Observation of the Superconductivity of High Temperature Superconductor, YBa Cu O δ

Transcription

1 Observation of the Superconductivity of High Temperature Superconductor, YBa Cu O Chih-pin Chuu Department of Physics Purdue University, West Lafayette, In Abstract: We used YO 2 3, CuO 2, BaCO 3 to produce YBa Cu O, which is a high temperature superconductor. By measuring the emf induced by the change of magnetic flux passing through the sample and its surrounding, we can decide the critical temperature of the YBa Cu O superconductor. This is an easy way to produce superconductors and observe superconductivity in lab

2 Content 1 Introduction 2 Fabrication of YBa Cu O Sample 3 Setting up the measuring circuit 4 Determining the critical temperature T c 5 Measuring the relation between the temperature and the normalized signal to determine the critical temperature for phase transition 6 Conclusion 7 Reference

3 1 Introduction: Since Bednorz, Muller, Hor, and Chu at el. [1] found the complex metal oxide product can reach a critical temperature T c to 77K, which is much higher than the low temperature superconductor, people started doing measurement on all kind of high temperature superconductor. The discovery of the yttrium-barium-copper-oxygen (YbaCuO) group with its key ratios of the three metals opened up the whole field of high temperature superconductivity, since it was the first compound discovered having a T c above the temperature of liquid nitrogen. So experimentalist need not use the expensive liquid Helium for high temperature superconductor experiment any more. Our purpose is to make YBa Cu O superconductor through the reaction: 1 Y2O 3 + 2BaCO3 + 3CuO YBa2Cu3Ox + 2CO2( ), observe the superconductivity. 2 YBa Cu O is a high temperature superconductor. It has a critical temperature T c is about 90K [1] to 93K [2]. YBa Cu O is one of the most commonly used materials, because of its cheap price, high T c and easy fabrication procedure. We attach the driving coil and pick-up coil on to the sample pellet. We run AC current through the driving coil. From Faradey s law: (ε is the induced em, φ is the magnetic field flux, B is the magnetic field, A is the cross area of the coil, i is the current passing through the coil, n is the number of terms per unit length.) dφ dba ( ) d( µ 0ni A) ε = = = dt dt dt = µ 0 na di dt

4 We know that the alternative current can induce emf. So we attach a pick-up coil at the other side of the sample to collect the changing magnetic field flux. From Meissner effect [3], we know that when the sample become superconductor, it will exclude all the magnetic field lines. However, YBa Cu O is a type II high energy superconductor [2]. This means that it will trap some quantity of magnetic field flux in the domains inside between H c1 and H c2. [2] But the magnetic field flux through the pick-up coil will still decrease. As a result, by observing the decrease of voltage signals picked up at pick-up coil, we can tell if the sample gain superconductivity or not. Before YBa Cu O becoming superconductor, the magnetic field passing through YBa Cu O sample; however, when it become a superconductor, it will repel most of the magnetic field passing through it. So both the magnetic flux across the coil and the induced em decrease. By measuring the induced voltage on the pick-up coil, we can decide the critical temperature T c precisely. 2 Fabrication of YBa Cu O Sample Mixing and grinding YO 2 3, CuO 2, BaCO 3 into homogeneous powders. The original weight of the materials we used is: YO 2 3 (1.53g), CuO 2 (5.33g), BaCO 3 (3.22), in order to make the total weight to be 9g before any further cooking. Less will make the pellet fragile when you moving it, and too thick will make the temperature of the sample to be non-uniform during the magnetization measurement.

5 Loading the powders into the alumina combustion boat. The boat should be cleaned first. We used a relative high temperature (1200 C) to clean the boat. There is still some residue on it. Locating the boat into the center of the heating tube. We cooked the powders in the boat by high temperature. The condition is as following: Increasing the temperature at a rate of 3 C/min until 975 C. Maintaining at the temperature for 24 hr. And cooling down by 1 C/min until 0 C and let cooled for 24 hr. After cooking, it became 5.40g. The pressure of flowing oxygen is 20 psi. It became very tough after being cooked. The programming instructions of the heater are as following: 1. Pr1 : 3 C/min (How fast we want to heat up the boat) 2. PL1 : 975 C (How high we want the maximum temperature to be) 3. Pd1 : 24hr ( How long we want the boat to stay in the hot tube) 4. PR2 : 1 C/min (How fast we want the boat to cool down) 5. PL2 : 0 C (What s the final temperature we want to cool down) 6. Pd2 : End (How long we want the boat to stay at the final temperature) The heating rate shouldn t be set to too fast. We made a mistake by setting the heating rate to be 30 C/min and cooling rate to be 10 C/min. The sample broke into two parts. It had superconductivity, but for measuring, we recommend slowly heating up. Then we grind the pellet to make it onto powder again. Pressing the powder into circular pellet. The pellet weight 8.51g. The pellet is 25 mm in diameter and 4mm thick. The coils are 12 mm in diameter and 4mm thick. After drying the pellet, we repeated once again the cooking procedure.

6 3 Setting up the measuring circuit Waiting until cool, then we used varnish glue to attach the driving coil and the pick-up coil onto the pellet and connected to the power supply, oscilloscope, multimeter, and resistor. Then attaching the sample with the holder by vacuum grease. The temperature sensor is attached to the sample by vacuum grease and wound by silicon tape. The tape can remain sticky under low temperature. The following is the measuring circuit. Multimeter Driving coil 5 Ω Resistor Oscillo Scope Power Supply Pick-up coil Multimeter Oscillo Scope

7 Setting up the whole system (including the cryostate and freezer) and lowering the temperature to around 50K, then we used pump/freezer and temperature controller to control the temperature and recorded by reading the value on multimeters. Pump / Freezer Cryostate (sample located inside) Temperature Controller 4 Determineing the critical temperature T c At the critical temperature when YBa Cu O transform from normal state to superconductor state, there is a discontinuity of thermal conductivity coefficient and specific heat. [1](pp.41- pp.42) Since all these phenomenon is related to the electron (transforming from single electron to Cooper pair), so we suggest the electromagnetic properties, such as the quantity of magnetic field flux passing through the pellet, will also have discontinuity. This is very reasonable. Since at the critical temperature of YBa Cu O, even the crystal structure will also change [2] (p. 53). While cooling down form room temperature to above the critical temperature, the electrons lose thermal kinetic energy, so become easier to be magnetized. As a result, more magnetic field flux can pass through the

8 pellet and we get an increasing normalized signal, which is the voltage signal of the pick-up coil divided by the current signal of the driving coil. However, when the temperature reach the critical temperature T c, all the electrons start to form Cooper pair and the superconductor allow only a limit magnetic field flux passing though certain domains inside the sample. And the number of single electrons which is available being magnetized decreases, so the normalized signal decreases. As a result, we decided to define the point on figure of normalized signal vs. temperature where there is a discontinuous slope (as marked in the figure) to be the critical temperature T c. 5 Measuring the relation between the temperature and the normalized signal to determine the critical temperature for phase transition Since the T c of YBa Cu O is around 90K [1]-93K [2], so we focused on the range from 100K to 85K. The temperature of cryostate dropped much faster than the YBa Cu O pellet. The signal (Voltage) of pick-up coil will drop dramatically when YBa Cu O reach its critical temperature. The signal changed really fast and should be recorded even by quarter minute. We did see the drop of the signal, which means there is superconductivity. The following three figures show the normalized signal vs. the temperature of the sample. The normalized signal is defined as the voltage signal of the pick-up coil divided by the current signal of the driving coil. The position of the sharp drop means the phase transition of the superconductor. We can see that the critical temperature T c shifts a little. It s

9 about 92K in figure 1, 89.7K in figure 2, and 89 K in figure 3. All of them are very close to the common critical temperature of YBa Cu O. We see that the signal gradually increase to a certain value just before the phase transition and obviously drop during the phase transition. The place where the normalized signal starts to drop is the critical temperature T c. normalized signal V (pick-up) / I (drive) YBaCuO (95K --> 85 We define the temperature at place where the normalized si start to drop dramatically to critical temperatur. 92K Temperature of YBaCuO Figure 1 We lowered the temperature from room temperature to about 50K and focused on the range of 95K 85K. T c is 92K in this figure. However, during the drop of the temperature, the decreasing rate is too fast so that we

10 don t have enough time to gather enough data points. So we decided to increase the temperature and record the data. normalized signal V (pick-up)/ I (drive) YBaCuO superconductivi 85K --> 95K Critical Temperature Starting phase transition Temperatur of YBaCuO Figure 2 The increasing rate of the sample is much slower than the decreasing rate, so we can collect more data points and have a close look at the transition of the phase. Before reaching the critical temperature, the normalized signal increase gradually first and drop dramatically at the critical temperature T c. The T c seems to be 89.7K, which is different from the T c when we lowered the temperature. The normalized signal is at about the same value as Figure 1.

11 YBaCuO (80K-->100K normalized signal V (pick-up)/ I (drive) Critical temperatur Temperatur of YBaCuO ( Figure 3 This is another measurement focusing at the range from 100K to 80K. We collected the data points while heating up the sample. The T c is about 89K, which is very similar to Figure 2. We clearly found the same shape for the curve and showing that there is superconductivity. 6 Conclusion The temperature sensor has 4 significant digit, though we record all of them, but only showing 2 significant digit on the plot. The driving coil s signal is recorded by current (ma), which has 5 digits, but we recorded 4 digits; the pick-up coil s signal is recorded by voltage (mv), which also has 5 digits, and we recorded 4 digits. The only thing we did is to write down the

12 value. As a result, if the system is set up correctly, the only error came from the non-synchronized recording of the temperature and the signals of the coils. This is because the signal changed so fast while the temperature decreased after the critical temperature. We couldn t catch up the change. So the voltage/current signal we wrote down for time t might actually be the value for t ± t. We recorded each 0.5K for decreasing and 0.25K for increasing the temperature. So t < 0.5K. This might cause a slight shift of the x-position on the plot and effect the critical temperature T c. The shift should be at the same order as t, which is about 0.1K 0.5K. However, we do observed the sudden drop of the normalized signal which showing the critical temperature of phase transition about superconductivity. 7 Reference: [1].High Temperature Superconductivity, by K.P.Shiha & S.L.Kakani, Nova Science Publishers, 1994 [2].High Temperature Superconductors (Processing and Science), by A. Bourdillon & N.X. Tan Bourdillon, Academic Press, 1994 [3].Introducttion to Superconductivity, IBC Technical Services Ltd, 1989