Nickel Chromite Research Articles

Nitrogen dioxide and volatile sulfide sensing properties of copper, zinc and nickel chromite

Polycrystalline powders of CuCr2O4, ZnCr2O4 and NiCr2O4 were prepared from the decomposition of metal nitrates with glycine, and their structures confirmed by X-ray diffraction. The three spinels were mixed with ethyl cellulose dissolved in a suitable solvent (80:20 toluene-ethanol) to form pastes which were applied to substrates and dried. The gas sensing properties of the resulting thick films were investigated with ppm levels of NO2, CH3SH and C2H5SH at 220 °C. It was found that the spinels can be used to detect the presence of these gases. Further experiments showed that by substituting some of the A or B cations with Li and Mg, respectively, the gas sensing properties of the spinels can be altered significantly.

Gibbs energy of formation of nickel chromite

The equilibrium partial pressure of oxygen for a system containing coexisting NiCr2O4, Cr2O3, and Ni was determined at 1000 to 1300 K using the electrochemical technique. Yttria-stabilized zirconia was employed as a solid-state electrolyte in the open-circuit electromotive force (EMF) measurements. Results were compared to the reported data in the literature. The standard Gibbs energy of NiCr2O4 at 1000 to 1300 K was determined to be ΔG°f(NiCr2O4)=-1327.2+0.314T±2.874 (kJ/mole)

Effect of oxygen on NO reduction over nickel chromite catalysts

Reduction rates of NOx in HOx+CO(H2)+O2 mixtures over a pure nickel chromite catalyst and samples supported on γ-Al2O3 and faience are high. At a space velocity of 10,000 h−1, the complete reduction of nitrogen oxides by hydrogen and by carbon monoxide is achieved at 400–450°C and 450–500°C, respectively. Hence these catalysts can be recommended as a basis to develop commercial catalysts for NOx removal from oxygen-containing exhaust gases.

Routes of nitrogen formation in reduction of nitrogen oxide by carbon monoxide and hydrogen over nickel chromite catalyst

The route of NO reduction to N2 over nickel chromite catalyst is dependent on the nature of the reducing agent. NO reduction by dihydrogen proceeds mainly through the step of N2O formation, whereas NO reduction to N2 by carbon monoxide is mainly direct without a step of N2O formation.

Porous structures of iron, cobalt and nickel chromites

The porous structures of FeCr 2O 4, CoCr 2O 4 and NiCr 2O 4 as well as of the products of their activation with potassium ions (up to 5 mol.% K 2O) were investigated. The nature of porosity of the above solids appeared to be polydispersive. Potassium ions, introduced during syntheses of these spinel-type compounds, caused an increase in the volume of the micropores and mesopores.

Channeling study of the co-operative Jahn-Teller effect in nickel chromite single crystal by backscattering and resonant nuclear scattering of helium ions

Using the channeling technique, the co-operative Jahn-Teller effect in nickel chromite single crystals has been studied by helium ion backscattering and the resonant nuclear scattering of O 16(α,α)O 16 at E α = 3.056 MeV. The results have shown that the chan dips for Ni + Cr and O atoms around 〈110〉, 〈111〉 and 〈111̄〉 axes become shallow at temperatures below the dynamic to static Jahn-Teller transition, which has been attributed to extra dechanneling due to the domain effect and explained by the (110) twin model. Also there is some indication that the Ni or Cr atoms (or both) are laterally displaced from the 〈111〉 or 〈111̄〉 axis at temperatures lower than the transition temperature. This differs from the usual expectation for the co-operative Jahn-Teller effect.

On the chemical transport of cobalt and nickel chromites

Abstract The chemical transport of CoCr 2 O 4 and NiCr 2 O 4 with chlorine as a transport agent has been investigated. On the basis of thermodynamic analysis of the transport reactions at 800–1500 K it has been established that the gas phase should contain Cl 2 , CrO 2 Cl 2 and CoCl 2 during transport of CoCr 2 O 4 , while Cl 2 , CrO 2 Cl 2 and NiCl 2 should be present when NiCr 2 O 4 is transported. Within the same temperature range cobalt chromite should be transported without decomposition whereas nickel chromite would probably decompose at high temperatures and a low total pressure in the transport system. Octahedral single crystals of CoCr 2 O 4 with a spinel structure have been obtained whereas NiCr 2 O 4 formed octahedra and plates with a tetragonal structure. The reasons of the differences observed with the systems CoCr 2 O 4  Cl 2 and NiCr 2 O 4  Cl 2 have been discussed.

The activity of nickel chromite catalyst

The nickel oxide—chromia gel powder solid-state reaction was followed at 400° – 1200 °C. The reaction fits kinetically the Jander and/or Ginstling-Brounshtein equation. Catalytic activity towards H 2O 2 decomposition was correlated with pertaining formation structural changes including tetragonal deformation of NiCr 2O 4.

An Exercise in X-Ray Diffraction Using the Polymorphic Transition of Nickel Chromite

Using x-ray diffraction techniques, it is possible to observe directly a polymorphic transition in nickel chromite (NiCr2O2. The low temperature of the transition (about 31°C) allows laboratory study without a special heating stage or heating camera. A heat lamp is sufficient to initiate the change in structure. Study of this polymorphic transition is well-suited to a course in either x-ray crystallography or mineralogy. The transition is explained in terms of the Jahn-Teller effect.

Application of representation analysis to the magnetic structure of nickel chromite spinel: erratum

In Bertaut & Dulac [Acta Cryst. (1972), 28, 580-588], for clerical reasons an error has crept in so that the numerical moment values of y and z components of Cr1 are to be interchanged. One should read in the Abstract and in relation (8.9): Sz(Cr1) = -0.73 (instead of -0.45); sy(Cr1) = 0.45 (instead of + 0.73).

Observation of a cooperative Jahn-Teller phase transition in nickel chromite by particle channeling

The cooperative Jahn-Teller phase transition in Nickelchromite (NiCr 2O 4) has been studied using the channeling method. The angular dependences of the normalized backscattering yields of 1.5 MeV He ions from a NiCr 2O 4 single crystal close to the [111], [110] and [11 1 ] crystallographic axes has been measured for various temperatures above and below the transition temperature. The standard backscattering geometry utilizing an annular detector was employed. Significant changes in the minimum yields and the widths of the angular dependences were found, as well as changes in the angles between the axes. The phase transition temperature of the crystal has been measured.

Application of particle channeling to the study of a cooperative Jahn-teller phase transition in nickel chromite

Development of particle channeling in single crystals has led to a number of important applications in studies of impurity ions in solids, crystal surface structures and decay of short-lived compound nuclei. In all of these cases the crucial property is the ability of channeled particles to distinguish spatial positions of crystal lattice atoms or impurity atoms with high accuracy. This property of channeled ions suggests potential application to studies of crystalline phase transitions, thermal motion of lattice atoms and investigation of the dynamic and static behavior of “off center” or Jahn-teller impurity ions. We report here on an experimental demonstration of these possibilities in a channeling study of a reversible cooperative Jahn-teller transition in nickel chromite. In these experiments the angular dependence of the normalized backscattering yields of 1.5 MeV helium ions from a NiCr 2O 4 single crystal close to the [111], [110], [11 1] , [211] and [12 1] crystallographic axes has been measured for va temperatures above and below and below the transition temperature. At the transition temperature an abrupt change in direction of the crystal axes was observed. In addition, the minimum yield of the channeling dips was found to change significantly.

Thermal decomposition of potassium chlorate in presence of chromium(III) oxide and nickel(II) chromite(III)

A study of thermal behaviour of intimate mixtures of different molar ratios of potassium chlorate and chromium(III) oxide, and potassium chlorate and nickel(II) chromite(III) was made by employing thermogravimetry, differential thermal analysis, chemical analysis, infrared spectroscopy and X-ray powder diffraction analysis. Potassium chlorate in presence of Cr(III), starts decomposing around 200°C which is much below the decomposition temperature of pure KClO 3. Each mole of Cr(III) takes up 8/3 moles of KClO 3 to become oxidized into potassium dichromate.

Thermal decomposition of potassium perchlorate in presence of chromium(III) oxide and nickel(II) chromite(III)

A study of the thermal decomposition of intimate mixtures of different molar ratios of potassium perchlorate and chromium(III) oxide, and potassium perchlorate and nickel(II) chromite(III) was conducted employing thermogravimetry, differential thermal analysis, chemical analysis, infrared spectroscopy and X-ray diffraction analysis. Upon heating to 400°C Cr(III), in 2:1 molar ratios, is oxidized to potassium dichromate. Only a stoichiometric amount of Cr(III) was oxidized from molar ratios less than 2:1, leaving behind excess Cr 2O 3 and NiCr 2O 4 whereas from molar ratios greater than 2:1, Cr(III) was completely oxidized and the excess KClO 4 started decomposing around 410°C which is much below the decomposition temperature of pure KClO 4.

Oxygen and Metal Activities of the Chromium‐Nickel‐Oxygen System Between 900° and 1100°C

The equilibrium oxygen pressures for Cr‐Ni alloys existing as the alpha (α) and gamma (γ) phases and coexisting with chromium oxide , nickel oxide , and nickel chromite were determined at 900°, 1000°, and 1100°C using an electrochemical cell with a calcia‐zirconia electrolyte. These pressures were related to compositional determinations of the solid phases and the Cr‐Ni‐O isotherms. The free energies of formation of chromium oxide and nickel chromite were determined. With increasing nickel content of the alloy, the oxygen pressure increases from the dissociation pressure of pure chromium oxide to that of the univariant phase field, , with chromium oxide containing <0.2 weight per cent (w/o) nickel. At higher oxygen pressures, the phase field extends up to the univariant phase field. Nickel chromite was essentially stoichiometric the nickel content of chromium oxide was ∼0.5 w/o, and the γ‐alloy contained only a very small chromium content, ∼10−4 w/o. No variation was detected in the spinel composition across the region but the chromium content of the alloy decreased to ∼10−8 w/o at the univariant phase field, with nickel oxide containing 4.5 w/o Cr.

Mechanism and Kinetics of Nickel Chromite Formation

Nickel chromite formation was studied using dense polycrystals and single crystals of and . The influence of time, temperature, and oxide purity on spinel structures and formation rates was determined. Experiments were performed by reacting pure pellets with pure and manganese‐doped pellets of . All reactions were done in air for times ranging from 24 to 640hr, and at temperatures from 1200° to 1500°C.The rates of nickel chromite formation in dense, crack‐free specimens are two orders of magnitude smaller than the rates of formation in porous specimens, or at surfaces of dense specimens. These solid‐state reaction rates are affected, and possibly controlled, by the rate of chromium dissolution in the nickel oxide, while the reaction rates at free surfaces are controlled bydiffusion of Ni2+ and Cr3+ through the spinel.

Elastic properties and cooperative Jahn-Teller effect in nickel chromite

The symmetry elastic constant c 11 − c 12 shows softening at the cooperative Jahn-Teller phase transition. The dominant mechanism for the transition is the strain coupling to the nickel ions. Für Temperaturen nahe am kooperativen Jahn-Teller Phasenübergang wird die Symmetrie elastische Konstante c 11 − c 12 weich. Der dominante Mechanismus für diesen Phasenübergang ist die Kopplung der Nickel Ionen an die Symmetrie Komponente der Dehnung.

Application of representation analysis to the magnetic structure of nickel chromite spinel

The magnetic structure of the tetragonal nickel chromite spinel (a0 = 5.76; c = 8.50 Å) has been solved by representation analysis in space group I41amd. Magnetic reflexions decompose into two sets; (a) ferrimagnetic ones, produced by a Néel mode along the x-axis and belonging to the two-dimensional Γ5g representation of wave vector k = [000]; (b) antiferromagnetic reflexions produced by non-colinear anticentered y and z modes belonging to a two-dimensional representation of wave vector k = [001]. The Shubnikov groups of the ferri- and antiferromagnetic modes considered separately are Imm′a′′ and Ip2′2′21 respectively. Their intersection has the very low symmetry P2′y. The antiferromagnetic mode of Ni (in 000 and 0 ½ ¼) has only y components (Sy = 0.58). The chromium spins decompose into two sets: Cr1 (in 0 ¼ 5/8 and ½ ¼ 1/8) has y and z components (Sy = + 0.73 for the former and −0.73 for the latter atom, Sz = −0.45): CrII (in ¼ ½ 7/8 and ¼ 0 3/8) has only z components (Sz = 0.86). The total spins are (Ni) = 1.0 and (Cr) = 1.11, and the moment values are μ(Ni) = 2.0 μB: μ(Cr) = 2.22μB. The figures are computed from neutron diffraction data given by Prince in 1961. An equivalent model (magnetic twin) has ferrimagnetism along Oy and antiferromagnetic x and z modes. Magnetic interactions are highly anisotropic.

Twin structure of NiCr2O4Single Crystal Due to Cooperative Jahn-Teller Distortion

The tetragonal phase of nickel chromite (NiCr 2 O 4 ) was investigated by X-ray diffraction method for single crystal. The {110} twin structure was found, below the transition temperature (29°C), as a result of the tetragonal domain formation due to the Jahn-Teller distortion. The twin angle was observed to vary in accord with the temperature dependence of c/ a and to be about 1.1° and 1.6° at T =21°C and T =-30°C, respectively.

Structure of Nickel Chromite

Nickel chromite is a normal spinel whose structure has a tetragonal distortion below about 310°K. The structure parameters have been determined by neutron diffraction at 77°K, at which temperature the c/a ratio of a face-centered unit cell is 1.04. The space group is I41/amd with four Ni ions in positions 4(a), eight Cr ions in positions 8(d), and 16 O ions in positions 16(h) with y = 0.239 and z = 0.392. The two independent Cr-O distances are 1.98 A and 1.99 A, indicating that the octahedron is not greatly distorted. The tetrahedron around the nickel is elongated by nearly 14% parallel to the c axis. Below a Curie temperature of about 65°K, nickel chromite has a small ferromagnetic moment. The neutron diffraction pattern at 4.2°K shows extra peaks which can be indexed on the basis of a primitive cell having the same dimensions as the body-centered chemical cell. The intensities of these peaks cannot be accounted for by any simple model for the magnetic structure. In fact, a systematic search of possible structures belonging to magnetic space groups with symmetries as low as classes m′m′2 and mm′2′ has failed to turn up any structure which is consistent with the diffraction data.