Volume 19, Issue 3
June 1996, pages 429-605
pp 429-435 June 1996
An effective method of growing single crystals of organic photoconductors such as phthalocyanine in the presence of doping impurity such as iodine by vacuum sublimation is discussed in this paper. This method is very useful especially when an organic material does not have a melting point but decomposes above a particular temperature. So far, doping has been done by exposing the previously grown organic single crystals to the dopant vapour, but growing the crystal in the presence of dopant vapour makes the doped-crystal more efficient and stable. The photosensitivity as well as the photoconductivity of the doped crystals by this method increases significantly.
pp 437-442 June 1996
Thin films of CdSe1−xTex were prepared by using electro deposition with varying doping concentrations of Te(x). Photoconductive studies as a function of applied field, time, wavelength, and intensity of incident light were carried out at room temperature between 1 and 3·5 eV. After establishing that the electrode contacts made using silver paint were ohmic, it was found that as the concentration of Te(x) increases in CdSe1−xTex, the band gap of the material decreases according to the empirical relationEg = (2 − 1·5x)eV.
pp 443-448 June 1996
Photoconductivity studies on cadium sulphide (CdS) crystals grown by chemical vapour transport method were carried out at room temperature (300°C) over the spectral range between the near ultraviolet and the near infrared. Three samples of CdS crystals, viz. undoped CdS crystal, 0·5 ppm zinc doped CdS crystal, and 1 ppm zinc doped CdS crystal, were used. The variation of photocurrent as a function of applied field, intensity of the incident light, response time, and incident wavelength was studied. It was observed that the band gap decreased linearly as doping concentration increased. This is interpreted as being due to doped impurity atoms acting as traps very close to the conduction band edge. The rise and decay times also decreased linearly as doping concentration increased. This has been interpreted due to more free charge carriers being created in the crystal with increase in doping concentration, thereby making the crystal more photosensitive.
pp 449-453 June 1996
The compound tin selenide was prepared from the constituent elements (Sn and Se) using the standard fusing technique and from X-ray diffraction studies it was identified as tin selenide. Thin films of SnSe were obtained on thoroughly cleaned glass substrates by vacuum sublimation on substrates maintained at 301 K. The photoconductive relaxation of these films was studied with oxidation. The results have been explained with the help of grain boundary potential barrier model.
pp 455-466 June 1996
In this paper we report on the growth of polycrystalline diamond films on Mo, W, and Ni substrates using oxy-acetylene combustion flame technique. Effect of substrate temperature on the growth of diamond films has been studied in the temperature range 600–1100°C. The deposits and their surface morphology has been characterized by X-ray diffraction and scanning electron microscopy (SEM). A short duration pretreatment of Mo substrates by outer zone of the oxy-acetylene flame at lower substrate temperatures, results in the improvement of quality and adherence of the films. Growth of diamond as well as other intermediate compounds depending on the nature of substrates and interface layers is discussed.
pp 467-473 June 1996
Micro-crystalline diamond (MCD) and diamond like carbon (DLC) thin films were deposited on silicon (100) substrates by hot-filament CVD process using a mixture of CH4 and H2 gases at substrate temperature between 400–800°C. The microstructure of the films were studied by X-ray diffraction and scanning electron microscopy. The low temperature deposited films were found to have a mixture of amorphous and crystalline phases. At high temperatures (> 750°C) only crystalline diamond phase was obtained. Scanning electron micrographs showed faceted microcrystals of sizes up to 2µm with fairly uniform size distribution. The structure of DLC films was studied by spectroscopic ellipsometry technique. An estimate of the amount of carbon bonds existing insp2 andsp3 form was obtained by a specially developed modelling technique. The typical values ofsp3/sp2 ratio in our films are between 1·88–8·02.
pp 475-481 June 1996
Yttrium iron garnet thin films have been prepared by low pressure metallo-organic chemical vapour deposition method (MOCVD). Dipivaloyl methanates of yttrium and iron have been used as the precursors in the MOCVD growth of the garnet films. Post deposition O2 annealing at 900°C is required to form a garnet phase which also shows orthoferrite and component Y2O3 and α-Fe2O3 phases. We show that a partial H2 reduction treatment minimizes secondary phases and stabilizes the garnet phase. These treatments also enhance the magnetic properties considerably.
pp 483-493 June 1996
Cadmium telluride crystal has been grown by solution method from Te-rich (Cd0·3Te0·7) melt. Ingots having 9 mm diameter and length up to 30 mm were grown by cooling the melt slowly (1°C/h) under a vertical temperature gradient of about 30°C/cm. As-grown ingots were characterized for optical transmission and resistivity. The middle portion of the ingots exhibited better optical transmission properties. Resistivity (p-type) was found increasing, towards the last-to-grow end, from 103 to 106 Θ-cm. Surface barrier type of detectors, made from low resistivity (≅ 104Θ-cm) materials, were found suitable for detection ofX- and low energy gamma radiations. In case of high resistivity (≅106Θ-cm) detectors, the performance was seen to be affected by polarization.
pp 495-504 June 1996
Praseodymium barium molybdate (PBM) single crystals grown by gel method, reveal multiple and isolated octahedral bipyramidal crystals as well as spherulites of PBM at different depths from the surface inside the gel. Oscillation X-ray diffraction pattern of the sample reveals that PBM is single crystalline in nature. Infrared absorption spectrum confirms the presence of molybdate (MoO4)2− ions in PBM, while the thermogravimetric analysis shows that PBM loses molybdenum oxide component around 420°C. Energy dispersive X-ray analysis establishes the presence of Pr, Ba and Mo in the sample. X-ray photoelectron spectroscopic studies of PBM establish the presence of Pr, Ba and Mo in their respective oxide states. An empirical structure has been proposed on the basis of these findings. The implications are discussed.
pp 505-512 June 1996
Ceramics (Pb0·75Ca0·25)[(Ni1/2W1/2)yTi1−y]O3 fory = 0 ·05, 0·10, 0·15 and 0·30 have been prepared using high temperature solid state reaction method. X-ray analyses show that these materials are of single-phase perovskite type tetragonal structure. To solve inaccuracy in finding para-ferroelectric phase transition temperatureTc(εmax), we have carried out the analyses of the data using a computer program. The role of Ni and W as modifiers on structural and electrical (dielectric and pyroelectric) properties is discussed in terms of the usefulness of these materials. Results indicate that ceramic Pb0·75Ca0·25Ni0·25W0·025Ti0·95O3 is a good candidate for pyroelectric applications.
pp 513-525 June 1996
Valence compensated perovskite system Sr1 −xLaxTi1 −xCoxO3 shows dielectric relaxor behaviour with very high value of dielectric constant in the composition range 0·20 <x < 0·40. In this paper the effect of composition on microstructure and the resulting electrical behaviour is reported. The compositions withx = 0·25, 0·30, 0·33, 0·35 and 0·37 have been synthesized by solid state ceramic method and dielectric measurements were made in the temperature range of 300–500 K and frequency range of 100 Hz to 1 MHz. Grain boundaries played an important role in their dielectric behaviour. Complex plane impedance and modulus techniques were used to separate out the contributions of grain and grain-boundaries to the resulting dielectric behaviour. It was observed that the bulk resistivity as well as the grain boundaries resistance decreased with increasingx. Furthermore, impedance analysis demonstrated that extremely high value of dielectric constant observed in these materials was due to barrier layers formation at grain-grain-boundaries interfaces.
pp 527-531 June 1996
The d.c. electrical resistivity ‘ρ’ and thermoelectric power ‘α’ are studied as a function of temperature for Mn substituted ferrites with general formula Zn0·3Ni0·7+xMnxFe2−2xO4. At lower Mn concentrations, the increase in d.c. resistivity is attributed to the hindering of Verwey mechanism Fe2+ ⇌ Fe3+ due to stable bonds of Mn3+ + Fe2+ pair. The decrease in resistivity at higher Mn concentrations (i.e. whenx > 0·15) is attributed to the formation of Mn3+ cluster and Ni2+ ⇌ Ni3+. The activation energy values show one to one correspondence with resistivity values. The compositional variation of thermoelectric power showsn-type behaviour for the samples withx < 0·2 whereasp-type behaviour for the samples withx ⩾ 0·2. Thep −n transition is attributed to the formation of Ni3+, Fe2+ + vacancies which act asp-type carriers. The temperature dependences ofα, ρ, and mobility clearly confirm the conduction mechanism to be due to polaron hopping.
pp 533-537 June 1996
Bismuth germanium oxide (BGO) has tremendous application potential due to its favourable electro-mechanical and electro-optic properties. The present paper describes some of our preliminary results on the variation of dielectric constant with applied high hydrostatic pressure and temperature. It is noted that BGO is a promising candidate for its application as a pressure sensor.
pp 539-548 June 1996
This paper discusses the preparation of samarium-cobalt (SmCo5) alloy powders by reduction-diffusion process. These powders were blended with equal weight percentages of soft metal/alloy powders, such as indium, tin and solder alloy (Pb-17Sn), to prepare bonded magnets. Important magnetic properties such as remanence, coercivity and energy product of these magnets were measured. Effect of matrix metal/alloy on the magnetic properties of processed magnets is outlined.
pp 549-557 June 1996
AA 2219 aluminium alloy is the material proposed for the construction of tanks for liquid cryogenic fuels like liquid oxygen and liquid hydrogen, in Indian launch vehicle programmes. To meet the material requirement, ISRO has developed the alloy AA 2219 indigenously on an industrial scale. Process development and structure-property correlation of auto TIG weldments of AA 2219 Al alloy reported in this paper establishes that postweld reaging of AA 2219 weldments in T62 condition enhances both strength and ductility.
pp 559-564 June 1996
Commercially available aluminium powder has been processed in a thermal plasma jet. The processed powder has been characterized by scanning electron microscopy (SEM) for particle size and morphology. Particle size distribution has been determined by laser scattering technique. Results show that, in contrast to the irregular shapes of the particles of the raw material, majority of the processed powder particles bear spherical or near-spherical morphology. The spherical morphology without sharp edges and corners and particle size distribution in a narrow range ensures free flow of the powder through the powder feed lines, and better spray efficiency, making it ideal for thermal spray applications.
pp 565-571 June 1996
The paper reports the synthesis of nanocrystalline NiAl by mechanical alloying of pure metal mixture and a mixture of prealloyed powder with Ni/Al. A large number of compositions have been studied to establish the phase field of NiAl in the milled state. The phase field of NiAl in the ball milled condition was found to be much wider (10–68 at.% Ni) than its equilibrium phase field (45–59 at.% Ni). The metastable equilibrium achieved by mechanical alloying was identical for a given composition irrespective of the starting ingredients. The crystallite size of NiAl reached a minimum (5 nm) at the phase boundary of NiAl/Ni3Al.
pp 573-579 June 1996
A new Ag+ ion conducting composite electrolyte system (1−x)[0·75 AgI: 0·25 AgCl]:xSnO2 using a quenched/annealed [0·75 AgI: 0·25 AgCl] as host compound in place of conventional host AgI, has been investigated. The effects of various preparation methods and soaking time are reported. The composition 0·8[0·75 Agl: 0·25 AgCl]:0·2SnO2 exhibited optimum conductivity (σ = 8·4 × 10−4S/cm) with conductivity enhancement of ∼ 101 from the annealed host at room temperature. Transport property studies such as electrical conductivity (σ) as a function of temperature using impedance spectroscopy technique, ionic transference number (tion) using Wagner’s d.c. polarization method and ionic mobility (μ) by transient ionic current technique were carried out on the optimum conducting composition. The mobile ion concentration (n) was calculated from ‘σ’ and ‘μ’ data.
pp 581-586 June 1996
Using the optical absorption spectra and the Judd-Ofelt theory, intensity parameters (Ωλ) are calculated. Radiative lifetimes (TR), branching ratios (β) and integrated absorption cross sections (Σ) are reported for certain excited levels of Nd3+ ion in lead acetate glass.
pp 587-600 June 1996
Several aspects of butyl (CIIR)-polychloroprene (CR) rubber blends viz (i) phase morphology evolution with respect to varied blend ratios and the effect of compatibilizer, (ii) processing characteristics, (iii) vapour impermeability and flame retardancy, (iv) mechanical properties, (v) ageing characteristics and (vi) the effect of addition of fillers into these blends have been studied. Scanning electron microscopy (SEM) has been used as a tool to investigate the phase morphology and also the failure mechanism. Blends based on CR and CIIR are likely to be important to Defence services for developing several rubber products with the above properties and characteristics.
pp 601-605 June 1996
The equations have been derived for the trigonal system to calculate the lattice strains produced by the non-hydrostatic pressure condition which arises when the sample is compressed between the anvils without any pressure transmitting medium.
Volume 42 | Issue 5
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