July. 191j
T H E JOCR-VAL OF I S D L - S T R I A L A S D ESGINEERIiVG C H E M I S T R Y
THE PREPARATION, PROPERTIES AND COMPOSITION OF SILUNDUM B y SAMUEL -4. TUCKER A N D ALEXAXDER LOWY Received M a y 3, 1915
‘
Silundum is a product of t h e electric furnace, a n d n-as discovered b y Bolling,’ a t Frankfort-on-the\lain: Germany. As early as 1900 he began t h e s t u d y of t h e use of t h e various silicon carbides as resistors, being led t o this work b y t h e fact t h a t these carbides c a n be subjected t o high temperatures without undergoing decomposition. His method was t o mix t h e silicon carbide powder with various binders in order t o hold t h e individual particles together. T h e mass is a non-conductor a t low temperatures, b u t at a b o u t joo--800° C. i t becomes conductive, t h u s being useless at n higher temperature. R\- I 9 0 4 a process was worked out a n d patented 111.- which i t is possible t o convert a n y piece of carbon, e. g . , a crucible. brick, rod, e t c . , into silicon carbide. B y t h i s method resistors without a n y binding material are obtained. which will s t a n d temperatures as high as 1 6 5 0 - 1 7 0 0 ° C. Previous t o 1904. silicon carbide h a d been known in t w o conditions onlyamorphous a n d crystalline-and i t was supposed t o be formed by t h e interaction of sublimed carbon a n d sublimed silicon. Bolling, however, discovered t h a t silicon penetrates carbon when both are in a highly heated condition, b u t a t t o o low a temperature for t h e carbon t o have a n y appreciable vapor tension. He claimed t h a t silicon at a b o u t 1600’ C. exists in t h e form of vapor. Carbon at this temperature has a n extremely small vapor pressure. According t o Bolling’s definition, silundum is t h e product t h a t is obtained when carbon is heated in a n atmosphere of silicon x-apor. -In electric furnace such as is used for t h e manufacture of carborundum was used for t h e preparation of silundum. Pieces of carbon pressed t o t h e desired shape 17-ere introduced- i n t o a mixture of sand a n d carbon. a n d this was heated b y using as a resistor a granular core of coke. T h e s a n d was reduced a n d t h e silicon. \-olatilizing, penetrated t h e carbon objects a n d t h u s “silundumized” t h e m . hIoyat? criticized Bolling’s work, a n d claimed t h a t silundum n-as not a new chemical individual, b u t merely a new variety of silicon carbide. Xcheson’s p a t e n t 3 No. 89 j , 5 3 1 is interesting: inasmuch as it deals with a similar product. I n order to protect crucibles, tuykres, bricks, muffles, a n d other articles of carbon from destruction a t high t e m peratures in t h e presence of oxygen, or when exposed t o molten metals, Xcheson proposes t o cover t h e m with a coating of a highly refractory siloxicon, or silicon oxycarbide. This prolongs their life a n d prevents disintegration. X portion of t h e carbon on t h e surface of t h e article itself is utilized in producing t h e siloxicon coating. -After having been shaped t o t h e form desired, t h e carbon or graphite article is embedded in a mixture of silica a n d carbon in t h e proportion of t w o p a r t s of s a n d t o five of carbon, a n d is
’ F.
Bolling, Chem. Zlg.. S2, 1104; Elecirochem. a n d M r l . Ind.. 7, 24. M o y a t , Chem. Z t g . , 32, 1166. .4cheson. U.S. P a t e n t S o . 895,531; Electrochem a n d - V e t . I n d . . 6 , 3 7 9 .
exposed t o t h e proper temperature at which silicon, oxygen, a n d carbon combine t o form siloxicon. T h e product obtained b y this method resembles in appearance t h e lighter-colored silundum, which is t o be discussed later in greater detail. Tucker a n d his co-workers’ in 1909 made a n experimental s t u d y of t h e formation of silundum. I n their experiments t h e charge was made u p according t o t h e proportion Si02 : 2SiC. This charge was heated in a carborundum furnace, t h e core of which, instead of being of granular carbon, consisted of a number of small carbon plates, pressed together between t w o horizontal graphite electrodes. Around t h e core n-as placed a charge of sand a n d carborund u m , in which were embedded, a t varying distances from t h e core. t h e carbon articles t o be silundumized. These workers also investigated t h e reactions between sand a n d carbon. Their experiments showed t h a t silundum can be produced b y either of t h e above reactions, i. e., b y t h e reaction of sand a n d carbon or of sand a n d carborundum. T h e y also showed t h e relative difficulty of oxidizing silundum. T w o plates, silundumized on t h e surface, were heated t o a high temperature b y means of a n electric current. T h e graphite cores were completely oxidized, leaving only box-like shells of silundum. Amberg-Bodio2 also prepared silundum, a n d states t h a t , “According t o t h e temperature a n d t h e time, there is obtained a gray- t o metallic-appearing silicon carbide, with a content of silicon greater or less t h a n t h a t which corresponds t o t h e formula S i c . T h e process m a y be interrupted a t a n y time a n d a coating of silundum of a n y desired thickness produced.” Within t h e past few years, F. J. Tone3 has o b tained four p a t e n t s for t h e preparation and manufact u r e of dense, compacted silicon carbide. F r o m his description, t h e t e r m “dense compacted silicon carbide” is synonymous with t h e t e r m “silundum.” Egly,4 early i n 1913, patented a substance called “Silit” which is prepared b y heating t o a fairly high temperature silicon carbide, silicon, a n d a binder, until a homogeneous mass results. “Silit” is a good resistor a n d has m a n y industrial applications. F. J. Tone5 describes a silicidized carbon which he calls “Silfrax.” I n order t o show t h e interior structure of t h e material, he gives microphotographs of i t . A survey of t h e literature a n d a careful preliminary examination of several samples of silundum, indicated t o t h e writers t h a t in all probability t h e material known as silundum exists in two modifications. T h e crudeness of t h e analytical methods used in determining t h e composition of these refractory materials made i t impossible t o draw a n y definite conclusion from t h e work already published. Moreover, little or no d a t a are available on such important conditions for t h e preparation of silundum as temperature,
’ Tucker, Kudlich a n d Heumann, Trans. A m . Electrochem. SOL.,16, 207. 3 4
Amberg-Bodio, Z. Elektrochem., 1 5 , 725-727; Chem. Rbs.. 4, 149. Tone, U. S. Patents Nos. 913,324, 992,698, 1,013,700, 1,013,701. Egly, Eleklrotechn. Z e i t . , 3 4 , 263-267; Chem. A b s . , 6 , 536; 7, 1142,
2357. 5
Tone, T r a n s . A m . Eleclvochem. S o c , 26, 181.
j66
T H E J O C R N A L OF I;VDL'STRIAL A N D ENGIXEERI.VG C H E M I S T R Y
proportions of reacting materials, a n d t i m e of heating. It seemed, therefore, t h a t b y a careful investigation into t h e best conditions of temperature for preparing silundum, with a n improved method of analysis a n d a careful examination of properties, considerable light might be thrown upon t h e question of t h e composition of silundum. With this purpose in mind t h e present investigation was undertaken. Silundum, according t o t h e conditions under which it is formed, exists in t w o modifications. One of these, a steel-gray variety, is formed at a temperature of about 1900' C. a n d analyzes very closely t o t h e formula S i c ; t h e other, a slate-green variety formed a t a b o u t 1600' C., is a n oxycarbide of silicon of t h e formula Si4C40. It has been determined t h a t silundum does not decompose below 2 2 0 0 ' C. At higher temperatures silicon distils off, leaving a graphite skeleton of t h e same form as t h e original silundum, which action would indicate t h a t t h e graphite does n o t vaporize at t h e temperature of decomposition, a n d would show, further, t h a t silundum is formed b y t h e reaction of silicon vapor a n d solid carbon, t h e temperat u r e of formation being lower t h a n t h e temperature of decomposition. T h e action which t a k e s place in t h e process of silundumization m a y be summarized as follows: T h e silicon which is liberated from silica b y t h e reducing action of coke- or sugar-carbon penetrates t h e solid carbon or graphite objects, a n d there reacts with t h e carbon, forming a steel-gray variety of silu n d u m . T h i s t a k e s place above 1800' C. Below this temperature t h e slate-green variety is formed, most likely b y t h e penetration of t h e carbon b y t h e silicon vapor a n d carbon monoxide. T h e stumbling block in t h e analysis of such refract o r y substances as silundum has been t h e difficulty of completely decomposing t h e material a n d burning all t h e carbon t o carbon dioxide. T h e silicon m a y be determined easily b y means of t h e well-known sodium carbonate a n d potassium nitrate fusion. I n order, therefore, t o overcome t h e difficulties hitherto encountered in t h e carbon determination, various fluxes a n d oxidizing agents were tried, with t h e object of replacing t h e inadequate sodium peroxide a n d magnesia method' previously used. After considerable experimentation litharge was found t o be a n excellent flux for decomposing t h e material a n d partially oxidizing i t , complete oxidation being effected b y means of a s t r e a m of oxygen. Using this method on carb o r u n d u m crystals, results were obtained t h a t were accurate within 0 . 3 per cent of theoretical. Considering t h e n a t u r e of t h e material treated, this was satisfactory. Analyses of silundum samples were made b y this new method, a n d t h e results checked on duplicates. Therefore, we consider this t o be a good method for t h e . accurate determination of carbon in refractory carborundum-like substances. DESCRIPTIOK O F EXPERIMENTS PREPARATION O F sILuNDullr-In order t o obtain sufficient amounts of t h e substance for t h e further s t u d y of i t s properties a n d composition, silundum 1
Fitzgerald, Elecfrochem. I n d . , 2, 443.
T'ol. 7, SO.7
was prepared in a coke resistance furnace (Experiments I t o 7 ) . Since this t y p e of furnace does n o t lend itself t o accurate temperature measurements, t h e temperatures of formation a n d decomposition were measured in a separate series of experiments (8 t o 13) in t h e Arsem vacuum electric furnace.' T h e coke resistance furnace2 was chosen because with i t a range of temperatures u p t o about 2 6 0 0 ' C. can be obtained by varying t h e a m o u n t of electrical energy. T h e furnace, diagrammatically represented in Fig. I , consists essentially of a coke resistor enclosed b y cemented fire-clay brick walls with magnesia lining. T h e graphite electrodes extend through t h e walls into t h e coke. T h e graphite crucible containing t h e charge a n d t h e articles t o be silundumized were embedded in t h e coke core. T h e energy was supplied by a jo kw. 9.C. generator. The measurements a n d t h e cI b 0 ~ 4 1 ~ - l 3 0 ~ ~ 4 1 j c l l 0 - - $
4
44t
Yiun ViLw FIG. RESISTANCE F U R N A C (ED I X E ~ ~ S I OI NS SM m . )
exact details of construction of t h e furnace are indicated in t h e diagram. Carbon a n d graphite rods varying in diameter from I / g t o 3 / g in., a n d 2 in. in length were used. I n a few experiments carbon tubes were used. T h e charge consisted of clean s a n d a n d coke. For t h e samples t h a t were t o be analyzed, special charges, consisting of precipitated silica a n d sugar-carbon were used, in order t o obtain a pure product. T h e objects t o be silundumized in these cases x e r e also made of graphite of t h e highest purity. EXPERIJIENTS IS C O K E RESISTASCE FTRS.4CE
Experiments I t o 7 -rere conducted in order t o find o u t how t h e variation of t h e different factors of time, energy consumption a n d composition of charge would affect t h e nature of t h e product formed. 1
2
Arsem, Trans. A m . Electrochem. SOC.,9 , 153. Tucker, I b i d . , 11, 307.
T h e readinks taken ,during these experiments appear in Table 1. EXPSRLMBNT No. I
A~~~~~~ voits
1o.m 10.10
10.20 10.30 10.40
m.so
ii.00 t1.10 !1.20
ii.30
iz.on
0 i5
ino 150 1x1 150 200
270
mo ma ann
40 38
36 34 34 34 28 26 26 26
26
still higher temperatures than were used in the previous expcrimcnts. 4 charge like t h a t in Experiment I
Tam& I - R s ~ m h - , i s TAKEN O ~ ~ R L N EXPERIMENTS Y IN Con6 R B S I S I A N CP~i ~ -nli~ci No. 2 ExFemMriwr N O . 1 nnPrarMalir XJlJrnBER 5 ~i~~ A~~~~~~ voles ~ i m e ~ m p e r e svolts mine ~ m n e vrd ~i s~ ~ m voltso EXPBIXMENT
1.30 1.45
1.5s
2.00 2.10 2.20
2.30 2.40 2.50 3.00
0 35 200 30 3 ~ ) 30 30 400 400 26 400 26 400 26
410 410 410
26
26 26
y.30
n
34
9.45 9.55
200 310
32 30 25 25
,n.oo
10.15 in.30 10.45 I I .on 11.30 1z.00
40.5 400
410 410
+mi 400
400
i2.30
400
1.00
400
0 75 100 200 250
2s
350
25 25 25 25 25 25
450
X X P E H I M E S T I-.- -A charge consisting of a mixture af 60 g. coke and r g o g. sand (the theoretical amount to produce silicon) was placed in a graphite crucible. Four carbon rods i/( in. in diameter a n d z in. long were then embedded in t h e charge a n d the crucible tightly covered with a graphite cover. Crucible a n d contents were placed in t h e furnace within t h e coke resistor, a n d the electric current turned on. After two hours t h e furnace was allowed t o cool, and examination of t h e contents of t h e crucible showed t h a t t h e rods were n o t silundumized. T h e sand was fused around t h e graphite rods. This experiment indicated t h a t a higher temperature was necessary for the accomplishment of the desired reaction. L X P E R I M E N T 2 was conducted in order t o s t u d y tlie effect of higher tcmperature on t h c reaction. T h e electrical energy consumption was increased, as indicated nlmvc. with t h e object of obtaining this desired higher temperature, T h e composition of t h e charge was t h c same as in Experiment I . T h e graphite rods upon fracture were found to be silundumized half way through. T h e line of demarcation between the silundumized portion a n d t h e portion not :ict,ccl upop was vcry sharp (see t h e microphotograph, Fig. 2 ) . T h e color of t h i s product was slate-green. I j a n d 6). \-I-More complete silundumization takes place when t h e object is embedded in t h e charge (see Expt. 4 ) .
1-01. 7 , No.
Temp. 3C 20 25 50 i5 100 125 150
175 200 225
THE TX.0
SLATE-GREESVARIETY Rod 1.871 Cm. Long and 0 635 Cm. in Diameter Measured Specific resistance resistance Ohms per cc. Ohms 0.1546 0.91450 0.1543 0.91270 0.1.522 0,90000 0.1506 0,89092 0.1491 0.88192 0,87212
0.86202 0.85392 0.84522 0.83530
0.1474 0.1457
0.1443 0.1429 0.1412
VARIETIES OF S I L U X D U M STEEL-GRAY VARIETY Rod 2.078 Cm. Long and 0.348 Cm. i.1 Diameter Measured Specific resistance resistance Ohms per cc. Ohms 0.2391 5.2250 5.1886 0.2374 4.9786 0.2278 0,2205 4 8186 0.2118 4.6291 0.2022 4.4191 0.1958 4.2791 0.1901 4.1541 0.185O 4.0441 0.1809 3.9541
t h e temperature was read by means of a n ordinary thermometer with its bulb in t h e heating chamber. Table I11 shows t h e measured resistances a n d t h e specific resistances in ohms a t t h e various temperatures. T h e specific resistances are plottfd against t h e temperatures in Fig. 3. 1 This was done in the Electrical Testing Latciidlory of Columbia University
4
Attempts were made t o spray the ends of t h e rods according t o t h e Schoop' .method, b u t when t h e y were heated, t h e brass 'contacts became loosened from t h e ends of t h e rods. I t is evident from the above results t h a t t h e resistance is practically a linear function of the. temperature. T h e temperature coefficient of resistance is negative. T h e specific resistance is much less t h a n t h a t for carborundum. being, a t 2 j 0 . 0.1 j 4 3 o h m per cubic centimeter for t h e slate-green variety. a n d 0 . 2 3 7 4 o h m per cubic centimeter for t h e steel-gray variety, while t h a t of carborundum is j o ohms per cubic centimeter a t 2 j '.? I t was a t t e m p t e d t o measure t h e ohmic resistance of the a b o r e rods a t higher temperatures b y clamping t h e m between platinum plates. T h e resistance varied from 7 j ohms t o a fraction of a n o h m , depending upon t h e pressure exerted upon t h e platinum plates, so t h a t measurements of t h e same order of accuracy as those
atmospheres of oxygen. in a gelatine capsule containing a mixture of benzoic acid a n d silundum. T h e gelatine a n d t h e benzoic acid were completely oxidized, b u t t h e silundum was unattacked. A C T I O X Of F U S E D S A L T S , ETc.---Situndum is not decomposed b y t h e following fused salts: sodium silicate, borax, a mixture of potassium chlorate and potassium nitrate. potassium acid sulfate, cryolite, potassium dichromate. Silundum is decomposed by fused sodium carbonate, sodium hydroxide, and potassium hydroside in presence of air, yielding t h e corresponding silicates and carbon dioxide. A C T I O N O F .kcIDs--Silundum is not acted upon by hydrochloric, nitric, or sulfuric acids, nor by a mixture of hydrochloric a n d nitric acids. a mixture of chromic and sulfuric acids: nor by fused boric acid. Commercial silundum, however. is slightly attacked h y hydrofluoric acid and by a mixture of nitric' and hydrofluoric acids, because of t h e presence of free silicon a n d silicon dioxide; b u t pure samples of silundum are not attacked b y these acids. XCTIOS O F O T H E R sussTascEs---Slolten sulfur lias n o action on silundum. Sodium peroside oxidizes it t o sodium silicate plus sodium carbonate. Idcad oside also oxidizes i t , producing metallic lead ant1 carbon dioside (pp. j 6 6 a n d 5 7 0 ) . Fused sodium in presence of air decomposes silundum. yielding sorliuni silicate and sodium carbonate. The action in this case is most likely due t o the presence of sodium peroside formed from the sodium. C 0 3 I P O S I T I O S OF S I L L S D V U
0g.iw ,150
~
/bo ,170
.i8o
~~~
,190
,200 ,210 ,220
.UO ,M
Frc 3
determined aljo\-e n.ith plated ends could not be made abo\-e 2 2 j" C. H A K D S E S S and specific gravity measurements are giyen in Tahle 11- for t h e t w o \-arieties of silundum and for Tone's and Bolling's products. S P E C I F I C G R A T - I T Y was determined by means of a specific gra\-ity bottle. care being t a k e n t o e s p i all air but)hles b y careful heating and subsequent cooling. T A B L E Iy--->I.4Rl3YESS
..1one's "Silifrax",
,
, ,
A S D SPCCIFlC G R A V I T Y OP s l L V S D l . > l
.
Boiling's silundum, . . , (:remish variety silundum. Steel-gray riiriety silundum. PROPERTIES
OF
, ,
. ,
BOTH
Speci.fic gravity , . 2 96 2.9; , , . 2 92 ., ... , 2.91 ,
vAXIETIES
OF
Hardness Rlohr's scale) 9 plus
9 8 to 9 9 minui
SILVSDI-31
0x1-gen, nitrogen, and hydrogen ha\-e no action o:i silundum heated t o temperatures ACTIOS
OF
G.J.SES
--
u p t o IIOOO (2. a t t e m p t was made t o oxidize silund u m in a born!> calorimeter under a pressure of 2 : - M r i . I n d . , 1914, p . + 5 7 .
THISJ O C K N A L , 4, 8 5 3 ; 5, 7 7 6 , 7 . 7 1 Pamphlet, "Chemical a n d Physical Properties or C.~rl,oruridum," published b y The Carhorundum C o , S i a g a r a Falls. S Y I O l 3 2
I n order t o obtain silundum as pure as possible for analysis. graphite rods \?-ere siluntlumized according t o t h e prei-iously described methods. in :i charge consisting of sugar-carbon and precipitated silica. The resulting silundum 1%-as broken u p into small pieces in a steel mortar, and su!xequently grounlcl t o n \very fine powder in an agate mortar. T h e pon-der n-as then purified. Depending upon the conditions of preparation, t h e product obtained carbide or silicon oxycarbide, with probable impurities either from t h e charge or from t h e reaction, or from t h e mortar in n-hich t h e material was ground. T h e impurities may lie silicon. silica, carbon, and iron. Silicon was removed with boi1in.g potassium hydroxide solution, a n y remaining silica by means of hytlrofluoric acid, and the uncombined carbon b y long oxidation with a strong blast lamp. Finally. in order t o assure complete osiclation t h e material \vas boiled n-ith chromic and sulfuric acids. and then with hydrochloric acid i n order t o remove a n y particles of iron which may h a w been abraded from the m o r t a r . The material vias then thoroughly n-nshetl. and whcn drictl x ~ a sready for analysis. Fur the silicon determination 0.3 t o 0 . 4 g. of the :tl.jove purified substance n-as weighed into a platinum crucible. T o this were added 4 g. of chemically pure sodium carbonate and L: small crystal of potassium nitrate. The contents were then ne11 mixed and the covered crucible heated, great caution 1)eing use(1 t o
T H E J O C R Y A L O F I L V D 1 7 S T R I A L AA-D E S G I N E E R I N G C H E M I S T R Y
Si0
avoid spattering. T h e heating was continued until a quiet fusion resulted, then strongly increased until all t h e carbon particles were oxidized, leaving a clear melt. This upon cooling gave a clear, white mass, which was dissolved in hydrochloric acid a n d evaporated t o dryness, dehydrated three times, a n d t h e determination of silica made as in rock analysis. T h e silica was weighed b y difference after repeated evaporati o n with hydro fl u or i c acid . 11upli c a t e a n a1y se s checked within 0.1j per cent. F r o m t h e a m o u n t of silica obtained t h e percentage of silicon was calculated. T h e results are given in Table V. TABLEY-ANALYSES PER CENT SILICONFOUND Steel-gray Slate-gray silundum silundum 69.46 63.16 69.59 63.28
OF
SILUXDUM FOR SILICON THEORETICAL PER CENT SILICON in silicon in carbide Si&O 70.22 63.88
...
...
T h e method' formerly employed for t h e determination of combined carbon in carborundum-like substances was t o fuse a b o u t 0 . 3 g. of t h e substance with 3 g. of calcined magnesium oxide a n d 6 g. of sodium peroxide in a nickel crucible. A blank test h a d t o be r u n at each set of experiments t o determine t h e a m o u n t of carbon dioxide present in t h e magnesia a n d sodium peroxide. Furthermore, it was necessary t o heat t h e crucible with a hydrogen flame until a vigorous reaction occurred, a n d t h e crucible while still h o t was transferred t o a desiccator charged with sodalime a n d allowed t o cool. T h e carbon dioxide was t h e n determined b y a n y suitable method. This scheme is subject t o grave error, owing t o t h e appreciable amounts of carbon dioxide present in t h e magnesia a n d sodium peroxide. T h e carbon dioxide content of t h e sodium peroxide continually increases, a n d therefore, even with t h e blank corrections, t h e results are subject t o error. T h e reaction is generally so violent t h a t spattering is almost unavoidable. Moreover, t h e process is long, a n d involves too m a n y operations t o yield absolutely quantitative results under t h e conditions given. T h e method worked o u t in this paper avoids these difficulties. After preliminary experiments with various oxides, such as pure cupric oxide, ferric oxide, a n d manganese dioxide, as oxidizing agents, it was found t h a t only molten litharge gave good results on carbon determinations in carborundum-like substances. hlolten litharge disintegrates t h e particles of silundum, a n d a t the same time oxidizes t h e carbon t o carbon dioxide. The reactions may be written a s follows: Sic qPbO+SiO? 4Pb COY, PbO SiOIPbSiOa, a n d aPb 02+ 2PbO. PROCEDURE-0.20 t o 0 . 2 j g. of the sample was weighed out in a watch glass. This was thoroughly mixed with j g. of litharge whose carbon dioxide content h a d been determined in a blank test carried o u t under t h e same conditions as t h e analysis. T h e mixture of litharge a n d sample was placed in a C. RI. Johnson combustion boat made of vitrified clay, 1 2 0 m m . long a n d 1 5 mm. wide.2 Experiments were
+
1
2
+
Fitzgerald, Loc. c i l . Eirner a n d Amend. S e w Vork
+
+
+
Yol. 7 , KO. 7
also made with combustion boats of porcelain, alund u m , nickel, iron, a n d copper, b u t all except t h e vitrified clay were attacked a n d fused b y t h e molten litharge. T h e boat with its charge was placed in t h e silica t u b e of a n electric combustion furnace, a n d t h e air in t h e furnace displaced b y oxygen t h a t h a d been thoroughly dried in a drying train. T h e current was t u r n e d on a n d t h e temperature raised slowly enough t o avoid too rapid decomposition, which, if i t occurred, would blow t h e charge o u t of t h e combustion boat. A gentle stream of oxygen was passed through t h e a p paratus throughout t h e determination, a n d t h e course of t h e reaction was followed b y observing t h e r a t e a t which t h e bubbles of gas passed through t h e absorption apparatus. As soon as t h e evolution of carbon dioxide tended t o become rapid, t h e electric current a n d t h e flow of oxygen were turned off until t h e reaction subsided, a n d no more bubbles of gas passed through t h e absorption apparatus. T h e n t h e current was turned on again, a n d a slow stream of oxygen passed through. T h e temperature inside t h e furnace was kept at a b o u t 600' C. for about 4 j minutes, a n d then gradually raised t o about rooo0. This was done t o
FIG.4
eliminate a n y possibility of carbon dioxide remaining i n t h e molten mass. T h e carbon monoxide t h a t may be given off at this temperature is converted into carbon dioxide b y passing over a heated coil of cupric oxide. T h e carbon dioxide evolved was absorbed in a Vanier K O H absorption apparatus. T h e arrangement of t h e a p p a r a t u s as set u p is indicated in Fig. 4. Analyses made b y this method are given in Table V I . TABLEVI-CARBON I N CARBORUNDUM AND SILUNDUM Correction for COz in PbO 0.0175 Gram TheoWeight Grams Weight Per cent retical C in P e r c e n t Sample COz Cot Grams evolved corrected sample Carbon SAMPLE L~~~~ carborundum crystals,, 0 . 2 0 0 4 0 . 2 3 3 0 0 . 2 1 5 5 2 9 . 3 3 in SIC ( 0 . 2 1 4 2 0.2488 0 . 2 3 1 3 2 9 . 4 5 2 9 . 7 8 0.2509 0 , 2 9 2 5 0 , 2 7 5 0 2 9 . 8 9 in Steel-gray silundum. . . . . . . . . ~ 0 ,.22211 8 4 7 0 , 2 65 0896 0 . 2 3 49 33 4 2 9 . 98 54 2 9SIC .78 ( 0 , 2 0 8 2 0,2308 0,2133 2 7 . 9 4 in Slate-green silundum . . . . . . . 1 00..22246153 0 , 2 64 38 38 0 ,. 2 4 3 51 83 2 7 . 8 75 6 SidAO 27.09 E
.
F r o m t h e examination of t h e d a t a in Tables V a n d V I as summarized in Table V I I , i t is evident t h a t t h e percentage of carbon a n d t h e percentage of silicon in t h e steel-gray variety of silundum a d d up nearly t o 100 per cent, indicating t h a t the accepted formula
July. 191 j
T H E J O C R S . I I , O F I.VDUSTRIAL A S D EAVGIXEERIiYG C H E M I S T R Y
S i c is correct for this modification or variety. On the other h a n d t h e silicon a n d carbon in t h e slategreen variety of silundum a d d u p t o only 91.07 per cent. This variety evidently does not correspond t o t h e formula S i c . TABLEVII-SCMMARY OF ANALYSES Carborundurn
poUND S i . . . . . . . . . . .
c . .. . . . . . . . .
29:39
TOTAL. . . . . . . . . . . . . . . . . . PI~K CXST THEORETICAL c,
,,. ,
,
,. J OTAL., . . . . . . . . . . . . . . . .
29,i8 " '
Steel-gray silundurn 69.53 29.89
__
99.42 Silicon carbide 70.22 29.78
__
100.00
Slate-green silundum 63.22 27.85
-
91.07 SirCiO 63.88 27.09
90.97
I t may h e assumed from t h e careful method of preparation, in which only pure silica. sugar-carbon, a n d graphite were used, t h a t t h e compound can contain only silicon, carbon, a n d oxygen. Since a temperature of approximately 1600' C. must be attained for t h e reaction t o begin, practically all t h e air is expelled from the covered crucible; a n d further, since carbon monoxide gas is formed by t h e reaction a n d sweeps out the last traces of air, t h e reaction product can consist only of carbon, silicon, a n d oxygen, a n d their compounds. T h e remaining 8.93 per cent may legitimately he assumed t o be oxygen. T h e percentage of oxygen corresponding t o t h e formula Si4C40 is 9.03 per cent; oxygen b y difference is 8.93 per cent, which is good agreement. .Attempts were made t o determine t h e oxygen content of t h e slate-green variety directly. T h e sample was placed with a pure metal, such as lead, steel, or copper in a vitrified clay boat in a combustion tube of a platinum-wound electric furnace, in which a temperature of 1300' C . can be easily attained. T h e air in t h e apparatus was displaced by extremely carefully purified nitrogen, prepared from ammonium nitrite. T h e idea in view was t h a t t h e molten metal would dissolve t h e substance a n d carbon monoxide or carbon dioxide might be liberated. Arrangement was macle for t h e monoxide t o be oxidized to dioxide by passing through a hot t u b e containing cupric oxide. Owing t o t h e fact t h a t t h e sample in each case floated on the surface of t h e molten metal, no reaction occurred, as evidenced b y t h e fact t h a t t h e K O H absorption apparatus did not gain in weight. .Is regards t h e relationship of silundum t o carborundum. results obtained in this paper indicate t h a t t h e steel-gray variety is a form of carborundum. The following facts may be mentioned: I-Silundum has t h e same chemical composition as car b or u nd u in. -.---Their chemical properties are similar.
