Sol-gel derived nanoparticles of Zn substituted lithium ferrite (Li0.32Zn0.36Fe2.32O4): magnetic and Mössbauer effect measurements and their theoretical analysis

Nanoparticles of Zn substituted lithium ferrite (Li_0.32Zn_0.36Fe_2.32O₄) have been prepared by a sol-gel method where the ultra-sonication technique has been adopted to reduce the agglomeration effect among the nanoparticles. The samples were heat-treated at three different temperatures and the formation of the nanocrystalline phase was confirmed by X-ray diffractograms (XRD). The average particle size of each sample has been estimated from the (311) peak of the XRD pattern using the Debye-Scherrer formula and the average sizes are in the range of 10-21 nm. The average particle size, crystallographic phase, etc. of some selected samples obtained from the high-resolution transmission electron microscopy are in agreement with those estimated from the XRD patterns. Static magnetic measurements viz., hysteresis loops, field cooled and zero field cooled magnetization versus temperature curves of some samples carried out by SQUID in the temperature range of 300 to 5 K clearly indicate the presence of superparamagnetic (SPM) relaxation of the nanoparticles in the samples. The maximum magnetization of the SPM sample annealed at 500 °C is quite high (68 Am²/Kg) and the hysteresis loops are almost square shaped with very low value of coercive field at room temperature (827.8 A/m). The particle size, magneto-crystalline anisotropy, etc. have been estimated from the detailed theoretical analysis of the static magnetic data. The dynamic magnetic behavior of the samples was also investigated by observing the ac hysteresis loops and magnetization versus field curves with different time windows at room temperatures. The different soft magnetic quantities viz., coercive field, magnetization, remanance, hysteresis losses, etc. were extracted from dynamic measurements. Dynamic measurements confirmed that the samples are in their mixed state of SPM and ordered ferrimagnetic particles, which is in good agreement with the results of static magnetic measurements. Mössbauer spectra of the samples recorded at room temperature (300 K) and at different temperatures down to 20 K confirmed the presence of the SPM relaxation of the nanoparticles of the samples.     

Synthesis of wide band gap nanocrystalline NiO powder via a sonochemical method

A sonochemistry-based synthesis method was used to produce nanocrystalline nickel oxide powder with ∼20 nm average crystallite diameter from Ni(OH)₂ precursor. Ultrasound waves were applied to the primary solution to intensify the Ni(OH)₂ precipitation. Dried precipitates were calcined at 320 °C to form nanocrystalline NiO particles. The morphology of the produced powder was characterized by transmission electron microscopy. Using sonochemical waves resulted in lowering of the size of the nickel oxide crystallites. FT-IR spectroscopy and X-ray diffraction revealed high purity well-crystallized structure of the synthesized powder. Photoluminescence spectroscopy confirmed production of a wide band-gap structure.     

Effect of nitrogen addition on hydrogen incorporation in diamond nanorod thin films

The effect of nitrogen addition in the feed gas on the finally incorporated amount of hydrogen in the diamond nanorods (DNRs) thin films has been investigated. The Raman spectroscopy measurements helped to understand the structural and quality changes with increasing nitrogen gas flow rate during CVD deposition. The hydrogen concentration was measured with 3.0 MeV He²⁺ beam using elastic recoil detection analysis technique and it was found that with the addition of nitrogen, the hydrogen concentration was increased. The results of non-Rutherford backscattering spectroscopy (NRBS) used to measure the amount of nitrogen in the DNRs thin films have shown that the incorporated nitrogen is below the detection limit of NRBS technique. Our results suggested that the addition of nitrogen has affected the overall quality of diamond films in two ways; increasing the thickness of diamond films by increasing the non-diamond carbon content and increasing the hydrogen impurity incorporation. The role of nitrogen additive on diamond growth and hydrogen incorporation is discussed.     

共掺杂的二氧化锆纳米材料中Yb3+和Tm3+上转换发光

用共沉淀法制备了二氧化锆掺Yb3 和 0 2 (0 4 ,1)mol%Tm3 的纳米材料 ;用发射波长为 980nm的激光激发样品 ,测量了掺 0 2mol%Tm3 的纳米材料在不同退火温度下的上转换光谱 ;测量了在相同退火温度、不同泵浦电流下的上转换发射光谱 ;并研究了蓝色上转换发射的过程。     

Light emission from Si nanocrystals embedded in CaF2 epilayers on Si(1 1 1): Effect of rapid thermal annealing

Visible photoluminescence (PL) of nanocrystalline silicon (nc-Si) embedded in single crystal CaF₂ formed on Si(1 1 1) has been studied and the influence of ex situ rapid thermal annealing (RTA) on the surface morphology and PL spectra has been studied. It has been found that the PL intensity and uniformity was improved by RTA with appropriate temperature and short annealing time.     

Enhancement of catalytic activity by increasing surface area in heterogeneous catalysis

The use of nanoclusters in systems with confined void spaces such as inside mesoporous or microporous solids appears to be an efficient way of preventing aggregation of nanoclusters in their catalytic application. Zeolite-Y is considered as a suitable host providing highly ordered supercages with a diameter of 1.3 nm. Intrazeolite metal(0) nanoclusters were prepared at room temperature by ion-exchange of metal cations with the extra framework Na⁺ ions in Zeolite-Y, followed by the reduction of the metal cations in the cavities of Zeolite-Y with sodium borohydride in aqueous solution, whereby the Zeolite-Y is reloaded with Na⁺ ions. Hence, host framework remains intact as shown by using a multi-prong approach. Intrazeolite transition-metal(0) nanoclusters were isolated by suction filtration and drying in vacuum at room temperature and characterized by a combination of analytical methods. Intrazeolite metal(0) nanoclusters were tested as catalyst in the hydrolysis of sodium borohydride and ammonia-borane, both of which have been considered as a promising hydrogen storage materials. High catalytic activity and the outstandingly long lifetime of intrazeolite transition-metal(0) nanoclusters catalyst in the hydrogen generation from the hydrolysis of both sodium borohydride and ammonia-borane is demonstrated. The results are attributed to the small size of the nanoclusters within the zeolite cages as well as prevention of agglomeration of the nanoclusters.     

Preparation and microwave-absorbing properties of NiFe2O4-polystyrene composites

NiFe₂O₄ nanoparticles were synthesized by the polyacrylamide gel method with acrylamide as the monomer and N,N′-methylenediacrylamide as lattice agent. The average crystallite sizes of the nickel ferrites annealed at 500, 600 and 800 °C are about 10, 30 and 50 nm, respectively. Ferrite-polystyrene composites were made by hot pressing, and microwave-absorbing properties of the composites with different contents of 35, 45, 55 and 65 wt% ferrite were investigated by testing complex permeability and complex permittivity in the X-band (8.2-12.4 GHz) frequency range. All the parameters, ε′, ε″, μ′ and μ″, increase with increasing ferrite content. The reflection losses were calculated based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 65 wt% ferrite content shows a minimum reflection loss of −13 dB at 11.5 GHz with a −10 dB bandwidth over the extended frequency range of 10.3-13 GHz for an absorber thickness of 2 mm.     

Ce-EU-1分子筛的合成和表征

采用水热法,以溴化六甲双铵（HMBr2）为模板剂,硅溶胶、偏铝酸钠为硅铝源,以六水硝酸铈为铈源合成了稀土Ce-EU-1分子筛,并通过XRD、FTIR、紫外-可见漫反射光谱、热重分析（TG-DTG）和氮气吸附-脱附等测试手段对合成样品的孔道结构、物化性能及Ce的存在状态进行了表征。XRD结果表明,合成样品不仅具有很高的纯度和结晶度,为典型的立方有序排列的EU-1结构,而且部分Ce已取代Si或Al进入微孔分子筛的骨架。样品的FTIR图谱在980cm-1附近有明显的Si—O—Ce特征吸收峰,证明Ce存在于分子筛的骨架中。紫外-可见漫反射图谱显示,在253nm附近出现O—Ce之间的电子跃迁特征峰,进一步证明了Ce进入了分子筛骨架。氮气吸附-脱附结果表明了Ce的掺入对分子筛的物化性能和孔道结构产生了影响。同时考察了镧系其他金属离子（Ln：La、Nd、Sm和Gd）对EU-1分子筛的影响,发现随着镧系离子半径的减小,Ln-EU-1分子筛的相对结晶度逐渐降低,晶胞体积减小。     

Synthesis and characterization of thermoluminescent Li2B4O7 nanophosphor

Lithium borate (Li₂B₄O₇) is a low Z_eff, tissue equivalent material that is commonly used for medical dosimetry using the thermoluminescence (TL) technique. Nanocrystals of lithium borate were synthesized by the combustion method for the first time in the laboratory. TL characteristics of the synthesized material were studied and compared with those of commercially available microcrystalline Li₂B₄O₇. The optimum pre-irradiation annealing condition was found to be 300 °C for 10 min and that of post-irradiation annealing was 300 °C for 30 min. The synthesized Li₂B₄O₇ nanophosphor has very poor sensitivity for low doses of gamma up to 10¹ Gy whereas from 10¹ to 4.5×10² Gy this phosphor exhibits a linear response and then from 4.5×10² to 10³ Gy it shows supralinearity. Thermoluminescence properties of Li₂B₄O₇ nanophosphor doped with Cu has also been investigated in this paper. It shows low fading and a linear response over a wide range of gamma radiation from 1×10² to 5×10³ Gy. Therefore the synthesized lithium borate nanophosphor doped with Cu may be used for high dose measurements of gamma radiations.     

Evidence for a solid phase of dodecahedral C 20

Evidence is presented for the formation of a solid phase based on the smallest fullerene, C₂₀, in thin diamond-like carbon films deposited by ultraviolet laser ablation from diamond onto nickel substrates at room temperature in the presence of 10^-4 torr of cyclohexane or benzene. Laser desorption mass spectrometry from the films shows the presence of C₂₀, C₂₁ and C₂₂ species, while micro-Raman spectroscopy and electron diffraction from selected particles together with first principle density-functional calculations, indicate a cubic solid with dodecahedral C₂₀ cages as building blocks. Unlike solid C₆₀ and fully protonated C₂₀, which are bound by van der Waals forces, the proposed structure is stabilized by linking of the C₂₀ dodecahedra with bridging carbon atoms at interstitial tetrahedral sites to form a face-centered-cubic lattice with 22 carbon atoms per unit cell. Received 10 October 2002 / Received in final form 24 December 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: zafar.iqbal@njit.edu