全文获取类型
收费全文 | 358篇 |
免费 | 57篇 |
国内免费 | 48篇 |
出版年
2024年 | 2篇 |
2023年 | 12篇 |
2022年 | 14篇 |
2021年 | 22篇 |
2020年 | 26篇 |
2019年 | 31篇 |
2018年 | 21篇 |
2017年 | 39篇 |
2016年 | 23篇 |
2015年 | 23篇 |
2014年 | 25篇 |
2013年 | 22篇 |
2012年 | 9篇 |
2011年 | 18篇 |
2010年 | 25篇 |
2009年 | 30篇 |
2008年 | 23篇 |
2007年 | 20篇 |
2006年 | 15篇 |
2005年 | 9篇 |
2004年 | 9篇 |
2003年 | 12篇 |
2002年 | 4篇 |
2001年 | 6篇 |
2000年 | 1篇 |
1999年 | 5篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 2篇 |
排序方式: 共有463条查询结果,搜索用时 15 毫秒
401.
A Conductive Molecular Framework Derived Li2S/N,P‐Codoped Carbon Cathode for Advanced Lithium–Sulfur Batteries 下载免费PDF全文
Li2S is one of the most promising cathode materials for Li‐ion batteries because of its high theoretical capacity and compatibility with Li‐metal‐free anode materials. However, the poor conductivity and electrochemical reactivity lead to low initial capacity and severe capacity decay. In this communication, a nitrogen and phosphorus codoped carbon (N,P–C) framework derived from phytic acid doped polyaniline hydrogel is designed to support Li2S nanoparticles as a binder‐free cathode for Li–S battery. The porous 3D architecture of N and P codoped carbon provides continuous electron pathways and hierarchically porous channels for Li ion transport. Phosphorus doping can also suppress the shuttle effect through strong interaction between sulfur and the carbon framework, resulting in high Coulombic efficiency. Meanwhile, P doping in the carbon framework plays an important role in improving the reaction kinetics, as it may help catalyze the redox reactions of sulfur species to reduce electrochemical polarization, and enhance the ionic conductivity of Li2S. As a result, the Li2S/N,P–C composite electrode delivers a stable capacity of 700 mA h g?1 with average Coulombic efficiency of 99.4% over 100 cycles at 0.1C and an areal capacity as high as 2 mA h cm?2 at 0.5C. 相似文献
402.
水生态功能区划流程:双关系树框架与概念模型 总被引:1,自引:0,他引:1
水生态区划和水生态功能区划是流域水环境管理与水生态系统合理利用的重要基础,国内已经研究十年有余.本文针对该领域的一些共性问题,通过对水生态区划分与水生态功能类型划分的系统分析,探讨了水生态区划与水生态功能区划的关联性,提出了基于双树结构的RFCH流程和钻石概念模型;并以辽河流域为例,参照其他研究者的区划方案,开展了以水生态三级区为基础的水生态功能分类,为流域水生态功能区划提供了一种可借鉴的流程框架. 相似文献
403.
Intertidal seagrass has been selected as a Biological Quality Element for the assessment of ecological status under the Water Framework Directive. In Ireland, two species of seagrass, Zostera marina and Z. noltei occur in intertidal habitats. This study presents the first comprehensive assessment of the distribution and Water Framework Directive status of intertidal seagrass in the Republic of Ireland and Northern Ireland. Most of the areas assessed, using the Water Framework Directive-compliant assessment tool, have a status of HIGH or GOOD. Only two areas were found to have a status less than GOOD and in both, the cause for the decline was smothering by opportunistic foliose green macroalgae. Linear regression showed a relationship between pressure index scores and Ecological Quality Ratio, showing the relevance of the index as a metric of anthropogenic pressure. Trace element concentrations were examined in Z. noltei tissues and Trace Element Pollution Index values were calculated. The relationship between Trace Element contamination and Water Framework Directive status was examined but the results showed little correlation. However, a relationship between the pressure index and trace element contamination was obtained. This assessment provides the most comprehensive overview of intertidal seagrass beds in Ireland and establishes a strong baseline for ongoing monitoring and assessment under the Water Framework Directive. The data provide key information on the pressures affecting these valuable habitats which will assist in the development of measures to improve and protect our transitional and coastal waters. 相似文献
404.
405.
Nishimura T 《Primates; journal of primatology》2003,44(1):41-49
The descent of the larynx is a key phenomenon not only in postnatal development, but also in the evolution of human speech.
The positional change of the larynx is affected by the descent of the hyoid bone in relation to the mandible and cranial base,
and that of the laryngeal framework in relation to the hyoid bone. The phylogeny of the spatial configuration of the hyo-laryngeal
complex is one of the most important sources of information for elucidating the evolution of laryngeal descent. In the present
study, the anatomy of the complex was examined in various species of anthropoids to compare the configuration, the shape of
the basihyal and thyroid cartilage, and the length of the lateral thyrohyoid ligaments. Non-human hominoids share most features
with humans, while cercopithecoids and ceboids have anatomical features that sharply contrast to humans, except for the form
of the thyroid cartilage in ceboids. The laryngeal framework in hominoids is well separated from and assured of mobility independent
of the hyoid. In cercopithecoids and ceboids, it is, by contrast, locked into and tied tightly with the hyoid so that the
hyo-laryngeal complex acts as a functional unit. This spatial configuration is considered to be significantly related to the
mechanism that prevents aspiration, including epiglottic movement and vestibular closure. Non-human hominoids are inferred
to share the mechanism with human adults, not with cercopithecoids and ceboids, although their larynx is located as high as
the latter. Consequently, it is hypothesized that the descent of the larynx evolved in two steps. The first step would have
been descent of the thyroid in relation to the hyoid for the evolution of the mechanism preventing aspiration, which occurred
in the common ancestor of hominoids. The second step, descent of the hyoid within the neck, occurred during hominid evolution
for human speech.
Electronic Publication 相似文献
406.
Eric Burkholder 《Inorganica chimica acta》2005,358(1):116-122
The hydrothermal reactions of MoO3, tetra-2-pyridylpyrazine (tpyprz) and M(CH3CO2)2 · 2H2O (M = Co, Ni) yielded the two-dimensional oxides [M2(tpyprz)(H2O)2Mo8O26] · xH2O [M = Co, x = 1.8 (1); M = Ni, x = 0.6 (2)]. However, the reaction of (NH4)6Mo7O24 · 4H2O, tpyprz and Cu(CH3CO2)2 · H2O produced [{Cu2(tpyprz)}2Mo8O26] · 2H2O (3 · 2H2O). The isomorphous structures of 1 and 2 are constructed from clusters linked through {M2(tpyprz)(H2O)2}4+ subunits into two-dimensional networks. While the structure of 3 is also two-dimensional, the molybdate building block is present as the δ-isomer and the secondary-metal/ligand component consists of a one-dimensional chain. The structure of 3 is compared to that of the previously reported three-dimensional material [{Cu2(tpyprz)}2Mo8O26] · 7H2O which contains clusters and structurally distinct chains. 相似文献
407.
Gradient perception describes the process by which information about the chemoattractant concentration field surrounding a cell is transformed into an internal signal which is responsible for directed cell motion. Recently, many important biochemical details in immobilized and mobile Dictyostelium cells have been uncovered regarding the roles of enzymes regulating phosphoinositide lipids on the cell membrane which are responsible for gradient perception. We report on a modeling framework that describes the relationship between the membrane concentration of the primary 3'phosphoinositide lipids and the enzymes which regulate them. The model takes the form of partial differential equations describing the membrane concentration of these lipids. Working within this framework, we describe mechanisms which can be responsible for spatial amplification of these lipids and which do not employ lipid-enzyme feedback. An analysis of a basic module underlying this process is also performed. 相似文献
408.
Mohammad R. Hoque Xavier Gabarrell i Durany Cristina Sendra i Sala Gara Villalba Méndez Laura Talens Peiró Teresa Vicent Huguet 《Journal of Industrial Ecology》2012,16(5):699-709
We used a thermodynamic framework to characterize the resource consumption of the construction sector in 2001 in Catalonia, the northeast region of Spain. The analysis was done with a cradle‐to‐product life cycle approach using material flow analysis (MFA) and exergy accounting methodologies to quantify the total material and energy inputs in the sector. The aim was to identify the limitations of resource metabolism in the sector and to pinpoint the opportunities for improved material selection criteria, processing, reuse, and recycling for sustainable resource use. The results obtained from MFA showed that nonrenewables such as minerals and natural rocks, cement and derivatives, ceramics, glass, metals, plastics, paints and other chemicals, electric and lighting products, and bituminous mix products accounted for more than 98% of the input materials in the construction sector. The exergy analysis quantified a total 113.1 petajoules (PJ) of exergy inputs in the sector; utilities accounted for 57% of this exergy. Besides exergy inputs, a total of 6.85 million metric tons of construction and demolition waste was generated in 2001. With a recycling rate of 6.5%, the sector recovered 1.3 PJ of exergy. If the sector were able to recycle 80% of construction and demolition waste, then exergy recovery would be 10.3 PJ. Hence the results of this analysis indicate that improvements are required in manufacturing processes and recycling activities, especially of energy‐intensive materials, in order to reduce the inputs of utilities and the extraction of primary materials from the environment. 相似文献
409.
Anazawa M 《Theoretical population biology》2012,81(2):158-167
This paper provides first-principles derivations of population models for competition involving multiple resources with different competition types, based on resource partitioning between individuals. The following two cases are investigated. The first is the case in which the resource competed for and its competition type change depending on life stages from scramble to contest competition, or from contest to scramble competition. The second is the case in which individuals compete for two resources simultaneously with scramble and contest types, respectively. In both cases, population models are derived analytically, and in particular, the Hassell model is derived in the second case. The nature of reproduction curves and the stability properties of three population models derived are compared with each other. These models provide three representative models for competition involving both scramble and contest types. 相似文献
410.
Shale gas fracturing is a complex system of continuous operation. If human errors occur, it will cause a chain reaction, from abnormal events to fracturing accidents, and even lead to abandonment of shale gas wells. The process of shale gas fracturing has many production stages that are complex and the consequence of any error is serious. The human error modes in shale gas fracturing process are mutative. Therefore, human error should be studied in a systematic way, and in a hybrid framework, that is, whole integration of identification, prioritization, reasoning, and control. A new structured identification method of human error in a hybrid framework for shale gas fracturing operation is presented in the article. First, human error is structurally identified based on the human HAZOP method. Second, fuzzy VIKOR method is applied to comprehensive prioritization. Finally, 4M element theory is used to analyze the human error and control its evolution. The method improves the consistency of the identification results through the standard identification step and the identification criterion. Results from a study of feed-flow process indicate that 34 kinds of human errors can be identified, and high-probability errors occur in the behavior of implementation and observation. 相似文献