Supported by the DFG: SFB 408.
The γ-γ-perturbed angular correlation technique (PAC) was used to study the transition from a precursor to mullit during an isochronous annealing program. The PAC is very sensitive to changes in the surroundings of built in probe-nuclei. The precursor was prepared by sol-gel-synthesis in a composition of 71.44 w.% Al2O3, 28.065 w.% SiO2 and 0.495 w.% HfO2. Then this material was irradiated by thermal neutrons to produce the PAC-nucleus 181Hf(181Ta) by a (n,γ) reaction from the stable 180Hf isotope in the component HfO2. Subsequently the sample was annealed for 1h at different temperatures. After each annealing step a γ-γ-PAC-measurement was carried out. From differential scanning calometrie (DSC) measurements it is expected that at 976°C the mullit phase should occur.
Least square fits to the data yielded a low (ω=0,1 Mrad/sec), a middle (ω=1000 Mrad/sec) and a high quadrupole interaction frequency. The latter is typical for strongly disturbed probe surroundings as found in other amorphous materials. At a temperature above 1000°C the surrounding of the 181Hf-atoms change indicating a first step towards recrystallization. A new QIF of the same magnitude (ω=1300 Mrad/sec) appeared. Corresponding changes are also seen in X-Ray diffraction spectra taken on identically treated samples. The reason for the delayed transition to the mullit-phase is not clear at present and requires further investigation.