Thermal Stability Of Nanocrystals Research Articles

Thermal Stability of Nanocrystals in SLM-printed Ti64 Alloy Treated by Laser Shock Peening and Plasma Nitriding

Thermal Stability of Nanocrystals in SLM-printed Ti64 Alloy Treated by Laser Shock Peening and Plasma Nitriding

A novel method for development of hard nano crystalline surface through SMAT and mechanical alloying

A hard Nano crystalline surface was produced by simultaneous surface mechanical attrition treatment (SMAT) and mechanical alloying (MA) with activated carbon, followed by low temperature annealing below 500°C. The thermal stability of surface Nano crystals produced by SMAT and low temperature diffusion of carbon into Nano crystals were also investigated by annealing 200°C to500°C for 1h. Ni-Cr-Mo casehardening steel was used in the present study. The formation of Nano crystals and surface alloying with activated carbon by SMAT, microstructural changes during annealing, thermal stability of the Nano crystals and the surface hardness were been investigated by using Optical microscope, SEM, XRD and microvicker’s hardness tester. Experimental results had showed 20-50 nm crystals by SMAT operation by enhancing 0.19C to 0.4C. Enhanced diffusion kinetics of Nano crystals observed during the annealing process even at 200°C. The simultaneous SMAT and MA with carbon increased the surface hardness from 1.5 to 3.0 GPa in 20 minutes and subsequent annealing further increased the hardness steadily up to 450°C. A maximum hardness of 4.5 GPa was achieved by annealing treatment. The hardness decreased considerably at 500°C indicating grain growth at thistemperature. The results had clearly demonstrated that the surface Nano crystals formed by SMAT were found stable up to 450°C.

Size-dependent thermal stability and the smallest nanocrystal

The lattice-type-sensitive model for the melting entropy and enthalpy of nanocrystals has been presented based on a model for the size dependency of the melting point. In other words, the effect of the lattice and surface structure on thermal stability of nanocrystals has been discussed. The presented equations of the melting entropy and enthalpy have been compared to the experimental data of In and molecular dynamic (MD) simulation results of Ag nanoparticles. In addition, a minimum possible size of crystalline nanosolids has been calculated and compared to the experimental and simulation results of Pb and Al nanowires. Also a relation between the average coordination number of nanoparticles and their melting entropy and enthalpy has been presented. Consequently a minimum possible value for the average coordination number of crystalline nanoparticles has been calculated.

Cryosol Synthesis of Nanocrystalline Alumina

In the present research we employed the newly developed cryosol technique for the preparation of nanocrystalline Al(OH)3. The technique yields sols of aluminum hydroxide stable under a wide range of pH values and concentrations. Freeze-drying of the sols results in the formation of aluminum hydroxide powders with extremely low density and small particle size. Thermal behavior and phase evolution of the cryosol-derived aluminum hydroxide has been studied. According to X-ray diffraction data, annealing of cryosol-derived aluminum hydroxide results in amorphous alumina stable up to 800 °C. However DTA, electron diffraction, and 27Al NMR studies indicate that the samples amorphous to X-ray diffraction are composed of crystalline nanoparticles. The thermal stability of nanocrystals is supposed to be due to the high uniformity of the particles size distribution, the latter resulting from the synthetic method employed.