CB and CNTs are both nanoscale structures of graphitic carbon, although they have very different shapes. CNTs have very large aspect ratios, whereas CB has a more compact conformation. CB has been used as an additive to polymers to improve their conductivity for many years whereas CNTs are only now starting to be produced in large quantities (tonnes/year). There is, however, much interest in producing polymer/CNT composites owing to the superior electrical conductivity properties of the CNTs. The purpose of this study was to make a direct comparison between the microstructures formed by these two different carbon structures and the effect on rheology and electrical conductivity. CNTs were grown by CVD in the Material Science Dept at Cambridge University, producing tubes which are larger than commercially available CNTs. CB was obtained from the Cabot company. Both CB and CNTs were suspended in UV-curing epoxy, a Newtonian fluid, using ultrasound.
In an effort to understand the origin of the viscosity enhancement, optical microscopy combined with a Cambridge Shear System shear cell was performed on low concentration CB and CNT samples. The shear cell consists of two parallel quartz plates, the top of which is fixed whereas the bottom can rotate. The gap size was set at 0.5 mm and the sample was viewed 7.5 mm from the centre of the plates. CB and CNTs have very different structures: CB particles form isotropic, approximately spherical shapes whereas CNTs are long and thin and therefore anisotropic. There is therefore much more scope for particle alignment with shear for CNTs than CB and it was hoped that this would be observed. CB and CNT suspensions were subjected to the same shear rate profile as shown in this sketch. The samples were sheared at 0.5 reciprocal seconds for 15 mins. The shear rate was then increased to 10 s-1 and then 100 s-1 for 10 s each. Video was taken for the last 10 s of the 0.5 s-1 shear and the entire period of shear at 10 s-1 and 100 s-1.
Consider first a 0.25 wt% suspension of CB: [start video], the dark regions are the CB and the light regions are the epoxy. After 15 mins at a relatively low shear rate, these large aggregates have been formed… As the shear rate is increased, these aggregates start to break up… and then are broken up even more when the shear rate is increased to 100 s-1 so that at the end of the experiment… they are considerable smaller than they were at the beginning of the video. The aggregate size correlates with the viscosity of the suspension: larger aggregates correspond to a higher viscosity.
Repeating the experiment with a 0.25 wt% suspension of CNTs gives similar results, although the video is less clear. Once again, the dark regions correspond to the CNTs and the light regions the epoxy. After long times at low shear rates, large aggregates have been formed and these aggregates are broken down as the shear rate is increased.
This image shows a [0.25 wt%] suspension of CB after a long time at low shear. As seen in the video, the CB particles have formed aggregates. These are relatively large compared to the CB particles themselves, and so there will be relatively large van der Waals forces between them. At higher concentrations, the aggregates will be larger, and so the interactions between them will be larger until ultimately an extensive “elastic network” will be formed, corresponding the low frequency plateau observed in G’. [click] This image shows a suspension of CB after a period of high shear. The aggregates have been broken down, corresponding to a reduction in the apparent viscosity of the sample. [click and pause] The shear thinning behaviour is clearly related to the size of CB aggregates, with viscosity enhancement at low shear rates being possibly due to interaction between the aggregates. [pause] [click] CNTs have a long, thin structure compared to the more isotropic CB particles. This may affect the way they behave in suspension [pause]. After long times at low shear rates, the CNTs also form aggregates. As well as the van der Waals forces holding the particles together, however, there is also entanglement of CNTs [point out]. [click and pause] [click] After shear at high shear rates, as with the CB suspension, the aggregates have broken down, and the smaller CNT structures have also aligned with the shear flow [click and pause].
Steady Shear experiments were also performed and, once again, the behaviour is very similar between the CB and CNT suspensions, although a larger concentration of CB is required to get the same rheological response. Both systems show shear thinning behaviour with the amount of viscosity enhancement at low shear increasing with increasing filler concentration.
CNT Nantes- 2011
Carbon Nanotubes (CNTs)The Good The Bad& the Ugly Malcolm Mackley, Anson Ma, Kat Yearsley Department of Chemical Engineering and Biotechnology
The Carbon FamilyGraphite Graphene Diamond CNT Carbon Black 2
CNTs; “The Good” Super potential properties Stiffness CNT 1000 GPa (steel 210 GPa) Electrical Conductivity CNT 106 10 S /m 7 (Copper 6 107 S /m) Thermal Conductivity CNT 3500 W/mK (Copper 385 W/mK) 3
CNTs; “The Bad” health risk University of Cambridge Health and Safety Office HSD060C July 2009Carbon nanotubes and other insoluble fibrousnanoparticles that have the potential to becomeairborne should be handled under HEPA‡filtered local exhaust ventilation (LEV). ‡ High Efficiency Particulate Air (HEPA) 4
Aggregation/orientation model , Paco Chinesta Diffusion equation ∂ dρ ∂ ∂ψ (ρ, n ) ∂ψ (ρ, n ) ∂ 2ψ (ρ, n ) − ψ (ρ, n ) + Dr ( n ) − 3vn + Dn =0 ∂ρ dt ∂ρ ∂ρ ∂n ∂n 2 Where n is population from n = 0 to n = 1. βα = vd vc vn = v c − vd Dn = vd n + vc (1 − n ) , Destruction rate due to shear creation rate Constitutive equation σ = − p I + 2η D + 2ηN p a : D 17
CNT Fibre makers Pasqualli Group Rice University Houston Windle team Cambridge University 19
Rice University Houston.;Anson Ma and Matteo Pasquali, Oct 2010 20
TRUE SOLUTIONS OF SWNTs 1 min 10 min 60 min CNTs dissolve spontaneously in chlorosulfonic acidDavis, Pasquali, et al, Nature Nanotech, 4, 830, 2009 21Pasquali et al, US Patent Application under review (2009)
BEHAVIOR AT HIGH CONCENTRATION 12% vol SWNT in chlorosulfonic, CROSS POLARS co hi ntra nc gh t e er ion ISOTROPICCONCENTRATED LIQUID CRYSTALLINE analyzer -60 -45OOOO -75 -30O 50 µm 0O -15 polarizer Davis, Pasquai, et al, Nature Nanotech, 4, 830, 2009 22
SPINNING NEAT SWNT FIBERSEricson, Pasquali, Smalley, et al., Science, 305, 1447 (2004)Smalley, Pasquali, et al., US Patent 7,125,502 (2006)Davis, Pasquali, et al., Nature Nanotech., (2009)Pasquali et al., patent application under review 23
SPINNING FROM CHLOROSULFONIC ACID 7 WT% SWNT in chlorosulfonic 7 wt% SWNT in 7 wt% SWNT in acid, coagulated in 96% H2SO4 chlorosulfonic chlorosulfonic 13 µm acid: Solvent acid, coagulated in evaporation dichloromethane 30 µ m 100 µm 100 µm 1 µm 1 µm 1 µm • Smooth, compact fibers via slow coagulation • Coagulants: dichloromethane, chloroform, ether, sulfuric acid (96%) 25 Davis, Pasquali, et al, Nature Nanotech, 4, 830, 2009
Carbon Nanotube Fibres andtheir Composites Alan Windle And Team acromolecular aterials ab New Museums Site, Pembroke Street Cambridge, CB23QZ, UK
Fibre spinning in furnace H2 Ethanol or Hexane Ferrocene 2% Thiophene 0.3% Nanotube 1100 - 1300`°C Smoke Its elastic! an aerogel Ya-Li Li, Ian Kinloch and Alan Windle, Science, 304, p 276, 9 April 2004 27
Department of Materials, Cambridge, Injection system Reactor Gas exchange valveFibre collection 28
Continuous wind up with drawing (10 - 50 m/min)n.b. 20m/min at 0.05 tex is only 1mg/min or ~ 1g/day 29
The knot test; (Get Knotted) Tensile strength not degraded by presence of a knot. c.f. For carbon fibre with a knot, strength can be only 10 % of unknotted sample 30
Wide angle X-ray of fibre ESRF synchrotron source Grenoble 002 Winding rate = 20 m/min Winding rate = 30 m/minVery good orientation of CNTs, graphitic component Unoriented 31but an unoriented component (particles ?)
Carbon nanotube fibre propertiesPhysical: Specific Gravity 0.5 – 1.1 (Diameter 10 micron)Mechanical: Strength 0.5 - 2.3 GPa/SG Stiffness 20 – 80 GPa/SGThermal: Conductivity 50 – 1000 Wm -1K-1Electrical: Conductivity 8 x 105 S/m (no influence of sample length, but only 1/60 ofcopper or 1/8 on a unit mass basis). Mix of metallic and 32
Carbon nanotube fibresCarbon fibre Polymer fibres Carbon A new sort Nanotube of stuff ? Fibres Yarns 33
Conclusions•CNTs (still) have exciting potential.•Handle with care.•Dispersion still a challenging problem. Next generation Composite reinforcement ? “Carbon Fibre” ? 34
The problem! Polymers - CNTsMolecular dynamics simulations of entangled polymers (or maybe CNTs?) Prof Aleksei Likhtman (University of Reading) 35