Volume 19, Issue 1
February 1996, pages 1-176
pp 1- February 1996
pp 1-1 February 1996
pp 3-14 February 1996
Hydrogen plays a crucial role in the properties of various materials, devices and in growth of hydrogenated amorphous materials. Therefore, its quantification and determination of concentration at various depths is of immense interest. Among various techniques of hydrogen depth profiling, the nuclear techniques being non-destructive in nature are widely used. Elastic recoil detection analysis (ERDA) with helium ions was first used for hydrogen depth profiling. Later on, the heavy ions were utilised due to certain advantages. It has been shown recently by us that H alongwith other elements in thin film up to Ca can be detected simultaneously with heavy ion ERDA, if the elements being detected are well separated in masses. It also has been shown that even the neighbouring elements such as C, N, O alongwith H can be detected provided a ΔE-E detector telescope is used in ERDA. The time of flight (TOF) with ERDA, nuclear reaction analysis (NRA), neutron TOF are other nuclear techniques such as hydrogen loss during the measurement, which must be taken into account for accurate and reliable results. A scenario of developments in nuclear techniques with suitable examples is given in this brief review.
pp 15-28 February 1996
A brief overview is given on low temperature diffusive behaviour of H in Nb(OH)x, and measurement of diffusion, primarily by neutron scattering. We go in some detail into recent work on calculations of neutron structure factor as well as the diffusion coefficient of hydrogen, influenced by two-level excitations. Both the electronic and phononic contributions are covered. The importance of similar two-level excitations in insulating and metallic glasses is also pointed out.
pp 29-38 February 1996
Hydrogen plays a crucial role in the growth of micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films grown by plasma assisted chemical vapour deposition (PACVD) processes. It selectively etches graphite phase and helps in stabilizing the diamond phase. The presence of various hydrocarbon species in the plasma and their reaction with atomic, excited or molecular hydrogen on the substrate surface decide the mechanism of diamond nucleation and growth. Several mechanisms have been proposed but the process is still not well understood.
Control of hydrogen and other deposition parameters in the PACVD process leads to deposition of yet another class of materials called diamond like carbon. By varying the concentration of hydrogen it is possible to produce purely amorphous carbon films on the one hand and amorphous hydrogenated carbon films (with as high as 60% hydrogen) on the other. Very hard, optically transparent and electrically insulating films characterize the diamond like behaviour. The proportion of hydrogen and its bonding with carbon or hydrogen in the film can be varied to obtain very hard to very soft films which could be optically transparent or opaque. The microstructure of these films have been investigated by a large number of techniques. The results show interesting situations.
This paper reviews the work on the role of hydrogen on the growth, structure and properties of MCD and DLC thin films.
pp 39-50 February 1996
The hydrogen in hydrogenated amorphous silicon (a-Si: H) makes it behave like a hydrogen glass. Above a temperatureTE, which is analogous to the glass transition temperature, the hydrogen is able to move more freely than belowTE. This motion of hydrogen is believed to be responsible for the observed thermal and light induced metastabilities in a Si: H. However, the changes in the microstructure of the bonded hydrogen upon thermal quenching are found to influence the electronic properties of a-Si: H, in a manner, which is different from light soaking. Our studies suggest that the light soaking changes the potential fluctuations in lithium doped a-Si: H, whereas the thermal quenching does not.
pp 51-60 February 1996
Hydrogen in crystalline semiconductors has become a recent curiosity because of its high diffusivity and strong chemical activity in such materials. In contrast to the proton motion in ionic materials which gives rise to an enhanced conductivity, hydrogen in electronic materials interact with structural disorders and chemical impurities to control the electronic flow. Deep gap states in crystalline semiconductors due to various disorders such as surface/interface, grain boundaries, dislocations, irradiation and implantation damage etc. have been removed due to hydrogen bondings.
Hydrogen incorporation is done by plasma and direct ion beam hydrogenation methods, implantation technique and by a novel technique of damage free introduction. The most studied materials are silicon and gallium arsenide.I - V,C - V, DLTS and IR studies have been carried out on hydrogenated semiconductors to characterize the electronic flow, gap states and the nature of chemical bonds. Improvement in ideality factors of diodes, reduction in free carrier concentration, removal or reduction of deep states and appearance of new bondings such as Si-H, P-H, B-H etc. have been observed from various techniques.
The present paper reviews the various features of hydrogenation studies in crystalline silicon and gallium arsenide and highlights our results of hydrogenation studies on Pd/semiconductor devices.
pp 61-72 February 1996
Dynamic and non-equilibrium effects involving interaction between deuterium and radiation produced defects were studied during deuterium implantation of Cu and Ti. The technique of neutron yield measurement during deuterium implantation was employed and theoretical analysis was made to study the dynamics. SIMS, GIXRD and SEM studies on deuterium implanted samples were employed to study the evolution of implanted deuterium profiles, structure of deuterides and surface topography respectively.
pp 73-82 February 1996
Hydrogen is the lightest element in nature, and so, its detection and quantitative analysis is difficult by the conventional methods utilized for other elements. In the recent years the technique of elastic recoil detection analysis (ERD) using 1–2 MeV He+ beam has been developed to quantitatively and simultaneously analyze hydrogen and its isotopes in solids. Such a facility has been set up using the 2 MeV Van-de-Graaff accelerator at IIT Kanpur. It facilitates H and D analysis in a material up to a depth of ∼ 1µm with a detection sensitivity of 0·1 at.% and depth resolution of about 300 Å. The application potential of this setup is illustrated by presenting the results of measurements performed on Al:H:D systems prepared by plasma source ion implantation and highTc YBCO pellets exposed to humid atmosphere.
pp 83-90 February 1996
The impurity induced charge density of proton in Al is calculated by solving the Schrödinger equation self-consistently. The lattice contribution and the lattice dilation are included through spherical solid model potential and Blatt correction, respectively. The proton is kept at an octahedral site. The resulting phase shifts have been used to estimate the residual resistivity for dilute Al-H system. This model does not favour the formation of AlH, i.c., hydrogen exists in Al matrix as a free ion.
pp 91-102 February 1996
The room temperature hydrogen embrittlement problem in iron aluminides has restricted their use as high temperature structural materials. Previous studies have established that surface films affect hydrogen embrittlement (HE). The effect of surface passive layer on the hydrogen embrittlement behaviour of iron aluminides has been critically reviewed in this presentation. The role of thermomechanical treatments in affecting the mechanical properties has been discussed from a processing-structure-properties correlation view point. The alloy development philosophy to yield ductile iron aluminides has been outlined based on this review. Novel iron aluminide intermetallics that are being currently synthesized and characterized along these lines at IIT Kanpur are finally introduced.
pp 103-114 February 1996
Susceptibility of aluminium and its alloys towards hydrogen embrittlement has been well established. Still a lot of confusion exists on the question of transport of hydrogen and its possible role in stress corrosion cracking. This paper reviews some of the fundamental properties of hydrogen in aluminium and its alloys and its effect on mechanical properties. The importance of hydrogen embrittlement over anodic dissolution to explain the stress corrosion cracking mechanism of these alloys is also examined in considerable detail. The various experimental findings concerning the link between hydrogen embrittlement and stress corrosion cracking are also discussed.
pp 115-121 February 1996
This paper describes the method of producing plasma sprayed coatings of hydroxy apatite (HA) on metallic substrates of Ti-6Al-4V. Hydroxy apatite is a material which has similar composition to that of the mineral phase of the human bone. Poor mechanical properties however inhibits its use in the load bearing applications. The powders prepared in our laboratory are sprayed using a plasma spray torch operating in ambient. The deposited coatings are charaterized by X-ray diffraction and found to retain HA in the coatings as per requirements.
pp 123-129 February 1996
Single crystalline oriented films of indium antimonide have been grown on cadmium telluride substrates by the pulsed laser deposition technique. The films were (111) oriented which is the substrate orientation. The composition of the grown films were found to deviate from that of the target owing to loss of antimony during evaporation. This deviation from stoichiometry led to film-substrate reaction, resulting in mixed interface. The antimony deficiency in the films were controlled by correcting the stoichiometry, which led to avoiding mixed interfaces. The stoichiometric films showed good surface morphology and well defined sharp interfaces. The IR transmission spectrum showed sharp band to band absorption and effective detection in the MWIR.
pp 131-138 February 1996
This work gives the results of influence of temperature and deformation degree on changes in the metal grain growth of drawn copper products, because this mutual dependence was observed. Wire samples of O.F.H.C. copper and copper tube samples, chemical content of 99·97% Cu and 0·024% P, were exposed to recrystallized annealing after drawing. The annealing was carried out at temperatures of 400, 450, 500, 550 and 600°C, for 30 min, in laboratory conditions. Investigation results show that after drawing with high cold deformation degree (96–99%), the annealing leads to the changes in the continuous grain growth with increased temperature. The smaller grain size appears at 550°C in comparison with the lower annealing temperature. Annealing has influence on mechanical characteristics of tested samples and during drawing of copper products these characteristics are adequately changed: with regard to the annealing at the 500°C, the characteristics of strength and plasticity increase as a result of decreased grain size. The increase of annealing temperature to 600°C leads to the increased grain size and decreased values of strength and plasticity characteristics.
pp 139-145 February 1996
Ellipsometric studies in the 1·5 to 5 eV range have been carried out on thin films of Cd1−xMnx Te and exhibitE0,E1 andE1+Δ1 transitions of the cubic semiconductors. The effect on these transitions upon irradiation by HeNe laser light, application of small alternating magnetic fields and the influence of both the fields, applied simultaneously, has been investigated. The observed critical point shifts have been interpreted on the basis of changes in the band structure of these thin films. This has been corroborated by theoretically calculating the effective number of electrons contributing to the transition per atom —Neff.
pp 147-154 February 1996
The photoconductive properties of a carotenoid polyene,β-Apo-8′ carotenal in polycrystalline form has been studied. The growth of the photocurrent shows an overshoot in the growth-time curve before steady state value is attained. This behaviour of photocurrent is proposed to be due to higher value of recombination coefficient than trapping coefficient. From the temperature dependence study it is observed that the steady state photocurrent, at first increases with increase of temperature, attains a maximum at a particular temperatureTmax and then decreases with temperature. TheTmax value agrees with the temperature above and below which steady state photocurrent is attained differently. Monomolecular and bimolecular recombination processes at two temperature regimes are proposed to account for the observed behaviour. The dependence of photocurrent with excitation light intensity and wavelength study provide information on the carrier generation processes. The fast decay of photocurrent have been observed at different temperatures and from this study the decay constant is calculated and it is found to be temperature independent.
pp 155-159 February 1996
Fe2O3 thin film was prepared using aqueous solution of FeCl3 by spray pyrolysis. The substrate temperature was 450°C. The lattice parametersa andc for different concentrations were calculated from X-ray diffraction study. Hexagonal structure of the Fe2O3 thin film was confirmed. Band gap values of Fe2O3 prepared from different concentrations were determined from optical transmission data.
pp 161-164 February 1996
This paper contains results of experiments carried out with the aim of assessing the influence of various casting parameters on the microstructure of an aluminium alloy. Castings have been made with and without vibration, in molds with different thermophysical properties.
pp 165-176 February 1996
The increasing use of thermosetting resins in fibre reinforced composites for structural and insulating applications has necessitated the need to understand the process of crosslinking and systematics of curing time of resins with and without the reinforcing fine fibre filaments. We have employed dynamical measurements of loss tangent and dielectric constant over the frequency range 1 kHz–100kHz for investigating cure characteristics of composites. After adding crosslinking agents to resinous matrices (epoxy and polyester styrenated alkyd resins) the time variation of dielectric parameters was recorded until the completion of curing. The resin flow, the onset of gelation and completion of curing were unambiguously mapped by variations in dielectric parameters. Onset of gelation coincided with precipitous fall of dielectric constant and resonant maximum of dielectric loss. Asymptotic stabilization of dielectric parameters signalled completion of curing. The gel time and curing time were found to be substantially larger for fibre reinforced resins as compared to pure resins. The variation of electrically determined gel and cure time with frequency of ac signal employed and on the libre fraction in the composite are discussed in the paper. The present study shows that if dielectric measurements are to be employed to infer the completion of curing, low frequency measurements would be needed to correctly infer onset of gelation and time for curing.
Volume 42 | Issue 5
Click here for Editorial Note on CAP Mode