Formation of iron sulfide fine particles by evaporation in argon gas

Journal of Crs stal Growth 34 (1976) 92—102 -, North—Holland Publishing (oinpanv

FORMATION OF IRON SULFIDE FINE PARTICLES BY EVAPORATION IN ARGON GAS T. OSAKA, 11. NAKAZAWA ‘i’, T. I-IATANO and K. SAKAGUCHI ,‘sational Inst inst(~tor Researches in Inorganic Materials .Sakura-niura. Tharaki, Japan Received 28 July 1975; revised manuscript received 26 January 1976

line particles of iron sulfides were prepared by means of evaporation in argon gas in the pressure range of 5 to 100 Torr in order to study the early stages of growth. Bright and dark field transmission electron microscopy images and scanning micro diffraction patterns show: (i) The iron sulfide particles grow in chains of three types, which are commonly observed to turin tar from the evaporation source through the experiments with different argon gas pressures. One of the chains is composed nt hollow particles, which are characterized by two parts of a hollow core and polyerystalline crust. The second chain is composed of well defined particles which have some facets and edges indicating hexagonal symmetry. The third chain is composed of particles which exhibit the characteristic shape of an octopus—pot. The hollow particles were mainly prepared in the lower argon pressure range (5, 35 Torr), while the other two types of particle whose size increased with the argon pressure were grown in the higher pressure range tip to 100 Torr. It was found that the respective particles were coalesced into one long chain by the ferrimagnetic interaction between the particles. (ii) Some film-like products were also observed in the immediate vicinity of the evaporation source.

1. Introduction

at low pressure. This method is suitable for the study of compound systems such as iron decomposition of the deposits. In the present experiments, a flash evaporation technique was applied to preserve the composition of the original iron sulfide within close limits.

Previous work on the low-temperature phase of iron sulfide were carried out on thin films prepared by vacuum deposilion 1 —-31. In the case of geigite, Fe 7S4. grown on a sodium chloride substrate, epitaxial Films exltihit a preferred orientation similar to thaI found ut Ilun films of gold and other fcc metals deposited on ionic crystals t21. Other studies showed that the th in filnis of iron sulfide in the early stages of growth might he composed of multiply twinned parlicles 14.51. Attempts to generate isolated iron sulfide particles by vacuum deposition, however, have not been successful because of an extremely low deposition rale 2]. Detailed studies on ntetal smoke particles were carried out by several workers in recent years [6---9]. The results showed that the fine particles formed by evaporation in inert gases are similar to those formed ott solid substrates in a vacuum. Accordingly, the present investigation was initiated in order to prepare fine particles of iron sulfide by evaporation in argon *

The purpose of the present paper is to describe the result of an examination, by means of electron microscopy and electron diffraction, of the resultant iron sulfide fine particles.

2. Experimental apparatus

Present address: Department of Geoscienees, Marburg-

Stoichiometric pyrrhotite, Fe1 .o5~was synthesized from mixtures of iron and sulfur (Fe/S = I / 1) by the usual sealed silica tube method. The cheniical cootposition of the products was then checked by the method described by Arnold et al. 101. Evaporation was carried out in an oil-diffusion pumped vacuum evaporation apparatus. A working chambe.r was made of stainless steel, of which the total volume was about 75 litres. The arrangement of the evaporation unit for producing fine particles is shown in fig. 1. When the endless tape is rotated at

Universitàt, D-3550 Marburg-Lahn, West Germany.

constant speed, e.g. 4 mm/see, the powder falls into 92

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ature decreased only to the extent of several tens of When degrees. the The evaporant time 5ofsec. fell the into smoke thewas basket, present the in temperthe system was about

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nism were carried iron sulfide out to two smoke supplementary particles: the formatmon experiments (i) As mechaasheet check on Furthermore, theof behaviour ofexamine sulfur vapour, a copper (10 X 10 X 1 rnnt) was fixed on the side wall of the chamber 250 mm from the evaporation source. (ii) To assist in the interpretations of the formation mechanism, use was made of a permanent magnet of 2500 gauss, whose position is near the FI-microgrid shown in fig. I.

a tungsten basket heater. Smoke particles are collected on several nickel microgrids at different distances from the evaporation source. Each grid, marked by the letters A to 0 in fig. 1, was fixed by silver paint on a holder which was made of a stainless steel tube 3 mm in diameter. The holder was cooled with running water in order to prevent the collected products from being decomposed by radiant heat from the evaporation source. The distance between the grids and the source ranged from 10 to 140 mm. The grids above the source were positioned a little off the vertical axis so as to avoid disturbing the smoke current.

3. Experimental procedures The work chamber was evacuated by an ordinary system to the pressure of 1 X 10—6 tori without baking. After closing the valve to the evacuation systern, argon was admitted slowly to the apparatus until pressures of 5, 35 or 100 torr (measured by mercury manometer) were achieved. The source ternperature was kept at 1900°Cbefore evaporation,

4. Experimental results The electron microscope revealed that the iron sulfide smoke particles were almost all crystals of pyrrhotite of nonstoichiometric composition Fei~S, except for some rare products described later. The appearance of the aggregated particles was quite different depending upon the collection position (fig. 1). For example, deposited film like textures were formed in the immediate vicinity of the evaporation source (C and D in fig. 1). Isolated particles were obtained from the second area (E, F, and G in fig. 1). Finally, the chain-aggregated particles were always observed in the G-position and the residual region. The collection ranges, within which the chainaggregated particles were absent varied with changes of the applied argon pressure as indicated by B—H, C—F and C—Din fig. 1 for 5, 35 and 100 torr, respectively. Since the smoke was invisible to the eye in these regions, particle aggregation is presumed to account for the visibility of the smoke in the more distant regions. The smoke particles are, therefore, roughly grouped into two groups, the chain-aggregated and the isolated particles.

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Fig. 2. (a) Electron micrograph of chain-aggregated hollow particles. (b) Electron diffraction pattern taken from chain-aggregated hollow particles. (c) Electron mnicrograph showing the contrasts between the inner and outer region of a hollow particle. (d) Electron micrograph of hollow particles including a high density core. The arrows indicate typical profiles of the core.

4.1. Ghain-aggregated particles The shapes of each particle were similar within each chain, but varied from chain to chain. The partides were classified into three types based on the electron microscopic observations,

4.1.1. Hollow particles Figs. 2a---2d show typical electron micrographs and a diffraction pattern of the smoke particles of the type collected on the L-microgrid in fig. 1 (35 torr). A characteristic of this type is that each particle is composed of two or three parts of quite different densities. The outer, dense crust of the partide was confirmed tobe a polycrystalline pyrrhotite by using a scanning micro diffraction technique at an accelerating potential of 60 kV (fig. 3b). Fig. 3a is the STEM image displayed simultaneously on the CRT when the microarea electron diffraction pattern was observed on the fluorescent screen. The inner, less dense core of the particles is, however, extremely porous or possibly empty, since the contrast between the inner and outer region of the particles is extremely high in the electron niicrographs (fig. 2c). Some of the particles include a higher density core as indicated by arrows in fig. 2d. This consisted of a single crystal as confirmed by a scanning micro diffraction (figs. 3c and 3d). This was also confirmed by dark-field image, which showed the part of the hollow particles contributing to a particular diffracted

beam to be the high density core. The X-ray spectrum obtained from the same field of the core indicated that the chemical composition is somewhat higher in iron content relative to that of the outer crust. The chemical inhomogeneity of this type of particle is obviously illustrated by the broadness of the powder diffraction pattern in which the d(102) line indicates the wide composition range around an average cornposition of about Fe

9S10. Most of the hollow chain particles were observed at the lower argon gas pressure. The variation of the dimensions of the particles within a single chain was small as shown in fig. 2a. In addition, the higher density core often appeared in the particles produced at 5—35 torr and rarely in those at 100 torr. The size of the particles changed with applied pressures in the ranges from 100 to 400 A, 150 to 800 A and 200 to 500 A at 5, 35 and 100 torr, respectively. The optimuni pressure for growth of the hollow particle is, therefore, fairly low, probably close to about 35 tori.

4.1.2. Faceted particles Figs. 4a—4c are a typical electron micrograph, a STEM image and a diffraction pattern of the par. tides of this type, respectively. This particle exhibits a well defined crystal habit, showing that they are single crystals with hexagonal symmetry. This was confirmed by a scanning rnicrodiffraction of some particles (fig. 4c). The powder diffraction pattern for many particles

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indicates that the particles are chemically hornogeneous with composition of about Fe 7S8, as estimated fromri the d(l02) values, which ranged between 2.06 and 2.07 A. The size of these particles increased with applied pressure in the ranges from 200 to 400 A, 200 to 600 A and 500 to 1000 A at 5, 35 and 100 torr, respectively.

4.1.3. Octopus-pot particles Particles of this type exhibit the characteristic shape of an octopus-pot as shown in fig. 5a. The characteristic is as follows: (i) Some poor facets appear partly on the surfaces of the particles (fig. 5a, A). (ii) Small bubbles are observed in the inside of the

particles (fig. 5a, B). (iii) The hollow or the denser part is present at the top of the pot-like shape (fig. 5a, C). A dark field image taken by using the d(102) line of the powder pattern proved that the particles are single crystalline except for the top of the pot-like shape (fig. Sb). The sharp rings observed in the selected-area diffraction pattern (fig. Sc) indicate that the particles are pyrrhotite with a rigid chemical composition of Fe7S8, as estimated from the d(102) lines of 2.06 A. The particle size increases with pressure in the ranges from 200 to 400 A, 300 to 600 A and 300 to 1000 A at 5, 35 and 100 torr, respectively. No significant correlation between the shapes of

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these three particle types and the positions where the particles were collected was found. Both chains of the octopus-pot and faceted particles, however, are short and become entangled. The lengths of chains varied from 0.3 to 5.0 pm, 5.0 to 10.0 pm and 0.2 to 0.3 pm for 5, 35 and 100 torr, respectively,

4.2. Isolated particles Figs. 6 and 7 are transmission electron micrographs showing iron sulfide particles deposited on an amorphous carbon substrate. The former particles (fig. 6) exhibit an island structure similar to that commonly

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both figs. 6 and 7 very small particles (all of the partides are ca. 100 A in diameter) are found. The island and the flower-like particles give diffraction patterns corresponding to the troihite structure which is mdicated by d(lOO) = 5.16 A based on the 2C-type superstructure line, as shown in l’ig. 8. For the S amid 35 lorr cases both of these particles appeared in the vicinity of the evaporation source, i.e. in C-F positiomis (5 torr) and C—D (35 torr), respectively, but f’or the 100 torr the flower-like particles disappeared in all regions, though the cause is not apparent yet. In addition to the island particles for the 100 torr evaporations, marcasite and pyrite, FeS, (figs. 9a. 9h) and a new iron sulfide with a cementite structure were

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atthepositionsE,FandGinfig.lunder35torr. They were theregard same as chain-aggregated particles with to the the foregoing morphology amid chemical composition. Fig. 10 shows a typical electron micrograph of this type of particle. Furthermore, mixtures of the isolated and the chain-aggregated particles at first appear in the position G at 35 torr.

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and in an inert gas [61,the observed iron sulfide fine particles are not related to multiply twimined particles as was expected of the epitaxial films of iron sulfide. However, two features of the observed particles were brought into relief; (i) nonstoichiometry of the particles, and (ii) chain formation.

5. Discussion Despite the structural similarities between fcc metal particles formed by vacuum deposition [4,5]

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very fine, island and flower-like particles as shown in figs. 6 and 7 were not observed within the residual region except the immediate vicinity of the evaporation source, indicates the growth of them on the substrate. As is apparent from the diffraction pattern of fig. 8, the above region is chemically most homogeneous at an optimum condition of 35 torr and is filled with the vapour of chemical composition corresponding to the starting materials. On the other hand, isolated particles (fig. 10) and chain-aggregated

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particles (figs. 2a—2c, 4a, 5a) grown in hollow, faceted and octopus-pot types within the second and the residual regions respectively, are composed of pyrrhotite except the 2C-type, i.e. somewhat high sulfur content in comparison with the island and flower-like

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particles. Validity of the result is supported by one of the supplementary experiments as stated before: a copper sheet, placed on the inside of the chaniber wall changed colour from metallic to dark violet during smoke forrnation. The film was covellite, CuS [13], which was identified by electron diffractiOn and electron probe microanalysis after being isolated from the surface by immersion in dilute nitric acid. Thus, sulfur vapour is liberated during smoke formation. The nonstoichiometry of the partides, therefore, is due to the presence of this free sulfur.

by the collision between isolated particles below the Curie point.

Acknowledgements The authors are indebted to Dr. M. Iwata and Dr. S. Horiuchi for valuable discussions throughout this work, to Dr. S. Aida and Mr. M. Suzuki, JEOL, Ltd., for their cooperation in producing the STEM images.

References 5.2. Chain formation Ill H. Nakazawa and K. Sakaguchi, Mineral. J. 6 (1972) The nonstoichiometric, Fei_~S(X = 0.065 0.125) exhibit ferrimagnetism when the temperature decreased below about 3 10°C[14]. The chain formation mechanism of iron sulfide fine particles is considered to be similar to that of the ferromagnetic metal particle [15—17]. Because the mechanism can be illustrated by the observation as shown in fig. 11: in the figure the elongated portion of the chain, which was collected on the H-microgrid during formation with the magnet, contacted frequently in the direction of the magnetic field. When the chains collected in the absence of magnetic field put into the field, parallel rearrangements to the field did not appear. It seems reasonable to assume that the final stage leading to the chain formation may be achieved

458. 121 H. Nakazawa,58 T.(1973) Osaka and Mineralogist 962.K. Sakaguehi, Ant. 131 H. Nakazawa, T. Osaka and K. Sakaguehi. Nature Phys. Sci. 242 (1973) 13.

141 S. lno, J. Phys. Soc. Japan 21(1966)346. 151 K. Mihania and Y. Yasuda, J. Phys. Soc. Japan 21 (1966) 1166. (1967). N. Wada, Japan. J. AppI. Phys. 6 (1967) 553. N. Wada, Japan. J. AppI. Phtys. 7 (1968) 1287. S. Yatsuya, S. Kasukabe and R. Uyeda, Japan. J. Appl. Phys. 12 (1973) 1675. R.G. Arnold and L.E. Reichen, Am. Mineralogist 47 U962) 105. JEOL News, lOe (4) (1973)2. T. Osaka and H. Nakazawa, Nature 259 (1976) 109.

161 K. Kimoto and I. Nishida, Japan. J. AppI. Phys. 6 171 181 191 110)

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131 AS’I’M (~ardNo. 6-0464. Haraldsen. Z. Anorg. Atlgem. Chem. 246 (1941) 169. 195. 15) A. 1’asaki, S. Toniiyama, S. hida, N. Wada and R. Uyeda. Japan. J. Appi. Phys. 4 (1965) 707.

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1161 T. Tanaka and N. Tamagawa, Japan. J. AppI. Phys. 6 (1967) 1096. 1171 K. Kusaka, N. Wada and A. Tasaki, Japan. J. Appl. Phys. 8 (1969)599.