K2O对氧化物原料合成Al8B4C7的影响

采用硼砂、金属铝和碳粉为原料一步合成Al8B4C7.在合成过程中掺入不同含量的K2O,研究杂质K2O对合成Al8B4C7的影响.将原料混合均匀,在200MPa的压力下压制成φ20mm×20mm的圆柱型试样,将干燥后的试样在不同温度下的流动氩气气氛中进行合成.利用化学分析、XRD和SEM等分析测试技术,研究了K2O含量以及烧结温度对反应合成材料物相组成和显微结构的影响.结果表明:尽管加入不同含量的K2O,但其高温烧结反应后,若K2O的质量分数在0.019%～0.028%之间,就不出现含K的新相.K2O的加入不利于Al8B4C7的合成,它可将Al8B4C7的开始生成温度提升至1 400℃,而且1 700℃时,Al8B4C7含量较不加入K2O时明显减少.添加K2O合成的Al8B4C7为1μm左右大小的无规则小颗粒.     

受激布里渊散射慢光系统中强度调制器的应用研究

基于Mach-Zehnder干涉式的强度调制器具有移频特性,用于受激布里渊散射的慢光系统可以用来产生具有布里渊频移的信号光.理论和实验分析了M-Z干涉式电光强度调制器的频移特性.受激布里渊散射慢光实验中利用12.5 Gbit/s的电光强度调制器产生了相对于泵浦光具有9.394 GHz布里渊频移,脉宽为50 ns的信号光.在泵浦功率20 mW的情况下,该脉冲在2.5 km长的高非线性光纤中获得了32 dB的非饱和增益,脉冲延迟了25 ns.     

稀释气体对甲烷层流预混火焰燃烧速度的影响

基于GRI-Mech 3.0详细化学反应机理,利用预混燃烧模型(PREMIX Code)研究了甲烷-空气-稀释气层流预混火焰燃烧特性及火焰结构.重点探讨了不同化学当量比(0.5～1.5)、初始压力(0.05～0.40 MPa)、稀释气体种类(N2,CO2及H2O)和稀释摩尔比(0～0.35)对甲烷-空气-稀释气混合气层...     

多波段光度计定标及误差分析

摘要根据Langley法定标的原理,在河北省灵寿县对4台多波段太阳光度计MICROTOPSⅡ进行定标,并采用最小二乘法得到了很好的定标结果。分别计算了仪器的视场角、通道的半波宽度、大气状况以及气体吸收等方面对定标结果造成的影响,并且对水汽吸收通道（936nm）采用修正的Langley法定标,得到4台仪器各自5个通道的定...     

斑马鱼SAA基因克隆及表达分析

利用巢式PCR(nest PCR)的方法克隆斑马鱼血清淀粉样蛋白A基因,并进行生物信息学分析.实时荧光定量PCR结果显示:SAA在斑马鱼胚胎发育时期的表达量呈现明显的“增长—稳定—增长”趋势.成鱼组织分布中,SAA在11个斑马鱼组织中均能检测到,在心脏和肝脏中相对高表达.     

浅谈三段式电流保护

当线路发生短路故障时,重要特征之一是线路的电流急剧增大,当电流流过某一预定值时,反应于电流升高而动作的保护装置叫过流保护。而三段式电流保护具有作为主保护和后备保护的全部功能,在实际中得到广泛应用。     

浅谈我国铁路通信信号发展趋势

随着信息时代的到来,铁路的发展也在日新月异的变化着,铁路通信信号的发展趋势正在向一体化、多信息化、功能集成化、高度自动化的方向前进。文章主要从计算机联锁设备、列车自动控制系统、调度指挥系统三个部分对信号系统进行了分析和阐述。     

Structure of a nanobody-stabilized active state of the β(2) adrenoceptor

G protein coupled receptors (GPCRs) exhibit a spectrum of functional behaviours in response to natural and synthetic ligands. Recent crystal structures provide insights into inactive states of several GPCRs. Efforts to obtain an agonist-bound active-state GPCR structure have proven difficult due to the inherent instability of this state in the absence of a G protein. We generated a camelid antibody fragment (nanobody) to the human β(2) adrenergic receptor (β(2)AR) that exhibits G protein-like behaviour, and obtained an agonist-bound, active-state crystal structure of the receptor-nanobody complex. Comparison with the inactive β(2)AR structure reveals subtle changes in the binding pocket; however, these small changes are associated with an 11?? outward movement of the cytoplasmic end of transmembrane segment 6, and rearrangements of transmembrane segments 5 and 7 that are remarkably similar to those observed in opsin, an active form of rhodopsin. This structure provides insights into the process of agonist binding and activation.     

Crystal structure of the β2 adrenergic receptor-Gs protein complex

G protein-coupled receptors (GPCRs) are responsible for the majority of cellular responses to hormones and neurotransmitters as well as the senses of sight, olfaction and taste. The paradigm of GPCR signalling is the activation of a heterotrimeric GTP binding protein (G protein) by an agonist-occupied receptor. The β(2) adrenergic receptor (β(2)AR) activation of Gs, the stimulatory G protein for adenylyl cyclase, has long been a model system for GPCR signalling. Here we present the crystal structure of the active state ternary complex composed of agonist-occupied monomeric β(2)AR and nucleotide-free Gs heterotrimer. The principal interactions between the β(2)AR and Gs involve the amino- and carboxy-terminal α-helices of Gs, with conformational changes propagating to the nucleotide-binding pocket. The largest conformational changes in the β(2)AR include a 14 ? outward movement at the cytoplasmic end of transmembrane segment 6 (TM6) and an α-helical extension of the cytoplasmic end of TM5. The most surprising observation is a major displacement of the α-helical domain of Gαs relative to the Ras-like GTPase domain. This crystal structure represents the first high-resolution view of transmembrane signalling by a GPCR.     

学习型组织知识创新的文化环境分析

知识创新是学习型组织的内在要求。学习型组织的知识创新是在社会系统中进行的,社会文化环境对其有重要的影响和作用。而社会环境文化的作用包括了外生变量和内生变量两种不同的方式,其中外生变量表现为适应环境的行为,内生变量表现为积极主动地创新。