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Likewise, the low-temperature modulus tests described by Clash and Berg (6),and Gehman, Woodford, and Wilkinson ( 7 ) , are much too short to be used as measures of static modulus. The transient character of the modulus measured in these tests has, of course, been recognized; but there has not been previously any suggestion or demonstration that the long-time static moduli may fall so extremely low as to become zero or negative. ACKNOWLEDGMENT
The authors of this paper are indebted to G. F. Schrappel for obtaining the experimental data contained herein. LITERATURE CITED
(1) Alfrey, T.,“Mechanical Behavior of High Polymers,” New York,
Interscience Publishers, Inc., 1948. (2) “A.S.T.M. Standards,” Designation D 39546T, Method B, p. 1001, Philadelphia, Pa., American Society for Testing Materials, 1946. (3) Beatty, J. R.,and Dnvies, J. M., J . Applied Phys., 20, 633 1949.
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(4) Boggs, F. W.,“Statistical Mechanics of Rubber,” paper presented a t the 58th Meeting of the Division of Rubber Chemistry, AMERICAN CHEMICAL SOCIETY, Washington, D. C. (5) Clash, R. F., Jr., and Berg, R. M., IND.ENG.CHEM.,36, 279 (1944). (6) Flory, P. J., J . Chem. Phvs., 15, 397 (1947). (7) Gehman, S. D.,Woodford, D. E., and Wilkinson, C. S., IND. ENG.CHEM.,39, 1108 (1947); Rubber Chem. and Technol., 21, 94 (1948). (8) Greene, H.E., and Loughborough, D. L., J . Applied Phys., 16, 3 (1945); Rubber Chem. and Technol., 18, 587 (July 1945). (9) “Rubber, Synthetic, Medium-Soft; Molded, Sheet, and Strip (For Airport, Hatch, and Watertighband-Airtight-Door Gaskets),” U. S. Govt. Printing Office, MIL-R-SOOA, 1950. (IO) Smith, W. H., and Saylor, C. P., J . Research Natl. Bur. Standards, 21, 257 (1938). (11) Wood, L.A., “Advances in Colloid Sciences,” Vol. 11, Rubber, New York, Interscience Publishers, Inc., 1946. RECEIVEDMarch 23, 1951, This work was supported by funds under contract W44-109QM-2030, Office of the Quartermaster General. Contribution 112 from the General Laboratories of the U. S. Rubber Co., Passaic. N. J. Presented a t the 58th Meeting of the Division of Rubber Chemistry, AMERICAN CHEMICAL SOCIETY, Washington, D. C., 1951.
Crystal Size Distri ution of Electrolytic Metal wders J
POWDERS FROM FUSED ELECTROLYTE BATHS CHUIC-CHING MA School of C h e m i c a l Engineering, T u l a n e University, N e w Orleans, La.
ru’ DETERMINING the usefulness of metal powders for powder metallurgy applications, it is important to study their chemical and physical properties. The influences of physical properties are considered to be equally important as those of chemical properties. It is not infrequent that a powder with a high degree of purity may not be suitable for a powder metallurgy process because of the inferiority of one of its physical properties. The workability of a green compact made from loose metal powders and the mechanical strength of the resulting article are affected to a marked degree by such physical characteristics as particle size, particle shape, crystal structure, and surface conditions. The apparent density as well as the compactability depend entirely on these fundamental physical properties. Based on a statistical analysis, there is a definite and close relationship between the physical characteristics of the metal powders and the ultimate properties of the finished products. It cannot be overemphasized, therefore, that the success or failure of a powder metallurgy process relies upon the physical character of the metal powders employed. Metal powders may be produced by a great variety of mechanical, physical, and chemical methods. At present, most of the powders employed for molding processes are made either by reduction of metallic oxides or halides, or by electrolysis. The powders of the same metal, but prepared by different processes, may differ in crystal size and shape. For instance, iron powders are of irregular dentritic structure when produced electrolytically, but of regdar spherical shape when they are obtained from thermal decomposition of iron carbonyls. It is a known fact that electrolytic metal powders are of nonuniform size. As explained later there is a n advantage in having a wide range of crystal size distribution for pressing. From the results of the present investigation, it appears that metal powders obtained from fused electrolyte baths are suitable materials for powder metallurgy processes. The desired sizes of metal pow-
ders may be obtained by controlling such variables as temperature, current density, and composition of the bath. The problem of correlating the crystal size distribution of electrolytic tungsten to the temperature, current density, and other variables was first attacked by Fink and Ma (5, 6),who studied
THERMOCOU PCE
-SILICA
SCALE
1
&”= 1”
Figure 1. Graphite Crucible
INSULAT \N6 TUBE
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I N D U S T R I A L A N D EN G I N E E R I N G C H E M I S T R Y
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EXPERIMENTS ON CRYSTAL SIZE DISTRIMETALPOWDERS FROM FUSEDELECTROLYTE BATHS
SUMMARY OF
Cathode Bath Current Series Bat$ Cathode Temp., Density, No." Composition Product O C. Amp./Sq.Dm. Conatant Tantalum Varied 1 Constant 800-950 20 1 part KzTaFv 1 part KCI 0 . 1 Dart TazOa Constant Tantalum Varied 2 Constint 800-050 so 1 part KzTaF7 1 part KCl 0 . 1 part TasOi Varied Constant Constant Tungsten 900-1050 50 2 moles NerrB107 1 mole WO: Varied Constant Tungeten Constant 900-1050 60 2 parts phosphate mixtureb 1 part WOa Varied Thorium Constant Constant 780-850 60 1 part KThF8 0 . 5 part NaCl 0 . 5 part KC1 Varied Tantalum Constant 6 Constant 10-40 900 1 part KzTaF7 1 part KCl 0 . 1 part Taros 7 Constant Varied Tungsten Constant 30-60 900 2 moles NarB4Or 1 mole WOI Varied Tungsten Constant 8 Constant 1000 30-60 2 moles NaeB407 1 mole WOI Varied Constant Tungsten 9 Constant 900 30-60 2 parts phosphate mirtureb 1 part WOI Varied Columbium Constant 10 Constant 20-40 800 1 part KzCbOFs 1 part KCl Constant 11 Varying amount of infusible Tungsten Constant 1000 60 impurities in borate baths Constant Constant 12 Varying amount of infusible Tungsten 1000 50 impurities in phosphate baths ... 0 4 runs for each series with the exception of Series Nos. 11 and 12 which oonsist of two runs each. b Phosphate mixture consists of 7 moles NatPzO7 and 3 holes NaPOa.
ki
G c
uI
2
the tungsten crystal growth in fused borate and fused phosphate baths. Driggs and Lilliendahl (3)studied the influence of the chloride ion on crystal size of tantalum powders in a fused double fluoride bath. However, all these investigations were limited to a particular metal powder. There is no correlated generalization which can be applied to all electrolytic metal powders to bring forth the relationship of crystal size apd crystal size distribution on the one hand and the controllable%ariables on the other. If these variables could be correlated, the crystal size of any metal powder obtained from any electrolytic process might be predicted to a greater or lesser extent. The pmpose of the present investigation is to establish these generalized relationships. Studies on the same approach but related to aqueous electrolyte baths are now in progress. APPARATUS AND PROCEDURE
As shown in Figure 1 a graphite crucible which served both as an electrolyzing cell and as a cathode was made by drilling a hole in a 5-inch diameter graphite electrode. It was packed tightly with granular carbon resistors in the center of a resistance furnace, Twelve series of experiments, consisting of 2 t o 4 runs each asI listed in Table I, were carried out in this setup for the preparation of tantalum, tungsten, columbium, and thorium crystalline powders. I n each run, 600 t o 800 grams of dry charge were thoroughly mixed and put into the crucible. The furnace was heated up t o the desired temperature by an alternating current of about 400 amperes at a volta e of 25 to 50 volts. The temperature of the bath was measurecf with a thermocouple inserted through a small hole of */pinch diameter drilled vertically in the center of the wall of the a hite crucible. As soon as the charge in the crucible was megecf and heated up t o the electrolyzing temperature, electrolysis was started and continued until enough current passed through the bath for disqharging about 75% of the total content of the refractory metal ion a t the cathode. A graphite anode was used for all runs. At the end of each run, the salt was dissolved out and the metal powder recovered from the sides and bottom of the crucible. The cr stal size of the metal powder from each run was investigate$ by screen analysis abcording t o the standard method recommended by the American Society for Testing Materials.
Crystal Size, Microns
Figure 2. Crystal Size Distribution of Tantalum and Tungsten Powders Obtained from Fused Baths at Varying Temperatures
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100 90 80
70 A
60
Tun den powder from fused borate baths Bath temperaturo: 1000~ c. Bath composition: 9 moles NrzBkO7, 1 mole
50
40
WOV
30 20 10
0 I100
G
+a
890
3 $ 8 0
fi ._(
$
70
B
g
O y
Tungsten powder from fused phosphate bathsl Bath temperature: 900' C. 9 Bath compoaition: pa* phosphate mixture,
. I
5
50
1 part WOa
40
-
30 20 10
100 30
50
7090 200 Crystal Size, Microns
300 400
Figure 3. Crystal Size Distribution of Thorium frbm Fused Baths a t Varying Temperatures, and Tantalum and Tungsten Powders Obtained from Fused Baths at Varying Current Densities
90
C Columbium owdor from fused oxv&oeolumbate baths Bath temperatwo: 800' C. Bath corn osition: 1 part KnCbgFs, 1 Dsrt KCI
80
70
Aset of U. S. Standard sieves of 40,60,80,100, 120, 140,170,200, 230, 270, and 325 mesh, respectively, was used for each experi-
80
ment. The cumulative weight per cent passing t,hrough these screens was reported in microns equivalent t o the opening of each screen.
50
EXPERIMENTAL DATA
The results of the screen analysis of'the crystalline powders of tantalum, tungsten, columbium, and thorium obtained from fused fluoride, fluoride-chloride, borate, and phosphate baths a t varying temperatures, varying current densities, and varying impurities were plotted in Figures 2 to 6. FiG.res 2C and 3C have been published previously ( 5 ) . For the purpose of comparison and with the permission of the authors, these two curves were included in this paper. DISCUSSION O F RESULTS
Temperature, current density, and bath composition are the primary variables that do affect the number of crystal nuclei and the subsequent growth of the freshly deposited metal in a fused salt bath. I n order to correlate the complex nature of metal nqclei formation, crystal growth, and the resulting crystal size dis-
40
60 80 100 200 Crystal Size, Microns
400
600
Figure 4. Crystal Size Distribution of Tungsten and Columbium Powders Obtained from Fused Baths at Varying Current Densities
tribution with these variables, it was essential to set two of these variables'at fixed values while varying the third one. Five series of runs a t constant current densities and constant bath compositions but varying temperatures were made t o establish a generalized relationship between temperature and crystal size distribution. For the purpose of setting forth the relations between current density or percentage of impurities in the bath on the one hand and crystal size distribution on the other, seven more experiments were carried out to establish these generalized rules. The curves in Figure 2 indicatk that higher temperatures favor the formation of coarse crystalline powders in a fused elertrolyte
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INDUSTRIAL AND ENGINEERING CHEMISTRY
bath. From the remaining seven curves, it is evident that the higher the current density or the higher the percentage of impurities, the finer will be the metal crystals. By contColling carefully any two or all three of these variables, it is possible to produce coarse or fine crystalline powder to suit any particular case. If a high or a low level of one of the variables, such as temperature, has to be used, its effect on the crystal size may be counteracted by the other variables, such as current density.
200
300
6m
I I1
Crystal Size, Microns
Figure 5. Crystal Size Distribution of Tungsten Powder Obtained from Fused Borate Baths of Varying Ferric Oxide Content Bath temperature: 1000° C. Cathodeourrent density: 50 amp./sq. dm.
The above distinct characteristics may be attributed to the formation of an alkali metal vapor film over the cathode surface. The existence of such a film and its consequent polarization effect have been actually observed in every one of the experiments. This polarization effect is more pronounced when operating a t a lower temperature than a t a higher temperature. The boiling point of potassium metal is 760" C. and therefore any cathode film of the potassium vapor formed in the 800" C. bath will be more tenacious than in the 900" or looOo C. bath. The same is true in the caae of sodium vapor film (boiling point of sodium, 878" C.) which existed in the baths of series 5. Moreover, the cathode current efficiencies of the various runs were noticed to vary with the thickness of the polarized cathode films. I n the caae of tungsten, these phenomena have been fully explained by the author and Fink in their previous publications (46). Based on the above alkali metal-vapor film theory, it is easy to conclude that the lower the temperature, the thicker will be the alkali vapor film and the greater will be its polarization effect. The increase of polarization effect will encourage the formation of new crystal nuclei and finer metal crystals will result. On the other hand, the decrease of polarization effect due to rising temperature will stimulate crystal growth and discouragenuclei formation. Likewise, the higher the current density, the greater will be the alkali metal-vapor film polarization effect and, therefore, the finer will be the metal crystals. Coarser crystals will be obtained if lower current density is used, The influence of partially fusible impurities upon the metal crystal size may be explained in the same manner. In the present study, borate, chloride, fluoride, and phosphate have been used as fused salts in a number of runs for the electrolytic deposition of tantalum, tungsten, columbium, or thorium. According to the law of probability, it is sufficient to derive the generalized rules from experimental data of these runs if there are no contradictory results obtained from them. No attempt has beemmade t o study the purity of products, percentage of recovery, and energy efficiency of any one of the above processes. For detailed records of these, results of other investigators (1-7) should be consulted.
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As a matter of faot, electrolytic metal powders are of polycrystalline dendritic shape and have nonspherical particles. Within the sieve range, the crystal size is measured only by whether the particle will pass the opening of a screen of standard woven square mesh. This kind of measurement may not represent the true average diameter of the particle under consideration. However, this conventional screen test method is justifiable in this study for determining the relative values of crystal size distribution from different baths. Based on the experimental results of the present study, it a p pears that metal powders obtained from fused salt baths lpve a wide range of crystal size distribution. As this range becomes wider and wider, an ideal packing with perfect and complete contact between the particles will approach the limit of the massive volume of the metal fitting into the same space. Although this can never be accomplished from the practical viewpoint, it is quite possible to choose the appropriate temperature and proper current density for metal deposition t o attain the widest range of distribution within the limit of certain desired sizes. Under this condition, an increase in packing density and a reduction in porosity can be achieved, because the voids or pores created by the larger crystals are filled in by the next smaller particles and the voids of the latter can be further filed in by particles of still smaller sise and so forth. The dense compact resulting from the intimate contact of various particles would ensure sufficient adhesion strength between the boundaries of the neighboring particles after the molding and the sintering processes. This is a n explanation of why electrolytic-metal powders are one of the best materials for powder metallurgy processes.
Crystal Size, Microns
Figure 6. Crystal Size Distribution of Tungsten Powder Obtained from Fused Phosphate Baths of Varying Ferric Oxide Content Bath temperature 1OOO" C. Cathodecurrent density: 50 amp./sq. dm.
Rletal powders from aqueous electrolyte baths may follow the same pattern as far as temperature or current density and crystal size or crystal size distribution relationships are concerned. However, it is not justifiable to draw a definite conclusion until the experimental facts have been established. CONCLUSIONS
The generalized relationships between crystal size distribution, temperature, current density, and impurities in the bath have been established by a careful study of the crystalline powders of various metals obtained from different fused electrolyte baths. In general, within the range of the conditions specifled herein, raising the temperature tends to yield larger metal crystals, while raising the current density tends to yield smaller crystals. Partially fusible impurities, such aa ferric oxide, also affect the crys-
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tal size, an increased concentration of such impurities tending to decrease the size of crystals. A new theory is developed based on the polarization effect of alkali metal vapor film deposited on the cathode surface. This theory readily accounts for the many phenomena-sometimes very perplexing-observed in the behavior of the various fused electrolyte baths, 118 well as the phenomena relative to the size and size distribution of the various metal crystals deposited a t the cathode. As there are no contradictory results brought forth by quite a number of runs in many fused salt baths of widely distinct physical and chemical properties, this new theory may be extended to include all electrolytic metal powders obtained from fused alkali salt baths. Finally, the term “metal powder” and the term “metal crystal” may be used interchangeably since all the electrolytic metal powders are polycrystalline powders or crystals.
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ACKNOWLEDGMENT
The author wishes to express his most sincere appreciation to his teacher, Colin G. Fink of Columbia University, who succeeded in awakening in him a deep and lasting interest in the fields of powder metallurgy and electrochemistry. He also wishes to thank Francis M. Taylor and Louis V. Caserta, both of Tulane University, for their valuable assistance. LITERATURE CITED (1) Balke, C. W., IND. ENG.CHEM.,2 7 , 1166-9 (1935). (2) Driggs, F. H., and Lilliendahl, W. C., Ibid., 22, 1302-3 (1930). (3) Ibid., 23, 634-7 (1931). (4) Fink, C. G., and Ma, C. C., Trans. Electrochem. SOC.,84, 33--52 (1943). (5) Ibid., pp. 53-63. (6) Fink, C. G., and Ma, C. C., U. S. Patent 2,463,367 (March 1 , 1949), (7) Ibid., 2,554,527 (May 29. 1951). RECEIVED Deoember 18, 1950.
DEFOAMERS ROY E. MORSE’ AND HENRY V. MOSS Food Technology Laboratory, Monsanto Chemical CQ., Anniston, Ala.
T
HE problem of foam control during the production of bakers’ yeast has been serious since Marquardt, following the teachings of Pasteur, introduced aeration into the process in 1879. The problem waa intensified by the introduction of the Hayduck process using molasses and ammonia as nutrients in 1915. The shortage of grains a t that time accelerated the changeover to the and Frey, Kirby, and Schultz (6) Hayduck process. Frey (4), described these developments in detail. The growth of microorganisms, especially yeast, is inevitably tied to the use of aeration because of fhe larger yields received. The exact mechanism whereby yields are increased has not been fully elucidated, but de Becze and Liebmann ( 1 ) in their all-encompassing review on aeration have listed most of the current theories. Gee and Gerhardt (6) have also attempted to throw light on this mechanism. Two general solutions to the foaming problem have been advanced-the use of mechanica1,foam breakers, such as are employed in the Waldhof fermenter, and the use of additives which destroy or control the foam. Ross (10, I I ) , and Ross and McBain ( l a ) have classified and illustrated the substances in common usage as additive defoamers. Equipment has been designed so that defomer addition is automatic as required. Stefaniak, Gailey, Brown, and Johnson (14) and Bilford, Scalf, Stark, and Kolachov (a)have described such equipment. Generally, additive defoamers rely upon the alteration in surface relationships, and Ross (11)has explained some of the factors involved. In the use of chemical additives as defoamers other important characteristics deserve consideration, I n yeast growth, yields may be seriously impaired, and in antibiotics manufacture, unit yields may be lowered. Goldshmidt and Koffler (7) have made a study of the effect of defoamers upon penicillin yield and have found that with the use of lard oils the yield of penicillin is increased significantly. They explained the enhanced yield as a result of the effect upon surface relationships of the organism itself, and show that it is not due simply to the mechanical destruction of foam, EXPERIMENTAL
A well-known and accepted technique for testing defoamers for use during yeast manufacture is not available. Goldshmidt and 1 Present sddrese, Direator of Research, Kingan and Co., Indianapolis 6, Ind.
Komer ( 7 ) have described a method for testing defoamers for use in penicillin manufacture and Sinsheimer ( I S ) has described a method for testing foam stability. The technique described herein is advanced as one suitable for use in testing yeast defoamers. The apparatus consists essentially of a device for passing cleaned, measured air through a mixture of yeast, molasses, and test defoamer at a controlled temperature; meanwhile, the time for defoamer failure is observed. The apparatus is shown in Figure I. Construction details are shown in Figures 2 and 3.
TESTPROCEDURE. With the foam column in place, the bath is brought to 30”C. Two grams of active dried yeast are added to 200 ml. of test molasses and the mixture is stirred mechanically a t medium speed for 5 minutes. Molasses is prepared by diluting dark table molasses to a s ecific gravity of 1.0390 (2Oo/2O0)after adjustment to pH 4.5. d r p l u s molasses may be preserved by processing in half-pint home canning jars a t 100’ C. for 25 minutes, followed by an air cool. This arrangement permits enough for one test per jar. Air flow is adjusted so that after adding the test mixture, the air rate is 0.125 cubic foot per minute. Air must be started before addition of test mixture. Foam forms immediately and starts to climb in the tower. When tower is half filled, 0.05 ml. of test defoamer is added. Defoamer should be near at hand when starting test, because foam rise in the tower is rapid. The time is observed and failure is considered t o be the point a t which foam fills the tower. The failure time is noted. At the conclusion of an 8-hour test run,. yeast is separated from the s ent molasses. The filtering device shown in Figure 4 was f o u n i t o facilitate this operation. After three washes with cold distilled water, odor and toxicity observations are made. PREPARATION OF FILTERIXG DEVICE.To a high-s eed wetted filter paper under suction on a Buchner funnel, a thicg slurry of highspeed filter aid is added. Sufficient filter aid is used to give a pad about 0.5 inch thick. Suction is continued and when the filter aid pad is formed, but before it is dry, another filter Paper is placed upon it. Water to a depth of about 1 inch is immediately added, which helps weld the “sandwich” into a unit. In his compilation of defoamers Ross (IO,1 1 ) has listed fatty acids as having value in controlling foam during yeast production. Tall oil seemed t o be an unexplored source of fatty acids for such an application. Dunlap, Hassel, and Maxwell ( 3 ) and
