Accuracy of 3D MR microscopy for trabecular bone assessment: a comparative study on calcaneus samples using 3D synchrotron radiation microtomography

Magnetic resonance (MR) imaging is attractive for a noninvasive and radiation-free assessment of in vivo trabecular bone architecture. However the quantitative evaluation of architectural parameters could be biased by the limited sensitivity of MR. The aim of this study was to determine the accuracy of trabecular bone architectural parameters obtained from 3D high-resolution MR images, by comparison to reference images obtained by high-resolution X-ray microtomography using synchrotron radiation, from 29 samples of human calcaneus. MR images were obtained with a 66 m×66 m×66 m voxel size, using a 8.5 T MR microscope. Microtomography images were acquired with a 10 m×10 m×10 m voxel size, from the same samples. 3D architectural parameters characterizing the morphometry, topology, anisotropy, and orientation were computed from both modalities and carefully compared. To avoid errors, an identical region of interest was selected in the two corresponding images, and the same algorithms were run at identical spatial resolution. Our results establish that network connectivity, orientation and anisotropy are reliable from the MR data. The bone volume fraction, and morphometric parameters measured from the MR data, were found to be biased with respect to their values from the microtomography data, although there was a significant correlation between the two modalities.An erratum to this article can be found at     

原子力显微镜"突跳"研究

“突跳”现象严重影响着原子力显微镜(AFM)的性能．根据Hamaker微观连续介质理论,建立了AFM针尖同试样面接触力包含斥力的数学模型;分析了AFM悬臂梁弹性力同针尖接触力的关系,发现了引起AFM“突跳”的本质是弹性力同接触力的不稳定平衡问题;通过增加AFM悬臂梁刚度,提出了一种避免“突跳”现象发生的方法,并给出了仿真结果。     

氧化锌压敏陶瓷伏安特性的微观解析

为从微观层面上解析ZnO压敏陶瓷MOV的宏观伏安特性,根据电镜和深能级瞬态谱(DLTS)的观测结果,结合试验实测几种规格MOV小电流和大电流下的试验数据,建立ZnO在小电流区和大电流区的微观集中参数等效电路模型,然后依据晶界势垒、电子陷阱等理论,微观解析了各区的导电特性。结果表明:随着外施电压的增加,电子的穿透能力不断增强,使ZnO在小电流区晶界层的非线性微观等效电阻不断增大,它与纯ZnO晶粒层的线性电阻共同作用使ZnO小电流区伏安特性呈现出3个不同的特性宏观区域即预击穿区、击穿区和回升区;随着外施瞬态冲击大电流幅值的加大,ZnO在大电流区微观等效电感值增加,使ZnO大电流区伏安特性宏观呈现缓慢上升区、快速上升区和迅速上翘区;晶界层厚度的不均匀性和晶界层中电子陷阱密度的差异性宏观表现为等效电阻的非线性变化,晶界层和纯ZnO晶粒层在小电流区和大电流区具有不同的微观作用机理使得ZnO压敏陶瓷在不同电流区呈现出不同的独特宏观伏安特性。     

Structural characterization of sputtered single-phase γ? iron nitride coatings

Single-phase γ?-FeN films were deposited by d.c. magnetron sputtering in a reactive Ar/N₂ atmosphere. The films were characterized by X-ray diffraction, scanning and transmission electron microscopies, electron energy-loss and Mössbauer spectroscopy. The average lattice parameter of the γ? cubic cell is 0.455 nm. Microstructure studies by electron microscopies revealed nanostructured columnar films with no preferential orientation. Diffraction peak intensities (X-ray and electron diffraction) are close to the theoretical values for a ZnS-type structure while profile analysis of the X-ray diffraction pattern using Rietveld refinement method demonstrated that the γ?-phase is of ZnS-type. The energy-loss near-edge structures of N K-edge of the γ?-phase is similar to those of the ZnS-type γ″-phase suggesting an identical local atomic environment for N atoms. On the contrary, Mössbauer spectra of both structures are different, which is understood as a consequence of numerous vacancies in the γ? structure. Investigations of magnetic properties showed that the γ?-FeN compound is paramagnetic from room temperature to 2 K. The main conclusion of this work is that the γ?-FeN phase is of ZnS-type, and not of NaCl-type as it is usually reported.     

高强高模PET纤维拉伸过程超分子结构形成机理

用红外显微镜技术直接对单根聚对苯二甲酸乙二酯(PET)高强高模低缩纤维进行非破坏性光谱测量,同时进行广角X-射线衍射(WAXD)分析。主要研究了1100～750 cm - 1范围的红外光谱。通过谱带分离技术测定出PET纤维非晶区反式构象异构体含量。阐明了高强高模PET工业纤维在多级拉伸过程中各种分子构象的变化及形态结构的发展。结果表明:纤维的初始模量和断裂强力随非晶相反式构象含量的提高而增加。由于在第二级拉伸结束和随后的热辊定型过程使拉伸过程中发展起来的非晶相反式构象部分松驰下降到21.7% ,从而限制了机械性能的进一步提高。而经局部加热拉伸定型的纤维展示出较高的非晶区反式构象含量(30.1% ),纤维的机械性能有较大提高     

Effects of Environments on the Noise of Atomic Force Microscopy Cantilevers

环境对原子力显微镜悬臂噪声的影响

句宏宇 张松 崔树勋

（西南交通大学 生命科学与工程学院 材料先进技术教育部重点实验室,四川 成都 610031）

创新点说明：

1）发现了原子力显微镜（AFM）在高真空环境中具有非常小的噪声;

2）发现了AFM在液体环境中的噪声与液体粘度和悬臂运动速率之间的关系。

研究目的：

得到AFM的噪声和使用环境、悬臂种类以及仪器参数设置之间的关系,找出减小AFM噪声的有效方法。

研究方法：

模拟基于AFM的单分子力谱实验过程,通过计算AFM在高真空、大气和不同粘度液体环境中不同运动速率下力信号数值的标准差（即噪声大小）,分析AFM噪声的影响因素。

研究结果：

1）V形悬臂在高真空环境中具有非常小的噪声,在大气环境中噪声有微小增大,在液体环境中噪声显著增大;

2）AFM具有较低的系统噪声,且系统噪声大小和悬臂到基底的距离相关;

3）随着悬臂运动速率的增加,V形悬臂在不同粘度液体环境中的噪声表现出相似的增大趋势。且液体黏度越大,噪声随速率增大的趋势越显著;

4）相同环境中矩形悬臂的噪声小于V形悬臂的噪声。

结论：

1）高真空环境中AFM具有较小的噪声和较高的分辨率,是进行单分子力谱实验的理想环境;

2） 在液体环境中为了获得较高的信噪比,应尽可能地减小悬臂的运动速率和液体粘度。

关键词：原子力显微镜;悬臂;噪声;高真空;粘度

     

纳米电子器件的研究进展与军事应用前景

首先,分析了纳米电子器件碳纳米管、单电子晶体管的结构特点及其应用;然后,指出了碳纳米管、单电子晶体管应用中有待解决的问题;最后,综合阐述了纳米电子学的纳米技术在军事及航天领域中的应用前景。     

Preparation of Gold Nanorods of High Quality and High Aspect Ratio

We report the synthesis of gold nanorods (NRs) by seed-mediated growth method. A small amount of different shapes such as triangles, hexagons and a large amount of rods are obtained by varying the proportion of seed to metal salt, adding NaOH to growth solution as well as using the seed solution of CTAB-capped agent. The gold nanorod (NR) formation yield is improved. Meanwhile, the growth mechanism of high yield gold NRs is discussed. The high quality single size NRs can be separated from polydisperse sampl...     

Oxidation kinetics and high temperature in-situ observation of the oxidation behaviour of NbSi_2 at 1023 K

Pest oxidation has been known for a long time in refractory transition-metal disilicides such as NbSi2 and MoSi2[1―4]. However, the origin of pesting reaction of these materials is still under debate. Although the pesting phenomenon in NbSi2 has been reported in several works[5―7], a direct study of the mechanism is scarce at the moment. Compared to NbSi2, pesting in MoSi2 has received relatively extensive attention. Mckamey et al.[8] showed fragmentation near 773K occurred easily in as…     

Silicon assistant carbothermal reduction for SiC powders

The silicon assistant method to increase the reaction yield of carbothermal reduction of silica at a lower temperature is reported. The effect of silicon on the carbothermal reduction process has been investigated in detail. Compared with traditional reduction, the introduction of silicon can change the reaction path and further increase the conversion of silicon carbide at a lower temperature. It is considered that the assistant reduction consists of three steps: vaporizing and melting of silicon, formation of silicon monoxide, and synthesis of silicon carbide. The morphology of the synthesized SiC powders through the silicon assistant method can be affected apparently by the experimental temperature.