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In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University. When he cooled it to the temperature of liquid helium, 4 degrees Kelvin (-269C), its resistance suddenly disappeared.
The phenomenon of superconductivity is manifested in the electrical resistance vanishing at a finite temperature called the critical temperature and denoted Tc (Fig. 1). The latest data show that the resistivity of a superconductor is below 10âˆ’27Î©.cm. This can be compared with the resistivity of copper (an excellent conductor), which is 10âˆ’9Î©.cm.
Figure : Temperature dependence of resistance of a normal metal and a superconductor
The next great milestone in understanding how matter behaves at extreme cold temperatures occurred in 1933. German researchers Walther Meissner and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field. A magnet moving by a conductor induces currents in the conductor. This is the principle on which the electric generator operates. But, in a superconductor the induced currents exactly mirror the field that would have otherwise penetrated the superconducting material - causing the magnet to be repulsed. This phenomenon is known as strong diamagnetism and is today often referred to as the "Meissner effect". The Meissner effect is so strong that a magnet can actually be levitated over a superconductive material.
In subsequent decades other superconducting metals, alloys and compounds were discovered. In 1941 niobium-nitride was found to superconduct at 16 K. In 1953 vanadium-silicon displayed superconductive properties at 17.5 K. And, in 1962 scientists at Westinghouse developed the first commercial superconducting wire, an alloy of niobium and titanium (NbTi). High-energy, particle-accelerator electromagnets made of copper-clad niobium-titanium were then developed in the 1960s at the Rutherford-Appleton Laboratory in the UK, and were first employed in a superconducting accelerator at the Fermilab Tevatron in the US in 1987.
Overview of High Temperature Superconductors (HTS)
HTS are defect perovskite like cuprate materials, which were discovered in 1986 by Bednorz and Müller  . These materials have Tcs as high as 90-125K (prior to this, Tc achieved was below 23.2 K, corresponding to that of Nb3Ge). Bednorz and Muller discovered high temperature superconductivity (Tc ~ 35 K) in defect perovskite like oxide material La2âˆ’xBaxCuO4. These materials are layered structures in which sheets of copper and oxygen atoms alternate with sheets of rare-earth (and oxygen) atoms. Soon after, Paul Chu and his coworkers  discovered the so called 123 oxides of the general formula (RE)Ba2Cu3O7âˆ’Î´ (RE = Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm or Yb) with Tc values in the 90K region. The discovery of materials with superconductivity above the liquid-N2 temperature raised much hope and prompted intensive search for new classes of oxides with still higher Tcs. Two series of compounds belonging to the Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems have been found to exhibit superconductivity between 60 and 125K  4.
Development of 1G HTS and 2G HTS
Low Temperature Superconductors (LTS), with Tcs not higher than 30K, were limited to Clinical magnetic resonance imaging (which was the first major commercial application of superconductivity and remains as the major market today). But the discovery of ceramic-based high temperature superconductors opened the possibility of applying superconductivity to electric power devices such as power transmission and storage, HTS motors and generators.
The most commonly used materials in First-Generation Superconductors (1G HTS) were bismuth-based, specifically Bi2Sr2CaCu2O8+x (Bi-2212) and Bi2Sr2Ca2Cu3O10+x (Bi-2223). Since the late 1980s, Bi-2223 wires with silver matrix have been developed with acceptable mechanical property and current density. However, there are two major problems with first generation HTS wires:
The large amount of expensive metals used for the matrix makes them expensive.
The irreversibility field, at which the Jc drops to zero, of bismuth-based HTS materials is very low at higher temperatures, which makes 1G HTS highly sensitive to magnetic fields. They lose superconductivity at very low applied fields (~0.2 T) at 77 K, which is problematic to the construction of superconducting magnets and other high field applications.
More recently, rare earth-based HTS material ((RE)BCO) have been developed. Rare earth elements include, but are not limited to, Yttrium, Samarium and Gadolinium. This 2G conductor (or wire) offers both performance (operates at higher temperatures and background magnetic fields) and cost benefits. A typical YBa2Cu3O7-X (YBCO) lattice has an orthorhombic perovskite structure displayed in Fig. 2. As can be seen in the figure, the oxygen and copper atoms are bound together in alternating chains and planes. This layered structure of Cu-O planes is responsible for the superconductivity and the strong anisotropic properties. The oxygen content in REBCO lattice is very important to their superconductivity as shown in Fig. 3. The parameter x in the YBCO formula varies from 0 to 1, determined by the conditions during the subsequent annealing process.
Figure : Orthorhombic perovskite molecular structure of YBCO 
Figure : Dependence of YBCO's superconductivity on its oxygen content
Fabrication of 2-G HTS
In the beginning, the preparation of the bulk superconducting oxide samples, using conventional ceramic techniques (grinding, firing and pelletizing), became well established and was reproduced by many researchers. One drawback of the polycrystalline versions in which high temperature superconductors were initially synthesized was low critical current density (Jc), which was about 102-103 Acmâˆ’2. The usual magnitude of Jc which, for example, is achievable from conventional superconductors is about 106 Acmâˆ’2. Such a value would be useful for most of the devices, particularly high current ones, like high field electromagnets, lossless transmission, levitating vehicles, MRI etc.
At the early stage, it was recognized that low Jc values were due to weak-link effects arising from the presence of high angle grain boundaries separating adjacent gains and also because of absence of suitable flux pinning centers within the grains. Since thin films are necessarily grown on substrates, by proper choice of substrates suitably oriented films without high angle grain boundaries could be prepared and since thin film formation is like vapor quenching, secondary phase forming flux pinning centers might get nucleated. Therefore, formation of thin films of high temperature superconducting materials held considerable promise. However, preparing thin films of these materials turned out to be a more difficult task. The first successful deposition using sputtering, from bulk materials, was reported for the âˆ¼40K superconductors  7, including the preparation of microcrystalline thin films  .
The first report on the formation of thin films of YBCO was through electron beam evaporation of individual ingredients by Chaudhari et al.  . Later on, other techniques like screen printing, on beam sputtering  and pulsed laser evaporation  were employed for the synthesis of thin films of these YBCO high temperature superconductors.
Flux Pinning in REBCO
Intrinsic Pinning in REBCO
Extrinsic Pinning Center
Angular Dependence and Thickness Dependence of REBCO Thin Film Critical Current
In-Field Angular Dependence
Thickness Dependence of Critical Current
BZO and RE2O3 Doping in REBCO Thin Film
Chapter II: Methodology and experimental setup
In this chapter, the MOCVD system and its operating procedure used towards the fabrication of REBCO thin films are introduced. Also, the post-deposition treatments including DC sputtering of a silver cap layer and the oxygenation annealing, configurations and operations of the measurement systems and the sample bridging method used in this work are presented.
MOCVD system configuration and REBCO thin film deposition
M1-MOCVD system is a cold wall, reel-to-reel and single liquid precursor source system. A schematic representation of M1 configuration is shown in Fig. 4. REBCO thin films were deposited on IBAD-MgO buffered tape (Hastelloy substrate/Al2O3/Y2O3/IBAD-MgO/MgO/LaMnO3) with a width of 12mm. In our MOCVD process, THD precursors (2, 2, 6, 6,-tetramethyl-3, 5-heptanedionate) of Zr, Gd, Y, Ba, and Cu were used as metal organic source materials. Before each run, the M (thd)n (M=RE, Ba, Cu) precursors were mixed together and dissolved in an organic solvent, tetrahydrofuran (THF). And based on the molar ratios from the recipe, the mass of precursor powder is calculated.
Here is an example of precursor calculation. A 15% Zr-doped (Gd, Y)BCO with the ratio of 0.6 : 0.6 : 2 : 2.3, which is proven to be standard recipe for REBCO film with a ratio of 0.6 : 0.6 : 2 : 3. A typical molarity of (d) 0.2mol/L with a volume (L) of 200ml is used. The total molar amount of (Gd,Y)BCO (nall) will be:
The molar amount of each element can then be calculated. Take Gd as an example, the molar value of Gd (nGd) is calculated as following:
The molar weight of each element and chemical composition is shown in Table 2-1.
Table 2 Molar weight and chemistry formation of different elements
707.0521 g/mol (mGd)
639.7079 g/mol (mY)
864.2724 g/mol (mBa)
430.0807 g/mol (mCu)
824.2934 g/mol (mZr)
Finally, the mass of Gd-thd precursor needed in this standard precursor can be calculated by:
Mass of Y, Ba, Cu precursor powders can be calculated in the same way. The results are MY = 2.789g, MBa = 12.571g, MCu = 7.094g.
The calculation for Zr is different from other elements. As a doping element, the mole amount of Zr precursor is not included inside the total mole value. The mole value of Zr precursor powder is given by:
As a result, the mass of Zr precursor powder (MZr) is given as following:
After weighing all of the precursor powder, 200ml THF solvent is mixed with the powder in a clean bottle. The prepared single precursor liquid will be stirred by a magnetic stirrer for about 20 minutes until the powders are completely dissolved in the THF solvent.
During the deposition process, liquid precursor was pumped into the delivery line and carried by argon gas to the evaporator. Then the precursor vapor mixed with oxygen was delivered to the reactor consisting of the showerhead and the susceptor to enable the deposition process on the heated buffer tapes that moved at a certain speed. After the REBCO thin film deposition of each recipe group was finished, the precursor vapor would be pumped out and replaced with precursors for the next group.
Figure : Schematic diagram of the MOCVD system designed by SuperPower Inc 
After the deposition process, REBCO samples were cut off from the leader tapes and sputtered with a layer of silver in a DC sputtering system. The silver cap layer provides a means of good electrical contact and excellent solderability. The thickness of the silver layer is about 1 Î¼m determined by the power output and deposition time.
An oxygenation heat treatment was applied to replenish the oxygen in REBCO lost during the MOCVD and silver sputtering processes. The sputtered REBCO coated conductor samples placed in a horizontal tube furnace were oxygenated at 500°C for 10h in flowing oxygen. The ramp up and ramp down rates were 200°C/h and 180°C/h, respectively. After the samples were cooled to room temperature, they were taken out of the furnace and used for the following electromagnetic measurements and materials characterization.
There are three kind of electromagnetic measurements that are performed over the samples produced by M1. All the samples are subject to self-field and critical temperature measurements and then, according to these results, we establish a priority list for the measurement of the in-field performance for some of the samples.
Self-field Ic measurements were performed at 77Kwithout an external magnetic field and are very useful for quality control and preliminary evaluation of the performance of the samples. The self-field measurements were done using the four-probe system. It consists of a sample stage with current cables and a computer hooked up with a power supply and a Keithley multimeter. After mounting a sample on the stage and immersing the stage into a Dewar containing liquid nitrogen, a LabVIEW program preloaded in the computer was used to perform the measurement very quickly.
Figure : Schematic illustration of sample mounting and the four-probe method used during the critical current measurements 
All the contacts between voltage detection wires and samples, samples and silver current leads, silver current leads and the copper current tabs shown in Fig. 5 were made with indium solder. Indium melts at 156 °C which is low enough to protect samples from the potential damage caused by heat. The detection voltage criterion used to determine the critical current was 3 microvolt per centimeter of spacing between voltage detection contacts. During each measurement, a direct current (DC) generated by the power supply was carried by the sample. Until the critical current is reached, no voltage drop is observed. After the current reaches the critical value, the sample becomes resistive and shows an Ohmic behavior. The current at the detection voltage criterion is defined as the critical current. A typical Current versus Voltage (I-V) curve of Ic measurement is shown in Fig. 6.
Figure : An I-V curve obtained from a self-field critical current measurement performed at 77K. A detection voltage criterion of 3 µV/cm was used to determine the Ic value
One of the biggest issues with the Ic measurements is the sample burnout problem. A standard REBCO coated conductor sample used in this work was 12 mm in width and about 0.8 to 1.1 Î¼m in thickness (for the superconducting layer). Such a tape have a typical zero field Ic of 400 - 500 A at 77K. Once the transition to the normal state occurs with this high transport current, a significant joule heating is generated which can cause the sample burnout and can damage it permanently. To overcome this problem, a sample bridging method to proportionally reduce the transport current was developed. The procedure for the sample bridging method is as follows:
After cutting a piece of 2~3 cm from the tape, a bridge of 1~2 mm in width is patterned on the sample with Kapton® tapes.
The patterned sample is dipped into the silver etching solution (50% ammonium hydroxide + 50% hydrogen peroxide) until the silver layer not covered by Kapton® tapes was etched away.
After rinsing the sample with flowing tap water, it is dipped into the REBCO etching solution (10% nitric acid) for about 10 seconds.
The sample is then rinsed with flowing water and all the Kapton® tapes are removed carefully.
Samples with and without bridging are displayed in Fig. 7 for comparison. And then to get the Ic over the full width, we use the following relation:
Figure : Pictures of the sample with (above) and without (below) the bridge
Critical Temperature measurements
The superconducting transition temperature is measured by Meissner Effect of the superconducting samples. A temperature sensor is contacted with the sample to measure the temperature. The sample is sandwiched between two copper wire coils with different numbers of turns. The coil with fewer turns generates a small magnet field, and the other coil works as an induction coil. First, the whole system is cooled down to 77K with liquid nitrogen. Then, when the temperature is stable, the sample is pulled out just above the level of liquid nitrogen. There will be no induction voltage until the temperature reaches transition temperature (Tc).
The in-field critical current measurements were performed with a custom-designed cryogen-free 9 Tesla electromagnetic characterization system by Cryogenic Ltd, denoted as the high-field system for convenience. This high-field system enabled programmable angular and field dependent measurements at various temperatures as low as 4 K and magnetic field up to 9 T with no need of feeding supplying cryogen (usually helium and nitrogen) to the cryostat.
The high-field system is mainly made up of three parts: cryostat, Ic probe and controlling system.
A high current sample rotation Ic probe is used with the high-field system. The Ic probe is integrated with a motorized rotating sample platform, 600 A current leads, a Hall sensor and a sample thermometer. The motorized rotator on top of the probe is manipulated by the control system and provides a moving range of 200 degrees for the sample platform shown in Fig. 8.
Figure 8: Sample stage and accessories of the Ic probe 
Chapter III: Results and discussions
3.1. MOCVD results
3.1.1. Research on Zr content
3.1.2. Rare earth substitution
3.2. In-field data processing
3.2.1. Automation and processing
3.2.2. Pinning analysis