• Volume 18, Issue 6

October 1995,   pages  637-829

• Foreword

• My experiments with thin films—the nanostate of matter

The black art of thin films exhibiting whimsical properties and thus called ‘the fourth state of matter’ enticed the author to a challenge to unravel the mysteries of matter createdab-initio on a nanoscale. Nucleation and growth studies of evaporated and sputtered films (both polycrystalline and epitaxial) and the effects of kinetic energy, angle of incidence, electric field, substrate agitation, formation of metastable and amorphous structures, etc led to exciting discoveries. The fields of amorphous metal alloy and semiconducting films were thus born. Electron and optical transport investigations on pure and doped metal, semiconducting, oxide and polymer films established a variety of new phenomena of which the mean free path effects and specular scattering of conduction electrons in epitaxial metal films, giant thermopower in quenched metal films, sputter quenching, giant photocontraction effect in amorphous Ge-chalcogenide films, enhancement of superconducting transition temperature in transition metal films, nanosize multi layer coating (the first known superlattices) are noteworthy. A variety of new deposition processes such as solution growth of polymer films, solution growth of chalcogenide and oxide films, spray pyrolysis, coprecipitation of multicomponent oxides (for thick films), activated reactive evaporation/magnetron sputtering were innovated. This lecture will share the excitement of the contributions by the author, his 60 Ph.D students and a number of post doctoral fellows and faculty members at the Thin Film Laboratory, IIT Delhi and the Microscience Laboratory at IIT Kharagpur.

• Electronic structure of amorphous semiconductors

The effect of light soaking and thermal quenching on the electronic structure of hydrogenated amorphous silicon (a-Si:H) and chalcogenide glasses was studied. It was found that lithium dopeda-Si:H shows both light and thermal induced changes in electronic transport properties. In contrast, chalcogenides do not show any effect of thermal quenching, although they exhibit changes upon light soaking. By analysing the conductivity and thermopower data we have concluded that the light soaking increases the potential fluctuations present in lithium dopeda-Si:H, whereas quenching does not change them. A model qualitatively explaining these effects is presented.

• Microwave ferrites for phase shifters

The electronically steerable phased array radars (PARs) are finding extensive use in military, communication and civilian applications. A PAR system typically comprises a regular arrangement of a large number of phase controlled radiating elements. Ferrite-based phase shifters in various designs are employed in these systems operating at frequencies above 5 GHz due to their superior power rating and insertion loss characteristics. Under DRDO’s integrated missile programme considerable success has been achieved in the country on the development of ferrite materials, phase shifters and array system.

The work done at SSPL on the development of ferrite materials for both X- and C-band phase shifters is described. The effect of various substituents in the chemical composition of the lithium-titanium ferrites and of the process parameters to evolve the grades with required magnetic, electrical and microstructural properties are discussed. The investigations carried out to enhance optimally the r.f. power handling capability of the material for meeting the desired specifications of the C-band phase shifter are also given.

• Cast metal matrix composites—challenges in processing and design

Development of metal matrix composites has extended the choice of materials particularly for the space, aero-space and automobile industries, with a view to reduce fuel consumption and operating cost by weight saving. Tailoring of a composite to suit a given application requires choice of constituents and promotion of desirable interfaces. Solidifcation processing of composites demands particular attention to wetting of dispersoids by alloys, defects like porosities and evolution of microstructures particularly in respect of changing the nature of interfaces in a composite.

• Novel high performance fibres

The need for high performance materials for aerospace and other structural engineering applications has led to the development of carbon fibres. At IIT Delhi, an attempt is being made to develop acrylic precursor fibres for manufacturing carbon fibres indigenously. The present study deals with the structural regulation of acrylic precursors during thermo-oxidative stabilization and subsequent carbonization. A new microporous acrylic fibre-acrysorb has also been developed which has high water imbibition and moisture sorption properties. Production of X-ray opaque and antistatic polyester fibres has also been high-lighted.

• Material science aspects of phase change optical recording

Chalcogenide thin films are used as the recording medium for phase change-type optical memory discs. The films are switched between amorphous and crystalline states using the heat of a focussed laser beam. Large reflectivity differences between amorphous and crystalline states are then used to store and retrieve the information. An active chalcogenide layer for this purpose should have a high optical absorption coefficient (α), and good structural and thermal stability. It should be possible to switch the chalcogenide layer between amorphous and crystalline states repeatedly within a short duration, the optical contrast should be high, and the material must have large cycling capability. Keeping the above requirements in mind, we have carried out systematic investigation of structural, optical and crystallization behaviour of thin films of various compositions of GaGeTe, Sb2Te3 and BiSe. These studies have shown that these materials can be good candidates for use as recording media in erasable phase-change optical recording.

• Thermal plasmas in material processing

Thermal plasmas are partially ionized gases at atmospheric pressures, characterized by temperatures in the range of 2000–20,000 K and charged particle number densities in the range of 1019–1021 per m3. Thermal plasmas are produced by plasma torches as a highly constricted jet. The high temperatures, enthalpies and heat fluxes in the plasma jet make it amenable to many chemical and metallurgical processes of industrial importance. The processing environment can be inert as in the case of argon or nitrogen plasmas or can be made reactive by introducing suitable gases. Reactive thermal plasma processing is a novel technique, wherein the plasma enters the reaction scheme, with ions and excited species opening up new channels. This technique is versatile in producing a wide variety of materials like oxides, carbides, borides, aluminides and coatings of diamond, superconductors and bioceramics. In this paper, the basic design of the plasma devices and some of the significant materials-related activities carried out recently at BARC are reported.

• Fatigue of brittle materials—A critical appraisal

Recent demands for high performance ceramics and glass for various applications from bioceramics to cutting tools under fluctuating stress conditions has focussed attention of the scientific community towards fatigue behaviour of brittle solids. Attention to fatigue phenomena in alumina ceramics phenomenological to metals, having an endurance dependent on applied stress with a limit at around 50% of the single cycle fracture stress, was first drawn by the author in late sixties. Slip assisted fatigue process was not considered to be dominant in ceramic materials due to the absence of appreciable crack tip plasticity. With the background of this general survey of fatigue behaviour some fatigue studies based on mode of testing, theoretical and experimental analyses and fractographic evidence have been presented. Studies have shown that there is a dormant period between each successive crack advancement during which the residual stress and a plastic component is built up in a cumulative manner leading to eventual failure. During fatigue$$\sigma _{p_j }$$ (plastic) and$$\sigma _{p_j }$$ (residual stress) components are predominant for ductile metals and brittle glass/ceramics respectively.

It is also apparent that dislocation assisted plastic component as a contributing factor in the failure of brittle materials under fatigue cannot be ruled out.

• Processing of materials—monolithic to composites

A multimillion rupee 500-ton hydraulic extrusion/forging facility established at NPL, New Delhi, has been used to undertake extensive studies in forming, the process of plastically deforming, which is the most important way of shaping materials. Wrought materials are used extensively for making useful products employing extrusion and forging, the two important secondary processing techniques used to convert materials into useful shapes and sizes and also to improve the mechanical and metallurgical properties. The success of these processes depends on the proper control and eventual optimization of different process variables.

Different ferrous and non-ferrous materials including 316 and 321 stainless steels (using glass lubrication), aluminium brass, admiralty brass and 70/30 and 90/10 cupro nickels have been extruded at a lab-scale production level to determine the influence of different extrusion variables such as temperature, strain rate, extrusion ratio, different die design and several different compositions of lubricants on pressure requirement and surface quality of the products.

In the case of ferrous extrusions, the choice of billet-container lubricant has a marked effect on the shape of pressure-distance curves by influencing the effective coefficient of friction and heat transfer coefficient. Temperature, extrusion ratio and strain rate mainly influence the level of pressure-distance curves. Significance of specially designed dish shaped dies, avoiding the dead metal zone and the importance of extrusion limit diagrams for different materials on the available press capacity has also been highlighted.

Deformation behaviour of the hi-tech materials; discontinuously reinforced metal matrix composites (MMCs) and aluminium-lithium alloys using hot extrusion and closed die forging, have been carried out. Parameters have been optimized to extrude rods and thin walled circular MMC tubes of aluminium alloy (2124/6061)-SiCp MMCs having varying volume fractions of SiCp. This developmental work has been carried out using powder metallurgy, liquid metallurgy and spray atomization and deposition routes, in joint collaboration with several R & D organizations.

Hot extrusion being an important production technique, results of these experiments can be directly transferred to industries. Similar deformation studies either by closed die or by isothermal forging have been made to develop components.

The pilot plant level facility is available for any industry for carrying out developmental R & D work. NPL welcomes such collaboration.

• A new materials processing—hydrothermal processing

The hydrothermal processing has emerged as the new materials processing for powder preparation. This describes definition, history, advantages, characteristics of the powder, ideal and hydrothermal powder.

• Synthesis of nano particle of inorganic oxides by polymer matrix

Ultrafine (⩽ 150 nm) powders of spinels [MFe2O4 where M = Ni(II), Co(II) and Zn(II)]; rare-earth orthoferrites [RFeO3 where R = Sm, Nd and Gd], and rare-earth garnets [R3Fe3O12 where R = Sm, Nd and Gd] with good purity and chemical homogeneity were prepared through two new versatile chemical routes. The first route involved the coprecipitation of the desired metal nitrates from their aqueous solution, in presence of a water soluble polymer-polyvinyl alcohol (PVA), by triethyl ammonium carbonate solution. The other process involved complete evaporation of a mixture of optimum amounts of PVA and the desired aqueous metal nitrate solutions, with and without the addition of optimum amounts of urea when the mixture was evaporated to a pasty mass. In addition, detailed study on the reported potassium ferricyanide route was also carried out for the production of the rare-earth orthoferrite powders. The various precursor as well as the heat-treated mixed-oxide powders, prepared through each of the routes, were compared by the physical characterization studies involving thermal gravimetry and differential scanning calorimetry, infrared spectroscopy, X-ray powder diffraction, transmission electron microscopy, and room temperature magnetic measurements.

• Bulletin of Materials Science

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