THERMAL OXIDATION KINETICS OF GRAPHENE-MICRO POWDERS IN OXYGEN FLOW

The oxidizing properties of graphene-micro powders in a purified oxygen flow were measured (v(O2) = 5.21.10(-4) mole . sec(-1) = const). The graphene powders were certified by the manufacturer (Center for Advanced 2D Materials, Innovation Development Laboratories, National University of Singapore). The following methods were used to test the powders: fractionated sequential oxidation over time at set temperature maxima of the CO(2)release rate and gradual heating, like that provided by DTA, at a rate of similar to 20 K/min. The amount of extracted carbon as CO2 was recorded every minute. The repeated sequence of fractionated temperature oxidation of parallel graphene samples determined the characteristic decomposition (oxidation) of three fractions at temperatures of 953, 1003, and 1043 +/- 10 K The oxidation results showed that the graphene powders were satisfactorily homogeneous, and their oxidation proceeded in the same way as in stacked, conical, and tubular carbon nanofibers at 923, 973, and 1013 +/- 10 K Three similar morphological components of graphene microparticles were visually identified in scanning electron microscopy images as planar, conical, and tubular. The average sizes of graphene microparticles were three orders of magnitude greater than the average diameters of the same fibrous nanoforms. The oxidation mechanism for carbon nanoforms and microforms in air has two states. The first stage involves peripheral oxidation of graphene particles (wait state): absorption of oxygen molecules by the surface of 2D graphene (>= 234 K), migration to the edge (perimeter) carbon atoms, recombination of oxygen molecules with perimeter carbon, and saturation of broken carbon bonds with bridged oxygen (2) bonds. The second stage (thermokinetic state) includes CO cleavage in heating and oxidation of CO to CO2 (oxidation, decay, combustion). The wait state is maintained by the thermokinetic process. The sizes of microparticles and the specific concentration of edge perimeter pairs of carbon and oxygen atoms on the periphery of graphene platelets influence the oxidation rate and oxidation temperature of powder fractions. The shift of oxidation temperatures for morphological graphene forms in comparison with fibrous nanoforms is +(30-50 K) on average. The procedure for purification of graphene powders promotes the transition of the most active planar particles into coiled forms. The kinetic temperature dependence for the oxidation of purified graphene represents a stepped S-shaped curve with saturation. The initial rates of carbon oxidation, v(oxC) = 4.57 x x 10(-8) mole . sec(-1), were recorded at 823 K In entry into the exponential range of measurements (873-983 K), the oxidation reaction rates v(oxC) of the graphene micropowder increase from 9.99 x x 10(-8) to 1.5 . 10(-6) mole . sec(-1) and the oxidation reaction rate constants k(oxC) from 1.91 . 10(-4) to 1.51 . 10(-3). The activation characteristics are E-a.oxC = 168 +/- 110 kJ . mole(-1) and frequency characteristics are A(0) = 6.06 . 105 to 7.40 . 10(6) sec(-1). The oxidation rate varied from 1.92 . 10(-6) to 1.06 . 10(-6) mole . sec(-1) after the inflection point at 983 K and up to 1073 K and from 8.89 . 10(-7) to 3. 30 10(-8) mole sec(-1) at 1093 K Subsequently, up to 1123 K, the carbon oxidation rate became zero when the sample burned completely. The known theoretically calculated activation energy of graphite oxidation is 172 kJ . mole(-1). The experimental results are within the theoretical values. Preliminary measurements for multiwalled nanotubes and intragranular inclusions of graphite and free nanosized carbon (onions, graphite platelets, conical fibers, and tubes) of commercial B15-xCx boron carbide powders are close to the above data.