Stability at high pressure, elastic behavior and pressure-induced structural evolution of “Al5BO9”, a mullite-type ceramic material

Abstract

Elastic behavior and pressure-induced structural evolution of synthetic boron-mullite ''Al 5 BO 9 '' (a = 5.678(2) A ˚, b = 15.015(4) A ˚and c = 7.700(3) A ˚, space group Cmc2 1 , Z = 4) were investigated up to 7.4 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase transition or anomalous compressional behavior occurred within the investigated P range. Fitting the P-V data with a truncated second-order (in energy) Birch-Murnaghan Equation-of-State (BM-EoS), using the data weighted by the uncertainties in P and V, we obtained: V 0 = 656.4(3) A ˚3 and K T0 = 165(7) GPa (b V0 = 0.0061(3) GPa -1 ). The evolution of the Eulerian finite strain versus normalized stress (f E -F E plot) leads to an almost horizontal trend, showing that a truncated second-order BM-EoS is appropriate to describe the elastic behavior of ''Al 5 BO 9 '' within the investigated P range. The weighted linear regression through the data points gives: F E (0) = 159(11) GPa. Axial compressibility coefficients yielded: b a = 1.4(2) 9 10 -3 GPa -1 , b b = 3.4(4) 9 10 -3 GPa -1 , and b c = 1.7(3) 9 10 -3 GPa -1 (b a :b b :b c = 1:2.43:1.21). The highest compressibilities observed in this study within (100) can be ascribed to the presence of voids represented by five-membered rings of polyhedra: Al1-Al3-Al4-Al1-Al3, which allow accommodating the effect of pressure by polyhedral tilting. Polyhedral tilting around the voids also explains the higher compressibility along [010] than along [001]. The stiffer crystallographic direction observed here might be controlled by the infinite chains of edge-sharing octahedra running along [100], which act as ''pillars'', making the structure less compressible along the a-axis than along the b-and c-axis. Along [100], compression can only be accommodated by deformation of the edge-sharing octahedra (and/or by compression of the Al-O bond lengths), as no polyhedral tilting can occur. In addition, a comparative elastic analysis among the mullite-type materials is carried out.

Boron mullite: Formation and basic characterization

“…Fisch and Armbruster [31] recently investigated the structural evolution of synthetic mullite-type Al 5 BO 9 and Al 4 B 2 O 9 with increasing temperature. The rigid body behavior of the BO 3 units agrees well with the results of Gatta et al [32] who investigated the pressure-induced structural evolution of Al 5 BO 9 and also give a comparative analysis of the elastic behavior of mullite-type materials. Details on the temperature-induced structural changes in B-doped mullite cannot be given yet as it was not possible to identify the B-position from powder X-ray diffraction data.…”

Thermal Expansion of Aluminoborates

“…For structures containing AlO 4 and AlO 6 polyhedra Hazen and Prewitt (1977) state that structural changes with increasing pressure correspond to those with decreasing temperature. Elastic behavior of Al 5 BO 9 was investigated by Gatta et al (2010) and the behavior of lattice parameters with increasing pressure (compression is highest along the b-axis, followed by the c-and a-axis) is in agreement with our results for expansion of lattice parameters with temperature.…”

Al5BO9: A Wide Band Gap, Damage‐Tolerant, and Thermal Insulating Lightweight Material for High‐Temperature Applications

“…Mullite fibers, whiskers, and mullite fiber reinforced mullite matrix composites (mullite‐based CMCs) are also promising as thermal protection systems for combustors in gas turbine engines as well as wave‐transporting components in hypersonic vehicles . Recently, B‐containing mullite compounds have attracted the attention of material scientists as well as physicists and chemists due to the fact that they have lower density than the mullite compounds in aluminosilicate system and are thus more attractive as lightweight insulating and thermal protecting materials for aerospace applications . Among the B‐containing mullite ceramics, Al 5 BO 9 is intriguing because it has a low theoretical density of 2.95 g/cm 3 and is promising for applications as substrates for catalytic converters, optically translucent ceramics for high‐temperature windows, neutron absorbers in nuclear plant, thermal sealing and insulating materials in hypersonic vehicles, and reinforcements for soft metals and ceramics .…”