The sintering characteristics of BeO

THE

SINTERING

CHARACTERISTICS IL

The effect

of

various

characteristics presence

parametrrs

of brryllium

of water

vapor

on

oxitlt,

OF Be0 j-

.AlTKEI\

A.

t,ht sint.rring

is tleseribecl.

in thr sintclring

Tttc~

at~mospherc

has been found t,o retard the dcnsiiicatSion of bcryllirlm oxide

compacts.

thr compacts

The sinterability

are predried

in a dry atmosphere. sintering cent

rate

MgO

Weryllinm

The eff&

is largely

is added

longer

and NiO,

compacts

times

‘/A mok~ per

calciricd at. 1250” C insteatf similar

nlt~imatc don&&

or higher

trf AIOl.5, (‘rOl.5,

increases thfl ultimate, cltGty to

compacts

are l)art,icularly

at.

t~Tnr)~,rat,~~rf~s.

of l/l molr l)er cent

relative

MgO and Al&y

if

sint,erinp rat,tA but, a,ppears t,o

of achieving

The present? MgO,

if

air and sintcrcti

of wat’cr vapor on the

rlirninatetl

oxide powder

sintering

’ (1’in

to the 13~4 compact.

of 900” C has a slower be capable

(WI be improv~~l

at 900

withortt

of IGO additivca.

~~ffi~c:t ivo at, incsrraxing

thu density.

Lrs

1.

poudres

d’oxytlt~

tit* hc~r\~lli~lrri c:alcin&s

A

Introduction

The sintering behavior of Be(.) has not been extensively studied beceute of the lack of high purity, low-calcined Be0 powder. Earlier studies 1,“) have indicated t,hat,cnlcining temperature aud t,he presence of certain iIl~~urit,ies, such as MgO, have a profound effect on the sint,ering characteristics of Be<). The,:e studies t

This work

was

done

iuidt,r

tht’

auspicf~s of

thr

AT(ll-I)-171. 302

C.S.

Atomic

Enfxyy

(Iommission

untlt~r (10n(;ra(:t

THE

SINTERINC3

are the principal measurements terize

the

sintering

paper a) is being initial

sintering

used to charae-

behavior.

published kinetics

A

companion

elsewhere of

CHARACTERISTICS

beryllium

on the oxide

OF

303

Be0

were stored in a closed container allow

eql~ilibration

compacts

within

the

overnjght compact.

to The

were pre-fired at 900” C for two honrs

in air t.o remove the water and decompose

the

which interprets in more detail many aspects of

salt. No measurable density change was observed

sintering

by the pre-firing

2. 2.1.

of Be0

described

here.

2.2.

Experimental

Be0 STARTING

MATERIAL

All material used in this study had a total impurity content of less than 500 ppm and was prepared by calcining beryllium oxalate in dry air. The oxalate was calcined at 900” C for 8 hours. In addition a small batch of powder was calcined at 1250” C for 2 hours by heating the 900’ C calcined powder at; the higher temperature. The average particle diameter “da” was increased from 0.4 (u to 0.9 /t by the 1250’ c) heat treatment (as determined by lineal analysis of electron photomicrographs). The calcined material was screened through 100 mesh prior to compacting. All compacts were made by pressing without binder at 20 000 psi in a steel die one-half inch in diameter. The resultant thickness of t,he compacts was abont 200 mls. The initial density was between 1.59 and 1.61 g/cc for material calcined at 900” C and between 1.43 and 1.45 g/cc for material

calcined

at 1250’ C. Because

it was

suspected that water vapor was affecting sinterability, all compacts were dried between 850” C and 900’ C in air except where specifically stated. Additives were introduced into the compact after pressing when it was found that an aqueous solution was readily absorbed by the compact and diffused uniformly through the compact. About 0.3 cc of solution was needed to effectively distribute the additive homogeneously through the compact. Solutions were made up from the nitrate salt, of the cation additive to a concentration that would be equivalent to r/4 mole percent, of the beryllium oxide present. A chemical dye was added when the solutions were colorless to trace its penetration through the compact. The compacts with the solutions

SIXTERINC

compacts

t’reatment. PROCEDURE

were sintered in three atmos-

pheric environments : hydrogen, oxygen. The furnace used for hydrogen

vacuum,

and

sintering was

externally wound with molybdenum around a morganite tube closed at one end. The hydrogen was introduced at the open end at a controlled dew point. The compacts were located in separated compartments of a molybdenum boat which was loosely covered. The compacts were inserted while the furnace was cool and raised up to the operating temperature at the rat(e of between 400” and 500” C per hour. All cornpa& were heated for two hours except where specifically stated. The temperature was recorded with a Re-W thermocouple located next to the molybdenum boat. The furnace used for vacuum sintering was heated by a molybdenum sleeve by direct resistance. A pressure below five microns was maintained at all t#emperatures. A graphite holder was used to suspend the Be0 compacts inside the molybdenum sleeve. The compacts were generally about

pre-fired

at a temperature

1200” C for two hours to remove

of any

trapped gases before being sintered. The pellets were raised to the sintering temperat~lre in about five minutes and maintained at that temperature f*or two hours. The temperature was recorded and maintained which previously

with a pyrometer-controller had been calibrated against

a Re-W thermocouple. The furnace used for oxygen sintering was heated by a 60 9/, Pt-40 y0 Rh internally wound furnace. Oxygen was passed slowly through the furnace during sintering. The compacts were located in a rhodium boat which was loosely covered and the boat was inserted while the furnace was operating at about 1500” C. The

temperature

of the furnace

the sintering temperature

was the11 raised to

in about 1.5 minutes.

The temperature was recorded wit,11a Pt-10 yb R thermocouple

up to 1600” (!. For higher tempe-

ratures

an optical

furnace

proved

to

pyrometer have

oxygen

at temperatures

cessive

volatilization

3.

was used.

a short

lifetime

This in

of 1800” C due to ex-

of plaOinum metal.

Results

The sintering temperatures ranged from 1600” to 1900” C. However, all specimens with various additives were not sintered over the entire range since the effect of the additive could be characterized usually by sintering at’ a few temperatures. Some additives were not used in certain atmospheres where the additive was known to be unstable such as NiO in hydrogen. UOZ, and Crz03 in air. A few experiment:;, however, are shown where metal was formed by reduction of the oxide to illustrate this effect. No attempt was made to study the effect, of concentration of the additive on the sintering of BeO. Usually, however, l/a mole y0 of additive was beyond t’he solubility limit as indicated by the second phases present in some of the photomicrographs. The density was measured by immersion in CC14and corrected where measurable absorphion occurred due to open porosity. The density was also corrected for the presence of the additive, however, this was usually negligible. 3.1.

EFFECT OF CALCINING

TEPTPERATURE

Separate compacts, one made from material calcined at 900” C and the ot,her from material calcined at 1250” C, were sintered in vacuum, simultaneously at various temperatures for two hours. Results indicate as shown in fig. 1 that the higher calcined material has a lower sintering rate but the ultimate density between the two different calcined materials differs very little after two hours at 1800’ C. The higher temperature calcined compact had a slightly lower initial density which required 10 percent more volume shrinkage to achieve the same ultimat’e density. Both grain size and shape of the two

P

/I

1500 Fig.

1.

A

Be0 CALCINED 9OO'C VACUUM DRIED 900°C

o

Be0 CALCINED 9OO'C NOT VACUUMDRIED

0

Be0 CALCINEO 1250-C VACUUM DRIED 9OO'C

1700 SINTEAING TEMPERA:%

Yrrccnt

theoretical

3.2.

were

EFFECT

I 2000

essentially

OF WATER

l°C1

density

t ~3-0hwm3 at temperature

compacts sintering.

I 1900

I

1

1600

I

of

UeO

afiler

in vacuum.

the

same

after

VAPOR

Water vapor has been found to affect strongly the sintering rate of beryllium oxide. Fig. 1 and table 1 show this effect. Fig. 1 demonstrates the importance of pre-drying the compacts at 900” C before vacuum sintIering at higher temperatures. The Be0 powder calcined at 900” C was found to absorb as much as one weight percent water vapor from laboratory air. If water vapor is trapped within t)he compact1 at the xintering temperature, Be(OH)z gas will be formed. It is TABLE Effwtj

of'

M&W

sintaring

vapor at

1

on

hytirogtxl

1700” C for two

atmosphere holux

THE

CHARACTERISTICS

OF

305

Be0

that the formation of Be(OHf2 gas

believed within

SINTERING

the pores is responsible

densities

through

for the lower

mechanisms

discussed

on

page 309.

Table 1 also shows hydrogen

atmosphere

t‘he influence of wet sint~ring on the ultimate

density even when the compacts were pre-dried. The presence

of MgO as an additive

in Be0

appears to reduce the influence of water vapor to a negligible

effect, as shown by the last two

experiments at the bottom of table 1. Pure Be0 compacts when sintered in dry hydrogen in the presence of other Be0 compacts containing certain additives were found to have lower densities than when sintered alone. The effect on the pure Be0 compact was attributed to water vapor since many of these additives such as magnesium oxide, chromium oxide and uranium oxide were reduced in hydrogen to the metal or to a lower valence oxide producing water vapor. The presence of silica in the hydrogen furnace also was found to increase the water vapor content such that the sinterability

I

0

-

DRY OXYGEN

I

TEMP("C I Fig. 2. Percent theoreticaldensity of ReO after two hours at temperaturein various dry atmospheres.

of Be0 was markedly decreased at high tempera-

BeO. The presence

tures.

the pores may have stopped shrinkage and grain growth as shown in fig. 3 where the grain structure of Be0 when vacuum sintered above 1700” C is smaller and larger voids are present between the grains in contrast to similar specimens sintered in hydrogen.

3.3.

MICROSTR~CTURES

OF SINTERED

BOO

COMPACTS

It was noted that higher densities obtained with dry hydrogen atmospheres

were than

with vacuum or dry oxygen. The same trend was noted when addit,ives were present. Fig. 2 shows the measured densit’y of pure Be0 sintered in

various

dry

atmospheres

at

a series

of

temperatures and Fig. 3 shows the corresponding microstructures. The grain shape appears to be equiaxed throughout the temperature studied and in all atmospheres.

region

The vacuum sintered compacts showed little increase in density beyond 1700” C. One set of compacts sintered at 2000” C (not shown) showed an extensive amount of fissures indicating a reaction had occurred. Since it is known that graphite reacts with Be0 at temperatures close to the melting point, sufficient reaction may have occurred at sintering temperatures used here to influence the late stages of sintering of

of volatile

molecules a) in

Be0 sintered in oxygen appeared to give the lowest densities ; however, it is di~cult to determine from the data whether the effect of an oxidizing

atmosphere

on the sintering

rate is

real since different rates of rise to temperature and different furnace characteristics were present for the various sintering atmospheres. Better control of the furnace charaoteristics and sintering schedules are needed before further interpretation can be made. 3.4.

INFLUENCE

OF ADDITIVES

Several different types of additives were int(roduced into Be0 in the manner described earlier. All additives except lithium fluoride, lithium nitrate and beryllium fluoride caused marked increases in the density of sjntered

j.ill

THE

SINTERING

CHARACTERISTICS

Fig.

3.

sintering

Ol? Be0

Photomicrographs in various

for two hours.

307

of Be0

atmospheres

250 x

compacts

and

Etchant-HF

after

temperatures 2 MIN.

compacts.

(!om~)aot-s wit,h hhe.ie three additives

had t(he same densit,? as unadult~erated beryllium oxide.

It is assumed

an early

stage

additives

which

temperature sintering

formed

t’hey evaporat’ed

wit’11 t,he

As a group

the Be0

lowest,

at) the

eutject’ic

ga\‘e t’he fastel

rate and larger grain size as shown in

the photomicrographs tives

that

of sintering.

of Be0

containing

of A1203, MgO. and NiO.

containing

additives

add-

CrO, .5 Clhroniium oxide forms compounds which

are isomorphous

syst,em. Eo evidence

The cornpa&

which form higher eut,ectic

with

of extensive

was not’ecl with Cr203 additive

wit’11Be0

t,he Xl&

-Be0

grain growt 11

as \vas observed

temperatures with He0 such as (jr&, appeared to be closer to pure Be0 compactIs in densit.) and grain structure. Fig. 4 compares the relative densities obtailletl with various additives sinCered at 1700” ( ’ ill

wit811A1203 sintered at, t’he xa)rne t8emperatIures. The grain size of Be0 containing I,‘_, mole ‘Ib of (‘rO1,5 wa’i about, the same ss with pure He0 after siliteritig at, temperat8ures of I X00” C! antI beloT+,. At l!~OO”t-1 in hydrogen the grniil size

three different, at,mosphere.+. The relative ilrfluence of t’he addit(ires is the same in all three sintering environment,s. The effect, of each

leas about

additive studied is described below in more detail. The concentration of the additive was present at about l/a mole y0 of the formula written.

Small amounts of aluminum increased the density of HeO, large grains containing possible were developed as indicated 100

oxide markedly and extremely amlealing twins by t,he phot80-

r

t8hree times

larger

than

\vitjh pure

Be0 a11tl a slight, indicat,ion of t~winnjlig at a few of grain houndaries n’as rioted as shown iti tig. 6. The additive concentration it1 t’his case was reduced t 0 l/l2 mole 9,; t’o diminish t’lie excekve amolult, of second phase present at 114 mole

(‘().

The presence of MgO as an additive improved t’he ultimate density of BeO. The resulting grain st’ructure was larger but equi-axed at t#he temperatures

studied

as shown in fig. 7.

NiO Kicakel oxide which should form a plia>e y>,st,cln witlt Be0 similar to Be0 -RlgO is trot.

NONE

Fig.

4.

Be0

containing

Percent

for two

theoretical various

hours

density

additives

in different

of cwmpacts

sintcretl

of

at 1700” C

atmospheres

Fig.

5.

Photomicrograph

i1101.5 uf%cr sintering 250

x

of Be0

witjh ‘/_I mole

Id

for two hours in Hz at, 1700” C. Etchant-HF

4 MIX.

THE SINTERING

CHARACTERISTICS diffusion

OF Be0

309

model used in other sintering studies

4,5,6) and the evaporation-condensation discussed

by Kingery

model

and Berg 6). The inter-

ference of water vapor with shrinkage is presumably

caused by a high rate of distillation

gaseous

beryllium

hydroxide

formed

at the contact

in the

compact.

into

the

of

neck

points between particles

The rapid

mass transfer

of

material into the neck decreases the curvature initially

Fig.

6.

(301.5

Photomicrograph

of Be0

with l/12 mole

?A

after sintering for two hours in Hz at. 1900” C. 12.5 x

Etchantj-HF

4 MIN.

stable in either vacuum or hydrogen at the sintering temperature reducing to the metal. One sintering experiment at 1700” C in oxygen indicated that the grain size of Be0 with NO as an additive was comparable to Be0 with MgO as an additive both of which were much larger in grain size than pure BeO. 4.

Discussion

4.1.

EFFECT OF WATER VAPOR

The effect

of water vapor

on the sint’ering

kinetics of beryllium oxide has been discussed in more detail in the companion paper 3) on initial sintering kinetics. The shrinkage behavior of Be0 may be interpreted in terms of the bulk

present at the neck faster than it can

be decreased by bulk diffusion. Since the curvature provides the driving force for bulk diffusion, the diffusion process is retarded by distillation. Only the bulk diffusion mechanism leads to shrinkage, therefore water vapor has the net effect of retarding the shrinkage rate, Pre-drying of compacts at 900” C appears to eliminate most of the entrapped water vapor, since the highest densities are observed with this treatment. The initial sintering kinetic measurements have shown that when water is absorbed in the compact of BeO, the shrinkage rate is effected very strikingly. If compact’s of Be0 are not dried, the absorbed water may create a sufficient vapor pressure of Be(OH)z gas at the sintering temperature to seriously effect the sinterability of the powder compact. The presence of an additive such as MgO appears to decrease if not eliminate the effect of water vapor on the sinterability. The presence of 114 mole o/o of MgO increases the sintering rate of Be0 compacts.

If the increased sintering

rate can be attributed to faster diffusion rates, the presence of magnesium oxide probably makes the bulk diffusion mechanism dominate over the evaporation-condensation mechanism such that no effect of water vapor is observed. 4.2.

EFFECT OF CALCINING TEMPERATURE

The fact that lower fired densities are observed

Fig.

7.

Photomicrograph

of Be0

with ‘/a mole

7;)

MgO after sintering for two hours at 1700” C in Hz. 2.50 x

Etchant-HF

4 MIN.

with pure Be0 calcined at higher temperatures is in agreement with the more extensive results reported both by Quirk I), and by Livey and Williams 2). They indicate that the maximum density is obtained with Be0 powder which is calcined between 900’ C and 1000” C. A higher calcining temperature produces a larger average

particle tvhkh

size wlticl~ slows ctowt the sinterillg tllieil

towc~~s t,he final

~int~ering r:tt,es. however.

tlemity.

nrerel~~ require

rake

Slower

longer

wintering time.3 or higher t~emperatnre* for given time a11d the final densitie::

of tu;bterial calcined *(.?s stm11ri ~~~~~)r(~~~~h at two (~ifferet~t ~e~~~J~eratlli each ot,her RS t.ht sinterirlg temperature is

increased. The densitjias slwu~tl ilr fig. 1 for the material caIcine(l at !IOO”(’ iltltl 1250” (! folio\\ t;his hehr?,Vior. &.:<. &WECT OF ADDITIVES From brief sLadies reported

here it, is dificult

t’o make specific interpretations of t,he results. All additives st,udied promoted sintering wit,hontv t.he presence of a liquid phase. (~e~~era~l~ the solid stat,e sintperiug rate is nccelerat,ed by creating va,eancies within t)he iatt,ice. Ry intzoducing additives whose cations are of a valence different from beryllium OJM might expect an increased sint,ering rate in agreement w&h t.he t~hser~~tiolls reported here. However. it. is not nnderstjood how t,he presetice of magnesium or nickel accelerates the sintering rat,e. Bot’h magnesium do not, s&iotl The Al&

and nickel are bivalent~ and therefore create vacancies throtqh charge compeiiwhen il~~~)r~~(~r~t,e~l into t.ht He0 lntStice. obtninetl witch 1mus~4 grails t,esttife additive re.