Versatile Landscape of Low-k Polyimide: Theories, Synthesis, Synergistic Properties, and Industrial Integration.

The development of microelectronics and large-scale intelligence nowadays promotes the integration, miniaturization, and multifunctionality of electronic and devices but also leads to the increment of signal transmission delays, crosstalk, and energy consumption. The exploitation of materials with low permittivity (low-k) is crucial for realizing innovations in microelectronics. However, due to the high permittivity of conventional interlayer dielectric material (k ∼ 4.0), it is difficult to meet the demands of current microelectronic technology development (k < 3.0). Organic dielectric materials have attracted much attention because of their relatively low permittivity owing to their low material density and low single bond polarization. Polyimide (PI) exhibits better application potential based on its well permittivity tunability (k = 1.1-3.2), high thermal stability (>500 °C), and mechanical property (modulus of elasticity up to 3.0-4.0 GPa). In this review, based on the synergistic relationship of dielectric parameters of materials, the development of nearly 20 years on low-k PI is thoroughly summarized. Moreover, process strategies for modifying low-k PI at the molecular level, multiphase recombination, and interface engineering are discussed exhaustively. The industrial application, technological challenges, and future development of low-k PI are also analyzed, which will provide meaningful guidance for the design and practical application of multifunctional low-k materials.

1D/2D Carbon Nanomaterial-Polymer Dielectric Composites with High Permittivity for Power Energy Storage Applications.

With the development of flexible electronic devices and large-scale energy storage technologies, functional polymer-matrix nanocomposites with high permittivity (high-k) are attracting more attention due to their ease of processing, flexibility, and low cost. The percolation effect is often used to explain the high-k characteristic of polymer composites when the conducting functional fillers are dispersed into polymers, which gives the polymer composite excellent flexibility due to the very low loading of fillers. Carbon nanotubes (CNTs) and graphene nanosheets (GNs), as one-dimensional (1D) and two-dimensional (2D) carbon nanomaterials respectively, have great potential for realizing flexible high-k dielectric nanocomposites. They are becoming more attractive for many fields, owing to their unique and excellent advantages. The progress in dielectric fields by using 1D/2D carbon nanomaterials as functional fillers in polymer composites is introduced, and the methods and mechanisms for improving dielectric properties, breakdown strength and energy storage density of their dielectric nanocomposites are examined. Achieving a uniform dispersion state of carbon nanomaterials and preventing the development of conductive networks in their polymer composites are the two main issues that still need to be solved in dielectric fields for power energy storage. Recent findings, current problems, and future perspectives are summarized.

Dual-Effect Coupling for Superior Dielectric and Thermal Conductivity of Polyimide Composite Films Featuring "Crystal-Like Phase" Structure.

To match the increasing miniaturization and integration of electronic devices, higher requirements are put on the dielectric and thermal properties of the dielectrics to overcome the problems of delayed signal transmission and heat accumulation. Here, a 3D  porous thermal conductivity network is successfully constructed inside the polyimide (PI) matrix by the combination of ionic liquids (IL) and calcium fluoride (CaF2 ) nanofillers, motivated by the bubble-hole forming orientation force. Benefiting from the 3D thermal network formed by IL as a porogenic template and "crystal-like phase" structures induced by CaF2 - polyamide acid charge transfer, IL-10 vol% CaF2 /PI porous film exhibits a low permittivity of 2.14 and a thermal conductivity of 7.22 W m-1 K-1 . This design strategy breaks the bottleneck that low permittivity and high thermal conductivity in microelectronic systems are difficult to be jointly controlled, and provides a feasible solution for the development of intelligent microelectronics.

High toughness, healable, self-cleaning polydimethylsiloxane elastomers with "rigid-while-flexible" mutual network structure.

Polydimethylsiloxane (PDMS) elastomers with high mechanical and healing properties are developed as smart materials for electrical power systems and electronic devices to address electrical or mechanical damage. However, the challenge is to reconcile the conflicting molecular mechanisms of mechanical and healing properties in the development of PDMS elastomers. This study adopts the "rigid-while-flexible" mutual network structure by copolymerizing the rigid polyimide (PI) with flexible segments with dynamic reversible crosslinking designed on the PDMS backbones. This elastomer (designated PSiPI) exhibits high toughness, tensile strength and elongation at break, as well as excellent healing efficiency and recyclability. Moreover, the PSiPI elastomer also exhibits good insulation and corona damage healing properties. Taking advantage of the recyclability and healing properties of PSiPI elastomers, healable superhydrophobic coatings with contact angles greater than 150° have been prepared by compositing PSiPI elastomers with SiO2. Likewise, combining the elastomer with conductive materials can create a healing flexible conductor. This "rigid-while-flexible" design approach provides important inspiration for the development of high-performance, sustainable and environmentally friendly PDMS elastomers for electrical and electronic applications.

Advanced Dielectric Materials for Triboelectric Nanogenerators: Principles, Methods, and Applications.

Triboelectric nanogenerator (TENG) manifests distinct advantages such as multiple structural selectivity, diverse selection of materials, environmental adaptability, low cost, and remarkable conversion efficiency, which becomes a promising technology for micro-nano energy harvesting and self-powered sensing. Tribo-dielectric materials are the fundamental and core components for high-performance TENGs. In particular, the charge generation, dissipation, storage, migration of the dielectrics, and dynamic equilibrium behaviors determine the overall performance. Herein, a comprehensive summary is presented to elucidate the dielectric charge transport mechanism and tribo-dielectric material modification principle toward high-performance TENGs. The contact electrification and charge transport mechanism of dielectric materials is started first, followed by introducing the basic principle and dielectric materials of TENGs. Subsequently, modification mechanisms and strategies for high-performance tribo-dielectric materials are highlighted regarding physical/chemical, surface/bulk, dielectric coupling, and structure optimization. Furthermore, representative applications of dielectric materials based TENGs as power sources, self-powered sensors are demonstrated. The existing challenges and promising potential opportunities for advanced tribo-dielectric materials are outlined, guiding the design, fabrication, and applications of tribo-dielectric materials.

Alicyclic Polyimide With Multiple Breakdown Self-Healing Based on Gas-Condensation Phase Validation for High Temperature Capacitive Energy Storage.

Polymer dielectrics with combined thermal stability and self-healing properties are specifically desired for high-temperature film capacitors. The high thermal stability of conventional polymers benefits from the abundance of aromatic rings in the molecule backbone, but the high carbon content sacrifices their self-healing properties. Here, analicyclic polyimide with a high glass transition temperature (256 °C) and wide energy bandgap (4.58 eV) is designed, which exhibits electric conductivity more than an order of magnitude lower than that of classical polyimide at high electric fields and high temperatures. As a result, alicyclic polyimide achieves a discharged energy density of 4.54 J cm-3 and a charge-discharge efficiency of above 90% at 200 °C, which is superior to existing dielectric polymers and composites. The alicyclic polyimide benefits from a low pyrolytic residual carbon rate, retaining 93% of the dielectric breakdown strength after four electrical breakdown cycles. Distinguishing from the current condensed-phase self-healing concept, for the first time, exploring the self-healing capability of high-temperature polyimide dielectric is presented based on dual self-healing mechanisms of gas-phase and condensed-phase. The high energy density at high temperatures and the superior self-healing capability of alicyclic polyimide further indicate the promise of polyimide dielectric film capacitors for extreme conditions.

High-toughness, extensile and self-healing PDMS elastomers constructed by decuple hydrogen bonding.

Elastomers are widely used in traditional industries and new intelligent fields. However, they are inevitably damaged by electricity, heat, force, etc. during the working process. With the continuous improvement of reliability and environmental protection requirements in human production and living, it is vital to develop elastomer materials with good mechanical properties that are not easily damaged and can self-heal after being damaged. Nevertheless, there are often contradictions between mechanical properties and self-healing as well as toughness, strength, and ductility. Herein, a strong and dynamic decuple hydrogen bonding based on carbon hydrazide (CHZ) is reported, accompanied with soft polydimethylsiloxane (PDMS) chains to prepare self-healing (efficiency 98.7%), recyclable, and robust elastomers (CHZ-PDMS). The strategy of decuple hydrogen bonding will significantly impact the study of the mechanical properties of elastomers. High stretchability (1731%) and a high toughness of 23.31 MJ m-3 are achieved due to the phase-separated structure and energy dissipation. The recyclability of CHZ-PDMS further supports the concept of environmental protection. The application of CHZ-PDMS as a flexible strain sensor exhibited high sensitivity.

Intrinsic-designed polyimide dielectric materials with large energy storage density and discharge efficiency at harsh ultra-high temperatures.

Polymer dielectric materials with excellent temperature stability are urgently needed for the ever-increasing energy storage requirements under harsh high-temperature conditions. In this work, a novel diamine monomer (bis(2-cyano-4-aminophenyl)amine) was successfully synthesized to prepare a series of cyano-containing polyimides (CPI-1-3), which possessed excellent dielectric properties and high thermostability. The maximum dielectric permittivity was up to 5.5 at 102 Hz for CPI-3, being 2.5 times higher than that of commercially used BOPP. In comparison, the CPI-1 exhibited an outstanding breakdown strength of 433 MV m-1 and a high energy density of 2.5 J cm-3 even at 250 °C, which was the highest value reported under the same conditions. The synthesized CPIs through such an intrinsic approach are potential candidate materials for energy storage and even other applications under simultaneously harsh electrical and thermal conditions.

Enhanced High-Temperature Capacitive Performance of a Bilayer-Structured Composite Film Employing a Charge Blocking Layer.

The great development potential of polymer dielectric capacitors in harsh environments urgently requires enhancing capacitive performance at high temperatures. However, the exponentially increased conduction loss at high temperature and high field results in a drastic drop in energy density and charge-discharge efficiency. Here, a bilayer-structured polyimide (PI) composite film containing a wide-band gap inorganic layer as a charge blocking layer is designed. The inorganic layer improves the charge trapping ability and regulates the charge mobility at the electrode/dielectric interface. The charge injection mechanism in the interface-optimized PI/boron nitride nanosheet (BNNS) composite films is investigated by finite element simulation, and the effect of the BNNS layer on high temperature conduction is further understood. An appropriate thickness of the charge blocking layer establishes an effective energy barrier. Therefore, the composite films exhibit significantly suppressed conduction loss and excellent capacitive performance at a high temperature. A high energy density of 4.37 J cm-3 with efficiency of 92% is obtained at 200 °C and 500 MV m-1, which is superior to reported high-temperature dielectric polymers and their composite films. This work provides a promising approach to improve the energy storage performance of polymer materials at high temperatures.

Rising of Dynamic Polyimide Materials: A Versatile Dielectric for Electrical and Electronic Applications.

Polyimides (PIs) are widely used in circuit components, electrical insulators, and power systems in modern electronic devices and large electrical appliances. Electrical/mechanical damage of materials are important factors that threaten reliability and service lifetime. Dynamic (self-healable, recyclable and degradable) PIs, a promising class of materials that successfully improve electrical/mechanical properties after damage, are anticipated to solve this issue. The viewpoints and perspectives on the status and future trends of dynamic PI based on a few existing documents are shared. The main damage forms of PI dielectric materials in the application process are first introduced, and initial strategies and schemes to solve these problems are proposed. Fundamentally, the bottleneck issues faced by the development of dynamic PIs are indicated, and the relationship between various damage forms and the universality of the method is evaluated. The potential mechanism of the dynamic PI to deal with electrical damage is highlighted and several feasible prospective schemes to address electrical damage are discussed. This study is concluded by presenting a short outlook and future improvements to systems, challenges, and solutions of dynamic PI in electrical insulation. The summary of theory and practice should encourage policy development favoring energy conservation and environmental protection and promoting sustainability.

Dynamic Covalent Adaptable Polyimide Hybrid Dielectric Films with Superior Recyclability.

Polyimides (PIs) used in advanced electrical and electronic devices can be electrically/mechanically damaged, resulting in a significant waste of resources. Closed-loop chemical recycling may prolong the service life of synthetic polymers. However, the design of dynamic covalent bonds for preparing chemically recyclable crosslinked PIs remains a challenging task. Herein, new crosslinked PI films containing a PI oligomer, chain extender, and crosslinker are reported. They exhibit superior recyclability and excellent self-healable ability owing to the synergistic effect of the chain extender and crosslinker. The produced films can be completely depolymerized in an acidic solution at ambient temperature, leading to efficient monomer recovery. The recovered monomers may be used to remanufacture crosslinked PIs without deteriorating their original performance. In particular, the designed films can serve as corona-resistant films with a recovery rate of approximately 100%. Furthermore, carbon fiber reinforced composites (CFRCs) with PI matrices are suitable for harsh environments and can be recycled multiple times at a non-destructive recycling rate up to 100%. The preparation of high-strength dynamic covalent adaptable PI hybrid films from simple PI oligomers, chain extenders, and crosslinkers may provide a solid basis for sustainable development in the electrical and electronic fields.

Dynamic Sustainable Polyimide Film Combining Hardness with Softness via a "Mimosa-Like" Bionic Strategy.

Dielectric polyimides (PIs) are ubiquitous as insulation in electrical power systems and electronic devices. Generally, dynamic polyimide is required to solve irreversible failure processes of electrical or mechanical damage, for example, under high temperature, pressure, and field strength. The challenge lies in the design of the molecular structure of rigid polyimide to achieve dynamic reversibility. Herein, a low-molecular-weight polyimide gene unit is designed to crosslink with polyimide ligase to prepare the smart film. Interestingly, due to the variability of gene unit and ligase combinations, the polyimide films combining hardness with softness are designed into three forms via a "Mimosa-like" bionic strategy to adapt to different application scenarios. Meanwhile, the films have good degradation efficiency, excellent recyclability, and can be self-healable, which makes them reuse. Clearly, the films can be used in the preparation of ultrafast sensors with a response time ≈0.15 s and the application of corona-resistant films with 100% recovery. Furthermore, the construction of polyimide and carbon-fiber-reinforced composites (CFRCs) has been verified to apply to the worse environment. Nicely, the composites have the property of multiple cycles and the non-destructive recycle rate of carbon fiber (CF) is as high as 100%. The design idea of preparing high-strength dynamic polyimide by crosslinking simple polyimide gene unit with ligase could provide a good foundation and a clear case for the sustainable development of electrical and electronic polyimides, from the perspective of Mimosa bionics.

Tailored high cycling performance in a solid polymer electrolyte with perovskite-type Li0.33La0.557TiO3 nanofibers for all-solid-state lithium ion batteries.

Solid polymer electrolytes (SPEs) have drawn considerable attention owing to their reliable safety performance, electrochemical stability and exceptional flexibility, which make them superior to conventional liquid electrolytes. Here, we report a novel composite electrolyte which is composed of homogeneously dispersed Li ion-conducting Li0.33La0.557TiO3 (LLTO) nanowires in a poly(ethylene oxide) (PEO)/LiClO4 matrix. It is demonstrated that only 3 wt% LLTO nanofibers are needed for the optimal performance of SPEs. The PEO-based composite electrolyte shows an excellent Li ion conductivity of 4.01 × 10-4 S cm-1 at 60 °C. In addition, it is worth mentioning that the all-solid-state lithium battery based on this composite electrolyte exhibits a specific capacity of 140 mA h g-1 and an excellent capacity retention of 92.4% after running 100 cycles at a rate of 1C and 60 °C. The study offers a superior alternative for the design of PEO-based solid composite electrolytes.

Improved thermal conductivity and flame retardancy in polystyrene/poly(vinylidene fluoride) blends by controlling selective localization and surface modification of SiC nanoparticles.

The effect of selective localization of silicon carbide (SiC) and polystyrene (PS)-coated SiC (p-SiC) nanoparticles on the thermal conductivity and flame retardancy of immiscible PS/poly(vinylidene fluoride) (PVDF) blends has been systematically studied. The scanning electron microscopy (SEM) images reveal that SiC and p-SiC nanoparticles have different selective localizations in the PS/PVDF blends. The melting and crystallization behaviors of the PVDF component investigated by using differential scanning calorimetry are consistent with the SEM results. To reduce the volume fraction of fillers in the composites, a cocontinuous structure of PS/PVDF has also been built up. The cocontinuity window for PS/PVDF blends is ∼30-70 vol % according to the selective solvent dissolution technique. The selective localization of SiC in the PVDF phase of the PS/PVDF 70/30 blends produces a slightly higher thermal conductivity than that of p-SiC in the PS phase of the PS/PVDF 30/70 blends. However, the composites with selective localization of p-SiC exhibit the best combined properties of thermal conductivity and flame retardancy.

Improved dielectric properties of nanocomposites based on poly(vinylidene fluoride) and poly(vinyl alcohol)-functionalized graphene.

In this work, two series of nanocomposites of poly(vinylidene fluoride) (PVDF) incorporated with reduced graphene oxide (rGO) and poly(vinyl alcohol)-modified rGO (rGO-PVA) were fabricated using solution-cast method and their dielectric properties were carefully characterized. Infrared spectroscopy and atom force microscope analysis indicated that PVA chains were successfully grafted onto graphene through ester linkage. The PVA functionalization of graphene surface can not only prevent the agglomeration of original rGO but also enhance the interaction between PVDF and rGO-PVA. Strong hydrogen bonds and charge transfer effect between rGO-PVA and PVDF were determined by infrared and Raman spectroscopies. The dielectric properties of rGO-PVA/PVDF and rGO/PVDF nanocomposites were investigated in a frequency range from 10² Hz to 107 Hz. Both composite systems exhibited an insulator-to-conductor percolating transition as the increase of the filler content. The percolation thresholds were estimated to be 2.24 vol % for rGO-PVA/PVDF composites and 0.61 vol % for rGO/PVDF composites, respectively. Near the percolation threshold, the dielectric permittivity of the nanocomposites was significantly promoted, which can be well explained by interfacial polarization effect and microcapacitor model. Compared to rGO/PVDF composites, higher dielectric constant and lower loss factor were simultaneously achieved in rGO-PVA/PVDF nanocomposites at a frequency range lower than 1 × 10³ Hz. This work provides a potential design strategy based on graphene interface engineering, which would lead to higher-performance flexible dielectric materials.

Surface-functionalized MWNTs with emeraldine base: preparation and improving dielectric properties of polymer nanocomposites.

A comparative study of the dielectric properties of poly(vinylidene fluoride) (PVDF) based nanocomposites with pristine multiwalled carbon nanotubes (MWNTs) and surface-modified MWNTs with core/shell structure (denoted as MEB) as fillers, was reported. Compared with MWNTs/PVDF composites, the MEB/PVDF composites exhibited lower loss tangent and higher dielectric permittivity. It is suggested that the conductive/nonconducting core/shell structure of the MEB filler is the main cause of the improved dielectric properties. Percolation based MWNTs networks is in charge of the improvement of dielectric permittivity, and the nonconducting emeraldine base layer of the MEB filler supports the low loss tangent and low conductivity in the MEB/PVDF composites.

Improving dielectric properties of BaTiO3/ferroelectric polymer composites by employing surface hydroxylated BaTiO3 nanoparticles.

Dielectric properties of poly(vinylidene fluoride) (PVDF) based nanocomposites filled with surface hydroxylated BaTiO(3) (h-BT) nanoparticles were reported. The h-BT fillers were prepared from crude BaTiO(3) (c-BT) in aqueous solution of H(2)O(2). Results showed that the dielectric properties of the h-BT/PVDF nanocomposites had weaker temperature and frequency dependences than that of c-BT/PVDF nanocomposites. Meanwhile, the h-BT/PVDF composites showed lower loss tangent and higher dielectric strength. It is suggested that the strong interaction between h-BT fillers and PVDF matrix is the main reason for the improved dielectric properties.