Flavell, W.R.

Geeson, D.A.

Golden, M.S.

Male, S.E.

International Migration, Integration and Social Cohesion online publications

UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl)UvA-DARE (Digital Academic Repository)Parallel, core and valence level photoemission studies of nominal(Bi0.9,Pb0.1)2Sr2Ca2Cu3010+xGolden, M.S.; Geeson, D.A.; Male, S.E.; Flavell, W.R.Publication date1989Published inSuperconductor Science & TechnologyLink to publicationCitation for published version (APA):Golden, M. S., Geeson, D. A., Male, S. E., & Flavell, W. R. (1989). Parallel, core and valencelevel photoemission studies of nominal (Bi0.9,Pb0.1)2Sr2Ca2Cu3010+x. SuperconductorScience & Technology, 2(3), 175-181.General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s)and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an opencontent license (like Creative Commons).Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, pleaselet the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the materialinaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letterto: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. Youwill be contacted as soon as possible.Download date:10 Mar 2023Supercond.  Sci.  Technol. 2 (1989) 173-176. Printed in the UK P Regnier, M Sapin  and C Thomas  de  Montpreville SRMP, Departement  de  Technologie, Comissariat a I'Energie Atomique, Centre d'Etudes Nucleaires  de Saclay, 91191 Gif sur Yvette Cedex, France Received 27 February 1989, in  final  form 31 May 1989 Abstract. It  is  shown that the  substitution of Y,O, with ZrO, in  the synthesis of YBaCuO leads  to a mixture of BaZrO, , BaCuO, , CuO and YBa,Cu,0,-6. For zirconia  percentages  less  than 50% the  mixtures  exhibit a conductor- superconductor transition at a temperature  higher  than 77 K, which  is  due  to  the presence of the  orthorhombic phase, YBa,Cu,O,-, with 6 less  than 0.2. Between 50 and 70% zirconia,  the  materials  are  still  superconductors at low  temperature, but semiconductors  above  the  transition,  which  then takes place  over a broad  range of temperatures  lower  than 77 K. Above 70% zirconia,  the  ceramics  are semiconducting  down  to 4.2 K, and  the YBaCuO present  is  tetragonal  with  an oxygen stoichiometry  pretty  close  to 6. presumably because it  increases the wettability of the  liquid phase present at the beginning of the  sintering process. It was  found that  zirconia  improves  the  densification of the  ceramic mixtures, 1. introduction In  a previous paper  [l], we have shown that the  partial replacement of Y 2 0 3  with ZrO, in the  basic  ingredients required for the  preparation of YBa2Cu307-,, via the reactive sintering technique, leads to dense mixtures of ceramics, which are mechanically much stronger and chemically more  resistant to  atmospheric  orrosion than YBa,Cu,O,-, itself, but are still superconducting at liquid nitrogen  temperature for substitution percent- ages as high as 40%. Similar resistivity-temperature curves were reported by others [2, 41 but in all cases, including ours, it was not established whether yttrium was really substituted with zirconium in the  crystallographic cell of YBa,Cu,O,-, or merely used to synthesise secondary phases. Because this  question is of relevance for both under- standing the mechanism of superconductivity in this class of materials and for deposition of YBa,Cu,O,-, thin films on zirconia  substrates, we have extended  the range of substitution percentages explored and carried out systematic  micrographies, x-ray images, electron microprobe local analysis and x-ray powder diffraction on various  ceramic  mixtures  over  the  complete  range of substitution percentages of Y 2 0 3  with ZrO, . 2. Experimental procedure All the specimens were prepared from appropriate mix- tures of Y20,,   CuO and BaO, (not BaCO, as in our previous paper) powders, which were first ground in an agate  grinder for 16 h  then  compacted into pellets that were 3 mm thick and 25 mm in diameter by a  force of 5 x lo4 N applied through a hydraulic press. These pellets were sintered for 72 h at 980°C under flowing oxygen and subsequently  annealed for 48 h at  400°C in the  same  run. The specimens so obtained were cut  into rods of about 20 mm in length  and cross section 1.5 x 1.5 mm2.  Their electrical resistance was measured in the temperature range 4-300 K by the four-probe technique, using high local pressure mechanical contacts. The tested rods were then diamond polished on a  dry felt and  examined by optical as well as scanning  electron microscopy to observe  their porosity and  the size and  shape of their grains. To identify the phases present, the distributions of Y, Zr, Ba and  Cu were systematically recorded  through the x-ray intensities emitted from 100 pm x 100 pm spots scanned by the beam of an electron microprobe. This first check allows for repartition of the various zones of the observed spots  into a few classes of relative abundance. Chemical analysis of several representative zones of each class of abundance was then attempted by stop- ping the beam in the  centre of them. Although electron microprobe analysis is a sensitive and reliable tech- nique, the results were rather scattered, because most often secondary phases were finely dispersed into each major phase, so that it was only possible to get an indi- cation of the composition of the  major phases. Never- theless, these indications,  plus  the knowledge of phase equilibrium diagrams and of the proportions of the starting ingredients, allowed us to formulate some 0953-2048/89/030173 + 04 $02.50 @ 1989 IOP Publishing  Ltd 173 P Regnier et a/  hypotheses on the composition of the present phases. To corroborate these hypotheses, each specimen was ground  into a fine powder, of which an x-ray diagram was taken and compared with standards. In addition, some thin foils were prepared via dimpling plus ionic thinning for observations and analysis by transmission electron microscopy. Finally, it was checked that the composition of the  various phases present, so deduced, was consistent with the observed volume fractions. 3. Results and discussion As shown in figure 1, for replacement percentages of Y,O, with ZrO, less than 5%, the curve of resistivity against  temperature is little affected, with the same  criti- cal temperature  and  almost  the  same resistivity value at 100 K of -3 x R cm, a value which is one of the lowest ever published for sintered YBa,Cu,O, . For medium replacement percentages of l(r50Y0 (see figure 2) the ceramics are still superconductors with an onset  temperature close to 90 K, but  the higher the 90 100 Temperature ( K 1  Flgure 1. Plots of resistivity against temperature for ceramics synthesised with the  replacement of less than 5% of Y,03 with ZrO,, J = 1 A cm-'. The amount of  ZrO, (YO) is given for each curve. 0 80 90 100 Temperature ( K 1  Flgure 2. Plots of resistivity against temperature for ceramics  synthesised  with  replacement percentages of Y,03 with ZrO, between 10 and 50%. The amount of ZrO, (%) is given for each curve. J = 1 A cm-'. 100 200 300 Temperature ( K )  Figure 3. Plot of resistivity against temperature  for  a ceramic synthesised with  the  replacement Of 70% Of y,o3 with ZrO,. J = 1 A cm-'. replacement percentage  the lower the offset temperature and the higher the resistivity above  90 K. This last set of results is in fair agreement with that of our first paper [l] obtained with BaCO, in place of BaO, as a  starting ingredient. The only major difference is that supercon- ductivity above 77 K was observed here up  to replace- ments of 50%, whereas previously it was restricted to 40%. It  should  also be noted that the onset temperature of the 50% ZrO, specimen was surprisingly high (nearly 100 K). For 60%  ZrO, (figure 3) the specimens were no longer conducting  but  semiconducting at high tem- perature, nevertheless their resistivity was still observed to  drop from about 3 R cm at 80 K to zero at 50 K. With 70% ZrO, and also with higher replacement percentages, the samples were observed to be more  and more resistive and semiconducting down to 4 K. For example, the resistivity of the specimens substituted at 70 and 90% were, respectively, 30 and 95 R cm at 300 K and 190 and 6 x lo5 R cm at 50 K. Metallographic  examination of all the specimens tested has indicated that the samples were nearly as porous  as YB~,CU,O,_~  itself for replacement  percent- ages lower than 5%,  but  turned  out to be quasi- compact for replacement percentages of 10 and higher (cf figure 4). In agreement with observations by light microscopy, the  measured  density of the  substituted samples was found to be high: 6.32 0.03 for the 30% ZrO, specimen, for example, but  his figure is  less significant than  that  obtained by the  metallographic examination since the specimen is multiphased. The grain size was also much affected by the presence of zirconia. Starting from the  elongated  slabs of about 10 x 100 pm, usually observed in plane  sections  of YBa,Cu,O, sintered at temperatures higher than 950°C,  the  grains become smaller and smaller with higher and higher zirconia content, with the exception of some large and very bright  ones seen by optical microscopy, which were identified as CuO by electron microprobe analysis (EMA). Because most of the grains were smaller than the  lateral  resolution of our EMA ( W  1 pm) and of different composition, our local analyses were  v y scattered; nevertheless we 174 Figure 4. Metallographic examination of a ceramic synthesised  with  the replacement of 10% of Y,03 with ZrO, . occasionally found  some results close to those of BaZrO, or BaCuO,.  This allowed us to formulate the hypothesis that whatever the  percentage of zirconia, the following chemical reaction held : nZrO, + ( T ) Y z O ,  + 2Ba0, + 3CuO I - n  + (1 - n)YBa,Cu,O,-, + nBaZrO, + nBaCuO, + 2nCuO + i ( 3  - n + 26 - 2n6)O,f w i thOGnG1.  This  hypothesis was fully confirmed by the  comparison of x-ray powder diffraction diagrams  recorded  on each specimen with those of pure YBazCu307, BaZrO,, BaCuO, and CuO which were specially prepared for that purpose. The identification of BaZrO, and CuO was  very accurate  and,  doubtless,  that of BaCuO,  a bit less, because of the very high natural  background given by this compound. As for YBaCuO, for zirconia per- centages <40%, the orthorhombic  structure corres- ponding to 0 < 6 < 0.2 [S] was observed. With higher zirconia content  he  structure became less and less orthorhombic with increasing 6 values, and finally turned out to be tetragonal for zirconia percentages >70%, with 6 N 1 according to [4]. Unfortunately, the variation of the crystallographic cell of YBaCuO with zirconia content  could  alternative- ly be attributed  to  the  incorporation of zirconium atoms in the  lattice of YBaCuO. Hence to test this  chal- lenging hypothesis  some  thin foils suitable for transmis- sion electron microscopy were prepared, observed and analysed for both the 30 and  the 80%  zirconia samples. Heavily twinned and elongated grains, typical of the orthorhombic YBa,Cu,O,-,, were easily recognised in the 30% specimen. Microdiffraction, lattice images and local analysis of these grains were also consistent with this assertion and  no  trace of zirconium was detected. Furthermore,  it is worth  noting  that most of the YBaCuO grains observed were connected, so that per- colation of superconducting  paths was evident. In the 80% specimen, no twin was observed, but colonies of a few elongated grains nicely packed along their longest apparent dimension were present in mod- ZrO, in place of Y,03 in YBa,Cu,O, synthesis erate abundance. Here again, microdiffraction, lattice images and local analysis were consistent with YBaCuO,  but because of the  absence of twins, it is clear that, in agreement with the x-ray diffraction powder diagrams, these grains are of the tetragonal YBaCuO phase and not of the orthorhombic YBaCuO phase. Most of the observed colonies in this foil were sur- rounded with BaZrO,,  BaCuO, or CuO grains, which led to the idea that  the  tetragonal  phase  does  not perco- late even in the bulk three-dimensional sample. The reason for the  tetragonal  phase formed during  sintering being retained  after  annealing for 48 h at 400°C  under flowing oxygen is far from easy to understand. In fact, all the tetragonal-phase grains of the sintered pellets which contribute to the x-ray diffraction patterns have a free surface and consequently  should be in equilibrium with oxygen despite the presence of neighbouring sec- ondary phases. Nevertheless, these secondary phases, which are  more  abundant  than  the YBaCuO, constitute a rigid skeleton which probably prevents the YBaCuO grains embedded in them from shrinking due  to extra absorption of oxygen. It must be concluded that in our samples yttrium was not  substituted with zirconium in the crystallo- graphic cell of YBaCuO. More generally no  atom was substituted with another in the  various phases present; only pure YBaCuO, BaZrO,, BaCuO, and  CuO were observed. Consequently,  there is no reason why partial replacement of some Y,03  with ZrO, should increase the critical temperature for superconductivity, as claimed by [3]. Nevertheless this replacement is bene- ficial for the  material since it  considerably  enhances its compactness and mechanical properties. The exact role of zirconia in the densification process is still under investigation in our laboratory. All we can say here is that, whatever the  zirconia percentage, the densification occurs mainly between 880°C  and 930 "C during heating and so is practically terminated before the steady-state  temperature  (980°C) is reached. This is because, whatever the zirconia percentage, the content of BaO, and CuO is constant. Hence there is always almost the same amount of liquid present to enhance the sintering process but the higher the zirconia the lower the amount of YBaCuO to be sintered, so the increased densification with increased ZrO,  content may be simply due  to  the relative increase in the amount of liquid available for sintering. Unfortunately the  composition of this liquid, the  nature  and  the amount of secondary phases and the cold pressing are more or less dependent  upon  the  exact zirconia content, which seriously complicates  the  situation.  In  particular, it is well known that zirconium, hafnium and titanium increase the wettability of the solders used for brazing oxide ceramics [S]. It is very likely that the  addition of zirconia improves  the  sintering because it increases the wettability of the liquid phase present at the beginning of the process. Hence increased wettability could be responsible for sintering  improvement,  but this analogy of a  metal melt with a solid oxide may not be applicable to  an oxide melt. 175 P Regnier et a/  It is worth  mentioning  that  the fact that we observed the tetragonal phase for the highest zirconia percentages could explain why it is not so easy to get good  superconducting  thin films of YBa,Cu,O, depos- ited on zirconia. The  interaction of ZrO, substrates with YBaCuO is an important topic from the stand- point of the  deposition of thin films and there is a quite extensive literature on this point ([7-101 for example). In  addition  to  the secondary  phases  reported here (BaZrO,,  BaCuO,  and  CuO), most of the relevant papers claim the presence of Y,BaCuO,,  the well known green phase, which is easily recognised by optical microscopy and x-ray diffraction.  This difference with our results is probably due to the large excess of ZrO, present  during  thin film deposition which increases the  amount of BaZrO, synthesised, leaves less BaO,  available for the  formation of YBa,Cu,O,-, and consequently  favours  the  formation of some  Y,BaCuO, (a  barium  poorer phase). 4. Conclusion The  partial  substitution of Y203  with ZrO, in  the syn- thesis of YBaCuO does not lead to the replacement of yttrium with zirconium in the crystallographic cell of YBaCuO. The superconductor-conductor transition observed above  liquid  nitrogen  temperature for substitution percentages lower than 50% is due  to the usual YBa,Cu,O,-, orthorhombic phase with a 6 value less than 0.2. Nevertheless, for reasons which are  not yet  fully understood  this  replacement  favours  the  densification of the  ceramics and gives materials which are mechanically much  more  resistant  than YBa,Cu,O, itself. For zirconia  percentages higher than 70% the YBaCuO  present is no longer orthorhombic  but tetragonal, which explains why the  ceramics are  no longer superconducting and sheds some light on the problems faced in  the  deposition of good  superconduct- ing  YBaCuO  thin films on zirconia. Acknowledgments We would like to acknowledge R P Gupta  for fruitful discussions and for his stimulating  interest in the research, L Boulanger for electron microscopy and A Zemskoff and F Maurice  for  electron  microprobe analysis. References [l] Regnier P,  Gupta R  P  and Truchot P 1988 J. Phys. C :  Solid State Phys. 21 L463-8 [2] Poddar A, Mandal  P,  Choudhury  P,  Das A N  and Ghosh B 1988 J .  Phys. C :  Solid State Phys. 21 3323-31 [3] Batrak A G, Ben’Ko V Yu, Nechiporenko I N  and Fedorchenko  L I 1988 Fiz. Nizk. Temp. 4 435-6 (in Russian) [4] Suzuki T, Yamazaki T, Koukitsu A and Seki H 1989 J .  Mater. Sci. Lett. 8 19-20 [S] Wong Ng, Roth R S, Swartzendruber  L J, Bennet L H, Chiang  C K, Beech F and  Hubbard C  R 1987 Adv. Ceram.  Mater.  2 3B [6] Nicholas M  G 1986 Br.  Ceram.  Trans. J. 85 144 [7] Cheung  C T and Ruekenstein E 1988 Mater.  Lett. 78 [S] Perrin A, Li Z Z, Peiia 0, Padiou J and Sergent M 1988 [S] Naito M, Hammond R H,  Oh B, Hahn M R, Hsu J W 172-7 Rev. Phys. Appl.  23 257-64 P, Rosenthal P, Marshall A F, Beasley M R, Geballe T H and Kapitulnik A 1987 J. Mater.  Res.  2 713-27 1988 Appl. Phys. Lett.  52 1746-8 [lo] Stamper A, Greve  D W, Wong D and Schlesinger T E 176 

English

Parallel, core and valence level photoemission studies of nominal (Bi0.9,Pb0.1)2Sr2Ca2Cu3010+x

https://pure.uva.nl/ws/files/1972169/29780_133730y.pdf

Parallel, core and valence level photoemission studies of nominal (Bi0.9,Pb0.1)2Sr2Ca2Cu3010+x

Abstract

Similar works

Full text

Available Versions

International Migration, Integration and Social Cohesion online publications