Chemistry of High-Temperature Superconductors II - American

Chapter 22

Chemical

Compatibility

Superconductors

with

of

High-Temperature

Other

Materials 2,3

R. Stanley Williams1,2 and Shiladitya Chaudhury

Downloaded by UNIV LAVAL on September 19, 2015 | http://pubs.acs.org Publication Date: September 26, 1988 | doi: 10.1021/bk-1988-0377.ch022

1

Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90024-1569 Solid State Science Center, University of California, Los Angeles, CA 90024-1569 Department of Physics, University of California, Los Angeles, CA 90024-1569 2

3

In any application, the new oxide superconductors will have to coexist in intimate contact with other materials. However, the copper-oxygen bonds common to all presently known oxide superconductors with T above 40K are relatively weak. This means that elements that form much stronger bonds with oxygen, such as Si, will be chemically unstable when in contact with the copper-oxide super conductors. In this paper, we will establish procedures and guidelines that will enable workers to choose materials that will not react chemically with the oxide super-conductors. These should enable researchers working in all applications areas to avoid unnecessary empirical searches for suitable substrate or host materials by eliminating the most thermodynamically unfavorable choices. In particular, we have determined which of the elemental metals should be most stable in contact with the copper-oxide semi-conductors, and examined schemes for integrating these superconductors into Si devices. c

There has been a great deal of research over the past year on the new oxide superconductors. Justifiably, most of that work has concentrated on the preparation of materials with optimized properties; i.e., maximized T and critical currents. This is the proper time to think in broad terms about how to successfully encorporate the new superconductors into structures that will serve useful purposes. Particular attention must be paid to the processing of these structures, especially if any of the steps involve sintering or a .mealing steps at reasonably high temperatures (i.e., 700K or above). 0097-6156/88/0377-0291$06.00/0 1988 American Chemical Society c

β

In Chemistry of High-Temperature Superconductors II; Nelson, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


292

CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS Π

The superconducting 1-2-3 phases are k n o w n to be chemically sensitive, and i n fact are thought to be metastable compounds under all conditions of temperature and oxygen partial pressure (1). Thus, it is imperative that any material i n intimate contact with 1-2-3 phases does not react with the component oxides to form more stable compounds, since such a reaction w i l l destroy the superconducting material. This is especially important for thin film applications, since the amount of superconductor is small compared to that of the substrate, and the diffusion path for potential solid state reactions, i.e., the film thickness, is very short. In this paper we w i l l concentrate on searching for materials that should be most stable i n contact with the 1-2-3 superconductors, since they appear to be more sensitive to chemical disruption than the more recently discovered B i - and Tl-based phases. The principles and much of the data presented here can be applied to any oxide superconductor, whether it is presently known or yet to be discovered. The reason for the chemical vulnerability of phases based on copper oxides is shown i n F i g . 1, w h i c h is a display of the heat of formation per gram-atom of the most stable solid oxides formed by the metallic and semiconducting elements i n the periodic table. To generate this table, compilations of thermochemical data were consulted (2,3), and these sources p r o v i d e d the enthalpies of formation used for the calculations presented later. The reason for expressing the data i n the manner shown i n F i g . l is to get an idea of the relative strengths of the M - O bonds that form the solid oxides. One can see immediately that very few elements form weaker bonds with Ο than does C u . Thus, most elements w i l l reduce C u O to elemental C u , and are very likely to react chemically w i t h any of the copper-oxide superconductors as w e l l . A l t h o u g h a positive heat of reaction, determined for interactions of the 1-2-3 superconductors with another material, is not a sufficient condition for chemical stability, it is almost certainly necessary. Consequently, researchers interested i n the processing of copper-oxide superconductors into structures should consider thermochemical properties carefully. In this investigation, we have collected the relevant heats of formation and phase diagrams to analyze the stability of 1-2-3 compounds w i t h respect to the solid elements and several oxides. To check that the thermochemical predictions are indeed correct, we have made use of reports of chemical reactivity i n the literature, and have also performed simple experiments to determine whether or not 1-2-3 w i l l react w i t h another material and identify the reaction products w h e n it does. The next section describes the experimental procedure followed i n this study; the results and conclusions to be drawn from this paper are presented i n the succeeding sections. Experimental Procedure The reactivities of various materials with respect to Y B a C u 0 7 _ w e r e determined by heating mixtures of carefully weighed powders sealed i n 2

3


x


HA Be

A H f (heat of formation) in kcal/gm atom for stable solid oxides Al

-60.8

B

ΠΓΒ

Si

c P

N S

O

VIDA

Fr 7

— ?

Po 7

Rn

Xe

I At

Kr

Ar

Br

Cl

f /

Er Tm Yb Lu Dy Ho Tb Gd Sm Eu -87.6 -82.5 -87.1 -87.3 -89.1 -89.9 -90.9 -89.9 -86.7 -89.9 No Lr Fm Md Es Cf Am Cm Bk Pu 7 7 7 7 7 7 7 7 7 7

No relevant oxide Noreliabledata

Ce Pr Nd Pm -87.1 -87.2 -86.4 ? Np Th Pa U 7 -97.7 -97.5 ?

Tl Pb Bi Au Hg >0 -10.9 -18.6 -26.2 -27.3

IB

Nie

He

vnjB

Figure 1. Periodic table of the elements showing the heat of formation (in kcal/gram-atom) of the most stable solid oxide of each element. The more negative the value of A H the more strongly the element bonds to oxygen. (Data were taken from Refs. 2 and 3.)

Ac Ra -62.5 7

Ir Pt Os Re Hf Ta Ba La Cs W -27.0 •61.1 •85.7 -88.7 -69.8 -50.3 -33.3 -23.5 -19.3 -5.6

IVA VA VIA VTIA

F

IVB VB VD3 VTIB

ΠΒ -80.2 -72.5 -50.9 -27.1 Ga Se As Ge Zn Ni Cu Κ Ca Mn Fe Co V Cr Τι Se -28.9 -75.8 -91.1 -72.7 -58.2 -54.0 -47.3 -39.2 -30.9 -28.8 •18.6 -41.9 -51.7 -46.2 -31.2 -18.0 Te In Sn Sb Cd Ag Rh Pd Ru Zr Nb Mo Tc Sr Y Rb -26.3 -70.7 -91.1 -87.7 -64.9 -46.8 -34.5 -24.3 -18.3 -13.4 -2.4 -31.0 -44.2 -46.2 -34.4 -25.8

-47.5 -72.7 Na Mg -33.1 -72.8 ΠΙΑ

Li

ΙΑ Η




294

quartz ampoules. The 1-2-3 material, prepared by the citrate precipitation process (4), was obtained from the research group of Prof. Richard Kaner at U C L A . The other materials were reagent grade chemicals obtained from our stockroom. Small amounts of the m i x e d powders were reserved for x-ray powder diffraction analysis so that the phases present before and after heating could be compared. The ampoules were placed i n a furnace at 1073K for four hours to initiate any reactions. The intent here was not to determine the equilibrium products of the multi-element systems, but just to see if indeed a reaction d i d occur. Both heated and unheated mixtures, as w e l l as control samples that were single compounds sealed i n ampoules and heated, were examined by x-ray powder diffractometry. The diffractometer was computer controlled, and data collection times as long as 12 hours were used to obtain a reasonable signal-to-noise ratio. These studies showed that the 1-2-3 material by itself d i d not react with the quartz tubes during the heating cycle, but mixtures of 1-2-3 with other materials often d i d attack the quartz. Chemical Stability of 1-2-3 with Respect to Elements There are only nine elemental metals that w i l l not reduce C u O : R u , R h , P d , A g , Os, Ir, Pt, A u , and H g . These are grouped with a bold border i n the periodic table of Fig. 1. N o solid oxide of A u is stable, and the oxides of P d , A g , Pt, and H g are just barely stable. The chemical formula of the oxides of R u , Os, and Ir is M 0 , so two equivalents of C u O w o u l d be required to oxidize one equivalent of these metals by the reaction M + 2 C u O — > M 0 + 2 C u . For R h , the oxide is R h 0 , so three equivalents of C u O w o u l d be required to produce one equivalent of R h 0 . Thus, C u O is also stable with respect to these last four metals because the heat of formation of two (or three) equivalents of C u O is larger i n magnitude than a single equivalent of the oxides of R u , Os, and Ir (or Rh). Therefore, these nine metals are the only elements that may not react c h e m i c a l l y w i t h 1-2-3 superconductors, a l t h o u g h each individual system should be checked to ensure that they are indeed stable. We can actually use the reactivity or lack thereof of the 1-2-3 com p o u n d s w i t h different elements to determine r o u g h l y the heat of formation of the superconductor. For the purposes of this paper, we w i l l assume that 1-2-3 is a stable compound and that it has the ideal formula Y B a C u 0 . Thus, the reaction 2

2

2

2

3

3

2

3

6 > 5

1 / 2 Y 0 + 2BaO + 3CuO - > Y B a C u 0 2

3

2

3

(1)

6 5

has an u n k n o w n but negative heat of reaction, A H , or the heat of formation w i t h respect to the component oxides. W e can place a lower limit o n this value by considering a reaction between the 1-2-3 material and an element that is just barely above the threshold for reaction w i t h C u O . The most suitable element for this purpose is T l , since f

2T1 + CuO - > T1 0 + Cu ; 2

ΔΗι = - 2.9(± 2.6) kcal


(2)

22. WILLIAMS & CHAUDHURY

Chemical Compatibility with Other Materials 295

However, because of our lack of facilities to handle toxic materials, we chose instead to try the next best choice, which is Pb: Pb + CuO _> PbO + Cu ;

ΔΗ = - 15.3 (± 1.0) kcal

(3)

2

If we consider the idealized reaction between Pb and 1-2-3, 3Pb + Y B a C u 0 2

3

1

->

6 < 5

/2Y °3 +

2

B

a

2

3

° +

P

b

°

+ 3

C

u

\

4

^3=?

()

and this reaction is exothermic, then we can obtain limits for A H as follows: Downloaded by UNIV LAVAL on September 19, 2015 | http://pubs.acs.org Publication Date: September 26, 1988 | doi: 10.1021/bk-1988-0377.ch022

f

0 > A H = 3ΔΗ - Δ Η > 3ΔΗ = - 46 kcal f

2

3

(5)

2

where the second inequality holds because minus Δ Η is a positive number. We have carried out the reaction indicated by Eq.(4), and we do observe the appearance of elemental C u i n the reaction products. A l t h o u g h the uncertainty i n the estimate that we obtain appears to be large, the limits that we place on A H d o allow us to make some quantitative predictions w i t h respect to the reactions of 1-2-3 w i t h other elements and compounds. If we predict that 1-2-3 w i l l react based on the above lower limit for Δ Η , we can be very confident i n our prediction since A H m a y even be slightly positive (1). 3

f

Γ

f

Chemical Stability of 1-2-3 with Respect to Si and Insulators For many applications, a noble or near-noble metal cladding for copperoxide superconductors may be perfectly acceptable. However, one of the primary uses envisioned for high-T superconductors is as a conductor on semiconductor devices. It is obvious that 1-2-3 w i l l not be stable as a thin film on a bare Si substrate, since c

Si + 2CuO _> S i 0 + 2Cu 2

; Δ Η = - 143 kcal

(6)

4

Thus, one may predict 3/2Si + Y B a C u 0 2

3

6 5

—> 1/2Y2°3 +

2

B

a

°

+

3/2Si0 + 3Cu 2

(7)

where - 97 kcal > Δ Η > - 1 4 3 kcal 5

(8)

In fact, mixing Si and 1-2-3 i n the proportions shown i n Eq.(7) and heating immediately yields a b r o w n powder, w h i c h contains elemental C u and various silicates.


296


A n obvious idea is to use a layer of inert insulating material between the Si and the superconductor, such as S i 0 . However, here we r i m into the problem of silicate formation v i a the following reaction: 2

S i 0 + 2BaO _> B a S i 0 2

2

;

4

Δ Η = - 64.5 kcal

(9)

6

w h i c h is more exothermic than the estimate of Eq.(5) for the heat of formation of 1-2-3 from its component oxides. A stabilizing layer of aluminum oxide does not appear to be much more useful, since 2 A 1 0 + 2BaO _> 2 B a A l 0 Downloaded by UNIV LAVAL on September 19, 2015 | http://pubs.acs.org Publication Date: September 26, 1988 | doi: 10.1021/bk-1988-0377.ch022

2

3

2

4

;

Δ Η = - 48.0 kcal

(10)

7

w h i c h means that 1-2-3 probably reacts with A 1 0 as well. Thus, any processing procedure that w i l l require a high temperature annealing of the superconductor after it has been deposited onto the S i wafer w i l l require careful thought to design a suitable buffer layer. Another possibility that comes to m i n d is to use Y 0 . However, this is also not a reasonable choice. Examination of the quasi-ternary phase diagram i n Fig.2 that has been generated for the C u O - B a O - Y 0 system (5) reveals that there is no tie-line connecting 1-2-3 w i t h Υ2 Ba(OH) ; ΔΗ = - 35.4 kcal

(11)

BaO + C0 _> BaC0

(12)

2

2

2

3

8

; ΔΗ9 = - 64.4 kcal

Thus, we see that the reactivity of BaO with other oxides is a major factor in the chemical sensitivity of 1-2-3. The high-T superconductors that do not contain BaO should be significantly more stable than 1-2-3, and most likely much easier to process in the presence of other materials. c

Acknowledgments We thank B. Dunn and R.B. Kaner for their advice and help. This work was supported in part by the Office of Naval Research. Literature Cited 1. Sleight, A.W. In Chemistry of High-Temperature Superconductors; Nelson, D.L.; Whittingham, M.S.; George, T.F., Eds.; ACS Symposium Series No. 351; American Chemical Society: Washington, DC, 1987; pp2-12.


302

CHEMISTRY OF HIGH-TEMPERATURE SUPERCONDUCTORS U


2. Kubaschewski, O; Alcock, C.G. Metallurgical Thermochemistry; Pergamon: New York, 1979; pp 267-323. 3. Wagman, D.D.; Evans, W.H.; Parker, V.B.; Schumm, R.H.; Halow, I.; Bailey, S.M.; Churney, K.L.; Nuttall, R.L. J. Phys. and Chem. Data 1982, 11, Supp. 2. 4. Chu,C.;Dunn, Β. J. Am. Ceram. Soc. 1987, 70, C-375. 5. Frase, K.G.; Clarke, D.R. Advan. Cer. Mat. 1987, 2, No.3B, 295. RECEIVED July 8, 1988


Chemistry of High-Temperature Superconductors II - American

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