• Om Parkash

      Articles written in Bulletin of Materials Science

    • Characterisation of bubble materials

      M J Patni Om Parkash D Bahadur

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      The rapid development of magnetic bubble technology has required growth to exacting specifications of a large number of magnetic films of garnets and some amorphous binary and ternary alloys of rare-earth and transition metals. The characterisation of these films is an essential part in the search for newer materials which hold out promise for better device performance and cost viability. Many methods of films characterisation have been reported from time to time. By and large, these methods can be divided into two groups: one, bulk measurements made on the film and two, measurements made on the domains. We have attempted to collate and briefly introduce various techniques to characterise magnetic bubble materials in this review.

    • Electron transport in hot pressed Y3−xGdxFe5O12

      D Bahadur OM Parkash Devendra Kumar

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      Electron transport properties of a few hot-pressed garnets of the series Y3−xGdxFe5O12 (wherex=0, 1 and 2·4) have been measured. For comparison, a normal sintered YIG has been studied to see the effect of porosity and microstructure. The electron transport properties have been discussed on the basis of the model suggested by Austin and Mott keeping in view the distortion caused by the substitution.

    • Microstructural and dielectric behaviour of glass ceramics in the system PbO-BaO-TiO2-B2O3-SiO2

      OM Parkash Devendra Kumar R Rajgopalan

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      Glasses in the system PbO-BaO-TiO2-B2O3-SiO2 with and without P2O5 as nucleant have been prepared. The glass samples were ceramized based ondta studies. The ferroelectric phase crystallizing out has been found to be BaTiO3 fromxrd. The optical and scanning electron micrographs show the presence of BaTiO3 as major phase. In these glass ceramic samples, dielectric constant and dissipation factor are approximately constant with temperature and frequency upto the glass transition temperatureTg and thereafter increase sharply with temperature and finally level off. The addition of P2O5 as nucleant and molar ratio of (PbO + BaO) to TiO2 has marked influence on the dielectric behaviour and composition of ferroelectric phase crystallizing out.

    • Glass ceramics containing ferroelectric phases

      Om Parkash Devendra Kumar Lakshman Pandey

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      Glass ceramics prepared by controlled crystallization of glasses produce fine dispersion of crystallites in a glassy matrix. Glasses containing a mjor portion of constituents of a ferroelectric phase produce crystallites of ferroelectric phase in glass through a suitable heat treatment. The amount of network former in the initial glass has a profound influence on its crystallization behaviour and microstructure of the resulting ferroelectric glass ceramics. The value of dielectric constant and the nature of ferroelectric to paraelectric transition depend on the crystallite size and volume fraction of the ferroelectric phase. These glass ceramics are transparent for crystallite size less than 0·1µm and exhibit large quadratic-electro-optic effect.

    • Electrical properties of the system lanthanum lead cobalt titanium oxide

      Devendra Kumar Ch Durga Prasad Om Parkash

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      Measurements of DC electrical resistivity and Seebeck coefficient on the perovskite system La1−xPbxδ Co1−xTixO3−δ for 0·2≤x≤0·9 have been made in the temperature range 300–800 K. AC conductivity,σa.c, of all the samples were measured as a function of temperature (300–573 K) and frequency (1 kHz-1 MHz). DC resistivity behaviour of all the samples is similar. However, the resistivity value, which varies over 5–6 orders of magnitude, depends on both the compositionx and the structure of the samples. All the samples exhibitp-type electronic conduction. The value of Seebeck coefficient,α, for samples withx≤0·5 initially increases with temperature up to a particular temperature. Above this temperature, the behaviour ofα for samples withx≤0·5 and for samples withx>0·5 over the entire temperature range is similar to that of La CoO3. All the samples exhibit frequency-dependent a.c. conductivity at low temperatures.

    • Dielectric properties of the system Ca1 −xLaxTi1 −xCoxO3

      Ch Durga Prasad H S Tewari Devendra Kumar Om Parkash

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      Dielectric behaviour of samples of the system Ca1 −xLaxTi1 −xCoxO3 withx ⩽ 0·20 has been studied in the temperature range 300–525 K as a function of frequency. The strong dispersion ofɛ andD observed in these materials indicate the significant contribution of interfacial polarisation to the observed dielectric properties. The interfacial polarisation arises due to the presence of microscopic chemical heterogeneities arising out of the slow diffusion-controlled solid state sintering process used for their preparation.

    • Studies on the spin-state equilibria of cobalt ions in the system La1−xNaxCo1−xNbxO3 (x⩽0·40)

      Om Parkash N Chaturvedi M Tiwari Devendra Kumar

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      The spin-state equilibria of cobalt ions in the system La1−xNaxCo1−xNbxO3 (x⩽0·40) has been studied by measuring its magnetic susceptibility as a function of temperature in the range 300–600 K. It is found that the behaviour of the samples withx⩽0·10 is similar to that of LaCoO3, while compositions withx⩾0·20 behave quite differently, exhibiting simple paramagnetic behaviour.

    • Crystallization and microstructural behaviour of strontium titanate borosilicate glass ceramics with Bi2O3 addition

      O P Thakur Devendra Kumar Om Parkash Lakshman Pandey

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      Glasses in the system (65 −x) [SrO·TiO2] − (35) [2SiO2·B2O3] − (x) [Bi2O3] wherex = 1, 5, 10 (wt%) prepared by melting in alumina crucible (1375–1575 K), were subjected to different heat treatment schedules followed by DTA studies. Crystallization study showed the formation of Sr2B2O5 as major phase at low temperature (≈950°C) heat treatment. At high temperatures, TiO2 and SrTiO3 with or without Sr2B2O5 crystallize out depending on heat treatment. In this paper, the influence of variation in composition, thermal treatment on the nature of crystallizing phases as well as on the resulting microstructures are investigated through XRD, IR and SEM. Uniform crystallization was achieved by suitable addition of Bi2O3 and proper heat treatment.

    • Evidence of two ferroelectric PTCR components in valence-compensated ceramic system Ba1−xLaxTi1−xCoxO3

      Lakshman Pandey Rajesh K Katare Om Parkash Devendra Kumar

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      The electrical behaviour of valence-compensated ceramic system Ba1−xLaxTi1−xCoxO3 has been studied as a function of temperature (300–600 K) and composition (x ⩽ 0·20), using the method of impedance spectroscopy. The necessary equivalent circuit models that represent the data best have been obtained using impedance and modulus formalisms and grain and grain boundary contributions have been separated out. The compositionsx = 0·20 and 0·10 show a negative temperature coefficient of resistance (NTCR) behaviour whereas a very small variation of the grain and grain boundary resistance with temperature is observed forx = 0·05. A positive temperature coefficient of resistance (PTCR) behaviour having two ferroelectric components is observed forx = 0·01. These results reveal limitations in current theories of the PTCR effect.

    • IR study of Pb–Sr titanate borosilicate glasses

      C R Gautam Devendra Kumar Om Parkash

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      The infrared spectra (IR) of various glass compositions in the glass system, [(Pb𝑥Sr1–𝑥)O.TiO2]– [2SiO2.B2O3]–[BaO.K2O]–[La2O3], were recorded over a continuous spectral range (400–4000 cm-1) to study their structure systematically. IR spectrum of each glass composition shows a number of absorption bands. These bands are strongly influenced by the increasing substitution of SrO for PbO. Various bands shift with composition. Absorption peaks occur due to the vibrational mode of the borate network in these glasses. The vibrational modes of the borate network are seen to be mainly due to the asymmetric stretching relaxation of the B–O bond of trigonal BO3 units. More splitting is observed in strontium-rich composition.

    • Dielectric and impedance spectroscopic studies of (Sr1–𝑥Pb𝑥)TiO2 glass ceramics with addition of Nb2O5

      C R Gautam Devendra Kumar Om Parkash

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      Glasses were made by melt-quench method in the system [(Sr1–𝑥Pb𝑥)O.TiO2]–[2SiO2.B2O3]–5[K2O–BaO] (0.0 ≤ 𝑥 ≤ 0.4) with addition of 1 mol% Nb2O5. Perovskite strontium lead titanate in solid solution phase has been crystallized in borosilicate glassy matrix with suitable choice of composition and heat treatment schedule. Addition of 1 mol% of Nb2O5 enhances the crystallization of lead strontium titanate phase in the glassy matrix. Scanning electron microscopy (SEM) is performed to study the surface morphology of the crystallites and crystalline interface to the glass. Dielectric properties of these glass ceramics were studied by measuring capacitance and dissipation factor as a function of temperature at a few selected frequencies. Nb2O5 doped strontium lead titanate glass ceramic shows a high value of dielectric constant. It is of the order of 10,000 while the dielectric constant of undoped glass ceramic sample is of the order of 500. Complex impedance and modulus spectroscopic techniques were used to find out the contributions of polarization of crystallites and glass crystal interfaces to the resulting dielectric behaviour.

    • Dielectric properties of CaCu2.9Co0.1Ti4O12 and CaCu3Ti3.9Co0.1O12 ceramics synthesized by semi-wet route

      K D Mandal Alok Kumar Rai Laxman Singh Om Parkash

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      The effect of Co+2 doping on Cu+2 and Ti+4 sites in calcium copper titanate, CaCu3Ti4O12, has been examined. The doped compositions, CaCu3−𝑥Co𝑥Ti4O12 and CaCu3Ti4−𝑥Co𝑥O12 (𝑥 = 0.10) ceramics, were prepared by novel semi-wet route. In this method, calcium, copper and cobalt salts were taken in solution form and TiO2 was used in solid form. XRD analysis confirmed the formation of single-phase materials. Structure of CaCu3Ti4O12 does not change on doping with cobalt either on Cu-site or Ti-site and it remains cubic. Scanning electron micrographs (SEM) show average grain size of CaCu2.9Co0.1Ti4O12 to be larger than CaCu3Ti3.9Co0.1O12 ceramic. Energy dispersive X-ray spectroscopy (EDX) studies confined the purity of parent and Co-doped CaCu3Ti4O12 ceramics. Dielectric constant (𝜀r) and dielectric loss (tan 𝛿) of CaCu2.9Co0.1Ti4O12 is comparatively higher than that of CaCu3Ti3.9Co0.1O12 ceramic at all measured frequencies and temperatures.

    • Structural and mechanical behaviour of 5% Al2O3-reinforced Fe metal matrix composites (MMCs) produced by powder metallurgy (P/M) route

      Pallav Gupta Devendra Kumar Om Parkash A K Jha

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      The aim of this paper is to investigate the effect of sintering temperature and time on the properties of Fe–Al2O3 composite (5 wt% Al2O3; 95 wt% Fe) prepared by powder metallurgy process. X-ray diffraction, microstructure, density, hardness and compressive strength of prepared samples have been investigated. XRD studies show the presence of Fe and Al2O3 along with iron aluminate phase. Iron aluminate is formed as a result of reactive sintering between iron and alumina particles. Microstructural examination of the specimen showed a dense structure with nanosize dispersion of the reinforcement of ceramic phase. Density as well as hardness of specimens depend on the formation of iron aluminate phase, which in turn depends on sintering temperature and time.

    • Effect of height to diameter ($h/d$) ratio on the deformation behaviour of Fe–Al$_2$O$_3$ metal matrix nanocomposites


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      The present paper reports the effect of height to diameter ($h/d$) ratio on the deformation behaviour of Fe–Al$_2$O$_3$ metal matrix nanocomposites (MMNCs) during bulk processing. Sintered compacts were machined to the required size with different $h/d$ ratios. Test specimens were subjected to deformation at room temperature under three different interfacial friction conditions such as dry, solid and liquid lubrications. Deformed specimensshow a significant improvement in the density and hardness. Results also revealed the formation of a nanosize iron aluminate phase due to reactive sintering, which in turn contributes to grain refinement. Experimental density of the specimens was also verified with the theoretical density using the standard equations. It is expected that the present work will be useful in designing and developing MMNC products with better quality at competitive cost.

    • Electromagnetic interference shielding effectiveness of MgO−Al$_2$O$_3$−SiO$_2$ glass–ceramic system


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      MgO−Al$_2$O$_3$−SiO$_2$ (MAS)-based glass–ceramic system was prepared using very-low-cost raw materials, i.e.,talc, calcined alumina and calcined china clay with titanium dioxide as a nucleating agent. Glass–ceramics were preparedby a two-step process. In the first step, raw materials were mixed in the required proportion and melted at 1450$^{\circ}$C followedby water quench into a glassy frit. In the second step, powdered glass frit was uniaxially dry pressed into pellets followedby sintering at 1200$^{\circ}$C for 3h. X-ray diffraction pattern of the sintered compact shows well-defined peaks of cordieritealong with some anorthite and magnesium titanium oxide. The microstructure study of sample shows the presence of crystallineand glassy phases. Permittivity and permeability measurements were performed in the microwave frequency range12.4–17 GHz. The permittivity value of 5.7–6.0 and the permeability value of $\sim$1 were obtained. The reflection and transmission measurements show that the material possesses a shielding effectiveness in the range 2–10 dB over the frequencyrange 12.4–17 GHz.

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