Molecular Weight and Distribution of Cellulose and Cellulose Microspheres

[ Objective ] This study aimed to provide basic data for studying the relationship between structure and property of cellulose microspheres by measuring molecular weight of cellulose and cellulose microspheres with viscosity method and gel permeation chromatography （GPC） method. [ Method] In viscosity method, cadmium ethylenediamine was used as the solvent, intrinsic viscosity η of the solution was determined at 25 ℃ by using a Ubbelohde viscometer, to calculate the molecular weight of cellulose; in GPC method, 8% LiC1 / N, N-dimethylacetamide （LiC1/DMAc） was used as the solvent and 0.5% LiC1/DMAc was used as the mobile phase to determine the relative molecular weight and distribution of cellulose and cellulose microspheres. In addition, the determination results were analyzed to compare these two methods. [ Result ] Viscosity-average molecular weight Mr/ of cellulose and cellulose microspheres determined with viscosity method were 224,532 and 16,686, respectively; weight-average molecular weight Mw of cellulose and cellulose microspheres determined with GPC method were 284,196 and 22,345, respectively. [ Conclusion] The determination results of （;PC method are relatively close to the actual value and could truly reflect the characteristics of molecular weialat distribution of eellulose and cellulose mierosr, heres.

The Field Shielding Effect of Mountain on the Lightning Electromagnetic Field

In this paper we have studied the effect of strike to a cone-shaped mountain surrounded by two chains of hills on the lightning vertical electric field and azimuthal magnetic field at different distances, by using two-dimensional finite-difference time-domain (2-D FDTD) method in cylindrical coordinate systems. In order to analyze the electrostatic shielding effect of strike to a cone-shaped mountain surrounded by two chains of hills on the lightning, we chose three mountains, and the left one is stroke by lightning, and the right one is near the obervation site, and the middle one with the top heights increasing from 0 to 2 km is between them. For the observed point, the electrostatic shielding effect of the right one on the lightning vertical electric field is the most serious, and the electric field is much less than that for strike to flat ground level;compared with the electric field, the shielding effect of the right one on the lightning azimuthal magnetic field becomes less, for most cases, the lightning magnetic field at the observed site is larger than that for flat ground cases mainly due to that of the increment caused by strike to the right mountain. With the increase of distance (e.g., 20 km from the lightning strike point), the shielding effect of the right mountain on the lightning field becomes less, because the reflected wave from the right mountain bottom plays a more important role over intermediate ranges, and the far radiation electromagnetic field peak value becomes larger due to such a reflecting wave. Therefore, in the lightning detecting technique, we should pay more attention to the effect caused by chains of hills on the lightning location and the estimation of lightning current peak.

Effects of physical crust on soil detachment by overland flow in the Loess Plateau region of China

Physical soil crust(PSC),a key component of surface soil structure,exists extensively in loess areas.PSC is considered to have a significant effect on soil detachment processes.However,the long-term effects and the corresponding mechanisms of PSC on soil detachment by overland flow are still not well understood,especially in natural environments.To investigate temporal variation in soil erosion resistance and the underlying factors during PSC formation,an 8×8-m soil plot was exposed to natural conditions in the Loess Plateau over a 524-day period spanning two rainy seasons and a winter between them.A flume test was conducted to determine soil detachment capacity(Dc)under six designed flow shear stress levels(5.66-22.11 Pa)using crusted(SC)and non-crusted(NSC)soil samples at different PSC development stages.Subsequently,two soil erosion resistance parameters,rill erodibility(K_(r))and critical shear stress(τ_(c)),were calculated.Over time,in the SC and NSC treatments,K_(r)decreased from 0.516 to 0.120 s m^(-1)and 0.521 to 0.223 s m^(-1),respectively,whileτ_(c)increased from 0.49 to 4.42 Pa and 0.26-2.46 Pa,respectively.Variation in soil erosion resistance was rapid in the first one to two months,and then slowed down,with slight fluctuations afterwards.In the SC treatment,K_(r)was 42%lower andτ_(c)was 67%greater than those in the NSC treatment.Soil properties changed greatly for both treatments.SCT increased from 0 to 7.09 mm in the SC treatment.Coh increased from 2.91 to 9.04 kPa and 3.01-4.78 kPa in SC and NSC treatments,respectively.Both soil erosion resistance parameters could be well predicted by SCT and Coh in the SC treatment(R^(2)≥0.82),while their best predictor was Coh in the NSC treatment(R^(2)≥0.90).The results demonstrate that PSC formation enhances soil erosion resistance in the soil detachment process in the loess region under natural conditions.Our study revealed the important role and complexity of PSC in the process of soil erosion,and provided theoretical and data support for accurate understanding and prediction of soil erosion.

Deformation control criterion of shield tunnel under lateral relaxation of soft soil

Metro shield tunnels under the lateral relaxation of soil(LRS)are susceptible to significant lateral deformations,which jeopardizes the structural safety and waterproofing.However,deformation control standards for such situations have not been clearly defined.Therefore,based on a specific case,a model test is conducted to realize the LRS of a shield tunnel in a sandy stratum to reveal its effect on segment liners.Subsequently,a deformation control criterion is established.The LRS is simulated by linearly reducing the loads applied to the lateral sides of the segment structure.During lateral unloading,the lateral earth pressure coefficient on the segment decreases almost exponentially,and the structural deformation is characterized by horizontal expansion at the arch haunches and vertical shrinkage at the arch vault and arch bottom.Based on the mechanical pattern of the segment structure and the acoustic emission,the deformation response of a segment can be classified into three stages:elastic and quasi-elastic,damage,and rapid deformation development.For a shield tunnel with a diameter of approximately 6 m and under the lateral relaxation of sandy soil,when the ellipticity of the segment is less than 2.71%,reinforcement measures are not required.However,the segment deformation must be controlled when the ellipticity is 2.71%to 3.12%;in this regard,an ellipticity of 3%can be used as a benchmark in similar engineering projects.

“中国精神”笔谈

在当前中国和世界的重要节点上,我们需要调整观念、凝聚共识,重新审视“中国精神”问题。写入经典的“理念”虽然较明确,却未必完全成为现实、主导现实,然则真正活在中国人身上和心中、影响其思维和言行的核心“精神”是什么,应该如何更好地建构?这里既有对传统价值的创造性转化,以及马克思主义中国化的观念沉淀,也有现代化全球化等背景下的重新生长,以及对某些不足之处的反思与改进。本专题邀请七位名家,从不同角度和层面对此进行探索和阐释。

随机突变文库构建与筛选研究进展

定向进化是一个循环过程,在构建多样化基因序列和筛选功能基因变体之间交替进行。该技术目前已被广泛应用于DNA序列、基因功能和蛋白质结构的优化和分析。定向进化包括随机基因文库的生成、基因在合适宿主中的表达和突变文库的筛选。构建基因文库的关键是库容量和突变多样性,而筛选变体的关键是高灵敏度和高通量。文中讨论了定向进化技术的最新进展,这些新技术极大地加速和简化了传统的定向进化过程,为定向进化的发展注入了强劲动力。

Exogenous and endogenous determinants of spatial aggregation patterns in tibetan Plateau meadow vegetation

Aims We aim to quantify the relative importance of various endogenous and exogenous processes influencing the spatial distribution of the individuals of plant species at different temporal and spatial scales in a species-rich and high-cover meadow in the eastern tibetan Plateau.Methods We calculated green’s index of dispersion to infer the spatial dis-tribution patterns of 73 herbaceous species at two scales(0.25 and 1.0 m2).We constructed a series of generalized linear models to test the hypotheses that different species traits such as mean plant stem density,per capita dry biomass,maximum plant height and mean seed mass contribute to their spatial distribution.We used the first principal component of soil C,N and P to explain abundance vari-ation across quadrats and sub-plots.Important Findingsthe individuals of the species studied were highly spatially aggregated.at both spatial scales,biomass and stem density explained the most variation in aggregation,but there was no evidence for an effect of mean seed mass on aggregation intensity.the effects of soil carbon,nitrogen and phosphorus at different depths affected plant abundance mostly at the broader spatial scale.our results demonstrate that self-thinning and habitat heterogeneity all contribute to determine the spatial aggregation patterns of plant individuals in alpine meadow vegetation in the eastern tibetan Plateau.