Poole Ch.P., Jr. Handbook of Superconductivity

666

Chapter

14:

Phase Diagrams

Table 14.3.

Initial melts of four-phase equilibrium volumes containing the 2212 phase of BSCCO, see Fig. 14.20.

Sample DTA Melt Composition

ID Four-phase equilibria T(~ Bi Sr Ca Cu

1 2212-2110-119x5-CuO 25 41.7 16.1 22.3 19.9

2 2212-0x21-2110-CuO 830 30.4 7.4 26.6 35.6

3 2212-0x21-CaO-2110 838 35.2 20.8 24.6 19.4

4 2212-014x24-CuO-119x5 856 27.6 25.1 8.8 38.5

5 2212-2110-119x5-CaO 856 37.8 18.0 19.6 24.6

6 2212-014x24-0xl 1-CuO 861 26.2 24.2 22.3 27.3

7 2212-O14x24-Oxll-Ox21

863 25.3 23.6 18.9 32.2

8 2212-119x5-2310-CaO 873 29.8 23.2 21.9 25.1

9 2212-4805-0x21-CuO 875 25.9 21.8 14.2 38.1

10 2212-2201-2310-014x24 877 23.9 32.8 11.7 31.6

11 2212-4805-0xl 1-CuO 877 26.4 26.3 12.7 34.6

12 2212-119x5-2201-2310 877 32.8 30.7 4.5 32.0

13 2212-119x5-014x24-2201 878 27.7 26.9 2.5 42.9

14 2212-0x21-014x24-2310 885 28.7 21.4 20.8 29.1

15 2212-4805--0xl 1-0x21 887 29.9 24.7 14.9 30.5

16 2212-2310-0x21-CaO 889 26.9 24.7 17.6 30.8

Fig. 14.21.

Mole FrocUon CuO

0.4 0.3 0.2

i

~ .... I ..... I ......... 1 ......

I I

/I \\ 1

l ]878/''" - ~ k I

,* I I__\

~e77"~! ~176

'c*,.~.~_X85s I

~.,.,I i 8~8'~ ..... ,,q' 87s /838 \ ~

fl

I I \

! / ~ 825

I I

o.,L

\// i

l.-"

Primary crystallization field of the Bi-Sr-Ca-Cu-O2212 phase using orthogonal coordinates

(Wong-Ng

et al.,

1998).

D. Representative Phase Diagrams

667

Fig. 14.22.

Mole F'roction CoO

o.2 0.2 0.1

030.4 ' 877'

//

-(

I i -

i

',

I .... / I

'

i"

1

.... \'~

0.3

o

o.2.~

:=E

0.1

View of the primary crystallization field of the Bi-Sr-Ca-Cu-O 2212 phase approximately

orthogonal to the view shown in Fig. 21 (Wong-Ng

et al.,

1998).

Bi2Sr2CaCu20 x composition and Ag. Liquid immiscibility between oxide and Ag

liquids in 8-98 at.% Ag range at all temperature is found (McCallum

et al.,

1994). The solubility of Ag in the specific 2212 composition was less than

detection limits, but that Ag depressed the melting temperature of mixtures

with the BizSrzCaCu20 x composition through the formation of a eutectic with

~4% Ag.

With the addition of excess silver (~30 wt%), the initial melting tempera-

tures of the 2212 four-phase equilibrium volumes were all lowered, by amounts

ranging from 4 to 22~ (Wong-Ng

et al.,

1998) Silver entered the melt, with a

mole fraction of 6 to 8%. In Fig. 14.24, the outline of the Ag-free and Ag-

containing melts are shown as projections (normalized in terms of the CaO, CuO,

and Bi203 contents). Ag addition is manifested in a reduced copper content of the

melts.

3. Phase Diagrams fir Three CuO2 Layers

Phase equilibrium investigations of the Pb-free high-T c three-layered super-

conductors are much less numerous than those of the Pb-containing system.

One such study in air is represented in Fig. 14.25 (Majewski

et al.,

1991),

668 Chapter 14: Phase Diagrams

Fig. 14.23.

1200

1100

1000

~P

v-"

flO0

800

i ! i I i" I" i I i i i ' i I i'

m

Uqui~

Uquidl

+

Llq. 2

_ -

..... }_ +,,..,,

! -

9 ..

OX + Uq< /

~-- ..../// OX +Ag+Uq., I _

. _

/ (.-,r~ : . I

~. ~,-

mscco + Ag

i I i I I I i I ,I I. I i

7~176 2 4 6 e 16 : 9e lOO

ah~sr=coc..~ At Z

BISCCO

Temperature-compostion phase relations in the system of Bi2Sr2CaCu2Os-Ag (McCallum

et al.,

1994). The symbol OX is used to represent oxygen.

showing the section of Bi203-1(SrO + CaO)-CuO at 850~ through the

quatemary system Bi203-SrO-CaO-CuO. The 2223 phase is also surrounded

by very flat two-, three-, and four-phase equilibria. Therefore, a small deficiency

of CuO and/or Bi203 during the preparation of the 2223-phase sample results in

a significant decrease of the volume content of the 2223 phase. On the other hand,

an excess of Bi203 and CuO results in the formation of the 2223 phase in

addition to CuO and liquid. At 850~ the 2212 and the 2223 Pb-free super-

conductors are in equilibrium with a liquid of composition close to a Ca-rich

2201 phase. The 2223 phase only exists above 840~

Figure 14.26 shows the schematic projection diagram of the solid solution

region of the Pb-2223 phase of composition

Bi2.27_xPbxSr2Ca2Cu301o+a

as a

function of temperature (Kaesche

et al.,

1995). A single phase region was

identified between x = 0.18 and x = 0.36. This schematic diagram is intended

to illustrate phase existence regions only. There is a relatively narrow temperature

range within which a single phase can form (about 838-860~ Below and above

this temperature range with x values of 0.1 > x > 0.36, a multiple phase region is

observed. For x > 0.36, 2223 was found in equilibrium with numerous phases,

including Pb4(Sr , Ca)5CuOx, indicating that the maximum Pb solubility was

exceeded. For x < 0.18, only small amounts of 2223 were detected with the 2212

Fig. 14.24.

"COO"

r)2Cu03

P

D. Representative Phase Diagrams

669

0 20 40 60 80 1 O0

1/2(Bi203)" Mol % "CuO"

Primary crystallization field of the Bi-Sr-Ca-Cu-O 2212 phase projected onto the temary

plane and shown in broken lines, (a) without the addition of Ag and (b) with Ag added (Wong-

Ng

et al.,

1998). The projection of the 2212 solid solution is also shown in broken lines.

phase predominant over the entire temperature range. Figure 14.27 illustrates the

solid solution region of a composition BiyPbxSr2Ca2Cu3010+d at 850~ and the

surrounding phase fields as a function of the x and y values. The solid solution

has a triangular shape, and Pb substitution has widened the homogeneity range of

the 2223 phase.

In another subsolidus equilibrium study (Wong-Ng

et al.,

1997), the Pb-

2223 phase was found to be in equilibrium with 11 phases: lx20 [(Ca, Sr)2PbO4],

0x21, CaO, CuO, 3221 [(Pb, Bi)3(Sr, Ca)5Ox] ), 0xl 1 [Ca-rich], 0xl 1' [Ca-poor],

2310, 119x5, and 2212. Equilibria involving the complete 29 subsolidus five-

phase volumes and their initial melting temperatures of the Pb2223 phase in the

(Bi,Pb)-Sr-Ca-Cu-O system are shown in Table 14.4. Because of the large

number of phases and closely spaced phase compositions, the Pb-2223 phase

compatibilities, similar to those of the 2212 and Pb-free 2223 phases, include a

number of relatively "flat," or shallow, five-phase equilibrium volumes. Among

the 29 five-phase volumes, 16 involve the 2223-2212 phases that are mutually

670

Chapter

14: Phase

Diagrams

Fig. 14.25.

1/2(1 SrO 4- 1CaO)

SrsCoBi209 + CaO

+ Sr2CuO 3

Srl.sCa~.sBi208

Srl.sCo

1.5BI208 +

Sr2CuOz + CoO

Srl.sCo1.sBi20s

-I;, Ce2Cu03

2212 + cao../. - ~\ ~,.~~ r\ 2212 + Co2Cu03

Bi2Srl.5 Ca!.SCu20x

0

Bi203

Sro.6sCao.33Cu02

/4- Co2Cu03

S r7C OTCU 24041 -x

..--- ~-..~ \

\',.... ~ ~.... "..,.

\ \ -- ',.. "-,,,~

/~/~ ,

~- ~--.. -9

/j~ j,. ~" Liquid

k. ~ ~'~- "'"- ""

\.-- " . -...~.~ -

\ ---

20 40 60 80 1 O0

Mol ~ CuO

Section Bi203-{(SrO + CaO)--CuO through the quaternary phase diagram of the system

Bi203-SrO-CaO-CuO at 850~ in air, showing the equilibrium of 2212 and 2223 with liquid

(Majewski

et al.,

1991).

stable in a topologically consistent manner. Because the 2212 and 2223 phases

have similar structures (being members of the same homologous series), and the

2212 phase is a precursor for the formation of the 2223 phase, their mutual solid-

state compatibilities are extensive.

e. TI-Ba-Ca-Cu-O Systems

Phase equilibrium studies in the T1-Ba-Ca-Cu-O system are not as extensive as

those in the BYC, BRC, or the BSCCO systems, partly because of the additional

processing parameters of vapor pressure, and also because of the toxicity of T1-

containing compounds. Figure 14.28 shows the four-component composition

tetrahedron for the T1Ox-BaO-CaO-CuO system (Siegal

et al.,

1997). The

nominal compositions of the presently known and confirmed superconducting

phases were found to lie in the plane determined by T1Ox, 2BaO.CuO, and

D. Representative Phase Diagrams 671

Fig. 14.26.

8g

i

870

p:

\

85O

83ff

0.45

No 2223

183o-89o=cl

mostly molten I mostly

........ _" j

~- 9 k 9 ] 2212 9 -

, o o .

i

.

I I __

I

!

9 9 9 1 9

four-phase region '1

mostly

-- 2223 exists

i

2212 h

i

9 9 9 I

I

,

.

formation IX, \ a \ 1 f F

~'-ofalead ~ \ \o \ I 9 I mostly 9

containing I ~ 222~ ~ I mostly I 2212 d

phase;2223~ ~ ~ "~ 2223 I I

exists

' " " '

- L_'~___~_.__~._.J J

I multi-phase region I

b 12223 exists c i

,P I _9 ..... I 9 1 ~ 9 ! I I 1.

0.35 0.25 0.15 0.05

x, At~ Pb

in

Bi2.27_xPbxSr2Ca2Cu3010+d

Plot of temperature vs lead content x showing phase regions for

Bi2.27_xPbxSr2Ca2Cu301O+d

(Kaesche

et al.,

1995).

CaO-CuO (Fig. 14.29). Furthermore, the nominal composition of each quaternary

superconductor phase lies at the intersection of one of the two lines originating at

the component CaO.CuO and terminating at the Tl-1201 and T1-2201 composi-

tions. A second set of lines originates at T10 x and terminates at the Tl-free

compositions 0212, 0223, and 0234. Therefore, traveling along any one of the

second family of lines corresponds to adding or deleting thallium from the

previous compound.

The phase diagram of the system containing the 20 K superconductor

TlzBazCuO 6 (2201) is shown in Fig. 14.30 (Jondo

et al.,

1993). The TlzBazCuO 6

phase exists in both tetragonal and orthorhombic forms, and is in equilibrium

with TlzBaO4, T16Ba4OI3, TlzBa2Os, BaCuO2, and CuO. The pseudobinary cut

of the system TlzBazOs-CuO is shown in Fig. 14.31. The TlzBa2CuO 6 phase

melts incongruently into TlzBa205 and L.

The solid solution extent of the T1-2212 phase is illustrated in the

BazCaCuzOx-Tl203 system as a function of temperature (Fig. 14.32) (Cook

et al.,

1990). A single phase region around the 2212 composition at higher

temperatures has been confirmed. It was also found that a sample with maximum

T c occurred with less T1203 than the 2212 stoichiometry.

672

Cha~ter 14: Phase Diagrams

Fig. 14.27.

0.5

"l I ...... I I = I l

.....

\ . /

- \\ 9 --.- - .- -.Pb4(Sr, Co~sCulZ + ~ -

\ (Sr, Co)14Cuz4041-x +

0.4- \ CuO + 2223

\

\ /

.--.

2223

...,, ...... -.--

u:. L ~ -

c

to ,u /14 24..4.1-x " %1 \ 2212 + 2223

x -CuO +

221:+22231 ~- 9 -

0.2 ( . ) +2212+2223"~

- St Co 2Cu03 + 2212 + 2223 9 -

(

Sr.Co)i4Cu24041_x ~----....

9 /

/ (Sr, Ca)lCuOz + -

0.1- /

/

(Sr, Co)14Cu24041-x

+

/

2212 + CuO

9 9 -

- i 9

I .i , !

,..I I

l I

0"(1.6 1.8 2.0 2.2 2.4

y in BiyPbxSr2Co2Cu301o+d

Phase fields surrounding the 2223 phase

(BiyPbxSr2Ca2Cu3Olo+a)

as a function of lead

content x vs bismuth content y at 850~ (Kaesche

et al.,

1995). Table 14.4 provides a set of 20

five-phase volumes of Pb-2223.

The phase equilibria in the T120-BaO-CaO-CuO system that are slightly

below the onset of melting (two-zone furnace) are shown in Fig. 14.33 (Aselege

et al.,

1994). The stable ranges of p(Tl20 ),

P(O2) ,

and sample composition for

individual T1 superconductors, including the 1122, 2122, 1223, and 2223 phases,

are shown in Figs. 14.33a to 14.33c. In these diagrams, the partial p(TlzO)-p(O2)

subsolidus phase relationships with the BaO : CaO :CuO ratios of 2 : 1:2, 2:2:3,

Table 14.4.

Set of 29 Pb-2223 five-phase volumes (7.5 vol% 02/92.5 vol% Ar) of the (Bi, Pb)-Si-Ca-Cu-O

system. See Fig. 14.27.

2223-2212-1x20-119x5-2310

2223-2212-O14x24-1x20-119x5

2223-2212-0xll-2310-119x5

2223-2212- lx20-231 0-CaO

2223-2212-0x21- lx20-CuO

2223-2212-1x20-0xl 1-3221

2223-2212-1x20-CuO-0xl 1

2223-2212-3221-CAO-2310

2223-1x20-0xl l'-CuO-0xl 1

2223-1x20-CuO-0x21-O14x24

2223-1x20-CuO-0xl 1'-01424

2223-1x20-CaO-Oxl 1'-2310

2223-1x20-2310-O14x24-Oxll'

2223-1x20-CaO-0xl 1'-3221

2223-0xll'-2310-119x5-014x24

2223-2212-2310-0xl 1-3221 2223-2212-119x5-0xl 1-CuO

2223-2212-014x24-119x5-CuO 2223-2212-0x21-014x24-CuO

2223-2212-O14x24-0x21-1x20

2223-2212-0xl 1-0xl l'-CuO

2223-2212-3221-CaO-lx20 2223-2212-0xl 1-0xl 1'-2310

2223--Oxll'-2310-Oxll-l19x5

2223-0xl 1'-CAO-2310-0xl 1

2223-0xl 1'-014x24-119x5-CuO 2223-0xl l'-CaO-0xl 1-3221

2223-2310-3221-CaO-0xl 1

2223-1x20-0xl 1'-3221-0x 11

D. Representative Phase Diagrams 673

Fig.

14.28.

]']01.5

BoO ....

CoO

Bo2Cu03

CuO

Mol %

Four-component composition tetrahedron for the T1Ox-BaO-CaO-CuO system. The nominal

compositions of the superconducting phases in the system all lie in the plane determined by

T10 x, 2BaO.CuO, and CaO.CuO [84]. (Siegal

et al.,

1997)

and 2:3:4, are illustrated. It was found that p(O2) is not a critical variable

controlling the relative thermodynamic stabilities of 2223 and 2212, and their

stabilities are controlled by p(Tl20 ). The 2212 phase is the stable superconductor

phase, independent of batch stoichiometry and p(O2) within the highest range of

p(T120 ) (Fig. 14.33a). Higher values of p(Tl20 ) produced Tl-rich, nonsupercon-

ducting phases. The 2223 phase is stable on both the 2:2 : 3 and 3:2 : 4 tie-lines

over a narrow, lower range of p(Tl20), independent of p(O2) between at least

8 • 103 Pa and 8 x 104 Pa (Fig. 14.33c). An excess of Ca and/or Cu is required to

prepare single-phase 2223. As p(Tl20 ) is reduced below about 5 x 102 Pa, the

m = 1 phases

(TlmCan_lBazCunOz(n+l)+m)

become more stable (Fig. 14.33b). On

the 2:2:3 and 3:2 : 4 tie-lines, 1223 intergrowth with 2223 is observed (the

amount of intergrowth ranges from 50 to 90%). The 1324 phase (also present as

an intergrowth) is stable at lower p(Tl20 ) than 1223 on both the 2:2:3 and

3:2 : 4-tie lines. The 1122 phase is observed on the 1:2:2 tie-line in 100% O2

with p(T120 ) < 5 • 10

2

Pa.

674 Chapter 14: Phase Diagrams

Fig. 14.29.

TIO

. 1.5

2201

1201

2212

"X',,~~223

2BaO.CuO

254

V .k v ,~

k

V ~',x

i. w i,

0212 022.-5 0234 CcO.CuO

Mol %

Pseudoternary composition diagram for triangular plane of Fig. 14.28 of the T1Ox-BaO-CaO-

CuO system, illustrating the location of the superconducting phases (Siegal

et al.,

1997).

f. {Nd, Ce}-CuO 4 Systems

In 1989, a family of superconductors in which the carriers of the superconducting

current are electrons themselves rather than electron vacancies, or "holes," was

discovered (Tokura

et al.,

1989). These superconductors are Ce4-doped

compounds, with the formula Rz_xCexCuO4_y, where R stands for Pr, Nd, or

Sm. The compounds have the Nd2CuO 4 (T'-phase) type structure, which is

composed of sheets of Cu-O squares.

An isothermal section at 1000~ of the phase diagram of the Nd-Ce-Cu-O

system is shown in Fig. 14.34 (Pieczulewski

et al.,

1990). The only ternary

compound in this system is the solid solution

(Nd2_xCex)CuO4,

which extends

from x = 0 to 0.2. Three sets of solid solution tie-lines were observed that

connected the (Ce, Nd)Ox solid solution, Nd203, and CuO to (Ndz_~Ce~)CuO 4.

Superconductivity was only observed in the range of solid solution of

(Nd2_xCex)CuO4,

with 0.14 < x < 0.18. The absence of superconductivity from

D. Representative Phase Diagrams 675

Fig.

14.30.

TIQI.5

TI2Ba04

TI4Ba401o,~

TI2Ba205

%

zC

600oC

_ P02 =

105 Pa

e

V __

0 20

BaO

Tet-TI2Bo2CuOs

V ~, V V ........ -'~

40 Ba(~u02 60 80 100

Mol % CuO

Phase diagram showing the subsolidus relationships at 600~ and 105 Pa 02 of the system of

BaO-CuO-TI203. Ortho = orthorhombic; Tet = tetragonal (Jondo

et al.,

1993).

the greater part of the diagram is a significant feature of this system, which is a

direct result of the narrow composition range in Nd2_xCexCuO 4 that is super-

conducting.

The dependence of the degrees of oxygen nonstoichiometry in

Ndl.85Ce0.15CuO4_ x on temperature and oxygen pressure have been reported in

the range of temperature 350-950~ and of oxygen pressure 10 .5 to 1 atm (Kim

and Gaskell, 1993). The stability field (Fig. 14.35) of Ndl.85Ce0.15CuO4_ x

contains subfields with and without superconducting properties. The two

subfields meet at the Cu20-CuO transition line (dashed line). The variance of

the value x within the Ndl.85Ce0.15CuO4_ x stability field is between 0.0 and

0.045. This is a very small value in comparison with the variance of x = 0.0-1.0

that occurs in the Ba2YCu306+ x compound. The maximum value of (4- x) in

Ndl.85Ce0.lsCUO4_ x was determined to be 3.995. Ndl.85Ce0.15CuO4_ x melts

incongruently above 1150~ and decomposes to Nd203 -s NdCeO3. 5 -k-Cu20.

The value of T c in the superconducting phase increases from 5 to 24 K as x

increases from 0.008 to 0.05.

Poole Ch.P., Jr. Handbook of Superconductivity

Подождите немного. Документ загружается.