Insulator Behavior Research Articles

Nanoelectronics Using Metal-Insulator Transition.

Metal-insulator transition (MIT) coupled with an ultrafast, significant, and reversible resistive change in Mott insulators has attracted tremendous interest for investigation into next-generation electronic and optoelectronic devices, as well as a fundamental understanding of condensed matter systems. Although the mechanism of MIT in Mott insulators is still controversial, great efforts have been made to understand and modulate MIT behavior for various electronic and optoelectronic applications. In this review, recent progress in the field of nanoelectronics utilizing MIT is highlighted. A brief introduction to the physics of MIT and its underlying mechanisms is begun. After discussing the MIT behaviors of various Mott insulators, recent advances in the design and fabrication of nanoelectronics devices based on MIT, including memories, gas sensors, photodetectors, logic circuits, and artificial neural networks are described. Finally, an outlook on the development and future applications of nanoelectronics utilizing MIT is provided. This review can serve as an overview and a comprehensive understanding of the design of MIT-based nanoelectronics for future electronic and optoelectronic devices.

Ultralow thermal conductivity and high thermopower of a novel high-entropy (Sr0.2Ba0.2La0.2Eu0.2Pb0.2)Nb2O6 with tungsten bronze structure

High-entropy oxides (HEOs) have been extensively investigated as potential materials for thermoelectric applications. Here, a novel high-entropy (Sr0.2Ba0.2La0.2Eu0.2Pb0.2)Nb2O6 with a tungsten bronze structure was synthesized by solid-state method at high temperature, and tested its thermoelectric properties. The XRD, SEM-EDS, and TEM results showcase the successful integration of multi-component cations into the A site, yielding a single-phase material. The intrinsic thermal conductivity of this material is relatively low, measuring 0.78 W·m−1·K−1 at 323 K, thereby demonstrating exceptional thermal insulation behavior. This can be attributed to the highly disordered arrangement of multi-component cations with a distinctive high entropy configuration. At T = 1073 K, the high-entropy ceramic possesses the maximum power factor of 426.50 mW·m−1·K−2. As a result, an impressive thermoelectric figure of merit, ZT = 0.26, was achieved at 1073 K due to synergistic improvements in both electron transport and phonon scattering enabled by the high-entropy engineering strategy.

Mechanical, thermal insulation, and ablation behaviors of needle-punched fabric reinforced nanoporous phenolic composites: The role of anisotropic microstructure

Needle-punched fabric reinforced nanoporous phenolic composite (NPC) is a kind of promising ablative thermal protection material for spaceflight. However, in practical applications, typically anisotropic microstructure of NPC may lead to different performances and damage mechanisms under various directional mechanical or thermal loads. Herein, NPC is prepared and cut into specimens along three typical plane directions including XY-plane (0°), Z-plane (90°), and transitional-plane (45°), and their mechanical, thermal insulation, and ablation behaviors are systematically investigated. Benefiting from the woven fabric in XY-plane, NPC in 0°-plane direction exhibits highest tensile strength (169.2 ± 12.6 MPa), and CT image-based simulation further verifies that woven fabrics are primary load-bearing structure. Meanwhile, NPC in 45°-plane direction shows highest compressive strength (443.1 ± 18.2 MPa) but low compressive stress at low strain, demonstrating a weak bonding between two layers of woven fabrics. Moreover, the heat transfer simulation indicates that horizontally stacked woven fabrics effectively protect the internal material from thermal erosion, thus NPC in 0°-plane direction exhibits optimal thermal insulation. The ablation testing and micro-CT observations further demonstrate that the angle between woven fabric and thermal load significantly influences the ablation mechanism. The present work will further promote the structural reliability and optimization of needle-punched composites.

Flower-like Ni-Al/LDH synergistic polyaniline anchored to the carbon sphere surface to improve the fire performance of waterborne epoxy coatings

In this study, phytic acid -doped polyaniline (PPA) was loaded on the surface of carbon spheres, resulting in a classical C-N-P flame retardant system, that has a beneficial function for the formation of residual carbon. Subsequently, using this phosphorus-nitride carbon sphere (CS/PPA) as a carrier, flower-like Ni-Al-layered double hydroxide (Ni-Al/LDH) were uniformly grown on its surface, thus obtaining a novel nano-fire retardant with a graded structure to heighten the fire-resistant properties of aqueous epoxy resin. Importantly, the intervention of Ni-Al/LDH nanosheets not only imparts a gas-phase fire retardant function to the coating, but also the presence of Ni and Al bimetals can effectively catalyze the formation of more residual carbon in the combustion process. The highest residual carbon (30.7 %) in thermal weight loss experiments of the CS/PPA@Ni-Al/LDH/EP composite coating confirmed this view. Tests on the thermal insulation behavior of coatings illustrated that the backside temperature (168.8 °C) for CS/PPA@Ni-Al/LDH/EP was significantly smaller than that of other coatings, and that this optimum thermal shielding is inextricably linked to the high residual carbon and the residual metal oxides. Furthermore, the CS/PPA@Ni-Al/LDH/EP specimen shown the greatest expansion height (25.1 mm), expansion ratio (19.61) as well as the smallest smoke density rating (39.1 %), proving that its expansion property and smoke suppression were effectively improved. Moreover, the microstructure for the expanded carbon layer demonstrated that the CS/PPA@Ni-Al/LDH filled EP possessed a more complete and dense residual carbon after burning. Moreover, the high degree of graphitization of the residual carbon for CS/PPA@Ni-Al/LDH/EP determined its high thermal stability. The lowest cracked gas phase products and CO release of the CS/PPA@Ni-Al/LDH/EP composite coating during TG-IR testing confirmed its lowest fire toxicity. The development of such a composite flame retardant with a graded structure could help to advance the progress of waterborne intumescent fire coatings.

Improving the transient thermal response and ablation properties of epoxy‐modified silicone rubber composites by adding fluxing agent

AbstractIn this work, transient thermal response and ablation behavior of liquid silicone rubber composites containing fluxing/ceramic forming fillers were investigated under different heat flows using an oxyacetylene flame. The results indicated that the introduction of zinc borate (ZB) and aluminum oxide (Al2O3) effectively reduced the temperature at various depths of the samples, and they improved the thermal insulation properties and lowered pyrolysis rates. The above finding was attributed to the heat absorption arising from water release and melt filling as well as the vitrified reaction of solid melt due to the decomposition of ZB. Besides, the melting and exfoliation of Al2O3 and the formation of aluminum silicate (Al2SiO5) caused heat absorption effect. Additionally, the mass ablation rates and line ablation rates increased with rising heat flows coupling with a decrease of compressive strength of the char layers. In a nutshell, the effect of adding ZB/Al2O3 on the thermal insulation behavior of epoxy‐modified vinyl silicone rubber (EMVSR) composites under different heat flows was elucidtaed. This work served as a reference for the design and preparation of flexible ablative materials for thermal protection applications.

Flexible, Thermally Stable, and Ultralightweight Polyimide-CNT Aerogel Composite Films for Energy Storage Applications.

Polyimide (PI) aerogels are promising in various fields of application, ranging from thermal insulators to aerospace. However, they are typically in the form of a bulk monolith, which suffers from a lack of conformability and drapability. Moreover, their electrical conductivity is limited, and they mainly display an insulative behavior. These shortcomings can limit the applications of PI aerogels in energy storage systems, which require ultralightweight flexible conductive films, which at the same time offer high thermal stability, ultralow density, and high surface area. To overcome these obstacles, the present study reports the fabrication of PI-carbon nanotube (PI-CNT) aerogel composite films with varying CNT content prepared through a sol-gel preparation method, followed by a supercritical drying procedure. Compared to pristine PI aerogels, which displayed a large shrinkage and density of 18.3% and 0.12 g cm-3, respectively, the incorporation of only 5 wt % CNTs resulted in a significant reduction of both shrinkage and density to only 11.5% and 0.10 g cm-3, respectively. This suggests the importance of CNTs in improving the dimensional stability of aerogels and creating a robust network. Further characterizations showed that incorporation of 5 wt % CNTs also resulted in the highest pore volume (1.25 cm3 g-1), highest surface area (324 m2 g-1), highest real permittivity (80), highest electrical conductivity (3 × 10-1 S m-1), and ultrahigh service temperature (575 °C). It was also shown that the aerogel films can withstand a large degree of bending, can be twisted, and can be fully rolled with no obvious cracks propagated in the structure. The combined outstanding properties of the developed aerogel composite films make them promising potential candidates for supercapacitor electrodes. Therefore, the electrochemical performance of the devices based on aerogel electrodes was further studied. The device demonstrated a high energy density of 2.6 Wh kg-1 at a power density of 303.8 W kg-1. The total capacitance after 5000 cycles was 91.8% of the initial capacitance, which indicated excellent stability and durability of the device. Overall, this work provides a facile yet effective methodology for the development of high-performance aerogel materials for energy storage applications.

An investigation into the distinctive properties of LiAuS and NaAuS topological insulators

Topological insulators (TIs) have garnered significant interest in condensed matter physics and materials science due to their unique properties. Recently, Half-Heusler compounds have been predicted to exhibit topological insulator behavior based on electronic structure. We conducted a study on potential alternatives for optoelectronic, photorefractive, and photovoltaic devices. Using density-functional-theory (DFT) calculations, a systematic comparison of the structural, electronic, and optical properties of LiAuS and NaAuS is performed. The geometrically optimized structures of cubic LiAuS and NaAuS were compared with theoretical values. The electronic band structure revealed a non-trivial zero band gap at the G-point, and the influence of different orbitals on the conduction band (CB) and valence band (VB) was analyzed. The optical properties including absorption, reflectivity, index of refraction, extinction coefficient, conductivity, and dielectric function were examined over the energy spectrum ranging from 0 to 30 eV. LiAuS displays a notable absorption peak at 7.95 eV, whereas NaAuS exhibits a peak at 6.79 eV. The reflectivity peak, on the other hand, is higher at 19.79 eV and 8.29 eV for LiAuS, and at 6.74 eV for NaAuS. The reflective indices for LiAuS and NaAuS are determined to be 3.5 and 7.03, respectively. It is observed that LiAuS and NaAuS possess elevated optical conductivity at 6.26 eV and 6.14 eV, respectively. The conductivities of LiAuS and NaAuS in terms of their visual properties have been claimed to be 8.0(1/fs) and 6.84(1/fs), individually. In view of its high dielectric constant, LiAuS exhibits significant absorption peaks at 0.91 eV, 4.63 eV, and 13.63 eV, whereas NaAuS displays peaks at 0.07 eV, 4.45 eV, and 13.12 eV. Owing to their optical characteristics, LiAuS and NaAuS possess the potential for implementation in photorefractive and optoelectronic devices.

Chopped Straw as an Insulation Material: The Influence of Different Blow-In Technologies and Flame Retardants on Hygrothermal Properties

The demand for renewable resources in building construction is increasing, and wheat straw is an excellent option due to its superior environmental performance compared to traditional insulation materials. However, the hygrothermal properties of chopped wheat straw insulation have remained largely unexplored. At the moment, blown-in straw is only blown in vertically, although horizontal blowing would be more efficient depending on the situation. This study investigates the effect of different blowing techniques on the thermal properties of chopped wheat straw insulation, focusing on the difference between vertical and horizontal blowing techniques. In-situ-measured thermal conductivities were compared with design values used in energy balances. In addition, the long-term hygrothermal behavior of chopped wheat straw insulation treated with flame retardants was investigated. The methodology included heat flow plate measurements, needle probe measurements and laboratory measurements using the hot plate method. The results show that there is no significant difference in thermal performance between the blowing techniques. The measured thermal conductivities were lower than expected, challenging the current general normative moisture surcharge on the thermal conductivity of natural fiber insulation. The addition of the flame retardant had no noticeable effect on the hygrothermal properties of the chopped straw. Chopped wheat straw can be regarded as a highly ecological insulation material with great potential for the future.

Pyrolysis mechanism of different thermal aging of PVC insulation copper cable for home decoration

Thermal aging produces irreversible aging to home decoration PVC insulation copper cables through high temperatures, and their pyrolytic behavior changes as a result. The thermogravimetric-Fourier transform infrared analysis, the model-free method and the Málek method were used to investigate the effect of thermal aging on the pyrolysis mechanism of domestic PVC insulation copper cables. It was found that pyrolysis is more easily to occur in aged cables, and 20 days of aging is the most dangerous threshold for the pyrolysis of ZR-BVR cable insulation. The pyrolysis release gases were mainly concentrated in the temperature intervals of 270-340 and 400-700°C. In addition, it should be noted that different degrees of thermal aging affect the pyrolysis behavior of cable insulation but have little effect on the pyrolysis mechanism.

Effects of Approximating Recorded Lightning Currents With CIGRE Waveforms on Computed Fast-Front Overvoltages and Critical Lightning Currents Causing Flashover to Overhead Transmission Lines

This work investigates through simulations with the ATP-EMTP program the application of recorded lightning current waveforms to the estimation of fast-front overvoltages and critical lightning currents causing flashover to overhead transmission lines. Typical lines are evaluated with both horizontal (single-circuit) and vertical (double-circuit) phase configurations and AC voltage from 66 kV up to 765 kV. Backflashover as well as shielding failure flashover simulations were performed for several recorded first return-stroke current waveforms of negative downward lightning flashes, as reported in literature. Their approximations with the widely-used CIGRE lightning current waveform were also employed in simulations. CIGRE waveforms, considering the statistical distributions of the front time, maximum steepness, and time to half value, were used as well. A comparison between double-peak waveforms, composed of seven Heidler functions, and their CIGRE approximations is made. The latter generally yield conservative values of overvoltages and critical backflashover currents. The leader development models used for assessing the behavior of insulators and long air gaps under fast-front overvoltages are discussed regarding the criterion for the termination of leader propagation yielding withstand of line insulation.

Photochromic transparent bamboo composite with excellent optical and thermal management for smart window applications

In the face of the frequent occurrence of many global climate anomalies in recent years, the concept of green and sustainable development has driven a growing interest among researchers in the development and utilization of renewable biomass resources. Transparent wood (TW) has become a hot research topic due to its transparency, sustainability, and multi-functionality. However, the long growth period of wood and the shortage of wood resources have hindered the large-scale application of TW. Although bamboo has also been shown to have the capability to prepare transparent materials, the preparation of transparent bamboo (TB) with advanced functionality and large dimensions has always been a challenge. In this study, a straightforward but efficient method for preparing photochromic transparent bamboo (PTB) with enlarged dimensions (100 mm × 100 mm × 0.9 mm) is introduced. The method involves pre-crosslinking the delignification of sliced bamboo veneer (SBV) and impregnating delignified bamboo (DB) substrates with epoxy resin mixed with photochromic pigments (PC). The PTB composites that emerged displayed impressive characteristics such as excellent optical performance, rapid photochromic response, thermal insulation behavior, and robustness. The PTB’s excellent overall performance is promising to be applied in some high-end fields, such as smart windows, information storage, smart decorative materials, etc.

Modeling and Predicting the Mechanical Behavior of Standard Insulating Kraft Paper Used in Power Transformers under Thermal Aging

The aim of this research is to predict the mechanical properties along with the behaviors of standard insulating paper used in power transformers under thermal aging. This is conducted by applying an artificial neural network (ANN) trained with a multiple regression model and a particle swarm optimization (MR-PSO) model. The aging of the paper insulation is monitored directly by the tensile strength and the degree of polymerization of the solid insulation and indirectly by chemical markers using 2-furfuraldehyde compound content in oil (2-FAL). A mathematical model is then developed to simulate the mechanical properties (degree of polymerization (DPV) and tensile index (Tidx)) of the aged insulation paper. First, the datasets obtained from experimental results are used to create the MR model, and then the optimizer method PSO is used to optimize its coefficients in order to improve the MR model. Then, an ANN method is trained using the MR-PSO to create a nonlinear correlation between the DPV and the time, temperature, and 2-FAL values. The acquired results are assessed and compared with the experimental data. The model presents almost the same behavior. In particular, it has the capability to accurately simulate the nonlinear property behavior of insulation under thermal aging with an acceptable margin of error. Since the life expectancy of power transformers is directly related to that of the insulating paper, the proposed model can be useful to maintenance planners.

Dynamic Properties of Timber–Concrete Composite Beams with Crossed Inclined Coach Screw Connections: Experimental and Theoretical Investigations

Due to the low density and stiffness of wood, traditional timber floor systems are prone to producing large vibration responses. By combining timber beams with concrete floors, timber–concrete composite (TCC) floor systems show stronger bearing capacity, higher bending stiffness, and better thermal and sound insulation behaviors when compared with traditional timber floor systems. In this study, the vibration performance of TCC beams with crossed inclined coach screw connectors was investigated using dynamic tests. The influence of the screw diameters and slab dimensions on the dynamic performance of the composite beams was evaluated. The test results demonstrated that TCC beams show good dynamic performance when used as a floor component and meet the preliminary requirements of floor vibration comfort for fundamental frequency. The fundamental frequency and damping ratio of TCC beams decreases with the increase in slab dimensions. The bending stiffness and natural frequency of TCC beams decrease smoothly when reducing the screw diameter from 16 to 12 mm. Additionally, two theoretical models were used to predict the natural frequencies of the TCC beams, and the predicted values show good consistency with the measured ones.

Study on the Behavior of Low-Voltage Cable Insulation Subjected to Thermal Cycle Treatment

Abstract The paper presents a study on the effects of thermal aging on a low-voltage cable. Cable samples with a length of 50 cm were subjected to thermal cycling from +140 °C to −20 °C in a climate chamber with controlled temperature. This treatment compares the short-term failure or the emergency overload of some cables under real operating conditions at low temperatures. The effect of the temperature cycle is investigated through electrical measurements such as dielectric spectroscopy, voltage response measurements, and mechanical measurements such as Shore D hardness investigation. The results revealed that the thermal cycles of accelerated aging caused the plasticizer to diffuse into the jacket and then be released into the environment. At the same time, the thermal shocks produced microcracks in the cable jacket, accelerating the removal of the plasticizer.

Structural and topological phase transitions in Se doping-controlled intrinsic magnetic topological material FeBi2Te4

Defects profoundly affect the magnetic, electronic structure and topological behaviour of intrinsic magnetic topological insulators. Here, we investigate the magnetic, structural stability and topological properties of different Te atomic sites in the intrinsic magnetic topological insulator FeBi2Te4 with Se substitution of the FM-z magnetic order. When Se replaces the outermost site of the septuple layer (the Te atomic layer connected by van der Waals bonds), the Se–Bi bond length formed with its nearest neighbour Bi is drastically shortened and the lattice constant and volume contract accordingly. We speculate that this defect-induced chemical bond enhancement is related to the strong electronegativity of Se over Te. In contrast, the system has lower formation energy, a more stable structure, and enhanced magnetic moment when Se replaces the Te atomic layer inside the septuple layer. Also, we reveal a variety of topological phase transitions due to substitution defects. FeBi2Te4 is considered to belong to the topological classification of higher-order topological insulators with z 4 = 2. The system after Se substitution can be transformed into Weyl semimetals, strong 3D topological insulators, and unknown topological materials without symmetry-base indicators, respectively.

Pyrolysis Behavior, Pyrolysis Kinetics, Heat Transfer and Pyrolysis Simulation of Almandine/Boron Phenolic Resin Ceramizable Composites

Ceramizable composites have attracted great interest for the thermal protection of hypersonic vehicles. In our research described here the pyrolysis process of almandine/boron phenolic resin ceramizable composites was investigated; we note it can be divided into three stages (room temperature to 300 °C, 300 °C to 700 °C and 700 °C to 1300 °C). In addition, the pyrolysis products were identified by an analytical pyrolyzer coupled with a gas chromatography/mass spectrometry set-up (Py-GC/MS), and they were composed of phenols (69.82%), aromatic hydrocarbons (23.89%), and aromatic ethers and aliphatic esters (6.29%). Furthermore, the possible pyrolysis routes were proposed. Moreover, the pyrolysis kinetic parameters were calculated, and the activation energy, order of the reaction and pre-exponential factor were 54.18 KJ/mol, 0.869 and 2.828 s−1, respectively. In addition, Comsol Multiphysics finite element software was used to simulate the heat transfer and pyrolysis behavior. After the composite (with a thickness of 10 mm) was heated for 200 s at a heat flux of 480 KW/m2, the surface and back surface temperatures were 1200.6 °C and 163.7 °C, respectively. The thicknesses of the carbonized, pyrolysis and original layers were 2.11, 3.67 and 4.22 mm, respectively, which reflects the good thermal insulation behavior of the composite. We suggest the combination of calculation and simulation results can provide a theoretical basis for the design of thermal protection systems for hypersonic vehicles which are able to meet long-term and large-area thermal protection requirements.

Study on interface insulation properties of Al2O3 epoxy composites using plasma jet‐fluorinated etching/SiOx deposition

AbstractDue to their superior mechanical and electrical characteristics, Al2O3 epoxy composites (Al2O3/EP) are extensively utilized in producing insulation devices. However, Al2O3/EP is prone to accumulating a large amount of charge under a direct current (DC), which threatens the operation of the power system. In this article, we conducted surface fluorinated etching and SiOx deposition on the material by atmospheric pressure plasma jet (APPJ). Various testing methods were used to investigate the impacts of plasma modification on the physicochemical characterization of Al2O3/EP, and the changes in superficial conductivity, flashover voltage, charge dissipation rate, and trap distribution were experimentally tested. The influences of two plasma modification methods on the physicochemical characterization of Al2O3/EP were analyzed. The enhancement mechanism of Al2O3/EP flash behavior by fluorinated etching and SiOx deposition is summarized. The findings demonstrate that the insulation behavior of Al2O3/EP results from a combination of a few dominant and multiple factors. Fluorinated etching enhances the degree of roughness and the relative proportion of the F element, provides chemically deep traps, and raises the flashover voltage by up to 19%. SiOx deposition boosts the dispersion of charges by forming SiOx layers on the surface, reducing the degree of roughness and increasing the content of the silicon–oxygen element, making it more difficult for charges to accumulate and increasing the flashover voltage by up to 15%. This study will reveal the internal relationship between gas–solid interface and surface flashover, and provide a possible scheme for suppressing surface flashover.Highlights Modifications substantially increase the flashover voltage. Modifications allow a wide range of interface parameters to be adjusted. The effects of modifications on physicochemical properties are compared. The enhancement mechanism of flashover voltage by modifications is compared. The overall effect of multiple factors on insulation properties is analyzed.

Validation of a Qualification Procedure Applied to the Verification of Partial Discharge Analysers Used for HVDC or HVAC Networks

The insulation condition of HVDC grids consisting of cable systems, GIS, and converters should be monitored by partial discharge (PD) analysers using artificial intelligence (AI) tools for efficient insulation diagnosis. Although there are many experiences of PD monitoring solutions developed for the supervision of the insulation condition of HVAC grids using PD analysers, there are no standardised requirements for their qualification available yet. The international technical specification TS IEC 62478 provides general rules for PD measurements using electromagnetic methods but does not define performance requirements for qualification tests. HVDC and HVAC PD analysers must be tested by unambiguous test procedures. This paper compiles experiences of using PD analysers with HFCT sensors in HVAC grids (cable systems, GIS, and AIS) to define a qualification procedure for HVAC systems. This procedure is applicable to HVDC grids (cable systems, GIS, AIS, and converters) because the particularities related to the insulation behaviour under HVDC voltage are also considered. Representative PD sources are discussed in HVAC and HVDC positive and negative polarity. The PD pulse trend of representative insulation defects in HVDC cable systems is quite different from that of HVAC grids. Special attention should be paid to the acquisition of PD signals in HVDC grids since few pulses appear in solid insulations, mainly during voltage changes (polarity reversals or surges), but rarely in continuous operation with constant direct voltage. A synthetic PD simulator has been developed to reproduce trains of PD pulses or noise signals, similar to those that can appear in the power network. A set of three functionality tests has been developed for qualification of the diagnostic capabilities of PD analysers working up to 30 MHz addressed to HVDC or HVAC grids: (1) PD recognition test, (2) PD clustering test, and (3) PD location test. This qualification procedure has been validated by means of a round-robin test performed by five research institutes (RISE, FFII, TUDelft, TAU, and UPM) using commercial and in-development AI PD recognition and clustering tools to demonstrate its robustness and applicability. Applying this qualification procedure, two PD methods for electrical detection and prevention of insulation defects have been approved, one for HVAC and the other for HVDC grids.

Tuning the band gap in graphene nanoflakes via various impurities

The electronic properties of GNFs are controlled by utilizing different impurities, such as boron, nitrogen and oxygen. The differences in the electronic properties of the pristine GNFs with and without various concentrations of B, N and O impurities in different sites are calculated by utilizing the DFT method. The results show that the pristine GNFs has insulator behavior. By using a single B, N or O impurity, the electronic band gap is reduced and the behavior is changed from insulator to semiconductor behavior. Our findings show that the electronic properties of single (B, N or O)-doped GNFs depended on these impurities and the location of the impurities. By increasing the concentrations of these impurities, the GNFs still have semiconductor behavior, but the electronic band gap is increased by increasing the number of C atoms between these impurities. These are very interesting results: the electronic properties depend on the number of C atoms between these impurities. The Fermi level is shifted up with all these impurities, except B2 case (c), B2 cases (a and b) and B4 cases (b and c) doped GNFs, which have the opposite behavior. These impurities make the GNFs more stable and less reactive due to the total energy being increased by the increasing the number of impurities and changing the sites of these impurities inside the GNFs, although the opposite happens for B cases (b and c), B2 cases (a, b, and d), and B4 (all cases) doped GNFs. In brief, the electronic properties depended on the concentrations and the sites of these impurities, which can be utilized in various applications.

Fabrication of one-step shape memory gradient sound absorber with wrinkled inner wall and closed-pore structure

Plastic foam sound-absorbing material is of great significance to improve indoor sound environment. However, since the preparation of foamed material is fundamentally induced to form a homogeneous pore structure, the sound absorption frequency range is relatively narrow, limiting the sound absorption performance to a great extent. In order to improve the sound absorption performance in a wide frequency range, this work designed the combination of macro-porous gradient foam (MGF) and closed-pore resonator, introduced part of polyimide (PI) components based on the one-step free foaming method of polyurethane foam (PUF), and proposed a straightforward route to prepare basic polymer gradient foam. Through the arrangement and combination of different bubble holes, the sharp hole structure and the Helmholtz resonance structure were constructed by analogy. Importantly, the internal structure of the foam system was reorganized under the action of surface tension due to the inward collapse of the viscous bubbles during the process of bursting, resulting in the formation of wrinkled structures and small bubbles, which enhanced the resonant sound absorption at lower frequencies. In 200–6400 Hz, the MGF1 sample had the highest sound absorption capacity, with a cavity size range of 8.27–4.14 mm and the ability to absorb 90% of the acoustic wave above 1100 Hz. In addition, MGF samples also exhibited heat insulation behavior (0.047–0.061 W/(m·K)), shape memory, and energy absorption capacity. This gradient compound resonance structure satisfies the convenience of process manufacturing and high-efficiency sound absorption performance, which is essential for the wide application of plastic foam sound-absorbing materials.