2014GdBCO相稳定低氧压

Stability phase diagram of GdBa 2Cu 3O 7Àd in low oxygen
pressures
Jung-Woo Lee a ,Soon-Mi Choi a ,Joo-Hyun Song a ,Jae-Hun Lee b ,Seung-Hyun Moon b ,Sang-Im Yoo a ,⇑
a Department of Materials Science and Engineering and Research Institute of Advanced Materials (RIAM),Seoul National University,Seoul,Republic of Korea b
Superconductor,Nano &Advanced Materials (SuNAM)Co.,Ltd.,Anseong,Republic of Korea
a r t i c l e i n f o Article history:
Received 22January 2014
Received in revised form 24February 2014Accepted 25February 2014Available online 6March 2014Keywords:GdBCO Gd 2O 3
GdBa 6Cu 3O y Phase stability
Peritectic decomposition Monotectic reaction
a b s t r a c t
We report the stability phase diagram of GdBa 2Cu 3O 7Àd (GdBCO)in the low oxygen pressure (P O 2)regime of 1–100mTorr.For this study,amorphous precursor films were deposited on the LaAlO 3(LAO)(001)substrates at 200°C by pulsed laser deposition (PLD),annealed at various high temperatures in low P O 2,and then quenched using a reel-to-reel tube furnace.By analyzing the phases and microstructures of the as-quenched films,the stability phase diagram of GdBCO could be accurately constructed.In the P O 2regime of 20–100mTorr,the GdBCO stability line on the log P O 2versus 1/T (K À1)diagram can be expressed by the equation of log P O 2(Torr)=10.85–13880/T (K),and unlike the well-known peritectic reaction of GdBCO M Gd 2BaCuO 5(Gd211)+Liquid (L)in high P O 2,a pseudobinary peritectic reaction of GdBCO M Gd 2O 3+Liquid (L 1)occurs at the stability line of GdBCO.In the P O 2regime of 1–10mTorr,the GdBCO stability line can be expressed by the equation of log P O 2(Torr)=9.263–12150/T (K),and a ternary peritectic reaction of GdBCO M Gd 2O 3+GdBa 6Cu 3O y (Gd163)+L 2occurs at the stability line of GdBCO.In addition,a monotectic reaction of L 1M L 2+Gd163occurs at the phase boundary between Gd 2O 3+L 1and Gd 2O 3+Gd163+L 2.
Ó2014Elsevier B.V.All rights reserved.
1.Introduction
Long-length REBa 2Cu 3O 7Àd (RE:Y and rare earth elements,REBCO)coated conductors (CCs)with high critical currents (I c )have been fabricated for electric power applications [1].Up to date,various technologies,including PLD [2,3],metal–organic chemical vapour deposition [4],metal–organic deposition [5,6],and reactive co-evaporation [7–9],have been developed for this purpose.Since REBCO CCs are normally produced in a reduced oxygen atmo-sphere,it is essential to know the stability limits of the REBCO phases in the low P O 2regime.Therefore,the stability lines of REBCO phases on the P O 2versus 1/T (K À1)diagram in low P O 2(<$2Torr)have been investigated by many groups [10–20]and used as a thermodynamic guideline for optimizing the fabrication processes of REBCO CCs [21–28].
Recently,we have reported a successful fabrication of long-length high-I c GdBa 2Cu 3O 7Àd (GdBCO)CCs by a reactive co-evapo-ration deposition and reaction (RCE-DR)process [29].Accurate information on the phase stability of GdBCO in low P O 2is in de-mand not only for the optimization of the RCE-DR processing parameters,but also for a fundamental understanding of the growth mechanism of the GdBCO film.However,this information has never been reported yet,and thus seeking this knowledge is a motivation of this study.Although Iguchi et al.[28]reported the stability line of the GdBCO phase on the P O 2versus 1/T (K À1)diagram for the GdBCO films prepared by the trifluoroacetate (TFA)-MOD process,it was simply assumed to be parallel to that of the DyBCO phase reported by Ishizuka et al.[16].Moreover,they did not report the equilibrium phases above the GdBCO stability line (i.e.,the decomposition products of the GdBCO phase).On the other hand,if the equilibrium phases above the stability line of GdBCO were similar to those of YBCO in low P O 2,Gd211parti-cles might be observable within the GdBCO matrix,because as YBCO was reported to be decomposed into Y 2BaCuO 5(Y211)and other phases [10–15].However,contrary to this expectation,Gd 2O 3particles were observed within the GdBCO matrix of GdBCO CCs produced by the RCE-DR process [29],implying that the Gd 2O 3phase is chemically compatible with the GdBCO phase.To under-stand the formation of Gd 2O 3particles further is another aim of the study.
In order to construct the stability phase diagram of GdBCO on the P O 2versus 1/T (K À1)diagram in the low P O 2regime ranging from 1to 100mTorr,we carefully determined the stability line of GdBCO and also identified its decomposition products through a quenching experiment.In previous reports [10–20]on the phase stability of REBCO in the low P O 2regime,REBCO powders were
/10.1016/j.jallcom.2014.02.170
0925-8388/Ó2014Elsevier B.V.All rights reserved.
⇑Corresponding author.Address:Bldg.131(Research Institute of Advanced Materials),Rm.407,1Gwanak-ro,Gwanak-gu,Seoul 151-744,Republic of Korea.Tel.:+8228805720;fax:+8228876388.
E-mail address:siyoo@snu.ac.kr (S.-I.Yoo).
commonly used as samples for the experimental determination of the REBCO stability phase diagrams.In the low P O2regime,how-ever,Lindemer et al.[13]pointed out that an oxygen gas release from the REBCO powder made it difficult to accurately determine the stability limits of REBCO.Similarly,we were able tofigure out in our preliminary experiments that the GdBCO powder was decomposed into Gd211and other phases in the low P O2regime. As previously mentioned,since these decomposition products were contradictory to our observation of Gd2O3particles within the GdBCO matrix[29],we used GdBCO amorphousfilms as sam-ples in this study to avoid the oxygen gas release problem due to the decomposition of the GdBCO crystalline powder orfilm.
2.Experimental
The amorphous and crystalline GdBCOfilms were deposited on the LAO(001) substrates by PLD using both Nd:YAG(k=355nm)and KrF excimer laser (k=248nm).The GdBCO target of1inch diameter was prepared by the conven-tional solid–solid reaction using Gd2O3(99.9%),BaCO3(99.99%)and CuO(99.9%) as the starting materials.Details on the target preparation are reported elsewhere [30].For the fabrication of amorphous GdBCOfilms with Nd:YAG laser,the deposi-tion temperatures(T s),working oxygen pressure,energy density,laser frequency, and target-to-substrate distance werefixed at200°C,400mTorr,2J/cm2,5Hz, and6cm,respectively.Detailed PLD conditions using KrF excimer laser for crystal-line GdBCOfilms are reported in our previous work[31].
To determine the phase stability of GdBCO experimentally,the GdBCOfilms were annealed at various high temperatures in the P O2regime ranging from1to 100mTorr,and then quenched using the reel-to-reel tube furnace as shown in Fig.1.By the reel-to-reel motion,the GdBCOfilms on the LAO substrate(silver-pasted on metal tapes)were transferred to a hot zone of the tube furnace for the annealing under a given P O2,and subsequently moved out of the furnace for a rapid quenching.In the case of amorphousfilms,those werefirst moved to zone1set to a target annealing temperature at a very low oxygen pressure($1Â10À5Torr)since pre-requisite experiments on amorphousfilms revealed that they were not crystal-lized even at860°C for that P O2.Subsequently,thefilms were moved to zone2of which P O2was controlled to have a target level byflowing pure oxygen gas into the quartz tube,annealed for5or30min,andfinally quenched in the same P O2or very low P O2($1Â10À5Torr)by quickly removing samples out of the furnace through zone1or2.Due to the hygroscopic nature of the as-quenched samples,they were carbon-coated to prevent moisture contamination in air.
X-ray diffraction(XRD)(Bruker,D8-Advance)and transmission electron micro-scope(TEM)(JEOL,JEM-3000F)were performed for phase analysis after heat treat-ments.TEM samples were prepared by focused ion beam(FIB)(SII Nanotechnology, SMI3050SE).Microstructures were observed using afield emission scanning elec-tron microscope(FE-SEM)(JEOL,JSM-6330F).Section3.4,all experimental results are combined to construct the stability phase diagram of GdBCO and also compared with those of previous works[10–15,28].
3.1.Pseudobinary peritectic decomposition of GdBCO into Gd2O3+L1 in the PO2regime of20–100mTorr
Fig.2shows the XRD patterns of samples.As shown in Fig.2(a), one can see that as-deposited precursorfilms at200°C are amor-phous since only two peaks of the LAO substrate are observed. The XRD patterns of as-quenchedfilms after annealing at828–900°C in the P O2range of20–100mTorr for5min(100mTorr) and30min(20,30mTorr)are represented in Fig.2(b)–(j).Com-pared with GdBCO(00l)peaks observed at828–832°C,very small GdBCO(00l)peaks are observed at836°C at the P O2of20mTorr as shown in Fig.2(b)–(d)(see that Fig.2is the semi-logarithm plot). Since these small GdBCO peaks are most probably due to the GdBCO phase formed as a by-product during rapid quenching, the GdBCO stability boundary is obviously located at834±2°C for P O2of20mTorr.In the same way,the GdBCO stability bound-ary point at P O2of30mTorr was determined to have the temper-ature of846±2°C as shown in Fig.2(e)–(g).Unlike the equilibrium phases of Gd2O3plus liquid above the GdBCO stability boundary in the P O2of20–30mTorr,unknown XRD peaks in addition to Gd2O3, BaCu2O2,and Cu2O(not represented here)were also observed after annealing for30min at900°C and P O2of100mTorr,which might be due to a severe reaction between the liquid phase and the LAO substrate.When we used a short annealing time of5min to mini-mize the reaction,the as-quenched sample was found to be com-posed of Gd2O3and liquid phase,which is in good agreement with the equilibrium phases above the GdBCO stability boundary for P O2of20–30mTorr.This also revealed that the GdBCO stability boundary point for P O2of100mTorr are located at898±2°C (Fig.2(h)–(j)).Here,the undercooling required for the growth of the GdBCO phase was assumed to be negligibly small because the LAO substrate could play a role of a heterogeneous nucleation center as evidenced by only GdBCO(00l)peaks in all samples of Fig.2(b)–(j).It is remarkable that Gd2O3,BaCu2O2,and Cu2O phases
schematic of a reel-to-reel tube furnace used for this study.The oxygen pressures of zone1,2connected through a slit were separately controlled and pumping with turbomolecular pumps(TMPs)in each zone.
J.-W.Lee et al./Journal of Alloys and Compounds602(2014)78–8679
amorphous precursorfilms always experience lower P O2at a given annealing temperature before they are annealed at an aimed P O To identify the decomposition products of the GdBCO phase, performed microstructure analyses of as-quenched samples.FE-SEM images of as-deposited amorphousfilm are also shown Fig.3(a)and(b)for a comparison.The amorphous precursorfilm with the thickness of1.2l m is found to have a rough surface with some outgrowth particles.The FE-SEM surface image and cross-sectional TEM brightfield image of an as-quenchedfilm after annealing at860°C for P O2of30mTorr for5min are shown Fig.3(c)and(d),respectively.From Fig.3(c),one can observe smooth surface with some microcracks,suggesting that a liquid phase was involved.As shown in Fig.3(d),Gd2O3particles in the as-quenched sample were normally found at the interface between the sample and LAO substrate.High resolution images of the interface between the LAO substrate andfilm in Fig.3(e)and its fast Fourier transformation(FFT)patterns in Fig.3(f)clearly reveal that the Gd2O3particles epitaxially grow on the LAO substrate. Consequently,from Figs.2and3,it is obvious that GdBCO is peritectically decomposed into Gd2O3+L1in the low P O2regime of20–100mTorr,which is different from the well-known peritec-tic decomposition into Gd211+L in high P O2[33].
In the case of using amorphousfilms,the GdBCO phase coex-isted with Gd2O3,BaCu2O2,and Cu2O below its stability limit as shown in Fig.2.Therefore,strictly speaking,it is still uncertain that this line is the real stability limit of GdBCO.To further confirm the stability limit and decomposition products of GdBCO,we per-formed additional quenching experiments for the GdBCOfilms grown on the LAO(001)substrate by PLD using KrF excimer laser. As-grown GdBCOfilms in Fig.4(a)were annealed at various high temperatures at P O2of30mTorr for30min and then quenched in the same P O2.The XRD data of as-quenched samples are repre-
2.XRD patterns of as-deposited amorphousfilm(a)and as-quenchedfilms
after annealing at(b)828°C,(c)832°C,(d)836°C at P O2of20mTorr for30min,
840°C,(f)844°C,(g)848°C at P O2of30mTorr for30min,and at(h)892°C,
°C,(j)900°C at P O2of100mTorr for5min.
FE-SEM images of the as-deposited amorphousfilm:(a)plan-view.(b)cross-sectional view.(c)FE-SEM image and(d)cross sectional brightfield TEM images films after annealing at860°C at P O2of30mTorr for5min.(e)High resolution TEM images of the interface between Gd2O3and LAO substrate and transformation patterns.
and Compounds602(2014)78–86
GdBCO grains with relatively large x at relatively low temperature like756°C.Therefore,this onset temperature of a-axis oriented GdBCO grains cannot denote the stability limit of GdBCO with x$0.In Fig.4(c)and(d),one can see that XRD patterns of thefilms annealed at756–844°C are not considerably altered.On the other hand,at848°C,just above the stability limit of GdBCO(846±2°C) determined by using amorphous precursorfilms,a very small un-known peak is detectable as shown in Fig.2(e).Moreover,the GdBCOfilm is not completely decomposed even at924°C in Fig.4(g)but fully decomposed at928°C in Fig.4(h),suggesting that it can exist up to the temperature of926±2°C much higher than its stability limit of846±2°C determined from amorphous precursorfilms.In addition,unknown peaks are also observable film,indicating that a severe chemical reaction occurred
thefilm and substrate.We do not regard the full decompo-temperature of926±2°C as the real stability limit of GdBCO 2
of30mTorr since the GdBCO phase up to this temperature not exist as a pure single phase,but coexists with phases including Gd2O3,BaCu2O2,and unknown phase.From perspective of increasing decomposition temperature of other phases coexist,similar overheating phenomena
reported for YBCO in air by the research group of Shanghai Jiao Tong University[34].They found that YBCOfilms on LAO, SrTiO3,and MgO single crystalline substrates were not fully decomposed even after annealing at a temperature higher than the stability limit of YBCO in air($1005°C),and also suggested that the low surface energy of coherent interfaces betweenfilms and substrates might result in this overheating phenomena.The ‘partial’decomposition of YBCO,however,cannot be simply ex-plained by the overheating of YBCOfilms due to low surface energy of the interfaces.According to their interpretation[34],the ele-ment doping from the substrate is not a main factor to affect the overheating of YBCOfilm.On the hand,if the La3+ions of the LAO substrate substituted for the Gd3+site of GdBCO through the liquid phase,the decomposition temperature of La-doped GdBCO would increase in comparison with that of pure GdBCO. Further study must be performed to reveal the origin of the‘over-heating’phenomenon.From these results,we can see that the GdBCOfilms epitaxially grown on the LAO substrate are inappro-priate for accurate determination of the GdBCO stability line on the P O2versus1/T(KÀ1)diagram in low P O2and also for the iden-tification of their decomposition products.
As briefly mentioned in Introduction,although the XRD data are not represented here,the GdBCO powder in a Pt crucible was found to be decomposed into Gd211and the liquid phase in the low P O2of 30mTorr regardless of its amount within a tube furnace.For in-stance,unlike the decomposition of the GdBCO crystallinefilms pre-viously described,the GdBCO powder was partially decomposed into Gd211and the liquid phase at840°C in the P O2of30mTorr for1h,which is below the phase stability point(846±2°C)of GdBCO determined using GdBCO amorphousfilms in this study. This result might also be attributed to the decomposition of Gd1+x Ba2Àx Cu3O7Àd-type solid solution with higher x into Gd1+x Ba2Àx Cu3 O7Àd-type solid solution with lower x plus Gd211plus the liquid phase since it is unavoidable to form the Gd1+x Ba2Àx Cu3O7Àd-type solid solution even though we try to synthesize the stoichiometric GdBa2Cu3O7Àd compound by the solid state reaction process.While
XRD patterns of(a)as-deposited GdBCOfilm and as-quenchedfilms
annealing at(b)752°C,(c)756°C,(d)844°C,(e)848°C,(f)860°C,(g)924
°C at P O2of30mTorr for30min.The peaks of a-axis oriented GdBCO
as¡.
as-quenchedfilms after annealing at(a)712°C,(b)716°C,(c)720°C,and(d)730°C at P O2of1mTorr for30min.
speed of0.5°/min.
and Compounds602(2014)78–8681
understood by the oxygen gas release from the GdBCO phase during its decomposition.In comparison with the ambient low P O 2,a rela-tively higher local P O 2can be induced at the surfaces of the GdBCO powder surrounded by the liquid phase due to the oxygen gas re-leased from the decomposition of GdBCO.If the local P O 2is not effectively removed through the liquid phase,it can be large enough to decompose the GdBCO phase into Gd211plus the liquid phase rather than Gd 2O 3plus liquid phase.Consequently,if we use the GdBCO powder or GdBCO crystalline films as the specimens,it is very difficult to accurately determine the stability line of the GdBCO phase in the low P O 2region of 1–100mTorr because of the existence of the Gd 1+x Ba 2Àx Cu 3O 7Àd -type solid solution,and also it is impossi-ble to obtain equilibrium decomposition products of the GdBCO phase due to the oxygen gas release problem.
3.2.Ternary peritectic decomposition of GdBCO into Gd 2O 3+Gd163+L 2in the PO 2regime of 1–10mTorr
In the P O 2regime of 1–10mTorr,our preliminary study re-vealed that the GdBCO decomposition reaction was very sluggish,suggesting that another solid phase in addition to Gd 2O 3might be involved in the GdBCO decomposition reaction.Since the recombination kinetics into GdBCO from two different solid phases plus the liquid phase must be very slow,the GdBCO phase as a by-product during the rapid quenching was undetectable in this P O 2regime.Therefore,the GdBCO stability boundary could be deter-mined by analyzing the existence of the GdBCO phase after the ra-pid quenching.
Fig.5shows XRD patterns of as-quenched films after annealing at 712–730°C in the P O 2of 1mTorr for 30min.As shown in Fig.5A,one can find that intensities of Gd 2O 3(00l )reflections increase with increasing the annealing temperature.However,since the GdBCO peaks are hardly observable in this figure due to a slow kinetics of GdBCO formation,the XRD analysis with the slow scan speed of 0.5°/min was performed for the same sample as shown in Fig.5B.It can be seen that GdBCO (005)reflection dis-appears in samples annealed at the temperatures higher than 720°C,indicating that the GdBCO stability limit is located at 718±2°C for P O 2of 1mTorr.Likewise,the GdBCO stability limit was deduced to be 758±2and 806±2°C at P O 2of 3and 10mTorr,respectively.
Referring to the YBCO stability phase diagram [11–15],a Ba-rich ternary compound should exist as another solid phase above the GdBCO stability line.However,its existence is ambiguous on the XRD patterns in Fig.5(c)and (d).According to the previous report [35–37]on Ba-rich (BaO >35%)ternary compounds in the Gd 2O 3–BaO–CuO z ternary system,there exist three different phases of GdBa 3Cu 2O y (Gd132)[35],GdBa 4Cu 3O y (Gd143)[36],and GdBa 6Cu 3O y (Gd163)[37].Among these compounds,only Gd163is known to be stable at 810°C in the low P O 2of 100Pa ($750mTorr)under CO 2-free conditions [38,39].Thus if the Ba-rich ternary compound exists in the P O 2regime of 1–10mTorr,it must be the Gd163phase.
In order to further confirm the existence of the Ba-rich compound above the GdBCO stability line in the P O 2regime of 1–10mTorr,as-quenched film after annealing at 820°C with P O 2of 3mTorr for 30min was analyzed by TEM.The cross-sectional TEM micrograph of as-quenched film is represented in Fig.6(a).From the HR-TEM image of the interface between the LAO substrate and film in Fig.6(b)and its FFT patterns in Fig.6(c),it is obvious that Gd 2O 3particles are epitaxially grown on the LAO substrate.In addition,several nanometer-sized particles are ob-served as shown in Fig.6(d).From their selected area FFT patterns represented in Fig.6(e),we could identify these particles as
the
Cross-sectional bright field TEM image of as-quenched films after annealing at 820°C at P O 2of 3mTorr for 30min.(b)HR-TEM image of the interface and LAO substrate,and (c)its FFT patterns.(d)HR-TEM image of the selected area in (a),and (e)FFT patterns of selected area in (d).
Gd163compound,which is in accordance with our expectation as previously mentioned.Consequently,we believe that GdBCO is decomposed into Gd2O3+Gd163+L2in the P O2regime of 1–10mTorr above its stability line.
3.3.Monotectic reaction of L1M L2+Gd163in the PO2regime of3–
mTorr
In order to determine the phase boundary between the phase region of Gd2O3+L1and Gd2O3+Gd163+L2as described Sections3.1and3.2,respectively,amorphous precursorfilms were annealed at840–860°C in the P O2regime of1–10mTorr.For the 2
of1mTorr,the montectic reaction boundary is ambiguous,be-cause after the annealing temperature of860°C thefilm is not crystallized as shown in Fig.7(a),despite the obvious liquid phase Fig.8(a)).As shown in Fig.7(b)–(e),Gd2O3,Gd163and Cu2O detectable after annealing under the conditions of850°C,3mTorr
2and840°C,10mTorr P O2,while Gd163peaks disappear at860
and850°C at these pressures respectively,indicating that a mono-
tectic reaction boundary of L1?L2+Gd163is located between850
and860°C in the P O2of3mTorr and840–850°C in the P O2of
10mTorr.From the surface morphologies of the as-quenchedfilms
shown in Fig.8(b)–(e),it was suggested that a liquid phase might
be involved in these annealing conditions.
In addition,monoclinic Gd2O3is found to form at850–860°C in
the P O2of3mTorr as shown in Fig.7(b)and(c).A polymorphic
transition of Gd2O3was reported by Zinkevich[40],indicating that
GdBCO can exist as both cubic and monoclinic forms in air.The
cubic-to-monoclinic phase transition of a Gd2O3film was reported
by Molle et al.[41].They found that Gd2O3was transformed from
cubic to monoclinic structure on a Ge(001)substrate with increas-
ing thefilm thickness,suggesting that a strainfield could cause the
phase transition.In our experiments,while cubic Gd2O3was grown
on the LAO substrate in most of annealing conditions,monoclinic
Gd2O3was formed at the specific annealing conditions of the low
P O2of3mTorr and high temperatures(850–860°C).
3.4.Stability diagram of GdBCO
On the basis of results described in Section3.1–3,the stability
diagram of GdBCO could be constructed as shown in Fig.9.With
the least-squarefitting,the GdBCO stability line in the P O2regime
7.XRD patterns of as-quenchedfilms after annealing at(a)860°C at P O2
mTorr,and at(b)850°C and(c)860°C at P O2of3mTorr,and at(d)840°C and
°C at P O2of10mTorr for30min.
FE-SEM images of as-quenchedfilms after annealing at(a)860°C at P O2of1mTorr,and at(b)850°C and(c)860°C at P O2of3mTorr,and at(d)840°C and 10mTorr for30min.
and Compounds602(2014)78–8683
the amorphous state was unaltered after
C for P O2of1mTorr as previously de-region of1mTorr–760Torr,we represented both GdBCO and YBCO phases in Fig.10
figure,the stability lines of GdBCO(a)
P O2regime are from Refs.[42,13],
P O2regime(<$2Torr),we selected
(c)which was in consensus by several groups[11,12,14,15].The stability line
P O2regime of10–100Pa reported by displayed.
First of all,we would like to compare the GdBCO phase with previous one [28].They suggested the equation of the P O2regime of10–100Pa as the following: log PO2ðPaÞ¼13:1À13100=TðKÞ
for GdBCO in low oxygen pressures ranging from1to100mTorr.(d GdBCO,Gd2O3+L1,Gd2O3
YBCO and GdBCO in oxygen pressures ranging from0.001to760Torr.(Black line for GdBCO(a)by Iida 100mTorr).Blue line for GdBCO(d)by Iguchi et al.[28].Red line for YBCO(b)by Lindemer et al.[13](P O
2 decomposition products of YBCO(P O2<$2Torr)are referenced from to the work of MacManus-Driscoll figure legend,the reader is referred to the web version of this article).
In this equation,they have not experimentally determined the slope of GdBCO stability line in the log P O2versus1/T(KÀ1)dia-gram but adapted it from that of DyBCO reported by Ishizuka et al.[16].Although it deviates from our value of13880only by 6%,their GdBCO stability line is shifted to lower temperature region($70°C)in comparison with ours as shown in Fig.10.This discrepancy in the location of the GdBCO stability line is probably due to the existence offluorine in theirfilms since an oxyfluorine liquid phase formed at a lower temperature might lower the decomposition temperature of GdBCO[43].
Next,compared with the stability line of YBCO in the low P O2 regime(<$2Torr)reported by several groups[11,12,14,15],that of GdBCO is shifted downward in the log P O2versus1/T(KÀ1)dia-gram as shown in Fig.10.Lindemer’s line[13](not shown here)is located in higher temperature region compared with other reports [11,12,14,15].As one can see,the GdBCO stability line is located in a higher temperature region,while the slope of each line is similar. In the high P O2regime(>$2Torr),the decomposition temperature of GdBCO is about40°C higher than that of YBCO[13,42],while the decomposition temperature of GdBCO is about95°C higher than that of YBCO in the low P O2regime(<$0.4Torr).
It must also be of interest to compare the stable phases above the GdBCO stability line(i.e.,equilibrium phases as the decomposi-tion products of GdBCO)with those of YBCO.Although the decom-position reaction of YBCO in the low P O2regime below$1Torr ($133Pa)reported by several groups[10–15]is not in consensus, the Y211phase is commonly observed as the decomposition prod-uct of YBCO in this low P O2regime when YBCO bulk or powder sam-ples are employed.In contrast to these reports[10–15],there are several reports on Y2O3inclusions within the YBCOfilms[44–46] prepared in the low P O2regime below$1Torr,implying the possi-bility that the decomposition products of YBCOfilms may be differ-ent from those of YBCO bulk or powder.The oxygen gas release from YBCO powder and its effects on the phase stability in low P O2below94Pa($700mTorr)have been reported by Lindemer et al.[13].They found that YBCO decomposition temperature below 94Pa disagreed with other reports.According to their interpreta-tion,the oxygen gas released from YBCO powder significantly af-fects the ambient oxygen pressure,especially in low P O2. Therefore,the equilibrium phases decomposed from YBCO above its stability line is still ambiguous in the low P O2regime,and thus further study must be performed to clarify this point.
4.Conclusions
We could accurately construct the stability phase diagram of GdBCO in the low P O2regime of1–100mTorr by carefully analyzing as-quenchedfilms after annealing amorphous precursorfilms of GdBCO,where the quenching experiment was performed using the reel-to-reel furnace.It can be understood that since the oxygen gas should be absorbed by amorphousfilms during their formation of crystalline phases,the serious problem of oxygen gas release from the sample can be avoided by using the amorphousfilms,and thus the equilibrium phases above the GdBCO stability line can be stabi-lized more reliably.In the P O2regime of20–100mTorr,the GdBCO stability line could be experimentally determined as the following: log P O2(Torr)=10.85–13880/T(K).Above the stability line of GdBCO,it was decomposed into Gd2O3+L1,which was different from the well-known peritectic decomposition of GdBCO into Gd211+L in the high oxygen partial pressure like air.It could also be confirmed that in the P O2regime of1–10mTorr,the GdBCO stability line can be expressed by the equation of log P O2 (Torr)=9.263–12150/T(K),corresponding to the decomposition reaction of GdBCO into Gd2O3+Gd163+L2.In addition,the monotectic reaction of L1into L2+Gd163could be identified between Gd2O3+L1and Gd2O3+Gd163+L2in the P O2regime of 3–10mTorr.
Acknowledgments
This research was supported by a Grant from the Power Generation and Electricity Delivery Program of the Korea Institute of Energy Technology Evaluation and Planning(KETEP)funded by the Ministry of Trade,Industry and Energy,Republic of Korea. (Grant No.20131010501800).
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