Structural variation in improving thermal stability of transition alumina with La dopant

Abstract Structural variation in alumina directly determines the thermal stability of transition alumina with La dopant at high temperature. In this work, the thermal stability and alumina structure from well-crystallized ultrafine boehmite doped with various La-bearing compounds were studied. La2O3, LaAlO3, and LaAl11O18 could not efficiently inhibit transition alumina changing into α-Al2O3, α-Al2O3 seed benefited α-Al2O3 formation in the presence of La dopant. La(NO3)3 doping into boehmite effectively improved the thermal stability of θ-Al2O3 above 1200 °C. The nano-diameter active spots with low melting point led to the formation of α-Al2O3 with a smooth edge and sintering neck from the blank boehmite, whereas La dopant contributed to rhombus θ-Al2O3 with considerable pores, jagged edge, and wedge at 1200 °C for 2 h. Moreover, the Al and La contents fluctuated asynchronously during calcination, and the atomic ratio of Al/O decreased with the addition of La dopant. These results are attributed to La migration, assembling in active spots and increasing their melting point. Furthermore, relative to nearly 100% AlO6 coordination in α-Al2O3, La dopant significantly increased the AlO4 content above 900 °C. Therefore, La dopant increased the melting point of the active spots, hindered AlO6 formation, and inhibited α-Al2O3 nucleation on the alumina surface, thereby improved the thermal stability of the transition alumina.