Preparation and characterization of CuZnAl catalysts by citrate gel process

CuZnAl catalysts with different Cu loading (1-23 wt%) and a Zn:Al atomic ratio nearly constant (Zn:Al≅0.6), were prepared by the citrate sol-gel method and characterized by different techniques such as TG, BET, TPR, XRD and FTIR. The final structure obtained was strongly influenced by the calcination temperature and metal precursor composition. XRD and quantitative Rietveld revealed Zn and Al species were mainly incorporated into the normal spinel matrix and copper predominantly forms CuO. The formation of a ZnAl₂O₄ spinel was favored by increasing Cu amounts and/or by increasing calcination temperature (from 500° to 700 °C). The spinel phase of the catalysts calcined at 700 °C, had a good thermal stability and it was preserved after TPR measurements. Under hydrogen atmosphere Cu²⁺ was fully reduced to Cu⁰. Although the composition and the calcination temperature have a strong influence on the phase nature in CuZnAl catalysts, the reducibility of Cu species changes in a non significant way.     

Characterization of partially inverse spinel ZnFe2O4 with high saturation magnetization synthesized via soft mechanochemically assisted route

ZnFe₂O₄ was prepared by a soft mechanochemical route from two starting combinations of powders: (1) Zn(OH)₂/α-Fe₂O₃ and (2) Zn(OH)₂/Fe(OH)₃ mixed in a planetary ball mill. The mechanochemical treatment provoked reaction leading to the formation of the ZnFe₂O₄ spinel phase that was monitored by XRD, TEM, IR and Raman spectroscopy. The spinel phase was first observed after 4 h of milling and its formation was completed after 18 h in both the cases of starting precursors. The synthesized ZnFe₂O₄ has a nanocrystalline structure with a crystallite size of about 20.3 and 17.6 nm, for the cases (1) and (2), respectively. In the far-infrared reflectivity spectra are seen four active modes. Raman spectra suggest an existence of mixed spinel structure in the obtained nanosamples. In order to confirm phase formation and cation arrangement, Mössbauer measurements were done. Estimated degree of inversion is about 0.58 for both starting mixtures. The magnetic properties of the prepared ZnFe₂O₄ powders were also studied. The results show that the samples have a typical superparamagnetic-like behavior at room temperature. Higher values of magnetization in the case of samples obtained with starting mixture (2) suggest somewhat higher degree of cation inversion.     

Effect of zinc substitution on magnetic and electrical properties of nanocrystalline nickel ferrite synthesized by refluxing method

Nanocrystalline Nickel ferrite (NiFe₂O₄) and Zn substituted nickel ferrite (NiZnFe₂O₄) have been synthesized by the refluxing method. These ferrites were characterized by XRD, TEM, Mossbauer spectroscopy and VSM in order to study the effect of zinc substitution in nickel ferrite. XRD diffraction results confirm the spinel structure for the prepared nanocrystalline ferrites with an average crystallite size of 14-16 nm. Lattice parameter was found to increase with the substitution of Zn²⁺ ions from 8.40 Å to 8.42 Å. TEM images confirmed average particle size of about 20 nm and indicates nanocrystalline nature of the compounds. A shift in isomeric deviation with the doublet was observed due to the influence of Zn substitution in the nickel ferrite. The Zn content has a significant influence on the magnetic behavior and electrical conductivity of NiFe₂O₄. Saturation magnetization drastically increased whereas room temperature electrical conductivity decreased due to the addition of Zn content in NiFe₂O₄, indicating super magnetic material with lesser coercivity.     

Epitaxial growth of InN on nearly lattice-matched (Mn,Zn)Fe2O4

We have grown InN films on nearly lattice-matched (Mn,Zn)Fe₂O₄ (111) substrates at low temperatures by pulsed laser deposition (PLD) and investigated their structural properties. InN films grown at substrate temperatures above 400 °C show poor crystallinity, and their in-plane epitaxial relationship is [10-10]InN//[11-2](Mn,Zn)Fe₂O₄, which means that their lattice mismatch is quite large (11%). By contrast, high quality InN films with flat surfaces can be grown at growth temperatures lower than 150 °C with the ideal in-plane epitaxial relationship of [11-20]InN//[11-2](Mn,Zn)Fe₂O₄, which produces lattice mismatches of as low as 2.0%. X-ray reflectivity measurements have revealed that the thickness of the interfacial layer between the InN and the substrates is reduced from 14 to 8.4 nm when the growth temperature is decreased from 400 °C to room temperature. This suppression of the interface reactions by reducing the growth temperature is probably responsible for the improvement in crystalline quality. These results indicate that the use of (Mn,Zn)Fe₂O₄ (111) substrates at low growth temperatures allows us to achieve nearly lattice matched epitaxial growth of InN.     

Fuel additives and heat treatment effects on nanocrystalline zinc ferrite phase composition

Nanocrystalline ZnFe₂O₄ powder was prepared by the auto-combustion method using citric acid, acetic acid, carbamide and acrylic acid as fuel additives. Pure spinel zinc ferrite with the crystallite size of about 15 nm can be obtained by using acrylic acid as fuel additive. Samples prepared using other fuel additives contain ZnO impurities. In order to eliminate ZnO impurities, the sample prepared with citric acid as fuel additive was annealed at different temperatures up to 1000 °C in air and in argon. Annealed powders have pure ZnFe₂O₄ phase when annealing temperature is higher than 650 °C in air. Sample annealed at 650 °C in air is paramagnetic. However, annealed powders become a mixture of Fe₃O₄ and FeO after annealing at 1000 °C in argon atmosphere due to Zn volatility and the reduction reaction.     

Synthesis of goethite from Fe(OH)2 precipitates: Influence of Fe(II) concentration and stirring speed

Magnetic methods are efficient tools in soil and environmental science. But in such natural environments, several magnetic minerals are generally present. So, synthetic standard samples are necessary for calibration of laboratory techniques. The aim of this study was to synthesise goethite free of magnetic impurities (concentration <∼1 μg kg⁻¹) with different crystal sizes. Goethite was prepared by oxidation of aqueous suspensions of Fe(OH)₂ precipitates. Final products were characterised by X-ray diffraction, infrared spectroscopy, scanning and transmission electron microscopy and magnetic methods. Goethite could be obtained in the absence of any trace of strong magnetic minerals using FeSO₄·7H₂O and NaOH as reactants with the following experimental conditions: temperature=45 °C, [FeSO₄·7H₂O]=0.50 mol L⁻¹, [NaOH]=0.20 mol L⁻¹, stirring speed=760 rpm. The Fe(II) concentration and the stirring speed were varied. It proved possible to modify the size of the goethite crystals by varying the Fe(II) concentration and the stirring speed, but important changes of these parameters induced the formation of other phases, lepidocrocite when the oxidation reaction was drastically accelerated and Fe₃O₄ when the reaction was slowed down. In any case, for weak magnetic fields, a low-coercivity magnetic mineral saturating at weak magnetic fields was observed. It may correspond to traces of δ-FeOOH or to domains structurally similar to δ-FeOOH inside the multidomainic crystals of δ-FeOOH.     

Characterizations of octahedral zinc oxide synthesized by sonochemical method

The ultrasonic reaction of zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and hexamethylenetetramine (C₆H₁₂N₄) was investigated by varying the concentration of the reactants, the irradiation time, and the type of sonicator. The morphology, composition, and phase structure of the products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) and ultraviolet-visible (UV-vis) spectroscopy. Octahedral zinc oxide (ZnO) micropowders were formed at low concentrations, 0.05 M, of Zn(NO₃)₂·6H₂O and C₆H₁₂N₄ in both lab-made sonicator and commercial ultrasonic bath. However, at concentrations between 0.1 and 1.0 M Zn(NO₃)₂-C₆H₁₂N₄ mainly plate-like zinc hydroxide nitrate hydrate (Zn₅(OH)₈(NO₃)₂(H₂O)₂) resulted with only a small fraction of ZnO, irrespective of the irradiation time employed, highlighting the sensitivity of the system to the concentration of the starting materials. Heat treatment of Zn₅(OH)₈(NO₃)₂(H₂O)₂ at 350 °C in air affords a ZnO phase of irregular morphology. Octahedral ZnO is found to exhibit slightly lower IR absorption and similar UV absorption to that of commercial prismatic hexagonal ZnO, although an extra peak due to small quantities of Zn₅(OH)₈(NO₃)₂(H₂O)₂ is observed.     

Immiscibility in the NiFe2O4-NiCr2O4 spinel binary

The solid solution behavior of the Ni(Fe_1−nCr_n)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 Cr₂O₃ 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 T_cs. 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.     

Doping effect of Mn on the magnetic behavior in Ni-Zn ferrite nanoparticles prepared by sol-gel auto-combustion

The ferrite samples of a chemical formula Ni_0.5−xMn_xZn_0.5Fe₂O₄ (where x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized by sol-gel auto-combustion method. The synthesized samples were annealed at 600 °C for 4 h. An analysis of X-ray diffraction patterns reveals the formation of single phase cubic spinel structure. The lattice parameter increases linearly with increase in Mn content x. An initial increase followed by a subsequent decrease in saturation magnetization with increase in Mn content is observed showing inverse trend of coercivity (H_c). Curie temperature decreases with increase in Mn content x. The initial permeability is observed to increase with increase in Mn content up to x=0.3 followed by a decrease, the maximum value being 362. Possible explanation for the observed structural, magnetic, and changes of permeability behavior with various Mn content are discussed.     

Electrochemical behaviour of nano-sized spinel LiMn2O4 and LiAlxMn2−xO4 (x=Al: 0.00-0.40) synthesized via fumaric acid-assisted sol-gel synthesis for use in lithium rechargeable batteries

Pristine spinel LiMn₂O₄ and LiAl_xMn_2−xO₄ (x=Al: 0.00-0.40) with sub-micron sized particles have been synthesized using fumaric acid as chelating agent by sol-gel method. The synthesized samples were subjected to thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and cyclic voltammetry (CV) and galvanostatic cycling studies. The TGA curve of the gel shows several weight-loss regions stepwise amounting to 55% till 800 °C attributed to the decomposition of the precursors. Calcination to higher temperatures (800 °C) yields pure-phase spinel (LiAl_xMn_2−xO₄), as it is evident from the high-intensity XRD reflections matching to the standard pattern. SEM and TEM studies confirm that the synthesized grains are of uniform regular surface morphology. FT-IR studies show stretching and bending vibration bands of Li-O, Li-Al-Mn-O. LiAl_0.1Mn_1.90O₄ spinel was found to deliver discharge capacity of 139 mA h/g during the first cycle with columbic efficiency of 97%. LiAl_0.1Mn_1.90O₄ spinel exhibits the high cathodic peak current indicating better electrochemical performance. Low doping (x=0.1) of Al is found to be beneficial in stabilizing the spinel structure.     

Gas-sensing properties of perovskite La0.875Ba0.125FeO3 nanocrystalline powders

La_0.875Ba_0.125FeO₃ nanocrystalline powders have been prepared by a sol-gel method. The structure, conductance and gas-sensing properties were investigated. La_0.875Ba_0.125FeO₃ crystallizes as a perovskite phase with the orthorhombic structure. The La_0.875Ba_0.125FeO₃ based sensor shows good sensitivity and selectivity to alcohol gas. The highest sensitivity to 500 ppm alcohol gas reached was 58 at 170 °C. The adsorption of O₂ on the La_0.875Ba_0.125FeO₃ (0 1 0) surface was studied with the first-principles calculation based on the density functional theory. The results show that the surface states are near the Fermi energy level and that the Fe ion plays an important role in the process of oxygen adsorption, which affects the gas-sensing properties.     

Characteristics of spherical-shaped Li4Ti5O12 anode powders prepared by spray pyrolysis

Spherical-shaped Li₄Ti₅O₁₂ anode powders with a mean size of 1.5 μm were prepared by spray pyrolysis. The precursor powders obtained by spray pyrolysis had no peaks of crystal structure of Li₄Ti₅O₁₂. The powders post-treated at temperatures of 800 and 900 °C had the single phase of spinel Li₄Ti₅O₁₂. The powders post-treated at a temperature of 1000 °C had main peaks of the Li₄Ti₅O₁₂ phase and small impurity peaks of Li₂Ti₃O₇. The spherical shape of the precursor powders was maintained after post-treatment at temperatures below 800 °C. The Brunauer-Emmett-Teller (BET) surface areas of the Li₄Ti₅O₁₂ anode powders post-treated at temperatures of 700, 800 and 900 °C were 4.9, 1.6 and 1.5 m²/g, respectively. The initial discharge capacities of Li₄Ti₅O₁₂ powders were changed from 108 to 175 mAh/g when the post-treatment temperatures were changed from 700 to 1000 °C. The maximum initial discharge capacity of the Li₄Ti₅O₁₂ powders was obtained at a post-treatment temperature of 800 °C, which had good cycle properties below current densities of 0.7 C.     

Microstructural,dielectric and magnetic behavior of spin-deposited nanocrystalline nickel–zinc ferrite thin films for microwave applications

Nanocrystalline nickel ferrite and zinc doped nickel ferrite thin films with general composition Ni_1−xZn_xFe₂O₄; x=0.0, 0.2 and 0.5 were fabricated by the spin-deposition technique. Citrate precursor method was adopted to prepare coating solution used for film deposition. This method resulted in single phase, transparent, homogeneous and crack-free nanocrystalline ferrite thin films at annealing temperature as low as 400 °C. The substrates used for film deposition were ITO-coated 7059 glass, fused quartz and Si (1 0 0). The thickness of films was found to be in the range ∼1000–5500 Å. The surface microstructure and morphology investigated by atomic force microscopy (AFM) confirmed the grain size of nickel–zinc ferrite films to be in nanometer range indicating nanocrystalline nature of the films. Dielectric properties such as the real (∈′) and imaginary parts (∈″) of complex permittivity were measured in the X-band microwave frequency region (8–12 GHz) by employing extended cavity perturbation technique. The M–H hysteresis measurements on the films annealed at 650 °C revealed narrow hysteresis curves with H_c and M_s varying for different compositions.     

Chemical spray pyrolysis deposition and characterization of p-type CuCr1−xMgxO2 transparent oxide semiconductor thin films

A chemical spray pyrolysis technique for deposition of p-type Mg-doped CuCrO₂ transparent oxide semiconductor thin films using metaloorganic precursors is described. As-deposited films contain mixed spinel CuCr₂O₄ and delafossite CuCrO₂ structural phases. Reduction in spinel CuCr₂O₄ fraction and formation of highly crystalline films with single phase delafossite CuCrO₂ structure is realized by annealing at temperatures ?700 °C in argon. A mechanism of synthesis of CuCrO₂ films involving precursor decomposition, oxidation and reaction between constituent oxides in the spray deposition process is presented. Post-annealed CuCr_0.93Mg_0.07O₂ thin films show high (?80%) visible transmittance and sharp absorption at band gap energy with direct and indirect optical band gaps 3.11 and 2.58 eV, respectively. Lower (∼450 °C) substrate temperature formed films are amorphous and yield lower direct (2.96 eV) and indirect (2.23 eV) band gaps after crystallization. Electrical conductivity of CuCr_0.93 Mg_0.07O₂ thin films ranged 0.6-1 S cm⁻¹ and hole concentration ∼2×10¹⁹ cm⁻³ determined from Seebeck analysis. Temperature dependence of conductivity exhibit activation energies ∼0.11 eV in 300-470 K and ∼0.23 eV in ?470 K region ascribed to activated conduction and grain boundary trap assisted conduction, respectively. Heterojunction diodes of the structure Au/n-(ZnO)/p-(CuCr_0.93Mg_0.07O₂)/SnO₂ (TCO) were fabricated which show potential for transparent wide band gap junction device.     

The effect of B2O3 addition to the microstructure and magnetic properties of Ni0.4Zn0.6Fe2O4 ferrite

The effects of 0.01 and 0.1 mol B₂O₃ addition to the microstructure and magnetic properties of a Ni–Zn ferrite composition expressed by a molecular formula of Ni_0.4Zn_0.6Fe₂O₄ were investigated. The toroid-shaped samples prepared by pressing the milled raw materials used in the preparation of the composition were sintered in the range of 1000–1300 °C. The addition of 0.01 mol B₂O₃ increased the grain growth and densification giving rise to reduced intergranular and intragranular porosity due to liquid-phase sintering. The sintered toroid sample at 1300 °C gave the optimum magnetic properties of B_r=170 mT, H_c=0.025 kA/m and a high initial permeability value of μ_i=4000. The increment of the B₂O₃ content to 0.1 mol resulted in a pronounced grain growth and also gave rise to large porosity due to the evaporation of B₂O₃ at higher sintering temperatures. Hence, it resulted in an air-gap effect in the hysteresis curves of these samples.     

XRD and XPS characterization of mixed valence Mn3O4 hausmannite thin films prepared by chemical spray pyrolysis technique

Spray pyrolysis technique has been employed successfully for the synthesis of single phase mixed valence spinel hausmannite (Mn₃O₄) thin films using alcoholic start solution of manganese acetate (Mn(CH₃COO)₂·4H₂O) on pyrex glass substrates at atmospheric pressure using air as a carrier gas. Thermal decomposition of the precursor in the temperature range 320-490 °C led to the formation of Mn₃O₄ phase as revealed from the thermogravimetry analysis. Prepared samples are characterized by X-ray diffraction that shows spinel structure with space group I4₁/amd. Pure and well crystallized specimen is subjected to X-ray photoelectron spectroscopy for the surface chemistry investigation of these systems at a molecular level. Surface Mn/O ratio is compared to the bulk composition of the sample. Atomic force micrographs revealed that the morphology and the surface grains of the films largely influenced by the substrate temperature.     

Syntheses and properties of the Fe-Co/Fe3O4 ferrites

The Fe alloy-ferrite composites Fe-Co/Fe₃O₄ are synthesized by using disproportion of Fe (II) and reduction of Co (II) by Fe⁰ in a concentrated and boiling KOH solution. The Fe alloy and ferrites are prepared in aqueous solution without any templet and surfactants at low temperature. Their structures and magnetic properties are investigated by X-ray diffractometer (XRD) and vibrating sample magnetometer (VSM). From the results of XRD, it is shown that the samples have b.c.c and f.c.c structure of Fe, and the spinel structures of the ferrite before calcinations; the samples have b.c.c and spinel structures after calcinations at 300 °C; and the samples have only f.c.c structure and the spinel structures calcined at 500 °C.     

Micromechanical and thermal behaviour of gel grown pure- and sodium-modified copper tartrate crystals

Results regarding micromechanical characteristics of gel grown pure- and sodium-modified copper tartrate crystals, bearing composition CuC₄H₄O₆·3H₂O, (Cu)_0.77(Na)_0.23C₄H₄O₆·3H₂O and (Cu)_0.65(Na)_0.35C₄H₄O₆·H₂O, as obtained on using indentation induced hardness testing technique are reported. Thermal behaviour of these crystals in the temperature ranging from room temperature (∼25 °C) to about 600 °C is also reported. Pure copper tartrate crystals are found to be thermally more stable than the sodium-modified ones. Dependence of Vickers’ hardness number H_v on load ranging from 0.049 to 2.94 N on two different planes for all the three compositions is analyzed. It is shown that after initial rise in the value of H_v, the same achieves saturation at a load of 0.49 N. Modification of copper tartrate crystal by introducing sodium in its lattice brings about a change in the micromechanical characteristics. The saturation value of H_v decreases with increase in the concentration of sodium ions. The results on (0 0 1) and (1 1 1) planes for both pure and modified copper tartrate crystals suggest hardness anisotropy. Relative difference of hardness between the two planes and yield strength for both pure and modified copper tartrate crystals is worked out. The experimental results are analyzed for applicability of Meyer’s law and Proportional Specimen Resistance Model. It is suggested that the experimental results indicating reverse ISE phenomenon may be explained in terms of the existence of a distorted zone near the crystal-medium interface. The integral method of Coats and Redfern approximation applied to the thermoanalytical data suggests “Random Nucleation Model” for the reaction kinetics of these crystals. Non-isothermal kinetic parameters such as activation energy, frequency factor and order of reaction are calculated.     

Metatungstate and tungstoniobate-containing LDHs: Preparation, characterisation and activity in epoxidation of cyclooctene

Polyoxometalates (POMs) H₂W₁₂O₄₀⁶⁻ and W₄Nb₂O₁₉⁴⁻ have been intercalated between the brucite-like layers of Mg, Al and Zn, Al hydrotalcites by anion exchange, starting from the corresponding nitrate precursors. The solids have been characterised by Powder X-ray Diffraction (PXRD), Fourier Transform infrared (FT-IR) spectroscopy, N₂ adsorption-desorption at −196 °C and thermogravimetric (TG) and differential thermal analyses (DTA), and have been tested in the epoxidation of cyclooctene using H₂O₂ or t-BuOOH as oxidants. The results show that both anions are effectively located in the interlayer space maintaining their pristine structures without depolymerisation. Upon intercalation of such large anions microporosity is developed and subsequently an increase in the specific surface areas is also observed. In general, the prepared materials possess catalase and epoxidation activity, with ZnAl-intercalated H₂W₁₂O₄₀⁶⁻ giving the best results in terms of epoxide yield (17% at 24 h). Product selectivity is different for the intercalated and free POMs, the latter yielding 1,2-cyclooctanediol as the only product, whereas the former produces only the epoxide. The epoxidation reaction seems to be catalysed in homogeneous phase by the POM.