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Thermally stimulated electron emission of YBaCuO sandwich sample
Vacuum 54 (1999) 285 — 288
Thermally stimulated electron emission of YBaCuO sandwich sample J. Chrzanowski Institute of Experimental Physics, University of Wroc!aw, Max Born Sq. 9, 50-204 Wroc!aw, Poland
Abstract Preparation of YBaCuO samples consisting of s—n—s or n—s—n sandwich structure, where s represents the superconducting layer and n the nonsuperconducting one, respectively, is presented. It is found that under the influence of the magnetic field the YBaCuO sandwich samples behave as superconducting material; this is independent on a type of sandwich structure. These YBaCuO samples are examined by thermally stimulated electron emission method. The I(T) emission images depend on the structure of a sample and are different from those for homogenous YBaCuO samples (Chrzanowski, Phys 1995, 15, 21). 1999 Elsevier Science Ltd. All rights reserved. Keywords: High ¹ superconductivity; Thermally stimulated electron emission
1. Introduction
Y O #2BaCO #3CuOPYBa Cu O #2CO . \V
In this work, investigations of the thermally stimulated electron emission (TSEE) of YBaCuO-high-temperature superconducting materials, in the temperature range 100—600 K, are carried out. The aim of the work was to check whether TSEE patterns can reveal differences between superconducting and nonsuperconducting materials as well as to investigate of the resistance of s—n—s or n—s—n sandwich sample. The resistance of the sample, total and partial pressure of gasses desorbed from the sample, were controlled as in Ref. [2]. The measurements were performed under 10\ Pa vacuum condition. The detection system (electron multiplier and grid), monopole mass spectrometer (UFM), and Bayard—Alpert head are situated in a molybdenum glass chamber. The main body and the measuring table of apparatus are made of stainless-steel 1H18N9T. The intensity of the electron emission is measured by an electron channel electron multiplier; the temperature of the sample is controlled by two copper-constantan thermocouples. The reference temperature is measured on the surface of a sample.
The high-purity chemical compounds containing 1.057 g of BaCO , 0.302 g of Y O and 0.639 g of CuO were used to fabricate 2 g YBaCuO initial samples. These materials were ground and mixed in order to produce homogeneous powder. Next, this grayish-white powder was compressed under 1.1;10 Pa. The disk-shaped samples, 20 mm diameter and 2 mm thick, were then obtained. After compressing, the samples were heated at 1180 K for 24 h. As a result of BaCO decomposition the CO is delivered at this temperature. Then, the samples were cooled down to 680 K and maintained at this temperature for an hour under intense air supply. We can suppose that, in the temperature range from 670 to 780 K, the large quantity of oxygen is absorbed by the YBaCuO structure. It is a temperature region in which the YBaCuO structure transforms from tetragonal phase into a more ordered orthorhombic phase [3]. The copper—oxygen chains are formed at the same time, and the quantity of oxygen absorbed during this transformation determines the value of index x in the YBa Cu O \V structure. The samples used in our experiments consisted of three, 1 mm thick, n—s—n or s—n—s layers. The so-called n-layer is thought as nonsuperconducting, and the slayer as a superconducting one, respectively. For fabricating the samples studied, the initial n- and s-type
2. Sample preparation procedure The YBaCuO samples were baked out according to the chemical reaction:
0042-207X/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 4 7 9 - 5
286
J. Chrzanowski / Vacuum 54 (1999) 285—288
materials were ground once more. Then the n—s—n or s—n—s samples with sandwich structure were produced by compressing and heating at 1180 K. The homogeneous
only s- or n-type samples were also obtained. It was found that the n-type samples behave as weak diamagnetic material, the samples with sandwich structure as
Fig. 1. Temperature dependence of the electron emission intensity (TSEE) for YBaCuO (1-2-3) sandwich sample of (a) n—s—n and (b) s—n—s type (n — nonsuperconducting layer, s — superconducting layer), obtained during the heating run.
Fig. 2. Resistance of the YBaCuO (1-2-3) sandwich sample measured during cooling (1) and during heating (2).
J. Chrzanowski / Vacuum 54 (1999) 285—288
well as s-type samples have strong diamagnetic properties, they levitate in the magnetic field.
3. Results TSEE measurements employed for the examination of superconducting samples were carried out in the following manner. The sample was excited at 100 K by electrons with an energy of 300 eV (I "5 lA) for 30 s. Next, the sample was heated up to 600 K (with an approximate heating rate of 10 K/min) and then cooled down to 100 K at a cooling rate of about 9 K/min. After each heating/cooling treatment cycle (100—600—100 K) the sample was excited again and the TSEE measurements were performed once more. During consecutive heating processes, the I(¹) emission images obtained for s or n (2 mm thick) samples were reproducible. For nonsuperconducting YBaCuO sample two maxima on I(¹) curves were observed at 140 and 370 K; for a superconducting one the maxima on I(¹) curves were observed at 225, 370 and 420 K [1].
287
In the case of n—s—n type samples the I(¹) curves exhibit a maxima at 210, 320, 370 and 510 K (Fig. 1a.); for s—n—s samples at 170 and 330 K (Fig. 1b). It is worth mentioning that the initial electron emission intensity registered after electron excitation at 100 K decreases quickly for n—s—n type samples and rather weakly for s—n—s ones, respectively (Fig. 1a and b). For all samples studied, the electron emission intensity registered during cooling processes was independent of temperature, and was about an order of two lower than that obtained during heating processes. Resistance versus temperature measurements were carried out. Electrical connections to the sample were made with standard four-probe indium contacts. The anomaly on R(¹) curve related with rather small oxygen index x is observed at 150 K. The temperature of the superconducting phase transition is about 80 K (Fig. 2.). Figs. 3a and b show, by way of example, the X-ray diffractograms obtained for superconducting samples in the system Y—Ba—Cu—O (Fig. 3a) and nonsuperconducting samples in the system Y—Ba—Cu—O (Fig. 3b). It is
Fig. 3. X-ray diffraction patterns of the (a) superconducting and (b) nonsuperconducting. ( — — Barium Yttrium Copper Oxide — Ba YCu O )
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J. Chrzanowski / Vacuum 54 (1999) 285—288
difficult, on the basis of the X-ray spectrograms obtained for the YBa Cu O , to determine the state of super \V conductivity in the specimens. Differences in peak height, which appear at an angle of 56 2h, may be responsible for the presence and distribution of oxygen atoms [4, 5]. It is seen (Fig. 3b) that the peak decreases for the non-superconducting sample. The structure of YBa Cu O , like other high-tem \V perature superconductors, is a lamellar structure. Its transition from the tetragonal to the orthorhombic one (only the latter is superconducting) takes place at the oxygen content between 0.5 and 0. Increasing oxygen concentration (i.e. decreasing the x) is accompanied by increasing temperature ¹ and increasing distortion of the rhombic unit cell. It is supposed that oscillation of copper—oxygen chains, if correlated with displacements of barium Ba, copper Cu and oxygen O atoms bring about superconductivity of the structure YBa Cu O . \V
face layer (n or s). The surface structure determines also the time decay of the electron emission intensity (Fig. 1). The polycrystalline samples obtained reveal considerable porosity of the structure. No difference in the partial gas analysis was registered for the n—s—n as well as s—n—s samples. The systematic increase of partial pressure of CO, CO , O, OH and H O was observed during heating process [7]. It seems that the systematic investigation of these sandwich structure — samples from the point of view of their emissive properties should be continued because they can be used as a new tool for the control of the fabrication processes in the case of the high-temperature superconducting materials.
4. Discussion
[1] Chrzanowski J, Sujak B. Scientific Report of the Technical University of Opole No. 209. Phys 1995;15:21. [2] Chrzanowski J, Nowicki M, Sujak B, Klimkiewicz R, Morawski AW. Carbon 1993;31(4):549. [3] Anderson Ph W, R Schrieffer. Phys Today 1991;6:54. [4] Engler EM, et al. J Amer Chem Soc 1987;109:2848. [5] Engler EM, et al. In: Nelson DL, Whittingham M, George TF, editors. Chemistry of high-temperature superconductors. ACS Symp. 1987, Ser. 351. [6] Verkin BJ, Dimitriev WM, Zvyagin AZ, Kosmyna MB, Kutko VJ, Litinskaya LS, Panfilov AS, Plurznikov VB, Prihodko OR, Soloviev AL, Christenko LYe, Czurilov GYe. Fiz. Niz. Temp 1987;13:853. [7] Chrzanowski J. Mol Phys Rep 1994;7:209.
It is stated that under magnetic field influence the samples with n—s—n as well as s—n—s sandwich structure behave as superconducting material independently of which of the layer s or n constitutes the sample surface. The observation is confirmed also by R(¹) measurements. This means that the superconductivity in the case of these materials can be realized by the formation of a network of superconducting channels [6]. The emission images obtained for the samples with sandwich structure depends on the kind of the sur-
References
Thermally stimulated electron emission of YBaCuO sandwich sample J. Chrzanowski Institute of Experimental Physics, University of Wroc!aw, Max Born Sq. 9, 50-204 Wroc!aw, Poland
Abstract Preparation of YBaCuO samples consisting of s—n—s or n—s—n sandwich structure, where s represents the superconducting layer and n the nonsuperconducting one, respectively, is presented. It is found that under the influence of the magnetic field the YBaCuO sandwich samples behave as superconducting material; this is independent on a type of sandwich structure. These YBaCuO samples are examined by thermally stimulated electron emission method. The I(T) emission images depend on the structure of a sample and are different from those for homogenous YBaCuO samples (Chrzanowski, Phys 1995, 15, 21). 1999 Elsevier Science Ltd. All rights reserved. Keywords: High ¹ superconductivity; Thermally stimulated electron emission
1. Introduction
Y O #2BaCO #3CuOPYBa Cu O #2CO . \V
In this work, investigations of the thermally stimulated electron emission (TSEE) of YBaCuO-high-temperature superconducting materials, in the temperature range 100—600 K, are carried out. The aim of the work was to check whether TSEE patterns can reveal differences between superconducting and nonsuperconducting materials as well as to investigate of the resistance of s—n—s or n—s—n sandwich sample. The resistance of the sample, total and partial pressure of gasses desorbed from the sample, were controlled as in Ref. [2]. The measurements were performed under 10\ Pa vacuum condition. The detection system (electron multiplier and grid), monopole mass spectrometer (UFM), and Bayard—Alpert head are situated in a molybdenum glass chamber. The main body and the measuring table of apparatus are made of stainless-steel 1H18N9T. The intensity of the electron emission is measured by an electron channel electron multiplier; the temperature of the sample is controlled by two copper-constantan thermocouples. The reference temperature is measured on the surface of a sample.
The high-purity chemical compounds containing 1.057 g of BaCO , 0.302 g of Y O and 0.639 g of CuO were used to fabricate 2 g YBaCuO initial samples. These materials were ground and mixed in order to produce homogeneous powder. Next, this grayish-white powder was compressed under 1.1;10 Pa. The disk-shaped samples, 20 mm diameter and 2 mm thick, were then obtained. After compressing, the samples were heated at 1180 K for 24 h. As a result of BaCO decomposition the CO is delivered at this temperature. Then, the samples were cooled down to 680 K and maintained at this temperature for an hour under intense air supply. We can suppose that, in the temperature range from 670 to 780 K, the large quantity of oxygen is absorbed by the YBaCuO structure. It is a temperature region in which the YBaCuO structure transforms from tetragonal phase into a more ordered orthorhombic phase [3]. The copper—oxygen chains are formed at the same time, and the quantity of oxygen absorbed during this transformation determines the value of index x in the YBa Cu O \V structure. The samples used in our experiments consisted of three, 1 mm thick, n—s—n or s—n—s layers. The so-called n-layer is thought as nonsuperconducting, and the slayer as a superconducting one, respectively. For fabricating the samples studied, the initial n- and s-type
2. Sample preparation procedure The YBaCuO samples were baked out according to the chemical reaction:
0042-207X/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 4 7 9 - 5
286
J. Chrzanowski / Vacuum 54 (1999) 285—288
materials were ground once more. Then the n—s—n or s—n—s samples with sandwich structure were produced by compressing and heating at 1180 K. The homogeneous
only s- or n-type samples were also obtained. It was found that the n-type samples behave as weak diamagnetic material, the samples with sandwich structure as
Fig. 1. Temperature dependence of the electron emission intensity (TSEE) for YBaCuO (1-2-3) sandwich sample of (a) n—s—n and (b) s—n—s type (n — nonsuperconducting layer, s — superconducting layer), obtained during the heating run.
Fig. 2. Resistance of the YBaCuO (1-2-3) sandwich sample measured during cooling (1) and during heating (2).
J. Chrzanowski / Vacuum 54 (1999) 285—288
well as s-type samples have strong diamagnetic properties, they levitate in the magnetic field.
3. Results TSEE measurements employed for the examination of superconducting samples were carried out in the following manner. The sample was excited at 100 K by electrons with an energy of 300 eV (I "5 lA) for 30 s. Next, the sample was heated up to 600 K (with an approximate heating rate of 10 K/min) and then cooled down to 100 K at a cooling rate of about 9 K/min. After each heating/cooling treatment cycle (100—600—100 K) the sample was excited again and the TSEE measurements were performed once more. During consecutive heating processes, the I(¹) emission images obtained for s or n (2 mm thick) samples were reproducible. For nonsuperconducting YBaCuO sample two maxima on I(¹) curves were observed at 140 and 370 K; for a superconducting one the maxima on I(¹) curves were observed at 225, 370 and 420 K [1].
287
In the case of n—s—n type samples the I(¹) curves exhibit a maxima at 210, 320, 370 and 510 K (Fig. 1a.); for s—n—s samples at 170 and 330 K (Fig. 1b). It is worth mentioning that the initial electron emission intensity registered after electron excitation at 100 K decreases quickly for n—s—n type samples and rather weakly for s—n—s ones, respectively (Fig. 1a and b). For all samples studied, the electron emission intensity registered during cooling processes was independent of temperature, and was about an order of two lower than that obtained during heating processes. Resistance versus temperature measurements were carried out. Electrical connections to the sample were made with standard four-probe indium contacts. The anomaly on R(¹) curve related with rather small oxygen index x is observed at 150 K. The temperature of the superconducting phase transition is about 80 K (Fig. 2.). Figs. 3a and b show, by way of example, the X-ray diffractograms obtained for superconducting samples in the system Y—Ba—Cu—O (Fig. 3a) and nonsuperconducting samples in the system Y—Ba—Cu—O (Fig. 3b). It is
Fig. 3. X-ray diffraction patterns of the (a) superconducting and (b) nonsuperconducting. ( — — Barium Yttrium Copper Oxide — Ba YCu O )
288
J. Chrzanowski / Vacuum 54 (1999) 285—288
difficult, on the basis of the X-ray spectrograms obtained for the YBa Cu O , to determine the state of super \V conductivity in the specimens. Differences in peak height, which appear at an angle of 56 2h, may be responsible for the presence and distribution of oxygen atoms [4, 5]. It is seen (Fig. 3b) that the peak decreases for the non-superconducting sample. The structure of YBa Cu O , like other high-tem \V perature superconductors, is a lamellar structure. Its transition from the tetragonal to the orthorhombic one (only the latter is superconducting) takes place at the oxygen content between 0.5 and 0. Increasing oxygen concentration (i.e. decreasing the x) is accompanied by increasing temperature ¹ and increasing distortion of the rhombic unit cell. It is supposed that oscillation of copper—oxygen chains, if correlated with displacements of barium Ba, copper Cu and oxygen O atoms bring about superconductivity of the structure YBa Cu O . \V
face layer (n or s). The surface structure determines also the time decay of the electron emission intensity (Fig. 1). The polycrystalline samples obtained reveal considerable porosity of the structure. No difference in the partial gas analysis was registered for the n—s—n as well as s—n—s samples. The systematic increase of partial pressure of CO, CO , O, OH and H O was observed during heating process [7]. It seems that the systematic investigation of these sandwich structure — samples from the point of view of their emissive properties should be continued because they can be used as a new tool for the control of the fabrication processes in the case of the high-temperature superconducting materials.
4. Discussion
[1] Chrzanowski J, Sujak B. Scientific Report of the Technical University of Opole No. 209. Phys 1995;15:21. [2] Chrzanowski J, Nowicki M, Sujak B, Klimkiewicz R, Morawski AW. Carbon 1993;31(4):549. [3] Anderson Ph W, R Schrieffer. Phys Today 1991;6:54. [4] Engler EM, et al. J Amer Chem Soc 1987;109:2848. [5] Engler EM, et al. In: Nelson DL, Whittingham M, George TF, editors. Chemistry of high-temperature superconductors. ACS Symp. 1987, Ser. 351. [6] Verkin BJ, Dimitriev WM, Zvyagin AZ, Kosmyna MB, Kutko VJ, Litinskaya LS, Panfilov AS, Plurznikov VB, Prihodko OR, Soloviev AL, Christenko LYe, Czurilov GYe. Fiz. Niz. Temp 1987;13:853. [7] Chrzanowski J. Mol Phys Rep 1994;7:209.
It is stated that under magnetic field influence the samples with n—s—n as well as s—n—s sandwich structure behave as superconducting material independently of which of the layer s or n constitutes the sample surface. The observation is confirmed also by R(¹) measurements. This means that the superconductivity in the case of these materials can be realized by the formation of a network of superconducting channels [6]. The emission images obtained for the samples with sandwich structure depends on the kind of the sur-
References