Ln–Si–Al–O–N 五元体系新型氧氮化物材料的制备、结构和性能研究 (Preparation, Structure and Properties of Novel (Oxy)Nitrides in Five-Component Ln–Si–Al–O–N System)

Abstract

In this thesis, the preparation, structure and properties of several novel (oxy)nitride materials in the five-component Ln–Si–Al–O–N system have been comprehensively investigaed. The main researches include: (1) the growth mechanism, morphological formation process and photoluminescence properties of JEM-phase Nd-Sialon orthogonal microcrystals; (2) single crystal structure determination of JEM-phase LnSi6-zAl1+zOzN10-z with various lanthanide dopants (Ln＝Ce, Pr, Nd and Sm), and the investigation of the effect of lanthanide element on its z value; (3) preparation of JEM-phase Sialon powder with dopants of Nd and Sm; (4) Low-temperature pressureless sintering of dense AlN-polytypoid composite and its microstructure, mechanical properties and fracture mechanism; and (5) preparation and crystal structure of novel “114” nitride with first-ever [AlN6] coordination in non-high-pressure synthesized inorganic compound. The growth mechanism of Nd-Sialon orthogonal microcrystals in JEM-phase is the combination of VLS and VS mechanisms, whereas the critical factor that controlling the formation of two distinct morphologies of needle and wedge crystals is the relative position of the Fe-rich droplet on the extremity of initial crystals. Besides, it was measured that the JEM-phase Nd-Sialon single crystals can emit blue light with peak wavelength of 405 nm when excited by UV light of 325 nm. Furthermore, the single-crystal structure study indicates that the JEM-phase Ln-Sialons are of orthogonal symmetry and space group Pbcn, where the lanthanide element Ln can be La~Sm. Both chemical analysis and nominal z value optimization suggest that the z values of JEM-phase Sialons are different and in narrowed range, and most importantly, the z value decreases from ~1.0 (of La-JEM) to ~0.2 (of Sm-JEM). The lattice parameters of JEM phase decrease with the decreasing ionic radius of lanthanide, with the calculated density increases correspondingly. The purer powder of Sm-JEM and Nd-JEM was obtained when nominal z value was 0.6 and 0.2, with purity of 90.7wt% and 97.1wt%, respectively. On the other hand, we successfully fabricated the dense AlN-polytypoid composite ceramic at 1650 °C by pressureless sintering (PLS) process via optimization of lanthanide oxides. Its sintering mechanism is transient liquid phase sintering. The system is optimized to be Gd–Si–Al–O–N. The significance of the contribution is significantly lower the sintering temperature of AlN-polytypoid ceramics and enables the use of pressureless sintering which is beneficial for industrial applications. This temperature is even over 100 °C lower than that of hot press (HP) sintering reported in the literature. The room-temperature mechanical properties of as-fabricated materials are comparable with that of HPed AlN-polytypoid ceramics. The microstructure of the composite is consistent of partially twinned platelet 15R, hexagonal β-Sialon and boundary U-phase Sialon. The fracture mechanism of the materials is concurrent of transgranular and intergranular fracture modes. Most importantly, the single crystals and relatively pure powders of novel “114”-type nitrides LnAl(Si4-xAlx)N7-y□yOδ were synthesized for the first time. This nitride is the very first example of non-high-pressure synthesized inorganic compound that unprecedently possesses the [AlN6] octahedral coordination. The single-crystal structure determination and neutron diffraction analysis of these “114”-type nitrides indicate that the crystal structure is hexagonal with space group of P63mc. In this structure, the anticuboctahedral interstitial site is occupied by Ln, the octahedral interstitial site is occupied by Al, and the tetrahedral interstitial sites are occupied by Si/Al, with small amount of O (δ) sitting in the octahedral cavities of the structure framework. The 27Al NMR data serve as the direct proof for the existence of [AlN6] coordination, with chemical shift of 1.6 ppm. In addition, the new compounds are characterized by X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microstructure (HRTEM), and Raman microspectroscopy. Lastly, the reported “114”-type compounds including mineral, oxides and nitrides, are summarized and their structures are compared. A sub-type regulation for telling the different of “114”-type compounds with swedenborgite structure is rationally proposed.