthermore, dynamic mechanical analysis shows PCHMA‐CNT has a good mechanical properties as well as ultrahigh electrical resistance.     

Adhesion and Percolation Parameters in Two Dimensional Pd–LSCM Composites for SOFC Anode Current Collection

This paper is concerned with palladium–(La_0.75Sr_0.25)_0.97Cr_0.5Mn_0.5O₃ (LSCM) composite current collectors for solid oxide fuel cells (SOFCs); the composites, which are in a 2D configuration (thickness of about 8–10 µm), are deposited upon an LSCM electrode layer on top of an yttria zirconia electrolyte substrate. The influence of the LSCM particle size on the adhesion between palladium and LSCM are reported and discussed. Compositions using four different LSCM particle sizes (0.21, 0.49, 0.64, and 0.81 µm) with sintered Pd particle sizes approaching 10 µm are investigated. The best bonding is obtained when smaller particles are used. The electrical dc conductivity of the composite is reported as a function of the palladium volume fraction for all used LSCM particle sizes. The measured experimental values present typical insulating–conductive percolation. However, the transition occurs at ～33% of the conductive phase, that is, a lower percentage than for 2D ideal systems and a higher percentage than for 3D ideal systems. This is consistent with lower‐dimension percolation for a system of large‐grained conductors and small‐grained insulators. The general effective media (GEM) equation is used to fit the experimental data, and the two main parameters (the threshold point ?_c and the exponent t) are defined.     

Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites

Films comprising Au nanoparticles and polyphenylene dendrimers (first and second generation) are deposited onto transducer substrates via layer‐by‐layer self‐assembly and characterized by atomic force microscopy and X‐ray photoelectron spectroscopy. Their sorption behavior is studied by measuring the uptake of solvents from the vapor phase with quartz crystal microbalances (QCMs). The resistance of the films is simultaneously monitored. Both sensor types, QCMs and chemiresistors, give qualitatively very similar response isotherms that are consistent with a combination of Henry‐ and Langmuir‐type sorption processes. The sorption‐induced increase in relative differential resistance scales linearly with the amount of analyte accumulated in the films. This result is in general agreement with an activated tunneling process for charge transport, if little swelling and only small changes in the permittivity of the film occur during analyte sorption (a first‐order approximation). The relative sensitivity of the films to different solvents decreases in the order toluene ≈ tetrachloroethylene > 1‐propanol ? water. Films containing the larger second‐generation dendrimers show higher sensitivity than films containing first‐generation dendrimers.     

Copper Sulfide (CuxS) Nanowire‐in‐Carbon Composites Formed from Direct Sulfurization of the Metal‐Organic Framework HKUST‐1 and Their Use as Li‐Ion Battery Cathodes

Li‐ion batteries containing cost‐effective, environmentally benign cathode materials with high specific capacities are in critical demand to deliver the energy density requirements of electric vehicles and next‐generation electronic devices. Here, the phase‐controlled synthesis of copper sulfide (Cu_xS) composites by the temperature‐controlled sulfurization of a prototypal Cu metal‐organic framework (MOF), HKUST‐1 is reported. The tunable formation of different Cu_xS phases within a carbon network represents a simple method for the production of effective composite cathode materials for Li‐ion batteries. A direct link between the sulfurization temperature of the MOF and the resultant Cu_xS phase formed with more Cu‐rich phases favored at higher temperatures is further shown. The Cu_xS/C samples are characterized through X‐ray diffraction (XRD), thermogravimetric analysis (TGA), transmission electron microscopy, and energy dispersive X‐ray spectroscopy (EDX) in addition to testing as Li‐ion cathodes. It is shown that the performance is dependent on both the Cu_xS phase and the crystal morphology with the Cu_1.8S/C‐500 material as a nanowire composite exhibiting the best performance, showing a specific capacity of 220 mAh g^?1 after 200 charge/discharge cycles.     

军校电工电子实验教学探究与实践

针对电工电子技术课程的特点，立足军校学员学历与任职融合培养新模式，深化实验教学改革，依托军事应用，优化实验教学内容，构建多层次实践教学，拓展第二课堂活动，突出学为中心，实现工程能力和信息化素养培塑。实践表明，相关措施方法在电工电子实验教学中发挥作用并取得成果。     

Nanoparticle Reshaping and Ion Migration in Nanocomposite Ultrafast Ionic Actuators: The Converse Piezo–Electro–Kinetic Effect

This article reports on the effect of silver nanoparticles (NPs), used as active fillers, on the piezoelectric response of polymer composites. In particular, it is demonstrated that the application of a periodic electric field drives a collective drift of surface atoms of the NPs along the field direction (“electrokinetic effect”) which, in turn, creates macroscopic reversible tensile states. Overdriving the system, in high‐field conditions, the electronic current is counterbalanced by a massive injection of Ag⁺ ions into the matrix, producing a metastable exceptional expansion of the device. For similitude with the converse piezoelectric effect, it has been called the converse piezo–electro–kinetic effect. By using in situ spectroscopy, vibrometric analysis, real‐time UV‐visible spectroscopy, in situ electrical transmission electron microscopy, and in qualitative form ab initio and finite element method numerical simulations, i) the injection of ions from the NPs to the matrix, ii) the surface migration‐induced NP reshaping, and iii) the NP migration and consequent percolation path adjustments are shown. The implications of this study are significant for the development of ultrafast soft ionic actuators and create the premises for a broad range of applications in smart materials and devices.     

Structural,Electrical, and Thermal Behavior of Graphite‐Polyaniline Composites with Increased Crystallinity

Conductive materials are at the forefront of materials science research because of the large number of applications that have been developed around their interesting and unique properties. This work reports for the first time a correlation between the structural, electrical, and thermal behavior of novel graphite‐polyaniline (G‐PANI) composites with electrical conductivities greater than either of the individual components. The G:PANI mass ratio was varied during synthesis of the composites (90:10, 95:5, 96:4, 97:3, and 98:2 G:PANI mass ratios) and the highest electrical conductivity was determined for the composite having a G:PANI mass ratio of 96:4. The structural changes related to this increase in electrical conductivity were clearly reflected by the Raman spectra of the new composites, which indicated an improved crystallinity through a better stacking along the c‐axis of graphite when PANI was present (as evidenced by the G and 2 × D modes at 1582 and 2684 cm⁻¹). X‐ray diffraction data showed a slight increase in the (0 0 2) graphite crystal plane distance that was associated with a dilute stage intercalation or a possible “pseudo‐intercalation” of the polymer species between the graphite layers facilitating charge transfer in the composites. It is proposed that polyaniline acts as a charge transfer component between basal planes of graphite. Thermogravimetric analyses of the samples showed similar trends for the thermal stability in accordance with the electrical conductivity, the Raman and X‐ray diffraction data. The potential impact of this work is evident in the many areas that utilize graphite as conductive filler in electrically conducting materials. The composites can be used for a large number of applications in nanoelectronics, electromagnetic interference shielding, rechargeable batteries or as other advanced nanocomposite materials with improved electrical, structural, and thermal properties.     

Electrical Behavior and Electron Transfer Modulation of Nickel–Copper Nanoalloys Confined in Nickel–Copper Nitrides Nanowires Array Encapsulated in Nitrogen‐Doped Carbon Framework as Robust Bifunctional Electrocatalyst for Overall Water Splitting

Probing robust electrocatalysts for overall water splitting is vital in energy conversion. However, the catalytic efficiency of reported catalysts is still limited by few active sites, low conductivity, and/or discrete electron transport. Herein, bimetallic nickel–copper (NiCu) nanoalloys confined in mesoporous nickel–copper nitride (NiCuN) nanowires array encapsulated in nitrogen‐doped carbon (NC) framework (NC–NiCu–NiCuN) is constructed by carbonization‐/nitridation‐induced in situ growth strategies. The in situ coupling of NiCu nanoalloys, NiCuN, and carbon layers through dual modulation of electrical behavior and electron transfer is not only beneficial to continuous electron transfer throughout the whole system, but also promotes the enhancement of electrical conductivity and the accessibility of active sites. Owing to strong synergetic coupling effect, such NC–NiCu–NiCuN electrocatalyst exhibits the best hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance with a current density of 10 mA cm^?2 at low overpotentials of 93 mV for HER and 232 mV for OER, respectively. As expected, a two‐electrode cell using NC–NiCu–NiCuN is constructed to deliver 10 mA cm^?2 water‐splitting current at low cell voltage of 1.56 V with remarkable durability over 50 h. This work serves as a promising platform to explore the design and synthesis of robust bifunctional electrocatalyst for overall water splitting.     

In Situ Electrical Network Activation and Deactivation in Short Carbon Fiber Composites via 3D Printing

Prior studies on carbon-filler based, conductive polymer composites have mainly investigated how conductive filler morphology and concentration can tailor a material's electrical conductivity and overlooks the effects of filler alignment due to the difficulty to control and quickly quantify the filler alignment. Here, direct ink write 3D printing's unique ability is utilized to control carbon fiber alignment with a single process parameter, velocity ratio, to instantaneously activate or deactivate the electrical network in composites. Maximum electrical conductivity is achieved by randomly aligning carbon fibers that enhances the chance of direct fiber-to-fiber contact and, thus, activating the electrical network. However, aligning the fibers by increasing the velocity ratio disrupts the electrical network by minimizing fiber-to-fiber contact that resulted in a drastic decrease in electrical conductivity by as much as five orders of magnitude in both short and long carbon fiber composites. With this study, this study demonstrates that electrically conductive or insulative composites can be fabricated sequentially with a single ink. This novel ability to instantaneously control the electrical conductivity of carbon fiber reinforced composites allow to directly embed conductive pathways into designs to 3D print multifunctional composites that are capable of localized heating and self-sensing.     

Single‐Walled Carbon Nanotube Networks: The Influence of Individual Tube–Tube Contacts on the Large‐Scale Conductivity of Polymer Composites

Over two decades after carbon nanotubes started to attract interest for their seemingly huge prospects, their electrical properties are far from being used to the maximum potential. Composite materials based on carbon nanotubes still have conductivities several orders of magnitude below those of the tubes themselves. This study aims at understanding the reason for these limitations and the possibilities to overcome them. Based on and validated by real single‐walled carbon nanotube (SWCNT) networks, a simple model is developed, which can bridge the gap between macroscale and nanoscale down to individual tube–tube contacts. The model is used to calculate the electrical properties of the SWCNT networks, both as‐prepared and impregnated with an epoxy‐amine polymer. The experimental results show that the polymer has a small effect on the large‐scale network resistance. From the model results it is concluded that the main contribution to the conductivity of the network results from direct contacts, and that in their presence tunneling contacts contribute insignificantly to the conductivity. Preparing highly conductive polymer composites is only possible if the number of direct, low‐resistance contacts in the network is sufficiently large and therefore these direct contacts play the key role.     

Natural Vermiculite Enables High‐Performance in Lithium–Sulfur Batteries via Electrical Double Layer Effects

Lithium–sulfur batteries with potentially high specific energy are viewed as very promising candidates for next‐generation lightweight and low‐cost rechargeable batteries. However, sulfur‐based cathodes suffer from dissolution of polysulfides causing shuttle effects. Here, in order to confine elemental sulfur and anchor the polysulfides, a novel host that is an inexpensive natural clay mineral, viz., vermiculite is proposed. When compared to regular carbon–sulfur composites, vermiculite–sulfur composites offer promising rate capability and much better cycling stabilities, displaying capacity retentions of ≈89 and ≈93% within 200 cycles at C/2 and 1 C, respectively, and ≈60 % at C/5 within 1000 cycles. Postmortem studies, advanced adsorption tests, density functional theory calculations, and zeta potential measurements in combination with intrinsic characteristics of the natural vermiculite provide insights into the new mechanism. The vermiculite contains naturally present surface cations which show a strong tendency to adsorb S_n^2? anions, hence protecting them from dissolution. The excess surface charge is most probably compensated by excess Li⁺ in the space charge zones which is beneficial for charge transfer and local conductivity. The reported results show that natural clay‐minerals are promising sulfur hosts being able to fixate sulfides via electrical double layer effects, thus enabling high‐performance in lithium–chalcogen batteries.     

Polyaniline Entrapped in Silver: Structural Properties and Electrical Conductivity

By employing the new methodology of entrapment of organic molecules within metals, we demonstrate the ability to modify the conductivity of a metal by suitable polymer entrapment. Specifically, polyaniline (PANI) in two molecular weights was entrapped in silver at different concentrations and a comprehensive comparison was preformed for a range of the composite properties, characterized by XRD, SEM, BET, TGA, and density measurements. Pressed films were utilized to measure the electrical conductivity of the composites in order to study the PANI‐silver interactions at the molecular level and to establish a correlation between the microscopic morphology and the film conduction. Such correlations have been identified, and are interpreted. This work extends the functional applications of the new metallic composites and offers insight on the polymer‐metal molecular level interactions.     

Cover Picture: Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites (Adv. Funct. Mater. 6/2007)

The cover shows chemiresistors and mass‐sensitive vapor sensors coated with Au‐nanoparticle/dendrimer composites. The Au nanoparticles provide the film with electrical conductivity and the dendrimers control the chemical selectivity, as reported by Nadjedja Krasteva and co‐workers on p. 881. Analyte sorption follows a combined Henry–Langmuir model, and measurements reveal that sorption‐induced increase in film resistance scales linearly with the concentration of analyte sorbed in the film. The background shows a silicon wafer with lithographically defined microelectrode structures for chemiresistor fabrication. Films comprising Au nanoparticles and polyphenylene dendrimers (first and second generation) are deposited onto transducer substrates via layer‐by‐layer self‐assembly and characterized by atomic force microscopy and X‐ray photoelectron spectroscopy. Their sorption behavior is studied by measuring the uptake of solvents from the vapor phase with quartz crystal microbalances (QCMs). The resistance of the films is simultaneously monitored. Both sensor types, QCMs and chemiresistors, give qualitatively very similar response isotherms that are consistent with a combination of Henry‐ and Langmuir‐type sorption processes. The sorption‐induced increase in relative differential resistance scales linearly with the amount of analyte accumulated in the films. This result is in general agreement with an activated tunneling process for charge transport, if little swelling and only small changes in the permittivity of the film occur during analyte sorption (a first‐order approximation). The relative sensitivity of the films to different solvents decreases in the order toluene ≈ tetrachloroethylene > 1‐propanol ? water. Films containing the larger second‐generation dendrimers show higher sensitivity than films containing first‐generation dendrimers.