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Title of Thesis

Nawazish Ali Khan
Institute/University/Department Details
Department of Physics/ Quaid-i-Azam University, Islamabad
Number of Pages
Keywords (Extracted from title, table of contents and abstract of thesis)
homologues, high temperature superconductors, cuprous cyanide, cyanogen gas, yttrium-based cuprate superconductors, infrared lattice vibrations, infrared absorption

Synthesis and characterization of the three known members of the homologous series Y1Ba2Cu6+n O6+n (n=1,2,3) of high temperature superconductors using a novel route forms the main theme of our study. Use of the low melting point cuprous cyanide [Cu2(CN2)] as the starting copper compound is the key element of our preparation route. The liquid-solid reaction during the firing stage and the presumed high chemical activity of Cu+1 state of copper in [Cu2(CN2)] is expected to lead to efficient and more complete chemical reaction during calcination as compared to the all-oxide based synthesis. This, in turn, would produce more homogeneous material, relatively free from un reacted impurity phases.

Characterization studies based on electrical resistivity, a.c. magnetic susceptibility, x-ray diffraction and infrared absorption on both [Cu2(CN2)] and CuO-based 123 superconductors confirm the above expectations. A detailed investigation of the effects of various preparation conditions has been carried out for our cyanide-based route and the optimal parameters of synthesis established. Alumina boat for calcination is found to adversely affect the superconducting quality of our material while quartz boat leads to the best results. X-ray diffraction and IR absorption analysis results point to the incorporation of Al from the alumina boat as the probable cause of deleterious effects of the alumina boat.

Variation of the ambient used during preparation from the completely oxidising atmosphere to fully reducing atmosphere leads to very interesting insights into the probable reasons for a strong variation in the superconducting to semi conducting characteristics of the material. Detailed analysis of the temperature dependence of the normal-state resistivity of the semiconducting phase material leads to the following picture. Shallow acceptor states in the band gap are introduced by defects associated with oxygen deficiency in the 123 crystal. Thermal activation of free carriers (holes) from these states leads to an increase in the electrical conductivity with temperature. With the increase in the oxygen content from x=0.13 to 0.71, a decrease in activation energy of these states from 15 meV to 3.3 meV is observed which is reflected in the increase in the value of the normal state conductivity with increase in the oxygen content. Thus the well known increase in the hole density with oxygen content seems to be the result of a decreasing activation energy of oxygen related shallow acceptors, according to our observations.

Following the same Cu2(CN)2 route we have succeeded in synthesizing the other two homologues YBa2Cu4O8 and Y2Ba4Cu7O15-x, The important point of our preparation of these superconductors is that we could achieve good quality materials without the use of high oxygen pressures. In our preparation procedure oxygen at normal atmospheric pressure is used. This is a great advantage of our route over the hitherto reported high pressure routes. Again the effects of the variation of the various preparation parameters are thoroughly investigated for these materials. We believe Cu2(CN)2 holds the key to this success - in addition to making the chemical reactions during calcination more efficient, it seems to act as a getterer of oxygen for all three superconductors prepared by us.

To investigate the role of Cu2(CN)2 in greater detail a separate study on the decomposition behaviour of Cu2(CN)2 has been carried out in our work. Thermal analysis data are combined with infrared absorption measurements to obtain insight into the nature of the products at various stages of this decomposition. These studies led to the conclusion that cuprous cyanide, before its decomposition, is converted into a cyclized product. This cyclized product is very reactive and it reacts with other constituents very rapidly with the formation of Cu2O, CuO and liberation of cyanogen gas in the atmosphere of air. In the atmosphere of flowing nitrogen it is converted to Cu metal and cyanogen gas.

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2603.28 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents 0
109.41 KB
2 1 Introduction 1
146.04 KB
  1.1 Brief History 1
  1.2 Family Of Yttrium-Based Cuprate Superconductors 5
  1.3 Motivation Of Our Work 8
  1.4 Scheme Of The Thesis 10
3 2 Structure And General Properties Of Superconducting Materials 13
182.71 KB
  2.1 d-c Electrical Resistance 13
  2.2 Perfect Diamagnetism 15
  2.3 Crystallography 16
  2.4 Infrared Lattice Vibrations 21
4 3 Methods Of Preparation Of Y - Ba -Cu-Based Superconductors - A Survey 30
273.29 KB
  3.1 Y -Ba-Cu-O (123) Superconductor 30
  3.2 Y -Ba-Cu-O (124) Superconductors 37
  3.3 Y -Ba-Cu-O (247) Superconductor 44
5 4 Experimental Details 51
99.87 KB
  4.1 Sample Preparation 51
  4.2 Characterization 52
6 5 Synthesis And Characterisation Of YBa 2 Cu 3 O 7x Superconductors Using Cuprous Cyanide 60
313.64 KB
  5.1 Introduction 60
  5.2 Experimental Procedure 61
  5.3 Results And Discussion Role Of The Boat Material In The Synthesis Of Superconductor From Cuprous Cyanide 65
7 6 Synthesis Of YBa 2 Cu 4 O 8 Superconductor At Atmospheric Oxygen Pressure 81
356.45 KB
  6.1 Introductions 82
  6.2 Procedure Of Preparation 82
  6.3 Characteristics 82
  6.4 Effect Of Preparation Parameters On The Synthesis Of YBa 2 Cu 4 O 8 86
  6.5 Discussion And Conclusions 105
8 7 Synthesis Of 247 Superconductor At Normal Oxygen Pressure 111
209.97 KB
  7.1 Introduction 111
  7.2 Preparation Of 247 Material 112
  7.3 Results 113
  7.4 Susceptibility Measurements 115
  7.5 X -Ray Diffraction 117
  7.6 Discussion 122
  7.7 Conclusions 125
9 8 High Temperature Superconductivity In Y - Ba -Cu-O System In Oxygen Deficient Atmosphere 129
310.39 KB
  8.1 Synthesis Of 123 Using Cuprous Cyanide In The Reducing Atmosphere Of N2 129
  8.2 Synthesis Of 123 Using CuO In The Reducing Atmosphere Of N 2 130
  8.3 Oxygen Contents Of 123 Superconductor 135
  8.4 Discussion 135
  8.4 Simultaneously Preparation Of CuO -Based And Cu 2 ( CN) 2 -Based Superconductors 137
  8.5 Mechanism Of Decomposition Of Cuprous Cyanide 143
10 9 Semiconducting Phases Of YBa 2 Cu 3 O 7-x , Infrared Absorption In Y - Ba -Cu-O Homologues 155
288.82 KB
  9.1 Introduction 155
  9.2 Infrared Absorption Properties Of 124 Superconductor 165
  9.3 Infrared Absorption Of Y 2 Ba 4 Cu 7 O 15-x Superconductor 170
  9.4 Comparison Of IR Absorption In 123, 124 And 247 Materials 173
  9.5 Discussion 176
  9.6 References 177
11 10 Conclusions 179
573.16 KB
  10.1 Appendix 183