Effect of synthesis atmosphere, wetting, and compaction on the purity of AlB2

AlB2 is attractive as an energetic fuel because of its large heat of combustion on a volumetric basis. The formation of AlB2 from its elements is thermodynamically limited by decomposition into liquid aluminum and α-AlB12 at temperatures approaching 950 °C, and kinetically limited in the reverse reaction, making synthesis of high-purity powders difficult. Minimizing unreacted Al and B makes AlB2 more resistant to moisture degradation. AlB2 yield for similarly prepared samples was observed to be significantly higher in He–6% H2 or vacuum compared to a high-purity Ar atmosphere. Lower AlB2 yield was generally associated with higher Al2O3 content in the reaction products, suggesting that poor Al wetting on oxide surfaces generated by the reaction of Al with B2O3 and other sources of oxygen prevented complete reaction. Compaction decreased the amount of oxygen available to the interior of the pellet, and the vacuum and He–6% H2 environments reduced the amount of oxide impurities in the system, possibly by reducing or subliming them. By optimization of synthesis parameters the purity of AlB2 was measured as 93 wt%, which was better than the previously reported bulk syntheses. Accounting for unreacted boron is important when using XRD to assess purity since the quantification of secondary phases, such as Al, Al2O3, and Al3BC, allow the amount of boron, which is not easily detected in the scan, to be calculated.

Atmosphere and compaction pressure affect AlB2 purity.Figure optionsDownload as PowerPoint slideHighlights
► Vacuum or He–6% H2 yielded higher AlB2 than Ar due to reduced Al2O3.
► Compaction at 25 MPa prior to heating improved the AlB2 yield.
► XRD can be used to quantify AlB2 if Al is used to assess the free B content.
► Elimination of oxide impurities is the key to making higher purity AlB2.