Production and Characteristics of the Carbides of Tungsten

PRODUCTION AND CHARACTERISTICS O F T H E CARBIDES O F TUNGSTEN BY MARY R. ANDREWS

As is well known, the commercial method of exhausting lamps involves the use of rotary oil pumps. The problem has often arisen as to the effect of residual vapors from the oil on the characteristics of the lamps. Under normal operating conditions any such traces of residual gases are “cleaned up” by the phosphorus or other reagent which is volatilized for this purpose when the lamp is flashed.l But, under imperfect manufacturing conditions, it is possible that an amount of hydrocarbon vapor may remain in the lamp which is greater ” than that which can be cleaned up during flashing. It therefore appeared that the results of an investigation of the reaction between hydrocarbon vapors and an incandescent tungsten filament might throw some light upon possible sources of trouble in lamp manufacture. As ordinary oil vapor consists of a mixture of aliphatic hydrocarbons whose composition changes with the history of the oil it was decided to work with a pure hydrocarbon. Since as has been pointed out by Langmuir2 “there are many advantages in studying heterogeneous reactions a t very low pressures” it appeared desirable to work with very low pressures of hydrocarbon vapor. Vapor Pressure of Naphthalene Naphthalene, CIOHs,possesses properties which made it especially suitable for the purposes of the present investigation and it was used chiefly in the york described in this paper. Vapor pressure measurements for this substance have been published for temperatures above its melting point by Nelson S. Dushrnan: “Methods for the Production and Measurement of High Vacua,” Gen. Electric Rev., 24,676 et seq. (1921). Jour. Ind. Eng. Chem., 7, 350 (1915).

Characteristics of the Carbides of Tungsten

271

and Senseman* and for lower temperatures by Barker2and by Allen.3 Data a t still lower temperatures were obtained some time ago in this laboratory by Miss Mary Daly by means of the ionization gage. These latter determinations are here published for the first time. Temp. ~

O C

I

Pressure in mms. Hg.

1

Pressure in bars

~~~

18 14 11 6 5 0.5 0 0 - 2.5 - 5 - 6 - 6.5 - 7 - 9.5 -10.5 - 11

63.5 x 10-3 51.5 x 10-3 3 9 . 4 . ~10-~ 24.3 x 10-3 27.5 x 10-3 15.7 x 10-3 15.8 x 10-3 14.6 x 10-3 11.7 x 10-3 9.7 x 10-3 8.9 x 10-3 8..0 x 10-3 7 . 7 x 10-3 5.7 x 10-3 5.5 x 10-3 4 . 8 x 10-3

84.5 68.5 52.4 32.3 36.6 20.9 21.0 19.4 15.6 12.9 11.8 10.5 10.2 7.59 7.32 6.38

Plotting the logarithms of the pressures against the reciprocals of the absolute temperatures, and giving weight to the values obtained at the extreme temperatures it is found that within the limits of experimental error all the data are best represented by the two straight lines shownin Fig. 1. This corresponds to a vapor-pressure relation of the form

As a matter of fact most vapor pressure data a t low pressures are found to be very satisfactorily represented by an equation of this form.

a

Jour. Ind. Eng. Chem., 14, 58 (1922). Zeit. phys. Chem., 71, 235 (1910). Jour. Chem. SOC., 77, 410 (1900). Dushman and Found: Phys. Rev., [2] 17,7 (1921).

'

Mary R. Andrews

272

By means of this plot (extrapolated for low temperatures) the following vapor-tension data are obtained for the range of temperatures used in the present investigation. ZOO0

10

10

2.0

.Of Fig. 1 Temp.

O

0

- 10 -20 - 30 -40

-30

C

I

Pressure in mrn Hg.

13 x 10-3 5 . 0 x 10-3 1.55 x 10-3 .ax 10-3 .12 x 10-3 .03 x 10-3

I

Pressure in bars

20 7.3 2.4 0.77 0.23 0.06


273

Apparatus and Method Most of the experiments were carried out in an apparatus of the type shown in Fig. 2. The system could be exhausted to less than half a bar by a Langmuir condensation pump backed up by suitable mechanical pumps. During exhaustion and baking out of the lamp (A) the naphthalene reservoir (B)

Fig. 2 A-Lamp containing tungsten filament. B-Reservoir of hydrocarbon. C-Liquid air trap. D-Mercury pump. E-Bulb for collecting hydrogen. F-Mercury seal.

,

was usually warmed to eliminate moisture and adsorbed gases. This was followed by aging of the filament at about 2400 O K to constant resistance, meanwhile keeping the naphthalene trap in liquid air. Resistance measurements at room temperature were made in the usual way with a Wheatstone bridge and galvanometer, care being taken that the current passing through the filament was too small to heat it appreciably.. With the four mil filaments ordinarily used in these experiments this meant less than five milliamperes. The vapor pressure a t which the reaction was to be investigated was obtained by submerging the naphthalene reservoir (B) in acetone kept a t the desired temperature by adding liquid air. The residual naphthalene vapor after passing through the lamp (A) was condensed in the liquid air trap (C).

M a r y R. Aizdrews

274

The filament was lighted for a measured time and the hydrogen evolved was determined by closing the trap (E) and observing the rise in pressure in the bulb (D) by a McLeod gage. Filament temperatures were estimated by color matching with a kungsten lamp standardized in this laboratory. Reaetion between Tungsten and Naphthalene A t the surface of an incandescent tungsten filament a t a temperature of 1600' K or higher naphthalene is decomposed into hydrogen and carbon, the latter forming carbide as it diffuses into the metal. Chemical analysis of carbonized filaments and determinations of the hydrogen evolved during carbonization showed that the decomposition of naphthalene into carbon and hydrogen was quantitative, the carbon being entirely taken up by the tungsten. Conductance a t room temperature was measured a t intervals during carbonization. The following are two typical determinations. Filament A Conductance* percent

% C calculated from Hz

74.5 35.3 7.1 8.1 17.6 20.9

0.806 2.05 3.12 3.27 4.28 4.5

evolved

yo C found by analysis

-

-

4.0

Filament B Conductance* percent

% C calculated from Hn

68. 31. 7.6 34.8

1.26 2.14 3.11 5.2

* Reported filament.

evolved

analysis

5.9

as percent of the conductance or the original uncarbonized

Characteristics of the Carbides of Tangsten

275

Since both hydrogen and conductance measurements are subject to appreciable error and since analytical determinations on such extremely small samples are very difficult the agreement between calculated and determined values for carbon are as good as could be expected. Figure 3 shows the relation between conductance and the percent of carbon in the filament. It will be observed that the conductance decreases linearly with increased carbon content until a minimum of about six percent of the original conductance is reached a t a composition of 3.16 percent carbon. As this corresponds exactly to the formula of W2Citis

Fig. 3

Change of conductance with carbon content of carbonized tungsten

reasonable t o conclude that at this point the filament is completely converted into a carbide of the above composition. As the carbon content is increased beyond this minimum theconductance increases linearly again until a composition of 6.12 percent carbon is obtained, corresponding to the complete conversion of the filament into the carbide WC. Further heating in hydrocarbon vapor results in the slow deposition of a shell of carbon on the filament, the conductance being affected only inappreciably. As a check on this curve which was derived as an average of several determinations such as are shown above, two fila-

Mary R. Andrews

276

Filament 11 Filament 12

Carbon by conductance

C by analysis

3.6 6.2

3.9 6.2

in their experiments with carbon and tungsten heated in the electric furnace or the arc thought they identified such a compound, as well as WC and probably W2C. Moissan? in his early work with the electric furnace had prepared W2C, and Williams3 prepared WC and from it W2C in the same way, but neither of these authors found the tri-tungsten carbide. Microscopic Structure. Mechanics of Carbonization Cross-sections of filaments partially carbonized to W2C show, under the microscope, the compound as a ring surrounding a core of pure tungsten. The outer ring was brittle and would not take a polish ; the core could be given a mirror surface. In carbonizing, carbon must penetrate the layer of carbide already formed and react with the tungsten a t the outer surface of the core. It is the rate of diffusion of carbon through this ring of carbide which limits the rate of carbonization unless a high filament temperature and a very low pressure of hydrocarbon vapor are maintained, in which case the speed of carbonization may be limited by the rate of decomposition of the hydrocarbon a t the surface of the filament. For example, with a four mil filament running a t 2300" K in a pressure of naphthalene greater than three bars, carbonization is limited by the rate of diffusion. This is apparent if conductance is a

Zeit. anorg. Chem., 85, 292 (1914). Comptes rendus, 123, 15 (1896). Ibid., 126, 1722 (1898).


277

plotted against the time 'during which the filament has been lighted (Curve A-Fig. 4). I n this case as the shell of tungsten carbide, W2C, becomes thicker the rate of penetration of carbon to the tungsten core becomes less and the rate of decrease in conductance becomes less and less, though the pressure of hydrocarbon vapor remains constant. If the temperature of the filament be increased, diffusion of carbon will occur much more rapidly and carbonization will proceed a t a constant rate (Curve B-Rig. 4), if the temperature be high enough, being limited in this case only by the rate of decomposition of hydrocarbon at the surface of the filament. It was found that decornFig. 4 position of hydrocarbons (at lene (ClaHa),4 mil. tungsten filaments. A-

thalene and toluene) pro- at 23000 K, Bpat 25000 K ceeded much more rapidly while the filament was being carbonized from pure tungsten t o W2C than while it was being converted from W2Cto WC. Apparently decomposition is more rapid a t a surface of W2Cthan a t one of WC or carbon. To test this idea we lighted a pure carbon filament in naphthalene and found the reaction (measured in this case by the hydrogen evolved) to be very slow and of about the same order of magnitude as when a filament corresponding to the composition of WC is lighted under the same conditions. It may be that decomposition on a surface of WC is as rapid as on one of W2C, but that diffusion of carbon through the former compound is slower than through the latter at the temperature used so that a carbon coating is formed on the filament which cuts down the rate of decomposition to that on carbon. This rate being very low, the carbon coating would

Mary R. Aiidrews

278

tend to thicken up very slowly, probably more slowly than carbon would diffuse into the filament. The coating would therefore remain of negligible thickness and the effective rate of decomposition would be the rate at which carbon is absorbed by the filament. This idea is borne out by the decreasing rate of reaction as a filament is carbonized (at constant temperature and pressure) from W2C to WC. (See the second arm of Curve B-Fig. 4.) In carbonizing with acetylene the gas was allowed to flow from a large reservoir through the lamp to the liquid air trap. The rate of flow ,and the pressure in the lamp were controlled by the length and size of tubing between reservoir and lamp, and between lamp and trap. We found that at a given initial pressure in the lamp the rate of carbonization varied directly as the rate of flow of the gas. In other words, all of the acetylene reaching the lamp was decomposed. In our work with naphthalene, using the type of bulb shown in Pig. 1, carbonization progressed with a speed of only three or four percent of the rate to be expected from the calculation of the amount of gas striking the filament per second. We assumed a t first that only this proportion of the molecules which struck the filament reacted; but when in later experiments we used a lamp in which the naphthalene reservoir was larger and affording a greater area of the solid for evaporation, the rate of carbonization increased to three-tenths of the rate calculated from the vapor pressure of the naphthalene. Possibly the filament decomposed the gas faster than it evaporated from the solid, so that, during any time the filament was lighted, the pressure of hydrocarbon in the lamp was considerably below the normal vapor-pressure corresponding to the temperature of the reservoir. This point of view is indirectly supported by data given by Barker in his determination of the vapor pressure of naphthalene. He found great difficulty in obtaining saturation of the gas which he caused to flow over the naphthalene even a t a very slow rate. He suggested that the surface of the naphthalene may become covered with a layer of the gas LOC.cit.


270

or an impurity. Langmuirl has found that reactions between tungsten or molybdenum vapors and the simple gases take place, if they occur at all, at every collision. But .in the case of solids reacting with vapors, as when tungsten is attacked by oxygen or by a mixture of oxygen and carbon monoxide, he fin& the conditions much more complex.2 I n these cases only a fraction of the molecules of oxygen which strike the filament reacts with it. This fraction he calls “E.” Apparently monomolecular layers of gas adsorbed on the surface of the filament play an important part in the value of “E” for many such reactions. In the case of naphthalene this may also be the case as it is quite conceivable that there may be a monomolecular layer of carbon atoms adsorbed on the surface. We have, moreover, in the case of naphthalene a large molecule which in decomposing to carbon and hydrogen must deposit ten carbon atoms on the filament from the two rings of its structure-a complicated condition. While we have no proof as yet that every molecule of naphthalene that strikes the tungsten of W2Csurface is not decomposed, it seems probable from data given in the earlier part of this article that not all molecules striking a carbon surface (possibly also a surface of WC) react, since the rate of carbonization of a W2Cfilament is less than that for a less carbonized one. There must be considered, also the large volumes of hydrogen formed a t the surface of the filament, through which fresh naphthalene must pass to reach the metal. Further work is being undertaken on this phase of the subject, particularly with a view to determining the value of “E.” A few experiments were made, using pure ethyl alcohol as the carbonizing agent. With this gas flowing from a reservoir, carbonization was one to two-tenths as rapid as with acetylene a t the same pressure and rate of flow. This would indicate that not every molecule striking the filament was successfully “Chemical Reactions at Low Pressures,” Jour. Am. Chem. SOC., 41, 167 (1919). 2 Ibid., 35, 105 (1913); 37, 1139 (1915).

M a r y R. Andrews

280

decomposed. The presence of the oxygen atom probably accounts for this slowing down of the carbonization of alcohol. .

Physical Characteristics: Conductance and Temperature Coefficient Both carbides are brittle. Due probably to the difference in the coefficient of expansion between tungsten and W2C highly carbonized filaments invariably show radial cracks. While the cold resistivity of W2C is about fifteen times that of tungsten the temperature coefficient of resistance is very much less. The following table gives the resistivity and the ratio of hot to cold resistivity (RJR,) for W2C for various temperatures, and for comparison the ratio for pure tungsten at the same temperatures. These figures, however, are only approximate as they are based on measurements of a single filament of W2C which may not have been entirely homogeneous. Color temperature

298' 1655' 1800' 1930' 2060' 2215'

K K K K K K

Resistivity of wzc

x x x x x

.8i 1.05 1.10 x 1.15 1.20 1-25

10-4 10-4 10-4 10-4 10-4 10-4

RH/% for W

-

-

1.26 1.33 1.40 1.46 1.51

8.4i 9.43 10.30 11.1i 12.30

Figure 5 shows temperature-conductance curves for filaments of pure tungsten of W2Cand of filaments partially carbonized to WiC. A partially carbonized filament shows a t any temperature a conductance which is in agreement with the conductance calculated from the amount of tungsten and of W2C present. The coefficient of resistance, therefore, of any partially carbonized filament is calculable from its composition and the temperature coefficients of the two components. We are


28 1

indebted to Dr. W. E. Forsythe, of the Research Laboratory at Nela Park, for the pyrometric data on which these curves and the preceding table are based. He finds also that the

COLOR TEMP - 'K I800

16W

I

rm

2000

emissivity of W2C is practically the same as of pure tungsten. Measurements made by Mr. G. M. J. Mackay, of this laboratory, confirm this observation.

Carbonizing Agents Among the hydrocarbons other than naphthalene, which have been used by the author or by associates in this laboratory are benzene, toluene, anthracene, acetylene, methane and 1 In using toluene we found i t necessary t o determine its vapor pressure. This was done with the ionization gage. The points determined were:

Temp. "C

-90 -80

-70

The freezing point is roughly -90" C.

Pressure

3 bars 31 ,, 49

a,

282

Mary R.Andrews

illuminating gas. Undoubtedly almost any hydrocarbon vapor will react with incandescent tungsten to form tungsten carbides.

Decarbonization Carbonized filaments heated in vacuum decompose a t rates depending on the temperature. Below 2400" K the rate of decomposition is extremely low, but as the temperature is raised the rate is increased. Carbon is volatilized without loss of tungsten. It is possible to drive out the carbon completely and obtain a tungsten filament of the same conductance as before carbonization. In 8 hours at about 2700" K a 4 mil filament which had been carbonized to W2C was entirely decarbonized, and the conductance increased to the same as that of the original tungsten filament. The magnified crosssection of a filament which had been carbonized almost entirely to W2C and then heated in vacuum until the cold conductance has risen to about fifty percent of the original tungsten showed a ring structure. The outer ring consists of pure tungsten, the core is undecomposed carbide. Carbon can also be taken out very quickly by flashing the filament in a considerable pressure of hydrogen. In conclusion, I wish to express my sincere thanks to Dr. Saul Dushman for his enthusiastic interest and help in the work presented here, to Dr. Irving Langmuir for his valuable suggestions and criticisms, and to Mr. C. G. VanBrunt for the careful analytical work done in determining carbon in the minute samples available.

Summary The reaction between incandescent tungsten and naphthalene vapor has been studied. The existence of two compounds W2Cand WC is shown by points of inflection of the resistaficecomposition curve. The mechanics of the reaction are discussed, Tungsten can also be carbonized by coating the cold filament with carbonaceous material such as dextrine, lamp-black, glycerine, graphite etc., and then heating it in vacuum t o incandescence.


283

the factors being rate of diffusion of carbon through tungsten carbide as a function of temperature, the vapor pressure and the value of “E.” Approximate resistivities of W2C a t different temperatures are given and the conductance of a partially carbonized tungsten filament is shown to be the sum of the conductances of its two components. Various carbonizing agents are mentioned. A complete vapor-pressure curve for to +200° is shown, and a few valnaphthalene from -500’ ues for the vapor-pressure of toluol a t low temperatures given. Decarbonization is discussed briefly. Research Laboratory General Eleclric Co. Scheneclady, N . Y .

‘

Production and Characteristics of the Carbides of Tungsten

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