Electrical conduction in CaAl2O4

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SILICATES INDUSTRIELS 198513-4

Electrical Conduction in CaA1204

(*)

A.M. HOEFSLOOT , P .H.F. THIJSSEN and R. METSELAAR.

Technological University - Eindhoven (The Netherands)

35

Electrical conductivity and the ionic and electronic transference numbers have been measured for sintered samples of undoped CaAlz

°

4 , Measurements were performed in the temperature range 1200-1450° Cat oxgen partial pressures between 1 and 10-15_{bar. For oxygen pressures} between 1 and 10-5 _{bar ionic conductivity dominates and at low pressures electronic conductivity dominates. A defect model is proposed.}

INTRODUCTION.

Calcium aluminates are often encountered in cement technology, so much is known about their hydration and related chemical properties and about the phase-diagram of the CaO-AI203 system [1].

There are 5 phases between CaO and Al203 : C3A (3CaO.AI20 3), CJ2A7, CA, CA2 and CA6. Of

these, CJ2A7is suspected not to exist in an absolutely H20-free state [2]. Our interest in the electrical pro-perties of Calcium-Aluminates sterns from the use of a «melt-glass », consisting mainly of calcium and aluminum oxide, in the production of high pressure Sodium-Iamps. This «melt-glass » which is in fact not a glass, but completely crystallized, is used as a seal between metal electrode and translucent polycrys-tallince alumina. During operation of these lamps part of the electric current flows through alumina tube and seal.

We also know that failure of the lamp always involves Na-Ioss. Ifthe conduction mechanism of the calcium aluminates is known a more intelligent attempt can be made to improve the properties, viz. electrical conductivity ::md reactivity with Na, of the

« melt-glass ». EXPERIMENTAL.

Powders were prepared using the technique of hot petroleum drying [3]. To this end calcium and alumi-num nitrate (Puratronic, Johnson Matthey Chemicals Ltd.) were mixed in aqueous solution. This solution was emulgated in petroleum with Span 80 as emulga-tor, and subsequently the emulsion was slowly dripped into hot petroleum (160° C). The water evaporates (*) Conference on «Electroceramics» -Brussels 1984.

rapidly and an extremely intimate mixing of the salts is achieved. The petroleum was distilled off and the powder was heated in air to 650° C. After this treat-ment a white powder was obtained, which was X-ray amorphous (surface area 10-17 m2_{/g). The powders} were pressed into pellets, slowly heated to 1500° C to remove traces of carbon and sintered at this tempera-ture for 2 hrs, to a final density of at least 95% of the theoretical value. According to X-ray diffraction pat-terns and SEM pictures the pellets were pure CaAl204 without a second phase.

For electrical measurements the surface of the samples was polished and covered with platinum paint contacts (Leitplatin 3ü8A Demetron). The sam-ples were mounted between spring-Ioaded platinum electrodes in an alumina sample holder (Al99), fitted with Pt/Pt-lO % Rh thermocouples at both ends of the sample [4]. The sample holder was shielded from stray fields by means of a Pt shield.

The partial pressure of oxygen around the sample was controlled by mixing O 2 and N2 for the range 1-10-5_{bar, or CO and CO}

2for the range 10-5-10-15

bar. The composition of the 0 21N2 mixtures was checked at the outlet of the measurement cell with a zirconia oxygen gange. The electrical conductivity was obtained by measuring the small-signal ac reponse in the frequency range 0.1 Hz-1 MHz (Solar-tron 1174 Frequency Response Analyser). The com-plex impedance plots almost invariably showed the low frequency part of a circle and a tail departing from the reaI axis at a low angle ('" 10°). For the bulk resistance we took the impendance at the point where the drcle intersects the reals axis.

manuaUy adjusted to equal that of the r·ight hand elec-trode. ~as pt wi re

L.~~~~

pt 10" Rh wi re ?ZZZ'Z2::zz::::z.z:Z'ZZz.z.22ZZ'ZZZZZZZ'Z2ZZZ'Z2ZZ2:Z.2~~~~1

I

l e l ec t rode (Pt pa s te ) pt seal

~

L-

pel let

1cm

1",",",1

Figure 1 : The heart of Dur e.m.j. cell. See text.

Kcith ley 642 Uectromcter 1"- _7"'" Pt j _Pt

~11

1_{\. ~}' Keithlcy gas I I Pt 1~)S 11-N _{-Pt 1()~, Rh} Data Precision 8200 Voltage Cal ibrator

Figure 2 : Schematic drawing of Dur e.m.j. measurement.

The function of the guard is to prevent the sample e.m.f. from being short-circuited by surface and gas conduction. To achieve this it is kept at the sameP0₂

and voltage as the right-hand electrode.

Our ceU (Fig. 1) is constructed from translucent polycrystalline alumina and fits into a miniature Rh-wound furnace. The peUet was sealed to the Ieft-hand

tube with a Pt ring that was compressed between polished surfaces and kept under pressure at 12000

C for two hours.

The electrodes were Pt-paste painted on polished surfaces and guard tube and wires are all spring-loaded. The reference gas used on the right-hand side was air and on the left-hand sideP0₂ was controlled

SILICATES INDUSTRIELS 198513-4 37

as before. The flow was maintained at approx. 120 Sccm at both sides.

The e.m.f. E of such a cell is :

Fig. 3 : Conductivity ofCaAl₂0₄measured as afunction ofpartial oxygen pressure at different temperatures.

(1)

- 2.51

whereR is the gas constant, T the temperature in K,

F the Faraday constant and ti = uil u is the ionic transference number. ti is obtained from the measu-red voltage by : - 3.0 00 - 3.5 4F RT dE (2)

Fig. 4 : Measured e.m.j at 13470

Cand ionic transference number ti at

13470 _{Cand 1190}0

C,both as afunction ofpartial oxygen pressure

If a difference in P0

2 is maintened across an oxide

sample exhibiting mixed conduction(0 ~ ti ~ 1)then a voltage will develop [eq. (1)] and an electronic and an ionic electric current will flow in opposite direc-tions through the sample. Ifthe conducting ion is oxy-gen then this results in a flow of oxyoxy-gen from the high po₂-side to the low po

2-side. If a cation is the moving species, then it will move in the opposite direction and the sample will grow on one side and shrink on the other, if this is the only cation present. In our case however the second cation would probably have a dif-ferent mobility and the voltage should diminish after some time, because the lattice can not accomodate more than a certain concentration of defects. This we never observed, so we will interpret ti as the oxygen transference number. - 4.0 - 4.5 T = 1193[ac] T = 1242(ac] - 15 -10 o T= 1308 [Oe] o T= 1358[°el -5 Log Po 2 o T = 1428[Oe] o 0 o0 o 0 o [a tm]

RESULTS AND DISCUSSIüN.

> E 400 LL' ~ ui 200

t

o

ti

t

0,5

o

x x x 134 7·C • 1190·C -10 -5 0 ---. log (Poz)(bar) -15 The results of the conductivity measurements on

CaA/204 are shown in Fig. 3. In this figure u-Po₂ cur-ves are given for five temperatures. In Fig. 4

e.m./.

values are plotted as a function of Po

2, at 1347

0 _C.

The straight curve gives the Nernst line i.e. the

e.m.!

calculated from Eq. (1) for ti = 1. The corresponding ionic transport numbers are also shown in this figure. From this figure it is seen that ti is close to one in the region between 1 and 10-5_{bar. At lower oxygen} pressu-res the total conductivity u increases with decreasing

P0₂, whiletidecreases. This indicates that electrons are

the dominating charge carriers at low values of Po

2•

This is confirmed by the Seebeck coefficient that gives n-type conductivity atP0

2 = 2.10-13 .The partial elec-tronic conductivity u_e can be calculated from u with

u_e = (l-tiJu. The results at 13580 _{Care shown in}

Fig. 5. At oxygen pressures above""" 10-5_{bar the} con-ductivity is dominated by a pressure independent ionic

where A' stands for a singly charged acceptor ion; square brackets indicate concentrations.

o

o

Fig. 6: EJectronic and ionic conductivity of CaAl₂0₄

as a function ofJIT.

-4

o

1358 C

- 5

L . . - - - ' - - - ' - " - - - '

-15

-10

-5

log (PO)

2 -4 -3

(4)

(8) [A'J = 2 [Vo

J

The superscript dots and dashes indicate effective positive and negative charges (Kröger-Vink notation). The electroneutrality condition can be approximated by:

for u_eone expects

u e = uO,eexp (-EelkT). The equilibrium defect reaction is :

00:::112 02(g)

+

Vo 0

+

2e' (3)

Ifthe equilibrium constant of reaction (3) is_Kox,

Kox

=

[VOO][e'j2pb: (5)

Combination with Eq. (4) gives :

Ie']

=

(2Kox)//2[A']//2p-~4 (6) Sinceui

=

2[V0 0]qp(V0 0)

and u_e

=

[e']qp(e'), with q the electronic charge and p the mobility of the charge carrier, it follows from

Eqs (4) and (6) that ti is independent ofP0

2 while

u e rxpÓ~4. From Fig. 5 we observe a slope of -0.24 for ue, in close correspondence with this model. From the

fact that p(eï ~~p(V

0

0)we see that (V

0 .)

~~ (eï. From Fig. 4 one sees that u increases slightly abovePo/'" 10-2 ; at the same time ti shows a ten-dency to decrease. This can be explained by an increa-sing contribution of holes to the conductivity. Since

r'] 'h 0] -//4

ie • i' = constant, we expect Uh OC P0

2 •

For uione expects a temperature dependence

U·₁

=

Uo_,11-Jexp (-KI kT)₁ (7)

Fig. 6 shows the corresponding plots of log uiT

and log U_{e ,} as a function of reciprocal temperature.

From these curves we finduO,i

=

2.75X104 Q-Jcm -J,

UO,e

=

1.65x106 Q-J cm_-J,Ei

=

1.74 eVand _{E e}

=

3.05 eV. The values of 1.74 eVis equal to the migra-tion energy of the

Voo.

From Eqs (3), (6) and (8) it follows thatE_eis half the value of the energy of for-mation of

Voo.

It is uncertain at this moment which is the domi-nant acceptor impurity in our samples. In a spectro-chemical analysis a number of impurities bath with acceptor and donor character are observed.

u o Ol o -5 6 ~-_o Ol o

SILICATES INDUS TRIELS 1985/3-4 CONCLUSIONS.

From the foregoing we conclude that CaA/204 is a mixed conductor exhibiting mainly oxygen vacancy conduction at high oxygen pressures and mainly elec-tronie conduction at low oxygen pressures. The con-centration of oxygen vacancies was fixed in our expe-riments by an unidentified acceptor. In further experi-ments we will try to identify the acceptor and to deter-mine the effect of a change in its concentration.

39

REFERENCES.

[1] LEA F.M., The Chemistry of Cement and Concrete. 2nd ed. New York, 1956.

[2] NURSE R.W., WELCH J.H. and MAJUMDAR A.WJ., Trans. Brit. Cer. Soé. 64409, 1965.

[3] REYNEN P.J.R., BASTlUS A., FAIZULLAH M. and KAMPTZ H.v., Ber. Dt. Keram. Ges., 5463, 1977. [4] LARSEN P.K. and METSELAAR R.,

Phys. Rev. B14 2520, 1976.

[5] YEE J. and KRÖGER F.A., J. Am. Cer. Soc. 56 189, 1973.

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