PARTICLE AGGLOMERATION I N TUNGSTEN POWDER
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(3) GREENHILL, E . B . : Trans. Faraday SOC. 46, 625 (1949). NORMAN, A N D SCHMIDT, H . R . : J . Phys. & Colloid Chem. 65, 629 (1949). (4) HACKERMAN, (5) M A N X C , . A., LAWER, B. P . , AND HULTIN,C. T . : Ind. Eng. Chem. 28, 159 (1936). (6) RALSTON, A. w., AND HOERB,C. w.:J. Org. Chem. 10, 170 (1945). (7) RHODES, F. H . , A N D KUHN,W. E.: Ind. Eng. Chem. 21,1066 (1929). (8) SMITH,H.A , , A N D FWZEK, J. F . : J . Am. Chem. SOC.88, 229 (1946). (9) STEPANEKO, S . , AGRANAT, v., ASD Nov1Kov.4, T.:Acta Physicochim. U.R.S.S. 20, 923 (1943);Chem. Abstracts 41, 7178d. (10) TINGLE,E.D . : Nature 160, 710 (1947). (11) ZISMAN,W.A . : J . Chem. Phys. B, 534,729, 789 (1941).
PARTICLE AGGLOMERATIOS IK TUKGSTEK METAL POWDER B E R S A R D KOPELXIAS
~ N D C.
C. GREGG
Sblvania Elecirzc Products, Inc., Metallurgical Laboralorzes, Bayside, New York Received April 87, 1950 INTRODUCTION
Of the several methods available for particle-size distribution or analysis, such as sedimentation, turbidimetry, gas or liquid elutriation, none will yield an analysis in agreement with the microscopic examination of those metal powders which are prepared by the reduction of their oxides at elevated temperatures. Microscopic examination of such powders, which have been carefully dispersed in a liquid by spatulation of the slurry on a slide, invariably reveals agglomerates as well as single particles. The agglomerates are much more numerous in the fines, say, those particles having diameters of 1-3 microns. These agglomerates may be due to two factors: ( I ) sintering of the fine particles at the elevated temperatures of reduction, resulting in an agglomerate whose intraparticle strength approaches that of a grain boundary in a metal; ( 2 ) flocculation due to electrical forces, the degree of flocculation increasing with a decrease in particle size. In many applications of such powders it is desirable to know whether theEe agglomerates are mere flocculates or clusters of strongly sintered particles, since the latter would behave as single particles while the former would not. It is the primary purpose of this paper to show that aggregates seen in the microscope, after the tungsten powder has been carefully dispersed with a spatula, are clusters of sintered particles. These clusters may be assumed equivalent to single particles of the same diameter. THEORY
I t has been known for a long time that hydrophilic solids will be deflocculated in liquids of high dielectric constant (2) and in liquids whose interfacial tension against water is small (1). Flocculation will then occur in liquids of low dielectric constant and high interfacial tension. To date, all these observations have
558
BERNARD KOPELMAN AND C. C. GREW
been based upon the determination of the settling volume. A more rigorous way of observing flocculation and deflocculation is by measuring the apparent particle size and the distribution of the powder while it is suspended in a medium. An increase in the apparent particle size in a second liquid would certainly indicate that agglomeration is more evident in this liquid. One would begin to believe that flocculation is rather slight when the particle size remains unchanged as the dielectric constant of the suspending medium is increased. Furthermore, one could conclude that deflocculation was essentially complete when, in addition to the above, the particle size and distribution, measured while the solid is suspended in the liquid, are in agreement with the average particle size as measured by an air permeability method (sub-sieve sizer’). In this latter method, yielding the average particle size only, the total surface is determined by measuring the resistance of a packed bed of the particles to a grts flow. The particle size may be calculated from the total surface area by assuming that the particles are smooth spheres. Offhand, one would not expect good “wettability” of tungsten powder by water. The surface tension of tungsten has been estimated to be of the order of 2500 dynes/cm. (3). Therefore, it is reasonable to expect a very high interfacial tension. Further, in an earlier paper (3) it wrts shown that tungsten and other refractory metal powders were wetted poorly by mixtures of water and organic liquids. The same behavior was exhibited by the lower oxides of tungsten. We can deduce therefore that tungsten powder, when, dispersed in water alone, will assume a skin of tungstic oxide which, according to this same paper, displays hydrophilic properties. I n the experimental work to be described, no surface-active agents were employed. In some preliminary tests, utilizing small percentages of several dispersants, anionic, cationic, and nonionic, no difference in the particle size as compared t o pure water could be detected. The Palo-Travis equipment, which is essentially a long tube calibrated to measure the rate of sedimentation, was used in the latter tests. This, of course, does not constitute proof of the inability of surface-active agents to effect the dispersion of tungsten powder. Perhaps other agents or different percentages of those used would have been more successful
.
EXPERIMENTAL METHOD
The apparent particle size of tungsten metal powder, prepared by the reduction of tungsten oxide with hydrogen at about 850”C., was determined in eleven liquids of different dielectric constant. The technique is described as follows: Particle-size data were obtained by examination of the powder suspensions with the photelometer. The method is based upon observation of the varying light transmission at a iixed level below the surface of a suspension of powder in a liquid, while the powder is allowed 1
Fisher Scientific Company, Pittsburgh, Pennsylvania.
PARTICLE AGGLOMERATION IN TUNGSTEN POWDER
559
to settle. It is assumed that the sedimentation proceeds according to Stokes’ law. The fundamental transmission equation is (4) : s d
=
C(l0g 1 0
- log I d )
(1)
where 8 d = surface area (cm.*) of all particles of diameter dp have settled past the reference level. From equation 1, using a derivation similar to that of Wagner (5), the weight w of a fraction containing particles d, to dl in diameter (d2 > d,) may be calculated as follows:
-
W Kd,(log I d l log I d t ) (2) K = constant, d, = average particle diameter of the fraction assumed to equal the arithmetical average (dl d2)/2, and and I d z = intensities of transmitted light when particles of diameter greater than dl and d? have passed the reference level.
where
+
Idl
The weight per cent, P , of each fraction may then be expressed as:
The average particle diameter of the sample, d., is calculated as follows: d a = Z -d 2 100 I n practice, all determinations were made using a standard volume of 12.5 ml. of the powder suspension and a corresponding height of fall of 18.7mm. from the surface of the suspension to the reference level. The Stokes’ law calculations were made using density and viscosity data for the liquids taken from the literature. However, in the case of nitrobenzene, the liquid density was determined experimentally. Tungsten powder samples of 50 mg. were used in preparing all suspensions. The dispersion technique consisted in spatulation of the powder sample for 3 min. with a stainless-steel spatula on a flat glass plate. The sample waa mbintained in the form of a thin paste by an initial addition of 1-2 drops of liquid to the powder and subsequent additions during the preparation. Spatulation was effected by working the paste with the flat edge of the spatula in a circular movement and with considerable pressure. The sample was then washed carefully into a beaker, and sufficient liquid added to yield a suspension with an initial light intensity of 30-40 on the photelometer scale. (The photelometer scale had been calibrated a t 100 with the
560
BERSARD KOPELMAN AND C. C. GREGG
liquid medium.) The sample was then shaken end-over-end for 1 min. to insure uniformity and a 12.5-ml. portion was immediately removed and transferred to an absorption cell. The cell was transferred to the photelometer carriage after shaking to insure uniformity of the suspension, moved immediately into the path of the light beam, and readings of transmitted light intensity recorded according to a predetermined time schedule. All determinations were made a t an ambient temperature of about 25OC. No attempt \vas made to maintain a constant temperature. The average diameter of the tungsten powder used in this investigation was also determined by an air permeability method, using a Fisher sub-sieve sizer, and was found to be 4.2 microns. TABLE 1 Particle diameter of tungsten powder
1
UQUW
Benzene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hexane. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbon tetrachloride.. . . . . . . . . . . . . Diethyl ether.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acetic a c i d . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Butyl alcohol .................... Ethyl alcohol.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acetone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methyl alcohol. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nitrobenzene. . .................... Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
,I
PAXTICLE DIAYETEP
D I E ~ ~ P CIOCN S T A ~
OF TUNGSTEN
2.29 1.8i 2.24 4.33 9.7 17.8 25.8 26.6 31.2 35.1 81.07
I 1 ~
I i ~
mwmx
101159 9.24 7.87 7.31 5.64 5.02 4.44 4.30 4.21 5.70 4.42
DISCUSSION OF RESULTS
The results are given in table 1 and plotted in figure 1. From the plot it may be seen that the data fall on a smooth curve, with apparent particle size increasing with decrease in dielectric constant.* It is seen that the curve in figure 1 flattens out at about ethyl alcohol. This means that complete deflocculation has occurred in this liquid, and that no further dispersion of the powder will occur in liquids of higher dielectric constant. A log-log plot (figure 2) of average particle diameter versus dielectric constant The deviation of the value in nitiobensene is not totally unexpected. Dispersion of hydrophilic solids in liquids of high dielectric constant seems t o occur primarily in those liquids which are associated or polymerized in long chains. This points t o the adsorption of hydroxyl groups. The large values of the dielectric constant of methyl alcohol and water are due t o the high degree of association of these liquids. The large value of the dielectric constant of nitrobenzene is due not t o association but rather t o the acceptance of two electrons by the nitro group from the benzene ring, yielding a structure of high dipole moment. Thus, nitrobenzene shows a high dielectric constant as well as high interfacial tension against water; this is contrary t o most liquids of high dielectric constant.
PARTICLE .4GGLOMERATION I N TUNGSTEN POWDER x
561
BENZENE
a lo-
z
FIG 1. Turbidimetric analysis of particle size of tungsten powder in varioua liquids
I
3 K
c”
IO-
x
09-
I
I
a
r u a
L
LOG
DIELECTRIC CONSTANT
FIG.2. Relationship between apparent particle size of tungsten powder and the liquid in which the measurements were taken.
shows a straight-line relationship over a wide range of dielectric constant. The equation describing this portion of the plot is found to be
where p is the observed average particle size in microns, D is the dielectric constant of the liquid, and K is a constant, in this case equal to 10. The con-
562
BERNARD KOPELMAN AND C. C. GREGG
stant K is undoubtedly temperature dependent, and experimental determination of K &s a function of temperature (say at 30" and 35°C.) would allow estimation of the heat of flocculation between particles by integrating the well-known equation: dlnK dT -
3
-
AH RT2
As pointed out previously, above a dielectric conEtant of about 25 the apparent particle size of the powder does not seem to change, implying complete dispersion in this liquid. This limiting value of particle size, about 4.30 f 0.10 microns is in excellent agreement with the average particle size of the powder as determined by the sub-sieve sizer, given earlier as 4.2 microns.
I-
P I-
00
-
bo-
$
H
40
-
20
-
Y
? 4 2
a a
2
4
6
8 IO I2 PARTICLE DIAMETER (Mera:)
14
16
FIQ.3. Particle-size distribution of tungsten metal powder in v:rrious liquids
In the log-log plot there is a rather abrupt break in the straight-line relationship at a dielectric constant value of about 2.25. Data at this end of the dielectric range are difficult to obtain photelometrically because of the very rapid fall of such large agglomerates. Thus the data for the particle diameters in benzene and hexane are in doubt. If the straight-line relationship is extrapolated to zero of the log dielectric constant scale ((vacuum (or air) with a dielectric constant of (about) l ) ) ,the average particle diameter would be about 10 microns. Assuming some simple type of packing of spheres of diameter 4.3 microns, this indicates that on the average the size of agglomerates which can exist contains some twenty or so particles. (It is realized that the tendency for agglomeration is greater the smaller the particles, and thus the finer particles in the distribution range can agglomerate with many more than twenty particles.) Figure 3 shows the particle-size distribution obtained in several of these liquids. The ordinates have the usual meaning for this type of plot. Thus, at any point on a curve, the value of the accumulated per cent on the ordinate
DEVELOPER ACTIVITY OF HYDROXYBENZOIC ACIDS
563
is the total percentage of all particles below the particle size corresponding t o the value on the abscissa. The increase in flocculation with decrease in polarity of the liquid is evident from this figure. SUMMARY
Analysis of the particle diameter of tungsten metal powder in liquids of various dielectric constants reveals: (1) that over a wide range of dielectric constant, the apparent particle size is inversely proportional to the fourth root of the dielectric constant of the medium; (2) that the surface bonds in the metal powder due to electrical attraction are completely broken in associated liquids whose dielectric constant is over 25; (3) that essentially complete agglomeration of the powder occurs in liquids of dielectric constant less than about 2; and (4) that the metal powder with an average particle size of 4.2 microns would be expected to behave in vacuum or air as a powder with an average particle diameter of about 10 microns. The photelometer has been shown to be a useful tool for the analysis of particle size and distribution and can aid in measuring the degree of flocculation present in a suspension. REFERENCES (1) DONALD, M. B.:Chem. Ind. 69, 1069 (1940). (2) HARKINS,W. D . , A N D GANS,D . M.: J. Phys. Chem. S6, 56-97 (1932). (3) KOPELMAS, B.,A S D GREGG,C. C.: Trans. Am. SOC.Metals 41.293 (1949). (4) STATES, 51. N . : Am. SOC. Testing Materials, Proc. S9, 795 (1939). (5) WAGNER, L. A.: Am. SOC.Testing ,Materials, Proc. 93, 11, 553 (1933).
DEVELOPER ACTIVITY OF DI- AND TRIHYDROXYBENZOIC ACIDS' T. H. JAMES Kodak Research Laboratories, Roehesler, New York Received May 8 , 1960
According to the literature (7), gallic acid and protocatechuic acid will not develop a silver bromide emulsion, although their esters and acid amides are developing agents of normal activity. Development by gentisic acid and 2,3dihydroxybeneoic acid has been observed only under extreme conditions (I), although the corresponding esters and acid amides are reasonably active developing agents. It has been suggested (1) that the presence of a carboxyl group in the ring 1
Communication No.1247 from the Kodak Research Laboratories
