Effect of magnesium oxide on sol–gel spun alumina and alumina–zirconia fibres

Alumina fibres and alumina–zirconia fibres were prepared by sol–gel process. The starting materials used for the preparation of alumina and zirconia sol were aluminium-tri-isopropoxide and zirconium oxychloride respectively. Alumina sol and zirconia sol were mixed in definite proportions, so that the final composition contains 10 wt.% ZrO2. Alumina and alumina–zirconia fibres were prepared from the alumina sol and mixed sol respectively. MgO was introduced in the fibres by adding MgNO3 into the sol. Sintered alumina fibre has α-Al2O3 phase and alumina–zirconia fibre has α-Al2O3 and t-ZrO2 phases. The phase transition to α-Al2O3 takes place at higher temperature in the alumina fibre with MgO, whereas the effect is less in alumina–zirconia fibre. The addition of MgO reduces the grain size and increases the tensile strength.