Thermal behavior for a nanoscale two ferromagnetic phase system based on random anisotropy model

Advances in theory that explain the magnetic behavior as function of temperature for two phase nanocrystalline soft magnetic materials are presented. The theory developed is based on the well known random anisotropy model, which includes the crystalline exchange stiffness and anisotropy energies in both amorphous and crystalline phases. The phenomenological behavior of the coercivity was obtained in the temperature range between the amorphous phase Curie temperature and the crystalline phase one.     

Microstructure and soft magnetic properties of Finemet-type ribbons obtained by twin-roller melt-spinning

Soft magnetic ribbons of Finemet-type (Fe_73.5Cu₁Nb₃Si_13.5B₉) alloys are synthesized by the twin-roller melt-spinning technique directly from the melt, at tangential wheel speeds of 15, 18, 19 and 20 m/s. The microstructure and the magnetic properties are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA), thermo-gravimetric analysis (TGA) and hysteresis loops measurements. Samples cooled at 20 m/s are amorphous, while those quenched at lower wheel speeds are partially crystalline. All samples studied present saturation magnetization values (150-160 A m²/kg) higher than the commercial Finemet alloys (∼135 A m²/kg), obtained by controlled crystallization of amorphous single-roller melt-spun alloys. Optimal soft magnetic properties - σ_S=(154±8) A m²/kg and H_C=(6.9±0.9) A/m - are found in samples quenched at 19 m/s, consisting of size-distributed bcc Fe-Si nanograins (∼18 nm in average) embedded in an amorphous residual matrix. A minority nanocrystalline magnetic phase (≤10 nm) is also detected.     

Er~(3+),Yb~(3+)共掺杂Y_2O_3纳米晶体粉末中1.5μm近红外发光增强现象的研究

用燃烧法制备了平均粒径为10和40nm的(Y0.96Er0.02Yb0.02)O3纳米晶体样品,并通过1200℃高温退火获得了同样组分的体材料样品。利用X射线衍射谱(XRD),傅里叶变换红外吸收光谱(FTIR),透射电镜(TEM)和透射电镜(SEM)照片对样品的晶体结构和形貌进行了表征。测量了不同样品980nm激发下的上转换发射光谱和近红外发射光谱。对实验结果的分析发现,随着粒径的减小,样品发射光谱中红光和近红外发射的成分增加。产生这一现象的原因是由于纳米材料具有比表面积大的特点,能够吸附更多的OH-(振动能量3200～3800cm-1),OH-数量的增加使电子从Er3+的4I11/2→4I13/2能级(能量差3600cm-1)的无辐射弛豫速率增大,这一无辐射弛豫过程减少了4I11/2上的电子布居数,使绿光发射减弱;同时增加了4I13/2上的电子布居数,使红光和近红外发射增强。40nm样品的1.5μm发射主峰强度是体材料的1.6倍,这一结果对纳米发光材料的实际应用是很有意义的。     

Synthesis of carbon-encapsulated nickel nanoparticles

Carbon-encapsulated metal nanoparticles (CEMNs) were obtained by the catalytic decomposition of hydrocarbons (CH₄, C₂H₆, C₂H₄) on nanocrystalline nickel. Nanocrystalline nickel was obtained by precipitation from nickel nitrate solution, followed by calcination and reduction under hydrogen. A small amount of structural promoters (aluminium and calcium oxide) was added to avoid recrystallisation of fine nickel particles at elevated temperatures. Reduction and carburisation of the samples was carried out in a differential reactor with mass control. The rate of decomposition of methane, ethane and ethylene on nanocrystalline nickel was measured and the apparent activation energy of the process was determined. The obtained samples were characterised using the XRD, SEM and HRTEM methods.     

Contact and non-contact magnetoimpedance in amorphous and nanocrystalline Fe73.5Si13.5B8CuV3Al ribbons

The giant magnetoimpedance (GMI) effect in amorphous and nanocrystalline Fe_73.5Si_13.5B₈CuV₃Al ribbons has been studied as a function of a dc magnetic field, by contact and non-contact methods. In the contact method, an MI of 19% was obtained in the nanocrystalline ribbon while it is of the order of 10⁴ in the non-contact method. The huge sensitivity of the non-contact method is promising with regard to the increase of the sensitivity of the MI sensors. Field dependence of MI has shown double peak behavior in contact method and single peak profile in non-contact method. The domains were found to lie normal to the plane of the ribbon, through MFM studies. All the experimental results were discussed using the Polivanov model.     

Low-temperature preparation of nanocrystalline anatase films through a sol-gel route

Nanocrystalline anatase (TiO₂) films were prepared at very low temperature through a sol-gel route using titanium isopropoxide and hydrogen peroxide in ethanol. Crystallization occurred after film deposition at 35°C in an atmosphere saturated with water vapor. Both thin and thick films of nanocrystalline anatase were prepared. Observed particle size in crystallized films is approximately 20–40 nm as measured with AFM. No residual organic material was apparent through FTIR after crystallization occurred. Dynamic light scattering studies performed on this system indicate that particle size measured in solution is strongly dependent on the amount of agitation samples received prior to measurement.     

Preparation,microstructure, and magnetic properties of a nanocrystalline Nd12Fe82B6 alloy by HDDR combined with mechanical milling

Nanocrystalline Nd₁₂Fe₈₂B₆ (atomic ratio) alloy powders with Nd₂Fe₁₄B/α-Fe two-phase structure were prepared by HDDR combined with mechanical milling. The as-cast Nd₁₂Fe₈₂B₆ alloy was disproportionated via ball milling in hydrogen, and desorption–recombination was then performed. The phase and structural change due to both the milling in hydrogen and the subsequent desorption–recombination treatment was characterized by X-ray diffraction (XRD). The desorption–recombination behavior of the as-disproportionated alloy was investigated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). The morphology and microstructure of the final alloy powders subject to desorption–recombination treatment were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results showed that, by milling in hydrogen for 20 h, the matrix Nd₂Fe₁₄B phase of the alloy was fully disproportionated into a nano-structured mixture of Nd₂H₅, Fe₂B, and α-Fe phases with average size of about 8 nm, and that a subsequent desorption–recombination treatment at 760 °C for 30 min led to the formation of Nd₂Fe₁₄B/α-Fe two-phase nanocomposite powders with average crystallite size of 30 nm. The remanence B_r, coercivity H_c, and maximum energy product (BH)_max of such nanocrystalline Nd₁₂Fe₈₂B₆ alloy powders achieved 0.73 T, 610 kA/m, and 110.8 kJ/m³, respectively.     

Role of polytypism and degree of hexagonality on the photoinduced optical second harmonic generation in SiC nanocrystalline films

Photoinduced optiсal second harmonic generation was studied in nanocrystalline SiC films prepared by the method of direct ion deposition. For the studies were chosen three types of polytypes (with different degree of hexagonality) – 24R with degree hexagonality G=25, 27R-G=44, 33R with – G=36. The bicolor photoinduced treatment was performed by the wavelengths 1064nm/532 nm by 15 ns YAG:Nd laser. The efficiency of the output SHG was evaluated by ratio of the corresponding signal intensities with respect to the references and by the time delay between the SHG and the fundamental maxima. Explanation of the observed effect is given within a framework of the occurrence of the nano-trapping levels in the film crystalline interfaces.     

EXAFS and Raman studies of mechanical alloyed Ni25Se75 mixture under high-pressure conditions

Extended X-ray Absorption Fine Structure and Raman studies were performed to follow the structural and vibrational behavior of a mechanical alloyed Ni₂₅Se₇₅ mixture, containing nanocrystalline pyrite NiSe₂ phase, when exposed to high-pressure conditions. An increase in the local structural order of the nanocrystalline phase with pressure increasing was observed by means of Debye-Waller factor analysis. The relative Ni nearest-neighbors distances were determined as a function of pressure, which were used to determine the inverse linear compressibility of the nanocrystalline pyrite NiSe₂ alloy as well as its derivate by means of Murnaghan's equation. The Raman results showed tentative NiSe₂ phonons dispersion with pressure that becomes a difficult task since the existence/photo-induced nucleation of an important amount of nanocrystalline Se, detected due to the observation of its pressure-induced phase transitions.     

Synthesis of oleylamine capped copper nanocrystals via thermal reduction of a new precursor

The present investigation reports the novel synthesis of copper nanocrystals using thermal reduction, and their physicochemical characterization. The copper nanocrystal powder has been prepared using [bis(2-hydroxyacetophenato)copper(II)] as a precursor. The effect of oleylamine and triphenylphosphine on the particle morphology has been investigated. Transmission electron microscopy (TEM) analysis has demonstrated that the copper nanocrystals have an average diameter of about 3 nm. The as-prepared copper nanocrystals were characterized by XRD, SEM, TEM, EDX, UV–Vis and FTIR.