Characteristics of carbon aerogel at variation in pyrolysis conditions

Carbon aerogel was obtained by pyrolysis of organic aerogel by ambient pressure drying technique. The effect of pyrolysis conditions on characteristics of carbon aerogel such as density, specific surface area and conductivity was studied. The properties and structure of carbon aerogel samples were investigated by nitrogen adsorption, four-point probe method and XRD diffraction. The results showed that carbon aerogel had structure between amorphous and graphite state. The highest specific surface area was 800 m2/g at pyrolysis temperature of 700 o C. The pore-size was distributed in microporous, with the maximum total pore volume of 0.44 cm3/g. The electrical conductivity of carbon aerogel was highest at pyrolysis temperature of 800-900 o C with the value in the range of 1.744-1.923 S/cm.


INTRODUCTION
Carbon aerogels were very interesting monolithic materials with a large of potential applications because of their versatile properties, such as low density, high surface area, high porosity, low electrical resistivity and controllable structure characteristics [1,2].
Carbon aerogels had been prepared from the different organic aerogel sources in many previous studies.Al-Muhtaseb and Ritter [3] synthesized carbon aerogel from resorcinolformaldehyde organic aerogel at different drying conditions.The experiments data showed that properties of carbon aerogel were affected of many factors such as the catalyst concentration, the initial gel pH, the concentration of solids in the sol, the drying method and the conditions of pyrolysis and activation.Carbon aerogel was synthesized from resorcinol-furfural based organic aerogels in the study of Rejitha et al. [4].Carbon aerogels obtained by using different catalysts showed BET surface area, average pore size, total pore volumes in the range of 438-496 m 2 /g, 17.9-22.4Å and 0.20-0.27cm 3 /g, respectively.Moreover, carbon aerogel was synthesized from polyureas, melamineformaldehyde and polyurethanes.Among these materials, the resorcinol-formaldehyde system was the most frequently studied and the reaction parameters were the most well understood [5].
Carbon aerogel are prepared by the polycondensation of resorcinol (R) with formaldehyde (F) under sodium carbonate as base catalyst and pyrolysis process.Generally, there were four steps in the preparation of carbon aerogels, namely sol-gel formation, solvent exchange, drying and pyrolysis, all of which affect the characteristics of carbon aerogels.The most important step that was influence on the properties of carbon aerogel was the drying procedure.The conventional drying method involved a supercritical drying, which was very complex, expensive and dangerous.To overcome these disadvantages, new preparation methods for producing carbon aerogels under ambient pressure drying conditions have been suggested [6][7][8].According to these methods, organic aerogel could be drying at ambient pressure with minimize volume shrinkage.
In previous study [9], carbon aerogel was synthesized from resorcinol-formaldehyde via ambient pressure drying method.The purpose of this paper was to further research the influence of pyrolysis conditions on properties of carbon aerogel.The properties of carbon aerogel such as specific surface area, pore-size distribution were investigated by nitrogen adsorption measurements.The electrical conductivity of these samples was evaluated by four-point probe method.In addition, the structure of carbon aerogel was characterized by XRD diffraction.

Preparation of carbon aerogel
Carbon aerogel (CA) was derived from pyrolysis of a resorcinol-formaldehyde (RF) aerogel.The molar ratio of formaldehyde (F) to resorcinol (R) was held at a constant value of 2. They were dissolved in distilled water with Na 2 CO 3 as a base catalyst, the mass percentage of the reactants in solution was set at RF = 40%, and the molar ratio of resorcinol to catalyst (C) was set at R/C = 1000.Sol-gel polymerization of the mixture was carried out in plastic moulds by holding the mixture at room temperature for 24 h, at 50 o C for 24 h, and at 80 o C for 72 h to obtain RF wet gels.The aqueous gels were then exchanged with acetone for 3 days.Subsequently, RF organic aerogels were prepared by directly drying RF wet gels at ambient temperature and pressure for 5 days.
Carbon aerogels were obtained via pyrolyzing RF organic aerogels at 800 o C in a continuous nitrogen atmosphere, flowing at a rate of 400 mL/min for 3 h and then in a flow of CO 2 for 2 h, flow rate of 200 mL/min.

Characterization methods
Surface area and pore-size distribution of carbon aerogels samples were characterized by analysis of nitrogen absorption-desorption isotherms measured by ASAP 2020 analyzer (Micrometrics Instruments Corp.).Brunauer-Emmett-Teller (BET) method was used for total surface area measurements, and t-plot method was used for estimating mesopore surface area.
Pore-size distribution was obtained by the Barret-Joyner-Halenda (BJH) method from desorption branch of the isotherms.Total porevolume was calculated from the adsorbed volume of nitrogen at P/P 0 =0.99 (saturation pressure).The bulk densities of the samples were estimated by measuring the dimensions and the mass of each monolithic sample.Electrical conductivities of carbon aerogel have been determined by using four-point probe method.Structure of carbon aerogel was characterized X-ray diffraction using a Bruker D8 Advance diffractometer with Cu-Kα radiation (λ=1.54060Å) operated at the voltage and current values of 40 kV and 40 mA respectively for the 2θ values in the range 5-70° at a scan speed of 1.2°/min.

Effect of pyrolysis conditions on properties of carbon aerogel
During pyrolysis, the RF aerogel structure was transformed into a relatively pure carbon structure by removing any remaining oxide and hydrogen groups at an elevated temperature.
Variations in the pyrolysis conditions caused significant changes in the properties of the carbon aerogel.A summary of these changes was shown in Table 1. Figure 1 showed the changes in mass loss and shrinkage of RFderived carbon aerogel via pyrolysis.Higher pyrolysis temperature tended to increase the mass loss of carbon aerogel.The mass loss was in the range of 49-63%.The shrinkage of the samples was almost constant at ~50% during increase the pyrolysis temperature from 600 to 900 o C.    The density of the samples was slightly increased with rising pyrolysis time because of density of graphite (2.09-2.23 g/cm 3 ) in structure of carbon aerogel.The higher pyrolysis time, the more graphitic structure obtained and it was the reason of increasing density of carbon aerogel.Moreover, activation time was also influence on electrical conductivity of carbon aerogel.According to the research of Al-Muhtaseb and Ritter [3], the electrochemical capacitance of carbon aerogel samples was Trang 92 increased with increasing the thermal activation time up to ~3 h and thereafter reduced it.The electrochemical double-layer capacitance of RF carbon aerogel was maximum after ~3 h of activation with CO 2 [11].Thus, the electrical conductivity of carbon aerogel increased with increasing the activation time up to ~3 h.

Characteristics of carbon aerogel at variations pyrolysis conditions
The pore structure of carbon aerogel samples was investigated by nitrogen adsorption

Trang 94
Moreover, the presence of a peak at ~10 o showed that the impurities were contained in the structure of carbon aerogel samples after pyrolysis.This peak was disappeared after activation process.The activation also improved the properties of carbon aerogel [14,15].

CONCLUSIONS
Carbon aerogel was synthesized at different pyrolysis conditions and it indicated that carbon aerogel samples had highest specific surface area at pyrolysis temperature of 700 o C, with surface area value of 800 m 2 /g.When increasing the pyrolysis temperature above 700 o C the surface area was slightly decreased.The electrical conductivity of carbon aerogel was optimal at pyrolysis temperature of 800-900 o C. The pore-size was distributed in range of microporous with the average pore-size of 18-22 Å.The total pore volume of carbon aerogel was in the range of 0.18 to 0.44 cm 3 /g.The structure of carbon aerogel was characterized by XRD methods indicated that carbon aerogel had structure between amorphous and graphite state.

Figure 1 .
Figure 1.Mass loss and shrinkage of RF-derived carbon aerogel.

Figure 2 .
Figure 2. Effect of pyrolysis temperature on electrical conductivity and density of carbon aerogel

Figure 4 .
Figure 4. Electrical conductivity and density of carbon aerogel at variation of pyrolysis time.

Figure 3 .
Figure 3. Specific surface area and electrical conductivity of carbon aerogel at different pyrolysis temperature.
isotherms of carbon aerogels pyrolyzed at 600, 800 and 900 o C, showed in Figure 5a, belong to Type IIa in IUPAC classification [12].This type of isotherms was identified with multilayer absorption on the surface of carbon aerogel, and was characteristic of nonporous or macroporous absorbents.The isotherms exhibit a narrow hysteresis loop, which is typical of thermodynamically irreversible adsorption processes [13].Figure 5b showed typical poresize distributions of carbon aerogel at various pyrolysis temperature.It was indicated that the pore-size distribution of all carbon aerogel samples were similar.The pore-size of these samples was distributed in the range of 7-9 Å and 11-30 Å, with average pore-size was 18-22 Å.According to nitrogen absorption measurements, properties of carbon aerogel were calculated and summarized in Table 2.The highest specific surface area of carbon aerogel reached 800 m 2 /g at pyrolysis temperature of 700 o C. The obtained carbon aerogel was contained micro-, meso-and macroporous with the percentage of 58-72, 11-23 and 15-20 %, respectively.The carbon aerogel particle was in the range from 77 to 128 Å.The total pore volumes of samples were in the range of 0

Figure 5 .
Figure 5. Nitrogen adsorption-desorption isotherms (a) and pore-size distributions (b) of carbon aerogel at variation of pyrolysis temperature.

Figure 6 .
Figure 6.X-Ray diffraction pattern of carbon aerogel

Table 1 .
Characteristics of carbon aerogel at different pyrolysis temperature.

Table 2 .
Properties of carbon aerogel at variation in pyrolysis conditions.