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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 23, No. 11
JACOB F. SCHOELLKOPF MEDAL A W A R D For achievement declared to constitute a major advance in engineer, and the successful business administrator. The value science and t o embody the spirit of research in industry Frank of his inventions is attested by the remarkable commercial J. Tone, president of The Carborundum Company, Pu’iagara development which has been built upon them. Falls, N. Y., was chosen as the first recipient of the Jacob F. The idea of founding the medal originated with Robert J. Schoellkopf Medal of the Western New York Section of the Moore in 1929, when he was vice chairman of the Western New AMERICAN CHEMICAL SOCIETY.The medal was formally pre- York Section of the SOCIETY Mr. Moore, now associated sented on September 2 , 1931, a t the public meeting in connection with the General Bakelite Corporation, Bloomfield, N. J., enwith the eighty-second convention of the AMERICAN CHEMICAL listed the interest of Jacob F. Schoellkopf, who agreed to create SOCIETY in Buffalo N. Y., by Moses Gomberg, of the Cniversity a trust fund for the purpose. The Jacob F. Schoellkopf medal is of Michigan, President of the SOCIETY. to be awarded annually by the Western New York Section Outstanding contributions of Mr. Tone (as cited by the jury for outstanding work in industrial chemistry. The face of the of award) include his work, with the late F. A . J. Fitzgerald, on medal carries a likeness of J . F. Schoellkopf against a background the production and commercial properties of silicon carbide, the of Niagara Falls, with whose power development Mr. Schoellkopf production of pure metallic silicon, and the industrial applica- has long been intimately associated. The reverse of the medal tion of electrochemistry. Nearly one hundred patents have been bears a wreath and the inscription: “Awarded by Western granted to Mr. Tone, who possesses t o an unusual degree the New York Section, American Chemical Society, t o Frank J. rare combination of the qualities of the pure scientist, the plant Tone.”
High-Temperature Products of Silicon Frank J. Tone THECARBORUNDUM
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COMPANY,
N 1891 when Acheson, with his plumber’s-pot furnace, produced the first silicon carbide crystals on the end of a carbon rod by the aid of a few amperes of electric current, he opened the door to vistas wider than he knew. This experimental furnace is popularly called the cradle of the artificial abrasive industry, but it was more than that. To it as a beginning may be traced the numerous progeny of which artificial graphite and siloxicon are the oldest children, followed later by recrystallized silicon carbide, silicon metal, silicon monoxide, siliconized silicon carbide, fibrous silicon oxycarbide, fused mullite, and other silicon compounds and allied products. ’ In reviewing the history of this large family of high-temperature products of silicon, what jumps to view above all else is Mother Nature’s readiness to disclose her secrets to the scientific worker if only he will play the role of apt pupil. The discoveries of the reaction products of silicon and carbon a t high temperature almost always were due to some furnace disorder, some accident or failure of operating control. Take, for instance, the first child of the silicon carbide furnace-graphite. Toward the end of the silicon carbide furnace run, portions of the product frequently became overheated. Down in the high-temperature zone next to the core, silicon carbide dissociated, silicon was vaporized, and the carbon remained as graphite. Once a piece of one of the amorphous carbon electrodes broke away from the terminal of the furnace, fell into the core, and was converted to graphite. In these two happenings Nature disclosed to the alert mind of Acheson the way to produce artificial graphite and graphitized electrodes. They have now become the basis of a great industry. It soon came about that, when something went wrong with the furnace and it showed symptoms of indigestion, we were a t once on the lookout for a new product. We came to welcome some of these manifestations of the ‘‘cussedness of inanimate objects” and to console ourselves after the fashion of Browning’s Rabbi Ben Ezra, reflecting with him:
NIACARA FALLS,N. Y .
Then welcome each rebuff That turns earth’s smoothness rough; Each sting that bids nor sit nor stand but go. Be our joys three parts pain; Strive, and hold cheap the strain.
In the following remarks I shall describe some of the vagaries of the silicon carbide furnace which proved such a stimulus in our research. Silicon Metal
A good example is the genesis of massive silicon metal. The normal working of the silicon carbide furnace is a quiet orderly affair, but it was not invariably so. Occasionally the core became displaced, owing to the uneven settling of the charge, and this caused high-resistance regions or hot spots to develop, giving rise to excessive temperatures. The current by-passed the conducting core and found a new path through the floor of the furnace. When the furnace was unloaded, it showed a very poor output of silicon carbide but a high output of fused bricks and moIten pavement, interspersed with large nodules and masses of a dark metallic product of bluish silver luster, which was soon identified as elemental crystalline silicon, now commercially called silicon metal. It was one of my first interests to devise a new furnace structure to keep the current in the straight and narrow path, to follow up the new product and determine how it was formed, and to develop a cheap commercial process for producing silicon metal as a main product. Up t o this time silicon was available only in the form of powder or small crystals and, a t a price of $100.00 an ounce ( 7 ) ,it found little use outside the laboratory. As stated in a paper delivered before this SOCIETY by F. S. Hyde in 1899, the preferred method of making silicon was to reduce silica with magnesium powder, fuse the reduction product with cryolite and aluminum to form an alloy of aluminum and silicon, and then treat with hydrochloric acid, leaving silicon in finely divided bright spangles.
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I N D USTRIBI, A N D ENCINEEltlNG C l B M I S T RY
Soveniber, 1931
De Chalntot, working from tlie ferrosilicon end, had produccd an alloy of FeSi2 and Si containing 69 per cent total silicon equivalent to 38 per cent free silicon. This he treated with sulfuric and hydrofluoric acids and obtained the free silicon in powdered form, but a t a cost which precluded any broad commercial use (8). The presence of elemental silicon as a chance product in the silicon carbide furnace made it clear that silicon was probably a product oithereaction,
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come nfincreasingimportance. Among tire most important of ttiesearcthealuminum-silicon alloyscoirtaining8 to Sjpercent silicon. Silicon does not combine with aluminum as it does with most common metals, and under normal crystallizing conditions it tends to produce a comparatively coarse structure having no great mechanical strength. With the remarkable discovery that the crystalline character and distribution of the silicon could be entirely altered by the addition of a minute amount of sodium (4).the alloys have assumed great commercial imSi02 2C = Si 2CO or Sios 2SiC = 3Si 2CO portance. I n their production, a high-purity silicon comparatively free frmn iron is essential, and this requirement makes necessary the use of high-purity raw materials or purification of the silicon with acids. The hardening effect of silicon 011 copper, and particularly on the copper a l l o y s c o n t a i n i n g a s m a l l amount of nickel, also gives promise of further conlmercial development (Sj. Silicon combines with both copper and nickel to form silicides, and, owing t o tlie fact that the alloysrespond to heat treatment, the hardening constituent can be dispersed throughout the alloy in a finely divided condition. Another important use tias been for the generation of hydrogen by means of caustic soda for the inflation of d i r i g i b l e s and balloons. Portable field generating plants are used and provide the most convenient and simple means of supplyratory furiiaco the continuous proF. J. Tone ing hydrogen in large quantities ductiou of silicon metal is hardly -. wit.h a minimum transportation of nossiblc. I n a 500-11. D. furnace raw materials. Thousands of tons ;lost of the difficulties disappear, although it is necessary to work with a charge of silicon carbide were so used during the World War. Other less important uses have been in cast.ingsfor chemical and silica. When we increased the size of the furnace to 1200 h. p. and later to 4000 h. p., the difficultiesdisappeared, and it ware, electrical resistors, and rectifiers. was possible to work with a straight charge of silica rock and Recrystallized Silicon Carbide coke. So, in some fields at least, tho laboratory worker is at a disAnother early discovery was recrystallized silicon carbide. advantage in making new discoveries. In Acheson’s original This was made by thelate Francis A. J. Fitsgerald, and I wish laboratory furnace he got minute crystals of silicon carbide and here to pay tribute to an old and valued associate, who connothing else. Tlicre was probably not a visible trace of graph- tributed so substantially to Acheson’s early work on silicon ite or silicon monoxide or other compound. T o bring these into carbide and graphite, and who was always a source of inspirabeing required the larger operation where there was a freer in- tion to ine and to all others who worked with him. Fitzterplay over wider areas of differences in temperature, in pres- gerald was a true scientist. He wanted facts andall thefacts. sure, in atmosphere, and in solid, liquid, and gaseous phases. IIe infused into all his work the simplicity, truthfulness, and Ce1t.ic charm so characteristic of the man. No other electroSilicon Metal in Industry chemist, professionally and personally, was more highly reOne nf the principal uses of silicon metal has Gcrn in the garded and widely respected. The discovery of recrystallized silicon carbide came about manufacttire of non-aging sheet steel for electric transformers. This steel requires a content of about 4 per cent silicon, and to through t,rouble we had in maintaining our furnace terminals. secure this by the addition of the ordinary grade of ferro- Fire brick surrounding and supporting the terminals had a silicon tended to chill the steel in the ladle, a difficulty which way of melting down and disappearing. Fitzgerald tried the silicon metal entirely overcame. The reduction of brick made from silicon carbide grain with a binder, and they liysteresis losses and the increased life of the electrical ap- werc successful, hut he also found that after a few runs the paratus by use of non-aging steel represents a saving of bonded silicon carbide brick had chauged to something elsethat they had iii fact recrystallized-and he discovered that, rnajor importance in the electrical industry. As a reducing a.gent, silicon plays an important role in the by molding to shape a mass of silicon carbide crystals with a Ueckett process for ihe production of low-carbon ferro alloys temporary binder and refiring to a high temperature in the electric furnace, the mass was recrystallized into a strong dense of vanadium and molybdenum. The uses of silicon have also been extended to the field of self-bonded product exactly retaining ita original form. It nonferrous metallurg)., and during the past few years the has the same physical and chemical properties as silicon nonferrous alloys containing silicon as a component have be- earbide and is highly refractory.
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INDUSTRIAL AND ENGINEERING‘ CHEMISTRY Silicon Carbide Heating Elements
Vol. 23, No. 11
fibrous feltlike material of greenish or gray-white color having a soft, pliant, resilient structure. Its apparent density varies from 0.005 to 0.015, and its real density, depending on its relative silicon carbide content, varies from 1.90 to 2.30. The approximate diameter of the fibers is 0.001 mm., or of the order of magnitude of the wave length of light. Its outstanding characteristic is its thermal resistivity, which is t h P highest of all known heat-insulating materials.
I n 1897 when The Carborundum Company began the commercial graphitizing of electrodes under the Acheson process, our first order came from the Castner Company for anodes for electrolytic alkali. These were round carbon rods 15 by 1.25 inches, and we placed them crosswise in the furnace to form a horizontal pile from terminal to terminal. This pile was surrounded by a thin layer of fine carbon and then by a layer of a silica-coke mixture. One day a workman neglected Silicon Monoxide to place the carbon layer properly, and, on opening the furnace I found the top layer of anodes partially converted into silicon Silicon monoxide was still another child of the silicon carbide instead of into graphite. On another occasion a carbide furnace. For a time it gave hope of valuable comworkman, by mistake, left in the furnace a pine board which mercial possibilities but has never fulfilled these early promises. had been used as a partition in placing the layer of fine carbon, When portions of the core became overheated, hot spots and this board was found completely converted to silicon developed in the charge and gases formed blowholes and burst carbide, portions of it being an exact replica of the wood and from the furnace in miniature volcanoes. The craters were showing the grain and other physical characteristics. found to consist of brown vitreous masses having a chemical These results led me to study the conversion cf carbon rods composition corresponding approximately to silicon monoxide and rods of molded silicon carbide in the regular atmosphere and, when the gases condensed in the furnace or were collected of the silicon carbide furnace. I soon discovered that the in chambers free from air provided for the purpose, the silicon rods so produced were good enough conductors to be used as monoxide was deposited in the form of a soft opaque brown electric heating elements. This early work on the production powder of impalpable fineness. of formed articles in the silicon carbide furnace became the Silicon monoxide is an intermediate reduction product basis of the present-day silicon carbide heating element, a formed when silica is heated with carbon, silicon carbide, or typical form of which is known under the trade name of silicon. -4vast amount of work was devoted to this product “Globar,” and which is composed of self-tonded cr recrystal- by Potter ( 6 ) ,and, although after contests in the patent office lized silicon carbide. The subsequent development of these the basic patents were awarded to me, the technical and comelements has extended over many years, and the electrical mercial development, so far as it was carried out, was due to properties of the element as now manufactured are unique in the highly creditable work of Potter. The uses developed the field of nonmetallic conductors. were as a pigment for paint and printing inks and as a heatBy research extending as far as special methods of producing insulating material. the silicon carbide which is used and by suitable manipulation There has always been some question as to whether silicon during the recrystallizing process, it has been possible to pro- monoxide is a true chemical compound, since the chemical duce an element composed entirely of silicon carbide, which, properties are what might be expected from a mixture of silihowever, has entirely different electrical properties from sili- con and silica. Potter produced considerable evidence as to con carbide in any other known form. The ordinary type of the constancy of composition of the so-called amorphous subrecrystallized piece has a high and erratic electrical resistivity limed material, even when the proportions of silicon and silica when cold and a sharply negative temperature-resistance in the original charge were varied over wide limits. Regardcoefficient. The same is true of either loose unbonded crystals less of whether the material is a definite compound, it possesses or those bonded in other ways to an even greater degree. The the number of properties which are rather unusual. The modern resistor, however, has a positive temperature-resist- vitreous material has an apparently homogeneous vitreous or ance coefficient over the operating temperature range and glassy fracture which appears definitely nonmetallic, yet, on requires no excess voltage to start the current flowing. It is immersion in a solution of copper sulfate and hydrofluoric acid, accurately supplied over a wide range of specific resistivities. copper is displaced over the fractured surface. The vitreous Thus there is available for the first time a commercially mass often contains many microscopic inclusions, and around feasible heating element which can be used a t temperatures of these inclusions are a large number of concentric rings, indifrom 1100’ to 1400’ C. without special protective atmospheres cating a possible reaction or solution of the inclusion in the or costly transformer equipment. melt. If the silicon is dispersed through the material in eleThe utilization of these heating elements is increasing mental form, it must be very finely divided and may be rapidly as their advantages are realized, and already ranges colloidal. The phenomenon may be similar to the well-known from small domestic rods in reflector room heaters to indus- “metal fogs” in fused electrolytes. trial elements dissipating around 25 kw. each a t 1400-1500” C. Their use in forging and heat-treating furnaces is particularly Artificial Mullite wide, as the possitiility of closely controlling furnace atmosphere a t high temperatures is of marked advantage in this While perfecting the commercial production of silicon, I industry. They are also used for glass annealing, laboratory turned my attention to the prcduction of an alloy of silicon furnaces, and other applications. and aluminum, working with a charge of aluminum silicate and carbon in place of silica and carbon. Judged by immediate Fibrous Silicon Oxycarbide results, the experiment was a failure. Working with an arc Another offspring of the silicon carbide furnace is fibrous furnace, I secured a very good reduction of the silica, but the silicon oxycarbide. One day an Irish member of the furnace alumina proved difficult to reduce. The resulting alloy congang came to me in much excitement and said “God forgive tained 15 per cent aluminum and 84 per cent silicon. When me for lying, but this furnace has worms. Look a t their the furnace was tapped, there came out with the alloy a deluge nests,” and he pointed out masses of a green cottonlike prod- of highly fused aluminum silicate depleted of a portion of its uct which had formed in a cavity at the furnace terminal. silica. This overflowed the ladle and formed in slabs on the When furnace vapors containing silicon and carbon monoxide brick floor, making rather perfect castings and reproducing are led out and caused to condense in a chamber closed to the every joint of the brick floor which served as the mold. The outside air, there may be formed as R condensation product a product was a uniform crystalline mass of great hardness and
I N D U S T R I A L -4iI-D ENGINhJERILVGCHEMISTRY
Sovember, 1931
compactness and possessed such interesting abrasiye and refractory properties that i t became the object of our continued research rather than the alloy. Its compctsition could be controlled, the alumina content varying from 60 to 90 per cent, depending upon the degree of silica reduction. KO difficulty was experienced in casting the material in all the various compositions u p to 90 per cent alumina. Since the identification of mullite by Bowen and Greig ( I ) , the interest in these fused mixtures has broadened tremendously, and they now bid fair to become one of the norld's most useful refractories. Formation, Recrystallization, and Dissociation of Silicon Carbide
I wish to devote a word, before closing, to the more modern views of the high-temperature reactions of silicon and carbon. As the development of silicon products has progressed, we have been obliged to revise or a t least enlarge upon some of our former theories. Acheson's original equation, Si02
+ 3C = Sic + 2CO
while still acceptable as representing the original reactants and final products, does not go very far in explaining the mechanism of the formation of silicon carbide. I would like to review briefly some of the more recent work, together with a few supplemental experiments carried out by J. A. Boyer in the laboratories of The Carborundum Company. The fact that silicon carbide is produced in the furnace in the form of perfectly developed crystals has led to the assumption by many that the reaction in which silicon carbide is formed must take place in the vapor phase, since it would be difficult to explain the aggregates of large perfectly formed crystals unless they were deposited from vapor. I n recent years there has keen considerable evidence to show that silicon carbide itself can be vaporized and redeposited, and the possibility of subliming silicon carbide has eliminated the necessity for considering the fundamental silicon carbide reaction as taking place entirely in the vapor phase. The vaporization of silicon carbide was first indicated in connection with the process of making silicon carbide rerractories known as recrystallization, I n this process a molded article composcd of silicon carbide grains is heated t o a very high temperature, and during the process the grains apparently grow together to form a strong coherent niasq. If the process is continued for a prolonged pericd of time, the crystals become much larger than thcse originally present in the mix, and the larger crystals seem to grow a t the expense of the smaller ones. When recrystallization is effected in a closed silicon carbide muffle, the articles often have large numbers of perfectly formed silicon carbide plates deposited over their entire suface. The formaticn of large silicon carbide crystals within a silicon carbide muffle containing only silicon carbide as a charge would he difficult to explain except by the vaporization and redeposition of the silicon carbide itself. The vaporization of silicon carbide in a n undissociated state mas also indicated by the experimental results of Ruff and Iionschak (8),who measured quantitatively the residue after d i c o n carbide had been exposed to vaporizing conditions a t various temperatures. =1 certain amount of dissociation occurred in every case, but the carbon content of the residue was much too low to be accounted for by the loss of silicon alone as a vapor. It seems possible that a considerable part of the large crystal formation in the furnace can be due to the vaporization and redeposition of silicon carbide itself, and that the phenomenon is analogou, to the crystal growth occurring in the recrystallization of si1 con carbide refractories. If this is the case, it is entirely possible that the product, when firqt formed
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in the furnace, is not highly crystalline, and that it can be produced without conversion of the reacting substances into vapor . Formation of Silicon Carbide
Various temperatures have been given in the literature for the formation of silicon carbide from sand and carbon, varying from about 1200°C. upwards. Even if the measurements were originally correct, the values would have little theoretical significance, since the question of whether formation is possible depends upon the partial pressure of the carbon monoxide gas in contact with the solid phases present. I n cases where the carbon monoxide formed is removed only by diffusion, this pressure is a rather variable quantity. The only "formation temperature" which has a definite meaning is the temperature where the carbon monoxide pressure becomes equal to atmospheric pressure, for at this temperature the carbon monoxide can push away the surrounding atmosphere and the reaction can go rapidly to completion. Ruff and Konschak ( 8 ) have measured the temperature where the pressure of carbon monoxide obtained by heating a sand-carbon mix is equal to 1 atmosphere and have found that it is about 1650"C. It is interesting to note that this temperature is considerably below the temperature where highly crystalline silicon carbide is produced. The actual mechanism of the silicon carbide reaction has not been determined with certainty. Ruff and Konschak ( 8 ) , on the basis of thermodynamic calculations, claim that the reaction is between silicon vapor and solid carbon, but the data upon which these calculations are based are not very convincing. Even if the reaction is between silicon vapor and bolid carbon, it would be necessary for the silicon carbide to vaporize and redeposit in order to produce large, perfectly formed crystals. I n this connection it is interesting to follow the course of the reaction by determining the loss in weight of the materials at various stages during formation. On heating a sand-carbon mix to a temperature of approximately 1800"C. for from 1t o 2 hours under reducing conditions, the material will lose approximately 60 per cent of its weight, indicating that the liberation of carbon monoxide is substantially completed. The product, however, is not crystalline as far as can be visually observed, but resembles the so-called amorphous silicon carbide, which is a greenish material insoluble in a mixture of nitric and hydrofluoric acids. On heating this lowtemperature product to about 22OC-2300" C., it recrystallizes and presents the appearance of the usual recrystallized silicon carbide. The loss in weight in this recrystallization stage is usually only about 5 per cent on the basis of the original mix, although somewhat higher weight losses have occasionally been observed. It seems entirely probable that similar conditions obtain in the silicon carbide furnace, and that the material originally formed is not highly crystalline, but recrystallizes on heating to a higher temperature. Recrystallization and Dissociation
I n the usual process of recrystallization, a sand-carbon mix is used to surround the article to be recrystallized, and it has been assumed that a silicon-vapor atmosphere is necessary to effect recrystallization. The process is also carried out over a prolonged period of timP, as, for example, from 24 to 36 hours. We have recently succeeded in recrystallizing small articles in approximately 2 minutes or less by merely inserting them in a graphite-lined high-temperature furnace. *4rticles were recrystallized a t temperatures up to 2100" C. nithout any apparent decomposition of the silicon carbide. I n accordance with the older views silicon carbide, when heated to about 2240" C., dissociates completely into silicon and carbon, the silicon being driven off as vapor and the
INDUSTRIAL AND EN( XNhZRING CHEMISTRY
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carbon remaining as graphite. The value of 2240" C. for the dissociation temperature was determined independently by Gilett (5),Tucker and Lampen (fO), and Saunders @), and the results were in close agreement. I n these measurements pure silicon carbide was not used as a starting point, but the temperature given represented that of the graphite zone in the furnace. Working with pure silicon carbide in a furnace giving substantially uniform heat distribution throughout the chamber, we have found that pure silicon carbide grain can he heated considerably above 2240"C. without decomposition. The true disyociation temperature may be defined as the temperature a t which the pressure of the vapor resulting from heating silicon carbide is equal to atmospheric pressure. The measurement of this temperature involves either a vapor-pressure nieasurement or detection of a change in the rate of vaporization rather than the heating of the material for a prolonged period of time. Ruff and Konschak on tlie basis of vaporpressure measurements of somewliat uncertain accuracy give this temperature as approximately 2700" C. We have made a number of experiments on the dissociation of very pure silicon carbide and, while the work ia not completed, some of the preliminary findings are of interest. We have found that, on heating silicon carbide grain, the extent to which the conversion to graphite takes place is dependent upon the conditions of the experiment, and particularly upon the facilities for vapor diffusion and the temperature of the zone immediately adjacent to the heated material. Keeping the temperature constrtnt a t temperRtures of approximat,ely 2500' C., we have been able to obtain residues varying from 4 to 98 per cent in graphite content, depending upon the facilities offered for condensation of the vapor. We were able to condense silicon carbide in considerable quantities during tliese experiments, but, whenever uniform temperature conditions were provided throughout the container so that very little condensation took place, this result was reflected in a much snialler loss in weight of the original sample and a much lower graphite content in the residue. These detenniiiations were presumably made at a temperature below that at which the vapor pressure was equal to atmospheric pressure, for otherwise diffusion and condensation would not have had such a great effect; yet the temperature was approximately 2500" C., so that the true dissociation temperature, as defiiied in terms of vapor pressure, would
THEELIXIR 01' LIFE by John A. Lomax (1857-is27 in)
John Arthur Lomax was an English genre-painter well known for his paintings of the Cavaliers of the Eighteenth Century in costumes of that period. Little is known of "The Elixir of Life" heyoyond the fact that it was included in the Exhibition of the Royal Academy in London in 1908. Efforts to determine its present whereabouts have failed.
Yol. 23, No. 11
presumably be above this value. The effect of impurities and tlie nature of the surrounding atmosphere on dissociation have not as yet been determined. Summary 1-Silicon carbide can apparently vaporize without complete dissociation, and it Beems probable that recrystallization and the growth of large crystals are primarily due to this cause. 2-The temperature a t which the pressure of carbon munoxide, produced by heating a sand-carbon mix, is greater than 1 atmosphere is considerably below that necessary for the formation of highly crystalline silicon carbide. A Iowtemperature product elraracterized by a lack of well-developed crystals can be formed at these temperatures. It is entirely possible that silicon carbide is originally formed in this condition and that large crystals are the result of recrystallization or crystal growth. 3--Thcre is no necessity for assuming that silicon carhide must be formed entirely in the vapor phase, if silicon carbide itself can be vaporiaed. 4--Recrystallhation of silicon carbide is ahnost instantaneous at very high temperatures, and a sand-carbon mix is not neccssmy to effect it. 5-The dissociation temperature of pure silicon carbide, defined as the temperature at which the pressure of the vapor resulting from heating silicon carbide is equal to atmosplieric pressure, is believed t.o he considerably higher than 2240" C., the value of 2700"C. given by Ruff and Konschak apparently being more nearly correct. Literature Cited (1) Boweil mid Creii, J. A m . Ccmm. Sor. I , 238-54 11924); Bowen, Greig, aiid7,ics.I. nrash. Acad. Sii., 14, 183-91 (1924). ( 2 ) Chalrnot. de, U. S. Patent 589,411 (1807). (3) Coison, Iron .is*, 119. 3 5 3 4 , 4, 21-4 (1827); Am. lnil. Mining Met. En&, Proc. Insl. lMdo1r Div., 1921, 435-50. (4) Edwards, I'nry. and Cllurehill. U. S. Patent 1,110,4G1 (Merch 21.
_""_,. *Llnni
(5) Giiett, J . Phyr. Cham., 16,213 (1911). (0) Potter, T r o w . A m . IiIcitrorhrm. So'., la, 191-228 (1807). (71 Richards. Chem. M d . Ens.. .. 16.. 26 119101. . isj Ruff %adKonschrk, 2. liiekiroihcm., ah, 515 (1926). (9) Saunders. %on$. A m . i?Jadrachcm. Soc., 21,425 (19121. (IO) Tirckrrnnd Lampen, J. A m . Chrm. SOL.,28, 853 (1906).
