- Email: [email protected]
Deformation and structure of YBCO ceramics and single crystals in the temperature range 300–1200 K
PflYSICA
Physica B 194-196 (1994) 2095-2096 North-Holland
Deformation and structure of YBCO ceramics and single crystals in the temperature range 300-1200 K. V.S.Bobrov, V.A.Goncharov, G.A.Emelchenco, L.S.Fomenko'. A.P.Ivanov. A.N.Izotov, Yu.A.Osipyan, N. S. Sidorov, E.V. Suvorov, V. Sh. Shekhtman, L.N.Zavelskaya, I.I.Zverkova. Institute of Solid State Physics, Russia Ac. Sci. 142432 Chernogolovka, Moscow distr.. Russia. *Institute of Low Temperature Physics, Ukraina Ac Sci 310164 Kharkov, Lenin av.,47,Ukraina. The data deformation and structure of YBCO ceramics and single crystals have been reported. It has been concluded that deformation of ceramic specimens is governed by processes of grain-boundary break-dmvn. In syngle crystal specimens and ceramic crystallites the deformation is followed by dislocation processes, alterration in the twin structure, and microcracking.
1.INTRODUCTION At low and moderate temperatures YBCO single crystals and ceramics are brittle (e.g.l-5). A noticeable plasticiB' of ceramics in observable only at elevated temperatures (5-8). Scarce data are available on macroscopic detbrmation in single c~,stals (5). Only some works (e.g. 2,5~8) report strnctural studies ofdetbrmed high-T superconductors. Further studies are required to elucidate the mechanisms of de~bnnation in these materials. In what follows we shall consider the results obtained in this field tbr YBCO ceramics and single crystals.
2.EXPERIMENTAL YBCO ceranctic sample (3x3x10 mm 3) almealed in oxygen had T =93-96 K. They were deformed by compression in air or in a helium atmosphere.The ortorombic single crystals of YBCO ( T ~ 70-90 K) in the foma platelets (~0.1 x2x4 ngll 3) with the developed (a,b) - basal plane were defornled in air by 3-point bending (fig. 3). The loading rate ~as 100 ~trrdmin in either case. Structural studies involved transmission electron microscopy (TEM), X-ray ditTractomet~ and polarization-optical technique.
ensued depended of the preparation conditions in the starting samples (5). These factors also affected the values of T at which the brittle-plastic transition took place. The TEM data have shoun that after the brittle fracture of the YBCO ceramics in the range T<_To some crTsrallites exhibit dislocation bands with pronounced edge and screw components (e.g fig 2a), which may be initiated bv stress concentrators in tile cristallite contact regions. These bands imersect file system of twins without tbnnmg macrosteps at their bomldaries. There are no singularities of the dislocation distribution in the band, and their generation bv secondary dislocation sources is not observed. This may suggest that the tx~in boundaries are not effective barriers tbr gliding dislocation. After the plastic flow of YBCO at elevated T there appear custallites in which twins are absent, and there arises a dislocation network (e.g.fig.2b). According to the TEM data (fig.2a) it should be noted that specific contrast after passing the dislocations m the acting reflex (020) is observed. It can be explained b.vdisplacement of the atoms with moving the dislocations (~ve mean these are the oxygen atoms). The basic phase composition remained unaltered at
A 3.RESULTS AND DISCUSSION. To the values of temperatures belmv the orthorhombtetragon phase transition region (To~1050-1100 K) the detbrmation of YBC'O ceramic samples occured in a quasielastic fashion and ternmmted in brittle fracture (e.g.1 fig. 1). Near T o the deformation pattern changed, a noticeable deviation from the quasi-elastic slope developed (e g.2 fig 1), and with T'_>To the plasticity of YBCO ceramic samples increased drastically. The defonuation curves in this case had a yield tooth (e.g. 3 fig. 1). The crack limit of the samples and the loading at which a noticeable plastic flow
Elsevier Science B.V. SSDI 0921-4526(93)1680-K
o.
180
0
180
0 t
180
360
[s]
FIGURE 1 Compression of YBCO ceramics in a helimn atmosphere: 1T~- 990, 2-1030, 3-1090 K. (P-loading, t-time).
2096
zo.E [oofl
L°'3Pm
large degree of plastic deformation of some samples, the 12-3 compounds decomposed practicall? completel> into C uo and BaCuO 2 (5). In the process of deformation the t~action of the superconducting phase diminished and T of the deformed samples decreased. These phase transtbnnations may, supposedly, be stimulated by the processes in the intergrain botmdary regions at c~smllite migration. In some xvorks high temperature plasticitT of YBCO ceramics is discussed from the vie~ poim of intergrain glide and difl'usional processes (5-8). In this respect it uould be interesting to examine the deformation of single c~stals of analogous composition. Fig. 3 demonstrates the deformation curve for bending perpendicular to the (a,b) - plane of a YBCO cr~stal ~ith T ~ l l 0 0 K (S~5%). Throuhout the temperature range the single c~'stals had a Io~ er plasticity against the ceramic samples. Their detbnnation is accompanied b? the crack formation, preferentially in the extension region. The microhardness of YBCO and other 1-2-3 single c~stals (1-4) is at the level of such solid and brittle materials as Si and Ge. The TEM data have shoun that the microhardness of crystal YBCO is accompanied b~ the tbnnation of dislocations in region of indentatious.(The similar data have been obtained in (11)).
FIGURE 2 TEM of YBCO ceramics cr~stallites: a - zone [201] at~er
4.CONCLUSION
brittle fracture, T~700 K; b - zone [001] aRer plastic flo~x,
An anahsis of the data indicates a special role of intergrain boundaries at plastic flox~ of YBCO ceramics. File plasticit> of ceramics c~stallites mid sinole crystals max be attributed to dislocation processes (e.g.fig.2) and vacanc\ difthsion, tbr instance, oxygen atoms (2,4). The effects of txvin structure alteration related x~ith these may be of consequence (2,9,10).
T-~ 1150 K
~t.l~ 0.5
AKNOWLEDGEMENTS
/
The authors thank V.LKulakov, Yu.D.Midila~hvili, S.S.Shevaga~ A.A.Shokhov and O.VDolgopolov for the assistance
REFERENCES
tN
20O
FIGURE 3 Bending o f a YBCO single crystal in air, T-~1100 K Dash elastic slope. -
brittle fracture, poorly developed plastic floxv and heating the samples without detbnning them. hnportantly, that developed plastic flo~ lead to a radical change of the structure. As suggested by the X-ray diffractomeLry after a
(1)R.F.Cook et a l , Appl.Ph.vs.Lett.,51(1987) 454. (2)V. S. Bobrov et al., t=iz. Tverd. Tela., 31 (1989) 93. (3)V. V.Demirskii et al., Sverkhprovodimost, 3 (1990) 1. t4)N.N Peschanska? a et a l , Yiz.Tverd.Tela, 30 (1988) 3503: 31 ,(1989)271. (5)V.S.Bobrov et al., ibid, 32 (1990) 826. (6)O.AKayibyshev et al., Dokladv Ac.Nauk, 305 (1989) 2079. (7)A.W. yon Sttmberg et al.. J. Appl.Phys., 66 (1989) 2079. (8)GBussed et al., J.Amer.Ceram. Soc., 72 (1989) 137. (9)L.ADorosmskii et al., JETP Lett., 49 (1989) 182. (10) LDebra et al., J.Mater.Res., 4 (1989) 745. ( 11 ) J.Rabier et al~ Rev.Phys.Appl., 55 (1990) 55.
Physica B 194-196 (1994) 2095-2096 North-Holland
Deformation and structure of YBCO ceramics and single crystals in the temperature range 300-1200 K. V.S.Bobrov, V.A.Goncharov, G.A.Emelchenco, L.S.Fomenko'. A.P.Ivanov. A.N.Izotov, Yu.A.Osipyan, N. S. Sidorov, E.V. Suvorov, V. Sh. Shekhtman, L.N.Zavelskaya, I.I.Zverkova. Institute of Solid State Physics, Russia Ac. Sci. 142432 Chernogolovka, Moscow distr.. Russia. *Institute of Low Temperature Physics, Ukraina Ac Sci 310164 Kharkov, Lenin av.,47,Ukraina. The data deformation and structure of YBCO ceramics and single crystals have been reported. It has been concluded that deformation of ceramic specimens is governed by processes of grain-boundary break-dmvn. In syngle crystal specimens and ceramic crystallites the deformation is followed by dislocation processes, alterration in the twin structure, and microcracking.
1.INTRODUCTION At low and moderate temperatures YBCO single crystals and ceramics are brittle (e.g.l-5). A noticeable plasticiB' of ceramics in observable only at elevated temperatures (5-8). Scarce data are available on macroscopic detbrmation in single c~,stals (5). Only some works (e.g. 2,5~8) report strnctural studies ofdetbrmed high-T superconductors. Further studies are required to elucidate the mechanisms of de~bnnation in these materials. In what follows we shall consider the results obtained in this field tbr YBCO ceramics and single crystals.
2.EXPERIMENTAL YBCO ceranctic sample (3x3x10 mm 3) almealed in oxygen had T =93-96 K. They were deformed by compression in air or in a helium atmosphere.The ortorombic single crystals of YBCO ( T ~ 70-90 K) in the foma platelets (~0.1 x2x4 ngll 3) with the developed (a,b) - basal plane were defornled in air by 3-point bending (fig. 3). The loading rate ~as 100 ~trrdmin in either case. Structural studies involved transmission electron microscopy (TEM), X-ray ditTractomet~ and polarization-optical technique.
ensued depended of the preparation conditions in the starting samples (5). These factors also affected the values of T at which the brittle-plastic transition took place. The TEM data have shoun that after the brittle fracture of the YBCO ceramics in the range T<_To some crTsrallites exhibit dislocation bands with pronounced edge and screw components (e.g fig 2a), which may be initiated bv stress concentrators in tile cristallite contact regions. These bands imersect file system of twins without tbnnmg macrosteps at their bomldaries. There are no singularities of the dislocation distribution in the band, and their generation bv secondary dislocation sources is not observed. This may suggest that the tx~in boundaries are not effective barriers tbr gliding dislocation. After the plastic flow of YBCO at elevated T there appear custallites in which twins are absent, and there arises a dislocation network (e.g.fig.2b). According to the TEM data (fig.2a) it should be noted that specific contrast after passing the dislocations m the acting reflex (020) is observed. It can be explained b.vdisplacement of the atoms with moving the dislocations (~ve mean these are the oxygen atoms). The basic phase composition remained unaltered at
A 3.RESULTS AND DISCUSSION. To the values of temperatures belmv the orthorhombtetragon phase transition region (To~1050-1100 K) the detbrmation of YBC'O ceramic samples occured in a quasielastic fashion and ternmmted in brittle fracture (e.g.1 fig. 1). Near T o the deformation pattern changed, a noticeable deviation from the quasi-elastic slope developed (e g.2 fig 1), and with T'_>To the plasticity of YBCO ceramic samples increased drastically. The defonuation curves in this case had a yield tooth (e.g. 3 fig. 1). The crack limit of the samples and the loading at which a noticeable plastic flow
Elsevier Science B.V. SSDI 0921-4526(93)1680-K
o.
180
0
180
0 t
180
360
[s]
FIGURE 1 Compression of YBCO ceramics in a helimn atmosphere: 1T~- 990, 2-1030, 3-1090 K. (P-loading, t-time).
2096
zo.E [oofl
L°'3Pm
large degree of plastic deformation of some samples, the 12-3 compounds decomposed practicall? completel> into C uo and BaCuO 2 (5). In the process of deformation the t~action of the superconducting phase diminished and T of the deformed samples decreased. These phase transtbnnations may, supposedly, be stimulated by the processes in the intergrain botmdary regions at c~smllite migration. In some xvorks high temperature plasticitT of YBCO ceramics is discussed from the vie~ poim of intergrain glide and difl'usional processes (5-8). In this respect it uould be interesting to examine the deformation of single c~stals of analogous composition. Fig. 3 demonstrates the deformation curve for bending perpendicular to the (a,b) - plane of a YBCO cr~stal ~ith T ~ l l 0 0 K (S~5%). Throuhout the temperature range the single c~'stals had a Io~ er plasticity against the ceramic samples. Their detbnnation is accompanied b? the crack formation, preferentially in the extension region. The microhardness of YBCO and other 1-2-3 single c~stals (1-4) is at the level of such solid and brittle materials as Si and Ge. The TEM data have shoun that the microhardness of crystal YBCO is accompanied b~ the tbnnation of dislocations in region of indentatious.(The similar data have been obtained in (11)).
FIGURE 2 TEM of YBCO ceramics cr~stallites: a - zone [201] at~er
4.CONCLUSION
brittle fracture, T~700 K; b - zone [001] aRer plastic flo~x,
An anahsis of the data indicates a special role of intergrain boundaries at plastic flox~ of YBCO ceramics. File plasticit> of ceramics c~stallites mid sinole crystals max be attributed to dislocation processes (e.g.fig.2) and vacanc\ difthsion, tbr instance, oxygen atoms (2,4). The effects of txvin structure alteration related x~ith these may be of consequence (2,9,10).
T-~ 1150 K
~t.l~ 0.5
AKNOWLEDGEMENTS
/
The authors thank V.LKulakov, Yu.D.Midila~hvili, S.S.Shevaga~ A.A.Shokhov and O.VDolgopolov for the assistance
REFERENCES
tN
20O
FIGURE 3 Bending o f a YBCO single crystal in air, T-~1100 K Dash elastic slope. -
brittle fracture, poorly developed plastic floxv and heating the samples without detbnning them. hnportantly, that developed plastic flo~ lead to a radical change of the structure. As suggested by the X-ray diffractomeLry after a
(1)R.F.Cook et a l , Appl.Ph.vs.Lett.,51(1987) 454. (2)V. S. Bobrov et al., t=iz. Tverd. Tela., 31 (1989) 93. (3)V. V.Demirskii et al., Sverkhprovodimost, 3 (1990) 1. t4)N.N Peschanska? a et a l , Yiz.Tverd.Tela, 30 (1988) 3503: 31 ,(1989)271. (5)V.S.Bobrov et al., ibid, 32 (1990) 826. (6)O.AKayibyshev et al., Dokladv Ac.Nauk, 305 (1989) 2079. (7)A.W. yon Sttmberg et al.. J. Appl.Phys., 66 (1989) 2079. (8)GBussed et al., J.Amer.Ceram. Soc., 72 (1989) 137. (9)L.ADorosmskii et al., JETP Lett., 49 (1989) 182. (10) LDebra et al., J.Mater.Res., 4 (1989) 745. ( 11 ) J.Rabier et al~ Rev.Phys.Appl., 55 (1990) 55.