lites. The first step in the substrate process is ball-milling to break up these aggregates. Increasing the amount of ballmilling would lead to greater breakup of the agglomerates and more crystallites. However, still further ball-milling could break up the crystallites which should fracture perpendicular 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 result from either selective or anisotropic grain growth. Variations 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 at the substrate surface, 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 100 grains. containing
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 basalplane 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 texture in AL03 substrates does not appear to affect the performance of current substrates. Therefore, the principal use of texture may be as a process control in the production of substrates. The mechanism of texture development in the unfired and fired states, which cannot be elucidated clearly at 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 are 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 ’ C. B. Barrett and T. B. Massalski, Structure of Metals, 3d .e: McGraw-Hill Book Co., New York, 1966. 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.,
191- 468-74 . 52 _ (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. “Preferred Orienratibn in Hot-Pressed Magnesia,” J . Amer. Ceram. S;C., 50 [lo] 550-51 (1967). T. L. Schock, Bell Laboratories, Allentown, Pa.; personal communication. ‘ J. J. Thompson, “Forming Thin Ceramics,” Amer. Ceram. 42  480-81 (1963). S ~ CBull., . 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 Errors in the X-Ray Reflection Method for the Quantitative Determination of Preferred Orientations,” J . Appl. Phys., 23 [ 3 ] 3411;45 (1952). A. Von Hippel and W. B. Westphal, “High Dielectric Constant Materials as Capacitor Dielectrics,” Tech. Rept. 145, Massachusetts Institute of Technology Laboratory for Insula:!it 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, “Comments on Inverse Pole Figure Methods,” Trans. AIME, 212 [l] 3 9 2 0 (1958). Y. Nakada and T. L. Schock, Bell Laboratories, Allentown, 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. ’
Phase Equilibria in the System CaO-BaO-WO, ERIC R. KREIDLER General Electric Lighting Research Laboratory, Nela Park, Cleveland, Ohio 44112 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 at the eutectic temperature (1340°+100C) and are 18 mol% CaWO, in BaWOn and 3.5 mol% BaWOl in CaW04. A phase diagram is given for the BaWOa-CaWOasystem. Evidence is presented which shows that Ba,WO, is a stable phase, and the BaO-WO, phase diagram is revised accordingly. There are 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.
HE phase diagram for the system CaO-BaO-WO, has not been reported in the literature, but some data on compound formation within the system are available. The com-
Presented at the 73rd Annual Meeting, The American Ceramic Society, Chicago, Ill., April 27, 1971 (Basic Science Division, No. 31-B-71). Received December 22, 1971; revised copy received May 26, 1972.
Phase Equilibria in the System CaO-BaO-WO, 515 Table I. Thermal Stability of BazWO,
pounds which have been reported are BaCa2WOe,' Ba6CaWz012,'*2 Bal.,Cal.,WOo,3and 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 system 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.
Heat treatment Temp. ("C) Time (h)
600 800 1000 1200 1400 1510
65 65 65 65 18 4
BaW04+BazWOs+BaCOa+ +BarWOo BaW04 Ba2WO6
Ba2WOs BazWOa Ba2W0, Ba2W0, Ba2W0, Ba,WO,
BazW05 BazW05 Ba2W05 Ba2W0,
%itially sample A= Ba2W0, and B = a 1:1 molar mixture of BaW04 and Ba3WOo. S e e discussion in Section I11 (5). I
11. Experimental Procedure The samples, which weighed ~ 1 g,0 were 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 at 800" to 900°C for 15 h. The powders thus obtained were reground under acetone and reheated at 1050' to 12OO0C for 15 h. These materials were used to make cylindrical pellets (3 mm high by 10 mm in diameter) for equilibration and melting experiments. Temperatures, which were measured to an 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 Pt crucibles to give deep purple products. The color results from reaction of BaO with P t (Ref. 10) and not from formation of reduced tungsten compounds, as is clearly demonstrated by the fact that identical samples, prepared under similar conditions 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 parameters were measured for samples which were equilibrated at the desired temperatures, quenched in air, and packed into an A1 sample holder. Part of the holder intercepted the X-ray beam, thereby giving reference peaks for correction of the diffraction angles. Although lattice parameters were not extrapolated to e = go", the procedures used gave results in agreement 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 parentheses are the standard deviations in the last significant figures). Some of the products were examined with a petrographic microscope, but the particles were too small to allow measurement of optical properties. Limits of solid solubility were determined 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 thermocouple 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.
111. Results and Discussion ( 1 ) System BaO-WOs Contrary to previously reported phase diagrams for the system Ba0-W03,4*'BaZW05was 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 observable heat effects between room temperature and 1600"C, and examination of the sample after the run showed that no meIting had occurred. The DTA curves of samples containing 30.0, 33.3, and 40.0 mol% WO,t are reproduced in Fig. 1. The 30.O%-WO3 sample consists of a mixture of Ba3WOeand Ba,WO,, whereas the 33.3%-w03sample contains only Ba2W0,. Rapid reversible heat effects were observed at 1385'25" and 149Oo~1O0Cin 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 as reversible polymorphic inversions in Ba,WO,.
*XRD-5, General Electric Co., Schenectady, N. Y. +Model 600, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del. $Molar percentages are used throughout.
Vol. 55, No. 10
Journal of T h e American Ceramic Society-Kreidler
I I I 4 6 8 MOLE % BaW04-
I 1 I 10 15 20 MOLE % C a W 0 4
I MOLE %
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 1320Ok 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 yand ~ , ~ 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 z1420°C. Samples subjected to higher temperatures 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, consistent 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-BaLWOswere 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 af.' The existence of barium polytungstates such as BaW207is an open question. Although it seems certain that such a compound 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.
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.
Lattice Parameters of Cal-,Ba,WO, Solid Solutions*
Lattice parameters a(A) c (A)
CaW04 CaW04(ss) CaWOdss)
5.241(2) 5.245(3) 5.24513)
11.379(5 11.389(71 11.398(9
0.050 0.100 0.80
cawo&sj CaW04(ss)+ BaW04(ss) BaWOdss) +CaW041ss)
5.250(3j 5.256(4) 5.56713)
ii.415(8) 11.42(1) 12.561(8)
0.90 0.95 1.oo
BaWOi(s4 BaW04(ss) BaW04
5.600(3j 5.615(3) . .
0.000 0.005 0.010 . ._.
*Samples were equilibrated at 1300°C for 24 h; lattice parameters were measured at room temperature. 1 (ss) =solid solution.
Phase Equilibria in the System CaO-BaO-WO,
517 co 0
\cowo4ss t -\
1 ' -1
1300 T O 0 --co wo4 ss 1200 TI
I 1100 03 I
Ca W04SS+ Ba W04
0 0 0 0 0 . i 0
I I -
B a 3 W 6 BazWOJ
BaW04 MOLE '10
(3) System CaO-BaO-WO, The phase relations for the system Ca0-Ba0-W03 were established 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 containers. Samples which were heated at 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,WOojoin, 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 at 1200" and 14OO0C. The 120OOC isothermal section is shown in Fig. 5(A). The only melting observed at 1200°C occurred within the CaW04BaW04-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 as an apex for 6 of the 8 compatibility triangles. The only phases which cannot exist in equilibrium with BazCaWOo(or its solid solutions) 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 compositions 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-CaOWO, at 1400°C; a rough isothermal section is shown in Fig.
0 MELTED 0
i c a 3 w 0 6
Table IV. No.?
3 4 5
Fig. 5. System CaO-BaO-WOs: ( A ) 12OO0C isotherm and ( B ) partial 1400°C isotherm.
*For Table 111, order ACSD-115 from Data Depository Service, American Ceramic Society, 65 Ceramic Drive, Columbus, Ohio 43214; remit $5.00 for photocopy.
Subsolidus Invariant Points in the System CaO-BaO-WO, at 1200°C Phases present
Ba,CaWO,:%+BaWOa+tr BaZWO, BazCaWOo* CaO +BaO BaW04*+Ba,CaWOo+CasWOo BaW04*+CaW04+CasWOs BaW04*+BazCaWOa+Ba2WOo
tSee Fig. 5(A). Cell parameters are for phases indicated by asterisks.
a=8.383( 2)A a=8.384(2) A V=393.6(4)A3 V=392.5(4) A3 V=400.8(4) A3
Lattice parameters of Ba2CaWOfl solid solutions. Samples prepared at 1200°C; parameters measured at 22°C.
5(B). Actual melting points were not determined, and only the presence or absence of a liquid phase is noted. The compositions 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 attempts 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 CaoWOoby solid-state reactions were unsuccessful, and no suitable explanation of the observations by Nassau and Mills' is available. S o l i d Solubility in IEa,WO,, BaiCaWOn,and BaZWO, The extent of solid solubility at 120OOC in Ba,WOs, BaiCaWOG, and Ba,WOa was studied. Although slight changes were observed in the interplanar spacings of Ca-doped BanWOo(Table V ) , the composition interval was too large to determine accurately 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 composition in Fig. 6. The breaks in the curve indicate that at 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 as a function of composition. (4)
(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 are 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 *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: 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 with a=8.65 A. cubic cells (a=8.62 A ) 3 are stacked to give tetragonal symmetry. On the basis of high-temperature X-ray measurements, Chang et aL4postulated a noncubic+cubic transition in BaXWOe at S3OO"C. The DTA curve of Ba,WOfl showed no heat effects 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, Ba3WOfldecomposes to a mixture of Ba2WO6and 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,WOflhas a lower-temperature limit of stability (in which case the compound would decompose 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 Ba3WOoheated in N, a t 600°C for 30 h did not decompose, showing that BaaWOois thermally stable at 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 oxidation 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 temperatures up to ~1450OC. 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 tungstates 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 confirmed 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-WOatriangle, which was not studied in detail. Tungsten bronzes of the type reported by Vandeven et aLi9should be observed under reducing conditions near the WO, apex.
Acknowledgments The writer thanks his colleagues at General .,Electric who assisted in this work. Samples were prepared by Barbara
Sintering of Thoz and Tho,-Y,O, with NiO
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,” Radiotekh. Elektron., 5, 1246-53 (1960). * A. A. Maklakov and E. P. Ostapchenko, “X-Ray Investigation of the Kinetics of the Formation of Barium Calcium Alum p t e s and Tungstates,” Zh. Strukt. Khim., 1 [Z] 178-82 (1960). E. G. Steward and H. P. Rooksby, “Pseudocubic AlkalineEarth Tungstates and Molybdates of the R3MX, Type,” Acta Ccystallogr., 4, 503-507 (1951). L. L. Y. Chang, M. G. Scroger, and Bert Phillips, “AlkalineEarth Tungstates: Equilibrium and Stability in the M-W-0 S p e m s , ” J. Amer. Ceram. SOC., 49  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 tam Main), 35 [I-31 133-38 (1962). R. Scholder and L. Brixner, “Alkaline Earth Molybdates,
Tungstates and Uranates of Valence States (IV), (V), and (VI),” Z . Naturforsch. B, 10, 178-79 (1955). E. S. Zhmud and E. P. Ostapchenko. “X-Ray Investigation of the Systems BaO-W03, Bad-Moo3, .and BaO-Ta,O,,” Zh. Strukt. Khim., 2 [I] 33-45 (1961). loS. J. Schneider and C. L. McDaniel, “BaO-Pt System in A:,’’ J. Amer. Ceram. SOC.,52  518-19 (1969). Powder Data File, Card No. 8-457. Joint Committee on Pqyder Diffraction Standards, Swarthmore, Pa. B. D. Cullity, Elements of X-Ray Diffraction; pp. 356-58. A$lison-Wesley Publishing Co., Inc., Reading, Mass., 1956. V. M. Zhukovskii, E. V. Tkachenko, and T. A. Rakova, “Equilibrium Diagrams of the Mo03-MMo04Systems (M =Mg, Cci: Sr, Ba),” Russ. J . Inorg. Chem., 15 [I21 1734-36 (1970). 0. A. Ustinov, G. P. Novoselov, M. A. Andrianov, and N. T. C$Fbotarev, “The BaO-MoOa System,” ibid.,  1320-21. Powder Data File, Card No. 13-76. Joint Committee on P2yder Diffracticn Standards, Swarthmore, Pa. Card No. 15-90 in Ref. 15. Bert Phillips and L., L. Y. Chang,,, “High-Temperature Stabilitv of Tungsten Oxide Structures, Trans. AIME, 230  1205-1206 (196l). L. L. Y. Chang and Bert Phillips, “Phase Relations in Refractory Metal-Oxygen Systems,” J. Amer. Ceram. SOC., 52 [lo] 527-33 (1969). D. Vandeven, J. Galy, M. Pouchard, and P. Hagenmuller, “Structural Evolution as a Function of Temperature of Several Tungsten Oxide Bronzes Low in the Insertion Element,” Mater. Res. Bull., 2  809-17 (1967).
Activated Sintering of T h o , and Tho,-Y,O, with NiO 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 shrinkage 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 concentration 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 are 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 responsible.
solid electrolytes are valuable for measuring thermodynamic quantities at high temperatures and as sensors for determining low oxygen concentrations in gases and liquid metals. These electrolytes are useful at oxygen concentrations below the region covered by solid electrolytes based on ZrO,. The sintering temperatures for these mixed electrolytes a r e normally above 200OoC,’a temperature much too high for some electrode fabrication procedures, especially the total encapsulation of an 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 densification process. HORIA-YTTRIA
The mechanism of activation of sintering is not well understood, 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 are irregularly shaped, have at least a 10-fold size distribution if they are <20 fim in size, and usually contain impurities which may play a vital role in densification. Although 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 at grain boundaries, providing a low-energy circuit for diffusion. In particular, 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. Halbfinger 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.