• DEVENDRA KUMAR

      Articles written in Bulletin of Materials Science

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

      R K Mandal Ch Durga Prasad O M Parkash Devendra Kumar

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      Glasses with varying molar ratios of PbO/BaO in the system BaO-PbO-TiO2-B2O3-SiO2 were prepared keeping (BaO + PbO)/TiO2 ratio equal to one. The glasses were ceramized by two-stage heat treatment. X-ray diffraction indicates that PbTiO3 crystallizes in lead-rich glasses while BaTiO3 precipitates in barium-rich compositions. Solid solution (Ba, Pb)TiO3 does not seem to crystallize over the entire range of compositions. Simultaneous presence of PbO and BaO in the initial glass composition reduces the yield of ferroelectric phase. Dielectric properties have been interpreted in terms of microstructural features.

    • Electrical conduction in the solid solution La1 −xNaxCo1 −xNbxO3 (0·01 ⩽x ⩽ 0·99)

      O M Parkash Devendra Kumar Rabindra Kumar

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      Seebeck coefficient and DC resistivity of the solid solution La1 −x NaxCo1 −xNbxO3 (0·01 ⩽x ⩽ 0·99) have been measured in the temperature range 300–900 K. Seebeck coefficient is positive for all compositions over the temperature range of measurements. Conduction is due to 3d electrons of cobalt ions in the compositions withx ⩽ 0·60. Conduction occurs among localized sites for compositions withx ⩾ 0·70.

    • Preparation and characterization of Ca1 −xLaxTi1 −xCoxO3 (0·00<x⩽0·50) system

      O M Parkash Ch Durga Prasad Devendra Kumar

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      The possibility of the formation of solid solution in the system Ca1 −xLaxTi1 −x CoxO3 forx⩽0·5 has been investigated. X-ray diffraction studies show that compositions withx=0·05, 0·1, 0·2, 0·3 and 0·5 prepared by the ceramic method are single-phase materials. All the compositions have a structure similar to CaTiO3 with a pseudo-cubic unit cell. Preliminary studies show that interfacial polarization contributes significantly to their dielectric constant.

    • 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.

    • Electrical behaviour of lanthanum- and cobalt-doped strontium stannate

      O M Parkash K D Mandal C C Christopher M S Sastry Devendra Kumar

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      Electrical properties of lanthanum- and cobalt-doped strontium stannate, SrSnO3, have been studied as a function of temperature. All the compositions investigated have cubic structure. Measurement of Seebeck coefficient in one of the cobalt-doped SrSnO3 sample shows that it exhibitsp-type conductivity. Results of AC conductivity measurements show that conduction occurs by hopping of charge carriers in lanthanum-doped samples and forx=0·01 in cobalt-doped sample. Inx=0·05 sample in cobalt-doped system, conduction seems to occur in extended states.

    • Equivalent circuit models for electronic ceramics

      Lakshman Pandey O M Parkash Rajesh K Katare Devendra Kumar

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      Complex immittance spectra of model equivalent circuits involving resistive and capacitive elements are calculated. A comparison of experimentally obtained complex immittance plots with these diagrams greatly facilitates the search for the most appropriate equivalent circuit representing the electrical properties of electronic ceramics.

    • Dielectric and microstructural behaviour of strontium titanate borosilicate glass ceramic system

      O P Thakur Devendra Kumar O M Parkash Lakshman Pandey

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      65(SrO·TiO2)−35(2SiO2·B2O3) wt% glass was synthesized. Differential thermal analysis study shows one exothermic peak which shifts towards higher temperature with increasing heating rate. Glass ceramics prepared by controlled crystallization of strontium titanate borosilicate glass produce uniform distribution of crystallites in a glassy matrix. Attempt was made to crystallize strontium titanate phase in this glass ceramic. Different phases precipitated out during ceramization have been identified by X-ray diffraction. It appears that due to high reactivity of SrO with B2O3, strontium borate crystallizes as principal phase followed by TiO2 (rutile) and Sr3Ti2O7 phases. Dielectric constant of these glass ceramics was observed to be more or less temperature independent over wide range of temperatures with low values of dielectric constant and dissipation factor.

    • Effect of 1 wt% CoO addition on dielectric and microstructural behaviour of (SrO·TiO2)-(2SiO2·B2O3) glass and glass ceramic

      O P Thakur Devendra Kumar O M Parkash Lakshman Pandey

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      Glass of the nominal composition 64 wt%(SrO·TiO2)·35 wt%(2SiO2·B2O3)-1 wt%(CoO) was prepared. The glass samples were subjected to heat treatment at 900 and 950 C. The phase progression in these glass ceramics from X-ray diffraction studies shows the formation of Sr2B2O5 as primary crystalline phase followed by rutile (TiO2), Sr3Ti2O7, SrB2Si2O8 and Sr3B2SiO8 as secondary phases. The first DTA exothermic peak of glass corresponds to the crystallization of Sr2B2O5, rutile and Sr3Ti2O7 phase while second crystallization peak may be assigned to the formation of SrB2Si2O8 and Sr3B2SiO8 phases. From microstructure studies we find that strontium borate grows with larger grain size whereas the other phases like Sr3Ti2O7, TiO2 appear smaller in size. Cobalt oxide content in the strontium titanate borosilicate glass ceramic gives the thermal stability to dielectric behaviour and decreases the dielectric loss.

    • Effect of composition on dielectric and electrical properties of the Sr1 −xLaxTi1 −xCoxO3 system

      Shail Upadhyay Devendra Kumar O M Parkash

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      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.

    • 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.

    • 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

      PALLAV GUPTA DEVENDRA KUMAR A K JHA OM PARKASH

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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.

    • Polymer-assisted co-precipitation route for the synthesis of Al$_2$O$_3$–13% TiO$_2$ nanocomposite

      NEERA SINGH RANABRATA MAZUMDER PALLAV GUPTA DEVENDRA KUMAR

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      The present investigation reveals the effect of processing parameters on the properties of alumina–titania (Al$_2$O$_3$–TiO$_2$) nanocomposites. A polymer-assisted (Pluronic P123 triblock co-polymer) co-precipitation route has been employed to synthesize Al$_2$O$_3$–TiO$_2$ nanoparticles. As a surfactant, pluronic P123 polymer exhibits hydrophobic as well as the hydrophilic nature simultaneously which detains the agglomeration and hence the nano size particle have been obtained. Effect of surfactant concentration on morphology and particle size of product has also been investigated. Thermal behaviour of the prepared powder samples have been studied using differential scanning calorimeter/thermal gravimetric analysis and dilatometer. Formation of aluminium-titanate (Al$_2$TiO$_5$) phase has been confirmed using X-ray diffraction analysis. It has been observed by field emission scanning electron microscopy analysis that the particle size reduced effectively (below 100 nm) when polymer-assisted co-precipitation route is used instead of the simple co-precipitation technique. A highly dense microstructure of sintered samples has been obtained, driven by reduced particle size.

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

      PREETI KUMARI PANKAJ TRIPATHI OM PARKASH DEVENDRA KUMAR

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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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