Deliverables

Deliverables for 2nd year (2014-01-01 - 2015-12-31)

D.1.1. Experimental samples of glasslike carbon foams and composite glassy carbon - graphite foams of different bulk densities and pore size; data on porosity, cell size and morphology of produced foams and their correlation with electrical conductivity;

Different types of carbon foams were prepared. First, tannin-based glasslike carbon foams were obtained according to a method described elsewhere [1-3]. Briefly, a resin was prepared by dissolving flavonoid tannin in water and mixing the solution with furfuryl alcohol, formaldehyde and a liquid blowing agent. When the mixture was homogenous, a small amount of catalyst was added to polymerize furfuryl alcohol with itself and with tannin, thus producing heat used for boiling the blowing agent. The resin thus foamed and hardened at the same time, leading to lightweight rigid foam whose density was controlled by the initial amount of blowing agent: the higher this amount, the higher the porosity. After drying, parallelepiped blocks were cut off these foams and were pyrolysed at 900°C under nitrogen flow in a tubular electric furnace. Carbon foams were produced at 9 different densities, ranging from 0.036 to 0.114 g/cm3.

In addition to this, composite carbon foams were prepared from the same kind of precursors as before, but using graphite powders as fillers in order to enhance their electrical conductivity [4,5]. For that purpose, the formulations were optimized so that foaming was still possible. Introducing graphite indeed drastically increased the viscosity of the initial resins from which the carbon foams were obtained, and a significant part of the porosity was lost accordingly. As a result, the obtained range of densities was shifted towards higher values, 0.13 – 0.59 g/cm3 or 0.13 – 0.59 g/cm3 using a fine graphite powder or exfoliated graphite as filler, respectively.

Finally, the same kind of composite foams were prepared from sucrose as precursor instead of phenolic compounds as above [6]. For that purpose, sucrose was dissolved in water and various acids were tested for catalyzing sucrose self-polymerization at 120°C. Graphite powders of various kinds and in different amounts were added to these mixtures until no foaming was possible. The mixtures were left for 1 to2 days in a ventilated oven at 120°C, during which sucrose was converted into a highly porous resin due to the release of a high amounts of water vapor. Such materials were then carbonized at 900°C, leaving sucrose-based composite carbon foams comprising graphite powders. A broad range of materials was therefore obtained, having a conductivity about 3 times higher than that of their counterparts made from tannin, whereas having exactly the same range of bulk densities.

A major advance was achieved when using nickel nitrate as foaming catalyst. This compound indeed not only behaves as an acid when decomposed in aqueous sucrose solutions at 120°C, but the residual nickel oxide was reduced to metal nickel during the pyrolysis of the foams. Now, nickel is a well-known graphitization catalyst and, at the contact of carbon, converted the disordered sucrose-derived char into a graphite-like material. As a consequence, higher conductivities were measured and proved to be induced by graphitization. The results were so good that the process and the best formulations, combining low cost, high porosity and high conductivity were patented [7].

Of course, as all these materials were not obtained at once and required long optimizations, all their structural and physical properties were measured so that it was possible to observe improvements or not when coming back to the formulations after characterization. All the data are available on demand.

[1]    W. Zhao, A. Pizzi, V. Fierro, G. Du, A. Celzard, Effect of composition and processing parameters on the characteristics of tannin-based rigid foams. Part I: Cell structure,” Material Chemistry and Physics, vol. 122, no. 1, pp. 175182, July 2010.

[2] W. Zhao, A. Pizzi, V. Fierro, G. Du, A. Celzard, Effect of composition and processing parameters on the characteristics of tannin-based rigid foams. Part II: Physical properties,” Material Chemistry and Physics, vol.  123, no. 1, pp. 210217, Sep. 2010.

[3] G. Tondi, V. Fierro, A. Pizzi, A. Celzard, “Tannin-based carbon foams,” Carbon,  vol. 47, no. 6, pp. 1480 –1492, May 2009.

[4]    P. Jana, V. Fierro, A. Pizzi, A. Celzard. “Biomass-derived, thermally conducting, carbon foams for seasonal thermal storage”, Biomass and Bioenergy, vol. 67, pp. 312-318, 2014.

[5]    P. Jana, V. Fierro, A. Pizzi, A. Celzard. “Thermal conductivity improvement of composite carbon foams based on tannin-based disordered carbon matrix and graphite fillers”, Materials and Design, vol. 83, 635-643, 2015.

[6]    A. Celzard, A. Szczurek, P. Jana, V. Fierro, M.C. Basso, S. Bourbigot, M. Stauber, A. Pizzi. “Latest progresses in the preparation of tannin-based cellular solids”. Journal of Cellular Plastics, vol. 51, pp. 89-102, 2015.

[7]    A. Celzard, P. Jana, V. Fierro. “Porous carbonaceous matrices for thermal energy storage”. French patent FR 13 59063, PCT/EP2014/069800

 

Deliverables for 1st year (2014-01-01 - 2014-12-31)

 

D.1.2. Experimental samples of powders having various grain morphologies and intrinsic electrical properties such as multi-walled CNTs with various mean outer diameters prepared via CVD and arc-discharge methods; exfoliated graphite and thick graphene, graphene nano-pates, activated carbons, etc;

The following carbon/graphite powers were used for analysis :

(i) Exfoliated graphite (EG). Accordion-like particles were produced through a thermal shock of natural graphite flakes intercalated with sulphuric acid, leading to a material of low packing density, around 0.003 g cm-3. Typically, the diameter of the EG particles is in the range 0.3-0.5 mm, and their aspect ratio is around 20.

(ii) Thick graphene (TG) was prepared by suspending EG particles in cyclohexane, and submitting the suspension to a series of grinding and ultrasonic dispersion steps. After freeze-drying, disk-shaped particles, having diameter and thickness around 10 and 0.1 µm, respectively, were obtained.

(iii) Three types of artificial graphite flakes, kindly supplied by Timcal G+T (Switzerland) under the name TIMREX® KS, were also used: coarse graphite (CG), medium graphite (MG), and fine graphite (FG), having mean flake diameters between 100 and 200 µm, between 44 and 75 µm, and between 15 and 44 µm, respectively. The aspect ratio of artificial graphites (ratio of the major to the minor axes of the ellipsoidal particles) is in the range 6.1 - 6.5, the higher the diameter, the higher the aspect ratio as the dimension of the minor axis changes much less.

(iv) Two types of activated carbons were also used for composites fabrication. These are ACF (Activated Carbon Fine, Norit SAUF (94016-7)): typical particle size 5 µm and ACC (Activated Carbon Coarse = Norit GL50 (94009-4)): typical particle size 20 µm.

(v) The effect of all these carbons on the composites properties was compared to that obtained by using natural graphite (NG) from Madagascar as filler. The latter is composed of shiny, rather large, flakes having typical diameters within the range 500 - 750 µm, and a typical aspect ratio of 10 – 11.

Along with all mentioned powders, graphene nanoplatelets (GNP) were produced by microcleavage exfoliation of expanded graphite. As raw material, we used Asbury® expandable graphite. This material is manufactured by treating flake graphite with various intercalation reagents that migrate between the graphene layers in a graphite crystal and remain as stable species. If exposed to a rapid increase in temperature, these intercalation compounds decompose into gaseous products, which results in high inter-graphene layer pressure. This pressure develops enough force to push apart graphite basal planes in the “c” axis direction.

 

Deliverables for 1st year (2014-01-01 - 2014-12-31)

 

D.1.3. Experimental samples of PyC nanometrically thin films on dielectric (SiO2) substrate of different thicknesses; thickness and surface characterization of produced samples using AFM, stylus profiler; structural characterization using Raman spectroscopy and SEM/TEM; data base on sheet resistance of PyC thin films.

PyC films were deposited on 0.5 mm thick quartz substrates by chemical vapor deposition (CVD). Briefly, the CVD chamber was heated to 700°C in hydrogen atmosphere at a pressure of ~ 10 mBar. At the temperature of 700°C the hydrogen atmosphere was replaced by CH4:H2 gas mixture. Next, in order to start the spontaneous methane decomposition, the CVD chamber was heated up to 1100°C at 10°C/min. The temperature of the chamber was kept at 1100°C for five minutes and then cooled down to 700°C during 80 minutes. At the temperature of 700°C the CH4:H2 gas mixture was replaced by hydrogen. The rest of the cooling was done in hydrogen atmosphere (~ 10 mBar). It is worth noting that there was no gas flow in the CVD chamber during the deposition process. This allowed us to reduce the gas consumption and also gave more time for polyaromatic hydrocarbon formation and cross linking. As a result, we got PyC films on the top of SiO2 substrate being 5-110 nm thick.

The homogeneity of the samples was controlled by SEM (LEO - 1455 Vand). PyC films thickness was measured by a stylus profiler (Veeco Instruments, Dektak 150) with an accuracy of 1.5 nm, and was also controlled by atomic force microscopy. The average film roughness was measured with atomic force microscope (Thermo Microscopes, AFM Autoprobe M5 for films thinner than 30 nm, and AFM Solver P47 PRO, NT-MDT for thicker films). The roughness of the fabricated PyC films shows a weak dependence on the thickness. The average and root-mean-square roughness were 1.1 ± 0.2 nm and 1.5 ± 0.3 nm, respectively. Raman spectroscopy measurements revealed that PyC films produced in our experimental conditions are composed of randomly oriented and intertwined graphene flakes with typical size less than 5 nm. The films also consist of small amounts of amorphous carbon and sp2-sp3 bonds.

 

Deliverables for 1st year (2014-01-01 - 2014-12-31)

 

D2.1. Experimental samples of epoxy resin composites filled with 0.25-2.0 wt.% of artificial graphite, natural graphite, exfoliated graphite (worm-like particles), thick graphene, carbon blacks having different surface areas, graphene nano-plates, activated carbon of different granulometries and CVD-made commercial SWCNT and MWCNT, as well as lab-made CVD and arc-discharge CNTs;

Two types of polymer composites samples were produced during the first reporting period of project implementation.

1)    EPIKOTE™ Resin 828 was used as composite matrix. EPIKOTE Resin 828 is a medium viscosity liquid epoxy resin produced from bisphenol A resin and epichlorhydrin. It contains no diluent. EPIKOTE 828 provides good pigment wetting and good resistance to filler settling and a high level of mechanical and chemical resistance properties in cured state. Several series of composite samples, using Epikote 828, a curing agent called A1 (i.e., a modified TEPA) and 0.25, 0.5, 1, 1.5 and 2 wt. % of various graphite/carbon fillers were fabricated as follows. The resin was degassed under vacuum (1–3 mbar) for 12-14 h, then was put into an oven at 65°C. In the meantime, the graphite was dispersed in propanol, and the suspension was submitted to an ultrasonic bath for 1.5 h. Afterwards, the alcoholic suspension of graphite was mixed with the resin. The obtained mixture was placed inside an oven at 130–150°C for evaporating the alcohol. The curing agent A1 was added to the mixture of resin and filler through slow manual mixing for about 7 min. The blend was then poured into moulds of dimensions 1 cm ´ 1 cm ´ 7 cm, and left as such for 20 h for the curing process at room temperature, and finally 4 h in an oven at 80°C. When the process was completed, the samples were removed from the moulds.

 

2)    Multi-walled carbon nanotubes synthesized by CVD technique [2] having external diameter ~ 30 nm and approximate length 10-20 μm were used. The D.E.R. 321 (Dow Chemical) chosen as a polymer matrix is ortho-cresyl glycidyl ether diluted standard bisphenol-A based liquid epoxy resin, of extremely low viscosity (500-700 mPa.s at 25oC). The polyethylene polyamine (PEPA) (viscosity 200-300 mPa.s at 25oC) was used as hardener. A simple sonochemical method was employed for in situ surface modification of carbon nanotubes in one step processing. Pristine MWCNTs were mechanically dispersed for 30 min at 9000 rpm in the liquid epoxy resin (which is a standard processing protocol), as well as in the polyethylene-polyamine hardener resulting in two types of dispersions: DER321/MWCNT and PEPA/MWCNT, containing various amounts of nanotubes. Then, intensive ultrasonic irradiation of the aforementioned dispersions at 250W for 60 min in 40oC temperature bath was applied to provoke grafting of epoxy and polyethylene-polyamine chains to the carbon nanotube surfaces, which resulted in epoxy-grafted (MWCNT-e) and amine-grafted (MWCNT-a) carbon nanotubes in the respective dispersions. Solid composites were then fabricated by curing the irradiated dispersions, DER321/MWCNT and PEPA/MWCNT, with the addition to them of an appropriate amount of the second component (hardener and epoxy resin, respectively), at the molar ratio 70:30 (DER321:PEPA). The process of curing took 2 h in room conditions, followed by post-curing for 2h at 100°C. As a result, bulk 1 mm-thick samples of epoxy-grafted (ER/MWCNT-e) and amine-grafted composites (ER/MWCNT-a) were prepared with different nanotube contents varying from 0.03 wt.%  to 0.3 wt.%.