- Email: [email protected]
Changes in the chemical reactivity of metals exposed to an inert gas glow discharge
Vacuum/volume 33/number Printed in Great Britain
Changes exposed D F Klemperer
5lpages 301 to 305/l
0042-207X/83/050301 -05$03.00/O Pergamon Press Ltd
983
in the chemical reactivity of metals to an inert gas glow discharge and
D J Williams,
Department of Physical Chemistry, University of Bristol, Cantocks Close,
Bristol BSB ITS, UK received 15 October 1982
From time to time the literature mentions curious effects on the chemical reactivity of metals due to inert gas ion bombardment: reactivity in corrosive environments is variously said to be inhibited or enhanced. Although there is no obvious explanation for such effects, some possible mechanisms have been suggested. We have carried out a few simple experiments designed to demonstrate that reactivity effects really do exist and to test such mechanisms as have been proposed. The results are qualitative because a glow discharge was used to implant the rare gas ions. Evaporated films of aluminium and nickel become amorphous after bombardment with xenon ions and the films resisted gaseous and liquid corrosion. On the other hand, aluminium foil bombarded with xenon ions in a Penning pump arrangement was attacked more heavily than unbombarded aluminium. We attribute passivation to the known lack of reactivity of amorphous metals. Glassy materials appear to lack the normal routes of attack between their subsurface regions and the attacking medium. On the other hand, when a metal surface is heavily ion bombarded the surface is probably damaged to such an extent that the attacking medium gains physical access to the interior and corrosion proceeds rapidly.
1. Introduction Glow discharges in various gases are being used to an increasing
extent in etching and sputtering applications. Bombardment with inert gas ions is also commonly used to clean surfaces and to enable depth profiles to be obtained. Any effect on the chemical reactivity that arises purely from the impact of ions is therefore of interest. Investigations over the last 20 yr have shown that metals frequently exhibit increased resistance to chemical attack after they have been bombarded with inert gas ions3’. A number of theories have been proposed to account for this phenomenon, but no one theory will explain all the results. The chief theories are: (1) A polymerized hydrocarbon film is deposited on metallic surfaces during ion bombardment’*‘. This film is formed when the ion beam polymerizes ambient gases over the surface3’. (2) Ion bombardment causes an increase in the thickness of the air-formed oxide layer. The factor responsible for enhanced film growth has not been identified3v4. (3) Inert gas occluded in the superficial regions of the metal crystal hinders migration to the reacting surface between metal and corroding medium. Alternatively, the epitaxial relationship between the metal and the growing product ofchemical reaction is altered’*‘*6. These theories cover experimental data for a range of metals for which chemical attack was inhibited by implanting them with He, Ne, Ar and Xe. In this paper we report similar resistance to chemical attack for clean evaporated films of Al and Ni after Xe ion bombardment in a glow discharge. Reduced reactivity is
linked with the amorphous structure that ion bombardment has conferred onto these metals. Bombardment with inert gas ions, however, can enhance, as well as inhibit the chemical reactivity of metals6”.2g. It appears therefore that bombardment can cause different types of structural alteration in metals and the bombardment parameters are important. With this in mind we also report an enhancement of chemical reactivity for Al foil after strong bombardment with Xe ions in a Penning pump arrangement. Increased reactivity is associated with severe radiation damage.
2. Experimental Aluminium and Ni films evaporated in ultrahigh vacuum apparatus were used in the passivation experiments. The evaporants were 99.9995% Al wire supplied by Goodfellow Metals Limited and Marz grade Ni wire supplied by Materials Research Corporation. Evaporant was simultaneously deposited onto the wall of a spherical 500 cm3 Pyrex glass vessel and onto microscope coverslips placed inside the vessel. The vessel was maintained at 273 K during evaporation in uacuo and 78 K during evaporation in Xe. Films were a few hundred A thick. Pure Research Grade Xe, supplied by British Oxygen Company, was incorporated into the film in two ways: (a) The film was evaporated in ULICUO. Then a 3 kV glow discharge was struck in 2 x 10S3 torr Xe between the centrally located evaporator and the film, with the film as cathode. The discharge flux was about 15 PA cmd2 of film. Electrical contact to 301
D F Klemperer and D J Williams: Changes
in the chemical reactivity of metals
the film was made with a small piece of Pt foil. The pressure
of Xe was manostatted with a reservoir of solid Xc at 78 K situated elsewhere in the apparatus. (b) The film was evaporated in 2 x low3 torr Xe onto the walls of the vessel cooled to 78 K. After evaporation, the film was warmed to room temperature. Bodys and Campbell” have shown that appreciable quantities of rare gas are retained in the metal by this technique and we’ have measured thermal desorption spectra for Xe on heating our films above room temperature. Indeed, as in the cast of metals bombarded with rare gas ions’“, the metal must bc melted to relcasc the last part of the incorporated rare gas. The filmed coverslips were used to test chemical reactivity. Aluminium film was dipped into aqueous 1 wt% HgCI, solution for 1 min, and washed. This amalgamates the oxidized Al surface with mercury and leads normally to the rapid and intense growth of fibrous alumina when left in moist air’ ‘. Nickel film was simply covered with a mixture of 1 part concentrated nitric acid and 4 parts water. Transmission electron diffraction patterns and electron probe microanalyses were obtained for the films in a Jeol JEM-100 CX electron microscope. Aluminium foil of 0.25 mm thickness (Marz grade, supplied by Materials Research Corporation) was used in the reactivity enhancement experiment. The foil was subjected to Xe ion bombardment by mounting it in place of the two Ti cathode plates in a small glass ion pump with an electrode arrangement of the the cathode plates type described by Hall I2. In this arrangement sandwich a cellular Ti anode and the whole is placed in a strong magnetic field. A 3 kV discharge was struck and manostatted in 2x 10-3 torr Xc. The discharge flux was about 15 ,rcA cm-’ of Al cathode. Cathodes assumed their characteristic etch pattern as Al was sputtered during the discharge. The etch pattern is made up of the etch pits which form where the high-density parallel ion beams defined by individual cells bombard the cathodes. Bombarded AI foil was tcstcd for chemical attack in the same way as AI film.
Figure 1. Oxidation of thin Al film. A and B were ion bombarded in a Xc glow discharge and have been passivated (metal shows up black). C was evaporated in Xeat 78 K and D wasevaporated in vacuum (metal that has oxidized away shows up white).
3. Results Figure 1 shows the result of a reactivity test on four Al films evaporated onto microscope coverslips. The photograph was taken a day after the HgCl, dip, although the result was essentially the same within a minute. Film A was evaporated in lo-’ torr and Film B in lo- 5 torr of residual gases; both films were subjected to a glow discharge for 30 min and neither shows much sign of oxidation. (Metal appears black and opaque: oxide appears white and transparent.) Film C was evaporated in Xc at 78 K; it readily oxidized away. Film D was uvaporarcd in IO -’ torr and scrvcd as a control; about a third of it oxidized right through. Figure 2 is the electron diffraction pattern for Film B after bombardment. It shows that the film contains regions of amorphous structure. In addition to the diffuse Al rings there is an inner ring which cannot be assigned (d = 3.6 + 0.3 A). Film C also gave an amorphous electron diffraction pattern (Figure 3); in this case the rings are so diffuse that no extra features can be discerned. Film D gave sharp diffraction rings which tallied exactly with Al and showed no extra features. Electron probe microanalysis of Film A revealed the presence of trace of W, Pt and Xe as impurities. Figure 4 shows the result of a reactivity test on three Ni films. The photograph was taken 3 h after addition of acid. Film E was evaporated in 10m9 torr and subjected to a glow discharge for 30 min. The film has not dissolved in acid to any appreciable 302
Figure 2. Elccrron dtffraction pattern Tram Al lilm aflcr Xc IOII bombardment.
extent. On the other hand Films F and G dissolved
readily. Film F was evaporated in Xe at 78 K and Film G was a control, evaporated in IO-” torr. Figure 5 is the electron diffraction pattern for Film E after bombardment. As in the case of bombarded Al, the film contains regions of amorphous structure and there are additional features which cannot be assigned. Inside the diffuse Ni rings there are at least one complete ring and two arced rings (for which the d values are about 2.5, 3.5 and 6.9 A respectively). In contrast to the case of AI, Film F, as well as Film G gave sharp diffraction rings which tallied exactly with Ni and showed no extra features.
D F Klemperer
and D J WiNjams:
Changes in the chemical reactivity of metals
Figure 3. Electron difliaction pattern from AI film evaporated in Xe onto glass at 78 K and warmed to room temperature.
Figure 5. Electron diffraction pattern from Ni bombardment.
film after Xe ion
Electron probe microanalysis of Al foil after bombardment revealed no impurities except for Ar, which was present in the original material’ j.
4. Discussion
Figure 4. Acid attack on thin Ni film. E was ion bombarded in a Xe glow discharge and has been passivated (metal shows up black; lifting has occurred). F was evaporated in Xe at 78 K and G was evaporated in vacuum (metal dissolved away).
Figures 6(a) and (b) show the result of a reactivity test on Al foil bombarded for 30 min in the Penning pump glow discharge. Figure 6(a) shows a piece of foil containing nine etch pits photographed from above. Abnormally heavy oxidation in the etch pits has led to the columnar growths shown in the side view, Figure 6(b). This growth may be compared with the normal fibrous growth on the unetched ‘cross-bars’ of the foil pattern (and on the reverse side of the foil, not shown). On the other hand, there is no oxidation at all in regions immediately surrounding the etch pits where only light Xe ion bombardment occurred. These shiny metallic regions, which stay bright indefinitely, appear white in Figure 6(a). The photographs were taken a few minutes after the HgCl, dip.
Aluminium Film A was prepared under ultra-high vacuum conditions and bombarded with pure Xe ions, yet this film resisted oxidation as did Al Film B which was prepared under poor (unbaked) vacuum conditions and then bombarded with an appreciable background r of common residual gases present. Passivation is therefore unlikely to be due to a protective layer of polymerized hydrocarbon. Again, no passivation was observed for Al Film C and Ni Film F, both of which contained Xe incorporated by the evaporation process. The inert gas per se cannot, therefore, be responsible for passivation. Our Al films were subjected to oxidation experiments in which the air-formed oxide layer ceases to protect the metal. Since Xe ion bombarded Al Films A and B were both passive, it is unlikely that any air-formed oxide layer is responsible for passivity. On the other hand, passive Al and Ni films that had been bombarded with Xe ions contained amorphous regions. It is not possible to distinguish between amorphous regions mixed with normal crystallites and crystallites whose surfaces are uniformly amorphous. Nevertheless the latter situation is more likely to arise in a glow discharge because all regions of the film surface are subjected to an ion flux. We conclude that ion bombardment made the surface of our films amorphous and that the amorphous metals become chemically unreactive. Nevertheless Al Film C, which was evaporated in Xe, was also amorphous yet it oxidized readily. We return to this seeming inconsistency below. It is well known that ion bombardment can make crystalline materials amorphous. Internal ionic bombardment by alpha particles, for instance, will make radioactive crystalline minerals amorphous14, producing the so-called metamict state15. It is also known that Ar ion bombardment of diamond and graphite 303
0 F Klemperer and D J Williams: Changes
6(b)
in the chemical reactivity of metals
.d
Figure 6. Enhancement and inhibition of chemical reactivity of thick AI foil by Xe ion bombardment. The Al foil was made the cathode of an ion pump with an ‘egg box’anode. The foil was then oxidized. (a) Top view, (b) side view. Abnormally heavy oxide growth (columns in (b)) has occurred in the nine etch pits (shown in (a)) where the ion flux was most intense. However, the metal remains unattacked and shiny around the etch pits (white areas in (a)) where the ion flux was light. Normally-oxidized metal shows up as the black background.
produces glassy carbon16. Poate17 and Grant’s have reviewed work demonstrating that implantation with a variety of ions can produce metastable amorphous alloys from crystalline metals, and implantation techniques have been used to obtain amorphous layers in direct contact with a single crystal substrate19. Evidence that amorphous metals are unreactive has also been accumulating3’. Chemically resistant amorphous Fe alloys (metallic glasses) can be prepared by very rapid quenching from the liquid” (splat cooling). Vapour quenched and sputtered metallic films have also been obtained in the amorphous phase and they show increased resistance to chemical attack2’, whilst glassy carbon is relatively unreactive to oxygenL6. Implantation with a variety of ions therefore appears to be capable of producing amorphous, chemically-resistant surface alloys of target and 304
implant atoms22, whilst bombardment with inert gas ions may inhibit the reactivity of an otherwise pure metal if an amorphous layer is formed as a result of bombardment. The questions now arise as to why amorphous metal should be unreactive and how ion bombardment can enhance as well as inhibit reactivity on adjacent areas of the same specimen. We believe that during ion bombardment the structure of the target metals changes to a porous amorphous structure (of low specific density) via a glassy or liquid-like state. The mechanism for these changes is thought to be the formation and rapid coolmg of thermal spikes so that the surface initially takes on the structure of a splat-cooled metallic glass. Then, as bombardment proceeds, this glass structure becomes progressively damaged forming a more open structure which is consequently very reactive because attacking species can easily diffuse into the bulk metal. It will be possible to obtain structures which are glassy (as in the cast of our ion bombarded metal films) and/or porous (as in the case of our ion bombarded foils) depending on the ion dose. The oxidation of Al begins with the chcmisorption of oxygen onto the metal surface. This rapid chemisorption is followed by a slower uptake whilst oxygen diffuses into the bulk and is incorporated as oxide. The amorphous structure is unlikely to affect the initial rapid chemisorption of oxygen, but it may affect the absorption of oxygen into the bulk metal. One model for slow uptake of oxygen on AlZ3 involves an activated place cxchangc between adsorbed oxygen atoms and underlying Al atoms, the whole being in a crystalline array. In this manner oxygen can diffuse into the bulk but the process will be inhibited if the crystalline structure of the metal is destroyed. Alternatively, the slow uptake of oxygen could be assisted by imperfections such as grain boundaries and dislocations running away from the surface and along which oxidation can proceed. Again, ion bombardment can serve to destroy such diffusion paths. Further or conccntratcd bombardment, however, causes radiation damage on such a scale that the migration of oxygen into the bulk is accelerated and oxidation is rapid. The extra features on our electron diffraction patterns for films bombarded with Xe ions suggest that the implanted gas is capable of introducing a new periodicity into the structure. Gas bubble superlattices are, in fact, known to form when He ions and protons arc implanted into a variety of metals at 300 K*‘. Helium implanted in fee metals like Ni, for instance, forms bubbles of about 20 ?, diameter arranged in a fee superlattice with a lattice constant of about 6.6 nmz5. Implanted Xe might also be located on a superlattice which gives rise to the extra diffraction features. Clearly more careful examination is needed but if we draw an analogy between occluded Xe atoms or microbubbles and point defects then it is worth noting that point-defect aggregation can be energetically more favourable than a random distribution of isolated point defects, even at low concentration. The situation brings to mind the crystallographic shear phases which have now been investigated in a wide range of nonstoichiometric materials26. Certainly the extra diffraction features cannot be assigned to any impurities known to be present. Theimpurities introduced to the film during theglow discharge, however, might conceivably affect the chemical properties of our specimens. The Pt impurity was due to sputtering of the electrical contact to the film and the W impurity found to be present in Al must be sputtered from the W evaporation filament by negative ions in the plasma. On the other hand, the introduction of impurities into our foil specimens during the glow discharge is much less likely. Any impurity is also unlikely both to enhance
D F Khmpwer md D J Williams: Cham
in the chemical raactivitY of metals
and to inhibit reactivity in juxtaposition on the same specimen of Al foil. We plan to repeat our bombardment experiments with evaporated &ns using a redesigned electrical contact, and an ion gun in place of the simple glow discharge set-up. Quantitative information concerning the numerous parameters which detcrmine the ion bombardment process, such as the charge of the incident ion, the ion energy, and the angle of incidence of ions on the target surface cannot be obtained by glow discharge mcthods2’. Some of the undesirable effects in a glow discharge such as multiple collisions of gas ions leading to reduced ion energies and random angles ofincidence of ions on the target are reduced but not eliminated in a Penning pump type of arrangement; this is capable of concentrating and intensifying the incident flux onto small areas of the cathode. Finally, we consider the structure of a metal film that has been evaporated in Xc. A diffuse diffraction pattern was obtained (at room temperature) for Al Film C which was evaporated in Xc at 78 K, yet a sharp pattern was obtained for Ni Film F, which was also evaporated in Xc at 78 K. Ready oxidation (rather than passivation) of the amorphous Al Film C is accounted for by drawing an analogy between the reactive, open structure of metal that has been subjected to concentrated ionic bombardment and the much-faulted, imperfect structure of a metal deposited at 78 K in Xc. As the 6lm deposits, so Xe atoms arc physically adsorbed on the growing layer and become occludedwith attendant intra- and extra-microparticle lattice mis6t.t. On warming, occluded Xc is not released smoothly. Electron microscopy and other techniques reveal that the Xc pressure peaks in thermal dcsorption mass spectra reflect structural and phase changes occurring in the material from which Xc is being r&ascd9*2E.In general, one of the Xc pressure peaks may be associated with the change from amorphous to crystalline structure as the sample is warmed. Aluminium and Ni evaporated in Xc arc both thought to be amorphous at 78 K. Xenon is dcsorbed from both metals as the 6lm is warmed from 78 K. This leaves a porous, reactive structure in the host metal. As Xc is progressively dcsorbed, the open structure becomes unstable and resorts to its crystalline state when a characteristic temperature is reached. This change has been observed at 373 K for Al9and we assume that it occurs below room temperature for Ni.
5. chnclusIons
(1) Metal surfaces subjected to a light Xc ion bombardment become amorphous and liquid-like in structure. Heavier, conccntrated bombardment damages the surface and physically opens it. (2) The amorphous liquid-like metal surface is chemically unreactive. After severe radiation damage the surface is readily attacked. (3) Metals evaporated in Xc at 78 K contain occluded Xc. Their structure can be amorphous at room-temperature but chemical reactivity is unimpaired.
ACkOOWkdg-
We thank Dr D W Thompson for the electron diffraction pictures and for electron probe microanalysis. We also thank Prof J M Thomas for his useful discussion. A Science Research Council maintenance award to DJW is gratefully acknowledged. Ref8rences ‘WJMoorr,SR~~LCtutha~dSNBrbwn,Lr~~~r Joniaue. Thtforieset ADD~~C~U~OUS, editions du CNRS, Paris, p 35 (1962). o&a&ova, soo Tech PhysLett, 2,199 (1976). aYMKhirnyfandLNK ‘VAuhworth.DButa.WAGMtRPMP~~~~dTCWellington, Ion Beam Im&Uat&n Semiwndds and Other Matrrirrl.9, PI&Urn, New York, p 367 (1975). ‘V~hwoRh,WACLGrPIlSRPMPrOCtMPDdTCWellington,Carosion Sci. 16,363 (1976). ’ E D Hondros and I JHnard, CR Acad Sci, Par&, 254,1043 (1962). 6 S Muhl, R A Collins and G Dearnaley, Applicarions oIlon Beam to MatrrioLr1975, Inst Phys Conferenoc Se&s No 28, London, p 147 (1976) ’ P D Goode, Applicationsof Ion Beams to Materials, 1975. Inst Phys Conference Series No 28, London, p 154 (1976). ’ J W S Bodys and K C Campbell, Iru J Appf R&at Isotopes, 24,107 (1973). 9 D F Kiomncrcr and D J Williams, to be published. r” C W ‘Tucira and F J Norton, I Nucl M-at, 2,329 (1960). I1 J H L Watson. A Valleio-Frcirc. P de S Santos and J Parso- KoUoid2. 154,4 (1957). ’ . l2 L D Hall, Science, 128,279 (1958). L D Hall, Reu Sci Insmen, 29,367 (1958). t3 We have found that Al from various sourocs contains s@ificant quantities of hr. Our Al foil (Mars grade, Materials Research Corporation) contained 0.2 wt% of Ar. Beer can ring-pulls contained as much as Zwt%ofAr. r4 R C Ewing Am Mineral, 68,728 (1975). ts A Pabst, Am Mineral, 37,137 (1952). t6 S Evans and J M Thomas, Proc Roy Sot, A3S3,103 (1977). ” J M Poatc, .I Vat Sci Technof, 15,1636 (1978). I6 W A Grant, J Vat Sci Technol, 15, 1644 (1978). I9 S S Lau, J Vat Sci Technof, l&l656 (1978). so K Hashimoto, K Gsada, T Masumoto and S Shimodaira, Corrosion Sci, 16,71(1976). K Asami, T Masumoto and S Shimodaira, CorrosionSci, 16,909 (19761 s’-P J Grundy and J M Marsh,J Mater Sci, 13,677 (1978). ” J K Hirvonen. J Vat Sci Technol. 15.1662 (19781. 23 h4 A H Lanyon and B M W TraPd&ProcR;y So;, AZ27,387 (1955). E E Huber and C T Kirk, St@ce Sci, S, 447 (1966). ** P B Johnson and D J Mazey, J Nucl Mater, 93 and 94,721 (1980). Is P B Johnson and D J May, Radiat Efects. 53,195 (1980). 26 R J D Tilley, Chemical Physicsof Solidsand theirSugkces (Editedby M W Roberts and J M Thomas). Royal Society of Chemistry, London, p 121 (1980). *’ M Kaminsky, Atomic and Ionic Phenomena on Metal Surjbces. Academic, New York (1965). *’ V Balek, J Mater Sci, 4,919 (1969). V Balek, J Mater Sci, 5,166 (1970). 29 V A&worth, R P M Procter and W A Grant, Ion Impkrmotbn (Edited bg J K Hirvoncn). Academic, London, p 175 (1980). ’ J A Davies, unpublished work It was recently shown to the authors, for instance, that uranium treated in an argon plasma can become more m&ant to moist air than nickel-plated uranium. The same effect was mentioned b P Haymann, Le Bombardment lonique, The’aies et Applications, ~tionsduCNRS,Paris,p25(1962). ‘t J Goodman, J PolymerSci, U,551(1960) H Yasuda and C E Lamarc, J Appl Palm Sci, IS, 2277 (1971). 32 K Hashimoto and T Masumoto,UltrarapidQuenchingofliquid Alloys (Editedby H Herman) Academic, London, p 291 (1981)
in
305
Changes exposed D F Klemperer
5lpages 301 to 305/l
0042-207X/83/050301 -05$03.00/O Pergamon Press Ltd
983
in the chemical reactivity of metals to an inert gas glow discharge and
D J Williams,
Department of Physical Chemistry, University of Bristol, Cantocks Close,
Bristol BSB ITS, UK received 15 October 1982
From time to time the literature mentions curious effects on the chemical reactivity of metals due to inert gas ion bombardment: reactivity in corrosive environments is variously said to be inhibited or enhanced. Although there is no obvious explanation for such effects, some possible mechanisms have been suggested. We have carried out a few simple experiments designed to demonstrate that reactivity effects really do exist and to test such mechanisms as have been proposed. The results are qualitative because a glow discharge was used to implant the rare gas ions. Evaporated films of aluminium and nickel become amorphous after bombardment with xenon ions and the films resisted gaseous and liquid corrosion. On the other hand, aluminium foil bombarded with xenon ions in a Penning pump arrangement was attacked more heavily than unbombarded aluminium. We attribute passivation to the known lack of reactivity of amorphous metals. Glassy materials appear to lack the normal routes of attack between their subsurface regions and the attacking medium. On the other hand, when a metal surface is heavily ion bombarded the surface is probably damaged to such an extent that the attacking medium gains physical access to the interior and corrosion proceeds rapidly.
1. Introduction Glow discharges in various gases are being used to an increasing
extent in etching and sputtering applications. Bombardment with inert gas ions is also commonly used to clean surfaces and to enable depth profiles to be obtained. Any effect on the chemical reactivity that arises purely from the impact of ions is therefore of interest. Investigations over the last 20 yr have shown that metals frequently exhibit increased resistance to chemical attack after they have been bombarded with inert gas ions3’. A number of theories have been proposed to account for this phenomenon, but no one theory will explain all the results. The chief theories are: (1) A polymerized hydrocarbon film is deposited on metallic surfaces during ion bombardment’*‘. This film is formed when the ion beam polymerizes ambient gases over the surface3’. (2) Ion bombardment causes an increase in the thickness of the air-formed oxide layer. The factor responsible for enhanced film growth has not been identified3v4. (3) Inert gas occluded in the superficial regions of the metal crystal hinders migration to the reacting surface between metal and corroding medium. Alternatively, the epitaxial relationship between the metal and the growing product ofchemical reaction is altered’*‘*6. These theories cover experimental data for a range of metals for which chemical attack was inhibited by implanting them with He, Ne, Ar and Xe. In this paper we report similar resistance to chemical attack for clean evaporated films of Al and Ni after Xe ion bombardment in a glow discharge. Reduced reactivity is
linked with the amorphous structure that ion bombardment has conferred onto these metals. Bombardment with inert gas ions, however, can enhance, as well as inhibit the chemical reactivity of metals6”.2g. It appears therefore that bombardment can cause different types of structural alteration in metals and the bombardment parameters are important. With this in mind we also report an enhancement of chemical reactivity for Al foil after strong bombardment with Xe ions in a Penning pump arrangement. Increased reactivity is associated with severe radiation damage.
2. Experimental Aluminium and Ni films evaporated in ultrahigh vacuum apparatus were used in the passivation experiments. The evaporants were 99.9995% Al wire supplied by Goodfellow Metals Limited and Marz grade Ni wire supplied by Materials Research Corporation. Evaporant was simultaneously deposited onto the wall of a spherical 500 cm3 Pyrex glass vessel and onto microscope coverslips placed inside the vessel. The vessel was maintained at 273 K during evaporation in uacuo and 78 K during evaporation in Xe. Films were a few hundred A thick. Pure Research Grade Xe, supplied by British Oxygen Company, was incorporated into the film in two ways: (a) The film was evaporated in ULICUO. Then a 3 kV glow discharge was struck in 2 x 10S3 torr Xe between the centrally located evaporator and the film, with the film as cathode. The discharge flux was about 15 PA cmd2 of film. Electrical contact to 301
D F Klemperer and D J Williams: Changes
in the chemical reactivity of metals
the film was made with a small piece of Pt foil. The pressure
of Xe was manostatted with a reservoir of solid Xc at 78 K situated elsewhere in the apparatus. (b) The film was evaporated in 2 x low3 torr Xe onto the walls of the vessel cooled to 78 K. After evaporation, the film was warmed to room temperature. Bodys and Campbell” have shown that appreciable quantities of rare gas are retained in the metal by this technique and we’ have measured thermal desorption spectra for Xe on heating our films above room temperature. Indeed, as in the cast of metals bombarded with rare gas ions’“, the metal must bc melted to relcasc the last part of the incorporated rare gas. The filmed coverslips were used to test chemical reactivity. Aluminium film was dipped into aqueous 1 wt% HgCI, solution for 1 min, and washed. This amalgamates the oxidized Al surface with mercury and leads normally to the rapid and intense growth of fibrous alumina when left in moist air’ ‘. Nickel film was simply covered with a mixture of 1 part concentrated nitric acid and 4 parts water. Transmission electron diffraction patterns and electron probe microanalyses were obtained for the films in a Jeol JEM-100 CX electron microscope. Aluminium foil of 0.25 mm thickness (Marz grade, supplied by Materials Research Corporation) was used in the reactivity enhancement experiment. The foil was subjected to Xe ion bombardment by mounting it in place of the two Ti cathode plates in a small glass ion pump with an electrode arrangement of the the cathode plates type described by Hall I2. In this arrangement sandwich a cellular Ti anode and the whole is placed in a strong magnetic field. A 3 kV discharge was struck and manostatted in 2x 10-3 torr Xc. The discharge flux was about 15 ,rcA cm-’ of Al cathode. Cathodes assumed their characteristic etch pattern as Al was sputtered during the discharge. The etch pattern is made up of the etch pits which form where the high-density parallel ion beams defined by individual cells bombard the cathodes. Bombarded AI foil was tcstcd for chemical attack in the same way as AI film.
Figure 1. Oxidation of thin Al film. A and B were ion bombarded in a Xc glow discharge and have been passivated (metal shows up black). C was evaporated in Xeat 78 K and D wasevaporated in vacuum (metal that has oxidized away shows up white).
3. Results Figure 1 shows the result of a reactivity test on four Al films evaporated onto microscope coverslips. The photograph was taken a day after the HgCl, dip, although the result was essentially the same within a minute. Film A was evaporated in lo-’ torr and Film B in lo- 5 torr of residual gases; both films were subjected to a glow discharge for 30 min and neither shows much sign of oxidation. (Metal appears black and opaque: oxide appears white and transparent.) Film C was evaporated in Xc at 78 K; it readily oxidized away. Film D was uvaporarcd in IO -’ torr and scrvcd as a control; about a third of it oxidized right through. Figure 2 is the electron diffraction pattern for Film B after bombardment. It shows that the film contains regions of amorphous structure. In addition to the diffuse Al rings there is an inner ring which cannot be assigned (d = 3.6 + 0.3 A). Film C also gave an amorphous electron diffraction pattern (Figure 3); in this case the rings are so diffuse that no extra features can be discerned. Film D gave sharp diffraction rings which tallied exactly with Al and showed no extra features. Electron probe microanalysis of Film A revealed the presence of trace of W, Pt and Xe as impurities. Figure 4 shows the result of a reactivity test on three Ni films. The photograph was taken 3 h after addition of acid. Film E was evaporated in 10m9 torr and subjected to a glow discharge for 30 min. The film has not dissolved in acid to any appreciable 302
Figure 2. Elccrron dtffraction pattern Tram Al lilm aflcr Xc IOII bombardment.
extent. On the other hand Films F and G dissolved
readily. Film F was evaporated in Xe at 78 K and Film G was a control, evaporated in IO-” torr. Figure 5 is the electron diffraction pattern for Film E after bombardment. As in the case of bombarded Al, the film contains regions of amorphous structure and there are additional features which cannot be assigned. Inside the diffuse Ni rings there are at least one complete ring and two arced rings (for which the d values are about 2.5, 3.5 and 6.9 A respectively). In contrast to the case of AI, Film F, as well as Film G gave sharp diffraction rings which tallied exactly with Ni and showed no extra features.
D F Klemperer
and D J WiNjams:
Changes in the chemical reactivity of metals
Figure 3. Electron difliaction pattern from AI film evaporated in Xe onto glass at 78 K and warmed to room temperature.
Figure 5. Electron diffraction pattern from Ni bombardment.
film after Xe ion
Electron probe microanalysis of Al foil after bombardment revealed no impurities except for Ar, which was present in the original material’ j.
4. Discussion
Figure 4. Acid attack on thin Ni film. E was ion bombarded in a Xe glow discharge and has been passivated (metal shows up black; lifting has occurred). F was evaporated in Xe at 78 K and G was evaporated in vacuum (metal dissolved away).
Figures 6(a) and (b) show the result of a reactivity test on Al foil bombarded for 30 min in the Penning pump glow discharge. Figure 6(a) shows a piece of foil containing nine etch pits photographed from above. Abnormally heavy oxidation in the etch pits has led to the columnar growths shown in the side view, Figure 6(b). This growth may be compared with the normal fibrous growth on the unetched ‘cross-bars’ of the foil pattern (and on the reverse side of the foil, not shown). On the other hand, there is no oxidation at all in regions immediately surrounding the etch pits where only light Xe ion bombardment occurred. These shiny metallic regions, which stay bright indefinitely, appear white in Figure 6(a). The photographs were taken a few minutes after the HgCl, dip.
Aluminium Film A was prepared under ultra-high vacuum conditions and bombarded with pure Xe ions, yet this film resisted oxidation as did Al Film B which was prepared under poor (unbaked) vacuum conditions and then bombarded with an appreciable background r of common residual gases present. Passivation is therefore unlikely to be due to a protective layer of polymerized hydrocarbon. Again, no passivation was observed for Al Film C and Ni Film F, both of which contained Xe incorporated by the evaporation process. The inert gas per se cannot, therefore, be responsible for passivation. Our Al films were subjected to oxidation experiments in which the air-formed oxide layer ceases to protect the metal. Since Xe ion bombarded Al Films A and B were both passive, it is unlikely that any air-formed oxide layer is responsible for passivity. On the other hand, passive Al and Ni films that had been bombarded with Xe ions contained amorphous regions. It is not possible to distinguish between amorphous regions mixed with normal crystallites and crystallites whose surfaces are uniformly amorphous. Nevertheless the latter situation is more likely to arise in a glow discharge because all regions of the film surface are subjected to an ion flux. We conclude that ion bombardment made the surface of our films amorphous and that the amorphous metals become chemically unreactive. Nevertheless Al Film C, which was evaporated in Xe, was also amorphous yet it oxidized readily. We return to this seeming inconsistency below. It is well known that ion bombardment can make crystalline materials amorphous. Internal ionic bombardment by alpha particles, for instance, will make radioactive crystalline minerals amorphous14, producing the so-called metamict state15. It is also known that Ar ion bombardment of diamond and graphite 303
0 F Klemperer and D J Williams: Changes
6(b)
in the chemical reactivity of metals
.d
Figure 6. Enhancement and inhibition of chemical reactivity of thick AI foil by Xe ion bombardment. The Al foil was made the cathode of an ion pump with an ‘egg box’anode. The foil was then oxidized. (a) Top view, (b) side view. Abnormally heavy oxide growth (columns in (b)) has occurred in the nine etch pits (shown in (a)) where the ion flux was most intense. However, the metal remains unattacked and shiny around the etch pits (white areas in (a)) where the ion flux was light. Normally-oxidized metal shows up as the black background.
produces glassy carbon16. Poate17 and Grant’s have reviewed work demonstrating that implantation with a variety of ions can produce metastable amorphous alloys from crystalline metals, and implantation techniques have been used to obtain amorphous layers in direct contact with a single crystal substrate19. Evidence that amorphous metals are unreactive has also been accumulating3’. Chemically resistant amorphous Fe alloys (metallic glasses) can be prepared by very rapid quenching from the liquid” (splat cooling). Vapour quenched and sputtered metallic films have also been obtained in the amorphous phase and they show increased resistance to chemical attack2’, whilst glassy carbon is relatively unreactive to oxygenL6. Implantation with a variety of ions therefore appears to be capable of producing amorphous, chemically-resistant surface alloys of target and 304
implant atoms22, whilst bombardment with inert gas ions may inhibit the reactivity of an otherwise pure metal if an amorphous layer is formed as a result of bombardment. The questions now arise as to why amorphous metal should be unreactive and how ion bombardment can enhance as well as inhibit reactivity on adjacent areas of the same specimen. We believe that during ion bombardment the structure of the target metals changes to a porous amorphous structure (of low specific density) via a glassy or liquid-like state. The mechanism for these changes is thought to be the formation and rapid coolmg of thermal spikes so that the surface initially takes on the structure of a splat-cooled metallic glass. Then, as bombardment proceeds, this glass structure becomes progressively damaged forming a more open structure which is consequently very reactive because attacking species can easily diffuse into the bulk metal. It will be possible to obtain structures which are glassy (as in the cast of our ion bombarded metal films) and/or porous (as in the case of our ion bombarded foils) depending on the ion dose. The oxidation of Al begins with the chcmisorption of oxygen onto the metal surface. This rapid chemisorption is followed by a slower uptake whilst oxygen diffuses into the bulk and is incorporated as oxide. The amorphous structure is unlikely to affect the initial rapid chemisorption of oxygen, but it may affect the absorption of oxygen into the bulk metal. One model for slow uptake of oxygen on AlZ3 involves an activated place cxchangc between adsorbed oxygen atoms and underlying Al atoms, the whole being in a crystalline array. In this manner oxygen can diffuse into the bulk but the process will be inhibited if the crystalline structure of the metal is destroyed. Alternatively, the slow uptake of oxygen could be assisted by imperfections such as grain boundaries and dislocations running away from the surface and along which oxidation can proceed. Again, ion bombardment can serve to destroy such diffusion paths. Further or conccntratcd bombardment, however, causes radiation damage on such a scale that the migration of oxygen into the bulk is accelerated and oxidation is rapid. The extra features on our electron diffraction patterns for films bombarded with Xe ions suggest that the implanted gas is capable of introducing a new periodicity into the structure. Gas bubble superlattices are, in fact, known to form when He ions and protons arc implanted into a variety of metals at 300 K*‘. Helium implanted in fee metals like Ni, for instance, forms bubbles of about 20 ?, diameter arranged in a fee superlattice with a lattice constant of about 6.6 nmz5. Implanted Xe might also be located on a superlattice which gives rise to the extra diffraction features. Clearly more careful examination is needed but if we draw an analogy between occluded Xe atoms or microbubbles and point defects then it is worth noting that point-defect aggregation can be energetically more favourable than a random distribution of isolated point defects, even at low concentration. The situation brings to mind the crystallographic shear phases which have now been investigated in a wide range of nonstoichiometric materials26. Certainly the extra diffraction features cannot be assigned to any impurities known to be present. Theimpurities introduced to the film during theglow discharge, however, might conceivably affect the chemical properties of our specimens. The Pt impurity was due to sputtering of the electrical contact to the film and the W impurity found to be present in Al must be sputtered from the W evaporation filament by negative ions in the plasma. On the other hand, the introduction of impurities into our foil specimens during the glow discharge is much less likely. Any impurity is also unlikely both to enhance
D F Khmpwer md D J Williams: Cham
in the chemical raactivitY of metals
and to inhibit reactivity in juxtaposition on the same specimen of Al foil. We plan to repeat our bombardment experiments with evaporated &ns using a redesigned electrical contact, and an ion gun in place of the simple glow discharge set-up. Quantitative information concerning the numerous parameters which detcrmine the ion bombardment process, such as the charge of the incident ion, the ion energy, and the angle of incidence of ions on the target surface cannot be obtained by glow discharge mcthods2’. Some of the undesirable effects in a glow discharge such as multiple collisions of gas ions leading to reduced ion energies and random angles ofincidence of ions on the target are reduced but not eliminated in a Penning pump type of arrangement; this is capable of concentrating and intensifying the incident flux onto small areas of the cathode. Finally, we consider the structure of a metal film that has been evaporated in Xc. A diffuse diffraction pattern was obtained (at room temperature) for Al Film C which was evaporated in Xc at 78 K, yet a sharp pattern was obtained for Ni Film F, which was also evaporated in Xc at 78 K. Ready oxidation (rather than passivation) of the amorphous Al Film C is accounted for by drawing an analogy between the reactive, open structure of metal that has been subjected to concentrated ionic bombardment and the much-faulted, imperfect structure of a metal deposited at 78 K in Xc. As the 6lm deposits, so Xe atoms arc physically adsorbed on the growing layer and become occludedwith attendant intra- and extra-microparticle lattice mis6t.t. On warming, occluded Xc is not released smoothly. Electron microscopy and other techniques reveal that the Xc pressure peaks in thermal dcsorption mass spectra reflect structural and phase changes occurring in the material from which Xc is being r&ascd9*2E.In general, one of the Xc pressure peaks may be associated with the change from amorphous to crystalline structure as the sample is warmed. Aluminium and Ni evaporated in Xc arc both thought to be amorphous at 78 K. Xenon is dcsorbed from both metals as the 6lm is warmed from 78 K. This leaves a porous, reactive structure in the host metal. As Xc is progressively dcsorbed, the open structure becomes unstable and resorts to its crystalline state when a characteristic temperature is reached. This change has been observed at 373 K for Al9and we assume that it occurs below room temperature for Ni.
5. chnclusIons
(1) Metal surfaces subjected to a light Xc ion bombardment become amorphous and liquid-like in structure. Heavier, conccntrated bombardment damages the surface and physically opens it. (2) The amorphous liquid-like metal surface is chemically unreactive. After severe radiation damage the surface is readily attacked. (3) Metals evaporated in Xc at 78 K contain occluded Xc. Their structure can be amorphous at room-temperature but chemical reactivity is unimpaired.
ACkOOWkdg-
We thank Dr D W Thompson for the electron diffraction pictures and for electron probe microanalysis. We also thank Prof J M Thomas for his useful discussion. A Science Research Council maintenance award to DJW is gratefully acknowledged. Ref8rences ‘WJMoorr,SR~~LCtutha~dSNBrbwn,Lr~~~r Joniaue. Thtforieset ADD~~C~U~OUS, editions du CNRS, Paris, p 35 (1962). o&a&ova, soo Tech PhysLett, 2,199 (1976). aYMKhirnyfandLNK ‘VAuhworth.DButa.WAGMtRPMP~~~~dTCWellington, Ion Beam Im&Uat&n Semiwndds and Other Matrrirrl.9, PI&Urn, New York, p 367 (1975). ‘V~hwoRh,WACLGrPIlSRPMPrOCtMPDdTCWellington,Carosion Sci. 16,363 (1976). ’ E D Hondros and I JHnard, CR Acad Sci, Par&, 254,1043 (1962). 6 S Muhl, R A Collins and G Dearnaley, Applicarions oIlon Beam to MatrrioLr1975, Inst Phys Conferenoc Se&s No 28, London, p 147 (1976) ’ P D Goode, Applicationsof Ion Beams to Materials, 1975. Inst Phys Conference Series No 28, London, p 154 (1976). ’ J W S Bodys and K C Campbell, Iru J Appf R&at Isotopes, 24,107 (1973). 9 D F Kiomncrcr and D J Williams, to be published. r” C W ‘Tucira and F J Norton, I Nucl M-at, 2,329 (1960). I1 J H L Watson. A Valleio-Frcirc. P de S Santos and J Parso- KoUoid2. 154,4 (1957). ’ . l2 L D Hall, Science, 128,279 (1958). L D Hall, Reu Sci Insmen, 29,367 (1958). t3 We have found that Al from various sourocs contains s@ificant quantities of hr. Our Al foil (Mars grade, Materials Research Corporation) contained 0.2 wt% of Ar. Beer can ring-pulls contained as much as Zwt%ofAr. r4 R C Ewing Am Mineral, 68,728 (1975). ts A Pabst, Am Mineral, 37,137 (1952). t6 S Evans and J M Thomas, Proc Roy Sot, A3S3,103 (1977). ” J M Poatc, .I Vat Sci Technof, 15,1636 (1978). I6 W A Grant, J Vat Sci Technol, 15, 1644 (1978). I9 S S Lau, J Vat Sci Technof, l&l656 (1978). so K Hashimoto, K Gsada, T Masumoto and S Shimodaira, Corrosion Sci, 16,71(1976). K Asami, T Masumoto and S Shimodaira, CorrosionSci, 16,909 (19761 s’-P J Grundy and J M Marsh,J Mater Sci, 13,677 (1978). ” J K Hirvonen. J Vat Sci Technol. 15.1662 (19781. 23 h4 A H Lanyon and B M W TraPd&ProcR;y So;, AZ27,387 (1955). E E Huber and C T Kirk, St@ce Sci, S, 447 (1966). ** P B Johnson and D J Mazey, J Nucl Mater, 93 and 94,721 (1980). Is P B Johnson and D J May, Radiat Efects. 53,195 (1980). 26 R J D Tilley, Chemical Physicsof Solidsand theirSugkces (Editedby M W Roberts and J M Thomas). Royal Society of Chemistry, London, p 121 (1980). *’ M Kaminsky, Atomic and Ionic Phenomena on Metal Surjbces. Academic, New York (1965). *’ V Balek, J Mater Sci, 4,919 (1969). V Balek, J Mater Sci, 5,166 (1970). 29 V A&worth, R P M Procter and W A Grant, Ion Impkrmotbn (Edited bg J K Hirvoncn). Academic, London, p 175 (1980). ’ J A Davies, unpublished work It was recently shown to the authors, for instance, that uranium treated in an argon plasma can become more m&ant to moist air than nickel-plated uranium. The same effect was mentioned b P Haymann, Le Bombardment lonique, The’aies et Applications, ~tionsduCNRS,Paris,p25(1962). ‘t J Goodman, J PolymerSci, U,551(1960) H Yasuda and C E Lamarc, J Appl Palm Sci, IS, 2277 (1971). 32 K Hashimoto and T Masumoto,UltrarapidQuenchingofliquid Alloys (Editedby H Herman) Academic, London, p 291 (1981)
in
305