Effect of calcination temperature on the structure and hydroxylation activity of Ni0.5Cu0.5Fe2O4 nanoparticles

Nanocrystalline Ni_0.5Cu_0.5Fe₂O₄ was synthesized by sol-gel method with varying calcination temperature over the range of 500-1000. The powders obtained were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). In addition, thermal analysis (TG-DTG-DTA) of the precursor was carried out. The study reveals the simultaneous decomposition and ferritization process at rather low temperature (280-350). For the crystalline structure investigated, single cubic spinel is gained when the precursor was decomposed at 800-1000, whereas separated crystal CuO formed when calcination temperature is below 800. The increase of calcination temperature favors the appearance of Fe_B³⁺, Cu_A²⁺ and O on the spinel surface. The hydroxylation activity is relative to the amount of Cu_B²⁺ species on the spinel surface. The lattice oxygen species on the spinel surface are favorable for the deep oxidation of phenol.     

The microstructure and optical properties of crystallized hydrogenated silicon films prepared by very high frequency glow discharge

A series of nc-Si:H films with different crystalline volume fractions have been deposited by very high frequency glow discharge in a plasma with a silane concentration [SiH₄]/([SiH₄] + [H₂]) varying from 2% to 10%. The nc-Si:H films have been characterized by Raman spectroscopy, XRD diffraction, and UV-vis-near infrared spectrophotometer. The deposition rate increases nearly linear with increasing the silane concentration while the crystalline volume fraction decrease from 58% to 12%. The refractive index and the absorption of the samples were obtained through a modified four-layer-medium transmission model based on the envelope method. It was found that the refractive indices and absorption coefficient increase with increasing silane concentration. Compared to the films deposited using conventional RF-PECVD with excitation frequency of 13.56 MHz, the samples prepared by very high frequency glow discharge have higher absorption coefficients, which is due to its better compactness and lower defect density.     

Structural and magnetic properties of Co50Ni50 powder mixtures

In the present work, morphological, structural, thermal and magnetic properties of nanocrystalline Co₅₀Ni₅₀ alloy prepared by high energy planetary ball milling have been studied by means of scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. The coercivity and the saturation magnetization of alloyed powders were measured at room temperature by a vibration sample magnetization. Morphological observations indicated a narrow distribution in the particle and homogeneous shape form with mean average particle size around 130 μm². The results show that an allotropic Co transformation hcp→fcc occurs within the three first hours of milling and contrary to what expected, the Rietveld refinement method reveals the formation of two fcc solid solutions (SS): fcc Co(Ni) and Ni(Co) beside a small amount of the undissolved Co hcp. Thermal measurement, as a function of milling time was carried out to confirm the existence of the hcp phase and to estimate its amount. Magnetic measurement indicated that the 48 h milled powders with a steady state particles size have the highest saturation (105.3 emu/g) and the lowest coercivity (34.5 Oe).     

A study of ultrasonic velocity and attenuation on nanocystalline MgCuZn ferrites

The nanocrystalline MgCuZn ferrites with particle size (∼30 nm) have been synthesized by microwave-hydrothermal (M-H) method at 160 °C/45 min. The powders were densified at 750-900 °C/30 min using microwave sintering method. The sintered samples were characterized using X-ray diffraction and scanning electron microscope. The grain sizes of the sintered samples are in the range of 60-80 nm. The ultrasonic velocities have been measured on MgCuZn ferrites using the pulse transmission method at 1 MHz. The ultrasonic velocity is found to decrease with an increase of temperature. A small anomaly is observed around the Curie temperature, 520 K. The anomaly observed in the thermal variation of longitudinal velocity and attenuation is explained with the help of magneto-crystalline anisotropy constant.     

The linear thermal expansion of bulk nanocrystalline Al and SS304 at low temperature

The linear thermal expansions (LTE) of bulk nanocrystalline (NC) Al, 304 stainless steel (SS304) and their conventional coarse-grained poly-crystalline counterparts (CCPC) were studied by the strain gage method in four directions within rolling plane (for bulk NC SS304 and Al) and one direction (for their CCPC) from liquid nitrogen temperature to 300 K. LTE of bulk NC Al and SS304 in four directions were equal to or less than those of their CCPC. This result was different from the fact that the smaller thermal expansions of many other bulk nanocrystalline materials were larger than those of their CCPC. We had to conclude that the less linear thermal expansions of bulk NC SS304 and Al were attributed to their larger defects at grain boundaries and residual strain. However, the larger linear thermal expansions of many other NC materials resulted from two factors.     

Effect of substitution of Co ions on the structural and electrical properties of nanosized magnesium aluminate

Cobalt substituted nanosized magnesium aluminates having a nominal composition MgAl_2−xCo_xO₄ where x = 0.0, 0.5, 1.0, 1.5, 2.0 were synthesized by the chemical co-precipitation method. Aluminium (Al³⁺) ions were completely and successfully substituted by Co²⁺ ions, which yielded an electron rich terminal compound MgCo₂O₄. All the samples were characterized by means of X-ray diffraction (XRD), thermogravimetry and differential thermal analysis (TG/DTA) and dc electrical resistivity measurements. The investigated samples were found to be spinel single phase cubic close packed crystalline materials as demonstrated by XRD data. Using the Debye Scherer formula, the calculated crystallite size was found Co²⁺ concentration dependent and varied between 7 and 19 nm. The lattice constant, X-ray density and bulk density were found to increase while percentage porosity decreases on increasing the Co²⁺ concentration. The dc electrical resistivity was found to decrease as a function of temperature as expected for a typical semiconductor. The doped Co²⁺ ions are believed to form small polarons and hopping of these small polarons between the adjacent sites seems to be partially responsible for conduction in the system. The activation energy of hopping of small polarons was also calculated.     

Synthesis, structural, magnetic and optical properties of nanocrystalline ZnFe2O4

Nanocrystalline zinc ferrite (ZnFe₂O₄) is synthesized by high-energy ball-milling after 12 h from a powders mixture of zinc oxide (ZnO) and hematite (α-Fe₂O₃) with balls to powders mass ratio of 20:1. X-ray diffraction, vibrating sample magnetometer (VSM), the Mössbauer spectrometry and photoluminescence (PL) are used to characterize the samples. Rietveld analysis and VSM measurements show that the powder has an average crystallites size of 10 nm and a ferrimagnetic behavior with a saturation magnetization of 30 emu/g. After annealing at 700 °C, the lattice parameter reduces from 8.448 to 8.427 Å and the sample transforms into a superparamagnetic behavior, which was confirmed as well by the room temperature Mössbauer spectrometry. Different mechanisms to explain the obtained results and the correlation between magnetism and structure are discussed. Finally, the broadband visible emission band is observed in the entire PL spectrum and the estimated energy band gap is about 2.13 eV.     

Effect of helium ion irradiation on the structure and electrical resistivity of nanocrystalline Cr–N and V–N coatings

The structural stability and electrical resistivity of nanocrystalline Cr–N and V–N coatings prepared by ion beam-assisted deposition were studied. The results showed that under helium ion irradiation up to doses of 1.0^.10¹⁷ ion/cm² the fine-crystalline objects successively increase their resistance without apparent structural changes. The subsequent dose increase leads to gas-vacancy void formation and chromium nitride structure destruction. The presence of the initial closed porosity in vanadium nitride favors its structural stability at investigated maximum damage doses.     

纳米二氧化钛的制备及Eu3＋掺杂发光研究

以钛酸四丁酯为前驱物,采用溶胶-凝胶法制备了四种不同配方Eu3+掺杂的TiO2纳米晶.利用扫描电镜（SEM)、EDS能谱、光致发光光谱对样品的形貌、成份及性能进行了表征.研究了退火温度、稀土Eu3+离子掺杂摩尔分数、溶剂乙醇量等对发光性能的影响,并对其发光机理进行了探讨.结果表明:稀土Eu3+掺杂TiO2纳米晶样品,掺杂均匀、颗粒大约在30~80 nm|从EDS能谱分析可得Ti:O原子个数比并不是按化学计量TiO2满足1:2,这是因为在TiO2中形成的是Ti-O-Ti键,Eu3+离子很可能取代了Ti4+离子,同时又形成了氧空位,表明稀土Eu3+离子进入TiO2晶格中|样品的主发射峰在614 nm(5D0→7F2)处发光最强,且在593 nm(5D0→7F1)处出现了属于磁偶极跃迁的发射峰,制备Eu3+∶TiO2纳米晶的组分、退火温度、溶剂乙醇的量不同,发射光谱的强度也不同.     

A practical grinding-assisted dry synthesis of nanocrystalline NiMoO4 polymorphs for oxidative dehydrogenation of propane

A practical two-stage reactive grinding-assisted pathway waste-free and cost-effective for the synthesis of NiMoO₄ has been successfully developed. It was demonstrated that proper design in synthetic strategy for grinding plays a crucial role in determining the ultimate polymorph of NiMoO₄. Specifically, direct grinding (DG) of MoO₃ and NiO rendered α-NiMoO₄ after annealing, whereas sequential grinding (SG) of the two independently pre-ground oxides followed by annealing generated β-NiMoO₄ solid solution. Characterizations in terms of Raman and X-ray diffraction suggest the creation of β-NiMoO₄ precursor in the latter alternative is the key aspect for the formation of β-NiMoO₄. The DG-derived α-NiMoO₄ tested by oxidative dehydrogenation of propane exhibited superior activity in contrast to its analog synthesized via conventional coprecipitation. It is suggested that the favorable chemical composition facilely obtained via grinding in contrast to that by coprecipitation was essential for achieving a more selective production of propylene.