- Effect of nitrogen substitution on the structural and magnetic ordering transitions of NiCr2O4
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The nitrogen (N) doped spinel NiCr2O4 has been synthesized at 773 K (N500) and 873 K (N600) by ammonolysis of NiCr2O4 powders to study the effect of anion doping on its structural and magnetic properties. The N contents are determined by thermogravimetric oxidation, yielding a composition that can be described as NiCr2O3.68N0.21 (N500) and NiCr2O3.55N0.30 (N600). X-ray photoelectron spectroscopic studies suggest that N3? species partly substitute the oxygen in the lattice and oxygen vacancies exist in the N doped samples. There is evidence that in the nitrided sample, the Cr ion is most likely in a mixed oxidation state. As the N content increases, the structure at room temperature changes from tetragonal to the cubic phase; N500 is only partially tetragonal; N600 is completely cubic. Such structural change is the consequence of the depression of the cooperative Jahn-Teller effect of Ni2+ in the tetrahedral A site caused by the presence of N3?. Combined heat capacity and temperature dependent magnetic susceptibility measurements give clear evidence of the magnetic and structural transitions in the N doped NiCr2O4. The Jahn-Teller transition temperatures decrease with increasing N content; this is likely due to increased covalency and hence enhanced contribution of the angular momentum and the spin-orbit coupling to local chemical bonding around Ni2+. Antiferromagnetic transitions are observed at TS = 23 K and 22 K for N500 and N600, respectively. Hence there is indeed a lowering of transition when compared to pure NiCr2O4 (28 K). The magnetic loops at different temperatures confirm that the material behaves as a paramagnet over a wide range of temperatures T ~ 80-350 K. The material also exhibits a canted ferrimagnetic structural transition between 30 and 70 K. We also report evidence for increased frustration and lowered correlation length in N doped compounds compared to the parent NiCr2O4. The present study on N? doping effects on the structure and magnetic properties of this NiCr2O4 is expected to be useful for tailoring the ferric phase transitions through anion substitutions.
- Liu, Xin,Yin, Nan,Thomas, Tiju,Yang, Minghui,Wang, Junhu,Shi, Quan
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p. 112140 - 112147
(2016)
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- Xanes and crystal field spectroscopy of five-coordinated nickel(II) in potassium-nickel phosphate
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The first spectroscopic data on KNiPO4 used as reference compound for 5-coordinated nickel in crystalline oxide compounds are presented. The Ni-K edge X-ray Absorption Near Edge Structure (XANES) spectrum is compared to those of 4- and 6-coordinated Ni in NiCr2O4 and CaNiSi2O6, respectively. The intensity of the pre-edge increases from 6- to 4-coordination while that of the main-edge decreases. Both features are thus sensitive indicators of the Ni coordination state. Crystal field spectra are also different for the three nickel coordination states. The KNiPO4 crystal field spectrum exhibits a position and relative intensity of the absorption bands similar to those found in the spectra of pentacordinated Ni complexes. They are interpreted using the electron energy level diagram of Ni(II), in a distorted square pyramid crystal field. Criteria for the use of XANES and crystal field spectroscopy to characterize the presence of 5-coordinated Ni in oxide compounds are given. The similar crystal field stabilization energy encountered in 5- and 6-coordinated nickel explains the stability of 5-coordinated nickel in phosphates.
- Galoisy,Calas
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- Structural transition of spinel compound NiCr2 O4 at ferrimagnetic transition temperature
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Magnetic properties and crystal structure of spinel compound NiCr2 O4 have been investigated by magnetization and high-resolution X-ray powder diffraction measurements. The structural transition from tetragonal to orthorhombic symmet
- Ishibashi,Yasumi
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- Dielectric relaxation and small polaron hopping transport in sol-gel-derived NiCr2O4 spinel chromite
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Herein, we have synthesized NiCr2O4 chromite by the sol-gel auto-combustion route. The refined X-ray powder diffraction pattern confirmed the single-phase formation of the material, which crystallized in a cubic normal spinel structure with space group Fd3m. The complex impedance study indicated the existence of relaxation phenomenon and predicted the semiconducting nature of the material. The frequency-dependent Nyquist plot modeled by (RGBCGB) (RGQG) equivalent circuit exhibited the effects of grain and grain boundary to the electrical response of the compound over the measured temperature range of 263-393 K. The experimental AC conductivity data followed Jonscher's double power-law behavior, and the frequency exponent profile suggested small polaron hopping as the most probable transport mechanism for the sample. The complex modulus analysis showed the non-Debye type behavior of the material. In contrast, the dielectric constant exhibited dispersion in the low-frequency regime with a large tangent loss directly related to temperature.
- Javed, Muhammad,Khan, Ayaz Arif,Kazmi, Jamal,Mohamed, Mohd Ambri,Khan, Muhammad Nasir,Hussain, Mubushar,Bilkees, Rehana
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- Oxygen-sensing properties of spinel-type oxides for stoichiometric air/fuel combustion control
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The temperature dependence of electrical resistance of several spinel-type oxides was measured in exhaust gases of air-methane combustion, to evaluate the potential of the oxides as an air/fuel stoichiometric oxygen sensor. The results are discussed in te
- Shimizu,Kusano,Kuwayama,Tanaka,Egashira
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- Magnetic structure of NiCr2O4 studied by neutron scattering and magnetization measurements
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The magnetic ordering of the normal spinel ferrimagnet NiCr 2O4 below TC = 74 K was reinvestigated by neutron scattering and magnetization measurements on a powder specimen. We found another magnetic transition at TS = 31 K besides TC in both experiments. The ordering of a ferrimagnetic (longitudinal) component and that of an antiferromagnetic (transverse) component occur at TC and TS, separately. A new magnetic structure model of NiCr 2O4 below TS with a spontaneous magnetization of about 0.3 μB/formula is proposed based on experimental neutron scattering intensity. 2004 The Physical Society of Japan.
- Tomiyasu, Keisuke,Kagomiya, Isao
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- Spin glass and exchange bias behavior in magnetically frustrated Ni1?xMgxCr2O4 (x = 0.0–0.50)
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Single-phase polycrystalline samples of Ni1?xMgxCr2O4 (x = 0.0–0.50) were prepared by sol gel route and their structural and magnetic properties were studied. Structural transformation from tetragonal (I41/amd) to cubic (Fd3-m) phase is observed at room temperature due to Mg substitution. Mg substitution gives rise to reduction in ferrimagnetic and antiferromagnetic transition temperatures along with a signature of spin glass like phase in samples of intermediate compositions, i.e., for x = 0.10, 0.20 and 0.30. The observed stretched exponential type relaxation of thermoremanent magnetization and the highly frustrated magnetic behavior confirm the glassy magnetic phase. Significant increase in exchange bias field under field cooled condition for the Mg substituted samples is observed. The origin of exchange bias in theses samples is explained by considering exchange anisotropy between the ferrimagnetic and antiferromagnetic components of canted spin. The training effect of exchange bias field is also observed.
- Borah, Ritupan,Ravi
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- New co-precipitation technique for the preparation of mixed-oxides
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Fine ceramic oxide powders of the composition MN2O4 [spinels, where M = Ni(II), Zn(II), Co(II), Cd(II) and N = Fe(III), Cr(III)], M3Fe5O12 [garnets, where M = Y, Gd] and RBa2Cu3O7+δ [ceramic superconductors, where R = rare earths] have been synthesized through a novel coprecipitation technique. The novelty of this route lies in its simplicity and cost-effectiveness. The coprecipitation has been done using a mixture of (NH4)2CO3/(NH4)HCO3, triethylamine and formaline in the presence of soluble polymers such as polyvinyl alcohol (PVA). The final product formation is confirmed and characterized by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX).
- Pathak,Mukhopadhyay,Pramanik
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- Catalytic Decomposition of Isopropanol over Chromite Spinels MCr2O4 (M=Ni, Mn and Mg)
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The decomposition of isopropanol over nickel, manganese and magnesium chromite spinel catalysts has been investigated in the vapour phase in an integral reactor.Its decomposition follows first-order kinetics.Kinetic and thermodynamic parameters have been calculated using the Arrhenius and Eyring equations.The activity pattern is found to be NiCr2O4>MnCr2O4>MgCr2O4.The chromite spinels have been characterised by X-ray studies, i.r. spectral analysis, conductivity and thermoelectric potential measurements.All three chromites were found to be p-type semiconductors in the temperature range 150-400 deg C.Exclusive dehydrogenetion is shown by NiCr2O4 and MnCr2O4, whereas MgCr2O4 functions as a dehydrogenation and dehydration catalyst.A linear correlation exists between the entropy of activation and the activation energy for electrical conduction for the chromite spinels studied.
- Balasubramanian, Krithivasa,Krishnasamy, Vengaimuthu
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- Subsolidus of the MO-Cr2O3-V2O5 systems, where M=Ni, Mg
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Phase relations in the system: MO-Cr2O3-V2O5, where M=Ni and Mg, were determined up to the solidus lines over the whole components concentration range. On the basis of this, a division of both systems into subsidiary subsystems was performed. The melting temperatures of individual areas were also established.
- Rychlowska-Himmel,Bosacka
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- [NiEn3]CrO4: Structure, thermal properties, and pseudomorphism
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The crystal structure of [NiEn3]CrO4 (En is ethylendiamine), C6H24CrN6NiO4 is studied: a = 9.0408(5) ?, c = 9.7466(4) ?, V = 689.92(8) ?3, space group P63/mmc, Z = 2, dx = 1.704 g/cm3, Ni–N 2.133(9) ?, ∠N–Ni–N 82.4(5)°. The thermal properties are determined. When it is heated in a hydrogen atmosphere, a mixture of nanocrystalline Ni and Cr2O3 powders with coherent scattering regions of 59 nm and 90 nm respectively is formed. It is demonstrated that thermal decomposition products consist of pseudomorphic particles that inherit the sizes and shapes of initial [NiEn3]CrO4 single crystals.
- Khranenko,Komarov, V. Yu.,Gerasimov, E. Yu.,Zadesenets,Maksimovsky,Gromilov
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- Enhanced magnetodielectric response in Dy modified NiCr2O4
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The chemically synthesized high purity spinel NiCr2?xDyxO4 (x?=?0, 0.1) samples have been characterized using magnetic and dielectric measurements in presence of high magnetic field. Crystal and magnetic structure of the samples have been determined by analyzing neutron diffraction data recorded between temperature of 6?K and 300?K. NiCr2O4 crystallizes in tetragonal phase with the space group of I41/amd whereas NiCr1.9Dy0.1O4 crystallizes in the mixed phase of cubic (space group of Fd3ˉm) and tetragonal phases at room temperature. An additional phase of DyCrO3 with orthorhombic structure has been found in the Dy doped compound. The lattice parameter a increases and the c decreases in tetragonal structure with the substitution of Dy in Cr site. Both the samples show superlattice reflection peak indicating the presence of long range AFM ordering (transverse component) below 40?K. But the saturation magnetization slightly increases after Dy doping. An anomaly observed near Curie temperature in ε′ (T) of NiCr2O4 and NiCr1.9Dy0.1O4 demonstrates the contribution of coupling between ferroelectricity and ferrimagnetism in the compounds. A linear correlation between the difference in dielectric constant and the field dependent squared magnetization for both the samples has been observed. The spin–spin interactions are most likely responsible for the observed magnetodielectric (MD) effect due to the magnetodielectric hysteresis in both parent and doped samples. Interestingly the MD% is found to increase with Dy doping.
- Mandal,Singh, Ripandeep,Das,Sarkar, Tarapada,Nath
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- Resonance-like magnetic excitations in spinel ferrimagnets FeCr 2O4 and NiCr2O4 observed by neutron scattering
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We performed powder neutron scattering experiments on spinel ferrimagnets ACr2O4 with magnetic and Jahn-Teller A2+ ions (A = Fe, Ni, Cu). In both FeCr2O4 and NiCr 2O4, although geometric frustration had been expected to vanish so far, resonance-like magnetic excitation modes were discovered around Q ? 1.4 A-1 and E ? 5 and 8 to 9 meV in a low-temperature magnetic order phase, similar to the local spin resonance modes early reported in highly frustrated spinel antiferromagnets ACr 2O4 with nonmagnetic and non-Jahn-Teller A2+ ions (A = Mg, Zn). In a high-temperature magnetic order phase the resonance-like inelastic scattering transformed into quasielastic scattering. In CuCr 2O4 no such diffusive scattering was observed. We interpret the resonance-like magnetic excitations as a dynamical spin-frustration effect, and discuss a necessary condition for the appearance of them. 2008 The Physical Society of Japan.
- Tomiyasu, Keisuke,Hiraka, Haruhiro,Ohoyama, Kenji,Yamada, Kazuyoshi
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- Structural and magnetic properties of Cu-Ni-Cr spinel oxides
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The compounds CuCr2O4 and NiCr2O4 crystallize at room temperature in a tetragonal distorted spinel structure, s.g. I41/amd, with axes ratio c/a1, respectively. The distortion is caused by the Jahn-Teller ions Cu2+ and Ni2+ which flatten or elongate their surrounding oxygen tetrahedron. CuCr2O4 and NiCr2O4 form a complete solid solution series Cu1-xNixCr2O4 where for 0.825a temperature-dependent crystallographic phase transition from cubic to tetragonal symmetry between 865 K (CuCr2O4) and 310 K (NiCr2O4). The phase Cu0.15Ni0.85Cr2O4 undergoes a second crystallographic transition to orthorhombic symmetry, space group Fddd, at T=300 K. The neutron diffraction experiments as well as SQUID measurements reveal magnetic ordering of the ions between 150 and 50 K which partially occurs as a two-step mechanism.
- Tovar,Torabi,Welker,Fleischer
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- Immiscibility in the NiFe2O4-NiCr2O 4 spinel binary
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The solid solution behavior of the Ni(Fe1-nCrn) 2O4 spinel binary is investigated in the temperature range 400-1200 °C. Non-ideal solution behavior, as exhibited by non-linear changes in lattice parameter with changes in n, is observed in a series of single-phase solids air-cooled from 1200 °C. Air-annealing for 1 year at 600 °C resulted in partial phase separation in a spinel binary having n=0.5. Spinel crystals grown from NiO, Fe2O3 and Cr 2O3 reactants, mixed to give NiCrFeO4, by Ostwald ripening in a molten salt solvent, exhibited single-phase stability down to about 750 °C (the estimated consolute solution temperature, T cs). A solvus exists below Tcs. The solvus becomes increasingly asymmetric at lower temperatures and extrapolates to n values of 0.2 and 0.7 at 300 °C. The extrapolated solvus is shown to be consistent with that predicted using a primitive regular solution model in which free energies of mixing are determined entirely from changes in configurational entropy at room temperature.
- Ziemniak,Gaddipati,Sander
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- Exchange bias and magnetization reversal in Ni(Cr1?xFex)2O4 (x=0–0.20)
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Exchange bias and magnetization reversal in single phase samples of Ni(Cr1?xFex)2O4 (x=0–0.20) were studied through magnetic measurements. Substitution of Fe for Cr changes the crystal structure at room temperature from tetragonal (space group: I41/amd) to cubic (space group: Fd3ˉm) form. Temperature variation of magnetization measurements show that these samples undergo ferrimagnetic transitions and the transition temperature (TC) increases from 73 K for x=0.0 to 314 K for x=0.20. An interesting magnetization reversal phenomenon was observed for x=0.06 sample with a magnetic compensation temperature of 49 K. M–H loop measurements at different temperature show the signature of presence of strong antiferromagnetic interactions especially at low temperature (Ta maximum exchange bias field of 5670 Oe is observed for x=0.06 sample and it is explained by considering the exchange anisotropy between the ferrimagnetic and the antiferromagnetic components. The exchange bias field and the vertical shift in magnetization decrease exponentially with increase in temperature.
- Barman, Junmoni,Babu,Ravi
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- Structure and magnetic phase transitions in (Ni1? xCox)Cr2O4 spinel nanoparticles
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NiCr2O4 and CoCr2O4 are two distinct members of the chromite family (ACr2O4) having unique structural and magnetic properties. This paper is a continuation of previous work [Mohanty et al., J. Magn. Magn. Mater. 451(2018)20] and is focused on the modification of ferrimagnetic properties in NiCr2O4 by substituting Co ions at Ni sites. In order to do so (Ni1? xCox)Cr2O4 (x = 0, 0.50, 0.75, 1.00) powder samples were synthesized using chemical co-precipitation techniques. Interest is primarily on (Ni0.5Co0.5)Cr2O4 and (Ni0.25Co0.75)Cr2O4 as characterization results on the end members have been reported. It was determined with in-situ temperature dependent X-ray diffraction measurements that these compositions become fully crystallized cubic spinels with space group Fd3-m above 700 °C. All samples were subsequently calcined at 900 °C to ensure phase purity and uniform crystallinity. Transmission electron microscopy showed non-uniform distribution of particle sizes with the majority of the particles having bi-pyramidal structures with flat tops for both samples. A comparative account of different ferrimagnetic Curie temperature, θf or TC, estimations are presented as determined from measured temperature dependences of the following techniques: Fits of the magnetic susceptibility data indicated a minimum value for the (Ni0.5Co0.5)Cr2O4 sample at 56.1 ± 0.9 K, followed by an increase to 76.8 ± 0.4 K for the (Ni0.25Co0.75)Cr2O4 sample; Neutron diffraction measurements gave values of 80 ± 1 K and 87 ± 1 K; Their paramagnetic Curie points, θp, were determined to be 73.3 and 84.9 K, respectively. The magnetic frustration index f = ΘCW/TC was found to decrease continuously over the series from 11.9 for NiCr2O4 to 7.1 and 6.5 for x = 0.5 and 0.75 respectively with (Ni0.5Co0.5)Cr2O4 being more frustrated than (Ni0.25Co0.75)Cr2O4. Magnetization as a function of applied magnetic field measured at 3 K demonstrates unusually high coercivity for (Ni0.5Co0.5)Cr2O4, supporting the notion for the higher degree of magnetic frustration in this sample. No exchange bias was observed for M (μ0H) in the (Ni0.25Co0.75)Cr2O4 sample measured at 3 K, but did however show a significant anomaly in its hysteresis loop at this temperature. The anomaly in hysteresis behaviour disappeared when measured at 1.7 K and also at higher temperatures. Frequency dependent ac-susceptibility measurements revealed no shift in the peak position with frequency refuting the existence of spin-glass like behavior in both these samples.
- Mohanty,Venter,Sheppard,Prinsloo
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- A magnetocaloric study on the series of 3d-metal chromites ACr2O4 where A = Mn, Fe, Co, Ni, Cu and Zn
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The 3d-metal chromites ACr2O4 where A is a magnetic ion, show the paramagnetic to ferrimagnetic phase transition at TC while for non-magnetic A-site ion, ACr2O4 show paramagnetic to antiferromagnetic phase transition at TN. In this report, we present the detailed study of magnetic and the magnetocaloric effect (MCE) of the 3d-metal chromites ACr2O4 (where A = Mn, Fe, Co, Ni, Cu, and Zn) near TC and TN. We find the magnitude of MCE (-ΔSM) decreases on decreasing the magnetic moment of A-site ion with an exception for CuCr2O4. Additionally, to know more about the order and nature of phase transition, we have made a scaling analysis of (-ΔSM) for all the chromites across the phase transition temperatures TC and TN.
- Ali, Anzar,Singh, Yogesh
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- Three-dimensionally ordered macroporous spinel-type MCr2O4 (M = Co, Ni, Zn, Mn) catalysts with highly enhanced catalytic performance for soot combustion
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Three-dimensionally ordered macroporous (3DOM) spinel-type MCr2O4 (M = Co, Ni, Zn, Mn) catalysts were successfully prepared by a colloidal crystal template strategy and characterized by means of XRD, FE-SEM, BET, FT-IR, H2-TPR and O2-TPD. The 3DOM catalysts exhibited superior activity to their corresponding bulk counterparts, which were used as reference benchmarks in this study, due to the enlarged contact between the catalyst and the soot particle and improved mass transfer caused by the unique well-defined 3DOM structures. Meanwhile, the factors influencing the catalytic activity of the 3DOM catalysts in soot combustion were also optimized, such as metallic cations and feed compositions. Furthermore, the 3DOM catalysts displayed strong durability against structural collapse due to the robust 3DOM structure, demonstrating their promising potential in view of the practical working conditions of diesel engines.
- Wang, Jinguo,Yang, Gaoyang,Cheng, Li,Shin, Eun Woo,Men, Yong
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p. 4594 - 4601
(2015/09/01)
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- Synthesis of bimetallic Ni-Cr nano-oxides as catalysts for methanol oxidation in NaOH solution
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Bimetallic Ni-Cr nano-oxide catalysts were synthesized by thermal decomposition method and were investigated as the anode electrocatalysts for the oxidation of methanol. The catalysts were characterized by X-ray diffraction and transmission electron microscopy. The electroactivity of the catalysts towards methanol oxidation in a solution containing 0.25 M NaOH and 1.0 M MeOH was examined using cyclic voltammetry and chronoamperometry. The results indicate that a mixture of rhombohedral-structured NiO and Cr 2 O3 nanocrystals generated at the calcination temperature of 500-700 ° C while octahedral-structured spinel NiCr 2 O4 formed at higher temperature. The influence of metallic molar ratio on the electrocatalytic performance of the catalysts was studied. The Ni-Cr nano-oxides prepared at comparatively low temperature displayed significantly higher catalytic activity and durability in alkaline solution toward electrooxidation of methanol compared with the pure nano NiO. The results indicate a synergy effect between NiO and Cr2 O3 enhancing the electrocatalytic properties of the bimetallic Ni-Cr nano-oxide catalysts. Meanwhile, NiCr2 O4 hardly increased the activity and durability of the catalyst. In addition, the Ni-Cr catalyst also exhibited excellent stability and good reproducibility. Therefore, Ni-Cr nano-oxide catalyst may be a suitable and cheap electrocatalyst for methanol oxidation in alkaline medium.
- Gu, Yingying,Luo, Jing,Liu, Yicheng,Yang, Haihong,Ouyang, Ruizhou,Miao, Yuqing
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p. 3743 - 3749
(2015/03/05)
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- Thermal, structural and magnetic studies on chromite spinel synthesized using citrate precursor method and annealed at 450 and 650 °c
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Chromite Spinel materials were synthesized in this study by the citrate precursor method using four divalent cations (Ni2+, Co2+, Zn2+, and Cu2+). Citrate precursors consisting of mixed chromium citrates were first subjected to a thermogravimetric (TG) analysis for determining optimum temperatures for annealing. TGof coprecipitated chromium(III) citrate-zinc citrate gel has been carri ed out separately in N 2 and O2 atmospheres. In both the cases, dehydration is followed by a four-step decomposition. The TG data were subjected to kinetic/mechanistic analysis, and the values of activation energy and Arrhenius factor were approximated. TG curves ofvarious powders which were obtained on annealing at the two temperature s did exhibit thermal instability when carried out in N2 atmosphere. A large coercivity of 2701.01 Oe was observed for NiCr2O4 at 650 °C. On the basis of the results, 450 °C has been chosen for annealing treatment of the four gels. The samples were accordingly annealed at two different temperatures (450 and 650 °C) in a muffle furnace for 1 h in each case. The annealed powders were characterized using X-ray diffraction (XRD), SEM, and vibrating sample magnetometer (VSM). The XRD patterns show that annealing of CuCr2O4, NiCr 2O4, and CoCr2O4 at 450 °C yields very small crystallites with poor Bragg reflections, although ZnCr 2O4 samples show better peaks in XRD data. Annealing at 650 °C resulted in particle size range of 8-89 nm in the four cases. In the case of ZnCr2O4, the particle size was 8 nm.
- Singh, Rakesh K.,Yadav, A.,Narayan, A.,Singh, Amrendra K.,Verma, L.,Verma, R. K.
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p. 197 - 204
(2012/03/07)
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- The catalytic behaviour of NiFe2-xCrxO4 (0.0≤×≤ 2.0) during the thermal decomposition of ammonium perchlorate, polystyrene and their composite propellants
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NiFe2O4, NiFeCrO4 and NiCr2O4 catalysts have been prepared by solid state reaction and characterised by using chemical analysis and X-ray diffraction techniques. Electrical conductivity and thermoelectric power measurements have shown that the catalysts are p-type semiconductors. The thermal decomposition of ammonium perchlorate, polystyrene and a composite propellant of ammonium perchlorate and polystyrene has been studied in the presence of these catalysts using thermogravimetric analyser. The effectiveness of the catalysts during the decomposition of ammonium perchlorate (AP), polystyrene (PS) and a composite propellant of AP+PS has the following order respectively: NiCr2O4 > NiFeCrO4 > NiFe2O4 NiFeCrO4 > NiCr2O4 > NiFe2O4 and NiFeCrO4 > NiFe2O4.
- Dubey,Singh,Srivastava,Ojha
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p. 841 - 847
(2007/10/03)
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- Oxidative Dehydrogenation of Propane on NixMg1-xAl2O4 and NiCr2O4 Spinels
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The NixMg1-xAl2O4, NiCr2O4, and MgCr2O4 spinels have been synthesized, characterized with the XRD and XPS methods, and tested in the oxidative dehydrogenation of propa
- Sloczynski, Jerzy,Ziolkowski, Jacek,Grzybowska, Barbara,Grabowski, Ryszard,Jachewicz, Dariusz,Wcislo, Krzysztof,Gengembre, Leon
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p. 410 - 418
(2008/10/08)
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- The chromate ion as a ligand. Physicochemical studies of nickel(II) chromate complexes with 2,2′-bipyridine
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The preparation of two novel nickel chromate complexes with 2,2′-bipyridine is described. The results of thermal, spectral, magnetic and scanning microscopic studies are analyzed. The data show that the complexes are monomers with pseudooctahedral arrangement around the nickel ion. The coordination of the chromate ion has been found in one of the complexes. The TG-DTG-DTA data show that the thermal decomposition of the complexes occurs in two and three stages in air and four stages in He atmosphere. The decomposition of the complexes has been discussed and compared to that known for other metal chromate complexes with bpy.
- Repka,Cieslak-Golonka,Surga,Zurowski
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- Preparation of fine particle chromites. A combustion approach
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Fine particle metal chromites, MCr2O4 where M = Mg, Mn, Fe, Co, Ni, Cu and Zn have been obtained by the combustion of a redox mixture containing corresponding divalent metal nitrate, chromium (III) nitrate and tetraformal trisazine (TFTA). An aqueous saturated solution of the redox mixture when rapidly heated at 350°C boils, foams and ignites to yield voluminous, fine particle chromite powder. Formation of chromite was confirmed by X-ray diffraction pattern and IR spectra. Fine particle nature of chromites was studied using particle size analysis, surface area measurements, TEM and SEM. The particle size of chromites is below lum and the surface area ranges from 13 to 30 m2/g.
- Manoharan,Kumar,Patil
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p. 731 - 738
(2008/10/08)
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- Early stages of oxidation of yttrium-implanted Ni-20% Cr
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The effect of ion-implanted yttrium on oxidation at elevated temperatures (500 to 1000° C) has been studied in Ni-Cr alloys using high-voltage electron microscopy (HVEM) of in situ oxidized specimens, electron microscopy of scales oxidized ex situ, and Ru
- Yang,Welsch,Mitchell
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p. 1724 - 1738
(2008/10/08)
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- INVESTIGATION OF A MECHANICALLY ROBUST CATALYST OF THE CONVERSION OF NATURAL GAS WITH STEAM.
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This work is devoted to a study of the interaction of the components of nickel-boron-chromium-aluminum oxide catalyst under various conditions, in comparison with a nickel-boron-aluminum oxide system. The influence of additions of boron and chromium oxides on the process of producing nickel-aluminum spinel has been elucidated - the formation of the latter is one of the main causes of deactivation of the methane conversion catalyst based on a nickel-alumina system. The increased degree of combination of nickel oxide in a nickel-boron-aluminum oxide sample on calcining it in air or argon is explained by the fact that the compound Ni//3B//2O//6 is formed as well as the aluminum-nickel spinel. In calcination in a medium of steam, the Ni//3B//2O//6 is unstable and the amount of the free nickel oxide in the sample is increased.
- Kochetkova,Anufrieva,Anokhin,Peregudov,Titova
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p. 1365 - 1368
(2008/10/08)
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