高通量研磨-X射线荧光光谱法测定不同植源性食品中硒含量北大核心

为了提高食品的样品检验效率,采用高通量研磨制备样品,利用X射线荧光光谱法对食品中硒含量进行检测。重点研究了样品含水率、样品颗粒粒径、研磨条件、检测条件等因素对检测结果的影响。结果表明:样品干基含水量小于20%,样品颗粒粒径小于100μm;高通量研磨条件为装样量3 g,频率30 Hz,研磨3 min;高精度X射线荧光光谱仪检测条件为检装样量0.2 g,PP膜压紧,检测90 s时的情况下,硒的检出限0.01 mg/kg,仪器的精密度小于5%,与粉碎法进行对比,相对偏差在-10.37%~8.05%。说明高通量研磨-X射线荧光光谱法检测食品中硒含量的方法快速、准确,可以满足批量样品现场检测分析需求。     

REINFORCEMENT OF POLYDIMETHYLSILOXANE NETWORKS BY NANOCALCIUM CARBONATE

Although a number of investigations have been devoted to the analysis of silica or carbon black filled elastomer networks, little work has been done on the reinforcement of CaCO3 filled elastomer network. In this work, the reinforcement of polydimethylsiloxane (PDMS) network by using CaCO3 nano-particles was investigated. We have found a simultaneous increase of tensile strength, modulus and elongation with the increase in nano-CaCO3 content, which suggests that nano-CaCO3 particles can indeed be used as a reinforcing agent, just like silica or carbon black. Interestingly, the tensile strength,modulus and elongation were seen to leave off for the first time when the content of nano-CaCO3 particles reaches to 80%.PDMS also showed an enhanced elastic modulus and storage modulus with the increase in nano-CaCO3 content, particularly for samples with high nano-CaCO3 content. SEM was used to investigate the dispersion of the filler in PDMS matrix. A better dispersion was found for samples with high nano-CaCO3 content. A great increase of viscosity was found for samples with higher filler content, which is considered to be the reason for the good dispersion thus the reinforcement, because high viscosity will be helpful for breaking the agglomerates of fillers into small size particles under effect of shear. Our work provides a new way for the reinforcement of elastomer by using an adequate amount of nano-CaCO3 particles instead of as mall quantity of silica, which is not only economically cheap but also very effective.     

电感耦合等离子体原子发射光谱法测定锰矿石中主次量元素

传统的化学分析方法分析矿石中主次量元素流程长,效率低。受干扰影响大,耗费大量人力物力,采用电感耦合等离子体原子发射光谱法测定技术对锰矿中的主次量元素进行了研究,主要测定了铝、铜、钙、铁、钾、镁、锰、钠、镍、磷、钛、锌等13种元素的含量。仪器采用射频功率1 150W,雾化器流速为0.70L/min,辅助器流速0.50L/min,观测高度12mm的测定条件。采用硝酸、盐酸、氢氟酸、高氯酸四酸溶矿,实验表明,溶液中分别共存200mg/L以下的钾、钠、钙、镁、钒、锰,对各微量元素的测定基本没有干扰。分析方法的检出限0.0002%~0.0065%。相对标准偏差(RSD)3.7%。均符合DZ/T0130—2006地质矿产实验室测试质量管理规范中精密度和准确度的控制指标。     

New Type of Conserved Quantities of Lie Symmetry for Nonholonomic Mechanical Systems in Phase Space

The new types of conserved quantities, which are directly induced by Lie symmetry of nonholonomie mechanical systems in phase space, are studied. Firstly, the criterion of the weak Lie symmetry and the strong Lie symmetry are given. Secondly, the conditions of existence of the new type of conserved quantities induced by the weak Lie symmetry and the strong Lie symmetry directly are obtained, and their form is presented. Finaily, an Appell-Hamel example is discussed to further illustrate the applications of the results.     

单面完整约束力学系统的形式不变性

研究单面完整约束力学系统的形式不变性.根据运动微分方程的形式在无限小变换下的不变性，给出了单面完整约束力学系统形式不变性的定义和判据，建立了系统的形式不变性与Noether对称性、Lie对称性之间的关系，并举例说明结果的应用. 关键词： 分析力学 单面约束 形式不变性 Lie对称性 Noether对称性 守恒量     

变质量Birkhoff系统的Lie对称性和非Noether守恒量

采用嵌入质量法建立了变质量系统的Birkhoff方程.根据Lie对称性理论给出了变质量Birkhoff系统的Lie对称性确定方程，得到了系统的Lie对称直接导致非Noether守恒量的存在条件和形成.举例说明结果的应用. 关键词： 变质量 Birkhoff系统 Lie对称性 非Noether守恒量     

求解微分方程的Hojman方法

首先，将Hojman用于求解二阶微分方程组守恒量的方法推广并应用于一阶微分方程组，特别是奇数维微分方程组的积分问题.然后，证明 Hojman定理是本文定理的特殊情形.最后，举例说明结果的应用. 关键词： 微分方程 Hojman定理 守恒量 积分     

自润滑弹带孔隙渗出油量的统计计算

从弹带的作用及特性出发,建立了自润滑弹带在挤进过程中孔隙渗出油量的数学模型,通过统计计算初步得出了渗出油量和行程之间的关系.这一问题的解决为自润滑弹带材料的选择提供了理论依据,也为研究身管延寿问题提供了新的方法.     

航空摄影胶片与CCD信息量的研究

为了探寻航空摄影胶片与CCD相比的优势所在，计算了航空摄影胶片和CCD的信息量，时间自由度数Nt，空间自由度数Ns，颜色自由度数Nc，偏振自由度数Nw等．结果指出，普通国产1124航空摄影胶片信息量是3K×3K面阵CCD信息量的61倍，是9K×9K面阵CCD信息量的6．7倍．并根据CCD的分辨极限和成品率低的特点，得出了CCD的信息量将无法超越航空摄影胶片的结论．     

A Unified Symmetry of Mechanical Systems with Variable Mass in Phase Space

In this paper, the definition and the criterion of a unified symmetry of the mechanical system with variable mass in phase space are given. The Noether conserved quantity, the generalized Hojman conserved quantity, and Mei conserved quantity deduced from the unified symmetry are obtained. An example is given to illustrate the application of the results.