Atmospheric effects on the thermally induced sintering of nanoparticulate gold films

Gold (Au) films were formed by sintering of Au nanoparticles (NPs) under gas flows of air, oxygen (O₂), nitrogen (N₂), or N₂ bubbled through formic acid (FA/N₂). The microstructure changes of the Au nanoparticulate films were studied when different atmospheres were applied. The Au film sintered under FA/N₂ showed the progressive agglomeration and grain growth with porosity in the film, while the film sintered under N₂ had NPs without participating grain growth. A necking between NPs was observed in the film, however, unnecked NPs were still found. The Au film sintered under O₂ atmosphere showed the NPs agglomeration with various sizes up to 50 nm. X-ray characteristic peaks of the (111)-preferred orientation were observed in all samples. All samples showed N–H stretching at 3200–3300 cm⁻¹ regardless of sintering atmosphere. Hydrocarbon chains (C–H) at 2850–3000 cm⁻¹ were detected in the film sintered under N₂. For the Au film sintered under O₂, C–H stretching at 2850–3000 cm⁻¹, C–H deformation at 1350–1470 cm⁻¹, and C–O stretching at 1200–1300 cm⁻¹ were observed. C–O stretching at 1600–1700 cm⁻¹ was observed for the film sintered under FA/N₂ atmosphere. The electrical resistance of the film was related with microstructures and organic residual materials left in the film. Even though either porosity or carbon residues were observed in the film, the sintering of NPs in FA/N₂ or N₂ showed the sheet resistance comparable to that of electroplated one.     

Photocatalytic degradation of indigo carmine dye using TiO<Subscript>2</Subscript> impregnated activated carbon

The photocatalytic degradation of indigo carmine dye was studied using hydrothermally prepared TiO₂ impregnated activated carbon (TiO₂: AC). A comparison between the degradation of the indigo carmine dye using commercial TiO₂ and TiO₂: AC revealed the efficiency of the title compound. The degradation reaction was optimized with respect to the dye concentration and catalyst amount. The reduction in the chemical oxygen demand (COD) revealed the mineralization of dye along with colour removal. The active compound like TiO₂ was impregnated onto the activated carbon surface under mild hydrothermal conditions (<250°C, P ∼ 40 bars). The impregnated activated carbon samples were characterized using powder X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Magnetic properties and methylene blue adsorptive performance of CoFe2O4/activated carbon nanocomposites

Owing to the unique microporous structure and high specific surface area, activated carbon (AC) could act as a good carrier for functional materials. In this paper, CoFe₂O₄/AC nanocomposites were prepared by a facile hydrothermal method for the adsorption of dyes in wastewater. The results indicated that CoFe₂O₄ nanoparticles presented the spinel structure and existed in the pores of AC. The saturation magnetization (Ms) increased with the CoFe₂O₄ content, while the surface area and pore volume decreased. For the larger magnetic moment, very few CoFe₂O₄ were needed to maintain the higher surface area of CoFe₂O₄/AC nanocomposites. The sample-5 (CoFe₂O₄:C = 1:200) possessed the surface area of 1096.85 m² g⁻¹ (close to 1243.35 m² g⁻¹ of AC) and Ms of 5.11 emu g⁻¹, which were sufficient for magnetic separation in wastewater treatment. 99% methylene blue could be adsorbed in 50 min, and then the CoFe₂O₄/AC nanocomposites could be separated from the solution easily by an outer magnet.     

Preparation of high-surface-area activated carbon from <Emphasis Type="Italic">Zizania latifolia</Emphasis> leaves by one-step activation with K<Subscript>2</Subscript>CO<Subscript>3</Subscript>/rarefied air

An activated carbon (AC) with high-porosity was prepared from Zizania latifolia leaves by a one-step method combining chemical and physical activation. K₂CO₃ was employed as a chemical reagent, and air as a physical agent. During the activation, several key parameters were discussed, including the effects of activation temperature, K₂CO₃ impregnation ratio, amount of introduced air on the surface area and pore volumes evolution of the ACs derived from the Zizania latifolia leaves. The synergistic effect between the chemical agent and the physical agent was also investigated. Under optimal activation conditions, the as-synthesized AC attained a maximum surface area up to 2481 m²/g, with 1.21 cm³/g pore volume, and it had a micro/meso porosity developed by the combining activation. The crystal sizes of the as-synthesized AC along the a- and c-axes were about 5 nm and 1–2 nm, respectively. The average thickness of the crystallites is 3–4 layers with about 0.37 nm interlayer spacing.     

Mesoporous activated carbon as electrode for electric double layer capacitor

Mesoporous activated carbon with high surface area (AC-Y-A) was prepared by carbonization followed by activation of vinylidene chloride copolymer containing yttrium acetylacetonate. Their performances as electrodes for electric double layer capacitor (EDLC) in tetraalkylammonium tetrafluoroborate (R₄NBF₄)/propylene carbonate (PC) or acetonitrile (AN) solutions were evaluated and compared with those of microporous activated carbons [AC and AC(CO₂)]. The specific EDLC capacitances of the mesoporous AC-Y-A are almost the same as those of microporous AC at low discharge current. However, at high discharge current, the specific capacitance of AC-Y-A is much higher than those of both AC and AC(CO₂) in PC and AN solutions. The capacitances decreased with increasing alkyl chain length of R₄ N⁺ ions and discharge current. These results suggest that EDLC capacitance depends on pore size of activated carbons and smooth movement of R₄ N⁺ ions in pores.     

Changes on the nanostructure of cementitius calcium silicate hydrates (C–S–H) induced by aqueous carbonation

The nanostructure of the main binding phase of the hydrated cements, the calcium silicate hydrates (C–S–H), and their structural changes due to aqueous carbonation have been characterized using TEM, nitrogen physisorption, and SAXS. Synthetic C–S–H has been used for this purpose. Two different morphologies were identified, similar to the high density and low density C–S–H types. When submitting the sample to a CO₂ flux, the low density phase was completely carbonated. The carbonation by-products, calcium carbonate, and silica gel were also identified and characterized. The precipitation of the silica gel increased the specific surface area from 95 to 132 m²/g, and its structure, formed by particles of ~5 nm typical radius, was observed by small angle X-ray scattering. In addition, the resistance of the high density C–S–H to carbonation is reported, and the passivating effect of the precipitated calcium carbonate is also discussed. Finally, the results have been compared with carbonation features observed in Portland cement carbonated experimentally at downhole conditions.     

Effects of activation agents and intrinsic minerals on pore development in activated carbons derived from a Canadian peat

Activated carbons (ACs) with very high specific surface areas up to approximately 900 m²/g and total pore volume up to 0.5 cm³/g were produced from a Canadian peat through chemical activation using either H₃PO₄ or ZnCl₂ as the activation agent, followed by activation/carbonization in air at 450 °C for 45 min. ZnCl₂ was found to be more effective for developing microporous structures in the ACs, while H₃PO₄ is more efficient in developing the mesopores. Demineralization of the AC precursor to remove intrinsic minerals greatly affected the development of pore structures during the activation process. The AC derived from the demineralized peat activated by ZnCl₂ attained the highest BET surface area with significantly increased micro-/mesopores.     

Perfect wettability of carbon by liquid aluminum achieved by a multifunctional flux

The wettability of carbon (graphite and glassy carbon) by liquid aluminum was studied. A special molten salt (flux) system was developed under which perfect wettability (a zero contact angle) of liquid aluminum was achieved on carbon surfaces. The principal component of the flux is K₂TiF₆ dissolved in a molten alkali chloride. K₂TiF₆ is a multifunctional flux component as it performs the following tasks: (i) dissolves the oxide layer covering liquid aluminum, (ii) through an exchange reaction with liquid aluminum it ensures the necessary amount of Ti dissolved in liquid Al, which is needed to cover the Al/C interface by TiC. As TiC is a metallic carbide, it is perfectly wetted by liquid Al–Ti alloys. In this paper, the conditions of perfect wettability of carbon by liquid Al under MCl–K₂TiF₆ molten salts (fluxes) are found as function of: (i) the basic component of the flux (MCl = LiCl, or NaCl–KCl or CsCl), (ii) K₂TiF₆ content of the flux, (iii) temperature, (iv) flux:Al weight ratio, (v) specific surface area of Al, and (vi) specific surface area of carbon. A simplified theoretical equation is derived to reproduce the experimental data.     

One-pot synthesis of nickel oxide–carbon composite microspheres on nickel foam for supercapacitors

Using glucose and Ni(NO₃)₂ as precursors, the nickel oxide–carbon (NiO–C) composites are directly formed on the nickel foam by a one-pot hydrothermal method. The product presents a spherical morphology with the carbon component in the composite being a non-graphitic phase. The presence of NiO provides additional pseudo-capacitance for the electrode materials and the composites exhibit superior specific capacitance to the pure carbon materials formed on the nickel foam. Apart from that, compared with the nickel sheet substrate, the utilization of the nickel foam benefits for the achievement of higher performance and stability owing to its unique 3D structure. The highest specific capacitance for the NiO–C–nickel foam electrode of 265.3 F g⁻¹ is obtained at a discharge current density of 0.25 A g⁻¹.     

Adsorption characteristics of activated carbon prepared from spent ground coffee

This paper reports results of a research project which attempts to produce low-cost activated carbon from agro-residue wastes. The ground coffee residue which is a by-product of coffee making was collected from coffee shops, prepared, and converted to activated carbon by a chemical activation method. The objective of this work is to investigate the effects of preparation conditions on properties of the activated carbon obtained. The preparation condition is defined by pyrolysis rate, concentration of ZnCl₂, impregnation time, and carbonization temperature. The pyrolysis rate was fixed at 10 °C min^?1 for 4 h with three concentrations of ZnCl₂ (5, 10, and 15 wt%), three durations of impregnation time (8, 12, and 24 h), and three carbonization temperatures (400, 450, and 500 °C). The morphology and specific surface area were, respectively, determined using SEM and BET techniques. It was found in this study that the activated carbon with the best properties was obtained at the preparation condition given by 15 wt% of ZnCl₂, impregnation time of 24 h, and 500 °C carbonization temperature. On average, the activated carbon had a pore diameter of 0.61 nm, specific surface area of 831 m² g^?1, and a total pore volume of 0.44 cm³ g^?1. It was also found that the adsorption isotherm of Cu (II) fitted well with Freundlich isotherm.     

Impregnation of ZnO onto activated carbon under hydrothermal conditions and its photocatalytic properties

Zinc oxide photocatalyst was impregnated onto the activated carbon under mild hydrothermal conditions (T=150°C, P = 20–30 bars) to form a ZnO:AC composite material. The ZnO:AC composite was characterized using powder X-ray diffraction (XRD), Fourier infrared spectroscopy (FTIR), BET surface area measurements and scanning electron microscopy (SEM). As-prepared ZnO:AC composite exhibited higher photocatalytic activity when compared to the commercial ZnO and untreated activated carbon; this was testified by the degradation of acid violet dye using ZnO:AC and commercial ZnO. The effect of various parameters such as initial dye concentration, catalyst loading, pH of the medium, source and intensity of illumination on the photocatalytic degradation of acid violet using ZnO:AC were investigated. Real time textile effluents have also been considered for the degradation using ZnO:AC composites. The reduction in the chemical oxygen demand (COD) values of the treated effluents revealed a complete destruction of the organic molecules along with the color removal.     

Porous carbons

Carbon in dense as well as porous solid form is used in a variety of applications. Activated porous carbons are made through pyrolysis and activation of carbonaceous natural as well as synthetic precursors. Pyrolysed woods replicate the structure of original wood but as such possess very low surface areas and poor adsorption capacities. On activation, these exhibit increased adsorption volumes of 0.5-0.8 cm³/gm and surface areas of 700–1800 m²/gm depending on activation conditions, whether physical or chemical. Former carbons possess mixed pore size distribution while chemically activated carbons predominantly possess micropores. Thus, these carbons can be used for adsorption of wide distributions of molecules from gas to liquid. The molecular adsorption within the pores is due to single layer or multilayer molecule deposition at the pore walls and hence results in different types of adsorption isotherm. On the other hand, activated carbon fibres with controlled microporous structure and surface area in the range of 2500 m²/gm can be developed by controlled pyrolysis and physical activation of amorphous carbon fibres. Active carbon fibres with unmatchable pore structure and surface characteristics are present and futuristic porous materials for a number of applications from pollution control to energy storage.     

Biomass-derived nitrogen-rich porous carbon composite for supercapacitor application

In this study, egg proteins are used as a nitrogen source for the synthesis of nitrogen-rich carbonaceous material through hydrothermal carbonization (HTC) for the electrochemical energy storage application. The composite of activated carbon with egg-derived protein (AC/EDP) is prepared by mixing untreated egg proteins in the aqueous dispersion of activated carbon, followed by HTC at 220 °C for 12 h in a Teflon-lined autoclave. The resultant composite is then directed to chemical activation with KOH and thermal activation at a temperature ranging from 500 to 700 °C. The nitrogen-doped activated carbon exhibited a microporous and mesoporous structure with a high specific surface area of 1660 m² g^?1, confirmed through BET analysis. The composite morphology was analyzed through scanning and high-resolution transmission electron microscopy. X-ray photoelectron spectroscopy indicates the presence of a considerable amount of pyrrolic, pyridinic, and quaternary nitrogen in AC/EDP, which improved the electrochemical performance. The composite activated at 700 °C exhibited the highest capacitance of 263 F g^?1 at a current density of 0.2 A g^?1. The highest energy density and power density values are 32 Wh kg^?1 and 7920 W kg^?1, respectively. The AC/EDP exhibited high cyclic stability, and the capacitance retention observed after 10,000 cycles is 98%.

     

Hydrothermal synthesis and characterization of micro to nano sized carbon particles

Experiments under hydrothermal conditions were carried out to study the formation of elemental carbon through the decomposition of β-Silicon carbide (β-SiC) in the presence of organic compounds. The organic compounds were known to dissociate at low P-T conditions and produce the C–O–H supercritical fluids, which will have great influence on the decomposition of β-SiC. These hydrothermal runs under the influence of supercritical fluids will not only help in dissociating the silicon carbide into SiO₂ and free elemental carbon but also control the type of carbon phase formation. The Scanning Electron Microscopic (SEM) studies of the run products indicate the formation of the micron sized carbon particles having characteristic shapes like spherical/ovoid, scaly with metallic lustre and in some runs we find the formation of nano sized octahedral crystallites. Micro Raman study of these carbon particles reveals that bulk of the carbon formed in these runs were disordered / sp²–hybridized carbon but we also noticed the formation of black scales over the spherical particles and the octahedral crystallites which exhibits the sp³-hybridization having the characteristic sharp band at 1332 cm^-1, comparable to that of diamond powder. The octahedral crystallites are formed at the inner walls of the hollow spherical carbon particles. In any of the experimental runs we haven't used either the metal catalysts or the diamond seed with the charge material. Thus, the nucleation and formation of nano sized octahedral crystallites of diamond (sp³-hybridized carbon) in the present study at lower P-T conditions compared to that of its formation in nature is a significant breakthrough in the study of diamond synthesis.     

Activated carbons derived from sugarcane bagasse for high-capacitance electrical double layer capacitors

Activated carbon (AC) from sugarcane bagasse was prepared using a simple two-step method of carbonization and chemical activation with four different activating agents (HNO₃, H₂SO₄, NaOH, and KOH). Amorphous carbon structure as identified by X-ray diffraction was observed in all samples. Scanning electron microscopy revealed that the AC had more porosity than the non-activated carbon (non-AC). Specific capacitance of the non-AC electrode was 32.58 F g^?1 at the current density of 0.5 A g^?1, whereas the AC supercapacitor provided superior specific capacitances of 50.25, 69.59, 109.99, and 138.61 F g^?1 for the HNO₃ (AC-HNO₃), H₂SO₄ (AC-H₂SO₄), NaOH (AC-NaOH), and KOH (AC-KOH) activated carbon electrodes, respectively. The AC-KOH electrode delivered the highest specific capacitance (about 4 times of the non-AC electrode) because of its good surface wettability, the largest specific surface area (1058.53 m² g^?1), and the highest total specific pore volume (0.474 cm³ g^?1). The AC-KOH electrode also had a great capacitance retention of almost 100% after 1000 GCD cycles. These results demonstrate that our AC developed from sugarcane bagasse has a strong potential to be used as high stability supercapacitor electrode material.

     

The effect of high-temperature heat-treatment on the strength of C/C–SiC joints

Joining of carbon fiber reinforced C–SiC dual matrix composite (denoted by C/C–SiC) is critical for its aeronautical and astronautical applications. Joining of C/C–SiC has been realized through a reaction joining process using boron-modified phenolic resin with micro-size B₄C and nano-size SiO₂ powder additives. The effect of the heat-treatment temperature on the retained strength of the joints, calculated by dividing the strength of the heat-treated joints by the strength of the joints before heat-treatment, was studied. The maximum retained strength of the joints is as high as 96.0% after the heat-treatment at 1200 °C for 30 min in vacuum, indicating good heat resistance of the joints. The thickness of the interlayer of the joint after the heat-treatment is about 18 μm and it is uniform and densified. There are no obvious cracks or pores at the interfaces. During the heat-treatment, carbon, oxygen, silicon, and boron diffuse at the interfacial area. The interlayer is composed of B₄C, SiO₂, glassy carbon, amorphous B₂O₃, and borosilicate glass. SiC appears in the interlayer of the joint heat-treated at 1400 °C for 30 min in vacuum. The addition of B₄C and SiO₂ powders contributes to the densification of the interlayer, the bonding at the interfaces and the heat resistance of the joints.     

Influence of Si:Al ratio on the microstructural and mechanical properties of a fine-limestone aggregate alkali-activated slag concrete

Three series of fine limestone aggregate, alkali-activated blast furnace slag (AAS) concretes were fabricated and tested; two through activation with waterglass/NaOH solution, of which one included NaCl as a retarding agent, and one activated by Na₂CO₃. Each of these series was made up of three formulae containing different amounts of Al₂O₃. The compressive strengths of the series activated by waterglass/NaOH after 28 days were ≈65 ± 5.3 MPa, a 22% increase compared to previously reported formulae containing no additional Al₂O₃. Increasing the amount of Al₂O₃ did not further increase strength, however. The Na₂CO₃-activated formulae had strengths of ≈35 ± 3 MPa after 28 days, representing no increase in strength over formulae not containing Al₂O₃ previously reported. X-ray diffraction showed the main binding phase to be calcium silicate hydrate (C–S–H) gel, as is commonly found in ordinary Portland cement (OPC). Fourier transform infrared spectroscopy showed little difference from the previously reported results for formulae not containing Al₂O₃ and strongly resemble the spectra reported elsewhere for C–S–H. Electron microscopy, coupled with energy dispersive spectroscopy, showed the cementing phase to be a single homogenous phase—not a mixed system of geopolymer and C–S–H gel—with a lower volume fraction of unreacted slag than formulae without Al₂O₃. The reason for the increase in strength of Al₂O₃-containing formulae is unclear, but is unlikely to be ascribed to the formation of large amounts of ‘geopolymers’ and may be related to a possible increase in reaction temperature of between 2 and 5°C, depending on amount of additive.     

Removal of lead (II) and nickel (II) ions from aqueous solution using activated carbon prepared from rapeseed oil cake by Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> activation

In this study, rapeseed oil cake as a precursor was used to prepare activated carbons by chemical activation with sodium carbonate (Na₂CO₃) at 600 and 800 °C. The activated carbon with the highest surface area of 850 m² g^?1 was produced at 800 °C. The prepared activated carbons were mainly microporous. The activated carbon having the highest surface area was used as an adsorbent for the removal of lead (II) and nickel (II) ions from aqueous solutions. The effects of pH, contact time, and initial ion concentration on the adsorption capacity of the activated carbon were investigated. The kinetic data of adsorption process were studied using pseudo-first-order, pseudo-second-order kinetic models and intraparticle diffusion model. The experimental data were well adapted to the pseudo-second-order model for both tested ions. The adsorption data for both ions were well correlated with Langmuir isotherm. The maximum monolayer adsorption capacities of the activated carbon for the removal of lead (II) and nickel (II) ions were determined as 129.87 and 133.33 mg g^?1, respectively.     

Electro-conductive porous ceramics prepared by chemical vapor infiltration of TiN

Using pressure-pulsed chemical vapor infiltration (PCVI) method, TiN was partially infiltrated at 850 °C from gas system of TiCl₄ (1%)–N₂ (10%)–H₂ into the highly porous carbon substrates prepared by the carbonization of cotton-wool, filter paper, and wood at 1,000 °C in Ar for 4 h. After 10,000 pulses of PCVI, electro-conductive porous ceramics having the three-dimensionally continuous current paths were obtained, which had the porosity of 80% and more, the resistivity of 10⁻⁵–10⁻⁶ Ω m, and the average pore sizes of 10–40 μm. The geometric surface area per unit volume of the sample was higher than that of the conventional foil-type current collector for lithium-ion battery. The surface area showed the highest value for the sample obtained from carbonized wood substrate.     

Mechanical properties and oxidation behaviors of carbon/carbon composites with C–TaC–C multi-interlayer

To improve the mechanical properties and oxidation-resistance properties, a C–TaC–C multi-interlayer structure was introduced in carbon/carbon (C/C) composites by chemical vapor infiltration. Compared with conventional C/C composites, a higher fracture toughness and strength have been achieved by using the C–TaC–C multi-interlayer. In addition, the composites also exhibit a higher preliminary oxidation temperature and a lower mass loss at high temperatures. The oxidation rate of the composites increases with temperature increasing in the range of 700–1300 °C, reaching a maximum value at 1300 °C, then decreases in 1300–1400 °C. A hexagonal structure of Ta₂O₅ phase is obtained when being oxidized at 700–800 °C, and it transforms to an orthorhombic phase at temperatures above 900 °C. The structures of C–TaC–C multi-interlayer are intact without cracks or porosities after being oxidized at 700–800 °C. In 900–1300 °C, the composites are oxidized uniformly with the formation of pores. At temperatures above 1300 °C, there are oxidation and non-oxidation regions with the oxidation process being controlled by diffusion.