*5.1.2 Laser evaporation*

*Transition Metal Compounds - Synthesis, Properties, and Application*

For the fabrication of high Tc-superconducting materials, the easiest approach

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

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

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

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

**5. Preparation and fabrications routes**

cooling [71–73].

to be required.

**5.1 HTS films and coated conductors**

*5.1.1 Vacuum co-evaporation*

evaporation and scattering are discussed here [71]:

**66**

synthesis [71].

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 irregularity.
