THE THERMODYNAMIC PROPERTIES L. R. BIDWELL,? OF F. E. COBALT-PALLADIUM RIZZO$ and J. V. SOLID SOLUTIONS* SMITHS The thermodynamic activities of cobalt in palladium, relative to pure f.c.c. ferromagnetic cobalt, have been determined from e.m.f. measurements on solid-electrolyte oxygen concentration cells over the temperature range SOO-1100°C. The activity of cobalt in palladium exhibits positive departures from ideal solution behavior in cobalt-rich alloys and negative departures in palladium-rich alloys. The activities of palladium, deduced from the experimental data, indicate negative departures from ideality at all compositions. Cobalt-palladium alloys were of particular interest because their unusually high Curie temperatures permitted compositions up to 40 at. % palladium to be studied in the ferromagnetic state. The experimental data were of sufficient precision that the ferromagnetic-paramagnetic transition could be detected in alloys containing nominally 20 and 30 at. ‘4 palladium. The relative from the changes in the partial molar free energies integral molar excess entropies, determined of mixing as a function of temperature, are positive for all the compositions studied. The heats of mixing are endothermic for cobalt-rich compositions and exothermic for palladium-rich compositions. An extrapolation of the experimental values of the partial molar free energies of cobalt to lower temperatures, suggests the presence of a at high du cobalt z%partir le palladium, au cobalt de f.e.m. B oxygene zt et 1100°C. du cobalt le palladium au comoortement de en en palladium. du palladium, & la un in&& de Curie B l’&at ferromag&tique, des alliages dont les cobalt, et nbgatifs il est jusqu’8 40% at. de la transformation ferromagn&ique-paramagn&ique puisse atre detectbe dans les alliages contenant normalement 20 & 30 de palladium. B partir des variations des Energies libres molaires partielles du en fonction de de melange en cobalt et exothermiques en palladium. du cobalt la presence de miscibilite de 600°C en cobalt. compositions vont in Palladium an festen Sauerstoffkonsentrationszellen im Palladium in kobaltreichen im in palladiumreichen zu ‘A Palladium im ferromagnetischen in Legierungen 30 At. ‘A Palladium in kobaltreichen in palladiumreichen das Vorhandensein einer Mischungsliicke unterhalb INTRODUCTION The normally ladium paramagnetic are radically altered properties METALLURGICA, VOL. zu tiefen 600°C bei hohen Kobaltkonzentrationen first-period of pure pal- by the addition of the * Received October 24, 1969; revised March 6, 1970. t Metals and Ceramics Division, Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio 45433. Formerly al,: Aerospace Research Laboratories, W-PAFB, Ohio. $ Department of Metallurgical Engineering, University of Texas at El Paso, El Paso, Texas 79999. Formerly at: Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio. eras ace Research Laboratories, Wright-Patterson Air For!e%aseFOhio 45433. ACTA zu erlauben. 18, SEPTEMBER 1970 transition ium and manganese, elements. auf hin. Alloys for example, with chrom- tend toward anti- ferromagnetic behavior, while those with iron, cobalt and nickel become ferromagnetic at very low concentrations of the 3d element. In the latter alloys, the bulk saturation magnetization is much larger than can be accounted for by the 3d element alone.(l) In order to rationalize the distribution of the atomic moments in these alloys, it is necessary to assume that magnetic atoms.(z) 1013 The moments neutron are induced in the palladium scattering measurements, ACTA METALLURGICA, VOL. the 18, 1970 magnetic cobalt-rich and structural transformations in alloys. EXPERIMENTAL PROCEDURE The chemical activities (a,,) and relative partial molar free energies (A(?,,) of cobalt in cobalt3 5 1000 . “\_ oz F lk.0 0 palladium a 0 0 1. 000. 5soo_ I- 00 0 CURIE TEMP electrical from the potentials of the cell Co, Co0 (Zr,.,,Ca,,,BO,.sjl Co-Pd, Co0 ‘-\\\\. 400\+ as functions ‘\ %/> \a -6‘ <.I> >, Ob.‘, CO 20 1 40 ATOMIC % ‘Y ’ 00 I 60 h in cobalt investigated,c5) magnetic potential by Cable et aZ.t3) indicate US,(~) rather unusual solutions at all E= RT ---lnaco= 2F - A%, ___ 2F (1) T is the absolute R is the gas constant and F is the Faraday and positive for nickel-rich of mixing, however, A theoretical compositions. The details the methods of the apparatus for purifying the electrolyte operating allows from the open-circuit where E is in volts, for to pure oxygen in that The excess entropies positive is equivalent by means of the relations nickel by one of were negative of oxygen of the cell electrolyte(6.7) and AC,,, to be determined properties was observed of the free energy with temperature solutions. state The essentially from the rate of change behavior the heats of mixing deduced palladium-rich the reference is correct. solid solutions containing in The reference ferromagnetic cobalt Since the solubility for cobalt. conductivity both in the presence and in the absence of an applied In an earlier study of the thermodynamic and temperature helium. is less than 0.06 at. ‘A at all temperatures unit activity ionic of palladium purified saturated with oxygen. a,, that this assumption of state for this cell is solid, f.c.c., Pd PALLADIUM field, conducted of both composition an atmosphere FIG. 1. Phase equilibrium diagram for the cobaltpalladium system (redrawn from Hansen and Anderko’5’). Alloys investigated are indicated on 900°C isotherm. were were determined open-circuit, concentration 63-01 200 solid solutions reversible, and the helium and preparing employed tablets, and the in this investigation in previous publica- to these properties indicated that the observed positive tions.(4ns) The alloys studied were prepared at nom- excess entropies inal analysis of the expected ferromagnetic could be almost contributions totally ascribed to have been adequately constant. and its calibration, cell electrode procedures temperature, intervals of 10 at. % from the results of electron-spin disordering. The present study ofcobalt-palladiumsolidsolutions grade was undertaken melted in high-purity mics of alloying. especially diagram(s) in the thermodyna- The cobalt-palladium attractive because, system was as shown in the phase in Fig. 1, the ferromagnetic state persists to relatively high temperatures and palladium contents. In addition, the system is similar to the nickelpalladium system at elevated temperatures, minima in the liquidus and solidus curves 50 at. % palladium solutions. and a complete At temperatures below series and from from the International as a further step in an investigation of the role of ferromagnetism cobalt with near of solid 6OO”C, however, the phase equilibria for cobalt-rich alloys are rather poorly defined. Past studies of this region have led to ambiguous results since the cobalt f.c.c. to h.c.p. transformation involves a great deal of hysteresis and does not occur isothermally. It was hoped that the e.m.f. technique employed in this investigation might be sufficiently sensitive to follow the course of both surface described ground, severely purity analysis obtained alumina crucibles, cold-worked, the solidus with a tungsten Chemical analyses Battelle Memorial reagent sponge Nickel Co. They were vacuum recrystallized for 24 hr at 50°C below and powdered palladium low-nickel carbide homogenized temperature, dental drill. of the alloys vr-ere performed by Institute. A spectrographic im- of the alloys and starting materials indicated that 0.2 wt. ‘A nickel was the major impurity. Reagent grade Co0 was used in the preparation metal-metal oxide cell electrodes. of the Prior to its use, the Co0 was heated at 1100°C for 24 hr in a stream of purified helium in order to remove oxygen that may have been present. any excess The temperature range over which reliable data were obtained (800-l 100°C) was established by several factors. Attempts to measure cell potentials for cobalt-rich alloys below 6OO”C, which presumably would have penetrated the two-phase field of the BIDWELL f.c.c. to h.c.p. unsuccessful. THERMODYNAMIC et al.: transformation Even (see Fig. with an especially ance circuit employed successfully PROPERTIES l), were data that could at low temperatures than be regarded 70 at. %, the e.m.f. dependent, i.e. they values and The irreversible to be time in e.m.f. of a thin electrical the was generally to be attributable blocking to the layer of oxide reference-electrode/electrolyte interface. 7: pp’ This situation can arise when the diffusion of oxygen within the reference electrode is not sufficiently rapid to zero, compensate electronic The magnitude with temperature maintain conductivity of this and within with excursions all compositions oxide to 1100°C 1000°C RESULTS taken are tabulated e.m.f.-temperature seven, with cobalt-30.7 reproducibility the experi- ments 90.3 at. % 1. experi- the extent of the The data cells, out of a total at. ‘A palladium Cell potential 0.082 0.217 0.307 -800°C 3.13 5.41 6.73 3.72 6.43 8.21 z? 1000°C 1100°C hVl°C) 4.32 7.50 10.7 4.92 9.24 13.3 5.97 17.7* 25.6? 0.401 0.510 0.600 10.3 23.6 36.0 13.9 29.0 42.8 17.5 34.3 49.4 21.2 39.6 56.2 36.4 53.4 67.2 0.714 0.800 0.903 73.7 140 226 82.4 151 241 91.1 162 256 99.8 173 271 87.0 109 147 * For T > 994°C; t For 21 > 906°C; 5 900°C for T < 994”C, dE/dT for T < 906”C, dE/dT = = of electrodes. dE (mV) randomly was in Fig. fl per 2, while conventional e.m.f. with respect to heating 10.1 ,JV/“C. 14.8 pV/“C. separate f3 Occasional cells, e.g. yielding excellent results by were unaccountably reproducibility cells of the same nominal per cent or better, accounted *0.2 or cent. standards, The temperatures. A typical in Fig. 2. B3 In general, the of the data for a given cell, for measure- taken cooling, cells fall on top of those for clarity. more erratic than the others, particularly TABLE 1. Cell potentials for cobalt-palladium alloys Npd at rates of change with curve, showing scatter in the data, is presented are for four separate shown and were omitted DISCUSSION in Table shown The data for the remaining to the smoothed mental data and their respective temperature be enough reported. AND from It can obtained 1100 ,“C FIG. 2. Partial e.m.f. data for a cobalt-30.7 at.% palladium alloy. Data are from four out of a total of seven cells. equilibrium the were TEMPERATURE cannot and avoiding with data 1000 900 8600 increases rapidly and temperatures Cell potentials of electrolyte. By limiting range reliable EXPERIMENTAL the current cell potential. metal-metal above alloy, transport if the oxygen interface. temperature palladium of leakage the electrode two-phase at the electrolyte long open-circuit by the very small, but non- a cell irreversibly distributed mental the that is permitted damage I / with time at a rate content. The 90.3 at. % decrease and appeared formation oxygen ml I -2 A alloy behaved in a similar fashion at 1000°C above. at 12I contents greater tended 1015 SOLUTIONS decreased that increased with palladium palladium SOLID O-A3 0 -A4 8-83 m-84 13 as thermodynamically At 1200°C and palladium Co-Pd ‘411 high imped- in a previous investigation of copper-platinum alloys,(g) the cells polarized easily and did not yield reversible. OF at the higher of data which in most for by small differences from composition was cases can be in composition of at. % or less. The Curie temperatures palladium alloys experimental are temperature shows the transition of the 21.7 and 30.7 at. % well within range.‘@ the 800-1100°C Figure 2 clearly from ferromagnetic to paramagWhile this transition netic behavior in the 30.7 o/oalloy. is of second order and the change (partial molar free energy) linear curve, are too the nonlinearity scattered to justify in cell potential should be a smooth non- is small and the data anything but a linear approximation. The two line segments in Fig. 2 were drawn the data from through all seven cells. The points of intersection occurred at 906°C for this alloy and at 994°C for the 21.7 at. ‘A palladium alloy. These temperatures are remarkably close to the 990°C and 900°C Curie temperatures reported by Grube and Kastnero’) for alloys containing nominally 20 and 30 at. % palladium closeness of the agreement respectively. is doubtlessly While the somewhat ACTA 1016 METALLURGICA, VOL. 18, 19’70 largely the results of gas equilibration except at three highest contents. The are from galvanic cell very similar those used this investigation. agreement between two studies excellent. If is assumed the temperature from the work are at 12OO”C, correction for difference in results in closer agreement. principal feature the chemical of cobalt a positive from ideality cobaltrich and a departure in rich solutions. shown in 4, this similar to behavior exhibited nickel in except a ATOMIC % PALLADIUM 7, FIG. 3. Activities of cobalt-palladium alloys. I-this vestigation, 1100°C; a--Sohwerdtfeger 1200°C.“e’ and inMuan, ( , , , , I I , , I I , I , fortuitous in view of the linear treatment of the data required in the present work, it nevertheless gives powerful support to the reliability and sensitivity of the data. The activities (a,,) and partial molar free energies (AB,,) of cobalt, relative to solid cobalt saturated with oxygen, were calculated from equation (1). The related quantities for palladium were evaluated from a graphical integration of the Gibbs-Duhem equation in the form : Nco log YPd = -%o’NCoNPd + s0 %o’~~co (2) where the N, are the constituent mole fractions, ypd is the activity coefficient (apd/Npd) of palladium and c(’ E log yc0/NPd2. The activities of cobalt determined at 1100°C are compared in Fig. 3 with values obtained by Schwerdtfeger and Muan at 1200°C. The data shown for these investigators are I CO 20 40 ATOMIC 60 I I 80 Pd % PALLADIUM FIG. 5. Entropies of mixing of cobalt-palladium alloys at 1000°C. that the positive deviations from ideality of cobalt are much larger. The activities of palladium indicate negative departures from ideality in both systems but significantly more so in the case of the cobalt alloys. The partial molar entropies (A&‘,,), excess entropies (ASooxs ) and enthalpies (Ai7,,) of mixing of cobalt were calculated from the rate of change of cell potential with temperature (see Table 1) using the relations Afl CO 02 20 40 60 00 ATOMIC % PALLADIUM Pd FIG. 4. Comparison of cobalt-palladium and nickelpalladium(4) activities at 900°C. A80,xs (3) = Asco + R In Nc,, (4) = AQco f T AS,, (5) and A&, BIDWELL THERMODYNAMIC et GE.: PROPERTIES’OF Co-Pd 20 SOLID 40 ATOMIC co 20 40 ATOMIC 60 80 Pd s Nco &$yoXs + () No,)2 (1 - 80 % PALLADIUM best ilIustrated by referring to the pertinent excess quantities. These are compared with the values previously obtained for nickel-palladium alloys(Q in Figs. 7-9. The deviations from ideal alloying behavior are similar for both systems, although subs~nt~a~~ larger for the cobalt dloys. The integral excess free energies of mixing or iron-palladium alloys, reported by Aukrust and Muan, exhibit a similar compositional dependency to that shown in Fig. 7, in&dmg the presence of an inflection point at low pa~dium contents and a maximum negative value at high palladium oontents. The absolute value of the latter exceeds 3000 cal/mol and suggests an increasingly negative departure from ideality for the iron-group elements in palladium in the order nickel, cobalt, iron. As indicated in Fig. 8, the positive excess entropies of mixing of cobalt alloys are nearly twice those for dNcoC6) and the free energies deduced from equation (2). The partial and integral molar properties calculated at 1000% are illustrated in Figs. 5 and 6. GENERAL 60 1017 FIG. 8. Compwison of the excess entropies of mixing of eob~t-palladia and nic~al-palladiuml*) alloys at 1000°C. % PALLADIUM The corresponding partial molar quantities for palladium were evaluated from a graphical integration of the Gibbs-Duhem equation in the farm SOLUTIONS DISCUSSION The s~g~~~a~t ~hara~~ristics of the the~od~amie properties of cobalt-palladium solid-solutions are - -600 1 i! \ jj-800 rI 0 Q-1000 400 -800 20 40 ATOMIC 60 80 Pd Y. PALLADlUM 7. Comparison of the excess free energies of mixing of cobalt-palladium and nick&-palladium~~) alloys at 1000°C. Fm. ’ ‘ 20 1 s 40 ATOMIC 8 ’ 60 s r 80 / ’ Pd % PALLADIUM Fm. 9. Comparison of the enthalpies of mixing of cobaltpalladium and nickal-pall~ium~6~ alloys at 1000°C. ACTA 1018 METALLURCICA, VOL. 18, 1970 3. the cobalt atoms in the alloys are fully polarized and consist of a mixture of electronic con~~uratio~ containing (0.7No,) - 3d8(s = 1) and (0.3NoJ 3dQ (s = +) states; and 4. the palladium atoms consist of a mixture of (nB WPd 1.7NoJ - nB + 4dg (s = &) and 1.7NoJ - 4iP0 states, where nB is the average number of Bohr magnetons/atom for the alloy at O’K. This model is based on localized or quasi-bound delectrons (generalized Heisenberg model) and assumes, co 20 40 60 80 Pd in keeping with the neutron scattering measurements ATOMIC % PALLADIUM of Cable et &.,(16) that the magnetic moments of the Fra. 10. Comparison of estimflted m.qnetic entropies of cobalt atoms are independent of composition. The mixing with experimental excess entropies of mixing for cobalt-palladium alloys at 1000°C. latter investigators also concluded that the palladium moments varied with composition, reaching a maxinickel-palladium alloys. While the excess entropy mum value of about 0.4 Bohr magnetonslatom at of the nickel alloys can be almost completely ascribed high palladium concentrations. The above model to electron-spin disordering,(4) a calculation of the is in good agreement, predicting a maximum of 0.41 magnetic contribution to the cobalt data is complicated Bohr magnetonsjatom at N,, = 0.7. by the fact that they cover alloys both above and If the estimated AS(Mag) are subtracted from the below their Curie temperatures and are relative to observed ASYs, the resulting ASxs (Nonmag), as cobalt and palladium in the ferromagnetic and parashown in Fig. 10, are negative with minima at both magnetic states, respectively. The statistical mechancobalt-rich and palladium-rich compositions. The ical value for the molar magnetic entropy arising from presence of the minima suggests a tendency toward the disorder of a ferromagnetic material is given by clustering or short-range atomic ordering in the two the expression :(14) composition ranges. The effects of overstating the probable degree of magnetic disorder in both pum S(~ag) = R In (2s + 1) (7) cobalt and the cobalt-rich alloys in calculating where s is the resultant spin quantum number of the AS(Mag) has been to decrease its value slightly, by an unpaired electrons. This value is actually only amount that increases with palladium content. approached as a limit at temperatures well above the Thus, the true ASKS (Nonmag) should be somewhat Curie temperature. While the spontaneous magnetizamore negative than shown in Fig. 10, with the most tion decreases to zero at the Curie temperature, pronounced effect being a deeper minima at high marking the disappearance of long-range magnetic palladium concentrations. The basic shape of the order, a significant amount of short-range order curve of ASxS (Nonmag) remains unchanged, remains.05) Despite the uncertainty introduced by however. Although no attempt has been made to dealing with an unknown degree of order, an estimate estimate the electronic and vibrational contributions of the magnetic contribution to the thermodynamic to the entropy of mixing, both of these factors are properties is useful in interpreting the alloying expected to be positive. Hence, it is expected that the behavior of cobalt-palladium alloys. configurational excess entropy of mixing is even more The magnetic entropies of mixing, AS(Mag), negat$ve than ASXS (Nonmag). estimated for cobalt-palladium alloys at 1000°C The heats of mixing found for cobal~palladium as a function of composition are shown in Fig. 10. alloys are similar to those observed for nickelThese values were calculated from equation (7), palladium alloys t4)but much more pronounced in their the magnetization data of Bozorth et al.,(ls) and an deviations from ideality (see Fig. 9). Darbyos) assumed ferromagnetic model incorporating the has recently confirmed the unusual behavior, i.e. following features : endothermic palladium-poor and exothermic palla1. pure cobalt is 55 per cent ordered at 1OOO”C!;(~~~dium-rich alloys in the nickel-palladium system by 2. the alloys, N,, > 0.1, are completely disordered differential liquid-metal solution calorimetry. In at 1000°C; view of the other similarities between the two systems, -0.8s BIDWELL et al.: THERMODYNAMIC PROPERTIES OF Co-Pd SOLID SUMMARY 1. AND CONCLUSIONS The relative thermodynamic palladium solid temperature solutions range 1019 SOLUTIONS properties of cobalt- were determined 800-l 100°C by solid in the electrolyte e.m.f. methods. 2. Cobalt-rich from ideality positive deviations for the solvent and negative solutions deviations for the solute. exhibit Palladium-rich alloys exhibit negative departures for the solvent and solute alike. The activity data are similar in character to those obtained for the and iron-palladium(13) systems. nickel-palladium(4) 3. The excess entropies of mixing are positive at all compositions. -600 electronic IO CO 20 ATOMIC FIG. 30 configurational 50 40 However, excess minima at cobalt-rich % PALLADIUM 4. The heats support to the relative enthal- pies found in the present investigation. palladium-rich enthalpies alloys and of mixing exhibited the positive values by seen alloys indicate that the presumably ASxS (config ura t’lonal) are to be interpreted content; inferring suggested by as tendencies toward tering, respectively. cross-sections Pd,Co The ordering and clus- The magnetic-diffuse-scattering support dependence these alloys exhibited of the former the possibility. cross-sections of short-range order rather than being the smoothly functions expected for random alloys. varying structure was greater alloy, that the amount for material stepwise of cooled from 900°C over a period of 12 days than for material quenched from lOOO”C, indicating positional While clustering or solid immiscibility has been noted in iron-rich palladium that the order is of a type. observed in the cobalt-palladium system. occur in the cobalt-rich has not been system, unmixing’lg) alloys in the similar iron- The possibility that this may also alloys of the present system can be inferred by extending the experimental data Isothermal plots of the relative to lower temperatures. partial molar free energies of cobalt, obtained by extrapolating the experimental data, are shown in Fig. 11. Unmixing below 600°C is clearly indicated but may actually occur at lower temperatures at all, due to the influence of the magnetic that are increasing with decreasing within this temperature range. the short-range within vary or not factors temperature from endothermic values with increasing palladium that the non-random presumed behavior configurational excess compositions ordering at palladium-rich and compositions. ACKNOWLEDGEMENTS The authors would like to thank Mr. D. F. Frank of the Aerospace Research Laboratories for preparing of the University Dayton for his assistance in developing programs used in this investigation. for structural features characteristic They noted, for the Pd,Co with compositions are non-random the alloys and Dr. J. E. Davision observed by Cable et aE.f3)for PdCo and compositions angular short-range that the are negative entropies is clustering at cobalt-rich for the cobalt-rich negative entropies and palladium-rich of mixing values to exothermic negative of magnetic, suggests these regions. mixing for cobalt in cobalt-ualladium allovs obtained y from experimental and extrapolated data. The factors and that the solid solutions 11. Isotherms for the partial molar free energy of this lends considerable consideration and vibrational of the computer REFERENCES 1. D. GERSTENBERG, Ann. Phys. 2, 236 (1958). 2. J. CRANOLE, Phil. Mag. 5, 335 (1960). 3. J. W. CABLE, E. 0. WOLLAN and W. C. KOEHLER, Phys. Rev. 138, A755 (1965). 4. L. R. BIDWELL and R. SPEISER, Acta Met. 13, 61 (1965). 5. M. HANSEN and K. ANDERKO, Constitution of Binary Alloys. McGraw-Hill (1958). 6. K. KUIKKOLA and C. WAGNER, J. electrochem. SOC. 104, 379 (1957). 7. W. &. KI~QERY, J. PAPPIS, M. E. DOTY and D. C. HILL, J. Am. Ceram. Sot. 42, 355 (1959). 8. F. E. RIZZO, L. R. BIDWELL and D. F. FRANK, Trans metall. Sot. A.I.M.E. 239, 593 (1967). 9. L. R. BIDWELL, W. J. SCHULZ and R. K. SAXER, Acta Met. 15. 1143 (1967). 227, 371 (1963). 10. R.-A. RAPP, Tians. metaZZ.Soc.A.I.M.E. 11. G. GRUBE and H. KASTNER, 2. Elecktrochem. 42. 159 (1936). 12. K. SCHWERDTEEQER and A. MUAN, Acta Met. 13, 509 11965). 13. &:. AUKRUST and A. MUAN, Acta Met. 10,555 (1962). North-Holland 14. E. A. GUGGENHEIM, Thermodynamics. (1957). 15. R. J. WEISS and K. J. TAUER, Phys. Rev. 102,149O (1956). 16. R. M. BOZORTH, P. A. WOLFF. D. D. DAVIS. V. B. COMPTONand J. I%. WERNICK, Ph&. Rev. 122,115+ (1961). 17. J. H. VAN VLECK, Magnetic Properties of Metals and Alloya, ASM Symp. (1958) Chapter 1. ASM (1959). 18. J. B. DARBY, JR., Presented at the 97th AIME Annual Meeting, New York (February 1968). 19. E. RAUB, H. BEESKOWand 0. LOEBICH,JR., Z. MetaZZk. 54, 549 (1963).
