effect of post-sintering quenching parameters on tc and crystal structure of the bi2sr2cacu2o8+δ...

Physica C 168 (1990) 591-598 North-Holland

EFFECT OF POST-SINTERING QUENCHING PARAMETERS ON Tc AND CRYSTAL STRUCTURE OF THE Bi2Sr2CaCu2Os+ 6 (2212) PHASE

Asok K. SARKAR and I. MAARTENSE University of Dayton Research Institute, 300 College Park Avenue, Dayton, OH 45469, USA

Received 9 February 1990 Revised manuscript received 18 April 1990

Effects of post-sintering cooling rate from liquid quenching to slow air cooling were investigated for the low-To 2212 phase in the Bi-Sr-Ca-Cu-O system. The fastest cooling rate yielded the 2212 granular phase with the highest superconductive To. The change in Tc is attributed mainly to the change in oxygen stoichiometry. The difference between the granular and bulk T¢'s was smallest after slow cooling, even though the granular T¢ then had its lowest value. No variation in lattice parameters was observed by powder X-ray diffraction. When the samples were quenched, structural imperfections were introduced; these could not be eliminated even by long-term annealing and slow cooling in air. The lowering of T~ was inhibited by the presence of these imperfections in the crystal structure of the 2212 phase. It is emphasized that the true granular superconductive properties are best observed by magnetic measurements since bulk resistivity measurements may lead to erroneous conclusions.

1. Introduction

Changes in the oxygen stoichiometry of the per- ovskite family of oxide materials have been studied extensively because of their reversibility. The high- T~ superconducting cuprates belonging to this family are no exception in this regard. In fact, the critical temperature, T¢, of the new superconductors is ex- tremely sensitive to the oxygen stoichiometry. In most of the recently discovered rare-earth containing superconductors, T¢ is optimized only at the highest oxygen content. This behavior has been linked to the oxidation state of the copper ion by some investigators. However, after the non-rare-earth-containing Bi -Sr -Ca-Cu-O superconductors were discovered, their insensitivity to oxygen treatment has been reported by many investigators. Major changes in T¢ were not noticed initially, after either slow cooling [ 1-4 ] or quenching [ 5 ] in air or oxygen, although contradictory results have been reported more recently.

There are basically two superconducting phases of interest in the Bi -Sr -Ca-Cu-O system. One is the low-To ( ~ 8 0 K), Bi2Sr2CaCu2Os+a (2212) phase and the other is the high-T¢ ( ~ 1 1 0 K), Bi2Sr2Ca2Cu3Olo+a (2223) phase. The first report

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of the interdependence of Tc on the Sr/Ca ratio and oxygen stoichiometry for these two phases was by Tallon et al. [ 6 ]. They concluded that the Tc incre- ment for the 2212 phase from 80 K to 91 K obtained by rapid quenching of bulk specimens was due to oxygen deficiency. Since then, several other investigators [ 7-11 ] have observed similar phenomena by varying the cooling rate from slow cooling in air to quenching the samples from temperature as high as 860°C into liquid nitrogen.

A variety of methods [ 12-18 ] including oxygen treatment have been employed to change the oxygen content in Bi-based superconductors, thereby alter- ing T¢ of the 2212 phase. The lowest T¢ value of 63 K first reported by Sarkar et al. [ 12 ] after oxygen annealing was later confirmed by Morris et al. [ 13 ] using high pressure oxygen treatment. A reduction in oxygen content to enhance T¢ of the 2212 phase has been accomplished by argon treatment [ 14 ], nitrogen treatment [ 18 ], hydrogen absorption [ 15 ], and also by thermal cycling [ 16 ]. In fact, a wide variation in T¢ has been reported for the 2212 phase, ranging from a lowest value of ~ 50 K for the Y- doped 2212 phase by Groen and De Leeuw [ 17 ] to a highest value of ~95 K [4,16].

It should be noted that a majority of the Tc values

592 A.K. Sarkar, L Maartense / Tc of quenched Bi2Sr2CaCu2Os+6

was measured magnetically. However, some investigators [ 6-10,11,14 ] measured bulk Tc values re- sistively instead of the more accurate magnetically determined granular T¢. This practice of indirect T¢ measurements led some investigators [9 ] erro- neously to interpret their results in correlating T¢ with oxygen stoichiometry. In any case, most reports stated that the T~ enhancement of the 2212 phase was max- imized by reducing the oxygen content. Cardwell et al. [ 19 ], however, failed to observe any Tc change of the 2212 phase even magnetically, by varying the post-sintering cooling rates.

An interesting fact reported by some investigators [ 9,11,12,17 ] was the shifting of peak positions in the powder X-ray diffraction (XRD) patterns of the 2212 phase. The changes were due to the lattice parameter variations of the 2212 phase associated with a change in oxygen stoichiometry. Still, others [ 8,13,18,19 ] did not observe any changes whatso- ever in the powder XRD patterns of their samples. Whenever a lattice parameter variation was observed, the c-axis length was found to increase with the oxygen deficiency and vice versa.

In this paper, we present our results obtained for the 2212 phase when bulk samples are cooled at various rates. It is shown that the variation in cooling rates affects not only the Tc of this phase but also some aspects of its crystal structure.

2. Experimental

The precursor powder with starting composition Bi2Sr2Ca2Cu3Ox was prepared by solid state reaction of stoichiometric amounts of reagent grade Bi203, SrCO3, CaCO3, and CuO. Details of the powder syn- thesis are given elsewhere [ 20 ]. Pellets, ~ 20 mm in diameter and ~ 3 mm thick, were uniaxially pressed in a tungsten carbide mold using ~ 5 gm of the powder under ~45 000 psi (310 MPa). These pellets were then sintered on alumina dishes placed on a small fire-brick block inside a silica-glass tube furnace at 870°C for 40 h in air. The temperature of the furnace was controlled within +_ 1 °C by placing the tip of the chromel/alumel thermocouple in the vi- cinity of the pellet. The pellets were then subjected to three types of cooling procedure. In the first type, called liquid quench, the block was quickly with-

drawn from the hot zone of the tube and the pellet was quenched in tetrachloroethylene to room temperature. In the second type of cooling, called air quench, the block was withdrawn as before and the pellet was placed on a refractory block to cool rap- idly to room temperature. In the third procedure, the pellet was cooled inside the furnace at a rate of ~ 2°C/min to 200°C and then cooled to room temperature by turning off the furnace. The pellets undergoing this procedure were designated slowly cooled. All pellets suffered weight loss of ~ 3.4%, presumably due to the loss of Bi from the system during sintering, and slightly increased volumetri- cally, yielding a final density lower than the green density.

The superconductive properties of the samples taken from inside each pellet were measured through AC magnetic susceptibility as described elsewhere [ 12 ]. The Tc of the superconducting granular phase was taken to be the onset temperature of the diamagnetic susceptibility signal. Unless otherwise noted, Tc here refers to that of the granular phase itself rather than that of the resistive transition of the bulk ceramic. When the transition was not sharp, then Tc was defined as the extrapolated zero of the susceptibility.

The phase identification via powder XRD was performed using a Rigaku Rotaflex X-ray diffrac- tometer fitted with a rotating copper anode. A por- tion of the pellet was finely ground in an agate mor- tar, sieved through 325 mesh screen and the resulting powder was thinly spread on a glass slide with a mix- ture of collodion and ethyl acetate.

3. Results and discussion

The temperature dependences of %'(T), and Z" (T) , the real and imaginary parts of the complex magnetic susceptibility, in various AC fields, h, for the three specimens are shown in figs. 1, 2 and 3. These data display the features commonly seen in sintered ceramic superconductors, i.e. a transition to bulk superconductivity identified by the onset of a large sensitivity to AC field strength at a temperature below that of the relatively field-independent granular transition. For the liquid- and air-quenched samples (figs. 1 and 2), only one superconducting

A.K. Sarkar, L Maartense I T~ of quenched Bi2SrzCaCuz08+~ 593

- 0 . 2

=o. = F/ / CURVE I - 0 . 0 4 v

_ 0 6 I_/;I / 2 - 0 . 1 6 ' 1 ~ / 3 - 0 . 4 0

" F / 4 - t o o - - 0 8 l-- / I 5 - 1 . 6 0 I ,,,, ~

• F/' ~- ~,.~6 ~oj 7 . - 3 . 6 0

- I . 0 I I I I I 0 20 40 60 80 tO0 120

TEMPERATURE (K)

0

- 0 . 2

.~ - 0 . 4

~ - 0 . 6

,¢

- 0 . 8

- 1 . 0 0

- - I I I f ~ + ~ ~ / / =o. :

. ////111 CURVE , - 0.04 . ~HI~I/ 2 - 0. , 6

- I l l~ I l l 3 - 0 .40 -

_ / / / 11 / i 4 - , . o o ~~Hill 5 - 1 . 6 o

- / / / / / / / 6 - 2.40

_ /////// . _

_- J- ///lll (o)

20 4 0 60 80 I00 120 TEMPERATURE ( K )

v

x

,¢

0.2

0.1

0 0

I I I I i

i

(b)

20 40 60 80 I00 120 TEMPERATURE (K)

4"

v

x

I I 0.3

0.2

0.1

0 - 0 20

I I

(b)

r I 40 60 80 I00 120

TEMPERATURE (K)

Fig. 1. Temperature dependence of AC susceptibility with varying AC field (h) for a Bi2SrlCa2Cu3Oy sample quenched in tetrachloroethylene; ( a ) real part, X'; ( b ) imaginary part (loss), X"-

granular phase is detected below 90 K. However, for the slowly cooled sample (fig. 3), in addition to the major superconducting phase at ~65 K, a trace amount ( ~ 2% of the full diamagnetic susceptibility) of the second phase (T¢~ 108 K) is also observed. As reported in an earlier study [ 20 ], the 108 K phase is the high-T¢, 2223 phase and the phase ap- pearing at ~ 90 K or below is the low-To, 2212 phase. It can be seen in these figures that the liquid- quenched sample has the 2212 phase with the highest T¢ (89 K) and that T¢ goes down to 75 K for the air-quenched sample. The lowest T¢, 65 K, is seen in the slowly cooled sample.

These variations are believed to be associated with the oxygen stoichiometry of the 2212 phase. During

Fig. 2. Same as fig. 1 but for air-quenched sample.

rapid cooling this phase cannot attain the maximum equilibrium oxygen concentration which the chemical composition and crystal structure can accom- modate. The equilibrium oxygen content of the 2212 phase at 870°C in air is low and it can be frozen-in metastably by fast quenching, resulting in an oxygen- deficient 2212 phase at room temperature. In other words, this method can reproduce the oxygen stoichiometry obtained by equilibrating the sample in a low partial pressure of oxygen, which has been reported by previous researchers.

The strong dependence of x (T ) on AC field strength seen in figs. l, 2, and 3 is associated with the weak-link intergranular structure. The onset temperature of this bulk-type behavior is essentially equal to the bulk Tc of the samples as defined by the zero- resistivity point [ 12,20 ]. In this connection, it is in-

594 A.K. Sarkar, L Maartense I T¢ of quenched BizSrzCaCuzOs+~

- 0 . 2

- 0 , 4

~ - 0 . 6

,¢ - 0 , 8

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0

- ' H= 6 ' I ~ ),, _ ,,oOe 2 / O C U R V E I - - 0 . 0 4

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(Q)

| I I I - 0 . 0 5 2 0 4 0 6 0 8 0 I 0 0 120

T E M P E R A T U R E (K)

0 . 4

0 . 3

0 . 2

O. I

i I I I i

(b)

0 I I ~ i I 0 2 0 4 0 6 0 8 0 I 0 0 120

T E M P E R A T U R E I K )

- 0 . 0 2 5

Fig. 3. Same as fig. 1 bu t for s lowly cooled sample ; the cu rve on the f ight is the upper 5% o f the full curves.

teresting to note that, although Tc of the granular superconducting 2212 phase reaches 89 K after liquid quenching, the bulk T~ is only ~ 40 K, and the qual- ity of the intergranular coupling is very low. This degradation, relative to the slowly cooled material, is indicated by the increased spreading of the x (T ) curves as the AC field is varied, at all temperatures below the bulk transition temperature, which indi- cates a very low current-carrying capability. This poor intergranular coupling is attributed to the severe mi- crocracking in the liquid-quenched sample; the pellets tended to be very brittle. The situation is better in the air-quenched sample, in which the bulk T~ ad- vances to 62 K with improved intragranular coupling behavior, despite a reduced granular T~ of 75 K. Although T~ of the granular 2212 phase in the

slowly cooled sample is only 65 K, the bulk Tc remains near 62 K, due to the much better intergranular coupling. These values of Tc are listed in table I, together with those of the annealed samples to be discussed later.

The foregoing discussion points out the need for achieving good coupling in order to obtain a bulk T~ very close to the T~ of the granular phase, even though the latter may not have the highest value attainable. It is also obvious that relying on the resistive or bulk T~ to monitor the effects of processing variables can lead to erroneous conclusions regarding the basic superconductive properties.

The XRD patterns for samples cooled to room temperature by these three different methods are shown in fig. 4. The majority of the peaks (marked in fig. 4 (a ) ) of the slowly cooled specimen can be matched with the published XRD pattern of the low- T~ 2212 phase [ 21 ]; impurity peaks due to insulat- ing Ca2CuO3 and CuO are also marked. In addition to these impurity peaks, there are a few very small peaks (marked (*)) that are not assigned to any phase and may be due to superlattice modulation, incommensuracy or cation order/disorder phenomena often present in the low-To phase. The high-To, 2223 phase, which was detected with the AC susceptibility technique, is below the detection limit ( ~ 5%) of the X-ray technique.

The XRD patterns of the liquid- and air-quenched samples (figs. 4(b) and 4 (c ) ) are very similary to fig. 4(a) . The majority of the peaks also belongs to the 2212 phase, but with a few prominent distinc- tions. First of all, the only impurity phase detected is CuO and no Ca2CuO3 is seen. However, many new peaks are now visible and a careful observation re- veals that some of these peaks are actually more intense peaks of the type marked (*) in fig. 4(a) . The relative intensities of the major peaks belonging to the 2212 phase in fig. 4(a) remain more or less the same in figs. 4(b) and 4(c) and any variation may be attributed to preferred orientation effects of the platy grains due to the XRD sample preparation.

In the determination of the crystal structure of the 2212 phase, many investigators have mentioned the difficulty of establishing the true average atomic positions and/or the true space group of the crystal lattice [ 21,22 ]. However, incommensurate superstructure modulations appear to be a very common feature

A.K. Sarkar, 1. Maartense / Tc of quenched Bi ~r zCaCu 2Os+6

Table I Effect o f cooling parameters upon the granular and bulk Tc values o f samples with nominal composit ion o f Bi2Sr2Ca2CuaO r

595

Sample Treatment Granular Tc (K) Bulk Tc (K)

Liquid-quenched 89 ~ 40 Liquid-quenched & annealed 75 ~ 45 Air-quenched 75 62 Air-quenched & annealed 70 59 Slowly cooled 65 62

I-

z

>- n* < n- I -

n,, <

>- I- N z p- z

L • 2 2 1 2

C • Co2CuO 3

L

I I

L

L

L

I

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(b)

(c)

4 14 2 4 3 4 4 4

DEGREE ( 2 8 ) ' - ~

Fig. 4. Powder X R D patterns for Bi2Sr2Ca2CuaOy samples; (a) slowly cooled in air, (b) quenched in tetrachloroethylene, (c) quenched in air.

of the 2212 structure and in some cases have been reported as satellites in the powder XRD pattern [22,23 ]. Unfortunately, the new peaks observed in the present study cannot all be matched with these

published patterns. Thus, the intense extra peaks we observe may be due not only to superstructure modulations but also to local changes in site symmetry. These extra peaks could not be matched with any other phases known to be present in the Bi-Sr-Ca- Cu-O system and thus their identification as impurity phases can be virtually ruled out. To be sure that this phenomenon is not peculiar to the "2223" starting composition, pellets were made from a 2212 nominal composition, heat treated as before and then subjected to the identical liquid quench procedure. These samples were found to produce very similar XRD patterns, as shown in fig. 5.

Another interesting observation made during this study was that no shift occurred in the positions of the main peaks in the XRD patterns of the 2212 phase resulting from different quenching techniques. Yet, during a previous study by Sarkar et al. [ 12 ], the 2212 phase produced by sintering and slow cooling in oxygen of a sample with starting cation ratios of 1112, a clearly visible shift of peaks to higher 20 values suggested a lattice contraction due to oxygen loading. The Tc of that 2212 phase was unequivo-

7-

rr

ae t-- I a,i a-

>- t-

z b.I I-- Z

I I I

I I 4 14 2 4

I 3 4 4 4

DEGREE ( 2 e ) - - - ~

Fig. 5. Powder XRD pattern for Bi2Sr2CaCu20 z sample quenched in tetrachloroethylene.

596 A.K. Sarkar, L Maartense / Tc of quenched Bi2Sr2CaCuzOs+,

cally determined to be 63 K. In the present case, even though T¢ of the slowly cooled sample was 65 K, no lattice contraction of the 2212 phase relative to that of the liquid ( T¢,,- 89 K) or air ( T~ ~ 75 K) quenched samples was observed. Thus, the primary effect of changes in the oxygen stoichiometry of the 2212 phase appears to be the variation of To, with the lattice parameter remaining unchanged, as reported by Morris et al. [ 13 ] and Namgung et al. [ 19 ]. When a change in lattice parameters does occur, it proba- bly is associated with processing variables which are only indirectly related to the oxygen content.

In order to isolate the effect of oxygen stoichiometry on T¢ of the 2212 phase, both the liquid- and air-quenched samples were annealed at 870 °C for 40

h in air and then put through the slow cooling procedure used for the third sample. These two specimens were reexamined via AC susceptibility, and the results are shown in figs. 6 and 7. It is seen that annealing has indeed lowered Tc of the granular 2212 phase from 89 K to 75 K and raised the bulk Tc from 40 K to 45 K in the liquid-quenched sample. For the air-quenched sample, similar treatment has lowered Tc of the 2212 phase from 75 K to ~70 K, at the same time lowering the bulk T¢ from 62 K to 59 K. All our Tc values are summarized in table I. Appar- ently, the Tc change in the 2212 phase is not totally determined by the oxygen content. Otherwise, the granular Tc would have been lowered all the way to 65 K as was the case for the original slowly cooled sample. Therefore, Tc of the 2212 phase depends on

- 0 . 2

~- 7 - 0 . 2

- 0 . 4

~ = - 0 . 4

iiii.ii I + ~ -0 .6

- ( a ) + - I

- 0 . 8 - 7

- - t . O ~ - ,

0 20 4 0 60 80 I 00 120 - 1.0

T E M P E R A T U R E (K) 0 20 4 0

I I I

H=O h in Oe

RVE I - 0 .04 2 - 0 . 1 6 3 - 0 . 40 4 - 1.00 5 - 1.60 " 6 - 2 .40 7 - 3 .60

(a)

I I I

60 80 100 120

K

k

0.2

O.I

I

0 I 0 20

I I I I

(b)

I I I " - ' t - - - 4 0 60 80 I 0 0 120

T E M P E R A T U R E ( K )

Fig. 6. Temperature dependence of AC susceptibility with varying AC field (h) for the liquid-quenched Bi2Sr2Ca2Cu3Oy sample of fig. 1 annealed at 870°C for 40 h and then slowly cooled in air at ~ 2 o C/min; (a) real part, X'; (b) imaginary part (loss), X".

TEMPERATURE (K)

[ x

0.3

0.2

0.I

0~- 0

I I I

(b)

~ L ; I 20 40 60 80 I 0 0 120

T E M P E R A T U R E ( K )

Fig. 7. Same as fig. 6 except the original sample was the air- quenched Bi2Sr2Ca2Cu3Oy sample of fig. 2.

A.K. Sarkar, L Maartense / Tc of quenched Bi:Sr2CaCu2Os+6 597

some structural factors as well. As far as direct structural influences on T¢ are con-

cerned, Tarascon and Bagley [24] have suggested that these superconductors are prone to having stacking faults which are influenced by the annealing temperature and the cooling rate, and that these stacking faults can lead to changes in T¢. In fact, our XRD patterns of the liquid- and air-quenched samples do show some broadening of the peaks and also an amorphous background which suggests the presence of stacking disorders and strains in the structure of the 2212 phase. Annealing of these samples would be expected to eliminate these effects.

We find that the powder XRD patterns of these annealed samples do not revert to that of the slowly cooled sample, even after an additional 120 h of annealing and slow cooling. For the liquid-quenched sample, only a few of the low intensity superstructure peaks present in the original specimen disap- pear after annealing and the ones that remain can still be attributed to the "satellite" reflections from the 2212 structure. The XRD pattern of the annealed air-quenched sample also remains more or less unchanged, apart from some intensity variations. The sharpness and strength of these extra reflections im- plies that the associated structural entities exist on an extended scale, and that their elimination could require a very long annealing time. In any case, it is clear that such quenched-in structures can limit the influence of oxygen stoichiometry on Tc of the 2212 phase, or at least restrict the full uptake of the equilibrium oxygen content.

The origin of these structural defects must lie in the kinetics of the formation of the high-T¢ phase in the pure Bi -Sr -Ca-Cu-O system and its stability near the sintering temperature. It is important to note that we have not found evidence of these defects in quenched samples of this system when Pb is substi- tuted for Bi in the batch composition. This observation bears directly upon the questi~m of what is the influence of lead in stabilizing the high-T¢ structure, and the difficulty of doing so in the absence of lead. However, that subject falls outside the scope of this paper.

It is generally accepted that T¢ of the superconducting cuprates is affected by the oxidation state of copper, which is itself related to the hole concentration determined by the St /Ca ratio. The change in

the oxygen content measured by Groen and De Leeuw [ 17 ] for the 2212 phases having different Tc's was small and they surmised that oxygen alone cannot affect the total density of mobile holes in the 2212 phase. The measurement of the oxygen content in this system again is a controversial one, but Udand and Tietz [25 ] have shown that the actual chemical composition of the 2212 phase can vary widely de- pending on the sintering and annealing parameters even when starting with the same nominal composition. They related the change in Tc and lattice constant c with the amount of Ca substituting for Sr in the 2212 crystal lattice without mentioning anything about a drastic structural change. They also found that T~ and the lattice constant c both decreased with increasing Ca content up to Ca= 1.4 mole/formula unit. An increase in hole concentration, as inferred from the actual chemical composition, can indeed cause a decrease in T~ of the 2212 phase [ 26 ]. Groen and De Leeuw [ 17 ] suggested that the change in T~ was mainly due to a different distribution of holes between the Bi202 layers and the CuO2 sheets. The c-axis contraction is caused by the collapse of the Bi2Oz layers with increasing covalency of the Bi-O bonds. So, the density of holes in the CuO2 planes depends on the nature of the Bi202 layers. The fact that no lattice contraction was clearly evident in this study and also by Morris et al. [ 13 ] may be due to the sample preparation conditions used in these two studies that do not, for some reason, facilitate the contraction of the Bi202 layers, and yet affect the Tc of the 2212 phase. In other words, we appear to be varying the oxygen concentration in the Cu-O planes more than that of the Bi202 layers.

4. Summary and conclusion

Ever since the 2212 phase was discovered in the Bi-Sr-Ca-Cu-O system, the determination of its true structure has been the subject of controversy. Not only can there be many imperfections in its structure in terms of stacking of various planes of atoms, but there can also be compositional variations due to atomic substitutions, interstitials, or oxygen vacan- cies. All of these factors, in fact, can have an effect on Tc of this 2212 phase in a very subtle, as yet un- known, way. Previous investigators have speculated

598 A.K. Sarkar, L Maartense / Tc of quenched BizSrzCaCu2Oa+n

on these effects. Many have clearly shown the effect o f the oxygen s to ichiometry on Tc. Our s tudy shows that by changing the cooling rates o f samples in the B i - S r - C a - C u - O system f rom a s inter ing temperature o f 870°C in air, T¢ o f t b e 2212 phase can be varied over the range o f 65 K to 90 K.

The change in T~ is due most ly to the oxygen stoi- ch iomet ry and due par t ly to the s tructural imperfec- t ions in t roduced by rap id cooling. The imperfec t ions could not be e l imina ted by long anneal ing and thus they imposed a l imi t on the lowering o f T¢ toward the value per t inent to the high oxygen content ob- ta ined by slow cooling o f an unquencbed sample. These s tructural defects indirect ly affect the hole concent ra t ion in the CuO2 planes o f the 2212 structure, thus restr ict ing the change in Tc o f the 2212 phase. We do not bel ieve that our sample prepara- t ion technique had a large effect on the oxygen content o f the Bi202 layers, since no var ia t ion in lat t ice parameters was observed.

We should emphasize that our conclusions arc based on the properties of the granular superconducting phases themselves, rather than on those of the bulk material. Inferring the superconductive properties from the bulk or resistive behavior is mis- leading. The bulk properties are dependent upon the intcrgranular interactions, which can be greatly influenced by the post-s inter ing cooling parameters , independent ly o f the intr insic mater ia l propert ies .

Acknowledgements

We are grateful for the suppor t and advice given by P.M. Hemenger and T.L. Peterson at the Air Force Mater ia ls Labora tory , Wright Research and Devel- opmen t Center, Wright -Pa t te rson Air Force Base, Ohio, where we pe r fo rmed the magnet ic and structural characterizat ions and which sponsored the work of I .M. under Contrac t No. F33615-g8-c-5423.

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effect of post-sintering quenching parameters on tc and crystal structure of the bi2sr2cacu2o8+δ...

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