Thin SolidFilms, CONDENSED
195 (1991) 357-366 MATTER
INVESTIGATION
FILM
357
BEHAVIOUR
OF GALLIUM
AND
INDIUM
DURING
MELTING
V. P. MAIBORODA I.N. Frcmtseairh Imtitulr
(Received
February
ofMoleriuls Science
Problems, Kieo (U.S.S.R.)
12, 1990; revised June 13. 1990: accepted
July IO, 1990)
It has been shown for gallium of 99.9999 wt.% purity that when small weighed amounts of the metal placed in special crucibles are superheated to 400°C in a vacuum of lo- ’ Pa, the acute-angled shapes of the starting sample are retained, and platelike protrusions appear on the surface in the case of partial coagulation. Structural changes in indium 99.98 wt.% purity were studied using transmission electron microscopy and microdiffraction during heating to a temperature of 190 “C above the melting point. We managed to superheat 50-100 nm thick film samples, having an ultrafinedispersion fragmentation into plates in the initial state, to a temperature well above the melting point, with the external shapes being retained. In the case of partial coagulation of the sample, the formation of thin films 3-10 nm thick was observed; the films were similar to solid films in appearance but had an electron diffraction pattern typical of a liquid, with texture elements. The size of the coherent scattering regions was about 2.5 x 15 nm. The chemical composition of the film corresponded with indium.
1.
INTRODUCTION
Diffraction studies of the liquid-metal structure’-3 revealed microscopic regions having short-range atomic ordering. These formed the basis for the development of a theory of a crystal-like liquid structure consisting of ordered microgroups and zones of disordered atoms ‘v3. It is evident that if the zone of disordered atoms is related to a structureless liquid, the microgroups must constitute a second phase, i.e. the single-component liquid is two phase. However, this is in contradiction with the phase rule if it is valid for ultrafine-crystalline systems, and the attempt to eliminate this contradiction by introducing the idea of a short lifetime of “clusters” is unconvincing. This is, above all, due to the fact that the scattering or intensity curves have a fine structure (aluminium, magnesium, silicon, indium, gallium, tin), which reflects not the probability-fluctuation distribution of atoms but a concrete ordering. Therefore, the long lifetime of the microgroups and hence the two-phase structure of a single-component metallic liquid are rather 0040-6090/91/$3.50
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358
V. P. MAIBORODA
admissible. This contradiction can be eliminated by either ruling out clusters from the liquid, or considering the disordered zone to be a compressed gas phase. This phase always coexists as saturated vapours with the crystal phase, and is also present when a large crystal breaks up in some way into extremely small elements. In this case, the saturated vapour pressure increases owing to the “capillary” effect4, which may produce an air cushion between submicrocrystals; the air cushion enables them to slide easily, which is equivalent to a decrease in liquid viscosity. Thus, the macroproperties and microproperties inherent in a “traditional” liquid correspond fully with a metal melt of crystal-like structure as well. One of these properties is the sphere-like smooth surface of a drop, which is due to surface tension forces. Therefore, to isolate structural elements from a superheated melt, their relative sliding, leading to metal coagulation, must be decreased. This can be attained by using a metal having a low saturated vapour pressure and by conducting the experiment at a low external pressure, owing to the fact that the higher the external pressure, the higher the partial pressure4. 2.
EXPERIMENTAL
DETAILS
The investigations were carried out with a scanning Auger electron microscope (JAMP-10s) equipped with an attachment for heating to 650 “C in a vacuum of less than or equal to 10m7 Pa (10e9 Torr). The sample was heated in a crucible (Fig. l), with a tapered surface for the metal to spread out after melting. The metal spreading process at the initial stage must expose the structural elements of the liquid, if any, in the form of protrusions. The oxide and carbon films were removed from the surface with argon ions.
Fig. 1. Crucible
with a sample for heating
The investigations were carried out on gallium of 99.9999 wt.% purity. The Tshaped sample was placed by its central core in the crucible hole and purified with argon ions for about 5 min before the investigation. As can be seen from Fig. 2, the
(b)
(a)
Fig. 2. Micrograph of the starting gallium surface of 99.9999 wt.“,, purity, and at 20’C more at different magnifications: (a) 200 x , (b) 1800 x
surface ofthe sample has “lacerated”edges, which are formed when cutting the metal with partial local melting and crystallization in the form of balls. Figure 3 shows micrographs of the metal surface at 380 “C (parts (a) and (b)) and 400 “C (part (c)), from which it follows that gallium superheated to a temperature 350 “C above the melting point retains a form typical of the solid state. Auger spectrum analysis of the
(a)
Fig. 3. Micrograph
of the same surface when heating
to different temperatures:
(a)(b) 380 C,(c) 400 -C.
V. P. MAIBORODA
360
surface shown in Fig. 3(b) is presented in Fig. 4 and indicates the presence of oxygen, elemental carbon and gallium. After purification with argon ions for 5 min at 380 “C, the oxygen and carbon content decreases to 4-5at.“/,. Oxygen in such quantities cannot form a continuous solid film at the above temperature. The depth of the zone near the surface, which forms the characteristic spectrum, is 0.8-l nm, and, therefore, if one assumes that oxide and carbon films do produce a certain relief on the surface, the height of this roughness is not resolved in the micrograph at the magnification given. Only protrusions that are higher by a factor of 103-lo4 than deposited films are resolved. Therefore, the observed configurations reflect the presence of platelet elements in the structure of the superheated gallium melt. It should be noted that the temperature rise to 400 “C resulted in some coagulation of the metal, but this involved the appearance of platelet elements (Fig. 3(c)) as a stack. A plate-type relief is also observed very distinctly on other surface portions. The melt solidified in the device, i.e. in a vacuum of less than or equal to lo-’ Pa, partially retains its shape and relief (Fig. 5). If one takes a new sample and brings up the melt temperature to 200-400 “C in a lo-’ Pa vacuum, then places the crucible in a sluice chamber, bleeds in air and holds the sample until it crystallizes, the drop becomes oval in shape, and the relief disappears. The purification of the surface with argon does not give rise to any relief. This indicates the samples to be really superheated in the investigation to a temperature well above the melting point. Examination of the data obtained gives certain grounds for believing a metallic liquid to consist, at the submicroscopic level, of microcrystals in the form of bars and
Fig. 4. Chemical composition of the gallium sample surface shown in Fig. 3 (b) at 380 ‘C after purification with argon for 2 min; Auger spectrum analysis. Fig. 5. Gallium
surface after cooling in IO-’ Pa vacuum (2700x).
MELTING
OF
Ga AND In
361
plates which are apparently fragmented into smaller elements, and of a compressed saturated vapour interlayer. Therefore, it is quite reasonable to study structural changes in metals by heating above the melting point using transmission electron microscopy. Such studies were performed on indium. An attempt to carry out the above experiment was first made by Palatnik and coworkers’ and Komnik6,‘. They used electron diffractometry to investigate indium, tin and bismuth films, which were obtained by the condensation method and had a thickness of 1-30nm. They showed the structure factor of scattering (intensity curves) to agree with those obtained in the case of X-ray diffraction from melts’, which was a cause for considering the films to be liquid. It should be taken into account, however, that films produced in a vacuum of approximately lO-3 Pa contain a substantial amount of oxygen and carbon, which, as impurities, can distort the state of the metal. The films were on a substrate during heating, which could hinder their coagulation in melting and melt superheating; there are no micrographs of the films. Finally, a number of intermediate thicknesses (50-100nm) were not examined, at which a different result may be obtained. Therefore, the aim of the present study was to analyse the sequence of structural changes in an indium film of 99.98 wt.% purity, which was thinned to 50-100 nm by the deformation method. This method prevents, in contrast to chemical thinning, the formation of oxide films, and disperses the structure to an extremely high degree. The investigations were carried out with the electron microscope 3MB-100JI equipped with a device for heating to 1000 “C (IIPOH-3Y9). As could be expected, the original substructure of the metal (Fig. 6(a)) is fineplate type, in accordance with which the microdiffraction pattern (Fig. 6(b)) contains “strands”. The plane spacing parameters are 2.72,2.46,2.29 A etc. and correspond to the tetragonal indium lattice (the instrument constant is 97 A mm). When the sample is heated in the microscope to 225 “C and held for at least 10 min at this temperature (the temperature was checked by a calibrated curve dependent on current), changes in the metal substructure are seen, and the two close reflexes assume a rectangular shape, with the “strands” being retained (Fig. 7(a) and(b)). When the holding time is increased and the temperature raised to 290-350 “C, a coagulation of the metal from the central hole to the periphery is observed, which is accompanied by the formation
(4 Fig. 6. (a) Substructure (18000x).
(b) and (b) microdiffraction
of starting
strained
indium
of 99.98wt.9,
purity
(4 Fig. 7. Micrograph
(b) of (a) an indium film and (b) its microdiffraction
at 225 ’ C (18000 x ).
of “transparent” films (Fig. 8(a)) as a single act; these films have electron diffraction patterns with indications of texture (Fig. 8(b)). The plane spacing of the texture maxima is 2.84 A. This value is lower than the plane spacing of the strongest line of In,O,, which is over 2.91 A at about 300°C. Besides, the oxide cannot be formed instantly. There is also a reflex splitting, indicating that the lattice tetragonality increases, and a ring-shaped line appears at low angles, which possibly relates to the
Fig. 8. Bright-field micrograph ofan indium film and its microdiffraction at 290 C, formed at 350 C ((a), (b)), and its dark-field micrograph in the( lOl),‘(l I I)-type double reflex (c)((a) 18000x,(c) 32000x).
MELTING
OF
363
Ga AND In
oxide. A distinctive feature of the “transparent” films (Fig. 8(a)) is the presence of a halo between bright reflexes (Fig. 8(b)), indicating the metal to be amorphous or ultrafine-crystalline. Analysing the image, it should be admitted that the films look like solid objects. However, the thin film state as solid state exists only at OC-5 solid-phase state of pure Tmelt 6-7’ Therefore, the statement that the amorphous, metal exists when it is superheated above the melting point and results from “melting” does not contradict the statement that the smallest structural elements of the liquid are thin films of 3-10 nm thickness, which exist as solids. Examination of dark-field micrographs showed (Fig. 8(c)) that the halo is inherent in the film and that the texture double of (lOl)/(l 1 I)-type reflexes were obtained from coherent scattering regions, which are seen in the dark-field micrograph as dashes joined together into rectilinear colonies. Microphotometering of the luminous spots shows them to be 0.07-0.16 mm wide and 0.5-1.0 mm long. This agrees with the size of the coherent scattering region which is about 2.5 nm wide and 15-30 nm long. There are good reasons to believe that the observed luminous spots correspond with clusters, which are elongate in form in this particular case*. The reflexes of the first ring (halo) correspond to a plane spacing of approximately 2.8 A, which agrees with the radius of the first coordination sphere for liquid indium, and cannot therefore relate to the oxide, which is crystalline up to approximately 850 “C. The dark-field image did not change during observation for 10min. This suggests that the observed coherent scattering regions exist as stable millimicrocrystals. No liquid phase as rolling drops, having an electron diffraction pattern in the form of halo, was found. After cooling the sample it was examined in a scanning Auger electron microscope with a probe microanalyser to check the chemical composition of the surface of the film formed. Figure 9 shows a scanning electron micrograph, where circles denote the places and diameter of probing. Before probing, the film was ionically purified. The chemical composition of the “transparent” film formed comprises 11.1 at.% elemental carbon, 81.2 at.% In and 7.7 at.% 0; the “thick” coagulant is composed of 11.3, 80.9 and 7.8 at.% respectively. These data indicate that the “transparent” amorphous film containing millimicrocrystals has the same surface composition as the more massive portion and consists mainly of indium. For subsequent examinations, the sample was transferred into a transmission electron
Fig. 9. Micrograph of the indium sample surface after high-temperature denotes the Auger probe analysis region (3500x).
studies and cooling.
The circle
364
V. P. MAIBORDDA
microscope JEM 1OOCX equipped with an attachment for probe analysis Link 860/500. Figure 10(a),(b) shows a micrograph of a “transparent” film and its electron diffraction pattern, which is similar to that from the liquid state (Fig. 8(b)) but is larger owing to the higher instrument constant, which is 1008, mm. The plane spacings of the first ring-shaped lines coinciding with the halo are 2.72, 2.46 and 2.29 A, which correspond to the indium lattice. There are no lines relating to oxides. Near the primary bundle there is a ring-shaped line corresponding to the 12.5 8, parameter, which appears to characterize the size of the “transparent” film microcrystalline elements. The substructure of the thick portion of the film (Fig. 10(c)) is fine-disperse and contains twins, as indicated by the “strands” in the electron diffraction pattern (Fig. 10(d)). The plane spacings at an instrument constant of 46 A mm correspond to the indium lattice. The X-ray spectrum (Fig. 11) of the zone shown by a circle in Fig. 10(c) refers to indium. The thin portion of the film gives no characteristic spectrum during probing and analysing with the Link system, apparently owing to its small thickness.
(a)
(c) Fig. 10. Substructure and microdiffraction metal film ((c), (d))((a) 14000x,(c) 19000x).
(b)
(4 of a solidified
“transparent”
film ((a), (b)) and of a matrix-
It should be pointed out that superheating well above the melting point with the retention of the external shapes can be achieved only on samples with a platetype ultrafine-disperse fragmentation, which gives rise to “strands” in the electron diffraction pattern (Fig. 6(b)). This substructural state is attained by forging or pressing a piece of metal down to 0.1 mm thickness. If the above foil thickness is
MELTING
Fig.
OF
365
Ga AND In
Il. X-ray spectrum of the zone of approximately
0.25 mm diameter
of an indium sample (Fig. 6(c)).
obtained by rolling, such samples exhibit no “strands” in the initial state; when a temperature 20-40°C below the melting point is reached, the film is observed to coagulate into a drop. In this case, thin transparent films are also formed at the initial stage, but the process is fast, and diffraction analysis of their elements is difficult. This circumstance was used as an additional method for monitoring the operation of the heating furnace. 3. RESULTS The results obtained give certain grounds for believing liquid metals to consist, at the submicroscopic level, of plate-type solid-like elements containing in some cases ultrafine-crystalline blocks and a compressed vapour-gas medium. It is in this case that the phase rule holds, which forbids the two-phase state in a liquid. At the same time, the above rule is valid when the volume ofthe phases under consideration is large, and surface phenomena can be neglected. In the case of an ultrafine-disperse system, such an assumption is not legitimate, i.e. a deviation from the phase rule is possible. The micrograph in Fig. 8(a) and the microdiffraction pattern with the darkfield image (Fig. 8(b), (c)) indicate “liquid” indium, when strongly superheated, to consist of two phases-millimicrocrystalline and amorphous (having a halo), i.e. there’ isl a ‘second phase equilibrium triple ‘point. The two-hump type oft phonon spectrum’, which is interpreted as a spectrum of two phases, should be compared with the crystalline and with the solid-like amorphous (ultrafine-crystalline) phases. In this case, however, the “liquid” phase as obtained by superheating by about 180 “C does not exist in nature at all at the submicroscopic level. This is possible if one remembers that the formation of bound microgroups by condensation between
366
V. P. MAIBORCDA
atoms of the same sort may be due to exchange interactions which are described by the electron pair model (Geitler, London) or by the molecular orbital model. Bond directionality and energy predetermine the formation of solid-like ultrafinecrystalline elements with greater probability than that of an almost structureless liquid in the ionic subsystem model and of almost free electrons. Such bonds also appear to be retained in melting. 4.
CONCLUSIONS
(1) The melting and superheating to 400 “C of a small weighted amount of gallium of99.9999 wt.‘:‘, purity in a vacuum ofless than or equal to lo-’ Pa result in the retention of the original acute-angled shapes; when there is partial coagulation, protrusions micrometres high and a plate-type relief, which is retained in crystallization in the above vacuum, appear on chemically pure surfaces. (2) When the external pressure is raised to atmospheric pressure, the “drop” assumes a spherical shape, and the surface becomes smooth. (3) When indium is superheated in an electron microscope well above the melting point, the formation of a solid-like, transparent indium film of amorphous or ultrafine-crystalline structure is observed, which has microdiffraction in the form of double reflexes and a halo and contains elongate crystals approximately 2.5 x 15 nm in size. REFERENCES
l 2 3 4 5 6 7 8
V. I. Danilov. Slrucruw and Crystallization ofLiquids, AN USSR Press. Kiev. I956 (in Russian), De Bur. U.spd~i Fiz. Nrruk. 51 (1953) 41. Y. I. Frenkcl, Kineric Theoremo/‘Liquidc. Nauka. Leningrad, 1975 (in Russian). M. K. Karapetyants. Chemicul Thernwd~namkr, Goschimizdat. Moscow, 1953. pp. 228-229 Russian). B. T. Boiko. L. S. Palatnik and N. I. Ridkina. FMM, 13 (1962) 555. Y. F. Komnik. FMM. 16 (1963) 867. Y. F. Komnik, Kri.ctolhgrcrfi?u. II (1966) 213. J. A. Aronovitz and M. G. Stephen, .I. P/T~.c.A. 20 (1987) 2539.
(in