| 其他摘要 | As an important group-III nitride, AlN has promising applications in field emission display, reinforcement and ultraviolet light-emitting diodes due to its low electron affinity, excellent mechanical and thermal properties, and direct wide bandgap. One-dimensional (1D) AlN nanostructures, compared with conventional polycrystalline AlN, are predicted to have better mechanical and physical properties and better application performance because of their high aspect ratio and low defect concentration. However, there are still several problems in this field at present: (i) the growth mechanism of 1D AlN nanostructures is not clear and their growth lacks of controllability; (ii) it is difficult to get AlN nanostructures desirable for specific device applications; (iii) the purity and yield of this structure need further improved; (iv) some fundamental properties of the as-synthesized AlN nanostructures have not been explored yet. In this study, controllable synthesis, physical properties and applications of 1D AlN nanostructures were investigated.
Based on the analysis of the thermal mechanic and kinetic conditions of the growth of AlN crystal, several 1D AlN nanostructures including nanofibers, nanoplatelets, nanobrush, nanobelts, nanoneedles, nanorods, nanowires and nanotips were synthesized by chemical vapor deposition (CVD) and mobile nitrogen arc-discharge methods. Through investigating the relationship between the synthesis conditions and the morphology of products, controllable synthesis of 1D AlN nanostructures was realized and their growth mechanism was proposed.
By adjusting the reactant concentration at different growth steps, Eiffel-tower-shape single-crystalline AlN nanotips were fabricated on Si substrate by a modified CVD method without using expensive photolithography process. By combining the catalyst-seeded method and vapor doping method, AlN nanoneedle array has been position-sited synthesized, meanwhile, the in-situ Si doping in AlN nanoneedles was successfully achieved. By etching Si substrate and controlling its surface morphology, vertically aligned single-crystalline AlN nanoplatelets were epitaxially grown on this substrate. Unlike the template-directed synthesis which usually results in polycrystalline nanostructures, AlN nanoplatelets achieved by this method was single crystalline. By improving the growth temperature and temperature gradient, single-crystalline mushroom-like AlN nanorod array was synthesized by the mobile nitrogen arc-discharge method.
The field emission (FE) properties of as-synthesized AlN nanostructures have been investigated. Field emission measurements showed that these arrays have much lower turn-on and threshold fields than those of conventional AlN films. Moreover, their FE current stability is obviously higher than that of carbon nanotubes, which indicates that they are promising materials for FE applications. Among the four 1D AlN nanostructure arrays mentioned above, the Si-doped AlN nanoneedles show the lowest turn-on field and the Eiffel-tower-shape nanotips have the highest FE stability, indicating the manipulating geometrical morphology and doping are two effective methods to optimize the FE properties of the 1D AlN nanostructures.
In addition, photoluminescence (PL) properties of AlN nanobelts and hierarchical brush-like AlN nanostructures have been investigated because of their unique morphologies. It is shown that their PL spectra are different from those of conventional AlN microparticles. Strong green (at ~570 nm) and yellow (at ~585 nm) light emissions were obtained from nanobelts and nanobrushes, respectively, suggesting their potential applications in light emission devices. Control experiments verified that these PL spectra were originated from the surficial oxygen defects.
Based on the synthesis of a sufficient amount of high-purity AlN nanowires, Al-based composites reinforced by AlN nanowires (AlN-NWs) were fabricated by hot-pressing. Microstructural observations reveal that the interface between reinforcement and matrix is clean, bonded well and no interfacial reaction product was formed at the boundary. Mechanical properties including yield and tensile strength of the composites were improved with AlN-NWs volume fraction changing from 5 to 15 vol.-%, and the maximum tensile and yield strengths of the composite were about 5 and 6 times, respectively, as high as those of Al matrix. AlN nanowires, compared with conventional powder AlN, have higher reinforcing efficiency, and loading-transfer mechanism was the main strengthening mechanism in the composites. In addition, AlN-NWs can effectively decrease the coefficient of thermal expansion (CTE) of composites, and the CTE of 15 vol.-% composite was about one half that of Al matrix, and the experimental results matched well with the prediction by “Schapery” model. These composites are expected to be utilized as packaging material with high strength and low thermal expansion. |
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