A STUDY OF THE STRUCTURE OF ELECTRODEPOSITED METALS. PART I1 BY L. B. HUNT
In an earlier paper’ the writer has adumbrated a theory of the interference with crystal growth by ions, molecules or particles, other than those which constitute the metal being deposited, but no detailed suggestions were put forward of the precise mechanism by which this interference occurred. The present communication constitutes an attempt to elucidate this mechanism by means of a further and more detailed consideration of the processes occurring at the interface between cathode and electrolyte, in conjunction with the published data on the inclusion of foreign bodies within electrodeposits. I n order to arrive at definite conclusions it has been necessary to start from the fundamental mechanisms of crystal growth and metallic cohesion, with particular reference to the concept of the two-dimensional lattice, and thence to develop a mechanical picture of the formation of this lattice from a more highly disperse phase during cathodic deposition. The ground having been cleared in this way, two further lines of thought have been developed. I n the first place the problem has been treated as a case of surface chemistry. The interface cathode-electrolyte presents unusual and complicating features which render generalisation difficult. The constant renewal of surface which is occurring is particularly favourable to adsorption, but various bodies are present which may be adsorbed to a greater or less extent. Several workers have ascribed the influence of substances such as gelatin to adsorption, but have given no clear indication of the manner of this adsorption. Secondly, the two-dimensional lattice idea has been further considered, and a new concept has been introduced of a two-dimensional lattice distortion, leading to changes of orientation and consequent refinement of grain. This concept has been applied to the data on the electro-deposition of alloys and of those metals with which hydrogen is co-deposited. Two distinct types of interference have thus been recognised, occurring respectively between, and during, the formation of the two dimensional lattices. It is fairly certain, however, that the transition from one to the other is not discontinuous in such cases as the discharge of complex cations and solvent sheaths, Le., when the interfering particles are already attached to the metal ions. There is also the possibility, of course, of both types of interference occurring in any one case. A number of investigators have referred to two other processes as influencing the structure of electrodeposited metals, via. cataphoresis, and “mechanical inclusion” of the electrolyte or of colloidal particles. Now although cataphoresis may account for the arrival of particles at the cathode, some such process as adsorption must be brought into play to cause their ultimate cohesion to the cathode, whilst mechanical inclusion is of doubtful
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existence, and in any case has no fundamental bearing on the problem in hand. These two processes have therefore been omitted from consideration a t the present time.
The Mechanism of Lattice Formation Current ideas on the subject of crystal growth are definitely in favour of a discontinuous process occurring by layers. It is a simpler matter to visualise t'he formation of a two-dimensional lattice in the process of crystallisation from the molten state, in which case the distribut,ion in the two phases is substantially the same. Any attempt to form a mechanical picture of this lattice formation during cathodic deposition, however, introduces several complications, chief among which is the very great difference in distribution of t'he metal ions in the two phases. Other factors tending to obscure the picture are the deposition of only one kind of ion, in cont,radistinction to the building up of crystals of inorganic salts, and the nature of the electrical forces involved which on a priori grounds makes any suggestion of a pause in the process of deposition difficult to uphold. More so than with any other process of crystal growth, it is obviously impossible for deposition to be a continuous process, or for a two-dimensional lat,tice to be laid down in its entirety. The only alternative is therefore a gradual laying down of the lattice in such a way as t'o satisfy the conditions of distribution and the supply of free electrons. Carrying the process of elimination a &age further, general considerations and experimental observations on crystal growth suggest the extreme unlikelihood of a random deposition of ions at more or less isolated points until a layer is complete. I n those cases of deposition referred to in Part I as "cathodic reproduction" very extensive and uniform layers have clearly been formed in order to continue the orientation of the underlying crystals without imperfection. Volmer2 postulat'es an adsorption layer of ions in a state intermediate between that of the electrolyte and that which they will take up in the lattice, but bases his ideas of the subsequent lattice formation on the assumption of the surface mobility of the adsorbed ions. This concept is obtained by analogy with t'he growth of mercury crystals by deposition from the vapour phase. This process, however, involves uncharged atoms only, and it is quite likely t>hatin this condition the mercury atoms arriving at the surface of the growing crystal may cohere primarily with van der Waals' forces and may possess lateral mobility. I n the case of electrodeposition, on the ot'her hand, positively charged ions are approaching a negat,ively charged surface, and the writer cannot agree that surface mobilit,y in these conditions of powerful binding is possible. Volmer found by oscillographic measurement's (during which the amount of current passing was so small that concentration polarisation was absent), that the polarisation accompanying the deposition of zinc on large well-formed zinc crystals was quite appreciable, whereas With zinc amalgam and finely crystalline zinc foil as cathodes, no percept,ible polarisat'ion was recorded. This result' he ascribed to a retardation on the part of the met'al ions entering the solid lattice, caused by fluctuations in
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thickness of the adsorbed layer, whilst in the case of the fluid electrodes, discharge and entry into the fluid phase constitute a single process. I n the case of the finely crystalline electrodes the actual surface area may have been considerably greater than the apparent area. The existence of an adsorption layer of metal ions on the surface of a cathode is a reasonably justifiable assumption in view of the work of Langmuir and others, coupled with the much older concept of a double layer due to Helmholtz and Lippmann. By making this assumption, however, the problem of the manner in which the layer is formed and maintained is still left unsolved. Presumably metal ions in the vicinity of the cathode will frequently be in collision with the cathode surface, and will tend to adhere thereto, prior to the passage of a current, although this will apply in greater or less degree to all ionic and molecular species present. On passing a current, however, the anions will be repelled violently, leaving an adsorbed layer of cations front which deposition can take place provided the potential requirements are satisfied. As deposition removes the metal ions the layer must be replenished by the process of diffusion. The process of deposition can therefore be roughly represented thus: Diffusion -+ Adsorption +Lattice formation. The final velocity will naturally be governed by the slowest stage in the process. The speed of diffusion will be determined by the concentration gradient across the diffusion layer, as also will the speed of adsorption. Increase of temperature, agitation of the solution and decrease of current density will thus promote diffusion, and adsorption from the diffusion layer. The concentration gradient, however, will depend upon the amount of deposition occurring, and thus on the current flowing. The delivery of electrons at the cathode surface is thus the determining factor, but certain complexities are introduced when the mechanism of this process is considered. The Electron Theory of Metals and Metallic Cohesion The majority of workers in this field have dealt mainly with the mathematical aspect of the problem, and it has become more and more apparent, as the wave nature of electrons has been established, that the formation of a mechanical picture should not be attempted. Certain general conclusions may, however, be drawn as to the inner structure of a metal as it affects the building up of a lattice a t the cathode. This process consists fundamentally of the passage of the conduction electrons through the cathode until on reaching the surface they are captured by the metal ions, arriving from the opposite direction. There are therefore two points on which information is desired; firstly, as to the manner in which the electrons arrive a t the cathode surface, and then as to the manner in which they cause the cohesion of the newly formed two-dimensional lattice to the bulk of the cathode. I n the first place, it is known that comparatively few of the electrons present in a metal are concerned in the conduction process, which comprises a relatively slow drift superimposed on the very rapid random movement of
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the electrons. The concepts of the manner in which the electrons exist in a metal vary from the interpenetrating lattice theory of Lindemann: supplemented by Sir J. J. Thomson4 to involve the movement of chains of electrons along the lattice, to the free electron-gas theory of S~rnrnerfeld,~ whilst Barlow6 has suggested a resemblance to an incompressible fluid rather than to a gas. Bloch’ has adopted the view that, whilst the conducting electrons are free, their motion is not completely haphazard, as postulated by Sommerfeld, but is a motion in a periodic field of force of which the period is the same as that of the atomic lattice. The electrons may then be regarded as both free and bound, i.e., as jumping from atom to atom and pausing for a time a t each. Lennard- Jones and M700d8have further discussed the electronic distribution in metals and have obtained a partial solution which is particularly interesting from the present point of view. A complete solution for a threedimensional metal was not attempted, but the problem was simplified to that of a hypothetical two-dimensional lattice. For further details and the resulting diagram the original paper must be consulted, but the authors concluded that regions of greater electron density exist between the nuclei, and that the electrons within these regions are to be regarded as shared between the four surrounding nuclei (still retaining the two-dimensional concept), since they cannot be definitely associated with any one nucleus. These shared electrons apparently constitute a lattice array interpenetrating that of the nuclei, similar to the suggestions of Lindemann and Sir J. J. Thomson, but differing in that the system is not static even a t the absolute zero. Those electrons which have sufficient energy will cross the equipotentional lines and be able to travel from end to end of the metal, causing a continual interchange of electrons. This, however, does not advance the solution of the problem of how the slow drift of the conduction electrons accounts for the cohesion of a metal ion on euery lattice poznt. Barlow, however, observes that the pressure of the applied E.M.F. will be sufficient to cause only a limited number of gaps to be bridged by the conducting electrons, so that their flow will be restricted t o paths that are continually changing. The electrons will always be attempting to move forward, but they must wait until an opportunity offers. It seems to the present writer that the most probable mechanism of the arrival of electrons at the surface, in the case of electrodeposition at all events, is in a succession of waves at right angles to the direction of flow, which will appear as somewhat analogous to a creeping barrage passing continually over the face of the cathode. It has already been said that the formation of a mental picture of this type is in msny cases unjustifiable, but there are definite objections to being too easily dissuaded from the particle concept in the present case, such as the relations expressed in Faraday’s laws, and the orderly arrangement of tho lattices resulting from the interaction of metal ions and electrons, despite the indeterminate nature of the conduction process. If an imaginary plane is taken through the cathode, normal to the direction of flow of the current, and similar to Lennard- Jones’ two-dimensional lattice, it is clear that all the conduction electrons cannot be moving simultaneously.
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There must, therefore, be either a random here and there movement (an idea which is immediately discounted), or a wave-like oscillation, having an amplitude of the same order as the mean free path of the electrons themselves, and a frequency dependent upon the applied E.M.F. The passage of this wave train through the adsorption layer will naturally govern the formation of the next adsorption layer, and the whole process will thus be governed by these electron waves. As regards the nature of metallic cohesion, Hume-Rothery8 suggests that the reason underlying the formation of this particular type of bond is that there are insufficient valency electrons for each atom to complete its octet by sharing one electron with each of several neighboring atoms. An atom of Group N will normally tend to complete its octet by sharing one electron with each of (8-N) other atoms. When, however, there are less than four available valency electrons, each atom can no longer share one electron with more than four other atoms. When this occurs a bond must be formed in which an electron can be associated with more than two atoms. This bond will thus resemble the normal covalent bond in which an electron is shared between two atoms, but differ in that a larger number of atoms will share a valency electron. The point then arises of the difference between the cohesive forces in adsorption and in lattice formation, Le., the difference between an adsorbed ion and a “discharged atom.’’ The obvious conclusion is that the difference is due to the presence of the excess valency electrons. If the supply of electrons were cut off, adsorption would also cease, since the adsorption layer would no longer be incorporated into the metal lattice, thus forming a new surface on which further adsorption can take place. It is probable, therefore, that the adsorption of metal ions comprises an electrostatic type of adhesion in which the adsorbed ions attach themselves by the mirror image forces to the surface, without interfering with the cohesion bonds in the lattice. The supply of electrons by conduction will then operate to incorporate the adsorbed layer by the covalent electron-sharing linkage. Reverting to the wave concept of electrons, the transition from ion to “atom” may then be considered not as a sharply defined process, but as a merging of one into the other. Interfacial Phenomena The existence of an adsorption layer a t the interface cathode-electrolyte has been the basis of the foregoing discussion, and it now becomes necessary to examine in more detail the conditions in this layer with respect to the various ionic and molecular species present in the solution. The constant renewal of surface which occurs cannot but be favourable to pronounced adsorption effects, and in these circumstances the experimental fact that adsorption of species other than the metal ion takes place to a very limited extent requires a word of explanation. The point of immediate interest is to consider the forces operating on a metal ion approaching a cathode. Firstly, there is the force due to the electrical field, which is responsible for the migration of the ions, and extends throughout the electrolyte. Secondly, there is the short
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range electrostatic force between the cathode and the ion, and also the van der Waals' cohesive forces. In t,he opposite direction there are the retardations due to the anionic atmosphere and the solvent sheath respect,ively. Now the ionic atmosphere, according to the Debye-Huckel theory, is continually buildihg up in front, and dying away in rear, of a moving ion. When a cation reaches a point a few angstroms away from the cathode, therefore, the building up must cease and the dying away become more complete, owing to the repulsive forces on the anions prohibiting their close approach to the cathode surface. The resultant, force on a metal ion passing through this
FIG.I Variation in potential energy with distance from cathode surface
part of its course will thus be first a slight retardation, followed by an increased attraction. The next event will be the repulsion of the solvent envelope, which will cause a further increase in potential energy of the ion, again followed by a sharp fall, to the point where the ion is held by the electrostatic forces, vibrating aboul the position of minimum potential energy, pending the arrival of one or more electrons t,o bring about ultimate cohesion. The potential energy of a metal ion as a function of its distance from the cathode surface is represented diagrammatically in Fig. I by curve A. This curve is at best a first approximation, largely owing t o the complexit,ies introduced by the superposition of an electrical field on the more or less normal adsorption phenomena, but it is now tentatively put forward as a basis for future discussion. Discharge of the ion will occur only if the neutralisation potential E, of Gurney,l0 be greater than the work function of the metal 6,less the applied potential difference V. This neutralisation potential is defined by
E=I-W
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where I is the ionisation potential of the ion, and W represents the energy of solvation, which must be replaced on discharge. The condition for discharge is then given by E = I - W > b - V Various other cations are usually present in the electrolytes employed for deposition, but except in cases of alloy formation, which will be dealt with later in the paper, they are never found in the deposit. The inference is that ions such as those of the alkali metals are not only incapable of being discharged, but on colliding with the cathode, have less tendency to be adsorbed, and are more likely to be reflected into the solution, in accordance with their relative ionisation potentials. This will result, however, in a building up of the concentration of alkali metal salts in the vicinity of the cathode, with a consequent decrease in the dissociation of the metal ions. The adsorption of molecules will not be affected by the electrical field, but the ions of the metal lattice will cause the formation of induced dipoles in the molecules, which together with the long range van der Waals' forces will bring about adsorption. The potential energy of a molecule or solvent dipole is represented in Fig. I by curve B. There will be a small minimum a t a greater distance from the surface than was the case with a metal ion, from which it is clear that molecules may be adsorbed in a layer above the ionic layer. They will, however, be comparatively loosely bound, and will thus possess an appreciable chance of escaping back into solution. They will also possers a certain amount of lateral mobility, according to Volmer's observat i o q 2 and it may be necessary for stable nuclei or groups to be formed in this way before adsorption becomes possible. Solvent sheaths present a transitional case, in that the molecules are already bound to the metal ions, and that they arrive in more or less stable groups. However, the chance of their survival as far as the adsorption layer is considerably reduced by the increased concentration of metal ions in the two dimensional state. The possibility of their inclusion is nevertheless not to be ignored, as was indicated in Part I of this paper.' The above explanation merely accounts for the infrequency of the process. I n this connection some results obtained by Aten, Hertog and Westenberg," in the absence of solvation are instructive. Experiments were carried out with fused electrolytes and it was found that although the grain-size of electrodeposited silver, copper and nickel, was still dependent upon temperature and composition of bath, the wide differences which exist between the structures of these metals when deposited from solution disappeared. In general the grain-size was much larger than in deposits from solution. Van LiemptI2 has also obtained coarsegrained deposits of tungsten from fused electrolytes. A similar transitional case is presented by the deposition of complex cations, such as FuseyaIs and his co-workers have investigated in copper sulphate electrolytes. Here again the molecular species arrive a t the cathode already bound to the metal ion, but their concentration will not be reduced to the same extent as will that of the solvent molecules. The probability of their
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becoming adsorbed and included is thus much greater. Fuseya ascribed his results to “discharge” of the complex ions, but his curves bear every resemblance to adsorption isothermals. Fuseya,I4 however, could not obtain the same results with lead and zinc electrolytes, although complex ion formation was found. This failure he ascribed to the decomposition of the complex ions in the film of more alkaline solution near the cathode.
The Inclusion of Basic Matter A more interesting phenomenon which has recently received attention concerns colloidal hydroxides or basic salts precipitated in the cathode film owing to the decrease in hydrogen ion concentration caused by the discharge
?
FIG.2
of hydrogen. This was first investigated by O‘Sullivan,lj who concluded that in the case of nickel sulphate electrolytes the hydroxide entered the deposit and influenced its structure. Macnaughtan and Hammond‘e have recently extended this line of work, using Brinell hardness as a criterion of grain size in the deposit. It was found that the acidity of the electrolyte exerted a marked influence upon the hardness and structure of the deposit. In general a low acidity favoured the formation of hard deposits having a finely crystalline structure, whilst greater acidity favoured the formation of softer deposits having a larger grain size. Further, it was found that the hardness did not increase uniformly with increasing pH, but rose suddenly after a certain critical pH value had been reached. The present writer,” however, has drawn attention to the somewhat illogical method used to represent the results of these workers, namely, the plotting of Brinell hardness against pH. By recalculating to hydroxyl ion concentration and plotting the results obtained
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against Brinell hardness the writer has obtained curves of a different type, shown in Fig. 2 which bring out more clearly the differences between the electrolytes containing ammonium ions and those free from ammonia, owing to the different types of buffer action exerted. It appeared that whereas with the electrolytes N and F containing boric acid and sodium fluoride the amount of inclusion increased rapidly with increasing hydroxyl ion concentration, in the solution y, containing ammonium sulphate the rate of increase of hardness fell away as hydroxyl ion concentration increased. While the latter effect might well be due, as suggested by the authors, to the tendency of the precipitated matter to redissolve in the ammoniacal solution in the vicinity of the cathode, the mechanism of the inclusion remains unexplained. Adsorption in the true sense of the term is clearly insufficient to account for the increase in the amount of inclusion with increasing hydroxyl ion concentration, which fact of itself suggests a more direct mechanism of inclusion. The writer wp,,ild therefore suggest that this mechanism consists of the discharge of a complex ion containing the metal hydroxide, or of an ion to which the colloidal basic matter is attached. I n this connection it is interesting to note that Brdicka’* has recently investigated the hydrolysis of cobaltous chloride solutions at the dropping mercury cathode, and has concluded that deposition may occur from the ion [Co(H20)6 OH]‘. The deposition of basic matter is also a factor of importance in the electrodeposition of chromium. Sargent’g stated that the first stage in the process of electrolysis of chromic acid was the formation of a film of basic chromium chromate on the cathode. This was followed by a partial reduction of chromic ions to chromous ions, and deposition ,from both types with simultaneous evolution of hydrogen. LiebreichZ0suggested that metallic chromium was obtained by secondary reduction of the divalent hydroxide Cr(OH)2, in the cathode film, basing his statement on the fact that the metal is found beneath a layer of hydroxide when deposited from almost neutral electrolytes. The smaller the OH’ concentration the greater would be the tendency for chromium to be deposited free from hydroxide. Mullerz1 agreed that the film at the cathode was composed of chromic oxide or chromic chromate, but maintained tha; deposition took place from the hexavalent ion. The sulphate radical was supposed to destroy the film and allow chromic acid to reach the cathode and be reduced, whilst in the absence of the sulphate or other radical the diaphragm (then invisible) was assumed to be “ideal.” Small ions such as the hydrogen ion would be able to pass through the diaphragm, but large anions would be unable to follow. This concept has been criticised by Ollard,22who showed that electrophoresis, which was held by Muller to be responsible for pressing the diaphragm against the cathode, was unfavourable in the chromic acid solutions employed. 4s regards the actual inclusion of basic matter in the deposit, Adcock3 found that chromium contained considerable amounts of oxygen in a form from which insoluble Cr2O8was produced by heating in vacuo. The original oxygenic matter dissolved in dilute acid without residue, suggesting that it was not CrZO3,and no evidence of the presence of the electrolyte could be
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found on crushing the deposit. The inference is that basic matter was included in the deposit in a manner analogous t80the inclusions found by O'Sullivan and by Macnaughtan and Kammond. Brit'ton and WestcottZ4have also recorded the deposition of basic mat,ter from solutions of chromic sulphate and chromic chloride. Furthermore these workersz5have recently found as much as 13% of hydroxide matter included in the apparently metallic portions of the deposit. The writer considers that the concept introduced above to explain t,he inclusion of basic matter in nickel deposits will also suffice to account for the presence of such inclusions in chromium deposits, and also for the observed diaphragm formation. The colloidal particles are considered to be attached to the metal. ions in the adsorption layer, and whilst this layer is static it will naturally not be visible. When deposition occurs, however, with the accompanying evolution of hydrogen, large amounts of basic matter will be brought up to the cathode. Small amounts will certainly remain adsorbed, and so become included in the deposit, but the major portion will be separated from the metal ions. The adsorbed layer will then thicken and become detached from the surface, thus becoming visible as a diaphragm in contradistinction t,o the breakdown of a molecular film, which merely results in molecular dispersion or solution, This concept appears to agree well with the results of Forster and Deckert'e who, working with stannous sulphate electrolytes to which m-cresolsulphonic acid had been added, found that whilst the effect of adsorbed molecules was prolonged, the colloidal matt'er was rapidly exhausted, the effect increasing with increasing current density. In the case of basic matter, of course, the supply of colloidal particles is continuous owing to t,he constant evolution of hydrogen causing a rise in p H of the cat,hode layer. The process will differ considerably from the normal phenomenon of adsorpt,ion in that the amount of inclusion will increase regularly with increasing concentration of basic matter in the cathode film. The number of metal ions having colloidal or basic matter attached will necessarily be only a small fraction of the total number of metal ions proceeding t o the cathode, this fraction being dependent on the hydroxyl ion concentration. The Deposition of Foreign Cations In certain cases of electrodeposition there are two principal cations entering the deposit. One of these may be the hydrogen ion, as is the case with the metah of the transitional group, or both may be metal ions, leading to the formation of an alloy. Now deposits formed in this way from two ions are well known t o possess finely crystalline structures. For instance, nickel, cobalt or iron, deposited from solutions of their simple salts, are invariably of much finer structure than metals such as copper and zinc deposited under similar conditions. Furt,her, deposits of two metals which normally yield coarsely crystalline deposits alone, are found to give fine-grained alloys. I n the foregoing discussion foreign bodies which presumably enter, or rather cause, the grain boundaries of the deposited metal, have been the chief concern, but it' is clear that foreign ions which enter the lattice have also to be
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taken into consideration. In the case of alloy deposition the variable factors such as current density, temperature, agitation, eto., instead of influencing the structure of the deposit, merely serve to bring about alterations in its composition. No explanation of this finely crystalline structure has as yet been forthcoming. The constitution of electrodeposited alloys has been investigated by a number of workers. NakamuraZ7found that the lattice constant of electrolytic brass containing 82% of copper showed no apparent difference from that of cast brass of the same composition. The deposit may therefore be presumed to consist of the normal a-solid solution of zinc in copper. Roux and Cournot2* examined electrodeposits of copper-zinc, cadmium-silver, cadmium-zinc, and cadmium-nickel alloys, and found that the X-ray spectra could not be reproduced by superposition of those of the constituent metals, indicating that compounds or solid solutions, or both, were formed. Similarly Fuseya and SasakiZ9examined their deposits of chromium-iron alloys and found them to consist of solid solutions. StillwelPo showed that in cadmium-silver alloys a number of phases consisting of both solid solutions and compounds were present. Unfortunately the systems hydrogen-metal have received very little attention, and it is as yet quite impossible to state whether these systems represent solid solutions of the substitutional type, or solid solutions of the interstitial type (in which the solute ions are situated in the spaces between those of the solvent) or compounds. Yamadaal and McKeehana2 obtained the characteristic X-ray pattern for palladium, but found that the lattice was uniformly expanded by the presence of hydrogen. Linde and Boreliusa3 concluded that both solid solution and compound formation occurred in the system palladium-hydrogen, whilst H a n a ~ a l tconsidered ~~ that the compound Pd-H was formed. Evidence of the formation of a solid solution was obtained by Hiittig and Brodorb,35 who found that the presence of hydrogen in electrolytic chromium caused a widening of the lattice without altering its form. It is now established that the ions of a metal entering into solid solution replace those of the solvent metal in the lattice, so that the two kinds of ions are situated at random in a common lattice. If, however, the limit of solid solubility is exceeded a new phase must be formed in which the crystal structure differs from that of the pure metals, and which may consist partly of an intermetallic compound. The details of the concept of substitution in solid solutions have been worked out by Rosenhaina6who in explaining the increased hardness of these phases suggested that solute atoms distort the lattice of the solvent metal, thus causing an increased resistance to slip. The writer proposes to adapt this hypothesis of lattice distortion to the case of a two dimensional lattice in course of formation. I n the solid three-dimensional state it is comparatively easy to visualise distortion producing hardness, but with a two-dimensional lattice the hardness factor is absent, and rigidity is greatly reduced. The writer suggests, therefore, that the presence of stranger ions in the two-dimensional lattice will cause distortion and breaking up of this
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lattice with consequent changes in orientation and grain refinement. If the limit of solid solubility is exceeded a new phase must be formed, which will again be conductive to grain refinement. This type of interference with lattice formation clearly occurs in a plane parallel to the cathode, in contradistinction to the other types of interference which have been considered. The result of lattice distortion is therefore the production of a fibrous type of deposit, such as is invariably found with deposits of the transitional metals and of alloys.
Conclusion The various types of interference with lattice formation which have been described are shown diagrammatically in Fig. 3, which represents the passage of the ionic and molecular species from the electrolyte to the cathode.
FIG.3 The processes occurring at a cathode
Continuous lines indicate the major processes, and broken lines the processes of infrequent occurrence. The branching lines indicate the removal of certain types of ions and molecules from the cathode layer. Thus the first event on the approach of a cation to the cathode will be the loss of its anionic atmosphere, which will be followed by the loss of the solvent sheath or the complex portion of the ion. The latter may, as shown, remain adsorbed to a slight extent and thus be included in the deposit. The difference between the removal of molecules and colloidal particles is also indicated. Molecules adsorbed from the cathode will return into solution, whereas colloidal matter will not, and may accumulate temporarily as a diaphragm a short distance from the cathode, prior to its redistribution by diffusion or electrophoresis. Further, certain colloidal particles, more particularly those consisting of basic matter, will be adsorbed more firmly and in greater amount owing to their attachment to the metal ion, and will behave in a similar manner to complex ions. The theory of interference with crystal growth proposed by the writer' receives considerable support from the lines of thought which have been developed in this paper. It is clear that no simple hypothesis, such as that
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based on changes in polarisation, can be expected to account for the variations in structure of electrodeposited metals. Each species present in the electrolyte must be considered on its own merits with respect to the forces acting upon it, and its behaviour under these forces. It is considered that the adoption of this method has resulted in a more detailed insight into the mechanism of interference with crystal growth, and although the writer hopes to be able to present experimental data and more exact information bearing on the relative potency of the interfering bodies, it is his opinion that the first essential is an attempt to present a clear and logical account of the subject. By applying existing knowledge in the co-ordination of the available data and the further development of the hypothesis it is hoped that this object has been achieved.
References Hunt: J. Phys. Chem., 36, roo6 (1932). 2 Volmer: 2. physik. Chem., 139, 597 (1928). 3 Lindemann: Phil. Mag., 29, 127 (1915). 4 Thomson: Phil. Mag., 44, 657 (1922). 6 Sommerfeld: 2. Physik, 47, I (1928). 8 Barlow: Phil. Mag., 8, 289 (1929). 7 Bloch: 2. Physik, 52, jjj (1928). 8 Lennard-Jones and Wood: Proc. Roy. SOC., 12OA, 734 (1928). 9 Hume-Rothery: Phil. Mag., 9, 65 (1930). 10 Gurney: Proc. Ro SOC.,134A, 137 (1931). Aten, Hertog andkestenberg: Trans. Am. Electrochem. SOC., 47, 265 (1925). 12Van Liempt: 2. Elektrochemie, 31, 249 (1925). '3Fuseya and Nagano: Trans. Am. Electrochem. SOC.,52, 249 ( I 27). 14 Fuse a and Yumoto: J. SOC.Chem. Ind. Japan, 31, 80B (19287. 15 O'Sufivan: Trans. Faraday SOC.,26, 89 (1930). 18 Macnaughtan & Hammoqd: Trans. Faraday SOC.,27, 633 (1931). 17 Hunt: Discussion on above. 18 Brdicka: Coll. Czech. Chem. Comm., 3, 396 (1931). 1 9 Sargent: Trans. Am. Electrochem SOC.,37, 479 (1920). 20 Liehreich: 2. Elektrochemie, 29, 208 (1923). 21 Muller: 2. Elektrochemie, 32, 299 (1926). Z2 Ollard: Metal Ind. (London). 31, 417 (1~27). 23 Adcock: J. Iron Steel Inst,,'ll5;3& (1627). 24 Britton and Westcott: Trans. Faraday SOC.,27, 809 (1931). 26 Britt,on and Westcott: Private Communication. 26 Forster and Deckert: 2. Elektrochemie, 36, 901 (1930). 27 Nakamura: Sci. Papers Inst. Phys. Chem. Research, Tokyo, 25, 287 (1925). 28 Roux and Cournot: Industrie electrique, 37, 4 5 (1928). 2 9 Fuseya and Sasaki: Trans. Am. Electrochem. io,., 59, 209 (1931). 30 Stillwell: J. Am. Chem. Soc., 53, 2416 (1931). 51 Yamada: Phil. Mag., 45, 241 (1923). 32 McKeehan: Phys. Rev., 20, 82 (1922). 33 Linde and Borelius: Ann. Physik, 84, 747 (1927). 34 Hanawalt: Phys. Rev., 33, 444 (1929). 35 Hiitti and Brodkorb: 2. anorg. Chem., 144, 341 (1925). 35Rosenhn: Proc. Roy. SOC.,99A, 192 (1921). 1
The Laboratory, VauxhaZZ Motors, Ltd., Lulon. February 22, 195'2.
