STUDY ON Bi
2Sr
2CaCu
2O
8/CoFe
2O
4COMPOSITES
A Dissertation submitted in partial fulfillment of the requirements for the degree of
Master of Science in
PHYSICS By BAMADEV DAS ROLL NO-410PH2123
Under the supervision of
Dr. Prakash Nath Vishwakarma
National Institute of Technology, Rourkela Rourkela-769008, Orissa, India
May-2012
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Department of Physics
National Institute of Technology, Rourkela Rourkela-769008, Orissa, India
CERTIFICATE
This is to certify that, the work in the report entitled “STUDY ON Bi2Sr2CaCu2O8/CoFe2O4 COMPOSITES” by Bamadev Das, in partial fulfillment of Master of Science degree in PHYSICS at the National Institute of Technology, Rourkela(Deemed University); is an authentic work carried out by him under my supervision and guidance. The work is satisfactory to the best my knowledge.
Dr. Prakash Nath Vishwakarma
Department of Physics NIT Rourkela .2 | P a g e
Declaration
I hereby declare that the project work entitled ―Study on Bi2Sr2CaCu2/CoFe2O4 Composites” submitted to the NIT, Rourkela, is a record of an original work done by me under the guidance of Dr. Prakash Nath Vishwakarma Faculty Member of NIT,Rourkela and this project work has not performed the basis for the award of any Degree or diploma/
associate ship/fellowship and similar project if any.
Bamadev Das
MSc, Physics
NIT, Rourkela
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ACKNOWLEDGEMENT
-
I would like to express my sincere thanks to Dr. Prakash Nath Vishwakarma for his valuable guidance and constant encouragement throughout this project work. I wish to record my special thanks to Mr. Achyuta Kumar Biswal (Ph.D), Miss. Jashashree Ray(Ph.D), Miss. Sanghamitra Acharya (M.Tech) for his valuable help in all respect of my project work.
I record my sincer thanks to Department of Metallurgical and Material Science for extending all facilities to carry out the XRD and SEM.
I express heartiest thanks to all the faculty members of Department of Physics, NIT Rourkela who have made direct or indirect contribution towards the completion of this project.
It gives me an immense pleasure to thank all my friends and all the research scholars of the Dept. of Physics
Date: Bamadev Das
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CONTENTS PAGE
NO.
Abstract CHAPTER-1
INTRODUCTION
1.1 Historical Review of Superconductor 1
1.2 Superconductivity 1 1.3 Properties of Superconductor 2
1.4 Meissner Effect in Superconductor 4
1.5.Classification of Superconductor 4
1.6 Theory of Low Temperature Superconductor (BCS Theory) 6 1.7 High Temperature Superconductor 7
CHAPTER-2 Bi
2Sr
2CaCu
2O
8 2.1.
Introduction 82.2. Bi2Sr2CaCu2O8 8
2.3. Crystal Structure Of Bscco 8
2.4. Literature Survey 9
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CHAPTER-3
EXPERIMENTAL TECHNIQUES
3.1. Introduction 10
3.2. Synthesis of Materials 10
3.3. Characterisation of Materials 14
CHAPTER-4 RESULT AND DISCUSSION
4.1. Introduction 174.2. X-ray diffraction (XRD) 17
4.3. Scanning Electron Microscope(SEM) 20
4.4. Temperature Dependance Resistivity 22
4.5 I-V Characteristics Curve 25
CHAPTER-5 CONCLUSION
26REFERENCES
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Abstract
Composites of Bi-2212 with Cobalt ferrite (CoFe2O4) have been prepared to study different properties like critical current density, transition temperature and surface morphology. Bulk Bi2Sr2CaCu2o8 (Bi-2212) superconductors are prepared by solid state route. Cobalt ferrite (CoFe2O4) samples are prepared via sol-gel method. The phases of both Bi-2212 and cobalt ferrite are studied by x-ray diffraction, which are found to be in match with JCPDS data. When the various concentrations of cobalt ferrite is added to Bi-2212, the transition temperature of Bi-2212 decreases drastically. Addition of higher concentration makes the sample semiconducting till the lowest temperature of investigation. The critical current density of Bi-2212 is measured below the transition temperature. It is observed that the critical current density of Bi-2212 decreases with increasing temperature.
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CHAPTER- I INTRODUCTION
1.1 HISTORICAL REVIEW OF SUPERCONDUCTOR
The field of superconductivity is one of the interesting area for the researchers because of its peculiar properties like resistivity, conductivity, etc. The phenomena
―superconductivity‖ was discovered first by a Dutch physicist Kamerlingh Onnes in 1911.[1]This phenomenon was observed first in Hg (mercury) which is a low temperature superconductor.
1.2 SUPERCONDUCTIVITY
The phenomenon of becoming electrical resistance zero at a certain temperature is called ―superconductivity‖, and the material having these properties is called
―superconductor‖. The invention of this phenomenon created considerable interest since the material is having no electrical resistance and hence negligible heat loss. For this property, it can be applied to fabricate powerful and economical devices. But so far, this phenomenon is possible only at low temperature. Due to this reason, it is not feasible to manufacture such devices. It is very difficult to maintain such temperature for a long time [2].
Fig 1.1 Temperature dependence resistivity of mercury (Hg) whose transition temperature is 4.2K. [4]
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1.3 PROPERTIES OF SUPERCONDUCTIVITY CRITICAL TEMPERATURE
When a superconductor is cooled, its electrical resistivity becomes zero at certain temperature. This temperature is called critical temperature. Above this temperature, the materials exhibit metallic behaviour. At this temperature, there is a phase transition from metallic to superconducting. The transition is not due to change in the crystal structure or properties of the crystal lattice. Hence superconducting transition can be interpreted as an electronic transition in which conduction electron enters into an ordered state [2]. The value of critical temperature of various elements, alloys and compounds discovered from 1911 to till date are shown graphically in fig (1.2).
Figure1.2 Historical development of critical temperatures in the
superconducting materials (not in exact scale) [4]
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CRITICAL MAGNETIC FIELD
The superconductors are highly diamagnetic i.e. magnetic field lines are repelled by the superconductor. This is called ―Meissner effect‖, after the discoverer of this phenomenon.
Above a certain value of magnetic field, the superconductor will not be able to repel the magnetic field and loses its superconducting property. This field is called critical magnetic field Hc(T) which is a function of temperature [3]. Above this field, the material behaves as a normal metal. Experiments conducted for pure superconductor shows that critical magnetic field Hc(T) is roughly parabolic and is given by Tuyn’s law
[7]:
(0) 1 22 )
( Tc
H T T Hc c
CRITICAL CURRENT DENSITY
When a high current is applied to the material (superconductor), its superconductivity behaviour is lost. The current at which the material is no more a superconductor is called critical current density.
The phase diagram of superconductor as a function of magnetic field, temperature and current density, is shown below. The material is superconducting only they are under the volume shown in fig 1.3
Fig1.3 phase diagram of superconductor which shows three critical parameter of superconductivity. [4]
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1.4 MEISSNER EFFECT IN SUPERCONDUCTOR
Meissner effect is discovered by Meissner and Ochsenfeld in 1933. This effect tells that a superconductor expels the magnetic flux applied externally, when it is cooled below critical temperature [2]. Hence the magnetic field inside a superconductor is zero
i.e B = μo(M + H)= 0 or, χ = M/H = –1
The susceptibility of superconductor is –1 which shows that the superconductor is perfectly diamagnetic material.
1.5 CLASSIFICATION OF SUPERCONDUCTOR
Basically, superconductors are of two types i.e. type-I and type-II. This classification is based on their behaviour in an external magnetic field.
Type-I Superconductor
This kind of superconductor strictly follows Meissner effect. These superconductor exhibits perfect diamagnetic below critical magnetic field. The critical field for most of the type-I material is 0.1tesla. These materials lose their superconductivity at low value of field.
Pure elements exhibit this type of behaviour.
Fig1.4 Magnetization curve for type-I superconductor which shows that the material is in superconducting state below Hc. [4]
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Type –II Superconductor
This superconductor does not follow Meissner effect strictly. This means the magnetic lines of force doesn’t penetrate in this material abruptly at critical field. From fig (1.5), the material exhibits perfect diamagnetic behaviour for fields less than Hc1 i.e. it follows Meissner effect. As the field increases from Hc1 , the fields penetrate the material. At Hc2, the field penetrates the material completely. The fields Hc1 and Hc2 are called lower and upper critical field respectively. The material in the region between Hc1 and Hc2 is said to have vortex state. In this state, there are some normal regions existing in the sample. This is a mixed state. These superconductors need higher field to lose its superconducting nature completely.
Fig 1.5 Magnetization curve for type-II superconductor in which the
material exhibits perfect diamagnetism below Hc
1and there exists a vortex state
between Hc
1and Hc
2. [4]
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1.6 THEORY OF LOW TEMPERATURE SUPERCONDUCTOR (BCS THEORY)
There are so many theories proposed in order to explain the phenomenon of superconductor. One of these theories is called BCS theory. It successfully explains the superconductivity. This theory is formulated by Bardeen, Cooper and Schrieffer. They were awarded Nobel Prize for their theory in 1972.
According to this theory, the two electrons can be attracted to each other and hence can occupy single quantum state. A pair of electron behaves like a boson. Thus they can occupy single quantum state.
The two electrons can attract each other via lattice vibration i.e. phonon. When an electron moves through a crystal, it distorts the +ve ion core called lattice and sets a slow forced oscillation. Since the electron move faster, it leaves the place much before the distortion dies off. If another electron moves through this region, it exerts an attractive force to the distorted lattice. This force lowers the energy of the second electron. Therefore, the attraction between two electrons is happened via phonon.
Fig 1.6 Schematic diagram of electron-electron interaction via lattice vibration i.e. phonon [4]
As described above, the two electrons interact with each other via phonon. This interaction is dominant over repulsive force that exists between two electrons. These two electrons form a pair called cooper pair. The energy of such pair is less than the energy of two free electrons. The difference in the energy is the binding energy of two electrons. The cooper pair forms new ground state having an energy gap from the lowest excited state.
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1.7 HIGH TEMPERATURE SUPERCONDUCTOR
High temperature superconductors (HTS) are the materials having high critical temperature. Johannes Bednorz and Karl Muller discovered first high temperature superconductor in 1986 on the La-Ba-Cu-O system. Thus, a new class of material was discovered whose critical temperature is higher than that of normal metallic superconductor.
These superconductors are called high Tc ceramic superconductor. Shortly after the first discovery of HTS, a series of HTS were synthesized.
YBa2Cu3O7–y (YBCO) obtained by Wu et al. [8] in 1987 was the first material having a transition temperature Tc higher than the boiling point of liquid nitrogen. For this compound, the oxygen content has a decisive influence on the superconducting properties, the highest transition temperature Tc = 93 K is reached for the composition YBa2Cu3O6.92, whereas for y <0.6 the material is not superconducting. Some other materials like BSCCO, iron based superconductor are also high temperature superconductor [5].
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CHAPTER-II BSCCO
2.1 INTRODUCTION
High temperature superconductor ceramics is essential for application in engineering system such as fault current limiter and energy storage. In such applications, the BSCCO system is one of the best candidates due to its high critical temperature, phase stability and excellent critical current density.
2.2 Bi2Sr2CaCU2O8
Bismuth strontium calcium copper oxide or BSCCO is high temperature superconductor having chemical formula Bi2Sr2Can-1Cun O2n+4+x[3]. It is the first HTS which did not contain a rare earth element [3]. BSCCO is an excellent candidate for the practical application like resistive superconductor current limiter due to its homogeneous voltage characteristics over the whole sample. However, these systems must be fabricated into composites with required structure in order to get high Tc for the practical application [4].
2.3 CRYSTAL STRUCTURE OF BSCCO
Fig 2.1 Crystal structure of Bi-2212 which shows that the structure is layered structure. [4]
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2.4 LITERATURE SURVEY
(I) Enhancement of electrical and mechanical properties in Nano size MgO added Bi2Sr2CaCU2O8, [6]
Here the effect of addition of MgO with parent Bi-2212 was studied. When MgO is added to BSCCO, the electrical and mechanical properties of Bi-2212/MgO is increased.
(II) Low magnetic field sensitivity of c-Axis transport in BSCCO (2212) single Crystals, [15]
In this paper, the sensitivity to low magnetic fields of c-axis transport in BSCCO (2212) bulk single crystals is studied by applying magnetic field to the sample.
(III) Synthesis of Bismuth-Strontium-Calcium-Copper Oxide powders for Silver Composite Wires with Uniform Micron Sized Filamen. [8]
Here a chemical precipitation method is utilized to synthesize ' bismuth-strontium-calcium copper oxide powder. Tetra-methyl ammonium hydroxide and tetra-methyl ammonium carbonate are used as a precipitant and for pH adjustment
IV) Impact of Pb substitution on the formation of high tc superconducting phase in Bi-2212 system derived through sol–gel process.[14]
Here, the Pb-Bi-Sr-Ca-Cu-O system is synthesized by sol-gel method. The maximum shrinkage of all samples occurs at 8500C.
COMPARATIVE STUDY OF THE SYNTHESIS TECHNIQUES TO OBTAIN
National Physical Laboratory, New-Delhi-ll0 0 A COMPARATIVE STUDY OF THE SYNTHESIS TECHNIQUES TO OBTAIN
HIGH T c SINGLE PHASE IN BISMUTH BASED SUPERCONDUCTORA COMPARATIVE STUDY OF THE SYNTHESIS TECHNIQUES TO OBTAIN
HIGH T c SINGLE PHASE IN BISMUTH BASED SUPERCONDUCTORS A COMPARATIVE STUDY OF THE SYNTHESIS TECHNIQUES TO OBTAIN
HIGH T c SINGLE PHASE IN BISMUTH BASED SUPERCONDUCTO
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CHAPTER—III
EXPERIMENTAL TECHNIQUES
3.1 INTRODUCTION
The details of experimental techniques used for the preparation of superconducting Bi2Sr2CaCu2 (BSCCO) powder/pellets, cobalt Ferrite (CoFe2O4) and temperature dependent resistivity measurement, I-V characterstics, powder x-ray diffraction of the sample, scanning electron microscope (SEM) and Electron dispersive spectroscopy (EDS) of the samples are briefly presented in this chapter.
3.2 SYNTHESIS OF MATERIALS
3.2.1 SYNTHESIS OF Bi2Sr2CaCu2O8 (BSCCO) POWDER & PELLETS
From the literature survey, we found that solid state route is one of the best methods for the preparation of BSCCO. The precursors taken are Bi2O3 (Bismuth Oxide), SrCO3
(Strontium Carbonate), CaCO3 (Calcium Carbonate) & CuO (Cupper Oxide) in proper stoichiometry ratio.
The method of preparation involves following steps
Thoroughly mixing of the precursors.
Sintering
Pelletization
The detailed procedure adopted is described below.
For the preparation of BSCCO, the constituent precursors are weighted in an electronic digital balance to an accuracy of 0.001 mg. The weighted precursors are mixed and grounded properly by using an agate mortar pestle for about 4 hours. The mixture is then transferred to a crucible and is sintered inside a furnace with temperature regulation arrangement for the phase formation of BSCCO. First heating is done at 7000C for 12 hours.
The furnace is then turned off and the sample is allowed to cool down slowly. The schematic diagram of the sintering process is shown below.
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After heating the precursors for 12 hours, it turned in black. The black mass is dislodge gently from the crucible to the mortar and grounded for 4 hours .The sample is then sintered at 7700C for 12 hours. The graphical representation of the sintering process is shown below.
After the second cycle heating, the black mass is grounded again for 4 hours to mix the powder homogeneously. The black mass is again sintered at 8200C for 20 hours. The graphical representation of the sintering process is shown below.
700
0C 12 HOURS
HEATING
770
0C 12 HOURS
HEATING
820
0C 20 HOURS
HEATING
300 K
300 K
300 K 300 K
300 K 300 K
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The black mass is obtained after heating the powder, is gently dislodged from the crucible to the mortar and grounded for 2 hours and then pellets are made in a die-set by applying 5 ton pressure in a hydraulic press. The pellets are sintered at 8800C for 12 hours.
The graphical representation of the sintering process is shown below.
300 K 300 K
Repeating the sintering process four times assured complete removal of CO2 by the reaction. All the heating are done in air. Slow cooling at the final stage of sintering, allows oxygen to be taken up by the sample and provides good oxygen stoichiometry. After cooling the pellets to room temperature, the pellets are preserved in a desiccator to avoid its degradation due to atmospheric moisture and CO2.
3.2.2 SYNTHESIS OF COBALT FERRITE (CoFe2O4)
Sol-gel process is found to be the best method for the synthesis of cobalt ferrite. For the preparation of cobalt ferrite, the chemicals taken are cobalt nitrate and iron nitrate.
Glycine is taken as the fuel for the synthesis. Both nitrates i.e. cobalt nitrate & iron nitrate are taken with proper stoichiometric ratio and dissolved in water in order to make clear solution.
The amount of water taken is 50 ml for the respective solution. All the solutions are added to the chelating agent (glycine) with proper stoichiometric ratio. The color of the solution is found to be sky blue. The final solution is taken in a beaker and heated slowly with continuous stirring for about 2-3 hours. The stirring of the solution is done by a magnetic stirrer. After 2-3 hours, gel is formed. Due to the presence of glycine, gel burnt vigorously and the precursor powder is formed. The color of the material is transformed to deep black color.
880
0C 12 HOURS
HEATING
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The powder is then grounded properly by using an agate mortar pestle. The grinding of powder is done for 2 hours and transferred to crucible for sintering process. The material is sintered inside furnace at 6000C for 3 hours. The graphical representation of the sintering process is shown below.
3.2.3 SYNTHESIS OF COMPOSITES (BSCCO+CFO)
For the synthesis of composites i.e. mixing of BSCCO & CFO, bulk material of BSCCO and CFO are taken. The two materials are mixed in different composition with proper stoichiometric ratio. The materials are then grounded properly by using an agate mortar pestle and the grinding is done for 1-2 hours so that the materials get mixed homogeneously. Palletisation of the material is done using a die set by applying proper pressure to the die set. The pellets are sintered at 8000C for 1 hour. The graphical representation of the sintering process is given below.
800
0C 1 HOUR
HEATING
300 K 300 K
300 K 300 K
600
0C 3 HOURS
HEATING
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3.3 CHARACTERISATION OF MATERIALS 3.3.1 X-RAY DIFFRACTION
X-ray diffraction technique is a versatile method to determine the different phases, crystal structure, lattice defects, lattice strain and the crystallite size (in case of nano- particles) with a great accuracy. The x-ray diffractometer works on the principle of Bragg’s diffraction law. Diffraction patterns are observed when x-ray impinges on periodic structure with geometrical variations on the length scale of the wavelength of the radiation.
In the present work, the freshly prepared bulk sintered pellets of BSCCO and powder samples of CoFe2O4 are characterized by powder x-ray diffraction at room temperature with Cu Kα radiation using PHILIPS PW 1830 HT X-Ray Generator.
The details of the XRD are listed below:-
Equipment name PHILIPS PW 1830 HT X-Ray Generator
Target used Copper(Cu Kα)
Range(2ϴ) 200 - 800
Scan rate 30 per minute
Wavelength of X-ray 1.54 A0
3.3.2 SCANNING ELECTRON MICROSCOPE (SEM)
Scanning Electron Microscope is used for the study of sample’s surface topography or morphology, compositional analysis of the material and other properties like electrical conductivity. It images the sample by scanning it with a beam of electron. The electron interacts with the atoms of the samples that make up the samples producing signals that contain information about the sample.
In the present work, the pellets of BSCCO and composites made by BSCCO and CoFe2O4 in various compositions are characterized by JEOL JSM – 6480 LV microscope.
The images of surface of the sample are taken in different magnification i.e. 2000, 5000, 7500 & 15000.
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The details of SEM experiment are given below:
Equipment name JEOL JSM – 6480 LV
Magnification 2000, 5000, 7500, 15000
3.3.3 TEMPARATURE DEPENDENT DC RESISTANCE
For the measurement of resistance of the sample, a standard four probe method (two inner probes as voltage leads and two outer probes as current leads) is employed. Advantage of using four probe technique is that it eliminates the lead resistance as well as the contact resistance which would otherwise influence the measurement of the sample resistance. A brief description of the measurement is given below.
a) SAMPLE LEAD CONNECTION
The pellets of bulk sintered BSCCO and composites of BSCCO & CoFe2O4 are cut out to approximately 1.5mm x 1.5mm x 10mm dimension. The sample is mounted inside the cooling chamber of the close cycle refrigerator. Four probe electrical contacts to the sample are made by highly conducting silver paint. The outer two probes are current leads whereas the two inner probes are voltage leads. The graphical representation of the four probe method is shown below.
Fig 3.1 Schematic diagram of four probe method in which the two outer probe are current source and the two inner probe are voltage source. [4]
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b) CONSTANT CURRENT SOURCE AND CURRENT SWITCHING
A Keithley constant current source (Keithley 6221) is used to supply the DC current to the sample. Thus the resistance of the sample is directly proportional to the voltage developed across the two voltage leads. The voltage is developed due to the application of current. The magnitude of the current is restricted to a maximum of 1mA in sintered rectangular slabs cut out of pellet.
C) DIGITAL NANOVOLTMETER
The voltage developed in the sample by applying current in the four probe setup, is measured by a digital nano voltmeter (Keithley Model 2182). This is a highly sensitive nano voltmeter having low noise with a sensitivity of 10 nV.
D) DATA ACQUISITION
The instrument such as the constant current source, the nano voltmeter and the temperature controller are interfaced to a computer for automatic data acquisition as a function of temperature and I-V measurements at different temperatures. The resistivity of the sample is taken from room temperature (around 3000C) to 8K.
3.3.4 I-V MEASUREMENT
For the measurement of I-V below Tc, we set a temperature below Tc using temperature controller and heater and the constant DC current is passed through the two outer probes in small steps from DC current source and the voltage developed is measured through the two inner probes connected to Nano voltmeter.
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CHAPTER-IV
RESULT & DISCUSSION
4.1 INTRODUCTION
The analysis regarding the structure, phases, average grain size, crystalline nature of BSCCO (2212) superconductor is broadly discussed by using x-ray diffraction data. From scanning electron microscope images, the analysis regarding surface morphology, grain boundary, etc is discussed. The temperature dependence of resistivity is also discussed.
4.2 X-RAY DIFFRACTION
The powder x-ray diffraction patterns of BSCCO, CFO and their composites taken in the range 200-800 are shown in the figure (4.1), (4.2), (4.3), (4.4). The peaks of BSCCO and CFO are perfectly matching with the JCPDS data (ICDD 46-0545). The indexing of the xrd data and the calculation of the lattice parameter are done by using Philips X-pert high score software
.
20 30 40 50 60
0 50 100 150 200 250 300 350
intensity(arb units)
2
BSCCO(2212)
(0 0 8) (1 0 5) (0 1 0) (1 0 7) (0 0 12) (1 1 10) (0 0 16) (1 0 15) (2 1 5) (2 1 7) (0 0 20)
Fig (4.1) x-ray diffraction pattern of pure BSCCO(2212)
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20 30 40 50 60
0 100 200 300 400 500 600
intensity(arb units)
2
CFO
(2 2 0) (3 1 1)(2 2 2) (4 0 0) (4 2 2) (5 1 1) (4 4 0)
Fig 4.2 X-ray diffraction pattern of CoFe
2O
420 30 40 50 60
0 100 200 300 400 500 600 700 800
Intensity(a.u)
2
-- BSCCO
-- CFO
Fig (4.3) x-ray diffraction of BSCCO+(0.5 %) CFO. The symbols * marks the
BSCCO phase and # marks the CFO peaks.
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20 30 40 50 60
0 100 200 300 400 500
Intensity(a.u)
2
---BSCCO
---CFO
Fig (5.4) x-ray diffraction pattern of BSCCO+(5%) CFO. The symbols * marks the BSCCO phase and # marks the CFO peaks.
By doing analysis of x-ray diffraction pattern of BSCCO(2212), we have concluded that the material is crystalline in nature. Various high intensity peaks of BSCCO(2212) are observed. These are the major peaks of BSCCO. Some low intensity peaks are also observed in the 2ϴ range 550-650. On comparing the results with the JCPDS database(file no- file no- 46-0545), the xrd pattern is found to be having good match with the tetragonal phase with lattice constants a=b=3.82 Å, c=30.6 Å. The volume of the unit cell is 446.5 cm3.
From fig(4.2), it is found that all peaks of CFO are in good match with JCPDS data (ICDD 22-1086). The sharpness of the peaks shows that the material is crystalline in nature.
The major phase of CFO is observed at (hkl) value (311). The material is a cubic system with lattice parameter a=b=c=8.392 Å. The volume of the unit cell is 591 cm3.
When we have made composite of BSCCO and CFO having concentration 0.5%
(CFO), we have found that the structure of the composites changes from tetragonal to orthorhombic having lattice parameter a=b=5.41 Å and c=30.8 Å. The volume of the unit cell increases to 902.63 cm3. Hence the material becomes denser with the addition of CFO. When we add 5% of CFO to BSCCO, the volume of the unit cell increases.
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4.3 SCANNING ELECTRON MICROSCOPE (SEM)
The scanning electron images of BSCCO, CFO and their composites with different composition are shown in fig (4.5), (4.6) and (4.7).
Fig (4.5) SEM image of BSSCO (2212) shows sharp grain boundaries with some voids that exist in the sample when sintered at 880 K for 12 hr.
Fig (4.6) SEM image of BSCCO + (0.5 %) CFO shows the agglomeration of grains and the densification of the sample when sintered at 800 K for 1 hr.
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Fig (4.7) SEM image of BSCCO + (5 %) CFO shows that the surface becomes smoother and densification of the sample occurs when sintered at 800K for 1hr.
The SEM image of pure BSCCO(2212) shows that it has layered structure. The grain size remains almost same having distinct and clear grain boundaries. Voids are observed in the sample.
As we add CFO with concentration 0.5% and 5%and sintered at 800 K, the surface of the sample become smoother and the voids disappear. The sample becomes denser. This may be due to the agglomeration of grains i.e. intergrain fusion. The morphological changes are caused due to the partial melting of individual grains.
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4.4 TEMPERATURE DEPENDENCE RESISTIVITY
The temperature dependence resistivity is measured using four probe method. The dc current supplied for the measurement of resistivity is 1mA. The variation of resistivity ρ(T) versus temperature curves of pure BSCCO and composites of BSCCO are shown in fig (4.8), (4.9), (4.10), (4.11), (4.12).
0 50 100 150 200 250 300 350
-0.002 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020
Resistivity(m)
TEMP(K)
resistivity(m)
TC-ONSET-- 77 k TC-FINAL-- 57 k TC -- 69 k
Tc onset Tc
Tc final
Fig (4.8) temperature dependence resistivity of pure BSCCO
0 50 100
-0.001 0.000 0.001
in the sample
d/dT
Temp(k)
derivative
Tc- 69 K
Tc-onset
-
77 KTc-Final-57 K
Fig (4.9) derivative of vs T curve of BSCCO
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0 50 100 150 200 250 300 350
0.00000 0.00001 0.00002 0.00003 0.00004 0.00005
RESISTIVITY(OHM M)
RESISTIVITY(OHM M)
TEMP(K)
TC-ONSET--77 K TC -final--23 K TC --40 K Tc onset
Tc Tc final
Fig (4.10) temperature dependence resistivity of BSCCO+(0.5 %) CFO
0 30 60 90 120
0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006
d/dT
Temp(k)
derivative
Tc-40K
Tc-onset-77K Tc-final-23K
Fig (4.11) derivative of ρ(T) vs T curve of BSCCO+(0.5 %) CFO
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0 50 100 150 200 250 300 350
-0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
0.08 RESISTIVITY(OHM M)
RESISTIVITY(OHM M)
TEMP(K)
Fig (4.12) Temperature dependence resistivity of BSCCO+(5 %) CFO
From fig (4.8), it is observed that when we decrease the temperature of the sample from room temperature (300K) , its resistance decreases. This means the sample shows metallic behaviour. However, the transition from metallic to superconducting starts at temperature 77 k. This temperature is called onset critical temperature. The complete phase transition occurs at temperature 57 K. This temperature is called final critical temperature. At this temperature, the resistance of the sample is zero.
When we made composites with 0.5% CFO, the transition temperature of the sample is 40 K. Due to the addition of CFO, the residual impurity of the sample increases and simultaneously the transition temperature decreases.
When we made composites with 5% CFO, the sample shows insulating behaviour.
This is due to addition of higher concentration of magnetic material, which suppresses superconductivity.
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4.5 I-V MEASUREMENT
The current-voltage (I-V) measurement of the sample below critical temperature in the range of current 0-12 mA is shown in fig (5.13) and (5.14).
0.00 0.02 0.04 0.06 0.08 0.10 0.12
0.0000 0.0001 0.0002 0.0003 0.0004 0.0005
voltage(v)
current(A) Ic--73 mA
Jc--39.9 mA/m2
Fig(4.13) I-V curve of BSCCO at 40 K
0.00 0.02 0.04 0.06 0.08 0.10 0.12
0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006
voltage(v)
Ic-- 43.2 mA Jc--23.76 mA/m2
current(A)
Fig (4.14) I-V curve of BSCCO at 55 K
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From the graph, it is clear that the graph is not linear. At low value of current, the voltage is zero. Hence the sample is in superconducting in nature. As the current increased till a critical value Ic, voltage is developed across the sample and hence resistance is developed in the sample. The current at which the sample is no more a superconductor is called critical current. The sample is in normal metal state beyond the critical current. The critical current densities for BSCCO at 40 K and 55 K are 39.9mA/m2 and 23.76 mA/m2.
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CHAPTER-V CONCLUSION
Bi2Sr2CaCu2O8 samples and Cobalt ferrite (CoFe2O4) are prepared by solid state method and sol-gel method respectively. Composites of Bi-2212 and cobalt ferrite with various compositions of cobalt ferrite are prepared to study various properties such as electrical resistivity, critical current density etc. The study regarding the phases and surface morphology of pure Bi-2212, cobalt ferrite and composites are also done.
By analysing the data from x-ray diffraction, it is found that the structure of Bi-2212 material is tetragonal. By addition of Cobalt ferrite with different concentration i.e. 0.5% &
5%, the volume of Bi-2212 increases and the structure changes to orthorhombic.
From SEM images it is found that there are some voids in the pure Bi-2212 sample.
By addition of Cobalt ferrite, the voids disappear; the grains agglomerate and the surface become smoother. Due to the agglomeration of grains, the density of the sample increases.
From R-T measurement, it is found that the transition temperature for pure BSCCO is 69K while the addition of 0.5% Cobalt ferrite with BSCCO decreases the transition temperature to 23K. The addition of 5% CFO makes the sample semiconducting.
From I-V curve, it is found that the critical current density at temperature 40K and 55K is found to be 39.9mA/m2 and 23.76mA/m2 respectively.
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REFFERENCES
1) C. kittle, Introduction to solid state physics ( 7
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2) R.K. Puri and V.K. Babbar , Solid state physics(8
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3) Wikipedia.org/wiki/high-temperature superconductor
4) http://www.google.co.in/search?hl=en&safe=active&biw=1382 5) http://books.google.co.in/books?id=yLwIYiWo0VYC&pg=PA271 6) Enhancement of electrical and mechanical properties in Nano size
MgO added Bi
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8, N. A. Hamid, N.F. Shamsudin, and K.W.
See Kamihara Y, Watanabe T, Hirano M and Hosono H (2008) J. Am Chem.Soc. 130, 3296
7) Takahashi H, Igawa K, Arii K, Kamihara Y, Hirano M and Hosono H (2008) Nature 453, 376,Bednorz J G, Müller K A (1986) Z. Phys. B 64, 189 Wu M K, Ashburn R J, Torng C J, Hor P H, Meng R L, Gao L, Huang Z J, Wang Y Z, Chu C W (1987) Phys. Rev. Lett. 58, 908
8) Synthesis of Bismuth-Strontium-Calcium-Copper Oxide powders for SilverComposite Wires with Uniform Micron Sized Filaments, S.
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9, NO. 2, JUNE 1999IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9, NO. 2, JUNE 1999
9) PROPERTIES OF THE MELT PROCESSED 2212 BSCCO IN
SILVER Ming Yang, I.Belenli, C.M.R.Grovenor, M.J.Goringe,
R.Jenkins*,H.Jones*,Mat. Res. Soc. Symp. Proc. Vol. 275. @1992
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10)Investigating the vortex melting phenomenon in BSCCO crystals using magneto-optical imaging technique.
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