**5. Preparation and fabrications routes**

For the fabrication of high Tc-superconducting materials, the easiest approach which involves mixing sintering is thermochemical (solid-state) reaction. Precursors, generally oxides and carbonates, in stoichiometric ratios are mixed and ground in fine powder through a Ball mill. Alternative methods to achieve a homogeneous mixture include coprecipitation, freeze-drying [71], and sol–gel techniques. The resulted in a homogeneous mixture is then subjected to an elevated temperature of about 800–950 °C for various hours and cooled down till room temperature, reground, and put to calcination again. To ensure homogeneity of extract repeat this process several times and afterward, powder is transformed into compact pellets for sintering. The key factor to synthesize better quality superconductors is sintering conditions such as annealing temperature, time, and rate of cooling [71–73].

A quaternary compound with composition YBa2Cu3O7 − x; was synthesized by calcination and sintering of precursor metal oxides Y2O3, CuO, and carbonate BaCO3 mixed and milled in an appropriate molar ratio. The reaction was carried out at 900–950 °C and sintered in an oxygen environment (950 °C). Appropriate stoichiometry of oxygen is essential to fabricate this superconducting YBa2Cu3O7 − x compound. During sintering, a semiconducting tetragonal phase of YBa2Cu3O6 is formed which converts into superconducting YBa2Cu3O7 − x material on gradual cooling in an oxygen atmosphere. The addition and removal of oxygen in YBa2Cu3O7 − x compound is a reversible process as a result both oxygenated orthorhombic YBa2Cu3O7 − x and deoxygenated tetragonal YBa2Cu3O6 phases are interchangeable at 700 °C [71, 74, 75]. Bi, Tl, and Hg-based HTS are difficult to prepare as compared to the synthesis of YBCO system. This hassle is attributed to the formation of similar layered structures in different phases of these compounds which results in the introduction of intergrowth (syntactic) and faults produced during synthesis which make isolation of single superconducting phase impossible. In case of Bi–Sr–Ca–Cu–O system, Bi-2212 (Tc ≈ 85 K) is easier to form as compared to Bi-2223 single-phase (Tc ≈ 110 K). Sintering for few hours at 860–870 °C is sufficient to prepare Bi-2212 phase while the formation of Bi-2223 phase requires extensive heating at 870 °C for more than one week [76]. It has been proposed that Pb substitution in Bi–Sr–Ca–Cu–O composites enhance growth of high-Tc phase, [77] but extended sintering for long-duration continues to be required.

#### **5.1 HTS films and coated conductors**

Films of HTS can be synthesized both via in-situ and ex-situ methods. During in-situ techniques, direct epitaxial crystallization takes place under applied conditions. In case of ex-situ synthesis, initially, low temperature is used to deposit films but this low temperature is not suitable for the required crystalline phase, thus deposited films are subjected to sintering under an oxygen environment to secure the necessary crystalline structure. Various physical means to deposit films inclusive evaporation and scattering are discussed here [71]:

#### *5.1.1 Vacuum co-evaporation*

As layers of HTS are precipitated they are evaporated by various sources like electron beam guns or resistive evaporators. This method applies to two-step synthesis [71].

**67**

*High Temperature Superconductors*

*5.1.2 Laser evaporation*

*5.1.3 Magnetron scattering*

*5.1.4 Chemical precipitation*

conformation

irregularity.

*DOI: http://dx.doi.org/10.5772/intechopen.96419*

This is an efficient technique for HTS thin film deposition. The main benefit of this method is same rate of evaporation for all chemicals present in a compound [78, 79]. Likewise, there are some disadvantages too such as (a) small portions of stoichiometric film are deposited (b) non-uniform film thickness, and (c) surface

Magnetron Scattering is used for one-stage HTS (YBCO films) deposition with the advantages of having a smooth surface and homogeneous thickness. In this method plasma with high energy electron and ions is generated which compensates the high-temperature requirement and helps to attain one-stage HTS films [80, 81].

In this technique from the stream of volatile metal–organic compounds, metallic components are mixed with gaseous oxidizer in a reactor. This stream of volatile mixture is transported further and oxide film precipitates on the substrate. This

a.More chance to get a homogeneous film with a large surface area of unplanned

method has following advantages over the previous ones:

b.enhanced rate of condensation with better quality

c.flexibility at initial stage of technical system [82].

Other most widely used techniques are as follow:

• Electro-phoretic deposition [84].

93], and Rf-sputtering [94, 95].

**5.2 BSCCO films, Tapes, and Wires**

• 2D texture film via ion-beam assisted-deposition [82, 83].

• MOCVD (Metal–organic chemical vapor deposition) [85].

• Coated conductors synthesis via MOD (metal–organic deposition) [86].

• Buffer layer can be prepared by surface oxidation epitaxy (SOE) [87, 88], electron beam evaporation, laser ablation, [89–91], ion beam sputtering [92,

Melt-processing, electrophoretic deposition, dip-coating, doctor-bladed, and organic precursor film are some techniques helpful in synthesizing Bi-2212 thick films over Ag and MgO substrates. In doctor-bladed practice, before heat treatment, a plane film is formed over the surface of glass slab. For this purpose slurry of powder blend is poured over the glass plate and is spread with the help of a straight-edge blade attached to the plate to form a smooth film. In dip-coating method, Ag foil is dipped in mixture, and film is set down over it. To prepare organic precursor films,
