Phase equilibria in the system cao bao w

Evans, “Standard X-Ray Powder Diffraction Patterns, Vol.. Introduction HE phase diagram for the system CaO-BaO-WO, has not T been reported in the literature, but some data on com- pound

514 Journal of The American Ceramic Society-Kreidler Vol 55, No 10

lites The first step in the substrate process is ball-milling to

break up these aggregates Increasing the amount of ball-

milling would lead to greater breakup of the agglomerates

and more crystallites However, still further ball-milling

could break up the crystallites which should fracture perpen-

dicular to the basal plane, thus reducing their platelike shape

This reasoning predicts that the texture in the unfired tape

should be strongest for intermediate amounts of ball-milling

Similarly, texture in the tape could be influenced by the

rheology of the slurry, which is affected by the proprietary

deflocculating agents, binders, and plasticizers

Enhancement of texture during firing would appear to re-

sult from either selective or anisotropic grain growth Varia-

tions in texture with the same firing schedule could result from

differences in grain-growth inhibitors Nakada and SchockI4

have suggested that selective grain growth occurs in grains

with low-surface-energy planes exposed a t the substrate sur-

face, resulting in a texture primarily in the surface grains

Such a model is consistent with their observation of a strong

effect of sintering atmosphere on texture However, in the

present work it was shown by both X-ray techniques and

transmitted polarized light that a strong texture extends through

the entire thickness of the substrate It is not clear that a

surface-controlled texture would extend through a thickness

containing > 100 grains

V Summary and Conclusions

High-density ALO, substrates have a significant preferred

orientation or texture The major component of this texture

is characterized as a basal-plane fiber texture with the fiber

axis normal to the surface of the substrate and the basal-

plane pole parallel to the fiber axis The strength and s h a r p

ness of the texture vary from lot to lot and among suppliers,

but its character remains essentially the same Unfired tapes

exhibit a weak texture that is enhanced during firing without

changing its character appreciably

The variation in the physical properties resulting from tex-

ture in AL03 substrates does not appear to affect the perform-

ance of current substrates Therefore, the principal use of

texture may be as a process control in the production of sub-

strates The mechanism of texture development in the un-

fired and fired states, which cannot be elucidated clearly a t

this time, is probably initiated from the preferred orientation

of the platelet-shaped initial crystallites in the unfired tape This preferred orientation is enhanced during firing by selective growth of specially oriented grains

Acknowledgments The writers a r e pleased to acknowledge the assistance of

G E Johnson (dielectric-constant measurements) and S E

Koonce (electron micrographs), the frequent discussions of texture and its determination with B C Wonsiewicz, and the careful review of the manuscript by T D Schlabach, M D Rigterink, and P R White

References

e:

’ C B Barrett and T B Massalski, Structure McGraw-Hill Book Co., New York, 1966 of Metals, 3d

D W Baker H R Wenk and J M Christie “X-Rav Analysis o f Preferred Orientatibn in Fine Grained Quartz A& gregates,” J Geol., 77, 144-72 (1969)

A H Heuer, D J Sellers, and W H Rhodes, “Hot-Working

of Aluminum Oxide: I.” J Amer Ceram SOC., 52 191 468-74 _ -

(14969)

J L Pentecost and C H Wright; pp 174-81 in Advances

in X-Ray Analysis, Vol 7 Edited by G R Mallett, Marie Fay, a?d W M Mueller Plenum Press, New York, 1964

Hideo Tapai Tuvia Zisner T Mori and E Yasuda “Pre- ferred Orienratibn in Hot-Pressed Magnesia,” J Amer Ceram

T L Schock, Bell Laboratories, Allentown, Pa.; personal

S;C., 50 [lo] 550-51 (1967)

communication

‘ J J Thompson, “Forming Thin Ceramics,” Amer Ceram

H E Swanson, M I Cook, Thelma Isaacs, and E H Evans, “Standard X-Ray Powder Diffraction Patterns, Vol

9,;’ Nut Bur Stand (U.S.), Circ No 539, 1960; 64 pp

Powder Data File, Joint Committee on Powder Diffraction Standards, Swarthmore, Pa

‘OW P Chernock and P A Beck, “Analysis of Certain Er- rors in the X-Ray Reflection Method for the Quantitative De-

termination of Preferred Orientations,” J Appl Phys., 23 [ 3 ]

A Von Hippel and W B Westphal, “High Dielectric Con-

stant Materials as Capacitor Dielectrics,” Tech Rept 145,

Massachusetts Institute of Technology Laboratory for Insula-

ti Research, Dec 1959

W F Brown, Jr.; pp 1-154 in Handbuch der Physik, Vol 17is Edited by S W Fluegge Springer-Verlag, Berlin, 1956

M H Mueller, W P Chernock, and P A Beck, “Com-

ments on Inverse Pole Figure Methods,” Trans AIME, 212 [l]

3 9 2 0 (1958)

Y Nakada and T L Schock, Bell Laboratories, Allentown,

S ~ C Bull., 42 [9] 480-81 (1963)

3411;45 (1952)

Pa.; personal communication

” J W Newsome, H W Heiser, A S Russell, and H C

Stumf, “Alumina Properties,” ALCOA Res Lab Tech Pap

No 10, 2d ed., 1960; 88pp

’

ERIC R KREIDLER

General Electric Lighting Research Laboratory, Nela Park, Cleveland, Ohio 441 12

The 1200°C isothermal section of the system Ca0-Ba0-W03 was

studied in detail The system contains one ternary compound,

Ba,CaWOo, which can exist in binary equilibrium with BaO,

CaO, Ba3WOo, Ba,WO,, BaWOa, and Ca3WOfl The composition

range of solid solutions based on the ternary compound extends

from Ba,CaWOo to Bai 8,Cai.i,W0, a t 1200°C Solid solubility

along the binary join BaWOrCaWOa was studied in the interval

1000” to 1340°C Maximum solid solubilities occur a t the eutec-

tic temperature (1340°+100C) and a r e 18 mol% CaWO, in

BaWOn and 3.5 mol% BaWOl in CaW04 A phase diagram is

given for the BaWOa-CaWOa system Evidence is presented

which shows that Ba,WO, is a stable phase, and the BaO-WO,

phase diagram is revised accordingly There a r e 3 polymorphs

of BaLWO, related by rapid reversible inversions a t 1385”&5O

and 149Oor1O0C The low-temperature form of Ba3WOo is

tetragonal (a=8.65(2) A and c=16.43(4) A), not cubic as p m viously reported The compounds CasWOo, BaW20T, BaCa,WOe, Ba,CaW,O,,, and Bai.,Cai.,WOo reported in earlier studies were

not confirmed

I Introduction

HE phase diagram for the system CaO-BaO-WO, has not

T been reported in the literature, but some data on com- pound formation within the system are available The com- Presented a t the 73rd Annual Meeting, The American Ce- ramic Society, Chicago, Ill., April 27, 1971 (Basic Science Division, No 31-B-71) Received December 22, 1971; re-

vised copy received May 26, 1972

HEATING CURVES

pounds which have been reported are BaCa2WOe,' Ba6CaWz012,'*2

Bal.,Cal.,WOo,3 and Ba2CaW0,.' Only BazCaWO, was confirmed

as a compound in the present study

The system CaO-WO,, which was studied by Chang et al.,'

contains the congruently melting compounds CaWO, and

Ca3WOo Nassau and Mills5 presented X-ray data for a third

phase identified as CaoWOI However, as Nassau and Mills

indicated, this phase was not obtained reproducibly and may

be metastable Baglio and Natansohn; who indexed the X-ray

pattern of Ca.,WOo, indicated that it is isostructural with Ca,UOo

The system BaO-WO, was studied by Purt,' whose work was

confirmed by Chang et al.' According to both studies, the sys-

tem contains the compounds BaW04 and BasWOo only The

present work, however, indicates that Ba2W05 also occurs in

the system Dibarium tungstate was discovered by Scholder

and Brixner' and was subsequently observed by Bondarenko

et al.' and by Zhmud and Ostapchenko? The X-ray powder

patterns of Ba,WO, and BaW207 were reported by the latter

workemD The data reported for BaW207 actually represent

a highly oriented pattern of BaW04 such that the 001 reflections

predominate Preferred orientation could be expected in light

of the manner in which the pattern was taken? Although the

existence of BaWz07 is possible, it is very doubtful that the

compound was observed by Zhmud and Ostapchenko

I I I I , , , I , , ,

COOLING CURVES

11 Experimental Procedure

The samples, which weighed ~ 1g, 0were prepared from

chemically pure BaCO,, CaC03, and blue tungstic oxide

(WO, ,,) The starting materials were weighed to the nearest

milligram, mixed thoroughly under acetone in glass mortars,

and heated a t 800" to 900°C for 15 h The powders thus ob-

tained were reground under acetone and reheated a t 1050' to

12OO0C for 15 h These materials were used to make cylindri-

cal pellets (3 mm high by 10 mm in diameter) for equilibration

and melting experiments Temperatures, which were mea-

sured to a n accuracy of a 5 " C with Pt-PtlORh thermocouples,

were held constant to within + 10°C during equilibration

Samples having high concentrations of BaO reacted with

P t crucibles to give deep purple products The color results

from reaction of BaO with P t (Ref 10) and not from forma-

tion of reduced tungsten compounds, as is clearly demonstrated

by the fact that identical samples, prepared under similar con-

ditions in ALO, crucibles, were white or cream-colored The

reactions with P t were not usually extensive enough to alter

the phase relations, but discolored samples were discarded,

and replacements were made in Alz03 crucibles

Phases were identified by standard X-ray powder techniques,

using a diffractometer" with CuKa radiation Lattice param-

eters were measured for samples which were equilibrated a t

the desired temperatures, quenched in air, and packed into a n

A1 sample holder Part of the holder intercepted the X-ray

beam, thereby giving reference peaks for correction of the dif-

fraction angles Although lattice parameters were not ex-

trapolated to e = go", the procedures used gave results in agree-

ment with previously reported values For example, the lattice

parameters of BaW0, were determined to be a=5.615(3) and

c=12.722(8) A, in good agreement with the accepted values

of a=5.6134 and c=12.720 A (Ref 11) (numbers in paren-

theses a r e the standard deviations in the last significant fig-

ures)

Some of the products were examined with a petrographic

microscope, but the particles were too small to allow measure-

ment of optical properties Limits of solid solubility were de-

termined from plots of unit-cell volume or lattice parameter

as functions of composition The method is outlined in detail

by CulIity.'* Differential thermal analyses were performed

on a thermoanalyzer? equipped with a Pt-Ptl3Rh thermo-

couple and a furnace capable of operation to 1600OC The

reference standard was a-Al,Oi, the sample size was 71 mg,

and the heating and cooling rates were 12"C/min

Table I Thermal Stability of BazWO,

Heat treatment Phases present

Samule B*

Temp ("C) Time (h) Sample A'

BaW04+BazWOs+BaCOa+

600 65 Ba2WOs Ba2WO6 + BarWOo + BaW04

Ba2W0, BazW05

Ba2W0, BazW05

Ba,WO, Ba2W0,

%itially sample A= Ba2W0, and B = a 1 : 1 molar mixture of

S e e discussion in Section I11 ( 5 ) BaW04 and Ba3WOo

0

111 Results and Discussion

( 1 ) System BaO-WOs

Contrary to previously reported phase diagrams for the sys- tem Ba0-W03,4*' BaZW05 was found to be a stable, reproducible phase which should be included in the diagram The stability

of Ba2W0, is indicated in Table I Samples initially consisting

of pure BanWO, (sample A ) showed no tendency to decompose

over the interval 600' to 151OoC, whereas samples initially consisting of a mechanical mixture of BaWOa and Ba3WO, (sample B) invariably reacted to give Ba2WOs The reaction was incomplete at 600' and 800'C and complete at higher temperatures Samples A and B were heated simultaneously

to ensure equal thermal treatments

Further evidence for the existence of Ba,WO, was obtained from DTA of several samples Pure Ba,WOo gave no observ- able heat effects between room temperature and 1600"C, and examination of the sample after the run showed that no meIt- ing had occurred The DTA curves of samples containing 30.0, 33.3, and 40.0 mol% WO,t a r e reproduced in Fig 1 The 30.O%-WO3 sample consists of a mixture of Ba3WOe and Ba,WO,, whereas the 33.3%-w03 sample contains only Ba2W0, Rapid reversible heat effects were observed a t 1385'25" and 149Oo~1O0C in both samples Since neither sample had melted (maximum temperature= 1600°C) and since no heat effects were observed in Ba3WOo, the heat effects a t 1385" and 1490'C are interpreted a s reversible polymorphic inver- sions in Ba,WO,

*XRD-5, General Electric Co., Schenectady, N Y

+Model 600, E I du Pont de Nemours & Co., Inc., Wilming-

$Molar percentages are used throughout

ton, Del

516 Journal of T h e American Ceramic Society-Kreidler Vol 55, No 10

8 0 0

0' B W t W ( T E T R A G 1

1

m

- 805 D

"'

~ + B , W ( T E T R A G )

I

Fig 2 System BaO-W03 The parts of the system from

0 to 25% and 40 to 100% wo3 are from Refs 4 and 7

L =liquid, c = cubic, tetrag and t = tetragonal, orthn =

orthorhombic, B=BaO, and W=WO:r (thus e.g B3W=

BaZWOo)

The 40.0%-W03 sample consists of a mixture of Ba,WOi and

BaWO, and exhibits reversible heat effects at 1320O k 5" and

1385Ok5OC Examination of the sample after it had been

heated through the first DTA peak only revealed that it had

partially melted The peak at 132OOC thus corresponds to the

eutectic melting observed p r e v i o ~ s l y ~ , ~ and the peak at 1385°C

to the inversion in the remaining Ba,WO, crystals Both heat

effects are observed on cooling if the temperature has not

exceeded z 1420°C Samples subjected to higher temper-

atures yielded a single peak on cooling (usually at ~ 1 3 0 0 ° C )

Presumably, complete melting occurs above 142OoC, and the

liquid thus formed undergoes such extensive supercooling

that, by the time crystallization occurs, the 1385°C inversion

in Ba2W0, has been bypassed

A revised phase diagram for the BaO-W03 system, con-

sistent with the preceding results, is given in Fig 2 In

agreement with common practice, the polymorphs of Ba2WOs

are designated p, and y in order of increasing temperature

The X-ray powder pattern of e-BazWO agrees well with that

given by Zhmud and Ostapchenko' and is not reproduced here

X-ray patterns of p- and y-BaLWOs were not obtained because

of the high temperatures involved The parts of the phase

diagram outside the interval 25 to 40% W03 are taken from

the work of Purt' and Chang et a

The existence of barium polytungstates such as BaW207 is

an open question Although it seems certain that such a com-

pound was not observed by Zhmud and Ostapchenko (see

Section I), compounds such as BaMo201 (Refs 13 and 14)

and BaU207 (Ref 15), which should be analogous to BaW20,,

have been reported The cornpounds would be expected to be

structurally similar, but no apparent relations exist between

the X-ray patterns which have been reported so far.89f3-'6

Furthermore, the two studies reporting BaMo,O, do not agree

satisfactorily en either the X-ray pattern or the melting point

(i.e incongruent, 653"*3OC (Ref 13) and incongruent, 715'C

(Ref 14)) More work is needed to prove or disprove the

existence of such phases

n

1300O

oa -

315 -

3

-I

-

-

I I I I I I

3120 CaW04 MOLE % BaW04-

4 0 0 A

I I 1 I 1 I

BaWO4 MOLE % C a W 0 4 -

Fig 3 Unit-cell volumes of scheelite-type solid solutions

( 2 ) System CaWO4-BaW04

Solid solubility was studied along the BaWOrCaW04 join X-ray measurements were made on samples which had been equilibrated for 24 h at the desired temperatures and quenched rapidly in air The change in unit-cell volume of the solid solutions as a function of composition is plotted in Fig 3, and the lattice parameters are given in Table 11 The limits of solid solubility determined from Fig 3 were used to construct the BaW0,-CaW04 phase diagram (Fig 4) The subsolidus phase boundaries, which are accurate to k1.0 mol'%, agree well with the observed phase assemblages The melting p i n t s

of pure BaW04 and CaW04 were taken from the l i t e r a t ~ r e , ~ and the eutectic temperature was determined by DTA of samples containing 50.0 and 70.0% BaW04

Table 11 Lattice Parameters of Cal-,Ba,WO,

Solid Solutions*

( X ) Phases presentt a(A) c (A)

0.000 CaW04 0.005 CaW04(ss) 0.010 CaWOdss)

5.241(2) 11.379(5 5.245(3) 11.389(71 5.24513) 11.398(9

0.100 CaW04(ss) + BaW04(ss) 5.256(4) 11.42(1) 0.80 BaWOdss) +CaW041ss) 5.56713) 12.561(8) 0.90 BaWOi(s4

0.95 BaW04(ss)

1 .oo BaW04

5.600(3j i2.67(lj 5.615(3) 12.722(8)

*Samples were equilibrated at 1300°C for 24 h; lattice

(ss) =solid solution

parameters were measured at room temperature

(L

W

!?J 1100

W

I-

1000

900

BOO

I

0

a

I

I

I

I -

I

I

\ -

I

03 0 0

I

I

r

I

Ca W04SS+ Ba W04 SS

-

1600

LIQUID

\

\cowo4ss t L I Q '\ \

1400

-

2 1300 T O 0

- co wo4 ss

W

n

s 1200 TI 0

a

1

I

\

-1

\

\cowo4ss t L I Q '\ \

1400

- \

-

2 1300 T O 0

- co wo4 ss

W

n

s 1200 TI 0

a

(3) S y s t e m CaO-BaO-WO,

The phase relations for the system Ca0-Ba0-W03 were es-

tablished from the data in Table 111% and the studies of the

BaO-WO, and BaW0,-CaW04 systems No particular problems

were encountered, except that samples containing uncombined

BaO could not be heated for long periods in Pt or AL03 con-

tainers Samples which were heated a t 1050°C for 15 h

frequently exhibited nonequilibrium phase assemblages This

fact, which is particularly true for samples with compositions

on or near the Ba,WOe-Ca,WOo join, may explain the confusion

in the literature concerning compound formation within the

system The nonequilibrium data are not included in Table

111 When homogeneous samples were used, equilibrium was

obtained in < 15 h a t 1200" and 14OO0C

The 120OOC isothermal section is shown in Fig 5(A) The

only melting observed a t 1200°C occurred within the CaW04-

BaW04-W0, triangle, but no attempt was made to map the

liquid field The single ternary compound, BaKaWO,,, is the

outstanding featwe of the system and serves a s an apex for

6 of the 8 compatibility triangles The only phases which

cannot exist in equilibrium with BazCaWOo (or its solid solu-

tions) are CaWOa and WO, The invariant points ( a , c, and

d in Fig 5 ( A ) ) were determined by measuring the unit-cell

parameters of the solid solutions (Table IV) and reading the

corresponding compositions from Figs 3 and 6 The com-

positions of the remaining invariant points ( b and e in Fig

5 ( A ) ) are fixed by the observed phase assemblages and the

requirements of the phase rule

Extensive melting was observed in the system BaO-CaO-

WO, at 1400°C; a rough isothermal section is shown in Fig

co 0

Ba 0 B a 3 W 6 BazWOJ BaW04

MOLE '10

Co 0

( B )

0 MELTED

0 NOT MELTED

i c a 3 w 0 6

BOO Ba3W06 BogW05 BaW04

MOLE %

wo3

Fig 5 System CaO-BaO-WOs: ( A ) 12OO0C isotherm and

( B ) partial 1400°C isotherm

*For Table 111, order ACSD-115 from Data Depository Ser- vice, American Ceramic Society, 65 Ceramic Drive, Columbus, Ohio 43214; remit $5.00 for photocopy

Table IV Subsolidus Invariant Points in the System CaO-BaO-WO, at 1200°C

Invariant

1

B&.oC&.IWO~(~

tSee Fig 5(A)

C e l l parameters are for phases indicated by asterisks

518 Journal of T h e American Ceramic Society-Kreidler

8.38 2

8.376

0.370

Vol 55, No 10

Table V Changes in Interplanar Spacings as a

Function of Composition for Ba3W0,

Interplanar spacings ( A )

613

Composition Phases present 415 440

Ba,.uzCao.oHWOo Ba3WOfl + BaiCaWOe 1.7570 1.5223 1.3619 Ba,.,,Cao,,WO, Ba.WO,, + Ba,CaWOs 1.7575 1.5229 1.3621

Ba, 7~Cao.naWOe Ba;WOfl+BaiCaWOe 1.7588 1.5241 1.3629

d (obs.) 1/11 hkl d (calc.)

Baz CaWO, Bo'1,86Ca1,,4W06

Baz-XCa I+ x WO6 (X)

Fig 6 Lattice parameters of Ba2CaWOfl solid solutions

Samples prepared at 1200°C; parameters measured at 22°C

5.02

*3.34 3.06

5

6

100

112

005 220.105

5.08 3.28 3.06.3.07

*2.239 2.165 1.986

~~ ~

1.663 1.529

9

7

512,416

440 1.662, 1.666 1.530

5 ( B ) Actual melting points were not determined, and only

the presence or absence of a liquid phase is noted The com-

positions selected for study characterize the melting behavior

along the binary joins and within the compatibility triangles

All the compounds in the system melt a t temperatures above

1400°C

The previously reported compounds, BaCazWOfl, Ba5CaW,OIz,

and Ba,.,Cai WOo, apparently resulted from nonequilibrium

conditions and misinterpretation of X-ray data Present at-

tempts to prepare these compounds were unsuccessful and

resulted in two-phase mixtures, in accord with Fig 5(A)

Further evidence against the existence of such compounds is

found in the reported X-ray patterns The pattern of

Ba,CaWlOv2 (Ref 16) can be interpreted as a mechanical

mixture of Ba,WOe and Ba2CaWOo The pattern of BaCa,WOG'k

is virtually identical to that of Ba,CaWO, and so is the lattice

parameter (a= 8.38 and 8.390 A, respectively) Such close

similarity is very unlikely in view of the large size difference

between Ca" and Ba'' ions No X-ray pattern of Ba,.5Ca1.5WOe

has been reported Attempts to prepare CaoWOo by solid-state

reactions were unsuccessful, and no suitable explanation of the

observations by Nassau and Mills' is available

( 4 )

The extent of solid solubility a t 120OOC in Ba,WOs, BaiCaWOG,

and Ba,WOa was studied Although slight changes were ob-

served in the interplanar spacings of Ca-doped BanWOo (Table

V ) , the composition interval was too large to determine ac-

curately the solid-solution limit However, the data indicate

a solid solubility of <2.T% Ca,WO, in Ba,WOn The lattice

parameter of Ba,CaWO,% is plotted as a function of composi-

tion in Fig 6 The breaks in the curve indicate that a t 1200°C

the composition range of the ternary compound extends from

Ba,CaWO, to Bai.H6Ca,.,rWOo The soIid solubiIity of Ba2CaWOG

and "Ca,WO," in Ba,WO, was concluded to be negligible, since

the interplanar spacings of Ba,WO, did not change a s a func-

tion of composition

(5) Bff3WOo

The X-ray pattern of BarWOo contains lines which cannot

be indexed on the basis of a cubic unit cell and which were

not included in previously reported An X-ray powder

pattern taken at a scanning rate of s=O.lO"/min is given in

Table VI The lines in question a r e marked by asterisks The

pattern was successfully indexed on the basis of a tetragonal

unit cell with a=8.65(2) and c=16.43(4) A The c/a ratio

is 1.898, which implies that two of Steward and Rooksby's

S o l i d Solubility in IEa,WO,, BaiCaWOn, and BaZWO,

*Card No 18-164, Joint Committee on Powder Diffraction

Standards, Swarthmore, Pa This card has been deleted from

the file since this report was submitted

~ NOTE:

CuKa radiation

with a=8.65 A

Tetragonal a=8.65(2) A, c=16.43(4) A, c/a=1.898,

*By omitting these lines, the pattern can be indexed as cubic

cubic cells (a=8.62 A ) 3 a r e stacked to give tetragonal sym- metry On the basis of high-temperature X-ray measurements, Chang et aL4 postulated a noncubic+cubic transition in BaXWOe

a t S3OO"C The DTA curve of Ba,WOfl showed no heat ef-

fects between 25°C and 1600°C (Section III( 1 ) ) ; however, the possibility of a second-order phase transformation cannot

be ruled out

As may be seen in Table I, Ba3WOfl decomposes to a mix- ture of Ba2WO6 and BaCO, when heated in air at 600"C, but there is no indication of such reaction at higher temperatures This behavior raises a question concerning the low-temperature stability of Ba,WOu Either Ba,WOfl has a lower-temperature limit of stability (in which case the compound would decom- pose to BazWOj and BaO with subsequent carbonation of BaO)

or CO, reacts directly with the compound to produce BaC0, and Ba2W0, (in which case the compound would be thermally stable at 600°C) A sample of Ba3WOo heated in N, a t 600°C for 30 h did not decompose, showing that BaaWOo is thermally stable a t 600°C in the absence of COz, and the BaO-W0, phase diagram was constructed accordingly (Fig 2 )

(6) Reduced Tungsten Compounds

The effects of the occurrence of tungsten in several oxida- tion states on the phase relations must be considered, The data on tungsten oxide^"*^' indicate that WO, (or a slightly reduced variety thereof) is the stable phase in air a t all tem- peratures up to ~ 1 4 5 0 O C The formation of alkaline-earth tungstates should further stabilize the Wfl+ ion and make it even more difficult to reduce Scholder and Brixner: who attempted to prepare reduced (W4+ or W6+) alkaline-earth tung- states by a variety of techniques, found them very difficult (if not impossible) to prepare In view of these considerations, it was anticipated that the system Ca0-Ba0-W0, could be studied

in air a t temperatures at least to 145OOC without formation

of reduced tungsten compounds This expectation was con- firmed by experiment In fact, reduced tungsten species were not observed at the highest temperature (160OOC) used in this study, except in samples containing uncombined WO, Such samples occur only within the BaWOa-CaWOa-WOa tri- angle, which was not studied in detail Tungsten bronzes of the type reported by Vandeven et aLi9 should be observed un- der reducing conditions near the WO, apex

Acknowledgments assisted in this work The writer thanks his colleagues a t General ,Electric who Samples were prepared by Barbara

Press, and X-ray patterns were obtained by Jeanette Cooper

and her staff Preliminary studies of solid solubility along

the BaW04-CaW04 join were conducted by N M Reminick

while a guest at this laboratory The project was suggested

by J F Sarver and W E Smyser and encouraged by R L

Hickok

References

’B V Bondarenko, E P Ostapchenko, and B M Tsarev,

“Thermionic Properties of Alkali Metal Tungstates,” Radio-

tekh Elektron., 5, 1246-53 (1960)

* A A Maklakov and E P Ostapchenko, “X-Ray Investiga-

tion of the Kinetics of the Formation of Barium Calcium Alu-

m p t e s and Tungstates,” Zh Strukt Khim., 1 [Z] 178-82 (1960)

E G Steward and H P Rooksby, “Pseudocubic Alkaline-

Earth Tungstates and Molybdates of the R3MX, Type,” Acta

Ccystallogr., 4, 503-507 (1951)

L L Y Chang, M G Scroger, and Bert Phillips, “Alkaline-

Earth Tungstates: Equilibrium and Stability in the M-W-0

S p e m s , ” J Amer Ceram SOC., 49 [7] 385-90 (1966)

K Nassau and A D Mills, “A New Calcium Tungstate:

C+WO0,” Acta Crystallogr., 15, 808-809 (1962)

J A Baglio and S Natansohn, “Crystal Structure of CasTeOo

a;d CarWOo,” J Appl Crystallogr., 2 [Pt 61 252-54 (1969)

G Purt, “Binary System BaO-W03,” Z Phys Chem

( F k f o r t am Main), 35 [I-31 133-38 (1962)

R Scholder and L Brixner, “Alkaline Earth Molybdates,

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GEORGE P HALBFINGER and MORRIS KOLODNEY

Department of Chemical Engineering, The City College of New York, New York, New York 10031

The effect of additives on the sintering of ThQ, and ThO,-Y,Oa

compacts and loose powders was studied by isothermal shrink-

age measurements and by scanning electron micrography

Small amounts of the oxides of Ni, Zn, Co, and Cu reduced the

sintering temperature The behavior of NiO a t a concentra-

tion of 0.8 wt% (2.5 mol%) was studied in detail and found to

yield high-density bodies a t temperatures below 1500°C The

presence of Y,O, as a separate phase increases the rate of

sintering of Thoz, but smaller amounts of NiO a r e much more

potent The major portion of the densification occurs very

rapidly and is followed by a much slower sintering process

typical of volume diffusion The fast early shrinkage may be

caused by the capillary forces of a liquid, but since no evidence

of melting was found, a solid-state mechanism may be re-

sponsible

I Introduction

HORIA-YTTRIA solid electrolytes are valuable for mea-

T suring thermodynamic quantities at high temperatures

and as sensors for determining low oxygen concentrations in

gases and liquid metals These electrolytes a r e useful a t

oxygen concentrations below the region covered by solid elec-

trolytes based on ZrO, The sintering temperatures for these

mixed electrolytes a r e normally above 200OoC,’ a temper-

ature much too high for some electrode fabrication procedures,

especially the total encapsulation of a n electrode within the

electrolyte with oxidation-resistant lead-throughs Therefore,

it would be highly desirable to sinter below the melting point

of Pt This reduction in temperature may be achieved by

adding small amounts of sintering aid to activate the densifi-

cation process

The mechanism of activation of sintering is not well under- stood, although sintering theory has received much attention since Kuczynskia proposed his sintering model The models deal almost exclusively with pure, monosized, regular-shaped, homogeneous powders instead of the usual industrial powders, which a r e irregularly shaped, have a t least a 10-fold size

distribution if they a r e <20 fim in size, and usually contain

impurities which may play a vital role in densification Al- though departures from ideality make the Kuczynski model and similar models unreliable in real case^,^^^ they may be used as guidelines in studying mechanisms Detailed analyses

of sintering phenomena have been provided by Thummler and Thomma6 and Coble and Burke:

Sintering aids probably produce a lower-energy path for mass transport They may alter the defect structure of the host solid, thereby increasing the diff usivity and assisting densification Additives may also segregate a t grain bound- aries, providing a low-energy circuit for diffusion In par- ticular, if these additives initially coat the major constituent particles and if they possess a lower sintering temperature, they may concentrate rapidly in the necks between particles The result may be greatly enhanced diffusion of vacancies

Received February 16, 1972; revised copy received April

21, 1972

Based in part on a thesis submitted by George P Halb- finger for the Ph.D degree at The City University of New

York, December 1971

Supported by the Faculty Research Award Program of The City University of New York and by the National Aeronautics and Space Administration Lewis Research Center under Grant

NO 33-013-017