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Microstructure of PtO2-doped melt-textured Y1Ba2Cu3O7-δ
Physica C 235-240 (1994) 465-466
PHYSICA
North-Holland
M i c r o s t r u c t u r e of P t O 2 - d o p e d m e l t - t e x t u r e d Y1Ba2Cu3OT_6 D.Miiller, M.Ullrich, K.Heinemann and H.C.Freyhardt Institut ffir Metallphysik, Universit/~t GSttingen, Hospitalstr. 3/7, 37073 GSttingen, Germany Melt-textured YIBa2Cu3OT-6 was processed by the VGF(vertical gradient freeze)-method. The precursor had an excess of 25mo1% Y2BaCuO5 (211) and 1.8mo1% PtO2. jc of these samples is enhanced compared to melt-textured YIBa2Cu3OT-~ with excess 211 but without PtO2 doping. In order to understand this enhancement in j~, a detailed microstructural study was performed by transmission electron microscopy. The twin spacings in these samples are reduced to 50-100nm compared to twin spacings of 200-300nm in melt-textured Y1Ba2Cu3OT_~ without PtO2 doping. The PtO2 doping leads to a reduced size (< l p m ) and an enhanced density (2× 101S/m 3) of the 211 particles. Most of the 211/123 interfaces are almost clean and free of crystal defects. Only a few of them were surrounded by defects like stacking faults. Therefore, we conclude that the the 123/211 interface is the dominant pinning center in the PtO2 samples. The Cu required for the stacking fault formation near the 211 particles originates from the 211 inclusions as confirmed by EDX. 1
Melt-texturing is a widely used procedure to get well aligned bulk Y1Ba2Cu3OT-~ (123) with high critical current densities. Melt-textured Y] Ba2Cu3OT-6 mainly consists of grains separeted by low-angle grain boundaries which do not act as weak-links. Therefore, there is only little need to improve the grain-boundary properties of this matieral but one has to optimize the intragranular properties. Many studies has been performed to find out which crystal defects might be effective pinning centers. It is still under discussion whether the Y2BaCuO5 inclusions, which remain in the 123 phase due to an incomplete peritectic reaction or due to excess 211 additions in the precursor, act directly or indirectly via the surrounding secondary defects as pinning centers. In order to get more detailed information about the influence of the 211 phase on the microstructure of melt-textured Y1Ba2Cu307_~, microstructural investigations were performed by transmission electron microscopy (TEM). 2
with an excess of 25mo1% 211 phase and 1.8mo1% PtO2. Magnetization measurements were performed with a Faraday balance to determine the critical current densities, jc, at various temperatures and magnetic fields by using the anisotropic critical Bean model. The TEM samples were thinned mechanically and by ion milling. A Philips 400T fitted with an EDX analyser and a Philips 420ST with a resolution of 0.3nm were used for the microstructural investigations. The surface morphology of the samples were studied by an analytical scanning electron microscope.
INTRODUCTION
3
RESULTS
AND
DISCUSSION
j~(77K,1T) = 5.5.104A/cm 2 of the melt-textured YiBa2Cu307-~ with excess 211 phase and PrO2 additions exceeds the critical current density of melt-textured 123 phase with excess 211 phase but without PtO~ additions by a factor of two. In order to understand these findings the microstructural changes of the melt-textured monoliths were investigated.
EXPERIMENTAL
Melt-textured Y1Ba2Cu3OT-6 was produced by the "vertical gradient freeze" (VGF) method [1]. Y1Ba2Cu307-~ samples with an excess of 25mo1% 211 phase were compared to samples 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00795-0
The diameters of the 211 inclusions are reduced from about 2-20pm [2] to 80nm-l.5#m in PtO2-doped samples compared to undoped samples while the density of the 211 inclusions is enhanced from 6.10]4/m 3 to 2.101S/m 3. This is attributed to the ability of Pt to act as a nucleation
466
I~. Mt:iller el al./Phy.sica (" 235 240 H 9 9 4 ) 465 46g)
center for l;}le 211 inclusions or to retard the ()sl watd ripening of the :211 i)articles. Whih' *he 123/211 interface in Y1 l:la.e('.u:~O7 ,, ~nonoliths with excess 211 phase t)u| w i t h o u l I'tO~ doping has an enhanced density of dislocations and stacking faults c o m p a r e d to the 1:23 matrix, the 123/211 interfaces ill lq.O._,-doped sanlt)les are clean and alH~ost free ()f defects (Fig. l). Only a few dislocations and stacking fauh,s were ~bserved in the 123 matrix of the Pt():, doped 123 phase. Mosl of the dislocations t)os sess b = ( 1 0 0 } or b=(OlO) Burgers vectors, t)ut also a few dislocations with b = ( 1/2)(1 10} are p,'('
s,,,t. The displacement vectors R=(~/S)(30 t) ,,r R=(1/(~)(031) of the stacking faults corresl)omt
d. T h e size of the twin domaias in Pt():, ,I,q,(~,l saml)les various t)('lwc('l/ 1 and l()plll. 'Fh,'s,' v;lll..~ fulfill an cquatic)n d,.riv,d hy Welch [:i]
d=
(t.IC,/cMO'e) 1/~
21 ~
,
l ! !
where g is the siz(' ~,f ltw twin domain a l , , n ~ lhe twin hounda.ry~>undary. c is a CCnlStalli. \1 i< (.h(' shear modulus and 0 is l.h(' orthorh~,lllhi,i~.~ (Fig. g). T h o u g h il is cl)ergetically faw)ural)l,. ),> form large twin domains, the PtO:2 doped semi)l('> show rather small 1,win domains. This s,'(.~s ~,, I>o due t<) heterogenous m . ' l e a t i o n <>f ttw l will ,1,, mains at lh(" :211 t)artich's
t(~ an extra CuO-plan('.
.... =L(:, i v
mr•
i i i
1
Figure l. High resolution T E M micrograph of'a sharp 123/211 interface in a PtO~-dol)e(I 123 sample. Whih~ the 211 particles are (h>deiicient as confirmed hy EDX and surrounded by a large nund)er ,)f stacking faults in the undoped samples [2], t,hc 21 I inclusions in lhe l)tO,,-doped samples exhibil n() ( h~ deficiency and ahnost no stacking faults al I,he interface. Therefore, we conclude that the (iu required for the stacking fault formation originates from the 211 particles. T h e stacking faults might form in order to compensate the different thermal contraction of the 123 matrix and d w 211 particles during the cooling period. The twin spacing, d, is reduced from a b o u t 200300nm in u n d o p e d samples to 50-100nm in PtO~doped saml>les. The twin thickness, t. is half of
Figure 2. Twin thickness versus twin d,,.~ai. size in I)t().e-doped samples. While twin boundaries are known to hay-,)nl~ little influence on flux pinning, 211 parl,icles ar~, known to ell_hance ttw crilical currem dol~sil3 Since the 1:23/:211 interfaces in the Pt()'e &,p,',l samples are sharp and there arc only a fi'w ,,lh~,l crystal defects than l.h~' :211 inclusions i. lh~ 123 matrix we suppuse thal in these samples lh, 1:23/:211 interfaces itselw~s acl as pinning cc=llol>, REFERENCES
l. M. Ullrich and ft.(l Freyhar(tt: P r - , ~,f the I C M A S ' 9 3 (1993) 2(15 2. D.Milller, M. Ullrich, K. lteinemann and II.(' FreyhardL Proc. of the E U ( : A S ' 9 3 (1993) xxx 3. I).O Welch, in: Phys. Rcv. B 39 (19g.()) 6(;67
PHYSICA
North-Holland
M i c r o s t r u c t u r e of P t O 2 - d o p e d m e l t - t e x t u r e d Y1Ba2Cu3OT_6 D.Miiller, M.Ullrich, K.Heinemann and H.C.Freyhardt Institut ffir Metallphysik, Universit/~t GSttingen, Hospitalstr. 3/7, 37073 GSttingen, Germany Melt-textured YIBa2Cu3OT-6 was processed by the VGF(vertical gradient freeze)-method. The precursor had an excess of 25mo1% Y2BaCuO5 (211) and 1.8mo1% PtO2. jc of these samples is enhanced compared to melt-textured YIBa2Cu3OT-~ with excess 211 but without PtO2 doping. In order to understand this enhancement in j~, a detailed microstructural study was performed by transmission electron microscopy. The twin spacings in these samples are reduced to 50-100nm compared to twin spacings of 200-300nm in melt-textured Y1Ba2Cu3OT_~ without PtO2 doping. The PtO2 doping leads to a reduced size (< l p m ) and an enhanced density (2× 101S/m 3) of the 211 particles. Most of the 211/123 interfaces are almost clean and free of crystal defects. Only a few of them were surrounded by defects like stacking faults. Therefore, we conclude that the the 123/211 interface is the dominant pinning center in the PtO2 samples. The Cu required for the stacking fault formation near the 211 particles originates from the 211 inclusions as confirmed by EDX. 1
Melt-texturing is a widely used procedure to get well aligned bulk Y1Ba2Cu3OT-~ (123) with high critical current densities. Melt-textured Y] Ba2Cu3OT-6 mainly consists of grains separeted by low-angle grain boundaries which do not act as weak-links. Therefore, there is only little need to improve the grain-boundary properties of this matieral but one has to optimize the intragranular properties. Many studies has been performed to find out which crystal defects might be effective pinning centers. It is still under discussion whether the Y2BaCuO5 inclusions, which remain in the 123 phase due to an incomplete peritectic reaction or due to excess 211 additions in the precursor, act directly or indirectly via the surrounding secondary defects as pinning centers. In order to get more detailed information about the influence of the 211 phase on the microstructure of melt-textured Y1Ba2Cu307_~, microstructural investigations were performed by transmission electron microscopy (TEM). 2
with an excess of 25mo1% 211 phase and 1.8mo1% PtO2. Magnetization measurements were performed with a Faraday balance to determine the critical current densities, jc, at various temperatures and magnetic fields by using the anisotropic critical Bean model. The TEM samples were thinned mechanically and by ion milling. A Philips 400T fitted with an EDX analyser and a Philips 420ST with a resolution of 0.3nm were used for the microstructural investigations. The surface morphology of the samples were studied by an analytical scanning electron microscope.
INTRODUCTION
3
RESULTS
AND
DISCUSSION
j~(77K,1T) = 5.5.104A/cm 2 of the melt-textured YiBa2Cu307-~ with excess 211 phase and PrO2 additions exceeds the critical current density of melt-textured 123 phase with excess 211 phase but without PtO~ additions by a factor of two. In order to understand these findings the microstructural changes of the melt-textured monoliths were investigated.
EXPERIMENTAL
Melt-textured Y1Ba2Cu3OT-6 was produced by the "vertical gradient freeze" (VGF) method [1]. Y1Ba2Cu307-~ samples with an excess of 25mo1% 211 phase were compared to samples 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00795-0
The diameters of the 211 inclusions are reduced from about 2-20pm [2] to 80nm-l.5#m in PtO2-doped samples compared to undoped samples while the density of the 211 inclusions is enhanced from 6.10]4/m 3 to 2.101S/m 3. This is attributed to the ability of Pt to act as a nucleation
466
I~. Mt:iller el al./Phy.sica (" 235 240 H 9 9 4 ) 465 46g)
center for l;}le 211 inclusions or to retard the ()sl watd ripening of the :211 i)articles. Whih' *he 123/211 interface in Y1 l:la.e('.u:~O7 ,, ~nonoliths with excess 211 phase t)u| w i t h o u l I'tO~ doping has an enhanced density of dislocations and stacking faults c o m p a r e d to the 1:23 matrix, the 123/211 interfaces ill lq.O._,-doped sanlt)les are clean and alH~ost free ()f defects (Fig. l). Only a few dislocations and stacking fauh,s were ~bserved in the 123 matrix of the Pt():, doped 123 phase. Mosl of the dislocations t)os sess b = ( 1 0 0 } or b=(OlO) Burgers vectors, t)ut also a few dislocations with b = ( 1/2)(1 10} are p,'('
s,,,t. The displacement vectors R=(~/S)(30 t) ,,r R=(1/(~)(031) of the stacking faults corresl)omt
d. T h e size of the twin domaias in Pt():, ,I,q,(~,l saml)les various t)('lwc('l/ 1 and l()plll. 'Fh,'s,' v;lll..~ fulfill an cquatic)n d,.riv,d hy Welch [:i]
d=
(t.IC,/cMO'e) 1/~
21 ~
,
l ! !
where g is the siz(' ~,f ltw twin domain a l , , n ~ lhe twin hounda.ry
t(~ an extra CuO-plan('.
.... =L(:, i v
mr•
i i i
1
Figure l. High resolution T E M micrograph of'a sharp 123/211 interface in a PtO~-dol)e(I 123 sample. Whih~ the 211 particles are (h>deiicient as confirmed hy EDX and surrounded by a large nund)er ,)f stacking faults in the undoped samples [2], t,hc 21 I inclusions in lhe l)tO,,-doped samples exhibil n() ( h~ deficiency and ahnost no stacking faults al I,he interface. Therefore, we conclude that the (iu required for the stacking fault formation originates from the 211 particles. T h e stacking faults might form in order to compensate the different thermal contraction of the 123 matrix and d w 211 particles during the cooling period. The twin spacing, d, is reduced from a b o u t 200300nm in u n d o p e d samples to 50-100nm in PtO~doped saml>les. The twin thickness, t. is half of
Figure 2. Twin thickness versus twin d,,.~ai. size in I)t().e-doped samples. While twin boundaries are known to hay-,)nl~ little influence on flux pinning, 211 parl,icles ar~, known to ell_hance ttw crilical currem dol~sil3 Since the 1:23/:211 interfaces in the Pt()'e &,p,',l samples are sharp and there arc only a fi'w ,,lh~,l crystal defects than l.h~' :211 inclusions i. lh~ 123 matrix we suppuse thal in these samples lh, 1:23/:211 interfaces itselw~s acl as pinning cc=llol>, REFERENCES
l. M. Ullrich and ft.(l Freyhar(tt: P r - , ~,f the I C M A S ' 9 3 (1993) 2(15 2. D.Milller, M. Ullrich, K. lteinemann and II.(' FreyhardL Proc. of the E U ( : A S ' 9 3 (1993) xxx 3. I).O Welch, in: Phys. Rcv. B 39 (19g.()) 6(;67