Aerosol generation and measurement of multi-wall carbon nanotubes

Mass production of some kinds of carbon nanotubes (CNT) is now imminent, but little is known about the risk associated with their exposure. It is important to assess the propensity of the CNT to release particles into air for its risk assessment. In this study, we conducted aerosolization of a multi-walled CNT (MWCNT) to assess several aerosol measuring instruments. A Palas RBG-1000 aerosol generator applied mechanical stress to the MWCNT by a rotating brush at feed rates ranging from 2 to 20 mm/h, which the MWCNT was fed to a two-component fluidized bed. The fluidized bed aerosol generator was used to disperse the MWCNT aerosol once more. We monitored the generated MWCNT aerosol concentrations based on number, area, and mass using a condensation particle counter and nanoparticle surface area monitor. Also we quantified carbon mass in MWCNT aerosol samples by a carbon monitor. The shape of aerosolized MWCNT fibers was observed by a scanning electron microscope (SEM). The MWCNT was well dispersed by our system. We found isolated MWCNT fibers in the aerosols by SEM and the count median lengths of MWCNT fibers were 4–6 μm. The MWCNT was quantified by the carbon monitor with a modified condition based on the NIOSH analytical manual. The MWCNT aerosol concentration (EC mass base) was 4 mg/m³ at 2 mm/h in this study.     

Tunable mechanical properties of MWCNT–glass fiber fabric reinforced epoxy composites by controlling MWCNTs dispersing conditions

A novel, simple and cost-effective method, which is capable of easily tailoring the dispersion of multi-walled carbon nanotubes (MWCNTs), was developed here to fabricate the MWCNT–glass fiber fabric (MWCNT–GFf) multiscale composites with tunable mechanical properties. MWCNTs were dispersed into the commercial GFfs through the combined effect of the ultrasound and amino silane (AS) firstly, followed by a resin infusion process. By tuning the ultrasonic power and AS concentration, it is possible to control the MWCNTs dispersion level and subsequently mechanical properties of resultant composite. Making use of optimal dispersion conditions, which involves the optimal combination of ultrasonic power and AS concentration, the interlaminar shear strength of MWCNT–GFf reinforced composites was dramatically increased by 40.5%, and the storage modulus in the glassy region and rubbery region was improved by 27.7% and 125.0%, respectively. The work demonstrates the great promise of this novel method toward practical, industrial application in manufacturing fiber-reinforced composites with superior mechanical properties.     

Development of high efficiency nanofilters made of nanofibers

Electrospinning is a fabrication process that uses an electric field to control the deposition of polymer fibers onto a target substrate. This electrostatic processing strategy can be used to fabricate fibrous polymer mats composed of fiber diameters ranging from several microns down to 100 nm or less. In this study, optimized conditions to produce nanofibers using Nylon 6 are investigated and the Nylon 6 nanofilters using nanofibers of 80–200 nm in diameter are designed and evaluated the filtration efficiency and pressure drop across the filter. When the Nylon 6 concentration is 15 wt.%, electrospun fibers have an average diameter of 80 nm, but there are many beads, and the concentration increases to 24 wt.%, the fiber diameter gradually thickens to 200 nm, but there are not any beads. When the spinning distance is small, the thinner nanofibers are produced and the more fibers are collected on the grounded electrode. The filtration efficiency of Nylon 6 nanofilters is 99.993% superior to the commercialized HEPA filter at the face velocity of 5 cm/s using 0.3 μm test particles. Even though the high pressure drops across the nanofilter, they show the potential to have the application of HEPA and ULPA grade high efficiency filter.     

Formation mechanisms of nanocomposite layers based on multiwalled carbon nanotubes and non-stoichiometric tin oxide

Nanocomposite layers based on multiwalled carbon nanotubes (MWCNTs) and non-stoichiometric tin oxide (SnO _x ) have been grown by magnetron deposition and CVD methods. In the case of the CVD method, the study of the structure and phase composition of obtained nanocomposite layers has shown that a tin oxide “superlattice” is formed in the MWCNT layer volume, fixed by SnO _x islands on the MWCNT surface. During magnetron deposition, the MWCNT surface is uniformly coated with tin oxide islands, which causes a change in properties of individual nanotubes. Electrical measurements have revealed the sensitivity of nanocomposite layers to (NO₂)⁻ molecule adsorption, which is qualitatively explained by a change in the conductivity of the semiconductor fraction of p-type MWCNTs.     

Bipolar diffusion charging of high-aspect ratio aerosols

Recent studies have raised concerns over applicability of the conventional charging theories to non-spherical particles such as soot aggregates and single-walled carbon nanotube aerosols of complex shape and morphology. It is expected that the role of particle structure and shape on particle diffusion charging characteristics may be significant in the submicron size range for carbon nanotubes (CNTs) and nanofibers (CNFs). In this study, we report experimental data on equilibrium charging characteristics of high-aspect ratio aerosol particles such as CNFs and multi-walled CNTs (MWCNTs) when exposed to a bipolar ion atmosphere. A neutral fraction was measured, i.e., the fraction of particles carrying no electrical charge. A differential mobility analyzer (DMA) was used to classify aerosols, leaving a bipolar radioactive charger to infer the bipolar charging characteristics at different mobility diameters in the submicron size range. The measured neutral fractions for CNF aerosol particles were lower than the corresponding Boltzmann values by 24.4%, 42.0%, and 45.8% for mobility diameters of 400 nm, 600 nm, and 700 nm, respectively, while the neutral fractions for measured aerodynamic diameters of 221 nm, 242 nm, and 254 nm were much lower than those expected by Boltzmann charge distribution, by 43.8%, 63.1%, and 67.3%, respectively. Neutral fractions of spherical particles of polystyrene latex (PSL) and diethylhexyl sebacate (DEHS) particles, measured under identical experimental conditions and procedure, agreed well with the Boltzmann charge distribution. The measured neutral fractions for MWCNT aerosol particles were lower than the corresponding Boltzmann values by 22.3%–25.0% for mobility diameters in the size range from 279 nm to 594 nm. Charging-equivalent diameters of CNF particles correlated well with either mobility diameter or equal-area diameter, which were found to be larger than their mobility or equal-area diameters by up to a factor of 5 in the size range of 400 nm–700 nm, while those of MWCNT particles were larger than the corresponding diameters by a factor of 2 in the size range of 279 nm–594 nm.     

Thermal and electrical conductivity of poly(l-lactide)/multiwalled carbon nanotube nanocomposites

Multiwalled carbon nanotubes (MWCNTs) are considered to be the ideal reinforcing agent for high-strength polymer composites, because of their fantastic mechanical strength, high electrical and thermal conductivity and high aspect ratio. Polymer/MWCNTs composites are easily molded, and the resulting shaped plastic articles have a perfect surface appearance compared with polymer composites made using usual carbon or glass fibers. Good interfacial adhesion between the MWCNTs and the polymer matrix is essential for efficient load transfer in the composite. The ultrahigh strength polymer composites demand the uniform dispersion of the MWCNTs in the polymer matrix without their aggregation and the good miscibility between MWCNT and polymer matrix. This approach can also be applied to biodegradable synthetic aliphatic polyesters such as poly(l-lactide) (PLLA), which has received a great deal of attention due to environmental concerns. In this study, PLLA was melt-compounded with MWCNTs. A high degree of dispersion of the MWCNTs in the composites was obtained by grafting PLLA onto the MWCNTs (PLLA-g-MWCNTs). After oxidizing the MWCNTs by treating them with strong acids, they were reacted with l-lactide to produce the PLLA-g-MWCNTs. The mechanical properties of the PLLA/PLLA-g-MWCNT composite were higher than those of the PLLA/MWCNT composite. The electrical conductivity of the composites was determined by measuring the volume resistivity, which is a value of the resistance expressed in a unit volume by two-probe method. The thermal diffusivity and heat capacity of composites was measured by laser flash method, and the effects of modification of the MWCNT in PLLA matrix are discussed.     

Electrical conductivity and electromagnetic interference shielding characteristics of multiwalled carbon nanotube filled polyacrylate composite films

Multiwalled carbon nanotubes (MWCNTs) were homogeneously dispersed in pure acrylic emulsion by ultrasonication to prepare MWCNT/polyacrylate composites applied on building interior wall for electromagnetic interference (EMI) shielding applications. The structure and surface morphology of the MWCNTs and MWCNT/polyacrylate composites were studied by field emission scanning microscopy (FESEM) and transmission electron microscopy (TEM). The electrical conductivity at room temperature and EMI shielding effectiveness (SE) of the composite films on concrete substrate with different MWCNT loadings were investigated and the measurement of EMI SE was carried out in two different frequency ranges of 100-1000 MHz (radio frequency range) and 8.2-12.4 GHz (X-band). The experimental results show that a low mass concentration of MWCNTs could achieve a high conductivity and the EMI SE of the MWCNT/polyacrylate composite films has a strong dependence on MWCNTs content in both two frequency ranges. The SE is higher in X-band than that in radio frequency range. For the composite films with 10 wt.% MWCNTs, the EMI SE of experiment agrees well with that of theoretical prediction in far field.     

A modified high-output,size-selective aerosol generator

A compact and commercial aerosol generator capable of generating narrowly size-distributed aerosols with high mass concentrations was designed, fabricated and tested. The aerosol generator, consisting of a Delavan simplex nozzle (Model 30610-4), an L-shaped settling chamber and a virtual impactor with a clean air core, was modified and improved from Chein and Lundgren's work [20] to be more compact and readily commercial. The performance of the aerosol generator was evaluated using corn-oil, sodium chloride and uranine solutions. The results indicated that the cornoil droplets produced by the generator had a mass medium aerodynamic diameter (MMAD) of 7.20 ± 0.32 μm with a geometric standard deviation (GSD) of 1.48 ± 0.01 and the aerosol generation rate was 13.8 ± 1.3 mg/min. Solid aerosols generated from NaCl solution were found to have an MMAD in the range 1.39–4.88 μm with a GSD of 1.34–1.47 with the volumetric solution concentration varying from 0.1% to 9%. At the same time, the aerosol generation rate varied from 0.27 ± 0.05 to 15.8 ± 1.8 mg/min. depending on the solution concentration and the particle size produced. In addition, a 0.01% uraniane solution was tested to generate a submicron aerosol with an MMAD of 0.93 μm and a GSD of 1.48.     

Morphology of single-wall carbon nanotube aggregates generated by electrospray of aqueous suspensions

Airborne single-wall carbon nanotubes (SWCNTs) have a high tendency to agglomerate due to strong interparticle attractive forces. The SWCNT agglomerates generally have complex morphologies with an intricate network of bundles of nanotubes and nanoropes, which limits their usefulness in many applications. It is thus desirable to produce SWCNT aerosol particles that have well-defined, unagglomerated fibrous morphologies. We present a method to generate unagglomerated, fibrous particles of SWCNT aerosols using capillary electrospray of aqueous suspensions. The effects of the operating parameters of capillary electrospray such as strength of buffer solution, capillary diameter, flow rate, and colloidal particle concentration on the size distributions of SWCNT aerosols were investigated. Results showed that electrospray from a suspension of higher nanotube concentration produced a bimodal distribution of SWCNT aerosols. Monodisperse SWCNT aerosols below 100 nm were mostly non-agglomerated single fibers, while polydisperse aerosols larger than 100 nm had two distinct morphologies: a ribbon shape and the long, straight fiber. Possible mechanisms are suggested to explain the formation of the different shapes, which could be used to produce SWCNT aerosols with different morphologies.     

Chemical aerosol detection and identification using Raman scattering

Early warning of the presence of chemical agent aerosols is an important component in the defense against such agents. A Raman spectrometer has been constructed for the purpose of detecting and identifying chemical aerosols. We report the detection and identification of a low‐concentration chemical aerosol in atmospheric air using 532‐nm continuous wave laser Raman scattering. We have demonstrated the Raman scattering detection and identification of an aerosol of isovanillin of mass concentration of 1.8 ng/cm³ with a signal‐to‐noise ratio of about 19 in 30 s for the 1116‐cm⁻¹ mode with a Raman cross section of 3.3 × 10⁻²⁸ cm² using 8‐W double‐pass laser power. Copyright © 2014 John Wiley & Sons, Ltd.     

Effects of gas composition on the growth of multi-walled carbon nanotube

This paper studies the effects of different gas compositions on the growth of multi-walled carbon nanotube (MWCNT) films by using an electron cyclotron resonance chemical vapor deposition (ECR-CVD) method. The Raman spectrum was employed to explore the composition of the MWCNT films grown under different mixtures of C₃H₈ and H₂. The results showed that the optimum relative intensity ratio of the D band to G band (i.e., I_D/I_G) is 2 for the cases considered in this study. In addition, the morphology and microstructure of the MWCNTs were examined by field emission scanning electron microscopy (FE-SEM) and field emission gun transmission electron microscopy (FEG-TEM). Furthermore, atomic force microscopy (AFM) and scanning thermal microscopy (SThM) were used to study the surface topography and thermal properties of the MWCNTs.     

Size response of an SMPS–APS system to commercial multi-walled carbon nanotubes

Carbon nanotubes (CNTs) are representative-engineered nanomaterials with unique properties. The safe production of CNTs urgently requires reliable tools to assess inhalation exposure. In this study, on-line aerosol instruments were employed to detect the release of multi-walled CNTs (MWCNTs) in workplace environments. The size responses of aerosol instruments consisting of both a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS) were examined using five types of commercial MWCNTs. A MWCNT solution and powder were aerosolized using atomizing and shaking methods, respectively. Regardless of the phase and purity, the aerosolized MWCNTs showed consistent size distributions with both SMPS and APS. The SMPS and APS measurements revealed a dominant broad peak at approximately 200–400 nm and a distinct narrow peak at approximately 2 μm, respectively. Comparing with field application of the two aerosol instruments, the APS response could be a fingerprint of the MWCNTs in a real workplace environment. A modification of the atomizing method is recommended for the long-term inhalation toxicity studies.     

Reduced graphene oxide/MWCNT hybrid sandwiched film by self-assembly for high performance supercapacitor electrodes

A reduced graphene oxide/multiwalled carbon nanotube (RGO/MWCNT) hybrid sandwiched film with different MWCNTs content was prepared by vacuum-assisted self-assembly from a complex dispersion of graphene oxide (GO) and MWCNTs followed by heat-treating at 200 °C for 1 h in a vacuum oven to reduce the GO into RGO. The free-standing RGO/MWCNT hybrid sandwiched film before heat-treatment showed a layered structure with an entangled network of MWCNTs sandwiched between the GO sheets. This unique structure not merely contribute to remove the oxygen-containing groups in GO during the heat-treatment, but also decrease the defects for electron transfer between RGO layers, which enhances the electrochemical capacitive performances of graphene-based films. A specific capacitance up to 379 F/g was achieved based on RGO/MWCNT with 30 % MWCNTs mass fraction at 0.1 A/g in a 6 M KOH electrolyte. The excellent performance of RGO/MWCNT hybrid sandwiched film signifies the importance of controlling the surface chemistry and sandwiched nanostructure of graphene-based materials.     

二维相关吸收光谱法研究多壁碳纳米管与腐植酸吸光组分间非均相吸附行为

溶解性有机质（DOM）是水环境中一类复杂的溶解性有机混合物,不仅可影响污染物归趋及生物有效性,且DOM自身属于消毒副产物（DBPs）前驱体。为此,如何高效去除水中DOM已成为当前环境污染控制与治理技术研究的热点问题之一。以商品化腐植酸（HA）为DOM典型代表物质,利用紫外可见吸收光谱结合二维相关光谱法（2D-COS）从动力学、吸附等温线及热力学等角度研究了原始多壁碳纳米管（MWCNT）、羟基化MWCNT及羧基化MWCNT与HA吸光组分间吸附行为。2D-COS提高了HA一维吸收光谱分辨率,经二维相关吸收光谱图分析显示3种MWCNTs对HA吸光组分的吸附变化顺序均为275 nm→400 nm,表明MWCNTs与HA吸光组分间为非均相吸附行为。动力学结果显示MWCNTs与275 nm处HA吸光组分间吸附速率高于MWCNTs与400 nm处HA吸光组分间吸附速率,表明275 nm处HA吸光组分可优先吸附至三种MWCNTs上。在25和35 ℃条件下,MWNCTs与HA吸光组分间吸附等温线表现为非线性,且Freundlich模型拟合决定系数R2大于Langmuir模型拟合决定系数,表明Freundlich吸附等温模型比Langmuir吸附等温模型更有利于描述MWCNTs与HA吸光组分间吸附等温线。275 nm处HA吸光组分的饱和吸附容量（q_max）及相同给定平衡浓度下单点吸附系数K_d高于400 nm处HA吸光组分,进一步表明MWCNTs与HA吸光组分间为非均相吸附。此外,当给定平衡浓度（c_e=0.5 cm-1和c_e=1.5 cm-1）越低,MWCNTs与HA吸光组分间结合能力越大,即HA吸光组分浓度较低时更易与MWCNTs上高能吸附位点相结合。相同平衡浓度下MWCNTs与特定HA吸光组分间吸附能力顺序表现为MWCNT8>MWCNT8-OH>MWCNT8-COOH,表明功能化基团可影响MWCNTs与HA吸光组分间吸附特性。另外,相同条件下单点吸附系数K_d与MWCNTs比表面积间无显著相关性（p>0.05）,表明比表面积不是造成MWCNTs与特定HA吸光组分间结合能力差异的主要因素;K_d与MWCNTs中孔孔隙度间呈显著正相关（p<0.05）而与微孔孔隙度间无显著相关性（p>0.05）,这主要是由于HA吸光组分可进入MWCNTs中孔而难以通过MWCNTs微孔。最后热力学分析结果显示Gibbs自由能变（ΔG0）<0、焓变（ΔH0）>0和熵变（ΔS0）>0,表明MWCNTs吸附HA吸光组分的过程为吸热反应,可自发进行,升温可促进MWCNTs对HA吸光组分的吸附,且吸附过程中固液界面的无序性增加。相同温度及吸光组分下,MWCNT8的ΔG0值小于MWCNT8-OH及MWCNT8-COOH,进一步表明MWCNT8与特定HA吸光组分结合能力强于MWCNT8-OH及MWCNT8-COOH。证明了2D-COS可识别HA中具有不同吸附行为的吸光组分,二维相关吸收光谱技术可作为研究MWCNTs与DOM间非均相吸附行为的有效工具。同时,有助于更好地理解MWCNTs与DOM间相互作用特性及机理,不仅可为水环境中DOM去除研究提供基础数据及新的研究思路,且有利于更好地预测自然环境中MWCNTs及DOM迁移传输及环境归趋。     

Atmospheric aerosols in Lombardy

Summary The paper deals with the study of atmospheric aerosols in some sites in Lombardy. Measurement campaigns and sampling methods are reported along with the data obtained with XRF and PIXE techniques on the aerosol mass concentration and its elemental composition. Paper presented at the IX Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 8–10, 1992, Rome.     

Electrospun cross linked rosin fibers

In this study, we describe the first reported preparation of rosin in fiber form through use of an electrospinning technique utilizing various solvent systems. The polymer concentration of the formed fiber was studied by using various solvents such as chloroform, ethanol, N-N dimethylformamide (DMF), tetrahydrofuran (THF), acetone, and methylene chloride (MC). An electrospray of the solution resulted in the beaded form of the rosin. By varying the polymer concentration with MC, we were then able to obtain uniform fibers. However, the fibers exhibited large diameter. We believe that it is possible to reduce the diameter of the rosin fibers through appropriate selection of electrospinning parameters. In addition, the morphological transitions from beads, to beaded fiber, to fiber were studied at different polymer concentrations. We propose a possible physical cross linking mechanism for the formation of rosin fibers during the electrospinning process. Our results demonstrate the feasibility of producing fiber nanostructures of rosin by using an electrospinning technique.     

Influence of air-oxidation on electric double layer capacitances of multi-walled carbon nanotube electrodes

In this work, air-oxidized multi-walled carbon nanotube (MWCNT) electrodes have been prepared from catalytically grown MWCNTs of high purity and narrow diameter distribution. The experimental results show that air-oxidation modifies the intrinsic structure of individual MWCNTs and a little improves the dispersity of the MWCNTs. The specific capacitances of the electrodes in electric double layer capacitors (EDLCs) based on oxidized MWCNTs are obviously improved through air-oxidation. The specific capacitance of 50 F/g is obtained in the air-oxidized MWCNTs at 600 °C on a single cell device with 35 wt% H₂SO₄ as an electrolyte. This is probably increased BET specific surface area and mesopore volume of the oxidized MWCNT electrode materials of EDLCs. These properties are, therefore highly desirable for the development of electrochemical capacitors with high power and long cycle life.     

Environmental pollution due to black carbon aerosols and its impacts in a tropical urban city

Black carbon (BC) has become the subject of interest in the recent years for a variety of reasons. BC aerosol may cause environmental as well as harmful health effects in densely inhabited regions. BC is a strong absorber of radiation in the visible and near-infrared part of the spectrum, where most of the solar energy is distributed. Black carbon is emitted into the atmosphere as a byproduct of all combustion processes, viz., vegetation burning, industrial effluents, motor vehicle exhausts, etc. In this paper, we present results from our measurements on BC aerosols, total aerosol mass concentration, and aerosol optical depth over an urban environment, namely Hyderabad during January-May, 2003. Diurnal variations of BC suggest that high BC concentrations are observed during 6:00-9:00 h and 19:00-23:00 h. Weekday variations of BC suggest that the day average BC concentrations increases gradually from Monday to Wednesday and gradually decreases from Thursday to Sunday. Fraction of BC to total mass concentration has been observed to be 7%. BC showed positive correlation with total mass concentration and aerosol optical depth at 500 nm. Radiative transfer calculations suggest that during January-May, diurnal averaged aerosol forcing at the surface was calculated to be −33 Wm⁻² and at the top of the atmosphere (TOA) it is to be +9 Wm⁻².     

Microwave-induced fabrication of copper nanoparticle/carbon nanotubes hybrid material

Copper nanoparticle/multi-walled carbon nanotube (MWCNT) hybrid material could be fabricated by microwave irradiation to a suspension of MWCNTs dispersed in copper precursor with the presence of ethylene glycol. Microscopic and spectroscopic analyses could confirm the uniform dispersion of copper oxide nanoparticles hybridized on the outer surface of MWCNTs. Reduction of Cu²⁺ ions to Cu₂O and Cu nanoparticles was ascribed to functional groups which were generated after the precursor was irradiated by microwave. Sufficient irradiation time of 5 min or longer played an important role to induce the agglomeration of copper oxide nanoparticles on the MWCNT surface.     

Plasma-polymerized organosilicones as engineered interlayers in glass fiber/polyester composites

The plasma polymerization technique was used to surface modify glass fibers in order to form a strong but tough link between the glass fiber and the polyester matrix, and enable an efficient stress transfer from the polymer matrix to the fiber. Plasma polymer films of hexamethyldisiloxane, vinyltriethoxysilane, and tetravinylsilane in a mixture with oxygen gas were engineered as compatible interlayers for the glass fiber/polyester composite. The interlayers of controlled physico-chemical properties were tailored using the deposition conditions with regard to the elemental composition, chemical structure, and Young's modulus in order to improve adhesion bonding at the interlayer/glass and polyester/interlayer interfaces and tune the cross-linking of the plasma polymer. The optimized interlayer enabled a 6.5-fold increase of the short-beam strength compared to the untreated fibers. The short-beam strength of GF/polyester composite with the pp-TVS/O₂ interlayer was 32% higher than that with industrial sizing developed for fiber-reinforced composites with a polyester matrix.