STRUCTURAL STUDIES AND CRYSTALLIZATION BEHAVIOUR OF ELECTROLESS Ni-P FILMS

Abstract

Amorphous metallic alloys are metals with no long range atomic order. They are also called glassy alloys or non-crystalline solids. These alloys have emerged as an important class of novel industrial materials which have received considerable attention of scientific community. The metallic glasses have unique mechanical, electrical transport and magnetic properties resulting-from its amorphous structure. There are two important classes of magnetic amorphous alloys, transition metal-metalloid alloys and the rare earth-transition metal alloys. The transition metal-metalloid alloys typically contain 75 to 90 atom percent iron, nickel, cobalt etc. and the remainder being a metalloid like boron, silicon , phosphorus or aluminium. The amorphous alloys can be prepared by various techniques like, splat cooling, sputtering. ion implantation. vapor deposition. electrolytic deposition and electroless deposition. In electroless deposition the alloy is deposited in the form of a film on a catalytic substrate by a reduction reaction. This technique is simple and so, is widely used in industries. However, this technique has not yet received adequate attention from research workers. The thermal stability'of various metastable phases formed - and the crystallization of amorphous alloys during annealing, is of prime technological importance since many of the unusual properties associated with amorphous state dete-riorate with even a small crystallization. The stability also depends upon the technique of preparation employed for preparing the alloys. One of the singular advantages of studies on amorphous alloys is that we can vary the ,composition continuously, to prepare homogeneous alloys which can be studied as a function of composition and-temperature without the complicating interference from structural phase transition. The present study is on the amorphous binary Ni-P alloy films made by electroless technique and its aim is to investigate the thermal stability and crystallization behaviour of the amorphous film. The first chapter introduces the concept of amorphous alloys, their potential in terms'of industrial applications and scientific research. The second chapter is a review of our present day knowledge of amorphous transition metal-metalloid system , with emphasis on Ni-P system. The amorphous Ni-P films are prepared by using electroless deposition technique also known as chemical reduction method. In this technique a bath containing nickel sulphate, ammonium sulphate. sodium citrate and sodium hypophosphite. is used to reduce nickel on an aluminium substrate. The substrate after sensitizing is dipped into the bath kept at 90°C. The fbl-I of the solution has a vital influence on the composition of the films deposited and, therefore, is maintained at a constant value in the range of 7-10. The procedures of preparing films, and other measurements are described in Chat ~r=3. After the films containing 10.8 to 23.4 atom per cent phosphorus . are prepared, they are subjected.to transmission electron microscopy and X-ray diffraction for structural studies. It is observed that the film containing lower phosphorus content are characterised by the presence of microcrystalline nickel. The films containing 10.8 atom per cent phosphorus exhibits the presence of micro-crystalline nickel. As the phosphorus content of the films increases to 23.4 atom per cent the amorphous nickel region goes on increasing. A number of metastable phosphides are observed to appear in minor amounts. The occurrence of these phosphides is attributed to the local fluctuations. of composition. The 'film having phosphorus contents of 10.8 , 14.3. 15.3, 17.8 , 19.8 , 22.4 and 23.4 atom per cent are then subjected to an annealing treatment at various temperatures such as 200. 250. 300. 400, and 600°C. The samples have been annealed in a vacuum of 10-3 mm of mercury. generally for 4 hours. Only the specimens annealed at 600°C have been subjected to an annealing treatment of 15 minutes. , iv The crystallization behaviour of electroless Ni-P films has been divided into two broad categories. First the alloys having microcrystalline nickel as a major constituent in the 'as-deposited' state and secondly, the alloys which are primarily amorphous and do not contain any significant amount of microcry-stalline nickel. A transition phase of Ni12P5 is observed to form from both the supersaturated microcrystalline Nickel and 1'Jl ariOrph4us on low temperature annealing and all the transition phase start disappering when stable Ni3P phase forms. Atthe same time there is precipitation of crystalline Nickel from the amorphous phase. The temperature at the start of precipitation of Ni12P5 increases with the increase in the phosphorus content, but the temperature at the start of precipitation of crystalline Nickel reduces. For films containing 234g 22.4 atom per cent phosphorus both the precipitations take place over the same temperature range. At temperatures above 354°C there exists only the stable phases - crystalline nickel and Ni3P, for all the films investigated irrespective of their composition. The studies on microstructural changes on annealing are reported in Chapter - 4. To. study the magnetic behaviour-of the amorphous Ni-P alloys,the films in the 'as-deposited' state are investigated in the vibrating sample magnetometer, under an applied field of 5 K Oe. The minor amounts of compounds of nickel-phosphorus in the 'as-deposited' state contain more phosphorus in comparison to the matrix. This relative reduction in phosphorus concentration in the matrix will show an enhanced magnetic moment. However, during annealing of the Ni-P films, two reactions causing an increase in magnetization are identified. The first reaction is the precipitation of a transition phase Ni12P5 li•wii amor-phous and/or crystalline phases in the as-deposited film, appearing as the first peak in the temperature variation of magnetic moment. The second reaction corresponds to precipitation ' of crystalline nickel from the amorphous phase disp- V laying a second peak in the temperature variation of magnetic moment. These results form the subject matter of Chapter - 5. After establishing two reactions during annealing it is of interest to investigate the kinetics of each of these reactions. For this purpose the magnetic behaviour during isothermal annealing is investigated and compared with Johnson-Mehl-Avrami equation to determine the values of A vram i exponent, n. and activation energies. The crystalli zation occurring at the second peak of magnetization curve has also been studied by using a differential scanning calorimeter and the values of 'n' and activation energies of three Ni-P films having phosphorus content of less than 20 atom per cent, 20 atom per cent and greater than 20 atom per cent have been determined.