全文获取类型
收费全文 | 640篇 |
免费 | 62篇 |
国内免费 | 177篇 |
出版年
2024年 | 1篇 |
2023年 | 24篇 |
2022年 | 20篇 |
2021年 | 33篇 |
2020年 | 24篇 |
2019年 | 39篇 |
2018年 | 25篇 |
2017年 | 26篇 |
2016年 | 20篇 |
2015年 | 31篇 |
2014年 | 38篇 |
2013年 | 58篇 |
2012年 | 39篇 |
2011年 | 60篇 |
2010年 | 39篇 |
2009年 | 49篇 |
2008年 | 33篇 |
2007年 | 40篇 |
2006年 | 43篇 |
2005年 | 33篇 |
2004年 | 34篇 |
2003年 | 19篇 |
2002年 | 17篇 |
2001年 | 22篇 |
2000年 | 10篇 |
1999年 | 12篇 |
1998年 | 9篇 |
1997年 | 10篇 |
1996年 | 7篇 |
1995年 | 5篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 6篇 |
1988年 | 2篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
排序方式: 共有879条查询结果,搜索用时 15 毫秒
1.
辽宁省菜田生态系统组分优势值与系统优势度分析 总被引:2,自引:1,他引:1
辽宁省菜田生态系统组分优势值与系统优势度分析周宝利,葛晓光李宁义(沈阳农业大学园艺系,110161)(沈阳市城建中专,110013)AnalysisonSuperiorityValueandDominanceDegreeofVariousCompon... 相似文献
2.
Stefano Carenini Dirk Montag Harold Cremer Melitta Schachner Rudolf Martini 《Cell and tissue research》1996,287(1):3-9
We have previously shown that mice deficient in the gene for the myelin-associated glycoprotein (MAG) develop normal myelin
in the peripheral nerves, but show axon and myelin degeneration at eight months of age, suggesting that MAG is involved in
the maintenance of axon-Schwann cell integrity. The search for molecules that might replace MAG during myelination revealed
an overexpression of the neural cell adhesion molecule (N-CAM) at those aspects where MAG is detectable in wild type mice.
To test whether N-CAM might compensate for MAG during myelination in MAG-deficient mice, double mutants deficient in both
MAG and N-CAM (MAG−/N-CAM−mice) were generated by cross-breeding the single mutants. Whereas alterations of myelin development were not detectable in
either of the single or double mutants, degeneration of myelin and axons occurred approximately 4 weeks earlier in MAG−/N-CAM−than in MAG−mutants. Furthermore, at 8 weeks of age, single fiber preparation and electron microscopy revealed that the number of profiles
indicative of degeneration was substantially increased in MAG−/N-CAM−mutants when compared to MAG−mice. These data suggest that in MAG-deficient mice N-CAM does not compensate for MAG in myelin formation but partially substitutes
for it in the maintenance of axon-myelin integrity.
Received: 20 May 1996 / Accepted: 19 July 1996 相似文献
3.
4.
Ecological processes that can realistically account for network architectures are central to our understanding of how species assemble and function in ecosystems. Consumer species are constantly selecting and adjusting which resource species are to be exploited in an antagonistic network. Here we incorporate a hybrid behavioural rule of adaptive interaction switching and random drift into a bipartite network model. Predictions are insensitive to the model parameters and the initial network structures, and agree extremely well with the observed levels of modularity, nestedness and node-degree distributions for 61 real networks. Evolutionary and community assemblage histories only indirectly affect network structure by defining the size and complexity of ecological networks, whereas adaptive interaction switching and random drift carve out the details of network architecture at the faster ecological time scale. The hybrid behavioural rule of both adaptation and drift could well be the key processes for structure emergence in real ecological networks. 相似文献
5.
Dr. Hans-Hermann Schmidt 《Archives Of Phytopathology And Plant Protection》2013,46(4):321-322
Information on the structure of the population of Phomopsis species in Ukraine, that describes the structure of Phomopsis spp. cultivated on medium: the morfological characteristics, properties of pathogenicity and phytotoxicity. It is described the method of evaluation and selection of unsusceptible forms of sunflower to phomopsis. It includes the optimal infection level, the forms of an inoculum that is effective and optimal way for artificial infection. 相似文献
6.
基于耦合协调度的黄土高原地区NDVI与降水关系的变异诊断 总被引:2,自引:0,他引:2
植被是地表生态系统的重要"指示器",在能量交换、水循环、碳循环、生物地球化学循环和维持中发挥着重要作用,降水是影响植被变化的主要气候因子,研究两者之间的作用关系具有重要的意义和价值。利用Mann-Kendall趋势检验法和Hust指数分析了黄土高原地区归一化植被指数(NDVI)的变化趋势,使用相对发展率(RDR)指数和重心转移模型分析了NDVI变化的时空差异,并构建了基于耦合协调度理论和Pettitt检验方法的NDVI与降水关系的变异诊断方法,识别了黄土高原地区NDVI与降水关系的突变点,探讨了降水对NDVI变化的影响以及造成NDVI与降水关系变化的原因。结果表明:(1)黄土高原地区73.49%面积的NDVI在1998-2017年有呈现显著增加趋势(P<0.05),大部分地区NDVI在未来依旧呈现增加趋势;(2)黄土高原地区丘陵沟壑区与高原沟壑区的NDVI增加幅度大于黄土高原地区整体的增加幅度,而北部风沙区和农灌区以及黄土高原地区边界区域的NDVI增加滞后于区域整体变化;(3) NDVI与降水耦合协调程度逐年增强,两者关系在2006年发生显著突变(P<0.05);(4) NDVI呈现显著增加区域降水明显高于不显著变化区域(P<0.05),降水对NDVI变化存在一定影响,在丘陵沟壑区、高原沟壑区北部和东部河谷及土石山区北部NDVI和降水存在显著正相关关系(P<0.05),然而黄土高原地区大部分区域的降水并不存在显著变化趋势(P>0.05),因此造成黄土高原地区NDVI与降水关系在2006年发生显著突变的主要原因应该是人类活动(P<0.05)。研究成果有助于进一步理解黄土高原植被变化与降水的相互作用,为黄土高原生态建设和水土流失治理提供一定的科学支撑。 相似文献
7.
8.
Cheon-Soon Bae Sung-Sang Hong Sang-Jeom Ahn Yang-Suk Jang Byung-Ki Hur 《Biotechnology and Bioprocess Engineering》2005,10(3):230-235
For the acellular pertussis vaccine with a high immunogenicity, the concentration, composition and characteristics of acellular
pertussis antigens are the crucial points to be considered. Nevertheless, it has not been proved yet whether or not the polymerization
degree, one of the characteristics of formalin-detoxified acellular pertussis antigens, has an influence on vaccine potency.
Thus, in the present study, the correlations among detoxification conditions of acellular pertussis bulks, their polymerization
degrees and their immunogenicities were examined. In addition, the relative importance of pertussis toxoid in vaccine immunogenicity
was also investigated. Results show that a lower lysine concentration during detoxification induces highly-polymerized antigens,
the immunogenicity has a great dependency on the polymerization degree of antigens, and also pertussis toxoid has a relatively
stronger influence on the immunogenicity than other antigens. Accordingly, in the aspect of the potency of detoxified acellular
pertussis vaccine, it can be demonstrated that the polymerization of antigens and its degree are the major factors affecting
the immunogenicity along with a relatively high content of pertussis toxoid. 相似文献
9.
M. Abou Hachem S. Bozonnet M. Willemoë s B. Kramh ft K. Fukuda B. C. B nsager M. T. Jensen J. N hr S. Tranier N. Juge X. Robert R. Haser N. Aghajari B. Svensson 《Biocatalysis and Biotransformation》2006,24(1):83-93
-Amylases are endo-acting retaining enzymes of glycoside hydrolase family 13 with a catalytic (β/)8-domain containing an inserted loop referred to as domain B and a C-terminal anti-parallel β-sheet termed domain C. New insights integrate the roles of Ca2 + , different substrates, and proteinaceous inhibitors for -amylases. Isozyme specific effects of Ca2 + on the 80% sequence identical barley -amylases AMY1 and AMY2 are not obvious from the two crystal structures, containing three superimposable Ca2 + with identical ligands. A fully hydrated fourth Ca2 + at the interface of the AMY2/barley -amylase/subtilisin inhibitor (BASI) complex interacts with catalytic groups in AMY2, and Ca2 + occupies an identical position in AMY1 with thiomaltotetraose bound at two surface sites. EDTA-treatment, DSC, and activity assays indicate that AMY1 has the highest affinity for Ca2 + . Subsite mapping has revealed that AMY1 has ten functional subsites which can be modified by means protein engineering to modulate the substrate specificity. Other mutational analyses show that surface carbohydrate binding sites are critical for interaction with polysaccharides. The conserved Tyr380 in the newly discovered 'sugar tongs' site in domain C of AMY1 is thus critical for binding to starch granules. Furthermore, mutations of binding sites mostly reduced the degree of multiple attack in amylose hydrolysis. AMY1 has higher substrate affinity than AMY2, but isozyme chimeras with AMY2 domain C and other regions from AMY1 have higher substrate affinity than both parent isozymes. The latest revelations addressing various structural and functional aspects that govern the mode of action of barley -amylases are reported in this review. 相似文献
10.
While aggregating the throughput of existing disks on cluster nodes is a cost-effective approach to alleviate the I/O bottleneck
in cluster computing, this approach suffers from potential performance degradations due to contentions for shared resources
on the same node between storage data processing and user task computation. This paper proposes to judiciously utilize the
storage redundancy in the form of mirroring existed in a RAID-10 style file system to alleviate this performance degradation.
More specifically, a heuristic scheduling algorithm is developed, motivated from the observations of a simple cluster configuration,
to spatially schedule write operations on the nodes with less load among each mirroring pair. The duplication of modified
data to the mirroring nodes is performed asynchronously in the background. The read performance is improved by two techniques:
doubling the degree of parallelism and hot-spot skipping. A synthetic benchmark is used to evaluate these algorithms in a
real cluster environment and the proposed algorithms are shown to be very effective in performance enhancement.
Yifeng Zhu received his B.Sc. degree in Electrical Engineering in 1998 from Huazhong University of Science and Technology, Wuhan, China;
the M.S. and Ph.D. degree in Computer Science from University of Nebraska – Lincoln in 2002 and 2005 respectively. He is an
assistant professor in the Electrical and Computer Engineering department at University of Maine. His main research interests
are cluster computing, grid computing, computer architecture and systems, and parallel I/O storage systems. Dr. Zhu is a Member
of ACM, IEEE, the IEEE Computer Society, and the Francis Crowe Society.
Hong Jiang received the B.Sc. degree in Computer Engineering in 1982 from Huazhong University of Science and Technology, Wuhan, China;
the M.A.Sc. degree in Computer Engineering in 1987 from the University of Toronto, Toronto, Canada; and the PhD degree in
Computer Science in 1991 from the Texas A&M University, College Station, Texas, USA. Since August 1991 he has been at the
University of Nebraska-Lincoln, Lincoln, Nebraska, USA, where he is Professor and Vice Chair in the Department of Computer
Science and Engineering. His present research interests are computer architecture, parallel/distributed computing, cluster
and Grid computing, computer storage systems and parallel I/O, performance evaluation, real-time systems, middleware, and
distributed systems for distance education. He has over 100 publications in major journals and international Conferences in
these areas and his research has been supported by NSF, DOD and the State of Nebraska. Dr. Jiang is a Member of ACM, the IEEE
Computer Society, and the ACM SIGARCH.
Xiao Qin received the BS and MS degrees in computer science from Huazhong University of Science and Technology in 1992 and 1999, respectively.
He received the PhD degree in computer science from the University of Nebraska-Lincoln in 2004. Currently, he is an assistant
professor in the department of computer science at the New Mexico Institute of Mining and Technology. He had served as a subject
area editor of IEEE Distributed System Online (2000–2001). His research interests are in parallel and distributed systems, storage systems, real-time computing, performance
evaluation, and fault-tolerance. He is a member of the IEEE.
Dan Feng received the Ph.D degree from Huazhong University of Science and Technology, Wuhan, China, in 1997. She is currently a professor
of School of Computer, Huazhong University of Science and Technology, Wuhan, China. She is the principal scientist of the
the National Grand Fundamental Research 973 Program of China “Research on the organization and key technologies of the Storage
System on the next generation Internet.” Her research interests include computer architecture, storage system, parallel I/O,
massive storage and performance evaluation.
David Swanson received a Ph.D. in physical (computational) chemistry at the University of Nebraska-Lincoln (UNL) in 1995, after which he
worked as an NSF-NATO postdoctoral fellow at the Technical University of Wroclaw, Poland, in 1996, and subsequently as a National
Research Council Research Associate at the Naval Research Laboratory in Washington, DC, from 1997–1998. In 1999 he returned
to UNL where he directs the Research Computing Facility and currently serves as an Assistant Research Professor in the Department
of Computer Science and Engineering. The Office of Naval Research, the National Science Foundation, and the State of Nebraska
have supported his research in areas such as large-scale scientific simulation and distributed systems. 相似文献