No abstract

Monolith-type titanium nitride/silicon nitride nanocomposites, denoted as TiN/Si N , have been prepared by a reaction of polysilazanes with a titanium amide precursor, warm pressing of the resultant polytitanosilazanes, and subsequent pyrolysis of the green bodies at 1400 °C. Initially, a series of polytitanosilazanes was synthesized and the role of the chemistry behind their synthesis was studied in detail by using solid-state NMR spectroscopy, elemental analysis, and molecular-weight measurements. The intimate relationship between the chemistry and the processability of these precursors is discussed. Polytitanosilazanes display the appropriate requirements for facile processing in solution and as a melt, but they must be treated with liquid ammonia to be adapted for solid-state processing, that is, warm-pressing, to design dense and mechanically stable structures after pyrolysis. We provide a comprehensive mechanistic study of the nanocomposite conversion based on solid-state NMR spectroscopy coupled with thermogravimetric experiments. HRTEM images coupled with XRD and Raman spectroscopy confirmed the unique nanostructural features of the nanocomposites, which appear to be a result of the molecular origin of the materials. The as-obtained samples are composed of an amorphous Si N matrix, in which TiN nanocrystals are homogeneously formed in situ in the matrix during the process. The hardness and Young moduli were measured and are discussed.

The series of the SrTi(1-x)Co(x)O(3-delta) polycrystalline samples has been prepared via solid-state reaction in air. The structural study shows that a solid solution exists for all compositions, such as 0 < or = x < or = 0.9, which discards the presence of ferromagnetic and metallic cobalt clusters for the low level of cobalt substitution. The existence of systematic extra peaks on the electron diffraction patterns for x > 0.5 indicates that the oxygen vacancy ordering is responsible for the superstructures (2a(p) x 2a(p) x 4a(p), where the subscript "p" refers to the perovskite subcell). The oxygen nonstoichiometry is confirmed by thermogravimetric analysis. The magnetic properties reveal a maximum in the magnetic moment per mole of cobalt for x approximately 0.10. However, the clear lack of strong ferromagnetism does not suggest that a diluted magnetic semiconductor (DMS)-type behavior is induced in SrTiO(3) as Co is substituted for Ti. The transport properties along the SrTi(1-x)Co(x)O(3-delta) line are explained by considering the substitution of a Co(3+)/Co(4+) mixed valency, which decreases the room-temperature resistivity by at least 5 orders of magnitude as one goes from SrTi(0.9)Co(0.1)O(3-delta) to SrTi(0.1)Co(0.9)O(3-delta).