Influence of nitrogen immersion on the mechanical properties of (NiO)x(Bi1.6 Pb0.4)Sr2Ca2Cu3O10-δ composite

(NiO)(x)(Bi-1.6 Pb-0.4)Sr2Ca2Cu3O10-delta composite, where 0.0 <= x <= 0.2 wt%., were prepared using solid state reaction method. The prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). Vickers microhardness measurements (H-V) were carried out at room temperature under different applied loads varying from 0.49 to 9.8 N, and dwell times (40 and 59 s). It was noted that dwell time and Vickers microhardness were inversely proportional. H-V values increase as x increases up to 0.1 wt%, and then they decrease with further increases in x. All samples exhibit indentation size effect (ISE) with normal trend, as Vickers microhardness decreases by increasing the applied loads. Also, Vickers microhardness measurements of the prepared samples were done during both loading forces up to 9.8 N and unloading downwards to 0.49 N. It was noted that unloading values of Vickers microhardness are slightly greater than loading values. The elastic/plastic deformation model (EPD) was used to interpret the loading and unloading Vickers microhardness results. It is clearly noted that values of d(o), the added elastic component the measured plastic indentation semi-diagonal (d), in the unloading results are much higher than those for loading data. The effect of liquid nitrogen immersion for 16 h on Vickers microhardness values was examined. A significant improvement in the Vickers microhardness of (Bi, Pb)-2223 samples immersed in liquid nitrogen was observed. Such behavior is attributed to the fact that nitrogen immersion increases the volume contraction of the superconductor matrix, causing the shrink of the pores and voids present in the samples. Different models were used to analyze the obtained results such as Meyer's law, Hays-Kendall (HK) approach, elastic/plastic deformation (EPD) model, and modified proportional specimen resistance (MPSR) model. The experimental results of Vickers microhardness of both samples without and with liquid nitrogen immersion are well fitted according to the MPSR model.