**5. New approaches**

*Nanorods and Nanocomposites*

Mara and co-workers investigated nanoparticle nanocomposites of Cu-Nb with thicknesses of 5 and 40 nm with magnetic sputtering techniques [54]. They observed the growth of layers was consistent with the planes of {111} Cu//{110} Nb

The experiments are given by three-dimensional Cu-Nb nanocomposites in which nano-sized drops of niobium are immersed into a copper matrix. The structure consists of coarse phase of copper and niobium-rich compounds, which are scattered throughout the amorphous phase. Degtyarenko showed such systems being superconducting even at Nb content lower than a percolation threshold, but its properties in comparison with the granular one fabricated at slow melt cooling have a distinction in kind [55]. The transport properties in this system are strongly affected by the size and proximity effects but free from a complicating anisotropy factor. In comparison with it, Degtyarenko et al. evaluated the Cu-Nb composites with a micron-scale structure. Samples of the first type represent the tapes prepared at extremely fast cooling of Cu-Nb melt. The niobium drops and spacing between them have a measure of about 10 nm, and the drops are immersed into the Cu matrix with low content (~1 at%) of Nb. The system is similar to a granular superconductor, but its

and with the constant control of uniform growth of the nano-layers.

structural sizes are less than coherence lengths of the materials forming it.

on the amorphous phase composition (**Figure 5**) [55].

The copper-rich or niobium-rich phases exhibit a positive resistance coefficient (TCR). In the Cu-90Nb composition, the first transition temperature is superconducting (TC~4.5 K). The amorphous film exhibits high room temperature resistance as well as negative TCR, and the transfer of superconductivity is dependent

*The temperature dependence of the resistivity in the vicinity of the superconducting transition of the slow cooled* 

**222**

**Figure 6.**

**Figure 5.**

*(a) and quenched (b) samples [55].*

*The stress-strain diagram of Cu-Nb composite which is true in different circumstances [56].*

Early success in manufacturing Cu-Nb bimetals at scale have come as threedimensional nanocomposites. At this scale, Shokrvash et al. [57] lined the Cu-Nb bulk nanostructure with a metal oxide precursor (Nb2O5) and Cu. These studies describe the successful synthesis of Cu-Nb nanocomposites by the electro-deoxidation method. Structural analysis of the obtained products reveals rapid reduction of Nb2O5 to Nb and the embedment of Cu and Nb within the bulk nanostructures of Nb and Cu, respectively. The authors have elaborately discussed the electrochemical route to process Cu-Nb composites. The solubility of Cu in Nb could be enhanced through this technique. They have adopted different characterization techniques to show the solubility, structure, and morphology of Cu-Nb composites synthesized by the electrochemical route. The oxygen-free nanostructured materials could be processed using this technique. This investigation deals with the study of the electrolysis synthesis of Cu-Nb nanocomposites, and developed to fabrication of nanoparticulate bulk materials with equilibrium processes to delivering improved electrical conductance (**Figures 7** and **8**) [58].

It has been proven in the Cu-Nb system that the immiscibility can be overcome and direct alloying can be realized between the constituent elements by a local electric field between the boundaries of nanoparticles [58]. The mechanical properties of as-synthesized materials have been investigated by nanoindentation testing. The indentation hardness and modulus have been measured using a maximum load of 10 mN with a loading rate of 20 mN/minute and holding and unloading rate 20 mN/ minute. The acquisition of data from five tests presented the ultimate strength for Cu-14%Nb and Cu-22%Nb nanocomposites ranging 4.42–4.82 GPa [59].

In these research, thermodynamic and kinetic model has been established for direct alloying between Cu and Nb, based on electromigration theory. Particularly, this opens up prospects for designing the new materials from immiscible metallic elements, which is the main topic in designing alloys as well as materials science progression.

#### **Figure 7.**

*FESEM image of Cu-22%Nb representing the composite structure (images by secondary electron mode) (with the permission of Shokrvash [57]).*

**Figure 8.** *The dark field TEM image of Cu-22%Nb nanocomposite (with the permission of Shokrvash [57]).*
