Reactive Shell Model for Boron Oxidation

Abstract

An approach suitable for quantitative description of oxidation of porous powders with a complex particle morphology is developed and applied for a 95% pure boron powder. It is shown that boron particles, which are agglomerates of nanosized, partially fused primary particles, can be represented as spheres with reactive porous shells and impenetrable cores. The oxidation of such powders is assumed to occur volumetrically within the reactive shells. The shell thickness is obtained from simple experiments with the actual powders. To determine the thickness, thermo-gravimetric measurements are performed and processed for powders with different but overlapping particle size distributions. The processing uses measured powder particle size distributions to assign fractions of the powder mass gain to individual particle size bins. Selection of the appropriate initial reactive shell thickness makes it possible to ensure that particles of the same size present in samples with different size distributions oxidize in the same way. The specific powder studied here was successfully described with a reactive shell thickness of about 1.28 μm. Surface area measurements can be used to interpret the reactive shell as comprising packed spherical primary particles. For the powder used here, the radius of such primary particles is 80 nm. The proposed reactive shell model is shown to be valid as long as the thickness of boron oxide grown on the primary particles does not exceed 15 nm. The approach is useful for describing reactions leading to ignition of porous powders and of powders consisting of irregular aggregates in particular. It is also expected to be applicable to describe corrosion, catalyst aging, and other phenomena involving surface reactions for such powders.

Effect of Iron Doping on Photocatalytic Activity of Boron Oxide Processed Using Polyvinyl Borate Precursor

“…The specified model is based on the assumption that B 2 O 3 particles are in regular spherical shape. However, similar to many studies in the literature, real B 2 O 3 particles have a complex morphology [6] (Figure 3). B 2 O 3 powder was composed of irregular shaped particles with sizes in the range from 100 to 1000 nm and the incorporation of Fe ions did not change the irregular shape and the average size of the B 2 O 3 particles.…”

“…B 2 O 3 powder was composed of irregular shaped particles with sizes in the range from 100 to 1000 nm and the incorporation of Fe ions did not change the irregular shape and the average size of the B 2 O 3 particles. Hence, the proposed model failed in describing the reaction mechanism of the B 2 O 3 synthesis [6]. SEM images of both samples show crystal layers on amorphous phase, which means that the precursor could not be completely converted to the B 2 O 3 crystal phase (Figure 3).…”

“…Apart from the specified techniques, B 2 O 3 powder can be synthesized using polyvinyl borate precursor through relatively low-temperature exothermic reactions [6][7][8]. The heat treatment of polyvinyl borate precursor provides the formation of B 2 O 3 powder, composed of submicron size particles.…”

“…It is known that B 2 O 3 synthesis reactions are governed by heterogeneous oxidation. The transport of the reactants through the oxide layer of B 2 O 3 may limit the oxidation reaction [6]. It is not easy to control both the B 2 O 3 content and the morphology of the product powder.…”

Surface-Functionalized Boron Nanoparticles with Reduced Oxide Content by Nonthermal Plasma Processing for Nanoenergetic Applications

“…The increased energy release occurs due to the reduction of the oxide layer, which eliminated the diffusion barrier for the oxidizer to access a greater volume fraction of the metallic boron in the BNPs. No significant exothermic peak is observed after ∼750 °C because of the presence of liquid boron oxide that clogs the porosity as the oxidation reaction progresses . This decreases the overall rate of oxidation of the boron sample.…”