Development and characterization of solid and porous polylactide‐multiwall carbon nanotube composites

This article describes the fabrication of solid and porous polylactide (PLA)‐multiwall carbon nanotube (MWNT) composites prepared using melt blending and subsequent batch processing of porous structures. The morphology and thermal, rheological and electrical properties of the PLA‐MWNT composites prepared with MWNT concentrations of 0, 0.5, 1, 2, and 5 wt% were characterized. The composite structure consisted of identifiable regions of MWNT aggregation and MWNT dispersion. Increasing MWNT content was found to increase the thermal stability and crystallization kinetics of PLA. The addition of MWNT to PLA significantly increased the melt viscosity and electrical conductivity of the composites. Based on rheological and electrical measurements, a continuous MWNT network structure in PLA was found to form when the concentration of MWNT is increased from 0.5 wt% (0.33 vol%) to 1 wt% (0.66 vol%). As many current day applications of polymers and polymer composites require lightweight and low‐density materials, porous PLA‐MWNT composites were fabricated from a batch porous structure processing technique. Porous PLA‐MWNT composites containing 2 and 5 wt% MWNT had lower relative densities, which is attributed to the higher viscosity of the composites suppressing collapse of the porous structure during processing. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Growth and morphology manipulation of carbon nanostructures on porous supports

Structural polymorphism of carbon grown on porous supports by chemical vapor deposition (CVD) is demonstrated. Combining three different supports (activated carbon, macroporous polymeric beads, and microporous aluminophosphate molecular sieves), with two catalysts (Ni and Fe), yielded a variety of carbon nanostructures ranging from coils and belts to fibers and tubes, without changing CVD temperature, time, and precursor composition and flow rate. Ion exchange between ammonium-impregnated supports and the metals was necessary in order to achieve a fine dispersion of the catalyst over the support surface. Metal-support interactions and the balance between ammonium and metal concentrations were investigated and found to considerably affect catalyst dispersion, shape, and crystallographic orientation, which in turn determined the morphological and structural characteristics, and yield of the carbon product. The catalyst-loaded supports and the resulting carbon materials were characterized by scanning and transmission electron microscopy, thermogravimetic analysis, X-ray diffraction, and Raman spectroscopy, while nitrogen and mercury porosimetry were used to characterize the supports and evaluate the degree of support pore blocking by the carbon deposits.     

One-pot hydrothermal synthesis of nitrogen-doped hierarchically porous carbon monoliths for supercapacitors

The nitrogen-doped hierarchically porous carbon monoliths (N-HPCMs) were successfully synthesized by using dicyandiamide (DCDA) as nitrogen source, phenolic resol as carbon precursor and mixed triblock copolymers as templates via a one-pot hydrothermal approach. The obtained carbon monoliths possess tunable mesopore size (4.3–11.4 nm), large surface area (552–660 m²/g), and high nitrogen content (up to 12.1 wt%). Ascribed to the nitrogen-doped frameworks and hierarchical porosity, N-HPCMs exhibit good electrochemical performance as the supercapacitor electrode with specific capacitance of 268.9 F/g (in 6 M KOH) at a current density of 1 A/g, and a 4.1 % loss of the specific capacitance after 5,000 charge–discharge cycles, indicating a long-term cycling stability. Such unique features make N-HPCMs promising electrode materials for high performance supercapacitors.     

Magnetically-separable hierarchically porous carbon monoliths with partially graphitized structures as excellent adsorbents for dyes

Magnetically-separable hierarchically porous carbon monoliths with partially graphitized structures were synthesized through confinement self-assembly in polyurethane (PU) foam associated with a direct carbonization process from triblock copolymer F127, phenolic resol and ferric nitrate. It was observed that the magnetic Fe nanoparticles were embedded in the walls of graphitic porous carbon matrix, and the resulting materials exhibited hierarchically porous structure with macropores of 100–450 μm, mesopore size of 4.8 nm, BET surface area of 723 m²/g, pore volume of 0.46 cm³/g, and saturation magnetization of 3.1 emu/g. Using methylene blue as model dye pollutant in water, the carbon monolith materials showed high adsorption capacity of 190 mg/g, exhibiting excellent adsorption characteristics desirable for the application in adsorption of dyes and easy separation under an external magnetic field.     

Electrically conductive porous alumina/graphite composite synthesized by starch consolidation with reductive sintering

This paper reports a facile and environment-friendly process to synthesize electrically conductive porous alumina/graphite composites by starch consolidation technique followed by reductive sintering. Green ceramic composites were consolidated with different starches and sintered at different temperatures in an argon atmosphere. Electrical measurements, carbon contents and Raman analyses of carbon structures determined an optimal sintering temperature of 1700 °C, which lead to a uniform formation of conductive graphitic networks at an optimal concentration of about 3.8 vol% in the porous composites. These carbon networks resulted into porous composites having high electrical conductivities measured in the range from 3 to 7 S/cm, which depended on the starch types and their porous properties. Correspondingly, the bulk porosities of the sintered composites were measured from 42 to 46%, with rounded micropores having diameters ranging from 14 to 39 μm. These porous properties of the sintered composites offer promising applications for conductive membrane and porous electrode.     

Fabrication of the nitrogen doped ordered porous carbon derived from amino-maltose with excellent capacitance performance

In this paper, pristine and nitrogen doped ordered porous carbon materials were fabricated by using maltose and amino-maltose synthesized by hydrothermal reaction as precursors via template strategy. The fabricated pristine ordered porous carbon (OPC) and nitrogen doped ordered porous carbon (NOPC) exhibit excellent textural properties and good capacitance performance, which specific surface area (S_BET) reach 1107 and 726 m² g^?1 for the pristine OPC and NOPC materials while the specific capacitance reach up to 139 and 183 F g^?1 under a current density of 0.5 A g^?1, respectively. The capacitance retention rate for the pristine OPC and NOPC reaches ca. 81 and 92% as the current density increased from 0.5 to 20 A g^?1, and no apparent capacitance decrease was observed after 5000 cycles. Although a sharp decrease of specific surface area was observed after N doping, the specific capacitance of NOPC was improved about 31% than that of the pristine OPC, the enhanced wettability and surface availability after N doping were found to be responsible for the enhanced capacitance performance of NOPC.     

Design and characterization of porous mullite based semi-vitrified ceramics

Three porous ceramic composites were prepared from readily available raw materials (kaolin, bauxite, feldspar and kyanite). The porous ceramic formulations were sintered at different temperatures ranging from 1200 to 1400°C. The fired specimens were characterized by determining their porosity, bulk density, flexural strength, thermochemical stability, microstructure, water and mercury permeability. Apparent porosity and bulk density in the range 15.57 ± 1.56–42.73 ± 2.28?vol% and 2.23 ± 0.31–2.68 ± 0.41?g?cm^?3 respectively were obtained after firing. The flexural strength was in the range of 32.31 ± 2.1–74.88 ± 2.57?MPa and the thermal expansion coefficient of 5–9 × 10^?6 C^?1. The values of water permeability were 745.4, 641.45 and 525.91?L/m² h?kPa respectively for PK3, PK4 and PK5. It was found that at high temperature (1400?°C), kyanite particles enhanced the porosity and thermal stability by reducing glass formation and improving crystallization. The presence of the interconnected pores with size between 0.03 and 4.50?µm, the high total volume of pores together with the high flexural strength and thermal stability make the synthesized porous ceramics suitable for high-pressure filtering applications.     

Expanded graphene-oxide encapsulated polyaniline composites as sensing material for volatile organic compounds

The expanded graphene-oxide (EGO) encapsulated PA composite materials are prepared by in-situ chemical oxidative polymerisation polyaniline (PA) where polymerization of aniline was carried out in presence of EGO using ammonium-persulphate in an acid medium. The synthesized samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrical conductivity measurements. The electrical conductivity get increases with temperature showing semiconducting behaviour and the conductivity is found to be 101.04 S/m at 413 K. The composite materials are exposed with various concentrations of vapours of different volatile organic compounds (VOCs) such as acetone, chloroform and carbon tetrachloride and compared with the pristine polymer. The oxidising VOCs like acetone on exposure to pristine polymer and PA/EGO composite is found to be decrease in resistivity by hydrogen bonding with the redox cites of the polymer. Among these VOCs, the sensitivity towards chloroform is found to be more in PA and its composites than the other two compounds.     

Aerosol-assisted synthesis of hierarchical porous titanosilicate molecular sieve as catalysts for cyclohexene epoxidation

Hierarchical titanosilicate composites (HTS) have been synthesized by an aerosol spray method, by which TS-1 nanoparticles were assembled with cetyl trimethyl ammonium bromide and tetraethylorthosilicate/tetra-n-butyl titanate to form a hierarchical porous composites with a core–shell structure. The structure, porosity, morphology and composition of the materials were characterized by X-ray diffraction, nitrogen adsorption isotherm, Raman spectroscopy, electron microscopy and X-ray fluorescence spectroscopy. The hierarchical porous molecular sieve combines the advantages of TS-1 and Ti-KIT-1, and shows high activity and selectivity to epoxide in cyclohexene epoxidation with H₂O₂. The yield of cyclohexene oxide on HTS is much higher than those on TS-1 and Ti-KIT-1. The selectivities to cyclohexene oxide and 1,2-cyclohexanediol can be adjusted by varying the titanium contents of the HTS samples.     

Highly ordered porous carbon/wax composites for effective electromagnetic attenuation and shielding

We have demonstrated a highly ordered porous carbon (HOPC) as an effective electromagnetic absorber. The unique porous structures allow HOPC to possess high surface area and establish effective three-dimensional (3D) conductive interconnections at very low filler loading, which is responsible for effective electrical loss in terms of dissipating the induced current in the corresponding wax composites. Owing to the 3D porous frame, the wax composites with 1 and 5 wt% HOPC have shown effective bandwidth ∼2 and ∼4.5 GHz, respectively, which is considerably competitive to the performance found in the carbon nanotube- (CNT) and graphene-based composites of much higher filler loadings. This concept based on porous absorbers demonstrates more advantages in the fabrication of lightweight microwave-absorbing materials. Furthermore, the composite with 20 wt% HOPC has exhibited highly effective electromagnetic shielding performance up to 50 dB, which competes well with what has already been achieved in the composites embedded with CNTs and graphene. The fundamental mechanism based on electrical conductivity and complex impedance suggests specific strategies in the achievement of high-performance composites for electromagnetic attenuation and shielding.     

Fabricating porous ceramic materials via phase separations in blends of cellulose acetate and ceramic nanoparticles

As applications of porous ceramic materials have gradually expanded, the novel technologies for the fabrication of porous ceramic materials with a delicate and controllable structure are still attractive. In this work, three types of porous monolithic ceramic materials, including Al₂O₃–SiO₂, TiO₂, and SiC, have been fabricated by thermally impacted and non-solvent-induced phase separations in blends of cellulose acetate and ceramic nanoparticles. These materials possessed three-dimensional interconnected porous structures with low densities, high porosities, and hierarchical pores ranging from 5 nm to 6 μm. The relationships between microstructures and phase separations were systematically investigated. Furthermore, electromagnetic shielding effectiveness of 20 dB from 5 to 18 GHz in porous SiC materials has been achieved, revealing that those materials have potential applications in the electromagnetic shielding. This work provides a powerful and general approach to fabricate porous monolithic ceramic materials with a wide range of various ceramic nanoparticles.     

Preparation and carbon dioxide uptake capacity of N-doped porous carbon materials derived from direct carbonization of zeolitic imidazolate framework

The preparation, characterization and CO₂ uptake performance of N-doped porous carbon materials and composites derived from direct carbonization of ZIF-8 under various conditions are presented for the first time. It is found that the carbonization temperature has remarkable effect on the compositions, the textural properties and consequently the CO₂ adsorption capacities of the ZIF-derived porous materials. Changing the carbonization temperature from 600 to 1000 °C, the composites and the resulting porous carbon materials possess a tuneable nitrogen content in the range of 7.1–24.8 wt%, a surface area of 362–1466 m² g⁻¹ and a pore volume of 0.27–0.87 cm³ g⁻¹, where a significant proportion of the porosity is contributed by micropores. These N-doped porous composites and carbons exhibit excellent CO₂ uptake capacities up to 3.8 mmol g⁻¹ at 25 °C and 1 bar with a CO₂ adsorption energy up to 26 kJ mol⁻¹ at higher CO₂ coverages. The average adsorption energy for CO₂ is one of the highest ever reported for any porous carbon materials. Moreover, the influence of textural properties on CO₂ capture performance of the resulting porous adsorbents has been discussed, which may pave the way to further develop higher efficient CO₂ adsorbent materials.     

Nitrogen-doped porous carbon microspherules as supports for preparing monodisperse nickel nanoparticles

N-doped porous carbon microspherules were developed through controlled carbonization of the copolymer of vinylidene chloride and acrylonitrite. The carbon precursors were prepared by an inorganic-organic hybrid route. Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) analysis showed the formation of N-doped carbon spherules. N₂ sorption analysis showed that the resultant carbon materials have a BET surface area of 692 m²/g. Nickel nanoparticles supported over them are kept in a well-dispersed state. Electron Probe Microanalysis (EPMA) and Transmission Electron Microscopy (TEM) showed nickel nanoparticles are quite monodisperse. Analysis of XPS spectra of the samples with different surface nitrogen atomic concentration demonstrated that nitrogen species on the carbon surfaces have a great impact on the dispersion state of the mounted metal nanocrystals.     

Structure in metallurgical cokes and carbons as studied by etching with atomic oxygen and chromic acid

Anisotropie carbons and cokes exhibit an optical texture or micro-texture in the size range 0.5–300 μm in polished surfaces using optical microscopy. Structure within this optical texture can be studied as the topography created by etching surfaces with atomic oxygen and chromic acid. Atomic oxygen preferentially etches an isotropic carbon layer which exists between the grains of the fine-grained mozaics. Chromic acid oxidizes or etches selectively the surfaces of anisotropic carbon to create fissures parallel to basal plane orientation. Structural components within petroleum cokes, carbon fibres and carbon/carbon fibre composites are revealed. Chromic acid oxidizes isotropic components in metallurgi-cal cokes more slowly and so reveals the structure of cokes as prepared from co-carbonizations of coal with petroleum pitch. It is considered that these etching techniques augment our knowledge of internal structure within carbons and cokes and of considerations of strength and fracture in these materials.     

Synthesis and characterization of a xonotlite fibers–silica aerogel composite by ambient pressure drying

Xonotlite fibers (XFs) reinforced silica aerogel composites were prepared by a sol–gel method under ambient pressure drying. XFs were synthesized through a dynamic hydrothermal route and had a noodle-like structure with length of 5–10 μm and average diameter of 150–200 nm. The microstructure analysis showed that XFs were inlaid in silica aerogel matrix by physical combination which contributed to restrict the volume shrinkage of alcogels and maintain the integrality aerogels during drying process. The physical, naonporous and thermal properties of the as prepared aerogel composites were investigated and discussed in detail. The new aerogel composites possessed porous nanostructure, which exhibited typical properties of 0.126 g/cm³ density, 4.132 cm³/g pore volume, and thermal conductivity of 0.0285 W/(m K). The results indicated that the introduced XFs didn’t significantly alter the porosity, hydrophobicity or thermal conductivity of aerogel matrix. It was also found that the aerogel composites had much more outstanding porosity than that of pure aerogel upon calcinations at 800 °C. This study fabricated XFs–silica aerogel composites and explored a new way for silica aerogels to endure and remain monolithic under ambient pressure drying.     

Hierarchical porous nitrogen-doped carbon materials derived from one-step carbonization of polyimide for efficient CO<Subscript>2</Subscript> adsorption and separation

Hierarchical porous nitrogen-doped carbon (HPNC) materials are synthesized through one-step carbonization of polyimide using triblock copolymer P123 as mesoporous template. The microstructure, chemical composition and CO₂ adsorption behaviors are investigated in detail. The results show that HPNC materials have hierarchical micro-/mesopore structures, high specific surface area of 579 m²/g, large pore volume of 0.34 cm³/g, and nitrogen functional groups (5.2 %). HPNC materials exhibit high CO₂ uptake of 5.56 mmol/g at 25 °C and 1 bar, which is higher than those of previously reported nitrogen-doped porous carbon materials. After 5 cycles the value of CO₂ adsorption uptakes is 5.28 mmol/g, which is approximately 95 % of the original adsorption capacity. The estimated CO₂/N₂ selectivity of HPNC materials is 17, revealing great promise for practical CO₂ adsorption and separation applications. The efficient CO₂ uptake and enhanced CO₂/N₂ selectivity are due to the combination of nitrogen-doped and hierarchical porous structures of HPNC materials.     

Preparation and characterization of porous crosslinked microspheres of new aromatic methacrylates

The aim of this work was to prepare a new group of aromatic methacrylate monomers, utilise them in preparation of porous microspheres and study the influence of their chemical structure on the textural properties of porous methacrylate microspheres. Polymeric microspheres were prepared by suspension-emulsion polymerisation of four aromatic monomers: methacryloiloxybenzene, 1,2-dimethacryloiloxybenzene, 1,3-dimethacryloiloxybenzene and 1,4-dimethacryloiloxybenzene with another crosslinking agent—trimethylolpropane trimethacrylate. Mass median diameters of obtained beads are in the range 22–35 μm. The polymerisation reactions were carried out in the presence of a pore forming diluent. The influence of the diluent system on the porous structure of microspheres was studied in detail. To determine the textural properties of the studied microspheres, nitrogen adsorption–desorption and inverse sized exclusion chromatography measurements were used. Specific surface area of the obtained microspheres achieves value from 185 to 510 m²/g. Since obtained polymeric materials can be used as chromatographic packings for HPLC their porous structure in a swollen state was investigated. Significant differences in the porous structure parameters for dry and swollen microspheres were observed.     

Thermal conductivity of composite catalysts containing metallic copper as a reinforcing component

The dependences of the electrical and thermal conductivities of porous composite materials containing a metallic component (copper) on the volume copper content are investigated experimentally. The measured thermal conductivities of samples prepared according to the proposed technique indicate that the thermal conductivity of monolithic catalysts with a copper content of no less than 15 vol % exceeds 1 W m^?1 K^?1. This corresponds to the formation of a connected cluster consisting of conducting spheres in a random packing of conducting and insulating spheres. A comparative analysis of the thermal and electrical conductivities of the composites demonstrates that, at a copper content of higher than 20 vol %, the thermal conduction through a percolation cluster formed by copper particles makes the dominant contribution. In employing composite materials containing a catalytically active component in exothermic catalytic processes (Fischer-Tropsch synthesis, steam conversion of carbon monoxide CO, etc.), their high thermal conductivity is an important advantage that makes it possible to decrease the temperature gradient across the porous composite catalyst bed. A semiempirical method for calculating the thermal conductivity of composites is developed. The results of the calculations performed using the proposed method are in good agreement with experimental data.     

New mesoporous carbon materials synthesized by a templating procedure

The new porous carbon materials were obtained by templating procedure using mesoporous silica (SBA-15) as template. The ordered mesoporous silica materials were synthesized by using Pluronic P123 (non-ionic triblock copolymer, EO₂₀PO₇₀O₂₀). SBA-15/cryogel carbon composites were obtained by sol–gel polycondenzation of resorcinol and formaldehyde in the presence of different amount of SBA-15. The polycondenzation was followed by freeze drying and subsequent pyrolysis. One set of SBA-15/sucrose carbon composites was prepared by using sucrose as carbon source. The silica template was eliminated by dissolving in hydrofluoric acid (HF) to recover the carbon material. The obtained carbon replicas were characterized by nitrogen adsorption–desorption measurements, X-ray diffraction and scanning electron microscopy (SEM). It was revealed that the samples have high specific surface (533–771 m² g^?1), developed meso- and micro-porosity and amorphous structure. Porous structure of carbon replicas was found to be a function of the carbon source, properties of SBA-15 and silica/carbon ratio. Room temperature adsorption of nitrogen and adsorption of phenol from aqueous solutions were investigated.     

Dual mesoporous carbon with high nitrogen doping level as an efficient electrode material for supercapacitors

This paper reports a dual mesoporous carbon (NDMC) with high nitrogen doping level derived from the amino production of the sucrose synthesized under hydrothermal condition. The S _BET and total pore volume of the reported materials reaches up to 1101 and 1.67 cm³ g^?1, the small mesopores center at about 3.22–3.31 nm while the larger mesopores locate at 8.98–12.58 nm. The doping content of the nitrogen heteroatoms is found to be more than 11.6 at.%, and depend on the carbonization temperature. The maximum specific capacitance of the reported materials reaches up to 512 F g^?1 due to the additional contribution of pseudo-capacitance induced by the nitrogen heteroatoms doping. The capacitance retention rate is found to be up to 95% after 1000 times cycles. The dual mesoporous structure, high specific area, additional pseudo-capacitance, enhanced wettability and conductivity are found to response for the superior capacitance performance of the reported materials.