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A procedure for the modification of undercooling of metal droplets
Volume
3, number
MATERIALS
9,lO
LETTERS
July 1985
A PROCEDURE FOR THE MODIFICATION OF UNDERCOOLING OF METAL DROPLETS D.H. RASMUSSEN, K. JAVED, M. APPLEBY and R. WITOWSKI Depariment Received
of Chemical Engineering,
Clarkson
University, Potsdam,
NY 13636, USA
5 April 1985
Droplet emulsification - a procedure for dispersion of fine liquid metal droplets in a second phase - is a powerful technique for the study of nucleation and growth during solidification. The dispersion of bulk liquid metals as fine droplets relegates the influence of impurity particles to a limited fraction of total sample mass. The chemistry and structure of the surface coating stabilizing the metal droplets controls heterogeneous crystal nucleation and the onset temperature for solidification. To be truly useful, the droplet emulsification technique must be applicable to the high melting metals and alloys of commercial utility and the surface oxide structures native to these metals must be modifiable. A procedure for surface coating and suspension of solid metal powders in solid ceramic powder matrices has been developed which permits the study of pure metals and commercially important alloy systems, with melting temperatures between room tempe ‘ature and 1500°C. The ceramic powder matrices have been shown to be non-catalytic to nucleation for the pure metals: Ag, Au, Cu and Ni and Co. In fact, undercoolings for these metals equal or exceed values reported by any other technique.
1. Introduction Droplet emulsification - a procedure for dispersion of fine liquid metal droplets in a second phase - is a powerful technique for the study of the nucleation and growth of crystals during solidification. The dispersion of bulk liquid metals as tine droplets relegates the influence of impurity particles to a limited fraction of the sample. The chemistry and structure of the surface coating stabilizing the metal droplets controls heterogeneous metal nucleaction and the onset temperature for solidification. The extraction of thermal energy from individual droplets under near isothermal conditions at constant reservoir temperature or by rapid solidification rate processing at high heat transfer conditions during quenching permits modification and control of the microstructure of the solidification product. To be truly useful, the droplet emulsification technique must be applicable to the high melting metals and alloys of commercial utility. The original technique, devised by Vonnegut [I] and used by Turnbull et al. [2-41 has undergone a number of modifications. Rasmussen [S] established that organic liquids could be used as carrier media for 344
metal droplets which both melt below 450°C and do not react with the organic liquid. A number of findings using this technique have been reported by Rasmussen, Perepezko and co-workers [6-91. These results have included larger undercoolings to nucleation for pure metals and alloys than previously expected, observation of metastable and amorphous solidification products, achievement of undercooling into liquid-liquid miscibility gaps, and observation of liquid-liquid demixing into metastable states, etc. The major limitation to application of the technique to materials of technological importance has been that of temperature. A variety of approaches have been attempted to create metal droplet emulsions for nucleation and quenching studies for metals melting at high temperature. Powell and Hogan [lo] created liquid metal droplet dispersions suspended in solid metal matrices by annealing binary alloy systems just above the eutectic or peritectic reactions. They studied the nucleation of the second crystalline solid in the presence of the first. Rasmussen [I I] and Perepezko [ 121 attempted to transfer the established low-temperature metal emulsification technique to intermediate and high temperatures by replacing the or0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 3, number 9,lO
MATERIALS
ganic carrier fluids with fused salts. This procedure has met with only moderate success because of corrosion problems with the emulsion generation systems and the difficulty in controlling the oxidizing environment within the fused salt at high temperature, Flemings [ 131 has achieved metal droplet emulsification at high temperatures by using an inorganic fused oxide as the suspending medium for the metal droplets. The liquid oxides provided fluxing of impurities in the liquid metal droplets but did not prevent metal droplet coalescence during setting due to density differences between the droplets and the liquid oxide. Flemings [ 131 has undertaken a program of study which will suspend metal droptets in liquid oxides at high tem~rature and at low gravity. Such a procedure may permit longer term stabilization of droplet dispersions in suspending media of highly different density than is possible in similar ground based emulsification studies. However, this technique is of limited “practical” importance even for scientific use without readily accessible low “g” environments. Therefore, it was necessary to develop a droplet emulsification technique which would provide for formation and stabilization of metal droplets which melt at a high tem~rature within a non-catalytic second phase suspending media. The properties of the resulting droplet dispersion would require stability for high-temperature processing, and modifiable surface coatings which permit large liquid metal undercooling. The present studies show that such a technique is not only feasible but will provide the opportunity to study both the interfacial interaction of liquid metals with inorganic solids and the undercooling, nucleation and solidification of metals and alloys which melt at high temperatures. The technique used to suspend the metal droplets and to keep them from coalescing on melting is &led the solid in solid suspension technique. That is, a dispersion of liquid metal droplets within the confines of a finely divided surface active or fluxed solid powder is achieved by prior mixing of two solid powders at room temperature followed by heating. T&s new method will permit the study of undercooling and nucleation of high melting temperature alloy systems. First, the metal is produced or obtained as a finely divided solid with a known droplet size distribution. Then, the metal particles are blended with an appropriate volume fraction of much finer particles of the suspend-
LE’M’ERS
July 1985
ing powder and the solid-solid suspension is heated within the appropriate gaseous en~ronment to ensure adherence of the suspender particles to the metal droplets. The criterion for such adherence as given by Kitchener [14] or Dean [15] is that the contact angle for the liquid metal on a ffat surface of the particulate material be fmite and greater than 90”. A positive contact angle prevents total wetting and ingestion of the powder in the liquid metal. The criterion of a greater than 90” contact angle is required to prevent micellization of the powder and gas entrapment within large coalesced liquid metal droplets, The selection of available suspender particles is based on the criteria that the liquid metal contact angle on the solid substrate be greater than 90” and that no chemical reaction between the suspender particle and the liquid metal may occur. The efficacy of a particular suspender to permit undercooling of the liquid metal is an additionally desirable feature, though no a priori criterion for such undercooling can be proposed at this time. It is convenient that a number of liquid metals display contact angles in excess of 90’ on alumina and thoria surfaces [16]. Alumina and thoria are available commer~a~y in more than one c~st~ographic modification - degree of dehydration - and a range of average particulate sizes. Because of their thermodynamic stability following annealing, availability in different crystallographic modifications and sizes, suspender powders prepared from these substrates were chosen to generate liquid metal droplet dispersions. For alumina, it will be noted in the following section that differences in crystallographic structure do impart some effect on the resulting liquid metal undercooling. Another fine particulate considered as a possible suspender powder was graphite, though carbide formation with certain high melting point liquid metals precluded its general use. The difference in bonding and crystallography make graphite a potentially less catalytic suspender to nucleation than more native metal oxides. Use of other reactive suspender powders, other than alumina containing agents for fluxing the droplet surfaces or chemically reacting with them to reduce or dissolve native oxides, has not yet been studied.
345
Volume 3, number 9,lO
2. Experimental method
~~-pu~ty metal particles have been used for preparation of solid in softi metal in ceramic suspensions. The metals Ag, Au, Cu, Ni and Co were purchased from Alfa Products Corporation in an atomised particulate state. Sample preparation was achieved by mixing the metal particles (all samples ranged between 5 and 20 pm average diameter) with the suspender particles (average size ranges from 0.05 to 1 pm diameter). The mixed particles are then pressed into a pellet for thermal analysis. The suspending powders were purchased from Buehler Ltd. The primary functions of the suspending powder were to keep the metal droplets physically separated, one from another and to adhere to the liquid metal surface as a dispersion stabilizer upon melting of the metal. Because of mixing difficulties (large density difference between the ceramic and the metal particles) it is necessary to have excess suspending powder present in the mixture and to fluidize the mixture in a secondary low boiling point liquid. For convenience we used reagent grade methanol for powder mixing and suspension. The use of methanol permitted solids con~ntration by fntering and~or evaporation of suspender solvent. The metal droplets were ~trasonic~y dispersed within a suspending powder in a 1: 3 or 1:4 volume ratio. In order to modify the surface coating on the metal droplets, a small amount of surfactant reagent, phosphomolybdic acid (PMA) was added to particular samples and the fluidized mixture allowed to settle or be filtered. The suspending liquid was evaporated from the sample at low temperature. Alumina powders were chosen for the preliminary study because they are: (1) once calcined, stable over the temperature range of the experiment; (2) inert with respect to o~dation-reduction reactions with the metal over the entire experimental temperature range; and, (3) obtainable in two different initial physical and chemical states and in different mesh sizes. The droplet distribution parameters were observed before and after thermal analysis. A Zeiss Videoplan photo analyser was used to determine the initial and subsequent distribution parameters, mean and standard deviation. The initial powder was never completely dispersed into 100% individual metal droplets. There was always a tendency for small cold welded or clumped aggregates to be retained within the disper346
July 1985
MATERIALS LETTERS
sion. The small primary aggregates contained usually less than 4 or 5 ~di~d~ droplets and were a variable percentage of the total metal mass. After first melting the clustered metal droplets coalesced to change the droplet size distribution to a new and stable value. Continued thermal cycling did not change this distribution. Unlike the suspensions of the droplets in glass technique, we have a dispersion of metal droplets with a determinable droplet size distribution which remains constant with time.
3. Experimental results The nucleation tem~ratures of copper, silver, gold, nickel and cobalt have been determined by this technique. A size distribution analysis of the metal droplets was obtained with a Zeiss Videoplan. The undercooling of the dispersions was determined by differential thermal analysis with a Perkin-Elmer DTA 1700. Thermograms for pure copper and pure gold are presented in fig. 1, For the copper dispersion, all of the
COPPER
+ 0
I
IQ
GOLD
+
0 I
800
900 TEMPERATURE,
1000
1100
“C
Fig. 1. Thermograms for solid in solid dispersions of copper and gold. Note the uniform undercooling in each case to a single exotherm at maximum undercooling. The copper undercooled 245%, the gold undercooled 220°C.
Volume 3, number 9,lO
r
COBALT T”
+ 0
I
)_
NICKEL
a
+ 0
I
J 1000
1100
July 1985
MATERIALS LE’I-TERS
1200 TEMPERATURE,
1300
1500
1400 OC
Thermogram for solid in solid dispersions of cobalt and nickel. Note the uniform undercooling to a single exotherm at 109O’C for the cobalt. The cobalt undercooled a maximum of 42O’C. The thermogram for nickel has multiple exotherms with the last peaking at 11OO’C and an undercooling of 340°C. Fig. 2.
metal undercools to an exotherm with a peak at 245 degrees of undercooling (T, = 828°C) and a maximum undercooling of about 266 degrees. For the gold dispersion, all of the metal undercools to an exotherm with a peak at 220 degrees of undercooling (T, =
843°C) and a maximum undercooling of 230 degrees. Two thermogram exothermic peaks are observed for the nickel dispersion in fig. 2, the first exothermic peak occurs at an undercooling of 290 degrees with maximum undercooling at 310 degrees and a second smaller exotherm is observed at 340 to 350 degrees of undercooling. The first of these peaks corresponds to the undercoolings reported by Cech and Turnbull [2] as maximum undercooling for nickel. The origin of the larger undercooling for a small but substantial fraction of the droplets of nickel will be further explored. A thermogram for pure cobalt is also presented in fig. 2. Cobalt uniformally undercooled about 400 degrees to 1080°C. This undercooling is at least 100 degrees greater than any reported undercooling for cobalt. It is not known whether this undercooling is limited by the surface coating, the suspending ceramic or by homogeneous nucleation. The undercooling, surface treatment , size distributions and purity levels of the five metals are summarized in table 1. No significant change in the particle size distribution was observed after a complex experiment which consisted of coating, melting, solidification and multiple thermal cycling. Maximum observed undercoolings for the metals studied are also presented in table 1. These maximum undertoolings pertain to metal droplets precoated with phosphotungstic acid, phosphomolybdic acid, silicic acid or boric acid and with these fluxes also added to the coating blend. Determining the maximum effects of particular fluxing agents will be detailed in a later study. It appears that the nature of the surface coating is the major controlling factor in the degree of undercooling for a given droplet size distribution, It is for this reason that it is believed that further study of the surface coating chemistry may provide for even larger
Table 1 Undercooling of metal droplets suspended in CU-AIZO~ Metal
Purity (%)
Diameter (pm)
Undercooling (‘C)
Surfactant
cu Ag
99.5 99.9
12+ 5 19+ 10
PMA PMA
Au Ni
99.95 99.999
12* 5 5*5
PMA PMA
co
99.99
5*5
PMA
pk 1 pk 2
to peak
max.
245 225
266 242
220 290 340 400
230 310 350 420
347
Volume 3, number 9,lO
MATERIALS LETTERS
undercoolings. While the absolute undercoolings are large they still are a much smaller percentage of the absolute melting temperature than the established undertoolings for low melting point metals. In the present preliminary study the bulk of the solidification occurs at the maximum undercooling, as evident from the thermograms shown in figs. 1 and 2. This consistency of the results indicates that excellent sample uniformity is attainable. Even if the undercooling is limited by the droplet surface coating, a uniform catalytic potency of the surface coating is achieved in a given droplet distribution in the presence of the suspender powder. Consistently higher undercooling is obtained with a-type AllO (0.05 pm diameter) as the suspender powder. The o-type alumina powder has an octahedral microcrystalline structure and is much finer than the y-type Al,O, alumina powder (0.3 pm) with rhombic crystal structure. This alumina will dehydrate and transform during heating and may affect the interface by interacting with the flux more than will the y-type alumina.
4. Discussion The droplet emulsification technique has been made applicable to high melting metals and alloys and the chemistry of the interface fluxing shown to yield large undercoolings. Using a straightforward and simple procedure for the treatment of metal droplet surfaces and the suspension of metal powders in solid ceramic powder matrices, the study of undercooling and nucleation in as-cast powders can be achieved. The ceramic powder matrices have been shown to be non-catalytic (as compared to published results) to metal crystal nucleation for the pure metals: Ag, Au, Cu, Ni and Co. In fact, undercoolings for these metals equal or exceed values reported by any other technique. ‘The procedure permits: (a) the retention of droplet dispersion size distributions of as-prepared powders or when cold welded droplet clusters exist, the redetermination of a modified stable distribution; (b) the control of the influence of the suspending ceramic powder on metal droplet nucleation by a suitable intervening amorphous layer; (c) the measurement of the nucleation temperature for as-received droplet dispersions; and (d) the modification of this undercooling by chemical etching and fluxing of the solid droplet surfaces. The most useful applications of this technology include the study of nucleation rate 348
July 1985
and nucleus composition versus temperature and the study of the influence of nucleation at low temperature versus rapid solidification processing on droplet microstructure. Indeed, quantitative information on the impact of high undercooling and nucleation at low temperatures on the structure of commercially important metals can be readily achieved.
Acknowledgement This program of research was supported by a grant from the National Science Foundation, DMR-8 107 117.
[l] [2] [3] [4] [S]
B. Vonnegut, .I. ColIoid Sci. 3 (1948) 563. D. Turnbull and R.E. Cech, J. Appl. Phys. 21 (1950) 804. J.H. Holloman and D. Turnbull, J. Metals (1951) 803. D. Turnbull, J. Chem. Phys. 20 (1950) 411. D.H. Rasmussen and C.R. Loper Jr., Acta Met. 23 (1975) 1215. [6] D.H. Rasmussen and C.R. Loper Jr., Acta Met. 24 (1976) 117. [7] D.H. Rasmussen, J.H. Perepezko and C.R. Loper Jr., in: Rapidly quenched metals, eds. N.J. Grant and B.C. Giessen (MIT Press, Cambridge, MA, 1976) p. 5 1. [8] D.H. Rasmussen, I.E. Anderson, C.R. Loper Jr. and J.H. Perepezko, in: Sheffield International Conference on Solidification and Casting (Met. Sot., London, 1979) p. 169. [ 91 J.H. Perepezko, C. Galaup and D.H. Rasmussen, in: Mat. Sci. Space Proc. Eur. Symp. (ESASP-SP, 1979) p. 375. [lo] G.L.F. Powell and L.M. Hogan, TMS-AIME 242 (1968) 2133. 1111 D.H. Rasmussen and M. Sivaramakrsihnan, in: Nucleation, growth and impurity effects in crystallization process engineering, ed. M.A. Farrell Epstein, AICHE Symposium Series, Vol. 215 (New York, 1982) p. 14. 1121 J.H. Perepezko, C. Galaup and K.P. Cooper, in: Materials processing in the reduced gravity environment of space, MRS Symp. Proc. Vol. 9, ed. G.E. Rindone (North-Holland, Amsterdam, 1982) p. 491. 1131 MC. Flemings, The Materials Processing Research Base of the Materials Processing Center, Annual Report 1982, NASA-CR-170174. [I41 J.A. Kitchener and P.R. Mussellwhite, in: Emulsion science, ed. P. Sherman (Academic Press, New York, 1968) p. 80. 1151 R.B. Dean, Modern colloids (Van Nostrand, Princeton, 1949) p. 250. 1161 L.E. Murr, Interfacial phenomena in metals and alloys (Addison-Wesley, Reading, 1975) p. 69.
3, number
MATERIALS
9,lO
LETTERS
July 1985
A PROCEDURE FOR THE MODIFICATION OF UNDERCOOLING OF METAL DROPLETS D.H. RASMUSSEN, K. JAVED, M. APPLEBY and R. WITOWSKI Depariment Received
of Chemical Engineering,
Clarkson
University, Potsdam,
NY 13636, USA
5 April 1985
Droplet emulsification - a procedure for dispersion of fine liquid metal droplets in a second phase - is a powerful technique for the study of nucleation and growth during solidification. The dispersion of bulk liquid metals as fine droplets relegates the influence of impurity particles to a limited fraction of total sample mass. The chemistry and structure of the surface coating stabilizing the metal droplets controls heterogeneous crystal nucleation and the onset temperature for solidification. To be truly useful, the droplet emulsification technique must be applicable to the high melting metals and alloys of commercial utility and the surface oxide structures native to these metals must be modifiable. A procedure for surface coating and suspension of solid metal powders in solid ceramic powder matrices has been developed which permits the study of pure metals and commercially important alloy systems, with melting temperatures between room tempe ‘ature and 1500°C. The ceramic powder matrices have been shown to be non-catalytic to nucleation for the pure metals: Ag, Au, Cu and Ni and Co. In fact, undercoolings for these metals equal or exceed values reported by any other technique.
1. Introduction Droplet emulsification - a procedure for dispersion of fine liquid metal droplets in a second phase - is a powerful technique for the study of the nucleation and growth of crystals during solidification. The dispersion of bulk liquid metals as tine droplets relegates the influence of impurity particles to a limited fraction of the sample. The chemistry and structure of the surface coating stabilizing the metal droplets controls heterogeneous metal nucleaction and the onset temperature for solidification. The extraction of thermal energy from individual droplets under near isothermal conditions at constant reservoir temperature or by rapid solidification rate processing at high heat transfer conditions during quenching permits modification and control of the microstructure of the solidification product. To be truly useful, the droplet emulsification technique must be applicable to the high melting metals and alloys of commercial utility. The original technique, devised by Vonnegut [I] and used by Turnbull et al. [2-41 has undergone a number of modifications. Rasmussen [S] established that organic liquids could be used as carrier media for 344
metal droplets which both melt below 450°C and do not react with the organic liquid. A number of findings using this technique have been reported by Rasmussen, Perepezko and co-workers [6-91. These results have included larger undercoolings to nucleation for pure metals and alloys than previously expected, observation of metastable and amorphous solidification products, achievement of undercooling into liquid-liquid miscibility gaps, and observation of liquid-liquid demixing into metastable states, etc. The major limitation to application of the technique to materials of technological importance has been that of temperature. A variety of approaches have been attempted to create metal droplet emulsions for nucleation and quenching studies for metals melting at high temperature. Powell and Hogan [lo] created liquid metal droplet dispersions suspended in solid metal matrices by annealing binary alloy systems just above the eutectic or peritectic reactions. They studied the nucleation of the second crystalline solid in the presence of the first. Rasmussen [I I] and Perepezko [ 121 attempted to transfer the established low-temperature metal emulsification technique to intermediate and high temperatures by replacing the or0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Volume 3, number 9,lO
MATERIALS
ganic carrier fluids with fused salts. This procedure has met with only moderate success because of corrosion problems with the emulsion generation systems and the difficulty in controlling the oxidizing environment within the fused salt at high temperature, Flemings [ 131 has achieved metal droplet emulsification at high temperatures by using an inorganic fused oxide as the suspending medium for the metal droplets. The liquid oxides provided fluxing of impurities in the liquid metal droplets but did not prevent metal droplet coalescence during setting due to density differences between the droplets and the liquid oxide. Flemings [ 131 has undertaken a program of study which will suspend metal droptets in liquid oxides at high tem~rature and at low gravity. Such a procedure may permit longer term stabilization of droplet dispersions in suspending media of highly different density than is possible in similar ground based emulsification studies. However, this technique is of limited “practical” importance even for scientific use without readily accessible low “g” environments. Therefore, it was necessary to develop a droplet emulsification technique which would provide for formation and stabilization of metal droplets which melt at a high tem~rature within a non-catalytic second phase suspending media. The properties of the resulting droplet dispersion would require stability for high-temperature processing, and modifiable surface coatings which permit large liquid metal undercooling. The present studies show that such a technique is not only feasible but will provide the opportunity to study both the interfacial interaction of liquid metals with inorganic solids and the undercooling, nucleation and solidification of metals and alloys which melt at high temperatures. The technique used to suspend the metal droplets and to keep them from coalescing on melting is &led the solid in solid suspension technique. That is, a dispersion of liquid metal droplets within the confines of a finely divided surface active or fluxed solid powder is achieved by prior mixing of two solid powders at room temperature followed by heating. T&s new method will permit the study of undercooling and nucleation of high melting temperature alloy systems. First, the metal is produced or obtained as a finely divided solid with a known droplet size distribution. Then, the metal particles are blended with an appropriate volume fraction of much finer particles of the suspend-
LE’M’ERS
July 1985
ing powder and the solid-solid suspension is heated within the appropriate gaseous en~ronment to ensure adherence of the suspender particles to the metal droplets. The criterion for such adherence as given by Kitchener [14] or Dean [15] is that the contact angle for the liquid metal on a ffat surface of the particulate material be fmite and greater than 90”. A positive contact angle prevents total wetting and ingestion of the powder in the liquid metal. The criterion of a greater than 90” contact angle is required to prevent micellization of the powder and gas entrapment within large coalesced liquid metal droplets, The selection of available suspender particles is based on the criteria that the liquid metal contact angle on the solid substrate be greater than 90” and that no chemical reaction between the suspender particle and the liquid metal may occur. The efficacy of a particular suspender to permit undercooling of the liquid metal is an additionally desirable feature, though no a priori criterion for such undercooling can be proposed at this time. It is convenient that a number of liquid metals display contact angles in excess of 90’ on alumina and thoria surfaces [16]. Alumina and thoria are available commer~a~y in more than one c~st~ographic modification - degree of dehydration - and a range of average particulate sizes. Because of their thermodynamic stability following annealing, availability in different crystallographic modifications and sizes, suspender powders prepared from these substrates were chosen to generate liquid metal droplet dispersions. For alumina, it will be noted in the following section that differences in crystallographic structure do impart some effect on the resulting liquid metal undercooling. Another fine particulate considered as a possible suspender powder was graphite, though carbide formation with certain high melting point liquid metals precluded its general use. The difference in bonding and crystallography make graphite a potentially less catalytic suspender to nucleation than more native metal oxides. Use of other reactive suspender powders, other than alumina containing agents for fluxing the droplet surfaces or chemically reacting with them to reduce or dissolve native oxides, has not yet been studied.
345
Volume 3, number 9,lO
2. Experimental method
~~-pu~ty metal particles have been used for preparation of solid in softi metal in ceramic suspensions. The metals Ag, Au, Cu, Ni and Co were purchased from Alfa Products Corporation in an atomised particulate state. Sample preparation was achieved by mixing the metal particles (all samples ranged between 5 and 20 pm average diameter) with the suspender particles (average size ranges from 0.05 to 1 pm diameter). The mixed particles are then pressed into a pellet for thermal analysis. The suspending powders were purchased from Buehler Ltd. The primary functions of the suspending powder were to keep the metal droplets physically separated, one from another and to adhere to the liquid metal surface as a dispersion stabilizer upon melting of the metal. Because of mixing difficulties (large density difference between the ceramic and the metal particles) it is necessary to have excess suspending powder present in the mixture and to fluidize the mixture in a secondary low boiling point liquid. For convenience we used reagent grade methanol for powder mixing and suspension. The use of methanol permitted solids con~ntration by fntering and~or evaporation of suspender solvent. The metal droplets were ~trasonic~y dispersed within a suspending powder in a 1: 3 or 1:4 volume ratio. In order to modify the surface coating on the metal droplets, a small amount of surfactant reagent, phosphomolybdic acid (PMA) was added to particular samples and the fluidized mixture allowed to settle or be filtered. The suspending liquid was evaporated from the sample at low temperature. Alumina powders were chosen for the preliminary study because they are: (1) once calcined, stable over the temperature range of the experiment; (2) inert with respect to o~dation-reduction reactions with the metal over the entire experimental temperature range; and, (3) obtainable in two different initial physical and chemical states and in different mesh sizes. The droplet distribution parameters were observed before and after thermal analysis. A Zeiss Videoplan photo analyser was used to determine the initial and subsequent distribution parameters, mean and standard deviation. The initial powder was never completely dispersed into 100% individual metal droplets. There was always a tendency for small cold welded or clumped aggregates to be retained within the disper346
July 1985
MATERIALS LETTERS
sion. The small primary aggregates contained usually less than 4 or 5 ~di~d~ droplets and were a variable percentage of the total metal mass. After first melting the clustered metal droplets coalesced to change the droplet size distribution to a new and stable value. Continued thermal cycling did not change this distribution. Unlike the suspensions of the droplets in glass technique, we have a dispersion of metal droplets with a determinable droplet size distribution which remains constant with time.
3. Experimental results The nucleation tem~ratures of copper, silver, gold, nickel and cobalt have been determined by this technique. A size distribution analysis of the metal droplets was obtained with a Zeiss Videoplan. The undercooling of the dispersions was determined by differential thermal analysis with a Perkin-Elmer DTA 1700. Thermograms for pure copper and pure gold are presented in fig. 1, For the copper dispersion, all of the
COPPER
+ 0
I
IQ
GOLD
+
0 I
800
900 TEMPERATURE,
1000
1100
“C
Fig. 1. Thermograms for solid in solid dispersions of copper and gold. Note the uniform undercooling in each case to a single exotherm at maximum undercooling. The copper undercooled 245%, the gold undercooled 220°C.
Volume 3, number 9,lO
r
COBALT T”
+ 0
I
)_
NICKEL
a
+ 0
I
J 1000
1100
July 1985
MATERIALS LE’I-TERS
1200 TEMPERATURE,
1300
1500
1400 OC
Thermogram for solid in solid dispersions of cobalt and nickel. Note the uniform undercooling to a single exotherm at 109O’C for the cobalt. The cobalt undercooled a maximum of 42O’C. The thermogram for nickel has multiple exotherms with the last peaking at 11OO’C and an undercooling of 340°C. Fig. 2.
metal undercools to an exotherm with a peak at 245 degrees of undercooling (T, = 828°C) and a maximum undercooling of about 266 degrees. For the gold dispersion, all of the metal undercools to an exotherm with a peak at 220 degrees of undercooling (T, =
843°C) and a maximum undercooling of 230 degrees. Two thermogram exothermic peaks are observed for the nickel dispersion in fig. 2, the first exothermic peak occurs at an undercooling of 290 degrees with maximum undercooling at 310 degrees and a second smaller exotherm is observed at 340 to 350 degrees of undercooling. The first of these peaks corresponds to the undercoolings reported by Cech and Turnbull [2] as maximum undercooling for nickel. The origin of the larger undercooling for a small but substantial fraction of the droplets of nickel will be further explored. A thermogram for pure cobalt is also presented in fig. 2. Cobalt uniformally undercooled about 400 degrees to 1080°C. This undercooling is at least 100 degrees greater than any reported undercooling for cobalt. It is not known whether this undercooling is limited by the surface coating, the suspending ceramic or by homogeneous nucleation. The undercooling, surface treatment , size distributions and purity levels of the five metals are summarized in table 1. No significant change in the particle size distribution was observed after a complex experiment which consisted of coating, melting, solidification and multiple thermal cycling. Maximum observed undercoolings for the metals studied are also presented in table 1. These maximum undertoolings pertain to metal droplets precoated with phosphotungstic acid, phosphomolybdic acid, silicic acid or boric acid and with these fluxes also added to the coating blend. Determining the maximum effects of particular fluxing agents will be detailed in a later study. It appears that the nature of the surface coating is the major controlling factor in the degree of undercooling for a given droplet size distribution, It is for this reason that it is believed that further study of the surface coating chemistry may provide for even larger
Table 1 Undercooling of metal droplets suspended in CU-AIZO~ Metal
Purity (%)
Diameter (pm)
Undercooling (‘C)
Surfactant
cu Ag
99.5 99.9
12+ 5 19+ 10
PMA PMA
Au Ni
99.95 99.999
12* 5 5*5
PMA PMA
co
99.99
5*5
PMA
pk 1 pk 2
to peak
max.
245 225
266 242
220 290 340 400
230 310 350 420
347
Volume 3, number 9,lO
MATERIALS LETTERS
undercoolings. While the absolute undercoolings are large they still are a much smaller percentage of the absolute melting temperature than the established undertoolings for low melting point metals. In the present preliminary study the bulk of the solidification occurs at the maximum undercooling, as evident from the thermograms shown in figs. 1 and 2. This consistency of the results indicates that excellent sample uniformity is attainable. Even if the undercooling is limited by the droplet surface coating, a uniform catalytic potency of the surface coating is achieved in a given droplet distribution in the presence of the suspender powder. Consistently higher undercooling is obtained with a-type AllO (0.05 pm diameter) as the suspender powder. The o-type alumina powder has an octahedral microcrystalline structure and is much finer than the y-type Al,O, alumina powder (0.3 pm) with rhombic crystal structure. This alumina will dehydrate and transform during heating and may affect the interface by interacting with the flux more than will the y-type alumina.
4. Discussion The droplet emulsification technique has been made applicable to high melting metals and alloys and the chemistry of the interface fluxing shown to yield large undercoolings. Using a straightforward and simple procedure for the treatment of metal droplet surfaces and the suspension of metal powders in solid ceramic powder matrices, the study of undercooling and nucleation in as-cast powders can be achieved. The ceramic powder matrices have been shown to be non-catalytic (as compared to published results) to metal crystal nucleation for the pure metals: Ag, Au, Cu, Ni and Co. In fact, undercoolings for these metals equal or exceed values reported by any other technique. ‘The procedure permits: (a) the retention of droplet dispersion size distributions of as-prepared powders or when cold welded droplet clusters exist, the redetermination of a modified stable distribution; (b) the control of the influence of the suspending ceramic powder on metal droplet nucleation by a suitable intervening amorphous layer; (c) the measurement of the nucleation temperature for as-received droplet dispersions; and (d) the modification of this undercooling by chemical etching and fluxing of the solid droplet surfaces. The most useful applications of this technology include the study of nucleation rate 348
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and nucleus composition versus temperature and the study of the influence of nucleation at low temperature versus rapid solidification processing on droplet microstructure. Indeed, quantitative information on the impact of high undercooling and nucleation at low temperatures on the structure of commercially important metals can be readily achieved.
Acknowledgement This program of research was supported by a grant from the National Science Foundation, DMR-8 107 117.
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