Iron Carbides: Preparation from Carbon-Rich Matrix Morton H. Litt' and Shad M. Aharoni Diuision of Macromolecular Science, Case Western Reseroe Uniuersity, Cleceiand, Ohio 14106 Preparation of iron carbides, from extremely fine iron oxide particles embedded in a carbonaceous matrix, i s described. Under low carbon monoxide pressure, a t 350"C, Fe3C i s formed. At temperatures of 500°C to 1000°C and under high carbon monoxide pressures, FezoCg-type carbides are formed. Treatment of the starting materials under vacuum u p to 800°C caused no chemical change.
T h e r e are four commonly used methods to prepare iron carbides: carburizing metallic iron with CO (Nagakura, 19593; reducing iron oxides with carburizing gases such as CO or CH, (Hume-Rothery, et al., 1942, Leslie et al., 1959, Petch, 1944, Shinjo e t al., 1964, Westgren and Phragmen, 1922) ; heating iron nitrides in a carburizing atmosphere to yield carbides and nitrogen gas (Jack, 1948a, Senateur and Fruchart, 1963); and electrolytic separation of carbidic precipitates from tempered steels (HumeRothery et al., 1942, Leslie et al., 1959, Petch, 1944, Shinjo et al., 1964, Westgren and Phragmen, 1922). I n the present work fine grains of magnetite (about 35 A in diameter) in a carbonaceous matrix were subjected to heating. Carbides were expected, but the details of the reactions and materials found were unexpected.
with a chromium target and half the photographic film covered with a vanadium foil filter. Static magnetic measurements were performed on a Null coil pendulum magnetometer, designed according to Bozorth et al., 1956. Results
Table I lists the samples in which iron carbides were found by X-ray techniques, the respective heat treatments, and the results of the magnetic measurements. The percent of iron in the samples, determined by element analyses, is given and the magnetic moment in emuigram iron. As all the samples contain excess carbon, this last figure is the most meaningful one. Some water sorption, from air humidity, on the large surface areas of the samples was found to cause some inaccuracies in the elemental analyses, manifested in about +3% fluctuation in the Experimental values of emuigram iron. Sample d40-700-90 was not analyzed fully and the 37.9% Fe seems to us to be too The starting material was a dispersion of very fine high. Based on other heat-treated samples, the iron in iron oxide particles in a carbonaceous matrix. The element analysis of Sample ~ 2 showed 6 it to be FelCRjH54N,170,1,~l, that sample should not exceed 35%. Sample 40 had the composition FelCiiH72NOe042, and but the average elemental composition for all the samples Sample #t41, FelC7GHL,NObOli. Three of the heat-treatwas about FelC,jaH.i.,,No,iO ii (except for Sample # 26, the oxygen of the starting material samples was determined by subtraction). The oxide w m mostly F e i 0 4 . The iron Table I. Correlation between Temperature of Heat oxide particles averaged 35 A , or smaller, in diameter, Treatment, Carbide Formed, and Its Magnetic Moment depending on the particular sample. The preparation of HeotEmuig the' starting material is described elsewhere (Aharoni, treatment Emu/g iron, 1969). Sample code % Fe conditions Product" sampleb zt3% To check the effects of oxygen availability on the prod12 2.9 ... 24.6 = 26 ucts, samples were heated for various times a t temp.3 45 FeC 34.0 2 26-350-10 peratures as defined in Table I , in a 25-ml long-necked 84 32.2 ' FeiC 38.3 2 26-350-10 Kjeldahl flask, with air diffusing very slowly t o the samples 48 18.2 ' FeX 37.7 *' 26-350-60 14 through a nitrogen atmosphere introduced into the flask 3.5 24.3 t* 41 154 65.6 " Fe& 43.4 -141-500-30 prior to heating. For heat treatment up to 500"C, the 134 81.8 61 .0 Ti 4 1-500-60 15-cm long, !/.-in. i.d. neck was tilted a t 45" and left 19 4.4 ... 23.2 F 40 open during the heating, but for 700°C heating, a 3-mm 181 68.5 37.9 FeXg, a 40-700-90 i d . , 10-cm long tube was added to slow oxygen diffusion. Fe 142 50.2 ' Fe& 35.3 For the sake of brevity, we shall use the following ~40-700-180 124 55.0 Fe& * 40-700-420 44.4 notation: Sample number 26, for example, heated a t 350" C * F e X Y No data No data 2 26-TGAN o data for 10 min, will be denoted by q26-350-10, and so forth. 1000 Whenever vacuum was applied, it was done by continuous 140-150 150-161 93.4 Fe C' 177 161 pumping with an oil pump to 1.5 mni mercury, with 90.5 FeiC the vacuum measured a t the reaction flask. The chemical "Nature of carbide determined from X-ray diffraction interplanar digestion of samples was done by boiling them for 15 spacings. * Determined at 3 0 P K and in an applied field of 15200 min in concentrated HC1, diluting, washing, repeating, Oe. 'Samples were heated up to 500°C under vacuum to remove volatiles. Flask sealed prior to cooling. Flask cooled open. ' 250and finally drying. X-ray patterns were obtained using M1 flask, cooled open. r: In TGA instrument, nitrogen atmosphere. a Picker Debye-F :herrer 57.3-mm radius powder camera
*
' To whom corrrmspondence should be addressed. 176
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
*Blum and Pauthenet, 1953; Guillaud, 1944; Hofer and Cohen, 1959. ' Hofer and Cohen, 1959.
Table II. lnterplanar Spacings of Carbides Obtained at Temperatures of G 41 -500-30
d, A
I
3.405
vvwb
2.530 2.384
\'W W
2.282 2.220
VW
2.076 2.024 1.983 1.861 1.801
m vs
1.481 1.429 1.327 1.269 1.223 1.214 1.190 1.170
.
4 1-500-60
~
d, A
_
_
I
_
S
_ -__
__
d, A
3.372
2.530 2.390
TGA
40-700-90
I
d,
500' C and Above
= 26- 1000
A
2.517 2.377 2.294
vw w
2.086 2.022 1.971 1.856 1.805 1.763
mw b
W
ms W
VW
mw 2.074 2.027
W
mwb mwb
mw mwb vwb vwh vwb vwb vvwb VS
1.799
1.429 1.268
1.170
2.110 2.065 2.022 1.974 1.864
W
1.760 1.699
vwb vvwb
1.605
vwb
W
W
1.470 1.429 1.324
mu' w w vvvw v vs mw
1.22:1 1.215 1.191 1.169 1.161
VVS
vw Vu' V
W
vs W
wb
w
v U'
1.681
vw
1.j86
VW
1.504
VV\W
1.428 1.325 1.276 1.224 1.214 1.191 1.170 1.163 1.152
vw
W
hkl
d, calcd
230 310 160 340 322 203 410 42 1 430 05 3 00 4 034 500 501 413 353 54 1 005 600 60 1 63 1 642 701 712 722 505 634 535 546
3.418 2.960 2.508 2.389 2.294 2.280 2.238 2.095 2.075 2.019 1.980 1.852 1.809 1.763 1.707 1.677 1.608 1.584 1.507 1.481 1.424 1.325 1.275 1.224 1.213 1.192 1.169 1.162 1.155
I
vs 2.947 2.517 2.353
Fej C 1
m vvw VVW
vvw vvwb wb wb
" Sample 7 40-700-180 yielded practically the same interplanar spacings and relative intensities as Sample 40-700-90. This reflection corresponds to the most intense graphite reflection, 002. ' T h i s reflection is very close to the most intense reflection of metallic iron,
ment products were analyzed fully, and gave the following elemental compositions: 340-700-180, Fe,C-,H,, I: i: 40-700-420, Fe C,& Ki,,,,Oa I; and 41-500-30, F e l C, Ho N,,,O, . On submitting each sample to heat treatment, under these conditions, each gradually loses carbon, oxygen, nitrogen, and hydrogen. T h e loss increases with the duration of the treatment and is larger in the 2 41-500 series than in the 40-700 series, probably because larger amounts of oxygen were present to facilitate formation of volatiles. The variations in composition of the samples are instructive. Allowing for the amounts of hydrogen and nitrogen t o be removed in the form of water and nitrogen, one finds that the rest of the oxygen was removed as. practically, only carbon monoxide. I n the k 40-700 series, with little air access, the oxygen in the sample removed some of the carbon; the latter was removed in such quantity as to indicate t h a t very little cxygen from the air was involved during the first 3 hr. I n the ~ 4 1 - 5 0 0series, however, making the same allowances for hydrogen and nitrogen, we find t h a t more carbon than can be removed by CO generated by the sample alone was missing. This means that in the presence of a greater supply of air, some oxygen reached t h e sample and removed some of the carbon as volatiles. Heating the same starting materials (iron oxide particles suspended in a carbonaceous matrix) under vacuum to temperatures up to 800" C for periods of up to 120 minutes produced no change in the materials, except for minor losses of carbon and hydrogen and lowered water sorption from the air. X-ray diffraction patterns which depended on the treat-
ments, were obtained. The ones obtained from the samples heated under vacuum had up to three very broad and diffuse reflections of Fe.i04, remaining unchanged from our starting materials (Aharoni, 1969). I n no case were there additional reflections that could have been indexed as graphitic or other carbonaceous substance. T h e X-ray diffraction patterns have therefore indicated t h a t the carbonaceous matrix was amorphous. Acid-digested samples of the starting materials lost t h e X-ray reflections they possessed prior to the digestion, and the carbon was completely amorphous. Element analyses, chemical wet tests, and magnetic measurements have all indicated t h a t after the digestion, the samples were practically 1007C carbon. The diffraction patterns obtained from samples heated a t 350°C could be indexed as F e l C . T h e patterns of samples treated a t 500",700", and 1000°C could not be indexed in this pattern, or in any other form of Fe:,C (Leslie et al., 1959; Jack, 1948a). However, when the orthorhombic unit cell given by Jack (1948b) was used for his iron percarbide with FeqCe composition ?nd unit cell dimensions of a = 9.043 A, b = 15.663 A, and c = 7.921 A, Samples g 41-500-30, 41-500-60, 4 40-70090, 4 40-700-180, i40-700-420, and the T G A product of S26-1000, each with about 20 reflections, indexed quite well. This iron percarbide, Fe2"Cq, was once referred t o as Hagg carbide and currently it is referred to as FeiCz (Nagakura and Oketani, 1968). Attempts to index this carbide according t o other unit cells, such as t-carbide (Jack, 1951), Fe2C (Hofer et al., 19491, and FeC (Eckstrom and Adcock, 1950), showed no fit. As can be seen from Table 11, the deviations between the observed and the calculated interplanar spacings were,
*
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
177
in almost all cases. ,,Cc, becomes the stable phase. This is exactly the opposite of the stabilities under vacuum, where the Fe&s ( F e X or Fe-C,) phase decomposes to yield F e C , iron, and graphite a t temperatures of 450'C to 550°C (Jack, 1948'0; Nagakura, 1959; Nagakura and Oketani, 1968; Senateur and Fruchart, 1963). Even though there are minor differences in the specific magnetization values of F e C , FeJoCs, and FeJC, in the literature (Blum and Pauthenet, 1953; Guillaud, 1944; Hofer e t al., 1949; Hofer and Cohen, 1959; Senateur and Fruchart, 1963), the values are in the range of 140-160 emulgram carbide with only minor deviations, a t room temperature. The magnetization values of our carbidecontaining samples, Table I. show that the samples treated a t 330" C were probably not completely converted to carbide, and Sample 340-700-90 shows such high magnetization that it probably contains some metallic iron. The rest of the samples give the right range of magnetization as the literature values when the latter are converted to emu /gram iron. I n conclusion it can be said: Upon heat treatment in the presence of varying amounts of carbon monoxide, small particles of magnetite embedded in a n amorphous carbon matrix react t o form iron carbides. The conversion is temperature-dependent. At 350" C, it is incomplete in the time allowed and results in FelC. At higher temperatures, from 500-1000" C, FejoC9(FesC?)-type carbides are formed as the stable phase, and the conversion seems t o approach completion. The details are in agreement with Cameron and with the accepted mechanism of magnetite reduction by carbon monoxide t o form carbides. literature Cited
Aharoni, S. M., MS thesis, Case Western Reserve University, Cleveland, Ohio, 1969. Blum, P., Pauthenet, R., Compt. Rend., 237, 1501-2 (1953). Bozorth, R. M., Williams, H. J., Walsh, D. E., Phys. Rec., 103, 572-8 (1956). Cameron, D . I . , J . Iron Steel Inst., 189, 251-5 (1958). Eckstrom, H. C., Adcock, W . A.. J . Amer. Chem. SOC., 72, 1042, 1043 (1950). Guillaud, C., Compt. Rend., 219, 614-16 (1944). Hofer, L. J. E., Cohen, E. M., Peehles, W. C., J . Amer. Chem. Soc., 71, 189-95 (1949). Hofer, L. J. E., Cohen, E. M., ibid., 81, 1576-82 (1959). Hume-Rothery, W., Raynor, G. V., Little, A. T., J . Zron Steel Inst., 145, 143P-9P (1942). Jack, K. H., ibid., 169, 26-36 (1951). Jack, K. H., Proc. Roy. SOC.,A195, 41-55 (1948a). Jack, K. H., ibid., 56-61 (194813). Leslie, W . C., Fisher, R. M., Sen, N., Acta Metal., 7, 632-44 (1959). Meroc, J. F., Boulle, A., Compt. Rend., 266, Ser. C., 17703 (1968). Nagakura, S., J . Phys. SOC.Japan, 14. 186-95 (1959). Nagakura, S., Oketani, S., Trans. Iron & Steel Inst., Japan, 8, 265-94 (1968). Petch, N. J., J . Iron Steel Inst., 149, 143P-5OP (1944). Senateur, J. P., Fruchart. R., Compt. Rend., 256, 311417 (1963). Shinjo, T., Itoh, F., Takaki, H., Nakamura, Y., Shikazono, N., J . Phys. SOC.Japan, 19, 1252 (1964). Westgren, A., Phragmen, G., J . Iron Steel Inst., 105, 24162 (1922). RECEIVED for review June 25, 1970 ACCEPTED February 4, 1971
