Formation of sodium aluminates at high temperatures under the influence of Mg and Ca

Solid State Ionics 16 (1985) 185-194 North-Holland Publishing Company

FORMATION

OF

C.M.P.M. Eindhoven

SODIUM

185

ALUMINATES

SARIS+ University

HIGH

Technology,

of

H. VERWEIJ Philips Research

AT

P.O.

Labs.,

Box

TEMPERATURES

P.O.

Box 513,

80.000,

UNDER

THE

5600 MB Eindhoven,

JA Eindhoven,

5600

INFLUENCE

OF

The

Mg AND

Ca

Netherlands

The Netherlands

The formation of sodium aluminates at high temperatures under the influence of Mg and Ca was studied to obtain insight into lifetime processes that occur in high pressure sodium (HPS) discharge lamps with translucent alumina envelopes. Powders with molar composition Na2C03-5A1203, mixed and fired at Na2CD3-MgO-5A1203 and 2Na2CO3-CaCO3-5A1203 were thoroughly Na2CO3-9A1203, temperatures up to 16OO’C for top-level times up to 100 hrs. Quantitative X-ray diffraction was used to study the composition of the quenched mixtures. In the systems Na2CO3-5A1203 and Na2CO3-9A1203 the compound Na20.Al203 is formed first at low coincident with the decomposition of carbonate. This compound subsequently reacts temperatures, with Al203 to form a mixture of E- and El”- Al203 in a weight ratio of 1:6. At temperatures above 14OO’C 6”-Al203 transforms to El-Al2O3. Remarkably this transformation is most complete in the Na2CO3-5A1203 mixture. In the Na2CO3-MgO-5A1203 mixture the compound Na20.Al203 is also formed ET- and 6”-Al203 are formed subsequently in a weight ratio first, directly followed by MgO.Al203. of about 1:1.6. At temperatures above 14OO’C stabilized 6”-Al203 is then formed almost completely. The reactions in the system 2Na2CO3-CaCO3-5A1203 are vary complex. The compound Na20.Al203 is formed first, directly followed by the ternary compound Na20.CaO.2A1203 which forms solid At about 1OOO’C the compound Na20.3Ca0.8A1203 solutions with Na20.Al203, starting at 115O’C. starts to form with a maximum around 12OO’C. At higher temperatures it is consumed again, resulting in a complete disappearance at 16OO’C. 6/e”-Al203 also starts to form at IOOO’C and is still present at 16OO’C, together with a solid solution of Na20.Ca0.2A1203 and Na20-A1203. Conclusions can be drawn from the present results that are relevant for an understanding of lifetime processes in HP5 lamps.

1.

In recent in

high

with

years

and

5000

inside

the

0.7-l

lamps

discharge

with

envelopes

the

In

wall

at the the

ratures

a very with

must

be

that

ends

but

steady

discharge

: Philips

decreases

degradation

sometimes

HPS

reaction

plasma

Glass

takes

Dev.,

TGA, has

(TGA)

HPS

range

material

in the

highest

There

range

alumina

of

tion

TGA

in

P.O.

BOX 218,

0 167-2738/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

are

of

to that

obtain

was

lamp and

translucent

the

that

A1203

almost

The Netherlands

in in

process

to

shown2s3

wall

present

sintering

with

tube

causes

processes a

added

w % MgO to

product

the

leakage.

Mg2+,

to

order

zone

Tube

In

5600 MO Eindhoven,

the

tube

lifetime

0.25

of

efficiency. leads

sodium,

“cold”

sodium

properties

a pressureless

about

sintered

of

on

It

of

the

influence

powder.

of a

colour

indications

an

in

in

and degradation

1BOO’C in H2, MgO is

tempe-

place.

of

lamps.

about

mid

walls

Disappearance

blackening

alumina

the

products

itself.

1.

To

a disappearance

tube

degradation

the

in the

the

excess

white

to 125O’C

with

wall

pressure

in for

are

temperatures

address

ref. sodium

gastight

tube

slow the

the

means

lamp zones

material

* Present

instance

tube

This

translucent

from 85O’C zone.

for

in

causes of

by reaction

lamps

reasonable

preferably

and efficiency

atm.

discharge

reaction

blackening

much interest

performance,

a lifetime see

been

(HPS)

colour

hours;

colour

has

sodium

white

obtain

of

there

pressure

a good

efficiency of

This

INTRODUCTION

at

starting an

addi-

may result theoretical

186

C.M.P.M. Saris, H. Vcrweij / Formation

density

and

ture.

This

subject

a

of

years;

relatively

beneficial many

a review

ring

effect

tions

the

defect

chemistry

grain

boundaries.

1.1.

solubility

1530, 90,

240,

of

solubility

of

MgO at

the

wet

of

refs

5-7

single-crystal

1.2.

pies

of

is

both

ration The also

although

are of

ref.

at

alu-

reported

to of

was reported.

dope

an

that

MgO.Al203

1.3.

Mg in Al203 7 was to

preci-

concluded lattice;

Mg’Al

by

on the It of

. It

formed

suggestion frequently

encountered

no direct

proof in

forma-

by

plausible

brotin

has

bivalent

in yet

the

H+

literature

been

transition

presence

of

and

similar

to

12.

A rough

Al2O3.

For

1 to

1

to

about

5

of

the

order

of

mag-

MgO solubility

segregation

below

of

Mg

in

of

Mg

the

boundary

reported. metal

the

more

is

was

surfaces.

sensitive

with

depth

of

a

that

these

rent

from

are

rapid

from sec-

normal

after of

Auger refs of

19

XPS in fracture

segregation

factor

Refs

that

18,

show

20-22

that

It

which

on grain is

are

argued diffe-

precipitates,

sintering

no

micropreci-

MgO.Al203 instead.

the

mea-

17,

of

but

MgD.Al203

decrease

in

to

microprecipitates,

upon cooling

on grain

polished

In

at.

occurs

formed

gene-

concluded

used

non-stoichiometric

phases

is

technique

2 and 4 was arrived

pitates

MgO-

as a non-

profiling

and

used

the

it

16 no Mg was found fracture

of the

solubility

phase

on

measurements

on

lot

14 and 15 Mg segrega-

boundaries

Mg segregation

the

second

In refs.

If

the

temperature

on SAM measurements

formed

Al2O3.

exceeds

the

a

regarding

microstructure

grain

combination

of

in

is

in

In ref.

between

literature

lines6.

surfaces

There

sintering

microhardness

and 24

the

MgO.Al203

the

segregation

14-24.

largely

at

at

boundary

Mg segregation

reflected

real

colou-

by

be

H2-sintered

bands

refs

in

limit

boundary

compensation

an MgO also

corresponds

real

grain in

rally

report

crystals.

defect

on

of

surements

(t’lgAl,h’)X

the

of the

10

is

boundary

controversy

suggests

possible very

This

observed

reported

tions.

that

conditions

sapphire

8

concluded7.9 is

are

tion

under

made

cause

the

vacancies

Mg’Al

refs

a

can be expec-

will

studies revealeo

in

of

TGA with

complex

this

Al2O3.

the

Studies

Mg occu-

the

IR

It

2 wppm O-H in

Grain

nature

refs

8

VO and

is

which

of

in

ref.

is

gave

-concentration

Defect

that

oxygen

oxidizing

Mg-doped

reported

0)x

found

-stoichiometric

sapphire.

formation

an O-H . . 0 com-

Al2O3

the

study

that

was argued

..O absorption i3-n'

of

wppm MgO in

Mg-

by a nucleation

of

surrounding. in Hp-sintered

laboratories

of

from

It the

least

compensation

14oo.c.7

was concluded

with

direct

triplets

nitude

particles

ref.

caused

Preliminary

estimate

The

In

H2 gas

(Mg’Al,O-H....

a triplet

be

and can

plot.

sapphire

on an Al site

at

A solubi-

was found’

the

Al;’

compensation

that

defect

6 the

agreement

also

conditions.

under

defects

ti+

are

in of

and

by ion

in the

ted

the

respectively

chemistry

MgO-doped

compensation

that

is

of

of

F-centres

Vi

in

powders found

In

lines

generally

sites

oxidizing defect

Al203

Arrhenius

chemistry

studies It

reduction

TGA at our

translucent

close

it

purpose

compensation tion

in

dislocation

chemistry

Al

Mg at

was studied

was

small

very

Defect

9.

Verneuil-grown

plex

the

at hig/~ tcmpevatures

ERR and IR measurementslo-13.

bivalent

mechanism.

chemistry and

are

in

136O’C

studies

The main defect

of

was studied.

by a single

and growth

Al203,

defect

sapphire

data

along

MgO in

were

H2,

7 the

13 wppm MgO at

occurred

assump-

wppm MgO.

of

of

on

solubilities

183O’C

lity

pitation

depend

segregation

1630-C

in

Verneuil

be described

25

sinte-

MqO-doped

and

250 wppm. In ref. -doped

5

and 1100

sintered

minas,

past

The

MgO in Al203

vacuum;

1730

540

doped

the

MgO in Al203

ref.

in

1630,

4.

aluminates

present.

In

were heated

the

ref.

of

the

of

The solubility 5-7.

in

discussed

and

microstruc-

MgO has been

during

given

Solubility

refs.

of

studies

is

models

about

regular

effect

ofsodium

as

solubility

are a result of

MgO

C.M.P.M. Saris, H.’ Verweij /Formation

with

decreasing

dance

with

the

of

real

absence ded In

in

solubility

clude

with in

that,

large of

boundary

refs

sence

in

are

of

in

the

con-

than

amount

Na

sence between

Cs

is

of

TGA

1.5.

on

The percentage

of

with

the

high

make

it

plausible

offers

-phase

Ns

in

to

with

is

at

pre-

a

boun-

evidence film

for in

grain TGA

such

is

some speculations

enhanced

ionic

diffusion

or

in

a

TGA. attsck

about with

explained

in

stabilizes

thermodynamic

some

terms

in which

phase

which

grain

boundary

tions

in

Na-Cs Ca

causes

terms

aluminate,

forms

a

mechanical

coherence.

TGA envelopes

in or

precursor

which in

systems

mentioned

transformation

sbout

stabilities

E-Al203

less

reac-

are

studies

ref.

31,

three

high at

cited

in

Na20.5$Mg0.14&

of

stabilized composition

tal

studies

part

It

exist

of

in

refs

Ca

are

was

with

composition with

the and

Single-crystal

coma

new

studies

in the

resulted

system 35 in

complementary;

known

theoreti-

single

crys-

in

theo-

the

Ns20-CaO-A1203 and

ref.

which at

high

36. 36

at

Both the

12OO’C

ternary

was

2Na20.3Ca0.5A1203

Na20.Al203

concluded

:A stsbilized

composition

Na20.A1203-Cs0.A1203-A1203

Na20.Cs0.2A1203,

sys-

reported

Na20.4Mg0.15A120334.

reported

The

the

been

Ns20.MgO.5A1203;

composition

6”-Al203

928 ) ,

have

32.

B “‘-Al203

been

with

compounds

binsry

resulted

be

than

by

uncertainties

Al203

Al2O3.

the

the

hampered

and

6”‘-Al203

with

of

of

are

generally

ref.

6”-Al203

cal

where

in

6”-Al2O3,

6 ““-Al203

l$

died.

Phsse

0.68Ns20.1.88Mg0.7.48A1203,

compound

the

temperatu-

temperatures

17OO’C

1~Ns20.1$MgO.1O&A1203;

retical

in

studies

compounds

of

by

relations

and Ca0.6A120330.

the

ternary

TGA attsck

lOOO-13OO’C

Na20. llA120327

in

the

especially

the

Na20.5$A120329)

into

relevant

of

of

I theor.

(theor.

of phase

kinetics

in

Ca concentra-

generally

kine-

the

insight

higher

studies

aluminate of

TGA in

systems

HP5 lamps.

Phase

kinetic

breakdown

Overall are

could

rates

reaction

the

position

Na+ 25726. of

the

of

range

in

slow

modification

TGA,

exchange

explain

paths

occurs;

applied

of

unclear,

have been

Enhancement

yet

in

litera-

still

there

in

The most

the

attsck

not

that

a second-

although

ionic

in

together

boundary

in

much higher

means that

Na20-MgO/CaO-A1203,

tem Na20-MgO-A1203

and

purity

to

corwppm,

by Ca.

needed

Phase

the

high

sodium

is

by segre-

present

slthough

tubes 1000

be

fundamental

regions.

are

res

surfaces16-19*23.

Ca

will This

diffusion

Auger

temperatures

Ca-containing

systems

A1203-rich

caused

Ca found,

enhanced

the

lamp

reaction

grain

be enhanced

that

of

the

attack

reletions

and

less

TGA,

the

the

about

Na concentration

of

a more

knowledge

and

that

of

observed

segregated

boundary

The mechanism

hours

This

fracture

no real

300

of

must be used

Phase

Na,

immersed

especially

structure,

grain

of

were

rste

sintering

Ca0.6A1203-like

samples Ca,

attack.

is

pre-

on

Na in

Ns20-MgO/CsO-A1203

demonstrsted

was found

and

thought

XPS measurements

for

the

Ca which

also

of

It

boundary

sttack

effect

where

9OO’C

the

concentrations

Cs is

levels

at

and

grain

gation

of

increased

Ns

severe

in

that

the

enhancement

To gain

a detrimental

100 hours.

of

rate

dissolved

grains.

TGA even

that

stages

arguments

tion

on grain

amount of

the Cs concentration.

tic

micropreci-

the

concentrations

means final

187

at high temperatures

to

resctions

various

liquid

responds which

present

MgO.Al203

a minor

experiments26

with

1OOO’C for

by

to

msinly

aluminates

50 wppm while

The

conclu-

we tend

indications

An effect

model

doped

ture

20-23

the

strong

Ca in

lifetime25.

dary

5-7.

TGA samples23.

TGA, Mg is

in

20 wppm, has

in

refs

actor-

was also

non-stoichiometric snd

in

Ca in TGA

There

in

of

precipitates;

and non-segregated

than

18,

normal

planes

1.4.

data

is

on thin-foil

MgO.Al203

pitetes

This

Mg segregation

a STEM study

agreement

in

temperature.

of sodium

have

studies

subsystem

was

stu-

compound as

reformulated forms

solid

temperatures.

solutions In

the

C.M.P.M. Saris, H. Verweij i Formation

188

high-alumina Na20.3Ca0.8A1203 results

a

area

of

was

ref.

36

new

found;

are

compound

the

phase

represented

in

of‘sodium

alumirmtes

2.

EXPERIMENTAL

study

fig.

The

1.

starting

Na2CO3 blic

ques

des

ture

content u,

Ca0.6A1203.

by

ture

range

loss

is

phase

information

quaternary

compounds

-Al203

known

are

1.6. To

Present see

tion

at

a very of

can

in

compounds and

and

ternary

matic

range the

of

is

by

literature.

-filled

followed Two

of

study

of

as

of

compositions were

tl-

chosen

and

B”-Al2O3;

another

Ca-content

studies

on

(1000-13OO’C)

was

stabilized

2Na2C03-Ca0-5A1203 in

the

systems

of

in

TGA.

No syste-

formation

of

the

temperature

are

known

com-

from

6

by

MPa

tempeeaweight

in

in

with

a

measured

lid

of

mocouples

the

The

to

The

level, top

of

levels

and

con-

(Degussa, A123)

Temperatures

and

the

was

200’C/hr

were

furna-

were

S-type

temperature

by

found

temperature

Germany,

material.

sample

followed

was

samples

the

of

heating

configuration.

stabilized

rate

MPa.

chamber

two

sample

heating

top

same

by

100

crucibles

Republic

using

close

the

a

580-925-C.

PID

cascade

alumina

Federal

of

inside

pre-

using

at

on

adaptive

in

half-

followed

bag

range

fired

hours

were

analysis

the

pre-

jars

PMMA die,

treatment

dense

were

a

in

5

Pellets

rubber

standard

heat

Frankfurt,

elements.

a

digital

a

for

carbonates

were

in the

a

in

were

pressing

thermogravimetric

direct

tained

level.

were CaCO3

the

mixing

balls.

of

pellets

During

below

powders

powders

uniaxial

completely

with

the

mixture

polyamide

pressing

The

of

two-sided

occur

with

the

theoretical

polyamide

decomposition

ces

m2/g.

w X in

the

the

using

control in

Na2CO3-MgO-5A1203 range

to

size

analysis.

44.3

mixtures

of

checked

Na2C03,

a function

binary

binary

uses

formation

of

mois-

particle

15.6

CO2 content

vibrating

by

The

area

of

A152,

average

640-81O’C;

with

pared

combina-

the

mixtures

low

interest

which

Na2C03-5A1203

and

related

TGA-Na

present

in

composition

the

in

Na20-CaO-MgO-

moisture

Syntheti-

France),

w X,

loss

g stock

pressure

a

the

time.

B”-Al203

pounds

in

composition

direct

or

literature.

happen

range

the

reflecting

system

powder

and

ternary in

temperatures

130,

Rubis

thermogravimetric

of

London,

44.0%.

isostatic

A1203

composition

chosen

the

approach

Na2CO3-9A1203

one

the

temperatures

temperature

the

from

pragmatic

MgO,CaCO3

in

high

study

whet

high

at

Les

surface

and

Ltd.,

JMC

Jarrie,

specific

isopropanol No

w X.

detectable

wX.

1.12

a weight

pared

0.14

no

Chem.

Kuhlmann,

Alpes,

moisture

75

content

specpure 2.7

(Ugine

checked

and

Matthey

Kingdom),

a-Al203

nas

&

CO2 content

showed

Repu-

content.

United

0.1

Federal

moisture

(Johnson

and

were:

Darmstadt,

suprapure>99.9%,

(idem),

MgO

used

AG,

Germany),

CaC03

FIGURE 1 1200’C in the system relations at Phase Na20.A1203-Ca0.A1203-A1203. The shaded area can be constructed in many ways, depending on the assumption of the relative stability of B-alumi-

Merck

analyse799.5%,

moisture

The

materials

(E.

of

zur

LO

at high temperutures

lOO’C/hr 900,

up

therfurnace

programmed to up

1000,

200-C to

top

1100,

C.M.P.M. Saris, H. Verweij /Formation

1200,

1300,

1400,

times

of

3,

1,

10,

set-point

rStUre

conformity O.l’C

conformity

an

using

(Nederlandse

a type

and CuKa

radiation.

tic

in

peaks

converted

the

into

responding

with

Ca-containing

of

to

-.

cor-

factors,

In the results the

were

the

calibration

owing

powder

spectra

mixtures. the

3 ;

Eindhoven)

quantities

mixture

::

characteris-

diffraction

using

be made quantitative the

of

standard

40

diffractometer

Intensities X-ray

the

samples

X-ray

b.v.,

components

obtained

The

by

PW 1050

2 -ii

was about

0.25%.

Eedrijven

relative

2’C;

case

of

could

not

complexity

of

Mixture

heat

of

the

It

and

of

with

that

its

to

The weight

mixtures

was estimated

results

are

at

top

level

From this has

of time

figure

completely and that

l4OO’C

the

At

centration the which

is

results the

is

also

can be seen

a weight

temperatures of

expense

B”-A1203 of

the are

literature.

the

given

accordance In ref.

-

x-x

I

\-

and

Na20.Al203 firing

between mixtures of

above

phase

at

the

decreases

is

l59O.C.

found

of con-

These

that

1~00

1500

1600

T(“Cl

FIGURE 2 Calculated compositon of Na2COj-9A120) mixtures after heat treatment with 10 hours top level time as a function of top level temperature.

at

B-Al2039

observations

with

1300

1200

and

ShOSt

B”/B-A1203

of

IWO 1lW

at

1200 is

l4OO’C

gradually

10 hrs

2.

that

ratio

37 it

Al,Oa

-*-8

reacted

for

after

the

B-

1

I

I

-x- +” A1203

patterns;

in fig.

concentration

dominant in

X-ray

at temperatures

with

6.

of the

fired

it

of

rather

Y/

of (NA

I.

mixture,

composition

constant,

the

the

X (WT%I

relatively

of

disappeared

1200’C

about

from

60

the

of

I

-+-=Na20.Al&13

70

at

is

is

a mixture

in Table

the

studied

reacts

80

below

Ne20.Al203

composition

given

The composition

only

formation

form

90

time

decomposition

subsequently

Al203

mixtures

top-level

was

that

after

B”-A1203.

the

of

I

I

ct- .AI,O,

dependence

temperatures

time

first

Na20.Al203

slowly

a strong

at

was found

formed

carbonate fast.

only

effect

llOO-l4OO’C. compound

Na2CO3-9A1203

showed

The influence

remarkable

l2OO’C;

the

treatment

on temperature. was

lOO(

Na2CO3-9A1203

The composition after

h

Table I. X-ray peak heights and calculated composition Na2COy9Al203 mixtures after heat treatment = Na20.Al203).

spectra. 2.1.

189

-

level

The tempe-

level

inspected

Philips

top

hours.

top

of

and

with

was within et

accuracy

air-quenched

diffraction

the

30 and 100

and stability

with

were

and 159O’C

1550

of sodium aluminates at high temperatures

transformation 14OO’C

is

in

not

stable

the

res

that

rate

of

infinitely with are

than

B”-Al203

and

slow

respect

higher

64

to at

that

E-Al203 the

below

B”-Al203 at

is

temperatu-

most 14OO’C.

The

190

C.M.P.M. Saris. H. Verweij / Formation

instability above

of

1550-C

also

found

in

The slight at

binary

in

the the

of

of

is

not

refs

was

38-41.

concentration

14OO’C can

which

at high temperatures

be ascribed

critical

to

for

the

study.

2.2.

Mixture

The

heat

for

the

position

for

the

treatment

are

Na20-VA1203

of

the

from

the

given

in Table

60

Na2C03-5A1203

results

after as

studies

aluminates

temperatures

Na20-A1203

the o-Al203

above

Na20 evaporation present

at

system

phase

increase

temperatures

6”-A1203

binary

of sodium

mixture.

reacted

X-ray

Na2CO3-5A1203 qualitatively

was

and

the

50

same

The weight

mixtures

patterns

mixture the

X[WT%)

com-

estimated

results

are

II. I

lo-

-.

-.

110O 1200

1300

1 hl h:

0

d

900 1000

/ 1500 16CKl

1.400

T(T) FIGURE 3 Calculated composition of Na2CO3-5A1203 mixtures after heat treatment with 10 hours top level time as a function of top level temperature.

O-Al203

at

159O’C

sodium-rich

ascribed of (NA

The

composition

of

the

top-level

time

is

The

composition

constant

in

of the

with

a 8”/8-Al203

sion

to E-Al203 The

below

it

is

atomic and

ratio is

is

overall

scale

is

of

about

6.

complete

that

the

independent

to

2.3.

the

reaction

The observation

The conver-

the

larger

by

ratio

temperature

suggests the

mechanism

between that

that on

Na20.Al203

conversion

to

in

overall

the

most

surprising

used

are

Na2CO3-MgO-5A1203

heat

time

below

but

mobility

if

can

be

in

the

and

tes

and

mation hr

first

at

sent.

the

that

its

not

the

where

were

unreacted

of

at

is

top-level

is

was

at

also

the

compound

the

carbona-

of

fast.

complete

MgO was still

after

top

time

It

rather

yet

of

temperatures

studied.

quantities obtained

mixtures

dependence

influence

only

Na20.Al203

formation

theoretical

and MgO.Al203

a strong

decomposition

MgOaA1203 was not

900’C The

influence was

after

that of

and

remarkable

159O’C again

formed

mixture

Na2CO3-MgO-5A1203 showed

again

was

12OO’C;

1550

of

treatment

temperature

level

observed

at 159O’C.

of

after on

: again

B”/8-Al203

both

10

3.

remarkably

1200-14OO’C

determined

of

with

in fig.

Na20 concentration

completely

a-A1203.

fired

given

mixtures of

almost

observation 14OO’C

and of

the

range

mixtures also

somewhat

mixture.

The composition

hours

more complete is

arguments

stoichiometry Table I X-ray peak heights and calculated composition Na2CO3-5A1203 mixtures after heat treatment = Na20.Al203).

is

mixture

of

For-

after

1

pra-

Na20.A1203

firing

for

10

C.M.P.M. Saris, H. Verweij /Formation

hours

at

react

compounds B/Ef11-A1203 the

At

1OOO’C.

mixtures.

incorporation

not

be

to

drawn

from

in

the

X-ray

position

of

the

reacted

diffractograma,

of

the

IY-Al203

to

due MgO is form

a

6”-Al203

phase.

No

aluminates

could

be

The weight

com-

estimated

from

mixtures,

are

could

diffractograma

diagrams.

given

in

at high temperatures

about

compounds

Moat

sodium

detected

the

X-ray

ternary

Mg-containing

form

to

conclusions

into

stabilized

both

Al203

these

limitations.

incorporated

partially other

Mg in

the

temperatures

with

Direct

of

resolution

probably

higher

further

of sodium aluminates

Table

III.

n

Kl

FIGURE 4 Calculated composition of Na20-MgO-5A1203 mixtures after heat treatment with 10 hours. top level times as a function of top level temperature.

found

in

the

binary

difference

Table III X-ray peak heights and calculated compositions of Na2CO3-MgO-5A1203 mixtures after heat treatment (NA = Na20.Al203 and MA = MgO.Al203).

strong at

with

increase

can be seen

appeared is

after

the

the

system

binary

is

Na20.Al203

at

a

The

after

present

10 hours

at

top

and

part

of

of

present,

of

level

top

quantity

the

time

is

temperature

position

of

stant

but

ratio

of

in 1.6

the this

range mixture, case

instead

of

of

llOO-14OO’C

again, with the

is

value

of

10 hra.

of

Mg. At a

catalyst

2.4.

has

The

Mixture

dependence

con-

weight

6 that

was

after

top

level

to

1400’C.

tion

position

effect

a top

of

of

level of

the

B”-A1203

by

Mg apparently

formation

of

heat on time

acts

B”-A1203

to

form of

2Na2CO3.CaCO3.5A1203

treatment

also

temperature

and

was remarkable

From high

experiments

starts

Mg-stabilized with

as and

2Na2CO3-CaCO3-5A1203

composition

given

the corn--

the

almost

a demonstration

temperatures

the

6”-Al203

B-A1203.

tures

in

partially

is

is

of

The mixture

at 159O.C

This

for

especially

mixtures

14OO’C. of

firing

noticeable

concentration

stabilization

lower

mixtures

almoat

a B”/B-Al203

time

the

4.

In the

consists after

Another

binary

the

above

B”-Al203

well-known

At 11OO’C 10 hours

initially

composition

which

temperature

mixtures.

considerable

already

dia-

llOO’C,

corresponding

and MgO.Al203,

disappeared. for

firing

stationary

when

B/B”-Al203

fig.

the

has completely

Na2CO3-A1203

becomes

time

fired

a-Al203

10 hours

below

lOO’C,

for

level

that

the of

temperatures

completely It

mixtures.

it at

the

was observed 700’C

Na2CO3, followed

mix-

a marked

influence

at temperatures

temperature

about

showed

X-ray that with

of up

diffracNa20.A.1203

the

decom-

by decomposition

of

C.M.P.M. Suris, H. Verweij / Formation

192

CaCO3

up

to

8OO'C.

Na20.3CaO.RA1203 starts

to

The form

component at

1OOO'C

of sodium aluminates

at high temperatures

study of ref. 36 if it is assumed that the limit of solubility of Na20.Al203 in Na20.Ca0.2A1203

together with the formation of B/B"-Al203; the

at 11OO'C is such that the overall composition

concentration of this component goes through a

of the mixture is within the phase triangle

maximum at firing temperatures of 12OO'C and is

Na20.Ca0.2A1203/Na20.A1203S.S. -

almost zero after firing at 16OO'C. Due to the

l3"-A1203-Na20.3CaO.I3Al203. This assumption can

complexity of

be made plausible from the observation that at

the

spectra

the

distinction

between E- and 6"-Al203 could not be made with

11OO'C and top level times less than 10 hrs a

lany accuracy. If b/D"-Al203 is counted as one

mixture of only s-s. and Al203 is formed. If it

phase, the number of phases found in the mix-

is assumed that the S.S. already has reached

tures fired at temperatures above 1OOO'C was

equilibrium the composition of the S.S. limit at

always 4, which conflicts with the phase rule.

11OO'C can be estimated to 32% Na20. Wualitati-

This discrepancy can be ascribed to the fact

vely the reaction rate of the components is

that Na20.Al203 and Na20.Ca0.2A1203 form com-

about the same as for the binary Na2CO3-A1203

plete solid solutions (s.s.1 at high temperatu-

mixtures. This gives an

res that decompose on cooling36. No attempt was

sluggish formation of B/B"-Al203 at this tempe-

indication that the

made to estimate the weight composition of the

rature is rate determining :

mixtures because of the complexity of the X-ray

cannot be formed directly from the solid solu-

patterns. The

relative peak

heights of

the

components found are given in Table IV.

Na20.3CaO.RA1203

tion and Al203 without the formation of another Na20-containing compound, which in this case is 6/6'l-Al203.

The course of the reactions at 1200, 1300 and 14OO'C as a function of top level time is qualitatively the same as for the reactions at 11OO'C except that the reaction rates are increasingly higher while the ratio (Na20.3CaO.l3A1203)/(R/B"-Al2O3) steadily decreases. This can be ascribed to a gradual expansion of the two-phase area s.s.-E"-Al2O3, at the expense of the two-phase area

s.s.-Na20.3CaO.BA1203,

towards

the

Na20.Ca0.2A1203 composition with increasing temperature. With this expansion the composition of Table IV Relative peak heights for 2 Na2CO3-CaO-5A1203 mixtures after heat treatment (N q Na20, C = CaO, A = Al2O3).

-6"-Al203 boundary, which means that at some

The reactions at 11OO'C were not yet stabilized

longer present. This phenomenon is indeed obser-

after 100 hours firing. At this temperature the

ved for mixtures, fired at 1600'C. The qualita-

the mixture gradually approaches the low s.s.-

temperature above 14OO'C these two phases will be in equilibrium and Na20.3CaO.RA1203 is no

combination Na20.Ca0.2A1203/Na20.A1203apparent-

tive course of the reactions after 10 hours at

ly reacts very slowly with A1203 to form the

top level as a function of temperature is given

high

alumina

compounds Na20.3CaO.RA1203 and

r'3/B81-A1203. This is in agreement with the phase

in figure 5.

193

C.M.P.M. Saris, H. Verweij / Formation of sodium aluminates at high temperatures probably

a concurrent

E/W’-Al203

the surface

at

reaction

grain

of

boundary

reaction

of

Al203

(3b);

6”-Al203

(and &A1203?)

is not certain

investigation.

uD0

1500

ml0

FIGURE

of

of

the maximum in concentration

the component Na203.CaO.RA1203 is

with

the

slow

but

kinetics

at low temperatures

sing

the

of

of reactions processes

relevant effect

of

for lifetime of reaction

dary coherence tion

1.

and with the cros-

planes. -

for

precursor

phase.

3.

REMARKS AND CONCLUSIONS

micro-precipitates

clusions

results

about

magnesium-

the

and

can be used to draw con-

mechanism of

calcium-containing

sodium vapour in the temperature -13OO’C.

These reactions

into three steps: tion

Reaction

Na20.Al203;

TGA

of with

range of lOOO-

can roughly be divided step 1 must be oxida-

of Na to Na20 by reduction

is s reaction

reactions

of TGA. Step 2

of Na20 and Al203 to the compound fast

formation

of this

compound from

Al203 and Na2C03 was always found in our experiments. Step 3, the actual

corrosion

process,

is

of

be most

The

as it

grain

is

boundary

grain boundary phase network for

instance,

the

Na20.3CaO.RA1203

presence

of

MgO.Al203

or normal second phase seems importance

as it

is

present

along the grain boundaries. of reactions

have to be the subject in which the present useful.

the

connected

only discontinuously Further details

for reac-

Ca seems to

along

for

to be of secondary

is destroyed

boundary corrosion

a perfectly of,

This step,

1. A secondary

becomes available

The Ca0.6A1203-like

formation

The present

rate-determining

is reaction

grain

binary at higher temperatures.

of lamp

lamps.

HP5

an

3 may be that the grain boun-

continuously

provides

be

that the reac-

of the TGA material

6”-Al203 FINAL

to

than the rates

current

The presence

important

S.S.

of further

conclusion

study is

present

Na20.Ca0.2A1203/Na20.A1203

Mg2+ or H+ or

2 and 3 are generally

rates

formation

fast

of reac-

tion

connected

increasingly

boundary lines.

is the subject

One general

so that more surface The occurrence

and a normal

by dissolved

this

means that the primarily

5

on the

drawn from the present

lifetime Relative peak heights of characteristic X-ray 2Na2CO3-CaCO3-5A1203 mixtures reflections of after heat treatment with 10 hours top level time as a function of top level temperature.

(3~)

on grain

order of magnitude larger

T(‘C)

and

grain-boundary

whether the kinetics

tion 1 are influenced

1300

6/B”-Al203

about the same as 3c with normal

other impurities;

1000 1100 1200

Ca0.6A1203-like

to

with B/B”-Al2O3 to stabi-

MgO.Al203 precipitates

900

boun-

MgO.Al203 microprecipitates

bulk reaction, It

a grain

a pseudo

grain boundary planes lized

a number of

(3a);

Na20.Al203,

and

Na20.3Ca0.8A1203

REL. PEAK HT(%l

of

: Reaction of Na20.Al203 with Al203 to

reactions dary

process

1, 2 and 3 will

of more detailed results

may appear

studies to be

C.M.P.M. Saris, H. Verweij 1 Formation

194

of sodium

alumirlates

at high temperatures

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