Showing papers by "Defence Metallurgical Research Laboratory published in 2015"

Effect of poling process on piezoelectric properties of sol–gel derived BZT–BCT ceramics

Abstract: Lead free piezoelectric ceramics barium zirconate titanate-barium calcium titanate, [xBZT-(1-x)BCT] (0.48≤ x ≤0.52), were synthesized by sol-gel method. Calcination of the as-synthesized precursor powders resulted in crystalline powders with single-phase perovskite structure at 700°C, which is significantly lower than that obtained by solid-state reaction. Solid-state sintering at 1450°C resulted in highly dense microstructure with ≥95% of the theoretical density. The effect of electric poling on the piezoelectric properties of the sintered [xBZT-(1-x)BCT] ceramics is studied with varying poling temperatures and poling fields. The results indicated that optimized poling conditions are required in enhancing the piezoelectric properties of [xBZT-(1-x)BCT] ceramics. The morphotropic phase boundary (MPB) composition, 0.5BZT-0.5BCT, showed a high remanent polarization (P r) of 12.2μC/cm2 and a low coercive field (E c) of ~0.14kV/mm. The optimized poling conditions resulted in high piezoelectric charge coefficient d 33 ~637pC/N, large electromechanical coupling coefficient k p ~59.6%, a large strain of 0.157%, a large piezoelectric voltage constant g 33 ~29mVm/N for (0.5BZT-0.5BCT) composition. In this report, the excellent piezoelectric properties of the sol-gel derived BZT-BCT ceramics has been analysed and correlated to its structure and poling conditions.

Origin of spin gapless semiconductor behavior in CoFeCrGa: Theory and Experiment

Abstract: Despite a plethora of materials suggested for spintronic applications, a new class of materials has emerged, namely spin gapless semiconductors (SGS), which offers potentially more advantageous properties than existing ones These magnetic semiconductors exhibit a finite band gap for one spin channel and a closed gap for the other Here, supported by electronic-structure calculations, we report evidence of SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (prototype LiMgPdSn) structure but exhibiting chemical disorder (${\mathrm{DO}}_{3}$ structure) CoFeCrGa is found to transform from SGS to half-metallic phase under pressure, which is attributed to unique electronic-structure features The saturation magnetization $({M}_{S})$ obtained at $8\phantom{\rule{016em}{0ex}}\mathrm{K}$ agrees with the Slater-Pauling rule and the Curie temperature $({T}_{C})$ is found to exceed $400\phantom{\rule{016em}{0ex}}\mathrm{K}$ Carrier concentration (up to $250\phantom{\rule{016em}{0ex}}\mathrm{K}$) and electrical conductivity are observed to be nearly temperature independent, prerequisites for SGS The anomalous Hall coefficient is estimated to be $185\phantom{\rule{016em}{0ex}}\mathrm{S}/\mathrm{cm}$ at $5\phantom{\rule{016em}{0ex}}\mathrm{K}$ Considering the SGS properties and high ${T}_{C}$, this material appears to be promising for spintronic applications

Effect of alloying addition and microstructural parameters on mechanical properties of 93% tungsten heavy alloys

Abstract: Liquid phase sintering, heat treatment and swaging studies on three tungsten heavy alloys, 93W–4.9Ni–2.1Fe (wt%), 93W–4.2Ni–1.2Fe–1.6Co (wt%) and 93W–4.9Ni–1.9Fe–0.2Re (wt%) were carried out in detail with respect to microstructure, tensile and impact properties. All the alloys were sintered and swaged to 40% deformation. The results indicate that Re addition reduces the grain size of the alloy compared to W–Ni–Fe and W-Ni-Fe-Co alloys. W–Ni–Fe–Re alloy shows superior tensile properties in heat treated condition as compared to W–Ni–Fe and W–Ni–Fe–Co alloys. SEM study of fractured specimens clearly indicates that the failure in case of W–Ni–Fe–Re was due to transgranular cleavage of tungsten grains and W–W de-cohesion. W–Ni–Fe and W–Ni–Fe–Co alloys also failed by mixed mode failure. However, in these cases, ductile dimples corresponding the failure of the matrix phase was rarely seen. Thermo-mechanical processing resulted in significant changes in mechanical properties. While W–Ni–Fe–Re alloy showed the highest tensile strength (1380 MPa), W–Ni–Fe–Co exhibited the highest elongation (12%) to failure. A detailed analysis involving microstructure, mechanical properties and failure behavior was undertaken in order to understand the property trends.

Constitutive modeling for predicting peak stress characteristics during hot deformation of hot isostatically processed nickel-base superalloy

Abstract: Hot flow behavior of hot isostatically processed experimental nickel-based superalloy is investigated over temperature and strain rate ranging from 1000–1200 °C and 0.001–1 s−1, respectively by carrying out constant true strain rate isothermal compression tests up to true strain of 0.69. True stress–true strain curves corrected for adiabatic temperature rise exhibited rapid strain hardening followed by flow softening behavior irrespective of temperature and strain rate regimes investigated, although anomalous flow behavior is observed at 1200 °C. Variation of peak flow stress with temperature is corroborated to the microstructural changes pertaining to the morphology and relative volume fraction of the phases present. From the experimental results, constitutive model incorporating the effects of strain rate, strain, and temperature is established to describe the hot flow behavior of investigated alloy. Dependence of peak flow stress on strain rate and temperature described by Zener–Hollomon (Z) parameter indicated increase in peak flow stress with Z. Additionally Cingara-Queen equation is employed to predict flow curve up to peak stress. The reliability of developed constitutive models is validated statistically and the results indicate reasonable agreement with experimental findings.

Compliments of confinements: substitution and dimension induced magnetic origin and band-bending mediated photocatalytic enhancements in Bi1−xDyxFeO3 particulate and fiber nanostructures

Abstract: The manifestation of substitution and dimension induced modifications in the magnetic origin and photocatalytic properties of Dy substituted bismuth ferrite (BDFOx) particulate and fiber nanostructures are reported herein. A gradual transformation from rhombohedral to orthorhombic structure is observed in BFO with the increasing concentration of Dy. Substitution induced size reduction in particulate and fiber nanostructures is evident from the scanning and transmission electron micrographs. Energy band structures of both particulate and fiber nanostructures are considerably influenced by the Dy substitution, which is ascribed to the formation of new energy states underneath the conduction band of host BFO. Field dependent and temperature dependent magnetic studies reveal that the origin of magnetism in pure BFO systems is due to the antiferromagnetic-core/ferromagnetic-shell like structure. On the other hand, it gets completely switched into ‘canted’ spin structures due to the substitution induced suppression of cycloidal spins in BFO, which is found to be the origin of magnetism in BDFOx particulate and fiber nanostructures. The visible light driven photocatalytic activity of BDFOx nanostructures is found to be enhanced with increasing concentration of Dy. Substitution induced band gap modification, semiconductor band bending phenomenon mediated charge transfer and reduced recombination resistances are attributed to the observed photocatalytic enhancements in these nanostructures.

Edge-carboxylated graphene anchoring magnetite-hydroxyapatite nanocomposite for an efficient 4-nitrophenol sensor

Abstract: The surface chemistry and physical properties of edge-carboxylated graphene (ECG) have to date been over looked in terms of understanding the real world practical applications. The accurate identification of each possible oxygenated group on the surface of the basal plane as well as the edges of ECG is necessary to understand the properties for their potential multifunctional applications. Herein, we report the use of a simple high energy ball mill to prepare a large scale production of ECG from natural graphite flakes through interaction with aspartic acid under solid conditions. These 2 dimensional ECG sheets were anchored with magnetite-hydroxyapatite (m-HAp) using a simple hydrothermal process. The prepared materials were systematically investigated by various analytical techniques to realize the structural, morphological, compositional and functional properties. These m-HAp dispersed ECG sheets can be further used to modify the glassy carbon electrode (GCE) for the sensitive and selective detection of 4-nitrophenol (4-NP) by cyclic voltammogram (CV) and differential pulsed voltammetry (DPV). The high specific surface area of 130 m2 g−1 for the m-HAp on ECG displays an excellent catalytic activity with reversible redox behavior of 4-NP. The modified electrode possesses a good detection limit and high sensitivity of 0.27 μM and 0.587 μA μM−1 cm−2, respectively, towards 4-NP, rendering practical industrial applications.

Effect of high temperature ageing on microstructure and mechanical properties of a nickel-free high nitrogen austenitic stainless steel

Abstract: The effect of ageing and oxidation at 850 °C for different durations viz., 1, 10, 100 and 200 h on microstructure and mechanical properties of a nickel free high nitrogen steel (Fe–Cr–Mn–N) was investigated. The microstructure of the alloy in as-received condition essentially consists of fine equi-axed austenite grains with annealing twins. However, ageing at 850 °C has resulted in the formation of very fine, globular and discrete Cr 2 N precipitates initially at grain boundaries for an ageing time of 1 h and subsequently within the grains with increasing ageing time beyond 1 h. The size of these precipitates has progressively increased with further ageing. In addition, coarse intermetallic precipitates of (Fe,Mn)Cr type with body centred tetragonal structure were observed in the samples aged for ≥100 h at 850 °C. The effect of these precipitates on mechanical properties of the alloy was studied. It was noticed that the mechanical properties deteriorated on ageing, which has been attributed to the depletion of solid solution strengthening elements like nitrogen, chromium and manganese from the matrix to form nitrides and intermetallic precipitates. Moreover the alloy aged in air (i.e., oxidised) exhibits marginally inferior tensile properties as compared to unoxidised alloy.

Poling electric field dependent domain switching and piezoelectric properties of mechanically activated (Pb0.92La0.08)(Zr0.60Ti0.40)O3 ceramics

Abstract: (Pb0.92La0.08)(Zr0.60Ti0.40)O3 (PLZT 8/60/40) ceramics were prepared via a combinatorial approach of high energy mechanical ball milling (mechanical activation), followed by cold isostatic pressing. Electrical properties of the piezoelectric ceramics are greatly influenced by the poling conditions (poling electric field, poling time and poling temperature). In this present study, the effect of the poling electric fields on the piezoelectric properties of PLZT 8/60/40 ceramics was investigated. It is a common practice to subject piezoelectric ceramics to electric fields well beyond their coercive fields, in order to pole them but here the measured values of piezoelectric (d33 and g33) and electromechanical coupling coefficients (kp, k33 and k31) at different poling fields show that a ferroelectric material can be poled at ~5 kV/cm (<0.5 Ec), far below the coercive field without compromising the induced high piezoelectricity. The results were discussed with the help of polarization, current and strain versus electric field curves.