Gowda, D S Sake

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GROWTH FEATURES OF SINGLE CRYSTALS OF SILVER AMALGAM IN THE PRESENCE OFCOPPERD. S. SAK E G O W DA a n d  N . M AD AIAHDepartment of I*hvsios, Banoalore University, BANaALORE-1.(Received February 19, 1968 ; Reauhmitkd July 18, 1968)(Plato 10)ABSTRACT. In the absence of copper, the growth of single crystals of silver amalgam from AgNOa solution and mercury drop takes place on the mercury surface. In the presence of copper, in the form of wire in contact with mercury drop placed in the solution of AgNOa, the single crystals do not appear on the droj) but grow radial to the wire. The growth features of these crystals depend on the strength of the solution as well os the quantity of mercury. There seems to be a critical concentration of AgNO  ^ solution above which the growth of crystals on the wire is favoured and below just the amalgamation.T N T K O D U 0 T 1 O NCrystals arc grown from solution, vapour and melt. The growth of crystals and the growth mechanism has been pursued by many from time to time. It appears from an article on metal filaments, reported by Hardy (1956), that Henkel and Wallerius, as early as 1720 noticed for the first time the growth of single crystals from a drop of mercury placed in the solution of silver salt and of silver filaments by heating Ag2S. The formation of silver whiskers by electrolysis by Aten et al (1920) has been studied. Generally the nucleation for the growth of whiskers from solution has been attributed to screw dislocation (Newkirk et al, 1955). Sears (1957) in his work on the fibrous growth of NaClOg has observed that surface tension and the consequent gradient in surface tension of the solution due to dif­ferent rates of evaporation as the cause for fibrous growth.The growth of single crystals of silver amalgam using N /50 AgNOg solution and a globule of mercury and a particular aspect of the effect of heat on the struc­ture of single crystals of silver amalgam has been published by Rama Swamy et al(1965). In  the present work an attempt has been made to study the growth peculiarities of single crystals of silver amalgam in the presence of copper using AgNOg solution o f different concentrations and mercury globules of different amounts. A  striking contrast has been observed in the growth of single crystals of silver amalgam in the presence of copper as compared to that in the absence of copper.34292sAKE GOWDA AND N. MADAIAH Indian journal of Physics Vol, 42, No. 5 PLATE 10 A\-ia \ ilidriKjlion photograph o f silver hlameiil(2X)Radial growth o f silver amalgam crystals in the presence o f  copper in contact with Mercury drop at extreme left.0.1')[ ig. 2. SiKer amalgam er>slals in the absence ol coppci.Fig. 4. Rotation picture o f silver amalgam crystals grown in the absence of copper.D. S. SAKE GOWDA and N. MADAIAH Indian Journal of .'hvsicj Vol. 42, No. S PLATE I OB'i . 'Fii;. 5. Rotation picture ol silver .inialgam ciyslals grown in the presence of eoppci.(36x)rig. 1. Streaks seen on eopper surface after the completion of reaction with AgNO , solution in the absence of mercury.(36.;)Fig. 8. Copper surface as seen after the compln"' of reaction in the presence of merctir.'.Growth Fmtures o f Silver Amalgam Crystal 2 9 3E X P E R I M E N T A LExperimental observations fall into three parts. In (A) we describe the growth of silver fibres from AgNOg solution and copper wire; in (B), the growth of silver amalgam crystals in the presence of copper and in (C), t|e growth of the same crystals by varying the concentration of silver nitrate solution and the quantity of mercury. ?f'.(A) When a copper wire of 0.162 cm. thick is kept iilolined in a crystallizing dish containing N/50 AgNO^ solution at room tcrnperatu]^, bright fibres of silver are seen all along the immersed portion of the wire withii|a few minutes and the growth is radial. Tlu^  fibres, separated after the completion of the reaction, are (loaned with distilled watc'r and dried. The X-ray diffraction photograph (figure ], plate lOA) of the silver filaments taken reveals that the powder lines are not continuous. Also the analysis of X-ray picture gives the structure as f.c.c. and the latti(3e constant as 4.095A which agrees with tliat of silver. This indicates that the silver filaments grown on the copper surface are due to the replacement (^ f copper by silver.(B) Long needle-like single crystals of silver amalgam shoot out from different points of the surface of the mercury droi3 (figure 2, plate lOA) placed in a dish containing N/50 AgNOg solution. The presence of copper wire touching the mercury drop in the same system has completely changed the initiation of nucleation on the surface of mercury. The fact is that no crystals appear on the mercury suifac^o (figun? 3, plate JOA) but on the immersed portion of the wire and radial to it. These crystals a few mms. in length are thin pointed needles. The rotation picture (figure 4, plate lOA) is of single crystals of silver amalgam grown in the absencti of copper and of (figure 5, plate lOB) is of the same crystals grown in the presence of copper. The layer line spots of the latter arc quite sharp compared to those of the other.(C) The observations made in the presence of copper are repeated with 5gms. and 10 gms. of mercury taking equal volumes (200 c.c.) of AgNOg solution of different concentrations in long identical cylinderical Jars 4 cms. in internal diameter. The crystals do not appear all along the length of the wire but only up to a certain point of the immersed wire. This reveals that the appearance of crystals on the wire as well as the surface nucleation length is a function of con­centration. The relationship is graphically represented in (figure 6, plate lOB) by the curves shown side by side to bring out the similarity between the two cases. In both cases the surface nucleation length is minimum at some concentration. With large quantity, this minimum occurs at a lower concentration. Above the294 D. S. Sake Oowda and N. Mndaiahconoentraiion at which the miniraum occurs, mostly micro-crystals grow and below that only the amalgamation of the surface with no crystals.Ooncontnition <»f N03 SnUilton0. Curvoa showing tho variation of surfaco niujli^ ation length with eoneentration of AgNO j solution.D I S C U S S I O NThe growth process of silver filaments can be explained as follows ; The replacement of copper by silver during chemical reaction between copper and AgNOj solution, renders possible the initiation of nuclcation on copper itself. Once the nuclcation occurs the growtli of silver filaments may be attributed to screw dislocation that is favoured by the transport of silver ions from the surround­ing medium towards the nuclcation sites by diffusion proct'ss and simultaneous replacement of coppiT. Observing under a microscope a well-cleaned surface of copper subjected to chcunical reaction using dilute solution of AgNOg, irregular streaks or channels (figure 7, plate lOB) are noticed along the immersed wire. T)uo to nonuniformity in the density distribution of silver ions as they approach the copper wire, those channels are not uniform either in depth or in width. This perhaps is an indication of randomness in rapid crystallization that results in the growth of silver filaments. Also the discontinuity of the lines in the X-ray picture of these filaments shows the tendency of the growth towards crystallinity.The radial growth of crystals along the wire and none on the mercury globule is a very striking observation which is explained below. The chemical reaction between AgNOj solution and copper is more predominant than nuclcation taking place first on the mercury surface. Consequently, the silver ions are adsorbed in the regions wherever copper is replaced. The mercury ions themselves creep along the channels formed on the wire and migrate into the adsorbed layer of silver (figure 8, plate lOB ). The creeping of mercury is very conspicuous even to the bare eyes. This is due to the existence of some kind of capillary forcesGrowth Featvres of Silver Amalgam Crystal 29 6probably related to interfacial tension that will produce acute angle of contact of mercury with the adsorbed layer of silver and not dii(3 to electrical forces since the standard electrode potentials of silver and mercury ar(' the same (— —0.799 volts). With the surface migration of meroury ions, the nuclcation for th(^  growth of silver amalgam crystals takes place and further growth rf q^uircs the presence of screw dislocation as suggested by Sears to provide self- perpetuating growth steps at which the atoms can be easily attached.With AgNOg solution of varying concentration there seems to be a critical concentration above which th(^  crystals ap])ear and beJow tio crystals. At higher concentrations, as nuTCury creeps up and single crystals shoot up radial to the wire, the tendency of the silver ions above' is to mov(' towards the (crystals already formed. From a similar observation made by Aten cf at (1920) in tlu'ir studies of silver whiskers formed by electrolysis using silver nitrate solution as electrolyte, (luring deposithm of silviT ions on cathod(‘ consisting of a ring on which silver fibres arc' already formed (by an earlier experiment) and a fresh flat disc in contact with the ring, most of the silvc'r ion current was towards the whiskers on the cathode ring.At low(T concentrations the availabk' silver ions are few in number and pro­bably just enough for the roplacf^ment of cnppei*. The proce ss is too slow and the nu'rcury creeps at a v('ry slow rate and spn'ads over the adsorbed layer of silver without the formation of crystals. Tlu' fact that the length of amalgamation incimses with further lowering of cioncentration seems to indicate tlie amlgama- lion process or otherwise surface rnicleatioii itself may be a prior stage for the growth of crystals.A C K N O W L E U (J E M E N TThe authors thank very much Dr. K. N. Kuclu‘la, Prob'ssor and Head of the Department of Physics, Bangalore University, Bangalore, for his help and useful discussions and also express their gratitude to Dr. S. Rama Swamy for his tm- eouragoment and suggestions during his stay as Professor of Physics under the scheme for Retired Professors.R E F E R E N C E SAten, A. II. W., and Boerlage, 1920, Report of the International Conference on Growth and perfection of crystals^ '' held at Cooperstownf New York, Aug. 1958, p. 85.Hardy, H. K., 1956, Progreaa in Metal Phyaica 6, 45.Nowkirk, J. B. and Sears, G. W., 1955, Acta Met., 8, 110.Rama Swamy, S. and Sake Gowda, D. S., 1965, Proc. Indian Acad. Sci. 61, 301.Sears, G. W., 1967, J. Chem, Phya., 28, 1549.

Growth Features of Single Crystals of Silver Amalgam in the Presence of Copper

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Growth Features of Single Crystals of Silver Amalgam in the Presence of Copper

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