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Serious microcracks often occur on the surface of nanohydroxyapatite (n-HAP) artificial bone scaffolds prepared by selective laser sintering (SLS) technology. In this study, we found that appropriate preheating before sintering can reduce and attenuate the cracks. The microstructure and morphology of sintered n-HAP were tested at different preheating temperature and laser sintering speed with scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). The experiments showed that the cracks gradually reduced and then disappeared when the preheating temperature increased from 0°C to 600°C while other parameters remain unchanged. The n-HAP particles gradually fused and grew up, while the grain size of sintered n-HAP will be attenuated with the increase of preheating temperature. As the thermal conductivity of n-HAP increases with increased preheating temperature, the temperature drops quickly, inhibiting greatly the grain growth of n-HAP. We obtained a group of optimum parameters when the sintered n-HAP still maintains nanostructure and possesses the optimal comprehensive performances, that is, laser power is 26 W, spot diameter is 4 mm, sintering speed is 200 mm/min, layer thickness is 0.4 mm, layer density is 852 kg/m³, and optimized preheating temperature is 600°C. These data illustrated that the cracks of sintered n-HAP can be eliminated at appropriate preheating temperature and sintering speed. This provided experimental optimal condition for the preparation of artificial bone scaffolds with nanohydroxyapatite ceramics.

A class of mixed boundary value problems (BVPs) arising in the study of scattering of surface water waves by the edges of floating structures comprising of elastic plates, with or without cracks, is examined for their solutions. It is observed that the simplest possible method of solution of such BVPs is the one that involves solution of an over-determined system of Linear Algebraic Equations. Such over-determined systems of equations are best solved by the method of least squares. Numerical results for useful practical quantities such as the "reflection" and "transmission" coefficients are obtained for one of the problems considered here.