Ultralow Density Research Articles

Ultralow-density carbon foam composites with bean-like Co-embedded carbon nanotube whiskers towards high-performance microwave absorption

We describe the design and fabrication of an ultralight carbon foam composite with special bean-like Co-embedded carbon nanotube whiskers used for high-performance microwave absorption materials. A facile approach was developed to create this carbon foam with the self-growth of carbon nanotubes on low-cost cobalt-decorated commercial melamine sponges (MF) via a simple thermal annealing process. It was found that a large number of bean-like carbon nanotubes whiskers (1D) embedded with Co nanoparticles (0D) grew on the surface of carbon skeleton (3D) under the action of surface autocatalysis. This unique hierarchical micro/nanostructure not only reduced the density of the materials but also effectively improved their thermal oxidation stability and microwave absorption ability. The resultant foam showed an excellent microwave absorption performance with the combined advantages of ultralow density (9.4 mg/cm3), strong absorption (minimum reflection loss value of −46.2 dB), wide absorption width (4.8 GHz), and high thermal oxidation stability (>350 °C). The RL values of the foam could be all below −10 dB at the frequency from 4 to 18 GHz covering 87% of the whole test frequency range. These remarkable results also provide a potentially scalable, simple, and cost-effective strategy for designing metal-embedded carbon composite nanostructures for catalysis, electrochemistry, and other applications.

Ultralight, hydrophobic, sustainable, cost-effective and floating kapok/microfibrillated cellulose aerogels as speedy and recyclable oil superabsorbents

Cellulose aerogels achieve excellent absorption of waste oil and organic pollutant, which has received lots of attention recently. It is still a big challenge to obtain aerogels with both high cost-effectiveness and advanced oil absorption performance, since it is a time-consuming, and environmentally unfriendly process to obtain cellulose, compared with direct usage of natural fibers. In this manuscript, we develop highly porous and hydrophobic kapok/microfibrillated cellulose (MFC) aerogels with a dual-scale hierarchically porous structure at micro-level as cost-effective, sustainable, and floating superabsorbents via simple vacuum freeze-drying and surface modification. Kapok, a natural hollow fiber, has been recently considered as a new sustainable resource for oil cleanup. By partially replacing MFC with chopped kapok fibers in MFC aerogels (MMAs), the resultant kapok/MFC aerogels (KCAs) exhibit ultralow density (5.1 mg/cm−3), ultrahigh porosity (99.58%) and hydrophobicity (140.1°) leading to advanced oil sorption (130.1 g/g) that is 25.3% higher than that of MMAs. In addition, these KCAs can rapidly and selectively absorb waste oil from oil-water mixture with ultrahigh absorption ability of 104–190.1 g/g, which is comparable to other environmentally unfriendly and high-cost aerogels. Furthermore, the KCAs own excellent reusability and sustainability. These benefits enable the KCAs a suitable alternative to clean oil spills.

Facile synthesis and electromagnetic wave absorption properties of silver coated porous carbon composite materials

Biomass porous carbon (PC) has become a novel microwave absorbing material because of its ultralow density, high dielectric loss capability and friendly to environment. In this study, we prepare Ag/PC composite materials by a simple hydrothermal method from 120 °C to 160 °C. By changing the reaction temperature, the electromagnetic parameters are adjusted and the absorption performance is optimized. XRD results prove the successful preparation of Ag/PC composite materials at various temperatures. SEM and TEM images show the Ag particles are attached on the outer surface and inner wall of PC. The compositing of Ag particles promotes the graphitization of porous carbon and forming a conductive network, which help to improve the complex permittivity. The Ag/PC composite material synthesized at 160 °C exhibits the optimal absorbing performance. The reflection loss value below −10 dB reaches in the frequency range of 10.4–12.4 GHz for the sample in 1.8 mm thickness, with a minimum value of −35.4 dB at 11.56 GHz. The results demonstrate that incorporating with Ag is an efficient way to improve the absorbing performance of PC. This work helps to develop high performance biomass porous carbon based microwave absorption materials.

Utilization of LTEx Feynman Gate in Designing the QCA Based Reversible Binary to Gray and Gray to Binary Code Converters

Aims: The Quantum-dot Cellular Automata explores a unique perspective in the arena of the architectural design of future quantum computers, precisely due to its ultra-low packing density, high operating speed, and low power dissipation. On the other side, reversible computing allows the implementation of extreme low power-consuming circuits by avoiding energy dissipation during the time of computation. Objective: In this paper, we have explored the QCA design of reversible binary to gray and gray to binary code converters based on the application of a unique model of Feynman gate using the layered T exclusive-OR module (abbreviated in this work as LTEx Feynman gate). Methods: We have proposed algorithms to produce multi-control reversible binary to gray and gray to binary code converters and to develop cost-efficient QCA layouts. Results: Our systematic literature survey on the existing QCA designs of reversible binary to gray and gray to binary code converters helped us to compare and analyze the proposed design with the existing ones and identify it as the best design in terms of reversible, and QCA design metrics. Conclusion: Significant improvements in design metrics owing to successful experimentations over the previous designs are reported while instantiating 3X3,4X4, and 8X8 counterpart layouts.

Demonstration of bright x-ray sources from solid and foam TiO2 targets at the Shenguang-III prototype laser facility

Enhancement of x-ray sources from laser-produced plasmas is significant in wide-ranging applications. Solid and foam TiO2 targets were both used to generate bright x-ray sources at the Shenguang-III prototype laser facility, with a total laser power of 3.2 TW. The new foam targets were with an ultra-low initial density of 8.1 mg/cm3 and a high Ti fraction of 33 sat. %. By absolute measurements, the multi-keV x rays of the 4 keV–7 keV band and 1.6 keV–4.4 keV band and the total x rays above 0.1 keV of the foam target have simultaneously shown conversion enhancements of 1.4, 3.1, and 2.3 times, respectively, compared with the solid target. A much larger emission volume and an average electron temperature of 3.2 keV by moving the foci of laser beams inner were obtained for the foam target, providing a good condition for multi-keV x-ray production.

Efficient macroporous adsorbent for phosphate removal based on hydrate aluminum-functionalized melamine sponge

Effective removal of phosphate from aqueous solution is of great significance in environmental protection. In this paper, the concept of developing a hierarchical and cost-effective aluminum functionalized melamine foam (MF) as phosphate adsorbent is presented. The material with 3D interconnected network, flexibility and ultralow density was fabricated via a simple adsorbing method based on hydrate aluminum and commercially available MF sponge. For use as an adsorbent, performance of the as-prepared MF-HA sponge was assessed with various pH, temperatures, adsorption time, initial P concentrations, ionic strengths and co-existing ions. The experimental results demonstrated that MF-HA performed well in a wide pH range of 4–10 and in the presence of foreign anions, with high maximum uptake capacity of 36.21 mg P/g. Furthermore, for the as-prepared material, the phosphate removal performance at different temperatures established the endothermic nature of the adsorption process. Moreover, MF-HA sponge could be directly separated from the solution and showed a satisfactory recyclability. The adsorbing strategy to design and fabricate the novel sponge with resultant high adsorption efficiency, cost-effectiveness, preferable selectivity, environmental friendliness, and easy separation from water may promote sponge functionalization for applications in extensive wastewater treatment areas.

Ultraflexible Integrated Organic Electronics for Ultrasensitive Photodetection

AbstractFlexible organic photodetectors can form seamless contact with human skin, enabling continuous health monitoring. However, developing flexible photodetectors with high detectivity remains challenging because of the high theoretical dark current of photodetectors using a bulk heterojunction (BHJ) structure. Herein, a simple approach is reported that integrates a field‐effect transistor (FET) and an organic photodiode (OPD) module on the same substrate to achieve an ultralow dark current density (3.0 × 10−8 mA cm−2), ultrahigh detectivity (1.7 × 1015 Jones), and excellent flexibility. In the integrated sensor, the light‐sensing process occurs in the OPD module, whose signal is used to control the gate of the FET where the current output process occurs, enabling to utilize the light‐sensing properties of the OPD's BHJ structure while bypassing its high dark current. The ultraflexible integrated sensor amplifies the photoplethysmogram signal intensity from the OPD module by a factor of ≈10, thereby confirming its potential as an indoor wearable biosensor.

Non-visible transformations of chaotic attractors due to their ultra-low density in AC–DC power factor correction converters

Recently, it has been shown that DC–AC and AC–DC power converters whose dynamics is governed by two vastly different frequencies lead to a special class of piecewise smooth models characterized by a practically unpredictable number of switching manifolds between partitions in the state space associated with different dynamics. In the previous publications, we have shown that transformations of chaotic attractors in such models can be caused by their interactions with unstable cycles occurring in the regions of the attractors associated with an ultra-low invariant density, which is beyond a practical observability in physical or numerical experiments. The appearance of such low-density regions has been explained by the so-called spiking phenomenon. In the present paper, we make the next step and ask the natural question what are the reasons behind the appearance of spikes. Applying a novel technique (cobweb diagrams for non-autonomous maps), we explain a connection between spiking and the presence of two vastly different frequencies.

Antibacterial Activity of Linezolid against Gram-Negative Bacteria: Utilization of ε-Poly-l-Lysine Capped Silica Xerogel as an Activating Carrier.

In recent times, many approaches have been developed against drug resistant Gram-negative bacteria. However, low-cost high effective materials which could broaden the spectrum of antibiotics are still needed. In this study, enhancement of linezolid spectrum, normally active against Gram-positive bacteria, was aimed for Gram-negative bacteria growth inhibition. For this purpose, a silica xerogel prepared from a low-cost precursor is used as a drug carrier owing to the advantages of its mesoporous structure, suitable pore and particle size and ultralow density. The silica xerogel is loaded with linezolid and capped with ε-poly-l-lysine. The developed nano-formulation shows a marked antibacterial activity against to Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus. In comparison to free linezolid and ε-poly-l-lysine, the material demonstrates a synergistic effect on killing for the three tested bacteria. The results show that silica xerogels can be used as a potential drug carrier and activity enhancer. This strategy could provide the improvement of antibacterial activity spectrum of antibacterial agents like linezolid and could represent a powerful alternative to overcome antibiotic resistance in a near future.

MXene-Modulated Electrode/SnO2 Interface Boosting Charge Transport in Perovskite Solar Cells.

Interface engineering is imperative to boost the extraction capability in perovskite solar cells (PSCs). We propose a promising approach to enhance the electron mobility and charge transfer ability of tin oxide (SnO2) electron transport layer (ETL) by introducing a two-dimensional carbide (MXene) with strong interface interaction. The MXene-modified SnO2 ETL also offers a preferable growth platform for perovskite films with reduced trap density. Through a spatially resolved imaging technique, profoundly reduced non-radiative recombination and charge transport losses in PSCs based on MXene-modified SnO2 are also observed. As a result, the PSC achieves an enhanced efficiency of 20.65% with ultralow saturated current density and negligible hysteresis. We provide an in-depth mechanistic understanding of MXene interface engineering, offering an alternative approach to obtain efficient PSCs.

Green preparation of 3D large-sized SiC nanowire aerogels

Due to the combination of ultralow density, high thermal insulation, high-temperature resistance and good chemical stability, SiC aerogels show considerable potential for applications even in harsh conditions. However, the difficulty and cost involved in manufacturing them in large sizes severely limited their practical applications. Herein, we developed a green and economic method for fabrication of 3D large-sized SiC nanowire aerogels using Si powder as silicon source and bread as template as well as carbon source. The prepared SiC nanowire aerogel possesses ultralow density, large size, excellent high-temperature stability, and low thermal conductivity (0.038 W m−1 K−1 at 200 °C in air).

Skyrmion lattice creep at ultra-low current densities

Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities jc to depin from the underlying atomic lattice. Above jc skyrmions exhibit well-controlled motion, making them prime candidates for race-track memories. In thin films thermally-activated creep motion of isolated skyrmions was observed below jc as predicted by theory. Uncontrolled skyrmion motion is detrimental for race-track memories and is not fully understood. Notably, the creep of skyrmion lattices in bulk materials remains to be explored. Here we show using resonant ultrasound spectroscopy—a probe highly sensitive to the coupling between skyrmion and atomic lattices—that in the prototypical skyrmion lattice material MnSi depinning occurs at {j}_{c}^{* } that is only 4 percent of jc. Our experiments are in excellent agreement with Anderson-Kim theory for creep and allow us to reveal a new dynamic regime at ultra-low current densities characterized by thermally-activated skyrmion-lattice-creep with important consequences for applications.

Solvent Vapor Strengthened Polyimide Nanofiber-Based Aerogels with High Resilience and Controllable Porous Structure.

Owing to the hierarchically three-dimensional (3D) network, ultralow density, and high porosity, nanofiber-based aerogels (NFAs) have drawn great attention recently. However, precise control of the porous structure and mechanical properties of NFAs, which have been proved to be extremely essential to the applications, still remains a major challenge. Herein, electrospun polyimide (PI) nanofibers were utilized as building blocks to construct NFAs through the solid-templating technique. The porous structure of PI nanofiber-based aerogels (PI-NFAs) could be adjusted by changing the processing parameters. By further welding the adjacent nanofibers at the contact sites with solvent vapor, high-resilience PI-NFAs were successfully prepared with comparable or higher recoverable, under compression, folding and torsion relative to other NFAs. The welded PI-NFAs showed ultralow density (minimum of 0.96 mg/cm3), high porosity (maximum of 99.93%), and tunable hierarchical structure. Therefore, this study brought a new perspective on the simple preparation of high-resilience nanofiber-based aerogels with tunable porous structures.

Excellent Passivation of n‐Type Silicon Surfaces Enabled by Pulsed‐Flow Plasma‐Enhanced Chemical Vapor Deposition of Phosphorus Oxide Capped by Aluminum Oxide

Phosphorus oxide (POx) capped by aluminum oxide (Al2O3), prepared by atomic layer deposition (ALD), has recently been introduced as a surface passivation scheme for planar n‐type FZ silicon. In this work, a fast pulsed‐flow plasma‐enhanced chemical vapor deposition (PECVD) process for the POx layer is introduced, making it possible to increase the POx deposition rate significantly while maintaining the POx/Al2O3 passivation quality. An excellent surface passivation is realized on n‐type planar FZ and Cz substrates (J0 = 3.0 fA cm−2). Furthermore, it is demonstrated that the POx/Al2O3 stack can passivate textured surfaces and that the application of an additional PECVD SiNx capping layer renders the stack stable to a firing treatment that is typically used in fire‐through contact formation (J0 = 12 fA cm−2). The excellent surface passivation is enabled by a high positive fixed charge density (Qf ≈ 4 × 1012 cm−2) and an ultralow interface defect density (Dit ≈ 5 × 1010 eV−1 cm−2). Finally, outstanding passivation is demonstrated on textured silicon with a heavy n+ surface doping, as is used in solar cells, on par with alnealed SiO2. These findings indicate that POx/Al2O3 is a highly suited passivation scheme for n‐type silicon surfaces in typical industrial solar cells.

Assembly of Ultralight Dual Network Graphene Aerogel with Applications for Selective Oil Absorption.

High-performance graphene aerogels with well-developed internal structures are generally obtained by means of introducing additive materials such as carbon nanotubes, cellulose, and lignin into the aerogel network, which not only enhances the cost but also complicates the preparation process. Therefore, tailoring the internal structure of pristine graphene aerogel in a feasible way to achieve high performance is of great significance to the practical applications. Herein, a novel cysteamine/l-ascorbic acid graphene aerogel (CLGA) was fabricated by a simple one-step hydrothermal method followed by freeze-drying. Through the creative combination of the reducing agent l-ascorbic acid and cross-linking agent cysteamine, a dual-network structure was constructed by both layered physical stacking and vertical chemical cross-linking. The addition of cysteamine not only enhanced the reduction degree but also assisted the formation of more vertical connections between graphene nanosheets, resulting in more abundant pores with smaller sizes compared with graphene aerogels prepared by the traditional hydrothermal reduction method. CLGA possessed an ultra-low density of 4.2 mg/cm3 and a high specific surface area of 397.9 m2/g. As expected, this dual-network structure effectively improved the absorption capacity toward a variety of oil and organic solvents, with an outstanding oil absorption capacity up to 310 g/g. Furthermore, CLGA possessed good mechanical properties and oil/water selectivity. The absorbed oil could be recovered by both continuous absorption-removal process and mechanical squeezing, making the as-prepared aerogel superior absorbent material for a variety of applications, such as selective oil absorption and water treatment.

Photo detection and modulation from 1,550 to 2,000 nm realized by a GeSn/Ge multiple-quantum-well photodiode on a 300-mm Si substrate.

A GeSn/Ge multiple-quantum-well (MQW) p-i-n photodiode structure was proposed for simultaneously realizing high detectivity photo detection with low dark current and effective optical modulation based on the quantum confined Stark (QCSE) effect. The MQW stacks were grown on a 300-mm Ge-buffered Si substrate using reduced pressure chemical vapor deposition (RPCVD). GeSn/Ge MQW p-i-n photodiodes with varying mesa diameters were fabricated and characterized. An ultralow dark current density of 16.3 mA/cm2 at -1 V was achieved as expected due to the low threading dislocation density (TDD) in pseudomorphic GeSn layer. Owing to the ultralow dark current density and high responsivity of 0.307 A/W, a high specific detectivity of 1.37×1010 cm·Hz1/2/W was accomplished at 1,550 nm, which is comparable with commercial Ge and extended-InGaAs photodetectors. Meanwhile, the bias voltage-dependent photo response was investigated from 1,700 to 2,200 nm. The extracted effective absorption coefficient of GeSn/Ge MQW shows a QCSE behavior with electric field-dependent exciton peaks from 0.688 to 0.690 eV. An absorption ratio of 1.81 under -2 V was achieved at 2 μm, which shows early promise for effective optical modulation. The high frequency response was calculated theoretically, and the predicted 3-dB bandwidth for the photodiode with a mesa diameter of 30 μm could reach 12 GHz at -2 V.

Novel Band-Edge Work Function Performance Modulation via NPT with PMOS1st/NMOS1st Laminated Stack for PMOS Low Power Target

In this paper, the band-edge work function performance is systematically investigated and modulated via novel nitrogen plasma treatment (NPT) with the advanced PMOS1st (TiN/TiN/TiAlC) and NMOS1st (TiN/TiN) laminated stacks for the fabricated PMOS capacitors. The basic multi-VT performance is strongly modulated by controlling NPT process. 1) Flatband voltage (VFB) shifts towards band edge are obtained as +120 mV (undiluted), +430 mV (diluted) for PMOS1st and +80 mV (undiluted), +210 mV (diluted) for NMOS1st. 2) By manipulating the NPT process from undiluted and diluted case, it can provide significant high band-edge effective work function ranging from 4.89 eV (undiluted) to 5.21 eV (diluted) for PMOS1st and 5.22 eV (undiluted) to 5.35 eV (diluted) for NMOS1st laminated stack, respectively. 3) NPT diluted with hydrogen is observed to maintain ultralow bulk trap density (1.11 × 1011 cm−2 for PMOS1st and nearly zero for NMOS1st) and interface trap density (3.34 × 1011 eV−1 cm−2 for PMOS1st and 6.45 × 1011 eV−1 cm−2 for NMOS1st). The significant band-edge work function modulation and very low bulk and interface trap density demonstrate the novel NPT with PMOS1st/NMOS1st laminated stack is very promising to achieve the target of PMOS low-power application in the further technology node.

Modulating electromagnetic interference shielding performance of ultra-lightweight composite foams through shape memory function

High-performance electromagnetic interference shielding (EMI) materials are desirable in aerospace, military and mobile electronics applications. However, fabrication of smart and ultra-lightweight EMI shielding materials with adjustable EMI shielding performance through an energy-efficient and environmental-friendly process is still challenging. Herein, we demonstrate using a conductive biomass-derived d-glucaric acid-chitosan/single-walled carbon nanotube (SWNT/GA-chitosan) polymer composite layer and a crystallized paraffin layer to fabricate ultra-lightweight polyurethane foams with shape memory and adjustable EMI shielding functions, of which the shape change via external stimulus enables the foams to adjust EMI shielding efficiency autonomously. The foam coated with synthesized conductive composite layers shows an exceptional EMI shielding effectiveness of 56 dB at an ultra-low density of 0.03 g/cm3 with only 0.171 vol% SWNT. The foam also exhibits ultra-high durability over 2000 compression-recovery cycles. Furthermore, introduction of paraffin layer as a reversible network results in shape memory foams with high fixity ratio >95% and recovery ratio >90%. These foams exhibit a wide range of adjustable EMI shielding efficiency from 18 dB to 30 dB by controlling their macroscopic shape from compression. Such smart foams with shape memory and adjustable EMI shielding functions fabricated by an environmental-friendly strategy represent a promising material candidate for the wide applications of EMI shielding.

Robust and lightweight biofoam based on cellulose nanofibrils for high-efficient methylene blue adsorption

Robust and ultralight biofoams had been successfully prepared using readily available and biocompatible cellulose nanofibrils (CNFs) as the matrix. The γ-glycidoxypropyltrimethoxysilane (GPTMS) was first added into the CNFs suspension to act as a crosslinker to form covalent linkages between cellulose chains. Then the gelatin was incorporated into the networks via reacting with the epoxy groups on GPTMS and forming hydrogen bonding with CNFs. The content of gelatin had a significant influence on the properties of the obtained foams. With the introduction of a small amount of gelatin, the foams exhibited significantly enhanced mechanical properties and stability in water in comparison with the CNF foams without GPTMS and/or gelatin. The foams containing the appropriate ratio of gelatin to CNFs possessed the high porosity (99.16%), ultralow density (0.0077 g/cm3), good mechanical properties, and abundant functional groups (hydroxyl and carboxylate groups). These features made it an ideal adsorbent toward methylene blue (MB) and the maximum adsorption capacity could reach up to 430.33 mg/g. Additionally, the cationic MB could be removed from the mixed cationic/anionic dye solutions with high selectivity, possibly due to the strong electrostatic interactions between MB and the adsorbent. Furthermore, the recycling test demonstrated the good reusability of this biofoam. Therefore, this work provides an environmentally benign method to fabricate robust CNF-based foams, which can be considered as green adsorbents for the treatment of water pollutants.

Strong, highly hydrophobic, transparent, and super-insulative polyorganosiloxane-based aerogel

Wide application of hybrid silica aerogels is limited by the costly preparation process and poor mechanical properties due to the inadequate initial connectivity at the interparticle regions. Most of the conventional solutions come at the cost of increased processing time, density and thermal conductivity. This paper reports a rapid and simply prepared 3-(trimethoxysilyl)propyl methacrylate (TMSPMA)-based aerogel with ultra-low density, high mechanical properties, extremely low thermal conductivity (10 mW·m−1 K−1), and high hydrophobicity. The overall connectivity state was tuned by manipulating the nano-architecture of the aerogel structure. This involved tailoring the constituent materials at the molecular level, the sequence of the reactions, thermodynamics and the processing parameters. Two novel approaches were explored. First, a post-gelation free radical self-curing (SC) method with no added organic moieties was examined. The reaction sequence and the precursor nature were then changed. The free radical polymerization was performed first to obtain a novel macro-precursor (PTMSPMA), followed by hydrolytic polycondensation. Adjustment of the thermodynamic variables during gelation led to two different structures, pearl necklace-like and reticulate. The reticulated PTMSPMA-derived aerogels exhibited the lowest density, lowest total thermal conductivity, and the highest compression modulus. Further modification of reticulated aerogel processing conditions surprisingly showed that the choice of alcohol has profound impact on the mechanical properties, transparency, and surface properties (hydrophobicity) of aerogels by tailoring the molecular assembly. While ethanol created a super hydrophilic aerogel, benzyl alcohol rendered a highly hydrophobic aerogel.