The effect of nano-Cr2O3 on solid-solution assisted sintering of MgO refractories

The effect of Cr₂O₃ particle size on the densification of magnesia refractories was investigated. Magnesia grains (<45 μm) were mixed with 2 wt% of micro-Cr₂O₃ (2 μm) and nano-Cr₂O₃ particles (10–20 nm) and sintered at 850–1450 °C, for 5 h in air. The progress of the densification and phase evolution of samples was studied with the support of X-ray diffraction phase analysis (XRD), Fourier transformer infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). It was shown that the densification of magnesia was enhanced by reducing the particle size of the added chromia to the range of 20 nm. According to the phase analysis results, the higher dissolution rate of Cr₂O₃ in MgO in the MgO–Cr₂O₃ system was responsible for the faster densification of nano-Cr₂O₃ containing mixes.     

Ordered CoFe2O4 nanowire arrays with preferred crystal orientation and magnetic anisotropy

CoFe₂O₄ nanowire arrays were fabricated by electrodeposition of Fe²⁺ and Co²⁺ into anodic aluminum oxide (AAO) templates and further oxidization. The phase structure of the nanowires is cubic spinel-type, and the XRD result exhibits perfect preferred crystallite orientation along the nanowire axes. Compared with CoFe₂O₄ nanowire arrays synthesized by other methods, the magnetic hysteresis loops demonstrate that the arrays of nanowires exhibit uniaxial magnetic anisotropy with easy magnetization direction along the nanowire axes owing to the large shape anisotropy. This approach provides a facile technology to fabricate oxide nanowires with uniaxial magnetic anisotropy.     

Synthesis of MnFe2O4 nanocrystals by wet-milling under atmospheric conditions

This work reports an original method for synthesis of well-crystallized manganese ferrite (MnFe₂O₄) nanoparticles via a high energy wet milling technique under atmospheric conditions, starting from metallic Mn and Fe powders in the presence of distilled water. The effects of milling conditions on the formation and magnetic properties of MnFe₂O₄ nanoparticles were investigated in detail. Fully stoichiometric MnFe₂O₄ nanocrystals with an average crystallite size of 14.5 nm were produced after 24 h of milling. As-synthesized MnFe₂O₄ nanocrystals were found to show soft magnetic behavior at room temperature with saturation magnetization of 53 emu/g. Due to reduced thermal effects, the saturation magnetization increased to 68 emu/g at 5 K. Results show that this method is simple and efficient for the mass production of MnFe₂O₄ nanoparticles.     

Corrosion mechanisms of Al2O3/MgAl2O4 by V2O5, NiO, Fe2O3 and vanadium slag

The effects of V₂O₅, NiO, Fe₂O₃ and vanadium slag on the corrosion of Al₂O₃ and MgAl₂O₄ have been investigated. The specimens of Al₂O₃ and MgAl₂O₄ with the respective oxides above mentioned were heated at 10 °C/min from room temperature up to three different temperatures: 1400, 1450 and 1500 °C. The corrosion mechanisms of each system were followed by XRD and SEM analyses. The results obtained showed that Al₂O₃ was less affected by the studied oxides than MgAl₂O₄. Alumina was only attacked by NiO forming NiAl₂O₄ spinel, while the MgAl₂O₄ spinel was attacked by V₂O₅ forming MgV₂O₆. It was also observed that Fe₂O₃ and Mg, Ni, V and Fe present in the vanadium slag diffused into Al₂O₃. On the other hand, the Fe₂O₃ and Ca, S, Si, Na, Mg, V and Fe diffused into the MgAl₂O₄ structure. Finally, the results obtained were compared with those predicted by the FactSage software.     

Enhanced cycling stability of LiMn2O4 cathode by amorphous FePO4 coating

The cycling performance of LiMn₂O₄ at room and elevated temperatures is improved by FePO₄ modification through chemical deposition method. The pristine and FePO₄-coated LiMn₂O₄ materials are characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy. Their cycling performances are thoroughly investigated and compared. The 3 wt.% FePO₄-coated LiMn₂O₄ exhibits capacity losses of only 32% and 34% at room temperature and 55 °C, respectively, after 80 cycles, much better than those of the pristine material, 55% and 72%. The cyclic voltammograms at 55 °C reveal that the improvement in the cycling performance of FePO₄-coated LiMn₂O₄ electrodes can be attributed to the stabilization of spinel structures. The separation of FePO₄ between active materials and electrolyte and its interaction with SEI (solid electrolyte interphase) film are believed to account for the improved performances.     

Temperature compensating ZnAl2O4-Co2TiO4 spinel-based low-permittivity microwave dielectric ceramics

TiO₂, CaTiO₃ and SrTiO₃ were added to the 0.79ZnAl₂O₄-0.21Co₂TiO₄ (ZACT in abbreviation) system to control its temperature coefficient of resonant frequency (τ_f). The effects of these additions on sinterability, phase compositions and microwave dielectric properties of the ceramics synthesized by the solid-state reaction were investigated. The results show that TiO₂, CaTiO₃ and SrTiO₃ can all reduce the densification temperature of the ZACT ceramics within the scope from 75 to 150 °C. CoTi₂O₅ second-phase with negative τ_f value appears in the TiO₂ doped ZACT system, which inhibits TiO₂ addition's function for adjusting τ_f value of ZACT ceramics. While, CaTiO₃ and SrTiO₃ can both tune effectively τ_f value to obtain temperature-stable materials.     

The effect of foreign ions on the reactivity of the CaO-SiO2-Al2O3-Fe2O3 system: Part II: Cations

The subject of this paper is the effect of foreign cations on the reactivity of the CaO-SiO₂-Al₂O₃-Fe₂O₃ system. One reference mixture and eighteen modified mixtures, prepared by mixing the reference sample with 1% w/w of chemical grade MnO₂, CuO, V₂O₅, PbO, CdO, ZrO₂, Li₂O, MoO₃, Co₂O₃, NiO, WO₃, ZnO, Nb₂O₅, CrO₃, Ta₂O₅, TiO₂, BaO₂ and H₃BO₃ were studied. The effect on the reactivity is evaluated on the basis of the free lime content in samples sintered at 1200 and 1450 °C. At 1200 °C, the reactivity of the mixture is greatly increased in the presence of Cu and Li oxides. Based on their effect at 1450 °C, the added elements can be divided into three groups. W, Ta, Cu, Ti and Mo show the most positive effect, decreasing the free CaO (fCaO) content by 30-60%, compared with the pure sample. Cr and B cause an increase of fCaO content, while the rest of the elements exhibit a marginal positive effect. According to their volatility at 1450 °C, the added compounds can be subdivided into three groups of low (Ti⁴⁺, Cu²⁺, Mo⁶⁺, W⁺⁶, V⁵⁺, Zn²⁺, Zr⁴⁺), moderate (Cr⁶⁺, Co³⁺, Ni²⁺, Mn⁴⁺) and high volatility (Cd²⁺, Pb²⁺). All burned samples, analyzed by means of X-ray diffraction, have a final mineralogical composition, which corresponds to the structure of a typical clinker.     

Multifuctional Fe3O4@C/YVO4:Dy nanopowers: Preparation,luminescence and magnetic properties

As-synthesized Fe₃O₄ nanoparticles were encapsulated with carbon layers through a simple hydrothermal process. Fe₃O₄/C nanoparticles were coated with YVO₄:Dy³⁺ phosphors to form bifunctional Fe₃O₄@C@YVO₄:Dy³⁺ composites. Their structure, luminescence and magnetic properties were characterized by XRD, SEM, TEM, HRTEM, PL spectra and VSM. The experimental results indicated that the as-prepared bifunctional composites displayed well-defined core–shell structures. The ∼12 nm diameter YVO₄:Dy³⁺ shell exhibited tetragonal structure. Additionally, the composites exhibited a high saturation magnetization (13 emu/g) and excellent luminescence properties, indicating their promising potential as multifunctional biosensors for biomedical applications.     

CoAl2O4 spinel catalyst for soot combustion with NOx/O2

Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NO_x + O₂ stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N₂ adsorption–desorption measurements. The CoAl₂O₄ spinel was able to oxidise soot as fast as the reference Pt/Al₂O₃ catalyst. Its catalytic activity can be attributed to a high NO_x chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO₂.     

The comparison of mechanical behavior of MgO-MgAl2O4 with MgO-ZrO2 and MgO-MgAl2O4-ZrSiO4 composite refractories

Mechanical properties of different compositions obtained from the additions of 5, 10, 20 and 30 wt.% zircon (ZrSiO₄) into the MgO-spinel composite refractories and ZrO₂ into MgO have been examined, the variations that occurred have been determined, and the parameters affecting those factors have been investigated with the reasons. The density, strength, Young's modulus, fracture toughness, fracture surface energy and work of fracture were measured and evaluated. Microstructural variations and fracture surfaces have been examined and the formation of new phases has been identified depending on the additive type and quantity. The relationships between mechanical properties and structural variations for different compositions have been examined. In MgO-spinel materials, strength, Young's modulus and fracture toughness values decrease up to 20% spinel addition and stay almost constant for further loads. ZrO₂ addition displays same trend but not as effective as spinel. Besides, since ZrO₂ is stable in cubic form, it does not show any toughening mechanism. Forsterite formation is the most important factor for 2-fold improvement in the mechanical properties of MgO-spinel-zircon refractories. The more the zircon addition, the more the mechanical properties improve. The generation of natural bonding between matrix particles with forsterite formation, on the other hand, causes the fracture path to turn to transgranular fracture with an increase in fracture surface energy and a decrease in work of fracture, among which the latter is considered as an indicator of thermal shock resistance of the materials being high.     

Synthesis and physical characteristics of ZnAl2O4 nanocrystalline and ZnAl2O4/Eu core-shell structure via hydrothermal route

Nanocrystalline zinc aluminate (ZnAl₂O₄) particles with a spinel structure were prepared by hydrolyzing a mixture of aluminum chloride hexahydrate and zinc chloride in deionized water. It was found that pH value and reaction temperature play critical roles in the formation of nano-sized ZnAl₂O₄. Depending on pH values in the precursor solution, ZnAl layered double hydroxide (ZnAl-LDH), ZnO, boehmite or gibbsite could be formed. At pH 7 and T>120 °C, the nanocrystalline ZnAl₂O₄ particles with average particle size of ∼5 nm are easily synthesized through ZnAl layered double hydroxide (ZnAl-LDH). After surface treatment with R-OH by using the cationic surfactant CTAB, the ZnAl₂O₄/Eu core-shell structure can be developed. The ZnAl₂O₄/Eu core-shell structure can show both emissions from ⁵D₀ to ⁷F₂ sensitivity energy level and ⁵D₂ to ⁷F₀ depth energy level.     

Effect of TiO2 on phase evolution and microstructure of MgAl2O4 spinel in different atmospheres

The effect of TiO₂ on the formation and microstructure of magnesium aluminate spinel (MgAl₂O₄) at 1600 °C in air and reducing conditions were investigated. Under reducing conditions, stoichiometric MgAl₂O₄ spinel shifted toward alumina-rich types owing to volatilization of MgO, resulting in an increase in the porosity of fired samples. Addition of graphite to mixtures of MgO and Al₂O₃ intensified the reducing conditions and accelerated the formation of non-stoichiometric MgAl₂O₄. For TiO₂-containing samples on addition of MgAl₂O₄, magnesium aluminum titanium oxide (Mg_xAl_2(1−x)Ti_(1+x)O₅, x = 0.2 or 0.3) was detected as a minor phase. Under reducing conditions, XRD peak shifts were smaller for TiO₂-containing samples than for samples without TiO₂ owing to the formation of a solid solution of TiO₂ in MgAl₂O₄ and establishment of alumina-rich spinel, which have opposite effects on increasing the lattice parameter. In bauxite-containing samples, MgAl₂O₄ spinel, corundum, magnesium orthotitanate spinel (Mg₂TiO₄) and amorphous phases were identified. Mg₂TiO₄ spinel formed a complete solid solution with MgAl₂O₄ spinel but Mg₂TiO₄ remained as a distinct phase owing to the heterogeneous microstructure of bauxite-containing samples. Also dense microstructure established in air fired TiO₂ containing samples. The results are discussed with emphasis on the application and design of alumina-magnesia-carbon refractory materials, which are used in the steel industry.     

A new regime for high rate growth of nanocrystalline diamond films using high power and CH4/H2/N2/O2 plasma

In this work, we report high growth rate of nanocrystalline diamond (NCD) films on silicon wafers of 2 inches in diameter using a new growth regime, which employs high power and CH₄/H₂/N₂/O₂ plasma using a 5 kW MPCVD system. This is distinct from the commonly used hydrogen-poor Ar/CH₄ chemistries for NCD growth. Upon rising microwave power from 2000 W to 3200 W, the growth rate of the NCD films increases from 0.3 to 3.4 μm/h, namely one order of magnitude enhancement on the growth rate was achieved at high microwave power. The morphology, grain size, microstructure, orientation or texture, and crystalline quality of the NCD samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction, and micro-Raman spectroscopy. The combined effect of nitrogen addition, microwave power, and temperature on NCD growth is discussed from the point view of gas phase chemistry and surface reactions.     

Preparation of columbite MgNb2O6 and ZnNb2O6 ceramics by reaction-sintering

Columbite MgNb₂O₆ (MN) and ZnNb₂O₆ (ZN) ceramics produced by the reaction-sintering process were investigated. Secondary phases Mg_0.652Nb_0.598O_2.25 and Mg_0.66Nb_11.33O₂₉ were found in MgNb₂O₆ pellets. After 1250 °C sintering for 2 h, a density 4.85 g/cm³ (97.1% of the theoretical value) was obtained in MgNb₂O₆ pellets. In ZnNb₂O₆ pellets, no secondary phase formed. The maximum density 5.55 g/cm³ (98.7% of the theoretical value) occurs at 1200 and 1180 °C sintering for 2 and 4 h, respectively.     

Crystal structure studies of thermally aged LiCoO2 and LiMn2O4 cathodes

LiCoO₂ and LiMn₂O₄ cathodes were studied by X-ray diffractometry (XRD) and electron diffraction after ageing in the charged state at elevated temperature. Some cathodes were stopped at different times during ageing and XRD measurements were taken to monitor changes in the crystal structure over ageing time. The results indicate that Li-ions intercalate into the cathodes lattice during ageing thus decreasing the available discharge capacity. Analysis of electron diffraction patterns of LiCoO₂ and LiMn₂O₄ retrieved from the cathodes after ageing shows that irreversible crystallographic transformations have taken place in both electrodes. Dark field imaging illustrates that LiCoO₂ forms a layer of spinel phase on its surface. In LiMn₂O₄ a tetragonal distorted spinel is observed when the cathode has been in the 3 V regime for considerable length of time.     

XANES study of polyaniline-V2O5 and sulfonated polyaniline-V2O5 nanocomposites

In this work, two materials for secondary lithium battery cathodes formed by polyaniline-V₂O₅ and sulfonated polyaniline-V₂O₅, which have a higher charge capacity than the V₂O₅ xerogel, were synthesized. X-ray absorption and Fourier transform infrared spectroscopies were employed to analyze the short-range interactions in these materials. Based on these experiments, it was possible to observe significant differences in the symmetry of the VO₅ units, and this was attributed to the intimate contact between V₂O₅ and the polymers, and to some flexibility of the VO₅ square pyramids due to the low range order of the nanocomposites.     

Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Effect of CeO2 addition on densification and microstructure of Al2O3-YSZ composites

Alumina (Al₂O₃) and alumina-yttria stabilized zirconia (YSZ) composites containing 3 and 5 mass% ceria (CeO₂) were prepared by spark plasma sintering (SPS) at temperatures of 1350-1400 °C for 300 s under a pressure of 40 MPa. Densification, microstructure and mechanical properties of the Al₂O₃ based composites were investigated. Fully dense composites with a relative density of approximately 99% were obtained. The grain growth of alumina was inhibited significantly by the addition of 10 vol% zirconia, and formation of elongated CeAl₁₁O₁₈ grains was observed in the ceria containing composites sintered at 1400 °C. Al₂O₃-YSZ composites without CeO₂ had higher hardness than monolithic Al₂O₃ sintered body and the hardness of Al₂O₃-YSZ composites decreased from 20.3 GPa to 18.5 GPa when the content of ZrO₂ increased from 10 to 30 vol%. The fracture toughness of Al₂O₃ increased from 2.8 MPa m^1/2 to 5.6 MPa m^1/2 with the addition of 10 vol% YSZ, and further addition resulted in higher fracture toughness values. The highest value of fracture toughness, 6.2 MPa m^1/2, was achieved with the addition of 30 vol% YSZ.     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

Dielectric properties and microstructures of CaSiO3 ceramics with B2O3 addition

The effects of B₂O₃ additives on the sintering behavior, microstructure and dielectric properties of CaSiO₃ ceramics have been investigated. The B₂O₃ addition resulted in the emergence of CaO–B₂O₃–SiO₂ glass phase, which was advantageous to lower the synthesis temperature of CaSiO₃ crystal phase, and could effectively lower the densification temperature of CaSiO₃ ceramic to as low as 1100 °C. The 6 wt% B₂O₃-doped CaSiO₃ ceramic sintered at 1100 °C possessed good dielectric properties: _r = 6.84 and tan δ = 6.9 × 10⁻⁴ (1 MHz).