Photopatternable Conductive PDMS Materials for Microfabrication

Conductive photodefinable polydimethylsiloxane (PDMS) composites that provide both high electrical conductivity and photopatternability have been developed. The photosensitive composite materials, which consist of a photosensitive component, a conductive filler, and a PDMS pre‐polymer, can be used as a negative photoresist or a positive photoresist with an additional curing agent. A standard photolithographic approach has been used to fabricate conductive elastomeric microstructures. Feature sizes of 60 µm in the positive photoresist and 10 µm in the negative photoresist have been successfully achieved. Moreover, as the conductive filler, silver powders significantly improve the electrical conductivity of the PDMS polymer, but also provide enhanced mechanical and thermal properties as well as interesting biological properties. The combined electrical, mechanical, thermal, and biological properties along with photopatternability make the PDMS‐Ag composite an excellent processable and structural material for various microfabrication applications.     

Designing Negative Differential Resistance Devices Based on Self‐Heating

There are a number of important emerging applications that would benefit from simple circuit elements that exhibit tunable current‐controlled negative differential resistance (NDR). The advent of such devices would enable, for example, key components for threshold logic and neuromorphic computing such as threshold switches, periodic and chaotic oscillators, and small signal amplifiers. Circuit elements that provide NDR with modifiable electrical characteristics would also be useful for creating optimized “selectors” that enable addressing of individual memory cells in large resistance‐based memory arrays. Currently, there are no simple, bipolar, two‐terminal commercial devices that exhibit current‐controlled NDR. This paper demonstrates that current‐controlled NDR can, in principal, arise from any electrical conduction mechanism that depends superlinearly on temperature, and that in practice a broad spectrum of materials can be utilized to yield NDR. A model is presented that accurately predicts conditions under which NDR can be observed and guidelines are provided for choosing materials that result in desired electrical characteristics. This model accurately predicts the behavior of some existing structures and can be used to tailor future circuit elements for emerging applications. It may also explain the onset of a number of “soft breakdown” phenomena.     

Thermoelectric Potential of Polymer-Scaffolded Ionic Liquid Membranes

Organic thin films have been viewed as potential thermoelectric (TE) materials, given their ease of fabrication, flexibility, cost effectiveness, and low thermal conductivity. However, their intrinsically low electrical conductivity is a main drawback which results in a relatively lower TE figure of merit for polymer-based TE materials than for inorganic materials. In this paper, a technique to enhance the ion transport properties of polymers through the introduction of ionic liquids is presented. The polymer is in the form of a nanofiber scaffold produced using the electrospinning technique. These fibers were then soaked in different ionic liquids based on substituted imidazolium such as 1-ethyl-3-methylimidazolium chloride or 1-butyl-3-methylimidazolium bromide. This method was applied to electrospun polyacrylonitrile and a mixture of polyvinyl alcohol and chitosan polymers. The ion transport properties of the membranes have been observed to increase with increasing concentration of ionic liquid, with maximum electrical conductivity of 1.20 × 10^?1 S/cm measured at room temperature. Interestingly, the maximum electrical conductivity value surpassed the value of pure ionic liquids. These results indicate that it is possible to significantly improve the electrical conductivity of a polymer membrane through a simple and cost-effective method. This may in turn boost the TE figures of merit of polymer materials, which are well known to be considerably lower than those of inorganic materials. Results in terms of the Seebeck coefficient of the membranes are also presented in this paper to provide an overall representation of the TE potential of the polymer-scaffolded ionic liquid membranes.     

Thermal filaments in vanadium dioxide

A relatively simple thin-film structure using vanadium dioxide is analyzed in detail to show that high-temperature, low-resistivity filaments can exist in VO2under appropriate electrical bias. These filaments will dominate both the static and dynamic electrical behavior of the structure, giving rise to V-I characteristics similar to those expected from materials exhibiting current-controlled bulk negative resistivities. The analysis indicates that these filaments may not only provide electrical features important from a device point of view, but also provide a tool for the measurement of fundamental material properties and for studying the dynamics of the phase transition in VO2.     

Laser processing of high-Tc superconductingthin films

High-T_c superconducting thin films can be deposited and processed by pulsed and CW lasers, and a respectable materials technology for the Y-Ba-Cu-O superconductor is rapidly emerging. The pulsed laser deposition technique is simple because it produces films with compositions nearly identical to those of the target pellets. A larger variety of substrates can be used, compared to other deposition technologies, because of the relatively low temperature requirements. The laser deposition mechanism has been investigated. As-deposited superconducting films, epitaxial films with smooth surfaces, and multilayer structures with abrupt interfaces have been produced. The electrical transport properties can be changed locally using a focused argon-ion laser by modifying the oxygen stoichiometry. This laser writing can be erased by room-temperature exposure to an oxygen plasma. Other laser patterning methods such as material removal, melt-quench, and direct pattern transfer are being developed     

The Dielectric Properties of Wet Materials

In microwave remote sensing, knowledge of the dielectric properties of the materials observed (vegetation, soils) is important for the interpretation of the recorded reflection or emission and for the design of models for this reflection or emission. Although there exists no simple law which describes the properties of a heterogeneous system when the properties of its components are known, boundaries can be indicated for the permittivity of such a system. Heterogeneous systems containing water, such as wet soils and plant material, have very complex dielectric properties due to the role of the conductivity. However, at the higher microwave frequencies, in particular above the S-band, these conductivity effects play only a minor role and the free water is most important. When knowing the permittivity of wet materials the penetration depth can be determined. In all practical cases this depth is smaller than the wavelength used for the observations.     

Universal Control on Pyroresistive Behavior of Flexible Self‐Regulating Heating Devices

Smart heating devices with reliable self‐regulating performances and high efficiency, combined with additional properties like mechanical flexibility, are of particular interest in healthcare, soft robotics, and smart buildings. Unfortunately, the development of smart heaters necessitates managing normally conflicting requirements such as good self‐regulating capabilities and efficient Joule heating performances. Here, a simple and universal materials design strategy based on a series connection of different conductive polymer composites (CPC) is shown to provide unique control over the pyroresistive properties. Hooke's and Kirchhoff's laws of electrical circuits can simply predict the overall pyroresistive behavior of devices connected in series and/or parallel configurations, hence providing design guidelines. An efficient and mechanically flexible Joule heating device is hence designed and created. The heater is characterized by a zero temperature coefficient of resistance below the self‐regulating temperature, immediately followed by a large and sharp positive temperature coefficient (PTC) behavior with a PTC intensity of around 10⁶. Flexibility and toughness is provided by the selected elastomeric thermoplastic polyurethane (TPU) matrix as well as the device design. The universality of the approach is demonstrated by using different polymer matrices and conductive fillers for which repeatable results are consistently obtained.     

Separation of low molecular siloxanes for electronic application byliquid-liquid extraction

Silicone resins are widely used for electronic packaging as potting and encapsulating materials. Silicone resins have many advantages for electronic packaging applications such as superior electrical properties, thermal stability, low water absorption, etc. Furthermore, silicone resins are not only used as protective materials for integrated circuit (IC) devices but also as conducting materials for interconnection. However, silicone resins have two big drawbacks: low adhesion strength and low molecular weight creep. A simple liquid-liquid extraction method has been developed to purify silicone resins, which will improve adhesion strength and eliminate low molecular weight creep. This paper describes the results of the liquid-liquid extraction method to remove low molecular weight cyclic siloxanes. Fourier transform-infrared (FT-lR) spectroscopy was used to monitor the removal rate of low molecular weight cyclic siloxanes. Thermogravimetric analysis (TGA) was used to evaluate the purity of silicone resin. Gas chromatography-mass spectrometry (GC/MS) was used to identify the low molecular weight cyclic siloxanes. Thermomechanical analyzer (TMA), dynamic mechanical analyzer (DMA), and die shear test were used for evaluate the properties of silicone resin     

Glass Fibers with Carbon Nanotube Networks as Multifunctional Sensors

A simple approach to deposit multiwalled carbon nanotube (MWNT) networks onto glass fiber surfaces achieving semiconductive MWNT–glass fibers is reported, along with application of fiber/polymer interphases as in‐situ multifunctional sensors. This approach demonstrates for the first time that the techniques of conducting electrical resistance measurements could be applicable to glass fibers for in situ sensing of strain and damage; the techniques were previously limited to conductive and semiconductive materials. The electrical properties of the single MWNT–glass fiber and the “unidirectional” fiber/epoxy composite show linear or nonlinear stress/strain, temperature, and relative humidity dependencies, which are capable of detecting piezoresistive effects as well as the local glass transition temperature. The unidirectional composites containing MWNT–glass fibers exhibit ultrahigh anisotropic electrical properties and an ultralow electrical percolation threshold. Based on this approach, the glass fiber—the most widely used reinforcement in composites globally—along with the surface electrical conductivity of MWNTs will stimulate and realize a broad range of multifunctional applications.     

On the Largest Possible Mobility of Molecular Semiconductors and How to Achieve It

A large database of known molecular semiconductors is used to define a plausible physical limit to the charge carrier mobility achievable within this materials class. From a detailed study of the desirable properties in a large dataset, it is possible to establish whether such properties can be optimized independently and what would be a reasonably achievable optimum for each of them. All relevant parameters are computed from a set of almost five thousand known molecular semiconductors, finding that the best known materials are not ideal with respect to all properties. These parameters in decreasing order of importance are the molecular area, the nonlocal electron–phonon coupling, the 2D nature of transport, the local electron–phonon coupling, and the highest transfer integral. It is also found that the key properties related to the charge transport are either uncorrelated or “constructively” correlated (i.e., they improve together) concluding that a tenfold increase in mobility is within reach in a statistical sense, on the basis of the available data. It is demonstrated that high throughput screenings, when coupled with physical models of transport produce both specific target materials and a more general physical understanding of the materials space.     

Harvesting vibration energy using piezoelectric material: Modeling,simulation and experimental verifications

Piezoelectric materials produce electrical charges when subjected to dynamic strain. These materials can be used to capture and store vibrational energy which later can be used to power up small devices. This paper presents an analytical estimation of voltage production of piezoelectric cantilever beam when subjected to base excitation, with and without attached proof masses. The beam is modeled using Euler–Bernoulli, also known as thin beam theory. As such, the model obtained here is applicable for micro- and nano-beams. The frequency response function (FRF) that relates the output voltage and transverse acceleration is identified for multi-mode vibration. These analytical predictions are then compared with experimental results and good agreement is obtained.     

Analytical circuit model for thin film electroluminescent devices

A simple equivalent circuit is developed for alternating current thin film electroluminescent devices. With sinusoidal excitation voltage an exact analytical solution is presented for calculating the device current, transferred charge and the internal voltage across the phosphor layer. The circuit is used for modeling EL devices with widely varying phosphor layer materials that include ZnS, CaS, SrS, CaF₂, SrF₂, ZnF₂ and multilayer structures prepared with Atomic Layer Deposition. The equivalent circuit gives a straightforward way to characterize the electrical properties of these materials. The fluorides have inferior breakdown properties compared to sulfides although they are much more symmetric. The strong asymmetry of SrS is attributed to its nonuniform defect distribution     

The influence of parameter dispersion of electrical cells on the array power output

Energy conversion by electrical (photovoltaic and chemical) cells are of relatively low power and low voltage levels. The cells are therefore connected in an array to produce the higher power and voltage levels desired. The dispersion of the cell parameters affect the array performance in such a way that the power output is lower than that desired, and hence additional cells have to be added. A larger dispersion of the parameters requires a greater amount of additional cells for the same power output requirement to make up for the so-called cell mismatch losses. This paper deals with the influence of parameter dispersions on array output power. The required number of cells in an array is determined analytically for a known distribution of the cell parameters. The nonidentical cell array can now be interchanged with an equivalent identical cell array permitting less complex calculations. The analysis in this paper is given for a large number of ideal electrochemical cells; a similar approach can be applied on other electrical cells.     

Analysis of electrically conductive silver ink on stretchable substrates under tensile load

Electrical conductors were printed by the screen printing method on stretchable PVC substrates and on fabrics. Polymer thick film silver ink was used as the conductive medium. The electrical performance and the structure of the ink film were investigated in unloaded conditions and under strain. In addition, the ink film morphology was examined. The goal of this study was to provide information for developing a strain sensor for large strain levels using the materials under investigation. An additional aim was to assist the integration of electronics into other structures. The results showed that strain sensitive structures can be made using the materials selected for this study and these materials provide an opportunity to develop strain sensors. The structures also tolerated large strain levels and thus they can be integrated into other materials which are exposed to strain.     

Scavenging Energy Sources Using Ferroelectric Materials

Ferroelectric materials have attracted interest for over a hundred years as a result of their spontaneous polarization and a polarization orientation that can be reversed by the application of an external electric field. In addition, the degree of polarization can be affected by external stimuli such as vibrations, stress, heat, and light. These properties enable ferroelectric materials to be used to fabricate nanogenerators, which are devices used in energy scavenging applications and provide an opportunity to obtain electrical energy from a variety of external stimuli. This review discusses the development of ferroelectric-based nanogenerators for scavenging mechanical, thermal, and solar energies through the piezoelectric effect, pyroelectric effect, and photovoltaic effect, respectively. The mechanisms of the effects and the pathways to optimize the output performance of the nanogenerators are analyzed in detail. Recent developments in energy harvesting using ferroelectric materials are discussed with the objective to motivate attention and efforts in this growing field.     

Evaluation of Half‐Heusler Compounds as Thermoelectric Materials Based on the Calculated Electrical Transport Properties

A theoretical evaluation of the thermoelectric‐related electrical transport properties of 36 half‐Heusler (HH) compounds, selected from more than 100 HHs, is carried out in this paper. The electronic structures and electrical transport properties are studied using ab initio calculations and the Boltzmann transport equation under the constant relaxation time approximation for charge carriers. The electronic structure results predict the band gaps of these HH compounds, and show that many HHs are narrow‐band‐gap semiconductors and, therefore, are potentially good thermoelectric materials. The dependence of Seebeck coefficient, electrical conductivity, and power factor on the Fermi level is investigated. Maximum power factors and the corresponding optimal p‐ or n‐type doping levels, related to the thermoelectric performance of materials, are calculated for all HH compounds investigated, which certainly provide guidance to experimental work. The estimated optimal doping levels and Seebeck coefficients show reasonable agreement with the measured results for some HH systems. A few HHs are recommended to be potentially good thermoelectric materials based on our calculations.     

Self‐Healable and Stretchable Organic Thermoelectric Materials: Electrically Percolated Polymer Nanowires Embedded in Thermoplastic Elastomer Matrix

Self‐healable and stretchable energy‐harvesting materials can provide a new avenue for the realization of self‐powered wearable electronics, including electronic skins, whose main materials are required to be robust to and stable under external damage and severe mechanical stresses. However, thermoelectric (TE) materials showing both self‐healing properties and stretchability have not yet been demonstrated despite their great potential to harvest thermal energy in the human body. As most existing TE materials are either mechanically brittle or unrecoverable after being subjected to damage, a novel approach is necessary for designing such materials. Herein, self‐healable and stretchable TE materials based on all‐organic composite system wherein polymer semiconductor nanowires are p‐doped with a molecular dopant and embedded in a thermoplastic elastomer matrix are reported. The polymer nanowires are electrically percolated in the matrix, and the resulting composite materials exhibit good TE performance. The composites also exhibit both excellent self‐healing properties under mild heat and pressure conditions and good stretchability. It is believed that this work can be a cornerstone for the design of self‐healable and stretchable energy‐harvesting materials as it provides useful guidelines for imparting electrical conductivity to insulating thermoplastic elastomers, which typically possess versatile and useful mechanical properties.     

激光制备新型石墨烯/铜基复合电触头

为了解决电力系统中关键材料电触头同时受到电弧烧蚀、动静触头相互碰撞产生的冲击载荷和摩擦以及电流产生焦耳热引起的熔焊的问题,提高电气设备的使用性能,发展新型高性能触头材料,提出了一种石墨烯/紫铜复合触头的新型制备方法。采用等离子体辅助加工手段,在质量分数为0.999的紫铜表面上制备出镍铜合金过渡层,在此过渡层上利用高功率连续激光原位制备石墨烯表面薄膜,全覆盖于紫铜基底表面,作为独立涂层来抵抗触头材料所受的破坏,探索了制备石墨烯复合触头材料过程中的等离子体辅助加工工艺以及激光加工工艺。结果表明,石墨烯/铜基触头材料具有优异的电工特性,电阻与紫铜相近,硬度为紫铜的1.8倍,摩擦系数仅为0.06。本研究可为电工材料提供新的解决思路和新的材料体系。     

Functional Nanomaterials on 2D Surfaces and in 3D Nanocomposite Hydrogels for Biomedical Applications

An emerging approach to improve the physicobiochemical properties and the multifunctionality of biomaterials is to incorporate functional nanomaterials (NMs) onto 2D surfaces and into 3D hydrogel networks. This approach is starting to generate promising advanced functional materials such as self‐assembled monolayers (SAMs) and nanocomposite (NC) hydrogels of NMs with remarkable properties and tailored functionalities that are beneficial for a variety of biomedical applications, including tissue engineering, drug delivery, and developing biosensors. A wide range of NMs, such as carbon‐, metal‐, and silica‐based NMs, can be integrated into 2D and 3D biomaterial formulations due to their unique characteristics, such as magnetic properties, electrical properties, stimuli responsiveness, hydrophobicity/hydrophilicity, and chemical composition. The highly ordered nano‐ or microscale assemblies of NMs on surfaces alter the original properties of the NMs and add enhanced and/or synergetic and novel features to the final SAMs of the NM constructs. Furthermore, the incorporation of NMs into polymeric hydrogel networks reinforces the (soft) polymer matrix such that the formed NC hydrogels show extraordinary mechanical properties with superior biological properties.     

Environmental protection of inkjet-printed Ag conductors

Electrically conductive silver nanoparticle ink patterns were fabricated using the inkjet printing method. Two different polymer films were used as the substrate materials. The patterns were exposed to humidity and salt fog and the electrical performance (sheet resistance and RF performance) as well as mechanical endurance (adhesion) were measured before and after the environmental tests. The electrical properties of the printed structures remained good in all the measurable samples. The adhesion between the ink and a substrate material appeared to be a greater challenge in harsh environments. Protection capabilities of one dip coated and one hot laminated barrier materials were evaluated during the environmental tests. The results showed that there is a need for environmental protection in printed electronics. Especially the laminated barrier films can offer a potential solution for shielding printed electronics in harsh environments as they can provide good mechanical protection, and can easily be integrated in roll-to-roll process.