Understanding the ageing process in RPC’s from an ion conductivity

approach.

C. Pecharromán1 , M. Morales2, G. Mata-Osoro1 , L. A. Díaz3 , V. Valcárcel4, J. A. Garzón2

1-Instituto de Ciencia de Materiales de Madrid (CSIC), Spain.

2-Centro de Investigación en Nanomateriales y Nanotecnología (CINN), (CSIC), Spain.

3-LabCaF, Facultad de Física, Universidad de Santiago de Compostela (USC), Spain.

4-Instituto de Cerámica de Galicia (ICG), Spain

Physical requirements for the resistive plate of a high rate

timing RPC (for instance CBM demands 20 kHz/cm2)

• Resistivity~ 109 m.

• Breakdown field Ev>>>1MVm-1 (1000V/mm)

• Thermal Stability.• Not a very high permittivity• I/V ohmic if possible (R field independent)• Easy to fabricate and not very expensive.

Conductivity of materials• Intrinsic materials are divided into insulator

and conductors.• Materials in the range of 10-9 -1m-1 are limited:

10-18

10-15

10-12

10-9

10-6

10-3

100

103

106

109

1012

Fu

sed

qu

arz

Tefl

on

Poly

eth

yle

ne

Am

ber

Para

ffin

Ru

bb

er

Su

lfu

rM

ica

Porcela

inS

tyro

foam

Ru

tile

(TiO

2)

Ru

tile

(TiO

2)(

Z)

Gla

ssM

arb

le Bak

eli

teD

isti

lled

wa

ter

Fu

sed

sil

ica

Beryll

ium

ox

ide(B

eO

)+In

trin

sic s

ilic

on

Sa

pp

hir

e+

Ferrit

e(F

e2O

3)+

Dry s

oil

Ple

xig

las+

Gall

ium

arse

nid

e (

GaA

s)+

Intr

insi

c g

erm

an

ium

Tell

uriu

mC

arb

on

Grap

hit

eC

ast

iron

Mercu

ry

Nic

hro

me

Sil

icid

es

Germ

an

sil

ver

Sil

icon

ste

el

Poly

sili

co

nL

ead

Tan

talu

mT

inP

lati

nu

mIr

on

Nic

kel

Zin

cT

un

gst

en

Brass

Alu

min

um

Gold

Cop

per

Sil

ver

Material

(S

)

Ceramic/Metal insulators• Physical Mixtures of substances with very different

conductivities.• The conductivity of these materials changes brusquely around

the percolation threshold.• Heterogeneities in composites with concentration close to the

percolation threshold spoil their properties.• Samples presented good homogeneity• Good conductivity values.• Samples can be machined.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.010

-14

10-11

10-8

10-5

10-2

101

104

107

Con

cuct

ivity

-1m

-1)

f

fc

C. Pecharromán et al. Adv. Mater, 12, 294 (2000)

An example, Ni/BaTiO3 composites• Schottky and Tunneling conductivity behavior.

• Detected areas of very large electric fields (hot spots)• Samples suffer breakdown process at a certain Vc value.

0 100 200 300 400 500 600 700 800 900 1000-18

-16

-14

-12

-10

-8

-6

-4

-2

0

2

4

f=0.12 1300ºC 2h f=0.25 1300ºC 2h f=0.25 1350ºC 16h f=0.27 1300ºC 2hf=0.29 1300ºC 2h

Log(

J (A

m2 ))

E1/2 (V/m)1/2

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.210-12

10-11

10-10

10-9

10-8

10-7 T = 403 K T = 383 K T = 363 K T = 343 K T = 323 K

I (A

)

V1/2 (V1/2) 10-2 10-1101

102

103

104

105

= E

loc/E

(fc - f)

0 2x105 4x105 6x105 8x105 1x10610-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

12 Ni vol. % 25 Ni vol. % 27 Ni vol. % 29 Ni vol. %

(-1

·m-1)

E (V/m)

C. Pecharromán et al. Adv. Mater, 13, 1541 (2001)R. Jimenez et al., Ferroelectrics 268 807 (2002)C Pecharromán et. al. Ferroelectric (2010)

Conduction models for metal/insulator interfaces

P r o c e s s M a th . E x p r e s s .

T u n n e ll in g A W e Vj c te W e V e

F ie ld e m is s io n 326 .8 3

2W

Ej E e

Ele

ctro

de

pro

cess

es

S c h o ttk y 2sW E

k Tj A T e

O h m ic gW

k TE

j eT

Io n ic 2

dWW

k TE

j eT

P o o le F re n k e l d P FW E

k Tj E e

Insu

lato

r p

roce

sses

S p a c e C h a rg e 2

3

9

8

Vj

d

T independent

Log(I)~V1/2

I~V, Arrhenius

Log(I)~V1/2

electronic

Metal/ceramic composites under strong electric fields.

• Metallic phase is a good electronic field• Potential only drops in insulator regions.

Metal

Insulator

Metal

Insulator MetalMetal

Electron conductivity

ion conductivity

Mo/Mullite Composites• Why Mullite/Molybdenum?

• Mullite is an aluminum-silicate.– Its crystalline structure includes oxygen

vacancies– It has a very high dielectric strength– It has the same expansion coefficient as Mo

• Mo is a refractaire metal.• It is relatively stable against oxidation.• Processing of powders is quite simple.

MulliteHartmut Schneider (Editor), Sridhar Komarneni (Editor)

Mo/Mullite Composites• Electrical properties of Mu/Mo composites• I/V curves seem to indicate a space charge displacement

mechanism. (I~V2).• Conductivity is very sensitive to temperature.• I/T indicates a thermal activated mechanism.• An ion conduction mechanism, at high fields is proposed.

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0-22

-20

-18

-16

-14

-12

-10

-8

-6

Mu/Mo 11 vol. %

log(

J) (

log(

Am

-2))

1000/T (K-1)

200 µm

0 1x105 2x105 3x105 4x1050.0

5.0x10-4

1.0x10-3

1.5x10-3

2.0x10-3

2.5x10-3

j (A

/m2 )

E (V/m)

11% 12% 13%

-1x105 0 1x105 2x105 3x105 4x1050

1x10-9

2x10-9

3x10-9

4x10-9

5x10-9

6x10-9

7x10-9

8x10-9

9x10-9

1x10-8

11 vol. Mo % 12 vol. Mo % 13 vol. Mo %

(

-1 m

-1)

E (V/m)

10 100 1000 10000 100000 10000000

5

10

15

20

25

30

Die

lect

ric

Con

stan

t

Frequency (Hz)

Mu/Mo 11 vol.% Glass

Mo/Mullite Composites

• Ohmic+space charge displacement model

250 500 750 10000.0

2.0x10-8

4.0x10-8

6.0x10-8

8.0x10-8

1.0x10-7

I (A

)

Volt (V)

310K Quadratic term Linear termo Fit

SCD

Ohm 1 2I I I

1

VI

R

2

2 3

8

9

VI

d

=3.6·108 m=3.13·10-19 m V-1

=4.9·1010 m=1.37·10-23 m V-1For float glass

Mo/Mullite Composites ageing• Ageing behaviour

– Ressitance presents a strong temperature dependant behaviour.– Slight temperature peaks induce ireverssible resistance increments.

0 2 4 6 8 10

0

1x108

2x108

3x108

4x108

5x108

6x108

0 2 4 6 8 1021

22

23

24

25

26

(·

m)

T (

ºC)

time (days)

Strutural images of mullite and glasses.

http://www.themolecularuniverse.com/ISAACS, Se´bastien Le Roux* and Valeri Petkov*, J. Appl. Cryst. (2010). 43, 181–185

E

Faradaic ion-defect currentElectrochemical Breakdown

Na+, K+ cations in glasses

Simple model for composite electrochemical breakdown

• Time to induce a structural atomic failure by faradic current

• conductivity stops and atomic structure will collapse when a critical fraction of carriers abandon their equilibrium position . The fraction between total charge QT and charge defects Q is called (tolerance to faults in the crystalline structure).

•

• For crystals =10-5 ~ 10-17. Assuming =109 m and =10-11 then t is around some days

• For a glass with =10-11 -1m-1 may vary many orders of magnitude. An intermediate value is =10-12 which results t around of years

Qjdt Et

S

T

Q

Q

iT A

i i

qQ Vol N e

Mw 3810 equiv/gi i

i i

q f

Mw

1

T A T

Q E t

Q d N e

9 32 10

~A TN e C mt

V V

Glass ageing• Ageing behaviour– Resistance presents a strong temperature

dependant behavior.– Slight temperature peaks induce irreversible

resistance increments.– The damage degree is smaller than in the case

of Mu/Mo composites

0 24 48 72 960.0

5.0x109

1.0x1010

1.5x1010

2.0x1010

2.5x1010

3.0x1010

3.5x1010

4.0x1010

(·

m)

time (h)

Equation y = a + b*x

Adj. R-Square 0.76774

Value Standard Error

Book1_E Intercept 3.15108E1 1.98784E7

Book1_E Slope 3.71745E7 410469.18948

20

24

28

T (

ºC)

0 2 4 6 8 10

0

1x108

2x108

3x108

4x108

5x108

6x108

0 2 4 6 8 1021

22

23

24

25

26

(·

m)

T (

ºC)

time (days)

Deffects and hot spots.• In a composites, processing deffects modify localy

the metal concentration, and therefore the conductivity

• In a near percolated system, in some points the electric field can reach very high values – Hot Spots.

• In those Hot Spots, the material can suffer dielectric breakdown in these area.

100 µm

Pecharroman C, PCCP, 11 5922 (2009)

Mu/Mo=3.13·10-19 m V-1

glass=1.37·10-23 m V-1

New proposal.

• Mixtures formed by insulators and electron conductors (transition metal oxides) slightly above the percolation threshold.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

10-14

10-11

10-8

10-5

10-2

101

104

107

Con

cuct

ivity

-1

m-1

)

f

fc

Metal/ceramic

Conductor Ceramic/Insulator ceramic

Electronic conduction from oxygen defects in transition metal oxides (Fe, Zn, Sn, Ni, Ti, etc).

Commercial ferrite behaviour

• Linear I/V response (Ohmic).• Conductivity does not vary so much

with T.• No ageing.• Too much conductivity (up to now)

1x105 2x105 3x105 4x105 5x105 6x105 7x105 8x1050.0

5.0x10-8

1.0x10-7

1.5x10-7

2.0x10-7

2.5x10-7

3.0x10-7

(

-1·m

-1)

E (V/m) 0 24 48 72 96 1200

1x106

2x106

3x106

4x106

5x106

6x106

7x106

0 24 48 72 96 12030

31

32

33

34

35

36

(·

m)

Time (h)

Temperature

T

empe

ratu

re (

ºC)

Time (h)

Conclusions• Mu/Mo composites, with metal concentration

below fc presents good values for RPC materials, nearly ohmic behavior and high breakdown field.

• However, electrochemical ageing increases its resistance until breakdown happens after several days.

• Ion conduction must be avoided for long term operation. Only defective-tolerant atomic structures may work reasonably (float glass)

• Possible solution: transition metal oxide ceramics (Fe, Ni, Ti, Zn, Sn, Co, etc) with conductor phases slightly above the percolation threshold.