Transport critical current in electron and ion irradiated sintered samples of YBa2Cu3O7

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Transport critical current in electron and ion irradiated sintered samples of YBa2Cu3O7

F. Rullier-Albenque, J. Ardonceau, R. Kormann

To cite this version:

F. Rullier-Albenque, J. Ardonceau, R. Kormann. Transport critical current in electron and ion irradi- ated sintered samples of YBa2Cu3O7. Journal de Physique I, EDP Sciences, 1991, 1 (3), pp.395-402.

�10.1051/jp1:1991141�. �jpa-00246339�

Classification

Physics

Abstracts

74.70V 74.60J 61.80

Transport critical current in electron and ion irradiated

sintered samples of YBa~CU~O~

F.

Rullier-Albenque (I),

J. Ardonceau

(I),

R. Kormann (~)

(')

Laboratoire des Solides Irradids, CEREM, Ecole

Polytechnique,

91128 Palaiseau Cedex, France

(~) ^LCR Thomson, Domaine de Corbeville, 91404

Orsay

Cedex, France

(Received

21

September

1990,

accepted

in

final form

5 December

1990)

Rksumk. ~ Des dchantillons

polycristallins

de

YBa2CU~O~

ont dtd irradids _par des Electrons de 2,5 MeV h basse

tempdrature

et par des ions

oxygdne

I

tempdrature

ordinaire. Los Evolutions de la

tempdrature critique

_{T~, de la} rdsistivit6 et du courant

critique

de transport j~ ont dtd mesurdes

en fonction de la fluence des

particules

et, dans le cas de l'irradiation _par des Electrons, des

tempdratures

de recuit

jusqu'i

300 K. Dans _ces deux irradiations, nous montrons que la vitesse de diminution de 7~ ^{est rel16e} aux nombres d'atomes

ddplacbs

_par chocs dlastiques. Nous mettons en Evidence des conditions

expdrimentales

pour

lesquelles

j~ augrnente respectivement de 5 9b et 16 9b pour les irradiations _aux 61ectrons et aux ions. Dans _ce demier _cas,

l'augrnentation

de j~ est

accompagnde

d'une rdduction de la rdsistivitd de l'dchantillon. Nous montrons que la

prdsence

des

joints

de

grains joue

_un rble dans _ces

phdnomdnes qui

sont dus I la

migration

de ddfauts crdds _sous irradiation. Tandis que

l'augmentation

de rdsistance rdsulte de l'endolnrnage-

ment des

propridtds intragranulaires

dans le _cas des irradiations _aux Electrons, la modification des

joints

de

grains

semble contribuer I l'augrnentation de la rdsistance

aprds

l'irradiation _aux ions O de 155 MeV.

Abstract.

Polycrystalline samples

of YBa~CU~O~ were irradiated by 2.5 MeV electrons at low temperature and

by

155 MeV oxygen ions at room temperature. The evolutions of critical temperature _T~,

resistivity

and transport critical current j~ were followed _{as a} function of

particle

fluences and, in the _case of electron

irradiation, annealing

temperatures. ^{It is} shown that decrease

rates of 2~ can be well described in terms of the number of atomic

displacements

in these two

irradiations. We find experimental conditions which lead to an increase of

j~

by

respectively

5 fb and 16 9b for electron and O irradiation. In the latter _case, the increase ofj~ is

accompanied

by a

reduction in the

resistivity

of the

sample.

We show that the _presence of

grain

boundaries

plays

a

role in these

phenomena

which _are due to mobile irradiation-induced defects. Whereas the resistance increase is found _to be

only

due _to the

damage

of

intragrain properties

for electron irradiation, modification of

grain

boundaries _seems to contribute to the resistance increase after 155 MeV O irradiation.

Introducfion.

In bulk sintered

samples

of

high

7~

YBa2CU~O~,

the

transport

^critical current

density

j~

is determined

by

the presence of weak finks at

grain

boundaries. Most

analysis _suggest

that

396 JOURNAL DE PHYSIQUE I bt 3

the low values of

j~ (<

1000

A/cm~

at 77

K)

_are due to weak

intergranular superconductive coupling

between the

grains resulting

from their random arrangement and

possibly

from the

extrinsic

properties

of the

grain

boundaries

[1, 2].

Another alternative is to _assume that

pinning

forces

acting

_on

Josephson

vortices _are weak

[3].

These

intergranular pinning

forces

are

thought

to

depend

_on the

degree

of disorder in the

coupling strength

between the

grains.

Irradiation induced defects _may act as additional

pinning

centers. An enhancement of the

intragrain

critical current was shown in irradiated

single crystals

of

YBa2Cu~07 [4-6].

However it has been found that irradiation

damages

the weak links and _so decreases the transport critical _current

through

the defects introduced at the

intergrain regions _{[7, 8].}

In this _{paper we}

investigated

the influence of _very low concentrations of irradiation induced defects _on the

transport

^critical current

density

of ceramic

samples

of

YBa2Cu307.

Two

different types ^of irradiations _were used 2.5 MeV electron irradiation at low temperature

(20 K)

followed

by annealing

_up to room temperature ^{and 155 MeV} oxygen irradiation carried out at _room

temperature.

^In both

irradiations,

_we found

experimental

conditions for which

j~

^{increases. It} appears that this increase is due _to mobile irradiation defects.

Moreover the evolution of the critical current under irradiation allows _us to

give

_some

information about the modifications of

grain

boundaries in irradiated

polycrystalline samples.

Previous results of 2.5MeV electron irradiation [9] ^{have shown that the decrease of}

T~

compared

to the relative increase of resistance is the _same in

poly-

and

single crystals

of

YBa~CU~O~, suggesting

that the variation of resistance in

polycrystals

_comes

mostly

from the

damage

of the

superconducting grains.

^Results

presented

in this paper establish that this conclusion is not valid for 155 MeV O ion irradiation.

Experiments.

SAMPLES. The YBaCUO precursor was

provided by

Criceram

(I).

It is _a blend of barium

carbonate,

_copper and

yttrium

oxides

synthesized using

oxalate route.

This

product

_was calcined at 900 °C for 15 h. After

deagglomeration

in mortar and

pestle,

the

powder

_was attrition milled for half an hour in

anhydrous ethyl

alcohol

using

zirconia balls. The calcined

powder

_was

shaped

^{into thin sheets}

using

_{a tape}

casting technique.

For

this,

the

powder

_was mixed with _a binder formulation

consisting

of

solvents,

binders and

dispersant.

The

organic

additives _were

carefully

chosen _to avoid any

powder decomposition

and

pollution during

the thermal _{treatment 11}

6, 17].

The

casting

conditions

(like

doctor blade

height)

_were determined in order to

get

^sintered

tapes

^{around 30} ~m.

The

temperature-time-oxygen partial

_pressure

procedure

_was chosen to avoid _any

powder decomposition

slow temperature ^rise

(I%mn)

ⁱⁿ low oxygen

partial

_pressure, decarbonation at 875 °C for 10

h, sintering

at 940 °C for 16 h. After

sintering

the

samples

_were

slowly

cooled

down to room

temperature (I%mn)

in pure oxygen with _a

step

at 500 °C for 4 h.

Depending

_on the

samples,

_7~ values at 90 fb of the

superconducting

transition _were located between 90.4 and 91K with transition width of 4 K.

MEASUREMENTS.

Transport

critical currents were determined at 20 K in

liquid hydrogen by measuring

the V-I _curves

using

d-c- current. The contacts to the

sample

_were made

by

attaching

Pt wires

using

silver

past

and then

annealing

the

sample

at 300 °C for _one hour.

Contact resistances _was lower than 0.I fl

allowing

current of 10 A without

heating

of the

sample

in

liquid hydrogen.

A I

~V/mm

criterion for electric field _was used for

defining j~.

For both kinds of

samples

values of

j~

are of the order of 200-300

A/cm~

_at 77 K and 000

A/cm~

at 20 K. It is worth

mentioning

here that

geometrical shapes

of the

samples

_were

(~) Criceram

Pechiney Company

C-R-V- BP 27, 38340

Voreppe.

chosen in such _{a way} that self-field effects _are

negligible.

Indeed in these

polycrystalline

oxide

superconductors

the

transport

^critical current

strongly depends

_on the

magnetic

field

[1, 2],

which leads to current limitation in _a

macroscopic sample

caused

by

the

magnetic

self-field.

For

tapes

^{of thickness less than 100} ~m it has been shown

[2]

that

j~

values do not suffer from self-field limitation. Measurements _were

performed

in the earth's

magnetic

field and _{care was} taken _{to ensure} that all the

samples

_were cooled down in the _same conditions.

IRRADIATIONS. 2.5 MeV electron irradiation _was

performed

in the VINKAC low

temperature

facility coupled

in the Van de Graaff accelerator installed in Palaiseau

[10].

During irradiation, samples

_were immersed in

liquid hydrogen

and the V-I _{curves were} measured in situ. At the end of the irradiation

corresponding

to an electron fluence of 1.15

x1019e/cm~, samples

_were located in _a fumace above the

hydrogen

bath and the resistive transition _curves and _ten minutes isochronnal

annealings

at different temperatures

(respectively 100, 150, 200,

250 and 300

K)

_were carried out.

155 MeV oxygen irradiations _were

performed

at the

post-accelerated

tandem in

Saday.

The irradiations _were done in _{a vacuum} irradiation chamber mounted _{on one} of the beam lines of the ion accelerator. This device consists of _a

sample

holder and _an ion beam

degrader

surrounded

by

two collimators

allowing

the irradiation of _a 3 _mm diameter surface. The

degrader

works like _a

paddle-wheel

of aluminium foils in rotation in front of the

sample,

the thickness and the number of these foils

being

calculated to _{assure an as}

homogeneous damage

as

possible throughout

the

sample.

In fact 155 MeV O range is 90 ~m in

YBa2Cu~07

which is much

higher

than

sample

thickness

(30 ~m).

The ion

degrader

is rather used here in order to increase

(by roughly

_a factor

two)

the

damage

rate. As _a matter of fact _oxygen ions _are also

implanted uniformly

in the

sample

but the concentration of

implanted

O ions

(0.

I ppm for the

highest

ion fluence

investigated)

is

negligible compared

to the number of atomic

displace-

ments. Accurate ion beam

dosimetry

_was achieved

by surrounding

the

sample

holder with _a

Faraday

_cup. The irradiation and the resistance measurements were carried out at room

temperature ^{with ion flux limited} to 5

x101°ions/cm~/s

in order to avoid

heating

of the

samples

above 60 °C. Irradiation at

increasing

fluences _were done _on different

samples

of the

same

batch,

the

superconducting

transition _curves and the V-I _{curves at} 20 K

being

measured before and after irradiation for all these

samples.

These two

types

^of irradiation result in different

damage configurations

_: electrons create

essentially point

defects

uniformly

and

homogeneously through

the

samples

while in oxygen ion irradiation defects _are

produced mostly

in

displacement

cascades. It is

possible

to estimate

the number of atomic

displacements

in these two cases. Calculations were

performed by

assuming

the _same

displacement

threshold _energy of 20eV for all the atoms

[10].

The calculated

displacement

cross-sections _are

respectively equal

to 350 barns for 2.5MeV electrons and 2.7

x106

bams for 155 MeV O ions.

Results and discussion.

ELECTRON IRRADIATION. As far _as the variations of the critical

temperature

T~ and the resistance R _are

concerned, tapes

^of

YBa~CU~O~ present exactly

the _same behaviour _as the standard

samples [9]

;

namely

the decrease rate of T~ ^is

equal

to 2.8 _x 10~ ^~9

K/(e/cm~)

_or

-8K/9bdpa

and the relation between _T~ decrease and the relative resistance increase measured at 100 K is the _{same as} that

previously reported. Figure

I

displays

the variation of the critical current

density j~

measured in situ at 20 K _{as a} function of the electron fluence.

After _a very

slight

increase at low fluence

(corresponding

to 3

x10-sdpa), _j~

decreases

linearly

with irradiation. At the end of irradiation

j~

is decreased

by nearly

28 f6 while the resistance at 100 K is

only

increased

by

16 f6.

398 JOURNAL DE

PHYSIQUE

I bt

850

800

~ .

_

750 ~00

"£

.

~ 7900

Q

⁷⁰⁰ .

~/

~ ~ ~ °

YBa~CU~O~ tape ^hmdiated by 2.5MeV elecUons at 20K

550

~ ~ ~ ~

Damage (lo"~

dpa)

Fig.

I. In situ measurement of the transport ^critical current

density

j~ as a function of the irradiation

damage

for _a

polycrystalline YBa2Cu307 sample

irradiated at 20.8 K by ^2.5 MeV electrons. The inset shows the

beginning

of the _curve.

After _a fluence of 1.13

x10~9e/cm~ (corresponding

to 4

x10-3dpa), _annealing

_{up to}

room temperature were

performed. Recovery

_curves of T~, p

(100 K)

and

j~

are

reported

in

figure

2. The main result of this

figure

is to show that the value of

j~

becomes

higher

than its initial value after

annealing

at 300K whereas the recovery of _T~ and R

(100 K)

remain

respectively equal

to 45 f6 and 80 fb. We _can

point

_out here that the _same

tendency

has been observed _on sintered

samples

of different

origins,

but with

nearly

similar initial characteristics.

Under

annealing

after 20 K electron

irradiation, percentages

^of recovery are

always

more

important

for

j~,

^{then for} R

(100 K)

and at last T~.

~

~

'',

b '.

~,,

$ '. R(i00K)

# '.. ",

~ ._

",,

$t Annealing of_aYBa~Cu~0~tape ^~~'..

bra~ated by 2.5MeV elecuons at 20K '.,

~~

ata fluence of 1.13xld~e/cm~ (3x10'~_{dpa) ",}

loo 150 200 250 300

Annealing temperature (K)

Fig.

2.

Percentages

of _recovery of 2~, p

(100 K)

and

j~ (20 K)

^of a

YBa2Cu307 Polycrystalline sample

irradiated at 20.8 K

by

2.5 MeV electrons. The irradiation induced variations _were

respectively equal

to

3.3 K, 280 ~LQ. cm and 227

A/cm~

for T~, p

(100

K) and j~ (20 K).

ION IRRADIATION.

Figure

3 shows the _room temperature

resistivity

in _an oxygen ion

irradiated

YBa~CU~07 tape

as a function of irradiation fluence. All the

samples

studied present ^the same behaviour _{: a} small decrease followed

by

_a linear increase of the

resistivity

with

increasing damage.

Similar results _were obtained _on thin films of

YBa~CU~O~

irradiated

1400

j

^{YB~CU~07 ~Pe} ~r&&alert

~ by 155MeV O

f

^{~ ~~}

~ l 2 0 0

f

wmple2

(

00

@

1000 ~

0 2 lo 4 10 6 lo 8 10

damage (dpa)

Fig.

3. Evolution of the resistance _{as a} function of the irradiation

damage

for _a

YBa2Cu307 polycrystalline

sample ^irradiated by 155 MeV _oxygen ions.

Dispersion

of the

points

_comes from variation of

sample

temperature ^due to fluctuations of irradiation _current. The _curve represents

points

taken with the beam off (T~~~~~ = 20 °C

).

The three

samples

studied _are indicated.

at room temperature

by

boron ions

[11].

Three

samples (1,2,3) initially

identical _were irradiated with O fluences of

1.8,

2.7 and 13.4 _x 1014

ions/cm~.

These different fluences _are indicated in

figure

3 and the low

temperature

^results are

reported

in

figure

4.

Interesting properties

_were obtained _on

sample

I _{at a} fluence of 3

x10-4dpa

for which the _room

temperature resistivity

is minimal _: the normal state

resistivity

decreases at all

temperatures (-

0.6 fb at _room

temperature

^{and 4 fb} at 100

K),

the critical

temperature

^{also decreases}

(by

about I

K)

and the critical _current increases

(16fb).

One also observes that the

p

(T)

_curve is

slightly

modified

by

irradiation

(see Fig. 5).

For the other

samples

the increase

of

resistivity

is

accompanied by

decreases of T~ and

j~

as

usually reported

for irradiated

polycrystalline samples.

One finds that T~ decreases

nearly linearly

with ion

damage,

its decrease rate

being equal

to

-10K/fbdpa.

This value is in _a

good agreement

with that

obtained in electron irradiation

(-

8

K/fbdpa),

which shows that T~

degradations

_can be described in these _two irradiations in terms of number of atomic

displacements

_as

already emphasized by

Summers et al.

[12]. Contrary

to that observed at _room

temperature,

one can see in

figure

4 that the resistance at 100 K increases _no

longer linearly

_{as a} function of ion

damage.

This _can be related to the fact that the

resistivity

_versus temperature p

T)

_curves are

0.2

,

0 ^' _' 0.8

' ,

~ ,

'

~~ ~

~Jc%co ,

'

~'~

04 -

~'

' 0A

0.6 '

' 0.2

,, ^{' '}

0.8

-j~~

⁰

o

-0.2

0 210'~ 410'~ 610'~ _810'~

Damage (dpa)

Fig.

4. Evolution of the critical temperature 2~, ^the

resistivity

measured at 100 K and the transport critical current

density

measured at 20 K for the three

samples

irradiated

by

155 MeV O ions. The full line is _a linear fit whereas the dashed lines _are

just guides

for _eyes.

400 JOURNAL DE

PHYSIQUE

I bt 3

s

%m@U

$

1.0

$

+ _%mjQ

£ ^{before kmdialion}

0.5

~

o-o

50 100 150 200 250 300

T (K)

Fig.

5.

Resistivity

_versus temperature curves for ion irradiated

samples

of

YBa2Cu307.

Resistivities

are normalized to their initial values at 300 K.

modified

by

ion irradiation _as shown in

figure

5 and

approach

a metal-insulator transition _at low temperature ^{in the} most irradiated

sample. Moreover,

^{in this} case, the decrease of

j~

is well correlated to the increase of

resistance, suggesting

that these modifications _may arise from the _same effect.

DIscussloN. Effects of electron and 155 MeV O irradiations _are summarized in

figure

6 where the relative variations of T~

(a)

and

j~ (b)

_are

plotted

_versus the relative increase of

resistivity

at 100 K. Different remarks _can be made about this

figure.

First,

in these two

experiments

_we observe _an increase of the transport ^critical current

density.

In electron irradiation

experiments,

this increase _comes

obviously

from recovery processes

during annealing.

From this result _we conclude that the increase of

j~

^observed on

sample

^I is also due to _some

migration

of defects

occurring during

_room

temperature

^ion irradiation.

In this latter _case the fact that the decrease of the resistance below its initial value is

accompanied by

an increase of the critical current whereas the critical temperature decreases suggests ^that processes take

place

at the

grain

boundaries. Indeed T~ ^is

thought

to

depend mostly

to the

crystallographic

arrangement inside the

grains

whereas transport ^critical current

is determined

by

weak links between the

grains.

Different

annealing experiments

_on

poly-

and

o.02 0.2

~~~ (b)

o it Ion irrndintlon 0 °,O

~ ;on irmdintion

~ O electron irredIntion

'o,

o ejectron lrr8dintion z

°.°~ ° ~"~ ~""~~'~"~

.j

^°.2

1~

~,~"~

~""~~""~

)"

^~'~~

~

0 4 '

~

'

0.06 ,

'

0.6 _'

,

0.08

'

~~0.2

_{0 0.2 0.4 0.6 0.8}

~~0.2

_{0 0.2 0.4 0.6 0.8}

AR/R~ AR/R~

Fig.

6. Relative variations of 2~ (a) ^and j~ (b) _versus relative variation of R (100 K) for YBa2Cu307

polycrystals

irradiated by 2.5 MeV electrons _at 20 K and annealed _up to 300 K

(*)

and

by

155 MeV O ions _{at room} temperature (o). The full line in (a) represents the relation obtained between

AT~/T~

and

AR/Ro

for low temperature electron irradiation [9].

single crystals

of electron irradiated

YBa~CU~O~

have shown that recovery ^rates of the resistance _are

higher

in the former than in the latter _ones

[9, 13],

which

suggests

^that

grain

boundaries may

provide

sinks for

point

^defects. In

particular

_oxygen interstitials which _are

probably

the most mobile defects in this structure can be

trapped

^into

grain

boundaries

leaving

their _own vacancies behind them. One _can

point

out here that Marwick _et al.

[ll]

propose that

ordering

of _oxygen vacancies in the basal

plane,

I-e- that which contains the Cu- O

chains,

_may

explain

the reduction of

resistivity

ⁱⁿ B-irradiated films of

YBa2Cu~07.

Nevertheless at the fluences considered here

(corresponding

to about

10-4dpa),

the

concentration of oxygen vacancies is

probably

too weak to be ordered at _a

macroscopic

scale.

Another idea is that

migration

of oxygen ^atoms towards

grain

boundaries

might improve

the electronic

qualities

of these

regions

which _are often

oxygen-deficient.

This

explanation

appears reasonable if _we take into account that modifications of R and

j~

are related each other. As far _as the increase of

j~

is

concerned,

it is also

possible

that defects

trapped

in

grain

boundaries may ^act as additionnal

pinning

centers for

Josephson

vortices.

From

figure

6a it also _appears that the relations between the variations of

ATJT~

and

AR/RJ

_are different after electron and O irradiation. We

pointed

out above that the decrease

rate of _{T~ can} be well understood in terms of number of atomic

displacements

in the

YBa2Cu~07

lattice for these two types of

projectiles.

In _a

previous experiment

_{[9] we} found that the relation between

AT~

and

AR/11o

was the _same for

poly-

and

single crystals

of

YBa2Cu~07

irradiated

by

2.5MeV electrons. This led _us to conclude that

polycrystal resistivity

increase under electron irradiation is

mostly

determined

by

disorder in the

grains.

However,

the fact that the relative increase of resistance is

equal

to the relative decrease of transport ^critical current for O-irradiated

samples

2 and 3 suggests ^that modification of

grain

boundaries is the

principal agent

of the

resistivity change

in this latter _case.

Indeed, according

to the

Ambegaokar-Baratoff expression

for _a SIS

Josephson junction,

the

transport j~

is

inversely proportional

to the normal-state resistance of the

junction.

In the _same way, the

metal-insulator transition observed in the most O-irradiated

sample (Fig. 5) might

result from the _same

origin,

I-e- carrier localization at the

macroscopic

scale of the

grains.

At this time it is _not clear

why

the

grain

boundaries _{are more} sensitive to defects created

by

ion than electron irradiations. Electron

microscopy

observations have shown the appearance of _an

amorphous layer

at the surface of the

grain

in _room

temperature

^{ion and 35 K electron}

irradiated

YBa2CU~O~ [14, 15].

In the latter _case

damage

_necessary to initiate the

amorphization

is much

higher (around

I

dpa)

than in the former _one. It is certain that the

high

concentration of defects inside _a

displacement

cascade created at the surface of _a

grain

will

strongly modify

the

properties

of the

grain boundary.

But the

probability

of such events is much too low to

explain

^{the observed} increase of resistance.

The different irradiation temperatures ⁱⁿ these

experiments

_room

temperature

for ions and low

temperature

^{for electrons could}

provide

in _our

opinion

_a better clue to understand the results. Ion-induced

grain boundary amorphization

was

previously

attributed

by

Clark et al.

[14]

to

grain boundary

defect accumulation

resulting

from irradiation-enhanced

diffusion and

segregation.

The different

mobility

of defects

might

then

explain

the different behaviours of resistance after _room and low

temperature

irradiation. Moreover at _a

given

temperature, one expects ⁱⁿ this

interpretation

that electrons will be _more efficient than

ions, point

defects

being usually

_more mobile than

agglomerated

defects.

Conclusion.

We have irradiated

polycrystalline tapes

^of

YBa~CU~O~ by

two very different

types

^of

projectiles.

We confirm that the decrease rate of 7~ is related to the number of atomic

displacements

and is

approximatively equal

to -10

K/9b dpa

for these two irradiations.

402 JOURNAL DE

PHYSIQUE

I bt 3

We show that

reorganization

of irradiation-induced defects _can lead to a reduction in the

resistivity

and _a concomitant increase of the

transport

^critical current

density.

It is clear that the presence of

grain

boundaries

play

_a crucial role in these processes,

probably by providing

sinks for

point

defects.

In the most O-irradiated

samples,

_we have _some evidences that the increase of

resistivity

is determined

by

the

degradations

of

grain

boundaries.

Clearly

in situ ion irradiation

measurements of

resistivity

_on

poly-

and

single crystals

of

YBa~CU~O~

would be very useful to elucidate this

point.

Acknowledgements.

We _are very

grateful

to D. Lesueur for calculations of atomic

displacements

and

helpful

discussions of the results and to C. de Novion for the critical

reading

of the

manuscript.

We thank G. Jaskierowicz and P.

Laplace,

and the team of the

post-accelerated

tandem _at

Saday

for their technical assistance

during

electron and O irradiations. Part of this work _was

suppported by

the MRT contract n 87-A0687.

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