INTRODUCTION:

In physics number of phenomena is consequences of activity of specific mutual interactions. E.g. relation between thermal motion of the atomic building of blocks of matter and ordering forces. As we go on increasing the temperature thermal motional energy becomes very high compared to relevant ordering energy. All phase transitions follow this rule. Therefore it is not surprising fact that unexpected properties of matter become important tool in development of technology. Superconductivity is one of the tools.

In the year 1908 Heike Kamerlingh-Onnes achieved liquefaction of Helium. He measured electric resistance of metals at liquefication temperature of Helium. Electrical resistance decreases linearly with temperature is found by measurements of metals. But there are 3 possibilities –

1)The resistance approaches to zero with decreasing temperature.

2) It will reach finite limiting value.

3)It could pass through a minimum and approach infinity at low temperature.

Kamerlingh-Onnes studied platinum and gold samples of high purity. He observed resistance reaches finite limiting value (Residual resistance) supporting second fact. But the value of residual resistance depends on purity. The purer the samples smaller was the residual resistance .So they expect zero resistance for pure platinum and gold in temperature of liquid helium. According to kamerlingh-Onnes resistance is due to motion of atoms. So they perform tests on mercury since mercury is well purified by means of multiple distillations. They found resistance rapidly approaches a zero value with his equipment which barely detects the resistance^[1].

In further experiments with improved apparatus they observed that resistance change took place within a temperature interval of few hundredths of a degree and from it is clear that it resembled more a resistance jump rather than continuous decrease.

Fig. 1 Schematics of temperature dependence of electric resistance at low temperature.

From fig.2 it is clear that below 4.2K mercury had passed into a new state which possesses extraordinary electrical properties may be called superconductive state. Soon it is clear that purity was unimportant. Superconductivity occurs in many metals .At atmospheric pressure niobium is element with transition temperature 9K.Now a days thousands of superconducting compounds have been found, and this development is going on ^[2].

Fig.2 The superconductivity of Mercury

In 1957 the theory of John Bardeen, Leon Neil Cooper, and John Robert Schrieffer (BCS theory) states that during transition to superconducting state electrons condense pairwise into a new state in which they form a coherent matter wave.

For more than 75 years superconductivity was a low temperature phenomenon. This changed in 1986; superconductors based on copper oxide were discovered by J.G. Bednorz and K.A. Muller.

In mid of 1960s scientists recovered an interesting superconductors in the Ba-Pb-Bi-O system. System of La-Sr-Cu-O system shows superconductivity above 40K.After that it was observed same properties for Y-Ba-Cu-O system above 80K.With superconductivity at temperatures above the boiling point of liquid nitrogen (T=77k), one could envision many important technical applications of this phenomenon.

Today we are familiar with “High – temperature superconductors” based on copper oxide. Examples are YBa2Cu3O7 and Bi2Sr2CaCu2O8 which shows property around 90K.

Fig.3 Evolution of superconducting transition temperature since the Superconductivity

From the fig.4 of evolution of superconducting transition temperatures where we can see jump – like increase due to the discovery of the copper oxides. The metallic compound MgB2 shows superconductivity with a transition temperature of 39K. This discovery had a great impact in physics and it turned out that MgB2 behaves similarly to the “classical” metallic superconductors.

Even after long period of time it is still unclear how Cooper pairing accomplished in these materials but seems likely that magnetic interactions play an important role. Due to discovery of the cuprates, the phenomenon of superconductivity is not restricted to a particular temperature range and research is going to found these properties at room temperature or even above it.

Superconductivity will enter our daily lives more and more, in the field of energy technology or microelectronics. For example, refrigerators and cold boxes are regular household items. Modern cryo-coolers today reliably reach temperatures of 30K and even 4.2K and lower.

Mostly magnetic field sensors are made from YBa2Cu3O7 are employed for the non-destructive testing of materials or for detecting magnetic cardiac signals. In field of energy technology, cables are made from high-temperature superconductors. High temperature superconductors can be kept in a well-stabilized state or above or below strong magnets. In this way a contact- free bearing and motion without friction can be achieved, which is highly attractive in many fields of technology.

Processible conjugated polymers from organic semiconductors to organic metals and superconductors

As we know conjugated polymers having spatially extended Π-bonding system which result in physical properties and since significant research efforts are made. Research has discovered that when this conjugated polymer in neutral (undoped) state behaves as semiconductors and can be used as active components of ‘plastic electronics’ such as polymer light-emitting diodes, photovoltaic cells etc. Then if there is redox and acid-base doping of conjugated polymers result in increase in electronic conductivity of the polymer. This paper is about discovery of organic polymer superconductor.

Before 1980s reports dealing with polyconjugated systems were very rare and research devoted to these materials were not systematic. The chemical nature of doping reactions results in polymers conductive was published and further spectroscopic studies were demonstrated for the transformation of polymer chains into polycarbocations. This important discovery led to various aspects of conjugated polymers. Focuses were given to synthesis of conjugated polymers in undoped (semi conducting state) and doping process which transforms ‘polymeric semiconductors’ to ‘polymeric metals’.

Various work were done to finally get the process of preparation of semi-conducting conjugated polymers by varying optical gap by appropriate functional group of conjugated backbone. In particular use of electron donating or electron withdrawing group result in bond alteration and change in band gap ^[3].

Processible organic conductors from doped conjugated polymers

1. Principles of conjugated polymers doping

There are two types of doping which are usually distinguished- the redox type and the acid-base one. Poly (acetylene), poly (p-phenylene) polyheterocyclic polymers and their derivative usually undergo the redox-type doping. P-type oxidative doping can be carried out chemically or electrochemically which either involves cathodic or oxidation of the polymer chains to polycarbonium cations with insertion of anions between polymer chains which neutralize the change of polycarbonium cations.

Fig.4 Unsubstituted conjugated polymers and optical gap in π conjugated systems

Fig.5 p-Type doping of poly (acetylene)

Fig.6 n-Type doping of poly (acetylene)

By hole-electron symmetry, one may postulate analogous picture for n-type doping. In heterocycliconjugated polymers, different charge configurations are formed

Also vibration, electronic and other properties of the polymer are robustly altered upon doping and its super molecular structure. Conjugated polymers reach the conductivity of metals with negative temperature coefficient ^[4].

Fig.7 conjugated polymers doping

2. Methods of conjugated polymers doping:

Conjugated polymers can be doped in bulk with large size which is carried during polymerization. It should be stressed that catalytic and electro catalytic properties of bulk-doped polymers are different from surface polymers. Usually their conductivity drops drastically even expose to ambient atmosphere.

When the polymers were reacted with Lewis acids and bases, distinguished optical switching and conductivity changes were observed, evidencing the outstanding case of efficient non oxidative doping. Remarkably, in previously reports works, coordination of Lewis acids cause band gap shift but not doping of the conductive polymer ^[5].

3. Doping induced processibility:

Presently conductive polymers those are made through combine good mechanical properties with high conductivity. They can be mixed with thermoplastics or elastomers or other polymeric matrices to give blends with low percolation threshold. Finally, they can be deposited as conductive layers on polyamide or polyester fibers ^[6].

4. Application of doped conjugated polymers:

The versatility of polymer materials is expanding because of the introduction of electro-active behaviour description related to some of them. The most interested development in this area is related to the discovery of intrinsically conductive polymers or conjugated polymers, which also include such examples as polyacetylene, polyaniline, polypyrrole, and polythiophene as well as their derivatives.

Conjugated polymers have a combination of properties- both metallic (conductivity) and polymeric; doping results in conjugated polymer’s semiconducting nature to a wide range of conductivity, from insulating to low conducting. The doping process is an effective test method for the production of conductive polymers as semiconducting substance, providing an alternative for inorganic semiconductors.

Polymeric Superconductors

The first organic superconductor was discovered in 1980s by means of electrochemical oxidation of tetra methyl tetraselena which give ion radical salt. The success in preparation of organic superconductors was on the basis of single crystals of sufficient quality to observe superconductivity on the electrode if sufficiently low current densities were used. The inorganic polymer can be changed to superconductor at extremely low temperature, i.e. Tc = 0.26K can be prepared in form of single crystals via solid-state polymerization.

First chains with highly regular microstructure must be prepared. Second, crystallize the ordered structures using special solution. Then chemical or electrochemical doping must be carried out for creation of free charge carriers. But this process is the weakest point of the whole procedure because of doping induces disorder even it was carried out in solution using counter ion induced processibility. Doped polymers by processing through solution are partially order. From this point of view, approaches of superconductivity of conjugated polymers were presented ^[7].

The vital aim in the discovery of conjugated polymer superconductivity was to introduce charge carriers electronically without doping through chemical or electrochemical doping. This was achieved through field – effect transistor (FET) configuration. Typically FET consists of three electrodes.

Two of them (source and drain) are deposited on semi conducting layer whereas third one (gate) is separated from the semiconductor material by a thin layer of a dielectric. The advantage of FET constitution is based on the reality that p-type charge carriers can be injected into the polymer layer electronically. Moreover their concentration can be specifically controlled over a very wide range by gate bias. At higher hole densities transition occur from semiconductor to metal and at last becoming superconductors at 2.35K.

Fig. 8(a) STM images (600 X 600 Aͦ²) of the long-range ordering in P3HT thin films on HOPG, (b) STM images (200X200 Aͦ²) of the long-range ordering in P3HT thin films on HOPG, (c) STM images (100X100 Aͦ²) of the long-range ordering in P3DDT thin films on HOPG, (d) STM images (200X200 Aͦ²) of the long-range in P3DDT thin films on HOPG.

Fig.9 Schematic structure of the regioregular poly (3-hexythiophene) FET

To date superconducting properties are not yet shown by doped conjugated polymer through chemically or electrochemically. This is probably due to the fact the doping and processing techniques developed to date do not lead to supramolecular structures ordered to guarantee the formation of a continuous arrangement of superconducting zones. It is hope that there would be progress in this area of materials for research which will lead to chemically doped conjugated polymer superconductors in near future.

High performance YBCO coated superconductor wires

After discovery of high temperature superconductors such as YBa2Cu3O7 (known as YBCO or Y-123) researchers tried to produce affordable flexible conducting wires with high current density ,at the cost of copper wire. Main obstacle for the commercial production of wires is that weak links phenomenon (grain boundaries formed by the misalignment of neighboring YBCO grains are known to form obstacles to current flow).By careful alignment of grains keeping low angle boundaries between superconducting YBCO grains allow more current to flow. Critical misalignment angle for YBCO is 4⁰,below which current density is same as YBCO films grown on single crystals.

Fig.10 the schematics of first and second generation wires are shown in figure (a) and (b)

Methods developed to obtain biaxially textured substrates suitable for high-performance YBCO films:-

1) ion-beam-assisted deposition (IBAD)

2) rolling-assisted biaxially textured substrate (RABiTS) process

3) inclined substrate deposition

The industry standard for characterizing second generation wire is to divide the current by the width of the wire. With either a 3 m thick YBCO layer carrying a critical current density Jc of 1 MA/cm2 or a 1 m thick YBCO layer carrying a Jc of 3 MA/cm2 the electrical performance translates to300 A/cm-width. Converting these numbers to the industry standard of 0.4-cm-wide HTS wire would correspond to 120 A in a 0.4-cm-wide tape, or 300A/cmwidth. This performance level is comparable to that of the commercial 1 generation wire. Further increases in thickness or critical current density, or finding a way to incorporate two layers of YBCO (either a double-sided coating or joining two YBCO tapes face to face) in a single-wire architecture would result in a performance exceeding first generation wires: a high overall engineering critical current density ,JE, at 77 K. “Engineering” critical current density includes the effect non superconducting substrates and buffers. Another advantage of second generation wires is that having better in field electrical potential at high temperatures ,lower processing costs, low ac losses.

1) Ion beam assisted deposition: - The ion beam is used to grow textured buffer layers onto a flexible but untextured metal, typically a nickel alloy. Initially yttrium-stabilized zirconia (YSZ) is used. Now a days IBAD templates of YSZ, gadolinium zirconium oxide (Gd2Zr2O7, or GZO), and magnesium oxide (MgO) are being used to make YBCO tapes.

Perovskite buffers such as LaMnO3, SrTiO3, and SrRuO3 have been found to be having been found to be similar in temperament with IBAD-MgO substrates.

MgO substrate achieves good texture after nucleation(approx.10nm thick Mg film is needed). But texture development template depends on smoothness of starting nickel alloy tapes(Electro polishing of nickel alloy substrate allows surface roughness <1 nm). In a typical IBADMgO template, a total of five buffer layers are involved: an Al2O3 barrier; amorphous Y2O3 as the nucleation layer; an IBAD-MgO layer; and a homoepitaxial-MgO layer, involving the growth of MgO without ion beam assist, followed by either SrTiO3 or LaMnO3. On IBAD-MgO templates,1.4-_m-thick YBCO films with Ic values of 109 A(3.8 m length) and 144 A(1.6 m length)have been achieved.

Fig.11 The schematic illustration of IBAD process

2) Rolling-assisted biaxially textured substrates (RABiTS) process:-

RABiTS process uses thermomechanical processing to obtain flexible biaxially oriented nickel or nickel alloy substrates. Buffers that transfer the texture of the metal substrate to the superconductor and prevent reaction between the substrate and the superconductor are deposited on the substrate. YBCO superconductors are deposited epitaxially on the buffer layer.

The starting substrate serves as a structural template for the YBCO layer, which has substantially fewer weak links than the substrate. In RABiTS method high purity silver used in first generation of wires replaced by low cost nickel or nickel alloy which allows fabrication of wires at affordable prize. The RABiTS architecture most commonly used consists of a starting template of biaxially textured Ni-W (3 at.% or 5 at.%) with seed layer of 75 nm Y2O3, a barrier layer of 75 nm YSZ, and a cap layer of 75 nm CeO2. In this architecture, all the buffers have been deposited by physical vapour deposition processes. Ic of 250–270 A/cm-width with a standard deviation of 2.0–4.0% was achieved.

Fig.12 Rolling assisted biaxially textured substrate process

3)Inclined substrate deposition:-

The textured buffer layers are produced by vacuum depositing material at a particular angle on an untextured nickel alloy substrate. After discovery of ISD-YSZ process texturing of reel to reel MgO buffer layer is improved.( Jc of 1.2 MA/cm2 at 77 K and self-field on short ISD-MgO templates with YSZ/CeO2 buffers using pulsed laser deposition of YBCO). Recently, the THEVA group has achieved 7⁰–8⁰ grain alignment in MgO-ISD tapes. By growing dysprosium barium copper oxide (DyBCO) films, they have achieved an improvement of 1⁰–2⁰ from the MgO layer using in situ electron-beam co-evaporation. A typical HTS coating thickness is 1.5–2.0m. They have also reported a critical current density of 2.3 MA/cm2 in 20-cm-long tapes with an Ic level of more than 400 A/cm-width. Several-meter-long tapes exhibited Jc values of around 1.4–1.5 MA/cm2 at 77 K and self-field. Both IBAD and RABiTS have more advantages over ISD ^[8].

Superconducting materials for large scale applications

Recent years various developments are made on properties of superconductor materials. This is made to an account for the usage in various electrical applications such as in cables, wires and tapes in a broad range.

In addition to that, a new material is discovered which is expected that it have potential to be used in various applications which are MgBr2. Basically superconducting materials are the main key for the development of various application by improving properties. HTS tapes are made of the materials which can be operating at temperature of 50K.

Previously, superconducting materials were develop by Nb-Ti (superconducting transition temperature Tc = 9K) and Nb3Sn (Tc = 18K). But the use of this materials got decrease after 1980’s with the discovery of layer cuprates superconductors. Manufacturing of the cuprates from the conductors was difficult, but there were first and second generation on the basis of silver- sheathed composite and biaxial coated conductors using YBa2Cu3O7(less anisotropic).Then the discovery of MgB2 below 39K in the 2000s lead to another applicant in large scale. Two different superconducting wires exist- the classical low-temperature magnet and plasma-containment magnets for various applications. Such as particle accelerator, fusion power, electrical power equipment such as motors, generator, synchronous condensers, power transmission cables, transformers.

Niobium-Titanium alloy: Nb-Ti alloy superconductors from past 40 years are known as “workhouse” materials in superconductor industry. This was discovered in 1960’s which have a high upper critical field of order 11T at 4.2K and 14T at 2K. They have good ductility. The discovery of twisting of wires results in the reduction of the filament coupling which leads to the development of wires with great improvements. In 1980’s using Nb-Ti introduce first superconducting accelerator ^[9].

1) BSCCO-2212 and BSCCO - 2223

BSCCO – need to be doped (hole) by an excess oxygen atom in order to superconduct. It was the first HTS material used for making “superconducting” wires. It has short coherence length. This result in grains in the polycrystalline wire of good contact and must be smooth. It is a good candidate because it can be aligned either by melt or mechanical deformation ^[10].

2) Magnesium Diboride

MgB2 is high critical temperature binary compound around 39K. This very common topic for research as it is a ‘two gap’ superconductivity. It was greatly influenced in the field of development of superconductors. At moderate magnetic fields, it can be used for transport media generally upto 5 teslas. Although, at higher magnetic fields better performance can be achieved and as we know it uses for thin films, bulk and wires.

Because of the MgB2 can operate at higher temperature, its system can be cooled by the modern cry cooling device. It is not costly, problematic or hazardous as for cooling liquid helium is used. Helium is available in poor quantity as a natural resource and not easy to found for scientific or industrial applications. This MgB2 can be treated as a major solution for industrial development ^[11].

CONCLUSION:

After the development of first conductor by Stephan Gray there has been lots of improvisation in it. Firstly research was done to reduce the size of the conductor but as soon as the demand increased scientist started developing materials which will have more capacity to conduct known as superconductor. Right now the researchers are working for the bulk production of the superconductors

REFERENCES:

[1] Werner Buckle, Reinhold Kleiner. Superconductivity: Fundamentals and Applications , May1997, 2^nd ed: pp- 84-89.

[2] K. H. Bennemann, J. B. Ketterson. History of Superconductivity: Conventional, High-Transition Temperature and Novel Superconductors. A review. Available from: http://www.springer.com/cda/content/document/cda_downloaddocument/9783540732525-c1.pdf?SGWID=0-0-45-546406-p173745028

[3] Adam Pron, Patrice Rannou. Processible conjugated polymers: from organic semiconductors to organic metals and superconductors. A review. Available from: http://www.sciencedirect.com/science/article/pii/S0079670001000430

[4] A. G. MacDiarmid, R. J. Mammone, R. B. Kaner, S. J. Porter, R. Pethig, A. J. Heeger, D. R. Rosseinsky. The Concept of `Doping' of Conducting Polymers. A review . Available from: http://rsta.royalsocietypublishing.org/content/314/1528/3

[5] Prof. A.K. Bakhshi. Fundamentals of Electrically Conducting Polymers. A review. Available from: http://nsdl.niscair.res.in/jspui/bitstream/123456789/486/1/Fundamentals%20of%20electrically%20conducting%20polymers.pdf

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[7] Dr. Steven G. Haupt, Dr. David R. Riley, Dr. John T. McDevitt. Conductive polymer/high-temperature superconductor composite structures. A review. Available from: http://onlinelibrary.wiley.com/doi/10.1002/adma.19930051017/abstract

[8] M. Parans Paranthaman and Teruo Izumi. High-Performance YBCO-Coated Superconductor Wires. A review. Available from: http://journals.cambridge.org/action/displayAbstract?fromPage=onlineandaid=7962256

[9] Scanlan, Ronald M, Malozemoff, Alexis P. Larbalestier, David C. Superconducting materials for large scale applications. A review. Available from: http://ieeexplore.ieee.org/xpl/login.jsp?tp=andarnumber=1335554andurl=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F5%2F29467%2F01335554

[10] A. Baldini, L. Bargioni, S. Conti, R. Garret. BSCCO-2223 Ag-sheathed tapes production. A review. Available from: http://link.springer.com/article/10.1007/BF03185446

[11] Cristina Buzea and Tsutomu Yamashita. Review of superconducting properties of MgB2. A review. Available from: http://inspirehep.net/record/1227309/files/arXiv%3Acond-mat_0108265.pdf

Received on 03.05.2016 Accepted on 06.06.2016

Research J. Engineering and Tech. 2016; 7(2): 67-74.

DOI: 10.5958/2321-581X.2016.00015.5

[1] Werner Buckle, Reinhold Kleiner. Superconductivity: Fundamentals and Applications , May1997, 2nd ed: pp- 84-89.