Papers by Arundhati Sengupta
International Journal of Nanoscience Vol. 10, Nos. 4 & 5 (2011) 591-595, 2011
Ferromagnetic nanoparticles when dispersed in a dielectric ceramic matrix offer a new series of p... more Ferromagnetic nanoparticles when dispersed in a dielectric ceramic matrix offer a new series of percolation composites with a metal-semiconductor or insulator interface that tailors the functional properties. In this purview, ZrO2 of a high dielectric constant (εr) is stabilized in a cubic (c) polymorph by incorporating CrO2 of strongly ferromagnetic nanoparticles. A selective composition 20CrO2–80ZrO2 is synthesized after heating a polymer precursor with glycerol at 500°C–800°C in air for 2 h. Broadening of X-ray diffraction peaks reveals Cr4+:c-ZrO2 with an average crystallite size 5 nm in the 500°C heated sample and that is grown to 8 nm on increasing the temperature to 800°C. They occur as bundles of thin platelets in electron microscopic images. Both the samples have ferromagnetic hysteresis loops with 794 and 211 Oe of coercivities, respectively. Variations of εr-value and the power loss are studied as a function of frequency f (50 Hz–100 kHz range) on cold-pressed pellets at selective temperatures, in the 30°C–500°C range, in describing the effects of the magnetic species.
ACS Sustainable Chem. Eng. 2017, 5, 1280−1286, 2017
A Zn2+–silica modified CoFe2O4 (CZFS) nanostructured composite, useful for adsorbing cationic pol... more A Zn2+–silica modified CoFe2O4 (CZFS) nanostructured composite, useful for adsorbing cationic pollutants from water, was prepared by a wet-chemical method. The composite comprises cubic spinel crystallites (average 18 nm size) with amorphous silica clusters decorated on the crystallites-surface. Improved surface area (59.8 m2/g) of CZFS over those of Zn2+ modified CoFe2O4, CZF (32.6 m2/g), and CoFe2O4, CF (42.8 m2/g), together with its high negative ζ-potential of −35.4 mV (from surface SiO–) provides CZFS with improved adsorption capacity for Methylene blue (MB) over that of CZF and CF. MB adsorption (initial adsorbate concentration C0 = 5–25 mg/L) conforms to the Langmuir isotherm model, with maximum monolayer adsorption capacity Qm = 25.6 mg/g. CZFS exhibits adsorption efficiency Ae ≥ 98% for removal of heavy metal ions Cr3+, Cu2+ and Pb2+ (C0 = 5 mg/L). High Ae = 99.9% for Pb2+ dropped only to Ae = 98.8% for higher C0 = 20 mg/L (Qm = 19.8 mg/g). Saturation magnetization of 39 emu/g enables easy magnetic separation of CZFS from water. Good reusability of CZFS adsorbent was observed for up to three cycles. In summary, CZFS efficiently removes MB as well as heavy metal ions (especially Pb2+) from contaminated water.
IEEE Xplore, Article No. 6167955, pp. 205-209, 2011
Inclusion of magnetic spins such as Cr 4+ (3d2 ) in cubic zirconia c-ZrO2 , a dielectric oxygen i... more Inclusion of magnetic spins such as Cr 4+ (3d2 ) in cubic zirconia c-ZrO2 , a dielectric oxygen ion conductor, is useful to tailor functional dielectric, electrical, and magnetic properties in a single system for several applications. A simple liquid polymer precursor containing Cr4+ and Zr4+ ions in a dispersed structure via glycerol is explored to prepare a solid solution CrO2 -ZrO2 at a moderate pressure 1.5 atm. A recovered powder when heated at moderate temperature 500-800°C in ambient air yields a single phase compound of small crystallites (5-10 nm) in a c-ZrO 2 like stabilized phase. In this process, as much CrO 2 as 20 mol% has been dissolved. At room temperature, a sample having 20 mol% CrO 2 (heated at 500°C) exhibits a magnetic hysteresis loop with large coercivity 794 Oe, but small saturation magnetization 1.64 emu/g and remanent ratio 0.253. At low frequencies such as ~1 kHz, the dielectric permittivity (ε r ) stays nearly constant ~30 (close to the value 35 in pure c-ZrO 2 ) over a wide temperature range 30-200°C before it increases regularly with temperature, showing a value 183 at 500 °C. Also stable ac conductivity in this temperature range rises progressively by heating above 200 °C, with as large as five-orders of enhanced value 10-3 Sm-1 by heating to 500 °C. In a model two phase system, the ε r -value arises from the conductivity. Both the properties reveal an inflection point in the Curie transition point near 275 °C.
IEEE Xplore, Article no. 6609340, pp. 443-446, 2013
A compound Zr 0.8 Cr 0.2 O 2 upon adding Cr 4+ (3d2) spins in a dielectric host ZrO 2 exhibits im... more A compound Zr 0.8 Cr 0.2 O 2 upon adding Cr 4+ (3d2) spins in a dielectric host ZrO 2 exhibits impedance, magnetic and other functional properties. Small Zr 0.8 Cr 0.2 O 2 crystallites (5 nm average size) of a cubic ZrO 2 -like phase are obtained by heating a polymer precursor at 425 K temperature in ~0.6 bar pressure in an autoclave and then at 775 K for 2 h in ambient air. The sample reflects the Curie point (TC) as a distinct peak/transition in the real (Z') and imaginary (Z") impedance. The TC shifts from 505 K to ~585 K with increasing the frequency from 0.1 kHz to 100.0 kHz. The Z'-value traces a plateau below ~20.0 kHz and that widens progressively from 555 K to 615 K. The Z"-value exhibits a relaxation peak at ~8 kHz at 555 K and that shifts to higher frequencies with increasing temperature, reaching ~50 kHz at 615 K. The activation energy of relaxation of charge carriers drops from 1.76 eV over 550-580 K to 0.63 eV over 580-625 K in spin-disorder just above the TC.
Journal of Environmental Chemical Engineering 5 (2017) 5285–5292, 2017
A mesoporous composite (FZ) comprising microsheets of hematite modified and decorated with tetrag... more A mesoporous composite (FZ) comprising microsheets of hematite modified and decorated with tetragonal zirconia nanocrystallites (∼4 nm size), useful for adsorbing cationic pollutants from water, is prepared by a hydrothermal method. It exhibits improved adsorption behavior for methylene blue (MB) and various heavy metal ions (> 99% efficiency for Cu2+, Cd2+ and Pb2+, initial concentration C0 = 5–20 mg/L) as compared to unmodified zirconia (Z0) and hematite (F0). This behavior of FZ is ascribed to a combination of surface features: its high zeta potential ζ = −50.7 mV (∼1.8 times more negative than ζ of Z0 or F0) and large number of mesopores (size distribution 1.8–33 nm, maxima at 3.1 nm) leading to a reasonably high surface area (100.5 m2/g). For MB adsorption (C0 = 5–40 mg/L), correlation coefficient (R2) values for different adsorption isotherm models are in the order Langmuir (0.990)> Temkin (0.956)> Freundlich (0.842), indicating homogeneous FZ surface which supports a monolayer of MB. Maximum monolayer adsorption capacity Qm, Freundlich constant KF (related to adsorption capacity) and Temkin constant bT (related to adsorption heat) are 44.7 mg/g, 12.8 (mg/g)(L/mg)1/f and 257.8 J/mol, respectively (1/f = heterogeneity factor). Pb2+ adsorption conforms well to both the Langmuir and Freundlich models (R2 > 0.99), with Qm = 34.3 mg/g. Much higher KF = 75.0 (mg/g)(L/mg)1/f and bT = 665.2 J/mol (R2 = 0.941 for Temkin model) in this case indicates a significantly favorable Pb2+ adsorption. FZ exhibits reasonably good reusability for 3–4 cycles of use for MB/Pb2+ adsorption.
Materials Chemistry and Physics 142 (2013) 717-725, 2013
A spin Cr4+(3d2) doped ZrO2 dielectric phase has tailored magnetism, ionic conductivity and optic... more A spin Cr4+(3d2) doped ZrO2 dielectric phase has tailored magnetism, ionic conductivity and optical properties. The synthesis is made with a polymer complex of Cr6+ and Zr4+ salts in glycerol that gives a controlled ionic conversion Cr6+ → Cr4+ in ambient air. At ∼320 K in open air, a CrO3 solution (orange color) forms a metal ion–polymer complex which darkens to a blackish color of Cr4+ species. A hydrogel dried at ∼330 K is heated in an autoclave at a small pressure ∼0.5 bar at 420–470 K so that it yields Zr1−xCrxO2 (x ≤ 0.2) buried in carbon which is burnt out when annealing at higher temperatures ≥673 K in air. Small crystallites, with D = 3–10 nm diameter, adapt a cubic ZrO2-type phase at ≤1073 K. As much CrO2 as 20 mol% (x = 0.2) is doped in this way. An optimal annealing at 773 K gives D = 5 nm sample plates with magnetization M = 9.08 emu g−1 (in the CrO2 part) and coercivity Hc = 54.2 Oe at 295 K, or M = 85.79 emu g−1 (Hc = 100 Oe) at 5 K in a magnetic hysteresis loop (not saturating until 60 kOe field). The Curie point ∼482 K is enhanced over CrO2 (387 K) by strong Cr4+–O2−–Cr4+ superexchange interaction. Spin-assisted electrical conductivity drops largely by 85–100%, depending on the applied frequency 10–100 kHz, in the paramagnetic → ferromagnetic order. Small core–shell crystallites facilitate both the magnetic and transport properties.
Applied Surface Science 363 (2016) 346–355, 2016
Stable hydro/oleo-phobic and superhydrophobic nanopowders, useful for self-cleaning applications,... more Stable hydro/oleo-phobic and superhydrophobic nanopowders, useful for self-cleaning applications,are synthesized at room temperature by modifying Zr(OH)4·nH2O with a very low surface-energymolecule—1H,1H,2H,2H-perfluorododecyltrichlorosilane whose long chain { (CH2)2(CF2)9CF3moiety(PFD)} serves as surface-protrusion. The PFD-content is varied over 3.6–18.7 wt% in optimizing ahydrophilic to hydro/oleo-phobic or even to superhydrophobic transformation. Two halos in the X-raydiffraction pattern of amorphous Zr(OH)4·nH2O are accompanied by a peak at 2 = 18.0◦which growsin intensity progressively as the PFD-content increases from 5.2 to 18.7 wt%. The peak corresponds toCF2CF2crystalline order (10–20 nm) at the PFD-functionalized surface. The microstructure showsZr(OH)4·nH2O as a cloud-like phase, bonded to plate-like sheaths (PFD moiety). The C F stretching bandsat 1150 and 1210 cm−1grow in intensity relative to that of O H stretching at 3460 cm−1in proportionto the PFD-content. An 18.7 wt% PFD-functionalized sample exhibits a high contact angle CA = 153◦forwater (contact angle hysteresis = 4◦and roll-off angle <4◦), together with CA = 132◦for glycerol, CA = 130◦for diethylene glycol, and CA = 113◦for n-hexadecane, supporting good superhydrophobicity and oleo-phobicity. Surface-energy reduction due to PFD moiety together with an optimal spacing between thesurface-protrusions explains the water/organic liquid repellency.
Journal of Nano Research Vol. 28 (2014) pp 101-108, 2014
A transition metal ion Cr4+(3d2 spins) doped ZrO2 of small crystallites presents tailored magneti... more A transition metal ion Cr4+(3d2 spins) doped ZrO2 of small crystallites presents tailored magnetic, electrical and optical properties useful for magnetodielectric and spintronic applications. A liquid polymer precursor gel (Cr4+ and Zr4+ complex with glycerol) when heated in an autoclave at a small pressure 0.6-0.7 atm at 420–470 K results in a compound Zr0.8Cr0.2O2 with an average 5 nm crystallite size of a stabilized cubic (c)-ZrO2-type phase (after 2 h annealing at 773 K in air). A broad Raman band is observed at ~300 cm-1 in acoustic and transverse optic phonons characteristic of the c-ZrO2-type structure, with a prominent O–Cr–O stretching band at 878 cm-1 in a ferromagnetic order. A CrO2/Cr2O3 surface layer exhibits two weak bands at ~1011 cm-1 and 1032 cm-1. The sample exhibits an ‘S’-shaped ferromagnetic hysteresis loop (does not saturate below 60 kOe fields) at 5 K, with a magnetization M = 85.79 emu/g (in the CrO2 part) at 60 kOe and coercivity Hc = 100 Oe. With warming above 5 K, the loop converges progressively with only a weak ferromagnetism at room temperature, M = 9.08 emu/g and Hc = 54.2 Oe. As a pinning barrier, the uncompensated spins in the surface layer supports coercivity at low temperature. A model magnetic structure describes the magnetic properties in correlation to the microstructure.
fromProceedings of DAEBRNS third international symposium on materials chemistry [en] Combustion m... more fromProceedings of DAEBRNS third international symposium on materials chemistry [en] Combustion method has been successfully used in preparation of a large number of technologically useful ceramics (refractory, magnetic, dielectric, semiconductor, insulator, etc.). Modifying a dielectric oxide by inclusion of ferromagnetic species allows tuning dielectric, electrical, magnetoelectric, and other properties by combining favorable interaction between the dielectric and magnetic counterparts in a single united system of a hybrid nanocomposite. Composting a stabilized ZrO 2 by incorporating CrO 2 (a ferromagnetic oxide having as high value of magnetic moment as 1.92 μ B per Cr 4+ at 10 K with a nearly 100% spinpolarization ) has drawn great attention owing to specific applications in magnetic devices of ultrahigh magnetic density, giant magnetoresistance sensors, and dielectric based systems with low power loss at high frequencies. In this investigation, we intend to synthesize different compositions in studying the percolation behavior of the additives on the magnetoelectric properties. As a dielectric medium, ZrO 2 supports CrO 2 magnetic properties useful for telecommunications, ionic conductor, gas sensor, thermal barrier coating, and structural ceramics. A cubic (c) ZrO 2 required for such applications can be made to exist at room temperature on suitable grainrefinement with selective additives. Here, we report part of our work on synthesizing a magnetic composite 20CrO 2 80ZrO 2 by a simple combustion method from a mixture of CrO 3 and ZrO(NO 3 ) 2 ·H 2 O with camphor in ambient air Primary SubjectMATERIALS SCIENCE (S36)
AIP Conference Proceedings 1536, 981 (2013), 2013
A partial Cr4+(3d2)→Zr4+(4d0) substituted compound Zr1-xCrxO2, which offers magnetic, dielectric,... more A partial Cr4+(3d2)→Zr4+(4d0) substituted compound Zr1-xCrxO2, which offers magnetic, dielectric, and other functional properties, is prepared from a polymer precursor consisting of Cr4+ and Zr4+ ions dispersed in a medium of glycerol. A precursor powder, after heating a gel at ∼425 K at ∼0.6 bar pressure in retaining the Cr4+ species, when annealed at temperatures 670–770 K in ambient air for 2 h yields the Zr1-xCrxO2(x≤0.2) crystallites, 3–5 nm average size, of a cubic-ZrO2-type crystal structure. The microstructure in a sample annealed at 670 K and 770 K reveals nanorods and nanoplates, respectively. Over 5–300 K, both samples exhibit ferromagnetic hysteresis loops that do not saturate-up even in a large field 60 kOe. At 300 K (x = 0.2), a magnetization M (at 60 kOe) 0.40 emu/g in the nanorods rises to 1.32 emu/g in the nanoplates, with an increase in coercivity (Hc) from 6 Oe to 55 Oe. A crystallite-growth from 3 nm (at 670 K) to 5 nm (at 770 K) in single domain particles supports the increase in the M and Hc-value. Lower shape anisotropy in nanoplates than in nanorods further favors an increase in the Hc-value
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Papers by Arundhati Sengupta