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Thermoelectric power measurements of CuCr2-xAlxO4 and Cu1-xMgxCr2O4 solid solution spinel oxides
Abstract
Thermoelectric power of the solid solution system CuCr2-xAlxO4 for x = 0-2 was determined between 300 and 673 K. CuCr2O4 shows p-type conduction and for x > 0.08 it changes to n-type. The results show that this is due to the presence of Cu1+tet ions produced during the redistribution of cations in the spinel lattice when aluminum is substituted for chromium ions. In the p-type samples charge hopping is between Cr4+oct and Cr3+oct and in the n-type, between Cu2+tet and Cu1+tet. The samples show n-type or p-type behavior depending on the dominant charge carrier present at a particular temperature.
... Å, c = 7.78 Å, c/a = 1.29) normal spinel at room temperature. 22,23 The tetragonal distortion is a consequence of the Jahn-Teller effect of Cu 2+ cations that removes the ground-state degeneracy resulting in the flattening of the CuO 4 tetrahedra to the lower-symmetry tetragonal phase. However, upon heating, it transforms to the cubic spinel phase near 600 • C, where the compression of the CuO 4 tetrahedra is removed by orbital melting (transition from an orbitalordered to orbital-disordered state). ...
We report the magnetic and optical properties of CuCr2O4thin films fabricated by atomic layer deposition(ALD) from Cu(thd)2, Cr(acac)3, and ozone; we deposit 200 nm thick films and anneal them at 700 °C in oxygen atmosphere to crystallize the spinel phase. A ferrimagnetic transition at 140 K and a direct bandgap of 1.36 eV are determined for the films from magnetic and UV-vis spectrophotometric measurements. Electrical transport measurements confirm the p-type semiconducting behavior of the films. As the ALD technique allows the deposition of conformal pin-hole-free coatings on complex 3D surfaces, our CuCr2O4films are interesting material candidates for various frontier applications.
Herein, we show the synthesis of high-capacity anode, InFeCoO4 spinel for lithium ion batteries (LIBs), by facile glycine-assisted chemical approach. The structure and morphology are evaluated by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS) and scanning electron microscopy (SEM) techniques, respectively. The pure phase formation of spinel InFeCoO4 is confirmed from XRD pattern, whereas the oxidation state of Co in 2+ is determined from XAS analysis. Electrochemical performance of InFeCoO4 in the half-cell configuration is evaluated by galvanostatic and cyclic voltammetry (CV) in the voltage window of 0.005–3.0 V vs. Li. When cycled at 60 mA g−1, it shows a high first cycle reversible capacity of 750 (±10) mA h g−1. However, slow capacity degradation is noticed upon cycling and reached 285 (±10) mA h g−1 after 40 cycles. An improved Li-storage performance is noticed under similar cycling condition, when the electrode is heat-treated. It shows first cycle reversible capacity of 880 (±10) mA h g−1 and reached 535 (±10) mA h g−1 after 40 cycles. The coulombic efficiency is >98 % during cycling. The improved Li-storage performance is possibly due to the distribution of PVDF (binder) in the active materials as well as better electrical contact after heat treatment.
Transition-metal spinels are efficient catalysts in a number of heterogeneous processes, such as CO oxidation, catalytic combustion of hydrocarbons and oxychlorination of methane. The properties of catalytic materials are highly dependent on the synthesis route. Spinels are often produced at high temperatures by the calcination of precursors such as powder mixtures, slurries or resins. Combustion synthesis is a cost-efficient method used to produce homogeneous and fine particles with high reproducibility. Cu0.8Ni0.2Cr2O4 spinel was obtained by the combustion of metallic nitrates using urea as fuel. The resulting powders were calcinated at different temperatures and characterized by thermogravimetric and particle size analyses, X ray diffraction, and scanning electron microscopy. The effect of urea on the control of the process and particle morphology was investigated. The results revealed the formation of porous powders with increasing crystallinity as the calcination temperature increased. Crystallization of spinel started at 700°C.
We report magnetization and ac susceptibility as functions of the temperature and frequency for CuCr2O4 spinel oxide from 2 K to 300 K. Bulk CuCr2O4 crystallizes at room temperature in a tetragonal distorted spinel and above 865 K its structure is cubic spinel; distortion is caused by Jahn–Teller Cu2+ ions. The magnetization data of the polycrystalline sample indicates ferrimagnetic order below T
C
=122 K. Magnetization isotherm resulted in an average magnetic moment of 0.08 μB/f.u. at 2 K values lowest to expected value. This discrepancy can be explained assuming a triangular configuration of spins Cr3+ and Cu2+. The ferromagnetic phase of the sample does not show glassy behavior. Its magnetic response can be explained simply from the domain wall dynamics of otherwise homogeneous ferrimagnet.
Transition-metal spinels are efficient catalysts in a number of heterogeneous processes, such as CO oxidation, catalytic combustion of hydrocarbons and oxychlorination of methane. The properties of catalytic materials are highly dependent on the synthesis route. Spinels are often produced at high temperatures by the calcination of precursors such as powder mixtures, slurries or resins. Combustion synthesis is a cost-efficient method used to produce homogeneous and fine particles with high reproducibility. Cu0.8Ni0.2Cr2O4 spinel was obtained by the combustion of metallic nitrates using urea as fuel. The resulting powders were calcinated at different temperatures and characterized by thermogravimetric and particle size analyses, X ray diffraction, and scanning electron microscopy. The effect of urea on the control of the process and particle morphology was investigated. The results revealed the formation of porous powders with increasing crystallinity as the calcination temperature increased. Crystallization of spinel started at 700 oC.
Electrical and magnetic measurement results show that Rh2O3-I is a p-type semiconductor showing temperature independent Pauli paramagnetism above 200 K. Below 200 K, this oxide shows an increase in the magnetic susceptibility with decreasing temperature, indicating a magnetic phase transition at 200 K. This transition is also reflected in the electrical resistivity measurements.
The effect of a polymer matrix on phase transitions in CuFe2O4spinel oxide has been studied. XRD results show that cubic CuFe2O4is obtained by quenching from 1173 K, while with CuFe2O4-PEEK composite samples, the cubic form is obtained by quenching from considerably lower temperatures (~733 K). The change in resistivity with temperature of the quenched composite samples shows that the cubic samples quenched from <1173 K undergo several phase changes during heating. Measurements of thermoelectric power indicate that the composite samples quenched from ≤1073 K shown-type conduction, and the sample quenched from 1173 K showsp-type conduction.
Electrical resistivity (ρ) and thermoelectric power (α) measurements were carried out in inert atmosphere on Fe2Mo1−xTixO4(0.0≤x≤1.0) spinel oxides in the temperature ranges 100–600 and 300–600 K, respectively. Thermal activation energy (Ea), carrier concentration (n), activation energy for carrier concentration generation (En), and mobility (μ) values have been calculated from resistivity and thermoelectric power data. The results show that conduction in titanium-substituted Fe2MoO4samples, i.e., Fe2Mo1−xTixO4, is carried out through the hopping of charge carriers, and the linear ln (ρ) vs 1/Tplots show a break at the magnetic phase transition temperature. Progressive substitution of molybdenum by titanium in Fe2MoO4shows that the ratio of Fe3+/Fe2+decreases and in Fe2TiO4the ratio is very small. The decrease in the amount of Fe3+with increasingxresults in the high resistivity and large lattice parameter of Fe2TiO4.
Electrical resistivity (rho) and thermoelectric power (alpha) measurements were carried out in air on Cu1-xCdxCr2O4 (0 less-than-or-equal-to X less-than-or-equal-to 1.0) spinel oxides in the temperature ranges 300-873 K and 300-723 K, respectively. Thermal activation energy (E(a)), carrier concentration (n), activation energy for carrier concentration generation (E(n)), and mobility (mu) values have been calculated from the resistivity and thermoelectric power data. The results show that conduction in cadmium-substituted CuCr2O4 samples is by hopping of charge carriers on the octahedral sites. The charge carrier hopping is, however, impeded at lower temperatures due to the larger C(oct)-Cr(oct) distance when the large Cd2+ ion (97 pm) is substituted for the Cu2+ ion (72 pm) in the tetrahedral site of the CuCr2O4 lattice. The temperature dependence of alpha shows a change in the sign of the slope at the temperature at which hopping conduction becomes evident. (C) 1994 Academic Press, Inc.
Thermogravimetry and X-ray analysis show that the iron titanate spinel sample is nonstoichiometric and has the formula Fe1.76Ti1.12O4. On heating in air below 573 K, it undergoes surface oxidation and on further heating DTA and TG show that it undergoes bulk oxidation in three stages. X-ray analysis and resistivity measurements show that while octahedral Fe2+ ions are oxidised, the spinel structure is retained and during Fe
tet2+ oxidation a metastable phase is formed and when the oxidation is complete, the pseudobrookite phase is formed.
Studies on the thermal stability of FeCr2O4 in air show that surface oxidation preceeds bulk oxidation. Electrical resistivity and thermoelectric power measurements show that above 47 K, Fe2+ ions are oxidised by the oxygen adsorbed on the surface and above 523K diffusion of the ions into the bulk occur. DTA and TG show that the bulk oxidation starts above 673 K and the change from the defect spinel phase to the rhombohedral phase occurs during oxidation and is not a sharp transition. © 1990 Wiley Heyden Ltd., Chichester and Akadémiai Kiadó, Budapest.
Electrical resistivity (ρ) and thermoelectric power (α) measurements were carried out in air on CuCr2-xAlxO4(0.06 ≤ x ≤ 0.10) spinel oxides in the temperature range 300-675 K. Carrier concentration (n), optical phonon frequency (γo), and mobility (μ) values have been calculated from resistivity and thermoelectric power data. The results show that CuCr2-xAlxO4 spinel oxides are compensated semiconductors and the total compensation of the hole carriers by electrons occur when 0.09 chromium ions are replaced by aluminium ions.
Rhodium-substituted CuCr2O4, i.e., CuCr2-xRhxO4, spinel oxides were prepared. X-ray analysis showed that single phase spinels were obtained for x = 0 to 0.8, 1.8, and 2.0. The other samples had an additional Cu2Cr2O4 phase. The temperature variation of electrical resistivity for all the single phase samples except CuRh2O4 was similar to that of CuCr2O4 and with the substitution of Cr (3d transition metal) by Rh (4d transition metal) the conduction process did not change gradually from CuCr2O4 type to CuRh2O4 type.
Composites of CuCr2O4 semiconductor with the polymer poly-aryl-ether-ether-ketone were prepared. Characterization of the composite materials by X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared (IR) spectroscopy indicates that no new compound is formed during composite formation between the semiconductor and the polymer. XRD and differential scanning calorimetry (DSC) results show that crystallinity of the polymer decreased with increasing amounts of CuCr2O4 in the composite. The results of the resistivity measurements show that the semiconducting property of CuCr2O4 did not change upon composite formation. Calculation of mobility of the charge carriers shows that, similar to CuCr2O4, the mobility of the composites changed linearly with temperature. On heating, the composite changed from extrinsic to intrinsic type, and Ea−in increased and mobility decreased upon increasing the polymer in the composite.
The catalytic effect on the thermal decomposition behavior of ammonium perchlorate (AP) of p-type nano-CuO and CuCr2O4 synthesized by an electrochemical method has been investigated using differential scanning calorimetry as a function of catalyst concentration. The nano-copper chromite (CuCr2O4) showed best catalytic effects as compared to nano-cupric oxide (CuO) in lowering the high temperature decomposition by 118 °C at 2 wt.-%. High heat releases of 5.430 and 3.921 kJ g−1 were observed in the presence of nano-CuO and CuCr2O4, respectively. The kinetic parameters were evaluated using the Kissinger method. The decrease in the activation energy and the increase in the rate constant for both the oxides confirmed the enhancement in catalytic activity of AP. A mechanism based on an electron transfer process has also been proposed for AP in the presence of nano-metal oxides.
The catalytic effects of doped or mixed CuO-Cr2O3 oxides on the thermal decomposition of ammonium perchlorate (AP) were investigated by using DTA, electrical conductivity and X-ray diffraction techniques. The results obtained revealed that the decrease in the defect electron of CuO catalyst doped with 1 at.% Cr3+ inhibited its activity, while the opposite effect was observed when Cr2O3 was doped with 1 at.% Cu2+. On increase of the concentrations of both oxides, the catalyst containing 70 at.% Cr3+ was found to be the most active during the decomposition of AP. The existence of CuCr2O4 at this ratio was demonstrated by X-ray diffraction. The activity of this spinel was explained on the basis of a hopping mechanism between Cr3+/Cr4+ active sites. Finally, the activation energies of different decomposition stages of AP alone and mixed with catalysts were calculated.Mittels DTA, Rntgendiffraktion und elektrischer Leitfhigkeit wurden die katalytischen Wirkungen von versetzten oder gemischten CuO-Cr2O3 Oxiden auf die thermische Zersetzung von Ammoniumperchlorat (AP) untersucht. Die erhaltenen Ergebnisse zeigen, da\ die Abnahme an Defektelektronen von CuO-Katalysator, versetzt mit 1 mol% Cr3+, seine Aktivitt vermindern, whrend ein entgegengesetzter Effekt bei Cr2O3 beobachtet wird, das mit 1 mol% Cu2+ versetzt ist.Durch Erhhung der Konzentrationen beider Oxide zeigt der Katalysator bei der Zersetzung von AP die gr\te Aktivitt bei einem Cr3+-Gehalt von 70 mol%. Mittels Rntgendiffraktion konnte bei dieser Zusammensetzung die Existenz von CuCr2O4 gezeigt werden. Die Aktivitt dieser Spinellstruktur wurde durch einen Hpfmechanismus zwischen Cr3+/Cr4+-aktiven Stellen erklrt. Weiterhin wurden die Aktivierungsenergien von verschiedenen Zersetzungsstufen von AP mit und ohne Katalysatoren berechnet.
The first synthesis of epitaxial spinel Cu Cr 2 O 4 thin films is reported, which exhibit enhanced magnetization in excess of 200% of the accepted bulk value when grown on single-crystal (110) Mg Al 2 O 4 substrates. Bulk Cu Cr 2 O 4 is a ferrimagnetic insulator with a low net magnetic moment of 0.5μB due to its tetragonal unit cell with a c/a ratio of 1.29 and frustrated moment configuration. It is shown that through epitaxial growth and substrate-induced strain, it is possible to enhance the magnetization of these films by reducing the tetragonality of its unit cell which, in turn, effectively decreases magnetic moment frustration allowing for a greater net moment.
The catalytic electrodeposition of a variety of metal ions M2+ (Zn2+, Cd2+, Hg2+, Ni2+, Pb2+, Cu2+, Ag+), all of environmental issue, was performed under visible light in aqueous air-equilibrated CuFeO2 suspensions. The delafossite CuFeO2 was synthesized by nitrate way and the surface/bulk ratio is greatly increased. In the dark, it exhibits a long term chemical stability with a corrosion rate of 1.3 × 10−8 mol m−2 year−1 in KCl (1 M). The black oxide, with a gap of 1.32 eV, determined from the reflectance diffuse spectrum, was characterized by photoelectrochemistry. The flat band potential was found to be 0.16 VSCE and a correlation exists between the redox potential of M2+/0 couple and the conduction band of CuFeO2 located at −1 VSCE. Ag+ is slightly photoelectrodeposited because of its low adsorption and its positive potential. By contrast, Cu2+ and Cd2+ are efficiently reduced owing to the strong dark adsorption and the large driving force at the interface. The quantum yield of M2+ deposition works out to be 0.1%; the metal deposition proceeds competitively with the water reduction and decelerates over time. The specie showing the high H2-evolution exhibits the lower M2+ reduction and the medium pH should make the M2+ deposition easier. The best photocatalytic H2-evolution (0.18 ml g−1 min−1) was obtained in solution containing Cd2+ (30 ppm). This is a surprising result and merits a more detailed investigation.
The study of electrical conductivity in (Fe2+AlCr)O spinels and γ(FeAlCr)2O lacunar spinels obtained by oxidation shows that the conduction is largely determined by octahedral sites and that reduction in the Cr–Cr distance either by aluminum substitution or by aluminum and vacancies brings in “foreign” ions in the chromium sublattice and that the activation energy increases. The profile of the σ = ƒ(t) curves during oxidation of those p-type spinels is related to the simultaneous presence of Fe2+ and Fe3+ ions on tetrahedral sites, which favors a process of electron hopping.L'étude de la conductivité électrique des spinelles (Fe2+AlCr) O et desspinelles lacunaires obtenus par oxydation γ(FeAl Cr)2O montre que la conduction est largement dominée par les sites octaédrizues et que la réduction de la distance Cr–Cr par substitution soit d'aluminium, soit d'aluminium et de lacunes introduit des ions “étrangers” dans le sous réseau du chrome et que l'énergie d'activation augmente. L'évolution des courbes σ = ƒ(t) lors de l'oxydation de ces spinelles de type p est reliée à la présence simultanée d'ions Fe2+ et d'ions Fe3+ sur les sites tétraédriques lesquels favorisent un processus de “hopping” d'électrons.
The compounds with spinel lattice structure (1)
CdxCu1-xMn2O4, (2)
CoxCu1-xMn2O4 and (3)
MgxCu1-xMn2O4 (0≤x≤1)
are prepared. Studies of crystal symmetry as a function of composition
and of electrical resistivity as a function of composition and
temperature, are reported. The compositions with tetragonal symmetry
exhibit thermal hysteresis of electrical resistivity, whereas those with
cubic symmetry display usual linear variation of logρ vs. 1/T. The
electrical conduction involves the hopping mechanism of electrons
between octahedral site Mn3+ and M4+ ions. It is
shown that there is major contribution to activation energy due to the
cooperative distorting forces in the tetragonally crystallized systems.
CuMn2O4 is shown to have cation distribution
Cuz1+Cu1-z2+[Mnz4+Mn2-2z3+]O42-
where (0<z<0.1).
The effects of substituting divalent metal ions (Mg, Ca, Sr, Ba) for Y in YCrO3 were investigated by electrical conductivity, Seebeck coefficient, and thermal conductivity measurements. The electrical conductivity results were consistent with the hopping-type conduction of a temperature-independent concentration of small polarons, with measured activation energies of 0.18-0.26 eV. The Seebeck coefficient increased nearly linearly with temperature and indicated p-type conductivity. Both electrical conductivity and Seebeck coefficient results show a strong dependence on dopant size (ionic radius) and indicate that the highest carrier concentrations were associated with Ca as the dopant, which is attributed to the similar ionic radii of Ca2+ and Y3+. The thermal conductivity decreased slightly with temperature and dopant concentration.
Electrical resistivity of CuCr2O4 spinel obtained by a coprecipitation method was measured at various temperatures from 350 to 923K in air. Oxidation of its surface chromium ions at various temperatures was determined by an iodometric method. The temperature dependence of resistivity and the presence of higher-valent chromium ions at higher temperatures indicated CuCr2O4 to be an extrinsic semiconductor. Log varrho vs 1/T plots yielded straight lines in the whole temperature range with two breaks, one around 550K and the other around 730K. The first break was attributed to the saturation of the surface with higher-valent chromium, while the second break was attributed to tetragonal-to-cubic phase transition in CuCr2O4. Log varrho vs 1/T plots obtained while cooling exhibited hysteresis near the second break, confirming this break to be due to a first-order diffusionless transition.
The cation distributions in copper ferrite-chromite series and the copper ferrite quenched from high temperatures are measured by X-ray diffraction method. The lattice parameters are also measured as a function of chromium ion content and of temperature. The critical fractions of copper ions on 16d and 8a sites for the occurrence of bulk crystal distortion from cubic to tetragonal symmetry are determined. The critical temperature for the distortion of copper ferrite is 360°C. This crystal distortion depends only upon the copper ion distribution. The abrupt appearance of distortion and coexistence of two phases, cubic and tetragonal, near the critical fraction and the critical temperature suggest that this distortion is of the first order as predicted by the statistical theory. From the aging of the distortion, the activation energy for copper ion to migrate from 16d site to 8a site is obtained. From the comparison of the critical fraction of copper ion in this series with those in other mixed spinels, the origin of this crystal distortion is discussed.
The preparation of a complete range of solid solutions between the cubic spinel nickel manganite, NiMn2O4, and the tetragonal spinel, Mn3O4, is described. Studies of the crystal symmetry, the semiconductor behavior and low-temperature magnetization are reported each as a function of composition. Analysis of the crystal-chemical and thermoelectric properties leads to a tentative chemical formula for the system which provides an explanation for the electrical conductivity and magnetization. The conductivity involves the “hopping” of electrons from octahedral-site Mn3+ ions to octahedral-site Mn4+ ions. The unpredictable magnetic moments are also accounted for by the properties of octahedral-site Mn3+ and Mn4+ ions.
Changes in crystallographic, electrical, and thermal properties of CuCr2O4 spinel were investigated by replacing Cu with Mg, i.e., Cu1-xMgxCr2O4, and Cr with Al, i.e., CuCr2-xAlxO4. The tetragonal distortion in CuCr2O4 disappeared with 60% replacement of Cu by Mg (x = 0.6) or 50% replacement of Cr by Al (x = 1.0). The temperature variation of electrical resistivity for all the tetragonal samples was similar to that of CuCr2O4. The first order, diffusionless phase transition was manifest in the hysteresis loops of log varrho vs 1/T plots. The resistivity and activation energy for conduction changed sharply near the phase transition composition. With the replacement of Cr by Al, the conduction in CuCr2O4 was found to change from p type to n type. The low thermal stability of the spinel was found to be due to a high concentration of tetrahedral Cu2+ ions (>80%) and compressed tetragonal distortion which strains the spinel lattice. This strain is removed by replacing either Cu with Mg or Cr with Al, whereby the spinel becomes stable.
The electrical resistivity and the Seebeck coefficients of Mn3O4, in the temperature range 1100-1700 K, have been measured under oxygen partial pressures of 1-10-6 atm. The resistivity is thermally activated with an activation energy of 1.3 eV for the tetragonal (low-temperature) phase and 0.65 eV for the cubic (high-temperature) phase. The thermoelectric power shows p-type behavior with an activation energy of 1.1 eV for the tetragonal phase and 0.3 eV for the cubic phase. The data can be explained satisfactorily in terms of small-polaron hopping.
The influence of the cation migration process on the electrical properties of MgMn2O4 is investigated. By measuring the temperature dependence of the electrical resistance and thermoelectric power, it is possible to study the migration process of the cations in the spinel lattice, and to establish the temperature range in which the tetragonal-cubic phasechange occurs. By using the thermoelectric power data it is possible to specify the valence formulae of samples which have been quenched or slow-cooled from high temperature. The electrical conduction occurs by a hopping mechanism between the octahedral-site Mn3+ and Mn4+ ions, and involves very low mobilities.
The distortions from cubic symmetry which occur in certain transition-metal oxides, particularly those with the spinel structure, are discussed in terms of crystal (ligand) field theory. It is shown that many such distortions, including all the large ones, are related in a simple fashion to the electronic configuration of the metal ion and may be considered to arise as a consequence of a Jahn-Teller type of distortion.
Electrical conductivity, Seebeck-coefficient, and Hall-effect measurements have been made on single-phase boron carbides, B(1-x)C(x), in the compositional range from 0.1 to 0.2 X, and between room temperature and 1273 K. The results indicate that the predominant conduction mechanism is small-polaron hopping between carbon atoms at geometrically inequivalent sites.