Closed Pore Structures Research Articles

Spatial confinement strategy modulated by kinetic diameters of gaseous molecules for sodium storage

Constructing closed pore structures is essential for improving the plateau capacity of high-capacity hard carbon (HC) anodes for sodium-ion batteries. However, the absence of a straightforward and efficient strategy for constructing closed pores has hindered the advancement of high-capacity HC anodes. Here, we have developed a spatial confinement strategy for constructing closed pore structures using pyrolytic carbon (PC) as substrate and pyrolysis gas as the carbon source for chemical vapor deposition. The deposition of pyrolysis gas effectively tightens the pore entrance, thereby preventing electrolyte infiltration and transforming the open pores in the PC into highly efficient sites for sodium storage. The obtained optimal anodes demonstrate a remarkable specific capacity of 324.6 mAh g-1. More importantly, we calculate the kinetic diameters of the carbon source molecules from their iso-electron density surfaces and correlate them with the mechanism of closed pore formation, which will effectively guide the fabrication of closed pores for sodium storage.

Multiphase Symbiotic Engineered Elastic Ceramic-Carbon Aerogels with Advanced Thermal Protection in Extreme Oxidative Environments.

Elastic aerogels can dissipate aerodynamic forces and thermal stresses by reversible slipping or deforming to avoid sudden failure caused by stress concentration, making them the most promising candidates for thermal protection in aerospace applications. However, existing elastic aerogels face difficulties achieving reliable protection above 1500°C in aerobic environments due to their poor thermomechanical stability and significantly increased thermal conductivity at elevated temperatures. Here, a multiphase sequence and multiscale structural engineering strategy is proposed to synthesize mullite-carbon hybrid nanofibrous aerogels. The heterogeneous symbiotic effect between components simultaneously inhibits ceramic crystalline coarsening and carbon thermal etching, thus ensuring the thermal stability of the nanofiber building blocks. Efficient load transfer and high interfacial thermal resistance at crystalline-amorphous phase boundaries on the microscopic scale, coupled with mesoscale lamellar cellular and locally closed-pore structures, achieve rapid stress dissipation and thermal energy attenuation in aerogels. This robust thermal protection material system is compatible with ultralight density (30mg cm-3), reversible compression strain of 60%, extraordinary thermomechanical stability (up to 1600°C in oxidative environments), and ultralow thermal conductivity (50.58mW m-1 K-1 at 300 °C), offering new options and possibilities to cope with the harsh operating environments faced by space exploration.

Rationally Regulating Closed Pore Structures by Pitch Coating to Boost Sodium Storage Performance of Hard Carbon in Low‐voltage Platforms

AbstractHard carbon (HC) materials with rich closed pore structures and nano‐scaled soft carbon coating layer have emerged as promising anode in sodium‐ion batteries (SIBs). However, it still remains a tremendous challenge to precisely regulate closed pore structures and soft carbon coating thicknesses for achieving excellent electrochemical performance in SIBs at low‐voltage platforms. Herein, PCHC‐10 with abundant and suitable‐sized closed pore size (0.45 nm) and nano‐scaled soft carbon coating layer has been accurately designed by chemical crosslink reaction between the pre‐oxidized phenolic resin and a small addition of pitch to form ester‐based bond. As anode, PCHC‐10 delivered large reversible capacity of 359.8 mAh g−1 within 0.001–2.5 V, and high capacity of 242.8 mAh g−1 in low voltage platforms (≤0.15 V). Besides, PCHC‐10 anode exhibits 91.4% capacity retention for 100 cycles, and Na3V2(PO4)3//PCHC‐10 full cell has superior rate performance and high energy density of 231.2 Wh kg−1. Furthermore, the detailed electrochemical storage behaviors and theoretical calculations revealed that the HC owning closed pore‐size of 0.45 nm has the strongest Na+ storage abilities in low‐voltage platforms. This work presents a novel insight for constructing HC with suitable‐sized closed pore structures and soft coating layer to boost Na+ storage capability in low‐voltage platforms.

ZrC-SiC closed-cell ceramics with low thermal conductivity: Exploiting unique spherical closed-cell structure through tape casting and CVI techniques

Porous ultra-high temperature ceramics (UHTCs) are recognized as novel candidates for fulfilling the requirements of thermal protection systems of hypersonic aircrafts, as they possess excellent high-temperature resistance and low thermal conductivity. Currently, the reported porous UHTCs predominantly exhibit an open pore structure. By contrast, closed-cell UHTCs, formed by employing ceramic hollow microspheres (HMs) as pore-forming agents, hold great potential for achieving superior thermal insulation performance. Unfortunately, the implementation of this strategy has been hindered by the scarcity of raw materials and preparation techniques.In this paper, ZrC-SiC closed-cell ceramics were first successfully prepared through a combination of tape casting and chemical vapor infiltration (CVI) techniques, utilizing the self-developed ZrC HMs as the primary raw material. The morphology, microstructure, and thermal insulation properties of the obtained ZrC-SiC closed-cell ceramics were investigated. The results indicate that when the content of ZrC HMs is 30 vol.%, the density of the prepared porous ceramics is 2.09 g cm–3, with a closed porosity of 14.05% and a thermal conductivity of 1.69 W (m K)–1. The results clearly prove that the CVI process can successfully convert ZrC HMs into closed pore structures within porous ceramics. The introduction of ZrC HMs suppresses the contribution of free electrons to thermal conductivity and brings about a large number of solid-gas interfaces, which increases the interfacial thermal resistance and significantly reduces the phonon thermal conductivity. Consequently, the as-prepared ZrC-SiC closed-cell ceramics show excellent thermal insulation properties. This study provides a new idea and method for the development of porous UHTCs and offers a more reliable material choice for thermal protection systems.

Lightweight cementitious composites incorporating fly ash cenospheres and perlite microspheres

The research on lightweight concrete has been an area of interest, especially in modular construction since low self-weight can ease the transportation process and speed up construction time. Past research found that conventional lightweight aggregate concrete (LWAC) developed using coarse lightweight aggregates and natural fine aggregates can fulfil the requirement of structural lightweight concrete. However, the dry densities achieved by these LWAC are relatively high, at around 1750–2070 kg/m3. To address this issue while producing structural LWAC with a lower density range, micro lightweight aggregates (MLWA) with closed pore structures are utilized. In this study, two types of MLWA: fly ash cenospheres (FAC) and perlite microspheres (PM) were used to produce two series of lightweight cementitious composite (LCC). The incorporation level of MLWA was 25%, 50%, 75% and 100% by volume of coarse silica sand which was utilized to optimize the content of MLWA. The performance of LCC was investigated through compressive strength, flexural strength, water absorption, drying shrinkage and thermal conductivity tests. Use of FAC and PM yielded satisfactory compressive strength of up to 46.7 MPa and 45.3 MPa respectively at low densities. While flexural strength of mixes with FAC and PM reached 8.0 MPa and 8.7 MPa respectively. Use of 75% FAC showed the lowest water absorption of 3.3%. Incorporation of 100% PM reduced thermal conductivity by as much as 82.4%. The optimum LCC mix in this study comprised 75% FAC and 25% coarse silica sand which achieved a specific strength of 29.0 kPa/kgm−3 and a dry density of 1406 kg/m3, while having an average compressive strength and flexural strength of 40.8 MPa and 6.3 MPa at 28 days, respectively. The overall results show that LCC made up of FAC is more suitable for structural applications, whereas LCC made up of PM exhibits better thermal insulation properties.

Revealing the effect of hard carbon structure on the sodium storage behavior by using a model hard carbon precursor

The sodium storage mechanism of hard carbon anodes still remains controversial. It is usually believed that there exists relationship between the plateau capacity and the closed pores of hard carbon. Herein, a sulfamethazine molecule is selected as a model precursor to prepare hard carbon. A molten state occurs during the carbonization process of this precursor, which reduces the formation of open pores. Hard carbon materials exhibiting diverse closed pore structures are synthesized by carbonizing at temperatures ranging from 1100 °C to 1700 °C. The closed pores and plateau capacity exhibit a linear correlation, which is based on their closed pore volume and sodium-ion storage capacities. The hard carbon material prepared at 1500 °C exhibits excellent electrochemical performance, which delivers a high reversible capacity of 292 mAh g−1 with a plateau capacity of 230 mAh g−1 at a current density of 0.05 A g−1, as well as decent cycling stability (83 % capacity retention at 0.1 A g−1 over 200 cycles). The model hard carbon confirms that the plateau capacity is attributed to the storage of sodium ions in the closed pores.

Highly dispersed red phosphorus produced by mechanical milling for enhanced flame retardancy of polyurethane-based foam

To improve the exothermic temperature and flammability of polyurethane foam (PUF), red phosphorus (RP) and diatomite (D) from mechanical milling were chosen as the flame retardants to be incorporated into the polyurethane/waterglass foam (PUHF). The maximum exothermic temperature and temperature distribution of PUF and PUHF were measured using an infrared thermal camera. The morphology and chemical structure of the flame retardants as well as the chemical structure, morphology, compressive strength, thermal stability and flame retardancy of different PUFs were comprehensively investigated by scanning electron microscope, energy dispersive spectrometer, Fourier transform infrared spectroscopy, X-ray diffraction, compression test, thermogravimetric analysis, limiting oxygen index, vertical burning combustion test, cone calorimetry and X-ray photoelectron spectroscopy. The obtained results showed that the maximum exothermic temperature and temperature distribution of PUHF were lower and more uniform than those of PUF. The addition of D increased the dispersion of RP in PUHF, and the uniform and closed pore structures and compressive property were well-retained. Moreover, RP/D/PUHF exhibited excellent thermal stability and flame retardant properties. In addition, a possible flame retardant mechanism of RP/D/PUHF was proposed.

Regulating closed pore structure enables significantly improved sodium storage for hard carbon pyrolyzing at relatively low temperature

AbstractThe closed pore has been considered as the key structure for Na ion storage in hard carbon. However, the traditional view is that closed pores can only be formed by the curling of graphite‐like crystallites in the case of high temperature carbonization. Ingenious designing of closed pore structures at lower temperature is still blank. Herein, for the first time, engineering the wall thickness and number of closed pores in waste rosewood‐derived hard carbon was successfully achieved at a low temperature of 1100°C by removing the lignin and hemicellulose components in wood precursor. When applied as an anode material, the optimum sample exhibits a high capacity of 326 mAh/g at 20 mA/g and a remarkable rate capability of 230 mAh/g at 5000 mA/g, significantly higher than those of the untreated sample (only 33 mAh/g at 5000 mA/g). The significantly improved Na storage performance should be attributed to abundant closed pores that provide sufficient spaces for Na storage and thin pore wall structure that is beneficial to the diffusion of Na+ in the bulk phase. This work provides a new idea for the future application of biomass‐based hard carbon for advanced Na ion batteries.

Alkali Metal Ion Insertion and Extraction on Non-Graphitizable Carbon with Closed Pore Structures

We compared the insertion and extraction behaviors of lithium and sodium ions in non-graphitizable carbon with closed pore structures heat treated 2073 K and non-graphitizable carbon without heat treatment to investigate the elemental difference of the charge-transfer reactions of alkali metal ions from a kinetic viewpoint. The lithium system has smaller kinetic parameters for the insertion/extraction reactions such as charge transfer resistance and activation energy compared to those in the sodium system. On the other hand, the diffusion coefficients of the lithium and sodium ions in non-graphitizable carbon are almost the same. Based on these results, we discuss the rate-determining process of charge-transfer reactions and provide a rational design of non-graphitizable carbon as an anode material for use in lithium ion and sodium ion batteries.

Metal-Organic Framework as a Functional Analyte Channel of Organic-Transistor-Based Air Pollution Sensors.

Air pollution sensors based on organic transistors have attracted much interest recently; however, the devices suffer from low responsivity and slow response and recovery rates for gas analytes. These shortcomings are attributed to the low charge-carrier mobility of organic semiconductors and to a structural limitation resulting from the use of a thick and continuous active layer. In the present work, we investigated the material properties of a multiscale porous zeolitic imidazolate framework, [Zn(2-methylimidazole)2]n (ZIF-8), and examined its potential as an analyte channel material inserted at an organic-transistor active layer. A series of carbonized zeolitic imidazolate frameworks (ZIFs) were prepared by thermal conversion of ZIF-8 and also studied for comparison. The microstructures, morphologies, and optical/electrical characteristics of polythiophene/ZIF-8 hybrid films were systematically investigated. Organic-transistor-type nitrogen dioxide sensors based on the polythiophene/ZIF-8 hybrid films showed substantially improved sensing properties, including responsivity, response rate, and recovery rate. The electrical conductivity of the carbonized ZIF-8s enhanced the field-effect mobility of the organic transistors; however, the sensing performance was not improved, because of the closed pore structures resulting from the carbonization. These results provide invaluable information and useful insights into the design of transistor-type gas sensors based on organic semiconductor/metal-organic framework hybrid films.

Properties of magnesium potassium phosphate cement-expanded perlite composites applied as fire resistance coating

Cement-based composites have been a commonly used fire resistance material. As a new cement, magnesium potassium phosphate cement (MKPC) has been a popular and important research object due to its various excellent properties including heat resistant. However, only limited studies have been conducted on its application as fire-resistant coating. In this paper, the thermal properties of MKPC blended with/without expanded perlite (EP) under fire exposure were investigated. Two types of EP with open-pore and closed-pore structures were used, and each type of EP contained two different sized particles including 212–270 μm and 270–550 μm. The fluidity and setting time of MKPC-EP composites and adhesive strength between MKPC-EP composites coating and plywood were measured. The fire resistance properties of MKPC-EP composites were studied, including loss of weight (LOW), generated cracks, phase changes, thermal analysis, residual adhesive strength, and microstructure changes. The obtained results were intended to reveal the feasibility of MKPC-EP composites applied as coating to protect the substrate against exposure to fire. The addition of EP decreased the fluidity, extended the setting time, and enhanced the adhesive strength before and after fire test. LOW and accumulative lengths of visible cracks after fire exposure reduced for certain formula and are dependent on the pore structures, sizes and dosage of EP. X-ray diffraction (XRD) investigation and thermal analysis-infrared spectroscopy reveal MgKPO4⋅6H2O decomposed to MgKPO4⋅H2O by losing crystal water stage by stage during elevating temperature. Scanning electron microscopic (SEM) observations show that EP refined the MgKPO4⋅6H2O, and, after exposure to flame, the MKPC-EP exhibited a much denser microstructure with interconnecting MgKPO4·H2O of radiative flakes, when compared to the MKPC paste.

Bubble-templated rGO-graphene nanoplatelet foams encapsulated in silicon rubber for electromagnetic interference shielding and high thermal conductivity

A facile bubble-templated method based on gelling behavior of GO is proposed to construct 3D interconnected graphene network for efficient EMI shielding and heat conduction rubber composites. This thought realizes high-efficiency EMI SE enhancement when compared with traditional methods, suggesting strong potential in EMI shielding materials. • Bubble-templated 3D graphene networks were constructed via a novel and facile foaming route. • 3D graphene network endow the PDMS composites with excellent EMI shielding performance. • The composites possess extremely low density and outstanding electrical and thermal conductivities. Introducing three-dimensional (3D) conductive network in polymer composites is a greatly potential way for acquiring desirable electromagnetic interference (EMI) shielding capacity to satisfy the requirements of next-generation mobile electronics devices. However, facile and scale production to achieve rational design of the microstructure of 3D conductive framework is still remain challenging. Here, we reported a facile foaming route to construct the bubble-templated 3D graphene network, which endow the polydimethylsiloxane (PDMS) rubber composites with excellent EMI-shielding effectiveness of ~86 dB at a thickness of 2 mm. The superior EMI-shielding property is ascribed to the abundant closed pore structures to reflect and absorb electromagnetic waves and high electrical conductivity from interconnected 3D graphene networks. Meanwhile, owing to the low filler loading content of only 18.1 wt%, the density of the obtained composite is just 1.1 g/cm 3 , which is about 200% lower than those of the commercial products with a similar EMI SE value. Moreover, this composites also have a high thermal conductivity of above 3 W/(m·K), showing great promise for applications of EMI shielding and efficient thermal management in aerospace, telecommunications, and mobile electronics devices.

Controlled Synthesis of Porous Carbon Nanostructures with Tunable Closed Mesopores via a Silica-Assisted Coassembly Strategy

Controllable fabrication of mesoporous carbon nanoparticles (MCNs) with tunable pore structures is of great interest, due to the remarkable effect of pore structure on electrochemical performance o...

Fabrication of mesoporous double-layer antireflection coatings with near-neutral color and application in crystalline silicon solar modules

Single-layer antireflection (SLAR) coatings have been applied in crystalline silicon solar modules for a decade, which evolved from open-pore silica at an early stage into closed-pore structures nowadays in industry. For improving photovoltaic and aesthetic characteristics of solar modules, not only AR properties but also color neutrality of AR coatings need to be intensively studied. In this work, the double-layer antireflection (DLAR) coatings were designed and their residual color was studied by comparing with conventional SLAR and DLAR coatings under both normal and oblique incidences. According to the design, we fabricated the mesoporous DLAR coatings, which exhibited a high average transmittance of Tave = 99.02% at visible wavelengths from 380 nm to 780 nm and a low chroma value of C = 3.35. Applying the mesoporous DLAR coatings to the crystalline silicon mini-modules, the power conversion efficiency was increased by 2.40% on average in relative terms. Besides the power output gain, durable DLAR coatings with neutral color are of great significance for not only imparting a natural and sheer-black appearance to photovoltaic modules without unwanted blueish color, but also finding practical applications in consumer electronics, building glazing, and display windows.

Fabrication of lightweight alumina containing fine closed pores by controlling the relationship between phase stress and superplasticity: Experimental and mathematical studies

This work aims to fabricate porous materials with fine closed pore structures. In this study, in order to increase the migration of grain boundaries, alumina-sol and CaO micropowder are added to micro-alumina powder before sintering. A mathematical model is proposed to calculate the induced phase stress and provide a quantitative description of the relationship between stress and superplasticity. The effect of the relationship between stress and superplasticity on the properties and microstructures of fabricated lightweight alumina samples is investigated. With the addition of CaO micropowder, the reaction between the CaO and Al2O3 produces phase stress, which can activate the superplasticity of alumina-sol and increase grain boundary migration. Pores are closed and divided during migration; therefore, closed porosity of the samples gradually increase, while the open porosity and pore size decrease. However, excessive CaO content results in the breaking of grain boundaries and cause the closed porosity to decrease and open porosity to increase. According to the results in this study, a phase stress of approximately 21.14 MPa should be produced to fabricate lightweight alumina with a low open porosity, high closed porosity, and small pore size.

Analyses of Young's modulus and thermal expansion coefficient of sintered porous alumina compacts

Abstract This paper reports the derivation of Young's modulus ( E ) and thermal expansion coefficient (TEC, β ) for a sintered porous structure with open pores. The theoretical E is affected by the number of grains and the grain boundary area in sintered ceramics. The measured E –porosity relationship for porous alumina compacts were compared with the theoretical E values that were derived for the present open-pore structure and also for the dispersed (closed)-pore structure treated previously. With decreasing porosity (50% → 10%), the scattered E values showed a gradually increasing tendency, which were located between two theoretical curves for the open-pore structure. The sudden increase of E values in the porosity range from 10% to 0% was well explained by the theoretical dependence of E on porosity for the open- or closed-pore structure. The β values for the porous alumina structures were independent of porosity and close to the β values reported for fully dense alumina compacts. This result was in accordance with the theoretical β –porosity relationships for the open-pore and closed-pore structures.

3D surface topology guides stem cell adhesion and differentiation

Polymerized high internal phase emulsion (polyHIPE) foams are extremely versatile materials for investigating cell–substrate interactions in vitro. Foam morphologies can be controlled by polymerization conditions to result in either open or closed pore structures with different levels of connectivity, consequently enabling the comparison between 2D and 3D matrices using the same substrate with identical surface chemistry conditions. Additionally, here we achieve the control of pore surface topology (i.e. how different ligands are clustered together) using amphiphilic block copolymers as emulsion stabilizers. We demonstrate that adhesion of human mesenchymal progenitor (hES-MP) cells cultured on polyHIPE foams is dependent on foam surface topology and chemistry but is independent of porosity and interconnectivity. We also demonstrate that the interconnectivity, architecture and surface topology of the foams has an effect on the osteogenic differentiation potential of hES-MP cells. Together these data demonstrate that the adhesive heterogeneity of a 3D scaffold could regulate not only mesenchymal stem cell attachment but also cell behavior in the absence of soluble growth factors.

Influence of alumina bubble particles on microstructure and mechanical strength in porous Cu–Sn–Ti metals

In order to obtain high porosity and satisfactory strength simultaneously for the porous metallic layer of the grinding tools, alumina bubble particles were added into Cu–Sn–Ti alloy powders to fabricate porous metals using a vacuum sintering method. The influence of the alumina bubble particles on the microstructure and the mechanical strength of the porous Cu–Sn–Ti metal blocks were investigated. Results show that adding alumina bubble particles into Cu–Sn–Ti alloy powder generate closed pore structures for the metal blocks. Good bonding interface between alumina bubble particles and Cu–Sn–Ti alloy is formed mainly dependent on the chemical resultants of TiAl and TiO. A relationship between the bending strength of the porous Cu–Sn–Ti metal blocks and the size and volume fraction of the alumina bubble particles is established.

Effects of Surface Cathode Properties on Biopolymer Structuring via Electrochemical Deposition

Electrochemical deposition is a versatile bottom up-approach for the realization of advanced surfaces in biomedical applications. Deposition of chitosan via cathodic polarization has been recently proposed as a simple, fast, and effective technique also for the preparation of cellular solids [1], by their peeling-off from the deposition substrate. Chitosan ((1,4)-linked 2-amino-2- deoxy-d-glucan) is a natural cationic polymer that has been widely applied in drug delivery and tissue engineering for its proven biocompatibility, biodegradability, and antibacterial properties. Its electrodeposition, based on transport phenomena and reduction reactions at the diffusion layer, offers a control of the process parameters through simple adjustment of processing time and applied potential. The morphology of the obtained structures, being in the range of interest for tissue engineering scaffolds, can be easily tuned by an appropriate selection of anionic species and their concentration. Here we show the effects of the selection of anionic species and the surface cathode properties on the morphology of deposited coatings in order to modulate the obtained porosity. Electrocoagulation processes strongly depend on local reduction at the cathode, in turn affected by the acid/base equilibrium in solution: we hence examined (i) different water-based acidic bath and (ii) surfaces with different overvoltages for the hydrogen evolution reaction. Different coating morphologies, in terms of pore size and interconnection, were obtained by modifying electrochemical bath composition and controlling the overvoltage for hydrogen evolution by selecting the proper substrate material and surface preparation. Gas evolution at the cathode surface determined the growth of pores in the deposit coatings and, depending on the process parameters, close vs. open porosities were observed. A mass increase of deposited chitosan by increasing pH was generally observed, even if this trend is not monotone by increasing the pH. An increase of film porosity has been, in fact, noticed by increasing the mass rate deposition and hydrogen evolution and entrapment in the deposited gels. The overall deposition process seems affected by different electrolytes mainly as a consequence of their acid/base equilibria. Porosity can also be tuned according to the surface properties of selected metallic substrates, based on the characteristic exchange current for hydrogen evolution reaction. An accurate control of this process, hence allows to modulate open vs. closed pore structures. The possibility of tuning surface morphology of this class of films paves the way for the design of a novel class of 3D structures supporting tissue engineering approach and advanced drug delivery.[1] L. Altomare, L. Draghi, R. Chiesa, L. De Nardo, Materials Letters 78 (2012) 18.

Novel mesoporous high-performance films derived from polycyanurate networks containing high-boiling temperature liquids

Novel mesoporous polycyanurate (PCN) films were generated by polycyclotrimerization of dicyanate ester of bisphenol E in the presence of varying amounts of inert high-boiling temperature liquids, i.e. dimethyl phthalate or dibutyl phthalate, followed by their quantitative extraction after complete formation of PCN crosslinked frameworks. The chemical structures of precursory networks and resulting porous thermosetting films were confirmed by FTIR analysis and gel fraction determination. The porosity of the latter films was examined by SEM and DSC-based thermoporometry. Average pore diameters were found to be lower than 50nm and pore volumes as high as 0.16cm3g−1 were determined. The density values of porous samples were found to be lower than that of non-porous pure PCN synthesized as a model system, thus suggesting the occurrence of closed pore structures. Moreover, the thermal properties of PCN-based films were investigated by DSC and TGA. The high-boiling temperature liquids decreased their glass transition temperatures (Tg) and their degradation temperatures by acting as plasticizers. This plasticizing effect logically disappeared after removal of pore-forming phthalates from the networks: the Tg values of the mesoporous films were even higher than that of non-porous neat PCN, while their thermal stability turned out to be nearly identical to that of neat PCN.