Synthesis of Aluminum Borate Whiskers by Molten Salt Method and Investigating the Effect of Their Content and Sizes in the Reinforcement of Alumina-Borosilicate Glass Composites

Abstract

The aim of the present investigation is two-fold: (i) to synthesize crystalline aluminum borate whiskers consist of the single phase Al18B4O33, which is most stable phase in Al2O3- B2O3 system and (ii) to reinforce alumina-borosilicate glass matrix with the synthesized Al18B4O33 whiskers and exploring the effect of whiskers content and sizes on the mechanical properties of the composites. A molten salt method involving monopotassium aluminum sulfate, boric acid and potassium sulfate as precursors was chosen to synthesize single-phase aluminum borate whiskers. The X-ray powder diffraction and scanning electron microscopy established that highly crystallized Al18B4O33 whiskers with orthorhombic structure were obtained at 1000oC. By varying B/Al ratio from 0.14 to 0.22 and reaction time from 1 to 6 hours, the length and lateral dimension of the whiskers could be largely controlled from 6–73 μm and 0.6-5 μm respectively. Instead of the well-known vapor-liquid-solid mechanism, a self-catalytic mechanism was proposed in order to explain the one-dimensional growth of Al18B4O33 whiskers. The phase evolution sequences were corroborated by DSC-TGA and subsequent X-ray analysis by heating the precursor powder mixtures at the corresponding endotherms. Three different sizes of Al18B4O33 whiskers(viz., L= 11.83 ± 5.26 μm, D=0.87 ± 0.27 μm, L=35.65 ± 9.64 μm, D=1.73 ± 0.38 μm and L=52.04 ± 21.22 μm, D=3.78 ± 1.08 μm respectively) with three different volume fractions (i.e. 10, 20 and 30% respectively) were used in the present work to reinforce alumina-borosilicate glass matrix. Improper dispersion of the whiskers inside the matrix hinders the densification of the final composites greatly (maximum 60% relative density was obtained). The poor dispersion is attributed to the non-equiaxed inclusions (i.e. whiskers) and large standard deviation of the whiskers sizes. Mechanical properties are parallel to the retarded densification of the porous composites and a highest splitting tensile strength of about 10 MPa was achieved due to interlocking effect of whiskers inside the matrix. Typical toughening mechanisms (i.e. fracture energy absorption processes) like whiskers bridging, whiskers pullout, crack deflection were not observed in the present work. The achieve strength may allow the present composite materials for structural applications (low load bearing areas), thermal insulation and filtering purposes.