Phase equilibria in the system cao bao w

Vol 55, No 10 Journal of The American Ceramic Society-Kreidler 514 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 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 [9] 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 [ ] 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] (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 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 T 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 October 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 Phases present Sample A' 65 65 65 65 18 Samule B* + 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 HEATING CURVES I I I , COOLING , , I , , , CURVES 11 Experimental Procedure The samples, which weighed ~ 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 " 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 ( ) 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 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 516 - 316 I I I I I - n oa 315 1300O - -I I 3120 CaW04 A 402 400 - 800 0' 805 ~+B,W(TETRAG) Ba I I "' I I I MOLE % BaW04- I I I I I 10 15 20 MOLE % C a W I I I I I I BWtW(TETRAG.1 D m I MOLE % WOa Fig System BaO-W03 The parts of the system from to 25% and 40 to 100% wo3 are from Refs and L =liquid, c = cubic, tetrag and t = tetragonal, orthn = orthorhombic, B=BaO, and W=WO:r (thus e.g B3W= BaZWOo) BaWO4 Fig 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 ~ 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 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, 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 I 25 3( Unit-cell volumes of scheelite-type solid solutions ( ) 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 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* Composition (X) Phases presentt 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) i2.67(lj 12.722(8) 0.000 0.005 0.010 _ *Samples were equilibrated at 1300°C for 24 h; lattice parameters were measured at room temperature (ss) =solid solution Phase Equilibria in the System CaO-BaO-WO, October 1972 517 co 1600 1500 1400 -2 W sna II ' - \ \cowo4ss t -\ LIQUID ' -1 LIQ '\ \ \ 1340f10 \ 1300 T O co wo4 ss 1200 TI I (L I 1100 03 I W ?J! W 0 I I I- Ca W04SS+ Ba W04 \- 0 0 i SS a 1000 r I I I I I - 900 I BOO I I I I I , I I I Ba B a W BazWOJ BaW04 MOLE '10 Co (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 of the 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 and 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 MELTED NOT MELTED (B) i c a w < Table IV No.? BOO Ba3W06 BogW05 BaW04 wo3 MOLE % Fig 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 $5.00 for photocopy Subsolidus Invariant Points in the System CaO-BaO-WO, at 1200°C Phases present Ba,CaWO,:%+BaWOa+tr BaZWO, BazCaWOo* CaO +BaO BaW04*+Ba,CaWOo+CasWOo BaW04*+CaW04+CasWOs BaW04*+BazCaWOa+Ba2WOo + tSee Fig 5(A) Cell parameters are for phases indicated by asterisks Cell parameter$ 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 Invariant point BazCaWOe(a Ba,CawO,(aj Bao.sC&.IWO,( B&.oC&.IWO~(~ BaW04(c) 518 Journal of T h e American Ceramic Society-Kreidler 8.388 I I I I l 1 Vol 55, No 10 Table V Changes in Interplanar Spacings as a Function of Composition for Ba3W0, Composition 8.382 Interplanar spacings (A) 613 415 440 Phases present Ba3WOo Ba,WOo Ba3WOfl BaiCaWOe Ba,.uzCao.oHWOo Ba,.,,Cao,,WO, Ba.WO,, +Ba,CaWOs Ba, 7~Cao.naWOe Ba;WOfl+BaiCaWOe + 8.376 Table VT 0.370 Baz CaWO, 8.364 I -.I6 I Bo'1,86Ca1,,4W06 I OO l L l I6 Baz-XCa I+x WO6 -32 I I 48 (X) 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