Synthetic Metals, 46 (1992) 71-77 71

Composites of polyaniline/superconductor YBa2Cu3OT_~: production and properties

E. N. Izakovich , V. M. Geskin* and S. V. S t epanov USSR Academy of Sciences Institute of Chemical Physics, Chernogolovka 142 432 (USSR)

(Received July 15, 1991; accepted July 17, 1991)

A b s t r a c t

Electrochemical behaviour of a high-temperature superconductor, YBaeCu3OT~, in the normal state in aqueous sulfuric, phosphoric, hydrofluoric and oxalic acids alone and in the presence of aniline has been studied. Polymerization of aniline yielding conducting polyaniline takes place on the surface of YBa2Cu~OT.8 in sulfuric media at open circuit and in hydrofluoric solution and its mixture with oxalic acid upon anodic polymerization, up to 80% superconducting phase being retained. Polyaniline is electroactive and exhibits protective properties.

I n t r o d u c t i o n

The s tudy and p r o d u c t i o n of ma te r i a l s c o m p o s e d of two p romis ing c o m p o n e n t s , h i g h - t e m p e r a t u r e s u p e r c o n d u c t o r s (HTSC) and e lec t rocon- duc t ing p o l y m e r s (EP) , and coa t ings on the i r ba se is o f a ce r ta in in teres t nowadays . E P conduc t iv i ty m a y be cont ro l lab ly c h a n g e d b y dop ing within a wide r a n g e of va lues cha rac te r i s t i c o f insu la tors ( ~ 1 0 - 8 - 1 0 -5 o h m -1 c m - ' ) and re l iable c o n d u c t o r s (101-105 o h m - ' c m - i) [ 1 ]. Such mate r ia l s could be u s e d as p r o t e c t i v e coa t ings of conduc t ing or insulat ing proper t i es . It should be n o t e d t ha t EP m a y be p r o d u c e d e l ec t rochemica l ly in an anodic (ox ida t ive) p r o c e s s r a t he r t han in a ca thod ic ( r educ t ion) p r o c e s s less f avourab le for HTSC, wh ich is e m p l o y e d for me ta l e lec t rodepos i t ion .

The YBa2Cu30~_~ coa t ing (YBCO) b y po lypyr ro l e was p e r f o r m e d in elec- t r o c h e m i c a l po lymer i za t i on in ap ro t i c m e d i a [2, 3]. The p o l y m e r film is o f sufficient e lec t roac t iv i ty and adhes ion . EP polyani l ine (PAn) is no t f o r m e d f r o m anil ine e i ther on YBCO [4] or on Pt [5] u n d e r s imilar condi t ions . The p r e s e n t p a p e r r e p o r t s on the p r o d u c t i o n and p r o p e r t i e s of the PAn/YBCO c o m p o s i t e .

PAn p r o d u c t i o n is k n o w n to r equ i re (see, e.g., ref. 6) the p r e s e n c e of a p ro t ic acid. Howeve r , m a n y ac ids a re cha rac te r i zed by high cor ros ive act ivi ty t o w a r d s YBCO, in par t icu lar , a q u e o u s so lu t ions of hydroch lo r i c [ 7 - 1 0 ],

*Author to whom correspondence should be addressed.

Elsevier Sequoia. All rights reserved

72

sulfuric [ 11 ], and trifluoroacetic [ 12 ] acids. Passivation may be expected to proceed only in the case when a metal of HTSC composition forms a weakly soluble salt with an acid anion. Such an effect was observed in vapour [13, 14] and solutions of HF (1% ethanolic [15], conc. aqueous [13], in aqueous solutions of oxalic and even sulfuric (1%) acids [16].

On the basis of these data we undertook an investigation of YBCO behaviour and the possibility of producing its composite with PAn in elec- trochemical polymerization of aniline in aqueous solutions of sulfuric, phos- phoric, hydrofluoric and oxalic acids. They all have anions that are difficult to oxidize (an obligatory requirement for an electrolyte of anodic synthesis) and form weakly soluble salts with some or all metals of YBCO composition, namely, barium sulfate, yttrium phosphate, barium and yttrium fluorides, barium, yttrium or copper oxalates.

Experimental

Rod-like YBCO compacts ( ~ 1 X 5 × 10 mm 3) obtained by a standard method [17] and flat samples ( ~ 1 × 5 mm 2) of ground tops of rods soaked with lacquer or melted paraffin were studied. A copper wire welded by a Wood alloy was a conducting substrate. The place of contact was also isolated from the solution by lacquer film or melted paraffin. PAn was deposited onto the electrodes electrochemically (ePAn) or as the solution of a soluble fraction of chemically synthesized PAn base in N,N-dimethylformamide (sPAn) [18]. Electrochemical procedures were carried out in an undivided three-electrode cell with the aid of a PI-50.1 potentiostat (USSR) with a PR-8 programming device. All the potentials cited here are against Ag/AgC1 reference electrode. Platinum foil or SnO2-covered glass were used as auxiliary electrodes.

The presence of copper in solution was qualitatively detected from the characteristic cyclic voltammogram of its cathodic deposition and further anodic dissolution on the Pt wire electrode.

Superconducting propert ies of the synthesized samples were evaluated from the temperature dependence of diamagnetic screening down to 77 K.

The content of a superconducting (SC) phase in the sample was monitored by means of a highly sensitive method enabling the detection of 1 0 - 6 - 1 0 - ~ g of the SC phase in the 1 mm s sample. The method is based on the measurements of non-resonance absorption of microwave energy by a su- perconductor placed into the cavity of the ESR spectrometer and cooled below its T¢ [19]. The samples were investigated in an ESR-300 spect rometer (Bruker) equipped with a thermostat, which allowed temperature variations of the sample within the range 4 - 3 0 0 K.

The data obtained from the sample modification were compared with those on the initial superconductor . The percentage of SC phase in the modified sample was evaluated from the signal drop in zero magnetic field.

73

R e s u l t s and d i s c u s s i o n

YBCO b e h a v i o u r at o p e n c i rcu i t In solutions of sulfuric and phospha t ic acids, YBCO potentials remain

high at open circuit (curves 1, 2 in Fig. 1) above the t he rmodynamic potential of water decompos i t ion , which p rovokes the format ion of gas bubbles on the surface of the sample, its bleaching, fast falling and comple te decom- posit ion.

In the p resence of aniline in solut ions of these acids, PAn format ion takes place on YBCO at open circuit at the expense of its own oxidat ion potential, which decreases after the po lymer-coa t ing format ion down to values unfavourable for water decompos i t ion (curves 5 - 7 ) and bubble format ion, sample decompos i t ion no t being observed for at least an hour. The t ime of protec t ive-coat ing format ion depends on the aniline concent ra t ion ( compare curves 6 and 7). Thus, some passivat ion of YBCO proceeds in the aniline- conta in ing solut ion of these acids after the PAn layer formation. However, the p rocess of not iceable cor ros ion is observed before the format ion of a sufficiently thick PAn layer. In solut ions of hydrofluoric and oxalic acids the YBCO potent ial is seen to drop fast (curves 8, 9), therefore there is no gas

ElY 4,t

l I

l'.. U l-

Ue . . . . . . 8fO

T~.me//#

Fig. 1. Open circuit potentials of YBCO in aqueous solutions: 1, 1 M HaPO4; 2, 1 M H2SO4; 3, 2 M HF; 4, 0.5 M H2C204; 5, 0.2 M aniline+ 1 M H3PO4; 6, 0.1 M aniline+ 1 M H2SO4; 7, 0.4 M aniline + 1 M H2SO4; 8, 0.2 M aniline + 2 M HF; 9, aniline (saturated) + 0.5 M H2C204.

74

evolu t ion and s a m p l e decompos i t i on does not p r o c e e d even a f te r con t inuous s torage . Never the less , the sur face is gradual ly cove red with a l ight-green film. PAn is sca rce ly fo rm ed in solut ions of these ac ids in the p r e s e n c e of anil ine at o p e n circuit. Apparent ly , the film of a weak ly soluble salt f o rmed d e c r e a s e s the sur face potent ia l and p r even t s anil ine oxidat ion.

In con t r a s t to solut ions of the oxalic acid, Cu ions are de tec ted in those of the hydrof luor ic acid in bo th the p r e s e n c e and a b s e n c e of aniline, which indica tes par t ia l decom pos i t i on of the e lec t rode . This m a y be exp la ined by c o p p e r f luoride solubility; however , c o p p e r is likely to be washed off f r o m the under - su r face layer only.

Polarization behaviour of YBCO and PAn electrodeposition As seen f r o m Fig. 2(a), YBCO behav iou r in solu t ion of hydrof luor ic acid

a p p e a r s to be like tha t of an inert e lec t rode within a wide r ange of potent ia ls , which al lows the s tudy of the possibi l i ty of anil ine e l ec t rochemica l poly- mer iza t ion to p r o c e e d on its surface . However , po lar iza t ion be low - 0 . 1 V is no t desirable , s ince the ca thodic cu r ren t inc reases sharp ly resul t ing in an i r revers ib le p e a k in the subsequen t anodic cycle, a t t r ibu ted to a soluble subs tance , s ince it dec reases wi thou t fu r the r ca thodic polymer iza t ion .

The t ime of HTSC p r e s e n c e in solut ion should be minimized dur ing PAn e lec t rosyn thes i s on YBCO. For this p u r p o s e high concen t r a t i ons of aniline were ma in ta ined to p r o m o t e the p rocess . The 0.2 M aniline + 2 M HF solut ion was found to be a favourab le m e d i u m for PAn deposi t ion , which was seen f r o m the a p p e a r a n c e and increase in the charac te r i s t i c p e a k s in the p r o c e s s of cycl izat ion (Fig. 2(b)) . On fur ther increase in anil ine concen t ra t ion (0.4 M anil ine + 2 M HF) (AF) the solut ion a round the e lec t rode b e c a m e yel low b e c a u s e of the o l igomer iza t ion p rocess . A dec rea se in acid concen t ra t ion (0.2 M aniline + 1M HF) l~d to fas t deve lop ing of e l ec t rosyn thes i s inhibition, p r o b a b l y caused by low conduct iv i ty of PAn f o r m e d at p r o t o n deficiency.

p e a k b A

s l i

/ , ! i

i ' #

1

(a) (b)

Fig. 2. Cyclic voltammograms of the YBCO electrode in 2 M HF at 100 mV s-l: (a) background; Co) containing 0.2 M aniline ( ), background after electrodeposition of PAn ( - - - , . . . . . ). The numbers on the curves are the potential cycle numbers.

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Copper ions were shown to be present in HF solution, though they were not detected in the oxalic acid medium and aniline oxidation did not proceed on YBCO, PAn formation being observed on the PT electrode under the same conditions. Thus, the YBCO electrode may be supposed to be blocked by oxalates formed on its surface. Addition of oxalic acid to hydrofluoric acid medium decreases copper dissolution, thus preventing PAn electrosynthesis blockage. PAn was electrodeposited from the 0.2 M aniline+ 2 M HF+ 0.25 M H2C204 solution (AFO). Electrochemical behaviour of YBCO in AF and AFD media is similar.

In the galvanostatic regime of synthesis, the potential passed its maximum and became constant at about 0.7 V. Such behaviour is typical and is explained by the presence of an inductive period, within which aniline oxidation on the non-occupied surface proceeds at overvoltage and further autocatalytic growth of PAn on the polymer layer formed [20l.

Electrochemical behaviour of YBCO in sulfuric and phosphoric acids appears to be of no interest, since the samples readily decompose in these media.

Elec t rochemica l behav iour o f PAn /YBCO Two anodic peaks (a and b) and corresponding cathodic peaks may be

seen on the ePAn/YBCO cyclic voltammograms (Fig. 2(b), at 0.4-0.45 and 0.6-0.7 V, respectively). An increase in scan rate results in the anodic peaks shifting to a more positive region and the cathodic peaks becoming more negative, which is characteristic of the systems with ohmic loss, all peaks being less pronounced. Peak b only is seen on the sPAn/YBCO cyclic voltammograms (Fig. 3).

It is of interest to compare the behaviour of PAn/YBCO and that of PAn on other electrodes. The cyclic voltammograms of ePAn/Pt in hydrofluoric medium are common (Fig. 4): peaks of 1 and 3 pairs belong to PAn redox processes and those of 2 to the products of its degradation because of splitting and/or cross-linkage of the polymer chains [20]. On the sPAn/Pt cyclic voltammograms (Fig. 5) pair 2 is absent in the first cycle since neither low-molecular admixtures (they are washed off from the polymer before dissolution) nor a cross-linked polymer (it is unsoluble) are contained in this coating.

When an ohmic loss is considerable (a thick layer of ePAn/Ti (Fig. 6) [21D, anodic peak 1 is shifted far into the anodic region.

The comparison shows that peak a lying in the region of pair 2 on the ePAn/YBCO cyclic voltammogram, and absent in the case of sPAnPYBCO, may also be assigned to the by-product conversions. The current absence in the region of peak 1 (common for ePAn/YBCO, sPAn/YBCO and ePAn/ Ti) is probably caused by a considerable ohmic loss resulting in the shift of the corresponding oxidation process to the region of peak b, whereas in the case of YBCO it probably corresponds to the superpositions of peaks 1 and 3. The latter fact is confirmed by a sharpening of the reduction peaks 3 and b at the transition from aniline-containing solution to a pure acidic

76

~'¢~ " 0 . / / /

2,:J] Fig. 3. Cyclic voltammogram of the YBCO electrode coated with soluble PAn in 2 M HF at 100 mV s -~.

Fig. 4. Cyclic voltammograms of the Pt electrode in 2 M HF at 100 mV s -1 containing 0.2 M. aniline ( ), background after electrodeposition of PAn ( - - - , . . . . . ). The numbers on the curves axe the potential cycle numbers.

j i . ~ , ,

Fig. 5. Cyclic voltammogram of soluble PAn coated onto SnO2 glass in 1 M H2SO 4 at 10 mV S - 1 .

Fig. 6. Cyclic voltammogram of thick PAn electrocoated onto titanium at 100 mV s -1.

m e d i u m . T h e s i m i l a r i t y o f t h e P A n / Y B C O a n d P A n / P t b e h a v i o u r m a y be a l s o

o b s e r v e d in c y c l i c v o l t a m m e t r y o f t h e i r d e g r a d a t i o n a t s c a n n i n g u p t o h i g h

a n o d i c p o t e n t i a l s (F igs . 2 ( b ) a n d 4).

Thus , a t a n o d i c p o l a r i z a t i o n o f YBCO in an a q u e o u s a n i l i n e - c o n t a i n i n g

s o l u t i o n o f h y d r o f l u o r i c a c i d a n d a t t h e a d d i t i o n o f o x a l i c ac id , PAn f o r m e d

77

is a n a l o g o u s in its e l e c t r o c h e m i c a l p r o p e r t i e s to t h o s e o b t a i n e d on o t h e r

e l e c t r o d e s .

S u p e r c o n d u c t i v i t y o f Y B C O a f t e r P A n e l e c t r o d e p o s i t i o n All t h e s a m p l e s u n d e r s t u d y w e r e s u p e r c o n d u c t o r s (To = 9 4 - 9 3 K), t h e

SC p h a s e lo s s n o t e x c e e d i n g 2 0 % in t h e s a m p l e s w i t h PAn d e p o s i t e d f r o m

A F a n d A F O .

X-ray ana ly s i s s h o w e d a s m a l l i n c r e a s e in t h e g e n e r a l b a c k g r o u n d o f

t h e d i f f r a c t o g r a m s ( ~ 5 % ) w i t h i n t h e a n g l e r a n g e 2 0 = 1 4 - 7 6 °, w h i c h c a n be

a s s i g n e d to an i l i ne p r e s e n c e o n t h e s u p e r c o n d u c t o r s u r f a c e [22] .

T h e p a r a m e t e r s o f t h e SP p h a s e do n o t c h a n g e o n PAn d e p o s i t i o n , t h e

d i s a p p e a r a n c e o f al l t h e i m p u r i t i e s in t he s u p e r c o n d u c t o r b e i n g o b s e r v e d

b e f o r e PAn d e p o s i t i o n .

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